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

School of Molecular Sciences(Chemistry)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.ope

In situ-generated metal oxide catalyst during CO oxidation reaction transformed from redox-active metal-organic framework-supported palladium nanoparticles

The preparation of redox-active metal-organic framework (ra-MOF)-supported Pd nanoparticles (NPs) via the redox couple-driven method is reported, which can yield unprotected metallic NPs at room temperature within 10 min without the use of reducing agents. The Pd@ra-MOF has been exploited as a precursor of an active catalyst for CO oxidation. Under the CO oxidation reaction condition, Pd@ra-MOF is transformed into a PdOx-NiOy/C nanocomposite to generate catalytically active species in situ, and the resultant nanocatalyst shows sustainable activity through synergistic stabilization.open4

Ordered mesoporous Co3O4 spinels as stable, bifunctional, noble metal-free oxygen electrocatalysts

We report the use of noble metal-free ordered mesoporous Co 3O4 spinels (meso-Co3O4), templated from KIT-6 mesoporous silica, as highly active and stable bifunctional electrocatalysts for both oxygen evolution and reduction reactions (OER and ORR, respectively). The meso-Co3O4 nanostructures showed high activity for OER in an alkaline medium (0.1 M KOH), which makes them comparable to the most active Ir/C catalyst and better than Co3O4 nanoparticles (NPs) and the Pt/C catalyst. Furthermore, meso-Co 3O4 exhibited enhanced stability, compared to Co 3O4 NPs. The enhanced activity and stability of meso-Co3O4 over Co3O4 NPs could be attributed to its high surface area and structural stability of the gyroid network structure in the meso-Co3O4 catalysts. The meso-Co3O4 nanostructures also showed promising activity for ORR and exhibited a methanol-tolerance superior to the Pt/C catalyst. The total overpotential of meso-Co3O4 for OER (at 10 mA cm-2) and ORR (at -3 mA cm-2) was 1.034 V, which is on a par with noble metal-based catalysts. This work demonstrates that directing metal oxides into mesostructures is a promising means of preparing highly active, stable, bifunctional oxygen electrocatalysts that can potentially replace expensive noble metal-based catalysts. This design strategy can be extended to other reactions relevant to energy conversion and storage applications.close17

Expression of Keratin 10 in Rat Organ Surface Primo-vascular Tissues

AbstractThe primo-vascular system is described as the anatomical structure corresponding to acupuncture meridians and has been identified in several tissues in the body, but its detailed anatomy and physiology are not well understood. Recently, the presence of keratin 10 (Krt10) in primo-vascular tissue was reported, but this finding has not yet been confirmed. In this study, we compared Krt10 expression in primo-vascular tissues located on the surface of rat abdominal organs with Krt10 expression on blood and lymphatic vessels. Krt10 protein (approximately 56.5 kDa) was evaluated by western blot analysis and immunohistochemistry. Krt10 (IR) in the primo-node was visualized as patchy spots around each cell or as a follicle-like structure containing a group of cells. Krt10 IR was also identified in vascular and lymphatic tissues, but its distribution was diffuse over the extracellular matrix of the vessels. Thus Krt10 protein was expressed in all three tissues tested, but the expression pattern of Krt10 in primo-vascular tissue differed from those of blood and lymphatic vascular tissues, suggesting that structural and the regulatory roles of Krt10 in primo-vascular system are different from those in blood and lymphatic vessels

Highly interconnected ordered mesoporous carbon-carbon nanotube nanocomposites: Pt-free, highly efficient, and durable counter electrodes for dye-sensitized solar cells

We report the preparation of highly interconnected ordered mesoporous carbon-carbon nanotube nanocomposites which show Pt-like dye-sensitized solar cell (DSSC) efficiency and remarkable long-term durability as DSSC counter electrodes.close413

Ordered mesoporous carbon-carbon nanotube nanocomposites as highly conductive and durable cathode catalyst supports for polymer electrolyte fuel cells

Ordered mesoporous carbon-carbon nanotube (OMC-CNT) nanocomposites were prepared and used as catalyst supports for polymer electrolyte fuel cells. The OMC-CNT composites were synthesized via a nanocasting method that used ordered mesoporous silica as a template and Ni-phthalocyanine as a carbon source. For comparison, sucrose and phthalocyanine were used to generate two other OMCs, OMC(Suc) and OMC(Pc), respectively. All three carbons exhibited hexagonally ordered mesostructures and uniform mesopores. Among the three carbons the OMC-CNT nanocomposites showed the highest electrical conductivity, which was due to the nature of their graphitic framework as well as their lower interfacial resistance. The three carbons were then used as fuel cell catalyst supports. It was found that highly dispersed Pt nanoparticles (ca. similar to 1.5 nm in size) could be dispersed on the OMCs via a simple impregnation-reduction method. The activity and kinetics of the oxygen reduction reaction (ORR), measured by the rotating ring-disk electrode technique revealed that the ORR over the Pt/OMC catalysts followed a four-electron pathway. Among the three Pt/OMC catalysts, the Pt/OMC-CNT catalyst resulted in the highest ORR activity, and after an accelerated durability test the differences in the ORR activities of the three catalysts became more pronounced. In single cell tests, the Pt/OMC-CNTbased cathode showed a current density markedly greater than those of the other two cathodes after a high-voltage degradation test. These results were supported by the fact that the Pt/OMC-CNT-based cathode had the lowest resistance, which was probed by electrochemical impedance spectroscopy (EIS). The results of the single cell tests as well as those of the EIS-based measurements indicate that the rigidly interconnected structure of the OMC-CNT as well as their highly conductive frameworks are concomitantly responsible for the OMC-CNT nanocomposites exhibiting higher current density and durability than the other two carbons.close17

Ultrathin titania coating for high-temperature stable SiO2/Pt nanocatalysts

The facile synthesis of silica supported platinum nanoparticles with ultrathin titania coating to enhance metal-support interactions suitable for high temperature reactions is reported, as thermal and structure stability of metal nanoparticles is important for catalytic reactions.close8

Ordered mesoporous porphyrinic carbons with very high electrocatalytic activity for the oxygen reduction reaction

The high cost of the platinum-based cathode catalysts for the oxygen reduction reaction (ORR) has impeded the widespread application of polymer electrolyte fuel cells. We report on a new family of non-precious metal catalysts based on ordered mesoporous porphyrinic carbons (M-OMPC; M = Fe, Co, or FeCo) with high surface areas and tunable pore structures, which were prepared by nanocasting mesoporous silica templates with metalloporphyrin precursors. The FeCo-OMPC catalyst exhibited an excellent ORR activity in an acidic medium, higher than other non-precious metal catalysts. It showed higher kinetic current at 0.9a�...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 atom and FeCo-OMPC compared to Pt/C. This effect and high surface area of FeCo-OMPC appear responsible for its significantly high ORR activity.open251

Alveolar Macrophages Treated With Bacillus subtilis Spore Protect Mice Infected With Respiratory Syncytial Virus A2

Respiratory syncytial virus (RSV) is a major pathogen that infects lower respiratory tract and causes a common respiratory disease. Despite serious pathological consequences with this virus, effective treatments for controlling RSV infection remain unsolved, along with poor innate immune responses induced at the initial stage of RSV infection. Such a poor innate defense mechanism against RSV leads us to study the role of alveolar macrophage (AM) that is one of the primary innate immune cell types in the respiratory tract and may contribute to protective responses against RSV infection. As an effective strategy for enhancing anti-viral function of AM, this study suggests the intranasal administration of Bacillus subtilis spore which induces expansion of AM in the lung with activation and enhanced production of inflammatory cytokines along with several genes associated with M1 macrophage differentiation. Such effect by spore on AM was largely dependent on TLR-MyD88 signaling and, most importantly, resulted in a profound reduction of viral titers and pathological lung injury upon RSV infection. Taken together, our results suggest a protective role of AM in RSV infection and its functional modulation by B. subtilis spore, which may be a useful and potential therapeutic approach against RSV

Superior patient survival for continuous ambulatory peritoneal dialysis patients treated with a peritoneal dialysis fluid with neutral pH and low glucose degradation product concentration (Balance)

BACKGROUND: In recent years, laboratory and clinical research has suggested the need for peritoneal dialysis fluids (PDFs) that are more biocompatible than the conventional PDFs commonly used today. Bioincompatibility of PDF has been attributed to low pH, lactate, glucose, glucose degradation products (GDPs), and osmolality. PDFs with neutral pH and low GDPs are now available commercially. In vitro and early clinical studies suggest that these solutions are indeed more biocompatible but, as of now, there is no evidence that their use improves patient outcome. METHODS: Using a dedicated database of over 2000 patients treated with PD in Korea, we were able to conduct a retrospective observational study comparing outcomes for incident continuous ambulatory PD patients treated with a standard, conventional, heat-sterilized PDF to the outcomes for patients treated with a novel, low GDP, neutral-pH PDF prepared in a dual-compartment, double-bag PD system (Balance; Fresenius Medical Care, St. Wendel, Germany). In an intention-to-treat analysis, patient and technique survival, peritonitis-free survival, and peritonitis rates were compared in 611 patients treated with Balance for up to 30 months and 551 patients with a standard PDF (stay . safe; Fresenius Medical Care) treated in the same era and with equivalent follow-up. RESULTS: The patients were well matched for most relevant characteristics except older age distribution for the patients treated with the standard PDF. Patients treated with Balance had significantly superior survival compared to those treated with the standard PDF (74% vs 62% at 28 months, p = 0.0032). In a multivariate Cox regression model including age, diabetes, and gender, the survival advantage persisted (relative risk of death for Balance 0.75, 95% confidence interval 0.56 - 0.99, p = 0.0465). Modality technique survival was similarin Kaplan-Meieranalysis for both PDFs. No differences were detected in peritonitis-free survival or in peritonitis rates between the two solutions. CONCLUSION: This study, for the first time, suggests that treatment with a novel biocompatible PDF with low GDP concentration and neutral pH confers a significant survival advantage. The exact mechanisms for such a survival advantage cannot be determined from this study. The usual criticisms of observational studies apply and the results reported here strongly warrant the undertaking of appropriately designed, randomized, controlled clinical trials