Development Of Novel Transition-metal-based Water Splitting Reaction Electrocatalysts For Photovoltaic-Electrocatalytic Water Splitting Application

Solar power is a clean and inexhaustible resource.Photovoltaic-electrocatalytic water splitting reaction is one of the most efficient methods to convert the solar energy into hydrogen.It is necessary not only to develop highly active hydrogen evolution reaction(HER)catalyst but also to synthesize efficient anodic catalyst for oxygen evolution reaction(OER)due to the kinetics limit of it.This dissertation is concentrated on the hotspot of transition-metal-based catalysts due to the easy modulation of their electronic structure.On the basis of the respective characteristics of OER/HER process,we developed a series of highly active electrocatalysts for water electrolysis reaction via the electronic-structure modulation of these electrocatalysts.The contents of studies are as following:(1)Amorphous Cox-Fe-B as efficient OER catalysts are synthesized to explore the roles of Fe and Co in metal borides.Both the Co and Fe sites are identified as active sites,and the synergistic effect of them improves the catalytic activity.The experimental results verify that Fe can propel the formation and stabilization of high valence metal-OOH species which is the key intermediate of OER,and Co accounts for the enhanced conductivity.The optimal sample(Co2-Fe-B)presents low overpotential of 298mV(J=10mA cm^-2),small Tafel slope(62.6mV dec^-1)and good stability.A solar-to-hydrogen(STH)conversion efficiency of 4.2%is achieved by connecting the electrodes to a solar cell.(2)The effect of 3d transition-metal codoping on the OER performance of WO_2.72catalyst is investigated.By codoping of Co and Fe,the alkaline-labile WO_2.72(WO)is transformed to an efficient and alkaline-solution-stable electrocatalyst(entitled as Co&Fe-WO).The codoping lowers the chemical valence of W to improve the durability of W-based catalyst,improves the electron-withdrawing capability of W and O to stabilize the Co and Fe in OER-favorable high-oxidation-state,and enriches the surface OH groups(reactive sites).The Co&Fe-WO catalyst shows very low overpotential(226mV,J=10mA cm^-2),small Tafel slope(33.7mV dec^-1)and good conductivity.The Co&Fe-WO electrode is applied to a photovoltaic-water splitting system to stably produce hydrogen for 50 h with high STH conversion efficiency of 16.9%.(3)The intermediate layer modulation is applied to the hierarchical microwire array electrode to improve the HER activity.The electrode comprises of the Ni foam substrate,the intermediate layer of Co(CO₃)_0.5(OH)_0.11·H₂O(CoMoCH)microwire array and the surface thin layer of NiCoP.The Mo doping not only enriches the electron density at active sites but also increases the amount of them.Besides,the conductivity of the electrode is obviously improved.Based on the above factors,the array electrode shows very low overpotential of 45mV(J=10mA cm^-2)and small Tafel slope of 63.0mV dec^-1.We construct a photovoltaic-water splitting device to realize practical STH conversion with the efficiency of 14.9%.