Preparation Of 3D Porous Transition Metal Heteroalloy Composite Catalysts By Electrodeposition And Its Electrocatalytic Total Water Splitting

Hydrogen generation with water electrolysis is a promising strategy for its high energy density and cleanliness with broad market prospect.However,the energy loss causing by the large overpotentials in the process of hydrogen evolution reaction and oxygen evolution reaction prevents the further industrialization of water electrolysis.Therefore,the fabrication of efficient,cost-effective and stable bifunctional electrocatalysts for electrocatalytic water splitting is of great significance for energy conversion and storage.In recent years,there have been many researches on bifunctional electrocatalysts but most of them are composed of single components and are diffult to possess high electrocatalytic activityies simultaneously for OER and HER.Based on the previous work,alloy composite bifunctional catalysts with nanoporous honeycomb-like structures,prepared by first-row transition metal with good electrocatalytic activity such as nickel,iron,cobalt and copper,have been investigated extensively in this paper.First,the nanotubular Ni(Cu)/NF alloy catalysts using nickel foam(NF)as the substrate via a electrodeposition-dealloying method were prepared.Then ultrathin NiFe alloy layers were coated on the surface and up-sidewalls of Ni(Cu)nanotubes by a selective electrodeposition and NiFe@Ni(Cu)/NF heterostructure composite catalyst was obtained.The results of SEM,TEM and EDX mapping showed that NiFe alloy layers were uniformly deposited on the surface and sidewalls of Ni(Cu).The nanotubes were interconnected through the NiFe layer and the tube diameter became smaller,about 80?150 nm.The characteristic peak of Fe0.64Ni0.36 was observed by XRD,which confirmed that NiFe exists in the form of alloy.The characteristic peaks of Ni and Fe hydroxide were observed in the XPS spectrum,indicating a small amount of hydroxide was formed in the co-deposition process.The NiFe@Ni(Cu)catalyst shows a Pt-like performance in hydrogen evolution reaction and efficient oxygen evolution reaction activity in alkaline medium(1 M KOH).The result of linear sweep voltammetry shows that it requires the HER and OER overpotential of 36 and 209 mV to achieve the benchmark current density of 10 mA cm-2,respectively,while the corresponding Tafel slope is only 41.3 and 36.7 mV dec-1.The 10 h chronopotentiometric curves carried out in different current densities show negligible degradation,revealing the remarkable stability.The reasons for the enhancing catalytic performance are explored.On the one hand,it is found that the deposition of NiFe keeps the unique honeycomb-like nanoporous structure of Ni(Cu),and the superlarge ECSA provides more catalytic active sites and increases the mass transfer.On the other hand,the HER-active composite(Ni(Cu))and OER-active composite(NiFe)are tightly coupled to form heterostructure.The strong synergistic effect between Ni(Cu)and NiFe enhances the intrinsic catalytic activity.EIS test shows that the charge transfer resistance decreases,indicating that the heterostructure improves the conductivity.The excellent HER and OER catalytic performance make it a bifunctional catalyst for overall water splitting.In the two-electrode cell voltage test,NiFe@Ni(Cu)/NF only needs 1.525 and 1.81 V to deliver 10 and 100 mA cm-2 with a good durability.In order to further improve the catalytic performance of NiFe@Ni(Cu)/NF,we introduced cobalt into the co-deposition and adjusted the doping amount of Co to obtain NiCoFe@Ni(Cu)/NF composite catalyst.SEM found that NiCoFe had a tendency to agglomerate and was uniformly deposited on the sidewalls of Ni(Cu)in the form of nanospheres.The granular nanoporous structure further increases the ECSA of the electrode and is beneficial to the rapid desorption of H2 from the electrode surface.The doping of Co further enhances the synergistic effect between NiCoFe and Ni(Cu)components,thereby increasing the intrinsic catalytic activity.The HER and OER overpotentials of NiCoFe@Ni(Cu)/NF are 25 and 218 mV at the current density of 10 mA cm-2.Specifically,NiCoFe@Ni(Cu)/NF exhibits far higher catalytic activity than NiFe@Ni(Cu)/NF at high current densities.The overpotentials of HER and OER required to reach the current density of 100 mA cm-2 are only 116 and 256 mV.In the two-electrode tests,electrolyzer using NiCoFe@Ni(Cu)/NF electrode as both anode and cathode(NiCoFe@Ni(Cu)/NF||NiCoFe@Ni(Cu)/NF)exhibits splendid bifunctional catalytic activity.The cell voltage is only 1.512 V at the current density of 10 mA cm-2.And it can catalyze overall water splitting with the voltage of 2.44 V at a current density of 500 mA cm-2,which provides the possibility for NiCoFe@Ni(Cu)/NF to be used in practical industrial production from water electrolysis.