Alternative Fuel Cell Technologies for Cogenerating Electrical Power and Syngas from Greenhouse Gases Meng Li, Bin Hua, and Jing-Li Luo* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada ABSTRACT: Increasing environment awareness and energy demands are the reasons for emerging energy technologies with ecofriendliness and high efficiency. Of the various candidates, the solid oxide fuel cell (SOFC) is very appealing because of its high efficiency and fuel flexibility. Traditionally, SOFCs directly convert the chemical energies of the readily available fuels into electricity with H2O and CO2 as the products, which is very promising in terms of the energy efficiency yet leads to CO2 emission in practice. In fact, SOFCs are able to allow in situ CO2−CH4 reforming and H2 selective electro-oxidation in their anodes. Such a process enables a sustainable path to produce electrical power and syngas from CO2 but is hindered by several issues. This Perspective discusses the main technical challenges of this process and available approaches achieved so far. The potential future directions for advancing this technology are also pointed out.
C
arbon takes many forms and moves through the environment via the carbon cycle. When fossil fuels are burnt in power industrial operations and/or supply energy for daily activities in human society, most of the carbon immediately enters the atmosphere as carbon dioxide (CO2), a major component of greenhouse gases (GHGs) that trap heat in the atmosphere. The excessive CO2 emission into the atmosphere has caused climate changes and global warming, which adversely impact our ecosystem and are held accountable for abnormal weather conditions and some natural disasters in various regions on our planet. Therefore, the worldwide battle against negative greenhouse effects is gaining momentum, and the search for clean energy sources to mitigate adverse climate change has intensified. Presently, fossil fuels provide the largest share of world total energy supply, and they will still account for 78% of energy use by 2040.1−3 Thus, development of an effective and efficient way to utilize these carbonaceous fuels is imperative for a low-carbon world. Solid oxide fuel cells (SOFCs) provide a viable way to simultaneously address the issues associated with clean energy sources and environmental pollution (Figure 1) because it has the potential to produce electrical power and syngas with little or zero GHG emissions of CO2 and CH4, the two primary components of global GHGs.4 Because of the high working temperature (500−800 °C) and fuel flexibility, SOFCs can directly convert the chemical energy in fossil fuels into electricity with high efficiencies (typically 50−80%),5,6 and more importantly, SOFCs are able to perform the in situ dry reforming of methane (DRM, eq 1) reaction and simultaneous H2 selective electrooxidation in their anode compartments. CH4 + CO2 ↔ 2CO + 2H 2 © 2017 American Chemical Society
The typical working temperature of an intermediate temperature (IT) SOFC ranges from 500 to 800 °C, which is beneficial for the DRM according to the theoretical calculation illustrated in Figure 2.7 Such working temperatures help achieve high CO2 equilibrium conversion (Figure 2a) while restraining the water− gas shift reaction (Figure 2b). One of the advantages of such a process is that it produces syngas, which is a raw material of the Fischer−Tropsch process for the synthesis of many chemicals and fuels. Second, the system provides net energy output. DRM can achieve a high conversion rate, albeit high energy input is required due to the extremely endothermic nature of the reaction. In an SOFC, on the other hand, the electrochemical oxidation reactions, such as CO and H2 oxidation reactions, are exothermic and, thus, are capable of compensating for the energy required by DRM. In fact, the energy compensation could be easily achieved by tuning the operating temperature and/or current density of an SOFC. Operation in this manner allows endothermic DRM and exothermic fuel oxidation to be carried out simultaneously in order to achieve a thermally neutral process. Last, coupling CH4 with CO2 could effectively mitigate the degradation of SOFC anodes from carbon deposition, an apparent advantage in comparison with direct utilization of methane. Despite all of the promising features, a number of technical challenges still need to be addressed before its practical applications. To “pair up” electricity generation with syngas production, the anode needs to be active for both DRM and electrochemical Received: May 8, 2017 Accepted: July 7, 2017 Published: July 7, 2017
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Figure 1. Schematic diagram showing an SOFC system for simultaneous chemical production and electricity generation.
Figure 2. (a) CO2 equilibrium conversion and (b) H2/CO ratio as a function of temperature at different CO2/CH4 ratios. Reprinted with permission from ref 7. Copyright 2011 Elsevier.
To “pair up” electricity generation with syngas production, the anode needs to be active for both DRM and electrochemical oxidation reactions.
CH4 ↔ Cads + 2H 2
(2)
2CO ↔ Cads + CO2
(3)
Additionally, most fuels, even industrial hydrogen, may contain sulfur contaminants to some extent.16,17 These impurities in the operating system, even at ppm levels, are detrimental to Ni-based anodes.18−22 To couple the dry reforming process and
oxidation reactions. Earlier studies have shown that various metal catalysts, such as palladium, platinum, ruthenium, rhodium, iridium, nickel, and copper, exhibited excellent activity toward DRM with high CO and H2 yields.8,9 However, the high cost and scarcity of the precious metals listed hinder their large-scale applications. Ni is not a precious metal and, therefore, a good choice for the anode component as the best trade-off between the activity and cost. Ni-based cermets are conventional anodes for SOFCs, which possess high electronic conductivity, excellent activity for fuel oxidation, and compatibility with common electrolyte materials. Nonetheless, Ni-based anodes are prone to deactivation in carbonaceous fuels caused by rapid carbon deposition.10−15 Carbon formation originating from methane cracking (eq 2) and carbon monoxide disproportionation (eq 3) is one of the main reasons for catalyst deactivation.
To couple the dry reforming process and fuel oxidation reactions in the anode chamber, carbon deposition and sulfur poisoning effects on the Ni-based anodes are two of the main issues causing major concern. fuel oxidation reactions in the anode chamber, carbon deposition and sulfur poisoning effects on the Ni-based anodes are two of the main issues causing major concern. In addition to the anodes, the electrolytes will have a remarkable influence on the proposed process because they determine 1790
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Figure 3. (a) Schematic drawing of coupled electricity and syngas production processes in a layered O-SOFC; (b) time-dependent CH4 conversion of the DRM process over a NiCu-ZDC catalyst at 800 °C; (c) time-dependent voltage of a H2S-pretreated SOFC with triple-layer anodes in 50 ppm of H2S-containing CH4−CO2 at 800 °C and 1.5 A cm−2 and the corresponding variations of anode exhaust compositions.23 Reproduced by permission of The Royal Society of Chemistry.
anode is prone to carbon and sulfur poisoning, resulting in performance degradation and structure failure. In the case of H-SOFC, H2 molecules obtained from the CO2−CH4 reforming reaction adsorb, dissociate, and are oxidized on the anode catalyst, and then the protons diffuse through the proton-conducting electrolyte into the cathode to react with O2, thus generating electricity as the electrons flowing through the external circuit. Unlike an oxygen ion-conducting SOFC, no oxygen ions diffuse into the anode to oxidize CO in a H-SOFC, limiting the concentration of CO2 in exhausts. Therefore, compared with O-SOFC, the main advantage of H-SOFC is the possibility to achieve high H2-selective electro-oxidation (eq 6) and sequester CO2 production due to its intrinsic property.
the working principles of SOFCs. Conventionally, SOFCs are categorized as oxygen-conducting SOFCs and proton-conducting SOFCs, denoted as O-SOFCs and H-SOFCs, respectively. The different properties of the electrolyte materials result in different fuel oxidation processes and varying degrees of carbon and sulfur tolerance. Commercial Y2O3-stabilized ZrO2 (YSZ) is the most widely used oxygen ion-conducting electrolyte, which transfers oxygen ions obtained in the cathode into the anode to oxidize adsorbed fuels. However, the catalytic activity of Ni-YSZ cermet for DRM is not quite satisfactory. CH4 conversion (eq 4) on the Ni-YSZ catalyst is only around 50%, while the CO selectivity (eq 5) is lower than 70% at 750 °C.23 CH4 conversion =
CO selectivity =
1/2[CO] × 100 (%) 1/2[CO] + [CH4]
[CO] × 100 (%) [CO] + [CO2 ]
(4)
H 2 selectivity =
[H 2 converted] × 100 (%) [CO converted] + [H 2 converted] (6)
Doped barium cerate perovskites are well-known proton conductors for their high proton conductivity in the IT range.24,25 Ni-doped BaCeO3 cermet anodes also have attracted broad interests due to their enhanced carbon deposition resistance in water-containing carbonaceous fuels.26−29 Unfortunately, the application of H-SOFC to DRM still remains a challenging task because the doped BaCeO3 perovskites are vulnerable to decomposition in an atmosphere with a high concentration of CO2
(5)
The O2− transported through the YSZ electrolyte could only reach the three phase boundaries (TPBs) where the electrochemical reactions can take place and react with adsorbed fuel molecules as well as the deposited solid C and S. C and S adsorbed on the Ni surface far away from TPBs are unlikely to be removed due to the absence of O2−. Therefore, a Ni-YSZ 1791
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Figure 4. (a) Schematic showing the configuration of the novel layered H-SOFC; (b) stability test of simultaneous DRM and power generation with layered H-SOFC fueled with CH4−CO2 at 700 °C: galvanostatic test under a constant load of 1 A cm−2, and exhaust gas composition records.37 Reproduced by permission of The Royal Society of Chemistry.
and/or H2S.25,30−32 Although doped BaCeO3 can adsorb H2O molecules owing to its hydrophilic property and facilitate Cads removal, it does not show superior carbon deposition resistance in dry hydrocarbon fuels.27 More importantly, due to the absence of oxygen ions, Ni-based anodes of H-SOFCs are more likely to catalyze carbon formation. Layered Structure. Recently, we have proposed two strategies to address these two issues for simultaneous chemical production and electricity generation from CH4−CO2 in SOFCs. The first tactic is fabrication of the layered anode structure, in which a conventional anode layer and a reforming layer are adopted for the electrochemical process and DRM, respectively. We employed a catalytic layer on top of the conventional Ni-based anodes, working as an on-cell microreformer. In this configuration, the CH4−CO2 feed stream is converted into CO and H2 in the on-cell reforming layer and the fuel electrochemical oxidation reactions will take place in the Ni-based anodes. The add-on layer needs to possess high catalytic activity for DMR along with sufficient coke and sulfur tolerance. Ni0.8M0.2 (M = Co, Cu, Fe)-doped CeO2 catalysts have excellent catalytic activities for the DRM reaction. The methane conversion was even higher than 90% at temperatures above 750 °C.23 Alloying Ni with transition metals could also achieve a higher carbon deposition resistance.33 We adopted a Ni0.8Cu0.2(NiCu)-Ce0.8Zr0.2O2 (ZDC) add-on layer to the Ni-YSZ support (Figure 3a). The catalytic activity of the NiCu-ZDC catalyst for dry reforming was much higher than that of conventional Ni-YSZ in the sweet CH4−CO2 atmosphere from 550 to 800 °C. More importantly, enhanced sulfur tolerance was achieved by incorporating the NiCu alloy. Cu has lower affinity to H2S than Ni;34 therefore, a NiCu alloy is thermodynamically more stable than pristine Ni in a H2S-containing atmosphere. The NiCu-ZDC catalyst was capable of achieving similar CO2 conversion and syngas yield before and after treatment in the H2S-containing atmosphere. It is interesting to find that a sulfur-prepoisoned NiCu-ZDC catalyst exhibited improved carbon deposition
resistance (Figure 3b). The NiCu-ZDC catalyst suffered performance degradation in sweet CH4−CO2 as a result of carbon deposition, but the sulfur-poisoned catalyst remained stable in both sweet and sour CH4−CO2 fuels, which is ascribed to the sulfur passivation effect. Most significantly, the sulfur-deposited Ni-YSZ anode was able to produce CO-enriched syngas through selective electro-oxidation of H2, an achievement toward lowering CO2 emission (Figure 3c). A Ni0.8M0.2-doped CeO2 catalyst can also be adopted to work as an on-cell reforming layer for H-SOFC. Alloying Ni with Co could achieve improved catalytic activity for DRM and carbon deposition resistance.35,36 A Ni0.8Co0.2-La0.2Ce0.8O1.9 (NiCo-LDC) reforming layer was fabricated on the surface of a Ni-BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) anode support (Figure 4a). Prior to diffusing into the Ni-BZCYYb anode functional layer, the CH4−CO2 feed is fully reformed in the NiCo-LDC catalyst layer, yielding CO and H2. The CO2 conversion of the NiCo-LDC catalyst was as high as 96.9%, which is more than 2-fold higher than that of a Ni-BZCYYb catalyst (48.3%). Moreover, the CO selectivity of the NiCo-LDC catalyst was also much higher than that of the latter one.37 Subsequently, the formed syngas diffuses into the Ni-BZCYYb functional layer, and H2 is selectively and electrochemically oxidized to generate power and release heat (eq 7). 2H 2 + O2 ↔ 2H 2O
(7)
The peak power density was up to 910 mW cm−2 at 700 °C, and CO-enriched syngas was obtained. The full cell exhibited excellent long-term stability at 1 A cm−2 and 700 °C for 100 h (Figure 4b). In conventional CO2 electrolysis and the dry reforming process, excessive potential and heat are applied in order to drive the reactions. Instead, CO-enriched syngas and electricity are simultaneously generated with high efficiency in our layered H-SOFC. In addition, the heat released by H2 oxidation reaction could completely compensate that required for dry methane reforming only if the utilization of H2 is higher than 1792
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We introduced PrBaMn2O5+δ (PBM) double perovskite NPs into a conventional Ni-YSZ anode for O-SOFC. The catalytic activity of the Ni-YSZ catalyst toward DRM was also improved by PBM NP decoration. Compared with the Ni-YSZ catalyst, the NPs + Ni-YSZ catalyst exhibited superior activity toward the CH4 dry reforming reaction with higher CH4 conversion and CO selectivity. PBM with a layered structure has mixed-valence transition metal cations (Mn4+/Mn3+/Mn2+) and fast oxygen ion diffusion channels. The PBM NPs can serve as the oxygen sources as well as the mediums for O2− diffusion to accelerate both the carbon removal and Sads desorption processes. PBM is also an active catalyst for hydrogen and hydrocarbon oxidation reactions.41 The formed PBM NPs/YSZ grains interfaces are active sites for electrochemical oxidation reactions, which remarkably enlarge TPBs of the Ni-YSZ anode. Therefore, the electrochemical performance is also expected to be improved via PBM NP decoration. The peak power density of a full cell with a Ni-YSZ anode in the CH4−CO2 fuel was remarkably improved by 25−30% at 750 and 800 °C after PBM NP decoration. Moreover, the NPs + Ni-YSZ cell did not suffer performance degradation when 50 ppm of H2S was introduced into the CH4− CO2 fuel due to the reduced sulfur poisoning effect by PBM. During a 100 h durability test (Figure 5b), the NPs + Ni-YSZ cell exhibited its potential for cogenerating electricity and syngas with stable performance output and high concentration of syngas yield. The concentrations of CH4 and CO2 in the exhausts were limited, indicating the high activity of the anode toward DRM. A CO-enriched syngas with the concentrations of CO and H2 at around 56 and 42%, respectively, was obtained. Therefore, the reformed H2 was selectively and electrochemically oxidized for electricity generation; hence, CO2 production was sequestered due to suppressed CO electrochemical oxidation. The syngas with a high CO/H2 ratio is a valuable raw material and can be used in Fischer−Tropsch synthesis of long-chain hydrocarbons.42 We discovered that the method of PBM NP decoration was also applicable to modify the proton-conducting BZCYYb perovskite in a Ni-BZCYYb anode for H-SOFC (Figure 6a).43 BZCYYb grains have a high concentration of oxygen vacancies on their surface, which can readily interact with H2O, forming hydroxyl groups (eq 8).27,44
52%. In the long-term stability test, the H2-selective oxidation remained at around 95% (eq 6).
CO-enriched syngas and electricity are simultaneously generated with high efficiency in our layered H-SOFC. Nanoparticle Decoration. The second strategy is decorating conventional Ni-based anodes with infiltrated nanoparticles (NPs), which serve as CO2 captors and promoters for Cads and Sads removal.38,39 C and S are prone to adsorbing on a Ni surface, blocking the active sites for both chemical and electrochemical reactions. If adsorbed C and S are not removed in time, they would dissolve into the Ni lattice and destroy Ni particles completely. In a conventional Ni-YSZ anode (Figure 5a), O2−
× • H 2O + V •• O + OO = 2OH O
Figure 5. (a) Proposed mechanisms showing that the infiltrated NPs do not merely resist the deposition of S/C species but also promote the S/C removal process; (b) time-dependent voltage and corresponding effluent gas compositions of a NPs + Ni-YSZ cell in dry-sour biogas (CH4/CO2 = 1:1, 50 ppm of H2S) at 800 °C and 1.5 A cm−2 during a 100 h stability test. Reproduced with permission from ref 40. Copyright 2017 Elsevier.
(8)
The aqueous PBM precursor solution tends to aggregate on the BZCYYb surface owing to its hydrophilic property. Infiltrated PBM NPs selectively cover the entire surface of BZCYYb grains, preventing decomposition of the BZCYYb perovskite phase in CO2-containing fuel. The 300 nm thick PBM nanoaggregates layer serves as an active catalyst for DRM as well, compensating for the insufficient catalytic activity of the Ni-BZCYYb cermet. For the DRM process on a single-phase PBM catalyst, there was still about 20% CO2 left in the exhaust gas. CO2 conversion on the PBM catalyst could be remarkably improved to exceed 97% when combined with a nanosized Ni4Co bimetallic catalyst. The detected concentration of residual CO2 was only about 1.7% for DRM on the NiCo/PBM catalyst. Therefore, we introduced Ni4Co catalyst into the PBM-modified Ni-BZCYYb anode to form a NiCo/PBM-Ni-BZCYYb anode. TEM results revealed that Ni4Co NPs, with a typical diameter of 20 nm, monodispersed on PBM grains. PBM grains were capable of preventing the aggregation of Ni4Co NPs upon heat treatment, and the high catalytic activity remained. The formed NiCo/PBM nanoarchitecture on the BZCYYb surface can effectively convert CO2
transported through electrolyte could only reach the TPBs and react with adsorbed fuel molecules, solid C and S. Cads and Sads adsorbed on a Ni surface far away from TPBs are not likely to be removed due to the absence of O2−. In the case of a Ni-YSZ anode decorated with NPs, the deposition of NPs on Ni grains efficiently decreases the surface area of Ni exposed to fuels. Consequently, interactions between Cads/Sads species and the Ni surface are inhibited. More importantly, impregnated NPs covering a Ni surface could largely extend the TPBs by conducting O2−, thus facilitating C/S species removal processes. Also, the YSZ/NPs interfaces formed on the previous inert YSZ surface are active for electrochemical reactions of fuels. This contributes to the improvement of SOFC performance output. 1793
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Figure 6. (a) Schematic preparation procedures of constructing the NiCo/PBM bifunctional nanoarchitecture on BZCYYb in the porous nickel cermet anode; (b) schematic diagram showing expanded TPBs by NiCo/PBM nanoarchitectures on the Ni-BZCYYb interfacial area; (c) polarization curves of different cells at 700 °C in 50 ppm of H2S−CH4−CO2; and (d) time-dependent voltages and the corresponding exhaust gas compositions of FC-NiCo/PBM in 50 ppm of H2S-containing CH4−CO2 at 1 A cm−2 and 700 °C. Reproduced with permission from ref 43. Copyright 2016 John Wiley & Sons.
molecules into CO, restraining interaction between BZCYYb and CO2. Moreover, both NiCo/PBM nanoarchitectures and the original Ni grains are active sites for proton generation. As a result, the TPBs for hydrogen electrochemical conversion are substantially increased (Figure 6b). The NiCo/PBM catalyst also exhibited outstanding catalytic activity toward DRM in sour CH4−CO2 (containing 50 ppm of H2S), with the concentration of CO2 in the exhaust gas as low as 3.6%. For H-SOFC with the conventional Ni-BZCYYb anode (FC), its performance was very poor when operating in sour CH4−CO2 fuel due to the low reforming activity of Ni cermet, rapid sulfur poisoning effect on Ni, and instability of BZCYYb in the CO2-containing atmosphere. Its peak power density at 700 °C was remarkably improved to be over 1.1 W cm−2 after modification of NiCo/PBM nanoarchitectures (FC-NiCo/ PBM), as shown in Figure 6c. Moreover, this engineered electrode enabled direct methane conversion in H-SOFC, resulting in effective CO2 utilization and the cogeneration of electrical power/synthesis gas. The H-SOFC with a NiCo/PBMdecorated Ni-BZCYYb anode showed stable power output of 0.81 W cm−2 in sour CH4−CO2 fuel while yielding exhaust gas with a high concentration of syngas (>95%) (Figure 6d). Conclusively, the selective deposition of secondary catalysts and the formation of bifunctional nanoarchitectures provide a promising strategy for powering H-SOFC with readily available H2S-containing CH4−CO2 fuel. In summary, we have demonstrated that SOFCs have a promising alternative application for CO-enriched syngas production from GHGs. Incorporating an on-cell reforming layer and impregnating NPs are two feasible ways to improve the reactivity as well as carbon/sulfur tolerance of the anodes, thus leading to the remarkable effectiveness of the proposed processes. Coupling Ni alloy with oxide composite catalysts enables excellent catalytic activity toward CH4−CO2 reforming, and such composites can work as on-cell or in-cell reformers for both H-SOFC and O-SOFC technologies.
The selective deposition of secondary catalysts and the formation of bifunctional nanoarchitectures provide a promising strategy for powering H-SOFC with readily available H2S-containing CH4−CO2 fuel. Although DRM has been well investigated in the catalytic field, the development of integrating it into an SOFC anode chamber is still in its infancy. Recently, there has been fruitful development in the structure design of Ni-based catalysts for DRM as the size and structures of catalysts have significant effects on their reactivity.45 Well-controlled structured Ni-based catalysts can be obtained by entrapping Ni particles in a mesoporous framework, achieving improved carbon resistance and CH4 conversion. These mesostructured materials have the potential to be adopted as SOFC anode materials for simultaneous DRM and electrochemical oxidation reactions. Their activity and stability under SOFC operating conditions need to be further investigated. Moving forward, future research work in this area should also be focused on scale-up of the single cells, suitable cell components, and the SOFC stacks before practical application of this technology takes place. The technological and customizable features of SOFCs discussed herein are expected to open up more exciting new opportunities in our search for more efficient, ecofriendly energy sources to reduce GHG emission for the benefit of human society via protecting our environment, developing the economy, and beyond.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bin Hua: 0000-0001-8329-5825 Jing-Li Luo: 0000-0002-2465-7280 1794
DOI: 10.1021/acsenergylett.7b00392 ACS Energy Lett. 2017, 2, 1789−1796
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Notes
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The authors declare no competing financial interest. Biographies Meng Li is a postdoctoral fellow in the Department of Chemical and Materials Engineering, University of Alberta, Canada. Her research is focused on advanced electrode materials for solid oxide fuel cells and solid oxide electrolysis cells. Bin Hua is a postdoctoral fellow in the Department of Chemical and Materials Engineering, University of Alberta, Canada. His research is focused on key materials for fuel cells, energy storage and conversion, and heterogeneous catalysis. Jing-Li Luo is a professor in the Department of Chemical and Materials Engineering, University of Alberta, Canada. She was elected to be a Fellow of the Canadian Academy of Engineering in 2016. Her research is focused on fuel cells, energy storage, clean energy technology, and corrosion control.
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ACKNOWLEDGMENTS This research is financially supported by the Climate Change and Emissions Management Corporation, Alberta, Canada, and the Natural Sciences and Engineering Research Council of Canada.
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