Panagiotis Trogadas

CeO2 surface oxygen vacancy concentration governs in situ free radical scavenging efficacy in polymer electrolytes.

Nonstoichiometric CeO(2) and Ce(0.25)Zr(0.75)O(2) nanoparticles with varying surface concentrations of Ce(3+) were synthesized. Their surface Ce(3+) concentration was measured by XPS, and their surface oxygen vacancy concentrations and grain size were estimated using Raman spectroscopy. The surface oxygen vacancy concentration was found to correlate well with grain size and surface Ce(3+) concentration. When incorporated into a Nafion polymer electrolyte membrane (PEM), the added nonstoichiometric ceria nanoparticles effectively scavenged PEM-degradation-inducing free radical reactive oxygen species (ROS) formed during fuel cell operation. A 3-fold increase in the surface oxygen vacancy concentration resulted in an order of magnitude enhancement in the efficacy of free radical ROS scavenging by the nanoparticles. Overall, the macroscopic PEM degradation mitigation rate was lowered by up to 2 orders of magnitude using nonstoichiometric ceria nanoparticles with high surface oxygen vacancy concentrations.

Hierarchically Structured Nanomaterials for Electrochemical Energy Conversion

Hierarchical nanomaterials are highly suitable as electrocatalysts and electrocatalyst supports in electrochemical energy conversion devices. The intrinsic kinetics of an electrocatalyst are associated with the nanostructure of the active phase and the support, while the overall properties are also affected by the mesostructure. Therefore, both structures need to be controlled. A comparative state-of-the-art review of catalysts and supports is provided along with detailed synthesis methods. To further improve the design of these hierarchical nanomaterials, in-depth research on the effect of materials architecture on reaction and transport kinetics is necessary. Inspiration can be derived from nature, which is full of very effective hierarchical structures. Developing fundamental understanding of how desired properties of biological systems are related to their hierarchical architecture can guide the development of novel catalytic nanomaterials and nature-inspired electrochemical devices.

Nature-inspired optimization of hierarchical porous media for catalytic and separation processes

Hierarchical materials combining pore sizes of different length scales are highly important for catalysis and separation processes, where optimization of adsorption and transport properties is required. Nature can be an excellent guide to rational design, as it is full of hierarchical structures that are intrinsically scaling, efficient and robust. However, much of the “inspiration” from nature is, at present, empirical; considering the huge design space, we advocate a methodical, fundamental approach based on mechanistic features.

The Effect of Uniform Particle Size Distribution on Pt Stability

A major barrier to proton exchange membrane fuel cell (PEFC) commercialization is catalyst durability. During long term fuel-cell operation, a loss of Pt active surface area at the cathode has been observed resulting in slower oxygen reduction reaction (ORR) (1-2) kinetics and hence a reduction in cell potential. The loss of Pt surface active area is due to three processes: Ostwald ripening of Pt nanoparticles, dissolution and diffusion of Pt, and corrosion of the carbon support.

A lung-inspired approach to scalable and robust fuel cell design

A lung-inspired approach is employed to overcome reactant homogeneity issues in polymer electrolyte fuel cells. The fractal geometry of the lung is used as the model to design flow-fields of different branching generations, resulting in uniform reactant distribution across the electrodes and minimum entropy production of the whole system. 3D printed, lung-inspired flow field based PEFCs with N = 4 generations outperform the conventional serpentine flow field designs at 50% and 75% RH, exhibiting a ∼20% and ∼30% increase in performance (at current densities higher than 0.8 A cm−2) and maximum power density, respectively. In terms of pressure drop, fractal flow-fields with N = 3 and 4 generations demonstrate ∼75% and ∼50% lower values than conventional serpentine flow-field design for all RH tested, reducing the power requirements for pressurization and recirculation of the reactants. The positive effect of uniform reactant distribution is pronounced under extended current-hold measurements, where lung-inspired flow field based PEFCs with N = 4 generations exhibit the lowest voltage decay (∼5 mV h−1). The enhanced fuel cell performance and low pressure drop values of fractal flow field design are preserved at large scale (25 cm2), in which the excessive pressure drop of a large-scale serpentine flow field renders its use prohibitive.

Optimization of mesoporous titanosilicate catalysts for cyclohexene epoxidation via statistically guided synthesis

An efficient approach to improve the catalytic activity of titanosilicates is introduced. The Doehlert matrix (DM) statistical model was utilized to probe the synthetic parameters of mesoporous titanosilicate microspheres (MTSM), in order to increase their catalytic activity with a minimal number of experiments. Synthesis optimization was carried out by varying two parameters simultaneously: homogenizing temperature and surfactant weight. Thirteen different MTSM samples were synthesized in two sequential ‘matrices’ according to Doehlert conditions and were used to catalyse the epoxidation of cyclohexene with tert-butyl hydroperoxide. The samples (and the corresponding synthesis conditions) with superior catalytic activity in terms of product yield and selectivity were identified. In addition, this approach revealed the limiting values of each synthesis parameter, beyond which the material becomes catalytically ineffective. This study demonstrates that the DM approach can be broadly used as a powerful and time-efficient tool for investigating the optimal synthesis conditions of heterogeneous catalysts.

Role of Particle Size Distribution on Pt Stability

This work aims to address the role of particle size distribution on Pt stability. A theoretical model is built based on modified Butler-Volmer equations, platinum oxide surface coverage and interparticle interactions between platinum oxide particles. Preliminary results demonstrate the change in platinum ions concentration across the electrode and membrane and reveal the effect of potential cycling on the particle size. Further research on the effect of particle size and potential cycling on electrochemically active surface area and its comparison with experimental data is in progress.

Hybrid Electrocatalysts for Degradation Mitigation in PEFCs

Pt/C/MnO2 hybrid catalysts were prepared by a wet chemical method. Rotating ring-disk electrode (RRDE) experiments were performed to estimate the amount of hydrogen peroxide (H2O2) formed during the oxygen reduction reaction (ORR) as a function of MnO2 content. Pt/C/ MnO2 (5% by weight of MnO2) hybrid electrocatalysts produced 50% less hydrogen peroxide than the baseline Pt/C electrocatalyst. The hybrid electrocatalysts were used to prepare membrane electrode assemblies that were tested at 90°C and 50% RH at open circuit with pure hydrogen as fuel and air as the oxidant. The concentration of F in the anode condensate over 24 hours was found to be reduced by a factor of 3-4 when Pt/C/MnO2 replaced Pt/C as the catalyst. Through cyclic voltammetry and RRDE kinetic studies, the lower ORR activity of the hybrid electrocatalysts was attributed to catalyst treatment with acid during MnO2 introduction.