Synthesis Of Mo,Co,Cu Based Materials Using Bacterial Cellulose As Substrate For Electrochemical Applications

The rapid increase of energy demand and the serious environmental problems have stimulated great research interest to develop renewable energy technologies.It has been proven experimentally and theoretically to be feasible using low-cost and earth-abundant transition metal catalysts to electrocatalytic splitting water to produce hydrogen(H2).Although H2 has been widely considered as a clean,environmentally friendly and efficient energy source,the high overpotential of hydrogen evolution reaction(HER)and sluggish reaction kinetics of HER catalysts have been the biggest limitation for its large-scale applications through electrocatalytic splitting water technology.To tackle these issues mentioned above,precious metal materials(e.g.,Pt,Rh,RuO2,etc.)have been widely used as electrocatalysts for electrocatalytic splitting water to generate H2,however,these shortcomings of precious metal catalysts such as expensive and source scarcity have limited their large-scale production applications.In recent years,transition metal based materials(Mo,Co,Cu etc.)with controllable defects,active doping sites and exposed active crystal faces have been promising candidates for water splitting to produce H2 with great potential to replace precious metal electrocatalysts.To further improve water splitting efficiency,the development of effective synthetic strategies to fabricate non-noble metal electrocatalysts(Mo,Co,Cu etc.)with controllable crystalline structure,composites and catalytic active sites is highly needed,thus realizing catalysts’ multifunctionality and high H2 generation efficiency.On the basis of the above discussions,this thesis work is mainly focused on the synthesis of transition metal(e.g.,Mo,Co,Cu etc.)electrocatalysts using bacterial cellulose as substrate with rich surface O-containing functional groups for electrocatalytic water splitting applications.Their catalytic active mechanisms have been discussed in detail based on the experimental and theoretical calculations results.The specific research contents and achievements are summarized as follows:1.Cobalt(Co)-doped MoS2 nanosheets with different Co contents(Co-MoS2)have been successfully prepared by a facile one-step hydrothermal method.The experimental results demonstrate that pure MoS2 nanosheets without Co doping as electrocatalyst exhibits certain HER activity but ignorable oxgen evolution reaction(OER)activity in both acidic and alkaline conditions.In contrast,the obtained Co-MoS2-0.5 shows superior bifunctional HER and OER activities with the onset potentials of 0.04/0.03 V(vs.RHE)for HER and 1.68/1.33 V(vs.RHE)for OER,and delivering low overpotentials at 10 mA cm-2 of 60/90 mV for HER and 540/190 mV for OER in 0.5 M H2SO4 and 1.0 M KOH,respectively.This superior bifunctionality of Co-MoS2 makes it to promise as cathode and anode catalyst to establish an overall water splitting electrolyzer with high efficiency.2.Utilizing bacterial cellulose(BC)as substrate to adequately adsorb Co and Mo precursors with different molar ratios,after freeze-drying,an in-situ vapor-phase hydrothermal method was employed to transform Co/Mo-adsorbed BC into Co-doped MoS2 nanosheets grown on bacterial cellulose derived carbon fibers(Co-MoS2/BCCF).The synthesized Co-MoS2/BCCF with an optimized Co content as electrocatalyst exhibits bifunctional HER and OER activities in 1.0 M KOH electrolyte with the onset potentials and the overpotentials at 10 mA cm-2 for HER and OER of 25 mV/1.36 V(vs.RHE)and 76 mV/200 mV,respectively.At the same time,the fabricated electrocatalyst also indicates high applicable stability and great potential as electrode material for supercapacitors.3.The layer structured MoS2 is a typical hydrogen evolution reaction(HER)electrocatalyst but possessing poor activity for oxygen evolution reaction(OER).In this section,we investigate a cobalt covalent doping approach capable of inducing HER and OER bifunctionality into MoS2 for efficient overall water splitting.Utilizing bacterial cellulose(BC)as substrate to adequately adsorb Co and Mo precursors with different molar ratios,after freeze-drying,Co-containing Mo2C nanostructures on BC derived carbon fibers(BCCF)(Co-Mo2C/BCCF)can be first achieved by a simple pyrolysis approach in N2 atmosphere.Subsequently,the synthesized Co-Mo2C/BCCF was converted into Co covalently doped MoS2 nanostructures on BCCF(Co-MoS2/BCCF)through a facile vapor-phase hydrothermal method.The results demonstrate that covalently doping cobalt into MoS2 can lead to obvious enhancement on HER activity and simultaneously inducing remarkable OER activity.The catalyst with optimal cobalt doping density can readily achieve HER and OER onset potentials of-0.02 and 1.45 V(vs.RHE)in 1.0 M KOH.Importantly,it can deliver high current densities of 10,100 and 200 mA cm-2 at low HER and OER overpotentials of 48,132,165 mV and 260,350,390 mV,respectively.The reported catalyst activation approach could be adapted for bi-functionalization of other transition metal dichalcogenides.4.Utilizing bacterial cellulose(BC)as substrate to adequately adsorb Co2+ precursors with different concentrations,after full freeze-drying,Co2+-adsorbed BC was pyrolytically converted into metallic Co loaded BCCF(Co/BCCF)with different Co particles sizes(2.0 nm～10.0 nm)and contents(0.4%～21.5%)in N2 atmosphere.As electrocatalyst,the synthesized Co/BCCF was systematically investigated for OER and selective chlorine evolution reaction(CER)in neutral conditions.The experimental results demonstrate that Co/BCCF-42 with an average Co particle size of 10.0 nm and Co content of 17.0%exhibits superior OER activities with an onset potential of 1.28 V(vs.RHE)and an overpotential of 90 mV at 10 mA cm-2.Importantly,the fabricated Co/BCCF-42 also indicates superior selective CER activities with an onset potential of 1.36 V(vs.RHE)and an overpotential of 170 mV at 10 mA cm-2.Corresponding catalytic active mechanisms have been discussed on the basis of experimental results.5.Utilizing bacterial cellulose(BC)as substrate to adequately adsorb Cu2+ precursors with different concentrations,after full freeze-drying,Cu2+-adsorbed BC was pyrolytically converted into metallic Cu loaded BCCF(Cu/BCCF)with different Cu particles sizes(4.0 nm～12.0 nm)and contents(0.6%～23.7%)in N2 atmosphere.As electrocatalysts,the obtained Cu/BCCF samples were studied for HER and OER bifunctional activities in alkaline media,thus overall water splitting application.The optimal Cu/BCCF catalyst shows better OER performance with the onset potential and overpotential of 10 mA cm-2 of 1.40 V((vs.RHE)and 250 mV,respectively.At the same time,the Cu/BCCF-42 catalyst also indicates superior HER activity with an onset potential of 18 mV(vs.RHE).This superior bifunctionality of Cu/BCCF catalyst demonstrates its great potential as cathode and anode catalyst for overall water splitting application.