Alkaline fuel cells disadvantages

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

One of the first fuel cell designs was low-temperature alkaline fuel cells (AFCs) used in the U.S. space program. They served to produce both water and electricity on the spacecraft. Some of their disadvantages are that they are subject to carbon monoxide poisoning, are expensive, and have short operating lives. The AFC electrodes are made of porous carbon plates laced with a catalyst. The electrolyte is potassium hydroxide. At the cathode, oxygen forms hydroxide ions, which are recycled back to the anode. At the anode, hydrogen gas combines with the hydroxide ions to produce water vapor and electrons that are forced out of the anode to produce electric current. 

Several types of fuel cells have been developed and are classified according to the electrolytes used alkaline fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells (PAFCs), PEMFCs, and solid oxide fuel cells (SOFCs). As shown in Figure 1.3, the optimum operation temperatures of these fuel cells are different, and each type has different advantages and disadvantages. 

There are five classes of fuel cells. Like batteries, they differ in the electrolyte, which can be either liquid (alkaline or acidic), polymer film, molten salt, or ceramic. As Table 1 shows, each type has specific advantages and disadvantages that make it suitable for different applications. Ultimately, however, the fuel cells that win the commercialization race will be those that are the most economical. 

Wang Y, Li L, Hu L, Zhuang L, Lu J, Xu B. 2003. A feasibility analysis for alkaline membrane direct methanol fuel cell Thermodynamic disadvantages versus kinetic advantages. Electrochem Commun 5 662. 

This cell works optimally at 80 °C using relatively inexpensive materials. When it is switched on in the cold, it produces about one-quarter of the power finally produced after it warms up. This is an advantage compared with other types of fuel cells operating at intermediate (200 °C) or high (650 to 1000 °C) temperatures, which need an auxiliary power source to start them and warm them up. The alkaline environment means that a wide range of electrode catalysts are available, while cells using acid solutions can only use noble metal electrode materials, which is a distinct economic disadvantage for terrestrial applications. 

The hydrogen-oxygen fuel cell has a standard emf of 1.23 V. What advantages and disadvantages there to using this device as a source of power compared to a 1.55-V alkaline battery ... 

Many different types of fuel-cell membranes are currently in use in, e.g., solid-oxide fuel cells (SOFCs), molten-carbonate fuel cells (MCFCs), alkaline fuel eells (AFCs), phosphoric-acid fuel cells (PAFCs), and polymer-electrolyte membrane fuel cells (PEMFCs). One of the most widely used polymers in PEMFCs is Nalion, which is basically a fluorinated teflon-like hydrophobic polymer backbone with sulfonated hydrophilic side chains." Nafion and related sulfonic-add based polymers have the disadvantage that the polymer-conductivity is based on the presence of water and, thus, the operating temperature is limited to a temperature range of 80-100 °C. This constraint makes the water (and temperature) management of the fuel cell critical for its performance. Many computational studies and reviews have recently been pubhshed," and new types of polymers are proposed at any time, e.g. sulfonated aromatic polyarylenes," to meet these drawbacks. 

Recent developments in AAEMs have opened up the possibiUty of an alkaline analog of the acidic solid polymer electrolyte fuel cell. This could utilize the benefits of the alkaline cathode kinetics and at the same time eradicate the disadvantages of using an aqueous electrolyte. As the AAEM is also a polymer electrolyte membrane (sometimes abbreviated as PEM), some clarity in abbreviations is required. In this chapter, PEM refers only to the proton exchange membrane fuel cells (acidic), AAEM refers to the anion exchange membrane H2/O2 fuel cells, and AFC exclusively refers to the aqueous electrolyte alkaline H2/O2 fuel cells. Anion exchange membranes are also employed in alkaline direct alcohol fuel cells, discussion of which will refer to them as ADMFC/ADEFC (methanol/ ethanol). 

Wang, Y., Li, L., Hu, L., Zhuang, L., Lu, J. and Xu, B. A feasibility analysis for alkaline membrane direct methanol fuel cell thermodynamic disadvantages versus kinetic advantages , Electrochem. Commun., 5 (2003) 662-666. Yang, J. and Xu, J.J. Nanoporous amorphous manganese oxide as electrocatalyst for oxygen reduction in alkaline solutions , Electrochem, Commun., S (2003) 306-311. 

The electrolyte within the cell can be acid or alkaline. When carbonaceous fuels are used, continuous operation of the cell requires acid electrolytes because alkaline electrolytes form carbonates with the carbon dioxide produced by the cell reaction. The necessity of working with acid electrolytes, however, brings considerable disadvantages, as many materials are less stable in an acid than in an alkaline medium they corrode or decompose. 

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