Three-Dimensionally Micro-Architected Protonic Ceramic Fuel Cells

Ceramic high-temperature fuel cells and electrolysers are efficient energy conversion systems for electrical power generation and hydrogen production. Their core components are constituted by a stack of electroactive Solid Oxide Cells (SOCs) in which the electrochemical reactions take place. Thanks to their flexibility, the same stack can be alternatively operated in both fuel cell and electrolysis modes. However, the insufficient durability of SOCs still constitutes a major limitation for the technology. The present study addresses this issue and aims to bring some new insights on the effect of the Solid Oxide Fuel Cell (SOFC) versus Solid Oxide Electrolysis Cell (SOEC) operating modes on degradation of a typical Ni-YSZ//YSZ//CGO//LSCF-CGO cell. The electrochemical degradations are generally attributed to several underlying phenomena such as electrode microstructural evolution, material chemical decomposition or electroactive sites poisoning by contaminants. Among them, it is generally considered that Ni agglomeration in the Ni-YSZ cermet and Lanthanum Strontium Cobalt Ferrite (LSCF) material destabilization are two prevalent mechanisms involved in the cell performance deterioration. Therefore, these two mechanisms have been specifically investigated by a coupled approach of long-term testing in both SOFC and SOEC modes (1000 ≤ t (h) ≤9000) and post-test characterizations. The experimental results have then been analyzed in the frame of an in-house multi-scale model with the purpose to interpret them and to quantify the effect of material ageing on cell performances. The extent of Ni agglomeration has been characterized by three-dimensional electrode reconstructions obtained by X-ray nano-holotomography at European Synchrotron Radiation Facility (ESRF) (on the new Nano-Imaging beamline ID16A-NI). The new set-up and protocol enable the reconstructions of valuable 3D volumes (Fig. 1) with a large field of view (~50 µm) along with a high spatial resolution (~50 nm). The electrode morphological properties, which have been measured on the 3D volumes, have revealed a substantial Ni coarsening over time at 850°C and 750°C, whereas no Ni depletion was detected at the electrolyte interface. The increase of the Ni particle size is found to induce a decrease in both (i) the density of Triple Phase Boundary (TPBs) lines and (ii) the interfacial surface area between Ni and gas. Moreover, it was found that the Ni/YSZ interfacial surface area does not evolve during the experiments. This statement indicates that the ceramic backbone in the cermet prevents a massive Ni agglomeration at the SOFC/SOEC operating temperature. The compilation of all experimental data have allowed fitting the parameters of a physically-based law for Ni coarsening that was introduced in the modelling framework. The simulations have revealed that Ni agglomeration explains around 20-25% of the electrochemical degradation at 850°C after 1000 hrs of operation. However, the electrode microstructural evolution is found not to be affected by the cell polarizations. Therefore, the mechanism cannot explain the higher degradation rates recorded in electrolysis mode compared to the fuel cell ones. To explain the impact of the operating modes (SOFC or SOEC) on the degradation rates, several post-test analyses (i.e. Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray µfluorescence and µdiffraction techniques) have been employed to investigate the phase reactivity in the region of the CGO barrier layer. The characterizations have revealed that Sr diffusion across the barrier layer and formation of SrZrO3 secondary phase occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode (Fig. 2). As a consequence, LSCF destabilization is found not to be involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. The post-test analyses have also revealed a diffusion and an accumulation of Co in the region of the barrier layer which is concomitant with the formation of SrZrO3. The formation of these Co-rich segregates in contact with SrZrO3 grains have been identified as cobalt-ferrite type compound. The in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF destabilization mechanism. The cell polarization curves and the local quantities within the O2 electrode have been computed in both fuel cell and electrolysis modes. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in the LSCF material. It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation has been detailed. Figure 1

Protonic ceramic fuel cells (PCFCs) have high energy conversion efficiencies and can be employed as next‐generation fuel cells. However, novel cathode materials with high electrochemical performances at low operating temperatures are required to practically apply PCFCs. BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3– δ (BCFZrY) is a promising material for PCFC cathodes owing to its low electrode polarization resistance ( R p ). Herein, instead of Zr doping, BaCo 0.4 Fe 0.4 A 0.1 Y 0.1 O 3– δ (BCF A Y) compounds, where A represents Ti, Mg, or Zn, were evaluated as PCFC cathode materials. BCF A Y ( A : Ti, Mg, or Zn) compounds have a higher oxygen permeability than BCFZrY. The electrochemical properties and power generation performance of the PCFCs with BCF A Y cathodes were measured and compared with those of PCFCs with BCFZrY cathodes. The PCFC with a BCFMgY cathode exhibited the lowest R p , leading to the highest power density, that is, 0.80 W cm −2 at 600°C. In addition, for all PCFCs with BCFY‐based cathodes, the reciprocal area‐specific resistances of R p and oxygen permeability fluxes of the cathode materials had an approximately proportional relationship. Therefore, it was confirmed that PCFC cathodes with a high oxygen permeability had a low cathode polarization resistance, which significantly enhanced the power generation performance of PCFCs.

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.

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

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

Carbon-neutral fuel fed solid oxide fuel cell (SOFC) system can achieve highly efficient power generation without any greenhouse gas emissions. In this paper, the influence of supplied fuel species on the SOFC performance has been evaluated by heat and mass balance analysis. The electrical efficiency of carbon-neutral fuel fed SOFCs is quantified and compared with that of conventional fuel fed SOFCs. Then, the effect of electrical efficiency increase on CO2 emission is also investigated. Furthermore, the effect of proton-conducting ceramic materials application as an electrolyte is also discussed to evaluate the influence on the electrical efficiency. The carbon-neutral fuel fed protonic ceramic fuel cells (PCFCs) had an advantage to enhance the electrical efficiency compared with the same fuels fed typical SOFCs. Due to the increase in electrical efficiency, the reduction in CO2 emission was clearly appeared as CO2 emission factor.

High temperature electrolysis is a promising option for carbon-free hydrogen production and huge energy storage with high energy conversion efficiencies from renewable and nuclear resources. Over the past few decades, yttria-stabilized zirconia (YSZ) based ion conductor has been widely used as a solid electrolyte in solid oxide electrolysis cells (SOECs). However, its high operation temperature and lower conductivity in the appropriate temperature range for solid electrochemical devices were major drawbacks. Regarding improving ionic-conducting electrolytes, several groups have contributed significantly to developing and applying LaGaO3 based perovskite as a superior ionic conductor. La(Sr)Ga(Mg)O3 (LSGM) electrolyte was successfully validated for intermediate-temperature solid oxide fuel cells (SOFCs) but was rarely conducted on SOECs for its high efficient electrolysis performance. Their lower mechanical strengths or higher reactivity with electrode compared with the YSZ electrolysis cells, which make it difficult to choose compatible materials, remain major challenges. In this field, SOECs have attracted a great attention in the last few years, as they offer significant power and higher efficiencies compared to conventional YSZ based electrolysers. Herein, SOECs using LSGM based electrolyte, their applications, high performance, and their issues will be reviewed.

An advanced bifunctional single-element-incorporated ternary perovskite cathode for next-generation fuel cells

Solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs) are promising energy conversion devices, on whose basis green hydrogen energy technologies can be developed to support the transition to a carbon-free future. As compared with oxygen-conducting cells, the operational temperatures of protonic ceramic fuel cells (PCFCs) and electrolysis cells (PCECs) can be reduced by several hundreds of degrees (down to low- and intermediate-temperature ranges of 400–700 °C) while maintaining high performance and efficiency. This is due to the distinctive characteristics of charge carriers for proton-conducting electrolytes. However, despite achieving outstanding lab-scale performance, the prospects for industrial scaling of PCFCs and PCECs remain hazy, at least in the near future, in contrast to commercially available SOFCs and SOECs. In this review, we reveal the reasons for the delayed technological development, which need to be addressed in order to transfer fundamental findings into industrial processes. Possible solutions to the identified problems are also highlighted.

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.

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.

The nickel(Ni)-patterned electrode enables researchers to quantify the triple-phase boundary (TPB) length and Ni surface area, exclude the influence of bulk gas diffusion and clearly separate the active regions of the chemical/electrochemical reactions. This tool has been studied for identifying the reaction mechanism in SOFC but rarely in SOEC. In this study, the Ni-patterned electrodes with the stripe width of 100 μm were tested in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H2O/H2 and further H2O/CO2/H2/CO. The experimental test shows the stability of the Ni-patterned electrode in the H2O/H2 atmosphere was much poorer than that in the CO/CO2 atmosphere due to the nickel reacting with steam. Thus, the Ni-patterned electrode needed to operate at a temperature of ≤700oC with the inlet H2/H2O molar ratio of >7 and the current-applied time shouldn’t be over 5 h to keep the accuracy of the TPB length and Ni surface area. The effects of the temperature, partial pressure of H2O and H2 are investigated. The activation energy of the Ni-patterned electrodes was 0.386 eV. The electrochemical performance had a positive correlation with both the partial pressure of H2 and H2O, especially showed higher sensitivity to the partial pressure of H2O. Further, a possible mechanism of the H2O/H2 electrochemical conversion was proposed according to the experimental data and existing literature, which contains two-step charge-transfer reaction: H(Ni)+O2-(YSZ)OH-(YSZ)+(Ni)+e- and H(Ni)+OH-(YSZ)H2O(YSZ)+(Ni)+e-. Based on the experiments, an analytical calculation was performed to investigate the rate-determining steps in the Ni-patterned electrodes by bridging the connection between the kinetic parameters and operating conditions. The results indicate the rate-limiting steps may be different for the SOFC and SOEC modes. In SOFC mode, the H2 electrochemical oxidation could depend more on the charge-transfer reaction H(Ni)+O2-(YSZ)→OH-(YSZ)+(Ni)+e-, however in SOEC mode, the rate-determining step of the H2O electrochemical reduction should be H2O(YSZ)+(Ni)+e-→OH-(YSZ)+H(Ni). In this way, a reversible mechanism could be applied for reversible SOFC (RSOFC) to describe the conversion between hydrogen and steam by different charge-transfer-reaction domination in the SOFC and SOEC mode. Figure 1

Lowering the operating temperature of high-temperature fuel cells has historically been motivated by stack and system cost reduction and durability considerations towards making fuel cells a more attractive distributed generation technology. Protonic ceramic fuel cells (PCFCs) operating beneath 600oC have emerged as a promising technology candidate for developing power generation systems. In this paper, we present an economic manufacturing model that was developed to estimate the overall expected cost of a PCFC stack and compare the results with solid oxide fuel cell (SOFC) technology for hydrogen and methane fuels. The costs of PCFC stacks operating on methane at 500oC (0.155 W/cm2) and 550oC (0.240 W/cm2) are estimated to be 4% to 38% lower, respectively, than SOFC stacks operating at 800oC (~0.300 W/cm2).

Ammonia has received considerable attention as a promising carbon-free hydrogen carrier. At temperatures above 400 °C, NH3 is thermodynamically unstable with respect to decomposition into nitrogen and hydrogen and is, thus, suitable for direct use in solid oxide fuel cells (SOFCs) without external reforming. However, poor catalytic activity for ammonia decomposition at the moderate temperatures of protonic ceramic fuel cell (PCFC) operation has resulted in low fuel cell power output relative to operation on hydrogen and likely contributes to reported cell degradation. Here we prepared cells based on a thermodynamically robust electrolyte, a high activity cathode, and an anode with a distinctive structure to overcome challenges of poor activity and stability. The cells delivered peak power densities of 0.59 and 0.44 W cm–2 under H2 and NH3, respectively, at 500 °C, excellent stability over a period of 200 h, and no detectable NOx in the anode exhaust gas.