Fuel Cell Mode Research Articles

Use of reduced-voltage EIS to establish a relation between oxygen concentration and EIS responses of large commercial PEM fuel cell modules

This paper employs equivalent circuit analysis of electrochemical impedance spectra (EIS) signatures of commercial proton exchange membrane fuel cell stacks (PEMFC) to establish a relation between oxygen concentration and EIS response within individual cells of the stack. This paper is part of an ongoing effort to use EIS for detecting and quantifying hydrogen crossover to understand and reduce hydrogen cathode exhaust emissions. Selected results from different sets of experiments performed on Ballard water-cooled and air-cooled PEMFC stacks are presented. A novel reduced-voltage EIS system with the ability to perform impedance measurements over different cell counts is employed to record EIS scans of these stacks. In the first set of experiments, hydrogen was injected into the cathode side of a 30 kW 150+-cell water-cooled module at various rates to emulate the phenomenon of hydrogen crossover at different levels. In the second set of experiments, nitrogen was injected into a target single-cell in an air-cooled stack with over 50 cells (50+-cell) to dilute the cathode available oxygen, and the impedance was measured over various numbers of cells spanning around this target cell. A novel equivalent circuit model with resistors, constant phase elements (CPEs), and a linear finite Warburg parameter was used to study the EIS data via an optimization-based fitting approach in python. Results of these experiments have shown there is a proportional relationship between the Warburg parameter (in the equivalent circuit model used) and hydrogen crossover (at nominal idle and low-current air flows), and that this Warburg parameter increases significantly with reduced cathode air flows. Also, it is concluded that this relationship can be further used, along with decomposition of the EIS scan of a stack into different cell-level scans, to estimate the number of large-leaky cells as well as the total hydrogen crossover rate.

Status of SOC Development at Chaozhou Three-Circle

Chaozhou Three-Circle (Group) Co., Ltd. (CCTC) is a leading company in development and manufacturing of electronic and ceramic components. CCTC has been focusing on SOFC R&D and manufacturing since 2004, covering the complete value chain from powder to power. Based on anode-supported cell technology (ASC in fuel cell mode), the C2-stack in planar design using metallic interconnect exhibits a high efficiency of more than 69% (DC, LHV) at 1.5 kW. Long-term stack tests using pipeline natural gas have been conducted for 20,000 hours, with low degradation rates of below 0.4%/kh.

Current Status of Ceres Electrolysis Programme

This paper reports on experimental testing and verification of Ceres solid oxide technology in electrolysis mode at all levels from small single cells to a 100-kW module. This programme has demonstrated that the electrochemical performance of Ceres core cell technology is broadly the same in fuel cell and electrolysis modes suggesting no fundamental difference when reversing the direction of current. Additionally, lifetime predictors such as degradation rate and resilience to thermal cycles and emergency stops, measured at stack level, show an equally high level of real-world robustness in electrolysis mode as in fuel cell mode. Finally, results from testing our first-of-a-kind 100kW electrolysis module show reliable operation, high electrical efficiency of >87% (LHV) or 38kWh/kgH2 and stacks performing at the same level as measured on test stands.

Scenario Evaluation of Reversible Solid Oxide Cells Integrated with Local Renewable Energy Sources: Assessment of Different Coupling Applications

This work is focused on the analysis of the integration of reversible Solid Oxide Cells (rSOCs) for power grid flexibility and renewable sources exploitation purposes. Firstly, the area with the highest mismatch between renewable generation and load in Italy has been identified, making sure that, from a technical point of view, the use of an rSOC system can prove to be advantageous. An experimental campaign on a 0.4 kW SOC short-stack was carried out, with the aim to replicate, by means of dedicated tests, the operating conditions of the larger scale SOC system designed to absorb the abovementioned misalignment.Technology-wise, the high potential of the rSOC for this application has been proven: the results show an efficiency of the system estimated to be over 55% in fuel cell mode, with a water conversion rate of 89%. Moreover, the designed system appears to fit perfectly into the real scenario, with an annual operation of 8700 hours.

Layout and Experimental Results of an 10/40 Kw rSOC Demonstration System

Forschungszentrum Jülich has been operating an rSOC system in the 10/40 kWAC power class since 2021. This system uses four 20-layer sub-stacks in the mark H20 stack design. During the test campaign, a power range from 1.7 to 13 kWAC could be shown in fuel cell mode. The highest efficiency in fuel cell mode of 63.3 % was achieved at a power output of 10.4 kWAC, related to the lower heating value (LHV) of hydrogen. With a power input of -49.6 kWAC, the highest efficiency of 71.1% (LHV) was achieved in electrolysis mode. At this point, 11.7 Nm³ h-1 of hydrogen were produced. The following manuscript shows the layout and the experimental results of the rSOC demonstration system.

Enhancing the Catalytic Activity of Ce(Mn, Fe)O2 on a Perovskite-Based Electrode via Spraying for High-Temperature CO2 Electrolysis

Solid oxide electrolysis cells (SOECs) have the potential to be highly efficient devices for high-temperature CO2electrolysis. However, commonly used Ni-based cathodes suffer from redox instability under CO/CO2 conditions. La(Sr)Cr(Mn)O3 (LSCM) is a promising alternative due to its coking resistance, but it has poor catalytic activity on CO2reduction. To address this, Ce(Mn, Fe)O2 (CMF) was added as a catalyst to accelerate CO2 adsorption. In this study, the non-touchable coating process by spraying was employed to fabricate the well-decorated embedded CMF on LSCM surface, without complicated pretreatment and infiltration. The performances of the highly decorated electrode surface with CMF nano catalyst were measured at 1123 K, exhibiting a maximum power density of 0.503 W cm-2 in fuel cell mode (H2) and electrolysis performance of 0.835 A cm-2 at 1.5 V in CO2 reduction. This approach shows potential for enhancing the catalytic activity of LSCM cathodes and advancing the development of SOECs for sustainable energy production.

Progress in Research and Development of Solid Oxide Cells, Stacks and Systems at Forschungszentrum Jülich

Forschungszentrum Jülich has been involved in the research and development of SOCs for more than 30 years. In this work, selected highlights on material, cell, stack and system development are presented, whereas improving performance and understanding degradation phenomena were in the focus. On cell level, operation in steam, co-electrolysis as well as pure CO2 electrolysis mode was researched. A hierarchical degradation model framework was developed that relates changes at the level of electrode particles to changes in electrode structure, resulting materials properties and overall lifetime-performance. On stack level, progress in clarification and optimization of performance and lifetime relevant processes was made. The role of contaminants, foremost silicon species and sulfur dioxide in feed gases, was investigated to support technical applications. On system level, an rSOC system with an output power of 10 kW in fuel cell mode and an input power of 40 kW in electrolysis mode was developed.

Recent Highlights on Solid Oxide Cells, Stacks and Modules Developments at CEA

Solid Oxide Cell (SOC) technology is considered as an efficient electrolysis technology to produce hydrogen at large scale. It can also operate in fuel cell mode using different fuels (carbon-based or non-carbon based like ammonia), and in reversible mode. Though proofs-of concept have been achieved at different relevant scales for those operating modes, some R&D works still need to be performed to improve performance, durability and cost. Improved and upscaled cells and stacks need to be developed, with a methodology combining multiscale and multiphysics modelling, electrochemical characterization in relevant conditions and advanced post-test analysis. Their integration into modules made of several stacks is also a stepping stone in order to reach multi-MW electrolysers. CEA is working on the whole value chain of SOC technology, from cell development and optimisation to module design and operation through stack upscaling. Recent achievements on those aspects will be presented.

Reversible Metal Supported Solid Oxide Cells

Solid oxide cells (SOCs) can operate in fuel cell and electrolysis mode, allowing for production of electricity and heat from a green fuel, and for storage of electricity as gas or use as fuel. Lifetime and costs are major factors enabling commercialization. Metal supported SOCs (MSCs) provide cost-competitive materials within the cell. The present study investigates the reversible operation of MSCs, fabricated by tape casting, lamination, and screen-printing, at 650 oC in 50/50 steam/hydrogen gas. In the initial few hundred hours, the MSC show a lower degradation rate compared to a state-of-the-art (SoA) fuel electrode supported cell in the electrolysis mode, while it is ca. comparable in fuel cell mode. Electrochemical impedance evaluation revealed that the degradation is due to a simultaneous increase of the serial and polarization resistances in the MSC while it is mainly due to increase of the polarization resistance in the SoA cell.

Development Progress of a 70% Efficient Hybrid SOFC-I.C. Engine Hybrid System for Stationary Applications up to 1 MW

Flexible and dispatchable, high-efficiency power generation supplied with carbon-neutral renewable fuels is needed to help enable defossilization of the electric grid. Pressurized, hybrid SOFC systems fueled with hydrogen, biogas, or renewable natural gas can generate clean power at ultra-high efficiency. In this summary, we provide an update on the development progress of a full-scale, hybrid system that targets low cost (<1000 $/kW) and ultra-high efficiency (70%-LHV) distributed power generation for applications up to 1 MW. The system features pressurized, metal-supported SOFC technology from Ceres Power which is integrated with a modified diesel engine which converts the residual chemical exergy in the anode tail-gas from the SOFC to drive auxiliaries and produce net additional power. Updates on critical hardware advancements around pressurized multi-stack, 30 kW fuel cell modules, low-speed high efficiency rotating equipment, and ultra-high efficiency power electronics are provided. A techno-economic outlook for such power generation systems in various stationary applications.

Microstructural Changes in Ni-YSZ Electrodes Operated in Fuel Cell and Electrolysis Modes: Effect of Gas Diffusion Limitations

Ni-YSZ electrode support symmetric cells were operated at 0, 0.75, 1.00, and 1.50 A/cm2 for 1000 h in 50% H2-50% H2O at 800 ˚C. Electrochemical fracture at the anode-electrolyte interface is observed to occur under high anodic overpotential. Ni migration is observed and quantified over time at the anode of the polarized cells; however, the cathode shows no migration compared to control. Gas diffusion calculations show that steam is significantly enriched and depleted at the anode and cathode respectively, leading to the formation or suppression of volatile Ni(OH)x species, which have been hypothesized as a transport pathway for Ni. However; gas flux calculations show that chemical evaporation alone is unlikely to be fast enough to induce the Ni loss observed.

Investigation of the Abnormal I-V Curve Shape of Hybrid Direct Carbon Fuel Cell Using Lanthanum Strontium Calcium Titanate-Based Anode

Solid oxide direct carbon fuel cells are being developed for use with waste derived materials. We have fabricated solid oxide fuel cells with La0.2Sr0.25Ca0.45MxTi1-xO3-δ (M=Ni, Fe, or Co) anodes, YSZ electrolyte, and LSM cathode. Electrochemical performance of these cells has been evaluated with solid carbon as the fuel and promising results have been recorded. For example, fuel cells with La0.2Sr0.25Ca0.45Ni0.05Ti0.95O3-δ anode tested in hybrid direct carbon fuel cell mode reached a maximum power density of 82.3 mW/cm2 measured in N2 purging gas at 850 °C. During hybrid direct carbon fuel cell testing with solid carbon fuel and nitrogen purging gas, I-V curves obtained showed an abnormal shape between 0.6V and 0.7 V. To investigate the origin of this line shape, we have used methods such as electrochemical impedance spectroscopy, potentiostatic study, and analysis of off gases from fuel cell using gas chromatography.

Integration of Reversible Solid Oxide Electrolysis (rSOC) to Wastewater Treatment Plants for Sustainable Green Gas Production and Balancing of Green Power

Biogas produced from anaerobic digestion (AD) of sewage sludge and organic wastes at wastewater treatment plants (WWTPs) can present an important energy contribution to a sustainable society. Raw AD biogas contains 50-65 molar% methane (CH4) and 50-35 molar% carbon dioxide (CO2) and can be upgraded to biomethane (with over 98 molar% CH4). During the CO2 separation step of biomethane upgrading, substantial amounts of CO2 are emitted to the atmosphere. A methanation unit can convert this CO2 into sustainable Synthetic Natural Gas (SNG) and replace the conventional CO2 separation unit for biomethane upgrading, but needs addition of hydrogen from water electrolysis. The water electrolysis can be built as a novel high-performance reversible Solid Oxide Electrolysis (rSOC), as demonstrated in the European H2020 project BALANCE. To highlight the zero-emission potential of the rSOC, this paper emphasizes author’s earlier results from the European H2020 project DEMOSOFC, demonstrating zero-local emission SOFC using raw biogas from WWTP sewage sludge and organic wastes. In this paper, profitability estimations for optimal rSOC and methanation investment and operation strategies at a modern city level WWTP in Finland, Europe, are presented. Power used in rSOC electrolysis mode (SOEC) origins from intermittent wind power, while power produced in rSOC fuel cell mode (SOFC) uses raw AD biogas. A dedicated Power-to-X optimization model is used, optimizing power market operations, dimensions of renewable power sources, plant dimensions and operational scheduling. High profitability is shown using European energy crisis market data 2021-2022, resulting in sustainable biomethane net production costs of 30-70 €/MWh.

Characterizing the Performance of kW-Scale Multi-Stack Solid Oxide Fuel Cell Modules through Modeling

The development of commercial SOFC systems often utilizes models of a single representative SOFC stack to simulate the system’s prime mover. In practice, however, these representative single-stack SOFC models fail to consider effects due to the reality of multi-stack power modules needed to construct commercial-scale systems. Effects such as flow maldistribution, inter-stack heat transfer, and area-specific resistance (ASR) variation between stack units within a single module will create deviations in performance from a single stack-based system simulation. The following outlines efforts to consider these thermofluidic effects and the associated performance deviations of a novel 30kWe SOFC multi-stack module using models in the gPROMS Modelbuilder® environment. Specifically, the impacts of gas manifolding, heat loss, and stack placement are investigated, and their potential impacts are discussed.

On the composition of Sr2Fe1.5Mo0.5O6-δ – Sm0.2Ce0.8O2-δ composite as fuel electrodes for hydrogen reversible solid oxide cells

Sr2Fe1.5Mo0.5O6-δ-Sm0.2Ce0.8O2-δ (SFMO-SDC) composites are high-efficiency fuel electrode materials for reversible solid oxide cells (rSOCs). Systematic variation in the SFMO-SDC composite shows that the composite with 40 wt% of SDC gives rise to the highest amount of oxygen vacancy and the highest number of electronic defects due to multiple electronic activation effects. Polarization resistance changes over the steam content show the parabolic trend with the optimal humidity at 20 vol% at open circuit voltage and in fuel cell mode, while in electrolyzer mode, highly humidified hydrogen is favored. Electrochemical studies and distributions of relaxation times analysis illustrate that the rate-determining step in fuel cell mode is the water desorption reaction whereas that in electrolyzer and at open circuit voltage (OCV) is surface chemical diffusion/oxide transport processes.

Electrical properties of BaZr0.5Ce0.3Y0.1Yb0.1O3-δ proton conductor for reversible proton-conducting solid oxide electrochemical cells

Proton-conducting oxide-based reversible solid oxide electrochemical cells (protonic-SOEC, P-SOEC) have a great potential to be the most efficient transition technology operating at intermediate temperatures. Here, we have introduced a perovskite-type proton conductor BaZr0.5Ce0.3Y0.1Yb0.1O3-δ (BZCYYb5311) and evaluated its electrochemical and transport properties. The chemical stability of BZCYYb5311 was confirmed by thermo-gravimetric analysis (TGA) under pure CO2 conditions and X-ray diffraction analysis. The total and partial conductivities were obtained using DC four-probe conductivity measurements under various pO2 and pH2O conditions. The ratio of proton conductivity to ionic conductivity including the oxygen-ion conductivity of this material was 98% at 600 °C, indicating that it is a relatively pure proton conductor with most of the ionic conduction almost exclusively carried out by protons. Chemical diffusivities were calculated from Fick's 2nd law by non-linear least squares fitting during oxidation/reduction and hydration/dehydration processes and were faster than other proton conductor BZCY (Y-doped BaCeO3-BaZrO3) series, which have a similar zirconium contents. In addition, the theoretical performance and efficiency of the reversible P-SOEC were determined from the spatial distribution of oxygen and hydrogen partial pressure in BZCYYb5311, calculating the maximum power density (4.5 Wcm−2 at 600 °C, 3% of water vapor pressure) for the fuel cell mode and current density (12.3 Acm−2 at 1.5 V, 600 °C, and 20% of water vapor pressure) for the electrolysis mode. The fuel consumption efficiency is high (99.1% at 0.6 V at 600 °C), while the electrolysis efficiency is low (72.3% at 1.6 V) due to the dominant p-type regime in the electrolyte during electrolysis. This low electrolysis efficiency can be enhanced by recovering the electrolyte regime by increasing pH2O or decreasing pO2 in the air electrode; however, it will still have low efficiency. Consequently, BZCYYb5311 is not an efficient electrolyte material for electrolyzers, but is promising for fuel cells because of its superior chemical stability, exceptional pure proton conductivity, and better bulk diffusion than those of the BZCY system.

Designing the nano-scale architecture of the air electrode for high-performance and robust reversible solid oxide cells

Herein, an innovative La0.6Sr0.4CoO3 (LSC) nanostructured air electrode, having superior catalytic activity and exceptional robustness against critical interfacial delamination-induced degradation, is reported. A ∼50 nm thin LSC air electrode decorated onto a porosity graded Gd0.1Ce0.9O2 backbone via ultrasonic-assisted infiltration showed significantly flourished ORR and OER kinetics with prolific electrochemical performance compared to the cell with conventional LSC air electrode. The SOC showed maximum power density of 2.24 W cm−2 in the fuel cell mode and maximum current density of 4.57 A cm−2 at an operating voltage of 1.6 V in the electrolysis mode at 750 °C. Results of the reversible cycling and galvanostatic stability tests indicate excellent durability, which is attributed to the elimination of detrimental O2 pressure evolved at the air electrode/electrolyte interface. Sited among the highest performances till date, this study provides a realization of a tremendously durable SOC design for the production of green hydrogen and electricity.

Experimental Results of a 10/40 kW-Class Reversible Solid Oxide Cell Demonstration System at Forschungszentrum Jülich

In 2018, a 5/15 kWDC reversible solid oxide cell system was developed and successfully operated by Forschungszentrum Jülich. Based on the knowledge gained with this first system, an optimized system in the power class of 10/40 kWAC was developed afterwards in 2019 that uses the well-established Integrated Module. This module consists of four 20-layer sub-stacks, two heat exchangers and five heating plates. It represents the main components of the system. The basic system layout was retained in general from the previous system and adjusted in accordance with the higher power level, as well as the supporting balance of plant components. The layout of the demonstrator system and its results are described. During the experimental evaluation in fuel cell mode, the system could provide an electrical output power from 1.7 to 13 kWAC. The maximum system efficiency of 63.3% based on the lower heating value (LHV) could be reached at a system power of 10.4 kWAC. This operating point was also analyzed regarding the temperature distribution in the stack and efficiency losses. In electrolysis mode, a maximum efficiency of 71.1% (LHV) was achieved with an electrical power input of −49.6 kWAC. At this operating point, about 11.7 Nm3 h−1 of hydrogen are generated. The stack temperature distribution and the efficiency losses are also analyzed for the electrolysis mode. Finally, the potential for the efficiency optimization through higher heat integration in this mode is experimentally evaluated and discussed.

Numerical Analysis of the Effect of Liquid Water during Switching Mode for Unitised Regenerative Proton Exchange Membrane Fuel Cell

As unitised regenerative proton exchange membrane fuel cell (URPEMFC) is progressing in terms of its performance, more emphasis should be placed on the understanding of the interaction between multiphase reactants and products and its effect during the switching mode. A 3D transient computational fluid dynamics model was utilised in this study to simulate the supply of liquid water into the flow field when the system switched from fuel cell mode to electrolyser mode. Different water velocities were investigated to identify their effect on the transport behaviour under parallel, serpentine, and symmetry flow fields. From the simulation results, 0.5 m·s-1 water velocity was the best-performing parameter in achieving optimal distribution. Among different flow field configurations, the serpentine design achieved the best flow distribution due to its single-channel model. Modification and refinement in the form of flow field geometric structure can be performed to further improve the water transportation behaviour in URPEMFC.

Constructing an active and stable oxygen electrode surface for reversible protonic ceramic electrochemical cells

The reversible protonic ceramic electrochemical cells (R-PCECs) can efficiently and cost-effectively store and convert energy at low-intermediate temperatures (400-700 oC). Their widespread commercialization is mainly limited by the challenges of oxygen electrodes due to the slow oxygen reaction kinetics and poor durability. In this study, we first enhance the reaction activity and surface stability of a double-perovskite PrBaCo2O5+δ (PBC) oxygen electrode by employing a fluorite-based Pr0.1Ce0.9O2+δ (PCO) catalyst coating. The PCO-coated PBC (PCO-PBC) oxygen electrode shows a much-reduced area-specific resistance of 0.096 Ωcm2 and good performance on a fuel-electrode supported single cell at 650 oC, displaying a typical peak power density of 1.21 Wcm-2 (in fuel cell mode) and a typical current density of 2.69 Acm-2 at 1.3 V (in electrolysis mode) with reasonable faradaic efficiencies and durability. PCO coating has significantly improved the surface exchange process, facilitated ion diffusion, and suppressed the Ba-segregation of PBC, as confirmed by the analyses of electrochemical performance and TEM.