Synthesis And Mechanism Of Highly Efficient Hydrogen Evolution Catalysis For Water Splitting

Since 21th century,more and more attention has been paid to the energy crisis and environment pollution problem.Obviously,it’s quite necessary to develop the new renewable green energy,in which the hydrogen energy is one of the most promising options with its high energy density and energy conversion efficiency.Water electrolysis in the alkaline environment is an important hydrogen production method which takes for great catalysts to promote the conversion efficiency.NiCo2S4 with excellent electrical conductivity,hybrid d orbitals and versatile redox nature,has displayed excellent catalytic performance for oxygen evolution and oxygen reduction reactions.Transition metal sulfide-NiCo2S4 has great stability,abundant empty d orbits and excellent redox properties.The unsatisfied catalytic activity is commonly attributed to the strong interaction between the adsorbed H(H*)and the strongly electronegative sulfur sites,which substantially hinders the H*from fleeing away to produce free H2.Moreover,given that the strong electronegativity is the essential feature of S,the unfavorable H*desorption behavior is also a common drawback for most of metal sulfides for HER catalysis.Although morphological engineering can improve the catalytic performance of metal sulfides to some extent by increasing the surface area,it cannot fundamentally alter the intrinsic catalytic properties of metal sulfides.To solve this,we replace some sulfur with the more electro-negative nitrogen resulting the decrease of the electricity density of nickel and cobalt atoms and the facilitation of the absorption and disassociation of water while at the same time,the electricity density of sulfur decreases and the interaction between the sulfur and hydrogen atom weakens which is for the separation of the hydrogen.We get to know specific structure,morphology,chemical state,composition and electrochemical catalytic properties through quantitative characterization and electrochemical test.In this passage,we will introduce our main researches and conclusions as below.(1)We synthesize Ni-Co-N nanowires through hydrothermal method with cobalt nitrate as cobalt source and nickel nitrate as nickel source as a precursor for the nitrogen doped nanowires.We search for the factors influencing the morphology and the variations include the ratios of nickel and cobalt,hydrothermal temperature,zaction time,the carrier gas and so forth.According to the result,the best preparation condition for the N-NiCo2S4 is to conduct the hydrothermal reaction under 120℃ for 12h with the nitrogen as the carrier gas and the sulfur source.N-NiCo2S4 has an overpotential of 41 mV under the current density of 10 mA Cm-2,the best among the known and the Volmer-HeyrovsKy mechanism for Tafel slope.(2)The XRD pattern shows no existence of new structure proving that nitrogen is doping into the material and the red shift of the peak diffraction angle belong the(400)plane indicating the increase of the lattice parameter.Based on the DFT calculations for different doping conditions,the doped nitrogen decrease the lattice while the nitrogen inInterstitial lattice do the contrary.Furthermore,EXAFS measurement also indicates the decreased coordination number of Co-S is attributed to the introduced N,which occupies the S sites to form Co-N.N with even stronger electronegativity than S can further withdraw electrons around metal atoms,resulting in the reduction of the electron density localization around the metal atoms.All above support the indication that the nitrogen existence as a replacement of sulfur.Further DFT calculation shows that bulk nitrogen could increase the conductivity of material.XPS and NEXAS test shows the change of the electricity density proving that the introduction of nitrogen decreases the electricity density of the cobalt and nickel which is for the absorption of the water and the separation of hydrogen atom,and the DFT calculation come to the same conclusion.The UPS exam shows that after the doping,the band gap of the sample is away from the Fermi indicating the decrease of electricity density of the sample surface and the sulfur promoting the separation of the absorbed hydrogen,the same conclusion could be reached from state density graphs(3)The DFT calculation shows that the catalytic reaction in the alkaline environment includes the disassociation of water,the separation of HO*to the OH-,the reaction between the H*and water,the generation and separation of H2*.The doping nitrogen mainly lower the energy barrier of the water disassociation from 0.74 eV to 0.56 eV and the second step from 1.63 eV to 1.61 eV.The nitrogen lowers the electricity density of the sulfur and the energy barrier while the bond length of the Co-N is much shorter than the Co-S one,further proving the strong interaction between the cobalt and nitrogen.From the electron density difference patterns,the lower electricity density in the sulfur site is also observed promoting the water splitting.Overall.N dopants can efficiently facilitate the rate determining steps of water dissociation and H-S breaking,which can be reasonably correlated with the improved HER reaction kinetics in N-NiCo2S4.The lower d-band center is relative to the Fermi level,the lower in energy the antibonding states are and the weaker bond strength.The weakened metal-S bond results in the decreased localized electron cloud densities on S site and thus weakens the S-H interaction and facilitates desorption of hydrogen to promote the HER activity.In addition,the calculated smaller formation energy of substitution(2.08 eV)than that of the insertion state(2.96 eV)further supports the substitutional N is favorable,which is consistent to the XPS and XANES results.(4)Given that the desorption of H*from sulfur sites is a common drawback for conventional metal sulfides for HER reaction,we further implemented this strategy on other commonly used metal sulfides,including FeS2,CoS2 and NiS2 for HER catalysis.Interestingly,the electrochemical performance of all the N doped metal sulfides has been significantly improved.,respectively.All these results clearly reveal that nitrogen doping could be a facile and general method to promote the HER catalysis of metal sulfides by modulating their surface electronic properties,providing more ideas for the modification of the sulfides in the future.