Controlled Synthesis Of Three-Dimensional Nickel-Iron Alloy@carbon Nanocomposite Catalysts And Their Electrolytic Water Performance Study

Electrolytic water to hydrogen technology is an effective way to solve the shortage of fossil energy and reduce carbon emissions,and its key lies in the development of low-cost,high-performance catalysts for hydrogen and oxygen precipitation reactions.Layered double hydroxides（LDHs）materials have attracted much attention in the field of electrocatalysis due to their structural diversity,low price and simple synthesis methods.However,the disadvantages of poor electronic conductivity and limited electrochemically active surface area still limit its application.Nickel-iron alloy materials exhibit excellent electrolytic properties due to the synergistic effect of nickel and iron,but they also suffer from easy agglomeration,few active sites,and poor stability.An effective solution is to encapsulate these metal nanoparticles into graphene layers or carbon nanotubes.However,most of the reported ones require nitrogen doping and can only be used for semi-reaction.Therefore,the synthesis of low-cost bifunctional catalysts is still challenging.In this work,a three-dimensional nickel-iron alloy carbon nanocomposite catalyst with high electrical conductivity and dispersion was controllably synthesized by solid-state pyrolysis using a layered double hydroxide with salicylic acid root intercalation as a single precursor,and its electrolytic water performance was investigated.The details of the study and the conclusions are as follows.1.In this work,three-dimensional carbon-covered nickel-iron alloy nanocomposite catalysts（Ni_xFe_y@C）were prepared by pyrolysis in N₂atmosphere using binary nickel-iron layered hydroxides（Ni_xFe_y-Sal LDHs）synthesized by self-assembly of salicylate intercalation in aqueous phase as a single precursor（x:y is the ratio of the amount of matter of the metal）.The precursors and Ni_xFe_y@C samples with different roasting temperatures were characterized by SEM,TEM,BET,XRD,ICP,Raman,FT-IR,in situ FT-IR,XPS and other tools,and the results showed that.The Ni₄Fe₁-Sal LDHs precursors exhibit a three-dimensional nano-flower-like morphology with a diameter of about 600 nm.the Ni₄Fe₁@C-500 nanocomposites obtained by pyrolysis at 500℃still maintain the three-dimensional nano-flower-like morphology and have a high metal loading（91.32%）,a large specific surface area(221.06 m²·g^-1)and an abundant pore structure.The nanosheets generate uniformly sized,highly crystalline carbon-covered nickel-iron alloy nanoparticles with a particle size of about 10 nm.the carbon layer consists of 1-3 layers of graphene with abundant functional groups（C-OH,C-M,C-O-M,etc.）on the surface.The C-M and C-O-M formed by the NiFe alloy and the carbon layer facilitate the stability between the cores and shells,improve the electron transport capacity,and enhance the synergy between the cores and shells.In the electrocatalytic hydrogen precipitation（HER）and oxygen precipitation（OER）reaction tests,excellent catalytic activity was demonstrated in alkaline electrolytes,with an overpotential of 81 m V for HER at a current density of10 m A·cm^-2,slightly higher than that of 54 m V for the noble metal Pt/C catalyst,and an overpotential of269 m V for OER,significantly better than that of 339 m V for the noble metal Ru O₂catalyst.In addition,Ni₄Fe₁@C-500 catalyst loaded on carbon paper was tested for full hydrolysis,providing a current density of10 m A·cm^-2with a cell voltage of only 1.57 V and remaining stable for280 h of continuous operation at certain voltage conditions.2.Based on the above research results,in this work,three-dimensionalcarbon-coatedNiFe-ZnO（NiFe-ZnO@C）nanocomposites were obtained by pyrolysis of ternary nickel-iron-zinc layered hydroxides（NiFe Zn-Sal LDHs）as a single precursor,and three-dimensional carbon-coated NiFe alloy（NiFe（ZnO）@C-500）nanocomposite catalysts were synthesized by dissolving its amphiphilic oxide ZnO using strong alkali solution.The above samples were characterized by SEM,TEM,EDX,XRD,Raman,XPS and other tools,and the results showed that.The NiFe（ZnO）@C-500 nanocomposite catalyst also has a three-dimensional nanoflower-like morphology as well as a core-shell structure.In contrast to theNiFe-ZnO@C sample,the ZnO in the NiFe（ZnO）@C-500 sample was almost completely dissolved and more defect sites were violated on the graphene surface,which improved the active center and intrinsic activity and further improved the OER and total water resolution performance.At a current density of 10 m A·cm^-2,the overpotential of OER was 245 m V,which was better than that of the noble metal Ru O₂catalyst and Ni₄Fe₁@C-500nanocomposite catalyst,respectively,and the overpotential of HER was98 m V,which did not decrease significantly with respect to the Ni₄Fe₁@C-500 nanocomposite catalyst.In addition,it was loaded on carbon paper for total water decomposition tests,providing a current density of 10 m A·cm^-2with a cell voltage of only 1.54 V and remained stable for 90 h of continuous operation at 1.65 V.All these results led to the preparation of NiFe（ZnO）@C superior non-precious metal-based bifunctional electrode materials for total water decomposition.In summary,in this paper,three-dimensional NiFe@C and NiFe（ZnO）@C nanocomposites were synthesized using NiFe-Sal LDHs and NiFe Zn-Sal LDHs as single precursors.The prepared materials possess hydrogen and oxygen precipitation properties comparable to those of noble metal bases and have excellent stability,which are of positive significance for hydrogen energy development and carbon emission reduction.