Hydroxylated Polyoxometalate with Cu(II)- and Cu(I)-Aqua Complexes: A Bifunctional Catalyst for Electrocatalytic Water Splitting at Neutral pH

Abstract

A sole inorganic framework material [Li(H2O)4][{CuI(H2O)1.5} {CuII(H2O)3}2{WVI 12O36(OH)6}]·N2·H2S·3H2O (1) consisting of a hydroxylated polyoxometalate (POM) anion, {WVI 12O36(OH)6}6–, a mixed-valent Cu(II)– and Cu(I)–aqua cationic complex species, [{CuI(H2O)1.5}{CuII(H2O)3}2]5+, a Li(I)-aqua complex cation, and three solvent molecules, has been synthesized and structurally characterized. During its synthesis, the POM cluster anion gets functionalized with six hydroxyl groups, i.e., six WVI–OH groups per cluster unit. Moreover, structural and spectral analyses have shown the presence of H2S and N2 molecules in the concerned crystal lattice, formed from “sulfate-reducing ammonium oxidation (SRAO)”. Compound 1 functions as a bifunctional electrocatalyst exhibiting oxygen evolution reaction (OER) by water oxidation and hydrogen evolution reaction (HER) by water reduction at the neutral pH. We could identify that the hydroxylated POM anion and copper-aqua complex cations are the functional sites for HER and OER, respectively. The overpotential, required to achieve a current density of 1 mA/cm2 in the case of HER (water reduction), is found to be 443 mV with a Faradaic efficiency of 92% and a turnover frequency of 4.66 s–1. In the case of OER (water oxidation), the overpotential needed to achieve a current density of 1 mA/cm2 is obtained to be 418 mV with a Faradaic efficiency of 88% and turnover frequency of 2.81 s–1. Diverse electrochemical controlled experiments have been performed to conclude that the title POM-based material functions as a true bifunctional catalyst for electrocatalytic HER as well as OER at the neutral pH without catalyst reconstruction.

Electrocatalytic Hydrogen Evolution by a Uranium(VI) Polyoxometalate: an Environmental Toxin for Sustainable Energy Generation

“…Nyquist plots of electrochemical impedance spectroscopy were collected in the HER region by employing the sinusoidal potential (−500 mV vs RHE). Differential pulse voltammetry (DPV) for compound 1 was performed at neutral pH as well as in acidic pH 3 with the 50 mV pulse for 100 ms time and 5 mV step height for 60 ms step width, and the integration time was 5 ms. A homemade electrochemical cell for bulk electrolysis has been used to determine the Faradaic efficiency of the catalyst from the volume (in mL) of the electrochemically generated hydrogen gas. − …”

Measurements of Dioxygen Formation in Catalytic Electrochemical Water Splitting

“…To minimize the problem of different increase of pressure in anodic and cathodic compartments, Das et al [16,17] designed an electrochemical cell to collect the generated oxygen and to measure its volume. This cell (two electrode configuration) resembles a double-jacked vessel in which the inner anodic chamber is connected to the outer cathodic chamber with a small hole to manage and maintain ionic transport.…”