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J. Phys. Chem. B 2006, 110, 6072-6080
Effect of Water on the Low Temperature Conductivity of Polymer Electrolytes Ana Siu,† Jennifer Schmeisser,† and Steven Holdcroft*,†,‡ Department of Chemistry, Simon Fraser UniVersity, Burnaby, British Columbia V5A 1S6, Canada, and Institute for Fuel Cell InnoVation, National Research Council Canada, 3250 East Mall, VancouVer, British Columbia V6T 1W5, Canada ReceiVed: June 10, 2005; In Final Form: October 6, 2005
The proton conductivity of radiation-grafted ethylenetetrafluoroethylene-grafted-poly(styrene sulfonic) acid (ETFE-g-PSSA) and Nafion 117 membranes between 25 and -37 °C is reported. The freezing of water in the membranes, which strongly depends on the internal acid concentration, results in a 4-fold decrease in proton conductivity. The activation energies before and after the freezing of the membranes are ∼0.15 and 0.4 eV, consistent with proton transport through liquid water and strongly bound water, respectively. Differential scanning calorimetry data show that up to 14 H2O molecules per H+/SO3- group remain unfrozen at subzero temperatures and are believed to be responsible for the low temperature conductivity that is observed. These results indicate that proton conductivity in membranes may be achieved via strongly bound and highly polarized water.
1. Introduction A membrane with high proton conductivity is desired for proton exchange membrane fuel cells (PEMFCs) in order to reduce Ohmic losses during their operation.1 To date, perfluorosulfonic acid membranes, in particular Nafion, have been the solid electrolyte of choice and the technology standard in the PEMFC industry because the material is commercially available and is also chemically and mechanically stable. Nafion is a polymer that consists of a perfluorinated backbone and pendent vinyl ether side chains terminated with SO3H groups. Small angle X-ray and neutron studies indicate that Nafion exhibits a nanophase separated morphology.2,3 Upon hydration, the hydrophillic regions within the materials coalesce to yield a continuous ionic/aqueous pathway4,5 that facilitates proton transport. Fully hydrated (or wet) Nafion exhibits a high proton conductivity due to its high proton mobility and high proton concentration. However, at lower humidity, the proton conductivity is much lower because of a concomitant drop in the proton mobility.6,7 The mechanism of proton conduction in Nafion has been a topic of discussion for several decades. Proton hopping (the Grotthus mechanism) and vehicular diffusion are believed to be the predominant modes of proton conduction.8,9 It has been suggested that the contribution of the Grotthus mechanism to conductivity occurs largely in the center of a water-swollen pore, and consequently, proton mobility is higher in this region.10,11 Counterviews suggests that such a distinction (Grotthus vs vehicular conduction) is not so clear-cut because charge-carrying protons are undistinguishable from the “sea” of background protons and water8 and “fixed”, “free”, or “excess” protons are not considered to exist per se: a proton, mobile in one moment of time, becomes part of the next water molecule and so on. Furthermore, the protonated clusters H3O+, H5O2+, and H9O4+ are considered mobile but short-lived. Kreuer9 estimates that * Corresponding author. Phone: 1-604-291-4221. Fax: 1-604-291-3765. E-mail:
[email protected]. † Simon Fraser University. ‡ National Research Council Canada.
hydrogen bond breaking and forming occurs at a rate of 1011 s-1. As this rate decreases with temperature, transport by the Grotthus mechanism is also believed to decrease. Still, the Grotthus mechanism is known to enhance proton transport because the transport of alkali metal cations through perfluorinated membranes, where the mechanism is absent, is much lower.12,13 Water is vital to the transport of protons through proton exchange membranes. In a real system, the distinction between the different states of water within the hydrophilic pores can be difficult to discern because the rate of proton exchange in acidic water is high, as illustrated by the single broad 1H NMR peak of hydrated membranes.14,15 However, the local environment of water in the ionic pores can still be identified from the temperature at which water in the membrane freezes. This method of analysis classifies water into either nonfreezable or freezable. Nonfreezable water is defined as water that is strongly bound to either the polymer backbone or the ionic groups that is associated with the polymer; nonfreezable water yields no characteristic thermal transition in differential scanning calorimetry (DSC) analyses. Water molecules that are highly polarized by virtue of being in close proximity to an ion exist in hydration shells and are unable to crystallize.16,17 Freezable water, whether only weakly polarized or liquidlike, exhibits similar thermal transitions to bulk water. A simplified schematic drawing of the different types of water within an ionic pore is shown in Figure 1. Nonfreezable water is situated along the pore walls, whereas freezable water is located near the pore center, as suggested by modeling, X-ray and neutron scattering work.4,11,18 Water furthest away from the SO3- (e.g., pore center) bears the closest resemblance to bulk water, so it is expected to crystallize first. The ice crystal will continue to grow with further decrease in temperature until the residual water molecules cannot reorient themselves and pack into a crystal lattice, giving rise to the nonfreezable water. Many studies show that water absorbed in hydrophilic polymer systems does not exhibit the same calorimetric,19,20 diffusive,10 or spectroscopic21,22 behavior as bulk water. In this
10.1021/jp0531208 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/10/2006
Low Temperature Conductivity of Polymer Electrolytes
Figure 1. Schematic diagram illustrating the different types of water in the hydrophilic pore of a membrane. Reducing the humidity reduces the amount of freezable water present in the pore and alters its size.
work, the focus will be on using calorimetric techniques. Low temperature DSC measurements have been extensively used to study water in polymers, but they have mainly been applied to hydrogels.23 A few reports are available for fuel cell membranes. Earlier work by Hietela20 and Gupta19 quantifies the amount of freezable water by integrating the areas under the DSC curves and comparing the enthalpies calculated to that found for pure water. Kim and co-workers24 showed that the different states of water can be measured indirectly by correlating the glass transition temperature of hydrated polymers, obtained from DSC measurements, with spin-spin relaxation times, T2, obtained from 1H NMR. This paper addresses two issues: (1) the nature of water inside the polymer membrane and how it can be influenced by temperature and humidity and (2) how these parameters jointly influence proton conductivity at subzero temperatures and under reduced humidity. Subzero, low temperature conductivity of PEMs25,26 has not been as extensively studied as high temperature conductivity (>100 °C)27,28 even though it has relevance to low temperature fuel cells and operation. Two types of membranes were examined: a series of experimental radiationgrafted ethylenetetrafluoroethylene-grafted-poly(styrene sulfonic) acid (ETFE-g-PSSA) membranes that contain varying ionic contents and commercially available Nafion 117 (N117) (Figure 2) for benchmark comparison. The former, while being chemically susceptible to degradation under standard fuel cell conditions, are available with a range of ion exchange capacities (IECs), so that data can be compared and the relationship between the nature of water and proton transport can be further understood. 2. Experimental Section Radiation-grafted ETFE-g-PSSA membranes were provided by K. Lovell and co-workers (Cranfield University, U.K.). A detailed description of the synthesis of ETFE-g-PSSA membranes is described elsewhere.29 Briefly, a porous, preformed ethylenetetrafluoroethylene (ETFE) film (Du Pont) is subjected to γ radiation and subsequently immersed in styrene solution to initiate the graft polymerization of styrene. The length and density of the graft polymers is controlled by the conditions of polymerization. The styrene units are sulfonated to various degrees to provide a systematic variance in the ionic content of the membrane. The IEC of each membrane (millimoles of SO3-/ gram of dry polymer) is provided in parentheses. Nafion membranes were boiled in 3% hydrogen peroxide solution for 30 min and then in water for another 30 min. The membranes were converted to the protonic form by boiling them in 0.5 M H2SO4 for 30 min. ETFE-g-PSSA membranes were however directly converted to the protonic form by soaking them
J. Phys. Chem. B, Vol. 110, No. 12, 2006 6073 in 2 M H2SO4, followed by rinsing and soaking them in Milli-Q (Millipore) water without boiling them in peroxide. The membranes were equilibrated from 99 to 85% relative humidity (RH) in an ESPEC SH-241 humidity chamber at 25 °C. Liquid-saturated membranes (wet) were equilibrated by immersion into Millipore water at room temperature. Water contents were measured using thermal gravimetric analysis (TGA). Membranes were pat-dried with tissue paper and placed on a tared DSC aluminum pan. No (significant) loss of water was observed from the sample during the 1 min assembly time (
