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Pt-Ag Catalysts as Cathode Material for Oxygen-Depolarized Electrodes in Hydrochloric Acid Electrolysis Artjom Maljusch, Tharamani Chikka Nagaiah, Stefanie Schwamborn, Michael Bron, and Wolfgang Schuhmann* Analytische Chemie - Elektroanalytik & Sensorik, Ruhr-University Bochum, D-44780 Bochum, Germany Pt-Ag nanoparticles were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and tested using cyclic voltammetry and scanning electrochemical microscopy (SECM) with respect to their possible use as catalyst material for oxygen reduction in 400 mM HCl solution. For comparison, a Pt catalyst was investigated under similar conditions. The redox competition mode of scanning electrochemical microscopy (RC-SECM) was adapted to the specific conditions caused by the presence of Clions and used to visualize the local catalytic activity of the Pt-Ag deposits. Similarly prepared Pt deposits were shown to dissolve underneath the SECM tip. Pt-Ag composites showed improved long-term stability toward oxygen reduction as compared with Pt even under multiple switching off to open-circuit potential in 400 mM HCl. The recovery of Cl2 employing the electrolysis of aqueous hydrochloric acid1 is an increasingly important industrial process since most of the primarily produced chlorine is used as a highly reactive intermediate compound yielding Cl- or HCl as a byproduct. Thus, the recycling of Cl2 from HCl can partly replace brine electrolysis especially at industrial sites where the formed HCl cannot be used directly in combined production processes. At pH 0 the difference in the standard potentials between the oxidation of 2 Cl- to Cl2 and the reduction of 2 H+ to H2 is 1.36 V. Taking into account the kinetic losses and the ohmic drop at high current densities, it is obvious that the high energy consumption of this electrolysis process possesses serious drawbacks. Furthermore, since all membranes exhibit an at least small permeability for both Clions as well as Cl2, uncontrolled shutdown of the electrolysis cell can lead to an accumulation of H2(g) and Cl2(g) in the cathode compartment, causing serious safety problems.2 These challenges can be overcome when the H2 formation at the cathode is replaced by an oxygen-depolarized cathode (ODC). In this case, the anodic reaction remains the same while at the cathode * To whom correspondence should be addressed. Tel.: +49 (234)-32-26200. Fax: +49 (234)-32-14683. E-mail:
[email protected]. (1) Federico, F.; Martelli, G. N.; Pinter, D. In Modern chlor-alkali technology: Gas-diffusion electrodes for chlorine related (production) technologies; Moorhouse, J., Ed.; Blackwell Science: London, 2001. (2) Ziegelbauer, J. M.; Gulla´, A. F.; O’Laoire, C.; Urgeghe, C.; Allen, R. G.; Mukerjee, S. Electrochim. Acta 2007, 52, 6282–6294.
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side oxygen will be reduced to water:3 The formal potential of oxygen reduction is 1.23 V at standard conditions, leading to a significant decrease of the potential difference between anode and cathode reactions of only 0.13 V. Thus, successful application of ODCs significantly reduces the cell voltage and consequently results in energy savings of up to 30%.2 Platinum is known to be the most effective catalyst for the oxygen reduction reaction (ORR) but has been shown to be of poor stability in the highly corrosive conditions of aqueous HCl electrolysis.2,4 The dissolution rate of Pt even in solutions of low Cl- concentration can be substantial. This leads to a strong catalyst dissolution/deactivation during cell shutdown, where the cathode is not under potential control and Cl2 penetrates to the cathode side of the electrolysis cell.5,6 At the same time, the strong adsorption of Cl- ions at the Pt surface can cause kinetic limitations for the ORR7 and thus enhance the formation of H2O2. H2O2 produced during the ORR can damage the polymer electrolyte membrane and thus lead to an increasing crossover of Cl- ions as well as Cl2 to the cathode compartment. Additional deactivation of catalyst can be caused by various organic contaminants dissolved in the electrolyte.8 Hence, there is an urgent need for the development of oxygen reduction electrocatalysts which are highly active and simultaneously stable under the corrosive conditions of HCl electrolysis. In recent years, Rh-based materials, specifically RhxSy,9,10 have been investigated as oxygen reduction catalysts for HCl electrolysis despite their high price (Rh, $90/g).11 The intrinsic activity of these catalysts is generally lower than that of state-of-the-art Ptbased catalysts. However, they offer much higher operational stability and are less inhibited by Cl- ions and organic compounds. (3) Gulla´, A. F.; Allen, R. G.; De Castro, E. S. Catalyst for oxygen reduction, De Nora S.p.A. (IT), WO/2004/106591, 2004.. (4) Federico, F.; Martelli, G. N.; Pinter, D. Modern Chlor-Alkali Techology; SCI: London, 2001; Vol. 8. (5) Gestermann, F.; Ottaviani, A. Modern Chlor-Alkali Techology; SCI: London, 2001; Vol. 8, p 49. (6) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 508 (1-2), 41–47. (7) Horanyi, G.; Wasberg, M. J. Electroanal. Chem. 1996, 404 (2), 291–298. (8) Gestermann, F.; Pinter, H.-D.; Speer, G.; Fabian, P.; Scannel, R. PCT Int. Appl. 2003, 17. (9) Allen, R. J.; Giallombardo, J. R. ; Czerwiec, D.; De Castro, E. S.; Shaikh, K. De Nora S.p.A. (IT), US 6,149,782, 2000. (10) Gulla´, A. F.; Gancs, L.; Allen, R. J.; Mukerjee, S. Appl. Catal., A 2007, 326, 227–235. (11) http://www.platinum.matthey.com. 10.1021/ac902620g 2010 American Chemical Society Published on Web 02/10/2010
Our strategy to overcome the challenges of stability and activity was to retain Pt because of its intrinsically high activity and to stabilize it with a second metal. To achieve this, we have chosen silver because of its lower standard potential of E°Ag+/Ag ) +0.8 V as compared with that of Pt (E°Pt2+/Pt ) +1.2 V) in Cl--free solution. During any uncontrolled shutdown of the electrolysis cell, Ag will be primarily oxidized and the formed Ag+ ions will precipitate on the catalyst surface under formation of insoluble AgCl, thus preventing catalyst dissolution. When operation is continued and the cathode is again polarized to reducing potentials, the AgCl will be reduced and the catalyst is again available for oxygen reduction. Reported methods for the synthesis of Pt-Ag catalysts, besides classical methods such as impregnation, include radiolysis,12,13 microwave heating of a solution of Pt and Ag salts in the presence of PVP,14 and DC sputtering.15 Our approach, however, was to use pulsed electrodeposition, that allows a wide variation of deposition parameters in order to adjust the properties of the resulting electrocatalysts. In a preceding publication, we have shown that pulsed electrodeposition from a solution containing 3 mM H2PtCl6 and 1.5 mM AgNO3 in the presence of ethylenediamine leads to partially alloyed Pt-Ag particles which are homogeneously distributed over the surface of a glassy carbon electrode.16 Usually, for electrocatalyst development classical electrochemical techniques such as cyclic voltammetry and rotating disk electrode measurements are employed for the evaluation of catalyst activity. However, recently also scanning electrochemical microscopy (SECM) was applied to evaluate localized catalytic activity in the sample generation-tip collection mode17 (SG-TC) or the tip generation-sample collection mode (TG-SC) of SECM.18 A higher spatial resolution can be obtained using the redox competition mode (RC-SECM).19,20 The redox competition mode is a bipotentiostatic experiment, in which the SECM tip competes with the sample for the molecular O2 dissolved in the electrolyte solution. Thus, if the tip is positioned above an active catalyst site, the tip current decreases. The present contribution addresses the synthesis and application of self-protected Pt-based (Pt/Pt-Ag on glassy carbon) electrocatalysts under conditions relevant to industrial HCl electrolysis using RDE measurements, evaluation of the local catalytic activity by means of RC-SECM, and long-term stability tests. (12) Doudna, C. M.; Bertino, M. F.; Blum, F. D.; Tokuhiro, A. T.; Lahiri-Dey, D.; Chattopadhyay, S.; Terry, J. J. Phys. Chem. B 2003, 107, 2966–2970. (13) Treguer, M.; de Cointet, C.; Remita, H.; Khatouri, M.; Mostafavi, M.; Amblard, J.; Belloni, J.; de Keyzer, R. J. Phys. Chem. B 1998, 102, 4310– 4321. (14) Patel, K.; Kapoor, S.; Purshottam, D.; Mukherjee, T. J. Chem. Sci. 2005, 117, 311–316. (15) Huang, H.; Holme, T.; Prinz, F. B. ECS Trans. 2007, 3 (32), 31–40. (16) Chikka Nagaiah, T.; Maljusch, A.; Chen, X.; Bron, M.; Schuhmann, W. ChemPhysChem. 2009, 10, 2711-2718. (17) Fernandez, J. L.; Bard, A. J. Anal. Chem. 2003, 75, 2967–2974. (18) (a) Fernandez, J. L.; Mano, N.; Heller, A.; Bard, A. J. Angew. Chem., Int. Ed. 2004, 43, 6355–6357. (b) Fernandez, J. L.; Walsh, D. A.; Bard, A. J. J. Am. Chem. Soc. 2005, 127, 357–365. (c) Fernandez, J. L.; Raghuveer, V.; Manthiram, A.; Bard, A. J. J. Am. Chem. Soc. 2005, 127, 13100–13101. (19) Eckhard, K.; Chen, X.; Turcu, F.; Schuhmann, W. Phys. Chem. Chem. Phys. 2006, 8, 5359–5368. (20) Karnicka, K.; Eckhard, K.; Guschin, D. A.; Stoica, L.; Kulesza, P. J.; Schuhmann, W. Electrochem. Commun. 2007, 9, 1998–2002.
EXPERIMENTAL SECTION Sample Preparation and Electrochemical Investigations. For the RC-SECM measurements, catalyst-modified glassy carbon (GC) plates (Sigradur G, HTW Hochtemperatur Werkstoffe, Thierhaupten, Germany) were used as working electrodes. Before each sample preparation, the GC surface was cleaned by polishing with different grades of alumina paste (3, 1, 0.3, and 0.05 µm, LECO, Kirchheim, Germany) to obtain a mirror finish and rinsed with triple-distilled water. An electrochemical droplet cell (adapted from Hassel et al. (ref 21)) was used for the deposition of catalyst spots. The droplet cell consists of a small borosilicate glass capillary (D ) 2.5 mm, wall thickness about 0.25 mm, Hilgenberg, Germany) pulled to a diameter of smaller than 500 µm at the end facing toward the GC working electrode. A Pt wire served as counter electrode and a Ag/AgCl/3 M KCl/agar gel as reference electrode. The GC plate used as working electrode was positioned below the droplet cell. Pt spots as test samples were prepared by pulsed electrodeposition from an aqueous 10 mM H2PtCl6 solution (Merck, Darmstadt, Germany), and Pt-Ag spots from a solution containing 3 mM H2PtCl6 and 1.5 mM AgNO3 using an Autolab (PGSTAT12) potentiostat/galvanostat. For Pt-Ag deposition, a pH of 11.0 was adjusted using ethylenediamine (EDA)/KOH. The solution is sufficiently basic to ensure that AgNO3 readily dissolves without AgCl precipitation. In order to prevent the depletion of the metal ion concentration in front of the GC surface and to achieve a more homogeneous metal deposition, pulsed electrodeposition was used. Pt was deposited by applying 10 times a potential pulse sequence of 0 mV for 500 ms and -600 mV for 200 ms. The electrodeposition of Pt-Ag was carried out using 30 pulses of 600 mV (2 s), -250 mV (5 ms), and -750 mV (3 s).16 For cyclovoltammetric (CV) investigations and chronoamperometric measurements, the catalyst was prepared on 1 mm diameter GC electrodes using the same parameters for pulsed electrodeposition. Glassy carbon rods sealed in glass tubes22 or embedded in a Teflon holder were used as working electrodes. The electrocatalytic activity toward oxygen reduction of all samples was determined by means of cyclovoltammetry at a scan rate of 50 mV s-1 using an Autolab (PGSTAT12) potentiostat/galvanostat in a single compartment three-electrode cell with a Ag/AgCl/3 M KCl reference electrode (RE) and a Pt foil as counter electrode (CE). A 400 mM HCl solution was used as electrolyte for all activity measurements. The solution was purged with argon for 20-30 min to remove any O2 from the electrolyte in order to obtain background voltammograms and with O2 before the evaluation of the electrocatalytic activity. Redox Competition Mode of SECM (RC-SECM). For SECM measurements, a Sensolytics Base SECM (Sensolytics, Bochum, Germany) with option HighRes was used, and the measurements were carried out using a bipotentiostat (PG 100, (21) Hassel, A. W.; Fushimi, K.; Seo, M. Electrochem. Commun. 1999, 1, 180– 183. (22) Smutok, O.; Ngounou, B.; Pavlishko, H.; Gayda, G.; Gonchar, M.; Schuhmann, W. Sens. Actuators, B 2006, 113, 590–598.
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Scheme 1. Schematic Representation of a Flow Cell Setup Used for Chronoamperometric Experiments To Measure the Stability of the Prepared Catalyst (Pt and Pt-Ag nanoparticles)
Figure 1. Cyclovoltammogram of a Pt-microelectrode (25 µm) in 400 mM HCl (a) solution and 200 mM H2SO4 (b) solution at a scan rate of 50 mV s-1, RE: Ag/AgCl/3 M KCl, CE: Pt wire. Inset: Region of oxygen reduction as recorded in air-saturated 400 mM HCl.
Jaissle, Waiblingen, Germany).23 In order to avoid background current shifts caused by uncompensated tilt angles between the scanning plane of the SECM tip and the sample, a software-based tilt correction was used. Experiments were carried out in a fourelectrode configuration with the sample acting as working electrode 1, the SECM tip as working electrode 2, a Pt-wire as counter electrode, and a Ag/AgCl/3 M KCl as reference electrode. The SECM tip was a glass-insulated Pt-microelectrode fabricated as described previously24 using 25 µm diameter Pt-wire (Goodfellow, Bad Nauheim, Germany). For electrical noise elimination, all measurements were carried out in a Faraday cage. Aqueous 400 mM HCl solution was used as electrolyte. The redox competition mode of SECM (RC-SECM) was employed to measure the electrocatalytic activity of the catalyst spot toward oxygen reduction. SECM experiments were performed with an increment of 50 µm at a distance of 10 µm above the surface of the catalyst. At each site of the scanning grid, a pulse potential profile is applied to the tip. The initial potential (P1) is a resting potential, which is applied for 500 ms. At this potential, no redox processes at the microelectrode take place and the restoration of equilibrium occurs. The value of P1 has to be adapted to the specific conditions of SECM in aqueous HCl as described in Results. The second potential (P2, -150 mV) is the measuring potential which is applied for 400 ms. At this potential, oxygen can be reduced to water. If the sample was simultaneously polarized to a potential, at which oxygen can be catalytically reduced, then the tip and the sample compete for the oxygen in the gap between them. Thus, the current at the tip during P2 reflects a measure for the local activity of the underlying sample. For background correction, the option “polynom flatten” in the software MIRA (G. Wittstock, Software and Consulting, Oldenburg, Germany) was used. The algorithm of this procedure is based on the subtraction of the polynom which was calculated as the best fit to the original data. For the subtraction of the tilted plane, a first-order polynom was chosen. (23) (a) Eckhard, K.; Etienne, M.; Schulte, A.; Schuhmann, W. Electrochem. Commun. 2007, 9, 1793–1797. (b) Kundu, S.; Chikka Nagaiah, T.; Xia, W.; Wang, Y.; Van Dommele, S.; Hendrik Bitter, J.; Santa, M.; Grundmeier, G.; Bron, M.; Schuhmann, W.; Muhler, M. J. Phys. Chem. C 2009, 113 (32), 14302–14310. (24) Kranz, C.; Ludwig, M.; Gaub, H. E.; Schuhmann, W. Adv. Mater. 1995, 7, 38–40.
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Stability Test. In order to investigate the long-term stability of the prepared bimetallic catalysts under conditions similar to industrial applications (400 mM HCl solution enriched with Cl2), a flow system was developed (Scheme 1). Two homemade flow cells were connected serially and a peristaltic pump (Minipuls3, Gilson, France) was used for continuously pumping of 400 mM HCl solution through the setup with a flow rate of 0.33 mL min-1. Two Pt-disk electrodes (d ) 1 mm) were mounted in the first flow cell, and a constant potential of 1.2 V vs Ag/AgCl/3 M KCl was applied with a potentiostat (MP 75, Bank, Go ¨ttingen, Germany) for permanent Cl2 production. The HCl solution enriched with Cl2 was transported to the second flow cell where two working electrodes (WE1 and WE2), a Ag/AgCl/3 M KCl as reference electrode and a stainless steel capillary as counter electrode, were placed. WE1 had only Pt deposited on the GC surface while WE2 was modified with a Pt-Ag catalyst. Measurements were performed with an electrochemical detector Biometra EP30 (Go ¨ttingen, Germany). For simulation of uncontrolled cell shutdown, a homemade relay card controlled by a PC was used to switch off the potential applied to the electrodes in flow-through cell two. RESULTS AND DISCUSSION RC-SECM Studies of Pt Deposits in HCl Solution. RCSECM experiments in 400 mM HCl were carried out on Pt catalyst spots deposited on a glassy carbon plate using an electrochemical droplet cell. The previously reported pulse profile at the SECM tip19 had to be modified because of the presence of Cl- ions and a different pH value. In order to find the optimum potentials for the SECM pulse profile, cyclovoltammetric experiments were carried out using a Pt microelectrode at a scan rate of 50 mV s-1 in 400 mM HCl solution. As shown in Figure 1 (curve a), as expected two oxidative processes were observed in the forward scan at a potentials higher than 1 V. The peak with a maximum current at +1.4 V is presumably due to Cl- oxidation, and the current increase at potentials above 1.7 V is due to water oxidation. For comparison, CVs were recorded in Cl--free solution of similar pH using 200 mM H2SO4 (Figure 1, curve b). The peak at 1.4 V is not observed, confirming the assignment to the oxidation of Cl- ions.
Figure 2. (A) Photography of the Pt sample after the RC-SECM measurement shown in Figure S1 (Supporting Information). The scan was performed with a Pt microelectrode (25 µm), the x- and y-increment of tip movement was 50 µm. (B) Enlarged section of A. (C) Photo of a measurement similar to that for A but with a base potential of 200 mV and 400 mM HCl (left) and 200 mM H2SO4 (right) as electrolytes.
Figure 3. (A) Chronoamperometric measurement in oxygen-free 400 mM HCl, Pt-microelectrode (25 µm), RE: Ag/AgCl/3 M KCl, CE: Pt wire. The WE potential was increased stepwise with tpulse ) 3 min. (B) Absolute current difference vs applied potential.
In the reverse scan oxygen reduction reaction in 400 mM HCl solution starts below 250 mV, as shown in the inset. On the basis of the above cyclovoltammetric studies, the pulse profile (Scheme S1B, Supporting Information) used for RC-SECM measurements in phosphate buffer (pH 7.0)19 was modified (Scheme S1C, Supporting Information). The base potential (P1) was initially set to +400 mV for 500 ms in order to avoid simultaneous oxygen reduction and Cl- oxidation. The oxygen injection pulse (P2) proposed in earlier studies19 could not be used because of concomitant Cl- oxidation. The potential of the measuring pulse (P3) was set to -150 mV for 400 ms. Despite the careful adaptation of the pulse potential, the electrocatalytic activity of the pure Pt deposits on GC could not be visualized using RC-SECM (Figure S1, Supporting Information). Inspection of the sample surface using a light microscope (Figure 2A) clearly revealed the reason for the impossibility to detect the local catalytic reaction at the catalyst spot. The Pt deposits were entirely removed at the grid points at which the SECM tip was positioned during SECM imaging. Evaluation of the observed pattern in the Pt deposits (Figure 2B) confirmed that the holes have the same diameter as the electrochemical active tip (25 µm Pt electrode) area and the distance between them is equal to the increment of tip movement (50 µm). Similar RC-SECM measurement with a base potential (P1) of +200 mV at the microelectrode caused the same pattern (Figure 2C, left). However, RC-SECM experiments in absence of chloride ions
did not affect the morphology of the Pt sample (Figure 2C, right). Obviously, the Pt deposits on the substrate were etched during the SECM experiment. In order to gain a deeper understanding of the processes occurring in the gap between the tip and the sample during RCSECM in 400 mM HCl, chronoamperometric experiments at the tip were carried out in the potential range between -100 mV and 400 mV in oxygen-free solution. The SECM tip was positioned 10 µm above a glass plate, and a sequence of potential pulses was applied with stepwise increasing potentials (Figure 3A). The absolute difference between the currents measured before the first pulse and at the end of all following pulses was calculated and plotted as a function of the applied potential (Figure 3B). A linear dependence of the current difference on the applied potential was observed at potentials below +100 mV. At higher potentials the current increase was exponential. We assume that Cl- oxidation to Cl2 already starts at above +100 mV. Rough calculation of the concentration of Cl2 at this potential from the Nernst equation indicates it should be extremely small. However, because of the restricted diffusion in the gap between tip and sample, the generated low amount of Cl2 may reach the Pt deposits and assist in Pt dissolution, leading finally to the observed etching pattern. It has to be taken into account that a small flux of Cl2 will be generated from the SECM tip to the sample during the potential pulse sequence applied to the SECM tip for the envisaged RC-SECM imaging. During the Analytical Chemistry, Vol. 82, No. 5, March 1, 2010
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Figure 4. (A) RC-SECM images of Pt-Ag nanoparticles spots in 400 mM HCl with different applied sample potentials: (a) -250 mV, (b) -200 mV, (c) -150 mV, and (d) -100 mV. The tip (Pt-microelectrode, 25 µm) was polarized at +50 mV during P1 and at -150 mV during P2. Images were taken after 368 ms during the measurement pulse. (B) Baseline-corrected RC-SECM x-line scans (at y ) 1000 µm) of the electrochemically deposited Pt-Ag spots, with the sample polarized at different potentials (400 mM HCl, Etip ) -150 mV, data acquisition after 368 ms of the detection pulse).
pulse, this flux of Cl2 is obviously sufficiently high to completely remove the Pt under formation of, for example, soluble PtCl42complexes. From the chronoamperometric measurements shown above, it becomes clear that the value for the base potential in the RC-SECM sequence has to be further decreased to a value of +50 mV even though at this potential, oxygen reduction at the tip already occurs. Electrodeposition of Pt-Ag Nanoparticles. With the aim of stabilizing Pt-based oxygen reduction catalysts using Ag, Pt-Ag nanoparticles have been prepared by pulsed electrodeposition. The hypothesis, as outlined above, is, that in the case of uncontrolled application of more positive potentials during oxygen reduction in HCl electrolysis (e.g., in the case of uncontrolled cell shutdown), 1894
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first the Ag is oxidized, forming an insoluble protecting AgCl deposit. When the potential is switched back to more negative potentials, the AgCl will be reduced under formation of the protective Ag deposits while liberating the Cl- ions. It is assumed that because of formation of the insoluble Ag salt the protective coating will not be lost despite its intermediate oxidation, and hence the stabilizing effect will be observed over prolonged times. Pt-Ag spots were deposited from a solution containing 3 mM H2PtCl6 and 1.5 mM AgNO3 at a pH value of 11 adjusted by means of ethylenediamine.16 Energy-dispersive X-ray analysis (EDAX) confirmed that both Pt and Ag were deposited on the GC surface while scanning electron microscopic (SEM) images indicated a homogeneous distribution of nanoparticles
with diameters in a range below 200 nm. The size and shape of the nanoparticles strongly depended on the ratio of Pt and Ag in the deposition solution as well as on the potentials applied during pulsed deposition. X-ray diffraction data (XRD) suggested that part of the Pt-Ag particles were deposited as alloys (Ag3Pt; JCDPS, 65-3258) with a lattice parameter a ) 3.894 Å. The peaks representing Pt, Ag, and the corresponding oxides of these elements are not visible. However, the nominal composition of the catalysts as derived by EDAX did not fit the ratio 3:1 for Ag3Pt, indicating that other X-ray amorphous phases must be present. Electrochemical Characterizations of Pt-Ag Catalysts. In order to evaluate the catalytic properties of the prepared Pt-Ag deposits, cyclovoltammetric experiments have been performed in 400 mM HCl and 200 mM H2SO4 (Figure S2; Supporting Information). Similar to the behavior of pure Pt, during the forward scan a peak attributed to Cl- oxidation can be observed at 1.5 V in aqueous HCl, whereas no such peak is observed in Cl--free solution. During the reverse scan in HCl, two peaks are observed at +0.62 V and at +0.11 V, which may be due to the reduction of oxidation products of Pt and Ag, respectively. RC-SECM experiments were carried out using Pt-Ag catalyst spots deposited on a GC plate in 400 mM HCl. When the SECM tip is scanned over a catalytically active region in the redox competition mode, the current at the tip will decrease. Figure 4A shows background current-corrected RC-SECM images of Pt-Ag deposits. Bright color indicates a smaller tip current and hence higher local ORR activity of the sample than the darker regions. In order to evaluate the influence of the sample potential on the visualization of ORR activity using SECM, the experiments have been carried out at different sample potentials of -250 mV (Figure 4a), -200 mV (Figure 4b), -150 mV (Figure 4c), and -100 mV (Figure 4d). At -150 mV a good contrast between the active spot and the significantly less active nonmodified GC surface is obtained. At more negative potentials (-200 mV and -250 mV), it seems that the catalyst became less active for oxygen reduction. It is known that the activity of catalysts for ORR can be influenced by potential dependent processes such as H+ and Cl- adsorption. With a decrease of the sample potential a higher amount of protons will be adsorbed on the catalyst surface and thus a smaller number of active catalyst sites will be available for the oxygen molecules. This leads to the observed decrease of the local catalytic activity. In an attempt to quantify the results from the SECM investigations, the absolute difference in reduction current at the tip over the active spots is plotted for various sample potentials and the line scan is shown in Figure 4B. The successful visualization of local catalytic activity of Pt-Ag deposits in HCl solution by means of RC-SECM gives a first hint on the expected improved stability of these deposits with respect to dissolution. An inspection of the sample after RC-SECM measurements did not reveal any visual dissolution of the catalyst deposits at the sites where the SECM tip was positioned. In order to further evaluate the stability of the catalysts under conditions similar to those encountered in industrial HCl electrolysis, measurements were carried out in a flow-through cell setup. For simulating unwanted cell shutdown, an interruption mode was developed consisting of a sequence of polarization of the catalyst-modified GC electrodes to oxygen reduction potential
Figure 5. Electrocatalytic stability test by chronoamperometric experiments of (a) Pt-Ag and (b) only Pt catalyst. The measurement was performed in a flow cell in 400 mM HCl at a flow rate of 0.33 mL/min and at a potential of -150 mV operated in an interruption mode with 10 min of operation and 2 min of interruption to open-circuit potential. CE: Stainless steel capillary, RE: Ag/ AgCl/3 M KCl.
for 10 min followed by disconnecting the working electrodes by means of a relay board for 2 min thus establishing open circuit potential (OCP). Aqueous 400 mM HCl solution was pumped through two consecutive flow-through electrochemical cells. In the first one, Cl2 was continuously produced by applying a potential of 1.2 V vs Ag/AgCl/3 M KCl between two Pt electrodes. Thus, a slightly Cl2-enriched solution is continuously pumped through the second flow-through electrochemical cell, in which two working electrodes modified with Pt nanoparticles alone and Pt-Ag nanoparticles were integrated. The current traces during the 10 min polarization segments to -150 mV at both electrodes were recorded potentiostatically. As seen from Figure 5A, the oxygen reduction current at the electrode modified with Pt-Ag nanoparticles reaches a constant plateau after 10 min while the current was constantly decreasing in case of the Pt catalyst. Figure 5B is a plot of the normalized oxygen reduction current versus the number of cell shutoffs to OCP. Even after 10 shutdown cycles, the Pt-Ag catalyst-modified electrode shows about 90% of its starting catalytic activity while the activity of the Pt catalyst drops down to only about 60%. CONCLUSIONS Pt and Pt-Ag nanoparticles have been deposited onto glassy carbon by pulsed electrodeposition. The local catalytic activity of the Pt-Ag nanoparticles was successfully visualized by means of a modified pulse profile during an RC-SECM experiment in 400 mM HCl while Pt deposits turned out to be unstable under the applied conditions. The Pt-Ag nanoparticles exhibit good catalytic Analytical Chemistry, Vol. 82, No. 5, March 1, 2010
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activity toward the oxygen reduction reaction and higher longterm stability than that of pure Pt catalysts.
Germany) for helpful discussions. The first two authors contributed equally to this work.
ACKNOWLEDGMENT The authors are grateful to Dr. Thomas Erichsen (Sensolytics, Bochum, Germany) for software adaptations and troubleshooting during SECM and flow cell experiments. Artjom Maljusch is grateful to the International Max Plank Research School SurMat for financial support, and Tharamani Chikka Nagaiah is grateful to the Alexander von Humboldt Foundation for a postdoctoral fellowship. The authors are grateful to Dr. J. Kintrup, Dr. R. Weber, and A. Bulan (Bayer Materials Science, Leverkusen,
SUPPORTING INFORMATION AVAILABLE SECM image of Pt spot; extended cyclovoltammetric characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review November 16, 2009. Accepted January 27, 2010. AC902620G
