Research Review on Synthesis of Biowaste Graphene Quantum Dots for Supercapacitor Applications

1. Introduction

Figure 1. Graphical abstract of Graphene Quantum Dots for Supercapacitor Applications.

Figure 1. Graphical abstract of Graphene Quantum Dots for Supercapacitor Applications.

The rapid advancement of technology and increasing energy demands have spurred significant interest in developing efficient and sustainable energy storage devices [1]. Supercapacitors have emerged as promising candidates due to their high power density, fast charge-discharge rates, and long cycle life[2]. However, to fully harness their potential, innovative materials with exceptional electrical properties are required[3].

Graphene quantum dots (GQDs) have garnered considerable attention for their unique attributes, including high surface area, excellent electrical conductivity, and tunable electronic properties[4]. These nanoscale fragments of graphene combine the advantageous characteristics of both graphene and quantum dots, making them ideal for a variety of applications, including energy storage, sensing, and optoelectronics[5].

Traditionally, the synthesis of GQDs involves complex and costly methods that often rely on hazardous chemicals[6]. In response to environmental and economic concerns, researchers have explored the use of biowaste as a precursor for GQD production. This approach not only provides a sustainable and cost-effective alternative but also addresses the pressing issue of biowaste management.

Biowaste, which includes agricultural residues, fruit peels, leaves, and other organic waste, is abundant and renewable[7]. Utilizing these materials for the synthesis of GQDs aligns with the principles of green chemistry and circular economy, promoting the conversion of waste into valuable products[8]. The process of converting biowaste into GQDs typically involves methods such as hydrothermal treatment, chemical exfoliation, carbonization, and microwave-assisted synthesis[9].

The biowaste-derived GQDs exhibit properties that are highly beneficial for supercapacitor applications[10]. They offer numerous active sites for charge storage, enhance electrical conductivity, and contribute to the structural stability of the electrodes[11]. As a result, supercapacitors incorporating GQDs demonstrate improved energy and power densities, as well as excellent cycling stability[12].

This review aims to provide a comprehensive overview of the synthesis of GQDs from biowaste and their application in supercapacitors. It will cover the various synthesis methods, the characterization of GQDs, and their performance in supercapacitor devices. Additionally, the review will address the challenges and future prospects in this burgeoning field, highlighting the potential of biowaste-derived GQDs to revolutionize energy storage technologies.

2. Synthesis Methods

The synthesis of GQDs from biowaste involves several methods, primarily categorized into top-down and bottom-up approaches. The selection of biowaste material and the synthesis method significantly influence the properties of the resultant GQDs[13].

2.1. Top-Down Methods

Table 1. Top-Down Methods.

Method	Process	Examples	Advantages	Challenges
Hydrothermal and Solvothermal Processes	Biowaste is dispersed in water or an organic solvent and heated in a sealed vessel (autoclave) under high-pressure and high-temperature conditions.	Orange peels, citric acid from fruit peels, spinach leaves.	Simple setup, environmentally friendly, controlled size and functional groups.	Requires precise control of reaction conditions, limited scalability due to autoclave capacity.
Chemical Exfoliation	Carbon-rich biowaste is treated with strong acids or bases to exfoliate it into graphene layers, which are then broken down into GQDs.	Cow manure, soybean waste, sugarcane bagasse.	High yield, straightforward process.	Use of hazardous chemicals, potential impurities or defects in GQDs.

Table 1. Top-Down Methods.

Method	Process	Examples	Advantages	Challenges
Hydrothermal and Solvothermal Processes	Biowaste is dispersed in water or an organic solvent and heated in a sealed vessel (autoclave) under high-pressure and high-temperature conditions.	Orange peels, citric acid from fruit peels, spinach leaves.	Simple setup, environmentally friendly, controlled size and functional groups.	Requires precise control of reaction conditions, limited scalability due to autoclave capacity.
Chemical Exfoliation	Carbon-rich biowaste is treated with strong acids or bases to exfoliate it into graphene layers, which are then broken down into GQDs.	Cow manure, soybean waste, sugarcane bagasse.	High yield, straightforward process.	Use of hazardous chemicals, potential impurities or defects in GQDs.

2.2. Bottom-Up Methods

Table 2. Bottom-Up Methods.

Method	Process	Examples	Advantages	Challenges
Carbonization and Pyrolysis	Thermal decomposition of biowaste at high temperatures in an inert atmosphere (nitrogen or argon) to form carbon structures, further processed into GQDs.	Chicken feathers, peanut shells, coconut shells.	Inexpensive, abundant materials, good electrical conductivity.	High energy consumption, potential non-uniform GQDs.
Microwave-Assisted Synthesis	Microwave radiation rapidly heats and decomposes biowaste into GQDs, involving mixing the biowaste with a solvent and irradiating with microwaves.	Banana peels, sugarcane bagasse, coffee grounds.	Fast, energy-efficient, scalable, desirable size and surface properties.	Requires precise control of microwave parameters to avoid over-heating or incomplete decomposition.

Table 2. Bottom-Up Methods.

Method	Process	Examples	Advantages	Challenges
Carbonization and Pyrolysis	Thermal decomposition of biowaste at high temperatures in an inert atmosphere (nitrogen or argon) to form carbon structures, further processed into GQDs.	Chicken feathers, peanut shells, coconut shells.	Inexpensive, abundant materials, good electrical conductivity.	High energy consumption, potential non-uniform GQDs.
Microwave-Assisted Synthesis	Microwave radiation rapidly heats and decomposes biowaste into GQDs, involving mixing the biowaste with a solvent and irradiating with microwaves.	Banana peels, sugarcane bagasse, coffee grounds.	Fast, energy-efficient, scalable, desirable size and surface properties.	Requires precise control of microwave parameters to avoid over-heating or incomplete decomposition.

3. Properties and Characterization

Table 3. Properties and Characterization.

Property	Description	Characterization Technique	Details
Size and Morphology	The dimensions and shape of the GQDs.	Transmission Electron Microscopy (TEM)	TEM provides detailed images, allowing for precise measurement of GQD size and observation of their morphological characteristics.
Crystalline Structure	The arrangement of atoms in the GQDs.	X-ray Diffraction (XRD)	XRD is used to assess the crystalline structure, determining the presence of graphitic layers and crystalline phases.
Degree of Graphitization	The extent to which the GQDs are composed of graphitic carbon.	Raman Spectroscopy	Raman spectroscopy evaluates the degree of graphitization by analyzing the D and G bands, which indicate defects and graphitic structure respectively.
Surface Functional Groups	Chemical groups present on the surface of the GQDs.	Fourier Transform Infrared Spectroscopy (FTIR)	FTIR identifies functional groups on the GQD surface, providing information on chemical bonding and surface chemistry.
Optical Properties	The behavior of GQDs in response to light.	Photoluminescence (PL) Spectroscopy	PL spectroscopy analyzes the emission of light from GQDs, which is crucial for applications in optoelectronics and bioimaging, by studying their luminescent properties.

Table 3. Properties and Characterization.

Property	Description	Characterization Technique	Details
Size and Morphology	The dimensions and shape of the GQDs.	Transmission Electron Microscopy (TEM)	TEM provides detailed images, allowing for precise measurement of GQD size and observation of their morphological characteristics.
Crystalline Structure	The arrangement of atoms in the GQDs.	X-ray Diffraction (XRD)	XRD is used to assess the crystalline structure, determining the presence of graphitic layers and crystalline phases.
Degree of Graphitization	The extent to which the GQDs are composed of graphitic carbon.	Raman Spectroscopy	Raman spectroscopy evaluates the degree of graphitization by analyzing the D and G bands, which indicate defects and graphitic structure respectively.
Surface Functional Groups	Chemical groups present on the surface of the GQDs.	Fourier Transform Infrared Spectroscopy (FTIR)	FTIR identifies functional groups on the GQD surface, providing information on chemical bonding and surface chemistry.
Optical Properties	The behavior of GQDs in response to light.	Photoluminescence (PL) Spectroscopy	PL spectroscopy analyzes the emission of light from GQDs, which is crucial for applications in optoelectronics and bioimaging, by studying their luminescent properties.

4. Supercapacitor Applications

A supercapacitor, also known as an ultracapacitor or electrochemical capacitor, is an energy storage device that combines the properties of traditional capacitors and batteries[14]. Unlike conventional capacitors, which store energy through electrostatic charge separation, supercapacitors store energy through both electrostatic and electrochemical processes[14]. This dual mechanism allows them to have much higher energy storage capacity than traditional capacitors, while also offering faster charge and discharge rates compared to batteries.

4.1. Key Characteristics of Supercapacitors

Table 4. Key Characteristics of Supercapacitors.

Characteristic	Description	Benefits
High Power Density	Ability to deliver and accept charge quickly.	Ideal for applications requiring rapid energy bursts, such as power tools and regenerative braking systems.
Long Cycle Life	Can withstand millions of charge-discharge cycles without significant degradation.	Offers much longer lifespans compared to batteries, reducing the need for frequent replacements.
Fast Charge and Discharge	Low internal resistance allows rapid charging and discharging.	Enables quick energy availability, useful in applications like emergency power backup and peak load leveling.
Wide Operating Temperature Range	Effective operation over a broad range of temperatures.	Suitable for use in extreme environments, from cold climates to hot industrial settings.
Environmentally Friendly	Often use less harmful materials and have simpler recycling processes compared to batteries.	Reduces environmental impact and aligns with sustainable and green technology goals.

Table 4. Key Characteristics of Supercapacitors.

Characteristic	Description	Benefits
High Power Density	Ability to deliver and accept charge quickly.	Ideal for applications requiring rapid energy bursts, such as power tools and regenerative braking systems.
Long Cycle Life	Can withstand millions of charge-discharge cycles without significant degradation.	Offers much longer lifespans compared to batteries, reducing the need for frequent replacements.
Fast Charge and Discharge	Low internal resistance allows rapid charging and discharging.	Enables quick energy availability, useful in applications like emergency power backup and peak load leveling.
Wide Operating Temperature Range	Effective operation over a broad range of temperatures.	Suitable for use in extreme environments, from cold climates to hot industrial settings.
Environmentally Friendly	Often use less harmful materials and have simpler recycling processes compared to batteries.	Reduces environmental impact and aligns with sustainable and green technology goals.

4.2. Applications

Table 5. Applications.

Application	Description	Examples
Energy Storage	Supplementing batteries in hybrid and electric vehicles, renewable energy systems, and grid storage.	- Hybrid and electric vehicles for regenerative braking and acceleration. - Solar and wind energy systems for smoothing out power fluctuations. - Grid storage for load balancing and peak shaving.
Consumer Electronics	Providing quick bursts of energy in various electronic devices.	- Cameras for flash systems. - Laptops for quick boot operations. - Portable speakers for high-power sound bursts.
Industrial Uses	Used in heavy machinery to provide quick energy for lifting and moving.	- Cranes and forklifts for lifting heavy loads. - Uninterruptible power supplies (UPS) for industrial systems.
Backup Power	Offering reliable power in case of short-term outages.	- Computers to prevent data loss during power interruptions. -Medical devices to ensure continuous operation. -Telecommunications equipment to maintain service during power failures.
Transportation	Enhancing performance and energy efficiency in various transport systems.	- Electric buses for energy recovery and acceleration. <br>- Trains for auxiliary power systems. <br>- Aerospace for fast-acting power needs.
Military and Defense	Providing robust and reliable power in critical defense applications.	- Powering communication devices. <br>- Energy systems in drones and other unmanned vehicles. <br>- Rapid-deployment power supplies for field operations.

Table 5. Applications.

Application	Description	Examples
Energy Storage	Supplementing batteries in hybrid and electric vehicles, renewable energy systems, and grid storage.	- Hybrid and electric vehicles for regenerative braking and acceleration. - Solar and wind energy systems for smoothing out power fluctuations. - Grid storage for load balancing and peak shaving.
Consumer Electronics	Providing quick bursts of energy in various electronic devices.	- Cameras for flash systems. - Laptops for quick boot operations. - Portable speakers for high-power sound bursts.
Industrial Uses	Used in heavy machinery to provide quick energy for lifting and moving.	- Cranes and forklifts for lifting heavy loads. - Uninterruptible power supplies (UPS) for industrial systems.
Backup Power	Offering reliable power in case of short-term outages.	- Computers to prevent data loss during power interruptions. -Medical devices to ensure continuous operation. -Telecommunications equipment to maintain service during power failures.
Transportation	Enhancing performance and energy efficiency in various transport systems.	- Electric buses for energy recovery and acceleration. <br>- Trains for auxiliary power systems. <br>- Aerospace for fast-acting power needs.
Military and Defense	Providing robust and reliable power in critical defense applications.	- Powering communication devices. <br>- Energy systems in drones and other unmanned vehicles. <br>- Rapid-deployment power supplies for field operations.

4.3. Importance of Biowaste-Derived GQDs in Supercapacitors

The integration of graphene quantum dots (GQDs) derived from biowaste into supercapacitors offers numerous advantages, including enhanced performance, cost-effectiveness, and environmental sustainability[15]. These GQDs provide a high surface area, excellent electrical conductivity, and good structural stability, which are crucial for improving the overall efficiency and capacity of supercapacitors[16]. By utilizing biowaste as a source material, this approach also promotes recycling and reduces waste, aligning with green chemistry principles and supporting the development of eco-friendly energy storage solutions[17].

4.4. Types of Supercapacitors

Supercapacitors are classified into various types based on their electrode materials and mechanisms of energy storage[18]. The main types include electric double-layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors[19]. Each type has distinct characteristics and advantages, making them suitable for different applications.

Table 6. Types of Supercapacitors.

Type	Description	Key Characteristics	Applications
Electric Double-Layer Capacitors (EDLCs)	Store energy through electrostatic charge separation at the electrode-electrolyte interface.	- High power density - Long cycle life - Fast charge and discharge - No chemical reactions involved	- Energy storage - Consumer electronics - Power backup - Electric vehicles
Pseudocapacitors	Store energy through fast and reversible redox reactions at the surface of the electrodes.	- Higher energy density than EDLCs - Involves Faradaic (redox) reactions - Moderate power density - Shorter cycle life compared to EDLCs	- Renewable energy systems - Portable electronics - Power tools - Sensors
Hybrid Capacitors	Combine the properties of EDLCs and pseudocapacitors, typically using different materials for each electrode.	- Balanced energy and power density - Improved performance - Combination of electrostatic and Faradaic mechanisms	- Electric vehicles - Grid storage - Industrial

Table 6. Types of Supercapacitors.

Type	Description	Key Characteristics	Applications
Electric Double-Layer Capacitors (EDLCs)	Store energy through electrostatic charge separation at the electrode-electrolyte interface.	- High power density - Long cycle life - Fast charge and discharge - No chemical reactions involved	- Energy storage - Consumer electronics - Power backup - Electric vehicles
Pseudocapacitors	Store energy through fast and reversible redox reactions at the surface of the electrodes.	- Higher energy density than EDLCs - Involves Faradaic (redox) reactions - Moderate power density - Shorter cycle life compared to EDLCs	- Renewable energy systems - Portable electronics - Power tools - Sensors
Hybrid Capacitors	Combine the properties of EDLCs and pseudocapacitors, typically using different materials for each electrode.	- Balanced energy and power density - Improved performance - Combination of electrostatic and Faradaic mechanisms	- Electric vehicles - Grid storage - Industrial

4.4.1. Electric Double-Layer Capacitors (EDLCs)

- Mechanism: EDLCs store energy by forming an electric double layer at the interface between the electrode and the electrolyte. This electrostatic separation of charges does not involve any chemical reactions[20].

- Materials: Commonly used materials include activated carbon, carbon nanotubes, and graphene.

- Advantages: High power density, long cycle life, and rapid charge-discharge capability.

- Disadvantages: Lower energy density compared to pseudocapacitors and batteries.

4.4.2. Pseudocapacitors

- Mechanism: Pseudocapacitors store energy through fast and reversible redox (Faradaic) reactions at the electrode surface. These reactions involve electron transfer and result in higher energy storage capacity[21].

- Materials: Typically use transition metal oxides (such as RuO2, MnO2) and conducting polymers (such as polyaniline, polypyrrole).

- Advantages: Higher energy density compared to EDLCs.

- Disadvantages: Lower cycle life and power density compared to EDLCs.

4.4.3. Hybrid Capacitors

- Mechanism: Hybrid capacitors combine the mechanisms of EDLCs and pseudocapacitors, using different electrode materials to optimize both energy and power density. They often use a carbon-based material for one electrode and a pseudocapacitive material for the other[22].

- Materials: Can include a combination of activated carbon and transition metal oxides or conducting polymers.

- Advantages: Balanced performance, offering a compromise between the high power density of EDLCs and the high energy density of pseudocapacitors.

- Disadvantages: Complexity in design and potentially higher costs.

5. Challenges and Future Perspectives

Supercapacitors have made significant strides in energy storage technology, but several challenges remain to be addressed for their widespread adoption and further advancement. Future research and development efforts are focusing on overcoming these challenges and unlocking the full potential of supercapacitors in various applications.

Table 7. Challenges and Future Perspectives.

Challenge	Description	Potential Solutions
Energy Density	Supercapacitors have lower energy density compared to batteries, limiting their use in long-duration energy storage.	- Development of advanced electrode materials - Innovations in electrode design and architecture
Cycle Life and Durability	Supercapacitors can degrade over time, especially under high cycling conditions, impacting their long-term reliability.	- Improved electrode materials and electrolyte formulations - Enhanced cell design for durability and stability
Cost-Effectiveness	High material and manufacturing costs hinder widespread adoption of supercapacitors in commercial applications.	- Scalable synthesis methods for cost-effective production - Recycling strategies for electrode materials
Temperature Range	Supercapacitor performance can be limited by extreme temperatures, affecting their applicability in harsh environments.	- Development of electrolytes and materials with wide temperature tolerance - Thermal management strategies
Integration into Systems	Efficient integration of supercapacitors into existing energy storage systems and electronic devices requires tailored designs.	- Collaborative efforts between material scientists, engineers, and system integrators - Standardized interfaces and protocols
Advancements in Nanomaterials	Continued research on advanced nanomaterials to enhance energy and power density while maintaining stability.	- Exploration of graphene, carbon nanotubes, and metal oxides for improved performance - Nanomaterial synthesis innovations
Hybrid Energy Storage Systems	Integration of supercapacitors with batteries for hybrid systems combining benefits of both technologies.	- Development of hybrid architectures for optimized performance - Control algorithms for seamless integration
Smart and Flexible Devices	Design of flexible and lightweight supercapacitors for wearable electronics and IoT applications.	- Development of flexible electrode materials and packaging - Integration with flexible electronics and sensors
Environmental Sustainability	Emphasis on sustainable manufacturing processes and recycling strategies to minimize environmental impact.	- Use of renewable materials and green synthesis methods - Recycling programs for spent electrode materials
Standardization and Commercialization	Establishment of industry standards and regulations to facilitate wider adoption and integration of supercapacitors.	- Collaborative efforts among stakeholders to define performance metrics and safety standards - Market-driven policies for incentivizing adoption and investment

Table 7. Challenges and Future Perspectives.

Challenge	Description	Potential Solutions
Energy Density	Supercapacitors have lower energy density compared to batteries, limiting their use in long-duration energy storage.	- Development of advanced electrode materials - Innovations in electrode design and architecture
Cycle Life and Durability	Supercapacitors can degrade over time, especially under high cycling conditions, impacting their long-term reliability.	- Improved electrode materials and electrolyte formulations - Enhanced cell design for durability and stability
Cost-Effectiveness	High material and manufacturing costs hinder widespread adoption of supercapacitors in commercial applications.	- Scalable synthesis methods for cost-effective production - Recycling strategies for electrode materials
Temperature Range	Supercapacitor performance can be limited by extreme temperatures, affecting their applicability in harsh environments.	- Development of electrolytes and materials with wide temperature tolerance - Thermal management strategies
Integration into Systems	Efficient integration of supercapacitors into existing energy storage systems and electronic devices requires tailored designs.	- Collaborative efforts between material scientists, engineers, and system integrators - Standardized interfaces and protocols
Advancements in Nanomaterials	Continued research on advanced nanomaterials to enhance energy and power density while maintaining stability.	- Exploration of graphene, carbon nanotubes, and metal oxides for improved performance - Nanomaterial synthesis innovations
Hybrid Energy Storage Systems	Integration of supercapacitors with batteries for hybrid systems combining benefits of both technologies.	- Development of hybrid architectures for optimized performance - Control algorithms for seamless integration
Smart and Flexible Devices	Design of flexible and lightweight supercapacitors for wearable electronics and IoT applications.	- Development of flexible electrode materials and packaging - Integration with flexible electronics and sensors
Environmental Sustainability	Emphasis on sustainable manufacturing processes and recycling strategies to minimize environmental impact.	- Use of renewable materials and green synthesis methods - Recycling programs for spent electrode materials
Standardization and Commercialization	Establishment of industry standards and regulations to facilitate wider adoption and integration of supercapacitors.	- Collaborative efforts among stakeholders to define performance metrics and safety standards - Market-driven policies for incentivizing adoption and investment

Future research should focus on optimizing synthesis methods, exploring new biowaste sources, and enhancing the understanding of the relationship between GQD properties and supercapacitor performance. Interdisciplinary approaches combining materials science, chemistry, and engineering will be crucial in overcoming these challenges.

6. Conclusion

The synthesis of graphene quantum dots from biowaste represents a sustainable and economically viable approach to producing high-performance materials for supercapacitor applications. By leveraging abundant and renewable biowaste resources, this strategy not only addresses environmental concerns but also advances the development of next-generation energy storage devices. Further research and technological advancements are needed to fully realize the potential of biowaste-derived GQDs in commercial supercapacitors.