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Research Papers

Mass Transport Characteristics of Cathodes in a Phosphoric Acid Polybenzimidazole Membrane Fuel Cell

[+] Author and Article Information
M. Mamlouk1

School of Chemical Engineering and Advanced Materials, Merz Court,  Newcastle University, Newcastle upon Tyne, NE17RU, UKmohamed.mamlouk@ncl.ac.uk

K. Scott

School of Chemical Engineering and Advanced Materials, Merz Court,  Newcastle University, Newcastle upon Tyne, NE17RU, UK

1

Corresponding author.

J. Fuel Cell Sci. Technol 8(6), 061003 (Sep 23, 2011) (15 pages) doi:10.1115/1.4004501 History: Received July 20, 2009; Revised June 30, 2011; Published September 23, 2011; Online September 23, 2011

A study of the effect of electrode parameters on the mass transport characteristics of cathodes used for oxygen reduction in a phosphoric acid loaded polybenzimidazole membrane fuel cell is reported. Mass transport characteristics were determined using chrono-amperometry to measure the dynamic response of electrodes. Mass transfer behavior was analyzed using equations for diffusion in finite lengths of thin film electrolytes covering the catalyst surface area. Electrode structure parameters were measured using SEM images of the cross section of the membrane electrode assemblies. Electrode mass transfer parameters were determined for cathodes using different catalyst Pt loadings and using cathodes which were heat treated to modify micro-structure and hydrophobicity. Analysis of data showed that the dynamic current response was not controlled simply by mass transport by diffusion of oxygen through an electrolyte film covering the catalysts surface, but by an interfacial mass transport at the gas (vapor)/electrolyte film interface. Electrodes which exhibited the better oxygen mass transfer and solubility characteristics also produced better cell voltage versus current density performance in fuel cell studies.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

SEM cross-sectional images of the liquid nitrogen fractured MEAs

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Figure 2

Variation of the current with the inverse of square root of time (based on the Cottrell equation for diffusion in semi-infinite length). 40% Pt/C.

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Figure 3

Panel (a) recorded current-time transient response for MEA utilizing 60% Pt/C cathode with standard membrane. Panel (b) recorded current-time transient response for MEA utilizing 40% Pt/C HT (heat treated) cathode with standard membrane. Panel (c) recorded current-time transient response for MEA utilizing 50% Pt/C cathode with standard membrane. Panel (d) recorded current-time transient response for MEA utilizing 40% Pt/C cathode with standard membrane.

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Figure 4

Log (i) versus Log (t) transient at 120 °C for 40% Pt/C and 40% Pt/C (heat treated) with various cathode feeds (fractal boundary)

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Figure 5

Panel (a) Recorded and modeled current versus time transient for 50% Pt/C LD (membrane with low loading level of 4PRU) with various cathode feeds and temperatures. Panel (b) Recorded and modeled current versus time transient for 40% Pt/C (standard membrane 5.6 PRU) with various cathode feeds and temperatures.

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Figure 6

Panel (a) The effect of Pt:C ratio or catalyst thickness on the observed limiting current density when operating with air at temperatures of 120, 150 and 175 oC. Panel (b) The effect of Pt:C ratio or catalyst thickness on the observed limiting current density when operating with heleox at temperatures of 120, 150 and 175 °C. Panel (c) The effect of Pt:C ratio or catalyst thickness on the observed limiting current density when operating with air 2 atm. Temperatures of 120, 150 and 175 °C.

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Figure 7

Panel (a) Cell performance with air at 150 °C of MEAs using 40, 50 and 60% Pt/C cathode electrodes utilizing 0.4 mgPt cm−2 with 40% wt PTFE [9]. Panel (b) Cell performance with oxygen (atm) at 120 °C of MEAs using 40, 50 and 60%Pt/C cathode electrodes utilizing 0.4 mgPt cm−2 with 40% wt PTFE [9].

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Figure 8

The effect of 30% Pt/C electrode’s acid content on the observed limiting current density when operating with air. Temperatures of 120, 150 and 175 °C.

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Figure 9

The effect of 50% Pt/C electrode acid content on the observed limiting current density when operating with air. Temperatures of 120, 150 and 175 °C. LD denotes membrane with low doping level of 4 PRU.

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Figure 10

Cell performance at 120 °C of MEAs using standard cathode electrode and heat treated cathode. 0.4 mgPt cm−2 40% Pt/C with 40% wt PTFE. A: oxygen, B: air 1bar (gauge) and C: air used at the cathode [9].

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Figure 11

The effect of PTFE content of 50% Pt/C electrode on the observed limiting current density when operating with heleox (He-21%O2). Temperatures of 120, 150 and 175 °C.

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