Improved Photocatalytic Water Splitting Activity of Highly Porous WO3 Photoanodes by Electrochemical H+ Intercalation

Ramunas Levinas,Tomas Murauskas,Natalia Tsyntsaru,Henrikas Cesiulis

doi:10.3389/fceng.2021.760700

Abstract

WO3 photoanodes are widely used in photoelectrochemical catalysis, but typically the as-synthesized material is annealed before application. It is therefore desirable to explore less energy-intensive treatments. In this study, WO3 films of up to 3.9 μm thickness were obtained by galvanostatic anodization of tungsten foil in a neutral-pH Na2SO4 and NaF electrolyte, also containing a NaH2PO2 additive (to suppress O2 accumulation on the pore walls). Additionally, the WO3 photoanodes were modified by applying a cathodic reduction (H+ intercalation) and anodic activation treatment in-situ. XPS spectra revealed that intercalation modifies WO3 films; the amount of W5+-O and O-vacancy bonds was increased. Furthermore, subsequent activation leads to a decrease of the W5+ signal, but the amount of O-vacancy bonds remains elevated. The as-prepared and reduced (intercalated &amp; activated) films were tested as OER photoanodes in acidic 0.1 M Na2SO4 media, under illumination with a 365 nm wavelength LED. It was observed that thinner films generated larger photocurrents. The peculiarities detected by XPS for reduced films correlate well with their improved photocatalytic activity. Photo-electrochemical impedance and intensity modulated photocurrent spectroscopies were combined with steady-state measurements in order to elucidate the effects of H+ intercalation on photoelectrochemical performance. The reduction results in films with enhanced photoexcited charge carrier generation/separation, improved conductivity, and possibly even suppressed bulk recombination. Thus, the intercalation &amp; activation adopted in this study can be reliably used to improve the overall activity of as-synthesized WO3 photoanodes, and particularly of those that are initially poorly photoactive.

Highlights

Oxide materials have long been thought to be the most promising photo-electrodes, as their corrosion resistance should ensure stable performance over long periods of time (Bak et al, 2002)
The focused ion beam (FIB) cross-sections revealed that their thickness increases with the anodization time, and all films have a porous morphology both on the surface and within the layer
The photoelectrochemical performance for oxygen evolution assessed in an acidic Na2SO4 electrolyte, reveals that the shorter anodization times (2 or 5 min) yield films with rather high photocatalytic activity that could reach ∼ 2 % photon conversion efficiency at 1.2 V under 50 mW cm−2

Summary

INTRODUCTION

Oxide materials have long been thought to be the most promising photo-electrodes, as their corrosion resistance should ensure stable performance over long periods of time (Bak et al, 2002). Tungsten oxide (VI) in particular has garnered substantial attention It is an n-type semiconductor, and has a narrow band gap, typically reported within the range of 2.5–2.8 eV, as measured by indirect optical transitions up to 3.38 eV for amorphous films (Patel et al, 2009; Gullapalli et al, 2010). As-anodized WO3 films are sometimes referred to as “bleached”, whereas after reductive intercalation they become “colored”, owing to the dark blue appearance of hydrogen tungsten bronze HxWO3 (Patel et al, 2010; Ou et al, 2012). The target of this study was the synthesis of highly porous tungsten oxide films by galvanostatic anodization and evaluation of their photoelectrochemical water splitting performance as asdeposited and reduced WO3 photoanodes. Non-stationary methods such as photo-electrochemical impedance spectroscopy and intensity modulated photocurrent spectroscopy were used to discern conductivity and spacecharge layer effects on the oxygen evolution reaction

MATERIALS AND METHODS

CONCLUSION

Findings

DATA AVAILABILITY STATEMENT