A review of advancements in commercial and non‑commercial Nafion‑PEM for DMFC

What is it about?

Nafion membranes are commercially available for the application of direct methanol fuel cells (DMFCs) due to their unique nano-porous structure, high wettability, high ion exchange capacity due to sulfonic groups and high mechanical strength. However, its high cost and high swelling in water result in high methanol crossover, low chemical stability and low ion conductivity at elevated temperatures that limit its usage. Moreover, in commercial membranes when the thickness increases, the ion conductivity compromises and when the thickness decreases, the fuel crossover increases which disrupts the performance of the fuel cell. The modification of pre-existing Nafion membrane such as Nafion 115, Nafion 117, Nafion 212, Nafion 112 and laboratory recasted Nafion membrane is a promising requirement for their future applications. Additives such as organic, inorganic nanoparticles and polymers apply to the Nafion membrane that not only tune the physical aspects of the membrane but also improve the electrochemical properties of the membrane. This review article focuses on advances in different Nafion commercial membranes and laboratory recasted non-commercial Nafion membrane that make under special conditions after modifications. This paper provides challenges, advantages, and disadvantages, as well as future advances in the application of composite membranes in direct methanol fuel cells.

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Why is it important?

Nafion is a multi-talented material that generates high performance in the DMFC while acting as a proton exchange membrane. Their sulfonic groups on the surface create a strong connection with water that clears the path for high proton conductivity and their nano-porous structure minimizes methanol permeability. However, when DMFC testing time increases, swelling of the Nafion increases which promotes the methanol crossover and the loss of water, which drops the conductivity. According to the manufacturing method, Nafion 212 is made of solution casting while Nafion 117, Nafion 115 and Nafion 112 are made of extrusion casting. Despite the thin quality of Nafion 212 and Nafion 112, Nafion 115 and Nafion 117 are widely used at the commercial level due to their high thickness which retards more methanol from crossing through the proton exchange membrane. However, the high thickness increases internal resistance and ultimately low ion conductivity degrades the overall performance of DMFC. The increase in thickness also increase the cost of the membrane. As shown in Table 7, A thin Nafion 212 membrane somehow fulfills this drawback of high cost with high ion conductivity but possesses a low capability of blocking methanol crossover as compared to other thick commercial membranes. The recasting of Nafion solution by casting and electrospinning method to make a membrane gives extra facility to maintain the cost, desired porosity, water uptake and controlled fuel crossover. However, the less efficient laboratory methods and the lack of facilities to control the environment limit the physical and electrochemical benefits of recasted Nafion membranes. Commercially available Nafion membrane are modified with nanoparticles and polymer coatings while recasted Nafion membranes are modified with the incorporation of polymers and nanoparticles. The polymer addition in Nafion develops the non-swelling walls of the narrow channels within the membrane by creating hydrogen bonding and controlling the movement of fuel crossover. Moreover, the addition of these nanomaterials in the Nafion membrane fulfills the goals of high ion conductivity by using their functional groups and hydrophilic nature and their pore size reduction property minimize the methanol permeability but their amount in the proton exchange membrane remains low due to their detachment and agglomeration problem which still hinder the full utilization of nanoparticles in the PEM. Moreover, the high swelling of polymers in the modified Nafion membrane still allow high methanol crossover at high temperatures. A zero methanol crossover policy is still under consideration, which needs to be fulfilled in order to get high performance from the DMFC. The following key points are addressed after careful evaluation of the recent progress in the proton exchange membrane of DMFC. • The interaction between commercial Nafion membrane and recasted Nafion membrane with additives must improve to emphasize proton conduction and Grotthuss/ Vehicular mechanisms. • The swelling of the proton exchange membrane needs to improve for minimum methanol crossover and the longlife of the membrane.Grafting of the nanoparticles with the commercial Nafion membrane needs to be further promoted because of its easy application. • The time and process of the nanoparticle dispersion in the polymer solution should be improved carefully. Table 7 shows that the overall performance of the Nafion 212 membrane is superior to the other commercial Nafion membranes (Nafion 115, Nafion 117 and Nafion 112) and recasted Nafion membranes due to its better electrochemical properties and interconnected porous structure but does not support fuel crossover due to its thin nature [198].