Preparation And Performance Studies Of Multi-element Transition Metal Compounds For Electrolytic Water Splitting

Hydrogen energy is considered to be a promising clean energy due to its abundant reserves,easy availability of raw materials,and non-polluting products.The electrocatalytic water splitting involves two half-reactions,namely,the oxygen evolution reaction（OER）and the hydrogen evolution reaction（HER）.Transition metal compounds have broad application prospects in the field of water electrolysis catalysts due to their abundant reserves,low prices,and excellent catalytic activity.However,single-component transition metal catalysts encounter bottlenecks in performance improvement.On the other hand,traditional powder transition metal catalysts need to be coated on glassy carbon electrodes,which will cause problems such as dead volume and easy aggregation of electrocatalysts to lead to the reduction of active sites.Therefore,the problems of insufficient performance,small number of active sites,and poor stability of single-component powder transition metal catalysts need to be solved urgently.Based on this,this study intends to in situ synthesize multi-element transition metal compounds on various substrates（carbon cloth,nickel foam）to prepare supported catalysts.Multivariate transition metal phosphides,oxides and nitrides are prepared by hydrothermal reaction or solvothermal reaction and subsequent phosphating or ammonia treatment.The main research contents are as follows:（1）Two-dimensional Ni Fe P nanosheets（Ni Fe P/CC）self-supporting electrocatalytic materials were in situ synthesized on carbon cloth by solvothermal method and subsequent phosphating treatment.Electrochemical data show that Ni Fe P/CC can reach a current density of 20 m A cm^-2 with only an overpotential of 281 m V in alkaline OER,and its Tafel slope is only 74 m V dec^-1,which is much lower than Ni P/CC(379 m V and 136 m V dec^-1),Fe P/CC(388m V and 85 m V dec^-1),Ni Fe O/CC(336 m V and 81 m V dec^-1),powdered Ni Fe P(350 m V and108 m V dec^-1)and the commercial Ru O₂(345 m V and 105 m V dec^-1).After 50 h of continuous OER testing,the current density of the Ni Fe P/CC electrode showed only a small decrease.XPS data showed that Ni O or Ni OOH appeared on the surface of Ni Fe P/nanosheet after stability test.The excellent OER performance of Ni Fe P/CC benefits from the synergistic effect between Ni Fe P and Ni O or Ni OOH,and the carbon cloth substrate plays a role in protecting the active sites of the nanosheets and improving the stability of the material.（2）In situ synthesis of Fe Co Ni Cu Mn N pentagonal high-entropy nitride（Fe Co Ni Cu Mn N/CC）with unique nanowire/nanosheet structure on carbon cloth by hydrothermal synthesis and Ar/NH₃ post-treatment.The as-prepared Fe Co Ni Cu Mn N/CC can be directly used as an alkaline HER self-supporting electrode material.The results show that the overpotential of Fe Co Ni Cu Mn N/CC to reach a current density of 10 m A cm^-2 is only 184m V,and its Tafel slope is 113 m V dec^-1.Under the same test conditions,the overpotential and Tafel slopes of Fe Co Ni N/CC,Fe Co Ni Cu N/CC,Fe Co Ni Mn N/CC and powder Fe Co Ni Cu Mn N electrodes are 220 m V and 147 m V dec^-1,252 m V and 147 m V dec^-1,222 m V and 213 m V dec^-1,500 m V and 195 m V dec^-1,respectively,indicating the high catalytic performance of multicomponent high-entropy nitrides.The continuous 50 h HER test shows that the current density of Fe Co Ni Cu Mn N/CC has been maintained at around 23 m A cm^-2,indicating its excellent corrosion resistance and oxidation resistance.The synergistic catalysis of multi-component high-entropy nitrides contributes to improving the HER performance of electrocatalytic materials.（3）Fe-doped Ni₃N nanospheres（Fe-Ni₃N/NF）were in situ synthesized on the surface of nickel foam by hydrothermal reaction combined with Ar/NH₃ post-treatment.Fe-Ni₃N/NF electrode achieves overpotentials of 277 m V and 40 m V to reach a current density of 10 m A cm^-2 and Tafel slopes of 55 m V dec^-1 and 71 m V dec^-1 in OER and HER,respectively.In contrast,the overpotentials of Ni₃N/NF without Fe doping at a current density of 10 m A cm^-2 are 376m V and 124 m V,respectively,and their Tafel slopes are 97 m V dec^-1 and 144 m V dec^-1,respectively.On the other hand,when the the current density reaches to 10 m A cm^-2,the overpotential of Ru O₂ in OER and Pt/C in HER need 314 m V and 25 m V,respectively.Compared with Ni₃N/NF,Fe-Ni₃N/NF has lower overpotential and Tafel slope,which indicates that the doping of the second component element can effectively change the electron density of transition metal nitrides,thereby reducing the adsorption energy of reactants and improving catalytic performance and efficiency of electrolyzed water.Because of this reason,the OER performance of Fe-Ni₃N/NF is better than that of Ru O₂ and its HER performance is very close to that of Pt/C.（4）Ni_0.2Mo_0.8N nanosheets were grown on the previously synthesized Fe-Ni₃N two-dimensional nanosheets to form a self-supporting electrode material with a hierarchical structure(Ni_0.2Mo_0.8N/Fe-Ni₃N/NF).In the hierarchical structure,small-sized Ni_0.2Mo_0.8N nanosheets are uniformly anchored on Fe-Ni₃N nanosheets.The Ni_0.2Mo_0.8N/Fe-Ni₃N/NF electrode material requires only 266 m V and 40 m V overpotentials in the OER and HER reactions,respectively,to reach a current density of 20 m A cm^-2,while the comparison samples Fe-Ni₃N/NF,Ni_0.2Mo_0.8N/NF and powder Ni_0.2Mo_0.8N require 292 m V and 88 m V,320 m V and48 m V,450 m V and 364 m V to reach 20 m A cm^-2,respectively.In addition,the performance of noble metal electrocatalysts has also tested in this chapter.Ru O₂ requires an overpotential of328 m V to reach 20 m A cm^-2 in OER,while Pt/C requires an overpotential of 40 m V to reach20 m A cm^-2.Electrochemical data show that the Ni_0.2Mo_0.8N/Fe-Ni₃N/NF composite catalyst exhibits excellent electrocatalytic activity in both OER and HER reactions,and its current density is only slightly decreased after 50 h of stability test,showed excellent electrochemical stability.The full water splitting test of Ni_0.2Mo_0.8N/Fe-Ni₃N/NF shows that its cell voltage is only 1.54 V(10 m A cm^-2),which has a very good application prospect for full water splitting.