| As a kind of clean and renewable energy source,hydrogen energy is expected to be an ideal alternative to traditional carbon-based fuels.Among many methods of hydrogen production,water electrolysis is considered as a promising route with low energy consumption,zero carbon footprints involvement in the reaction（2H2O→O2+2H2）,and high purity H2.It consists of a hydrogen evolution reaction（HER）at the cathode and an oxygen evolution reaction（OER）at the anode.Since the OER process involves 4e-transfer,the reaction kinetics are sluggish,resulting in the formation of O-O bonds requiring a high thermodynamic potential（>1.23 V）,which greatly reduces the overall efficiency of water splitting.Replacing OER with other thermodynamically more favorable reactions（such as urea,methanol,ethanol and glycerol oxidation,etc.）is a low-energy production method of H2 that has received considerable attention in recent years.Compared with OER,urea oxidation reaction（UOR）has a lower thermodynamic potential（0.37 V）,coupling UOR with HER to construct a hybrid water electrolysis system（HER||UOR,CO（NH2）2+H2O→N2+3H2+CO2）.It can not only improve the overall H2 production efficiency,but also alleviate the environmental pollution problems caused by urea-rich wastewater.However,in HER||UOR system,the UOR at the anode（CO（NH2）2+6OH-→N2+5H2O+CO2+6e-）is a 6e-transfer process with sluggish reaction kinetics,requiring efficient and stable catalysts to enhance the overall efficiency.Nickel-based catalysts are one of the most promising non-precious metal catalysts due to their high abundance and active properties.Among them,nickel-based compounds have recently received great attention in HER,OER and UOR.Compared with oxygen,sulfur and selenium in the same group,tellurium formed nickel telluride has poor electrocatalytic performance for water splitting due to weak cleavage of H2O molecules and too long Te-H bonds during the reaction,but nickel-based telluride has excellent electrical conductivity,which facilitates rapid electron transfer and is also easily modified.Based on this,this thesis investigates the electrocatalytic performance of nickel telluride in urea electrolysis by introducing cations and anions to modulate the surface interface of nickel telluride.The details are as follows:（1）NiTe precursors were synthesized by hydrothermal method,followed by doping cation Ru and high temperature annealing to form Ru-Ni2.86Te2/NF catalysts.The physical characterization results show that the morphology of the nanosheets did not change significantly after Ru doping,but the surface became rough from smooth,which may be due to the presence of Ru in the form of atomic clusters on the surface of the nanosheets.Meanwhile,the introduction of Ru can change the crystal structure of nickel telluride to form a non-stoichiometric Ni2.86Te2.The electron cloud density at the surface interface is also changed to promote the charge transfer in the electrocatalytic process.The electrochemical activity test results show that Ru-Ni2.86Te2 has a large ECSA and a small Tafel slope.The HER and UOR performance of Ru-Ni2.86Te2 is much better than that of Ni2.86Te2.In HER||UOR system,only a cell voltage of 1.468 V is required to achieve 100 m A cm-2,which is far superior the noble-metal catalyst（Pt/C||Ru O2/NF,1.631 V）.In addition to the fact that the introduction of the cation Ru in NiTe can modulate its crystal structure and change the surface electronic state of the catalyst,thus significantly enhancing the catalytic performance of HER/UOR,this section employs the modification of homologous nonmetals with similar properties to Te to explore the effects of the introduction of the anions O,S,and Se on the structure of NiTe and its electrocatalytic performance.NiTe was used as the precursor and O,S,and Se were infiltrated by the high-temperature gas phase.The results showed that,unlike the introduction of the cation Ru,the introduction of O and S did not affect the crystalline phase transition of NiTe,but S could form Ni3S2 with Niand the introduction of Se would induce Ni2.86Te2 to generate a NiTe phase unfavorable to HER in addition to generating a new species NiSe2.Therefore,Ni3S2/Ni2.86Te2/NF exhibited the most excellent HER and UOR activity in electrochemical performance tests,which will be further investigated in the next section.（3）The crystal phase,morphology,electron valence state and electrochemical performance of Ni3S2/Ni2.86Te2/NF were systematically studied.XRD results show that the introduction of S presents as Ni3S2 phase;electron microscopy images show that the generated Ni3S2 exists in the form of nanorods,Ni3S2/Ni2.86Te2/NF is rich in defect sites;after sulfidation can regulate the electron distribution around Ni2.86Te2 and Ni3S2,which provides a favorable electric field environment for the electrocatalytic reaction.The electrocatalytic activity results show that Ni3S2/Ni2.86Te2/NF has excellent UOR and HER activity in 1 mol L-1 KOH+0.3 mol L-1 urea solution.When the current density reaches 100 m A cm-2,the UOR and HER require only 1.37 V and 0.227 V,respectively.In addition,the also has smaller Tafel slope and Rct.This excellent electrochemical performance is related to the abundant defect sites formed after the introduction of sulfur,which can expose more active sites and contribute to the adsorption of reaction molecules;meanwhile,the synergistic effect between the different components of Ni3S2and Ni2.86Te2 is also beneficial to enhance the electrocatalytic performance.Therefore,this work provides a new idea for modifying similar inert catalysts. |
