Direct methanol fuel cells water management

Direct methanol fuel cell (DMFC) was developed in 1950s-1960s, based on the liquid alkaline or aqueous acid solution as the electrolyte. It converts the methanol directly into electricity, instead of using indirectly produced hydrogen from methanol through the reforming process. Today, DMFC commonly refers to as the one that employs PEM as the electrolyte. Fuel for DMFC is a dilute solution of methanol, usually 3-5 wt% in water. The size of DMFC can be considerably smaller than PEMFC because of the elimination of fuel processor, and complex humidification and heat management systems. The performance of DMFC is relatively low compared to that of PEMFC. 

Abstract Most of the transport processes of a fuel cell take place in the gas diffusion media and flow fields. The task of the flow fleld is to uniformly distribute the reactant gases across the electrochemically active area and at the same time ensure an adequate removal of the reactant products, which is water on the cathode side in both polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). Gas diffusion media are required to supply the reactant under the land areas of the flow fleld at the same time, the gas diffusion media has to ensure a good thermal as well as water management to avoid any non-optimum conditions. Characterization tools for gas diffusion media are presented, flow fleld types and design criteria are discussed and the effect of both components on the performance of a fuel cell are highlighted. System aspects for different fuels (hydrogen, vapor-fed DMFCS, liquid fed DMFCs) are compiled and the different loss contributions and factors determining the performance of a fuel cell system are shown. 

An interesting consideration is a possible tradeoff in the design of the fuel cell component and the fuel source. In the case of the direct methanol fuel cell (DMFC), for example, water is required for the reaction of methanol. The discharge product of the fuel cell, water, can be used if the water management or recovery is incorporated in the fuel cell a one-time cost of increased size, weight and complexity of the fuel cell. Or water can be added to the fuel source at the expense of a recurring lower specific energy of the diluted fuel source. 

The main purpose of many investigations is to improve water management in proton-exchange membrane fuel cells (PEMFCs) and direct methanol fnel cells (DMFCs) operating at temperatures below 120°C, at which water is present in the liquid as well as in the gaseous state. 

Reforming reaction takes place at high temperatures. Typically, in vehicle systems utilizing methanol reformation, hydrogen is directly injected into the cell where methanol and water are vaporized to form H2, CO, and C02 [6], This reformation takes place at approximately 280°C [6], Materials that operate at these temperatures are quite costly. In addition, separate loop cooling systems (currently in the form of humidifiers) are required to keep the overall cell temperature down and the water management under control for a PEM fuel cell. An aqueous membrane is required to prevent boiling, temperatures below 100°C are necessary [6],... 

In 1997 and 1999 U.S. patents of Wilkinson et al. it was suggested that a multilayer electrode be used as the anode in DMFCs. The first layer consists of catalyst applied to carbon paper. Most of the methanol is oxidized in this layer. The second catalytic layer, applied directly to the membrane, already has a diluted methanol solution. Other ways of limiting methanol crossover are discussed in the next section when we discuss improvements in water management in fuel cells. 

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