The Fabrication Of Copper Sulfides And Their Applications In Electrochemical Energy Storage And Conversion

To utilize renewable resources via electrochemical energy storage and conversion technology is a promising solution to the energy crisis and environment issues.The physical,chemical,electrochemical reactions occurred on the electrode-electrolyte interface are important components of an electrochemical process.Thus the design of high-performance and durable electrode materials is vital to electrochemical energy storage and electrocatalysis.This thesis focuses on the investigation of copper sulfides based electrodes.At the very beginning,we proposed a simple and quick grinding method for the synthesis of high-purity CuS nanosheet,which was further studied on its application in aqueous aluminum ion batteries,polysulfide-iodide flow batteries and CO2 reduction reactions.As for aqueous aluminum ion batteries,the Al-storage mechanism of CuS nanosheets was investigated.And by analyzing the troubles we encountered when applying CuS nanosheets in polysulfide-iodide flow batteries and CO2RR,we re-designed the copper sulfides based electrocatalysts for better performance,and corresponding catalytical mechanisms were discussed.1.High-purity CuS nanosheets have been successfully synthesized at room temperature via a quick,simple,and efficient grinding approach by using CuCl2·2H2O,thiourea,and NaOH as starting materials.The X-ray diffraction(XRD)patterns and Raman spectra indicated that as-prepared CuS was of high purity and scanning electron microscope(SEM)images showed the sheet-like nanostructure.The specific area of as-synthesized CuS nanosheets was obtained with a N2 adsorption/desorption isotherm,and the value is 23.18 m2·g-1,which is higher than that of CuS fabricated by the traditional method.The bandgap of the as-synthesize CuS was calculated to be around 1.8 eV corresponding to the UV-vis diffuse reflection spectra,showing a potential prospect in photocatalysis and solar cells.A 10-g-level production also has been achieved in our laboratory,and the yield was over 95%based on the Cu source.The grinding approach can be a promising method for the industrial-scale production of high-purity CuS nanosheets.2.By adding high-concentrated LiCl to AlCl3 solution,we obtained a novel electrolyte for aqueous aluminum ion batteries,in which the electrochemical Al-storage performance of CuS nanosheets was tested.Meanwhile,it's found that Al metal was corroded severely in such electrolyte,thus copper foil was used as anode.To the electrolyte was then added CuCl to trigger on the deposition/strapping process of copper.With CuS nanosheets as cathode,the soft-pack Cu-Al dual-ion batteries were fabricated and tested.The results show that CuS nanosheets delivered high Al-storage capacity and high-concentrated Cl-helps facilitate the reaction between Al3+ and CuS cathode.After 200-cycle charging-discharging test,88.6%of the pristine capacity of the dual-ion battery was maintained,showing great stability.3.By studying the performance of CuS nanosheets in polysulfide anolyte,it's found that CuS irreversibly changed to aggregated Cu7S4 particles.The aggregated structure hindered the activity of Cu7S4.Therefore,we designed Cu7S4/CNT composites as electrocatalyst for aqueous polysulfide anolyte.It's also found that Cu7S4/CNT composites could also effectively absorb iodide ions and facilitate corresponding redox reactions.Thus polysulfide-iodide flow battery based on Cu7S4/CNT composites electrodes was fabricated and tested.Polysulfide-iodide flow batteries with Cu7S4/CNT loaded electrode can deliver a peak power density of 84.6 mW·cm-2,which is more than 13 times higher than that of bare CNT electrode.Moreover,the flow cell has run for over 500 h under 30 mA·cm-2 without obvious efficiency decay,showing great potential for practical application.4.By analyzing the performance of CuS nanosheets in CO2RR,it's found that CuS was irreversibly reduced to Cu0 and core-shell-like particles formed.HR-TEM,in-situ Raman spectra and ex-situ XPS showed that the edge of as-formed particles was elemental Cu and the inner part was residual CuS.Analysis on lattice fringe confirmed a Cu(l11)/CuS(102)structure.The following LSV tests exhibits higher current density,which inspired us that the elemental Cu formed during the reduction of CuS may serve as active sites for catalyzing CO2RR.Further electrochemical measurements and corresponding product analysis indicated that the core-shell-like catalyst delivered a current density of 50 mA/cm2 at-0.7 V(vs RHE)and a formate Faradaic efficiency of 65%,and remained stable in a 20-cycle chronoamperometry test.showing much better catalytical performance and higher selectivity than elemental Cu catalyst.It was found that residual S beneath the Cu0 layer lowered the binding energies of intermediates HCOO*and*COOH,promoting their desorption and the successive formation of HCOOH or HCOO-,thus regulating the high selectivity of the product.This work provides a convenient and economical method for developing a highly active electrocatalyst for producing HCOOH in CO2 electroreduction,and it is beneficial to understand the mechanism of enhanced selectivity to HCOO-for CuS electrode materials.