Effect Of Surface Co-Containing Oxides On Kinetics Of Hydrogen Production From Alkaline Water Electrolysis On Co₃N Cathode

The hydrogen energy,as one of the cleanest energy sources,is one of the best solutions for mankind to solve global warming and energy shortage in the 21st century,which will be the main way for humans to obtain energy in the future.The large-scale production of hydrogen is limited by the dependence on fossil energy sources,low energy conversion efficiency,and high price.In order to solve the urgent need for large amounts of hydrogen energy,hydrogen production from water decomposition,as an ideal and scalable method to obtain high-purity hydrogen,has become a research hotspot.As an electrode reaction,efficient electrode materials are the key to obtaining large amounts of hydrogen at low cost,and the reaction kinetic is the focus of rapid preparation of hydrogen production.The current electrocatalytic materials are mainly concentrated in the transition metal nitrides,sulfides,phosphides and other non-metallic elements,supplemented by two-dimensional exfoliation,manufacturing defects and other preparation methods to regulate the catalytic activity,but there are still some problems such as high hydrogen evolution overpotential,poor stability.The transition metal nitrides have special physical and chemical properties.The filled states of d-band are narrowed after formation of nitrides,resulting in the similar electronic structure of noble metals up to the Fermi level.On the other hand,the unfilled states of d-band,which energies above the Fermi level,are broadened after formation of nitrides,giving rise to a greater density of empty levels for nitrides than parent metal,thus,making nitrides better electronic acceptors than those of the patent metals.However,phenomenon of in-situ oxidation occurs when nitrides are catalyzed for HER in alkaline solution,therefore,partial oxides are formed on the catalytic interface.It is important to understand reasonably that the change of the catalytic interface affects the reaction.In order to avoid the influence of the preparation of the nanomaterial and the fabrication of the working electrode on the experimental data.Based on this,a highly efficient electrocatalytic Co3N plate electrode material prepared by a simple method on the Co plate electrode is taken as the research object.In this dissertation,the effect of nitriding process on hydrogen production and the effect of cobalt oxide formed on the in-situ oxidation of electrode surface on the hydrogen production are systematically studied.In 1 M KOH solution,the overpotential of Co3N electrode at 10 mA cm-2 is only 230 mV,while the overpotential of Co and CoOx electrodes are 400 mV and 442 mV respectively,which are significantly higher than that of the Co3N electrode.The Tafel slope,characterizing the rate-determining step of the catalytic reaction,shows that the Tafel slope of the Co3N electrode is 101 mV dec-1,indicating that the rate limiting step of Co3N electrode is the Volmer step-the dissociation reaction of H2O.The activation energy of Co3N electrode for HER is only 26.6 kJ mol-1,which is much lower than that of Co electrode 64.3 kJ mol-1,which is higher than 11.69 kJ mol-1 of Pt electrode.The stability test shows that the catalytic activity of Co3N electrode can maintain up to 100 h at 10 mA cm-2,and the overpotential of the Co3N electrode at 10 mA cm-2 only decreases by 20 mV after 100 h.The XPS test shows that the surface of the Co3N electrode after 100 h undergoes in-situ oxidation to form cobalt oxide,Co0 is observed in Co3N electrode after 100 h HER.The Tafel slope of the Co3N electrode after 100 h test is reduced to 79.5 mV dec-1,indicating that the cobalt oxide formed on the catalytic interface promote the Volmer step and greatly increase the catalytic reaction rate.We use the SCN-ion,which can coordinate with cobalt ions,occupy adsorption sites on cobalt ions and reduce the catalytic reaction activity,which proved that the cobalt oxides formed on the catalytic interface promote the catalytic rate of HER.The DFT calculations find that the N atom in Co3N can optimize the Gibbs free energy of the H*at the catalytic interface,the Hollow site on the Co3N(002)where an N atom is located directly below the H*adsorption site is the most optimal site for absorbing H*.