Using a hydrogen pump cell, water flux, current density, and membrane resistanceacross a Nafion 115 membrane were measured as functions of temperature, water activity,and applied potential. Water flux increased with increase in water activity difference across themembrane. When a potential was applied, (i) the current increased with increase in appliedpotential up to an applied potential around 0.3V; (ii) the current did not increase with furtherincreases in applied potential for applied potentials between 0.3V to 1V. The water fluxcontinually increased with increase in applied potential, even when the current did not increasefor applied potentials > 0.3V. Electro-osmotic drag coefficients were evaluated as a function ofapplied potential, water activity and temperature. The electro-osmotic drag coefficient (numberof water molecules transported with the protonic current) was < 0.15 at low applied potentialsand increased to around 0.35 at applied potentials of 1V. The results indicate that theconduction mechanism for protons in Nafion changes from proton hopping to electric fieldassisted motion of hydrated proton aggregates as the applied potential was increased.
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