Preparation And Catalytic Properties Of Electrocatalysts For Water Splitting Based On Transition Metallic Carbides

With the increasing energy demand and the serious pollution caused by the burning of fossil fuels,the new pollution-free and clean energy reserves have broad prospects.Hydrogen is considered as the cleanest and most harmless zero-emission fuel,and hydrogen can be obtained through the electrolysis of water,which has the potential to produce pure hydrogen in an environmentally friendly manner.The hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）are two important semi-reactions involved in water splitting.The half-reaction of water splitting to generate oxygen is a four-electron process with a theoretical potential of 1.23 V.For electrolyzing water,the activity of the catalyst plays a vital role in water spilitting.Currently,many transition metal electrocatalysts,such as sulfides,selenides,nitrides,and borides,have been investigated as possible replacements for noble metal electrocatalysts.In addition to these materials,transition metal carbides exhibit surprising properties.The similarity of d-band electronic structure between the carbides endows metal carbides with superior hydrogen evolution properties.It is worth noting that combining transition metal carbides with catalyst supports with excellent electrical conductivity can greatly improve their electrochemical activity and stability due to the low electron transfer resistance.Besides,due to the electron-rich nature of transition metal,the catalytic performance of transition metal electrocatalysts can also be improved by introduction of metallic dopants or alloying metallic elements,which can effectively reduce the number of d orbitals not occupied,thus further synergistically enhancing activity.In this work,we use the simple and easy method to prepare three-dimensional bimetallic Co₆W₆C and aporous nitrogen-doped composite of carbon-wrapped Co/Mo₂C nanorods composites,respectively.The material was cast on carbon cloth to fabricate the catalytic electode for water splitting.The thesis mainly includes the following two works:1.The three-dimensional bimetallic catalyst with a ginger-like morphology consisting of Co₆W₆C was prepared via a simple hydrothermal synthesis and pyrolysis method.Co₆W₆C was prepared by hydrothermal and pyrolysis using WO₃ and cobalt gluconate as precursor.The influence of the precursor proportion on the composition and structure of the synthesized product was elaborated by changing the proportion of Co:W in the precursor.At the same time,the effect of pyrolysis atmosphere（ammonia and nitrogen）on the composition of the product was studied.The experimental results show that the catalyst Co₆W₆C（1/4-NH₃）obtained when the ratio of Co:W is 1:4 and the atmosphere is ammonia gas has uniform structure morphology and large surface roughness.Therefore,it can effectively provide more catalytic active sites and electron transport capacity.In addition,in the ammonia and nitrogen atmosphere,the calcined products were 1/4-NH₃（Co₆W₆C）and 1/4-N₂（CoWO₄）,respectively.It was also found that the electrocatalytic performance of bimetallic carbides was much better than that of oxides.Co₆W₆C has an outstanding overpotential for HER at 10 mA cm^-2of 101 mV vs.RHE;for OER,the overpotential is 343 mV vs.RHE at 10 mA cm^-2.In addition,the stability performance of the Co₆W₆C maintains an excellent percentage of 91% after 60,000 s for HER and holds a prominent ratio of 95% after 45,000 s for OER.For water splitting of two electrodes,the Co₆W₆C maintains 89% of the initial current after 40,000 s in 1 M KOH.The catalyst performs well in HER and also performs satisfactorily in OER,supporting the potential for using bimetallic carbides in the splitting of water.The work has been published in the International Journal of Hydrogen Energy.2.CoMoO₄ nanorods were used as metal precursors for the coating of polymer polypyrrole（PPy）on their surface.Aporous nitrogen-doped composite of carbon-wrapped Co/Mo₂C composites were synthesized by polymerization and pyrolysis.The influence of the ratio of precursor and pyrolysis temperature on the structure and electrocatalytic properties of Co/Mo₂C composites were investigated by changing the ratio of CoMoO₄ to PPy and the pyrolysis temperature.The synergistic heterointerface interactions created by carbon-encapsulated Co/Mo₂C composites could considerably enrich the active sites and greatly promote the electronic transfer,and greatly facilitate electronic transfer;the active Co-N sites favor the HER catalysis;the nitrogen-doped carbon facilitates electron transport and prevents the agglomeration of particles and corrosion in alkaline solution.In 1 M KOH,Co/Mo₂C composites achieve a low overpotential of 157 mV vs.RHE for HER.For OER,Co/Mo₂C composites show a low overpotential of 366 mV vs.RHE at 10 mA cm^-2.After 45,000 s stability test,Co/Mo₂C composites can maintain 94% and 93% of its initial current for HER and OER,respectively.Towards the overall water splitting via two-electrode system,the Co/Mo₂C composites also show stable catalytic performance,maintaining 91% of original current through 40,000 s stability test.