The HER performance of colloidal Pt nanoparticles incorporated in polyaniline

Electrochimica Acta 45 (2000) 4171– 4177 www.elsevier.nl/locate/electacta

The HER performance of colloidal Pt nanoparticles incorporated in polyaniline Maria Grzeszczuk *, Piotr Poks Faculty of Chemistry, Uni6ersity of Wroc*aw, l 14 F.Joliot-Curie St, 50 -383 Wroc*aw, l Poland Received 2 October 1999; received in revised form 28 December 1999

Abstract Dc polarization curves and admittance spectra of PANI electrodes having incorporated Pt nanoparticles are recorded at H2 evolution reaction (HER) potentials, and analyzed in order to determine the kinetic parameters of the process. The physical and equivalent circuit models for the H2 evolution process in aqueous solutions of hydrochloric acid are found similar for PANI with Pt nanoparticles and bulk Pt electrodes. The coverage of the electrode by H atoms, and the rate constants for the two kinetic steps involved in the process of H2 evolution at Pt nanoparticles incorporated with a thin polyaniline film have been determined. The rate constants for the forward Volmer and Heyrovsky reactions are 2 ×10 − 11 and 3×10 − 11 mol s − 1 cm − 2, respectively. The coverage of the electrode by adsorbed hydrogen atoms in the Tafel region is ca. 0.5 and the double layer capacitance decreases with overpotential from 18 to 11 mF cm − 2 (over the range of − 0.2 to − 0.6 V vs. SCE). Thus, the electrochemically available area of the catalytically active surface of the Pt-polyaniline composite electrode is only slightly smaller than the geometrical area of the corresponding bulk Pt electrode. Furthermore, some effects due to the electropolymerization and metal electrodeposition procedures on the electrode properties of the Pt-particles incorporated with thin films of polyaniline are presented. The low overpotential admittance data show deviations from predictions of the classical kinetic model of HER. Together with the Tafel region results, this may indicate specific transport properties and/or nonequivalent adsorption sites for adsorbed H. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Pt-particles; Polyaniline; Thin polymer films; Cathodic H2 evolution; Impedance spectroscopy

1. Introduction Nanometer-size electrode materials have attracted considerable attention in recent years due to the fundamental aspects of their properties as well as many potential applications, including molecular catalysis [1]. Platinum catalyses many processes important for technologies of energy production and environment protection. Specific interactions between the metal catalyst * Corresponding author. Tel.: +48-71-3204336; fax: 48-713282348. E-mail address: [email protected] (M. Grzeszczuk).

and its support are known to introduce new routes of chemical reactivity [2]. The H2 evolution reaction (HER) was selected for investigations in which the catalytic properties of platinum dispersed in polyaniline film electrodes, performing in an aqueous acid solution, were carried out. Particles having sizes ranging from nano- to micrometers were studied. The primary goal was to examine whether interaction of polyaniline with platinum affects the mechanism and kinetics of HER. It was found in part I of these studies [3] that the Pt-particles, grown under the potentiostatic conditions on the thin film polyaniline electrode from millimolar solutions of hexachloroplatinic acid, provided, as expected, the active

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sites for the cathodic evolution of H2. On the other hand, no Pt centers, active for proton discharge, were detected when the potentiodynamic procedure involving cycling dc potential between the oxidized and reduced states of polyaniline was used. The latter method was expected to lead to a distribution of the atomic sized Pt-particles inside the polymer layer. The catalytically active Pt-particles grown on the surface of the polymer layer had a maximum diameter of the order of 10 − 1 mm. The HER performance of the colloidal Pt-particles characterized by diameters of the order of 100 –101 nm [4], and incorporated with polyaniline during its electrosynthesis, is presented in this study.

2. Experimental

2.1. Cell and electrodes The substrate-working electrode was a glassy carbon (gc) disc sealed in Teflon. The auxiliary electrode was a gc rod, not in a separate compartment. The carbon electrodes (Metrohm) were polished and cleaned according to conventional procedures. The reference elec-

trode was a SCE. All electrochemical experiments were conducted in a nitrogen atmosphere and in a cell termostated at 25 9 1°C. Preparation of colloidal suspensions of platinum was based on the procedure described in [4]. A dilute solution of sodium citrate and hexachloroplatinic acid was heated for about 4 h at 90°C and then transferred to the solution used for preparation of the polyaniline electrode. Polyaniline electrodes with incorporated platinum particles were prepared by two methods. The electrode designated as gc/(PANI+ Pt)1 was grown potentiostatically from a 4 M hydrochloric acid + 2 M aniline+ 2 mM platinum — containing solution using a stationary gc electrode held for 10 s at + 0.82 V and then 60 s at +0.70 V. The chronoamperometric curve corresponding to the already described procedure is shown in Fig. 1A. It was observed that the polymer film grows very fast in response to the first potential step. The estimated thickness of the polyaniline layer of gc/(PANI+ Pt)1 was 0.73 mm. The electrode indicated as gc/(PANI+ Pt)2 was generated by means of a potentiodynamic regime. Ten linear potential scan cycles taken between − 0.10 and +0.85 V were applied to a gc electrode rotated at 1500 rpm in 3 M HCl + 1 M aniline+ 4 mM platinum solution (see Fig. 1B). The estimated thickness of the polymer film of gc/(PANI+Pt)2 was 0.12 mm. The thicknesses of the deposited films were estimated from the charge involved in the reversible redox process of the polymer [3]. The Pt load of the electrode is not known but expected to be less than the polyaniline load in the composite. The latter is ca. 1 mg for the gc/ (PANI+ Pt)2 electrode that gives ca. 10 mg cm − 2. All parameters were calculated using the geometric surface area of the substrate electrodes (Ageom = 0.07 cm2).

2.2. Apparatus and chemicals The general-purpose electrochemical system and frequency response analyzer (AUTOLAB electrochemical instruments) were employed for the dc and ac measurements. Trisodium citrate (p.a. POCH-Gliwice) was used without purification and other chemicals were as described in [3].

2.3. Procedures

Fig. 1. Current traces recorded during the electrochemical polymerization of polyaniline with incorporated Pt-nanoparticles, (A) for the gc/(PANI+ Pt)1 electrode; (B) for the gc/ (PANI+Pt)2 electrode (the last cycle voltammogram) (see Section 2 for details).

Measurements of the cell impedance were performed at several dc potentials in the frequency range 0.1 Hz– 50 kHz, resulting in about 100 frequency data points per spectrum. The amplitude of the alternating potential was 5 mV and the dc potential step was 25 mV. The equilibration time before ac measurements at a given dc potential was 600 s and ac response data,

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persed platinum particles being too small to be detected at the maximum resolution attainable with the microscope ( ×104).

3. Results and discussion

3.1. Ac and dc results

Fig. 2. SEM images of the electrodes, magnifications, 3000 × (A) gc/(PANI+ Pt)1, (B) gc/(PANI+ Pt)2.

corresponding to a fifth sine wave, were recorded. The dc polarization curves were recorded using potential scan rate 0.001 V s − 1 and they were assumed to correspond to the steady state conditions.

2.4. SEM micrographs The polymer electrodes were examined ex-situ by scanning electron microscopy (SEM). SEM micrographs shown in Fig. 2 were obtained using a Stereoscan 180 microscope (Cambridge instruments). The photos correspond to electrodes that differ in the thickness of the polymer layer and the method of preparation of the polymer. The main difference between the electrodes seems to be in the degree of roughness, the thinner layer having a smoother surface. At higher resolution, it can be seen that the thin layer is in-homogeneous in composition. Based on comparison to micrographs of other polyaniline electrodes [3,5] it can be assumed that the white features observed in Fig. 2b correspond to oxidized polyaniline, the dis-

Impedance spectra and current-potential curves were recorded in 1 M HCl in order to determine the kinetic parameters of the H2 evolution process. As far as the dc data are concerned, the electrodes studied in this work differed considerably from each other. Although, the relevant characteristics were time dependent due to slow aging/poisoning effects, the main difference between the two electrodes concerns their behavior at low overpotentials. In the Tafel region, the slopes of the current– potential curves are, in general, smaller for the electrode having the thicker polymer film, i.e. gc/ (PANI+ Pt)1. This would indicate a different mechanism of the electrochemical process as compared with that for the gc/(PANI+Pt)2 electrode. The representative data are shown in Fig. 3. Impedance spectra of the electrodes showed a depressed semicircle in the first quadrant of the complex plane plots. Furthermore, for the gc/(PANI+ Pt)2 case, two additional semicircles were observed in the fourth quadrant of the plane thus featuring a pseudo-inductive behavior of the electrode system. An example is shown in Fig. 4, where the experimental and simulated spectra are compared. The simulation plots were made according to the equivalent circuit model R(LR)(Q(RL)R). This involves two inductances L, a constant phase element Q and four resistances R. The circuit code used is described in [6]. The high frequency pseudo-inductive behavior was found to depend on geometry and/or the overall impedance of the electrode cell, especially that of the auxiliary electrode. Similar properties of other electrode systems have been reported recently [7,8]. The most probable cause of the pseudo-inductive behavior at high frequencies is electrical coupling between cell elements. Therefore, we assume that only the low frequency inductance represents the intrinsic behavior of the electrochemical process at the working electrode. The admittance of the working electrode Ye is: Ye = Ydl + Yf

(1)

where, Ydl is the double-layer admittance and Yf is the faradaic admittance. The behavior of the electrical double-layer is described well by a CPE [9], thus: Ydl = Y0( j…)n

(1a)

The faradaic admittance of the HER can be described by [10]:

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Fig. 3. Dc polarization curves recorded in 1 M HCl under potential control conditions, (1) gc/(PANI+Pt)1; (2) gc/(PANI+Pt)2.

Yf =A+

B C +j…

(1b)

In the above equations, … is the angular frequency, j= − 1, and Y0, n, A, B, C are frequency-independent parameters. Results of ac measurements can be compared with the dc data using the following formula: i=

RT 1 F Rf,… “ 0

(2)

where, Rf,… “ 0 is the faradaic resistance at sufficiently low frequencies and other symbols have their usual electrochemical significance. A comparison of the cur-

rent density versus potential data, obtained using Eq. (2) with that from the steady state dc measurements, is shown in Fig. 5. For the gc/(PANI+Pt)2 electrode, good agreement between the two sets of data was at the higher overpotentals. Results of the measurements for this electrode were subjected to further quantitative analysis in order to determine the rate constants of contributing reaction steps. Furthermore, the double-layer impedance of the gc/ (PANI+ Pt)2 electrode was purely capacitive at almost all studied electrode potentials over the range of − 0.2 to − 0.6 V. The double-layer capacitance for this electrode had values between 18 and 11 mF cm − 2, decreasing as E decreases.

Fig. 4. Complex plane impedance spectra for the gc/(PANI+ Pt)2 electrode in 1 M HCl at −0.475 V. The simulation corresponds to R(LR)(Q(RL)R) electrical circuit model.

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Fig. 5. The experimental dc polarization curve for gc/(PANI+Pt)2 electrode in 1 M HCl compared with the dc current data calculated from the corresponding ac data at low frequencies.

3.2. Kinetic model The mechanism of H2 evolution in aqueous solutions involves formation of an adsorbed H atom intermediate in the so-called Volmer adsorption step, an electrochemical desorption of H, as H2, into solution (Heyrovsky step), and a chemical recombinative desorption of H as H2 (Tafel mechanism [11]. In addition, some electrode materials undergoing specific interactions with H atoms exhibit absorption of H into the electrode that leads to a more complex mechanism of the electrode process [12,13]. The ac behavior for a generalized two-step electrochemical reaction with one adsorbed intermediate was treated first by Armstrong et al. [14] and later by Harrington et al. [10]. The kinetic equations for three steps of the H2 evolution process, involving the overpotential-deposited hydrogen atoms, are as follows [15]:



61 =ki(1− q) exp −

 

(1 −i)F(E− E 0%) RT

(3)

iF(E−E 0%) RT

(4)

×q exp + 62 =k2q exp − 63 =k3q 2

n n

iF(E−E 0%) −k − i RT

n

and other symbols have their usual significance. The rate Eqs. (3)– (5) corresponds to the process without diffusion control and negligible contributions of the backward directions of the Heyrovsky and Tafel steps. The latter is expected to apply at higher overpotentials and/or for ki  k − i. Furthermore, the concentration terms for the reactants are included within the rate constants, and the rates are expressed in units of i/F (where i and F are the current-density and the Faraday constant, respectively). At higher overpotentials, Tafel behavior of the parameters of the electrode admittance can be observed, assuming that the coverage of the reaction sites by adsorbed H atoms is constant. Indeed, at sufficiently high overvoltages, values of q will be driven to the limiting value q= 1 if the Tafel reaction is the rate-determining step in production of H2. However, it will reach only a limiting value q 51 if the Heyrovsky reaction is the principal desorption pathway [12]. Using definitions of parameters A, B, and C [10], one obtains from Eqs. (3)– (5), the following equations corresponding to the potential region where a linear dependence of E on log(P) is observed, where P= A, B, C, as follows:

n

0.5F 2 0.5F(E− E 0) [k1 − k1q +k2q] exp − RT RT

B=

F(E−E 0) 0.5F 3 (k1 − k2)(k2q −k1 + k1q) exp − RT q1RT

(5)

where, the Eqs. (3)–(5) represent rates of the Volmer, Heyrovsky and Tafel reactions, respectively. The k-coefficients are rate constants for the corresponding forward (‘i’ subscripts) and backward (‘ −i’ subscripts) reactions. They are defined as a product of the apparent standard rate constant of the reaction and a concentration term. q is the coverage by the adsorbed H atoms



A=

C=





0

0.5F(E− E ) F (k1 + k2)exp − RT q1

n

(6)

n (7) (8)

where q1 is the charge required for a deposition of a monolayer of H.

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Fig. 6. Parameters of the faradaic admittance for HER at gc/(PANI+ Pt)2 in 1 M HCl in the Tafel region. Points — experimental data, lines — linear regression fits to the experimental data.

The dependencies observed experimentally are shown in Fig. 6 and the kinetic parameters obtained from the analysis of admittance data are collected in Table 1. The rate parameter derived from the steady state dc measurements, using Eq. (9) in the potential region where the Tafel relation is observed, is quoted here for comparison, viz.



i= 2Fk2q exp −

0.5F(E −E 0’) RT

n

(9)

It was assumed, in the usual way, that i=0.5 in Eqs. (6)–(9). The i-values shown in Table 1 were determined from the analysis of the experimental data, and support the above assumption. A considerable deviation of log(C) from the straight line drawn in Fig. 6 can be considered to be due to a contribution from the Tafel reaction competing with the Heyrovsky reaction. Taking into consideration Eq. (8) with an additional term representing a contribution from the recombination of two H atoms results in k3 values having no physical significance (i.e. they are

mostly negative numbers). Thus, it may be assumed that the linear approximation used in the analysis of C-parameter is poor and arises from the scatter in experimental data determining C, i.e. from the low-frequency inductive loop in the complex-plane impedance plots. Furthermore, the Eqs. (6), (7) and (9) were combined to determine the values of k’s and q. The results are as follows: k1 = 2×10 − 11 mol s − 1 cm − 2, k2 = 3×10 − 11 mol s − 1 cm − 2, q =0.4. These values were calculated assuming i= 0.5 and the charge required for a deposition of a monolayer of H was q1 = 210 mC cm − 2 [10]. The k-values quoted above confirm the usefulness of Eq. (8) for the C-parameter analysis, making the proposed kinetic model self-consistent. The surface coverage was calculated assuming that it is a potential-independent parameter over the observed Tafel region. Thus, the value given above can only be considered as an estimate of q and can only be calculated [13] on the basis of the rate constants of the reactions. Then, under all the assumptions referred to above, q =0.6 is obtained. Therefore, a final estimate

Table 1 Rate parameters and symmetry coefficients determined from the electrode admittance and dc polarization data in the cathodic Tafel region for the gc/(PANI+Pt)2 electrode in 1 M HCl Equation

Rate parameter

Rate parameter in units of k(mol s−1 cm−2)

i

R-coefficienta

Eq. Eq. Eq. Eq.

k1−k1q+k2q k1−k2)(k2q−k1+k1q) (k1+k2) k2q

2×10−11 2×10−23b 4×10−11 1×10−11

0.51 0.47 0.51 0.47

0.9981 0.9780 0.8514 0.9984

a b

(6) (7) (8) (9)

R-coefficient for the linear regression fitting of log(P) vs. E and log(i ) vs. E. Rate parameter in units of k 2(mol s−1 cm−2)−2.

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of the surface coverage, q, by adsorbed H at high overpotentials is 0.5 90.1. The polyaniline film electrodes with confined Pt-particles bear some resemblences to electrodes that possess the ability for H atoms to be transported through their bulk, e.g. as with Pd. Interactions of polyaniline with protons and/or hydrogen atoms will depend on its oxidation state [16]. Protonated polyaniline, as with other ammonium cations, can itself be a proton donor for HER [17]. Radical sites along a PANI chain may influence reactivity and transport of the hydrogen atom intermediate. The diffusional process(es) indicated by the impedance characteristics of the polyaniline-Pt(micro) electrode at low overpotentials [3] are, may be, related to such properties [18]. The mechanism of the H2 evolution process at the polyaniline-Pt(nano) electrodes seems to be complex. To find a kinetic model for HER at the low overpotentials, one requires a numerical approach [19]. The electrical model used here could still consist of two subcircuits that can be related to the H2 evolution process but the number of parameters determining the rate of HER would increase. Furthermore, complexity of the system also increases as the faradaic impedance deviates considerably from the theory for an electrode process with one adsorbed intermediate (Eq. (1b)). In order to obtain satisfactory fitting of the experimental data to the model, the pseudocapacitance Cp = −A 2/B has to be replaced by the constant phase element with n$0.70 (EB −0.2, \ −0.4 V). Such behavior can indicate nonequivalent adsorption sites and/or specific transport properties for the adsorbed H. Furthermore, dc polarization curves at low overpotentials, which differ considerably for the two types of electrodes characterized by different thickness and morphology, support such interpretation.

4. Conclusions The composite Pt-polyaniline electrodes studied in the present work were assumed to contain nanoparticles of the metal distributed in the polymer film matrix. Some electrochemical characteristics of the two types of electrodes were similar, e.g. the behavior of the double-layer where the impedances were found to be capacitative at almost all the potentials of HER. A similar behavior had also been observed before for the composite electrode with Pt-microparticles [3]. This must be due to the homogeneous nature of the electrode surface for the double-layer formation at the PANI-Pt electrodes. The present study was focused on the faradaic processes, namely the kinetics of HER. Difficulties with a quantitative analysis of the data were overcome by assuming a simplified mechanism of the process at overpotentials beyond −200 mV. The rates of H2 evolution at the thin-layer, composite Pt-polyaniline

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electrode were to be determined by the rates of proton discharge and electrochemical desorption of hydrogen. The fractional coverage of the composite electrode by adsorbed H atoms was estimated to be 40 – 60% of the underpotential deposited monolayer at a corresponding bulk Pt electrode. The q-value is close to that found for active Pt in 0.5 M NaOH, the rate constants being, however, much lower [15]. Moreover, the Cdl-values may indicate that the electrochemically active surface area of the electrode is similar to the geometric one. The rate constants of the proton discharge and electrochemical desorption appear to have similar values thus indicating a mixed kinetic control of the process by the two reactions, and a high probability of observing pseudo-inductive behavior of the faradaic impedance [19].

Acknowledgements Financial support from KBN (Scientific Research Committee, Poland), through grant No. 2 P303 111 07, is greatly acknowledged.

References [1] G. Schmid, Chem. Rev. 92 (1992) 1709. [2] J.O.M. Bockris, S.U.M. Khan, Surface Electrochemistry, Plenum Press, New York, 1993. [3] M. Grzeszczuk, Electrochim. Acta 39 (1994) 1809. [4] C.S.C. Bose, K. Rajeshwar, J. Electroanal. Chem. 333 (1992) 235. [5] M. Grzeszczuk, G. Z: abin´ska-Olszak, J. Electroanal. Chem. 359 (1993) 161. [6] B.A. Boukamp, Equivalent circuit user manual, 1988– 1989, University of Twente. [7] A. Kisza, Pol. J. Chem. 68 (1994) 613. [8] A. Losch, J.W. Schultze, J. Electroanal. Chem. 359 (1993) 39. [9] G.J. Brug, A.L.G. Van den Eeden, M. Sluyters-Rehbach, J.H. Sluyters, J. Electroanal. Chem. 176 (1984) 225. [10] D.A. Harrington, B.E. Conway, Electrochim. Acta 32 (1987) 1703. [11] S. Trasatti, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemical Science and Engineering, vol. 2, VCH, 1992, p. 17. [12] B.E. Conway, G. Jerkiewicz, J. Electroanal. Chem. 357 (1993) 47. [13] A. Lasia, Pol. J. Chem. 69 (1995) 639. [14] R.D. Armstrong, R.E. Firman, H.R. Thirsk, Faraday Disc, Chem. Soc. London 56 (1973) 244. [15] L. Bai, D.A. Harrington, B.E. Conway, Electrochim. Acta 32 (1987) 1713. [16] M. Kajali, L. Nyholm, L.M. Peter, J. Electroanal. Chem. 313 (1991) 271. [17] B. Wie˛cek, Pol. J. Chem. 67 (1993) 2227. [18] W.C. Conner Jr, J.L. Falconer, Chem. Rev. 95 (1995) 759. [19] L. Bai, B.E. Conway, Electrochim. Acta 38 (1993) 1803.