Controlling the Activity and Stability of Electrochemical Interfaces

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Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species Venkateshkumar Prabhakaran, Zhongling Lang, Anna Clotet, Josep M. Poblet, Grant Edward Johnson, and Julia Laskin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06813 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species Venkateshkumar Prabhakaran,1,* Zhongling Lang,2,* Anna Clotet,2 Josep M. Poblet,2,& Grant E. Johnson,1 and Julia Laskin3,# 1Physical

Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA.

2Department

de Quı́mica Fı́sica Inorgànica, Universitat Rovira i Virgili, Marcel·lí Domingo 1, Tarragona 43007, Spain.

3Department

of Chemistry, Purdue University, West Lafayette, IN 47907, USA.

KEYWORDS: In situ electrochemistry, Keggin polyoxometalate, mixed-addenda, electrode-electrolyte interface, ion soft landing, redox supercapacitor Corresponding Authors: #[email protected], [email protected]

ABSTRACT: Understanding the molecular-level properties of electrochemically-active ions at operating electrode-electrolyte interfaces (EEI) is key to the rational development of high-performance nanostructured surfaces for applications in energy technology. Herein, an electrochemical cell coupled with ion soft landing is employed to examine the effect of “atomby-atom” metal substitution on the activity and stability of well-defined redox-active 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 calculations is that substitution of only (1-3) tungsten by molybdenum atoms in the PW12O403- anions results in a substantial spike in their first reduction potential. Specifically, PMo3W9O403- showed the highest redox activity in both in situ electrochemical measurements and as part of a functional redox supercapacitor device, making it a “super-active redox anion” compared to all other PMoxW12-xO403- species. Electronic structure calculations showed that metal substitution in PMoxW12-xO403- causes the lowest unoccupied molecular orbital (LUMO) to protrude out locally making it the “active site” for reduction of the anion. Several critical factors contribute to the observed trend in redox activity including: (i) multiple isomeric structures populated at room temperature, which affect the experimentally-determined reduction potential, (ii) substantial decrease of the LUMO energy upon replacement of W atoms with more electronegative Mo atoms, (iii) structural relaxation of the reduced species produced after the first reduction step. Our results illustrate a path to achieving superior performance of technologically-relevant EEIs in functional nanoscale devices through understanding of the molecular-level electronic properties of specific electroactive species with “atom-by-atom” precision.

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Mechanistic understanding of molecular-level processes taking place at nanostructured electrode-electrolyte interfaces (EEI) is important to the development of clean and sustainable electrochemical technologies for applications in energy generation and storage as well as water and chemical separation and purification.1-5 However, the complexity of operating EEI makes it difficult to obtain an accurate molecular-level description of charge transfer kinetics, which affect the observed reduction/oxidation potentials. One of the major challenges is understanding the effect of different charge and ion transfer pathways (diffusion, migration and convection) and ionic interactions on the activity and stability of the EEI.6-7 Several in situ and operando electrochemical and physical characterization approaches combined with theoretical modeling have been used to study molecular mechanisms at operating EEIs.8-9 For example, photon- and neutron-based scattering, nuclear magnetic resonance, infrared spectroscopy and electron microscopy-based approaches8-12 have been developed to study EEIs with a particular emphasis on investigating processes that are detrimental to the performance of EEI in electrochemical devices. Another promising research direction is focused on preparation of precisely-defined EEI with exquisite control over the ions of interest, which enables understanding of the intrinsic activity of electroactive species at operating conditions. Soft landing of mass-selected ions enables the uniform deposition of complex ions with specific composition, charge state, and kinetic energy.13-20 Soft landing also provides atomically-precise control over the chemical properties of the EEI that cannot be achieved using conventional electrode preparation methods.21-24 For example, soft landing was used to achieve superior performance of solid-state EEIs comprised of redox active PMo12O403- polyoxometalate (POM) on carbon nanotube (CNT) electrodes compared to those prepared using ambient solution dropcasting and electrospray deposition.22 The superior performance of the solid-state EEI prepared by ion soft landing was attributed to uniform deposition of POM anions and complete elimination of strongly-coordinating inactive counterions and solvent molecules that negatively alter the stoichiometry and chemistry of active ions at the EEI. In situ characterization of EEI is key to understanding structure-function relationships that underlie the rational design of improved EEI for a broad range of electrochemical technologies. For example, Anderson et al. examined the electrochemical activity of size-selected platinum clusters soft landed on glassy carbon electrodes using a specially-designed in-vacuum liquid cell.25-26 They demonstrated substantial electrode damage during electrochemical cycling following exposure of the supported clusters to air and water, indicating the importance of the in situ measurements.25, 27 We recently introduced an experimental approach for in situ electrochemical characterization of ions soft landed on solid-state EEI either in vacuum

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or at a controlled partial pressure of a reactive gas.21, 23-24, 28 This approach employs thin nanostructured ionic liquid membranes (IL) with excellent ion transport properties, which may be specially designed for studying either redox or reactive electrochemistry at EEI. For redox electrochemistry, we showed that the in situ electrochemical cell combined with ion soft landing facilitates molecular-level understanding of electron transfer processes occurring in redox-active PMo12O40. The oxygen reduction reaction (ORR) activity of mass-selected ~Pt30 clusters was also studied as a model system for reactive electrochemistry. Collectively, these studies demonstrated the advantages of characterizing electrochemical processes at well-defined EEIs prepared using ion soft landing under carefully-controlled conditions. Our previous studies indicate that the structure and electronic properties of electroactive species play an important role in determining their activity at EEI. Herein, we report a study of the effect of the chemical composition of Keggin POM anions on the properties of well-defined EEI. Keggin POM anions are stable metal-oxide clusters with multielectron redox activity (e.g., PMo12O403- can accept up to 24 electrons without losing its structural integrity).29 These distinctive properties of POMs make them promising materials for applications in rechargeable batteries, redox-active electrodes for electrochemical separations such as water purification, and photo-/electro-chromic devices.4, 28-31 Using gas-phase anion photoelectron spectroscopy, Waters et al. demonstrated that the highest occupied molecular orbital (HOMO) of PW12O403- is stabilized by 0.35 eV in comparison to PMo12O403- suggesting that PMo12O403- is more reactive and less stable than PW12O403-.32 This experimental observation agrees with theoretical calculations.33-34 [Hereafter, the abbreviations W12POM and Mo12POM will be used to refer to the single metal analogs PW12O403- and PMo12O403-, respectively, and Mo12-xWx POM will be used to refer to PMo12-xWxO403- (x = 0, 1, 2, 3, 6, 9, 12) species]. Based on this previous work it is plausible that specific mixed-addenda molybdenum-tungsten POM anions (i.e., PMoxW12-xO40) may offer the combined advantages of both higher redox activity and stability.35-38 We hypothesized that investigation of atom-by-atom substitution of molybdenum with tungsten in the PMo12O403anion may identify a more stable and reactive mixed-addenda POM, which will be beneficial for the development of high-performance and stable energy storage devices. To test this hypothesis, we used a combination of the in situ electrochemical cell, ion soft landing, and density functional theory (DFT) calculations to examine and model the electrochemical properties of mass-selected POM anions at nanostructured EEIs. We demonstrate a pronounced and unexpected effect of the stoichiometry of mixed-addenda POM anions on their redox activity and stability at the operating EEI. A dramatic increase in the reduction potential of the MoxW12-x POM anions was observed when only one tungsten atom was replaced by a molybdenum atom and a similar increase was propagated for the next 2 and 3 metal substitutions. Strikingly, we found both experimentally and through theoretical calculations

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Both the theoretical and experimental results show that the reduction potentials of the mixed-addenda POMs do not vary monotonically with an increasing number of substituted Mo atoms. Rather, a substantial increase in the first reduction peak potential of ~ 600 mV was observed for the Mo1W11 anion compared to W12POM, in which a W6+ ion was substituted by Mo6+. The substantial change induced in the 1st redox potential is associated with the fact that the electron added to the mixed-addenda POM anion in the reduction step is localized on the Mo center, since Mo6+ in a fully oxidized POM has a greater electron affinity than W6+.34-36 Indeed, in both Mo12POM and W12POM, all of the metal centers have an equal probability to accept additional electrons during reduction. However, when a W 6+

(5, 7) (4, 8)

(a,b) = PMoaWbO402-/ MoaWb2-

600

800

1000

(3, 9)

(5, 7)

1200

(4, 8)

(7, 5) (6, 6)

(8, 4)

(9, 3)

(2, 10)

The cyclic voltammograms (CVs) of selected mixed-addenda MoxW12-x (x = 3) POM anions are compared with those of their single metal analogs – Mo12POM and W12POM, in Figure 2a. The overall trend in the first electron reduction potentials for a range of mixed-addenda POM anions obtained from both calculations and experimental results are plotted with respect to Mo content in Figure 2b. The reduction potentials are given relative to the potential for W12POM. The measured reduction potential of W12POM is -100 mV vs Ag. Therefore, the experimental reduction potentials of all mixed-addenda POMs shown in Figure 2b are shifted by +100 mV. CVs and the reduction and oxidation potentials of all the redox steps of the mixedaddenda POM anions studied in this work are presented in the SI section (Figure S2-S12).

(a,b) = PMoaWbO403-/ MoaWb3-

(3, 9)

A mixed-stoichiometry distribution of Mo12-xWx POM anions obtained by electrospray ionization mass spectrometry (ESI-MS) of a solution prepared with a molar ratio of 1:1 of Na3(PMo12O40) to Na3(PW12O40) is presented in Figure 1. Soft landing was used to deposit a precisely-controlled amount of triply-charged mixed-addenda POM anions of selected stoichiometry (x = 0, 1, 2, 3, 6, 9, 12) on the in situ electrochemical cell to study the redox properties of these anions as a function of metal substitution in the absence of complications arising from contaminants, solvent, counterions, and other POMs. Unless specified otherwise, 1 × 1014 ions were deposited in each soft landing experiment.

(6, 6)

REDOX ELECTROCHEMISTRY OF MIXED-ADDENDA PMoXW12-XO403-

(7, 5)

RESULTS AND DISCUSSION

ion is replaced by a Mo6+ as in W11MoPOM, the additional electron has a higher probability of going to the Mo-centered orbital. In other words, the more electronegative Mo6+ decreases the LUMO energy level of the W11MoPOM making it more easily reducible compared to W12POM.33 Interestingly, the reduction potential of W11MoPOM is very close to that of Mo12POM and even ~50 mV higher. A similar increase in the first reduction potential of 550, 620, 600 and 570 mV was also observed for Mo2W10, Mo3W9, Mo6W6 and Mo9W3 POMs with respect to W12POM. DFT calculations provide insight into the factors that contribute to the experimentally-observed trend in reduction potentials with Mo substitution. In particular, we found that the contribution of multiple isomeric structures, the solvent effect, and the extent of structural relaxation after electron transfer to POM must be accurately described in the modeling to obtain good agreement between the experimental and calculated reduction potentials. These factors are discussed in detail below.

(8, 4)

that Mo3W9 POM shows the largest structural relaxation among all mixed addenda POM, which makes it the most oxidizing species. Collectively, our results demonstrate that high-performance nanostructured electrochemical interfaces may be rationally designed by combining in situ electrochemical characterization of precisely-selected electroactive species at EEI with a high-level theoretical description of their electronic properties. We anticipate that this molecular-level approach, made possible by ion soft landing, will play an increasing role in the development of future high performance energy technologies.

(10, 2) (9, 3)

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Abundance

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1400

m/z

Figure 1. Representative negative mode ESI mass spectrum of a mixed-addenda POM solution prepared with molar ratio of 1:1 of Na3(PMo12O40) and Na3(PW12O40), showing the presence of triply (blue text labels) and doubly (red text labels) charged mixed-addenda POM anions with a distribution of stoichiometries (PMo12-xWxO40n-, x = 0-12; n=3,2); the bracket notation (a,b) designates the number of molybdenum (a) and tungsten atoms (b), respectively in the mixed-addenda POM anions. First, we consider the contributions of different isomers that are populated at room temperature.39-40 There are many possible locations to substitute W with Mo when more than one Mo is introduced into the W 12POM framework. For example, for x = 0, 1 and 12 there is only one lowenergy isomer per anion, whereas for x = 2, 3, 6 and 9, the number of possible isomers per anion is 5, 13, 46, and 13, respectively. Two series of calculations were performed to evaluate the reduction potentials of mixed-addenda POM shown in Figure 2b: i) using the structures with lowest calculated free energy (red points) and ii) using the structures of isomers that are most populated at room temperature (blue squares). In the latter case, the theoretically-calculated population of each isomer was estimated using the Boltzmann distribution from the free energies computed in the EMIMBF4 ionic liquid solution and the degeneracy

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Current, (A/cm2)

of each distinct isomer (Table S1). The present DFT calculations reproduce well the relative reduction potentials obtained experimentally. In particular, as shown in Figure 2b, the trend for the calculated potentials is fully consistent

W12 POM Mo3W9 POM Mo12 POM

0.2

(a)

r

0.0

** * -0.2

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+ r/r' - Cc /Cc : Cobaltocenium/Cobaltocene

-0.4

(internal reference redox couple)

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Relative 1st Reduction Potential (V)

Potential, V vs. Ag 1.5 Experimental (relative V vs. W12POM) Calculated (highest population isomer) (b) Calculated (most stable isomer)

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Pure Mo12POM

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(c)

DEox|red

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DGox|red

-3.5 -4.0 -4.5 0

2

4 6 8 x in PMoxW12-x

10

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Figure 2. (a) CVs of selected soft landed mixed-addenda POM anions, W12POM, Mo3W9, Mo6W6 and Mo12POM. Scan rate: 10 mV s-1. Approximately 1 x 1014 ions were deposited in each case. The first reduction peak in each CV is

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highlighted with an asterisk (*) (b) Comparison of the experimental reduction potentials of the first reduction step obtained from the CVs and those calculated theoretically for a range of mixed-addenda POM anions relative to the reduction potential of WPOM. Both the experimentally observed potentials vs. Ag bulk (open symbols) and the values relative to W12POM (filled symbols) are shown for comparison; (c) Comparison of the calculated lowest unoccupied molecular orbital (LUMO) energies of the fully oxidized MoxW12-x (x = 0, 1, 2, 3, 6, 9, 12) POM anions (isomers with highest population) with respect to the reduction electronic energies, vertical reduction energies, and Gibbs free energy associated with the first reduction step. with the experimental values when they are based on the isomers with highest populations. For the x = 2 and 3 species, the most stable structures also correspond to those with greatest population. In contrast, the isomer population in PMo6W6 is distributed due to a large number of isomers, with the highest population of any one isomer only reaching 11.8%. The low degeneracy (ng = 4) of the isomer Mo6W6-1 (Figure 3 and Table S1) has led to an equal population (11.8%) with respect to isomer Mo6W6-29 that is about 1 kcal·mol-1 higher in energy (ng = 24). The first reduction potential of the most stable Mo6W6 isomer is calculated to be 580 mV, which is 150 mV lower than that of the most populated Mo6W6 isomer (737 mV) in solution and even smaller than the potential computed for Mo9W3 (650 mV). Therefore, it is important to note that the computed values obtained using the most stable isomer do not reproduce the experimentally observed trend in reduction potential. It follows that the redox properties simulated for the anion Mo6W6 are more reliable when the isomer population is taken into account. The only exception was Mo9W3, for which the most stable and highest populated isomers showed almost the same reduction potentials.

THEORETICAL INVESTIGATION OF ELECTRONIC STRUCTURE CHANGES OF PMOxW12-xO403- vs. REDOX PROPERTIES To further understand the relevance of selected POM structures to their redox properties, we performed a detailed analysis of the five isomers of the representative anion Mo2W10 (Figure S13 and Table S2). The relative stability of the five isomers depends on the surrounding electrolyte media. Hence, in the gas phase, isomer-2, also known as isomer (1,5) is the most stable. However, in ionic liquid solution, isomer-4 (1,7) becomes the lowest in energy. If the calculations are performed in water, the anions are much more stabilized because of the higher dielectric constant of the solvent. As shown by the values in Table S2, the reduction potential depends on the isomer in a range up to 80 mV according to present calculations in ionic liquid media. The computed reduction potentials for isomers (1,2), (1,5) and (1,12) of Mo2W10 are found to be even more oxidizing than the neighboring anion Mo1W11, whereas the reduction potential computed for isomer (1,7), the most stable and

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highest populated isomer in solution, is fully consistent with the experimental value. Therefore, it is reasonable to propose that isomer (1,7) may be dominant for the anion Mo2W10 present in the experiment.

Figure 4. 3D representation of the LUMOs calculated for the mixed addenda POM anions (MoxW12-x, x = 1, 2, 3, 6, 9). The corresponding LUMO energies are shown in parentheses.

Figure 3. Ball-and-stick representation of the most stable calculated structures for mixed-addenda POM anions (MoxW12-x, x = 1, 2, 3, 6, 9). For x = 6 and 9 the most populated (MP) isomer is different than the most stable (MS) one and both structures are shown in the figure. The isomer number according to Table S1 numeration is given in bold. Color code: Mo (Cyan), W (blue), P (orange), O (red). Next, we examine the calculated frontier molecular orbitals and other structural and electronic factors that affect the redox properties of different POM anions. The oxidizing ability of the substituted POM anions depends on the nature and energy of the LUMOs (Figure 4). Due to the higher electron affinity of the molybdenum ion, the substitution of W6+ by Mo6+ stabilizes the relative energy of the LUMO from –3.29 eV in W12POM to –3.72 eV in Mo1W11. The LUMO of Mo1W11 has a larger participation (47%) of Mo(d) orbitals in comparison to the delocalized contribution of metal in pure W12POM and Mo12POM. This implies that the electron added to Mo1W11 should be efficiently localized on the Mo center. With increasing numbers of Mo centers in the range of mixed-addenda POM, it is observed that the energies of the LUMOs are slightly changed without a clearly defined pattern, as shown in Table S3 and Figure 2c. The LUMO energies are fully consistent with the calculated vertical reduction energies, in which only the electronic contribution is included and the geometry is not relaxed after 1e- reduction of the fully oxidized POM (Table S3 and Figure 2b). Therefore, the energies of the LUMO and the vertical reduction energies do not explain why Mo3W9 and Mo2W10 are the most oxidant anions in this study.

In contrast, when the structures of the anions are allowed to relax after 1e- reduction, the energies involved become substantially more negative (from –0.44 to –0.66 eV) in Mo substituted anions and only shift by –0.21 eV in the pristine PW12POM. The structural differences between polyoxotungstates and polyoxomolybdates for oxidized and reduced anions have been reported previously.51 It is worth mentioning that among all these species, Mo3W9 shows the largest structural relaxation and becomes one of the most oxidant species according to the electronic energy (ΔEox|red). When the additional terms that contribute to the free energy are considered, the energy changes are much less important in absolute values, but collectively, they transform Mo3W9 to the anion with the most negative free energy (–4.46 eV). As a result, Mo3W9 is predicted to have the most positive electron reduction potential (798 mV (Table S3) ) in the series of mixed-addenda POM anions examined in this study, which is in agreement with the experimental findings (Figure 2b). Our results indicate that accurate description of the first reduction potentials of POM anions using DFT calculations must take into account the electronic properties of the isomeric structures that are populated at the temperature of the experiment, solvent stabilization of the fully oxidized anion, and structural relaxation of the reduced species after 1e- reduction.

MULTI-ELECTRON PROPERTIES OF PMOxW12-xO403The above discussion of the effect of mixed POM stoichiometry on the first reduction potential provides valuable information about the interplay between structure, electronic effects, and solvent stabilization in these systems. In order to obtain additional insights into the multi-electron

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Number of electrons transferred during oxidation, x 1014

properties of mixed-addenda POM anions, each of the oxidation and reduction peaks observed in the CVs of all mixed-addenda POM anions were analyzed. The total number of electrons transferred in each redox step was calculated from the charge obtained by integrating each peak using Faraday’s law (see SI section for details on calculations). The cumulative number of electrons transferred sequentially along the redox steps during oxidation and reduction scans of mixed-addenda POM anions with ~ 1 x 1014 soft landed ions is presented in Figures 5a and 5b, respectively. The total number of electrons transferred during CV scans of Mo12POM is greater than that of W12POM confirming that Mo12POM is more redox active than W12POM. It is 2.5

Mo12 Mo9W3 Mo6W6 Mo3W9 Mo2W10 Mo1W11 W12

(a) 2.0 1.5 1.0 0.5 0.0

Cumulative

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other mixed-addenda POM anions which further corroborates the superior redox activity of this species in comparison with other POMs observed experimentally and calculated theoretically in this study.

REDOX-SUPERCAPACITOR STABILITY

PERFORMANCE

AND

Based on the observed trend in the redox activity of POM anions and on the known trend in their stability reported in the literature, we hypothesized that specific mixed-addenda POM anions containing only a few Mo atoms may increase the performance of a redox-supercapacitor device compared to that possible with pure W12POM or Mo12POM. To test this hypothesis, we fabricated redox-supercapacitors by soft landing 1 x 1014 anions of a range of mixed-addenda POMs onto CNT electrodes. The electrodes were assembled into supercapacitor devices and tested using Galvanostatic charge-discharge (GCD) as described in the experimental section. The first and last three cycles of GCD curves of all mixed-addenda anions are presented in the Figure S14. The total specific capacitance of mixed-addenda anions before and after 1000 cycles of GCD measurements between 0 to 1 V at a rate of 8 A/g is shown in Figure 6. The total specific capacitance presented in this figure is the sum of the Faradaic capacitance derived from the redox activity of POM anions and non-Faradaic capacitance

1.0

Potential, V

500

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Mo12 Mo9W3 Mo6W6 Mo3W9 Mo2W10 Mo1W11 W12

(b) 2.0 1.5 1.0 0.5

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Number of electrons transferred during reduction, x 1014

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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First cycle After 1000 cycles

450 400 350 300 250 200 150 100 50

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also interesting to note that Mo12POM exhibits a higher number of electrons transferred in the first three redox steps, whereas it is the opposite case for W12POM which has a higher number of electrons transferred in the last redox steps. In contrast, the mixed-addenda anions showed a more steady increase in the number of electrons transferred as a function of potential. Overall, Mo3W9 showed the highest number of electrons transferred among all

2

3

6

9

12

x in MoxW12-x

Potential, V Figure 5. Cumulative number of electrons transferred during (a) oxidation and (b) reduction scans of a range of soft landed mixed-addenda POM anions on the in situ electrochemical cell. Total number of POM soft landed in each case is 1 x 1014 ions.

1

Figure 6. Comparison of the total specific capacitance before (black bars) and after (blue bars) 1000 GCD cycles performed between 0 to 1 V at a rate of 8 Ag-1 observed with redox supercapacitor devices containing a range of soft landed mixed-addenda POM anions. Total number of POM soft landed in each case is 1 x 1014 ions. derived from the electrolyte ions at the CNT electrode (~112 ± 12 F/g). The non-Faradaic capacitance is the same for the series of redox supercapacitors examined in this study. It follows that the observed trend in the total capacitance shown in Figure 6 is attributed to the differences in redox activity of different POM anions.

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Consistent with observations obtained from the in situ electrochemical measurements, the device fabricated using W12POM shows the lowest capacitance across the series of mixed-POM anions. For example, the total specific capacitance of the Mo1W11 device is 370 F/g which is ~115 % higher than that of the W12POM device (~ 174 F/g). All other mixed-addenda anions MoxW12-x (x = 2, 3, 6, 9, 12) exhibit a similar total specific capacitance in the initial cycles even though the specific capacitance of Mo3W9 is slightly higher than the others. After 1000 GCD cycles, the capacity retention of mixed addenda MoxW12-x anions was found to be 95.3%, 97.1 %, 97.5%, 97.7%, 93.6%, 88.1%, 84.3% for x = 0, 1, 2, 3, 6, 9 and 12, respectively (Figure 5). W12POM shows higher capacity retention than Mo12POM which is consistent with the previously established observation that W12POM is more stable than Mo12POM.32 Furthermore, the capacity retention is higher for MoxW12-x (x = 1, 2 and 3) than their single-addendum analogs. Meanwhile, Mo3W9 exhibits the highest capacity retention. These observations establish that substitution of Mo atoms in W12POM anions plays a substantial role in the formation of a superior redox-active anion (Mo3W9) with higher redox activity than single metal POMs. The results presented in this study demonstrate that the intrinsic properties of redox-active species obtained from in situ electrochemical experiments provide the basis for the rational design of high-performance electrochemical devices. Ion soft landing provides a controlled way to deposit well-defined redox-active species for in situ characterization of their intrinsic properties at EEI. Meanwhile, complementary electronic structure calculations provide detailed insights into the variation in structure and electronic properties of POM anions in the electrolyte solution with substitution of addenda atoms, which helps rationalize the observed trend in redox activity of mixed-addenda POMs. Theoretical results revealed substantial variation in the reduction potentials of different isomers of mixed-addenda POM anions and demonstrated that the most populated isomers at experimental conditions are not always the most stable species. In addition to electronic effects, structural relaxation of the anions after 1e- reduction was found to be a key factor determining the special properties of the Mo3W9 anion, which was found to be the most oxidant species in the series of POM anions examined in this study.

CONCLUSION In this work, we examined the redox electrochemistry of mass-selected mixed-addenda MoxW12-x POMs with precisely-defined stoichiometry (i.e., x = 0, 1, 2, 3, 6, 9, 12) experimentally and computationally. An in situ electrochemical cell combined with soft landing of mass-selected ions provided detailed insights into the redox activity of electroactive species at operating EEIs. Our joint experimental and theoretical findings establish that a single substitution of a W atom by Mo is sufficient to substantially alter the electronic structure of Keggin W12POM.

tential making the mixed-addenda POM species substantially more redox active than W12POM. Furthermore, we estimated the total number of electrons transferred sequentially during the redox steps observed in the CV scans. In situ electrochemical measurements showed that Mo3W9 has the highest redox potential and transfers the largest number of electrons making this anion the most oxidant species in the range of mixed-addenda POM anions studied. These experimental observations are supported by complementary theoretical calculations of the structures and reduction potentials of the mixed addenda POMs in the ionic liquid electrolyte. Theoretical calculations also show that single substitution of a W atom by Mo is sufficient to substantially alter the electronic structure of the Keggin W12POM. Consistent with the experimental data, the tri-substituted anion (Mo3W9) is calculated to be more oxidant than the other members of the series. The superior redox properties of this anion are attributed to the large relaxation energy associated with the 1e- reduction step. The high-level theoretical calculations, therefore, provide information on the critical factors that affect the redox activity of a range of mixed addenda POMs. These include: (i) the relative stability and population of multiple isomeric structures of POMs at experimental conditions, which affect the reduction potential, (ii) decrease of LUMO energy after replacement of W atoms with more electronegative Mo atoms, (iii) stabilization of the redox-active species by solvation in the electrolyte solution (ionic liquid, water), and (iv) structural relaxation of the redox species occurring after 1e-reduction. Finally, we demonstrate that the molecular-level understanding of the intrinsic redox activity of the mixed-addenda POM anions may be directly translated to the complex operating environment of redox supercapacitor devices. Specifically, the superior stability characteristic of W12POM and the higher capacitance of Mo12POM. In particular, redox supercapacitors prepared by soft landing MoxW12-x (x = 1, 2, 3) onto technologically-relevant CNT electrodes showed higher specific capacitance and capacity retention compared to other mixed addenda species with the most redox active Mo3W9 species producing the highest-performing supercapacitor device. These combined atomic-level insights allowed us to design a high-performance stable EEI in an energy storage device using Mo3W9 as a “super electroactive anion” of this class of Keggin POMs. The molecular-level insights into the processes occurring at EEI obtained from this work will aid the design of improved sustainable electrochemical energy technologies.

METHODS PREPARATION OF MIXED-ADDENDA POM ANIONS The mixed-addenda MoxW12-x (x = 0, 1, 2, 3, 6, 9, 12) anions were prepared in a one-pot synthesis using specific molar ratios of salts of PMo12O40 : PW12O40 (1:11, 2:10, 3:9, 1:1, 9:3) as detailed in SI section II.41-42

We found that substitution of a single W6+ with Mo6+ in W12POM resulted in a dramatic increase in the 1st redox po-

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IN SITU ELECTROCHEMISTRY OF MASS-SELECTED MIXEDADDENDA POM ANIONS The intrinsic redox activity of mixed-addenda POM anions was evaluated by cyclic voltammetry (CV) measurements acquired on soft landed triply-charged anions deposited directly onto specially-designed in situ thin film electrochemical cells integrated in our custom-built ion soft landing instrument (See SI section III, IV and V and Figure S1).23, 43-44 Distinguishable redox peaks originating from both the Cc/Cc+ (internal reference redox couple) and the deposited anions were observed in the CVs. To correct for the additional electrode polarization of the metallic Ag reference electrode (RE), the obtained CV curves were manually adjusted to maintain the reduction potential of Cc/Cc+ at a value of -1.52 V. Furthermore, to correct for the difference between the reference potential determined by CV measurements and potentials predicted using theoretical calculations, the corrected potentials in all CVs were further adjusted so that the first electron reduction peak of PW12POM was at 0 V. This process makes the first electron reduction peak potential of PW12POM a reference potential for all CVs and related analysis presented in this work.

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where ngi is the degeneracy that is defined as the number of different atom substitutions leading to identical geometric isomers, DGi is the relative free energy to the most stable type of structure for a given x, T is the temperature with 298.15 K, and kB is the Boltzmann constant. A data set collection of the computational results is available in the ioChem-BD repository and may be accessed via https://doi.org/10.19061/iochem-bd-2-23.49 PERFORMANCE EVALUATION OF REDOX SUPERCAPACITORS FABRICATED USING MIXED-ADDENDA POM ANIONS Redox supercapacitor devices were fabricated by sandwiching an 1-ethyl-3-methylimidazolium tetrafluoroborate and poly(vinylidene fluorideco-hexafluoropropylene) (EMIMBF4 / PVDF-HFP) ionic liquid based electrolyte membrane between two 1 x 1 cm as-prepared soft landed electrodes. The specific capacitance of the as-prepared redox supercapacitor device was determined using Galvanostatic charge/discharge experiments (GCD) (See Section VII).

ASSOCIATED CONTENT THEORETICAL CALCULATIONS OF ELECTRONIC STRUCTURES AND REDUCTION POTENTIALS OF MIXEDADDENDA POM ANIONS The reduction potentials of the range of mixed-addenda POM anions were theoretically determined by computing the free energy associated with the process shown in equations 1-3. All calculations were performed in the Gaussian09 code with the B3LYP functional under the tight convergence criteria and the ultrafine grid. 45-47 See SI section VI for the detailed description of computational methods employed in theoretical calculations presented in this work. POMox3-(aq) + ne- → POMred4-(aq), 298.15 K ENHE = −(

ΔGox|red nF

− ESHE )

--- (1) --- (2)

Supporting Information. The SI section consists of descriptions of experimental and theoretical methods, cyclic voltammograms of soft landed MoxW12-x-(x = 0, 1, 2, 3, 6, 9, 12), calculated geometries of the five distinct isomers of anion Mo2W10, list of degeneracies, relative free energies and population of each isomer, computed values for the five distinct isomers of the PMo2W10O403- anion, and calculated free energies and electronic energies associated with the first electron reduction step for the range of mixed addenda POMs studied in this work. This material is available free of charge via the Internet at http://pubs.acs.org. The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author # &

where ΔGox|red is the free energy change of the reduction process, F is the Faraday constant, and ESHE is the potential corresponding to the absolute free energy of the half-reaction shown in equation (3).48 ½H2(g) → H+(aq)+1e–(ΔGSHE = +4.24 eV, ESHE = ‒4.24V) - (3)

[email protected] [email protected]

Author Contributions *These authors contributed equally The manuscript was written through contributions of all authors. All authors have given approval for the final version of the manuscript.

ACKNOWLEDGMENT To estimate the isomer distribution of calculated POM structures, the weight (pop%) of each type of structure for a given x was computed by considering the free energy of each type of structure and the number of isomers using equation (4): −∆𝐺𝑖

pop% =

ng𝑖 𝑒 𝑇𝑘𝐵

−∆𝐺𝑖

∑𝑖 ng𝑖 𝑒 𝑇𝑘𝐵

∗ 100%

--- (4)

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division. The research was performed using EMSL, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for DOE under Contract DE-AC05-76RL01830. This work was also supported by the Spanish Ministerio de Ciencia e Innovación

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(Project No. CTQ2017-87269-P) and the Generalitat de Catalunya (2014SGR199 and XRQTC). J.M.Poblet. thanks the ICREA foundation for an ICREA ACADEMIA award.

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