O₂-Fe²⁺ Slow Etching Nickel Foam Supplemented By Phosphating （Sulfurization） To Prepare Electrocatalyst For Water Splitting

The massive consumption of fossil fuels has triggered a global environmental crisis,and the non-renewable nature of traditional fossil fuels has seriously affected future sustainable development.As a clean energy,hydrogen has the characteristics of high energy density and non-polluting combustion.Electrocatalytic hydrolysis of water has high efficiency and high purity and is one of the most promising hydrogen production methods at present.To date,the benchmark catalysts for the two half-reactions oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）for water electrolysis are Pt/C catalysts and RuO₂/IrO₂,respectively.The high cost and low reserves of these noble metal-based catalysts greatly restrict their industrialization.Therefore,it is of great significance to develop an inexpensive,stable,simple-to-prepare,and excellent-performance catalyst for water electrolysis.At present,the preparation process of bifunctional catalysts for total water splitting is complicated.This paper conducts research on this issue,and the main contents are as follows:（1）The nickel foam（NF）is directly oxidized and then phosphated,the degree of phosphating is low,and the obtained catalyst has low catalytic activity.Etching NF with hydrochloric acid can obtain bifunctional nickel-based catalysts.Although a single nickel-based catalyst has certain catalytic activity,it is far from the effect of noble metal-based catalysts.Fe³⁺can rapidly etch NF and can achieve Fe doping on nickel-based catalysts,but its strong oxidizing property makes it impossible to build nanostructures on the surface of NF and obtain a high active surface area,and the activity of the catalyst also cannot reach the standard of noble metal-based catalysts.Therefore,a new strategy was proposed in this chapter,namely,oxygen-assisted Fe²⁺etching of nickel foam（NF）followed by low-temperature phosphating to obtain in situ chrysanthemum-like nanostructured Fe-Ni₂P@NF on NF.Herein,Ni foam was used as the electrode substrate and also served as a nickel precursor,and Fe²⁺was an etching agent and simultaneously a dopant.The electrode fabrication did not require other special agent and additional energy consumption.Ni₂P in-situ grew from the NF electrode,ensuring robust adhesion and minimal electron-transfer resistance between the Ni₂P catalyst and the NF substrate.Benefiting from the nanostructure and electronic modulation of Fe doping in Ni₂P,the as-prepared Fe-Ni₂P@NF electrode demonstrated excellent bifunctional catalytic activity in alkaline media,with low overpotential of 70 m V for HER at 10 m A cm^–2and 254 m V forOER at 50 m A cm^–2in1.0 M KOH.When the electrocatalytic electrode was applied to an electrolyzer,only1.76 V cell potential was achieved at 50 m A cm^–2,superior to the Pt/C@NF||RuO₂@NF benchmark（1.85 V）.This study may provide a simple and large-scale fabrication method to construct promising bifunctional water-splitting electrodes for practical use.And besides nickel phosphide,the developed etching method can be extended to fabricate Fe-doped nickel sulfide and nickel nitride,and more electrocatalytic applications can be found.（2）To verify the universality of the strategy proposed in（1）,the phosphorus source was replaced with a sulfur source,and the in-situ grown nanostructured Fe-Ni₃S₂@NF was obtained after low-temperature vacuum sulfidation.Compared with Ni₃S₂obtained by hydrochloric acid etching and sulfurization,the electronic structure of Fe-doped Ni₃S₂is regulated,the morphology is changed,and theOER activity is greatly improved.The Fe-Ni₃S₂@NF electrode exhibits good bifunctional catalytic activity in an alkaline media.In 1.0 M KOH,the overpotential of HER was113 m V at 10 m A cm^-2and the overpotential ofOER was 301 m V at 50 m A cm^-2.When the Fe-Ni₃S₂@NF electrode is applied to the electrolyzer,only a cell potential of 1.81 V is required at 50 m A cm^-2,which is better than that of the electrolyzer composed of Pt/C@NF||RuO₂@NF（1.85 V）,better than the Ni₃S₂@NF||Ni₃S₂@NF electrolyzer（1.92 V）.The successful preparation of Fe-Ni₃S₂@NF demonstrates the generality of the strategy proposed in the previous section.