In the Laboratory
A Simple Student Experiment for Teaching Surface Electrochemistry: Adsorption of Polyoxometalate on Graphite Electrodes
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David Martel, Neso Sojic, and Alexander Kuhn* Laboratoire d’Analyse Chimique par Reconnaissance Moléculaire, École Nationale Supérieure de Chimie et de Physique de Bordeaux, 16 avenue Pey Berland, 33607 Pessac, France; *
[email protected] Motivation Electrochemistry is an indispensable part of modern chemical education (1). Besides well-adapted courses there is also a great need for new laboratory experiments that allow students to understand recent trends in electrochemical research. One area of high research activity is modified electrodes. Many different modification procedures have been developed, and electrochemical studies allow better insight, for example, into the electronic properties, the stability, and eventually the catalytic activity of the modified interface (2). Students should be introduced to this field of research by exciting experiments that are conceptually uncomplicated, simple to execute, and time- and cost-effective (3). We describe a very simple experiment, using inexpensive materials, that illustrates the electrochemical behavior of a surface-adsorbed redox species. The data are obtained by classic cyclic voltammetry and the study completes the traditional laboratory courses on electroactive species in solution. It fits easily in a 4-hour class and in our case is performed by fourthyear chemistry students. The students remember this experiment especially because of the originality of the working electrode, the uncomplicated surface modification procedure, and the easily understandable setup. The students will become familiar with the following techniques and concepts: cyclic voltammetry capacitive current spontaneous chemisorption calculation of theoretical and experimental surface coverage determination of the number of electrons involved in the redox process relationship between peak current and scan rate electrocatalysis with surface confined molecules
Below are brief theoretical descriptions of the abovementioned points that students will learn by performing the corresponding parts of the experiment. Experimental Procedure The students use a simple one-compartment glass cell. Cyclic voltammetry is performed with a three-electrode setup: one silver/silver chloride reference electrode, a platinum counter electrode, and the working electrode. The latter should be made from a carbon-containing material because the studied adsorption is best on such a material. Glassy carbon or edge-plane graphite can be used, but in that case the electrode preparation is quite tedious. To avoid this and to illustrate that an electrode can be something very simple,
we have chosen a graphite pencil as working electrode. Instead of complicated polishing procedures, a new and clean electrode surface is easily obtained with a pencil sharpener. To establish a good electric contact with the graphite rod inside the pencil, the wooden part was removed at the upper end over a length of approximately one centimeter. The supporting electrolyte is 0.5 M H2SO4. The electroactive species that adsorbs readily on the graphite surface is phosphomolybdic acid (H3PMo12O40). A 30% stock solution of hydrogen peroxide was used for the catalysis experiments. Hazards The chemicals used in this experiment present a small hazard due to either their acidity or their oxidizing character, and therefore when handling sulfuric acid, phosphomolybdic acid, and hydrogen peroxide, the experimenter should wear gloves and have suitable eye protection. There are no other significant hazards. Results Cyclic voltammetry is one of the popular tools used to study surface-confined redox species. The potential of the working electrode is changed linearly with time, starting from a potential at which no electrode reaction occurs. The potential is changed to values where reduction or oxidation of the adsorbed species takes place followed by a scan back to the initial potential. Very valuable information concerning the amount of adsorbed material, the formal potential of the involved redox couples and the kinetics of the electron transfer can be obtained. The integrated area under each wave represents the charge Q associated with the reduction or oxidation of the adsorbed layer: Q = nFA Γ
(1)
where n is the number of electrons, F is the Faraday constant, A is the electrode surface area, and Γ is the surface coverage in moles of adsorbed molecules per surface area. The peak current Ipeak is proportional to scan rate v, which contrasts with the v 1/2 dependence observed for freely diffusing redox species (4 ). In the present case the adsorbed species belongs to the family of polyoxometalates. These molecules can undergo several successive electron transfer reactions (5) and are the subject of many research efforts because of their interesting properties related to catalysis, molecular magnets, electrochromic devices, and other applications (6 ). Some polyoxometalates are able to adsorb strongly and irreversibly on carbon surfaces (7 ) but still show a highly reversible electrochemistry.
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In the Laboratory 10
0.8
A
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I / µA
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(E vs Ag/AgCl) / V
(E vs Ag/AgCl) / V
Figure 2. Cyclic voltammogram of the electrode modified with PMo12O403᎑ (thick line) compared to the unmodified electrode (thin line). Supporting electrolyte 0.5 M H2SO4, scan rate 100 mV/s.
1.0
B 0.8
0.6
I / µA
The typical value obtained for the graphite pencil is 8.7 µF. Taking into account the geometric surface area 0.4
A = πrᐉ
(surface of a cone with radius r = 1 mm at the base and a side length ᐉ = 5 mm), one can calculate a specific capacitance of 27 µF/cm2. Even though the real electroactive surface area is greater than the geometric area, this is a reasonable value for this kind of surface.
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25
50
75
100
v / (mV s᎑1)
Figure 1. A: Background current of the bare graphite pencil in 0.5 M H2SO4 supporting electrolyte. Scan rate of 10, 20, 50, 75, and 100 mV/s, starting from 600 mV and going negative. B: Plot of the total capacitive current measured at 325 mV vs Ag/AgCl against scan rate.
We use this chemisorption to attach a monolayer of PMo12O403᎑ to the surface of our pencil electrode.
Capacitive Current First a cyclic voltammogram of the clean pencil surface is recorded in pure supporting electrolyte. The thick line in Figure 1A shows this background current Icap at v = 10 mV/s. In a first-order approximation we can assume that it is constant over the whole potential range. Its magnitude is proportional to the scan rate (thin lines) and to the capacitance Cd of the interface: Icap = v Cd
(2)
Plotting the total Icap as a function of v gives a straight line with a slope equal to twice the capacitance (Fig. 1B). 350
(3)
Chemisorption The pencil is dipped in the 5 mM solution of H3PMo12O40 for 10 s. After rinsing the electrode with distilled water a cyclic voltammogram between +600 and ᎑100 mV vs Ag/AgCl is recorded in pure supporting electrolyte (Fig. 2, thick line). A striking change of the signal is observed, corresponding to a 3 × 2 electron transfer reaction (8) involving the chemisorbed molecules. Surface Coverage The recorded signal can be integrated and a global charge under the two most positive peaks of 10᎑5 C is obtained. Knowing that four electrons are involved we can evaluate the amount of electroactive material on the surface. A typical value measured by students is 1.6 × 1013 molecules. Renormalizing by the geometric electrode surface area, an experimental surface coverage of Γ = 1.6 × 10᎑10 mol/cm2 can be calculated. To check whether this corresponds to a monolayer, the students estimate the theoretical surface coverage by considering the polyoxometalate as a square with d = 10.8 Å (9). A dense arrangement of these molecules should lead to a coverage of Γ = 1.4 × 10᎑10 mol/cm2. This confirms that, in the limit of the approximations made, the electrode is covered with a monolayer of polyoxometalate.
Journal of Chemical Education • Vol. 79 No. 3 March 2002 • JChemEd.chem.wisc.edu
In the Laboratory 2
10
A
0
I / µA
I / µA
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0
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(E vs Ag/AgCl) / V
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Figure 4. Catalysis of the electroreduction of H2O2. Cyclic voltammogram at 10 mV/s of a PMo12O403᎑-modified pencil in pure 0.5M H2SO4 (thick line). After the addition of 150 mM and 300 mM H2O2 respectively (thin lines).
B
4
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(E vs Ag/AgCl) / V
8
6
0.2
Ip ox
Ip / µA
2
0
-2
Ip red
-4
-6
-8 0
25
50
75
100
v / (mV s᎑1)
Figure 3. A: Cyclic voltammogram of the modified electrode at 10, 20, 50, 75, and 100 mV/s showing only the two most positive redox peaks. B: Plot of the most positive oxidation and reduction peak current as a function of scan rate.
Number of Electrons Another valuable piece of information that can be extracted from the recorded signal is the peak width at halfheight. For a Nernstian reaction its value is given by ∆Ep,1/2 = 90.6/n mV. In our case we find a mean value of 55 mV (for the two most positive oxidation peaks), which is not too far from the theoretical value of 45.3 mV expected for a two-electron process. I = f(v) Since the monolayer is perfectly stable, it is possible to record a series of voltammograms with different scan rates and thus to verify the theoretical relation (eq 4): Ipeak= n 2F 2 v A Γ (4RT )᎑1
(4)
Figure 3A shows the voltammograms obtained for different scan rates. Plotting the peak currents Ipeak for the most
positive oxidation and reduction peak against scan rate yields two straight lines with opposite slope (Fig. 3B). The linear correlation confirms the presence of a surface-confined redox couple, and using these slopes we can verify the number of electrons involved in the redox process. A value of 1.8 is obtained by using eq 4. When the number of electrons is known from the literature, as in our case, the eq 4 can also be taken to calculate the surface coverage. The slopes of Ipeak = f (v) being +0.077 and ᎑0.075 µA s mV ᎑1, one can determine a surface coverage of 1.3 × 10᎑10 mol cm᎑2. This value is slightly lower than the result obtained by integrating the voltammogram but is still in quite good agreement with a monolayer coverage.
Electrocatalysis Modified electrode surfaces can be used for catalytic applications and promising possibilities lie on the horizon in fields like fuel cells or biosensors. Polyoxometalates have not only been shown to be good catalysts in general, but one can also use them as electrocatalysts attached to electrodes (10). One reaction that is very difficult on bare graphite is the reduction or oxidation of hydrogen peroxide. Large overpotentials have to be applied and therefore catalysis is needed to detect this compound at reasonable potentials. Polyoxometalate monolayers containing molybdenum are able to catalyze the electroreduction of H 2O 2 at unusual potentials (11). The PMo 12O 403᎑modified pencil is very well suited to explain the principles of electrocatalysis to students. Without addition of H2O2 the adsorbed polyoxometalate shows two symmetrical reduction and oxidation waves (Fig. 4, thick line). After addition of hydrogen peroxide the less positive redox wave becomes asymmetric. The reduction current increases, whereas the oxidation current tends to disappear (two thin lines for different H2O2 concentrations). This typical behavior can be explained by a chemical reaction of the four-electron reduced PMo12O407᎑ with H2O2.
JChemEd.chem.wisc.edu • Vol. 79 No. 3 March 2002 • Journal of Chemical Education
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In the Laboratory
If this reaction is fast enough, no more PMo12O407᎑ is available for the electrochemical reoxidation, but the chemically regenerated PMo12O405᎑ can be reduced electrochemically several times during one scan, leading to an enhancement of the reduction current. In conclusion, this very simple experimental setup facilitates teaching students some essential terms in the context of surface electrochemistry and may motivate them to enter deeper into this fascinating field of contemporary research. Acknowledgments We would like to thank the University of Bordeaux I and the École Nationale Supérieure de Chimie et Physique de Bordeaux for financial support. Supplemental Material CAS registry numbers and safety information, notes for the instructor, and student data forms are available in this issue of JCE Online. W
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Literature Cited 1. J. Chem. Educ. 1983, 60 (4); special issue on electrochemistry. 2. Murray, R. W. Molecular Design of Electrode Surfaces; Wiley: New York, 1992. 3. Holder, G. N. J. Chem. Educ. 1999, 76, 1478. 4. Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. 5. Pope, M. T.; Müller, A. Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity; Kluwer: London, 1994. 6. Chem. Rev. 1998, 98 (1); special issue on polyoxometalates. 7. Klemperer, W. G.; Wall, C. G. Chem. Rev. 1998, 98, 297. 8. Tanaka, N.; Unoura, K. Inorg. Chem. 1983, 22, 1963. 9. Barteau, M. A., Song, I. K.; Kaba, M. S. J. Phys. Chem. 1996, 100, 19577. 10. Sadakane, M.; Steckhan E. Chem. Rev. 1998, 98, 219. 11. Martel, D.; Kuhn, A. Electrochim. Acta 2000, 45, 1829.
Journal of Chemical Education • Vol. 79 No. 3 March 2002 • JChemEd.chem.wisc.edu
