In the Laboratory

JChemEd.chem.wisc.edu • Vol. 77 No. 9 September 2000 • Journal of Chemical Education 1195

One of the most important parameters in heterogeneouscatalysis, especially in electrocatalysis, is the real surface areaof the catalyst or electrode, which will determine the catalystactivity (1). The electrocatalysis mechanism is based on theelectrode–electroactive species charge transfer through theelectrode surface. So the reaction rate, and consequently theelectric current, is proportional to the electrode real surface area.

The human eye is not able to observe an electrode’smicrotopography. As a result, surface area measuring instru-ments designed for macroscopic systems can determine onlythe electrode’s geometric surface area, not the real surface areainvolved in microscopic processes. Figure 1 shows differentsurface images depending on the observation scale. From amacroscopic viewpoint the electrode surface may appearperfectly flat, but the presence of surface steps, holes, kinks,and terraces yields a real surface area that is larger than theapparent one.

To measure an electrode real surface area we need touse a microscopic probe. One such probe is the adsorptionof gaseous atoms or molecules onto the solid surface. A gasmolecule may be adsorbed on a solid surface by weak inter-actions (physisorption) or by strong interactions (chemisorp-tion) (2). If physisorption takes place, the gas particles carpetthe electrode surface forming a monolayer on which successivegas layers may adsorb (Fig. 2a). In this case the real surfacearea can be evaluated by measuring the amount of adsorbedgas and the area occupied by each gas atom or molecule. Thenthe real surface area (Sr) can be calculated as

Sr = naNAaa (1)

where na is the amount (in moles) of gas adsorbed in themonolayer, NA is Avogadro’s number, and aa is the area occu-pied by each adsorbed particle.

When the gas is absorbed by chemisorption, each atomor molecule of gas is attached by a real covalent bond to anadsorption site. In the case of a metallic surface this adsorptionsite is an atom (Fig. 2b). The evaluation of the real surface arearequires the determination of both the amount of adsorbedgas and the surface atom density of the theoretically flat metalfrom crystallographic data. Sr can be calculated as

S r =na NA

dm(2)

where dm is the surface metal atom density.In this paper we propose an easy experiment for physical

chemistry, electrochemistry, chemical engineering, and analyti-cal chemistry students, in which the real surface area of anelectrode is determined by cyclic voltammetry. An explanationof this experiment can be found in ref 3. The theoretical basisand various applications of the cyclic voltammetry techniquecan be found in refs 4–9. Previous knowledge about general

physical chemistry and physicochemical techniques is requiredfor good comprehension of this experiment. The experimenthas been used in a course on physicochemical techniquesfor students of marine sciences. Students have previouslytaken several courses in physics (e.g., General Physics, FluidMechanics), chemistry (e.g., General Chemistry, PhysicalChemistry, Organic Chemistry), and mathematics (e.g.,Algebra, Calculus). But to perform this experiment, they onlyneed basic concepts in general chemistry, electrochemistry,and mathematics.

Experiment Fundamentals

Hydrogen adsorption on the platinum electrode surfaceis achieved by application of sufficiently negative potentialsto the electrode when this is in contact with an aqueoussolution. Three regions may be distinguished in the cyclicvoltammetric curve of a Pt electrode in contact with an acidsolution (Fig. 3) (9). The “oxygen region” is found at positivepotentials. During the positive sweep prior to O2 evolution,a hydrated Pt oxide monolayer is formed (anodic current).

Determination of the Real Surface Area of Pt Electrodesby Hydrogen Adsorption Using Cyclic VoltammetryJosé M. Doña Rodríguez,* José Alberto Herrera Melián, and Jesús Pérez PeñaDepartamento de Química, Universidad de Las Palmas de Gran Canaria, Islas Canarias, Spain;*jose.dona@quimica.ulpgc.es

baa

Figures 2. Different adsorbate particles packaging in (a) physi-sorption and (b) chemisorption processes.

a

10 mm

10 µm

10 nm

Figure 1. Differences in detail of solid surface images depending onthe order of magnitude of the measurement.

In the Laboratory

1196 Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu

In the center of the voltammetric curve is a region whereonly low currents (positive anodic for the positive sweep andnegative for the negative sweep) can be found. This is thedouble-layer region where only capacitive processes take place.Finally, the “hydrogen region” is found at negative potentials.This region will be studied in more detail. As more negativepotentials are applied, the reduction of H+ and the adsorptionof H atoms become stronger:

H+(aq) + e� + site → H(ad) (3)

This process continues as electrode potentials become moreand more negative, until the formation of a H(ad) monolayeris achieved. Once the Pt surface is fully covered by hydrogenatoms, the adsorption of H2 molecules will take place:

2H(ad) → H2(ad) (4)

These adsorbed molecules come together to form hy-drogen bubbles, that will leave the Pt electrode surface whenthey have grown large enough:

nH2(ad) → nH2(g) + 2n-sites (5)

At this moment a high cathodic potential is applied onthe electrode and many free sites are exposed to the solution.Immediately steps 3, 4, and 5 take place successively at a highrate, giving rise to the sharp cathodic current increase knownas the hydrogen evolution. The formation of the H(ad) mono-layer can be easily detected at the potential where the cathodiccurrent rapidly rises. When the potential is reversed, theopposite processes (anodic currents in the hydrogen region)take place.

This is a chemisorptive process and eq 2 can be used tocalculate the Pt electrode real surface area. The value of dmcan be evaluated using the Anderson criterion (10), whichproposes that for aged polycrystalline surfaces, the lowerMiller index planes (100), (110), and (111) predominate, ina proportion of 33% for each one. In this case for Pt, dmtakes a value of 1.3 × 1015 cm�2 (11).

Taking into account that this is a charge-transfer processin which the adsorption of a hydrogen atom on a Pt site in-volves one electron, the moles of adsorbed hydrogen atomscan be calculated as

na =Qm

F(6)

where Qm is the charge associated with the formation of amonolayer and F is the Faraday constant. Then eq 2 can berewritten as

S r =Qm NA

Fdm

=Qm

edm(7)

where e is the electron charge. In this case, e × dm = (1.602 ×10�19 C) × (1.3 × 1015 cm�2) = 208 µC/cm2.

At this point, the main problem is the Qm calculation.Taking into account that

I =dQdt

(8)

then

Q = I dt

t i

tf

(9)

where ti is the time when the hydrogen adsorption starts andtf is the moment when the monolayer is completed. Forcyclic voltammetry the potential applied to the electrode inthe negative sweep varies as

V = V0 – vt (10)

where V0 is the initial potential, v is the potential sweep rate(dV /dt), and t is the time. Then

ti =V0 – Vi

v , tf =V0 – Vf

v (11)

and

dt = dVv (12)

and eq 9 can be rewritten as:

Q = 1

v I dVV0–V i / v

V0–V f / v

(13)

In this way, Qm can be calculated by integrating the volt-ammetric curves between the potentials where the cathodiccurrent deviates from the double layer current and the hy-drogen evolution starts. However, the capacitive current isnot restricted to the capacitive region, but extends over thewhole range of the electrode potential. For this reason Qmmust be calculated as

Q = 1

v I dV – QdlV0–V i / v

V0–V f / v

(14)

where Qdl is the charge associated to double layer capacitiveprocesses in the hydrogen adsorption region. The inset inFigure 3 shows the Qm and Qdl graphic evaluation.

Figure 3. Cyclic voltammetric current potential curve for a platinumelectrode in contact with a 0.5 M H2SO4 solution. The electrodepotential is referenced to the mercury sulfate reference electrode(MSE). The inset shows the different charge contributions in thehydrogen region.

Double layer region

Double layer region

Oxygen regionHydrogen

region

Hydrogenevolution

Hydrogen evolution charge

Double layercapacitivecharge

Hydrogenadsorptioncharge

Potential / mV

Cur

rent

/ µA

0-200-400-600

20

10

0

-10

-20

-30-800 200 400 600 800

In the Laboratory

JChemEd.chem.wisc.edu • Vol. 77 No. 9 September 2000 • Journal of Chemical Education 1197

The Experiment

The experiment was performed in a thermostated three-electrode cell (9) with a Hg/HgSO4/K2SO4(sat) referenceelectrode (MSE). Although the MSE reference electrode isnot a common one, the use of more popular reference elec-trodes such as Ag/AgCl or Hg/Hg2Cl2 is not recommendedbecause of possible Cl� ion diffusion to the working electrodesurface. The Cl� ions, even at very low concentrations,strongly adsorb on the Pt electrode surface. This blocks theactive surface sites and thereby masks the real electrode surface.On the other hand, the electrode potentials referenced to theMSE are easily converted to normal hydrogen electrode(NHE) referenced potentials by adding 640 mV. This kindof reference electrode is very safe under normal operatingconditions. However, some safety rules must be observed incase of electrode breakage:

CAUTION: Mercury and mercury compounds are poisonousand teratogenic (12). If metallic mercury is spilled, a silverbar can be used to collect it and deposit it in a propercontainer. Positively charged metal ions can be removedfrom solution by using Amberlite IR-120(plus) or Dowex50X8-100, strongly acid gel-type resins with a sulfonicacid functionality. Solutions of HgCl2 that contain nega-tively charged species can be decontaminated usingAmberlite IRA-400(Cl) or Dowex 1X8-50 (13).

The working electrode was a platinum sheet with ageometric surface area of 0.2 cm2 (taking into account bothsides). The counter electrode was a platinum helical wire. Thetest solution was 0.5 M H2SO4, which was deaerated withN2 gas for 10 minutes before the experiment. A N2 flow wasmaintained over the solution to avoid air exchanges with theenvironment. The voltammetric experiment was carried outusing a Princeton-Applied-Research Model 263 Potentiostat/Galvanostat and the voltammetric curves were recorded usinga Linseis LY 18100 recorder.

The voltammetric curves were obtained at 25 °C between800 and �665 mV vs MSE, at a scan rate of 50 mV/s. Ini-tially, continuous cyclic scans between the limits were appliedto the electrode until a reproducible voltammetric curve wasobtained (about 10 minutes). This treatment, called electrodeactivation, irreversibly oxidizes adsorbed organic impuritieson the electrode surface. Sometimes the electrode activationis not able to eliminate hard-to-oxidize impurities. In thiscase it is necessary to extend the electrode potential sweeplimit to more positive potentials during a couple of scans toensure the complete elimination of impurities (14 ).

In general, two hours is enough for the experiment,including the solution preparation and electrode activation.

Results

After successive scans are reproducible, one of them isrecorded (Fig. 3) and Qm is determined by integrating thecathodic part of the hydrogen region. Many computer pro-grams provide an easy way to treat and integrate these curves.In our case, the EG&G Headstart Creative ElectrochemistrySoftware was used for both experiment control and the datatreatment.

Once Qdl is subtracted from the calculated Q value, avalue of about 62.5 µC is obtained for Qm. By eq 7 we cal-culated an electrode real surface area of about 0.3 cm2, whichis 50% higher than the 0.2 cm2 value of the geometric sur-face area. About 90% of the students obtained good agree-ment between their results and the expected ones (less than10% difference). When erroneous results were obtained,students were questioned about the certainty of their valuesand asked to repeat the measurement once again. These casesillustrate the importance of performing the determinationtwice in triplicate.

Conclusions

Measuring the I/V voltammetric curves in aqueous solu-tions is an easy way to determine real surface area by hydrogenadsorption. Clearly, the real surface area differs highly fromthe geometric one, indicating the importance of electrodemicrotopography in processes that involve the electrode surface.This is clearly perceived by the students.

Remarks

Mercury is a very common electrode in polarographic andvoltammetric experiments. It is well known that hydrogenand oxygen do not adsorb on mercury and this method isnot suitable to determine the Hg electrode real surface area.However, as the mercury electrode is liquid, surface rugositiesare not expected to be present. So in this case, the real surfacearea will coincide with the geometric one.

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