Elod Lajos Gyenge

Defect engineering is a promising strategy for tailoring the electronic structure and charge distribution of catalysts, which could achieve unexpected physical and chemical properties to enhance the catalytic activity and durability. Recently, defect engineering is widely used in the design of carbon-based catalysts for electrochemical reactions. However, most of the research involving carbon defects relates to the heteroatom defects. The concept of topological carbon defects has been proposed and investigated in oxygen reduction reaction (ORR), but the exploration of topological carbon defects is still in its infancy. The design of carbon materials with high density homogeneous topological carbon defects and their extended application in other key electrochemical reactions are still big challenges. In this thesis, an efficient NH3 thermal treatment strategy is proposed for thoroughly removing pyrrolic N and pyridinic N dopants from N-enriched porous carbon particles, to create high-density topological carbon defects in carbon frameworks. The as-prepared 3-dimensional topological defected porous carbon (DPC) particles are investigated by near-edge X-ray absorption fine structure measurements and local density of states analysis, and the defect formation mechanism is revealed by reactive molecular dynamics simulations. The resultant DPC is used as an electrocatalyst in electrochemical carbon dioxide reduction (ECR), yielding a superficial current density of -2.8 mA cm⁻² (-28 A m⁻²) with Faradaic efficiency of 95% for CO generation at 25 °C and 101 kPa in 0.1 M KHCO₃ aqueous solution. The role of the topological carbon defects is analyzed in ECR. Then the carbon-based material enriched with topological defects is selected to serve as a substrate material to investigate the heterojunction effect of Pt and topological carbon defects. Both experimental characterizations and theoretical simulations reveal that the strong Pt-defect interaction can modify the electronic structure and charge distribution on the interface of the catalyst. Different Pt-defect coupling catalysts are prepared and the influence of the topological carbon defects on Pt are analyzed. Pt supported on DPC (Pt-DPC) catalyst achieves a 55 mV positive shift of half-wave potential for ORR in O2 saturated 0.1 M HClO4 electrolyte compared with commercial Pt catalyst on graphitized carbon at 25 °C and 101 kPa.

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Catalyst durability in renewable energy systems (including CO₂ electroreduction and Hydrogen fuel cells) is vital to the overall lifetime of the systems. To protect hydrogen fuel cells during cell voltage reversal, oxygen evolution reaction (OER) catalysts (typically RuO₂ or IrO₂) are added to the anode of proton exchange membrane fuel cells (PEMFCs). In this thesis, the durability of these OER catalysts was investigated by measuring the mass changes of heat-treated IrOx powders (350, 450 and 550 °C) drop cast on an electrochemical quartz crystal microbalance (EQCM). IrOx catalysts showed difference in frequency response due to the differential uptake of water and formation of oxyhydroxide species during cyclic voltammetry (CV) experiments (1.2, 1.4 and 1.6 to 0.05 V vs. RHE) confirmed by XPS. Platinum on carbon catalyst (Pt/C) suffered from carbon corrosion during potential holds at 1.8 and 2.0 V. The addition of IrOx powders to Pt/C protected the layer against carbon corrosion creating a simulated reversal tolerant anode.Additionally, OER catalysts were ranked by OER activity and dissolved Ir³⁺ in the electrolyte after an ex situ accelerated stress test (AST) in a representative PEMFC anode environment. The ex situ results are compared with reversal times obtained in a single cell PEMFC subjected to anode accelerated stress test (AAST). Generally, catalysts with higher OER mass activity and Ir³⁺ dissolution had longer reversal times. Heat treatment of unsupported IrOx increased OER durability in the fuel cell anode environment.Catalyst instability strongly affects the viability of electroreduction of CO₂ (ERC) systems as well. To improve the durability and efficiency of ERC catalysts, five Bimetallic Sn-Pb catalyst compositions were electrodeposited (from fluoroborate or oxide media) on two carbon supports. Sn majority catalysts with 15 to 35% Pb generated stable faradaic efficiencies (FE) up to 95% during constant potential electrolysis whereas pure Sn experienced an extensive (up to 30%) decrease in FE. After electrolysis, XRD analysis showed a SnO₂ phase present on 35% Pb catalysts but not on pure Sn catalysts. It is proposed that the presence of Pb in Sn majority catalysts stabilized SnO₂ and enhanced faradaic efficiency durability during ERC.

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High efficiencies and reduced greenhouse gas emissions promote proton exchange membrane fuel cells (PEMFCs) as a promising energy conversion technology. However, its widespread commercial application is hampered by certain cost, performance and durability limitations. The interface between the MPL (microporous layer) and the cathode CL (catalyst layer) plays an important role in a PEMFC’s overall performance, since it houses the reaction sites for the oxygen reduction reaction. The interface may furthermore significantly affect mass transport behavior, ohmic contributions and the hydration state of the membrane at different humidities. The main objective of the study was therefore to advance PEMFC research through the development of alternative MPLs offering dual-functional improvements: enhanced interfacial characteristics and improved operational flexibility via suitability for low cathode humidity applications. Alternative MPLs were evaluated based on extensive material characterization and single cell performance testing. Graphene was demonstrated to be a promising alternative. The material displays beneficial interfacial characteristics (a stacked planar morphology, superior conductivity, adhesive behaviour, and improved electrical connectivity with the CL) and furthermore results in improvements in the kinetic and ohmic polarization regions, compared to the conventional CB (carbon black) MPL. Although graphene MPLs also suffer from mass transport limitations, the problem can be addressed through the addition of CB. The addition increases the MPL’s water permeability, which helps to establish a balance between water removal (for the prevention of flooding) and water retention (for membrane hydration) at high and low RH (relative humidity). For graphene tested under one-dimensional control, this results in synergistic performance enhancements, showing a 30% and 80% increase in the maximum power density at 100% and 20% cathode RH. In addition to increased water permeability, other common effects resulting from the creation of CB composites (also observed for reduced graphene oxide and graphite) include decreased surface wettability and through-plane resistance. For the application to low loaded CCMs (0.1 mg cm-²), the potential to improve performance with graphene-based MPLs appears restricted by the catalyst loading itself. Nevertheless, graphene helps to improve performance preservation of low loaded CCMs at low humidity conditions, as also demonstrated for conventionally loaded CCMs (0.4 mg cm-²).

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The electrocatalysis of oxygen reduction and evolution reactions (ORR and OER, respectively) on the same catalyst surface is among the long-standing challenges in electrochemistry with paramount significance for a variety of electrochemical systems including regenerative fuel cells and rechargeable metal-air batteries. Non-precious group metals (non-PGMs) and their oxides, such as manganese oxides, are the alternative cost-effective solutions for the next generation of high-performance bifunctional oxygen catalyst materials. Here, initial stage electrocatalytic activity and long-term durability of four non-PGM oxides and their combinations, i.e. MnO₂, perovskites (LaCoO₃ and LaNiO₃) and fluorite-type oxide (Nd₃IrO₇), were investigated for ORR and OER in alkaline media. The combination of structurally diverse oxides revealed synergistic catalytic effect by improved bifunctional activity compared to the individual oxide components. Next, the novel role of alkali-metal ion insertion and the mechanism involved for performance promotion of oxide catalysts were investigated. Potassium insertion in the oxide structures enhanced both ORR and OER performances, e.g. 110 and 75 mV decrease in the OER (5 mAcm-²) and ORR (-2 mAcm-²) overpotentials (in absolute values) of MnO₂-LaCoO₃, respectively, during galvanostatic polarization tests. In addition, the stability of K⁺ activated catalysts was improved compared to unactivated samples. Further, a factorial design study has been performed to find an active nanostructured manganese oxide for both ORR and OER, synthesized via a surfactant-assisted anodic electrodeposition method. Two-hour-long galvanostatic polarization at 5 mAcm-² showed the lowest OER degradation rate of 5 mVh-¹ for the electrodeposited MnOx with 270 mV lower OER overpotential compared to the commercial γ-MnO₂ electrode. Lastly, the effect of carbon addition to the catalyst layer, e.g. Vulcan XC-72, carbon nanotubes and graphene-based materials, was examined on the ORR/OER bifunctional activity and durability of MnO₂ LaCoO₃. The highest ORR and OER mass activities of -6.7 and 15.5 Ag-¹ at 850 and 1650 mVRHE, respectively, were achieved for MnO₂-LaCoO₃-multi_walled_carbon_nanotube-graphene, outperforming a commercial Pt electrode. The factors affecting the durability of mixed-oxide catalysts were discussed, mainly attributing the performance degradation to Mn valence changes during ORR/OER. A wide range of surface analyses were employed to support the presented electrochemical results as well as the proposed mechanisms.

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For applications that require small amounts of H₂O₂ or have economically difficult transportation means, an alternate, on-site H₂O₂ production method to the current industrial anthraquinone auto-oxidation process is needed. Thus far neutral production of H₂O₂ has been limited to bench-top laboratory scaled research with low yield of H₂O₂ [1]. To produce neutral H₂O₂ on-site and on-demand for drinking water purification, the electroreduction of oxygen at the cathode of a solid polymer electrolyte (SPE) cell could be a possible solution. The work presented here has utilized a SPE cell operating in either fuel cell mode (power generating) or electrolysis mode (power consuming) to produce H₂O₂. The SPE cell reactor is operated with a continuous flow of cathode carrier water flowing through the cathode to remove the product H₂O₂. Two catalysts were chosen for further study in this work, one is the inorganic cobalt-carbon composite catalyst, to be used in both fuel cell mode and electrolysis mode operation. The other is the riboflavin-anthraquinone-carbon composite catalyst, to be used in only the electrolysis mode operation. Through parametric experiments in both modes of operation, the Co-C catalyst was able to achieve peroxide production rate of ~200 μmol hr-¹ cm-² and 4 mW cm-² operating at a cell temperature of 60°C with a current density of 30 mA cm-² and 30% current efficiency in fuel cell mode operation. Long term recycle experiments over a period of 72 hours showed an accumulated H2O2 concentration of over 1400 ppm. Investigation of both catalysts in electrolysis mode operation showed that the AQ-C catalyst achieved maximum H₂O₂ production of 580 μmol hr-¹ cm-² operating at 40°C and a current density of 240 mA cm-² with an 8% current efficiency; while the Co-C catalyst had a maximum H₂O₂ production rate of 360 μmol hr-¹ cm-² operated at 240 mA cm-² with 8% current efficiency. Long term recycle study of both catalysts in electrolysis mode generated maximum H₂O₂ concentrations of over 3000 ppm in 72 hours. Water sample analysis showed no degradation of the catalysts in either mode of operation.

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Integrated sensing and biosensing microfluidic systems often require sealing between polydimethylsiloxane (PDMS), glass, and gold interfaces. Studying substances that can self-organize onto glass and gold surfaces may achieve these goals and pave the way for new technological advances. Work presented in this thesis focuses on characterizing the adsorption of N-[(3-trimethoxysilyl)propyl]ethylene-diamine triacetic acid (or TMS-EDTA) on Au and applying this knowledge to construct leak-free PDMS-based electrochemical cells. First, surface analysis of TMS-EDTA-modified Au surfaces was conducted using various techniques. Water contact angle measurements and X-ray photoelectron spectroscopy confirm that the carboxylated silane can chemically modify Au surfaces. Atomic force microscopy studies indicate that a uniform surface coverage with monolayer thickness is formed. Infrared spectroscopy studies indicate that there is little evidence of siloxane cross-linking. Surface plasmon resonance results suggest that the carboxylates on TMS-EDTA-modified Au are available for streptavidin immobilization. Second, electrochemistry was used to determine the Gibbs free energies of adsorption of TMS-EDTA on Au under aqueous conditions. Electrochemical differential capacitance measurements reveal that the potential-dependent free energies of adsorption are ∼ - 20 to - 30 kJ/mol (for potentials between - 0.5 and 0.2 V) in the complex electrolyte solution used. Furthermore, at highly negative potentials ( ∼ - 1.1 V), TMS-EDTA adsorbs minimally onto the Au surface. Third, PDMS surfaces were functionalized to present primary amino groups, and glass or gold slides were functionalized to present carboxyl groups. Strong bonding was achieved by bringing the two surfaces in contact and reacting at room temperature. Shear tests reveal that the novel carboxyl-amine bonding strategy achieved a comparable bond strength as the conventional methods. Subsequently, TMS-EDTA was applied to construct leak-free PDMS-based electrochemical cells. Pressure leak tests were conducted to provide a more realistic measure of the bond strengths under aqueous conditions. A method to electrochemically remove the adsorbed TMS-EDTA layer off of the Au electrode, while maintaining the sealed cell chamber, was also developed. The characterization studies and fabrication strategy presented have led to the development of leak-free PDMS-based electrochemical devices that are suitable for sensing and biosensing applications.

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Biochar, a by-product of biomass pyrolysis, was investigated as a carbon-based electrode material for a water treatment method based on electrostatic adsorption/desorption of ions in electric double layers (EDLs) formed on the charged electrodes (capacitive deionization, CDI). Surface area, porous structure, and functional groups of biochar were developed, and corresponding effects on EDL capacitive performance were studied. A novel method was explored to tailor the micro- and meso-porous structures of activated biochar by exploiting the interaction between pre-carbonization drying conditions and carbonization temperature (475–1000 C) in a thermo-chemical process (KOH chemical activation). The mechanism of porosity development was investigated; results suggest that the conversion of KOH to K₂CO₃ under different drying conditions has a major role in tailoring the structure. The resultant surface area, micro- and meso-pore volumes were: 488–2670 m² g-¹, 0.04–0.72 cm³ g-¹, and 0.05–1.70 cm³ g-¹, respectively. Tailored biochar samples were investigated using physico-chemical surface characterization and electrochemical methods. For electrochemical testing, activated biochar was sprayed onto Ni mesh current collectors using Nafion® as binder. The majorly microporous activated biochar showed promising capacitances between 220 and 245 F g-¹ when 0.1 mol L-¹ NaCl/NaOH was used as the electrolyte. Addition of mesoporous structure resulted in significantly reduced electrode resistance (up to 80%) and improved capacitive behaviour due to enhanced ion transport within the pores. CDI of NaCl and ZnCl₂ solutions was investigated in a batch-mode unit through the use of tailored biochar electrodes. For NaCl removal, all samples showed promising capacity (up to 5.13 mg NaCl g-¹) and durability through four consecutive cycles. In contrast, in the case of ZnCl₂, the microporous sample showed a considerable drop in removal capacity (>75%) from cycle 1 to 4, whereas the combined micro- and mesoporous sample exhibited relatively small electrosorption capacity. Interestingly, the sample with mostly mesoporous structure has shown the highest removal capacity (1.15 mg ZnCl₂ g-¹) and durability for Zn²⁺ removal. These results emphasize the importance of tailoring the porous structure of biochar as a function of the specific size of adsorbate ions to improve the CDI performance.

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This thesis first aims at developing an electrochemical approach for low temperature, simple, and cost-effective synthesis of graphene microsheets (GNs) using graphitic electrodes in ionic liquid (IL) medium. The second major focus involves the products application as cathode-modifying microporous layers (MPLs) in proton exchange membrane fuel cells (PEMFCs) as well as anode-modifying materials in microbial fuel cells (MFCs). For the electrochemical exfoliation, a novel IL/acetonitrile electrolyte is introduced, and investigated with low concentration of ionic liquids. Using iso-molded graphite rod as the anode, up to 86% of exfoliation was achieved with the majority of the products as graphene flakes in addition to smaller quantities of carbonaceous particles and rolled sheets. Moreover, the simultaneous anodic and cathodic GN production was developed here with a synergistic exfoliation effect. When graphitic anode and cathode were subjected to a constant cell potential, up to 3 times higher exfoliation yields were generated compared to single-electrode studies on each side (~6-fold improvement in total). Thorough materials characterization confirmed the production of ultrathin GNs (
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Capital and operating costs of fuel cell systems must be reduced before they can be competitive with conventional energy conversion technologies. This dissertation concerns the development of an unconventional fuel cell aimed at meeting that challenge.Presented here, for the first time, is a novel cylindrical Swiss-roll mixed-reactant fuel cell (SR-MRFC) that eliminates expensive and failure-prone components of conventional fuel cells. The proof-of-concept of the SR-MRFC was performed both in monopolar and bipolar architectures. In the monopolar case 3D anodes with platinum or with osmium catalysts were coupled to a gas-diffusion MnO₂ cathode in a 20×10-⁴ m² single-cell SR-MRFC, operated with a two-phase mixture of 1 M NaBH₄/2M NaOH(aq) + O₂(g). Instead of a Nafion® membrane, a porous diaphragm was employed. At 323 K, 105 kPa(abs), the peak superficial power densities of the SR-MRFC with the platinum and osmium anode catalysts were up to respectively 2230 and 1880 W m−² with good performance stability during 3 hr continuous operation. These values are the highest power densities ever reported for MRFCs operating under similar conditions and match the highest reported values for conventional dual chamber PEM direct borohydride fuel cells. Scale up of the single-cell SR-MRFC to 100×10-⁴ m² and 200×10-⁴ m² gave corresponding peak superficial power densities of 900 and 700 W m-², while the 20×10-4 m² bipolar reactors produced peak volumetric power densities of 267 and 205 kW m-³.This work also explored the feasibility of electroreduction of N₂O on Pt and Pd in the cathode of a MRFC to generate electricity from N₂O in the tail gases of industrial processes. Here the SR-MRFC was operated using two-phase fuel + oxidant mixtures of 1 M NaBH₄ / 2M NaOH(aq) + N₂O(g) and 0.5 M CH₃OH/2 M NaOH(aq) + N₂O(g). At 323 K, 105 kPa(abs) the peak superficial power densities for the mixed NaBH₄- and MeOH-N₂O systems were respectively 340 W m-² (Pt anode/Pd cathode) and 38 W m-² (PtRu anode/Pd cathode). This work demonstrates for the first time that co-generation of electricity and abatement of N₂O may potentially compete with thermochemical processes of N₂O capture currently under development.

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Direct borohydride fuel cells (DBFC) are a promising technology for meeting increasing energy demands of portable electronic applications. The objective of this dissertation was to contribute to the understanding of borohydride (BH₄⁻) electro-oxidation and the development of the DBFC anode; a component which can influence both the performance and cost of a DBFC system. The first part of the investigation involves the elucidation of the BH₄⁻ electro-oxidation mechanism on Pt. The BH₄⁻ electro-oxidation mechanism was studied by correlating the results obtained by the electrochemical quartz crystal microbalance technique (EQCM) and the rotating disk electrode technique (RDE) with density functional theory (DFT) calculations from the literature. It was found that BH₄⁻ electro-oxidation on Pt resulted in the adsorption of reaction intermediates, such as BH₂OHad and BOHad, which required high oxidizing potentials to desorb/ oxidize from the catalyst surface. It was also found that the BH₄⁻ oxidation mechanisms (Langmuir – Hinshelwood versus Eley - Rideal) were dictated by the availability of Pt-sites and the competitive adsorption of OH⁻ and BH₄⁻.The second part involves an investigation of the performance of three different carbon black supported anode catalysts: Pt, PtRu, and Os, with a focus on Os catalysts. Fundamental electrochemical methods combined with fuel cell experiments revealed that osmium nanoparticles are kinetically superior and stable catalysts for BH₄⁻ electro-oxidation compared to Pt and PtRu. It was also found that supported Os electrocatalysts appear to favour the direct oxidation of BH₄⁻ in comparison to Pt, and PtRu electrocatalysts. The final section of this dissertation focuses on the effect of electrocatalyst support and anode design on the performance of the DBFC anode. It was found that the Vulcan® XC-72 supported catalyst alleviated mass transfer related problems associated with hydrogen generation from BH₄⁻ hydrolysis. The most significant improvement was obtained when using the graphite substrate supported catalysts (three-dimensional anodes). Fuel cell studies revealed power densities of 103 mW cm⁻² to 130 mW cm⁻² achieved by 1.7 mg cm⁻² Os and ~1 mg cm⁻² PtRu three-dimensional electrodes respectively at 333 K, using an O₂ oxidant at 4.4 atm (abs), and a 0.5 M NaBH₄ – 2 M NaOH anolyte composition.

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No abstract available.