Gradient-distributed NiCo₂O₄ nanorod electrode for redox flow batteries: Establishing the ordered reaction interface to meet the anisotropic mass transport

In this work, a gradient-distributed NiCo₂O₄ nanorod-composed graphite felt electrode (G-NiCo₂O₄-GF) was proposed and developed to achieve an ordered reaction toward the nonuniform mass transport in flow batteries. To this end, the ordered-nucleation hydrothermal method was designed, for which the NiCo₂O₄ nanorod was grown concentrated in one side of electrode, and gradually decreased along the through-plane direction until hardly any in the other side. Theoretical calculating results indicated that the G-NiCo₂O₄-GF electrode yielded an obviously lower difference of local current density along through-plane direction, nearly one third of the uniformly-enhanced electrode. Flow battery assembled with the as-prepared electrode exhibited high energy efficiency (88.81 % at 100 mA cm⁻²), superior operation current density (500 mA cm⁻²), and long cycling stability (162 h, 200 cycles). This work provides an effective approach to develop the high-performance electrode for flow batteries.