The Study Of Exogenous Nanomaterials Modified NiCoFe Based Oxides For Water Splitting Catalysts

In recent years,research on renewable and clean energy has become an important issue because of environmental pollution and fossil energy consumption.In this process,hydrogen is considered a potential alternative to fossil fuels because of its zero carbon emissions and high energy density.At present,hydrogen is still produced mainly by steam reforming.But it produces a lot of carbon dioxide in the process and requires high temperatures.In such a situation,attention needs to be paid to other potential hydrogen production methods.Therefore,electrochemical water splitting for hydrogen production has been extensively studied.Electrochemical water splitting is mainly composed of two half-reactions:the oxygen evolution reaction（OER）at the anode and the hydrogen evolution reaction（HER）at the cathode.In the actual reaction process,both reactions are extremely slow because of the energy barrier.Especially for the oxygen evolution reaction,because four electrons are involved in the transfer process,the overpotential required by OER is much higher than that of HER.Therefore,it is a feasible idea to use catalysts in the reaction to reduce energy consumption.It is also feasible to use urea oxidation reaction instead of oxygen evolution reaction.But urea oxidation involves the transfer of six electrons and requires the introduction of a catalyst to help the reaction proceed.At present,Pt-based catalysts and Ir O₂ materials are still considered as the most effective catalysts for OER and HER.However,the scarcity and instability of noble metal-based catalysts limit their large-scale application.Therefore,it is challenging and urgent to develop efficient,stable and low-cost bifunctional electrocatalysts for water splitting.Based on the above considerations,different kinds of catalysts for electrolytic water were prepared and studied in this thesis,using Ni,Co and Fe transition metal elements,which are common and abundant in the earth,as followers.（1）we prepared hydrothermal method and calcination to design a new Ce O₂ decorated Co Fe₂O₄ electrocatalyst.Concretely,the Ce O₂@Co Fe₂O₄/NF material showed a small overpotential of 213 m V at the current density of 100 m A cm^-2 in a 1.0 M solution of KOH.Meanwhile,the Tafel slope of Ce O₂@Co Fe₂O₄/NF is 64.33 m V dec^-1,which is lower than that of Co Fe₂O₄/NF(84.33 m V dec^-1).Under the influence of excellent OER activity,the UOR activity was also evaluated.In a 1.0 M solution of KOH（with 0.5 M urea）,the Ce O₂@Co Fe₂O₄/NF material had a potential of 1.4 V at the current density of 100 m A cm^-2.Experimental results showed that the electrochemical activity of the Ce O₂@Co Fe₂O₄/NF material was ascribed to the optimized electronic structure and enhanced efficiency of electron transfer.We also utilized DFT to investigate the high catalytic activity of the material by modeling it.The results showed that Co Fe₂O₄ acts as a water adsorber,while the presence of Ce O₂ promotes the electron transfer efficiency.（2）According to the literature reports,doping,heterogeneous interface construction and other strategies could improve the electrocatalytic performance of materials.Here,we had done some work in finding highly efficient and stable catalytic materials by building heterogeneous interfaces.A series of P-Co₃O₄@Ni Co-LDH/NF hybrid materials were in situ obtained on nickel foam through a three-step synthesis method involving hydrothermal,high temperature calcination and electro-chemical deposition approaches.The obtained P-Co₃O₄@Ni Co-LDH-2/NF material with a prickly pear shape exhibits excellent HER electrocatalytic activity.When the current density is 100 m A cm^-2,the overpotential is only 181 m V,which is one of the best catalytic activities reported so far.This work provided some reference value to construct heterogeneous interfaces for improving the catalytic performance.（3）Based on the above considerations,we firstly designed a three-step method including hydrothermal,calcination and hydrothermal to synthesize Ni Co₂O₄@Ni-MOF on nickel foam as high efficiency OER catalyst.Among them,Ni Co₂O₄ was rich in active sites and polymetallic oxidation states,and had good electrocatalytic activity by itself,but the potential for further development exists.Ni-MOF has a porous structure,highly dispersed metal composition and high specific surface area.Therefore,it was introduced into the material to enhance the electrolytic water performance of Ni Co₂O₄.The Ni Co₂O₄@Ni-MOF electrode present excellent OER activity,it displayed a low overpotential of 340 m V at the current intensity of 100 m A cm^-2 in potassium hydroxide solution environment.Meanwhile the catalyst owns an outstanding UOR activity too,whose potential is 1.31 V at the same current intensity in 1.0 M KOH with 0.5 M urea.This work provided research strategies for the preparation of highly efficient OER and UOR catalysts to achieve water splitting under alkaline media.