Edward F. Holby

Pt nanoparticle stability in PEM fuel cells: influence of particle size distribution and crossover hydrogen

This work demonstrates the essential role of particle size and crossover hydrogen on the degradation of platinum polymer electrolyte membrane fuel cell (PEMFC) cathodes. One of the major barriers to implementation of practical PEMFCs is the degradation of the cathode catalyst under operating conditions. This work combines both experimental and theoretical techniques to develop a validated and thermodynamically consistent kinetic model for the coupling of degradation and the catalyst particle size distribution. Our model demonstrates that, due to rapid changes in the Gibbs–Thomson energy, particle size effects dominate degradation for ∼2 nm particles but play almost no role for ∼5 nm particles. This result can help guide synthesis of more stable distributions. We also identify the effect of hydrogen molecules that cross over from the anode, demonstrating that in the presence of this crossover hydrogen surface area loss is greatly enhanced. We demonstrate that crossover hydrogen changes the surface area loss mechanism from coarsening to platinum loss through dissolution and precipitation off of the carbon support.

Experimental Observation of Redox-Induced Fe–N Switching Behavior as a Determinant Role for Oxygen Reduction Activity

The commercialization of electrochemical energy conversion and storage devices relies largely upon the development of highly active catalysts based on abundant and inexpensive materials. Despite recent achievements in this respect, further progress is hindered by the poor understanding of the nature of active sites and reaction mechanisms. Herein, by characterizing representative iron-based catalysts under reactive conditions, we identify three Fe-N4-like catalytic centers with distinctly different Fe-N switching behaviors (Fe moving toward or away from the N4-plane) during the oxygen reduction reaction (ORR), and show that their ORR activities are essentially governed by the dynamic structure associated with the Fe(2+/3+) redox transition, rather than the static structure of the bare sites. Our findings reveal the structural origin of the enhanced catalytic activity of pyrolyzed Fe-based catalysts compared to nonpyrolyzed Fe-macrocycle compounds. More generally, the fundamental insights into the dynamic nature of transition-metal compounds during electron-transfer reactions will potentially guide rational design of these materials for broad applications.

Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst

Replacing platinum in air-fed fuel cells Replacing expensive and scarce platinum catalysts in polymer electrolyte membrane fuel cells for the oxygen reduction reaction (ORR) with ones based on non-noble metals would speed up the adoption of hydrogen fuel vehicles. Most of the candidate replacement catalysts that have shown high performance do so only when running on pure oxygen. Chung et al. developed an iron-nitrogen-carbon catalyst from two nitrogen precursors that forms a high-porosity structure and exhibits high ORR performance when running on air. The proposed catalytically active site is FeN4. Science, this issue p. 479 A hierarchically structured iron-nitrogen-carbon catalyst for the oxygen reduction reaction is highly active in air. Platinum group metal–free (PGM-free) metal-nitrogen-carbon catalysts have emerged as a promising alternative to their costly platinum (Pt)–based counterparts in polymer electrolyte fuel cells (PEFCs) but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen (H2)–air fuel cell. Herein, we demonstrate H2-air performance gains achieved with an iron-nitrogen-carbon catalyst synthesized with two nitrogen precursors that developed hierarchical porosity. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ~0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared. The proposed catalytic active site, carbon-embedded nitrogen-coordinated iron (FeN4), was directly visualized with aberration-corrected scanning transmission electron microscopy, and the contributions of these active sites associated with specific lattice-level carbon structures were explored computationally.

In Situ Anomalous Small-Angle X-ray Scattering Studies of Platinum Nanoparticle Fuel Cell Electrocatalyst Degradation

Polymer electrolyte fuel cells (PEFCs) are a promising high-efficiency energy conversion technology, but their cost-effective implementation, especially for automotive power, has been hindered by degradation of the electrochemically active surface area (ECA) of the Pt nanoparticle electrocatalysts. While numerous studies using ex situ post-mortem techniques have provided insight into the effect of operating conditions on ECA loss, the governing mechanisms and underlying processes are not fully understood. Toward the goal of elucidating the electrocatalyst degradation mechanisms, we have followed Pt nanoparticle growth during potential cycling of the electrocatalyst in an aqueous acidic environment using in situ anomalous small-angle X-ray scattering (ASAXS). ASAXS patterns were analyzed to obtain particle size distributions (PSDs) of the Pt nanoparticle electrocatalysts at periodic intervals during the potential cycling. Oxide coverages reached under the applied potential cycling protocols were both calculated and determined experimentally. Changes in the PSD, mean diameter, and geometric surface area identify the mechanism behind Pt nanoparticle coarsening in an aqueous environment. Over the first 80 potential cycles, the dominant Pt surface area loss mechanism when cycling to 1.0-1.1 V was found to be preferential dissolution or loss of the smallest particles with varying extents of reprecipitation of the dissolved species onto existing particles, resulting in particle growth, depending on potential profile. Correlation of ASAXS-determined particle growth with both calculated and voltammetrically determined oxide coverages demonstrates that the oxide coverage is playing a key role in the dissolution process and in the corresponding growth of the mean Pt nanoparticle size and loss of ECA. This understanding potentially reduces the complex changes in PSD and ECA resulting from various voltage profiles to a response dependent on oxide coverage.

Activity of N-coordinated multi-metal-atom active site structures for Pt-free oxygen reduction reaction catalysis: Role of *OH ligands

We report calculated oxygen reduction reaction energy pathways on multi-metal-atom structures that have previously been shown to be thermodynamically favorable. We predict that such sites have the ability to spontaneously cleave the O2 bond and then will proceed to over-bind reaction intermediates. In particular, the *OH bound state has lower energy than the final 2 H2O state at positive potentials. Contrary to traditional surface catalysts, this *OH binding does not poison the multi-metal-atom site but acts as a modifying ligand that will spontaneously form in aqueous environments leading to new active sites that have higher catalytic activities. These *OH bound structures have the highest calculated activity to date.

Growth Trajectories and Coarsening Mechanisms of Metal Nanoparticle Electrocatalysts

Functional ensembles of nanoscale dimension particles are of great importance in many areas of science and technology; for example, in the field of catalysis. The structural properties of an individual nanoparticle, such as particle size or shape, closely correlate with its catalytic activity. Within a particle ensemble, structural properties can be distributed over a range of values, making the observed catalytic activity a complex convolution of individual contributions. This complexity is why a fundamental understanding of structure–activity relationships of particle ensembles requires a statistically reliable and accurate measurement of the distribution of relevant particle properties. Understanding the underlying atomic-scale mechanisms and processes that cause performance degradation of supported, catalytically active Pt nanoparticles is of utmost importance for the design of durable electrocatalytic energy conversion devices, such as polymer electrolyte membrane fuel cells (PEMFCs). Electrode performance depends largely on the total available electrochemically active Pt particle surface area (ECSA), which in turn depends on the size of the catalytically active particles. Hence, an accurate experimental description of the time changes of the Pt particle size distribution (PSD) during electrochemical operation is a key requirement to better understand how and why distributed Pt particle electrodes loses active surface area and activity. A common method to measure PSDs of electrocatalysts involves direct space TEMderived histograms. TEM, however, is an invasive analytical method, which makes in situ observations of PSD evolutions difficult. Previous work has identified a small number of basic mechanisms that control active surface-area loss under electrochemical conditions: first, Pt dissolution off the conductive support (dissolution mass loss mechanism); second, Pt particle Ostwald ripening (Ostwald coarsening mechanism); third, active area decrease attributed to particle migration followed by particle coalescence (coalescence coarsening mechanism) or else, fourth, Pt surface area loss owing to support corrosion leading to the detachment of Pt particles (detachment mass loss mechanism). Accurate discrimination between competing particle growth mechanisms, such as Ostwald ripening and migration/coalescence has been of great interest early on. Wynblatt and Gjostein derived detailed kinetic expressions for Ostwald and coalescence growth, and showed differences in the expected kinetic exponent. Unfortunately, the experimental errors in the particle growth trajectories often limited a reliable conclusion as to the dominant growth mechanism. Granqvist and Buhrmann claimed that the shape of the PSD can provide clues as to the growth mechanism. A more detailed understanding of the role of PSD in ECSA loss and a clarification of which coarsening and loss mechanisms are mainly affecting the PSD in the course of electrochemical treatment has become a recent focus of research. Building on early approaches ever more sophisticated Pt coarsening models have been proposed to provide insight in experimental changes in PSD and concomitant ECSA losses. Since modeling of TEM-derived data is often limited to the initial and final PSD data, X-ray derived PSD data is an interesting alternative. However, combined experimental and modeling approaches have been missing. Small angle X-ray scattering (SAXS) can be used to noninvasive measure the time evolution of Pt nanoparticle PSDs. It can be applied in situ and provides statistically reliable average structural information of nanoscale objects in the 1– 100 nm range. Haubold et al. pioneered the use of SAXS to investigate growth and transformations of carbon-supported Pt nanoparticle electrocatalysts. Later, Stevens and Dahn used ex situ SAXS to determine Pt PSDs. More recently, anomalous SAXS was used to study structural details of bimetallic core-shell electrocatalysts, whereas in situ SAXS studies revealed a time-resolved look at Pt particle growth under potential cycling. Here, we present for the first time a comprehensive computational and experimental approach to investigate and understand the structural stability of Pt nanoparticle electrocatalysts through in situ time-resolved measurements and modeling. In situ SAXS is used to measure changes in the PSD (growth trajectories) under electrochemical conditioning mimicking a PEMFC cathode. The PSD trajectories are compared to computational predictions using particle dissolution/coarsening model. Variation of the initial PSD and the applied electrode [a] Dr. C. Yu Department of Chemical and Biomolecular Engineering University of Houston Houston, TX 77204 (USA) [b] Dr. E. F. Holby, Prof. D. Morgan Department of Materials Science and Engineering University of Wisconsin Madison, WI 53706 (USA) [c] Dr. R. Yang, Dr. M. F. Toney Stanford Institute of Materials and Energy Sciences and Stanford Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory Menlo Park, CA 94025 (USA) [d] Prof. Dr. P. Strasser Department of Chemistry Chemical Engineering Division Technical University Berlin 10623 Berlin (Germany) E-mail : pstrasser@tu-berlin.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200090.

Critical role of intercalated water for electrocatalytically active nitrogen-doped graphitic systems

Removal of intercalated water within graphitic sheets is critical to achieving high-performing oxygen reduction reaction catalysts. Graphitic materials are essential in energy conversion and storage because of their excellent chemical and electrical properties. The strategy for obtaining functional graphitic materials involves graphite oxidation and subsequent dissolution in aqueous media, forming graphene-oxide nanosheets (GNs). Restacked GNs contain substantial intercalated water that can react with heteroatom dopants or the graphene lattice during reduction. We demonstrate that removal of intercalated water using simple solvent treatments causes significant structural reorganization, substantially affecting the oxygen reduction reaction (ORR) activity and stability of nitrogen-doped graphitic systems. Amid contrasting reports describing the ORR activity of GN-based catalysts in alkaline electrolytes, we demonstrate superior activity in an acidic electrolyte with an onset potential of ~0.9 V, a half-wave potential (E½) of 0.71 V, and a selectivity for four-electron reduction of >95%. Further, durability testing showed E½ retention >95% in N2- and O2-saturated solutions after 2000 cycles, demonstrating the highest ORR activity and stability reported to date for GN-based electrocatalysts in acidic media.

Structurally Defined 3D Nanographene Assemblies via Bottom-Up Chemical Synthesis for Highly Efficient Lithium Storage

Functionalized 3D nanographenes with controlled electronic properties have been synthesized through a multistep organic synthesis method and are further used as promising anode materials for lithium-ion batteries, exhibiting a much increased capacity (up to 950 mAh g-1 ), three times higher than that of the graphite anode (372 mAh g-1 ).

New Understanding of Pt Surface Area Loss in PEMFC's: Temperature Effects

Loss of nanoparticle surface area under polymer electrolyte membrane fuel cell (PEMFC) operating conditions is an important barrier to the practical application of PEMFCs. We have developed an electrochemical rate theory model to better understand a number of aspects of Pt surface area loss, including the effects of particle size distribution, crossover molecular hydrogen, and Pt oxidation. As a specific example we here discuss some initial efforts to understand the effects of temperature on Pt nanoparticle stability. We compare our results to a previously developed first-order kinetic model and use this comparison to evaluate the role of the kinetic activation enthalpy for Pt dissolution and precipitation on overall surface area loss.