Highly reduced and protonated aqueous solutions of [P2W18O62]6− for on-demand hydrogen generation and energy storage


As our reliance on renewable energy sources grows, so too does our need to store this energy to mitigate against troughs in supply. Energy storage in batteries or by conversion to chemical fuels are the two most flexible and scalable options, but are normally considered mutually exclusive. Energy storage solutions that can act as both batteries and fuel generation devices (depending on the requirements of the user) could therefore revolutionize the uptake and use of renewably generated energy. Here, we present a polyoxoanion, [P2W18O62]6−, that can be reversibly reduced and protonated by 18 electrons/H+ per anion in aqueous solution, and that can act either as a high-performance redox flow battery electrolyte (giving a practical discharged energy density of 225 Wh l−1 with a theoretical energy density of more than 1,000 Wh l−1), or as a mediator in an electrolytic cell for the on-demand generation of hydrogen.

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Fig. 1: Structure and basic electrochemistry of [P2W18O62]6−.
Fig. 2: Reversible multi-electron redox chemistry of [P2W18O62]6−.
Fig. 3: Hydrogen production, on demand, from solutions of reduced polyoxometallates.
Fig. 4: Application of Li6[P2W18O62] in a redox flow battery.


  1. 1.

    Dunn, B., Kamath, H. & Tarascon, J.-M. Electrical energy storage for the grid: a battery of choices. Science 334, 928–935 (2011).

    CAS  Article  PubMed  Google Scholar 

  2. 2.

    Cook, T. R. et al. Solar energy supply and storage for the legacy and nonlegacy worlds. Chem. Rev. 110, 6474–6502 (2010).

    CAS  Article  PubMed  Google Scholar 

  3. 3.

    Roger, I., Shipman, M. A. & Symes, M. D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat. Rev. Chem. 1, 0003 (2017).

    CAS  Article  Google Scholar 

  4. 4.

    Hanley, E. S., Amarandei, G. & Glowacki, B. A. Potential of redox flow batteries and hydrogen as integrated storage for decentralized energy systems. Energy Fuels 30, 1477–1486 (2016).

    CAS  Google Scholar 

  5. 5.

    Halls, J. E. et al. Empowering the smart grid: can redox batteries be matched to renewable energy systems for energy storage? Energy Environ. Sci. 6, 1026–1041 (2013).

    CAS  Article  Google Scholar 

  6. 6.

    Posada, J. O. G. et al. Aqueous batteries as grid scale energy storage solutions. Renew. Sustain. Energy Rev. 68, 1174–1182 (2017).

    CAS  Article  Google Scholar 

  7. 7.

    Alotto, P., Guarnieri, M. & Moro, F. Redox flow batteries for the storage of renewable energy: a review. Renew. Sustain. Energy Rev. 29, 325–335 (2014).

    CAS  Article  Google Scholar 

  8. 8.

    Olah, G. A., Prakash, G. K. S. & Goeppert, A. Anthropogenic chemical carbon cycle for a sustainable future. J. Am. Chem. Soc. 133, 12881–12898 (2011).

    CAS  Article  PubMed  Google Scholar 

  9. 9.

    Le Formal, F., Bourée, W. S., Prévot, M. S. & Sivula, K. Challenges towards economic fuel generation from renewable electricity: the need for efficient electrocatalysis. Chimia 69, 789–798 (2015).

    Article  CAS  PubMed  Google Scholar 

  10. 10.

    Carmo, M., Fritz, D. L., Mergel, J. & Stolten, D. A comprehensive review on PEM water electrolysis. Int. J. Hydrog. Energy 38, 4901–4934 (2013).

    CAS  Article  Google Scholar 

  11. 11.

    Symes, M. D. & Cronin, L. Decoupling hydrogen and oxygen evolution during electrolytic water splitting using an electron-coupled-proton buffer. Nat. Chem. 5, 403–409 (2013).

    CAS  Article  PubMed  Google Scholar 

  12. 12.

    Rausch, B., Symes, M. D., Chisholm, G. & Cronin, L. Decoupled catalytic hydrogen evolution from a molecular metal oxide redox mediator in water splitting. Science 345, 1326–1330 (2014).

    CAS  Article  PubMed  Google Scholar 

  13. 13.

    Guinot, B. et al. Techno-economic study of a PV–hydrogen-battery hybrid system for off-grid power supply: impact of performances’ ageing on optimal system sizing and competitiveness. Int. J. Hydrog. Energy 40, 623–632 (2015).

    CAS  Article  Google Scholar 

  14. 14.

    Pellow, M. A., Emmott, C. J. M., Barnhart, C. J. & Benson, S. M. Hydrogen or batteries for grid storage? A net energy analysis. Energy Environ. Sci. 8, 1938–1952 (2015).

    CAS  Article  Google Scholar 

  15. 15.

    Peljo, P. et al. All-vanadium dual circuit redox flow battery for renewable hydrogen generation and desulfurization. Green Chem. 18, 1785–1797 (2016).

    CAS  Article  Google Scholar 

  16. 16.

    Mulder, F. M., Weninger, B. M. H., Middelkoop, J., Ooms, F. G. B. & Schreuders, H. Efficient electricity storage with a battolyser, an integrated Ni–Fe battery and electrolyser. Energy Environ. Sci. 10, 756–764 (2017).

    CAS  Article  Google Scholar 

  17. 17.

    Pope, M. T. Heteropoly and Isopoly Oxometalates (Springer, Heidelberg, 1983).

    Google Scholar 

  18. 18.

    Papaconstantinou, E. & Pope, M. T. Heteropoly blues. III. Preparation and stabilities of reduced 18-molybdodiphosphates. Inorg. Chem. 6, 1152–1155 (1967).

    CAS  Article  Google Scholar 

  19. 19.

    Launay, J. P. Reduction de l’ion metatungstate: stades eleves de reduction de H2W12O40 6−, derives de l’ion HW12O40 7− et discussion generale. J. Inorg. Nucl. Chem. 38, 807–16 (1976).

    CAS  Article  Google Scholar 

  20. 20.

    Way, D. M., Bond, A. M. & Wedd, A. G. Multielectron reduction of α-[S2Mo18O62]4– in aprotic and protic media: voltammetric studies. Inorg. Chem. 36, 2826–2833 (1997).

    CAS  Article  PubMed  Google Scholar 

  21. 21.

    Bond, A. M., Vu, T. & Wedd, A. G. Voltammetric studies of the interaction of the lithium cation with reduced forms of the Dawson [S2Mo18O62]4– polyoxometallates anion. J. Electroanal. Chem. 494, 96–104 (2000).

    CAS  Article  Google Scholar 

  22. 22.

    Takamoto, M., Ueda, T. & Himeno, S. Solvation effect of Li+ on the voltammetric properties of [PMo12O40]3− in binary solvent mixtures. J. Electroanal. Chem. 521, 132–136 (2002).

    CAS  Article  Google Scholar 

  23. 23.

    Grigoriev, V. A., Cheng, D., Hill, C. L. & Weinstock, I. A. Role of alkali metal cation size in the energy and rate of electron transfer to solvent-separated 1:1 [(M+)(acceptor)] (M+ = Li+, Na+, K+) ion pairs. J. Am. Chem. Soc. 123, 5292–5307 (2001).

    CAS  Article  Google Scholar 

  24. 24.

    Ueda, T. et al. Voltammetric behavior of 1- and 4-[S2VVW17O62]5− in acidified acetonitrile. Dalton Trans. 44, 11660–11668 (2015).

    CAS  Article  PubMed  Google Scholar 

  25. 25.

    Ji, Y., Huang, L., Hu, J., Streb, C. & Song, Y.-F. polyoxometallates-functionalized nanocarbon materials for energy conversion, energy storage and sensor systems. Energy Environ. Sci. 8, 776–789 (2015).

    CAS  Article  Google Scholar 

  26. 26.

    Wang, H. et al. In operando X-ray absorption fine structure studies of polyoxometallates molecular cluster batteries: polyoxometallates as electron sponges. J. Am. Chem. Soc. 134, 4918–4924 (2012).

    CAS  Article  PubMed  Google Scholar 

  27. 27.

    Nishimoto, Y., Yokogawa, D., Yoshikawa, H., Awaga, K. & Irle, S. Super-reduced polyoxometallates: excellent molecular cluster battery components and semipermeable molecular capacitors. J. Am. Chem. Soc. 136, 9042–9052 (2014).

    CAS  Article  PubMed  Google Scholar 

  28. 28.

    Chen, J.-J. et al. High-performance polyoxometallates-based cathode materials for rechargeable lithium-ion batteries. Adv. Mater. 27, 4649–4654 (2015).

    CAS  Article  PubMed  Google Scholar 

  29. 29.

    Huang, Q. et al. A highly stable polyoxometallates-based metal–organic framework with ππ stacking for enhancing lithium ion battery performance. J. Mater. Chem. A 5, 8477–8483 (2017).

    CAS  Article  Google Scholar 

  30. 30.

    Chen, J.-J. et al. Design and performance of rechargeable sodium ion batteries, and symmetrical Li-ion batteries with supercapacitor-like power density based upon polyoxovanadates. Adv. Energy Mater. 7, 1701021 (2017).

    Google Scholar 

  31. 31.

    Hartung, S. et al. Vanadium-based polyoxometallates as new material for sodium-ion battery anodes. J. Power Sources 288, 270–277 (2015).

    CAS  Article  Google Scholar 

  32. 32.

    Pratt, H. D. III, Hudak, N. S., Fang, X. & Anderson, T. M. A polyoxometallates flow battery. J. Power Sources 236, 259–264 (2013).

    CAS  Article  Google Scholar 

  33. 33.

    Pratt, H. D. III & Anderson, T. M. Mixed addenda polyoxometallates ‘solutions’ for stationary energy storage. Dalton Trans. 42, 15650–15655 (2013).

    CAS  Article  PubMed  Google Scholar 

  34. 34.

    Chen, J.-J. J. & Barteau, M. A. Molybdenum polyoxometallates as active species for energy storage in non-aqueous media. J. Energy Storage 13, 255–261 (2017).

    Article  Google Scholar 

  35. 35.

    VanGelder, L. E., Kosswattaarachchi, A. M., Forrestel, P. L., Cook, T. R. & Matson, E. M. Polyoxovanadate-alkoxide clusters as multi-electron charge carriers for symmetric non-aqueous redox flow batteries. Chem. Sci. 9, 1692–1699 (2018).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  36. 36.

    Liu, Y. et al. An aqueous redox flow battery with a tungsten–cobalt heteropolyacid as the electrolyte for both the anode and cathode. Adv. Energy Mater. 7, 1601224 (2017).

    Article  CAS  Google Scholar 

  37. 37.

    Kato, C. et al. Quick and selective synthesis of Li6[α-P2W18O62]·28H2O soluble in various organic solvents. Dalton Trans. 42, 11363–11366 (2013).

    CAS  Article  PubMed  Google Scholar 

  38. 38.

    Bernardini, G., Wedd, A. G. & Bond, A. M. Reactivity of one-, two-, three- and four-electron reduced forms of α-[P2W18O62]6− generated by controlled potential electrolysis in water. Inorg. Chim. Acta 374, 327–333 (2011).

    CAS  Article  Google Scholar 

  39. 39.

    Pope, M. T. & Papaconstantinou, E. Heteropoly blues. II. Reduction of 2:18-tungstates. Inorg. Chem. 6, 1147–1152 (1967).

    CAS  Article  Google Scholar 

  40. 40.

    Harmalker, S. P., Leparulo, M. A. & Pope, M. T. Mixed-valence chemistry of adjacent vanadium centers in heteropolytungstate anions. I. Synthesis and electronic structures of mono-, di-, and trisubstituted derivatives of α-[P2W18O62]6−. J. Am. Chem. Soc. 105, 4286–4292 (1983).

    CAS  Article  Google Scholar 

  41. 41.

    Keita, B. & Nadjo, L. New aspects of the electrochemistry of heteropolyacids part IV. Acidity dependent cyclic voltammetric behaviour of phosphotungstic and silicotungstic heteropolyanions in water and N,N-dimethylformamide. J. Electroanal. Chem. 227, 77–98 (1987).

    CAS  Article  Google Scholar 

  42. 42.

    Prenzler, P. D., Boskovic, C., Bond, A. M. & Wedd, A. G. Coupled electron- and proton-transfer processes in the reduction of α-[P2W18O62]6− and α-[H2W12O40]6− as revealed by simulation of cyclic voltammograms. Anal. Chem. 71, 3650–3656 (1999).

    CAS  Article  PubMed  Google Scholar 

  43. 43.

    Bernardini, G., Zhao, C., Wedd, A. G. & Bond, A. M. Ionic liquid-enhanced photooxidation of water using the polyoxometallates anion [P2W18O62]6− as the sensitizer. Inorg. Chem. 50, 5899–5909 (2011).

    CAS  Article  PubMed  Google Scholar 

  44. 44.

    Wang, M., Zhao, F. & Dong, S. A single ionic conductor based on Nafion and its electrochemical properties used as lithium polymer electrolyte. J. Phys. Chem. B 108, 1365–1370 (2004).

    CAS  Article  Google Scholar 

  45. 45.

    Millet, P. et al. PEM water electrolyzers: from electrocatalysis to stack development. Int. J. Hydrog. Energy 35, 5043–5052 (2010).

    CAS  Article  Google Scholar 

  46. 46.

    Xu, C., Ma, L., Li, J., Zhao, W. & Gan, Z. Synthesis and characterization of novel high-performance composite electrocatalysts for the oxygen evolution in solid polymer electrolyte (SPE) water electrolysis. Int. J. Hydrog. Energy 37, 2985–2992 (2012).

    CAS  Article  Google Scholar 

  47. 47.

    Huskinson, B. et al. A metal-free organic–inorganic aqueous flow battery. Nature 505, 195–198 (2014).

    CAS  Article  PubMed  Google Scholar 

  48. 48.

    Lin, K. et al. Alkaline quinone flow battery. Science 349, 1529–1532 (2015).

    CAS  Article  PubMed  Google Scholar 

  49. 49.

    Janoschka, T. et al. An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials. Nature 527, 78–81 (2015).

    CAS  Article  PubMed  Google Scholar 

  50. 50.

    Li, L. et al. A stable vanadium redox-flow battery with high energy density for large-scale energy storage. Adv. Energy Mater. 1, 394–400 (2011).

    CAS  Article  Google Scholar 

  51. 51.

    Mohamed, M. R., Sharkh, S. M. & Walsh, F. C. Redox flow batteries for hybrid electric vehicles: progress and challenges. IEEE Veh. Power Propuls. Conf. 09, 551–557 (2009).

    Google Scholar 

  52. 52.

    Leung, P. et al. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Adv. 2, 10125–10156 (2012).

    CAS  Article  Google Scholar 

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The authors thank Q. Zheng (University of Glasgow) for assistance with mass spectrometry and NMR. M.D.S. thanks the Royal Society for a University Research Fellowship. The authors acknowledge funding from the EPSRC (grant nos. EP/H024107/1, EP/J00135X/1, EP/J015156/1, EP/K021966/1, EP/K023004/1 and EP/L023652/1), the EC (318671 MICREAGENTS) and ERC (project 670467 SMART-POM).

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L.C. conceived the concept and, together, L.C., J.J.C. and M.D.S. expanded the hypothesis, planned experiments and wrote the paper. J.J.C. performed all the electrochemistry experiments and analysis.

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Correspondence to Mark D. Symes or Leroy Cronin.

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Supplementary Methods, Supplementary Characterization, Supplementary Data, Supplementary Figures 1–26, Supplementary Tables 1–5

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Chen, J., Symes, M.D. & Cronin, L. Highly reduced and protonated aqueous solutions of [P2W18O62]6− for on-demand hydrogen generation and energy storage. Nature Chem 10, 1042–1047 (2018). https://doi.org/10.1038/s41557-018-0109-5

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