Mechanism of water splitting based on NiCoP bonded by carbon quantum dots
Hydrogen energy is a green,efficient and sustainable energy that can be an important way to solve the current global"double carbon"problem.The development of low-carbon,clean,efficient and long-term stable water-splitting hydrogen production technology will be beneficial to the sustainable development of the economy and society.Water splitting reactions are currently limited due to their high overpotentials and slow kinetics,including the anodic oxygen evolution reaction(OER)and the cathodic hydrogen evolution reaction(HER).Therefore,it is of great practical value to develop high-efficiency,low-cost non-noble metal catalysts to overcome the energy barrier of the water splitting reaction and achieve rapid and efficient complete water splitting to produce hydrogen.Transition metal phosphides are expected to replace noble metals as overall water splitting catalysts due to their good electrochemical properties.However,their catalytic activity and reaction kinetics are lower than those of noble metals in overall water splitting,due to the high surface proton adsorption energy in the HER process and weak adsorption of hydroxyl radicals in the OER process.Transition metal phosphides are prone to metal atom leaching and surface reconstruction during the catalytic process,thereby reducing their structural stability and shortening the service life of the catalyst.The previous reports have shown that improved processes include atomic doping,defect construction,interface control and component matching.These methods mainly obtain non-precious metal catalysts with high activity and long-term stability by modulating the surface nanostructure of the catalyst,optimizing the electronic structure of the catalyst,and balancing the relationship between catalytic activity and structural stability.Herein,this work reports the design and preparation of carbon quantum dot-bonded nickel cobalt phosphate as catalysts for water splitting based on the strong interaction between carbon quantum dots and the surface functional groups of nickel cobalt phosphate.We describe the reaction energy barriers,possible pathways,intermediate state adsorption and desorption processes,and changes in reaction free energy of this catalyst during the water splitting process from the perspective of nano-micro interface bonding between quantum dots and transition metal phosphide.Density functional theory(DFT)calculation results show that the P-Co and P-Ni bond lengths on the transition metal phosphide surface shrink after the introduction of carbon quantum dots,thereby inhibiting the leaching of nickel and cobalt atoms.Ab initio molecular dynamics(AIMD)simulation results show that the catalyst system has low energy and small temperature fluctuations,indicating that the bonded structure has good dynamic stability.In the HER process,carbon quantum dots can improve the surface charge distribution of nickel cobalt phosphate,reduce the energy barrier for water molecules to decompose on its surface,and enhance the ability of surface hydrogen to overflow.In the OER process,carbon quantum dots can adjust the d-band center of the cobalt site and reduce its theoretical overpotential.Carbon quantum dot bonded nickel cobalt oxide was prepared by combining hydrothermal method and chemical vapor deposition method.The overpotentials of this catalyst in HER and OER reactions are 101 and 250 mV,respectively.The water decomposition potential of the assembled symmetrical alkaline electrolyzer(1.56 V)is 80 mV lower than the decomposition potential of the precious metal electrolyzer(Pt||RuO2).After 14 h of continuous electrolysis,the current density retention rate was 92.18％.Therefore,it is possible to develop a clean,low-carbon,efficient and long-lasting stable overall water splitting catalyst by activating catalytic activity and strengthening catalytic stability by bonding quantum dots on the surface of transition metal phosphide.
carbon quantum dotssurface bondingNiCoPcatalytic mechanismoverall water splitting
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