Deborah J. Myers

Synthesis-structure-performance correlation for polyaniline-Me-C non-precious metal cathode catalysts for oxygen reduction in fuel cells

In this report, we present the systematic preparation of active and durable non-precious metal catalysts (NPMCs) for the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs) based on the heat treatment of polyaniline/metal/carbon precursors. Variation of the synthesis steps, heat-treatment temperature, metal loading, and the metal type in the synthesis leads to markedly different catalyst activity, speciation, and morphology. Microscopy studies demonstrate notable differences in the carbon structure as a function of these variables. Balancing the need to increase the catalyst’s degree of graphitization through heat treatment versus the excessive loss of surface area that occurs at higher temperatures is a key to preparing an active catalyst. XPS and XAFS spectra are consistent with the presence of Me–Nx structures in both the Co and Fe versions of the catalyst, which are often proposed to be active sites. The average speciation and coordination environment of nitrogen and metal, however, depends greatly on the choice of Co or Fe. Taken together, the data indicate that better control of the metal-catalyzed transformations of the polymer into new graphitized carbon forms in the heat-treatment step will allow for even further improvement of this class of catalysts.

Effect of Voltage on Platinum Dissolution Relevance to Polymer Electrolyte Fuel Cells

One of the processes responsible for performance degradation of a polymer electrolyte fuel cell (PEFC) is the loss of the electrochemically active surface area of the platinum-based electrocatalysts, due in part to platinum dissolution. The long-term dissolution behavior of polycrystalline platinum and high-surface-area carbon-supported platinum particles was studied under potentiostatic conditions relevant to PEFC cathode conditions. The equilibrium concentration of dissolved Pt was found to increase monotonically from 0.65 to 1.1 V (vs SHE) and decrease at potentials >1.1 V. Dissolution rates measured at 0.9 V were comparable for the two types of electrodes (1.4 and 1.7 × 10 -14 g/cm 2 s).

Evidence for lithium superoxide-like species in the discharge product of a Li–O2 battery

We report on the use of a petroleum coke-based activated carbon (AC) with very high surface area for a Li-O(2) battery cathode without the use of any additional metal catalysts. Electrochemical measurement in a tetra(ethylene) glycol dimethyl ether-lithium triflate (TEGDME-LiCF(3)SO(3)) electrolyte results in two voltage plateaus during charging at 3.2-3.5 and 4.2-4.3 V versus Li(+)/Li. Herein we present evidence from Raman and magnetic measurements that the lower plateau corresponds to a form of lithium peroxide with superoxide-like properties characterized by a low temperature magnetic phase transition and a high O-O stretching frequency (1125 cm(-1)). The magnetic phase transition and the high O-O stretching frequency disappear when charged to above 3.7 V. Theoretical calculations indicate that a surface superoxide structure on lithium peroxide clusters and some lithium peroxide surfaces have an unpaired electron and a high O-O stretching frequency that help explain the observations. These results provide evidence that the form of the lithium peroxide discharge product is important to obtaining a low charge overpotential, and thus improving the round-trip efficiency between discharge and charge.

A carbon-nanotube-supported graphene-rich non-precious metal oxygen reduction catalyst with enhanced performance durability

A non-precious metal catalyst for oxygen reduction in acid media, enriched in graphene sheets/bubbles during a high-temperature synthesis step, has been developed from an Fe precursor and in situ polymerized polyaniline, supported on multi-walled carbon nanotubes. The catalyst showed no performance loss for 500 hours in a hydrogen/air fuel cell. The improved durability is correlated with the graphene formation, apparently enhanced in the presence of carbon nanotubes.

Activity–Stability Trends for the Oxygen Evolution Reaction on Monometallic Oxides in Acidic Environments

In the present study, we used a surface-science approach to establish a functional link between activity and stability of monometallic oxides during the OER in acidic media. We found that the most active oxides (Au ≪ Pt < Ir < Ru ≪ Os) are, in fact, the least stable (Au ≫ Pt > Ir > Ru ≫ Os) materials. We suggest that the relationships between stability and activity are controlled by both the nobility of oxides as well as by the density of surface defects. This functionality is governed by the nature of metal cations and the potential transformation of a stable metal cation with a valence state of n = +4 to unstable metal cation with n > +4. A practical consequence of such a close relationship between activity and stability is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate is neither too fast nor too slow.

Performance Durability of Polyaniline-derived Non-precious Cathode Catalysts

This research has focused on performance durability of the newly-developed polyaniline (PANI)-derived non-precious metal cathode catalysts. These catalysts show high oxygen-reduction activity in electrochemical and fuel cell testing, reflected by the onset and half-wave (E1/2) potentials of oxygen reduction in RDE testing of 0.90 V and 0.77 V, respectively. Best-performing catalysts also exhibit insignificant H2O2 yield of less than 1%. Catalyst performance in fuel cell testing strongly depends on the choice of nitrogen precursors, transition metals used, and carbon supports. As expected, catalyst stability is affected by the operating voltage of the fuel cell, with more stable performance observed at low operating voltage and open cell voltage, than at intermediate voltages. Physical and electrochemical characterization of the catalysts, also in the presence of hydrogen peroxide, has been carried out to provide insight into the origin of possible degradation mechanisms.

Using Surface Segregation To Design Stable Ru‐Ir Oxides for the Oxygen Evolution Reaction in Acidic Environments

The methods used to improve catalytic activity are well-established, however elucidating the factors that simultaneously control activity and stability is still lacking, especially for oxygen evolution reaction (OER) catalysts. Here, by studying fundamental links between the activity and stability of well-characterized monometallic and bimetallic oxides, we found that there is generally an inverse relationship between activity and stability. To overcome this limitation, we developed a new synthesis strategy that is based on tuning the near-surface composition of Ru and Ir elements by surface segregation, thereby resulting in the formation of a nanosegregated domain that balances the stability and activity of surface atoms. We demonstrate that a Ru0.5Ir0.5 alloy synthesized by using this method exhibits four-times higher stability than the best Ru-Ir oxygen evolution reaction materials, while still preserving the same activity.

Stability and Dissolution of Platinum Surfaces in Perchloric Acid

Stability and dissolution of platinum single-crystal surfaces were investigated with atomic force microscopy and inductively coupled plasma mass spectrometry. Both low-index surfaces and nanofaceted surfaces were investigated. A clear difference was observed between the large low-index surfaces and the nanofaceted surfaces. In the low-index surfaces, the platinum oxide formation passivates the surfaces, resulting in a lower dissolution rates at higher potentials. The nanofaceted surface dissolves faster at a higher potential, indicating the edges and comers are the main sources of dissolution. The differences in the dissolution behaviors between the low-index surfaces, (111), (100), and (110), are also discussed.

Bimetallic Pd-Cu oxygen reduction electrocatalysts

A series of Vulcan carbon-supported Pd-Cu catalysts with various molar ratios of Pd to Cu was prepared by co-impregnation followed by a reduction in a hydrogen atmosphere at three different temperatures. The degree of alloying between the two metals, alloy composition, and particle size and size distribution were characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The electrocatalytic activity for the oxygen reduction reaction (ORR) for these various compositions was determined using the thin-film rotating disk electrode technique. Our study reveals that the Pd-Cu bimetallic electrocatalysts, with a suitable degree of alloying, offer a greatly enhanced ORR activity compared to the Pd monometallic electrocatalyst. The best electrocatalytic activities were observed for the bimetallic catalysts that showed alloy nanoparticles with a Pd-Cu molar ratio of approximately 1:1.

In situ small-angle X-ray scattering observation of Pt catalyst particle growth during potential cycling.

A unique type of inorganic-organic hybrid semiconductor bulk material is capable of emitting direct white light. Their photoluminescence properties can be tuned precisely and systematically by modifying structures and composition. They could be used as a single-material light-emitting source in high efficiency white-light-emitting diodes.