About Perfluorosulfonic Acid Membrane
Perfluorosulfonic acid (PFSA) membrane, or perfluorosulfonic acid ion exchange membrane (PFSIEM), is a perfluoropolymer membrane with a polytetrafluoroethylene structure as the backbone and an alkenyl ether structure with sulfonate groups at the end as side chains. It has good chemical stability and thermal stability and is a good ion exchange carrier. Therefore, PFSIEM has been widely used in the fields of chlor-alkali, fuel cells, and various membrane processes. In particular, one of the most critical components in all-vanadium redox flow batteries, proton exchange membrane fuel cells and ion membrane chlor-alkali process devices is the perfluorosulfonic acid ion exchange membrane.
Perfluorosulfonic Acid Membrane Selection Guide
PFSA Membranes Selection List
Structure of Perfluorosulfonic Acid Ion Exchange Membrane
PFSA membrane is composed of a hydrophobic fluorocarbon skeleton and hydrophilic sulfonic acid groups. In recent years, a variety of PFSA membranes have been gradually developed. They may have various other chemical compositions, different equivalent weights and side chain lengths.
Structure of PFSA membrane. [1]
Many researchers have proposed different models for PFSIEM microstructure, such as:
- Ion cluster network model
The microstructure of PFSIEM after water swelling is spherical ion clusters embedded in the fluorocarbon skeleton. spherical
The ion clusters are connected by small channels, and ions or water molecules pass through the ion clusters through the small channels to achieve conductivity.
- "Sandwich" microstructure model
In the methanol-water system, two perfluorosulfonic acid units form the outer layers on both sides, and a water-methanol or ion channel is formed in the middle. The complex passage of water-methanol or ions through the perfluorosulfonic acid membrane is composed of these simple units.
Microscopic model of PFSA membrane. [2]
Key Features of PFSA Membranes
- High proton conductivity, especially at low humidity and high temperature.
- Low gas permeability, low electronic conductivity, high chemical stability, and low water transport rate through diffusion and electro-osmosis.
- PFSA membranes offer chemical and thermal stability, high ion conductivity, and high cation transport selectivity.
- PFSA membranes have limitations include conductivity dependence on environmental humidity and a restricted operating temperature range.
How Does Perfluorosulfonic Acid Membrane Achieve Ion Selectivity?
The ion selectivity of perfluorosulfonic acid (PFSA) membranes is achieved through their unique chemical structure and microstructure.
The chemical structure of PFSA membranes comprises a perfluorinated backbone with pendant sulfonic acid groups. These sulfonic acid groups provide the membrane with its ion-selective properties. They allow for the selective transport of cations, particularly protons, due to the negatively charged sulfonic acid groups that attract and facilitate the passage of positively charged ions while excluding or impeding negatively charged ions.
The microstructure of PFSA membranes, specifically their clustered and channel-like morphology, plays a significant role in ion selectivity. The clustered structure consisting of hydrophilic domains enables the preferential transport of cations through water-mediated transport channels. This microstructure restricts the passage of larger anions, contributing to the membrane's ion selectivity.
Applications of PFSA Membranes
- Batteries, including direct methanol fuel cells, proton exchange membrane fuel cells, redox flow batteries, and metal sulfur batteries.
- Chlor-alkali electrolysis
- Sensors
- Pervaporation
- Gas separation
- Photocatalysis
References
- Safronova, et al. Membranes, 2023, 13(8), 721.
- Wanzhong Lang, et al. Membrane Science and Technology, 2005, 25(6), 69-74.
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