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

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

Asynchronous Crystal Cell Expansion during Lithiation of K⁺‑Stabilized α‑MnO₂

α-MnO₂ is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li⁺. The overall electrochemical performance of α-MnO₂ is determined by the tunneled structure stability during its interaction with Li⁺, the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal–orthorhombic–tetragonal symmetric transition during lithiation of K⁺-stabilized α-MnO₂ is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 × 1 and 2 × 2 tunnels exist along <i>c</i> ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the ⟨100⟩ projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of α-MnO₂’s tetragonal unit cell along <i>a</i> and <i>b</i> lattice directions, which results in a tetragonal–orthorhombic–tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 × 2 tunnels are the preferred sites for Li⁺ occupancy. The sequential Li⁺ filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/α-MnO₂ batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO₂ as an intercalated electrode

Asynchronous Crystal Cell Expansion during Lithiation of K⁺‑Stabilized α‑MnO₂

α-MnO₂ is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li⁺. The overall electrochemical performance of α-MnO₂ is determined by the tunneled structure stability during its interaction with Li⁺, the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal–orthorhombic–tetragonal symmetric transition during lithiation of K⁺-stabilized α-MnO₂ is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 × 1 and 2 × 2 tunnels exist along <i>c</i> ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the ⟨100⟩ projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of α-MnO₂’s tetragonal unit cell along <i>a</i> and <i>b</i> lattice directions, which results in a tetragonal–orthorhombic–tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 × 2 tunnels are the preferred sites for Li⁺ occupancy. The sequential Li⁺ filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/α-MnO₂ batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO₂ as an intercalated electrode