Expanded 3D Electrode Architecture for Low Temperature Direct Liquid Fuel Cells
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Expanded 3D Electrode Architecture for Low Temperature Liquid Fuel Cells R. Craig Urian Naval Undersea Warfare Center, 1176 Howell St., Newport, RI 02841, U.S.A. ABSTRACT The US Navy continues to pursue electrochemical power sources with high energy density for undersea applications. The direct electro-oxidation and electro-reduction of sodium borohydride and hydrogen peroxide are being investigated to meet these goals. In an effort to minimize polarization losses and increase power density, a novel carbon microfiber array (CMA) electrode in combination with a carbon cloth electrode is being investigated. INTRODUCTION The carbon microfiber array electrode (CMA) is composed of graphite fibers that protrude from a current collector like blades of grass. The CMA was developed for the direct reaction of peroxide in the Mg-H2O2 semi fuel cell. [1] There, the high surface area of the microfiber cathode reduces peroxide concentration polarization, resulting in increased power and energy density. This electrode architecture is also very useful in systems where particulate mater and gas bubble formation are components of the circulating electrolyte. For this work the CMA architecture was adapted into a novel membrane electrode assembly and evaluated in the direct borohydride / hydrogen peroxide fuel cell. BH4- + 8OH- Æ BO2- + 6H2O + 8e- 1.24V HO2- + H2O + 2e- Æ 3OH0.87V
(1) (2)
The unique feature of this architecture vs. traditional membrane electrode assemblies (MEAs) is how all three components of the triple boundary interface are optimized: electrical connectivity, ionic connectivity and mass transport. EXPERIMENT Carbon microfiber deposition The fabrication process is described else where in detail [1]. Briefly the pitch based carbon microfibers were 10 µm diameter (Mitsubishi Chemical) and were cut to 750 µm in length by Engineered Fibers Technology, LLC. (Shelton, CT) The carbon microfibers were electrostacticly deposited (flocked) onto Carbone 2020 graphite electrodes coated with a wet carbon-based adhesive ink (112-48, Creative Materials, Tewksbury, MA) using a Maag Flockmaschinen Model SPG 1000. The graphite electrode (5.079 cm x 10.160 cm) had an active area dimension of 3.0 cm by 8.335 cm (25 cm2 active area) with a 810 µm deep cavity to accommodate the vertically oriented, flocked fibers and carbon cloth electrode. The fiber density of the electrodes was 30,000 fibers/cm2. To complete the modification, Spectracarb 2225
activated carbon cloth was hot pressed to N115 and laid over the flocked fibers. The combined active area of the modified CMFA was 175 cm2 which is a 7 times amplification of the projected working area. Catalyst Deposition The simultaneous deposition of Pd and Ir to form the alloy PdIr 1:1 onto the electrodes was carried out by potential pulse deposition. The potential was pulsed between open circuit and -200 mV vs. Ag/AgCl. The electrode assembly was mounted into a membrane separated flow cell consisting of separate anode and cathode compartments (i.e. fuel cell). The Ag/AgCl refe
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