A direct carbon fuel cell (DCFC) is the only fuel cell type that converts the chemical energy stored in solid carbon, which could be obtained from coal and biomass, into electricity directly via an electrochemical route with a higher energy efficiency and less pollution than conventional coal-fired power plants ^{[1, 2]}. In this work, the effect of the surface properties of carbon fuel on its electrochemical oxidation reactivity is investigated. Activated carbon (AC) is pre-treated with HNO₃ and NaOH, respectively, both of which decrease the graphitization degree and increase the oxygen content of AC. The amount of hydroxyl groups on the surface of AC increases after the treatments with HNO₃ and NaOH, and then decreases during the subsequent heating process in an inert atmosphere. On the contrary, the carbonyl groups on the surface of AC-HNO₃ remain stable during the heating process. AC-HNO₃ shows the highest reactivity towards oxidation and reverse Boudouard reactions due to its lowest graphitization degree and highest oxygen content. The performance of the cell supported by a 380-μm-thick yttria stabilized zirconia electrolyte layer with 60 mol% La_0.6Sr_0.4Co_0.2Fe_0.8O₃-40 mol% Gd₂O₃ doped CeO₂ as the electrodes is investigated with various fuels. The cell with AC-HNO₃ as the fuel exhibits the highest maximum power density of 81 mW cm^-2 at 800 ^oC.

Fig. 1. I-V and I-P curves of single cells with various carbon fuels at 800 ^oC

References

[1] Jiang C, Ma J, Corre G, et al. Chemical Society Reviews, 2017, 46(10): 2889-2912

[2] Jang H, Park Y, Lee J. Chemical Engineering Journal, 2017, 308: 974–979.