Well-Aligned Channels Derived from Wood for Electron and Mass Transportation in Aqueous Flow Batteries

Nature has provided scientists and engineers with tremendous inspirations and resources to design new materials, architectures, and devices. Inspired by the transportation of water and ions in trees, for the first time, a natural wood-derived freestanding electrode with well-aligned channels was designed as an alternative for the pricey electrode in aqueous vanadium redox flow battery (VRFB). The microscale vertically well-aligned channels (the largest channels size are 60 µm) with pit pores on the channel walls work as efficient mass transfer pathways in VRFB. Meanwhile, the designed freestanding wood carbon (WC) electrode is chemically stable, electrically conductive, and has a large specific surface area of 542.28 m2g-1 for fast charge transfer and electrochemical reaction. Furthermore, to improve the wettability and electrochemical catalytic activity, carboxylic groups were introduced on WC via nitric acid (HNO3) treatment (N-WC). Negatively charged carboxylic groups (with a zeta potential of -50.6 mV for N-WC suspension) provide extra driving force for the adsorption of vanadium cations towards the electrode surface, and therefore further enhance the mass transfer. The discharge capacity retention for N-WC electrode is 83.58 % after 100 cycles at 100 mA cm-2, whereas, the corresponding capacity retention is 57.21 % for WC electrode. Moreover, we found that the N-WC electrode has a predominant effect on the positive side of VRFB with the associated half-cell presents voltage efficiency of 92.10 % and energy efficiency of 88.23 % at current density of 40 mA cm-2. Notably, the discharge capacity for the positive half-cell has an excellent cycle stability of 90.19 % retention after 100 cycles at 100 mA cm-2. This work opens a new strategy to apply Nature derived structure and materials for mass and charge transfer in energy storage areas.