A. John Appleby

High energy efficiency and high power density proton exchange membrane fuel cells: Electrode kinetics and mass transport

Abstract The development of proton exchange membrane (PEM) fuel cell power plants with high energy efficiencies and high power densities is gaining momentum because of the vital need of such high levels of performance for extraterrestrial (space, underwater) and terrestrial (power source for electric vehicles) applications. Since 1987, considerable progress has been made in achieving energy efficiencies of about 60% at a current density of 200 mA/cm2 and high power densities (⪢ 1 W/cm2) in PEM fuel cells with high (4 mg/cm2) or low (0.4 mg/cm2) platinum loadings in electrodes. This article focuses on: (i) methods to obtain these high levels of performance with low Pt loading electrodes — by proton conductor impregnation into electrodes, localization of Pt near front surface; (ii) a novel microelectrode technique which yields electrode kinetic parameters for oxygen reduction and mass transport parameters; (iii) demonstration of lack of water transport from anode to cathode; (iv) modeling analysis of PEM fuel cell for comparison with experimental results and predicting further improvements in performance; (v) recommendations of needed R&D for achieving the above goals.

Performance and endurance of a PEMFC operated with synthetic reformate fuel feed

Widespread implementation of polymer electrolyte membrane fuel cell (PEMFC) powerplants for stationary and vehicular applications will be dependent in the near future on using readily available hydrocarbon fuels as the source of the hydrogen fuel. Methane and propane are ideal fuels for stationary applications, while methanol, gasoline, and diesel fuel are better suited for vehicular applications. Various means of fuel processing are possible to produce a gaseous fuel containing H2, CO2 and CO. CO is a known electrocatalyst poison and must be reduced to low (10s) ppm levels and CO2 is said to cause additional polarization effects. Even with no CO in the feed gas a H2/CO2/H2O gas mixture will form some CO. Therefore, as a first step of developing a PEMFC that can operate for thousands of hours using a reformed fuel, we used an anode gas feed of 80% H2 and 20% CO2 to simulate the reforming of CH4. To investigate the effect of reformate on cell performance and endurance, a single cell with an active area of 58 cm2 was assembled with a membrane electrode assembly (MEA) furnished by Texas A&M University using IGTs internally manifolded heat exchange (IMHEX™) design configuration. The MEA consisted of a Nafion 112 membrane with anode and cathode Pt catalyst loadings of 0.26 and 1.46 mg/cm2, respectively. The cell was set to operate on a synthetic reformate–air at 60°C and 1 atm and demonstrated over 5000 h of endurance with a decay rate of less than 1%/1000 h of operation. The cell also underwent four successful thermal cycles with no appreciable loss in performance. The stable performance is attributed to a combination of the IGT IMHEX plate design with its inherent uniform gas flow distribution across the entire active area and MEA quality. The effects of temperature, gas composition, fuel utilization (stoics) and thermal cycle on cell performance are described.

ANALYSIS OF ACTIVE MATERIAL AND ADDITIVE DISTRIBUTIONS IN A NICKEL HYDROXIDE ELECTRODE BY SEM/EDX TECHNIQUES

Abstract Cobalt and cadmium additive distributions in the active material across the thickness of a nickel electrode were determined by using scanning electron microscopy and energy dispersive X-ray analysis techniques. The nickel oxide/hydroxide active material is impregnated in small pores (10 to 15 μm) of the nickel substrate of the electrode. To avoid possible interference by the nickel metal substrate, the additives and ionic nickel were analyzed on micro-spots in the active material on a cross section of the electrode. The atomic ratio of nickel to oxygen at each analytical point was used to confirm that the analyses were carried out at spots constituting pure active materials only, without any contamination from the nickel substrate/metal. Results showed that cobalt and cadmium additives were more concentrated at the surface than the bulk for electrodes prepared by the electrochemical impregnation technique. Trends in the distributions of the individual elements (additives) did not change after cycling of the electrodes (up to about 50 cycles) in flooded cell configurations.