Tim S. Olson

Cross-Laboratory Experimental Study of Non-Noble-Metal Electrocatalysts for the Oxygen Reduction Reaction

Nine non-noble-metal catalysts (NNMCs) from five different laboratories were investigated for the catalysis of O(2) electroreduction in an acidic medium. The catalyst precursors were synthesized by wet impregnation, planetary ball milling, a foaming-agent technique, or a templating method. All catalyst precursors were subjected to one or more heat treatments at 700-1050 degrees C in an inert or reactive atmosphere. These catalysts underwent an identical set of electrochemical characterizations, including rotating-disk-electrode and polymer-electrolyte membrane fuel cell (PEMFC) tests and voltammetry under N(2). Ex situ characterization was comprised of X-ray photoelectron spectroscopy, neutron activation analysis, scanning electron microscopy, and N(2) adsorption and its analysis with an advanced model for carbonaceous powders. In PEMFC, several NNMCs display mass activities of 10-20 A g(-1) at 0.8 V versus a reversible hydrogen electrode, and one shows 80 A g(-1). The latter value corresponds to a volumetric activity of 19 A cm(-3) under reference conditions and represents one-seventh of the target defined by the U.S. Department of Energy for 2010 (130 A cm(-3)). The activity of all NNMCs is mainly governed by the microporous surface area, and active sites seem to be hosted in pore sizes of 5-15 A. The nitrogen and metal (iron or cobalt) seem to be present in sufficient amounts in the NNMCs and do not limit activity. The paper discusses probable directions for synthesizing more active NNMCs. This could be achieved through multiple pyrolysis steps, ball-milling steps, and control of the powder morphology by the addition of foaming agents and/or sulfur.

Predictive modeling of electrocatalyst structure based on structure-to-property correlations of x-ray photoelectron spectroscopic and electrochemical measurements.

Chemical structure and catalytic activity of nonplatinum porphyrin-based electrocatalyst for oxygen reduction is characterized by combination of X-ray photoelectron spectroscopy (XPS) and rotating disk electrode. The goal of the study is to show how modifications in the molecular structure affect catalytic characteristics and how to use these structural modifications in a purposeful manner to increase catalytic activity. Initial correlation of structure to electrochemical performance is achieved through the application of principal component analysis (PCA) to curve-fits of high-resolution XPS spectra combined with results of electrochemical measurements. Furthermore, a predictive model that describes this correlation is build using the combination of genetic algorithm (GA) and multiple linear regression (MLR). Based on structure-to-property correlations, two types of active sites responsible for the catalytic activity, i.e., Co associated with pyropolymer and Co particles covered by oxide layer, are determined, and a dual-site for oxygen reduction on cobalt porphyrins is hypothesized, allowing for designing a catalyst structure with optimal performance characteristics.

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.

Effect of Halide-Modified Model Carbon Supports on Catalyst Stability

Modification of physiochemical and structural properties of carbon-based materials through targeted functionalization is a useful way to improve the properties and performance of such catalyst materials. This work explores the incorporation of dopants, including nitrogen, iodine, and fluorine, into the carbon structure of highly-oriented pyrolytic graphite (HOPG) and its potential benefits on the stability of PtRu catalyst nanoparticles. Evaluation of the changes in the catalyst nanoparticle coverage and size as a function of implantation parameters reveals that carbon supports functionalized with a combination of nitrogen and fluorine provide the most beneficial interactions, resulting in suppressed particle coarsening and dissolution. Benefits of a carefully tuned support system modified with fluorine and nitrogen surpass those obtained with nitrogen (no fluorine) modification. Ion implantation of iodine into HOPG results in a consistent amount of structural damage to the carbon matrix, regardless of dose. For this modification, improvements in stability are similar to nitrogen modification; however, the benefit is only observed at higher dose conditions. This indicates that a mechanism different than the one associated with nitrogen may be responsible for the improved durability.

Composition- and Morphology-Dependent Corrosion Stability of Ruthenium Oxide Materials

Ruthenium oxide materials were evaluated as possible non-carbon-based supports for fuel cell catalysts. The effects of composition and morphology of ruthenium oxide materials on the conductivity and corrosion stability in the gas-diffusion electrode (GDE) configuration were thoroughly investigated. The compositions of the bulk and surface of three ruthenium oxide materials, along with the surface area and surface morphology, were compared. We have found that all tested ruthenium oxide powders exhibited higher corrosion stability compared to carbon. Full conversion of RuO(2).nH(2)O to the RuO(2) phase by postreduction in a hydrogen atmosphere leads to improved conductivity and corrosion stability.

Functional DMFC Cathode Catalysts and Supports Based on Niobium Oxide Phase

Composite electrocatalysts consisting of platinum nanophase supported on carbon black decorated with niobium oxide phase were prepared by a two step precipitation/reduction procedure. The intrinsic catalytic activity of these materials was evaluated for the oxygen electroreduction reaction, measured in the absence and presence of methanol. It was found that the Nb x O y -composite materials are more hydrophilic and thus initially have lower intrinsic catalytic activity for oxygen reduction. However, materials containing Nb x Oy offer better overall platinum mass utilization, achieved through much better dispersion of the platinum nanophase. Such advantageous dispersion is observed even for high metal loadings. Another advantage of these materials is minimal effect of poisoning with the intermediates formed during methanol oxidation that results in the enhanced methanol tolerance in direct methanol fuel cells (DMFC). .

Gold-Decorated Flow-Through Electrodes: Effect of Electrochemical Time Constant on Electrodeposition of Au Particles on Reticulated Vitreous Carbon

Here, we investigate the effect of the time constant on the ED of Au particles on RVC substrates. Potentiostatic ED was performed on two RVC samples with electrochemical accessible surface areas of 1.0 and 750 cm 2 . Progressive nucleation was observed on both substrates. At the same applied potentials, the large ohmic drop on the high surface area sample (750 cm 2 ) cannot be neglected and results in a significantly reduced nucleation density (∼30%) and a larger mean diameter. Knowledge of how the electrochemical cell time constant (resistance-capacitance time constant) affects particle coverage and size allows one to optimize ED conditions on substrates with high surface areas. The scaling of ED conditions is important for several technological applications.

The role of nitrogen doping on durability in the Pt-Ru/HOPG system.

Improving catalytic activity and durability are two major issues that must be addressed for fuel cells to become commercially viable. Surface modifications and doping of the catalyst support has been shown to effectively address both of these issues through significant improvements in the catalyst-support interactions [1, 2]. In this work we discuss the role of nitrogen doping via ion-implantation on the stability of a Pt-Ru nanoparticle catalyst phase supported on model highly-oriented pyrolytic graphite (HOPG) substrates.

A core-level spectroscopic investigation of the preparation and electrochemical cycling of nitrogen-modified carbon as a model catalyst support

The synthesis and electrochemical cycling of platinum–ruthenium nanoparticles sputtered onto nitrogen-implanted highly-oriented-pyrolytic-graphite (HOPG) was studied with soft X-ray spectroscopy. The near edge X-ray absorption fine structure (NEXAFS) of the carbon 1s, nitrogen 1s, and oxygen 1s transitions were measured as a function of sample preparation and electrochemical cycling. The NEXAFS of the C 1s edge indicate defect formation in the graphitic (sp2) network of the carbon support due to implantation. The primary nitrogen species include pyridinic, nitrilic, and graphitic with no evidence of pyrrolic nitrogen. Upon exposure to ambient conditions, the carbon defects react and produce both –CO and –C–OH species. Sputtering Pt : Ru and subsequent air exposure introduces more defects that react with ambient oxygen to increase the number of –CO species. The samples also show signs of oxidization after implantation. Electrochemical cycling of the samples restores the C 1s fine structure associated with graphitic (sp2) carbon and alters the concentration of nitrogen species associated with the nitrile functional groups. The cycling also induces platinum oxidation and ruthenium loss, determined from X-ray photoelectron spectroscopy (XPS) of the Pt 4f, Ru 3d and Ru 3p. The results provide useful evidence of the types of nitrogen species that are present after electrochemical processes which can be used in the rational design of future electrocatalyst systems.

The Influence of Surfaces and Deposition Processes on Pt Structure and Properties

Transparent conductive oxides (TCOs), In-Zn-O (IZO) and Ga-Zn-O (GZO), on glass are used as model substrates to study the effect of surface treatments and deposition processes on Pt growth and nanostructure. The TCO type and surface treatments appear to affect Pt nucleation and growth. Ar and O2 plasma surface treatments significantly lowered the contact angle of water measured on TCOs compared to trimethylaluminum surface treatment and samples without surface treatment. Annealing TCO samples in oxygen resulted in lower IZO conductivity and higher contact angle; while annealing in vacuum or hydrogen resulted in increased carbon on the surface, which appears to be related to higher water contact angles and higher conductivity. Higher amounts of zinc and carbon (probably due to contamination from the annealing chamber) on the IZO surface seem to correlate with lower water contact angles and lower conductivity.