Bifunctional hydrous RuO₂ nanocluster electrocatalyst embedded in carbon matrix for efficient and durable operation...

Ruthenium oxide (RuO₂) is the best oxygen evolution reaction (OER) electrocatalyst. Herein, we demonstrated that RuO₂ 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 RuO₂ (i.e. hydrous and anhydrous, respectively h-RuO₂ and ah-RuO₂) to show different ORR and OER electrocatalytic characteristics. Thus, h-RuO₂ required large ORR overpotentials, although it completed the ORR via a 4e process. In contrast, h-RuO₂ triggered the OER at lower overpotentials at the expense of showing very unstable electrocatalytic activity. To capitalize on the advantages of h-RuO₂ while improving its drawbacks, we designed a unique structure (RuO₂@C) where h-RuO₂ nanoparticles were embedded in a carbon matrix. A double hydrophilic block copolymer-templated ruthenium precursor was transformed into RuO₂ nanoparticles upon formation of the carbon matrix via annealing. The carbon matrix allowed overcoming the limitations of h-RuO₂ by improving its poor conductivity and protecting the catalyst from dissolution during OER. The bifunctional RuO₂@C catalyst demonstrated a very low potential gap (ΔE _OER-ORR = ca. 1.0 V) at 20 mA cm^-2. The Zn||RuO₂@C cell showed an excellent stability (i.e. no overpotential was observed after more than 40 h).