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Abstract
The heterogeneous performance of polymer electrolyte fuel cell in space and time was discussed for operation at high current density. The cell voltage, detected by a segmented electrode, varies along a gas flow channel from upper to bottom stream and oscillates in time, which is referred as a respiration mode. At the higher current, the cell voltage at different positions started to be synchronized, as the current density increases. In order to investigate a role of water on the respiration, we employed a new method of contrast variation for small-angle neutron scattering (SANS) using deuterium (D2) gas as a fuel. By using D2, we introduce special scattering contrast in a polymer electrolyte film (Nafion®), when the film is originally swollen by H2O. After switching from H2 to D2 gas (humidified with H2O), we found that SANS intensity significantly decreases about 40% at the q-position of scattering maximum (qm) originating from the water-microdomains in the polymer electrolyte. After quantitative analyses of the scattering intensity, it was elucidated that 20 wt% of the total water is occupied by D2O as a steady state. At around the average intensity, SANS intensity oscillates with a time interval ∼100sec, which corresponds to the respiration mode found for voltage. The respiration behavior is considered as a non-linear & non-equilibrium phenomenon in an open system, where water flooding plays a role of feedback to decelerate fuel transportation and chemical reaction of water generation.
Highlights
A polymer electrolyte fuel cell (PEFC) is a sustainable and functioning system continuing power generation with moderate burn of hydrogen and oxygen gas
The cell voltage, detected by a segmented electrode, varies along a gas flow channel from upper to bottom stream and oscillates in time, which is referred as a respiration mode
We expect that PEFC during operation, an open system in a non-equilibrium & steady state, allow us to explore a boundary between materials and life
Summary
A polymer electrolyte fuel cell (PEFC) is a sustainable and functioning system continuing power generation with moderate burn of hydrogen and oxygen gas. At a higher limit of current, corresponding to oxygen diffusion loss, the respiration at different positions starts to be synchronized in a plane direction. This non-equilibrium behavior is related to water flooding at a reaction site of cathode. Small-angle neutron scattering (SANS), which is an essential method for materials science, plays a key role to observe membrane structure and water distribution ranging in nano-scales selectively in a cell stack. We report that PEFC using deuterium gas exhibits higher cell performance (5% in voltage) as compared to normal PEFC using H2 This is due to difference in Gibbs free energy between H2 and D2. We operate a polymer electrolyte fuel cell (PEFC) using deuterium (D2) gas as a fuel (deuterium fuel cell), as well as hydrogen gas (H2)
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