Preparation And Electrocatalytic Performance In Overall Water Splitting Of NiCo-based Electrocatalysts

With the rapid economic development,environmental pollution and energy poverty has become global ecological crisis.Hydrogen energy,as an ideal pollution-free and renewable new energy,has attracted more and more attention.The overall water splitting is an ideal route for hydrogen production because of its mature technology and produced hydrogen with high purity.Although noble metals such as Pt,Ir and their compounds exhibit excellent catalytic performance,the scarcity of reserves and the high cost limit their development and application.Therefore,the design and synthesis of stable,efficient and cheap non-noble-metal electrocatalysts is of great significance for improving the utilization of electricity in water splitting industry.The overall water splitting reaction can be divided into two half-reactions,the hydrogen evolution reaction?HER?at the cathode and the oxygen evolution reaction?OER?at the anode.Due to the completely different reaction pathways in the process of water reduction and oxidation,the development of bifunctional electrocatalysts for both HER and OER is greatly limited.The transition metal Ni,Co have rich element composition and electronic valence states,showing excellent catalytic activity and low cost.Carbon cloth is a kind of textile with high conductivity and good flexibility.Using carbon cloth as support electrode in the water splitting reaction can make the prepared samples have higher electron transfer rate and can improve the utilization of active materials.In this paper,based on the excellent transition metal Co-Ni alloys and their compounds with high catalytic activity,low overpotential and good stability for the water splitting,we systematically investigated the preparation conditions,the structure characteristics and the influence on the catalytic activity and stability of HER and OER.The detail contents are as follows:1)Dispersed Ni-Co precursor nanoneedle arrays uniformly grew on carbon cloth?CC? fibers by a hydrothermal method using Ni,Co salts and urea as reagents.Then (Ni_0.33Co_0.67)S₂ network structure with cross-linked pores was formed via a sulfuration process at 350?[(Ni_0.33Co_0.67)S₂ NWs/CC].Serving as a self-supporting electrode,(Ni_0.33Co_0.67)S₂ NWs/CC exhibited excellent catalytic performance for both HER and OER.Its excellent performance could be attributed to the three-dimensional porous structure exposing fully the active sites,the synergistic catalytic effect of Ni and Co,the effective regulation of the morphology and the electronic structure of Ni and Co due to the introduction of S, and the increased electron transport rate by the carbon substrate.Based on the efficient bifunctional catalytic performance of(Ni_0.33Co_0.67)S₂ NWs/CC,a two-electrode system constructed by(Ni_0.33Co_0.67)S₂ NWs/CC was used in overall water splitting,and only a low cell voltage of 1.57 V was needed to approach a current density of 10 mA cm^-2.2)A series of CoNi alloys with different Co/Ni ratios were prepared via a hydrothermal reduction method.It was found that the Co_0.75Ni_0.25 alloy with hierarchical dendrite structure showed high catalytic activity for HER.Its special hierarchical structure could exposes more active sites,enlarge the contact area with electrolytes,and facilitate the binding and transport of proton.Based on the dendrite structure,the Co_0.75Ni_0.25 alloy dendrite was further carbonized at 600? to obtain a core-shell architecture with the N-doped carbon shell coated CoNi alloy/CoNiO₂dendrite?CoNi/CoNiO₂@NC-600?.CoNi/CoNiO₂@NC-600 showed significantly improved activity and stability for OER.The complementary system for overall water splitting was constructed with Co_0.75Ni_0.25 alloy dendrite and CoNi/CoNiO₂@NC-600 dendrite,and only a cell voltage of 1.51 V was needed to approach a current density of 10 mA cm^-2,showing excellent catalytic performance.3)The tiny NiMo alloy nanoparticles grew uniformly on the surface of carbon cloth via a galvanostatic electrochemical deposition method.Then the polystyrene?PS? microspheres were self-assembled on the surface of NiMo alloy nanoparticles to obtain the template.The Co?OH?₂ nanosheets grew in the void of the microsphere template by the second galvanostatic method.Finally,The composite composed of Co?OH?₂ cavity array coated NiMo alloy on carbon cloth?NiMo-Co?OH?₂ CA/CC?was obtained after the dissolution of PS microspheres.The interaction between NiMo alloy and Co?OH?₂ resulted in the transformation of electronic structure on the surface of NiMo alloy.And the cavity array structure with the increased specific surface area of the electrode material could distribute a large number of active sites.Thus,NiMo-Co?OH?₂ CA/CC exhibited high catalytic activity and stability for both HER and OER.In the test of overall water splitting,only a cell voltage of 1.54 V was needed to approach a current density of 10 mA cm^-2.