Substituent Impact on Quinoxaline Performance and Degradation in Redox Flow Batteries.

Aqueous redox flow batteries (RFBs) are attractive candidates for low-cost, grid-scale storage of energy from renewable sources. Quinoxaline derivatives represent a promising but underexplored class of charge-storing materials on account of poor chemical stability in prior studies (with capacity fade rates >20%/day). Here, we establish that 2,3-dimethylquinoxaline-6-carboxylic acid (DMeQUIC) is vulnerable to tautomerization in its reduced form under alkaline conditions. We obtain kinetic rate constants for tautomerization by applying Bayesian inference to ultraviolet-visible spectroscopic data from operating flow cells and show that these rate constants quantitatively account for capacity fade measured in cycled cells. We use density functional theory (DFT) modeling to identify structural and chemical predictors of tautomerization resistance and demonstrate that they qualitatively explain stability trends for several commercially available and synthesized derivatives. Among these, quinoxaline-2-carboxylic acid shows a dramatic increase in stability over DMeQUIC and does not exhibit capacity fade in mixed symmetric cell cycling. The molecular design principles identified in this work set the stage for further development of quinoxalines in practical, aqueous organic RFBs.

Understanding capacity fade in organic redox-flow batteries by combining spectroscopy with statistical inference techniques.

Organic redox-active molecules are attractive as redox-flow battery (RFB) reactants because of their low anticipated costs and widely tunable properties. Unfortunately, many lab-scale flow cells experience rapid material degradation (from chemical and electrochemical decay mechanisms) and capacity fade during cycling (>0.1%/day) hindering their commercial deployment. In this work, we combine ultraviolet-visible spectrophotometry and statistical inference techniques to elucidate the Michael attack decay mechanism for 4,5-dihydroxy-1,3-benzenedisulfonic acid (BQDS), a once-promising positive electrolyte reactant for aqueous organic redox-flow batteries. We use Bayesian inference and multivariate curve resolution on the spectroscopic data to derive uncertainty-quantified reaction orders and rates for Michael attack, estimate the spectra of intermediate species and establish a quantitative connection between molecular decay and capacity fade. Our work illustrates the promise of using statistical inference to elucidate chemical and electrochemical mechanisms of capacity fade in organic redox-flow battery together with uncertainty quantification, in flow cell-based electrochemical systems.

Correlating Stability and Performance of NaSICON Membranes for Aqueous Redox Flow Batteries.

Aqueous redox flow batteries (RFBs) are promising candidates for low-cost, grid-scale energy storage. However, the polymer-based membranes that are used in most prototypical systems fail to prevent crossover of small-molecule reactants, which results in high rates of capacity fade. In this work, we explore the feasibility of a von Alpen sodium superionic conductor Na3.1Zr1.55Si2.3P0.7O11 (NaSICON) as an RFB membrane by examining its resistance, permeability, and interfacial morphology as a function of electrolyte composition and temperature. The resistance of NaSICON is stable for several weeks while immersed in neutral to strongly alkaline ([OH-] = 3 M) aqueous electrolytes, and its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. The glassy phase of the NaSICON microstructure at the membrane-electrolyte interface is susceptible to some etching while in contact with aqueous electrolytes containing sodium ions. This etching becomes more extensive when potassium ions are present in the electrolyte, leading in certain instances to complete disintegration of the membrane. A ∼0.7 mm-thin NaSICON membrane can nevertheless support over three weeks of cycling of a ferrocyanide|permanganate flow cell in a strongly alkaline electrolyte ([OH-] = 3 M), with apparently negligible reactant crossover and very low capacity fade (<0.04%/day). NaSICON's area-specific resistance also decreases dramatically with increasing temperature and decreasing membrane thickness; there is a 5.6× reduction from a 1.19 mm-thick membrane at 18 °C (101 Ωcm2) to a 0.61 mm-thick one at 70 °C (18 Ωcm2). Lowering the thickness of the membrane to 0.1 mm or lower will result in power densities at above ambient temperatures that are comparable to power densities of polymer membrane-containing flow cells. This work highlights the promise of ceramic membranes as effective separators in RFBs operating under neutral pH to strongly alkaline pH conditions.

Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review.

Aqueous organic redox flow batteries (RFBs) could enable widespread integration of renewable energy, but only if costs are sufficiently low. Because the levelized cost of storage for an RFB is a function of electrolyte lifetime, understanding and improving the chemical stability of active reactants in RFBs is a critical research challenge. We review known or hypothesized molecular decomposition mechanisms for all five classes of aqueous redox-active organics and organometallics for which cycling lifetime results have been reported: quinones, viologens, aza-aromatics, iron coordination complexes, and nitroxide radicals. We collect, analyze, and compare capacity fade rates from all aqueous organic electrolytes that have been utilized in the capacity-limiting side of flow or hybrid flow/nonflow cells, noting also their redox potentials and demonstrated concentrations of transferrable electrons. We categorize capacity fade rates as being "high" (>1%/day), "moderate" (0.1-1%/day), "low" (0.02-0.1%/day), and "extremely low" (≤0.02%/day) and discuss the degree to which the fade rates have been linked to decomposition mechanisms. Capacity fade is observed to be time-denominated rather than cycle-denominated, with a temporal rate that can depend on molecular concentrations and electrolyte state of charge through, e.g., bimolecular decomposition mechanisms. We then review measurement methods for capacity fade rate and find that simple galvanostatic charge-discharge cycling is inadequate for assessing capacity fade when fade rates are low or extremely low and recommend refining methods to include potential holds for accurately assessing molecular lifetimes under such circumstances. We consider separately symmetric cell cycling results, the interpretation of which is simplified by the absence of a different counter-electrolyte. We point out the chemistries with low or extremely low established fade rates that also exhibit open circuit potentials of 1.0 V or higher and transferrable electron concentrations of 1.0 M or higher, which are promising performance characteristics for RFB commercialization. We point out important directions for future research.

Insights into Electrochemical Oxidation of NaO2 in Na-O2 Batteries via Rotating Ring Disk and Spectroscopic Measurements.

O2 reduction in aprotic Na-O2 batteries results in the formation of NaO2, which can be oxidized at small overpotentials (<200 mV) on charge. In this study, we investigated the NaO2 oxidation mechanism using rotating ring disk electrode (RRDE) measurements of Na-O2 reaction products and by tracking the morphological evolution of the NaO2 discharge product at different states of charge using scanning electron microscopy (SEM). The results show that negligible soluble species are formed during NaO2 oxidation, and that the oxidation occurs predominantly via charge transfer at the interface between NaO2 and carbon electrode fibers rather than uniformly from all NaO2 surfaces. X-ray absorption near edge structure (XANES), and X-ray photoelectron spectroscopy (XPS) measurements show that the band gap of NaO2 is smaller than that of Li2O2 formed in Li-O2 batteries, in which charging overpotentials are much higher (∼1000 mV). These results emphasize the importance of discharge product electronic structure for rationalizing metal-air battery mechanisms and performance.

Correction: Revealing instability and irreversibility in nonaqueous sodium-O2 battery chemistry.

Correction for 'Revealing instability and irreversibility in nonaqueous sodium-O2 battery chemistry' by Sayed Youssef Sayed et al., Chem. Commun., 2016, 52, 9691-9694.

The effect of water on discharge product growth and chemistry in Li-O2 batteries.

Understanding what controls Li-O2 battery discharge product chemistry and morphology is key to enabling its practical deployment as a low-cost, high-specific-energy energy conversion technology. Several studies have recently shown that the addition of substantial quantities (hundreds to thousands ppm) of water and weak acids to dimethoxyethane (DME)-based electrolytes can significantly increase Li-O2 battery discharge capacity, without substantially changing the discharge product chemistry, which remains Li2O2. The exact mechanisms behind these device-level improvements, however, are not yet understood. In this study, we show that the presence of water in a DME-based electrolyte decreases the rate of Li2O2 nucleation on the electrode surface during Li-O2 battery discharge, using potentiostatic electrochemical measurements, and direct, ex situ observations of Li2O2 particles. We also show that adding water to an acetonitrile (MeCN)-based electrolyte results in LiOH upon discharge, as opposed to only Li2O2. Using first principles calculations, we propose that this change in discharge product chemistry is attributable to increased proton availability, as shown by a lower pKa for water in MeCN than in DME. This study combines kinetic and morphological analyses with first principles calculations, and elucidates relationships among electrolyte composition, discharge product chemistry and growth mechanisms for the rational design of efficient metal-air batteries.

Revealing instability and irreversibility in nonaqueous sodium-O2 battery chemistry.

Charging kinetics and reversibility of Na-O2 batteries can be influenced greatly by the particle size of NaO2 formed upon discharge, and exposure time (reactivity) of NaO2 to the electrolyte. Micrometer-sized NaO2 cubes formed at high discharge rates were charged at smaller overpotentials compared to nanometer-sized counterparts formed at low rates.

Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium-Oxygen Batteries.

Fundamental understanding of growth mechanisms of Li2O2 in Li-O2 cells is critical for implementing batteries with high gravimetric energies. Li2O2 growth can occur first by 1e(-) transfer to O2, forming Li(+)-O2(-) and then either chemical disproportionation of Li(+)-O2(-), or a second electron transfer to Li(+)-O2(-). We demonstrate that Li2O2 growth is governed primarily by disproportionation of Li(+)-O2(-) at low overpotential, and surface-mediated electron transfer at high overpotential. We obtain evidence supporting this trend using the rotating ring disk electrode (RRDE) technique, which shows that the fraction of oxygen reduction reaction charge attributable to soluble Li(+)-O2(-)-based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measurements of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li2O2 morphologies observed at different discharge overpotentials: formation of large (∼250 nm-1 µm) toroids, and conformal coatings (<50 nm) at higher overpotentials. These results highlight that RRDE and EQCM can be used as complementary tools to gain new insights into the role of soluble and solid reaction intermediates in the growth of reaction products in metal-O2 batteries.

Experimental and Computational Analysis of the Solvent-Dependent O2/Li(+)-O2(-) Redox Couple: Standard Potentials, Coupling Strength, and Implications for Lithium-Oxygen Batteries.

Understanding and controlling the kinetics of O2 reduction in the presence of Li(+)-containing aprotic solvents, to either Li(+)-O2(-) by one-electron reduction or Li2 O2 by two-electron reduction, is instrumental to enhance the discharge voltage and capacity of aprotic Li-O2 batteries. Standard potentials of O2 /Li(+)-O2(-) and O2/O2(-) were experimentally measured and computed using a mixed cluster-continuum model of ion solvation. Increasing combined solvation of Li(+) and O2(-) was found to lower the coupling of Li(+)-O2(-) and the difference between O2/Li(+)-O2(-) and O2/O2(-) potentials. The solvation energy of Li(+) trended with donor number (DN), and varied greater than that of O2 (-) ions, which correlated with acceptor number (AN), explaining a previously reported correlation between Li(+)-O2(-) solubility and DN. These results highlight the importance of the interplay between ion-solvent and ion-ion interactions for manipulating the energetics of intermediate species produced in aprotic metal-oxygen batteries.

Rate-Dependent Nucleation and Growth of NaO2 in Na-O2 Batteries.

Understanding the oxygen reduction reaction kinetics in the presence of Na ions and the formation mechanism of discharge product(s) is key to enhancing Na-O2 battery performance. Here we show NaO2 as the only discharge product from Na-O2 cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Sodium peroxide dihydrate was not detected in the discharged electrode with up to 6000 ppm of H2O added to the electrolyte, but it was detected with ambient air exposure. In addition, we show that the sizes and distributions of NaO2 can be highly dependent on the discharge rate, and we discuss the formation mechanisms responsible for this rate dependence. Micron-sized (∼500 nm) and nanometer-scale (∼50 nm) cubes were found on the top and bottom of a carbon nanotube (CNT) carpet electrode and along CNT sidewalls at 10 mA/g, while only micron-scale cubes (∼2 µm) were found on the top and bottom of the CNT carpet at 1000 mA/g, respectively.

Chemical Instability of Dimethyl Sulfoxide in Lithium-Air Batteries.

Although dimethyl sulfoxide (DMSO) has emerged as a promising solvent for Li-air batteries, enabling reversible oxygen reduction and evolution (2Li + O2 ⇔ Li2O2), DMSO is well known to react with superoxide-like species, which are intermediates in the Li-O2 reaction, and LiOH has been detected upon discharge in addition to Li2O2. Here we show that toroidal Li2O2 particles formed upon discharge gradually convert into flake-like LiOH particles upon prolonged exposure to a DMSO-based electrolyte, and the amount of LiOH detectable increases with increasing rest time in the electrolyte. Such time-dependent electrode changes upon and after discharge are not typically monitored and can explain vastly different amounts of Li2O2 and LiOH reported in oxygen cathodes discharged in DMSO-based electrolytes. The formation of LiOH is attributable to the chemical reactivity of DMSO with Li2O2 and superoxide-like species, which is supported by our findings that commercial Li2O2 powder can decompose DMSO to DMSO2, and that the presence of KO2 accelerates both DMSO decomposition and conversion of Li2O2 into LiOH.

The influence of transition metal oxides on the kinetics of Li2O2 oxidation in Li-O2 batteries: high activity of chromium oxides.

Reducing the energy loss associated with Li2O2 electrochemical oxidation is paramount to the development of efficient rechargeable lithium-oxygen (Li-O2) batteries for practical use. The influence of a series of perovskites with different eg filling on the kinetics of Li2O2 oxidation was examined using Li2O2-prefilled electrodes. While LaCrO3 is inactive for oxygen evolution upon water oxidation in alkaline solution, it was found to provide the highest specific current towards Li2O2 oxidation among all the perovskites examined. Further exploration of Cr-based catalysts showed that Cr nanoparticles (Cr NP) with an average particle size of 40 nm, having oxidized surfaces, had comparable surface area activities to LaCrO3 but much greater mass activities. Unlike Pt/C and Ru/C that promote electrolyte oxidation in addition to Li2O2 oxidation, no evidence of enhanced electrolyte oxidation was found for Cr NP relative to Vulcan carbon. X-ray absorption spectroscopy at the O K and Cr L edge revealed a redox process of Cr(3+) ↔ Cr(6+) on the surface of Cr NP upon Li2O2 oxidation, which might be responsible for the enhanced oxidation kinetics of Li2O2 and the reduced charging voltages of Li-O2 batteries.