Research On Electrocatalytic Performance Toward Water Splitting Of Several Doped Cobalt-based Compounds

As we all know,with the rapid growth of national economy and population,environmental pollution and natural resources shortage are becoming increasingly serious.Therefore,there is an urgent demand for human to explore clean energy to replace traditional fossil fuel.Among the alternatives to fossil fuels,hydrogen energy is generally regarded as a promising renewable energy due to its high energy density and clean combustion products,which has attracted the attention of numerous researchers in the past decades.Compared with other methods,hydrogen production by electrochemical water splitting has higher conversion efficiency and purity,which is one of the best ways to produce hydrogen.Electrochemical water splitting contains two half reactions:hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.In theory,the decomposition voltage of water electrolysis is 1.23 V.However,due to the slow HER and OER processes,the actual decomposition potential of electrocatalytic water splitting is usually higher than the theoretical potential.Hence,it is necessary to develop efficient and durable catalysts to reduce the overpotential of cathode or anode,and diminish energy consumption.Up to now,some noble metal materials（such as Pt,Pd,Ir,Ru and their compounds）are widely considered as efficient electrocatalysts for HER and OER due to their excellent electrochemical performance and suitable adsorption energy for intermediates.Unfortunately,the expensive price and scarcity of precious metals have seriously hindered their large-scale application.Thus,it is urgent to develop efficient and cheap electrocatalysts to replace precious metal materials.Co-based electrocatalysts with the characteristics of low cost and adequate reserves,which is widely considered as a potential substitute for noble metal materials.However,the low intrinsic activity and insufficient number of active sites lead to a large gap between their activity and that of noble metal catalysts.Based on these,this paper aims to solve the inherent shortcomings of cobalt-based materials through the heteroatom doping strategy,which is helpful to regulate the electronic structure and increase the number of active sites of cobalt-based materials.At the same time,the influence of heteroatom doping on the adsorption ability of reaction intermediates at the active sites was deeply studied by theoretical calculation.The main research contents are as follows:（1）In this paper,a metallic and non-metallic ion co-doped Co S₂ based catalyst forefficient electrocatalytic oxygen evolution reaction was reasonably designed and constructed.Co S₂ nanoparticles co-doped with N and Fe catalyst（N,Fe-Co S₂）was prepared on the hydrophilic carbon paper by two-step electrodeposition and low-temperature calcination.The experimental results showed that the introduction of Fe increased the electrochemical active surface area（ECSA）and the amount of Co³⁺for N,Fe-Co S₂,thus promoting the exposure of more active sites and enhancing its OER activity.The incorporation of N atoms prevented the oxidation of S element on the catalyst surface,which strengthened the stability of the catalyst and reduced the charge transfer resistance of the material.Under the synergistic effect of N and Fe,the catalyst demonstrated an excellent OER activity and a long-term stability in the alkaline electrolyte.N,Fe-Co S₂electrocatalyst only required 196 m V OER overpotential to drive10 m A·cm^-2,and can stably electrolyze for 160 hours at this current density.Meanwhile,only a 250 m V OER overpotential was need to achieve 100 m A·cm^-2.DFT theoretical calculations unveiled that the co-doping of Fe and N optimized the adsorption energy of oxygen-containing intermediates for Co sites,significantly reduced the reaction energy barrier of the rate determination step,and improved the intrinsic activity of the active sites.（2）In view of the previous work,the catalyst only possesses alkaline OER performanceand the synthesis steps are relatively complicated,which is difficult to meet the industrial demand for a simple synthesis method and dual-functional electrolytic water catalyst.Therefore,in this chapter,vanadium doped cobalt carbonate hydroxide nano-sheet catalyst（V-Co CH）for efficient water splitting was successfully synthesized on carbon paper by a simple one-step hydrothermal method.In the early stage of OER process,the V species in the catalyst were partially dissolved in the alkaline electrolyte,inducing the catalyst surface self-etching,which was helpful to the catalyst surface reconstruction,resulting in more plentiful active sites and oxygen vacancies.Synergy of V doping and oxygen vacancies was quite favor of improvement of the activity.V-Co CH with rich oxygen vacancies displayed an excellent OER catalytic performance with an ultra-low overpotential（183 m V）and a robust long-term stability（160 hours）at 10 m A·cm^-2.The overall water splitting cell composed of V-Co CH only acquired 1.52 V to drive 10m A·cm^-2 and exhibited a prominent stability for 250 hours at this current density without obvious degeneration.Theoretical calculations revealed that the V atoms of V-Co CH regulated the electron density around active sites,rather than itself served as an active site.The induced oxygen vacancies had an important effect on improvement of the OER performance via not only the exposure of more active sites but also modulation of electronic structures.This work provides an effective strategy for the design and construction of efficient and durable electrocatalysts.