Synthesis Of Cobalt Phosphide-based Electrocatalysts And Its Catalytic Performance For Hydrogen Evolution From Water Splitting

The design of an efficient,non-precious and stable electrocatalyst for hydrogen production from water splitting is crucial for the renewable energy conversion process,yet still a difficult task.For the upcoming clean and renewable energy system,it is important to find non-precious metal-based electrocatalysts which with high efficiency and durability for the hydrogen evolution reaction(HER).Transition metal phosphides(TMPs)has attracted the interest of many researchers because of their abundant content on the earth,low price,diverse synthetic methods and excellent catalytic performance for hydrogen evolution.In this paper,cobalt phosphide-based electrocatalysts of Ce-doped CoP CNTs,N-doped CoP MSs,and B-doped CoP NPs were synthesized by simple experiments.The detail contents are as follows:(1)The cerium doped CoP nanoparticles(NPs)embedded in carbon nanotubes(CNTs)for efficient and durable hydrogen evolution was developed.The detail preparation process was described as the followings:First,cerium was introduced into ZIF-67 to form Ce-doped ZIF-67 by a joint nucleation method.Then,the Ce-doped Co-CNTs was synthesized by carbonization of Ce-doped ZIF-67.During this process,Co2+was reduced to form Co NPs and the nanostructure of CNTs was formed by the catalytic effect of Co NPs.Finally,by using Ce-doped Co CNTs as the precursor,the target catalyst(Ce-doped CoP CNTs)was obtained through a chemical vapour deposition(CVD)process in the presence of NaH2PO2.Results of the transmission electron microscopy(TEM)and scanning electron microscopy(SEM)showed that the target catalyst maintained the original rhombic dodecahedron morphology of ZIF-67 and the Ce-doped CoP NPs embedded in CNTs distributed uniformly throughout the catalyst.In electrochemical measurements,the catalyst with Ce doping amount of 0.05 mmol(Ce0.05-doped Co CNTs)showed the best performance for HER in 0.5 M H2SO4 solution.The onset potential,Tafel slope,electron transfer resistance(R,t)and double-layer capacitance(Cd1)of Ce0.05-doped Co CNTs was 49 mV,78 mV dec-1,19.2Ω and 10.5 mF cm-2,respectively.Meanwhile,the catalyst yielded a current density of 10 mA cm-2 merely at an overpotential of 146 mV.Furthermore,it maintained 90%of the original current density in a chronoamperometry measurement and showed no obvious decay even after 2000 cycles scans in a long-term durability test.(2)The catalyst of N-doped Nanosheet self-assembled microspheres(N-doped CoP MSs)with an excellent HER catalytic property was prepared by hydrothermal reaction,high temperature pyrolysis and CVD,using cobalt acetate,polyvinylpyrrolidone(K30)and ethylene glycol as raw materials,The morphology and phase composition of the catalyst were characterized by XRD,TEM,SEM and XPS.HER electrocatalytic performance of N-doped CoP MSs was tested in a 0.5 M H2SO4 solution.When the current density was 10 mA cm-2,the corresponding overpotential was 122 mV,and the Tafel slope was 65 mV dec-1.The catalyst maintained 60%of the original current density in the chronoamperometry measurement even after 11 hours.After 2000 cycles of cyclic voltammtetry scanning,the werpotential at 10 mA cm-2 was even no change.(3)Using Prussian blue analog(PBA)as precursor,boron(B)was doped into PBA by co-precipitation method,then B-doped CoP NPs were generated after pyrolysis in nitrogen and CVD phosphating.By changing the feed ratio of boron source,the effect of different doping amount of B on the catalytic perforrmce of HER was studied.Through a series of characterization and electrochemical tests,it was found that when the molar ratio of B to Co was 1(B1-doped CoP NPs),the catalyst showed the best catalytic perfomance XRD,TEM,SEM,XPS and other characterization methods were used to analyze the crystal phase structure,morphology and element valence of the catalyst When Bi-doped CoP NPs applying for HER in 0.5 M H2SO4 solution,the polarization curve showed that when current density was 10 mA cm-2,the corresponding overpotential was 123 mV and the Tafel slope was 75 mV dec-1.The I-t curve showed that after 9 hours of ehronoamperometry test,the catalyst maintained 74%of the original current density.In addition,after 2000 cycles of cyclic voltammetry test,the overpotential at 10 mA cm-2 almpost no changed.