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TECHNICAL PAPERS

# Investigation of Atomistic Scale Transport Phenomena of the Proton Exchange Membrane Fuel Cell

[+] Author and Article Information
Chin-Hsien Cheng

Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

Che-Wun Hong1

Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwancwhong@pme.nthu.edu.tw

1

Corresponding author.

J. Fuel Cell Sci. Technol 4(4), 474-480 (Apr 13, 2006) (7 pages) doi:10.1115/1.2756845 History: Received December 08, 2005; Revised April 13, 2006

## Abstract

This paper studies the transport phenomena inside the electrolyte of proton exchange membrane fuel cells (PEMFCs) using atomistic simulation techniques. The investigated material of the electrolyte is $Nafion®$, which is the most widely adapted polymer membrane in low-temperature fuel cells. The molecular dynamics simulation system includes part of the Nafion structure, numerous water molecules, and the transporting cations. The cations are assumed to be hydroxoniums $(H3O+)$, which are a hydrogen proton combined with a water molecule. Simulation results indicated that the electrostatic energy dominated the other potential energies in the total internal energy analysis. Clusters of water molecules tend to move toward the sulfonic acid group in the Nafion fragment, where the hydrophilic/hydrophobic characteristics can be observed. The transport phenomena of hydroxoniums are classified into two categories—continuous migration and noncontinuous hopping. The self-diffusion coefficients of the hydroxoniums and the water molecules in the membrane were evaluated to be $3.476×10−5cm2∕s$ and $4.993×10−5cm2∕s$ respectively, based on the Einstein relation. The calculated self-diffusion coefficients are of the same order of magnitude as the experimental results, which indicates this atomistic simulation is reaching more and more practical in engineering analysis.

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## Figures

Figure 1

Chemical structure of the Nafion, subscripts x equals to 5–10, y nears 1000, and z is about 1–2, respectively

Figure 2

(a) Simplified structure of a single Nafion fragment used in the simulation and (b) charge distribution of the single Nafion fragment (in the unit of e)

Figure 3

Three-dimensional molecular structure of the single Nafion fragment used in the simulation as the initial configuration

Figure 4

Configuration and charge distribution of (a) a water molecule and (b) a hydroxonium

Figure 5

Total internal energy of the simulation system

Figure 6

Convergence and magnitude of the inter-/intrapotential energies of the simulation system

Figure 7

Temperature fluctuations during the 1ns MD simulation

Figure 8

Initial arrangement of the simulation system, including six Nafion fragments (in dark gray), six hydroxoniums (in black), and water molecules (in light gray)

Figure 9

Snapshots of the simulation results at the time step of (a)6.25ps, (b)12.5ps, and (c)37.5ps

Figure 10

Trajectory of a specific hydroxonium during the 1ns MD simulation

Figure 11

Radial distribution functions of (a) the C–OW pair, (b) the F – OW pair, where OW is the oxygen atom of the water molecule, C and F are the major component of the Nafion backbone

Figure 12

Radial distribution functions of (a) the O – OW pair and (b)O−H3O+ pair, where OH3 is the oxygen atom of the hydroxonium

Figure 13

Radial distribution functions of (a) the S−H2O pair and (b) the S−H3O+ pair

Figure 14

Radial distribution functions of (a) the H3O+−H2O pair and (b) the H3O+−H3O+ pair

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