Towards New Micro-Scale Supercapacitors: Pseudocapacitance of Hydrous Ruthenium Oxide Investigated By Multi-Scale Time-Resolved Electrochemical Methods
Authors: Razzaghi Fatemeh , Perrot Hubert , Sel Ozlem 
Published: February 2020

In recent years, the use of metal oxide thin films with nanoscale porosity for diverse electrochemical applications, e.g. energy storage, EC devices, etc. has become a subject of growing interest.
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Among these materials, ruthenium oxide is a particular metal oxide which can offer a type of electrochemical capacitance termed pseudocapacitance which also involves faradaic charge-transfer reactions.  RuOx. nH2O stands as an electroactive material with very high surface area which prosecute the fast reversible redox transitions in the potential window of charge and discharge. The amorphous and hydrous form exhibits noticable pseudocapacitance higher than its crystalline counterpart.
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 For the improvement of the performance of metal oxide based electrodes, main attention has been attributed to their nanostructuration with various morphologies due to the unique properties and functionalities that can effectively be exploited in electrochemical devices. Among various synthesis methods such as sol-gel, spray pyrolysis and hydrothermal synthesis, electrochemical synthesis remains to be a fast, simple, low-cost and low-temperature technique which may also lead to the formation of homogenous nanoscale films with desirable qualities. Moreover, the incorporation of surfactant molecules in the electrogenerated film results in the production of thin nanostructured films by using potential-controlled self-assembly of surfactant-inorganic aggregates at solid-liquid interfaces
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. The resulted nanostructures, with small sizes, and large surface area to volume ratios, are expected to facilitate the ion intercalation/extraction process. These improvements are attributed to a facilitated transfer and short diffusion length for ions transport, a high electrode/electrolyte contact area, and a better stress/strain management of the material during ion intercalation/adsorption.

Despite the morphological information obtained by some in situ or ex situ characterization techniques such as

X-ray photoelectron spectroscopy, small and wide-angle X-ray scattering, none of these methods alone provide the information on the exact identification of the transferred ionic species, their dynamics of transfer at the interfaces, as well as the role of electrolyte composition and the effect of ions solvation on the different electrochemical phenomena.

Classical EQCM (Electrochemical Quartz Crystal Microbalance) was largely used to overcome these problems but its performances are not enough to extract pertinent results. Therefore, here, an alternative characterization tool was proposed to overcome the limitations of these classical EQCM. Specifically, the pseudocapacitive behavior related to the cation transfer in hydrous RuOx thin films was investigated by coupled time resolved characterization methods  A
c-electrogravimetry, consisting of in-situ coupling of electrochemical impedance spectroscopy (EIS) and fast quartz crystal microbalance (QCM), was used here: it measures simultaneously the electrochemical impedance, ΔE/ΔI (ω), and the mass/potential transfer functions (Δm/ΔE (ω)) It provides the access to the relevant information on the kinetics of species transferred at the solid/solution interfaces, and their transport in the bulk of the materials, the nature of these species as well as their relative concentration within the material. This coupling dominates over the limitations of QCM technique and has the ability to deconvolute the global mass variations provided by QCM measurements. Specifically, it detects the contribution of the charged or uncharged species and to identify anionic, cationic, and the free solvent contributions during various redox processes.
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In the present work, this approach is exploited to study the ion transfer behaviour of amorphous hydrous RuOx in both mostly compact and rather porous structures. An electrochemical pathway was chosen, so-called surfactant-assisted electrodeposition is used for their preparation. In addition, we particularly focus on the amorphous materials rather than crystalline ones to emphasize its role in the electrochemical performance of such films. The electrochemical performance and ion intercalation/adsorption mechanisms were studied in aqueous H2SO4, Li2SO4 and Na2SO4 electrolytes by electrogravimetric methods (EQCM, and ac−electrogravimetry). Special attention is attributed to understand the nature of the ions involved in the charge compensation, solvation and the role of the electrolytes and the dynamic information of ions transfer at the electrode/electrolyte interfaces, ac−electrogravimetry as a gravimetric probe is used to study the complex H+ or Li+ intercalation/extraction mechanisms and to extract subtleties unreachable with classical tools.

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