Karen E. Swider-Lyons

Experimental Methods for Quantifying the Activity of Platinum Electrocatalysts for the Oxygen Reduction Reaction

A tutorial is provided for methods to accurately and reproducibly determine the activity of Pt-based electrocatalysts for the oxygen reduction reaction in proton exchange membrane fuel cells and other applications. The impact of various experimental parameters on electrocatalyst activity is demonstrated, and explicit experimental procedures and measurement protocols are given for comparison of electrocatalyst activity to fuel cell standards. (To listen to a podcast about this article, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html.).

Impact of Sulfur Dioxide on the Oxygen Reduction Reaction at Pt/Vulcan Carbon Electrocatalysts

The poisoning of the oxygen-reduction reaction (ORR) by adsorbed sulfur-containing species was quantified for platinum fuel-cell materials using rotating ring disk electrode methodology. Electrodes of Pt on Vulcan carbon (Pt/VC) were contaminated by submersion in SO 2 -containing solutions. The initial sulfur coverage of the Pt was determined from the total charge consumed as the sulfur was oxidized from S° at 0.05 V (vs a reversible hydrogen electrode) to water-soluble sulfate (SO 2- 4 ) at >1.3 V. Electrodes were then evaluated for their ORR activity. Significant (33%) loss in Pt mass activity was measured when approximately 1.2% of the Pt surface had adsorbed the sulfur-containing species. Sulfur coverage of 14% caused a 95% loss in mass activity. When 37% of the Pt surface was covered with sulfur, the reaction pathway of the ORR on the Pt/VC catalyst changed from a 4-electron to 2-electron process reaction for peroxide, a reagent which can aggressively attack Nafion. We conclude that adsorbed sulfur is not removed under typical steady-state operating conditions of a proton exchange membrane fuel cell, so it will affect operation by decreasing mass activity of the catalysts and by enhancing formation of the deleterious H 2 O 2 by-product.

Improved lithium capacity of defective V2O5 materials

We demonstrate that point defects may be introduced into a metal oxide to increase its Li-ion capacity by using various heat treatments to modify the defect structure of polycrystalline V2O5 and then comparing the Li capacity of the materials. The V2O5 that is heated under O2/H2O at 460 jC has a 23% higher Li capacity than the as-received material despite no change to its longrange structure. Other heating conditions lower the Li capacity of the V2O5. We infer that heating under O2/H2O introduces defects, such as cation vacancies associated with lithiated oxygen sites, which can electrochemically exchange Li ions and serve as additional charge-storage sites. This study may also explain how metal oxides synthesized from sol–gels, such as xerogels and aerogels, insert lithium ions without concomitant reduction of transition-metal-ion sites—high-surface-area metal oxides are likely to be nonstoichiometric and rich with surface point defects which can serve as additional charge-storage sites. D 2002 Published by Elsevier Science B.V.

High-Performance Solid Oxide Fuel Cell Cathodes with Lanthanum-Nickelate-Based Composites

Lanthanum nickelate (LN, La 2 NiO 4-δ ) is a mixed ionic and electronic conductor with oxygen self-diffusion and surface exchange properties that predict that it will be a good cathode catalyst for solid oxide fuel cells. We show that LN performs poorly (0.3 W cm -2 ) when used as a single-phase cathode in yttria-stabilized-zirconia-based air-H 2 dime cells at 800°C. Power densities up to 2.2 W cm -2 are measured only when LN is used in a composite bilayer cathode. Parametric modeling shows that the kinetics of the LN-based cathodes is relatively poor, but their low ohmic resistance leads to their high power density.

Enhanced Oxygen Reduction Activity in Acid by Tin-Oxide Supported Au Nanoparticle Catalysts

Gold nanoparticles supported on hydrous tin-oxide (Au-SnO{sub x}) are active for the four-electron oxygen reduction reaction in an acid electrolyte. The unique electrocatalytic of the Au-SnO is confirmed by the low amount of peroxide detected with rotating ring-disk electrode voltammetry and Koutecky-Levich analysis. In comparison, 10 wt % Au supported on Vulcan carbon and SnO{sub x} catalysts both produce significant peroxide in the acid electrolyte, indicating only a two-electron reduction reaction. Characterization of the Au-SnO{sub x} catalyst reveals a high-surface area, amorphous support with 1.7 nm gold metal particles. The high catalytic activity of the Au-SnO is attributed to metal support interactions. The results demonstrate a possible path to non-Pt catalysts for proton exchange membrane fuel cell cathodes.

Direct-write planar microultracapacitors by laser engineering

We have successfully employed laser direct write and micromachining to fabricate high capacity hydrous ruthenium oxide (RuO x H y or RuO 2 .xH 2 O) microultracapacitors. A laser direct-write process is used to deposit uniform pads of RuO 2 .0.5H 2 O in sulfuric acid under ambient temperature and atmospheric conditions. Ultraviolet laser micromachining is used to tailor the shape and size of the deposited material into planar electrodes. The specific capacitance of the laser-deposited materials is comparable to reported values of ∼720 F/g. The microultracapacitors demonstrate linear charge and discharge behavior at currents below 1 mA, as expected for an ideal capacitor. By studying the charge storage and power output as a function of discharge current, the power can be successfully modeled assuming only simple ohmic losses. Parallel and series combinations of these microultracapacitor cells provide the expected addition of capacitance. Maximum discharge currents of 50 mA are applied to two cells in parallel without damage to the microultracapacitor cells. The microultracapacitors exhibit high specific power and specific energy with over 1100 mW/g at approximately 9 mWhr/g for an 80 μg cell with a footprint of 2 mm 2 and a thickness of 15 μm.

Oxygen Reduction Reaction on Platinum/Tantalum Oxide Electrocatalysts for PEM Fuel Cells

We investigate platinum supported on tantalum oxide as a possible catalyst for oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells. Three synthetic routes are evaluated to compare activities of tantalum-oxide-supported platinum fuel cell electrocatalysts: (i) deposition of platinum colloids on tantalum oxide followed by mechanical grinding with Vulcan carbon (VC); (ii) deposition of tantalum oxide on VC, followed by the deposition of platinum colloids; and (iii) deposition of Pt colloids on VC, followed by deposition of tantalum oxide. These are compared to a Pt/VC standard made with the same Pt colloids. The area-specific activities for the ORR at 0.9 V are a factor of 1.5 higher for catalysts synthesized via preparation route (ii) compared to a Pt/VC standard. The area-specific activities of the catalysts synthesized via routes (i) and (iii) are close to that of Pt/VC. The higher area-specific activity of the catalyst synthesized by route (ii) may be due to the preferential adsorption of OH groups to the oxide vs platinum surface.

FIB Damage in Silicon: Amorphization or Redeposition?

The structural and chemical heterogeneity of 2.5-nm Pt50Ru50 electrocatalysts was studied by transmission electron microscopy using selected area diffraction, lattice imaging, electron-energy loss spectroscopy, and energy-dispersive X-ray spectroscopy. The catalysts with the highest methanol oxidation activities exhibit oxidation-induced phase separation on the nanoscale to from Pt-rich metal embedded in Ru-rich hydrous and anhydrous oxide. Reduction of the oxide-on metal samples produces a true bimetallic face-centered cubic Pt50Ru50 alloy, with 275 times lower oxidation activity.

Platinum-Iron Phosphate Electrocatalysts for Oxygen Reduction in PEMFCs

Proton exchange membrane fuel cells (PEMFCs) depend on platinum at the cathode to catalyze the oxygen reduction reaction (ORR) and maintain high performance. This report shows that the electrocatalytic activity of Pt is enhanced when it is dispersed in a matrix of hydrous iron phosphate (FePO). The Pt-FePO has 2 nm micropores with Pt dispersed as ions in Pt 2+ and Pt 4+ oxidation states. Increased ORR performance is demonstrated for the Pt-FePO + Vulcan carbon (VC) materials compared to a standard 20 wt % Pt-VC catalyst on rotating disk electrodes with Pt-loadings of 0.1 mg(Pt) cm -2 . The improvement in the ORR is attributed to the adsorption/storage of oxygen on the FePO, presumably as iron-hydroperoxides. The ORR activity of the Pt-FePO in air is close to that in oxygen at low current density, and therefore this catalyst has a distinctly unique behavior from Pt-VC. Contrary to Pt-VC, the Pt-FePO catalyst shows activity towards hydrogen and CO oxidation, but does not exhibit their characteristic adsorption peaks, suggesting that Pt ions in the iron phosphate structure are less sensitive to poisoning than metallic Pt. The results present opportunities for new low-Pt catalysts that extend beyond the current capabilities of Pt-VC.

Oxygen Reduction Reaction Kinetics of SO2-Contaminated Pt3Co and Pt/Vulcan Carbon Electrocatalysts

Sulfur dioxide, SO2, is a common impurity in air that is known to deactivate electrocatalysts for the oxygen reduction reaction ORR at proton exchange membrane fuel cell cathodes. The SO2 poisoning of a Vulcan-carbon-supported platinum cobalt alloy Pt3Co/VC is compared to that of a standard platinum Pt/VC electrocatalyst using cyclic voltammetry CV and rotating ring-disk electrode RRDE methodology at controlled concentrations of SIV in an oxygen-free solution. The CV and RRDE measurements show that for electrodes with the same Pt loading, the Pt3Co/VC is two times more active than the Pt/VC. Upon exposure to SIV solutions, the Pt3Co/VC nanoparticle electrocatalysts are more poisoned than the Pt/VC ones, and their initial sulfur coverage is higher. The poisoning of both catalysts is accompanied by an increase in the amount of H2O2 production, as adsorbed sulfur species inhibit the four-electron ORR. The Pt3Co/VC electrocatalyst loses 80% activity in a 0.0001 M SIV compared to a 30% loss by the Pt/VC electrocatalysts. The adsorbed sulfur species are more easily removed from the Pt3Co/VC than the Pt/VC by potential cycling, implying a weaker bonding between S x species and Pt3Co/VC. We conclude that Pt3Co is