DMFC, design

In addition to these smaller applications, fuel cells can be used in portable generators, such as those used to provide electricity for portable equipment. Thousands of portable fuel cell systems have been developed and operated worldwide, ranging from 1 watt to 1.5 kilowatts in power. The two primary technologies for portable applications are polymer electrolyte membrane (PEM) and direct methanol fuel cell (DMFC) designs. 

Figure 3.55. Carbon nanohorn substrates used for catalyst backing in NEC s passive DMFC design. (From Y. Kubo (2004). Micro fuel cells for portable electronics. In Proc. 15 World Hydrogen Energy Conference, Yokohama. Used with permission.)...

The chapter is concluded with a brief discussion of the guidelines for an optimal PEFC/DMFC design, based on the functional map of the cell and the established laws of feed consumption. 

Oorja Protonics commercializes a 1,000 W DMFC designed for use in forklifts, and other vehicles in the materials handling industry. The DMFC is fitted inside the battery tray of the existing machinery and continuously charges the vehicle battery [50, 65]. In Table 9.3 are summarized the performance characteristics of DMFC portable generators. 

The DMFC design is quite similar to that of a PEFC (Figure 1.10). As in a PEFC, in a DMFC the anode and cathode are separated by a polymer electrolyte membrane, typically Nafion . These cells also operate in the temperature range of 30-90 °C. 

There are four principle ways that DMFC designers use to reduce fuel crossover, and there are other ideas that are more at the experimental stage. The four key established methods are the following ... 

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. 

Polyfuel (USA), a spin-off of SRI International, is developing direct methanol fuel cells to replace Lithium ion batteries in wireless, handheld and portable devices, based on patented, proprietary technology. PolyFuel s membranes are based on hydrocarbon polymers, rather than perfluorinated and are considered to be best-in-class for portable direct methanol fuel cells (DMFC) designed for portable electronic devices such as laptops, PDAs or cell phones. 

The Jet Propulsion Laboratory and Giner Inc. have an on-going collaboration to develop electrochemical DMFC stacks. A 5-cell stack (with an active area of the electrode of 25 cm ) was designed and constructed for operation with unpressurized air. " The performance characteristics of the stack at two operating temperatures (60 and 90 °C) and two 1 M methanol flow rates (5 and 2 liter/min), are rather good 2 V at 250 mA/cm at 90 C. The variation in cell-to-cell performance was very small. Efforts are being made at several other laboratories (e.g., LANL, H-Power) to design, construct, and test DMFC stacks. 

Scott et al. [33] designed a DMFC with stainless steel mesh as the anode FF plate that was able to remove the carbon dioxide gas effectively. Later, the same research group was able to demonstrate that using similar meshes as DLs in the anode side also improved the overall gas removal [26,34] (wet-proofed CFP was used as the DL on the cathode side). These meshes were used on the anode side and were made out of catalyzed Ti because similar meshes have been used extensively as catalyzed electrodes in other industries, such as the chlor-alkali industry [26]. 

Metal foams have been used in the past in the development of FF plates. However, Gamburzev and Appleby [53] used Ni foams as both a DL and a flow field plate with an MPL layer on one of its surfaces. They observed that such a design had high contact resistance between the nickel foam and the MPL and also increased gas diffusion resistance due to the required MPL thickness. Arisetty, Prasad, and Advani [54] were able to demonstrate that these materials can also be used as potential anode diffusion layers in DMFCs (see Figure 4.10). In fact, the nickel foam used in this study performed better than a carbon cloth (Avcarb 1071HCB) and a stainless steel mesh. However, it was recognized that a major drawback for these foams is their susceptibility to corrosion. 

In another study, Chen and Zhao [55] demonstrated that by using a Ni-Cr alloy metal foam as the cathode DL (and current collector), instead of a CFP or CC, the performance of a DMFC can be enhanced significantly due to the improvement of the mass transfer of oxygen and overall water removal on the cathode side. Fly and Brady [56] designed a fuel cell stack in which the distribution layers were made out of metal foams (open cell foams). In addition, more than one foam (with different porosity) could be sandwiched together in order to form a DL with variable porosity. 

For this type of fuel cell, a number of reports studying anode MPLs have been published. Neergat and Shukla [124] used a hydrophobic MPL on the cathode (carbon black and PTFE) and a hydrophilic MPL on the anode (carbon black and Nafion) (see Section 4.3.2). Different types of carbon particles were used (Vulcan XC-72, acetylene black, and Ketjenblack) and it was concluded that Ketjenblack was the carbon that showed the best performance when it was used on both the anode and cathode MPLs with 10 wt% Nafion and 10 wt% PTFE, respectively. A similar design was also used by Ren et al. [173] in a passive DMFC. Improvement of the DMFC performance by using a hydrophilic MPL, as discussed previously, was also demonstrated by Lindermeir et al. [125]. They compared both hydrophilic and hydrophobic MPLs for the anode DL, and it was observed that the former improves the mass transport of the MEA. 

The optimum amount of PTFE in the anode MPL depends on the operating conditions and design of the DMFC. Dohle et al. [176] used a 500 W DMFC stack and observed that anode MPLs with 13 wt% PTFE had the best performance. Peled et al. [177] designed anode MPLs that had around 20-40 wt% PTFE and determined that the layers with lowest PTFE content performed the best. 

To reduce methanol crossover and improve water back diffusion through the membrane in passive DMFCs, Kim et al. [179] designed an MEA with... 

The efficiency of PEMFCs ranges from about 40 to 50%, and operating temperature is about 255 K. The PEMFCs and direct methanol fuel cells (DMFCs) are considered to be promising power sources, especially for transportation applications. The PEMFCs with potentially much higher efficiencies and almost zero emissions offer an attractive alternative to the internal combustion engines for automotive applications. This fuel cell has many important attributes such as high efficiency, clean, quiet, low-temperature operation, capable of quick start-up, no liquid electrolyte and simple cell design (Hu et al., 2004). 

DMFC modeling thus aims to provide a useful tool for the basic understanding of transport and electrochemical phenomena in DMFC and for the optimization of cell design and operating conditions. This modeling is challenging in that it entails the two-phase treatment for both anode and cathode and that both the exact role of the surface treatment in backing layers and the physical processes which control liquid-phase transport are unknown. 

Despite the fact that much effort has been made to model the DMFC system, considerable work remains, particularly in support of the emerging portable designs and systems. Few have treated the dominating effects of two-phase flow. No model to date has sufficient detail to provide a microfluidic theory for portable systems including effects of channel geometry and wettability characteristics of the GDL on fluid flow in the anode or cathode. Modeling studies are needed to fully elucidate the intricate couplings of methanol, water, and heat-transport... 

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