The proton-exchange membrane (PEM) is an integral component of solid polymer electrolyte fuel cells. The membrane acts as a separator to prevent mixing of reactant gases and as an electrolyte for transporting protons from anode to cathode. 1 High proton conductivity, mechanical strength, and chemical stability of the membrane are factors that affect fuel cell performance. 2,3 Many PEM materials, including the widely studied Nafion series of membranes, are ionomers that consist of a hydrophobic backbone possessing pendant cation exchange sites such as SO 3 -. According to the Eisenberg-Hird-Moore (EHM) model, 4,5 the ionic sites aggregate to form multiplets. To explain conductivity through Nafion, Gierke proposed a "cluster-network" model in which clusters of ions, separated by a polymer backbone matrix, are connected via short, narrow channels (∼1 nm in diameter). 6,7 However, it has yet to be proven that the formation of cluster networks is necessary to achieve high conductivity in PEMs. That is, for a given ion content, is proton conductivity served better by having the ions form a cluster network rather than having them dispersed homogeneously throughout the membrane?To examine the relationship between structure, morphology, and conductivity, we report the synthesis, film formation, and properties of a novel class of well-defined graft polymers comprising styrene and sodium styrene sulfonate (PS-g-PSSNa). This system was chosen because it enabled control of both the copolymer's ion content and morphology. PS-g-PSSNa was prepared via the stable free radical polymerization of sodium styrene sulfonate (SSNa), 8-10 and the "living" terminus of PSSNa reacted with divinylbenzene (DVB) to yield PSSNa possessing a terminal vinyl group (PSSNa-DVB). The degree of polymerization, molecular weight, and polydispersity of PSSNa were 32, 6.5 × 10 3 , and 1.25, respectively. The resultant macromonomer was emulsion-copolymerized with styrene as illustrated in Scheme 1. PSSNa-DVB inserts into the propagating polystyryl chains (PS), as graft chains, because the DVB terminus is located in the core of the polymerizing micellar particles. By adjusting the feed ratio of PSSNa-DVB to styrene, a series of copolymers with uniform graft chain length and controlled graft density was

No abstract

The length of graft chains in graft polymers is controlled in order to dictate the formation of a nanochannel network of ions in a non‐ionic matrix. Graft polymers were prepared by copolymerization of styrene with poly(sodium styrene sulfonate) (PSSNa) macromonomers. The latter were prepared with controlled molecular weight and narrow polydispersity by stable free radical polymerization. Phase separation of ionic aggregates occurs to a greater extent in films prepared from amphiphilic polymers possessing longer graft chains. Films prepared from polymers containing low ion content comprise of isolated ionic domains and exhibit low ionic conductivity. Increasing the ion content with the membrane, by increasing the number density of ionic graft chains in the polymer, results in ionic domains that coalesce into a network of nanochannels, and a dramatic increase in ion conductivity is observed. The ionic network is developed to a greater extent for films based on longer ionic graft chain polymers; an observation explained on the basis of phase separation.

Proton-exchange-membrane fuel cells (PEMFCs) have received considerable interest as a reliable power source due to their ability to attain high power densities with high energy efficiency. 1 A vital component of the PEMFC is the proton exchange membrane (PEM), which provides the ionic path between the anode and the cathode while separating the two reactant gases. The PEM material most frequently used for this type of application is Nafion ® due to its chemical and mechanical stability and its commercial availability. Since Nafion is relatively costly, much research is being directed toward developing less expensive membrane materials. 2 One such alternative class of membrane derives from studies of radiation-grafted polymerization of styrene monomer into matrices such as fluorinated ethylene propylene (FEP), ethylenetetrafluoroethylene (ETFE), and poly(vinylidene fluoride) (PVDF) with subsequent sulfonation of the polystyrene. 3-6 The grafting and sulfonation processes allow the introduction of ionic conductivity while maintaining the mechanical characteristics of the base polymer. The process uses prefabricated commercial films and thus circumvents difficulties in obtaining thin films of uniform thickness. Parameters such as cross-linking density and membrane thickness can also be controlled. 3,7 The first step in the radiation-grafting technique is irradiation of the base polymer membrane usually with a gamma-source or electron-beam to form radicals on the polymer backbone. This is followed by introduction of the monomer, subsequent copolymerization, and concurrent attachment to the base polymer as a graft chain. Finally, the grafted chains are sulfonated. 3 Scherer's group has studied the radiation grafting approach extensively. They have explored the influence of synthetic conditions on the degree of grafting, structure, and physicochemical properties of FEP-g-polystyrenesulfonic acid (PSSA) films and evaluated their performance, stability, and degradation in PEMFCs. 3,4,7-9 From these studies they have been able to identify important membrane properties, for example: optimum thickness, cross-linking density, and specific resistance required for fuel-cell applications. 3 The performance of PEMFCs is dependent on many factors, but three are prominent and involve the PEM: (i) the ohmic overpotential due to membrane resistance, (ii) the activation overpotential due to slow kinetics of the oxygen reduction reaction (ORR) at the electrode/membrane interface, and (iii) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surface. 10 In order to understand fully the applicability of PEMs in PEMFCs, it is important that the materials be systematically investigated to establish correlations between membrane composition and electrochemical properties. A better understanding of these properties at a fundamental level should lead to further advances in membrane development for applications in PEMFCs.Lehtinen et al. have studied the electrochemical properties of PVDF-g-PSSA membrane...

Voltage cycling is one of the most damaging stressors for automotive PEMFC. Understanding of the effects of voltage cycling on performance degradation is crucial to improve PEMFC durability for automotive applications. This study focuses on the interaction between upper potential limit (UPL) and lower potential limit (LPL) on the stability of PEMFC. A well-defined peak of degradation rate is observed when the LPL is ∼0.8 V with UPL of 1.35 V. A mathematical model was developed to understand the observed relationship between degradation rate and lower potential. Modeling results suggest that when cycling to a lower potential of ∼0.8 V, almost all dissolved Pt migrate from the catalyst layer to the membrane with negligible re-deposition, resulting in a peak of degradation rate at ∼0.8 V. The amount of Pt in the membrane (PITM) measured at end of life (EOL) samples correlates with degradation rates and is in agreement with modeling results.An automotive fuel cell needs to withstand thousands of operational hours. During normal operation of the vehicle, while load cycles occur, there will be fast dynamic change of cathode potential in the range of ∼0.6-0.95 V. During startup and shutdown, the cathode potential can be higher than 1.35 V. 1 It is well known that Pt is electrochemically unstable during voltage transitions, especially when the potential goes above 1.0 V. 2,3 The reaction of Pt to form ions, Eq. 1, is a typical failure mode during voltage cycling and it has been extensively studied by many groups. 4-10 Some researchers have shown that Pt will go into ion form during an anodic potential sweep. 11-13 It is also known that Pt can be oxidized to PtO or PtO 2 at the potentials above 1.15 V. [11][12][13] The reactions are shown as below Eq. 2 and 3.During the cathodic sweeps, Pt oxide can be electrochemically dissolved by Eq. 4 or chemically dissolved by Eq. 5. 11,14,15 Repeated anodic and cathodic voltage sweeps will accelerate Pt dissolution. As a result, permanent Pt loss could occur and ultimately lead to the fuel cell stack decay. The dissolution of Pt is further exacerbated with higher upper potentials. Imai et al. carried out in-situ measurements of Pt oxide at different potentials in aqueous media. 16 They observed place exchange phenomenon when the potential was higher than 1.4 V, suggesting Pt could suffer more severe dissolution at such upper potentials. Therefore, it is of great interest to study the impact of parameters in voltage cycling on Pt durability.The Pt durability can be dependent on many factors, including voltage window (upper potential, lower potential), operation conditions (relative humidity (RH), temperature, gas pressure, etc). The impact of RH and temperature has been investigated and discussed by some publications from Bi et al. 17,18 The importance of upper potential on platinum dissolution during voltage cycles has also been studied by many groups. Wang et al. conducted experiments to measure the equilibrium concentration of dissolved Pt at different upper potentials in acidic s...