Preparation And Hydrogen Evolution Properties Of Ni-based Composite Cathode Materials

At present,the energy crisis and environmental pollution are becoming more serious due to the burning of a large amount of fossil fuels.Hydrogen energy as a pure and pollution-free renewable power energy carrier has attracted much attention.Microbial electrolytic cells（MECs）are a kind of hydrogen production technology which use microorganisms to decompose organic matter in wastewater and produce hydrogen in the cathode in recent years.The traditional cathode materials are precious metals such as platinum,palladium and rhodium,which cannot be used on a large scale,because of their high price,easy poisoning and deactivation.Therefore,it is very important to explore low cost and high activity cathode material.This paper mainly studies the MEC cathode catalyst.Carbon paper was used as the substrate for all samples.Ni/Mo/Cu was electrodeposited on carbon paper by conventional pulse voltammetry;Ni（OH）₂/Ni/g-C₃N₄/C composite was prepared by means of in situ chemical reduction method by reducing and in situ growth of Ni onto graphitic carbon nitridate（g-C₃N₄）,and then dripping the composite onto CP.The samples were physically characterized by Scanning Electron Microscope（SEM）,Transmission Electron Microscope（TEM）,Diffraction of X-rays（XRD）,X-ray Photoelectron Spectroscopy（XPS）and BET tests.In order to further explore the mechanism of hydrogen evolution reaction（HER）,Linear Sweep Voltammetry（LSV）,Electrochemical Impedance Spectroscopy（EIS）,Tafel,Chronoamperometry（CA）and Chronopotentiometry（CP）were used to conduct electrochemical tests.Finally,it was tested for hydrogen production by MEC.1.Ni/Mo/Cu/C electrode prepared by electric pulse method and its HER performanceNi/Mo/Cu was electrodeposited on carbon paper substrates by conventional pulsed voltammetry.SEM images show that the surface of the Ni/Mo/Cu/C electrode was stacked with several hundred nanometers of spherical particles.Compared with the Ni/C and Ni/Mo/C electrodes,the spherical structure exposes more active sites and the specific surface area of the electrode is larger,so that the Ni/Mo/Cu/C electrode can contact the electrolyte more fully and significantly improve the hydrogen evolution performance of the electrode.EDS results show that the atomic percentages of Ni,Mo and Cu in the sample was 22.03%,15.5%and 3.07%.XPS results illustrated that the coexistence of Ni,Mo and Cu elements in Ni/Mo/Cu/C electrode;the percentage of Cu⁰ and Cu²⁺was 63.77%and 27.23%;the ratio of Mo⁴⁺and Mo⁶⁺was 2:1.XRD patterns showed that Ni and Cu in the Ni/Mo/Cu/C electrode were typical cubic crystal structure,and Mo element presented amorphous structure.The results of LSV,CA and CP tests revealed that the Ni/Mo/Cu/C electrode has low overpotential of hydrogen evolution and good stability.EIS tests showed that Rct（4.6Ω）and Rs（16.4Ω）were both smaller than Pt mesh,the electron transfer rate was the fastest and the mass transfer impedance was the lowest.The Tafel curve tests showed that the Tafel slope of Ni/Mo/Cu/C was 23.87 m V·dec^-1,which possessed the best hydrogen evolution kinetics,and the HER on the Ni/Mo/Cu/C electrode follows the Volmer-Tafel reaction path,and the rate-determined step was Tafel reaction.In the MEC tests,the gas production of the Ni/Mo/Cu/C electrode was 18.3 m L,of which H₂ accounted for 93.7%.By calculated,R_cat,R_H2,Q_H2,η_w,η_w+s and FE of the Ni/Mo/Cu/C electrode were 7.89%,208.47%,0.171 m₃-H₂·m^-3·d^-1,16.70%,18.85%and 64.18%,which were all higher than the Pt cathode.2.In-situ chemical reduction preparation of Ni（OH）₂/Ni/g-C₃N₄/C electrode and its HER performanceNi（OH）₂/Ni/g-C₃N₄/C electrode was prepared by in-situ chemical reduction and in-situ growth of Ni onto g-C₃N₄,which was then loaded on carbon paper.SEM and TEM images show that Ni（OH）₂ and Ni particles are interlaced in the two-dimensional layered structure inside g-C₃N₄,forming flower-like nano-clusters,which was lower than the lamellar accumulation on the surface of g-C₃N₄.This feature was conducive to HER,increased the diffusion of hydrogen atoms.The XRD spectrum showed that there were characteristic peaks of Ni（OH）₂,Ni and g-C₃N₄.XPS results proved that the existence of Ni²⁺,Ni⁰,and C,N elements,which strongly supports the result that Ni（OH）₂ and Ni particles were embedded in g-C₃N₄.The BET test showed that the specific surface area of Ni（OH）₂/Ni/g-C₃N₄ was 72.38m²·g^-1,while the specific surface area of g-C₃N₄was 39.53 m²·g^-1,indicating that Ni（OH）₂/Ni/g-C₃N₄ possessed larger specific surface area and better adsorption activity than g-C₃N₄.LSV test showed that when the molar ratio of Ni element to g-C₃N₄ is 2:5 and the loading on the carbon paper is 4 mg·cm^-2,the composite exhibited the most excellent hydrogen evolution performance.Tafel test showed that the Tafel slope was 44.3 m V·dec^-1,which was slightly higher than that of the Pt mesh(45.81 m V·dec^-1),indicating that it had strong hydrogen evolution kinetics.The CP diagram illustrated that it possessed good electrocatalytic stability,so the electrode was suitable for practical applications.The MEC hydrogen production tests showed that the maximum current density of Ni（OH）₂/Ni/g-C₃N₄ was 16.75 A·m^-2,and the gas production was 16.5 m L,of which H₂ accounted for 90.32%,which were better than Pt mesh.