Chapter 11
Single Cell Empirical Data
11.1 Design and Construction of Cells and the Materials Employed
Concentration cells offer opportunities of many design forms. The materials employed can be fashioned in many shapes and sizes. There are no basic problems in producing devices with very special characteristics. Materials are most easily taken as flat sheets for electrodes because that configuration lends itself to a host of design application requirements.
The simple parallel plate electrode design approach is probably the easiest to describe and represent in drawings. For that reason alone, the configuration descriptions employed here will be confined to such designs. Other cell and battery shapes are certainly possible if required for particular applications where, for example, cylindrical forms would offer advantages over rectangular structures. The following are the principal components and materials for construction:
- Carbon for electrodes
- Porous plastic sheets as separators
- Cation exchange membranes
- Plastic containers for cell or battery assemblies
- Electrical contacts for external connections
- Microporous carbon with large surface areas.
The electrodes are basically graphite plates with a “surfacing” of microporous carbon frequently referred to as activated charcoal. This layer of active carbon provides very large surface areas for storing reactants as well as electrochemical reaction sites.
This general design is shown as an edge view of a cell in Figure 11.1. The simple sandwich structure has the entire intervening space between electrodes, other than the presence of a separator, occupied by porous carbon. The prime function of the separator, as mentioned earlier, is to keep the carbon particles from electronically short circuiting the cell and prevent, or at least retard, the bulk mixing of electrolytes in the two cell compartments. A very porous, non-conductive material such as porous polyethylene or PVC can serve that purpose. Unfortunately, mechanically porous materials will not prevent the diffusion of electrolytes sufficiently to give cells high charge retention. Hence, in most instances sheets of ion exchange materials are employed rather effectively. Electrical resistance is increased, but the benefits appear to warrant that increase. As the development of concentration cells progresses, it may be possible to eliminate relying mainly upon separators or membranes to retard diffusion (primarily of S= ions) from migrating to lower concentration sides. This may be accomplished by making greater use of the reactant storage within the cell in the adsorbed state and, perhaps, as solids waiting to become solubilized when needed for charging and discharging.
Figure 11.1 Edge of an assembled cell.
An exploded view of this cell is provided in Figure 11.2, and typical dimensions for laboratory test cells are shown in Figure 11.3.
Figure 11.2 Exploded view of cell components.
Figure 11.3 Internal dimensions typical of present cells.
Cells of the above dimensions are the easiest to construct and are the most practical as experimental vehicles for studying their basic characteristics. Inter-electrode spacing is amenable to the insertion of porous carbon granules even after cell assembly if an open top is provided. In most laboratory test cells, the carbon is contained within a plastic frame. The cells are assembled in a series of steps with the array being in a horizontal position.
The first step is to adhere a frame to the graphite plate electrode using either an epoxy resin or RTV sealant, depending on whether the cell is to be used as a permanent structure or as a structure that can be disassembled at a later time to either change components or materials, such as the type of porous carbon or membrane. There are advantages to both designs. For example, a cell that is capable of being disassembled also provides the opportunity to examine its internals after cycling. These differences are shown in Figures 11.4 and 11.5.
Figure 11.4 Open top disassemblable test cell.
Figure 11.5 Encapsulated laboratory cells.
The next step is to fill the space in the frame with the particles of carbon. Then, the membrane or separator is adhered to the frame. The second frame is then glued to the membrane, and the void provided by the second membrane is filled with carbon particles. The final step is to adhere the second electrode to the frame. If needed, the entire assembly can be clamped together in order to maintain integrity, and the cell can be stood upright for normal operation. At some point during the fabrication of the cell, wires are attached to the carbon plates by metal screws and wire lugs through holes drilled in the plates.
Two holes are provided in the tops of the frames (the top when in the upright position) for filling with electrolyte prior to electrical cycling. In some cases, especially where the initial electrolyte contains solids such as sodium sulfide and free sulfur, the electrolyte composition is physically mixed with the porous carbon and placed into the frame voids prior to assembly.
11.2 Experimental Data
Over fifty cells have been fabricated and tested over a five-year period. Some of these cells employed graphite plates as described above, and others used electrodes fabricated by compression molding of graphite and rigid plastic binders such as high density polyethylene, PVC, polypropylene, and ABS.
This electrical performance data is presented here in order to give the reader a better idea of the behavior of such concentration cell systems. A typical, small laboratory cell performance is shown in Figure 11.6. The total volume of the cell is about 5 in3 and the working electrode area is 10 in2. Charge and discharge curves of cell volts and cell voltage versus time are reproduced. In this instance, the discharge is into a constant resistive load while the charging takes place at near-constant voltage. As can be observed, the voltage drops off rather rapidly, and the amperage climbs proportionately. These curves are not as sharp as those obtained by maintaining a constant power level at discharge because the demands on the current are considerably less.
Figure 11.6 Sodium sulfide cell of 10 sq. in. electrode area.
The very rapid decline in cell potential during the beginning of discharge is evident. This is due to the rapid dissipation of sulfide ions (and the consequent drop in concentration) on the concentrated side while the diluted side quickly experiences an influx of sulfide by diffusion across the separator as well as by the creation of sulfides due to the discharge electric current. Not much change is required in these concentrations at the peak of full charge to make a great change in the ratio of ion concentration. Hopefully, these conditions will be significantly improved as we begin to rely more upon the storage of reagents in reservoirs in the solid state.
Figure 11.7 shows that open circuit voltages were automatically recorded at different intervals during cycling. The mode of charge/discharge should be obvious upon inspection. This data was taken at constant load, and the peaks of the brief open circuit blips indicate little polarization effects.
Figure 11.7 Cycle number 76 of test cell #S–11.
Until the techniques for reagent storage are developed to a practical stage, the electrical characteristics will be about the same. Merely charging the cells to greater concentration differentials by producing very high concentrations at one electrode and very low concentrations at the other with all reagents in the liquid phase (dissolved state) will not result in significant improvements. There is very little equivalent storage capacity at the low concentration electrode, and molecular diffusion will quickly raise its concentration level. In order to achieve higher energy storage capacities, it is necessary to develop highly dependable, solid-state storage, as discussed in previous chapters.