Mn(II)-aqua-decavanadate: a bifunctional homogeneous electrocatalyst for hydrogen evolution- and oxygen reduction-reaction

Abstract. Although the organic-inorganic hybrid Mn-containing decavanadate single crystals are reported, surprisingly, there is no report on organic-free Mn-containing decavanadate-based single crystals. In this work, we have achieved isolating pure inorganic single crystals of manganese containing decavanadate compound K2[Mn(H2O)6]2[V10O28] (1). This water-soluble compound (1) has unambiguously been characterized by single-crystal X-ray crystallography, including routine spectroscopic and microscopic analyses. Interestingly, compound 1 acts as a bifunctional homogeneous electrocatalyst for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in an acidic aqueous solution without its decomposition (or electrodeposition). Notably, in the case of HER, homogeneous solutions (identical condition) of individual components [V10O28]6- (i.e., Na6[V10O28]) and [Mn(H2O)6]2+ (e.g., MnCl2‧6H2O) do not exhibit electrocatalytic HER activity, even though these two components are the primary sub-units of compound 1. However, the synergy of [V10O28]6- with [Mn(H2O)6]2+ within the crystal matrix enhances the stability and significantly increases the homogeneous electrocatalytic HER activity of compound 1. Experiments involving ORR utilizing a rotating ring-disc electrode (RRDE) have validated the electrochemical production of H2O through four-electron reduction of molecular oxygen. Compound 1 shows a better electrocatalytic ORR activity as compared with those of homogeneous solutions (identical condition) of individual [V10O28]6- and [Mn(H2O)6]2+components of compound 1 indicating a synergy between [V10O28]6- and [Mn(H2O)6]2+ within the crystal matrix.