Electrochemically Induced Templated Sol-Gel Deposition of Mesoporous Silica and Nanocomposite Thin Films
Authors: Vanheusden Genis , De Taeye Louis , Lopez Baez Rodrigo , Vereecken Philippe M. 
Published: December 2024

Electrochemically induced sol-gel depositions have recently been gaining scientific interest for their ability to synthesize highly ordered mesoporous silica thin films on electrode substrates.1 The process works by using cathodic reactions to electrogenerate OH- ions through the reduction of a pro-base (e.g., water) and form an alkaline diffusion layer near the electrode surface. Within this alkaline zone, the gelation reactions of a pre-hydrolyzed, kinetically-inhibited silica sol-gel precursor solution are catalyzed, thus triggering the growth of a gel film onto the electrode. The addition of surfactant templates allows for the formation of mesoporous thin films with highly organized pore structures that have a wide range of applications in a wide range of fields. The advantages of this electrochemically induced process include the ability to control the deposition using electrochemical parameters, the faster gelation compared to conventional sol-gel, the option to omit high-temperature treatment/solidification steps and the ability to coat 3D structured electrodes. Furthermore, the rapid gelation is limited to the zone within the alkaline diffusion layer during film growth. Recently, we demonstrated improved process control by tuning the diffusion layer thickness using rotating disk electrodes2, thereby inhibiting the formation of aggregated particle byproducts, and provided an in-depth overview of the mechanism and relevant aspects of the film growth3.

In the present work, the same electrochemically induced sol-gel process is performed using an ionic liquid as the template, leading to the controlled deposition of a functional ionic-liquid-templated silica gel layer that can be dried to an ionically conductive nanocomposite with applications for sensors, supercapacitors and solid-state Li-ion batteries. The nanocomposite under study is based on recent works wherein a Li-ion ionic liquid electrolyte (ILE) was nanoconfined in an inert silica matrix, synthesized via sol-gel reactions.4–6 The nanoconfinement of the ILE in the mesoporous silica matrix results in a monolithic solid structure, while the Li+-ions retain liquid-like high ionic conductivities exceeding that of a pure ILE when the ion diffusion along silica wall/ILE interface dominates.4,6 The conventional sol-gel process used to synthesize the ionic liquid-templated silica requires several days to solidify into the product. By using the electrochemically induced sol-gel process, we demonstrated the ability to controllably grow coatings of 1 to 35 micrometers thickness on electrode surfaces in mere seconds to minutes. The obtained films show comparable properties as films prepared through conventional gelation.

This presentation starts from an overview of the electrochemically induced sol-gel process for mesoporous silica thin films and outlines the methodology used to electrodeposit ionic-liquid-templated silica nanocomposite coatings. The functionality of the coatings as thin-film electrolyte was demonstrated in all-solid-state thin-film test-cells using TiO2, LiMnO2 (LMO) and LiNi0.5Mn1.5O4 (LNMO) as cathodes with Li metal as the anode. Since these thin-film model systems allowed the formation of single, well-defined interfaces between the different components, the underlying electrochemical processes during charge-discharge cycles could be carefully monitored. In this way, this work demonstrates the flexibility of the electrochemically induced sol-gel process by extending the methodology to grow composite coatings, in this case useful for thin-film solid-state batteries.

References

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