Roy G. Gordon

The invention features redox flow batteries and compound useful therein as negolytes or posolytes. The batteries and compounds are advantageous in terms of being useable in water solutions at neutral pH and have extremely high capacity retention. Suitable negolytes are diquaternized bipyridines, suitable posolytes are water-soluble ferrocene derivatives.

Climate change caused by the accumulation of anthropogenic CO 2 emissions motivates the development and deployment of cost-effective, scalable, modular, and energetically efficient techniques to capture CO 2 from point or diffuse sources. Electrochemically-driven CO 2 capture processes utilizing redox-active organics in aqueous flow chemistry and operating at ambient temperature and pressure show promise for nonflammability, continuous-flow engineering, and the possibility of being driven at high current density by inexpensive, clean electricity. [1-2] We show that the deprotonated hydroquinone-CO 2 adducts, whose insolubility limits the utility of the quinone-hydroquinone redox couple, become soluble when alkylammonium cations are introduced. Consequently, we introduce alkylammonium groups to anthraquinone via covalent bonds, making the resulting bis[3-(trimethylammonio)propyl]-anthraquinones (BTMAPAQs) soluble. [3] We report the first aqueous quinone flow electrochemistry induced CO 2 capture/release process, which occurs at ambient temperature and pressure. We show that the electrochemically reduced BTMAPAQs are both Lewis bases and Brønsted bases, thus capturing CO 2 via both a pH-swing and a nucleophilicity-swing mechanism. Among the 1,4-, 1,5-, and 1,8-BTMAPAQ isomers, 1,5-BTMAPAQ reaches the theoretical capture capacity of two CO 2 molecules per quinone from 1-bar CO 2 -N 2 mixtures for which the CO 2 partial pressure is as low as 0.05 bar, or the applied current density is as high as 100 mA/cm 2 , or the organic concentration is as high as 0.4 M. The energetic cost ranges from 48 to 140 kJ/molCO 2 . In a crude simulated flue gas composed of 3% O 2 , 10% CO 2 , and 87% N 2 with a flow rate of ~12 mL/min, 1,5-BTMAPAQ electrolyte reversibly captured and released 50% of the theoretical capacity during an exposure of over 4 hours. It outperforms its isomeric counterparts 1,4-, and 1,8-BTMAPAQ in capture capacity and O 2 tolerance, demonstrating a substituent position effect on the reactivity of isomers with CO 2 and O 2 . The results provide fundamental insight into CO 2 capture with aqueous quinone flow electrochemistry. The oxygen tolerance of reduced quinones may be significantly advanced through molecular engineering by introducing steric and/or electronic effects, intra-molecular interactions, elevating or lowering the oxidation potentials of reduced quinones. [1] Jin, S.; Wu, M.; Gordon, R. G.; Aziz, M. J.; Kwabi, D. G. pH swing cycle for CO 2 capture electrochemically driven through proton-coupled electron transfer, Energy Environ. Sci. , 2020 , 13 , 3706-3722. [2] Jin, S.; Wu, M.; Jing, Y.; Gordon, R. G.; Aziz, M. J. Low energy carbon capture via electrochemically induced pH swing with electrochemical rebalancing, Nature Communications , 2022 , 13 , 2140. [3] Jing, Y.; Amin, K.; Xi, D.; Jin, S.; Alfaraidi, A.; Kerr, E.F.; Gordon, R.G.; Aziz, M.J, Electrochemically induced CO 2 capture enabled by aqueous quinone flow chemistry, ChemRxiv , 2023 , DOIs:10.26434/chemrxiv-2023-nfg6z-v2; 10.26434/chemrxiv-2023-nfg6z.

Aqueous organic redox flow batteries (AORFBs) have emerged as potentially disruptive technologies for the storage of electrical energy from intermittent renewable sources. With the goal of cost-effective, safe, and scalable stationary long duration energy storage systems, AORFBs could become preferred over Li-ion batteries for grid-scale stationary storage due to their inherent non-flammability, lack of materials scarcity fluctuations, and intrinsic decoupling of energy and power capacities. Our group has demonstrated that calendar life, rather than cycle life, limits molecular lifetimes in AORFBs due to various molecular instabilities that lead to side reactions, thus inhibiting performance [1]. To accurately determine molecular fade rates, we utilize potentiostatic cycling to avoid artifacts caused by drifts in internal resistance and employ volumetrically unbalanced compositionally symmetric cell configurations to distinguish molecular fade from membrane crossover or cell unbalancing. Redox-active organic molecule stability has improved to the point that the most stable chemistries degrade at less than 1% per year [2]. With further lifetime increases, the measurement of lower capacity fade rates necessitates higher precision coulometry methods and thermally accelerated degradation protocols to determine which stabilizing approaches are most effective without waiting for multi-month cycling tests to quantify capacity fade. We developed a high-throughput setup for cycling AORFBs at elevated temperatures, providing a new dimension in the flow battery characterization space to explore [3,4]. Capacity fade rates of previously published redox-active organic molecules, as functions of temperature, were evaluated in the high-throughput setup. Demonstrated Arrhenius-like behavior in the temporal capacity fade rates of multiple AORFB electrolytes provided the ability to extrapolate fade rates to lower operating temperatures. Complemented by open source zero-dimensional modelling that incorporates redox-active material degradation [5], we explored temporal capacity evolution in symmetric cells driven by different capacity fade mechanisms such as active species degradation, self-discharge [6], and membrane crossover. Collectively, these results highlight the relevance of electrochemical techniques to understand molecular degradation and expedite the screening process of candidate molecules for long lifetime AORFBs, which may enable massive grid penetration of intermittent renewable energy. References [1] M.-A. Goulet and M. J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods,” Journal of The Electrochemical Society , 165 , A1466 (2018). [2] M. Wu, Y. Jing, A. A. Wong, E. M. Fell, S. Jin, Z. Tang, R. G. Gordon, M. J. Aziz, “Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors,” Chem , 6 , 1432 (2020). [3] E. M. Fell and M. J. Aziz, “High-Throughput Electrochemical Characterization of Aqueous Organic Redox Flow Battery Active Material,” Journal of The Electrochemical Society , 170 , 100507 (2023). [4] E. M. Fell, T. Y. George, Y. Jing, R. G. Gordon, M. J. Aziz, “Leveraging Temperature-Dependent (Electro)Chemical Kinetics for High-Throughput Flow Battery Characterization,” Journal of The Electrochemical Society , 171 , 040501 (2024). [5] E. M. Fell, J. A. Fell, M. J. Aziz, “RFBzero: A Python package for zero-dimensional simulation of redox flow battery cycling,” Journal of Open Source Software , in review (2024). Software available at https://pypi.org/project/rfbzero/ [6] E. M. Fell, D. De Porcellinis, Y. Jing, V. Gutierrez-Venegas, T. Y. George, R. G. Gordon, S. Granados-Focil, M. J. Aziz, “Long-Term Stability of Ferri-/Ferrocyanide as an Electroactive Component for Redox Flow Battery Applications: On the Origin of Apparent Capacity Fade,” Journal of The Electrochemical Society , 170 , 070525 (2023).

We study the capacity fade rate of a flow battery utilizing 2,6-dihydroxyanthraquinone (DHAQ) and its dependence on hydroxide concentration, state of charge, cutoff voltages for the discharge step and for the electrochemical regeneration (oxidation of decomposition compounds back to active species) step, and the period of performing the electrochemical regeneration events. Our observations confirm that the first decomposition product, 2,6-dihydroxyanthrone (DHA), is stable, but after electro-oxidative dimerization, the anthrone dimer decomposes. We identify conditions for which there is little time after dimerization until the dimer is rapidly reoxidized electrochemically to form DHAQ. Combining these approaches, we decrease the fade rate to 0.02%/day, which is 18 times lower than the lowest rate reported previously of 0.38%/day, and over 200 times lower than the value under standard cycling conditions of 4.3%/day. The findings and their mechanistic interpretation are expected to extend the lifetime and enhance the effectiveness of in situ electrochemical regeneration for other electroactive species with finite lifetimes.

Electrochemically driven CO 2 capture processes utilizing redox-active organics in aqueous flow chemistry show promise for nonflammability, continuous-flow engineering and the possibility of being driven at a high current density by inexpensive, clean electricity. We show that deprotonated hydroquinone–CO 2 adducts, whose insolubility limits the utility of the quinone–hydroquinone redox couple, become soluble when alkylammonium cations are introduced. Consequently, we introduced alkylammonium groups to anthraquinone via covalent bonds, making the resulting bis[3-(trimethylammonio)propyl]anthraquinones (BTMAPAQs) soluble. We report the first aqueous quinone flow chemistry-enabled electrochemical CO 2 capture/release process, which occurs at ambient temperature and pressure, and show that it proceeds via both pH-swing and nucleophilicity-swing mechanisms. 1,5-BTMAPAQ reaches the theoretical capture capacity of two CO 2 molecules per quinone from 1-bar CO 2 –N 2 mixtures, for which the CO 2 partial pressure is as low as 0.05 bar, or the applied current density is as high as 100 mA/cm 2, or the organic concentration is as high as 0.4 M. The energetic cost ranges from 48 to 140 kJ/mol CO 2 . In a crude simulated flue gas composed of 3% O 2, 10% CO 2, and 87% N 2, 1,5-BTMAPAQ electrolyte reversibly captured and released 50% of the theoretical capacity during an exposure of over 4 h. It outperforms its isomeric counterparts 1,4-, and 1,8-BTMAPAQ in capture capacity and O 2 tolerance, demonstrating a substituent position effect on the reactivity of isomers with CO 2 and O 2 . The results provide fundamental insight into electrochemical CO 2 capture with aqueous quinone flow chemistry and suggest that the oxygen tolerance of reduced quinones may be significantly advanced through molecular engineering.

CO2 capture based on a pH swing driven electrically through the reversible proton-coupled electron transfer of organic molecules can be powered entirely by clean electricity. A major technical challenge is the reversible chemical oxidation of the reduced organics by atmospheric O2, penalizing energy efficiency and capture capacity. We report the development of a hybrid phenazine flow cell system that employs a pH-swing facilitated direct air capture (DAC) process, utilizing redox-active cyclic poly(phenazine sulfide) fabricated solid electrodes. The system maintains a separation between the air and the O2-sensitive reduced phenazine, enabling stable and effective CO2 capture from gas mixtures containing O2. This hybrid flow cell demonstrated significant oxygen compatibility, exhibiting a coulombic efficiency of 99% and requiring only 73 kJ molCO2-1 for simulated flue gas and 126 kJ molCO2-1 for DAC. The hybrid cell design strategy of isolating vulnerable species offers an efficient pathway for DAC and may be broadly applicable to avoiding undesirable side reactions in other electrochemical devices.

Abstract Wearable power sources are envisioned since their development promises to speed up the widespread application of wearable devices in different areas, such as healthcare, smart‐city management, and robotics. Here, the 4′‐((9,10‐anthraquinone‐2‐yl)oxy)butyrate (2‐BEAQ), an anthraquinone derivative, is synthesized, and further applied for producing a redox‐active shear‐thinning hydrogel (BEAQ‐gel). The gel comprises cylindrical aggregates of 2‐BEAQ molecules dispersed within a water matrix, interconnected through ionic, ion‐dipole, and hydrogen bonding interactions. BEAQ‐gel also presents interesting rheological characteristics and composition tunability since it can be produced in a large range of concentrations. This improved redox 3D hierarchical network also makes the network capable of retaining high quantities of potassium hydroxide, thereby enhancing conductivity. By coupling BEAQ‐gel with Ferricyanide the development of a wearable battery is demonstrated, which exhibits an output voltage of 0.89 V even when bent at a 180° angle, making it suitable for powering wearable devices. This work presents an innovative alternative for the production of wearable devices, from the design of the anode and cathode materials to the wearable casing and demonstrates the use of redox‐active low‐molecular‐weight‐gel (LMWG) as an active material of a microbattery giving a valuable glimpse to the further development of wearable energy storage devices.

The library of redox-active organics that are potential candidates for electrochemical energy storage in flow batteries is exceedingly vast, necessitating high-throughput characterization of molecular lifetimes. Demonstrated extremely stable chemistries require accurate yet rapid cell cycling tests, a demand often frustrated by time-denominated capacity fade mechanisms. We have developed a high-throughput setup for elevated temperature cycling of redox flow batteries, providing a new dimension in characterization parameter space to explore. We utilize it to evaluate capacity fade rates of aqueous redox-active organic molecules, as functions of temperature. We demonstrate Arrhenius-like behavior in the temporal capacity fade rates of multiple flow battery electrolytes, permitting extrapolation to lower operating temperatures. Collectively, these results highlight the importance of accelerated decomposition protocols to expedite the screening process of candidate molecules for long lifetime flow batteries.

Establishing a pH difference between the two electrolytes (pH decoupling) of an aqueous redox flow battery (ARFB) enables cell voltages exceeding the 1.23 V thermodynamic water-splitting window, but acid–base crossover penalizes efficiency and lifetime. Here we employ mildly acidic and mildly alkaline electrolytes to mitigate crossover, achieving high round-trip energy efficiency with open circuit voltage >1.7 V. We implemented an acid–base regeneration system to periodically restore electrolytes to their initial pH values. The combined system exhibited capacity fade rate <0.07% per day, round-trip energy efficiency >85% and approximately 99% Coulombic efficiency during stable operation for over a week. Cost analysis shows that the tolerance of acid–base crossover could be increased if the pH-decoupling ARFB achieved a higher voltage output and lower resistance. This work demonstrates principles for improving lifespan, rate capability and energy efficiency in high-voltage pH-decoupling ARFBs and pH recovery concepts applicable for pH-decoupling systems. Establishing pH differences in aqueous flow batteries widens their voltage window, but acid–base mixing shortens their lifespan. In this study, the authors introduced a pH recovery system to address crossover issues, ensuring long-lasting, high-voltage pH-decoupled flow batteries.

The library of redox-active organics that are potential candidates for electrochemical energy storage in flow batteries is exceedingly vast, necessitating high-throughput characterization of molecular lifetimes. Demonstrated extremely stable chemistries require accurate yet rapid cell cycling tests, a demand often frustrated by time-denominated capacity fade mechanisms. We have developed a high-throughput setup for elevated temperature cycling of redox flow batteries, providing a new dimension in characterization parameter space to explore. We utilize it to evaluate capacity fade rates of aqueous redox-active organic molecules, as functions of temperature. We demonstrate Arrhenius-like behaviour in the temporal capacity fade rates of multiple flow battery electrolytes, permitting extrapolation to lower operating temperatures. Collectively, these results highlight the importance of accelerated decomposition protocols to expedite the screening process of candidate molecules for long lifetime flow batteries.

We study the capacity fade rate of a flow battery utilizing 2,6-dihydroxyanthraquinone (DHAQ) and its dependence on hydroxide concentration, state of charge, cutoff voltages for the discharge step and for the electrochemical regeneration (oxidation of decomposition compounds back to active species) step, and period of performing the electrochemical regeneration events. Our observations confirm that the first decomposition product, 2,6-dihydroxyanthrone (DHA), is stable but after electro-oxidative dimerization, the anthrone dimer decomposes. We identify conditions for which there is little time after dimerization until the dimer is rapidly re-oxidized electrochemically to form DHAQ. Combining these approaches, we decrease the fade rate to 0.02%/day, which is 18 times lower than lowest rate reported previously of 0.38%/day, and over 200 times lower than the value under standard cycling conditions of 4.3%/day. The findings and their mechanistic interpretation are expected to extend the lifetime and enhance the effectiveness of in-situ electrochemical regeneration for other electroactive species with finite lifetimes.

This is a correction for DOI 10.1149/1945-7111/ad1295.

We present two novel experimental techniques designed to quantify the contributions of nucleophilicity-swing and pH-swing mechanisms to carbon capture in the electrochemical aqueous quinone-based CO2 capture process. Through thermodynamic analysis, we elucidate the intricate interplay between these two mechanisms, and emphasize the critical role of understanding this interplay in the material discovery cycle for carbon capture applications. This insight prompts the development of two innovative in situ techniques. The first technique capitalizes on discernible voltage signature differences between quinone, and quinone-CO2 adducts. By incorporating a reference electrode into the carbon capture cell setup, we apply this method to investigate bis[3-(trimethylammonio)propyl]-anthraquinones (BTMAPAQs). Our findings reveal the isolated contributions of nucleophilicity-swing and pH-swing mechanisms to overall carbon capture capacity under varying wait times and CO2 partial pressures. The second method is developed based on our finding that the adduct form of the quinone exhibits a fluorescence emission from an incident light at wavelengths distinct from the fluorescence of the reduced form, enabling differentiation through optical band-pass filtering at each unique fluorescent signature. Thus, we introduce a non-invasive, in situ approach using fluorescence microscopy, providing the unique capability to distinguish between oxidized, reduced, and adduct species with sub-second time resolution at single digit micrometer resolution. This powerful technique holds significant promise for studying such systems, representing an advancement in our ability to understand carbon capture processes.

Proton-coupled electrochemical reactions as proton pumps can facilitate in situ or ex situ pH recovery and capacity rebalancing within single-membrane pH-decoupling batteries.

Climate change caused by the accumulation of anthropogenic CO2 emissions motivates the development and deployment of cost-effective, scalable, and energetically efficient techniques to capture CO2 from point or diffuse sources. Electrochemically-driven CO2 capture processes utilizing redox-active organics in aqueous flow chemistry show promise for nonflammability, continuous-flow engineering, and the possibility of being driven at high current density by inexpensive, clean electricity. We show that the deprotonated hydroquinone-CO2 adducts, whose insolubility limits the utility of the quinone-hydroquinone redox couple, become soluble when alkylammonium cations are introduced. Consequently, we introduce alkylammonium groups to anthraquinone via covalent bonds, making the resulting bis[3-(trimethylammonio)propyl]-anthraquinones (BTMAPAQs) soluble. We report the first aqueous quinone flow chemistry-enabled electrochemical CO2 capture process, which occurs at ambient temperature and pressure, and show that it proceeds via both a pH-swing and a nucleophilicity-swing mechanism. 1,5-BTMAPAQ reaches the theoretical capture capacity of two CO2 molecules per quinone from 1-bar CO2-N2 mixtures for which the CO2 partial pressure is as low as 0.05 bar, or the applied current density is as high as 100 mA/cm2, or the organic concentration is as high as 0.4 M. The energetic cost ranges from 48 to 140 kJ/molCO2. In a crude simulated flue gas composed of 3% O2, 10% CO2, and 87% N2, 1,5-BTMAPAQ electrolyte reversibly captured and released 50% of the theoretical capacity during an exposure of over 4 hr. It outperforms its isomeric counterparts 1,4-, and 1,8-BTMAPAQ in capture capacity and O2 tolerance, demonstrating a substituent position effect on the reactivity of isomers with CO2 and O2. The results provide fundamental insight into electrochemical CO2 capture with aqueous quinone flow chemistry and suggest that oxygen tolerance of reduced quinones may be significantly advanced through molecular engineering.

Investigations are ongoing to determine whether electrochemically-driven CO 2 capture with inexpensive redox-active molecules in aqueous electrolyte in a scalable flow cell system has the potential to cost-effectively reach impactful scale. Water-soluble redox organic molecules “Q” can potentially capture CO 2 from diffuse and point sources in an electrochemical flow cell. For example, proton-coupled electron transfer of these molecules (Q+2H + +2e - → QH 2 ) can raise the electrolyte pH, leading to the capture of CO 2 from air; subsequent electrochemical oxidation of the reduced molecules (QH 2 → Q+2H + +2e - ) acidifies the electrolyte, resulting in the release of pure CO 2 [1-2] . The thermodynamic analysis indicates that an idealized cycle requires the minimum work input varying from 16 to 75 kJ/mol CO 2 as throughput per cycle increases, with the potential to go substantially lower if CO 2 capture or release is performed simultaneously with electrochemical reduction or oxidation. The method appears safe and scalable, as it utilizes non-toxic, non-corrosive, non-volatile and potentially low-cost organic molecules. An alternative CO 2 capture mechanism, which appears to operate in parallel to this pH swing mechanism, is a nucleophilicity swing in which the electrochemically reduced species spontaneously forms a CO 2 adduct, Q(CO 2 ) 2 2 - , which releases its CO 2 upon electrochemical oxidation. In spite of the advantages in thermodynamic minimum and sustainable electrochemical processes, oxygen intolerance of the reduced molecules (QH 2 ) has limited its application to direct air capture (DAC) due to reversible chemical oxidation of QH 2 back to Q by atmospheric oxygen. Here, we report reduced Q species with improved resistance to oxidation by air. The air tolerance of the reduced Q with captured CO 2 was evaluated under air exposure and tracked by 1 H NMR. From day-1 to day-9, an NMR sample was loosely capped and stored in lab air. There was no sign of chemical changes in the NMR spectra for the first 5 days, and the peaks of Q in the oxidized state did not appear before the ninth day. The air stability of Q(CO 2 ) 2 2 - within the first five days suggests the feasibility of utilizing Q for a CO 2 capture/release cycle in the presence of oxygen that takes &lt;&lt; 5 days. We will report the results of molecular engineering efforts to further improve molecular stability of the reduced form under oxygen. [1] Jin, S.; Wu, M.; Gordon, R. G.; Aziz, M. J.; Kwabi, D. G. pH swing cycle for CO2 capture electrochemically driven through proton-coupled electron transfer, Energy Environ. Sci. , 2020 , 13 , 3706-3722. [2] Jin, S.; Wu, M.; Jing, Y.; Gordon, R. G.; Aziz, M. J. Low energy carbon capture via electrochemically induced pH swing with electrochemical rebalancing, Nature Communications , 2022 , 13 , 2140.

The development of devices capable of capturing and sequestering carbon dioxide emissions directly from air or at point sources is motivated by mitigating climate change. Electrochemically driven carbon capture and release at ambient temperature and pressure through the formation of complexes with organic redox active molecules [1,2] or via pH swing caused by proton-coupled electron transfer during molecular redox events [3,4] has shown promising results for carbon capture and release in terms of energy cost, scalability and safety compared to the state of the art amine scrubbing technologies [5]. Quinones are organic molecules containing an unsaturated six-member ring with two carbonyl groups. They exhibit two potential mechanisms of CO 2 capture. (a) Upon electrochemical reduction, the carbonyl groups within quinones provide sites for nucleophilic addition of carbon dioxide which results in the formation of a quinone-CO 2 adduct (CO 2 reversibly bound by quinones). (b) Depending on the pK a of the quinone and the local pH, the electrochemical reduction of the compound can be proton-coupled, creating hydroxide, which captures CO 2 as carbonate or bicarbonate. In this work, we introduce in situ electrochemical characterization and in situ fluorescence microscopy to separately quantify the two aforementioned contributions toward capture of CO 2 at partial pressures ranging from 0.1 bar to 0.4 mbar. The non-invasive in situ analytical methods introduced in this work are used to distinguish, with sub-second time resolution, between the oxidized, reduced and adduct species by their electrochemical and fluorescence signatures. The results of this study permits us to understand the underlying simultaneous contributions of quinone-CO 2 adduct path and the pH-swing path toward carbon capture and release and, additionally, introduces powerful non-invasive methods for gathering insights on similar systems. 1 X. Li, X. Zhao, Y. Liu, T.A. Hatton, and Y. Liu, "Redox-tunable lewis bases for electrochemical carbon dioxide capture", Nature Energy 7 , 1065 (2022). 2 Y. Liu, H.Z. Ye, K.M. Diederichsen, T. Van Voorhis, and T.A. Hatton, "Electrochemically mediated carbon dioxide separation with quinone chemistry in salt-concentrated aqueous media", Nat Commun 11 , 2278 (2020). 3 S. Jin, M. Wu, Y. Jing, R.G. Gordon, and M.J. Aziz, "Low energy carbon capture via electrochemically induced pH swing with electrochemical rebalancing", Nat Commun 13 , 2140 (2022). 4 S. Jin, M. Wu, R.G. Gordon, M.J. Aziz, and D.G. Kwabi, "pH swing cycle for CO 2 capture electrochemically driven through proton-coupled electron transfer", Energy &amp; Environmental Science 13 , 3706 (2020). 5 G.T. Rochelle, "Amine scrubbing for CO 2 capture", Science 325 , 1652.

Aqueous redox flow batteries require long-term stable redox molecules for electrical energy storage. Anthraquinones, especially ether bond-decorated ones, experience two dominant decomposition pathways, including side-chain loss and anthrone formation. With the aid of DFT calculations, we designed an anthraquinone (3-NH 2 -2-2PEAQ) bearing an ether substituent and a neighboring NH 2 group, which suppresses both of these decomposition mechanisms and exhibits a high solubility of 1.1 M. When paired with ferrocyanide in a full cell, this anthraquinone, at a concentration of 1.0 M, can operate at greater than 1 V with an extremely low capacity fade rate of 0.01%/day and a Coulombic efficiency above 99.8% while cycling for over a month. The synergistic effects of the ether and parent amino substituents are extremely sensitive to the precise substitution pattern of the anthraquinone. This study demonstrates the effectiveness of the judicious use of DFT-based prediction in rapidly identifying promising candidate electrolytes from a large chemical space.

Abstract A novel iron‐based posolyte redox species are presented for an aqueous redox flow battery, (Tetrakis(2‐pyridylmethyl)ethylenediamine)iron(II) dichloride, which is obtained by a simple synthetic route, shows a high redox potential of 0.788 V versus SHE, and exhibits exceptional aqueous solubility of 1.46 M. Paired with bis(3‐trimethylammonio)propyl viologen tetrachloride at neutral pH, the battery demonstrates an open‐circuit voltage of 1.19 V and delivers good cycling performance, with a capacity fade rate of 0.28% per day and coulombic efficiency of 99.3%. Postmortem chemical and electrochemical analyses of the posolyte species suggest future routes for stabilization of the complex. Among all the iron complexes with a redox potential above 0.4 V versus SHE, this compound exhibits the highest solubility. These results offer valuable insights that can be applied to the development of future posolyte species for sustainable energy storage solutions.

Designing electrolytes based on mixture of different organic redox active molecules brings the opportunity of enhancing the volumetric energy density of flow batteries and removes the requirement of high solubility for individual organic species in the mixture. In the present work, we conduct computational and experimental analysis to investigate the electrochemical performance of mixed redox-active organic molecules. A zero-dimensional transient model is employed to investigate the changes in the half-cell potential and the concentrations and partial currents of individual redox reactions in a mixture of organic molecules over time. The model demonstrates the effects of individual properties of species such as kinetic rate constants, mass transfer coefficients, concentration ratios and standard redox potentials and reports the effect of energy-losing homogenous chemical redox reaction on the voltage efficiency and concentration ratios of the mixed species. Pairs of anthraquinone negolyte species were selected for an experimental case study. A mixture of 2,6-N-TSAQ and 2,6-DHAQ showed 40% increase in the volumetric energy density compared to the performance of 2,6-DHAQ alone. Based on the results of the experimental and computational analysis, we propose guidelines for the design of suitable mixed redox-active organic species.

Climate change caused by the accumulation of anthropogenic CO2 emissions motivates the development and deployment of cost-effective, scalable, and energetically efficient techniques to capture CO2 from point or diffuse sources. Electrochemically-driven CO2 capture processes utilizing redox-active organics in aqueous flow chemistry show promise for nonflammability, continuous-flow engineering, and the possibility of being driven at high current density by inexpensive, clean electricity. We show that the deprotonated hydroquinone-CO2 adducts, whose insolubility limits the utility of the quinone-hydroquinone redox couple, become soluble when alkylammonium cations are introduced. Consequently, we introduce alkylammonium groups to anthraquinone via covalent bonds, making the resulting bis[3-(trimethylammonio)propyl]-anthraquinones (BTMAPAQs) soluble. We report the first aqueous quinone flow chemistry-enabled electrochemical CO2 capture process, which occurs at ambient temperature and pressure, and show that it proceeds via both a pH-swing and a nucleophilicity-swing mechanism. 1,5-BTMAPAQ reaches the theoretical capture capacity of two CO2 molecules per quinone from 1-bar CO2-N2 mixtures for which the CO2 partial pressure is as low as 0.05 bar, or the applied current density is as high as 100 mA/cm2, or the organic concentration is as high as 0.4 M. The energetic cost ranges from 48 to 140 kJ/molCO2. In a crude simulated flue gas composed of 3% O2, 10% CO2, and 87% N2, 1,5-BTMAPAQ electrolyte reversibly captured and released 50% of the theoretical capacity during an exposure of over 4 hr. It outperforms its isomeric counterparts 1,4-, and 1,8-BTMAPAQ in capture capacity and O2 tolerance, demonstrating a substituent position effect on the reactivity of isomers with CO2 and O2. The results provide fundamental insight into electrochemical CO2 capture with aqueous quinone flow chemistry and suggest that oxygen tolerance of reduced quinones may be significantly advanced through molecular engineering.

Designing electrolytes based on mixture of different organic redox active molecules brings the opportunity of enhancing the volumetric energy density of flow batteries and removes the requirement of high solubility for individual organic species in the mixture. In the present work, we conduct computational and experimental analysis to investigate the electrochemical performance of mixed redox-active organic molecules. A zero-dimensional transient model is employed to investigate the changes in the half-cell potential and the concentrations and partial currents of individual redox reactions in a mixture of organic molecules over time. The model demonstrates the effects of individual properties of species such as kinetic rate constants, mass transfer coefficients, concentration ratios and standard redox potentials and reports the effect of energy-losing homogenous chemical redox reaction on the voltage efficiency and concentration ratios of the mixed species. Pairs of anthraquinone negolyte species were selected for an experimental case study. A mixture of 2,6-N-TSAQ and 2,6-DHAQ showed 40% increase in the volumetric energy density compared to the performance of 2,6-DHAQ alone. Based on the results of the experimental and computational analysis, we propose guidelines for the design of suitable mixed redox-active organic species.

The attraction of aqueous organic redox flow batteries (AORFBs) lies in the potential for low mass-production cost and long lifetime of the organic molecules. To reach cell potentials &gt;1.0 V, several AORFBs have employed the ferri/ferrocyanide redox couple as posolyte in alkaline conditions. Recent works have reported significant amounts of capacity fade of this redox couple at high pH, attributed either to chemical decomposition associated with cyanide ligand dissociation and irreversible hydroxylation of the iron complex [1,2], or due to cell unbalancing associated with electrochemical oxygen evolution reaction (OER) [3]. We assess the chemical and electrochemical stability of ferri/ferrocyanide utilizing a volumetrically unbalanced, compositionally symmetric cell method [4]. A series of electrochemical and chemical characterization experiments was performed to distinguish between “real” capacity fade (redox-active is structurally damaged) and “apparent” capacity fade (redox-active remains structurally intact and active), when ferri/ferrocyanide electrolytes are used in the capacity-limiting side of a flow battery. Our results indicate that, in contrast with previous reports [1,2], no chemical decomposition of ferri/ferrocyanide occurs at tested pH values as high as 14 in the dark or in diffuse indoor light. Instead, an apparent capacity fade takes place due to a chemical reduction of ferricyanide to ferrocyanide, via chemical OER. We find that this parasitic process can be further enhanced by carbon electrodes, with apparent capacity fade rates at pH 14 increasing with an increased ratio of carbon electrode surface area to total amount of ferricyanide in solution. Based on these results, we report a set of operating conditions that enables the cycling of alkaline ferri/ferrocyanide electrolytes. We further demonstrate how apparent capacity fade rates can be engineered by the initial cell setup and employ a zero-dimensional model [5] to explain cell behavior. If protected from direct exposure to light, the chemical stability of ferri/ferrocyanide anions allows for their practical deployment as electroactive species in long duration energy storage applications at alkaline pH values up to at least 14. [1] J. Luo, A. Sam, B. Hu, C. DeBruler, X. Wei, W. Wang, and T.L. Liu, “Unraveling pH dependent cycling stability of ferricyanide/ferrocyanide in redox flow batteries,” Nano Energy, 42, 215 (2017). [2] M. Hu, A. Wang, T.L. Liu, “Cycling Performance and Mechanistic Insights of Ferricyanide Electrolytes in Alkaline Redox Flow Batteries,” ChemRxiv, (2022), DOI: 10.26434/chemrxiv-2022-lqms7-v3 [3] T. Paéz, A. Martínez-Cuezva, J. Palma, E. Ventosa, “Revisiting the cycling stability of ferrocyanide in alkaline media for redox flow batteries,” Journal of Power Sources, 471, 228453 (2020). [4] M-A. Goulet, M.J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”, Journal of the Electrochemical Society 165, A1466 (2018). [5] S. Modak, D.G. Kwabi, “A Zero-Dimensional Model for Electrochemical Behavior and Capacity Retention in Organic Flow Cells,” Journal of the Electrochemical Society 168, 080528 (2021). Figure caption: Potentiostatic cycling of a 0.1 M ferri-/0.1 M ferrocyanide pH 14 symmetric cell with SGL electrodes. The volume of the capacity-limiting side (CLS) was 6 mL and that of the non-capacity-limiting-side (NCLS) was 11 mL. Each refresh is a NCLS replacement of 11 mL of fresh 50% SOC electrolyte. Figure 1

Redox flow batteries (RFBs) are large-scale energy storage technologies that are attractive due to their safety, flexible design, and long life. Among different types of RFBs, aqueous organic systems have gained a lot of attention due to the possibility of tailoring the redox properties by modifying the organic chemical structure. We introduce 2-2-propionate ether anthraquinone (abbreviated 2-2PEAQ), which is a singly substituted anthraquinone that is synthesized via a facile synthetic route from potentially inexpensive precursors. 2-2PEAQ has the lowest molecular weight (MW = 296.07 g/mol) among the anthraquinone derivatives reported to date that can deliver an extremely low capacity fade rate (&lt;0.02%/day), low membrane permeability (&lt;1 x 10 -13 cm 2 /s), high solubility (≥1 M transferrable electrons) and acceptable redox potential (OCV &gt;~1.0 V vs. Fe(CN) 6 3-/4- ) for use in aqueous organic flow batteries. Having a smaller MW per mole transferrable electrons in species that can be synthesized via simple synthesis routes is highly preferable because both attributes correlate with low mass production cost. We will discuss the degradation mechanisms of 2-2PEAQ and report its performance in the negolyte of a redox flow battery under practically relevant conditions, advancing the prospects for commercialization of grid-scale aqueous organic redox flow batteries.

Redox flow batteries are promising technologies for energy storage and conversion applications. Among different types of flow batteries, iron-based redox-active materials can have cost and environmental advantages. We introduce a high-potential tris(2,2'-bipyridine-4,4'-diyldimethanol) iron dichloride compound as a posolyte species with a redox potential more than 0.5 V more positive than that of ferro/ferricyanide. The new species, abbreviated Fe(Bhmbp) 3 , operating at near-neutral pH, was paired with bis(3-trimethylammonio)propyl viologen tetrachloride as the negolyte species. The resulting aqueous flow battery exhibits an open-circuit voltage of 1.36 V and it shows excellent cycling stability with a capacity fade rate of 0.0007% per cycle or 0.07% per day. In comparison with previously reported organic and metalorganic compounds, Fe(Bhmbpy) 3 exhibits a superior combination of high positive redox potential and low capacity fade rate. We investigated the decomposition pathways during cell cycling to an unprecedented level of depth for an aqueous posolyte. We found that the compound degrades through self-discharge, dimerization and ligand dissociation. The proposed decomposition pathways explain the capacity fade phenomena and offer opportunities for future improvement of metalorganic posolytes with high redox potential and good cycling stability

Aqueous redox flow batteries (ARFBs) are being developed for grid scale electricity storage. However, the 1.23 V water splitting window makes the development of a long lifetime high-voltage ARFB challenging. Though the pH-decoupling design, with an acidic posolyte and an alkaline negolyte has been demonstrated to broaden the operating voltage window, long term operational stability and overall energy efficiency are penalized because of acid-base leakage along the pH gradient. Mild acid and base are used as supporting electrolytes to diminish proton and hydroxide crossover rates, resulting in higher overall energy efficiency. An acid-base in-situ regeneration system is implemented to restore the negolyte and posolyte pH to starting values. The capacity fade rate achieved is &lt; 0.08%/day, with &gt; 75% overall energy efficiency, and the Coulombic efficiency is around 99%. This work demonstrates a possibility of solving the critical issues (lifespan, rate capability, long term practicability and energy efficiency) for pH-decoupling ARFBs and guides the design for next generation high voltage ARFBs.

Carbon dioxide capture technologies will be important for counteracting difficult-to-abate greenhouse gas emissions if humanity is to limit global warming to acceptable levels. Electrochemically mediated CO2 capture has emerged as a promising alternative to conventional amine scrubbing, offering a potentially cost effective, environmentally friendly and energy efficient approach. Here we report an electrochemical cell for CO2 capture based on pH swing cycles driven through proton-coupled electron transfer of a developed phenazine derivative, 2,2′-(phenazine-1,8-diyl)bis(ethane-1-sulfonate) (1,8-ESP), with high aqueous solubility (>1.35 M) over pH range 0.00–14.90. The system operates with a high capture capacity of 0.86–1.41 mol l−1, a low energetic cost of 36–55 kJ mol−1 and an extremely low capacity fade rate of <0.01% per day, depending on organic concentration. The system charge–discharge cycle provides an electrical energy storage function that could be run only for storage when called for by electricity market conditions. Electrochemical carbon capture is a promising way to electrify CO2 emissions mitigation, but capacities are often low due to poor solubility of the redox-active organic molecules at the heart of the process. Here the authors report a high-capacity and high-stability electrochemical CO2 capture system based on a phenazine derivative they have developed.

Aqueous redox flow batteries (ARFBs) constitute a promising technology for grid-scale electricity storage, but it is challenging to implement cell voltages exceeding the 1.23 V thermodynamic water splitting window with high Coulombic efficiency and long lifetime. pH decoupling – the creation of a pH difference between the negolyte and posolyte – can broaden the operating voltage window and improve long-term operational stability. This penalizes the efficiency, however, due to acid-base crossover induced by the pH gradient. As the voltage of the water splitting window varies linearly with pH whereas crossover fluxes vary exponentially, we employed mildly acidic and mildly basic electrolytes to develop a cell with high round-trip energy efficiency at an open-circuit voltage &gt; 1.7 V. Moreover, we implemented an in situ acid-base regeneration system to periodically restore the negolyte and posolyte pH to their initial values. The combined system exhibits a capacity fade rate of less than 0.07% per day, a roundtrip energy efficiency of over 85%, and a Coulombic efficiency of approximately 99%. This work demonstrates principles for addressing critical issues such as lifespan, rate capability, long-term practicability, and energy efficiency in pH-decoupling ARFBs, providing guidance for the design of the next generation of high-voltage ARFBs.

Aqueous redox flow batteries (ARFBs) constitute a promising technology for grid-scale electricity storage, but it is challenging to implement cell voltages exceeding the 1.23 V thermodynamic water splitting window with high Coulombic efficiency and long lifetime. pH decoupling – the creation of a pH difference between the negolyte and posolyte – can broaden the operating voltage window and improve long-term operational stability. This penalizes the efficiency, however, due to acid-base crossover induced by the pH gradient. As the voltage of the water splitting window varies linearly with pH whereas crossover fluxes vary exponentially, we employed mildly acidic and mildly basic electrolytes to develop a cell with high round-trip energy efficiency at an open-circuit voltage &gt; 1.7 V. Moreover, we implemented an in situ acid-base regeneration system to periodically restore the negolyte and posolyte pH to their initial values. The combined system exhibits a capacity fade rate of less than 0.07% per day, a roundtrip energy efficiency of over 85%, and a Coulombic efficiency of approximately 99%. This work demonstrates principles for addressing critical issues such as lifespan, rate capability, long-term practicability, and energy efficiency in pH-decoupling ARFBs, providing guidance for the design of the next generation of high-voltage ARFBs.

We assess the suitability of potassium ferri-/ferrocyanide as an electroactive species for long-term utilization in aqueous organic redox flow batteries. A series of electrochemical and chemical characterization experiments was performed to distinguish between structural decomposition and apparent capacity fade of ferri-/ferrocyanide solutions used in the capacity-limiting side of a flow battery. Our results indicate that, in contrast with previous reports, no structural decomposition of ferri-/ferrocyanide occurs at tested pH values as high as 14 in the dark or in diffuse indoor light. Instead, an apparent capacity fade takes place due to a chemical reduction of ferricyanide to ferrocyanide, via chemical oxygen evolution reaction. We find that this parasitic process can be further exacerbated by carbon electrodes, with apparent capacity fade rates at pH 14 increasing with an increased ratio of carbon electrode surface area to ferricyanide in solution. Based on these results, we report a set of operating conditions that enables the long-duration cycling of alkaline ferri-/ferrocyanide electrolytes and demonstrate how apparent capacity fade rates can be engineered by the initial system setup. If protected from direct exposure to light, the structural stability of ferri-/ferrocyanide anions allows for their practical deployment as electroactive species in long duration energy storage applications.

Organic reactants are promising candidates for long-lifetime redox flow batteries, and synthetic chemistry unlocks a wide design space for new molecules. Minimizing crossover of these molecules through ion exchange membranes is one important design consideration, but the ways in which the crossover rate depends on the structure of the crossing species remain unclear. Here, we contribute a systematic evaluation of size- and charge-based effects on dilute-solution small molecule permeability through the Nafion NR212 cation exchange membrane. We found that increasing the magnitude of charge number z with the same sign as membrane fixed charges, achieved here by successive sulfonation of quinone redox cores, results in more than an order of magnitude permeability reduction per sulfonate. Size-based effects, understood by comparing the Stokes radii of the quinones studied, also reduces permeability with increasing effective molecule size, but doubling the effective size of the redox reactants resulted in a permeability decrease of less than a factor of three.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Transparent, electrically conductive and infrared-reflective films of zinc oxyfluoride are produced by chemical vapor deposition from vapor mixtures of zinc, oxygen and fluorine-containing compounds. The substitution of fluorine for some of the oxygen in zinc oxide results in dramatic increases in the electrical conductivity. For example, diethyl zinc, ethyl alcohol and hexafluoropropene vapors are reacted over a glass surface at 400.degree. C. to form a visibly transparent, electrically conductive, infrared reflective and ultraviolet absorptive film of zinc oxyfluoride. Such films are useful in liquid crystal display devices, solar cells, electrochromic absorbers and reflectors, energy-conserving heat mirrors, and antistatic coatings.

Abstract An extremely stable, energy‐dense (53.6 Ah L −1 , 2 m transferrable electrons), low crossover (permeability of &lt;1 × 10 −13 cm 2 s −1 using Nafion 212 (Nafion is a trademark polymer from DuPont)), and potentially inexpensive anthraquinone with 2‐2‐propionate ether anthraquinone structure (abbreviated 2‐2PEAQ) is synthesized and extensively evaluated under practically relevant conditions for use in the negolyte of an aqueous redox flow battery. 2‐2PEAQ shows a high stability with a fade rate of 0.03–0.05% per day at different applied current densities, cut‐off voltage windows, and concentrations (0.1 and 1.0 m ) in both a full cell paired with a ferro/ferricyanide posolyte as well as a symmetric cell. 2‐2PEAQ is further shown to have extreme long‐term stability, losing only ≈0.01% per day when an electrochemical rejuvenation strategy is employed. From post‐mortem analysis (nuclear magnetic resonance (NMR), liquid chromatography–mass spectrometry (LC‐MS), and cyclic voltammetry (CV)) two degradation mechanisms are deduced: side chain loss and anthrone formation. 2‐2PEAQ with the ether linkages attached on carbons non‐adjacent to the central ring is found to have three times lower fade rate compared to its isomer with ether linkages on the carbon adjacent to the central quinone ring. The present study introduces a viable negolyte candidate for grid‐scale aqueous organic redox flow batteries.

An aluminum oxide film is deposited on a heated substrate by CVD from one or more alkylaluminum alkoxide compounds having composition R.sub.n Al.sub.2 (OR').sub.6-n, wherein R and R' are alkyl groups and n is in the range of 1 to 5.

Fims of fluorine-doped zinc oxide are deposited from vaporized precursor compounds comprising a chelate of a dialkylzinc, such as an amine chelate, an oxygen source, and a fluorine source. The coatings are highly electrically conductive, transparent to visible light, reflective to infrared radiation, absorbing to ultraviolet light, and free of carbon impurity.

In a photovoltaic cell structure containing a visibly transparent, electrically conductive first layer of metal oxide, and a light-absorbing semiconductive photovoltaic second layer, the improvement comprising a thin layer of transition metal nitride, carbide or boride interposed between said first and second layers.

We report an electrochemical cell for CO2 capture based on pH swing cycles driven through proton-coupled electron transfer of a newly developed phenazine, 2,2'-(phenazine-1,8-diyl)bis(ethane-1-sulfonate) (1,8-ESP), which exhibits high aqueous solubility, &gt; 1.35 M, over pH range 0.00–14.90. The system operates with a high capture capacity of 0.86–1.41 mol/L, a low energetic cost of 36.4–55.2 kJ/mol, and an extremely low capacity fade rate of &lt;0.01%/day, depending on organic concentration. The system charge-discharge cycle provides an electrical energy storage function that can be run efficiently only for storage when called for by electricity market conditions. These results demonstrate the great potential of electrochemically-driven pH swing cycles based on proton-coupled electron transfer of redox-active organics for CO2 capture.