Composite And Doping Modification Towards Fe₇S₈ To Improve The Performance For Electrocatalytic Water Splitting

Overall water splitting to produce hydrogen or oxygen with high-performance electrocatalysts is an important research field for the development of sustainable clean energy systems.Among them,lowering the corresponding overpotential for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）is the key technical problem,which is related to the practical application process.Till now,precious metals Pt and RuO₂/IrO₂ are the most excellent catalytic materials for HER and OER,but their high cost and low storage limit their wide application.Therefore,the development of one abundant-reserve,high-activity and durable non-noble metal material for electrolytic water splitting has become a research hotspot in recent years.Fe-based sulfides are not only earth-abundant and cheap but have high electrocatalytic activity and stability in the process of electrocatalysis.Typically,pyrrhotite（Fe₇S₈）and pentlandite(Fe_4.5Ni_4.5S₈)are especially abundant in natural nickel ore and can be used as efficient catalysts for HER and OER.The resource production area of nickel ore is located in Panshi City,Jilin Province.Combining the major strategic demand of energy development of our courtry and the local economic and industrial advantages of Jilin Province,it is of great significance to develop an efficient catalyst material for electrolytic water splitting in natural mineral products,and to develop hydrogen energy related industries in Jilin Province.To achieve the above purpose,natural nickel ore was firstly obtained from Hongqiling,Jilin Province.Then Fe_4.5Ni_4.5S₈/Fe₇S₈ was extracted from nickel ore after crushing,ball milling,flotation and thermal-alkali treatment.Electrochemical tests show the HER performance of Fe_4.5Ni_4.5S₈/Fe₇S₈ is very excellent,but there are still some problems such as narrow applicable range of p H,poor OER performance,more impurities and small specific surface area.Therefore,single-phase pyrrhotite（Fe₇S₈）nanosheets were synthesized and used as a bifunctional electrocatalyst in alkaline solution.It showed satisfactory performance for HER and OER.On this basis,we then regulated the electronic structure of Fe₇S₈ through element doping to improve its adsorption capacity for water molecules in alkaline media,thus enhancing the ability of Fe₇S₈ for electrochemical water splitting.Based on the above ideas,bulk phase Fe_4.5Ni_4.5S₈/Fe₇S₈ was extracted from nickel ore and Co-doped pyrrhotite(Fe_7-xCo_xS₈)ultrathin nanosheets were prepared in this paper.Then their electrocatalytic performance was analyzed and evaluated.The specific research contents are as follows:（1）Fe_4.5Ni_4.5S₈/Fe₇S₈ composites were obtained by ball milling,flotation and thermal-alkali treatment.Through a series of characterization,it is proved that Fe_4.5Ni_4.5S₈/Fe₇S₈ composite is submicron particles with a diameter of 400～700 nm.At the same time,three kinds of Fe_4.5Ni_4.5S₈/Fe₇S₈ materials（denoted as PnPo-1,PnPo-2 and PnPo-3,and their molar ratios were 1:1,1.1:1 and 1.3:1,respectively）were obtained by changing the dosage of DETA.It was found that when the molar ratio of Fe_4.5Ni_4.5S₈ and Fe₇S₈ was 1.1:1,the electrochemical performance of PnPo-2composite was best.By comparing the XPS spectra of single-phase Fe₅Ni₄S₈,Fe₇S₈ and PnPo-2composite,it was found that the 2p orbital of Fe and Ni in PnPo-2 has a shift of electron energy compared with those in single phase,which indicated that there exists electron transfer at the interface of the two phases in PnPo-2.By comparing the S 2p XPS and EPR spectra before and after activation,it was found that a large number of sulfur vacancies were generated during the electrolysis process of PnPo-2.And the experimental results showed that the sulfur vacancies were the real catalytic active sites.Then,the HER performances of Fe₅Ni₄S₈,Fe₇S₈ and PnPo-2 in 0.5M H₂SO₄electrolyte were tested.It was found that the overpotential,Tafel slope and charge transfer impedance of PnPo-2 for HER were much lower than those of Fe₅Ni₄S₈ and Fe₇S₈,which demonstated that the electron inteaction at the interface of PnPo-2 could immensely enhance the conductivity of the material and facilitate the electron transfer rate at the interface.As a result,PnPo-2 showed similar catalytic properties to Pt-based materials towards HER,demanding an overpotential of 48 mV to reach a current density of 10 mA cm^-2.In addition,the electrocatalyst possessed an ultralow Tafel slope(43 mV dec^-1)and was capable of catalysing continuously for 100 h,providing excellent stability.（2）Co-doped pyrrhotite(Fe_7-xCo_xS₈)ultrathin nanosheets were designed.It was proved that the doping of cobalt atoms can regulate the chemical environment around iron atoms in pyrrhotite,thus enhancing the adsorption energy of water molecules on the surface of Fe_7-xCo_xS₈,and significantly improving its catalytic performance.The optimal doping ratio of cobalt atom was determined to be 13.6%,and the sample was denoted as Fe6.05Co0.95S8.SEM and TEM characterizations showed that a series of Fe_7-xCo_xS₈ were a well-dispersed and uniform ultrathin nanosheets.XPS analysis showed that the addition of cobalt atoms changed the surrounding chemical environment of iron atoms in Fe_7-xCo_xS₈ samples,while the chemical environment of cobalt and sulfur atoms remained basically unchanged.In addition,the contact Angle measurement results show that Fe_6.05Co_0.95S₈ has a better wettability to water molecules than Fe₇S₈.These results indicate that the doping of cobalt atoms can regulate the chemical environment around iron atoms,thus enhancing the adsorption energy of pyrrhotite to water molecules in alkaline media,thus significantly improving the electrocatalytic efficiency of overall water splitting.Therefore,employed as OER catalyst in basic electrolyte,the Fe_6.05Co_0.95S₈ performed a low overpotential of 311 mV at 10 mA cm^-2with a small Tafel slope of 50 mV dec^-1.For HER,Fe_6.05Co_0.95S₈ showed a low overpotential of 284 mV at 10 mA cm^-2 with a small Tafel slope of 88 mV dec^-1coupled with high stability.The assembly of the catalyst into a symmetrical device Fe_6.05Co_0.95S₈||Fe_6.05Co_0.95S₈ was used for water electrolysis,and it demanded a cell voltage of 1.80 V to obtain a current density of 10 mA cm^-2.