Surface Chemistry of WC Powder Electrocatalysts Probed In Situ with NAP-XPS

Abstract

Tungsten carbide (WC) is a renowned compound catalyst material for electrochemical water splitting, and its high electrocatalytic activity toward the hydrogen evolution reaction (HER) has been repeatedly reported. However, its susceptibility to oxidation raises the fundamental question of the underlying reason for its high activity, especially since passivation and thus potential deactivation can occur not only in air but also during reaction. Hence, the investigation of the surface chemistry under true operating conditions is crucial for a fundamental understanding of the electrocatalytic process. In this work, we use electrochemical X-ray photoelectron spectroscopy (EC-XPS) to revisit the surface chemistry of WC powder electrodes in alkaline electrolyte in situ and under full potential control. Our results show that although the surface is initially covered with oxide, this passive film dissolves in the electrolyte under electrochemical reaction conditions. This clarifies the active surface termination during the HER and highlights the potential of laboratory-based EC-XPS to study applied energy conversion materials.

Keywords: Carbide; Electrocatalysis; Hydrogen evolution reaction; ICP‐MS; In situ XPS.

Figure 1

Morphology, chemistry, and electrocatalytic performance of the bare WC compound material. a) Layered scanning electron microscopy (SEM) image and energy dispersive X‐ray (EDX) maps of the WC powder, showing a particle size of ∼200 nm. b) UHV XPS high‐resolution spectrum of the W 4f region with significant WO₃ contribution. c) Faradaic and ionic currents from differential electrochemical mass spectrometry (DEMS) of WC, WO₃, and Pt/C powder inks in 0.1 M NaOH (scan rate 2 mV s⁻¹).

Figure 2

a) EC‐XPS in situ cell for powder materials. EC‐XPS PEEK cell with indent for the powder ink working electrode (WE), grounded via its glassy carbon (GC) current collector, platinum wires as quasi reference and counter electrodes (RE, CE). A tilted geometry allows for a < 20 nm thick 0.1 M NaOH electrolyte film connected to the bulk electrolyte reservoir. The measurement principle relies on cooling of the electrolyte to 2°–8 °C, leading to a vapor pressure of ∼7–10 mbar H₂O and backfilling of the chamber with ∼10 mbar H₂O. Details of the cell with the corresponding 3D drawing are given in the Supporting Information. b) Cyclic voltammogram (CV) of a WC powder ink in 0.1 M NaOH (scan rate: 50 mV s⁻¹). Grey shaded regions indicate the potentials at which charge transfer takes place. Arrows indicate scan direction. The dotted line shows the thermodynamic potential of the HER.

Figure 3

EC‐XP spectra and peak shifts of the WC powder ink electrode in 0.1 M NaOH. a) O 1s high‐resolution spectra with liquid phase water (LPW) and gas phase water (GPW) signals, showing distinct shifts in binding energy (BE) with altered potential. b) W 4f high‐esolution spectra with WC and W⁶⁺ components, where WC shows negligible shift in BE with potential, while the W⁶⁺ peak positions significantly change. In a) and b), the spectral responses to the first applied potential are shown in the top panels; the potential changes are carried out from top to bottom. c) Quantitative evaluation of the BE shifts of all components calculated corresponding to Equation (1); the dashed line indicates the expected −1eV V⁻¹ shift in the double layer region, where no charge transfer reactions occur. The grey shaded areas highlight the potential regions in which charge transfer reactions take place.

Figure 4

Online ICP‐MS analysis of the WC powder ink electrode in 0.1 M NaOH. W dissolution rate (bottom panel) at the corresponding potentials (top panel) versus time during potential sweeps (three cycles) with a scan rate of 2 mV s⁻¹. The upper vertex potential was increased from 1.0 V_RHE (first two cycles) to 1.2 V_RHE (third cycle).