  The development of high-power density vanadium redox flow batteries (VRFBs)
with high energy efficiencies (EEs) is crucial for the widespread dissemination
of this energy storage technology. In this work, we report the production of
novel hierarchical carbonaceous nanomaterials for VRFB electrodes with high
catalytic activity toward the vanadium redox reactions (VO2+/VO2+ and V2+/V3+).
The electrode materials are produced through a rapid (minute timescale)
low-pressure combined gas plasma treatment of graphite felts (GFs) in an
inductively coupled radio frequency reactor. By systematically studying the
effects of either pure gases (O2 and N2) or their combination at different gas
plasma pressures, the electrodes are optimized to reduce their kinetic
polarization for the VRFB redox reactions. To further enhance the catalytic
surface area of the electrodes, single-/few-layer graphene, produced by highly
scalable wet-jet milling exfoliation of graphite, is incorporated into the GFs
through an infiltration method in the presence of a polymeric binder. Depending
on the thickness of the proton-exchange membrane (Nafion 115 or Nafion XL), our
optimized VRFB configurations can efficiently operate within a wide range of
charge/discharge current densities, exhibiting energy efficiencies up to 93.9%,
90.8%, 88.3%, 85.6%, 77.6%, and 69.5% at 25, 50, 75, 100, 200, and 300 mA cm-2,
respectively. Our technology is cost-competitive when compared to commercial
ones (additional electrode costs < 100 euro m-2) and shows EEs rivalling the
record-high values reported for efficient systems to date. Our work remarks on
the importance to study modified plasma conditions or plasma methods
alternative to those reported previously (e.g., atmospheric plasmas) to improve
further the electrode performances of the current VRFB systems.
