Developing DMFCs

From the above discussion, it is clear that the main objectives of fundamental investigations on the direct methanol fuel cell are to  

These main objectives can be reached only by modifying the structures and compositions of primarily the anode (methanol electrode) and secondarily the cathode (oxygen electrode) as discussed in Sections 111 and IV, respectively. In addition. Section IV discusses the conception of new proton exchange membranes with lower methanol permeability in order to improve the cathode characteristics. Section V deals with the progress in the development of DMFCs, while in Section VI the authors attempt to make a prognosis on the status of DMFC R D and its potential applications. 

ELECTRODE KINETICS AND ELECTROCATALY SIS OF METHANOL OXIDATION—ELECTROCHEMICAL AND SPECTROSCOPIC INVESTIGATIONS 

Methanol can be considered as a hydrogen carrier in a fuel cell. Conventionally, methanol has been reformed/shift converted to produce hydrogen. A low concentration of carbon monoxide formed during this process leads to a strong poisoning of the anode, and even after cleaning of the 

worldwide efforts have focused on the elucidation of the reaction mechanism. For this purpose, knowledge about the following items is vital (1) identification of reaction products and the electrode kinetics of the reactions involved, (2) identification of adsorbed intermediate species and their distribution on the electrode surface, and (3) dependence of the electrode kinetics of the intermediate steps in the overall and parasitic reactions on the structure and composition of the electrocatalyst. It is only after a better knowledge of the reaction mechanisms is obtained that it will be possible to propose modifications of the composition and/or structure of the electrocatalyst in order to significantly increase the rate of the reaction. 

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The enthusiasm for developing DMFCs (the fuel cell researcher s dream) evolved in the 1960s, which was really the boom period for R D activities on all types of fuel cell technologies, mainly because of NASA s vital need for fuel cell power plants for space vehicles. As early as the 1960s it was recognized that the major challenges in developing DMFCs... 

As will be reported, as result of continuous efforts to optimize cell components and cell structure in developing DMFCs for portable power applications, we were able to demonstrate 30-cell DMFC stacks operated at 60 °C and fed with ambient air at 2-3 times faradaic stoichiometry, generating a power density of 320 W (active stack volume) during at least the initial week of testing in our laboratory. 

Until recently (i.e., till early 1990s), most of the efforts to develop DMFCs has been with sulfuric acid as the electrolyte. The recent success with a proton conducting membrane (perfluorosulfonic acid membrane) in PEMFCs has steered DMFC research toward the use of this electrolyte. The positive feature of a liquid feed to a DMFC is that it eliminates the humidification subsystem, as required for a PEMFC with gaseous reactants. Another positive point is that the DMFC does not require the heavy and bulky fuel processor. Two problems continue to be nerve-wracking in the projects to develop DMFCs (1) the exchange current density for methanol oxidation, even on the... 

SFC Energy AG (formerly Smart Fuel Cell) is a world leader in developing DMFC systems as battery chargers for various applications. It has achieved sales of more than 25,000 units, which is one of the world s highest fuel cell sales figmes. Its product portfolio includes the EFOY 600, 900, 1200, 1600,... 

Despite advancement in the development of direct methanol fuel cells (DMFCs), some restrictions still inhibit their large-scale commercialization. This chapter has discussed one of the primary constraints, that is, identification of appropriate membrane materials. Nafion membranes that dominate the market of polymer electrolyte membranes allow methanol permeation from the anode to the cathode side of a DMFC. This results in serious negative consequences. Three approaches have been pursued in order to resolve the methanol permeation problem. These include Nafion membranes modification, development of alternative membranes and provision of high activity anode catalysts or methanol tolerant cathode catalysts. All the three options have achieved certain degree of success in solving the problan. Of particular interest are the Nafion membranes modification and development of alternative membranes in which membranes with permeability values of 10 to 70 times lower than the pure Nafion membranes have been developed. In general, based on the tremendous research efforts being made to develop DMFCs membranes with the best qualities, we are optimistic that very soon the issue of methanol permeation shall become a history. 

In the United States, the Department of Defense (DOD) and the Department of Energy (DOE) promoted in 1992 the Defense Advanced Research Project Agency (DARPA) program to develop a DMFC for portable and mobile applications. Several institutions are involved (IFC, JPL, LANE, Giner, Inc.) and small stacks (up to 10 elementary cells) were built by IFC and JPL. The performances are quite encouraging, with power densities of 250 mW/cm at 0.5 V. More details are given in Section V.2. 

The electrocatalytic oxidation of methanol has been thoroughly investigated during the past three decades (see reviews in Refs. 21-27), particularly in regard to the possible development of DMFCs. The oxidation of methanol, the electrocatalytic reaction, consists of several steps, which also include adsorbed species. The determination of the mechanism of this reaction needs two kinds of information (1) the electrode kinetics of the formation of partially oxidized and completely oxidized products (main and side products) and (2) the nature and the distribution of intermediates adsorbed at the electrode surface. 

One may conclude from all these studies that the loss in fuel utilization and Coulombic efficiency in a DMFC due to methanol crossover is still a major barrier in the development of such types of electrochemical power sources. 

The Jet Propulsion Laboratory and Giner Inc. have an on-going collaboration to develop electrochemical DMFC stacks. A 5-cell stack (with an active area of the electrode of 25 cm ) was designed and constructed for operation with unpressurized air. " The performance characteristics of the stack at two operating temperatures (60 and 90 °C) and two 1 M methanol flow rates (5 and 2 liter/min), are rather good 2 V at 250 mA/cm at 90 C. The variation in cell-to-cell performance was very small. Efforts are being made at several other laboratories (e.g., LANL, H-Power) to design, construct, and test DMFC stacks. 

Coutanceau C, Koffi RK, Leger JM, Marestin C, Mercier R, Nayoze C, Capron P. 2006. Development of materials for mini DMFC working at room temperature for portable applications. J Power Sources 160 334. 

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

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