School of Energy and Chemical Engineering (Chemical Engineering)ope

Baek, Du San

School of Energy and Chemical Engineering (Chemical Engineering)Environmental issues induced by fossil-fuel-based energy have emerged as the most important issue to be solved. Hence, clean and renewable energy technologies have played pivotal roles in complying with environmental regulations while reducing reliance on fossil fuels. They include renewable energy cycles using the production, transportation, and exploitation of hydrogen (H2), the so-called H2 economy, and the development and commercialization of eco-friendly H2 production technologies using water electrolysis. Alkaline water electrolyzers (AWEs) can produce H2 in larger scales compared to polymer electrolyte membrane water electrolyzers. They can also be employed in the chlor-alkali process using unlimited seawater. However, in the case of the hydrogen evolution reaction (HER), the half-reaction of the AWE, the kinetics for the alkaline HER is significantly lower than that of the acidic HER, which reduces the efficiency of the entire water electrolyzer system. Thus, this dissertation presents the development of catalysts for highly efficient alkaline HER using ordered mesoporous metal carbides and carbons.

In this work, thermodynamically metastable ordered mesoporous molybdenum carbide (MMC) with alpha-phase (a-MoC1???x) was generated using the chemical vapor deposition (CVD) method of methane gas. The a-MoC1???x has a higher water dissociation capability than the beta-phase (b-Mo2C) of the thermodynamically stable molybdenum carbide. It has also recently been in the spotlight for heterogeneous catalysis, such as alkaline HER, water-gas shift reaction, and aqueous methanol-reforming reaction, which require high water dissociation capability. In Chapter 2, we identified activity descriptors and recent descriptor-guided nanoscale catalyst design strategies for alkaline HER. Through investigation of recent efforts on alkaline HER, the hydrogen binding energy (HBE), water dissociation capability, water reorganization energy, and hydroxide binding energy were identified as the four important factors for alkaline HER activity. These descriptors are interconnected; therefore, the hydrogen and hydroxide binding energies and water dissociation capability should all be considered. Based on the understanding of activity descriptors, we investigated new design strategies for high-performance alkaline HER catalysts. Among them, interfacing two catalytic components is the most prevalent strategy for improving the alkaline HER activity; it involves interfacing one catalytic material that exhibits optimal HBE with another that has high water dissociation ability.

In Chapter 3, a structure-performance relationship was established in the alkaline HER by controlling the number of graphitic carbon layers present on the surface of MMC. The carbon overlayer formed on the surface of the MMC increased proportionally with carburization temperature and decreased with H2 post-annealing. As a result of H2 post-annealing, the MMC-H2 showed significantly increased alkaline HER activity. Using XRD, TEM, XAS, and XPS analyses, we confirmed that only the partial carbon layers on the MMC were removed while maintaining the chemical and structural properties of the MMC parts.

In Chapter 4, highly efficient alkaline HER catalysts were synthesized using MMC as a catalytic support for Pt and Ir nanoparticles (NPs). First, the higher water dissociation capability of a-MoC1???x compared to b-Mo2C was investigated by scrutinizing their electrochemical behavior and theoretical calculation results. Next, Pt NPs were highly dispersed onto the MMC (Pt/MMC) using the impregnation-reduction method of the Pt precursor. The synthesized Pt/MMC catalyst showed higher mass and specific activity for alkaline HER than commercial Pt/C catalysts, and the excellent performance was demonstrated using XPS and XAS analyses. In addition, the MMC support was used as a support for Ir catalysts (Ir/MMC), confirming the generally applicability of the synergistic effect of MMC for alkaline HER.

In Chapter 5, we developed a new synthetic method for 3D ordered mesoporous tubular graphene-like carbon (OMGC) through selective segregation of the surface carbon layers of MMC. In metal carbide synthesis using the CVD method, carbonaceous gases, such as methane, are first decomposed to generate carbon at high temperatures. The generated carbon is dissolved into metal precursors to form carbide phases. However, owing to limited interstitial sites, an excess amount of carbon is segregated onto the carbide surfaces, generating carbon layers. In this study, the carburization and in situ carbon generation were conducted inside a confined condition of a mesoporous silica template, thereby generating ordered mesoporous tubular graphene-like carbon with a 3D interconnected structure. In addition, the general applicability of the synthetic method was explored by adjusting various carburization conditions. The OMGC showed 80 times higher electrical conductivity than previously synthesized ordered mesoporous carbons (OMCs), and had a high pore volume and specific surface area.

In Chapter 6, the OMGC was used as a catalytic support for the Ru NP catalyst (Ru/OMGC) in the alkaline HER. Ru catalysts are cheaper than Pt catalysts, and have excellent water dissociation capability owing to their high oxophilicity, which facilitates the initiation step of alkaline HER. The Ru/OMGC catalyst showed remarkable activity and kinetics for the alkaline HER. In addition, the Ru/OMGC catalyst was used as a cathode catalyst for the water electrolyzer single-cell, which exhibited higher activity compared to commercial Ru/C and Pt/C and other Ru/OMC catalysts.

In Chapter 7, the size effect of Ru NPs on the alkaline HER was investigated. The size effect of NP catalysts is a powerful tool for infering major active sites for catalytic reactions. Ru NP catalysts with sizes ranging from 0.6 to 4.1 nm were prepared by controlling their Ru contents in the Vulcan carbon support (Ru/VC). The size effect could be exclusively determined through TEM, XRD, and XPS analyses of the synthesized Ru/VC catalysts. The mass and specific activity of the alkaline HER gradually increased as the Ru size decreased from 4 to 1 nm; however, the activity decreased at the sub-nanometer scale. Thus, we deduced that the corner sites of the Ru NPs were the major active sites for the alkaline HER.ope

Ordered Mesoporous Metal Carbide-Derived Electrocatalysts for the Alkaline Hydrogen Evolution Reaction

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