Effects of Anionic Surfactant Sodium Dodecyl Sulfate on Electrochemical Performances of Zinc-Air Flow Batteries
Authors: Hosseini Soraya , Kheawhom Soorathep 
Published: February 2020

Zinc-air batteries are promising for various future energy application. Nevertheless, low efficiency of zinc utilization in zinc-air batteries limited their broad applicability. Therefore, extensive efforts have been carried out to address this issue by either improving a zinc anode or an electrolyte. Different approaches were reported to enhance the performance such as utilizing gel-type electrolytes, coating zinc with various chemicals and polymers and employing organic additives of the electrolyte. The electrolyte has a significant impact on the performance and cycle life of the batteries.
Potassium hydroxide (KOH) has been extensively used as the electrolyte for Zn-air batteries due to its high ionic conductivity, low viscosity, and high oxygen diffusion coefficient. However, the usage of KOH revealed several technical issues such as corrosion and passivation of the zinc anode and carbonates formation leading to the battery performance deterioration.
This work aims to examine the effect of utilization an anionic surfactant (sodium dodecyl sulfate, SDS) in 7 M KOH aqueous solution electrolyte on the discharge performance of zinc-air flow battery using granular zinc anode. SDS was implemented in two methods. The first method is to directly add SDS (0.0173 - 0.268 mM) to the electrolyte (zinc-KOH/SDS). The other method is to coat zinc granular using 1 M SDS (SDS/zinc-KOH). The effects of both methods on electrochemical properties and discharge performance of the batteries were then investigated. By using zinc-KOH/SDS, an 11% increase in discharge capacity over the electrolyte without SDS was observed. However, SDS/zinc-KOH had adverse effects on the battery performance by preventing zincate formation and leading to a significant drop in discharge voltage and capacity.
A comparison of polarization characteristics showed that the presence of SDS caused an increase in the current for both methods. Nevertheless, by adding SDS to KOH solution ranging from 0.0173 to 0.268 mM, an increase of discharge current from 160 to 212 mA at 1 V was observed with an increase from 14 to 51% of discharge current. Besides, a 14% increase in discharge current was revealed for SDS coated zinc granular. The optimal amount of SDS was then determined. Cyclic voltammetry of three types zinc-KOH, zinc-KOH/SDS and SDS/zinc-KOH showed the main difference between two methods. Zinc-KOH and zinc-KOH/SDS demonstrated similar anodic peaks including zinc dissolution corresponding to zincate formation and zinc oxide formation whereas for SDS/zinc-KOH the peak related to zincate formation was not observed. Moreover, the anodic peak slightly shifted to the positive potential direction, and the peak intensity of zincate formation increased when SDS was introduced to the electrolyte. Electrochemical impedance spectroscopy revealed that the presence of SDS for both methods increased the charge transfer resistance in comparison with Zinc-KOH, confirming that aggregation of surfactant molecules on the zinc surface or interface KOH/zinc raised the charge transfer resistance.

Figure 1