Charge Transfer of Transition Metal Ions for Flow Battery Applications

Abstract

Redox flow batteries (RFBs) are one of the most promising technologies for grid scale energy storage. RFBs store and release energy by charge transfer (CT) reactions in active species flown on electrode surface. Over the years, many RFBs using transition metal ions as active species have been demonstrated, among which vanadium RFBs (VRFBs: VO2+/VO2+ // V2+/V3+) are the most commercialized. The slow CT kinetics of transition metal ions at electrodes introduces voltage losses that lower RFBs energy storage efficiency, thereby increasing RFBs costs. More than 70 electrolyte and electrode treatments have been shown to improve the CT kinetics of vanadium redox couples used in VRFBs alone over the last two decades. However, there is still a lack of mechanistic understanding of how CT of transition metal ions occurs, which has prevented a targeted approach for rational design of materials that enhance CT kinetics to lower RFBs cost.

In this dissertation, I present my efforts to understand the CT mechanism of transition metal ions at the electrode surface and identify electrode and electrolyte properties that can be tailored to design materials with improved CT kinetics of transition metal ions for development of low-cost RFBs. I use V2+/V3+ as the probe redox couple because the slow V2+/V3+ kinetics is shown to in part limit the performance of most commercialized VRFBs. I elucidate the structure of hydrated and complexed vanadium ions in different acidic electrolytes. I identify two key properties using kinetic and spectroscopic measurements, microkinetic modeling, and adsorption energy calculations that act as descriptors of V2+/V3+ kinetics. I demonstrate that the identified descriptors also explain CT kinetics of several other transition metal ion redox couples used in RFBs.

I conduct V2+/V3+ kinetic measurements in different acidic electrolytes (H2SO4, HCl, HBr, HI, and HClO4) to isolate the effect of anions and on different metal electrodes (Au, Ag, Cu, Bi, and W) to isolate the effect of the electrode on CT kinetics. I show that the V2+/V3+ is an inner sphere reaction and the V2+/V3+ kinetics in different acidic electrolytes correlates with the calculated adsorption energy and desorption barrier of the vanadium intermediate. The anions in the electrolyte serve as bridges for CT between the electrode and the vanadium ions, altering the energy of the vanadium intermediate. I show that the d-band center of the electrode linearly correlates with V2+/V3+ kinetics on different metal electrodes. The d-band electronic structure controls the kinetics by changing the adsorption energy and desorption barrier of the vanadium intermediate.

I demonstrate that the desorption barrier of the transition metal ion intermediate correlates with the kinetics of several other metal ion redox couples including Cr2+/Cr3+, Cd0/Cd2+, and Fe2+/Fe3+ in the presence of anions and serves as a descriptor to understand the influence of anions on transition metal ion CT kinetics. I show that the d-band center linearly correlates with CT kinetics of several Cr, Fe, and Co-based complexes on metal electrodes, indicating d-band center is a descriptor for transition metal ion CT kinetics on electrodes. The desorption barrier of the transition metal ion intermediate can be tuned by altering metal ions’ coordination structure and d-band center of the electrode can be modified by alloying or nano structuring and can serve as design principles for development of new electrolytes and electrodes with enhanced transition metal ion CT kinetics for low-cost RFBs.