One-pot preparation of the fluorenone and fluorenol polymer

The fluorenone and fluorenol polymers were moulded via a cross-linking reaction of a bifunctional fluorenone or fluorenol derivative, 2,7-bis[2-(diethylamino)ethoxy]-9-fluorenone or -fluorenol, and a three-functional cross-linker, 1,3,5-tris(bromomethyl)benzene (Fig. 1c; equimolar feed ratio of the functional groups), in a Teflon boat by simply heating (90 °C) their N-methylpyrrolidone solution and washing it with N-methylpyrrolidone, water and methyl alcohol (Supplementary Methods). A typical yield of the insoluble network or gel part (gel fraction) was 86% (the median of five specimens). The chemical structures of fluorenone, fluorenol and quaternized ammonium moieties represented in Fig. 1c were supported by 13C cross-polarization magic angle spinning–nuclear magnetic resonance (CP/MAS NMR) spectroscopy (Supplementary Fig. 1). A network structure of the polymer was analysed by dynamic mechanical measurement (Supplementary Fig. 2). The storage modulus was much larger than the loss modulus, suggesting a mechanically tough gel formation with adequacy for a pocketable or a facile transportable hydrogen carrier (for the moulded example of a bendable sheet via the one-pot reaction, see Fig. 1a,b). Standing experiments of the fluorenone and fluorenol polymers in an air atmosphere at 80 °C for 30 days did not show any degradation or any chemical structure change detected in 13C CP/MAS NMR spectroscopy (and the amount of evolved hydrogen gas from the fluorenol polymer was not significantly decreased after the standing), to support durability of the polymers as a hydrogen carrier. Neither the fluorenone nor fluorenol polymers were soluble in water, but were hydrophilic and swollen; they uptook water with 25–35 wt% on the air of a humidity of 30% and with 50–60 wt% in water to be the hydrogels.

Figure 1: Hydrogen fixing and releasing by the fluorenone/fluorenol polymer. (a) A sheet of the fluorenone and fluorenol hydrogel on a 5 g scale and the fluorenol sheet sealed up with a gas-barrier bag (after hydrogen releasing). (b) Schematic representation of hydrogen-fixing and -releasing cycle. (c) Preparation scheme of the fluorenone and fluorenol polymer. Full size image

Electrolytic hydrogenation or hydrogen fixation

We have previously reported that the polymers of aromatic ketone derivatives, such as anthraquinone polymers, are reversibly reduced and applicable as an anode-active material in organic-based rechargeable devices18,19,20,21. Fluorenone is an aromatic ketone compound and a two-step reversible wave in the cyclic voltammogram was preliminarily reported in water-free solvents22, suggesting two negative charge storage per fluorenone unit. We have found, in this study, that fluorenone turns back to fluorenol through the very facile electrolytic two-electron reduction and two-proton addition in water.

In situ preparation of the fluorenone polymer on a glassy carbon substrate gave an electrode homogeneously coated with the fluorenone polymer with ca. 1 μm layer thickness. The fluorenone polymer layer was swollen but did not elute out in water, acetonitrile (AN), or their mixtures. In the AN electrolyte (0.1 M (C 4 H 9 ) 4 NPF 6 )), the fluorenone polymer electrode exhibited two quasi-reversible redox waves at −1.3 and −1.7 V (versus Ag/AgCl) in the cyclic voltammetry (CV; Supplementary Fig. 3a). The electrolytic reduction capacity of the fluorenone polymer layer reached 67 mA h g−1, which was 92% of the calculated capacity for the two-electron reduction: almost all fluorenone units stored two negative charges throughout the whole polymer layer (Fig. 2). This charging proceeded quantitatively, even at a rapid charging rate of 10 C (or charging for 6 min), indicating that charge propagated efficiently throughout the polymer layer based on the redox gradient-driven and the rapid charge self-exchange reaction among the fluorenone units (we have previously described a similar charge propagation and storage for redox-active polymers23,24,25).

Figure 2: Charge storage and electrolytic hydrogenation of the fluorenone polymer. Charging/discharging curves of the fluorenone polymer at a rate of 10 C measured in the AN electrolyte. Inset: schematic image of charge propagation in the polymer layer and conversion plots of the fluorenone to fluorenol unit in the fluorenone polymer layer electrolytically reduced in the AN/water electrolyte. Theoretical capacity of the fluorenone polymer-coated electrode was 52 mC per 0.2 mg polymer. The dashed line indicates the theoretical conversion with the passed charge; the error bars for the s.d. calculated from five samples. Full size image

The electrochemical study of the fluorenone polymer layer was then carried out in protic electrolyte solutions. The addition of a drop of water to the AN electrolyte immediately caused, in the CV, disappearance of the oxidation peaks at negative potential (Supplementary Fig. 3b). The fluorenone polymer layer was monitored with infrared by applying −1.5 V in AN/water (vol 5/1) electrolyte: The absorption ascribed to ν C=O at 1,710 cm−1 decreased as the passed charge increased (Fig. 2 inset), indicating the decrease in fluorenone content in the polymer through the electrolytic reduction, or protonation or hydrogenation. The coulombic efficiency was, for example, 87% after the passage of equivalent charge, estimated with the conversion of the fluorenone unit.

Bulk reduction of the monomeric fluorenone dissolved in the AN/water (vol 5/1) electrolyte was examined by using a glassy carbon working electrode and applying −1.5 V. The electrolytic reduction progressed almost quantitatively (coulombic efficiency 97%). The product was isolated (yield 95%) and identified to be fluorenol by 1H NMR spectroscopy (Supplementary Fig. 4). The same reduction of fluorenone was carried out by using the AN/D 2 O solution, to almost quantitatively yield two-deuterated fluorenol C 13 OH 8 D 2 (m/z=calcd for 184.23). The deuterated fluorenol was heated at 80 °C with the aqueous iridium catalyst (described in the succeeding section). The retention time of the gas evolved from the deuterated fluorenol was identical to that of D 2 in gas chromatography, clearly supporting the electrolytic deuteration (hydrogenation) of fluorenone with D 2 O (water) and deuterium (hydrogen) evolution from the fluorenol.

To discuss the rapid and quantitative hydrogenation reaction in the fluorenone polymer layer, we investigated the 1H NMR of the mixture of fluorenol and the fluorenone dianion (the latter was prepared by the electrolytic reduction of fluorenone) (Supplementary Methods). The peaks of hydroxyl (Ha) and cyclopentane (Hb) protons of fluorenol were broadened and shifted downfield by increasing the dianion content (Fig. 3a), indicating a proton-exchange reaction between the fluorenol and the fluorenone dianion. The rate constants of the exchange were estimated with the relaxation time (T) (Supplementary Fig. 5 and Table 1). These proton-exchanging rate constants are in the order of 103–104 M−1 s−1 for fluorenol/fluorenone dianion and were large enough to explain the proton exchange in the fluorenone polymer layer.

Figure 3: Proton exchange reaction of fluorenol/fluorenone dianion. (a) 1H NMR measurements of fluorenol in the presence of the fluorenone dianion were conducted in AN-d 3 . Inset, expanded cyclopentane and hydroxyl proton spectra of fluorenol. (b) Schematic image of proton propagation in the polymer layer. Full size image

Table 1 Hydrogen evolution rate and yield from alcohols. Full size table

Similar proton propagation had been examined using hydroquinone polymers26,27; however, most of those were electrochemically inactive in aqueous media because of the very slow electron/proton hopping and of hydrophobic property of the polymers, and the redox active examples have been restricted only to the very thin quinone layer coated on a glassy carbon27. The fluorenone polymer hydrogel caused both an electrochemically effective charge and proton propagation or the hoppings (Fig. 2, inset and Fig. 3b), probably due to an appropriate network structure of the fluorenone units and the hydrophilic gel state. It should be noted here that the electrolytic reduction and the successive hydrogenation of the fluorenone polymer progressed in the presence of aqueous electrolyte at room temperature.

Hydrogen evolution from the fluorenol polymer

The fluorenol polymer (10 g) was soaked in an aqueous solution (15 ml) containing the iridium catalyst (157 mg of aqua (6,6′-dihydroxy-2,2′-bipyridine)(pentamethylcyclopentadienyl)iridium(III) bis(triflate)28) and heated at 80 °C. Rapid gas evolution from the polymer was ascribed to the elimination of H 2 by gas chromatography analysis, which amounted to 309 ml (94% of the formula weight-based mobile hydrogen amount) after 5 h (Table 1 and Supplementary Fig. 6). The fluorenol polymer specimen (5 g) containing the aqueous iridium catalyst was sealed up with a gas-barrier bag and was heated (photo in Fig. 1); the evolved pure hydrogen gas was also analysed by gas chromatography.

Hydrogen evolution rates from the polymer are listed in Table 1, along with those from the monomeric alcohols in the presence of the same iridium catalyst at 80 °C. The rate from the fluorenol polymer was 30 times larger than that from the monomeric fluorenol (entries 1 and 2), which is explained by the highly populated fluorenol units in the polymer and by the low solubility of fluorenol in the aqueous solution (footnotes of Table 1).

Among the alcoholic derivatives as hydrogen donors, fluorenol gave the higher rate of hydrogen evolution in comparison with those of other secondary alcohols (entries 4–7), although fluorenol is an aromatic alcohol. X-ray crystallographic structures of fluorenol and fluorenone (Supplementary Fig. 7) depicted that the hydroxyl group and hydrogen atom bound to cyclopentane carbon are located out of the planar (slightly bent) fluorene skeleton, which is in contrast to the totally planar fluorenone structure. This suggests that the elimination of two hydrogens of the secondary alcohol fluorenol to form the π-conjugated planar fluorenone could be thermodynamically favourable.

After the hydrogen evolution, the colourless fluorenol polymer turned a reddish colour (Supplementary Fig. 8a,b). Infrared spectra supported that almost all fluorenol units in the polymer turned into fluorenone units through the dehydrogenation, to yield the fluorenone polymer (characterization in Supplementary Fig. 8c).

Cycle of electrolytic hydrogenation and hydrogen evolution

A composite sheet (14.3 g) of the fluorenol polymer with carbon nanofibre as a conductive additive was prepared, to perform the cycling of hydrogen fixing and releasing (Supplementary Methods). The composite sheet was soaked in the AN/water electrolyte and −1.5 V was applied till a charge of 390 C per g polymer was passed. The sheet was transferred into 15 ml of water containing the iridium catalyst and heated at 80 °C for 5 h. The hydrogen evolution was almost similar to that of the neat fluorenol polymer and the evolved hydrogen (296 ml) almost reached the theoretical hydrogen density held in the composite sheet (entry 3 in Table 1). The composite sheet was then hydrogenated again and this cycle was performed 50 times without any significant deterioration in the hydrogen evolution capacity (Supplementary Fig. 9).