Electrocatalysis in Solid Acid Fuel Cells

Solid state electrochemical reactions play a crucial role in many energy conversion devices, yet the pathways of many reactions remain unknown. The elusiveness of the reaction mechanisms is due, in part, to the complexity of electrochemical reactions; because electrochemical reactions require the interaction of many species (e.g., ions, electrons, and adsorbates) across multiple phases (e.g., electrolyte, catalyst, and gas phases), elucidation of the reaction pathway can quickly become complicated. In this work, we develop and utilize model catalyst | electrolyte systems, that is, structures of reduced complexity, to study electrode reactions in solid acid fuel cells which operate at intermediate temperatures of ~ 250 ºC. We employ AC impedance spectroscopy to explore the reaction pathway for hydrogen electro-oxidation over Pt thin films sputter-deposited atop the proton-conducting solid acid electrolyte CsH2PO4. We observed that hydrogen electro-oxidation occurs by diffusion of hydrogen through Pt, taking advantage of the entire Pt | CsH2PO4 interfacial area rather than being confined to the triple-phase sites. This insight opens up new avenues for developing high performance electrodes with low Pt loadings by eliminating the requirement that Pt-based electrodes be comprised of high triple-phase site densities long considered to be critical for Pt electrocatalysis. Indeed, even for flat, planar electrodes of very thin Pt films, we obtained a Pt utilization that is significantly higher than in typical composite electrodes. We also demonstrate the efficacy of a new tool for probing the spatial heterogeneity of electrochemical reactions at the metal | electrolyte interface. We characterized oxygen electro-reduction kinetics at the nanoscale Pt | CsHSO4 interface at ~ 150 ºC using conducting atomic force microscopy in conjunction with cyclic voltammetry and AC impedance spectroscopy. Not only did we find the electrochemical activity for oxygen electro-reduction to vary dramatically across the electrolyte surface but the current-voltage data, when analyzed in the Butler-Volmer framework, exhibited a strong counter-correlation between two key kinetic parameters, the exchange coefficient and exchange current. Specifically, the exchange current spanned five orders of magnitude while the exchange coefficient ranged between 0.1 and 0.6. Such a correlation has not been observed before and points to the power of atomic force microscopy for electrochemical characterization at electrolyte | metal | gas boundaries in general. As reduction in microstructural complexity is a key advantage in model electrode | electrolyte systems, we also sought to understand the bulk properties of solid acid compounds, specifically, the relationship between microstructure and the superprotonic phase transition, the latter of which lends solid acid compounds their high proton conductivities at intermediate temperatures. We found a correlation between phase transformation hysteresis and crystallographic compatibility of the high- and low-temperature phases of the Cs1–xRbxH2PO4 solid solution series. Therefore, it is to be expected that hysteresis, and therefore microcrack formation, can be minimized during phase transformation via the principle of crystallographic compatibility. This is confirmed in single crystals of CsHSO4, which was found to have higher crystallographic compatibility, lower hysteresis, and significantly fewer microcracks formed during phase transition compared to CsH2PO4. The apparent applicability of the theory of crystallographic compatibility implies a new tool for identifying solid acid compounds with suitable microstructures for fuel cell application and for model electrode | electrolyte systems. </p

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

LANDSAT D local user terminal study

The effect of the changes incorporated in the LANDSAT D system on the ability of a local user terminal to receive, record and process data in real time was studied. Alternate solutions to the problems raised by these changes were evaluated. A loading analysis was performed in order to determine the quantities of data that a local user terminal (LUT) would be interested in receiving and processing. The number of bits in an MSS and a TM scene were calculated along with the number of scenes per day that an LUT might require for processing. These then combined to a total number of processed bits/day for an LUT as a function of sensor and coverage circle radius

Letter from Louie W. Strentzel to Nellie [surname unknown] , 1871 Nov 28.

I am also gathering the last leaves to skeleton and bleach for Phantom Bouquets. Have you ever tried to make them? It is very tedious work but when finished, nothing can be more exquisitely beautiful. Nellie, I should like very much to have you come down and make me visit this winter. Could not you and No. [3] who is I believe Miss Blanche come by sometime with your father on his way from Sacramento or from San Francisco, which is perhaps the most convenient route? And Let us know a day or two before so that we can meet you at the wharf in [Martinez] and we will all endeavor to make it as pleasant for you as possible And now if you will only be good and forg[illegible], and answer this very soon, so that I may be fairly started in the way of correspondence before the Old Year is gone- I am superstitious about that - I shall be most thankful and will try also to smooth out the cramped lines which my pen has of late been wont to make. Father and mother join me in kindest regards to you all Truly Your friend Louie W. Strentzel [4] [1] Alhambra, Nov. 28, 1871.Dear Nellie, The long-hoped for clouds and rain and storm have at last come down upon us, bringing, for me at least, the rest and quiet which here is not to be known all the summer long. The past summer especially has been a busy time for me. We have had company almost continually, and the hur- ried work of the fruit season, together with mother\u27s ill health, has given more care and work to my head and hands than I have been able to go through with and not be tired out. But now, with leisure to remember my neglected duties, there comes again before me a long list of letters unwritten which have for a long time persisted in haunt- ing me most uncomfortably. And indeed, Nellie, it is so long since I have written any but the shortest notes, that I find myself almost as bereft of idea in be- ginning this letter as I was when first ordered to write a school composition. So I can only hope that you will be a little less wroth over the unanswered letter [1] 06382 of last spring, when I say that it is a year and more since I wrote a line to my aunt and cousins in Texas. My Uncle Erwin who came out with his family last June, is living down opposite Vallejo, but his youngest child Janie has been staying here with us the past three months. They are all still very much dissatisfied with California and want to return to their Texas home. Mother has been away since Thursday, having taken Janie and gone up the Stockton river to visit Mr. Kimbell\u27s family who live near Wells\u27 Landing. They are old Texas friends whom your mother used to know very well. Wouldn\u27t it be splendid if for once the old home acquaintances could all be collected together, say at a quilting or Christmas festival of the old fashion! Just fancy the wonderful recital of experiences, recollections, and perhaps thrilling ad- ventures that we younger folks might come to hear at such a time. How should you like to go back to our native State, Nellie, and see everything for yourself? By and by, when an iron steed goes flying across the plains and [2]on to Honey Grove, I shall want to be there for a little while; but know I could never be satis- fied to consider any other but California as my true home. And it is here in our own Golden State that I want most to travel, to see its wonders and grand scenery, and its people as well. As it is I have seen so very little; go very seldom even to San Francisco, an then for a day or two only: but for some months, we have been hoping and planning over the promised coming of the Railroad through our town, when it will be less trying and tedious a journey to go beyond the encircling hills of Alhambra Valley. Our valley has seemed very beautiful though, the last month with its trees and vines tinted with hues of autumn. Some of the foreign grape vines especially have shown in masses of yellow and crimson and flame, but the wonder was short lived, for the beating rain came and left only bare branches. I have been dry- ing a quantity of the leaves, and there is such variety of marking and color, that after being varnished, they will make beautiful wreaths. [3

Letter from [Louie Strentzel] to [recipient unknown], [ca. 1877].

The railroad passenger trains com- menced running between Oakland and Martinez the first of July and at last it is reasonable to expect that they will continue to run on and on even until the end of time unless superseded by cars aerial. The superintendent promises to have the cars going through to Sacramento by the first of September and it seems as if no traveling could be more delightful than this 50 mile ride along the shore with pleasant hills and the sea 04049[Page 2]waters always in sight. [ I?] We intend to go up on the first train to see Mrs. Carr, the Doctor was very sick after returning to Sacramento and it has been very hard for them both. 0404

A clinical evaluation of the Borish vectrographic nearpoint chart

A clinical evaluation of the Borish vectrographic nearpoint char

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