1. Barber, J. Photosystem II: the water splitting enzyme of photosynthesis and the origin of oxygen in our atmosphere. Q. Rev. Biophys. 49, e14 (2016).

2. Hunter, B. M., Gray, H. B. & Muller, A. M. Earth-abundant heterogeneous water oxidation catalysts. Chem. Rev. 116, 14120–14136 (2016).

3. Seh, Z. W. et al. Combining theory and experiment in electrocatalysis: insights into materials design. Science 355, eaad4998 (2017).

4. Galloway, J. N. et al. Transformation of the nitrogen cycle: recent trends, questions, and potential solutions. Science 320, 889–892 (2008).

5. Sheehan, S. W. et al. Commercializing solar fuels within today’s markets. Chem 3, 3–7 (2017).

6. Smolinka, T., Ojong, E. T. & Garche, J. in Electrochemical Energy Storage for Renewable Sources and Grid Balancing 103–128 (Elsevier, 2015).

7. 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).

8. Wallace, A. G. & Symes, M. D. Decoupling strategies in electrochemical water splitting and beyond. Joule 2, 1390–1395 (2018).

9. 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).

10. Chen, L., Dong, X., Wang, Y. & Xia, Y. Separating hydrogen and oxygen evolution in alkaline water electrolysis using nickel hydroxide. Nat. Commun. 7, 11741 (2016).

11. Landman, A. et al. Photoelectrochemical water splitting in separate oxygen and hydrogen cells. Nat. Mater. 16, 646–651 (2017).

12. Bode, H., Witte, J. & Dehmelt, K. Zur kenntnis der nickelhydroxidelektrode—I. Uber das nickel (ii)-hydroxidhydrat. Electrochim. Acta 11, 1079–1087 (1966).

13. Bode, H., Witte, J. & Dehmelt, K. Zur kenntnis der nickelhydroxidelektrode. II. Über die oxydationsprodukte von nickel(ii)‐hydroxiden. Z. Anorg. Allg. Chem. 366, 1–21 (1969).

14. Mao, Z. & White, R. E. The self-discharge of the NiOOH/Ni(OH) 2 electrode constant potential study. J. Electrochem. Soc. 139, 1282–1289 (1992).

15. Winsel, A. & Fischer, C. New apparatus for the measurement of the self-discharge of the nickel hydroxide electrode. J. Power Sources 34, 331–338 (1991).

16. Diaz-Morales, O., Ferrus-Suspedra, D. & Koper, M. T. M. The importance of nickel oxyhydroxide deprotonation on its activity towards electrochemical water oxidation. Chem. Sci. 7, 2639–2645 (2016).

17. Macdonald, D. D. & Challingsworth, M. L. Thermodynamics of nickel-cadmium and nickel-hydrogen batteries. J. Electrochem. Soc. 140, 606–609 (1993).

18. Cox, N. et al. Electronic structure of the oxygen-evolving complex in photosystem II prior to O–O bond formation. Science 345, 804–808 (2014).

19. Dau, H. et al. The mechanism of water oxidation: from electrolysis via homogeneous to biological catalysis. ChemCatChem 2, 724–761 (2010).

20. Mcbreen, J. in Handbook of Battery Materials (eds Daniel, C. & Besenhard, J. O.) 149–168 (Wiley-VCH, 2011).

21. Snook, Ga, Duffy, N. W. & Pandolfo, A. G. Evaluation of the effects of oxygen evolution on the capacity and cycle life of nickel hydroxide electrode materials. J. Power Sources 168, 513–521 (2007).

22. Oliva, P., Laurent, J. F. & Leonardi, J. Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides. J. Power Sources 8, 229–255 (1982).

23. Armstrong, R. D., Briggs, G. W. D. & Charles, E. A. Some effects of the addition of cobalt to the nickel hydroxide electrode. J. Appl. Electrochem. 18, 215–219 (1988).

24. Ortiz, M. G., Castro, E. B. & Real, S. G. Effect of cobalt electroless deposition on nickel hydroxide electrodes. Int. J. Hydrog. Energy 39, 6006–6012 (2014).

25. Wang, X. et al. Surface modification and electrochemical studies of spherical nickel hydroxide. J. Power Sources 72, 221–225 (1998).

26. Wang, X., Luo, H., Yang, H., Sebastian, P. J. & Gamboa, S. A. Oxygen catalytic evolution reaction on nickel hydroxide electrode modified by electroless cobalt coating. Int. J. Hydrog. Energy 29, 967–972 (2004).

27. Portemer, A., Delahaye-Vidal, A. & Figlarz, M. Characterization of active material deposited at the nickel hydroxide electrode by electrochemical impregnation. J. Electrochem. Soc. 139, 671–678 (1992).

28. Johnson, B. A., Ferro, R. E., Swain, G. M. & Tatarchuk, B. T. High surface area, low-weight composite nickel fiber electrodes. J. Power Sources 47, 251–259 (1994).

29. Ng, P. K. & Schneider, E. W. Distribution of nickel hydroxide in sintered nickel plaques measured by radiotracer method during electroimpregnation. J. Electrochem. Soc. 133, 17–21 (1986).

30. Becker, M. D., Garaventta, G. N. & Visintin, A. Pulse-current electrodeposition for loading active material on nickel electrodes for rechargeable batteries. ISRN Electrochem. 2013, 732815 (2013).

31. Jamesh, M. I. & Sun, X. Recent progress on Earth abundant electrocatalysts for oxygen evolution reaction (OER) in alkaline medium to achieve efficient water splitting—a review. J. Power Sources 400, 31–68 (2018).

32. Meites, L. Handbook of Analytical Chemistry (McGraw-Hill, 1963).

33. Licht, S. pH measurement in concentrated alkaline solutions. Anal. Chem. 57, 514–519 (1985).

34. Oshitani, M., Takayama, T., Takashima, K. & Tsuji, S. A study on the swelling of a sintered nickel hydroxide electrode. J. Appl. Electrochem. 16, 403–412 (1986).

35. Chen, J., Bradhurst, D. H., Dou, S. X. & Liu, H. K. Nickel hydroxide as an active material for the positive electrode in rechargeable alkaline batteries. J. Electrochem. Soc. 146, 3606–3612 (1999).

36. LeRoy, R. L. & Bowen, C. T. The thermodynamics of aqueous water electrolysis. J. Electrochem. Soc. 127, 1954–1962 (1980).

37. Esswein, A. J., Surendranath, Y., Reece, S. Y. & Nocera, D. G. Highly active cobalt phosphate and borate based oxygen evolving catalysts operating in neutral and natural waters. Energy Environ. Sci. 4, 499–504 (2011).

38. Esposito, D. V. Membraneless electrolyzers for low-cost hydrogen production in a renewable energy future. Joule 1, 651–658 (2017).

39. Millet, P. in Hydrogen Production: Electrolysis (ed. Godula-Jopek, A.) 63–116 (Wiley‐VCH, 2015).

40. Guillet, N. & Millet, P. in Hydrogen Production: Electrolysis (ed. Godula-Jopek, A.) 117–167 (Wiley‐VCH, 2015).

41. Seitz, L. C. et al. A highly active and stable IrO x /SrIrO 3 catalyst for the oxygen evolution reaction. Science 353, 1011–1014 (2016).

42. Zhou, H. et al. Highly active catalyst derived from a 3D foam of Fe(PO 3 ) 2 /Ni 2 P for extremely efficient water oxidation. Proc. Natl Acad. Sci. USA 114, 5607–5611 (2017).

43. Dong, B. et al. Two-step synthesis of binary Ni–Fe sulfides supported on nickel foam as highly efficient electrocatalysts for the oxygen evolution reaction. J. Mater. Chem. A 4, 13499–13508 (2016).

44. Zhou, H. et al. Water splitting by electrolysis at high current densities under 1.6 volts. Energy Environ. Sci. 11, 2858–2864 (2018).

45. Han, L., Dong, S. & Wang, E. Transition-metal (Co, Ni, and Fe)-based electrocatalysts for the water oxidation reaction. Adv. Mater. 28, 9266–9291 (2016).

46. Chen, C., Khosrowabadi Kotyk, J. F. & Sheehan, S. W. Progress toward commercial application of electrochemical carbon dioxide reduction. Chem 4, 2571–2586 (2018).

47. Subbaiah, T., Mallick, S. C., Mishra, K. G., Sanjay, K. & Das, R. P. Electrochemical precipitation of nickel hydroxide. J. Power Sources 112, 562–569 (2002).

48. Zhu, W., Zhang, D. & Ke, J. Electrochemical impregnation and performance of nickel hydroxide electrodes with porous plaques of hollow nickel fibres. J. Power Sources 56, 157–164 (1995).

49. Batchellor, A. S. & Boettcher, S. W. Pulse-electrodeposited Ni–Fe (oxy)hydroxide oxygen evolution electrocatalysts with high geometric and intrinsic activities at large mass loadings. ACS Catal. 5, 6680–6689 (2015).

50. Conway, B. E. & Bourgault, P. L. The electrochemical behavior of the nickel–nickel oxide electrode: part I. kinetics of self-discharge. Can. J. Chem. 37, 292–307 (1959).

51. Barnard, R., Randell, C. F. & Tye, F. L. Studies concerning charged nickel hydroxide electrodes. I. Measurement of reversible potentials. J. Appl. Electrochem. 10, 109–125 (1980).

52. Nickell, R. A., Zhu, W. H., Payne, R. U., Cahela, D. R. & Tatarchuk, B. J. Hg/HgO electrode and hydrogen evolution potentials in aqueous sodium hydroxide. J. Power Sources 161, 1217–1224 (2006).

53. Bienvenu, G. Method for co-generation of electric energy and hydrogen. US patent 8,617,766 (2013).

54. Chen, G. et al. Accelerated hydrogen evolution kinetics on NiFe‐layered double hydroxide electrocatalysts by tailoring water dissociation active sites. Adv. Mater. 30, 1706279 (2018).