(340ax) Improving Charge Transfer in Metal Ions for Aqueous Redox Flow Batteries

Integrating renewables like solar and wind to meet the growing energy demand has led to a search for low-cost energy storage technologies. Aqueous redox flow batteries (RFBs) are a promising technology for storing energy on a large scale due to its ability to decouple energy and power. RFBs store energy in active species in different oxidation states, which undergo charge transfer at the electrodes surface to store and release energy. Several 3d transition metal ion (Ti to Zn) based RFBs have been demonstrated, but they suffer from low voltage efficiencies due to slow charge transfer at the electrode surface. Most of these RFBs use acidic electrolytes to dissolve the active species and porous carbon felts due to their high surface areas. However, the lack of mechanistic understanding of charge transfer has prevented the design of electrolytes and electrocatalysts to improve the charge transfer and reduce RFBs capital costs.

Here, I will discuss my research focusing on understanding charge transfer in metal ions in aqueous phase. We use V²⁺/V³⁺ reaction as a probe reaction because the slow V²⁺/V³⁺ charge transfer at the negative electrode in part limits the performance of most commercialized vanadium-based RFBs. We conduct kinetic measurements in carefully chosen electrolytes on different electrocatalysts (metals, carbon, etc.) under controlled conditions (electrochemically active surface area, mass transfer, etc.) to develop a simplistic microkinetic model explaining the observed behavior.^1,2 We identify the structure of the reacting species in solution by extended X-Ray absorption and UV-vis spectroscopy, and detect the structure of adsorbed intermediate by surface enhanced Raman spectroscopy. Finally, we show that the V²⁺/V³⁺ reaction kinetics correlates to energy of the identified adsorbed vanadium intermediate.

We use this improved understanding of V²⁺/V³⁺ reaction to explain charge transfer in other redox couples (Fe²⁺/Fe³⁺, Cr²⁺/Cr³⁺, etc.) with flow battery applications. From the available kinetic data for other redox couples in open literature on electrode surfaces, we show that the reaction kinetics for all these redox couples correlate with the intermediate’s adsorption energy.³ This work identifies that the existence of optimum adsorption energy at which charge transfer can be maximized. Further, this opens a new avenue of electrolyte and electrocatalysts development with ideal intermediate adsorption energy that would improve the voltage efficiency of RFBs, thereby reducing their capital costs, and making them more feasible for large scale energy storage.

Research Interests:

Electrocatalysis, Energy Storage, Batteries, Sustainable Energy

References:

1. Agarwal, H., Florian, J., R. Goldsmith, B. & Singh, N. V²⁺/V³⁺ Redox Kinetics on Glassy Carbon in Acidic Electrolytes for Vanadium Redox Flow Batteries. ACS Energy Lett. 4, 2368–2377 (2019).

2. Agarwal, H., Florian, J., Goldsmith, B. R. & Singh, N. The Effect of Anion Bridging on Heterogeneous Charge Transfer for V²⁺/V³⁺. Cell Reports Phys. Sci. 2, 100307 (2021).

3. Florian, J., Agarwal, H., Singh, N. & Goldsmith, B. R. in preparation (2021).