COMMENT pubs.acs.org/JPCC
Reply to Comment on “Electrochemical Quartz Crystal Microbalance Study of Borohydride Electro-Oxidation on Pt: The Effect of Borohydride Concentration and Thiourea Adsorption” V. W. S. Lam, D. C. W. Kannangara, A. Alfantazi, and E. L. Gyenge* The University of British Columbia Vancouver, BC, Canada
T
he electro-oxidation of borohydride on platinum is a complicated reaction due to competing electrocatalytic and thermocatalytic pathways, some of them generating hydrogen gas. Our study is the first one to our knowledge that employed the electrochemical quartz crystal microbalance technique (EQCM) to investigate this reaction.1 Repeating selected experiments a number of times, we are confident that within normal experimental error limits our results are representative. We welcome other experimental studies repeating our experiments under similar conditions and discussing comparatively the results. In the comments to our paper, Varela et al. do not present or refer to any of their own original experimental data with this system which would have been important for a fair and comprehensive discussion of our experimental results.2 Instead, the comments are basically related to two issues: one concerning the state of the Pt surface in our experiments (mostly with respect to Figures 2 and 3) and the second the sensitivity of the QCM measurements. Some of the questions probably originated from the novelty of our work and lack of comparative results in the literature. Hereby, we will present our answers to the issues raised.
1. Pt SURFACE IN 2 M NAOH To start with, we must emphasize that the borohydride oxidation was performed on the voltametrically stabilized Pt surface which under the conditions explored in the paper was achieved after 10 or 11 consecutive cyclic voltammetry scans in the potential domain indicated in the Experimental Section of our paper. This is mentioned on page 2733, second paragraph, where the stabilization of the hydroxide region of Pt is indicated.1 We have recently repeated the baseline experiments under the experimental conditions shown by Figures 2b and 3, by scanning the sputtered Pt resonator electrode in 2 M NaOH in the absence of borohydride up to 200 times. These repeated runs confirmed that the CV behavior achieved after 10 or 11 scans can be considered representative for the voltametrically stable surface. The authors of the comment emphasize the need of a “time invariant” Pt surface. We are not sure what the authors of the comment mean by time invariant surface, and furthermore, we do not think there is such a surface in any electrocatalytic system. An electrode surface can present a voltametrically stable behavior for a certain time period, but this does not mean that it is time invariant. Surfaces in catalysis are dynamic, and they undergo time and experimental condition-dependent reorganization and restructuring. Comments were made also with respect to the mass changes of the Pt surface between the 1st and 11th cycles in the absence r 2011 American Chemical Society
of borohydride (Figure 3),1 considering these mass changes “unreasonable”. First of all, it is essential to remember that the quartz crystal microbalance measures mass changes relative to the mass of the surface at the start of the scan. Therefore, the time-dependent evolution of the Pt EQCM in 2 M NaOH in Figure 3 for the first 11 cycles should be looked upon as follows: the start of scan nr. (N þ 1) with Δm = 0 corresponds to the mass of the Pt surface at the end of scan nr. N plus any measurable adsorption that occurred on the electrode exposed to the electrolyte at open-circuit during the time between the two scans. Stated briefly, Figure 3 shows that carrying out a mass balance between the anodic and cathodic scans for all the scans 1 to 11, the mass gains virtually equal the mass losses. Thus, as mentioned before, after the 11th scan the surface became voltametrically stable, and we confirmed this separately by carrying out up to 200 scans. In those scans, the anodic mass gain was similar to the value given by Figure 3 run 11, and the anodic mass gain was equal to the cathodic mass loss (for instance, for scan 200). Issues were also raised regarding the cleanness of the measurements with respect to the H-upd and hydroxide/oxide region.2 Our sputtered polycrystalline Pt resonator-electrodes were chemically and electrochemically cleaned as discussed in the Experimental Section.1 Figure 2a in the paper shows clearly the double peak response in the correct potential region due to H-upd.1 The hydrogen upd peaks on Pt in alkaline media are strongly dependent on the crystal facets of Pt (e.g., (111), (110), (100)), the surface area, and the state of the Pt surface which is affected by both the scanned potential window and the number of cycles. Morallon et al. (reference 32: in our original paper) show nice examples with respect to the issues mentioned above for various single-crystal Pt in 0.1 M NaOH.3 They show clearly the effect of Pt crystallography and the anodic switching potential (up to 1.4 V vs RHE) on the shape of the voltammograms (e.g., compare Figures 3, 5, and 6 in ref 3). On Pt(111) for example, the hydrogen-upd behavior can vary between virtually no distinct peak response, just a broad wave in the respective potential domain when using low anodic switching potential and somewhat sharper hydrogen-upd peaks when using high switching potentials (1.4 V vs RHE), which could have also caused some surface restructuring after several cycles. Pt(110) on the other hand presents a more pronounced peak H-upd response compared to Pt(111).3 Note we used more concentrated electrolyte (2 M NaOH) which could also have some influence. Other Received: February 8, 2011 Revised: April 21, 2011 Published: April 29, 2011 10312
dx.doi.org/10.1021/jp201282f | J. Phys. Chem. C 2011, 115, 10312–10313
The Journal of Physical Chemistry C studies point to these issues as well, such as those by Drazic et al.4 and Tripkovic et al.5 Both papers have been also referenced in our original publication (see ref 29 and 31, respectively).1 We did not study the crystallography of the polycrystalline Pt resonatorelectrode and its impact on hydrogen-upd. It would have been beyond the objective here. In the comment, it is stated that it is not expected to observe broad waves in the hydroxide/oxide region, and the broad reduction wave in Figure 2a is due to impurities in the electrolyte. We disagree. Others have reported as well Pt cyclic voltammograms in alkaline media with broad waves in this potential region.3,4,6 A good example is given in Figure 1 of ref 4, where broadening of the reduction wave in the hydroxide region of Pt(111) was due to change of the switching potential from 0.9 V vs 1.2 V vs SHE. Other examples of what could be called broad hydroxide/oxide reduction peaks on Pt in 0.1 M KOH can be found in Mayrhofer et al.7,8 Comparing the cyclic voltammograms of Pt in alkaline media from refs 38, one could easily see that there is variation in the shape of Pt voltammograms as a function of various experimental conditions: electrode pretreatment, crystallography, electrolyte composition, switching potentials, number of cycles, etc. We believe our Figure 2 describes adequately the baseline for Pt in 2 M NaOH in the hydroxide/ oxide region as well. A number of researchers, including Appleby,9 Drazic et al.,4 Tripkovic et al.,5 and others proposed the formation of two types of OHad species on Pt in alkaline media, namely, “weakly” (or reversibly) adsorbed and “strongly” (or irreversibly) adsorbed. These papers are referenced in our study. Furthermore, Conway in his review of Pt electrochemistry10 referring to his original work with Angertstein-Kozlowska (ref 30 in our paper1) attributed the overlapping anodic peaks in the hydroxide region to different PtOH stoichiometries such as PtOH, Pt2OH, and Pt5OH (Figure 4 in ref 9 from the work of Angertstein-Kozlowska et al.). It is therefore natural to assume that some of these surface hydroxide structures proposed by Conway et al. could be either weakly or more strongly adsorbed as discussed above.
COMMENT
would not change the essence of the measurements, as it would be only a cosmetic change. In conclusion, ours being the first investigation of borohydride oxidation by EQCM, we are eager to see original experimental results obtained by researchers for in-depth discussions to advance the understanding of this complex electrochemical reaction on various catalyst surfaces. Only by carrying out the EQCM experiments on this system some of the issues and points mentioned above can be fully appreciated. While there could be differences in the EQCM behavior of the Pt as a function of surface conditions (e.g., oxidation), crystallography, electrode cleaning and pretreatment procedures, electrolyte composition, and investigated electrode potential range, we are confident that our EQCM data within the above inherent experimental variations correctly represent the case of borohydride oxidation and that the conclusions of our work are sound.
’ REFERENCES (1) Lam, V. W. S.; Kannagara, D. C. W.; Alfantazi, A.; Gyenge, E. J. Phys. Chem. C 2011, 115, 2727. (2) Varela, H.; Sitta, E.; Machado, E. J. Phys. Chem. C 201110.1021/ jp2009385. (3) Morallon, E.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1990, 288, 217. (4) Drazic, D. M.; Tripkovic, A. V.; Popovic, K. D.; Lovic, J. D. J. Electroanal. Chem. 1999, 466, 155. (5) Tripkovic, A. V.; Popovic, K. D.; Lovic, J. D. J. Serb. Chem. Soc. 2001, 66, 825. (6) Morallon, E.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1992, 334, 323. (7) Mayrhofer, K. J. J.; Crampton, A. S.; Wiberg, G. K. H.; Arenz, M. J. Electrochem. Soc. 2008, 155, P78. (8) Mayrhofer, K. J. J.; Wiberg, G. K. H.; Arenz, M. J. Electrochem. Soc. 2008, 155, P1. (9) Appleby, A. I. J. Electrochem. Soc. 1970, 120, 1205. (10) Conway, B. Prog. Surf. Sci. 1995, 49, 331.
2. ON THE MASS CHANGES AND SENSITIVITY OF THE EQCM MEASUREMENTS In response to the comments regarding the low sensitivity of our mass change by EQCM (9 MHz crystal),2 we disagree. As one could see from our EQCM calibration experiment in Figure 1b employed to calculate the Sauerbrey constant, the EQCM response was as expected based on previously published experimental and theoretical work (references included).1 Deviations from the Sauerbrey constant would have revealed any problems with the setup. Studying the borohydride system by EQCM is difficult due to fine mass changes. Therefore, to minimize the EQCM data scatter, it was more suitable to carry out measurements over longer gate times which meant averaging the data per potential steps as shown by the figures. This is the reason for the stepwise response seen in most of the EQCM figures, and it did not cause any loss of valuable information. In the case of the borohydride system, the EQCM data traced nicely the peaks of the voltammograms. For the sake of consistency with our borohydride system measurements, the background scans on Pt were also performed using the same instrument settings and data collection times. Further, mathematical data smoothing procedures could be applied to the stepwise response, but this 10313
dx.doi.org/10.1021/jp201282f |J. Phys. Chem. C 2011, 115, 10312–10313
