Solutions of Polyoxometalates: Ionic Association and Oversaturation as the Factors Affecting Electrode Surface Modification
Authors: Tsirlina Galina 
Published: February 2020

Solutions of polyoxometalates (POM) are widely applied for modification of electrode surfaces with 2D adlayers, multilayer compositions and rechargeable 3D films. All currently existing and future applications (sensors, electrochromic devices, electrocatalytic processes, molecular electronics) are related to operation in solutions. Correspondingly, wet fabrication techniques are preferable to adapt the material to operation medium from the very beginning. The majority of wet techniques involving oxoanions of tungsten, molybdenum, vanadium and some other transition metals utilize POM solutions. Even if the simplest oxocompounds are used as the starting components of acidic and neutral solutions, they always undergo complex transformations with formation of isopolyanins. Under these circumstances POMs become direct molecular precursors of substances responsible for functional properties of modified electrodes, and exact molecular characterization of these precursors should be considered as the principle research task.
For heteropolycompounds synthesized beforehand, the main problems are (1) pH-induced transformations to lacunary anions and (2) association with solution cations. Both problems are closely related to more general problem of (3) multistep protonation. For majority of POMs, no data on protonation and association constants are available, and serious efforts (both experimental and computational) are typically required to characterize solutions properly [1, 2].
For isopolyanins, additional problem is (4) speciation resulting in coexistence of numerous anions under equilibrium or (more usually) in metastable solution equilibrating very slowly [3-6]. The latter is most typical for highly acidic solutions demonstrating ca. 2-3 orders formal oversaturation with respect to low soluble hydrated oxides. Just this unexpectedly high concentration of isopolyanions is favorable for immediate oversaturation with reduction products and subsequent nucleation of non-stoichiometric oxides with rather attractive rechargeable and electrochromic properties.
Finally, general problem consists in (5) adsorption effects on ionic equilibria. Namely, additional protonation and/or ionic association can take place in POM adlayers formed at electrified interfaces of both ideally polarizable [1,2] and catalytically active [7,8] surfaces.
Original results of electrochemical techniques, in situ Raman and UV-vis spectroscopy are presented to illustrate how the problems (1) – (5) can be solved, with illustrations related to importance of ionic speciation and its dynamics in solution for surface modification procedures.
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