High temperature proton exchange membranes with enhanced proton conductivities at low humidity and high temperature based on polymer blends and block copolymers of poly(1,3-cyclohexadiene) and poly(ethylene glycol)

Abstract

Hot (at 120 °C) and dry (20% relative humidity) operating conditions benefit fuel cell designs based on proton exchange membranes (PEMs) and hydrogen due to simplified system design and increasing tolerance to fuel impurities. Presented are preparation, partial characterization, and multi-scale modeling of such PEMs based on cross-linked, sulfonated poly(1,3-cyclohexadiene) (xsPCHD) blends and block copolymers with poly(ethylene glycol) (PEG). These low cost materials have proton conductivities 18 times that of current industry standard Nafion at hot, dry operating conditions. Among the membranes studied, the blend xsPCHD-PEG PEM displayed the highest proton conductivity, which exhibits a morphology with higher connectivity of the hydrophilic domain throughout the membrane. Simulation and modeling provide a molecular level understanding of distribution of PEG within this hydrophilic domain and its relation to proton conductivities. This study demonstrates enhancement of proton conductivity at high temperature and low relative humidity by incorporation of PEG and optimized sulfonation conditions.

Trifluoromethanesulfonimide-based hygroscopic semi-interpenetrating polymer network for enhanced proton conductivity of nafion-based proton exchange membranes at low humidity

“…These results indicate that protons transfer though the composite membrane is more facile due to the introduction of low-energy-barrier pathways. [17,45] This phenomenon is attributed to three factors: 1) the super acidity of MPTI significantly increases the local acid concentration in the ionic channels, optimizing the chemical environment for proton conduction; 2) incorporating the hydrophilic MPTI SIPN into the membrane increases the water uptake ( Figure 6 and Figure 7), facilitating the dissociation of acid groups, which provides more protons as charge carriers, and the construction of hydrogen networks, which promotes high-speed proton conduction by the Grotthuss mechanism; [14,[46][47][48][49][50] and 3) the pendent HNTf2 groups on the interconnected networks can act as proton carriers to further accelerate proton conduction, with optimization of the microphase separation structure allowing the formation of unrestricted and well-defined ionic pathways for proton conduction (Figure 5j). [35] To further evaluate the potential of the hybrid membranes for operation at low humidity, it is of significance to investigate the relationship between the proton conductivity of the membranes and RH, as shown in Figure 8c.…”

“…[9,10] In the hydrophilic phases, water facilitates the dissociation of acid groups and swelling to form well-connected ionic channels. [11][12][13][14] On the other hand, to realize proton diffusion, water acts as an ion carrier by forming H9O4 + (Eigen cation) or H5O2 + (Zundel cation) (vehicle mechanism) and as a conveyor for rapid delivery of protons by the continuous breaking and forming of hydrogen bonds (Grotthuss mechanism). [15,16] Typically, PEMFCs are expected to operate at high temperatures and low humidities to overcome the problems associated with low electrochemical reaction rates, catalyst poisoning, and complex water management systems.…”

“…[1,11,15,18,20] Hygroscopic materials can hydrate PEMs by capturing the moisture in feed gases and retaining the water generated in electrode reactions. Great efforts have been devoted to exploiting high-performance hygroscopic materials, with typical materials including high concentrations of sulfonated groups, [21] phosphoric acid groups, [1,18] polyethylene glycol (PEG) segments, [14,17] N-heterocyclic groups, [15,20] and Brønsted acids [22]. For example, a composite membrane was fabricated by mixing PEG-modified graphene oxide with a Nafion matrix.…”

Phosphonated polyimides: Enhancement of proton conductivity at high temperatures and low humidity

“…Therefore, numerous efforts have been made recently to develop highly efficient membranes to support these promising energy devices [4][5][6][7][8].…”

Ab Initio Molecular Dynamics Simulations of an Excess Proton in a Triethylene Glycol–Water Solution: Solvation Structure, Mechanism, and Kinetics

“…Polyethylene glycol (PEG) (also known as poly­(ethylene oxide) (PEO) in its long chain form) is a water-soluble polymer that has been widely studied for its interesting properties in aqueous environments. These properties include interactions of its ethylene oxide (EO) chains in aqueous solutions, − the ability to adsorb on surfaces and interfaces, and proton conductivity in polymer electrolytes. − The interaction of water with the PEG chain backbone has been widely studied to date, both by experiment ,, and simulation ,− due to the interesting ability of PEG to form bridged hydrogen bonds (HBs) from EO monomer units to water molecules to other EO monomer units within the same chain . These properties give PEG a wide variety of applications such as biphasic (two-phase) liquid–liquid catalysis, electrochemical energy conversion as a proton exchange membrane additive, − , and in drug delivery via modification of therapeutic molecules (known as PEGylation). − …”