Multiscale Pore Construction Via Ice-template Method And Oxygen Evolution Performance Of Ni₇Fe₃ Anode

In face to the global strategic demands of"carbon peaking"and"carbon neutrality",producing hydrogen by water electrolysis with the advantage of zero carbon dioxide emission is one of the most promising renewable energy conversion methods.Due to the thermodynamic constraints of the oxygen evolution reaction（OER）and the slow kinetics of the electrolysis process,the search for anodes and OER catalysts with excellent electrocatalytic performance and stability has become an essential topic in water electrolysis research.In this paper,porous Ni₇Fe₃anodes with a large surface area and layered structure,which were prepared by the ice-template method,were taken as the research object,and their pore structure and intrinsic catalytic activity were optimized and enhanced by starch-ice dual template method and atmospheric pressure HNO₃hydrothermal method,respectively.A self-supporting multi-scale porous Fe OOH/Ni₇Fe₃ anode with excellent OER catalytic activity and stability was constructed.The structure-activity relationship between pore structure and OER properties and its implementation effect are also expounded.The main research contents and conclusions of this paper are as follows:（1）Three-dimensional layered Ni₇Fe₃ anode was constructed by the ice-template method,and its OER performance and mechanism were studied.The regulation mechanism and influence rule of the ice-template method on the pore structure of the Ni₇Fe₃ anode was also discussed.The results showed that the regulation law of porous Ni₇Fe₃ anode structure by the ice-template method was as follows:the thickness of the Ni₇Fe₃ anode layer increased with the increase of solid phase content,and the porosity could be controlled in the range of 48-77%by the initial solid phase content of slurry The content of slurry binder could control the variation of pore structure from three-dimensional ordered layer to disordered cellular structure.When the cold finger temperature ranges from-50℃to-10℃,its pore size and wall thickness could be accurately regulated in the range of 8-43μm and 5-23μm,respectively.The Ni₇Fe₃ anode with the initial solid phase content of 15 vol.%had the best OER activity,the overpotential corresponding to the current density of 10 m A cm^-2 and 100 m A cm^-2 was246.1 m V and 317.1 m V,respectively.And the Tafel slope was 47.2 m V dec^-1.Both Ni and Fe were the active sites of OER,and the OER of the Ni₇Fe₃ anode was dominated by Ni（OH）₂.During the oxygen evolution process,surface self-remodeling occurred,and a Ni/Fe（O）OH layer with a thickness of about 40 nm was formed.（2）A transport linear model was used to describe the electrochemical impedance spectroscopy response of the Ni₇Fe₃ anodes,and the mass and charge transfer resistance was quantified.Combining the statistical methods,the structure-activity relationship between the pore structure and its OER performance was illustrated.The results showed that the Pearson correlation coefficients of ion diffusion resistance,charge transfer resistance and electrochemical active surface area with Tafel slope of porous Ni₇Fe₃ anode were 0.734,0.613 and-0.339,respectively,indicating that reducing ion diffusion resistance and charge transfer resistance had a high probability of improving OER performance.The improvement of anodic OER performance did not depend on the increase of electrochemical active surface area.Among Ni₇Fe₃ anodes with different structures,the dominant role of ion diffusion and charge transfer resistance in limiting OER performance changed dynamically.Introducing small pores,which were significantly different from the size of large pores between layers,could effectively improve the ion transport path of the porous anode and reduce its diffusion resistance.However,excessive pores would damage the continuity of the structure and adversely affected the charge transfer.Therefore,there was an equilibrium point for reducing the resistance of mass and load transfer to the pore structure.Finally,the experimental results verified the importance of balanced ion diffusion and charge transport resistance in the pore structure of the Ni₇Fe₃ anode to optimize OER performance.（3）A starch-ice dual template method was developed to optimize the pore structure of the porous Ni₇Fe₃ anodes.The results showed that the starch-ice dual template method could accurately control the porosity and microstructure of porous materials.The macrostructure of porous Ni₇Fe₃anode could be controlled by using the rheological properties of slurry and the effect of cold field temperature on the growth of ice crystals.The surface microstructure morphology could be effectively controlled by combining the repulsion and eutectoid characteristics of starch capsules and solid particles during the freezing process.And a new pore structure with high porosity and high connectivity with the characteristics of large interlamellar pores and micropores on the lamellar surface could be constructed.This structure effectively improved the charge and mass transfer of porous Ni₇Fe₃ anode and provided a larger electrochemical reactive area and more active sites,significantly improving the OER performance.The Tafel slope of the Ni₇Fe₃ anode prepared by starch-ice dual template method was 46.9 m V dec^-1,and the overpotential at 10 m A cm^-2 and 100 m A cm^-2 current densities were 246.3 m V and 306.5 m V,respectively.Compared with the Ni₇Fe₃ anode prepared by the ice-template method,it decreased by 7.6%and 13.3%,respectively.（4）In-situ construction of nano Fe OOH on the surface of Ni₇Fe₃anode was realized by atmospheric pressure HNO₃ hydrothermal method,and multi-scale porous Fe OOH/Ni₇Fe₃ anode was developed,which had interlamellar macropores and lamellar micropores and in-situ growth of nanocrystals.The results showed that the multi-scale porous anode was obtained by hydrothermal treatment of Ni₇Fe₃ at 70-90℃with 0.1 M HNO₃ for 4 h,and its multi-scale pore structure,including interlamellar macropores,lamellar micropores and micro-nano pits,could effectively improve the resistance of ion diffusion and charge transfer and provide a larger reactive surface.The nano Fe OOH loaded on the surface was OER intermediate active site,which could further enhance OER activity.Moreover,the binder-free self-supporting structure made the electron transfer in the body and electrode-electrolyte interface easier.It achieved long-term stable OER at high current density.The Tafel slope of the multiscale pore Fe OOH/Ni₇Fe₃ anode was only 21.6 m V dec^-1,and its overpotential was only 193.6 m V and 257.1 m V at the current densities of10 m A cm^-2 and 100 m A cm^-2,respectively.Compared with the Ni₇Fe₃anode prepared by the ice-template method,the anode decreased by 26.4%and 27.2%,respectively.The Ni and Fe in the Ni₇Fe₃ matrix diffused to the outer surface of the Fe OOH nanocrystals during the OER process,and in-situ formed the Ni/Fe（O）OH layer with a thickness of about 100-200 nm.The main phases were Ni（OH）₂ and Fe（OH）₃.Moreover,Ni（OH）₂ was further oxidized to high-valence hydroxyl oxide Ni OOH,and the Ni/Fe（O）OH layer and Fe OOH nanocrystals promoted the OER process.