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titanium composite electrodes for electrochemical gas generation from aqueous electrolytes
PERGAMON
Electrochimica Acta 44 (1998) 525±532
Preliminary Note
Boron doped diamond/titanium composite electrodes for electrochemical gas generation from aqueous electrolytes F. Beck a, *, H. Krohn a, W. Kaiser a, M. Fryda b, C. P. Klages b, L. SchaÈfer b a
UniversitaÈt Duisburg-Gesamthochschule, Fachgebiet Elektrochemie, Lotharstrasse 1, D-47057 Duisburg, Germany b Fraunhofer-Institut fuÈr Schicht- und Ober¯aÈchentechnik (IST), D-38108 Braunschweig, Germany Received 28 January 1998
Abstract Titanium substrates were coated by 2±5 mm thick boron doped diamond layers through the HFCVD-process from % an activated H2/CH4 gas phase. The doping level was 50±230 atom ppm B. The surface was saturated with $CH2 ÿ2 groups. A TiC interphase was formed in situ. Stationary current±voltage curves up to j2= 0.1 A cm were measured in three aqueous electrolytes, 1 M H2SO4, 1 M NaOH and 3 M NaCl. Very high overvoltages of vZv = 1± 2 V at 0.1 A cm ÿ 2 were found for the relevant gas evolution reactions. An appreciable, potential dependent portion of the Galvani voltage seems to be due to a voltage drop in a space charge layer in the semiconductor. Tafel plots % yield no straight lines therefore. An additional reason is the inertness of the electrode surface due to the $CH2 % groups in the negative and $C.O groups in the positive region. Electrochemical transformations between these surface structures seem to be possible. # 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Diamond/titanium electrode; Boron doping; Hot Filament CVD; Gas evolution; Electrochemical window
1. Introduction The overwhelming majority of stoichiometric chemical compounds is due to only one element, carbon. From a viewpoint of bond theory, these compounds can be classi®ed as sp3 and sp2 types, corresponding to aliphatic and aromatic systems, respectively. Electrochemical electrodes based on carbon and graphite have been known for a long time. Their relatively high electronic conductivity has allowed a wide practical application of these nonmetallic systems. However, diamond as the other C-modi®cation has only recently found interest as an electrochemical electrode. It complements the well known elemental semiconductors Si and Ge. This late development is mainly due to its very low electrical conductivity and its high material costs. However, when doped with boron, the material attains an appropriate electronic conductivity of 0.01± 0.1 S cm ÿ 1. The insulator is converted to a practical
* Author to whom correspondence should be addressed. Fax: +49 2033 7925 40; e-mail: [email protected]
electrode material in this way [1]. Secondly, the development of hot ®lament chemical vapor depositions (HFCVD) promises the realization of practical electrode designs with an intrinsic possibility for scaling up to industrial sizes [2]. An important consideration is the material of the substrate. It should be conducting and inert against corrosion and anodic dissolution. The open literature indicates clearly that silicon is the preferred substrate. Among 15 publications [3±5] are cited as examples. Other substrate materials are tungsten (®ve publications), e.g. Ref. [6], and molybdenum [7]. Titanium has not yet been considered as a substrate material in spite of the fact that this valve metal has been widely used in industrial electrochemistry for nearly 30 years in DSA's (dimensionally stable anodes, Ti/RuO2, TiO2, SnO2, . . .) following the innovation of Henry Beer [8]. It should be mentioned that Ti sheets coated by diamond are known for general corrosion protection and physical designs [9±11]. In addition self supporting diamond ®lms are mentioned for electrochemical investigations [12].
0013-4686/98/$19.00 # 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 1 1 5 - 7
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F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
We report for the ®rst time results in detail for the electrochemical behaviour of these titanium/B-doped diamond composites in aqueous electrolytes after some preliminary information [2]. The electrode reactions are the generation of simple gases (H2, O2, Cl2) rather than solid products. The formation of solid phases is excluded. But the formation of covalent bonds is involved, where at other electrodes the necessity of some catalytic activity of the electrode is considered to achieve low overvoltages.
2. Experimental The electrodes were circular round disks of titanium with a 3 mm layer of boron-doped diamond. The deposition of these layers was performed by the hot ®lament chemical vapor deposition (HFCVD) process. Deposition parameters were as follows: ®lament temperature, 2000±28008C; substrate temperature, 500± 10008C; pressure, 10±100 mbar and deposition rate, 0.3±6 mm/h. The process is described in detail elsewhere [2, 13]. The electrodes, which were employed in this work are described in Table 1. According to this, boron concentrations ranged from 50±230 ppm and resistivities from 10±150 O cm. The boron concentrations were analytically determined by secondary ion mass spectroscopy (SIMS), in conjunction with a quantitative depth pro®le (down to 1± 2 mm) by sputtering. A constant pro®le for boron was found at a level indicated in Table 1. The disks were mounted horizontally in an electrode holder of polypropylene, according to Fig. 1. A similar con®guration can be found in Ref. [14] and, for vertical electrodes, in ref. [15]. An O-ring provides a perfect seal against the brass contact. The free area exposed to the electrolyte was 1.1 cm2. The counter electrode was a platinum sheet. The reference electrode was a saturated calomel electrode (SCE). Potentials measured vs this reference are de®ned as UK. In some cases, these potentials are transformed to potentials vs SHE (DU = 250 mV) and such potentials are labeled as UH.
Table 1. Characterization of electrode samples: boron doped HFCVD-diamond on titanium
Fig. 1. Schematical representation of the design of Ti/diamond (B) electrodes. Cell components: (1) working electrode, (2) Luggin capillary, (3) counter electrode, (4) O-ring, (5) polypropylene housing, (6) brass contact, (7) steel ball and (8) glass cell.
The aqueous electrolytes were 1 M H2SO4, 1 M NaOH and 3 M NaCl. The electrolytes were prepared from thrice distilled water and analytical grade reagents. The temperatures were 358C for the former two, but 708C for the latter to meet eventual industrial conditions. All measurements were done under argon. The voltammetric runs were performed at a voltage scan rate of 20 mV s ÿ 1. Starting from the rest potential UR, the ®rst two cycles were in the negative potential range between UR up to a current density of ÿ100 mA cm ÿ 2 (hydrogen evolution). Cycles 3 and 4 were in the positive potential region between UR and +100 mA cm ÿ 2 (oxygen evolution). Thereafter, the whole potential interval was explored between H2- and O2-evolution. These cycles 5 and 6 are depicted in Figs. 2±5 to be representative for the steady state. Both start with the negative potential range. A potentiostat (HEKA PG 284) with an integrated voltage scan generator was employed.
No.
B-Conc. (atom-ppm)
k (S cm ÿ 1)
3. Results
1 2 3 4 5 6
200 130 50 230 50 50
0.016 0.014 0.007 0.10 0.007 0.007
3.1. Quasi steady state current±voltage curves Fig. 2 displays a quasi steady state current±voltage curve in aqueous 1 M H2SO4. As already mentioned, only the complete cycles (5 and 6) are shown. A broad electrochemical window can be identi®ed, which is lim-
F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
527
Fig. 2. Quasi-stationary current±voltage curves in 1 M H2SO4 at 358C for electrode No. 2, 130 ppm B doping level, 20 mV s ÿ 1, Ar (ÐÐÐ). For comparison: blank Ti in 1 M NaOH (- - -). Only cycles 5 and 6 are displayed.
ited at the negative end by the hydrogen branch and at the positive end by the oxygen branch. The current rises there relatively steeply. No anodic polarization due to the eventual presence of an n-conducting TiO2 interlayer can be found. The overvoltages at 0.1 A cm ÿ 2 with reference to the equilibrium potentials, indicated by U 0H2 and U 0O2 , are rather high, namely ÿ1.15 and +2.2 V, respect-
ively. However, a limiting anodic current density of some mA cm ÿ 2, starting at about +1.8 V vs SCE, can be seen in the positive forward scan, but it has disappeared in the back scan. The behaviour of a blank titanium sheet electrode is shown in addition as a dashed curve. The electrolyte is 1 M NaOH. Clearly, an anodic valve metal character is found in this case and only the steep rise of cathodic
Fig. 3. Quasi-stationary current±voltage curves in 1 M NaOH at 358C for electrode No. 1, 200 ppm B doping level, 20 mV s ÿ 1, Ar.
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F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
Fig. 4. Inner cycles for the measurements, which are shown in Fig. 3 (ÐÐÐ). A result with p-Si as base electrode is shown in addition (- - -).
current densities is found again, while anodic currents are nearly inhibited up to 10 V. A comparison to the curve for Ti/diamond in NaOH in Fig. 3 shows a near coincidence of both curves in the same electrolyte. Fig. 3 shows the analogous curves in 1 M NaOH. The potentials are shifted generally by about ÿ0.83 V according to the Pourbaix diagram. Theoretically, a shift of 14 ÿ 0.061 = ÿ 0.85 V was to be expected. The overvoltages are nearly the same as in Fig. 2, namely Z = ÿ 0.9 and +1.85 V at 0.1 A cm ÿ 2 for the
H2- and O2-branch, respectively. This time, however, a much more pronounced anodic step can be observed, but only in the forward scan, coming from the hydrogen branch. As shown in Fig. 4, ``inner cycles'' between +1 and ÿ1.6 V can be obtained according to a limitation of the current density of voltage scan reversal to only 9 mA cm ÿ 2. Fig. 4 discloses another interesting detail as dashed lines, which hold for a substitution of titanium by p-silicon. Much ¯atter curves are obtained
Fig. 5. Quasi-stationary current±voltage curves in 3 M NaCl at 708C for electrode No. 1, 200 ppm B doping level, 20 mV s ÿ 1, Ar.
F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
Fig. 6. Cathodic Tafel plots for 1 M H2SO4, various electrodes, two independent runs for each. r, W electrode No. 6, 50 ppm B; w, * electrode No. 2, 130 ppm B; q, Q electrode No. 4, 230 ppm B.
due to the presence of a base electrode material of a higher resistivity. Finally a neutral solution of 3 M NaCl yields the quasi stationary current±voltage curves of Fig. 5. The measurements were performed at 708C. The overvoltage for H2 is higher in this case, ZH = ÿ 2 V at 0.1 A cm ÿ 2. This estimation is based on a pH-value of 7. For unbuered solutions at lower pH-values, ZH would be even higher. On the other hand, Z + is much lower for Cl2 evolution, only 1.2 V at 0.1 A cm ÿ 2. Chlorine can be smelled.
529
Fig. 8. Anodic Tafel plots for 1 M H2SO4, various electrodes, two independent runs for each. r, W electrode No. 6, 50 ppm B; w, * electrode No. 2, 130 ppm B; q, Q electrode No. 4, 230 ppm B.
than those with the higher B doping. Analogous behaviour is observed in 1 M NaOH, cf. Fig. 7. In the case of anodic Tafel plots (cf. Fig. 8), such a discrimination cannot be found and the curves are spread over a potential interval of more than 1 V for two decades of current density, independent of the boron doping level and by analogy with the cathodic curves for the low doping level.
3.2. Tafel plots
4. Discussion
Cathodic Tafel plots are shown in Figs. 6 and 7. The measurements in the back scan were evaluated in all cases. Fig. 6 starts with 1 M H2SO4 as an electrolyte. No straight Tafel lines are obtained in this and all other cases. The measurements are well reproducible. Electrodes with a low B doping level exhibit much ¯atter curves
4.1. Surfaces and interphases The HFCVD diamond coating proceeds in a H2/ CH4-gas phase (1±4% CH4) at high temperatures [2, 13, 16]. The C-source is methane, and intermediates such as CH3, :CH2, etc. are present. In case of a Ti substrate, at least three reactions can be discussed with the H-atoms in the gas phase. The nTiO2 layer will be removed: Ti=TiO2 4H ÿ4Ti=Ti 2H2 O
1
No barrier eect of eventually present n-TiO2 could be detected. In the case of Ti/RuO2, this barrier action is neutralized according to a totally dierent mechanism. The metal conductor RuO2 dissolves in the isomorphic TiO2 and ¯oods the solid with electrons. The blank Ti may also dissolve some hydrogen: Ti xH g ÿ4TiHx
2
Indirectly, the C-atoms may react with the Ti to form titanium carbide: Fig. 7. Cathodic Tafel plots for 1 M NaOH, various electrodes, two independent runs for each. q, Q electrode No. 3, 50 ppm B; r, (diamond with cross) electrode No. 5, 50 ppm B (coincidence); w, * electrode No. 1, 200 ppm B.
Ti yC g ÿ4TiCy
3
As a matter of fact, XRD-measurements at pristine probes of Ti/diamond (B) composites disclose the fol-
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F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
This surface will behave hydrophobically and may be subject to anodic oxidation to form carbonyl groups, cf. Fig. 9. In the course of diamond ®lm growth, the surface may be subject to dynamic disruption under the in¯uence of the H-atoms in the activated gas [6]
4
but a rapid resaturation can be foreseen. The ex situ measurement of the real surface compositions in the steady state by XPS is scheduled for the near future.
4.2. Chemical composition and other bulk properties of the coating
Fig. 9. Schematic representation of carbon surface groups for diamond and graphite and their anodic conversion.
lowing three components, but no TiO2. The theoretical data are shown in brackets: 1. Diamond: Y = 43.98; d = 2.059 (2.060); I = 400 (100). 2. TiC (rhambrabaevite): Y = 41.88; d = 2.157 (2.164); I = 200 (100). 3. Ti (substrate): Y = 40.28; d = 2.241 (2.244); I = 430 (100). TiC is a metallic conductor. Under these conditions, a H-terminated diamond surface will be expected, as shown in the upper part of Fig. 9. The surface carbon atoms will be saturated by hydrogen. The sp3 bonding in the bulk will be contin% ued at the surface ($CH2). Depending on the crystal % plane, another con®guration: 0 $C±H may be possible.
As already mentioned in Section 2, the chemical composition of the diamond coatings down to a depth of about 1±2 mm was determined by SIMS. Hydrogen was found to be present as the second most frequent element after carbon at a concentration of about 1 at.%. This is the reason for the relatively low resistivity of undoped CVD diamond thin ®lms, namely 102±108 O cm [17] vs 1014 O cm of pure diamond, which is an insulator according to its large band gap of 5.5 eV. Other impurities were identi®ed in relatively small concentrations and in all cases the surface concentrations (®rst values) decreased towards the nearly constant bulk concentrations: Oxygen: 10000 4 100 at.-ppm Nitrogen: 3000 4 100 at.-ppm Titanium (?): 100 4 2 at.-ppm It is not clear why Ti is present at the surface. Samples No. 3 and 6 (cf. Table 1) displayed a strong rise in the depth range of 1±2 mm upto some at.%, but the others did not. The presence of graphitic (C sp2) impurities will be discussed in Section 4.3.
Table 2. Overvoltages Z at j = 0.1 A cm ÿ 2 for electrode reactions at HFCVD-diamond layers on Ti in aqueous electrolytes. Electrodes: Ti/diamond (B) for (1)±(3), but W/diamond (B) for (4) No.
Electrolyte
Branch
Z 100 (V)
(1)
1 M H2SO4
(2)
1 M NaOH
(3)
3 M NaCl
(4)
0.5 M H2S04
ÿ + ÿ + ÿ + ÿ +
ÿ1.15 + 2.2 ÿ0.9 + 1.85 ÿ2.0 + 1.2 ÿ1.9 + 1.7
Remark with reference to the back scan with reference to the back scan after Martin et al. [6]
F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
4.3. Electrode kinetics, overvoltages A striking feature is the observation of high overvoltages Z for these electrodes. Our ®ndings are summarized in Table 2 and the Z-values of 1±2 V are in agreement with other reports [2, 5, 6, 18, 19]. These exceptionally high overvoltages are frequently suggested to correlate with the hydrogen terminated surface, at least for cathodic hydrogen evolution. The ``inert'' electrode surface does not allow for the adsorption of H-atoms and such a step is involved in all traditional mechanisms for the hydrogen electrode. It should be mentioned that our measurements in Figs. 2±5 and in Table 2 as well as those of Martin et al. [6] are based on diamond with a higher B-doping level. Smaller vZv values are observed sometimes and they are attributed by Martin et al. [6] to the presence of graphitic (C sp2) impurities. An identi®cation via Raman spectroscopy has been tried by some authors. However, the presence of other impurities cannot be ruled out. On the other hand, the very high values of vZv for H2-evolution in NaCl, cf. Table 2 and [5] may be due to a speci®c adsorption of the halogenide. At the bare anode, on the other hand, Clad may be present. In conventional mechanisms for halogen anodes, adsorbed halogen atoms play an important role too. This may be an explanation for the low Z + in this case. Anodic steps are observed in Figs. 2 and 3. They may be due to the oxidation of the CH2-®lm to form surface bound CO-groups, cf. Fig. 9. The electrode kinetics should however be very sluggish in the absence of an electrocatalyst, cf. Ref. [20]. The speci®c charge, which is contained in the pronounced anodic step of Fig. 3, is QA31 C cm ÿ 2, which is 1000 times larger than that of a monolayer. The surface factor is known to be about 10. In the back scan, the steps have disappeared totally. Presumably the preceding high anodic potentials are responsible for an eective puri®cation % of the diamond surface ($CO 4 C±COOH 4 CO2, C(sp2) 4 CO2). The blank diamond surface is ®lled % again by $CH2 in the negative branch. Another category of oxygen-containing surface groups on C is due to graphite- and carbon electrodes (sp2-bonds), cf. Fig. 9 (lower part). They have been known for a long time [21±23]. A possible anodic conversion to o-quinones is also indicated in Fig. 9. The overvoltages for such reactions should be much lower than for the sp3-case. The electrolytes do have high conductivities. IR corrections at 0.1 A cm ÿ 2 are in the order of 20 mV and can be neglected. The measurement of galvanostatic transients under appropriate conditions is under way. The double layer capacitance seems to be relatively small, 1±3 mF cm ÿ 2 [24]. CD = 5 mF cm ÿ 2 can be de-
531
rived from a CV curve shown as Fig. 1 in Ref. [24]. But these data remain somewhat uncertain due to poorly de®ned surface factors. The texture gained by SEM displays relatively coarse, well faceted diamond crystals [2, 3, 7]. The average slope of semilogarithmically plotted current±voltage curves (Figs. 6±8) are unusually high, between 200 and 500 mV per decade. z-potential eects are surely absent due to the high electrolyte concentrations. But two possibilities remain for a correction of the Galvani voltage Dj at the phase boundary into an kinetically relevant part Dj kin and an inactive part Djo: Dj Djkin Djo
5
One possibility is due to the semiconductor behaviour of the electrode, cf. Section 4.4. The other becomes important in the case of adsorbed electroactive moieties, cf. an example in the ®eld of organic electrosynthesis in Ref. [25]. But it is unlikely that the small (hydrated) hydrogen ion exhibits such an eect. 4.4. Semiconductor electrode behaviour Boron-doped diamond must be regarded as a p-semiconductor. Thus, the electrochemical results must be discussed in terms of semiconductor physics [26] and semiconductor electrochemistry [27]. The electrode reactions can be classi®ed according to valence bond (VB)- or conduction band (CB)-mechanisms. As an example, the anodic dissolution of germanium proceeds according to a VB-mechanism [27, 28]. As a consequence, a diusion limited current±voltage curve of the holes as the minority carriers is observed for nGe [28]. Another electrode reaction is cathodic hydrogen evolution, and in case of a CB-mechanism a diusion limited current±voltage curve should be found for p-Ge, but this is not observed [27, 28]. The reason may be a high overvoltage for this reaction. However, for GaAs, p-type electrodes exhibit this eect, in contrast to n-type [29]. No such limiting currents were found in the present case of B-doped diamond/Ti-electrodes. Moreover, the trend of doping is quite the opposite. According to Figs. 6 and 7, the hydrogen cathode has a much higher overvoltage in the case of a low boron concentration than for high boron doping levels. It is not clear at present how the hole concentration aects this hydrogen electrode. Negative redox systems are often found to react according to a VB-mechanism [30]. A possible explanation for the large overvoltages, which are observed in general with these diamond electrodes, may be attributed to the semiconductor behaviour. Djo of Eq. (5) then has two components, namely a general kinetic part (Z + > Z ÿ ) and a part due to a compaction/dilution of the holes at negative/
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F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
positive potentials, which again yields Z + > Z ÿ . EIS (capacitance) measurements in the potential region of the electrochemical window are currently performed in order to evaluate these relationships.
5. Practical aspects Diamond is an extremely hard and corrosion-resistant material in general. This is also re¯ected in an excellent electrochemical stability. The electrodes were anodically stable in the course of an electrolysis in 1 M H2SO4 or 3 M NaCl at some 100 mA cm ÿ 2 for about 100 hs [2]. No damage of the diamond surface could be detected thereafter. Swain [31] reports a superior inertness of a diamond (1000 ppm B doping level)/Sielectrode in 1 M HNO3/0.1 M NaF at 508C in comparison to HOPG-(graphite) or glassy carbon. An analytical feature was repeatedly stressed in the work of Swain et al. [3, 17, 31], namely the low electrochemical double layer capacitance. It leads to a low level of capacitive base currents in electroanalytical work. It assists in an improvement of the accuracy of such measurements. As already mentioned above, an appreciable portion of the Galvani voltage at the diamond electrode is within the semiconductor and it is therefore kinetically not ecient. Reports of ecient electrode processes due to the virtually high overvoltages at these electrodes, cf. Ref. [32], should therefore be handled with caution.
Acknowledgements Performance of the XRD-measurement by J. Ibsch, UniversitaÈt Duisburg and of SIMS determinations by Dr Peter Willich, IST Braunschweig, are gratefully acknowledged. We are indebted to Dr B. Wermeckes of this laboratory for helpful electroorganic discussions.
References [1] M. Werner, R. Job, A. Zaitzev, W. R. Fahrner, W. Seifert, C. Johnston, P. R. Chalker, Phys. Status Solidi (A) 154 (1996) 385. [2] M. Fryda, A. Dietz, T. MattheÂe, L. SchaÈfer, C.-P. Klages, Nachr. Chem. Tech. Lab. 45 (1997) 379. [3] G. M. Swain, Adv. Mater. 6 (1994) 388. [4] A.Ya. Sakharova, Yu.V. Pleskov, F. Di Quarto, S. Piazza, C. Sunseri, I. G. Teremetskaya, V. P. Varnin, J. Electrochem. Soc. 142 (1995) 2704. [5] N. Vinokur, B. Miller, Y. Avyigal, R. Kalish, J. Electrochem. Soc. 143 (1996) L238.
[6] H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson, J. C. Angus, J. Electrochem. Soc. 143 (1996) L133. [7] R. Ramesham, M. F. Rose, Diamond Relat. Mater. 6 (1997) 17. [8] H. B. Beer, GDR-Pat. 55 223 (Prior. 12.5.1965), Belg. Pat. 710 551 (2/1967); cf. O. de Nora, German Oen. 1 814 567 (12/1968). [9] C.-P. Chang, D. L. Flamm, D. E. Ibbotson, J. A. Mucha, J. Appl. Phys. 63 (1988) 1744. [10] H. Shiomi, H. Nakahata, T. Imai, Y. Nishibayashi, Y. Fujimori, Jpn. J. Appl. Phys. 28 (1989) 758. [11] R. C. McCune, R. E. Chase, W. R. Drawl, Surf. Coat. Technol. 39±40 (1989) 223. [12] B. Miller, R. Kalish, L. C. Feldman, A. Katz, N. Moriya, K. Short, A. E. White, J. Electrochem. Soc. 141 (1994) L41. [13] R. KroÈger, L. SchaÈfer, C.-P. Klages, R. Six, Phys. Status Solidi (A) 154 (1996) 33. [14] F. Beck, H. Krohn, S. Rashwan, I. Litzenberger, J Power Sourc. 32 (1990) 287. [15] F. Beck, A. Pruss, Electrochim. Acta 28 (1983) 1847. [16] J. C. Angus, A. Argoitia, R. Gat, Z. Li, M. Sunkara, L. Wang, Y. Wang, Philos. Trans. R. Soc. London Ser. A: 342 (1993) 195. [17] G. M. Swain, R. Ramesham, Anal. Chem. 65 (1993) 345. [18] H. B. Martin, A. Argoitia, J. C. Angus, A. B. Anderson and U. Landau, in Applications of Diamond Films and Related Materials, Third International Conference, eds. A. Feldman, Y. Tzeng, W. A. Yarbrough, M. Yoshikawa and M. Murakawa. 1995. [19] A. Argoitia, H. B. Martin, E. J. Rozak, U. Landau and J. C. Angus, in Diamond for Electronic Applications, Mater. Res. Soc. Symp. Proc. 416, eds. D. L. Dreifus, A. Collins, T. Humphrey, S. K. Dars and P. Pehrsson. Pittsburgh, PA, 1996, p. 349. [20] F. Beck, Elektroorganische Chemie. Verlag Chemie, Weinheim, 1974, pp. 189±190. [21] V. A. Garten, D. E. Weiss, Aust. J. Chem. 8 (1955) 68. [22] V. A. Garten, D. E. Weiss, Aust. J. Chem. 10 (1957) 309. [23] K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties. Wiley, New York, 1988. [24] S. Alehashem, F. Chambers, J. W. Strojek, G. M. Swain, R. Ramesham, Anal. Chem. 67 (1995) 2812. [25] B. Wermeckes, F. Beck, Electrochim. Acta 30 (1985) 1491. [26] W., Jr. Crawford Dunlap, An Introduction to Semiconductors. John Wiley and Sons, New York, 1957. [27] H. Gerischer, in Physical Chemistry: An Advanced Treatise, Vol. IXA Electrochemistry, eds. H. Eyring, D. Henderson and W. Jost. Academic Press, New York, 1970, pp. 463±542. [28] F. Beck, H. Gerischer, Ber. Bunseges. Phys. Chem. 63 (1959) 500. [29] H. Gerischer, I. Mattes, Z. Physik. Chem. (Frankfurt) 49 (1966) 112. [30] F. Beck, H. Gerischer, Ber. Bunseges. Phys. Chem. 63 (1959) 943. [31] G. M. Swain, J. Electrochem. Soc. 141 (1994) 3382. [32] R. Tenne, K. Patel, K. Hashimoto, A. Fujishima, J. Electroanal. Chem. 347 (1993) 409.
Electrochimica Acta 44 (1998) 525±532
Preliminary Note
Boron doped diamond/titanium composite electrodes for electrochemical gas generation from aqueous electrolytes F. Beck a, *, H. Krohn a, W. Kaiser a, M. Fryda b, C. P. Klages b, L. SchaÈfer b a
UniversitaÈt Duisburg-Gesamthochschule, Fachgebiet Elektrochemie, Lotharstrasse 1, D-47057 Duisburg, Germany b Fraunhofer-Institut fuÈr Schicht- und Ober¯aÈchentechnik (IST), D-38108 Braunschweig, Germany Received 28 January 1998
Abstract Titanium substrates were coated by 2±5 mm thick boron doped diamond layers through the HFCVD-process from % an activated H2/CH4 gas phase. The doping level was 50±230 atom ppm B. The surface was saturated with $CH2 ÿ2 groups. A TiC interphase was formed in situ. Stationary current±voltage curves up to j2= 0.1 A cm were measured in three aqueous electrolytes, 1 M H2SO4, 1 M NaOH and 3 M NaCl. Very high overvoltages of vZv = 1± 2 V at 0.1 A cm ÿ 2 were found for the relevant gas evolution reactions. An appreciable, potential dependent portion of the Galvani voltage seems to be due to a voltage drop in a space charge layer in the semiconductor. Tafel plots % yield no straight lines therefore. An additional reason is the inertness of the electrode surface due to the $CH2 % groups in the negative and $C.O groups in the positive region. Electrochemical transformations between these surface structures seem to be possible. # 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Diamond/titanium electrode; Boron doping; Hot Filament CVD; Gas evolution; Electrochemical window
1. Introduction The overwhelming majority of stoichiometric chemical compounds is due to only one element, carbon. From a viewpoint of bond theory, these compounds can be classi®ed as sp3 and sp2 types, corresponding to aliphatic and aromatic systems, respectively. Electrochemical electrodes based on carbon and graphite have been known for a long time. Their relatively high electronic conductivity has allowed a wide practical application of these nonmetallic systems. However, diamond as the other C-modi®cation has only recently found interest as an electrochemical electrode. It complements the well known elemental semiconductors Si and Ge. This late development is mainly due to its very low electrical conductivity and its high material costs. However, when doped with boron, the material attains an appropriate electronic conductivity of 0.01± 0.1 S cm ÿ 1. The insulator is converted to a practical
* Author to whom correspondence should be addressed. Fax: +49 2033 7925 40; e-mail: [email protected]
electrode material in this way [1]. Secondly, the development of hot ®lament chemical vapor depositions (HFCVD) promises the realization of practical electrode designs with an intrinsic possibility for scaling up to industrial sizes [2]. An important consideration is the material of the substrate. It should be conducting and inert against corrosion and anodic dissolution. The open literature indicates clearly that silicon is the preferred substrate. Among 15 publications [3±5] are cited as examples. Other substrate materials are tungsten (®ve publications), e.g. Ref. [6], and molybdenum [7]. Titanium has not yet been considered as a substrate material in spite of the fact that this valve metal has been widely used in industrial electrochemistry for nearly 30 years in DSA's (dimensionally stable anodes, Ti/RuO2, TiO2, SnO2, . . .) following the innovation of Henry Beer [8]. It should be mentioned that Ti sheets coated by diamond are known for general corrosion protection and physical designs [9±11]. In addition self supporting diamond ®lms are mentioned for electrochemical investigations [12].
0013-4686/98/$19.00 # 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 1 1 5 - 7
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We report for the ®rst time results in detail for the electrochemical behaviour of these titanium/B-doped diamond composites in aqueous electrolytes after some preliminary information [2]. The electrode reactions are the generation of simple gases (H2, O2, Cl2) rather than solid products. The formation of solid phases is excluded. But the formation of covalent bonds is involved, where at other electrodes the necessity of some catalytic activity of the electrode is considered to achieve low overvoltages.
2. Experimental The electrodes were circular round disks of titanium with a 3 mm layer of boron-doped diamond. The deposition of these layers was performed by the hot ®lament chemical vapor deposition (HFCVD) process. Deposition parameters were as follows: ®lament temperature, 2000±28008C; substrate temperature, 500± 10008C; pressure, 10±100 mbar and deposition rate, 0.3±6 mm/h. The process is described in detail elsewhere [2, 13]. The electrodes, which were employed in this work are described in Table 1. According to this, boron concentrations ranged from 50±230 ppm and resistivities from 10±150 O cm. The boron concentrations were analytically determined by secondary ion mass spectroscopy (SIMS), in conjunction with a quantitative depth pro®le (down to 1± 2 mm) by sputtering. A constant pro®le for boron was found at a level indicated in Table 1. The disks were mounted horizontally in an electrode holder of polypropylene, according to Fig. 1. A similar con®guration can be found in Ref. [14] and, for vertical electrodes, in ref. [15]. An O-ring provides a perfect seal against the brass contact. The free area exposed to the electrolyte was 1.1 cm2. The counter electrode was a platinum sheet. The reference electrode was a saturated calomel electrode (SCE). Potentials measured vs this reference are de®ned as UK. In some cases, these potentials are transformed to potentials vs SHE (DU = 250 mV) and such potentials are labeled as UH.
Table 1. Characterization of electrode samples: boron doped HFCVD-diamond on titanium
Fig. 1. Schematical representation of the design of Ti/diamond (B) electrodes. Cell components: (1) working electrode, (2) Luggin capillary, (3) counter electrode, (4) O-ring, (5) polypropylene housing, (6) brass contact, (7) steel ball and (8) glass cell.
The aqueous electrolytes were 1 M H2SO4, 1 M NaOH and 3 M NaCl. The electrolytes were prepared from thrice distilled water and analytical grade reagents. The temperatures were 358C for the former two, but 708C for the latter to meet eventual industrial conditions. All measurements were done under argon. The voltammetric runs were performed at a voltage scan rate of 20 mV s ÿ 1. Starting from the rest potential UR, the ®rst two cycles were in the negative potential range between UR up to a current density of ÿ100 mA cm ÿ 2 (hydrogen evolution). Cycles 3 and 4 were in the positive potential region between UR and +100 mA cm ÿ 2 (oxygen evolution). Thereafter, the whole potential interval was explored between H2- and O2-evolution. These cycles 5 and 6 are depicted in Figs. 2±5 to be representative for the steady state. Both start with the negative potential range. A potentiostat (HEKA PG 284) with an integrated voltage scan generator was employed.
No.
B-Conc. (atom-ppm)
k (S cm ÿ 1)
3. Results
1 2 3 4 5 6
200 130 50 230 50 50
0.016 0.014 0.007 0.10 0.007 0.007
3.1. Quasi steady state current±voltage curves Fig. 2 displays a quasi steady state current±voltage curve in aqueous 1 M H2SO4. As already mentioned, only the complete cycles (5 and 6) are shown. A broad electrochemical window can be identi®ed, which is lim-
F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
527
Fig. 2. Quasi-stationary current±voltage curves in 1 M H2SO4 at 358C for electrode No. 2, 130 ppm B doping level, 20 mV s ÿ 1, Ar (ÐÐÐ). For comparison: blank Ti in 1 M NaOH (- - -). Only cycles 5 and 6 are displayed.
ited at the negative end by the hydrogen branch and at the positive end by the oxygen branch. The current rises there relatively steeply. No anodic polarization due to the eventual presence of an n-conducting TiO2 interlayer can be found. The overvoltages at 0.1 A cm ÿ 2 with reference to the equilibrium potentials, indicated by U 0H2 and U 0O2 , are rather high, namely ÿ1.15 and +2.2 V, respect-
ively. However, a limiting anodic current density of some mA cm ÿ 2, starting at about +1.8 V vs SCE, can be seen in the positive forward scan, but it has disappeared in the back scan. The behaviour of a blank titanium sheet electrode is shown in addition as a dashed curve. The electrolyte is 1 M NaOH. Clearly, an anodic valve metal character is found in this case and only the steep rise of cathodic
Fig. 3. Quasi-stationary current±voltage curves in 1 M NaOH at 358C for electrode No. 1, 200 ppm B doping level, 20 mV s ÿ 1, Ar.
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Fig. 4. Inner cycles for the measurements, which are shown in Fig. 3 (ÐÐÐ). A result with p-Si as base electrode is shown in addition (- - -).
current densities is found again, while anodic currents are nearly inhibited up to 10 V. A comparison to the curve for Ti/diamond in NaOH in Fig. 3 shows a near coincidence of both curves in the same electrolyte. Fig. 3 shows the analogous curves in 1 M NaOH. The potentials are shifted generally by about ÿ0.83 V according to the Pourbaix diagram. Theoretically, a shift of 14 ÿ 0.061 = ÿ 0.85 V was to be expected. The overvoltages are nearly the same as in Fig. 2, namely Z = ÿ 0.9 and +1.85 V at 0.1 A cm ÿ 2 for the
H2- and O2-branch, respectively. This time, however, a much more pronounced anodic step can be observed, but only in the forward scan, coming from the hydrogen branch. As shown in Fig. 4, ``inner cycles'' between +1 and ÿ1.6 V can be obtained according to a limitation of the current density of voltage scan reversal to only 9 mA cm ÿ 2. Fig. 4 discloses another interesting detail as dashed lines, which hold for a substitution of titanium by p-silicon. Much ¯atter curves are obtained
Fig. 5. Quasi-stationary current±voltage curves in 3 M NaCl at 708C for electrode No. 1, 200 ppm B doping level, 20 mV s ÿ 1, Ar.
F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
Fig. 6. Cathodic Tafel plots for 1 M H2SO4, various electrodes, two independent runs for each. r, W electrode No. 6, 50 ppm B; w, * electrode No. 2, 130 ppm B; q, Q electrode No. 4, 230 ppm B.
due to the presence of a base electrode material of a higher resistivity. Finally a neutral solution of 3 M NaCl yields the quasi stationary current±voltage curves of Fig. 5. The measurements were performed at 708C. The overvoltage for H2 is higher in this case, ZH = ÿ 2 V at 0.1 A cm ÿ 2. This estimation is based on a pH-value of 7. For unbuered solutions at lower pH-values, ZH would be even higher. On the other hand, Z + is much lower for Cl2 evolution, only 1.2 V at 0.1 A cm ÿ 2. Chlorine can be smelled.
529
Fig. 8. Anodic Tafel plots for 1 M H2SO4, various electrodes, two independent runs for each. r, W electrode No. 6, 50 ppm B; w, * electrode No. 2, 130 ppm B; q, Q electrode No. 4, 230 ppm B.
than those with the higher B doping. Analogous behaviour is observed in 1 M NaOH, cf. Fig. 7. In the case of anodic Tafel plots (cf. Fig. 8), such a discrimination cannot be found and the curves are spread over a potential interval of more than 1 V for two decades of current density, independent of the boron doping level and by analogy with the cathodic curves for the low doping level.
3.2. Tafel plots
4. Discussion
Cathodic Tafel plots are shown in Figs. 6 and 7. The measurements in the back scan were evaluated in all cases. Fig. 6 starts with 1 M H2SO4 as an electrolyte. No straight Tafel lines are obtained in this and all other cases. The measurements are well reproducible. Electrodes with a low B doping level exhibit much ¯atter curves
4.1. Surfaces and interphases The HFCVD diamond coating proceeds in a H2/ CH4-gas phase (1±4% CH4) at high temperatures [2, 13, 16]. The C-source is methane, and intermediates such as CH3, :CH2, etc. are present. In case of a Ti substrate, at least three reactions can be discussed with the H-atoms in the gas phase. The nTiO2 layer will be removed: Ti=TiO2 4H ÿ4Ti=Ti 2H2 O
1
No barrier eect of eventually present n-TiO2 could be detected. In the case of Ti/RuO2, this barrier action is neutralized according to a totally dierent mechanism. The metal conductor RuO2 dissolves in the isomorphic TiO2 and ¯oods the solid with electrons. The blank Ti may also dissolve some hydrogen: Ti xH g ÿ4TiHx
2
Indirectly, the C-atoms may react with the Ti to form titanium carbide: Fig. 7. Cathodic Tafel plots for 1 M NaOH, various electrodes, two independent runs for each. q, Q electrode No. 3, 50 ppm B; r, (diamond with cross) electrode No. 5, 50 ppm B (coincidence); w, * electrode No. 1, 200 ppm B.
Ti yC g ÿ4TiCy
3
As a matter of fact, XRD-measurements at pristine probes of Ti/diamond (B) composites disclose the fol-
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F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
This surface will behave hydrophobically and may be subject to anodic oxidation to form carbonyl groups, cf. Fig. 9. In the course of diamond ®lm growth, the surface may be subject to dynamic disruption under the in¯uence of the H-atoms in the activated gas [6]
4
but a rapid resaturation can be foreseen. The ex situ measurement of the real surface compositions in the steady state by XPS is scheduled for the near future.
4.2. Chemical composition and other bulk properties of the coating
Fig. 9. Schematic representation of carbon surface groups for diamond and graphite and their anodic conversion.
lowing three components, but no TiO2. The theoretical data are shown in brackets: 1. Diamond: Y = 43.98; d = 2.059 (2.060); I = 400 (100). 2. TiC (rhambrabaevite): Y = 41.88; d = 2.157 (2.164); I = 200 (100). 3. Ti (substrate): Y = 40.28; d = 2.241 (2.244); I = 430 (100). TiC is a metallic conductor. Under these conditions, a H-terminated diamond surface will be expected, as shown in the upper part of Fig. 9. The surface carbon atoms will be saturated by hydrogen. The sp3 bonding in the bulk will be contin% ued at the surface ($CH2). Depending on the crystal % plane, another con®guration: 0 $C±H may be possible.
As already mentioned in Section 2, the chemical composition of the diamond coatings down to a depth of about 1±2 mm was determined by SIMS. Hydrogen was found to be present as the second most frequent element after carbon at a concentration of about 1 at.%. This is the reason for the relatively low resistivity of undoped CVD diamond thin ®lms, namely 102±108 O cm [17] vs 1014 O cm of pure diamond, which is an insulator according to its large band gap of 5.5 eV. Other impurities were identi®ed in relatively small concentrations and in all cases the surface concentrations (®rst values) decreased towards the nearly constant bulk concentrations: Oxygen: 10000 4 100 at.-ppm Nitrogen: 3000 4 100 at.-ppm Titanium (?): 100 4 2 at.-ppm It is not clear why Ti is present at the surface. Samples No. 3 and 6 (cf. Table 1) displayed a strong rise in the depth range of 1±2 mm upto some at.%, but the others did not. The presence of graphitic (C sp2) impurities will be discussed in Section 4.3.
Table 2. Overvoltages Z at j = 0.1 A cm ÿ 2 for electrode reactions at HFCVD-diamond layers on Ti in aqueous electrolytes. Electrodes: Ti/diamond (B) for (1)±(3), but W/diamond (B) for (4) No.
Electrolyte
Branch
Z 100 (V)
(1)
1 M H2SO4
(2)
1 M NaOH
(3)
3 M NaCl
(4)
0.5 M H2S04
ÿ + ÿ + ÿ + ÿ +
ÿ1.15 + 2.2 ÿ0.9 + 1.85 ÿ2.0 + 1.2 ÿ1.9 + 1.7
Remark with reference to the back scan with reference to the back scan after Martin et al. [6]
F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
4.3. Electrode kinetics, overvoltages A striking feature is the observation of high overvoltages Z for these electrodes. Our ®ndings are summarized in Table 2 and the Z-values of 1±2 V are in agreement with other reports [2, 5, 6, 18, 19]. These exceptionally high overvoltages are frequently suggested to correlate with the hydrogen terminated surface, at least for cathodic hydrogen evolution. The ``inert'' electrode surface does not allow for the adsorption of H-atoms and such a step is involved in all traditional mechanisms for the hydrogen electrode. It should be mentioned that our measurements in Figs. 2±5 and in Table 2 as well as those of Martin et al. [6] are based on diamond with a higher B-doping level. Smaller vZv values are observed sometimes and they are attributed by Martin et al. [6] to the presence of graphitic (C sp2) impurities. An identi®cation via Raman spectroscopy has been tried by some authors. However, the presence of other impurities cannot be ruled out. On the other hand, the very high values of vZv for H2-evolution in NaCl, cf. Table 2 and [5] may be due to a speci®c adsorption of the halogenide. At the bare anode, on the other hand, Clad may be present. In conventional mechanisms for halogen anodes, adsorbed halogen atoms play an important role too. This may be an explanation for the low Z + in this case. Anodic steps are observed in Figs. 2 and 3. They may be due to the oxidation of the CH2-®lm to form surface bound CO-groups, cf. Fig. 9. The electrode kinetics should however be very sluggish in the absence of an electrocatalyst, cf. Ref. [20]. The speci®c charge, which is contained in the pronounced anodic step of Fig. 3, is QA31 C cm ÿ 2, which is 1000 times larger than that of a monolayer. The surface factor is known to be about 10. In the back scan, the steps have disappeared totally. Presumably the preceding high anodic potentials are responsible for an eective puri®cation % of the diamond surface ($CO 4 C±COOH 4 CO2, C(sp2) 4 CO2). The blank diamond surface is ®lled % again by $CH2 in the negative branch. Another category of oxygen-containing surface groups on C is due to graphite- and carbon electrodes (sp2-bonds), cf. Fig. 9 (lower part). They have been known for a long time [21±23]. A possible anodic conversion to o-quinones is also indicated in Fig. 9. The overvoltages for such reactions should be much lower than for the sp3-case. The electrolytes do have high conductivities. IR corrections at 0.1 A cm ÿ 2 are in the order of 20 mV and can be neglected. The measurement of galvanostatic transients under appropriate conditions is under way. The double layer capacitance seems to be relatively small, 1±3 mF cm ÿ 2 [24]. CD = 5 mF cm ÿ 2 can be de-
531
rived from a CV curve shown as Fig. 1 in Ref. [24]. But these data remain somewhat uncertain due to poorly de®ned surface factors. The texture gained by SEM displays relatively coarse, well faceted diamond crystals [2, 3, 7]. The average slope of semilogarithmically plotted current±voltage curves (Figs. 6±8) are unusually high, between 200 and 500 mV per decade. z-potential eects are surely absent due to the high electrolyte concentrations. But two possibilities remain for a correction of the Galvani voltage Dj at the phase boundary into an kinetically relevant part Dj kin and an inactive part Djo: Dj Djkin Djo
5
One possibility is due to the semiconductor behaviour of the electrode, cf. Section 4.4. The other becomes important in the case of adsorbed electroactive moieties, cf. an example in the ®eld of organic electrosynthesis in Ref. [25]. But it is unlikely that the small (hydrated) hydrogen ion exhibits such an eect. 4.4. Semiconductor electrode behaviour Boron-doped diamond must be regarded as a p-semiconductor. Thus, the electrochemical results must be discussed in terms of semiconductor physics [26] and semiconductor electrochemistry [27]. The electrode reactions can be classi®ed according to valence bond (VB)- or conduction band (CB)-mechanisms. As an example, the anodic dissolution of germanium proceeds according to a VB-mechanism [27, 28]. As a consequence, a diusion limited current±voltage curve of the holes as the minority carriers is observed for nGe [28]. Another electrode reaction is cathodic hydrogen evolution, and in case of a CB-mechanism a diusion limited current±voltage curve should be found for p-Ge, but this is not observed [27, 28]. The reason may be a high overvoltage for this reaction. However, for GaAs, p-type electrodes exhibit this eect, in contrast to n-type [29]. No such limiting currents were found in the present case of B-doped diamond/Ti-electrodes. Moreover, the trend of doping is quite the opposite. According to Figs. 6 and 7, the hydrogen cathode has a much higher overvoltage in the case of a low boron concentration than for high boron doping levels. It is not clear at present how the hole concentration aects this hydrogen electrode. Negative redox systems are often found to react according to a VB-mechanism [30]. A possible explanation for the large overvoltages, which are observed in general with these diamond electrodes, may be attributed to the semiconductor behaviour. Djo of Eq. (5) then has two components, namely a general kinetic part (Z + > Z ÿ ) and a part due to a compaction/dilution of the holes at negative/
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F. Beck et al. / Electrochimica Acta 44 (1998) 525±532
positive potentials, which again yields Z + > Z ÿ . EIS (capacitance) measurements in the potential region of the electrochemical window are currently performed in order to evaluate these relationships.
5. Practical aspects Diamond is an extremely hard and corrosion-resistant material in general. This is also re¯ected in an excellent electrochemical stability. The electrodes were anodically stable in the course of an electrolysis in 1 M H2SO4 or 3 M NaCl at some 100 mA cm ÿ 2 for about 100 hs [2]. No damage of the diamond surface could be detected thereafter. Swain [31] reports a superior inertness of a diamond (1000 ppm B doping level)/Sielectrode in 1 M HNO3/0.1 M NaF at 508C in comparison to HOPG-(graphite) or glassy carbon. An analytical feature was repeatedly stressed in the work of Swain et al. [3, 17, 31], namely the low electrochemical double layer capacitance. It leads to a low level of capacitive base currents in electroanalytical work. It assists in an improvement of the accuracy of such measurements. As already mentioned above, an appreciable portion of the Galvani voltage at the diamond electrode is within the semiconductor and it is therefore kinetically not ecient. Reports of ecient electrode processes due to the virtually high overvoltages at these electrodes, cf. Ref. [32], should therefore be handled with caution.
Acknowledgements Performance of the XRD-measurement by J. Ibsch, UniversitaÈt Duisburg and of SIMS determinations by Dr Peter Willich, IST Braunschweig, are gratefully acknowledged. We are indebted to Dr B. Wermeckes of this laboratory for helpful electroorganic discussions.
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