Metal‐free Fabrication of Nitrogen‐doped Vertical Graphene on Graphite Felt Electrodes with Enhanced Reaction Kinetics and Mass Transport for High‐performance Redox Flow Batteries

Abstract

Graphite felt is commonly used in redox flow batteries, but the low specific surface area and poor catalytic activity cause unsatisfactory mass transfer and reaction kinetics. Here, nitrogen‐doped vertical graphene is in‐situ grown on graphite felt via a metal‐free chemical vapor deposition method, which exhibits a high specific surface area and remarkable catalytic activity due to abundant exposed high‐density sharp graphene edges and nitrogen doping. Multiphysical simulations reveal that the vertical‐standing nanostructure promotes the accessibility of vanadium ions to electrode/electrolyte interfaces, effectively decreasing the mass transport resistance of active species. Density functional theory calculation evidence shows that nitrogen doping aids in the improvement of catalytic activity via boosting vanadium ions’ adsorption and redox. Consequently, the nitrogen‐doped vertical graphene/graphite felt electrode shows an energy efficiency of 87.1% at 200 mA cm−2, significantly higher than that of pristine (65.9%) and air‐oxidize (73.1%) electrodes, an energy efficiency over 80.2% at 300 mA cm−2 during 1500 cycles, and a high‐peak power density of 1308.56 mW cm−2, which are superior to previously reported carbon nanomaterial decorated electrodes for flow batteries. Significantly, the synthesis process only involves gas‐phase reactions without metal catalysts to avoid hydrogen evolution reactions. This work provides an exciting pathway for developing high‐performance electrodes for flow batteries.

The design engineering of nanocatalysts for high power redox flow batteries

“…What is more, Zhao et al innovatively engineered nitrogen-doped vertical graphene @graphite felt (NVG@GF). 69 This electrode manifests a porous characteristic (Fig. 2d), favourably contributing to enhanced mass transfer.…”

“…Furthermore, nano-sized metal nitrides, metal carbides, and nitrogen-doped carbon compounds with abundant active sites and excellent conductivity also showed enormous potential in all-vanadium RFBs. 68,69 Within this realm, Zhou's team has proposed the implementation of an interface electric field using Mo 2 C and Mo 2 N nanocatalysts. 68 As illustrated in Fig.…”

In Situ Growth of Amorphous MnO2 on Graphite Felt via Mild Etching Engineering as a Powerful Catalyst for Advanced Vanadium Redox Flow Batteries

“…Currently, graphite felts (GFs) are widely used as electrodes for VRFBs due to their benign mechanical property, low cost, excellent acid resistance, and electrochemical stability. 21,22 However, the GFs usually possess poor hydrophilicity with the vanadium electrolyte and show a poor electrocatalytic activity toward vanadium ion redox reactions, resulting in a lower energy efficiency and power density for VRFBs. 23,24 In this regard, various strategies have been applied to enhance the electrochemical properties of GFs.…”

“…For example, the oxygencontaining groups were introduced on the electrode surface to enhance the hydrophilic performance of GFs via surface oxygen functionalization, 19,25,26 such as acid treatment, 27 heat treatment, 28 electrochemical oxidation, 29 and oxygen plasma treatment. 30 In addition, some functional material deposi- tions, 31 including metal/metal oxide deposition 32−35 and carbon-based material deposition, 21,36,37 were used to improve the electrocatalytic activity and electronic conductivity of GFs.…”

“…Currently, graphite felts (GFs) are widely used as electrodes for VRFBs due to their benign mechanical property, low cost, excellent acid resistance, and electrochemical stability. , However, the GFs usually possess poor hydrophilicity with the vanadium electrolyte and show a poor electrocatalytic activity toward vanadium ion redox reactions, resulting in a lower energy efficiency and power density for VRFBs. , In this regard, various strategies have been applied to enhance the electrochemical properties of GFs. For example, the oxygen-containing groups were introduced on the electrode surface to enhance the hydrophilic performance of GFs via surface oxygen functionalization, ,, such as acid treatment, heat treatment, electrochemical oxidation, and oxygen plasma treatment .…”