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Characterisation of Pd electrode surface modified by phase transformation-induced plastic deformation using fractal geometry
Journal of Electroanalytical Chemistry 514 (2001) 118– 122 www.elsevier.com/locate/jelechem
Short Communication
Characterisation of Pd electrode surface modified by phase transformation-induced plastic deformation using fractal geometry Jeong-Nam Han a, Masahiro Seo b, Su-Il Pyun a,* a
Department of Materials Science and Engineering, Korea Ad6anced Institute of Science and Technology, 373 -1 Kusong-Dong, Yusong-Gu, Taejon 305 -701, South Korea b Graduate School of Engineering, Hokkaido Uni6ersity, N13W8, Kita-ku, Sapporo 060 -8628, Japan Received 17 May 2001; received in revised form 16 July 2001; accepted 11 August 2001
Abstract The surface change of a Pd electrode developed by plastic deformation due to the phase transformation of a-PdH to b-PdH has been characterised by fractal geometry. The b-PdH phase formed in the matrix electrode of a-PdH phase causes the plastic deformation of the electrode and hence increases the surface roughness as well as the surface area. The fractal dimension of the Pd electrode surface modified by the plastic deformation was determined from the diffusion-limited peak current density during linear sweep voltammetric measurements. The surface fractal dimension obtained is increased from 1.97 to 2.09 with increasing fraction of b-PdH phase in the matrix electrode from zero to unity. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pd electrode; Phase transformation; Fractal dimension; b-PdH phase; Surface roughness
1. Introduction During hydrogen injection into and extraction from a Pd electrode, the electrode usually undergoes the phase transformation of a-PdH into b-PdH and vice versa, respectively [1,2]. A typical relative volume expansion due to the phase transformation has been reported to be of the order of 10% [3]. Consequently, the Pd electrode that has experienced phase transformation once, is plastically deformed and hence changes its surface morphology markedly. Considering that surface modification incurred by the plastic deformation can greatly affect the electrochemical response of the electrode [4], it is necessary to know how the surface morphology of the electrode is modified with the phase transformation of a-PdH to b-PdH in the electrode. Fractal geometry is an efficient tool for characterising an irregular surface [4 – 12]. Imre et al. [7] investi* Corresponding author. Tel.: + 82-42-869-3319; fax: +82-42-8693310. E-mail address: [email protected] (S.-I. Pyun).
gated fractured carbon steel surface prepared by the Charpy impact test to determine the surface fractal dimension Df. Roberge and Trethewey [4] also characterised corroded aluminium surface by introducing fractal geometry. Recently, Stromme et al. measured the Df of a Sn oxide film [10], a Li insertion electrode [11] and Rf-sputtered In oxide and Sn oxyfluoride films [12]. Electrochemical methods encompassing chronoamperometry [7,8], ac-impedance spectroscopy [11,13] and cyclic (or linear sweep) voltammetry [9–12] have been used for determining Df. Chronoamperometry requires an almost ideal electrolyte with very small resistance losses, and the extracted fractal dimension shows an unphysical variation with the size of the potential step [14]. The interpretation of ac-impedance spectroscopy is not straightforward: the frequency dependence of capacitance on solid electrodes is due to the atomic inhomogeneities rather than the geometry aspects of roughness [15]. For more routine measurement of Df, cyclic (or linear sweep) voltammetry has been preferred. In this work, the surface morphology modified by the plastic deformation due to the phase transformation of
0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 1 ) 0 0 6 2 9 - 5
J.-N. Han et al. / Journal of Electroanalytical Chemistry 514 (2001) 118–122
a-PdH to b-PdH has been investigated using the fractal geometry. The Df was determined by analysing a linear sweep voltammogram: the diffusion-limited anodic peak current density of Fe2 + /Fe3 + redox reaction was measured as a function of scan rate w.
2. Experimental The specimen was cut from a 75 mm thick Pd sheet (Leico Co.) to give a foil of 1× 1 cm dimension. The as-received pure 99.9% Pd foil specimen was annealed under a vacuum of 10 − 2 Pa at 650 °C for 2 h, followed by furnace-cooling. The annealed specimen was mechanically polished with 0.05 mm alumina emulsion. The naked Pt wire was spot-welded onto the Pd foil to provide electrical conduction. First, hydrogen was galvanostatically injected into the Pd electrode at 13.63 mA cm − 2 for various times in 0.1 M NaOH solution to modify the surface morphology by the plastic deformation induced due to the phase transformation of a-PdH to b-PdH. Subsequently, hydrogen was then completely extracted from the electrode by jumping the electrode potential between − 1.2 and − 1.4 V (SCE) to 0 V (SCE) to exclude completely all effects of any possible hydrogen retained in the electrode on the subsequent Fe2 + /Fe3 + redox reaction. Galvanostatic hydrogen injections at a constant current density of 13.63 mA cm − 2 for 0, 9.0×102, 1.8× 103 and 2.7× 103 s gave the composition of the electrode specimens of Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. Hydrogen content l in PdHl electrode pre-charged with hydrogen was calculated by integrating the anodic current transient resulting during subsequent potentiostatic hydrogen extraction with respect to time. All potentials quoted in this work are referred to the value of the potential of the saturated calomel electrode (SCE), which was measured to be about 0.90 V vs. RHE (reversible hydrogen electrode) in 0.1 M NaOH solution (E(SCE) =E(RHE) − 0.90 V) [16]. A Pd electrode, a platinum wire and a SCE were used as the working, counter and reference electrodes, respectively. Two kinds of electrolyte were employed in the present work: an aqueous solution of 0.1 M NaOH for current transient measurements and a mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 for linear sweep voltammetric measurements. The two electrolytes were deaerated by bubbling with purified argon gas before and during the electrochemical experiments. The surface morphology for the plastically deformed and subsequently hydrogen-discharged Pd electrode specimen was observed with an atomic force microscopy (AFM). After that, linear sweep voltammograms were recorded on the same Pd electrode
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specimen in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 within the potential range −0.5 to 1.0 V (SCE) at various scan rates ranging from 10 to 200 mV s − 1. In all electrochemical experiments, both sides of the foil electrode specimen were exposed to the electrolyte at room temperature so that a total exposed area of about 2 cm2 was used.
3. Results and discussion Fig. 1(a), (b), (c) and (d) show the three-dimensional AFM image of the plastically deformed and subsequently hydrogen-discharged Pd electrode specimen, with compositions of the pre-charged state Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. Here, it should be stressed that the surface modification incurred by the plastic deformation due to the phase transformation of a-PdH to b-PdH still remains unchanged even after hydrogen is completely extracted from the electrode. We can readily estimate the mole fraction of a-PdH and b-PdH phases in the pre-charged state electrode from the maximum l of a-PdH phase 0.03 and the minimum l of b-PdH phase 0.6 [3]. From Fig. 1(a), (b), (c) and (d), it is observed that as the fraction of b-PdH phase in the matrix electrode of a-PdH phase is increased from zero through 0.38, 0.77 to unity, the value of the surface roughness is progressively increased. This indicates that the plastic deformation developed by the phase transformation of a-PdH to b-PdH in the electrode increases the surface roughness as well as the surface area. Actually, the increment of surface roughness with the amount of b-PdH in the electrode can be readily monitored by the naked eye. Fig. 2 depicts linear sweep voltammograms obtained from the Pd electrode specimen, with the composition of the pre-charged state PdH0.6, in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 at various scan rates ranging from 10 to 200 mV s − 1. The anodic current arises when the Fe(CN)46 − ions diffuse towards the electrode surface and undergo oxidation. As the scan rate w is increased from 10 to 200 mV s − 1, the anodic peak current density jp,anod is increased from 1.88 to 9.67 mA cm − 2 and at the same time the peak shifts in the positive direction. Similar sets of linear sweep voltammogram were obtained for the Pd electrode specimens, with the composition of the precharged states Pd, PdH0.23 and PdH0.46. The values of jp,anod ( ) obtained from Fig. 2 are plotted on a logarithmic scale against w in Fig. 3(d). The values of jp,anod obtained from the Pd electrode specimens, with the composition of the pre-charged states Pd (), PdH0.23 ( ) and PdH0.46 (), are plotted on a logarithmic scale against w in Fig. 3(a), (b) and (c),
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Fig. 1. Three-dimensional AFM image of the Pd electrode specimen subjected to a series of galvanostatic hydrogen injections and subsequent potentiostatic hydrogen extractions in 0.1 M NaOH solution. The electrode composition of the pre-charged state corresponds to (a) Pd, (b) PdH0.23, (c) PdH0.46 and (d) PdH0.6.
Fig. 2. Linear sweep voltammograms obtained from the Pd electrode specimen in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 at various scan rates ranging from 10 to 200 mV s − 1. The specimen was previously subjected to a series of galvanostatic hydrogen injection and subsequent potentiostatic hydrogen extraction in 0.1 M NaOH solution. The electrode composition of the pre-charged state corresponds to PdH0.6.
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Fig. 3. Plot of anodic peak current density jp,anod against scan rate w, obtained from the linear sweep voltammogram measured from the Pd electrode specimen in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 at various scan rates ranging from 10 to 200 mV s − 1. The specimen was previously subjected to a series of galvanostatic hydrogen injection and subsequent potentiostatic hydrogen extraction in 0.1 M NaOH solution. The electrode composition of the pre-charged state corresponds to (a) Pd, (b) PdH0.23, (c) PdH0.46 and (d) PdH0.6.
respectively. The values of the parameter h, which equals the slope of log jp,anod versus log w plot, were calculated to be 0.48590.008, 0.5059 0.015, 0.5219 0.008 and 0.54590.006 from the linear fit (solid line) to the data points for each Pd electrode specimen. The calculated values of h correspond to the composition of the pre-charged states Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. The standard deviations were found to be 9.2× 10 − 3, 1.8×10 − 2, 9.7 × 10 − 3 and 7.3×10 − 3 for Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. When the measured voltammetric current is limited by diffusion of the electroactive species towards the electrode surface, jp,anod resulting from the linear sweep
voltammetric measurement is proportional to w h, and h is related to the Df of the working electrode by [8–12] D −1 h= f (1) 2 where the parameter h is as defined above. From Eq. (1), the values of the parameter h of 0.485, 0.505, 0.521 and 0.545 were easily converted into the fractal dimensions Df 1.97, 2.00, 2.04 and 2.09, respectively. The value of Df increases with an increase of the fraction of b-PdH phase in the matrix electrode of the a-PdH phase. Bearing in mind that the Df of a fractured carbon steel surface prepared by the Charpy impact test was reported to be in the range of 2.1 and
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2.2 [7], the values of Df obtained in the present work are considered to be reasonable. However, the fractal dimension obtained from this work is smaller than that measured from columnar gold electrodeposits by using chronoamperometric technique [17] and STM/AFM methods [18]. We should note that the value of Df of Pd is less than 2. This indicates that the entire surface is not electrochemically active, but is partially inactive. In the Pourbaix diagram for the system Pd– H2O at 298 K [19], one observes the threshold potential is 0.15 V (RHE) for the equilibrium of a-PdHl and Pd. Above this threshold potential, metallic Pd is thermodynamically stable. Chierchie et al. [20] reported that the PdOH layer is formed on the Pd electrode surface in the potential range 0.7–0.9 V (RHE) and it is transformed into a more stable layer of PdO above 0.9 V (RHE). As a consequence, it is inferred that the formation of PdO at the hydrogen extraction potential 0.9 V (RHE) makes some of the electrode surface inactive. Since the hydrogen extraction conditions of the four electrode specimens (Pd, PdH0.23, PdH0.46 and PdH0.6) are the same, we assume that the effect of PdO formation on the change in the surface roughness is the same for the four electrodes and the change in Df is attributed mainly to the plastic deformation due to the formation of b-PdH in the electrode.
4. Conclusions In the present work, the Pd electrode surface modified by the plastic deformation induced during the phase transformation of a-PdH to b-PdH in the electrode has been characterised by the fractal geometry. By analysing the diffusion-limited anodic peak current density of Fe2 + /Fe3 + redox reaction during linear sweep voltammetric measurements, the fractal dimension Df of Pd electrode surface modified by the plastic deformation incurred due to the phase transformation of a-PdH to b-PdH has been determined. As the fraction of b-PdH phase in the matrix electrode of a-PdH phase is increased from zero to unity, the value of Df is increased from 1.97 to 2.09.
Acknowledgements The present work has been carried out under the auspices of the joint program of Korea Science and Engineering Foundation (KOSEF) and Japan Society for the Promotion of Science (JSPS) 1999/2001. The authors are indebted to KOSEF for the financial support of this work. Furthermore, this work was partly supported by the Brain Korea 21 project.
References [1] H. Peisl, in: G. Alefeld, J. Vo¨ lkl (Eds.), Hydrogen in Metals, vol. I, Springer-Verlag, Berlin, 1978, p. 53. [2] E. Wicke, Z. Phys. Chem. N.F. 143 (1985) 1. [3] M. Mueller, J.P. Blackledge, G.G. Libowitz, Metal Hydrides, Academic Press, New York, 1968, p. 547. [4] P.R. Roberge, K.R. Trethewey, J. Appl. Electrochem. 25 (1995) 962. [5] L. Nyikos, T. Pajkossy, Electrochim. Acta 31 (1986) 1347. [6] U. Rammelt, G. Reinhard, Electrochim. Acta 35 (1990) 1045. [7] A. Imre, T. Pajkossy, L. Nyikos, Acta Metall. Mater. 40 (1992) 1819. [8] Y. Dassas, P. Duby, J. Electrochem. Soc. 142 (1995) 4175. [9] M. Stromme, G.A. Niklasson, C.G. Granqvist, Solid State Commun. 96 (1995) 151. [10] J. Isidorsson, M. Stromme, R. Gahlin, G.A. Niklasson, C.G. Granqvist, Solid State Commun. 99 (1996) 109. [11] M. Stromme, G.A. Niklasson, C.G. Granqvist, Phys. Rev. B 54 (1996) 2968. [12] M. Stromme, G.A. Niklasson, C.G. Granqvist, Phys. Rev. B 54 (1996) 17884. [13] M. Stromme, J. Isidorsson, G.A. Niklasson, C.G. Granqvist, J. Appl. Phys. 80 (1996) 233. [14] K.A. MacDonald, J.M. Bell, J. Barczynska, V. Voelkel, Proc. Soc. Photo-Opt. Instrum. Eng. 2017 (1993) 95. [15] Z. Kerner, T. Pajkossy, Electrochim. Acta 46 (2000) 207. [16] J.-N. Han, S.-I. Pyun, T.-H. Yang, J. Electrochem. Soc. 144 (1997) 4266. [17] P. Ocon, P. Herrasti, L. Vazquez, R.C. Salvarezza, A.J. Arvia, J. Electroanal. Chem. 319 (1991) 101. [18] P. Herrasti, P. Ocon, R.C. Salvarezza, J.M. Vara, L. Vazquez, A.J. Arvia, Electrochim. Acta 37 (1992) 2209. [19] M. Pourbaix, Atlas of Electrochemical Equilibria, Cebelcor, Brussels, 1974, p. 358. [20] T. Chierchie, C. Mayer, W.J. Lorentz, J. Electroanal. Chem. 135 (1982) 211.
Short Communication
Characterisation of Pd electrode surface modified by phase transformation-induced plastic deformation using fractal geometry Jeong-Nam Han a, Masahiro Seo b, Su-Il Pyun a,* a
Department of Materials Science and Engineering, Korea Ad6anced Institute of Science and Technology, 373 -1 Kusong-Dong, Yusong-Gu, Taejon 305 -701, South Korea b Graduate School of Engineering, Hokkaido Uni6ersity, N13W8, Kita-ku, Sapporo 060 -8628, Japan Received 17 May 2001; received in revised form 16 July 2001; accepted 11 August 2001
Abstract The surface change of a Pd electrode developed by plastic deformation due to the phase transformation of a-PdH to b-PdH has been characterised by fractal geometry. The b-PdH phase formed in the matrix electrode of a-PdH phase causes the plastic deformation of the electrode and hence increases the surface roughness as well as the surface area. The fractal dimension of the Pd electrode surface modified by the plastic deformation was determined from the diffusion-limited peak current density during linear sweep voltammetric measurements. The surface fractal dimension obtained is increased from 1.97 to 2.09 with increasing fraction of b-PdH phase in the matrix electrode from zero to unity. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pd electrode; Phase transformation; Fractal dimension; b-PdH phase; Surface roughness
1. Introduction During hydrogen injection into and extraction from a Pd electrode, the electrode usually undergoes the phase transformation of a-PdH into b-PdH and vice versa, respectively [1,2]. A typical relative volume expansion due to the phase transformation has been reported to be of the order of 10% [3]. Consequently, the Pd electrode that has experienced phase transformation once, is plastically deformed and hence changes its surface morphology markedly. Considering that surface modification incurred by the plastic deformation can greatly affect the electrochemical response of the electrode [4], it is necessary to know how the surface morphology of the electrode is modified with the phase transformation of a-PdH to b-PdH in the electrode. Fractal geometry is an efficient tool for characterising an irregular surface [4 – 12]. Imre et al. [7] investi* Corresponding author. Tel.: + 82-42-869-3319; fax: +82-42-8693310. E-mail address: [email protected] (S.-I. Pyun).
gated fractured carbon steel surface prepared by the Charpy impact test to determine the surface fractal dimension Df. Roberge and Trethewey [4] also characterised corroded aluminium surface by introducing fractal geometry. Recently, Stromme et al. measured the Df of a Sn oxide film [10], a Li insertion electrode [11] and Rf-sputtered In oxide and Sn oxyfluoride films [12]. Electrochemical methods encompassing chronoamperometry [7,8], ac-impedance spectroscopy [11,13] and cyclic (or linear sweep) voltammetry [9–12] have been used for determining Df. Chronoamperometry requires an almost ideal electrolyte with very small resistance losses, and the extracted fractal dimension shows an unphysical variation with the size of the potential step [14]. The interpretation of ac-impedance spectroscopy is not straightforward: the frequency dependence of capacitance on solid electrodes is due to the atomic inhomogeneities rather than the geometry aspects of roughness [15]. For more routine measurement of Df, cyclic (or linear sweep) voltammetry has been preferred. In this work, the surface morphology modified by the plastic deformation due to the phase transformation of
0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 1 ) 0 0 6 2 9 - 5
J.-N. Han et al. / Journal of Electroanalytical Chemistry 514 (2001) 118–122
a-PdH to b-PdH has been investigated using the fractal geometry. The Df was determined by analysing a linear sweep voltammogram: the diffusion-limited anodic peak current density of Fe2 + /Fe3 + redox reaction was measured as a function of scan rate w.
2. Experimental The specimen was cut from a 75 mm thick Pd sheet (Leico Co.) to give a foil of 1× 1 cm dimension. The as-received pure 99.9% Pd foil specimen was annealed under a vacuum of 10 − 2 Pa at 650 °C for 2 h, followed by furnace-cooling. The annealed specimen was mechanically polished with 0.05 mm alumina emulsion. The naked Pt wire was spot-welded onto the Pd foil to provide electrical conduction. First, hydrogen was galvanostatically injected into the Pd electrode at 13.63 mA cm − 2 for various times in 0.1 M NaOH solution to modify the surface morphology by the plastic deformation induced due to the phase transformation of a-PdH to b-PdH. Subsequently, hydrogen was then completely extracted from the electrode by jumping the electrode potential between − 1.2 and − 1.4 V (SCE) to 0 V (SCE) to exclude completely all effects of any possible hydrogen retained in the electrode on the subsequent Fe2 + /Fe3 + redox reaction. Galvanostatic hydrogen injections at a constant current density of 13.63 mA cm − 2 for 0, 9.0×102, 1.8× 103 and 2.7× 103 s gave the composition of the electrode specimens of Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. Hydrogen content l in PdHl electrode pre-charged with hydrogen was calculated by integrating the anodic current transient resulting during subsequent potentiostatic hydrogen extraction with respect to time. All potentials quoted in this work are referred to the value of the potential of the saturated calomel electrode (SCE), which was measured to be about 0.90 V vs. RHE (reversible hydrogen electrode) in 0.1 M NaOH solution (E(SCE) =E(RHE) − 0.90 V) [16]. A Pd electrode, a platinum wire and a SCE were used as the working, counter and reference electrodes, respectively. Two kinds of electrolyte were employed in the present work: an aqueous solution of 0.1 M NaOH for current transient measurements and a mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 for linear sweep voltammetric measurements. The two electrolytes were deaerated by bubbling with purified argon gas before and during the electrochemical experiments. The surface morphology for the plastically deformed and subsequently hydrogen-discharged Pd electrode specimen was observed with an atomic force microscopy (AFM). After that, linear sweep voltammograms were recorded on the same Pd electrode
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specimen in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 within the potential range −0.5 to 1.0 V (SCE) at various scan rates ranging from 10 to 200 mV s − 1. In all electrochemical experiments, both sides of the foil electrode specimen were exposed to the electrolyte at room temperature so that a total exposed area of about 2 cm2 was used.
3. Results and discussion Fig. 1(a), (b), (c) and (d) show the three-dimensional AFM image of the plastically deformed and subsequently hydrogen-discharged Pd electrode specimen, with compositions of the pre-charged state Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. Here, it should be stressed that the surface modification incurred by the plastic deformation due to the phase transformation of a-PdH to b-PdH still remains unchanged even after hydrogen is completely extracted from the electrode. We can readily estimate the mole fraction of a-PdH and b-PdH phases in the pre-charged state electrode from the maximum l of a-PdH phase 0.03 and the minimum l of b-PdH phase 0.6 [3]. From Fig. 1(a), (b), (c) and (d), it is observed that as the fraction of b-PdH phase in the matrix electrode of a-PdH phase is increased from zero through 0.38, 0.77 to unity, the value of the surface roughness is progressively increased. This indicates that the plastic deformation developed by the phase transformation of a-PdH to b-PdH in the electrode increases the surface roughness as well as the surface area. Actually, the increment of surface roughness with the amount of b-PdH in the electrode can be readily monitored by the naked eye. Fig. 2 depicts linear sweep voltammograms obtained from the Pd electrode specimen, with the composition of the pre-charged state PdH0.6, in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 at various scan rates ranging from 10 to 200 mV s − 1. The anodic current arises when the Fe(CN)46 − ions diffuse towards the electrode surface and undergo oxidation. As the scan rate w is increased from 10 to 200 mV s − 1, the anodic peak current density jp,anod is increased from 1.88 to 9.67 mA cm − 2 and at the same time the peak shifts in the positive direction. Similar sets of linear sweep voltammogram were obtained for the Pd electrode specimens, with the composition of the precharged states Pd, PdH0.23 and PdH0.46. The values of jp,anod ( ) obtained from Fig. 2 are plotted on a logarithmic scale against w in Fig. 3(d). The values of jp,anod obtained from the Pd electrode specimens, with the composition of the pre-charged states Pd (), PdH0.23 ( ) and PdH0.46 (), are plotted on a logarithmic scale against w in Fig. 3(a), (b) and (c),
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Fig. 1. Three-dimensional AFM image of the Pd electrode specimen subjected to a series of galvanostatic hydrogen injections and subsequent potentiostatic hydrogen extractions in 0.1 M NaOH solution. The electrode composition of the pre-charged state corresponds to (a) Pd, (b) PdH0.23, (c) PdH0.46 and (d) PdH0.6.
Fig. 2. Linear sweep voltammograms obtained from the Pd electrode specimen in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 at various scan rates ranging from 10 to 200 mV s − 1. The specimen was previously subjected to a series of galvanostatic hydrogen injection and subsequent potentiostatic hydrogen extraction in 0.1 M NaOH solution. The electrode composition of the pre-charged state corresponds to PdH0.6.
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Fig. 3. Plot of anodic peak current density jp,anod against scan rate w, obtained from the linear sweep voltammogram measured from the Pd electrode specimen in the mixed aqueous solution of 0.5 M Na2SO4 and 0.02 M K4Fe(CN)6 at various scan rates ranging from 10 to 200 mV s − 1. The specimen was previously subjected to a series of galvanostatic hydrogen injection and subsequent potentiostatic hydrogen extraction in 0.1 M NaOH solution. The electrode composition of the pre-charged state corresponds to (a) Pd, (b) PdH0.23, (c) PdH0.46 and (d) PdH0.6.
respectively. The values of the parameter h, which equals the slope of log jp,anod versus log w plot, were calculated to be 0.48590.008, 0.5059 0.015, 0.5219 0.008 and 0.54590.006 from the linear fit (solid line) to the data points for each Pd electrode specimen. The calculated values of h correspond to the composition of the pre-charged states Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. The standard deviations were found to be 9.2× 10 − 3, 1.8×10 − 2, 9.7 × 10 − 3 and 7.3×10 − 3 for Pd, PdH0.23, PdH0.46 and PdH0.6, respectively. When the measured voltammetric current is limited by diffusion of the electroactive species towards the electrode surface, jp,anod resulting from the linear sweep
voltammetric measurement is proportional to w h, and h is related to the Df of the working electrode by [8–12] D −1 h= f (1) 2 where the parameter h is as defined above. From Eq. (1), the values of the parameter h of 0.485, 0.505, 0.521 and 0.545 were easily converted into the fractal dimensions Df 1.97, 2.00, 2.04 and 2.09, respectively. The value of Df increases with an increase of the fraction of b-PdH phase in the matrix electrode of the a-PdH phase. Bearing in mind that the Df of a fractured carbon steel surface prepared by the Charpy impact test was reported to be in the range of 2.1 and
122
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2.2 [7], the values of Df obtained in the present work are considered to be reasonable. However, the fractal dimension obtained from this work is smaller than that measured from columnar gold electrodeposits by using chronoamperometric technique [17] and STM/AFM methods [18]. We should note that the value of Df of Pd is less than 2. This indicates that the entire surface is not electrochemically active, but is partially inactive. In the Pourbaix diagram for the system Pd– H2O at 298 K [19], one observes the threshold potential is 0.15 V (RHE) for the equilibrium of a-PdHl and Pd. Above this threshold potential, metallic Pd is thermodynamically stable. Chierchie et al. [20] reported that the PdOH layer is formed on the Pd electrode surface in the potential range 0.7–0.9 V (RHE) and it is transformed into a more stable layer of PdO above 0.9 V (RHE). As a consequence, it is inferred that the formation of PdO at the hydrogen extraction potential 0.9 V (RHE) makes some of the electrode surface inactive. Since the hydrogen extraction conditions of the four electrode specimens (Pd, PdH0.23, PdH0.46 and PdH0.6) are the same, we assume that the effect of PdO formation on the change in the surface roughness is the same for the four electrodes and the change in Df is attributed mainly to the plastic deformation due to the formation of b-PdH in the electrode.
4. Conclusions In the present work, the Pd electrode surface modified by the plastic deformation induced during the phase transformation of a-PdH to b-PdH in the electrode has been characterised by the fractal geometry. By analysing the diffusion-limited anodic peak current density of Fe2 + /Fe3 + redox reaction during linear sweep voltammetric measurements, the fractal dimension Df of Pd electrode surface modified by the plastic deformation incurred due to the phase transformation of a-PdH to b-PdH has been determined. As the fraction of b-PdH phase in the matrix electrode of a-PdH phase is increased from zero to unity, the value of Df is increased from 1.97 to 2.09.
Acknowledgements The present work has been carried out under the auspices of the joint program of Korea Science and Engineering Foundation (KOSEF) and Japan Society for the Promotion of Science (JSPS) 1999/2001. The authors are indebted to KOSEF for the financial support of this work. Furthermore, this work was partly supported by the Brain Korea 21 project.
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