Surface Modification And Design Of Highly Efficient Cobalt-Based Compounds For Water Splitting

Hydrogen is considered as a promising renewable energy source due to its abundance,releases high heat,clean and renewable.Water splitting is used as strategy engineering for hydrogen production.Development of efficient electrocatalysts to reduce the overpotential of water splitting is the key to improving energy efficiency and hydrogen production rate.In this dissertation,focusing on the the reaction mechanism of water splitting,sticking to the essential structure-activity relationship,and emphasize the design and optimizing of electrocatalytic performance at atomic scale.The related surface modification method such as vacancy,doping and interface was used to improve the performance of the electrocatalyst.Details are listed as below:First,the hydrogen adsorption plays an important role in optimizing acidic HER activity.This work focus on the establishment of the relationship between the electronic structure and the hydrogen adsorption energy?G_H?by comparing the changes of electronic structure of MoS₂ before and after hydrogen adsorption.The results show that G_H of MoS₂is closely related to the"electron filling energy".On this basis,a series of TMD were investigated to verify the linear relationship.Finally,according to the structure-activity relationship and the electronic structure of MoS₂,the G_H of MoS₂ is adjusted by non-metal doping.This work provides an appropriate direction for optimizing HER activity.Secondly,the slow kinetics of Volmer step limits the alkaline HER activity of cobalt sulfide?CoSx?.In this work,interface structure Ni?OH?₂/CoS_x was designed to balance the adsorption energy of*H₂O and*H.Experimentally,a three-dimensional Ni?OH?₂/CoSx array was prepared by a simple hydrothermal method,and it exhibited high hydrogen evolution catalytic activity under alkaline conditions with a low 72 mV@10 mV cm^-1 and an small Tafel slope of 79 mV dec^-1.Meanwhile,the number of active sites of Ni?OH?₂/CoS_x decreases and the improvement of performance attribute to increase of intrinsic activity,which suggesting that Ni?OH?₂ and CoS_x synergistic effects accelerate alkaline HER.This work provides a direction for optimizing the basic hydrogen evolution reaction by balancing*H₂O and*H adsorption energy.Thirdly,the scaling relationship between the reaction intermediate?*OH,*O and*OOH?in the OER process constrain the OER activity.In this work,combined with the first-principles calculation and related experimental,the effects of oxygen vacancies on the OER of CoOOH were discussed.DFT reveals that the introduction of oxygen vacancies changes the valence state of cobalt and increases the adsorption energy of H₂O.Moreover,the overpotential of OER can be greatly reduced by the synergistic effect of two actions sites.Experimentally,the synthetic CoOOH with oxygen vacancies exhibited lower overpotential0.269 V and Tafel slope 46 mV dec^-1.Through this work,synergistic promoting the OER by two action sites is proposed.Finally,this work propose a strategy of lower valence-state doping to stabilize the oxygen vacancies?Vo?.DFT indicates that the formation energy of Vo on CoOOH is decreased apparently after lower valence-state Zn doping.Even more importantly,the Jahn-Teller distorted can reduces the thermodynamic overpotential.The synthetic Zn-CoOOH possesses a faster OER kinetic with a low Tafel slope of 44 mV dec^-1,and exhibits excellent electrochemical stability.Thus,the lower valence-state doping is promising strategy for fast-stabilized electrocatalytic water oxidation.We use Ni?OH?₂/Co₉S₈ and Zn-CoOOH to achieve overall water splitting.The performance of Ni?OH?₂/Co₉S₈|Zn-CoOOH electrolytic cell with an operating voltage 1.59 V for current density 10 mV cm^-2,which is close to that of electrolytic noble metal cell Pt/C||RuO₂.