Fuel cell, as an alternative green power source for automobiles and portable electronics, has attracted worldwide attention due to its desirable properties such as high energy density and low greenhouse gas emission. Despite great progress in the past decades, several challenges still remain as obstacles for the large-scale commercialization. Among them, the high cost of Pt-based electrode material is considered as a major barrier, while the life span or stability of electrode catalysts is another concern since the electrocatalysts can be easily poisoned during the fuel cell operation. In order to overcome these issues, nanostructured carbon materials, especially carbon nanotubes (CNTs), are studied as catalyst support. In addition, recent research also suggests that the coupling of a second metal element with Pt can effectively protect the electrocatalysts from being poisoned and thus improve their long-term durability. The objective of the present work was to demonstrate an efficient synthetic method for the preparation of CNTs supported binary PtM (M=Ru, Sn) electrocatalysts. In this project, a polymer wrapping technique along with an in-situ polyol reduction strategy was adopted to decorate well-dispersed binary PtM nanoparticles on the surface of modified-CNTs. The unique nanostructures as well as the excellent catalytic activities of the as-prepared nanohybirds were investigated through a diversity of physiochemical and electrochemical characterization techniques. This fabrication method provided a simple and convenient route to assemble Pt-based catalyst on carbon substrates, which is useful for the further development of high-performance fuel cell catalysts.

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Dendrimer-encapsulated nanoparticles (DENs) were prepared, characterized, and immobilized on solid surfaces. The resulting films were applied as electrocatalysts for the oxygen reduction reaction (ORR). First, the synthesis, physical and chemical properties, and stability of Pd DENs prepared within poly(amidoamine) (PAMAM) dendrimers were studied in aqueous solution. In this part of the study, the following new findings were reported: (1) the maximum Pd ion loading in the dendrimer was correlated to the number of interior amines available for complexation; (2) Pd DENs could be synthesized within amine-terminated Pd DENs by controlling the solution pH; (3) the oxidative stability of Pd DENs was significantly improved by removing solution-phase impurities; (4) exposure to hydrogen gas reversibly converts partially oxidized Pd DENs back to the zerovalent state. Second, Pt and Pd DENs were prepared using amine-terminated PAMAM dendrimers, and then the free amine groups on the periphery were used to immobilize Pt and Pd DENs onto Au surfaces via an intermediate self-assembled monolayer. The resulting DEN films were more robust and had higher coverages of DENs compared to the DEN films prepared via physisorption. Third, Pt DENs were prepared and immobilized on glassy carbon electrodes using an electrochemical coupling method. The resulting films were electrochemically active for the ORR. These electrocatalytic monolayers were also robust, surviving up to 50 consecutive electrochemical scans for ORR and sonication in acid solution with no significant change in activity. Finally, PtPd bimetallic nanoparticles containing an average of 180 atoms (~1.8 nm in diameter) and composed of seven different Pt:Pd ratios were prepared within sixth-generation, hydroxyl-terminated PAMAM dendrimers. Transmission electron microscopy and single-particle energy dispersive spectroscopy confirmed the sizes and compositions of the particles. These DENs were immobilized on glassy carbon electrodes, and their electrocatalytic properties were evaluated as a function of composition using cyclic voltammetry and rotating disk voltammetry. The results showed that the maximum rate for the ORR occurs at a Pt:Pd ratio of 5:1, which corresponds to a relative mass activity enhancement of 2.5 compared to otherwise identical monometallic Pt nanoparticles.

Dendrimer-encapsulated nanoparticles (DENs) were prepared, characterized, and immobilized on solid surfaces. The resulting films were applied as electrocatalysts for the oxygen reduction reaction (ORR). First, the synthesis, physical and chemical properties, and stability of Pd DENs prepared within poly(amidoamine) (PAMAM) dendrimers were studied in aqueous solution. In this part of the study, the following new findings were reported: (1) the maximum Pd ion loading in the dendrimer was correlated to the number of interior amines available for complexation; (2) Pd DENs could be synthesized within amine-terminated Pd DENs by controlling the solution pH; (3) the oxidative stability of Pd DENs was significantly improved by removing solution-phase impurities; (4) exposure to hydrogen gas reversibly converts partially oxidized Pd DENs back to the zerovalent state. Second, Pt and Pd DENs were prepared using amine-terminated PAMAM dendrimers, and then the free amine groups on the periphery were used to immobilize Pt and Pd DENs onto Au surfaces via an intermediate self-assembled monolayer. The resulting DEN films were more robust and had higher coverages of DENs compared to the DEN films prepared via physisorption. Third, Pt DENs were prepared and immobilized on glassy carbon electrodes using an electrochemical coupling method. The resulting films were electrochemically active for the ORR. These electrocatalytic monolayers were also robust, surviving up to 50 consecutive electrochemical scans for ORR and sonication in acid solution with no significant change in activity. Finally, PtPd bimetallic nanoparticles containing an average of 180 atoms (~1.8 nm in diameter) and composed of seven different Pt:Pd ratios were prepared within sixth-generation, hydroxyl-terminated PAMAM dendrimers. Transmission electron microscopy and single-particle energy dispersive spectroscopy confirmed the sizes and compositions of the particles. These DENs were immobilized on glassy carbon electrodes, and their electrocatalytic properties were evaluated as a function of composition using cyclic voltammetry and rotating disk voltammetry. The results showed that the maximum rate for the ORR occurs at a Pt:Pd ratio of 5:1, which corresponds to a relative mass activity enhancement of 2.5 compared to otherwise identical monometallic Pt nanoparticles.

ABSTRACT CuZn Alloy- Based Electrocatalyst for CO2 Reduction Amira Alazmi Carbon dioxide (CO2) is one of the major greenhouse gases and its emission is a significant threat to global economy and sustainability. Efficient CO2 conversion leads to utilization of CO2 as a carbon feedstock, but activating the most stable carbon-based molecule, CO2, is a challenging task. Electrochemical conversion of CO2 is considered to be the beneficial approach to generate carbon-containing fuels directly from CO2, especially when the electronic energy is derived from renewable energies, such as solar, wind, geo-thermal and tidal. To achieve this goal, the development of an efficient electrocatalyst for CO2 reduction is essential. In this thesis, studies on CuZn alloys with heat treatments at different temperatures have been evaluated as electrocatalysts for CO2 reduction. It was found that the catalytic activity of these electrodes was strongly dependent on the thermal oxidation temperature before their use for electrochemical measurements. The polycrystalline CuZn electrode without thermal treatment shows the Faradaic efficiency for CO formation of only 30% at applied potential ~−1.0 V vs. RHE with current density of ~−2.55 mA cm−2. In contrast, the reduction of oxide-based CuZn alloy electrode exhibits 65% Faradaic efficiency for CO at lower applied potential about −1.0 V vs. RHE with current density of −2.55 mA cm−2. Furthermore, stable activity was achieved over several hours of the reduction reaction at the modified electrodes. Based on electrokinetic studies, this improvement could be attributed to further stabilization of the CO2•− on the oxide-based Cu-Zn alloy surface.

This thesis explores the synthesis, characterization, and electrochemical behavior of nanocomposites composed of cobalt (Co) and molybdenum (Mo) deposited onto graphene oxide (GO), COOH-functionalized multi-walled carbon nanotubes (CNT-COOH), and blends of these two graphitic nanomaterials. The study aims to investigate the structural, morphological, and electrocatalytic properties of these nanocomposites synthesized via a hydrothermal method. Using a combination of analytical techniques including Raman Spectroscopy, Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS), Cyclic Voltammetry (CV), Chronoamperometry, and UV-vis Spectroscopy, the nanocomposite structures were comprehensively characterized. SEM imaging demonstrated differential deposition of CoMo particles, demonstrating higher affinity and deposition on CNT-COOH compared to GO. EDX and XPS findings confirmed successful deposition of Co and Mo sulfides and oxides on both supports. According to XPS data, cobalt sulfides, molybdenum dioxide, and molybdenum disulfide were the dominant species synthesized in CoMo-CNT-COOH and CoMo-GO, whereas the prevalent species in CoMo-CNT-COOH-GO were cobalt and molybdenum sulfides. Electrochemical analyses, particularly CV tests, unveiled unique electro-oxidative activity of CoMo-CNT-COOH for Methyl Orange (MO) analyte at -0.4 V. CoMo-CNT-COOH exhibited approximately 68% optimum electrooxidation of MO after 5 hours for 100 mL solution initially made of 30 ppm MO and 0.5 molar sulfuric acid, indicating potential for environmental remediation applications. / Thesis / Master of Chemical Engineering (MChE)

Glycerol is a very promising alternative fuel to hydrogen in fuel cells. However, the utilisation of glycerol as a fuel requires a good catalyst, due to the slow kinetics of glycerol electrooxidation. Gold has been identified as a promising catalyst due to its high activity and stability for glycerol electrooxidation – although the overpotentials are higher than on platinum and palladium. Modification of a nano-Au/C catalyst by the addition of MnO2, in an attempt to further improve the activity and lower the overpotential for glycerol oxidation, was therefore first explored. This was followed by investigations into the effects of gold particle size and loading. Finally, the effect of gold particle size on oxidation of gold-catalysed glycerol oxidation intermediates was also briefly explored. Studies into MnO2 addition showed that the pre-deposition of MnO2 yielded catalysts with smaller, more uniform gold particles, and catalysts with MnO2 contents of 5 and 9 wt % had higher mass activities and lower onset- and peak- potentials than Au/C. All the Au/xMnO2/C catalysts were more active than the palladium- and platinum-based catalysts reported in literature, which effectively demonstrated the advantage of using a gold-based catalyst for glycerol oxidation – especially when supported by MnO2 which lowered the overpotential for glycerol oxidation over gold. For the study into gold particle size, small gold particles of average diameter ≤ 4.7 nm had higher gold mass-based activities than medium-sized (14.7 nm) particles and were at least twice as active as catalysts containing large (≥ 43 nm) gold particles. The small gold particles also gave lower glycerol oxidation onset potentials, which was attributed to the predominance of Au(110) planes on those particles. Glycerol oxidation also appeared to proceed further along the oxidation pathway over small gold particles, which was confirmed in preliminary studies into the oxidation of glycerol oxidation intermediates. However, specific activity increased with increasing gold particle size, due mainly to the higher intrinsic activity of the Au(111) plane, which increased relative to Au(110) with increasing gold particle size. The important requirements for fuel cell applications are factors such as high mass activity, low overpotentials and high stability – all of which were met by the catalysts containing small gold particles defined by predominantly Au(110) facets. Investigations into the gold loading effect showed similar mass- and specific- activities for catalysts with 5-20 % gold loading. However, only the catalysts with higher gold loadings (15-20 %) did not deactivate early during CV, indicating that a larger gold surface area is necessary to resist poisoning at high potentials. On the basis of low onset potentials, high mass activity, and stability at low overpotentials, a minimum gold loading of 12.5 % appears to be necessary for a supported gold catalyst with small gold nanoparticles; although even higher loadings may be preferable for a higher power output in a fuel cell. Importantly, the insights gleaned from this study on the fundamental properties required for early activation, activity and stability of the gold catalysts could lead to a more intelligent design of gold-based catalysts in future.

Molecular hydrogen has been proposed as an energy store to help meet the world's ever increasing demand for clean energy because the oxidation product (produced by either combustion or in a fuel cell) results in the formation of water. To realize this goal, energy efficient catalysts comprised of earth abundant elements must be used. The work in this dissertation describes investigations of diiron dichalcogen catalysts used for proton reduction. These complexes are inspired by the active site of the [FeFe]-hydrogenase enzyme. Catalysts were extensively studied with cyclic voltammetry in conjunction with photoelectron spectroscopy and density functional theory calculations in order to determine the effects that bridging ligands and 2Fe2E (E = S or Se) core substitutions have on the electronic structure and catalytic ability of these complexes. The complex µ-(pyrazine-2,3-dithiolato)diironhexacarbonyl (pyrazine-cat) was prepared and found to catalyze proton reduction at a -0.49 V overpotential, which represents a 16% decrease over the previously studied complex µ-(benzene-1,2-dithiolato)diironhexacarbonyl (benz-cat). Electrochemical investigations in conjunction with DFT calculations indicated the possibility of two mechanisms for proton reduction, both of the ECEC type. The first mechanism is Fe-based and analogous to the mechanism reported for benz-cat. The second is a nitrogen-based mechanism which occurs at more negative potentials than the Fe-based mechanism. Overall, pyrazine-cat maintained the ability to mediate successive redox states similar to benz-cat and the electron withdrawing nature of the pyrazine caused the initial reduction to occur at a lower potential than benz-cat. Ultimately this results in the decreased overpotential for catalytic proton reduction by pyrazine-cat. Investigations of the electronic structure and catalytic ability of complexes of the type (µ-ECH₂XCH₂E-µ)Fe₂(CO)₆ where E = S or Se and X = CH₂, S or Se were also carried out. All complexes were found to catalyze H₂ production from acetic acid in acetonitrile. DFT calculations indicate that when X = S or Se the HOMO changes character from predominatly metal based (X = CH2) to containing significant chalcogen lone pair character. The presence of the chalcogen lone pair character helps to facilitate a rotated structure in either the oxidized or reduced forms of these complexes. Through computations it was found that oxidation of the X = S or Se complexes results in a CO ligand rotating into a semi-bridging position, which opens a vacant site on one of the Fe-centers. The bridgehead bends toward this vacant site donating electron density greatly stabilizing the cation and more interestingly forming a structure which strongly resembles the active site of the [FeFe]-hydorgenase. Complexes which contain a chalcogen in the bridgehead undergo potential inversion, leading to a two-electron initial reduction. This is in part due to electron-electron repulsion between chalcogen lone pair electrons and the reduced Fe-centers, which leads to the formation of a rotated dianion.Complexes with the general structure (µ-E (CH₂)nE-µ)Fe₂ (CO)₆ where E = S or Se and n = 3, 4, or 5 were investigated using cyclic voltammetry, photoelectron spectroscopy, and DFT calculations. Substitution of Se in the 2Fe2E core for S resulted in a lengthening of the FeFe bond. As the linker length increased from n =3 to 5, one of the apical CO's is pushed down due to a steric interaction creating a more obtuse Fe-Fe-C angle. Larger effects of the linker length were seen in the oxidation and reduction chemistry. CV and UPS show that linker length has little effect on the oxidation potential or onset ionization energy. Computations predict that the oxidized structure is rotated, and as the linker length increases there is an agostic interaction which forms between a methylene proton and the vacant site on the rotated Fe-center. Reduction potentials for these complexes are found to decrease with increasing linker length, which was attributed to the steric interaction between the alkane linker and the apical CO helping to facilitate rotation of the anion. Interestingly catalytic potentials were found to depend almost entirely on chalcogen character in the 2Fe2E core, with S-containing catalysts having a lower catalytic potential than Se-containing catalysts. The long known complex [η⁵-CpFe(CO)SMe]₂ was investigated as both a proton reduction and H₂ oxidation catalyst. Reduction of [η⁵-CpFe(CO)SMe]₂ revealed that the complex undergoes a two electron irreversible reduction and the reduced species precipitates onto the glassy carbon electrode surface. The new species on the electrode surface facilitates proton reduction at a -0.3 V overpotential, which is significantly lower (0.9 V) than the most similar complex Fp₂. Unlike previous catalysts of this type, [η⁵-CpFe(CO)SMe]₂ catalytic current does not decrease as overpotential decreases. [η⁵-CpFe(CO)SMe]₂ was also shown to undergo two one-electron oxidations, and in the presence of H₂ and the dication, appears to oxidize H₂. The ability of [η⁵-CpFe(CO)SMe]₂ to both oxidize H₂ and reduce protons to H₂ addresses a known deficiency for catalysts mimicking the function of the active site of the [FeFe]-hydrogenase.

>Magister Scientiae - MSc / This study focused on binary PtRu and PtSn electrocatalyst, synthesized using the polyol approach and supported on MWCNTs, TiO2 and MoO2 materials, after synthesis part of the resultant electrocatalyst was heat treated to improve alloying of the secondary metal to the primary platinum metal catalyst and also to enhance the stable distribution and uniform dispersion of the nanoparticles on the support material. Physical characterization of the supported catalyst was done using XRD, HRTEM, HRSEM and EDS for elemental analysis. For electrochemical characterization RDE-CV and RDE-LSV were employed. The homeprepared electro-catalysts were then compared to the Pt/C, PtRu/C and PtSn/C commercial electro-catalysts accordingly. XRD confirmed that the binary electro-catalyst for both the commercial and home-prepared display characteristic patterns similar to that of the standard Pt/C electro-catalyst, an indication that all catalysts have prevailed the Pt face-centred-cubic (fcc) crystal structure. Particle size and size distribution examined using HRTEM showed that Pt/C and PtSn/C was uniformly dispersed on the carbon support and that all electrocatalyst supported on MWCNTs showed small particle size known to enhance the activity of the catalyst. However, after heattreatment the particle size increased for all prepared supported electrocatalyst as was expected from literature. SEM micrographs showed that all electrocatalyst were decorated on the support material with agglomerates on some parts of the samples, agglomeration was more pronounced for catalysts supported on MoO2. The metal loading for the home- prepared electrocatalyst was examined using EDS and it was observed to be closer to that of the commercial catalysts. It was also observed that there were changes on the loading of the electrocatalysts after they were subjected to heat treatment and depending on the support material the metal loading of the catalyst was either more or less. This study found PtSn/C to be the most active commercial catalyst for methanol tolerant and oxygen reduction. For the home-prepared electrocatalyst supported on MWCNTs, PtSn/MWCNT-HT was found to be the most active catalyst while for catalyst supported on metal oxides PtSn/MoO2 was found to be more active than the rest of the Pt-based electro catalyst supported on metal oxides. Results showed that PtSn is more active than PtRu and could function as a methanol tolerant oxygen reduction electro-catalyst for the cathode of a direct methanol fuel cell. Furthermore, in terms of durability, the home-prepared electrocatalyst proved to be more durable than the commercial electro-catalyst supported on carbon black, with catalyst supported on MWCNTs showing more stability than other supported electro-catalyst. Multi-walled carbon nanotubes have therefore proven in this study to be the best supporting material for electro-catalyst as catalyst supported on them showed to be more stable than commercial catalyst supported on carbon black.

The work in this thesis addresses the improvement of electrocatalyst performance in hydrogen PEM fuel cells. An agglomerate model for a catalyst layer was coupled with a one dimensional macroscale model in order to investigate the fuel cell performance. The model focuses on the agglomerate scale and the characteristic length in this study was 0.4 µm. The model was validated successfully with the experimental data. Based on the analysis of variance method at a 99% confidence level, the variation in the average fuel cell voltage was significantly sensitive to that in the volume fraction of electrolyte in an agglomerate. The effect of changing electrolyte film thickness was observed to have a significant impact only in the mass transport limited region, whereas the effect of changing agglomerate radius was found over the entire range of current density. An analysis comparing the effect of agglomerate shape at a constant platinum loading, a constant characteristic length and assuming the semi-finite structure was suitable for this study. Sphere, cylinder and slab agglomerate geometries were considered. The behaviour of the utilisation effectiveness was discovered to be strongly affected by the agglomerate shape. The improvement in the utilisation effectiveness was non-linear with current density. The advantage of the slab geometry in distributing reactant through the agglomerate volume was reduced and consequently the increase in utilisation effectiveness for slab-like agglomerates diminishes in the high current density region. At 0.85 Acm−2, the maximum improvement of the catalyst utilisation effectiveness in slab was 27.8% based on the performance in sphere. The improvement in fuel cell maximum power density achieved using slab-like agglomerate was limited to around 3%. The improvement in the overall fuel cell performance by changing the agglomerate shape was not significant. To achieve significant improvements in fuel cell performance will require changes to other features of the catalyst layer.