Paratacamite doped polypyrrole for effective hydrogen storage

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Paratacamite doped polypyrrole for effective hydrogen storage S. Padmapriya a, V. Sudha b, S. Harinipriya a,b,* a

Electrochemical Systems Laboratory, SRM Research Institute, SRM Institute of Science and Technology, Kattankulathur, 603203, India b Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, 603203, India

article info

abstract

Article history:

Paratacamite doped Polypyrrole (PDPpy) is synthesized by wet chemical method employing

Received 7 September 2018

CuCl2 as oxidant. BET, optical and structural analysis revealed mesoporous polymer doped

Received in revised form

with [Cu2(OH)3Cl]. Electrochemical analysis of PDPpy in 0.01 M H2SO4 indicated excellent

3 January 2019

hydrogen storage ability and steady hydrogen evolution kinetics. The maximum capacity

Accepted 22 January 2019

of hydrogen storage is found be 59.1 mAh/g via galvanostatic charge/discharge experi-

Available online 13 February 2019

ments. Optical studies confirmed the stability of the PDPpy after HER. Plausible mechanism for hydrogen storage and evolution is deduced from 1HNMR and XPS analysis. Analogous

Keywords:

studies have been carried out in Polypyrrole (Ppy) synthesized using FeCl3 and Ammonium

Polypyrrole

per disulphate (APS) oxidants to understand the ability of hydrogen storage in the absence

Paratacamite

of paratacamite.

1

H NMR

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

XPS Hydrogen storage BET

Introduction Many metal microparticles were used as electrocatalyst for Hydrogen Evolution Reaction (HER) employing conducting polymers as support [1]. Conducting polymers usually are synthesized by chemical or electrochemical methods by mixing of monomer and oxidant in aqueous as well as organic solution [2]. Among the conducting polymers, Polypyrrole (Ppy) is especially promising for commercial applications due to its good ecological stability, facile synthesis and higher conductivity than many other conducting polymers [3]. The most commonly used oxidants are APS, FeCl3, H2O2, K2Cr2O7

and CeSO4. Although the investigations on CuCl2 initiated polymerization of pyrrole suggested a new approach for the chemical synthesis [4], the transition-metal-ion doping enhanced the electrochemical properties of Ppy. The dopant such as Cu2þ, Zn2þ, and Fe2þ, act as redox active catalysts and improved the capacitance behaviour of Ppy [5]. In the literature, it is demonstrated that CuCl2 as oxidant involves doping of mineral called paratactamite (Cu2(OH)3Cl) in Ppy, trapped during polymerization [6]. The mineral, paratactamite (dicopper chloride trihydroxide) is rhombohedral, with several polymorphs involving oxygen and hydrogen bonding structures [7]. It is well known that minerals with high surface area can play a significant role in hydrogen storage mechanism

* Corresponding author. Electrochemical Systems Laboratory, SRM Research Institute, SRM Institute of Science and Technology, Kattankulathur, 603203, India. E-mail addresses: [email protected], [email protected] (S. Harinipriya). https://doi.org/10.1016/j.ijhydene.2019.01.191 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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[8,9] and immobilization of metals on Ppy retains good electrocatalytic properties [10e12]. Hydrogen can be stored via numerous methods, such as physisorption of hydrogen on absorbent materials, compressed gas in its liquid form and hydrogen intercalation in metals and complex hydrides [9]. Inspite of the ambiguity in the reproducibility, the use of conducting polymers in hydrogen storage applications was first introduced in the year 2002 by storing 8 wt% of H2 in commercial PANI and Ppy [13,14]. Electrochemical hydrogen storage in porous materials is a more suitable method, in comparison to the conservative hydrogen storage methods due to the process occurring at relatively low as well as high temperature and pressure. The hydrogen storage capacity of inorganic spinels such as BaAl2O4, CuO/Al2O3, Zn2SnO4, and CoAl2O4 were determined in the literature based on their electrochemical performances [15e20]. Hybrid metal/conducting polymer composites such as Ni/PANI or Ni/Ppy were used as electrocatalysts for hydrogen production in PEM H2 generator [21]. Mesoporous Ppy showed 2.2 wt% of H2 storage capacity at 77 K and high pressure [22]. Researchers had attempted hydrogen storage in Ppy by doping Fe2O3 [23] using electrochemical methods. In addition Ppy-CuCl2 nanocomposites were demonstrated for supercapacitor applications [5]. HER on Ppy and PANI/Ppy coated on Pt surface in acidic solutions is not new to researchers [24,25]. Several catalysts including noble metals were examined as electrocatalyst for efficient H2 production [26e30]. So far, Pt is the best electrocatalyst, due to its high catalytic effect and low overpotential for HER in acidic medium [31]. However, the major bottle neck is the non-availability of efficient Pt-based electrocatalysts for HER. Novel materials as electrocatalysts for HER to reduce or replace the use of noble metals had been extensively investigated in the literature [32e34]. Ppy show good electrocatalytic properties towards HER in alkaline medium and the feasibility of using Ppy coated electrodes for the electrolytic production of hydrogen is well demonstrated [35]. Although Ppy coated electrodes were studied for H2 production in alkaline medium, very few investigations were carried out in acid medium. Eventhough there are several investigations on hydrogen storage capacity of Ppy as mentioned previously, attempting the same using PDPpy by electrochemical methods in acidic medium is not known in the literature. The major novelty of this work is to study the electrochemical hydrogen storage capacity of PDPpy. In addition, PDPpy formation, the reaction mechanism for the formation of the PDPpy in HCl and hydrogen storage studies were analyzed. Thus the present work aims to (i) synthesize PDPpy using CuCl2, (ii) understand the effect of specific surface area of PDPpy on the electrode kinetics of HER, (iii) obtain the maximum charge stored on PDPpy employing electrochemical analysis and (iv) confirmation of doping by 1H NMR and X-ray Photoelectron Spectroscopy (XPS) analysis.

shaking for 30s. Subsequently, the mixture is allowed to settle for 2 h and the obtained product is washed with deionized water and dried in vaccum for 24 h at 50  C (cf Scheme 1). This proportion of precursor ratio is based on the preparation of PANI/Ppy reported in the literature for effective hydrogen storage [36]. Since polymerization is an exothermic process, the reaction is carried out at low temperatures with one reactant slowly added into the other under vigorous stirring. But the resulting Ppy synthesized by the mentioned procedure were agglomerated and therefore not effective for many applications. In order to prepare dispersible Ppy, we preferred continuous stirring throughout polymerization to enhance the processability of Ppy [37]. CuCl2 is taken in larger proportion than py on account of its weak oxidation ability [3] and also to facilitate formation and doping of Cu2(OH)3Cl during polymerization of py [6].

Electrode preparation Copper sheet of dimension 1  2 cm2 is cleaned with ethanol. PDPpy is mixed in ethanol and sonicated for 15min. Copper substrate is dip coated with PDPpy paste and dried at room temperature.

Material characterisation Morphology of PDPpy is studied using FESEM F E I Quanta, Model-FEG 200 in the magnification range of 0.5e3 mm under high vacuum. The structural analysis of PDPpy is carried out employing Xpert Pro with copper target (Cu Ka) in X-ray wavelength of 1.54
A, from 100 to 900. UVevis Spectroscopy is done using Evolution 220 PC spectrophotometer in the wavelength range of 250e1200 nm. For the determination of BET surface area, N2 adsorptionedesorption measurements were conducted at 77 K by Quantachrome. PDPpy is degassed in vacuum at 150  C for 24 h prior to the measurement. Thermogravimetry is carried out with LABSYS EVO TG-DSC 1600  C analyser. Non-isothermal experiments were carried out at 5  C min1 from 27 to 300  C. Ar atmosphere is maintained throughout the experiment. 1H NMR (Bruker: 500 MHz) is used to characterize the molecular structure of the polymer samples. The polymers were dissolved in DMSO‑d6 to prepare the NMR samples. Electrochemical experiments were done employing 3-electrode assembly using Zahner Zennium electrochemical workstation. The electrodes utilized are Ag/

Experimental section PDPpy is synthesized by chemical oxidative polymerization using pyrrole monomer. Freshly distilled pyrrole (60 drops) and CuCl2 (25.57 g) were dissolved in 1 M HCl in separate beakers. The two solutions were simultaneously mixed with

Scheme 1 e Illustrative representation of PDPpy synthesis.

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AgCl (reference electrode), platinum (counter) and PDPpy coated copper (working). All experiments were carried out in 0.01 M H2SO4 at different scan rates in the potential range of 1 to 1 V. EIS is measured in the frequency range from 100 kHz to 100 mHz at 10 mV amplitude. The XPS analysis is carried out by ESCA LAB 250X1 Base system with UPS and XPS image mapping.

Results and discussion The morphological, structural, optical, 1H NMR, XPS and thermal analysis of synthesized Ppy using FeCl3, APS and thermal analysis of PDPpy are provided in Supporting Information (S1). PDPpy is subjected to BET, electrochemical, 1 H NMR and XPS analysis to understand the surface area, hydrogen storage ability, maximum charge stored and the dopant nature. Fig. 2 e SEM image of PDPpy before HER.

Material analysis The N2 adsorption-desorption isotherms of PDPpy exhibit characteristics between type II and type IV behaviors [38]. The rationale behind this behaviour is the condensation of gases in the pores of the adsorbent at pressure below the saturated pressure (PS) [38]. The hysteresis noticed in Fig. 1 at high relative pressure, provide evidence for mesoporous material. The formation of mesoporous material is also confirmed by SEM analysis (Fig. 2). This unique mesoporous nature of PDPpy could be attributed to the entrapment of paratacamite as dopant in Ppy. Fig. 3 represents SEM of PDPpy after HER, here the formation of mesoporous chains seen in Fig. 2 is absent and PDPpy exists as spherical particles. FTIR further confirms the presence of dopant via the broad peak between 3000 and 3500 cm1 corresponding to the hydrogen bond (eOeH….NeH) stretching. The hydrogen of the hydroxyl groups from paratacamite, H-bonds with the hetero nitrogen of Ppy (cf. Fig. 4). This broad peak is absent in the case of Ppy synthesized from FeCl3 and APS as shown in Supporting Information S1. The BET surface area of PDPpy is calculated as 36.671 m2/g which is 1.15 times higher than the literature

Fig. 1 e BET analysis of PDPpy.

value of 32 m2/g of Ppy [39]. Surface area of Ppy synthesized using different oxidants are in the order (CuCl2>FeCl3 >APS). As the surface area increases, the catalytic activity increases. Hence the BET results explicitly supports that PDPpy can be a better catalyst than FeCl3 or APS based Ppy. BET analysis of Ppy synthesized from FeCl3 and APS are provided in Supporting Information S2. A strange similarity had been noticed between the surface area of PDPpy with that of paratacamite reported in the literature [40]. Paratacamite possess surface area of 38 m2/g [40] whereas that of PDPpy is 36.67 m2/g. This similarity clearly supports the mesoporous polymer formation with linear as well as branched chains (cf.Scheme 2). XRD pattern for PDPpy shown in Fig. 5 indicate amorphous product. The broad peak around 22.5 indicates short range arrangement of Ppy chains leading to scattering of X-rays at the interplanar spacing [41]. Analogous to the literature [42], varying oxidants leads to shift of the peaks towards higher 2q (cf. Supporting Information S1). XRD analysis also support the entrapment of paratacamite in

Fig. 3 e SEM image of PDPpy after HER.

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Fig. 5 e XRD of PDPpy. Fig. 4 e FTIR of PDPpy. Ppy via enhancement in the peak intensity associated with peak shift towards lower q. The increased peak intensity for PDPpy is caused by improvement in the crystallinity of the polymer. The improved crystallinity is due to Cu2(OH)3Cl entrapment which is absent in the case of Ppy synthesized from FeCl3 and APS (cf.Supporting Information S1). FTIR spectra (Fig. 4) of PDPpy shows peak at 682.06 cm1 assigned to C-Cl stretching, 798.80 cm1 to C-N stretching and 1212.57 cm1 caused by C-C stretching. 932.38 cm1 is due to stretching vibration of C]C bonds. The peaks at 1295.93 and 1570.13 cm1 could be attributed to C-H deformation and stretching vibration of C-C bonds in py respectively [42]. Broad peak between 3000 and 3500 cm1 represent O-H vibrations of paratacamite which is attached to hetero nitrogen of Ppy ring via hydrogen bond and provides evidence for the entrapment of paratacamite in Ppy chain [6]. The UVevis spectra of PDPpy is shown in Fig. 6(aed). In general conducting polymers show resonance structures between aromatic and quinoid forms. In the neutral state, it is insulating in nature (z1010 S/cm) whereas in the oxidized state, it becomes conducting (103e10 S/cm) [43,44]. The charge is delocalized on the repeating units of polymers and results in a radical cation (polaron) or dication (bipolaron) [45]. The charge in the chain converts aromatic form into quinoid structure. The obtained bandgap via Tauc's plot at 1.35eV is at lower end of the literature reported range of 1.3e2.32 eV [46]

Scheme 2 e Polaron-Bipolaron structure of Ppy.

for Ppy. The band gap of pure paratacamite is 2.67 eV [40]. The observed band gap of 1.35eV indicates high conductivity of the polymer and could be attributed to the entrapment of paratacamite during polymerization. The entrapped Cu2(OH)3Cl increases the electronic percolation due to conjugation in the polymer via H-bonding and hence reduces the bandgap. The dopants interact with polarons and bipolarons to increase the electrical conductivity of PDPpy [43,44]. The polaron - bipolaron states (c.f Scheme 2) explains electrochemical behaviour of conducting polymers. After HER, the band gap is found to be 1.12eV which is less than the band gap of PDPpy before HER. During the Cyclic Voltammetry (CV) experiments, continuous diffusion of protons from the bulk of the electrolyte to PDPpy is generated leading to increased protonation of PDPpy after HER. This increased protonation, increases the conductivity of PDPpy as well as reduces the bandgap after HER by 0.23eV cf. Figs. 8 and 9. This reduction in band gap indicates the stability of PDPpy without degradation after HER. Fig. 7 (a) shows 1H residual solvent chemical shift at 2.49 ppm for DMSO‑d6 and 1H HOD chemical shift at 3.3 ppm accounts for DMSO‑d6. Broad peak observed in between 7.0 and 7.2 ppm is attributed to the hetero nitrogen bonded to hydrogen (cf. Fig. 7) [6]. To confirm that the 7e7.2 ppm peak is due to Cu2(OH)3Cl entrapped in Ppy, 1H NMR spectra of the PDPpy is recorded in the presence of NaOH. Identical broad peak around 5e6 ppm is noticed indicating the removal of paratacamite from Ppy via precipitation as Cu(OH)2. In Fig. 7(b) the peak occurs at lower chemical shift of 5e6 ppm, due to formation of H-bonds between free OH interaction and hetero nitrogen atom. This experiment confirms the entrapment of Cu2(OH)3Cl in Ppy which is precipitated as Cu(OH)2 in the presence of excess OH ions due to common ion effect. The common ion effect is expected to shift the OH peak to lower ppm. As anticipated, the OH proton peak is shifted to 5e6 ppm from 7 to 7.2 ppm in the presence of NaOH solution. The 1H NMR spectra for Ppy synthesized by other oxidants are provided in Supporting Information S3. The high-resolution XPS spectra of C1s, N2s, O1s and Cl2p is shown in Fig. 8. The C1s core level spectra in Fig. 8(a) exhibits peak at 283.87 eV attributed to the C-H and C-C groups in the Ppy backbone and the other peaks situated at 284.8 and

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Fig. 6 e (a) UVevis Spectra of PDPpy before HER, (b)Tauc's plot for PDPpy before HER, (c)UVevis spectra after HER and (d) Tauc's plot for PDPpy after HER.

287.01 eV representing the doped Ppy [47]. The N1s core level spectrum (Fig. 8(b)) is deconvoluted into three components. The first peak at the lower binding energy (397.8 eV) corresponds to nitrogen in imine state of the oxidized Ppy. The high intensity peak at 398.9 eV indicates secondary amine nitrogen of the Ppy backbone. Third signal with higher binding energy of 399.5eV is assigned to positively charged nitrogen (Nþ) of the doped PDPpy backbone. In addition to the implicit confirmation by XRD, FTIR, UVevis spectra, explicit confirmation is carried out via 1HNMR and XPS analysis. The highresolution Cu2p peak from paratacamite core level XPS spectra is shown in Fig. 8(c). The peak fitting of core Cu 2p3/2 level with binding energy of 934.2eV is attributed to the Cu (II) state. The peak fitting shows Cu2p level consists of two peaks at 943 and 953.5 eV, both corresponding to Cu (II). The presence of Cu (II) peaks in Cu2p XPS spectra confirms the doping of Cu2(OH)3Cl in Ppy and this is in agreement with the literature [4]. The Cl2p core level spectrum in Fig. 8(d) appeared at 197.32, 198.95 and 201.24 eV. The first two peaks at 197.32 and

198.95eV accounts for the covalently bonded chloride ions with Ppy polaron [48], whereas the 201.24eV peak is due to CuCl bonding. In general, free Cu-Cl XPS peak appear at 200.8eV [49]. In the present scenerio a slight shift towards higher binding energy (201.24eV) could be due to the presence of rhombohedral paratacamite, with hydroxyl groups bridging the two Cu(II) ions and the Cl covalently attached to first Cu(II) gets H-bonded with hydroxyl group of second Cu(II) [6]. This is also supported by FTIR analysis (cf.Fig. 4). The O1s spectrum (Fig. 8(e) fit into three components with binding energies of 530, 531.8 and 533.02 eV and contributed 19.8% of the total area. These were assigned to Cu(I)eO and Cu(II)eO, OHeCueOH bonds [50]. The presence of Cu(I)-O could be justified based on the structure of paratacamite. As the two Cu2þ ions are connected by eOH bridge, each Cu2þ ions is sensed as partially polarized Cu(I)-O. TheNC ratio from XPS is estimated to be 5.81. In general for Ppy, if NC is greater than 5, it indicates formation of branched polymer [4]. In the present case, asNC >5, branched Ppy chain formation is justified. The

Fig. 7 e 1H NMR spectra of PDPpy in (a) DMSO and (b) with NaOH in DMSO.

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Fig. 8 e High resolution XPS spectra of PDPpy.

mass percentage of Cu is measured as 1.14 and accounts for around 10e12% yield of PDPpy as reported in the literature [4]. Thus, from XPS analysis, we can infer the formation of branched PDPpy linked to linear Ppy (cf.Scheme 3). Elemental composition of PDPpy is provided in Supporting Information S4. Fig. 8(f) represents the full XPS spectrum of PDPpy.

Electrochemical analysis CV of PDPpy as electrocatalyst in 0.01 M H2SO4 is investigated within the potential window of 1 to þ1 V at different load ranging from 1 to 100 mV/s as shown in Fig. 9, to understand the hydrogen storage ability and hydrogen evolution kinetics. Fig. 9(a) shows reduction peaks of PDPpy resulting in H2 evolution. PDPpy happens to maintain electroneutrality during HER in the polymer via diffusion of protons from the bulk which is also confirmed by the UVevis spectroscopy of PDPpy after HER. The band gap of

PDPpy decreases after HER by 0.23eV, due to its protonation via continuous supply of protons from the electrolyte. During synthesis of PDPpy, HCl is used as dopant and after HER H2SO4 (electrolyte) acts as the protonating agent. Since H2SO4 is more acidic than HCl and can supply 2 mol of protons the rate of protonation is faster in H2SO4 after HER. The UVevis studies of PDPpy after HER confirmed stability of PDPpy after HER without degradation. The reduction peak shifts between 0 and 0.25 V with the applied load and the peak shift towards negative voltage indicates HER [51]. Fig. 9(b) depicts the peak current for HER on PDPpy surface at different applied loads. The ip vs load graph shows very negligible change in peak current with increase in load. Thus from CV analysis it can be concluded that, PDPpy involves steady rate of HER irrespective of the load applied. The CV analysis of HER on Ppy synthesized using FeCl3 and APS are provided in Supporting Information S5. The linear correlation obtained from Fig. 9(b) is

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In the present case, PDPpy shows three regions corresponding to different oxidized or reduced states (cf. Fig. 10(a)). Region I: involves more negative potential with positive slope and accounts for the adsorbed proton on PDPpy. Region II is caused by protonation of Ppy from the bulk resuting in Pyrrolium cation and is a constant plateau region1. Region III, corresponds to HER on PDPpy resulting in more positive slope on high current and potential region. Thus the three regions of LSV supports the mechanism predicted for HER on PDPpy in acidic medium (Scheme 5). The LSV analysis of HER on Ppy synthesized from FeCl3 and APS are provided in Supporting Information S6. As charge stored is calculated by voltammetric techniques for conducting polymers at different energy regions [53], the equilibrium charge stored on PDPpy is determined from LSV analysis. The maximum charge stored as protons on PDPpy is 2.13 kF/g (cf.Supporting Information S7). The capacitance decreases with increase in load due to fast supply of protons from bulk solution to the electrode surface. These results demonstrate that PDPpy act as a better electrocatalyst for hydrogen storage. The Ppy synthesized employing FeCl3 and APS resulted in 50% charge storage in comparison with PDPpy (cf.Supporting Information S5). Thus from CV and LSV analysis we can infer that PDPpy possess better ability to store hydrogen in the form of proton. The charge storage capacity of Ppy synthesized by FeCl3 and APS are provided in the Supporting Information S8. Heterogeneous electron transfer processes are those involved in the transfer of electrons between redox active chemical species and a solid-state electrode. The electrochemical heterogeneous electron transfer rate constants khet can be calculated using exchange current density as follows

Fig. 9 e (a) Cyclic Voltammetry of PDPPy coated on Cu and(b) Current density io (Amps/cm2) Vs loadU in 0.01 M H2SO4.

4

6

ip ¼ 1:7*10 ±3:9*10




5

load þ 0:00223±3:3*10




io ¼ nFCkhet

(1)

The intercept value of (0.00223±3.3*105) in Amperes corresponds to average peak current for HER on PDPpy. Based on the CV analysis, the mechanism of HER on PDPpy is deduced as shown in Schemes 4 and 5. The mechanisms for HER on metal surface in acidic medium [25] were represented via Volmer-Heyrovsky-Tafel mechanism. H3Oþ þ e / Hads þ H2O (Volmer)

(2)

2Hads / H2 (Tafel)

(3)

H3Oþ þ e þ Hads / H2 þ H2O (Heyrovsky)

(4)

A reminiscent of Heyrovsky mechanism is observed as plausible mechanism of proton adsorption on PDPpy. Step I: PDPpy react with Hydronium ion and produces Pyrrolium cation; Step II:The doped Pyrrolium cation is further protonated from the bulk of the electrolyte and leads to evolution of H2; Step III: Depleted protons on PDPpy is replenished from bulk as hydronium ion. LSV for PDPpy coated on copper substrate is studied at different applied load (1e100 mV/s), in the potential range of 1 to þ1 V. In general, LSV is employed in the literature to identify polaron-bipolaron states of the conducting polymers such as PANI [52].

(5)

where khet is heterogeneou reaction rate constant (cm/s), io is exchange current density2 (A/cm2), n is number of electrons transferred, F is Faraday constant (96500C/mol), C is bulk concentration of the electrolyte (0.01 M H2SO4). The average khet for HER is in the order Ppy (FeCl3)>Ppy (APS)>PDPpy. The linear correlation of Fig. 10(b) leads to

khet ¼ 2:3*107 ±9:01*109 load þ 4:64*106 ±7:61*108 -6

(6)

8

The intercept of (4.64*10 ±7.61*10 ) in cm/s corresponds to average khet for HER on PDPpy. This low rate of HER on PDPpy in comparison to noble metal electrocatalysts supports the mechanism proposed in Scheme 4, where proton stored is high on PDPpy. The HER analysis on Ppy synthesized from FeCl3 and APS are provided in Supporting Information S9. EIS is carried out before and after HER to understand (i) the charge transfer and (ii) diffusion of ions from bulk of the electrolyte. Fig. 11 show the Nyquist plots of PDPpy in 0.01 M H2SO4 and associated equivalent circuits. Electrical Equivalent Circuit (EEC) models were used to explain the impedance before and after HER on PDPpy in 0.01 M H2SO4, where R1e diffusion Resistance, Rs-solution resistance, R2-charge transfer resistance, Q1,Q2, Q3,Q4 are 1

The current stays constant till protonation of the whole chain of polymer. Hence Region II is a constant plateau region. 2 io is approximately taken as peak current/Area from CV in A/ cm2, where Area of the electrode A ¼ 2 cm2.

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Scheme 3 e Mechanism of polymerization of pyrrole associated with paratacamite entrapment leading to mesoporous structure.

constant phase elements (CPES) and W is Warburg impedance. For a ¼ 1 and a ¼ 0.5; the system corresponds to capacitor and Warburg component caused by diffusion controlled process respectively [53]. The CPEs before HER on PDPpy are Qy1 0.0056U1 S0.5, Qy2 ¼ 0.0086U1 S0.3 with its exponent parameter (Qa1, Qa2) below 0.5. Thus Qy1 and Qy2 account for the diffusion resistance. The rationale behind this conclusion is the fact that the reaction between pyrrole and CuCl2 oxidizes pyrrole to pyrrolium cation radical and reduces Cu (II) to Cu(I). Cu(I) combines with chloride ion to form insoluble cuprous chloride (CuCl). As CuCl is not stable in oxygen and water, it oxidizes to stable Cupric trihydroxy

chloride (Cu2(OH)3Cl) and this insoluble salt get doped into the polymer during polymerization [6]. The diffusion resistance (R2) for PDPpy in 0.01 M H2SO4 after HER is 13.992 and could be attributed to the formation of PDPpy-Hþ intermediate species as well as the hopping of protons on PDPpy surface. From Table 1, we can conclude that Qy1AH ¼ 10Qy1 BH (AH is after HER, BH is before HER). Rct or R2 value observed for PDPpy is very low and is due to the high surface area as revealed by SEM and BET analysis. The presence of Cu2(OH)3Cl dopant facilitates electronic percolation via conjugation and helps in electron transfer on the Ppy surface. Rct ¼ 0.242U indicates more facile and continuous charge transfer on the PDPpy,

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Scheme 4 e Illustrative representation for HER on PDPpy coated on copper in acidic medium.

Scheme 5 e Plausible mechanism for HER on PDPpy coated on copper in acidic medium.

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with the electronic percolation ðDG# EC Þ on PDPpy surface is calculated from Eq. (4) as 0.1 eV. The ionic conductivity of PDPpy is calculated4 from the diffusion resistance (R1) of doped Ppy. The ionic conductivity before HER is 0.0274S/cm and after HER being 0.1602 S/cm. The ionic conductivity increases by an order of magnitude for PDPpy after HER. Before HER, electronic percolation predominated on the surface of PDPpy and hence higher electrical conductivity is observed. After HER, as the PDPpy is saturated with protons, surface hopping of Hþ ions predominates. Thus the PDPpy possess high ionic conductivity after HER. The activation energy associated with proton hopping ðDG# IC Þ is calculated from Eq. (7) using ionic conductivities as # 0.0453eV. Thus,DG# EC ¼ 2:21DGIC . The obtained values of activation energy for ionic and electrical conductivity is in satisfactory agreement with the proton hopping activation energy reported in the literature via Grotthus mechanism [54]. The obtained energies are close to proton breaking energy in H-bonds [54]. Thus the proton interaction with PDPpy is of hydrogen bonding in nature thereby supporting our proposed mechanism in Scheme 4. Exchange current density (io) for HER on PDPpy can be calculated from EIS studies as Rct ¼ RT=nFio

(8)

According to Arrhenius equation Fig. 10 e (a) Linear Sweep Voltammetry of PDPpy coated on Cu and (b) khetvs applied load in 0.01 M H2SO4. caused by transfer of H3Oþ ions from bulk to electrode/electrolyte interface. The Electrical and Ionic conductivity of PDPpy before and after HER in 0.01 M H2SO4 is calculated using Arrhenius model as follows: s ¼ so exp

DG# RT

(7)

where so is conductivity of PDPpy before HER, s being conductivity of PDPpy after HER. The electrical conductivity before HER is calculated3 using charge transfer resistance (Rct or R2) component as 4.132 S/cm. This value is in satisfactory agreement with the literature [6]. The electrical conductivity after HER is calculated from appropriate R2 as 0.0715 S/cm. Two orders of magnitude decrement in the electrical conductivity after HER indicates the reduction in the electronic percolation on PDPpy after HER. The rationale behind this observation is that during HER, PDPpy gets saturated with protons and the supply of protons after HER is spontaneous. Thus all available lone pair and unbonded electrons including those available with Cu2(OH)3Cl is utilized for protonation. The activation energy associated

3 For before HER, R2 ¼ 0.242U; Resistivity r ¼ R2A/l; A ¼ Area of electrode in cm2; l ¼ length of electrode in cm; r ¼ 0.242 U cm so¼1/r ¼ 4.132 S/cm. For after HER, R2 ¼ 13.992U; Resistivity r ¼ R2A/l; A ¼ Area of electrode in cm2; l ¼ length of electrode in cm; r ¼ 13.992 U cm so¼1/r ¼ 0.0715 S/cm.

khet ¼

kb T DG# exp h RT

(9)

where kb, T,h,  DG# , R, khet represents Boltzmann constant in J/K, Temperature in 298 K, Plancks constant in Js, activation energy in kJ/mol, Gas constant in J/K/mol, heterogeneous rate constant (cm/s) respectively. Using Eqs. (7)e(9), the calculated free energy of activation for charge transfer process on PDPpy before and after HER is almost identical (cf.Table 2). PDPpy shows lower activation energy after HER indicating facile protonation after HER. This supports the controlled HER rate on PDPpy, as the Rate of protonation>Rate of HER (ca. PDPpy possess excess protonation than required for HER). Thus from EIS analysis, we can conclude that PDPpy is a better electrocatalyst for hydrogen storage (as protons) and involves steady HER irrespective of the load applied. The EIS analysis and khet values of Ppy synthesized from FeCl3 and APS are provided in Supporting Information S10.

Galvanostatic charge -discharge studies Hydrogen storage capacity can also be determined by charge/ discharge profile [20]. The potential is maintained between 0.5 and 0.5 V vs standard Ag/AgCl electrode. Charge/ discharge profile of PDPpy in 0.01 M H2SO4 is recorded for 100 cycles at 1 A/g cf. (Fig. 12(a)). In general, specific capacity is calculated as follows

4 For before HER, R1 ¼ 36.499U; Resistivity r ¼ R2A/l; A ¼ Area of electrode in cm2; l ¼ length of electrode in cm; r ¼ 36.499 U cm so¼1/r ¼ 0.0274S/cm. For after HER, R2 ¼ 6.244U; Resistivity r ¼ R2A/l; A ¼ Area of electrode in cm2; l ¼ length of electrode in cm; r ¼ 6.244 U cm so¼1/r ¼ 0.1602 S/cm.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 7 7 3 e6 7 8 6

Fig. 11 e (a) Nyquist plot before HER, (b) after HER and its equivalent circuit fitting, (c) before and (d) after HER in 0.01 M H2SO4

Table 1 e Equivalent circuit parameters for PDPpy in 0.01 M H2SO4. Parameter Before HER After HER

C¼

Rs (U) 22.899 35.906

Qy1

Qa1 3

5.6  10 6.18  104

0.559 0.774

R1 (U)

Qy2

36.499 6.2440

I*Dt m

0.0086 2.22  108

(10)

where I in mA is discharge current density (10 mA), Dt in seconds denote discharging time, and m represents the weight of PDPpy coated on Cu substrate (10 mg). PDPpy demonstrated initial capacity 59.1 mAh/g as shown in Fig. 12(b). The obtained capacity is in the same order and is ~11% higher than Ppy- Fe2O3 nanocomposite reported in the literature [23]. The reason behind this enhanced hydrogen storage capacity being the possibility of dimer and trimer formation during synthesis of PDPpy using CuCl2, as oxidant [55]. The existence of dimer and trimer configurations increases the hydrogen adsorption sites [55] and thus PDPpy shows more hydrogen storage than Ppy-Fe2O3. In Fig 12(b), at

Table 2 e Rct, io, khetvalues calculated form Nyquist plot of HER on doped Ppy.

Before HER After HER

0.242 13.99

0.1061 1.8  103

khet (cm/s)

Activation energy for HER (kJ/mol)

5.49  105 9.32  107

290.7 286.3

0.339 0.997

R2 (U) 0.242 13.992

W e 99.960

Qy3 8.5  10 e

Qa3 6

4.221 e

7th cycle slight increase followed by decrease in the capacity is noticed and it is atrributed to the continuous protonation of PDPpy after HER from the bulk of the electrolyte. The reproducibility of this increase in capacity after 7th cycle is due to the existence of different hydrogen adsorption sites in the PDPpy such as Ppy and Cu2(OH)3Cl and the adsorbed hydrogen is in equilibrium with the electrolyte. From 60 to 100 cycles, the capacity reduces to 10 mAh/g indicating the reduction in the stability of PDPpy (Fig 12(c)). Although PDPpy does not degrade upon HER, the coating on Cu surface corrodes after 100 cycles leading to fall in the capacity. The maximum capacitance by Ppy-metal nanocomposite is 266.7 F/g [23] and CuCl2 doped Ppy/MWCNTs being 312F/g [5]. Specific capacitance calculated from this charge/discharge of PDPpy in the present studies being 212.9F/g C¼

Rct (U) Calculated io (A/cm2)

Qa2

I*Dt mDV

(11)

So far researchers used Ppy composite with highly conducting materials such as MWCNTs and graphene to achieve the capacitance of nearly 300F/g but PDPpy reaches capacitance nearly 213F/g without any other dopant influence. Capacitance of Ppy doped with 2 wt% of CuCl2 shows 213F/g [5] which is identical to the capacitance achieved in the present studies. This proves the ability of PDPpy to store hydrogen and

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 7 7 3 e6 7 8 6

Fig. 12 e (a) Time vs potential, (b) Capacity of PDPpyin 0.01 M H2SO4 is recorded for 100 cycles and (c) Charging and discharging capacity of PDPpy vs No. of Cycles.

upon further optimizing the process further enhancement in the hydrogen storage capacity can be achieved.

Conclusion PDPpy was synthesized by wet chemical method using CuCl2 as oxidant. Structural, morphological and optical properties of the synthesized polymer had been studied. BET reveals that PDPpy shows higher surface area of 36.67 m2/g. Electrochemical studies such as CV, LSV, EIS of dip coated PDPpy on Cu demonstrated good hydrogen storage and evolution kinetics in 0.01 M H2SO4. Capacity of hydrogen storage is found be 59.1 mAh/g via galvanostatic charge/discharge and is approximately 11% higher capacity than Fe2O3 doped Ppy. These results revealed that PDPpy can act as better electrocatalyst for hydrogen storage and controlled evolution of hydrogen.

Acknowledgements The authors thank Department of Science and Technology (DST/INSPIREFELLOWSHIP/2015/IF150980), Government of India for funding the Senior Research Fellow with DSTINSPIRE fellowship. Authors thank Nanotechnology Research centre for SEM results and Department of Physics and

Nanotechnolgy, SRM IST for FTIR and XRD results. The authors thank Dr. Pintu Kumar Kundu for UVevis Spectroscopy facility. The authors thank Prof. Dr. G. S. Vinod Kumar for TGA characterisation. The fruitful discussion on polymorphs of Cu2(OH)3Cl with Prof. Dr. G. T. Senthil Andavan, Department of Chemistry, SRM IST, Kattankulathur is gratefully acknowledged. The authors deeply acknowledge the valuable comments and suggestions by the reviewers.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.01.191.

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