Experimental Demonstration of the Electric Efficiency with BZYb based Protonic Ceramic Fuel Cell

Protonic Ceramic Fuel Cell (PCFC) is expected to be the next-generation fuel cell for its higher energy conversion efficiency and lower operation temperature. BaZrO 3 based oxide are potential materials as PCFC electrolyte because they have a high chemical stability under CO2 condition and high proton conductivity. We have developed PCFC using BaZr 0.8 Yb 0.2 O x (BZYb) as the electrolyte and performed max power density 0.49 W cm –2 at 873 K. For practical use of PCFC, it is needed to conduct the higher electric efficiency and to improve the durability. Especially, improving the electric efficiency of PCFC might be difficult due to the internal current leakage by the hole creation. In this work, we have evaluated the electric efficiency using a Φ60mm anode supported PCFC (active electrode area: Φ45mm, 15.9 cm 2 ). Thin and dense BZYb electrolyte is supported on porous Ni-BZYb cermet anode and La 0.6 Sr 0.4 CoO x cathode. The effect of fuel utilization on PCFC cell was examined and the electric efficiency of PCFC was calculated as follows: η = V / (-ΔH / nF) × Uf where the n and F are the mole number and the Faraday constant, respectively. ΔH is the standard combustion enthalpy of hydrogen or methane. The value of -ΔH / nF is 1.24 V or 1.04 V in the case of hydrogen supply or methane supply respectively. V is the cell voltage and Uf is the fuel utilization. Φ60mm anode supported PCFC was fabricated by tape-casting process used for the industrial production of ceramic electronic devices. NiO-BZYb anode slurry and BZYb electrolyte slurry were prepared by mixing the powder, binder, solvents and plasticizer. And then, these slurries were tape-casted respectively. The anode green sheets and the electrolyte green sheets were stacked and pressed. The anode electrolyte half-cell was obtained by firing this laminate at 1673–1773 K for 2 h. La 0.6 Sr 0.4 CoO 3-δ (LSC) paste was applied to the surface of the half-cell by screen–printing, and then, fired at 1223 K for 2 h to form a cathode. The electrochemical performance of the Φ60mm anode supported PCFC was investigated at 873 K under the conditions of a humid fuel gas (i.e., 3% H 2 O, 48.5% N 2 and 48.5% H 2 ) and a humid air (i.e. 3% H 2 O and 97% air). Fuel flow rate were determined by the operational current and the fuel utilization (Uf), and air flow rate were constant at 264 cc min –1 , which the air utilization (Uair) was 30% at the current of 0.3 A cm –2 . Figure 1 shows the comparison of the current-voltage (I-V) and current-power (I-P) characteristics of Φ60mm and Φ20mm BZYb cell. Fuel flow rate was 100 cc min –1 at this experiment. The same electrochemical performance was observed, and the maximum power reached 7 W at 12.9 A in the case of the Φ60mm cell. Figure 2 shows the cell performance of 0.3 A cm –2 current density under the various Uf condition. The theoretical voltage line was also drawn in the figure, which was calculated by Nernst equation used the P H 2 in the anode outlet gas, P O 2 and P H 2 O in the cathode outlet gas. Cell voltage followed the same path as the theoretical voltage in the low Uf condition (Uf≦70%). On the other hand, the gap between the cell voltage and theoretical one was larger in the higher Uf condition (Uf≧80%). Anode gas diffusion overvoltage might be increased by P H 2 decreasing in anode gas or the imbalanced gas flow due to decreasing the anode gas flow rate. We demonstrated that BZYb based PCFC could be operated at high Uf condition (i.e., Uf=90%) and the maximum electric efficiency was 55% (DC). This presentation is partially based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1