Recent Advances in High Temperature Electrolysis Cells using LaGaO3-based Electrolyte

The electrochemical reduction of CO2 was studied via solid oxide electrolysis cell (SOEC), a type of device that can work reversely into a solid oxide fuel cell (SOFC) to generate electricity. Application of SOEC for CO2 electrolysis possesses potential rewards both in energy and environmental aspects, as it offers a way to recycle CO2 into chemicals and value-added fuels, which helps to reduce the accumulation of atmospheric CO2 and realize the carbon neutral cycling of fuels. Secondly, SOEC techniques provide a means to utilise the intermittent renewable sources, such as wind, tide, etc., as energy input to store excess electricity in the form of H2, CO and hydrocarbons and use these chemicals when necessary. Yet it is a challenging task to realize an efficient reduction of CO2 by SOEC due to the non-polar nature of CO2 fuels which are hard to be chemically absorbed and activated in high temperature range. The CO product from CO2 reduction is also demanding for the choice of fuel electrode (i.e. cathode in SOEC and anode in SOFC) materials, which has been an issue for the CO/hydrocarbon-fuelled SOFCs. To date, the CO2 electrolysis by SOEC is still at the starting point, and the mechanisms on the electrochemical reduction of CO2through SOECs are not fully understood. Extensive efforts need to be dedicated to the material developments, mechanism study, and system designs etc. Effort was made in our lab to find a highly performed, long-term durable cathode material for electrochemical reduction of CO2 by SOEC and to obtain more understandings of the mechanism of CO2 electrochemical reduction process. Different cathode materials were employed, including Ni-8 mol% yttria stabilised zirconia (YSZ) cermet, (La,Sr)(Cr,Mn)O3 (LSCM)-YSZ composite, and LSCM-(Gd, Ce)O2 (GDC) composite. Focus was casted on LSCM based cathodes which were found to be carbon-resistive, and the microstructure of LSCM-based cathodes was tuned to high performance and efficient CO2 electrolysis by the strategy of applying a gradient composite cathode and of adopting wet impregnation as cell fabrication procedures. The electrochemical performance of CO2 electrolysis was characterized in various CO2-CO mixtures and applied potentials in 900-750oC with the aid of impedance spectroscopy, on YSZ electrolyte supported three-electrode SOECs. In this paper, impedance behaviour of the electrochemical reduction reaction of CO2 from different cathode SOECs was correlated with the variations in gas composition, operating temperature and loading potential. As both charge transfer and surface adsorption/desorption equilibration and surface diffusion of activated species were found to be dominant processes taking place in the scope of cathodes under examination, with the later being significant in CO2-rich fuels, areas of discussion will be the effects of operational conditions on the impedance arcs associated with these steps, and how the cathode microstructure impacts the impedance behaviour and the corresponding elementary steps from CO2electrochemical reduction. By introducing a LSCM-YSZ 30-70/LSCM-YSZ 60-40 graded cathode and by incorporating extra catalyst into LSCM-GDC composite, the cathode performance was greatly improved, and the surface activity was profoundly accelerated. However, the most effective way to promote cathode performance was to introduce the cathode components in separate steps via wet impregnation. A competitive CO2 electrolysis performance to Ni-YSZ cermet was obtained from the GDC impregnated LSCM-YSZ cathode with 0.5% Pd extra catalyst, which also showed a comparable performance between SOEC and SOFC when operating in CO2-CO 50-50 mixture.

Introduction With the recent enhancement of renewable energy utilization, large scale energy conversion and storage become more important to control the surplus electric power due to the unsteady power generation. Reversible hydrogen energy storage system via solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) are expected as a high efficient device because of its high operating temperature (600℃~1000℃). It is possible to unify them to one device as reversible system (RSOFC/EC), but it is necessary to understand the mechanism of FC/EC reactions and design the device with a good balance between FC and EC in order to apply them for an actual energy system. While SOFC reaction models and many electrochemical data of electrodes have been developed and reported by many researchers, there are few reports to investigate the reaction model of SOEC. Thus, it is possible to predict the performance of SOEC based on a comprehensive SOFC/EC models and experimental data of SOFC, because the reactions of SOFC and SOEC are a reversible relationship and the equal value of IR loss can be applied. However, the comprehensive SOFC/EC electrochemical model has not been established. A series of experimental data of anodic and cathodic current density vs. the potential should be investigated at various conditions to develop the SOFC/EC model. In this work, a series of anodic and cathodic electrode performance was taken using a SOFC in various conditions. Furthermore, attempting to predict SOEC performance from SOFC performances, we proposed and discussed a new model of electrolysis based on our developed SOFC model. Experimental The Disks of 8 mol% Y2O3-ZrO2 (YSZ; 0.25 mm thickness, 20 mm diameter) were used as the electrolyte and the anode was a Ni/YSZ or Ni/GDC composite, and the cathode was a La0.8Sr0.2MnO3/ScSZ composite. We measured the dependence of the current density of anodes on oxygen activity with wide range to get both power generation and electrolysis data for various gas composition (PH2=0.3~0.99 atm, PH2O=0.01, 0.03, 0.05 atm). Results and Discussion 1. Method to fit parameters on the SOFC/EC model We have developed the electrochemical reaction model on the fuel electrode (=anode) based on the equilibrium of competitive adsorption and oxygen activity, ao , specified by anode potential (Eq.1, Table.1) [1]. The SOFC model was constructed by assuming competitive adsorption equilibrium of H2, H2O and O on Ni surfaces at three-phase boundary and the rate-determining step to be Langmuir-type reactions of H with O (Table.1). We obtained an equation of the electrical current density, i, as a function of the oxygen activity (ao ) calculated from the anode potential which is simply measured from experiments. To confirm effectiveness of the model, we fitted the model to actual experimental data described by i vs. ao . From the equations of model on Table.1, each current density of SOFC (ia ) and SOEC(ic ) are described as Eq.2. To draw the plot of i vs. ao on the model, the rate constants (ka, kc ) and the equilibrium constants (KO, KH, KH2O ) need to be defined as reliable value. Thus, from the equilibrium potential vs. 1 atm O2 (Eq.3), we defined the rate constant of kc as Eq.4. When the kc became a dependent variable of ka , then we fitted ia to power generation experimental data described by i vs. ao so that the values of KHKO/KH2O were fitted approximately. Finally, i c model could be drawn on the plot of i vs. ao and the maximum value of i c was obtained by Eq.5 which is independent of ao and Eq.6 was obtained using Eq.4. 2. Experimental results and parameter fitting on the SOFC/EC model As the expreimental results, the maximum power density of SOFC with Ni/YSZ was 588.5 mW/cm2 for FC mode at PH2=0.99 atm, PH2O=0.01 atm, and ic(max) was 0.285 A/cm2 for EC mode at PH2O=0.03 atm. However, ic(max) calculated by the model (Eq.6) was 0.050 A/cm2. Also, in case of SOFC with Ni/GDC, ic(max) was 0.400 A/cm2 for EC mode at PH2O=0.03 atm, while ic(max) calculated by the model (Eq.6) was 0.055 A/cm2. The electrolysis current density measured by our RSOFC/EC showed 6 times higher than model values approximately. This result suggested the possibility that additional parallel reaction path was existed. In this presentation, we will discuss the possible electrochemical paths to explain the disagreement between the model and the experimental data. Finally, the comprehensive SOFC/EC model will be proposed to predict EC performance using SOFC data.

Composite electrodes comprising a rare-earth nickelate phase and a doped rare-earth ceria phase with a high level of rare-earth doping have been studied as oxygen electrodes. While rare-earth nickelates have shown great promise as oxygen electrodes in both solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOECs), they become unstable in contact with rare-earth doped ceria. In this work we describe a strategy to eliminate this instability and enable the use of these electrodes as stable high performance oxygen electrodes in both SOFC and SOEC applications. The performance of such composite electrodes is explored using a symmetrical cell approach, and through the electrical conductivity relaxation (ECR) method and compare it to state-of-the-art oxygen electrodes comprising strontium doped lanthanum manganite (LSM) – yttria stabilized zirconia (YSZ) composite electrodes. The symmetrical cell tests which reveal the stabilized nickelate-ceria electrodes to be superior, are also well-correlated with the results of testing complete cells in both SOFC and SOEC modes. Based on these results we make the case for deploying these stabilized nickelate-ceria composite electrodes as oxygen electrodes in reversible SOFC-SOEC systems.

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The solid oxide fuel cells (SOFCs) have been considered as important alternative energy technology, due to a variety of advantages, such as their high energy conversion efficiency, free noxious emissions, and direct use of a wide range of hydrocarbon fuels. Especially, the reverse operation of SOFCs, which is called solid oxide electrolysis cell (SOEC), is currently one of the most promising technologies for fuel production. However, their high-temperature operation (typically >700°C) leads to the rapid degradation of electrodes and necessitates a high balance of plant costs. To make SOFCs more economically feasible and viable, further reduction of operating temperature is inevitable. To this end, the utilization of proton-conducting oxides, which exhibit relatively high ionic conductivity with the low activation energy (<0.5eV), enables protonic ceramic fuel cells (PCFCs) to operate at an intermediate temperature regime (IT, 400–600°C), where the aforementioned benefits of SOFCs can also be accomplished. Moreover, PCFCs prevents the dilution of the fuel during the operation and provides higher resistance of carbon cocking on hydrocarbon fuels.Although the PCFCs have distinct advantages against traditional oxygen conducting SOFCs, the lower-than-predicted performance of PCFCs and scalability issues caused the considerable skepticism of the successful applicability of PCFCs. Most importantly, unlike traditional SOFCs, the development of mixed oxide ion and electron conductors (MIECs) for oxygen reduction reaction (ORR) at the cathode of PCFCs has shown marginal success due to the absence of protonic conduction and still needs to be improved. At the cathode, the ORR is limited to triple phase boundary (TPB), where the electrolyte, gas, and cathode mutually contact, and its geometrical enhancement at the interface area between cathode and electrolyte layers is crucial to improve the performance of PCFCs. Therefore, electrode-electrolyte interface architecting via micro-patterning is one of the most attractive approaches, since it significantly expands the catalytic active sites of TPB near the interface, where ORR preferentially occur. Moreover, this architecting approach also facilitates the use of currently viable state-of-art cathode materials of SOFCs; Lanthanum Strontium Cobalt Ferrite (LSCF), Barium Strontium Cobalt Ferrite (BSCF). Therefore, patterns with a high aspect ratio, which maximize the interface area, are required for significant enhancement of electrochemical performance.Here, for the first time, we describe a three-dimensionally micro-architecting process for anode-supported PCFCs, demonstrating dramatic performance enhancement from the expanded interface area between cathode and electrolyte. The various patterns with high ratios were exactly transferred from the original PUA (polyurethane acrylate) molds to the thin and dense protonic conducting electrolyte layer. The patterns maintained the same aspect ratios during the sintering process and finally increased the interface area of more than 30%. The morphology of the patterned surface was characterized by scanning electron microscope (SEM) and atomic force microscopy (AFM). The AC electrochemical impedance spectroscopy was utilized to examine the electrochemical performances of patterned cells. Our unique patterning method opens the new pathway for further performance enhancement of PCFCs. Figure 1

High temperature electrolysis (HTE) cells struggle to reach lifetimes in the thousands of hours for hydrogen production from steam. While oxygen-conducting solid oxide electrolysis cells (o-SOECs) are a more mature technology than proton-conducting (p-SOECs) counterparts, both types of cell suffer from multiple decomposition modes including: delamination of layers, atomic migration, secondary phase formation, and reaction at the triple-phase boundaries at the electrodes. While the macroscopic results of these decomposition modes have been reported, the mechanisms driving onset and evolution of these decomposition modes is not well understood. In this work, we report on the results of a collaborative effort between four national laboratories to investigate the onset of decomposition in aged HTE cells using transmission X-ray microscopy (TXM) at SSRL at SLAC of o-SOEC and p-SOEC cells. Using TXM we volumetrically image, in 3D, the density of aged cells with 30 nm x 30 nm x 30 nm voxel (3D pixel) resolution. Using this technique, we investigate the nucleation of interfacial voids and the near-environment around those voids; revealing insight into the mechanism driving void formation. SSRL’s TXM also enables the use of X-ray absorption near edge structure (XANES) measurements at each voxel, which yields stoichiometric and oxidation state maps of key elements. This XANES data, combined with TXM, allows identification of how elemental migration, and oxidation state changes, drive the formation of voids. Similarly, the evolution of the triple-phase boundaries at the electrodes is investigated using a combination of TXM and XANES. Together, this work demonstrates a powerful technique for investigating the onset and evolution of decomposition in a range of HTE cells.

Different solutions adopted in producing Solid Oxide Cells (SOCs) have proven to be scalable and the technology is now approaching commercialization. The high energy conversion efficiency, coupled with the fuel flexibility and the reduced use of expensive catalyst materials promoted by the favorable thermodynamics at the operative temperatures, are making the technology increasingly attractive and stacks are finding applications in more fields. Large scale power-to-X plants, particularly the ones devoted to hydrogen production (Solid Oxide Electrolysis Cells – SOEC), are among the solutions that are gaining most momentum in the energy market. X-to-power (Solid Oxide Fuel Cells – SOFC) solutions such as decentralized heat and electricity production, auxiliary power systems and heavy duty transport propulsion are spreading as well, facing challenges such as power density and dynamic load operation.Advancement in the SOC technology is opting at substituting ceramic materials with mechanical support function, in favor of metal. A lower amount of ceramic materials not only reduces the raw material costs, but also allows rapid thermal cycling without compromising the mechanical integrity of the cells – hence allowing faster start-ups and load regulations. Furthermore, our recent studies demonstrated a scalable and innovative layer integration process that allows to fabricate complex 3D multilayer structures with internal gas channels and cavities of different geometries, using conventional tape casting. The monolithic SOC stacks rising from this process, combined with the integration of metallic parts in the great majority of their volume, may achieve groundbreaking power densities and the dynamic load operation desired for the mentioned applications, while decreasing CapEX and resources necessary to produce high performing SOCs of the future.The monolithic SOC stacks obtained within this work are made of three standard repetitive units (SRUs). Each SRU consists of a SOC backbone, composed by two porous zirconia based electrode backbones to be infiltrated with appropriate electro-catalysts, a dense thin zirconia based electrolyte, two sets of ferritic steel gas channel structures – one at the fuel side and one at the oxygen side – conferring enhanced gas diffusion to the triple phase boundaries, mechanical support and electronic conductivity and composite metal/ceramic interconnect plates connecting in series the SRUs. The total number of layers co-fired to produce monoliths within this work is 19 and the monoliths are denoted as “MNx3” (nomenclature defined for a monolith stack with 3x SRUs). The cross section is showed in Figure, where scanning electron microscopy is complemented by qualitative energy dispersive x-ray spectroscopy (EDS) to highlight the material composition of the three SRUs. This work presents the challenges and a solid framework for the solutions adopted to fabricate monolithic stacks aimed at hydrogen production, using the conventional tape casting shaping technique integrated with the innovative assembly process developed. Among the challenges, focus is given to i) the shaping of internal gas channels, ii) structural integrity during fabrication and operation and iii) the use of non-critical raw materials. Detailed electrochemical characterization is currently in progress and the results will be presented at the conference. Besides the real initial performance of the stacks at this development stage, it is speculated that SRU cells performance could get close to the ones of state of the art metal supported cells, also relying on electrocatalyst infiltration. Figure 1

A novel system for the production of pure hydrogen from natural gas based on solid oxide fuel cell–solid oxide electrolyzer

To improve solid oxide electrolysis cell (SOEC) system designs and fabrication quality, endurance testing of SOEC systems provides a proving ground on which real-world usage can be simulated, and quality assurance issues can be identified early on. A test facility at Idaho National Laboratory (INL) was designed, built, and operated to facilitate endurance testing of a reversible system by Oxeon Energy. Designated Emerald, the test facility is currently the only test facility at INL designed to allow both SOEC and solid oxide fuel cell (SOFC) testing of the same module. To safely operate a reversible SOEC test facility, several safety features are required, and a hazard and operability analysis is of Emerald is discussed here. Design challenges due to differences in the endothermic SOEC and exothermic SOFC reactions were addressed in the process recuperators. Controls for the Emerald test facility were built in an open-source JavaScript runtime environment, Node.js and executed on a Linux-based industrial controller. The Oxeon Energy module has been successfully brought up to SOEC ready conditions. Initial operations of the Emerald test facility are being improved to eventually achieve non-stop operations and thousands of hours of testing of both SOEC and SOFC.

Electrochemical performance and long-term durability of a 200 W-class solid oxide regenerative fuel cell stack

Introduction Reversible solid oxide cells (r-SOCs) are attractive electrochemical energy devices that can operate as both solid oxide fuel cells (SOFCs) for power generation, and solid oxide electrolysis cells (SOECs) for steam electrolysis (1). Renewable power generation, such as wind and solar, depends on daily weather conditions and so that it is desirable to adjust the supply of fluctuating electricity to match demand using reversible energy devices such as r-SOCs. The operational concept of r-SOCs is schematically described in Figure 1. The aim of this study is to examine the electrochemical properties and durability of r-SOCs using alternative fuel electrode materials. Experimental Various r-SOCs were prepared using conventional and alternative fuel electrode materials. As the conventional materials, Ni-cermet with Sc2O3-doped ZrO2 (ScSZ) was used for the fuel electrode; dense 200 mm thick ScSZ membrane for the electrolyte; and La0.6Sr0.4Co0.2Fe0.8O0.3 oxide (LSCF) air electrode with Gd-doped CeO2 (Gd0.1Ce0.9O2) acting as a buffer layer between the ScSZ electrolyte and the LSCF air electrode. As alternative fuel electrode materials, GDC and/or La0.1Sr0.9TiO3 (LST) were used as the fuel electrode backbone. By applying the co-impregnation procedure of Ni and GDC, fuel electrodes with highly dispersed nanoparticles of Ni catalysts and GDC, denoted as Ni-GDC/LST-GDC, were prepared (2,3). A Pt-based reference electrode was deposited beside the air electrode, such that the fuel electrode potential was measured as the voltage between the fuel and reference electrodes.Current-voltage characteristics were characterized, including voltage cycle tests performed by switching the direction of current simulating r-SOC operation. Long-term durability tests in SOEC and SOFC modes were conducted at 800°C. Air was supplied to the air electrode, while 50%-humidified hydrogen gas was supplied to the fuel electrode. The current density in the SOFC and SOEC modes is denoted as positive and negative values, respectively. Results and discussion Figure 2 shows fuel electrode potential as a function of current density in both SOFC and SOEC modes. The cells with the Ni-zirconia cermet, GDC, LST, LST-GDC, and Ni-GDC/LST-GDC could be operated in both modes. In the SOFC mode, the conventional Ni-zirconia cermet exhibited the highest fuel electrode voltage among these materials studied. In contrast, in the SOEC mode, the GDC-containing materials exhibited lower (i.e. better) fuel electrode voltage compared to Ni-zirconia cermet even without Ni catalysts, indicating enhanced electrocatalytic activity of mixed-conducting GDC for the electrode reactions with water vapor at the fuel electrodes. Voltage cycle durability up to 1,000 cycles and long-term durability up to 1,000 hours could be measured. Latest results on the durability with various electrode materials will be presented. Acknowledgements A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Project No. JPNP20005). References Q. Minh and M. B. Mogensen, Electrochem. Soc. Interface, 22, 55 (2013).Futamura, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, J. Electrochem. Soc., 164 (10), F3055 (2017).Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, International J. Hydrogen Energy, 44 (16), 8502 (2019). Figure 1

Hydrogen energy has been hailed as a versatile energy of the future that could aid the transition to net-zero emissions. It has large potential as an alternate fuel source for mobility and power, heating and feedstock for the industry. The energy carrier is produced from water electrolysis technology using different types of electrolyzer (i.e., Proton Exchange Membrane (PEM) or Solid Oxide Electrolysis Cells (SOECs)) for hydrogen separation process. In this energy research journey, PETRONAS has jointly worked with SLB to evaluate the feasibility of using SOECs for green hydrogen production. The study's focus is to explore integrating SOECs into existing industrial facilities creating hybrid systems, specifically with available flue gas from combustion process e.g. power generation. This allows for the utilization of waste heat from existing processes to enhance overall energy efficiency in hydrogen production. To understand hydrogen production and its utilization in the existing facility, the full system including existing gas turbines, solid oxide electrolyzer cells (SOECs), a hydrogen compression train, and a temperate cooling water system were modeled in a process simulation software. The model also included a heat recovery steam generator (HRSG) to recover the energy from gas turbine exhaust gases to heat water feed to produce saturated steam which will be the feed to SOEC. Several configurations considering heat integrations were explored, including heat recovery from SOEC product stream. Various sensitivities were run for different configurations of the process to study hydrogen purity, hydrogen blending, overall power required, water required in the overall process, and power produced by the hydrogen generated from SOEC and CO2 emission values in all the configuration. The results were compared and evaluated to help assess how variation in parameters affect the performance or outcome. The results were evaluated for blending the produced hydrogen with the existing export gas before and after export gas compression and how it influences the hydrogen purity. Hydrogen blending with the fuel gas to the gas turbine also helped in overall reduction of CO2 emission. It was observed that additional cooling of the hydrogen stream to achieve better hydrogen purity did not significantly help in reducing CO2 emissions when hydrogen is used as fuel gas; therefore, cooling with temperate water system can be considered. All the cases were compared and evaluated for expanding the application of green hydrogen technologies in the facility considering viability and reduction in emissions. The study was a part of a collaborative research initiative by the industry players to accelerate the development and deployment of SOEC technologies in existing facilities. The results from the study provided an overall understanding of adding SOEC in the facility and valuable insights to the stakeholders to make well-informed choices on the configuration.

Electrolyte degradation in anode supported microtubular yttria stabilized zirconia-based solid oxide steam electrolysis cells at high voltages of operation

Developing safe, reliable, cost-effective, and efficient hydrogen-electricity co-generation systems is an important step in the quest for national energy security and minimized reliance on foreign oil. This project aimed to, through materials research, develop a cost-effective advanced technology cogenerating hydrogen and electricity directly from distributed natural gas and/or coal-derived fuels. This advanced technology was built upon a novel hybrid module composed of solid-oxide fuel-assisted electrolysis cells (SOFECs) and solid-oxide fuel cells (SOFCs), both of which were in planar, anode-supported designs. A SOFEC is an electrochemical device, in which an oxidizable fuel and steam are fed to the anode and cathode, respectively. Steam on the cathode is split into oxygen ions that are transported through an oxygen ion-conducting electrolyte (i.e. YSZ) to oxidize the anode fuel. The dissociated hydrogen and residual steam are exhausted from the SOFEC cathode and then separated by condensation of the steam to produce pure hydrogen. The rationale was that in such an approach fuel provides a chemical potential replacing the external power conventionally used to drive electrolysis cells (i.e. solid oxide electrolysis cells). A SOFC is similar to the SOFEC by replacing cathode steam with air for power generation. To fulfill the cogeneration objective, a hybrid module comprising reversible SOFEC stacks and SOFC stacks was designed that planar SOFECs and SOFCs were manifolded in such a way that the anodes of both the SOFCs and the SOFECs were fed the same fuel, (i.e. natural gas or coal-derived fuel). Hydrogen was produced by SOFECs and electricity was generated by SOFCs within the same hybrid system. A stand-alone 5 kW system comprising three SOFEC-SOFC hybrid modules and three dedicated SOFC stacks, balance-of-plant components (including a tailgas-fired steam generator and tailgas-fired process heaters), and electronic controls was designed, though an overall integrated system assembly was not completed because of limited resources. An inexpensive metallic interconnects fabrication process was developed in-house. BOP components were fabricated and evaluated under the forecasted operating conditions. Proof-of-concept demonstration of cogenerating hydrogen and electricity was performed, and demonstrated SOFEC operational stability over 360 hours with no significant degradation. Cost analysis was performed for providing an economic assessment of the cost of hydrogen production using the targeted hybrid technology, and for guiding future research and development.

Power generation and steam electrolysis characteristics of an electrochemical cell with a zirconia- or ceria-based electrolyte