Timothy P. Holme

First principles study of doped carbon supports for enhanced platinum catalysts

Highly oriented pyrolytic graphite (HOPG) implanted with N, Ar and B is studied as a support for platinum nanoparticle catalysts for fuel cells. Experimentally, we find that Pt supported by N-HOPG is more disperse, more catalytically active and suffers less particle ripening than native HOPG, while Pt supported on Ar-irradiated HOPG is slightly more active but ripens more than Pt on native HOPG. Defective HOPG supports are modeled by density functional theory (DFT) calculations that confirm and explain the above experimental results. First, defect energetics are studied to demonstrate that nitrogen doping at high doses likely causes agglomerated nitrogenous defect clusters, and irradiation with Ar ions creates vacancies that agglomerate in vacancy clusters. Second, Pt catalyst particle nucleation and agglomeration is studied. For Pt clusters supported on HOPG with nitrogen defects, calculations show a greater driving force for nucleation and greater particle tethering. For Pt clusters supported on HOPG with vacancy aggregations, this study shows a strong driving force for nucleation and a much enhanced tendency for particle ripening. Third, the electronic structure of Pt clusters on different supports is calculated. Finally, reaction energetics are calculated for two likely reaction pathways over Pt clusters supported on different HOPG substrates. Pt-N-HOPG shows modified electronic structure of the Pt catalyst and increased activity towards oxygen. Pt-Ar-HOPG shows slightly enhanced catalytic activity towards oxygen. In all respects, the findings agree with experiment. The calculations attribute the catalytic activity changes primarily to changes in the workfunction and secondarily to the d-band structure of supported Pt particles.

Nitrogen: unraveling the secret to stable carbon-supported Pt-alloy electrocatalysts

Nitrogen functionalities significantly improve performance for metal-based carbon-supported catalysts, yet their specific role is not well understood. In this work, a direct observation of the nanoscale spatial relationship between surface nitrogen and metal catalyst nanoparticles on a carbon support is established through principal component analysis (PCA) of electron energy loss spectral (EELS) imaging datasets acquired on an aberration-corrected scanning transmission electron microscope (STEM). Improved catalyst–support interactions correlated to high substrate nitrogen content in immediate proximity to stabilized nanoparticles are first demonstrated using model substrates. These insights are applied in direct methanol fuel cell prototypes to achieve substantial improvements in performance and long-term stability using both in-house and commercial catalysts doped with nitrogen. These results have immediate impact in advanced design and optimization of next generation high performance catalyst materials.

Interpretation of Low Temperature Solid Oxide Fuel Cell Electrochemical Impedance Spectra

Electrochemical impedance spectroscopy was performed on low temperature solid oxide fuel cells with yttria-stabilized zirconia electrolytes and different electrode materials and morphologies. Three loops are seen in a Nyquist plot; the high frequency loop is attributed to the electrolyte and series resistance. The intermediate and low frequency loops are influenced by the material and morphology of both electrodes. To clarify which elementary processes contribute to each loop, kinetic Monte Carlo simulations of a solid oxide fuel cell were performed to calculate the reaction rates for each elementary process. The rates fall into three groupings, allowing the identification of processes with corresponding features in the impedance spectra. Vacancy diffusion processes occur at the highest frequency, agreeing with the usual assignment of the high frequency loop with series resistance. Chemical reactions at the anode have an intermediate frequency, suggesting that the intermediate frequency loop is dominated by anode reactions. Low frequency reactions include electrochemical reactions, chemical reactions at the cathode, and water formation and desorption at the anode. This agrees with the experimental findings of the strong dependence of the low frequency loop on the bias voltage and the dominance of the cathode reactions in the low frequency regime.

Increased Cathodic Kinetics on Platinum in IT-SOFCs by Inserting Highly Ionic-Conducting Nanocrystalline Materials

One of the crucial factors for improving intermediate-temperature solid oxide fuel cell (SOFC) performance relies on the reduction in the activation loss originating from limited electrode reaction kinetics. We investigated the properties and functions of the nanocrystalline interlayer via quantum simulation and electrochemical impedance analyses. Electrode impedances were found to decrease several folds as a result of introducing a nanocrystalline interlayer and this positive impact was the most significant when the interlayer was a highly ionic-conducting nanocrystalline material. Both exchange current density and maximum power density were highest in the ultrathin SOFCs (fabricated with microelectromechanical systems (MEMS) compatible technologies) consisting of a 50 nm thick nano-gadolinia doped ceria (GDC) interlayer. Oxygen vacancy formation energies both at the surface and in the bulk of pure zirconia, ceria, yttria-stabilized zirconia, and GDC were computed from density functional theory, which provided insight on surface oxygen vacancy densities.

Kinetic Monte Carlo Simulations of Solid Oxide Fuel Cell

The kinetic Monte Carlo technique was employed to simulate an entire solid oxide fuel cell (SOFC) during operation to gain insight into the electrode kinetics and rate-limiting steps in the intermediate temperature range. By combining the quantum simulation studies of oxide ion migration in the fuel cell electrolyte with the experimental studies of the cathode and anode reaction rates, a complete SOFC can be modeled. To study the effect of triple phase boundaries and the size of the catalyst, simulations were performed for different sizes of Pt clusters on the electrolyte surface. The results confirm that the charge-transfer reaction rates depend on the catalyst size. The fuel cell with smaller catalyst particles produces higher power density as expected. The reaction rates of each process were recorded as a function of time. The overpotentials were subsequently determined as a function of catalyst size. The results show that oxygen adsorption is the slowest step on the cathode, while water formation is the slowest step on the anode. The methodology can be used to optimize the catalyst size on both electrodes to reduce the activation loss in intermediate temperature SOFCs.

Energy transfer between quantum dots of different sizes for quantum dot solar cells

Exciton recombination and slow charge carrier transport, major limitations of advanced photovoltaic cells, may be mitigated by designing cells with strong electric fields in the active regions. This may be done by combining quantum dots (QDs) of different Fermi levels in close proximity. While previous reports of quantum dot solar cells utilizing QDs of different sizes indicate that electrons and holes are transferred together from large bandgap QDs to small bandgap quantum dots, lowering the efficiency of the solar cell, we report a mechanism that may be able to use different bandgap QDs to split excitons and drive charge carrier transport, increasing the efficiency of solar cells. Quantum simulations of band structures of QDs show indications of this behavior, and experiments on solar cells with quantum dots of different sizes separated by thin insulating layers show improved photocurrent compared to solar cells with QDs of the same size.

Atomic Layer Deposition of PbS-ZnS quantum wells for high-efficiency solar cells

Quantum confinements such as quantum wells, wires, and dots posses several advantages for next-generation solar cells. In this study, we present results on quantum confinement in PbS-ZnS quantum wells deposited by Atomic Layer Deposition (ALD). Materials selection criteria are presented with a focus on the properties of the well and barrier material. PbS quantum wells embedded in thin ZnS barrier layers are shown to demonstrate quantum confinement effects through scanning tunneling microscopy (STM). The band gap of the PbS films has been varied from 0.4–1.0 eV by varying the number of ALD cycles. The bandgap variation with film thickness is well matched to results predicted by effective-mass theory.

Design of Heterogeneous Catalysts and the Application to the Oxygen Reduction Reaction

A method is developed to use ab initio calculations to predict what materials will have high catalytic activity for heterogeneous dissociative adsorption reactions. This method may be used to restrict the combinatorial possibilities of materials selection to a reasonable search space. The method is demonstrated for the test case of the oxygen reduction reaction, and it is shown that an Ag–Pt compound is superior to the standard Pt catalyst. Density functional theory was used to evaluate dissociative adsorption of oxygen on Ag n (n = 4, 6, 8, 14), Pt m (m = 2, 4, 8), and Ag n Pt m [(n, m) = (4, 2), (6, 2), (4, 4)] clusters. Stable adsorbed, dissociated, and activated states and energies were found. The AgPt compounds show enhanced performance over Ag and Pt clusters of comparable size. Calculated energy of associative and dissociative adsorption on Pt and Ag is in broad agreement with experiment. DFT models of oxygen adsorption and dissociation on slabs of Ag, Pt, and PtAg x were found to agree with experiments and cluster models. A model is given to explain the reactivity of oxygen with Pt and Ag. A Pt/Ag bilayer and a random alloy are examined through experiment and simulation to show that it is possible to fine tune electronic properties, and therefore reactivity for oxygen dissociation. The reactivity of these compounds toward oxygen is generally intermediate to that of pure Ag and Pt; thus a AgPt alloy is expected to be a better low-temperature catalyst for oxygen dissociation than pure Ag or Pt under nearly reversible conditions. Indeed, experiments confirm an Ag3Pt2 alloy to show superior activity at lower than half the platinum loading. Stress analysis confirms that the altered electronic structure, and thus the enhanced catalytic activity, must be due to an alloying effect rather than a strain effect from lattice expansion.