Controlling the Interfacial Environment in the Electrosynthesis of MnO_x Nanostructures for High-Performance Oxygen...

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_x) 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 Mn²⁺ and Mn³⁺ concentrations at the outer Helmholtz plane (Frumkin effect on the electrodeposition kinetics) and templating of the MnO_x nanostructure. CTAB produces MnO_x 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^-2) 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 Mn₃O₄ or SDS deposited catalyst containing a combination of α- and β-MnO₂, MnOOH, and Mn₃O₄.The interaction effects among the deposition variables (surfactant type and concentration, anode potential, Mn²⁺ concentration, and temperature) reveal the optimal strategy for high-activity bifunctional MnO_x catalyst synthesis. Mass activities for OER and ORR up to 49 A g^-1 (at 1556 mV_RHE) and -1.36 A g^-1 (at 656 mV_RHE) are obtained, respectively.