Synergy of {Co(H₂O)₆}²⁺ with a Polyoxometalate Leads to Aqueous Homogeneous Hydrogen Evolution: Experiments and Computations

Abstract

In this work, we have described a polyoxometalate (POM)-based inexpensive and easily synthesizable compound [Co(H 2 O) 6 ] 2 [{K(H 2 O)} 2 V 10 O 28 ]•2H 2 O (1), which exhibits electrocatalytic hydrogen evolution in its aqueous solution without its decomposition (or electrodeposition), acting as a rare homogeneous electrocatalyst. Even though the compound [Co(H 2 O) 6 ] 2 [{K(H 2 O)} 2 V 10 O 28 ]•2H 2 O (1) (soluble in water) shows electrocatalytic hydrogen evolution reaction (HER) activity because of the Coulombic attraction, including H-bonding interactions, between the [Co(H 2 O) 6 ] 2+ cationic species and [{K-(H 2 O)} 2 V 10 O 28 ] 4− anionic species, the individual homogeneous solutions of [V 10 O 28 ] 6− (source: Na 6 [V 10 O 28 ]•18H 2 O) and [Co(H 2 O) 6 ] 2+ (source: CoCl 2 •6H 2 O) do not show any electrocatalytic HER activity. We have thus established that the synergy of [V 10 O 28 ] 6− with [Co(H 2 O) 6 ] 2+ in crystal matrix as well as in the aqueous solution of 1 makes the compound 1 a stable and highly active electrocatalyst for homogeneous HER in an aqueous solution. In order to corroborate these homogeneous HER studies, we performed density functional theory (DFT) calculations to show that decavanadate cluster anion [V 10 O 28 ] 6− interacts with hexa-aqua complex cation [Co(H 2 O) 6 ] 2+ via strong H-bonding interactions, leading to a synergy effect that enables the cobalt center of [Co(H 2 O) 6 ] 2+ to be an active site of HER in the present work.