Unveiling the Formation of Solid Electrolyte Interphase and its Temperature Dependence in "Water-in-Salt" Supercapacitors

Toward Ultrahigh-Rate Energy Storage of 3000 mV s-1 in Hollow Carbon: From Methodology to Surface-to-Bulk Synergy Insights

Despite great efforts on economical and functionalized carbon materials, their scalable applications are still restricted by the unsatisfying energy storage capability under high-rate conditions. Herein, theoretical and methodological insights for surface-to-bulk engineering of multi-heteroatom-doped hollow porous carbon (HDPC), with subtly designed Zn(OH)F nanoarrays as the template are presented. This fine-tuned HDPC delivers an ultrahigh-rate energy storage capability even at a scan rate of 3000 mV s-1 (fully charged within 0.34 s). It preserves a superior capacitance of 234 F g-1 at a super-large current density of 100 A g-1 and showcases an ultralong cycling life without capacitance decay after 50 000 cycles. Through dynamic and theoretical analysis, the key role of in situ surface-modified heteroatoms and defects in decreasing the K+-adsorption/diffusion energy barrier is clarified, which cooperates with the porous conductive highways toward enhanced surface-to-bulk activity and kinetics. In situ Raman aids in visualizing the reversibly dynamic adsorption/releasing of the electrolyte ions on the tailored carbon structure during the charge/discharge process. The potential of the design concept is further evidenced by the enhanced performances in water-in-salt electrolytes. This surface-to-bulk nanotechnology opens the path for developing high-performance energy materials to better meet the practical requirements in the future.

Understanding the Behavior of Supercapacitor Materials via Electrochemical Impedance Spectroscopy: A Review

Energy harvesting and energy storage are two critical aspects of supporting the energy transition and sustainability. Many studies have been conducted to achieve excellent performance devices for these two purposes. As energy-storing devices, supercapacitors (SCs) have tremendous potential to be applied in several sectors. Some electrochemical characterizations define the performance of SCs. Electrochemical impedance spectroscopy (EIS) is one of the most powerful analyses to determine the performance of SCs. Some parameters obtained from this analysis include bulk resistance, charge-transfer resistance, total resistance, specific capacitance, response frequency, and response time. This work provides a holistic and comprehensive review of utilizing EIS for SC characterization. Overall, researchers can benefit from this review by gaining a comprehensive understanding of the utilization of electrochemical impedance spectroscopy (EIS) for characterizing supercapacitors (SCs), enabling them to enhance SC performance and contribute to the advancement of energy harvesting and storage technologies.

Toward Ultrahigh-Rate Energy Storage of 3000 mV s-1 in Hollow Carbon: From Methodology to Surface-to-Bulk Synergy Insights

Despite great efforts on economical and functionalized carbon materials, their scalable applications are still restricted by the unsatisfying energy storage capability under high-rate conditions. Herein, theoretical and methodological insights for surface-to-bulk engineering of multi-heteroatom-doped hollow porous carbon (HDPC) is presented, with subtly designed Zn(OH)F nanoarrays as the template. This fine-tuned HDPC delivers an ultrahigh-rate energy storage capability even at a scan rate of 3000 mV s-1 (fully charged within 0.34 s). It preserves a superior capacitance of 234 F g-1 at a super-large current density of 100 A g-1 and showcases an ultralong cycling life without capacitance decay after 50 000 cycles. Through dynamic and theoretical analysis, the key role of in situ surface-modified heteroatoms and defects in decreasing the K+ -adsorption/diffusion energy barrier is clarified, which cooperates with the porous conductive highways toward enhanced surface-to-bulk activity and kinetics. In situ Raman further aids in visualizing the reversibly dynamic adsorption/releasing of the electrolyte ions on the tailored carbon structure during the charge/discharge process. The potential of the design concept is further evidenced by the enhanced performances in water-in-salt electrolytes. This surface-to-bulk nanotechnology opens the path for developing high-performance energy materials to better meet the practical requirements in future.

Oil palm leaf-derived hierarchical porous carbon for "water-in-salt" based supercapacitors: the effect of anions (Cl- and TFSI-) in superconcentrated conditions

This study investigates the use of a hierarchical porous carbon electrode derived from oil palm leaves in a "water-in-salt" supercapacitor. The impact of anion identity on the electrical performance of the carbon electrode was also explored. The results show that the prepared carbon had a hierarchical porous structure with a high surface area of up to 1840 m2 g-1. When a 20 m LiTFSI electrolyte was used, the carbon electrode had a specific capacitance of 176 F g-1 with a wider potential window of about 2.6 V, whereas the use of a cheaper 20 m LiCl electrolyte showed a higher specific capacitance of 331 F g-1 due to the smaller size of the Cl- anion, which enabled inner capacitance. Therefore, the anion identity has an effect on the electrochemical performance of porous carbon, and this research contributes to the understanding of using "water-in-salt" electrolytes in carbon-based supercapacitors. The study's findings provide insights into developing low-cost, high-performance supercapacitors that can operate in a wider voltage range.

Chitosan-reinforced gelatin composite hydrogel as a tough, anti-freezing, and flame-retardant gel polymer electrolyte for flexible supercapacitors

Hydrogel-based electrolytes for flexible solid-state supercapacitors (SSCs) have received significant attention due to their mechanical robustness and stable electrochemical performance over a wide temperature range. However, achieving flame retardancy in such SSCs at subzero temperatures to increase their practical utility remains challenging. Furthermore, there is a need for sustainable and bio-friendly SSCs that use natural polymer-based hydrogel electrolytes. This study reports a novel approach for developing a chitosan-reinforced anti-freezing ionic conductive gelatin hydrogel to meet these demands. Immersion of chitosan-containing gelatin hydrogels in salt solutions caused chitosan precipitation, resulting in composite hydrogels. The precipitated chitosan contributes to the reinforcement of the gelatin hydrogel network, resulting in a high mechanical toughness of up to 3.81 MJ/m³, a fracture energy of 26 kJ/m², anti-freezing properties (below -30 °C), and excellent flame retardancy without softening. Furthermore, the hydrogel exhibits excellent electrochemical performance, with an ionic conductivity ranging from 72 mS/cm at room temperature (26 °C) to 39 mS/cm at -30 °C. The proposed hydrogel exhibits potential for use in SSC as a gel polymer electrolyte. This study demonstrates a novel strategy for controlling the mechanical, thermal, and electrochemical characteristics of flexible supercapacitors using biological macromolecules.

Spatial Confinement Strategy for Micelle-Size-Mediated Modulation of Mesopores in Hierarchical Porous Carbon Nanosheets with an Efficient Capacitive Response

Commercial supercapacitors using available carbon products have long been criticized for the under-utilization of their prominent specific surface area (SSA). In terms of carbonaceous electrode optimization, excessive improvement in SSA observed in the gaseous atmosphere might have little effect on the final performance because cracked/inaccessible pore alleys considerably block the direct electrolyte ion transport in a practical electrochemical environment. Herein, mesopore-adjustable hierarchically porous carbon nanosheets are fabricated based on a micelle-size-mediated spatial confinement strategy. In this strategy, hydrophobic trimethylbenzene in different volumes of the solvent can mediate the interfacial assembly with a carbon precursor and porogen segment through π-π bonding and van der Waals interaction to yield micelles with good dispersity and the diameter varying from 119 to 30 nm. With an increasing solvent volume, the corresponding diffusion coefficient (3.1-14.3 m2 s-1) of as-obtained smaller micelles increases, which makes adjacent micelles gather rapidly and then grow along the radial direction of oligomer aggregates to eventually form mesopores on hierarchically porous carbon nanosheets (MNC150-4.5). Thanks to the pore-expansion effect of trimethylbenzene, the mesoporous volume can be adjusted from 28.8 to 40.0%. Mesopores on hierarchically porous carbon nanosheets endow MNC150-4.5 with an enhanced electrochemically active surface area of 819.5 m2 g-1, which gives rise to quick electrolyte accessibility and a correspondingly immediate capacitive response of 338 F g-1 at 0.5 A g-1 in a three-electrode system. Electrolyte transport through pathways within MNC150-4.5 ultimately enables the symmetric cell to deliver a high energy output of 50.4 Wh kg-1 at 625 W kg-1 in a 14 m LiOTF electrolyte and 95% capacitance retention after 100,000 cycles, which show its superior electrochemical performance over representative carbon-based supercapacitors with aqueous electrolytes in recent literature.

In Situ Growing BCN Nanotubes on Carbon Fibers for Novel High-Temperature Supercapacitor with Excellent Cycling Performance

Carbon nanomaterials have elicited much research interest in the energy storage field, but most of them cannot be used at high temperatures. Thus, a supercapacitor with high energy and desired stability at high temperatures is urgently required. Herein, BCN nanotubes (BCNNTs) with excellent performance at high temperatures are generated on carbon fibers by optimizing the ratio of B and N. The nanotubes' morphology can effectively alleviate the structural damage caused by the rapid adsorption/desorption of the electrolyte during long-time charge/discharge cycles at high temperatures, thus improving the high-temperature cycle stability. The symmetric supercapacitors that are assembled with the binder-free BCNNT electrode in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM·BF₄ ) exhibited a high areal capacitance of 177.1 mF cm^-2 at a current density of 5 mA cm^-2 , and capacitance retention is maintained up to 86.1% for 5000 cycles at 100 °C. Moreover, the flexible supercapacitor based on BCNNTs in poly(vinylidenefluoride hexafluoropropylene)/EMIM·BF₄ /succinonitrile gel electrolyte also exhibits good volumetric capacitance (1.98 mWh cm^-3 at a current density of 5 mA cm^-2 ) and cycling stability (92.6% retention after 200 charge/discharge cycles) at a temperature of 100 °C. This work shows that binder-free BCNNTs are promising materials for high-temperature flexible energy storage devices.

The Electrochemical Behaviour of Quaternary Amine-Based Room-Temperature Ionic Liquid N4111(TFSI)

In this study, we used the in situ X-ray photoelectron spectroscopy (XPS), in situ mass spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy methods, for the first time, in a detailed exploration of the electrochemical behaviour of a quaternary amine cation-based room-temperature ionic liquid, butyl-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide (N4111(TFSI)), at the negatively and positively polarised molybdenum carbide-derived micro-mesoporous carbon (mmp-C(Mo2C)) electrodes that can be used as high surface area supporting material for electrocatalysts. The shapes of the C 1s, N 1s, O 1s, F 1s and S 2p XPS spectra were stable for N4111(TFSI) within a very wide potential range. The XPS data indicated the non-specific adsorption character of the cations and anions in the potential range from −2.00 V to 0.00 V. Thus, this region can be used for the detailed analysis of catalytic reaction mechanisms. We observed strong adsorption from 0.00 V to 1.80 V, and at E > 1.80 V, very strong adsorption of the N4111(TFSI) at the mmp-C(Mo2C) took place. At more negative potentials than −2.00 V, the formation of a surface layer containing both N4111+ cations and TFSI− anions was established with the formation of various gaseous compounds. Collected data indicated the electrochemical instability of the N4111+ cation at E < −2.00 V.

High temperature supercapacitors using water-in-salt electrolytes: stability above 100 °C

The high temperature performance of water-in-salt electrolytes was investigated using a carbon-based electrode with commercial cell components. Supercapacitors using 21 m Li bis(trifluoromethylsulphonyl)imide (TFSI) and 21 m LiTFSI + 7 m Li trifluoromethanesulphonyl electrolytes are shown to operate at a voltage of 2 V at 100 °C and 120 °C, respectively, with gravimetric capacitances exceeding 100 F g-1.