Low temperature DMFCs for portable applications

Technological improvements in direct methanol fuel cells (DMFCs) are promoted by their exciting perspectives of application in portable, transportation and stationary devices. Direct Methanol Fuel Cells were in the past mainly designed for relatively high temperature (>90°C) operation in order to enhance the methanol electro-oxidation kinetics and to increase the ionic conductivity of the polymeric membrane. New composite membranes have been developed to further extend the operating temperature up to 150°C. High operating temperatures are especially favourable for transportation applications. However, the most promising short term application of DMFC appears now to involve the field of portable power sources[1-4] where they can suitably to replace or support batteries. In this regard, increasing interest is devoted towards the miniaturisation of these fuel cell devices in order to replace the current Li-ion batteries. Theoretically, methanol has a superior specific energy density (6000 Wh/kg) in comparison with the best rechargeable battery, lithium polymer and lithium ion polymer (600 Wh/kg) systems. This advantage translates into much longer conversation times using mobile phones, longer time of continuous operation for laptop computers and more power available on these devices to support consumer demand. In relation to consumer convenience, another significant advantage of the DMFC over the rechargeable battery is its potential for instantaneous refuelling. Unlike rechargeable batteries that require hours for charging a depleted power pack, a DMFC can have its fuel replaced in minutes. These significant advantages make DMFC a promising device in the portable electronic market [5-7]. The research activity of this thesis is addressed towards the development of low temperature (30-60°C) DMFCs for portable applications. The different working conditions of portable system i.e. passive operation mode, air breathing etc., introduce new issues with respect to DMFCs operating at high temperatures; thus development of a system for portable applications requires further efforts. In this thesis, three fundamental aspects have been mainly addressed, these are: I. Pt loading: analysis of the influence of the Pt loading on the electrochemical behaviour of a DMFC; II. Methanol tolerant cathode catalysts: analysis of the catalytic activity for the oxygen reduction reaction in DMFC for Pt alloy with transition metals; III. Water management: optimization of electrode properties; IV. Passive mode operation. The aim of the present work is to contribute to the comprehension of the main issues associated to the low DMFC operation temperature. Such an investigation was carried out by using both physico-chemical and electrochemical methods. Specific tests were carried out by using diagnostic techniques in order to individuate potential solutions to drawbacks affecting DMFCs.