Solid Oxide Fuel Cells (SOFC) use an electrolyte consisting of a solid oxide material which conducts negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the oxygen ions with hydrogen or carbon monoxide takes place on the anode side. This kind of fuel cell operates at very high temperatures (500 - 1,000°C) and does not need expensive platinum catalysts which are vulnerable to carbon monoxide catalyst poisoning. However, while SOFCs hold great promise for providing highly efficient, clean energy for a low-carbon future, current materials suffer from certain drawbacks that can often lead to premature deterioration of the fuel cell. For example, sulphur poisoning is a problem commonly encountered in SOFCs and there are other forms of degradation particularly concerning anode materials which can limit their applicability, such as accumulation of carbon deposits if the fuel is carbon-based. These carbon deposits may even cause the SOFC to fail altogether.
Now (2021), scientists at the National Energy Technology Laboratory are researching a novel class of advanced materials called high-entropy alloys (HEAs) for anodes that may lead to the manufacture of more resilient solid oxide fuel cells (SOFCs).
The ceramic anode layer of SOFCs has to be porous so that the fuel can flow towards the electrolyte which is why granular matter is often selected for anode fabrication procedures. The anode usually is the thickest and strongest layer in a cell, since its function is to use the oxygen ions that diffuse through the electrolyte to oxidize the hydrogen fuel. The most commonly-used material is a cermet consisting of nickel mixed with the ceramic material, usually yttria stabilized zirconia.
As the formation of solid carbon is a very frequent occurrence in commercial SOFCs, the research team tried to counteract the deposition of carbon and other problems by developing HEAs that were able to replace the materials typically used for anode manufacture. HEAs generally consist of five or more alloys that all contribute significantly to the overall element mix, which results in a more stable structure. The scientists created the HEA alloys using oxide and chemical precursors by conventional sintering and sol-gel processing techniques. The HEAs with the optimised formulation were synthesized by precipitation method. For this purpose, nitrate precursors such as copper(ii) nitrate trihydrate, nickel (ii) nitrate hexahydrate, cobalt (ii) nitrate hexahydrate, or manganese (ii) nitrate tetrahydrate and 10% gadolinium-doped ceria were mixed in deionised water and then heated to 120°C. Ammonia was added to adjust the pH value to about 7-9. The solution was stirred all night to homogenise it. The next day the liquid was heated in a box furnace with an attached ventilation system. The furnace temperature was 500°C which let the ammonia and nitrates burn off while calcination was taking place. The obtained product was then ground to a powder to be incorporated in a fuel cell.
Scientists have long sought to improve solid oxide fuel cells. In 2016, scientists investigated the effects of calcination and milling of 8 mol% yttria stabilized zirconia used in a nickel- yttria stabilized zirconia anode on the performance of anode supported tubular fuel cells. Two different types of cells were prepared. For the anode preparation, a suspension was prepared by combining nickel oxide and yttria stabilized zirconia with 30 vol.% graphite as the pore former. The electrochemical results proved that optimisation of the microstructure of the fuel electrode was important for the optimal distribution of gas within the support of the cell, especially under electrolysis condition where the performance for an optimized cell could be improved by a factor of two.
In 2019, a study investigated the potential of a redox-stable ceramic anode-supported LT-SOFC showing high performance and redox stability. The anode-supported configuration was able to improve the high ohmic loss associated with conventional ceramic anodes and achieved a high open circuit voltage of ∼0.9 V as well as a peak power density of 500 mW/cm2 at 600 °C in hydrogen. Also, the ceramic anode-supported SOFCs remained stable over many redox cycles under harsh operating conditions. The research showed that oxygen nonstoichiometric of SFCM compensated for the dimensional changes occurring during redox cycles.
The new anode design with high-entropy alloys was able to improve several drawbacks of common SOFCs and also exhibited improved efficiencies. The alloys created by the team also worked well in the SOFC during testing. Carbon formation was not witnessed during operation. Moreover, the HEA anode had lower reformation rates than traditional nickel-based anodes, which may contribute to improving the durability of the SOFC by decreasing internal temperature differences that can lead to thermally induced stresses and mechanical failure.
The project will be completed at the end of September and hopefully make a contribution to creating more sustainable and efficient solid oxide fuel cells. The scientists have already applied for a provisional patent, and also fuel cell manufacturers have shown interest in using the technology.