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. © (2012) Trans Tech Publications, Switzerland

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/cm² 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/cm², 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

Electrophoretically‐deposited metal‐decorated CNT nanoforests with high thermal/electric conductivity and wettability tunable from hydrophilic to superhydrophobic

A single‐step, room‐temperature, and scalable electrophoretic deposition process is reported to form nanocomposites on any electrically conductive surface with metal nanoparticle decorated carbon nanotubes (CNTs). The contact angles (CAs) can be easily tuned from ≈60° to 168° by varying the deposition voltage, while hydrophobicity and superhydrophobicity surprisingly arise from the hydrophilic CNTs being deposited. The relatively high voltage tends to vertically align CNTs during deposition, leading to architectural micro/nanoscale roughness on the surface. The combination of the multiscale roughness along with the low surface energy of hydrocarbon functional groups on the CNT surface has enabled facile wettability control, including the Petal and Lotus effects. Further, the relatively vertical orientation of the CNTs, without any coating, allows for current and heat transfer along their axis with superior conductivity. Similar behavior in terms of CA control is seen for all three divalent metal ions in the deposition solution (i.e., Cu2+, Ni2+, and Zn2+) that are used to charge the CNTs while eventually getting co‐deposited. This implies that this method could possibly be extended to other metals by selecting appropriate charging salt. A patterning technique is also demonstrated for facile fabrication of superhydrophobic CNT‐metal islands surrounded by hydrophilic CNT coating

Optofluidic Lasers with Monolayer Gain at the Liquid–Liquid Interface

We report optofluidic lasers with a monolayer gain material that self-assembles at the two-phase liquid–liquid interface. The self-assembly process deterministically introduces the gain at the surface of a microdroplet optical cavity, where the lasing mode has maximal interaction with the gain medium. A complete monolayer gain can be achieved in this surface-gain geometry, giving a surface density on the order of 10¹⁴ cm^–2, which proves to be difficult, if not impossible, to achieve in the monolayer gain created at the solid–liquid interface via the surface immobilization method. We demonstrated that the lasing characteristics are drastically different between the gain material that is confined to the liquid–liquid interface and that homogeneously distributed in the bulk liquid solution. Our study reveals the unique capabilities of the surface-gain geometry optofluidic laser, which can be developed into a novel sensing platform to study biophysical and biochemical processes at the molecular level and has vast applications in biomedical diagnostics

Scalable high-power redox capacitors with aligned nanoforests of crystalline MnO \u3c inf\u3e 2 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/cm2 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/cm2, 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. © 2013 American Chemical Society