Dane A. Boysen

Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes

The compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature (200-300 degrees C) fuel cells. In order to settle the question of the high temperature behavior of this material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (p[H2O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 degrees C, with the conductivity rising to a value of 2.2 x 10(-2) S cm(-1) at 240 degrees C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH2PO4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH2PO4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm(-2). Thus, for fuel cells in which the supported electrolyte membrane was only 25 microm in thickness and in which a peak power density of 415 mW cm(-2) was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH2PO4, a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

Polymer solid acid composite membranes for fuel-cell applications

A systematic study of the conductivity of polyvinylidene fluoride (PVDF) and CsHSO4 composites, containing 0 to 100% CsHSO4, has been carried out. The polymer, with its good mechanical properties, served as a supporting matrix for the high proton conductivity inorganic phase. The conductivity of composites exhibited a sharp increase with temperature at 142°C, characteristic of the superprotonic phase transition of CsHSO4. At high temperature (160°C), the dependence of conductivity on vol % CsHSO4 was monotonic and revealed a percolation threshold of ~10 vol %. At low temperature (100°C), a maximum in the conductivity at ~80 vol % CsHSO4 was observed. Results of preliminary fuel cell measurements are presented.

Alcohol Fuel Cells at Optimal Temperatures

High-power-density alcohol fuel cells can relieve many of the daunting challenges facing a hydrogen energy economy. Here, such fuel cells are achieved using CsH2PO4 as the electrolyte and integrating into the anode chamber a Cu-ZnO/Al2O3 methanol steam-reforming catalyst. The temperature of operation, ~250°C, is matched both to the optimal value for fuel cell power output and for reforming. Peak power densities using methanol and ethanol were 226 and 100 mW/cm^2, respectively. The high power output (305 mW/cm^2) obtained from reformate fuel containing 1% CO demonstrates the potential of this approach with optimized reforming catalysts and also the tolerance to CO poisoning at these elevated temperatures.

Airborne Nanoparticle Concentrations in the Manufacturing of Polytetrafluoroethylene (PTFE) Apparel

One form of waterproof, breathable apparel is manufactured from polytetrafluoroethylene (PTFE) membrane laminated fabric using a specific process to seal seams that have been sewn with traditional techniques. The sealing process involves applying waterproof tape to the seam by feeding the seam through two rollers while applying hot air (600°C). This study addressed the potential for exposure to particulate matter from this sealing process by characterizing airborne particles in a facility that produces more than 1000 lightweight PTFE rain jackets per day. Aerosol concentrations throughout the facility were mapped, breathing zone concentrations were measured, and hoods used to ventilate the seam sealing operation were evaluated. The geometric mean (GM) particle number concentrations were substantially greater in the sewing and sealing areas (67,000 and 188,000 particles cm−3) compared with that measured in the office area (12,100 particles cm−3). Respirable mass concentrations were negligible throughout the facility (GM = 0.002 mg m−3 in the sewing and sealing areas). The particles exiting the final discharge of the facilitys ventilation system were dominated by nanoparticles (number median diameter = 25 nm; geometric standard deviation of 1.39). The breathing zone particle number concentrations of the workers who sealed the sewn seams were highly variable and significantly greater when sealing seams than when conducting other tasks (p < 0.0001). The sealing workers’ breathing zone concentrations ranged from 147,000 particles cm−3 to 798,000 particles cm−3, and their seam responsibility significantly influenced their breathing zone concentrations (p = 0.03). The finding that particle number concentrations were approximately equal outside the hood and inside the local exhaust duct indicated poor effectiveness of the canopy hoods used to ventilate sealing operations.

Proton (deuteron) conductivity in Cs1.5Li1.5H(SO4)2 and Cs1.5Li1.5D(SO4)2 single crystals

Abstract Calorimetric (DSC), NMR and electrical studies of Cs 1.5 Li 1.5 X(SO 4 ) 2 (X=H, D) single crystals have been performed in the temperature range from 300 to 533 K. No phase transitions are observed upon heating and the conductivity follows an Arrhenius temperature dependence to the point of decomposition at ∼470 K. Despite the high protonic conductivity of Cs 1.5 Li 1.5 X(SO 4 ) 2 ( σ ∼10 −3 S cm −1 ) at high temperature, these compounds cannot be classified as superprotonic because of the large value of the activation energy ( E a ∼1 eV). The proton NMR studies confirmed that two crystallographic proton sites exist in Cs 1.5 Li 1.5 H(SO 4 ) 2 , presumably corresponding to two minima in the single, crystallographically distinct (and asymmetric) hydrogen bond. A measurable isotope effect in the conductivity data confirmed that protons/deuterons, as opposed to lithium ions, are the mobile species. The high activation energy for proton transport in Cs 1.5 Li 1.5 X(SO 4 ) 2 most probably results from the asymmetry of the hydrogen bonds, and from electrostatic repulsion between the protons and the Li + ions which likely hinders reorientation of XSO 4 − groups.