Direct methanol fuel cell cathode loadings

H+) Produced by the dissociation of Hj at the anode to the cathode, (3) Prevention of the associated electron flow through the membranes forcing them to flow in the external circuit to the cathode to produce DC current, and (4) Support for the catalyst loaded on the electrodes. When Hj is replaced by methanol as a fuel in liquid form in direct methanol fuel cell (DMFC), the dissociation of methanol solution at the anode produces protons that are transported through the hydrated PEM to the cathode, where a reduction of O2 produces water in the presence of the protons. To qualify PEM for commercial application in PEMFC and DMFC, it should have a combination of properties including (Maiyalagan and Pasupathi 2010 Neburchilov et al. 2007 Nagarale et al. 2010) ... 

Direct methanol fuel cells (DMFCs) are attracting much more attention for their potential as clean and mobile power sources for the near future [1-8], Generally, platinum (Pt)- or platinum-alloy-hased nanocluster-impregnated carbon supports are the best electrocatalysts for anodic and cathodic fuel cell reactions. These materials are veiy expensive, and thus there is a need to minimize catalyst loading without sacrificing electro-catalytic activity. Because the catalytic reaction is performed by fuel gas or fuel solution, one way to maximize catalyst utilization is to enhance the external Pt surface area per unit mass of Pt. The most efficient way to achieve this goal is to reduce the size of the Pt clusters. 

Figure 13.8. DMFC polarization (a and c) and power density (b and d) curves of various catalysts at 75 °C (a and b) and 90 °C (c and d), respectively. Methanol concentration at the anode 1 mol with a flow rate of 1.0 mL min oxygen at cathode non-htunidified oxygen at a pressure of 0.2 MPa with a flow rate of 200 seem. Except for Pt-Ru black and Pt-Ru black (JM) with a Pt loading of 2.66 mg cm, the Pt loading of other anode catalysts is 1.33 mg cm. 40 wt% Pt/XC72 (Johnson Matthey) used as the cathode catal57st with a Pt loading of 1.4 mg cm [35]. (Reprinted from Journal of Power Sources, 168, Guo J, Sun G, Sun S, Yan S, Yang W, Qi J, et al. Polyol-synthesized PtRu/C and PtRu black for direct methanol fuel cells, 299-306, 2007, with permission from Elsevier.)...

Diard, J.-R, Glandut, N., Landaud, R, Gorrec, B. Le, and MonteUa, C. 2003. A method for determining anode and cathode impedances of a direct methanol fuel cell running on a load. Electrochim. Acta. 48, 555-562. 

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). 

Figure 4.2. DMFC polarization behavior using aqueous methanol and solid polymer electrolyte (SPE Nafion 117) [52]. a) Anode half-cell polarization comparison between SPE and 0.5 M H2SO4, PtRu/C 0.5 mg cm, b) fuel cell polarization curve using 2 M CH3OH in water effect of temperature. PtRu/C coated Nafion 117 membrane. Note the anode and cathode catalyst loads in the case of fuel cell experiments were not specified in [52]. (Reproduced Ifom Journal of Power Sources, 47(3), Surampudi S, Narayanan SR, Vamos E, Frank H, Halpert G, LaConti A, Kosek J, Surya Prakash GK, and Olah GA, Advances in direct oxidation methanol fuel cells, 377-85, 1994, with permission fi-om Elsevier.)...

Yu and Scott prepared membrane assembly electrodes (MEAs) with Pt loadings in both anode and cathode CLs of about 2 mg cm for a direct methanol alkaline fuel cell using an anion exchange membrane. They found that the cell performance increased dramatically with an MEA that did not include the GDL on the anode, because of lower reactant mass transfer resistance [94]. They also used platinized Ti mesh as the anode and current collector in the AEMFC. The cathode was a standard gas diffusion electrode with a Pt loading of approximately 2 mg cm , which consisted of a backing layer (carbon paper), a GDL and a CL. The novel anode showed higher catalytie aetivity than the conventional Pt/C electrode, and gave stable fuel cell performance [95]. 

Fig. 22 Current density-cell voltage characteristics for 2 M methanol and 3 M KOH solution at different loading of anode catalyst in direct alcohol alkaline fuel cell at 25°C, Anode Pt-black Cathode Mn02.

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