10.1021/acsomega.8b01143.s001 Jie Yin Jie Yin Fangfang Di Fangfang Di Junxue Guo Junxue Guo Kaixuan Zhang Kaixuan Zhang Wenli Xu Wenli Xu Yunying Wang Yunying Wang Shaozhen Shi Shaozhen Shi Ning Chai Ning Chai Chaofan Chu Chaofan Chu Jiazhen Wei Jiazhen Wei Wenzhi Li Wenzhi Li Xin Shao Xin Shao Xipeng Pu Xipeng Pu Dafeng Zhang Dafeng Zhang Xiaozhen Ren Xiaozhen Ren Jie Wang Jie Wang Jinsheng Zhao Jinsheng Zhao Xianxi Zhang Xianxi Zhang Xinting Wei Xinting Wei Fang Wang Fang Wang Huawei Zhou Huawei Zhou Tuning Ni-Foam into NiOOH/FeOOH Heterostructures toward Superior Water Oxidation Catalyst via Three-Step Strategy American Chemical Society 2018 NiOOH water oxidation catalysts water oxidation Three-Step Strategy Splitting RuO x catalysts Hydrogen-to-oxygen volume ratios water splitting Superior Water Oxidation Catalyst water oxidation activity heterostructure charge transfer resistance 2018-09-11 17:54:22 Journal contribution https://acs.figshare.com/articles/journal_contribution/Tuning_Ni-Foam_into_NiOOH_FeOOH_Heterostructures_toward_Superior_Water_Oxidation_Catalyst_via_Three-Step_Strategy/7074752 Splitting of water into hydrogen and oxygen has become a strategic research topic. In the two semi-reactions of water splitting, water oxidation is preferred to the four-electron-transfer process with a higher overpotential (η) and is the decisive step in water splitting. Therefore, efficient water oxidation catalysts must be developed. IrO<i><sub>x</sub></i> and RuO<i><sub>x</sub></i> catalysts are currently the most efficient catalysts in water oxidation. However, the limited reserve and high prices of precious metals, such as Ir and Ru, limit future large-scale industrial production of water oxidation catalysts. In this study, we tune inert Ni-foam into highly active NiOOH/FeOOH heterostructures as water oxidation catalysts via three-step strategy (surface acid-treating, electroplating, and electrooxidation). NiOOH/FeOOH heterostructures as water oxidation catalysts only require η of 257 mV to reach a current density of 10 mA cm<sup>–2</sup>, which is superior to that of IrO<sub>2</sub>/Ni-foam (280 mV). The high electrochemically active surface area (72.50 cm<sup>2</sup>) and roughness factor demonstrate abundant interfaces in NiOOH/FeOOH heterostructures, thus accelerating water oxidation activity. The small value (4.8 Ω cm<sup>2</sup>) of charge transfer resistance (<i>R</i><sub>ct</sub>) indicate that fast electronic exchange occurs between NiOOH/FeOOH heterostructures catalyst and reaction of water oxidation. Hydrogen-to-oxygen volume ratios (approximately 2:1) indicate an almost overall water splitting by the double-electrode system. Faraday efficiency of H<sub>2</sub> or O<sub>2</sub> is close to 90% at 2:1 hydrogen-to-oxygen volume ratio. NiOOH/FeOOH heterostructures exhibit good stability. The results provide significance in fundamental research and practical applications in solar water splitting, artificial photoelectrochemical cells, and electrocatalysts.