Consideration of thermodynamic, transport, and mechanical properties in the design of polymer electrolyte membranes for higher temperature fuel cell operation

Abstract

A phenomenological theory is provided for water sorption and proton transport in polymer electrolyte membranes (PEMs) as well as in polymer-inorganic nanocomposite membranes (NCPEMs) that not only serves to rationalize the sorption and conductivity behavior of conventional PEMs such as Nafion but also provides a framework for rational design of improved PEMs and NCPEMs. The thermodynamic model, which considers the effect of osmotic pressure on the activity of free water within the membrane pores, predicts the entire sorption isotherm and provides a plausible explanation for the so-called Schroeder's paradox. The transport model incorporates the various mechanisms of proton transport, namely, surface hopping, Grotthuss diffusion, and en masse diffusion. As the design of alternate PEMs suitable for effective proton transport under hot and dry conditions is a key current technological goal, the rational design of NCPEMs for this purpose is considered here in detail on the basis of an extension of the transport model to account for the influence of the inclusion of functional additives in NCPEMs. The results also point to the reason that Nafion is an excellent PEM, because the hydrophobic nature of its backbone induces water away from surface into pore bulk where efficient proton diffusion occurs.

Polymeric organic–inorganic proton-exchange membranes for fuel cells produced by the sol–gel method

“…It is noted that the vehicle mechanism predominates to a greater degree in the moist membrane whereas the Grotthus mechanism becomes dominant in the dried Nafion membrane. The increase of the conductivity in the organic-inorganic membranes due to retention of an additional amount of water by the inorganic component is also supported by data in [31]. The presence of unhydrolyzed alkoxysilyl groups resulting from incomplete realization of the sol-gel process prevents the formation of uninterrupted channels, and this worsens the transfer of the proton.…”

Influence of coion and counterion size on multi-ionic solution nanofiltration

“…This is approximately 8% of the permeate concentrations of the other ions. The proton diffuses through several mechanisms, including the vehicular and the Grotthuss ones [34]. Its diffusion and convection hindrance factors cannot be calculated from Equations A1-A5 and other fitting parameters would have to be included in the model resolution [20], as the theoretical background has not been established.…”

Hyperbranched poly(ether sulfone)s: preparation and application to ion-exchange membranes

“…The mechanism of proton conduction in Nafion and other protonexchange membranes involves the formation of water-filled pores and channels within the film 82,83 and proton conduction occurs through a hopping mechanism. The pores are generated by the rearrangement of polymer chain, which bring hydrophilic sulfonic acid groups into contact with water and allow hydrophobic segments to migrate away.…”

“…The pores are generated by the rearrangement of polymer chain, which bring hydrophilic sulfonic acid groups into contact with water and allow hydrophobic segments to migrate away. 83 However, the mobility of the polymer chains is problematic at elevated temperatures. The management of liquid water, which is critical for proton hopping, is more difficult as the temperature approaches the boiling point.…”