| Oxygen reduction reaction（ORR） electrocatalysis is an extremely important research topic owing to its practical applications in electrochemical energy devices such as fuel cells and metal–air batteries. Until now, platinum（Pt）-based catalysts including Pt alloys have been widely used as the mostefficient catalysts to catalyze the inherently sluggish ORR. The high cost of Pt together with its limited natural abundance has however hindered the widespread commercial success of these electrochemical energy technologies. Significant efforts have focused on the development of alternative non-Pt catalysts that are based on non-precious metals and/or various heteroatom-doped carbonaceous materials. Among the catalysts studied, M-Nx/C catalysts have been explored as promising candidates due to their high activity, good stability and excellent methanol tolerance, as well as abundant cheap precursors. However, their catalytic activities in acidic medium still need further improvement, particular for use in proton-exchange membrane（PEM） fuel cells. In this thesis, we have designed and prepared a series of non-precious metal M-Nx/C and studied their electrochemical activities towards ORR in both alkaline and acidic medium. TEM, low temperature nitrogen absorption/desorption, XRD, EDX, Raman and XPS are used to characterize the catalysts’ structure and compositions. The kinetic of ORR has been studied using CV, RDE and RRDE techniques.We try to find the key parameters that affect ORR and the possible structure of active sites. The main points obtained are summarized as follows:（1） We have developed an efficient ORR catalyst based on nitrogen doped porous graphene foams through a hard templating approach using dicyandiamide as nitrogen precursors. The obtained catalyst exhibits both remarkable ORR activity and long term stability in both alkaline and acidic solutions. The ORR onset potential of PNGF is 1.02 V vs. RHE and limiting current density is 7 mA cm-2 in alkaline solution. Its ORR activity is even better than that of the Pt-based catalyst. In acidic solution, the onset potential and limiting current density of PNGF is 0.83 V vs. RHE and 7.5 mA cm-2, respectively. PNGF also exhibits an ORR process that occurs mainly through a 4-electron transfer pathway in both alkaline and acidic medium. The electrochemical poisoning tests indicate that the residual iron in the catalysts may participate in the ORR in acidic medium, while it has little influence on the ORR performance of the catalysts in alkaline medium. In addition, the PNGF catalyst also exhibits a high selectivity for the ORR with excellent tolerance to methanol poisoning effects and high stability in both alkaline and acidic solution. The outstanding electrochemical performance is due to the unique 3-D macroporous morphology and a high surface area. The introduction of silica during the fabrication process can effectively decrease the aggregation of graphene sheets and thus increase the surface area. The large surface area and porous structure have an advantage of active sites exposure and rapid transportation of electro-reactants/products.（2） Nitrogen precursors and the type of transition metal precursors play an important role in the catalysts’ performance. By selecting proper nitrogen precursors（cyanamide, melamine and urea）, hierarchical porous N-doped graphene foams（HPGFs） functioned by a transition metal were successfully prepared using silica nanoparticles as a template. The effects of transition metal precursors on the performance of HPGFs are also investigated. The HPGF-1 prepared with cyanamide and FeCl2 as precursors exhibits the best elecrocatalytic activity, even better than that of PNGF. HPGF-1 has a high onset potential of 1.03 V and large diffusion-limiting current density of 9 mA cm-2 in alkaline solution, better than that of commercial Pt/C catalyst. In acidic solution, the onset potential and diffusion-limiting current density of PNGF is 0.81 V vs. RHE and 10 mA cm-2, respectively. The large diffusion-limiting current density is benefit from the high surface area and appropriate porous structure. HPGF-1 has high energy conversion efficiency with 4epathway dominated ORR process in both alkaline and acidic solution. Moreover, HPGF-1 outperforms commercialPt/C catalyst in methanol tolerance and long term stability. With a proton exchange membrane, the fuel cell had an open circuit voltage of 0.83 V. A peak power density of 21.6 mW cm-2 was achieved. Upon integration into zinc-air batteries, very inspiring cell performance was observed. The open-circuit voltage（OCV） of the single cell with HPGF-1 as the cathode catalyst was 1.40 V and a peak power density of 327.5 mW cm-2 was obtained.（3） Furthermore, heteroatoms（B and P） doped porous graphene foams was investigated. With orthoboric acid as boron precursor, triphenylphosphine as phosphorus precursor, cyanamide as nitrogen precursor, hierarchical porous doped graphene foams（HPGFs） functioned by a transition metal were successfully prepared through a hard templating approach using silica nanoparticles as a template. It was found that the added transition metal salt can significantly improve the ORR performance of doped graphene catalysts, especially in acidic solution. The ORR catalytic activity can also be enhanced by binary and ternary doping. However, compared with nitrogen doped porous graphene catalysts, the obtained materials exhibit lower ORR catalytic activities. Exploiting synergistic effects still remains a difficult challenge.（4） A facile and economical method to large scale production of non-precious metal catalysts was reported. With N,N’-bis（salicylidene）ethylenediamine（salen） as a new nitrogen precursor, CoSO4 as transition metal precursor and Vulcan as carbon support, we synthesize Co-N-S/C catalysts through a simple solid-sate reaction. The effect of heat-treat temperature on the ORR performance of Co-N-S/C catalysts has been discussed. High temperature pyrolysis has a significant influence on the ORR performance. Heat treatment can decompose the Co-salen complex to metallic Co, which then agglomerates to form large particle sizes when the temperature further increases. The main form of metal in the catalysts is Co and Co9S8. Co-N-S/C-700 shows the best ORR activity. Co-Nx may be the active sites in the catalysts and it can be destroyed above 800 oC. Graphitic N may also act as theORR active site and contribute to higher catalytic activity. The overall electron transfer number for the catalyzed ORR was determined to be3.63.9, with 3.719.9% H2O2 production over the potential range, suggesting thatthe ORR catalyzed by Co-N-S/C catalysts is dominated by a 4-electron transfer pathway from O2 to H2 O. In addition, these catalysts exhibit superior methanol tolerance to commercial 40% Pt/C catalyst, thus the Co-N-S/C catalysts are promising for use as electrocatalysts in direct methanol fuel cells. |
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