Origins of Water State and Ionic Cluster Morphology for High Proton Conductivity of Short Side-Chain Perfluorinated Sulfonic Acid Membranes

Abstract

The property of perfluorinated sulfonic acid (PFSA) membranes depends not only on the ion exchange capacity (IEC), but also on the chemical structure of the functional side-chain and the phase-separation morphology. Two PFSA membranes, the long side-chain (LSC) and the short side-chain (SSC), have been investigated to study the structure−property relationship, covering the ionic domain structure and the proton transport. The proton conductivity of the SSC PFSA membrane is 143 and 209 mS/cm at 30 °C and 80 °C in water, which is 30− 40% higher than that of the LSC PFSA membrane (103 and 161 mS/cm). The bound-to-free water ratio in the hydrated membranes was analyzed by differential scanning calorimetry and Raman spectroscopy, which show that a higher ratio accounts for the improved proton conductivity of the SSC PFSA membrane. The chain mobility was analyzed by solid-state nuclear magnetic resonance, which reveals that the side chain of the SSC membrane more readily self-assembles. This result was verified by the morphology from transmission electron microscopy. The small-angle X-ray scattering results show that the SSC PFSA membrane exhibits smaller domain spacing between the ionic clusters in dehydrated membranes. These observations, a larger ionic cluster and smaller domain spacing in the dehydrated SSC membrane, indicate a reduced size of the hydrophobic assembly feature domains, and the ionic channel connectivity is better in the SSC, which can be another key issue for its improved proton conductivity, in addition to the higher IEC and higher proton mobility.

High-temperature low-humidity proton exchange membrane with “stream-reservoir” ionic channels for high-power-density fuel cells

“…The procedures for preparation of membranes were previously described in ( 35 ). The polymers were dissolved into water/alcohol mixed solvent to form ~10 wt % solutions.…”

Atomistic Simulations of Hydrated Sulfonated Polybenzophenone Block Copolymer Membranes

“…Whereas, the diffusion coefficients of H 3 O + ions in block polymer matrix is significantly higher (∼3 times) than reported [52] in the phenylated sulfonated poly ether ether ketone ketone (Ph‐SPEEKK) membrane matrix. The mobility of ions in PFSA membrane matrix is greatly influence by side chain pendants length/flexibility of terminal sulfonate group [53,54] and hydrophilic clusters around the pendant [55,56] . Whereas, the sulfonategroup in this work is connected directly to the aromatic ring of polymer chain backbone of Block‐A.…”

“…The mobility of ions in PFSA membrane matrix is greatly influence by side chain pendants length/ flexibility of terminal sulfonate group [53,54] and hydrophilic clusters around the pendant. [55,56] Whereas, the sulfonategroup in this work is connected directly to the aromatic ring of polymer chain backbone of Block-A. The overall conductivity performance of the block-copolymer membrane is consist of vehicular diffusion (in this work from classical MD simulations) and structural diffusion of the proton (i. e. Grotthuss mechanism).…”

Rational Materials and Structure Design for Improving the Performance and Durability of High Temperature Proton Exchange Membranes (HT‐PEMs)

“…[ 43 , 44 ] Subsequently, a series of SSC PFSAs with different ionic exchange capacities (IEC) and side chain lengths have been developed, and systematic studies on their physical properties and performances have been conducted. [ 13 , 45 , 46 , 47 ] Compared with the traditional Nafion membranes, the SSC PFSAs showed an improved T g , good hydration capacity, and high proton conductivity, which enabled potential applications in moderate temperature (100–120 °C) and low‐relative humidity fuel cells. [ 12 ] Recently, our research group has prepared homogeneous SSC PFSA membranes and achieved excellent single‐cell performance and durability at high temperatures.…”

“…Therefore, under the typical operating temperature range of HT‐PEMFCs, the PFSA membranes would not experience significant thermal degradation. [ 46 , 110 ] For chemical degradation, it generally comes from two aspects: [ 111 ] 1) the fuel crossover results in the direct reaction between H 2 and O 2 , and then produces hydroxyl (HO·) and hydroperoxyl (HOO·) radicals, 2) the two‐electron reaction of ORR in the cathode catalyst layers can also lead to the formation of HO· and HOO· radicals. These radicals will attack the vulnerable bonds (C─S, C─O, and C─F bond) in the PFSA structures ( Figure 7 a ), resulting in the loss of membrane integrity and thickness thinning.…”