Carbon-based Nanoarchitectures As Non-precious Metal Bifunctional Electrocatalysts For Oxygen Electrode Reactions

Oxygen electrode electrocatalysts are among the main bottlenecks for the commercialization of fuel cells and water electrolysis technologies. At present, Pt and its alloys are still the irreplaceable electrocatalysts for oxygen reduction reaction (ORR) at the cathodes of the fuel cells, and RUO2and IrO2are the best electrocatalysts for oxygen evolution reaction (OER) at the anodes of water electrolysis devices. In recent years, nanoncarbons doped with various metals and non-metals are intensively explored as oxygen electrode electrocatalysts. This thesis work aimed at developing Fe-N doped nanocarbon composites of the graphene, carbon nanotube and carbon black and investigating their bifunctional electrocatalytic properties for oxygen electrode reactions. The main contents and results are summarized as follows.1. Carbon nanospheres/carbon nanotubes/graphene sheets composite catalystsFe-N doped carbon nanotube/graphene sheet (FeN-CNT/GS) binary composite have been prepared by high temperature treatment of a precursor mixture of graphene oxide (GO) and melamine in the present of Fe salts, resulting in carbon nanotubes (CNTs) with diameters from28to41nm growing on graphene sheets. During the high temperature treatment, the reduction and heterotope atoms doping of GO are accompanied by the nucleation and growth of CNTs on the reducing GO sheets. The in-situ growth of CNTs on GS can prevent the aggregation and stacking of GS.Ternary Fe-N doped cabon nanospheres/carbon nanotubes/graphene sheets (FeN-CS/CNT/GS) by introducing CS into the precursor mixture so that they can intercalated between GO sheets, which results in formation of CNTs with more uniform diameter around40nm and further improved electrochemical active surface area.The binary FeN-CNT/GS exhibits ORR and OER activities significantly higher than the two moieties of FeN-CNT and N-GS, while the FeN-CS/CNT/GS ternary composite delivers further enhanced activity over the binary FeN-CNT/GS. For ORR, FeN-CS/CNT/GS shows activity close to the commercial Pt/C in acidic medium with only-40mV gap in half-wave potential and superior to Pt/C in alkaline medium, as well as superior stability at the constant potential and methanol tolerance. For OER, the activities of FeN-CS/CNT/GS are approaching to the commercail IrO2in the acidic and alkaline media.Combined XPS, XRD and Mossbauer characterization show various Fe-Nx species in FeN-CS/CNT/GS, FeN-CNT/GS, FeN-CNT/CS and FeN-CNT materials. The total contents of Fe-N4and C-Fe-N2species are found to be～60%and～75%respectively in FeN-CNT/GS and FeN-CS/CNT/GS, but only～10%in FeN-CNT. However, the contents of the inactive elemental Fe and Fe-C species in FeN-CNT is as high as～70%. Furthermore, reasonable linear relationships of the activities of ORR represented by the dynamic current density (ik) at0.85V (vs. RHE) and OER represented by the current values (ik) at1.8V (vs. RHE) with the percentage amounts of the possible active FeN4and C-Fe-N2species have been found for the corresponding catalysts, suggesting that FeN4and C-Fe-N2are the active sites for the Fe-N doped non-precious carbon catalysts.By replacing Fe with Co, Ni and Mn respectively, various metal-N doped binary and ternary carbon composites are prepared and their electrocatalytic abilities toward ORR and OER are investigated. It is found that that the capability of metals in catalyzing CNT formation follows a trend of Fe>Co>Mn≈Ni, the ORR activities of the corresponding carbon composites follows a trend of Fe>Co>Mn>Ni, and the OER activities follows a trend of Fe>Mn>Ni>Co.2. The iron phthalocyanine based composite carbon catalystsInspired by the above implication that Fe-N4structure might be the active sites, we expect that improved catalytic activity can be achieved by highly dispersing the Fe-N4contained ion phthalocyanine (FePc) on carbon materials of high surface areas. A series of FePc-based composites are synthesized by ball-milling the mixtures of FePc with various carbon blacks and graphenes in ethanol. Stacked nanorods ca.100nm in length are obtained by ball-milling the FePc alone, while nanorods of～50nm in length uniformly dispersed on graphene sheets are obtained when ball-milling mixtures of FePc and graphene sheets. Upon replacing graphene sheets with carbon black, a novel core-shell nanostructure with FePc layer as shell can be constructed.UV-Vis and XPS results indicate that the interaction between FePc and the carbon support is found to be enhanced with the dispersion of FePc. Electrochemical measurements show that a weight ratio of1:1of FePc and carbon materials gives the most optimized electrocatalytic activity of the prepared composite. The EC600@FePc and FePc/GS composites show superior catalytic activity, stability and methanol tolerance to the commercial20%Pt/C at the same loading for ORR, with the mass and specific activities being-5and-2.2times higher at the potential of0.9V (vs. RHE). In addition, these FePc-based composites exhibit considerable OER electrocatalytic activities.H2-O2alkaline polymer fuel cells respectively with the as-prepared EC600@FePc and the commercial20%Pt/C as cathode catalysts are constructed at the same catalyst loading of100μg/cm2. The EC600@FePc single cell exhibits a peak power density of112mW/cm at the current density of300mA/cm, which is higher than that of84mW/cm2for Pt/C single cell.3. Polyaniline/graphene composite catalystsUsing micelles formed by various surfactant self-assemblies in water as soft-template, polyaniline (PANI) of a variety morphologies, such as the1-D semi-nanotube, nanorods and nanowires, as well as irregular nanoparticles. The semi-nanotubes possess uniform diameters of-80nm with dentations of-10nm grown on the skin. These morphologies can be maintained in high-temperature treatment. The optimized Fe-N doped composite shows good ORR activity and the considerable OER activity in alkaline medium.