Bilayer Cell Model and System Design of Highly Efficient Protonic Ceramic Fuel Cells

Abstract

Protonic ceramic fuel cells (PCFCs) are promising devices for highly efficient next-generation fuel cell systems. PCFCs provide several benefits. Water formation at the cathode can improve fuel utilization. Also, lowering operation temperature using proton-conducting solid electrolyte membranes will enable a long lifetime and low system costs. On the other hand, ionic and electronic transport properties, i.e., proton, oxide ion, hole, and electron conductions, in PCFCs induce leakage current in electrolyte membranes and decrease energy conversion efficiency. Therefore, controlling the transport properties and designing cell structures to prevent them are important issues for developing highly efficient PCFCs. In this study, a comprehensive design approach was conducted from a bilayer cell to a PCFC system.It has been reported that bilayer electrolytes can improve the interface between cathode and electrolyte and also prevent nickel diffusion (1). Additionally, it has been reported that bilayer electrolytes can suppress leakage current by using a hole blocking layer (2, 3). Therefore, the use of bilayer electrolytes can help improve the efficiency and performance of PCFCs. In this study, a bilayer PCFC consisting of BaZr0.8Y0.2O3−δ (BZY) and BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) was modeled by considering the transport properties of each electrolyte. The main objective of the modeling was to determine the best cell design and operating conditions that maximize PCFC efficiency. Calculations were done by solving the set of integral equations described by Choudhury and Patterson (4) but extended for bilayer electrolytes (3) to obtain the cell potential, and the total and proton current densities.It was found that higher efficiencies were obtained when using a thin layer of BZY with a thicker layer of BZCYYb. The calculations suggest that the thin BZY layer acts as an electron-blocking layer. For example, leakage currents of less than 5% can be achieved by using a BZY(1 µm)|BZCYYb (19 µm). It was also found that the PCFC efficiency increased with decreasing temperature. Thus, higher efficiencies were obtained at 500°C than at 600°C.In addition, the energy conversion efficiency of a 5kW(DC)-class PCFC system was evaluated. In this study, the fuel in an anode was assumed to be hydrogen obtained by steam reforming of methane. Gas compositions in the streams of the PCFC system were calculated based on thermodynamic equilibrium at atmospheric pressure. The operating temperature of PCFC module was set as 500°C-600°C, and the external current density was assumed to be 300 mA/cm2. Here, the leakage current ratio was defined as a percentage of the leakage current to the total ion current density. The system efficiency was defined as an extracted power of the PCFC to the combustion heat of methane (25°C, LHV).The results are described as a standard case of 600°C as follows. A system efficiency of 69.5％ (LHV, DC) was attainable, assuming the cell voltage of 0.85 V, leakage current ratio of 1%, and fuel utilization of 94.2％. In this case, the steam-to-carbon ratio (S/C) was 2.5. In addition, a system efficiency of over 70％ (LHV, DC) was obtained under the cell voltage of 0.9 V, leakage current ratio of less than 5%, and fuel utilization of over 90％. Based on the above results, a cell design with bilayer electrolyte was discussed considering electrode/electrolyte materials and physical properties (e.g., transport number and conductivity). Acknowledgments This work was supported by a project, (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and JSPS KAKENHI JP21H04938 and JP21J14251.