The Modern Student laboratory Using Electrochemical Principles
An Electrochemistry Experiment: Hydrogen Evolution Reaction on Different Electrodes D. Marin, F. Mendicuti, and C. Teijeiro Departamento de Quimica Fisica, Universidad de Alcald de Henares, AicalA de Henares 28871, Madrid, Spain
Electrochemical laboratory classes are frequently limited to processes that follow the Nernst equation. However, most electrochemical processes take place at potentials far from the equilibrium potential, that is, with an overvoltage. This paper presents a simple laboratory experiment designed to acquaint the student with overvoltage in the hydrogen evolution reaction. The experiment is done in two stages. The first shows that hydrogen evolution takes place a t different potentials depending on the electrode material (mercury, platinum, or silver). The overvoltage on each material is calculated. In the second stage the exchange current density, a kinetic parameter, is obtained from the Tafel equation. Theory In this section we present relevant aspects of the theory of ovemotential(13). The ~otentialE a t which a reaction takes place depends on the nature and physical state ofthe electrode surface: E is different from the reversible Downtial E, given by the Nernst equation. The difference between these potentials is a direct measure of the irreversibility of the electrochemical process, and is known as the overvoltage or overpotential, q. q=E-E, (1)
Overvoltage is related to c m n t densityj by the empirid Tafel equation (eq 2) for cathodic or reduction processes.
cause accidental spills are then easier to pick up. If the use of mercury is considered to be too dangerous, the expe"ment can be done with only platinum and silver electrodes. The mercurv electrode was made of a c a ~ i l l a wtube connected by ruhbcr tubing to a mercury resemoir. Adrop area of 3 x 10 cm2was determined from the weieht of a known number of drops, considering the merc;ry drop as a sphere. The platinum electrode is made of a platinum wire (length 0.5 cm, diameter 0.08 cm) in a glass tube. Asurface area of 0.13 cm2was obtained by simple approximation to a cylinder. Before use, this electrode must be kept immersed in an acidic aqueous solution of ferric sulfate and ferrous sulfate to avoid oxide and hydrogen adsorption on the surface. The silver electrode was made in the same way as the platinum electrode using a silver wire or a small silver plate. Before use, this electrode must be treated with a n aqueous NH40H solution and then washed with distilled water to clean the surface. A saturated calomel electrode (SCE) was used as the reference electrode. It was built as follows. A couple of drops of mercury were added to Hg2C12and mixed thoroughly in a mortar until the mixture turned gray. At that moment, a few drops of saturated KC1 solution were added to make the pasty mixture. Mercury was poured into a test tube, filling the spherical bottom. The pasty mixture was
where b, Tafel's slope, gives information on the mechanism of the electrode reaction and where current I is usually expressed as current density, I = electrode area
At equilibrium, 11 = 0, and the exchange current density isj, (log j. = alb). The value of j. depends on both the reaction and the electrode. At equilibrium, although the net current is zero, equal cathodic and anodic currents are flowing. Exchange current densities range from 10 A/cm2 to pA/cmZ. The lower the exchange current density, the higher the overvoltage that must be applied to create a significant net current flow. Exoerimental Procedure Electrodes Caution: Mernvy is easy to spill, and its vanor is noisonous. so it must be handled with ear;. We recommend handling the Figure 1. I-E curves (polarograms)in H S 0 4 aqueous solutions at pH 3.21 obtained with mercury over a wooden or plastic tray, be- platinum, silver, and mercury electrodes.
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then transferred to the test tube containing the mercury to a level of approximately 1.5 cm. Next the saturated KC1 solution was added to the test tube, which was then left to stand for 24 h. The circuit was closed hy a sdt bndge immersed in the KC1 solution and by a platinum contact connected to the mercury & the bottom. Alternatively, a comercial SCE can be used. Aplatinum electrode must be used as the auxiliary electrode if a three-electrode cell is used.
y I/
Solutions Aqueous solutions of HzS04at pH 3.0 to 4.5 were used. Solutions were prepared by diluting a stock solution and measuring the pH directly in the cell. Nitrogen was bubbled through the solutions for 10 min to remove dissolved oxygen before each measurement.
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Apparatus " I The current-potential (I-E) curves were obtained with a polarograph. However, a simple potentiostat, with Or without an x9 re- Figure 2. Tafelplotsfor different H S 0 4 aqueous soIutio"s with (A) platinum (pH:03.05, corder, can also be used. W 3.35, 3.55, 3.79, and A4.20) and (6) mercury (pH:03.20, W 3.35, A 3.50, 4.01. and 4.35) electrodes. Analysis of the 1-E Curves E ("8. SCE)= E (VS.NHE) - 0.24 V (7) Tafel plots were constructed from the I-E values on the rising portion of the polarographic wave (e.g., Fig. 1)at apResults and Discussion proximately 20% of the limiting current where the process Fikuw 1 shows curves obtained with platinum, silver, and is controlled by the charge-transfer rate. Current densities mercury electrodes in a H2S0, solution at pH 3.2. The hydrowere calculated from the current (in and the electrode gen di&harge begins at-approximately-4.35, -0.80; and area (in em2).The overpotential is obtained from E, calcu-1.30 V (vs. SCE). At more negative potentials more current lated with the Nernst equation (eq 3), flows until a limiting value is reached. Accordmg to eq 6, the hydrogen discharge overvoltage on m e w , silver, and platinum electrodes is +0.08,-0.37, and -0.87 V Now that the overvoltage is known, the kinetic parameters can be obtained. Pairs of E-I values may be taken and the measured E value as in eq 4. from plots as in Figure 1at different pH values and calculated values ofn - i. Anlot ofn vs. loe i for the mercurv and platinum electrodes at several pH values is given in Figure 2. TheTafel analvsis was not done with the silver electrode The potential E a t which hydrogen evolution takes because it is diflfcult to keep the surface clean. place has been unequivocally shown ( 4 )to he independent The i. values obtained for the m e r n w electrode ranof hydrogen pressure, so eq 4 can be rewritten as shown in from 1?3 to 10-14A/cm2. For the electrodej,ls eqs 5 and 6, around lo4 A/cm2. The j, value is pH-dependent on the mercury electrode, whereas it is pH-independent on the platinum electrode because the reaction mechanism for hydrogen evolution is different. It would also be interesting to examine copper and lead. The exchange current density for copper is about 10.' Akmz and for lead about 10-14A/cm2.All the results presented were obtained by students in an undergraduate where 11, is the overvoltage at 1atm of hydrogen because laboratory course.
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Literature Cited under this condition. Usually, q, is the overvoltage mentioned; it is used in this paper. The experimental E value is obtained vs. SCE, but the E, value is calculated vs. NHE, so E must be corrected with eq 7. A278
Journal of Chemical Education
1. Bard. A. J.; Fadkner, L. R. Elpdmchemlml Methmls; J. W i k y and Sone: New Ymk, 1980;pp 19.91. and 105. 2. Bocktis, J. OM.:Reddy. A. K N. M o d c m E h t m h e r n i s t r y ; Plenum: New Yo&, 1977; Val. 2, pp 879 and 892. 3. Costa.J. M . Fundomsnloa &Ekt&~.AlhambraUnive*iidad: Madrid.. 1981:... no6
and 65.
4. Erdey-Druz, T.Kiwlier ofElelmdD Prapsses; Wiley-lnterseienee: New York, 1972: p 170.
