Carbon-based Aerogel Electrocatalysts Toward The Oxygen Reduction Reaction:Preparation,Structure Optimization And Catalytic Applications

Electrocatalytic oxygen reduction reaction(ORR),as the core cathode reaction of a variety of clean energy devices,is of great significance to realize the "two-carbon" goal.Since ORR involves the multiple electron-transfer process with sluggish kinetics,among which the four-electron(4e-)ORR pathways to generate water(H2O)is dominated in fuel cells and zinc-air batteries(ZABs),and the two-electron(2e-)ORR with the merits of green,in-situ and on-demand generation of hydrogen peroxide(H2O2)has also attracted attention.As a result,the ORR needs catalyst to improve activity and selectivity.At present,noblemetals have the highest reported ORR kinetic activity,but the scarcity and insufficient stability largely impede their widespread applications.Therefore,the development of cheap and efficient noble-metal-free alternatives is the focus of current research,but it still remains a grand challenge.As the ORR occurs at the triple-phase boundaries,the catalytic performance is closely related to the coordination environment and the accessibility of the active sites at the interface.Therefore,regulating the electronic structure and microstructure of a catalyst is the key to improving the catalytic performance.Based on the above content,this paper focusses on the carbon-based aerogels with the advantages of low cost,easy functionalization and abundant pore channels.By making use of the regulation strategies including the atomic-level engineering and morphology engineering,we design and build a series of efficient single atom and metal-free carbon-based aerogels and explore their applications in ZAB s(4eORR)and H2O2 electrosynthesis(2e-ORR).Meanwhile,combined with experiments and various characterization techniques,the relevant catalytic mechanism and modulation rules are elaborated,which provide valuable reference for the design of advanced functional materials used in the field of energy and catalysis.The specific research contents and conclusions are as follows:(1)In order to solve the problem of low ORR activity and poor mass transfer of current catalysts in fuel cells and ZABs,we developed a morphology and pore engineering strategy to adjust the electronic and geometric structure concurrently in a catalyst(termed as eFe-N3/PCF).We explored the structure-activity relationship by combining multi-group comparison experiments,theoretical calculations and in-situ nitrite reduction quantified active site density tests.eFeN3/PCF catalyst has combined merits of high intrinsic activity originated from the under-coordinated and edge-hosted Fe-N3 moieties,improved mass transport efficiency and utilization of active sites(～4-fold enhancement)as a result of the the multiscale porous carbon framework.Benefiting from above advantageous features,eFe-N3/PCF exhibits an outstanding ORR activity in alkaline medium(the onset potential and half-wave potentials are up to 1.090 V and 0.934 V,respectively),and delivers a much smaller mass transfer resistance in 500 mA cm-2 in a gas diffusion electrode as well as a peak power density of 294 mW cm-2 in a ZAB.This study provides guidance for the rational design and synthesis of high-performance single-atom catalysts for basic exploration and practical applications.(2)In order to investigate the selective effect of strain engineering on the ORR reaction pathway,a P-modified single-atom Co catalyst(CoN4-PC)was synthesized via a polymerization-and-pyrolysis strategy,and for the first time,the strain effect induced by P doping for tuning the coordination environment of Co and the ORR pathway from 4e-to 2e-was systematically studied.Combining the X-ray absorption spectrum of synchrotron radiation and theoretical calculations results,it is found that the introduce of P dopants can stretch the Co-N bond of the CoN4 sites,decrease the electron density of the Co atoms and weaken the OOH*adsorption strength on the active sites of CoN4-PC.As a result,this catalyst delivers an exceptional performance for the 2e-ORR with 97%H2O2 selectivity and a large turnover frequency(TOF=2.36 s-1)in 0.1 M KOH.Moreover,when evaluated in a practical flow cell,an unprecedent H2O2 production rate of 11.2 molH2O2 gcat.-1h-1 was achieved during a 110-h long-term durability test.This work broadens new strategies for the synthesis of highly efficient single-atom catalysts,and provides new insights for the tunning of electronic structure at the atomic level.(3)In order to solve the problem of low activity and insufficient stability of 2e-ORR catalyst at high current densities,a Ni-based single-atom catalyst(NiO-C)with a super-coordinated NiO6 configuration was synthesized.The sixcoordinated O atoms synergistically optimize the electronic structure of central Ni atoms,and the porous conductive carbon aerogel network reduces the resistance of concentration polarization.The electrochemical test results show that Ni-O-C has the highest intrinsic activity and longest cycle durability in 0.1 M KOH,with outstanding TOF(3.67 s-1)and no attenuation in performance after 50000 cycles.Moreover,after 3 1 h of continuous operation at a high current density of 120 mA cm-2,the Faraday efficiency can still be maintained at 90%.The robust super-coordinated structure promotes the possibility for large-scale of H2O2 electrosynthesis,and also provides reference for the construction of highperformance materials.(4)In order to explore the influence of deformation of metal-free nanomaterials on their surface structure properties and ORR performance,three carbon-based aerogel catalysts with different curvatures were designed.The results indicate that when the curvature increases,the strain in the nanocarbon increases,decreasing the work function,which helps to accelerate the electron transfer from the catalyst to O2.At the same time,the increase of curvature enriches the pore structure of the catalyst and promotes the timely removal of the generated H2O2 from the active sites.Compared with planar low-curvature graphene,the half-wave potential of high-curvature nanocarbon(HCC)increases by 100 mV and the kinetic current density improves by 14 times.In addition,HCC exhibits excellent stability(30 h,100 mA cm-2),which exceeds all of the metal-free catalysts reported so far.This study is of great significance for revealing the electronic/geometric structure of graphene regulated by deformation.