Exfoliated Graphene Composite Membrane for the All-Vanadium Redox Flow Battery

Abstract

Vanadium redox flow batteries are emerging as a promising grid storage solution. Unlike competing flow battery concepts, these utilize vanadium in both the catholyte and anolyte chambers which enables easy regeneration and balancing of the cell upon crossover of species through the membrane during long-term use. To increase the time between regeneration cycles and to improve the overall efficiency of vanadium flow batteries, we investigate the use of an ultrathin, graphene coating on the surface of various Nafion membranes. Electrochemically exfoliated graphene (EEG) was dispersed at the air−water interface of a Langmuir−Blodgett trough, compressed, and transferred to Nafion 117 (180 μm thickness) and Nafion 115 (127 μm) membranes. Single-cell vanadium redox flow batteries assembled with the coated membranes led to significantly higher energy efficiency (increased by 13%), power density (by 67%), and discharge capacity (by 17.5%) over 100 cycles compared to uncoated Nafion. The graphene layer was stable over cycling, and electrochemical impedance spectroscopy and self-discharge experiments indicated that the improved battery performance is due to a combination of reduced vanadium crossover and enhanced electrochemical activity provided by the graphene at the electrode surface.

Ammonium Bifluoride‐Etched MXene Modified Electrode for the All−Vanadium Redox Flow Battery

“…A significantly lower charge transfer resistance, indicated by the reduced diameter of the semi-circles, was observed for the VRFB using the NH 4 HF 2 -etched MXene modified negative electrode. The EIS results also revealed that the charge transfer resistance for the negative electrode R CT-(assumed to be the larger of the overlapping semicircles [3,72] was significantly decreased for the MXene modified electrode from 0.65 Ω to 0.17 Ω (Table S1). The observations from EIS are consistent with the polarization, charge-discharge, and CV data.…”

“…The ohmic resistance of the cell (R s ) is indicated by the high frequency intercept of the Nyquist plot with the horizontal axis, and the charge transfer resistances (R CT ) of the two electrodes are related to the diameter of the overlapping flattened semi-circles. [3,71] As shown in Figure 6b, the VRFB using a NH 4 HF 2 -etched MXene modified negative electrode exhibited a slightly lower ohmic resistance R s (indicated by high frequency intercept) of 0.15 Ω, compared to the with the thermal treated electrode. This decrease in ohmic resistance is likely due to an increase in the conductivity of the carbon paper due to the addition of the conductive MXene.…”

“…The shape of the Nyquist plot indicates two overlapping semi-circles, consistent with previous studies that have associated these semi-circles with the positive and negative electrode reactions. [3,70] Following these previous studies, the EIS data was fitted to a simple equivalent circuit representative of the physical system (shown in Figure 6b). Although alternative equivalent circuits could be used to fit the data, this circuit enables comparison of the behavior of the VRFB with different electrode materials.…”

“…A Nafion 117 membrane was used to separate the two electrodes. Although cells using carbon paper have been reported to have a lower power density than those using carbon or graphite felt, and alternative membrane materials can also offer higher performance, [3,14] the objective of this study is not to optimize the cell design or maximize the VRFB performance. Like-for-like comparison of MXene modified electrode materials using the same cell design and electrode substrate enable investigation of the impact of the etching method on the VRFB cell behavior.…”

“…The vanadium redox flow battery (VRFB) is the most intensively studied redox flow battery (RFB) technology, and commercial VRFBs are available for large-scale energy storage systems (ESS). [1][2][3] In an RFB, the electrical energy is stored using dissolved redox active species within the liquid electrolyte. The electrolytes are pumped through the porous electrodes in the cell, which are separated by a membrane to prevent cell shortcircuit and mixing of the anolyte with the catholyte.…”