Stabilization of cobalt-polyoxometalate over poly(ionic liquid

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Stabilization of cobalt-polyoxometalate over poly(ionic liquid) composite for efficient electrocatalytic water oxidation Subhasis Das Adhikary, Aarti Tiwari, Tharamani C. Nagaiah, and Debaprasad Mandal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12592 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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ACS Applied Materials & Interfaces

Stabilization of Cobalt-Polyoxometalate over Poly(ionic liquid) Composite for Efficient Electrocatalytic Water Oxidation Subhasis D. Adhikary, Aarti Tiwari, Tharamani C. Nagaiah* and Debaprasad Mandal*

Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India

KEYWORDS: Electrocatalysis • oxygen evolution reaction • polyoxometalate • ionic polymer • alkaline media

ABSTRACT: The key to unlock a renewable, clean and energy dense hydrogen fuel lies in designing an efficient oxygen evolving catalyst exhibiting high activity, stability and cost-effectiveness. This report addresses an improved activity towards oxygen evolution by a composite of cobalt-polyoxometalate [Co4(H2O)2(PW9O34)2]10-, CoPOM) and an ionic

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polymer, poly(vinyl butyl imidazolium) (PVIM) in highly alkaline media. PVIM provides a stable platform for CoPOM and acts as conductive linker between CoPOM and the electrode surface forming a concrete solid composite which balances the multi-negative charge of CoPOM synergistically. This improved stability and conductivity of CoPOM by PVIM in the PVIM-CoPOM composite performs remarkable electrocatalytic water oxidation with a very low over potential of 0.20 V and a very high current density of 250 mA/cm2 (at 1.75 V vs. RHE) with TOF of 52.8 s-1 in 1M NaOH.

INTRODUCTION Economical hydrogen production to address the increased demand for developing sustainable energy system is one of the most challenging aspects. Splitting of water into hydrogen and oxygen would be the most efficient process. The electrocatalytic water oxidation (EWO) process is most viable wherein hydrogen evolution reaction (HER) occurs at the cathode and oxygen evolution reaction (OER) at the anode. However, the most critical step of EWO is the oxidative half-cell reaction of oxygen evolution due to


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slow kinetics, large potential (1.23 V) and instability of catalysts.1-3 Mostly, precious metal oxide catalysts, including ruthenium and iridium oxides, have been reported to be amongst the most efficient for water oxidation under both acidic and basic media.4-11 However, high cost, limited resources and instability of these catalyst hinders their largescale implementation.12-13 Earth-abundant 1st-row transition metal oxide catalysts have also been explored but are considered to be of lower electrocatalytic activity for water oxidation with moderate overpotential.14-15 Contrary to this a few catalysts such as NiyFe116-19 yOx,

Mn3O4,20-23 bimetallic phosphides24 and CoxOy 25-28 are known to exhibit high

activity for water oxidation while retaining their original structure. Cobalt-based OER catalysts in particular, are attractive due to their high activity, earth abundance, and good stability in alkaline conditions.27-30 Recently, polyoxometalates (POM) have shown its potential ability towards energy storage, electrocatalysis and water oxidation due to their multi-electron redox properties without much structural change and high stability in harsh conditions.31-36 A tetracobalt sandwiched POM [Co4(H2O)2(PW9O34)2]10- (CoPOM) has been reported as highly efficient catalyst for water oxidation.37 The activity of CoPOM was found to increase with increase


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in pH of the solution and attained one of the fastest OER at pH 8 with a TOF of 5 s-1.37 However, the CoPOM is not stable at above pH 838 and stabilizing the CoPOM at higher pH is one of the biggest challenge. Despite extensive research on transition metal oxide catalyst at highly alkaline media, to the best of our knowledge no literature reported on molecular catalyst towards OER at pH 8 or higher. In addition, the poor conductivity of POMs hinder the electrocatalytic water oxidation and the CoPOM was shown to have an OER at a very high overpotential of 690 mV in pH 7.39 A few literatures have been reported on enhancement of the conductivity thereby activity of polyoxometalate molecular catalysts using various carbon supports like graphene, multi-walled carbon nanotubes and mesoporous carbon however, the OER activity have been reported only under neutral pH.40-42 So far, CoPOM over NCNT and RuPOM on graphene are the best known molecular catalyst reported with an overpotential of 0.37 V and 0.35 V respectively in pH 7.40, 43 Therefore, in the present paper we aim to enhance OER activity by improving the conductivity and simultaneously the stability of CoPOM under highly alkaline condition.


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Recently, we have shown that the ionic polymer are very effective towards the enhanced conductivity of polyoxometalates in electrocatalysis.44 Here, we report an efficient electrocatalytic water oxidation using a composite of CoPOM with PVIM [poly(vinyl butyl imidazolium)] at pH 14. The ionic polymer, PVIM balances the multi-negative charge of CoPOM and provides a better electronic conductivity by shuttling the electrons at the electrode electrolyte interface.

EXPERIMENTAL SECTION Catalyst Preparation: Initially, K10(CoPOM) was synthesized using reported45 procedure followed by cation exchange with PVIM to yield a concrete white solid of the PVIMCoPOM composite (detailed in SI). This composite was subsequently characterized physiochemically and electrochemically towards water oxidation (detailed in SI). Electrochemical Studies: Electrochemical analysis was carried out in a single compartment three-electrode electrochemical cell set-up. The glassy carbon (GC, Ø3 mm) electrode was employed as working electrode hosting the catalyst and Hg/HgO/1 M NaOH as reference electrode (RE) with a Pt-mesh counter electrode (CE). The GC


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working electrode was polished to a mirror finish before analysis using alumina slurry of various grades ranging from 3 to 0.05 µm (PINE instrument, USA) over the Nylon polishing cloth (SM 407052, AKPOLISH). The GC was further ultrasonicated in deionised water for 15 min to remove any alumina particles sticking over the surface following which it was rinsed thoroughly. The CoPOM-PVIM catalyst slurry was prepared by dispersing fine particles of the composite powder (1.25 mg) in a mixture of isopropyl alcohol (IPA, 20 µL) and deionized water (480 µL, 12 MΩ) and further sonicated for 30-40 min. and 20 µL of the obtained slurry (50 µg) was drop-casted over polished GC electrode (Ø3 mm) which served as working electrode (WE). The electrochemical experiments were performed using an Autolab 302N modular potentiostat/ galvanostat run by Nova 1.11 software. Electrocatlytic activity towards OER was performed in deaerated 1 M NaOH electrolyte (pH 14) or in 0.2 M sodium phosphate (NaPi) buffer of pH 8 at a scan rate of 50 mV/s. The hydrodynamic analysis was carried out by utilizing a rotation speed controlling unit (AFMSRCE, Pine Research Instrument Inc., USA) and measurements were performed at various rotation rate from 100 to 1600 rpm at a scan rate of 25 mV/s.


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RESULTS AND DISCUSSION In this study, we have shown that PVIM cationic polymer matrix provides a stable platform for CoPOM to perform remarkable electrocatalytic water oxidation in alkaline media. The PVIM-CoPOM composite shown to have a very low over potential (η) of 0.20 V with a TOF of 52.8 s-1 in 1M NaOH (pH 14) and also exhibits a very high current density of 250 mA/cm2(at 1.75 V vs. RHE). Our approach was to improve the conductivity of CoPOM and enhance its electrocatalytic activity towards OER and stability in 1M NaOH using PVIM polymer matrix.


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Figure 1. (a) Crystal structure of Na4K6[Co4(H2O)2(PW9O34)2] (CoPOM) and counter ions are omitted for clarity, (b) Schematic representation of EWO on PVIM-CoPOM composite, (c) AFM and (d) EFM image of PVIM-CoPOM composite over an area of 2 µm × 2 µm; (e) corresponding cross-sectional amplitude (mV) profile as indicated in (d). The highly resilient tetracobalt polyoxometalate anion, [Co4(H2O)2(PW9O34)2]10(CoPOM) was synthesized and recrystallized under slow evaporation of the aqueous mixture [detailed in supporting information (SI)]. The molecular structure of resulting crystal was analysed by Single Crystal X-ray diffraction measurement and is represented in Figure 1a and Table S1 (SI, CCDC no. 1511657). The composite of CoPOM and PVIM was prepared using slight excess of PVIMBr in order to replace the K ions in K10[CoPOM] resulting in a concrete white solid where PVIM acts as a binder (detailed in SI). The obtained PVIM-CoPOM composite was characterized in depth using FT-IR, powder XRD, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopic (AFM) techniques. FT-IR analysis reveals that all the W-O characteristic stretching frequencies and the peak at 1160 cm-1 corresponding to the C-N stretching and in plane C-H bending vibration of the imidazolium ring of PVIM remains unchanged in PVIM-CoPOM composite compared to K10[CoPOM] and PVIMBr (Figure S3, SI) except a decrease in W-Ot by 6 cm-1 indicating weak interaction with PVIM due to large size of both the ions. The


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morphology by AFM and SEM images (Figure 1c and S4, SI) shows that the composite exhibit granular nature and was further entailed by electrostatic force microscopy (EFM). The EFM image (Figure 1d) shows darker granular region over the bright area when the positively biased tip (5 V) was scanned over the PVIM-CoPOM composite surface indicating that the negatively charged CoPOM is distributed uniformly over PVIM as witnessed from potential profile diagram (Figure 1e). DFT calculations reveal that the LUMO orbitals in [CoPOM]10-; [Me2Imd]4[CoPOM]2- (1,3 dimethyl imidazolium cation) and [PVIM][CoPOM]2- are primarily the d-orbitals of four Co centers with some mixing from two W centers as depicted in Figure S5 (SI). On the other hand, the HOMO and HOMO-1 has contribution from four Co4Ox centers and PO4 orbitals. Even after introduction of [Me2Imd]4 to CoPOM the orbital contributions in the respective HOMO and LUMO remained almost same (Figure S6, SI) and are rather involved in balancing the charge of [CoPOM]10- to facilitate the electron transfer.


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Figure. 2. a) Cyclic voltammogram of PVIM, CoPOM and PVIM-CoPOM composite in 1M NaOH (pH 14) at a sweep rate of 50 mV/s, b) Chronopotentiometric curves (E-t) of PVIMCoPOM composite in 1M NaOH (pH 14) at 10 mA/cm2, c) PVIM-CoPOM composite at pH 14 and pH 8 electrolyte (inset, zoomed for pH 8), d) Nyquist plot for PVIM-CoPOM composite in 5 mM K4[Fe(CN)6].3H2O at pH 14 and 8 (at 0.35 V and 0.30 V) respectively (inset, Zoomed for pH 14), CE: Pt mesh; RE: Hg/HgO/1M NaOH.


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The electrocatalytic activity of PVIM-CoPOM composite towards OER was studied by cyclic voltammetry (CV), rotating disc electrode (RDE), rotating ring disc electrode (RRDE), chronoamperometric, and electrochemical impedance spectroscopic (EIS) techniques. In addition, the local electrocatalytic activity towards OER was studied by scanning electrochemical microscopic (SECM) technique using ultra microelectrode. Initially, EWO was studied by CV in 1M NaOH (pH 14) electrolyte wherein PVIM-CoPOM composite over glassy carbon (GC) as working electrode (WE), with a Pt mesh and Hg/HgO/1M NaOH as counter and reference electrode respectively. As observed from Figure 2a, well-defined oxidation process occurs at a potential of 1.43 V ( = 0.20 vs. RHE) and reaches to a very high current density (cd, jk) of 250 mA/cm2 (1.75 V vs. RHE), a closer inspection of this CV in the potential region 1.4-1.5 V discloses an overpotential () of 0.26 V at 10 mA/cm2.47 A comparison with commercial RuO2 measured in similar conditions shows an overpotential of 0.29 V at 10 mA/cm2 and a cd (jk) of 250 mA/cm2 in the first cycle, which is almost similar to the composite. But the activity steeply drops down upon subsequent cycling owing to its instability at pH 14 (Figure S7a, SI). On the other hand, CV of K10[CoPOM] alone shows less activity and PVIM did not show any characteristic feature for OER (Figure 2a). Further, the activity of the composite was evaluated by an alternative CV and chronopotentiometric study at a fixed current density of 10 mA/cm2.9 As can be seen in Figure 2b the potential required to reach 10 mA/cm2 is 1.49 V (vs. RHE) and remained constant till the end of the measurement (24 h) demonstrating that the over potential


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required to reach 10 mA/cm2 is 0.26 V which is consistent with the CV studies. It is noteworthy to mention that, the enhanced OER activity was observed both in terms of onset potential and current density as revealed from CV (Figure S7b, SI) taken periodically during chronopotentiometric measurement (Figure 2b). The activity of the composite is significantly better than the reported benchmarking transition metal oxide OER catalysts under basic media.4, 8, 17-19, 43 However, there is no report on molecular catalyst in a similar condition. CoPOM is known to be an efficient catalyst for chemical water oxidation at pH 8 in presence of Ru(bpy)3, hence, CV measurements were performed at pH 8 (0.2M NaPi).37 Interestingly, activity of the composite at pH 8 (1.5 V vs. RHE) is also better than the reported

molecular

catalyst

RuPOM/graphene40

and

CoPOM/CNT41

whose

overpotentials are 0.35 and 0.37 V respectively at pH 7. However, the activity is very less compared to pH 14 both in terms of onset potential and a current density which is only 8.66 mA/cm2 (Figure 2c). The differential activity of PVIM-CoPOM composite in this two pH could be due to the kinetics of the interfacial charge transfer process and was studied by EIS in both pH 14 and 8 electrolyte towards OER. As evident from Figure 2d (Figure S9, Table S2, SI), both Rs (solution resistance) and Rp (polarization resistance) are lower at pH 14 electrolyte and the Rct (charge transfer resistance) is 20 times lower (ca. 11 Ω) compared to pH 8 (ca. 222 Ω). This higher Rct at pH 8 indicates prevalence of sluggish kinetics prior to mass transport limitation due to resistance at electrode-electrolyte


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interface. This emphasizes the fact that the catalyst exhibits faster kinetics towards OER at pH 14 due to facilitated electron transport at the catalyst surface. The quantitative estimation of oxygen evolution was performed using gas chromatography by collecting O2 at a constant potential of 1.75 V (vs. RHE) which yielded a TOF of 52.8 s-1 (detailed in SI). The measured TOF is much higher compared to the Hill’s Na10[CoPOM] molecular catalyst which is one of the fastest homogeneous WOCs catalyst (pH 8) with a TOF of 5 s-1 obtained using [Ru(bpy)3]3+ as the redox mediator.37 The key difference between these are only the pH of the medium. More importantly, PVIM-CoPOM composite shows higher TOF (20.7 s-1 at η = 300 mV; Figure S11c, SI) even compared to one of the fastest known OER electrocatalyst in basic media Ni0.9Fe0.1Ox has a TOF of 2.8 s-1.16 The mass activity of PVIM-CoPOM is found to be 1355 A g-1 at η = 300 mV which is higher than the Ni0.9Fe0.1Ox (1065 A g-1) and related oxide materials like ball-milled BSCF nanoparticles (500 A g-1),15 CoOOH nanosheets (66.6 A g-1),45 IrO2 (~7 A g-1)4 and RuO2 (~10 A g-1).8 These findings demonstrate that the PVIMCoPOM composite is electrochemically highly active towards OER. It is well known that larger the electrochemical surface area (ECSA) higher will be the electrocatalytic activity towards OER. The ECSA was calculated by double layer pseudo-capacitance measurement and was found to be 2 cm-2 (Figure S10, SI).


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Figure. 3. a) Chronoamperometric measurement in 1M NaOH, the WE potential was increased step wise after every 2 min from 0 to 0.7 V (vs. Hg/HgO), Inset: absolute current difference vs. applied potential and b) RRDE analysis for oxygen evolution at disk and simultaneous collected oxygen reduction at the ring (0.33 V vs. RHE)) for 200 cycles at 1000 rpm and a sweep rate of 25 mV/s, CE: Pt mesh; RE: Hg/HgO/1M NaOH.

In order to intricately study the OER activity, chronoamperometric measurements were performed by applying a series of incremental potential pulses, each for 2 min and the corresponding current density (jk) were recorded (Figure 3a). The plot of an absolute jk

vs. applied potential (Figure 3a, inset) shows a rapid increase in current density at a potential of 1.43 V (vs. RHE) at pH 14 indicating that the oxygen evolution starts at this potential ( = 0.20 V vs. RHE). Further, the kinetics of OER was studied using RDE and


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RRDE measurements under hydrodynamic condition. The polarization curves obtained from RDE data reveals a steep increase in disk current (jd) at 1.45 V (vs. RHE) due to oxygen evolution. Simultaneously, the evolved O2 during OER was reduced at Pt-ring (at 0.33 V vs. RHE) resulting in a ring current density (jr) of 19.3 mA/cm2 and 1.4 mA/cm2 for pH 14 and 8 (Figure S11a, S12a, SI) respectively. This 15-fold increase in ring current density signifies that PVIM-CoPOM composite is highly active for OER at pH 14. The Tafel slope (Figure S11a, SI; inset) extracted from RDE data was found to be 30.88 mV/dec at pH 14 (115.60 mV/dec at pH 8, Figure S12b, SI). In addition, the rate was determined from RDE data (Figure S13, SI) by evaluating the kinetic current density (ik) [𝑖𝑘 = (𝑖𝑙𝑖𝑚. ―𝑖)/(𝑖𝑙𝑖𝑚. × 𝑖)] followed by rate constant [𝑖𝑘 = 𝑛𝐹𝐴𝑘𝑐∞] (Detailed in SI).46 The rate constant of the composite at pH 14 was found to be 0.00108 cm/s (at 1.75 V; Figure S11b, SI) indicating a fast kinetics of oxygen evolution reaction. More importantly, negligible changes in the ring current even after 200 cycles under hydrodynamic condition (Figure 3b) demonstrates stable performance towards oxygen evolution. This was additionally supported from sequential CV cycling (1000 cycles, Figure S8b, SI), where in a slight decrease in current density was observed even under 1000 rpm rotation.


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Figure. 4. (a) Schematic representation of SG-TC mode where catalyst-coated GC plate evolves oxygen which is detected at the Pt-microelectrode (Ø 10 µm) by reducing oxygen back to water, (b) 3D SECM image of PVIM-CoPOM in 1 M NaOH at Es = 1.76 V (vs. RHE) and Etip = 0.72 V (vs. RHE) and (c) CV of post OER electrolyte of PVIM-CoPOM, K10[CoPOM], PVIMBr and Co(OH)2 added in 1 M NaOH at a sweep rate of 50 mV/s. CE: Pt wire; RE: Hg/HgO/1M NaOH.


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In addition, we have also evaluated the local catalytic activity of the PVIM-CoPOM composite towards OER by scanning electrochemical microscopy (SECM) using ultra microelectrode. The visualization of the OER activity was performed by sample generation tip collection (SG-TC) mode of SECM wherein PVIM-CoPOM composite over glassy carbon plate as sample (WE1) and Pt-microelectrode (Ø 10 µm, WE2) as tip. The SG-TC mode allows the evolution of oxygen at the composite and the reduction of evolved O2 was collected at the Pt-tip in a single scan (Figure 4a). SECM experiments were performed in 1M NaOH where in, the sample was polarized at a potential of 1.75 V (vs. RHE) and a Pt-tip was kept at 0.72 V (vs. RHE) in order to invoke oxygen reduction at a sample-to-tip distance of only 15 µm to efficiently detect the evolved oxygen. When tip passes over the active composite spot, where the electrolyte was rich in oxygen (since oxygen evolved during OER was electrochemically reduced at the tip) high reduction current (-0.7 nA, Figure 4b) was observed, while less current was observed over the unmodified region. The colour changes from red through yellow, green to dark blue with increase in the reduction current demonstrating the higher activity towards OER at pH 14. The uniform dip in current at the tip across the composite with a high reduction current indicates that the active sites of the PVIM-CoPOM composite are uniformly distributed in the catalyst spot. Besides activity, the catalyst is very stable in 1M NaOH witnessed from the measurement performed in a 1500 µm × 1500 µm area, at a 20 µm/s scan speed, which consumes roughly 24 h per measurement and the analysis was repeated thrice, but still the catalyst spot was intact and active.


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These findings demonstrate an outstanding OER activity of the PVIM-CoPOM composite in an alkaline media (pH 14), this could be ascribed to synergistic effect of PVIM with CoPOM wherein PVIM, the cationic polymer electrostatically attracts the OH- ions and facilitates oxidation at CoPOM center. Further, it ensures the stability of CoPOM and acts as a conductive matrix as well as binder which facilitate the shuttling of the electron at the electrode-electrolyte interface. To understand the enhanced activity towards OER, whether is due to the PVIM-CoPOM composite or any cobalt species that could leach out in the electrolyte during oxygen evolution, in-depth analysis of the post water oxidation electrolyte (after 500 cycles of OER+15 h of polarization at 1.75 V) was evaluated by fast scanning electrochemical technique using carbon fiber ultra-microelectrode (Ø7 µm) as working electrode and Microwave Plasma Atomic Emission Spectroscopy (MPAES) analysis. The CV measurements of the post water oxidation electrolyte (Figure S15, detailed in SI) shows no redox peak in the entire potential range (0 to 2.4 V vs. RHE; Figure S15b, SI) indicating no detectable active catalyst (cobalt species) present in the post water oxidation electrolyte (1M NaOH). In addition, several control experiments were performed by purposefully adding K10[CoPOM] and Co(OH)2 into fresh 1 M NaOH separately and the obtained CV clearly shows a well-defined redox behavior due to the presence of added cobalt-species in the electrolyte (Figure S15e and 15f, SI). Besides, PVIMBr (in 1M NaOH) lacks any redox response as depicted in Figure 4c.


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In addition, to confirm the role of PVIM in holding cobalt species in 1M NaOH under OER condition a control experiments were performed with Co(OH)2-PVIM composite (replacing CoPOM with Co(OH)2) and evaluated OER activity in similar condition (1M NaOH). The obtained CV reveals that Co(OH)2-PVIM composite requires 50 cycles for activation towards OER and the activity was found to degrade rapidly after 100 cycles (Figure S16a) whereas it was retained for PVIM-CoPOM composite (Figure S15a). The analysis of the post water oxidation electrolyte for Co(OH)2-PVIM when tested by ultramicroelectrode did not result in any redox peak corresponding to cobalt species in an electrolyte demonstrating that PVIM acts as a versatile binder holding both the CoPOM and Co(OH)2. These control studies clearly signifies that the observed OER activity in case of PVIMCoPOM composite is solely due to the composite and the active catalyst does not leach out in 1M NaOH. This was further supported by MPAES analysis where no cobalt species were detected in the post water oxidation electrolyte (calibration range of 1 to 40 ppm and a calibration correlation coefficient limit=0.999). However, it is still unclear how the PVIM is activating and stabilizing the active catalyst, a plausible explanation could be the abstraction of C2-proton of [PVIM]+ by OH- and neutralizing the OH- attack on CoPOM ion-pair of [PVIM-OH] inside the solid [PVIMCoPOM] composite matrix. The 1H NMR of [PVIM]Br shows an instant abstraction of C2-H in 1M NaOH (Figure S19, SI). Further the counter cation PVIM in PVIM-CoPOM was replaced with tetrabutyl ammonium (TBA) and the OER activity was analysed to ensure the role of PVIM as a support for enhanced conductivity and stability in 1M NaOH. Initially, the activity of TBACoPOM was comparable to PVIM-CoPOM composite however, with subsequent cycles a drastic decrease in the OER activity was observed in terms of both onset potential and current density (Figure S16C, SI). This control experiments demonstrate that although both the TBA-CoPOM and PVIM-CoPOM composites shows good OER activity, but TBA can’t stabilize the CoPOM in longer run signifies the importance of imidazolium cationic polymer.


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The stability and reusability of the catalyst is highly important to have a sustainable energy system. Therefore, the composite was analysed after 1000 cycles of CV measurement. The SEM and AFM (Figure S17, SI) shows neither any changes in the morphology nor any particles on the surface that could be due to the degradation of the PVIM-CoPOM composite which is further supported by EDX analysis where the concentration of Co and W remains same before and after EWO (Table S4, SI). Even, no changes in the νC-N and W-O stretching frequencies were observed in FT-IR spectra (Figure 5a). The XP survey spectra of before and after the EWO shows a strong peak at 284.5 eV which is attributed to C 1s (Figure 5 and Figure S18, SI). The peaks at 32-38, 400, 532 and 780-796 eV are attributed to the presence of W 4f, O 1s, N 1s and Co 2p (+2 oxidation state) respectively. No impurities were observed before or after EWO. The comparison of surface atomic concentration of various elements before and after EWO confirms that the composite is highly stable and the structure is not destructed during EWO.


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Figure 5. (a) FT-IR spectra and deconvoluted XP spectra of (b) Co 2p; and (c) O 1s for the PVIM-CoPOM composite.

CONCLUSIONS

In this study, we have demonstrated that an ionic polymer matrix, PVIM provides a stable platform for CoPOM to perform outstanding electrocatalytic water oxidation with very low overpotential of 0.20 V and a very high current density of 250 mA/cm2 (at 1.75


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V) with TOF of 52.8 s-1 in 1M NaOH. The activity of PVIM-CoPOM catalyst towards the electrochemical water oxidation has been investigated by employing a number of complementary

electrochemical

methods,

including

cyclic

voltammetry,

chronoamperometric, RDE, RRDE, SECM, and GC analysis. The stability of the catalyst has been evidenced from ultrafast microelectrochemistry, MPAES and several control experiments. This conductive polymer matrix acts as a multifunctional material which provides both physical and chemical stability in highly alkaline media and simultaneously accelerated the kinetics. By utilizing this polymer, we have successfully developed a highly sustainable catalyst for OER which are cost effective exhibiting activity.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website. The following files are available free of charge.


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Material synthesis, physical and electrochemical characterization, computational results, TOF calculation and Tables S1-S4.

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT T. C. Nagaiah thanks Department of Science and Technology (DST) for Ramanujan Fellowship (SR/S2/RJN-26/2012). D. Mandal thank Department of Atomic Energy (DAE), India (2013/37C/57/BRNS) and DST India (SB/FT/CS-046/2012); S.D. Adhikary and A. Tiwari thanks IIT Ropar for Fellowship.

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SYNOPSIS

A polycationic matrix assisted molecular catalyst (PVIM-CoPOM) towards efficient electrocatalytic water oxidation.


32

Stabilization of cobalt-polyoxometalate over poly(ionic liquid

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