Engineering Hexagonal/Monoclinic WO₃ Phase Junctions for Improved Electrochemical Hydrogen Evolution Reaction

Electrochemical hydrogen evolution reaction (HER) is one of the most promising green methods used to produce renewable and sustainable energy. The development of a highly efficient Pt-free electrocatalyst for HER is a crucial point for the ecosustainability and cost reduction of this method. Herein, WO3 nanorods were synthesized by a hydrothermal method and calcinated in air at 400 °C for different times (30, 60, and 90 min). Experimental investigation involved SEM, TEM, XRD, and electrochemical analyses such as linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Mott Schottky analysis. Calcination at 400 °C induces a peculiar crystal phase transition driven by the formation of hexagonal/monoclinic WO3 phase junctions. The best HER performance (170 mV overpotential for 10 mA/cm2) is obtained when WO3 nanorods show comparable volumes of hexagonal and monoclinic phases (after 60 min annealing). The effect of phase junction on HER catalysis sustained by WO3 nanorods is investigated in detail, opening the route of efficient, Pt-free catalysts for HER application.