Methanol fuel cell anode losses

For isolating the overpotential of the working electrode, it is common practice to admit hydrogen to the counter-electrode (the anode in a PEMFC the cathode in a direct methanol fuel cell, DMFC) and create a so-called dynamic reference electrode. Furthermore, the overpotential comprises losses associated with sluggish electrochemical kinetics, as well as a concentration polarization related to hindered mass transport ... 

Methanol (MeOH) crossover from the anode to the cathode in the direct methanol fuel cell (DMFC) is responsible for significant depolarization of the Pt cathode catalyst. Compared to Pt-based catalysts, NPMCs are poor oxidation catalysts, of methanol oxidation in particular, which makes them highly methanol-tolerant. As shown in Fig. 8.25, the ORR activity of a PANI-Fe-C catalyst in a sulfuric acid solution is virtually independent of the methanol content, up to 5.0 M in MeOH concentration. A significant performance loss is only observed in 17 M MeOH solution ( 1 1 water-to-methanol molar ratio), a solution that can no longer be considered aqueous. The changes to oxygen solubility and diffusivity, as well as to the double-layer dielectric environment, are all likely to impact the ORR mechanism and kinetics, which may not be associated with the electrochemical oxidation of methanol at the catalyst surface. Based on the ORR polarization plots recorded at... 

The above analysis can also be used in rotating disc electrode (RDE) test, where the difference in half wave potentials is used as AE or for the positive electrode of an electrolysis cell or for both the anode and the cathode of an alkaline fuel cell and direct methanol fuel cell, where the anode also suffers high overpotential losses. 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

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. 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

The high theoretical efficiency of a fuel cell is substantially reduced by the finite rate of dynamic processes at various locations in the cell. Substantial efficiency losses at typical operating temperatures occur already in the anodic and cathodic catalyst layers due to the low intrinsic reaction rates of the oxygen reduction and, in the case of the DMFC, of the methanol oxidation reaction. (The catalytic oxidation of hydrogen with platinum catalysts is very fast and thus does not limit PEFC performance.) In addition, at low temperatures, turnover may be limited by noble metal catalyst poisoning due to sulfur... 

At the beginning of the 1980s, the interest in DMFC arose and methanol was used as fuel for high-temperature fuel cells [12]. Nowadays, PEM fuel cells with Nafion as electrolyte appear to be well suited for the direct oxidation of methanol. There are, however, some major problems in adapting a PEMFC to operate with methanol. The catalytic material of the anode has to be improved in order to avoid the loss of activity because of the formation of by-products. As in the case of a PEMFC using hydrogen from a reformer as fuel, a method to improve the anodic material is the use of Pt-Ru or Pt-Ru-Sn mixtures [8]. A particular problem of the DMFC is cross-diffusion of methanol through the electrolyte. 

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