Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes

Diamond and Related Materials 10 Ž2001. 620᎐626

Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes M. Yoshimuraa,U , K. Hondaa , R. Uchikado a , T. Kondo a , Tata N. Rao a , D.A. Tryk a , A. Fujishimaa , Y. Sakamoto b, K. Yasui b, H. Masudab a

Department of Applied Chemistry, School of Engineering, The Uni¨ ersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Department of Industrial Chemistry, Faculty of Engineering, Tokyo Metropolitan Uni¨ ersity, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397, Japan

Abstract Oxygen plasma-etched nano-honeycomb diamond thin film electrodes were examined for electrochemical capacitor applications in non-aqueous electrolytes. As-deposited and nano-honeycomb diamond electrodes in 0.5 M TEABF4rPC both exhibited a wide potential window Žapprox. 7.3 V., similar to that of glassy carbon electrodes. For as-deposited diamond, the impedance behavior was found to be similar for non-aqueous and aqueous electrolytes, and the double-layer capacitance was found to be 21.8 ␮F cmy2 , almost the same as that obtained in aqueous electrolytes. For the honeycomb diamond electrodes, however, the impedance behavior observed in non-aqueous electrolytes was significantly different from that in aqueous electrolyte and indicated that the ac signal cannot penetrate to the bottom of the honeycomb pores in the non-aqueous electrolytes due to low conductivity, and that not all the surface may contribute to the double-layer capacitance. This result was verified by mathematical simulation. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrochemistry; Nano-honeycomb diamond; Non-aqueous electrolyte; Electrical double-layer capacitor

1. Introduction Conductive boron-doped diamond electrodes have gained tremendous interest for various electrochemical applications including electroanalysis w1,2x, electrosynthesis w3x, electrochemical treatment of waste water w4x and electrochemical double-layer capacitors w5x. Recently, for the first time, we reported the use of nanoporous diamond electrodes for aqueous electrochemical capacitor applications w5x. Diamond electrodes are attractive for double-layer capacitor applications due to their wide potential windows Ž3.54 V vs. AgrAgCl., which are wider than those Ž3.04 V vs. U

Corresponding author. Tel.: q81-3-5841-7245; q81-3-5812-6227. E-mail address: [email protected] ŽA. Fujishima..

AgrAgCl. of carbon electrodes, and their small residual current densities. From the as-deposited diamond films, nano-honeycomb structures can be fabricated through oxygen plasma etching. Oxygen plasma treatment does not influence the potential window of nano-honeycomb diamond films. However, along with increasing the electrode area, this treatment increases the double-layer capacitance by factors up to 400. In this result, the energy density for the pore type 400 nm = 3 ␮m electrode Žpore diameter = depth. was estimated to be 76.5 J gy1 w5x, which is larger than that obtained for an activated carbon electrode Ž18᎐72 J gy1 w6x. in aqueous electrolyte. The energy densities for activated carbon electrodes can be greatly increased through the use of non-aqueous electrolytes, due to the wide potential windows, the values reaching 1400᎐2100 J gy1 w7,8x.

0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 3 8 1 - 2

M. Yoshimura et al. r Diamond and Related Materials 10 (2001) 620᎐626

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However, the only study of diamond electrodes in nonaqueous electrolytes thus far is that of Wu et al. w9x who examined the electrochemistry of C 60 in acetonitrilertoluene. Otherwise, the fundamental properties of diamond electrodes in non-aqueous electrolytes have been little studied. In the present study, we have examined the potential window in non-aqueous electrolytes. In addition, we consider the double-layer capacitor application using nano-honeycomb diamond electrodes, comparing the behavior in non-aqueous electrolytes with that in aqueous.

2. Experimental Highly boron-doped diamond films were grown by microwave-assisted plasma chemical vapor deposition ŽCVD. with a commercial microwave plasma reactor ŽASTex Corp., Woburn, MA. w10x. The morphology of the films was examined with a scanning electron microscope ŽSEM, JEOL Model JSM-5400 LV.. Nanohoneycomb structures were fabricated from polished diamond films Žroughness approx. 1 nm, Namiki Precision Jewel Co., Ltd., Tokyo, Japan.. Nanohoneycomb structures were prepared by oxygen plasma etching through anodic alumina masks with various pore dimensions w11,12x. In the present work, oxygen plasma etching was carried out Ž13 min for with 60-nm pore diameter and 30 min for 400-nm pore diameter. in a plasma etching apparatus ŽSAMCO, BP-1.. The operating oxygen pressure was 20 Pa, and the plasma power was 150 W. The glassy carbon ŽGC: 0.071 cm2 , GC-20, Tokai Ltd.. and highly oriented pyrolytic graphite ŽHOPG, ZYA grade, Advanced Ceramics Corp.. electrodes were prepared by standard procedures, as reported elsewhere w5x. The electrochemical and impedance measurements were carried out with standard equipment and procedures, as reported elsewhere w5x. The electrochemical studies were performed in a single-compartment glass cell. The planar working electrode was mounted on the bottom of the glass cell by use of a glass O-ring joint and Viton O-ring. An organic solvent used was propylene carbonate ŽPC., and tetraethylammonium tetrafluoroborate ŽTEABF4 , 0.5 mol dmy3 ., which have come into favor due to its better solubility and conductivity, was added.

3. Results and discussion 3.1. Scanning electron microscopy Fig. 1 shows SEM top-view images of two types of

Fig. 1. SEM images of highly boron-doped nanohoneycomb diamond electrodes. Ža. pore type 60 = 500 nm, Žb. pore type 400 nm = 1.8 ␮m Ždiameter = depth..

diamond nano-honeycomb films that were fabricated from the polished diamond films. Uniform, well-ordered holes, with a hexagonal close-packed pattern, are clearly seen in these figures. The pore type 60 = 500 nm has particularly very well defined cylindrical holes ŽFig. 1a.. The surface area was estimated to be a factor of 10.9 times larger for the honeycomb film compared to a flat, polished surface. On the other hand, for pore type 400 nm = 1.8 ␮m, due to high pore depth of 1.8 ␮m, the roughness factor was 9.60. 3.2. Cyclic ¨ oltammetry In this study, the working potential window is defined as the range between the potential at which the anodic and cathodic current densities reach 2 mA cmy2 w11x. Fig. 2 shows the CVs obtained on three kinds of electrodes in organic and aqueous solutions. The non-aqueous electrolyte exhibits a potential window ŽFig. 2a᎐c. that are 1.5᎐2.5 times wider than that of the aqueous acid electrolyte ŽFig. 2d.. The potential windows for the diamond electrodes were almost the same as those for GC. The reaction that controlled the potential window, i.e. those involving the oxidation and reduction of the organic solvent, do not appear to

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M. Yoshimura et al. r Diamond and Related Materials 10 (2001) 620᎐626

Fig. 2. Cyclic voltammograms obtained for a. as-deposited diamond film electrode in 0.5 M TEABF4 rPC; Žb. pore type 60 = 500 nm in the same electrolyte; Žc. GC in the same electrolyte; and Žd. as-deposited diamond in aqueous 1 M H 2 SO4 ; sweep rate: 10 mV sy1 .

involve adsorbed intermediates but instead to involve outer-sphere electron transfers. This result will be discussed in a separate publication. 3.3. Electrical double-layer capacitance and energy density The impedance ŽCole᎐Cole. plots for as-deposited diamond in the non-aqueous electrolytes exhibit a straight line that approaches the origin at high frequency, as in aqueous solution w11x. This behavior is considered with a simple series resistance ᎐capacitance

network, indicative of the electrical double-layer properties in both cases. Capacitances were determined at various frequencies in the low frequency range from the low frequency imaginary component of the impedance Ž0.01 Hz. using the relation Z s yjrŽ ␻C . w5,11x ŽTable 1.. The double-layer capacitance, Cdl , determined for the as-deposited diamond electrode in PC was 21.4 ␮F cmy2 . This value is on the same order as that obtained in an aqueous solution Ž12.9 ␮F cmy2 .. In contrast, the capacitance for a honeycomb electrode Žpore type 60 = 500 nm. in an aqueous solution Ž1.83= 10 3 ␮F cmy2 . was approximately 150 times larger than that for the as-deposited diamond surface ŽTable 1.. This value results from the increased roughness factor Ž10.9. together with the effect of the oxygen plasma etching. The capacitance increases approximately 20 times ŽTable 1. in changing the termination of the diamond surface from hydrogen to oxygen. This increase can also be observed in non-aqueous electrolytes. The capacitance for pore type 60 = 500 nm in PC, however, was estimated to be 187 ␮F cmy2 , approximately one-tenth of the value obtained for an aqueous solution. The value Ž3.80= 10 3 ␮F cmy2 . for pore type 400 nm = 1.8 ␮m in an aqueous solution is also approximately 150 times larger than that for the as-deposited diamond surface. The factor is thought to be due to the needle-like nanostructure at the bottom of the honeycomb pore. In PC, the capacitance for pore type 400 nm = 1.8 ␮m was 666 ␮F cmy2 , which is approximately 300 times larger than that for the as-deposited dia-

Table 1 Roughness factor

0.5 M TEABF4rPC

1 M H2 SO4

a

as-deposited Direct etched diamond Glassy carbon Pore type 60 = 500 mm Pore type 400 nm = 1.8 ␮m Activated carbon as-deposited diamond Direct etched diamond Pore type 60 = 500 nm Pore type 400 nm = 1.8 ␮m Activated carbon

4.0 4.0 10.9 9.60

4.0 4.0 10.9 9.60

Potential window, ⌬V, Va

Electrical double-layer capacitance Cdl , ␮F cmy2 Žgeometric.b

7.3 7.3 7.3 8.0 8.0 5.3e

21.4 208 44.7 187 666

3.5 3.6 3.1 2.9 1.2e

12.9 238 1830 3800

Electrical double-layer capacitance, Cdl , F gy1c

1.48 12.7 60᎐400

14.5 72.5 100᎐400

Energy density Edl , J gy1 Žgeometric.d

11.9 101.6 210᎐1400f

17.9 74.3 18᎐72

Values obtained from cyclic voltammograms. The definition of a potential window is defined by the potential values at which the current density was less than 2 mA cmy2 . b Values obtained by AC impedance analysis at y0.2 V vs. AgrAgq. c Specific capacitance for a hypothetical through-hole diamond membrane. d The specific energy was estimated from the equation, Edl s 1r2 = Cdl = Ž ⌬Vr2. 2 . e Values obtained with HOPG. f Taken from w7,8x.

M. Yoshimura et al. r Diamond and Related Materials 10 (2001) 620᎐626

mond surface but is one-sixth of the value obtained for aqueous solution. These differences result from a combination of the effects of differences in the electrical conductivities of the solutions and of the molecular radii of the solvents and solutes. The electric conductivity measured in 0.5 M TEABF4rPC was 7.7 mS, which is one-sixth of the value Ž45.8 mS. measured in 1 M H 2 SO4 . In addition, the radii of PC and TEABF4 are larger than those of H 2 O and H 2 SO4 , respectively, which effects the magnitude of the capacitance. As a result, in non-aqueous electrolyte, the potential oscillations cannot be felt over the entire length of the pore and cannot access all of the available surface area. We can calculate the capacitances and energy densities per unit weight for the honeycomb electrodes if we assume that we can fabricate free-standing diamond films thin enough so that we could make through-hole films with the same dimensions as those used here, i.e. with the film thickness the same as the pore depth. Table 1 shows that the specific capacitance values estimated for the various films in PC range from 1.48 F gy1 to 72.5 F gy1 . In a similar fashion, we can take the capacitance values Ž Cdl . from the impedance measurements and the potential window values Ž ⌬V . from the CV measurements, and calculate energy densities Žper unit weight. for the hypothetical through-hole diamond honeycomb double-layer capacitors by use of the formula Edl s 0.5= Cdl = Ž Vr2. 2 . The value for pore type 400 nm = 1.8 ␮m in PC was 101.6 J gy1 , which is larger than that Ž74.3 J gy1 . in an aqueous solution. However, this value cannot be compared to those for commercial activated carbons in non-aqueous electrolytes Ž210᎐1400 J gy1 w7,8x.. 3.4. Cole᎐Cole plots Fig. 3 shows experimental Cole᎐Cole plots for the pore type 400 nm = 1.8 ␮m film obtained in 1 M H 2 SO4 and 0.5 M TEABF4rPC. The plots in aqueous solution ŽFig. 3a. exhibit two distinct domains: a high frequency domain, where the behavior is similar to that for a cylindrical pore electrode, with a characteristic linear portion at a 45⬚ angle, and a low frequency domain, where the penetration depth of the potential oscillations approaches the pore depth and the electrode then starts to behave like a flat electrode w13᎐15x. The plots obtained in PC ŽFig. 3b., however, exhibit only a single type of behavior, to the high frequency behavior in aqueous solution. In this case, even at low frequencies, the penetration depth of the potential oscillations remains less than the pore depth and therefore the measured double-layer capacitance does not correspond to the full honeycomb surface area. This behavior was observed in the plots for pore type 60 =

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Fig. 3. Complex-plane ŽCole᎐Cole. impedance plots for the pore type 400 nm = 1.8 ␮m honeycomb electrode obtained in Ža. 1 M H 2 SO4 at q0.4 V vs. AgrAgCl, and Žb. 0.5 M TEABF4 rPC at ᎐0.2 V vs. AgrAgq wexperimental data points Ž`.x. The simulated curves Žsolid lines. were calculated on the basis of the equivalent circuit in Fig. 4 with the parameters in Table 2.

500 nm in PC. Even for pore type 400 nm = 1.8 ␮m, whose pore diameter is five times larger, the AC signal was not able to reach the bottom of the honeycomb pores in PC. 3.5. Numerical simulation In this section, we simulated the impedance using the equivalent circuit in order to understand the mobility of the ions in the honeycomb pores in detail. The impedance of porous electrodes can be simulated using the transmission line model w16x. To this model, we have added Cdext and R sext to the transmission line model ŽFig. 4. w5x. From the geometric parameters of the cylindrical pores Ži.e. diameter, depth and number density., which are obtainable from SEM observation, the impedance can be evaluated. The calculated impedance curves for pore type 400 nm = 1.8 ␮m are shown together with the experimental curves in Fig. 3. It can be verified that the equivalent circuit employed is able to reproduce the impedance plots for the honeycomb electrodes. Table 2 summarizes the values of the fitting parame-

Fig. 4. Equivalent circuit based on the transmission line model for the double-layer charging of a honeycomb diamond electrode. R ext s s external series resistance; R spore s pore series resistance; Cdext s external differential capacitance; Cdpore s pore differential capacitance.

624

Electrode type

Series resistance for external surface, Rsext , ⍀ cm2

Differential capacitance for external surface Cdpore , ␮F cmy2

0.5 M TEABF4rPC Pore type 60 = 500 nm 400 = 1.8 ␮m

163 149

80 200

1 M H2 SO4 Pore type 60 = 500 nm 400 = 1.8 ␮m

213 136

60 290

Series resistance for pore, Rspore , ⍀ cmy2

7.81= 104 2.20= 103

71.0 305

Differential capacitance for pore Cdext , ␮F cmy2

Time constant for pore ␶pore , s

Pore density n, cmy2

390 390

111.0 37.6

1.0= 1010 4.8= 108

140 420

5.15 1.65

1.0= 1010 4.8= 108

Electrolyte ␬, mS cmy1

0.42 4.7

15 220

Average relative Ž%.

24.3 17.4

9.75 7.74

M. Yoshimura et al. r Diamond and Related Materials 10 (2001) 620᎐626

Table 2 Parameters used for fitting the impedence results in the complex plane plots

M. Yoshimura et al. r Diamond and Related Materials 10 (2001) 620᎐626

ters and the average relative errors for the calculated curves. As one example, pore type 400 nm = 1.8 ␮m, the R spore value obtained in aqueous solution was 2.2 k ⍀ cm2 , approximately seven times larger than that in aqueous solution Ž305 ⍀ cm2 .. The electrolyte conductivity ␬ calculated in PC Ž4.7 mS cmy1 . was quite similar to the experimental value Ž7.7 mS cmy1 ., obtained with a conductivity bridge. In contrast, for pore type 60 = 500 nm, R spore obtained in PC was approximately 1000 times larger than that in aqueous solution. The ␬ calculated in PC was 0.42 mS cmy1 , as compared with the experimental value of 7.7 mS cmy1 . From these results, we can conclude that the pore diameter can influence the apparent conductivity significantly, as we have also found in aqueous electrolyte. Specifically, the mobility of the ions is decreased in very small pores. The reasons for this effect are a topic of continuing investigation.

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Fig. 6. Ragone plots for pore type Ž1. 60 = 500 nm, Ž2. 400 nm = 1.8 ␮m obtained in 1 M H 2 SO4 and Žb. 0.5 M TEABF4 rPC. Power densities Ž Pave . and energy densities Ž E . for each plot were estimated from the equations Pave s average ŽV. = I and Es ⌺ Ž V = I = ⌬ t ., respectively. The V, I, t values used were obtained from Fig. 5.

3.6. Gal¨ anostatic measurements

Fig. 5. Constant-current discharge curves for pore type Ž1. 60 = 500 nm and Ž2. 400 nm = 1.8 ␮m. Electrolytes: Ža. 1 M H 2 SO4 ; and Žb. 0.5 M TEABF4rPC; geometric surface area: 0.071 cm2 ; capacitances ŽC. were estimated for each curve from the equation C s Ž ⌬ t = i .r⌬V; current density: 70 ␮A cmy2 . The vertical axis is the potential referred to the respective open circuit potentials, 0.4 V vs. AgrAgCl in 1 M H 2 SO4 , and ᎐0.2 V vs. AgrAgq in 0.5 M TEABF4 rPC.

To assess the behavior of the honeycomb diamond electrode under conditions closer to those of an operating electrochemical capacitor, galvanostatic measurements were carried out w17,18x. Fig. 5 shows the constant-current discharge curves for both of the honeycomb electrodes. The low conductivity in PC has a large influence on the discharge curves ŽFig. 5 1-b,2-b., i.e. in which the potential drops much more quickly than in aqueous solution ŽFig. 5 1a,2a.. Therefore, in spite of the wide potential window in PC, the practically useful discharge times were smaller than those in aqueous solution. The power density Pave and energy density E were calculated from the discharge curves obtained at various current densities using the formula Pave s average Ž V . = I and Es ⌺Ž V = I = ⌬ t ., respectively. Fig. 6 shows the Ragone plots based on these values. In the plot obtained in aqueous solution for pore type 400 nm = 1.8 ␮m ŽFig. 6 2a., Pave values were almost the same as those for pore type 60 = 500 nm ŽFig. 6 1a.. However, the E values obtained for pore type 400 nm = 1.8 ␮m were smaller than those for pore type 60 = 500 nm, even though the latter has much smaller double-layer capacitance on a geometric area basis ŽTable 1.. This results from differences in the pore structures. As seen in the SEM images ŽFig. 1., the relative wall thickness for the larger pore size honeycomb is small, leading to an increased resistance in the solid electrode phase. This effect is accentuated due to the larger pore depth. The E values obtained for pore type 60 = 500 nm in PC ŽFig. 6 2b. were the smallest of all. In addition to the low electrical conductivity of the solution, the low responsible capacitance for pore type 60 = 500 nm Ž187 ␮F cmy2 ; Table 1., due to the pore field penetration,

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M. Yoshimura et al. r Diamond and Related Materials 10 (2001) 620᎐626

can be regarded as responsible. For pore type 400 nm = 1.8 ␮m, the Pave and E values in PC ŽFig. 6 2b. were estimated to be on the same order as for those in aqueous solution. These results indicate that the large resistances in non-aqueous electrolytes offset the effect of wide potential windows.

non-aqueous electrolytes do not offer any advantage over aqueous electrolytes for diamond electrodes. For pore type 400 nm = 1.8 ␮m in non-aqueous electrolyte, the power and energy densities could reach only to almost the same values as those in aqueous electrolytes. Therefore, we conclude that the combination of pore type 400 nm = 1.8 ␮m and aqueous electrolyte is the best of those examined thus far.

4. Summary and conclusions Boron-doped diamond films used in non-aqueous electrolytes exhibited 1.5᎐2.5 times wider potential windows Ž7.3 V. than those in aqueous electrolytes. GC and nano-honeycomb diamond exhibited almost the same potential windows as that for as-deposited diamond. On as-deposited diamond electrodes, the electrical double-layer capacitance in non-aqueous electrolyte Ž21.9 ␮F cmy2 . was as large as those obtained in aqueous electrolytes. However, two types of nanohoneycomb diamond exhibited smaller capacitance values than those in aqueous electrolytes due to pore field penetration. The energy density for pore type 400 nm = 1.8 ␮m used in 0.5 M TEABF4rPC reached a value of 101.6 J gy1 . However, this value was less than half of the values using activated carbon electrodes. The differences in the capacitances observed in aqueous and non-aqueous electrolytes were due to the low electrical conductivities of the latter, together with large radii of the corresponding ions. The impedance results indicated that the AC signals could not penetrate to the bottom of the honeycomb pores and thus all of the surface area could not contribute to the formation of the electrical double-layer. These results were confirmed by the simulation using the transmission line model. We have considered the application of nano-honeycomb diamond electrodes as electrical double-layer capacitors. Although diamond exhibits a wider potential window than that of GC in an aqueous electrolyte, the window is almost the same for both the electrodes in non-aqueous electrolytes. Moreover, the large electrolyte resistance of organic electrolytes offsets the additional width of the potential windows, indicating that

Acknowledgements This research was supported by the Japan Society for the Promotion of Science ŽJSPS., Research for the Future Program ŽExploratory Research on Novel Materials and Substances for Next Generation Industries .. References w1x T.N. Rao, A. Fujishima, Diam. Relat. Mater. 9 Ž2000. 3. w2x M.C. Granger, G.M. Swain, J. Electrochem. Soc. 146 Ž1999. 12. w3x F. Okino, H. Shibata, S. Kawasaki et al., Electrochem. SolidState Lett. 2 Ž1999. 382. w4x M. Fryda, D. Herrmann, L. Schater et al., New Diam. Frontier Carbon Technol. 9 Ž1999. 229. w5x K. Honda, T.N. Rao, D.A. Tryk, A. Fujishima, M. Watanabe, K. Yasui, H. Masuda, J. Electrochem. Soc. Žsubmitted.. w6x H. Shi, Electrochim. Acta 41 Ž1996. 1633. w7x I. Tanahashi, A. Yoshida, A. Nishino, Bull. Chem. Soc. Jpn. 63 Ž1990. 2755. w8x K. Hiratsuka, K. Sanada, T. Morimoto, K. Kurihara, Denki Kagaku 59 Ž1991. 607. w9x Z. Wu, T. Yano, D.A. Tryk, K. Hashimoto, A. Fujishima, Chem. Lett. Ž1998. 503. w10x G.M. Swain, J. Electrochem. Soc. 141 Ž1994. 3382. w11x K. Honda, T.N. Rao, D.A. Tryk et al., J. Electrochem. Soc. 147 Ž2000. 659. w12x H. Masuda, M. Watanabe, K. Yasui, D.A. Tryk, A. Fujishima, Adv. Mater. 12 Ž2000. 444. w13x J-P. Candy, P. Foulloux, M. Keddam, H. Takenouti, Electrochim. Acta 26 Ž1981. 1029. w14x F.M. Delnick, C.D. Jaeger, S.C. Levy, Chem. Eng. Commun. 35 Ž1985. 23. w15x J. Rishpon, S. Gottesfeld, J. Electrochem. Soc. 131 Ž1984. 1960. w16x R. De Levie, in: P. Delahay ŽEd.., Advances in Electrochemistry and Electrochemical Engineering, 6, John Wiley & Sons, New York, 1967, pp. 329᎐397. w17x V. Srinivasan, J.W. Weidner, J. Electrochem. Soc. 146 Ž1999. 1650. w18x A.M. Johnson, J. Newman, J. Electrochem. Soc. 118 Ž1971. 510.