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

Abstract

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, WO 3 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 WO 3 phase junctions. The best HER performance (170 mV overpotential for 10 mA/cm 2 ) is obtained when WO 3 nanorods show comparable volumes of hexagonal and monoclinic phases (after 60 min annealing). The effect of phase junction on HER catalysis sustained by WO 3 nanorods is investigated in detail, opening the route of efficient, Pt-free catalysts for HER application.

WO3 Nanorods Decorated with Very Small Amount of Pt for Effective Hydrogen Evolution Reaction

“…WO 3 nanorods were synthesized using the hydrothermal route, and the post-synthesis thermal annealing (400 °C for 60 min) was conducted with the aim of improving the HER catalytic activity according to our previous work [ 14 ]. A partial hexagonal to monoclinic phase transition was achieved after the thermal annealing, as shown in the XRD pattern of Figure S2 , in which the XRD pattern of the WO 3 nanorods is compared with the characteristic XRD patterns of the hexagonal and monoclinic WO 3 .…”

“…All the reagents were purchased from Sigma-Aldrich, (St. Louis, MO, USA) and used without further purification. WO 3 nanorods were hydrothermally synthesized according to our previous work [ 14 , 23 ]. In brief, the precursor solution was prepared using sodium tungstate (Na 2 WO 4 , 0.825 g) dissolved in deionized water (19 mL).…”

“…The powders were calcinated on a hot plate in air at 400 °C, as described in our previous work [ 13 ], to obtain a partial hexagonal to monoclinic phase transition until reaching similar contents. The annealing time was set as 60 min, so that the hexagonal and monoclinic crystallites phases had comparable volumes [ 14 ].…”

“…Several earth-abundant TMO-based electrocatalysts have been studied for the activation of efficient HER in acidic conditions [ 4 , 6 , 11 , 12 ]. Among them, WO 3 represents a suitable candidate thanks to its large availability and high electrochemical stability at low pH, especially in the nanostructured form (nanorods, nanowires, nanosheets) [ 13 , 14 , 15 ]. Unfortunately, in their pristine form, WO 3 nanostructures possesses a poor electron transport ability and few active sites for hydrogen ion absorption, which result in a poor HER ability [ 7 , 16 , 17 ].…”

Engineering of Nanostructured WO3 Powders for Asymmetric Supercapacitors

“…Instead, well-defined typical diffraction peaks of the hexagonal structure are shown in the XRD pattern of WO 3 _Ann70, which appear at 2Ɵ = 14.00°, 24.36°, 26.84°, 28.22°, 33.62°, 36.58°, and 49.95° thus confirming its pure hexagonal phase [ 21 ]. Moreover, peaks clearly related to the monoclinic crystal structure appear at 2Ɵ = 23.00°, 23.50°, 24.28°, 33.12°, 33.54°, 33.84°, 34.04°, 49.74°, 55.71° when the WO 3 powder is annealed at 400 °C, by suggesting a partial phase transition and the coexistence of hexagonal and monoclinic WO 3 phases ( h -WO 3 and m -WO 3 ) in WO 3 _Ann400 [ 23 ].…”