Design Of Cobalt-based Composites And Their Catalytic Applications In Water Splitting

To promote the sustainable development,it is necessary to explore new green energy and reduce the usage of fossil fuel.Hydrogen energy is one of the most promising energy carriers as alternatives to finite fossil fuels due to its high energy density,the wide raw materials and the pollution-free combustion product.Generating hydrogen by alkaline water electrolysis has attracted great attention to avoid acid-fog contamination and achieve high purity hydrogen?nearly 100%?for a moderate energy input.The dissociation of water involves two half-reactions:the oxygen evolution reaction?OER?and the hydrogen evolution reaction?HER?and they both are crucial in improving the overall efficiency of water splitting.To expedite the reaction rate and reduce the overpotential,many superior catalysts applied in HER and OER have been designed and explored.Currently,state-of-the-art hydrogen evolution catalysts?RuO2,IrO2?and OER electrocatalysts?Pt,Ru,Rh.etc.?remain to be noble metals.However,their widespread application is limited by the scarcity and high cost.Co-based transition metal hydroxides with large layered structure,good conductivity,low price and easy preparation and thus are ideal electrocatalyst for OER.However,these hydroxides are unstable and their activities need to be further improved.Moreover,Transition metals especially cobalt with moderate H adsorption energy have been designed to catalyze HER.Transition metals?Co,Ni and Fe?as additional species can also regulate the d-band electronic state density of parent materials and exhibit markedly promotional effect for HER.However,the conductivity of pure metals is poor and easy to accumulate during the reaction process,which hamper the transfer of electrons or protons and decrease the active sites.On the contrary,it is desirable to prepare Co-based composites with porous carbon as support due to its large surface area,high conductivity,varied structure and strong resistance to acid/alkaline environments.Since HER and OER are crucial in accelerating overall efficiency of water splitting,it is necessary to develop bifunctional catalysts for O2 evolution and H2 generation which can make full use of the redox characteristics of metal materials and reduce the cost.Based on the above consideration,this thesis synthesized Fe-incorporated a-Co?OH?2,Co engineered WN and WC and cobalt carbon composites with various valence cobalt.Furthermore,this dissertation gives more in-depth analysis of their catalytic activities and reaction mechanism.The oxygen evolution reaction?OER?via a multistep four-electron oxidation is kinetically sluggish,and thus is the bottleneck of water splitting.Herein,in the second part,we firstly explored high-efficient and durable OER electrocatalyst.Alpha cobalt hydroxide??-Co?OH?2?was prepared by a simple and easy scalable coprecipitation method.But a-Co?OH?2 was extremely unstable and easy to transform into ?-Co?OH?2,therefore we introduced Fe into ?-Co?OH?2 and improved the activity and stability.The ?-CoFe?OH?x had a platelet-like shape.The average thickness examined by measuring 150 upstanding nanosheets using HRTEM was 2.8 nm.The catalytic ability of Fe incorporated Co?OH?2 was remarkably higher than the corresponding Co?OH?2?a-Co4Fe?OH?x compared with ?-Co?OH?2,?-Co4Fe?OH?x relative to ?-Co?OH?₂?,indicating the promotion effect of Fe towards OER.Additionally,the turnover frequency?TOF?of Fe incorporated Co?OH?2?the ratio of Co/Fe=4?was 11 and 5 times larger than ?-Co?OH?2 and IrO2,respectively.Fe can be incorporated into?-Co?OH?2 forming LDH structure,Fe3+ cations substituting for Co2+ into the lattice of a-Co?OH?2.To balance the charge,more cations inserted the layers,enlarging the distance between the sheets,increasing the ECSA,and thus improving the activity.What's more,Fe modified the electronic structure of?-Co?OH?2 and made Co2+ oxidation more difficult and thus improved the stability.The theoretical overpotential of ?-Co1-mFem?OH?2 in terms of catalysing OER was significantly below ?-Co?OH?2's based on density functional theory?DFT?calculations.Electrochemical experiments further indicated that the relation between Fe content in ?-Co?OH?2 and OER activity presented as a volcano curve.To enhance overall efficiency of water splitting,the reaction rate of OER need to be improved,it is imperative to explore the high-efficient HER catalysts.In the third part,we designed a "?-to-?" phase transformation strategy by first introducing Co-based metal-organic frameworks?MOFs,ZIF-67?into tungsten precursor,taking advantage of the MOFs container effects to form truncated octahedral morphology,and then transforming the mixtures into?-WN/Co composites through NH3 calcination treatment.The calcination temperature and the molar ratio of Co/W for the formation of ?-WN are crucial and which also further influence the catalytic performance of WN/Co for HER.The ?-WN/Co2.45?synthesized at 700 ? and under Co/W molar ratio of 2.45?manifests the best catalytic activity among all WN/Co catalysts.EXAFS and XRD results confirmed the phase transformation from ?-WN to ?-WN promoted by metallic Co.The experiments and theoretical calculation confirmed that the 8-WN exhibited superior HER performance compared to ?-WN,i.e.23-fold increase in TOF.The synergistic effect between Co and 8-WN increased the HER activity of 8-WN/Co2.45,only required 76 mV overpotential to achieve 10 mA cm-2,which was comparable to or even better than the performance of other non-noble metal-based HER catalysts.Impressively,not only metal cobalt can induce the formation of ?-WN,but also metal nickel and iron facilitate the production of ?-WN.Both ?-WN/Ni and 8-WN/Fe also demonstrated considerable catalytic activity towards HER.Metallic cobalt could induce the phase transformation and it could also optimize the electron structure of parent materials around Fermi level?EF?.In the fourth part,we successfully doped cobalt into WC nanoparticles supported on N-doped carbon and optimized the electron features of WC by facile annealing CoW-based MOF precursors in an inert atmosphere.The formed nanoparticles?NPs?with an average size of 9.5 nm on CoW/CN are uniformly dispersed on N-doped carbon.XRD,XPS and XAFS verified part Co were introduced into WC,leading to the contraction of WC lattice and part existed in the form of isolated metallic cobalt.The calcination temperature and the molar ratio of Co/W had important influence on the electronic structure of WC,which also further influence the catalytic performance of WC for HER.The CoW/CN?synthesized at 800 ? and under Co/W molar ratio of 3?manifests the best catalytic activity,only requiring onset potential of 31 mV.The CoW/CN presented a remarkably improved activity in comparison with pure WC and metallic cobalt,indicating the cooperation effect of Co and WC.DFT demonstrated the incorporation of Co remarkedly increased the state density of WC at EF originated from the electron transfer from Co to W by forming the W-C bond,resulting in the moderate H2O and H adsorption energy toward optimized HER kinetics.Similarly,Ni and Fe can also modulate the electronic structure of WC and improve the HER activity.Co-based catalysts could be applied in HER?metallic cobalt?and OER?cobalt hydroxide?,could we design bifunctional catalysts for simultaneously generating H2 and O2 by taking advantage of polyvalent cobalt?In the fifth part,CoO,@CN was achieved by simple thermal treatment of the inexpensive starting materials?Melamine,CoNO3·6H2O and D?+?-Glucosamine hydrochloride?at 800 ? in N2 atmosphere.The flake-like morphology of the resulting hybrids was observed.The formed cobalt nanoparticles?NPs?with average size of 12.3 nm are uniformly dispersed on N-doped carbon sheets.XRD and XPS verified that CoOx@CN was composed of metallic cobalt encapsulated by N-doped carbon,cobalt oxide and N-doped carbon and they played an important role in HER process.Co metal has been calculated to have a proper binding energy for H atom,promoting the generation of hydrogen.CoO and Co3O4 can be hydroxylated to dissociation water,facilitating the volmer reaction.N-doped carbon coated Co nanoparticles,improving the conductivity and stability of CoOx@CN for HER.The incorporation of nitrogen into carbon architecture improved the catalytic activity by increasing the proportion of Co0 due to the electron donating effects.The improved catalytic ability of CoOx@CN after HER towards OER arose from the improved percentage of Co2+.When applied as water electrolysis catalysts,CoOx@CN reached 20 mA cm-2 current density at 1.55V.To sum up,we synthesized Co-based HER and OER catalysts by utilizing polyvalent cobalt.The introduction of Fe into a-Co?OH?2 improved the activity and stability and made a-Co?OH?2 itself better OER electrocatalyst.The electronic effect induced the phase transformation of WN,modified the state density of WC at EF level,and thus prepared low-cost and high-efficient catalysts.The preparation methods applied in this paper is simple,facile and thus it provides a possibility for the large-scale preparation of catalysts.The raw materials source are extensive.Furthermore,this dissertation deeply studied the factors that affects HER performance by DFT calculation from atomic level.This paper provides guidance for rationally designing highly active,cheap and stable non-noble metallic catalysts,and also promotes the advance of water splitting industry.