Study On Low-cost Metal Sulfide As Efficient Electrocatalyst For Overall Water Splitting

Since the 21st century,the excessive development and combustion of traditional fossil fuels have caused serious environmental problems and energy crises.It is urgent to explore newly sustainable clean energy.Hydrogen,as a clean energy which has no secondary pollution and can be recycled,has become the best candidate the replace fossil fuel.It is considered to be one of the most efficient ways to produce hydrogen by electrolyzing water.The water splitting process includes two partial reactions,that is the oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）.The theoretical voltage required for hydrogen production from electrolyzed water is 1.23 V.However,due to the kinetic effect,the water splitting reactions require an additional potential,and is denoted as overpotential.The noble metal catalysts（Ru,Ir,Rh,etc.）exhibit excellent catalytic activity in overall water splitting,however,their applications are limited by the scarce resources and high price.Consequently,the transition metal sulfides,as a newly non-noble metal materials,has attracted increasing attentions in the field of electrocatalysis.By employing nickel foam as the substrate,the number of active sites and the electron transfer rate could be effectively promoted,while the synthesis cost could also be reduced.This thesis firstly reviewed the recent studies of transition metals sulfide electrocatalysts,specifically,focusing on the optimization strategy of the electrocatalysis performance and the active in reported literatures.Based on the progress and defect of the existing researches,the present thesis rationally selects low-cost transition metal sulfides with excellent structural and electrochemical properties as the research object.By employing strategies of surface decorations and metal ions dopants,the electrocatalytic activity of cobalt based sulfides was enhanced,which is closer to those of noble metal-based catalysts.The main work and results are as follows:（1）For the first time,we synthesized Co₃O₄@Co₃S₄/NF and used it as a bifunctional electrocatalyst for water splitting reactions.The growth mechanism of Co₃O₄@Co₃S₄/NF was studied by controlling the reaction time.The experimental results show that the morphology of Co₃S₄ does not change with time,but the number of Co₃O₄ surface is increased.The specific activity of water oxidation(270 mV@100 mA cm^-22 and 143 mV@20 mA cm^-2)was also investigated,and these values are also the minimum reported for electro-driven water splitting to date.When used as a monolithic total hydrolysis catalyst,its activity(1.53 V@10mA cm^-2)is comparable to that of precious metal catalysts.It is proved to be a true catalytically active substance by stability experiments.（2）A Cu-doped Co₉S₈ electrocatalytic material was designed by a simple three-step hydrothermal method.First,Cu-Co₉S₈-6 h was used as a catalyst for OER and HER.In OER,only 260 mV was achieved to drive the current density of 50 mA cm^-2,which requires only 62mV in the HER produces a current density of 10 mA cm^-2.When Cu-Co₉S₈-6 h was used as a self-made two-electrode system,a current density of 10 mA cm^-22 was achieved with only 1.49V,and Cu-Co₉S₈-6h exhibited long-term stability in 1 M KOH.Density functional theory（DFT）calculations show that the electrocatalytic activity of Cu-doped Co₉S₈-6 h is mainly attributed to the increase of water adsorption energy during the doping process of metallic Cu.By comparing Co²⁺,Cu²⁺and Co³⁺,it was proved that active Cu²⁺replaced inert Co²⁺,and the abundant high-valence Co³⁺sites exhibited excellent catalytic activity for Cu-doped Co₉S₈-6h.（3）M（Fe,Ni,Zn,Mo）-Co₉S₈ was synthesized by simple hydrothermal method using metal organic frameworks（MOFs）.Firstly,the adsorption energy of doped metal cations on water was compared by density functional theory（DFT）.The DFT calculations show that the superior electrocatalytic activity of Ni-Co₉S₈ is attributed to the increase of conductivity and the increase of water adsorption energy caused by Ni doping.By comparing the water adsorption energy of Co²⁺,Ni²⁺and Co³⁺,Ni³⁺,it is proved that active Ni²⁺substitution Inert Co²⁺enriches the double high valence state Co³⁺and Ni³⁺sites;secondly,the presence of Ni³⁺enhances the chemical adsorption of OH-,promotes the transfer of electrons to the catalyst surface;finally,under the support of MOFs organometallic framework,The resulting surface rough hollow nanostructures further improve the reaction surface area and promote mass transfer.Moreover,we have measured the electrochemical performance of each catalytic material in detail.Ni-Co₉S₈ exhibits excellent electrochemical performance in OER(300mV@100 mA cm^-2)and HER(90mV@10mA cm^-2).Ni-Co₉S₈ was used as a bifunctional water-splitting catalyst and exhibited excellent electrocatalytic activity(1.45 V@10 mA cm^-2)and stability,which was consistent with the results of DFT calculation.