Fuel cell comparison with batteries

Shen, Y., Kordesch, K. Aronson, R.R. A comparison between the polymer electrolyte membrane fuel-cells and the alkaline fuel-cells (AFC) with liquid electrolyte. The NPC 99 Nagoya International Battery and Power Sources Conference, September 1999, Nagoya, Japan, 1999. 

EFCs typically generate low power, in comparison with conventional batteries and fuel cells, and with their initial prototypes energy density and energy utilization from source fuels tend to be low. With further development, however, the advantages of EFCs are expected to provide competitive benefits for certain applications over the conventional electrochemical power sources. In hybrid approach, EFCs may be combined with conventional batteries, fuel cells, and supercapacitors to form advanced power sources with improved performance for specific applications. 

For closed-cycle applications, such as for spacecraft, submarines, or transportation vehicles, the combinations of lightweight, reasonable power density, and compact size are favorable features in comparison with equivalent-capacity battery-based systems. In the International Space Station, for example, both electricity and water are provided by fuel cells. Fuel cells have not only been used in space exploration, but also in submarines (because they generate no noise or vibration). They have also been used to recover the energy from methane that is generated by wastewater, by garbage dumps, and more recently in automobiles as an alternative to the IC engine. 

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... 

The price of the fuel cell stack is determined by the cost of the materials and the production technology. These costs are strongly determined by the number of units produced. For various components a strong decrease in the price can be expected as the demand increases. As a consequence, reliable cost estimations are difficult. However, the material expenses and some estimates of production costs can be compared with the actual market prices of the competing technologies, namely primary batteries, rechargeable batteries and the internal combustion engine. The comparison shows that fuel cell systems are still too expensive for most applications and a reduction of the cost by a factor of 10-50 is necessary to achieve competitiveness. 

Most commercial applications for EDLCs and pseudocapacitors are for backup and pulse power sources for electronic devices. More recently, they have been explored as a power source for hybrid vehicles when used in combination with fuel cells or batteries. Their role is in load levehng, start-up, and acceleration (28). The traditional electrodes used in EDLCs are high surface area carbon (1000 m /g) which have specific capacitances around 100 F/g for a single electrode (5). Low-voltage devices (2.3 V) with capacitance values of 470, 900, and 1500 F are available commercially (29). By comparison, the specific capacitance of Ru02 electrode pseudocapacitors is in the range of 720 to 900 F/g (30-33). For the most part, ECs based on this material have yet to reach the market. 

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. 

The combination of favorable properties of PANI and TiO opens the possibility for various applications of PANI/TiO nanocomposite materials, such as piezoresistivity devices [41], electrochromic devices [99,118], photoelectrochemical devices [43,76], photovoltaic devices/solar cells [44,50,60,61,93,119], optoelectronic devices/UV detectors [115], catalysts [80], photocatalysts [52,63,74,75,78,84,87,97,104,107,121,122,125], photoelectrocatalysts [122,123], sensors [56,61,65,69,85,86,95,120,124], photoelectrochemical [110] and microbial fuel cells [71], supercapacitors [90,92,100,109,111], anode materials for lithium-ion batteries [101,102], materials for corrosion protection [82,113], microwave absorption materials [77,87,89], and electrorheological fluids [105,106]. In comparison with PANI, the covalently bonded PANI/TiO hybrids showed significant enhancement in optical contrast and coloration efficiency [99]. It was observed that the TiO nanodomains covalently bonded to PANI can act as electron acceptors, reducing the oxidation potential and band gap of PANI, thus improving the long-term electrochromic stability [99]. Colloidal... 

Wee JH (2007) A feasibility study on direct methanol fuel cells for laptop computers based on a cost comparison with lithium-ion batteries. J Power Sources 173 424-436... 

An electric double-layer capacitor, also known as supercapacitor, pseudocapacitor, electric double layer capacitor (EDLC), supercapacitor or ultracapacitor is an electrochemical capacitor with relatively high energy density. Compared to conventional capacitors the energy density is typically on the order of thousands of times greater than an electrolytic capacitor. In comparison with conventional batteries or fuel cells, EDLCs have lower energy density but a much higher power density. 

Table 1.6 Comparison of electric cars with batteries and with fuel cells [132].

An important criterion for the comparison of different fuel-cell hybrid systems is the maximum fuel-cell current ramp rate. Current changes that are too rapid could lead to fuel starvation, which would result in increased degradation of the fuel cell. In [85] a fuel-cell triple hybrid corresponding to concept 2.0 for a forldift truck is analyzed. For a characteristic driving cycle, the maximum fuel cell current ramp was 600 A s in the case of a pure fuel-cell system. By hybridizing the fuel cell with only a battery according to concept 1.0, this could be reduced to about 200 A s , which was also very fast Only by combining the battery with supercapacitors was a further reduction achieved. For a 20F supercapacitor, the calculated current ramp rate was 100 A s. To obtain a current ramp rate of about 20 A s, a 165 F supercapacitor was needed. 

A further point is that the method does not stand very good comparison with metal air batteries. If the user is prepared to use quantities of water, and is prepared to dispose of water/metal hydroxide mixtures, then systems such as the aluminium/air or magnesium/air battery are preferable. With a salt-water electrolyte, an alumininm/air battery can operate at 0.8 V at quite a high current density, producing three electrons for each aluminium atom. The electrode system is much cheaper and simpler than a fuel cell. 

In some cases the models for designing such hybrid systems are quite well developed, because they have already been used for solar panel-battery hybrid systems. These are very widely used in telecommunications and navigation equipment. In comparison, the model needed for a fuel cell-based system is much simpler, since with the solar panel both the consumption and generation of power vary in a mixture of random and predictable ways. With fuel cell-based hybrid systems, the primary generation of electrical power can at least be leUed upon and accurately modelled. 

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