Preparation Of Novel Carbon Materials And Investigation On Their Applications As Fuel Cell Electrocatalysts

In order to reduce greenhouse gas emissions and smog pollution generated from combustion of fossil fuels,polymer electrolyte membrane fuel cells?PEMFCs?have been extensively studied in different applications.A fuel cell is a device which transforms chemical energy stored in fuel into electrical energy directly.It is generally accepted that fuel cells offer the ultimate solution to energy issues owing to their high efficiency,zero emission,quiet operation process and unlimited renewable fuel sources.PEMFCs with the advantages of low operating temperature and quick start up have attracted much attention worldwide due to their potential application to replace internal combustion engines.In recent years,many efforts have been devoted to improve fuel cell efficiency.To date,the most effective catalyst employed in a PEMFC is platinum.The catalytic efficiency of Pt catalysts is highly dependent on their particle size and their dispersion on the support.Due to the resource scarcity and high cost of Pt,a large-scale deployment of PEMFCs is prohibited.An ideal catalyst support for fuel cells preferably features with a high surface area to anchor active components,robust chemical/electrochemical stability to resist corrosion and excellent electrical conductivity.Targeted at synthesis of alternative novel carbon supports to the commercially available carbon black,three types of carbon materials are prepared and investigated on their applications as fuel cell electrocatalysts in this thesis.Firstly,aiming at improving the Pt utilization and stability of a fuel cell catalyst,carbon nanotubes?CNTs?,which functioned as spacers,are wedged between graphene sheets via electrostatic self-assembly.The as-prepared catalyst possesses an interconnected 3D sandwiched structure which prohibits graphene restacking,leading to catalysts with high performance.The catalysts are extensively characterized by X-ray diffraction?XRD?,transmission electron microscopy?TEM?,scanning electron microscopy?SEM?,Fourier transform infrared spectroscopy?FTIR?,Raman spectrum,Brunauer-Emmett-Teller?BET?.It is found that negatively charged CNTs and positively charged Pt/graphene are self-interacted into ordered nanostructures.An enhancement factor of 92%for Pt utilization efficiency and electrochemical surface area?ESA?is found for the optimized Pt/rGO-CNT relative to that of Pt/rGO.Compared with the state-of-the-art Pt/C,the hybrid catalyst show markedly enhanced stability,i.e.,with an ESA retention of 61%after cycling the catalyst in 0.5 M H₂SO₄ from-0.2 to 1.0 V up to 2000 scans,whereas the ESA retention for commercial Pt/C is only 19%.Secondly,carbon encapsulated copper nanoparticle catalysts with fine copper nanoparticles of a mean size of 4.9 nm were prepared using a one-pot hydrothermal process.The as-prepared Cu@C was then heat-treated in hydrogen atmosphere at 500?and then used to catalytically decompose carbon precursor during CVD process.The confined growth of copper catalysts by the encapsulated carbon at high temperatures favored yielding CSCNTs with small and uniform diameter of 42 nm distribution.It was found that the morphology and physical properties of the CSCNTs could be controlled and tuned by tailoring the carbon-encapsulated catalysts?e.g.,the particle size or types of metal catalysts?.XRD,TEM,SEM,Raman,BET were employed to characterize these materials.The optimized synthesis temperature was at 600 ?,which is lower than that required for conventional CNTs due to the space confinement induced heat confinement for the exothermic reaction.PtRu bimetallic nanoparticles were supported on the as-prepared CSCNTs and demonstrated as fuel cell catalysts.Thirdly,hydrazine hydrate was used as a nitrogen source via a hydrothermal method in order to prepare N-doped graphene materials(with a large specific surface area of 203 m² g^-1).They were characterized by XRD,Raman,XPS and BET.It was found that the introduction of nitrogen into graphene reduced the agglomeration of graphene and created a few layers of graphene sheets with interconnected open pores.XPS analysis revealed that nitrogen was successfully doped into the materials.N-6 and N-Q bond with two and three sp² C atoms,respectively,and can contribute one pair of electrons to the conductive?-system.Both of them can introduce one more electron to the?-system,which can be expected to greatly enhance the conductivity of rGO sheets.The as-constructed materials provided an effective pathway for charge transport and high concentration active sites for efficient ion adsorption/desorption demonstrated great potential applications in energy storage systems.