D. A. Stevens

High Capacity Anode Materials for Rechargeable Sodium‐Ion Batteries

Electrochemical techniques have been used to study the reversible insertion of sodium into hard‐carbon host structures at room temperature. In this paper we compare these results with those for lithium insertion in the same materials and demonstrate the presence of similar alkali metal insertion mechanisms in both cases. Despite the gravimetric capacities being lower for sodium than lithium insertion, we have achieved a reversible sodium capacity of 300 mAh/g, close to that for lithium insertion in graphitic carbon anode materials. Such materials may therefore be useful as anodes in rechargeable sodium‐ion batteries.

Ex Situ and In Situ Stability Studies of PEMFC Catalysts Effect of Carbon Type and Humidification on Degradation of the Carbon

We explore ex situ and in situ fuel cell catalyst degradation test methods and the impact of catalyst degradation on fuel cell performance. A series of platinum-loaded carbons with two different carbon supports are aged ex situ in an isothermal oven and in situ using a 1.2-V fuel cell accelerated test. The ex situ combustion test and in situ 1.2 V accelerated fuel cell test both show that the rate and extent of carbon combustion for samples with the same platinum loading increases as the surface area of the carbon increases, presumably because platinum deposits as smaller particles, covering more of the carbon surface. In addition, the rate of reaction/loss of carbon is shown to increase significantly if humidity is introduced into the environment, a concern for long-term polymer electrolyte membrane fuel cell (PEMFC) operation because PEMFCs operate in a hot, humidified, oxygenated environment. Finally, graphitized carbon black supports with the same surface area and platinum loading as ungraphitized supports show much greater stability in both the ex situ and in situ tests, suggesting that carbon surface chemistry plays a strong role in oxidative stability under fuel cell conditions.

Studies of Transition Metal Dissolution from Combinatorially Sputtered, Nanostructured Pt1 − x M x (M = Fe, Ni; 0 < x < 1 ) Electrocatalysts for PEM Fuel Cells

The dissolution of Fe and Ni from Pt 1 - x M x (M = Fe, Ni; 0 0.6, the lattice constant expands indicating that transition metals dissolve also from the bulk. X-ray photoelectron spectroscopy results show complete removal of surface Ni (Fe) after acid treatment at 80°C for all compositions. The results of the acid treatments compare well to the composition changes that occur when a Pt 1 - x Fe x or Pt 1 - x Ni x combinatorial catalyst library is used in an operating PEM fuel cell.

Electrochemical Characterization of the Active Surface in Carbon-Supported Platinum Electrocatalysts for PEM Fuel Cells

A series of carbon-supported nanosized platinum electrocatalysts for use in proton exchange membrane (PEM) fuel cells have been characterized electrochemically in acid solution. This analysis was performed under galvanostatic conditions. in contrast to the more traditional potentiodynamic methods that generate cyclic voltammograms (CVs). The galvanostatic method used has the advantage of collecting data more efficiently than a typical CV protocol. The resultant differential capacity curves (equivalent to a CV) for the samples analyzed showed that the electrochemical response changed as the platinum loading on a given carbon support increased. In particular, peaks developed in the low potential (hydrogen adsorption/desorption) regions while peaks corresponding to the desorption of oxygen-containing species shifted to higher potentials. A model is proposed to explain this behavior based on changes to the size of the platinum particles. [DOI: 10.1149/1.1573195] All rights reserved.

Fuel Cell Studies on a Non-Noble Metal Catalyst Prepared by a Template-Assisted Synthesis Route

The catalytic activity of a templated catalyst material (Fe-N-C), prepared using FeCl 3 , pyrrole, and a mesoporous silica gel as template, was investigated. The oxygen reduction reaction activity of the catalyst material was measured using the rotating ring-disk electrode technique, revealing an onset potential of about 0.85 V vs a reversible hydrogen electrode. The catalytic activity of the prepared sample was also tested in a 5 cm 2 fuel cell for different catalyst loadings using oxygen or air atmospheres. Fuel cell polarization measurements were recorded at 75°C. The power densities varied between 0.032 and 0.095 W/cm 2 for different catalyst loadings when oxygen was used. In addition, durability tests were carried out by keeping the cell at 0.6 V for 16 h. The current densities decayed to about 25% of the initial value during this period. The results are compared to state-of-the-art results from the literature. The templated catalyst shows comparable activity to the best catalysts in the literature but its durability requires improvement.

Characterization and PEMFC Testing of Pt1 − x M x ( M = Ru , Mo , Co , Ta , Au , Sn ) Anode Electrocatalyst Composition Spreads

Pt 1-x M x (M = Ru,Mo,Co,Ta,Au,Sn) random alloy samples, covering most of the binary composition range, have been prepared via magnetron sputtering. The alloys were deposited through shadow masks onto 3M nanostructured thin-film catalyst support for testing in a 64-electrode polymer electron membrane fuel cell (PEMFC). CO stripping voltammograms and hydrogen oxidation polarization curves with pure hydrogen and with reformate containing up to 50 ppm CO were measured on all the samples. In agreement with reports in the literature, Ru, Mo, and Sn were found to improve the CO tolerance of Pt, although the intrinsic hydrogen oxidation activity of Pt decreased significantly as the Sn content increased. The addition of Co to Pt had no impact on CO tolerance, possibly because of loss of surface Co through dissolution in the fuel cell. The addition of Au to Pt led to an increase in hydrogen oxidation overpotential when CO was present. Small amounts of Ta gave a small reduction in hydrogen oxidation overpotential in the presence of CO, but the overpotentials were still too high for practical application in a reformate-fed fuel cell.

On the determination of platinum particle size in carbon-supported platinum electrocatalysts for fuel cell applications

Abstract Using a conventional laboratory diffractometer, small angle X-ray scattering (SAXS) was used to determine the average platinum particle size in samples of carbon with a range of platinum loadings. The results obtained were compared with those obtained from wide-angle X-ray diffraction (WAXS) studies. SAXS was more effective than WAXS for determining the average platinum particle size in samples where the grains were so small that the resultant diffraction peaks in the WAXS profiles were too broad to accurately determine the peak width for use in the Scherrer equation.

Magnetron Sputtered Fe – C – N , Fe – C , and C – N Based Oxygen Reduction Electrocatalysts

Thin-film libraries of Fe x C 1-x-y N y (0 < x < 0.06, 0 < y < 0.5) have been prepared by combinatorial sputter deposition in a 40/60 N 2 /Ar process gas. The libraries were subsequently annealed between 800 and 1000°C to induce structural and compositional changes. These libraries contained/retained more nitrogen than those sputtered in lower N 2 partial pressures. Physical and electrochemical properties of these combinatorial libraries were studied using scanning electron microscopy, X-ray photoelectron spectroscopy, and rotating ring-disk electrode cells. Maximum catalytic activity for oxygen reduction reaction in acid was achieved for the libraries annealed at 800°C and approaches the best activity of Fe-C-N-based electrocatalysts reported in the literature to date.

RDE Measurements of ORR Activity of Pt1 − x Ir x ( 0 < x < 0.3 ) on High Surface Area NSTF-Coated Glassy Carbon Disks

Layered Pt 1-x Ir x (0 < x < 0.3) and Pt films were sputtered onto 0.05 μm mirror-polished and nanostructured thin film (NSTF)-coated glassy carbon (GC) disks. Rotating disk electrode (RDE) studies of oxygen reduction reaction (ORR) activity were completed for all prepared disks. The Pt 1-x Ir x film was prepared by depositing alternating layers of Pt (constant thickness) and Ir (gradient), finished with a 5 nm Pt top layer. The NSTF-supported catalysts had much higher active surface areas and reached the diffusion-limited current at higher potentials than the bare GC supported catalysts. The surface enhancement factor (SEF) of Pt on NSTF-coated disks was approximately 14. The SEF increased (reaching a maximum of 22 at x = 0.2 in Pt 1-x Ir x ) as the Ir content increased for the Pt 1-x Ir x samples on NSTF. The kinetic ORR current density also increased with increasing Ir content. A similar trend was not observed for the same catalyst coated onto bare GC disks. All of the catalyst/support combinations had identical Tafel slopes and area-specific current densities, suggesting that Pt is the active catalytic ingredient. Catalysts coated on NSTF-coated GC disks can be used to accurately examine both catalytic activities and the effects of high surface area supports in a single measurement.

Enhanced CO-Tolerance of Pt–Ru–Mo Hydrogen Oxidation Catalysts

A ternary composition spread, (Pt 1-x Ru x ) 1-y Moy, 0 < x < 1; 0 < y < 0.3, was prepared through sputter deposition onto a nano-structured thin film support. The film was found to be reasonably stable when exposed to acid at 80°C, although there was evidence for loss of some Mo, presumably through a corrosion mechanism. The catalytic activity towards hydrogen oxidation of this composition range was measured simultaneously in a 64-electrode proton exchange membrane fuel cell with emphasis on performance in the presence of CO. The addition of either Mo or Ru to Pt led to a reduction in hydrogen oxidation overpotential for a simulated reformate gas stream containing up to 50 ppm CO. The best performance under CO-containing reformate was found for compositions containing both Ru and Mo, e.g., Pt 0.40 Ru 0.35 Mo 0.25 . The performance observed was significantly better than that measured on compositions containing Pt and Ru only. The use of air bleed was found to be most beneficial for compositions containing predominantly Pt and Ru.