The development of green flexible micro-supercapacitors (MSCs) is one of the biggest challenges in future wearable electronics. Flexible MSCs are mainly produced from non-biodegradable synthetic polymers, resulting in massive electronic waste. Moreover, complex multi-step fabrication increases their production cost. Here, the direct fabrication of highly conductive, intrinsically flexible, and green microelectrodes from naturally fallen leaves in ambient air using femtosecond laser pulses without any additional materials is reported. Hierarchically porous graphene is patterned on different types of leaves via a facile, mask-less, scalable, and one-step laser writing. Leaves consist of biominerals, which decompose into inorganic crystals that serve as nucleation sites for the growth of 3D mesoporous few-layer graphene. The femtosecond laserinduced graphene (FsLIG) microelectrodes formed on leaves have lower sheet resistance (23.3 Ω sq −1 ) than their synthetic polymer counterparts and exhibit an outstanding areal capacitance (34.68 mF cm −2 at 5 mV s −1 ) and capacitance retention (≈99% after 50 000 charge/discharge cycles). The FsLIG MSCs on a single leaf could easily power a light-emitting diode or a table clock and could be applied in wearable electronics, smart houses, and Internet of Things.
The evolution of the phase and morphology of FeOOH nanorods prepared by a hydrothermal method is studied via X-ray diffraction (XRD) and in situ transmission electron microscopy. The FeOOH nanorod with a tetragonal structure (β-FeOOH) is gradually converted into a rhombohedral FeO nanorod by a simple thermal treatment. The existence of an intermediate FeOOH structure with high lattice strains during the phase transition is identified by Rietveld analysis using XRD. The electrochemical properties of the nanorods are investigated based on the crystal phases to elucidate their relative catalytic activities. The strained-FeOOH nanorods exhibited enhanced catalytic water oxidation activity and stability. Typically, the strained-FeOOH nanorods showed high electrochemical stability under neutral conditions, while tetragonal FeOOH nanorods under the same conditions showed rapid deactivation for water oxidation reaction.
The addition of water initiates the phase transition of hexagonal CoO to Co(OH) nanocrystals. Inducing the phase transition of h-CoO on various substrates results in efficient chemical bonding between Co(OH) and the substrate. The efficient deposition of Co(OH) is widely applicable for electrochemical and photoelectrochemical water oxidation reactions.
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