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

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+y4 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 cmeCSA) than the ordinary Mn3O4 electrode (0.0088 A g-1 cmeCSA). 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.

Automated summary

In this study, sulfur-doped manganese pre-catalysts were prepared via electrodeposition method and supported by carbon paper. The effect of different anions on the structure and oxygen evolution reaction (OER) activity of the catalysts was analyzed. The results showed that sulfur and water induce the formation of manganese oxide/hydroxide during the electroreduction process, promoting the OER activity. This study provides insights into the structure-activity evolution of manganese-based pre-catalysts during electrodeposition and OER catalytic process.