Research on renewable energy sources such as solar and wind has dramatically dropped the cost of electricity generated from these sources. Renewables such as solar photovoltaics, wind turbines and geothermal are producing more and more electricity than ever before.¹ Yet, being intermittent in nature prevents the use of such renewables as the main source for grid-scale electricity unless combined with an appropriate electrochemical energy storage (EES). Vanadium flow batteries (VFBs) are preferred choice as EES due to their elongated life cycle, quick response, ability to go for deep discharge, low cost, and independent energy-power outputs.² However, most VFBs operate at low power densities, which increase the stack size significantly. In addition, costly Nafion® membrane is a key component regarding the high stack cost³; therefore, low power densities significantly increase the cost per kWh capacity. There are reports with improved power densities by modifying cell architecture⁴; still there is scope for cost reduction by replacing Nafion® completely with low-cost polybenzimidazole (PBI) membranes. PBI membrane has enticed attention thanks to its amphoteric properties, which repel vanadium ions in acid solutions.² However, due to its high resistance, morphology and chemical structure of the membrane need to be tuned to achieve high power densities.

We report the strategies to tune the PBI membrane properties including acid treatment and thickness reduction. The objectives of these treatments are to improve the proton – sulfate ion transport while keeping permeability of vanadium ions low. Our preliminary results indicate that PBI membranes are capable of operating at higher power densities, if the transport properties and morphology are fine-tuned. Indeed, acid treated PBI membrane showed >550 mWcm^-2 of peak power density, which is comparable to that of Nafion® membranes. Furthermore, the capacity decay and long term cycling performance of these PBI membranes in VFBs will be presented at the conference. To the best knowledge of the authors, this is the first instance to report the high peak power densities for PBI membranes in VFBs.

Acknowledgement

This work is supported by Laboratory Directed Research & Development, Los Alamos National Laboratory.

References:

(1) EIA - Electricity Data https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_1_01_a (accessed Nov 16, 2017).

(2) Maurya, S.; Shin, S.-H.; Lee, J.-Y.; Kim, Y.; Moon, S.-H. RSC Adv. 2016, 6 (7), 5198–5204.

(3) Minke, C.; Turek, T. J. Power Sources 2015, 286 (Supplement C), 247–257.

(4) Aaron, D. S.; Liu, Q.; Tang, Z.; Grim, G. M.; Papandrew, A. B.; Turhan, A.; Zawodzinski, T. A.; Mench, M. M. J. Power Sources 2012, 206, 450–453.