Evidencing Fast, Massive, and Reversible H⁺ Insertion in Nanostructured TiO₂ Electrodes at Neutral pH. Where Do Protons Come From?

Abstract

Ongoing developments of sustainable energy technologies based on high-surface-area semiconductive metal oxide electrodes operating under mild and safe aqueous conditions require deep understanding of proton and electron transfer/transport throughout their porous structure. To address this issue, we investigated the electrochemical reductive protonation of high surface area nanostructured amorphous TiO 2 electrodes (produced by glancing angle deposition) in both buffered and unbuffered aqueous solutions. Quantitative analysis of the two charge storage mechanisms was achieved, allowing proper deconvolution of the electrical double-layer capacitive charge storage from the reversible faradaic one resulting from the proton-coupled reduction of bulk TiO 2. We evidence that this latter process occurs reversibly and extensively (up to an intercalation ratio of 20%) not only under strongly acidic pH conditions but also, more interestingly, under neutral pH with the intercalated proton arising from the buffer rather than water. Moreover, we show that in comparison with reductive Li + intercalation the proton-coupled electron charge storage occurs more rapidly (in a few seconds). This important finding suggests that a high-rate and high-power charge storage device could potentially be achieved with the reversible H +-coupled charge/discharge process in TiO 2 at neutral pH, opening thus new opportunities to the development of eco-friendly batteries for electrical energy storage.

Accessing the Two‐Electron Charge Storage Capacity of MnO2 in Mild Aqueous Electrolytes

“…It most likely results from a capacitive effect, as demonstrated for other metal oxide semiconductors in the absence of ion intercalation, [ 55,56 ] notably for TiO 2 . [ 37,57 ] It is worth noting that the potential window investigated here lies within the conduction band of MnO 2 , [ 58 ] so it behaves as an ohmic conductor which Fermi level is governed by the underlying ITO potential. The density of delocalized charge carriers in MnO 2 is consequently modulated through the filling of the conduction band by charge injection from ITO.…”

“…This is very analogous to what we recently demonstrated for the reversible insertion of protons in TiO 2 in the presence of a weak acid. [ 37,38 ] From these considerations, we can thus propose the following global proton‐coupled electron transfer reaction at the metal oxide/electrolyte interface. where AH and A − are the weak acid and conjugated base of the buffer, respectively.…”

“…Moreover, their semi‐transparency allows for their quantitative characterization by UV/vis spectroelectrochemistry, a methodology we previously used to investigate the proton‐coupled electron charge storage at nanostructured TiO 2 electrodes. [ 37,38 ] In the present case, spectroelectrochemical analysis offers the additional opportunity to quantify in situ the amount of MnO 2 on the electrode, allowing for real‐time monitoring the active material that dissolves and redeposits along the discharging/charging cycles. In order to deconvolute the exact role of the different chemical species contained in the electrolyte, the electrodes were cycled in buffered and unbuffered aqueous electrolytes of different nature, composition, and pH.…”

Caesium for Perovskite Solar Cells: An Overview

“…LEDs, luminescent solar concentrators and related photonic technologies represent other platforms where Cs‐based perovskites can behave as core materials in combination with other recently emerged advanced concepts . In this scenario, Mn 2+ doping is currently considered one of the leading strategies to modify the optical and magnetic functionalities of nanocrystals of various chalcogenide and oxide semiconductors .…”

“…LEDs, luminescent solarc oncentrators and related photonic technologiesr epresent other platforms where Cs-based perovskites can behavea sc ore materials in combination with other recently emerged advanced concepts. [244][245][246][247][248][249][250][251][252][253] In this scenario, Mn 2 + doping is currently considered one of the leadings trategies to modify the opticala nd magnetic functionalitieso f nanocrystals of variousc halcogenide ando xide semiconductors. [254] An interesting synergy between these materials chemistry features was proposed by Yuan et al,w ho synthesized as eries of Mn 2 + -doped CsPbCl 3 nanocrystals using reaction temperature and precursor concentration to tune Mn 2 + content up to 14 %.…”