Determination of the Real Surface Area of Pt electro-catalyst by Hydrogen Under-potential (UPD) Deposition

Transcription

1 Experiment 1: Determination of the Real Surface Area of Pt electro-catalyst by Hydrogen Under-potential (UPD) Deposition 1. Motivation [1] based on: Journal of Chemical Education, 77, (2000), pp: [2] Anderson, J. R. Structure of Metallic Catalysts; Academic: London, (1975). [3] Biegler, T.; Rand, D. A. J.; Woods, R. J. Electroanal. Chem. (1971), 29, pp: 269. [4] Lewis, R. J. Sr. Sax s Dangerous Properties of Industrial Materials, 8th ed.; Van Nostrand Reinhold: New York, (1992), pp: [5] Lunn, G.; Sansone, E. B. Destruction of Hazardous Chemicals in the Laboratory, 2nd ed.; Wiley: New York, (1994), pp: One of the most important parameters in heterogeneous catalysis, especially in electrocatalysis, is the real surface area of the catalyst or electrode, which will determine the catalyst activity [1]. The electrocatalysis mechanism is based on the electrode electroactive species charge transfer through the electrode surface. So the reaction rate, and consequently the electric current, is proportional to the electrode real surface area. The human eye is not able to observe an electrode s microtopography. As a result, surface area measuring instruments designed for macroscopic systems can determine only the electrode s geometric surface area, not the real surface area involved in microscopic processes. Figure 1 shows different surface images depending on the observation scale. From a [1] macroscopic viewpoint the electrode surface may appear perfectly flat, but the presence of surface steps, holes, kinks, and terraces yields a real surface area that is larger than the apparent one. To measure an electrode real surface area we need to use a microscopic probe. One such probe is the adsorption of gaseous atoms or molecules onto the solid surface. A gas molecule may be adsorbed on a solid surface by weak 1 [1]

2 interactions (physisorption) or by strong interactions (chemisorption) (2). If physisorption takes place, the gas particles carpet the electrode surface forming a monolayer on which successive gas layers may adsorb (Fig. 2a). In this case the real surface area can be evaluated by measuring the amount of adsorbed gas and the area occupied by each gas atom or molecule. Then the real surface area (S r ) can be calculated as: SS rr = nn aa NN AA aa aa (1) where n a is the amount (in moles) of gas adsorbed in the monolayer, N A is Avogadro s number, and a a is the area occupied by each adsorbed particle. When the gas is absorbed by chemisorption, each atom or molecule of gas is attached by a real covalent bond to an adsorption site. In the case of a metallic surface this adsorption site is an atom (Fig. 2b). The evaluation of the real surface area requires the determination of both the amount of adsorbed gas and the surface atom density of the theoretically flat metal from crystallographic data. S r can be calculated as: where d m is the surface metal atom density. What Students will learn? ss rr = nn aann AA dd mm (2) Measuring the I/V voltammetric curves in aqueous solutions is an easy way to determine real surface area by hydrogen adsorption. Clearly, the real surface area differs highly from the geometric one, indicating the importance of electrode microtopography in processes that involve the electrode surface. 2. Experiment Fundamentals Hydrogen adsorption on the platinum electrode surface is achieved by application of sufficiently negative potentials to the electrode when this is in contact with an aqueous solution. Three regions may be distinguished in the cyclic voltammetric curve of a Pt electrode in contact with an acid solution (Fig. 3). The oxygen region is found at positive potentials. During the positive sweep prior to O 2 evolution, a hydrated Pt oxide monolayer is formed (anodic current). In the center of the voltammetric curve is a region where only low currents (positive anodic for the positive sweep and negative for the negative sweep) can be found. This is the double-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 negative potentials are applied, the reduction of H + and the adsorption of H atoms become stronger: HH + (aaaa) + ee + ssssssss HH(aaaa) (3) 2

3 This process continues as electrode potentials become more and more negative, until the formation of a H(ad) monolayer is achieved. Once the Pt surface is fully covered by hydrogen atoms, the adsorption of H 2 molecules will take place: 2HH(aaaa) HH 2 (aaaa) (4) These adsorbed molecules come together to form hydrogen bubbles that will leave the Pt electrode surface when they have grown large enough: nnnn 2 (aaaa) nnhh 2 (gg) + 2nn ssssssssss (5) At this moment a high cathodic potential is applied on the electrode and many free sites are exposed to the solution. Immediately steps 3, 4, and 5 take place successively at a high rate, giving rise to the sharp cathodic current increase known as the hydrogen evolution. The formation of the H(ad) monolayer can be easily detected at the potential where the cathodic current rapidly rises. When the potential is reversed, the opposite processes (anodic currents in the hydrogen region) take place. This is a chemisorptive process and eq 2 can be used to calculate the Pt electrode real surface area. The value of d m can be evaluated using the Anderson criterion [2], which proposes that for aged polycrystalline surfaces, the lower Miller index planes (100), (110), and (111) predominate, in a proportion of 33% for each one. In this case for Pt, dm takes a value of cm -2 [3]. Taking into account that this is a chargetransfer process in which the adsorption of a hydrogen atom on a Pt site involves one electron, the moles of adsorbed hydrogen atoms can be calculated as: nn aa = QQ mm FF (6) where Q m is the charge associated with the formation of a monolayer and F is the Faraday constant. Then eq 2 can be rewritten as: ss rr = QQ mmnn AA = QQ mm (7) FFFF mm mm PPPP mm PPPP eeee mm where e is the electron charge. In this case, e d m = ( C) ( cm -2 ) = 208 μc/cm 2, and m Pt is catalyst mass (g). At this point, the main problem is the Q m calculation. Taking into account that: II = dddd dddd (8) then tt ff QQ = IIIIII (9) tt ii 3

4 where t i is the time when the hydrogen adsorption starts and t f is the moment when the monolayer is completed. For cyclic voltammetry the potential applied to the electrode in the negative sweep varies as: VV = VV 0 θθθθ (10) where V 0 is the initial potential, v is the potential sweep rate (dv/dt), and t is the time. Then and tt ii = VV 0 VV ii θθ dddd = dddd θθ, tt ff = VV 0 VV ff θθ (11) (12) and eq 9 can be rewritten as: QQ = 1 θθ (VV 0 VV ff )/θθ (VV 0 VV ii )/θθ IIIIII (13) In this way, Q m can be calculated by integrating the voltammetric curves between the potentials where the cathodic current deviates from the double layer current and the hydrogen evolution starts. However, the capacitive current is not restricted to the capacitive region, but extends over the whole range of the electrode potential. For this reason Q m must be calculated as: QQ = 1 θθ (VV 0 VV ff )/θθ (VV 0 VV ii )/θθ IIIIII QQ dddd (14) where Q dl is the charge associated to double layer capacitive processes in the hydrogen adsorption region. The inset in Figure 3 shows the Q m and Q dl graphic evaluation. 4

5 Figure 3. Cyclic voltammetric current potential curve for a platinum electrode in contact with a 0.5 M H 2 SO 4 solution. The electrode potential is referenced to the mercury sulfate reference electrode (MSE). The inset shows the different charge contributions in the hydrogen region [1]. 3. Materials and Apparatus 0.1 M HClO 4 (made form ultra-pure water 18 MOhms) N 2 gas Pt/C electrocatalysts Isopropanol ultra-pure water 18 MOhms Nafion 5 wt% solution in methanol (with dilution 1:100 v:v) 5

6 Potentiostat/Galvanostat type: MaterialsM 515 thermostated three electrode cell (Figure 4 ) Pt wire counter electrode (CE) Hg/Hg 2 SO 4 (E = V) reference electrode (RE) Glassy carbon disk: working electrode substrate (WE) + thin layer of catalyst ink Automatic pipette Weighting paper, spatula, vial, balance, ultrasonic bath, glass pipette 6

7 Reference electrode (Hg/Hg 2 SO 4 ) Counter electrode (Graphite rod or Pt wire) Working electrode (GC disk) 7

8 Figure 4. Electrochemical 3-electrode cell 4. Conducting Experiment 1. Fill electrochemical cell with 0.1 M HClO 4, place counter and reference electrodes (rinse RE in DI water several time to avoid K 2 SO 4 contamination) 2. Purge electrolyte with N 2 for 20 min prior to measurement 3. Prepare Pt/C catalyst ink: 10 mg of Pt/C (20wt% Pt) ml DI water ml isopropanol place in glass vial and sonicate for 20 min (at max. 25 o C) 4. Cast 12 µl (in portion of 3 x 4 µl) on 5.00 mm of diameter GC disk with diameter cm 2 - let dry each portion of the ink and drop-cast next and dry under the laboratory lamp (usually 10 min is sufficient) 5. 7 µl of Nafion solution in methanol (1:100) drop cast on the top of dried catalyst layer let it dry for 5 min under lamp 6. Place working electrode in electrochemical cell, observe WE area remove air bubbles if necessary using gentle N 2 flow 7. Connect electrodes to potentiostat (Figure 5): ask instructor to check connection before you applied voltage! 8

9 Figure 5. Electrode connection to potentiostat in 3-electrode system. 8. Design electrochemical experiment in software VoltaScope (Figure 6): Cyclic voltammetry a) Potential range: V to V vs. RE*, scan rate 100 mv/sec, number of scans 50 electrode activation, irreversibly oxidizes adsorbed organic impurities on the electrode surface, if reproducible voltammograms are obtained perform step b b) Potential range: V to V vs. RE, scan rate 20 mv/sec, number of scans Turn off potentiostat, reassembly electrochemical cell: dispose WE (active part, not holder), rinse CE and RE in DI water, clean electrochemical cell and fill in with DI water, place CE and WE in proper containers ask your teacher for instruction 10. Copy txt. or csv. file to your folder for analysis of results (Origin Lab for the peak integration). For this purpose go to File (see panel below) select export parameters and separator (choose txt. or csv. and export). File can be open in MS Excel or Origin lab. 9

10 Attention! Open (import file) in Origin Lab and perform peak integration for the single (last) CV recorded at 20 mv/sec 5. Analysis of results 1. Determine Q m by integrating the cathodic and anodic parts of the hydrogen region (Q m will be an average; Q m = (Q anodic +Q cathodic )/2) and subtracting Q dl according to eq. 14, consider potential scan rate (20 mv/sec) in your calculation Use Origin Lab or VoltaScope software 2. Calculated electrochemically active surface area of the catalysts by eq Compare electrochemically active surface area (point 2) with geometric surface area of the catalyst (based on geometry of your working electrode deposit) 10

11 Attention! CV spectrum stabilizes after 6-8 scans (conditioning electrode taking place, e.g. an opening of pores and the electrode OCV equilibration) Use last CV scan recorded at scan rate of 20 mv/sec for your calculations! Figure 6. Setting of CV in VoltaScope software: go to File Save measure (after measurement remember to export your data to open for calculations! Go to File set export format and export) 6. Safety precaution Mercury and mercury compounds are poisonous and teratogenic [4]. If metallic mercury is spilled, a silver bar can be used to collect it and deposit it in a proper container. Positively charged metal ions can be removed from solution by using Amberlite IR-120(plus) or Dowex 50X8-100, strongly acid gel-type resins with a sulfonic acid functionality. Solutions of HgCl 2 that contain negatively charged species can be decontaminated using Amberlite IRA-400(Cl) or Dowex 1X8-50 [5]. Lab coat, laboratory goggles, gloves are mandatory for the Praktikum in Energy Systems! Report has to be submitted no later than 4 weeks after experiment to responsible instructor. Everybody is obligated to pass entry colloquium before starting the experiment. 11