Design and optimization of single particle electrodes for the kinetic analysis of hydrogen evolution and absorption on hydrogen storage alloys

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Design and optimization of single particle electrodes for the kinetic analysis of hydrogen evolution and absorption on hydrogen storage alloys J.E. Thomas a,*, R.M. Humana a, S.G. Real a, R.H. Milocco b, E.B. Castro a a

Instituto de Investigaciones Fisicoquı´micas Teo´ricas y Aplicadas (INIFTA), Fac. de Ciencias Exactas, UNLP, CCT La Plata-CONICET, C.C. 16 Suc. 4, CP 1900 La Plata, Argentina b Grupo Control Automa´tico y Sistemas (GCAyS), Depto. Electrotecnia, Facultad de Ingenierı´a, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuque´n, Argentina

article info

abstract

Article history:

In the literature, there is a large discrepancy between reported values of electrochemical,

Received 26 August 2011

kinetic and transport parameters of hydrogen storage alloys. These discrepancies arise,

Received in revised form

because in most cases, electrodes are prepared with the powdered alloy supported within

28 December 2011

a porous matrix, constituted by carbon and additive binders such as PTFE. The main

Accepted 31 January 2012

drawback, of this preparation technique, for the identification of kinetic parameters, is the

Available online 28 February 2012

uncertainty in the specific active area value, where the hydrogen evolution and absorption processes take place. To overcome the disadvantages described, a new type of electrode,

Keywords:

was designed, using a single particle of AB5 and AB2 hydride forming alloys. The data

Hydrogen

obtained from electrochemical impedance measurements were adjusted in terms of

Metal hydride

a physicochemical model that takes into account the processes of hydrogen evolution and

Kinetic study

absorption coupled to hydrogen diffusion. From the study it can be concluded that the differences in the behavior of the AB5 and AB2 alloys, presenting the first best performance during the activation and operation at high discharge currents, are mainly due to higher values of the exchange current density and the diffusion coefficient of H for the AB5 alloy. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The performance of the metal hydride electrode, during the battery operation, is related to the rate of mass and charge transport processes and of the charge transfer interfacial reactions related to the electrochemical hydriding process. Accordingly, knowledge of kinetic and transport parameters such as the exchange current density (i0) and the diffusion coefficient of hydrogen (DH) is essential in the design of new electrode materials. In the literature, there is a large discrepancy between reported values of electrochemical, kinetic and

transport parameters of hydrogen storage alloys, these discrepancies arise, because in most cases, electrodes are prepared with the powdered alloy supported within a porous matrix, constituted by carbon, nickel sponge or copper powder and additive binders such as PTFE, used to improve cycling stability. The main drawback, of this preparation technique, for the identification of kinetic parameters, is the uncertainty in the specific active area value (aa), where the hydrogen evolution and absorption processes take place. In this case it is necessary to estimate aa, using several simplifying assumptions, such as the assumption of spherical alloy particles of

* Corresponding author. Tel.: þ54 2214257430; fax: 54 2214254642. E-mail address: [email protected] (J.E. Thomas). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.181

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Nomenclature aa Ap AB5 AB2 Cdl Cmax DH E F f fc H i i0 j JH Jp Ki Keq L

Active area per electrode unit volume Electrode geometric area Hydride forming alloy Hydride forming alloy Double layer capacitance Maximum H concentration in the alloy Diffusion coefficient of H in active material Measured potential Faraday constant. Frequency Characteristic frequency Hydrogen Current density Exchange current density pffiffiffiffiffiffiffi imaginary number 1 Radial flow of H. Cost function Reaction constant of step i Equilibrium constant of H absorption reaction (HAR). Electrode thickness

mean radius. This procedure gives rise to a large discrepancy in the parameter values published in different papers [1]. In recent works single particle electrodes have been used to study the kinetics of metal hydride electrodes [2,3]. In these works the system is assembled by attaching a single particle to a carbon fiber by means of a micropositioner, spherical geometry supposition is also required to solve mass transport equations. In the present work a new single-particle electrode design is presented. The electrode is constructed using a single particle of hydride forming alloy, assembled in such way that a definite geometric surface area of about 1  103 cm2 is in contact with the electrolyte. AB5 and AB2 alloys were used to study the kinetics of the hydrogen evolution and absorption processes. The parametric identification was performed by fitting electrochemical impedance (EIS) data in terms of a physicochemical model based on the classical theory of porous flooded electrodes [4] and considering the mechanism of hydrogen evolution and absorption occurring at the electrochemical interface associated with the specific active area.

2.

Experimental

Working electrodes were prepared with AB5 and AB2 alloys with nominal composition: LmNi4.1Co0.4Mn0.4Al0.5 and Zr0.9Ti0.1NiMn0.5Cr0.25V0.25, respectively. The electrodes were constructed using micropipette tips as external support, inside of which a single alloy particle, with a diameter less than 1 mm, was placed. A good electric contact was attained using a Ni wire and making pressure on the particle with a threaded PTFE support. After this assembly, the tips were loaded with a liquid epoxy resin by vacuum, to avoid the formation of air bubbles within the resin. Once the resin solidified on the tips, the electrodes were worn out until an

M MHad R T S SHab vi xs Zdl ZF Zf Zi Zp

Active site in the alloy surface Adsorbed H atom on a surface Gas constant Temperature Interstitial place in the alloy An H atom in an interstitial site Reaction rate of step i Fraction concentration of H atoms in the active material Double layer capacitance impedance per unit volume Faradaic reaction impedance per unit volume Faradaic impedance per unit area Interfacial impedance per unit volume Porous impedance function

Greek letters b Symmetry factor k Effective electrolyte conductivity q Fractional surface concentration of Had u Angular frequency G Maximum surface concentration of Had

alloy surface of approximately 1  103 cm2 was exposed. The electrodes were subsequently polished to mirror with alumina and washed with distilled water for 5 min in an ultrasonicator. A schematic diagram of the device is presented in Fig. 1. SEM measurements were used to check the electrode surface. Electrochemical measures were carried out in a threecompartment cell, using a NiOOH counter electrode and a Hg/ HgO reference electrode, all potentials in the text are referred to the Hg/HgO reference electrode. The electrolyte was KOH 6 M solution previously deaerated with N2 bubbling and with a surface flow of N2 during the experiments. All measures were carried out at a controlled temperature of 30  C. The electrodes were cycled for a few cycles at 50 mV/s between 1 and 0.4 V to check the electric contact and to eliminate any possible impurity of the surface of the alloy. Electrochemical Impedance Spectroscopy (EIS) measures were carried out at constant potential, in the following order of potentials E ¼ 0.8, 0.85, 0.875, 0.925, 0.95, 1, 1.05 V, waiting 15 min before the frequency sweep, in order to attain steady-state current values, being I z 0 for E > 1 V, (equilibrium condition). EIS spectra were recorded between 100 kHz > f > 1 mHz with 5 points per decade and a 10 mV amplitude. After EIS experiments chargeedischarge cycles at constant current were carried out. Charging curves were performed at 50 mA for 8000 s and for the discharge a 5 mA discharge current with a cut off potential of 0.6 V was applied. After 5 charge/ discharge cycles the electrode surface was checked by SEM.

3.

Results and discussion

Fig. 2 shows the front view of a polished electrode, previous to the experiments. In Fig. 2 the outer circumference is the tip with the epoxy resin holding the alloy particle. Before

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Fig. 1 e Electrode assembly.

conducting EIS experiments, voltammetric runs were performed in order to check the electrode response and conductivity. As depicted in Fig. 3, the electrodes provide appreciable hydrogen evolution currents. No peaks related to the formation of hydride were detected, probably due to the high voltammetric scan rate. It is also noted that the electrodes have a good conductive response and a fairly flat double layer region with no impurity peaks. In general, all electrodes prepared by this technique have good electrical and electrochemical responses, acceptable for conducting EIS experiments. Figs. 4 and 5 present EIS spectra for AB5 and AB2 alloys recorded at different potential values. The impedance responses show similar features for both electrode materials. At more anodic potentials E > 0.95 V, the diagrams show a complex capacitive response related to the competitive processes associated with absorption/diffusion and H2 evolution. At E < 1 V, H2 evolution dominates the impedance response of the system in accordance with the presence of a single capacitive time constant whose magnitude decreases with cathodic potential, indicating a kinetic mechanism regulated by the Volmer-Heyrovsky route for the H2 evolution reaction [5].

Fig. 2 e Front view of the electrode.

Fig. 3 e Voltammetric scan response of AB5 electrodes previous to EIS experiments (v [ 50 mV/s).

The comparison between Figs. 4 and 5, indicates that the EIS response of the AB2 alloy is associated with impedance values more than 3 times higher than those related to the AB5 alloy. Fig. 6 shows an enlargement of Fig. 4 in the high frequency region (1  105 Hz > f > 100 Hz). The figure shows a decrease in the impedance phase angle with increasing cathodic potential. This transition, from phase angle values close to 900 at high frequencies, to values close to 450, indicates a surface transformation of the electrode, which changes from a rough surface to a surface associated with a porous structure, whose impedance response is related to a 450 phase angle at high frequencies and a transition to 900, below the so called “characteristic frequency” [4,6]. In order to identify the parameters of the system, a fitting procedure of the experimental impedance data in terms of the proposed model (see Appendix section) was accomplished. A fitting program was developed, based on the NeldereMeade simplex search algorithm [7]. The fitting was considered acceptable when, the cost function, Jp < 5.103. Fitting results corresponding to the AB5 electrode, calculated using eqs. (6) and (7), are depicted in Fig. 7. For the AB5 electrode, the parameters, derived from the fitting procedure, used in the calculation of Zp, are depicted in Table 1. The following parameters were considered constant [8]: Ap ¼ 2e3 cm2, Cmax ¼ 0.06 mol/cm3, G ¼ 1e9 mol/cm2, bi ¼ 0.5, Cdl ¼ 5e5 F/cm2. The parameters Keq and k1 were not identifiable [9], as the same Jp values were obtained for values of Keq and k1 changing in a wide range, accordingly xs values could not be calculated. Fitting results corresponding to E ¼ 1 V are not reliable as the equilibrium condition I ¼ 0, is not strictly met. Fitting results corresponding to the AB2 electrode, calculated using eqs. (1), (3)e(5), (7), are depicted in Fig. 8. For the AB2 electrode, the parameters, derived from the fitting procedure, used in the calculation of Zp, are depicted in Table 2. L ¼ 0.05 cm was considered, this assumption leads to less reliable fitting results.

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Fig. 4 e Nyquist plots, corresponding to AB5 electrode, recorded at different potentials.

The following parameters were considered constant [8]: Ap ¼ 3e3 cm2, L ¼ 0.05 cm, Cmax ¼ 0.09 mol/cm3, G ¼ 1e9 mol/ cm2, bi ¼ 0.5, Cdl ¼ 5e5 F/cm2. The parameters Keq and k1 were not identifiable [9], as the same Jp values were obtained for values of Keq and k1 changing in a wide range, accordingly xs values could not be calculated. Fitting results corresponding to E ¼ 1.05 V are not reliable as the equilibrium condition I ¼ 0, is not strictly met. The identified parameters assembled in Tables 1 and 2 indicate lower values of i0 and DH for the AB2 alloy with respect to the AB5 alloy, resulting i0 about 2 orders of magnitude and DH, 6 orders of magnitude larger for the AB5 alloy. This fact is consistent with better performance of AB5 alloys, in relation to its fast activation and higher “rate capability”. The value of i0 in hydride forming alloys is related to the variable H concentration, x, according to ref. [10]. In our work it was not possible to establish the relation between i0 and x, as x could not be identified.

Fig. 6 e Enlargement of the high frequency region of Fig. 5.

Fig. 7 e Experimental and theoretical Nyquist diagrams, corresponding to the AB5 electrode.

After the EIS experiments the electrodes were analyzed with an optical microscope and the image depicted in Fig. 9 expose the roughening of the surface, due to H absorption, predicted by EIS measurements at high frequencies. These electrodes were afterward subjected to chargeedischarge cycles at constant current. Charging curves were performed at 50 mA for 8000 s and for the discharge

Table 1 e Parameters derived from the fitting procedure, electrode AB5.

Fig. 5 e Nyquist plots, corresponding to AB2 electrode, recorded at different potentials.

E/V

i0/A cm2

D/cm2s1

0.875 0.900 0.925 0.950 1.000

5.5  104 6.2  104 2  104 2.5  104 1.4  103

5.0 2.5 1.0 8.0 1.5

    

109 109 109 1010 109

k2/mol s1 cm2

ST/cm2

2.6  109 3.3  109 6  109 8  109 2.9  108

0.070 0.126 0.200 0.189 0.040

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Fig. 8 e Experimental and theoretical Nyquist diagrams, corresponding to the AB2 electrode.

Table 2 e Parameters derived from the fitting procedure, electrode AB2. E/V

i0/A cm2 DH/cm2s1

0.850 0.875 0.900 0.950 1.050

1.0 4.1 6.7 7.0 1.0

    

106 106 106 106 104

4.0  3.2  1.5  4.0  4.0 

1016 1016 1016 1016 1016

k2/mol s1 cm2 2.5  5.7  1.0  4.0  7.2 

1013 1013 1012 1012 1011

aa/cm1 K/U1 cm1 9900 6890 6730 1  104 8800

0.0010 0.0012 0.0013 0.0014 0.0010

a 5 mA discharge current with a cut off potential of 0.6 V was applied. After 5 charge/discharge cycles the electrode surface was checked by SEM. The results of these experiments show that the unique particle electrode responds in a similar way as porous electrodes with a large amount of alloy when subjected to chargeedischarge cycling. The discharge capacities

Fig. 10 e SEM photographs of (a) AB5 and (b) AB2 electrodes after 5 charge-discharge cycles.

increase as the particles break and provide greater surface area exposed to electrolyte. During the first five charge/discharge cycles, the activation process of the particle takes place. Fig. 10 shows photographs of the electrodes after five complete charge/discharge cycles, showing important changes in the exposed active area, with high roughening of the surface and some cracks perpendicular to the surface.

4.

Fig. 9 e Images of electrode AB5 after EIS experiments.

Conclusions

This paper presents a new design and assembly of electrodes for the determination, of physicochemical and kinetic parameters of hydride-forming alloys. Using the EIS technique and fitting experimental results in terms of a theoretical physicochemical model, values for exchange current and diffusion coefficients of H, were determined, at different potentials in the region of hydriding and evolution of H2, without the need to assume values of active areas and without the interference of the conductive supports or binders such as Teflon.

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frequencies, in this case it is necessary to use the complete Zp function given by eq. (1).

Acknowledgments This work has been financed by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET) of Argentina, the Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica and Universidad Nacional de La Plata.

Appendix A. Theoretical analysis In the following section a physicochemical model is developed from which the theoretical impedance function of the electrochemical systems, Zp, is derived. Zp was fitted to experimental EIS data, in order to identify the structural and kinetic parameters of the analyzed systems. The electrode/electrolyte interface was modeled as a flooded porous structure. The corresponding impedance function, Zp, for highly conductive solids, given by eq. (1), has been derived in previous publications [4,6,8]. Zp ¼

  L 1 Ap k n tanh n

(1)

A.1. Derivation of Zf In the derivation of Zf the classical kinetic mechanism, given by the Volmer(I)-Heyrovsky(II)-Hydrogen Absortion Reaction (HAR) (III) scheme, is considered [5,9]. The HAR Reaction (III) is followed by H transport into the bulk of the active metal. H transport is described in terms of Fick’s laws [11]. As the impedance measurements were recorded close to the equilibrium condition, i z 0, we shall assume additionally that the potential perturbation does not disturb the equilibrium of the HAR step (III) [7]. The function, Zf, for this system has been derived in previous publications [7,12]: ART  Zf ðuÞ ¼ RT þ  1B F C þ Gju þ VMðuÞ

where G corresponds to the maximum surface concentration of adsorbed hydrogen, Had. M(u), is the mass-transfer function [9,13] and it is derived solving Fick’s laws for the corresponding geometry and boundary conditions. In this case semiinfinite planar geometry is assumed:

where:  1=2 1 Zi1=2 n¼L k

MðuÞ ¼ (2)

Being, Ap, the electrode geometric area, L, the porous structure thickness, k the effective conductivity of the electrolyte (S/cm) and Zi the interfacial impedance per unit volume (U cm3). Zi implies the double layer capacitance impedance (Zdl) linked in parallel with the faradaic reaction impedance (ZF), i.e. 1 Z1 ¼ Z1 i dl þ ZF

1 juCdl aa

(4)

Zf ZF ¼ aa

(8)

Cmax corresponds to the maximum admissible H concentration, in the alloy and DH is the diffusion coefficient of H. In eq. (7) the different constants correspond to: 1 i0 F ¼ RT RT

(9)

A ¼ FðK1 þ K1  K2  K2 Þ

(10)

B ¼ 2ðF=ðRTÞÞðK2 qÞ

(11)

C ¼ 2ðK2 þ K2 Þ

(12)

 2 . Keq V ¼  Keq q þ ð1  qÞ

(13)

Keq ¼ K3 =K3 :

(14)

(5)

Being, aa the interfacial area per unit vol. (cm1) and Zf the faradaic impedance per unit area (U cm2). The other symbols have the usual meaning [7]. Using eq. (1) it can be demonstrated that at low frequencies, below a certain “characteristic frequency”, fc (Fig. 6), dependent on the values of K, L and Zi, it is possible to consider the porous structure as a “planar” electrode, of area ST, ST ¼ ApLaa Accordingly, Zp, is given by: Zp ¼

1 pffiffiffiffiffiffiffiffiffiffiffi DH ju

Cmax

(3)

where Zdl ¼

(7)

1 juCdl ST þ

ST Zf

(6)

This procedure applies to the AB5 electrode response, below fc ¼ 100 Hz. The identification of ST is easily performed, fitting EIS data in terms of eq. (6). For the AB2 electrode, it is not possible to detect this transition in the phase angle at high

Being, i0 the exchange current density, Ki the reaction rates of steps I, II and III, q the fractional surface concentration of adsorbed hydrogen, Had and xs ¼ CsH =Cmax , CsH is the H concentration in the alloy, just below the metal surface. The steady-state equilibrium condition, iF ¼ 0, imposes additional relations which allow to reduce the independent parameters in the fitting procedure [9]. The theoretical impedance function may be calculated in terms of eqs. (6) and (7) (“planar” surface, AB5 electrode), or eqs. (1), (3), (4), (5) and (7) (porous surface, AB2 electrode), giving adequate values to the different parameters of the system. For the AB2 electrode, it is not possible the identification of i0 and DH, due to the uncertainty in the value of L. Nevertheless even in the case of the AB2 alloy, a fitting procedure was performed, assuming a constant value of, L ¼ 0.05 cm, smaller than the length of the alloy particle.

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