Design and Preparation of Electrocatalysts Based on Ordered Mesoporous Carbons for Oxygen Reduction Reaction

The research presented in this dissertation is aimed at the development of electrocatalysts for the oxygen reduction reaction (ORR) based on ordered mesoporous carbons (OMCs). The ORR is a key reaction in electrochemical energy devices such as fuel cells and metal-air batteries. Because of its sluggish kinetics compared to its counterpart reaction (i.e., hydrogen oxidation reaction in fuel cells), ORR needs to be catalyzed by a precious metal such as platinum, to achieve favorable reaction kinetics. However, the high cost and scarcity of Pt limit the large-scale application of these systems. Therefore, tremendous efforts have been devoted to developing highly active, cost-effective electrocatalysts for the ORR. In this regard, this thesis presents multiple approaches to develop efficient electrocatalysts based on OMCs, from supported Pt catalysts to heteroatom-doped, non-precious metal catalysts. The first part of this thesis presents OMC-supported platinum catalysts for the ORR. We investigated the effect of different framework structures of OMCs on the activity and durability for the ORR by comparing the electrochemical behaviors of Pt nanoparticle catalysts supported on these different OMC supports. For this purpose, three representative OMCs were used as support materials: CMK-3, CMK-3G, and CMK-5. These OMCs with the same hexagonal mesostructure have different carbon frameworks and graphiticities, which can affect their surface areas and microporosities. Pt/CMK-3G exhibited the highest electrochemically active surface area, kinetic current density, mass activity, and half-wave potential, whereas Pt/CMK-3 showed the lowest values. Pt/CMK-3G also showed the highest ORR activity after an accelerated durability test, with a minimal shift in half-wave potential. The higher ORR activity of Pt/CMK-3G is attributed to the formation of highly crystalline Pt particles as well as its highly graphitic, crystalline carbon structure, which causes the weak adsorption of surface oxides and a strong interaction between the Pt particles and the support. In addition to investigation of the effect of different framework structures of OMCs on the performance in the ORR, we developed highly conductive and durable OMC-based nanocomposites. Ordered mesoporous carbon-carbon nanotube (OMC-CNT) nanocomposites, were synthesized via a nanocasting method that used ordered mesoporous silica (OMS) as a template and Ni-phthalocyanine as a carbon source. For comparison, two OMCs with varying degrees of conductivity, OMC(Suc) and OMC(Pc), were also prepared using sucrose and phthalocyanine, respectively. Among the three Pt/OMC catalysts, the Pt/OMC-CNT catalyst showed activity that was superior to those of the Pt/OMC(Suc) and Pt/OMC(Pc) catalysts. This trend was even more pronounced after accelerated durability tests (ADTs), which were performed to test the durability of the catalysts. In single-cell tests that are more relevant with respect to the practical applications, the Pt/OMC-CNT catalyst showed a current density that was higher than those of the other two catalysts after high-voltage degradation tests. The half-cell and single-cell tests using the Pt/OMC catalysts indicated that the rigidly interconnected