Increasing the lifetime of fuel cell catalysts

In this thesis, I discuss a novel idea of fuel cell catalyst regeneration to increase lifetime of the PEM fuel cell electrode/catalyst operation and, therefore, reduce the catalyst costs. As many of the catalyst degradation mechanisms are difficult to avoid, the regeneration is alternative option to prolong catalyst lifetime. In the thesis, I investigate fundamental aspects of Pt catalyst regeneration ex situ, which can be potentially applied in situ later on. The regeneration strategy consists of two steps: (i) Full or partial dissolution of coarsened Pt catalyst nanoparticles and (ii) Chemical or electrochemical redeposition of the dissolved Pt. I discuss dissolution aspects of Pt nanoparticles in mild and carbon-preserving conditions, which are essential for the subsequent redeposition of the nanoparticles on the same support and potential redeposition strategies. For the redeposition of Pt nanoparticles, we investigate electrodeposition technique and chemical reduction in a templated medium, such as microemulsions.Chemical EngineeringApplied Science

Synthesis of Durable Platinum Catalyst for Proton Exchange Membrane Fuel Cells in Bicontinuous Microemulsion (abstract)

ChemE/Chemical EngineeringApplied Science

Stimulated-healing of proton exchange membrane fuel cell catalyst

Platinum nanoparticles, which are used as catalysts in Proton Exchange Membrane Fuel Cells (PEMFC), tend to degrade after long-term operation. We discriminate the following mechanisms of the degradation: poisoning, migration and coalescence, dissolution, and electrochemical Ostwald ripening. There are two ways to tackle this problem. The first option involves formulation of durable catalyst, which can resist harsh fuel cell conditions, and this is the conventional route. The second option is reactivation by dissolution and then redeposition of the catalyst nanoparticles, which is an unprecedented method for platinum catalyst regeneration/stimulated-healing and the one we shall discuss. Dissolution of platinum can be achieved electrochemically, by potential cycling of the fuel cell electrode impregnated with platinum nanoparticles in oxygen enriched acidic electrolyte according to following reactions [1]: Pt + H2O?PtO + 2H+ + 2e- (1) PtO + 2H+?Pt2+ + H2O (2) During the potential cycling, platinum oxides are formed at each positive cycle and subsequently dissolved as platinum ions in the electrolyte on the negative cycle. These cycles are alternated continuously. The partial dissolution of platinum nanoparticles results in a decrease in particles size and oxidation of the poisonous species on the platinum surface. The process of dissolution is monitored in-situ via cyclic voltammetry technique. The concentration of dissolved platinum is measured with Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The next step of the regeneration procedure is redeposition of the dissolved platinum back onto the carbon support of the fuel cell electrode. This can be realized by means of electrodeposition. A negative potential is applied to an electrode from where the platinum was dissolved and this results in a reduction of the dissolved platinum ions. Regenerated nanoparticles are characterized by AFM, TEM and XRD. The activity of the catalyst will be checked via voltammetric techniques.ChemE/Chemical EngineeringApplied Science

Reversible Nanoparticle Formation As a Potential Strategy for PEMFC Catalyst Regeneration (abstract)

ChemE/Chemical EngineeringApplied Science

Bicontinuous microemulsions for high yield, wet synthesis of ultrafine nanoparticles: A general approach

The design of a synthesis strategy for metal nanoparticles by templating dense microemulsions is proposed. Particle size is controlled by surfactant size rather than by microemulsion composition. The strategy was demonstrated with various systems with different surfactant: cationic, anionic and non-ionic and of different sizes. Formulations were determined using the microemulsion phase diagrams. Synthesis was demonstrated for platinum nanoparticles with some examples for gold. The nanoparticles were subsequently extracted from the microemulsion by absorption onto a carbon support, after which the surfactant was recycled.Chenical EngineeringApplied Science

Covalently, Non-Covalently and Non Functionalized Networked Graphitic Structures as robust catalyst support in PEM electrodes (abstract)

ChemE/Chemical EngineeringApplied Science

Experimental and molecular dynamics characterization of dense micro emulsion systems morphology, conductivity and SAXS

Microemulsions are exciting systems that are promising as tuneable self-assembling templating reaction vessels at the nanoscale. Determination of the nano-structure of microemulsions is, however, not trivial, and there are fundamental questions regarding their design. We were able to reproduce experimental data for an important microemulsion system, sodium-AOT-n-heptane-water, using coarse-grained simulations involving relatively limited computational costs. The simulation allows visualization and deeper investigation of controversial phenomena such as bicontinuity and ion mobility. Simulations were performed using the Martini coarse-grained force field. AOT bonded parameters were fine-tuned by matching the geometry obtained from atomistic simulations. We investigated several compositions with a constant ratio of surfactant to oil while the water content was varied from 10 to 60% in weight. From mean square displacement calculation of all species, it was possible to quantify caging effects and ion mobility. Average diffusion coefficients were calculated for all charged species and trends in the diffusion coefficients were used to rationalize experimental conductivity data. Especially, the diffusion coefficient of charged species qualitatively matched the variation in conductivity as a function of water content. The scattering function was calculated for the hydrophilic species and up to 40% water content quantitatively matched the experimental data obtained from small angle X-ray scattering measurements. For higher water contents, discrepancies were observed and attributed to a nearby phase separation. In particular, bicontinuity of water and oil was computationally visualized by plotting the coordinates of hydrophilic beads. Equilibrated coarse-grained simulations were reversed to atomistic models in order both to compare ion mobility and to catch finer simulation details. Especially, it was possible to capture the intimate ion pair interaction between the sodium ion and the surfactant head group

Experimental and molecular dynamics characterization of dense microemulsion systems: Morphology, conductivity and SAXS

Microemulsions are exciting systems that are promising as tuneable self-assembling templating reaction vessels at the nanoscale. Determination of the nano-structure of microemulsions is, however, not trivial, and there are fundamental questions regarding their design. We were able to reproduce experimental data for an important microemulsion system, sodium-AOT–n-heptane–water, using coarse-grained simulations involving relatively limited computational costs. The simulation allows visualization and deeper investigation of controversial phenomena such as bicontinuity and ion mobility. Simulations were performed using the Martini coarse-grained force field. AOT bonded parameters were fine-tuned by matching the geometry obtained from atomistic simulations. We investigated several compositions with a constant ratio of surfactant to oil while the water content was varied from 10 to 60% in weight. From mean square displacement calculation of all species, it was possible to quantify caging effects and ion mobility. Average diffusion coefficients were calculated for all charged species and trends in the diffusion coefficients were used to rationalize experimental conductivity data. Especially, the diffusion coefficient of charged species qualitatively matched the variation in conductivity as a function of water content. The scattering function was calculated for the hydrophilic species and up to 40% water content quantitatively matched the experimental data obtained from small angle X-ray scattering measurements. For higher water contents, discrepancies were observed and attributed to a nearby phase separation. In particular, bicontinuity of water and oil was computationally visualized by plotting the coordinates of hydrophilic beads. Equilibrated coarse-grained simulations were reversed to atomistic models in order both to compare ion mobility and to catch finer simulation details. Especially, it was possible to capture the intimate ion pair interaction between the sodium ion and the surfactant head group.ChemE/Chemical EngineeringApplied Science

Continuous electrochemical oxidation of biomass derived 5-(hydroxymethyl)furfural into 2,5-furandicarboxylic acid

Abstract: A continuous electrochemical process with integrated product separation has been developed for production of 2,5-furandicarboxylic acid (FDCA) by oxidation of 5-(hydroxymethyl)furfural (HMF) in aqueous alkaline media on non-noble Ni/NiOOH foam electrodes at ambient conditions. Initially, voltammetry studies were performed in both, acid and alkaline media, on various catalyst materials: Au, Au3Pd2, Pt, PbO2, Ni/NiOOH and graphite. Preparative electrolysis was performed on Au, Au3Pd2, Pt, PbO2, Ni/NiOOH electrodes in a divided glass cell and Ni/NiOOH showed the best performance with an FDCA yield of ≈ 90% and a Faradaic efficiency of ≈ 80%. The electrolysis conditions were then optimized to industrially relevant conditions in a filter-press type flow reactor with Ni/NiOOH foam anode. HMF concentrations as high as 10 wt% were converted to FDCA at pH 12 in a buffer free 0.1 M Na2SO4 electrolyte with continuous addition of NaOH to maintain constant pH. An FDCA separation yield up to 95% was achieved via pH shift crystallization. The electrolysis and FDCA separation results were used for the design and construction of a bench-scale system where continuous FDCA production, including integrated product separation, was tested and reported in this work. This publication for the first time presents a continuous electrochemical FDCA production system with integrated product separation at industrially relevant product concentrations, 10 wt% HMF, and utilizing non-noble electrode materials. Graphical Abstract: [Figure not available: see fulltext.