High-Performance Near-Neutral Zinc-Air Batteries Enabled By Two-Step Electrochemical Pre-Treatment
Authors: Lai Chi-Yu , Tseng Kai-Yu , Hu Chi-Chang 
Published: December 2024

The rapid development of portable electronics and emerging electric vehicles has created a pressing need for electrical energy storage systems. Among these systems, rechargeable zinc-air batteries (ZABs) have emerged as a compelling candidate due to their high theoretical energy density of 1218 Wh k-1, coupled with inherent safety features, cost-effectiveness, and environmental sustainability. The efficiency of ZABs is significantly boosted by alkaline electrolytes, known for their high ionic conductivity, which improves charging and discharging processes. However, the stability and cycle life of ZABs suffer due to issues like zinc dendrite growth and carbonate salt formation, arising from the side reaction with electrolytes. Recently, the substitution of traditional alkaline electrolytes with near-neutral electrolytes has gained attention, offering a pathway to alleviate the aforementioned drawbacks.

A pivotal area of research in advancing near-neutral zinc–air batteries (ZABs) centers on the development of bi-functional catalysts. This focus stems from the significantly reduced catalytic efficiency for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in near-neutral electrolytes compared to their performance in alkaline counterparts. Notably, these electrocatalysts are typically deposited onto a gas diffusion layer (GDL) to facilitate both mass and electron transport. The characteristics of the GDL play a critical role in influencing the battery's overall efficiency due to its involvement in the three-phase reaction. Despite its importance, there has been a scarcity of research dedicated to exploring the properties and optimization strategies of the GDL, highlighting a gap in the current understanding and development of high-performance near-neutral ZABs.

In this study, we introduce a straightforward two-step electrochemical pre-treatment method aimed at enhancing the functionality of both the commercial GDL and the catalyst. Initially, the commercial GDL undergoes electroactivation in an acidic environment, as evidenced by an uptick in –OH and –C=O signals in Fourier-transform infrared spectroscopy, indicative of a rise in oxygen-containing functional groups. This alteration significantly decreases the electrolyte's contact angle with the GDL from 130 to 60 degrees, signifying enhanced hydrophilicity. Further analysis via Raman spectroscopy reveals an augmentation in defective surface areas, advantageous for electrochemical processes. Subsequently, the bi-functional catalyst, Ru0.7Sn0.3O2, was drop-coating on the activated GDL and underwent a secondary electroactivation pre-treatment. The SEM results show that the modification substantially generates the porous structure of the catalyst and thus increases the electrochemically active surface area, dramatically enhancing the activity of the catalyst.

To assess the impact of the electrochemical pre-treatment, we performed electrochemical evaluations on ZAB full cells, comparing air cathodes that underwent pre-treatment (activated ZAB, denoted as ac-ZAB) with those that did not (original ZAB referred to as o-ZAB), within a chloride-based near-neutral electrolyte environment. Cyclic voltammetry testing highlighted the enhanced bifunctional catalytic efficiency of Ru0.7Sn0.3O2, with ac-ZAB showing stronger cathodic ORR and anodic OER signals in comparison to the o-ZAB. Furthermore, the galvanodynamic discharge curve along with its power density curve (Figure a) revealed that the ac-air has a much higher power density of 38 mV cm-2, surpassing the o-ZAB (15 mV cm-2). It is noteworthy that ac-ZAB exhibits much smaller activation polarization in comparison to o-ZAB, indicating the effectiveness of the pre-treatment. In long-cycle galvanodynamic charge-discharge tests (Figure b), the ac-ZAB showcased improved performance, demonstrating lower charging voltages and higher discharging voltages (1.98 V and 1.15 V, respectively), resulting in a reduced voltage gap of approximately 0.83 V. The battery incorporating ac-ZAB also exhibited remarkable stability, maintaining performance over 10 days. These outcomes underscore the pre-treatment’s ability to significantly boost bifunctional oxygen electrocatalytic activity, surpassing untreated counterparts and setting new benchmarks in the field.

To evaluate the wide application of this pre-treatment approach, we assembled a range of battery models incorporating the pre-treated cathode. Notably, the flexible near-neutral ZABs exhibited a significant enhancement in power density (25 mV cm-2) alongside a narrowed voltage gap (0.98 V). Additionally, its application to alkaline ZABs also significantly improves the battery performance, affirming the pre-treatment's utility across different battery chemistries. Various GDLs and catalysts were also subjected to the pre-treatment process. The subsequent material analyses and electrochemical performance assessments indicated successful activation, as evidenced by improved functional characteristics. This investigation offers a straightforward yet innovative methodology for augmenting the air electrode design, contributing to the development of high-performance ZAB.

Figure 1