List of abbreviations
Chapter 2: Literature Review
2.4. Summary of literature review
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polarization curve. Table 2.6 compares some critical parameters of nafion composite membranes developed with methanol impermeable and/or proton conducting additives.
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Table 2.6 Comparison of critical properties of nafion composite membranes with Pd and Pt additives Reference Preparation
technique
Water uptake (%)
Proton conductivity (S/cm)
MCO DMFC performance
Ma et al., 2003
Sputtering NR Improved (value not
reported)
Reduced (value not reported)
Maximum of 2.8 mW/cm2 for 1 µm Pd- Ag/nafion, and 0.9 mW/cm2 for nafion-117 (1 M methanol, ambient temperature)
Kim et al., 2003
Casting NR 4.9 for nafion-117,
and 3.2 for 0.90 mg/cm2 Pd/nafion
3.2 × 10−6 cm2/s for nafion-117, 4.3× 10−7 cm2/s for 0.90 mg/cm2 Pd/nafion
Maximum of 60 mW/cm2 for nafion-117 and 65 mW/cm2 for 0.90 mg/cm2 Pd/nafion (2 M methanol, 30 °C) Maximum of 34 mW/cm2 for nafion-117 and 79 mW/cm2 for 0.90 mg/cm2 Pd/nafion (10 M methanol, 30 °C)
Prabhuram et al., 2005
Sputtering NR NR NR 300 mA/cm2 for nafion-
115, 220 mA/cm2 for Pd/nafion, 280 mA/cm2 for (60:40) Pd-Cu/nafion at 0.12 V (2 M methanol, 70 °C)
Lee et al., 2008
Cation exchange
22.93 for pure cast nafion and 41.98 for 2% Pt/nafion
0.033 for pure cast nafion, and 0.038 for 2% Pt/nafion
NR Maximum of 77.7
mW/cm2 for pure cast nafion, and 100 mW/cm2 for 2% Pt/nafion (1 M Methanol, 70 °C)
NR: Not reported
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Modification of nafion with zeolite, and their derivatives could increase thermal and mechanical properties and reduce MCO. However, increasing impermeable filler content usually resulted in a simultaneous reduction in proton conductivity and mechanical failure. The composite membranes containing the impermeable additives were found to have increased MCO when loading increased, attributed to lack of interactions between the filler and the polymer creating a crude dispersion impairing the transport trend. Pd modified nafion membranes could lower MCO while maintaining the proton conductivity and also allowed usage of higher methanol concentrations without depreciating the fuel cell performance. However, it has been indicated that prolonged hydride formation and its conversion into protons during cell operation can lead to hydrogen embrittlement in the Pd thin layer, which can eventually cause mechanical degradation of the Pd layers from the surface of the membrane. Alloying Pd with other metals could alleviate the problem of hydrogen embrittlement but the composite showed increased MCO than Pd/nafion membranes. Modification of nafion with methanol oxidizing catalyst like Pt is a good strategy. However, the performances of the composite membranes are limited by factors such as high methanol concentration and high flow rate of methanol. Besides, Pt is very expensive and would increase the cost of the composite membranes.
Therefore, the search for a nafion composite possessing the physic-chemical properties of nafion with the additional property of methanol selectivity is still going on and there have been efforts globally by the researchers to meet the set criterion. For a proper assessment of the nafion composites a detailed characterization of the composite membranes both under ex-situ environment and fuel cell conditions is necessary. Another significant point to be noted is that DMFC has applications for automotive, stationary, and portable devices. Hence, as pointed out by Zang and Shen, (2012), it would be impractical to
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expect that a single type of membrane can meet all the requirements. If the application of DMFC is for portable devices like camera, laptops, thermometers, i-pods, mobile phones, etc., which are operated at ambient temperature, performance of the composite membranes at ambient temperature is noteworthy rather than that at high temperature.
Thus, for enhanced DMFC performance at ambient temperature both the parameters namely, low MCO and high proton conductivity will play a crucial role. The mechanical properties of the composite membranes namely, tensile strength and swelling have also not been taken into consideration while studies have shown that the mechanical properties of the membranes are extremely important to the performance and longevity of the fuel cell. Nafion is a polymeric membrane, which means that it responds to stress in a time dependant manner. The membrane of a fuel cell is subject to various stresses, including those due to solvent mass uptake and clamping between flow plates. Hence, an accurate knowledge of the physico-chemical properties of the synthesized nafion composite membrane under a range of environmental conditions is crucial to identify the potential composite membranes for DMFC. Moreover, the degradation of nafion in polymer electrolyte membrane fuel cells has caused widespread concern. However, there are barely any reports on the analysis of nafion composites for chemical stability or oxidative stability.
It is an undeniable fact that in the foreseeable future, it will be difficult to completely replace nafion membranes, especially at temperatures less than 80 °C, because of their excellent oxidative stability and superior proton conductivity. Besides, the nafion membranes reinforced by filling pores offer a cost-effective option. So, in order to improve the performance of nafion composites, there is the need for extensive research with new type of additives that would contribute to low MCO, acceptable proton
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conductivity, high thermal, chemical and mechanical durability, moderate swelling and, high power density. It is also desirable for the additives to have some kind of bond with the nafion matrices so as to enhance the durability of the composites. Therefore, the objective of the current research is set as per the above literature survey and discussed in the next section.