Passive reactant supply

A further problem in DMFC operation is due to the evolution of gaseous CO2 at the anode (Ye et al 2005b). Then gas bubbles that can locally interfere with the flow of the aqueous methanol solntion may form in the flow field on the anodic side of the bipolar plates. This leads to a nonuniform distribntion of the reaction (and thus current) across the MEA snrface. This effect is particularly noticeable when the solution is snpplied passively (e.g., by free flow from a tank above). To overcome it, one shonld nse an active reactant supply at flow rates several times in excess of the stoichiometric requirements (Cowart, 2005). This raises the question of how to dimension the means of pumping the solution through (and what energy they wonld consnme). The dimensions would depend on the pressure drop within the flow field channels between solution input and outlet (Yang et al., 2005). 

Direct reaction is feasible for the IVA-VA compounds. This is surprising, since a low vapor pressure element, such as silicon, would be expected to passivate on exposure to phosphorus or arsenic. The valve metal analogy to electrochemical reactions may be invoked here fresh silicon probably diffuses rapidly through the compound, providing a continuous supply of reactant. A cold zone with a temperature of less than 500°C supplies phosphorus to the silicon, kept slightly near the 1 1 SiP melting point of 1166°Ck... 

The State of the art regarding mass transport in DMFC has been discussed by Zhao and coworkers in two recent reviews, including the effect of operating conditions, flow field design and diffusion layer morphology on the transport of methanol and water [67], and the transport of methanol, water and oxygen in small DMFC with passive supply of reactants [68],... 

Zhao TS, Chen R, Yang WW, Xu C (2009) Small direct methanol fuel cells with passive supply of reactants. J Power Sources 191 185-202... 

Zhao et al. [22] reviewed with passive supply of reactants, emphasizing that the mass-transport mechanisms of reactants and products in passive DMFCs that have... 

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