Bifunctional hydrous RuO 2 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).