Olga A. Baturina

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.

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.

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

Effect of SO2 on the Performance of the Cathode of a PEM Fuel Cell at 0.5–0.7 V

The effect of 1 ppm SO 2 in air on the performance of the proton exchange membrane (PEM) fuel cell cathodes was studied at operating voltages of 0.5, 0.6, and 0.7 V at 80°C and 100% relative humidity. The coverage of the cathode catalyst (50% Pt/Vulcan carbon) with sulfur species θ S after exposure to SO 2 in air at different cell voltages is evaluated from the hydrogen desorption region on cyclic voltammetry (CV) curves and from ohmic-resistance-corrected polarization curves. The θ s calculated from CVs after exposure to SO 2 in air is compared to those of cathodes exposed to SO 2 in nitrogen. The θ S decreases as the cathode potential increases in both air and nitrogen. The cathodes poisoned in air have 2.5 times lower sulfur coverage than those poisoned in nitrogen. The cells poisoned at 0.6 V have the highest performance vs those poisoned at 0.5 and 0.7 V at constant current density; even though the sulfur coverage is lowest in the cell poisoned at 0.7 V, adsorbed hydroxy groups and sulfate/bisulfate ions on the platinum surface are likely to impede the oxygen reduction reaction at 0.7 V. The sulfate/bisulfate ions can be removed from the surface by holding the cathode at its open-circuit voltage in nitrogen.

Products of SO2 Adsorption on Fuel Cell Electrocatalysts by Combination of Sulfur K-Edge XANES and Electrochemistry

Electrochemical adsorption of SO(2) on platinum is complicated by the change in sulfur oxidation state with potential. Here, we attempt to identify SO(2) adsorption products on catalyst coated membranes (CCMs) at different electrode potentials using a combination of in situ sulfur K-edge XANES (X-ray absorption near-edge structure) spectroscopy and electrochemical techniques. CCMs employed platinum nanoparticles supported on Vulcan carbon (Pt/VC). SO(2) was adsorbed from a SO(2)/N(2) gas mixture while holding the Pt/VC-electrode potential at 0.1, 0.5, 0.7, and 0.9 V vs a reversible hydrogen electrode (RHE). Sulfur adatoms (S(0)) are identified as the SO(2) adsorption products at 0.1 V, while mixtures of S(0), SO(2), and sulfate/bisulfate ((bi)sulfate) ions are suggested as SO(2) adsorption products at 0.5 and 0.7 V. At 0.9 V, SO(2) is completely oxidized to (bi)sulfate ions. The identity of adsorbed SO(2) species on Pt/VC catalysts at different electrode potentials is confirmed by modeling of XANES spectra using FEFF8 and a linear combination of experimental spectra from sulfur standards. Results on SO(2) speciation gained from XANES are used to compare platinum-sulfur electronic interactions for Pt(3)Co/VC versus Pt/VC catalysts in order to understand the difference between the two catalysts in terms of SO(2) contamination.

Comparison of the Sulfur Poisoning of PBI and Nafion PEMFC Cathodes

The poisoning effect of H 2 S and SO 2 in air is compared for phosphoric-acid-doped polybenzimidazole (PBI) membrane and perfluorosulfonic acid (Nafion) proton exchange membrane fuel cells (PEMFCs). The cathodes of PBI PEMFCs are about 70 times more resistant to 1 ppm of H 2 S or S0 2 than the Nafion PEMFCs. The PBI PEMFCs only lose <2% in cell performance when exposed to 1 ppm of H 2 S or S0 2 and have 5.2 and 7.1% losses with 5 and 10 ppm H 2 S over 24 h, respectively. Purging the poisoned PBI cells with neat air leads to complete performance recovery.

Insights into PEMFC Performance Degradation from HCl in Air

The performance degradation of a proton exchange membrane fuel cell (PEMFC) is studied in the presence of HCl in the air stream. The cathode employing carbon-supported platinum nanoparticles (Pt/C) was exposed to 4 ppm HCl in air while the cell voltage was held at 0.6 V. The HCl poisoning results in generation of chloride and chloroplatinate ions on the surface of Pt/C catalyst as determined by a combination of electrochemical tests and ex-situ chlorine K-edge X-Ray absorption near-edge structure (XANES) spectroscopy. The chloride ions inhibit the oxygen reduction reaction (ORR) and likely affect the wetting properties of diffusion media/catalyst layer, while the chloroplatinate ions are responsible for enhanced platinum particle growth most likely due to platinum dissolution-redeposition. The chloride ions can cause corrosion of the Pt nanoparticles in the presence of aqueous HCl in air even if no potential is applied. Although the majority of chloride ions are desorbed from the Pt surface by hydrogen treatment of the cathode, they partially remain in the system and re-adsorb on platinum at cell voltages of 0.5-0.9 V. Chloride ions are removed from the system and fuel cell performance at 0.5-0.7 V is restored by multiple exposures to low potentials.

Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis

We use plasmonic Au-TiO2 aerogels as a platform in which to marry synthetically thickened particle-particle junctions in TiO2 aerogel networks to Au∥TiO2 interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H2) generation under both broadband (i.e., UV + visible light) and visible-only excitation. In doing so, we elucidate the dual functions that incorporated Au can play as a water reduction cocatalyst and as a plasmonic sensitizer. We also photodeposit non-plasmonic Pt cocatalyst nanoparticles into our composite aerogels in order to leverage the catalytic water-reducing abilities of Pt. This Au-TiO2/Pt arrangement in three dimensions effectively utilizes conduction-band electrons injected into the TiO2 aerogel network upon exciting the Au SPR at the Au∥TiO2 interface. The extensive nanostructured high surface-area oxide network in the aerogel provides a matrix that spatially separates yet electrochemically connects plasmonic nanoparticle sensitizers and metal nanoparticle catalysts, further enhancing solar-fuels photochemistry. We compare the photocatalytic rates of H2 generation with and without Pt cocatalysts added to Au-TiO2 aerogels and demonstrate electrochemical linkage of the SPR-generated carriers at the Au∥TiO2 interfaces to downfield Pt nanoparticle cocatalysts. Finally, we investigate visible light-stimulated generation of conduction band electrons in Au-TiO2 and TiO2 aerogels using ultrafast visible pump/IR probe spectroscopy. Substantially more electrons are produced at Au-TiO2 aerogels due to the incorporated SPR-active Au nanoparticle, whereas the smaller population of electrons generated at Au-free TiO2 aerogels likely originate at shallow traps in the high surface-area mesoporous aerogel.

Development of a sustained fluoride delivery system.

OBJECTIVE To develop a novel delivery system by which fluoride incorporated into elastomeric rings, such as those used to ligate orthodontic wires, will be released in a controlled and constant manner. MATERIALS AND METHODS Polyethylene co-vinyl acetate (PEVA) was used as the model elastomer. Samples (N = 3) were prepared by incorporating 0.02 to 0.4 g of sodium fluoride (NaF) into previously prepared PEVA solution. Another group of samples prepared in the same manner were additionally dip-coated in PEVA to create an overcoat. Fluoride release studies were conducted in vitro using an ion selective electrode over a period of 45 days. The amount of fluoride released was compared to the optimal therapeutic dose of 0.7 microg F(-)/ring/d. RESULTS Only coated samples with the highest fluoride content (group D, 0.4 g of NaF) were able to release fluoride at therapeutic levels. When fluoride release from coated and uncoated samples with the same amount of NaF were compared, it was shown that the dip-coating technique resulted in a fluoride release in a controlled manner while eliminating the initial burst effect. CONCLUSIONS This novel fluoride delivery matrix provided fluoride release at a therapeutically effective rate and profile.