No abstract

Aqueous redox flow batteries are potentially well suited for grid scale energy storage for their uncoupled power and energy, safety, cost-effectiveness and longevity.[1] Although organic aqueous flow batteries with quinone redox active species have demonstrated promising results, including high stability, > 1 V open-circuit potential and high solubility, during operation they tend to be solubility-limited at moderate pH and, due to proton-coupled electron transfer, tend to swing the electrolyte pH to extreme values during cycling.[2-5]Viologens, another class of redox active organic molecules, are soluble regardless of solution pH and their redox reactions do not involve coupled protons or hydroxides, thus enabling stable pH during cycling. However, previously reported viologen-based flow batteries suffer from high capacity fade rates, high active material permeability, or low power density.[6-9] Here we present a highly stable phosphonate-functionalized viologen as the redox-active species in or aqueous redox flow batteries (ARFBs) operating at nearly neutral pH. The solubility is 1.23 M and the reduction potential is the lowest of any substituted viologen utilized in a flow battery, reaching -0.462 V vs. SHE at pH 9. The negative charges in both the oxidized and the reduced states of 1,1'-bis(3-phosphonopropyl)-[4,4'-bipyridine]-1,1'-diium dibromide (BPP - Vi) effect low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery pairing BPP-Vi with a ferrocyanide-based posolyte across an inexpensive, non-fluorinated cation exchange membrane at pH = 9 exhibits an open-circuit voltage of 0.9 V and an extremely low capacity fade rate of 0.016%/day or 0.00069%/cycle. Overcharging leads to viologen decomposition, causing irreversible capacity fade. This work introduces extremely stable, extremely low-permeating and low reduction potential redox active materials into near neutral ARFBs. Figure Inserted Here Figure Caption: Extended cycling at 40 mA cm-2 of a 1 m BPP-Vi | Fe(CN)6 flow cell. Electrolytes comprised 6.2 mL of 1.0 m BPP - Vi titrated with 14 m NH4OH to pH = 9 (negolyte) and 40 mL of 0.3 m K4Fe(CN)6 and 0.3 m K3Fe(CN)6 in 2 m NH4Cl at pH = 9 (posolyte). The average coulombic efficiency was >99.95%. The fitted discharge fade rate was 0.016%/day or 0.00069%/cycle. References 1T. Nguyen and R.F. Savinell, "Flow Batteries", The Electrochemical Society Interface 54 (2010). 2B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, "A Metal-Free Organic-Inorganic Aqueous Flow Battery", Nature 505, 195 (2014). 3D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. De Porcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, "Alkaline Quinone Flow Battery with Long Lifetime at pH 12", Joule 2, 1907 (2018). 4Y. Ji, M.A. Goulet, D.A. Pollack, D.G. Kwabi, S. Jin, D. Porcellinis, E.F. Kerr, R.G. Gordon, and M.J. Aziz, "A Phosphonate‐Functionalized Quinone Redox Flow Battery at near‐Neutral pH with Record Capacity Retention Rate", Advanced Energy Materials 9, 1900039 (2019). 5S. Jin, Y. Jing, D.G. Kwabi, Y.L. Ji, L.C. Tong, D. De Porcellinis, M.A. Goulet, D.A. Pollack, R.G. Gordon, and M.J. Aziz, "A Water-Miscible Quinone Flow Battery with High Volumetric Capacity and Energy Density", ACS Energy Letters 4, 1342 (2019). 6E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon, and M.J. Aziz, "A Neutral pH Aqueous Organic-Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2, 639 (2017). 7B.H. Camden DeBruler, Jared Moss, Jian Luo, T. Leo Liu, "A Sulfonate-Functionalized Viologen Enabling Neutral Cation Exchange, Aqueous Organic Redox Flow Batteries toward Renewable Energy Storage", ACS Energy Letters 3, 663 (2018). 8J. Luo, B. Hu, C. Debruler, Y.J. Bi, Y. Zhao, B. Yuan, M.W. Hu, W.D. Wu, and T.L. Liu, "Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries", Joule 3, 149 (2019). 9T. Janoschka, N. Martin, M.D. Hager, and U.S. Schubert, "An Aqueous Redox-Flow Battery with High Capacity and Power: The TEMPTMA/MV System", Angew Chem Int Ed Engl 55, 14427 (2016). Figure 1

Aqueous organic redox flow batteries are promising candidates for large-scale energy storage. However, the design of stable and inexpensive electrolytes is challenging. Here, we report a highly stable, low redox potential, and potentially inexpensive negolyte species, sodium 3,3',3'',3'''-((9,10-anthraquinone-2,6-diyl)bis(azanetriyl))tetrakis(propane-1-sulfonate) (2,6-N-TSAQ), which is synthesized in a single step from inexpensive precursors. Pairing 2,6-N-TSAQ with potassium ferrocyanide at pH 14 yielded a battery with the highest open-circuit voltage, 1.14 V, of any anthraquinone-based cell with a capacity fade rate <10%/yr. When 2,6-N-TSAQ was cycled at neutral pH, it exhibited two orders of magnitude higher capacity fade rate. The great difference in anthraquinone cycling stability at different pH is interpreted in terms of the thermodynamics of the anthrone formation reaction. This work shows the great potential of organic synthetic chemistry for the development of viable flow battery electrolytes and demonstrates the remarkable performance improvements achievable with an understanding of decomposition mechanisms.

Aqueous organic redox flow batteries (AORFBs) offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables. However, molecular decomposition is the major barrier preventing AORFBs from being commercialized. Structural modifications can improve molecular stability at the expense of increased synthetic cost and molecular weight. Utilizing 2,6-dihydroxy-anthraquinone (DHAQ), without further structural modification, we demonstrate that electrochemical regeneration could be a viable route to achieve low-cost, longlifetime AORFBs. In situ (online) NMR and EPR and complementary electrochemical analyses reveal that decomposition compounds i.e., 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL), can be converted back to DHAQ in two steps: first DHA(L) 2− are oxidized to the dimer (DHA)2 4− at -0.32 V vs. SHE by one-electron transfer; subsequently, the (DHA)2 4− is oxidized to DHAQ 2− at +0.57 V vs. SHE by three-electron transfer. Electrochemical regeneration rejuvenates not only DHAQ 2− , but also the positive electrolyte -rebalancing the states of charge of both electrolytes without introducing extra ions. We demonstrate the repeated capacity recovery with DHAQ | potassium ferro-/ferricyanide flow battery in basic conditions, and show the approach is also effective for anthraquinone-2,7-disulfonate in acid. Electrochemical regeneration strategies may extend the useful lifetime of many water-soluble organic molecules with anthraquinone core structures in electrochemical cells.

No abstract

The effect of particle size on the water-soluble substances and microscopic structure of sorghum straw powder were investigated. Sorghum straw powder with four particle size (300 ~ 450μm, 125 ~ 150μm, 97 ~ 105μm, 330 ~ 420nm ) were studied for analysing changes of water-soluble substances and microscopic structure. The results showed that with the particle size decreasing the pH value of water-soluble substances decreased, the concentration of reducing sugar in water increased firstly then began to decrease when it reached to a certain value, the crystallinity of sorghum straw powder decreased, and the degree of polymerization of sorghum straw was lowered ,respectively.

This paper used respective ratios of 50%, 60%, 70%, 80%, 90% and 100% of straw and pig excrement as raw materials to produce biogas and fertilizer. The test focused on gas production, the contents of available nutritional elements nitrogen, phosphorus and potassium, and the organic matter content. The experimental results demonstrate the fermentation can produce more biogas, nitrogen, phosphorus, potassium, and organic mass under the conditions which the fermentation cycle is 15 days and the ratio of straw and excrement is 70%.

Water-soluble anthraquinones (AQs) hold great promise serving as redox-active species in aqueous organic redox flow batteries. Systematic investigations into how the properties of redox molecules depend on the water-solubilizing groups and the way in which they are bound to the redox core are, however, still lacking. We introduce water-solubilizing groups linked to anthraquinone by C=C bonds via Heck cross-coupling reactions and convert C=C bonds to CC bonds through hydrogenation. The anthraquinone and the ending groups are connected via branched or straight chains with either unsaturated or saturated bonds. We investigate the influence of water-solubilizing chains and ionic ending groups on redox potentials of molecules and identify three important trends. (1): The electron-withdrawing ending groups can affect redox potentials of AQs with two unsaturated hydrocarbons on the chains through π-conjugation. (2): For chains with two saturated or unsaturated straight hydrocarbons, water-solubilizing ending groups increase redox potentials of the AQs in the order of PO32 <CO2<SO3. (3): AQs with saturated and unbranched chains at high pH possess desirably low redox potentials, high solubilities, and high stability. Disproportionation leads to the formation of anthrone, which can be regenerated to anthraquinone. Tautomerization results in the saturation of alkene chains, stabilizing the structure. We utilize these observations to identify a potentially low-cost and long-lifetime negolyte that demonstrates a temporal fade rate as low as 0.0128%/day when paired with a potassium ferrocyanide posolyte.