| Hydrogen energy has the advantages of high energy density,high calorific value,renewability and zero emission,which is conducive to building a green and wonderful society,and is considered as the most promising clean energy in the future.Fuel cell(FC)technology can realize green and efficient conversion of hydrogen energy,but its sluggish ORR intrinsic kinetics directly restricts the output power and energy density.At present,commercial Pt/C still have some problems,such as high price,scarce reserves,low selectivity and easy dissolution in acidic electrolyte,which seriously hinder the large-scale application of fuel cells.Therefore,it is a challenge to design and develop new non-noble metal electrocatalysts with low cost,high activity,high selectivity and acid resistance.As a substitute for Pt/C,transition metal chalcogenides(TMDCs)have attracted much attention because of their unique characteristics,such as low price,abundant resources and high conductivity.Transition metals(such as Fe,Co,Ni,etc.)have unfilled 3d-orbital and show good ORR catalytic activity.A large number of research results show that when the same chalcogen elements(X=S,Se or Te)coordinate,the ORR intrinsic activity of transition metal follows the following order:Co>Ni>Fe.In addition,compared with oxygen(O)and sulfur(S),selenium(Se)has smaller electronegativity and coordinates with transition metal(M)to form a more covalent M-X bond,thus showing higher metallicity.Therefore,focusing on the design and construction of highefficiency ORR electrocatalysts,it is great significance to study the controllable preparation and electrocatalytic performance of cobalt selenide material for promoting the green and efficient conversion of hydrogen energy.In this paper,aiming at improving the ORR reaction rate in acidic medium,a series of CoSe2/C catalysts with high activity and stability were prepared by alkali etching and chemical vapor deposition based on CoAl-LDH.The relationship between crystal structure,morphology,pore structure and surface electronic structure and the performance of ORR was systematically discussed.Furthermore,based on the controllability of type and content of cations toward LDH materials,NiCoAl-LDH is regarded as a precursor.By investigating the changes of physical characteristics and ORR performance of CoSe2 after introducing Ni,the relationship between the structure of the materials and ORR performance was revealed.It can be found that the activity and stability of(NiCo)Se2 are upgraded.The research contents and results are as follows:(1)According to the controllability of the type and content of cations in LDH materials,cobalt nitrate and aluminum nitrate were used as divalent(MⅡ)and trivalent(MⅢ)metal salts,respectively,and urea was used as precipitant to prepare CoAl-LDH precursor(nCo/Al=2:1)by one-step hydrothermal method.Then,using alkali etching method to prepare intermediate with different etching time(x=2,4 and 8 h).Finally,a series of CoSe2/C-X were prepared by physically mixing intermediate with carbon black and selenizing them by chemical vapor phase method.The research results show that COSe2 with two crystal phases can be obtained by adjusting the alkali etching time and changing the content of Al metal element in the CoAl-LDH.The existence of Al element induced the phase transition of CoSe2 crystal structure,from c-CoSe2 to o-CoSe2 and c-CoSe2 mixed phase.Furthermore,CoSe2/C-X catalysts prepared by different etching time,the mass ratio of c-CoSe2/o-CoSe2 also shows some differences.In addition,compared with c-CoSe2 prepared by selenizing Co(OH)2 as precursor,mixed phase CoSe2(c-CoSe2 and o-CoSe2)prepared by selenizing CoAl-LDHs precursor under the same conditions showed higher ORR activity in 0.5 M H2SO4 electrolyte,indicating that the synergistic effect on the two crystalline phases is beneficial to improving the reaction activity of the catalyst.(2)In order to solve the problem of relatively poor ORR activity of single active component,using NiCoAl-LDH as precursor to conduct carbonsupported nickel-cobalt selenides with high activity were prepared,namely(Ni1Co2)Se2/C,(Ni1Co1)Se2/C and(Ni2Co1)Se2/C,and the effects of different ratio of Ni/Co and extra-metal element to ORR performance were systematically investigated.The results show that the introducing of Ni metal can change crystal structure and electronic structure of CoSe2,and enhance the interaction between metal active components and selenium element.In 0.5 M H2SO4,(Ni1Co1)Se2/C catalyst showed the best ORR performance,and its onset potential(Eonset)and half-wave potential(E1/2)were 0.76 V and 0.67 V(vs.RHE),respectively.In the range of 0.2-0.6 V,the yield of hydrogen peroxide was less than 5%,and the electron transfer number was close to 4 and E1/2 only decreased by 5 mV after 3000 cycles of cyclic scanning.At the same time,in 0.1 M KOH,the catalyst also showed higher ORR activity,Eonset and E1/2 were 0.92 V and 0.82 V(vs.RHE),respectively,the yield of hydrogen peroxide was lower than 2%in 0.2-0.8 V.It proved that the introducing Ni element into CoSe2 is beneficial to improve ORR performance. |
