Direct methanol fuel cells anode kinetics

Figure 6.66. DMFC anode impedance plots obtained under pure kinetic control (+) 500 mA/cm2, ( ) 300 mA/cm2, (A) 200 mA/cm2, ( ) 100 mA/cm2 [53], (Reprinted from Journal of Power Sources, 84(2), Mueller JT, Urban PM, Holderich WM. Impedance studies on direct methanol fuel cell anodes, 157-60, 1999, with permission from Elsevier.)...

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 ... 

Xing L, Scott K, Sun Y-P (2011) Transient response and steady-state analysis of the anode of direct methanol fuel cells based on dual-site kinetics. Int J Electrochem Article 853261... 

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... 

There is another fuel cell working under the ambient condition, that is, direct methanol fuel cells (DMFCs). Difference in the PEFCs and DMFCs is their anode fuels (the cathode fuel is oxygen in both cases). In the DMFCs, methanol (CH3OH) is supplied to the anode instead of the hydrogen for the PEFCs and this difference is crucial for their ceU performances. Although the Pt is known to be an active catalyst for both HOR and methanol oxidation reaction (MOR), kinetics of the MOR is much slower than that of the HOR and ORR on the Pt catalyst, which increases anode overpotential and gives an inferior cell performance in the DMFCs as demonstrated in Fig. 1. Therefore, an important research topic is... 

Abstract One of the most critical fuel cell components is the catalyst layer, where electrochemical reduction and oxidation of the reactants and fuels take place kinetics and transport properties influence cell jjerformance. Fundamentals of fuel cell catalysis are explain, concurrent reaction pathways of the methanol oxidation reaction are discussed and a variety of catalysts for applications in low temperature fuel cells is described. The chapter highlights the most common polymer electrolyte membrane fuel cell (PEMFC) anode and cathode catalysts, core shell particles, de-alloyed structures and platinum-free materials, reducing platinum content while ensuring electrochemical activity, concluding with a description of different catalyst supports. The role of direct methanol fuel cell (DMFC) bi-fimctional catalysts is explained and optimization strategies towards a reduction of the overall platinum content are presented. 

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%. 

However, DMFCs do suffer some drawbacks such as lower electrical efficiency and higher catalyst loadings as compared to H2 fuel cells. Efforts should be continued on developing anode catalysts with improved methanol oxidation kinetics, cathode catalysts with a high tolerance to methanol, membranes with lower methanol permeation rates, and strategies to reduce the methanol crossover rate. Attention should also be given to other direct-feed fuel cells using other liquid fuels (such as formic acid). 

Considering (Mily the thermodynamics of the DMFC (used here as a representative of direct alcohol fuel cells), methanol should be oxidized spraitaneously when the potential of the anode is above 0.05 V/SHE. Similarly, oxygen should be reduced spontaneously when the cathode potential is below 1.23 V/SHE, identical to a H2-O2 fuel cell. However, kinetic losses due to side reactions cause a deviation of ideal thermodynamic values and decrease the efficiency of the DMFC. This is presented in Fig. lb, which includes various limiting effects as kinetics, ohmic resistance, alcohol crossover, and mass transport. The anode and cathode overpotentials for alcohol oxidation and oxygen reduction reduce the cell potential and together are responsible for the decay in efficiency of approximately 50 % in DMFCs [13, 27]. 

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