Portable power sources

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... 

Table 4 and Fig. 18 illustrate the performance levels achieved by the active players in DMFC R D. The main goal in DMFC research in the U.S. and European programs is to achieve a stable performance level of 200 mW/cm at a cell potential of 0.5 to 0.6 V. It is because of the relatively low activity of the electrocatalyst for methanol electrooxidation that this power level is less than half that of a PEMFC with Hj as a fuel. A higher power level of the DMFC is essential for a transportation application, but the present power level goal is quite adequate for small portable power sources. 

The DMFC is the most attractive type of fuel cell as a powerplant for electric vehicles and as a portable power source, because methanol is a liquid fuel with values for the specific energy and energy density being about equal to half those for liquid hydrocarbon fuels (gasoline and diesel fuel). 

Our crystal-ball predictions are that DMFCs will first be commercialized as portable power sources for military and civilian applications before the year 2010 and that there will then be a quantum jump in the technology to be in a position to drive DMFC-powered electric vehicles 10 years thereafter. 

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. 

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). 

Portable systems must operate on a portable power source. Often this simply means batteries. When heating becomes a part of the operating sequence, batteries can be consumed quickly. A well-defined energy budget is necessary early in the design process. 

By far the largest sector of the battery industry worldwide is based on the lead-acid aqueous cell whose dominance is due to a combination of low cost, versatility and the excellent reversibility of the electrochemical system, Lead-acid cells have extensive use both as portable power sources for vehicle service and traction, and in stationary applications ranging from small emergency supplies to load levelling systems. In terms of sales, the lead-acid battery occupies over 50% of the entire primary and secondary market, with an estimated value of 100 billion per annum before retail mark-up. 

Cogeneration Back-up power Portable power Source F. Barbier (2005) ... 

In view of the abundance of natural gas resources found since the 1980s, and natural gas being a considerably cleaner fuel than petroleum or coal for the fuel processors, the main goals of the major worldwide fuel cell programs are to develop fuel cell power plants and portable power sources using natural gas or natural gas-derived fuel cells. A chart of all types of fuel cells, using natural gas as a fuel, is presented in Scheme 9.2 the applications being considered for the different types of fuel... 

Many types of rechargeable batteries have been developed as portable power sources for small electronic devices, such as watch, calculator, video camera, computer and so on. Lead-acid battery, Ni-Cd battery, Ni-Metal hydride battery, and lithium battery are well known and used in some portable electronic devices. Lithium batteries are the most attractive with regard to energy density or power density. Recently, a new rechargeable lithium battery, that is a so-called Lithium Ion Battery , was proposed by Sony Company [5]. In this battery, carbon materials... 

The DMFC is showing promise as a portable power source, but the bulky fuel is a problem (see several papers in Williams, 2002). Vehicle application of the DMFC is some way away. The advent of better ways of processing natural gas, for example the hydrogen mine, will cheapen methanol substantially, and move the DMFC towards a competitive position with the PEFC, which it does not yet occupy. There are no DMFC buses being demonstrated, and no DMFC stationary power plants for sale. 

At 1-10 W (watts), fuel cells could be used as battery replacements at 100 W to 1 kW, fuel cells could find military applications which require lightweight portable power sources for communications and weapon power at 1 - 10 kW, fuel cells could supply power to residential buildings and serve as auxiliary power units in vehicles and trucks. At higher power levels, the solid oxide fuel cell (SOFC) could be an effective approach for the distributed power generation and the cogeneration (i.e., combined heat and power). Above 1 MW, the SOFC could be integrated with a turbine power plant to improve the overall efficiency of power generation and reduce emissions. ... 

DMFCs have potential near-term applications mainly in the portable power source market, as they are smaller, lighter, simpler, and cleaner than conventional batteries. Liquid methanol is consumed directly in a DMFC, which implies a higher energy density of the fuel cell system. But the power densities achievable with state-of-the-art DMFCs are still smaller in comparison... 

Specific applications Portable power sources Quiet power sources Remote power sources Space explorations Military applications On-site or on-board fuel cells for stationary, mobile, and portable systems... 

Potential Applications of DMFCs in Portable Power Sources... 

Carbon monoxide is a common impurity in hydrogen that has been generated by a reforming process. Portable power sources, used by the military for energy generation in remote locations, use a methanol reforming reaction as shown in the following equation ... 

This chapter reviews selected topics related to p-DMFCs and MEMS technologies for portable power sources emphasis is on the results of recent studies carried out to clarify the factors affecting the performance of p-DMEC as a new power source. 

With nano-SDC infiltrated LSM cathode, the single cell performance was obviously improved. Our micro-stack results proved that the gas flow geometry critically affects the stack performance. The performance of linear micro-stacks decreased with increasing cell number, probably due to the uneven gas/temperature distribution. The symmetric star-shaped design exhibited attractive output, making it very promising for portable power sources. 

DMFCs have potential near-term applications mainly in the portable power source market, as they are smaller, lighter, simpler, and cleaner than conventional batteries. Liquid methanol is consumed directly in a DMFC, which implies a higher energy density of the fuel cell system. But the power densities achievable with state-of-the-art DMFCs are still very small in comparison to hydrogen-fuelled PEMFCs. One of the major problems lies in the use of liquid methanol solution on the anode of the DMFC, which, on the one hand, keeps the ionomeric membrane water saturated (and thus no humidification is needed) but, on the other hand, does not keep fuel (methanol or any other organic fuel, e.g., formic acid, ethanol) and water from permeating to the cathode side, since the basic PFSA membranes are permeable to both methanol and water. - The fuel and water crossover from anode to cathode hampers the performance of the air cathode. 

---