Modified PFSA membranes

Composite and Modified PFSA Membranes for Intermediate Temperature PEM Eiectroiysis... 

To achieve high-temperature operation, three approaches have been developed to modify PFSA membranes (1) swelling the membrane with nonaqueous and low-volatility solvents (2) reducing the membrane thickness and (3) impregnating the membrane with hygroscopic oxide nanoparticles or solid inorganic proton conductors. 

The polymer membranes (other than the perfluorinated membranes) are classified into three groups, viz. (a) modified PFSA membranes, (b) alternate sulfonated hydrocarbon polymers and their inorganic composite membranes and (c) acid-base complex membranes. 

Considerable efforts are being made to modify the PFSA membranes to achieve high temperature operation. In one approach the water is replaced with non-aqueous and low volatile media. This approach has met with limited success. The other approach is to develop methods to improve water management. The water balance in a PEMFC involves the following mechanisms ... 

Styrenic polymers, which are easy to synthesize and modify, were studied extensively in the early literature. One example is BAM made by Ballard Advanced Materials (see chemical structure below). This membrane is 75 pm thick and has an ion exchange capacity of about 1.1 to 2.6 meg/g. Its chemical stability is not as good as PFSA even with its perfluorinated backbone. Ballard claimed that this membrane could last for several hundred hours under low RH operating conditions. It is no longer in production due to its high cost and the lack of availability of the monomer. 

One approach is to modify the PFSA side-chains such that they cany more than one acid site since the ciystallinify and morphological properties arise essentially from the ratio of non-substituted TFE to functionalised I FE of the backbone polymer repeat unit, multi-acid side-chain ionomer membranes have the potential to demonstrate the mechanical properties of a higher EW polymer (characteristic of a single acid site per side-chain), and the proton conduction properties of a lower EW material (conferred by the presence of multi-acid sites per side-chain). This direction is being followed at 3M, where introduction of... 

It is for these reasons that rather broad efforts are made in many laboratories (a) to modify membranes based on PFSA-type polymers so that the foregoing defects may be overcome, (b) to design new types of membranes from different polymers or other materials, and (c) to design alkaline membranes exhibiting hydroxyl ion conduction. 

High-temperature proton exchange membrane fuel cells (HT-PEM fuel cells), which use modified perfluorosulfonic acid (PFSA) polymers [1—3] or acid-base polymers as membranes [4—8], usually operate at temperatures from 90 to 200 °C with low or no humidity. The development of HT-PEM fuel cells has been pursued worldwide to solve some of the problems associated with current low-temperature PEM fuel cells (LT-PEM fuel cells, usually operated at <90 °C) these include sluggish electrode kinetics, low tolerance for contaminants (e.g. carbon monoxide (CO)), and complicated water and heat management [4,5]. However, operating a PEM fuel cell at >90 °C also accelerates degradation of the fuel cell components, especially the membranes and electrocatalysts [8]. 

There are efforts to modify Nafion-based membranes to improve their properties. Various composite materials to improve disadvantageous properties of the PFSA ionomer are introduced later in another section. 

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