Metal-supported Cells Research Articles

(Invited) Dynamic Operation of Solid Oxide Electrolyzers

Electrolysis of steam to generate hydrogen is expected to be a highly dynamic operation if renewable resources such as wind and solar are used as the electrical power input. Rapid fluctuation of input power and intermittent operation of the solid oxide electrolysis stack will result in transient operating conditions at the cell level. All operating variables such as temperature, steam content, voltage, flow rate, etc. may change rapidly in these conditions.To address this situation, this work operates Ni/YSZ-supported and metal-supported cells under highly dynamic conditions, with individual variables isolated. The cells are remarkably robust to dynamic operation. A safe range is found for each operating variable, with accelerated degradation observed outside that range in certain cases. For example, cycling Ni/YSZ-supported cells between 1.3 V and 1.6V at 750°C and 50:50 H2O:H2 is tolerated well, but increasing the upper voltage to 1.7 V and higher leads to increased degradation. Additionally, Steam:hydrogen ratio is cycled between 3:97 and 75:25. Temperature is cycled between 150 and 800°C. Operation is cycled between fuel cell and electrolysis modes, with synchronized steam content. Metal-supported cells tolerate steam cycling, voltage cycling, temperature cycling and redox cycling, and results for button cells and 50 cm2 planar cells will be reported. In many cases, hold times are 1 minute, allowing thousands of cycles to be accumulated over hundreds of hours of operation.Figure 1. Dynamic cycling of a Ni/YSZ-supported button cell at 750C. (Top) Cycling between 1.3 and 1.7V with 1 min holds and 50/50 H2/H2O. (Bottom) Cycling between various steam contents at 1.3 V. Figure 1

Self-assembly and low-temperature sintered La0.6Sr0.4CoO3-δ @ Gd0.1Ce0.9O2-δ composite cathode with rich electrode/electrolyte interface and enhanced performance in harsh condition

In order to improve cost competitiveness and the further application of metal-supported cells, it is important to explore cathodes suitable for low-temperature sintering. However, decreasing cathode sintering temperature while maintaining robust interface of cathode/electrolyte and decent electrochemical performances is challenging for solid oxide fuel cells. Herein, by combining the Pechini method with one-pot method, La0.6Sr0.4CoO3-δ @ Gd0.1Ce0.9O2-δ (LSC@GDC) composite cathodes are prepared and investigated, compared with mechanically mixed LSC-GDC cathodes. At the cathode sintering temperature of 800 °C, the LSC@GDC cathode single cells perform better performances, with deceased resistances, improved power density outputs and better feasibility to wide oxygen partial pressures from 0.21 atm down to 0.03 atm. The optimum composition of 70:30 wt% for LSC@GDC is proposed. The anode supported single cell with LSC@GDC30 cathode exhibits a maximum power density of 0.789 W/cm2 and 0.545 W/cm2 at 750 °C, under cathode oxygen partial pressure of 0.21 atm and 0.03 atm, respectively. Moreover, the low temperature sintered LSC@GDC50 cathode performs decent durability during repeatedly controlled thermal cycling over 10 times.

Performance optimization of metal-supported solid oxide fuel cells using cathode and full cell impregnation with La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ electrode.

In this study, precursor solutions of La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN) symmetric electrode were prepared, and the applications of cathode impregnation and full cell impregnation in the preparation and performance optimization of four-layer metal-supported solid oxide fuel cells (MSCs) were thoroughly investigated. Test results indicate that the polarization impedance of cathode impregnated MSCs under H2 and CH4 atmospheres at 750 °C is approximately 0.1 Ω cm2 and 0.41 Ω cm2, respectively, with power densities of 1115 mW cm-2 and 700 mW cm-2, respectively. Meanwhile, the polarization impedance of full cell impregnated MSCs under the same conditions is 0.12 Ω cm2 and 0.40 Ω cm2, with power densities of 945 mW cm-2 and 840 mW cm-2, respectively. Remarkably, MSCs full cell impregnated LSCFN exhibit outstanding stability performance under CH4 atmosphere in a 100 h stability test. Research on the application of impregnation method for performance optimization of metal-supported cells is relatively scarce. The results reveal the feasibility of simplifying MSCs preparation steps using full cell impregnation method, further promoting the widespread application of metal-supported overall cells.

Machine learning based analysis of metal support co-sintering process for solid oxide fuel cells

Porous metals are promising substrate supports of solid oxide fuel cell for the applications of mobile and high-power density. The process conditions for a suitable mixture of pore former and stainless-steel particles, and also for the co-sintering with oxide layers of SOFC are investigated by machine learning (ML) methods. Half-cell metal supported cell (MSC) process with ScSZ electrolyte and NiO-ScSZ anode layer coated on the metal substrate of SUS430 was predicted by two ML methods of random forest (RF) and neural network (NN) were investigated. RF of multi output methods predicted the target variables, such as shrinkage, bend and pore structures with the better accuracy compared to NN, revealing that not only sintering temperature but also pore former and degreasing temperature of metal support influenced the process by feature importance analysis. The power generation performance of MSC tested from 550 °C to750 °C, and the relation to the microstructure of MSC is discussed.

Recycling and Reuse Strategies for Ceramic Components of Solid Oxide Cells

Fuel Cell and Hydrogen (FCH) applications will become crucial to enable the transition towards decarbonatization and meet the EU's zero net greenhouse gas emission targets to be achieved by 2050 (The European Green Deal, European Commission, 2019). As one part of novel FCH technologies, Solid Oxide Cells (SOCs) can be used as fuel cells and electrolyzers, enabling a fuel-flexible and adaptable range of applications.However, the Technology Readiness Level (TRL) of SOCs is currently assessed at 5–7 (H2-international, October 2022), which is lower compared to most of the technologies mentioned above. In order to achieve their market breakthrough, SOCs require scalable and cost-efficient manufacturing trails. This involves an adequate End-of-Life (EoL) material treatment, minimizing environmental impact, and avoiding landfill disposals. EoL strategies for FCH applications (including the SOC) are currently in the early stages and have not been adequately addressed. Until now, existing novel technologies and their materials are reviewed based on hazardousness, scarcity and cost. Initial considerations directly for SOC material recovery are given in two very recent publications. In these two studies, the focus was on the ceramic cell part of an SOC, aiming for the recovery of the most valuable cell fractions in a (semi-) closed loop scenario.Challenges in cell recycling arise from the diversity of structures and materials of established stack and cell designs. For industrial applications, planar stack geometry is likely to prevail, further subdivided based on the mechanical support used (fuel electrode-supported cells, FESC; electrolyte-supported cells, ESCs; metal-supported cells, MSCs). As a part of the German government-funded technology platform “H2Giga”, we are working on the re-integration of EoL FESC-type SOCs into the cell manufacturing process.The concept for FESC-recycling (Figure 1.) is based on the separation of the air-side perovskite materials (air-side electrode and contact layer) from the remaining predominant cell fraction (mechanical support, fuel electrode, electrolyte, and diffusion barrier layer).[1] Separation can be achieved by exploiting the chemical resistance of NiO and YSZ to suitable leachants such as hydrochloric acid or nitric acid. In comparison, the structure of the conventional perovskites used is more vulnerable to acid corrosion. The remaining solid fraction then undergoes a re-dispersion step and is incorporated into newly manufactured substrate. The recycled substrate is characterized in terms of electrical conductivity, mechanical stability, and microstructure. Critical components (Co, La) in the separated perovskite liquid fraction are to be recovered from the solution by precipitation.The presentation will guide the audience through the concept of multi-step recovery of the predominant cell fraction Ni(O)/YSZ, and will provide insides of the experimental results, ranging from the hydrometallurgical separation of cell fractions to suitable reprocessing techniques.[1] Sarner, S., Schreiber, A., Menzler, N. H., & Guillon, O. (2022). Recycling Strategies for Solid Oxide Cells. Advanced Energy Materials, 12(35), 2201805. Figure 1

Flatness Enhancement of Metal-Supported Solid Oxide Fuel Cells with Additional Compensation Layer

Solid oxide fuel cells are high-temperature fuel cells which offer several advantages: high efficiency, fuel flexibility, high-quality waste heat, and the use of non-noble metal catalysts. Metal-supported solid oxide fuel cells, in which the ceramic mechanical support layer is replaced with the metal support, are considered as next-generation solid oxide fuel cells because of their enhanced mechanical/thermal ruggedness compared to conventional ceramic-supported solid oxide fuel cells. However, different thermomechanical behavior of metal support and ceramic layers during sintering makes metal-supported cells vulnerable to warping. Cell warpage must be minimized because it reduces not only uniformity of wet-chemical coating, but also actual contact area between electrode and interconnect, increasing the contact resistance.In this study, causes of metal-supported cell warpage are identified, and the design that can minimize cell warpage is suggested for successful scale-up of metal-supported solid oxide fuel cells. Through thermomechanical and residual stress analyses, it is found that residual stress derived from coefficient of thermal expansion mismatch between electrode layers and the metal substrate is the main cause of the warpage of metal-supported solid oxide fuel cells. Based on these results, a novel design that has thicker metal protection layer on the opposite side of the electrolyte is proposed for minimizing cell warpage due to residual stress. Through the design modification, vertical deformation of 2-inch metal-supported solid oxide fuel cell can be successfully controlled to 17.36% of the previous design. Furthermore, electrochemical evaluation showed a 21% decrease in ohmic ASR and a 25% increase in peak power density after the design modification, implicating reduction of contact resistance as a result of increased contact area by enhanced flatness.

Low Temperature Performance and Durability of Solid Oxide Fuel Cells with Titanate Based Fuel Electrodes Using Reformate Fuel

The Ni/YSZ composite electrode is conventionally used for solid oxide cells, in electrolysis (SOEC) as well as fuel cell (SOFC) operation. For enhanced electrochemical performance at low temperature, mechanical durability, and impurity tolerance, alternative fuel electrode materials and cell configurations are required. In this paper we have studied a metal supported cell (MSC) with a titanate based fuel electrode (La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3, LSFNT) for its fuel cell performance using carbon containing fuel and compared to a state of the art (SoA) fuel electrode supported cell with a Ni/YSZ fuel electrode. In hydrogen fuel, the cells showed similar performance at intermediate and low temperatures (750 °C to 650 °C), although the ASR is slightly higher for the MSC at all temperatures and steam/hydrogen ratios. Additionally, the MSC showed fair initial performance in reformate type fuel compositions (CO/steam and CO/steam/hydrogen), i.e. the fuel electrode possesses activity for the water gas shift reaction, which opens the possibility to use such cells with hydrocarbon fuels after a pre-reformer. Durability testing in pre-reformed fuel gas revealed that further fuel electrode tailoring is required to minimize cell degradation in carbon containing fuels.

Study on the cell configurations and mechanical strength of planar-type solid oxide fuel cells

For solid oxide fuel cells, the following cell configurations have been developed: electrolyte-supported cell (ESC), anode-supported cell (ASC), and metal-supported cell (MSC). In this study, to compare the mechanical load tolerance corresponding to each of the abovementioned cell configurations, the curvature and residual stress were calculated using the Euler–Bernoulli beam theory. The results indicated that when the cells were flattened under the application of an external bending moment, the stress gradient in each layer of the cell disappeared, and the maximum stress in the cell was shown to decrease. Eventually, the failure probability of the cell substrate was demonstrated to take the smallest value. The allowable bending moments for the cell substrates were as follows: −0.17 to 0.08 Nm for ESC, −0.18 to 0.11 Nm for ASC, and −0.29 to 0.40 Nm for MSC at room temperature. Thus, MSC was found to possess the highest bending moment tolerance.

(Plenary) U.S. DOE EERE Activities in High Temperature Electrolysis and Reversible Fuel Cells

The Hydrogen and Fuel Cell Technologies Office (HFTO) within the U.S. Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy (EERE) supports a range of solid oxide-based research, development, and demonstration efforts that fall under the areas of high temperature electrolysis (HTE) and reversible fuel cells (RFCs). These efforts range from earlier stage R&D on cell components to demonstrating early system technology. Particularly when used in conjunction with high temperature process heat, HTE technology shows promise for highly efficient, cost effective, sustainable hydrogen production at high volumes. This potential has led to its inclusion in multiple HFTO initiatives, including two national lab-led consortia. HTE is one of four pathways being supported under DOE’s HydroGEN Energy Materials Network (EMN) Consortium on Advanced Water Splitting Materials (AWSM) for H2 production. The HydroGEN EMN offers an extensive collection of materials research capabilities at 5 core national laboratories for addressing AWSM R&D challenges in efficiency, durability, and cost. HydroGEN focuses on next generation materials including proton conducting ceramics and metal-supported cell architecture for HTE. H2NEW (H2 from Next-generation of Electrolyzers of Water) is a recently launched national lab consortium focused on overcoming technical barriers to enable affordable, durable, and efficient low- and high-temperature electrolyzers to achieve <$2/kg hydrogen production. Unlike HydroGEN, the emphasis is not on new materials development but on addressing components, materials integration, and manufacturing R&D. Durability is the most critical, initial focus of the H2NEW HTE effort.HTE also has a role in DOE’s H2@Scale energy system vision. This initiative is bringing together diverse stakeholders to advance affordable wide-scale hydrogen production, transport, storage, and utilization to unlock revenue potential and value across multiple sectors and applications. The use of low-cost electricity to affordably split water into hydrogen and oxygen is central to implementation of the H2@Scale concept. Reversible fuel cells also have a role in H2@Scale as well as DOE’s Energy Storage Grand Challenge as a promising technology for long duration energy storage. RFC stack and system technical targets to help guide ongoing and future R&D efforts were recently developed. In addition to an overview of these initiatives including the role solid oxide technologies will play, highlights of HFTO-supported R&D and analysis work from the cell to system level for both HTE and RFCs will be provided.

Performance of Metal Supported SOFCs Operated in Hydrocarbon Fuels and at Low (

The Ni/YSZ composite electrode is conventionally used for solid oxide cells, in electrolysis (SOEC) as well as fuel cell (SOFC) operation. For enhanced durability and C-tolerance, alternative fuel electrode materials are needed. In this paper, we compare the performance of two distinct Ni:CGO electrocatalyst coated A-site deficient lanthanum doped strontium titanate (La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3, LSFNT) based anodes, integrated into metal supported cells (MSCs), to the SoA Ni/YSZ anode supported cell in fuel cell mode for the first time. The three cells were investigated electrochemically by impedance spectroscopy (EIS) and performance under applied current (iV-curves) at temperatures 750˚C, 700˚C, 650˚C, and at novel 620˚C in steam/hydrogen and methane/steam. Additionally, one of the MSCs was investigated in CO/steam. Furthermore, galvanostatic durability tests were conducted in 4% steam/hydrogen for the MS-cells at low temperature (650˚C) at medium-high fuel utilization (50%).

(Invited) Metal-Supported Solid Oxide Fuel Cells and Electrolyzers for Low-Cost, Robust, Rapid-Start Systems

This talk will provide an overview of performance, durability, and applications of metal-supported solid oxide fuel cell and electrolysis cell technology developed at Lawrence Berkeley National Laboratory (LBNL). The unique LBNL symmetric cell architecture design, with thin zirconia ceramic backbones and electrolyte sandwiched between porous metal supports, offers a number of advantages over conventional all-ceramic cells, including low-cost structural materials (e.g. stainless steel), mechanical ruggedness, excellent tolerance to redox cycling, and extremely fast start-up capability.MS-SOFC performance with a variety of fuels will be presented, including hydrogen, ethanol, natural gas, and simulated reformates. The impact of the presence of carbon and internal reforming catalysts on the durability of the cells will be examined. In particular, oxidation of the stainless steel supports in the presence of carbon is analyzed. Scale-up from button cells to 50cm2 will be presented.The case for use of MS-SOECs for dynamic electrolysis operation will be made. Detailed analysis of degradation modes over 1000h of operation with 50/50 steam/H2 informed efforts to improve durability through the use of coatings and catalyst processing techniques.In addition to our long-standing development of zirconia-based metal-supported cells, recent efforts on metal-supported proton-conducting cells are ongoing. Co-sintering of 400-series stainless steel and BZCY-type proton conductors presents a number of challenges, including: Ba evaporation in reducing atmosphere, over-densification of steel at the high temperatures (>1400°C) required for BZCY processing, mismatch of metal and ceramic sintering behavior, and migration of Cr and Si from the steel support into the BZCY layers. Feasibility of new approaches to overcome these challenges will be discussed.

High Temperature Elastic Modulus of Proton Conducting Ceramics Y-Doped Ba(Zr,Ce)O3

Proton conducting ceramics (PCC) cells are promising energy conversion devices that enable high efficiency energy conversion at lower temperature range, solving the challenge of conventional solid oxide cells (SOCs) due to the high operating temperature. Electrochemical performance and chemical stability of PCC electrolyte has been investigated in recent studies, suggesting that rare-earth doped Ba(Zr,Ce)O3 perovskite-type ceramics are optimal materials exhibiting high proton conductivity and chemical stability during operations. On the contrary, mechanical stability of these PCC electrolyte materials has not been evaluated despite the fact that the mechanical properties are critically important for achieving long-term stable operation as fuel cells or electrolyser cells. For the development of conventional SOCs, mechanical stability during high temperature operation was one of the most significant challenges to deal with, which was attained as a result of detailed studies on in-situ elastic properties of composing materials such as oxygen ion conducting electrolytes and residual stresses. Similarly, for PCC cells, mechanical properties of cells and composing materials have been of significant interest in order to achieve mechanically stable long-term operation, even though PCC cells operate at lower temperature than SOCs. Furthermore, the metal-supported (MS) structure which provides superior mechanical robustness compared to anode-supported (AS) structure is expected to be applied effectively to PCC cells, which are called proton conducting ceramics – metal-supported cells (PCC-MSCs), leading to greater necessity of the mechanical evaluation of the cells and composing materials.Electrolyte is the most crucial component in an electrochemical cell and must be mechanically stable because ion transport and gas tightness made by electrolyte determines electrical performance. However, there has been important concern that larger thermal stresses might be introduced in PCC cells compared to SOCs, resulting from the thermal expansion coefficient (TEC) mismatch between the electrode and electrolyte and from the chemical expansion by the hydration that occurs in a certain temperature range. The PCC electrolyte is highly in need of investigation on in-situ mechanical properties, especially on elastic properties. In this study, elastic properties of electrochemically promising PCC, Y-doped Ba(Zr,Ce)O3 perovskite-type ceramics, were investigated under high temperature conditions. Elastic moduli such as Young’s modulus and Poisson’s ratio were measured by the method that we previously developed for elastic investigation in high temperature conditions using ultrasonic waves. This method enables highly accurate and repetitive examination of elastic properties at high temperatures in materials with poor sinterability including PCC by measuring ultrasonic sound velocities in pellets typically fabricated for electrochemical tests.Pellets of BaZr1-xYxO3-δ (BZY) with different concentrations of doped yttrium, BaZr0.9Y0.1O3-δ (BZY10), BaZr0.85Y0.15O3-δ (BZY15), and BaZr0.8Y0.2O3-δ (BZY20), were fabricated. Additionally, pellets of BaZryCe1-yY0.1O3-δ (BZCY) with different ratio of Ce to Zr, BaZr0.7Ce0.2Y0.1O3-δ (BZCY721) and BaZr0.8Ce0.1Y0.1O3-δ (BZCY811) were fabricated. Powders of PCCs above were consolidated to be thick rounded shape and sintered in air. Each prepared sample was set in an electric furnace in laboratory air atmosphere and sound velocities were measured with the sample slowly heated up to 700 °C and subsequently cooled down to room temperature to calculate elastic moduli at each measuring point. In the first series of heating and cooling measurements for as-sintered samples, hysteresis on elastic moduli in intermediate temperature range was observed. We repeatedly conducted a series of heating and cooling measurements several times, and then the hysteresis was not observed any further. Fig.1 shows final state Young’s modulus of BZCY721, BZCY811, and BZY10 (BZCY901) without hysteresis.Elastic moduli at room temperature have not changed through multiple heating and cooling measurements, and crystal structures and lattice parameters were also confirmed to remain constant by x-ray diffraction (XRD) analysis. The hysteresis found in a specific temperature range suggests that elastic moduli were influenced presumably by a change in defect structure of PCC caused by hydration or defect association of oxygen vacancies and dopants. At room temperature, Young’s modulus decreased with the increment of Ce concentration by 16 % from BZY10 to BZCY721. When materials have the same crystalline structure, Young’s modulus generally decreases as mean atomic volume of the base crystal increases. Because BaCeO3 has larger mean atomic volume than BaZrO3, this observation is qualitatively reasonable. However, in high temperatures, the difference became significant only for BZCY721, Young’s modulus decreased by 30 % from that at room temperature in BZCY721. These results suggest that Ce substitution causes different high temperature dependences. Figure 1

Development of Various Solid Oxide Cell Technology and Stack Platforms at Nexceris, LCC

With over 25 years of experience in solid oxide fuel cell (SOFC) materials, components and stack development, Nexceris has been rapidly expanding the technology portfolio. Development in Fuel Cell Materials, FlexCell and ChromLok technology are only a few examples. Stack testing capability up to 5 kW has been developed. Modular high-performance stack platforms that accommodate cells from 40 cm2 to 250 cm2 are being utilized, depending on customer requirements and system needs.Recently, Nexceris expanded its R&D efforts into solid oxide electrolysis cell (SOEC) technology. Initial performance of -1.5 A/cm2 and 500-hour durability tests at 1.3-1.4 V and 800 °C in short stacks show promising platform for further SOEC performance and durability improvements. Besides anode- and electrolyte- supported cell development, metal-supported cells (MSCs) have also been successfully developed at Nexceris and scaled to 250 cm2. The structure, performance and durability of MSCs are being continuously improved. Short 5-cell stacks are currently being demonstrated and the results will guide further stack development. A wide range of cell and stack technology developed at Nexceris provides capability to tailor cells and stacks based on user applications, whether those include continuous non-interrupted stationary operation or dynamic operation such as rapid thermal and redox cycling in mobile applications.

The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes

Detailed insight into electrochemical reaction mechanisms and rate limiting steps is crucial for targeted optimization of solid oxide fuel cell (SOFC) electrodes, especially for new materials and processing techniques, such as Ni/Gd-doped ceria (GDC) cermet anodes in metal-supported cells. Here, we present a comprehensive model that describes the impedance of porous cermet electrodes according to a transmission line circuit. We exemplify the validity of the model on electrolyte-supported symmetrical model cells with two equal Ni/Ce0.9Gd0.1O1.95-δ anodes. These anodes exhibit a remarkably low polarization resistance of less than 0.1 Ωcm2 at 750 °C and OCV, and metal-supported cells with equally prepared anodes achieve excellent power density of &gt;2 W/cm2 at 700 °C. With the transmission line impedance model, it is possible to separate and quantify the individual contributions to the polarization resistance, such as oxygen ion transport across the YSZ-GDC interface, ionic conductivity within the porous anode, oxygen exchange at the GDC surface and gas phase diffusion. Furthermore, we show that the fitted parameters consistently scale with variation of electrode geometry, temperature and atmosphere. Since the fitted parameters are representative for materials properties, we can also relate our results to model studies on the ion conductivity, oxygen stoichiometry and surface catalytic properties of Gd-doped ceria and obtain very good quantitative agreement. With this detailed insight into reaction mechanisms, we can explain the excellent performance of the anode as a combination of materials properties of GDC and the unusual microstructure that is a consequence of the reductive sintering procedure, which is required for anodes in metal-supported cells.

Ethanol internal reforming in solid oxide fuel cells: A path toward high performance metal-supported cells for vehicular applications

Internal reforming of ethanol fuel was investigated on high-performance metal-supported solid oxide fuel cells (MS-SOFCs) with infiltrated catalysts. The hydrogen concentration and internal reforming effects were evaluated systematically with different fuels including: hydrogen, simulated reformate, anhydrous ethanol, ethanol water blend, and hydrogen-nitrogen mixtures. A simple infiltration of Ni reforming catalyst into 40 vol% Ni-Sm0.20Ce0.80O2-δ (Ni-SDCN40) and fuel-side metal support leads to complete internal reforming, as confirmed by comparison to simulated reformate. The performance difference between hydrogen and fully-reformed ethanol is attributed entirely to decrease in hydrogen concentration. High peak power density was achieved for a range of conditions, for example 1.0 W cm−2 at 650 °C in ethanol-water blend, and 1.4 W cm−2 at 700 °C in anhydrous ethanol fuel. Initial durability tests with ethanol-water blend show promising stability for 100 h at 700 °C and 0.7 V. Carbon is not deposited in the Ni-SDCN40 anode during operation.

(Invited) Progress in Metal-Supported Solid Oxide Cells with Infiltrated Electrodes

This talk will provide an overview of performance, durability, and applications of metal-supported solid oxide fuel cell and electrolysis cell technology developed at Lawrence Berkeley National Laboratory (LBNL). The unique LBNL symmetric cell architecture design, with thin zirconia ceramic backbones and electrolyte sandwiched between porous metal supports, offers a number of advantages over conventional all-ceramic cells, including low-cost structural materials (e.g. stainless steel), mechanical ruggedness, excellent tolerance to redox cycling, and extremely fast start-up capability. The metal-supported solid oxide cells achieve high performance at 700°C: >1.5 W/cm2 with 3% humidified hydrogen fuel >1.2 W/cm2 with internal reforming of ethanol fuel >2.6 A/cm2 electrolysis current density at 1.3V and 50%steam/50% hydrogen With infiltrated catalysts, there is a tradeoff between initial performance and long-term stability, as extremely high surface area promotes high electrochemical reaction rates, but also provides high surface energy leading to coarsening and facilitates Cr deposition. Recent approaches to mitigating catalyst coarsening and Cr deposition within the cathode include coatings to prevent Cr evaporation from the stainless steel components, and optimization of infiltrated catalyst processing to stabilize the microstructure during operation. The degradation rate has improved to 2.3% kh-1 in fuel cell mode, compatible with the requirements for electric vehicle range extenders. Electrolysis operation, however, results in higher degradation rate. Efforts to identify and mitigate the additional degradation mechanisms in electrolysis mode will be discussed. Scale-up from button cells to 50cm2 is ongoing, and highlights will be presented. In addition to our long-standing development of zirconia-based metal-supported cells, recent exploratory effort has focused on compatibility between stainless steel metal supports and proton-conducting ceramics. Co-sintering of 400-series stainless steel and BZCY-type proton conductors presents a number of challenges, including: Ba evaporation in reducing atmosphere, over-densification of steel at the high temperatures (>1400°C) required for BZCY processing, and migration of Cr and Si from the steel support into the BZCY layers. Feasibility of new approaches to overcome these challenges will be discussed.

Development of a Versatile, High-Performance Solid Oxide Fuel Cell Stack Technology

A high-performance solid oxide fuel cell (SOFC) stack technology is being developed, based on a design concept that incorporates prime-surface metallic interconnects with the versatility of stacking different types of single cells (sintered cells or metal-supported cells). A preliminary design of the prime-surface interconnect based on an egg-carton shape configuration has been advanced for this stack concept. Laboratory-scale prime-surface interconnects have successfully been fabricated by stamping and evaluated for mechanical loading, flow distribution and current collection properties. A process based on sputtering has been developed for fabricating supported thin-film (several micrometers thick) cells for this stack technology. Fabricated thin-film cells have shown desirable structural characteristics and exceptional performance on different fuels at reduced temperatures (550º-650ºC). Sputtered cells (with yttria stabilized zirconia (YSZ) for the electrolyte, gadolinia-doped ceria (GDC) for the electrolyte/cathode interlayer, nickel-YSZ for the anode and lanthanum strontium cobalt iron perovskite (LSCF)–YSZ for the cathode) have exhibited peak power densities of ~1.7 W/cm2 and ~2.1 W/cm2 with hydrogen fuel and air at operating temperatures of 600ºC and 650ºC, respectively.

Progress in Metal-Supported Solid Oxide Fuel Cells and Electrolyzers with Symmetric Metal Supports and Infiltrated Electrodes

The LBNL metal-supported solid oxide cell architecture contains zirconia electrolyte and porous backbones co-sintered between porous stainless steel supports. Advantages of this design include low-cost structural materials, mechanical ruggedness, excellent tolerance to redox cycling, and extremely fast start-up capability. With infiltrated catalysts, high performance is also achieved: 1.5 W/cm2 with hydrogen fuel and 1.3 W/cm2 with internal reforming of ethanol fuel at 700 °C; 2.6 A/cm2 electrolysis current density at 1.3V and 50% steam/50% hydrogen at 700°C. Recent approaches to mitigating catalyst coarsening and Cr deposition within the cathode stabilize the microstructure during operation. The degradation rate is improved to 2.3% kh-1 at 700°C in fuel cell mode. Electrolysis operation, however, results in higher degradation rate. Preliminary effort to fabricate metal-supported cells with proton conducting electrolyte is successful for La0.99Ca0.01NbO4 electrolyte, and specific challenges for BaZr0.7Ce0.2Y0.1O3-δ electrolyte are determined.

Proton-Conducting Ceramics for Metal-Supported Solid Oxide Fuel Cells and Electrolysis Cells

Proton conducting oxide electrolyte materials could potentially lower the operating temperature of metal-supported solid oxide cells (MS-SOCs) to the intermediate range 400 to 600 °C. The porous metal substrate provides the advantages of MS-SOCs such as high thermal and redox cycling tolerance, low-cost of structural materials, and mechanical ruggedness. The cell structure for fuel cell and electrolysis operation is shown in Figure 1. To incorporate proton-conductors into metal-supported cell configuration through co-sintering fabrication, a key factor is that the proton conductors must be compatible with reducing atmosphere sintering, as the porous metal substrate can not be fired in oxidizing atmosphere. In this work, viability of co-sintering fabrication of metal-supported proton conducting solid oxide cells is investigated. Candidate proton-conducting ceramics including perovskite-type oxides BaZr0.7Ce0.2Y0.1O3-δ (BZCY), SrZr0.5Ce0.4Y0.1O3-δ (SZCY), and Ba3Ca1.18Nb1.82O9-δ (BCN), pyrochlore-type oxides La1.95Ca0.05Zr2O7-δ (LCZ) and La2Ce2O7 (LCO), and acceptor doped rare-earth ortho-niobates La0.99Ca0.01NbO4 (LCN) were synthesized via solid state reactive synthesis or sol-gel synthesis. These ceramics are sintered at 1450°C in reducing environment alone and supported on Fe-Cr alloy metal support, and their key characteristics such as phase formation, sintering property, and chemical compatibility with metal support are determined. Most electrolyte candidates suffer from one or more challenges identified for this fabrication approach, including: phase decomposition in reducing atmosphere, evaporation of electrolyte constituents, contamination of the electrolyte with Si and Cr from the metal support, and incomplete electrolyte sintering. BZCY is particularly interesting because of its high proton conductivity, but it is found to suffer from reaction with Si and Cr present in the metal support, and loss of Ba via evaporation during sintering. Various approaches to overcome these limitations are proposed, and preliminary assessment indicates that the use of barrier layers, low-Si-content stainless steel, and sintering aids warrant further development. In contrast, La0.99Ca0.01NbO4 is found to be highly compatible with the metal support and co-sintering processing in reducing atmosphere, allowing dense electrolyte and porous electrode made of LCN to be co-sintered on porous stainless steel, Figure 2. A metal-supported cell is fabricated with La0.99Ca0.01NbO4 electrolyte, ferritic stainless steel support, Pt air electrode and nanoparticulate ceria-Ni hydrogen electrocatalyst. The total resistance is 50 Ω∙cm2 at 600 °C. This work clearly demonstrates the challenges, opportunities, and breakthrough of metal-supported proton-conducting solid oxide cells by co-sintering fabrication. Figure 1

3D multiphysics modeling aided APU development for vehicle applications: A thermo-structural investigation

Solid oxide fuel cells (SOFC) are suitable for on-board electricity generation as Auxiliary Power Unit (APU) to support the electric power supply in heavy-duty vehicles. In order to satisfy the requirements of a lightweight fuel cell stack for mobile applications, thin-walled components must be used for the stack structure. This necessity is associated with material, process and design difficulties that must be solved in order to achieve a successful utilization. In this work a novel lightweight SOFC stack design with metal-supported cell was studied both numerically and experimentally. The metallic components are made from the Intermediate Temperature Metal (ITM), a high performance, high chromium ferritic stainless steels alloy. The multiphysics modeling approach (fluid dynamics, heat transfer, structural mechanics) was utilized in this work to predict the temperature distribution and the thermo-structural behavior of the new developed design. Geometric details of the fuel cell stack components as well as appropriate nonlinear, temperature and time-dependent constitutive models were developed to describe the material behavior. Experimental data were used to determine the material model parameters and validated the simulation results. The three-dimensional stress and deformation distributions in the individual stack components were evaluated and their maximum values for elements at risk were identified. Thus, the developed model enables the investigation of sustainability and serviceability of the structural elements to ensure a reliable operation of the stack. The developed computational model can be used as a design tool for parametric studies and optimization analysis to investigate the effects of process boundary conditions, material properties as well as geometrical design parameters and their variation on the induced thermal stresses.