Rolled Electrodes

Raney nickel electrodes prepared in this way were used in many of the fuel cell demonstrations mentioned in the introduction to this chapter. Often Raney nickel was used for the anode and silver for the cathode. This combination was also used for the electrodes of the Siemens alkaline fuel cell used in submarines in the early 1990s (Strasser, 1990). They have also been used more recently, in a ground up form, in the rolled electrodes to be desCTibed in Section 5.4.4 (Gnlzow, 1996). 

With a non-platinum catalyst, such electrodes are readily available at a cost from about 0.01 per cm, or around 10 per ft, which is very low compared to other fuel cell materials. Adding a platinum catalyst increases the cost, depending on the loading, but it might only be by a factor of about 3, which still gives, in fuel cell terms, a very low-cost electrode. However, there are problems. 

One problem is that the electrode is covered with a layer of PTFE, and so the surface is non-conductive, and thus a bipolar plate cannot be used for cell interconnection. Instead, the cells have to be edge-connected. This is not so bad, as with the nickel mesh running right through the electrode, its surface conductivity is quite good. A more serious problem 

When using such carbon-supported electrodes, it is clear that the carbon dioxide must be removed from the air. Another approach to the problem is the use of a different type of rolled electrode, which does not use carbon-supported catalyst. Gulzow (1996) describes an anode based on granules of Raney nickel mixed with PTFE. This is rolled onto a metal net in much the same way as the PTFE/carbon-supported catalyst. A cathode can be prepared in much the same way, only using silver instead of nickel. It is claimed that these electrodes are not damaged by CO2. 

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It should be noted that the rechargeable cells discussed later have the same construction and differ only in separator type, electrode composition and cathode / anode balance. For comparison, Fig. 3 shows the design of an AA-size lithium cell. The construction with a spirally rolled electrode increases the power output. 

A further electrical method, the use of which is restricted to film (and which indeed is little used at present) is the electro-contact treatment process proposed by Rothacker13 and described by Smith.8 In this, the film is passed over a metal roll and contacted by a number of free-rolling electrodes. A direct current potential difference is applied between the roll and the electrodes, and an a.c. voltage superimposed over the d.c. The flow of electrons thus induced brings... 

Today, the most commonly used electrode is the PTFE-bonded GDE, and various procedures have been described for their production. The preparation procedure for rolled electrodes will be described in detail to illustrate the composition of PTFE-bonded gas electrodes. Rolled electrodes are composites of a metal catalyst and PTFE supported by a metal grid on the back side of the electrode. The metal grid is responsible for high electrical conductivity and also for mechanical support and stability. 

After reactive mixing of the different materials for the electrodes, the PTFE powder, the copper powder, and the oxidized nickel catalyst in a knife mill, the powder is rolled into a self-carrying strip using a calender. This self-carrying strip is then rolled on to a metal grid to form the electrode. The rolled electrode has a thickness of 100-350 am. 

Because of the PTEE film, only a low porosity can be measured at the surface of the rolled electrode, and the active surface is therefore too small for acceptable performance. The pore system and the specific surface area of the AFC anodes... 

Figure 15.6 Section through a lithium/manganese dioxide cylindrical cell with rolled electrodes.

Figure 5.8 Photograph shows the structure of a rolled electrode. The catalyst is mixed with a PTFE binder and is rolled onto a nickel mesh. The thin layer of PTFE on the gas side is shown partially rolled back.

The cylindrical batteries use a spirally wound (jelly-roll) electrode construction, and the batteries are either crimped or hermetically sealed. Their construction is similar to the cylindrical spiral-wound electrode design of the Li/MnOz battery shown in Fig. 14.40. The larger cells are provided with low-pressure safety vents. 

This type of battery has a spiral-wound electrode pack, made from rectangular foil electrodes. Lithium foil is rolled on to an expanded metal mesh current collector as the negative electrode, and is separated from the similarly supported cathode by a polypropylene separator. Two types of cell construction are used jelly-roll electrodes in crimp-sealed or hermetically sealed cylindrical cells, and large 20-100Ah 12V flat-plate electrodes in large reserve batteries. It is a relatively high-pressure system and cells must have salety vents to avoid explosion in the event of accidental incineration (see Part 2 for further details of construction). 

Two typical cell constructions are used jelly-roll electrodes in crimp sealed on hermetically sealed cylindrical cells, and large 20-100Ah, 12 V flat-plate electrodes in large reserve batteries. 

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