Limits of DMFC

One of the most important limitations of direct methanol fuel cell is the low catalytic activity of electrodes, especially anodes and at present, there is no practical alternative to Pt based catalysts. High noble metal loadings on the electrode [158,159]

and the use of perfluorosulfonic acid membranes significantly contribute to the cost of devices. An efficient way to decrease the loadings of precious platinum metal catalysts and higher utilization of Pt particles is to better disperse the desired metal on suitable supports [160]. In general, small particle size and high dispersion of platinum on the support will result in high electrocatalytic activity. Carbon materials possess suitable properties for the design of electrodes in electrochemical devices. Carbon is an ideal material to support nano-sized metallic particles in the electrode for fuel cell applications. No other material except carbon has the essential properties of electronic conductivity, corrosion resistance, surface properties and the low cost required for fuel cell commercialization. In general, the conventional supports namely carbon black is used for the dispersion of Pt particles.

To improve the catalyst utility, the material with high surface area and high electron conductivity is required for the catalyst support. However, the high chemical resistance to acid or alkaline media, the possibility to control up to certain limits, the porosity and the surface chemistry made carbon based materials preferred for catalyst supports. Carbon possesses unique electrical and structural properties to be used in fuel cells. Various forms of carbon, such as graphite, carbon black and other

composite materials have been chosen for catalyst supports. Among them, carbon nanotubes represent a distinctive class of catalyst supports, exhibiting high surface area and many available adsorption sites. In single-walled carbon nanotubes, bundle adsorption sites are represented either by the grooves formed between adjacent tubes or by the nanotube interior, the interstitial channels between tubes and the outer bundle surface. For multi-walled carbon nanotubes, adsorption occurs in aggregated pores inside the tube or on the external walls. Besides these catalyst-related structural properties, carbon nanotubes are more stable to oxidation, feature an increased wear-resistance and possess a good thermal stability. Their metallic characteristics promotes them as good support for metal particles, but chemically functionalized nanotubes can support other catalysts as well, such as bimetallic nanoparticles and organo-metallic complexes. There are three other advantages for carbon nanotubes as catalyst supports. First, the high purity of the material prevents self-poisoning, a common problem of conventional catalysts. Next, the mere nature of these supports can be of interest for liquid-phase reactions and thus limits the mass transfer. Finally, the catalytic activity and its selectivity can directly benefit from specific metal–support interactions. An overall result of the above features tells that catalytic studies on carbon nanotube-based systems have confirmed increased loadings and good dispersion of catalyst particles with respect to other supports. When used as a catalyst support, carbon nanotubes lead to a typical activity and selectivity in several catalytic reactions such as hydrogenation of olefins and nitrobenzene into anilines or selective hydrogenation of double carbon bonds in unsaturated aldehydes. Also of interest are hydroformylation of olefins to aldehydes and partial dehydrogenation reactions. Another applications target catalysts for redox reactions and catalytic decompositions.

2.2.6 Synthesis of Pt catalyst by more efficient method

In recent years, some research teams have reported the synthesis of Pt catalysts by using microwave-assisted heating polyol process [161,162]. The reactant mixture of Pt or other metal precursor, support materials, ethylene glycol and different kinds of additives in a beaker was placed in the center of a household microwave oven and heated for 5s irradiation on and 60s irradiation off for six times (so called

“intermittent microwave heating”, IMH), or heated only for tens of seconds. These studies showed that microwave synthesized metal nanoparticles were very uniform in size and well dispersed on support material. The process indeed has advantages of simplicity, short reaction time and high energy efficiency. However, the most important key factor of a chemical reaction is still unknown, that is, the actual reaction temperature of metal precursor to precipitate as metallic nanoparticles in a household microwave oven is hard to be controlled or only be recorded during experiments. Besides, the loading amount (<20 wt%) and the dispersion density of Pt particles on support, carbon black, are still lower than that (about 40~60 wt%) of convention methods.

There are many factors that affect the efficiency of proton exchange membrane fuel cell (PEMFC). Direct methanol fuel cell (DMFC) with Pt/C catalyst is one effective factor for the transformation of hydrogen in methanol to water [163,164]. It is well known that the catalytic activity of Pt/C catalyst is strongly related to the Pt particle size, the particle size distribution degree and the dispersion of Pt on carbon support [165]. Therefore, Pt catalyst with suitable size, narrow size distribution and high dispersion should have high electro-catalytic activity in fuel cell application. As a result, considerable attention has been laid upon the synthesis and characterization of catalyst nanoparticles [166-168]. The simplest method, wet impregnation, used to synthesize Pt nanoparticles on electro-catalyst support is to impregnate the support

(usually carbon black) with a platinum precursor, and then heat the support above 300°C in the hydrogen atmosphere. However, the control of particle size and size distribution by this method is rather limited [169]. Hence, there are continuing efforts to investigate alternative methods to synthesize highly dispersed supported Pt particles with size-control and uniform size, such as micro-emulsions [170], supercritical fluid [171], sono-chemistry [172,173], polyol process [174,175] and sputtering[176]. All these methods generate colloids and clusters on the nanoscale and with greater uniformity. However, these methods are either time-consuming and complex in multi-step process, or low loading of Pt nanoparticles dispersed on supports.

In this present work, temperature-controlled microwave heating system was used to demonstrate the temperature effect on polyol process, and the effects of additives, reaction-time and other parameters on particle size, size distribution, loading amount and dispersion were also investigated under temperature controlled microwave heating process to obtain high dense Pt clusters on CNTs.