Redox (reduction-oxidation) capacitors are hybrids between physical supercapacitors, which release a burst of energy and discharge quickly, and batteries, which store more energy and release it gradually over a longer period. Typically, redox (chemical) capacitors have more energy and less power than the physical ones.

MTU scientist Dennis Desheng Meng theorized that the situation could be improved if manganese dioxide—which is attractive as an energy storage material—were made into nanorods. However, a stumbling block has been making manganese dioxide nanorods with the right set of attributes. Until now, researchers have been able to grow nanorods that either have the best crystalline structure or were aligned, but not both.

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.

—Santhanagopalan et al.

The chemical capacitors made with Meng’s manganese dioxide nanorods hold more energy plus they yield even more power than a comparable carbon-based physical capacitor.

The electrodes show very high power density (340 kW/kg at an energy density of 4.7 Wh/kg) and excellent cyclability (more than 92% capacitance retention over 2000 cycles). High areal specific capacitances of around 8500 μF/cm² were obtained for each electrode with a two-electrode device configuration. More than 93% capacitance retention was observed when the cycling current densities were increased from 0.25 to 10 mA/cm₂, indicating high rate capabilities of the fabricated electrodes and resulting in the very high attainable power density.

The researchers attributed the high performance of the electrodes to the crystallographic structure, 1D morphology, aligned orientation, and low contact resistance.

Resources

• Sunand Santhanagopalan, Anirudh Balram, and Dennis Desheng Meng (2013) Scalable High-Power Redox Capacitors with Aligned Nanoforests of Crystalline MnO₂ Nanorods by High Voltage Electrophoretic Deposition. ACS Nano 7 (3), 2114-2125 doi: http://pubs.acs.org/doi/abs/10.1021/nn3044462