COMMENT pubs.acs.org/JPCC
Comment on “Electrochemical Quartz Crystal Microbalance Study of Borohydride Electro-Oxidation on Pt: The Effect of Borohydride Concentration and Thiourea Adsorption” H. Varela,*,†,‡ E. Sitta,† and E. G. Machado† † ‡
Institute of Chemistry of S~ao Carlos, University of S~ao Paulo CP 780, CEP 13560-970, S~ao Carlos, SP, Brazil Ertl Center for Electrochemistry and Catalysis, GIST Cheomdan-gwagiro 261, Buk-gu, Gwangju 500-712, South Korea
n a recent paper, Gyenge and co-workers1 investigated the electro-oxidation of borohydride on a platinum electrode by means of combined electrochemical and microgravimetric experiments. The main goal was to study the effects of borohydride concentration and of thiourea adsorption in terms of the correlation between electrochemical and gravimetric data. Herein, we comment on some critical points we found when reading this paper. Overall, we observed many experimental and conceptual problems that, to our understanding, are critical and somewhat make their interpretation doubtful. In the following, we present some of these concerns. (a) The mass resolution is considerably low and actually unexpected for the setup used, i.e., a Seiko/PAR EQCM and 9 MHz quartz crystal. There are a number of references in both acidic2 5 and alkaline media,4,6,7 showing far superior resolution under nearly identical conditions. This is obviously a key issue when dealing with the complicated problem of interpreting the physical meaning of the interfacial mass changes. (b) Figures 1(a) and 2(a) show the voltammetric signatures of their platinum surface in both acidic and alkaline media. As normally accepted, such base cyclic voltammograms are well-known and often used to certify the cleanness and quality of the system as whole.8 10 The profiles presented in these two figures are simply not comparable to highstandard ones. In both cases, the region under potentially deposited (upd) hydrogen is ill defined, and the whole cyclic voltammograms are both characterized by a positive slope which makes the cathodic and anodic contributions very unsymmetrical. The argument that such profiles are acceptable for deposited platinum thin films can be readily contested in light of previously published data.3,4,11,12 (c) The authors claim that the shoulder prior to the main reduction peak in Figure 2(a) is caused by “two types of Pt OH”. We believe the initial stages of the platinum electro-oxidation are still a matter of debate. Nevertheless, in spite of the vast literature available,13 we are not aware of any similar type of cyclic voltammogram, and the reduction is known to proceed via a single and welldefined peak, at least in the conditions under which those experiments were carried out. (d) There is a huge hysteresis in the mass profile between the beginning and the end of the sweep in most voltammograms. Our experience shows that this hysteresis is likely
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to reflect a transient behavior, and many cycles are needed to get time-invariant profiles. This aspect also contributes to the uncharacteristic voltammetric profiles already mentioned. The authors invoked a mechanism that might operate during the transient, but no data or discussion is provided on the fact that at some point the current and mass change profiles must become stationary: the electrode surface can not gain or lose mass forever during the cycling! In most cases, until a time-invariant profile is reached, no reliable information can be extracted. Interpreting experimental data under transient conditions is a far from trivial task and, in the present case, very problematic. (e) Concerning the magnitude of the mass changes, the difference between the two oxidation processes depicted in Figure 2(a) and (b) seems unreasonable. In fact, the mass change for the surface oxidation process (say, between 0.1 and 0.6 V vs SHE) in the first cycle is about five times larger than that for the 10th cycle given in (b). It is important to recall here that the oxidation process remains nearly unaffected by the difference in the cathodic potential, as clearly seen in the voltammetric profiles. This similarity should certainly be present in the mass change profiles. In summary, we believe that the main conclusions in the paper by Lam et al.1 are not supported by the data presented. EQCM is a very powerful in situ technique and can help solve complex interfacial problems, such as, for instance, the ones associated to the electro-oxidation of borohydride on platinum. However, good experimental data are mandatory for any consistent and reliable interpretation. We hope the present discussion can trigger future experimental work on this system.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Lam, V. W. S.; Kannangara, D. C. W.; Alfantazi, A.; Gyenge, E. L. J. Phys. Chem. C 2011, 115, 2727. (2) Zolfaghari, A.; Conway, B. E.; Jerkiewicz, G. Electrochim. Acta 2002, 47, 1173. Received: January 28, 2011 Revised: March 23, 2011 Published: May 02, 2011 10310
dx.doi.org/10.1021/jp2009385 | J. Phys. Chem. C 2011, 115, 10310–10311
The Journal of Physical Chemistry C
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(3) Jerkiewicz, G.; Vatankhah, G.; Lessard, J.; Soriaga, M. P.; Park, Y. S. Electrochim. Acta 2004, 49, 1451. (4) Sitta, E.; Santos, A. L.; Nagao, R.; Varela, H. Electrochim. Acta 2009, 55, 404. (5) Santos, M. C.; Miwa, D. W.; Machado, S. A. S. Electrochem. Commun. 2000, 2, 692. (6) Shimazu, K.; Kita, H. J. Electroanal. Chem. 1992, 341, 361. (7) Gloaguen, F.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 1999, 467, 186. (8) Angerstein-Kozlowska, H.; Conway, B. E.; Barnett, B.; Mozota, J. J. Electroanal. Chem. 1979, 100, 417. (9) Tilak, B. V.; Conway, B. E.; Angerstein-Kozlowska, H. J. Electroanal. Chem. 1973, 48, 1. (10) Hamann, H.; Hamnett, A.; Vielstich, W. Electrochemistry; Wiley-VHC Verlag: Weinheim, 2007. (11) Vatankhah, G.; Lessard, J.; Jerkiewicz, G.; Zolfaghari, A.; Conway, B. E. Electrochim. Acta 2003, 48, 1613. (12) Boscheto, E.; Batista, B. C.; Lima, R. B.; Varela, H. J. Electroanal. Chem. 2010, 642, 17. (13) Conway, B. E. Prog. Surf. Sci. 1995, 49, 331.
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