Direct Methanol Fuel Cell Systems

There is increased interest in the use of Ru-based systems as catalysts for oxygen reduction in acidic media, because these systems have potential applications in practicable direct methanol fuel cell systems. The thermolysis of Ru3(CO)i2 has been studied to tailor the preparation of such materials [123-125]. The decarbon-ylation of carbon-supported catalysts prepared from Ru3(CO)i2 and W(CO)6, Mo(CO)is or Rh(CO)is in the presence of selenium has allowed the preparation of catalysts with enhanced activity towards oxygen reduction, when compared with the monometallic ruthenium-based catalyst [126],... 

Gottesfeld S, Minas C (2008) Optimization of direct methanol fuel cell systems and their mode of operation. In Kaka S, Pramuanjaroenkij A, Vasiliev L (eds) Mini-micro fuel cells. Springer, Dordrecht, pp 257-268... 

Kang K, Lee G, Gwak G, Choi Y, Ju H (2012) Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system. Int J Hydrogen Energy 37 6285-6291... 

Narayanan SR, Valdez TI (2003) Portable direct methanol fuel cell systems. In Vielstich W, Lamm A, Gasteiger H (eds) Handbook of fuel cells fundamentals, technology and applications, vol 4. Wiley, Chichester, England, Part 1, pp 1133-1141... 

YoonSK, Na YS, Joung Y, ParkJ, Kim Y, Hu L, Song I, Cho H (2009) Direct methanol fuel cell systems for portable applications, fuel cell seminar exposition 18 Nov 2009, Pahn Springs... 

Figure 4. Schematic view of the components and their relationships in a direct methanol fuel cell system. The thick arrow lines indicate how the air and fuel flow from one part to others and the thin solid lines express the relationships between two components coimected by a line.

As has already been mentioned, as of 2011, the commercial deployment into this space has been very Hmited. Here the market leader, or to be more accurate the market pioneer, is SEC Energy. Along with its distributor partner in the UK, UPS Systems, they have installed 20 EFOY units at the Elan Valley Reservoir to power telemetry equipment. The direct methanol fuel-cell systems use methanol, and with their 281 canisters there is sufficient energy to power the equipment 24 h per day for a minimum of 300 days. This is the largest project of its kind to date. 

Scott K, Taama WM, Argyropoulos P (1999) Engineering aspects of the direct methanol fuel cell system. J Power Sources 79 43-59. doi 10.1016/ S0378-7753(98)00198-0... 

Serenus H3-350W from Serenergy AS (Fig. 4b) with triple volumetric and double gravimetric power density, respectively, as compared to state-of-the-art direct methanol fuel-cell systems in the same power class, to be used either standalone or in 24 V hybrid battery—fuel-cell systems, (c) Aerospace Industry... 

Hotz et al. compared the efficiency of a small PEM fuel cell system, which had a methanol fuel processor composed of a fixed-bed steam reformer and preferential oxidation reactors, with a direct methanol fuel cell system [439]. Both systems were in the power range of 2 Wei. The steam reformer was designed as a tubular bundle filled with copper/zinc oxide catalyst, operated at a S/C of 1.1 and heated by the combustion gases of the afterburner. Because the methanol conversion was assumed to be only 45%, most of the methanol was combusted in the afterburner, which generated sufficient energy to keep the system on temperature. However, the efficiency of such a system will be low. The exergetic efficiency (which is in fact the system efficiency as defined in Section 2.2) ofthe fuel processor/fuel cell system was calculated to be 30%. 

As discussed in Section 9.1, in reality a fuel processor/fuel cell system with a power output below 5 Wei will have a much lower efficiency than 20%. The size of the PEM fuel cell and of the direct methanol fuel cell was assumed to be in the same range, which, from practical experience, is rather unrealistic. The efficiency of the direct methanol fuel cell system was calculated to be 25%. 

Besides chemical catalytic reduction of carbon dioxide with hydrogen, which is already possible in the laboratory, we are exploring a new approach to recycling carbon dioxide into methyl alcohol or related oxygenates via aqueous eleetrocatalytic reduction using what can be called a regenerative fuel cell system. The direct methanol fuel cell... 

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. 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

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. 

Therefore, methanol is the top candidate because of its low price, less toxicity, high energy density and easy handling. Although direct methanol fuel cells may need an auxiliary system to treat unoxidized or partially oxidized fuel in the exhaust gas, direct methanol fuel cells are still a very attractive system as a portable power source. 

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]. 

A particular version of the PEFC is the direct methanol fuel cell (DMFC). As the name implies, an aqueous solution of methanol is used as fuel instead of the hydrogen-rich gas, eliminating the need for reformers and shift reactors. The major challenge for the DMFC is the crossover of methanol from the anode compartment into the cathode compartment through the membrane that poisons the electrodes by CO. Consequently, the cell potentials and hence the system efficiencies are still low. Nevertheless, the DMFC offers the prospect of replacing batteries in consumer electronics and has attracted the interest of this industry. 

Fuel cells are electrochemical devices that convert the chemical energy of the fuels directly into electrical energy, and are considered to be the key technology for power generation in stationary, automotive, portable and even microscale systems. Among all kinds of fuel cells, direct methanol fuel cells have really exhibited the potential to replace current portable power sources and micropower sources in the market (Yao et al., 2006). 

Yao, S.C., Tang, X., Hsieh, C.C., Alyousef, Y, Vladimer, M., Redder, G.K., Amon, C.H., 2006. Micro-electro-mechanical systems (MEMS)-based micro-scale direct methanol fuel cell development. Energy 31 636-649. 

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol. 

Moore, R. M. (Bob). 2000. Direct Methanol Fuel Cells for Automotive Power Systems. Report No. 2000-01-0012. Warrendale, Pa. Society of Automotive Engineers. 

---