Alkaline fuel cells conductivity

A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with nonprecious, relatively inexpensive metals. To date, most low cost catalyst development work has been directed towards Raney nickel powders for anodes and silver-based powders for cathodes. The essential characteristics of the catalyst structure are high electronic conductivity and stability (mechanical, chemical, and electrochemical). 

Reasonable performance is exhibited by alkaline cells operated at low temperatures (room temperature up to about 70°C). This is because the conductivity of KOH solutions is relatively high at low temperatures. For instance an alkaline fuel cell designed to operate at 70°C will reduce to only half power level when its operating temperature is reduced to room temperature (20). 

Alkaline fuel cell (AFC) working at 80 °C with concentrated potassium hydroxide as electrolyte, conducting by the OH anion. This kind of fuel cell, developed by IFC (USA), is now used in space shuttles. 

Alkaline fuel cells (AFC), with concentrated KOH (in asbestos matrices) electrolyte, conduct OH -anions, generated at an 02/H20 exposed cathode to electro-oxidize H2 (fuel) at the anode at moderate temperatures, and... 

A second class of fuel cells employs hydroxide-conducting (alkaline) electrolytes, again either in form of a solid membrane (alkaline membrane fuel cells) or a liquid electrolyte (alkaline fuel cells). While the modem era of fuel cells began with the latter type, the former type is under intense research today because a stable, highly conducting alkaline membrane with good C02 tolerance has remained elusive to date. 

Alkaline fuel cell (AFC) was used for Apollo and Space Shuttle program. Alkaline fuel cell employs liquid alkaline, e.g., KOH, as an electrolyte so that fuel, as well as air or oxygen, should be free of CO2 because the strong alkaline electrolyte reacts with CO2 to form carbonates, which reduces the ionic conductivity. Electrodes, e.g., Ni, Ag, and metal oxides, are relatively inexpensive compared to that of other fuel cells. 

The monolithic structure, good mechanical strength, high surface area, and electrical conductivity of these carbon materials make them attractive as electrodes for various electrochemical applications. As hydrogen oxidation (or oxygen reduction) catalysts may be incorporated to such porous materials, one specific application to consider is the use of this type of material as alkaline fuel cell electrode. 

In an alkaline fuel cell, which contains an anionically conductive membrane, the exchange media is a hydroxyl radical, ("OH). At the anode, hydrogen is oxidized through a redox reaction, (Equation 3.3), producing water and... 

The reaction product of this reaction is water, which is formed at the cathode in acidic fuel cells. It can be formed at the anode, if an oxygen ion (or carbonate) conducting electrolyte is used, as in the case of high temperature fuel cells or in the case of the liquid alkaline fuel cell (see Section 8.1.4). The reaction product water has to be removed from the cell. 

Apart from the large volume of research and design work for PEMFC and DMFC, many studies of improved high-temperature fuel cells, SOFC and MCFC, have been conducted since 1990. A marked rise in the number of power plants based on MCFC was seen between 2003 and 2005. The volume of work concerned with alkaline fuel cells has strongly declined as the late 1980s. As to PAFC, the literature of recent years has offered only a few indications of research in this area. 

The cathode consists of lithiated nickel oxide. Nickel oxide is a p-type semiconductor, having a rather low conductivity. When doped with lithium oxide, its conductivity increases tens of times, owing to a partial change of Ni + to Ni + ions. The lithiation is accomplished by treating the porous nickel electrode with a lithium hydroxide solution in the presence of air oxygen. The compound produced has a composition given as Lij +Nii j( Nijj +0. This lithiation of nickel oxide was first applied in 1960 by Bacon in his alkaline fuel cell. 

PEMFC and alkaline fuel cells operate at temperatures below 100 °C, while all other types of fuel cells need higher temperatures for their electrolytes to become ion-conducting. The operating temperatures play a crucial role for the flexibility of a power plant. 

Recently, in our group, we evaluated the potentiality of a poly(iniide) (PI)/ organically-modified montmorillonite (O-MMT) nanocomposite membrane for the use in alkaline fuel cells [73]. Both X-ray diffraction and scanning electron microscopy revealed a good dispersion of O-MMT into the PI matrix and preservation of the O-MMT layered structure. When compared to the pure PI, the addition of O-MMT improved thermal stability and markedly increased the capability of absorbing electrolyte and ionic conductivity of the composite. Based on these results, the PI/ O-MMT nanocomposite is a promising candidate for alkaline fuel cell appUcations. 

Three approaches to reach those purpose have been reviewed and discussed in this Chapter the improvement of Nafion with fillers that reduce alcohol permeability, the synthesis of new polymers and blends with better alcohol selectivity, and the development of AEM with good conductivities for use in alkaline fuel cells where the alcohol crossover is not important. 

Yu EH, Scott K, Reeve RW (2006) Application of sodium conducting membranes in direct methanol alkaline fuel cells. J Appl Electrochem 36 25-32... 

Seo DW, Hossain MA, Lee DH, Lim YD, Lee SH, Lee HC, et al. Anion conductive poly(arylene ether sulfone)s containing tetra-quatemary anunonium hydroxide on flu-orenyl group for alkaline fuel cell application. Electrochim Acta 2012 86 360-5. 

Tanaka M, Fukasawa K, Nishino E, Yamaguchi S, Yamada K, Tanaka H, et al. Anion conductive block poly(arylene ether)s synthesis, properties, and application in alkaline fuel cells. J Am Chem Soc 2011 133(27) 10646-54. 

PI with an organically modified Montmorillonite nanocomposite was tested as membrane for alkaline fuel cells [108], In comparison to pure PI, the addition of orgaiucally modified Montmorillonite improves the thermal stability and increases the capability of absorption of electrolyte and increases the ionic conductivity of the composite. 

There are several types of fuel cells, which are classified primarily by the kind of electrolyte they employ. The materials used for electrolytes have their best conductance only within certain temperature ranges (Hirschenhofer 1994). A few of the most promising types include phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), alkaline fuel cell (AFC), proton exchange membrane fuel cell (PEMFC), and direct methanol fuel cell (DMFC). 

Alkaline fuel cells (AFCs) with a hydroxide conducting electrolyte,... 

Inasmuch as the use of epichlorohydrin concept, Agel et al. [13] developed a new and cheap type of anion exchange membranes (AEM) by preparing the polyepichlorohydrin (PECH) graft quaternary amines (DABCO, TEA) for use in alkaline cells. It s a quasi-gas impervious polymer membrane. The ionic conductivity was much improved to 10 S cm due to the low crystallinity and the anion exchange between Cf and OH ions on the polymer side chains. For the first time, the alkaline SPE employed in alkaline fuel cell, the test results exhibited good performance and could tolerate at high temperature up to 120°C. 

While increasing intrinsic conductivity in anion exchange membranes is still focus of research, a major factor influencing effective conductivity is carbon dioxide (CO2). In contrast to alkaline fuel cells (AFCs) with liquid KOH electrolyte, in which carbonate precipitation occurs, in alkaline membrane fuel cells there is no place for salt precipitation, since in solid electrolytes there are no free cation species available for precipitation. However, CO2 still affects the alkaline membrane fuel cell general performance by decreasing the effective conductivity of the anion exchange membranes [15-18], as it is indicated below ... 

Lin B, Qiu L, Qiu B, Peng Y, Yan F (2011) A soluble and conductive polyfluorene ionomer with pendant imidazolium groups for alkaline fuel cell applications. Macromolecules 44 9642-9649. doi 10.1021/ ma202159d... 

---