DMFC, mixed potential

The DMFC, based on a polymer electrolyte fuel cell (PEFC), uses methanol directly for electric power generation and promises technical advantages for power trains. The fuel can be delivered to the fuel cell in a gaseous or liquid form. The actual power densities of a DMFC are clearly lower than those of a conventional hydrogen-fed polymer electrolyte fuel cell. In addition, methanol permeates through the electrolyte and oxidizes at the cathode. This results in a mixed potential at the cathode (Hohlein et al., 2000). 

The major problem associated with the operation of DMFCs is the gradual diffusion of methanol through the membrane - known as methanol crossover - that leads to the establishment of a mixed potential at the cathode and, consequently, to a decrease of the working voltage of the cell. Because of the methanol crossover phenomenon, the maximum methanol concentration used in DMFCs is about 2M and membranes as thick as 175 pm are used. 

Even more important is the fact that most PEM materials (see below) are also quite permeable for water and methanol. Thus, thin membranes lead to substantial transport of these molecules from the anode side to the cathode (e. g., [25-29]). The permeation of methanol in the DMFC is undesirable for the obvious reason that it reduces the cell power ( mixed potential formation ), because no electrical work is generated in a cathodic oxidation reaction. Furthermore methanol on the cathode is unfavorable because it can block adsorption sites needed for the oxygen reduction reaction. The presence of methanol may even alter the rate constant of the oxygen reduction reaction. A typical solution to the problem of methanol transport is the use of dilute aqueous solutions of methanol, which assures almost complete oxidation when the anodic catalyst loading is high enough [30],... 

White et al. proposed an one-dimensional, isothermal model for a DMFC [168]. This model accounts for the kinetics of the multi-step methanol oxidation reaction at the anode. Diffusion and crossover of methanol are modeled and the mixed potential of the oxygen cathode due to methanol crossover is included. Kinetic and diffusional parameters are estimated by comparing the model to experimental data. The authors claim that their semi-analytical model can be solved rapidly so that it could be suitable for inclusion in real-time system level DMFC simulations. 

Noninteracting processes of oxygen reduction and methanol oxidation at the DMFC cathode result in a mixed potential corresponding to increase of cathode overpotential at some given cell current. 

Moreover methanol cross-over through the polymer membrane reduces the efficiency of the Fuel Cell by decreasing the cell voltage as a result of a mixed potential at the oxygen cathode. Improved membranes with low crossover have to be developed. Besides, in order to increase the reaction rate of methanol oxidation (and of oxygen reduction), some works propose to operate the DMFC at higher temperatures with temperature-resistant membranes... 

Unexpectedly, however, Voc depends on stoichiometry with the growth of A, Voc increases (Figure 4.26(b)). At A = oo, the cell open-circuit voltage reaches the value where E < 100 mV is the voltage loss under open-circuit conditions (see below). The effect of V(,c lowering due to methanol crossover (mixed potential) is well known in DMFC studies. The model above allows us to explain the dependence of OCV on feed molecule stoichiometry A. 

A. A. Kulikovsky. On the nature of mixed potential in a DMFC. J. Electrochem. Soc., 152(6) A1121-A1127, 2005b. 

Fuel crossover occurs to some degree in all low-temperature fuel cells, particularly in DMFCs. For a DMFC, methanol crossover not only results in additional fuel consumption, but also reduces the cell voltage by the effect of mixed potential, and... 

The permeation of methanol through the membrane from the anode to the cathode (cross-over) represents a source of severe performance loss. The combined reaction of methanol and oxygen on the cathodic platinum catalysts (or platinum-based alloys) leads to a mixed potential formation that reduces the maximum achievable potential considerably by up to 200 mV. Two strategies are followed to circumvent this phenomenon. On the one hand the general avoidance of methanol permeation through the membrane allows the use of standard catalysts. The other strategy is the use of methanol-tolerant catalysts for the cathode of DMFCs these materials are characterized by a complete inactivity towards methanol, which does not react on these catalyst surfaces. For the development of methanol-tolerant catalysts, several prerequisites have to be fulfilled to obtain competitive products the new material not only has to be as active as a comparable catalyst (platinum-based) but also the stability and cost aspect have to be considered. Recently, selenium-modified rathenium particles (RuSe ) were found to be a suitable alternative to platinum-based alloys where the addition of selenium increases the activity of pure rathenium particles to shift these catalysts in a competitive regime. ... 

Figure 4.31 shows a simplified system layout for a liquid-fed DMFC system. In order to benefit from the high energy density of methanol, it is necessary to operate the DMFC system from a supply of pirre methanol. However, since water is required in the electrochemical reaction, and to suppress the formation of mixed potentials at the cathode, the anode should be fed with aqueous methanol. Therefore, a mixing imit for water and methanol needs to be present in the system. 

Resistance of fuel transport through it. (This is a concern in a DMFC, in which methanol crossover takes place, and gets oxidized at the cathode. This reduces the cell voltage by formation of mixed potential at the cathode.)... 

Those critical functions of membrane for DMFC are simple but most important. Required functions are ionic conductivity, electrical insulation, gas and liquid (especially methanol) tightness, and chemical and mechanical stability. As indicated in Fig. 13.2, ohmic polarization is mainly due to the ionic resistance of membranes, but the low open circuit potential of cathode is also mainly coming from the voltage drop by mixed potential made of fuel crossover through the membrane. The low cost of material and process is also another factor in terms of commercialization. Especially for mobile applications, membranes have the additional function for mass balance of liquid fuel and water products circulated out of or through the membrane. In this manner, alternative membranes are under development and researchers are focused on four types perfluorinated and partially fluorinated membranes hydrocarbon and composite and other ionomer modifications inorganic materials. The current state of the art and technical approaches to these materials are discussed in detail elsewhere in this volume. 

Proton exchange membranes (PEMs) in DMFC should transport protons as an electrolyte and prevent fuel and oxidant mixing as a separator. Proton transport capacity affects the resistance and performance of fuel cells. The ability to separate influences the long-term stabiUty and fuel efficiency. The insufficiency of function in separation, which is called methanol crossover, leads to deterioration of cathode catalysts, and thus generates mixed potential and decreases the perfoimance and fuel efficiency of DMFC. 

Usually, methanol concentrations of 2 M or less are used in DMFCs, due to the serious problem of crossover of methanol through the electrolyte membranes. When this occurs, the transported methanol reacts directly on the cathode and seriously reduces the DMFC voltage. As a result of catalyst poisoning and mixed potential loss at the cathode the energy density using low concentration, methanol fuel cannot match that of current batteries. The anode reaction is ... 

Methanol Crossover There is a high crossover rate of methanol from anode to cathode because of the high concentration of methanol at the anode side. This results in a mixed potential at the cathode from crossover methanol oxidation and greatly reduces the open-circuit voltage of the DMFC from the theoretical value of 1.2 V to around 0.7-0.8 V. 

A mixed potential on the cathode and oxidation of methanol at this location both poison the cathode catalyst, consume oxygen, and greatly reduce the OCV, even more so than hydrogen crossover in H2 PEFC systems. Typical OCVs of DMFCs are significantly below 0.8 V. 

This mixed potential is explained in Fig. 5 through an Evans diagram. In an operating fuel cell, along with this polarization close to open circuit voltage (OCV), there are losses due to hydrogen permeation into cathode electrode from anode chambers in PEMFC and methanol crossover in direct methanol fuel cell (DMFC). In a half-cell system, the crossover losses do not exist, but the polarization due to the carbon oxidation or any other contaminant participating in a side-reaction depresses the OCV. 

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