Designing Binary Ru–Sn Oxides with Optimized Performances for the Air Electrode of Rechargeable Zinc–Air Batteries

Because of the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), binary ruthenium–tin oxides synthesized by a hydrothermal method with postannealing at 450 °C for 2 h are first proposed as bifunctional catalysts for these two reactions on the air electrode of rechargeable zinc–air batteries. The binary Ru–Sn oxides in various compositions show the typical oxide solid solution in the rutile phase. Among all binary Ru–Sn oxides, RuSn73 (70 atom % RuO2 and 30 atom % SnO2) and RuSn37 (30 atom % RuO2 and 70 atom % SnO2) show the highest catalytic activities toward the OER and ORR, respectively. Consequently, a novel design of the air electrode consisting of a RuSn37 coating on the carbon paper and a Ti mesh coated with RuSn73 (denoted RuSn(37-C|73-Ti)) is proposed to possess the optimal charge–discharge performances. A unique cell employing such an air electrode has been demonstrated to exhibit a very low charge–discharge cell voltage gap of 0.75 V at 10 mA cm–2. This cell with a peak power density of 120 mW cm–2 at the current density of 235 mA cm–2 also shows an outstanding charge–discharge stability over 80 h. This cell also exhibits an exceptionally high charge rate capability at 150 mA cm–2 with a low charging voltage of 2.0 V.