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

Abstract

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 structure and the highly conductive frameworks of the OMC-CNT nanocomposites were concomitantly responsible for their enhanced durability and single cell performance. The second part describes our approach to develop metal-free electrocatalysts for the ORR. A recent study showed that nanostructured carbon materials doped with a variety of heteroatoms have promising ORR activity, yet understanding of the underlying working principles of these materials has been limited to theoretical prediction. In this regard, we prepared a series of heteroatom-doped OMCs for a systematic study on the dopant effects in the ORR. The triple-doped N,S,O-OMC exhibited superior catalytic activity and reaction kinetics in the ORR in an alkaline medium when compared with the dual-doped (N,O-OMC and S,O-OMC) and the mono-doped (O-OMC) OMC catalysts. We found a systematic variation in the work functions, measured by surface-sensitive Kelvin probe force microscopy, depending on the type of dopant used. Significantly, the work functions of these heteroatom-doped OMCs displayed a strong correlation with the activity and reaction kinetics for the ORR. The last part addresses the transition metal and nitrogen-doped OMCs as high-performance catalysts for fuel cell and metal-air battery applications. We developed a new type of non-precious metal catalyst based on ordered mesoporous porphyrinic carbons (M-OMPC, M = Fe and/or Co) with high surface areas and tunable pore structures, which were prepared by nanocasting OMS templates with metalloporphyrin precursors. The FeCo-OMPC catalyst exhibited excellent ORR activity in an acidic medium, higher than those of other non-precious metal catalysts. It showed a higher kinetic current at 0.9 V than Pt/C catalysts, as well as superior long-term durability and MeOH-tolerance. Density functional theory calculations in combination with extended X-ray absorption fine structure analysis revealed a weakening of the interaction between oxygen atoms and FeCo-OMPC compared to Pt/C. This effect and the high surface area of FeCo-OMPC appear to be responsible for its significantly high ORR activity. We extended our approach to develop a new type of non-precious metal electrocatalyst using macrocyclic compounds as precursors. Nanocasting of OMS by the use of Ni- and Fe- phthalocyanine precursors yielded graphitic nanoshell-embedded mesoporous carbon (GNS/MC) nanohybrids. The GNS/MC exhibited very high activity and durability for the oxygen evolution reaction (OER) and ORR in an alkaline medium. The oxygen electrode activity of the GNS/MC was as low as 0.72 V, which represents one of the best performances among non-precious metal bifunctional oxygen electrocatalysts. The GNS/MC also exhibited very high long-term durability for the OER and ORR. The high electrocatalytic performance of the GNS/MC can be ascribed to the contributions of residual transition metal (Ni and Fe) entities, nitrogen-doped defect-rich graphitic nanoshells, and the high surface area of the mesoporous structure. Significantly, in aqueous Na-air battery tests, the GNS/MC-based cell exhibited superior performance to Ir/C- and Pt/C-based cells and demonstrated the first example of a rechargeable aqueous Na-air battery.