Preparation Of Carbon-based Noble-metal-free Catalysts And Their Electrocatalytic Activity For Oxygen Reduction Reaction

Low-temperature fuel cells as advanced energy technologies are considered as promising power supply of the future, due to their advantages of high energy conversion efficiency, environmental benign, fast starting, quiet and no rely on the fossil fuels. However, the commercialization of the fuel cells is obstructed by the some factors, one of them is the application of precious Pt as catalyst, due to the sluggish oxygen reduction kinetics in low-temperature fuel cells, a large amount of platinum-based catalysts should be applied at the cathode to accelerate the cathode oxygen reduction, resulting the high cost of the fuel cells due to the scarce reserve and high price of platinum. Recently, carbon-based materials have been considered as potential replacements for Pt-based catalysts for the ORR in fuel cells, which could be attributed to their distinct advantages, including extensive sources, low cost, high catalytic activity, good long-term stability, and excellent methanol tolerance. Actually, doped carbon catalyst has been recognized as one of the most promising non-platinum catalysts for fuel cells.In this thesis, we have designed and synthesized a series of high-performance noble-metal-free carbon-based oxygen reduction catalysts though co-doping with dual metal-free heteroatoms or multielements（including transition metal）, layer intercalated assembling, loading with transition metal-based nanoparticle, and investigated these catalysts systematically, the main works and achievements could be described as follows:（1） A metal-free nitrogen and phosphorus dual-doped reduced graphene oxide catalyst（NP-rGO） was successfully synthesized by a pyrolyzing procedure（at 900 ℃, under Ar protection） using graphite oxide and diammonium hydrogen phosphate（（NH4）2HPO4） as precursors. The specific surface area of NP-rGO is up to 376.2 m² g^–1, and it has a relatively high P-doping level（1.02 at. %） and a trace amount of N（0.35 at. %）. The catalyst exhibited enhanced catalytic performances for oxygen reduction reaction（ORR） via a dominated four-electron reduction pathway, as well as superior long-term stability, better tolerance to methanol crossover than that of commercial Pt/C catalyst in an alkaline solution.（2） A metal-free nitrogen and fluoride dual-doped reduced graphene oxide catalyst（N–F/rGO） has been successfully prepared by a facile thermal annealing method（at 850 ℃, under Ar protection） using graphene oxide and ammonium fluoride（NH4F） as precursors. The BET specific surface area of N–F/rGO is up to 412 m² g^–1, the N and F contents in N–F/rGO is 2.51 and 0.74 at%, respectively. The catalyst exhibited excellent activity toward the ORR, comparable to that of commercial Pt/C in an alkaline medium. The additional doping with F considerably enhanced the performance of the catalyst, which could be attributed to its high electronegativity and presence in the form of most active semi-ionic C-F, and the synergetic effect of N and F co-doping.（3） A Fe, N and S multielements co-doped reduced graphene oxide（Fe-N-S/rGO） was successfully prepared by directly pyrolyzing a mixture of graphene oxide, FeCl3, melamine and sulfur in N2 flow, during which deoxidization of GO and multielements co-doping are realized simultaneously. This catalyst shows excellent oxygen reduction activity in an alkaline medium, with an onset potential of 0.952 V（vs. RHE） and a kinetic-limiting current density of 4.78 m A cm^–2 at 0.682 V（vs. RHE）, which is superior to that of commercial Pt/C. Furthermore, it exhibits long-term stability, excellent methanol tolerance and high selectivity for the four-electron reduction pathway. We also studied the effect of N, S, and Fe on the ORR performance. Nitrogen and sulfur co-doped graphene yields a performance superior to that of mono-heteroelement doped catalysts, due to the summational effect of having two types of active centers, and the synergetic effect between N and S that arises from introducing larger asymmetrical spin and charge density, resulting from the doping of N and S. The strong promotion of Fe may result from three factors:（i） the Fe may act as a catalyst during the thermal expansion process, selectively promoting the formation of specific N-C sites;（ii） the Fe residue in the catalyst may form new types of active centers（Fe-Nx-C）.（4） A Nitrogen, phosphorus and Fe multielements co-doped carbon nanospheres catalyst（（Fe）N-P/Cns） was successfully synthesized by a facile method in which polyacrylonitrile nanospheres were pyrolyzed in the presence of diammonium phosphate and iron trichloride hexahydrate under N2 protection. The specific surface area of the catalyst is high up to 771.3 m² g-1, and it has a hierarchical micro-meso-macroporous structure. In an alkaline medium, the catalyst exhibits high electrocatalytic activity towards the oxygen reduction reaction（ORR） as well as excellent stability and methanol tolerance—superior in each case to commercial Pt/C catalyst. The effects that adding Fe salt and phosphorus on the structure and performance of the catalyst are also investigated. We suggest that the catalyst’s excellent electrocatalytic performance may be attributed to:（i） the synergistic effect, which provides more catalytic sites for the ORR, due to the nitrogen and phosphorus co-doping;（ii） the strong promotion by trace Fe residues; and（iii） the high surface area and excellent mass transport rate arising from the hierarchical porous structure.（5） To resolve the agglomeration problem of graphene, a three-dimensional（3D） nitrogen-doped graphene material containing a trace amount of cobalt and intercalated with conductive carbon nanospheres（Co-N-GCI） was successfully prepared by thermal annealing a mixture of graphene oxide, cobalt（II） acetate tetrahydrate, melamine and Carbon nanospheres. The insertion of the carbon nanospheres considerably enhanced the catalyst’s performance. In an alkaline medium, Co-N-GCI exhibited superior ORR performance than that of commercial Pt/C, and it also have excellent long-term durability. This carbon nanospheres act as “spacers” between the graphene sheets and thereby increase the graphene’s accessible surface area; provide abundant electrolyte channels, which one would expect to facilitate the diffusion of reactive species to catalytic active sites; and serve as “shortcuts” for interplanar electron transport, thus guaranteeing the material’s good conductivity.（6） A cobalt sulfide hollow nanospheres mounted on hole reduced graphene oxide co-doped with nitrogen and sulfur（CoxSy/N-S-rGO） was successfully prepared via a facile pyrolysis method using graphene oxide, Cobalt（II） acetate tetrahydrate, 1,10-phenanthroline monohydrate（phen）, and sublimed sulfur as precursor. In an alkaline medium, the catalyst exhibited superior ORR performance than that of commercial Pt/C, and it also have excellent long-term durability and methanol crossover effect. Detailed investigation clarified that the material’s excellent electrocatalytic performance is attributable to:（i） a synergistic effect, induced by the presence of multiple types of active sites, including cobalt sulfide hollow nanospheres, nitrogen and sulfur dopants, and possibly Co-Nx-C sites;（ii） the enriched edge carbon produced by cobalt sulfide etch carbon; and（iii） the high surface area and efficient mass transfer arising from the hierarchical porous structure.