In this article, the different approaches reported in the literature for modelling electrode processes in redox flow batteries (RFBs) are reviewed. Models for RFBs vary widely in terms of computational...
Hybrid
redox flow cells (HRFC) are key enablers for the development
of reliable large-scale energy storage systems; however, their high
cost, limited cycle performance, and incompatibilities associated
with the commonly used carbon-based electrodes undermine HRFC’s
commercial viability. While this is often linked to lack of suitable
electrocatalytic materials capable of coping with HRFC electrode processes,
the combinatory use of nanocarbon additives and carbon paper electrodes
holds new promise. Here, by coupling electrophoretically deposited
nitrogen-doped graphene (N-G) with carbon electrodes, their surprisingly
beneficial effects on three types of HRFCs, namely, hydrogen/vanadium
(RHVFC), hydrogen/manganese (RHMnFC), and polysulfide/air (S-Air),
are revealed. RHVFCs offer efficiencies over 70% at a current density
of 150 mA cm–2 and an energy density of 45 Wh L–1 at 50 mA cm–2, while RHMnFCs achieve
a 30% increase in energy efficiency (at 100 mA cm–2). The S-Air cell records an exchange current density of 4.4 ×
10–2 mA cm–2, a 3-fold improvement
of kinetics compared to the bare carbon paper electrode. We also present
cost of storage at system level compared to the standard all-vanadium
redox flow batteries. These figures-of-merit can incentivize the design,
optimization, and adoption of high-performance HRFCs for successful
grid-scale or renewable energy storage market penetration.
Common issues aqueous-based vanadium redox flow batteries (VRFBs) face include low cell voltage due to water electrolysis side reactions and highly corrosive and environmentally unfriendly electrolytes (3 to 5 M sulfuric acid). Therefore, this investigation looks into the comparison of a highly conductive ionic liquid with a well-studied deep eutectic solvent (DES) as electrolytes for non-aqueous VRFBs. The latter solvent gives 50% higher efficiency and capacity utilization than the former. These figures of merit increase by 10% when nitrogen-doped graphene (N-G)-modified carbon papers, via a one-step binder-free electrophoretic deposition process, are used as electrodes. X-ray computed tomography confirms the enhancement of electrochemical surface area of the carbon electrodes due to N-G while electrochemical impedance spectra show the effect of its higher conductivity on improving RFB performance. Finally, potential strategies for the scaling-up of DES-based VRFBs using a simple economical model are also briefly discussed. From this study, it is deduced that more investigations on applying DESs as non-aqueous electrolytes to replace the commonly used acetonitrile may be a positive step forward because DESs are not only cheaper but also safer to handle, far less toxic, non-flammable, and less volatile than acetonitrile.
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