DMFCs

The schematic diagram of the liquid-feed direct methanol fuel cell (DMFC) is shown in Figure 13.1. 

Fuel cells can run on fuels other than hydrogen. In the direct methanol fuel cell (DMFC), a dilute methanol solution ( 3%) is fed directly into the anode, and a multistep process causes the liberation of protons and electrons together with conversion to water and carbon dioxide. Because no fuel processor is required, the system is conceptually vei"y attractive. However, the multistep process is understandably less rapid than the simpler hydrogen reaction, and this causes the direct methanol fuel cell stack to produce less power and to need more catalyst. 

Steady-state and Dynamic Operations of 3W DMFC stack... 

The dynamic behavior of fuel cells is of importance to insure the stable operation of the fuel cells under various operating conditions. Among a few different fuel cell types, the direct methanol fuel cell (DMFC) has been known to have advantages especially for portable... 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

In this paper we report the effect of varying loads on a small size DMFC stack (10 cells with 9 cm active-area each). The transient responses of the stack voltage have been investigated upon variable current load conditions to obtain the information on the dynamic characteristics of the stack. Also, the transient responses of the stack current upon changing fuel flow rates have been monitored to obtain the optimal operating conditions for the staek. 

The DMFC used in this study was a 10-cell stack with 3cm X 3cm active area (total active... 

DMFC stack fabricated in this study, the minimal operating conditions are the flow rates of 3 ml/min and 2L/min for 2M methanol and air, respectively. 

TMs study has shown the dynamic behavior of a 5W DMFC stack when the current loads have changed by pulses and steps. In order to determine the optimum operating conditions of the stack, the dynamic behavior of the stack current has been studied under a constant voltage output of 3.8V, varying the flow rate of 2M methanol solution and air. For the stable operation of the 5W stack, the minimal fuel flow rates are found to be 3 ml/min and 2L/min for 2M methanol and air, respectively. 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

Although ORR catalysts for DMFCs are mostly identical to those for the PEM fuel cell, one additional and serious drawback in the DMFC case is the methanol crossover from the anode to the cathode compartment of the membrane electrode assembly, giving rise to simultaneous methanol oxidation at the cathode. The... 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

DMFC CH30H 25-150°C 0.1-10 30-45 50-200 1,000-10,000 1000 Portable power, standby power, transportation ( )... 

PAFC, phosphoric acid fuei ceii MCFC, moiten carbonate fuei ceii SOFC, soiid oxide fuei ceii PEMFC, proton exchange membrane fuei ceii DMFC, direct methanoi fuei ceii AFC, alkaiine fuel cell. 

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 main attractions of the DMFC are its high specific energy (Ws) and high energy density (W ), the values of which are calculated as follows ... 

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. 

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. 

The quantitative analyses of reaction products due to partial or complete oxidation can be performed by different methods. This type of determination is essential to improve electrode composition. Apart from a decrease in the Coulombic efficiency (see Section 11.2), the formation of partially oxidized products can be deleterious for the DMFC application because some of these products (e.g., formic acid) may be in liquid form and are corrosive. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

The effects of dispersion of the electrocatalyst and of particle size on the kinetics of electrooxidation of methanol have been the subject of numerous studies because of the utilization of carbon support in DMFC anodes. The main objective is to determine the optimum size of the platinum anode particles in order to increase the effectiveness factor of platinum. Such a size effect, which is widely recognized in the case of the reduction of oxygen, is still a subject of discussion for the oxidation of methanol. According to some investigators, an optimum of 2 nm for the platinum particle size exists, but studying particle sizes up to 1.4 nm, other authors observed no size effect. According to a recent study, the rate of oxidation of methanol remains constant for particles greater than 4.5 nm, but decreases with size for smaller particles (up to 2.2 nm). 

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