Pioneering work in DMFC technology was undertaken by Shell, Exxon-Alsthom, Allis Chalmers, and Hitachi during the 1960s and 1970s [1,6]. Research focused on developing noble metal catalysts in liquid acid and alkaline electrolytes [1]. During this period, the mechanistics of methanol oxidation at Pt-based catalysts were studied [1,7,8]. While fundamental understanding of methanol oxidation became more clear, maximum current densities remained low. It was thought the limitation on current density was largely due to inadequate ionic conduction and stability of the PEMs employed in the fuel cells.

Membrane Development

In the mid 1960s, DuPont introduced the perfluorinated superacid membrane Nafion®. It was considered a major advancement in PEM materials [3]. Earlier, less effective PEM materials included polystyrene-based ionomers and heteroge­neous sulfonated divinylbenzene cross-linked polystyrene [2,3]. These early PEMs had poor long-term chemical stability and low proton conductivity. Nafion performance was considerably better than these other materials, however, it was quickly recognized that methanol crossover through Nafion would limit its use­fulness as a separator in DMFCs [9]. Crossover diminishes cell efficiency and occurs when fuel that is fed to the anode crosses through the membrane to the cathode and reacts directly with the oxidant. The process also poisons the cathode electrocatalyst with methanol oxidation products.

In the mid-1980s, Nafion membranes became more widely available and solubilized Nafion was introduced to the market. As Nafion became more widely available, PEFC research began in earnest and has since continued. The most notable improvement is a dramatic reduction in catalyst loading at ever-increasing power outputs [6]. Another important discovery was that the stability of the Pt electrocatalyst is greatly enhanced when Nafion is added to the electrocatalyst later. These developments set the stage for a revival of interest in low-temperature DMFCs during the late 1990s [6,10,11,12]. Figure 9.3 demonstrates the increase in DMFC research activity over this period.

As will be seen in later sections of this chapter, current efforts in DMFC research include minimizing methanol crossover through the separator of DMFCs while maintaining high proton conductivity, developing methanol tolerant oxygen reduc­tion catalysts, and identifying more cost-effective methanol oxidation catalysts [13].