Bifunctional hydrous RuO nanocluster electrocatalyst embedded in carbon matrix for efficient and durable operation of rechargeable zinc-air batteries.

Ruthenium oxide (RuO) is the best oxygen evolution reaction (OER) electrocatalyst. Herein, we demonstrated that RuO can be also efficiently used as an oxygen reduction reaction (ORR) electrocatalyst, thereby serving as a bifunctional material for rechargeable Zn-air batteries. We found two forms of RuO (i.e. hydrous and anhydrous, respectively h-RuO and ah-RuO) to show different ORR and OER electrocatalytic characteristics. Thus, h-RuO required large ORR overpotentials, although it completed the ORR via a 4e process. In contrast, h-RuO triggered the OER at lower overpotentials at the expense of showing very unstable electrocatalytic activity. To capitalize on the advantages of h-RuO while improving its drawbacks, we designed a unique structure (RuO@C) where h-RuO nanoparticles were embedded in a carbon matrix. A double hydrophilic block copolymer-templated ruthenium precursor was transformed into RuO nanoparticles upon formation of the carbon matrix via annealing. The carbon matrix allowed overcoming the limitations of h-RuO by improving its poor conductivity and protecting the catalyst from dissolution during OER. The bifunctional RuO@C catalyst demonstrated a very low potential gap (ΔE  = ca. 1.0 V) at 20 mA cm. The Zn||RuO@C cell showed an excellent stability (i.e. no overpotential was observed after more than 40 h).