Fe-Doped Ni-Based Catalysts Surpass Ir-Baselines for Oxygen Evolution Due to Optimal Charge-Transfer Characteristics
Mai‐Anh Ha, Shaun M Alia, Andrew G. Norman, Elisa M. Miller
Abstract
High Resolution Image Download MS PowerPoint Slide Ni-based catalysts with Co or Fe can potentially replace precious Ir-based catalysts for the rate-limiting oxygen evolution reaction (OER) in anion-exchange membrane (AEM) electrolyzers. In this study, density functional theory (DFT) calculations provide atomic- and electronic-level resolution on how the inclusion of Co or Fe can overcome the inactivity of NiO catalysts and even enable them to surpass IrO 2 in activating key steps to the OER. Namely, NiO resists binding the key OH* intermediate and presents a high energetic barrier to forming the O*. Co- and Fe-substitution of Ni active sites allows for the stronger binding of OH* and preferentially activates O*/O 2 * formation, with Fe-substitution increasing the OER activity substantially as compared to Co-substitution. Whereas IrO 2 requires an activation energy of 0.34–0.49 eV to form O 2, this step is spontaneous on Fe sub -NiO. Electrodeposition of polycrystalline electrodes and synthesized nanoparticles exploit the Co or Fe presence, with Fe particularly exhibiting greater activity: Tafel slopes indicate a significant change in the mechanism as compared to pure NiO, validating the theoretical predictions of OER activation at different steps. High-performing synthesized nanoparticles of 25% Fe–Ni exhibited a 4.6 times improvement over IrO 2 and a 34% improvement over RuO 2, showcasing that non-platinum group metal catalysts can outperform platinum group metals. High-resolution transmission electron microscopy further highlights the advantages of Fe–Ni oxide synthesized nanoparticles over commercial catalysts: small, randomly oriented nanoparticles expose greater edge sites than large nanoparticles typical of commercially available materials.