Advanced Tri-Layer Carbon Matrices with π-π Stacking Interaction for Binder-Free Lithium-Ion Storage

Enabling materials with distinct features toward achieving high-performance energy storage devices is of huge importance but highly challenging. Commercial carbon cloth (CC), because of its appealing chemical and mechanical properties, has been proven to be an excellent conductive substrate for active electrode materials. However, its performance is notably poor when directly used as an electrode in energy storage, due to its low theoretical capacity and surface area. Herein, we successfully endow the CC with enhanced storage capacity via formation of a π-π stacking interaction by integrating electrochemically activated CC (denoted CC/ACC) with biomass-derived carbon (BMDC) (denoted π-CC/ECC@BMDC). The π-CC/ECC@BMDC electrode displays excellent storage performance with a high capacity of 2.53 mAh cm^-2 under 0.2 mA cm^-2 when used as anode material for lithium ion batteries (LIBs). Due to the induction energy, the negatively charged molecules of the CC/ACC functional groups interact with the BMDC during carbonization, creating the π-π stacking interaction. Based on first-principles calculations, the structural design of the tri-layer carbon enables the movement of electrons around the π-π stacking interaction, which significantly facilitates rapid transportation of electrons, creates three-dimensional (3D) ion tunnels for fast transportation of ions, and improves the electrode's mechanical and electronic properties.

Synthesis and electrochemical performance of an electroactive nitrogen-doping SnO2 nanoarray supported on carbon fiber

An electroactive nitrogen-doping tin dioxide nanorod array (N-SnO2 NRA) is designed as an effective energy-storage electrode material for supercapacitor applications. N-SnO2 supported on a carbon fiber substrate is prepared using SnCl4 as a precursor through hydrolysis, hydrothermal growth, and an NH3-nitriding process. Electroactive N-SnO2 is formed by an N-doping reaction between Sn(OH)4 and NH3, revealing a high nitrogen-doping level of 12.5% in N-SnO2. N-SnO2/carbon fiber reveals a lower ohmic resistance and charge transfer resistance than SnO2/carbon fiber, which is consistent with its higher current response and lower voltage drop in electrochemical measurements. N-SnO2 NRA has an independent nanoarray structure and a small side length of a quadrangular nanorod, contributing to a more accessible interspace, reactive sites, and feasible electrolyte ion diffusion. The N-SnO2/carbon fiber NRA electrode shows higher specific capacitance (105.4 F g−1 at 0.5 A g−1) and rate capacitance retention (45.0% from 0.5 to 5 A g−1) than a SnO2/carbon fiber NRA electrode (58.6 F g−1, 38.4%). Significantly, the cycling capacitance retention after 2000 cycles increases from 78.1% of SnO2/carbon fiber to 98.8% of N-SnO2/carbon fiber, presenting a superior electrochemical cycling stability. The N-SnO2 supercapacitor maintains stable power working at an output voltage of 1.6 V. The specific capacitance decreases from 75.2 to 55.1 F g−1 when the current density increases from 1 to 10 A g−1. The corresponding energy density decreases from 24.23 to 9.81 Wh kg−1, presenting a reasonable rate capability. So, the prepared N-SnO2 nanorod array demonstrates superior capacitance performance for energy-storage applications.

Fabrication and charge storage capacitance of PPY/TiO2/PPY jacket nanotube array

A PPY/TiO2/PPY jacket nanotube array was fabricated by coating PPY layer on the external and internal surface of a tube wall-separated TiO2 nanotube array. It shows coaxial triple-walled nanotube structure with two PPY nanotube layers sandwiching one TiO2 nanotube layer. PPY/TiO2/PPY reveals much higher current response than TiO2. The theoretical calculation indicates PPY/TiO2/PPY reveals higher density of states and lower band gap, accordingly presenting higher conductivity and electroactivity, which is consistent with the experimental result of a higher current response. The electroactivity is highly enhanced in H2SO4 rather than Na2SO4 electrolyte due to feasible pronation process of PPY in an acidic solution. PPY/TiO2/PPY could conduct the redox reaction in H2SO4 electrolyte which involves the reversible protonation/deprotonation and HSO4 − doping/dedoping process and accordingly contributes to Faradaic pseudocapacitance. The specific capacitance is highly enhanced from 1.7 mF cm−2 of TiO2 to 123.4 mF cm−2 of PPY/TiO2/PPY at 0.1 mA cm−2 in H2SO4 electrolyte. The capacitance also declines from 123.4 to 31.7 mF cm−2 when the current density increases from 0.1 to 1 mA cm−2, presenting the rate capacitance retention of 26.7% due to the semiconductivity of TiO2. A PPY/TiO2/PPY jacket nanotube with high charge storage capacitance is regarded as a promising supercapacitor electrode material.

Electrochemical performance of activated carbon fiber with hydrogen bond-induced high sulfur/nitrogen doping

The sulfur/nitrogen co-doped activated carbon fiber (S/N-ACF) is prepared by the thermal treatment of thiourea-bonded hydroxyl-rich carbon fiber, which can bond the decomposition products of thiourea through hydrogen bond interaction to avoid the significant loss of sulfur and nitrogen sources during the thermal treatment process. The sulfur/nitrogen co-doped carbon fiber (S/N-CF) is prepared by the thermal treatment of thiourea-adsorbed carbon fiber. The doping degree of the carbon fiber is improved by reasonable strategy. S/N-ACF shows a higher amount of S/N doping (4.56 at% N and 3.16 at% S) than S/N-CF (1.25 at% N and 0.61 at% S). S/N-ACF with high S/N doping level involves highly active sites to improve the capacitive performance, and high delocalization electron to improve the conductivity and rate capability when compared with the normal S/N co-doped carbon fiber (S/N-CF). Accordingly, the specific capacitance increases from 1196 mF cm⁻² for S/N-CF to 2704 mF cm⁻² for S/N-ACF at 1 mA cm⁻². The all-solid-state flexible S/N-ACF supercapacitor achieves 184.7 μW h cm⁻² at 350 μW cm⁻². The results suggest that S/N-ACF has potential application as a CF-based supercapacitor electrode material.

Sulfur/nitrogen co-doped activated carbon fiber is prepared by thermal treatment of thiourea-bonded hydroxyl-rich carbon fiber, which achieves high doping level and electrochemical performance.