Fe-Treated Heteroatom (S/N/B/P)-Doped Graphene Electrocatalysts for Water Oxidation

Anodic water splitting is driven by hydroxide (OH-) adsorption on the catalyst surface and consequent O-2 desorption. In this work, various heteroatoms (S/N/B/P) with different electronegativities and oxophilicities are introduced to alter the catalytic activity of reduced graphene oxide (RGO) as a catalyst for the oxygen evolution reaction (OER). It is found that, surprisingly, S-doped RGO outperforms the other RGOs doped with more electropositive or electronegative and more oxophilic heteroatoms, and this effect becomes more prominent after Fe treatment of the respective catalysts. Herein, we evaluate the OER activity of a series of Fe-treated mono-heteroatom (S/N/B/P)-doped RGO (Fe-X-G) catalysts, among which interestingly S-doped RGO catalyst treated with Fe (Fe-S-G) is found to show better OER activity than the well-known active Fe-N-C catalyst, demonstrating the best activity among all of the prepared catalysts, close to that of the state of the art IrO2/C catalyst, along with pronounced long-term stability. Density functional theory (DFT) calculations indicate that the OER activity highly depends on the electroneutrality and oxophilicity of doped heteroatoms and doping-induced charge distribution over RGO, demonstrating that S with mediocre electronegativity and the least oxophilicity exhibits optimal free energy for the adsorption of the OER intermediate and desorption of the final OER product. Furthermore, it is found that Fe treatment greatly helps in enhancing the number of active sites through the regeneration of reduced catalytically active S sites and improving the conductivity and surface area of the S-doped RGO, which are found to be key factors to furnish the Fe-S-G catalyst with the capability to catalyze the OER with high efficiency, even though Fe is found to be absent in the final catalyst.