Controllable Synthesis Of Cobalt Based Nanostructured Eletrocatalysts For High-Efficiency Overall Water Splitting

With the increasing of energy crisis and environmental pollution,there is an urgent need to develop renewable and sustainable new energy sources.Hydrogen is considered to be the most promising secondary energy source because of its clean,efficient,storable,inexpensive and readily available.Hydrogen produced by electrolyzed water is a simple,green and efficient method.However,the slow reaction kinetics of electrolyzed water leads to high overpotential and low efficiency,so it is necessary to design and develop an electrolyzed water catalyst with high catalytic activity and high stability.Water splitting undergoes two reactions at the cathode and the anode,namely,cathodic hydrogen evolution reaction?HER?and anodic oxygen evolution reaction?OER?.The most commonly used catalysts are precious metals?Pt?and noble metal oxides?IrO₂ and RuO₂?,but their high cost and limited reserves limit their applications.Cobalt-based catalysts have the advantages of low cost,environmental friendliness and abundant raw material sources,and have been widely used in electrocatalytic reactions.However,cobalt-based catalysts still have problems of low catalytic activity and poor stability.In this paper,two kinds of high-catalytic activity and long-life cobalt-based phosphide and nitride composite bifunctional catalysts are designed and synthesized.The catalytic properties of HER,OER and overall water splitting are studied.The main contents and innovations are as follows:1.W-doped Co?CO₃?_0.5?OH?·0.11H₂O nanoneedle precursors are synthesized on a conductive carbon cloth substrate by hydrothermal method,then W-doped CoP is prepared?W-CoP/CC?by phosphating.Electrochemical tests show that W-CoP/CC exhibits excellent HER catalytic activity in 0.5 M H₂SO₄ and 1 M KOH electrolytes,and the overpotential are32 and 77 mV at the current density of 10 mA cm^-2,respectively.The Tafel slopes are 57 and65 mV dec^-1,respectively.In addition,the W-CoP/CC catalyst also exhibits outstanding OER activity in 1 M KOH electrolyte,with an overpotential of 252 mV at the current density of 10mA cm^-22 and a corresponding Tafel slope of 74 mV dec^-1.W-CoP/CC is used as a bifunctional catalyst for overall-water splitting.The water splitting voltage of W-CoP/CC is 1.59 V at the current density of 10 mA cm^-2.And the voltage of the test cell dose not change substantially after 20 h at a current density of 20 mA cm^-2.Density functional theory calculations show that the doping of W makes the water adsorption energy of CoP larger,and the Gibbs free energy of hydrogen adsorption is closer to 0 eV,showing higher catalytic activity.The W-CoP/CC nanoneedle is grown in situ on a conductive carbon cloth substrate,avoiding the increase in series resistance,reduction of active sites,and slowing of ion diffusion rate during the use of the binder.At the same time,the open structure provides sufficient space to promote the overflow of bubbles and the diffusion of ions,which promotes the reaction kinetics.The doping of W regulates the adsorption energy of water on the surface of the catalyst,promotes the activation of water and accelerates the transfer of protons/electrons as well as hydrogen desorption in the Volmer process.Therefore,W-CoP/CC exhibits excellent electrocatalytic performance.2.We designed and synthesized a composite structure(VN-Co_5.47N/CC)in which Co_5.47N nanoparticles are segregated on a porous VN substrate,which grown vertically on a carbon cloth substrate.Co_5.47N nanoparticles are segregated in situ on the porous VN nanosheets,the conductive porous two-dimensional VN substrate can provide a larger contact area of the electrode material with the electrolyte and the fast electron transport channel;In situ segregation of Co_5.47N nanoparticles on VN sheets increases the electrochemically active sites;The porous two-dimensional structure facilitates the overflow of bubbles and the diffusion of ions during the catalytic process.The VN-Co_5.47N/CC catalyst has better OER and HER catalytic activity in 1 M KOH electrolyte.The overpotential at 10 mA cm^-2 current density are310 and 118 mV,respectively,and the corresponding Tafel slopes are 61 and 114.2 mV dec^-1.It is used as a bifunctional catalyst for electrolyzing water.The cell had a water splitting voltage of 1.645 V at a current density of 10 mA cm^-2.And the voltage of the test cell dose not change substantially after 20 h at a current density of 10 mA cm^-2,which shows excellent catalytic stability.