Preparation And Properties Of Ni Co Fe Based-hydroxide/sulfide Catalysts For Oxygen Evolution Reaction

Hydrogen?H₂?is predicted to be a promising energy storage medium or carrier,with the advantages including high energy density,environmental friendliness with less carbon emission,and clean product of water from combustion.Electrochemical water electrolysis is recognized to be one of the best powerful methods for hydrogen production.Electrochemical water splitting consists of two half-reaction including hydrogen evolution reaction?HER?at cathode and oxygen evolution reaction?OER?at anode.OER is a reaction of four-electron transfer and is the rate-determining step ultimately depending on the activity of OER electrocatalysts.The development of high efficient electrocatalytic materials with high energy conversion efficiency and low catalytic overpotential is the basis of accelerating OER process and electrolytic water reaction.Transition metal compounds,such as sulfides,oxides and layered double hydroxides?LDH?,are considered as substitutes of precious metal catalyst and have been widely studied in the field of electrocatalysis.Transition metal hydroxides,the most easily obtained precipitates in aqueous solution,have electrocatalytic activity in the electrolysis of water.Compared with single metal hydroxide,bimetallic hydroxides usually show more abundant valence state and better conductivity due to synergistic effect,and have the better electrochemical activity.Traditional coprecipitation is usually carried out at high temperature?>60 ^oC?conducive to crystal growth.However,the particles of product tend to agglomerate easily at high temperature,resulting in the decrease of specific surface area and"active site".Low-temperature coprecipitation inhibit the growth of crystal nucleus,result in the formation of the particles with higher specific surface area and porosity.The main works of this paper are as follows:Firstly,we developed a facile low-temperature coprecipitation approach to synthesize porous Co Fe layered double hydroxide?Co Fe LDH?.The effect of Fe³⁺content on the growth behavior of Co Fe LDH nanostructures and the electrocatalytic activity on oxygen evolution reaction?OER?are discussed.The results show that the morphology,crystallinity and pore structure of the product can be facilely tuned by the addition of Fe³⁺.Porous Co Fe LDH nanosheets with large total pore volume and enriched active sites were achieved under an optimal Co/Fe feed ratio.The obtained crystalline Co_0.66Fe_0.33LDH nanosheets reaches a current density of 10 m A·cm^-2with an overpotential of 296 m V and a Tafel slope of 33.3m V·dec^-1,and show good cycle stability.Secondly,Ni Fe layered double hydroxide?Ni Fe LDH?was also synthesized via low-temperature coprecipitation approach.By tuning Ni/Fe feed ratio and using Na₂CO₃as precipitant,mesoporous Ni Fe LDH particles were obtained.The special physicochemical features of these Ni Fe LDH particles,including high crystallization degree,large surface area and abundant crystalline/amorphous interfaces,synergistically endowed the catalyst with enriched active sites.Consequently,the optimal Ni_0.66Fe_0.33LDH electrocatalyst showed excellent activity toward OER in alkaline media,with a low overpotential of 248 m V to deliver a current density of 10 m A·cm^-2,a small Tafel slope of 46.9 m V·dec^-1,and strong durability.Finally,using the Ni_0.66Fe_0.33LDH with lare specific area and high activity as precursor,the Ni Fe S particles were obtained after sulfurization in a tubular furnace at 300 ^oC for 1 h.XRD results show that the main phase of Ni Fe S particles is Ni₃S₂,and the coexistence of Ni²⁺and Fe³⁺in the same crystal phase is conducive to the charge transfer between Ni?II?and Fe?III?.Nyquist plots show that the charge transfer resistance(R_CT)of Ni Fe S is much smaller than that of Ni_0.66Fe_0.33LDH.Compared to Ni_0.66Fe_0.33LDH,the Ni Fe S particles exhibit mor excellent catalytic activity for OER with a low overpotential of 220 m V at 10 m A·cm^-2and 284 m V at100 m A·cm^-2,possesses outstanding kinetics(the Tafel slope of 38.1 m V·dec^-1),and long-term stability.