Numerical Analysis of the Local Current Efficiency Distribution in a Protonic Ceramic Fuel Cell Considering Gas Concentration Distributions
Authors: Li Kunpeng , Suito Fumiyasu , Ryogo Takatera , Atsuhito Ota , Araki Takuto , Okuyama Yuji , Mori Masashi 
Published: July 2021

Abstract

Protonic ceramic fuel cells (PCFCs) are expected as highly efficient energy conversion devices due to the high proton conductivity at relatively lower operating temperature (400 ºC~600 ºC) compared to solid oxide fuel cells (SOFCs). The water steam is generated at air side so that there is less fuel dilution at fuel side. Therefore, PCFCs could operate with higher fuel utilization. However, besides the main proton conductivity, some other chargers including electron holes, oxygen ions, and electrons can also be conductive in PCFCs leading a performance decrement. Especially, the electron hole conductivity cannot be neglected inducing a decrease of current efficiency under some operating conditions. The conductivities in PCFCs are not only affected by the operating temperature, but the gas concentrations also strongly influence their values [1]. The conductivities of proton and hole are respectively show positive relationship to steam and oxygen concentrations [2-3]. These properties of PCFCs cause complex for investigating the performances.

In this study, the defect transports in a PCFC electrolyte with BaZr0.8Yb0.2O3-δ (BZYb20) material were numerical solved with a quasi-two-dimensional Nernst-Planck-Poisson (NPP) model and the effect of gas concentration on the cell performance was discussed. For the NPP model calculation algorithm, the Nernst-Planck equation (or flux continuity equation) and the Poisson equation are successively solved by finite difference method (FDM) until convergence criteria are obtained with the defect concentration boundary conditions. The defect concentrations at electrolyte membrane surfaces (boundary conditions of NPP model) are derived by equilibrium assumptions of the reactions such as hydration and oxidation reactions. The charge neutrality condition is also added to the boundary conditions for the exact solutions [4]. Subsequently, the local defects (i.e., proton and electron hole) concentrations are revealed, so that the local current efficiency distribution in the PCFC can be revealed.

The utilizations of fuel and oxygen are set at 80% and 30%, respectively. The vapor concentration at anode and cathode side are assumed as 3%. The electrolyte thickness is 8 μm in the present study. A constant temperature distribution is assumed in the numerical model. An example result under co-flow gas supply including proton current density and current efficiency distributions is shown in the following figure. The proton current density along fuel flow direction is found to increase from inlet to outlet. The reason can be considered as the influences of water steam distributions. The water steam concentration (or partial pressure) at both fuel and air side increase from inlet to outlet. The current efficiency at downstream are higher than that of upstream. This is contributed by the increase of oxygen in gas flow direction. Moreover, the influences of gas flow direction on cell performances have also been investigated.

Acknowledgements

This research is primarily based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) Grand Number JPNP20003, Japan. We also extend out special thanks to members of AIST Solid State Ionics Materials Group for useful information and discussions.

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Figure 1