Hydrophilicity as a Key Factor in Enhancing Oxygen Evolution Reaction on Rhombohedral Boron Monosulfide/Ni-Foam Electrodes with Carbon Material

In this study, we systematically investigated the impact of carbon support properties on the oxygen evolution reaction (OER) activity of rhombohedral boron monosulfide (r-BS), a two-dimensional catalyst composed solely of main group elements. Specifically, we examined the effects of crystallinity (crystallite size), defect density (Raman ID/IG ratio), and wettability (water contact angle) for four carbon materials: graphene, graphite, carbon black, and multiwalled carbon nanotube (MWCNT). A large number of independent r-BS/carbon material/Ni-foam electrodes were fabricated, and the average current density at 1.8 V vs RHE was evaluated through statistical analysis. The OER activity followed the order: graphene ≈ graphite > carbon black > MWCNT. However, no clear correlation was found with crystallite size or ID/IG ratio. In contrast, the contact angle showed a clear inverse relationship with OER activity, with measured values of 0, 55, 99, and 113° for graphene, graphite, carbon black, and MWCNT, respectively. The graphene-based composite showed the highest current density, although with large variability. In contrast, the graphite-based composite demonstrated similarly high activity with significantly better reproducibility. These findings highlight that the hydrophilicity of the carbon support, which governs the detachment of oxygen gas bubbles from the electrode surface, is a dominant factor in high-current-density OER performance. Overall, this study underscores the importance of contact angle engineering as a practical design principle for optimizing nonprecious-metal-based catalyst–support composites.