Alkaline-DMFC

Miiller reported the first detailed studies of the electrochemical oxidation of methanol and other organic compounds at platinum anodes in aqueous alkaline electrolytes in 1922 [11]. Some years later Tanaka [12] extended these studies for methanol electroxidation on Pt, Pd, Rh, and Au in a sodium hydroxide electrolyte. Based on these studies Kordesch and Marko [13] first recognized in 1951 the possibility of building an alkaline DMFC, as that schematized in Fig. 1.3, whose major trouble was that the cross-leakage of the methanol to the air cathode damages the noble metal... 

Fig. 1.3 Left. Scheme of the first alkaline DMFC built by Kordesch and Marko showing Can (—) (i), Pt-fuel electrode (2), Screen (3), Pt-cat. Carbon Tube (4), Separator (5), CH3OH + KOH (6), Filler (7), Metal Cap (+) (8). Right, part of a 60-W methanol-air battery with cylindrical air diffusion electrodes built by Brown Boveri (Reproduced from Refs. [13] and [18] with permission...

Yang CC (2007) Synthesis and characterization of the cross-linked PVA/Ti02 composite polymer membrane for alkaline DMFC. J Membr Sci 288 51-60... 

Yang CC, Lin CT, Chiu SJ (2008) Preparation of the PVA/HAP composite polymer membrane for alkaline DMFC application. Desalination 233 137-146... 

Modestov et al. (2009) proposed using alkali-doped polybenzimidazole (PBI) as a membrane material for alkaline DMFCs. A cell with such a membrane, a pyrolyzed PMPPCo cathode, a Ru-V anode, and a 10 M KOH +2 M EtOH solution as an electrolyte achieved a power density of 100 mW/cm at a temperature of 80°C and a voltage of 0.8 V. 

Methanol fuel works in both the alkaline FC considered in Chapter 5 and the PEMFC of Chapter 4. However, the reaction between the product carbon dioxide and the electrolyte of the alkaline (electrolyte) fuel cell (AFC) is a major problem, which would appear to be insoluble. The anode reaction of the alkaline DMFC is... 

The electrolyte thus inevitably and steadily loses its alkalinity, and has a very limited life. In alkaline DMFCs, it is not possible to see any product carbon dioxide, as it all reacts according to eqnation 6.3. For this reason the alkaline DMFC is not practical, even though it could have many advantages in terms of cost and performance in comparison with PEM-based systems. 

The result is the same as for the alkaline DMFC - six electrons per molecnle of methanol fuel. However, the reaction of 6.4 above does not proceed simply, but takes place in stages, which can take a variety of routes. The oxidation reactions of methanol have been extensively stndied, and they are described in much more detail in the literatnre... 

Equation (1.19) clearly implies the efficiency of methanol as a fuel because 6 electrons are produced per methanol molecule. However, as mentioned in Sect. 1.2.1.3, carbon dioxide will react with hydroxide resulting in the formation of the carbonate, which decreases the hydroxide concentration. The major problem for alkaline DMFC is that no carbon dioxide is produced as a product as all of the CO2 reacts with the hydroxide i.e.. 

Hence, these alkaline DMFC are practically discarded. Therefore, the main focus is on the polymer electrolyte membrane DMFC. 

There are six different types of fuel cells (Table 1.6) (1) alkaline fuel cell (AFC), (2) direct methanol fuel cell (DMFC), (3) molten carbonate fuel cell (MCFC), (4) phosphoric acid fuel cell (PAFC), (5) proton exchange membrane fuel cell (PEMFC), and (6) the solid oxide fuel cell (SOFC). They all differ in applications, operating temperatures, cost, and efficiency. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

SOFC = solid oxide fuel cell MCFC = molten carbonate fuel cell PAFC = phosphoric acid fuel cell AFC = alkaline fuel cell PEMFC = proton exchange membrane fuel cell DMFC = direct methanol fuel cell SAMFC = Solid alkaline membrane fuel cell. 

Several types of fuel cell are currently under development, using different electrolyte systems phosphoric acid (PAFC), alkaline, molten carbonate (MCFC), regenerative, zinc-air, protonic ceramic, (PCFC), proton exchange membrane (PEM), direct methanol (DMFC), and solid oxide (SOFC). The last four contain solid electrolytes. 

Note PAFC phosphoric acid fuel cell PEMFC proton exchange membrane fuel cell/polymer electrolyte membrane fuel cell MBFC microbiological fuel cell DMFC direct methanol conversion fuel cell AFC alkaline fuel cell MCFC molten carbonate fuel cell SOFC solid oxide fuel cell ZAFC zinc air fuel cell. 

Direct methanol fuel cell (DMFC) was developed in 1950s-1960s, based on the liquid alkaline or aqueous acid solution as the electrolyte. It converts the methanol directly into electricity, instead of using indirectly produced hydrogen from methanol through the reforming process. Today, DMFC commonly refers to as the one that employs PEM as the electrolyte. Fuel for DMFC is a dilute solution of methanol, usually 3-5 wt% in water. The size of DMFC can be considerably smaller than PEMFC because of the elimination of fuel processor, and complex humidification and heat management systems. The performance of DMFC is relatively low compared to that of PEMFC. 

In the past two decades, fuel cells and in particular imi-exchange membranes have become a top priority topic in material research. Fuel cells are seen as promising alternative energy conversion systems replacing the combustion-based techniques. Among the various types of fuel cells, the low-temperature fuel cells like the polymer electrolyte membrane fuel cell (PEMFQ, DMFC, or alkaline fuel cell (AFC) are the most flexible ones concerning range of appUcations e.g. portable, automotive, and stationary. 

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