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

Abstract

Although organic–inorganic hybrid Mn-containing decavanadate single crystals have been reported, surprisingly, there are no reports on organic-free Mn-containing decavanadate-based single crystals. In this work, we have achieved the isolation of pure inorganic single crystals of a 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 and 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 the HER, homogeneous solutions (under identical conditions) of individual components [V₁₀O₂₈]⁶⁻ (i.e., Na₆[V₁₀O₂₈]) and [Mn(H₂O)₆]²⁺ (e.g., MnCl₂·4H₂O) 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 the ORR utilizing a rotating ring-disk electrode (RRDE) have validated the electrochemical production of H₂O through four-electron reduction of molecular oxygen. Compound 1 demonstrates better electrocatalytic ORR activity as compared with that of homogeneous solutions (under identical conditions) of the 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.