Highly Active W2C-Based Composites for the HER in Alkaline Solution: the Role of Surface Oxide Species

Abstract

The hydrogen evolution reaction (HER) is a crucial electrochemical process for the proposed hydrogen economy since it has the potential to provide pure hydrogen for fuel cells. Nowadays, hydrogen electroproduction is considerably expensive, so promoting the development of new non-noble catalysts for the cathode of alkaline electrolyzers appears as a suitable way to reduce the costs of this technology. In this sense, a series of tungsten-based carbide materials have been synthesized by the urea-glass route as candidates to improve the HER in alkaline media. Moreover, two different pyridinium-based ionic liquids were employed to modify the surface of the carbide grains and control the amount and nature of their surface species. The main results indicate that the catalyst surface composition is modified in the hybrid materials, which are then distinguished by the appearance of tungsten suboxide structures. This implies the action of ionic liquids as reducing agents. Consequently, differential electrochemical mass spectrometry (DEMS) is used to precisely determine the onset potentials and rate-determining steps (RDS) for the HER in alkaline media. Remarkably, the modified surfaces show high catalytic performance (overpotentials between 45 and 60 mV) and RDS changes from Heyrovsky-Volmer to Heyrovsky as the surface oxide structures get reduced. H₂O molecule reduction is then faster at tungsten suboxide, which allows the formation of the adsorbed hydrogen at the surface, boosting the catalytic activity and the kinetics of the alkaline HER.

Keywords: AEMWE; HER; ionic liquid; suboxide; tungsten carbide.

Figure 1

XRD patterns of commercial W₂C (black line), WCU2 (green line), and WCU4 (red line) materials. Cubic tungsten (•, 00-004-0806; turquoise line) and hexagonal W₂C (, 00-035-0776; magenta) from the PDF2 database are also depicted for comparison purposes. The signals related to WC traces in the commercial carbide are marked with .

Figure 2

(A) Raman spectra, where δ indicates the O–W–O bending vibrational mode while ν_a and ν_s indicate the symmetric and antisymmetric W–O stretching mode, respectively. (B–F) High-resolution XPS scans recorded in the W 4f region for W₂C, WCU2, WCU4, WCU4-OPy and WCU4-EPy. (E,F) show contributions from F 2s in the same region.

Figure 3

(A) LSVs (top panel) with corresponding MSLSVs for m/z = 2 (bottom panel) signals obtained in the DEMS experiments performed with W₂C, WCU2, WCU4, WCU4-OPy and WCU4-EPy catalysts. The LSVs were recorded starting at 0.1 V_RHE and going toward reduction potentials at 1 mVs^–1 in 0.1 M NaOH (IUPAC convention) using a GC rod as counter electrode, a KCl_sat Ag/AgCl reference electrode and a GC disk as substrate for the working electrode material. The procedure for the modification of the substrate can be found in the experimental section. (B) Tafel plot for the HER obtained from MSLSVs.

Figure 4

(W²⁺ + W⁴⁺)/W⁶⁺ surface ratio (black line and left axis) and onset overpotential for the HER determined by DEMS (blue line and right axis) for all catalysts studied.

Figure 5

Chronopotentiometry experiments recorded for W₂C, WCU2, WCU4, WCU4-OPy, and WCU4-EPy catalysts. The catalysts were immersed in the solution under an applied potential of 0.1 V_RHE for 20 s and then −5 mA cm^–2 were applied for 15 h in a 0.1 M NaOH solution (IUPAC convention). A GC rod was used as a counter electrode, a KCl_sat Ag/AgCl was used as a reference electrode, and a GC disk was used as a substrate for the immobilization of the working electrode material. The procedure for the modification of the substrate can be found in the experimental section.