Designing Polyoxometalate based Layer-by-Layer Thin Films on Carbon Nanomaterials for Pseudocapacitive Electrodes

Multi-walled carbon nanotube (MWCNT) electrodes modiﬁed with mixtures of PMo 12 O 403– (PMo 12 ) and PW 12 O 403 − (PW 12 ) Keggin ions are investigated for electrochemical capacitor (EC) applications. We have discovered that when these polyoxometalate (POM) ions are combined in solution, they do not just mix physically, but instead react to form the PMo 12-x W x mixed addenda chemistries.BysimplymixingPMo 12 andPW 12 indifferentratios,avarietyofmixedaddendaions,eachwithuniqueelectrochemical properties, can be easily synthesized. This control over POM redox behavior afforded by the novel mixed addenda synthesis is used along with layer-by-layer (LbL) deposition to design molecular coatings that demonstrate desired pseudocapacitive characteristics. The best performance was achieved with a coating that superimposed a 3:1 PMo 12 O 403 − -PW 12 O 403 − mixed layer on a 1:1 GeMo 12 O 404 − -SiMo 12 O 404 − mixed layer, which resulted in an 11X capacitance enhancement over unmodiﬁed CNT. This dual layer electrode also demonstrated a close to rectangular CV proﬁle due to the overlapping redox features of the POM combination. The design of custom POM coatings to engineer the electrode surface represents a promising strategy for the development of high performance pseudocapacitive electrodes. ©

Increased demand for reliable high power energy storage devices has led to growing interest in the development of novel electrode materials for electrochemical capacitors (ECs). [1][2][3] One of the most promising approaches in EC electrode design involves the fabrication of nanostructured composites in which pseudocapacitive materials are immobilized on electrochemical double layer capacitive (EDLC) carbon substrates. There is synergy in this combination as the pseudocapacitive material allows for large charge storage capacity due to surface redox reactions, while the carbon substrate imparts mechanical stability, improved conductivity, and physical capacitive effects. [4][5] These types of composite materials including RuO 2 -CNT, 6 MnO 2 -graphene, 7 V 2 O 5 -CNT, 8 and conductive organic polymer (COP)-CNT 9 have been investigated extensively for pseudocapacitive EC electrodes.
Although there are many pseudocapacitive alternatives, polyoxometalates (POMs), nanoscale transition metal-oxygen clusters, have emerged as promising building blocks for nanocomposite electrodes. [10][11][12][13][14] POMs have an unmatched range of physical and chemical properties which arise from their seemingly endless variety of molecular structures and sizes. 15 One of the most common and widely researched classes of POM molecules is the Keggin cluster which has the general formula [MX 12 O 40 n− ] in which the central heteroatom (i.e. P, Si, or Ge) is surrounded by twelve addenda atoms (i.e. Mo or W) and forty oxygen atoms. [16][17] These molecules demonstrate high stability of their redox states and participate in fast reversible multielectron transfer reactions, making them ideally suited for pseudocapacitive applications. They also offer tremendous versatility as their electrochemical behavior can be tuned with small adjustments to the molecular chemistry. 17 The electrochemical activity of POMs can be combined with the stability and high surface area of carbon substrates to create robust nanocomposite EC electrodes. These POM-carbon composites can be prepared by a variety of techniques including electrodeposition, 18 chemisorption, 19 and layer-by-layer (LbL) self assembly. [20][21][22] The improved capacitive properties of MWCNT electrodes modified with PMo 12 O 40 3− (PMo 12 ) through chemisorption have been well documented. 3,11 Furthermore, Gomez-Romero has shown that POMs can be effectively immobilized on activated carbon electrodes to achieve enhanced capacitance as well as dramatic improvements in power density. 12,23 Most recently, reports of high performance graphene-POM nanocomposite EC electrodes have also emerged. [24][25] These studies demonstrate the potential for POMs as effective pseudocapacitive materials; however, these studies only explored a few commercially available POM chemistries like PMo 12 or PW 12 O 40 3− (PW 12 ). There are hundreds of POM molecules, each with unique properties that have yet to be explored for EC applications. Investigating a wide range of these molecules is very important, as the use of individual POM chemistries presents challenges for EC applications. The cyclic voltammograms (CVs) of Keggin POMs like PMo 12 or PW 12 do not show the ideal rectangular profile demonstrated by EDLC materials or mimicked by the overlapping redox peaks of pseudocapacitors like RuO 2 or MnO 2 . Instead, the CVs of Keggin molecules display narrow redox features with little overlap, resulting in current densities that can vary dramatically with voltage. 22,26 In order to overcome this limitation, we have shown that the LbL technique can be used to superimpose different POM molecules on the same electrode. With this method, multiple POM chemistries with different redox peak potentials can be combined to achieve overlapping redox features and a more ideally capacitive CV profile. 22,26 In addition to the multi-layer technique, a mixed layer coating approach can also be used to combine the effects of multiple POMs. For this method, different POM molecules are combined in an aqueous solution and coated on the electrode simultaneously in a single active layer. This simple process minimizes the electrochemically inert polycation layers and in certain cases, POM mixtures have been shown to result in synergistic combinations. 27 For instance, when PMo 12 and PW 12 are combined in a mixed layer electrode, the resulting composite does not display a simple combination of PMo 12 and PW 12 redox behavior, but instead exhibits its own unique electrochemical activity. This interesting electrochemical phenomenon was reported in our recent communication, 27 however, the underlying cause of this behavior was not well understood. In this study, we expand upon this work to gain a better understanding of how these molecules interact in the mixture and how these interactions affect the electrochemical properties of the composite electrode. We will also demonstrate how these POM mixtures can be used in conjunction with multi-layer modification to design molecular coatings that overcome the limitations of individual POM chemistries to achieve enhanced charge storage and a more ideally capacitive CV profile.  (GeMo 12 ) was synthesized according to a previously described method. 26 The mixed addenda H 3 PMo 9 W 3 O 40 (PMo 9 W 3 ) was prepared by the conventional etherate method. Briefly, 15.0 g NaMoO 4 · 2H 2 O, 6.82 g Na 2 WO 4 · 2H 2 O, and 1.85 g Na 2 HPO 4 · 2H 2 O were dissolved in deionized water and the pH was adjusted to 1.5 with H 2 SO 4 . The mixture was stirred continuously for 4 hours at 80 • C. The cooled solution was extracted with ether and the isolated solid was washed with deionized water and dried for 5 hours at 50 • C yielding a yellow crystal product. All reagents for the synthesis were purchased from Alfa Aesar.
Electrode fabrication: layer-by-layer (LbL) process.-MWCNT electrode films were fabricated by combining the "as-received" MWCNTs 28 with 4 wt% PTFE binder and rolling into thin films of approximately 100 μm thickness. The POMs were dissolved in deionized water to form 10 mM coating solutions. The mixed coating solutions were also prepared on a 10 mM basis (i.e., the 3:1 PMo 12 -PW 12 mixture was 7.5 mM PMo 12 and 2.5 mM PW 12 ). The LbL process used to fabricate the composite MWCNT-POM electrodes proceeded according to the following steps: Characterizations.-Electrochemical Analysis was performed on the bare and composite MWCNT films in a 3-electrode system. A cavity microelectrode 29 (CME) was used as the working electrode, while saturated Ag/AgCl and Pt mesh were used as the reference and counter electrodes respectively. The CME had a volume of about 1.9 × 10 −6 cm 3 . At least ten different tests were performed for each sample to ensure the stability of the process. The electrolyte for all tests was 1 M sulfuric acid (H 2 SO 4 ) and electrochemical measurements were conducted using an EG&G 273 potentiostat. X-Ray Diffraction (XRD) was conducted using a Philips XRD system with a monochromatized Cu-Kα anode. SEM micrographs were obtained using FEI Quanta FEG 250 large stage environmental SEM.

Results and Discussion
Polyoxometalate mixtures.-Electrochemical behavior.-We have explored the electrochemical properties of MWCNT electrodes modified with a mixed polyoxometalate active layer combining molybdenumcontaining PMo 12 with tungsten-containing PW 12 . The cyclic voltammogram of a CNT electrode modified with an equimolar mixture of PMo 12 and PW 12 is shown in Figure 1 along with the profiles for bare, pure PMo 12 -modified, and pure PW 12 -modified MWCNT for comparison. When two polyoxometalate molecules are combined equally in a mixed active layer, one could reasonably expect the resulting CV profile to be an equally weighted combination of the pure component CVs. A 50-50 weighted combination of the pure PMo 12 and PW 12 CV curves was calculated and plotted (gray line) along with the experimental data in Figure 1. With this simple model as a reference, it is clear that the actual CV of the PMo 12 -PW 12 mixture (red line) does not resemble an equal combination of the pure PMo 12 and PW 12 profiles. This was somewhat surprising, as X-Ray Photoelectron Spectroscopy (XPS) confirmed that Mo and W were deposited on the electrode surface in an equal molar ratio. 27 Nonetheless, the mixed active layer appears to have its own characteristic electrochemical activity. When compared to the model combination, the actual mixed layer CV displays much broader redox waves occurring at different redox peak potentials. This behavior suggests that an effect beyond simple mixing is at play when PMo 12 and PW 12 are combined.
To better understand the reasons for this mixture phenomena and how to leverage it for pseudocapacitive applications, a series of mixed active layer electrodes with varying ratios of PW 12 to PMo 12 were evaluated. The composition of each mixed layer electrode and their redox peak potentials are summarized in Table I with the cyclic voltammograms shown in Figure 2. The CV of the pure PMo 12 layer displayed three reversible redox peaks (labelled I, II, and III in Figure 2), each corresponding to the characteristic 2 electron transfer processes of the PMo 12 molecule. 17 When PW 12 is introduced to the active layer incrementally, a consistent trend appears in the CV profiles; the redox peak current densities all decrease while the peak widths broaden significantly. This broadening of the redox waves becomes more dramatic at higher PW 12 concentrations (above 37.5%) as the first redox peak develops a shoulder at higher potentials and the second and third redox waves begin to merge together. Even more interesting is that while the potential of the first redox peak remains relatively constant for all mixtures, the second and third redox peaks shift consistently to lower potentials in a manner proportional to the amount of PW 12 introduced to the mixture. In fact, when the second and third redox peak potentials are plotted against the amount of tungsten POM in the active layer ( Figure S1), a linear trend develops in which the second redox peak shifts by approximately 35 mV per 10% increment of added PW 12 . The trend for the third redox peak is not quite as linear, however, there is still a progressive negative shift in redox potential for each additional increment of PW 12 in the mixture.  It is clear from these results that none of the mixed active layers display a weighted combination of pure PMo 12 and PW 12 redox features. Rather, each mixture has its own characteristic electrochemical behavior, similar to that of PMo 12 , but with much broader redox waves shifted to lower peak potentials. These dramatic changes in electrochemical properties suggest that when PW 12 is added to the coating solution, it does not just mix with PMo 12 molecules, but instead promotes a chemical change. The electrochemical activity of the Keggin molecule is very sensitive to the addenda atom chemistry; the oxidation and reduction of these addenda atoms is what produces the Keggin redox features. The redox potential of the tungsten atom is much lower than that of molybdenum. Thus, the highly proportional negative shift in the redox peak potentials of the mixture with added PW 12 suggest that the change promoted by the PW 12 addition may affect the addenda atom stoichiometry of the Keggin cluster.
Structural analysis.-X-Ray Diffraction (XRD) was conducted on the PMo 12 -PW 12 mixtures to investigate potential structural changes that result from the combination of these molecules in solution. An equimolar aqueous mixture of PMo 12 -PW 12 (the same type used for the LbL deposition process) was prepared and dried at 70 • C for 8 hours to recover the solid powder. The XRD pattern of this solution-prepared solid mixture was compared to that of a physical (dry) mixture of equimolar PMo 12 and PW 12 powders and is shown in Figure 3  of a mixed addenda POM molecule. In this scenario, illustrated in Figure 4A, the combination of PMo 12 and PW 12 clusters in solution promotes the rearrangement of the constituent elements as a mixed addenda Keggin species combining both tungsten and molybdenum addenda atoms on the same molecule. This process is very sensitive to the stoichiometry of the mixture as the molar ratio of PMo 12 to PW 12 in solution will determine the addenda atom composition of the product molecule. For instance, a solution combining PMo 12 and PW 12 in a 3:1 molar ratio would result in the synthesis of PMo 9 W 3 which has a 3:1 ratio of Mo to W addenda atoms. This would explain why the molar ratio of PW 12 in the coating solution has such a consistent and dramatic effect on the electrochemical behavior of the mixed coating layer.
In order to confirm the proposed mixed addenda synthesis described above, the electrochemical behavior of a 3:1 molar mixture of PMo 12 and PW 12 was compared to that of PMo 9 W 3 O 40 3− (PMo 9 W 3 ) species synthesized by the conventional etherate method. 31 Figure 4B shows the CV of the 3:1 PMo 12 -PW 12 mixed layer-modified MWCNT electrode compared with that of the PMo 9 W 3 -modified electrode. The CV profile of the mixture is almost identical to that of the synthesized mixed addenda species, with both showing three sets of redox waves with very identical peak current densities and potentials. It should be noted that POM molecules especially mixed addenda species, display a complex equilibrium in aqueous solutions. 31 Thus, even when using the conventional etherate synthesis method, the product may not be pure PMo 9 W 3 , but rather a complex mixture of different mixed addenda species. Nevertheless, the comparison in Figure 4B shows that the simple mixing approach yields the same complex mixture as the conventional mixed-addenda POM synthesis method. These are not just random mixtures; increasing the proportion of tungsten in the PMo 12 -PW 12 solution caused a consistent and proportional decrease in redox peak potential, most likely caused by the lower reduction potential of the newly substituted tungsten addenda atoms. These results suggest that mixing PMo 12 and PW 12 in different molar ratios results in varying amounts of addenda atom substitution and a product that is very similar to that prepared by the conventional standard method for mixed addenda synthesis. Thus, with this approach, tuning POM redox behavior through addenda atom substitution becomes a simple and efficient process. This ability to easily make predictable and significant adjustments to POM redox properties is invaluable when designing electrodes to achieve desired pseudocapacitive characteristics.

Multi-layer Polyoxometalate Electrodes
Surface engineering with Mo based polyoxometalates.-POM mixtures can be used in conjunction with multi-layer modification to combine the effects of multiple POMs for increased charge storage and a more ideally shaped CV profile. To be effective, this surface engineering approach requires POM chemistries that show different yet complementary redox features. For instance, we have previously reported the synthesis of GeMo 12 O 40 4-(GeMo 12 ), a custom POM chemistry that shows charge storage complementary to that of the commercially available SiMo 12 O 40 4-(SiMo 12 ) molecule. To take advantage of this complementary behavior, GeMo 12 can be combined with SiMo 12 in an equimolar mixed coating layer. The resulting CV for the GeMo 12 -SiMo 12 mixed layer-modified CNT is shown in Figure 5A along with the pure component CVs for comparison. Since GeMo 12 and SiMo 12 both contain molybdenum heteroatoms, there is no possibility for mixed addenda synthesis so the mixed layer CV appears as a weighted combination of GeMo 12 and SiMo 12 profiles. Combining the overlapping GeMo 12 and SiMo 12 redox peaks makes for a broader mixed layer profile. However, the GeMo 12 -SiMo 12 CV still does not have an ideal rectangular shape and there are clear gaps in the charge storage centered around +0.15 and -0.10 V.
To improve upon this performance, an additional POM layer can be superimposed in order to add more overlapping redox features to the profile. The task becomes more difficult as this additional POM chemistry must show redox features complementary to those of both GeMo 12 and SiMo 12 . Figure 5B shows the CV of the GeMo 12 -SiMo 12 electrode overlaid with that of PMo 12 , another molybdenum based molecule. Our previous results have shown that the CVs of multi-layer POM electrodes demonstrate an additive combination of constituent layers. 26 To predict the CV shape of a dual-layer electrode with a PMo layer superimposed on a GeMo 12 -SiMo 12 layer, a simple model showing the direct addition of the GeMo 12 -SiMo 12 and PMo 12 CV curves is plotted along with the experimental data in Figure 5B (gray line). The charge storage of these constituent layers is not complementary; rather the redox peaks of PMo 12 almost directly overlay those of the GeMo 12 -SiMo 12 electrode. The result is a model of the dual layer CV that shows enhanced capacitance, but retains a shape that is no more rectangular than that of the single layer, still showing prominent gaps in charge storage at +0.15 and -0.1 V. The redox behavior of PMo 12 is too similar to that of the other molybdenum based POMs. There is clearly a limit to the amount the POM redox properties can be tuned without changing the addenda atom. However, this limitation may be overcome by introducing the tungsten addenda atom via simple solution-based mixed addenda synthesis.  allows for more significant adjustments to the electrochemistry of the molecule. With the mixing approach these adjustments can be made in a simple yet precise manner, enabling the design of more effective molecular coatings. For instance, by mixing PW 12 into the PMo 12 coating solution and promoting addenda atom substitution, the redox peaks of PMo 12 can be easily adjusted to smooth the peaks and fill in the charge storage vacancies demonstrated in the model shown in Figure 5B. Figure 6 shows the CV of the GeMo 12 -SiMo 12 electrode overlaid with that of the 3:1 PMo 12 -PW 12 electrode. As demonstrated, this PMo 12 -PW 12 mixture results in addenda atom substitution, which shifts the second and third redox features of PMo 12 to lower potentials, creating a series of almost perfectly overlapping redox peaks when overlaid with the GeMo 12 -SiMo 12 CV. As a result, the predicted dual layer CV model (gray line in Figure 6) shows no sharp peaks or gaps, but rather a broad profile with charge storage that is very well distributed throughout the voltage window. When the actual dual layer GeMo 12 -SiMo 12 (1:1)//PMo 12 -PW 12 (3:1) electrode was fabricated through LbL deposition, the resulting CV (black line in Figure 6) was quite similar to that of the model in terms of both current density and shape. The dual layer profile demonstrates relatively constant charge and discharge currents allowing for a near rectangular CV that mimics the ideal shape of benchmark pseudocapacitive materials such as RuO 2 .
The capacitance values of the bare, single layer-modified, and dual layer-modified electrodes as a function of scan rate are summarized in Figure 6B. At 50 mV s −1 , the bare MWCNT electrodes have a volumetric capacitance of 16.4 F cm −3 . With just a single POM active layer capacitance can be increased five to six fold, as demonstrated by the capacitance of 84.8 and 101.2 F cm −3 of the mixed PMo 12 -PW 12 (3:1) and mixed PMo 12 -GeMo 12 (1:1) electrodes, respectively. After dual layer modification, not only does the electrode show a more ideally shaped CV profile, but it also demonstrates further enhanced charge storage with a capacitance of 181.2 F cm −3 , over eleven times larger than the EDLC capacitance of the bare CNT. As the majority of this capacitance is achieved over a relatively modest potential window of 0.7 V, the POM composites may be best suited to asymmetric device configurations. There is also the potential for exploring new POM chemistries with more positive redox potentials in an effort to extend the charge storage window. The methodology presented above is a promising approach for leveraging POMs to engineer the surface of carbon materials for desired performance.
In addition to large capacitance, the POM-modified electrodes also demonstrate excellent high rate performance. Even at 1 V/s the dual layer electrode maintains a capacitance of 141.2 F cm −3 , close to 78% of its low-rate value. This type of high rate capability, rare for pseudocapacitive electrodes, results from the fast electron transfer kinetics of the POM cluster. The good overall conductivity of the electrodes may also be due to the intimate contact between the carbon substrate and the POM active layer. Figure 7 shows the SEM images of bare, single layer PMo 9 W 3 -modified, and dual layer GeMo 12 -SiMo 12 (1:1)//PMo 12 -PW 12 (3:1)-modified MWCNT. The coated electrodes still maintain the nanotube morphology, but show an obvious increase in diameter from 14 nm in the bare condition to 19 and 24 nm after single and dual layer POM modification, respectively. This implies an approximate thickness of 5 nm for the PDDA-POM bilayer, which appears to show relatively uniform surface coverage. Thus, the dramatic improvement in EDLC capacitance mentioned above can be achieved with just one to two POM active layers of nanometer thickness. The intimate contact between substrate and POM layer afforded by the LbL technique enables enhancement of capacitance to be achieved without significant sacrifices in electrical conductivity.

Conclusions
When PMo 12 and PW 12 solutions are used to modify MWCNT, the resulting mixed layer electrodes demonstrated their own characteristic redox activity, similar to that of PMo 12 but with broader redox peaks that were shifted to lower potentials. This behavior can be attributed to a chemical change that takes place when PMo 12 and PW 12 are combined in solution which results in the formation of the mixed addenda PMo 12-x W x molecules. For instance, a mixed solution of PMo 12 and PW 12 in a 3:1 molar ratio results in the synthesis of PMo 9 W 3 which has a 3:1 ratio of Mo to W addenda atoms. This simple mixing approach may not result in complete conversion to PMo 9 W 3 ; however the mixed addenda product is very similar that achieved by the current literature standard method for mixed addenda synthesis. The detailed mechanism behind this addenda atom substitution and it's applicability to alternative POM chemistries will be explored in future work.
The electrochemical behavior of the PMo 12 -PW 12 mixtures demonstrated that the addenda atom substitution causes the shifting of redox peaks. As more W is incorporated into the molecule, its second and third redox peaks shift proportionally to lower potentials. This precise control over POM redox behavior enables a more effective surface engineering methodology. New mixed addenda chemistries demonstrating highly tunable redox features can be quickly and easily synthesized via the simple mixing approach. Basic models can then be used to screen these new chemistries to determine the POM combinations that show the most complementary redox behavior and the most promising capacitive performance.
The multi-layer approach can then be employed to modify a carbon substrate with the developed nanometer scale molecular coatings which incorporate the optimal POM combination. This methodology was used to fabricate dual layer electrodes which superimposed a mixed 3:1 PMo 12 -PW 12 layer on a mixed 1:1 GeMo 12 -SiMo 12 layer. The dual layer electrode had a capacitance over eleven times larger than the EDLC, while demonstrating a close to rectangular CV shape due to the overlapping redox features of the POM combination. The POM composite also showed excellent performance at scan rates up to 1 V/s, owing to the highly reversible redox reactions of the POM molecule and their intimate contact with the carbon substrate. These results demonstrate that POM surface engineering, streamlined by the novel mixed addenda synthesis, is a promising strategy for designing molecular coatings to achieve desired pseudocapacitive characteristics.