Date of Award

24-4-2024

Document Type

Thesis

School

School of Chemical & Biotechnology

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

S.Devaraj

Keywords

Physical Science, Chemistry, Electrochemistry, Energy Storage, Zinc Air Batteries

Abstract

The spontaneous reaction between zinc and oxygen in an alkaline medium is exploited in non-rechargeable aqueous zinc-air batteries (ZABs) to generate useful electricity. The primary ZAB has a rich history as it was used in tramways, railway signalling and it is being used in hearing aids, pacemakers, etc. The success of primary ZABs led to the development of two types of rechargeable ZAB, namely, mechanically rechargeable ZAB (mrZAB) and electrically rechargeable ZAB (erZAB). In mrZABs, the exhausted anode and electrolyte are replaced periodically. Whereas the chemistry of primary ZAB is reversed using suitable electrocatalysts in erZABs. Electrocatalysts used in erZABs catalyse the oxygen evolution reaction (OER) during recharging and the oxygen reduction reaction (ORR) during discharging and thus improve the overall performance. Noble metals, metal oxides and carbonaceous materials are the most studied electrocatalysts for erZABs. Among these electrocatalysts, noble metals are considered a benchmark. However, the high cost and scarcity of noble metals have limited their large-scale production. This has triggered the research on developing highly active bifunctional electrocatalysts from earth-abundant materials.

On the other hand, the coulombic efficiency of erZAB is always less than 100% due to hydrogen evolution reaction (HER) and concurrent corrosion of zinc (Reaction 1 and 2). This is attributed to the fact that the standard reduction potential of Zn/ZnO (-1.26 V vs SHE) is more negative than that of HER (-0.83 V vs SHE) at pH=14.

2H2O(l) + 2e- → 2OH-(aq) + H2(g) Reaction 1

Zn(s) + H2O(l) → ZnO(s) + H2(g) Reaction 2

The concurrent corrosion of zinc not only decreases the performance of erZAB but also affects the cycling stability. Hence, the overall objective of the thesis is to develop bifunctional electrocatalysts for the erZABs and to inhibit the zinc corrosion thus improving the coulombic efficiency.

MnO2 is one of the most extensively studied transition metal oxides for erZABs and it exhibits polymorphism. There are a few reports on the effect of the crystal structure of MnO2 on the kinetics of ORR. However, the textural properties of various forms of MnO2 are not similar. In addition, the effect of crystal structure on the OER activity is not documented. The experimental conditions were carefully optimized to get different forms of MnO2 with similar textural properties and the effect of the crystal structure of MnO2 (, , ,  and ) on the kinetics of both ORR and OER was carefully investigated. The ORR activity follows the order: α-MnO2 > δ-MnO2 > λ-MnO2 > β-MnO2 > γ-MnO2 and the OER activity follows the order: α-MnO2 > δ-MnO2 > γ-MnO2 ≈ λ-MnO2 > β-MnO2. Among the five forms of MnO2, α-MnO2 with a large tunnel along with a higher percentage of oxygen-based functionalities on the surface provides more active sites for adsorption and facilitate electron transfer, thereby outperforming other forms of MnO2 towards ORR and OER.

Among various crystallographic forms, α-MnO2 exhibited the best catalytic activity towards ORR and OER. However, its performance is still inferior to the benchmark ORR and OER catalysts. To improve the electrocatalytic activity of α-MnO2, the surface oxygen vacancies (OVs) were tailored by heat treatment and by doping. On heating in air, the surface OVs of -MnO2 are found to increase. For instance, the surface OVs of -MnO2 heated at 500 °C in the air is more than 3 times of surface OVs of as-prepared MnO2. While the ORR activity of -MnO2 heated at 500 °C in air is comparable to the benchmark ORR catalyst, Pt/C, the OER activity is inferior to the benchmark OER catalyst, RuO2. The surface OVs were further increased by doping with Cu2+. 2 wt% Cu2+-doped MnO2 outperforms both the ORR and OER benchmark catalysts.

Activated carbon (AC) is used by various industries such as chemical, agricultural, pharmaceutical, oil refineries, metallurgical, etc., for purification of water, air, deodorization, decoloration, decontamination, separation and purification of solvents and gases, sewage treatment, as energy storage materials, catalysts and catalytic supports, etc. Some of the spent activated carbon (SAC) from the industries are either regenerated or co-fired with coal to generate energy. But, much of the waste is either illegally burnt, thereby contributing to air pollution and smoke haze, or simply left to decay in dedicated landfills emitting potent greenhouse gases. Following the principles of sustainable development goals, a process to convert SAC waste to nanocomposites imbued with electrocatalytic activities is demonstrated in chapter 4. The SAC from an exhausted water filter is converted into MnO2/C nanocomposites by hydrothermal reaction with KMnO4 of different concentrations and used as electrocatalysts for ORR and OER. MnO2/C nanocomposite synthesized using KMnO4:SAC ratio of 2:1 demonstrates a good ORR (onset potential: 0.842 V and current density: -8.91 mA cm-2) and OER (overpotential of 425 mV to reach 10 mA cm-2)activities.

Surgical facemasks (SFMs) were used extensively to protect from COVID-19 during a pandemic. The used SFMs are a serious threat to the environment. In general, used SFMs are incinerated or dumped in landfills. Current methods like incineration, steam treatment, plasma treatment and microwave treatment yield minimal carbon. Therefore, there is a growing need to upcycle waste SFMs into value-added products through safe, environmentally benign processes. Given this, the research community explored a few methodologies to repurpose waste SFMs, tackling their environmental impact. Although these methods are effective, most of them utilize only the outer layers of SFMs. In addition, the surface area and porosity are less compared to the commercial AC. In chapter 5, a method to upcycle waste SFMs into very high surface area carbon is demonstrated. The electrocatalytic activities of carbon derived from waste SFMs towards ORR were studied in an alkaline medium. On increasing the activation temperature, the porosity and ORR activity increases. Consequently, carbon derived from upcycling of SFMs with an activation temperature of 950 °C exhibits a very high surface area of 2163 m2 g-1 and excellent ORR activity, similar to benchmark, ORR catalyst, Pt/C.

The performance of erZABs not only depends on the bifunctional electrocatalyst deployed at the air-cathode but also on the HER occurring at the anode. While the rate of ORR and OER improve the performance of ZAB, HER causes zinc corrosion and thereby affects the performance of ZAB. To mitigate HER and concurrent Zn corrosion, the onset and overpotential of HER are increased by the uniform dispersion of carbon nanodots or metal oxide nanoparticles in the native electrolyte (6 M KOH), thereby reducing zinc corrosion. Dispersion of carbon nanodots or 0.1 wt% of SiOx or ZnO nanoparticles increases the HER overpotential to 321, 547 and 679 mV, respectively. The microscopic investigations confirm the inhibition of corrosion and dendritic growth of zinc. Besides these, nanofluid electrolytes enhance the kinetics of ORR and OER, thereby improving the performance of erZABs. Also, nanofluid electrolytes are stable over three months.

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