High overpotential and metal leaching-induced deactivation are two major challenges for oxygen evolution reaction (OER) electrocatalysts operating in corrosive acid. OER involves multiple reaction intermediates (e.g., *O, *OOH, *OH, shown in Figure 1). Their adsorption energies follow the scaling relationship (ΔG_OOH = ΔG_OH + 3.2±0.2 eV), which cannot be independently modulated, resulting in a much larger voltage than what thermodynamics authorizes. Previously, our group has been focused on optimizing the oxygen-binding strength, ranging from electronic manipulation to crystal phase engineering. However, the AEM-guided electrocatalysts from single-atom-based (Pt doped NiO¹, Ag doped MnO₂²) to nanoparticle-based (Au/MnO₂³) heterogeneous electrocatalysts, can hardly surpass the AEM-proposed minimum overpotential. To confront this technical dilemma, a new design principle and reaction mechanism should be considered.

One possible route for bypassing the extra reaction intermediate *OOH is to utilize the direct O-O radical coupling, namely oxide path mechanism⁴ (OPM) for heterogeneous catalysis and oxo-oxo coupling mechanism⁵ (I2M) for homogeneous catalysis, which has never been realized to date for designing and fabricating acid-stable OER electrocatalyst due to its stringent requirements for the geometric configuration of active metal sites. To be sharply different for the OPM pathway, the schematic illustration of simplified mechanisms has been listed in Figure 1.

Therefore, we report a crystalline α-MnO₂ nanofiber-supported Ru electrocatalyst (Ru/MnO₂) that fits the OPM design rule. The catalyst preparation is based on a one-step cation exchange method with Ru atoms substituting surface Mn atoms. The positions of Ru atoms follow the periodic arrangement of Mn sites in the crystalline α-MnO₂, resulting in the formation of small regularly arranged Ru ensembles (e.g., atom array). The cation exchange reaction also happens in-situ during OER, which not only triggers reconstruction of small Ru ensembles into big patches of Ru atom arrays but also avoids metal leaching-induced catalyst deactivation by capturing leached Ru ions back to support. The Ru atom array consists of symmetric Ru sites that are highly favorable for the OPM-type OER. The optimized Ru/MnO₂ delivers an overpotential of a mere 161 mV at a current density of 10 mA cm^-2 along with outstanding long-term durability (>200 h). The intrinsic OER activity of Ru/MnO₂ (Turnover frequency TOF = 1192.30 h^-1 at an overpotential of 165 mV) is more than 600 folds higher than that of RuO₂ (1.96 h^-1). The overall performance of Ru/MnO₂ is better than that of the state-of-the-art OER electrocatalysts. Extensive in-situ and ex-situ characterizations together with theoretical calculations verify the OER proceeded on the Ru/MnO₂ via OPM, in which the key step involves direct O-O radial coupling. Such a unique reaction pathway allows the Ru/MnO₂ to go beyond the overpotential ceiling set by the conventional mechanism.

The present work not only provides new insights for designing electrochemically stable active catalysts but also establishes a deterministic step to resolve bottlenecks in electrolytic hydrogen production.

Finally, this joint research was made satisfactory by various support and efforts with many researchers. Especially Professor Xiaopeng Li, who is currently Shanghai Distinguished Professor at Donghua University. He did his Ph.D. study at Max Planck Institute of Microstructure Physics and Martin Luther University of Halle Wittenberg. He has published more than 60 papers in academic journals such as Nat. Catal., Joule, Matter, and Chem. Sci. His main research topic is metal-air battery, electrolyzer, environmental catalyst.

For more information, please read our paper in Nature Catalysis. “In-situ Reconstructed Ru Atom Array on α-MnO₂ with Enhanced Performance for Acidic Water Oxidation”

https://www.nature.com/articles/s41929-021-00703-0

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