Steam electrolysis by solid oxide electrolysis cells (SOECs) with proton-conducting oxides.

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Steam electrolysis by solid oxide electrolysis cells (SOECs) with proton-conducting oxides Lei Bi, Samir Boulfrad and Enrico Traversa* Energy crisis and environmental problems caused by the conventional combustion of fossil fuels boost the development of renewable and sustainable energies. H2 is regarded as a clean fuel for many applications and it also serves as an energy carrier for many renewable energy sources, such as solar and wind power. Among all the technologies for H2 production, steam electrolysis by solid oxide electrolysis cells (SOECs) has attracted much attention due to its high efficiency and low environmental impact, provided that the needed electrical power is generated from renewable sources. However, the deployment of SOECs based on conventional oxygen-ion conductors is limited by several issues, such as high operating temperature, hydrogen purification from water, and electrode stability. To avoid these

Received 8th June 2014

problems, proton-conducting oxides are proposed as electrolyte materials for SOECs. This review paper

DOI: 10.1039/c4cs00194j

provides a broad overview of the research progresses made for proton-conducting SOECs, summarizing the past work and finding the problems for the development of proton-conducting SOECs, as well as

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pointing out potential development directions.

1. Introduction 1.1

Steam electrolysis by solid oxide electrolysis cells (SOECs)

Renewable and clean energies have attracted much attention worldwide as the consumption of limited fossil fuels in the current world not only causes serious energy crisis, but also Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. E-mail: [email protected]; Tel: +966-12-8084752

After obtaining his PhD in Materials Science in 2009 at University of Science and Technology of China (USTC), Dr Lei Bi joined National Institute for Materials Science (NIMS) in Japan as a research associate. Now he is a research scientist in Prof. Enrico Traversa’s group, at King Abdullah University of Science and Technology (KAUST). His research interests are materials for energy conversion and storage. Current research activities are Lei Bi focused on the development of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) with stable proton-conducting oxides.

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leads to environmental problems due to emission of harmful gases.1 Although solar power, wind power and tidal power are promising renewable energy sources, they are site-specific and intermittent, which is not suitable for continuous energy supply; therefore storage or energy carriers are needed.2–4 Hydrogen, which is transportable and storable, could serve as an attractive option for energy carrier, as a precursor for synthetic fuels when combined with CO2, as well as a potential clean fuel for many applications, such as for heating, electricity and vehicles. Therefore, the production of hydrogen has gained

Dr Samir Boulfrad received his PhD in 2007 in Materials Science and Engineering from Institut National Polytechnique de Grenoble (France). From 2007 to 2010, he was a Research Fellow with Prof. John Irvine at University of St-Andrews (UK). In 2010 he joined King Abdullah University of Science and Technology (KAUST) in Saudi Arabia as a Research Scientist. He is currently a member of the group of Prof. Enrico Traversa working Samir Boulfrad essentially on solid-state electrochemical means for energy conversion and storage. His main focus is advanced electrode microstructures for SOFC & SOEC, and hydrocarbon tolerant anodes for SOFCs.

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a significant interest during the past years.5–7 Hydrogen is a clean fuel, generating only H2O during combustion, provided that it is produced from water using renewable energy. In fact, hydrogen is not a primary energy source and H2 is very rare in nature. As a result, H2 production usually requires splitting H-containing compounds with energy. Currently, most of the H2 is produced by steam reforming of fossil fuels at temperatures usually higher than 500 1C. Due to the presence of sulfur in hydrocarbons a pre-desulfurization process is required to avoid the poisoning of the catalysts.3,8 Considering the influence on the environment, this conventional way of producing H2 is neither sustainable nor eco-friendly, since fossil fuels are not renewable sources and CO2 is emitted during this process contributing to global warming. Photoelectrolysis is being considered as a cheap and sustainable method for H2 production. In addition to the needed post separation of H2 and O2, the main drawback of this technique is now limited by its low efficiency8 that requires very large surfaces for any economically viable generation plant. Nowadays, it is important to develop a technology for producing H2 with high efficiency and low impact on the environment. Electrolysis cells based on fuel cell technology9,10 provides a solution in which H2 is produced from water and O2 is the only by-product, according to the following equation: H2O - H2 + 12O2

(1.1)

Although proton-exchange membrane (PEM) electrolysis cells, which split water molecules to produce H2 and O2 with electricity at low temperatures (70 to 80 1C), are already commercially available, the solid oxide electrolysis cells (SOECs)

In 1986 Enrico Traversa received his PhD in Chemical Engineering from the University of Rome La Sapienza. In 2013 he joined the King Abdullah University of Science and Technology (KAUST), Saudi Arabia, as Professor of Materials Science and Engineering, after being the Director of the Department of Fuel Cell Research at the International Center for Renewable Energy, Xi’an Jiaotong University, China. He joined the Enrico Traversa University of Rome Tor Vergata, Italy, in 1988, where he is a Professor of Materials Science and Technology, now on leave of absence. From January 2009 to March 2012, he was a Principal Investigator at the International Research Center for Materials Nanoarchitectonics (MANA) at the National Institute for Materials Science (NIMS), Tsukuba, Japan, leading a unit on Sustainability Materials. His research interests include nanostructured materials for environment, energy, and healthcare, with special attention to sustainable development, and he was elected as a Fellow of the Electrochemical Society in 2013.

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Fig. 1 Electric, thermal and total energy demand for H2O electrolysis as a function of temperature, showing the electric energy demand decreasing considerably which is compensated by the thermal energy with increasing working temperatures.

working at high temperatures could be more favorable for H2-production due to the following reasons. First, it is thermodynamically advantageous for electrolysis cells operating at high temperatures.11 The total energy demand (DH) of eqn (1.1) can be expressed as: DH = DG + TDS, where DG is the electrical energy demand and TDS is the heat energy demand. Fig. 1 shows thermodynamic data of steam electrolysis at a steam pressure of 1 atm. The total energy demand (DH) of the reaction drops considerably above 100 1C, when water shifts from the liquid to gas phase, then it remains almost constant. The electrical energy demand (DG) drops significantly with the compensation of thermal energy (TDS). Therefore, SOECs working at high temperatures can lead to reduced cost for hydrogen production with less consumption of electricity, also considering that the required heat energy can be provided from external sources, such as from waste heat from high-temperature industries. In contrast, since PEM electrolysis cells usually operate below 100 1C, a large amount of electricity is demanded for electrolysis.12 Second, PEM electrolysis cells use PEM fuel cell technology, inheriting both its advantages and disadvantages such as high capital costs. Noble metals are used in PEM cells as catalysts, resulting in electrode poison problems in the fuel cell mode and also leading to high costs. The cost could be even larger in PEM electrolysis cells as the carbon catalyst carrier tends to be oxidized at the oxygen electrode in the electrolysis operating mode. Therefore, PEM electrolysis cells could be more appropriate for small-scale hydrogen production and specific purpose applications.13 SOECs which are solid oxide fuel cells (SOFCs) running in a reversible mode10 allow high temperature electrolysis for producing hydrogen, benefiting from the reduced electricity demand and avoiding the use of noble metals as electrode catalysts. SOECs can serve as tools for storing electrical energy and heat energy as chemical energy (H2) from various renewable sources for trying to solve the intermittent nature of,


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Fig. 3 Sketch of the working mechanism of conventional SOECs with an oxygen-ion conducting electrolyte.

Fig. 2 Concept diagram of applications of a sustainable energy system based on SOEC/SOFC technology.

for instance, wind energy, solar energy and wasted heat. The stored chemical energy can be used for generating electricity when power from renewable sources is not available by simply operating SOECs in the reverse fuel cell mode. Fig. 2 shows a conceptual diagram of sustainable energy system applications based on SOEC/SOFC technology. Wind energy and solar energy, when produced in excess, may provide electricity for steam electrolysis to SOEC devices, coupled with heat energy from waste heat, for producing hydrogen that can be used for many industrial applications as well as an energy storage means. When electricity is needed, SOECs can work reversely in the fuel cell mode and the stored hydrogen can be used as a fuel for SOFCs, generating electricity from several W to MW, for different applications, ranging from portable devices to power plants. In this whole energy conversion and consumption process, only H2O will be consumed (in SOEC) and the only chemical product is H2O (in SOFC), with no consumption of fossil fuels and also no emission of greenhouse gases. The state-of-the-art commercial or lab-studied SOECs use yttria-stabilized zirconia (YSZ, the conventional high temperature oxygen-ion conductor) as the electrolyte material, as it is the case also for conventional SOFCs.14–16 For solid oxide cell electrodes working in both SOEC and SOFC modes, since the anode and cathode are opposite in these two modes, to avoid confusion we will refer to air electrodes for the SOEC anodes and SOFC cathodes, while hydrogen (or fuel) electrodes refer to the SOEC cathodes and SOFC anodes. Fig. 3 shows a sketch of the working mechanism of a SOEC with YSZ electrolyte. Steam is fed to the porous hydrogen electrode. When the required potential is applied to the cell, electrolysis starts to occur and water molecules are split to form hydrogen and oxygen-ions. The produced hydrogen gas, which is the desired product, is collected at the same electrode, while oxygen ions pass through the dense electrolyte film and reach the air electrode side where they are subsequently


oxidized to oxygen gas. The chemical reactions occurring can be expressed as: Hydrogen electrode: H2O + 2e - H2(g) + O2 Air electrode: O2 - 12O2(g) + 2e

(1.2) (1.3)

YSZ shows sufficient conductivity only at high temperatures, and therefore conventional SOECs have to be operated at those high temperatures (such as 800–1000 1C).17 Running SOECs at these temperatures is advantageous from the thermodynamic point of view to reduce the electricity demand for steam electrolysis; however, such high operation temperatures cause several drawbacks, such as poor long-term cell stability, interlayer diffusion, fabrication and materials problems.18,19 For these reasons, the development of electrolysis cells working at intermediate temperatures (500 to 700 1C) has gained much attention recently.20–22 Furthermore, operating SOECs in the intermediate temperature range opens the possibility to use many unexploited heat sources from industry for providing heat energy.23 Therefore, intermediate temperature SOECs are quite promising for applications due to their improved cell reliability, reduced materials issues, and a wider choice of heat energy sources for reducing the total SOEC energy demand. 1.2

Why using proton-conducting oxides

The conventional SOEC has to be operated at high temperatures due to the limited ionic conductivity of the available electrolytes, and thus the first concern for lowering its working temperature is developing electrolyte materials with higher ionic conductivity at intermediate temperatures.20,24 Alternative oxygen-ion conductors like doped CeO2 (ref. 25) and La0.9Sr0.1Ga0.8Mg0.2O3d (LSGM)26 have been tested owing to their good ionic conductivity at intermediate temperatures. However, high temperature proton conductors offer some advantages over oxygen-ion conductors. First, high temperature proton conductors show higher ionic conductivity compared with that of oxygen-ion conductors in the intermediate temperature range.24,27 Second, proton-conducting oxides show good chemical compatibility with Ni, which is the most used hydrogen electrode for both SOECs and SOFCs.28 In contrast, LSGM reacts with Ni, making its use still challenging.29–31

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Third, electrolysis cells with proton-conducting electrolytes show sufficient current efficiency. The first requirement of an electrolyte material for solid oxide cells is that it should show good ionic conductivity with negligible electronic conductivity under working conditions, and high temperature proton conductors meet this requisite. Although doped CeO2 shows good ionic conductivity and good chemical compatibility with most of the electrode materials, and indeed it is the most popular electrolyte material for intermediate temperature SOFCs,32 the high applied potential under SOEC working conditions inevitably leads to the occurrence of the reduction of Ce4+ to Ce3+, resulting in an increase in electronic conductivity and a decrease in ionic transference number.33,34 As a result, the current efficiency can only reach a few percent for the SOEC with doped CeO2 as the electrolyte,34 whereas the current efficiency has been demonstrated to maintain sufficiently high (50–95%) for SOECs with proton-conducting electrolytes, even under high applied potentials.35 In addition to the merits of proton-conducting materials, the SOEC system with proton-conducting electrolytes also shows several advantages compared with the SOECs using oxygen-ion electrolytes. First, only pure and dry hydrogen is produced at the hydrogen electrode side for proton-conducting SOECs and no further gas separation is needed.36 Fig. 4 shows the working mechanism of a SOEC with a proton-conducting electrolyte. Water (steam) is fed to the air electrode side, where it is electrochemically split into oxygen and protons. The protons migrate through the dense electrolyte layer to the hydrogen electrode side, where the protons combine with electrons to form H2, which is the desired product. As the proton-conducting electrolyte membrane is non-permeable to both oxide ions and molecular gases, only protons can pass through the electrolyte leading to a product consisting of only pure and dry hydrogen at the hydrogen electrode side. Second, the cell reliability would be better for proton-conducting SOECs. Ni is the most widely used hydrogen electrode for SOECs,15,37,38 but Ni is always at risk of being oxidized in the oxygen-ion SOECs as steam is produced at the hydrogen electrode side for oxygen-ion SOECs, and the high steam concentration tends to oxidize Ni particles.34,39,40 The Ni oxidation by steam has already been reported to lead to cell performance degradation.39,40 In contrast, this oxidation will not happen in proton-conducing SOECs as steam is produced at the air

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electrode side; therefore, the Ni electrode is only exposed to dry H2, leading to a better electrode stability. Third, the concept of reversible SOFC is more feasible with proton-conducting solid oxide cells. Solid oxide cells can switch from the fuel cell mode to the electrolysis mode upon applying potential values higher than the cell open circuit potential (OCP), and switch back from the electrolysis mode to the fuel cell mode upon applying potential values lower than the OCP.41 The whole device is called reversible SOFC (R-SOFC), which is able to produce electricity at the time of electricity shortage and produce hydrogen, and store the electrical power to chemical energy at the time of electricity surplus. To optimize the cell performance by reducing the ohmic resistance, the state-of-the-art solid oxide cells have to use film electrolytes in which the electrolyte layer is supported either on the air electrode substrate or on the hydrogen electrode substrate.20,42 Electrochemical modeling studies43 reveal that the hydrogen electrode supported cell configuration is the most favorable design for proton-conducting cells both in SOFC mode and SOEC mode for achieving high energy conversion efficiency, whereas the hydrogen electrode supported cell configuration is more favorable in SOFC mode and the air electrode supported cell configuration is more favorable in SOEC mode for cells with oxygen-ion electrolytes, implying that the cell has to change the cell configuration to achieve electrochemical optimization in these two modes, which is practically not realistic. In other words, the hydrogen electrode supported configuration is the optimized cell configuration for proton-conducting R-SOFCs working in both fuel cell and electrolysis modes. In contrast, R-SOFCs with oxygen-ion electrolyte have to suffer high overpotential in either fuel cell mode or electrolysis mode no matter which electrode supporting configuration is selected. Therefore, SOECs with proton-conducting electrolytes would be a promising technology for hydrogen production as well as for utilization of renewable energies. Since Iwahara et al.44–46 have discovered that some perovskite oxides show proton conductivity at intermediate temperatures, the research for proton-conducting solid oxide cells has made an impressive progress. It is interesting to find out that the first application of proton conducting oxides in SOECs was reported about 30 years ago.35 Both electrolyte and electrode materials for proton-conducting cells have been intensively studied during the past three decades.47–49 While proton-conducting SOFCs have been reviewed,24 to the best of our knowledge, there is no paper to document the research progress and the recent research activities for proton-conducting SOECs, since the reviews for high-temperature electrolysis are mostly focused on oxygen-ion conducting SOECs.13,33,50 In this review, we discuss the development of the state-of-the-art SOECs with proton-conducting electrolytes. The prospects and challenges for proton-conducting SOECs are also discussed.

2. Proton-conducting electrolyte materials for SOECs Fig. 4 Sketch of the working mechanism of proton-conducting SOECs.

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The development of high temperature proton-conducting oxides has been thriving during the past three decades; the most


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intensively studied materials are ABO3 (A = Ba, Sr; B = Ce, Zr) perovskite-type oxides, since they possess the best proton conductivity, which is due to the fundamental presence of trivalent acceptor doping ions at the B site.24 Taking Y-doped BaCeO3 as an example, oxygen vacancies are created with the substitution of the trivalent Y for tetravalent Ce at the B site, as described by eqn (2.1): 0


Y2 O3 ! 2YCe þ 3O o þ Vo

(2.1)

In a wet atmosphere, water molecules are incorporated into oxygen vacancies to form protons, which bind to oxygen ions as hydroxide defects, according to (2.2): H2 O þ Vo þ O o , 2OH

(2.2)

Protons can hop from one oxygen atom to another by breaking the oxygen–hydrogen bonds and forming new oxygen–hydrogen bonds with the adjacent oxygen atoms. Therefore, the water-rich working environment in steam electrolysis would be more beneficial for proton-conducting electrolytes compared with their use in SOFCs, since the high water content may accelerate the formation of proton charge carriers, according to (2.2), thereby resulting in proton conductivity enhancement.51,52 In addition, the water-rich environment can extend the operating temperature range for high temperature proton-conducting electrolytes,53 enabling proton-conducting SOECs to operate at a wider temperature range than the fuel cell counterparts. The first work on steam electrolysis with a proton-conducting oxide electrolyte has been performed by Iwahara et al., using Sc-doped SrCeO3 as an electrolyte having a thickness of 0.5 mm.35 Obvious hydrogen evolution was observed and the current efficiency reached up to 50–95% in the 0.1–0.8 A cm2 current range, with water vapor supplied at 1 atm. The SOEC with a 10% Sc-doped SrCeO3 electrolyte led to a hydrogen evolution rate of 2.5 mL min1 cm2 at a current of 0.4 A cm2 at 900 1C. This hydrogen evolution rate was almost equal to that measured for a SOEC with the oxygen-ion electrolyte under similar conditions.54 SrCeO3-based proton conductors with other dopants (such as Yb and Y)55,56 were also used as electrolytes in SOECs for hydrogen production, achieving similar hydrogen evolution behavior. A constant current electrolysis (0.2 A cm2) was attained for the SOEC with a relatively thick SrCe0.95Yb0.05O3d electrolyte (0.5 mm) at an applied potential of 2.8 V, and the current efficiency, determined by the ratio between the actual hydrogen evolution rate and the theoretical value, was above 95%.56 Iwahara et al.57,58 also fabricated a bench-scale electrolyzer using the SrCe0.95Yb0.05O3d electrolyte, which enables the production of pure and dry hydrogen at a rate of B3 L h1 at 750 1C. In fact, the hydrogen gas produced by protonconducting electrolysis cells is dry and therefore does not require further gas separation procedures, while water separation is needed for the hydrogen gas produced with oxygen-ion conducting electrolysis cells. These pioneering results indicate that the hydrogen production performance is not limited by the proton-conducting electrolyte, confirming the advantages of the proton-conducting electrolytes discussed in the introduction section.


A series of ABO3 perovskite proton-conducting oxides have been proposed as electrolyte materials in electrolysis cells.59 Despite the development of new proton-conducting oxides, such as Ba3Ca1.18Nb1.82O9d60 and LaNbO4,61 owing to their relatively low conductivity these new materials have been rarely used in electrolysis cells, still leaving the prominence to ABO3 proton-conducting electrolytes, and especially to cerates, such as doped BaCeO3 and SrCeO3, which exhibit both high proton conductivity and easy processability.62 BaCeO3 is more suitable than SrCeO3 as an electrolyte for solid oxide cells due to its higher ionic conductivity,49,58 and it is indeed the most utilized electrolyte material for proton-conducting solid oxide cells.63–66 Concerning the application in SOECs, BaCe0.9Nd0.1O3d sintered dense pellets were firstly developed as the electrolyte for electrolysis cells.44 With Pt as both air and hydrogen electrodes, the single electrolysis cell reached a current density of 0.2 A cm2 at 800 1C. Later, Y-doped BaCeO3 has been discovered to show the highest conductivity with respect to other dopants, and thus it has been proposed as the SOEC electrolyte.44,67 Stuart et al.68 fabricated an electrolysis cell using a BaCe0.9Y0.1O3d electrolyte with a thickness of 0.45 mm and tested it at different temperatures. The cell performance improved with the increase in the operating temperature and the current density of the cell reached 0.12 A cm2 at 800 1C with the applied potential of 2 V. As stated above, both doped SrCeO3 and BaCeO3 electrolyte materials have been successfully tested for electrolysis cells, but the large electrolyte thickness affected the cell performance; application of high potentials was necessary for hydrogen production to overcome the sizeable ohmic losses for these cells caused by the thick electrolytes.57 Therefore, the need to fabricate electrolyte films dense enough to prevent gas leakage was proposed to reduce the cell ohmic resistance. It has been estimated that relatively low applied voltages of 1.08 V at 0.4 A cm2 and 1.32 V at 0.6 A cm2 were required for SrCe0.9Sc0.1O3d electrolyte films with a thickness of 10 mm,55 while to achieve the same current densities under the same working conditions voltages of 7.2 V and 8.8 V were required for the cell with a 0.5 mm thick SrCe0.9Sc0.1O3d electrolyte. In addition, the reduced ohmic losses of the electrolyte film allowed the electrolysis cells to operate at lower temperatures.69,70 In fact, lowering the SOEC working temperature to the intermediate temperature range is one of the original aims of developing protonconducting SOECs. Therefore, the development of protonconducting SOECs using electrolyte films is a critical and necessary step towards applications. Although the preparation of proton-conducting electrolyte films (mainly BaCeO3 electrolyte) with thickness down to tens of microns has been achieved using various techniques,71–73 the reports on proton-conducing SOECs with electrolyte films are relatively scarce. The main concern for SOEC electrolyte films is about their chemical stability in a water environment, since the electrolyte has to be exposed to high steam concentrations during operation. From one side, as already stated the high steam concentration has the advantages to improve the hydrogen production performance of electrolysis cells and to expand the application temperature range for proton-conducting

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oxides, since high water concentration depresses the formation of other charge carriers than protons at high temperatures.53 On the other side, the high concentration of steam requires good stability of the electrolyte materials with respect to water, since the possible reactions between the electrolyte and water may lead to easy degradation of the whole electrolyte film, while this problem may be less important for the thick pellets. Considering this important issue, cerate proton conducting oxides are not suitable for use in SOEC electrolyte films due to their chemical instability in H2O. Tanner and Virkar74 have pointed out that BaCeO3 is thermodynamically unstable under H2O-containing conditions (B430 Torr) at temperatures lower than B800 1C. Similar thermodynamic calculation results were obtained for SrCeO3 indicating its chemical instability in H2O.75 The instability of cerates in H2O has been also confirmed experimentally,76 suggesting that more stable electrolyte materials are necessary for practical applications of electrolysis cells. In contrast to the instability of cerate proton-conducting oxides, zirconate proton-conducting oxides (such as SrZrO3 and BaZrO3) have been predicted to be thermodynamically stable in H2O,75,77 since the standard Gibbs free energy change (DG0) values for the possible reactions SrZrO3 + H2O - Sr(OH)2 + ZrO2 and BaZrO3 + H2O - Ba(OH)2 + ZrO2 are positive down to temperatures as low as B50 1C. Since DG0 values increase upon increasing the temperature, the chemical stability in water of SrZrO3 and BaZrO3-based materials improves with increasing temperatures accordingly. Therefore, SrZrO3 and BaZrO3-based materials could be a better choice as materials for protonconducting SOECs with electrolyte films due to their chemical stability against H2O. Sakai et al.78 fabricated a steam electrolyzer with a SrZr0.9Y0.1O3d electrolyte with a thickness of 0.5 mm. The cell with Pt electrodes showed a current density of 25 mA cm2 at 800 1C, which was almost one order of magnitude smaller than that of the electrolysis cells with BaCeO3 or SrCeO3 electrolytes; the performance was even lower when the working temperature was down to 600 1C. The application of Pt electrodes was found to be the major reason for such a poor performance, nonetheless using the optimized electrode materials a current density of only B60 mA cm2 was obtained, showing still a poor performance. The ohmic loss was found to be high due to the thick electrolyte pellet used in this research, which should be responsible for the low cell performance. Y-doped BaZrO3 was also suggested to be an electrolyte for proton-conducting SOECs because of its excellent chemical stability. Stuart et al.68 attempted to fabricate an electrolyzer with a BaZr0.9Y0.1O3d electrolyte, but no cell performance data were reported which was probably due to the high resistance of the BaZr0.9Y0.1O3d pellet.79,80 It is reasonable to consider the application of this material in electrolyte films for these cells, to reduce the cell ohmic losses and boost the cell performance accordingly. However, SOECs with BaZrO3 (or SrZrO3) film electrolyte have not been reported yet due to the processing difficulties that we will clarify in the following discussion. Considering the low conductive nature of strontium cerate/zirconate46 and the poor chemical stability of barium cerate,81 we can stress that doped-BaZrO3

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would be the most promising electrolyte material for solid oxide cells because of its high bulk conductivity, which is the highest among all the existing proton-conducting oxides,82 and its excellent chemical stability.79,81 However, BaZrO3 is also well known for its very poor sinterability and a large content of highly resistive grain boundaries, which hindered the application of BaZrO3 in solid oxide cells for several years.83,84 Its poor sinterability requires a high sintering temperature (such as 1800 1C)79 to obtain dense electrolytes. For preparing electrolyte films for solid oxide cells, the electrolyte layer has to be co-sintered together with the substrate electrode; after co-sintering at such a high a temperature, it is almost impossible to avoid the reaction and inter-layer diffusion between BaZrO3 and the electrode material,85 as well as difficult to keep sufficient porosity in the substrate electrode,86 leading to technical difficulties in preparing doped-BaZrO3 electrolyte films. Furthermore, a large volume of grain boundaries due to poor grain growth makes the electrolyte quite resistive and thus decreases cell performance, despite this the electrolyte thickness was successfully reduced to B15 mm.86 Therefore, it is generally considered that the fabrication of high performance solid oxide cells with BaZrO3-based electrolyte films is still a big challenge, although the situation has been much improved in the very recent years.47 Because of the refractory nature of barium zirconate protonconducting oxide, the existing literature usually compromises the properties of zirconates and cerates by substituting Ce to Zr in BaCeO3, to improve the chemical stability of the electrolyte material.79,81,87 This strategy decreases the sinterability and grain growth for the BaCeO3-based samples but maintains these two properties at an acceptable level to avoid the problems happening for the Ce-free BaZrO3-based materials, making the application in electrolyte films possible. The low Zr-doping level (such as 10%) was reported to improve the BaCeO3 chemical stability,27 though the improvement is limited and could be observed only in mild environments; thus this material cannot be used in electrolysis due to the water rich environment.88 Irvine et al.89 proposed a higher Zr-containing composition, BaCe0.5Zr0.3Y0.16Zn0.04O3d (BCZYZn), as the electrolyte material for a proton-conducting SOEC. ZnO was added in the composition to improve the sinterability. They fabricated a BCZYZn electrolyte film (40 mm) on a La0.8Sr0.2MnO3 air electrode substrate with a thin buffer layer of La0.8Sr0.2Cr0.5Mn0.5O3d to prevent the reaction between the electrolyte and the substrate. With a palladium hydrogen electrode, the electrolysis cell with the electrolyte film was tested at 605 1C and an observable amount of hydrogen was produced. Although no much technical details have been disclosed in this report, this preliminary result indicated the possibility of using barium zirconate electrolyte films in electrolysis cells, enabling the cell to work at an intermediate temperature. Comparing the air electrode supported configuration, the hydrogen electrode supported configuration would be more desirable for electrolysis cells due to its lower polarization resistance.36 Indeed most of the research concerning electrolysis cells deals with fabricating electrolyte films on supported hydrogen electrode substrates, which is usually made


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of Ni-based cermet. Gan et al.90 used a BCZYZn film electrolyte (75 mm) deposited on a NiO-BCZYZn hydrogen electrode for steam electrolysis, resulting in a large electrolysis current density of 2 A cm2 that corresponds to the theoretical amount of hydrogen produced up to 13.9 mL min1 cm2 at 700 1C, with an applied potential of 2 V. This output is significantly larger than that obtained for the electrolysis cell using a thick BCZYZn electrolyte (2 mm), which only reached 59 mA cm2 at 800 1C in a similar testing environment.91 The much improved electrolysis cell performance clearly shows the utter advantage of using electrolyte films for steam electrolysis, which reduced cell ohmic losses and greatly improved the cell performance. He et al.69 used BaCe0.5Zr0.3Y0.2O3d (BCZY) as the electrolyte for electrolysis cell and fabricated an electrolyte film of 20 mm thickness on a NiO-BCZY hydrogen electrode substrate. Electrolysis current densities at a cell potential of 1.5 V were 326, 565, and 830 mA cm2 at 600, 650, and 700 1C, respectively. The cell performance is comparable with that of the early reports for conventional proton-conducting SOECs with thick cerate electrolytes,44,55 but the electrolyte film enables the cell to work at lower temperatures and to apply much lower electrolysis potential, as well as providing improved chemical stability. Although BaCe0.5Zr0.3Y0.2O3d and BaCe0.5Zr0.3Y0.16Zn0.04O3d electrolyte materials have been successfully used in electrolysis cells, some studies indicate that the 30 at% doping concentration of Zr in BaCeO3 cannot provide sufficient chemical stability under aggressive conditions (such as in high steam concentration), while a higher Zr doping level up to 40 at% or 50 at% is necessary to have better endurance in a water rich environment.77,92 Azimova and McIntosh93 fabricated a reversible proton-conducting solid oxide cell that can work in both fuel cell and electrolysis modes using a 45 mm thick BaCe0.48Zr0.4Yb0.1Co0.02O3d (BCZYbCo) electrolyte film on a NiO-based hydrogen electrode. The cell with this electrolyte material was shown to work stably in a water rich environment under a water partial pressure (pH2O) up to 0.5 atm, with a steady production of hydrogen at 600 1C in the electrolysis mode, and a higher pH2O under the steam electrolysis conditions resulted in a higher electrolysis current, which suggested a higher hydrogen production rate. It is also interesting to find that the I–V curves measured for fuel cells remained unchanged after each SOEC test, indicating that no performance loss was found during the switch from the electrolysis mode to the fuel cell mode; this suggests that this proton-conducting electrolyte could be a suitable electrolyte material for reversible protonconducting SOFCs. However, the current density for this cell was generally limited, reaching only B100 mA cm2 at 600 1C. A large ohmic resistance from the electrolyte determined by impedance analysis measurements was one of the main factors for the low electrolysis cell performance, despite the electrolyte thickness being reduced to 45 mm. One may speculate that the trade-off relationship between the conductivity and chemical stability of the materials hinders the application of protonconducting oxides in SOECs, which is not wholly correct. To date, Y-doped BaZrO3 (BZY) is the only proton-conducting electrolyte material that combines both large bulk conductivity


Review Article Table 1 Summary of the composition and conductivity of H2O-tolerant proton-conducting electrolyte films reported in the literature

Ref.

Composition of the electrolyte films

Electrolyte film thickness (mm)

Film conductivity (S cm1, at 600 1C)

102 60 103 104 105 106 107 99 100 108 109

La0.99Ca0.01NbO4 Ba3Ca1.18Nb1.82O9d BaCe0.4Zr0.4Y0.2O3d BaCe0.7Ta0.1Y0.2O3d BaCe0.7Nb0.1Sm0.2O3d BaZr0.8Y0.2O3d–CaO BaZr0.8Y0.16Zn0.04O3d BaZr0.9Y0.1O3d BaZr0.8Y0.2O3d BaZr0.7Pr0.1Y0.2O3d BaZr0.7Sn0.1Y0.2O3d

20 15 20 25 15 25 20 20 30 12 12

4.9 104 (at 800 1C) 1 103 (at 700 1C) 2.1 103 2.3 103 1.4 103 1.8 103 1.7 103 2.3 103 2.7 103 2.3 103 2.6 103

and good chemical stability.49 Therefore, development of BZY is not compromising, but requires tackling the problems of poor sinterability and poor grain growth. In fact, research on BZY materials has re-gained considerable attention in the past few years and many strategies have been proposed to solve these problems for BZY, by the addition of sintering aids94,95 or the adoption of new doping strategies.88,96 In particular, the successful preparation of BZY electrolyte films97–101 has been demonstrated to boost the cell performance in the fuel cell mode, implying their potential application in SOECs. Table 1 summarizes the conductivity of the proton-conducting electrolyte films that have been reported to be stable in a water rich environment. One can see that most of the stable electrolyte films are made of modified-BaCeO3 materials or BaZrO3-based materials, and the electrolyte film thicknesses are all in the range of the tens of microns, which is desirable for practical applications. Although the modification of doped BaCeO3 with new dopants (such as Ta and Nb) or with a large amount of Zr (such as 40 at%) makes the materials stable against water, the film conductivities are lower than that of Y-doped BaCeO3 films, as expected.70 Moreover, some BaZrO3-based films showed even larger conductivity than that of these modified BaCeO3 films, but providing much better chemical stability.77,100,104 The use of BaZrO3-based films as electrolytes for SOECs seems to be a better choice considering both film conductivity and stability. These electrolyte films have been successfully tested in fuel cell applications, and their performance under electrolysis conditions is expected to be even better as the waterrich environment can improve the proton transference number.53 It should be noted that grain-boundary free BZY films prepared by pulsed laser deposition (PLD) show the largest conductivity ever reported for BZY samples, reaching 0.11 S cm1 at 500 1C.82 Fig. 5 shows the comparison of conductivity between the BZY film and other electrolyte materials. We can see that the conductivity of the BZY film is significantly higher than that of the polycrystalline BZY pellet due to the elimination of the highly resistive grain boundaries. It is also found that the conductivity of the BZY film is almost one order of magnitude larger than that of the mainstream intermediate temperature ionic conductors, including LSGM, Gd-doped CeO2(GDC) and Y-doped BaCeO3 (BCY). The high conductivity of this BZY film not only suggests its great potential application in micro solid

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Fig. 5 Comparison of the conductivity of a grain boundary free BZY film with other electrolyte materials, showing a significant higher conductivity of the BZY film in the whole temperature range. The SEM micrograph of the corresponding BZY film is also shown. Reprinted by permission from Macmillan Publisher Ltd: [Nat. Mater.] (ref. 82), copyright (2010).

oxide cells110 but also indicates there is still room for improving the performance of BZY polycrystalline electrolyte films by reducing the density of its grain boundaries.83,111 Ba3Ca1.18Nb1.82O9d (ref. 112) and Ca-doped LaNbO4 (ref. 113) also show good chemical stability, but their lower conductivity makes them less favorable than BaZrO3-based materials for SOEC applications, and they may be used only when the electrolyte thickness is reduced to below a few micrometers. Although the development of electrolyte materials for protonconducting SOECs benefits from the development of electrolytes

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for proton-conducting SOFCs, usually sharing similar compositions, the different working mechanism in electrolysis and fuel cells makes the electrolyte materials design somewhat different. In proton-conducting SOFCs, water is produced at the air electrode where a certain amount of CO2 in air as the oxidant is also present, while humidified fuel exists at the hydrogen electrode side, together with produced CO2 when running with hydrocarbons. Therefore, both sides of the electrolyte material are exposed to a H2O and/or CO2-containing atmosphere, which requires both sides of the electrolyte to be chemically stable for preventing the erosion of acidic gases (such as H2O and CO2) to the electrolyte layer.114 In contrast, in proton-conducting SOECs the electrolyte at the air electrode is exposed to high water concentrations, whereas the electrolyte at the hydrogen electrode side is only exposed to dry hydrogen without the co-existence of any acidic gas. Aggressive conditions at the air electrode side and mild conditions at the hydrogen electrode side make the design of bi-layer electrolytes desirable for electrolysis cells.115 Fig. 6(a) shows the design of the bi-layer electrolyte, consisting of a highly conductive but chemically unstable electrolyte layer (such as BCY) with the protection from a very thin layer of electrolyte material with high chemical stability (such as BZY). The protective layer can effectively shield the whole electrolyte layer against the attack of water at the air electrode side under the electrolysis conditions, while protons can pass through the electrolyte and finally form hydrogen at the hydrogen electrode. This bi-layer design has

Fig. 6 (a) Design of a bi-layer electrolyte structure, which can protect the inner conductive but unstable electrolyte layer against H2O; (b) SEM micrograph of the BZY/BCY bi-layer structure electrolyte and its electrochemical performance showing only a slight electrochemical loss compared with the conductive but unstable BCY electrolyte, but much improved conductivity with respect to the BZY electrolyte alone. Reproduced from ref. 115 with permission from The Royal Society of Chemistry; (c) the design of a bi-layer electrolyte structure with a thin LaNbO4 film which only allows proton transfer and blocks other charge carriers.

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already been proposed by Fabbri et al.115 for proton-conducting SOFCs, as shown in Fig. 6(b). A thin BZY layer (B1 mm) was deposited by PLD on a BCY pellet to protect the BCY pellet against acidic gases with only a slight loss in electrochemical performance. In addition, with the selection of appropriate materials for the protective layer, this layer may also have a function of blocking any other charge carriers except for protons, promoting the current efficiency and guaranteeing purity of hydrogen produced with electrolysis cells. Although BaCeO3 and BaZrO3 have been regarded as the best proton-conducting oxides, their proton conductivity is usually dominant below 700 1C53,58,88 and other charge carriers (such as electron holes and oxygen ions) are formed with higher temperatures decreasing the current efficiency for the electrolysis cells. In contrast, LaNbO4-based materials show good chemical stability and pure proton conductivity up to 800 1C in wet atmospheres,113 making them an attractive choice for the protective layer. The deposition of a very thin film as a protective layer can minimize the influence of the low conductivity for LaNbO4-based materials.116 Fig. 6(c) illustrates the schematic diagram of the bi-layer electrolyte with a LaNbO4 protective layer. The thin LaNbO4 layer, in addition to protecting the inner highly conductive electrolyte, will allow only protons (H+) to pass through the electrolyte layer, while blocking oxygen ions (O2) and electron holes (h ), due to the high proton transference number of doped-LaNbO4, thereby resulting in an increase in the current efficiency of electrolysis cells. Concerning the Sr-based proton-conducting system, surprisingly the conductivity of the SrZr0.9xCexY0.1O3d (x = 0–0.9) solid solutions does not monotonously increase upon increasing the Ce content.117 The maximum conductivity was found for the SrZr0.5Ce0.4Y0.1O3d (SZCY) composition, for which the reason is still unclear. Matsumoto et al.117 investigated the steam electrolysis with both thick (0.5 mm) and film (22 mm) SZCY electrolytes, finding that reducing the electrolyte thickness can significantly improve the cell performance, with the electrolysis voltage being as low as 1.2 V at 0.1 A cm2 for the cell with the electrolyte film. The voltage was translated to be a relatively low value of 1.4 V for the production of 100% hydrogen at 1 bar, showing an advantage over conventional alkaline water electrolysis and PEM cells for water electrolysis required to be operated at 2 V and 1.6 V, respectively. However, the electrolysis current was partially limited by the electrolyte resistance despite the thickness being reduced to 22 mm, and thus a further reduction of the electrolyte thickness may improve the cell performance. One has to consider that the conductivity of the SrCeO3–SrZrO3 system is relatively low compared with that of the BaCeO3–BaZrO3 system. The best conductivity for SZCY was B3.2 103 S cm1 at 600 1C, which is significantly smaller than the values reported for the Ba-based protonconducting system,79,81 hindering the electrolysis density of the cells. However, the high proton transference number59 of SrCeO3-based proton conductors even at high temperatures makes these materials promising for steam electrolysis working at temperatures above 800 1C.


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3. Electrode materials for proton-conducting SOECs 3.1

Air electrode materials

Air electrodes for proton-conducting SOECs have to be porous to allow the diffusion of steam and also to provide sufficient active sites for H2O oxidation, which is critical for the SOEC performance. The rate of H2O oxidation to protons would be lower than that of the H2 formation from protons, using a lowperforming air electrode material, leading to a decrease in proton concentration and an increase in hole concentration, thus lowering the steam electrolysis efficiency.118 Therefore, the development of high performance air electrode materials for proton-conducting SOECs is of paramount importance. Early studies concerning proton-conducting SOECs report the use of Pt as an electrode material.35 However, Pt is not preferred for practical applications due to its high cost. Moreover, the Pt electrode is found to show large polarization losses for high Zr-containing electrolytes (such as SrZrO3 and BaZrO3),81,117 probably due to wettability issues, which makes its use difficult with chemically stable electrolyte materials. Matsumoto et al.117 have compared the performance of Pt and Sm0.5Sr0.5CoO3 (SSC) as air electrodes for proton-conducting SOECs with a SZCY electrolyte, indicating that the SSC electrode shows much smaller electrode overpotential than Pt under the SOEC operating conditions. Other materials, such as La0.8Sr0.2CoO3d (LSC),93 La0.6Sr0.4Co0.2Fe0.8O3d (LSCF),91 Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF)91 and (La0.75Sr0.25)0.95Cr0.5Mn0.5O3d (LSCM),90 have been reported as air electrode materials for protonconducting SOECs. However, all these materials are used as air electrode materials for oxygen-ion conducting cells,119–121 and they are just directly applied in proton-conducting cells without considering the specific reactions occurring at the interface with the proton-conducting electrolytes. He et al.69 investigated the conduction mechanism of the air electrode for proton-conducting SOECs, finding the following steps in the air electrode reaction for proton-conducting SOECs: Step 1: H2O(g) - H2Oad Step 2: H2Oad - OHad + Had+ Step 3: OHad - Oad2 + Had+ Step 4: Oad2 e - Oad Step 5: Oad e - Oad Step 6: 2Oad - O2(ad) Step 7: O2(ad) - O2 Step 8: Had+ - HTPB+ Step 9: HTPB+ - Helectrolyte+ Step 1 to Step 3 describe the dissociation of H2O to O2 and + H ; Step 4 to Step 7 describe the formation and desorption of O2; Step 8 and Step 9 describe the proton migration to triple phase boundary (TPB) sites, which are regarded as the reaction active sites. Oxygen-ions do not participate in the reactions at TPB, while only protons do. The proton transfer process, including the proton transfer from decomposed H2O to TPB (Step 8) and the proton migration to the electrolyte from the

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Fig. 7 Working mechanism of different air electrode materials for proton-conducting SOECs. (a) Single phase electron-conducting (such as Pt) or mixed electron and oxygen-conducting (such as LSCF) electrodes with limited active sites at the air electrode/electrolyte interface; (b) composite electrode consisting of a proton conductor and an electron conductor with extended active sites; (c) single phase air electrode with simultaneous proton and electron conduction which extends the reaction sites to the whole electrode surface.

TPB sites (Step 9), is the rate-limiting step. This mechanism is quite different from that of SOFCs (even different from that for proton-conducting SOFCs), in which oxygen-ions also have to diffuse to the TPB area. As a result, the improvement of proton conduction rather than oxygen-ion conduction is critical for high performance air electrode materials for proton-conducting SOECs, indicating that the direct application of air electrode materials used for oxygen-ion cells for proton-conducting cells is not a rational approach, since these materials only show good electronic and oxygen-ion conductivities, which is not helpful for the whole air electrode reaction. In fact, in this case the reaction sites for the air electrode are only restricted to the interface between the air electrode materials and the electrolyte layer, as shown in Fig. 7(a). In order to extend the reaction sites, a rational approach is to make a composite air electrode material by mixing a protonconductor with an electronic or electronic/oxygen-ion mixed conductor.69 In this case, there are pathways in the electrode for both electrons (e) and protons (H+). The reaction active sites are extended to the whole connection area between the protonconductor and electron conductor, as shown in Fig. 7(b). However, the active sites are still restricted to part of the electrode

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layer. In addition, chemical compatibility is another problem to be considered for the composite electrodes.85 Recently, a single phase air electrode that simultaneously possesses both good electronic conductivity and adequate protonic conductivity was proposed for proton-conducting SOECs.122 The reaction mechanism of this single phase air electrode is shown in Fig. 7(c). The single phase electrode simultaneously provides both pathways for protons (H+) and electrons (e), enabling the electrochemical reactions to occur at the whole electrode surface. The reaction active area is much extended, even compared with the currently most utilized composite air electrodes. As a result, it is expected that this type of single-phase, mixed proton–electronic conducting air electrode can dramatically improve the SOEC performance. However, the design and search for electrode materials that show both high electronic conductivity and good proton conductivity is still a challenge, and only a few materials have been observed to show this property. BaCe0.5Bi0.5O3d,123 BaCe0.9Yb0.1O3d (ref. 124) and BaCe0.8xPrxO2.9 (ref. 125) have been proved to show both protonic conductivity and electronic conductivity, but the poor chemical stability of the BaCeO3-based material makes them inappropriate for practical exposure of the air electrode to high water concentrations in proton-conducting


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SOECs. Rao et al.122 have investigated cobalt doped BaZrO3 air electrodes, as cobalt is reported to be able to introduce electronic conductivity in BaZrO3 without eliminating the protonic conductivity. In their study, different cobalt doping concentrations were investigated, and the optimal composition was BaZr0.6Co0.4O3d showing the electronic conductivity of 5.24 S cm1 and ionic conductivity of 1.2 103 S cm1 at 700 1C. This air electrode shows a polarization resistance as low as 0.19 O cm2 at 700 1C, which is almost 65% lower than that obtained using a conventional SSC composite air electrode under the same conditions.126 This result suggests that the extended reaction sites by using mixed proton–electronic single phase air electrodes are beneficial for the air electrode performance. Although BaZr0.5Pr0.3Y0.2O3d (BZPY30)127 shows mixed proton–electronic conductivity as well as good chemical stability, its electronic conductivity only reaches an order of 102 S cm1 at 600 1C, which is too low to allow its use as an electrode alone; for air electrode reactions, this material has to be mixed with other electronic conductors to provide enough electronic conductivity. Fabbri et al.128 have tested LSCF–BZPY30 composite materials under wet air conditions and the polarization resistance of this electrode was as low as 0.011 O cm2 at 700 1C. This value is almost one order of magnitude smaller than that of the composite air electrodes made of an electron-conducting phase and a proton-conducting phase, suggesting that the mixed protonic–electronic conduction of BZPY30 provides more active sites extending the reaction area, and thus improves the electrode performance. The excellent performance of the LSCF–BZPY30 composite electrode was demonstrated in SOFC studies, and it is reasonable to assume that this air electrode could also be promising in proton-conducting SOECs, taking advantage of more pathways for electrons and protons favored by the extended reaction area. Besides the design of novel materials with both proton and electronic conductivity, another proposed approach for developing high performance air electrode materials was the search for proton conductivity in existing air electrode materials for oxygen-ion conducting electrolytes. Grimaud et al.129 have screened several well-known oxides, including La0.6Sr0.4Co0.2Fe0.8O3d (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF), PrBaCo2O5+d (PBCO) and Pr2NiO4+d (PNO), which are normally applied as air electrodes for oxygen-ion cells. However, it was found that BSCF, PBCO and PNO showed, in addition to the well-recognized oxygen-ion conductivity and electronic conductivity, some proton conduction in wet atmospheres, for which they can be labeled as triple conducting (O2/H+/e) oxides. The simultaneous conduction of protons and electrons could be beneficial for the air electrode reaction under electrolysis conditions. Li and Xie91 have investigated the performance of LSCF and BSCF as air electrodes for proton-conducting SOECs, and found that BSCF shows a much better performance than LSCF. The polarization resistance of LSCF air electrode reaches 0.36 O cm2 at 800 1C, while the polarization resistance value can be reduced to 0.075 O cm2 under the same conditions by using BSCF. The significantly reduced polarization resistance suggested that some proton conductivity in BSCF may help the proton migration to TPB sites, accelerating the air electrode reactions.129


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3.2

Hydrogen electrode materials

Pt is again the most used hydrogen electrode material in the early studies.57 Ni was alternatively proposed as a hydrogen electrode material to reduce costs for practical applications.117 Although both Pt and Ni perform well with BaCeO3- and SrCeO3-based proton-conducting electrolytes, their performance becomes poorer with increasing amount of Zr in the electrolyte composition.78,81,117 This means that Pt or Ni alone is not an appropriate choice for chemically-stable electrolytes for proton-conducting SOECs. One reason for the poor performance of these electrodes was attributed to the limited TPB area: Sakai et al.130 proposed to fabricate plated electrodes instead of conventional pasted electrodes to decrease the electrode particle size. Fig. 8 shows the schematic model of the TPB area for the conventional pasted electrodes and for the plated electrodes. The smaller electrode particle size for the electrodes prepared using the plating method provides more TPB sites per unit area than the conventionally screen-printed electrodes, leading to a better electrode performance and a higher H2 evolution rate at the same temperature. Nonetheless, the TPB area is still restricted to the electrolyte/electrode interface. In addition, the tight arrangement of small particles for the plated electrodes reduces the electrode layer porosity, which would hinder the diffusion of the produced H2.130 At present, the most used strategy to extend the TPB to a broader electrode area is the application of composite hydrogen electrodes consisting of metal and electrolyte oxide.90,93 Ni-based composite materials might be ideal as hydrogen electrodes for proton-conducting SOECs due to the following reasons. First, for SOEC devices based on electrolyte films, hydrogen electrodes are typically used as supports, and the relatively low cost of Ni makes it suitable for practical applications. Second, Ni shows very high electronic conductivity providing pathways for electron transfer under the operation conditions. Third, Ni shows good chemical compatibility with most of the existing proton-conducting oxides, even at temperatures as high as 1400 1C,99,131 facilitating the fabrication of proton-conducting SOECs with Ni-based hydrogen electrodes. Finally, proton-conducting SOECs produce only H2 at the hydrogen electrode side, which makes ambient conditions very mild for both Ni and proton-conducting oxides in the composite electrodes. Ni is thus free of the oxidation problems that occur for SOECs using oxygen-ion electrolytes. Proton-conducting oxides can also remain stable, as there is no acidic gas in the hydrogen electrode chamber. Therefore, Ni-based composite electrodes in principle can stably work under the proton-conducting SOEC operating conditions.

Fig. 8 TPB area of (a) the conventional pasted electrode and (b) the plated electrode, showing an improvement in active sites with the plated electrode.

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As a matter of fact, Ni-based composite materials have been recently investigated the most as hydrogen electrode materials for proton-conducting SOECs, especially for cells based on electrolyte films. Ni–SrZr0.5Ce0.4Y0.1O3d,117 Ni–BaCe0.5Zr0.3Y0.2O3d,69 Ni– BaCe0.5Zr0.3Y0.16Zn0.04O3d,90 and Ni–BaCe0.48Zr0.4Yb0.1Co0.02O3d (ref. 93) composite materials have been successfully used as hydrogen electrodes for proton-conducting SOECs. Since the hydrogen electrodes are used as the cell supports, relatively thick electrode layers have to be fabricated. Therefore, the microstructures for the composite Ni-based hydrogen electrodes have to be designed for allowing smooth diffusion of the produced H2. As proposed for tailoring SOFC electrodes, graded electrode design could be beneficial for both the H2 diffusion and electrochemical reactions.103 A relatively dense but thin functional layer close to the electrolyte, made of fine powders, can provide more pathways for electrons and protons, while a thicker and more porous external layer provides pathways for produced H2 diffusion. The application of a functional hydrogen electrode layer has been reported for oxygen-ion conducting SOECs, showing long-term stability,132 but the research is still lacking for proton-conducting SOECs deserving further studies to identify the optimal hydrogen electrode microstructures. Overall, it can be stated that the unique features of proton-conducting electrolytes for SOECs make the optimization of the hydrogen electrode microstructure already a very promising approach to improve the performance, without the need for finding novel electrode materials.

4. More than steam electrolysis There are several applications other than H2 production that can be deployed using proton-conducting electrolyzer cells. Iwahara et al.133 have used a proton-conducting ceramic as a solid electrolyte for electrochemical dehumidification, based on steam electrolysis. Fig. 9 describes the operating principle of electrochemical desiccation. Water vapor contained in the humidified target gas is electrolyzed at the air electrode side to form protons, which pass through the electrolyte layer reaching the hydrogen electrode side and forming H2 gas. This device

Fig. 9 Working principle of electrochemical dehumidification: the H2O content in the gas can be electrolyzed with proton-conducting SOECs to produce a dried gas.

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is actually a kind of steam electrolysis cell, in which the water content in the target gas is decreased during the electrolysis process, thereby resulting in gas desiccation. In their study, a cell made of a SrCe0.95Yb0.05O3d proton-conducting electrolyte and porous Pt as electrodes can dehumidify Ar gas with a small water content (PH2O = 45 Pa) down to PH2O E 20 Pa, at 700 1C at an applied current of 30 mA cm2. An even drier gas can be obtained at PH2O = 6.7 Pa with a larger applied current of 150 mA cm2, indicating that even a small amount of water content in the gas could be effectively removed by electrolysis cells using a proton-conducting electrolyte. Proton-conducting SOECs have also been tested for electrochemical reduction of CO2. In the report of Xie et al.,134 H2O and CO2 were fed separately to the air electrode and hydrogen electrode chambers. In the electrolysis mode, protons were formed, passed through a 60 mm thick BaCe0.5Zr0.3Y0.16Zn0.04O3d electrolyte film, and reacted with CO2. The reactions happening at the air electrode and hydrogen electrode can be written as: Air electrode: 2H2O - 4H+ + O2 + 4e

(4.1)

H2 - 2H+ + 2e

(4.2)

Hydrogen electrode: 2H+ + 2e + CO2 - CO + H2O (4.3) 8H+ + 8e + CO2 - CH4 + 2H2O

(4.4)

In their study, CO2 was successfully reduced with a conversion rate of 65%. The main product was CO (61%), with smaller amounts of H2 (8%) and CH4 (1.2%), as well as unreacted CO2 (29%). The CO2 conversion rate was much higher than that in the reverse gas water shift reaction, 37%, and also significantly improved comparing with that for oxygen-ion SOECs,135 only 11.5%, suggesting that electrochemical reduction with proton-conducting SOECs would be an effective method for CO2 reduction. It is worth noting that CH4 was also synthesized using the electrolysis process, with a better efficiency than in the conventional Fischer–Tropsch synthesis for CH4.134 The authors also suggested that lowering the operation temperature and utilizing higher current would improve the CH4 yield. In addition, a very recent study showed that a proper selection of electrode materials can further improve the performance of CO2 and H2O co-electrolysis with protonconducting membranes.136 In fact, iron-based electrodes showed a much improved performance than the copper and nickel-based electrodes. Furthermore, addition of Pt into the electrode can considerably improve the performance, indicating the importance of searching proper electrode materials for the application of CO2 reduction by protonconducting SOECs. Proton-conducting SOECs also show a good performance in NO reduction. NO is one of the most harmful compounds emitted from internal combustion engines and its reduction is becoming paramount for keeping the environment safe and clean. Kobayashi et al.137 have found that a steam electrolysis cell using a proton-conducting electrolyte can effectively reduce NO. A stable SrZr0.9Yb0.1O3d electrolyte was used because of


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the acidic nature of NO. Under the electrolysis conditions, the produced H+ can react with NO according to the reactions: 2NO(g) + 4H+ + 4e - N2(g) + 2H2O(g)

(4.5)

2NO(g) + 2H+ + 2e - N2O(g) + H2O(g)

(4.6)

NO(g) + 5H+ + 5e - NH3(g) + H2O(g)

(4.7)

The main products were N2O and N2 at low current densities, while N2 and NH3 were the main products at high current densities. A high NO removal efficiency was achieved by using the proton-conducting cells and the removal efficiency can reach 100% at current densities larger than 1.2 mA cm2 at 450 1C. By using SOECs with proton-conducting electrolytes, NO removal can even be achieved with the co-existence of O2 with NO, which is closer to the real application conditions, since engine exhaust gases usually contain a certain amount of O2 together with NO, while the cell with the oxygen-ion conducting electrolyte cannot reduce NO in an atmosphere containing O2.138 This result indicates that the proton-conducting SOECs show a great advantage over the conventional oxygen-ion conducting SOECs in NO reduction. Moreover, the described results allow us to speculate that operating proton-conducting SOECs under suitable conditions may also drive ammonia electrosynthesis.

5. Conclusions Proton conduction in oxides was discovered over 30 years ago being the first proposed application in electrolysis cells. The main aim of using a steam electrolyzer is to produce H2, and early research studies have demonstrated that SOECs using proton-conducting electrolytes are able to show H2 production rates similar to that of conventional oxygen-ion conducting SOECs, but offered additional advantages, such as production of pure and dry H2, no oxidation of Ni-based fuel electrodes, high current efficiency, and low operation temperatures. However, the development of proton-conducing SOECs is still lagging behind the oxygen-ion SOECs. The main reason is the dilemma in searching for an appropriate electrolyte material. The BaCeO3 and SrCeO3-based materials in the early studies showed good cell performance with a high H2 production rate, but their poor chemical stability makes their practical applications difficult, due to the high H2O content under electrolysis conditions. Although chemically stable electrolyte materials such as BaZrO3 were regarded in early studies to show too low conductivity for use as electrolytes, considerable efforts have been made to improve the conductivity until doped-BaZrO3 electrolyte films with good conductivity have been successfully fabricated, making a big step towards the practical application of proton-conducting SOECs. The development of stable electrolyte films is a critical step for proton-conducting SOECs but should be accompanied by the design of appropriate electrode materials for good cell performance. The unique advantages of proton-conductive electrolyzer cells make straightforward the optimal choice of Ni-based cermets as the hydrogen electrode materials. Though, the design of the air


electrode materials with good electrochemical performance still remains a great challenge, especially at low operating temperatures. An ideal air electrode material has to simultaneously possess good electronic conductivity and high proton conductivity, as well as good chemical stability towards H2O. The design of air electrode materials for proton-conducting solid oxide cells has been tackled very recently by tailoring the materials to obtain mixed proton and electronic conductivities, indicating an encouraging route for developing high performance air electrodes. Although developments of both electrolyte and electrode materials have made big progresses in the past years and make the use of proton-conducting SOECs promising for practical applications, significant efforts are needed to further explore new materials (especially air electrode materials), and to optimize the microstructure of both electrodes to fully utilize the advantages of proton-conducting SOECs.

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