Optimal Design of a Binder-Free Manganese/Cobalt Bilayer Bifunctional Catalyst for Rechargeable Zinc–Air Batteries

Abstract

We have developed a bilayer film comprising cobalt oxyhydroxide (CoOOH) underlayer and manganese dioxide (MnO2) upper layer, which are active toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), respectively. The bilayer bifunctional catalyst is synthesized by electrodepositing cobalt hydroxide (Co(OH)2) on a porous carbon paper (CP) and subsequently immersing the obtained Co(OH)2/CP in a potassium permanganate (KMnO4) solution without binders or conductive additives. Specifically, electron transfer between the already-deposited Co(OH)2 and MnO4 – proceeded in the solution, yielding MnO2, until all the Co ions become trivalent, after which self-terminates. The proposed method only allows for the construction of the minimum required bifunctional catalyst only at the reaction site of the gas-diffusion electrode, i.e., at the so-called three-phase interface, thus remarkably increasing catalyst utilization while improving reactant and product diffusions. The developed catalyst shows stable MnO2/CoOOH cycles at |20| mA cm–2 with a minimal difference (0.764 V) between the OER and ORR potentials, reflecting the structural advantage of the proposed catalyst. This work proposes efficient bifunctional catalysts having spatially separated OER/ORR reactive sites that can be synthesized via the simple and scalable electrochemical method, which does not require the skill and optimization of binder and electron-conducting additives.