Design, Performance And Regulation Mechanism Of Novel Non-Precious Metal Electrocatalysts For Oxygen Reduction Reaction In Acidic Medium

Fuel cell represents one of the most promising power sources for automotive transportation with the advantages of high-efficient energy output and low environmental impact. Compared with the alkaline fuel cells that suffer from the unavoidable reaction between the electrolyte and omnipresent CO2, the proton exchange membrane fuel cells (PEMFCs) are naturally CO2 tolerant with great convenience of oxygen supply directly from air, thus are closer to practical applications. Now the biggest challenge for large-scale applications of PEMFCs is the scarce resource, expensive cost and limited operational stability for the commercial Pt-based catalysts. Therefore, developing non-precious metal (NPM) electrocatalysts to replace Pt, especially the ones for cathodic oxygen reduction reaction (ORR) with sluggish kinetics, is of particular significance. Based on the state of the art progress in the field of NPM materials as well as the research basis in our own group, this dissertation concentrates on the design, synthesis, characterization and ORR performance of the novel carbon based NPM electrocatalysts including alloyed cobalt-molybdenum nitride (CoxMo1-xNy), iron/nitrogen/carbon (Fe/N/C), ⅧB metal nitrides/carbides, sulfur and nitrogen codoped carbon tubes (SNCT). The main contents of this dissertation are summarized as follows:1. Alloyed Co-Mo nitride electrocatalyst supported on nitrogen-doped carbon nanocages (NCNCs) which combines the merits of cobalt nitride and molybdenum nitride, shows the high activity comparable to cobalt nitride and the progressively enhanced stability with the increasing of the Mo ratio. The typical Coo.5Moo.5Ny/NCNCs catalyst demonstrates excellent ORR performance in acidic medium with high onset potential of 808 mV (vs. RHE), superior stability (>80% retention after 100 h continuous testing in 0.5 mol L-1 H2SO4), a dominant four-electron catalytic process and good immunity to methanol crossover. Together with the convenient and scalable preparation as well as the low cost, the alloyed Co-Mo nitride electrocatalyst shows great potential in application for fuel cells. This study also suggests a promising strategy to develop NPM ORR electrocatalysts in acidic medium, i.e., to construct the alloyed compounds by combining the substances with respective high activity and high stability.2. A new MnOx-induced strategy is designed to construct the Fe/N/C with highly exposed Fe-Nx active sites. This strategy involves the homogeneous dispersion of MnOx nanoparticles on the hierarchical N-doped carbon nanocages due to nitrogen participation, the subsequent uniform spreading of polyaniline by a MnOx reactive-template polymerization, followed by the successive iron incorporation and polyaniline pyrolysis. The obtained Fe/N/C demonstrates an excellent ORR performance including an onset potential of 920 mV (vs. RHE), four electron selectivity, superb stability and immunity to methanol crossover. The excellent performance is well correlated with the greatly enhanced surface active sites of the catalyst stemming from the unique MnOx-induced strategy. This study provides an efficient approach for exploring the advanced ORR electrocatalysts by increasing the exposed active sites.3. VDIB metal nitrides supported on NCNCs and carbon nanocages (CNCs) present the different ORR activities in acidic medium. By systematic comparison on their ORR activities prepared with or without nitrogen source, we found that only the former is highly active for ORR while the latter is quite poor despite their similar phases. This result indicates that the M-Nx (M=Fe, Co) related species play crucial role in the high ORR activity of metal-based catalysts, similar to the case of Fe/N/C catalysts. Density functional theory calculations demonstrate that Fe-N4/C moieties are responsible for the high ORR activity, far superior to the cases for Fe2N and Fe3C. The experimental and theoretical results mutually support that the high activities of the Fe-based catalysts originate from Fe-Nx/C moieties rather than Fe2N or Fe3C phases, which is significant for exploring advanced Fe-based electrocatalysts for ORR.4. A new route is applied to prepare S and N codoped carbon tubes by annealing the mixture of cysteine and amorphous carbonaceous tubes prepared by the MnOx template. In acidic medium, the optimized SNCT demonstrates excellent ORR performance with a record high onset potential of 851 mV (vs. RHE), dominant 4e" process and superior stability. By excluding the interference of Mn impurities, the excellent ORR activity of the SNCT catalyst could be attributed to the unique S and N codoping configurations stemming from our route. The progress in this study suggests the great potential of carbon-based metal-free electrocatalysts in acidic medium.