Elsevier

Catalysis Today

Volume 370, 15 June 2021, Pages 66-74
Catalysis Today

Interplay of the functional units of a binder in the oxygen reduction process of zinc-air battery

https://doi.org/10.1016/j.cattod.2020.09.022Get rights and content

Highlights

  • Functionalised Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene (f-SEBS) is developed and used as binder.

  • Anionic binders, Fumion and f-SEBS substitutes the expensive Nafion binder in alkaline ZAB.

  • Hydrophobic filament, hydrophilic branching with ionic conductivity formulates an effective cathodic binder.

  • Fumion is the most suitable binder for the state-of-art Pt/C catalyst.

Abstract

Zinc-air battery (ZAB) technology is a strong competitor in the automobile sector for powering automotive. Alas, there are a few issues that restrain them from being commercialized. One prominent issue which has not been discoursed thus far in alkaline ZAB is the impact of the type and content of the binder. The present work incorporates a study conducted to get insights on the impact of eleven different polymeric binders in the air electrode of an alkaline zinc-air battery. They include anion and cation conducting polymers, hydrophilic and hydrophobic polymers. The galvanostatic polarisation curves reveal a huge variation depending on the type of binder. Fumion ionomer, an anion conducting material, shows the highest current density of about 163 mA cm−2 at 0.6 V owing to the right proportion of hydrophilic-hydrophobic segments and ion-conducting nature. A specific capacity of 770 mA h g−1 was obtained when Fumion was employed as a binder with 1 mgPt cm−2. Eventually, the upshot of the experiment motivated us to try a home-made less commonly known anionic binder, functionalised-Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene, which performed on par with Nafion™ and Fumion at a practical operating voltage of ∼1.2 V.

Introduction

There is a general thrust for the development of electric vehicles which can serve as a good solution to reduce our dependency on gasoline. Currently, lithium-ion batteries (LIBs) has been conceived as the energy storage medium in electronic devices. However, the practical energy density of LIBs is not sufficed to achieve the long-term goal of the Electric Vehicle (EV) sector. Therefore, enormous efforts have been invested in research to hit upon novel energy storage technologies with higher energy densities. Accordingly, zinc-air batteries have been the centre of attraction with a theoretical energy density of 1089 Wh kg−1 (including oxygen) which is about five times higher than that of state-of-the-art LIBs (200–250 W h kg−1) [[1], [2], [3]]. Amongst the various metal-air batteries that have been explored, ZAB is a smart choice because of the abundant availability of environment-friendly zinc metal on the earth's crust, their high stability towards moisture and air leading to an easier and cheaper manufacturing process [[4], [5], [6]]. Moreover, the employment of less volatile and non-combustible materials in ZAB brings down the safety hazards. Despite many boons, sluggish kinetics and high overpotential of the oxygen reduction reaction (ORR) poses serious challenges that hamper the advancement of ZAB [7,8]. From the commercialization point of view, a lot of attention is paid on developing precious metal-free electrocatalyst. On that note, a report by Wang et al. sheds light on the promising nanocarbon based air catalyst [9]. But the downside associated with such carbon electrodes is their corrosive trait. Perhaps, substituting the corrosive carbon supports by non-corrosive and chemically stable supports is likely to tackle this snag. On the basis thereof, Devendrachari et al. had developed a more stable, durable and corrosion-resistant air cathode by exploiting TiN as catalyst support [10]. Apart from the slow kinetics, one another prominent matter associated with the air cathode, which is less explored, is the binder. Binders are polymeric substances, mainly recognized as an adhesive, employed to secure the active materials onto a current collector. Besides that, it plays a vital role by rendering an ion-conducting pathway for the migration of charge carriers. Additionally, to avoid the concentration polarisation at the air cathode, binders should be able to dissolve the gaseous reactant, O2. In general, the Nafion™ binder is being widely opted for air cathodes of ZAB, but there is no evidence reported in the literature supporting the choice of selection of the binder and it is an expensive material. There are earlier reports on examining the role of binder in the air cathodes of Na-air and Li-O2 battery. However, all metal-air battery systems despite following the same principle, do not share any common electrolytic medium and electrochemical behaviour. Sun et al. [11] had conducted a study with polymeric binders viz. polyethylene (PE), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), poly (methyl methacrylate) (PMMA). Of the four binders, PE was identified to be the best for Na-air as PE air cathode was stable against the discharge products contributing to the improved cycle life. Gainey et al. [12] have found PEO to perform well in Li-O2 battery amongst polyvinylidene difluoride (PVDF), PVP, PEO and polytetrafluoroethylene (PTFE) binders and this was reflected in terms of high discharge capacity. A similar binder effect is also been explored in microbial fuel cells (MFC). Cheng and his team [13] have examined the effect of binder on the power density of MFC and observed a decrease in power density from 480 ± 20 mW m−2 to 360 ± 10 mW m−2 with the replacement of Nafion™ by PTFE. Saito et al. [14] have explored the influence of the hydrophilic character of the binder on cathode performance of MFC and they have experienced nearly 15 % enhancement in power density with a hydrophilic binder (polystyrene-b-poly(ethylene oxide)) compared to hydrophobic binder (polystyrene-OH). These insights bring in the necessity to study every battery system individually concerning the binder. On the other hand, the development of a binder-free electrode [15,16] is also probed simultaneously. But omitting the binder cannot favour a battery in achieving good performance as it may lead to sophisticated fabrication, loss of flexibility of the electrode, increase in contact resistance due to poor adhesion property of the catalyst thereby lowering the efficiency of the electrode. Thus, it is noteworthy to analyse the properties of binders as the type of the binder can greatly influence the overall electrochemical performance of the batteries.This report brings in a detailed study on the effect of differently functionalised binders, falling into the category of either anionic, cationic or non-ionic. Fumion [17], Nafion™, [[18], [19], [20]], PVDF [21], PTFE [22], carboxymethylcellulose (CMC) [23], polypropylene (PP) [24], PE [25], Chitosan [26], PMMA [27], and PVP [28] were considered for the evaluation on the electrochemical performance of ZAB. Understanding the functional group of the binders and its interaction with catalyst material and discharge products is very vital in the selection and development of binders for ZAB. Based on the understanding gained we now introduce the least known, functionalised, anionic binder called Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene (f-SEBS) for zinc-air application.

Section snippets

Materials

Ethanol (Changshu Hongsheng fine chemicals, 99.9 %), Methanol (Finar, 99 %), Chloroform (Spectrochem, 99 %) Chlorobenzene (Spectrochem, 99 %), Dimethyl acetamide (DMAc (Spectrochem, 99 %)), Dimethylformamide (DMF (Spectrochem, 99 %)), Acetic acid (Qualigens, 99.5 %), N-Methyl-2-pyrrolidone (NMP (Spectrochem, 99 %)), Trimethylamine (TMA (Spectrochem, 30 % aqueous solution %)), chlorotrimethylsilane (Spectrochem, 98 %), Tin (IV) chloride (Anhydrous, Spectrochem, 98 %) and p-Xylene (Sigma Aldrich,

Results and discussion

Table 2 summarizes the contact angle data of the cathode and the ionic conductivity of the binders. Fig. 3 contains images of the contact angle measurement. Electrodes made using the binders viz. PP, PE, Nafion™, PVDF, Fumion, f-SEBS, PTFE, and PMMA have displayed a contact angle above 100° and prove themselves to be hydrophobic while PVP, CMC and chitosan-based ones have angles below 50° that confirm their hydrophilic nature. A few interesting properties projected out by the functional groups

Conclusion

Electrochemical investigations have revealed a huge variation in the ZAB performance concerning binder. The right combination of hydrophobic filament, hydrophilic branching and fine ionic conductivity of cathodic binder can synergistically reinforce a cathodes performance in ZAB. On switching to Fumion from the conventionally used Nafion™, we have observed less activation polarisation accompanied by a 4.6 % up-gradation in specific capacity of the ZAB. On learning the point that anionic binder

CRediT authorship contribution statement

L.K. Nivedha: Resources, Data curation, Writing - original draft. M. Raja: Conceptualization, Formal analysis, Investigation, Methodology. Kothandaraman Ramanujam: Funding acquisition, Supervision, Validation, Visualization, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author LKN greatly acknowledges the financial support from the Ministry of Human Resource and Development (MHRD), India, Author KR acknowledges the funding through grant no. DST/TMD/MECSP/2K17/20 from Department of Science and Technology (DST), India and Exploratory Research Project from Indian Institute of Technology Madras, Chennai, grant No. CY1920897RFER008477. Author MR acknowledges the National Post Doctoral Fellowship (NPDF) grant no. DST-SERB-NPDF (No. PDF/2017/001756), R. Rajaram

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