Sustainable Non-Noble Metal Bifunctional Catalyst ... - ACS Publications

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Sustainable Non-Noble Metal Bifunctional Catalyst for OxygenDepolarized Cathode and Cl2 Evolution in HCl Electrolysis Vikram Singh, Subhasis D. Adhikary, Aarti Tiwari, Debaprasad Mandal,* and Tharamani C. Nagaiah* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab-140001, India S Supporting Information *

ABSTRACT: The composite of polyoxometalate [WZn 3 (H 2 O) 2 (ZnW9O34)2]12− (ZnPOM) with polyvinylidene-butyl-imidazolium cation (PVIM) and oxidized carbon nanotubes (OCNT) as non-noble metal bifunctional catalyst has been studied for oxygen-depolarized cathode (ODC) and Cl2 evolution in HCl electrolysis for the first time. The cyclic voltammetry and rotating disk electrode measurement analysis reveals superior activity of the composite as bifunctional catalyst for ODC and Cl2 evolution. Chronoamperometric experiments show high long-term stability, comparable to the state-of-art catalyst, even under multiple shutdown to open circuit potential. X-ray photoelectron spectroscopic studies after electrolysis (48 h) confirms no degradation of the composite and, hence, appears to be stable. Scanning electrochemical microscopy (SECM) measurements indicate that, even after 72 h of electrolysis, the composite retains high activity, similar to fresh composite.

1. INTRODUCTION

cathode: 2H+ + 2e− → H 2(g )

Chlorine is a key building element for manufacturing important industrial chemicals and engineering materials,1 such as polymers, resins, and elastomers. The requirement of chlorine has risen appreciably in the last few decades, because of its increased demand for the preparation of chlorine-free end materials, such as polyurethanes (PU) and polycarbonates (PC), which are produced using chlorine chemistry as well as chlorinated polymers (e.g., PVC). Generally, ca. 50%1 of the Cl2 employed commercially ends up forming HCl and chloride salts as a byproduct during the course of their manufacturing, particularly those using phosgene and isocyanates as a carbonylating agent, which are key precursors for the production of PU and PC. The increased production of excess HCl as a byproduct cannot be utilized effectively only by employing it in the production of PVC or for other small manufacturing industries. Moreover, many small-scale industries in India and other developing countries simply quench the produced HCl with lime. The option of neutralizing the excess HCl is inadmissible for obvious reasons. Therefore, an intelligent way of valorizing the HCl produced as a byproduct is the recycling strategy for its conversion into high-purity Cl2 in order to make the associated processes sustainable. Chlorine production at present is primarily based on HCl electrolysis, which is now a substantially imperative methodology that involves the generation of hydrogen at the cathode (E0 = 0.00 V) and chlorine at anode (E0 = 1.36 V) with an overall reaction potential of −1.36 V.2,3 © 2017 American Chemical Society

E = 0.00 V vs NHE

(1)

E = 1.36 V vs NHE

(2)

anode: 2Cl− → Cl 2(g ) + 2e−

overall: 2HCl → H 2(g ) + Cl 2(g )

E = −1.36 V vs NHE

(3)

Under normal conditions, HCl electrolysis yields metric tons of chlorine with a power consumption of ∼1500 kWh.4 The considerable high energy consumption (accounting for ∼50% of the production costs), along with the safety concerns arising from the production of H2 gas, represent serious drawbacks for this electrolytic process. The critical problem associated with HCl electrolysis is observed during uncontrolled shutdown of the process, wherein accretion of H2 and Cl2 gas occurs in the cathodic compartment, since all the membranes have small penetrability to both chlorine (Cl2) and chloride (Cl−) ions, this creates serious safety problems and also leads to sluggish kinetics of the overall reaction3,5,6 resulting in poor energy utilization in HCl electrolysis. These aspects become even more alarming in relation to the present energy crisis and raises concerns about the carbon footprint. An alternative feasible solution is the replacement of hydrogen production at the cathode (eq 1) by an oxygenconsuming cathode known as oxygen depolarized cathode Received: January 24, 2017 Revised: April 17, 2017 Published: April 17, 2017 4253

DOI: 10.1021/acs.chemmater.7b00269 Chem. Mater. 2017, 29, 4253−4264

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Chemistry of Materials (ODC) and retaining Cl2 evolution at the anode. The formal potential of oxygen reduction under standard conditions is 1.23 V (eq 4),4 which results in a decrease in the overall cell potential to −0.13 V (eq 5). cathode: 4H+ + O2 (g ) + 4e− → 2H 2O(l)

E = 1.23 V vs NHE

(4)

overall: 4HCl + O2 (g ) → 2Cl 2(g ) + 2H 2O(l)

E = −0.13 V vs NHE (5)

Therefore, successful implementation of the ODCs substantially reduces the overall cell potential and ultimately results in an energy savings of ∼30%.4 Platinum has been the most effective catalyst so far for oxygen reduction reaction, but catalyst deactivation, poor stability, and dissolution of the catalyst at the cathode, even in a low concentration of Cl− ions, are major concerns.4 Only a few reports are available that also focus on noble metal-based catalysts, such as RhxSy,2,7 Pt−Ag,4,8 and RhxSy on CNTs.5 These catalysts exhibit high stability but reduced activity toward oxygen reduction, compared to Pt catalysts. In the current technology by ThyssenKrupp Uhde on HCl electrolysis, RuO2/ TiO2 is the state-of-art catalyst for chlorine evolution and RhxSy/C is used as an ODC catalyst.9 However, commercializing such catalysts in the mainstream market remains challenging, because of their high expense.10 Hence, there is great demand for the development of sustainable catalysts from earth-abundant non-noble metals that are eco-friendly, costeffective, but stable toward HCl electrolysis and exhibit heightened activity for being an oxygen depolarized cathode catalyst. This has eventually become an appealing challenge. Recently, polyoxometalates (POM) have shown tremendous potential application in water oxidation,11 electrocatalysis12,13 and energy storage14 applications, because of their multielectron redox activity and high stability under harsh conditions.15,16 However, POMs are naive contenders toward HCl electrolysis, which have never been explored so far. But, the electron transfer between poorly conductive POMs and external circuit is a challenging issue.17 Thus, to overcome this challenge, in the present work, we have studied [WZn3(H2O)2(ZnW9O34)2]12− (ZnPOM) supported over oxidized carbon nanotubes (OCNTs), which, in turn, enhances the electron transfer during HCl electrolysis. The ZnPOM catalyst (see Figure 1) entails Zn/W sandwiched between two POM fragments (ZnW9O34), wherein both Zn and W metals are non-noble, cost-effective, and eco-friendly. Moreover, the synthesis of this ZnPOM is cost-effective and can be scaled up easily for mass production and has been utilized only in some organic transformations (more precisely alcohol oxidation).18−20 However, both POM (inherently bear high negative charge) and OCNT are negatively charged species leading to poor interaction between them. In order to overcome this lacuna in connectivity, we have introduced a cationic polymer (ionomer), viz, polyvinylidene-butyl-imidazolium (PVIM), as a conductive linker that strongly holds both OCNT and POM while allowing uniform distribution over an OCNT support (Scheme 1). To the best of our knowledge, this composite is the first nonnoble metal bifunctional catalyst to demonstrate HCl electrolysis. Here, we report the development of bifunctional catalyst for ODC (O2 reduction) and Cl2 evolution using

Figure 1. Single-crystal X-ray structure of [WZn 3 (H 2 O) 2 (ZnW9O34)2]12− (ZnPOM).

Scheme 1. Schematic Representation of Interactions between ZnPOM, PVIM, and OCNT

uniformly distributed ZnPOM over OCNTs without aggregate formation using PVIM ionomer as an important breakthrough towards attaining superior activity with unique mechanistic and stability features in HCl electrolysis.

2. RESULTS AND DISCUSSION Sodium-containing ZnPOM was prepared from Na2WO4 and Zn(NO3)2 at pH ∼7.5. The white crystals were obtained from the reaction mixture upon standing for 2 days. The molecular structure of ZnPOM was revealed using single-crystal X-ray diffraction (XRD) measurement. The “Diamond” diagrams of molecular structure, along with selected numbering, are shown in Figure 1. As revealed from single-crystal analysis (Figure 1), the structure of ZnPOM consists of two [ZnW9O34] units with three octahedral Zn atoms and one octahedral W atom sandwiched between two POM units (ZnW9O34), resulting in a Keggin assembly with C3v symmetry. The formula was found to be Na8Zn2[WZn3(H2O)2(ZnW9O34)2]·16H2O, in which two Zn(II) and eight Na are present as counter cations. All the Na and Zn counterions are associated with terminal (Na1, Na2) and bridging (Na3) oxygens of the polyoxoanion, as well as with water molecules of the lattice. Zn2 and W10 atoms are sharing the same sites due to substitutional disorder. The three Zn centers are connected to each other by oxo-bridges. Zn1 at 4254


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Figure 2. HOMO−LUMO molecular orbitals of [WZn3(H2O)2(ZnW9O34)2]12− (ZnPOM).

Figure 3. (a) AFM image of OCNT-PVIM-ZnPOM over an area of 3 μm × 3 μm; (b) electrostatic force microscopy (EFM) image and (c) corresponding amplitude (mV) profile diagram (cross-sectional) from marked regions in panel (b).

stretching frequencies remained unchanged in PVIM-ZnPOM conjugate and the electrostatic interaction does not affect the W−O bond strengths. Furthermore, the strong interaction between OCNT and PVIM-ZnPOM conjugate is clearly witnessed from the shift in carbonyl group stretching frequency. As observed in Figure S1, the carbonyl peak for OCNT appeared at a frequency of 1710 cm−1 but shifted to a higher frequency (1730 cm−1)21 in the OCNT-PVIM-ZnPOM composite. Similarly, νCN (PVIM) also shifted from ∼1155 cm−1 toward higher frequency (1171 cm−1), confirming an interaction between them. 2.1. Density Functional Theory (DFT) Studies. Both [ZnPOM]12− and [Me2Imd]2[ZnPOM]10− (1,3-dimethyl imidazolium cation) were fully optimized in the DFT method using the B3LYP functional, considering the water solvation effect in the CPCM model. The results are in good agreement with the single-crystal X-ray structure, where the difference in bond distances are within the range of 0.01−0.07 Å. Interestingly, it was observed that the lowest unoccupied molecular orbital (LUMO) wave functions in both structures are primarily localized on the W center and has a dz2 character (Figure 2) and has no contribution from any of the three

the center of POM units (ZnW9O34) is in tetrahedral geometry, while all Zn2, Zn3, and W10 of the sandwich and Zn4 and Zn5 of counterions are in octahedral geometry. Furthermore, a PVIM-ZnPOM conjugate was prepared by using a slight excess of PVIMBr in order to replace the Na in Na12[ZnPOM], which resulted in the formation of concrete white powder, wherein PVIM acts as a binder. The obtained PVIM-ZnPOM was physically mixed (grinding) with OCNT to give an OCNT-PVIM-ZnPOM composite [see the Supporting Information (SI)]. The obtained OCNT-PVIM-ZnPOM composite was characterized in detail using Fourier transform infrared (FT-IR) spectroscopy, powder XRD, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM) techniques. As observed from FT-IR studies [see Figure S1 in the SI] the Na12[ZnPOM] shows peaks at 923 and 868 cm−1 corresponding to the stretching frequency of WOt and W−Oc−W (Otterminal oxygen, Oc corner-sharing WO6 octahedral), respectively. The peaks at 730 and 694 cm−1 corresponds to W−Oe− W (Oe edge-sharing octahedral), whereas the peak at 657 cm−1 is attributed to W−Ob−W (Ob represents the metal (Zn)−O sharing or sandwich). Interestingly, all the W−O characteristic 4255


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Figure 4. Cyclic voltammograms of (a) OCNT-PVIM-ZnPOM, PVIM-ZnPOM, and OCNT in 0.4 M HCl electrolyte, and (b) OCNT-PVIMZnPOM in both 0.4 M HCl and 0.2 M H2SO4 electrolyte vs RhxSy/C (30%) and Pt/C (20%) in N2 saturated 0.4 M HCl electrolyte at a scan rate of 10 mV s−1; counter electrode (CE): Pt mesh; reference electrode (RE): Ag/AgCl/3 M KCl.

Figure 5. (a) Nyquist plot of OCNT-PVIM-ZnPOM and PVIM-ZnPOM using 3 mM K3[Fe(CN)6] in 0.1 M KCl electrolyte; (b) chronoamperometric analysis of OCNT-PVIM-ZnPOM at various oxidative potentials of 1.27, 1.57, and 1.77 V and RhxSy/C at 1.77 V; (c) linear sweep voltammograms before and after chronoamperometric measurements at 1.77 V for 500 s at 1000 rpm in O2-saturated 0.4 M HCl electrolyte at a scan rate of 5 mV s−1. CE: Pt mesh; RE: Ag/AgCl/3 M KCl.

OCNT, PVIM, and ZnPOM within the composite, SEM imaging (see Figure S3 in the SI) and AFM imaging (see Figure 3, as well as Figure S4 in the SI) were performed. As observed from Figure 3a, the PVIM-ZnPOM is uniformly distributed over the OCNT surface without any aggregation. In addition, electrostatic force microscopy (EFM) images suggest that, when the positively charged tip was scanned over the OCNTPVIM-ZnPOM surface (Figure 3b), it shows a darker region over the tubes, indicating that the surface is negatively charged, similar to OCNT. Furthermore, as witnessed from potential (amplitude) profile diagram (Figure 3c), the composite is more negative (darker region) than OCNT surface due to the more negatively charged ZnPOM. These findings indicate that PVIM acts as a bridge between OCNT and PVIM-ZnPOM (Scheme 1), resulting in OCNT-PVIM-ZnPOM composite in which

sandwiched Zn centers. This is further indicated from LUMO +1 to LUMO+5, exhibiting similar W and Zn center characteristics. On the other hand, the highest occupied molecular orbital (HOMO) was localized on the oxygen atoms and the HOMO−LUMO gap was 4.134 eV. Even after introducing Me2Imd to ZnPOM, the orbital contributions in the respective HOMO and LUMO remain the same, having the HOMO−LUMO gap of 4.134 eV. Comparison between the above two structures reveals that there is no contribution from the imidazolium (Me2Imd) cation in both HOMO and LUMO (see Figure S2 in the SI). These findings indicate that PVIM cations are only involved in bridging OCNT and ZnPOM and facilitates the electron transfer. 2.2. AFM Analysis. In order to understand the morphology and topographical behavior, as well as the interaction between 4256


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Figure 6. (a) Cyclic voltammogram of OCNT-PVIM-ZnPOM in both N2- and O2-saturated 0.4 M HCl at a scan rate of 10 mV s−1. (b) Linear sweep voltammograms of OCNT-PVIM-ZnPOM, OCNT, and PVIM-ZnPOM at 1000 rpm with the inset showing an enlargement of the onset region in oxygen-saturated 0.4 M HCl electrolyte at a scan rate of 5 mV s−1. CE: Pt mesh; RE: Ag/AgCl/3 M KCl. (c) Koutecky−Levich (K-L) and (d) Tafel plots of the corresponding above-mentioned catalyst extracted from the RDE measurements in panel (b).

However, the cyclic voltammogram of PVIM-ZnPOM at a GCE shows no peak corresponding to Cl− oxidation in aqueous 0.4 M HCl electrolyte. This is attributed to the relatively high charge-transfer resistance of PVIM-ZnPOM (Rct = 2800 Ω, Figure 5a), which is due to very poor transfer of electrons at the electrode/electrolyte interface. Furthermore, experiments were carried out under similar conditions, using noble metal-based catalysts, such as RuO2/ TiO2 (30%), RhxSy/C (30%), and Pt/C (20%). Interestingly, as observed from Figure S5c in the SI) and Figure 4a, the oxidative peak current at 1.77 V is comparable to those obtained for the OCNT-PVIM-ZnPOM composite. This clearly indicates that being non-noble, the OCNT-PVIMZnPOM composite is highly electroactive toward chlorine evolution. This could be due to the synergistic effect resulting from a strong π−π interaction between OCNT and imidazolium cations of PVIM, along with electrostatic interaction between PVIM and ZnPOM, which, in turn, facilitates the electron transfer from ZnPOM to the electrode via PVIM and OCNT (Scheme 1), which are in agreement with FT-IR and EFM studies. In addition, the porous structure of OCNT facilitates fast electron transfer between the electrode/ electrolyte interface, resulting in less charge-transfer resistance, thereby reducing the overall Rct value for the OCNT-PVIMZnPOM composite (Rct = 415 Ω), which is witnessed from its small semicircle behavior in electrochemical impedance spectroscopy (EIS) studies (Figure 5a). 2.4. Stability Studies. To evaluate the stability of the OCNT-PVIM-ZnPOM composite for chlorine evolution under catalytic turnover conditions, electrolysis experiments were performed at a fixed potential of 1.77 V, where the chlorine evolution is maximum. All the measurements were performed

negatively charged ZnPOM is mostly exposed to the outer surface. This interaction is well complimented by FT-IR studies (Figure S1), wherein νCO and νCN show a shift in absorption frequency by 20 cm−1, compared to OCNT and PVIMZnPOM, respectively. 2.3. Electrocatalytic Studies. The electrocatalytic properties of these catalysts were evaluated using cyclic voltammetry (CV) measurements in deaerated aqueous 0.4 M HCl electrolyte at a scan rate of 10 mV s−1 (see Figure 4). As observed from Figure 4a, the cyclic voltammogram of a glassy carbon electrode (GCE) coated with an optimized loading of OCNT-PVIM-ZnPOM composite (see Figure S5a in the SI) shows a well-defined oxidation process when the potential was swept from an open circuit potential to a more anodic potential (positive) with Eox p at 1.77 V and can be attributed to the oxidation of chloride (Cl−) ions to chlorine (Cl2), as indicated by the evolution of chlorine gas. When the potential was swept back cathodically, a small reduction peak (Ered p ) at 1.22 V was observed, which is attributed to the reduction of Cl2 to Cl− ions. The cyclic voltammogram of OCNT in the aqueous 0.4 M HCl electrolyte is similar to those observed at OCNT-PVIMZnPOM but with a smaller oxidative peak current (iox). The OCNT exhibits oxidative peak current of ca. 462 mA cm−2 mg−1, which is lower compared to OCNT-PVIM-ZnPOM (1350 mA cm−2 mg−1). Furthermore, CV analysis of OCNTPVIM-ZnPOM was performed in the Cl− free electrolyte of similar pH using aqueous 0.2 M H2SO4 (see Figure 4b, as well as Figure S5b in the SI). In contrast, no oxidative peak at 1.77 V (black dotted line) was observed in the chloride-free electrolyte, confirming that the peak at 1.77 V corresponds to chlorine evolution. 4257


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Furthermore, RDE measurements were performed to evaluate the kinetic parameters responsible for the dioxygen reduction. The LSV were performed under hydrodynamic condition at a rotation rate of 1000 rpm, wherein the potential spanned a range from 1.0 V to −0.35 V (diffusion-limited region). All the catalysts show activity toward O2 reduction, which is clearly witnessed from the increase in the reduction current, as seen in the polarization curves (Figure 6b) extracted from RDE measurements. However, a shift in the onset potential (where the current start to decrease) and a significant change in reduction current density (jk) are clearly visible. The LSV analysis shown in Figure 6b confirms that O2 reduction for the PVIM-ZnPOM conjugate occurs at an onset potential of ca. 0.30 V at jk = −1.4 mA cm−2 mg−1. Interestingly, OCNT shows more positive onset potential (0.37 V), compared to the PVIMZnPOM conjugate. As expected, the onset potential for the OCNT-PVIM-ZnPOM composite shifted anodically from 0.37 V (OCNT) to 0.47 V. More importantly, the electrocatalytic activity toward O2 reduction is witnessed from its jk in the diffusion-limited region, wherein jk increases from −32.6 mA cm −2 mg−1 (PVIM-ZnPOM) to −81.2 mA cm −2 mg −1 (OCNT) and −86 mA cm−2 mg−1 for the OCNT-PVIMZnPOM composite, which is comparable to the commercial RhxSy/C (ca. −89 mA cm−2 mg−1), as shown in Figure S8d in the SI). As the rotation rate was varied from 100 rpm to 1300 rpm, the ORR current increases (Figure S8) according to the Koutecky−Levich (K-L) equation,

under stirred conditions in order to remove the bubble formation at the electrode surface and to avoid the change in local pH that would occur during long-term experiments. As shown from the i−t curves in Figure 5b, a rapid decrease in current is observed at the beginning of electrolysis (ca. t < 30 s) and later remained constant over time. This could be due to the formation of bubbles on the electrode surface at the early stage of electrolysis, which blocks certain active sites of the composite, resulting in current decay. This phenomenon is evidenced by repeated experiments on the same electrode surface after the removal of bubbles (by rinsing with water) on the surface showing no current decay, compared to the freshly prepared electrode (black line, overlapping with red line in Figure 5b). Furthermore, rotating disk electrode (RDE) measurement after electrolysis confirms that there is no decay in the reduction current and the current is comparable to that observed before electrolysis (Figure 5c). With the objective of quantitative estimation of Cl2, linear sweep voltammetry (LSV) was performed for 500 cycles, wherein the potential spanned a range of 1.27−1.87 V in 0.4 M HCl electrolyte. The amount of Cl2 evolved during electrolysis was determined by iodometric titration with 0.1 mM Na2S2O3. The amount of Cl2 evolved per 50 μg of OCNT-PVIMZnPOM composite was found to be 5.82 mg L−1 or 5.82 ppm (see Table S1 in the SI). Furthermore, SEM, AFM, and EDX measurements were performed for the composite catalyst on GCE surface after 500 cycles of LSV and after chronoamperometric experiments at a fixed potential of 1.77 V. As expected, neither the AFM nor SEM images (Figure S6 in the SI) revealed any deformation that could be produced from the degradation of the PVIM-ZnPOM conjugate. Furthermore, the EDX data confirm that the W and Zn contents are analogous to those found before electrolysis. These results indicate that the composite has good long-term stability in this highly corrosive medium on the time scale of the number of hours and also further confirms that the composite dispersed on GCE are not desorbed from the surface, even under stirred conditions for a number of cycles. Being a non-noble metal, the turnover frequency (TOF) of the OCNT-PVIM-ZnPOM composite for chlorine evolution at a potential of 1.77 V was found to be 3844 s−1, compared to 3330 s−1 for RhxSy/C (30%) (see Figure S7 and Table S2 in the SI). In addition, DFT studies (Figure 2) reveals that the orbital contribution of LUMO and LUMO1, LUMO2, and LUMO3 contains only the W atom, suggesting that, during the HCl oxidation, the electrons would be transferred to the W atom only and other centers remain unaffected. This is further supported from XPS studies. 2.5. Oxygen-Depolarized Cathode (ODC) Catalyst. The catalytic reduction of oxygen was investigated for various catalysts OCNT, PVIM-ZnPOM, and OCNT-PVIM-ZnPOM composite in O2 saturated 0.4 M HCl electrolyte for successful application as ODC material. Preliminary experiments were performed in deaerated 0.4 M HCl electrolyte by bubbling N2 for 30 min to remove the dissolved oxygen. The cyclic voltammogram shown in Figure 6a reveals that there is no peak corresponding to the reduction of oxygen for the composite. However, a steep increase in the irreversible electrocatalytic reduction current was observed at ca. 0.52 V for OCNT-PVIMZnPOM after the electrolyte was purged with O2, indicating that the composite is active toward the oxygen reduction reaction (ORR).

1 1 1 = + i ik id

where i is the measured current, while ik and id are the kinetic and diffusion limiting currents, respectively (i d = 0.62nFAD2/3ν−1/6CO2, where A is the surface area, D the oxygen diffusion coefficient (D = 1.34 × 10−6 cm2 s−1), v the kinematic viscosity (1.1 × 10−2 cm2 s−1), CO2 the bulk concentration of oxygen in the electrolyte (CO2 = 1.61 × 10−6 mol cm−2), and n the number of electrons transferred).22 The number of electrons involved were calculated from the slope of the K-L plots (1/jk vs 1/υ1/2; see Figure 6c) by linearly fitting the data obtained from the polarization curve extracted from RDE measurements. From the slope of the K-L plot at a potential of −0.13 V, the number of electrons that are involved in ORR was determined to be 3.6 for the OCNT-PVIMZnPOM composite, indicating that the oxygen reduction follows a four-electron pathway either by reducing oxygen directly to water or by reducing it to peroxide, which is subsequently reduced to water. However, the number of electrons for OCNT is 2.4. followed by 2 for the PVIMZnPOM conjugate, which suggests that these catalysts facilitate the formation of peroxide during ORR. Furthermore, the enhanced high electrocatalytic activity of the OCNT-PVIMZnPOM composite is witnessed from 2-fold increase in the kinetic current, as observed from the Tafel plots in Figure 6d. The TOF of OCNT-PVIM-ZnPOM for O2 reduction in the diffusion-limited region was found to be 244 s−1 (at −0.32 V), compared to 285 s−1 for RhxSy/C (30%) (at 0.16 V; see Figure S7 and Table S2 in the SI). 2.5.1. Stability Studies Similar to Industrial Conditions. Furthermore, to evaluate the stability of the OCNT-PVIMZnPOM composite as an ODC catalyst, chronoamperometric measurements were performed in 0.4 M HCl electrolyte in a 4258


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Figure 7. (a) Electrocatalytic stability test by chronoamperometric experiments of OCNT-PVIM-ZnPOM, RhxSy/C (30%), and Pt/C (20%) catalyst in O2-saturated 0.4 M HCl electrolyte at a potential of 0.12 V with 10 min of operation and 2 min of interruption to open-circuit potential; and (b) their normalized current versus interruption cycles derived from panel (a). CE: Pt mesh; RE: Ag/AgCl/3 M KCl.

Figure 8. (a) Chronoamperometric measurement in deaerated 0.4 M HCl electrolyte, the WE potential was increased stepwise after every 2 min from 0.32 V to 0.72 V; (b) absolute current difference versus applied potential. CE: Pt mesh; RE: Ag/AgCl/3 M KCl.

flow cell setup. Measurements were performed in Cl2-saturated 0.4 M HCl electrolyte in which Cl2 was generated at an applied potential of 1.2 V using two Pt electrodes in one flow cell and subsequently pumped to another electrochemical flow cell. The second electrochemical cell consisted of four electrodes: GCE coated with OCNT-PVIM-ZnPOM as WE(1) and Pt/C or RhxSy/C as WE(2), Pt mesh as a counter electrode (CE), and Ag/AgCl/3 M KCl as a reference electrode (RE). In order to mimic the conditions similar to industrial HCl electrolysis, such as unwanted cell shutdown, an interruption mode was established using NOVA software, wherein samples (WE1 and WE2) were continuously polarized at an oxygen reduction potential (−0.22 V) for 10 min of operation and 2 min interruption to open circuit potential (OCP) (shutdown) and the resulting currents were measured at both WE1 and WE2 electrodes. As observed from Figure 7a after 10 min of operation, the current reaches a constant value for OCNTPVIM-ZnPOM, while it continues to decrease relatively in the case of the Pt/C or RhxSy/C catalyst. Figure 7b represents a plot of normalized current versus the number of OCP (cell shutdown) extracted from chronomperometric measurements, which indicates that even after 10 shutdown cycles, OCNTPVIM-ZnPOM retained its catalytic activity more than 90% from its original value, while the activity of Pt/C reduces to 45% and the activity of RhxSy/C reduces to 83%. Our composite is exceptionally stable, even under extremely harsh and mechanically dynamic conditions, as witnessed from continuous Cl2 evolution (see Figure S10 in the SI). The Cl2 evolution was observed even after 3 days of exposure of the

catalyst coated on the GCE under continuous rotation at 1000 rpm in the corrosive HCl medium. The long-term activity was further studied for 360 h by chronoamperometric measurement (see Figure S11 in the SI), which shows that our catalyst is profoundly stable and can be considered further. We have also evaluated the activity and stability of the composite in a highly acidic medium (5 M HCl). The OCNTPVIM-ZnPOM composite shows similar electrocatalytic activity and stability, which is comparable to that of RhxSy/C and RuO2 toward Cl2 evolution (see Figures S12a and 12b in the SI). Furthermore, the activity remains the same either in the presence or the absence of oxygen (Figure S12c in the SI). In order to intricately study chlorine evolution, chronoamperometric experiments were performed in the potential range of 0.32−0.72 V in deaerated 0.4 M HCl electrolyte. A sequential increase in potential pulse was applied in a stepwise fashion and the corresponding current was measured with time (Figure 8a). The obtained data in Figure 8a was further processed to obtain absolute current4 (which is defined as the absolute difference between the current measured before the first pulse and at the end of each sequential pulse), as a function of applied potential (Figure 8b).4 From Figure 8b, it is clear that linearity between the absolute current and applied potential was observed until 0.42 V, indicating that the chlorine evolution starts from 0.42 V and a rapid enhanced chlorine generation takes place at higher applied potentials, as depicted by an exponential increase in current. 2.6. Local Catalytic Activity Using RC-SECM. To visualize the local catalytic activity toward the ORR redox 4259


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Figure 9. RC-SECM images of OCNT-PVIM-ZnPOM over GC plate for ORR in 0.4 M HCl electrolyte at various sample potentials of (a) −0.13 V, (b) 0.07 V, and (c) 0.42 V. (d) RC-SECM X-line scan (at Y = 400 μm) of the OCNT-PVIM-ZnPOM spot taken from panels (a), (b), and (c). The Pt tip (10 μm Ø) was polarized at 0.12 V. CE: Pt mesh; RE: Ag/AgCl/3 M KCl.

Figure 10. Deconvoluted XPS spectra of (a) W 4f, (b) O 1s, and (c) C 1s for the OCNT-PVIM-ZnPOM composite.

Furthermore, the local electrocatalytic activity was analyzed at various sample potentials between −0.13 V to 0.42 V. Notably, at all of the potentials, a good contrast between active catalyst spot and unmodified (GC) region was observed, even when the sample was polarized at very high positive potential (0.42 V), where the Cl2 evolution starts (Figure 8b). Furthermore, an optical microscopy photograph of the catalyst spot (Figure S13 in the SI), taken after 48 h of SECM measurement, shows no dissolution or degradation of the composite spot, indicating that composite is highly stable in a corrosive medium and is not desorbed from the surface. The successful visualization of the local catalytic activity (Figure 9) of the composite spot at various potentials indicates that the composite is highly active toward ORR and is not affected by Cl− adsorption. The line scan in Figure 9d represents the quantification of obsolete tip current versus the composite spot at various potentials. 2.7. Post-Electrolysis Characterization. To make HCl electrolysis sustainable under industrial conditions, the longterm stability and reusability of the composite is very essential.

competition mode of scanning electrochemical microscopy (RC-SECM) was performed on the OCNT-PVIM-ZnPOM composite catalyst on a GC plate. SECM measurements were performed by polarizing the SECM tip at a potential of −0.12 V, where a diffusion-controlled reduction of oxygen occurs in 0.4 M HCl electrolyte. The sample was polarized at a potential (−0.13 V), which is sufficiently negative for O2 reduction on the catalyst spot. During scanning, when the tip passes over the active catalyst zone, where the electrolyte was depleted of oxygen (due to electroreduction by the composite), lower tip currents (higher reduction current at spot) were observed, while higher tip currents were observed over the zones that were rich in oxygen. The RC-SECM three-dimensional (3D) image in Figure 9 shows good contrast between the active OCNT-PVIM-ZnPOM composite and the unmodified region (GC). The image colors change from blue-violet through green to red, and the red color indicates smaller tip current and, hence, high ORR activity for the OCNT-PVIM-ZnPOM composite than the darker (blue-violet) region. 4260


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Figure 11. (a) 3D RC-SECM and (b) 2D RC-SECM images of OCNT-PVIM-ZnPOM over a GC plate for ORR after 72 h of continuous sample polarization in 0.4 M HCl electrolyte [at a sample potential of 0.07 V, with the Pt tip (10 μm in diameter) held at 0.12 V]; (c) photomicrograph of the composite spot taken after this SECM measurement (CE: Pt mesh; RE: Ag/AgCl/3 M KCl).

Hence, further characterization of the OCNT-PVIM-ZnPOM composite after constant potential electrolysis (1.77 V) for 48 h and after 500 cycles of LSV is important. As discussed earlier, no deformation was observed in both AFM and SEM images (Figure S6) that could be produced from the degradation of the PVIM-ZnPOM conjugate. Furthermore, EDX data confirm that the W and Zn contents are analogous to that found before the electrolysis (Table S5 in the SI). Furthermore, FT-IR spectra (Figure S14 in the SI) shows only a small shift in νCO (OCNT) and νCN (PVIM) but, no changes were observed with respect to WOt and sandwiched Zn−O−W, indicating that ZnPOM is not affected by Cl− ions, which is in agreement with DFT studies. 2.7.1. XPS Analysis. In addition, any changes in the surface concentration of OCNT-PVIM-ZnPOM composite before and after constant potential electrolysis (48 h) were evaluated by high-resolution XPS. Both XP survey spectra (Figure S15 in the SI) show a strong peak at a binding energy of 284.5 eV, which corresponds to C 1s. The presence of N, O, Zn, and W were confirmed by the N 1s peak at 400.0 eV,23 the O 1s peak at 532 eV, the Zn 2p peak at ∼1020.0 eV,24 and the W 4f peak at 32− 38 eV,25 respectively. Impurities were not detected on the composite surface. The deconvoluted XP spectra for W 4f in Figure 10a show two doublet peaks, 4f7/2 and 4f5/2, which arises due to the spin−orbit coupling and is attributed to W6+. The deconvoluted XP spectrum of O 1s (Figure 10b) results in three peaks26 which originates from OCNT and ZnPOM. The C 1s deconvoluted spectrum shows three peaks originating from OCNT and PVIM (Figure 10c). The deconvoluted N 1s spectrum (Figure S15) exhibits two peaks: one is observed at 398.0 eV and another is observed at 400.0 eV, originating from imidazolium nitrogen of PVIM in the composite. Comparison of surface atomic concentration (Table S4 in the SI) of different elements in OCNT-PVIM-ZnPOM composite before and after electrolysis shows no variation in the concentration of any element which signifies that the composite is highly sustainable in this corrosive medium. 2.7.2. SECM Analysis. Three-dimensional (3D) and twodimensional (2D) RC-SECM images (Figures 11a and 11b) reveal the local catalytic activity of the OCNT-PVIM-ZnPOM composite after 72 h of electrolysis. Surprisingly, a good contrast in the color of these images, similar to that observed before electrolysis, was observed, indicating profound activity toward ORR (ODC) even after 72 h of continuous sample polarization. The examination of the catalyst spot (Figure 11c) on GC, as viewed under an optical microscope after the SECM measurement, reveals that the composite is highly active, is

extremely stable, and is comparable to that observed before electrolysis.

3. CONCLUSIONS The OCNT-PVIM-ZnPOM composite has been explored as a non-noble metal bifunctional catalyst for O2 reduction (ODC) as well as for Cl2 evolution in HCl electrolysis. This composite shows superior electrocatalytic activity toward HCl electrolysis, which is comparable to state-of-art catalyst. The PVIM ionomer in the composite acts as a binder as well as a bridge between OCNT and ZnPOM and facilitates electron transfer, which, in turn, enhances the overall activity and stability toward HCl electrolysis. Chronoamperometric studies show high long-term stability, compared to Pt/C and RhxSy/C under similar to industrial condition even under multiple shutdown to OCP by retaining its activity more than 90% of its original value compared to Pt/C (43%) and RhxSy/C (83%). XPS studies after electrolysis (48 h) confirms no degradation of the composite and is highly stable. Local catalytic activity of the composite was successfully visualized, even after 72 h of continuous electrolysis, using RC-SECM, indicating that the composite is highly sustainable in this corrosive medium. The composite is exceptionally active for a few hundreds of hours and stable under extremely harsh and mechanically dynamic conditions. The OCNT-PVIM-ZnPOM composites, being non-noble metals, pose themselves as a promising candidate for HCl electrolysis. 4. EXPERIMENTAL SECTION 4.1. Materials. All the reagents were used as received. Na2WO4· 2H2O (98%), Zn(NO3)2·6H2O (98%) α,α′-azoisobutyronitrile (98%), 1-vinylimidazole (99%), HCl, H2SO4 (Alfa Aesar); Pt/C (20%), E‑Tek; RhxSy/C (30%), De Nora; RuO2 (Sigma-Aldrich). Aqueous solutions were prepared using deionized water from Millipore system (>12 MΩ cm−1). 4.2. Synthesis of Poly(1-vinylimidazolium bromide) (PVIMBr). 4.2.1. Preparation of Poly(1-vinylimidazole) (1). A Schlenk tube was charged with 1-vinylimidazole (0.941 g, 10.00 mmol), AIBN (azobis(isobutyronitrile); 1.0 wt %, 0.013 g) and 4.0 mL of dry toluene. The mixture was degassed in three freeze−thaw cycles under vacuum, argon was purged for 30 min in order to remove oxygen. Reaction mixture was heated at 70 °C for 24 h resulting in a solid precipitate. The obtained solid was purified by precipitation in diethyl ether and dried under vacuum to yield 1 as white powder (0.750 g, 80%). The synthesized polymer is soluble in water and methanol but insoluble in chloroform, THF, and toluene. 1H NMR (D2O, δ ppm): 7.06−6.64 (broad, 3H, imidazole ring proton), 3.74− 2.57 (broad, 1H), 2.12−1.9 (broad, 2H). 4261


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Chemistry of Materials 4.2.2. Synthesis of Poly(1-vinylbutyl-imidazolium bromide) (2). A Schlenk tube fitted with condenser was charged with poly(vinyl imidazole) 1 (0.339 g, 3.62 mmol), n-butyl bromide (0.543 g, 3.98 mmol) and dry methanol. The reaction mixture was heated at 60 °C for 48 h and poured in acetone to give precipitate of 2 (0.772 g, 92.3%). 1H NMR (DMSO-d6, δ ppm): 9.61 (broad, 1H, NCHN), 7.83−7.73 (broad, 2H, NCHCHN), 4.12−3.84 (broad, 4H), 2.51− 2.49 (broad, 2H), 1.84 (broad, 2H), 1.33 (broad, 2H), 0.94 (broad, 2H). 4.2.3. Synthesis of Na12[WZn3(H2O)2(ZnW9O34)2]·[Na12(ZnPOM)]. A two-neck round-bottom flask fitted with a condenser was charged with Na2WO4·2H2O (7.599 g, 23.04 mmol) and dissolved in 24 mL of water. The solution was treated with 1.75 mL HNO3 (10 N) at 80−85 °C under vigorous stirring until the initial precipitate dissolved completely. A solution of Zn(NO3)2.6H2O (1.784 g, 6.00 mmol) in 6 mL of distilled water was added dropwise, under vigorous stirring and heating at 90−95 °C (without boiling). A white precipitate was formed which redissolved immediately. The addition was controlled in such a way so that reaction mixture remained clear until the end of addition. The pH of reaction mixture was ∼7.5. The reaction mixture was allowed to cool to room temperature, and the clear solution was evaporated to half the volume. White needlelike crystals were formed on standing for 2 days. Crystalline compounds were collected by filtration. The compounds were recrystallized from water and dried under vacuum to give Na12[ZnPOM] (1.562 g, 0.29 mmol (50.9%)). Thermogravimetric analysis (TGA) shows that the obtained compound is highly stable until 900 °C (92% residue) and released only 15 H2O molecules (Figure S21 in the SI). 4.2.4. Synthesis of (PVIM)[WZn3(H2O)2(ZnW9O34)2] (PVIM-ZnPOM Conjugate). A Schlenk tube was charged with Na12[ZnPOM] (0.350 g, 0.07 mmol) and dissolved in 3 mL of water and a solution of PVIMBr (0.204 g, 0.88 mmol) dissolved in 4 mL of water was added slowly. A white emulsion suspension was formed immediately, which was heated at ∼80 °C for 2 h. The emulsion suspension was cooled to room temperature and filtered through frit and dried under high vacuum to give PVIM-ZnPOM (0.338 g, 79.4%). TGA graph shows that the PVIM-ZnPOM is stable up to 235 °C (see Figure S22 in the SI). 4.3. Single-Crystal XRD. Single-crystal X-ray data were collected using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) on a Bruker D8 SMART APEX2 CMOS diffractometer at 100 K. Data integration was performed using SAINT.27 Routine Lorentz and polarization corrections were applied to all structures, and empirical absorption corrections were performed using SADABS.28 SIR9729 were used to solve the structure and the refinement was performed using SHELXL 2013. Zn were found in the counterion from the electron density Fourier map. The pertinent crystal data and refinement parameters are compiled in Table S6 in the SI. CIF files for the subject compound are deposited with the Cambridge Crystallographic Data Centre (CCDC No. 1495754). 4.4. Density Functional Theory (DFT) Calculation. The density functional theory (DFT) calculations were performed with the Gaussian 09 programs using Becke’s three-parameter hybrid exchange functional 30 and the Lee−Yang−Parr correlation functional (B3LYP).31 The double-ζ basis set of Hay and Wadt (LanL2DZ) with an effective core potential (ECP) was used for W, Zn to represent the innermost electrons of these atom32 and the main group elements (C, N, H, O) were described using the 6-31G(d,p) basis sets. The calculations were performed considering the solvation effects of water in CPCM model. The closed-shell geometry optimization calculations in [WZn3(H2O)2(ZnW9O34)2]12− and (Me2Imd)2[WZn3(H2O)2(ZnW9O34)2]10− were performed using the atomic coordinates provided by the X-ray structures of the [WZn3(H2O)2(ZnW9O34)2]12− anion. For the sake of simplicity, we have taken only two imidazolium cations (Me2Imd), instead of PVIM, to interact with the [ZnPOM]12− unit. 4.5. Electrocatalytic Activity Studies. 4.5.1. Sample Preparation and Electrochemical Investigations. Electrochemical experiments were performed in a single compartment glass cell consisting of a three-electrode assembly where the glassy carbon electrode (GCE, 3

mm in diameter) was used as a working electrode, Pt mesh was used as a counter electrode (CE) and Ag/AgCl (3 M KCl) was used as the reference electrode (RE). Prior to each experiment, the GCE surface was cleaned with a nylon polishing cloth (Catalog No. SM 407052, AKPOLISH) using different grades of alumina slurry (3, 0.3, 0.05 μm, PINE Instruments, USA). The working electrode (GCE) was prepared by drop-casting 20 μL of the sample slurry, prepared by grinding 2.5 mg of OCNT (70 wt %) and PVIM-Zn-POM (30 wt %) in a mortar and pestle for half an hour, and the mixture was dispersed in 40 μL of isopropyl alcohol (IPA) under sonication for 30 min. To obtain a fine dispersion, it was sonicated for another hour after increasing the volume to 1.0 mL with deionized water (12 MΩ). The electrochemical and kinetic parameters were analyzed using an Autolab 302N modular potentiostat/galvanostat; the data were recorded using Nova 1.11 software. The preliminary cyclic voltammetric investigations were performed in 0.4 M HCl electrolyte at a scan rate of 10 mV s−1. Rotating disk electrode (RDE) measurements were performed using a speed controlling unit (AFMSRCE, Pine Research Instrument, Inc., USA) at different rotations (100, 400, 700, 1000, 1300 rpm) at a scan rate of 5 mV s−1. Dioxygen reduction analysis was performed in O2-saturated 0.4 M HCl electrolyte, and chlorine evolution was performed in N2-saturated 0.4 M HCl electrolyte. 4.5.2. Stability Test. In order to evaluate the stability of the prepared catalyst while keeping in mind the industrial applications, a saturated HCl solution (0.4 M HCl electrolyte which is saturated with oxygen and chlorine), was used, maintaining the level of chlorine more or less the same during the chronoamperometric measurements. Chlorine gas was continuously generated in analysis media by connecting two Pt wires with a regulated DC power supply (Aplab, Model L3220) at a constant potential of 1.2 V. To obtain the chronoamperometric response, we employed a four-electrode configuration, among which two were working electrodes, one was coated with the standard catalyst (RhxSy/C (30%) and Pt/C (20%)), and another was coated with the composite (OCNT-PVIM-ZnPOM); a Pt mesh was used as a counter electrode (CE), while Ag/AgCl (3 M KCl) was used as a reference electrode (RE). In order to maintain uniformity during the entire experiment, the HCl medium was continuously stirred to maintain homogeneity for both working electrodes throughout the experiment. To mimic the uncontrolled cell shutdown, the procedure was designed using Nova software in such a way that 10 min of operation was followed by 2 min of interruption to open circuit potential (OCP). 4.5.3. Electrochemical Impedance Measurements. Electrochemical impedance spectroscopy was performed to inspect the electrical properties of both PVIM-ZnPOM conjugate and OCNT-PVIMZnPOM composite. The impedance spectra were recorded in 3 mM K3[Fe(CN)6] using 0.1 M KCl as a supporting electrolyte. Impedance measurement was carried out by applying a DC potential of 0.47 V over and above an AC perturbation of 10 mV for variable frequencies ranging from 0.1 Hz to 100 kHz in a logarithmic step and the detailed analysis is given in the SI. 4.5.4. SECM Studies. The local catalytic activity of composite catalyst for ORR was analyzed using Sensolytics Base SECM (Sensolytics, Bochum, Germany) in combination with bipotentiostat (Autolab 204, metrohm) and stepper-motor-driven x-y-z stages. The SECM experiments were carried out using four electrode assembly with the sample being OCNT-PVIM-ZnPOM on a GC plate as working electrode 1 (WE1), the SECM tip (10 μm in diameter, Sensolytics, Bochum, Germany) as working electrode 2 (WE2), Ag/ AgCl/3 M KCl as the RE, and a Pt foil as the CE. The redox completion mode of SECM (RC-SECM) was used to measure the local electrocatalytic activity of the OCNT-PVIMZnPOM composite catalyst toward O2 reduction (for ODC). SECM scans were performed with predetermined sample area with an increment of 10 μm in the X-direction, which was slow enough to avoid convection effects and Y-direction at an increment of 20 μm in a “fast comb” mode and at a tip-to-sample distance of 20 μm. At each point of the predefined x,y grid, a potential was applied to the tip (0.12 V) in which O2 can be reduced to water (H2O). During 4262


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Chemistry of Materials scanning, the sample was continuously polarized to a potential (from −0.13 V to 0.42 V), where it consumes oxygen, the tip and the sample compete for the available oxygen in the gap between sample and tip, and, thus, the current measured at the tip reflects the local activity of the underlying composite catalyst for oxygen reduction reaction (ORR). All measurements were carried out in a Faraday cage to facilitate the noise elimination. Data treatment was performed using Gwyddion and Origin 8.5 software (Northampton, USA). All the electrochemical measurements, unless specified otherwise, were repeated at least five times. 4.6. Physical Characterization. 4.6.1. Fourier Transform Infrared (FT-IR) Spectroscopy. FT-IR spectra were recorded using a Bruker Tensor-27 spectrometer in the range of 600−4000 cm−1, with spectral resolution of 4 cm−1. FT-IR data were collected and analyzed by OPUS. 4.6.2. Thermal Analysis. Thermogravimetric analysis (TGA) was performed in a TGA/DSC 1 instrument with an SDTA sensor (Mettler−Toledo). Thermal data were analyzed in STARe software. The temperature, weight, and tau lag was calibrated using an aluminum/zinc standard sample. To avoid external atmosphere effects, high-purity nitrogen gas (99.99%) was passed over the sample at a flow rate of 40 mL min−1 throughout the experiment. The experiments were performed using an alumina pan as a sample holder. Thermal stability was performed by heating the sample from 25 °C to 1000 °C at a rate of 10 °C/min. 4.6.3. Morphological and Elemental Analysis. The elemental components of the catalysts were analyzed using X-ray diffraction (XRD) data (PANalytical, Model X’Pert Pro MPD, with Cu Kα radiation) (see Figure S23 in the SI) and energy-dispersive X-ray analysis (EDAX) (Oxford Instruments, Model INCAx-act, 51ADD0013) measurements. Furthermore, scanning electron microscopy (SEM) imaging (JEOL, Model JSM-6610 LV) was performed to examine the morphology of the catalyst. AFM and EFM analysis were performed using a Bruker Multimedia 8 system that was operating in noncontact mode during AFM analysis and tapping mode during EFM measurement. During the first scan, topography was analyzed by noncontact mode, while, during the second scan, the positively biased tip (690 mV) was scanned over the selected area by keeping the tip-tosurface distance constant. 4.6.4. X-ray Photoelectron Spectroscopy (XPS). An ultrahigh vacuum (UHV) setup equipped with a monochromatic Al Kα X-ray source (hυ = 1486.6 eV) was used to perform X-ray photoelectron spectroscopy (XPS) measurements. A high-resolution PHI VersaProbe II Spectrometer with AES operating at 15 kV, 35 mA, and a base pressure of 7 × 10−10 mbar was employed for analysis. Measurements were performed in the fixed transmission mode with a pass energy of 376 eV. Spectra were obtained for C 1s, O 1s, N 1s, W 4f, and Zn 2p by considering the sp2-hybridized C 1s line from graphitic carbon at 284.5 eV as a reference for recalibrating the binding energy scales.

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Tharamani C. Nagaiah: 0000-0003-3545-6668 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.C.N. thanks Department of Science and Technology (DST) for Ramanujan Fellowship (No. SR/S2/RJN-26/2012) and Council of Scientific & Industrial Research (CSIR) India (No. 01(2786)/14/EMR-II). D.M. thanks Department of Atomic Energy (DAE), India (No. 2013/37C/57/BRNS) and DST India (SB/FT/CS-046/2012). V.S. gives thanks to UGC and S.D.A. and A.T. thank IIT Ropar for the Fellowship.

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(1) Pérez-Ramírez, J.; Mondelli, C.; Schmidt, T.; Schlüter, O. F.-K.; Wolf, A.; Mleczko, L.; Dreier, T. Sustainable chlorine recycling via catalysed HCl oxidation: from fundamentals to implementation. Energy Environ. Sci. 2011, 4, 4786−4799. (2) Ziegelbauer, J. M.; Gullá, A. F.; O’Laoire, C.; Urgeghe, C.; Allen, R. J.; Mukerjee, S. Chalcogenide electrocatalysts for oxygendepolarized aqueous hydrochloric acid electrolysis. Electrochim. Acta 2007, 52, 6282−6294. (3) Jin, C.; Nagaiah, T. C.; Xia, W.; Spliethoff, B.; Wang, S.; Bron, M.; Schuhmann, W.; Muhler, M. Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes synthesized by coating with polyaniline. Nanoscale 2010, 2, 981−987. (4) Maljusch, A.; Nagaiah, T. C.; Schwamborn, S.; Bron, M.; Schuhmann, W. Pt−Ag Catalysts as Cathode Material for OxygenDepolarized Electrodes in Hydrochloric Acid Electrolysis. Anal. Chem. 2010, 82, 1890−1896. (5) Jin, C.; Xia, W.; Nagaiah, T. C.; Guo, J.; Chen, X.; Li, N.; Bron, M.; Schuhmann, W.; Muhler, M. Rh−RhSx nanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygen reduction reaction. J. Mater. Chem. 2010, 20, 736−742. (6) Vidakovic-Koch, T.; Gonzalez Martinez, I.; Kuwertz, R.; Kunz, U.; Turek, T.; Sundmacher, K. Electrochemical membrane reactors for sustainable chlorine recycling. Membranes 2012, 2, 510−528. (7) Allen, R. J.; Giallombardo, J. R.; Czerwiec, D.; De Castro, E. S.; Shaikh, K. Rhodium electrocatalyst and method of preparation. U.S. Patent No. 6,149,782, 2000. (8) Weber, R.; Kintrup, J.; Schuhmann, W.; Bron, M.; Maljusch, A.; Tharamani, C. N. Electrode material, electrode, and method for hydrogen chloride electrolysis. U.S. Patent No. 20110139629, 2009. (9) Weber, R.; Kintrup, J.; Bulan, A.; Kämper, F. Processes for the production of chlorine from hydrogen chloride and oxygen. U.S. Patent No. 20080029404A1, 2008. (10) Johnson Matthey, http://www.platinum.matthey.com/ (accessed September 2016). (11) Lv, H.; Geletii, Y. V.; Zhao, C.; Vickers, J. W.; Zhu, G.; Luo, Z.; Song, J.; Lian, T.; Musaev, D. G.; Hill, C. L. Polyoxometalate water oxidation catalysts and the production of green fuel. Chem. Soc. Rev. 2012, 41, 7572−7589. (12) Adler, S. B. Factors governing oxygen reduction in solid oxide fuel cell cathodes. Chem. Rev. 2004, 104, 4791−4844. (13) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P. A review of anode catalysis in the direct methanol fuel cell. J. Power Sources 2006, 155, 95−110. (14) Nishimoto, Y.; Yokogawa, D.; Yoshikawa, H.; Awaga, K.; Irle, S. Super-reduced polyoxometalates: excellent molecular cluster battery components and semipermeable molecular capacitors. J. Am. Chem. Soc. 2014, 136, 9042−9052. (15) Quintana, M.; López, A. M.; Rapino, S.; Toma, F. M.; Iurlo, M.; Carraro, M.; Sartorel, A.; Maccato, C.; Ke, X.; Bittencourt, C.; Da Ros, T.; Van Tendeloo, G.; Marcaccio, M.; Paolucci, F.; Prato, M.; Bonchio, M. Knitting the catalytic pattern of artificial photosynthesis to a hybrid graphene nanotexture. ACS Nano 2013, 7, 811−817.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00269. Synthesis and characterization of ZnPOM, PVIMZnPOM, and OCNT-PVIM-ZnPOM (FT-IR, DFT, SEM, EDAX, AFM, electrochemical, SECM, XPS, TGA, NMR, PXRD, and SC-XRD analyses); detailed calculation for iodometric titration and TOF (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (D. Mandal). * E-mail: [email protected] (T. C. Nagaiah). ORCID

Debaprasad Mandal: 0000-0003-4701-543X 4263


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