Controlling the Interfacial Environment in the Electrosynthesis of MnO Nanostructures for High-Performance Oxygen Reduction/Evolution Electrocatalysis.

High-performance, nonprecious metal bifunctional electrocatalysts for the oxygen reduction and evolution reactions (ORR and OER, respectively) are of great importance for rechargeable metal-air batteries and regenerative fuel cells. A comprehensive study based on statistical design of experiments is presented to investigate and optimize the surfactant-assisted structure and the resultant bifunctional ORR/OER activity of anodically deposited manganese oxide (MnO) catalysts. Three classes of surfactants are studied: anionic (sodium dodecyl sulfate, SDS), non-ionic (t-octylphenoxypolyethoxyethanol, Triton X-100), and cationic (cetyltrimethylammonium bromide, CTAB). The adsorption of surfactants has two main effects: increased deposition current density due to higher Mn and Mn concentrations at the outer Helmholtz plane (Frumkin effect on the electrodeposition kinetics) and templating of the MnO nanostructure. CTAB produces MnO with nanoneedle (1D) morphology, whereas nanospherical- and nanopetal-like morphologies are obtained with SDS and Triton, respectively. The bifunctional performance is assessed based on three criteria: OER/ORR onset potential window (defined at 2 and -2 mA cm) and separately the ORR and OER mass activities. The best compromise among these three criteria is obtained either with Triton X-100 deposited catalyst composed of MnOOH and MnO or SDS deposited catalyst containing a combination of α- and β-MnO, MnOOH, and MnO.The interaction effects among the deposition variables (surfactant type and concentration, anode potential, Mn concentration, and temperature) reveal the optimal strategy for high-activity bifunctional MnO catalyst synthesis. Mass activities for OER and ORR up to 49 A g (at 1556 mV) and -1.36 A g (at 656 mV) are obtained, respectively.