Platinum thin film anodes for solid acid fuel cells

Hydrogen electro-oxidation kinetics at the Pt | CsH_2PO_4 interface have been evaluated. Thin films of nanocrystalline platinum 7.5–375 nm thick and 1–19 mm in diameter were sputtered atop polycrystalline discs of the proton-conducting electrolyte, CsH_2PO_4, by shadow-masking. The resulting Pt | CsH_2PO_4 | Pt symmetric cells were studied under uniform H_2-H_2O-Ar atmospheres at temperatures of 225–250 °C using AC impedance spectroscopy. For thick platinum films (>50 nm), electro-oxidation of hydrogen was found to be limited by diffusion of hydrogen through the film, whereas for thinner films, diffusion limitations are relaxed and interfacial effects become increasingly dominant. Extrapolation to vanishing film thickness implies an ultimate interfacial resistivity of 2.2 Ω cm^2, likely reflecting a process at the Pt | H_(2(g)) interface. Films 7.5 nm in thickness displayed a total electro-oxidation resistivity, R, of 3.1 Ω cm^2, approaching that of Pt-based composite anodes for solid acid fuel cells (1–2 Ω cm^2). In contrast, the Pt utilization (R^(−1)/Pt loading), 19 S mg^(−1), significantly exceeds that of composite electrodes, indicating that the thin film approach is a promising route for achieving high performance in combination with low platinum loadings

Nanoscale Electrodes by Conducting Atomic Force Microscopy: Oxygen Reduction Kinetics at the Pt|CsHSO_4 Interface

We quantitatively characterized oxygen reduction kinetics at the nanoscale Pt|CsHSO_4 interface at ~150 °C in humidified air using conducting atomic force microscopy (AFM) in conjunction with AC impedance spectroscopy and cyclic voltammetry. From the impedance measurements, oxygen reduction at Pt|CsHSO_4 was found to comprise two processes, one displaying an exponential dependence on overpotential and the other only weakly dependent on overpotential. Both interfacial processes displayed near-ideal capacitive behavior, indicating a minimal distribution in the associated relaxation time. Such a feature is taken to be characteristic of a nanoscale interface in which spatial averaging effects are absent and, furthermore, allows for the rigorous separation of multiple processes that would otherwise be convoluted in measurements using conventional macroscale electrode geometries. The complete current-voltage characteristics of the Pt|CsHSO_4 interface were measured at various points across the electrolyte surface and reveal a variation of the oxygen reduction kinetics with position. The overpotential-activated process, which dominates at voltages below -1 V, was interpreted as a charge-transfer reaction. Analysis of six different sets of Pt|CsHSO_4 experiments, within the Butler-Volmer framework, yielded exchange coefficients (α) for charge transfer ranging from 0.1 to 0.6 and exchange currents (i_0) spanning 5 orders of magnitude. The observed counter-correlation between the exchange current and exchange coefficient indicates that the extent to which the activation barrier decreases under bias (as reflected in the value of α) depends on the initial magnitude of that barrier under open circuit conditions (as reflected in the value of i_0). The clear correlation across six independent sets of measurements further indicates the suitability of conducting AFM approaches for careful and comprehensive study of electrochemical reactions at electrolyte-metal-gas boundaries

Vibration isolation for line of sight performance improvement

Diagrams of the Reaction Wheel Assembly (RWA) are presented along with charts and graphs illustrating jitter error model, induced vibration tests, radial displacement transfer function, and axial displacement power spectra density. The RWA isolator specification requirements are listed

Preparation of (Pb,Ba)TiO3 powders and highly oriented thin films by a sol-gel process

Solid solution Pb1-xBaxTiO3, with particular emphasis on Pb0.5Ba0.5TiO3, was prepared using a sol-gel process incorporating lead acetate trihydrate, barium acetate, and titanium isopropoxide as precursors, acetylacetone (2,4 pentanedione) as a chelating agent, and ethylene glycol as a solvent. The synthesis procedure was optimized by systematically varying acetylacetone: Ti and H2O:Ti molar ratios and calcination temperature. The resulting effects on sol and powder properties were studied using thermogravimetric analysis/differential scanning calorimetry, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller analysis, and x-ray diffraction (XRD). Crystallization of the perovskite structure occurred at a temperature as low as 450 °C. Thin films were prepared by spin coating on (100) MgO. Pyrolysis temperature and heating rate were varied, and the resultant film properties investigated using field-emission scanning electron microscopy, atomic force microscopy, and XRD. Under optimized conditions, highly oriented films were obtained at a crystallization temperature of 600 °C

Defect chemistry and transport properties of BaxCe0.85M0.15O3-d

The site-incorporation mechanism of M3+ dopants into A2+B4+O3 perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A2+-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B4+-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose here, on the basis of x-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are BaxCe0.85M0.15O3-d (x = 0.85 - 1.20; M = Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba0.919Nd0.081)(Ce0.919Nd0.081)O3, (Ba0.974Gd0.026)(Ce0.872Gd0.128)O2.875, and Ba(Ce0.85Yb0.15)O2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs

Ionic and Electronic Conductivity of Nanostructured, Samaria-Doped Ceria

The ionic and electronic conductivities of samaria doped ceria electrolytes, Ce_(0.85)Sm_(0.15)O_(1.925−δ), with nanometric grain size have been evaluated. Nanostructured bulk specimens were obtained using a combination of high specific-surface-area starting materials and suitable sintering profiles under conventional, pressureless conditions. Bulk specimens with relatively high density (≥92% of theoretical density) and low medium grain size (as small as 33 nm) were achieved. Electrical A.C. impedance spectra were recorded over wide temperature (150 to 650°C) and oxygen partial pressure ranges (0.21 to 10^(−31) atm). Under all measurement conditions the total conductivity decreased monotonically with decreasing grain size. In both the electrolytic and mixed conducting regimes this behavior is attributed to the high number density of high resistance grain boundaries. The results suggest a possible variation in effective grain boundary width with grain size, as well as a possible variation in specific grain boundary resistance with decreasing oxygen partial pressure. No evidence appears for either enhanced reducibility or enhanced electronic conductivity upon nanostructuring

Engineering the Next Generation of Solid State Proton Conductors: Synthesis and Properties of Ba_(3−x)K_(x)H_(x)(PO_4)_2

A new series of compounds with general chemical formula Ba_(3−x)K_(x)H_(x)(PO_4)_2 has been successfully prepared. This particular stoichiometry was targeted as a candidate solid-state proton conductor because of its anticipated structural similarity to known M_(3)H(XO_4)_2 superprotonic conductors (M = Cs, Rb, NH4, K; X = Se, S) and to the known trigonal compound Ba_(3)(PO_4)_2. The materials were synthesized from aqueous solution using barium acetate, dipotassium hydrogen phosphate, and potassium hydroxide as starting materials. Through variations in the initial solution stoichiometry or the synthesis temperature, the final stoichiometry could be controlled from x ~ 0.5 to ~1. X-ray powder diffraction, energy dispersive spectroscopy chemical analysis, ^(1)H magic angle spinning (MAS) nuclear magnetic spectroscopy, and thermogravimetric analysis were all employed to establish potassium and proton incorporation. The diffraction data confirmed crystallization of a trigonal phase, and chemical analysis showed the (Ba+K):P ratio to be 3:2, consistent with the target stoichiometry. The conductivity of the Ba_(3−x)K_(x)H_(x)(PO_4)_2 materials, as measured by A.C. impedance spectroscopy, is about 3 orders of magnitude greater than the end-member Ba_(3)(PO_4)_2 material with only a slight dependence on x, however, it is substantially lower than that of typical superprotonic conductors and of the M_(3)H(XO_4)_2 materials in particular. The close proximity of Ba to the hydrogen bond site is proposed to explain this behavior. At 250 °C, the conductivity is 2.4 × 10^(−5) S/cm for the composition x = 0.80, which, when combined with the water insolubility and the relatively high thermal stability, may render Ba_(3−x)K_(x)H_(x)(PO_4)_2 an attractive alternative in selected electrochemical applications to known superprotonic conductors

Composite nanostructured solid-acid fuel-cell electrodes via electrospray deposition

Stable, porous, nanostructured composite electrodes were successfully fabricated via the inexpensive and scalable method of electrospray deposition, in which a dissolved solute is deposited onto a substrate using an electric field to drive droplet migration. The desirable characteristics of high porosity and high surface area were obtained under conditions that favored complete solvent evaporation from the electrospray droplets prior to contact with the substrate. Solid acid (CsH_2PO_4) feature sizes of 100 nm were obtained from electrosprayed water–methanol solutions with 10 g L^(−1) CsH_2PO_4 and 5 g L^(−1) Pt catalyst particles suspended using polyvinylpyrrolidone (PVP). Alternative additives such as Pt on carbon and carbon-nanotubes (CNTs) were also successfully incorporated by this route, and in all cases the PVP could be removed from the electrode by oxygen plasma treatment without damage to the structure. In the absence of additives (Pt, Pt/C and CNTs), the feature sizes were larger, 300 nm, and the structure morphologically unstable, with significant coarsening evident after exposure to ambient conditions for just two days. Electrochemical impedance spectroscopy under humidified hydrogen at 240 °C indicated an interfacial impedance of ~1.5 Ω cm^2 for the Pt/CsH_2PO_4 composite electrodes with a total Pt loading of 0.3 ± 0.2 mg cm^(−2). This result corresponds to a 30-fold decrease in Pt loading relative to mechanically milled electrodes with comparable activity, but further increases in activity and Pt utilization are required if solid acid fuel cells are to attain widespread commercial adoption