Alkaline fuel cells electrode materials

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. 

The first key component of a membrane fuel cell is the membrane electrolyte. Its central role lies in the separation of the two electrodes and the transport of ionic species (e.g. hydroxyl ion, OH , in an AEM), between them. In general, quaternary ammonium groups are used as anion-exchange groups in these materials. However, due to their low stability in highly alkaline media [43,44], only a few membranes have been evaluated for use as solid polymer electrolytes in alkaline fuel cells. 

Fuel cells operate in a manner reverse to that of electrolysis, discussed in Chapter 2, combining fuel to make electricity. The basic design consists of two electrodes separated by an electrolyte. The oldest type of fuel cell is the alkaline fuel cell where an alkaline electrolyte like potassium hydroxide is used. The hydrogen enters through the anode compartment and oxygen through the cathode compartment. The hydrogen is ionized by the catalytic activity of the anode material and electrons are released into the external circuit. The protons react with the hydroxyl ions in the electrolyte to form water. The reaction can be written as ... 

Alkaline Fuel Cell The electrolyte for NASA s space shuttle orbiter fuel cell is 35 percent potassium hydroxide. The cell operates between 353 and 363 K (176 and 194°F) at 0.4 MPa (59 psia) on hydrogen and oxygen. The electrodes contain platinum-palladium and platinum-gold alloy powder catalysts bonded with polytetrafluoro-ethylene (PTFE) latex and supported on gold-plated nickel screens for current collection and gas distribution. A variety of materials, including asbestos and potassium titanate, are used to form a micro-porous separator that retains the electrolyte between the electrodes. The cell structural materials, bipolar plates, and external housing are usually nickel, plated to resist corrosion. The complete orbiter fuel cell power plant is shown in Fig. 27-62. 

The Alkaline Fuel Cell, AFC, is the most efficient low-temperature fuel cell presently available with a very high power density, therefore ideally appropriate for mobile applications. The AFC shows a similar performance as the PEFC, but with a much more demanding process control which is complicated because of the requirements of fuel purity (no CO2) and of the corrosive liquid electrolyte. Efficiencies of more than 60 % have been achieved with clean hydrogen and oxygen and noble electrode materials. The AFC was demonstrated to also work with a hydrogen-air system, in a 1 kW stack in Japan and in a 6 kW Russian system with more than 5(XX) h lifetime [34]. PEFC and AFC may play a... 

Multicomponent alloys of nickel and aluminum activated by Ti, Mo are most widespread and wide by used materials for hydrogen electrodes of low temperature alkaline fuel cells. To make hydrogen electrodes skeletal nickel prepared by alkali-soluble of alloy with composition 50 %Ni -i- 47 % A1 -i-3 % Ti is used. Raney catalyst is processed by 20 % suspense of Fluoroplast F-4 D with following drying in vacuum at 50 °C that permits pyrophoric catalyst to protect against self combustion and serves hydrophobic binder to form electrodes [5]. 

Apart from exhibiting electrocatalytic activity towards the electrode reactions, the electrocatalysts must be stable within the working cell. For the alkaline fuel cell (AFC) this is relatively easy since many electrocatalytic materials are adequately stable in alkaline solutions. The fact that the AFC is very sensitive to the presence of CO2, either in the fuel stream or in the air stream, has limited its application substantially to those simations where very pure hydrogen and very pure oxygen can be supplied. 

The ORR catalysts are used in either acidic or basic electrolyte solutions, and when they are used in either acidic or alkaline fuel cells in the presence of high oxidizing oxygen, electrochemical stabilities of both the catalysts and their support materials are very important for their practiced applications. In the presence of O2, the potential of electrode coated with a catalyst (or the catalyst s potential) will be higher than 1.2 V vs RHE. This potential is higher than the oxidation potentials of almost all metal and carbon materials, or in other words, almost all metal and carbon materials are thermodynamically unstable in the sense of electrochemistry. However, due to the slow kinetics of the oxidation process, or the oxidation product is less soluble, the carbon and Pt-based materials can still be used as ORR catalysts for fuel cells even at acidic or basic environment and high temperatures such as 70-80 °C. 

This method was also applied by other authors to hydrogen absorption in a porous electrode consisting of spherical particles of ABs-type material [458] to describe a whole polymer electrolyte fuel cell with gas diffusion in pores [459], alkaline fuel cells [460], and intercalation electrodes [461]. Several authors attempted to fit the experimental impedances to their models [456-461]. 

In 1923, Schmid [8, 9] improved fuel-cells electrode, and Scharf [10] developed the hydrogen diffusion electrode. A major advance in alkaline electrochemistry was achieved by Raney [11], who developed the weU-known Raney nickel (high surface-area Ni) catalyst. With this high surface-area electrode, it was possible to achieve a technically relevant performance with a simple catalyst material. 

The latter classificatirMi distinguishes between acidic and alkaline fuel cells. Acidic fuel cells with conduction require other electrode materials than alkaline... 

Carbon-supported catalysts (e.g., those employed in fuel cell electrodes) can show potentially very different behavior for the ORR than that of the bulk metal. Yang et al. [37] showed that Pd/C catalysts in alkaline had high activity for ORR, and that the carbon support itself is active for the two-electron reduction of O2 to peroxide (which can then migrate to the Pd particles for subsequent two-electron reduction to water). It is shown that all carbon materials have some ORR activity in alkaline solution (but none in acid), normally for the two-electron reduction to peroxide, although some oxidized carbon surfaces can complete the serial reduction to water at higher overpotentials [24,38-40]. 

This chapter attempts to provide a critical review of the work carried out on alkaline fuel cell, which directly uses hydrogen rich liquid fuel and oxygen or air as an oxidant. The subjects covered are electrode materials, electrolyte, half-cell analysis and single cell performance in alkaline medium. Koscher et al. (2003) brought out elaborate review work on direct methanol alkaline fuel cell. Earlier Parsons et al. (1988) reviewed literature on anode electrode where, the oxidation of small organic molecules in acid as well as in alkaline conditions was considered. A review work on electro-oxidation of boron compounds was done by Morris et al. (1985). However, in this chapter use of three specific fuels, e.g., methanol, ethanol and sodium borohydride in alkaline fuel cell is discussed. 

The anode electrode-catalyst is one of the important components of the alkaline fuel cell as it helps in the electro-oxidation of fuel. It is desirable that the anode electrode-catalyst provides faster reaction kinetics and 100% oxidation of fuels to CO2 and H2O. The most widely used catalyst, without doubt, is platinum. Platinum seems to be the best choice for acidic solutions, but other metallic alloy with platinum or other metals can match its performance in alkaline medium because of the favorable fuel oxidation in alkaline medium. Different anode materials based on Pt (Prabhuram et al. 1998, Moralldn et al. 1995, Tripkivic et al. 1996), Pt-Ru (Wang et al. 2003, Manoharan et al. 2001), Co-W alloys (Shobba et al. 2002), sintered Ag/ PdO (Koscher et al. 2003), spent carbon electrodes impregnated with Fe, Fe" or Ag (Verma 2000), nickel impregnated silicate-1 (Khalil et al. 2005) and nickel dimethylglyoxime complex (Golikand et al. 2005) are some of the catalysts studied for the electro-oxidation of methanol in alkaline medium. 

Alkaline solutions are generally known to lead to better catal5Tic activities than acidic solutions for many relevant electrode reactions. However, owing to the paucity in the development of suitable electrolyte materials, such as alkaline membranes, there has been much less fundamental work in the area of fuel cell catalysis in alkaline media. Nevertheless, there are a few hopeful developments in new alkaline polymer membranes [Varcoe and Slade, 2005] that are currently stirring up interest in smdying fuel cell catalytic reactions in alkalme solution. 

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