In this work, Ti3C2Tx MXene was investigated as electrocatalyst material for the anodic V2+/V3+ reaction in vanadium redox flow batteries (VRFBs). A simple drop coating process was established using additive-free, aqueous MXene dispersions to fabricate MXene-coated carbon paper electrodes. The performance of Ti3C2Tx as an anodic electrocatalyst was studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode cell. Furthermore, flow battery testing was performed to determine the performance of the modified electrodes. At a current density of 50 mA cm−2, the electrode with Ti3C2Tx loading of 0.2 mg cm−2 enabled a 7% higher energy efficiency and 22% higher electrolyte utilization rate than the pristine electrode. At a higher current density (100 mA cm−2), the energy efficiency and electrolyte utilization were increased by 17% and 46%, respectively. At 50% SOC, the coated electrode was able to reach a limiting current density of 220 mA cm−2 while maintaining a voltaic efficiency above 80%, whereas the pristine electrode could only reach up to 160 mA cm−2 at the same voltaic efficiency. The improved performance was mainly attributed to the enhanced electrode kinetics, increased electrochemically active surface area, and improved wetting properties due to the addition of Ti3C2Tx nanoflakes.
Understanding the
percolation characteristics of multicomponent
conducting suspensions is critical for the development of flowable
(semi-solid) electrochemical systems for energy storage and capacitive
deionization with optimal electrochemical and rheological performance.
Despite its significance, not much is known about the impact of the
selected particle morphology on the agglomeration kinetics and the
state of dispersion in flowable electrodes. In this study, the impact
of the conductive additive morphology on the electrochemical and rheological
response of capacitive flowable electrodes has been systematically
investigated. Critical viscosity limits have been determined for common
carbon additives that offer slurry formulations with improved electrochemical
and rheological performance. For instance, at the same electrical
conductivity of 60 mS cm–1, higher aspect ratio
particles, such as graphene and carbon nanotubes, offered 4 and 2.4
times lower viscosity compared to carbon black due to the improved
packing and conformity of the high aspect ratio particles. On the
other hand, thixotropic measurements showed that the flowable electrodes
with carbon black exhibit the fastest agglomeration kinetics, offering
25 % less time to recover from the applied shear due to spherical
morphology and facile agglomeration kinetics. Overall, our findings
show that the particle morphology has a significant impact on the
electrochemical and rheological performance of flowable electrodes
with up to 40 % difference in capacitance for similar viscosity suspensions.
Furthermore, a direct correlation between the rheological and the
electrochemical properties was established, offering morphology-independent
practical guidelines for formulating slurries with optimal performance.
In this manner, particles that can achieve the highest density of
packing before the critical limit were found to offer the optimal
balance between electrochemical and rheological performance.
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