Archimedes-Based Glycerol Displacement for Electrode Porosity Measurement in Lead-Acid Batteries

This video demonstrates the porosity measurement of lead acid battery plates using the glycerol displacement method based on Archimedes'principle. The lead acid battery is one of the oldest commercial rechargeable batteries. When a French physicist, Gaston Plante in 1859, demonstrated the working principle of the battery by having two lead plates submerged in sulfuric acid, it was only 20 years later when improvements in the technology was made, when pasting of the plates was a possibility and one could discharge and charge these plates reversibly.

Even at these early stages of the invention, the importance of the exposed surface area and the porosity of the material in the battery were realized, where the greater the surface area of the material would give a battery with more capacity. At the time, the commercialization of the battery found a unique application in emerging automotive industry, where it managed to help start the internal combustion engine of not only passenger vehicles, but also larger engines that were used in the military, such as tanks and aeroplanes. In the 1950s, we saw the introduction of the sealed lead asset battery that made use of either gel or glass mat separators.

The technology helped to limit the emissions of hydrogen gas during the recharging process. These types of batteries can also be manufactured into various sizes, from small batteries for computer backup systems to large truck batteries or solar farms. As manufacturing technologies improved over the years, so that the advancements in chemical analysis and the understanding of the processes that occur in the battery.

There will always be a need to improve the manufacturing process to ensure that a consistent product that is not only price competitive, but also of good quality that can provide electricity for long periods of time. In terms of the chemistry that occurs in the battery, it is referred to as a double sulfate theory and unique when compared to other battery types. In this case, the electrolyte, or the acid, forms part of the reaction mechanism during the discharge and charge process.

Analysis of the active material inside a battery has always been challenging, since by its very nature, the analysis is destructive. The battery has to be cut open and the plates disassembled. This can then only provide a snapshot in time of what the battery state or condition is.

This usually relates to the scientists wanting to know the battery state of charge and state of health. Besides a large range of chemical analysis that can be done on the materials, physical properties include the strength of the plates, the surface area, and the porosity of the active material. Surface area analysis is typically done by nitrogen absorption techniques, and in this study, we will look at measuring the porosity of the plates used in the lead acid battery.

Porosity relates to the available free space that is in the active material for the acid to move freely in and to be in contact with the active sites that will allow for an electrochemical reaction to take place. Traditionally, porosity of solid materials that have micro mesoporous are measured by mercury porosimetry. This required the active material to be physically removed from the plates in order for it to fit into the instrument sample holder.

This has a number of disadvantages, namely, only a representative sample from a plate area can be taken. The sample is such as disconnected from the grid or current collector, thereby inducing additional cracks and cavities into the material, and many laboratories have moved away from the use of mercury due to its toxicity. The advantage of the technique is that it can give you an idea of the pore size distribution of the material within a certain range.

The glycerol displacement method is a technique that is based on Archimedes'principle, and it uses glycerol as displacement medium. According to Archimedes'principle, the apparent loss in mass of an immersed body is equal to the mass of liquid displaced. From this, we can determine the volume of the immersed plate, the volume of the pores, and calculate the average porosity of the active material on the plate.

It is a simple and reliable method that makes use of non-toxic substances and can be used to measure the average porosity of an entire lead acid battery plate. It can measure a range of plates, types, and sizes, and it's suitable for use both in an academic lab and in an industrial lab. In addition to what is shown in this protocol, microstructural analysis using powder x-ray diffraction helps to better understand the electroactive materials inside the battery.

Begin by carefully disassembling the cold lead acid battery. Remove each battery cell and separate its components, ensuring the positive and negative plates are extracted without any alteration. After rinsing the plates with water to remove the residual acid, dry the negative plate in an oven under inert atmosphere, such as nitrogen, to prevent the oxidation of lead.

Dry at 110 degrees centigrade for six hours or until constant mass is achieved. Dry the positive plates in the oven at 110 degrees centigrade for 24 hours. Remove the plates from the oven and allow to cool in a desiccator to prevent moisture absorption.

Cut up the log of the plates and ensure the plates are dry before commencing the porosity measurement in order to record accurate masses. Fit an electronic balance in a sturdy frame structure above a container that contains glycerol of known density. Remove the dry plate from the desiccator.

Place onto the tared electronic balance and record the mass as mass of dry plate. Attach the telescopic crocodile clip to the top of the lead grid. Submerge the plate completely in glycerol, ensuring that the extended portion of the clip is kept above the liquid.

Bubbles that are trapped in the plate start to emerge. Put the lead onto the container and insert the stopper. Connect the setup to a low vacuum supply and apply a vacuum of approximately 100 mbar until the sides of the container show a slight indent, then turn off the vacuum.

This is to ensure that glycerol penetrates all the pores. Keep the plate on the vacuum for at least 15 minutes. If air bubbles are still visibly emerging from the lead plate after this period, extend the vacuum time for a little longer.

Place the balance attachment onto the tared scale and record the mass. Remove the plate from the glycerol and attach to the two crocodile clips on the balance attachment as shown. Make sure the plate is level and adjust if necessary.

Place the balance attachments with a soft plate onto the scale and let the plate hang above the glycerol for eight minutes to remove the excess glycerol that is on the surface of the plate. Wipe off any drops on the bottom of the plate Place the balance attachment on the scale and record the mass as the mass of glycerol saturated plate. Slowly raise the glycerol container by using the laboratory jack until the top of the plate is just submerged.

Record this mass as the mass of glycerol submerged plate. Repeat the experiments in triplicates. The parameters required for analyzing the percentage porosity and their formulas are defined in rows two to 12, mass of the dry plate, mass of glycerol saturated plate to be measured in triplicate, mass of glycerol submerged plates to be measured in triplicate, mass of the grid alloy, density of the grid alloy, density of glycerol, active material mass, active material absolute density, specific pore volume, bulk density, and finally, the percentage porosity, all to calculated using the formulas in column D.Input the parameters listed above in the corresponding column as shown.

Input the mass of the dry plate A into the column A.Click and drag to copy value to the adjacent cells for the triplicate. Input the triplicate masses obtained for the glycerol, saturated, and submerged plate into columns B and C respectively. Into columns D, E, and F, input the respective grid mass, grid density, and glycerol density masses.

Click and drop to copy each value to the adjacent cells for the triplicates. Input the formula for the active material mass into column G, where subtracted the grid mass from the mass of the dry plate. Click and drag to copy the value to the adjacent cells.

Input the formula for determining the absolute density of the active material into column H to compute the corresponding values. Click and drag to copy value to the adjacent cells. Input the formula for determining specific pore volume into column I to compute the corresponding values.

Click and drag to copy value to the adjacent cells. Input the formula for determining bulk density of the active material into column J to compute the corresponding values. Click and drag to copy value to the adjacent cells.

Finally, input the formula for calculating the percentage porosity of the active material mass on the plate into column K to compute the corresponding values. Click and drag to copy value to the adjacent cells. Then determine the average percentage porosity and the standard deviation of the plates by using the appropriate function on the spreadsheet.

The porosity data obtained for the negative and positive plates of three different batteries are tabulated. The bar chart shows the porosities obtained for the batteries, while the arrow bar indicates the standard deviations across the number of plates analyzed, demonstrating that the method is highly repeatable. Exile the diffraction patterns of the active materials from the positive and negative battery plates, showing chemical face compositions and their quantifications are also presented.

The glycerol displacement porosity measurement technique, notable for its ability to measure entire plates, as well as multiple plates from one or more cells, is demonstrated. The method provides a comprehensive assessment of the plates as opposed to analyzing only a portion of a single plate, making the results highly reliable.