Study On Nitrogen-doped Carbon Supported Non Precious Metal Cathode Catalysts For Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFCs) are becoming attractive, but there are critical issuesinvolved in the commercialization with regard to the core technologies of catalyst, methanolcrossover. More importantly, platinum, the best and most frequently used catalysts for theoxygen-reduction reaction (ORR) until now, have limited DMFCs commercialization due totheir price (¹500$·oz-1in2012) and scarcity. Pt-based materials, however, as the catalystsfor the ORR, limit DMFCs performance due to the incomplete4-electron transfer reaction forORR and the low methanol tolerance. Therefore, partial or complete replacement of Pt metalfor ORR has attracted a lot of interest to nonprecious catalysts. The main research contents ofthis thesis is to develop nitrogen-doped carbon supported non precious metal cathode catalysts(MeNC), which have the simple preparation technology, low-cost, high activity for ORR andgood methanol tolerant ability. Study the crystal structure, particle seize and chemicalspeciation of transition metal, by means of various electrochemical methods and materialsmeasurements. The home-made MeNC catalysts were used as the cathode catalysts to preparemembrane electrode assemblies (MEAs) and a single DMFC. The effect of MeNC catalystson the performance of DMFC was investigated.Co-based catalysts have been prepared by pyrolyzing melamine-formaldehyde(MF)aerogel, melamine-formaldehyde resin and prepolymer with cobalt acetate. The effect ofdifferent precursors on ORR catalytic activity of CoNC was investigated. Homogenousspherical carbon structures was formed in melamine-formaldehyde carbon aerogels supportedcobalt catalysts. The largest SBETreached800.17m²Â·g^-1. The CoNC catalysts prepared bymelamine-formaldehyde resin and prepolymer formed amorphous carbon, which SBETwasabout50m²Â·g^-1. MF carbon aerogels supported cobalt catalyst prepared by CO₂supercriticaldrying possessed the best ORR catalytic activity. The onset potential for oxygen reductionalmost was0.49V (vs. SCE), the number of electrons transferred per molecule of oxygen inthe overall reaction n=3.7. For a direct methanol fuel cell system, the maximum output powerdensity reaches39.34mW·cm^-2at65℃. The results of rotating disk electrode(RDE) andsingle cell tests revealed that the order of decreasing activities of the different CoNC catalystswas CoNC-supercritical＞CoNC-prepolymer＞CoNC-xerogel＞CoNC-freeze＞CoNC-resin.A series of CoNC catalysts were synthesized from the prepolymer of melamine-formaldehyde resin and cobalt acetate, then heat-treated at high temperature. The effect ofdifferent heat-treated temperatures (500-900℃) and different cobalt contents(0.5-4%) on their activity for the oxygen reduction reaction (ORR) was examined. SBETand pore valumeof CoNC increased as temperature rise and would be steady at high temperature.Heat-treatment at temperatures higher than500℃could lead to the reduction of cobalt to itsmetallic state. The cobalt particle size increased as heat-treated temperature rise. The cobaltparticle size on the CoNC prepared under700℃were centered around3nm, and would bebigger under900℃. Some cobalt particles were encapsulated by carbon substrate. XPSindicated the presence of Co(0), Co(II) and pyridinic, pyrrolic types of nitrogen atoms at700℃. SBETand pore valume of CoNC decreased with the increasing of cobalt loading. SEMshowed some pores in CoNC were clogged. The cobalt particle size increased with theincreasing of cobalt loading. The resulting catalysts (CoNC) all showed good catalytic activitytoward the ORR and well methanol tolerant ability. The overall electron transfer number forthe catalyzed ORR was determined to be2.8-3.65with little H₂O₂production, suggesting thatthe ORR catalyzed by CoNC catalysts is a mixture of2-and4-electron transfer pathways.The catalyst heat-treated at700℃and Co loadings5.32%yielded the best ORR activity, themaximum output power density reached39.29mW·cm-2, open circuit voltage was0.566V fora direct methanol fuel cell system at75℃.A series of FeNC catalysts were prepared from the prepolymer of melamine-formaldehyde resin and iron(II) acetate by heat-treated at high temperature in argon. Theeffect of different heat-treated temperatures (600-900℃) and different iron loadings (0-1.5%)on FeNC catalytic activities was investigated. SBETand pore valume of FeNC increased astemperature rise and could be steady. Ferric oxide could be reducted to metallic iron atheat-treatment temperatures exceeded600℃. The iron particle size increased as heat-treatedtemperature rise. Fe2p XPS revealed that the iron in FeNC included Fe(0) and Fe(III). TheN1s XPS narrow-scan spectra were deconvoluted into two components, which were assignedto pyridinic nitrogen, pyrrolic nitrogen. With the iron loading increasing, the iron particle sizeincreased, SBETand pore valume of FeNC first increased and then decreased. The RDE/RRDEindicated the FeNC catalysts had good ORR catalytic activity and methanol tolerant ability.The overall electron transfer number for ORR was detected to be2.9-3.6with little H₂O₂production, signifing that the ORR catalyzed by FeNC catalysts is a mixture of2-and4-electron transfer pathways. When the heat-treatment temperature exceeded700℃, ohmicresistance (Rs), interface resistance between electrode and electrolyte (Rc) and electrodereaction resistance (Rct) increased as temperature rise, but inductive reactance decreased. Thecatalyst prepared at700℃andFe loadings1.17%yielded the best ORR activity, in0.5mol·L-1H2SO4+0.2mol·L-1CH3OH, the overall electron transfer number for ORR was3.6, the maximum output power density reached37.67mW·cm-2, open circuit voltage was0.56Vfor a direct methanol fuel cell system at75℃.