Transition Metal Sulfides Derived From LDHs/PBAs Dual Precursors And The Electrocatalytic Performance As Oxygen Evolution Reaction Electrocatalysts

The global energy crisis and environmental problems necessitate reducing our dependence on traditional fossil fuels such as natural gas,oil and coal,and also reducing the emissions of carbon dioxide.Great efforts have been devoted to searching for abundant alternative energy sources on earth,and designing and synthesizing efficient energy-storage devices,such as fuel cells,metal-air batteries,and water splitting.Electrochemical water splitting is one of the most attractive technologies to store the renewable energy,which can convert the potential electric energy from renewable energy sources(such as solar energy and wind energy)to the contamination-free and sustainable hydrogen energy.The oxygen evolution reaction(OER),i.e.,water oxidation,plays a vital role in water splitting devices.At present,ruthenium dioxide(RuO2)/iridium dioxide(IrO2)noble metal electrocatalysts show the ideal electrocatalytic performance for OER in various electrolytes,however,the high price and scarcity of RuO2/IrO2 noble metals electrocatalysts have greatly hindered their commercial and large-scale applications.In this regard,the development of electrocatalysts for OER,which have the advantages of stability,high-efficiency,abundant reserves,low cost,and more importantly,can operate at a low overpotential(?),has been attractive in the industrial production of hydrogen(H2)with achieving efficient and inexpensive.In this thesis,two different types of two-dimensional layered double hydroxides(LDHs)/three-dimensional Prussian blue analogues(PBAs)composites were synthesized as precursors by a hydrothermal process.Transition metal sulfide composites(microflower-like NC/CoS2@Fe-CoS2 composites and hydrangea-like NC/FeS2@Fe-NiS2 composites)were successfully obtained through a sulfurization process.The morphology,structure and composition of the synthesized transition-metal sulfide composites were well examined,and were further used as OER electrocatalysts for electrochemical performance test.The main contents of this thesis are presented as follows:(1)Design and synthesis of micronflower-like NC/CoS2@Fe-CoS2 composites and the electrocatalytic performance for OERThe metanilate-Co(OH)2 hydrotalcite microflowers were synthesized as the precursors by a simple hydrothermal synthesis method,on which cobalt-iron Prussian blue analogue(CoFe PBA)nanocubes were decorated via a hydrothermal synthesis process.The obtained microflower-like metanilate-Co(OH)2@CoFe PBA composite precursors were then subjected to the sulfurization process to give rise to microflower-like NC/CoS2@Fe-CoS2 composites.Electrocatalytic testing shows that the microflower-like NC/CoS2@Fe-CoS2 composites have good electrocatalytic performance of both electrocatalytic activity and stability for OER,with an overpotential of 300 mV at the current density of 10 mA cm-2,and an overpotential of 330 mV at the current density of 20 mA cm-2,which are significantly better than those of the counterparts of NC/CoS2,NC/Fe-CoS2,CoS2.In addition,during the oxygen evolution reaction,NC/CoS2@Fe-CoS2 composites were observed to be partially oxidized to CoFeOOH/CoFeOH,and finally formed the NC/CoS2@Fe-CoS2/CoFeOOH/CoFeOH heterojunction,in which the CoFeOOH/CoFeOH layer can be used as the electrocatalytic active sites and the remaining NC/CoS2@Fe-CoS2 can be used as the conductive channel to facilitate the rapid transfer of electrons,so as to improve the electrocatalytic performance and long-term stability for OER.Considering the great flexibility in tuning the cations of LDHs layers and also in varying the types of Prussian blue analogues,the LDHs/PBAs precursor-based synthesis route can be easily be extended to the preparation of two-dimensional nanomaterials combined with three-dimensional nanomaterials with other elements for environmental and energy related materials such as metal sulfides,selenides,phosphides and nitrides.(2)Design and synthesis of hydrangea-like NC/FeS2@Fe-NiS2 composites and the electrocatalytic performance for OERPrussian blue nanocubes(PB)with uniform size were prepared by a hydrothermal synthesis method;And followed by a decoration of NiFe-LDH nanosheets vertically grown on the surface of the PB nanocubes during a hydrothermal synthesis process;The resulting hydrangea-like PB@NiFe-LDH composite precursors were then subjected to a sulfurization process,thus yielding the hydrangea-like NC/FeS2@Fe-NiS2 composites.The SEM observation shows that the obtained NC/FeS2@Fe-NiS2 still retained the original hydrangea-like morphology.The hydrangea-like NC/FeS2@Fe-NiS2 electrocatalyst exhibited excellent catalytic activity and long-term stability for the OER,with an overpotential of only 260 mV at the current density of 20 mA cm-2 and a Tafel slope of 49 mv dec-1,which are obviously superior to those of counterparts of Fe-NiS2,NC/FeS2 and the commercial IrO2.Moreover,the NC/FeS2@Fe-NiS2 composites were partially oxidized to NiFeOOH/NiFeOH during the oxygen evolution reaction,and giving rise to the NC/FeS2@Fe-NiS2@NiFeOOH/NiFeOH heterojunction,in which the NiFeOOH/NiFeOH layer severs as the catalytic active centers for the water oxidation process,the remaining NC/FeS2@Fe-NiS2 behaviors as a conductive channel to boost the rapid transfer of electrons,thus significantly improving the electrocatalytic activity and long-term stability.On the basis the unique advantages of modulating morphology and cationic types of LDHs and PBAs,this simple and cost-effective synthesis method can be easily extended to the preparation of other PBAs and LDHs combined,efficient and stable electrochemical energy conversion of hierarchical electrode micro-/nanomaterials.