Preparation And Properties Of Cobalt Phosphide/graphene Bifunctional Oxygen Electrocatalysts

Energy is an important foundation for promoting social progress,a key force for promoting economic development,and a serious problem that mankind is currently facing.The energy situation of our country is very severe,the consumption of resources is also huge,the supply of energy is very unbalanced,and the emergence of problems such as serious pollution to the environment has forced us to develop new energy that is cleaner,more efficient and recyclable.As far as we know that oxygen reduction and oxygen evolution reactions(ORR/OER)play crucial roles in energy conversion and storage devices.In this work,two kinds of stable oxygen electrocatalysts(CoP/HNBs@NrGO and CoP-HNTs@m-rGO)with hollow structures and highly active components were prepared to obtain excellent ORR/OER electrocatalytic active.Their material composition,element valence,specific surface area,pore size and apparent morphology were analyzed by a large number of physical characterization methods.The properties of ORR and OER were investigated by means of electrochemical performance tests.The catalytic mechanism of the catalysts was reasonably deduced.A hollow nanobox with CoP nanoparticles embedded in the carbon wall was obtained using ZIF-67 nanocube as template etched with tannic acid and then phosphated with sodium hypophosphite.Finally,a composite electrocatalyst(CoP/HNBs@NrGO)with excellent ORR/OER bifunctional activity was obtained after introducing a nitrogen-doped carbon source and annealing at high temperature.NrGO with high electrical conductivity can produce a strong synergistic effect with well-dispersed CoP nanoparticles,thereby enhancing oxygen adsorption/reduction and fast electron transfer.The carbon framework with hollow structure endows the CoP/HNBs@NrGO-2(mass ratio of GO to CoP/HNBs is 2)nanocomposites with sufficient active sites and electron transfer pathways,as well as enhanced structural stability.CoP/HNBs@NrGO-2exhibited a more positive half-wave potential of 0.88 V than commercial Pt/C(0.85 V),with only a negative shift of 28 mV in the half-wave potential after 5000 CV cycles.CoP/HNBs@NrGO-2 also exhibited excellent OER activity,requiring only a low potential of 1.59 V to reach a current density of 10 mA cm^-2.It also has satisfactory stability(?E_{j 10}=21 mV after 1000 CV cycles).In addition,we obtain a nanocomposite of a novel integrated architecture,which is composed of 1D hollow CoP nanotubes(CoP-HNTs)derived from the nanowire precursors(self-template method)and N-doped carbon layer(m-rGO,as a functional wrapping layer for CoP-HNTs)through a facile hydrothermal-phosphorization-annealing strategy(CoP-HNTs@m-rGO-0.4).Morphology of CoP-HNTs@m-rGO-0.4(mass ratio of GO to CoP is 0.4)shows that the exterior surface of hollow CoP-HNTs is firmly coated by nitrogen doped carbon with a layer thickness of 5-8 nm.?E(E_j10(1.58 V,OER)-E_1/2(0.91 V,ORR))of CoP-HNTs@m-rGO-0.4 is as low as 0.67 V for ORR/OER,which outperforms commercial Pt/C and RuO₂.Excellent activity and stability of CoP-HNTs@m-rGO-0.4 for ORR/OER benefit from synergistic effects between CoP and m-rGO,enhanced mass transfer via 1D hollow structure,and abundant active sites exposed on the interior/exterior surfaces of nanotube wall.Density functional theory calculations confirm that strong coupling interactions lead to interfacial charge polarization to remarkably promote the ORR/OER activities.This paper focuses on the role of hollow structure and interfacial effect in oxygen electrocatalysis,and synergistically integrates favorable thermodynamics with efficient kinetics through modification of electronic structure,which opens up a new way for the preparation of highly efficient and stable bifunctional electrocatalysts.