Proton conduction facilitates the hydrogen evolution reaction in heptamolybdate-protected SnO₂ nanocore

Recently, the Anderson-type heptamolybdate [Mo₇O₂₄]^6- ion has been used as an inorganic ligand to protect various metal oxy-hydroxides (FeOOH and Ni_xFe_1-xOOH) and oxides (SnO₂ and CeO₂), and these unique varieties of materials are employed as electrode materials for water oxidation and nitrate reduction. Herein, [Mo₇O₂₄]^6- covalently connected to a SnO₂ nanocore, namely Mo_x@SnO₂, is prepared through a hydrothermal route and has been fabricated on a 1 cm² surface of a conducting carbon support to study proton conduction and electrocatalytic proton reduction. The Mo_x@SnO₂ fabricated carbon cloth (CC) exhibits the maximum proton conductivity of 15.34 × 10⁻³ S cm^-1 at pH 3.65 and room temperature, which increased linearly with the increment in cell temperature and reached 32.47 × 10⁻³ S cm⁻¹ at 318 K. Under similar conditions, the bare SnO₂ fabricated on CC shows relatively low proton conductivity, highlighting the importance of POM ligands on Mo_x@SnO₂ to facilitate proton conduction with an extremely low activation energy of 0.13 eV, indicative of proton shuttling through the Grotthuss pathway. Further, the facile proton conduction on Mo_x@SnO₂ and the redox participation of POMs, as depicted from cyclic voltammetric studies, enable electrocatalytic hydrogen evolution reaction (HER) under mild acidic conditions, delivering an onset potential of -0.4 V vs RHE and a nearly two-fold higher catalytic current at -0.9 V compared to that observed for SnO₂. The electrokinetic study has confirmed that the improved performance of Mo_x@SnO₂ over SnO₂ is due to enhanced charge-transfer kinetics. This study showcases a new role for covalently attached heptamolybdate anions on SnO₂, facilitating proton relay and reduction.