Incorporation of Ruthenium Ions into Nanometer-Thick Films of Copper Sulfide by Anodization for Proton-Coupled Electron Transfer Reactions

Herein, we report a facile anodization-assisted direct fabrication of a ruthenium-ion-incorporated nanostructured copper sulfide thin-film (Ru-CuxS) cathode for electrocatalytic hydrogen evolution in a protic electrolyte. High-resolution transmission electron microscopy with elemental mapping reveals the ruthenium ions' incorporation and their uniform distribution in the copper sulfide thin film. The Ru-CuxS exhibits a cathodic overpotential of −121 mV vs the reversible hydrogen electrode (RHE) to attain the benchmark current density of −10 mA cm–2 with a very low electrocatalyst loading of 0.7–1 mg cm–2 as a thin film. In contrast, an anodically grown nanopetal-like copper sulfide (CuxS) thin film on a copper substrate requires −286 mV vs RHE to achieve a similar current density. After the prolonged hydrogen evolution, the electrochemically active surface area was further increased due to the exposure of more active sites. Also, the Ru-CuxS shows good protic stability with a low Tafel slope value of 77 mV dec–1 with better kinetics for hydrogen evolution than CuxS thin films. The thin film remains relatively stable for a long duration, even at a very high hydrogen evolution (at a constant current density of −350 mA cm–2). Furthermore, the doping of ruthenium ions to the copper sulfide displays nearly 10–15 times enhanced p-type photocurrent responses, and it is relatively stable in protic electrolytes compared to the p-type copper oxide photocathode. Ru-CuxS exhibits an enhanced oxygen reduction current density along with a significant shift in the onset reduction potential, being 250 mV more anodic than the CuxS thin film.