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Research Papers

Nanofiber Cathode Catalyst Layer Model for a Proton Exchange Membrane Fuel Cell

[+] Author and Article Information
Dennis O. Dever, Richard A. Cairncross

Department of Chemical
and Biological Engineering,
Drexel University,
Philadelphia, PA 19104

Yossef A. Elabd

Department of Chemical
and Biological Engineering,
Drexel University,
Philadelphia, PA 19104
e-mail: elabd@drexel.edu

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF FUEL CELL SCIENCE AND TECHNOLOGY. Manuscript received March 2, 2013; final manuscript received July 24, 2013; published online April 17, 2014. Editor: Nigel M. Sammes.

J. Fuel Cell Sci. Technol 11(4), 041007 (Apr 17, 2014) (13 pages) Paper No: FC-13-1026; doi: 10.1115/1.4026985 History: Received March 02, 2013; Revised July 24, 2013

The cathode catalyst layer in a proton exchange membrane fuel cell is now known to contain ionomer nanofibers and experiments have demonstrated a fuel cell performance increase of ∼10% due to those nanofibers. The experiments demonstrate that ionomer nanofibers have proton conductivities that exceed those of the bulk form of the ionomer by more than an order of magnitude. A new model of a proton exchange membrane fuel cell is presented here that predicts the effect of nanofibers on cell performance in terms of the enhanced nanofiber proton conductivity and other relevant variables. The model peak cell power density is ∼7% greater for the case with 10% of the cathode catalyst layer ionomer in nanofiber form versus the same case without nanofibers. This difference is consistent with trends observed in previously published experimental results. These results are significant since they suggest alternative methods to reduce platinum loading in fuel cells and to optimize fuel cell performance.

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Figures

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Fig. 1

Diagram of the CCL of the PEMFC showing the schematics and scanning electron micrographs (SEM) and transmission electron micrographs (TEM) of the Nafion® nanofibers, agglomerates, and other components. The SEM and TEM images are from references [38] and [54], respectively.

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Fig. 2

Nanofiber model voltage performance calculations (solid lines) compared to the experimental data. The model predictions are listed for two different values of the mass transfer coefficient for oxygen diffusion through the cathode GDL. The model predicts the data well for the coefficient of 0.164 cm/s. The results are based on a cathode/anode/cell temperature of 80 °C, pressure of 2.7 atm, and Pt loading of 0.14 mg/cm2 and 0.43 mg/cm2 in the anode and cathode and water activity of 1.0 in the channels.

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Fig. 3

Reaction rate variation within the cathode catalyst layer (CCL) of the fuel cell in terms of the volumetric consumption of oxygen for the base case (10% nanofibers) and no-fiber case (0% nanofibers) at a cell current of 1.0 A/cm2. Insets show the dimensionless oxygen concentration profile (Co(r)) within agglomerates at two locations for both cases. Here, Co(r) equals the ionomer oxygen concentration divided by the ionomer oxygen concentration that would occur if it was in equilibrium with the surrounding bulk gas. The domain of the radial coordinate r is displayed from the agglomerate center (0) to the surface ra.

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Fig. 4

Oxygen concentration throughout the cathode catalyst layer (CCL) of the fuel cell for the base case (10% nanofibers) and the no-fiber case (0% nanofibers) at a cell current of 1.0 A/cm2

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Fig. 11

PEMFC performance as a function of the oxygen mass transfer rate through the cathode gas diffusion layer (GDL) (10% nanofibers in the CCL). The displayed values are the mass transfer coefficient (Γoc) in cm/s, where 0.63 cm/s is the base case value. The results are based on a cathode/anode/cell temperature of 80 °C, pressure of 3.0 atm, and Pt loading of 0.1 mg/cm2 and 0.4 mg/cm2 in the anode and cathode. Other PEMFC conditions are listed in Table 3.

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Fig. 5

Overpotential throughout the cathode catalyst layer (CCL) of the fuel cell for the base case (10% nanofibers) and the no-fiber case (0% nanofibers) at a cell current of 1.0 A/cm2

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Fig. 6

PEMFC performance results as a function of the volume fraction of the ionomer in nanofiber form in the cathode catalyst layer (CCL). The results are based on a cathode/anode/cell temperature of 80 °C, pressure of 3.0 atm, and Pt loading of 0.4 mg/cm2 in the cathode and 0.1 mg/cm2 in the anode. Other PEMFC conditions are listed in Table 3.

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Fig. 7

Peak PEMFC power density as a function of the fraction ionomer in nanofiber form. The results are based on a cathode/anode/cell temperature of 80 °C, pressure of 3.0 atm, and Pt loading of 0.4 mg/cm2 in the cathode and 0.1 mg/cm2 in the anode. Other PEMFC conditions are listed in Table 3.

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Fig. 8

PEMFC performance as a function of the platinum loading in the CCL and nanofiber content in CCL: (a) 10% nanofibers, and (b) 0% nanofibers. The results are based on a cathode/anode/cell temperature of 80 °C, pressure of 3.0 atm, and Pt loading of 0.1 mg/cm2 in the anode. Other PEMFC conditions are listed in Table 3.

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Fig. 9

PEMFC performance as a function of the product of the effective platinum surface (Ao),ORR rate constant (k), and the nanofiber content in the CCL: (a) 10% nanofibers, and (b) 0% nanofibers. The results based on a cathode/anode/cell temperature of 80 °C, pressure of 3.0 atm, and Pt loading of 0.1 mg/cm2 and 0.4 mg/cm2 in the anode and cathode. Other PEMFC conditions are listed in Table 3.

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Fig. 10

PEMFC performance at different water vapor saturation levels in the CCL (10% nanofibers in the CCL). The results are based on a cathode/anode/cell temperature of 80 °C, pressure of 3.0 atm, and Pt loading of 0.1 mg/cm2 and 0.4 mg/cm2 in the anode and cathode. Other PEMFC conditions are listed in Table 3.

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