The Effect of Surfactant Functional Groups on Corrosion and General Performance in Zinc-Air Batteries
General Overview
Electricity, and by extension the means we store it, are essential for the functioning of modern society; without it, the very computer you are using to view this site would be unable to run, nor would the lights in our offices or the transportation of our cities. However, where we obtain this energy from is an issue; we are dependent on the combustion of fossil fuels, which produces carbon dioxide that contributes to global warming, whose effects we are already seeing with events like Hurricane Maria, rising sea levels, and record heat waves. Renewable energy is promising, as it is readily available and growing cheaper each year, but the fact that power output varies wildly with the weather requires massive battery banks to stabilize overproduction and underproduction. Additionally, electric vehicles, another way to reduce carbon dioxide emissions, still face the problem of high cost and low range. Metal-air batteries, specifically zinc-air batteries, are a cheap, environmentally safe solution, being 2-5 times more energy dense than lithium-ion batteries. However, zinc-air batteries corrode internally, decreasing discharge time, shelf life, voltage, and current output, making zinc-air batteries currently non-viable.
​
With these problems in mind, the purpose of this research was to improve the performance of zinc-air batteries by applying surfactants to the basic electrolyte. Surfactants are known to have anti-corrosion properties on other metals, and with their adsorption to the zinc anode by their hydrophobic tails, the hydrophilic head, and whatever functional group may be there, controls the flow of H2O, which causes corrosion, and OH-, which reacts with zinc to produce desired electrical current.
Left: Diagram of a zinc-air battery.
Center: Sodium dodecyl sulfate, an example surfactant (sodium not shown).
Right: The general reactions in a zinc-air battery. Note that H2O causes corrosion.
As part of the procedure, electrodeposition of MnO2 as a cathodic catalyst onto the carbon gas diffusion layer (GDL) cathode was first tested and ensured, albeit with inconsistent results. Following that, cases were either constructed from polycarbonate plastic or 3D printed. The carbon GDL was glued on afterwards, and it was later discovered that an additional Porex micropore layer needed to be glued on top as well since the GDL alone is not waterproof, voiding the first round of data collection. Additionally, KOH could not be added to the battery, as it refused to form a homogeneous solution with surfactants, even with only 5 mM surfactant concentration, which is highly unusual. Regardless, a control trial with no surfactants had voltages and currents near zero. For sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS), currents and voltages increased slightly over 3-5 hours and then decreased continuously, while sodium monododecyl phosphate (SMP) varied so much as to make any patterns indiscernible. Amp-hours and watt hours per gram of zinc anode were found to compare specific capacity and energy density, respectively, and while SDBS had the highest of both, significantly intersecting standard error bars cast doubt on the data. Some surfactants, namely cetylpyridinium chloride (CPC) and lauryl glucoside, could not be tested.
​
SDBS appears to be the most promising surfactant for reducing corrosion in zinc-air batteries and improving performance, but since this was done in the absence of dissolved KOH, this must be replicated with the dissolution issue resolved. Applying the surfactant as part of an anode slurry may suffice, as might investigating other anodes with university equipment. Despite the setback, zinc-air batteries hold significant potential as a competitor against lithium-ion batteries, being cheaper, safer, environmentally friendly, and more powerful, and these developments help it become a viable solution to renewable energy grid issues and improve electric vehicles.
