Electronic Structure Regulation And Oxygen Evolution Reaction Performance Of Water Splitting For Co-Based Transition Metal Electrocatalysts Supported By Carbon Spheres

Producing hydrogen by water electrolysis is a clean technology to produce high purity hydrogen without carbon emission,thus alleviating contemporary energy problems.However,the complex four-electron reaction of oxygen evolution reaction(OER)process for generating O2 leads to a slow reaction dynamic,thus resulting in a high overpotential,which seriously affects the efficiency of electrolytic water.In this regard,the porous carbon sphere and N-doped porous carbon sphere supporters are firstly prepared based on micro-nano structure design,then Co-based transition metal nano materials are introduced into carbon sphere supporters,and finally Co-Based transition metal electrocatalysts supported by carbon spheres composite materials(CoM-N/C)are constructed.The electronic structures of CoM-N/C electrocatalysts,such as p-d orbital coupling,coordination bonding,d-band center position and antibonding are further modulated by coordination and positive/anion modification strategies,thus OER performances of CoM-N/C are significantly improved in terms of increasing intrinsic activity and apparent activity.Meanwhile,Meanwhile,the cooperative regulatory mechanism of micro-nano structure and electronic structure has been explored,the synergistic enhancement mechanism of the activity and stability of CoM-N/C electrocatalysts has been revealed,and the relationship model of micro-nano/electronic structure-reaction kinetics-OER performance has been developed to provide a new idea for further enhancing the electrocatalyst performance and boosting the decelopment of CoM-N/C electrocatalyst materals.The main conclusions are as follows:(1)The strongly coupled g-C3N4/carbon spheres ternary hybrid system using a single atom as a "bridge" are constructed by the synthetic strategy of protonated g-C3N4 dispersion impregnating Co/carbon spheres.Experimental results and DFT calculations show that the bonding of single atom Co induces the coupling of p-d orbit,bring the efficient light trapping and photogenerated charge separation,fast electron transport paths and improved surface reaction dynamics and thus achieving high photoelectrocatalysis OER performance.The photocurrent density reaches 5.08 mA cm-2 at the overpotential of 1.23 V and the incident photon-to-current conversion efficiency arrives at 27.5%at 350 nm under AM 1.5 G illumination.(2)Highly dispersed Co atoms are successfully prepared by using an ultrathin g-C3N4@carbon sphere as the support,and subsequently the well-defined Co-N and Co-O coordination bonds on the atomic level are controllably constructed by adjusting the calcination atmosphere.The Co atoms with Co-O and Co-N bonds exhibit excellent OER performance in alkaline media at low and high overpotentials,respectively.DFT calculation coupled with HAADF-STEM and XPS techniques reveal that the high activities mainly originate from the precise O-Co-N and N-Co-N coordination in the g-C3N4@carbon sphere support.(3)A melamine-bridged self-construction strategy is proposed to in-situ embed Co-based bimetallic nanoparticles in the body of N-doped porous carbon spheres(CoM-e-PNC,M=Ni,Fe,Mn,and Cu),and achieve the controllable tailoring of the d-band center position by alloying of Co and another transition metal M.Result show that the enrichment and exposure of the active sites in the body interior of porous carbon spheres,and the best balance between the adsorption of OH species and the desorption of O2 induced by optimizing the d-band center position,collectively enhance the OER performance.Meanwhile,the relationship of d-band center position and OER activity is found to exhibit the volcano curve rule,where the CoNi-e-PNC catalyst shows optimal OER activity and stability.The overpotential is only 240 mV at the current density of 10 mA cm-2,and the decay of current density is about 4.4%after 100 h of continuous test.(4)Ultrafine Co nanoparticles embedded in the surface layer of N-doped carbon microspheres are prepared through an in-situ co-coordination strategy,and its d-band is modulated by introducing different Ni amounts.Results show that the introduction of Ni in the Co crystal lattice can tune the d-band centre and unpaired electrons,which collectively result in an enhancement of OER activity and kinetics.By investigating catalysts with Ni content from 0 at%to 75 at%,it was concluded that the materials with 25 at%Ni shows optimal OER activity as well as lower overpotential(285 mV at 10 mA cm-2)and high current densities(73.75 mA cm-2 at 1.63 V).Moreover,it also exhibits good stability with the negligible decline on current densities after 3000 CV cycles,and the current density could remain 85%after 100 h of continuous test under a high overpotential(310 mV).(5)The layer double hydroxides(M1M2OH-CS,M1M2=CoCu,CuNi and FeNi)possessing the nanoarray structures are successfully prepared using carbon spheres as a template,and surface S modified layer double hydroxides(S-M1M2OH-CS)are also prepared by surface in-situ sulfuration.The O element is replaced by S element,which induces the increase of electron concentration near the transition metal elements and thus decreases the d-band center position,eventually enhancing the interactions between antibonding orbitals and oxygen-containing adsorbents.The nanoarray structures effectively enlarge the contact area of catalysts and electrolyte,thus exposing a large number of surface active sites and speedinig up the charge transfer process.Thus,S-FeNiOH-CS catalyst shows the best OER performance with an overpotential of 328 mV and a Tafel slope of 42.8 mV dec-1 at the current density of 10 mA cm-2.