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C: Energy Conversion and Storage; Energy and Charge Transport

Double Metal Ions Exchanged Mesoporous Zeolite as an Efficient Electrocatalyst for Alkaline Water Oxidation: Synergy between Ni-Cu and Their Contents in Catalytic Activity Enhancement Subhajyoti Samanta, Santimoy Khilari, Kousik Bhunia, Debabrata Pradhan, Biswarup Satpati, and Rajendra Srivastava J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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The Journal of Physical Chemistry

Double Metal Ions Exchanged Mesoporous Zeolite as an Efficient Electrocatalyst for Alkaline Water Oxidation: Synergy between Ni-Cu and Their Contents in Catalytic Activity Enhancement Subhajyoti Samanta†, Santimoy Khilari†, Kousik Bhuniaǂ, Debabrata Pradhanǂ, Biswarup Satpati┴, and Rajendra Srivastava*†

†

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

ǂ

Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal-721302,

India ┴

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar,

Kolkata 700 064, India

E-mail: [email protected] Phone: +91-1881-242175; Fax: +91-1881-223395

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__________________________________________________________________ ABSTRACT The kinetics of total water splitting is mostly hampered by the sluggish oxygen evolution reaction (OER) at the anode of the electrolyzer. Herein, we put our focus to design a costeffective porous OER catalyst for the efficient water to fuel production. A simple metal ionexchange protocol is adapted to implant electroactive metal centres in the mesoporous architecture of ZSM-5. OER active Ni is incorporated as catalytic sites in the mesoporous ZSM5. Further, simultaneous incorporation of both Ni2+ and Cu2+ into the mesoporous ZSM-5 (MesoZ) matrix significantly boosted the OER catalytic activity. The optimization of Ni and Cu contents (1.04 wt% Ni and 0.44 wt% Cu) in the catalyst is found to be the essential to achieve high catalytic activity. The Cu content influences the onset potential, and Ni content determines the catalytic current during OER. Among developed catalysts, Ni2Cu1-Meso-Z offers the best performance even better than the state-of-art OER catalyst IrO2. The Ni2Cu1-Meso-Z delivers a current density of 10 mA/cm2 at an overpotential of 407 mV and exhibits a low Tafel slope of 55 mV/decade, and high electrochemical active surface area of 6.26 higher and roughness factor of 89.42. Moreover, the Ni2Cu1-Meso-Z retains 92% of its initial current density after 1000 potential cycles of a test run. The best performing Ni2Cu1-Meso-Z offers a Faradaic efficiency of 92 % whereas state-of-art IrO2 efficiency was decreased by 22 % at the similar experimental condition. Further Ni2Cu1-Meso-Z modified anode exhibits better performance in its single cell than IrO2 in which Pt was used as cathode. The excellent OER catalytic activity of double metal ion-exchanged Meso-Z is attributed to the large surface area of mesoporous ZSM-5, hydrophilicity, fast diffusion of water molecules through the favorable interaction with Si-OH groups, and optimum binding & dissociation of different oxygeneous OER intermediates on the catalyst surface. Excellent current density and sustainable performance suggest that the double metal ion-exchanged mesoporous zeolite can serve as a potential candidate to improve the overall water splitting in the electrolyzer. ______________________________________________________________________________


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INTRODUCTION Hydrogen (a high energy density fuel) is considered as a viable energy carrier for the supply of terawatt global energy required in the near future.1,2 In this context, production of hydrogen by electrocatalytic water splitting becomes a promising eco-friendly approach that can mitigate the dependency on depleting fossil fuel stock and minimize the environmental issues. Polymer electrolyte membrane (PEM) alkaline electrolyzer is commonly used for the water to hydrogen production.3 This technology becomes popular for its environmental benign nature and possibility to use renewable electrical energy generated from solar, wind, tidal etc. sources.3 However, the efficacy of the PEM electrolyzer relies on the electrode kinetics of cathodic and anodic half-cell reactions, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Mostly, the high overpotential and sluggish nature of OER cost significant energy loss.4 To improve the multi-electronic OER process, various electrocatalysts have been explored in the past decades. Among the tested OER catalysts, the performance of precious metals and their oxides lay on-the-top of the activity line. Further, RuO2 and IrO2 are considered as the state-ofart electrocatalyst for OER.4 In recent time, scientific community focuses their attention on the development of inexpensive and noble metal-free electrocatalysts for attaining high OER activity at low overpotential.5 These efforts include the development of metal oxides, metal hydroxides/oxyhydroxides, metal phosphides, and perovskites or their composites.6,7,8,9 Metal oxides and hydroxides have shown good OER catalytic activity in alkaline medium with considerable stability for long-term application. Among different transition metal based OER electrocatalysts, Ni-based oxide and hydroxides have been paid considerable attention as a lowcost alternate to the expensive IrO2 or RuO2 in recent years.10 However, the uncontrolled agglomeration of these metal oxides based catalysts resulting in an inferior catalytic activity.11 Therefore, the design and development of Ni-based electrocatalyst for practical application still require significant improvement. Several strategies have been considered to overcome these limitations which include (i) thin film formation, (ii) increasing specific surface area, (iii) introduction of porosity, and (iv) the use of porous catalyst support.11,12 Among these strategies, development of a high surface area porous electrocatalyst for OER becomes popular in recent years. Ma et al. reported metal-organic framework (MOF) derived Co3O4-carbon based porous hybrid electrocatalyst for OER in alkaline medium.12 A porous nanowires arrays of metallic Co4N on a carbon cloth was developed by Chen et al. and used as an electrode for OER.13 Qi et 3 ACS Paragon Plus Environment


al. reported a porous Nickel-Iron oxide as a promising OER catalyst.14 However, synthesis of theses porous catalysts require controlled synthesis procedure such as high temperature treatment (~450-600 °C), inert atmosphere, template removal from the precursor, usage of structure directing agents, sophisticated reaction environment. These parameters limit facile and large scale synthesis of such porous materials, and hence restrict their wide applications. Therefore, there is a plenty of scope for the economical and reproducible design of porous OER catalyst with excellent efficiency. Herein, we have considered mesoporous zeolite ZSM-5 (Zeolite Socony Mobil-5) as a porous support for OER application. The major advantage of mesoporous ZSM-5 includes its facile synthesis, high surface area (~550-600 cm2/g) with inter/intra-crystalline mesoporous voids and surface Si-OH groups that facilitate the diffusion of analytes at the vicinity of electrode/electrolytes and thereby leading to faster reaction kinetics.15 Large external surface area and mesopore volume of mesoporous ZSM-5 facilitate the diffusion of reactant/product molecules and could improve the mass transfer. Sufficient numbers of surface silanol groups present on the external surface of mesoporous ZSM-5 facilitate the adsorption of OH- to generate active sites on the surface. Further, large external surface area of mesoporous ZSM-5 provides highly dispersed active sites for the catalytic reaction. Additionally, availability of large numbers of surface silanol groups provides optimum wettability that provides appropriate contact between the electrocatalyst and the electrolyte. In the recent past, several reports were appeared in the literatures for zeolites based materials for various electrochemical applications.16 Among different framework of zeolites, the development of mesoporous ZSM-5 attracted scientific community in recent years to explore as electrode materials/support for various electrochemical processes such as methanol oxidation, heavy metal detection, and biomolecules recognition etc.16,17,18 Cui et al. designed a SnO2 nanocrystal decorated mesoporous ZSM-5 for methanol electrooxidation in alkaline medium.19 Additionally, Ni(OH)2, NiCo2O4, and CeO2/mesoporous ZSM-5 for methanol oxidation have been reported recently.20,21,17 Further, the applications of transition metal ion-exchanged ZSM-5 for electrocatalytic methanol oxidation, and electrochemical detection of ascorbic acid, dopamine, uric acid and tryptophan have also been demonstrated.22 Mesoporous ZSM-5 supported metal oxide composites exhibited improved electrochemical activity as compared to pristine metal oxide. The incorporation of the electroactive components into the mesoporous ZSM-5 framework could be an interesting, 4 ACS Paragon Plus Environment

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sustainable, and reproducible method to achieve the desired activity. In this direction, metal ionexchanged mesoporous ZSM-5 could be a simple and meaningful approach to fabricate electrode materials. Considering the noteworthy OER activity of Ni-based electrocatalysts, Ni2+ was incorporated in the mesoporous ZSM-5 matrix via the ion-exchange process. Again, literature reports reveal that bimetallic and multi-metallic electrocatalysts exhibited improved activity than their monometallic counterpart.23,24 Recently, bimetallic layered double hydroxides such as NiCo LDH, Ni-Fe LDH, Co-Mn LDH23,25,26 have been reported as promising OER catalysts with significantly improved performance to their monometallic layered hydroxide. These studies motivated us to incorporate two metals into the mesoporous ZSM-5 framework. Further, Cu doping on IrO2, LaFeO3, and TiO2 have significantly boosted the OER catalytic activity.27,28,4 Therefore, incorporation of Cu2+ as a second metal into the Ni2+-exchanged mesoporous ZSM-5 could further improve the electrocatalytic activity. Therefore, in the present study, we have considered following aspects to develop a costeffective OER catalyst. First, the economical OER active Ni2+ is incorporated into the mesoporous ZSM-5 matrix. Second, a co-exchange protocol was adopted for the simultaneous incorporation of Ni2+ and Cu2+ ions in the mesoporous ZSM-5 framework. Third, the influence of Ni2+ and Cu2+ ions concentration in the resultant ion-exchanged mesoporous ZSM-5 was systematically investigated by varying the stoichiometry of ion-exchange solution. Finally, the OER catalytic activity of the developed catalyst is compared in terms of onset potential, catalytic current density, and stability with the state-of-art OER catalyst, IrO2. The developed double metal (Cu2+ and Ni2+) exchanged mesoporous ZSM-5 (Ni2Cu1-Meso-Z) exhibits a promising OER catalytic activity.

2. MATERIALS AND METHODS 2.1 Synthesis of Metal Ion Exchanged Meso-ZSM-5 Mesoporous ZSM-5 (Zeolite Socony Mobil-5) zeolite (hereafter designated as Meso-Z) was prepared by following a reported procedure using the molar gel composition: 90 TEOS/10 PrTES/2.5 Al2O3/3.3 Na2O/25 TPAOH/ 2500 H2O.29 For the preparation of M-Meso-Z (M = Ni and Cu) [Ni-Meso-Z/Cu-Meso-Z], 1 g of Meso-Z was cation-exchanged with the desired metal source (50 mL of 1 M aqueous solution) at 343 K for 4 h resulting in the formation of metal ion5 ACS Paragon Plus Environment


exchanged Meso-Z. The ion-exchange process was performed thrice to ion-exchange the maximum amount of H+ ions from the matrix. For the synthesis of Ni2Cu1-Meso-Z, 1 g of MesoZ was treated with 50 mL of metal-ions aqueous solution containing 0.66 M NiCl2.6H2O and 0.33 M CuCl2.6H2O at 343 K for 4 h. This ion-exchange process was performed thrice, and the resulting material is designated as Ni2Cu1-Meso-Z. In the similar manner Ni1Cu2-Meso-Z was synthesized by taking appropriate amount of Ni and Cu metal sources. Similarly, Ni1Cu1-Meso-Z was prepared by the ion-exchange process (thrice) using equimolar concentration of (0.5 M NiCl2.6H2O and 0.5 M CuCl2.6H2O) both the metal sources at 343 K for 4 h. The content of metals was estimated by MP-AES.

2.2 Catalyst Characterization The crystalinity and phase purity of the catalysts were studied by powder X-ray diffraction (PXRD) analysis. The textural properties of the samples were evaluated from Nitrogen adsorption-desorption studies. The surface morphology of the samples was analyzed by field emission scanning electron microscopy (FESEM). The nanostructure of samples was studied by Transmission electron microscopy (TEM) measurement. The bulk chemical composition of samples was estimated from microwave plasma atomic emission spectroscopy (MP-AES). The surface chemical composition and oxidation states of different constituent elements of the samples were studied using X-ray photoelectron spectroscopy (XPS). The details of instruments, sample preparation and methods adopted in the characterization are provided in supporting information (SI).

2.3 Elecetrochemical Analysis Electrochemical measurements were performed using a CHI 660D electrochemical work station attached to an ALS RRDE rotator system. All the electrochemical measurements were carried out in a standard three electrode system consist of electrocatalyst coated glassy carbon (GC) electrode, saturated calomel electrode (SCE), and Pt wire as working, reference, and counter electrode, respectively. Prior to electrocatalyst mounting, the glassy carbon electrode was polished properly to a mirror like surface with alumina slurry and then ultrasonicated in ethanol and deionized water for 30 min, each. The working electrode was prepared by drop casting 20 μL aliquot of active materials over pre-cleaned glassy carbon electrode. Further, the aliquot was 6 ACS Paragon Plus Environment

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prepared by dispersing 1 mg of active material in 1 mL ethanol containing 5 µL of 5 wt % Nafion solution. The electrochemical activities of the catalyst were measured by cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) in 1 M KOH aqueous electrolyte solution. The OER catalytic activity was studied from the LSV plot recorded in the potential window of 0 to 1.0 V at a scan rate of 10 mV/s in a N2-saturated electrolyte. The working electrode was rotated at 1600 rpm during the potential scan to spin off the evolved oxygen. The Faradiac efficiency of the catalyst was estimated using a catalyst coated GC disk and a Pt ring consisting rotating ring disk electrode (RRDE) setup. The RRDE based analysis was executed by operating the potentiostat under a bipotentiostat mode. The Pt ring electrode fixed at a constant potential of 0.2 V vs RHE and the potential of disk electrode was scanned over a desired potential window. A scan rate of 10 mV/ s was applied for the potential scan of disk electrode with constant rotation at 1600 rpm. The electrolysis experiment was carried out to estimate the evolved oxygen during OER. Prior to electrolysis a catalyst slurry was loaded on the 1 cm2 area of a graphite paper electrode (purchased from Nickunj Eximp Enterprise Private Limited, Mumbai, India) and uncoated area of graphite sheet was protected by coating Teflon tape. Subsequently, the dried catalyst modified cathode was placed in one compartment, and Pt counter electrode was placed in the other compartment. Further, the two compartments were separated by a previously made PVA-PDDA anion exchange membrane.30 Subsequently, 1 M KOH was introduced to both the compartments. Nitrogen was purged for 30 min prior to the electrolysis. The whole cell was made gastight, and headspace (70 mL) was filled with nitrogen. The electrolysis was carried out by applying 10 mA anodic current for 360 min. The O2 generated in the cathode compartment was quantified using gas chromatography. For this purpose, the gas sample was collected from cathode compartment using a gastight syringe. The gas chromatography of gas sample was done in an Agilent Technologies 7890B gas chromatograph equipped with a packed column and a thermal conductivity detector. The single cell experiment was carried out by taking Ni2Cu1-Meso-Z as a anode and Pt wire as a cathode and LSV was operated at a scan rate of 10 mV/s in 1 M KOH. The same experiment was also carried out by taking IrO2 as cathode just by replacing Ni2Cu1Meso-Z and other conditions remained same as stated earlier.



3. RESULTS AND DISCUSSION 3.1

Physio-chemical Characterization

The phase and crystal structure of the catalysts were analyzed using powder X-ray diffraction (PXRD). The intense diffraction features reflect excellent crystallinity of the synthesized samples. The PXRD pattern of Meso-Z well agreed with the MFI framework structure of the ZSM-5 reported in the literature (Figure 1a).29 The single or double metal ion-exchanged Meso-Z samples exhibited similar diffraction patterns in their powder XRD patterns like pristine Meso-Z indicating the fact that the ion-exchange process did not affect the zeolite framework structure (Figure S1). The absence of any extra diffraction feature ruled out the formation of additional metal oxide or hydroxide phase. The specific surface area and pore size of all the synthesized materials were investigated by N2-sorption measurements. A type IV isotherm and an H2 hysteresis were observed in the N2-adsorption desorption isotherms for all the samples (Figure 1b). The sharp increase in the adsorbed volume in the P/P0 range below 0.05 reflects the presence of micropores in the samples. Further, the gradual increase of adsorption was noticed above this pressure. Moreover, increase in the N2 adsorption in the region 0.4 Ni1Cu2-Meso-Z> Ni2Cu1Meso-Z (Figure 6a). Thus, the onset potential for the present Ni-containing OER catalysts follows the similar trend as oxidation potential for Ni2+ to Ni3+ process at individual electrodes. The electrochemically active surface area (ECSA) and roughness factor (RF) of electrode play key roles in the electrocatalytic activity of an electrode. Therefore, ECSA is determined for the electrocatalysts by considering their double layer capacitance (Cdl) in the nonfaradic region of CV profile in 1 M KOH. A high ECSA offers higher number of electrocatalytic sites for OER thereby improves the catalytic efficiency of an electrocatalyst.37 The ECSA and RF values are obtained using following eqs. ECSA = Cdl/Cs

(1)

and RF = ECSA/Sgeo

(2)

where, Cdl, Cs and Sgeo represent double layer capacitance, specific Cdl of an atomically smooth oxide surface, and geometric surface area of the GC electrode, respectively. In the present study, Cs is considered to be 40 µFcm-2 for OER catalysts in 1 M KOH.38 To obtain the Cdl of different electrocatalysts, CV is recorded at different scan rate (10-100 mV/s) in the non faradic region (Figure 6b, Figure S8 (SI)) and charging current (ic) at 1.1 V vs RHE are measured. Subsequently, the Cdl value for each electrode is estimated using following relationship ic = vCdl

(3)

where, v denotes the scan rate. The estimated ECSA values of the as-prepared electrodes follow the order of Ni2Cu1-Meso-Z (6.26 cm2) > Ni1Cu2-Meso-Z (1.64 cm2) > Ni1Cu1-Meso-Z (1.37 cm2) > Ni-Meso-Z (0.51 cm2) > Cu-Meso-Z (0.39 cm2) (Figure 6c). This finding reflects the highest amount of catalytic sites available on Ni2Cu1-Meso-Z among the prepared electrocatalysts. Further, the roughness factor (RF) of an electrode offers valuable information 16 ACS Paragon Plus Environment

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about its electrocatalytic activity. Earlier reports suggest that the electrode with high RF value exhibits better OER catalytic activity.37 Similar to ECSA, the RF values of the catalysts show the similar trend (Table S3). It is important to mention that the ECSA and RF values of double metal ion-exchanged Meso-Z are much higher than single metal ion-exchanged Meso-Z which corroborates the essentiality of the incorporation of both Ni and Cu in the Meso-Z matrix. This superiority attributes to the increase of Cdl in double metal exchanged Meso-Z. Similar improvement of electrocapacitive behaviour is noticed for bimetallic hydroxide or oxide electrode as compared to their monometallic analogue in supercapacitor study.

Figure 6. (a) Cyclic voltammograms recorded at 20 mV/s scan rate in 1 M KOH with different OER catalysts, (b) cyclic voltammograms recorded at different scan rate with Ni1Cu2-Meso-Z OER catalyst, (c) scan dependent current production rate profile of different electrodes (Error bars in graph indicates ± SD (n=3)) and (d) Tafel polts of different OER catalysts.



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The kinetics of electrocatalytic process is extensively studied by the Tafel plot analysis. The Tafel plots of an electrode can be extracted from polarization plots using Tafel equation. Furthermore, the Tafel equation is derived from the Butler-Volmer equation, and the final form of the equation can be expressed as 2 η = a + b log(i/i0)

(4)

where ‚'η' refers to the overpotential, 'i' is the measured current density, 'i0' is the exchange current density, 'b' indicates Tafel slope and 'a' is a constant. The Tafel slope is estimated from the linear portion in the low-potential region, which refers to the activation-controlled current density region. A low value of Tafel slope reflects higher catalytic activity. The electrodes considered in present study exhibit Tafel slope of 289, 93, 55, 76, 83, 177, and 58 mV/decade for Meso-Z, Ni-Meso-Z, Ni2Cu1-Meso-Z, Ni1Cu1-Meso-Z, Ni1Cu2-Meso-Z, Cu-Meso-Z, and IrO2 respectively (Figure 6d). Among studied catalysts, Ni2Cu1-Meso-Z exhibits minimum Tafel slope implying its superior electrocatalytic activity over the other catalysts investigated in the present study. Further, the Tafel slope obtained with Ni2Cu1-Meso-Z is found to be comparable or superior to many previously reported OER catalysts (Table S2). Most importantly, the Ni2Cu1Meso-Z electrode offers lower Tafel slope than state-of-art IrO2 OER catalyst. This finding reveals the potentiality of Ni2Cu1-Meso-Z for practical implementation in alkaline water splitting or in general in alkaline water electrolyzer. Faradaic efficiency (FE) of the best performing Ni2Cu1-Meso-Z OER catalyst is estimated by considering an OER – oxygen reduction reaction (ORR) study. The OER-ORR study is executed with the bipotentiostat mode of the CHI 660D electrochemical work station. A rotating ring disk electrode (RRDE) consists of a catalyst coated glassy carbon disk and a Pt ring was used for the OER-ORR experiment. The O2 generated on the catalyst coated glassy carbon disk (OER) is reduced at the Pt ring electrode (ORR) during the OER-ORR experiment. The potential of the disk is scanned over a definite potential window and the potential of Pt ring held at a constant potential of 0.2 V vs RHE (Fig. 7).39 Moreover, the FE of the Ni2Cu1-Meso-Z is calculated using following equation 38 FE = 2ir/(id.N)

(5)


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where, ir, id, and N signify the ring current, disk current and collection efficiency (20 %) respectively. The estimated FE of Ni2Cu1-Meso-Z catalyst is found to be 92 % at onset potential of OER (1.561 V vs RHE) and 85 % at 1.637 V vs RHE (potential producing 10 mA/cm2 catalytic current density). A similar decrease of FE with increasing potential is reported previously by Swesi et al.39 Further, this decrease can be attributed to the generation of large O2 bubble at higher potential which remain less dissolved resulting in a lower diffusion O2 to the Pt

3 2 1

80 60 40 20 0 1.50

1.55

1.60

1.65

-0.4 -0.6 1.70

-0.8

Potential vs RHE (V)

0 1.0

0.0 -0.2

Potential=potential@ 2 10 mA/cm current

4

Disk Ring Potential=onset potential

5

Faradaic efficiency (%)

6

1.2

1.4

1.6


Ring current (mA)

ring.

Disk current (mA)

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1.8

Figure. 7 OER at Ni2Cu1-Meso-Z modified disk electrode in N2 saturated 1 M KOH and evolve oxygen molecules reduction (ORR) at Pt ring electrode (held at 0.2 V vs RHE). The ORR current (ring current) plotted as a function of disk potential (Error bars in graph indicates ± SD (n=3)). The various possible steps of OER at present double metal ion-exchanged Meso-Z is presented in Scheme 1.40,41 The magnitude of Tafel slope determines the rate-limiting step of OER. A higher Tafel slope of 120 mV/decade suggests first discharge of OH- ion (equation 6) is the rate-limiting step. Tafel slopes reflect that the surface species formed in the step just before the rate limiting step.41 In the present case, pristine Meso-Z and Cu-Meso-Z follow this path. However, the presence of Ni significantly reduces the Tafel slope suggesting different OER mechanism. A 19 ACS Paragon Plus Environment


Tafel slope lower than 120 mV/decade and near 60 mV/decade signifies second step (equation 7) controls the overall OER kinetics.40 Further, this low Tafel slope ascertains that the surface adsorbed species is generated in the early stage of the OER. The Ni-Meso-Z, Ni2Cu1-Meso-Z, Ni1Cu1-Meso-Z, Ni1Cu2-Meso-Z and IrO2 exhibit such kind of kinetics where the second step becomes rate limiting. The Ni-containing catalysts catalyze OER via the formation of Ni3+ active sites which is isoelectronic with Co4+.6 The OER catalytic activity of both Ni3+ and Co4+ has been extensively studied in the recent past.42 Moreover, the high valent metal centre is expected to boost the electrophilicity of the adsorbed O and facilitates the formation of hydroperoxy (OOH) species and further conversion to O2 molecules.42 The electrocatalytic water oxidation on a transition metal doped catalyst can be understood on the basis of d-band theory.4,42 According to the d-band theory, the catalytic activity of an OER catalyst is determined by the metal d state near Fermi level. The eg orbital of transition metal ions involves in the σ-bond formation with the anions adsorbed on the surface. Thus, the d electron filling in the eg orbital becomes a key to tune the bond strength of the oxygeneous intermediates for improving OER kinetics.42 The presence of an optimum amount of Ni and Cu together offers more favourable bond strength between the active metal ion and oxygeneous intermediate resulting in a good OER performance. Previously, Roy et al. demonstrated a lowering of overpotential and better current generation with Cu2+ doped TiO2 as compared to Fe or Co ion doped counterpart. The low overpotential is believed to be originated from the higher adsorption and decomposition of reactive intermediates (OOH, OH, O) at the catalyst surface.4 In the present study, the incorporation of Cu also enhances the OER catalytic efficiency of Ni. This enhancement can be attributed to the synergistic contribution of Ni2+ and Cu2+ at an optimum level in the Meso-Z matrix. Electrochemical impedance spectroscopy study is carried out to understand the charge transfer characteristics at the electrode-electrolyte interface during OER. The experimental findings are plotted regarding Nyquist plot; a complex impedance plot is used to evaluate the resistive and capacitive characteristics. The Nyquist plot for each electrode consists of an impedance arc at highfrequency region followed by a straight line in the lower frequency region. Further, the arc diameter refers to the Faradic charge transfer resistance (Rct) of the electrode-electrolyte interface. Moreover, an equivalent circuit (inset Figure 8a) consists of solution resistance (Rs), Rct and constant phase element used to simulate the experimental results and measured different resistive and capacitive elements. The Rct values of different electrodes follow the trend of Meso20 ACS Paragon Plus Environment

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Z (213.94) > Cu-Meso-Z (126.58) > Ni1Cu1-Meso-Z(69.56) > Ni-Meso-Z(28.29)> Ni2Cu1Meso-Z(20.42) (Figure 8a). The Ni2Cu1-Meso-Z exhibits the lowest charge transfer resistance which helps to attain efficient charge transfer from the electrode to electrolyte thereby enhances electrode kinetics. The Cu incorporation into the Ni-Meso-Z significantly reduces the Rct of the electrode (Figure 8a). However, the incorporation of a higher amount of Cu does not show the pronounced effect on Rct reduction. This reflects the essentiality of optimization of Cu content for achieving high OER activity. Further, influence of the catalyst loading on current density is also examined by varying the amount of the best electrocatalyst Ni2Cu1-Meso-Z (Figure 9a).

Scheme 1. A Plausible mechanism of alkaline water oxidation at Ni2Cu1-Meso-Z electrocatalyst. This analysis is carried out at the over potential of 407 mV where 10 mA/cm2 current density is achieved. The results indicate that initially the current density is increased with increasing the amount of the catalyst and gets saturated at 0.35 mg/cm2 (Figure 9a). But further increment in the catalyst amount leads to the reduction in the current density. At higher loading, Ni2Cu1-Meso-Z forms thick coating on the electrode surface that hamper the diffusion of O2 away from the 21 ACS Paragon Plus Environment


electrode surface. Based on the results one can conclude that 3.5 mg/cm2 is the ideal condition for the electrocatalytic measurement. Furthermore, influence of catalyst loading on AC impedance is evaluated with the electrochemical impedance spectroscopy measurements (Figure 9b). The measurements are carried out with four different amounts of Ni2Cu1-Meso-Z. All the EIS analysis exhibits only one type of semi-circle in its Nyquist plot (Figure 9b). The diameters of the semicircles which are the characteristics of the charge transfer resistance (Rct) exhibit a direct proportionality behavior with the catalysts amount. The obtained results indicate that R ct increases with increase in the amount of the catalyst during OER. Based on the results one can

st

Ni2Cu1-Meso-Z 1 cycle

2

-Zimaginary(ohm)

2000

Current density (mA/cm )

conclude that 1 mg catalyst loading is the ideal condition for the electrocatalytic measurement.

(a)

80

1600

Ni-Meso-Z Ni2Cu1-Meso-Z

800

st

IrO2 1 cycle

20

Ni1Cu2-Meso-Z Cu-Meso-Z Meso-Z

0 0

400

800

1200 1600 2000

Zreal(ohm)

Evolve oxygen (mol)

IrO2

11 2

)

Ni2Cu1-Meso-Z

10 9 8 7 6

(c)

5 0

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IrO2 1000 cycle

40

Ni1Cu1-Meso-Z

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(b)

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1200

Current (mA/cm

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

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36000

Time (sec)

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1.2

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1.6


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Ni2Cu1-Meso-Z

100

IrO2 80

(d) 60 40 20 0 0

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80

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160

Figure 8. (a) Nyquist plots of different OER catalysts modified electrodes, (b) polarization plots of first cycle and 1000th cycles of test run, (c) chronoamperometric (i-t) stability for a test run of 10 hours at a potential of 1.6 V, and (d) evolved oxygen gas at a test run of 3 hours at Ni 2Cu1Meso-Z and IrO2 (Error bars in graph indicates ± SD (n=3)). 22 ACS Paragon Plus Environment

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Practical implementation of an OER catalyst depends on its durability during long-term operation. Thus, to evaluate the robustness of the best performing Ni2Cu1-Meso-Z electrocatalyst, 1000 potential cycles of the test run is performed (Figure 8b). Only 8 % decrease in the current density with slight increase of the overpotential is observed. Moreover, the stability of the presently investigated OER catalyst is also examined through the microstructure evaluation after 1000 potential scans. Morphology is retained as evident from the TEM and STEM-HAADF analysis (Figure S9) which directly indicates that this OER catalyst has high stability even after 1000 potential scans which is one of the desired features of an electrocatalyst for the practical implementation. However, the state-of-art IrO2 catalyst current density is decreased by 22 %.

(a)

-Zimaginary (ohm)

11

@407 mV over potential)

2

10

Current density (mA/cm

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10 9 8 7 6 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 2

2.5 mg 2.0 mg 1.5 mg 1.0 mg

(b) 9 8 7 6 5 4 5

10

Catalyst loading (mg/cm )

15

20

25

Zreal (ohm)

30

35

Figure 9. (a) Catalyst loading-activity relationship (Error bars in graph indicates ± SD (n=3)), and (b) impedance-catalyst loading activity for Ni2Cu1-Meso-Z. This long-term stability and excellent OER activity corroborate the potential of the present investigated catalyst for its practical implementation in alkaline water electrolyzer. The oxygen evolved from Ni2Cu1-Meso-Z modified anode is estimated quantitatively. The quantity of evolved oxygen vs time profile is presented in (Figure 8d). The quantity of evolved O2 linearly increases with time which agreed well with the previous reports.44 The evolved O2 is more in the case of Ni2Cu1-Meso-Z when compared to state-of-art IrO2 (Figure 8d).



In order to apply the developed OER catalyst in real water electrolysis a complete single cell consists of Ni2Cu1-Meso-Z modified anode and Pt cathode is designed. Further, the single cell study is also carried out with bench mark IrO2 modified anode and Pt cathode. Figure 10 shows the polarization plots obtained with a two electrode configuration in the single cell. The Ni2Cu1-Meso-Z cell exhibits higher current density than the bench mark IrO2. Further, the operating cell voltage (potential for achieve 10 mA/cm2 current density) for Ni2Cu1-Meso-Z cell was found to be 1.65 V which is lower than the cell voltage required for IrO2 (1.71 V).45 This corroborates that the synthesized catalyst not only shows good performance in half cell study but also exhibits good performance in single cell study. 2

Current density (mA/cm )

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

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60

Ni2Cu1-Meso-Z (+)//Pt(-)

50

IrO2 (+)//Pt(-)

40 30 20 2

10 mA/cm

10 0 0.0

0.5

1.0

1.5

Potential (V)

2.0

Figure 10. Single cell polarization curve for Ni2Cu1-Meso-Z and IrO2 anode with Pt cathode. Polarization plots recorded at a scan rate of 10 mV/s in 1 M KOH.

4. CONCLUSIONS In conclusion, a very simple strategy was demonstrated to fabricate a cost-effective, metal ionexchanged mesoporous ZSM-5 electrocatalyst for the alkaline water oxidation. The incorporation of Ni2+ into the zeolitic matrix showed a prominent improvement in OER. Further, simultaneous incorporation of Ni2+ and Cu2+ into the mesoporous ZSM-5 matrix revealed to be a 24 ACS Paragon Plus Environment

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potential route to achieve the remarkable OER catalytic efficiency. The performance of Ni 2Cu1Meso-Z was found to be the best (onset potential of 407 at 10 mA/cm2) among the studied catalysts and even superior to the state-of-art IrO2 catalyst (onset potential of 446 at 10 mA/cm2). The Ni2+ content determined the catalytic current density whereas Cu2+ content influenced the onset potential for OER. Higher OER activity of Ni2Cu1-Meso-Z when compared to other catalysts correlated well with the generation of Ni3+ at lower overvoltage. Further higher electrochemical active surface area (6.26 cm2) reflected the higher amount of catalytic sites available on Ni2Cu1-Meso-Z among the prepared electrocatalysts. Higher roughness factor RF value (89.42) was consistent with this finding. The Ni2Cu1-Meso-Z exhibited more stability in its chronamperometic (i-t) test when compared to the state-of-art IrO2 catalyst for a 10 hours test run. Also, the Faradiac efficiency of Ni2Cu1-Meso-Z catalyst was found to be 92 % at onset potential of OER (1.561 V vs RHE) (potential producing 10 mA/cm2 catalytic current density). However, the state-of-art IrO2 catalyst current density was decreased by 22%. This long-term stability and excellent OER activity suggest the potential of the present investigated catalyst for its practical implementation in alkaline water electrolyzer. Further a complete single cell consists of Ni2Cu1-Meso-Z modified anode and Pt cathode was designed. The operating cell voltage (potential for achieve 10 mA/cm2 current density) for Ni2Cu1-Meso-Z cell was found to be 1.65 V which is lower than the cell voltage required for IrO2 (1.71 V) in a complete fuel cell. The hydrophilic nature, availability of surface hydroxyl groups, large surface area, and mesoporosity of ZSM-5 are responsible for the noteworthy catalytic activity of metal ion-exchanged mesoporous ZSM-5. Furthermore, this study provides a novel direction to develop a low cost, efficient, and sustainable OER catalyst by implanting active metal ions in the porous zeolitic matrix. The presently investigated novel catalyst produces encouraging and better performance for OER activity than many of the Ni and Cu based electrocatalysts reported earlier. Moreover, this strategy can be adopted to design various cost-effective electrocatalysts by considering multi-metal ion-exchange in other ion-exchange catalytic materials.

ASSOCIATED CONTENT Supporting Information 25 ACS Paragon Plus Environment


The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Supporting information includes detail of materials, catalyst characterization techniques, methods of electrochemical analysis, elemental analysis of various catalysts, XRD patterns of different catalysts, FESEM images of synthesized catalysts, EDAX spectra of different catalysts, HRTEM image, elemental mapping and HAADF-STEM image of Meso-Z, TEM images of recovered catalyst, and comparative assessment for OER performance with various reported catalysts.

AUTHOR INFORMATION Corresponding Author *

Rajendra Srivastava. Email: [email protected]

Phone: +91-1881-242175; Fax: +91-1881-223395 ORCID Subhajyoti Samanta: 0000-0002-4461-1319 Dr. Rajendra Srivastava: 0000-0003-2271-5376 Dr. Debabrata Pradhan: 0000-0003-3968-9610 Dr. Biswarup Satpati: 0000-0003-1175-7562 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Financial support provided by Nano Mission, DST, New Delhi through the research grant (SR/NM-NS-1054/2015) is gratefully acknowledged. SS sincerely thanks, MHRD, New Delhi, for a senior research fellowship. SK is grateful to the Director IIT Ropar for providing him with an Institute postdoctoral fellowship. We are grateful to the DST-FIST funded XPS facility at Department of Physics, IIT Kharagpur for XPS analysis. Authors acknowledged the characterization facilities availed from CRF, IIT Kharagpur and Mr Amit Kumar, JEOL INDIA PVT. LTD. for FESEM/EDAX analysis. Authors are thankful to the Director IIT Ropar for his constant encouragement in Interdisciplinary Research. AUTHOR CONTRIBUTIONS


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S. S. and R.S. designed the work plan. S.S. synthesized all the catalysts and carried out all the analytical characterizations. K.B. analyzed the electrochemical measurements. B. S. contributed in the HRTEM analysis. S. K., S.S., D.P., and R.S. carried out the data analysis. S.S., S.K., and R.S. wrote the manuscript. The final version of the manuscript was approved by all the authors.

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Double Metal Ions Exchanged Mesoporous Zeolite ... - ACS Publications

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