Sunand Santhanagopalan

Scalable High-Power Redox Capacitors with Aligned Nanoforests of Crystalline MnO2 Nanorods by High Voltage Electrophoretic Deposition

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.

High-Voltage Electrophoretic Deposition for Vertically Aligned Forests of One-Dimensional Nanoparticles

Deposition of aligned forests of 1D nanoparticles (carbon nanotubes and MnO(2) nanorods) on conductive, including flexible and transparent, substrates has been achieved at room temperature. The process, named high-voltage electrophoretic deposition (HVEPD), has been enabled by three key elements: high deposition voltage for alignment, low dispersion concentration of the nanoparticles to avoid aggregation, and simultaneous formation of a holding layer by electrodeposition. The effects of key parameters are investigated. The alignment on the vertical direction has been revealed by scanning electron microscopy of the samples, their superhydrophobicity, electrochemical performance, and capability to electrically connect two separated electrodes. Compared with their randomly oriented counterparts, the aligned nanoforests showed higher electrochemical capacitance, lower electrical resistance, and the capability to achieve superhydrophobicity, implicating their potential in a broad range of applications.

Enhanced Oxygen Evolution Reaction Electrocatalysis via Electrodeposited Amorphous α-Phase Nickel-Cobalt Hydroxide Nanodendrite Forests

We demonstrate an electrodeposition method to rapidly grow novel three-dimensional nanodendrite forests of amorphous α-phase mixed nickel-cobalt hydroxides on stainless steel foil toward high performance electrocatalysis of the oxygen evolution reaction (OER). The proposed hydrogen bubble-templated, diffusion-limited deposition process leads to the unprecedented dendritic growth of vertically aligned amorphous metal hydroxides, induced by the controlled electrolysis of the tuned water content in the primarily alcohol-based deposition solution. The hierarchical nature of these binder-free, amorphous metal hydroxide deposits leads to their superhydrophilic nature and underwater superaerophobic behavior. The combination of all of these qualities leads to exemplary catalytic performance. When directly grown on planar stainless steel substrates, these nanoforests show high OER activity with overpotentials as low as ∼255 mV to produce a current density of 10 mA cm-2 over 10 000 accelerated stability test cycles. This work demonstrates a novel fabrication technique that can simultaneously achieve a dendritic hierarchical structure, vertical alignment, superaerophobicity, amorphous crystal structure, and intimate contact with the substrate that leads to high catalytic activity with excellent durability.

High Voltage Electrophoretic Deposition of Aligned Nanoforests for Scalable Nanomanufacturing of Electrochemical Energy Storage Devices

High voltage electrophoretic deposition (HVEPD) has been used to obtain forests of aligned multi-walled carbon nanotubes (MWCNTs) on long strips of flexible, conductive substrates. Successful design and integration of a continuous HVEPD setup has enabled scalable fabrication of electrodes for electrochemical energy storage. The mechanism of continuous HVEPD has been investigated to ensure appropriate alignment. Well-aligned forests of MWCNTs were obtained using a conductive holding layer which helped reduce internal resistance and enhance the electrochemical performance of the electrodes.

Dynamic Observation of Tunnel-driven Lithiation Process in Single Crystalline a-MnCh Nanowires

1. Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Dive, Houghton, Michigan 49931, United States 2. Department of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, 1400 Townsend Dive, Houghton, Michigan 49931, United States 3. Department of Materials Science and Engineering, Shandong University, 17923, Jingshi Road, China. 4. Chemical Science and Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States 5. Department of Mechanical and Aerospace Engineering, University of Texas at Arlington, 500 West 1 Street, Arlington, Texas 76019, United States 6. Department of Physics, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, United State