Study on current efficiency of steam electrolysis using a partial protonic conductor SrZr_0.9Yb_0.1O_3-α

The current efficiency of steam electrolysis was measured in the temperature range from 460 to 600 °C using a steam electrolysis cell constructed with a partial protonic conductor SrZr_0.9Yb_0.1O_3-α as an electrolyte and Pt cermet electrodes. The efficiency was increased with increasing partial pressure of water vapor and temperature. The results were considered in relation to the reaction rates at the anode and cathode. Under the operating conditions of steam electrolysis, the reaction rate of producing H₂ from protons at the cathode was found to be faster than that of oxidizing water vapor into protons and O₂ at the anode. Therefore, the average concentration of protons in the partial protonic conductor during electrolysis decreased. On the other hand, the average concentration of holes increased. This is considered to decrease the efficiency of steam electrolysis. It was found that the effective transport numbers of charge carriers in the partial protonic conductor were controlled by the reaction rates at the electrodes at relatively low temperatures at which the equilibria between the atmosphere and defects in the partial protonic conductor were difficult to obtain.