Rational Construction Of Novel Electrocatalysts Based On Carbon Nanocages For Efficient Energy Conversion

The development of renewable clean energy is becoming more and more urgent to cope with the increasingly severe energy and environmental problems.The hydrogen economy coupling hydrogen production via water electrolysis with electricity generation via fuel cells can transform the intermittent solar and wind energies into steady electricity via the "energy carrier" of hydrogen,which exhibits many advantages such as high efficiency,environmental friendliness and good recyclability in energy utilization.This sustainable technology has become one of the key strategic development field in our country.However,serious electrode polarization usually presents in the energy conversion reactions of water electrolysis and fuel cells,and thus suitable electrocatalysts are needed to reduce the overpotential,boost the reaction kinetics and increase the energy efficiency.Precious metal electrocatalysts usually show high activity and stability,but suffer from their scarce resource and high cost.Therefore,the development of high-efficient,stable and cheap electrocatalysts is very important for improving the water electrolysis and fuel cell technologies and promoting the development of hydrogen economy.This dissertation concentrates on rational design of low Pt,single atomic Pt and precious metal-free electrocatalysts by taking advantages of hierarchical carbon-based nanocages featuring large specific surface area,micro-meso-macroporous coexistence,high conductivity,tunable nitrogen content and high stability.The energy conversion performance and regulation mechanism of these electrocatalysts have been studied in depth.The important progress of this dissertation are concluded below.1.Direct ethanol fuel cells are attractive alternative power sources due to the usage of liquid fuels featuring high energy density,low toxicity,easy storage and biomass-derived production.To date,most Pt-based electrocatalysts are still limited by the low mass activity and high susceptibility to poisoning for ethanol oxidation reaction(EOR)in acidic medium.Herein,the ternary platinum/tin oxide/carbon nanocages(Pt-SnOx/hNCNC,hCNC)electrocatalysts for EOR are constructed by microwave-assisted ethylene glycol(EG)reduction.CO electrooxidation from the stripping experiments is used as a sensitive indicator to evaluate the anti-poisoning capability of the catalysts.The correlation of the CO resistibility with the ternary geometrical configuration has been well established for the catalysts.It is revealed that the efficient ternary synergism of Pt-SnOx/hNCNC via the heterointerfaces is the key for high CO resistibility,which could facilitate the oxidative removal of CO species at Pt sites by the adsorbed OH species generated at neighbouring SnOx sites,thereby the facile regeneration of Pt active sites.Accordingly,the synergistic catalyst has been optimized which shows the excellent EOR performance in acidic medium with a superb mass activity and high durability.The consistence in the evolutions of EOR performances,ECSAs and CO resistibility with increasing Sn/Pt ratio indicates that CO electrooxidation can not only act as the sensitive indicator for anti-poisoning capability,but also be used to predict the EOR performance for this series of catalysts.This study provides a new approach of exploring advanced EOR electrocatalysts for potential applications.2.A facile impregnation-adsorption strategy has been developed to construct single atom catalysts(SACs)by synergic micropore trapping and N-dopant anchoring with the hNCNC support.The optimal Pt1/hNCNC exhibits a record-high electrocatalytic hydrogen evolution performance with ultralow overpotential,ultrahigh mass activity and ultra-long stability in acid medium.Experimental and theoretical studies have shown that the micropore confinement and electrostatic adsorption interaction with heteroatom in hNCNC support promote the formation of Pt single atom,and the strong interaction between Pt and doped N atoms makes Pt1/hNCNC have a strong catalytic stability in hydrogen evolution process.This finding suggests a new strategy to construct durable SACs of precious metals by combining micropore trapping and dopant anchoring effects with doped-carbon supports full of micropores.3.M/N/C(M=Fe,Co)catalysts have exhibited excellent catalytic performance toward oxygen reduction,but their practical applications still suffer from some shortcomings such as low density of active sites.Considering the high nitrogen content and abundant pore defects of graphitic carbon nitride(g-C3N4),the low-conductive g-C3N4 was uniformly loaded on the surface of high-conductive hierarchical carbon nanocages(hCNC)by the impregnation and pyrolysis process,leading to the formation of g-C3N4/hCNC composite.Co2+ ions were then captured by surface g-C3N4 species owing to the aboundant lone pair electrons of nitrogen and subnanometer cavities,and the subsequent pyrolysis led to the CoNx/C catalysts with highly exposed active sites,high conductivity and balanced pore structure.The optimized catalyst obtained at 800 C exhibits excellent oxygen reduction performance with a high onset potential(0.97 V)superior to commercial Pt/C catalyst,good immunity to methanol crossover and high stability in alkaline medium.This study demonstrates an effective strategy for the construction of high-efficient M/N/C catalysts with highly exposed active sites.