6.1.1 Expected scale of impact

On the basis of the 2010 annual report of US-DOE Idaho National Laboratory,²⁰ the co-electrolysis process, when coupled with nuclear energy systems, can save up to 50% of the carbon from the direct use of liquid fuels, and entail 99% less CO₂ emissions than the ‘standard’ coal-to-liquid fuel conversion. To illustrate the issues and potential pathway for the integration of renewable energy sources to the electric grid, consider the case of wind energy. While today wind electricity with very few exceptions is uneconomic without subsidies, wind resources are large and the production economics are improving. This leads to the question, if the production cost of wind electricity continues to decrease, is it possible for wind to become an economic large-scale source of energy? Low wind electricity production costs do not imply low-cost electricity to the customer unless that electricity is generated when the customer needs electricity. Unfortunately, wind energy production does not match customer demand either during the day or seasonally. In parts of the United States with deregulated grids, power producers pay the grid to take electricity at times of very low electricity demand. The economic viability of wind depends upon integrating wind into a larger utility system and addressing the time mismatch between production and consumption. A recent example that demonstrates the potential impact from reversible SOFCs/SOECs is reported by Forsberg et al., who examined alternative system designs to enable the large-scale economic use of wind energy.^21,22 To minimize costs, these systems produce electricity from wind, natural gas and nuclear power, to supply the total electricity demands for the Midwest electricity grid and hydrogen to Chicago or Alberta refinery markets. The Midwest electricity grid includes the central United States and parts of Canada and has the best wind resources in the United States. Real electricity demand and wind data were used. The economics were found to be favorable assuming (1) significant reductions in wind turbine costs, (2) successful commercialization of high-temperature electrolyzers that can also operate in reverse as fuel cells and (3) higher priced natural gas. These were considered reasonable assumptions looking a few decades into the future. To realize such large-scale impact, it is important to advance our knowledge and discover materials that can enable highly efficient and durable high-temperature electrolyzers that also operate in reverse as fuel cells, and can couple to various energy sources such as wind, solar and nuclear, to produce a mix of electricity and fuels. This strategy makes it possible to integrate a larger range of clean energy sources than we currently have in the electric grid.

6.2.1 Operating principles

Solid oxide electrolysis cells can be seen in simple terms as the reverse operation of SOFCs.^1,23 Therefore, reversible cells operate through a solid oxide electrocatalytic system, which functions reversibly in both the electrolytic and the fuel cell modes. An SOEC consists of three functional solid-state layers, composed of a porous high-surface-area cathode, a dense electrolyte and a porous high-surface-area anode. The basic reactions taking place in the SOEC and SOFC mode are illustrated in Fig. 6.2. In SOEC mode, steam is introduced at the cathode side (hydrogen electrode), where it is reduced to release hydrogen gas and oxide ions in the process. Oxide ions migrate through the cathode and the electrolyte into the anode (oxygen electrode), and get oxidized at the anode. This is accompanied by the release of electrons and combination of oxygen atoms to form oxygen molecules that are carried away from the cell. In a co-electrolytic mode, the electrochemical reduction of steam and carbon dioxide takes place together on the cathode, both processes releasing oxide ions, and forming a mixture of hydrogen and carbon monoxide gases. While in SOFC, hydrogen and carbon monoxide can serve as the fuel that is oxidized, in SOEC the steam and carbon dioxide are reduced electrochemically. The anode and cathode must possess high ionic and electronic conductivity and high activity for surface reactions. The electrolyte must have high ionic conductivity and electronic insulation.

Initially, SOECs were developed as tubular cell structures to avoid sealing problems, which is a major issue in segregating the produced H₂ and O₂ gases in planar cells, particularly over multiple thermal cycles.²⁵ Today, with advances in sealing technologies, the trend in cell design is to employ planar structures. A tubular and a planar solid oxide cell and its microstructure are shown in Fig. 6.3. The conventional electrode microstructure comprises of nano-to-micro-scale percolated particles, with high surface area and with ionic and electronic connectivity. A high packing density and significantly smaller hot volume in the planar cells than that attainable by a tubular design renders the planar configuration more attractive.¹³ Additional advantages to the planar design include lower manufacturing costs and shorter current paths that reduce the ohmic cell resistance.²⁶ Planar SOECs, similar to planar SOFCs, are designed as either electrolyte-supported or electrode-supported. The former employs the electrolyte as the thick support structure for the cell and is more suitable for operation at high temperatures which reduces the often large ohmic resistance associated with a thick electrolyte. In an electrode-supported cell one of the electrodes is the thickest part of the cell, supporting a film electrolyte of about 10 μm or less in thickness. Such a design has been developed in order to minimize ohmic resistances in SOFCs operating at intermediate temperatures, and may also be applied to intermediate-temperature SOECs. During the operation of an SOEC system, heat is generated in the stack as the result of irreversible losses due to ohmic resistance and electrode overpotentials. The magnitude of these energy losses depend on operating conditions such as the stack temperature and average current density, as well as materials selection and cell and stack geometry. Finally, it is important also to note that the SOECs are fully scalable because the main functionality of the system depends on each individual cell. To attain the desired rate of hydrogen production rate, the cells are stacked together to form ‘stack modules’ and stack modules are integrated together to form a plant on the basis of cell and system functionality. Therefore, decreasing the cost of each ‘cell’ through higher activity and increased durability of materials has direct impact on the overall viability of these systems.

6.2.2 Functional materials

The efficiency of solid oxide cells depends on the reaction kinetics on both the anode and the cathode, and the oxygen transport kinetics through the electrolyte. The chosen materials and structures for reversible solid oxide cells must enable fast kinetics for the electrode reactions and for transport of oxygen ions through the electrolyte. The high efficiency of the present SOFCs and SOECs is, in part, enabled by their high operating temperatures, typically 800–1000°C. They do not require expensive platinum catalyst materials, as is currently necessary for low-temperature (polymer electrolyte membrane) fuel cells. However, the use of oxide materials for electrolyte and electrode functionality renders the conductivity and activity acceptably high thus far only at elevated temperatures. In advancing the SOFC technologies, there is increased interest in intermediate-temperature operation in the 823–1073 K range to allow for a wider range of materials, more cost-effective fabrication methods and increased durability. The reduction in operating temperature has also been observed for the SOEC field in the limited number of experimental projects reported over recent decades,^28,29 where the temperature at which the cells are tested has decreased from 1273 to 1073 K.

Here the most commonly used electrolyte and electrode materials in SOEC tests are summarized, and the challenges associated with them and new opportunities in functional SOEC materials design are discussed in the following sections. The state-of-the-art SOFC materials have formed a starting point for the SOEC development. Typical electrolytes used are made of doped zirconia, for example, yttria-stabilized zirconia (YSZ) or scandia-stabilized zirconia (ScSZ), which is a fluorite-structured oxide. SOEC cathodes are made of composites of zirconia and Ni metal catalyst, Ni/YSZ. Doped perovskites based on LaCoO₃ 1 for example La_{1 – x}Sr _x Co_yFe_{1 – y} O_{3 – δ} (LSCF), and composite electrodes made of a perovskite and a solid electrolyte such as La_{1 – x} Sr_xMnO_{3 – δ} (LSM)/Y_xZr_{1 – x} O_{2 – δ} (YSZ) have been proposed and used as an alternative to the conventional LSM for anodes in SOECs.³⁰ These materials present high electronic conductivity, high oxygen ion conductivity, and reasonably high oxygen surface exchange coefficient for fast reaction kinetics at the gas/electrode interface. Both in the YSZ and in the LSM and LSCF structures, the incorporation and transport of oxygen relies on the availability and mobility of oxygen vacancies,³¹ as illustrated in Fig. 6.4. However, the reaction mechanisms and the reaction steps that govern the kinetics of oxygen evolution on the anodes has not yet been studied, contrary to the increasing interest and state of knowledge on elucidating the cathodic reactions on the same materials in SOFCs.³²

An important motivation for use of the traditional SOFC materials in SOECs has been that a positive correlation between the initial performance of air electrodes in both SOFC and SOEC modes was observed.^27,33 However, there are contradictions in the literature about the performance, in particular, of LSM/YSZ in SOEC mode. Some authors have reported that LSM/YSZ electrodes show poor performance under electrolysis conditions.^34,35 Wang et al.³⁴ concluded that the enhancement associated with cathodic polarization is lost during electrolysis due to anodic polarization. In contrast, the best results in SOEC cells to date were reported by Jensen et al.⁴ using LSM/YSZ composite oxygen electrodes. Laguna-Bercero et al.³⁶ recently addressed some of these contradictions by consistently testing the reversible anodic cathodic performance of LSCF and LSM/YSZ composite electrodes. It was found that both types of compositions show a good initial performance with low area-specific resistances, ASRs (shown in Fig. 6.5), in good agreement with Jensen et al.’s results, and with the performance of LSCF better than that of LSM/YSZ. SOFC–SOEC galvanostatic experiments were also performed for both samples and the ASR values of 0.93 and 0.79 Ω.cm² were obtained for the LSM and the LSCF samples, respectively, showing a non-linearity of the cell response under anodic and cathodic polarization at temperatures up to 800°C. This non-linearity is important to note because it indicates different governing mechanisms on the same material when polarized cathodically versus anodically. Microstructurally, no evident differences between these electrodes were found after the combined SOFC–SOEC experiments; however, the LSM/YSZ was associated with an evident degradation rate.

6.3.1 Delamination of the oxygen electrode

The delamination of the anode (oxygen electrode) from the electrolyte has been observed as a major problem leading to SOEC performance degradation (Fig. 6.7).⁵² The build-up of a high oxygen partial pressure at the electrode/electrolyte interface while pumping the oxygen flux into the anode from the electrolyte is thought to be one main reason for the detachment of the electrode layer from the electrolyte. According to Virkar’s model,⁵⁴ one of the principal modes of failure of solid-state anode/electrolyte interface is related to the generation of high chemical potentials and corresponding ultrahigh pressure of neutral species within the electrolyte in the vicinity of the interface. For example, in an oxygen-ion-conducting solid electrolyte such as YSZ, under suitable thermodynamic conditions, the oxygen chemical potential within the solid electrolyte, , may exceed the chemical potential of gaseous oxygen. That is, it is thermodynamically possible to achieve > in SOEC operating conditions. Because the chemical potential of gaseous reactants is a function of the partial pressure, a higher in YSZ may be achieved compared to that the anode can accommodate and result in electrode delamination. An interpretation of this problem solely based on ex situ visual observations, without a theoretical basis, may be misleading especially because these observations are gathered as aftermath results of some critical damage to the cell materials. While an in situ observation and proof of this mechanism is not yet available, it is consistent with the hillock-like delamination of dense thin-film LSM micro-electrodes from YSZ electrolyte upon anodic polarization reported by Brichzin et al.^55,56 It is important to recall that the typical electrode materials tested in SOECs thus far are made of the perovskite oxides such as LSM and LSCF, which accommodate oxygen vacancies and rely on them for oxygen transport and oxygen exchange. However, they do not accommodate oxygen excess in a stable manner in the crystal structure. Therefore, the mitigation of this problem may consider anode or interface materials which can also accommodate oxygen excess, as will be discussed in Section 6.6.

6.3.2 Poisoning of the hydrogen and oxygen electrodes by contaminants

The transport of contaminants, such as Cr, Ni and Si, from interconnects and seals followed by their deposition on the electrode materials either blocks the active electrocatalytic sites or results in the formation of secondary phases that are inactive and insulating local phases.¹ Research to date has enabled a better understanding of the effect of these poisoning species in SOFC degradation kinetics and has helped control the degradation rate in SOFC stacks down to lower than 2%/1000 h.⁵⁷ On the other hand, investigation of these species in the context of degradation mechanisms in SOECs is still in its early phases.^38,58,59 The source of contaminants and their transport mechanism, poisoning and deactivation of reaction sites and their relative impact on degradation is important to understand and control for the viability of reversible solid oxide electrolytic and fuel cells.

6.3.3 Cation segregation and phase separation

Cation interdiffusion and segregation can adversely affect the performance of the cells through local changes in the composition and structure. Interdiffusion of cations between the oxygen electrode and the electrolyte was shown to lead to the formation of an electrochemically insulating layer or microstructure in SOFCs and has been the subject of detailed studies.⁵⁰ Similar to interdiffusion, but different in location, the segregation of cations on especially the cathode surfaces in SOFCs is widely documented and has immense importance for determining the oxygen exchange activity on SOFC cathodes and SOEC anodes. If the extent of such cation interdiffusion and segregation were only small, it would not result in drastic changes in the bulk microstructure, and the original bulk phases could be retained, while the adverse effects would be confined to ‘critical’ interfaces where the segregation accumulates. However, if the movement and flux of cations is significant over the microstructure, these processes can compromise the phase stability and the global electrochemical properties of the cell materials (like the LSC microstructure tested in SOEC anode/contact layer discussed in Section 6.2).

For complex oxides, such as the perovskite family of relevance to solid oxide cells, the surface structure and chemistry (i.e., cation concentration and oxygen non-stoichiometry) are driven dynamically by the surrounding harsh environment at high temperature, in oxygen partial pressure and under electrochemical potentials.^{45−47,49,65−67} Under these conditions, electrode surface chemistries may vary very richly, and the atomic structure of the surface segregation, not only its composition, is important in determining the oxygen reduction and evolution activity. As a result of cation segregation, the surface could become a ‘different material’ than the bulk phase. Cation segregation is now a commonly observed and acknowledged phenomenon on the surface of the SOFC cathode materials, such as on SrTiO₃ (STO),⁴⁵ (La,Sr)MnO₃ (LSM),⁴⁶⁻⁴⁸ (La,Sr)CoO₃ (LSC)⁶⁸ and SrTi_{1 – x}Fe_xO₃ (STF).^49,69 Such deviations from the bulk nominal composition and phase can directly alter the surface electronic structure and the electrocatalytic activity of the surface. A clear example to this is presented by Liang and Bonnell.⁷⁰ Their work showed that the segregated Sr-rich islands that form on the annealed SrTiO₃ single-crystal surface differ from the unsegregated surface both in atomic and in electronic structure. A much wider band gap (6 eV) was found on the Sr-rich segregated islands compared to that on the unsegregated pristine surface (3.2 eV). Direct evidence to the modified valance and conduction band electronic structure of segregated surfaces under various conditions was also reported on for SrFeO₃,⁷¹ LSM,⁷² STO^73,74 and (La,A)MnO₃ (A = Ca, Sr, Ba).⁷⁵

Despite the wide-ranging observations on cation segregation on perovskite-type oxides, the answer to how such surface segregation layers influence the oxygen reduction and evolution kinetics remains controversial. Taking Sr segregation as an example, it has been reported that Sr segregation on the surface leads to degradation of oxygen reduction activities on (LSC)⁶⁸ LSM⁷⁶ and STF.⁴⁹ In contrast to these results, other groups have observed the enhancement of oxygen exchange kinetics by purposefully decorating (via deposition) SrO onto the surface of LSC⁷⁷ and Fe-doped STO.⁷⁸ However, the physical origin behind these empirical observations is not understood. An important motivating point is the lack of understanding of how the surface structure in the presence of cation segregation affects the oxygen reduction and evolution activity in SOFCs and SOECs. The specific open questions of importance for further research are: (1) What are the non-bulk-like structures that form on these segregated surfaces of perovskite oxides at elevated temperatures (e.g., the La- and Sr-rich surface of LSC shown in Fig. 6.8b)? (2) How is the electronic structure affected by the formation of a new surface composition and structure? (3) How does the segregated surface electronic structure affect the oxygen reduction and evolution activity of the electrode?

6.3.4 Red–ox stability of the hydrogen electrode

Reduction and oxidation (red-ox) cycling in SOFC anodes has been identified as an issue that leads to the instability of the Ni/YSZ electrode. Volume changes in Ni–YSZ electrodes due to reduction and oxidation of Ni catalyst particles result in mechanical stresses and consequently to microstructural changes in the electrode, which degrades its activity. NiO formation is another factor that deteriorates the surface activity of the catalyst. SOEC tests at Ris0 National Laboratory have also found significant microstructural degradation in the Ni–YSZ cathode at the high steam partial pressure conditions typical of electrolysis, especially when the cells were run at high current densities. The loss in performance for the high current density, hightemperature SOEC tests at − 2 A/cm², 950°C and ρ_H2o = 0.9 was dominated by a 200% increase in the polarization resistance of the cathode over only 68 h (∆Rp = 21%/68 h).⁵⁸ Postmortem microchemical analysis of these cells showed significant structural changes at the hydrogen electrode/electrolyte interface due to the relocation of Ni (mechanism illustrated in Fig. 6.9). A 2–4 μm thick dense layer of Ni on YSZ formed at the interface, caused by the relocation of Ni particles. Such a layer completely blocks the oxygen transfer from the cathode into the electrolyte. For SOECs aimed at these conditions, different hydrogen electrode materials, potentially all-ceramic-based ones, need to be considered.

6.4.1 Advanced oxygen electrode materials

The majority of the tested oxygen electrode materials in the electrolytic or reversible solid oxide cells have been based on the experience gained from the SOFC-related research, most commonly using the perovskite structured La_{1 – x}Sr_xMnO₃ and La_{1 – x}Sr_xCo_{1 – y}Fe_yO₃.⁸² As discussed in Section 6.5, while these materials are acceptably stable under relatively reducing conditions of SOFCs, they exhibit severe delamination of the oxygen electrode from the electrolyte in SOEC mode.³⁸ This is because these perovskites are not very flexible in maintaining phase integrity in oxygen poor as well as in oxygen-rich conditions.⁸³ In SOECs, oxygen flux accumulates at the electrode-electrolyte interface, builds up an internal oxygen pressure, and the typically oxygen-deficient perovskites cannot inherently accommodate oxygen excess, for example in the form of interstitials. Due to this inherent structural limitation, the critical interface between the SOEC anode and electrolyte eventually ruptures.^38,39,53 Therefore, a key requirement for the oxygen electrode in reversible solid oxide cells is a flexible structure that accommodates oxygen excess in SOEC mode and is also stable when oxygen poor conditions prevail in SOFC mode.

A report by Laguna-Bercero et al presents La_{2 – x}Sr_xCo_{0 5}Ni₀.₅O_{4 ± s} (LSCN) as a novel electrode for reversible solid oxide cells.⁸³ From electrochemical tests, LSCN was found to perform well and stable in both the electrolytic and fuel cell operation. ASR values obtained from the slope of the I-V curves (Fig. 6.10a) showed that at higher temperatures (800–850°C) the cell is performing equally in both operation modes. ASR values at 850°C were 1.92 and 2.05 Ωcm² for SOFC and SOEC, respectively. At lower temperatures, 700°C, on the other hand, lower ASR values were obtained under electrolysis mode (2.60 Ωcm²) in comparison with those obtained under SOFC mode (4.13 Ωcm²). It is remarkable that these results for SOEC performance were obtained at a low steam partial pressure (ρ_H2O = 0.03 atm), and thus, even lower ASR values are expected at the higher steam concentrations and at high current densities. Furthermore, even an activation of the LSCN electrode was observed under anodic polarization lasting up to about 2 h, as seen in the AC impedance data in Fig. 6.10b.

The key to the successful demonstration of this material in reversible electrolytic fuel cell functionality is its structure that can accommodate both oxygen interstitials and vacancies depending on the surrounding thermodynamic conditions. LSCN is a Ruddlesden-Popper phase with the K₂N_iO₄ structure, which is a layered perovskite-related system. The advantage of this material structure for the solid oxide cell oxygen electrode is its stability over a wide range of oxygen pressures. It can adopt a range of possible oxygen non-stoichiometries including both oxygen-rich states (Ln₂N_iO_4 + δ) and oxygen-deficient states (Ln₂N_iO_4-δ), and thus can perform stable in SOEC and in SOFC modes of operation.⁸⁴ Especially its capability to house oxygen-rich stoichiometry, in the form of oxygen interstitials, renders this structure as a good candidate to avoid the electrode-electrolyte interface delamination problem in electrolytic operation. Furthermore, components of this family of materials were shown to exhibit very fast oxygen diffusion as well as fast oxygen exchange kinetics, due to a layered and flexible structure that permits oxygen interstitials to migrate easily. Recent theoretical and experimental research on one member of this structure family, the (La,Sr)₂CoO₄ exemplified in Fig. 6.11, explained and demonstrated the favorable oxygen reaction sites and diffusion paths in this material.⁸⁵⁻⁸⁷ In contrast to the vacancy-mediated surface reactions and oxygen transport in LSM and LSCF (as illustrated in Fig. 6.4), LSCN and other K₂N_iO₄-structured materials incorporate and transport interstitials by an interstitialcy mechanism in oxygen-rich conditions as at the anode-electrolyte interface in electrolytic mode. This structure, with alternative cations at the A- and B-site that can further improve surface oxygen exchange kinetics, presents a promising route for development of stable and highly active oxygen electrodes for reversible solid oxide cells.

6.4.2 Advanced hydrogen electrode materials

Conventionally tested hydrogen electrodes in electrolytic and reversible modes are also based on the most widely studied and used anodes of SOFCs, in particular the composite Ni/YSZ. Instability of the Ni/YSZ against red-oxcycling⁵⁸ and in high steam partial pressures is discussed in Section 6.5. This problem in practice necessitates the feeding of a small quantity of hydrogen at the inlet of the cathode of the SüEC and complicates the cell design and decreases the cell efficiency.

In contrast to the metal/oxide composite Ni/YSZ, single-phase oxide ceramics are generally stable to red-ox cycling and could therefore be operated without the complication of hydrogen feed in the electrode inlet. Tsekouras and Irvine showed that the oxygen-excess La₀.₃Sr₀.₇TiO_3 + δ, A-site-deficient La_0.2Sr₀.₇TiO₃ and undoped SrTiO₃ perovskite oxides can serve as very active and stable hydrogen electrodes (cathodes) in SüEC, with favorable chemical, mechanical, thermal and red-ox stability.⁸⁸ Oxygen-excess compound is characterized by local oxygen-rich defects randomly distributed within the perovskite framework that induce fast conduction.⁸⁹ The A-site deficient compound, as part of the La_xSr_{1 – 3}x_/2TiO₃ series (x = 0.2), has been shown to be also a promising SOFC anode material.⁵¹ Steam electrolysis performance of these materials was largely independent of the presence or absence of hydrogen in the cathode inlet (as seen in Fig. 6.12), reflecting the red-ox stability of these perovskites and representing a possible advantage over the state-of-the-art Ni/YSZ composite cathode. Such favorable performance was realized particularly because this family of materials has good electronic conductivity by forming Ti^{3 +}/Ti^+ 4 couples in the reducing conditions of an SüEC cathode, in addition to surface area effects and good ionic mobility. Doping of the B-site in this material with Fe would be a good strategy to enable good conductivity also in the oxidizing conditions the SOFC operation, as would be required in a reversible cells.

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