Synthesis Of Ion-Doped Nickel-and Cobalt-Based Electrocatalysts And Their Catalytic Performance For Oxygen Evolution Reaction

Electrochemical water splitting is an ancient hydrogen production technology,but in the context of developing and utilizing primary renewable energy,it has been given a new definition as a key technology for clean energy conversion.Using surplus electrical energy generated by renewable energy to drive water electrolysis provides a green and environmentally friendly way to produce hydrogen while providing a preferred option for the efficient use of primary energy.Developing low-cost oxygen evolution reaction（OER）electrocatalysts with both high activity and good durability is a key technical challenge in developing efficient water electrolysis hydrogen production technology.Years of research have shown that non-precious metal catalysts such as Ni-based and Co-based catalysts have better OER activity than precious metal catalysts under alkaline conditions.In addition,alkaline water electrolysis has lower corrosion resistance requirements for hydrogen production system materials than acidic water electrolysis.From the perspectives of economy and operability,alkaline water electrolysis has more application prospects.Under the conditions of alkaline electrolyte and high oxidation potential of OER,the surface of non-precious metal catalysts will spontaneously transform into hydroxide oxides.According to current understanding of the mechanism,hydroxide is the active phase of non-noble transition metal OER catalysts,and the metal cations on its surface are the active sites for OER catalysis.It is a common method to improve the intrinsic activity of non-noble metal OER catalysts by introducing heteroatoms into hydroxyl oxides to modulate the electronic properties of metal cation active sites,thereby regulating the adsorption strength of OER reaction intermediates（OOH*,O*,OH*）at active sites.However,due to the lack of in-depth understanding of the catalytic mechanism and rational design principles for catalyst composition,most researchers follow the traditional empirical trial-and-error method to explore the modification effects of transition metal cations,while the potential modification functions of anions are not given enough attention.In view of this research status,this paper aims to develop high-performance and low-cost anode electrocatalysts for water electrolysis,focusing on the modification effects and mechanisms of metal cation doping,anion（F）doping,and anion/cation co-doping on Ni-based and Co-based OER catalysts.The main research results of this paper are as follows:（1）A Fe-doped CoOOH catalyst was prepared using a hydrothermal combined with electrochemical anodization method.The research results show that Fe-doped CoOOH amorphous layer can be formed on the surface of Co MoO₄·n H₂O by anodic oxidation of Co MoO₄·n H₂O/CF precursor in an electrolyte containing Fe²⁺.The Co MoO₄·n H₂O@Co_1-xFe_xOOH/CF catalyst prepared under the optimized conditions has better alkaline OER activity than most reported Co-based OER catalysts.It requires an overpotential of 230 m V at a current density of 10 m A cm^-2 and shows good stability.According to the design experiments and DFT theoretical calculation,the high OER activity of Co MoO₄·n H₂O@Co_1-xFe_xOOH/CF catalyst should be mainly attributed to the partial substitution of Fe³⁺for Co³⁺in CoOOH,resulting in a decrease in the free energy of the OER rate limiting step（*O→*OOH）.This work confirms that electrochemical anodization is a simple new method for the synthesis of metal ion-doped hydroxyl oxides,which provides a technical basis for the development of high-performance OER catalysts.（2）A Ni-based oxide precursor with higher activity than Co-based oxides was selected and subjected to an anodic oxidation treatment in an electrolyte containing F ions to prepare the target catalyst NiO（OH）_1-xF_x/Ni（OH）₂/NF.Combined experimental and theoretical calculations showed that during the anodic oxidation process,a few-nanometer-thick amorphous NiO（OH）_1-xF_x layer was formed on the surface of Ni（OH）₂.The substitution of F^-for OH^-in NiOOH altered the rate-limiting step of the OER and lowered the free energy change,thus improving the intrinsic activity of NiOOH for alkaline OER.The optimized NiO（OH）_1-xF_x/Ni（OH）₂/NF catalyst exhibited overpotentials that were reduced by 32 m V and 80 m V at current densities of10 and 100 m A cm^-2,respectively,compared to the non-F-doped NiOOH/Ni（OH）₂/NF catalyst.Moreover,NiO（OH）_1-xF_x/Ni（OH）₂/NF showed good OER stability in slightly basic electrolytes containing small amounts of F salt,with no decay in activity after 100 hours of constant current testing.This work reveals that F ion doping can significantly enhance the alkaline OER activity of NiOOH and proposes a simple and effective approach to improve the stability of F-doped OER catalysts,providing new ideas and methods for developing high-performance OER electrocatalysts（3）By combining hydrothermal,calcination,and anodic oxidation methods,a（Fe,F）-co-doped Ni_1-xFe_xO（OH）_1-yFy/NiO/NF catalyst was designed and synthesized.Through comprehensive analysis techniques such as XAS and XPS,it was found that Fe³⁺and F^-entered NiOOH in the form of doping ions,partially replacing Ni³⁺and OH^-.The optimized Ni_1-xFe_x（OH）_1-yF_y/NiO/NF catalyst achieved an overpotential of 186 m V at a current density of 10m A cm^-2 in alkaline electrolyte,reaching an advanced level compared to existing OER catalysts.Moreover,the Ni_1-xFe_x（OH）_1-yF_y/NiO/NF catalyst exhibited excellent stability,with only±15m V potential fluctuation after 100 hours of constant current testing at a current density of 500m A cm^-2 in slightly basic electrolytes containing small amounts of F ions.Using a commercially available polycrystalline silicon solar cell panel with a rated voltage of 1.5 V,the Ni_1-xFe_xO（OH）_1-yF_y/NiO/NF║Ni₁₀Mo/MoO_3-x/NF electrolytic cell can continuously produce hydrogen.This work reveals the significant modification effect of the co-doping of anions and cations on hydroxide OER catalysts,laying a technical foundation for developing high-performance OER electrocatalysts.（4）Based on the investigates the modification effect of introducing a second transition metal cation simultaneously in the anodic oxidation step using Ni_1-xFe_xO（OH）_1-yF_y/NiO/NF as the base material.It was found that the（Fe,Co,F）co-doped catalyst has the optimum OER activity with an overpotential of only 172 m V at 10 m A cm^-2,which is one of the most active basic OER catalysts reported so far.The potential fluctuation was 12 m V in an alkaline electrolyte containing a small number of F ions at a constant current density of 500 m A cm^-2for 100 h.Ni_1-x-yFe_xCo_y（OH）_1-zF_z/NiO/NF is a low-cost oxygen precipitation catalyst with both high activity and good stability,which has potential application for alkaline water electrolysis applications.