Oxidation of porous stainless steel supports for metal-supported solid oxide fuel cells

Oxidation behavior of porous P434L ferritic stainless steel, used for the fabrication of metal-supported solid oxide fuel cells (MS-SOFC), is studied under anodic and cathodic atmospheres. Temperature- and atmosphere-dependence is determined for as-sintered and pre-oxidized stainless steel. Pre-oxidation reduced the long-term oxidation rate. For pre-oxidized samples, the oxidation rate in air exceeds that in humid hydrogen for temperatures above 700 °C. The influence of PrOx, LSCF-SDC, and Ni-SDC coatings is also examined. The coatings do not dramatically impact oxide scale growth. Oxidation in C-free and C-containing anodic atmospheres is similar. Addition of CO₂, CH₄, and CO to humidified hydrogen to simulate ethanol reformate does not significantly impact oxidation behavior. Cr transpiration in humid air is greatly reduced by the PrOx coating, and a PrCrO₃ reaction product is observed throughout the porous structure. A dense and protective chromia-based scale forms on steel samples during oxidation in all conditions. A thin silica enriched oxide layer also forms at the metal-scale interface. In general, the oxidation behavior at 700 °C is found to be acceptable.     

Progress in metal-supported solid oxide electrolysis cells: A review

There is increasing global interest in using solid oxide electrochemical cells to perform electrolysis. Metal-supported solid oxide electrolysis cells (MS-SOEC) are being developed with stainless steel and Ni-based supports. The use of porous metal to support the electrochemically-active layers is anticipated to improve mechanical strength, decrease cost, and increase tolerance to aggressive operating conditions, including rapid thermal excursions. This review summarizes and analyzes the previous decade of progress in MS-SOEC development, and identifies critical needs for future work.     

Improvement of corrosion properties of porous alloy supports for solid oxide fuel cells

The development of protective coatings for porous metal supports is critical for sufficient life time for the fuel cells by enabling improved oxidation resistance, reduced chromium evaporation, and increased conductivity of the protective oxide scale. The oxidation of coated and non-coated substrates has been compared, and shows that it is possible to increase the oxidation resistance at 600 °C in air by a factor of 10 and in wet hydrogen by a factor of 1000, after vacuum coating of the porous metal supports by infiltration of a lanthanum–manganese–cobalt solution and fast curing in air at 900 °C. Chromium evaporation is also lowered by a factor of 10 in air at 600 °C. The experiments on pre-coated porous metal supports verify that the coating is well suited to use for metal supported fuel cells prepared by a low temperature fabrication route (below 1100 °C). An alternative coating procedure for coating of the metal supports after co-sintering of the anode and electrolyte has also been investigated and is well suited for the high-temperature fabrication route. For the high temperature fabrication route, the oxidation tests at 600 °C for 500 h in air and 100 h in wet hydrogen showed that post-coating is better than the pre-coating approach since the cell sintering steps has a detrimental effect on the pre-coated samples.     

Performance and durability of metal-supported solid oxide electrolysis cells at intermediate temperatures

Metal-supported solid oxide electrolysis cells (MS-SOECs) operating at 600–700 °C are attractive for storage of intermittent renewable electricity from solar and wind energy due to their advantages of easy sealing and fast startup. This paper reports on the fabrication of MS-SOECs consisting of dense scandium stabilized zirconia (SSZ) electrolytes, Ce_0.8Sm_0.2O_2−δ (SDC)/Ni impregnated 430L/SSZ cathodes and SmBa_0.5Sr_0.5Co₂O_5+δ (SBSCO) impregnated SSZ anodes supported on porous 430L alloys. Such cells demonstrated excellent electrolysis performance with current densities at 650 °C as high as 0.73 A⋅cm⁻² at 1.3 V in 50% H₂O-50% H₂ and 0.95 A⋅cm⁻² at 1.5 V in 90% CO₂-10% CO. Electrochemical impedance measurements indicated that the cell performance was largely limited by the ohmic losses for steam electrolysis and by the cathodic reduction reactions for CO₂ electrolysis, especially at reduced temperatures. Pronounced degradation was observed for both steam and CO₂ electrolysis over the preliminary 90-h stability measurements at 600 °C. SEM examination and EDS mapping of measured cells showed significant aggregation and coarsening of impregnated Ni particles, resulting in smaller activities for H₂O and CO₂ reduction reactions. As evidenced by the almost unaltered ohmic resistances over the measurement durations, the 430L stainless steel substrates demonstrate excellent resistances against corrosions from H₂O and CO₂ and thus show great promise for applications in reduced-temperature MS-SOECs.     

Hydrogen production by electrolysis of a phosphate solution on a stainless steel cathode

The catalytic properties of phosphate species, already shown on the reduction reaction in anaerobic corrosion of steels, are exploited here for hydrogen production. Phosphate species work as a homogeneous catalyst that enhances the cathodic current at mild pH values. A voltammetric study of the hydrogen evolution reaction is performed using phosphate solutions at different concentrations on 316L stainless steel and platinum rotating disk electrodes. Then, hydrogen is produced in an electrolytic cell using a phosphate solution as the catholyte. Results show that 316L stainless steel electrodes have a stable behaviour as cathodes in the electrolysis of phosphate solutions. Phosphate (1 M, pH 4.0/5.0) as the catholyte can equal the performance of a KOH 25%w solution with the advantage of working at mild pH values. The use of phosphate and other weak acids as catalysts of the hydrogen evolution reaction could be a promising technology in the development of electrolysis units that work at mild pH values with low-cost electrodes and construction materials.     

Progress in metal-supported solid oxide fuel cells: A review

Metal-supported solid oxide fuel cells provide significant advantages over conventional ceramic cells, including low materials cost, ruggedness, and tolerance to rapid thermal cycling and redox cycling. Various metal-supported cell designs have been developed, utilizing a range of electrolyte, electrode, and support materials prepared by various fabrication and deposition techniques. This paper reviews the current state of metal-supported cell technology and suggests opportunities for further development.     

Steam electrolysis performance of intermediate-temperature solid oxide electrolysis cell and efficiency of hydrogen production system at 300 Nm h

The steam electrolysis performance of an intermediate-temperature solid oxide electrolysis cell (SOEC) was measured at 650 °C at various steam concentrations. The cell voltage decreased with increasing steam concentration, which was attributed to a decrease in the steam electrode polarization. The highest performance of the SOEC was 1.32 V at 0.57 A cm⁻². On the basis of the electrolytic characteristics of this cell, the efficiency of a hydrogen production system operating at a capacity of 300 N m³ h⁻¹ was estimated. The system efficiency reached a higher heating value (HHV standard) of 98% due to the effective recovery of thermal energy from exhaust gas.     

Analysis of pressurized operation of 10 layer solid oxide electrolysis stacks

High temperature steam electrolysis using solid oxide electrolysis cell (SOEC) technology can provide hydrogen as fuel for transport or as base chemical for chemical or pharmaceutical industry. SOECs offer a great potential for high efficiencies due to low overpotentials and the possibility for waste heat use for water evaporation. For many industrial applications hydrogen has to be pressurized before being used or stored. Pressurized operation of SOECs can provide benefits on both cell and system level, due to enhanced electrode kinetics and downstream process requirements. Experimental results of water electrolysis in a pressurized SOEC stack consisting of 10 electrolyte supported cells are presented in this paper. The pressure ranges from 1.4 to 8 bar. Steady-state and dynamically recorded U(i)-curves as well as electrochemical impedance spectroscopy (EIS) were carried out to evaluate the performance of the stack under pressurized conditions. Furthermore a long-term test over 1000 h at 1.4 bar was performed to evaluate the degradation in exothermic steam electrolysis mode. It was observed that the open circuit voltage increases with higher pressure due to well-known thermodynamic relations. No increase of the limiting current density was observed with elevated pressure for the ESC-stacks (electrolyte supported cell) that were investigated in this study. The overall and the activation impedance were found to decrease slightly with higher pressure. Within the impedance studies, the ohmic resistance was found to be the most dominant part of the entire cell resistance of the studied electrolyte supported cells of the stack. A constant current degradation test over 1000 h at 1.4 bar with a second stack showed a voltage degradation rate of 0.56%/kh.     

Oxidation behavior of (Co,Mn)3O4 coatings on preoxidized stainless steel for solid oxide fuel cell interconnects

Ceramic coatings are being explored to extend the lifetime of stainless steel interconnects in planar Solid Oxide Fuel Cells (SOFCs). One promising coating is Co_1.5Mn_1.5O₄ spinel, which is deposited using various techniques, resulting in different coating thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation (0, 3, 10 and 100 h in 800 °C lab air) prior to coating. Samples were coated with 2 μm CoMn alloy using magnetron sputtering and were subsequently annealed in 800 °C air for 0, 10, 100 or 1650 h. Oxidation behaviors were evaluated as a function of these exposures, as well as in dual atmospheres and during area specific resistance (ASR) measurements in 800 °C lab air. Preoxidation was found to inhibit Fe and Cr transport from the stainless steel into the coating and preoxidized samples exhibited a substantially thinner surface layer after oxidation. After ASR testing for 1650 h in 800 °C air, the trend of the preoxidized sample values remained level while trend of the non-preoxidized sample values showed an increase. Observed oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects are presented and discussed.     

Fabrication and characterization of metal-supported solid oxide fuel cells

Metal-supported solid oxide fuel cells (SOFCs) are an acceptable approach to solving the serious problems of SOFC technology, such as sealing and mechanical strength. In this work, commercial stainless-steel plates, STS430, are used as supporting bodies for a metal-supported SOFC in order to decrease the number of fabrication steps. The metal support for a single-cell has a diameter of 28 mm, a thickness of 1 mm, and a channel width of 0.4 mm. A thin ceramic layer, composed of yttria-stabilized zirconia (YSZ) and NiO/YSZ, is attached to the metal support by using a cermet adhesive. La_0.8Sr_0.2Co_0.4Mn_0.6O₃ perovskite oxide serves as the cathode material because of its low impedance on the YSZ electrolyte, according to half-cell tests. The maximum power density of the cell is 0.09 W cm⁻² at 800 °C. The effects of temperature, oxygen partial pressure, and current collection by pastes are investigated. The oxygen reduction reaction at the cathode dominates the overall cell performance, according to experimental and numerical analyses.     

Manufacturing and characterization of metal-supported solid oxide fuel cells

A metal-supported solid oxide fuel cell design offers competitive advantages, for example reduced material costs and improved robustness. This paper reports the performance and stability of a recently developed metal-supported cell design, based on a novel cermet anode, on a 25 cm² (1 cm²/16 cm² active area) cell level. An electrochemical performance comparable to state-of-the-art anode-supported cells is demonstrated.Detailed electrochemical analysis allowed assignment of the overall polarization losses quantitatively to gas diffusion in the metal support, electrooxidation in the anode functional layer, oxygen reduction in the mixed ionic-electronic conducting cathode and an additional polarization process with a rather high relaxation frequency, which may be assigned to an insulating corrosion interlayer.The durability of the cells was investigated by means of galvanostatic operation for periods of up to 1000 h as well as the dynamic behavior, such as redox-, load- and thermal cycling tests.The galvanostatic stability tests indicated a fair, but significant degradation rate (∼5% decrease in cell voltage/1000 h at 650 °C and 0.25 A cm⁻²). Furthermore, the metal-supported cells underwent an endurance test of 100 redox cycles at 800 °C without severe degradation nor total failure.     

Performance and degradation of metal-supported solid oxide fuel cells with impregnated electrodes

Metal-supported solid oxide fuel cells (MS-SOFCs) containing porous 430L stainless steel supports, YSZ electrolytes and porous YSZ cathode backbones are fabricated by tape casting, laminating and co-firing in a reducing atmosphere. Nano-scale Ni and La_0.6Sr_0.4Fe_0.9Sc_0.1O_3−δ (LSFSc) coatings are impregnated onto the internal surfaces of porous 430L and YSZ, acting as the anode and the cathode catalysts, respectively. The resulting MS-SOFCs exhibit maximum power densities of 193, 418, 636 and 907 mW cm⁻² at 650, 700, 750 and 800 °C, respectively. Nevertheless, a continuous degradation in the fuel cell performance is observed at 650 °C and 0.7 V during a 200-h durability measurement. Possible degradation mechanisms were discussed in detail.     

Theoretical study on pressurized operation of solid oxide electrolysis cells

In this paper the influence of pressure on the performance of solid oxide electrolysis cells is theoretically analyzed in a pressure range between 0.05 and 2 MPa. A previously validated electrochemical model of a solid oxide fuel cell stack is used to predict electrolysis behavior. The effect of pressure on thermodynamics, kinetics and gas diffusion is discussed. It is shown that thermodynamics are negatively influenced by an increase in pressure whereas kinetics and gas transport are improved. Overall pressure effects are therefore only small. At low current density the electrolysis cell shows better performance at low pressure whereas performance improves with pressure at high current densities.     

High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers

Metal-supported solid oxide fuel cells are believed to have commercial advantages compared to conventional anode (Ni-YSZ) supported cells, with the metal-supported cells having lower material costs, increased tolerance to mechanical and thermal stresses, and lower operational temperatures. The implementation of a metallic support has been challenged by the need to revise the cell fabrication route, as well as electrode microstructures and material choices, to compete with the energy output and stability of full ceramic cells.The metal-supported SOFC design developed at Risø DTU has been improved, and an electrochemical performance beyond the state-of-the-art anode-supported SOFC is demonstrated possible, by introducing a CGO barrier layer in combination with Sr-doped lanthanum cobalt oxide (LSC) cathode. Area specific resistances (ASR) down to 0.27 Ω cm², corresponding to a maximum power density of 1.14 W cm⁻² at 650 °C and 0.6 V, were obtained on cells with barrier layers fabricated by magnetron sputtering. The performance is dependent on the density of the barrier layer, indicating Sr²⁺ diffusion is occurring at the intermediate SOFC temperatures. The optimized design further demonstrate improved durability with steady degradation rates of 0.9% kh⁻¹ in cell voltage for up to 3000 h galvanostatic testing at 650 °C and 0.25 A cm⁻².     

Personal power using metal-supported solid oxide fuel cells operated in a camping stove flame

A complete stand-alone product prototype providing combined cooking and power is fabricated by retrofitting a commercial camping stove with a stack of metal-supported solid oxide fuel cells (MS-SOFCs) delivering power to microelectronic LED driver and voltage boost circuits. The 5-cell stack produces 2.7 W (156 mW cm^?2) while cooking on the stove, and is demonstrated to produce LED lighting and mobile phone charging while operating outdoors. Cooking efficiency is minimally impacted by the presence of the MS-SOFCs. It is found that vertical orientation of the cells is critical to maintain separation of fuel and air when a pot is placed on the stove.     

The use and optimization of stainless steel mesh cathodes in microbial electrolysis cells

Microbial electrolysis cells (MECs) provide a high-yield method for producing hydrogen from renewable biomass. One challenge for commercialization of the technology is a low-cost and highly efficient cathode. Stainless steel (SS) is very inexpensive, and cathodes made of this material with high specific surface areas can achieve performance similar to carbon cathodes containing a platinum catalyst in MECs. SS mesh cathodes were examined here as a method to provide a higher surface area material than flat plate electrodes. Cyclic voltammetry tests showed that the electrochemically active surface area of certain sized mesh could be three times larger than a flat sheet. The relative performance of SS mesh in linear sweep voltammetry at low bubble coverages (low current densities) was also consistent with performance on this basis in MEC tests. The best SS mesh size (#60) in MEC tests had a relatively thick wire size (0.02 cm), a medium pore size (0.02 cm), and a specific surface area of 66 m²/m³. An applied voltage of 0.9 V produced a high hydrogen recovery (98 ± 4%) and overall energy efficiency (74 ± 4%), with a hydrogen production rate of 2.1 ± 0.3 m³H₂/m³d (current density of 8.08 A/m², volumetric current density of 188 ± 19 A/m³). These studies show that SS in mesh format shows great promise for the development of lower cost MEC systems for hydrogen production.     

Optimization of metal-supported solid oxide electrolysis cells with infiltrated catalysts

Metal-supported solid oxide electrolysis cells (MS-SOECs) are being developed for steam-to-hydrogen electrolysis, especially for utilization of dynamic or intermittent electrical power from renewable sources. Various aspects of the electrocatalyst processing and composition, and metal support structure were explored. Catalyst materials, infiltration temperature and infiltration cycles were optimized for high performance and durability. Numerous catalyst materials were screened for both oxygen and steam electrodes. The oxygen catalyst had moderate impact on both initial cell performance and durability. Reducing Ni content in the steam electrode had little effect on durability, but reduced initial performance. Ex-situ XRD analysis and cell assessment of catalyst infiltration temperature revealed that the optimal range is 750–850 °C. The best cell performance and durability was achieved with LSCF-SDC oxygen electrocatalyst and SDC-Ni (60:40 vol%) steam electrocatalyst infiltrated 11 times at 800 °C and operated at 700 °C. At low steam content, a significant mass transport limitation on the steam side results in limiting current behavior. Thinner and more porous metal supports were implemented, and found to improve steam mass transport at low steam content, relevant for SOECs operating under high H₂ recycle rate or high steam utilization.     

Development of three-layer intermediate temperature solid oxide fuel cells with direct stainless steel based anodes

Metal-supported solid oxide fuel cells (SOFCs) are usually four-layer structure consisting of the metal support, the anode, the electrolyte and the cathode. This communication reports a simplified three-layer design without the anode interlayer. The novel design is demonstrated by co-firing yttria-stabilized zirconia electrolytes and 430L stainless steel substrates, where Ni and doped ceria are impregnated to increase the catalytic activity toward electrochemical oxidation. Peak power density as high as 246 mW cm⁻² is obtained at 700 °C, and good tolerance to complete redox cycles is also initially demonstrated, suggesting that this design is feasible for high performance metal-supported SOFCs.     

High performance metal-supported solid oxide fuel cells fabricated by thermal spray

Metal-supported solid oxide fuel cells (SOFCs) have been fabricated and characterized in this work. The cells consist of porous NiO-SDC as anode, thin SDC as electrolyte, and SSCo as cathode on porous stainless steel substrate. The anode and electrolyte layers were consecutively deposited onto porous metal substrate by thermal spray, using standard industrial thermal spray equipment, operated in an open-air atmosphere. The cathode materials were applied to the as-sprayed half-cells by screen-printing and heat-treated at 800 °C for 2 h. The cell components and performance were examined by scanning electron microscopy (SEM), X-ray diffraction, leakage test, ac impedance and electrochemical polarization at temperatures between 500 and 700 °C. The half-inch button cells exhibit a maximum power density in excess of 0.50 W cm⁻² at 600 °C and 0.92 W cm⁻² at 700 °C operated with humidified hydrogen fuel, respectively. The half-inch button cell was run at 0.5 A cm⁻² at 603 °C for 100 h. The cell voltage decreased from 0.701 to 0.698 V, giving a cell degradation rate of 4.3% kh⁻¹. Impedance analysis indicated that the cell degradation included 4.5% contribution from ohmic loss and 1.4% contribution from electrode polarization. The 5 cm × 5 cm cells were also fabricated under the same conditions and showed a maximum power density of 0.26 W cm⁻² at 600 °C and 0.56 W cm⁻² at 700 °C with dry hydrogen as fuel, respectively. The impedance analysis showed that the ohmic resistance of the cells was the major polarization loss for all the cells, while both ohmic and electrode polarizations were significantly increased when the operating temperature decreased from 700 to 500 °C. This work demonstrated the feasibility for the fabrication of metal-supported SOFCs with relatively high performance using industrially available deposition techniques. Further optimization of the metal support, electrode materials and microstructure, and deposition process is ongoing.     

High temperature water electrolysis in solid oxide cells

Hydrogen production via high temperature steam electrolysis is a promising technology as it involves less electrical energy consumption compared to conventional low temperature water electrolysis, as consequence of the more favourable thermodynamic and electrochemical kinetic conditions for the reaction. This paper reports on the Solid Oxide Electrolyser Cell (SOEC) performance as function of the operating parameters temperature, humidity and current density. Current–voltage measurements are coupled with impedance spectroscopy, in order to identify the different loss terms in the cell behaviour coming from the electrolyte resistance and the electrode processes. Remarkably high electrical-to-hydrogen energy conversion efficiencies are achieved (e.g., cell voltages of 1.0 and 1.25 V at −1 A cm⁻² and 900 and 800 °C, respectively). Results obtained, moreover, show that an important limitation for the electrolysis reaction, at least at moderate absolute humidity values below about 70 vol.% can be the steam diffusion in the hydrogen/steam electrode.