PNNL

Abstract

Understanding the molecular-level properties of electrochemically active ions at operating electrode-electrolyte interfaces (EEI) is key to the rational development of superior electrochemical energy technologies. Herein, we demonstrate that such an under-standing of the multi-electron electrochemical activity of Keggin polyoxometalate anions (POMs) can be used to design high-performance EEIs for energy storage. A novel in situ electrochemical cell coupled with ion soft landing is employed to examine the effect of “atom-by-atom” metal substi-tution on the redox activity of well-defined POM anions, PMoxW12-xO403- (x = 0,1,2,3,6,9,12) at nanostructured ionic liquid EEI. A striking observation made by in situ electrochemical measurements and further supported by theoretical calcula-tions is that substitution of only (1-3) tungsten by molybdenum atoms in the PW12O403- anions results in a substantial increase in the first reduction potential. Specifically, PMo3W9O403- showed the highest redox activity compared to all other mixed-addenda POMs, therefore, making it as a “super active redox anion” for this class of redox species. These molecu-lar-level understandings translate well into device-level performance resulting in superior capacity and capacity retention when PMo3W9O403- is incorporated into a functional macroscopic redox supercapacitor device. Electronic structure calcu-lations showed that there are pronounced energy changes in the lowest unoccupied molecular orbital (LUMO) as a result of metal substitution in PMoxW12-xO403-. This causes the LUMO to protrude out and essentially become the “active site” for reduction on the mixed-addenda POM. Furthermore, the calculations enabled us to assess several critical factors that contribute to the observed trend in the first reduction potentials of the mixed-addenda POM anions such as: (i) the pres-ence of multiple isomeric structures populated at room temperature, which affects the experimentally-determined reduc-tion potential, (ii) substantial decrease of the LUMO energy upon replacement of W atoms with more electronegative Mo atoms, (iii) stabilization of the fully oxidized anion in the ionic liquid and (iv) structural relaxation of the reduced species produced after the first reduction step. Our results provide a path to achieving superior performance of technologically-relevant EEIs through understanding of the molecular-level electronic properties of specific electroactive species with “atom-by-atom” precision.