Bacterial Nanocellulose-Based Materials for Use in Supercapacitors.
The increasing demand for energy storage, driven by urbanization and scarcity of fossil resources, has generated interest in renewable sources and devices such as supercapacitors. Nanocellulose (NC), derived from cellulose, is explored as an eco-friendly and economical alternative to traditional materials in energy storage devices.
With properties such as high aspect ratio and good thermal stability, NC is ideal for electrodes and separators in supercapacitors.
This review highlights the advancement in the use of NC for supercapacitors, comparing them with conventional cellulose-based devices and highlighting the increase in research in this field.
SUPERCAPACITOR: A PROMISING OPTION FOR STORING ENERGY.
Energy density and power of devices.
Supercapacitors are energy storage devices that offer a balance between energy density and power, represented by the Ragone graph.
This graph compares different storage systems, showing energy density versus power density on a logarithmic scale. Supercapacitors are situated between devices that release energy in microseconds (such as capacitors) and those that release energy in hours (such as batteries).
The integration of nanocellulose (NC) and graphene-based materials into supercapacitors significantly improves their electrochemical properties. However, there is still room for improvement in the energy and power density of these devices.
Types of supercapacitors.
Electrochemical Double Layer Capacitors (EDLC): These store energy by transferring charge at the electrochemical interface between the electrode and the electrolyte, offering high power density and a long cycle life, but with lower energy density.
Pseudocapacitors: These use materials such as metal oxides and conducting polymers that involve redox reactions, providing higher energy density, but with a shorter cycle life and lower speed.
Hybrid Supercapacitors: These combine the principles of EDLC and pseudocapacitors to overcome the limitations of both. These hybrids can be symmetrical (similar electrodes), asymmetrical (different electrodes) or battery-type (combination of a supercapacitor electrode with a battery electrode), offering an optimal balance between energy density and power. Hybrid supercapacitors, in particular, show superior performance compared to individual EDLCs and pseudocapacitors.
Supercapacitor Materials
The choice of materials for supercapacitors is crucial to optimizing their performance. Electrode materials fall into three categories:
·        Carbon Materials: These include carbon nanotubes, graphene, and activated carbon.
·        Inorganic Materials: Such as manganese oxide, titanium oxide, and lithium-based materials.
·        Conductive Polymers: Examples are polyaniline (PANI) and polypyrrole (PPY), which allow a higher specific capacitance by storing and releasing charges through redox reactions in their polymer structure.
Cellulose vs. NanoCellulose in supercapacitors.
Cellulose Transformed into Nanocellulose (NC)Â NC offers advantages such as increased specific surface area, improved crystallinity, purity, and aspect ratio, as well as improved biocompatibility, surface chemical reactivity, and mechanical strength. These changes result in superior performance of supercapacitors made from NCÂ compared to those made from conventional cellulose.
NANOCELLULOSE (NC) SYNTHESIS METHODS
The synthesis of nanocellulose (NC)Â is carried out main steps:
Pretreatment: The cellulose is isolated from the lignocellulosic material.
Transformation: Cellulose is converted into nanostructured forms such as cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs).
CNC: It is obtained by acid hydrolysis and requires additional treatments such as solvent removal, neutralization, and drying. CNCs have a cylindrical structure with a width of 4-70 nm and a length of 100-6000 nm.
CNF: It is produced by chemical, mechanical and enzymatic treatments, resulting in long and wide fibers (20-100 nm wide and >10,000 nm long) with lower crystallinity than CNCs.
Bacterial Nanocellulose (BNC): It is synthesized in two stages by bacteria, through the production and crystallization of β-1,4-glucan chains. An example is the use of the K. sucrofermentans strain in a specific medium.
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Global Nanocellulose Market Size.
The global nanocellulose (NC) market is projected to grow from USD 297 million in 2020 to USD 783 million in 2025, with an annual growth rate of 21.3%. This increase is due to the demand for sustainable products and the scarcity of resources.
Europe leads the market, especially in pulp and paper, with government financial support.
The main applications of NC include pulp and paper, composites, packaging, biomedical, pharmaceutical, cosmetic and electronics.
Cellulose nanofibrils (CNFs), which account for more than 50% of the market, are noted for their low oxygen permeability and are expected to grow at a CAGR of 20.5%
NANOCELLULOSE (NC)-BASED SUPERCAPACITORS
Nanocelluloses (NCs) outperform macroscale cellulose fibers in supercapacitors due to their high surface-to-volume ratio, mechanical strength, thermal stability, and biocompatibility.
They are used in the manufacture of electrodes, separators, and other components, and NC-derived carbon materials improve the specific capacitance, energy density, and power density of supercapacitors. The porosity and lightness of NC facilitate better ion and electron transfer, increasing capacitance and optimizing device performance.
Nanocellulose (NC)-based electrodes:
This is accomplished in two main ways:
1)Â coating NC with conductive materials using techniques such as deposition, or
2) mixing NC directly with conductive materials such as carbon, metal particles, or conductive polymers through polymerization or in-situ mixing. These techniques allow NC to become a conductive material, improving stability, performance and reducing costs in electrode manufacturing.
Combination of NC with conductive polymers.
Conductive polymers such as polypyrrole (PPy) and polyaniline (PANI) are combined with nanocellulose (NC) to improve their conductivity and mechanical properties. The combination is mainly done by in-situ polymerization, where NC acts as the matrix and the conductive polymer is incorporated as a filler.
This method overcomes the solubility and manufacturability limitations of conductive polymers.
The interaction between the polymer's functional groups and the NC surface, such as TEMPO oxidation, improves the affinity and stability of the hybrid. In-situ polymerization also offers advantages such as ease of fabrication, low cost, and creation of 3D network structures, which improves the electromechanical properties of the composite material.
Combination of NC with conductive carbons.
The combination of nanocellulose (NC) with carbon materials, such as carbon nanotubes (CNTs) and graphene, significantly improves the conductivity of the compounds. CNTs and graphene are highly conductive and offer excellent mechanical and thermal properties. NC/carbon compounds, manufactured by coating or blending, exhibit better mechanical and conductive properties than compounds based on conductive polymers. Mixing carbon particles in the NC matrix is usually more effective than coating, as it allows for greater incorporation of conductive particles and provides a connected conductive network, improving the electrical conductivity and electromechanical stability of the material.
NC combination with metal particles
The combination of nanocellulose (NC) with metal particles, such as silver, significantly improves the conductivity of the composite material. The metal particles, which have a high intrinsic conductivity (~105 S/cm), are integrated into the NC matrix by coating or mixing. Techniques such as inkjet printing and high-temperature treatment boost conductivity, allowing highly conductive circuits to be created with low resistance and high-performance compounds to be manufactured.
NC-BASED ELECTROLYTES AND SEPARATORS.
NC-based electrolytes.
Definition and Role: Electrolytes enable the flow of charge between electrodes in energy storage devices. Liquid electrolytes offer high ionic conductivity but present safety risks. Alternatives such as gel and solid electrolytes (polymeric and inorganic) offer better stability and energy density.
Solid Polymer Electrolytes (SPE): Incorporating NC into SPEs improves their mechanical strength without greatly affecting conductivity. Examples include the use of NC in polyethylene matrices and other polymers, increasing dimensional stability and ion load capacity.
Gel Electrolytes (GPEs):Â GPEs, which combine liquid electrolytes with polymeric matrices, also benefit from the use of NC. The high polarity of NC derivatives improves ionic conductivity and electrolyte absorption. Examples include PVDF matrices with NC cyanoethylated and NFC nano paper to improve stability and porous structure
NC-based membranes/separators
Function of Separators: Separators maintain electrical insulation between the cathode and anode in energy storage devices, preventing internal short circuits and improving safety. They also allow ion transport through their pores filled with liquid electrolytes.
Limitations of Traditional Separators: Polyolefin separators (such as polyethylene and polypropylene) are common but present problems such as low melting point, high thermal shrinkage, and low wettability, which can negatively affect efficiency in high-power applications.
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Advantages of NC-Based Separators:
Improved Properties: NC-based separators offer better thermal stability, superior mechanical properties, and greater electrolytic wettability. This results in better ionic conductivity and lower thermal shrinkage compared to traditional separators.
Porous Structure: NC membranes have a porous structure that improves ionic conductivity. Examples include CNF films with labyrinthine nanostructures that exhibit high ionic conductivity (0.77 mS/cm) and better thermal stability than polyolefin separators.
Additional Innovations: The incorporation of silica nanoparticles (SiO2) and the use of functionalized materials, such as terpyridine (TPY), can control the porous structure and reduce adverse effects on the electrolyte, significantly improving cycle time and safety.
Advanced Applications: Solid-state supercapacitors have been developed using NC as an array for electrolytes and separators, showing high capacitance and resistance, and avoiding liquid electrolyte leakage problems.
TECHNICAL ASPECTS
Flexible Supercapacitors: Small, high-performance supercapacitors are required to be developed for applications in smart textiles and wearable electronics, using advanced nanostructures. Examples include doped graphene devices and stretchable supercapacitors with CNT electrodes.
Hybrid Devices: Hybrid supercapacitors combine the advantages of batteries and supercapacitors, offering high power and energy density. Examples include sodium-ion, potassium, and zinc supercapacitors, which show excellent performance and long service life.
Commercial Applications
Growing Demand: Supercapacitors are becoming increasingly relevant for wearable devices, telecommunications, electric vehicles, and clean energy due to their low equivalent series resistance (ESR), high cycle capacity, and wide operating temperature range.
Challenges in Supercapacitor Development.
Technical Problems: Supercapacitors have a lower energy density than batteries (less than 20 Wh/kg vs. 30-200 Wh/kg). Increasing energy density is crucial and research is underway using new materials and electrolytes with larger voltage windows.
Electrical Model Parameters:Â Supercapacitors typically operate at low voltages (less than 2.7 V), requiring series connections that can affect their lifespan if not properly managed.
Consistency Detection:Â It is essential to monitor the constant voltage in series-connected supercapacitors to avoid overloads that reduce their lifespan.
Industry Standard: Lack of uniform industry-wide standards for measuring and evaluating supercapacitors, which prevents consistent and standardized development in the industry. A set of international technical standards needs to be established.
CONCLUSION AND FUTURE PROSPECTS
Nanocellulose (NC) offers unique properties that make it valuable in energy storage, acting as a matrix or filler in supercapacitors and other devices. It is used to improve the electrochemical properties of carbon materials and as a separator or filler in gelled polymeric electrolytes (GPEs). Although significant progress has been made in its use for supercapacitors and ion batteries, challenges such as the production and conversion of NC, and its application in less common metal-ion batteries, remain. NC is expected to play an increasing role in the transition to greener energy.
REFERENCES:
This information is issued from Article: Review on nanocellulose-based materials for supercapacitor applications, Authors: Gedefaw Asmare Tafete, Metadel Kassahun Abera, Ganesh Thothadri.
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