The production of cost-effective catalysts for the production of hydrogen by electrolysis of water is important for clean energy production. In this work we report on a study of molybdenum disulfide (MoS2) as catalyst for the hydrogen evolution reaction (HER). Nitrogen doped carbon nanotubes (NCNTs) directly synthesized onto carbon paper have been decorated with MoS2. The electrodes utilize the improved conductivity of the NCNTs and the carbon paper for electron transport, combined with the high catalytic activity of MoS2. The NCNTs were successfully decorated with co-axial nano-flakes of MoS2 by a single step solvothermal process using Dimethylformamide (DMF) and ammonium tetrathiomolybdate. MoS2 was also prepared with alternative methods for comparison. The effects of supporting MoS2 on NCNTs were studied by simulations with density functional theory (DFT). The most active adsorption sites for hydrogen on MoS2 were identified and were on the edges. The catalyst showed competitive activity with other earth-abun- dant catalysts with an onset potential of 170 mV and a small Tafel slope of 40 mV/dec. The improved catalytic activity of HER by having NCNTs as support was confirmed by DFT and experimental results.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22278号 / 理博第4592号 / 新制||理||1659(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 北川 宏, 教授 竹腰 清乃理, 教授 吉村 一良 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Magnetic field effects (MFE) on electrochemical systems have been of interest to researchers for the past 60 years. MFEs on mass transport, such as magnetohydrodynamics and magnetic field gradients effects are reported, but MFEs on electron transfer kinetics have been rarely investigated. Magnetic modification of electrodes enhances electron transfer kinetics under conditions of high concentrations and low physical diffusion conditions, as shown by Leddy and coworkers. Magnetic microparticles embedded in an ion exchange polymer (e.g., Nafion) applied to electrode surfaces. Rates of electron transfer reactions to diffusing redox probes and to adsorbates are markedly enhanced. This work reports MFEs on hydrogen evolution on illuminated p-Si; MFEs on hydrogen evolution on noncatalytic electrodes; a model for MFEs on homogeneous self-exchange reactions; and a convolution based voltammetric method for film modified electrodes. First, a MFE on the photoelectrochemical hydrogen evolution reaction (HER) at p-Si semiconductors is demonstrated. The HER is an adsorbate reaction. Magnetic modification reduces the energetic cost of the HER by 400 - 500 mV as compared to Nafion modified electrodes and by 1200 mV as compared to unmodified p-Si. Magnetically modified p-Si achieves 6.2 % energy conversion efficiency. Second, from HER on noncatalytic electrodes, the MFE on photoelectrochemical cells arises from improved heterogeneous electron transfer kinetics. On glassy carbon electrodes, magnetic modification improves heterogeneous electron transfer rate constant, k₀,for HER 80,000 fold. Third, self exchange reaction rates are investigated under magnetic modification for various temperatures, outersphere redox probes, and magnetic particles. Arrhenius analyses of the rate constants collected from the experiments show a 30 - 40 % decrease in activation energy at magnetically modified electrodes. A kinetic model is established based on transition state theory. The model includes pre-polarization and electron nuclear spin polarization steps and characterizes a majority of the experimental results. Lastly, a convolution technique for modified with uniform films electrodes is developed and coded in Matlab (mathematical software) for simple and straightforward analysis of Nafion modified electrodes.

Molybdenum disulfide (MoS2) has been demonstrated to be a potential catalyst in the hydrogen evolution reaction (HER). Due to its highly active edge site, abundance, and low cost, it rivals Pt. However, the potential activity of the MoS2 basal plane has largely been ignored. The physical characteristics of MoS2 and its corresponding band structure change significantly with decreasing thickness, especially at the monolayer limit. Thus, an investigation on the thickness dependence may provide important insights into the MoS2 basal plane activity. In this thesis, the layer dependent electrocatalytic performance is investigated with mono-, bi- and multilayer MoS2 based individual nanodevices. Three conclusions were reached. (1) Monolayers showed exchange current densities more than one order of magnitude higher than that of the multilayers, 0.12 mA/cm2 and 8.7 mA/cm2, respectively. Furthermore, the onset potential of the monolayer was several hundred millivolts lower than that of the multilayer, about 0.2 V vs RHE for the monolayer versus 0.5 V vs RHE for the multilayer. The Tafel slope of 100-200 mV/dec revealed that the rate limiting step was the adsorption of hydrogen. (2) Interestingly, the bilayer sample exhibited an increase in its exchange current density from 0.3 mA/cm2 to 8 mA/cm2 when cycled extensively. This is suspected to be caused by intercalation of hydrogen between the atomic layers. (3) Additionally, the back-gate voltage is applied to tune the Fermi level of the material and the catalytic performance. It was found that the back-gate voltage induces an irreversible change in all samples, increasing the exchange current density by an order of magnitude. The superior basal plane performance of the monolayers to that of the multilayers reveals a new way to optimize the performance of MoS2 as a HER catalyst. In addition, the results above illuminate the yellow brick road to potential improvements in other layered materials as well.

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Philosophiae Doctor - PhD / The widespread use of fossil energy has been most convenient to the world, while they also cause environmental pollution and global warming. Therefore, it is necessary to develop clean and renewable energy sources, among which, hydrogen is considered to be the most ideal choice, which forms the foundation of the hydrogen energy economy, and the research on hydrogen production and fuel cells involved in its production and utilization are naturally a vital research endeavor in the world. Electrocatalysts are one of the key materials for proton exchange member fuel cells (PEMFCs) and water splitting. The use of electrocatalysts can effectively reduce the reaction energy barriers and improve the energy conversion efficiency.

Conversion of renewable energy resources (such as solar and wind) through water splitting to hydrogen and oxygen has attracted increasing attention. The sole product of hydrogen combustion is water, rendering a carbon-neutral energy cycle. Water splitting consists of two redox half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Both of these two transformations involve multi- electron/proton movement and thus are kinetically sluggish. In order to accelerate the reaction rates for practical application, efficient catalysts are needed. State-of-the-art catalysts for water splitting are usually composed of noble metals, such as platinum, ruthenium, and iridium, whose scarcity and high cost limit their wide employment. Consequently, it is of critical importance to develop competent and non-precious catalysts via low-cost preparation. Owing to the thermodynamic convenience and potential application in proton exchange membrane and alkaline electrolyzers, traditionally, most HER catalysts were developed under strongly acidic conditions while OER catalysts under strongly alkaline conditions. In order to accomplish overall water splitting, the coupling of HER and OER catalysts in the same electrolyte is mandatory. This thesis will summarize our recent efforts towards developing 1st-row transition metal-based sulfides and phosphides for electrocatalytic water splitting under ambient conditions.

The booming worldwide demand for energy and the increasing concerns about global warming due to fossil fuel consumption have urged the development of techniques for storing and converting renewable and clean energy resources. Electrocatlytic or photoelectrocatalytic water splitting to generate green energy carrier H2 with sustainable energy input, like solar, has been regarded as an attractive strategy for carbon-neutral energy needs. However, the sluggish kinetics for both half reactions (HER and OER), high overpotentials and thermodynamic requirements, and H2 and O2 gas crossover have been regarded as the major challenges, which limit its widespread application. On account of high efficiency and fast reaction rate, proton exchange membrane electrolyzer (PEME) has been developed as a mature technology for water splitting under acidic conditions. Nonetheless, it requires noble metals as robust and competent catalysts (like Pt for HER and IrO2 for OER), which is economically unfavorable. Owing to the thermodynamic convenience for OER and the integration of HER and OER in the same electrolyte, anion exchange membrane electrolyzer (AEME) has also been explored under alkaline conditions, utilizing first-row transition metals as bifunctional catalysts. However, for both PEME and AEME, H2 and O2 are generated simultaneously. Even though “gas impermeable” membranes are employed, the formation of H2/O2 mixture is inevitable. So one part of my research introduced a new strategy to couple HER with more thermodynamically favorable biomass-derived upgrading in alkaline solution, which requires lower energy input than overall water splitting and produces more valuable and non-gas products. However, the solubility of biomass-derived organic compounds as well as the competing reaction of water oxidation limits the catalytic current density. Therefore, we further introduce the concept of redox mediator (RM) to divide conventional water splitting into two separate steps. This allows H2 and O2 to be produced at different times as well as in different spaces and reduces the energy input required to conduct a productive step. This strategy not only prevents H2/O2 mixing but also reduces the voltage input as the redox potential of RM+/0 will be within the HER and OER thermodynamic potentials, hence allowing water splitting to be driven by photovoltaic cells with small photovoltage.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20657号 / 理博第4322号 / 新制||理||1621(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 寺西 利治, 教授 島川 祐一, 教授 小野 輝男 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

<p>    </p> <p>Surface functionalization of metallic nanoparticles is an attractive route to tailor the ensemble geometry and redox properties of active sites in heterogeneous catalysts. However, it is challenging to generate well-defined interfaces through conventional impregnation and one-pot colloidal synthesis methods. In this work, we utilize ligand exchange reactions for post synthetic surface modification of colloidal nanoparticles to generate unique core-shell and surface alloy structures. We use halometallate and metal chalcogenide complexes to create surface sites that are active for electrocatalytic hydrogen evolution reaction (HER). </p> <p>We synthesize a self-limiting monolayer of metal chalcogenides on colloidal Au nanoparticles through biphasic ligand exchange reaction between ammonium tetrathiomolybdate (NH₄)₂MoS₄ complex and Au nanoparticles. Through a combination of spectroscopy techniques and computational methods, we show that strong Au-S interactions introduce electronic and geometric distortion to the geometry and bond metrics of MoS₄^2-complex. Moreover, proximal MoS₄ units adsorbed on the Au surface interlink to form small MoSx oligomers with highly active bridging disulfide sites. Consequently, these core-shell AuMoS₄ nanoparticles exhibit significantly higher HER activity than MoS₄^2- supported on non-interacting carbon supports under highly acidic electrolyte conditions. Although post catalysis characterization reveals partial hydrolysis of surface adsorbed MoSx species, stable HER activity under bulk electrolysis condition indicates that active sites remain persistent. </p> <p>In an effort to extend these ligand exchange reactions to create metal/metal interfaces on other coinage metal nanoparticles such as Ag, we design metal-ligand coordination complexes to mitigate undesired galvanic replacement reactions. By varying the strength and number of coordinating ligands, we fine-tune the redox potential of oxidized noble metal precursors and confine the deposition of noble metals to a few surface layers of the Ag nanoparticles. We utilize organic amine and phosphine ligands to generate Ag@AgM core-shell nanoparticles, where M = Pd, Pt, and Au. Surface alloy or pure metal shells of Pd and Pt on Ag nanoparticles generated through this ligand-based strategy exhibited high precious metal atom utilization in electrocatalytic hydrogen evolution reaction. </p>