Efficiency of Proton Ceramics Fuel Cells Considering Mixed Conduction in Solid Electrolyte

Abstract

Proton ceramics fuel cells (PCFCs) using high temperature protonic conductors (HTPCs) as a solid electrolyte are expected because of their possibilities of intermediate temperature operation and high energy conversion efficiency [1]. Some of provskite oxides such as BaZrO3 are known to exhibit significant proton conductivity by doping lower valence cation and exposing to humidified atmosphere. But they are also known to have electron hole conduction when they are used in relatively high oxygen partial pressures p(O2) and high temperatures. Such electron hole conduction is of concern to reduce the efficiency of PCFCs because of the internal leakage current through the electrolyte. The influence of electronic conduction in the electrolyte on the energy efficiency and the output power of fuel cells have been already discussed by Choudhury and Patterson [2]. According to their theoretical treatment, the efficiency and the output power can be evaluated from the ionic and electronic conductivities by assuming local equilibrium in the electrolyte. In this work, we applied this theoretical treatment to discuss the efficiency and the output power of PCFCs. As an example, a PCFC with a BaZr0.8Y0.2O3- d electrolyte was discussed by using the ionic and electronic conductivity data by Nomura et al. [3]. Figure 1 shows the energy efficiency of the PCFC. Here, the gasses in the cathode and the anode were oxygen and hydrogen moisturized with 0.042 bar of water vapour, respectively, and we assumed, as an extreme case, that they supplied sufficiently enough to keep the partial pressures constant, meaning the utilization ratios of the gasses were zero. In addition, the influences of the electrode polarization were neglected, and only the efficiency due to the ionic transport in the electrolyte was considered. Figure 1 was obtained while considering the situation where 0.1 A‧cm-2 of the external current is extracted at 600 and 800°C. It is noted that the electrolyte thickness in the horizontal axis in Fig. 1 inversely decreases when the extracted current increases. It is clearly demonstrated in Fig. 1 that there exists the optimal electrolyte thickness showing the maximum efficiency. At the optimal electrolyte thickness, the loss due to the decrease in the ohmic resistance balances the loss due to the increase in the internal current leakage. It was also found that the efficiency of the PCFC using BaZr0.8Y0.2O3- d considerably degrades due to the electronic conduction, and is only 60% in maximum at 800°C. Since the contribution of the electronic conduction to the total conductivity becomes less significant at lower temperature, the maximum efficiency increases as the operation temperature decreased, although it is still 75% at 600°C. These calculations of the efficiency suggested that the electronic conduction in HTPCs should be taken into account when the structures and the operation conditions of PCFCs are designed. In the presentation, the comparison of the efficiencies of PCFCs and SOFCs will be also discussed. [1] Y. Matsuzaki, Y. Tachikawa, T. Somekawa, T. Hatae, H. Matsumoto, S. Taniguchi, K. Sasaki, Sci. Rep., 5, 12640 (2015). [2] N. S. Choudhury and J. W. Patterson, J. Electrochem. Soc., 118, 1398 (1971). [3] K. Nomura and H. Kageyama, Solid State Ionics, 178, 661 (2007). Figure 1