High Mass-Loading Biomass-Based Porous Carbon Electrodes for Supercapacitors: Review and Perspectives

Biomass-based porous carbon (BPC) with renewability and flexible nano/microstructure tunability has attracted increasing attention as efficient and cheap electrode materials for supercapacitors. To meet commercial needs, high mass-loading electrodes with high areal capacitance are preferred when designing supercapacitors. The increased mass percentage of active materials can effectively improve the energy density of supercapacitors. However, as the thickness of the electrode increases, it will face the following challenges including severely blocked ion transport channels, poor charging dynamics, poor electrode structural stability, and complex preparation processes. A bridge between theoretical research and practical applications of BPC electrodes for supercapacitors needs to be established. In this review, the advances of high mass-loading BPC electrodes for supercapacitors are summarized based on different biomass precursors. The key performance evaluation parameters of the high mass-loading electrodes are analyzed, and the performance influencing factors are systematically discussed, including specific surface area, pore structure, electrical conductivity, and surface functional groups. Subsequently, the promising optimization strategies for high mass-loading electrodes are summarized, including the structure regulation of electrode materials and the optimization of other supercapacitor components. Finally, the major challenges and opportunities of high mass-loading BPC electrodes in the future are discussed and outlined.

Pentafluoropyridine functionalized novel heteroatom-doped with hierarchical porous 3D cross-linked graphene for supercapacitor applications

The fabrication with high energy density and superior electrical/electrochemical properties of hierarchical porous 3D cross-linked graphene-based supercapacitors is one of the most urgent challenges for developing high-power energy supplies. We facilely synthesized a simple, eco-friendly, cost-effective heteroatoms (nitrogen, phosphorus, and fluorine) co-doped graphene oxide (NPFG) reduced by hydrothermal functionalization and freeze-drying approach with high specific surface areas and hierarchical pore structures. The effect of different heteroatoms doping on the energy storage performance of the synthesized reduced graphene oxide is investigated extensively. The electrochemical analysis performed in a three-electrode system via cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance spectroscopy (EIS) demonstrates that the nitrogen, phosphorous, and fluorine co-doped graphene (NPFG-0.3) synthesized with the optimum amount of pentafluoropyridine and phytic acid (PA) exhibits a notably enhanced specific capacitance (319 F g-1 at 0.5 A g-1), good rate capability, short relaxation time constant (τ = 28.4 ms), and higher diffusion coefficient of electrolytic cations (Dk+ = 8.8261 × 10-9 cm2 s-1) in 6 M KOH aqueous electrolyte. The density functional theory (DFT) calculation result indicates that the N, F, and P atomic replacement within the rGO model could increase the energy value (G T) from -673.79 eV to -643.26 eV, demonstrating how the atomic level energy could improve the electrochemical reactivity with the electrolyte. The improved performance of NPFG-0.3 over NFG, PG, and pure rGO is mainly ascribed to the fast-kinetic process owing to the well-balanced electron/ion transport phenomenon. A symmetric coin cell supercapacitor device fabricated using NPFG-0.3 as the anode and cathode material with 6 M KOH aqueous electrolyte exhibits maximum specific energy of 38 W h kg-1, a maximum specific power of 716 W kg-1, and ∼88.2% capacitance retention after 10 000 cycles. The facile synthesis approach and promising electrochemical results suggest this synthesized NPFG-0.3 material has high potential for future supercapacitor application.

Nile Blue Functionalized Graphene Aerogel as a Pseudocapacitive Negative Electrode Material across the Full pH Range

The pursuit of new negative electrode materials for redox supercapacitors with a high capacitance, boosted energy, and high rate capability is still a tremendous challenge. Herein, we report a Nile Blue conjugated graphene aerogel (NB-GA) as a negative electrode material with excellent pseudocapacitive performance (with specific capacitance of up to 483 F g^-1 at 1 A g^-1) in all acidic, neutral, and alkaline aqueous electrolytes. The contribution from capacitive charge storage represents 93.4% of the total charge, surpassing the best pseudocapacitors known. To assess the feasibility of NB-GA as a negative electrode material across the full pH range, we fabricated three devices, namely, a symmetric NB-GA||NB-GA device in an acidic (1.0 M H₂SO₄) electrolyte, an NB-GA||MnO₂ device in a pH-neutral (1.0 M Na₂SO₄) electrolyte, and an NB-GA||LDH (LDH = Ni-Co-Fe layered double hydroxide) device in an alkaline (1.0 M KOH) electrolyte. The NB-GA||NB-GA device exhibits a maximum specific energy of 22.1 Wh kg^-1 and a specific power of up to 8.1 kW kg^-1; the NB-GA||MnO₂ device displays a maximum specific energy of 55.5 Wh kg^-1 and a specific power of up to 14.9 kW kg^-1, and the NB-GA||LDH device shows a maximum specific energy of 108.5 Wh kg^-1 and a specific power of up to 25.1 kW kg^-1. All the devices maintain excellent stability over 5000 charge-discharge cycles. The outstanding pseudocapacitive performances of the NB-GA nanocomposites render them a highly promising negative electrode material across the entire pH range.

Biomass-Derived Porous Carbon Materials for Supercapacitor

The fast consumption of fossil energy accompanied by the ever-worsening environment urge the development of a clean and novel energy storage system. As one of the most promising candidates, the supercapacitor owns unique advantages, and numerous electrodes materials have been exploited. Hence, biomass-derived porous carbon materials (BDPCs), at low cost, abundant and sustainable, with adjustable dimension, superb electrical conductivity, satisfactory specific surface area (SSA) and superior electrochemical stability have been attracting intense attention and highly trusted to be a capable candidate for supercapacitors. This review will highlight the recent lab-scale methods for preparing BDPCs, and analyze their effects on BDPCs' microstructure, electrical conductivity, chemical composition and electrochemical properties. Future research trends in this field also will be provided.

Mesoporous activated carbon from starch for superior rapid pesticides removal

Pesticides contamination of water has caused considerable concern due to the potential hazard to human health. For the first time, mesoporous activated carbon from starch (ACS) was applied to remove pesticides from water. ACS could remove 11 pesticides rapidly (shake five times). The adsorption rates of ACS (>80%) for the 11 pesticides were higher than those of other adsorbents, including commercial activated carbon (AC), graphitised carbon black (GCB), C18, and primary secondary amine adsorbent (PSA). The mechanisms of the adsorption process for pyraclostrobin were also investigated. The pseudo-second-order model could better describe the adsorption for pyraclostrobin (R² = 0.99950). Langmuir model gave the best fit for the isotherm data (R² = 0.99899). Our findings demonstrate that oxygen-containing functional groups, N atom and π-bonding network of benzene promoted the adsorption. The adsorption efficiency of the ACS for 11 pesticides was still over 80% after five cycles.

Nitrogen and sulfur co-doped graphene aerogel for high performance supercapacitors

The development of high energy density and power density supercapacitors is very necessary in energy storage and application fields. A key factor of such devices is high-performance electrode materials. In this work, nitrogen and sulfur co-doped graphene aerogels (N/S-GA) were synthesized using graphene oxide as the precursor and 2-mercapto-1-methylimidazole as both the reducing agent and the N/S doping agent. The pore size distribution of the as-prepared N/S-GA was measured and the N/S-GA possesses a hierarchical porous structure. As an electrode material of supercapacitors, the N/S-GA could provide a suitable structure for charge accommodation and a short distance for ion transport. When 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4) ionic liquid was used as the electrolyte, the specific capacitance of the N/S-GA electrode material reached 212 F g-1 and 162 F g-1 at the current densities of 1 A g-1 and 10 A g-1, respectively. And the energy density and average power density of the N/S-GA based supercapacitor could reach 117 W h kg-1 and 1.0 kW kg-1 at 1 A g-1, 82 W h kg-1 and 9.5 kW kg-1 at 10 A g-1, respectively. It is believed that the N/S-GA material can be used in high-performance supercapacitors.