(Invited) Development and Characterization of Polyoxometallate-Based Systems for Aqueous Redox Flow Batteries
Authors: Kulesza Pawel J. , Rutkowska Iwona A. , Janiszewska Claudia , Di Noto Vito , Vezzu Keti , Negro Enrico 
Published: July 2022

Redox flow batteries are of potential importance to both large scale energy storage and powering the electrical vehicle. It is now accepted that flow batteries are the battery technology with the greatest potential to be one of the key elements in the energy transition to a sustainable electricity supply. Among critical challenges are low volumetric energy density of redox electrolytes, high cost and the maintenance limitations that greatly impede the wide application of conventional flow batteries. In this respect, the redox-active charge-storage material has a significant impact on the performance of a flow battery. The concentration of redox centers and their reaction kinetics have an influence on the available current densities and, thus, the power of the device.
Many inorganic and organic electroactive systems have been proposed as alternatives to vanadium species in redox flow batteries. In the study, we explore the concept that highly concentrated solutions of the polyoxometallates of molybdenum and tungsten can serve as model examples of multi-electron systems for all-liquid redox flow batteries and related fundamental investigations. Polyoxometallates are polynuclear inorganic materials with well-defined multi-electron reversible electrochemistry and electrocatalytic properties [1]. Among other important characteristics of heteropolyacids are that they exhibit very strong Brønsted acidity, act as proton conductors, and undergo fast, reversible, multi-electron electron transfers leading to the formation of highly conducting, mixed-valence (e.g. tungsten(VI,V) or molybdenum(VI,V) heteropoly blue) compounds. The polyoxometallate-based redox electrolytes have different chemical identities, and they could be considered as anolytes or catholytes, depending on their redox potentials but, typically, their use would require formation of an asymmetric system with different-type redox species. The scope of existing inorganic and organic electroactive materials can be expanded due the possibility of their functionalization and structural modification. Recent developments in the area of the transition-metal-derivatized polyoxometallates are also promising [2] because they imply the feasibility of formation of the bi-redox polyoxmetallate-based electrolytes. In particular, the systems containing such metals as copper, iron, ruthenium, nickel or tin could of interest. Such features as the feasibility of reversible multi-electron redox processes, the improved potential output and cycling performance, sufficiently high solubility and reasonable stability will be examined here. An alternative approach can be based on the preparation of robust zeolite-type cesium salts of polyoxometallates, Cs2.5H0.5PMo12O40, Cs2.5H1.5SiMo12O40, Cs2.5H0.5PW12O40, and Cs2.5H1.5SiW12O40. It is noteworthy that, at certain contents of cesium (or rubidium), these porous salts are characterized by fast charge propagation. They can be considered for application in a form of colloidal suspensions.
While kinetics of electrochemical processes has an influence on the systems’ current densities, the viscosity of the electrolyte and the mass transport dynamics are also affected by the choice of the redox-active material and its concentration. Trying to develop useful electroanalytical diagnostic criteria, we are going to extend the historical concepts of charge propagation in semi-solid or semi-liquid systems developed for mixed-valence redox polymers and polynuclear materials to the development of redox electrolytes. Fundamental electroanalytical approaches utilizing ultramicrodisk electrodes and interdigitated arrays will be adapted to characterization of solid suspensions synthesized in a form of stable colloidal solutions utilizing redox active centers capable of exhibiting fast electron transfers according to electron hopping mechanism. Of additional interest is the dynamics of electron transfer at the electrolyte/electrode interface, considered at the molecular and monolayer scales.
[1] I.A. Rutkowska, P.J. Kulesza, “Metal Oxide Cluster and Polyoxometallate Supports for Noble Metal Nanoparticles in Efficient Electrocatalysis” in Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier, vol. 5, pp 207–216, 2018.
[2] J. Goura, B.S. Bassil, J.K. Bindra, I.A. Rutkowska, P.J. Kulesza, N.S. Dalal, U. Kortz, Chemistry - A European Journal 26 (2020) 15821 – 15824.