Nickel-containing Electrocatalysts For Urea Oxidation:Synthesis,Structure,and Catalytic Mechanism

The development of clean and renewable new energy sources can help to solve a series of problems such as the energy crisis and environmental pollution caused by using of fossil fuels.Among all clean and renewable new energy sources,electrocatalytic water-splitting to produce hydrogen has the advantages of low cost and easy operation.The electrocatalytic water-splitting is composed of two half-reactions:oxygen evolution reaction?OER?on anodic and hydrogen evolution reaction?HER?on cathodic.Among them,OER has a large applied reaction potential,which greatly affects application of electrocatalytic hydrogen production.The urea oxidation reaction?UOR?has a lower theoretical potential and is expected to become an alternative reaction to the anode OER.UOR can purify industrial,agricultural,and domestic wastewater,thereby it may achieve the dual goals of producing clean energy and solve environmental problems.However,the low activity of UOR electrocatalysts and the unclear reaction mechanism limit the development of UOR.Therefore,in this article,structure,catalytic activity and mechanism of UOR electrocatalysts have been studied in depth.Based on this,UOR and HER double function electrocatalyst,high-activity electrocatalysts have been further developed,and the electrocatalysts have been modified for industrial requirements.The specific content is:?1?A MoNi₄/MoO_x@NF electrocatalyst with MoNi₄ alloy particles supported on oxygen vacancy-rich MoO₂ nanorods was synthesized on nickel foam by two steps of hydrothermal and thermal reduction reaction.It is found that MoO_x exhibits pseudo-octahedral coordination,where the electron cloud density on Mo comes higher,and the electron orbit hybrid phenomenon between Ni-O is more obvious.When the thermal reduction time increases from 0.5 to 4 h,the oxygen vacancy content and metal-to-metal interaction of the samples changed.At the same time,the electrocatalytic performance is increased first and then decreased,the UOR activity shows a volcanic curve.When the thermal reduction time is 2 h,the samples showed excellent electrocatalytic performance at both anode UOR and cathode HER.In a mixed solution of 1 M KOH and 0.33 M urea,UOR and HER need 1.29 and 0.01 V only to achieve a current density of 10 mA cm^-2;when the current density is increased by an order of magnitude to 100 mA cm^-2,requires 1.34 and 0.16 V only.The electrocatalytic performance is better than the precursor NiMoO₄@NF and most electrocatalysts.After12 h of continuous catalytic operation,the catalyst still maintained extremely high catalytic performance.The excellent catalytic performance is mainly due to the special catalyst surface state,oxygen vacancies and strong metal-to-metal interactions caused by thermal reduction.There is a strictly linear relationship between the catalytic performance and the degree of metal-to-metal interaction.These results provide a new way to predict and explain the structural properties of alloy particles supported electrocatalysts.?2?NiO-WO_x@NF electrocatalyst obtained from precursor Ni₄W₆O₂₁?OH?₂·4H₂O,where NiO as the core,and oxygen vacancy rich WO₂ as the shell through the two steps of hydrothermal and thermal reduction.The study finds out that after thermal reduction,WO₂ with a very low surface free energy was coated on NiO with a relatively large surface free energy,thus exhibiting the characteristics of adapting to both alkaline and acidic catalytic conditions.In alkaline environment,UOR and HER can reach a current density of 10 mA cm^-2 with only 1.34 and 0.04 V;HER in an acidic electrolyte can reach the same current density with 0.05 V only.More importantly,the core-shell electrocatalyst can maintain the catalytic performance of more than 50 h and 3000 CV cycles under various catalytic environments.The results show that the bimetallic oxidants with W and Ni as precursors could be expected to prepare electrocatalysts with excellent electrocatalytic performance and stability,which is helpful for the design and synthesis of advanced electrocatalysts that could adapt to a variety of application environments.