Department of Energy EngineeringRuthenium 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 for 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 (??EOER-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).
Additionally, partially hydrous RuO2 nanocluster embedded in carbon matrix (x-RuO2@C with x = hydration degree = 0.27 or 0.27@C) was presented as a bifunctional catalyst of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting. Symmetric water electrolyzers based on 0.27-RuO2@C for both electrodes showed smaller potential gaps between HER and OER at pH 0, pH 14 and even pH 7 than conventional asymmetric electrolyzers based on two different catalysts (Pt/C || Ir/C) that have been known as the best catalysts for HER and OER respectively. Moreover, 0.27-RuO2@C showed another bifunctional electroactivity for fuel cell electrochemistry including hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) that are backward reactions of HER and OER respectively. Pt-level HOR electroactivity was obtained while its ORR activity was inferior to that of Pt with 200 mV higher overpotential required. The tetra-functionality of 0.27-RuO2@C issued the possibility of single-catalyst regenerative fuel cells.ope
