Mixed addenda polyoxometalate “solutions” for stationary energy storage

Abstract

A series of redox flow batteries utilizing mixed addenda (vanadium and tungsten), phosphorus-based polyoxometalates (A-α-PV₃W₉O₄₀⁶⁻, B-α-PV₃W₉O₄₀⁶⁻, and P₂V₃W₁₅O₆₂⁹⁻) were prepared and tested. Cyclic voltammetry and bulk electrolysis experiments on the Keggin compounds (A-α-PV₃W₉O₄₀⁶⁻ and B-α-PV₃W₉O₄₀⁶⁻) established that the vanadium centers of these compounds could be used as the positive electrode (PV^IV₃W^VI₉O₄₀⁹⁻/PV^V₃W^VI₉O₄₀⁶⁻), and the tungsten centers could be used as the negative electrode (PV^IV₃W^VI₉O₄₀⁹⁻/PV^IV₃W^V₃W^VI₆O₄₀¹²⁻) since these electrochemical processes are separated by about 1 V. The results showed that A-α-PV₃W₉O₄₀⁶⁻ (where A indicates adjacent, corner-sharing vanadium atoms) had coulombic efficiencies (charge in divided by charge out) above 80%, while the coulombic efficiency of B-α-PV₃W₉O₄₀⁶⁻ (where B indicates adjacent edge-sharing vanadium atoms) fluctuated between 50% and 70% during cycling. The electrochemical yield, a measurement of the actual charge or discharge observed in comparison with the theoretical charge, was between 40% and 50% for A-α-PV₃W₉O₄₀⁶⁻, and ³¹P NMR showed small amounts of PV₂W₁₀O₄₀⁵⁻ and PVW₁₁O₄₀⁴⁻ formed with cycling. The electrochemical yield for B-α-PV₃W₉O₄₀⁶⁻ decreased from 90% to around 60% due to precipitation of the compound on the electrode, but there were no decomposition products detected in the solution by ³¹P NMR, and infrared data on the electrode suggested that the cluster remained intact. Testing of P₂V₃W₁₅O₆₂⁹⁻ (Wells–Dawson structure) suggested higher charge density clusters were not as suitable as the Keggin structures for a redox flow battery due to the poor stability and inaccessibility of the highly reduced materials.