Deep eutectic solvent mediated synthesis of Mn-based hybrid electrocatalyst for oxygen evolution reaction: Insights into the effect of anion on the evolution of structure-activity

Abstract

In this work, sulfur-doped manganese (S-Mn/CP) pre-catalysts supported by carbon paper (CP) were prepared from choline chloride/ethylene glycol-based deep eutectic solvent containing different manganese salts via electrodeposition method. The effect of anions, including NO3–, Cl-, SO42-, CH3COO–, and crystal water (H2O), on the evolution of the structure and OER activity of as-prepared Mn-based pre-catalysts was analyzed. It turns out that the NO3– and H2O can be reduced during the electroreduction of Mn(II) species and induce the formation of manganese oxide/hydroxide. Thiourea (TU), used as a sulfur source, decomposes at the cathode and releases S2-. The TU and H2O replace the Cl- in the [Mn(Cl)6]4- to form [Mn(Cl)6-x(H2O)x]x-4 and [Mn(Cl)6-x-y(TU)y(H2O)x]x+y-4 complexes, promoting the electroreduction of Mn(II) species. The OER intrinsic catalytic activity of the amorphous Mn3O4 pre-catalyst is 16.3 times lower than that of the metallic Mn0 pre-catalyst in a 1.0 M KOH solution. After annealing in an argon atmosphere, the surface of metallic Mn0 is in-situ converted into Mn3O4 and exhibits superior activity (0.0395 A g−1 cm-2ECSA) than the ordinary Mn3O4 electrode (0.0088 A g−1 cm-2ECSA). This study brings insights into the effect of anion on the evolution of structure-activity of Mn-based pre-catalysts during the electrodeposition and OER catalytic process.