Atomic Layer Deposited Catalysts for Fuel Cell Applications

The micro direct methanol fuel cell (µDMFC) has been proposed as a candidate to power portable applications. The device can operate at room temperature on inexpensive, energy-dense methanol fuel, and it can be easily "recharged" by fuel refilling. Microfabrication techniques could be one route for the realization of such tiny devices. It is a mature technology, suitable for mass production, where versatile structuring is available at the micro and nano regime. Carbon black supported catalysts synthesized by wet chemistry methods are not readily applicable for standard microfabrication techniques. Atomic layer deposition (ALD), on the other hand, is a highly suitable and still relatively unexplored approach for the synthesis of noble metal catalysts. It is a vapor phase growth method, primarily used to deposit thin lms. ALD is based on self-limiting chemical reactions of alternately injected precursors on the sample surface. Its unique growth characteristic enables conformal and uniform lms of controlled thickness and composition. In certain conditions ALD commences by island growth, resulting in discrete nanoparticle formation, which is generally preferred for catalytic applications. Pt-Ru is the best catalyst toward the methanol oxidation reaction (MOR). In the work described in this PhD dissertation, two series of Pt-Ru ALD catalysts supported on nitrogen-doped multi-walled carbon nanotubes (N-CNTs) have been evaluated toward the CO oxidation and MOR at room temperature in a three-electrode electrochemical cell. The first series was comprised of Pt-Ru ALD catalysts of various Ru compositions, between 0 and 100 at.%. For the compositions investigated, the best catalyst had a Ru composition of 29 at.%. In the second series Ru-decorated Pt catalysts of various Ru loadings, i.e., various Ru ALD cycles (1, 2, 5, 10 and 20), were investigated. The Pt nanoparticles decorated with 2 Ru ALD cycles exhibited highest catalytic activity, which also outperformed the best catalyst of the first series. In addition, a Si-based fuel cell design with ALD catalysts is presented, and its anode was evaluated toward the MOR

Proton Exchange Membrane Fuel Cells (PEMFCs)

The proton exchange membrane fuel cell is an electrochemical energy conversion device, which transforms a fuel such as hydrogen and an oxidant such as oxygen in ambient air into electricity with heat and water byproducts. The device is more efficient than an internal combustion engine because reactants are directly converted into energy through a one-step electrochemical reaction. Fuel cells combined with water electrolyzers, which electrochemically split water into hydrogen and oxygen using renewable energy sources such as solar, mitigate global warming concerns with reduced carbon dioxide emissions. This collection of papers covers recent advancements in fuel cell technology aimed at reducing cost, improving performance, and extending durability, which are perceived as crucial for a successful commercialization. Almost all key materials, as well as their integration into a cell, are discussed: the bus plates that collect the electrical current, the gas diffusion medium that distributes the reactants over catalysts promoting faster reactions, and the membrane separating oxygen and hydrogen gases and closing the electrical circuit by transporting protons. Fuel cell operation below the freezing point of water and with impure reactant streams, which impacts durability, is also discussed

Methanol and proton transport through chitosan‐phosphotungstic acid membranes for direct methanol fuel cell

Composite chitosan-phosphotungstic acid membranes were synthesized by ionotropic gelation. Their liquid uptake is higher for thin membranes (23 ± 2 μm), while it is lower (~70%) for thicker membranes (50-70 μm). Polarization curves recorded using single module fuel cell at 70°C allowed to estimate a peak power density of 60 mW cm−2 by using 1 M as methanol and low Pt and Pt/Ru loadings (0.5 and 3 mg cm−2) at the cathode and at the anode, respectively. Electrochemical impedance spectroscopy was used to estimate the membrane conductivity and to model the electrochemical behavior of methanol electrooxidation inside the fuel cell revealing a two-step mechanism mainly responsible of overall kinetic losses. Transport of methanol inside the membrane was studied by potentiostatic measurements, allowing to estimate a methanol diffusivity of 3.6 × 10−6 cm2 s−1

Mass transport aspects of polymer electrolyte fuel cells under two-phase flow conditions

Die Visualisierung und Quantifizierung von Flüssigwasseransammlungen in Polymerelektrolytmembran-Brennstoffzellen konnte mittels Neutronenradiographie erreicht werden. Dank dieser neuartigen diagnostischen Methode konnte erstmals die Flüssigwasseransammlung in den porösen Gasdiffusionsschichten direkt nachgewiesen und quantifiziert werden. Die Kombination von Neutronenradiographie mit ortsaufgelösten Stromdichtemessungen bzw. lokaler Impedanzspektroskopie erlaubte die Korrelation des inhomogenen Flüssigwasseranfalls mit dem lokalen elektrochemischen Leistungsverhalten. Systematische Untersuchungen an Polymerelektrolyt- und Direkt-Methanol-Brennstoffzellen verdeutlichen sowohl den Einfluss von Betriebsbedingungen als auch die Auswirkung von Materialeigenschaften auf die Ausbildung zweiphasiger Strömungen

Nanomaterials-Based Electrodes for Lithium-Ion Batteries and Alcohol Fuel Cells

This dissertation describes my research on surfactant-free synthesis of nanomaterials with applications for alcohol fuel-cell electrodes, and design and fabrication of nanomaterials-based current collectors that improve the performance of lithium-ion batteries (LIBs) by replacing existing current collectors. Chapter 1 provides a background on the electroanalytical tools used in this research, and an introduction to fuel cells and LIBs. Chapter 2 describes a novel synthesis method for fabricating gold-graphene composites by laser ablation of a gold strip in water. A well-known limitation in the fabrication of a metal-graphene composite is the use of surfactants that strongly adsorb on the metal surface and consequently reduce the catalytic activity of the metal catalyst. I developed a laser ablation-based one-pot synthesis to decorate graphene with gold nanoparticles (AuNPs) in water without using any surfactants. This linker-free gold-graphene composite was successfully tested as an electrode for the electrocatalytic oxidation of alcohols. A novel electrochemical method for depositing a porous gold-polycurcumin (Au-Polycurcumin) nanocomposite on conducting surfaces is presented in chapter 3. Au-Polycurcumin showed an excellent electrocatalytic activity for oxidation of small organic molecules such as ethanol, and methanol. In chapter 4, I demonstrate that reducing the resistance at the current collector active material interface (CCAMI) is a key factor for enhancing the performance of LIBs. I show that carbon nanotubes (CNTs), either directly grown or spray-coated on Al foils, are highly effective in reducing the CCAMI resistance of traditional LIB cathode materials (LiFePO4 or LFP, and LiNi0.33Co0.33Mn0.33O2 or NMC). The vertically aligned CNT-coated electrodes exhibited energy densities as high as (1) ∼500 W h kg–1 at ∼170 W kg–1 for LFP and (2) ∼760 W h kg–1 at ∼570 W kg–1 for NMC, both with a Li metal anode. In chapter 5, I demonstrate a surfactant-free spray coating process to coat commercial cellulose-based paper with CNTs. The prepared paper-CNTs are capable of replacing the conventional aluminum foil used in LIBs. Paper-CNTs were coated with LiFePO4 as the active material and used as cathodes with Li as the anode, and the assembled LIBs showed a high energy density of 460 Wh kg-1 at a power density of 250 W kg-1