Numerical Analysis of Humidification Condition Effects on Protonic Ceramic Fuel Cell Performance Considering Concentration Overpotential

Abstract

Protonic ceramic fuel cells (PCFCs) show relatively high performance at intermediate temperature range (400–600 °C) due to high proton conductivity in electrolyte. However, besides proton, other charges such as electron hole and oxygen vacancy can also be conductive in PCFCs. Especially, the hole conductivity will induce a hole current leakage so that current efficiency and power density will decrease. Besides the operating temperature, the conductivities of proton and hole are also strongly affected by the partial pressures of steam and oxygen in the PCFC [1,2]. Higher steam partial pressure leads higher proton conductivity, so that a higher power density can be expected. However, other influences such as concentration overpotential may be non-negligible. Furthermore, the proton and hole current density are affected by defect concentrations differences between two surfaces of the electrolyte membrane. Numerical simulation method is powerful tool for dealing with the complicated conditions in a PCFC mentioned above.In the present study, the influences of steam partial pressure on a BaZr0.8Yb0.2O3− δ (BZYb20) electrolyte are investigated. Specifically, to reveal the currents induced by the electrostatic potential and gas concentrations, Nernst-Planck model is selected. To relatively obviously reveal the steam concentration influences on the PCFC performances, low (3%) and high (69%) humidification conditions of H2 fuel gas are adopted. The results in Fig. 1(a) and 1(b) show the simulation and experimental current-voltage characteristics under 3% and 69% humidification conditions, respectively. The green solid and dotted lines are external and proton current densities considering concentration overpotential influence, respectively. The blue lines express external current density when no concentration overpotential in the numerical model. The red circles are the measured data. At low steam partial pressure, the simulation results well reproduced the experimental data without concentration overpotential consideration. However, an obvious deviation was found between simulation and experimental results when neglecting concentration overpotential in the simulation model. The performances such as open circuit voltage (OCV) and hole current leakage (difference between proton and external current density) are influenced by the steam partial pressure.