Nitrate Coordination Expedites Low-Valence Charge Transition Cycle of Ni for Stable Oxygen Evolution

The traditional oxygen evolution reaction (OER) always involves high-valence metal sites, which are generated at high potential and are difficult to stabilize for a long time, greatly constraining OER activity and stability. To avoid high-valence Ni4+, a pathway following the low-valence charge transition cycle, involving the electrochemical oxidation of Ni(OH)2 to NiOOH and a spontaneous exergonic chemical reduction of NiOOH to Ni(OH)2 to produce O2, has been designed and expedited by NO3– ligands, which is validated in nickel hydroxynitrate (NiOHNO3) coordinated with stoichiometric nitrate at 90 °C. The rich nitrate ligands decrease the energy barrier of Ni2+/Ni3+ evolution by not only breaking the strong hydrogen bonds in the crystalline framework by expanding the interlayer tunnels but also facilitating the hydrogen detachment and intercalation, serving as proton transfer relay stations. More intriguingly, this mechanism concerns only the transition between two valence states, which, together with strong electronic interaction between NO3– and Ni, can effectively prevent NiOHNO3 from leaching owing to excessive reconfiguration to unstable Ni4+. NiOHNO3 can operate at 200–1000 mA·cm–2 for 100 h. In addition, the overpotential at 10 mA·cm–2 considerably decreased (71 mV) compared with 4e–-OER (203 mV), which was much superior to other reported Ni-based electrocatalysts and commercial RuO2. This work provides a theoretical and experimental basis for the design and mechanism study of OER catalysts based on low-valence Ni.