High Temperature Electrolysis in Alkaline Cells, Solid Proton

Review pubs.acs.org/CR

High Temperature Electrolysis in Alkaline Cells, Solid Proton Conducting Cells, and Solid Oxide Cells Sune Dalgaard Ebbesen,* Søren Højgaard Jensen, Anne Hauch, and Mogens Bjerg Mogensen Department of Energy Conversion and Storage, Technical University of Denmark, Risø Campus, Frederiksborgvej 399, P.O. Box 49, DK-4000 Roskilde, Denmark 6.1.3. Electrolyte 6.2. Performance of H2O Electrolysis 6.3. Performance of CO2 Electrolysis 6.4. Performance for Coelectrolysis of H2O and CO2 6.5. Cell Degradation During H2O, CO2 and Coelectrolysis of H2O and CO2 at Mild Operating Conditions 6.6. Cell Degradation During H2O, CO2 and Coelectrolysis at High Current Densities 6.7. High Temperature Electrolysis in Solid Oxide Cells at Increased Pressure 6.8. High Temperature Electrolysis in Solid Oxide Cell Stacks 7. Reversible Operation of High Temperature Electrochemical Cells 8. Modeling the Performance of High Temperature Electrolysis Cells 9. Estimates of Production Cost of Renewable Synthetic Fuels via Electrolysis 10. Summary Author Information Corresponding Author Notes Biographies Acknowledgments Nomenclature Materials References

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CONTENTS 1. Introduction 1.1. CO2 Capture: Closing the Cycle 2. Electrolysis Cells 3. Electrolysis of H2O and CO2 3.1. Thermodynamics of Electrolysis 3.2. Electrolysis Efficiency 4. High Temperature Electrolysis in Solid Proton Conducting Electrolysis Cells 5. High Temperature Electrolysis using Alkaline Cells 5.1. Materials 5.2. Performance 6. High Temperature Electrolysis in Solid Oxide Cells 6.1. Electrode Materials and Structures for Solid Oxide Electrolysis Cells 6.1.1. Fuel Electrode (Cathode) 6.1.2. Oxygen Electrode (Anode) © 2014 American Chemical Society

1. INTRODUCTION The widespread use of fossil fuels within the current energy infrastructure is considered as one of the largest sources of CO2 emissions, which is argued to cause global warming and climate changes. One of the top energy priorities is therefore exploring environmentally friendly alternatives to fossil fuels (e.g., wind or solar) and thereby reduce CO2 emissions.1−4 Renewable energy technologies are sometimes criticized for not providing the necessary security of supply because the production cannot be tuned to fit the consumers’ needs. Thus, it is necessary to store vast amounts of energy by, e.g., converting surplus electricity to chemical energy in the form of compounds such as hydrogen, methane, methanol, or dimethyl ether (DME). In this form the energy is easily stored and can be used in the transport sector. Countries like Denmark with a large share of

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Received: February 14, 2014 Published: October 6, 2014 10697

dx.doi.org/10.1021/cr5000865 | Chem. Rev. 2014, 114, 10697−10734

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process (FT diesel).45−47 Alternatively, when applying alkaline cells, both low and high temperature, which can only electrolyze steam to form hydrogen, then synthesis gas and methane can be produced by reacting the hydrogen with CO2. By coelectrolysis of H2O and CO2, it is in principle possible to produce synthetic fuels from renewables without emitting CO2 and without the need for major modifications of existing infrastructure in the transport sector. For competitive electricity storage the electrolysis processes has to compete with, for example, batteries, for which efficiencies up to 75−80% can be reached,48 but the cost of energy storage in terms of price per kWh stored is much higher for batteries than for hydrocarbons. Furthermore, the energy density is about 100 times lower for batteries compared to synthetic gasoline; this means that the battery weight per kWh stored is about 100 times higher than the weight of the synthetic gasoline per kWh stored. Beside the importance of hydrogen and carbon monoxide in the production of synthetic fuels, both hydrogen and carbon monoxide are used in large quantities in the chemical industry.49,50 The most common use of hydrogen is the reaction with nitrogen in the Haber−Bosch ammonia process51 (ammonia commonly used for, e.g., fertilizer production), and in hydrogenation processes.50 Large quantities of aldehydes, acids, and ketones are produced by carbonylation (reaction with carbon monoxide) in the fine chemical industry.49,50,52 Also, methanol (one of the top ten petrochemicals, used for the manufacturing of synthetic hydrocarbons) is produced from hydrogen and carbon monoxide (and carbon dioxide).53 The wide use of hydrogen and carbon monoxide in the chemical industry may provide an additional market for production of clean carbon monoxide or hydrogen via high temperature electrolysis.

renewable energy supply will need cheap electricity storage in the near future to balance intermittent wind power supply. This gets increasingly important if the fraction of electricity supplied by wind power continues to increase as planned. 50% of the electricity is planned from wind power by 2025 in Denmark,5 compared to today’s wind supply of around 30% of the electricity consumption.6 Production of, e.g., methane via electrolysis may provide a means for electricity storage in the future as methane can easily be stored in the natural gas grid in most countries.7,8 Hydrogen offers significant promise as a basis for a future energy technology, and is argued to be the most versatile, efficient, and environmentally friendly fuel, although handling of hydrogen may be problematic.9 Today most hydrogen is produced by steam reforming of natural gas because of the high efficiency (on the order of 60−85%10) and low price of natural gas compared to electricity. In this process, natural gas is combined with high temperature steam to produce hydrogen along with carbon monoxide, and carbon dioxide.11,12 The unwanted CO and CO2 are removed from the mixture, and the end product is pure hydrogen. The disadvantages of hydrogen production from fossil fuels are the environmental problems associated with natural gas production, emission of CO2, and the purity of the produced hydrogen. Besides production of hydrogen from fossil fuels, hydrogen may be produced via biological processes,13−17 thermochemical cycles such as zinc− zinc oxide and iodine−sulfur cycles,18−20 photochemical processes,21−25 water electrolysis,26−29 and steam electrolysis.1,30−34 Hydrogen produced by electrolysis, even at higher production prices, may be implemented in certain markets due to the high purity of the produced hydrogen. To decrease the cost of “renewable” hydrogen, a massive research effort in the technologies of harvesting renewable energy as well as in the conversion technologies is necessary.35 Very enthusiastic but not necessarily realistic visions for a hydrogen economy have been published, where transition to a hydrogen economy within the following decades was proposed.36,37 Opponents to the hydrogen society have argued that the difficult handling of hydrogen and the big losses in converting electricity into hydrogen and back to electricity are potential show-stoppers.38,39 The argument against the hydrogen society based on the high losses converting electricity to hydrogen and back to electricity is correct with round trip efficiencies on the order of 20−25%,38−41 although the exact efficiency of the electrolysis process may be higher for future electrolyzers. Since hydrogen is an energy carrier (like electricity) and not an energy source, along with the fact that hydrogen will be difficult to use in the conventional transport sector, the conversion to a hydrogen based infrastructure will require huge investments and take decades to realize due to the difficulties in changing from one energy infrastructure to another. In order to circumvent the difficult handling of hydrogen, a methanol economy has been suggested in which methanol replaces fossil fuels as a means of energy storage and transportation fuel.42−44 SPCECs and SOECs can electrolyze not only H2O but also a mixture of H2O and CO2 to produce synthesis gas (a mixture of H2 and CO). Synthesis gas can be converted into various types of synthetic fuels,45−47 of which methane, methanol, and dimethyl ether are the simplest and cheapest to produce. The production of methane directly in the cell is possible with SPCECs and, if operated at increased pressures, also for SOECs. Also, synthetic petrol and diesel can be produced from synthesis gas via the Fischer−Tropsch

1.1. CO2 Capture: Closing the Cycle

CO2 capture from air and recycling or reuse of CO2 from industrial sources54−58 would be an attractive alternative to storage of CO2 in the underground (sequestration) and would in combination with coelectrolysis of CO2 and H2O provide CO2 neutral synthetic hydrocarbon fuels. The concept of production of synthetic fuels from atmospheric CO2 via electrolysis is not new, and the production of synthetic fuels such as methane59 and methanol60 from atmospheric CO2 combined with low temperature electrolysis was proposed in the 1960s60 and 1970s.59,61−63 The economy of methanol production was treated by Shell in the early 1970s,64 and later patented.65−67 Extracting CO2 from the exhaust of industrial plants was also treated.63 A renewed interest in the production of synthetic fuels from CO2 extracted from the atmosphere in combination with hydrogen production either via low or high temperature electrolysis has arisen since the late 1990s.68−79 Electrochemical conversion of CO2 (released from mineral carbonates, indirect air capture) to CO in SOECs followed by reacting the CO with steam to produce hydrocarbon fuels has also been proposed.80 Production of syngas via coelectrolysis in SOEC for the production of methanol81 and synthetic fuels71 appeared for the first time in the 1990s and has during the past decade received an increased interest.82−93 Capture of CO2 can be achieved by absorption processes employing amines or hydroxides as absorbents. The regeneration includes heating of the absorbent; therefore, reduction of the required energy is a determining factor for realizing CO2 recycling. From the viewpoint of energy saving in regeneration of the absorbent, hydroxides are preferable to amine solutions, 10698


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Figure 1. CO2 neutral energy cycle utilizing CO2 capture from air with the process involving a humidity swing100 in combination with a SOEC.

to store renewable energy. One very recent study has shown the feasibility of biogas upgrading using SOECs.103 The possibility of leaving some sulfur in the biogas feed to the SOEC, to reduce the steam reforming activity without sacrificing too much of the electrochemical electrolysis activity, was investigated. It was found that the presence of sulfur reduces the steam reforming activity to a stable level at sulfur concentrations above ∼6 ppm without a significant decrease in the electrochemical activity, showing the feasibility of upgrading biogas using SOEC.103 Alternatively, hydrogen from steam electrolysis via, e.g., low temperature alkaline cells or high temperature cells (HT-AECs, SPCEC and SOECs) can be added to the biogas. Both when applying steam electrolysis or coelectrolysis, the produced synthesis gas is methanated. Waste heat from the methanator may be used as feed for the high temperature cells.104 In a comparison of biogas upgrading via either steam electrolysis or coelectrolysis in SOECs, it was found that the route via steam electrolysis is slightly more efficient due a more favorable steam balance,104 although methanation of CO2 operates at slightly higher pressures than methanation of CO.104,105 Another option to increase biomass utilization is by coupling biomass gasification and high temperature steam electrolysis.106 In this process, the produced hydrogen is added to the gasified biomass to produce synthesis gas with the desired hydrogen to carbon monoxide ratio.106 A high efficiency can be achieved using the heat from the gasification to heat the steam and electrolysis unit, and the produced oxygen from the electrolysis reaction is used in the gasifier.106 In the proposed production of synthetic fuel, the fuel catalyst must be operated at elevated pressure to achieve a sufficiently high conversion ratio of synthesis gas to synthetic fuel. Pressurization also decreases the internal resistance of the SOECs,89 and this can be used to improve the efficiency of the electrolysis reaction and in turn reduce the synthetic fuel

since the energy requirement for CO2 removal in the hydroxide process is about half of that of the amine process94 and has been recognized as a potentially promising route for permanent and safe storage of CO2.95 Calcium hydroxide is a well-known CO2 absorbent,96 whereas also the less used magnesium hydroxide or calcium magnesium hydroxide (dolomite) may be employed. CO2 capture/recycling using magnesium hydroxide/ carbonate can be operated at significantly lower temperatures compared to the use of calcium hydroxide/carbonate and is therefore preferable. The heat necessary for the regeneration may come from the high temperature electrolysis reaction, decreasing the overall energy consumption. Capture of CO2 can be performed by solid adsorbents such as carbonates,97−99 or, as recently shown, a new proces involving a solid adsorbent regenerated by a humidity swing or temperature-vacuum swing, which significantly reduces the energy demands and cost of the process.100,101 A cycle for CO2 capture and recycling in combination with an SOEC for production of synthetic fuel is definitively technically possible, but the practical and economic aspects have to be assessed to determine the most suitable methods and operating conditions. A CO2 neutral energy cycle utilizing CO2 capture from air with the humidity swing100 in combination with an SOEC33 is sketched in Figure 1. Biogas produced from livestock manure or other types of biomass contains CO2 as a byproduct. If the CO2 is electrolyzed into CO by means of SOEC and wind or solar power, more biogas can be produced from less biomass which is appealing since biomass is a limited resource.102 In this case, the added biogas (i.e., synthetic fuel) is CO2 neutral. If the biogas is upgraded to pipeline quality, the present constraining tie to combined heat and power production can be removed, and new markets, eventually including the transport sector, can be made accessible. Production of synthetic fuel by upgrading biogas by means of SOEC and wind or solar power can also act 10699


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1 O2 + H 2O + 2e− 2

low efficiencies

cheap cell materials

OH− NaOH KOH nickel 80 °C commercially available

2OH− →

2H 2O + 2e− → H 2 + 2OH−

1 O 2 2

AEC

H 2O → H 2 +

H2O

1 O2 + 2H+ + 2e− 2

low electrode durability expensive platinum electrodes

H+ H2SO4 H2PO3 Pt/C/IrO2 150 °C expensive electrode materials

H 2O →

2H+ + 2e− → H 2

1 O 2 2

acid

H 2O → H 2 +

H2O

1 O 2 2

PEMEC

expensive materials

not commercially available

1 O2 + 2e− 2

low electrode and electrolyte durabilitya expensive platinum electrodes increased hydrogen crossover at elevated temperatures

O2− ceramic

O2 − →

nickel, ceramic 750−900 °C high efficiencies and the possibility to produce synthesis gas cheap cell materials

1 O2 + 2H+ + 2e− 2

1 O2 + 2H+ + 2e− 2

nickel, ceramic 400−600 °C high efficiencies and the possibility to produce synthesis gas pure H2 produced no H2 separation step cheap cell materials not commercially available

H+ ceramic

H 2O →

CO2 + 8H+ + 8e− → CH4 + 2H 2O

CO2 + 2e− → CO + O2 −

CO2 + 2H 2O → 2O2 + CH4

1 O 2 2

SPCEC

2H+ + 2e− → H 2

1 O2 2

CO2 → CO +

H 2O → H 2 +

H2O (CO2)

H 2O + 2e− → H 2 + O2 −

1 O 2 2

SOEC

H 2O → H 2 +

H2O CO2

Pt/C/IrO2 80 °C commercially available

H+ polymer

H 2O →

2H+ + 2e− → H 2

H 2O → H 2 +

H2O

a The applied polymer membrane and carbon supported electrodes cannot withstand long-term operation in the strong oxidizing conditions at the anodic polarization at high temperature.32 Other materials, e.g., based on titanium alloys may be used as catalyst carrier.109

disadvantages

electrodes temperature advantages

charge carrier electrolyte

anode reaction

cathode reaction

overall reaction

reactant

Table 1. Specifications of Alkaline Electrolysis Cells (AEC), Phosphoric Acid and Sulfuric Acid Electrolysis Cells (Acid), Polymer Electrolyte Membrane Electrolysis Cell (PEMEC), Solid Oxide Electrolysis Cells (SOEC), and Solid Proton Conducting Electrolysis Cells (SPCEC)

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CO2 is fed to the negative electrode where it is split into H2 and/or CO and oxide ions (O2−). The oxide ions are conducted through the solid oxide electrolyte from the negative electrode to the positive by the applied electric field. At the positive oxygen electrode the oxide ions recombine to gaseous oxygen. In the case of proton conducting ceramic electrolytes, the H2O gas is supplied to the positive oxygen electrode, the protons are conducted through the electrolyte, and the formed hydrogen is the only gas present in the negative electrode compartment. If CO2 is supplied to the negative electrode during the electrolysis, this may react with the produced H2 to form CH4 (synthetic natural gas, SNG) and H2O. If the electrolyzer is operated at 500 °C and pressurized to 30 bar, then a high yield of CH4 may be achieved. Further details are given in section 4. For high temperature alkaline cells, gas diffusion electrodes may be used. In this case, direct CO2 electrolysis is not possible since the charge carrier is OH−. Applying gas diffusion electrodes for alkaline electrolysis of steam at high temperature, similar to those used in high temperature solid oxide cells, circumvents the formation of gas bubbles. By circumventing the formation of gas bubbles the increase of the electrolyte resistance due to the gas bubbles is avoided, see section 5 for details.

production cost. This review deals with high temperature electrolysis to clarify the advantages and disadvantages of the specific technologies, and to identify the areas where further research and development is necessary.

2. ELECTROLYSIS CELLS Several different types of fuel cells have been developed, and can in general be used as electrolysis cells as well. The cell consists of two electrodes and an electrolyte. The different cells are typically divided into groups on the basis of the electrolyte. The electrolyte may be a liquid (alkaline or acid solution) or a solid (polymer or solid oxide). The electrolyte serves to conduct ions (the charge carrier), produced at one electrode, to the other. In order to avoid a short circuit inside the cell, the electrolyte has to be electron insulating. The overall electrolysis reaction (reactions 1 and 2, section 3) is a sum of two electrochemical reactions (also called half-cell reactions, Table 1), which occur at each of the two electrodes. The electrode where the reduction of reactants or intermediates takes place is called the cathode. The anode is the electrode where oxidation of reactants or intermediates takes place. Known types of fuel cells are polymer electrolyte membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFC), alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), polymer electrolyte membrane fuel cell (PEMFC), and solid oxide fuel cells (SOFC). The corresponding electrolysis cells are listed in Table 1. They are all capable of electrolyzing H2O to produce H2. Until recently, only SOECs were regarded as capable of electrolyzing CO2 to produce CO. Molten carbonate cells have been assumed insufficient as electrolysis cells for H2 and CO production because CO2 is involved in the oxygen electrode reaction, which implies CO2 must be added at the oxygen electrode and transferred as carbonate through the electrolyte. This increases the complexity of the system which in turn may lead to a lower overall efficiency. However, recently, a breakthrough in electrolysis of CO2 into CO using molten carbonate electrolytes has occurred.107,108 The cell types used for molten carbonate fuel cells are not practically usable for electrolysis. Basically, the break-through is that it has been shown to be feasible to use pure molten Li2CO3 above 800 °C as electrolyte, Ti-Al-metal alloy as cathode (CO evolution), and graphite as anode (O2 evolution). The most commonly used, and commercially available, electrolyzers are low temperature (∼80 °C) alkaline electrolysis cells. To achieve an acceptable hydrogen production rate, these cells are usually operated at rather high cell voltages (1.7−1.9 V). This causes a heat production in spite of the electrolysis process being endothermic. Increasing the temperature increases the electrode reaction rate to an acceptable level with low enough cell voltages that the cell is self-cooled. Solid oxide cells (SOCs) have been applied for electrolysis of H2O at high temperatures (750−1000 °C), whereas research on high temperature electrolysis using alkaline electrolysis cells (AEC) is rather limited so far.

H 2O + electric energy (ΔG) + heat (T ΔS) 1 → H 2 + O2 2

(1)

CO2 + electric energy (ΔG) + heat (T ΔS) 1 → CO + O2 2

(2)

To split H2O or CO2, the electric voltage applied over the two electrodes must exceed a minimum value: the so-called reversible voltage (also denoted decomposition voltage). The reversible voltage (Erev) is determined the by Gibbs free energy and is thereby a function of both pressure and temperature. At 25 °C and 1 atm of the reactants, the Erev is 1.23 V for H2O splitting and 1.33 V for CO2 splitting. Both Erev and the electrode polarization resistances decrease substantially with increasing temperature, which makes it advantageous to operate the electrolysis cells at elevated temperatures. Production of synthesis gas by coelectrolysis of H2O and CO2 in SOECs is a combination of reactions 1 and 2 to yield both H2 and CO. Coelectrolysis is more complicated than the two separate electrolysis reactions (reactions 1 and 2) because the water−gas shift (WGS), reaction 3, may occur in parallel with the electrolysis reactions: H 2O + CO ⇌ CO2 + H 2

(3)

At temperatures above ∼820 °C the equilibrium of the WGS reaction (reaction 3) is shifted toward CO and H2O (reverse water−gas shift, R-WGS, reaction).110 Electrolysis of CO2 without the addition of H2O in SOECs may lead to coke formation on nickel based fuel electrodes as nickel is known to catalyze dissociation of carbon containing gases.111−113 Coke formation is undesirable as carbon may build up in the electrodes and reduce the number of active sites for the electrolysis reaction which in turn will reduce cell performance. The formation of carbon can result in nickel removal from the electrode (dusting), and even more important, significant

3. ELECTROLYSIS OF H2O AND CO2 The electrolysis reaction is electrochemical splitting of the reactants by passing an electric current through two electrodes separated by an electrolyte. Electrolysis of both H2O (steam) and CO2 convert electric energy and heat into H2 and CO as specified in reactions 1 and 2. For SOECs the overall process for H2O and CO2 electrolysis is the same: gaseous H2O and/or 10701


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Figure 2. Thermodynamics of H2O and CO2 electrolysis at atmospheric pressure, calculated from data in ref 155.

synthesis gas in another reactor by reacting the produced hydrogen with CO2).

pressures can arise within the electrode and thereby fracture the electrode.114 The regions for catalytic formation of coke in SOCs have been treated in detail and were shown to occur at very high CO concentrations only at atmospheric pressure.115 At realistic CO2/CO concentrations during CO2 electrolysis (T > 800 °C and pCO < 0.9 bar), the equilibrium of the Boudouard reaction (2CO ⇌ C + CO2) is shifted toward CO, i.e., away from coke formation. It has been shown that the degradation observed during CO2 electrolysis is usually not associated with formation of coke within the Ni-YSZ electrode, but rather a consequence of impurities in the gas stream.116−118 Furthermore, coke formation is hardly critical during coelectrolysis at atmospheric pressure and low conversions, as it is well-known that addition of steam suppresses coke formation during, e.g., reforming.119 At increased pressure and high conversion degrees, formation of coke may be a problem.120−125 When operating at high pressure for methane production, the impregnation of an anticarbon deposition catalyst such as ruthenium may prevent carbon formation.126 The development of fuel electrode materials without nickel that are either resistant to carbon formation/deposition or are able to oxidize the formed carbon is another approach to circumvent the carbon formation. However, these materials generally have a lower electrocatalytic activity than nickel. The addition of an oxidation catalyst such as ceria has shown to increase the electrochemical activity without carbon deposition when operated with dry methane, butane, and synthetic diesel.127−130 The application of ceramic cells has been shown to be applicable both for the oxidation of hydrocarbons131−133 and for the electrolysis of CO2, without carbon deposition.134,135 Other oxides, such as strontium titanates, strontium vanadate, and lanthanum manganite, have been tested for steam electrolysis performance136−146 and may be applicable in carbon containing atmospheres.141,145,147−152 H2O and CO2 could be electrolyzed separately; hereafter, CO and H2 could be mixed to synthesis gas, thereby avoiding some of the complexity of coelectrolysis. Another option is electrolyzing H2O to hydrogen and reacting with CO2. There are, however, significant advantages of electrolyzing H2O and CO2 simultaneously, i.e., coelectrolysis. Focusing only on the electrolysis step, coelectrolysis is more energy efficient than the two separate electrolysis processes because the energy consumption for coelectrolysis is slightly lower than for CO2 electrolysis alone,83,90,117,153,154 and only one reactor is necessary to produce the synthesis gas (contrary to first producing hydrogen in one reactor and thereafter producing

3.1. Thermodynamics of Electrolysis

In examining the thermodynamics of the electrochemical process, the electrolysis cell is assumed to be ideal (no side reactions, short cuts etc.), consisting of a reversible anode/ cathode either immersed in a solution of potassium hydroxide (AEC, alkaline electrolysis cell) or separated by a solid electrolyte (SOEC, solid oxide electrolysis cell). Electrolysis of both H2O and CO2 becomes increasingly heat consuming with increased temperature. At elevated temperatures a significant part of the total energy demand can be provided as heat according to Figure 2. This provides an opportunity to utilize the Joule heat that is inevitably produced when electrical current is passed through the cell due to the internal resistance in the cell. In this way, the overall consumption of electrical energy is reduced, and the efficiency for production of H2 and/or CO can be increased. The endothermic heat consumed by the splitting of H2O and/or CO2 is related to the enthalpy of formation of H2O or CO2, ΔHf, as described by eq 4: ΔHf = ΔGf + T ΔSf

(4)

The total energy (ΔHf) needed for H2O and/or CO2 splitting consists of a fraction of electrical energy (the Gibbs free energy, ΔGf) and a fraction of heat energy (TΔSf) as described by eq 4 and shown in Figure 2. Electrolysis of both H2O and CO2 (reaction 1 and 2) is endothermic; consequently, the electrolyzer will cool unless heat (TΔSf) is supplied. The thermoneutral voltage (also denoted enthalpy voltage) is defined as the minimum thermodynamic voltage at which a perfectly insulated electrolyzer would operate, if there were no net inflow or outflow of heat. The value of the thermoneutral voltage (Etn) is thereby a thermodynamic quantity, a function of the operating conditions of electrolyte temperature, and total pressure. Etn corresponds to the change in enthalpy during the electrolysis process, at the operating temperature:

Etn =

1 ΔHf nF

(5)

where ΔHf is the molar enthalpy of formation of either H2O or CO2, n is the number of electrons involved per reaction for the electrolysis reaction (n is equal to 2 in the case of both H2O and CO2 electrolysis), and F is Faraday’s constant (96 485 Coulomb/mol). For H2O electrolysis, Etn is 1.48 and 1.29 V at 25 and 850 °C, respectively. For electrolysis of CO2, Etn is 1.47 and 1.46 V at 25 and 850 °C, respectively. Practically, the 10702


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superheating water is expected to be cheaper than pressurization by means of electric pumps.

numbers are for ratios of H2O/H2 and CO2/CO of 1 in the cathode compartment and 1 atm oxygen in anode compartment in the case of an oxide ion conducting electrolyte. In the case of other electrolytes the corresponding detailed analysis should be done in each case (even though the overall reversible thermodynamics are the same); e.g., there is no steam on the cathode side in a cell with proton conducting ceramic electrolyte. Thus, it should be noted that all thermodynamic values have to be calculated for the actual local conditions; e.g., the value of Etn also varies along the flow direction of the steam in an active electrolyzer situation. Erev is the minimum voltage required for splitting of either H2O or CO2 and is related to the standard Gibbs energy of formation. At standard conditions (25 °C and 1 atm) the Erev is θ Erev =

1 ΔGfθ nF

3.2. Electrolysis Efficiency

The literature presents a number of theoretical and experimental studies on the efficiency of high temperature electrolysis.120,121,123,158−170 Many discrepancies are found because calculated efficiency numbers are not based on the same assumptions. If the efficiency, η, for the electrolysis process is calculated on the basis of the amount of energy needed to split the reactants, the efficiency should be based on the higher heating value (HHV, the formation enthalpy, −ΔHθf of either CO or H2 oxidation depending on the electrolysis reaction). Then, the efficiency is defined as ΔHθf divided by the energy consumption, W, used to produce 1 mol of the product. The energy consumption includes the electricity and heat used for the electrolysis reaction plus any energy losses:

(6)

HHV HHV = W electricity + heat + loss HHV = UnF + heat + loss

The standard Gibbs free energy of formation of H2O is θ = −229.8 kJ/mol, and in the case of CO2, ΔGθf,CO2 = ΔGf,H 2O −250.1 kJ/mol. At 25 °C the reversible voltage at standard conditions for electrolysis of H2O is 1.23 and 1.33 V for electrolysis of CO2. The reversible voltage at any pressure and temperature can be expressed by the Nernst equation, which provides the relationship between the standard potential (Eθrev) for the cell reaction and the reversible potential (Erev) at varying partial pressures of reactants and products: Erev =

1 1 ΔGfθ − RT ln Q nF nF

η=

(8)

Here U is the cell voltage. If the efficiency is based on the low heating value (LHV) the efficiency will be 85% when there are no losses in the electrolyzer. This is inadequate, since (theoretical) lossless operation normally is associated with 100% efficiency. If the electrolysis cell is operated at or above the thermoneutral potential (Etn), all the heat for the electrolysis reaction is supplied by Joule heat produced within the cell. Hence, eq 8 can be rewritten as shown in eq 9.

(7) 1/2

Here, Q is the reaction coefficient, Q = pH2O/(pH2 + pO2 ) for H2O electrolysis and Q = pCO2/(pCO + pO21/2) for CO2 electrolysis, which at standard conditions is 1 for all reactants and products, thus, Q = 1 and ln Q = 0. As can be seen from Figure 2, an increase in temperature reduces the reversible potential substantially. Thus, it can be advantageous to operate the electrolysis cells at elevated temperatures. Furthermore, it is noted that with increasing pressure the reversible potential increases corresponding to the increase in free energy of the product gases. As expected, operation at increased pressure was found to increase the cell kinetics.89,156 In the case of alkaline electrolysis cells, increasing the pressure is beneficial as it results in reduction of the relative volume of the gas bubbles, and thereby increases the conductivity of the electrolyte and reduces the internal resistance. In solid oxide cells the impinging frequency, which is related to the chance for collision of the gaseous reactant with the electrochemically active sites of the electrodes, increases with pressure which theoretically reduces the electrode resistance.157 Only recently activities on pressurized SOEC have been initiated, and a reduction of the electrode resistance was obtained for pressurized SOECs.89,156 For AECs, pressurization can be achieved by electrolyzing liquid water, at which gases evolve, and thereby increasing the pressure. The reaction only carries on if the cell voltage exceeds the reversible voltage at the higher pressure; otherwise, the reaction stops. For SOECs, an increase in gas pressure can be achieved by heating the inlet steam. At 213 °C, the vapor pressure is 20 atm, and at 287 °C it is 70 atm. Such “low temperature” heat may be cheaper than electricity, especially if the heat is produced onsite by exothermal chemical or physical reactions (e.g., heat from nuclear power and/or catalytic reaction of syngas into methane). Hence, pressurization of the electrolyzer by means of

ηat or above E = tn

L′ =

loss nF

Etnθ nF Etnθ ΔHfθ = = UnF + loss UnF + loss U + L′ (9)

For all the cell types, designs can be chosen so losses such as heat loss to the surroundings, electrical leakage through the cell, gas leaks, etc. are quite small. Hence, if the cell is operated at the Etn or slightly above, the cell can be operated with efficiency close to 100%. In practice SOECs have been shown to operate at electrical efficiencies exceeding 95%.158,171−173 High conversion efficiency is of course beneficial; however, cost efficient hydrogen/synthesis gas production is usually more important. In order to optimize the overall production economy a high fuel production rate is necessary. Increasing the cell voltage increases the current density and in turn the hydrogen/synthesis gas production rate. Operating the electrolysis cell above the thermoneutral potential will cause a production of surplus heat, and the efficiency decreases according to eq 9. As already mentioned, today’s alkaline electrolysis cells are normally operated above the thermoneutral potential (1.7−1.9 V) to optimize the fuel production economy at the expense of production efficiency (equivalent to electric efficiency 78−87%). On top of this come system losses of, e.g., electrolyte circulation.

4. HIGH TEMPERATURE ELECTROLYSIS IN SOLID PROTON CONDUCTING ELECTROLYSIS CELLS Solid proton conducting electrolysis cells (SPCECs), have been studied for electrolysis of water and recently electrolysis of water combined with electrochemical conversion of CO2.174,175 10703


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Figure 3. Example of microstructure for gas diffusion electrodes for a solid proton conducting electrolysis cells (SPCECs) with the configuration NiBCZYZ|BCZYZ|BCZYZ-LSCM. Reprinted with permission from ref 203. Copyright 2012 The Electrochemical Society, Inc.

Figure 4. Working principle of solid proton conducting electrolysis cells (SPCECs) for (A) steam electrolysis, (B) electrolysis of water combined with electrochemical conversion of CO2, and (C) production of hydrogen with simultaneous upgrading of methane to higher hydrocarbons.

High temperature proton conducting oxides applied for electrolysis are predominantly ABO3‑δ type perovskites mainly based on SrCeO3, BaCeO3, and BaZrO3. To achieve proton conduction, a fraction of the B atoms is replaced by trivalent cations such as Y, Yb, Nd, etc.,176 resulting in the formation of oxygen vacancies, which take up water vapor. Water then reacts with oxides ions forming OH−, and proton conduction is induced.177−179 Proton conduction is possible in a material like yttria-doped barium zirconate, BaZr0.85Y0.15O2.925, because the substitution of Zr4+ with Y3+ causes oxygen vacancies in the crystal structure and these vacancies may take up water. If the material is fully saturated with H2O, then it becomes BaZr0.85Y0.15H0.15O3. The protons in the water then distribute homogeneously with only one proton (H+) on each oxygen; i.e., OH− ions are formed in the crystals. The protons are “free” to hop by thermal activation from one O2− ion to another. When a proton conductor is used as the electrolyte for an electrolysis cell (Figure 4), steam is introduced into the anode and hydrogen is electrochemically extracted from water in the form of protons, which are conducted to the cathode by the electric field.

H 2O →

anode reaction

cathode reaction

1 O2 + 2H+ + 2e− 2

2H+ + 2e− → H 2

(10) (11)

The advantage of this type of electrolysis cell is that pure hydrogen without water vapor can be obtained directly in contrast to the electrolysis cell based on oxide ion conduction (SOECs, Figure 9), and consequently, no fuel circulation is necessary in the SPCECs because water molecules are not generated at the fuel electrode. As mentioned above, the SPCECs have recently been applied for electrolysis of water combined with electrochemical conversion of CO2174,175 (Figure 4B). This process is often mentioned as coelectrolysis of CO2 and steam, although in the mechanism (reactions 12−15) only steam is involved in the electrolysis reaction. CO2 is electrochemically converted to CO by reaction with the proton (reaction 13). Another option for SPCECs is production of hydrogen with simultaneous upgrading of methane to higher hydrocarbons180,181 as illustrated in Figure 4C. Using proton conducting oxides as electrolyte and suitable electrodes, e.g., BCZYZ (BaCe0.5Zr0.3Y0.16Zn0.04O3‑δ) as electrolyte with iron based composite fuel electrode,182 it is possible to 10704


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convert 1−2% of the CO2 into methane at 650 °C following an overall reaction of the type: anode reaction cathode reaction

4H 2O → 2O2 + 8H+ + 8e−

but recently ceramic fuel electrodes have been demonstrated as well.182

5. HIGH TEMPERATURE ELECTROLYSIS USING ALKALINE CELLS Commercial alkaline electrolysis cells are generally operated at temperatures below 100 °C. Increasing the operating temperature influences both the thermodynamic of the system and the electrode performance. In the following section, only operation at elevated temperatures, above 150 °C, is treated. H2O is split into H2 and O2 by an electric current that is passed between two electrodes, which in classical alkaline electrolysis are submerged in an aqueous electrolyte as shown in Figure 5.

(12)

CO2 + 8H+ + 8e− → C H4 + 2H 2O (13)

overall reaction

CO2 + 2H 2O → 2O2 + CH4

(14)

However, as the equilibrium partial pressure of CH4 is low at 650 °C, most of the CO2 is converted into CO following the overall cathode reaction CO2 + 4H+ + 4e− ⇌ CO + 2H 2O

(15)

To date, most investigations on SPCEC focus on exploring electrolyte materials, and only a few systematic investigations of the electrode materials and reactions have been reported. The electrode structures of SPCEDs are similar to those applied for SOCs, i.e., gas diffusion electrodes. An example of an SPCEC, full cell with the configuration Ni-BCZYZ|BCZYZ|BCZYZLSCM, is shown in Figure 3. The first183 application of proton conducting electrolytes for steam electrolysis to produce hydrogen was on doped strontium cerate183,183−193 [e.g., yttria-doped, SCY (ytterbium-doped strontium zirconate, SZYb, has also been applied for electrolysis of nitrogen oxide194)], doped strontium zirconate181,189,195,196 (e.g., yttria-doped, SZY), and doped strontium cerate partial substituted with zirconia196 (e.g., yttriadoped, SCZY). In the early studies the electrodes consisted of platinum.183 Also, doped barium cerate has been investigated for electrolysis applications (e.g., yttria-doped, BCY195,197−199), and doped barium cerate partial substituted with zirconia (e.g., yttria-doped, BCZY200) as well as BCZY with the addition of cobalt (BCZYCo201) or zinc (BCZYZ182,202,203). When applying doped strontium cerate, the observed hydrogen production seems stable187,191 with a yield close to the theoretical as calculated from Faraday’s law,183,193,204,205 but high electrolyte resistance and cell potentials were observed for these SrCeO based proton conducters.187,191 Higher production rates are observed by substituting ceria with zirconia (SZY) and with increasing current efficiencies at higher water content.189,196 Doped barium cerates (e.g., yttria-doped, BCY) generally exhibit higher proton conductivity than SrCeO based proton conductors. However, these materials are unstable at high temperature in the presence of CO2 and steam.206,207 The stability of the doped barium cerates may be improved by the introduction of Zr at the B site.208,209 Doped barium cerates are to date one of the most investigated electrolytes for SPCECs,210−213 and zirconia-doped barium cerate partially substituted with zinc (BaCe0.5Zr0.3Y0.16Zn0.04O3‑δ),214 shows reasonably good proton conductivity at temperatures in the range 400−600 °C215,216 comparable to yttria-doped barium zirconate (BaZr0.9Y0.1O2.9).208,209 The oxygen electrode is usually perovskites such as strontium-doped lanthanum cobaltite (LSC)191 (and LSCBCZYbCo201), strontium-doped lanthanum ferrite partially substituted with cobalt (LSCF),181,217 strontium-doped lanthanum manganite partially substituted with chromium (LSCM; LSCM-BCZYZ202,203), samaria-doped strontium cobaltite (SSC)196 (and SSC-BCZY200), and the fuel electrode is usually Ni-based182,191,196,200,202,203 (Ni-BCZY,200 NiBCZYZ,182,202,203 or Ni-BCZYbCo201) in similarity to SOEC,

Figure 5. Working principle of alkaline electrolysis.

Water is a very poor ionic conductor, and ions must be added in order to form a conductive electrolyte so the reaction can proceed without resistance that is too high. Both alkaline and acidic solutions can be used. In the following only alkaline electrolysis will be described, where mainly potassium and sodium hydroxide solutions are used. Of potassium and sodium hydroxide, potassium hydroxide possesses the lowest resistance in the electrolyte and the lowest cell polarization is achieved. Therefore, in most commercial water electrolyzers, potassium hydroxide solutions (25−30 wt %) are used. Noble metal electrodes exhibit a lower hydrogen evolution overpotential, i.e., a lower resistance than nickel electrodes.218,219 However, electrodes of noble metals are too expensive for commercially competitive electrolysis cells. Therefore, nickel electrodes are applied in most commercial alkaline electrolyzers.218,220 By applying nickel-plated steel the cost can be reduced even further.167 Increasing the operating temperature influences the thermodynamic of the system, as described above. Only a few older papers provide experimental results on electrolyzers operated at elevated temperatures above 150 °C.221−226 As mentioned, conventional alkaline electrolyzers are operated at around 80 °C and a voltage of 1.7−1.9 V; therefore, more electrical energy than thermodynamically necessary is consumed (operation above the thermoneutral voltage). The excess energy must be removed from the system by, e.g., cooling water. The ohmic loss in the electrolyte significantly contributes with ∼25% of the total loss in the cell at −0.5 A/cm2.165 The conductivity of the electrolyte (both NaOH227 and KOH227−231) increases approximately linearly with temperature up to and above 150−200 °C (Figure 6); i.e., 10705


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Ni−Ir, Ni−Ru, Ni−Mo243) show slightly lower overpotentials than Ni. For pure nickel electrodes, mainly Raney-nickel, i.e., leached nickel is used, because of the high active surface area and porosity. Other highly porous, but expensive, materials such as platinum supported on carbon have been suggested as alternatives to the nickel electrodes.244−246 Of more advanced materials, spinel and perovskites (Co3O4, NiCo2O4, LaNiO3, and La0.5Sr0.5CoO3) were shown to perform better as oxygen electrode than pure nickel,237,241,247 although Raney-nickel was shown to provide the lowest overpotential, i.e., lowest internal cell resistance in one study.248 Contrary to this, other studies showed that the mixed metal oxides, and especially spinel and perovskites (Co3O4, NiCo2O4, LaNiO3, and SrCoO3), were the preferred anode materials.237,247 Most electrode materials have been tested at temperatures up to 160 °C with a slightly increased thermal degradation at high temperature.218,220,237,242,247 Conventional nickel electrodes were shown to withstand temperatures of at least 200 °C, as they were used for the Apollo Fuel Cell System, which was operated at 204 °C and successfully tested at temperatures of 260 °C.249 The separators/diaphragms (Figure 3) serve to separate the product gases. Separation of the product gases becomes increasingly important at high pressure where oxygen become highly soluble in the electrolyte (KOH). For low temperature electrolyzers, the separator/diaphragm can be nickel oxide, asbestos, or polymer. Both the polymer and asbestos become unstable at temperatures above 120 °C.227 Therefore, at high temperature electrolysis new separator/diaphragm materials should be developed. Oxide-ceramic diaphragms such as ceramics of titanates (BaTiO 3 and CaTiO 3 ) or even NiO227,250,251 may be suitable substitutes for the polymeric or asbestos separators/diaphragms at high temperatures. The construction materials for the electrolyzer have to withstand the enhanced corrosion at high temperature operation. Stable operation over several years of titanate based alkaline electrolyzers at temperatures above 200 °C remains to be proven.

the ohmic losses in the electrolyte decrease with increasing temperature.

Figure 6. Temperature dependence of the electrical conductivity of 35 wt % aqueous solutions of KOH at pressures from 25 to 40 bar. Reprinted with permission from ref 229. Copyright 2012 International Association for Hydrogen Energy.

Because of the increased conductivity with temperature, a rise in the operation temperature would allow for significant energy savings. For NaOH, the conductivity passes through a maximum slightly above 150 °C;227 an operating temperature above this limit would no longer be beneficial regarding the electrical conductivity of the electrolyte solution. On the other hand, for 50 wt % KOH, the highest conductivity is found at temperatures above 200 °C.227−231 Because of the high vapor pressure for the electrolyte solution, at elevated temperature it is necessary to operate the electrolyzer at high pressure, as the boiling point for a 50 wt % KOH electrolyte solution is around 180 °C at 1 atm pressure.232 With increasing pressure the relative volume of the formed hydrogen or oxygen gas bubbles is also lowered. The bubbles cause extra ohmic losses, since the electrical conductivity of gases is zero. Thus, the ohmic losses caused by bubbles are minimized at high pressure. On the other hand, with increasing pressure, gas accumulation within the porous electrode materials may occur. Gas bubbles within the porous electrode decrease the fraction of the active electrode surface, thereby decreasing the efficiency.233 Consequently, highly porous electrode materials are of great interest for electrolyzers operated at high pressure. Even though the Erev increases with pressure, the cell voltage decreases (when the cell is operating at realistic conditions) because the pressure lowers the ohmic loss due to the decreased relative volume of the gas bubbles and increased kinetics of the electrode processes.

5.2. Performance

As described above, increasing the operating temperature would obviously increase the efficiency of H2O electrolysis. However, work reporting on electrochemical studies of hydrogen and oxygen evolution reactions in alkali solutions at elevated temperatures is scarce. To the best of our knowledge only very limited experimental data for alkaline electrolysis at temperatures above 150 °C have been reported.221−223,240,241 Nevertheless it was proven that the efficiency of H2O electrolysis for the production of hydrogen on polished nickel electrodes increases significantly by high temperature operation as shown in Figure 7, where the electrolysis performance as a function the operation temperature (in 50 wt % KOH) is shown. While heating the alkaline electrolyte solution, the pressure increased from 1 atm at room temperature to 14.3 atm at 264 °C.222 The dual region of the Tafel slope observed in Figure 7 was explained by a change in mechanism due to the magnetic properties of nickel, 222 although at present the exact mechanism is not understood. Regardless of the interpretation of the Tafel slopes, it is evident that the increase in temperature significantly improves the kinetics of both oxygen and hydrogen evolution222 (Figure 7), and electrolysis at elevated temperatures would be beneficial.

5.1. Materials

The materials developed for low temperature alkaline electrolysis have been treated in several reviews,234−239 whereas only limited research has shown performance of materials developed for high temperature alkaline electrolysis.221−223,240,241 Several alloys (Ti−Ni,242 Ni−Co,241 and 10706


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Figure 8. Microstructure for a gas diffusion electrode for high temperature alkaline electrolysis cell (HT-AEC) with the configuration Ni|SrTiO3|Inconel. Adapted with permission from ref 253. Copyright 2013 Elsevier.

Such an electrolysis cell where the electrolyte is immobilized (by capillary forces) has the advantage that they can be operated at temperatures above the boiling point of water, while the electrolyte remains in the liquid phase due to the higher boiling point of the concentrated electrolyte (in the present case, KOH253), and most important, the application of gas diffusion electrodes allows operation without the formation of gas bubbles and thereby decreases the ohmic resistance. On the other hand, the lower volume of the liquid electrolyte decreases the electrolyte conductivity and thereby leads to an increased ohmic resistance. Nevertheless, the ohmic resistance can be reduced by reducing the thickness of the porous structure. For the application of the gas diffusion electrode for alkaline electrolysis, it is important to operate at increased pressure to avoid evaporation of the electrolyte, but at pressures lower than condensation of steam.253 For this kind of high temperature alkaline cell, although with an anode of silver deposited on nickel and a cathode of Inconel, a current density as high as −1A/cm2 at the thermoneutral voltage (1.48 V) was reported at 240 °C and 37 bar.253

Figure 7. Electrolysis performance as a function of the operation temperature for alkaline electrolysis on polished nickel electrodes in 50 wt % KOH solutions. The figure is compiled from data published in ref 222.

The increase in temperature has a more pronounced effect on the oxygen evolution than the hydrogen evolution reaction.222 Increasing the temperature significantly shifts the oxygen evolution reaction to lower potentials, which was explained by the slow kinetics of the oxygen electrode reaction.222 For the hydrogen evolution reaction, only small shifts in potential were observed at temperatures above 150 °C at low current densities. Nevertheless, substantial overpotentials are found for the hydrogen evolution reaction on nickel electrodes in alkaline solutions at lower temperatures.222 For the oxygen evolution reaction the potential at a current density of −0.25 A/cm2 (which is a typical current density for commercial electrolyzers252) decreases from 1.76 to 1.27 V by increasing the temperature from 80 to 264 °C, whereas for the hydrogen evolution reaction, a slightly smaller decrease was observed from −0.43 V at 80 °C to −0.19 V at 264 °C applying polished nickel electrodes at −0.25 A/cm2 versus a hydrogen reference electrode.222 The cell voltage decreases from 2.19 V at 80 °C to 1.46 V at 264 °C (Figure 7,222). At a current density of −0.50 A/cm2 the relative decrease in cell voltage is similar, where the cell voltage decreases from 2.29 V at 80 °C to 1.55 V at 264 °C (Figure 7222). Recently a new type of high temperature alkaline electrolysis cells based on metal foam gas diffusion electrodes wherein the potassium electrolyte was immobilized in porous strontium titanate (SrTiO3) has been demonstrated.253,254 The microstructure of such a cell with a gas diffusion electrodes is shown in Figure 8.

6. HIGH TEMPERATURE ELECTROLYSIS IN SOLID OXIDE CELLS The reversibility of solid oxide fuel cells (SOFCs), i.e., also operating as a solid oxide electrolysis cell (SOEC), was demonstrated for both H2O154,161,255−261 and CO2154,261−266 in the early 1980s, and research was focused on the use of heat from solar concentrators or waste heat from power stations/ nuclear reactors.257,267 Although the cells are reversible, slightly lower performance is normally observed in electrolysis mode. For both H2O/H2 and CO2/CO mixtures a marginally higher activity toward oxidation (operation in fuel cell mode) is found in comparison to reduction (operation in electrolysis mode).83,116,268−271On the basis of a theoretic study, it was suggested that this difference between oxidation and reduction lies in the slower diffusion of H2O compared to H2.163 A recent study has shown that the lower performance in electrolysis mode may be caused by an increased conversion resistance in electrolysis mode as well as a local temperature decrease in electrolysis mode as compared to fuel cell mode due to the exothermic nature of the oxidation and endothermic nature of the reduction.272 It was further speculated that the increased conversion resistance was caused by diffusion to the active NiYSZ electrode (see Figure 10), depleting the reactants at the active Ni-YSZ electrode, and that the conversion resistance reflects the gas composition at the active Ni-YSZ electrode,272 10707


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although previously, the conversion resistance was believed to be based on the difference in inlet and outlet gas composition. Besides the increased diffusion, the lower performance may be related to the increased polarization resistance in dry conditions.153,273 The results for the performance of SOEC up to the 1980s were mainly obtained on tubular SOECs with a porous support layer inside the oxygen electrode tube.154,172,255,257,258,261,274−277 Although investigations on cells of tubular design are still performed (both tubular cells278−280 and microtubular cells276,281−286), mainly planar SOECs have been reported in recent years, of which both electrolyte supported 287−289 and fuel electrode supported88,268,290−293 cells are investigated. In a comparison of the tubular and planar cell configurations, the planar cells exhibit higher performance (due to better gas distribution), and the production costs are lower.276 To avoid gas diffusion limitations within the fuel electrode, which may be introduced at high current densities and steam concentrations,31 also oxygen electrode supported planar cells have been investigated, although the cell showed nonoptimal performance. 294 Starvation is generally observed at lower current densities and degrees of conversion in electrolysis cell mode as compared to fuel cell mode, and diffusion limitations can clearly not be neglected for electrolysis cells. It has been shown that increasing the porosity for an scaffold/backbone structured Ni-YSZ|YSZ|LSF-YSZ where Ni and strontium-doped lanthanum ferrite (LSF) are impregnated onto a YSZ scaffold/ backbone decreased the steam starvation limitations.295 Upon an increase of the diameter of the channels in the YSZ scaffold/ backbone from approximately 10 to 75 μm, the current density where starvation was observed increased from −0.4 to −1.4 A/ cm2, which corresponded to an increase from 40% to 95% steam conversion.295 H2O electrolysis is often examined for hydrogen production only, although utilization of the produced oxygen would increase the overall economics of the electrolysis process.296,297 From the 1960s the use of SOECs for production of oxygen via electrolysis of CO2 (and coelectrolysis of H2O and CO2) was intensively investigated by NASA for oxygen production in space and on Mars.154,263,265,266,298−313 In this case CO was treated as an undesired side product and converted into carbon and CO2 (CO2 was returned to the SOEC) over an iron catalyst. Oxygen regeneration via SOECs for space missions and for Navy and ocean research submersibles, etc., is today investigated by Paragon Space Development Corporation.314 Beside production of hydrogen, SOECs have also been investigated for decomposition of tritiated water for recovery of tritium produced in the deuterium−tritium fusion reactor systems.315−317 In the SOEC gaseous H2O and/or CO2 is fed to the negative fuel electrode and oxygen is evolved at the positive electrode (Figure 9). The electric field (energy) forces electrons to the negative electrode, indicated as a wind turbine generator. This forces oxide ions (from H2O or CO2) to be conducted through the electrolyte from the negative electrode to the positive. The use of natural gas or biogas as “sweep gas” on the oxygen electrode (so-called fuel-assisted electrolysis) has been examined to increase the overall hydrogen yield.318−324 In this case, the produced oxygen is consumed in either partial or total oxidation of the natural gas. In the case of partial oxidation of the natural gas, hydrogen is also produced on the anode side (i.e., oxygen electrode side). From an overall energy efficiency

Figure 9. Working principle of a solid oxide electrolysis cell (SOEC).

point of view such a process does not make sense, but clean hydrogen is produced, and if the natural gas is available and cheap it may possibly increase the profit. 6.1. Electrode Materials and Structures for Solid Oxide Electrolysis Cells

As for high temperature alkaline cells and solid proton conducting cells, gas diffusion electrodes are applied for solid oxide cells/solid oxide electrolysis cells. To date, Ni-YSZ based cells with different oxygen electrodes are the most investigated. An example of a Ni-YSZ based cell with LSM-YSZ composite oxygen electrodes is shown in Figure 10.

Figure 10. Microstructures of gas diffusion electrodes for a solid oxide cell (SOC)/solid oxide electrolysis cell (SOEC) with the configuration Ni-YSZ|YSZ|LSM-YSZ. Reproduced by permission from ref 325. Copyright 2010 Sune Dalgaard Ebbesen.

6.1.1. Fuel Electrode (Cathode). As for alkaline electrolysis cells, both nickel and platinum326 electrodes may be applied on SOECs, and have been investigated for both H2O and CO2 electrolysis.154,171,262−266,327 Electrocatalytic investigations of model electrodes, mainly at OCV, indicate that, at temperatures above 700 °C, Pt|YSZ has a lower exchange current density in H2/H2O mixtures, i.e., a higher polarization resistance than Ni-YSZ|YSZ, with Ni being a better electrode material than Pt in this context.270,328−330 The effect of mixing nickel and platinum (as well as nickel and iron330,331) has been tested.330 There are, however, very contradicting reports in the literature, and lower overpotential for H2 evolution on porous Pt|YSZ than on Ni-YSZ|YSZ has been observed with porous electrodes on half-cells sintered at very high temperature of 1400−1600 °C.332 Here, it should be noted that it is extremely difficult to compare the electrocatalytic activity of well-defined model electrodes with that of porous electrodes having a much less well-defined structure. Another problem is in this context 10708


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Ni-YSZ|YSZ half-cells were reported to 0.29 Ω cm2 at the same operation conditions (800 °C, 50% H2O−50% H2).342 These Ni-cermet electrodes also show excellent CO2 electrolysis performance.83,88,116−118,153,154,343 Nickel particles dispersed on samaria-doped ceria (SDC) have also been applied for cathodes.3,331,344,345 However, the Ni-SDC performance did not meet that of the Ni-YSZ electrode. Recently, also SFM,346 lanthanum-doped strontium vanadate (LSV),138 and strontiumdoped lanthanum manganite partially substituted with chromium (LSCM)139−146,148,149,151,152,347 electrodes have been applied for the fuel electrode for both H2O139−146 and CO2141,145,148,149,151,152,348 reduction with reasonable activity. These ceramic electrodes exhibiting good conductivity in both oxidizing and reducing environments circumvent the need for a reducing gas.139,148,149 Increased performance in electrolysis mode compared to fuel cell mode was reported for LSV,138 niobium-doped strontium titanates (STN),136 a composite electrode of lanthanum-doped strontium titanate LST-ceria,269 and several molybdates.349 Moreover, the electrolysis performance of the LST-ceria composite based fuel electrode was found to be superior to that of the Ni-YSZ electrode.269 However, a significant reversible decrease in initial performance of the LST-ceria based electrode upon increasing the H2O/H2 ratio in feed gas to the electrode was reported.269 6.1.2. Oxygen Electrode (Anode). The oxygen electrode has to withstand highly oxidizing environments. Thus, only noble metals such as platinum and gold or electronically conducting mixed oxides can be applied. Similar to the fuel electrode, noble metals are rejected because of cost consideration. Oxygen electrodes are composites of typically electronic conductors, an electronic conductor/ionic conductor composite, or mixed ionic and electronic conductors (MIEC). For the electronic or electronic/ionic composite oxygen electrodes, the oxygen evolution is confined to the active triple phase boundary similar to the Ni-YSZ fuel electrode. For MIEC oxygen electrodes the oxygen evolution spread out over some of or (in case of a relatively low oxygen surface exchange rate) the entire gas exposed MIEC surface within the electrochemical active volume, which extends over many particles into the oxygen electrode.350 The high operating temperature and oxidizing atmosphere make metal oxides the favored material for oxygen electrodes in SOCs. Most oxygen electrodes consist of mixed conducting perovskite type oxides of the general composition ABO3‑δ. The A-site usually is occupied by a large trivalent lanthanide ion and often partially substituted by divalent alkali earth ions to increase both electronic and ionic conductivity. The B-site is often occupied by one or more small tri- or tetravalent 3d transition metal ions. The perovskite oxide is oxygen deficient at lower pO2 and oxygen stoichiometric or cation deficient at higher pO2. Materials with high oxygen deficiency tend to be more active electrocatalysts, but are also less stable when tested as SOFCs. The local pO2 influences the oxygen electrode conductivity (ionic and electronic), electrocatalytic activity, and mechanical stability, which affect both the performance and durability of the oxygen electrode. In early studies on SOECs, the most commonly used anode materials were mixed oxides with perovskite structure, such as LSM, and to date the most studied SOECs consist of a Ni-YSZ fuel electrode, YSZ electrolyte, and a LSM-YSZ composite oxygen electrode, often denoted Ni-YSZ|YSZ|LSM-YSZ. LSMs are widely applied as oxygen electrodes for the evolution of oxygen and show high

the strong tendency for segregation of components and impurities to the surfaces and interfaces of these solid electrolytes and electrodes.333 Similar to alkaline electrolysis, the use of noble metals as electrode material is too expensive for commercial SOECs. Nickel exhibits high activity, but mainly conducts electrons, and consequently, electrochemical charge-transfer reactions occur in the vicinity of the active triple phase boundary (TPB). For a Ni-YSZ electrode, the TPB is defined as the boundary where the Ni network, the YSZ network, and open pores meet to form percolation paths for transport of electrons, oxide ions, and gas, respectively.334−336 Hydrogen/protons have a low solubility and high mobility in Ni. This makes transport of H-species in the bulk materials of the Ni-YSZ composite possible over short distances of about 0.1 μm. The TPB may therefore extend up to 100 nm adjacent to the physical TPB.337 In the case of carbon-species (in CO2/ CO mixtures) this is not possible. To increase the reaction area in the fuel electrode, nickel particles are typically mixed with ionic conducting particles of the solid electrolyte material. This type of composite electrodes is called cermet electrodes. This type of fuel electrode has been investigated by many research groups over several decades in particular as SOFC electrode. An SOEC should be capable of handling pure H2O and/or CO2, but at the operating temperature, nickel oxidizes rapidly when exposed to these gases. Recycling of a small part of the product gases may provide a technical solution to this problem. A notable advantage of using platinum electrodes for electrolysis is that the electrode can be operated at conditions where nickel electrodes would undergo oxidation, thereby avoiding recycling of H2 or CO; however, 99% steam can be applied to the NiYSZ electrode at 850 °C without oxidation of the electrode.338 In order to circumvent the need for recycling of H2 or CO, all ceramic electrodes may be applied,139,148,149 see below. Of cells that are sensitive toward oxidation and are metal-supported cells, at least early cells cannot withstand high steam concentrations. A metal-supported SOC has been tested for electrolysis performance and durability but showed significant oxidation of the metal support structure.339 The application of metal-supported electrolysis cells should not be ruled out; one has to think that, in electrolysis mode, the oxidizing potential of the inlet gas will not be much different than the oxidizing potential of the outlet gases in fuel cell mode. Further, in electrolysis mode, the oxidizing stream will be reduced at the TPB, which may limit the oxidation at the active electrode. Thus, if the metal-supported cells can be operated in fuel cell mode with limited degradation, there should be no obstacles toward operating these cells as electrolysis cells. At 950 °C, the area specific resistance (ASR) for these NiYSZ based cells (Ni-YSZ|YSZ|LSM-YSZ) was reported to be as low as 0.15 Ω cm2 at 50% H2O−50% H2.268 Decreasing the temperature to 850 and 750 °C caused an increase in ASR to 0.27 Ω cm2 and 0.60 Ω cm2, respectively.268 Both fuel electrode supported cells and YSZ electrolyte supported cells as well as button cells have shown a slightly higher activity toward oxidation of H2 than reduction of H2O.268,340,341 The configuration of the cell greatly influences the cell performance. At the same fuel composition, a slightly lower initial performance was obtained for electrolyte supported button cells with Ni-YSZ electrodes (ASR of 0.35 Ω cm2).287,288 At 800 °C, an ASR of around 0.9 Ω cm2 was reported for a 500 μm Ni-YSZ electrode supported cell,269 and the ASR values for 10709


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performance.268,276,288,332,340,351−356 Recently, as development for oxygen electrodes for SOFC has advanced, MIEC electrodes have also been tested for SOECs, and even higher performance was reported when substituting LSM with lanthanum cobaltite (LC),340 strontium-doped lanthanum cobaltite (LSC),103,332,340,356−359 LSF,269,356,357,360−365 strontium-doped lanthanum ferrite partial substituted with cobalt (LSCF),269,358,360,362,363,366−374 strontium-doped lanthanum ferrite partial substituted with cupper (LSCuF),269 or strontium-doped barium ferrite partial substituted with cobalt (BSCF),357,375−377 and even neodymium (NNO),378−380 lanthanum (LNO),381 praseodym (PNO)381,382 nickelates or strontium-doped lanthanum nickelate partial substituted with cobalt (LSCN).383 Also, for the oxygen electrode, modification of the LSM-YSZ electrode by impregnating with CGO384 has shown increased performance when operated with anodic polarization. Unfortunately, the high strontium content in LSC decreases the chemical stability.340 Above ∼750 °C, the nickelates as well as LSF, LSC, and LSCF electrodes require a reaction barrier layer (usually made of doped ceria) to avoid formation of insulating secondary phases at the electrode| electrolyte interface.385−388 Also, a PrCoO3 oxygen electrode has been proposed,389 but due to the TEC mismatch with the YSZ electrolyte, this electrode cannot withstand thermal cycling, although this can be circumvented by impregnation of the active electrocatalyst. These oxygen electrodes generally show better initial performance than the LSM electrode. Figure 11 shows the dc

increased performance in fuel cell mode as compared to electrolysis mode383 similar to other electrode materials.83,116,268−271 It should be mentioned that, in all comparisons, air was flowing to the oxygen electrode,378,380,383 and in some cases only 3% H2O380,383 flowed to the fuel electrode. In these cases, symmetry across OCV is not expected. Especially, when flowing air (instead of pure oxygen) to the oxygen electrode (the examined electrode in this case), the apparent increase in performance may simply be a result of an increased oxygen partial pressure at the oxygen electrode due to the production of oxygen in electrolysis mode (it is well-known that increased pO2 at the oxygen electrode increases the oxygen electrode performance390−393). 6.1.3. Electrolyte. ZrO2 stabilized with 8 mol % Y2O3 (8YSZ) is usually used as electrolyte material for SOCs, since it possesses relatively high ionic conductivity. Stabilizing ZrO2 with only 3 mol % Y2O3 (3YSZ) results in a less fragile electrolyte, but it possesses a lower ionic conductivity (σ3YSZ = 0.05 S/cm and σ8YSZ = 0.09 S/cm at 1000 °C395). A variety of other electrolyte materials such as ScYSZ, CGO, SDC, etc. have been examined.3,288,289,332,351,396 Besides influencing the ohmic resistance for the cell, it was shown that the choice of electrolyte material may influence the electrode performance. The resistance of an Ni-SDC cathode was reported to be significantly lower when stabilizing zirconia with scandia (ScSZ) instead of yttria.3 A positive effect on the electrode overvoltage in electrolysis mode was also observed when substituting the YSZ electrolyte with a samaria-doped CeO2 electrolyte, but due to high electronic conductivity of ceria at very reducing conditions doped ceria may not be used as an electrolyte in an electrolyzer due to internal short-circuiting of the cell.332 For the production of synthetic fuels in SOECs, a low operation temperature would be advantageous as, e.g., methane can be produced directly in the SOEC147 (below 700 °C as described above) where YSZ is not an optimal electrolyte. Alternative electrolytes for such purpose could be doped lanthanum gallate, e.g., doped with strontium on the lanthanum site and magnesium on the gallium site (strontium-doped lanthanum gallate partial substituted with magnesium, LSGM) could be used. However, this will require modifications of the nickel based fuel electrode to avoid the formation of LaNiO3397−401 and fractures due to TEC mismatch between electrolyte and electrodes.

Figure 11. Polarization characterization for the planar Ni-YSZ based cells: Ni-YSZ|YSZ|LSM-YSZ, Ni-YSZ|YSZ|CGObarrier|LSCF-CGO, and Ni-YSZ|YSZ|CGObarrier|LSC-CGO. Operating conditions: 800 °C, 50% H2O−50% H2, oxygen was supplied to the oxygen electrode. The figure is compiled from data published in refs 358, 374, and 394.

6.2. Performance of H2O Electrolysis

The Faradaic efficiency for H2O electrolysis in SOECs has been shown to be close to 100% over a period of 1000 h;257 i.e., there are practically no parasitic reactions as long as the polarization of the cell is not inducing any electronic conductivity into the YSZ, i.e. below a cell voltage of ca. 1.6 V for usually foreseen operation conditions.402,403 Considering this together with the endothermic nature of the electrolysis process means that the H2 or CO efficiency will be 100% minus the heat loss from the electrolyzer to the surroundings. Nevertheless, significantly different performance results have been reported for SOECs with electrodes of same composition.261,268,332,340,404,405 Until recently, Ni-YSZ based fuel electrode supported SOECs with a YSZ electrolyte and an LSM oxygen electrode (Ni-YSZ|YSZ|LSM-YSZ) exhibited the highest performance for full cells.88,268,338 As described above, the ASR for these cell was shown to be 0.15 Ω cm2 at 950 °C with a fuel gas composition of 50% H2O−50% H2.338 At 850

characteristics for three SOCs, all produced and tested at Department of Energy Conversion and Storage, DTU (former Risø DTU). At OCV, the standard Ni-YSZ|YSZ|LSM-YSZ SOC shows a low performance (0.27 Ω cm2), compared with Ni-YSZ|YSZ|CGObarrier|LSCF-CGO (0.19 Ω cm2) and Ni-YSZ| YSZ|CGObarrier|LSC-CGO (0.15 Ω cm2), see Figure 11. Interestingly, although the cell with the LSC-CGO oxygen electrode shows better performance in SOFC mode than the cell with the LSCF-CGO oxygen electrode, the performance in SOEC mode is close to identical for the two cells (Figure 11). It has been shown that the nickelates show higher performance in electrolysis mode as compared to fuel cell mode at 800 °C and temperatures below.378,380,383 Contrary to this, at higher temperatures, the nickelates have shown 10710


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°C, the ASR was around 0.20 Ω cm2 when data to −0.25 A/cm2 are included for the calculation of the ASR.338 To date, Ni-YSZ is still the preferred “fuel electrode”, although electrodes such as Ni-ScYSZ,103,289,406,407 NiCGO,3,360,408 Cu-YSZ,409,410 NiCu-YSZ,411 doped strontium titanates such as lanthanum or niobium-doped strontinum titanates (LST137,147 and STN136), and composite electrodes LST-ceria,269,412 as well as nickel impregnated LST-ceria,413 have been tested for electrolysis performance. Also, modification of the Ni-YSZ electrode by impregnating the Ni-YSZ structure with CGO414 or molybdenum-doped ceria (MDC)415 has shown increased performance.414,415 The new materials are often accompanied by a change in electrolyte also. In such, CGO electrodes are often applied with ceria based electrotytes.332 Both scandia and yttria codoped zirconia as well as ceria based electrolytes have a higher electronic conductivity than YSZ, which may be problematic when operating the electrolysis cells at high cell voltages, since this will increase the electronic conductivity,403,416 consequently lowering the efficiency, and most critically possibly leading to degradation of the cell. However, it should be mentioned that there is a great debate whether the electronic conductivity protects or damages the cell, see section 6.6. Compared individually, the advanced materials have shown improved performance, although to date, the Ni-YSZ electrode shows the best performance,88,268,338 most likely due to decades of structural optimization. This improved initial performance of the alternative oxygen electrodes such as LC, LSC, LSF, LSCF, LSCuF, BSCF, compared to an LSM-YSZ oxygen electrode (which can be operated at low current densities without degradation), may be rapidly lost due to degradation, especially when operated at low current densities103,361,366,375,417 (see section 6.6). The increased degradation of the LSC-CGO oxygen electrode and barrier layer at low current densities may be caused by the mechanical weaker YSZ|CGO(barrier layer) interface compared to the YSZ|LSM-YSZ interface.103 At high current densities, the LSM-YSZ electrode was found to degrade significant.369,418 A comparison of the durability of the Ni-YSZ|YSZ|LSM-YSZ cell with a cell consisting of Ni-YSZ|YSZ|CGO(barrier layer)|LSCFCGO showed that the LSCF-CGO electrode was electrochemically more stable than the LSM-YSZ electrode when operated at −1.5 A/cm2 at 800 °C,369 although some microcracks were observed between the YSZ electrolyte and the CGO barrier layer. The LSM-YSZ electrode seems more stable at low current densities,103,417 whereas at high current density, the advanced electrodes, i.e., LSC and LSCF, are more stable.369The application of a barrier layer to limit zirconate formation at the interface between the oxygen electrode and electrolyte is necessary, but may cause increased degradation due to its lower mechanical strength.103

performed on ceramic cells with CGO cathodes134,135 (and CuCGO429) with an initial performance of 3.8 Ω cm2 at 850 °C (95% CO2).134 The performance for electrolysis of CO2 is often reported for CO2/H2 mixtures,92,263 although this is clearly misleading as this mixture will be in WGS-RWGS equilibrium during operation,83,153,430−432 and the observed electrolysis performance will therefore contain information on electrolysis of both H2O and CO2 (coelectrolysis). As will be described below, the performance for CO2 electrolysis is lower than the performance for H2O electrolysis.83,116,153,154 Consequently, characterizing the cells for CO2 electrolysis in mixtures containing H2 will result in unrealistically high performance. The resistance for CO2 electrolysis is found to be significantly higher than the resistance for H2O electrolysis at identical conditions83,116,153,154(Figure 13) and may be due to the slower diffusion of CO2/CO compared to H2O/ H2163,433−437 and the significantly different reaction mechanisms for the two electrolysis reactions153,270,438,439 resulting in a larger TPB resistance in CO2/CO mixtures compared to the TPB resistance in H2O/H2 mixtures.153 6.4. Performance for Coelectrolysis of H2O and CO2

Coelectrolysis of H2O and CO2 was initially studied in the 1960s and early 1970s by NASA for production of oxygen, and has recently started to receive renewed attention as a means for producing synthesis gas and thereby synthetic fuels. Within recent years, the performance for coelectrolysis was mainly reported for Ni-YSZ based SOECs.83,90,93,117,126,153,408,440−442 The measured ASR for CO2 electrolysis in the Ni-YSZ based SOEC produced at DTU is significantly higher than for both H2O electrolysis and coelectrolysis at both 750 and 850 °C (Figure 12). The lowest ASR in electrolysis mode was observed

Figure 12. Polarization characterization for a planar Ni-YSZ| YSZ|LSM-YSZ SOEC at 850 °C in a mixture of 50% CO2−25% CO−25% Ar (CO2 electrolysis), 25% CO2−25% H2O−25% CO−25% Ar (coelectrolysis), and a mixture of 50% H2O−25% H2−25% Ar (H2O electrolysis). Reprinted with permission from ref 153. Copyright 2012 The Electrochemical Society, Inc.

6.3. Performance of CO2 Electrolysis

for reduction of H2O, showing that electrolysis of both H2O and CO2 occur153 and that the WGS reaction cannot be neglected. 153,436 Similar trends can be found in the literature,92,126 whereas other studies have shown comparable performance for steam and coelectrolysis90,442,443 (Figure 13). On the basis of this observation, the authors claimed that CO is produced solely via the RWGS reaction and that no electrolysis of CO 2 occurs during coelectrolysis in nickel based SOCs.90,442,443 This statement was based on similar ASRs measured for steam and coelectrolysis, which were lower than

As mentioned, besides H2O electrolysis, SOECs are also capable of electrolyzing CO2 to CO and oxygen. Even though electrolysis (electroreduction) of CO2 in liquid phase has been studied extensively on many different electrode materials including palladium, platinum, copper, and nickel,419−424 only limited studies have been reported for electrolysis of CO2 in the gas phase. These studies were based on metal cermet electrodes of palladium, platinum, or nickel (refs 83, 88, 116, 148, 153, 154, 262−266, and 425−428). Recently, CO2 electrolysis was 10711


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800 °C.263 These results were reported as CO2 electrolysis, but CO2 would inevitably be converted to H2O and CO by the water−gas shift reaction, and the results thereby reflect coelectrolysis performance with a thermodynamic equilibrium composition of 64% CO2−16% H2O−16% CO−4% H2. In studies treating coelectrolysis specifically, the highest performance is found for Ni-YSZ based SOECs. ASR values as low as 0.22 Ω cm2 were reported for Ni-YSZ based SOEC operated at 800 °C with a fuel gas composition of 25% CO2− 50% H 2O−25% H292 (the thermodynamic equilibrium composition is 19% CO2−56% H2O−6% CO−19% H2). At more relevant operating conditions, an ASR of 0.26 Ω cm2 was reported for Ni-YSZ when operated at 850 °C with a fuel gas composition of 45% CO 2 −45% H 2 O−10% H 2 83 (the thermodynamic equilibrium composition is 40% CO2−50% H2O−5% CO−5% H2). 6.5. Cell Degradation During H2O, CO2 and Coelectrolysis of H2O and CO2 at Mild Operating Conditions

Relatively limited long-term electrolysis tests of SOECs have been reported on “state-of-the-art” Ni-YSZ electrode based cells.87,290,338,403,446−448 Even though the initial performance may be quite similar in electrolysis and fuel cell mode, the cells were found to degrade much faster in electrolysis mode than in fuel cell mode.338,449−451 At current densities below approximately 1 A/cm2 the degradation on the oxygen electrode can be limited and the main part of the degradation occurs on the Ni-YSZ cathode.87,338 We will refer to operation at current densities below 1 A/cm2 as “mild conditions”. SOEC with a relatively coarse Ni-YSZ electrode and a relative low performance was reported to be stable for thousands of hours at 995 °C.257 The higher performing NiYSZ based SOECs were reported to have relatively low degradation when operated at electrolysis current densities up to 0.75−1.00 A/cm 2 in 50% H 2 O−50% H 2 (at 850 °C),87,338,407,446 although increasing the steam concentration may induce degradation.452,453 Further, an increase in the steam conversion degree may increase the degradation rate.454,455 When operating the SOEC fuel electrode at mild conditions the most common degradation phenomena is segregation of impurities to the triple phase boundary.116,290,338,456−460 The performance loss was in some cases attributed to a glassy phase containing silica impurities at the triple phase boundaries of the fuel electrode originating from the applied glass seal.460 For cell testing, contamination of the Ni-YSZ electrode from “external” source, i.e., the glass seal, could be eliminated by applying a seal-less test setup461,462 or applying an inert sealing material such as gold, platinum, or nickel.83,117,153,446 For commercial use, the application of scavengers463−465 or a change in the properties of the glassy phase466,467 could be a solution. In fact, recent studies on stack performance have shown that glass sealing that does not negatively influence the stack durability can be developed.440 A cell and test system completely free of impurities is hardly an option especially when costs are not considered. The degradation for CO2 electrolysis was found to be comparable to the initial passivation/degradation rate of H2O electrolysis at similar conditions for identical SOECs (refs 87, 116−118, 268, 290, 338, and 446). In accordance with thermodynamics it was shown that the degradation during CO2 electrolysis is not associated with formation of coke within the Ni-YSZ electrode,116 but rather a consequence of impurities in the gas stream.116−118 The conclusion was based on the fact

Figure 13. Polarization characterization for H2O electrolysis (54.8% H2O−22.5% H2−22.7% N2), H2O/CO2 coelectrolysis (54.9% H2O− 22.6% CO2−22.5% H2), and CO2 electrolysis (100% CO2), with mean ASR values obtained at 800 °C on a 10 cell stack. The cells in the stack were planar electrolyte supported cells with a Ni cermet fuel electrode, a scandia-stabilized electrolyte, and a strontium-doped manganite oxygen electrode.442,443 The balance of the gas mixtures supplied to the fuel electrode was made with nitrogen. Reprinted with permission from ref 442. Copyright 2009 International Association for Hydrogen Energy.

for CO2 electrolysis, when characterizing the SOC in 54.8% H2O−22.5% H2−22.7% N2, 100% CO2, and 54.9% H2O− 22.6% CO2−22.5% H2 at 800 °C.90,442,443 At first, it should be noted that operating the SOC on pure CO2 may lead to nickel oxidation in the Ni-YSZ electrode as a CO2 concentration above 99.5% will theoretically lead to oxidation of nickel. The thermodynamic equilibrium composition for the coelectrolysis mixture used in the study is calculated to be 60% H2O−18% CO2−18% H2−4% CO at 800 °C.444 In these mixtures, the conversion resistance (calculated according to ref 445) is approximately 1.2 times higher for steam electrolysis compared to coelectrolysis. With a correction for the different conversion resistance, the electrochemical resistance for coelectrolysis may in this case also lie between that of steam and CO2 electrolysis, showing that both CO2 and H2O electrolysis occur in the nickel based SOCs. ASR in both electrolysis and fuel cell mode in 50% CO2− 25% CO was reported to be significantly higher than in 50% CO2−25% H2. This indicates that CO2 reacts with H2 to produce H2O and CO via the R-WGS reaction (reaction 3), increasing the H2O content at the active Ni-YSZ electrode and thereby decreasing the ASR for “CO2 electrolysis” because the ASR for H2O electrolysis is lower than the ASR for CO2 electrolysis. The mechanism for coelectrolysis therefore involves both H2O and CO2 electrolysis, and some CO2 reacts with H2 to form CO and H2O via the R-WGS reaction. The initial work by NASA was performed on platinum based cells with an ASR during coelectrolysis (80% CO2−20% H2O) of 0.26 Ω cm2 at 900 °C.262 Cheaper nickel based cells have also been tested, of which an ASR of 1.13 Ω cm2 was reported for an Ni based SOEC in a mixture of 80% CO2−20% H2 at 10712


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Figure 14. Cell voltage measured during electrolysis of H2O, CO2, and coelectrolysis of H2O and CO2 for Ni-YSZ|YSZ|LSM-YSZ cells when operated with cleaned gases. The cells were operated at 850 °C and with an electrolysis current density 0.25−0.50 A/cm2. Reprinted with permission from ref 117. Copyright 2010 The Electrochemical Society, Inc.

for electrolysis purposes, e.g., if the CO2 originates from biological sources. Another option, similar to circumventing coke deposition, is the application of, e.g., Cu-CGO485 or addition of ceria or niobium to Ni-YSZ486,487 which have shown increased durability in sulfur containing atmosphere in fuel cell mode.

that the cell could not be regenerated when operated at conditions that would oxidize carbon (fuel cell operation).116 On the other hand, aromatic hydrocarbons were shown to deactivate the SOC when operated as a fuel cell.468 This shows that deposition of coke/hydrocarbons may not necessarily be removed when operated in fuel cell mode. Removal of impurities will most likely be an important step on the way to efficient SOECs for CO2 electrolysis. The practical and economic aspects have to be assessed to determine the most suitable operating conditions and thresholds for acceptable degrees of cell degradation. A few studies have presented short-term durability of coelectrolysis.92,441,469 Long-term durability for more than a thousand hours have been reported for both platinum and nickel based SOECs.83,117,262,440 For the platinum based SOEC, a degradation rate of 70−100 mV per 1000 h of operation was observed at a current density of −0.2 A/cm2, 80% CO2−20% H2O, at 900 °C.262 The degradation rate for nickel based SOEC was 0−6 mV per 1000 h of operation at an current density of −0.25 A/cm2 to −0.75 A/cm2 and at 850 °C (45% CO2−45% H2O−10% H2).83,117,440 The degradation of the nickel based SOECs was concluded to be caused by segregation of impurities to the triple phase boundary.83,117,440 By cleaning the inlet gases the observed degradation at mild conditions can be eliminated, and operation without degradation is possible117,440 as shown in Figure 14. Instead of cleaning the inlet gases, another option is the development of impurity tolerant electrode materials. Steam and CO2 used for electrolysis may contain trace quantities of silicon, sodium, or chlorine, and in the case of CO2 collected from biomass or industrial sources, sulfur and phosphor.470−474 It is generally believed that cells with an all ceramic fuel electrode (like strontium titanates) have an increased tolerance to impurities, although these cells have mainly been tested for tolerance toward sulfur impurities,475−477 where obviously they show increased durability as sulfur does not adsorb on the oxides. These cells may be applied for electrolysis with increased durability, but impurities like chlorine and phosphor may interact with the oxide structure,478−484 limiting the durability of oxide cells applied

6.6. Cell Degradation During H2O, CO2 and Coelectrolysis at High Current Densities

Operating the SOECs at more “harsh” conditions, i.e., at current densities above −1 A/cm2 at 850 °C, may cause structural degradation of both the fuel electrode and the oxygen electrode. It have been shown that the current density significantly influences the (structural) degradation of the cells,91,373,374,394,418,441,455 and that degradation at the oxygen electrode (LSM-YSZ) is observed at lower current densities than degradation at the fuel electrode (Ni-YSZ).91 Since the degradation is most likely related to the electrode overpotential, the current density limit for “harsh” operating conditions will decrease with decreasing temperature; i.e., degradation will be observed at lower current densities at lower temperature. Several degradation phenomena have been observed when operating the SOECs at harsh conditions. For the nickel electrode, redistribution of nickel due to sintering446 or the formation of nanoparticles488,489 has been observed. For the electrolyte, reduction of the electrolyte when operated at high cell voltage,490 there is formation of oxygen bubbles at the grain boundaries close to the oxygen electrode which in extreme cases is observed together with a delamination of the oxygen electrode.418 Since the structural degradation is greatly influenced by stresses in the structure, the “safe” operating limit varies for nominally same structures, but produced by different routes. For example, the LSM based oxygen electrode produced at DTU only shows structural degradation if operated at or above −1 A/cm2 at 850 °C,118,418,440,441 whereas significant structural degradation (delamination) of the LSM electrode tested/produced at INL/Ceramatec shows significant structural degradation, already at −0.3 A/cm2 at 800−830 °C.447,491 The delamination indicates a weakening of the LSM10713


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Figure 15. SEM micrograph of the Ni-YSZ fuel electrode of a Ni-YSZ|YSZ|CGObarrier|LSCF based SOEC after operation for more than 900 h at −1 A/cm2 at 778 °C.499 It should be noticed that the cell was operated for 10 h under electrolysis conditions without steam supply, in which the cell voltage increased drastically resulting in a significant electronic conduction.367 Reprinted with permission from ref 499. Copyright 2013 Elsevier.

YSZ|YSZ interface, although it has to be pointed out that the observed delamination most likely occurs during dismounting the cell from the test house [otherwise the increase in serial (electrolyte resistance) would increase more than observed]. Microstructural changes occurring at the electrolyte close to the oxygen electrode, and/or directly at the interface, causing, in the most severe cases, delamination of the oxygen electrode, are presently the most discussed degradation phenomena occurring when operating the SOECs at harsh conditions.340,402,404,418,441,492−503 The delamination was argued to be caused by a pressure build-up occurring upon oxygen evolution in closed pores in the electrode/electrolyte interface,340,495 with formation of oxygen in YSZ grain boundaries close to the oxygen electrode402,418,495 (which may be promoted by cation diffusion418,504). The oxygen formation is probably caused by an increased electromotive potential (induced by the increased polarization drop across the oxygen electrode) in the YSZ electrolyte adjacent to the oxygen electrode. For comparison, one study showed similar pore formation in the electrolyte close to the anode when operated as an oxygen pump505 at current densities of 4.5 A/cm2 at 1045 °C, for extended periods.496 Increased activity of the oxygen electrode (i.e., decreasing the polarization voltage of the oxygen electrode) would limit the oxygen pressure, and thus the oxygen delamination. This means that cells with improved oxygen electrodes such as LC,340 LSC,103,332,340,356 LSF,356,360−363 LSCF,360,362,363,366−370 and BSCF375,376 which exhibit a lower polarization resistance should in turn also show better durability toward oxygen electrode delamination. In contrast, lower durability is often observed, which might be related to a lower mechanical strength of the barrier layer necessary for these advanced materials and/or the active oxygen electrode. Only a few studies show reasonable long-term performance of improved oxygen electrodes.363,367,369

Although containing an LSCF oxygen electrode with low overpotential, one study still showed significant degradation [3.4% during the entire test (9000 h) when operated at −0.28 to −1.0 A/cm2 at 780 °C].367 Other studies showed a clear improvement in the cell durability by exchanging the LSM-YSZ oxygen electrode with either a higher performing LSCF-CGO (and CGO barrier layer) oxygen electrode369 or a higher performing LSF or LSCF (with CGO barrier layer) oxygen electrode.362,498 This increased durability may be attributed to a lower overpotential at the oxygen electrode.340,369,384 The relation between overpotential and degradation rate can be analyzed by fixing the electrode composition while changing the oxygen activity. A study of LSM-YSZ electrodes with varying performance due to change in the porosity, but produced by the same production method, showed a clear relation between increasing performance (overpotential) and increasing degradation rate.340 Similar results have been observed when impregnating the LSM-YSZ electrode with CGO which simultaneously showed an increased performance and a decrease in the delamination.384 Other causes for degradation of the LSM oxygen electrode may be the formation of new phases at the interface with the electrolyte148,506−514 or partial oxidation/reduction of manganese.515 As a means to circumvent the oxygen electrode delamination or oxygen bubble formation, it has been proposed to allow for some electronic conductivity of the electrolyte which is claimed to lower the internal oxygen pressure avoiding oxygen electrode delamination,501,516 and this approach has been attempted to be proved experimentally.501 However, the electronic conductivity lowers the efficiency, and most important, since some electronic leak current is transferred through the electrolyte, the overpotential at the oxygen electrode lowers for a given current density. The lower overpotential at the oxygen electrode is most likely the reason for circumventing the delamination, and not the electronic conductivity as such. The 10714


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same effect could thus be obtained by applying a nonelectronic conducting electrolyte and operating the cell at lower current density, thereby increasing the efficiency of the system. Nickel sintering has been observed when operating the NiYSZ based SOECs at very harsh conditions (−2.0 A/cm2, 950 °C, and 90 H2O−10% H2 supplied to the Ni-YSZ electrode), causing a decrease in the active area (TPB length).446 The mobility of nickel was suggested to be a consequence of formation of Ni(OH)2 which would be drawn toward the electrolyte interface when polarizing the cell.446 The high steam concentration normally used during electrolysis operation has been claimed to enhance the degradation of SOCs,117,446,517−519 both in the form of nickel sintering,446,519 and formation of surface hydroxides,517,518 but also because of an increased mobility of impurities117,519 as discussed in section 6.5. Recently, also percolation losses368 and the formation of nanoparticles at the nickel surface within the Ni-YSZ structure were observed during both steam electrolysis and coelectrolysis of H2O and CO2 (Figure 15).488,489 In one study, the authors claim that, despite the significant structural change, the cell degradation was limited to approximately 3% per 1000 h operation at −1 A/cm2 for the main part of the test.488 For the last 1000 h of the test, the cell was operated at cell voltages at which some electronic conductivity may be expected, and operated for periods without steam supply.367 The operation at high cell voltages and with increased electronic conductivity certainly enhances the microstructural changes, and the postmortem analyses most likely mainly reflect the operation at high cell voltages without steam supply. Nevertheless, the results clearly show the effect of operating the cell at harsh conditions. It has to be mentioned that there is at present no evidence that operation in either steam, carbon dioxide, or in mixtures of steam and carbon dioxide (coelectrolysis) affects the degradation mechanism, as long as clean gases are used and carbon formation is avoided.

Figure 16. Polarization characterization for a planar Ni-YSZ| YSZ|LSM-YSZ SOEC at 750 °C in 50% H2O−50% H2 and at pressures ranging from 0.4 to 10 atm. Reprinted with permission from ref 89. Copyright 2010 International Association for Hydrogen Energy.

circuit voltage.89 With a higher initial performance of the cell, it is expected that the point of interception (equal cell voltage at different pressure) will be moved to higher current densities, and a noticeable effect on the SOEC performance at increased pressure is expected. At stack level there was a similar decrease of 20% in resistance for a 10-cell stack when increasing the pressure to 6.9 bar;156 the specific cell type was not stated in the publication. For this stack test also the short-term durability was also examined, in which the stack voltage increased around 5% for the first 60 h (corresponding to an increase of around 80%/1000 h of operation). Hereafter, the stack voltage decreased, indicating cell damage; indeed some cracked cells and broken seals were observed after test.156 6.8. High Temperature Electrolysis in Solid Oxide Cell Stacks

As mentioned above, mainly single SOECs have been investigated, but performance and durability of high temperature electrolysis stacks are attracting more and more attention.1,30,90,258,275,289,405,440,447,469,491,536−539 Idaho National Laboratory in collaboration with Ceramatec Inc. has been very active in testing the use of SOC stacks for high temperature electrolysis of steam for H2 production and coelectrolysis for syngas production. Testing has been conducted at various scales, including stack modules consisting of up to two 60-cell stacks.469 As a part of a demonstration project, the produced synthesis gas was fed through a Fischer−Tropsch reactor to produce liquid hydrocarbon fuels.539,540 In general the stacks are found both to perform worse and also to degrade faster than single cells due to chromium deposition from the interconnects491 and due to thermal imbalance.469,541 It is general accepted that the stack components may have a negative influence on the cell/stack degradation due to interconnected oxidation, deposition of impurities, as well as contact losses.363,455,541−543 In contrast to this, some stacks from Topsoe Fuel Cell (TOFC)368,440,537,544 and recently also some from Ceramatec363 (Figure 17) have shown similar durability as single cells117/button cells545 when operated at low current densities (below an electrolysis current density of 1 A/cm2). Initial rapid passivation of the cells/stack was observed followed by longterm activation. One study suggests that the lower stack performance, compared to cell performance, should be attributed to lower gas flow rates, thereby increasing the gas conversion resistance.440 The electrode performance of individual repeating units (RU) in the examined stack showed a performance compared to single-cell tests.440 The application of chromium

6.7. High Temperature Electrolysis in Solid Oxide Cells at Increased Pressure

Although several research institutes/companies have shown the effect of pressurization when operating solid oxide cells as fuel cells,157,520−534 examination of the performance of SOECs at increased pressure has only recently been initiated.89,156,532 Pressurized operation of the SOEC stack is energetically favored and will increase the efficiency by 3−5%.122,535 A significant decrease of approximately 20% in the internal resistance in both fuel cell and electrolysis cell mode of a NiYSZ based cell (Ni-YSZ|YSZ|LSM-YSZ) was found by increasing the pressure from 1 to 10 atm, when operated with 50% H2O−50% H2 supplied to the Ni-YSZ-electrode and O2 supplied to the LSM-YSZ-electrode89 (Figure 16). This decrease in cell resistance is due to the frequency of reactants hitting the triple phase boundary, and subsequently, conversion to products increases with increasing pressure,530,531 and decreased diffusion resistance.156,532 The open circuit voltage was found to increase with increasing pressure as predicted by the Nernst equation (eq 7, section 3). The exact same gain in performance is observed in electrolysis mode as compared to fuel cell mode (equal decrease in ASR as a function of pressure), but this is obscured by the observed increase in OCV. Although the internal resistance of the cell was found to decrease, the overall performance of the cell (operating cell voltage) was close to unchanged due to the increase in the open 10715


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Figure 17. Steam electrolysis durability of a 10-cell stack operated at −0.25 A/cm2 and 800 °C. The intermediate voltage was measured five places within the 10-cell stack. The cells within the stack consisted of Ni-Ce|ScSZ|CLF (CLF being lanthanum-doped cobalt ferrite). The stack was operated with 56% H2O−22% H2−22% N2 to the Ni-Ce electrode and air at the CLF electrode. Reprinted with permission from ref 363. Copyright 2013 International Association for Hydrogen Energy.

energy production and storage was proposed in the early 1980s (H2O154,161,256−258,275,536,546 and CO2154,262−266), and the principle of reversible operation of solid oxide cells for electricity storage, as well as constant operation in electrolysis mode for electricity storage as chemical energy in the form of hydrogen, was patented already in the mid-1990s.547,548 Characterizations of “reversible solid oxide cells/stacks” have been published extensively during several decades since electrolysis researchers normally characterize the cells in fuel cell mode also. Reversible long-term operation or even development of dedicated reversible cells is fairly new.138,295,362,503,549−560 SOCs operated at elevated temperature have the potential to provide large-scale electricity storage at a lower cost than other storage methods such as batteries (as discussed in the Introduction), which would enable high penetration of intermittent renewable electricity sources like solar and wind power. Reversible operation of the SOCs for electricity storage may have a limited economic benefit at present498,561 but may increase with increasing share of intermittent energy sources in the energy supply grid. Denmark in particular will need affordable electricity storage in the near future to aid intermittent wind power supply if the fraction of electricity supplied by wind power continues to increase from today’s 30% to 50% by 2025 in Denmark.5,6 Production of, e.g., methane via electrolysis may provide a means for electricity storage in the future as methane can easily be stored in the natural gas grid in most countries.6−8 The SOECs were found to degrade significantly when operated for long periods at high current density, due to oxygen precipitation at the interface to the oxygen electrode (as described earlier). One advantage of operating the SOCs reversibly or switching between idle operation and electrolysis operation is a potential durability improvement of the cells.562 Various degradation mechanisms may decrease since the polarization is switched forth and back

containing interconnect plates and the TOFC proprietary glass sealing did not negatively influence the stack durability when operated at 850 °C, and it was found that the Ni-YSZ based SOEC stack could be operated without degradation at current densities up to at least −0.75 A/cm2.368,440,544

7. REVERSIBLE OPERATION OF HIGH TEMPERATURE ELECTROCHEMICAL CELLS Several types of electrochemical cells can be operated reversibly, storing electricity as fuel in electrolysis cell mode, and producing electricity from fuel in fuel cell mode. As such, reversible electrochemical cells can be viewed as a type of battery which is charged during electrolysis cell operation and discharged during fuel cell operation. Thus, the solid oxide cell can be operated reversibly in both fuel cell mode and electrolysis cell mode. There are a number of terms used in the literature for SOCs operating reversibly, including reversible solid oxide fuel cell (R-SOFC), regenerative solid oxide fuel cell (also denoted R-SOFC), and solid oxide regenerative fuel cell (SORFC). NASA tends to use the term regenerative solid oxide fuel cell to stress that the system is a “closed loop” and the reactants are regenerated and stored for later use.295 The solid oxide cell itself can be operated reversibly in both fuel cell mode and electrolysis cell mode, and the term reversible/ regenerative SOFC (R-SOFC) is therefore somewhat misleading unless investigated at system level. It has to be stressed that the reversible SOCs (or R-SOFC) that have so far been tested are the exact same cells as have been tested as either fuel cells (SOFCs) or electrolysis cells (SOECs) and are not newly developed cells, although attempts to tailor SOCs for reversible operation have started to receive attention. In principle all high temperature cells operating on steam can be operated reversibly, even high temperature alkaline cells, and with close to identical performance in fuel cell mode and electrolysis cell mode. Reversible operation of high temperature SOCs for 10716


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Figure 18. Cell voltage measured during reversible operation (daily cycles) of an planar fuel electrode supported Ni-YSZ based cell566 operated at 750 °C in pure hydrogen and a current density of 0.25 A/cm2 in fuel cell mode and in 50% H2O−50% H2 with a current density of 0.50 A/cm2 in electrolysis cell mode.455 It has to be noted that the degradation rate stated in the figure was measured in fuel cell mode, whereas the degradation rate stated in the text was measured in electrolysis mode. Reprinted from ref 455 with permission from Versa Power Systems.

(current density of 0.5 A/cm2) and fuel cell mode (current density of either 0.5 or 0.25 A/cm2) as compared to electrolysis operation alone.455,555,558,559 For the study on reversible solid oxide cells, Versa Power Systems performed an extensive cell development and tested more than 20 different cell design.455 The development resulted in a decreased degradation rate at constant electrolysis conditions from 91 mV/1000 h to 19 mV/ 1000 h (operated at 750 °C in 50% H2O−50% N2 and an electrolysis current density of 0.50 A/cm2).455,559 Further, the degradation rate when operated reversibly with day−night cycles in electrolysis cell and fuel cell mode was decreased from ∼70 mV/1000 h558 to ∼15 mV/1000 h455,559 (measured in electrolysis mode) as shown in Figure 18. (It has to be noted that the degradation rate stated in Figure 18 was measured in fuel cell mode, whereas the degradation rate stated in the text is measured in electrolysis mode.) It has to be stressed that the operating conditions were not the same in both cases and can therefore not be compared directly. In the old study (∼70 mV/ 1000 h degradation), the cell was operated at 750 °C in 50% H2−50% N2 (in fuel cell mode) and in 50% H2O−50% N2 (in electrolysis cell mode) with a current density of ±0.50 A/cm2 in both fuel cell mode and electrolysis cell mode,558 whereas in the more recent study (∼15 mV/1000 h degradation), the cell was operated at 750 °C in pure hydrogen and a current density of 0.25 A/cm2 in fuel cell mode and in 50% H2O−50% H2 with a current density of 0.50 A/cm2 in electrolysis cell mode,455,559 Figure 18. From the recent study, it can be seen that the degradation rate of the reversible operation was not enhanced as compared with constant electrolysis operation. Alternately operating a single SOC (one single repeating unit, RU) in electrolysis mode and leaving idle at OCV showed either a durability increase, or that the durability was unaffected by cycles of either 10 s or 10 min as compared to constant electrolysis operation at −1A/cm2.553,564 One study on a single cell (Ni-YSZ|YSZ|LSCF-CGO) claimed no increased degradation when operating the cell switching between SOEC operation (in 70% H2O−30% H2 at −0.44 A/cm2, 700 °C)

rather than held steady in electrolysis mode. It should be mentioned that this improvement of cell durability may only be observed for degradation mechanisms depending on the applied current, and will not apply to some types of impurity induced degradation that was shown to be independent of the current density when operated periodically at OCV or constant electrolysis conditions.116 Since real reversible long-term operation, i.e., switching between electrolysis and fuel cell operation of SOC, is fairly new, only limited durability data are available, and have shown completely different results; i.e., some studies have shown a decreased durability when operated reversibly, whereas some studies have shown an increase in durability (see below). An LSV-YSZ half-cell test demonstrated the feasibility of reversible operation of SOCs.138 Also, symmetrical cells of LSM-YSZ have been tested successful at alternating current, showing a decreased degradation rate when alternating between SOFC and SOEC mode, at a high current density (−1.5 A/ cm2), and short cycles showed a larger improvement as compared to long cycles551 (during tests of these symmetrical cells of LSM-YSZ the application of a silver current collector was used, which significantly decreased the durability550). Real reversible long-term testing has to the best of our knowledge only been shown in a few publications, one comparing different oxygen electrodes, which makes it difficult to interpret the results, but the results indicate that the stack degradation is either unaffected or increased when operating in reversible mode as compared to electrolysis/fuel cell mode alone.362 A single cell study on a scaffold/backbone structured Ni-YSZ| YSZ|LSF-YSZ cell has shown an increase in degradation rate from around zero at constant electrolysis operation to around 30%/1000 h when operated intermittently by switching between electrolysis and fuel cell mode.295 The most extended reversible testing of an SOC has been performed at cell and stack level at Versa Power Systems455,555−560,563 and showed an increased degradation, up to 9 times, when operated at 750 °C and with an alternating current between electrolysis mode 10717


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the electrode reaction is very simple) a lot of disagreement about the actual detailed electrode reaction mechanism(s).593−595 Like in the case of the Ni-YSZ fuel electrode, both the dependence of the reaction rate on oxygen partial pressure and the apparent activation energies vary considerably.582,585,587,588,591,596−607 Finally, the partial pressure dependency may depend both on the initial performance of the cell and level of degradation272 as well as on the electrode microstructure.601,602,604 This indicates that interphases play a somewhat overseen role which could explain why materials fraction, sintering procedure, and segregation of components and impurities of the electrode materials to the surface and TPB region (see, e.g., section 6.5) have significant impact on cell performance and thereby explain the observed variations in published experimental results. The Butler−Volmer expression is often used to interpret the experimental results, also in the case of cermet electrodes, although the Butler−Volmer expression is describing a case with a single charge transfer across a barrier as the single ratedetermining step. The first general argument against the use of the Butler−Volmer equation is that the overpotential varies with the distance from the bulk electrolyte surface of the actual TPB reaction site due to iR drop in the electrolyte part of the composite electrode. Thus, the kinetics of the cermet electrodes cannot be modeled by the Butler−Volmer equation.608,609 It has also been shown through modeling that, due to the fact that more than one electrode process is rate determining in the SOECs, the Butler−Volmer equation is not valid.610,611 Further, an increasing number of papers points out that a significant part of the impedance of a composite electrodes (both fuel and oxygen electrodes) can and should be described using the finite-length Gerischer expressions.612−614 In this case the iV curve cannot follow a Butler−Volmer expression because even the iR drop corrected iV curve will be significantly influenced by ohmic resistance. Further, it is very difficult to accurately model electrode geometry as it is the result of many factors, e.g., percolation of phases, wetting of particles, and tortuosity. The random packed sphere model has been applied with some success; i.e., it has been possible to model the trends of the effect of various parameters such as ratio of Ni/YSZ and effect of particle size.615−619 When describing the cell performance, besides the electrode kinetics also gas phase kinetics have to be taken into account. The gas phase kinetics include the change in the Nernst potential as a consequence of an altered gas composition due to diffusion of species through the porous structure and the conversion of reactants and formation of products. In the literature the polarization resistance caused by diffusion is often denoted concentation polarization163,591,620 and is modeled in several different ways.433 It has to be stressed that this concentration polarization only treats polarization through the structure although the conversion of species is certainly also related to the concentration, and thus, the term concentation polarization (when only treating diffusion limitations) is somewhat misleading. In principle, both the diffusion and conversion should be straightforward, although in practice there is some disagreement about the actual detailed understanding. Theoretically, the diffusion resistance caused by gas diffusion limitations within the porous electrode is directly related to the gas composition change due to diffusion limitations through the porous electrode, causing a change in the Nernst potential which can be directly translate to the diffusion resistance.621 The actual diffusion length through the porous structure is

and leaving it idle at OCV although some nickel oxidation was observed (which was most likely due to an unsealed test configuration).554 The degradation rate in both constant and transient mode was 5%/1000 h corresponding to ∼70 mV/ 1000 h.554 Two studies have shown lower degradation when cycling between fuel cell and electrolysis cell mode as compared to electrolysis operation only.549,565 In the most recent study, a significant increase in durability by carefully operating a NiYSZ|YSZ|LSM-YSZ reversible was observed, where no longterm degradation was observed compared to a significant degradation during constant electrolysis operation of a similar cell. The study concluded that the increased durability was a consequence of elimination of the microstructural degradation that in constant electrolysis mode occurs near the oxygenelectrode/electrolyte interface.549 Considering the long-term cell stability needed for commercialization, the fact that intermittent operation might increase durability is highly promising in terms of application of SOC technology for large-scale energy storage.

8. MODELING THE PERFORMANCE OF HIGH TEMPERATURE ELECTROLYSIS CELLS Most research on high temperature electrolysis (both steam and carbon dioxide and coelectrolysis of steam and carbon monoxide) is experimental in nature. Studies on modeling the experimental cell, stack, and system performance have however increased in recent years but have so far been focused on high temperature electrolysis in solid oxide cells, and modeling electrolysis in proton conducting cells is rather limited.567−569 When modeling the electrode performance in electrolysis cells, the specific cell materials and, not least, the electrode structures are of great importance. A lot of experimental as well as modeling work has been carried out to identify the mechanism(s) of the given electrode reaction. This is valid for both the fuel electrode (mainly Ni-YSZ) as well as for the oxygen electrode (including LSM, LSCF, and LSC). Unfortunately there is very little agreement between the published experimental results. The apparent activation energy, current− voltage relationship (iV curve), measured impedance, and dependence of the iV curve on partial pressure of reactants and products vary hugely.570,571 Therefore, many scientists, who primarily are developing electrode and cell models, are of the opinion that these very different results cannot all be true. Yet, it might well be that all reported results are true. A large number of rate-determining steps have been identified for the Ni-YSZ electrode when operating in both fuel cell and electrolysis cell mode in mixtures of hydrogen, carbon monoxide, steam, carbon dioxide, and methane153,270,438,439,572−577 which are very dependent on electrode structure and composition. The rate-determining processes include, among others, the conductivity of ions through the electrolyte, the electrode reactions, gas diffusion, and gas conversion. Assumptions on structure and composition of the interfaces (including surfaces) are rarely explained, and many papers give the impression that the authors believe that the interfaces are simple cuts of the clean bulk crystal structures with one single charge-transfer process as the rate-determining step which can be modeled by the Butler−Volmer equation. However, reported activation energies as well as H2, CO, H2O, and CO2 partial pressure dependencies vary greatly273,578−592 which points to significant differences in the details of the reaction mechanisms from case to case even for nominally equal electrodes. For the oxygen electrode, there is (although 10718


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different losses in the specific cell for the given operating condition. It has to be stressed that these models will not be general as mentioned above (due to the cell variations) and will only apply for a specific cell. Nevertheless, these models are applicable to understand the electrochemical behavior of a cell, to determine appropriate operating conditions and to optimize the cell performance with respect to the operating conditions. More general engineering models, without concern about the actual mechanisms in details, have been developed for both steam electrolysis163,435,591,611,618,637−645 and carbon dioxide electrolysis343,434,646 and coelectrolysis of carbon dioxide and steam.436,647−649 These models have mainly been used to study the thermal effects (temperature gradients, etc.) during steam electrolysis278,435,611,637−645 as well as predict the iV curve, electrical losses, fuel production, and cell/system efficiency. Despite the level of detail in these models, and the fact that they are mostly fitted with the Butler−Volmer equation (which is linear over most of the relevant overpotential range), they provide an insight in many operation parameters. On the basis of these studies it has been suggested that oxygen electrode supported electrolysis cells are more favorable compared to fuel electrode supported cells due to diffusion limitations,163,591,620 that thin electrodes and electrolytes should be applied,591 and that the cells should be operated at increased temperature591 and pressure434,591 to increase the performance. Regarding diffusion limitations there is a great deal of inconsistency with some studies showing that diffusion has a large influence on cell performance591 (and especially for carbon dioxide electrolysis due to the high molar mass of CO/CO2 compared to H2/ H2O434) and others showing that diffusion limitations are insignificant.435,637 On the basis of significant diffusion limitations, particle size graded electrodes have been proposed.618 It has further been shown that the difference observed in the marginally higher activity toward oxidation than reduction (higher performance in fuel cell mode compared to electrolysis mode) in H2/H2O mixtures may be due to the slower diffusion of H2O compared to H2.163 As previously mentioned, from a thermodynamic point of view, coke formation within the porous structure may be detrimental; this phenomenon has also been modeled for both carbon dioxide343 and coelectrolysis of steam and carbon dioxide.648 Modeling has been used to study the degradation of SOECs,650−653 and the observed bubble formation/delamination at the oxygen electrode observed when operating the SOECs at high current densities. It has been suggested that the delamination is a result of an increased oxygen pressure within the electrolyte close to the oxygen electrode/electrolyte interface650,651 or being related to the temperature gradients across the cell and the different thermal expansion coefficients (TEC) for the electrode and electrolyte.652 It has to be mentioned that delamination is not only observed at the interface, which TEC differences might account for, but also inside the electrolyte, which TEC differences cannot account for. Although most models are steady state, recent models have included transient operation focusing on control strategies as well as thermal effects.637−640,654 Also, modeling at stack and system level has been conducted, where the distribution of current density, temperature, gas composition, as well as system efficiencies have been modeled.90,405,642,643,655−664 These studies provide important information on stack and system performance and may be useful for design optimization. The most extensive system modeling has been focused on the production of synthesis gas

extremely difficult to estimate, and the tortuosity factor is used to describe that the diffusion path through the porous structure is longer than the direct path (electrode/support thickness). Often, the tortuosity factor is modified to explain the deviations in the measured diffusion resistances, and examples of tortuosity factor of up to 10−15 can be found in the literature.582,622 Such high tortuosity does not seem physically reasonable when the porosity of the support layer is usually higher than 30% and when compared to tortuosity for porous catalyst particles, which is usually below 5.623 It has to be pointed out that the porous catalyst particles and the Ni-YSZ cermets are produced in significantly different ways; the catalyst particles are basically porous sintered ceramics impregnated with the active metal, whereas the cermets are produced by mixing the ceramics and oxidized metal after which the cell is cast and sintered, and at start-up the metal oxide is reduced to metal. Thus, the fabrication method of the cermets may induce higher tortuous paths through the support structure. Further, microstructural studies (using, e.g., focused ion beam scanning electron microscopes) of fuel electrode supported cells can provide essential information regarding the structure,624−631 and the tortuosity for typical fuel electrode supported SOCs with a porosity around 30% has been determined to be between 1 and 3.624−628 Similar to the diffusion resistance, theoretically, the conversion resistance caused by gas conversion is directly related to the gas composition change due to the conversion, causing a change in Nernst potential which is reflected as the conversion resistance.445 Simple expressions for the conversion resistance solely rely on the difference between the inlet and outlet gas composition have been proposed;445,632 however, the actual conversion resistance (as measured by impedance spectroscopy, local current voltage relation, local slope of the iV curve) is also related to the local conversion inside the cell,272 and models combining both diffusion and conversion have been proposed.633 In general, the SOC performance seems to depend on the cell history, materials purity, the detailed chemical compositions, and the physical properties of the electrolyte and electrode material. Therefore, a simple static universally valid mathematical expression cannot describe SOC electrode kinetics. Even though a simple valid universal model cannot be determined, a deep understanding of the electrodes and the electrode reactions is very useful for optimization of the SOCs. As described above, searching for one universal rate limiting reaction step for a given type of SOC composite electrode may not be very useful, and a simple charge transfer from/to a clean well-defined electrode surface where all TPBs are of equal potential to/from a clean electrolyte surface as the rate limiting reaction does not occur.608 Unfortunately, the models developed to date omit such considerations of the reaction kinetics, mostly due to a lack of kinetic data and the complicated patterns of the reactions that occur. The problem of limited knowledge of the reaction path is omitted in developed models by the simple assumption that the reaction is instantaneous and conversion is either assumed or is fitted to match experimental data. Describing and modeling the electrochemical electrolysis cell performance via impedance breakdown and physical equivalent circuit models272,634,635 may be successful in describing both the electrode and gas phase kinetics. This approach has been used to describe the electrolysis performance and the difference between fuel cell and electrolysis cell performance.272,273,635,636 These models provide quantitative information about the 10719


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and synthetic fuels in combination with nuclear energy.90,656,660 Also, the combination with other energy sources has been investigated, such as combining hydrogen production with geothermal heat657 as well as combining SOEC with photovoltaic and solar heat659 in order to increase the system efficiency. For a combination of photovoltaic and solar heat, an efficiency of 80−90% was predicted, although it has to be mentioned that the modeling was not validated experimentally. Other studies have shown similar high efficiencies for production of hydrogen via dynamic operation.643 Unfortunately, very few studies are thoroughly validated with experimental results, and in most cases, validation has been limited to only one iV curve if done at all. At system level, reversible operated systems have been modeled.658 In this specific case, the electricity and heat produced when operated in fuel cell mode (operated on methane) were supplied to the electrolysis unit, and the unreacted fuel was reacted (combustion) with the produced oxygen (from the SOEC) in order to heat the electrolysis cell. 658 Combustion of the unreacted fuel instead of recirculation seems inefficient, especially considering that the electrolysis stack may be operated exothermically, and in fact results in a low efficiency of around 45%. Although the high temperature electrolysis is normally aimed for hydrogen production, carbon monoxide, synthesis gas, and oxygen production have also been modeled.661,662

although it has to be mentioned that a low electricity price of 1.4−3.7 €¢/kWh has been used for the calculation of the production price. With a more realistic electricity price of 8−10 €¢/kWh673 (average electricity price in Europe in 2012, excluding taxes, but including transmission, distribution, and other services), the production price would be around 4−6 €/kg H2, which is higher than the target for hydrogen production setup by U.S. Department of Energy. The process for hydrogen production from methane by steam reforming has been improved significantly, and operates close to the theoretical thermodynamic efficiency.11 The high efficiency for steam reforming of methane is reflected in the low production cost for hydrogen, which is around 1−1.3 €/kg H2.674 Consequently, neither low temperature nor high temperature electrolysis can produce hydrogen at competitive prices. A similar conclusion is that hydrogen produced from water electrolysis will cost about 2−3 times that of hydrogen produced via steam reforming of natural gas, and has previously been made.675,676 The capital cost for methane steam reforming decreases significantly with increasing size. Increasing the production capacity from 100 m3/h to 1000 m3/h decreases the hydrogen production price 5-fold.677 At a hydrogen production capacity below ∼1000 m3/h methanol reforming becomes competitive to methane reforming, and electrolysis may be competitive at lower production capacities since the production price via electrolysis scales linearly with the electricity price irrespective of production size. On the other hand, high temperature electrolysis cannot compete with hydrogen production via steam reforming at current electricity prices. A very detailed analysis of the economy for production of FT diesel by coelectrolysis reported possible production prices around 1.0 €/L diesel678 (with an electricity price of 4.4 € ¢/kWh). A commercial study showed a production price of 1.0 €/L diesel679,680 whereas others have shown suggested prices as low as 0.4 €/L diesel33 (electricity price of 1.5 €¢/kWh) and 0.88 €/L diesel (at 1.52 €¢/kWh) and others as high as 3.00 €/L diesel (at 10.61 €¢/kWh).681 The predicted production cost of diesel via high temperature coelectrolysis in combination with the Fischer−Tropsch synthesis is comparable with today’s diesel prices of 1.1−1.2 €/L diesel.682 Operation of the solid oxide electrolysis cell stacks at higher current densities up to at least −0.75 A/cm2 without degradation may enable cost competitive production of synthetic hydrocarbon fuels without the consumption of fossil fuels.

9. ESTIMATES OF PRODUCTION COST OF RENEWABLE SYNTHETIC FUELS VIA ELECTROLYSIS If high temperature electrolysis should be taken outside the research laboratory it must be cost competitive to other hydrogen, carbon monoxide, or synthesis gas production technologies. The synthesis gas may be catalyzed into a synthetic hydrocarbon fuel, but again this is only interesting if the synthetic fuel is cheaper than the fossil fuels or if the renewable energy is exempt from taxes via a political drive for renewable energy. Renewable fuel costs will become more comparable to fossil fuel costs, if environmental costs and CO2 taxes are included.72 The estimated production costs of hydrogen and synthetic fuels via either low and high temperature electrolysis have been shown to depend strongly on electricity prices; in fact, many studies show that the electricity consumption accounts for more than 80% of the final hydrogen production price.498,655 Thus, investment costs (stack/system cost) for electrolysis systems are of less importance as compared to fuel cell systems. The hydrogen production prices for low temperature alkaline electrolysis were estimated to be 1.6−5 €/kg H2665,666 (when applying the H2A economic analysis methodology,667 developed by U.S. Department of EnergyHydrogen Program, the production price was estimated to 4.5 €/kg,666 based on year 2000 dollars and exchange rates), based on a low electricity price of 1.4−3.7 €¢/kWh. Similarly, the hydrogen production price applying high temperature electrolysis was estimated to be 1.1−1.8 €/kg H2.498,655,657,668 A very detailed sensitivity and economics study for a nuclear powered hydrogen production plant applying the H2A economic analysis methodology with a hydrogen production price of 2.6 €/kg H2 was estimated at an electricity price of 2.4 €¢/kWh.669,670 These production prices are below or very close to the target setup by U.S. Department of Energy of 2.0−3.0 $/gallon (gallon ∼3.785 L)671 (2005 as reference year) (1.6−2.5 €/kg H2) and later 2.0−4.0 $/gallon672 (1.6−3.3 €/kg H2) (2007 as reference year),

10. SUMMARY High temperature operation of water electrolysis significantly increases the performance of alkaline electrolysis cells (AECs), solid proton conducting electrolysis cells (SPCECs), and solid oxide electrolysis cells (SOEC). Increasing the operation temperature for AECs from the conventional 80−90 °C to above 250 °C significantly increases the electrolysis performance and thereby the efficiency. An obstacle for high temperature operation of the AECs is the lower stability of the materials, especially at high current densities. At present, suitable cell and separator materials for high temperature AECs, which are not more expensive than low temperature alkaline electrolyzer materials, have been identified, but the necessary long-term stability remains to be proven. Another advantage of SPCECs and SOECs is the possibility of coelectrolysis of H2O and CO2 yielding synthesis gas (CO + H2) which can be converted catalytically to various types of 10720


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AUTHOR INFORMATION

synthetic hydrocarbon fuels. The possibility to produce hydrogen and carbon dioxide neutral synthetic hydrocarbon fuels for the transport sector from renewable energy sources may create a future market for efficient high temperature electrolysis. Compared to SOECs, SPCECs have the advantage that pure hydrogen without water vapor can be obtained directly in contrast to the SOECs, and consequently, no fuel circulation is necessary in the SPCECs. On the other hand, most investigations on SPCEC have so far focused on exploring electrolyte materials, and the exploration of electrode materials is therefore necessary. Further, only very limited data on the durability of such cells have been presented. For SOECs operating at 750−850 °C, efficiencies exceeding 95% (using higher heating value) are realistic, and the initial performances reported for SOECs and stacks are promising. Operation at high current densities increases the production rate of hydrogen, carbon monoxide, or synthesis gas and thereby improves the overall economy. However, the state-ofthe art SOECs suffer from significant degradation (increase in cell resistance) at high current densities. When operated at high current densities, structural degradation is observed for both the fuel (cathode) and oxygen (anode) electrode. For the oxygen electrode, weakening of the electrolyte|electrode interface due to oxygen evolution occurs (which in most severe cases lead to delamination), whereas, at the fuel electrode, loss of percolation as well as the formation of nanoparticles are observed. Development of higher performing cells may circumvent these degradation phenomena by lowering the overpotential, and thereby the driving force for the degradation. This has been at least partly proven for the oxygen electrode, whereas for the fuel electrode the exact degradation mechanism still has to be resolved, and a means to circumvent degradation at high current densities has to be developed. For both electrodes, the main degradation modes have been identified, and the lifetime at high current densities is the main issue to be addressed before the technology can become commercially viable. In general, the hydrogen production price using high temperature electrolysis is competitive with low temperature alkaline electrolysis, provided long lifetimes. The estimated production costs of hydrogen and synthetic fuels have been shown to depend strongly on electricity prices (the electricity consumption account for more than 80% of the final hydrogen production price), and the investment costs (stack/system cost) become of lesser importance. With the current electricity prices, and the fact that production costs depend strongly on the electricity prices, high temperature electrolysis cannot compete with hydrogen production via steam reforming of natural gas. Fischer−Tropsch-diesel can be produced via coelectrolysis of CO2 and H2O at a price, which is higher than the production price of hydrogen produced via H2O electrolysis (comparing the production cost per MJ of total energy for the two energy carriers), but comparable with today’s diesel production price using fossil fuels. Today, significant resources are allocated to electricity-gridstabilization, and with an increasing share of renewable energy supply, the demand for grid-stabilization is expected to increase. Operating the electrolysis cells such that they are used both for grid-stabilization and fuel production could result in more competitive synthetic fuel production prices.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Dr. Ir. Sune Dalgaard Ebbesen is a senior research scientist and workgroup leader for the evaluation of Solid Oxide Electrolysis Cells at the Department of Energy Conversion and Storage at the Technical University of Denmark (DTU). His research is focused on electrochemical testing and development of solid oxide cells applied for high temperature electrolysis of H2O and/or CO2 for hydrogen production and synthetic fuel production. He performed his Doctoral research in heterogeneous catalysis at University of Twente, The Netherlands, and received his Doctor’s degree in 2007. He is author and coauthor of more than 25 scientific articles specifically on high temperature electrolysis in international peer reviewed journals and has two patent applications. In 2008 he was appointed the membership of the international expert group under the International Energy AssociationHydrogen Implementation Agreement Task 25: “High Temperature Process for Hydrogen Production”.

Søren Højgaard Jensen is a senior scientist and project leader at the Department of Energy Conversion and Storage at the Technical University of Denmark (DTU). His research is focused on high pressure solid oxide electrolysis cell testing, Li-battery degradation mechanisms, and 3D printing of electrochemical cells. He studied physics and mathematics at Copenhagen University and got his M.Sc. in 2003. In 2006, he finalized his Ph.D. at the Technical University of Denmark, Physics Department. He is author and coauthor of 20 scientific papers, mainly about electrochemical cell testing. 10721


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the Danish Council for Strategic Research, via the Strategic Electrochemistry Research Center (SERC, www.serc.dk), and by the Energy Technology Development and Demonstration Programme (EUDP) under the Danish Energy Agency via the project “Green Natural Gas”. The authors would like to acknowledge Christopher Graves, scientist at DTU energy conversion, for valuable discussions and comments to complete this review. Finally, the authors would like to thank Louise Persson for creating the cover art.

NOMENCLATURE AEC alkaline electrolysis cell AFC alkaline fuel cell ASR area specific resistance DME dimethyl ether Erev reversible voltage (also denoted decomposition voltage) Etn thermoneutral voltage (also denoted enthalpy voltage) F Faraday’s constant (96 485 Coulomb/mol) FT diesel Fischer−Tropsch diesel GTL gas to liquids (the conversion of gases to liquid fuels, diesel) HHV higher heating value HT-AEC high temperature alkaline electrolysis cell LHV lower heating value MCFC molten carbonate fuel cell MIEC mixed ionic and electronic conductors PAFC phosphoric acid fuel cell PEMEC polymer electrolyte membrane electrolysis cell PEMFC polymer electrolyte membrane fuel cell Q reaction coefficient R-SOFC reversible SOFC RU repeating units in the stack, consisting of interconnect and cell R-WGS reverse water−gas shift SNG synthetic natural gas SOC solid oxide cell SOEC solid oxide electrolysis cell SOFC solid oxide fuel cell SPCEC solid proton conducting electrolysis cell synthesis gas mixture of CO and H2 (in literature also denoted syngas) TEC thermal expansion coefficients TPB triple phase boundary U cell voltage WGS water−gas shift H efficiency

Anne Hauch is senior scientist at the Department of Energy Conversion and Storage, Technical University of Denmark. She received her Ph.D. degree in 2007 based on the thesis “Solid Oxide Electrolysis CellsPerformance and Durability” supervised by associate professor T. Jacobsen, senior researcher J. Bilde-Sørensen, and research professor M. B. Mogensen. Her research focuses on single cell testing and characterization of solid oxide fuel cells and electrolysis cells, primarily electrochemical and microstructural characterization. She is author and coauthor of more than 20 scientific articles on solid oxide fuel cells and electrolysis cells in international peer reviewed journals and the holder of a patent on sealing solid oxide electrolysis cell purposes.

Mogens Bjerg Mogensen is research professor at the Department of Energy Conversion and Storage, Technical University of Denmark, DTU, with 40 years in electrochemistry. He obtained his M.Sc. in Chemical Engineering in 1973 and Ph.D. in Corrosion in 1976 from the Department of Metallurgy, DTU. He has been manager of numerous projects and of the Strategic Electrochemistry Research Center (SERC). As of November 2013, there are 225 papers in his name registered by Web of Science, cited more than 7265 times (average of 32 citations per item). He has >350 scientific publications in peer reviewed journals, books, and conference proceedings and 20 patents/applications. He has been editor or coeditor for 9 books, 20 special reports, and 16 popular articles, with h-index 44. His honors include The Christian Friedrich Schonbein Medal of Honour at the eighth European Fuel Cell Forum 2008, and The Science of Hydrogen & Energy Award 2012, Sixth Hydrogen & Energy Symposium, Stoos, Switzerland.

Materials

BCY BCZY

yttria-doped barium cerate BaCexY1−xO3‑δ yttria-doped barium cerate partially substituted with zirconia BaCexZry‑xY1−yO3‑δ BCZYb ytterbium-doped barium cerate partially substituted with zirconia BaCexZry‑xYb1−yO3‑δ BCZYbCo ytterbium-doped barium cerate partially substituted with zirconia and cobalt BaCexZry‑x(YbCo)1−yO3−δ BCZYCo yttria-doped barium cerate partially substituted with zirconia and cobalt BaCexZry‑x(YCo)1−yO3−δ BCZYZ yttria-doped barium cerate partially substituted with zirconia and zinc (in literature also denoted BCZYZn) BaCexZry‑x(YZn)1−yO3−δ

ACKNOWLEDGMENTS This work was financially supported by the Danish Public Service Obligation programme (PSO) under Energinet.dk via the project “Pre-investigation of Electrolysis” and supported by 10722


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strontium-doped barium ferrite partially substituted with cobalt Ba1−xSrxCo1−yFeyO3‑δ gadolinium-doped ceria (in literature also denoted GDC) Gd1−xCexO2‑δ lanthanum-doped cobalt ferrite La1−xCoxFeO3‑δ lanthanum cobaltite LaCoO3‑δ lanthanum nickelate La2NiO4+δ strontium-doped lanthanum cobaltite La1−xSrxCoO3‑δ strontium-doped lanthanum ferrite partially substituted with cobalt (in literature also denoted LSCoF) LaxSr1−xCoyFe1−yO3‑δ strontium-doped lanthanum manganite partially substituted with chromium LaxSr1−xCryMg1−yO3‑δ strontium-doped lanthanum nickelate partially substituted with cobalt La2‑xSrxCo1−yNiyO4+δ strontium-doped lanthanum ferrite partially substituted with copper LaxSr1−xCuyFe1−yO3‑δ strontium-doped lanthanum ferrite LaxSr1−xFeO3‑δ strontium-doped lanthanum gallate partially substituted with magnesium LaxSr1−xGayMg1−yO3‑δ strontium-doped lanthanum manganate (in literature also denoted LSMO) LaxSr1−xMnO3‑δ lanthanum-doped strontium titanates (in literature also denoted LSTO and SLTO) LaxSr1−xTiO3‑δ lanthanum-doped strontium vanadate La1−xSrxVO3‑δ molybdenum-doped ceria Mo1−xCexO2‑ neodymium nickelate Nd2NiO4+δ praseodym nickelate Pr2NiO4+δ scandia-stabilized zirconia (in literature also denoted SDZ) Zr1−xScxO2‑δ yttria-doped strontium cerate SrCexY1−xO3‑δ sc a n d i a- d o p ed y t t r ia -s t a bi liz e d zi r co n ia Zr1−x(YSc)xO2‑δ yttria-doped strontium cerate partially substituted with zirconia SrCexZry‑xY1−yO3‑δ samaria-doped ceria Sm1−xCexO2‑δ strontium-doped iron molybdate SrFexMo1−xO3‑δ samaria-doped strontium cobaltite SmxSr1−xCoO3‑δ niobium-doped strontium titanate NbxSr1−xTiO3‑δ yttria-doped strontium zirconate SrZrxY1−xO3‑δ ytterbium-doped strontium zirconate SrZrxYb1−xO3‑δ yttria-stabilized zirconia Zr1−xYxO2‑δ zirconia stabilized with 3 mol % Y2O3 zirconia stabilized with 8 mol % Y2O3

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