Critical Review of Carbon Conversion in âCarbon Fuel Cells

Review pubs.acs.org/CR

Critical Review of Carbon Conversion in “Carbon Fuel Cells” Turgut M. Gür* Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States Biography Acknowledgments References

1. INTRODUCTION Conversion of carbonaceous solid fuels in carbon fuel cells (CFC) especially those that operated at moderately elevated temperatures is of great interest as concerns about the need for efficient and sustainable energy technologies and clean environment grow in importance on the global agenda. Essentially, CFCs are adaptations of common fuel cells (see Table 1) that are specially configured for handling and effective utilization of solid fuels. More than 80% of the global primary energy demand is met by fossil fuels. Coal-fired power plants typically operate at 30− 35% conversion efficiency and spew out copious quantities of CO2 and other contaminants into the environment. Such plants typically employ air for combustion, which then necessitates post separation of CO2 from nitrogen to capture it. To ensure energy security in a carbon-constrained world, there is pending urgency in developing new technologies especially for coal conversion that offer not incremental but significantly higher conversion efficiencies than 30−35%. Despite low efficiency and high emissions, coal-based power technology is predicted by all credible accounts to continue to dominate electrical power generation into the foreseeable future.1−3 This is partly because coal is arguably the most abundant and cheap (per energy basis) fossil fuel on this planet. Coal is responsible for more than 45% of the electricity generated in the U.S. currently and possesses a significantly higher share in others especially in populated and rapidly growing countries such as China and India where it stands at 75%. The recent “Carbon Pollution Standard for New Power Plants” proposal4 released on March 27, 2012 by the U.S. Environmental Protection Agency aims to place stringent measures on CO2 emission limits for new coal-fired power plants and bring them on par with current emission levels of natural gas plants. This will imply that coal-fired power generation needs to lower its CO2 emissions by nearly 40%, which would require pushing net efficiencies of plants from the current 30−35% to significantly more than 50%, which may be attainable only by invoking fuel cells in the conversion process (see sections 1.1. and 7 below). It must be kept in mind that the energy requirement for carbon capture and storage when implemented will further reduce net conversion efficiencies of today’s coal-fired power plants by 5−8 percentage points to around 27−28%.

CONTENTS 1. Introduction 1.1. Why Convert Carbon in Fuels Cells? 1.2. Is “Direct” Electrochemical Conversion Achievable in CFCs? 2. Mechanistic Delivery Modes in CFCs 2.1. Oxygen Delivery Modes 2.2. Carbon Delivery Modes 2.2.1. Steam versus Dry Gasification 3. Pretreatment of Carbons for CFCs 4. Carbon Utilization and Conversion in CFCs 4.1. Molten Hydroxide-Based CFCs (MH−CFC) 4.2. Molten Carbonate-Based CFCs (MC−CFC) 4.3. Hybrid CFCs 4.4. Molten Metal Anode-based CFCs (MA-CFC) 4.5. Solid Oxide-Based CFCs (SO−CFC) 4.5.1. Direct-Contact Cells 4.5.2. Rechargeable Pyrolytic Carbon Cells 4.5.3. Detached Cells 4.5.4. Fluidized Bed Carbon Fuel Cells (FBCFC) 4.5.5. Thermochemical Considerations in the Presence of Solid Carbon 4.5.6. Electro-oxidation of CO 4.5.7. Steam-Carbon Cell 5. Technical Challenges 5.1. Melt Stability 5.2. Wetting Issues in Molten Medium 5.3. Percolation and Electrical Connectivity 5.4. Ash 5.5. Cell Degradation by Sulfur and Other Impurities 5.6. Coking or Carbon Deposition 6. Potential Impact of CFCs on U.S. CO2 Emissions 7. Economic Prospects 8. Concluding Remarks Author Information Corresponding Author Notes © XXXX American Chemical Society

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A B D D E E F G G H I J K N N P P Q R S S T T T U U V V W W X Y Y Y

Received: February 2, 2013

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dx.doi.org/10.1021/cr400072b | Chem. Rev. XXXX, XXX, XXX−XXX

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Table 1. Comparison of Common Fuel Cell Types fuel cell type

cell temp (°C)

typical electrolyte

mobile species

alkaline (AFC)

60

aqueous KOH

OH−

direct methanol (DMFC)

60−80

polymer exchange membrane (PEMFC) phosphoric acid (PAFC) molten carbonate (MCFC)

80 200 650−750

sulfonated polymer (Nafion) sulfonated polymer (Nafion) H3PO4 Li2CO3−K2CO3 eutectic

solid oxide (SOFC)

650−1000

8 mol % Y2O3 doped ZrO2 O2−

cathode reaction

anode reaction H2 + 2OH− = H2O + 2e−

H+

1/2O2 + H2O + 2e− = 2OH− 3/2O2 + 6H+ + 6e− = 3H2O

H+

1/2O2 + 2H+ + 2e− = H2O

H+ CO32−

1/2O2 + 2H+ + 2e− = H2O 1/2O2 + CO2 + 2e− = CO32− 1/2O2 + 2e− = O2−

CH3OH + H2O = CO2 + 6H+ + 6e− H2 = 2H+ + 2e− H2 = 2H+ + 2e− H2 + CO32− = H2O + CO2 + 2e− H2 + O2− = H2O + 2e−

issues and reviews recent advances made mostly in the past decade in this nascent area of research that promises environmentally friendly solutions to coal power generation. Attractive incentives as well as the origins of inherent difficulties in effectively utilizing and converting solid fuels in fuel cells are discussed, and various approaches adopted to overcome or mitigate these barriers are grouped systematically under organized strategies. A critical account of the strengths and shortcomings of CFCs are provided, and knowledge gaps are identified and discussed for further research. Also, alternative viewpoints and arguments are offered to explain or refute speculative claims of direct electrochemical oxidation made in the literature, and a set of criteria for a unified CFC terminology is proposed for consideration of the scientific community. Carbon conversion is a natural prelude to the inherently more complicated and technically more challenging task of coal conversion in CFCs. Accordingly, most studies to date have been limited to processed carbons, due to additional problems and complications otherwise posed by coal contaminants. This paper shall explicitly focus on conversion in fuel cells that operate at moderately high temperatures between 500 and 1000 °C. To keep the discussions general, the word “carbon”, unless stated explicitly, is used in this paper as an encompassing generic name in collectively describing various forms of carbon, and their major reactions and conversion processes.

Therefore, it is imperative to explore and develop advanced coal conversion technologies that offer a dramatic increase in conversion efficiency coupled with proportionately less CO2 emission. Carbon conversion is the natural first step toward gaining a fundamental understanding necessary to build the foundation of efficient and advanced fuel cells for coal conversion. The quest for carbon conversion in fuel cells is not new and has been pursued intermittently for nearly 150 years.5 Recent interest and research activity in this area is fueled in part by concerns over energy and environment but, more importantly, by the realization that CFCs potentially offer two critically important advantages, namely, significantly higher conversion efficiencies and concentrated CO2 product streams. High efficiencies and low emissions are imperative for sustainability. In this regard, CFCs, if developed successfully, promise significant opportunities for next generation coal power technology. However, carbon conversion in fuel cells possesses inherent difficulties and technical challenges. Much progress was made in the past decade in both understanding fundamental issues as well as improving cell performance due to advances made in fuel cells. The present review addresses these areas in detail, organizes fundamental concepts, provides an authoritative analysis of carbon conversion, and groups various approaches to overcome barriers to carbon conversion under distinct strategies. Until very recently, only one brief prior review of CFCs existed with a device level focus and narrow scope6 and another short book chapter focused primarily on molten carbonate based CFCs,7 both published in 2007. Two recent reviews8,9 coauthored by the same group of researchers from CSIRO in Australia are now available. One of these reviews8 is more heavily weighted on describing the compositions, properties, pretreatment methods, and analysis of various coals and carbons for fuel cell applications, including the analytical methods and tools that have been utilized for their characterization. The second article9 from the same CSIRO group provides an extensive technology status review of both the common types of fuel cell systems that typically operate on gaseous fuels such as hydrogen and the “direct carbon fuel cells”. It also provides performance results from the literature and presents the status as well as commercial development efforts in CFC technology. The present article expands beyond these reviews and is focused on the fundamental concepts, introduces organizing themes to accomplish carbon conversion, and offers an authoritative assessment of literature results presented in interfacial groupings. It discusses carbon conversion in CFCs from the point of view of mechanistic strategies and materials

1.1. Why Convert Carbon in Fuels Cells?

Fuel cells are electrochemical devices that convert the chemical energy stored in the bonds of fuels into electrical energy. Electrochemical conversion offers inherently higher efficiency than is possible by chemical conversion into electrical energy. During power generation, coal is typically combusted in air or gasified by steam where irreversible mixing of the reactants and products is followed by a series of individual process steps to eventually generate electricity albeit at low efficiency. Such power plants commonly operate between temperature and pressure limits that generally dictate their efficiency, the theoretical maximum generally defined by the Carnot constraint. By contrast, electrochemical oxidation of carbon inside a CFC occurs isothermally and isobarically, where the reactant and product streams are kept separate. This further eliminates entropic losses due to mixing, and increases efficiency. Accordingly, the theoretical efficiency, Πth, for electrochemical conversion is defined by thermodynamic state functions as Π th = ΔG /ΔH = {1 − (T ΔS /ΔH )} B

(1)


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where T is the isothermal conversion temperature and ΔG, ΔH, and ΔS denote changes in Gibbs energy, enthalpy, and entropy, respectively. The Gibbs energy represents the maximum available work potential one can extract under constant temperature and pressure conditions using a reversible process, and is related to the equilibrium (Nernst) cell potential, E, by the simple relation E = −ΔG /nF

In the case of the full oxidation reaction (5), the values for Gibbs energy change are −394.4 kJ/mol at 300 K and −396.1 kJ/mol at 1200 K. The corresponding values for the entropy change within the practical fuel cell operating temperatures remain quite small (e.g., 2.88 J/K mol at 300 K and 0.58 J/K mol at 1200 K). This has important implications. The equilibrium cell potential of 1.02 V vs air, and the electrical cell conversion efficiency (Πcell) are both independent of temperature for all practical purposes. Most importantly, the small value for the entropy change results in comparable values of Gibbs energy and enthalpy changes in eq 1, yielding a conversion efficiency of unity (i.e., 100%). This high ceiling value of 100% for efficiency theoretically available at useful operating temperatures provides a major advantage for electrochemical conversion of carbon over chemical processes, although it is usually difficult to achieve direct conversion. Naturally during actual fuel cell operation, values for operating cell voltages and conversion efficiencies are expected to be lower than theoretical predictions. Practical systems invariably exhibit irreversible losses due to finite rates for electrochemical reactions, heat and mass transport, current collection and distribution in the cells and stacks, and other system losses. Common types of fuel cells operate mostly on gaseous fuels, such as hydrogen, where the extent of fuel conversion (or, utilization) can be determined by analysis of incoming fuel and outgoing anode gaseous products. The electrical conversion efficiency for these cells is typically given by,

(2)

where n is the number of electrons involved in the charge transfer reaction and F is Faraday’s constant. Provided that equilibrium conditions prevail at both electrodes, E is equal to the open circuit voltage (OCV) of the cell. Fuel cells that are specially modified or adapted to operate on solid fuels are commonly referred to in the literature as direct carbon fuel cells (DCFC). For reasons discussed in the next section, the author proposes the more appropriate name, carbon fuel cells (CFC). An idealized CFC schematically depicted in Figure 1, consists of an ionically transporting but

Πcell = (Π th)(Π voltage)(Πcurrent)(Π fuel)

The terms in parentheses denote respectively, the theoretical efficiency as defined by eq 1, voltage efficiency accounting for cell polarization losses, current (or Coulombic) efficiency taking into account losses due to parasitic reactions at the electrodes, and finally the efficiency of fuel utilization. Unfortunately, this definition for conventional fuel cells requires modification due to the presence of the solid carbon fuel in the CFC proper, where carbon activity remains unity during the oxidation reaction irrespective of the extent of conversion. In contrast, the activity (or, the concentration) of gaseous fuels in conventional fuel cells changes with extent of conversion. Accordingly, a general definition10 of electrical cell efficiency, Πcell, for CFCs is given by

Figure 1. Generic illustration of an idealized carbon fuel cell (CFC), where solid fuel is electrochemically converted to electrical power in a single process step while producing only CO2. The schematic also depicts the desired half-cell reactions.

electronically insulating solid, liquid or molten electrolyte (or, membrane). The solid fuel is housed inside the anode compartment where electrochemical oxidation is achieved by reaction of carbon with the oxide ions supplied through the electrolyte. The idealized net anode and cathode reactions for CFC are, respectively 2−

C (s) + 2O

Πcell = Πout /mchar HHVchar

(3) 2−

O2 (g) + 4e (electrode) = 2O (electrolyte)

(4)

The letters/words in parentheses denote either the state of the species or the phase in which the species resides, as applicable. The overall net reaction is C(s) + O2 (g) = CO2 (g)

(5)

Although the desired anode reaction is clearly eq 3, the mechanistic origins of the anode reactions are more complicated regardless of the fuel cell arrangement employed by different CFC approaches. It is likely in most cases that the anode reaction also involves partial oxidation, namely, C(s) + O2 −(electrolyte) = CO(g) + 2e−(electrode)

(8)

where Πout is the power output of the cell, mchar is the mass consumption rate of char, and HHVchar is its higher heating value. In most CFCs, however, experimental difficulties do not permit accurate determination of the amount of carbon (or, char) consumed. This is especially true for CFCs that employ a molten medium either in the form of a molten metal anode or a molten electrolyte, which makes it difficult to separate out the solid fuel and determine weight loss in a reliable manner. Moreover, full oxidation of carbon to the desired product CO2 may not always be realized. Not only partial oxidation of carbon to CO but also the Boudouard reaction may independently occur in the cell. Hence, the anode product gases generally contain not only CO2 but also CO depending on the type of CFC arrangement and its operating conditions. Naturally, for high cell efficiency the anode product gas is desired to be primarily CO2. Alexander et al.11 proposed a generalized expression for cell electrical efficiency that takes into account the product

−

(electrolyte) = CO2 (g) + 4e (electrode)

−

(7)

(6) C


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distribution in the anode exhaust. The mass consumption rate of char is expressed in experimentally accessible and measurable quantities of electrical current, I, through the cell, and the respective molar fractions of anode product gases such that

Also, the equilibrium cell potential of 1.02 V is expected to be practically independent of temperature over a wide range, in accordance with complete oxidation reaction (5) for carbon. In reality, however, no CFC study has provided experimental validation of this basic premise over a sufficiently wide temperature regime. For one, many CFC studies indicated open circuit potentials that are significantly different from the theoretically expected value of 1.02 V, suggesting that the cell potential is established primarily by the local equilibrium prevailing at the anode different from that corresponding to reaction 3. Most studies also observed strong temperature dependence of the cell potential. Several studies on carbon conversion in molten carbonate fuel cell arrangements reported open circuit potentials that sometimes range up to 1.4 V,15,16 suggesting that interactions involving the alkali metal in the melt contribute to the observed cell potential. Irvine and coworkers suggested15,17 the mediating roles of the CO32‑/O2‑ and CO2/CO pairs to account for the high OCVs up to 1.4 V reported in these studies. Even in studies claiming direct electrochemical oxidation, open circuit potentials of 0.3 and 0.7 V at cell temperatures between 900 and 1000 °C,18 and 0.83 and 0.65 V at 600 and 800 °C, respectively,19 were reported. These OCV values are clearly far from the expected potential of 1.02 V and also show strong temperature dependence contrary to expectations. As suggested earlier by Gür,13,14,20 the use of the term “direct” in CFC may only be appropriate if it is intended to refer to or indicate that overall conversion is accomplished in a single process chamber but carries no mechanistic implications. This argument was also in line with the suggestion made earlier by McIntosh and Gorte21 regarding “direct utilization” of gaseous and liquid fuels in fuel cells. Alternatively, the author suggests a less ambiguous and less contentious, but more encompassing generic name “carbon fuel cell” (CFC) to describe this family of fuel cells. CFC conveys no mechanistic inference and adequately describes the wide variety of CFC approaches employing various configurations and types of electrolytes and anodes that are adopted for efficient oxidation of carbon, while producing a flue gas stream highly concentrated in CO2.

(xc,charmchar /Mc)[2(xCO + 2xCO2)/4(xCO + xCO2)] = I /2F (9)

This equation allows the determination of the char consumption rate or, in other words, the rate at which char needs to be replenished or supplied to a cell in continuous operation. Here, xc,char denotes the fraction of carbon in the char, Mc is the molecular weight of carbon, xCO and xCO2 denote the molar fractions of CO and CO2 in the anode product gases respectively, and F is the Faraday constant. The only source for oxygen in CFCs is oxygen supplied by transport through the electrolyte. Hence, the numerator in the ratio inside the brackets represents the number of electrons corresponding to oxygen atoms carried by CO and CO2 in the product gases, while the denominator gives the total number of electrons expected to result from each carbon atom that is consumed in the solid fuel. For clarity reasons, the corresponding stoichiometric coefficients are left inside the equation without further simplification. Ideally, high conversion efficiency is desired at practically useful cell performance values. Although high conversion efficiency of 80% was previously estimated,7 practically high values for cell performance obtained at such high efficiencies have not been experimentally demonstrated yet. Recent modeling work has in fact indicated a trade off between performance and efficiency as expected.11 1.2. Is “Direct” Electrochemical Conversion Achievable in CFCs?

For historical reasons, the name “direct” in “direct carbon fuel cells” has been used widely in this topical area.6 However, this terminology may lead to misunderstanding if not used in the proper context. In most cases, “direct” electrochemical reaction of solid carbon by the “native” oxide ion (i.e., O2‑) in accordance with idealized reactions 3 and 4 is difficult to achieve in the true mechanistic sense of elementary reactions. In fact in practice, either the oxygen is supplied as a “nonnative” ion in the form of OH−, CO32‑, or a metal oxide, or alternatively, the carbon is provided as gaseous fuel such as CO and H2 to the electrochemical reaction site in order to achieve conversion in CFCs. Moreover, the desired anode reaction (3) involves the transfer of four electrons for every carbon atom, and most likely does not proceed in a single elementary reaction step, but instead involves multistep transfer of electrons in a sequence of elementary reactions. Likewise, it was suggested that direct electrochemical oxidation of even the simplest hydrocarbon (methane) in solid oxide fuel cells occurs in multiple reaction steps that likely involve oxidation of cracked species such as carbon. 12 The primary challenge in achieving carbon conversion in CFCs is how to effectively bring together the two reactants of reaction 5, namely, the solid carbon fuel particles and the gaseous oxygen in intimate contact at the electrochemical reaction site (ERS). In fact, almost all CFC efforts seek to design and devise the most effective strategy to deliver either the solid fuel or the gaseous oxygen in an appropriate chemical form to the electrolyte/electrode interface where electrochemical oxidation can take place.13,14

2. MECHANISTIC DELIVERY MODES IN CFCS In electrochemical systems, ERSs are necessarily restricted to the electrode/electrolyte interface, and this constraint presents major challenges for solid carbon conversion. Generally, ERSs are not readily and widely accessible to either the carbon particles or oxygen. Accordingly, all CFC configurations aim to overcome the physical barriers to electrochemical oxidation by providing effective delivery mechanisms to bring either the oxygen or carbon together at ERS in a transformed chemical vehicle.13,14 This a priori transformation of either the oxygen or the carbon to a suitable chemical delivery vehicle necessarily points to an indirect process that further questions the validity of “direct” in direct carbon fuel cell. The author organizes the delivery modes devised for different CFC arrangements under two distinct strategies based on specific mechanistic routes. This is illustrated in Table 2. The first strategy involves transforming oxygen into a chemical vehicle in the form of an oxygen donor, which delivers it in a suitable ion to the carbon particle at the ERS. Approaches adopted in this group place the ERS at the solid/liquid interface between the solid carbon fuel and the molten medium, either in the form of electrolyte or metal, in order to ensure intimate D


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Table 2. Summary of Two Delivery Strategies to Facilitate and Achieve Carbon Conversion in High Temperature Fuel Cellsa chemical form of fuel solid carbon solid carbon solid carbon (HC + Δ =) C (C + H2O =) CO + H2 (C + CO2 =) 2CO

(pyrolysis) (steam gasif ication) (dry gasif ication)

chemical form of oxygen CO32− OH− MOx

(in MC-CFC) (in MH-CFC) (in MA-CFC)

O2− O2− O2−

(in SO-CFC) (in SO-CFC) (in SO-CFC)

HC denotes hydrocarbon, and Δ is the thermal energy necessary to pyrolyze the hydrocarbon.

a

contact. The second strategy, by contrast, involves delivering carbon in a transformed chemical vehicle, usually in a gasified molecular form, to oxygen at the ERS. This group of approaches that usually employ a solid oxide electrolyte in a modified solid oxide fuel cell (SOFC) arrangement places the ERS at the gas/solid interface between the solid oxide electrolyte and the gasified carbon fuel, usually in the form of CO and/or H2, which can gain ready access to the triple phase boundary (TPB) via gas diffusion. To gain a better understanding of the mechanistic details of the half-cell reactions and their elementary steps in CFCs, detailed gas analysis and spectroscopic studies are needed to support and supplement electrochemical experiments, in a manner similar to the successful employment of FTIR, Raman, and X-ray photoelectron spectroscopy (XPS) for in situ characterization of electrode behavior and surface reactions at elevated temperatures in solid oxide fuel cells (SOFC) utilizing gaseous fuels.22−24 Also, it is helpful to bring in advanced computation and simulation tools25,26 to bear on studies of surface and interfacial reaction kinetics to guide experimental work in CFCs. Use of such tools have provided gainful insights regarding relative energetics for reaction pathways during oxidation of gaseous fuels in SOFCs27 as well as guiding experimentation and catalytic materials development.28

Figure 2. Schematic illustration depicting the delivery of oxygen to ERS in molten hydroxide or carbonate electrolyte with dispersed carbon fuel. Participation in the electro-oxidation reaction is limited only to the ERSs at the interface between the molten electrolyte and the electrically connected carbon particles, while unconnected carbons remain inactive. The yellow bubbles depict the evolving oxidation product CO2 gas, which can potentially be disruptive to cell operation and current stability if not controlled properly. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

carbonate (CO32−) or a hydroxyl (OH−) ion that is compatible with the type of molten electrolyte used, or as an oxide (O2−) ion in the form of a metal oxide. The oxygen donor ion supplied to ERS residing at the carbon/molten medium interface reacts with the carbon, provided that electrical connectivity (i.e., electron pathway) is properly maintained at all times. Naturally, connectivity (or percolation) of the carbon particles is much less of an issue in the case of sacrificial bulk carbon anode than for particulate carbon dispersed in the molten medium.

2.1. Oxygen Delivery Modes

2.2. Carbon Delivery Modes

In the case of CFC arrangements based on molten electrolytes, ERS may not be readily accessible either to oxygen or to electrons. In the first case, oxygen from air may be chemically incompatible with the electrolyte, or may have only limited solubility in the electrolyte. Similarly, electrons may have difficulty in accessing ERS because (i) the electrolyte is necessarily insulating and cannot provide a conduction pathway for electrons, (ii) there may be lack of sufficient connectivity or percolation among carbon particles, (iii) carbon particles may have poor electronic conductivity, (iv) there may be gradual separation and loss of electrical contact among carbon particles in the electrolyte as carbon mass is depleted via oxidation, or (v) there is insufficient wetting of carbon particles by the electrolyte. This is schematically illustrated in Figure 2, where only those interfacial sites fully accessible to electrons, carbon particles, and the electrolyte simultaneously through proper connectivity and wetting do actually serve as active ERSs, where electro-oxidation can be accomplished. CFCs that adopt the first strategy generally employ a molten medium in direct contact with carbon either in bulk form (i.e., as consumable anode) or as dispersed particles inside the molten medium. In these arrangements, oxygen from air is reduced at the cathode into a suitable oxygen donor such as a

In the context of solid oxide fuel cells (SOFC), ERS and TPB are synonymous and denote the same sites where charge transfer reaction takes place. Even for ultrafine carbon particles less than 1 μm in size, accessibility to ERS is necessarily limited. This is illustrated in Figure 3, which depicts a solid electrolyte membrane with porous electrodes impeding direct access of the solid fuel particle to the ERS. So this second strategy involves conversion of carbon particles into a convenient chemical delivery vehicle in the form of a gaseous fuel that can readily be supplied to the ERS by diffusion. The advantage of this approach, which is commonly employed in the context of solid oxide electrolytes, is to circumvent the slow kinetics for the electro-oxidation of solid carbon, to gain ready access to the ERS (i.e., TPB), and to avoid the constrained electronic pathway between the TPB at anode/electrolyte interface and the solid fuel particle. This approach is the most common strategy employed for CFCs based on SOFC arrangements, where solid-to-solid contact at the TPB is necessarily poor. It is accomplished typically by steam gasification of the solid fuel to produce syngas (i.e., a mixture of H2 and CO), which can be cleaned up to remove contaminants if necessary. The syngas generally undergoes a E


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C(s) + 2H 2O(g) = 2H 2(g) + CO2 (g)

(12)

Alternatively, gasification of solid carbon can be achieved without the use of steam, i.e., via dry gasification43,45 using the Boudouard reaction C(s) + CO2 (g) = 2CO(g)

The enthalpy change in (13) is slightly more endothermic than in (12), with a value of +169 kJ/mol at 1100 K. Moreover, Gibbs free energy change indicates that reaction 13 is thermodynamically favored above 1000K. In the presence of solid carbon, the respective partial pressures for CO and CO2 for a total pressure of 1 atm can be calculated49 from the Boudouard equilibrium (13) as a function of temperature. The corresponding values provided in Table 3 below are in excellent agreement with others.50

Figure 3. Schematic illustration depicting delivery of the fuel to ERS (TPB) on solid oxide based fuel cell via gasification of carbon by either steam or carbon dioxide. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

Table 3. Temperature Dependence of Boudouard Constant KB and the Respective Equilibrium Partial Pressures for CO2 and CO at 1 atm Total Pressure, Calculated from Thermochemical Data49

catalytic water−gas shift reaction in case H2 is the desired fuel to supply to the fuel cell. This particular approach called “integrated gasification fuel cell”, or IGFC, is currently being developed by the U.S. Department of Energy (DOE) under the Solid State Energy Conversion Alliance (SECA)29 program that is currently testing 25k W SOFC stacks for power generation. Alternatively, Gür and co-workers30−32 have proposed dry gasification for this purpose and successfully employed CO2 to gasify carbon,13,14,20,33−37 coal,20,38 and biomass10,20,39 to produce CO, which is then supplied to TPB at the fuel cell anode for electrochemical oxidation. 2.2.1. Steam versus Dry Gasification. There is extensive literature on oxyfuel40 and air combustion41,42 and gasification of carbon and coal.43−46 Recently, steam gasification using oxygen was adopted by advanced clean coal technology programs, which also aim at capture-ready CO2 production in order to avoid the otherwise expensive and energy intensive postseparation of nitrogen. DOE’s SECA29 and FutureGen47 in the U.S.A and GreenGen48 in China, are among the larger and more visible programs. Steam gasification is employed to generate H2 and CO, or syngas, via the reaction C(s) + H 2O(g) = H 2(g) + CO(g)

T (K)

KB (atm)

PCO2 (atm)

PCO (atm)

800 1000 1200

0.01 1.76 53.75

0.91 0.29 0.02

0.09 0.71 0.98

As expected, CO2 formation is thermodynamically favored at low temperatures while CO is the predominant species at elevated temperatures. Nevertheless, there is under equilibrium conditions nearly 9% CO even at temperatures as low as 800 K. It should be noted here that this is of particular interest to the discussion on “claims of direct electrochemical conversion” presented later in section 4.5.1.1. In any case, there is clear thermodynamic incentive to perform dry gasification at elevated temperatures in order to achieve fast equilibration and maximum conversion of carbon to CO for subsequent utilization at the fuel cell anode. The two fuels regardless of the type of gasification, when electrochemically oxidized at the anode TPB of the SOFC will undergo the corresponding reactions 2H 2(g) + 2O2 −(electrolyte)

(10)

= 2H 2O(g) + 4e−(electrode)

Upon removing contaminants such as sulfur, cleaned syngas readily diffuses into the anode microstructure and undergoes electrochemical oxidization at the TPB to CO2 and H2O. Alternatively, clean syngas can be water-shifted to produce additional hydrogen, which can then be electrochemically oxidized at the anode. The water−gas shift is an endothermic reaction with an overall enthalpy change of +102 kJ/mol at 1100 K. Steam gasification reaction (10) of carbon is an endothermic process with an enthalpy of +136 kJ/mol at 1100 K.49 So usually, oxygen (or air) is injected into the gasifier along with steam to sacrificially burn part of the carbon in coal in order to supply the heat necessary to drive the endothermic gasification reaction. Catalytic shifting of syngas is accomplished by further addition of steam to produce more H2 for the fuel cell. CO(g) + H 2O(g) = H 2(g) + CO2 (g)

(13)

(14)

2CO(g) + 2O2 −(electrolyte) = 2CO2 (g) + 4e−(electrode)

(15)

Energetics of H2 and CO oxidation reactions 14 and 15, calculated from thermochemical data,49 indicate that the enthalpy change for the CO oxidation reaction is significantly more exothermic than for the oxidation of H2 (i.e., −562 kJ·mol−1 of O2 versus −495 kJ·mol−1 of O2, respectively). Considering the enthalpy changes for reactions 12−15, it becomes obvious that there is no significant energetic difference between steam and dry gasification. Moreover, reactions 14 and 15 exhibit similar values for the standard Gibbs energy (i.e., −185 kJ·mol−1 of O2 versus −186 kJ·mol−1 of O2, respectively at 850 °C), indicating that almost identical work potentials are offered by the oxidation of either fuel. Although energetically similar, however, steam gasification process requires large quantities of water, making dry

(11)

The net overall reaction for steam gasification after steps 10 and 11 is given by F


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reported to increase functional oxygenated groups such as phenols and ethers, HNO3 treatment gives rise to carbonyl, carboxyl, and nitrate groups on the surface, and NaOH treatment results in surface hydroxyl groups.54 Similarly, Dicks and co-workers pretreated activated carbon and carbon black by acid washing and air plasma have indicated high electrochemical reactivity for HNO3-treated carbons due to higher population of oxygenated surface functionalities.55 Since reactivity was assessed by CO2 and CO release during temperature programmed desorption (TPD), the exact chemical identity of the oxygenated surface groups is not clear. Nevertheless, size and mesoporosity of the carbon particles seem to be important considerations concerning both wetting and electrochemical reactivity, indicated by linear increase in the maximum power density with mesoporous surface area.56 It is likely that the removal of inorganic constituents by acid or base treatments may result in the formation of surface carbon sites with low coordination numbers that are expected to be highly reactive and, hence, shifting the oxidation potential to more negative values in Figure 4. Clearly chemical pretreatment of carbon surfaces alter their reactivity, especially in molten electrolyte CFCs. Pretreated carbon surfaces also exhibit improved wetting by the molten electrolyte. Naturally, chemical pretreatment is expected to add to the cost of fuel. However, it is not obvious what advantages such pretreatment may offer for other CFCs operating at high temperatures, especially in SOFC based CFCs. Indeed, earlier work in SOFC based systems using chars that have undergone no chemical treatment has suggested that coal38 and biomass10 chars are more reactive than activated carbon. In this regard, one needs also to consider the role of heat pretreatment on the reactivity of carbons and chars besides surface treatment. Numerous studies57 undertaken to understand the effect of heating rate on pore structure, degree of disorder, and reactivity have reported higher heating rates correlating with higher reactivity.58,59 This consideration has practical implications in the way carbon may be supplied to the fuel cell proper to facilitate the formation of highly reactive char, which is desirable to obtain high power densities. Also, once inside the cell, physical and chemical interaction of the carbon fuel in bulk or in fine particulate form with the cell’s physical environment must also be considered for practical applications.60

gasification an attractive option for locations where water resources are scarce.

3. PRETREATMENT OF CARBONS FOR CFCS Studies to date have employed carbon in various forms for conversion in CFCs. These include activated carbons, synthetic carbons, amorphous carbon, carbon blacks, pyrolytic carbons, graphite, turbostratic carbon, etc. They also include various pretreatment methods with the intent to improve the physical and chemical properties of carbons and increase their reactivity for electrochemical conversion. Activated carbons derived mostly from forest waste such as wood contain inorganic constituents including Si and Al mostly as part of their ash content, as well as alkali metals such as Ca and Mg, and transition metals such as Fe and Mn. Many of these inorganic constituents are susceptible to chemical leaching or dissolution during pretreatment and may influence the porosity and surface chemistry of carbons. Zhu et al.51 reported that the surface of activated carbons shows basic characteristics when untreated, neutral when treated with HCl, and acidic when treated with HNO3. A careful study52 of structure−reactivity relationship for various nanostructured graphene-based carbons also reported the important role of surface oxygenated species in the formation of CO2. There is general agreement that the nature and population of surface groups affect chemical reactivity as well as the physical properties of carbons. Indeed, a recent study by Cao et al.53 indicated that chemical pretreatment of activated carbon particles by soaking in strong acids (HF and HNO3) or base (NaOH) significantly improved the wetting of the carbon by Li2CO3−K2CO3 melts. As shown in Figure 4, they have reported acid treatments were more

Figure 4. Current−voltage behavior of 150−350 mesh activated carbon (AC) indicating the importance of pretreatment in acids (HF and HNO3) or base (NaOH) and showing significant improvement in polarization loss of more than 0.4 V compared to untreated activated carbon. The measurements were made in 62 mol %Li2CO3-38 mol % K2CO3 eutectic electrolyte at 750 °C with carbon-to-carbonate mass ratio of 1:2 using a scan rate of 0.020 V/s. Reprinted with permission from ref 53. Copyright 2010 Elsevier.

4. CARBON UTILIZATION AND CONVERSION IN CFCS Pursuit for efficient conversion of carbon in fuel cells has a long history dating back more than a century5,61−71 with long periods of inactivity between spurts of efforts. Much of the early work in CFCs was reviewed by Liebhafsky and Cairns.72 In this paper, more recent studies reported mostly in the past decade are critically reviewed and discussed. The presentation of this work is organized with respect to cell configurations and focuses on moderate-to-high temperature regime (500−1000 °C) for CFC operation where reasonable rates and cell performance can be achieved. It should be noted, however, that many studies conducted at relatively low operating temperatures and generally in aqueous electrolytes73,74 are left outside the scope of this review in large part due to the exceedingly slow kinetics and large irreversible cell losses that do not provide clear incentives for practical prospects.

effective than base treatments and resulted in more than 0.4 V improvement in the cell open circuit voltage for HF-treated carbon compared to untreated samples. This was correlated with increases in both the surface area and pore volume of carbon. Moreover, acid and base treatments not only leach out and remove the inorganic impurities or matter that alter the pore microstructure inside the particles but more importantly may change the chemical nature of internal and external surfaces of carbon particles. For example, HF treatment was G


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(17) to form the oxygen donor OH−, nor the 4-electron anodic oxidation reaction (16) is likely to occur in a single elementary step. Hence, the name “direct” in this CFC approach does not truly signify direct electrochemical oxidation in the mechanistic sense, but instead meant to indicate a single process step accomplished in a single electrochemical chamber.77 Zecevic et al.78 adopted the molten hydroxide approach and reported successful conversion of cylindrical graphite rods used as consumable anodes immersed in reagent grade NaOH melt. Open circuit voltages ranged between 0.7 and 0.85 V, significantly lower than 1.02 V expected for the full oxidation of carbon. Cell performance at 630 °C with Ni foam cathode showed a maximum current density of 2500 A/m2 at about 0.15 V (or 370 W/m2), while with mild steel doped cathode with 2 wt % Ti (Fe2Ti) conversion efficiency was 60% at 500 A/m2. Cathodic losses were high due to mass transport limitations as well as ohmic losses that increased with increasing bubbling in the melt. While stirring reduced cathode polarization, gas bubbling increased ohmic resistance of the electrolyte, indicating a trade off between the two. In this cell configuration, however, one of the major concerns is the stability of the hydroxide melt against carbonation given by the reaction

4.1. Molten Hydroxide-Based CFCs (MH−CFC)

Physical limits imposed by poor solid-to-solid contact between the solid fuel and the anode surface can be circumvented by the use of molten electrolytes such as molten hydroxides, where solid/liquid interfaces can readily be formed. This expands the electrochemical reaction surface area and increases the population of ERS. Consequently, this results in improved cell performance and efficiency. Interestingly, the general concept of the molten hydroxide based CFC dates back more than a century to Jacques.75 The molten hydroxide electrolyte approach to carbon conversion offers several advantages. Several hydroxide eutectics melt at much lower temperatures than their molten carbonate counterparts, expanding the CFC operation regime to lower temperatures. For example, the NaOH-KOH eutectic melts at about 170 °C, while the individual hydroxides themselves melt at much higher temperatures, i.e., 318 and 360 °C, respectively. Oxygen is highly soluble in molten hydroxides in the form of peroxide (i.e., O22−) and superoxide (i.e., O2−) ions, which are expected to participate in the carbon oxidation reaction. Also, high ionic conductivity (∼300 S/m)76 of hydroxide melts allows such cells to operate at intermediate temperatures in the range 500−650 °C, where full oxidation to CO2 is thermodynamically favorable as opposed to the partial oxidation product CO. Furthermore, low cost materials such as Ni or Fe have sufficient stability in molten hydroxides and can be employed as cathode materials.

2CO2 (g) + 4OH−(electrolyte) = 2CO32 −(electrolyte) + 2H 2O(g)

As the solubility limit of the ion in the hydroxide melt is exceeded, it precipitates out as alkali metal carbonate inside the melt. Naturally, this parasitic reaction 18 is undesirable and consumes the OH− ions in the molten electrolyte. The solid precipitate may also clog the cell and its components. As a result, the performance of this CFC configuration is expected to degrade rapidly over time. A thermodynamic study79 proposed to employ instead an aqueous alkaline-carbonate electrolyte at 300 °C in order to mitigate carbonate formation and the resulting loss of OH− ions from the electrolyte. Invoking the Le Chatelier argument, Zecevic et al.78 proposed to increase the water content of the hydroxide melt by humidification in order to mitigate the driving force for carbonation. Indeed, they have experimentally shown that the rate of carbonate formation via reaction 18 depended strongly upon the water content. They also argued that keeping a humid atmosphere above the hydroxide melt helps reduce the corrosion rate of the Ni cathode, which is susceptible to oxidation by reaction with the peroxide and superoxide ions in the melt that were produced by molecular oxygen at the cathode through the respective reactions

Figure 5. MH−CFC schematic for electro-oxidation of bulk carbon anode. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

In this arrangement shown schematically in Figure 5, the net half-cell reaction for the electro-oxidation of carbon at the anode is

O2 (g) + 4OH−(electrolyte) = 2O2 2 −(electrolyte) + 2H 2O(g)

C(s) + 4OH−(electrolyte) = CO2 (g) + 2H 2O(g) + 4e−(electrode)

(18)

CO32−

(19)

3O2 (g) + 4OH−(electrolyte)

(16)

= 4O2−(electrolyte) + 2H 2O(g)

At the cathode, oxygen is reduced to hydroxide ions by O2 (g) + 2H 2O(g) + 4e−(electrode) = 4OH−(electrolyte)

(20)

Increasing the water content shifts the chemical equilibrium toward left and reduces the tendency for peroxide and superoxide formation. Indeed, Hackett et al.60 demonstrated improved cell performance over those reported by Zecevic et al.78 by employing prehumidified air at 70 °C supplied to the cathode compartment of the molten NaOH electrolyte cell operating at 600−700 °C. Open circuit voltage values for

(17)

Naturally, the desired net cell reaction is the full oxidation of carbon to CO2 according to reaction 3. Note that the anodic oxidation of carbon in this scheme occurs via reaction with hydroxide ions, which serve as the chemical vehicle to deliver oxygen to reaction 16. Neither the 4-electron cathode reaction H


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Inspired by early work on oxidation of carbon in aqueous84,85 and molten carbonate69 cells, Vutekakis et al.86 studied electrochemical oxidation of various carbonaceous solid fuels dispersed in a 32.1 wt % Li2CO3/34.5 wt % K2CO3/33.4 wt % Na2CO3 ternary molten carbonate fuel cell arrangement at temperatures between 500 and 800 °C. At 700 °C, they reported about 13% carbon efficiency indicating that 87% is chemically converted to CO by the Boudouard reaction 13. Moreover, they and others15,87 have reported a strong dependence of OCV on temperature as well as OCV values up to 1.4 V (see Figure 3in ref 86), both of which were contrary to expectations of truly “direct” conversion. Using various forms of pyrolytic carbon, Cooper and coworkers7,88−92 reported cell power densities between 400 and 840 W/m2 at 0.8 V and 800 °C, where biochar-derived carbons with high degree of lattice disorder demonstrated relatively higher power densities than others. Although no clear correlation was established between cell performance and carbon particle size or surface area, it was postulated that both high electrical conductivity and high degree of crystallographic disorder in the carbon lattice (i.e., turbostratic carbon) seem to enhance electrochemical reactivity of carbon particles. Generally, however, graphitic carbons have high electronic conductivity but poor reactivity, while disordered carbons exhibit poor electrical conductivity but high reactivity. Other studies also reported moderate power densities in the 400− 1400 W/m2 regime and emphasized the importance of carbon microstructure and surface chemistry.93−96 As expected, reactions occurring at the anode are rather complex and may involve multiple species in both ionic (e.g., CO32−, O2−, and C2O52−) and dissolved form (CO2, O2, and CO) in the molten electrolyte as well as multiples of surface sites and groups on the carbon surface. Mechanistically, the elementary details of the anode reactions are not fully established or understood yet. Cooper and Selman87 recently proposed possible elementary steps for reactions at the anode. However, the mechanistic details of the anode reaction have not been experimentally verified yet. Although the desired anode reaction is

graphite ranged between 0.705 and 0.788 V, which were lower than expected, while the maximum power density was 840 W/ m2. Clearly, for practical application of MH−CFC approach there is need to greatly improve and optimize cell performance, as well as address the stability issues due to the highly reactive nature of the molten hydroxide melt. 4.2. Molten Carbonate-Based CFCs (MC−CFC)

The natural progression to eliminate carbonate formation is to move from hydroxide melts to carbonate melts, which are chemically stable in the presence of solid carbon. MC−CFC is a modified configuration of molten carbonate fuel cell (MCFC) that typically contains a molten carbonate electrolyte that is a binary or ternary eutectic mixture of Li2CO3−Na2CO3−K2CO3, a porous NiO cathode and a Ni−Cr anode. However, NiO is not stable80 in the MCFC environment due to the dissolution reaction NiO(s) + CO2 (g) = Ni 2 +(electrolyte) + CO32 −(electrolyte)

(21)

Because NiO is less soluble in Na2CO3 than in K2CO3, Li− Na carbonate melts are preferable when compared to Li2CO3− K2CO3 melts. On the other hand, contact angle measurements indicated superior wetting characteristics for Li2CO3−K2CO3 binary eutectic over its Li2CO3−Na2CO3 counterpart,81,82 complicating the electrolyte selection process. A simplified depiction of a MC−CFC is shown in Figure 6. Utilization and conversion of carbon in a molten carbonate

C (s) + 2CO32 − (electrolyte) = 3CO2 (g) + 4e− (electrolyte)

(22)

other reactions also need to be considered for the anode compartment. Peelen et al.50 suggested the reactions

Figure 6. MC−CFC schematic for electro-oxidation of the carbon fuel particles dispersed in the molten carbonate electrolyte. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

CO32 −(electrolyte) = CO2 (g) + O2 −(electrolyte)

(23)

C(s) + CO32 −(electrolyte) = CO(g) + CO2 (g) + 2e−(electrode)

electrolyte has several inherent advantages. Unlike molten hydroxide electrolyte, carbon is stable in the carbonate environment. By selecting from a wide range of binary or ternary alkali carbonates, one can tailor the desired eutectic temperature of the molten electrolyte and thus CFC operating temperature. Upon wetting, molten carbonate electrolyte provides an extended solid/liquid interface and consequently, a high population of electrochemical reaction sites. Alkali metal ions in the carbonate melt also help promote gasification of carbon.83 On the other hand, MC−CFC systems require a complex CO2 management system and cathode materials that are tolerant to carbonate attack, which is a challenging problem at these elevated temperatures.

(24)

Note again that the anode reactions involve the carbonate ion, which delivers the oxygen required to oxidize the carbon fuel electrochemically. Similarly, oxygen is converted at the cathode into carbonate ions by O2 (g) + 2CO2 (g) + 4e−(electrode) = 2CO32 −(electrolyte)

(25)

The anode and cathode reactions clearly indicate that the oxidation of carbon by oxygen in MC−CFC involves an indirect pathway, again making use of an oxygen delivery vehicle. I


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Also, reaction 25 requires that 2/3 of the CO2 generated at the anode must be recycled into the cathode compartment to facilitate the formation of carbonate ions. The overall desired cell reaction is the conversion of carbon to CO2 given by (5). It was reported that the charge transfer reaction at the anode is the slow step with charge transfer resistances up to 60 ohm·cm2 at 750 °C and up to 220 ohm·cm2 at 650 °C50, while anode overpotentials up to 0.28 V were observed at 100−1200 A/m2 in the temperature range 650−850 °C.97 Similarly, the exchange current density at the anode was reported in the range 1−61 A/m2. This value is more than 2 orders of magnitude lower than for hydrogen fuel cells where the exchange current density is usually in the several thousands of A/m2. Reports on direct measurements and assessment of carbon efficiencies via gas analysis are scarce. Instead, Cooper7 calculated an electrical conversion efficiency of 78−80% for rates up to 1000 A/m2 at 800 °C based on estimates of cell losses. This exceptionally high efficiency value was in good agreement with 78% efficiency estimated theoretically by Hemmes et al.98 for the same system at 800 °C. Contrary to such high estimates, experimental measurements56 indicate the difficulty in achieving high conversion efficiencies due to the significant role played by the Boudouard reaction in MC−CFCs, in agreement with similar reports.86 Naturally, such chemical losses due to Boudouard consumption dramatically affect carbon conversion efficiency in molten carbonate based fuel cell arrangements. A modification of the MC−CFC approach involved the use of a fluidized bed anode made on Ni powder in flowing N2 gas.99 The advantages of the fluidized bed anode include high heat transfer rates and enhanced transfer of ionic species, both of which help reduce losses. Using a 62 mol % Li2CO3-38 mol % K2CO3 eutectic electrolyte at 650 °C, bamboo- and oak wood-derived activate carbon fuels gave current densities of 959 and 945 A/m2, respectively.

Figure 7. Hybrid-CFC schematic illustrating the binary solid YSZ/ molten carbonate electrolyte pathway for electro-oxidation of carbon fuel particles dispersed in the molten electrolyte. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

compartment as required in MC−CFC. Physical separation of the cathode and anode compartments by the YSZ ceramic membrane also eliminates the possibility of cathode corrosion by the molten carbonate electrolyte.80 This, of course, allows wider materials selection options for the cathode. It also offers easier management of the carbonate electrolyte since the cathode, which is physically separated, cannot be starved or flooded by the carbonate melt. However, it is known that alkali metal carbonate melts are highly corrosive, and the chemical stability of YSZ and other oxide ion conducting ceramics in molten carbonates is known to be poor, especially under reducing conditions.100 Naturally, such stability issues pose major challenges for extended operation of hybrid CFCs. Moreover, mechanistic understanding of the anode reactions is incomplete in the hybrid CFC arrangement. The situation is complicated by the fact that the nature and mechanism of ionic handshake between O2− and CO32− during serial transport across the solid electrolyte/molten carbonate interface is unclear. Furthermore, the solubility of O2− in the molten carbonate electrolyte is limited. Most of the studies in hybrid CFCs employed serial arrangement of the binary electrolyte.15−17,100−102 The majority of these studies yielded peak power densities of 500−600 W/ m2 or less, although one of them achieved power densities up to 1200 W/m2 at 950 °C using various acetylene blacks.16 The cathode reaction in this hybrid CFC arrangement is the same as previously given by (4). However, the reactions at the anode are complex and their mechanistic steps are not understood yet. Recently, Jiang and Irvine103 have proposed the following set of electrochemical and chemical reaction steps for consideration at the anode. Although these reactions were given in earlier sections, they are provided below for completeness and coherency.

4.3. Hybrid CFCs

Hybrid CFC circumvents an important shortcoming of MC− CFCs by combining MCFC and SOFC arrangements, where the molten carbonate and solid oxide ceramic electrolytes are in direct physical contact with each other. Unlike MC−CFC where the cathode reaction requires a 2:1 ratio of CO2:O2, the cathode reaction in the hybrid CFC configuration consists simply the reduction of oxygen from air to O2− ions that are incorporated into the yttria-stabilized zirconia (YSZ) electrolyte, a well-known oxide ion conductor. Oxide ions are transported across the electrolyte toward the molten carbonate holding the dispersed carbon slurry inside the anode compartment. Configurations of the composite binary electrolyte implemented for hybrid CFCs are discussed below under two strategies with respect to the transport pathways of the charge carrying ionic species, which are primarily O2− and CO32−, respectively, in the YSZ and carbonate electrolytes. The first strategy employs a serial connection of the binary electrolytes along the ion transport pathway, such that charge is transported first by oxide ions then by carbonate ions, respectively. This serially connected hybrid CFC arrangement schematically shown in Figure 7 provides several advantages over MC−CFCs. It employs air at the cathode as the oxygen source, greatly simplifying the cathode reaction and eliminating the need to recycle 2/3 of the anode CO2 gas into the cathode

C(s) + 2O2 −(electrolyte) = CO2 (g) + 4e−(electrode) (3)

C(s) + O2 −(electrolyte) = CO(g) + 2e−(electrode)

(6)

CO(g) + O2 −(electrolyte) = CO2 (g) + 2e−(electrode) (15) 2−

−

C(s) + 2CO3 (electrolyte) = 3CO2 (g) + 4e (electrode) (22) J


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CO32 −(electrolyte) = CO2 (g) + O2 −(electrolyte)

(23)

C(s) + CO2 (g) = 2CO(g)

(13)

factors including the use of highly catalytic cathode, low cell impedances and polarization losses, flowing air at the cathode, and faster kinetics and transport rates at these temperatures. However, during long-term stability tests conducted over 14 h, fuel cell performance at the cell potential of 0.7 V (vs air) showed a drop in current density from the initially high value of nearly 8000 to 2000 A/m2 within the first hour of operation, suggesting rapid deterioration and stability problems. The second hybrid CFC strategy based on a parallel combination of the binary electrolyte was proposed by Jia et al.96 that aimed to extend the contact between the carbon particles and the electrolyte, thus increasing the population of electrochemical reaction sites. In this approach, the solid oxide electrolyte is a dispersed (or porous) phase in the molten carbonate such that ionic charge carriers O2− and CO32− transport in tandem through parallel paths in the composite electrolyte. They employed oxide ion conducting samariumdoped ceria (SDC) particles dispersed in a 2:1 mole ratio Li2CO3−Na2CO3 eutectic carbonate electrolyte. Activated carbon was added to the composite binary electrolyte such that carbon-to-electrolyte weight ratio was 1:9. The overall anode reactions in this parallel-connected hybrid CFC arrangement are given by

Clearly, the presence of chemical steps in the proposed reaction sequence point to losses toward achieving high conversion efficiency and suggest indirect pathways are also operative. Moreover, experimentally backed arguments were not provided as to which of the reactions steps above clearly govern overall cell performance. In several studies,16,17,101 high OCV values of 1.2−1.5 V were reported, which are significantly larger than the thermodynamically expected value of about 1 V. These high OCV values suggest the intervening role of CO and/or the alkali ions in the molten carbonate participating in the anode reaction. Indeed, a cell potential of 1.42 V at 700 °C was estimated17 for the net cell reaction 2CO(g) + O2 (g) + 2Li 2O(s) = 2Li 2CO3(s)

(26)

103

It was also reported that the carbon-to-carbonate ratio in the melt plays a role in the observed OCV values, such that heat treatment of carbon in 50 mol % carbonate slurry resulted in OCV values of 1.57 V at 550 °C and 1.33 V at 700 °C. Although cell performances in the literature have consistently been at modest levels at best with hybrid CFCs, significant progress was recently made in this regard by Jiang et al.,104 who reported a record high power density of 8780 W/m2 at 750 °C as shown in Figure 8. This was achieved using a carbon fuel

C(s) + 2O2 −(electrolyte) = CO2 (g) + 4e−(electrode) (3) 2−

−

C(s) + 2CO3 (electrolyte) = 3CO2 (g) + 4e (electrode) (22)

Of course, contributions from CO to the anode reactions can also be expected. The cathode reactions in this parallel scheme are given by, O2 (g) + 4e−(electrode) = 2O2 −(electrolyte)

(4)

O2 (g) + 2CO2 (g) + 4e−(electrode) = 2CO32 −(electrolyte)

(25)

Planar cells were tested between 600 and 750 °C using different cathode gases, namely, 2:1 and 1:1 mol ratio CO2/O2 mixtures as well as air and pure O2. As expected, the nature of the cathode gas had a major impact on cell performance since it directly impacts the concentration and identity of the charge transporting ionic species. Peak power densities of this parallel hybrid CFC design measured for air, pure O2 and 2:1 CO2/O2 mixture at the cathode were 250 W/m2, 400 W/m2, and 1000 W/m2, respectively at 700 °C. The 1:1 CO2/O2 cathode gas mixture gave a slightly lower peak power of 900 W/m2 at this temperature. These results suggest that the parallel addition of the oxide ion conducting solid electrolyte to the molten carbonate may not offer significant advantage in cell performance. Due to the significantly higher ionic conductivity of the carbonate melt than that of the SDC solid electrolyte and the limited solubility of oxide ions in the molten carbonate melt, the contribution from the solid electrolyte component to overall cell performance is necessarily marginal in this parallel arrangement, and offers no significant advantage.

Figure 8. Hybrid CFC performance at 750 °C using pyrolyzed medium density fiberboard char. 8780 W/m2 is the highest power density reported for any CFC so far in the literature and is achieved in a CFC employing a La−Sr−Co-O cathode, YSZ electrolyte with Gd− Ce−O interlayer between YSZ and the cathode, and a Ni cermet supporting-anode. Reprinted with permission from ref 104. Copyright 2012 Royal Society of Chemistry.

derived from the pyrolysis of medium density fiberboard in their hybrid CFC, with a carbon fuel to Li2CO3−K2CO3 electrolyte weight ratio of 4:1. They also employed a highly catalytic La0.6Sr0.4CoO3 (LSC) cathode separated from the 5− 10 μm thick YSZ electrolyte by a thin interlayer of Ce0.9Gd0.1O2 (GDC) in order to avoid the solid state reaction between LSC and YSZ that tend to form a blocking zirconate layer. Such remarkably high performance is unprecedented in CFC literature to date, and is an excellent example to highlight the potential prospects of CFCs for practical applications. Jiang et al.104 attributed high cell performance to a combination of

4.4. Molten Metal Anode-based CFCs (MA-CFC)

Maintaining percolation and sufficient electrical connectivity among the carbon particles in the molten carbonate electrolyte at all times is a major operational challenge suffered by both MC- and hybrid CFCs. This problem can be eliminated entirely by using an electronically conducting molten metal anode, K


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otherwise expected of a cell voltage of 1 V expected for the electrochemical oxidation of carbon. Fortunately for these cells, the polarization loses at the anode can be low, and partially compensate for the voltage deficit. Moreover, the open circuit cell potential governed by the molten metal/oxide equilibrium is independent of the extent of conversion. In the CFC configuration shown in Figure 9, the YSZ electrolyte is in physical contact with a molten tin anode bath

which provides excellent conductivity among the dispersed carbon particles in the melt. The basic operating principle of MA-CFC can be represented by the reactions of the metal, M, at the anode compartment, M + xO2 −(electrolyte) = MOx + 2x e−(electrode)

(27)

C(s) + MOx = COx + M

(28)

Here COx represents the oxides of carbon, namely, CO and CO2 that are formed during the reduction of the metal oxide MOx. Clearly, reaction 27 is electrochemical while reaction 28 is chemical in nature. The cathode reaction is the same as (4) given previously. In other words, this scheme delivers the oxygen needed to oxidize carbon using “chemical looping”, where oxygen is exchanged chemically between carbon and the metal oxide, which serves as the delivery vehicle. This particular approach was originally conceived nearly four decades ago by Anbar,105,106 who proposed a fuel cell arrangement where coal char is dispersed in a molten carbonate electrolyte in direct contact with a molten Pb anode bath. Molten Pb is electrochemically oxidized at the anode interface, upon which the carbon subsequently reduces the Pb oxide chemically to its metallic form, thus completing the “loop”. This general approach was later modified first by Gür107 and then by others108−118 using instead, a solid oxide ionconducting electrolyte such as YSZ in direct contact with various molten metal anodes. There are several advantages offered by this modification over that by Anbar. Certainly, use of YSZ ceramic membrane avoids stability and corrosion issues generally encountered with molten carbonate electrolytes. Unlike molten metal anodes that readily conduct electrons, the electronically insulating molten carbonate electrolyte used by Anbar required the necessity to use a current collector at the anode even under fully percolated conditions. There have been various metal/metal oxide couples explored for MA-CFCs in the literature. Discussion of these studies are grouped under four distinct strategies based on the nature and physical property of the metal anode: (1) Molten metal anode that forms an oxide at the anode/ YSZ interface that is both ionically and electronically blocking (e.g., Sn/SnO2).107,109,110,114 (2) Molten metal anode that forms an oxide at the anode/ YSZ interface that is ionically transparent but blocking electronically (e.g., Bi/Bi2O3).107,111 (3) Molten metal anode that forms an oxide at the anode/ YSZ interface that has a melting temperature below that of the molten bath temperature (e.g., Sb/ Sb2O3).112,115,116 (4) Molten metal anode that is stable against oxidation, dissolves considerable amount of oxygen, and exhibits high oxygen diffusion rate in the molten bath (i.e., Ag), thus eliminating the formation of an oxide layer at the anode interface.117,118 With the exception of strategy #4 above, the open circuit cell potentials for other strategies are dictated primarily by the equilibrium between the molten metal and its stable oxide that forms at the molten metal/YSZ interface upon passage of current, and not by the carbon/oxygen equilibrium. Indeed, the reported values for open circuit potentials range between 0.48 V for Bi, to 0.75 V for Sb, and 0.78 V for Sn at the cell operating temperatures. These values are significantly less than

Figure 9. MA-CFC schematic illustrating the metal oxide “chemical looping” mechanism for delivering oxygen to the carbon fuel particles dispersed in the molten metal anode that also provides electrical connectivity among the carbon particles. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

containing a dispersion of solid carbon particles. Alternative designs place the Sn bath containing carbon particles in a separate compartment or chamber. In any case, the cell potential is governed by the Sn/SnO2 equilibrium. Indeed the current−voltage (I−V) behavior of a MA-CFC using a one-end closed YSZ tubular cell element immersed into a molten Sn anode bath is shown in Figure 10. The cell is operated at 855

Figure 10. Current−voltage (I−V) behavior of a molten Sn anode cell (MA-CFC) operating at 855 °C and employing a one-end closed YSZ tubular cell immersed in the molten Sn bed containing dispersed fine particles of carbon. The break in the slope of the I−V likely corresponds to the coverage of the blocking oxide film at the YSZ/ molten Sn interface. Adapted with permission from ref 107. Unpublished work 1980.

°C and had a porous Pt cathode painted on the inside surface at the closed end of the YSZ tube. The observed OCV of 0.93 V vs air for this cell is in good agreement with the Sn/SnO2 equilibrium potential of 0.87 V (vs air) calculated from established thermochemical data.49 Under current carrying conditions, oxide ions exiting the YSZ electrolyte oxidize the L


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higher than the melting point of Fe, which is 1535 °C. Interestingly, oxides of iron have melting temperatures similar to Fe, i.e., 1377 °C for FeO, and 1566 °C for Fe2O3, respectively. Although the advantage of using a molten metal whose oxide melts in a similar temperature regime was not explicitly articulated by Yentakakis et al.,115 this concept was recently proposed by Gorte and co-workers, 116 who studied anodic oxidation behavior of relatively low melting metals under inert gas (He and N2 ) environment. In the absence of carbon fuel, they demonstrated low open circuit potentials of 0.5 V at 900 °C for Pb and 0.75 V at 700 °C for Sb. Recently they reported significant power generation in a SOFC arrangement employing a scandia-stabilized zirconia (SSZ) solid electrolyte in contact with molten antimony anode containing dispersed carbon. 112 There is clear advantage in employing a system where the metal anode and its oxide are both in the molten state. It is to be expected that the transport rates in such a system will be faster than a system where the oxide is a solid phase at the cell operating temperature. Nevertheless, the cell potential in this system is governed by the metal/metal oxide equilibrium, and not by the carbon/oxygen equilibrium. The reported open circuit cell potential of 0.75 V versus air obtained at 700 °C is in reasonable agreement with the thermodynamic equilibrium between Sb and Sb2O3 at this temperature, which is above the melting points for both Sb (MP: 630 °C) and Sb2O3 (MP: 656 °C). Using sugar char as the solid fuel they reported a peak power density of 3600 W/m2 at 700 °C obtained at a cell voltage of 0.4 V. Under galvanostatic operation at 6000 A/ m2, they were able to maintain stable power of 3000 W/m 2 over a 12-h period. This result is encouraging and notable with implications toward practical prospects for molten metal anode systems, where peak power densities that have previously been reported seldom exceeded 1000 W/m2. As an interesting potential application, the reversibility of the Sb/Sb2O3 molten anode cell was recently demonstrated for energy storage purposes by Javadenkar et al.,121 where molten Sb was employed as the fuel from an external tank and the molten reaction product Sb2O3 was subsequently electrolyzed during the recharging cycle. They reported open circuit cell voltage of 0.75 V at 700 °C with a near-linear I−V behavior as well as encouragingly low specific impedances of about 0.15 ohm·cm2 for the electrodes under both fuel cell and electrolysis modes of operation. This system may offer attractive opportunities for large scale energy storage due to it high energy density for Sb (1020 Wh/kg)121 which is nearly 2 orders of magnitude larger than offered by flow batteries based on vanadium redox chemistry that are commercially being developed for stationary energy storage applications. The fourth strategy originally proposed by Gür117 employs Ag as the molten metal anode that is nonreactive, nonconsumable, and thermodynamically stable against oxidation at the cell operating temperature, and also possesses considerable solubility for oxygen in the melt. Despite its cost, Ag indeed offers many advantageous properties. Its melting point of 960 °C is ideally suited for the cell operating temperature of 1000 °C, where the Ag anode is in a molten state. Moreover, oxidation of Ag is not thermodynamically favored above 230 °C,49 so molten Ag anode offers thermodynamic stability at the cell operating temperatures up to 1000 °C. Furthermore, Ag is chemically inert to carbon and does not form carbide. These are critically important considerations to maintain a stable and coherent interface between the ionically conducting solid

molten tin, and deposit SnO2 at the molten anode/YSZ interface. The abrupt change in the slope of the I−V behavior around 0.7 V in Figure 10 is likely due to the onset of full coverage of the oxide layer at the YSZ/molten Sn interface. Beyond the break point, further increase in cell resistance at higher overvoltage is due to the growing SnO2 blocking layer at the interface. Using electrochemical impedance spectroscopy, the effective diffusion coefficient of SnO2 in molten tin at 1000 °C was estimated119 to be 0.198 cm2/s. However, such remarkably high diffusivity is questionable when compared to typical diffusion coefficients of 10−4−10−5 cm2/s that are commonly observed in liquid systems. In fact there is no experimental evidence that the SnO2 layer diffuses back into molten Sn to free up the anode interface. On the contrary, SnO2 has limited solubility in molten Sn, so it tends to build up at the anode interface, blocking both ionic and electronic transport. A related consideration is the limited solubility of dissolved oxygen in molten Sn and its production rate at the anode. The latter is dependent on the cell current density. Naturally, such limitations adversely affect cell performance, and eventually stop fuel cell operation when the anode/YSZ interface is fully covered with the SnO2 blocking layer. One strategy that was proposed109 to avoid blockage by SnO2 is to operate the cell at sufficiently low overvoltages such that SnO2 formation is not favored thermodynamically. At a cell temperature of 1000 °C, such thermodynamic consideration would require keeping the cell operating voltage above the theoretical threshold of 0.78 V vs air. This corresponds to the equilibrium potential for the Sn/SnO2 binary system above which the formation of SnO2 becomes thermodynamically unfavorable at this temperature. Consequently, this particular CFC approach provides the flexibility to be operated either like a battery or as a fuel cell. This arrangement is recently demonstrated also for gaseous and liquid (JP-8) fuel conversion.120 Other strategies outlined in #2, 3, and 4 above are adopted primarily to overcome or circumvent the difficulty posed by a blocking oxide layer at the YSZ interface that eventually impedes the anode reaction. The first of these strategies (i.e., #2 above) employs a molten metal anode such as Bi,107,111 whose oxide, δ-Bi2O3, is a known oxide ion conductor that can facilitate fast transport of oxide ions across the anode interface. A recent study comparing the oxidation of Sn and Bi molten anodes in flowing He observed significantly less anode impedance for Bi than with Sn as expected.111 At 700 °C the anode impedance for the molten Sn cell increased steadily from about 3 ohm·cm2 to nearly 250 ohm·cm2 over the course of the experiment indicating the formation and increasing coverage of the SnO2 layer, while the anode impedance for the Bi cell was considerably less and remained stable at 0.25 ohm·cm2. The corresponding open circuit voltages at 700 °C for the molten Sn and Bi cells were 0.93 and 0.48 V vs air respectively, in general agreement with the expected potentials for their respective metal/metal oxide equilibria. Another strategy (i.e., #3 above) is to operate the CFC at a sufficiently high bath temperature above the melting point of its oxide such that the metal oxide remains in a molten state and likely diffuse away from the YSZ interface. A paper-study by Yentekakis et al.115 simulated the expected performance for gasification of carbon particles dispersed in a molten Fe anode bath in direct contact with a YSZ solid electrolyte tube. The operating temperature of such a cell would necessarily be M


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the form of pressed pellets or in free powder form. Using this scheme, Horita et al.123 have investigated internal oxidation of carbon over metal carbide anodes in a solid oxide fuel cell arrangement at 1000 °C. The open circuit voltages were around 1 V vs air, in good agreement with values expected for the metal carbide/metal oxide equilibrium. At a cell voltage of 0.6 V, they reported a current density of about 100 A/m2 for VC and 10 A/ m2 for TiC anodes. Cell polarization studies enabled by a reference electrode showed higher anodic polarization for the metal carbide anodes than for pure graphite. Under flowing Ar purge, no CO2 evolution was observed via chromatographic gas analysis even at high overvoltage, indicating that the dominant reaction was oxidation of carbon to CO. Under CO2 flow, anode polarization curves suggested two separate reaction regimes. One was attributed to carbon oxidation to CO and the other for CO oxidation. However, the authors failed to discern the contribution from the Boudouard reaction to overall cell performance. In an effort to catalyze the Boudouard reaction, carbon oxidation study125 in the presence of 5 wt % Fe gave a power density of 4240 W/m2 at 0.5 V and OCV of 1.1 V at 850 °C, but rapid and significant degradation in cell performance was observed, which could not be attributed to fuel starvation. Based on the premise that specific gravity of solid carbon is nearly 4 orders of magnitude higher than that of a gaseous fuel, Nürnberger et al.,18,126 and Guzman et al.127 argued that poor contact between the solid carbon particle and the anode surface may not pose sufficient limitation to dismiss the concept of direct contact configuration. Indeed the former study18,126 employed porous pressed pellets of carbon fuel that were mounted directly on the bare free surface of 100 μm thick 3 mol % Y 2O3-stabilized zirconia electrolyte with porous YSZ/ LSMO composite cathode. The carbon pellets were made either from amorphous carbon of particle size 5−10 μm, or from graphitic carbon of particle size 40−60 μm. Using flowing N2 purge gas in the anode compartment, maximum power density achieved with amorphous carbon at 1000 °C was 400 W/m2 at 0.4 V, whereas this was less than 10 W/m2 for graphitic carbon at this temperature. Similar results were reported by Kulkarni et al.,19 who employed carbon black and micronized graphite in a flat SOFC arrangement consisting of 20 mol % Gd2O3-doped ceria (20GDC) electrolyte with La0.6Sr0.4Co0.2Fe0.8O3±δ (LSCFO) anode and LSCFO-Ag cermet cathode. The open circuit voltages measured between 600 and 800 °C decreased with increasing temperature and were significantly lower by up to 0.3 V than expected. The authors attributed low OCV to increased electronic contribution to total conductivity of the GDC electrolyte, known to take effect at elevated temperatures and in reducing environments. 128 They reported a maximum power density of 420 W/m2 at a cell voltage of nearly 0.35 V at 800 °C for carbon black in He purge gas at the anode, while the corresponding value in CO2 purge gas was about 520 W/m2 at 0.38 V. Improvement in cell performance in CO2 was attributed to additional contribution from CO supplied by the Boudouard reaction. Moreover, carbon black provided better cell performance than the graphitic fuel, in agreement with the relative stability of graphitic carbons to thermal oxidation compared to disordered carbons with lower degree of crystallinity, as is the case with carbon black.88,129,130 The role of the solid electrolyte was studied in the temperature regime between 600 and 880 °C using 8 mol % Y2O3-stabilized zirconia (8YSZ), 9 mol % Sc2O3-stabilized zirconia (9SSZ), and 20GDC sintered disks that were 2 mm

electrolyte and the molten Ag anode. Otherwise, any reaction product forming at this interface has the potential of impeding or blocking the charge transfer reaction, ultimately increasing anodic polarization and degrading the performance and efficiency of the CFC. Silver also offers high solubility and fast transport for oxygen. Even in solid form, diffusion coefficient for oxygen in Ag is rather high and measured122 to be 10−5cm2/s at 700 °C. Despite these significant advantages, Ag is an expensive metal and cost may become a bottleneck for progress in this direction. In a paper study, Gopalan et al.118 recently modeled a regenerative cell design based on molten Ag anode that is in physical contact with graphite or mixed conducting oxide current collector on one side, and YSZ solid electrolyte membrane on the other. Under power generation mode, it was envisioned that this cell employs H2 as fuel at the Ni/YSZ cermet anode and oxygen at the molten Ag cathode. In fuel cell mode, oxygen dissolved in molten Ag is reduced at the YSZ/Ag interface and transported to the anode side where it electrochemically reacts with H2. In this configuration, the sacrificial graphite current collector undergoes anodic reaction by the oxygen transported through both the YSZ and the molten Ag anode, to produce a mixture of CO and CO2. This modeling study demonstrated that it is possible to achieve ionic current densities on the order of 104 A/m2 in this configuration. 4.5. Solid Oxide-Based CFCs (SO−CFC)

Solid oxide fuel cell arrangements eliminate or simplify many of the issues inherent to molten electrolyte and molten anode systems including percolation, bubbling, wetting, multiphase interfaces, corrosion and materials stability, and kinetic and transport limitations in low viscosity media. Instead, conversion in SOFCs happens at gas/solid interfaces, and interactions at gas/solid and solid/solid interfaces are in general simpler to study, characterize, and control than similar phenomena at triple phase gas/solid/liquid interfaces prevalent in molten media CFCs. SOFC arrangements also circumvent problems related to flooding of ERS at the anodes by molten media or CO and CO2 gas bubbles that form in the viscous medium during carbon oxidation. As expected, gas bubbles block and hinder the active sites and prevent turnover for the next unit reaction, lowering conversion efficiency and cell performance. On the other hand, SOFC arrangements introduce their own specific challenges and shortcomings. One is due to poor solidto-solid contact between solid fuel particles and the anode surface. This restricts the current flow to discrete reactions spots (or contact points) that often gives rise to large polarization losses. The other challenge is posed by the dimensional incompatibility between the size of the solid fuel particles, which can be from tens of micrometers up to several millimeters, and the nanoscale dimensions of the electrochemical reaction sites (or, triple phase boundaries (TPB)) located at the solid electrolyte/anode interface. Most of these difficulties are overcome by gasification of the solid fuel by either oxygen,36,37,123,124 steam,29,67,68 or by dry CO2,20,38 whereby the product syngas readily diffuses to the TPB, where it is electrochemically oxidized. While gasification provides obvious benefits including direct access to TPB by the syngas, the direct contact approach was also pursued by many groups. 4.5.1. Direct-Contact Cells. In this geometry, the solid carbon fuel is in direct contact with the anode surface either in N


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thick.131 Respective powders of the electrolyte and carbon black were mixed with the addition of Ni and Ag powders to catalyze the reaction. The composite fuel particles placed in the anode compartment gave poor performance between 10 W/m2 and 110 W/m2 even at the highest test temperature of 880 °C, likely due to the thick electrolyte resulting in large ohmic losses. Best performance obtained with the 20GDC composite fuel mix was attributed to the lattice oxygen provided by GDC for the anodic reaction. Ag catalyst addition to carbon black was more effective than Ni at similar loading levels. A possible reason to explain this behavior may be the higher solubility and diffusivity of oxygen in Ag than in Ni. Also, Ag does not form carbide but Ni is known to react with carbon to form Ni3C. However, Ni3C most likely is not stable in the anode environment and may react with the lattice oxygen to give off CO. Others also took advantage of the higher ionic conductivity offered by Sc2O3 doped zirconia (SSZ),132 and employed a Ni-YSZ cermet anode-supported tubular cell geometry with Ni-SSZ active anode layer coated with a thin SSZ electrolyte film, and LSMO-SSZ composite cathode at the air side.133 Carbon black powder of 2−10 μm average particle size was filled inside the tubular cell in direct contact with the anode surface. The maximum power density was 1040 W/m2 obtained at a cell voltage of 0.4 V and 850 °C. Unfortunately, the authors did not conduct gas analysis to study the dominant reactions in their cells. 4.5.1.1. Alternative Accounts for Claims of Direct Electrochemical Oxidation. Several studies claimed to have achieved direct electrochemical oxidation of carbon in direct-contact SOFC arrangements.18,19,127,134 However, sufficient and convincing experimental evidence to back up such claims were scarce. For example, Nürnberger et al.18 estimated that only 60% of the amorphous carbon fuel is oxidized electrochemically at 900 °C and the remaining carbon was converted chemically to CO via the Boudouard reaction. Kulkarni et al.19 indicated that using 20GDC electrolyte they achieved direct oxidation of carbon black on La0.6Sr0.4Co0.2Fe0.8O3±δ anode at 600 °C and demonstrated 120 W/m2 of peak power density in flowing He. They had implicitly assumed no CO should be available or form at this temperature to fuel the anodic reaction. Relying on an independent study135 of cell performance with varying He carrier gas flow rate, Guzman et al.127 have discounted the role of CO oxidation and attributed power generation primarily to direct electrochemical oxidation of solid carbon at the anode. Based on a volumetric estimate of the triple phase boundary density, they argued that the rate of solid carbon oxidation at the anode is significantly faster than the oxidation of H2 on a per TPB site basis. Studies claiming direct electrochemical oxidation of carbon involve experiments mostly carried out at 600 °C and in inert carrier gas. It was argued in these studies that the mere generation of electrical power in an inert gas environment and at a temperature where the Boudouard reaction is not favorable, by itself constitutes sufficient evidence for direct oxidation. However, this is not a convincing argument. It is worth noting several considerations and arguments that may collectively support a different conclusion, that instead, the “CO shuttle mechanism” proposed originally by the author13,14 may indeed be the active driver primarily responsible for power generation under these conditions. This shuttle mechanism is schematically depicted in Figure 11, and illustrates the role of the nearby carbon in providing a supply of CO to the anode.

Figure 11. Schematic illustration of the “CO shuttle” mechanism depicting the continuous cycle of CO electro-oxidation at the anode to form CO2 that subsequently undergoes Boudouard reaction with the nearby carbon to form additional CO. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

First, commercial ultra pure He gas, unless deoxygenated by hot metal chips upstream of the cell, contains trace levels of O2 as well as moisture that may initiate the oxidation or gasification reaction of fine carbon black particles during cell heat up. Subsequent build up of CO through the Boudouard reaction may sustain the cell, as suggested earlier.37 The formation of CO2 or CO by impurities in He may start the CO shuttle cycle,14 which gives rise to power generation. Indeed, strong temperature dependence of open circuit potentials attained in these studies even at temperatures as low as several hundred degrees Celsius is indicative of CO/CO2 mixture present at the anode. Second, carbon black is inherently an excellent adsorbent and can form various stable surface complexes including oxygenated species when exposed to the environment prior to administering it into the cell proper. Moreover, the native chemical structure of carbon also contains such surface complexes and their acidity was found to be equivalent to the amount of CO2 evolved.136 Indeed, the reactivity of carbon is closely related to the presence of oxygenated surface groups, also called “surface oxides”.137 It is well-known that CO, CO2, and H2O are the primary products of thermal desorption from carbon surfaces, where CO2 desorbs at low temperatures and CO at high temperatures as expected.138 However, temperature programmed desorption (TPD) studies coupled to mass spectrometry and X-ray photoelectron spectroscopy clearly indicate that CO desorption may start at temperatures as low as 600 °C on graphitic carbon blacks139 and on Spherocarb carbon,140 at 650 °C on activated carbon,141 and about 700 °C from activated carbon derived from coconut char.33 Desorption of CO2 ceases above 800 °C, after which the primary product of desorption was CO.142 In short, CO and CO2 may readily be released from carbon surfaces during heating of the cells in these reports. Third, although the Boudouard reaction is thermodynamically favorable above ∼700 °C, this does not necessarily mean that there can be no CO formation at lower temperatures as seemingly assumed in studies that claim direct electrochemical oxidation. Indeed, Atamny et al.139 reported nearly 10% CO formation measured at 600 °C, which is in good agreement with the thermochemically predicted values in Table 3. Hence, the CO released in this manner may be sufficient to initiate the O


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“CO shuttle mechanism”,14 which then feeds into the power generation process. It has long been known that alkali metal salts as well as iron oxide are effective catalysts for gasification of carbon.143−145 As expected, adding iron catalyst to the Boudouard gasifier may significantly enhance the extent of gasification, where in one study the CO content of gasifier product increased nearly 6 fold to over 60% at 700 °C,146 and in another study, nearly 1 order of magnitude for catalyzed graphite, and up to 2 orders of magnitude for activated carbon at gasification temperatures around 700 °C.147 The CO shuttle mechanism is expected to be even more dominant on cell performance for cells operating at higher temperatures. Indeed, it was reported that at 900 °C direct oxidation was in competition with the Boudouard reaction.126 Hence, by all accounts CO is likely the active participant in the oxidation reaction at the anode as opposed to the proposed direct electrochemical oxidation of solid carbon to CO2. Likewise, the prevalent CO/CO2 ratio at the anode establishes the cell OCV, and not the C/CO2 equilibrium since this would exhibit practically no temperature dependence, whereas strong temperature dependence of OCV was observed in these reports. This points to the dominant role of CO and CO2 at the anode in these studies. As further evidence, a recent study that investigated the effect of physical contact between the anode surface and the carbon fuel found no evidence of direct oxidation of carbon, and no distinction for the anode reactions between the physically contacting and detached cell arrangements, where CO oxidation is the dominant mechanism in the latter case.148 These arguments question the conclusions regarding direct electrochemical oxidation made by various studies and rightfully demand that more convincing and incontrovertible experimental evidence is required to assert the validity of such claims. One possible approach may involve running experiments using carbon char that has been thermally pretreated at temperatures in excess of 900 °C in an inert atmosphere in order to remove all volatiles and adsorbed surface species, and then transferred to the test cell while avoiding any contact with oxygen or oxygenated species including H2O and CO2. Cell open circuit measurements should clearly indicate values close to the expected value of 1.02 V that remains practically independent of temperature over a sufficiently wide temperature regime. Moreover, spectroscopic studies of surface species at the anode and detailed gas analyses of the anode exhaust as a function of cell current density would add supporting evidence and valuable information to assess direct conversion. 4.5.2. Rechargeable Pyrolytic Carbon Cells. The premise of this approach is to greatly circumvent the poor solid-to-solid contact encountered in direct-contact schemes by depositing solid carbon in close proximity of the TPB. It offers an effective way to supply the solid fuel to the TPB by thermally depositing carbon deep inside the micropores of the anode structure close to the YSZ interface where there is access to the oxide ions. This approach was originally proposed by Ihara and his co-workers,149 who have deposited elemental carbon at 900 °C via thermal pyrolysis of hydrocarbon gases such as propane150 and methane151 inside the pores of the Ni/ YSZ and Ni/GDC cermet anode structures coated on 0.3 mm thick YSZ electrolyte disks with LSMO cathodes. After carbon charging, deposited carbon is then oxidized and consumed in fuel cell mode. Upon depletion, the cycle begins again. In this respect, this arrangement may resemble more a rechargeable

battery than a fuel cell. The maximum power densities were reported to be 520 W/m2 for carbon derived from propane pyrolysis and 550 W/m2 for carbon from methane. The proposed anode reaction mechanism involves full and partial oxidation of carbon to CO2 and CO, respectively, by reaction with the lattice oxygen, as well as the Boudouard reaction for carbon particles deposited away from the TPB. A similar study employed pyrolytic carbon derived from propane in a bilayer anode structure made of Ni/GDC and Ni/YSZ and reported to produce 56.5 W/m2 for about 60 s in flowing CO2 carrier gas.152 Naturally, this type of CFC necessarily operates in batch mode, where the amount of carbon deposited by pyrolysis is rather limited, and hence provides only limited energy capacity. To overcome this shortcoming, more recent work153 proposed a rechargeable operation mode that involved pulsed jet injection of iso-octane on to the anode at a relatively high frequency of 1 pulse/s. They demonstrated current densities up to 6000 A/m2 at 900 °C. This scheme provides a quasi-steady state fuel cell operation. Both the quantity and the particle size of deposited pyrolytic carbon are important considerations for this CFC approach. Thermogravimetric measurements of methane pyrolysis at 800 °C on Ni powders gave only 1% weight increase while no appreciable weight change was observed for YSZ powder, indicating that during pyrolysis, carbon seems to prefer Ni surfaces,154 where the carbon particles were found to be in the size range of 1−10 μm. Obviously, carbon particles of this size likely have limited opportunity to undergo direct electrochemical oxidation. For this type of carbon fuel cell, the location of the carbon deposit is an important consideration. Using electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy, Li et al.134 have studied the distribution and reactivity of pyrolytic carbon inside a Ni cermet anode structure deposited from methane at 800 °C. They postulated that direct electrochemical reaction requires ready access of the carbon particle to oxide ions as well as an electronic pathway to conduct the electrons. They too observed that carbon deposition prefers Ni surface sites as opposed to YZS surface, but the electrochemical oxidation is most likely if carbon is located at TPB, less so on YSZ, and rather limited on Ni. 4.5.3. Detached Cells. Poor, insufficient contact between the surfaces of carbon particles and the anode provides the motivation to search for alternative schemes that supply the fuel more effectively to TPB. Physically separating the SOFC element from the gasifier for generation of syngas from carbonaceous sources was originally suggested in 1960s by Archer and Zahradnik,67,68 who proposed to employ coalderived gases in a solid oxide fuel cell power plant. A similar approach has been adopted about a decade ago in the Solid State Energy Conversion Alliance (SECA)29 program supported by the U.S. Department of Energy, where coal undergoes steam gasification, followed by syngas cleanup and water gas shift processing. The resulting hydrogen is then supplied to a SOFC stack to generate power. In all detached cell arrangements, carbon fuel particles are physically separated from the anode surface, and the fuel supplied to TPB is generally in the form of CO, or a mixture of H2 and CO. Lack of physical contact avoids possible abrasion problems and wear of anode surface or microstructural damage during prolonged use. In the case of carbonaceous fuels with high ash contents, the detached cell scheme also minimizes P


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thermogravimetric experiments carried out between 250 and 1100 °C, no significant interaction between CO and Spherocarb was observed.157 The fuel cell element is made of YSZ electrolyte with a catalytic cermet anode such as Ni/YSZ and a mixed conducting cathode such as LSMO. Oxygen from air is reduced to oxide ions at the cathode, transported through the YSZ lattice, and electrochemically reacts with the CO at the anode TPB. The corresponding reactions at the anode side are

chemical interactions and possible mass transfer between ash and the anode surface, changing or damaging its microstructure and deactivating its catalytic reactivity. Gür and Huggins36 demonstrated power production from a tubular SOFC element physically separated from powdered carbon source such that the carbon fuel and the SOFC element resided in independently controlled temperature zones in a sealed quartz enclosure. They achieved a maximum current density of 320 A/m2 and OCV values between 1.0 and 1.1 vs air above 800 °C cell temperature. Although not understood at the time, it is now clear that their cell operated via the Boudouard reaction, where the CO formed at the carbon source was oxidized electrochemically to CO2 at the Pt anode. Similarly, a 4-mm block of charcoal was used as solid fuel in a SOFC arrangement where the charcoal pellet was separated by 5 mm from the YSZ electrolyte disk with porous Pt electrodes.124 The cell operated between about 800 and 1000 °C with OCV values between 0.936 and 1.10 V. Maximum current density of 223 A/ m2 was achieved at 1002 °C, where the product gas analysis indicated 98.4% CO and 0.2% CO2, with the remainder primarily N2 originally present in the chamber. Graphite powder was employed as the solid fuel in a 1.5 mm thick tubular YSZ electrolyte SOFC arrangement with porous Pt electrodes.155 The graphite bed was physically detached from the anode surface. The effect of fuel particle size on cell performance between 650 and 950 °C for graphite powers in the ranges >32 μm, 90−150 μm, and 150−180 μm indicated that performance was improved for smaller particles, where maximum power density of 168 W/m2 and OCV of 1.115 V was obtained at 950 °C. A similar study156 employed carbon black impregnated by K, Ca, and Ni catalysts using aqueous solution process with a loading ratio of 10:1 for carbon-to-metal atom, corresponding to a metal-to-carbon weight ratio of 7−8% for each of K, Ca, and Ni. For pure carbon black, cell performance at 0.7 V gave average power densities of 976 and 1543 W/m2 at 900 and 1000 °C, respectively. Similarly at the cell voltage of 0.7 V, power densities at 750 °C were 1477 W/m2 for K-catalyzed carbon black, 1123 W/m2 for Ni-catalyzed carbon black, and 1034 W/m2 for Ca-catalyzed carbon black. Efficacy of catalysts toward CO2 gasification was ranked in the descending order of K > Ni > Ca. A qualitative comparison of power densities indicated that addition of K to catalyze the Boudouard reaction seem to lower the cell operating temperature by nearly 200 °C for similar power densities, while the corresponding numbers are 150 °C for Ni and 130 °C for Ca. This suggests that the cell operating temperature can significantly be reduced by addition of appropriate catalysts to the Boudouard gasifier. Alternatively, cell performance can significantly be improved by catalyst addition. 4.5.4. Fluidized Bed Carbon Fuel Cells (FB-CFC). The possibility of converting solid carbonaceous fuels to electricity in a fluidized bed solid oxide fuel cell arrangement was first proposed by Gür and Huggins30,36 two decades ago. More recently, the fluidized bed carbon fuel cell (FB-CFC)31,32 approach was investigated further by Gür and co-workers using various carbons,13,14,20,33−37 coals,20,38 and biomass.10,20,39 FBCFC employs a SOFC element that is physically and thermally integrated with a dry gasifier.31,32 The anode reaction product CO2 is recirculated through the carbon bed minimally fluidizing the carbon particles while producing CO via the Boudouard reaction. As the CO rise through the carbon bed, its interaction with solid carbon is minimal and can be ignored. In fact, in

C(s) + CO2 (g) = 2CO(g)

(13)

2CO(g) + 2O2 −(electrolyte) = 2CO2 (g) + 4e−(electrode)

(15)

The reaction at the cathode is O2 (g) + 4e−(electrode) = 2O2 −(electrolyte)

(4)

The net cell reaction is given by C(s) + O2 (g) = CO2 (g)

(5)

The FB-CFC process is schematically depicted in Figure 12, which illustrates the individual reaction steps. The reactor

Figure 12. Conceptual cell schematic of the FB-CFC process, where the anode product gas CO2 is partly recycled through the carbon bed to fluidize and generate CO, which is subsequently oxidized at the anode. Adapted with permission from ref 14. Copyright 2010 The Electrochemical Society.

chamber that houses the fuel cell and the Boudouard gasifier typically operates between 800 and 900 °C. With an active cathode area of 5 cm2, maximum power density of 2200 W/m2 at 0.68 V was achieved35 with activated carbon at 905 °C as shown in Figure 13. A similar FB-CFC arrangement with 24 cm2 active cathode area demonstrated an attractive power density of 4500 W/m2 for a single cell power of 11 W at a practical cell operating voltage of 0.64 V at 850 °C using untreated Alaska (Waterfall Seam Kenai-Cook Inlet County, AK) coal powder as the solid fuel in the Boudouard gasifier.38 Electrical conversion efficiencies measured experimentally were around 50%,20,38 slightly lower than potentially offered by molten hydroxide or carbonate based CFCs. A thermodynamic analysis on various types of solid fuels including coals, biomass, Q


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Figure 14. Temperature dependence of the theoretical open circuit cell potentials for partial and full oxidation of carbon indicated by light solid lines compared to oxidation of hydrogen (dashed line), where gas phase compositions assume unit activity. The dark solid lines indicate the theoretical open circuit cell potentials for the C−O and C−H−O equilibrium for reactions 30 and 31, respectively, in the presence of solid carbon such that x ≫ 1. Clearly, the presence of solid carbon of unit activity significantly increases the expected cell potential, while the difference between C−H−O (steam gasification) and C−O (dry gasification) is only marginal, where steam gasification offers a small advantage of merely 0.02−0.04 V. Adapted with permission from ref 35. Copyright 2009 Elsevier.

Figure 13. Voltage-current-power (V−I−P) behavior at 905 °C of an anode-supported FB-CFC configuration of 5 cm2 active area operating on untreated activated carbon in direct contact with the anode surface. Cell performance indicated power density of 2200 W/m2 at 0.68 V versus air. Reprinted with permission from ref 35. Copyright 2009 Elsevier.

and waste, predicted system level conversion efficiencies in the range of 50−58%, which can easily be boosted to 60−65% by adding a Rankine bottoming cycle.35 By all accounts, these system level conversion efficiencies and cell performance values are sufficiently attractive to justify opportunities for further investigation and development. 4.5.5. Thermochemical Considerations in the Presence of Solid Carbon. The fluidized bed CFC arrangement can be realized in two schemes, namely, immersed bed and detached bed. In the first scheme, the SOFC elements may be fully immersed in the carbon bed inside the Boudouard gasifier such that the carbon bed is in direct physical contact with the anode surface. In the second scheme, the carbon bed may be detached and physically separated from the SOFC element, but still reside in the same thermal zone or reaction chamber. Both schemes have advantages and shortcomings. Although the detached bed configuration eliminates concerns regarding abrasion of the anode surface microstructure by solid fuel particles, effective transfer of heat required to sustain the endothermic Boudouard reaction may pose even more severe engineering challenges than the immersed bed configuration. In both cases, the enthalpy for the exothermic heat of CO oxidation at the SOFC anode is considerably larger than that needed for the Boudouard reaction (e.g., −560 kJ/mol versus +170 kJ/mol at the cell operating temperatures, respectively). On the other hand, it is desirable to carry out the gasification process at or near Boudouard equilibrium, which is favored by high temperatures (see Table 3). To maintain the gasifier temperature preferably in the range 800−1000 °C, effective transfer of the heat content of the anode product gases is required. This is more challenging to realize in the detached configuration than for the immersed scheme. The latter also offers a significant thermodynamic advantage, namely, that the presence of solid carbon fuel of unit activity shifts the CO equilibrium of the reaction gases to yield a higher OCV.35 This is depicted in Figure 14, which shows how the cell OCV varies with the equilibrium composition of the reaction products at 1 atm. The relevant reactions for Figure 13 are C(s) + O2 (g) = CO2 (g)

C(s) + 1 2 O2 (g) = CO(g)

(29)

CO(s) + 1 2 O2 (g) = CO2 (g)

(30)

H 2(g) + 1 2 O2 (g) = H 2O(g)

(31)

xC(s) + O2 (g) = C(s), CO(g), CO2 (g), O2 (g)

(32)

xC(s) + H 2O(g) = C(s), CO(g), CO2 (g), O2 (g), H 2(g), H 2O(g), CH4(g) (33)

As expected, the direct electrochemical oxidation of carbon in reaction 5 gives an OCV of about 1 V, nearly independent of temperature. Oxidation of CO results in a lower cell potential at elevated temperatures, since reaction 30 assumes equal activities, i.e., CO/CO2 = 1. However, if CO oxidation is carried out in the presence of solid carbon as in reaction 32, then it leads to substantially higher OCVs as indicated by the dark solid line labeled “C/O Equil” in Figure 14. A similar consideration for H/O ratio of 2 as indicated by dark solid line labeled “C/H/O Equil” is valid when the anode reactions are supplied by steam gasification carried out in the presence of solid carbon fuel as in 33, instead of oxidation of hydrogen via reaction 31. Comparison of the two equilibrium lines for C/O and C/H/O indicate a slight advantage of 0.02− 0.04 V higher OCV for steam gasification over dry gasification. However, this minor increase in OCV comes at the cost of using water, which is increasingly becoming a scarce resource. This again validates the minor difference between adopting dry versus steam gasification for SOFC-based CFCs. In either gasification schemes, however, the presence of solid carbon helps increase the CO/CO2 ratio at the anode yielding higher OCV than dictated otherwise by gaseous chemical equilibria of equal activities. In case of the detached cell configuration where only gaseous species dictate equilibria at the anode, the CO/CO2 ratio monotonically decreases in the

(5) R


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gas flow direction along the anode surface. Naturally these considerations are valid for true equilibrium conditions, and the actual cell performance and OCV values are dictated by the relative rates of gasification and anode processes. 4.5.6. Electro-oxidation of CO. Hydrogen is the most studied fuel in fuel cells, and its oxidation kinetics on SOFC cermet anodes is considered to be much faster than CO. This is attributed to the higher activation losses associated with anodic oxidation of CO.158−160 Many of these studies, however, were made on small button cells with less than 1 cm2 active electrode area, and they reported up to 2-fold higher power densities for hydrogen than for CO.159 By contrast, a recent study161 using anode-supported tubular SOFC design with a large active electrode area of 24 cm2 achieved power densities of 6700 W/ m2 for pure CO and 7400 W/m2 for pure H2, both measured at 104 A/m2 and 850 °C using the same exact cell. The difference between cell performances on H2 and CO oxidation is only 10%. Since both studies employed the same cell compositions and microstructure, such a wide difference in the relative levels of cell performance is likely related to geometric effects since individual electrode kinetics is believed to be comparable in these cells. In a similar study, Li et al.156 reported at 850 °C power densities of 2468 W/m2 for H2 and 1630 W/m2 for CO that is derived from carbon dry gasification. A difference of nearly 50% lower performance for CO is in general agreement with the earlier study161 and is significantly smaller than the 2fold difference reported in small cell studies. The dominant role of current collection losses typical for larger cells may account partly for the small difference observed between H2 and CO oxidation on similar cell compositions and structures, but of larger sizes than commonly employed by other studies. Indeed, a recent modeling study supports this possibility.162 Such losses during current collection from large cells may overshadow activation losses due to reaction kinetics of individual fuels. Nevertheless, it fails to fully explain it and points to insufficient understanding of the kinetic mechanism for CO electrooxidation. A recent study163 of CO and H2 oxidation on Cu/ ceria cermet anodes revealed identical power densities of 3050 W/m2 at 700 °C, pointing to the fact that catalytic properties of the anode material is also an important consideration. There is general agreement among studies of heterogeneous oxidation of CO to CO 2 on platinum group metal catalysts164−167 that under slightly reducing conditions CO oxidation on Pt, Rh, and Pd proceeds via the Langmuir− Hinshelwood (L-H) mechanism, where both CO and oxygen from the gas phase adsorb on the catalyst surface prior to the oxidation reaction. In the case where the metal catalyst is supported on a reducible oxide substrate such as ceria, which can supply lattice oxygen to the catalytic site for CO oxidation, the catalyst support may also be directly involved in the reaction mechanism. This so-called Mars-van Krevelen (M-K) mechanism168 is relevant to SOFC cermet anodes where lattice oxygen is supplied to the catalytic sites through vacancy transport in the oxide ion conducting electrolyte. However, the elementary steps for the CO oxidation mechanism on transition metal cermet anodes are not fully understood. A recent study by Sunde and co-workers169,170 examined several possible scenarios on Ni electrode, including oxidation of the gaseous CO via adsorbed atomic oxygen, oxidation of adsorbed CO via adsorbed atomic oxygen, and oxidation of adsorbed CO via lattice oxygen. They proposed that the electrochemical oxidation of CO likely involves adsorbed intermediates, namely,

adsorbed atomic oxygen and adsorbed molecular CO, in general agreement with the L-H mechanism. In an effort to better understand CO oxidation kinetics on Ni/YSZ cermet anodes, a computational modeling study171 investigated a detailed reaction mechanism involving the Ni surface reactions, YSZ surface reactions and the charge transfer reactions. The model suggested that at high CO partial pressures, charge transfer occurs via the steps 34 and 35 O2 −(YSZ) = O−(YSZ) + e−(Ni)

(34)

O2 −(YSZ) + *(Ni) = O(Ni) + *(YSZ) + e−(Ni)

(35)

Here, the subscripts Ni and YSZ denote free surface sites on Ni and YSZ, respectively. Reaction 34 represents a single electron transfer step, and (35) indicates the spillover of OYSZ− ion onto the Ni surface, simultaneously transferring its electron to Ni. At low CO partial pressures, spillover rate via (35) decreases due to site blocking and charge transfer occurs via a combination of reaction 34 and the additional reaction step 36. O−(YSZ) = O(YSZ) + e−(Ni)

(36)

This reaction involves electron transfer from the OYSZ− ion to Ni, resulting in the formation of lattice oxygen on YSZ surface. Naturally, the proposed mechanistic framework as well as the change in the CO oxidation mechanism with CO/CO2 ratio requires experimental verification. Nevertheless, such computational methods to predict possible reaction pathways and associated energetics are useful tools to provide guidance and understanding of CO electro-oxidation kinetics. 4.5.7. Steam-Carbon Cell. As an extension of the FB-CFC platform, the concept of exploiting the carbon−oxygen equilibria to drive the electrochemical reduction of steam to obtain carbon-free hydrogen was proposed earlier by Gür and Duskin.172 Ordinarily, steam electrolysis is thermodynamically uphill. Instead, the steam-carbon fuel cell concept provides a thermodynamically downhill process for spontaneous generation of hydrogen (and cogeneration of electrical power). More recently, the steam-carbon fuel cell concept was demonstrated experimentally,173−175 where the carbon compartment is physically separated from the H2/steam stream by the impervious YSZ solid electrolyte, thus requiring no further need for expensive separation processes otherwise required to remove CO and CO2 from the H2 stream as is the case for reformation processes. The steam-carbon fuel cell concept is schematically illustrated in Figure 15. CO-free hydrogen is critically important for many hydrogen based power generation and storage applications. Even at trace quantities, CO is known to poison Pt catalytic anodes in low temperature polymer exchange membrane fuel cells (PEMFC) that are under development by car manufacturers for transportation power sources. The new steam-carbon fuel cell concept takes advantage of the downhill thermodynamic driving force that is the result of the Gibbs free energy difference between steam reduction and carbon oxidation reactions, which produces up to 0.5 V of open circuit cell potential at 900 °C, depending upon the H2/H2O ratio at the cathode. The results of modeling the operation of such a cell indicated that it is possible to maintain practically significant current densities in the several thousand A/m2 using this new concept for efficient production of carbon-free hydrogen.175 S


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5.1. Melt Stability

Ionic salts employed in the molten state in CFCs based on molten carbonate eutectics and molten hydroxides offer extensive choices in tuning the melting point and chemical properties of the resulting melt simply by varying the composition of the melt. However, they also present serious challenges in terms of the chemical stability of both the melt itself as well as the surrounding materials in contact with the melt. It is well-known that molten salts have a high capacity to solvate other materials as well as gases due to the ionized nature of the melt. Accordingly, concerns related to hot corrosion, dissolution and chemical reaction of the surrounding cell materials with the melt expectedly narrow the choices available for cell construction materials and usually limit them to specialized and expensive materials. The melts may also interact chemically with some of the constituents of solid fuels such as mineral ash, and other soluble matter in coals, biomass, and biomass derived carbons. The resulting chemical changes and interactions undesirably alter the melt properties, and also damage cell components and degrade cell performance over time. Furthermore, dissolution of gases (e.g., oxygen) or moisture in these melts may result in highly reactive species. For example, it is well-known that oxygen or water when dissolved in alkali metal hydroxides such as NaOH or KOH form peroxide and superoxide ions that readily dissolve metals, even noble metals, and ceramics. As discussed earlier in this review, molten hydroxide melts also are susceptible to carbonate formation (see reaction 18) by CO2 evolution during electro-oxidation of the solid carbon anode or by interaction with ambient air.

Figure 15. Schematic depiction of the steam-carbon fuel cell concept, where steam is supplied to the cathode and solid carbon to the anode (top). This scheme is an extension of the FB-CFC platform and provides a downhill chemical potential gradient (bottom) for oxygen across the cell resulting in considerable driving force for effective cogeneration of hydrogen and electrical power simultaneously and spontaneously. By contrast in conventional steam electrolysis where steam is at the cathode and air is at the anode side, the resulting uphill oxygen chemical potential barrier of nearly 1 V must otherwise be overcome to split H2O at elevated temperatures.

5.2. Wetting Issues in Molten Medium

For many CFC configurations, the nature of surface groups play an important role in wetting of the pores and external surfaces of carbon particles with the molten medium. Complete wetting is important for the proper establishment of the electrical double layer at the molten electrolyte/carbon (or coal) interface, which is critically important for the charge transfer reaction (e.g., reaction of carbon with the CO32− ion). Proper wetting of the solid fuel by the molten medium in molten anode and electrolyte systems is of profound importance to achieving high cell performance and efficient conversion. Wetting in these systems takes place gradually over time, and there are numerous reports of long soaking times required sometimes up to a day in order to achieve a stable cell potential.87,96,176 This is indicative of initially poor wetting, which over time spreads and eventually penetrates the carbon

5. TECHNICAL CHALLENGES Clearly, CFCs offer many attractive and important opportunities to address and respond to the energy needs as well as the environmental concerns facing this century, but not without technical challenges, knowledge gaps, and engineering issues yet to be understood and resolved. Such challenges are also very much dependent upon the particular CFC system, where individual CFC configurations present different challenges. Table 4 presents a summary of broader issues facing individual CFC configurations reviewed above. A more detailed discussion of select challenges is provided below.

Table 4. Progress and Critical Issues for Various CFC Configurations progress and critical issues solid fuels tested

ash in electrolyte wetting of carbon tolerance to contaminants stability of electrolyte percolation requirement cell performance cell electrical efficiency

carbon coal char biomass removal interaction sulfur others

MH-CFC

MC-CFC

hybrid CFC

MA-CFC

SO-CFC

yes no no difficult likely concern unknown unknown low N/A low moderate

yes no no difficult likely concern high high moderate high low high

yes no yes difficult likely concern high high moderate high moderate high

yes yes yes difficult unlikely concern high high high N/A moderate moderate

yes yes yes easy unlikely N/A low low high N/A high moderate

T


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surface by molten metals in CFC arrangements employing solid oxide electrolytes is another important consideration for effective charge transfer during cell operation. This is particularly true for molten Sn anode CFC. In penetration experiments employing porous ceramic separator, liquid Sn did not wet the ceramics due to its high surface tension (>4 × 10−5 J/cm2) and its large contact angle (>90°), and hence did not penetrate pore sizes below 100 μm even at the elevated temperature of 1000 °C and at a pressure head of 0.9 psia.179 Similar results were reported for molten Sn anode CFC that wetting of the YSZ surface was poor as manifested by a high anode impedance,111 whereas this was not the case for molten Sb suggesting better wetting.112

pores. Addition of Rb and Cs carbonates to molten carbonate electrolyte lower the surface tension and improves wetting.176,177 Regardless of the type of molten hydroxide and carbonate electrolytes used for many CFC configurations, the charge transfer reaction occurs at the interface between the carbon particle and the molten electrolyte, provided there is a pathway for electrons to flow. Both the structure of the double layer at the carbon/electrolyte interface and the effectiveness of the charge transfer process at this interface depend largely on how well the surfaces of the solid fuel particles are wetted by the molten electrolyte. A recent study81 attempted to quantify and compare the wetting properties of Li2CO3−K2CO3 melts with Li−Na carbonate eutectics using contact angle measurements on an inert Au electrode, and reported that Li2CO3−K2CO3 melts exhibited smaller contact angles indicating better wetting properties. In a similar study, Yoshikawa et al.82 studied electrochemical and wetting behavior of Ni alloy anodes in Li/ Na and Li/K carbonate electrolytes at 650 °C, and estimated the contact angles as 61° and 40° for Li/Na and Li/K carbonate electrolytes, respectively, indicating better wetting properties of Li2CO3−K2CO3 melts. Another study53 demonstrated chemical pretreatment in strong acids (HF and HNO3) or base (NaOH) significantly improves wetting of activated carbon by Li2CO3−K2CO3 melts, and reported a significant shift in the oxidation potential for HF-treated carbon to lower values by nearly 0.4 V. It is expected that HF treatment increases oxygenated groups on the surface of the solid fuel particle,54 which promote wetting of the carbon or coal particles by the molten electrolyte and establishing the double layer, which facilitate the charge transfer reaction for the anodic oxidation of carbon. Similarly, Peelen et al.50 have reported up to 10-fold decrease in the charge transfer resistance when they immersed their graphite anodes for 24 h in the 62/38 mol % Li/K-carbonate eutectic melt at temperatures 650−750 °C. They explained this behavior by increase in the electroactive surface area due to surface roughening effects. Alternatively, a more plausible explanation may involve increased wetting of the graphite anode by the carbonate melt driven likely by capillary forces as well as diffusion during immersion, and increasing the effective surface area for electro-oxidation. Difficulty in wetting of the solid fuel is an important consideration for molten metal anode CFC systems also. Unfortunately, information about wetting characteristics of carbon is rather scarce in the literature for many molten metals. Nevertheless, some reports qualitatively indicate that wetting is critically important. For example, Jayakumar et al.112 observed poor contacting between the molten Sb anode and the solid fuel particles of carbon black and rice starch. Most of the carbon black suspended above the molten anode due partly to density difference, while the ash built up from the rice starch prevented anode reaction. To overcome this problem, Jayakumar et al. proposed separating the fuel cell compartment physically from the reaction vessel, where the electroactive species Sb2O3 formed at the fuel cell anode is regenerated chemically by reduction by the carbon in the solid fuel. Such a scheme, however, requires recirculation of hot, high density, high viscosity, two-phase molten medium between two process chambers, which is not an insignificant challenge. A further challenge is posed by the difficulty of wetting of ceramics surfaces178 by molten metals, as often encountered during brazing. Hence, achieving proper wetting of the ceramic

5.3. Percolation and Electrical Connectivity

Molten hydroxide and carbonate electrolytes are ionically conducting but electronically insulating and hence require the use of a current collector, usually made of nickel, to transport the electrons and facilitate the anodic oxidation reaction of carbon. However, nickel is not stable in the corrosive carbonate environment and gradually dissolves. Moreover, complete electrical connectivity and percolation among carbon particles must be maintained at all times to achieve high conversion efficiency. As the oxidation reaction proceeds and consumes carbon, the carbon particles are reduced in size and may get physically disconnected. This leads to loss of percolation and electrical connectivity among carbon particles in the molten electrolyte. Those carbon particles that are not connected electronically cannot participate in the electrochemical reaction, which is a major limitation for CFC schemes employing molten electrolytes. This can partially be overcome by maintaining high carbon loading at all times and using carbons that have high electronic conductivity such as graphite. However, the latter exhibits very poor reactivity in return. So not all forms of carbon may be suitable for this approach. Naturally, the use of a molten metal anode CFC configuration eliminates all concerns related to percolation and electrical connectivity among carbon particles in the anode compartment, since the molten metal anode provides excellent electronic conductivity to collect the electrons from the oxidation reaction. In addition, control of the evolution rate and buoyant flow of CO2 bubbles forming at the electrochemical interfaces through the carbonate melt also pose practical problems such as disruption to mass transport and current collection. Moreover, the ternary gas/liquid/solid system including gaseous CO2, solid carbon fuel, and the carbonate melt present multiplicity of interfaces inside the cell, which are challenging to characterize, understand, and control. Such interfaces govern rates and the effectiveness of critical processes including wetting, surface reactivity, extent of electrochemical reaction interface, mass and charge transport, and electron supply and collection. 5.4. Ash

Unlike coal, ash content of most carbons is typically quite low, usually below 1 wt %. Hence, chemical interactions of ash constituents with either the molten hydroxide electrolyte or the molten metal anode in CFCs may not present major operational problems in terms of loss of electrolyte, and precipitation or clogging of CFC components. In fact Weaver et al.180 reported that adding fly ash up to 10 wt % into the molten carbonate electrolyte of their MCFC-based CFC formed a separate phase of fine particulates but did not result in significant changes in the cell polarization behavior. On the U


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dissolved in the molten metal bath is electrochemically oxidized to SO2 gas, thus serving more as a fuel rather than poison. In a molten Sn anode cell, it was demonstrated108 that stable fuel cell operation could be achieved using liquid JP-8 fuel with a high sulfur content of 1350 ppm for 200 h without any fuel processing and reforming.

other hand, certain ash components, in particular silicates, may lead to deactivation of the anode by blocking active surface sites. However, there have been no systematic studies reporting on anode deactivation by ash in molten media CFCs. Cooper7 compared several industrial cleaning and deashing processes for coal and suggested that pretreatment for ash removal may be economically feasible. The separation of ash from the molten hydroxide and carbonate electrolytes can be accomplished by dissolution of the melt. Some carbonate eutectics such as K2CO3−Na2CO3 (but not Li2CO3) and hydroxides are soluble in aqueous solutions, and hence can be leached out to separate from the ash content. This of course, is not the case for molten metal anode based CFC, where separation of ash pose severe challenges and loss of the molten metal entrained with the solid ash. Impact of ash in SOFC-based CFCs may present less serious operational problems, since it is relatively easier to separate and handle solid particles in gaseous environments than in molten media. However, silicates generally found in ash can pose serious effects on the catalytic activity of anode materials. For these reasons, issues related to ash separation and removal as well as interaction of ash with cell components and materials all need to be investigated, understood and addressed before realizing the utilization and conversion of ash-containing solid fuels in CFCs.

5.6. Coking or Carbon Deposition

Excluding the colder regions of the gas manifolding and the CFC chamber where the reverse Boudouard reaction may lead to carbon deposition, coking is less of a problem in general for molten electrolyte or molten metal anode based CFCs. Under certain conditions, however, use of solid carbonaceous fuels in SOFC-based CFCs may potentially lead to carbon deposition at the anode, giving rise to catalytic deactivation and degradation of CFC performance. Ternary C−H−O phase equilibria as shown in Figure 16, can effectively predict the tendency for coking.194,195 The solid lines

5.5. Cell Degradation by Sulfur and Other Impurities

Solid carbon, depending on its origin may contain trace quantities of contaminants including sulfurous and silicate compounds. These impurities may poison the catalytic anode and adversely affect the durability and stable operation of CFCs. Among these, sulfur especially poses major challenges. Recent reviews181,182 discuss mechanisms of degradation and developments in sulfur-tolerant anode materials for solid oxide fuel cells that employ gaseous and liquid hydrocarbons, which normally contain varying quantities of sulfur. Outside of sulfur, there are other contaminants including As and P, which are known to rapidly degrade SOFC anodes made of Ni cermet.183−190 There is also evidence for synergistic effects that the presence of sulfur in the form of H2S magnifies and accelerates the cooperative deleterious effects of As and P on cell performance through Ni anode degradation.191,192 In a steam gasification environment, the formation of gasified contaminants mostly in the form of volatile hydrides such as AsH3 and PH3 and their deleterious effect on Ni anodes are well established.183−188 These studies have indicated that AsH3 and PH3 even at 1 ppmv level react with Ni irreversibly to form various Ni-phosphide and Ni-arsenide phases that deactivate the Ni anode. In the case of dry gasification, the formation of volatile hydrides is not likely. So the only mechanism for these contaminants to pass into the gaseous phase and then deposit on the Ni anode may be through volatilization. In a related study, Tao et al.193 investigated the impact of coal impurities dissolved in molten Sn baths. For such molten metal anode CFC systems, sulfur seems to be comparatively less of a problem against the stability of the metal anode than in molten carbonate and molten hydroxide based CFC systems. This is partly because sulfur, similar to oxygen, has considerable solubility in the molten metal at elevated temperatures (e.g., 8% solubility in Sn at 1000 °C). Although some sulfur tends to form metal sulfides via chemical reaction and necessitates addition of make up metal to the anode bath, most of the sulfur

Figure 16. Ternary equilibrium diagram for the C−H−O system at 1 atm total pressure indicating the location of the carbon deposition boundary as a function of temperature. Red heavy dashed line between CO and the H-apex marks the approximate boundary for carbon deposition at 850 °C, the operating temperature used in many SO− CFC studies. The red heavy arrow along the C−O edge indicates the direction the overall gas composition is shifted as CO is oxidized to CO2 at the anode. Adapted with permission from ref 195. Copyright 2003 The Electrochemical Society.

emanating from the H-apex mark the boundary of the carbon deposition region at 800 and 1000 °C. All nominal gas phase compositions that fall within the carbon deposition region have the thermodynamic tendency for coking. CFCs usually involve gas compositions that lie close to or along the C−O edge of the diagram. Accordingly, the oxidation of either the carbon or CO to CO2 moves the overall composition along the C−O edge toward the oxygen apex, i.e., away from the carbon deposition boundary. For most common hydrocarbon fuels, thermodynamic studies196,197 predict the possibility for carbon deposition under open circuit conditions. Recent studies reported infrared (IR) spectroscopy of oxygen species23,24 on SOFC cathode surfaces and in situ Raman spectroscopy of surface intermediates of CO and hydrocarbons, as well as sulfur, on Ni/cermet anodes under various cell operating conditions.198,199 Under open circuit condition, CO oxidation on V


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porous Ni/YSZ anode at 715 °C includes both graphitic carbon and Ni-COO intermediates. In current carrying mode under fuel cell conditions, however, a clear reduction was observed in the Raman peak for carbon, which seems to disappear completely in a matter of 10 min indicating complete removal from the anode surface.200 This is shown in the Raman spectra of Figure 17. Hence, the process of carbon deposition and removal seems to be dependent on both time and the cell operating conditions.

CFC and up to 65% conversion efficiency for various coals and carbons20,35,38 for a FB-CFC arrangement were estimated in recent studies, indicating that it may be possible to double the conversion efficiency of conventional coal power generation. Considering a hypothetically optimistic best case scenario, had all the coal fired power plants in the U.S. converted overnight to CFC based power generation, annual CO2 emissions would be reduced by nearly 1 Gt in the U.S. alone. This is an exceedingly large reduction that is equivalent to about 50% of the total CO2 emissions from the entire transportation sector in the U.S., or commissioning 136 of 1 GW nuclear power plants, or 270 000 of 1 MW wind turbines, or 750 GW of photovoltaic solar farms, which is nearly 2 orders of magnitude greater than the currently installed global capacity.202 Needless to say, this hypothetical scenario is admittedly unrealistic and unattainable, but it nevertheless demonstrates the grand scale at which CFCs may impact issues of CO2 emissions and capture toward mitigating environmental adversity and achieving cleaner power generation. Given credible expectations that the amount of coal use will continue to increase around the world over the next several decades, there is obvious incentive and imperativeness to advance CFC research and development to achieve this goal.

Figure 17. In situ Raman spectra from a Ni/YSZ cermet anode surface at 715 °C. Spectra at right show carbon deposition and build up from exposure to butane under OCV conditions (or, at −1.06 V vs air). Gradual removal of carbon by oxygen evolution under current carrying conditions is clearly evident at cell voltages of −0.46 V (left spectra), −0.76 V (center). The featureless spectrum encircled with a dashed trace in the middle top row is from a clean, reduced Ni/YSZ anode surface. D and G modes of graphite are indicated at 1350 and 1585 cm−1, respectively. At the cell voltage of −0.46 V, the G mode appears 1 min after the cermet anode is exposed to 5 cm3 butane (second row from top) and disappears after passage of current (third row from top). Similarly at −0.76 V, the G and D peaks that form within 1 min of exposure to butane eventually disappear completely after several minutes (fourth row from top). By contrast, D and G peaks steadily increase with butane exposure time under OCV conditions indicating growth and build up of surface carbon. Reprinted with permission from ref 200. Copyright 2007 American Chemical Society.

7. ECONOMIC PROSPECTS Advances in competing technologies for power generation such as oxyfuel combustion and gasification technologies that employ pure oxygen, and not air, offer incrementally high efficiencies in the low 40% not including carbon-capture and compression and produce a nitrogen-free concentrated CO2 flue stream. When the energy burden for carbon capture and compression is taken into account, the conversion efficiency reduces to about 32%, as estimated29,203 by the U.S. Department of Energy (DOE). Moreover, all process steps in next generation coal power technologies are physically separate, leading to thermal losses, which lower system efficiency. Oxyfuel combustion40 and gasification technologies43,45,143 also require up front production of pure oxygen by separation from air, which is an energy intensive process. Energy wise, oxygen separation from air typically costs more than 10% of the electrical power generated in the plant. By including a separate fuel cell conversion step in the process stream, the U.S. DOE estimates indicate that the improvements in system level conversion efficiencies may potentially be quite significant, increasing the efficiency from 28.4% for pulverized coal fire plants to 51.1% for an integrated gasification fuel cell (IGFC) process running at atmospheric pressure on coal-derived gases. Both efficiency figures include the energy costs of CO2 capture, compression and storage.29 Naturally, increased efficiency also produces proportionately smaller carbon footprint and pollutant emissions to the environment. So there is great potential for CFCs to be an economically viable efficient solution for third generation power generation. However, CFC technology is yet in its infancy. This makes it difficult to assess its economic viability and prospects. A more meaningful consideration may involve benchmarking with advanced power generation technologies such as integrated gasification combined cycle (IGCC) and IGFC, where credible estimates exist. IGFC is a modification of IGCC where a solid oxide fuel cell (SOFC) replaces the turbine as the major power generation engine. In essence, IGFC is almost identical to DOE’s SECA program29 but on a grand scale.

In agreement with the spectroscopic results, no carbon deposition was observed over a 375-h galvanostatic study of CO oxidation on Ni cermet anode at 4140 A/m2 and 850 °C.161 Accordingly, even if carbon deposition indeed occurs initially during open circuit operation of the cell, this surface carbon is removed by lattice oxygen carried by the current, leaving a clean anode surface.

6. POTENTIAL IMPACT OF CFCs ON U.S. CO2 EMISSIONS The U.S. currently emits about 5.8 Gt of CO2 into the atmosphere, and this is expected to rise to 6.9 Gt in 2035 at the current rate. More than 1/3 of CO2 emissions in the U.S. is by coal fired power plants. Correspondingly, CO2 emissions from U.S. coal power plants exceeded 2 Gt in 2007.201 Typically, every ton of coal used for power generation produces about 2.7 tons of CO2 emissions. As coal power plants are point source producers of CO2 emissions, CFCs are ideally suited to mitigate emissions from centralized power generation. This of course assumes successful development, commercialization, and large scale adoption of CFC technology for coal power generation. Currently, pulverized coal fired plants operate at conversion efficiencies in the range 30−35% depending upon their age, with most of the installed capacity operating in the low 30%. By contrast, 78% efficiency for carbon conversion88 for a MC− W


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Several attempts29,203−205 have been made to assess the expected costs of these advanced coal power technologies and compare them with existing pulverized coal fired power generation process. Their analyses also took into consideration the capture, compression and sequestration of CO2 from these power plants, which considerably lower the system level efficiencies while boosting up the costs. The results of four independent studies are compiled and presented in Table 5

promise superior opportunities for third generation coal power production.

8. CONCLUDING REMARKS As energy is indispensable and essential to maintain our standard of living, electricity sustains our life style. By all projections, coal will continue to be a major energy resource for decades to come, and its ∼40% share in the global electrical power production will remain practically unchanged and unchallenged through at least 2030.201 In our carbonconstrained world, effective and efficient utilization of solid fuels, especially coal, has become all too critically important to reduce the carbon footprint on the environment while maintaining energy security. Advances made in understanding efficient conversion of carbon in CFCs may ultimately pave the road to environmentally cleaner use of coal for power generation. By all accounts, recent progress on CFCs has demonstrated the promise of achieving the following advantages: • high conversion efficiency, potentially 60−70% • significantly reduced CO2 emissions • concentrated CO2 product stream requiring no need for separation • negligible NOx emissions due to • exclusion of nitrogen from entering the process stream • operating temperatures less than 1000 °C • reduced requirement for water, preserving a precious resource • system modularity not limited by economies of scale • suitable for both central base-load and distributed power generation • fuel flexibility due to largely carbon-specific chemistry • high quality waste heat, suitable for combined heat and power generation Understanding and optimizing carbon conversion in CFCs is the first natural and necessary step toward development of new strategies for effective utilization and efficient conversion of coal for power generation. It is not possible at this time to make an accurate prediction or judgment on which of the various CFC approaches or configurations reviewed here offer the most likelihood of success toward a practical system. They all exhibit varying degrees of complexities, complications and challenges that need to be overcome or understood through fundamental studies. The molten electrolyte and molten metal anode approaches seem to exhibit less susceptibility toward coal contaminants than solid oxide based CFCs. Moreover, molten carbonate based cells offer the advantage of extending the electrochemical reaction zone into a three-dimensional network via the dispersed carbon inside the molten bath. This should in principle lead to better cell performance. On the other hand, solid oxide based CFCs may be simpler to handle and work with since the primary processes in these systems involve gas/ solid interfaces that can be characterized, understood and controlled more effectively than the multitude of gas/liquid/ solid interfaces encountered in the molten systems. Moreover, electrode kinetics and mass transport rates in gaseous systems are naturally expected to be faster than in molten media. Similarly, separation and removal of coal ash may be simpler from the gas/solid environment of the solid oxide based CFCs than in molten media based CFCs. Although, significant advances were made in CFCs in the past decade, more targeted

Table 5. Estimated Cost and Performance Comparison of IGCC and IGFC with Pulverized Coal (PC) Power Generation Process advanced power systems with CO2 capture, compression, and storage PC

IGCC

IGFC

IGFC

baseline

baseline

atmos

press.

ref

efficiency HVV (%)

28.4 27.2 27 3570 2870 3820

cost of electricity (COE) c/kWh

15.0 11.6 9.9

51.1 42.8 43 49.4 2150 1991 3046 2000 10.8 8.5 7.7 8.8

57.0 57.3

capital cost $/kW

32.6 32.5 31 32.5 3330 2390 3780 2400 15.1 10.6 9.7 10.2

29 203 204 205 29 203 204 205 29 203 204 205

56.2 2100 1667 1800 10.3 7.3 7.9

below, which compares power generation technologies on the basis of expected efficiencies at the system level, capital cost for plant construction, and the levelized cost of electricity. As expected there are slight variations in the results, which are likely due to the methodology, input parameters, and assumptions employed in their cost analyses. Despite the slight spread in numbers, the results of these four independent studies show the same trend in general agreement with each other. Clearly, IGCC looks more attractive than the conventional pulverized coal power (PC baseline column in Table 5) technology, which provides the least desirable outcomes. Considering that a very large fraction of existing power plants in the U.S. and in the world utilize pulverized coal technology, IGCC is definitely a step forward in the right direction and represents the logical choice for next generation power production. The most striking result across the board, however, is the superiority of IGFC over the others on all three categories of comparison. Clearly, IGFC is not only significantly more efficient but is also more cost-effective. Although the fuel cell system cost is currently more expensive than a conventional combustion turbine, this is more than offset by the large decrease of equipment and operational costs downstream due to the significantly large conversion efficiency offered by IGFC. As opposed to IGFC where all individual process steps are physically and thermally separated from each other, CFCs aim to achieve conversion in a single process chamber. Integration of these process steps into a single thermal enclosure (or reactor vessel) give CFC clear advantages, as well as many technical challenges. If successful, however, it is reasonable to expect that CFCs will offer even higher conversion efficiencies than IGFC, comparatively smaller real estate, and lower costs of construction and operation, all leading to economically more attractive capital costs and cost of electricity. Clearly, CFCs X


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Biography

studies using coal in such cells are needed to further expand our understanding and appreciation of the technical barriers to technology development. Recent progress in SO-CFCs has already demonstrated practical power densities of 4500 W/m2 at a useful cell operating voltage of 0.64 V using untreated coal38 as fuel with experimental cell efficiencies around 50%,20,38 whereas a similar power density value of 4650 W/m2 was reported for activated carbon,125 and significantly higher values up to 8780 W/m2 for pyrolyzed carbon derived from medium density fiberboard in a hybrid-CFC.104 In modeling studies and estimates, cell efficiencies up to 78− 80% were reported. 6,7,95 CFCs also promise favorable economics 203−205 and a cost-effective pathway toward commercial prospects.29 However, for this nascent technology to advance in the technology pipeline before it gains practical interest for real applications, many scientific issues and technical hurdles need to be overcome, and for this it will require significant support for research. To this end, there are a multitude of fundamental issues and knowledge gaps. Surely, the mechanistic details and rates of carbon electro-oxidation reactions in molten electrolyte, molten anode, and solid oxide fuel cell arrangements are vastly different and poorly understood. Materials and chemical stability issues in CFCs have to be investigated systematically to ensure long service life. New materials with superior properties for cell components will help overcome some of the technical challenges. Among such critical problems is the poisoning effect of contaminants in coal and other solid fuels on cell components but, most importantly, on anode activity, stability, performance, and lifetime.206,207 Progress on this front will require not only wide scale investigation of the impact of these contaminants to understand how they react or interact with the anode but also to develop superior anode materials and current collectors resistant or tolerant to such contaminants. It is also important to consider processes aimed at removing substantial amounts of the contaminants before they reach the anode, preferably inside the CFC proper. Development of predictive models and use of computational tools will help test and design CFC arrangements and materials tailored to the particular type and composition of solid fuels.25,27,28 Another important issue relates to the mechanistic understanding and control of reactivity and kinetics of various solid fuels in dry and wet gasification environments, as well as in molten electrolyte or metal anode media. This point is important to expand the solid fuel beyond carbon in to coals and others such as biomass, municipal waste, agricultural waste, and waste plastics. In summary, the nascent technology of CFCs has the potential to offer efficient conversion of solid fuels such as coal, biomass, and solid carbonaceous waste to electrical power in either distributed or centralized fashion, and help solve many critically important issues facing the need for energy security and sustainable environment. However, it is only possible to achieve this outcome through increasing our fundamental understanding of the underlying materials issues and rate processes involved in solid fuel conversion.

Turgut M. Gür is a Consulting Professor of Materials Science and Engineering at Stanford University and the Executive Director of Stanford’s DOE-EFRC Center on Nanostructuring for Efficient Energy Conversion. Previously, he has served as the Technical Director for the NSF-MRSEC Center for Materials Research and, later, as the founding Technical Director for Geballe Laboratory for Advanced Materials at Stanford. He has been an Associate Editor for the Journal of the American Ceramic Society, organized and chaired multiple symposia and conferences, and served on several industrial, professional, and nonprofit boards including ten years on the Board of the International Society for Solid State Ionics. He holds BS and MS degrees in Chemical Engineering from Middle East Technical University in Ankara/Turkey, and three graduate degrees including a Ph.D. in Materials Science and Engineering from Stanford University. He holds 8 U.S. patents and has authored or coauthored 140 technical articles related to various aspects of energy conversion materials and processes including fundamental studies on fuel cells and electrocatalysis, phase equilibria and ion transport in oxide ceramics, thin film materials and fabrication, coal and hydrocarbon conversion, sensors, and separation membranes.

ACKNOWLEDGMENTS The author is grateful to co-workers R. E. Mitchell, A. C. Lee, and B. Alexander of Stanford University, M. Homel of Materials and Systems Research, Inc., and A. V. Virkar of the University of Utah for many helpful discussions. Partial support from a former DOE Grant No. DE-NT0004395 is also acknowledged. REFERENCES (1) U.S. Energy Information Administration: Annual Energy Outlook 2010, Annual Energy Outlook 2009: With Projections to 2030, Energy Information Administration, DOE/EIA-0383, 2009. (2) The Future of Coal: Options for a Carbon-Constrained World; Massachusetts Institute of Technology: Cambridge, MA, 2007; also available at: http://web.mit.edu/coal/The_Future_of_Coal.pdf. (3) COAL: Research and Development to Support National Energy Policy; NRC Report, June 2007. (4) EPA Proposal for Carbon Pollution Standards for New Power Plants; March 27, 2012; see: http://epa.gov/carbonpollutionstandards/ actions.html). (5) Williams, K. R. An Introduction to Fuel Cells; Elsevier Publishing Company: Amsterdam, 1966; Chapter 1. (6) Cao, D.; Sun, Y.; Wang, G. J. Power Sources 2007, 167, 250. (7) Cooper, J. F. Direct Conversion of Coal Derived Carbon in Fuel Cells. In Recent Trends in Fuel Cell Science and Technology; Basu, S., Ed.; Anamaya Publishers: New Delhi, India, 2007, p 1. (8) Rady, A. C.; Giddey, S.; Badwal, S. P. S.; Ladewig, B. P.; Bhattacharya, S. Energy Fuels 2012, 26, 1471.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. Y


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