Abstract
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 € 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.
Highlights
We proposed a low-pressure combined gas plasma treatment in an inductively coupled radio frequency reactor to produce highly catalytic electrodes for vanadium redox flow batteries (VRFBs)
By investigating different gas plasma pressures, the electrodes were optimized to reduce their kinetic polarization toward VRFB redox reactions
The proposed low-pressure combined gas plasma treatments were further validated on hierarchical electrodes produced by decorating graphite felts (GFs) fibers with single-/few-layer graphene flakes produced through a scalable wet-jet milling exfoliation process
Summary
Advanced large-scale energy storage systems (ESSs) are needed to meet the worldwide energy demand by exploiting renewable energy resources,[1−5] such as solar[6−8] and wind energies.[9−11] the intermittency and the instability of renewable power outputs have to be efficiently counterbalanced by the capability of ESSs to ensure a safe and reliable power supply continuously or on-demand.[12,13] In this context, redox-flow batteries (RFBs)[14−20] represent a promising stationary ESS technology because of their outstanding storage capability and output power[21−29] combined with prospective low costs,[30−38] easy scalability,[39,32,40] long lifetime,[41,42] low maintenance,[43,44] safety[44,45] and environmental friendliness.[44,45] Contrary to case-enclosed batteries, RFBs store the energy in the redoxactive material-based electrolytes, filling external reservoirs.[14−18] The electrolytes flow from the reservoirs to the electrode surfaces, where the redox reactions occur rapidly compared to those in metal (e.g., Li, Na, K, etc.)-ion batteries.[46,46,47] As a result, the overall RFB capacities can be adapted to industrialscale applications by enlarging the volume of the reservoirs independently by the power characteristics, which are defined by the size and number of cells in a module unit.[48−50,47] The energy density of a RFB is usually determined by three factors: (1) the concentration of the redox-active materials;[23,51−53] (2) the number of transferred electrons in the redox reactions;[54] and (3) the RFB voltage.[55,56] Among the RFBs, aqueous vanadium (V) redox flow batteries (VRFBs)[57−60] have been commercialized[61−64] thanks to their relevant energy and power performance coupled with the use of V-based species in both half-cells. Together with its low additional costs (
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