Oxygen Evolution Mechanism On A Nickel Electrode In Alkaline Media And The Catalytic Enhancement By Phytate

The oxygen evolution reaction（OER）is an important electrochemical reaction in the fields of energy and environment.Efficient catalysts are required to accelerate the sluggish kinetics of water oxidation.The recent interest in renewable energy storage has spurred a renaissance of the studies nonprecious OER catalysts.Nickel-based compounds are the most promising earth-abundant OER catalysts,attracting ever-increasing interest due to high activity and stability,but the reaction mechanisms remain highly controversial.For example,one understanding is that the catalytic active component of nickel to oxygen evolution reaction is?-NiOOH or?-NiOOH,and another is that the catalysis of nickel to oxygen evolution reaction is caused by the trace Fe impurities contained in KOH.In this paper,we study the OER on a nickel electrode using the conventional electrochemical and spectroelectrochemical methods,including the addition of soluble components（Fe（III）,Ni（II）ions and sodium phytate）in the electrolyte to explore its influence on the OER.The anodic process of nickel in NaOH was measured to investigate the activation and dissolution of the Ni（OH）₂/NiOOH film and the roles of dissolved species such as Fe（III）and Ni（II）in OER.The（oxy）hydroxide thin films formed on Ni surface were characterized by using FE-SEM,XRD and XPS.Through the controlled addition of Fe（III）and Ni（II）,we proved that the Fe（III）impurities in the electrolyte are insufficient to considerably improve the OER activity of the NiOOH catalyst.Alternatively,the added Ni（II）ions can significantly reduce OER activity.In situ thin-layer spectroelectrochemistry reveals that the chemical dissolution of Ni（OH）₂/NiOOH film produces a soluble Ni（II）species that is electrochemically stable in the whole tested potential region.Stirring the electrolyte promotes the diffusion of the Ni（II）species off the electrode surface,leading to an acceleration of electrode activation and ultimately an increase in the OER activity.According to the theoretical analysis of the experimental results,an OER mechanism based on aqueous Ni species is proposed.Our observations highlight the importance of recognizing and accounting for the roles of soluble species in heterogeneous electrocatalysis.The addition of excessive sodium phytate in the electrolyte can fully transform the trace Fe（III）ions in the solution into complexing state,but unexpectedly,it leads to the enhancement of oxygen evolution activity.The results further prove that Fe（III）ion impurities in the solution have no effect on OER activity.The addition of 0.01 mol L^-1phytate in 0.1 mol L^-1 NaOH led to a significant increase in the rate of the OER.The exchange current density of OER was increased from 5.08?10^–11 A cm^-2 to 3.95?10^–9 A cm^-2,and the overpotential at 5 mA cm^-2 was decreased by 70 mV.The catalytic effect did not decay over a period of at least 8 h electrolysis.The kinetic information of the reaction process of oxygen evolution was obtained by spectroelectrochemistry,which further indicated that phytic acid is a green,stable and efficient nickel-based oxygen evolution electrocatalyst.