Pyrite-type cobalt phosphosulphide bifunctional catalyst for aqueous and gel-based rechargeable zinc-air batteries

Bifunctional electrocatalysts with favorable electrode kinetics towards oxygen reduction and oxygen evolution reactions are the main pre-requisites to achieve practical utility of metal-air systems. Herein, ternary pyrite type nanosized cobalt phosphosulphide is reported as an excellent oxygen reduction/oxygen evolution catalyst in alkaline medium with a favorable reversible oxygen electrode index (difference between E-10mA for oxygen evolution and E-3mA for oxygen reduction) of 0.69 V. Four-electron reduction with low overpotential of similar to 240 mV to observe a current density of 10 mA cm(-2) is reported. The assembled zinc-air batteries with both aqueous liquid- and gel-based electrolytes display stable low charge-discharge voltage polarization over a long duration. Aqueous primary zinc-air battery exhibits an impressive peak power density of 385 mW cm(-2) at 560 mA cm(-2) and a current density of 220 mA cm(-2) at 1.0 V. The zinc-air battery in the air-breathing, rechargeable mode operates for over 100 h at 10 mA cm(-2) and over a few hundred cycles under a drain current of 50 mAcm(-2). The rechargeable battery has been investigated for both long term (4 h) and short term (10 min) cycles under different drain currents (10, 20 and 50 mA cm(-2)). Round trip efficiencies of-64% have been achieved. Use of CoPS electrodes with gel-based electrolytes has also been demonstrated. The primary gel battery displays a specific capacity of 803 mA cm(-2) corresponding to an energy density of 891 Wh kg(-1). The high efficiency and durability of the catalyst establish its feasibility as a cost-effective alternative to precious metal catalysts.