Nanostructured WO₃ photoanodes for efficient water splitting via anodisation in citric acid

In this work we report the production of nanostructured WO₃ photoanodes for solar water splitting produced via anodisation using for the first time citric acid (CA), a safer and more environmentally friendly alternative to fluoride-based electrolytes. Photoelectrochemical solar water splitting has shown potential as a renewable method for hydrogen production, a key ingredient to advance the de-carbonisation of our economy. Many methods to produce WO₃ photoanodes are time-consuming and require high temperatures and/or toxic chemicals, such as fluoride-based electrolytes. Here we report on a systematic investigation of the anodisation of tungsten using CA to establish a relation between (i) anodisation parameters (current, time and electrolyte), (ii) the resulting nanostructured morphology and (iii) its performance as a photoanode for water splitting. Characterisation was carried out by X-ray diffraction, scanning electron microscopy, linear sweep voltammetry, and ultraviolet-visible spectroscopy. After optimisation, the obtained WO₃ photoanodes produced a photocurrent of 0.88 mA cm^-2 at 1.0 V vs. Ag/AgCl in 0.5 M aqueous H₂SO₄ under AM1.5 solar irradiation. At low applied potentials (below 0.67 V vs. Ag/AgCl), closer to practical conditions, the photoanodes produced in CA outperformed a conventional counterpart made using a NH₄F electrolyte. The CA-anodised photoanodes also showed higher stability, retaining 90% of their activity after 1 h of chopped solar illumination. This work demonstrates the promise of anodisation in citric acid as an efficient and more sustainable method for the production of WO₃ photoanodes for solar water splitting.