Advancing sustainable energy systems: Aqueous batteries/ supercapacitor hybrids and triboelectric nanogenerators

Abstract

Amid growing concerns about climate change due to excessive use of fossil fuels, world leaders from 196 countries adopted a set of targets under the Paris Climate Agreement in 2015, aiming to limit the global average temperature rise to well below 2°C above pre-industrial levels with efforts to keep it below 1.5°C and to achieve net zero carbon emissions by the second half of the century, with many countries targeting 2050. Achieving these ambitious goals necessitates transformative advancements in clean energy generation, storage, and utilization. Batteries are at the forefront of the clean energy transition, with a current global market size estimated at USD 136.6 billion and a projected compound annual growth rate (CAGR) of 16.4% through 2030. However, the specific capacity of cathode materials (such as LiCoO2, LiNiMnCoO2, and LiFePO4) in currently commercialized Li-ion batteries is typically limited to less than 250 mAh g-1. This limitation highlights the need for alternative high-capacity cathode materials. Sulfur seems to fill this void perfectly with its high theoretical specific capacity (1675 mAh g-1), along with other advantages such as low cost, high abundance, and environmentally friendly nature. Moreover, to truly transition to clean energy, we need to direct attention again toward the development of aqueous batteries, which provide safer energy systems with significantly less environmental impact compared to current non-aqueous batteries. However, the high chemical reactivity of alkali and alkaline-earth metals such as Li, Na, K and Mg limits their direct application in aqueous environments, often necessitating complex safety measures that increase system cost. In this context, Fe emerges as an attractive anode material owing to its excellent stability in aqueous electrolytes, natural abundance (4th most abundant element) and ultralow cost (110 $ ton-1). In addition, it offers a high theoretical specific capacity of 960 mAh g-1, and its volumetric capacity (7558 mAh cm-3) is approximately 3.6 times greater than that of Li (2061 mAh cm-3). These advantages make the combination of Fe anode with a sulfur cathode in an aqueous electrolyte a compelling strategy for the development of high-performance, low-cost, and inherently safe battery systems. Meanwhile, although batteries have several advantages, their relatively low power density limits their use in applications requiring a rapid surge of energy such as high-performance electric vehicles during acceleration, or energy storage systems that must respond to sudden spikes in demand. Meanwhile, supercapacitors (SCs) have ultrastable cyclic stability backed up with the high power density reaching upto 10 kW kg-1. Therefore, strategies to integrate the advantages of batteries and supercapacitors are an effective solution for designing high-performance energy systems. While the development of energy systems is important, it is even more important to keep waste generation in check as technology improves. According to the recent reports, over 2 billion metric tons of municipal solid waste is generated annually, with biowaste, plastics, and electronic waste (E-waste) as the major components. The improper dumping of waste has dangerous consequences for land animals, sea creatures and the environment raising the serious threats to the sustainability of the planet. Therefore, effective approaches that reduce waste accumulation and promote its recycling and repurposing for energy generation are desirable. This thesis presents a holistic approach to addressing above mentioned challenges through the design and development of sustainable, cost-effective, and high-performance energy storage and harvesting devices. It is structured around three critical themes: advanced aqueous iron-sulfur battery technologies, battery-supercapacitor hybrid (BSH) systems, and waste-driven energy harvesting via triboelectric nanogenerators (TENGs). The Chapter 1 of this thesis discusses the present energy concerns and establishes the solutions for the current major concerns for the sustainable energy systems. Chapter 2 details the synthetic and characterization methodologies involved in this thesis. Chapter 3 introduces the dimethyl sulfoxide (DMSO) as the electrolyte additive in aqueous Fe-S battery to limit the hydrogen evolution reaction (HER) side reactions at anode and promote the stripping/plating efficiency of iron by modifying the solvation shell. The motivation for selecting DMSO is primarily its high Gutmann donor number (29.8), which enables it to preferentially solvate iron ions over water molecules. Moreover, DMSO’s ability to form hydrogen bonds suppresses water activity and stabilizes the electrolyte against the HER. The use of DMSO suppressed the HER by 6.7 times and reduced the corrosion of iron by 2.2 times characterized by various electrochemical and spectroscopic techniques. The battery achieved high specific capacity of 1145 mAh g-1 and achieved 400 cycles of stability for the first time for aqueous Fe-S battery. Chapter 4 addresses the problem of poor sulfur utilization in Fe-S batteries originating from the inability of sulfur cathode to accommodate the structural changes during electrochemical cycling as the sulfur amount in the cathode host increases. The use of high sulfur content is limited by the need to reserve sufficient free volume to accommodate the substantial (~33%) volume expansion associated with solid-solid phase transformations during cycling. To overcome this challenge, high-sulfur-content cathodes were synthesized through the in-situ incorporation of sulfur into a porous matrix of nitrogen-doped carbon spheres (NCS). The sulfur/carbon composite (i-S@NCS) exhibited a high sulfur content of 86% (weight %). The i-S@NCS_86 electrode delivered an impressive specific capacity of 1260 mAh g-1 at a current density of 0.1 A g-1, corresponding to 75% sulfur utilization based on the theoretical capacity of sulfur (1675 mAh g- 1). The second section of thesis (Chapter 5) is dedicated to engineering high-capacity, cobalt-free BSH devices using transition metal phosphides and phosphates (NiVP/Pi) with high electronic conductivity as the battery type electrode. Further, high capacity battery type electrodes were integrated into flexible substrates with industrially relevant mass loadings (>10 mg cm-2). The design criterion for the electrode materials involved choosing readily available and cost effective substrate materials such as paper and cotton cloths. Electroless and electrodeposition methods were optimized to obtain highly conductive flexible and ultradurable electrodes (NiFeP@Ni/NiB-FP and NiP@Ni/NiB-CC). While, NiFeP@Ni/NiB-FP showed record high areal capacity for paper-based electrodes with 1329 hour long lasting electrochemical stability, NiP@Ni/NiB-CC showed excellent performance among the textile-based electrodes and also served as ultrastable current collector for capacitive electrodes lasting for 40,000 cycles with only 6% loss. The final section of thesis (Chapter 6) focuses on energy harvesting technologies by upcycling waste materials including plastic, E-waste, and biowaste into triboelectric components for powering small electronics through ambient mechanical motions. The triboelectric nanogenerator (e-Cu@WPP-TENG) designed from plastic and E-waste materials was able to produce high open circuit voltage (Voc) of 550 V under human hand pressing movements. While the TENG designed from combining biowaste materials (banana peels, orange peels, mango seeds and coconut shells) with biodegradable polymer (PVA) achieved the high Voc upto 1030 V. Both devices demonstrated excellent energy harvesting capabilities, enabling them to power various electronic equipment. The ultimate goal was to promote effective waste-to-energy transformation, thus reducing the burden of waste management.