The Voltage Drop in the Electrolyte
The picture below depicts an anode immersed into the electrolyte, the average distance of the anode bottom from the metal pad, called ACD, Anode to Cathode Distance, the thickness of the CO2 bubbles layer underneath the anode, indicated with the greek letter d and the thickness of the layer of bubbles adhering to the anode, da.
This layer of bubbles is very important because it affects several aspects of the pot performance like:
- Current Efficiency
Talking about the voltage drop in the electrolyte, it is fundamental to point out that the current does not flow inside the CO2 bubbles. This implies that in calculating the voltage drop in the electrolyte we need to consider the actual areas available for the current to flow between the anode and the metal pad.
As depicted above, we have a layer of bubbles adhering to the anodes with a thickness da, while the thickness of the whole bubble layer is equal to d.
Therefore, using the first and the second Ohm’s law and indicating the electrolyte conductivity with χ, the electrical resistivity ρ=1/ χ and the line current as I, we have:
- Aa is the total area of the anodes, i.e. the area of a single anode multiplied by the total number of anodes present in the pot
- Aab is the total bottom area of the anodes free from bubbles adhering to them, considering all the anodes present in the pot
- Afb is the total average cross-sectional area of electrolyte free from bubbles, considering all the anodes present in the pot
In some papers the amount of anode area covered by bubbles, Aab, adhering to them and the cross sectional area available for the current to flow in the bubble layer, Afb, is expressed as a function of the alumina concentration in the electrolyte.
More in general, the dynamic of the CO2 gas bubbles escaping from underneath the anode is very complicated and depends not only from the electrolyte composition but also from the single anodes dimension and spacing of one to the other.
From the equation above, we can understand how, with the same ACD, the presence of the bubbles layer increases the Vb voltage component through the decrement of the actual area available for the current to flow into the electrolyte.
This is the reason why in recent years most companies have developed the so called “slotted anodes” technology with the aim to reduce the bubbles layer thickness, hence reducing the VB voltage component or, working with the same VB, increase the ACD to improve the current efficiency.