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

Abstract

Crystalline structures and lattice water molecules are believed to strongly influence the ability of metal oxides to reversibly and rapidly insert protons in aqueous batteries. In the present work, we performed a systematic analysis of the electrochemical charge storage properties of nanostructured TiO<sub>2</sub> electrodes composed of either anatase or amorphous TiO<sub>2</sub> in a mild buffered aqueous electrolyte. We demonstrate that both materials allow reversible bulk proton insertion up to a maximal reversible gravimetric capacity of ~150 mA·h·g<sup>-1</sup>. We also show that the TiO<sub>2 </sub>crystallinity governs the energetics of the charge storage process, with a phase transition for anatase, while having little effect on either the interfacial charge-transfer kinetics or the apparent rate of proton diffusivity within the metal oxide. Finally, with both TiO<sub>2</sub> electrodes, reversible proton insertion leads to gravimetric capacities as high as 95 mA·h·g<sup>-1</sup> at 75 C. We also reveal two competitive reactions decreasing the Coulombic efficiency at low rates, <i>i.e.</i> hydrogen evolution and a non-faradaic self-discharge reaction. Overall, this work provides a comprehensive overview of the proton-coupled electrochemical reactivity of TiO<sub>2</sub> and highlights the key issues to be solved in order to truly benefit from the unique properties of protons as fast charge carriers in metal oxides.

Interplay Between Charge Accumulation and Oxygen Reduction Catalysis in Nanostructured TiO2 Electrodes Functionalized with a Molecular Catalyst

“…where AH and A À are the acid and the conjugate base of the HEPES buffer, respectively, and x the maximal amount of protons, which can be reversibly stored within the TiO 2 lattice (previously established to x = 0.45 for the amorphous GLAD-TiO 2 electrodes). [39,40] In the CV of the MnÀ P/GLAD-TiO 2 electrode, a small sharp and irreversible cathodic peak is also detected at E = À 0.68 V. Its attribution to the Mn III !Mn II reduction of the immobilized chromophore can only be definitely assessed from the simultaneous absorbance increase at 432 nm, a wavelength corresponding to the maximal absorbance difference between the light absorption bands of reduced and oxidized MnÀ P (see Figure 1B). It is worth noting, the intensive Soret band of Mn(II) porphyrins is obviously observed at 425-435 nm.…”

“…where AH and A − are the acid and the conjugate base of the HEPES buffer, respectively, and x the maximal amount of protons, which can be reversibly stored within the TiO 2 lattice (previously established to x =0.45 for the amorphous GLAD‐TiO 2 electrodes) [39,40] …”

“…[36,37] For a long time, this proton-coupled charge storage process was assumed to be limited to a near-surface region, and it is only recently that it was demonstrated to occur within the bulk of nanostructured TiO 2 electrodes, notably in buffered as well as unbuffered mild aqueous electrolytes containing Brønsted weak acids (i. e. proton donors). [38][39][40] To date, little is known about electrochemical charge accumulation in TiO 2 under catalytic turnover conditions, except in a work published by Reisner's group mentioning a lower steady-state concentration of electrons in the conduction band under catalytic production of H 2 by an immobilized Ni(II) bis (diphosphine) catalyst. [18] The interplay between the catalytic processes and the charge storage processes within modified semiconductive TiO 2 matrices thus remains largely overlooked, despite its potentially significant consequences in photoelectrocatalytic applications.…”

“…For a long time, this proton‐coupled charge storage process was assumed to be limited to a near‐surface region, and it is only recently that it was demonstrated to occur within the bulk of nanostructured TiO 2 electrodes, notably in buffered as well as unbuffered mild aqueous electrolytes containing Brønsted weak acids ( i. e . proton donors) [38–40] …”

Hydrogen-Atom Binding Energy of Structurally Well-defined Cerium Oxide Nodes at the Metal−Organic Framework-Liquid Interfaces

“…Indeed, a few reported H-atom binding energies that were experimentally measured on binary materials have proven to be distinct from and have a wider distribution as compared to the computationally derived values. 14,19,[27][28][29] The apparent enhancement in electrocatalytic activity that involves multiple PCET reactions (e.g., water oxidation) upon surface amorphization of many metal oxides further questions whether the in-silico derived ΔG°H values are representative of the actual catalytic sites. [30][31][32][33] Metal−organic frameworks (MOFs) with redox-active inorganic nodes offer a unique opportunity to examine the thermodynamics of H-atom binding with atomic-level structural precision.…”