Three-dimensional crisscross porous manganese oxide/carbon composite networks for high performance supercapacitor electrodes

Joule Heating in Controlled Atmospheres to Process Nanocarbon/Transition Metal Oxide Composites and Electrodes

Composites of nanocarbons and transition metal oxides combine excellent mechanical properties and high electrical conductivity with high capacitive active sites. These composites are promising for applications such as electrochemical energy conversion and storage, catalysis, and sensing. Here, we show that Joule heating can be used as a rapid out-of-oven thermal processing technique to crystallize the inorganic metal oxide matrix within a carbon nanotube fabric (CNTf) composite. We choose manganese oxide and vanadium oxide as model metal oxides and show that the Joule heating process is rapid and enables accurate control over the temperature and phase transitions. Next, we use thermogravimetric analysis and Joule heating experiments in controlled atmospheres to show that metal oxides can actually catalyze thermal degradation and reduce the thermal stability of the CNTs, which could limit processing of many oxides. We solve this by using a reducing hydrogen atmosphere to successfully extend the Joule processing window and thermal stability of the CNTf/metal oxide composite to ∼1000 °C.

Flexible quasi-solid-state supercapacitors for anti-freezing power sources based on polypyrrole@cation-grafted bacterial cellulose

Polypyrrole (PPy)/cellulose nanofiber (CNF) composites have been widely used in flexible energy storage devices because of their lightweight, inherent mechanical flexibility and large specific surface area. However, it is still a challenge to obtain PPy/CNF composite electrodes with high cycling stability. Herein, an electrostatic self-assembly strategy was adopted to deposit anion-doped PPy onto cationic poly(methacryloxyethyltrimethyl ammonium chloride)-grafted bacterial cellulose (BCD) nanofibers. The optimized PPy@BCD electrode demonstrated a high areal capacitance of 6208 mF cm-2 at a current density of 0.5 mA cm-2 and superior cycling stability (a capacitance retention of 100 % after 10,000 charge-discharge cycles at 10 mA cm-2). A quasi-solid-state anti-freezing flexible supercapacitor (AF-FSC) was designed by employing polyacrylamide organohydrogel electrolyte, yielding an areal capacitance of 2930.6 mF cm-2 at 1 mA cm-2 and a capacitance retention of 92.2 % after 1000 cycles at -20 °C. The present AF-FSC is expected to serve as a power source in real-life low-temperature applications.

MOF-Derived MnO/C Nanocomposites for High-Performance Supercapacitors

As ordered porous materials, metal-organic frameworks (MOFs) have attracted tremendous attention in the field of energy conversion and storage due to their high specific surface area, permanent porosity, and tunable pore sizes. Here, MOF-derived MnO/C nanocomposites with regular octahedral shape were synthesized using a Mn-based analogue of the MIL-100 framework (Mn-MIL-100, MIL: Matérial Institut Lavoisier) as the precursor. Using aberration-corrected environmental transmission electron microscopy (ETEM), MnO nanocages with a diameter of approximately 20 nm were recognized in the MnO/C nanocomposites fabricated, dispersed in a microporous carbon matrix homogeneously. The nanocages are composed of MnO nanoparticles with a diameter of approximately 2 nm and with a single crystal structure. The specific surface area of the as-prepared MnO/C octahedra decreases to 256 m² g^-1 from 507 m² g^-1 of the Mn-MIL-100 precursor, whereas the total pore volume increases to 0.245 cm³ g^-1, which is approximately 29% higher than that of the precursor (0.190 cm³ g^-1). Additionally, when utilized as an electrode for supercapacitors, the MOF-derived MnO/C nanocomposite demonstrates a towering specific capacitance of 421 F g^-1 at 0.5 A g^-1 and good cycle stability (94%) after 5000 cycles. Our work reveals that the MnO nanoparticles in MOF-derived MnO/C nanocomposites exhibit nanocage structure characteristics, which might be inherited from the Mn-MIL-100 precursor with analogous supertetrahedron units.

Polypyrrole/SnCl2 modified bacterial cellulose electrodes with high areal capacitance for flexible supercapacitors

Polypyrrole (PPy)/bacterial cellulose (BC) composite membranes are a promising kind of lightweight and flexible electrodes for supercapacitors. Herein, we explored a facile and efficient electrostatic self-assembly approach to uniformly depositing anion-doped PPy onto positively charged SnCl₂-modifed BC (SBC). The obtained PPy@SBC electrode exhibited a high areal capacitance of 5718 mF cm^-2 at a current density of 0.5 mA cm^-2, a desirable capacitance retention of 83.1% at 5.0 mA cm^-2 and excellent cycling stability (a capacitance retention of 86.8% after 10,000 cycles at 10 mA cm^-2). A symmetric flexible supercapacitor was further assembled with the PPy@SBC electrodes, which delivered outstanding mechanical flexibility with negligible capacitance decay under different bent states. This study shows impressive potential in fabricating high-performance electrodes for flexible supercapacitors.

Molten-Salt-Assisted Synthesis of Hierarchical Porous MnO@Biocarbon Composites as Promising Electrode Materials for Supercapacitors and Lithium-Ion Batteries

Biomass-derived carbon composites (e.g., metal oxide/biocarbon) have been used as promising electrode materials for energy storage devices owing to their natural abundance and simple preparation process. However, low loading content/inhomogeneous distribution of metal oxides and inefficient cracking of biocarbon (BC) are intractable obstacles that impede the efficient utilization of biomass. In this work, hierarchical porous MnO/BC composites were prepared by a facile molten-salt-assisted strategy based on the superior salt-water absorption ability of agaric. The addition of NaCl induces a liquid reaction medium by formation of a molten salt mixture at high temperature to effectively realize the activation and cracking of the bulk carbon, and it also acts as a recyclable sacrificial template to form mesopores and macropores in the as-prepared hierarchical porous MnO/BC composites. The highly porous and uniform BC framework effectively enhances ion diffusion and electron-transfer ability, serves as a protective layer to prevent fracturing and agglomeration of MnO, and thus enables superior rate performance and cycling stability of the MnO/BC composite for both supercapacitor electrodes (94 % capacity retention at 20 mA cm^-2 after 5000 cycles) and lithium-ion battery anodes (783 mA h g^-1 after 1000 cycles). Notably, considering the simple and low-cost preparation process, this work opens a promising avenue for the large-scale production of advanced metal oxide/BC hybrid electrode materials for electrochemical energy storage.