Passively Operating DMFC

An interesting solution might be to operate with methanol vapor so that only a one-phase gas flow appears at the anode [54]. Different methods for evaporating methanol passively or actively in micro DMFCs have been described [55, 56). 

With vapor-fed micro DM FCs, a higher fuel efficiency, which also means a higher energy density and a better performance in addition to a higher power density, could be achieved [56, 57]. 

To realize micro fuel cells, there is a wide diversity of technological approaches beginning with a variety of primary fuels, different fuel cell technologies, and a range of manufacturing possibilities. Much work has already been done in the field of micro fuel cells, which have contributed to the understanding of the processes, effects, and interdependencies in fuel-cell operation. Commercialization of micro 

Mishler, J., Cho, S.C., and Adroher, X.C. (2011) A review of polymer electrolyte membrane fuel cells technology, applications, and needs on fundamental research. Appl. Energy, 88 (4), 981-1007. 

Butler, J. (2009) Portable Fuel Cell Survey 2009, Fuel Cell Today, Royston, available at http //www.fuelceUtodoY.com (last accessed 29 November 2011). 

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Direct ethanol fuel cells (DEFCs) produce power directly from ethanol without prior reforming. Compared with methanol used as the fuel for DMFCs, ethanol is nontoxic, environmentally friendly, and universally available, and making the handling easy. Since ethanol is also a liquid alcohol like methanol, the technological issues of crossover, discharge of carbon dioxide, and passive operation are comparable. 

A special case of Eq. (62) arises when the DMFC anode is (methanol) dead-ended, that is, all methanol fed into the anode at a current equivalent rate of /feed. either reacts at the anode or recombines with oxygen at the cathode. These operation conditions apply in 1-3 W, passive DMFC platforms developed for consumer electronics applications described later in this section. Under these conditions, rjf = /cell//fuel feed... 

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],... 

DMFC operated at temperatures above 100 °C exhibit the highest power densities, from 195 to 320 mW.cm at 110 °C up to 240-420 mW.cm at 145 °C, and they correspond to inorganic fillers such as Si02 and binary Si02/ hetropolyacids [30-32], zeolites [52, 53], montmorillonites [61], and zirconium phosphate [75], which prevent Nafion dehydration upon. All these membranes, whose methanol selectivity were not determined in most of the cases, exhibit moderated but systematic DMFC performance improvement (1.1 < RMPD < 1.4) compared to pristine Nafion in both, passive and active cells. 

Other types of DMFCs have been also addressed by modeling, as for example a passive Uquid-feed DMFC with neither external liquid pumps nor gas blowers by Chen and Zhao [184] (Fig. 8.16). An one-dimensional model is developed by considering coupled heat and mass transport, along with the electrochemical reactions occurring in passive DMFCs. The analytical solutions predicting the performance of the cell operating with different methanol concentrations are... 

Moreover, a new structure of passive DMFC with two methanol reservoirs separated by a porous medium layer was designed and a corresponding mathematical model was presented by Cai et al. [185], This type of DMFC can be directly fed with highly concentrated methanol solution or neat methanol. Modeling and experiments are used by the authors to optimize the porosity of the components. It was found that the new designed DMFC can be continuously operated for about 4.5 times longer than a conventional passive DMFC with the optimum parameters. 

In Chap. 1 (Sects. 1.5.5 and 1.5.6) we described the active and passive mode of DAFC operation and different configurations reported in R D works. Here we will briefly summarize the configuration and operation modes of DMFC prototypes, which are closely related to the fuel management and the miniaturization of the system. 

MTI Micro introduced in 2004 the integrated direct methanol fuel cell technology Mobion , and in 2007 the Mobion Chip . This small device, with a volume of only 9 cm, is fed with pure methanol to produce 84 mW.cm. These prototypes are passive DMFC and their design include a fuel conditioning system which allows operate in a temperature range of 0 0 °C, independent of the humidity and cell position, which is particularly useful in portable applications, such as mobile phones [40]. 

Baglio V, StassiA, MateraFV, Di Bias A, AntonucciV, Arico AS. Optimization of properties and operating parameters of a passive DMFC mini-stack at ambient temperature. J Power Sources 2008 180(2) 797—802. 

Litterst, C., Eccarius, S., Hebling, C., Zengerle, R., and Koltay, P. (2006) Increasing mu DMFC efficiency by passive CO2 bubble removal and discontinuous operation. J. Micromech. Microeng., 16 (9), S248-S253. 

C. Litterst, S. Eccarius, C. Hebhng, R. Zengerle, P. Koltay, Increasing /bubble removal and discontinuous operation , J. Micromech. Micmeng. 16 (2006), 248-253. 

The electrical performance flgures of active DMFCs are much higher than those of passive DMFCs. Yet owing to the need to use many ancillary pieces of equipment, such as pumps, valves, and controllers, power plants with active DMFCs are much more complex and in a number of cases are less reliable in their operation. Also, part of the electrical energy generated by them is used up... 

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). 

Together with often complex aspects of DMFC system design (applicable to both passive and active DMFC system concepts), durable operation of the fuel cell stack and the ability to deliver the required power for several thousand hours are generally considered crucial to the ultimate large-scale commercial success of DMFCs. Other key requirements include cell (stack) operation at a highest achievable voltage and cathode operation at ambient pressure, low airflow, and natural air humidification. 

Fiquid fuel used in the anode results in lower parasitic pumping requirements compared to gas flow. In fact, many passive DMFC designs operate without any external parasitic losses, instead relying on natural forces such as capillary action, buoyancy, and diffusion to deliver reactants. 

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