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

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.