Supercritical Fluid-assisted Synthesis Of Metal-N-C Nanomaterials And Their Electrocatalytic Application

Hydrogen fuel cell and metal-air batteries show great potential in area of new energy conversion and storage due to their low emissions,high efficiency and safety factors.However,the large-scale application of these advanced devices is liimited by high-cost noble-metal catalysts in cathode.Subnano cluster and single-atomic site catalysts have emeged as one of the most promising candicates for oxygen reduction reaction(ORR)because of their theoretical atomic utilization efficiency of 100%,low metal consumption and adjustable catalytic activity,and hold vast potential for developing sustainable energy.We focus on exploiting new strategies to sythesize carbon-supported transition metal clusters and single-atom catalysts(SACs),enhancing electrocatalytic activity via optimization of carbon structure,regulation of coordination environment and construction of high-density metal active sites,mainly studying the design and controllable preparation of high-performance SACs,deeply understanding the corresponding mechanism of oxygen electrocatalysis.The optimal catalysts will be used as cathode in liquid or flexible solid-state Zn-air batteries for evaluating its application.The research results are summerized as follow:(1)The dual phase carbon anchored high density subnano clusters was obtained by supercritical fluid-assisted method,in which the Fe precursors uniformly dispersied in cavities of ZIF-8 via supercritical CO₂(scCO₂)fluid impregnation and the dodecahedrons were incised,then the partial fracture surfaces of ZIF-8 formed carbon nanotube(CNT)under the catalysis of Fe in pyrolysis.The subnano Fe clusters were furhter coordinated by N in carbon matrix,and dual phase carbon anchored high density subnano clusters was obtained.The Fe-cluster/N-C catalyst shows superior alkaline ORR activity and stability,the half-wave potential is recorded to be 0.90 V and exceeds that of commericial Pt/C.On account of strong interaction between cluster and CNT that endows the higher occupied state of Fe d-orbit and CNT provides better conductivity,the Fe cluster sites supported on CNT possess higher activity compared to those in amorphous carbon and therefore emeged as primary active sites for ORR.(2)The porous carbon anchored Fe SACs and clusters catalyst was synthesized by optimized previous scCO₂ impregnation strategy.In preparation,the supercritical fluid incised dodecahedrons of ZIF-8 into uniform blocks and expanded partial micropores to mesopores,thus promoting the mass transfer of catalyst and activating inner SAC sites of carbon matrix.Furthermore,the Fe cluster nearby the SAC sites could weaken hybridization effect between d-orbits of Fe and 2p-orbits of adsorbed oxygen,thereby benefiting for adsoption of H⁺and desoption of OH^-.The resulted decline of thermodynamic energy barrier for ORR could enhance intrinsic activity of Fe N₄ moiety.The scCO₂-Fe _C@Fe NC catalyst shows ORR half-wave potential of 0.91 V in alkaline electrolyte,outperforming that of commericial Pt/C.Moreover,the liquid Zn-air battery(ZAB)using scCO₂-Fe _C@Fe NC as air cathode exhibits a peak power density of 175.9 m W cm^-2.(3)The ZIF-8 consisted of partial Co nodes was used as carrier and adsorbed Fe(acac)₂and glucose molecules by scCO₂ fluid impregnation,in which spatial confinement of ZIF-8and physical separation of glucose molecule prevent aggregation of Fe atoms in pyrolysis,thus porous carbon anchored uniformly dispersed Fe,Co dual single-atoms and adjacent Fe Co particles(M@Fe Co-N-C)catalyst was prepared.Importantly,Fe Co alloy particles could regulate the d-orbital electron distribution of adjacent single-atomic sites,optimize the adsorption and desorption of intermediates,thereby significantly enhancing the ORR intrinsic activity of Fe N₄ site and OER intrinsic activity of Co N₄ site.The M@Fe Co-N-C catalyst presents ORR half-wave potential of 0.95 V and OER overpotential of 340 m V(10 m A cm^-2),and the corresponding potential difference is only for 0.62 V.The rechargeable liquid and flexible solid-state ZABs using M@Fe Co-N-C catalyst as air-cathode respectively show the peak power density of 267.1 m W cm^-2 and 126.2 m W cm^-2,which are superior to those of Pt/C+Ru O₂ catalyst.Moreover,Flexible battery maintains stable charge and discharge current in bending experiment,showing application potential of flexible electronic devices.(4)The ZIF-8 derived carbon was used as carrier and doped Co,B and P elements via scCO₂ fluid impregnation combined with high-temperature pyrolysis,thus B,P-doped porous carbon anchored Co SACs(Co-NBP)was obtained.The introduced B and P atoms could regulate d-orbital electron distribution of Co sites and optimize the adsorption and desorption of intermediates,thereby promote charge transfer efficiency for ORR and effectively enhance ORR activity of Co SAC.The Co-NBP catalyst exhibit excellent ORR activity and durability in both of alkaline and acidic electrolytes.The alkaline and acidic ORR half-wave potentials were measured as 0.89 V and 0.79 V,respectively.(5)The cavities in ZIF-8 were optimized by the first supercritical reaction and adsorbed metalloporphyrin molecules with large size by the second supercritical reaction,the obtained composite pyrolyzed with thiourea to synthesize S-doped porous carbon anchored high-density Fe/Co/Ni SACs(Fe/Co/Ni-NSC).S-doping could optimize the adsorption and desorption of intermediates by regulating the charge distribution of single-atom sites,and reduce the thermodynamic energy barrier for ORR,as well as increase the occupied electrons in d-orbits of metal sites to accelerate charge transfer for ORR,and thus effectively enhancing ORR activity of metal SACs.The Fe-NSC shows the highest alkaline and acidic ORR activity among prepared catalysts,surpassing or closing to activity of commercial Pt/C.The ORR half-wave potentials were respectively measured as 0.935 V and 0.834 V in 0.1 M KOH and 0.1 M HCl O₄electrolytes.