Scalable High-Power Redox Capacitors with Aligned Nanoforests of Crystalline MnO₂Nanorods by High Voltage Electrophoretic Deposition

Abstract

It is commonly perceived that reduction-oxidation (redox) capacitors have to sacrifice power density to achieve higher energy density than carbon-based electric double layer capacitors. In this work, we report the synergetic advantages of combining the high crystallinity of hydrothermally synthesized α-MnO2 nanorods with alignment for high performance redox capacitors. Such an approach is enabled by high voltage electrophoretic deposition (HVEPD) technology which can obtain vertically aligned nanoforests with great process versatility. The scalable nanomanufacturing process is demonstrated by roll-printing an aligned forest of α-MnO2 nanorods on a large flexible substrate (1 inch by 1 foot). The electrodes show very high power density (340 kW/kg at an energy density of 4.7 Wh/kg) and excellent cyclability (over 92% capacitance retention over 2000 cycles). Pretreatment of the substrate and use of a conductive holding layer have also been shown to significantly reduce the contact resistance between the aligned nanoforests and the substrates. High areal specific capacitances of around 8500 μF/cm(2) have been obtained for each electrode with a two-electrode device configuration. Over 93% capacitance retention was observed when the cycling current densities were increased from 0.25 to 10 mA/cm(2), indicating high rate capabilities of the fabricated electrodes and resulting in the very high attainable power density. The high performance of the electrodes is attributed to the crystallographic structure, 1D morphology, aligned orientation, and low contact resistance.

Fast Supercapacitors Based on Graphene‐Bridged V2O3/VOx Core–Shell Nanostructure Electrodes with a Power Density of 1 MW kg−1

“…We emphasize that Meng and co-workers [ 46 ] recently reported MnO 2 nanorods-based EC with a power density of 340 kW kg −1 at an energy density of 4.7 Wh kg −1 , and more interestingly, Wang et al [ 47 ] reported 3D graphene/VO 2 nanobelt composite hydrogels as electrodes of supercapacitor that had high-capacitance and high-rate capability with a power density at least 300 kW kg −1 at an energy density of 12 Wh kg −1 . These recordlevel power density is close to, if not higher than the ordinary electrolytic capacitors that are regarded much fast than ECs but with a tiny energy density.…”

The influence of large cations on the electrochemical properties of tunnel-structured metal oxides

“…α-MnO 2 nanowires were synthesized hydrothermally using KMnO 4 and MnSO 4 as reactants in aqueous solution14. Specifically, 0.9878, g of KMnO 4 and 0.4226, g of MnSO 4 ·H 2 O were dispersed in 80 ml of deionized (DI) water under constant stir for 30 min to form a purple solution.…”

“…In addition, Mn-based oxides are low cost, environmentally friendly and safer with respect to over-charge conditions compared with Co-based oxide electrodes7. Consequently, α-MnO 2 has been extensively studied as a promising cathode material for lithium (sodium and magnesium) ion batteries891011, lithium air batteries1213 and supercapacitors141516.…”

Oxygen Evolution Assisted Fabrication of Highly Loaded Carbon Nanotube/MnO2 Hybrid Films for High‐Performance Flexible Pseudosupercapacitors

“…Previous studies mainly play balance between capacitance, conductivity, and flexibility. For examples, hierarchical nanostructures based on carbon cloths and metallic foils were used as the large‐specific‐area substrates for the coating of MnO 2 nanostructures, but the areal capacitances of these electrodes were usually lower than 200 mF cm −2 due to the low loading mass of MnO 2 , and the capacitances of assembled supercapacitors were lower than 50 mF cm −2 , far from the practical application requirements. This problem should be attributed to the low specific area and high density of these scaffolds.…”