Application of Redox Non‐Innocent Ligands to Non‐Aqueous Flow Battery Electrolytes

Abstract

It is widely accepted that increasing global energy demand should be met with renewable sources that do not contribute to the accumulation of greenhouse gases. Several abundant sources meeting these criteria, however, including solar and wind energy, are intermittent due to factors such as day/night cycles and weather variations. [ 1 ] This problem is amplifi ed by the poor match between peak/off-peak use of electricity and the generation capability of these renewable sources. Grid-scale strategies to mitigate these challenges, such as load-shifting, require energy storage components to supplement the current infrastructure, which comprises instantaneous generation and distribution of electricity. [ 2 ] Among several strategies for grid-scale energy storage, redox fl ow batteries (RFBs) are considered a promising technology. [ 3 ] In these systems, soluble, redox-active electrolytes are circulated between external storage tanks and current collectors. This approach has the benefi t of decoupling energy content and power output, both of which are independently scalable. [ 4,5 ] Aqueous RFBs are the current state-of-the-art, with commercial demonstration in place. [ 5 ] However these systems are limited to energy densities of ≈25 W h Kg −1 due to an open-circuit potential (OCP) that is bounded by water oxidation on one side and proton reduction on the other. [ 6,7 ] Further technical challenges include limited solubilities of electrolytes and narrow windows of temperature stabilities. [ 3 ] Non-aqueous systems, on the other hand, should allow for improved energy density due to the accessibility of much wider potential windows. Several such systems have been reported to date. [8][9][10][11] Nevertheless, the development of electrolytes with multiple, widely spaced redox events, and high solubility in organic solvents remains an important challenge.Herein we report details of a new, nonaqueous RFB electrolyte based on tetramethyl-and tetraethylammonium salts of tris(mnt)vanadium(IV) ([V(mnt) 3 ] 2− ; mnt = (NC) 2 C 2 S 2 2− ), 1a and 1b , respectively, including their battery performance in a static (i.e., non-fl owing) cell. The dithiolate moieties in this complex have been previously demonstrated to be redox noninnocent ligands (NIL), as they are known to participate in electrochemical events separate from those of the vanadium center to which they are bound. It was fi rst recognized through electron paramagnetic resonance spectroscopy in the 1960s that the singly occupied molecular orbital (MO) in [V(mnt) 3 ] 0 is ligand in character. [ 12 ] This fact was recently elaborated upon, whereby a suite of spectroscopic and computational methods unambiguously demonstrated that sequential one-electron reductions of [V(mnt) 3 ] 2− add electrons to the metal center, but one-electron oxidation removes an electron from the ligands, yielding an antiferromagnetically coupled, singlet-diradical ground state for [V(mnt) 3 ] 1− . [ 13 , 14 ] This ligand-based redox activity is a powerful approach to increasing the charge storage ...

“Wine-Dark Sea” in an Organic Flow Battery: Storing Negative Charge in 2,1,3-Benzothiadiazole Radicals Leads to Improved Cyclability

“…Such a rate performance is comparable to neutral electrolyte (e.g. NaCl)-based aqueous flow cells, 22, 40 and significantly better than most nonaqueous flow cells41…”

Recent Developments and Trends in Redox Flow Batteries

“…While the metal center provides the electrochemical activity of [ML n ], the coordinating ligands plays a pivotal role in determining a number of key parameters of the complex, such as the solubility, diffusivity, reversibility, and redox potential of each electron transfer event. Moreover, non-innocent (redox active) ligands may store additional charge thus increasing the intrinsic capacity of the complex [65]. A multi-step redox process with a sufficient potential difference enables the use of the same redox active compound as both the high potential and the low potential compound, which alleviates the concern of cross contamination.…”

“…2− ] were investigated for their potential application in non-aqueous RFBs [65]. It has also been demonstrated that the reduction of [V(mnt) 3 ] 2− adds one electron to the vanadium (IV) center and the oxidation of [V(mnt) 3 ] 2− removes one electron from the (mnt) 2− ligand.…”

Review Article: Flow battery systems with solid electroactive materials

“…22 The second group of compounds are ligand-modified ions, which may be dissolved in aqueous or organic solvents. 22,65,178,[180][181][182] For example, additional capacity has been reported from the ligands of a vanadium complex. 180 More complex organic redox compounds can be modified by chemical functionalization to optimize both the redox potential and solubility.…”

“…22,65,178,[180][181][182] For example, additional capacity has been reported from the ligands of a vanadium complex. 180 More complex organic redox compounds can be modified by chemical functionalization to optimize both the redox potential and solubility. 19,178 Beyond soluble compounds in RFBs, more recently research activity has increased toward solid active material tailored for RFBs.…”