Recent Advances in Redox Flow Batteries Employing Metal Coordination Complexes as Redox-Active Species

Abstract

Redox flow batteries (RFBs) that employ sustainable, abundant, and structure-tunable redox-active species are of great interest for large-scale energy storage. As a vital class of redox-active species, metal coordination complexes (MCCs) possessing the properties of both the organic ligands and transition metal ion centers are attracting increasing attention due to the advantages of multielectron charge transfer, high structural tailorability, and reduced material crossover. Herein, we present a critical overview of RFBs that employ MCCs as redox-active materials in both aqueous and nonaqueous mediums. The progress is comprehensively summarized, including the design strategies, solubility characteristics, electrochemical properties, and battery cycling performance of MCCs. Emphasis is placed on the ligand selection and modification strategies used to tune the critical properties of MCCs, including their redox potential, solubility, cycling stability, and electron transfer redox reactions, to achieve stable cycled RFBs with a high energy density. Furthermore, we discuss the current challenges and perspectives related to the development of MCC-based RFBs for large-scale energy storage implementations.

Graphical abstract

Keywords

Energy storage / Redox flow battery / Metal coordination complex / Electrochemistry / Redox noninnocent ligands

References

[1] Dunn B, Kamath H, Tarascon JM. Electrical energy storage for the grid: a battery of choices. Science, 2011, 334: 928-935

[2] Alotto P, Guarnieri M, Moro F. Redox flow batteries for the storage of renewable energy: a review. Renew. Sustain. Energy Rev., 2014, 29: 325-335

[3] Noack J, Roznyatovskaya N, Herr T, et al.. The chemistry of redox-flow batteries. Angew. Chem. Int. Ed., 2015, 54: 9776-9809

[4] Park M, Ryu J, Wang W, et al.. Material design and engineering of next-generation flow-battery technologies. Nat. Rev. Mater., 2017, 2: 1-18

[5] Skyllas-Kazacos M, Chakrabarti MH, Hajimolana SA, et al.. Progress in flow battery research and development. J. Electrochem. Soc., 2011, 158: R55

[6] Wang W, Luo QT, Li B, et al.. Recent progress in redox flow battery research and development. Adv. Funct. Mater., 2013, 23: 970-986

[7] Ye RJ, Henkensmeier D, Yoon SJ, et al.. Redox flow batteries for energy storage: a technology review. J. Electrochem. Energy Convers. Storage, 2018, 15: 010801

[8] Huskinson B, Marshak MP, Suh C, et al.. A metal-free organic-inorganic aqueous flow battery. Nature, 2014, 505: 195-198

[9] Lin KX, Chen Q, Gerhardt MR, et al.. Alkaline quinone flow battery. Science, 2015, 349: 1529-1532

[10] Soloveichik GL. Flow batteries: current status and trends. Chem. Rev., 2015, 115: 11533-11558

[11] Bartolozzi M. Development of redox flow batteries. A historical bibliography. J. Power Sources, 1989, 27: 219-234

[12] Jiang HR, Sun J, Wei L, et al.. A high power density and long cycle life vanadium redox flow battery. Energy Storage Mater., 2020, 24: 529-540

[13] Zeng YK, Zhou XL, An L, et al.. A high-performance flow-field structured iron-chromium redox flow battery. J. Power Sources, 2016, 324: 738-744

[14] Wu MC, Zhao TS, Wei L, et al.. Improved electrolyte for zinc-bromine flow batteries. J. Power Sources, 2018, 384: 232-239

[15] Pan F, Wang Q. Redox species of redox flow batteries: a review. Molecules, 2015, 20: 20499-20517

[16] Lourenssen K, Williams J, Ahmadpour F, et al.. Vanadium redox flow batteries: a comprehensive review. J. Energy Storage, 2019, 25: 100844

[17] Wang W, Sprenkle V. Redox flow batteries go organic. Nat. Chem., 2016, 8: 204-206

[18] Kowalski JA, Su L, Milshtein JD, et al.. Recent advances in molecular engineering of redox active organic molecules for nonaqueous flow batteries. Curr. Opin. Chem. Eng., 2016, 13: 45-52

[19] Wei XL, Pan WX, Duan WT, et al.. Materials and systems for organic redox flow batteries: status and challenges. ACS Energy Lett., 2017, 2: 2187-2204

[20] Meda L, Oldani F, Tozzola G, et al.. Searching for new redox-complexes\in organic flow batteries. Solid State Ion., 2018, 317: 142-148

[21] Liu B, Tang CW, Jiang HR, et al.. An aqueous organic redox flow battery employing a trifunctional electroactive compound as anolyte, catholyte and supporting electrolyte. J. Power Sources, 2020, 477: 228985

[22] Liu B, Tang CW, Jiang HR, et al.. Carboxyl-functionalized TEMPO catholyte enabling high-cycling-stability and high-energy-density aqueous organic redox flow batteries. ACS Sustain. Chem. Eng., 2021, 9: 6258-6265

[23] Winsberg J, Hagemann T, Janoschka T, et al.. Redox-flow batteries: from metals to organic redox-active materials. Angewandte Chem. Int. Ed., 2017, 56: 686-711

[24] Leung P, Shah AA, Sanz L, et al.. Recent developments in organic redox flow batteries: a critical review. J. Power Sources, 2017, 360: 243-283

[25] Ding Y, Yu GH. Molecular engineering enables better organic flow batteries. Chem, 2017, 3: 917-919

[26] Fischer P, Mazúr P, Krakowiak J. Family tree for aqueous organic redox couples for redox flow battery electrolytes: a conceptual review. Molecules, 2022, 27: 560

[27] Luo J, Wang AP, Hu MW, et al.. Materials challenges of aqueous redox flow batteries. MRS Energy Sustain., 2022, 9: 1-12

[28] Fan H, Hu B, Li HB, et al.. Conjugate-driven electron density delocalization of piperidine nitroxyl radical for stable aqueous zinc hybrid flow batteries. Angew. Chem., 2022, 134: e202115908

[29] Yang L, Hao YH, Lin JD, et al.. POM anolyte for all-anion redox flow batteries with high capacity retention and coulombic efficiency at mild pH. Adv. Mater., 2022, 34: e2107425

[30] Pan MG, Gao LZ, Liang JC, et al.. Reversible redox chemistry in pyrrolidinium-based TEMPO radical and extended viologen for high-voltage and long-life aqueous redox flow batteries. Adv. Energy Mater., 2022, 12: 2103478

[31] Hu B, Hu MW, Luo J, et al.. A stable, low permeable TEMPO catholyte for aqueous total organic redox flow batteries. Adv. Energy Mater., 2022, 12: 2102577

[32] Feng RZ, Zhang X, Murugesan V, et al.. Reversible ketone hydrogenation and dehydrogenation for aqueous organic redox flow batteries. Science, 2021, 372: 836-840

[33] Liu B, Tang cw, Zhang C, et al.. Cost-effective, high-energy-density, nonaqueous nitrobenzene organic redox flow battery. Chem. Mater., 2021, 33: 978-986

[34] Singh V, Kim S, Kang J, et al.. Aqueous organic redox flow batteries. Nano Res., 2019, 12: 1988-2001

[35] Kwabi DG, Ji YL, Aziz MJ. Electrolyte lifetime in aqueous organic redox flow batteries: a critical review. Chem. Rev., 2020, 120: 6467-6489

[36] Zhang CK, Li XF. Perspective on organic flow batteries for large-scale energy storage. Curr. Opin. Electrochem., 2021, 30: 100836

[37] Chen HN, Cong GT, Lu YC. Recent progress in organic redox flow batteries: active materials, electrolytes and membranes. J. Energy Chem., 2018, 27: 1304-1325

[38] Luo J, Hu B, Hu MW, et al.. Status and prospects of organic redox flow batteries toward sustainable energy storage. ACS Energy Lett., 2019, 4: 2220-2240

[39] Zhan XW, Lu XC, Reed DM, et al.. Emerging soluble organic redox materials for next-generation grid energy-storage applications. MRS Commun., 2020, 10: 215-229

[40] Liu YH, Chen QR, Zhang X, et al.. Degradation of electrochemical active compounds in aqueous organic redox flow batteries. Curr. Opin. Electrochem., 2022, 32: 100895

[41] Perry ML, Rodby KE, Brushett FR. Untapped potential: the need and opportunity for high-voltage aqueous redox flow batteries. ACS Energy Lett., 2022, 7: 659-667

[42] Chen QR, Lv YG, Yuan ZZ, et al.. Organic electrolytes for pH-neutral aqueous organic redox flow batteries. Adv. Funct. Mater., 2022, 32: 2108777

[43] Zhao ZM, Zhang CK, Li XF. Opportunities and challenges of organic flow battery for electrochemical energy storage technology. J. Energy Chem., 2022, 67: 621-639

[44] Cameron JM, Holc C, Kibler AJ, et al.. Molecular redox species for next-generation batteries. Chem. Soc. Rev., 2021, 50: 5863-5883

[45] Li ZJ, Lu YC. Material design of aqueous redox flow batteries: fundamental challenges and mitigation strategies. Adv. Mater., 2020, 32: 2002132

[46] Gong K, Fang QR, Gu S, et al.. Nonaqueous redox-flow batteries: Organic solvents, supporting electrolytes, and redox pairs. Energy Environ. Sci., 2015, 8: 3515-3530

[47] Hogue RW, Toghill KE. Metal coordination complexes in nonaqueous redox flow batteries. Curr. Opin. Electrochem., 2019, 18: 37-45

[48] Ding Y, Zhao Y, Li YT, et al.. A high-performance all-metallocene-based, non-aqueous redox flow battery. Energy Environ. Sci., 2017, 10: 491-497

[49] Wang DY, Liu RL, Guo W, et al.. Recent advances of organometallic complexes for rechargeable batteries. Coord. Chem. Rev., 2021, 429: 213650

[50] Karmakar A, Mukundan R, Yang P, et al.. Screening of metal complexes and organic solvents using the COSMOSAC-LANL model to enhance the energy density in a non-aqueous redox flow cell: an insight into the solubility. Phys. Chem. Chem. Phys., 2021, 23: 21106-21129

[51] Ding Y, Zhang CK, Zhang LY, et al.. Pathways to widespread applications: development of redox flow batteries based on new chemistries. Chem, 2019, 5: 1964-1987

[52] Ding Y, Zhang CK, Zhang LY, et al.. Molecular engineering of organic electroactive materials for redox flow batteries. Chem. Soc. Rev., 2018, 47: 69-103

[53] Palmer TC, Beamer A, Pitt T, et al.. A comparative review of metal-based charge carriers in nonaqueous flow batteries. Chemsuschem, 2021, 14: 1214-1228

[54] Yang ZG, Zhang JL, Kintner-Meyer MCW, et al.. Electrochemical energy storage for green grid. Chem. Rev., 2011, 111: 3577-3613

[55] Hogue RW, Armstrong CG, Toghill KE. Dithiolene complexes of first-row transition metals for symmetric nonaqueous redox flow batteries. Chemsuschem, 2019, 12: 4506-4515

[56] Eisenberg R, Gray HB. Noninnocence in metal complexes: a dithiolene dawn. Inorg. Chem., 2011, 50: 9741-9751

[57] Matsuda Y, Tanaka K, Okada M, et al.. A rechargeable redox battery utilizing ruthenium complexes with non-aqueous organic electrolyte. J. Appl. Electrochem., 1988, 18: 909-914

[58] Laramie SM, Milshtein JD, Breault TM, et al.. Performance and cost characteristics of multi-electron transfer, common ion exchange non-aqueous redox flow batteries. J. Power Sources, 2016, 327: 681-692

[59] Mun J, Oh DJ, Park MS, et al.. Highly soluble tris(2,2ʹ-bipyridine) metal bis(trifluoromethanesulfonyl)imide complexes for high energy organic redox flow batteries. J. Electrochem. Soc., 2018, 165: A215-A219

[60] Benazzi E, Cristino V, Caramori S, et al.. Electrochemical characterization of polypyridine iron(II) and cobalt(II) complexes for organic redox flow batteries. Polyhedron, 2018, 140: 99-108

[61] Cammack CX, Pratt HD, Small LJ, et al.. A higher voltage Fe(II) bipyridine complex for non-aqueous redox flow batteries. Dalton Trans., 2021, 50: 858-868

[62] Kim JH, Kim KJ, Park MS, et al.. Development of metal-based electrodes for non-aqueous redox flow batteries. Electrochem. Commun., 2011, 13: 997-1000

[63] Mun J, Lee MJ, Park JW, et al.. Non-aqueous redox flow batteries with nickel and iron tris(2,2ʹ-bipyridine) complex electrolyte. Electrochem. Solid-State Lett., 2012, 15: A80

[64] Cabrera PJ, Yang XY, Suttil JA, et al.. Evaluation of tris-bipyridine chromium complexes for flow battery applications: impact of bipyridine ligand structure on solubility and electrochemistry. Inorg. Chem., 2015, 54: 10214-10223

[65] Dong S, Cabral DM, Pringle JM, et al.. Exploring the electrochemical properties of mixed ligand Fe(II) complexes as redox couples. Electrochim. Acta, 2020, 362: 137109

[66] Xing XQ, Zhao YC, Li YD. A non-aqueous redox flow battery based on tris(1,10-phenanthroline) complexes of iron(II) and cobalt(II). J. Power Sources, 2015, 293: 778-783

[67] Xing XQ, Zhang DP, Li YD. A non-aqueous all-cobalt redox flow battery using 1,10-phenanthrolinecobalt(II) hexafluorophosphate as active species. J. Power Sources, 2015, 279: 205-209

[68] Yang CZ, Nikiforidis G, Park JY, et al.. Designing redox-stable cobalt-polypyridyl complexes for redox flow batteries: spin-crossover delocalizes excess charge. Adv. Energy Mater., 2018, 8: 1702897

[69] Kolesnichenko CX, Pratt HDIII, Small LJ, et al.. Elucidating instabilities contributing to capacity fade in bipyridine-based materials for non-aqueous flow batteries. ChemElectroChem, 2022, 9: e202101490

[70] Cabrera PJ, Yang XY, Suttil JA, et al.. Complexes containing redox noninnocent ligands for symmetric, multielectron transfer nonaqueous redox flow batteries. J. Phys. Chem. C, 2015, 119: 15882-15889

[71] Ruan WQ, Mao JT, Yang SD, et al.. Communication: tris(bipyridyl)iron complexes for high-voltage aqueous redox flow batteries. J. Electrochem. Soc., 2020, 167: 100543

[72] Li X, Gao PY, Lai YY, et al.. Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery. Nat. Energy, 2021, 6: 873-881

[73] Chen C, Zhang S, Zhu YZ, et al.. Pyridyl group design in viologens for anolyte materials in organic redox flow batteries. RSC Adv., 2018, 8: 18762-18770

[74] Armstrong CG, Toghill KE. Cobalt(II) complexes with azole-pyridine type ligands for non-aqueous redox-flow batteries: tunable electrochemistry via structural modification. J. Power Sources, 2017, 349: 121-129

[75] Sevov CS, Fisher SL, Thompson LT, et al.. Mechanism-based development of a low-potential, soluble, and cyclable multielectron anolyte for nonaqueous redox flow batteries. J. Am. Chem. Soc., 2016, 138: 15378-15384

[76] Skavenborg ML, McPherson JN, Pasadakis-Kavounis A, et al.. Leveraging coordination chemistry in the design of bipolar energy storage materials for redox flow batteries. Sustain. Energy Fuels, 2022, 6: 2179-2190

[77] Duarte GM, Braun JD, Giesbrecht PK, et al.. Redox non-innocent bis(2,6-diimine-pyridine) ligand-iron complexes as anolytes for flow battery applications. Dalton Trans., 2017, 46: 16439-16445

[78] Braun JD, Gray PA, Sidhu BK, et al.. Zn-Templated synthesis of substituted (2,6-diimine)pyridine proligands and evaluation of their iron complexes as anolytes for flow battery applications. Dalton Trans., 2020, 49: 16175-16183

[79] Elgrishi N, Chambers MB, Artero V, et al.. Terpyridine complexes of first row transition metals and electrochemical reduction of CO₂–CO. Phys. Chem. Chem. Phys., 2014, 16: 13635-13644

[80] Wang H, Sayed SY, Zhou YQ, et al.. Water-soluble pH-switchable cobalt complexes for aqueous symmetric redox flow batteries. Chem. Commun., 2020, 56: 3605-3608

[81] Chakrabarti MH, Dryfe RAW, Roberts EPL. Evaluation of electrolytes for redox flow battery applications. Electrochim. Acta, 2007, 52: 2189-2195

[82] Liu QH, Sleightholme AES, Shinkle AA, et al.. Non-aqueous vanadium acetylacetonate electrolyte for redox flow batteries. Electrochem. Commun., 2009, 11: 2312-2315

[83] Sleightholme AES, Shinkle AA, Liu QH, et al.. Non-aqueous manganese acetylacetonate electrolyte for redox flow batteries. J. Power Sources, 2011, 196: 5742-5745

[84] Bamgbopa MO, Shao-Horn Y, Almheiri S. The potential of non-aqueous redox flow batteries as fast-charging capable energy storage solutions: demonstration with an iron-chromium acetylacetonate chemistry. J. Mater. Chem. A, 2017, 5: 13457-13468

[85] Liu QH, Shinkle AA, Li YD, et al.. Non-aqueous chromium acetylacetonate electrolyte for redox flow batteries. Electrochem. Commun., 2010, 12: 1634-1637

[86] Zhang DP, Lan HJ, Li YD. The application of a non-aqueous bis(acetylacetone)ethylenediamine cobalt electrolyte in redox flow battery. J. Power Sources, 2012, 217: 199-203

[87] Shamie JS, Liu CH, Shaw LL, et al.. New mechanism for the reduction of vanadyl acetylacetonate to vanadium acetylacetonate for room temperature flow batteries. Chemsuschem, 2017, 10: 533-540

[88] Shinkle AA, Sleightholme AES, Griffith LD, et al.. Degradation mechanisms in the non-aqueous vanadium acetylacetonate redox flow battery. J. Power Sources, 2012, 206: 490-496

[89] Shinkle AA, Sleightholme AES, Thompson LT, et al.. Electrode kinetics in non-aqueous vanadium acetylacetonate redox flow batteries. J. Appl. Electrochem., 2011, 41: 1191-1199

[90] Herr T, Noack J, Fischer P, et al.. 1,3-Dioxolane, tetrahydrofuran, acetylacetone and dimethyl sulfoxide as solvents for non-aqueous vanadium acetylacetonate redox-flow-batteries. Electrochim. Acta, 2013, 113: 127-133

[91] Escalante-García IL, Wainright JS, Thompson LT, et al.. Performance of a non-aqueous vanadium acetylacetonate prototype redox flow battery: examination of separators and capacity decay. J. Electrochem. Soc., 2014, 162: A363-A372

[92] Suttil JA, Kucharyson JF, Escalante-Garcia IL, et al.. Metal acetylacetonate complexes for high energy density non-aqueous redox flow batteries. J. Mater. Chem. A, 2015, 3: 7929-7938

[93] Kucharyson JF, Cheng L, Tung SO, et al.. Predicting the potentials, solubilities and stabilities of metal-acetylacetonates for non-aqueous redox flow batteries using density functional theory calculations. J. Mater. Chem. A, 2017, 5: 13700-13709

[94] Hart, J.R.: Ethylenediaminetetraacetic acid and related chelating agents. In: Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (2011). https://doi.org/10.1002/14356007.a10_095.pub2

[95] Bae CH, Roberts EPL, Dryfe RAW. Chromium redox couples for application to redox flow batteries. Electrochim. Acta, 2002, 48: 279-287

[96] Bae C, Roberts EPL, Chakrabarti MH, et al.. All-chromium redox flow battery for renewable energy storage. Int. J. Green Energy, 2011, 8: 248-264

[97] Robb BH, Farrell JM, Marshak MP. Chelated chromium electrolyte enabling high-voltage aqueous flow batteries. Joule, 2019, 3: 2503-2512

[98] Waters SE, Robb BH, Scappaticci SJ, et al.. Isolation and characterization of a highly reducing aqueous chromium(II) complex. Inorg. Chem., 2022, 61: 8752-8759

[99] Wen YH, Zhang HM, Qian P, et al.. Studies on iron (Fe³⁺∕Fe²⁺)-complex/bromine (Br 2∕B^r–) redox flow cell in sodium acetate solution. J. Electrochem. Soc., 2006, 153: A929

[100] Waters SE, Davis CM, Thurston JR, et al.. Maximizing vanadium deployment in redox flow batteries through chelation. J. Am. Chem. Soc., 2022, 144: 17753-17757

[101] Schröder P, Obendorf D, Bechtold T. Electrochemistry of iron(II/III)-N,N'-ethylene-bis-(o-hydroxyphenylglycine) complexes in aqueous solution indicates potential for use in redox flow batteries. ChemElectroChem, 2019, 6: 3311-3318

[102] Modiba P, Matoetoe M, Crouch AM. Electrochemical impedance spectroscopy study of Ce(IV) with aminopolycarboxylate ligands for redox flow batteries applications. J. Power Sources, 2012, 205: 1-9

[103] So YM, Leung WH. Recent advances in the coordination chemistry of cerium(IV) complexes. Coord. Chem. Rev., 2017, 340: 172-197

[104] Modiba P, Matoetoe M, Crouch AM. Kinetics study of transition metal complexes (Ce-DTPA, Cr-DTPA and V-DTPA) for redox flow battery applications. Electrochim. Acta, 2013, 94: 336-343

[105] Yu X, Song YX, Tang A. Tailoring manganese coordination environment for a highly reversible zinc-manganese flow battery. J. Power Sources, 2021, 507: 230295

[106] Waters SE, Robb BH, Marshak MP. Effect of chelation on iron-chromium redox flow batteries. ACS Energy Lett., 2020, 5: 1758-1762

[107] Cappillino PJ, Pratt HDIII, Hudak NS, et al.. Energy storage: application of redox non-innocent ligands to non-aqueous flow battery electrolytes. Adv. Energy Mater., 2014, 4: 1300566

[108] Gong K, Xu F, Grunewald JB, et al.. All-soluble all-iron aqueous redox-flow battery. ACS Energy Lett., 2016, 1: 89-93

[109] Noh C, Chung Y, Kwon Y. Optimization of iron and cobalt based organometallic redox couples for long-term stable operation of aqueous organometallic redox flow batteries. J. Power Sources, 2021, 495: 229799

[110] Shin M, Noh C, Kwon Y. Stability enhancement for all-iron aqueous redox flow battery using iron-3-[bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid complex and ferrocyanide as redox couple. Int. J. Energy Res., 2022, 46: 6866-6875

[111] Shin M, Oh S, Jeong H, et al.. Aqueous redox flow battery using iron 2,2-bis(hydroxymethyl)-2,2ʹ,2ʹ-nitrilotriethanol complex and ferrocyanide as newly developed redox couple. Int. J. Energy Res., 2022, 46: 8175-8185

[112] Noh C, Chung Y, Kwon Y. Highly stable aqueous organometallic redox flow batteries using cobalt triisopropanolamine and iron triisopropanolamine complexes. Chem. Eng. J., 2021, 405: 126966

[113] Noh C, Chung Y, Kwon Y. Organometallic redox flow batteries using iron triethanolamine and cobalt triethanolamine complexes. J. Power Sources, 2020, 466: 228333

[114] Arroyo-Currás N, Hall JW, Dick JE, et al.. An alkaline flow battery based on the coordination chemistry of iron and cobalt. J. Electrochem. Soc., 2014, 162: A378-A383

[115] Lê A, Floner D, Roisnel T, et al.. Highly soluble Fe(III)-triethanolamine complex relevant for redox flow batteries. Electrochim. Acta, 2019, 301: 472-477

[116] Wen YH, Zhang HM, Qian P, et al.. A study of the Fe(III)/Fe(II)-triethanolamine complex redox couple for redox flow battery application. Electrochim. Acta, 2006, 51: 3769-3775

[117] Shin M, Noh C, Chung Y, et al.. All iron aqueous redox flow batteries using organometallic complexes consisting of iron and 3-[bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid ligand and ferrocyanide as redox couple. Chem. Eng. J., 2020, 398: 125631

[118] Ruan WQ, Mao JT, Yang SD, et al.. Designing Cr complexes for a neutral Fe–Cr redox flow battery. Chem. Commun., 2020, 56: 3171-3174

[119] Tsitovich PB, Kosswattaarachchi AM, Crawley MR, et al.. An Fe^III azamacrocyclic complex as a pH-tunable catholyte and anolyte for redox-flow battery applications. Chem. A Eur. J., 2017, 23: 15327-15331

[120] Sharma S, Andrade GA, Maurya S, et al.. Iron-iminopyridine complexes as charge carriers for non-aqueous redox flow battery applications. Energy Storage Mater., 2021, 37: 576-586

[121] Stauber JM, Zhang SY, Gvozdik N, et al.. Cobalt and vanadium trimetaphosphate polyanions: synthesis, characterization, and electrochemical evaluation for non-aqueous redox-flow battery applications. J. Am. Chem. Soc., 2018, 140: 538-541

[122] Huang HB, Howland R, Agar E, et al.. Bioinspired, high-stability, nonaqueous redox flow battery electrolytes. J. Mater. Chem. A, 2017, 5: 11586-11591

[123] Hwang S, Kim HS, Ryu JH, et al.. Ni(II)-chelated thio-crown complex as a single redox couple for non-aqueous flow batteries. Electrochem. Commun., 2017, 85: 36-39

[124] Kim HS, Hwang S, Mun J, et al.. Counter anion effects on the energy density of Ni(II)-chelated tetradentate azamacrocyclic complex cation as single redox couple for non-aqueous flow batteries. Electrochim. Acta, 2019, 308: 227-230

[125] Andrade GA, Popov IA, Federico CR, et al.. Expanding the potential of redox carriers for flow battery applications. J. Mater. Chem. A, 2020, 8: 17808-17816

[126] Chu T, Popov IA, Andrade GA, et al.. Linked picolinamide nickel complexes as redox carriers for nonaqueous flow batteries. Chemsuschem, 2019, 12: 1304-1309

[127] Li Y, Geysens P, Zhang X, et al.. Cerium-containing complexes for low-cost, non-aqueous redox flow batteries (RFBs). J. Power Sources, 2020, 450: 227634

[128] Lu CC, Bill E, Weyhermüller T, et al.. Neutral bis(α-iminopyridine)metal complexes of the first-row transition ions (Cr, Mn, Fe Co, Ni, Zn) and their monocationic analogues: mixed valency involving a redox noninnocent ligand system. J. Am. Chem. Soc., 2008, 130: 3181-3197

[129] Kim HS, Yoon T, Jang J, et al.. A tetradentate Ni(II) complex cation as a single redox couple for non-aqueous flow batteries. J. Power Sources, 2015, 283: 300-304

[130] Montero J, Navalpotro P, D́Epifanio A, et al.. Redox-active coordination polymers as bifunctional electrolytes in slurry-based aqueous batteries at neutral pH. J. Electroanal. Chem., 2021, 895: 115442

[131] Fernández-Benito A, Rivera-Gálvez FJ, Cisneros-Ruiz P, et al.. Novel ferrocene-containing amphiphilic block copolymer nanoparticles as high-capacity charge carriers in aqueous redox flow systems. Mater. Today Chem., 2023, 27: 101271

[132] Friedl J, Holland-Cunz MV, Cording F, et al.. Asymmetric polyoxometalate electrolytes for advanced redox flow batteries. Energy Environ. Sci., 2018, 11: 3010-3018

[133] Ai F, Wang ZY, Lai NC, et al.. Heteropoly acid negolytes for high-power-density aqueous redox flow batteries at low temperatures. Nat. Energy, 2022, 7: 417-426

[134] Zhao Y, Ding Y, Song J, et al.. Sustainable electrical energy storage through the ferrocene/ferrocenium redox reaction in aprotic electrolyte. Angew. Chem. Int. Ed., 2014, 53: 11036-11040

[135] Wei XL, Cosimbescu L, Xu W, et al.. Batteries: towards high-performance nonaqueous redox flow electrolyte via ionic modification of active species. Adv. Energy Mater., 2015, 5: 1400678

[136] Hu B, Liu TL. Two electron utilization of methyl viologen anolyte in nonaqueous organic redox flow battery. J. Energy Chem., 2018, 27: 1326-1332

[137] Cosimbescu L, Wei XL, Vijayakumar M, et al.. Anion-tunable properties and electrochemical performance of functionalized ferrocene compounds. Sci. Rep., 2015, 5: 1-9

[138] Kim HS, Yoon T, Kim Y, et al.. Increase of both solubility and working voltage by acetyl substitution on ferrocene for non-aqueous flow battery. Electrochem. Commun., 2016, 69: 72-75

[139] Song H, Kwon G, Citek C, et al.. Pyrrolinium-substituted persistent zwitterionic ferrocenate derivative enabling the application of ferrocene anolyte. ACS Appl. Mater. Interfaces, 2021, 13: 46558-46565

[140] Cong GT, Zhou YC, Li ZJ, et al.. A highly concentrated catholyte enabled by a low-melting-point ferrocene derivative. ACS Energy Lett., 2017, 2: 869-875

[141] Zhang CK, Qian YM, Ding Y, et al.. Biredox eutectic electrolytes derived from organic redox-active molecules: high-energy storage systems. Angew. Chem., 2019, 131: 7119-7124

[142] Zhang CK, Zhang LY, Yu GH. Eutectic electrolytes as a promising platform for next-generation electrochemical energy storage. Acc. Chem. Res., 2020, 53: 1648-1659

[143] Hwang B, Park MS, Kim K. Ferrocene and cobaltocene derivatives for non-aqueous redox flow batteries. Chemsuschem, 2015, 8: 310-314

[144] Chen H, Niu ZH, Ye J, et al.. Multicore ferrocene derivative as a highly soluble cathode material for nonaqueous redox flow batteries. ACS Appl. Energy Mater., 2021, 4: 855-861

[145] Milton M, Cheng Q, Yang Y, et al.. Molecular materials for nonaqueous flow batteries with a high Coulombic efficiency and stable cycling. Nano Lett., 2017, 17: 7859-7863

[146] Montoto EC, Nagarjuna G, Moore JS, et al.. Redox active polymers for non-aqueous redox flow batteries: validation of the size-exclusion approach. J. Electrochem. Soc., 2017, 164: A1688-A1694

[147] Friedl J, Lebedeva MA, Porfyrakis K, et al.. All-fullerene-based cells for nonaqueous redox flow batteries. J. Am. Chem. Soc., 2018, 140: 401-405

[148] Hwang S, Kim HS, Ryu JH, et al.. N-ferrocenylphthalimide; a single redox couple formed by attaching a ferrocene moiety to phthalimide for non-aqueous flow batteries. J. Power Sources, 2018, 395: 60-65

[149] Hwang S, Kim HS, Ryu JH, et al.. N-(α-ferrocenyl)ethylphthalimide as a single redox couple for non-aqueous flow batteries. J. Power Sources, 2019, 421: 1-5

[150] Xu DH, Zhang CJ, Zhen YH, et al.. Ferrocene/phthalimide ionic bipolar redox-active molecule for symmetric nonaqueous redox flow batteries. ACS Appl. Energy Mater., 2021, 4: 8045-8051

[151] Zhen YH, Zhang CJ, Yuan JS, et al.. Ferrocene/anthraquinone based bi-redox molecule for symmetric nonaqueous redox flow battery. J. Power Sources, 2020, 480: 229132

[152] Hu B, DeBruler C, Rhodes Z, et al.. Long-cycling aqueous organic redox flow battery (AORFB) toward sustainable and safe energy storage. J. Am. Chem. Soc., 2017, 139: 1207-1214

[153] Beh ES, De Porcellinis D, Gracia RL, et al.. A neutral pH aqueous organic-organometallic redox flow battery with extremely high capacity retention. ACS Energy Lett., 2017, 2: 639-644

[154] Zhu YZ, Yang F, Niu ZH, et al.. Enhanced cyclability of organic redox flow batteries enabled by an artificial bipolar molecule in neutral aqueous electrolyte. J. Power Sources, 2019, 417: 83-89

[155] Luo J, Hu MW, Wu WD, et al.. Mechanistic insights of cycling stability of ferrocene catholytes in aqueous redox flow batteries. Energy Environ. Sci., 2022, 15: 1315-1324

[156] Chen QR, Li YY, Liu YH, et al.. Designer ferrocene catholyte for aqueous organic flow batteries. Chemsuschem, 2021, 14: 1295-1301

[157] Yu JZ, Salla M, Zhang H, et al.. A robust anionic sulfonated ferrocene derivative for pH-neutral aqueous flow battery. Energy Storage Mater., 2020, 29: 216-222

[158] Zhao ZL, Zhang BS, Schrage BR, et al.. Investigations into aqueous redox flow batteries based on ferrocene bisulfonate. ACS Appl. Energy Mater., 2020, 3: 10270-10277

[159] Yao YW, Xu H, Tian ZZ, et al.. Simple-synthesized sulfonated ferrocene ammonium for aqueous redox flow batteries. ACS Appl. Energy Mater., 2021, 4: 8052-8058

[160] Schrage BR, Zhang BS, Petrochko SC, et al.. Highly soluble imidazolium ferrocene bis(sulfonate) salts for redox flow battery applications. Inorg. Chem., 2021, 60: 10764-10771

[161] Li YY, Xu ZA, Liu YH, et al.. Functioning water-insoluble ferrocenes for aqueous organic flow battery via host-guest inclusion. Chemsuschem, 2021, 14: 745-752

[162] Chopra R, de Kock C, Smith P, et al.. Ferrocene-pyrimidine conjugates: synthesis, electrochemistry, physicochemical properties and antiplasmodial activities. Eur. J. Med. Chem., 2015, 100: 1-9

[163] Tappe K, Knochel P. New efficient synthesis of taniaphos ligands: application in ruthenium- and rhodium-catalyzed enantioselective hydrogenations. Tetrahedron Asymmetry, 2004, 15: 91-102

[164] Borchers PS, Strumpf M, Friebe C, et al.. Aqueous redox flow battery suitable for high temperature applications based on a tailor-made ferrocene copolymer. Adv. Energy Mater., 2020, 10: 2001825

Funding

Research Grants Council, University Grants Committee(T23-601/17-R)

Hong Kong University of Science and Technology

RIGHTS & PERMISSIONS

The Author(s)