Design of Fe3+-Rich, High-Conductivity Lignin Hydrogels for Supercapacitor and Sensor Applications

A bio-based Janus hydrogel from cellulose and lignin with bilayer structure and asymmetric adhesion for accurate and sensitive human motion monitoring

Janus hydrogels for human motion monitoring are thriving due to their conductivity, flexibility, anisotropy and self-adhesion, etc. However, most of them face challenges such as complex processes, interlayer detachment, and surface contamination, which degrade their sensing accuracy and sensitivity. Hence, this study proposes a facile strategy using the cellulose and lignin as building blocks to construct a Janus hydrogel for accurate and sensitive sensing. The proposed process involves sequential pouring and in-situ thermal-induced polymerization. Initially, the cellulose dissolved in ZnCl2 solution, along with acrylic acid (Cel/ZnCl2-AA), forms the antifouling and conductive precursors in the top layer, while lignosulfonate and acrylamide (LS-AM) function as the adhesive precursor in the bottom layer. The viscosity difference of the two precursors allows local diffusion and polymerization at the interlayer interface, thereby developing a bilayer structure with strong interface bonds. Consequently, the Janus hydrogel exhibits high conductivity (1.2 S/m), excellent asymmetric adhesion, good stretchability (520 %) and compressive strain (∼70 %). These properties enable the hydrogel to accurately monitor both large (elbow and wrist bending) and small (swallowing and frowning) human movements with accurate sensitivity (gauge factor up to 2). This work offers new insight and synthesis strategy to design the bilayer hydrogels for practical applications in electronic skin and flexible sensing.

Poly (vinyl alcohol)/carboxylated cellulose nanofibers composite hydrogel flexible strain sensors

This study introduces a method for constructing a dual cross-linked hydrogel network via combined chemical and physical processes. Carboxylated cellulose nanofibers (CNF-C) and tannic acid (TA) were integrated into a borax-polyvinyl alcohol (PVA) matrix, followed by the incorporation of metal cations (Al3+) to fabricate PVA/CNF-C composite hydrogels. The PVA-TA@CNF-C-Borax-Al3+ hydrogel forms a multi-crosslinked 3D network through dynamic borate ester bonds between PVA and borax, coordination bonds between TA and Al3+, and hydrogen bonds from CNF, endowing the hydrogel with excellent mechanical properties. The PTCB(PVA-TA@CNF-Borax) hydrogel, with a TA to CNF-C mass ratio of 1:4, exhibits superior mechanical strength(1.6 MPa), robust conductivity(1.7 × 10-2 S/cm), and stable thermal properties(95 % at 60 °C). Furthermore, the influence of different valence ions on the hydrogel's properties was systematically investigated through the introduction of Na+, Zn2+, and Al3+ cations. It was found that Al3+ can effectively enhance the tension and elasticity of the crosslinked network, improving the mechanical adaptability and sensitivity of the hydrogel. Additionally, this hydrogel system exhibits excellent strain-sensing capabilities. When employed as a self-powered triboelectric nanogenerator sensor, it can generate a stable open-circuit voltage of 2 V.

Rational design of antifreeze and flexible chitosan-based hydrogels for integration device of supercapacitors electrodes and wearable strain sensors

Hydrogels with excellent flexibility are widely used in flexible sensors and supercapacitors, but their sensitivity and operating temperature range limit their application. In this study, chitosan (CS)/polyaniline (PANI) hydrogel with interpenetrating network structure is designed, in which phytic acid is used as crosslinking agent and antifreeze. The obtained CS/PANI hydrogel exhibits excellent mechanical properties, excellent sensing performance (Gauge Factor = 5.25), fast electrochemical response, high specific capacitance (383.7 F/g at 0.5 A/g) and good cycle stability, which may be due to the interpenetrating network structure formed between phytic acid cross-linked PANI and CS molecular chains. Due to these properties, CS/PANI hydrogels can be used as flexible sensors and supercapacitor electrodes materials. Because of the electrostatic interaction between the anionic and cationic groups in phytic acid, it also has certain frost resistance. The CS/PANI hydrogel can provide a high specific capacitance of 330 F/g at -40 °C. Compared with room temperature, the capacitance retention rate is as high as 87 %. It is believed that this CS/PANI hydrogel will be used as a new multifunctional material in many fields such as flexible electrodes, sensors and wearable devices in low temperature environments.

Transparent cellulose-lignin films containing Fe3+ with high UV absorption for thermal management

In this paper, cellulose-lignin films containing Fe3+ were prepared by the codissolution-precipitation method, and the films have high transparency as well as high UV absorption. In this process, kraft lignin chelates with Fe3+ and then bonds with cellulose through hydrogen bonding, evenly distributing within the film. The morphological results showed that the kraft lignin chelated with Fe3+ bound tightly linked to cellulose within the Fe@cellulose-lignin composite films. The addition of Fe3+ made the composite film (C-Fe-0.7-L) maintain a high transmittance of 66.9 % at 600 nm with excellent UV absorption. Excellent UV absorption performance, completely shielded from both Ultraviolet A (UVA), Ultraviolet B (UVB), and Ultraviolet C (UVC), and improved the thermal stability of the film. Under 100 mW cm-2 solar irradiation, the temperature of C-Fe-0.7-L rose rapidly to 60.7 °C. Based on this property, solar energy is transformed into kinetic energy, which realizes the photothermal conversion of solar energy. This work realizes the use of transparent films for energy utilization.

A Robust Lignin-Derived Moisture-Enabled Electric Generator with Sustained and Scalable Power Output

Leveraging ubiquitous moisture and abundant biomass in nature to convert chemical energy into electrical energy holds great promise for meeting energy demands. Herein, we report a simple, green, low-cost, and high-performance lignin-derived moisture-enabled electric generator (LMEG). An LMEG device with an area of 0.25 cm2 can give a stable open-circuit voltage of 1.26 V, a high short-circuit current density of 439.36 μA cm-2, and a maximum power density of up to 32.73 μW cm-2. Moreover, the LMEG exhibits continuous electrical output for at least 2 months, demonstrates high tolerance to a wide range of working environments and mechanical deformations, and is recyclable. Besides, a near-linear increase in electric production can be achieved by integrating LMEG devices in series or parallel, significantly with a current of 1.45 mA obtained by connecting only 18 LMEG units in parallel. The integrated LMEG can successfully drive various electronic products, including LED arrays, electronic watches, and hygrometers, highlighting its potential in self-powered systems and sensing applications.

Hydrogel-Based Sensor for the Detection of Iron Ions and Excessive Alerting

The excessive content of heavy metal ions in water has attracted increasing worldwide attention due to the potential harm to human health and ecological systems. Therefore, it is particularly significant to research and develop a sensitive and straightforward water quality monitoring method. Here, a dual-network designed hydrogel poly(acrylic acid-co-N-methylolacrylamide)/poly(3,4-ethylenedioxythiophene): polystyrenesulfonate, P(AA-co-N-MA)/PEDOT: PSS, was constructed and used for Fe3+ detection. Compared with previous reports, the P(AA-co-N-MA)/PEDOT: PSS hydrogel has a much more sensitive and convenient detection of immune iron ions (Fe3+), is applied to monitor a trace of over-the-standard (the lower limit of detection is 0.52 ppm corresponding to 0.06 S/m for human health monitor, and the threshold for water quality monitoring is 0.16 S/m). The relationship between Fe3+ content and the conductivity of the hydrogel allows it to integrate with other electron devices to provide an early warning function for Fe3+ in the human body, which would be of great significance to human health monitoring.

In situ growth of MnO2 on graphitized cellulose nanocrystal: A novel coaxial heterostructures with unblocked conductive networks as advanced electrode materials for supercapacitor

Manganese dioxide (MnO2) with high theoretical capacitance and natural abundance has attracted great attention but is still challenging for use in supercapacitors, due to the limited conductivity and lower electron and ion migration. Here, a facile in situ growth strategy is developed to prepare heterostructural graphitized cellulose nanocrystal (GCNC)/MnO2 with unblocked conductive networks through a hydrothermal reaction. The GCNC with highly ordered graphitic carbon layers as conductive support framework solves the agglomeration problem of MnO2 and increases effective specific surface areas in contact with electrolyte ions. Meanwhile, the stable connection of MnOC covalent bonds enhances the interface bonding, which effectively reduces the contact resistance at the interface of heterostructural GCNC/MnO2 and enhances the interface firmness. The resulting GCNC/MnO2 electrode shows a remarkable specific capacitance of 528.2 F g-1 at 0.5 A g-1. Furthermore, the symmetric supercapacitor based on GCNC/MnO2-15 mM exhibits high rate capability and excellent cycle stability of 100 % capacitance retention after 3000 cycles. This work provides a new path to designing high performance electrode material for sustainable energy technologies.

Recent advances in moisture-induced electricity generation based on wood lignocellulose: Preparation, properties, and applications

Moisture-induced electricity generation (MEG), which can directly harvest electricity from moisture, is considered as an effective strategy for alleviating the growing energy crisis. Recently, tremendous efforts have been devoted to developing MEG active materials from wood lignocellulose (WLC) due to its excellent properties including environmental friendliness, sustainability, and biodegradability. This review comprehensively summarizes the recent advances in MEG based on WLC (wood, cellulose, lignin, and woody biochar), covering its principles, preparation, performances, and applications. In detail, the basic working mechanisms of MEG are discussed, and the natural features of WLC and their significant advantages in the fabrication of MEG active materials are emphasized. Furthermore, the recent advances in WLC-based MEG for harvesting electrical energy from moisture are specifically discussed, together with their potential applications (sensors and power sources). Finally, the main challenges of current WLC-based MEG are presented, as well as the potential solutions or directions to develop highly efficient MEG from WLC.

Flexible high electrochemical collagen/lignin composite hydrogel for sensing and supercapacitor applications

Synthesis of polymer-based highly conductive hydrogels from natural and renewable sources with robust mechanical performances in flexible electronics remains a great challenge. In this research, a dynamic redox system is designed by using collagen (CL), sulfonate lignin (SL), acrylic acid (AA), and Al3+ to synthesize CL/PAA/SL/Al hydrogels. The formation of effective complexes of Al3+ with the abundant functional groups of CL, SL and PAA, the prepared hydrogel delivers various specific properties, for example, excellent ionic conductivity (4.61 S·m-1), stretchability and antimicrobial performance. The CL/PAA/SL/Al hydrogel demonstrates good mechanical strength, while the maximum tensile strength of the hydrogels is ∼604 kPa at a stretching of 1254 %, and the maximum compressive strength is ∼0.45 MPa, with the maximum stretching of 59.6 %. The CL/PAA/SL/Al hydrogel acts as a flexible strain sensor with high sensitivity. Enough hydroxyl and carboxyl groups in the hydrogels are essential for delivering the maximum 191 mV of open circuit voltage (Voc) rendered during moisture spraying. The supercapacitor assembled from CL/PAA/SL/Al hydrogel manifests specific capacitance (Cs), maximum energy density (Ed) and power density (Pd) of 268.75 F·g-1, 23.89 Wh·kg-1 and 2.4 kW·kg-1, respectively. The supercapacitor can retain its capacitance of 95.8 % after 5000 consecutive charge-discharge cycles.

Renewable symmetric supercapacitors assembled with lignin nanoparticles-based thin film electrolyte and carbon aerogel electrodes

Lignin as a natural biopolymer is becoming increasingly in demand due to its eco-friendly properties, while lignin-based electrolyte with high conductivity and reliable durability for applications in supercapacitors is still challenging. Herein, a facile method to prepare lignin nanoparticles (LNPs)-based solid electrolyte thin film (LF) was proposed through chemical cross-linking reaction. The fabricated LF exhibited a distinctive spongy porous structure with the ionic conductivity of 3.26 mS cm-1, demonstrating the exceptional flexibility and favorable mechanical properties. Moreover, the assembly of all-LNPs-based symmetric supercapacitor (SSC) devices was achieved using LF electrolyte and LCA electrodes for the first time, confirming the LF3 electrolyte superior to commercial cellulose separator in capacitive behaviour. This SSC device exhibited a specific capacitance of 122.7 F g-1 at 0.5 A g-1 and the maximum energy density of 17.04 W h kg-1. Furthermore, the incorporation of sodium alginate (SA) significantly enhanced the ionic conductivity of SA/LF3 electrolyte, and the resulting SSC device delivered a higher specific capacitance of 174.5 F g-1 at 0.5 A g-1 and the maximum energy and power densities of 24.24 W h kg-1 and 5023 W kg-1, respectively. This study proposes a promising approach for sustainable utilization of lignin in energy storage applications.

Binary Biomass-Based Electrolyte Films for High-Performance All-Solid-State Supercapacitor

Solid-state electrolytes have received widespread attention for solving the problem of the leakage of liquid electrolytes and effectively improving the overall performance of supercapacitors. However, the electrochemical performance and environmental friendliness of solid-state electrolytes still need to be further improved. Here, a binary biomass-based solid electrolyte film (LSE) was successfully synthesized through the incorporation of lignin nanoparticles (LNPs) with sodium alginate (SA). The impact of the mass ratio of SA to LNPs on the microstructure, porosity, electrolyte absorption capacity, ionic conductivity, and electrochemical properties of the LSE was thoroughly investigated. The results indicated that as the proportion of SA increased from 5% to 15% of LNPs, the pore structure of the LSE became increasingly uniform and abundant. Consequently, enhancements were observed in porosity, liquid absorption capacity, ionic conductivity, and overall electrochemical performance. Notably, at an SA amount of 15% of LNPs, the ionic conductivity of the resultant LSE-15 was recorded at 14.10 mS cm-1, with the porosity and liquid absorption capacity reaching 58.4% and 308%, respectively. LSE-15 was employed as a solid electrolyte, while LNP-based carbon aerogel (LCA) served as the two electrodes in the construction of a symmetric all-solid-state supercapacitor (SSC). The SSC device demonstrated exceptional electrochemical storage capacity, achieving a specific capacitance of 197 F g-1 at 0.5 A g-1, along with a maximum energy and power density of 27.33 W h kg-1 and 4998 W kg-1, respectively. Furthermore, the SSC device exhibited highly stable electrochemical performance under extreme conditions, including compression, bending, and both series and parallel connections. Therefore, the development and application of binary biomass-based solid electrolyte films in supercapacitors represent a promising strategy for harnessing high-value biomass resources in the field of energy storage.

Tough Supramolecular Hydrogels Crafted via Lignin-Induced Self-Assembly

Supramolecular hydrogels are typically assembled through weak non-covalent interactions, posing a significant challenge in achieving ultra strength. Developing a higher strength based on molecular/nanoscale engineering concepts is a potential improvement strategy. Herein, a super-tough supramolecular hydrogel is assembled by gradually diffusing lignosulfonate sodium (LS) into a polyvinyl alcohol (PVA) solution. Both simulations and analytical results indicate that the assembly and subsequent enhancement of the crosslinked network are primarily attributed to LS-induced formation and gradual densification of strong crystalline domains within the hydrogel. The optimized hydrogel exhibits impressive mechanical properties with tensile strength of ≈20 MPa, Young's modulus of ≈14 MPa, and toughness of ≈50 MJ m⁻3, making it the strongest lignin-PVA/polymer hydrogel known so far. Moreover, LS provides the supramolecular hydrogel with excellent low-temperature stability (<-60 °C), antibacterial, and UV-blocking capability (≈100%). Interestingly, the diffusion ability of LS is demonstrated for self-restructuring damaged supramolecular hydrogel, achieving 3D patterning on hydrogel surfaces, and enhancing the local strength of the freeze-thaw PVA hydrogel. The goal is to foster a versatile hydrogel platform by combining eco-friendly LS with biocompatible PVA, paving the way for innovation and interdisciplinarity in biomedicine, engineering materials, and forestry science.

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.

Development of lignin hydrogel reinforced polypyrrole rich electrode material for supercapacitor and sensing applications

The preparation of natural polymer-based highly conductive hydrogels with reliable durability for applications in supercapacitors (SCs) is still challenging. Herein, a facile method to prepare alkaline lignin (AL)-based polypyrrole (PPy)-rich, high-conductive PPy@AL/PEGDGE gel was reported, where AL was used as a dopant, polyethylene glycol diglycidyl ether (PEGDGE) as a cross-linking agent, and PPy as a conducting polymer. The PPy@AL/PEGDGE gel electrode materials with hollow structures were prepared by electrochemical deposition and chemical etching method and then assembled into sandwich-shaped SCs. Cyclic voltammetry (CV), galvanotactic charge discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycling stability tests of the PPy@AL/PEGDGE SCs were performed. The results demonstrated that the SCs can achieve a conductivity of 25.9 S·m-1 and a specific capacitance of 175 F·g-1, which was 127.4 % higher compared to pure PPy (77 F·g-1) electrode. The highest energy density and power density for the SCs were obtained at 23.06 Wh·kg-1 and 5376 W·kg-1, respectively. In addition, the cycling performance was also higher than that of pure PPy assembled SCs (50 %), and the capacitance retention rate can reach 72.3 % after 1000 cycles. The electrode materials are expected to be used as sensor and SCs devices.

Supramolecular polymers with dual energy storage mechanism for high-performance supercapacitors

In this paper, we prepared the supramolecular polymers (MWCNT-APP-s) with a dual energy storage mechanism as the electrode materials by the coordination of four transition metal ions with the small molecule chelator (APP) and functionalized carbon nanotubes, respectively. Among four MWCNT-APP-s, MWCNT-APP-Fe has the characteristics of moderate micropore/mesopore, significant hydrophobicity, redox property and functional groups. Interestingly, the redox reaction of Fe3+/Fe2+ and -CN-/-CN- transformation give MWCNT-APP-Fe an energy storage basis of pseudocapacitance, while MWCNTs and the micro/mesopore structure in MWCNT-APP-Fe provide a double-layer energy storage platform. As expected, on base of the dual energy storage mechanism, the symmetric supercapacitor assembled with MWCNT-APP-Fe has a higher specific capacity (Cs, 421 F g-1 at 1 mV s-1) as well as a long-lasting stability of 94.8% capacity retention with 99% Coulombic efficiency after 10,000 cycles at 20 mV s-1. More notably, the relevant aqueous Zn2+ hybrid supercapacitor provides a high capacity (Cm) of 191 mAh g-1 at 0.5 A g-1 and a long duration of over 2000 cycles at 50 A g-1, with a capacity retention of 92.4%. In summary, MWCNT-APP-Fe with a dual energy storage mechanism enables a potential application as an electrode material for high-performance supercapacitor.

Rapid fabricated in-situ polymerized lignin hydrogel sensor with highly adjustable mechanical properties

Conductive hydrogels have been widely used as sensors owing to their tissue-like properties. However, the synthesis of conductive hydrogels with highly adjustable mechanical properties and multiple functions remains difficult to achieve yet highly needed. In this study, lignin hydrogel characterized by frost resistance, UV resistance, high conductivity, and highly adjustable mechanical properties without forming by-products was prepared through a rapid in-situ polymerization of acrylic acid/zinc chloride (AA/ZnCl2) aqueous solution containing lignin extract induced by the reversible quinone-catechol redox of the ZnCl2-lignin system at room temperature. Results revealed that the PAA/ZnCl2/lignin hydrogel exhibited mechanical properties with tensile stress (ranging from 0.08 to 3.28 MPa), adhesion to multiple surfaces (up to 62.05 J m-2), excellent frost resistance (-70-20 °C), UV resistance, and conductivity (0.967 S m-1), which further endow the hydrogel as potential strain and temperature sensor with wide monitor range (0-300 %), fatigue resistance, and quick response (70 ms for 150 % strain). This study proposed and developed a green, simple, economical, and efficient processing method for a hydrogel sensor in flexible wearable devices and man-machine interaction fields.

Ultrastretchable and highly conductive hydrogels based on Fe3+- lignin nanoparticles for subzero wearable strain sensor

Conductive hydrogels have attracted considerable interest for potential applications in soft robotics, electronic skin and human monitoring. However, insufficient mechanical characteristics, low adhesion and unsatisfactory electrical conductivity severely restrict future application possibilities of hydrogels. Herein, lignin nanoparticles (LNPs)-Fe3+-ammonium persulfate (APS) catalytic system was introduced to assemble Poly(2-hydroxyethyl methacrylate)/LNPs/Ca2+ (PHEMA/LNPs/Ca) hydrogels. Due to the abundant metal coordination and hydrogen bonds, the composite hydrogel displayed ultrahigh stretchable capacity (3769 %), adhesion properties (248 kPa for skin) and self-healing performance. Importantly, hydrogel sensors possess with high durability, strain sensitivity (GF = 8.75), fast response time and freeze resistance (-20 °C) that could be employed to monitor motion signals in low-temperature regime. Therefore, the LNPs-Fe3+ catalytic system has great potential in preparing hydrogel for various applications such as human-computer interaction, artificial intelligence, personal healthcare and subzero wearable devices. At the same time, incorporation of natural macromolecules into polymer hydrogels is tremendous research significance for investigating high-value utilization of lignin.

Rapid and Controllable Preparation of Multifunctional Lignin-Based Eutectogels for the Design of High-Performance Flexible Sensors

Currently, there is a limited amount of research on PEDOT:LS (poly(3,4-ethylenedioxythiophene):sulfonated lignin)-based hydrogels. While the addition of PEDOT:LS can enhance the conductivity of the gel, it unavoidably disrupts the gel network and negatively affects its mechanical properties. The preparation process and freezing resistance of the hydrogels also pose significant challenges for their practical applications. In this study, we have developed a novel self-catalytic system, PEDOT:LS-Fe3+, for the rapid fabrication of conductive hydrogels. These hydrogels are further transformed into eutectogels by immersing them in a deep eutectic solvent. Compared with conventional hydrogels, the eutectogels exhibit improved elongation, mechanical strength, and resistance to freezing. Specifically, the eutectogels containing 2 wt % PEDOT:LS as conductive fillers and catalysts demonstrate exceptional stretchability (∼460%), self-adhesion (∼14.6 kPa on paper), UV-blocking capability (∼99.9%), and ionic conductivity (∼1.2 mS cm-1) even at extremely low temperatures (-60 °C). Moreover, the eutectogels exhibit high stability and sensitivity in flexible sensing, successfully detecting various human motions. This study presents a novel approach for the rapid preparation of the hydrogels by utilizing lignin in the conductive PEDOT polymerization process and forming a self-catalytic system with metal ions. These advancements make the eutectogels a promising candidate material for flexible wearable electronics.

Transparent, High Stretchable, Environmental Tolerance, and Excellent Sensitivity Hydrogel for Flexible Sensors and Capacitive Pens

The prospect of ionic conductive hydrogels in multifunctional sensors has generated widespread scientific interest. The new generation of flexible materials should be combined with superior mechanical properties, high conductivity, transparency, sensitivity, good self-restoring fatigue properties, and other multifunctional characteristics, while the current materials are difficult to meet these requirements. Herein, we prepared poly(acrylamide-acrylic acid) (P(AM-AA))/gelatin/glycerol-Al3+ (PG1G2A) ionic conducting hydrogel by one-pot polymerization under UV light. The prepared PG1G2A ionic conductive hydrogel had high tensile strength (539.18 kPa), excellent tensile property (1412.96%), good fast self-recovery and fatigue resistance, high transparency (>80%), excellent moisturizing, and antifreezing/drying properties. In addition, the ionic conductive hydrogel-based strain sensor can respond to mechanical stimulation and generate accurate, stable, and recyclable electrical signals, with excellent sensitivity (GF 5.81). In addition, the PG1G2A hydrogel could be used as flexible wearable devices for monitoring multiple strain and subtle movements of different body parts at different temperatures. Interestingly, the PG1G2A hydrogel capacitive pen embedded in the mold can be used to write and draw on the screen of a phone or tablet. This new multifunctional ionic conducting hydrogel shows broad application prospects in E-skin, motion monitoring, and human-computer interaction in extreme environments.

Waste dry cell derived photo-reduced graphene oxide and polyoxometalate composite for solid-state supercapacitor applications

In the modern era, realizing highly efficient supercapacitors (SCs) derived through green routes is paramount to reducing environmental impact. This study demonstrates ways to recycle and reuse used waste dry cell anodes to synthesize nanohybrid electrodes for SCs. Instead of contributing to landfill and the emission of toxic gas to the environment, dry cells are collected and converted into a resource for improved SC cells. The high performance of the electrode was achieved by exploiting battery-type polyoxometalate (POM) clusters infused on a reduced graphene oxide (rGO) surface. Polyoxometalate (K5[α-SiMo2VW9O40]) assisted in the precise bottom-up reduction of graphene oxide (GO) under UV irradiation at room temperature to produce vanadosilicate embedded photo-reduced graphene oxide (prGO-Mo2VW9O40). Additionally, a chemical reduction route for GO (crGO) was trialed to relate to the prGO, followed by the integration of a faradaic monolayer (crGO-Mo2VW9O40). Both composite frameworks exhibit unique hierarchical heterostructures that offer synergic effects between the dual components. As a result, the hybrid material's ion transport kinetics and electrical conductivity enhance the critical electrochemical process at the electrode's interface. The simple co-participation method delivers a remarkable specific capacity (capacitance) of 405 mA h g-1 (1622 F g-1) and 117 mA h g-1 (470 F g-1) for prGO-Mo2VW9O40 and crGO-Mo2VW9O40 nanocomposites alongside high capacitance retentions of 94.5% and 82%, respectively, at a current density of 0.3 A g-1. Furthermore, the asymmetric electrochromic supercapacitor crGO//crGO-Mo2VW9O40 was designed, manifesting a broad operating potential (1.2 V). Finally, the asymmetric electrode material resulted in an enhanced specific capacity, energy, and power of 276.8 C g-1, 46.16 W h kg-1, and 1195 W kg-1, respectively, at a current density of 0.5 A g-1. The electrode materials were tested in the operating of a DC motor.

Skin-adhesive lignin-grafted-polyacrylamide/hydroxypropyl cellulose hydrogel sensor for real-time cervical spine bending monitoring in human-machine Interface

Developing a straightforward method to produce conductive hydrogels with excellent mechanical properties, self-adhesion, and biocompatibility remains a significant challenge. While current approaches aim to enhance mechanical performance, they often require additional steps or external forces for fixation, leading to increased production time and limited practicality. A novel lignin-grafted polyacrylamide/hydroxypropyl cellulose hydrogel (L-g-PAM/HPC hydrogel) with a semi-interpenetrating polymer network structure had been developed in this research that boasted exceptional adhesion to the skin (∼68 kPa) and stretchability properties (∼1637 %) compared to PAM-based hydrogels. By incorporating conductive additives such as silver nanowires and carbon nanocages to construct a bridge-like structure within the hydrogel matrix, the resulting AgC@L-g-PAM/HPC hydrogel exhibited impressive electrical conductivity, surpassing that of other PAM-based hydrogels relying on MXene, with a maximum value of 0.76 S/m. Furthermore, the AgC@L-g-PAM/HPC hydrogel retained its efficient electrical signal transmission capability even under mechanical stress. These make it an ideal flexible strain sensor capable of detecting various human motions. In this study, a smart real-time monitoring system was successfully developed for tracking cervical spine bending, serving as an extension for monitoring human activities.

Electron beam irradiation modified carboxymethyl chitin microsphere-based hemostatic materials with strong blood cell adsorption for hemorrhage control

Timely control of coagulopathy bleeding can effectively reduce the probability of wound infection and mortality. However, it is still a challenge for microsphere hemostatic agents to achieve timely control of coagulopathy bleeding. In this work, the CCM-g-AA@DA hemostatic agent based on carboxymethyl chitin microspheres, CCM, was synthesized using electron beam irradiation-induced grafting polymerization of acrylic acid and coupling with dopamine. Irradiation grafting endowed the microspheres with excellent adsorption performance and a rough surface. The microspheres showed a strong affinity to blood cells, especially red blood cells. The maximum adsorption of red blood cells is up to approximately 100 times that of the original microspheres, the CCM. The introduction of dopamine increased the tissue adhesion of the microspheres. At the same time, the microspheres still possessed good blood compatibility and biodegradability. Furthermore, the CCM-g-AA@DA with Fe3+ achieved powerful procoagulant effects in the rat anticoagulant bleeding model. The bleeding time and blood loss were both reduced by about 90% compared with the blank group, which was superior to that of the commercially available collagen hemostatic agent Avitene™. In summary, the CCM-g-AA@DA hemostatic agent shows promising potential for bleeding control in individuals with coagulation disorders.

Preparation of lignin-based hydrogels, their properties and applications

Polymers obtained from biomass are a concerning alternative to petro-based polymers because of their low cost of manufacturing, biocompatibility, ecofriendly and biodegradability. Lignin as the second richest and the only polyaromatics bio-polymer in plant which has been most studied for the numerous applications in different fields. But, in the past decade, the exploitation of lignin for the preparation of new smart materials with improved properties has been broadly sought, because lignin valorization plays one of the primary challenging issues of the pulp and paper industry and lignocellulosic biorefinery. Although, well suited chemical structure of lignin comprises of many functional hydrophilic and active groups, such as phenolic hydroxyls, carboxyls and methoxyls, which provides a great potential to be applied in the preparation of biodegradable hydrogels. In this review, lignin hydrogel is covered with preparation strategies, properties and applications. This review reports some important properties, such as mechanical, adhesive, self-healing, conductive, antibacterial and antifreezing properties were then discussed. Furthermore, herein also reviewed the current applications of lignin hydrogel, including dye adsorption, smart materials for stimuli sensitive, wearable electronics for biomedical applications and flexible supercapacitors. Overall, this review covers recent progresses regarding lignin-based hydrogel and constitutes a timely review of this promising material.

Synthesis and Characterization of Thermally Stable Lignosulfonamides

Lignin, a highly aromatic macromolecule building plant cells, and cellulose are two of the most commonly occurring natural polymers. Lignosulfonate is a grade of technical lignin, obtained as a by-product in the paper and wood pulping industries, a result of the used lignin isolation method, i.e., sulfite process. In this work, sodium lignosulfonate is used as a starting material to manufacture sulfonamide derivatives of lignin in a two-step modification procedure. Since this direction of the lignin modification is rather rarely investigated and discussed, it makes a good starting point to expand the state of knowledge and explore the properties of lignosulfonamides. Materials obtained after modification underwent characterization by FTIR, SS-NMR, WAXD, SEM, and TGA. Spectroscopic measurements confirmed the incorporation of dihexylamine into the lignin structure and the formation of lignosulfonamide. The crystalline structure of the material was not affected by the modification procedure, as evidenced by the WAXD, with only minute morphological changes of the surface visible on the SEM imaging. The obtained materials were characterized by improved parameters of thermal stability in relation to the raw material. As-prepared sulfonamide lignin derivatives with a potential application as a filler in biopolymeric composites may become a new class of functional, value-added, sustainable additives.

Conductive Hydrogels Based on Industrial Lignin: Opportunities and Challenges

The development of green materials, especially the preparation of high-performance conductive hydrogels from biodegradable biomass materials, is of great importance and has received worldwide attention. As an aromatic polymer found in many natural biomass resources, lignin has the advantage of being renewable, biodegradable, non-toxic, widely available, and inexpensive. The unique physicochemical properties of lignin, such as the presence of hydroxyl, carboxyl, and sulfonate groups, make it promising for use in composite conductive hydrogels. In this review, the source, structure, and reaction characteristics of industrial lignin are provided. Description of the preparation method (physical and chemical strategies) of lignin-based conductive hydrogel is elaborated along with their several important properties, such as electrical conductivity, mechanical properties, and porous structure. Furthermore, we provide insights into the latest research advances in industrial lignin conductive hydrogels, including biosensors, strain sensors, flexible energy storage devices, and other emerging applications. Finally, the prospects and challenges for the development of lignin-conductive hydrogels are presented.

Lignin-containing hydrogels with anti-freezing, excellent water retention and super-flexibility for sensor and supercapacitor applications

We developed a highly conductive and flexible, anti-freezing sulfonated lignin (SL)-containing polyacrylic acid (PAA) (SL-g-PAA-Ni) hydrogel, with a high concentration of NiCl₂. Ni²⁺ contributes multi-functions to the preparation of the hydrogel and its final properties, such as fast polymerization reaction as a result of the presence of redox pairs of Ni³⁺/Ni²⁺ and hydroquinone/quinone, and anti-freezing properties of the hydrogel due to the salt effects of NiCl₂ so that at -20 °C the hydrogel shows similar properties to those at the room temperature. Thanks to the effective coordinations of Ni²⁺ with catecholic groups and carboxylic groups, as well as the rich hydrogen bonding capacity, the resultant hydrogel possesses excellent mechanical properties. High ionic conductivity (6.85 S·m^-1) of the hydrogel is obtained due to the supply of high concentration of Ni²⁺. Moreover, the ionic solvation effect of NiCl₂ in the hydrogel imparts excellent water retention ability, with water retention of ~93 % after 21-day storage. The SL-g-PAA-Ni hydrogel can accurately detect various human motions at -20 °C. The supercapacitor assembled from SL-g-PAA-Al hydrogel at -20 °C manifests a high specific capacitance of 252 F·g^-1, with maximum energy density of 26.97 Wh·kg^-1, power density of 2667 W·kg^-1, and capacitance retention of 96.7 % after 3000 consecutive charge-discharge cycles.

Recent advances in lignosulfonate filled hydrogel for flexible wearable electronics: A mini review

With the rapid development of flexible wearable devices, various polymer hydrogels have gained immense progress due to their adjustable mechanical properties, high conductivity, super sensitivity, good biocompatibility and adaptable wearability. Lignosulfonate (LS), generating from the sulfite pulping industry, was emerged as a promising filler in polymer hydrogels with great potential for multifunctional wearable electronics. Herein, we comprehensively review the latest research progress associated with LS-based hydrogels. Firstly, the function mechanism of lignosulfonate in diverse polymer hydrogels was introduced in detail. Then, the rational design strategies of LS filled multifunctional hydrogels was summarized as toughening filler, adhesive agent, conductive filler dispersant, UV protectant and catalysts. Finally, the future development of LS filled hydrogel for flexible wearable electronics was proposed.

High lignin containing hydrogels with excellent conducting, self-healing, antibacterial, dye adsorbing, sensing, moist-induced power generating and supercapacitance properties

Herein, we design a dynamic redox system of using high contents of lignosulfonate (LS) and Al³⁺ to prepare poly acrylic acid (PAA) (LS-g-PAA-Al) hydrogels. The presence of high LS and Al³⁺ contents, in combination with the effective Al³⁺ complexes formed, renders the resultant hydrogel with some unique attributes, including excellent ionic conductivity (as high as 7.38 S·m^-1) and antibacterial activity; furthermore, a very fast gelation (in 1 min) was obtained. As a flexible strain sensor, the LS-g-PAA-Al hydrogel with high conductivity demonstrates superior sensitivity in human movement detection. In addition, the rich anionic hydrophilic groups, such as sulfonic groups, phenolic hydroxyl groups, in the hydrogels impart the resultant hydrogels with excellent adsorption capacity for cationic dyes: when using Rhodamine B (RB) as a model cationic dye, the adsorption capacity of the resultant hydrogel reaches 334.64 mg·g^-1; as a moist-induced power generator, it generates maximum 150.5 mV open circuit voltage with moist air flow. When the hydrogel electrolyte is assembled into a supercapacitor assembly, it shows high specific capacitance of 245.4 F·g^-1, with the maximum energy density of 21.8 Wh·kg^-1, power density of 2.37 kW·kg^-1, and capacitance retention of 95.1% after 5000 consecutive charge-discharge cycles.