An Organofunctionalized Polyoxovanadium Cluster as a Molecular Model of Interfacial Pseudocapacitance Eric Schreiber, ∥,† Niamh A. Hartley, ∥,† William W. Brennessel, † Timothy R. Cook, ‡ James R. McKone,* ,§ and Ellen M. Matson* ,† † Department of Chemistry, University of Rochester, Rochester, New York 14627, United States ‡ Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States § Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States *S Supporting Information ABSTRACT: To design new materials for efficient and energy-dense electrochemical energy storage, it is critical to understand the interactions between metal oxides and alkali ions. Here, we discuss the solution-phase interactions of lithium, sodium, potassium, and alkylammonium cations with the Lindqvist-type polyoxovanadate alkoxide (POV alkoxide) cluster, [V 6 O 7 (OCH 3 ) 12 ]. In all cases, the presence of alkali cations positively shifts the half-wave potentials of the reduction events of the POV alkoxide cluster relative to alkylammonium. In contrast, the two cluster oxidation events are not affected by the presence of alkali ions, indicating that the observed changes in reduction potentials are the result of unique interactions with charge-compensating cations. Fur- ther analysis of the shift in reduction potential shows that the energetics of cation binding to the reduced cluster depend both on the charge state of the complex and the charge density of the compensating ion. Single-crystal X-ray diffraction studies indicate that two {Li} + ions undergo site-selective coordination to opposite faces of the octahedron upon complete reduction, manifesting in sluggish reoxidation of this tightly associated, ion-paired species. Thus, this single molecular complex demonstrates redox behavior that spans the range from nonspecific to highly specific cation binding, which is directly analogous to the transition from double-layer capacitance to pseudocapacitance in bulk energy storage electrodes. KEYWORDS: polyoxometalate, electrochemistry, charge compensation, capacitance, pseudocapacitance, lithium, vanadium, electrochemical energy storage ■ INTRODUCTION The interactions between cations and redox-active metal oxides are foundational to many electrochemical technolo- giesranging from secondary batteries to electrocatalysts to environmental and biological sensors. 1−3 Nonetheless, defining the molecular basis of charge compensation processes remains a key challenge that is complicated by the interfacial heterogeneity of solid electrode materials. 4 To this end, the study of molecular analogues of solid state oxides can provide crucial insights into the salient thermodynamics and kinetics of cation-coupled electron transfer processes, thereby enabling the design of improved electroactive materials for diverse applications. Polyoxometalates (POMs) are a class of discrete inorganic assemblies made up of three or more transition metal oxide subunits linked together by bridging oxide ligands (Figure 1). 5−8 Given the molecular composition of these clusters and their rich electrochemical profiles, researchers have turned to these soluble entities as homogeneous models for redox-active, solid state oxides. 8−12 The facile electron transfer associated with these metal oxide clusters has been attributed to the fact that POMs comprise multiple electron deficient metal ions (most commonly tungsten, molybdenum, and vanadium) that, like their bulk metal oxide analogues, accommodate changes in oxidation state by redistributing charge across multiple metal sites. 13 Over the past several decades, systematic studies of factors that influence the redox chemistry of POMs have resulted in a growing understanding of the role that pH, 14−16 solvent, 17−20 embedded heteroatoms, 21−24 surface transition metal substitutions, 25−27 and organofunctionalization of the cluster surface 28 play in dictating the electrochemical profile of metal oxide clusters. Likewise, prior investigations have Received: November 18, 2019 Accepted: November 21, 2019 Published: November 21, 2019 Article www.acsaem.org Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acsaem.9b02239 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX Downloaded via 5.101.220.156 on December 14, 2019 at 10:56:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.