ZnO-assisted synthesis of lignin-based ultra-fine microporous carbon nanofibers for supercapacitors

Co/CoO hetero-nanoparticles incorporated into lignin-derived carbon nanofibers as a self-supported bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries

The large-scale application of rechargeable Zn-air batteries (ZABs) necessitates the development of high-efficiency and cost-effective bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, the density functional theory calculations were performed to reveal the charge redistribution induced by the Co/CoO heterojunction integrating with N-doped carbon, which could optimize the d-band center, thereby accelerating O2 transformed into OOH* in the ORR and the conversion of O* into OOH* in OER. Guided by theoretical calculations, Co/CoO hetero-nanoparticles-decorated lignin-derived N-doped porous carbon nanofibers (Co-LCFs-800) were synthesized to use as an advanced self-supported bifunctional oxygen electrocatalyst. Consequently, Co-LCFs-800 shows a half-wave potential of 0.834 V in ORR and an overpotential of 354 mV at 10 mA cm-2 in OER. The Co-LCFs-800-based liquid ZABs afford an admirable performance with a large specific capacity of 780.8 mAh g-1, and the Co-LCFs-800-based solid-state ZABs exhibit satisfactory mechanical flexibility and cycling stability. The results suggest that the integration of hetero-nanoparticles into carbon nanofibers holds promise for oxygen cathode in ZABs.

Development of Oleylamine-Directed Nano-OZIF-67 Induced Microporous Carbon Nanofiber Conjugated with Tiny Fibrous PEDOT Polymer for Hybrid-Capacitor Electrodes

Making nanostructured metal oxides intertwined with graphitic carbon materials is critically important to construct hybrid supercapacitors with high-performance, cost-affordable, and free-standing 1D hybrid electrode material and is of foremost importance to responding positively to the impending energy crisis. Fine-tuned nano-sized ZIF-67 particles of 5 - 25 nm width (OZIF-67) are achieved using oleylamine-involved synthesis and are seeded into fibrous poly-PAN-PVP. Porous N-doped carbon nanofibers impregnated with OZIF-67-derived Co3O4 (oz-Co3O4-PNCNF) are obtained by calcining the electrospun polymer. The surface area of oz-Co3O4-PNCNF is exceptionally high, 429 m2 g-1, with the pore size predominantly ≈2 - 5 nm. Thin conducting polymer films of poly-(3,4-ethylenedioxythiophene) are formed onto oz-Co3O4-PNCNF (oz-Co3O4-PNCNF/PEDOT). A porous, long nanofibrous structure (≈100 nm thick) of oz-Co3O4-PNCNF and a spongy needle-like network of PEDOT interconnecting outer layers are established from FESEM images. The material has shown excellent energy storage capability with the specific capacitance (CS) of 417 F g-1 (oz-Co3O4-PNCNF/PEDOT). Free-standing supercapacitor device delivers excellent performance in asymmetric assembly (oz-Co3O4-PNCNF/PEDOT//PNCNF) with 141.1 Wh kg-1 and 84% CS at the end of 5000 charge-discharge cycles. The excellent porous network, combined with finely-tuned nano-sized Co3O4 particles and an overlying thin electropolymer film, work together to establish high energy, efficient, and stable supercapacitors.

ZnO-MnO2 co-modified hierarchical porous carbon nanofiber film electrodes for high-energy density supercapacitors

This study focuses on preparing composite hierarchical porous carbon nanofiber film that includes ZnO and MnO2. Using electrospinning technology, hierarchical porous structure was introduced into the nanofibers, enhancing energy density through the synergistic effect of zinc oxide and manganese dioxide. The zinc-manganese dioxide co-modified hierarchical porous carbon nanofiber film (ZnMnO-HPC) exhibits outstanding electrochemical performance when used as supercapacitor electrode, with a specific capacity reaching 401.77 C/g at 0.5 A/g, and 201.29 C/g at high current density of 5 A/g. ZnMnO-HPC also exhibits remarkable energy density when assembled with activated carbon electrode into asymmetric capacitor, reaching 38.37 Wh/kg at a power density of 407 W/kg and 19.5 Wh/kg at a power density of 12,800 W/kg, indicating promising applications in the high-energy-density supercapacitor field.

Development of 1D-Metalloid-Induced Highly Porous Carbon Nanofiber Conjugated with PEDOT Polymer Through Concurrent Selenization of ZIF-67 for Energy Storage and Green H2 Production

Manufacturing high-performance and cost-affordable non-metallic, electroactive 1D carbon material for energy storage and hydrogen evolution reaction (HER) is of foremost importance to respond positively to the impending energy crisis. Porous N-doped carbon nanofiber (PNCNF) is successfully synthesized by electrospinning, using selenium nanoparticles as a sacrificial template (where Se is reutilized for ZIF-67 selenization as a bi-process, and the surface of PNCNF is modified with poly(3,4-ethylenedioxythiophene) (PNCNT/PEDOT) by electropolymerization. The prepared materials are found ideal for energy storage (supercapacitor) and electrocatalysis (HER). The bi-functional material has shown excellent energy storage capability with the specific capacitance (CS) of 230 F g-1 (PNCNF) and 395 F g-1 (PNCNF/PEDOT), and the symmetric supercapacitor device, PNCNF/PEDOT//PEDOT/PNCNF, exhibits 32.4 Wh kg-1 energy density at 14400 W kg-1 power density with 96.6% Coulombic efficiency and 106% CS at the end of 5000 charge-discharge cycles. The rate capability of the symmetric supercapacitor cell of PNCNF/PEDOT is 51% for the current density increase from 1 to 8 A g-1, while that of PNCNF is a meager 29% only. Electrocatalytic HER at the PNCNF electrode is achieved with an overpotential of 281 mV@10 mA cm-2 relative to the Pt/C electrode and a low Tafel slop value of 96 mV dec-1.

Research progress in the preparation of lignin-based carbon nanofibers for supercapacitors using electrospinning technology: A review

With the development of renewable energy technologies, the demand for efficient energy storage systems is growing. Supercapacitors have attracted considerable attention as efficient electrical energy storage devices because of their excellent power density, fast charging and discharging capabilities, and long cycle life. Carbon nanofibers are widely used as electrode materials in supercapacitors because of their excellent mechanical properties, electrical conductivity, and light weight. Although environmental factors are increasingly driving the application of circular economy concepts in materials science, lignin is an underutilized but promising environmentally benign electrode material for supercapacitors. Lignin-based carbon nanofibers are ideal for preparing high-performance supercapacitor electrode materials owing to their unique chemical stability, abundance, and environmental friendliness. Electrospinning is a well-known technique for producing large quantities of uniform lignin-based nanofibers, and is the simplest method for the large-scale production of lignin-based carbon nanofibers with specific diameters. This paper reviews the latest research progress in the preparation of lignin-based carbon nanofibers using the electrospinning technology, discusses the prospects of their application in supercapacitors, and analyzes the current challenges and future development directions. This is expected to have an enlightening effect on subsequent research.

High-performance electrode materials of heteroatom-doped lignin-based carbon materials for supercapacitor applications

Supercapacitors are the preferred option for supporting renewable energy sources owing to many benefits, including fast charging, long life, high energy and power density, and saving energy. While electrode materials with environmentally friendly preparation, high performance, and low cost are important research directions of supercapacitors. At present, the growing global population and the increasingly pressing issue of environmental pollution have drawn the focus of numerous researchers worldwide to the development and utilization of renewable biomass resources. Lignin, a renewable aromatic polymer, has reserves second only to cellulose in nature. Ten million tonnes of industrial lignin are produced in pulp and paper mills annually, most of which are disposed of as waste or burned for fuel, seriously depleting natural resources and polluting the environment. One practical strategy to accomplish sustainable development is to employ lignin resources to create high-value materials. Based on the high carbon content and rich functional groups of lignin, the lignin-based carbon materials generated after carbonization treatment display specific electrochemical properties as electrode materials. Nevertheless, low electrochemical activity of untreated lignin precludes it from achieving its full potential for application in energy storage. Heteroatom doping is a common modification method that aims to improve the electrochemical performance of the electrode materials by optimizing the structure of the lignin, improving its pore structure and increasing the number of active sites on its surface. This paper aims to establish theoretical foundations for design, preparation, and optimizing the performance of heteroatom-doped lignin-based carbon materials, as well as for developing high-value-added lignin materials. The most reported the mechanism of supercapacitors, the doping process involving various types of heteroatoms, and the analysis of how heteroatoms affect the performance of lignin-based carbon materials are also detailed in this review.

Biomass-based controllable morphology of carbon microspheres with multi-layer hollow structure for superior performance in supercapacitors

The electrochemical properties of corn starch (CS)-based hydrothermal carbon microsphere (CMS) electrode materials for supercapacitor are closely related to their structures. Herein, cetyltrimethyl ammonium bromide (CTAB) was used as a soft template to form the corn starch (CS)-based carbon microspheres with radial hollow structure in the inner and middle layers by hydrothermal and sol-gel method. Due to the introduction of multi-layer hollow structure of carbon microsphere, more micropores were produced during CO2 activation, which increased the specific surface area and improved the capacitance performance. Compared to commercial activated carbon, the four different morphologies of corn starch CMS had better electrochemical performances. Consequently, the proposed CO2-(CTAB)-CS-CS exhibits a high discharge specific capacitance of 242.5F/g at 1 A/g in three-electrode system with 6 M KOH electrolyte, better than commercial activated carbon with 208.5F/g. Moreover, excellent stability is achieved for CO2-(CTAB)-CS-CS with approximately 97.14 % retention of the initial specific capacitance value after 10,000 cycles at a current density of 2 A/g, while the commercial activated carbon has 86.96 % retention. This implies that the corn starch-based multilayer hollow CMS could be a promising electrode material for high-performance supercapacitors.

Recent advances in biopolymers-based carbon materials for supercapacitors

Supercapacitors as potential candidates for novel green energy storage devices demonstrate a promising future in promoting sustainable energy supply, but their development is impeded by limited energy density, which can be addressed by developing high-capacitance electrode materials with efforts. Carbon materials derived from biopolymers have received much attention for their abundant reserves and environmentally sustainable nature, rendering them ideal for supercapacitor electrodes. However, the limited capacitance has hindered their widespread application, resulting in the proposal of various strategies to enhance the capacity properties of carbon electrodes. This paper critically reviewed the recent research progress of biopolymers-based carbon electrodes. The advances in biopolymers-based carbon electrodes for supercapacitors are presented, followed by the strategies to improve the capacitance of carbon electrodes which include pore engineering, doping engineering and composite engineering. Furthermore, this review is summarized and the challenges of biopolymer-derived carbon electrodes are discussed. The purpose of this review is to promote the widespread application of biopolymers in the domain of supercapacitors.

Recent advances in lignin-based carbon materials and their applications: A review

As the most abundant natural aromatic polymer, tens of million of tons of lignin produced in paper-making or biorefinery industry are used as fuel annually, which is a low-value utilization. Moreover, burning lignin results in large amounts of carbon dioxide and pollutants in the air. The potential of lignin is far from being fully exploited and the search for high value-added application of lignin is highly pursued. Because of the high carbon content of lignin, converting lignin into advanced carbon-based structural or functional materials is regarded as one of the most promising solutions for both environmental protection and utilization of renewable resources. Significant progresses in lignin-based carbon materials (LCMs) including porous carbon, activated carbon, carbon fiber, carbon aerogel, nanostructured carbon, etc., for various valued applications have been witnessed in recent years. Here, this review summarized the recent advances in LCMs from the perspectives of preparation, structure, and applications. In particular, this review attempts to figure out the intrinsic relationship between the structure and functionalities of LCMs from their recent applications. Hopefully, some thoughts and discussions on the structure-property relationship of LCMs can inspire researchers to stride over the present barriers in the preparation and applications of LCMs.

Biopolymer-based electrospun fibers in electrochemical devices: versatile platform for energy, environment, and health monitoring

Electrochemical power tools are regarded as essential keys in a world that is becoming increasingly reliant on fossil fuels in order to meet the challenges of rapidly depleting fossil fuel supplies. Additionally, due to the industrialization of societies and the growth of diseases, the need for sensitive, reliable, inexpensive, and portable sensors and biosensors for noninvasive monitoring of human health and environmental pollution is felt more than ever before. In recent decades, electrospun fibers have emerged as promising candidates for the fabrication of highly efficient electrochemical devices, such as actuators, batteries, fuel cells, supercapacitors, and biosensors. Meanwhile, the use of synthetic polymers in the fabrication of versatile electrochemical devices has raised environmental concerns, leading to an increase in the quest for natural polymers. Natural polymers are primarily derived from microorganisms and plants. Despite the challenges of processing bio-based electrospun fibers, employing natural nanofibers in the fabrication of electrochemical devices has garnered tremendous attention in recent years. Here, various natural polymers and the strategies employed to fabricate various electrospun biopolymers are briefly covered. The recent advances and research strategies used to apply the bio-based electrospun membranes in different electrochemical devices are carefully summarized, along with the scopes in various advanced technologies. A comprehensive and critical discussion about the use of biopolymer-based electrospun fibers as the potential alternative to non-renewable ones in future technologies is briefly highlighted. This review will serve as a field opening platform for using different biopolymer-based electrospun fibers to advance the electrochemical device-based renewable and sustainable technologies, which will be of high interest to a large community. Accordingly, future studies should focus on feasible and cost-effective extraction of biopolymers from natural resources as well as fabrication of high-performance nanofibrous biopolymer-based components applicable in various electrochemical devices.

Biopolymers-Derived Materials for Supercapacitors: Recent Trends, Challenges, and Future Prospects

Supercapacitors may be able to store more energy while maintaining fast charging times; however, they need low-cost and sophisticated electrode materials. Developing innovative and effective carbon-based electrode materials from naturally occurring chemical components is thus critical for supercapacitor development. In this context, biopolymer-derived porous carbon electrode materials for energy storage applications have gained considerable momentum due to their wide accessibility, high porosity, cost-effectiveness, low weight, biodegradability, and environmental friendliness. Moreover, the carbon structures derived from biopolymeric materials possess unique compositional, morphological, and electrochemical properties. This review aims to emphasize (i) the comprehensive concepts of biopolymers and supercapacitors to approach smart carbon-based materials for supercapacitors, (ii) synthesis strategies for biopolymer derived nanostructured carbons, (iii) recent advancements in biopolymer derived nanostructured carbons for supercapacitors, and (iv) challenges and future prospects from the viewpoint of green chemistry-based energy storage. This study is likely to be useful to the scientific community interested in the design of low-cost, efficient, and green electrode materials for supercapacitors as well as various types of electrocatalysis for energy production.

High mass load of oxygen-enriched microporous hollow carbon spheres as electrode for supercapacitor with solar charging station application

Carbon materials modified with pores and heteroatoms have been pursued as promising electrode for supercapacitors due to the synergic storage of electric double-layer capacitance (EDLC) and pseudocapacitance. A vital problem that the actual effect of pores and heteroatoms on energy storage varies with the carbon matrix used presents in numerous carbon electrodes, but is ignored greatly, which limits their sufficient utilization. Moreover, most of modified carbon electrodes still suffer from severe capacitance degeneration under high mass load caused by the blocked surface and inaccessible bulk phase. Here, we shape an interconnected hollow carbon sphere (HCS) as the matrix by regulating and selectively-etching low molecular weight component in the inhomogeneous precursors, accompanied with the decoration of rich oxygen groups (15.9at%) and micropores (centering at 0.6-1.4 nm). Finite-element calculation and energy storage kinetics reveal the modified HCS electrode exposes accessible dual active surface with highly-matched electrons and ions for pores and oxygen groups to improve both EDLC and pseudocapacitance. Under a commercial-level load of 11.2 mg cm^-2, the HCS exhibits a high specific capacitance of 288.3 F g^-1 at 0.5 A g^-1, performing a retention of 91.8% relative to 314 F g^-1 under 2.8 mg cm^-2 load, applicable for solar charging station to efficiently drive portable electronics.

In-situ ZnO template preparation of coal tar pitch-based porous carbon-sheet microsphere for supercapacitor

Three-dimension (3D) porous carbon-sheet microspheres (PCSMs) are prepared through coating coal tar pitch on basic zinc carbonate microspheres followed by in situ ZnO template carbonization and KOH activation. The as-prepared PCSMs show microsphere morphology composed of petal-like carbon nanosheets, which have large specific area (1359.88-2059.43 m² g^-1) and multiscale pores (mainly micropores and mesopores). As the supercapacitor electrodes, the 3D PCSMs present a good electrochemical performance with a large specific capacitance of 313 F g^-1 at 1 A g^-1 and high rate capability of 81.9% capacitance retention when increasing the current density up to 50 A g^-1 in a three-electrode system. In addition, the energy density can reach up to 18.79 Wh kg^-1 at a high power density of 878.4 W kg^-1 for PCSMs-0.2a symmetrical supercapcitor in 1 M Na₂SO₄ electrolyte.

Lignin Derived Porous Carbons: Synthesis Methods and Supercapacitor Applications

Lignin, one of the renewable constituents in natural plant biomasses, holds great potential as a sustainable source of functional carbon materials. Tremendous research efforts have been made on lignin-derived carbon electrodes for rechargeable batteries. However, lignin is considered as one of the most promising carbon precursors for the development of high-performance, low-cost porous carbon electrode materials for supercapacitor applications. Yet, these efforts have not been reviewed in detail in the current literature. This review, therefore, offers a basis for the utilization of lignin as a pivotal precursor for the synthesis of porous carbons for use in supercapacitor electrode applications. Lignin chemistry, the synthesis process of lignin-derived porous carbons, and future directions for developing better porous carbon electrode materials from lignin are systematically reviewed. Technological hurdles and approaches that should be prioritized in future research are presented.

Hydrogen-bonded frameworks crystals-assisted synthesis of flower-like carbon materials with penetrable meso/macropores from heavy fraction of bio-oil for Zn-ion hybrid supercapacitors

The application of biomass-based carbon materials in electrode materials are usually subject to their deficient adsorption sites as well as sluggish diffusion of electrolyte ions. Herein, flower-like carbons are obtained from the heavy fraction of bio-oil with the auxiliary of Hydrogen-bonded frameworks (HOFs) crystals. During the co-carbonization of the both, the HOFs crystals are removed on account of its poor stability, which directs the formation of flower-like morphology and generates the penetrable meso/macropores across petal-like carbon nanosheets. In addition, the pyrolysis gases serve as the agents for activation to enrich the active sites without the further activation. The degree of graphitization and the contents of pyridine nitrogen for carbon materials could be flexibly adjusted with the contents of HOFs. Owing to the beneficial 3D flower-like structure, high specific surface area (1076 m²/g), large pore volume (2.59 cm³/g), and rational N species, the assembled Zn//BH-4 hybrid supercapacitor reaches a superior energy density of 117.5 Wh/kg at 890 W/kg and maintains 60.7 Wh/kg even at 16.2 kW/kg.

Acid-directed preparation of micro/mesoporous heteroatom doped defective graphitic carbon as bifunctional electroactive material: Evaluation of trace metal impurity

Extensive research to explore cost-effective carbon materials as electrocatalysts and electrode materials for energy conversion and storage has been conducted in the recent literature. This raised a crucial question regarding the origin of this electrocatalytic activity from the heteroatom doping/ hierarchical porous defect-rich architecture and/ or the trace metal impurities introduced during synthesis/ inherent to the precursor. In this work, an insight into this issue is provided by considering a lignocellulosic biowaste, Euryale Ferox (foxnut) shells as a precursor to derive micro/ mesoporous defective graphitic carbon sheets by the phosphoric acid (H₃PO₄) dictated in-situ carbonization for oxygen reduction reaction (ORR) and supercapacitor applications. The sample synthesized at 900 °C (FP900) shows an onset potential of 0.98 V vs. reversible hydrogen electrode (RHE), ORR current density of 5.5 mA cm^-2, and current stability of 93% (in 10 h measurement) in 0.1 M KOH. In addition, a symmetric supercapacitor device is assembled using the prepared material and the specific capacitance of 207.5 F g^-1 at 1 A g^-1 is obtained. An attempt to explain the origin of the electrochemical performance is made by establishing parallels with the physicochemical characterizations. The inherently doped heteroatoms give rise to electroactive functionalities and the wide enough pore size distribution improves the active sites utilization efficiency by enhancing the accessibility to electrolytic ions resulting in better electrochemical performance. Furthermore, the contribution from the intrinsic trace metal impurities is evaluated using X-ray fluorescence (XRF) spectroscopy. The presented research clarifies the non-contributing nature of trace metal species owing to the inaccessibility of active sites and lower abundance in F900 and FP900, respectively.

Multifunctional Lignin-Based Composite Materials for Emerging Applications

Lignin exhibited numerous advantages such as plentiful functional groups, good biocompatibility, low toxicity, and high carbon content, which can be transformed into composites and carbon materials. Lignin-based materials are usually environmentally friendly and low cost, and are widely used in energy storage, environment, electronic devices, and other fields. In this review article, the pretreatment separation methods like hydrothermal process are illustrated briefly, and the properties and categories of technical lignin are introduced. Then, the latest progress of lignin-based composites and lignin-derived carbon materials is summarized. Finally, the current challenges and future developments were suggested based on our knowledge. It is expected that this review paper favored the applications of composites and lignin-derived carbon materials in the future.

Construction of hierarchical sea urchin-like manganese substituted nickel cobaltite@tricobalt tetraoxide core-shell microspheres on nickel foam as binder-free electrodes for high performance supercapacitors

Construction of binder-free electrodes with hierarchical core-shell nanostructures is considered to be an effective route to promote the electrochemical performance of supercapacitors. In this work, the porous Ni_0.5Mn_0.5Co₂O₄ nanoflowers anchored on nickel foam are utilized as framework for further growing Co₃O₄ nanowires, resulting in the hierarchical sea urchin-like Ni_0.5Mn_0.5Co₂O₄@Co₃O₄ core-shell microspheres on nickel foam. Owing to the advantages brought by unique porous architecture and synergistic effect of the multi-component composites, the as-prepared electrode exhibits a high specific capacitance (931 F/g at 1 A/g), excellent rate performance (77% capacitance retention at 20 A/g) and outstanding cycle stability (92% retention over 5000 cycles at 5 A/g). Additionally, the assembled Ni_0.5Mn_0.5Co₂O₄@Co₃O₄//AC (activated carbon) asymmetric supercapacitor achieves a high energy density (50 Wh/kg at 750 W/kg) and long durability (88% retention after 5000 cycles).