(690d) Active Species Concentration Effects On Performance for a Non-Aqueous All-Vanadium Redox Flow Battery

Authors: 
Shinkle, A. A., The Dow Chemical Company
Monroe, C. W., University of Michigan


        Redox flow batteries
(RFBs) are being developed for large-scale energy storage, uninterruptable
grid-independent power supplies and load-leveling systems for solar or wind
power.  Aqueous chemistries are used for all current commercial RFBs, therefore, cell potentials are limited to the
stability range of water (0?1.23 V vs. SHE).  Since a battery's cell
potential determines its energy and power density, aqueous chemistries limit
the possibilities for the performance of RFBs.

        Several non-aqueous systems have been
reported recently, including systems based on ruthenium, uranium, chromium,
manganese, and vanadium coordination complexes [1-5].  In the case of
vanadium acetylacetonate, the use of a non-aqueous electrolyte results in a stable
cell potential window of 2.2 V [5], more than 50% wider than the aqueous
single-metal vanadium chemistry (1.26 V [6]).  However, very few results
from battery charge/discharge experiments have been shown for this non-aqueous
vanadium system.

        This talk will examine the
charge/discharge profile for the vanadium acetylacetonate (V(acac)3)
RFB system in acetonitrile, with tetraethylammonium tetrafluoroborate (TEABF4)
support.  The effects of active-species concentration on battery
performance were examined.  Linear sweep voltammetry will be used to
determine the maximum charge/discharge rate of the battery cell as a function
of electrolyte composition.  Figure 1 shows the first five cycles of a
charge/discharge result for 0.4 M V(acac)3,
0.5 M TEABF4 in acetonitrile.  Coulombic, energy, and power
efficiency for charge/discharge experiments were examined as a function of
concentration to illustrate the effects of active species crossover. 
Efficiency and energy density were balanced to determine an optimal active
species concentration range for operation of the battery.  Lastly, the
non-aqueous system was compared to the optimized aqueous all-vanadium system to
identify areas of future work.

Figure
1:

First five cycles of a charge/discharge experiment for an all-vanadium
non-aqueous redox flow battery chemistry.  The system was charged at 3 mA until 50% theoretical state
of charge and discharged at 2 mA until 0.4 V. 
The system shown is 0.4 M vanadium acetylacetonate supported by 0.5 M
tetraethylammonium tetrafluoroborate in acetonitrile solvent.  An H-cell
configuration was used with graphite plate electrodes and a Neosepta
AHA anion exchange membrane.

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