Solid Oxide Fuel Cells Based on Proton Electrolyte: Advantages, Problems and Achievements
An interest to proton conductors is primarily aroused because in an SOFC based on proton electrolytes (SOFC-H) fed with hydrogen the fuel utilization (FU) is close to 100% whereas in the SOFC based on the oxygen-ion electrolyte (SOFC-О) FU is not more than 85 %. It was shown that the FU in the SOFC-H fed with methane is also high and exceeds 90% . Thermodynamic analysis shows that an average electromotive force (EMF) in the SOFC-H is significantly higher than that in the SOFC-O . Overall, the electrical efficiency of the former is by 15-20% higher than of the latter. It was shown that the efficiency of the methane fed SOFC-H is very high, especially at moderate relative power and can reach 80%.
However, proton electrolytes have some peculiarities that influence on the SOFC-H power and efficiency. Firstly, proton conductivity depends on pH2O. Secondly, they have partial hole conductivity under oxidizing atmosphere. Our calculations show that increasing in hydrogen humidity leads to increase of the SOFC-H power whereas the air humidification does not affect the power. The calculations also show that even small hole conductivity leads to drastic decrease of the SOFC-H efficiency. So, if an average hole transfer number is 0.1, the SOFC-H efficiency is not higher than 50%.
In the frame of the Megagrant project, we fabricated half-sells using a tape calendering method (TCM) with subsequent sintering . The single cell consisted of the 30 mm BaCe0.5Zr0.3Y0.2O3 (BCZY) electrolyte, the Y0.8Ca0.2BaCo4O7 cathode and the reduced 40%BCZY + 60%NiO and 45%BCZY + 55%NiO support and functional anode layers. It was found that the cell with BCZY electrolyte exhibited very high open circuit voltage (OCV) values (1.141 and 1.104 V at 600 and 700 oC) and therefore predominant ionic transport (average ions transport number 0.995 and 0.98 at 600 and 700 oC, correspondingly). These values are significantly higher than ones reported in literature for the cells with the similar electrolyte. For instance, the highest OCV values were obtained in  and they were by 0.1 V lower than ours. It is found that the maximal power densities of the fabricated cell (174 and 280 mW cm2 at 600 and 700 oC, respectively) are acceptable for an intermediate-temperature range of operation and comparable with those reported in literature. The tape calendering method can be considered as effective strategy for SOFC-H fabrication, ensuring the formation of gas-tight electrolytes and acceptable electrochemical properties.
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