Direct Liquid Fuel Cells

Microporous Layers in Direct Liquid Fuel Cells..246... 

As discussed previously, a number of different materials have been considered as potential candidates to be used as diffusion layers in PEMFCs and direct liquid fuel cells (DLFCs). The two materials used the most so far in fuel cell research and products are carbon fiber papers and carbon cloths, also known as carbon woven fabrics. Both materials are made from carbon fibers. Although these materials have been quite popular for fuel cells, they have a number of drawbacks—particularly with respect to their design and model complexity—that have led to the study of other possible materials. The following sections discuss in detail the main materials that have been used as diffusion layers, providing an insight into how these materials are fabricated and how they affect fuel cell performance. 

In electrochemical systems, metal meshes have been widely used as the backing layers for catalyst layers (or electrodes) [26-29] and as separators [30]. In fuel cells where an aqueous electrolyte is employed, metal screens or sheets have been used as the diffusion layers with catalyst layers coated on them [31]. In direct liquid fuel cells, such as the direct methanol fuel cell (DMFC), there has been research with metal meshes as DLs in order to replace the typical CFPs and CCs because they are considered unsuitable for the transport and release of carbon dioxide gas from the anode side of the cell [32]. 

In direct liquid fuel cells, the use of MPLs is also very popular and most of the details explained earlier also apply to the liquid fuel cells. However, some of the parameters differ from those in PEM fuel cells because there are other mass transfer-based issues in DLFCs, especially on the anode side related to methanol crossover and CO2 production. 

As stated earlier, CEP and CC are the most common materials used in the PEM and direct liquid fuel cell due fo fheir nature, it is critical to understand how their porosity, pore size distribution, and capillary flow (and pressures) affecf fhe cell s overall performance. In addition to these properties, pressure drop measurements between the inlet and outlet streams of fuel cells are widely used as an indication of the liquid and gas transport within different diffusion layers. In fhis section, we will discuss the main methods used to measure and determine these properties that play such an important role in the improvement of bofh gas and liquid transport mechanisms. 

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

Shipments of fuel cell-equipped mobile devices could grow very rapidly if they can eliminate the need for frequent recharging of current battery-powered models. The Medis 24/7 Power Pack in April 2007. It is a portable, disposable power source for small electronic devices such as cell phones and MP3 players. Manufactured by Medis Technologies, it is based on Direct Liquid Fuel cell technology, and may be of particular utility in military applications. Elsewhere, MTI MicroFuel Cells manufactures a power pack for portable electronics that is based on direct methanol fuel cell technology that it calls Mobion. 

In this book the focus is on PEMFCs therefore, in the following sections we will only discuss several major types of PEMFCs, such as H2/air (02) fuel cells, direct liquid fuel cells, PAFCs, and alkaline fuel cells. PEMFCs, also called solid polymer electrolyte fuel cells, use a polymer electrolyte membrane as the electrolyte. They are low-temperature fuel cells, generally operating below 300°C. 

There are several types of direct liquid fuel cells, such as direct methanol fuel cells (DMFCs), direct formic acid fuel cells (DFAFCs), and direct ethanol fuel cells (DEFCs), the most popular being the DMFC, which is the focus of this section. A schematic DMFC system is shown in Figure 1.7. 

DIRECT LIQUID FUEL CELLS WITH GASEOUS, LIQUID, AND/OR SOLID REAGENTS... 

Wilkinson and coworkers [40, 41] introduced the concept of a mixed-reactant direct liquid fuel cell where the air cathode was substituted with a metal-ion redox couple. This type of cell, which in the case of methanol is called mixed-reactant direct methanol redox fuel cell (MR-DMRFC) has the advantage of cathode selectivity, avoiding Pt group metals as cathodic catalysts, minimize flooding in the cathode and allows the use of larger fuel concentrations. 

Hide AB, Wilkinson DP, Fatih K, Girard F (2008) High fuel concentration direct-liquid fuel cell with a redox couple cathode. J Electrochem Soc 155 B1322-B1327... 

Abstract Direct liquid fuel cells for portable electronic devices are plagued by poor efficiency due to high overpotentials and accumulation of intermediates on the electrocatalyst surface. Direct formic acid fuel cells have a potential to maintain low overpotentials if the electrocatalyst is tailored to promote the direct electrooxidation pathway. Through the understanding of the structural and environmental impacts on preferential selection of the more active formic acid electrooxidation pathway, a higher performing and more stable electrocatalyst is sought. This chapter overviews the formic acid electrooxidation pathways, enhancement mechanisms, and fundamental electrochemical mechanistic studies. 

Demirci investigated the degree of segregation and shifting of d-band centers by metal alloy combinations to improve the direct liquid fuel cell catalyst activity through electronic promotion of the dehydrogenation pathway [57]. He focused on Pt- and Pd-based catalyst for formic acid electrooxidation and looked at the potential impact of surface adatom adsorption of other 3d, 4d, and 5d transition metals. The criteria he imposed for improved catalytic activity on Pt and Pd... 

Masel Rl, Zhu Y, Khan Z, Man M (2006) Low contaminant formic acid fuel for direct liquid fuel cell. US Patent 20060059769... 

In recent years, interest in the development of direct liquid fuel cell (DLFC) has increased considerably due to its advantages easy handling and storing of the liquid fuel, no need for reforming, and favorable power capability for the use portable electronics powered by miniature fuel cells. Most investigators are... 

Table 1.8. Comparison of selected properties for PEM fuel cells with different fuels under standard condition (25 C, 1 atm) [91]. (Reprinted from Journal of Power Sources, 154(1), Qian W, Wilkinson DP, Shen J, Wang H, Zhang J, Architecture for portable direct liquid fuel cells, 202-13, 2006, with permission from Elsevier.)...

The interest in these eompounds stems from the idea that they posses Pt-like eleetronie behavior in eatalytie reactions, whieh theoretically could make them attractive candidates as catalysts for direct liquid fuel cells [208, 209]. One of the first issues to be considered, however, is the stability of these materials under anodic conditions. Zellner and Chen recorded the cyclic voltammograms of WC and W2C deposited on carbon paper in 0.5 M H2SO4 (Figure 4.41) [210]. WC was stable up to 0.6 V vs. SHE while W2C was susceptible to oxidation forming WxOy starting from about 0 V and giving a peak at 0.5 V vs. SHE. 

Dodelet and co-workers did not employ the Pt-MWCNT/carbon paper as a direct liquid fuel cell anode. However, oxygen electroreduction experiments... 

The catalytic (supported or unsupported) interface in the vast majority of direct liquid fuel cell studies is realized in practice either as a catalyst coated membrane (CCM) or catalyst coated diffusion layer (CCDL). Both configurations in essence are part of the electrode design category, which is referred to as a gas diffusion electrode, characterized by a macroporous gas diffusion and distribution zone (thickness 100-300 pm) and a mainly mesoporous, thin reaction layer (thickness 5-50 pm). The various layers are typically hot pressed, forming the gas diffusion electrode-membrane assembly. Extensive experimental and mathematical modeling research has been performed on the gas diffusion electrode-membrane assembly, especially with respect to the H2-O2 fuel cell. It has been established fliat the catalyst utilization efficiency (defined as the electrochemically available surface area vs. total catalyst surface area measured by BET) in a typieal gas diffusion electrode is only between 10-50%, hence, flie fuel utilization eflfieieney can be low in such electrodes. Furthermore, the low fuel utilization efficiency contributes to an increased crossover rate through the membrane, which deteriorates the cathode performance. 

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