| As one of the most promising energy conversion and energy storage methods for large-scale production of high-purity hydrogen fuel,electrochemical water splitting driven by renewable energy has been widely regarded as a crucial clean energy production technology and has become the research hotspots around the world.The complementary combination of hydrogen energy and electric energy can allow renewable energy to be developed and utilized on a large scale.Theoretically,only 1.23 V potential is needed to electrochemically split water,but in fact,to overcome the reaction barrier,a higher potential needs to be applied,which hinders the efficient progress of the water electrolysis reaction.In order to minimize the overpotential of OER and HER and improve the energy conversion efficiency,highly efficient electrocatalysts must be used to reduce the energy barrier of each half-reaction.Noble metal-based exhibit outstanding catalytic activities for water splitting.However,the high cost,rarity and poor stability of these catalysts are significant restraints for industrial applications.Transition metals Ni,Co,Fe,Mo-based materials,due to their abundant reserves and low prices,have been developed as highly efficient HER and OER electrocatalysts.They have become the most promising catalysts to replace noble metals catalysts and have huge potential in practical applications.In order to develop non-noble metal electrocatalysts working at high current density,it is vital to overcome several challenges such as violent gas release,intensive surface reconstruction,and low charge transfer efficiency.Therefore,it is necessary to design self-supported catalysts with excellent intrinsic activity and conductivity.Among numerous non-noble metal catalysts,NiFe LDH materials have been extensively studied due to their low price,controllable morphology,and unique layered structure.However,due to low intrinsic conductivity and insufficient edge active sites,their catalytic performancefor practical applications is limited.In order to solve the above problems,in-situ self-supporting three-dimensional core-shell structure NiCoS@NiFe LDH composite electrode has been prepared as OER electrocatalyst through hydrothermal treatment,vulcanization and electrodeposition process due to the superior electrical conductivity,mechanical properties and catalytic performance of NiCoS nanorods.A series of characterizations proved that due to its unique three-dimensional core-shell structure,the self-supporting composite electrode has large specific surface area,efficient electron transmission and rapid gas release,which allows it to expose more active reaction sites.Therefore,it exhibits excellent performance in alkaline electrocatalytic oxygen evolution system.When the current density is 10 mA cm-2 and 100 mA cm-2,the overpotential of three-dimensional core-shell structure NiCoS@NiFe LDH composite electrode is only 214 mV and 254 mV for OER process,respectively,and the Tafel slope is 33.7 mV dec-1.After the electrode continuously works for 24 hours,the catalytic performance remained without significant decay.At high current density of 1000 mA cm-2,the overpotential of the electrode is only 340 mV,much better than that of the commercial noble metal oxides IrO2 electrode.Besides,among numberous non-noble metal catalysts for HER,transition metal phosphides(TMPS)have become the research hotspots due to their outstanding conductivity,catalytic activity and stability,and easy to adjust hydrogen adsorption free energy(ΔGH).Because HER process involves two steps,the dissociation of water and the release of hydrogen,constructing dual sites can separately control OH*adsorption free energy and H*adsorption free energy.Therefore,in this thesis,Ni and Mo elements that are easy to adsorb OH*and Co elements that are easy to adsorb H*are selected to promote water dissociation and hydrogen release simultaneously.Moreover,the combination of NiP,CoP and MoP can jointly regulate ΔGH through the weak ligand effect produced by M-P bond.Based on the above analysis,in-situ self-supporting porous ternary NiCoMoP nanosheets array electrode has been successfully prepared as HER electrocatalyst through hydrothermal and one-step phosphating processes.The uniform and dense pores increase the electrochemical specific surface area and the number of active sites of the material.Moreover,the combination of ternary metal phosphides to regulate the electronic structure improves the intrinsic activity of each active site,thus improving the HER catalytic activity and stability of the catalyst.The overpotentials are only 92,232,366 and 467 mV used to drive HER at the current densities of 10,100,500 and 1000 mA cm-2,respectively.The Tafel slope is 51.33 mV dec-1,and it can work stably for 24 hours at current densities of 10 and 100 mA cm-2,respectively,without significant decay.Compared with the commercial noble metal Pt/C electrode,the performance of this material is comparable.Moreover,among all the transition metal phosphide(TMPs)materials published recently,this porous ternary NiCoMoP nanosheet array electrode exhibits excellent performance,especially at high current density.NiCoS@NiFe LDH composite anode electrode and NiCoMoP nanosheet array cathode electrode have been assembled for the alkaline two-electrode overall water splitting system.Under the applied voltage of 1.52,1.76,1.89 and 1.99 V,the current density of the overall water splitting system can reach 10,100,300 and 500 mA cm-2,respectively.Besides,the cell can work stably for 40 hours at a current density of 10 mA cm-2.This work provides a new possibility for the design of highly efficient nonnoble-metal nanocomposite catalysts for alkaline overall water splitting. |
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