Hydronium Ions Stabilized in a Titanate-Layered Structure with High Ionic Conductivity: Application to Aqueous Proton Batteries

Abstract

Proton chemistry is a fascinating field with both fundamental and applied aspects. The development of solid-state proton conductors relying on abundant elements could help bring these two aspects. In this scope, we synthesized a disordered structure which, as revealed by the real-space refinement of the pair distribution function, has been identified to be the trititanate arrangement. The layered structure is stabilized by the presence of hydronium ions and water molecules located in the interlayer space. This compound displays a high ionic conductivity of 4·10–2 S/m with an activation energy of 0.24 eV, assigned to H+ mobility as shown by broadband dielectric spectroscopy. Proton mobility was further evidenced by solid-state proton nuclear magnetic resonance. Density functional theory calculations revealed that proton transfer occurs both within the interlayer space and with terminal oxide of the titanate framework through a Grotthuss-based mechanism rationalizing the high conductivity measured experimentally. Finally, we investigated the electrochemical properties with respect to the proton as a charge carrier using proton-free (KCl) and proton-donor (buffer acetic acid) electrolytes. The results showed that the structure can reversibly intercalate protons at a very high rate opening existing perspectives in the development of negative electrode materials for aqueous proton batteries. Overall, this study helps better understand the proton transfer mechanism occurring in a confined layered-type structure.

Evidence of Bulk Proton Insertion in Nanostructured Anatase and Amorphous TiO2 Electrodes

“…23,32,33 The results also call into question the recently proposed notion that bulk proton insertion in titanium oxides requires hydrated or protonated layered titanate structures. 23,24 Indeed, this is not consistent with our present findings where bulk proton insertion is shown to be effective even with the nonlayered and nonhydrated anatase structure (i.e., a 3D network of edge-sharing TiO2 octahedra). Additionally, one should note that the gravimetric capacities and CEs we have obtained here by CV (140 mA•h•g -1 at 10 mV/s with CE = 68 % for anatase GLAD-TiO2) significantly exceed those previously achieved with layered titanates (82 mA•h•g -1 with CE = 45% for H2Ti3O7 or 40 mA•h•g -1 for Ti3O7(H2O)2 at the same rate of 10 mV/s).…”

“…This corresponds to a remarkable rate performance, allowing the charge or discharge of a capacity close to 100 mA•h•g -1 in less than 30 s. Such a rate performance is better than that recently reported with layered titanates (33 mA•h•g -1 at 4.2 A•g -1 ). 24 It also definitely highlights the fast bulk proton insertion in TiO2 electrode materials, even when the electrode is composed of a non-layered structure. Figure 6.…”

“…Additionally, one should note that the gravimetric capacities and CEs we have obtained here by CV (140 mA•h•g -1 at 10 mV/s with CE = 68 % for anatase GLAD-TiO2) significantly exceed those previously achieved with layered titanates (82 mA•h•g -1 with CE = 45% for H2Ti3O7 or 40 mA•h•g -1 for Ti3O7(H2O)2 at the same rate of 10 mV/s). 23,24 Electrochemical performances of the GLAD-TiO2 electrodes Proton insertion in GLAD-TiO2 electrodes was next investigated by galvanostatic cycling at a rate of 0.36 mA•cm -2 (equivalent to 1.4 A•g -1 ). The shapes of the galvanostatic charge/discharge curves ( Figure 4A) corroborate well with the reversible waveforms recorded by CV.…”

“…12 So far, the majority of materials examined for their reversible proton insertion properties are hydrated transition metal oxides (mostly layered) such as WO3•nH2O, [13][14][15] H2W2O7, 16 MoO3, [17][18][19] HxIrO4, 20,21 VO2, 8,22 V2O5, 9 and H-titanates. 23,24 In many of these studies, it has been proposed that the presence of ions and/or structural water in the interlayer space or in the tunnel structure of the metal oxide material facilitates the transport of protons within the solid phase.…”

“…It is only recently that layered titanates were proposed for bulk electrochemical H + intercalation, with the underlying assumption that water molecules or structural protons in the interlayer space facilitate the bulk insertion of protons. 23,24 This contrasting view between layered and nonlayered titanate materials is however challenged by the relatively high proton charge storage gravimetric capacity of 118 mA•h•g -1 (equivalent to x = 0.35) we have recently obtained for a nonlayered amorphous TiO2 electrode immersed in a buffered aqueous electrolyte. 5 These conflicting results raise fundamental questions about the structural features that a metal oxide phase has to meet in order to allow for a fast and reversible bulk proton insertion.…”