Flexocatalytically driven water splitting: unprecedented hydrogen production using symmetry invariant ZnIn2S4 nanosheets

Abstract

Flexocatalytic water splitting offers an intriguing avenue for “sustainable hydrogen” generation and has the potential to overcome several inherent limitations of photocatalytic, electrocatalytic, and even piezocatalytic methods. In particular, flexocatalysis does not necessitate a non-centrosymmetric crystal structure, unlike piezocatalysis, which allows choosing from a large materials database. In this context, centrosymmetric 2D zinc indium sulfide (ZnIn₂S₄) nanosheets were utilized as an active material for flexoelectrically-driven water-splitting. Notably, by utilizing methanol as a sacrificial agent at an ultrasonic frequency of 40 kHz, an unprecedented H₂ evolution rate of 60 mmol g⁻¹ h⁻¹ (0.120 mmol h⁻¹) was achieved without using a co-catalyst, demonstrating the practical viability of the ZnIn₂S₄ (ZIS) nanosheets. FEA (finite element analysis) simulation reveals that induced flexoelectric polarization developed over the ZIS nanosheets due to inhomogeneous stress distribution that facilitates the progressive water-splitting reaction. A series of control experiments with electrochemical impedance spectroscopy and surface potential studies were conducted to gain mechanistic insights into the flexocatalytically-driven water splitting over ZIS nanosheets. Density functional theory corroborates the experimental findings, revealing that applying stress on the catalyst lowers the Gibbs free energy, promoting H₂ production over ZIS nanosheets. Thus, the present study presents a symmetry invariant pathway for transforming mechanical energy into H₂ production, thereby paving a potential way for sustainable and economically feasible H₂ production.