Research On Ni, Fe, Co-based Non-precious Metal Sulfides/phosphides As Oxygen Evolution Electrodes At High Current Densit

With the rapid development of economy,human’s excessive consumption of fossil energy makes the environmental pollution problem increasingly aggravated.Hydrogen（H₂）is regarded as an ideal green energy alternative to traditional fossil fuels due to its high energy density,clean,renewable characteristics.Among the various H₂production methods,H₂production by water electrolysis is one of the most efficient and environmentally friendly technology.Water electrolysis consists of two half reactions,namely the oxygen evolution reaction（OER）at the anode and H₂evolution reaction（HER）at the cathode.Among them,OER is a multi-electron transfer process,with a slow kinetic process that requires a high overpotential to drive the reaction,which greatly increases the energy consumption of electrolytic water and reduces the efficiency of H₂production by water electrolysis.Therefore,it is very important to develop efficient electrocatalysts for OER.Commercial precious metal catalysts such as RuO₂and IrO₂can exhibit high catalytic activity towards OER.Nevertheless,the scarcity,high price and poor stability of these noble metal materials seriously hinder their application in industrial water electrolysis.Moreover,these powder catalysts often require binder adhesion to the electrode surface,which severely hinders the mass and charge transfer during the catalytic process.In recent years,Ni,Feand Co-based non-noble metal catalysts has attracted extensive attention.Their catalytic activity at small current density(10 m A cm^-2)is comparable to or even better than that of noble metal catalysts such as RuO₂and IrO₂,but its activity and stability at high current density are rarely reported.In this thesis,three non-noble metal sulfide/phosphor catalysts with high catalytic activity and stability under high current density were designed and prepared,and their catalytic performance for OER or water electrolysis under high current density were investigated.The main research contents and results are as follows:1.Nickel foam（NF）-supported spherical Ni_xFe_yS catalyst was prepared by one-step hydrothermal sulfuration,and the effects of sulfuration temperature on the OER performance of the catalyst were investigated in 1.0 M KOH.Results show that the electrocatalyst has the best electrocatalytic activity when the sulfide temperature was 80℃.The NiFeS/NF catalyst displays an excellent OER performance at high current density,requiring extremely low overpotentials of 297 and 307 m V to reach500 and 1000 m A cm^-2,which are much lower than those on commercial RuO₂（476and 544 m V）.At the same time,the catalyst can stably catalyze the OER for 35 h at either a current density at 100 or 500 m A cm^-2,and the OER catalytic activity shows almost no attenuation.Compared with before OER test,the morphology and crystal structure of the catalyst after OER test did not change significantly,showing good chemical and structural stability.The catalytic activity and stability of Ni_xFe_yS/NF catalysts exceed those of most previously reported transition metal sulfide electrocatalysts,which provides an approach for the exploration of high-performance OER catalysts.2.Based on the previous work,the synergistic effect of the dual-active component catalyst in the catalytic process of OER was further investigated.A self-supporting（FeCo）P-（NiFeCo）S/NF electrode（denoted as（FC）P-（NFC）S/NF）with coexistence of crystalline and amorphous phase was prepared through hydrothermal and a subsequent electrodeposition methods.The（FC）P-（NFC）S/NF catalyst shows excellent catalytic activity and high stability for OER especially at high current densities.When the current density reaches 500 and 1000 m A cm^-2,the overpotentials of OER are only 274 and 280 m V respectively,which are much lower than those on the reference（FeCo）P and（NiFeCo）S catalysts,as well as commercial RuO₂and most of the non-noble metal catalysts reported in literatures.The results of chronopotentiometry（CP）showed that the potential only increased by 31 m V and 87m V after continuous working for 40 h at 100 and 500 m A cm^-2,respectively.In addition,compared to the catalyst before the OER test,the morphology and crystal structure of the catalyst did not change significantly.In addition to its larger surface area and higher electrical conductivity,the improvement of catalytic activity is also related to the synergistic effect between crystalline phase（NiFeCo）S and amorphous phase（FeCo）P.The two-component（FC）P-（NFC）S/NF catalysts with both crystalline and amorphous phases have certain reference significance for understanding the catalytic synergistic effect,designing and developing non-noble metal electrocatalysts for OER with high activity and stability.3.Another important part of this thesis is to design and prepare a bifunctional catalyst that can efficiently catalyze the cathodic and anodic reactions of water electrolysis at the same time.Ce₂O₃@Ni Co P/NF with heterogeneous structure andmulti-layered nanosheet morphology was synthesized using cobalt nitrate,nickel nitrate and cerium nitrate as metal sources and sodium hypophosphite as phosphorus source.The catalytic properties for HER,OER and water electrolysis were investigated in 1.0 M KOH.The catalyst requires only 183（HER）and 304（OER）m V of ultra-low overpotentials to achieve a high current density of 1000 m A cm^-2.Meanwhile,when the catalyst is used as anode and cathode for water electrolysis,it requires only 1.87 V of decomposition voltage to achieve a high current density of1000 m A cm^-2,which is far lower than commercial RuO₂and Pt/C catalysts and most bifunctional catalysts reported in the literatures.In addition,after 50 h of stability test at a high current density of 500 m A cm^-2,the morphology and composition of the catalyst were almost unchanged,showing a high stability.Therefore,the reasonable construction of the heterostructure of metal oxide and metal phosphide is another effective avenue to design and prepare high performance catalysts for water electrolysis.