Electrode, Bacon fuel cell

Fig. 13.16. Structure of porous electrode in the Bacon-Pratt and Whitney fuel cell. (Reprinted from J. O M. Bockris and S. Srinivasan, Fuel Cells Their Electrochemistry, Fig. 22, copyright 1969. Reproduced with permission of the McGraw-Hill Companies.)...

The optimization of interfaces for low temperature fuel cells has always depended on the availability of specific materials to control the porosity and wetting behavioiu" of the respective pores by the electrolyte. For example, one of the breakthroughs in the development of AFCs was the work of Bacon at a time when the chemically stable hydrophobizing agent, PTFE, was not yet available [4]. Bacon managed interface control with a dual-layer, dualporosity electrode, made out of Ni powders of different grain size. The later availability of PTFE opened up new possibilities for improved GDE designs. 

Other workers gradually went to less concentrated alkali (30-40% KOH) than found in Bacon s and P W s batteries. For the space shuttle program, United Technology Corporation (UTC-Power) developed a battery of alkaline fuel cells where 35% KOH immobilized in an asbestos matrix was used as the electrolyte. The electrodes contained a relatively large amount of platinum catalysts, so that at a temperature of 250°C it was possible to work at very high current densities, of up to 1 A/cm. ... 

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. 

The electrodes in Bacon s battery measured 370 cm, their thickness was 1.8 mm. At current densities from 200 to 400 mA/cm, the voltage of an individual cell in the battery was 0.90-0.95 V, which is considerably higher than that in phosphoric acid fuel cell (PAFC) and in modern proton exchange membrane fuel cell (PEMFC). A variety of corrosion problems were responsible for the total lifetime of Bacon s battery not reaching more than a few hundred hours. 

The discovery of the fuel cell followed soon after Faraday developed his laws of electrolysis. In 1839, Grove showed that the electrolysis of water was partially reversible. Hydrogen and oxygen formed by the electrolysis of water were allowed to recombine at the platinum electrodes to produce a current or what appeared to be reverse electrolysis. Using the same fundamental principles but somewhat more advanced technology, Bacon in 1959—after about 20 years of intensive effort—produced a 6 kW power unit that could drive a small truck. 

Fig. 9.12 Bacon hydrogen-oxygen fuel cell with gas-diffusion electrodes...

To facilitate the rapid attainment of equilibrium, a liquid gas-diffusion electrode was developed whereby concentration polarization could be minimized. The ohmic polarization (the RI drop between the electrodes, which gives rise to an internal resistance) is also minimized when the anode-to-cathode separation is reduced. The apparatus of the hydrogen-oxygen fuel cell developed by Bacon with gas-diffusion electrodes is shown in Fig. 9.12. The operating temperature of 240" C is attained with an electrolyte concentration of about 80% KOH solution, which with the high pressures of about 600 psi for H2 and O2, allows high current densities to be drawn with relatively low polarization losses. Units such as these with power of 15 kW have been built and used successfully for long periods. 

In the Bacon ceU, the first successful modem fuel cell, porous nickel was used as a material for the electrodes, doing double duty as conductor and catalyst for the current-producing electrochemical reactions (in the cathode the nickel was lithiated). A little later, Justi introduced Raney nickel and Raney silver as the catalytic electrodes nickel for the hydrogen anode and silver for the oxygen... 

This electrode structure was also used in the Apollo mission fuel cells. Such structures may or may not be combined with catalysts. In the Apollo and Bacon cells, the anode was formed from the straightforward nickel powder as described above, whereas the cathode was partially lithiated and oxidised. ... 

As already indicated, a flat, layer construction is normally used in fuel cell batteries, and an individual cell, usually not more than 1 cm thick, will be somewhat as shown in figure F.2. Low-temperature cells need the most effective catalyst, which is platinum, although silver can be used at the oxygen electrode. To prevent the cost being prohibitive, efforts are made to restrict the platinum coating to the actual region of contact between gas and liquid. In medium-temperature cells, such as the Bacon cell, which operate at about 470 K, nickel is used as catalyst... 

---