SOEC Mode Research Articles

A-site-deficiency range identified for in situ exsolution from (La0.4Sr0.6)1-αTi0.95Ni0.05O3±δ electrodes for SOFC and SOEC.

Modulating the A-site deficiency is a useful method to achieve the exsolution of nanoparticles on the surface of perovskite, improving the catalytic activity. However, rules for designing the deficiency value and its roles on the structure and performance remain unclear. In this study, a wide range of A-site deficiencies of (La0.4Sr0.6)1-αTi0.95Ni0.05O3±δ (LSTN, α = 0.00, 0.13, 0.15, and 0.18) titanate perovskite materials was designed to systematically investigate their crystal structure, binding energy, oxygen vacancy concentration, exsolution process, and electrochemical performance. An extremely high conductivity (e.g., 331.75 S cm-1@800 °C, 5% H2/Ar) was obtained in parallel with enhanced catalytical activity in SOFC and SOEC modes. The A-site-deficient samples displayed a higher conductivity, oxygen vacancy concentration, and power output than the stoichiometric samples (α = 0.00). The best maximum power density of 78.74 mW cm-2 and the highest population density of 25 particles per μm2 were obtained on the deficient LSTN with α = 0.13. These findings suggest that LSTN is an exceptionally promising material for solid oxide cell (SOC) electrodes.

In Situ Electrochemical Recovery: Sediment Transformation under Chromium Poisoning in Reversible Solid Oxide Cells with La0.6Sr0.4Co0.2Fe0.8O3-σ-Based Oxygen Electrodes.

Reversible solid oxide cells (RSOCs) are an all-solid-state electrochemical device, which can convert H2 into electricity in the fuel cell (SOFC) mode and electrolyze H2O into fuel gas in the electrolytic cell (SOEC) mode, exhibiting good application prospect in the development of carbon neutrality. However, the degradation of the air electrode caused by Cr-containing steel interconnects is a major obstacle that limits the broader application of RSOCs. Herein, the Cr poisoning effect on La0.6Sr0.4Co0.2Fe0.8O3-σ (LSCF)-based oxygen electrodes under the electrolysis mode was systematically investigated. The phase transition of the sediment during the chromium poisoning process was captured and monitored. When tested under the presence of Fe-Cr interconnects at 800 °C for 40 h, SrCrO4 on the surface of LSCF was clearly identified through XRD and Raman analysis as the main deposition, and with the prolonged operating time, LaCrO3 slowly emerged. Due to the much higher electrical conductivity of LaCrO3 compared to SrCrO4, the negative effect induced by Cr poisoning was offset along with test progressing due to the deposition transition phenomenon. Inspired by the interesting discoveries, transition from SrCrO4 to LaCrO3 can be artificially facilitated by switching the operating mode to the SOEC mode, which can partially recover the dramatic degradation caused by the Cr poisoning effect under the SOFC mode. The feasibility of the in situ electrochemical recovery method was also verified by the experimental results. The peak power density of the cells decreased from 0.829 to 0.505 W/cm2 when operating under the SOFC mode with an Fe-Cr metal connector, and after in situ electrochemical recovery in the SOEC mode, the peak power density recovered to 0.630 W/cm2. This study provides a new strategy for achieving high performance and stability of RSOCs.

Improved Reversibility of Thin-Film Solid Oxide Cells at 500 °C by Tailoring Sputtering Processes for Depositing Yttria-Stabilized Zirconia Electrolyte.

The present study investigates the impact of sputtering configurations on the microstructure and crystallinity of thin-film yttria-stabilized zirconia electrolytes for anodized aluminum oxide-supported all-sputtered thin-film reversible solid oxide cells. Employing various sputtering parameters, such as target-substrate distance and substrate rotation speed, the present study reveals distinct surface characteristics and crystalline structures of thin-film yttria-stabilized zirconia. The microstructure analysis includes scanning electron microscopy and atomic force microscopy examinations, uncovering the influence of the process parameters on the surface morphology, roughness, and grain size. X-ray diffraction data illustrate the texture preferences and crystallite characteristics. The electrochemical characterization of the reversible solid oxide cells demonstrates that the optimized sputtering configuration significantly outperforms the others in both SOFC and SOEC modes, showing exceptional current densities of 964 mA/cm2 at 1.3 V in electrolysis mode at 500 °C. Electrochemical impedance spectroscopy further reveals improved charge transfer reactions at the interface of the electrolyte. The enhanced electrochemical performance is attributed to the unique microstructure and crystallinity of the thin film of yttria-stabilized zirconia. The record-breaking electrolysis performance of this work at 500 °C underscores the potential of tailored sputtering parameters in optimizing the reversible solid oxide cell performance.

Improved surface activity of lanthanum ferrite perovskite oxide through controlled Pt-doping for solid oxide cell (SOC) electrodes

The development of multi-functional and highly mixed ionic-electronic conductive perovskite oxide-based electrodes is becoming an established trend for designing and developing SOFC/SOEC reversible cells. Ideally, the same material can be employed at both electrodes provided that structural stability, high conductivity and catalytic activity are preserved in different operational atmospheres. Here, controlled Fe substitution with a small extent (0.5 mol%) of platinum at the B-site of a lanthanum strontium ferrite is proposed as an effective method to enhance the original oxide properties as both air and fuel electrode for solid oxide cells. The effects of low Pt-doping on La0.6Sr0.4FeO3-δ structure, morphology and electrocatalytic activity are investigated and discussed. La0.6Sr0.4Fe0.995Pt0.005O3-δ (05P-LSF) is first tested as air electrode, displaying lower area specific resistance as compared to the Pt-free perovskite. 05P-LSF structural stability and conductivity are assessed in 100 % CO2 and 50 % CO2–50 % CO environments. Symmetric devices are then tested as SOECs in 100 % CO2, obtaining a current density output of 1.08 A/cm2 at 1.5 V. Electrochemical impedance spectroscopy (EIS) with distribution of relaxation times analysis (DRT) are used to provide insights on the electrode operation. Pt nanoparticle exsolution at the fuel electrode is induced by voltage application. The device stability under applied voltage is assessed for over 120 h. SOFC/SOEC characterization in a 50 % CO2/50 % CO mixture at 850 °C is also provided, obtaining a maximum power density of 270 mW/cm2 in SOFC mode, and a current density of 0.91 A/cm2 at 1.5 V in SOEC mode.

Performance and Degradation of Electrolyte Supported SOECs with Advanced Thin-Film Gadolinium Doped Ceria Barrier Layers in Long-Term Stack Test

Electrolyte supported Solid Oxide Cells (ESCs) with advanced thin-film Gd-doped ceria diffusion barrier layers between electrolyte and electrodes were assembled and electrochemically investigated in steam electrolysis mode in a so-called “rainbow” stack with 30 repeat units (RUs). The barrier layers were deposited onto the electrolyte supports via electron-beam physical evaporation deposition (EB-PVD) method at 600 °C. In this paper, the investigation mainly focuses on the electrochemical characteristics of RUs containing the EB-PVD thin-film GDC layers. At the initial stage of the SOEC operation, the stack reached a high performance with an electrical efficiency of 99.65% at 75% steam conversion and a total power input of 1.98 kW. A long-term stack test was performed in SOEC mode for over 5000 h and demonstrated a low voltage degradation of approx. +11.3 mV·kh–1 per RU (+0.9% kh–1). The RUs with EB-PVD GDC thin-films revealed similar initial performance and degradation rate to the state-of-the-art cells with screen printed GDC layers.

Ni-doped Fe-based perovskite to obtain multifunctional and highly efficient electrocatalytic active IT-SOFC electrode

This paper systematically studies Ni doping at Fe sites in La0.6Sr0.4Co0.3Fe0.7O3-δ (LSCF) as an intermediate temperature solid oxide fuel cell (IT-SOFC) electrode. The electrocatalytic activity of Ni-doped electrodes in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) modes was analyzed, rather than individual cathode or anode. The results indicate that Ni doping increases the internal vacancies, ORR activity, and charge transfer ability of La0.6Sr0.4Co0.3Fe0.6Ni0.15O3-δ (LSCFN15). In addition, compared to LSCF, LSCFN15 also reduces the energy barriers for surface oxygen exchange and bulk diffusion, thereby increasing the overall ORR kinetic rate. Combined with the distribution of relaxation time (DRT) analysis, it was confirmed that Ni-doped LSCFN15 increased the adsorption and dissociation rates of oxygen. At 750 ℃, the area-specific resistance (ASR) of LSCFN15 reached 0.045 Ω cm2, which is only 58.4 % of LSCF (0.077 Ω cm2). As a SOFC cathode, the peak power density (PPD) reached 1175 mW cm−2, 1.31 times that of LSCF (898 mW cm−2). As a SOFC anode, the current exchange density of LSCFN15 in OER mode reaches 164 mA cm−2, which is higher than the 125 mA cm−2 of LSCF, indicating that LSCFN15 also has the potential to serve as an anode for SOFC. In addition, Ni doping reduces the thermal expansion coefficient of the electrode and improves its thermal compatibility with traditional electrolytes. As an electrode, LSCFN15 exhibits excellent long-term stability in both SOFC and SOEC modes. Overall, Ni-doped Fe-based perovskite can improve the ORR and OER activities while enhancing the stability of cell operation and is expected to obtain multifunctional SOFC electrode materials.

Achieving excellent CO2/H2O conversion performance in symmetrical solid oxide electrolysis cells through in situ exsved Ni–Fe alloy nanoparticles

Symmetric solid oxide electrolysis cells (SSOECs) are widely studied because they can simplify the preparation process and save costs. In this study, high-performance symmetric electrode material La0.5Sr0.5Fe0.8Ni0.1Nb0.1O3-δ (LSFNiNb) is proposed. A heterogeneous structure with high catalytic activity is constructed via in-situ exsolution of Ni–Fe alloy nanoparticles (NPs). LSFNiNb shows the highest electrical conductivity of 157 S cm−1 in the air at 650 °C. In an atmosphere of 5% H2–95% N2, the polarization impedance is 0.302 Ω cm2 at 800 °C. While the current density is 0.767 A cm−2 in 60% CO2–20% H2O-20% H2, at 800 °C and 1.5 V. In addition, there is no significant degradation in SOEC mode at a current density of 0.2 A cm−2 for 110 h.

Controlled Pore Size of NiO-YSZ Tubular Substrate for Improved Performance of Reversible Solid Oxide Cell Using LaGaO3 Electrolyte Film

Effects of channel size in NiO-YSZ porous substrate were studied on power density in solid oxide fuel cell mode and electrolysis current in steam electrolysis mode. It was found that the cell deposited on anode substrate with larger pore diameter shows a superior performance. The LSGM cell prepared on Ni-YSZ tube with average channel diameter of ca. 2.5 μm shows the maximum power density of 0.36 W cm−2 in SOFC mode and 0.42 A cm−2 at 1.6 V in SOEC mode at 873 K. Spike potential noise which may be caused by insufficient gas diffusion in NiO-YSZ porous substrate was observed under constant current electrolysis condition in case of NiO-YSZ tube with narrow channel and the spike noise is suppressed by increasing channel size. NiO-YSZ tube with large channel size is also effective for increasing long term stability in electrolysis mode.

Theoretical study of catalytic activity modifications in CO2 methanation induced by an electric field in solid-oxide cells

We performed DFT calculations for hydrocarbon species on Ni(111) with direct electric fields and co-adsorbed O atoms, respectively. The latter effect is dominant in NEMCA mechanism on CO2 methanation and the overall reaction accelerates in SOEC mode.

A Study on Electrochemical Properties of Fuel-Electrode-Supported Reversible Solid Oxide Cells

Introduction Reversible solid oxide cells (r-SOCs) enable both power generation and steam electrolysis, and have attracted attention as a solid state electrochemical energy device for a decarbonized society (1). R-SOCs have the advantage of high efficiency due to high temperature operation, but there exist various technical issues remained in materials selection, start-up and shutdown, and long-term durability. Therefore, reducing operating temperature is desirable for practical applications (2,3). Electrode-supported cells, in which the electrode acts as a structural support, enable lower temperature operation by reducing the thickness of the electrolyte, which has certain electrical resistivity. Here in this study, the current-voltage characteristics and reversible cycle durability under r-SOC operating conditions are evaluated using fuel-electrode-supported cells, with the aim of developing r-SOCs capable of highly efficient fuel cell power generation and steam electrolysis. Experimental For the experiments, fuel-electrode-supported cells were fabricated using fuel electrode-supported half-cells (Japan Fine Ceramics, Japan), schematically shown in Fig. 1. The half-cell consists of scandia-stabilized zirconia (ScSZ: 10 mol%Sc2O3-1mol%CeO2-89 mol%ZrO2) or yttria-stabilized zirconia (YSZ: 8 mol%Y2O3-92 mol%ZrO2) electrolyte and a Ni-cermet fuel electrode, such as Ni-ScSZ or Ni-YSZ. (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was applied for the air electrode, and Gd0.1Ce0.9O2 (GDC) was inserted between the electrolyte and the air electrode to suppress interdiffusion and chemical reactions. Four types of cells were used with different combinations of the electrolyte component (YSZ or ScSZ) in the electrolyte layer or the supporting fuel electrode.In electrochemical tests, 50%-humidified hydrogen (100 ml min-1) was supplied to the fuel electrode, and air (150 ml min-1) was supplied to the air electrode. R-SOC initial performance tests were conducted using an electrochemical analyzer (1255B, Solartron). Cell voltage and impedance were measured at 700-800°C at current densities ranging from -0.5 A cm-2 to +0.5 A cm-2. Positive current density means the value in SOFC mode, while negative current density means the value in SOEC mode. In r-SOC 1,000 cycle durability tests, cycles of switching between SOFC and SOEC operation were repeated 1,000 times by varying current density. The range of current density in the cycle tests was between -0.2 A cm-2 and +0.2 A cm-2. The cell impedance was measured before the cycling test and every 100 cycles. After each test, the cells were analyzed by using a focused-ion beam scanning electron microscopy (FIB-SEM) to observe and evaluate the electrode microstructure. Results and discussion Figure 2 (a) shows the r-SOC initial performance of each fuel-electrode-supported cell. The cell with the Ni-YSZ fuel electrode and the YSZ electrolyte showed better current voltage characteristics than other cells. Figure 2 (b) shows the r-SOC 1000-cycle durability of the cell with the Ni-YSZ fuel electrode and the YSZ electrolyte. The results showed a decrease in power generation and electrolysis performance with increasing the number of cycles in both SOFC and SOEC modes. Possible degradation mechanisms will be discussed. Acknowledgments This paper is based on results obtained from a project (Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration), JPNP20005, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Collaborative support by Prof. H. L. Tuller and Prof. B. Yildiz at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References (1) Venkataraman, M. Pérez-Fortes, L. Wang, Y. S. Hajimolana, C. Boigues-Muñoz, A. Agostini, S. J. Mcphail, F. Mar échal, J. V. Herle, and P. V. Aravind, J. Energy Storage, 24, 100782 (2019).(2) Subotić, S. Pofahl, V. Lawlor, N. H. Menzler, T. Thaller, and C. Hochenauer, Energy Proc., 158, 2329 (2019).(3) B. Mogensen, Current Opinion Electrochem., 21, 265 (2020). Figure 1

Degradation Issues in Solid Oxide Electrolysers

One of the crucial aspects to achieving long lifetime and high-efficient SOCs (Solid oxide cells) is to enhance durability of the current devices.Degradation issues are typically associated with problems in both electrodes. For example, nickel-yttria stabilized zirconia (Ni-YSZ) hydrogen electrodes present under extreme conditions of high steam partial pressures, a low durability due to Ni oxidation resulting in lowered electronic conductivity and catalytic activity of the electrode. Ni agglomeration, depleting and micro crack in Ni-containing hydrogen electrode can also occur leading to a lower activity of the Ni-based electrodes as an SOEC fuel electrode. Furthermore, carbon deposition and sulfur poisoning on the Ni surface are also lead to cell performance degradation and poor durability.In addition, the performance of the oxygen electrode is also particularly more important in electrolysis mode than in fuel cell mode, as it is well established that electrochemically induced oxygen pressure increase at the electrolyte-oxygen electrode interface and subsequent membrane failure have been theoretically predicted and experimentally observed in SOEC mode. For example, several examples in the literature reported the presence on voids at the YSZ electrolyte, leading to delamination of the oxygen electrode. In this sense, lanthanide nickelates (Ln = La, Nd, Pr) have received considerable interest as materials for IT-SOFC electrodes and oxygen separation membranes, and they seem to be very attractive for electrolysis applications. The hyperstoichiometry of some oxygen electrode materials such as these Ruddlesden-Popper phases is believed to be favourable for effective oxygen evolution, as performance of these electrodes is enhanced in SOEC mode.Different strategies to develop optimized electrode structures as well as controlled operating conditions will be discussed in order to improve the durability of SOCs.

Preparation and performance evaluation of low temperature SOEC using lithium compounds as electrodes

Previous studies have found that in ceramic fuel cells using Ni0·8Co0·15Al0·05LiO2 (NCAL) as the electrode, lithium compounds such as LiOH generated by reduction of NCAL anode by H2 diffuse into oxide electrolyte membranes such as Gd0.1Ce0·9O2 (GDC), and the “GDC-lithium compounds” composite electrolyte formed online is an excellent proton conductor. In this paper, the low-temperature electrolysis performance of proton conductor type solid oxide electrolysis cell (P-SOEC) with GDC as electrolyte and NCAL as electrode for direct electrochemical conversion of H2O into H2 was investigated. It is found that at operation temperature of 550 °C, the current density of the SOEC prepared in this paper can reach 2.263 A cm−2 at 1.6 V. The composite electrolyte in the electrolysis cell reaches an ionic conductivity of 0.572 S cm−1 in the SOEC mode. The excellent hydrogen production performance proves that this new type of electrolysis cell with lithium compounds as electrodes is a promising and potential proton conductive low-temperature SOEC.

Electrochemical-energy- exergy analysis of reversible solid oxide cell-based small-scale stand-alone energy storage system

The present renewable energy systems should shift towards more storage-based systems due to their inherent intermittency. This study examines the electrochemical, energy, and exergy performances of a Reversible Solid Oxide Cell (ReSOC) based stand-alone energy storage system “with a pressurized gas tank”. The system operates in the fuel cell mode (SOFC) for power generation and electrolysis cell mode (SOEC) for syngas production. The exergy analysis gives a detailed insight into the irreversibility points in both system operation modes. The ReSOC system model is based on a validated ReSOC stack electrochemical model and system components’ mass-energy balance model. The power extraction level in the ReSOC stack was shown to determine the thermal management required through the electrochemical analysis. The energy analysis resulted in a system roundtrip efficiency of 51 % at the considered operating conditions. In addition, the highest points of exergy loss/destruction are the exhaust air (SOFC: 32 %, SOEC: 34 %), ReSOC stack (SOFC: 25 %, SOEC: 28 %), and pressure regulation valve (SOFC: 11 %, SOEC: 17 %). The results also show that the SOEC mode performed better than the SOFC mode in the energy and exergy analyses. This study may be used as a basis for stand-alone system performance improvement leading to higher efficiency.

Advanced Nanoscale Characterizations of Solid Oxide Cell Electrodes

The growing interest in the transition towards a fossil-free energy system has accelerated research and development on hydrogen-based energy conversion technologies. In this context, solid oxide cells (SOCs) have attracted attention as they can be operated in both electrolysis (SOEC) and fuel cell (SOFC) modes providing a clean and sustainable way of producing both green hydrogen and electricity at high efficiency. Despite their great potential, the current SOC technology faces some challenges due to the significant cell degradation upon operation. The performance loss has been attributed to several factors, such as microstructural evolution in the electrodes [1], and inter-diffusion of elements between the cell layers associated to material decomposition and reactivity [2–4]. Despite several efforts, all these degradation phenomena are still not precisely understood as they involve complex and intricate processes arising at different length scales. To address this issue, durability tests have been performed in representative operating conditions at high current density and humidity at the air side for both SOFC and SOEC modes. In addition, the tested cells were analyzed with advanced characterization techniques to study the material degradation down to the nanometer and the atomic scales.In this work, standard fuel electrode supported cells have been studied. They are composed of a La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) current collecting layer (CCL) and a LSCF/Ce0.8Gd0.2O2-δ (LSCF/GDC) composite functional layer (FL) for the air electrode, GDC for the barrier layer, (Y2O3)0.08(ZrO2)0.92 (YSZ) for the electrolyte and Ni-YSZ for the fuel electrode. The samples were aged for 2000 h in various operating conditions: temperature (750, 800, 850 °C), humidity in the air flow (0%, 3% and 8%) and operating mode (SOEC and SOFC at ±1 A cm-2). The electrochemical behaviour of the cells has been characterized using electrochemical impedance spectroscopy (EIS) and polarization curves. The advanced post-mortem characterizations include techniques such as synchrotron X-ray nano-diffraction and nano-fluorescence (nano-XRD and nano-XRF) spectroscopy performed at the European Synchrotron Radiation Facility (ESRF, ID16B), scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDX), and atom probe tomography (APT). A larger range of information was acquired by using complementary techniques, including focused ion beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), and high-resolution secondary ion mass spectroscopy (SIMS) imaging.The electrochemical characterizations (Fig. 1a) have shown that the degradation was aggravated by increasing the operating temperature, the presence of humidity at the air side and the operation in electrolysis mode. The impedance spectra have been interpreted using a physically-based multiscale model [5]. The analysis has revealed that the performance loss was mainly due to the fuel electrode degradation, while the air electrode degradation remained limited, except when operated under humid conditions in the SOEC mode. The post-mortem characterizations have shown that such degradation in the SOEC mode was mainly related to a strong Ni depletion, which became more pronounced with increasing operating temperature. For the cell operated under humid air in the SOEC mode, a significant amount of SrCO3 phase was observed on top of the air electrode. On the contrary, the cell operated under humid air in the SOFC mode did not show such phase formation. Advanced characterization techniques (nano-XRD, nano-XRF, STEM-EDX, and APT) put in evidence an intermixing of elements in the air electrode (LSCF/GDC) at the atomic scale (Fig. 1b). Moreover, the presence of an inter-diffusion layer (IDL) between GDC and YSZ phases was observed (Fig. 1c) for all samples, including the freshly prepared sample. Nevertheless, neither the mixing of elements in the air electrode nor the presence of the IDL seem to have a major influence on the cell degradation. It can be speculated that these phenomena make the air electrode more stable against phase decomposition by inhibiting the formation of commonly observed insulating phases such as SrZrO3 and SrO.

Strontium Free Cu-Doped La2NiO4 Oxides as Promising Oxygen Electrodes for Solid Oxide Electrolysis Cells

In this work, strontium-free oxygen electrodes for electrode-supported SOFC/SOEC cells were evaluated. Strontium is known to react with volatile Cr from the metallic interconnect to form Sr-Cr oxides at the surface of the electrode, thus reducing the cell performance. Strontium-free cobalt- or copper-doped nickelates, La2Ni1-xBxO4 (B = Co or Co; x=0.1-0.5) were synthesized using solid state synthesis method. The crystal structures of the oxides were studied by Rietveld refinement analysis of the powder X-ray diffraction data. Similar to the parent compound, the Ruddlesden-Popper (RP) La2NiO4 oxide, the Co- and Cu-doped compositions were confirmed to crystallize with the tetragonal crystal structure with I4/mmm space group.The electrochemical performance of the La2Ni1-xBxO4 electrodes was studied using 1 inch in diameter button cells in SOEC mode and compared to that of cells with the state-of-the-art (La0.6Sr0.4)0.95Co0. 0.8O3-δ (LSCF)- ceria composite electrodes. The oxide inks were screen printed onto the pre-fired ceria barrier layers on the YSZ electrolytes. Short- and long-term tests were conducted at 650, 750, and 800oC with the Ni-YSZ hydrogen electrode exposed to 50-98% steam in hydrogen. Scanning electron microscopy with electron diffraction spectroscopy (SEM/EDS) was used to analyze the electrode microstructures before and after testing.In general, the performance of the La2Ni1-xBxO4 electrodes was slightly lower than that of LSCF. However, the electrode microstructures were not optimized, and further improvements could be made. The Co-substituted nickelate performances showed some degradation in in time. Cu-doped nickelates exhibited more performance during the same tests and the degradation rate was insignificant. The composition with 20% Cu appeared to be more active as well as more stable.

Performance and Degradation of Electrolyte Supported SOECs with Advanced GDC Thin-film Layers in Long-term Stack Test

Solid oxide electrolyser (SOE) based on electrolyte-supported cell (ESC) architecture is proven to be highly efficient and reliable technology for green hydrogen production via high temperature electrolysis. Operating under industrial conditions, the performance, lifetime and robustness of the cells and stacks belong to the most important factors for an up-scaled penetration of the SOE technology into energy and chemistry sectors. A typical ESC consists of scandia- or yttria-stabilized zirconia electrolyte, Ni-cermet fuel electrode, lanthanum strontium cobalt ferrite (LSCF) oxygen electrode and either one or two Gd-doped ceria (GDC) layers with a thickness of 5-7 µm between electrolyte and electrodes. At the oxygen electrode side, the GDC layer prevents interdiffusion (mainly of Sr), whereas at the fuel electrode side, the GDC layer may improve the electrode adherence to electrolyte and reduce the interfacial resistance. State-of-the-art fabrication route of the GDC layers is screen-printing followed by partially sintering at high temperature between 1200 °C to 1300 °C. This fabrication process usually results in porous structure in the GDC layers and residual stresses which reduce the mechanical strength of the cells.Aiming to improve the mechanical strength of ESCs and the properties (e.g. porosity and electrical conductivity) of GDC layers, the conventional screen-printed GDC layers either only at the oxygen electrode or at both electrodes were replaced by 0.5 µm thick thin-film GDC layers fabricated by electron-beam physical evaporation deposition (EB-PVD) method at 600 °C. These EB-PVD-fabricated thin-film GDC layers were electrochemically characterized in a so-called “rainbow” stack with 30 repeat units (RUs), including 4 RUs with EB-PVD GDC layers at the oxygen electrode and screen-printed GDC at fuel electrode, 3 RUs with EB-PVD GDC layers at both electrodes and 23 RUs with state-of-the-art screen-printed GDC layers at both electrodes. The stack was operated for over 5000 h in SOEC mode under a current density of -410 mA cm−2 and 75 % steam conversion, with a feed gas composition of 80 % steam + 10 % H2 + 10 % N2 on the fuel electrode and air on oxygen electrode.In this paper, the investigation focuses on the electrochemical characteristics of RUs containing the EB-PVD thin-film GDC layers during the SOEC stack testing. The performance and degradation behavior of the stack and representative RUs were investigated and compared by means of current-voltage curves and electrochemical impedance spectroscopy (EIS). The initial performance and the degradation of the voltage and resistances of the stack and the RUs with EB-PVD GDC thin-film layers were determined and discussed in order to evaluate the effectiveness of the modifications in the scenario of long-term SOEC stack testing.The German Federal Ministry for Education and Research (BMBF) is acknowledged for the financial supports within the project H2GiGa-HTEL (grant no. 03HY124E).

Evaluation of Electrolytic Characteristics with a Single Cell Developed as SOFC

In recent years, it is concerned that a power supply and demand balance of power grid become unstable because of large introduction of variable renewable energy such as solar photovoltaic power generation. The reversible solid oxide cell (rSOC) can operate both SOFC and SOEC modes reversibly. Therefore, the rSOC can be used as a flexibility device for the power supply and demand adjustment by operating reversibly in response to power grid balance.Furthermore, CO2 electrolysis and co-electrolysis, which can generate synthesis gases such as H2 and CO, which are raw gases for methane or liquid fuels, from CO2 and H2O by adding an electric power, are promising carbon recycling technologies.In this study, the electrolytic characteristics were evaluated with a single cell developed as SOFC. It was confirmed in CO2 electrolysis test that CO2 can be directly electrolyzed to CO with about 100% current efficiency. In addition, the electrolysis voltages were clarified under various gas conditions in co-electrolysis and the cell performance factors were analyzed by our original method. In this method, the cell performance factors can be described by the internal resistance loss (IR loss, measured value), the fuel electrode overvoltage, the oxygen electrode overvoltage, and the Nernst loss (calculate value). As a result of this analysis in co-electrolysis mode before and after all tests, it was shown that the increase in IR loss was the main factor of the decrease in cell performance.

Electrochemical and microstructural characterization of the high‐entropy perovskite La0.2Pr0.2Nd0.2Sm0.2Sr0.2CoO3‐δ for solid oxide cell air electrodes

AbstractStrontium segregation (coupled to phase decomposition and impurity poisoning) and electrode delamination are two of the most important degradation mechanisms currently limiting the long‐term stability of solid oxide fuel cell and electrolysis cell (SOFC and SOEC) air electrodes. The present study aims to demonstrate that air electrodes made of entropy‐stabilized multi‐component oxides can mitigate these degradation mechanisms while providing excellent cell performance. A SOEC utilizing La0.2Pr0.2Nd0.2Sm0.2Sr0.2CoO3‐δ (LPNSSC) as an air electrode delivers −1.56 A/cm2 at 1.2 V at 800°C. This performance exceeds that of a commercial cell with La0.6Sr0.4CoO3‐δ (LSC) air electrode, which reaches −1.43 A/cm2. In a long‐term electrolysis test, the LPNSSC cell shows stable performance during 700 h, while the LSC cell degrades continuously. Post‐mortem analyses by scanning electron microscopy‐energy dispersive X‐ray spectroscopy indicate complete delamination of the LSC electrode, while LPNSSC shows excellent adhesion. The amount of secondary phases formed (esp. SrSO4) is also much lower in LPNSSC compared to LSC. In conclusion, the high‐entropy perovskite LPNSSC is a promising option for air electrodes of solid oxide cells. While LPNSSC can compete with ‒ or even outperform ‒ LSC air electrodes in terms of electrochemical performance, it could be particularly advantageous in terms of long‐term stability in SOEC mode.

Evaluation of Electrolytic Characteristics with a Single Cell Developed as SOFC

The reversible solid oxide cell (rSOC) can operate both SOFC and SOEC modes reversibly. It is necessary for developing the rSOC to clear the reversible operation problems by evaluating cell performance of SOFC and SOEC modes. Based on our original performance evaluation method of SOFC which was previously developed, a new performance evaluation method of SOEC which can be adapted under various temperature conditions has been developed in this report. The cell, which was developed as SOFC, performance before and after the CO2 direct electrolysis test (CO2+2e-→CO+O2-) was compared by using the performance evaluation methods of SOEC and SOFC. As a result, the IR loss and the cathode overvoltage increased after the CO2 direct electrolysis test in both SOEC and SOFC modes.

Advanced Nanoscale Characterizations of Solid Oxide Cell Electrodes

Solid oxide cells (SOCs) have attracted a growing attention thanks to their high efficiency and ability to operate in both electrolysis (SOEC) and fuel cell (SOFC) modes. Despite its great potential, the current SOC technology faces significant cell degradation during long-term operation. The degradation phenomena are still not well understood as they involve complex and intricate processes arising at different length scales. Additionally, it has been shown that the degradation rate under SOEC operation is generally higher than in SOFC mode. To address this issue, durability tests were performed in SOEC mode at 750, 800 and 850 °C for 2000 h at -1 A cm-2 in 10/90 vol.% H2/H2O and dry air. The electrochemical performance of the cells was assessed using electrochemical impedance spectroscopy. In addition, advanced characterization techniques with nanometer and atomic resolution have been used to study material degradation after long-term testing.