Positive electrode assembly

The positive electrode assembly consists of two electrode feed-throughs, and a circular mesh electrode. 

Each positive electrode feed-through consists of  

Each electrode feed-through has two of these silicone rubber washers, for a total of four in the positive electrode assembly. 

---

Mark the port holes for the positive electrode assembly on liner... 

One threaded Teflon insert for each feed-through (total two in the positive electrode assembly). 

The next step is to make washers for the positive electrode assembly. These are two nickel alloy washers and two silicone rubber washers. Use a 1" diameter washer as a model to trace the circumference and center hole onto the rubber and the nickel alloy sheet. Use a punch to cut out the center hole in the rubber sheet, and a punch or drill to cut out the center hole on the nickel alloy. The circumference of the rubber sheet can be cut with an exacto knife and the circumference of the nickel washer can be cut with tin snips. 

In the design of membrane-type fuel cell stacks (batteries), membrane-electrode assemblies (MEAs) are used, which consist of a sheet of membrane and of the two electrodes (positive and negative) pressed onto it from either side. 

In ED, cation-exchange membranes are alternated with anion-exchange membranes in a parallel manner to form compartments 0.5 to 1.0 mm thick. The entire membrane assembly is held between two electrodes. When an electrical potential is applied to the electrodes, all positive ions tend to travel towards the negative electrode, and all negative anions tend to move toward the positive electrode. 

Fig. 3.23. Conductivity cell with phial magazine Q containing phials and magnetic pusher. The electrode assembly E is that shown in Fig. 3.22. S is a stirrer shaft with a propeller at one end and a glass-enclosed magnet N at the other. The stirrer shaft is held in position by the PTFE bearings Tf Tf, and Tf and the glass tube spacer G Th ha. thermocouple pocket and B a magnetic breaker for the phial P. The propeller, driven by the rotating magnet M, pumps the cell contents around the loop L, so that when P is broken there is very fast mixing.

Cells are assembled in the fully charged state and the positive electrode mix contains antimony trichloride (29%), sodium chloride (15%), aluminium chloride (48%), carbon black (8%) and small amounts of sulphur, which are added to give a higher average discharge voltage. 

Because of the low operating temperature, PTFE can be used for the cell seals which simplifies cell assembly. The high operating voltage, however, leads to problems of corrosion and the only current collector material which has been found to show any corrosion resistance is tungsten. If sulphur is not added to the melt, a nickel current collector can be used, but omission of sulphur adversely affects the operation of the positive electrode. The problems encountered with the positive electrode were so severe that the development of this cell was discontinued. 

The cell design is shown schematically in Fig. 8.22. Two nickel components are sealed to a metallized j3-alumina collar by thermocompression bonding the 0-alumina tube is then attached to the alumina collar by a glass seal. This sub-assembly can then be welded to the prismatic metal can. A metal wick inserted in the steel can, before the sub-assembly is welded in place, acts as the cathode for sodium deposition when the cell is charged. The positive electrode is added as a mixture of nickel powder, salt and aluminium powder, and after addition of the molten sodium chloro-aluminate the cell is sealed by welding on the positive terminal... 

The electrode assembly shown (Figure 1) is fairly versatile and has been used by the submitter in flasks with electrolyte volumes of ca. 40 mL to 4 L. Additionally, the platinum electrodes may be replaced by other electrodes that fit directly into the thermometer adaptor, e.g., commercially available A in. graphite or stainless-steel rods. In the present example the electrodes are positioned vertically and are of opposite polarity. In other cases they may be positioned horizontally (parallel to a mercury cathode) and are both anodic. 

The cell consists of a commercially available four-necked, 500-mL, round-bottomed flask equipped with a 34/45 standard-taper joint electrode assembly (Note 1), a 24/40 standard-taper joint purge and vent assembly, a mercury pool cathode (Note 2), a cathode contact (Note 3), a magnetic stirring bar (Note 4), and thermometer (inserted in a 10/18 standard-taper joint neck). The two platinum anodes of the electrode assembly (Note 1) are positioned in a horizontal plane ca. 1 cm above (Note 4) the mercury (Cathode) surface. 

The electrode assembly (Note 1) is constructed the platinum electrodes are positioned vertically, parallel, and about 5 mm apart by careful bending of the lower platinum wire connections. 

---