Electrode-assisted catalytic water oxidation by a flavin derivative Ekaterina Mirzakulova 1 , Renat Khatmullin 1 , Janitha Walpita 1 , Thomas Corrigan 1 , Nella M. Vargas-Barbosa 2 , Shubham Vyas 3 , Shameema Oottikkal 3 , Samuel F. Manzer 3 , Christopher M. Hadad 3 and Ksenija D. Glusac 1 * The success of solar fuel technology relies on the development of efficient catalysts that can oxidize or reduce water. All molecular water-oxidation catalysts reported thus far are transition-metal complexes, however, here we report catalytic water oxidation to give oxygen by a fully organic compound, the N(5)-ethylflavinium ion, Et-Fl 1 . Evolution of oxygen was detected during bulk electrolysis of aqueous Et-Fl 1 solutions at several potentials above 11.9 V versus normal hydrogen electrode. The catalysis was found to occur on glassy carbon and platinum working electrodes, but no catalysis was observed on fluoride-doped tin-oxide electrodes. Based on spectroelectrochemical results and preliminary calculations with density functional theory, one possible mechanistic route is proposed in which the oxygen evolution occurs from a peroxide intermediate formed between the oxidized flavin pseudobase and the oxidized carbon electrode. These findings offer an organic alternative to the traditional water-oxidation catalysts based on transition metals. C onversion of the sun’s energy into electricity is a promising approach for the development of renewable-energy sources 1 . However, as a result of temporal and spatial fluctuations in the availability of sunlight on the Earth’s surface, it is not feasible to achieve a steady production of electricity; thus, storage of the elec- trical energy in the form of fuels, such as molecular hydrogen, is needed. Inspired by photochemical water splitting in natural photo- synthesis 2 , significant scientific efforts are aimed towards the devel- opment of solar fuel cells 3 . These devices are envisioned to use sunlight to split water into H 2 and O 2 . The recombination of these gases in a fuel cell can produce electricity whenever and wherever power is needed. Photochemical water splitting appears deceptively simple, but its realization requires the develop- ment of appropriate photoelectrodes 4 , membranes 5 and gas- evolving catalysts 6,7 . Oxygen-evolving catalysts are particularly difficult to design, mostly for the following reasons 8–10 : (1) the oxidation potential of the catalyst needs to be slightly above the thermodynamic potential for the oxidation of water to oxygen; (2) to avoid high-energy inter- mediates, oxidation of the catalyst must be coupled with proton transfer, followed by efficient O−O bond formation; (3) the catalytic reaction should occur with a high rate; (4) the catalyst should not be prone to oxidative damage at the potentials required to evolve oxygen from water and (5) the catalyst should not operate at extre- mely high pH values. All the molecular water-oxidation catalysts developed so far are transition-metal complexes, with Ru (refs 11–15), Ir (ref. 16), Os (ref. 17), Mn (ref. 18), Fe (ref. 19) or Co (refs 6,20) centres. Fully organic compounds that catalyse water oxidation were not found, possibly because of their tendency to undergo chemical damage under the strongly oxidizing conditions required for water oxi- dation. However, the potential advantage of organic water-oxidation catalysts is their low manufacturing cost because they are made of earth-abundant elements (C, H, O, N). For example, the low cost of conjugated polymers is the driving force for the development of organic light-emitting diodes 21 and solar cells 22 , even though the devices based on inorganic semiconductors, which are generally more expensive to manufacture, exhibit greater stability and superior performance. Furthermore, a detailed understanding of unwanted degradation chemistry in organic water-oxidation catalysts can lead to the development of self-repairing systems, as illustrated in previous electrochemical 23 and photoelectrochem- ical catalysts 24 . The model compound presented in this work is the N(5)-ethyl- flavinium ion (Et-Fl þ in Fig. 1a), which reacts reversibly with water to form the corresponding pseudobase derivative Et-FlOH. An initial electrochemical study investigated the possibility that a two-electron oxidation of Et-FlOH would lead to catalytic water oxidation, but this was found not to be the case 25 . This article reports that at high potentials (þ1.9 V versus normal hydrogen electrode (NHE)) the oxidation of Et-Fl þ itself leads to catalytic water oxidation to form molecular oxygen. The catalysis was mediated by the oxides formed on the carbon electrode. To our knowledge, this is the first report of catalytic water oxidation by an exclusively organic scaffold. Here we describe the electrochemical experiments that showed the catalytic behaviour of Et-Fl þ towards water oxidation, and density functional theory (DFT) calculations aimed at investigating the mechanism of catalysis. Results and discussion Cyclic voltammetry. Figure 1b shows cyclic voltammograms of Et-Fl þ obtained at different scanning directions. In analogy to the redox behaviour of natural flavins, the reduction peaks are assigned to the conversion of Et-Fl þ to the semiquinone Et-Fl † (þ0.2 V) and the reduction of Et-Fl † to a fully reduced hydroquinone Et-Fl 2 (20.5 V) (ref. 26). At positive potentials (red line, Fig. 1b), Et-Fl þ undergoes chemically irreversible oxidation at about þ1.9 V and the current produced during the anodic oxidation of Et-Fl þ is much stronger than the background current obtained in the absence of Et-Fl þ (black line, Fig. 1b). 1 Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, USA, 2 Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16801, USA, 3 Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA. *e-mail: kglusac@bgsu.edu ARTICLES PUBLISHED ONLINE: 26 AUGUST 2012 | DOI: 10.1038/NCHEM.1439 NATURE CHEMISTRY | VOL 4 | OCTOBER 2012 | www.nature.com/naturechemistry 794 © 201 2 M acmillan Publishers Limited. All rights reserved.