Electrochemical Preparation Of Nickel-based Electrode Materials In Cholinium Ionic Liquids And Their Performances For Hydrogen Evolution Reaction

With the development of society,the increasing depletion of fossil fuels and many environmental problems push people to seek a new clean energy.Hydrogen energy,as an excellent alternative,has the advantages of high energy density,environmental friendliness and high abundance of the earth.It is recognized as a green energy carrier.At present,the alkaline water electrolysis is one of the most promising methods for sustainable hydrogen production.However,the energy consumption of water electrolysis is quite high,so the cathode materials with low overpotential and high catalytic activity are the key to reduce its energy consumption.Platinum group metal?such as Pt and Pd?based catalysts have the advantages of low overpotential,low Tafel slope,high exchange current density and good stability of hydrogen evolution reaction?HER?.However,the high cost and low earth reserves limit the commercial application of noble metal-based catalysts.The transition metal-based catalysts with high abundance and low cost also show great catalytic potential under alkaline conditions.Among them,Ni based cathode materials have a good catalytic activity for HER and good corrosion resistance as well as stability in alkaline solution.They are ideal materials to replace precious metal materials,but their catalytic activity still needs to be improved,and their cycle stability still needs to be optimized.In order to improve the catalytic activity for HER of nickel-based materials,nickel-based materials with different morphologies were designed from two aspects of energy and geometric factors.Using ionic liquids as a new type of electrolyte,the short process preparation of self-supported nickel-based materials,which is environmentally friendly,low energy consumption,additives-free and conductives-free,is realized by electrochemical technology,so as to study the relationship and law between the microstructure of nickel-based electrode materials and the catalytic performance for HER.In this paper,firstly,the self-supported micro/nanostructured nickel electrode materials were prepared in two common deep eutectic solvents?DESs?media:choline chloride-urea?ChCl-Urea?and choline chloride-glycol?ChCl-EG?.At the same time,the activated carbon?AC?and 50 nm carbon particles were added respectively in a ChCl-EG DES that can produce cathode materials with better performance for HER,and the Ni-AC and Ni-C composite electrode materials with three-dimensional flower-like morphology were prepared by composite electrodeposition method,while using different counte rand reference electrodes prepared Ni-AC composite electrodes containing Ag and Pt.Besides,the Ni-Co electrode materials were prepared from precursors of NiCl₂ and CoCl₂.Activated carbon?AC?,50 nm carbon particles or carbon nanotubes?CNT?were added in a DES bath respectively,and then Ni-Co-AC/NF,Ni-Co-C/NF and Ni-Co-CNT/NF composite electrode materials were composite electrodeposited on the nickel foam substrate.In addition,Ni-Co-Pt-AC/NF composite electrode materials were prepared by Pt counter electrode dissolution method.In this paper,the electrochemical reduction behavior of Ni?II?complex anions in ionic liquids has been studied systematically.The composition,morphology and electronic structure of various electrode materials have been analyzed.The influence mechanism of carbon particles on the dissolution of platinum counter and silver reference electrodes in the process of composite electrodeposition has also been proposed.The relationship between the microstructure of the electrode materials and the catalytic performance of hydrogen evolution has been discussed.The main research results are as follows:?1?The reduction process of Ni?II??Ni?0?in two media?1ChCl:2Urea and1ChCl:2EG?is a diffusion controlled quasi-reversible process,and is also a one-step two electron transfer process:Ni²⁺+2e^–?Ni.At 328 K,the diffusion rates of Ni?II?in 1ChCl:2Urea and 1ChCl:2EG media are 3.66×10^–8 cm² s^–1 and 8.67×10^–8 cm² s^–1,respectively.By changing the types of ionic liquids and adjusting the cathodic potential,self-supported Ni electrode materials with different micro/nano morphologies can be obtained.Compared with 1ChCl:2Urea media,the Ni electrode material with nanoneedle shape obtained in 1ChCl:2EG-NiCl₂·6H₂O?0.2 M?electrolyte at cathode voltage of–0.6 V has higher catalytic activity for HER(?₁₀₀=324 m V),and its Tafel slope is 162 m V dec^–1.The value double-layer capacitance(C_dl)of the electrodeposited nano-Ni coating is 1.53 m F cm^–2,which is about 7 times of that of commercial Ni sheet,indicating that the nanoneedle-like Ni electrode material has relatively larger electrochemical active surface area.?2?The dispersion of AC particles in 1ChCl:2EG-NiCl₂·6H₂O?0.2 M?electrolyte is better than that in aqueous solution.The use of ionic liquid in composite electrodeposition avoids the use of other additives and simplifies the experimental process.self-supported Ni-AC and Ni-C composite electrodes were prepared in1ChCl:2EG-NiCl₂·6H₂O?0.2 M?electrolyte with carbon particles?AC or 50nm C?.The addition of carbon particles with different morphology will not change the phase and surface chemical state of the composite coating,but will affect its morphology.In addition,the formation of three-dimensional flower morphology can be controlled by changing the amount of carbon particles.The Ni-AC and Ni-C composite electrodes with flower structure have higher specific surface area and catalytic activity for HER than pure Ni electrodes.The overpotential of Ni-0.3AC composite electrode reaches114 m V and 257 m V when the current density is 10 m A cm^-2 and 100 m A cm^-2respectively,which exhibits good catalytic performance.It shows that the addition of carbon particles is conducive to increase the actual surface area of the catalyst for HER,which effectively reduces the overpotential in the HER process and thus reduces energy consumption.?3?After adding AC particles,self-supported Ni-AC composite electrode containing Ag and Pt was prepared by Pt counter electrode and Ag reference electrode?CE-Pt,RE-Ag?in 1ChCl:2EG-NiCl₂·6H₂O?0.2 M?electrolyte.It has better catalytic activity for HER than Ni-AC composite electrode prepared by Ni counter electrode and nonaqueous Ag reference electrode?CE-Ni,RE-NAg?.Further research found that the main reason is that after the addition of AC particles,the Pt counter and Ag reference electrodes were dissolved during the composite electrodeposition process,resulting in the formation of a Ni-Ag-Pt-AC composite coating on the cathode,which has better catalytic activity.The Ni-0.5AC?CE-Pt,RE-Ag?with 3D cauliflower-like structure with excellent HER performance has a relatively larger specific surface area,a large C_dlvalue of 22.2 m F cm^–2,and a relatively small overpotential of 48 m V and 135 m V at current densities of 10 m A cm^-2 and 100 m A cm^-2,respectively.The addition of AC particles with different contents has a great influence on the morphology of Ni-AC composite coating and the contents of Pt and Ag.The DFT calculation further confirmed that Pt/Ag electrode surfaces can interact strongly with Cl^-,and the addition of AC carbon particles further caused it to dissolve.Based on this,a conjecture mechanism for AC particles to promote the dissolution reaction of the Pt/Ag electrode surface was proposed.It is suggested that AC particles play a role of transport,which promotes the forward reaction of Pt/Ag electrode surface and makes Pt/Ag electrode dissolve.These results provide a new method for the preparation of composite materials containing platinum,and also suggest that the dissolution behavior of platinum counter electrode and silver reference electrode should be considered as a comprehensive factor in the process of composite electrodeposition.?4?Self-supported Ni-Co/Cu and Ni-Co/NF electrode materials were prepared by electrodeposition in 1ChCl:2EG-NiCl₂·6H₂O-CoCl₂·6H₂O on copper sheet and foam nickel respectively at different cathodic potentials.The results show that Ni-Co electrode materials with different Ni,Co contents and different morphology structures could be obtained at different cathodic potentials.Compared with the Ni/Cu electrode,Ni-Co/Cu and Ni-Co/NF electrode materials have better catalytic activity for HER.Moreover,the three-dimensional Ni-Co electrode material with nickel foam as substrate has better catalytic activity for HER and larger specific surface area than Ni-Co/Cu electrode material.For Ni-Co/NF electrode,the overpotential at 10 m A cm^-2 and 100m A cm^-2 current density is 133 m V and 247 m V respectively,and the C_dl value is 4.7m F cm^–2.?5?Three-dimensional foam nickel was used as the substrate,self-supported Ni-Co-AC/NF,Ni-Co-C/NF and Ni-Co-CNT/NF composite electrode materials were prepared by the composite electrodeposition technology in the 1ChCl:2EG-NiCl₂·6H₂O-CoCl₂·6H₂O with adding carbon particles?AC,50 nm C or CNT?.In addition,using the Pt electrode as the counter electrode,combined with the idea of Pt counter electrode dissolution,self-supported Ni-Co-Pt-AC/NF composite electrode material was prepared in the 1ChCl:2EG-NiCl₂·6H₂O-CoCl₂·6H₂O electrolyte with the addition of AC.Compared with Ni-Co/NF electrode materials,the addition of carbon particles will affect the morphology and structure of the composite coating and will not change the phase composition and chemical state of surface.Self-supported Ni-Co-AC/NF,Ni-Co-C/NF and Ni-Co-CNT/NF composite electrode materials show higher catalytic activity for HER than Ni-Co/NF electrode materials.Especially for the Ni-Co-AC/NF composite electrode material,the overpotential of is 51 m V and 197 m V at current densities of 10 m A cm^-2 and 100 m A cm^-2,respectively,and has a relatively higher electrochemical active surface area with a C_dl value of 19.7 m F cm^–2.The Ni-Co-Pt-AC/NF was prepared by using Pt counter electrode has better catalytic activity for HER,requiring the overpotential of 50 m V and 117 m V to reach current densities of 10 m A cm^-2 and 100 m A cm^-2,respectively.The results show that the use of Pt counter electrode can successfully prepare platinum-containing electrode materials.Based on the three-dimensional porous substrate structure and the introduction of active elements Co and Pt,the second phase particles are added to further increase the real surface area,which is conducive to promoting the HER and effectively reducing the overpotential,so that the composite material has excellent catalytic activity for HER.