CHEMCATCHEM FULL PAPERS DOI: 10.1002/cctc.201300287
Time Evolution of the Stability and Oxygen Reduction Reaction Activity of PtCu/C Nanoparticles Chinnaya Jeyabharathi,[a, c] Nejc Hodnik,[a, d] Claudio Baldizzone,[a] Josef C. Meier,[a] Marc Heggen,[b] Kanala L. N. Phani,[c] Marjan Bele,[d] Milena Zorko,[d] Stanko Hocevar,*[d] and Karl J. J. Mayrhofer*[a]
Crystalline Cu3Pt nanoparticles supported on graphitized carbon are synthesized by using a modified sol–gel method, and subsequent thermal annealing leads to alloying of Pt with structure. ElecCu and formation of a partially ordered Pm3m trochemical dealloying under potentiodynamic conditions (potential cycling) induces not only changes from rather spherical high-index faceted to more cuboctahedral low-index faceted core–shell structures for particles in a size range of 10–20 nm but also percolation for some particles larger than 20 nm. In contrast, during dealloying under potentiostatic conditions (potential hold) the semispherical shape of small particles is
completely retained and extensive porosity is formed on all particles larger than 20 nm. Other degradation processes are not observed on performing an additional accelerated aging test; hence, the high specific and mass activity of the catalyst decreases only slightly, mainly owing to continuing Cu leaching. The difference in dealloying protocols and their effect on the structure of the catalysts as well as their activities, considering the promising porosity formation, are discussed and indicate future directions for a rational design of active and stable oxygen reduction reaction catalysts.
Introduction Growing energy demand and serious environmental problems due to burning of fossil fuels urge the development of sustainable and green energy solutions. Therefore, solar, wind, and other forms of renewable power sources are currently experiencing a rapid ascent and are deployed worldwide. In parallel, the demand for the efficient conversion of electrical energy into chemical energy and vice versa for portable applications and large-scale energy storage increases. Low-temperature polymer electrolyte membrane fuel cells (PEMFCs) are an attractive choice for this purpose, in particular for the use of chemical energy in local stationary or automotive applications. PEMFCs using hydrogen as a fuel are already developed to a high level; however, some issues related to the material costs as well as catalyst activity and stability still hamper their market introduction. [a] C. Jeyabharathi, Dr. N. Hodnik, C. Baldizzone, J. C. Meier, Dr. K. J. J. Mayrhofer Max-Planck-Institut fr Eisenforschung GmbH Max-Planck Str. 1, 40237 Dsseldorf (Germany) E-mail:
[email protected] [b] Dr. M. Heggen Ernst Ruska-Centrum und Peter Grnberg Institut Forschungszentrum Jlich GmbH 52425 Jlich (Germany) [c] C. Jeyabharathi, Dr. K. L. N. Phani CSIR Central Electrochemical Research Institute Karaikudi 630 006, Tamil Nadu (India) [d] Dr. N. Hodnik, Dr. M. Bele, M. Zorko, Dr. S. Hocevar National Institute of Chemistry Hajdrihova 19, Ljubljana (Slovenia) E-mail:
[email protected]
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Typically, carbon-supported Pt nanoparticles are used for catalyzing the anodic (hydrogen oxidation reaction; HOR) and cathodic (oxygen reduction reaction; ORR) reactions. Although the kinetics of the HOR on Pt-based catalysts is fast and close to ideal, the kinetics of the ORR is 6 or more orders of magnitude slower and causes a cathodic overpotential of approximately 0.3 V at acceptable Pt loadings. A well-proven way to minimize the Pt loading is to alloy Pt with less noble transition metals, such as M = Ni, Co, Cu, Fe.[1–13] As a consequence, these alloys improve the ORR activity by a factor of 2–10 compared to standard Pt/C catalysts through electronic (ligand effect) and/or geometric alterations (strain effect), which seems to be sufficient for reducing Pt loadings according to certain development targets. However, the more critical concern of the state-of-the-art cathode materials, that is, their inability to sustain their activity during the extended fuel cell operation, is not circumvented but often improved by this approach. Thus, more detailed fundamental studies on the degradation processes of alloy nanoparticles are required to guide the development of next generation PEMFC catalysts. In addition to typical degradation processes of high-surface area, carbon-supported Pt (Pt-HSAC) catalysts,[14–19] alloy catalysts are prone to selective dissolution, that is, dealloying, of the less noble metal from the homogeneous bulk, which leads to the formation of noble metal-rich structures. Depending on the material properties such as size, composition, structure, and morphology, as well as on operating conditions, the dealloying of nanoparticles can follow different pathways.[20–26] Oezaslan et al.[21] showed that Cu-rich Pt alloy nanoparticles tend to form single core–shell particles in a size range of 10–15 nm, ChemCatChem 2013, 5, 2627 – 2635
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CHEMCATCHEM FULL PAPERS multi-core–shell particles up to 30 nm, and porous multi-core– shell particles larger than 30 nm. The phenomenon of porosity formation for PtCu particle catalysts above a certain characteristic particle diameter was also described by using the terms “Swiss cheese”-type structure or “spongy structure”.[12, 22] The size-dependent dealloying was confirmed by Snyder et al.[26] for Pt25Ni75 nanoparticles, in which nanoparticles smaller than 13 nm result in core–shell structures and nanoparticles larger than 13 nm result in porous structures. The formation of a Pt shell covering the Pt bimetallic alloy for smaller particles can be due to the inverse Gibbs–Thompson effect,[23] which suggests that smaller particles demonstrate higher surface atom mobility and thus have a better ability for surface passivation. In addition to the critical parameters of size and composition, the importance of the synthesis protocol for porosity formation was described in a recent work by Strasser et al. Larger porous nanoparticles were formed when leaching was performed in oxygen-saturated acid, whereas leaching in nitrogen saturated acid resulted in the formation of core–shell structures, which generally showed a higher electrocatalytic activity.[19] Moreover, various studies report how the synthesis protocol can alter the structure and nanoscale composition and thus their electrochemistry and degradation.[27–29] Although the importance of the dealloying of nanoparticles has already been acknowledged by several groups, the effect of electrochemical faceting of nanoparticles and its dependence on operating conditions have not been considered in experimental studies on alloys so far. Tian et al.[30] reported the formation of tetrahexahedral plain Pt nanoparticles faceted by high Miller index planes at the cost of sacrificial Pt nanoparticles during a square-wave potential treatment based on a dissolution/redeposition mechanism. In an in situ electrochemical STM study, Xu et al.[31] tracked the formation of a Pt skeleton structure followed by restructuring into a smooth (111) surface through a surface diffusion mechanism upon potential cycling between 0.06 and 1.0 VRHE (RHE = reversible hydrogen electrode) at a scan rate of 0.2 V s 1, which is the thermodynamically stable form. McCue et al.[23] demonstrated theoretically the possibility of the formation of pseudo-equilibrium Wulff structures during the early dealloying process on particles of the sizes from 4 to 17 nm via a surface diffusion mechanism. Such an effect would be highly important for Pt alloys as it could help achieve excellent activity combined with improved longterm stability; this has been demonstrated for ex situ prepared nanoparticles with a particular orientation.[32–34] Herein, we study the dealloying of a PtCu/C catalyst, the stability after dealloying, and the electrocatalytic activity for the ORR in acid solutions. The electrochemical studies are complemented by identical location (IL) electron microscopy after both dealloying and degradation to obtain a direct insight into the mechanism of degradation processes, such as dissolution, particle detachment, and carbon corrosion.[18, 35–37] Our results describe the size-dependent faceting on smaller alloy particles in parallel to the formation of porosity and core–shell structures. Furthermore, we shed light on the effect of the most important operation parameters on the final morphology and
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www.chemcatchem.org composition, factors that have a significant effect on the activity of the catalyst.
Experimental Section Catalyst synthesis and structural analysis PtCu nanoparticles supported on VULCAN XC72R carbon were synthesized by a using modified sol–gel method. Alloying of Pt with Cu was achieved through thermal annealing, as described in the patent application and a previous article.[38, 39] Annealing has already been proved to be beneficial with regard to the activity and stability of Pt alloy particles; however, it can produce large particle size distributions.[12] The catalyst composite consisted of Cu ( 24 wt %) and Pt ( 26 wt %). The catalyst with a high degree of structure was obtained by setting the temperathe ordered Pm3m ture below the order–disorder transition temperature close to 500 8C so that the system attains a thermodynamic equilibrium.[40] The powder XRD measurements and qualitative and quantitative analysis of the XRD patterns were described elsewhere.[39] The powder XRD patterns of the annealed PtCu/C are shown in Figure 1. The diffraction peaks at the angles of 24, 34, 55, 61, 77, 82, and 978 were characteristic of the ordered phase of PtCu (Pm3m). All other diffractions were due to solid solutions of Pt and phase). The phase Cu with a random distribution of atoms (Fm3m composition was obtained from the Rietveld refinement and corre phase. sponded to 66.5 % Fm 3m phase and 33.5 % Pm3m
Figure 1. XRD patterns of the annealed PtCu/C catalyst. The inset shows the phase. atomic arrangement of Pt (*) and Cu (*) in the Pm3m
Electrochemical experiments A three-electrode cell made of Teflon was used for the electrochemical measurements. The working electrode consisted of a glassy carbon (GC) disk of 5 mm diameter, which was mounted in a Teflon tip of a rotating disc electrode shaft (Radiometer Analytical, France). A graphite rod and an Ag/AgCl electrode (3 m KCl; Metrohm, Germany) were used as a counter and a reference electrode, respectively. However, if not explicitly stated otherwise, all potentials are referred to the RHE potential, which was determined separately before each measurement. To avoid Cl contamination from the reference electrode during extended stability investigations, the counter and reference electrodes were separated from the rest of the cell through a special construction including Nafion ChemCatChem 2013, 5, 2627 – 2635
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CHEMCATCHEM FULL PAPERS membranes.[41] A Gamry’s Reference 600 potentiostat–galvanostat was used and controlled in combination with the rotator and the gas system by using in-house developed LabVIEW software.[42] All the electrochemical experiments were performed in HClO4 solutions (0.1 m), which were prepared from conc. HClO4 (Suprapur, Merck) and ultrapure water (18.2 MW·cm; ELGA, Germany). Solution resistance was compensated by a positive feedback mechanism in such a way that the residual uncompensated resistance is less than 3 W in all experiments. The catalyst suspension was prepared by adding the catalyst (1 mg) to ultrapure water (2 mL), followed by ultrasonication for 1 h and drop casting the suspension (40 mL) on the freshly polished GC of the rotating disc electrode. The solvent was evaporated in a desiccator, and then freshly prepared Nafion (5 mL, 0.1 %) in 2propanol was coated over the catalyst layer. The final loading on the electrode was adjusted to 26 mgPt cm 2. The dealloying of the catalysts was performed in two ways: (1) by sweeping the potential of the electrode from 0.05 to 1.2 V for 500 cycles at a scan rate of 0.2 V s 1 and (2) by holding the potential at 1.2 V for 15 min or 2 h in a deaerated electrolyte. In both cases, the potential was held at 0.05 V before starting the dealloying process to avoid leaching of Cu at the open circuit potential. Accelerated degradation tests were performed by cycling the dealloyed PtCu/C catalysts in a deaerated electrolyte from 0.6 to 1.2 V for 7000 cycles at a scan rate of 1 V s 1. For studies on the ORR activity, the working electrode was polarized from 0.05 to 1.2 V at a scan rate of 0.05 V s 1 in oxygensaturated electrolyte with varying rotation rate (400, 900, 1600, and 2500 rpm). To obtain true ORR activities, the non-faradaic background current obtained from cyclic voltammetry in argon-saturated solutions was subtracted from the ORR current. The electrochemically active surface area (ECSA) after dealloying and degradation was determined by using the CO stripping method and used to estimate specific activities of the catalysts. A few milliliters of the electrolyte solution were extracted after the dealloying and the degradation process and analyzed by using inductively coupled plasma mass spectrometry (NexION 300, Perkin– Elmer, Germany) to quantify the amount of ions leached from the catalyst.
IL electron microscopy The catalyst suspensions for electron microscopy samples were prepared freshly by dispersing the catalyst particles in ultrapure water under bath sonication. A drop of a dilute suspension was pipetted onto a gold finder grid, coated with a carbon film, and then removed carefully from the grid with a tissue to keep the particle loading low (and thus avoid overlapping of multiple carbon aggregates). TEM investigations were performed with a JEOL JEM2200FS instrument at the operating voltage of 200 kV. Then, the gold finder grid was brought into contact with the GC disc electrode and cycled as mentioned for dealloying and degradation. After the electrochemical treatment, TEM micrographs of the dealloyed and degraded catalyst were obtained on ILs.[18] For IL-SEM investigations,[37] a graphite holder (Ted Pella, Inc., Graphite specimen mount, 12.7 mm diameter head) was used as a substrate to study the catalyst through a microscope (FE-SEM SUPRA 35 VP, Carl Zeiss, Germany) and as a working electrode in the electrochemical treatment. A few milligrams of the catalyst in powder form were attached to the surface of the SEM graphite holder. No signs of beam damage were observed in the IL-SEM and IL-TEM studies according to independent control experiments, as has also been reported in other previous works.[43] 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemcatchem.org Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) studies were performed before dealloying and after the above-mentioned potential treatment, but not on ILs. Micrographs were recorded with an FEI TITAN 80-300 scanning transmission electron microscope operating at 300 kV and equipped with a high-angle annular dark field (HAADF) detector and Gatan’s Tridiem 866 ERS image filter. “Z-contrast” conditions were achieved with a probe semiangle of 25 mrad (1 mrad = 0.0578) and a detector inner collection angle of 70 mrad. Under these conditions, the image intensity associated with an atom column is roughly proportional to the square of the mean atomic number of the atoms in the column for sufficiently thin objects. Other experimental conditions used are the same as described previously.[17] The concentration profiles of Pt and Cu were obtained by integrating the background-subtracted EELS curves in the energy window of 2155–2400 eV (Pt M4,5 edges) and 938–968 eV (Cu L2,3 edges).
Results and Discussion Electron microscopy The degradation of supported Pt particles can occur owing to the dissolution (with successive redeposition) and/or detachment of Pt particles and agglomeration or corrosion of the support, which results in the decrease in the Pt active surface area and specific activity.[14–18] Alloy particles can also undergo different dealloying processes leading to the formation of core–shell structures, multi-core–single shell particles with pits on the surface, or percolated particles (less noble metal-rich alloys)[9, 19, 21, 22, 27–29, 44] and even hollow particles (Pt-rich alloys).[45] To monitor these changes in alloy particles and to gain more insight into the dealloying mechanism, IL electron microscopy is used on particular locations before and after the dealloying and degradation processes. The IL-TEM and IL-SEM micrographs of PtCu/C—as-annealed, dealloyed by potential cycling, and after 7000 and 5000 degradation cycles, respectively—are shown in Figure 2. The catalyst initially consists of quasispherical and polydispersed particles, with particle sizes ranging from a few nanometers to more than 50 nm. Although eventually not optimal for applications, this large particle size distribution has the advantage that it enables us to study the size-dependent effect in the dealloying process for certain electrochemical treatments on one single sample, which improves the comparability and reliability of the investigations. After dealloying and degradation cycles, different stages and pathways of dealloying can be observed depending on the size of the particles. Particles in a size range of 10–20 nm change their shape and form faceted structures, but do not indicate any percolation. In contrast, the intriguing faceting of small particles, which may be induced by an initial surface dealloying, is not so pronounced for larger particles. Although all the faceted particles retain their structure after extended degradation cycles, for many larger particles above 20 nm in diameter another process, for example, porosity formation, is clearly observed. Notably, for some larger particles no complete percolation is observed in TEM micrographs even after degradation whereas all SEM micrographs indicate at least the initiation of this process by significant surface roughening. ChemCatChem 2013, 5, 2627 – 2635
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sufficient to cover up pores originating from Cu dissolution as well as to form stable core–shell structures on these length scales.[19, 23, 26] Wang et al.[22] observed “spongy” particles for a PtCu catalyst after a chemical treatment by immersing it in aerated acid whereas the potential treatment (cycling) did not lead to “spongy” structures. This indicates a certain similarity between the chemical treatment at open circuit potential and the controlled potential hold experiment. However, if the open circuit potential is adjusted by deaerating the acid solution, different effects corresponding to a controlled hold at lower potentials can be observed.[19] A combination of complex Figure 2. IL-TEM micrographs of the PtCu/C nanoparticles : A) as-annealed, B) after 500 dealloying cycles (0.2 V s 1 degradation mechanisms known between 0.05 and 1.2 V), and C) after 7000 degradation cycles (1 V s 1 between 0.6 and 1.2 V). IL-SEM micrographs of the same catalyst: D) as-annealed, E) after 500 dealloying cycles, and F) after 5000 degradation cycles. for typical Pt-HSAC, such as agglomeration, particle detachment owing to carbon corrosion, The IL-TEM micrographs of PtCu/C—as-annealed, dealloyed by or major dissolution of Pt, which lead to a decrease in the potential hold, and after additional 7000 accelerated degradaECSA and in turn activity,[17] was not observed for these alloys tion cycles—are shown in Figure 3. Because the potential is under the applied aging conditions. After dealloying and accelconstantly held above the reversible potential of Cu and Pt, erated aging, all particles are intact on the carbon matrix and the structural and compositional changes differ from transient no evidence for particle detachment or agglomeration was experiments. Particles smaller than 20 nm retain their spherical found. Moreover, no indication for particle growth owing to shape and do not form facets. Moreover, larger particles demthe redeposition of Pt in the sense of an Ostwald ripeningonstrate massive porosity even before the degradation prototype mechanism (with a dissolution of smaller particles and recol is applied and it seems that all of them are affected, which deposition of Pt on larger particles) could be observed, irreis in contrast to the case of potential cycling in which some spective of the method of dealloying, although the expressiveparticles remain intact. Holding the potential at 1.2 V for exness of IL-TEM for the latter is limited.[18, 37] Notably, the imtended periods of time does not induce surface rearrangement proved stability concerning these aspects is not due to alloyor dissolution/redeposition of Pt atoms, but rather their passiing, but rather due to the larger average particle diameters vation and thus immobilization. Potential cycling between recompared to the 2–6 nm of Pt–HSAC[47] and their preparation. gions of Pt oxidation and reduction instead leads to Pt dissoluThe HAADF micrographs and EELS concentration profiles of tion/redeposition[46] and/or Pt ad-atom movement, which is the PtCu/C nanoparticles before dealloying are shown in Figure 4. The uniform contrast of all particles indicates that both Pt and Cu are homogeneously distributed within each particle, irrespective of their size, which is confirmed by the EELS concentration profiles of selected particles. Although all particles are generally rich in Cu, the relative concentration could be different for individual particles. The representative HAADF micrographs and EELS concentraFigure 3. IL-TEM micrographs of the PtCu/C nanoparticles : A) as-annealed, B) after potential hold at 1.2 V for 2 h, tion profiles of the PtCu/C nanoand C) after 7000 degradation cycles (1 V s 1 between 0.6 and 1.2 V). 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4. HAADF micrographs and EELS concentration profiles of the PtCu/C nanoparticles before dealloying: A) 10 nm, B) 20 nm, and C) 30 nm. The sizes correspond to the particles confined in the yellow square, which are subjected to EELS. c: Cu; c: Pt. Arrows indicate the direction of line scans.
particle surface. On other particles, bright and dark regions appear throughout the volume after dealloying (Figure 5 b and c), which is in accordance with the IL-TEM measurements. The EELS measurements on these larger particles demonstrate that the intensity of both Pt and Cu increases in the bright region and decreases in the dark region. This confirms that contrast variations are not due to compositional variations according to different Z contrast of Pt and Cu, but rather due to the formation of porosity. Notably, the Cu content in these particles decreases significantly, particularly in regions of low contrast and Pt content, or even vanishes completely. Oezaslan et al.[21] observed the formation of Cu-rich inclusions, which gave rise to similar contrast variations and the formation of dark regions observed in HAADF micrographs after a significantly milder treatment (0.06 and 1.00 VRHE), with similar results on PtNi[27] and PtCo[28] catalysts. Complementary electrochemical investigations
Dealloying is performed on PtCu/C by using two protocols for approximately the same amount of time (2 h). The 3 D waterfall plots of cyclic voltammograms (CVs) recorded during the dealloying process are shown in Figure 6. Although the Figure 5. HAADF micrographs and EELS concentration profiles of the PtCu/C nanoparticles after dealloying by transition of the surface compo1 sweeping the potential of the electrode from 0.05 to 1.2 V for 500 cycles at a scan rate of 0.2 V s and consesition from a Cu- to a Pt-rich surquently degraded by cycling at a scan rate of 1 V s 1 between 0.6 and 1.2 V for 7000 cycles. c: Cu; c: Pt. Arrows indicate the direction of line scans. face is extended over the 500 cycles (Figure 6 a), it seems to be already completed after a few minutes of potential hold at 1.2 V (Figure 6 b). An intense particles after dealloying and degradation cycles are shown in Cu peak at 0.4 V with a broad shoulder up to 1.0 V initially Figure 5. In some of the dealloyed particles, a bright contrast dominates the CVs, which can be attributed to a Cu-rich suron the circumference is observed (Figure 5 a) whereas the inteface phase that could have segregated during the thermal anrior of the particles has a homogeneous contrast. Under Z-connealing process and to slightly stronger bond Cu in the vicinity trast conditions, the bright contrast feature indicates the forof Pt surface atoms, respectively.[48] Notably, no separate peaks mation of a thin Pt layer around a Pt-rich alloy core, which is different from the as-prepared catalysts. This supports the for a pure Cu phase have been detected in XRD patterns. Both notion that the rearrangement of Pt after the removal of surfeatures decrease significantly over the first 20 cycles, or within face Cu owing to potential cycling leads to a passivation of the the first 5 min of potential hold, owing to the dissolution of 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 6. A) First few cycles and every 20th CV of the continuously recorded scans during the dealloying of the PtCu/C catalyst with a scan rate of 200 mV s 1. B) CVs recorded after every 5 min of potential hold for a total period of 2 h. In addition, three dealloying stages during potential cycling are highlighted in the insets: from cycles C) 1–20, D) 40–100, and E) 200–500.
those surface Cu atoms (see also Figure 6 c). The same is true for the reversible reduction peaks visible in the negative scan of the potential cycling, which can be attributed to the underpotential and regular deposition of the fraction of dissolved Cu ions that did not diffuse into the electrolyte bulk. At the end of the 20th cycle, the Cu signals have completely vanished and first discernible features of hydrogen under potential deposition (Hupd) typical to a Pt surface start to appear. From the 20th cycle onward, the Hupd current increases continuously until after approximately 300 cycles a quasi-steady state consisting of a broad shoulder is reached. In parallel, a new peak arises at approximately 0.8 V in the positive and negative scans owing to Cu dissolution directly from Pt (Cuupd)[48] and exposure of more Pt sites on the surface, probably in the form of surface rearrangement and/or roughening (Figure 6 d). Because the CV after 100 cycles does not completely resemble the CV of pure Pt, it is inferred that Cu is still present in the near-surface region, which imposes a ligand effect on the Pt surface atoms. In contrast, the lower OH adsorption potential and the rather undefined Hupd region could be attributed to low-coordinated Pt atoms originating from surface roughening. Both explanations also hold for the changes from the 200th to the 500th cycle if the anodic peak at 0.8 V gradually disappears again and common polycrystalline Pt surface features, that is, irreversible Pt oxidation/reduction and defined Hupd peaks, develop (Figure 6 e). Thus, at the end of the cycling, Cu is completely leached from near-surface 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
layers and/or extended facets are formed, which can also be observed by using electron microscopy. The dynamic behavior during the potential hold experiment is overall similar; however, dealloying occurs much faster. A comparison of the final CVs recorded after dealloying is shown in Figure 7. In the case of potential cycling, three slightly pronounced Hupd peaks indicate a polyfaceted,[49] core–shell structure[21] whereas only two overlapping peaks remain after the potential hold. This difference is interesting in the light of the TEM results, according to which smaller faceted particles are formed during cycling but not after the hold experiment. Moreover, the capacitance and other adsorption features are higher after a potential hold than after cycling; the determined surface area is 32 and 23 m2 gPt1, respectively. The rapid dealloying of the catalysts at 1.2 V and the concomitant formation of highly porous structures for larger nanoparticles (see Figure 3) leads to a certain accessibility of the inner part of the catalyst particles for the electrolyte. This can be attributed to the formation of stable Pt oxide at these high potentials and thus a reduced mobility of the surface Pt atoms, which are unable to passivate the surface and block Cu dissolution. In contrast, the potential variation during cycling induces not only Cu dissolution but also Pt oxide reduction accompanied by dissolution/redeposition[30] and/or an increased surface mobility of Pt.[20] As a consequence, this enables reordering of the Pt surface, giving rise to faceting, and prevents further dealloying. Porosity is thus observed only on larger particles in the ChemCatChem 2013, 5, 2627 – 2635
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Figure 7. Comparison of A) CV of PtCu/C catalysts after dealloying by a) potential cycling (c) and b) potential hold (c) in deaerated 0.1 m HClO4 solution and B) corresponding CO stripping curves (scan rate = 50 mV s 1).
case of extended potential cycling after 7000 degradation cycles. The activity of the catalysts toward the ORR after dealloying via potential cycling and after an additional degradation treatment is studied by using the thin-film rotating disc electrode method.[50, 51] All values, including the mass activity and the Cu leached from the catalyst during degradation, are summarized in Table 1. The ORR curves at different rotation rates on deal-
Figure 8. A) Rotating disc electrode polarization curves for ORR on the PtCu/ C catalyst with different rotation rates recorded in 0.1 m HClO4 solution after dealloying (c) and at 1600 rpm after degradation (g); scan rate is 50 mV s 1 in positive scan direction. The inset shows the Koutecky´–Levich plot. B) Tafel plot of ORR curves at 1600 rpm: a) after dealloying by cyclic voltammetry and b) after degradation of the same electrode.
loyed PtCu/C and one at 1600 rpm after degradation of the same catalyst are shown in Figure 8 A. The corresponding Tafel plot of the ORR curve at 1600 rpm is shown in Figure 8 B. The Koutecky´–Levich plot is linear at various potentials, with a slope corresponding to a four-electron process; hence, the reduction is first order with respect Table 1. Surface area and electrocatalytic properties of the PtCu/C catalyst after dealto oxygen and leads to water as expected for a Pt loying by potential cycling and potential hold at 1.2 V, that is, before and after degradation by 7000 cycles between 0.6 and 1.2 V. The total percentage of Cu leached surface. The specific activity measured after dealloyfrom the catalyst, as determined from inductively coupled-plasma mass spectrometry ing, that is, 2.5 mA cmPt2, is approximately two times analysis, is given for various stages. higher than Pt black and five times higher than 3– 5 nm Pt-HSAC.[52] After 7000 degradation cycles, the MA[b] ECSA[c] Cu leached Dealloying SA[a] [mA cmPt2] [A mgPt1] [m2 gPt1] [%] method specific activity decreases slightly to 2.0 mA cmPt2 Before After Before After Before After Before After while the active surface area remains the same. Cyclic 2.5 0.2 2.0 0.2 0.58 0.06 0.42 0.05 23 2 21 2 56 1 71 4 Because the dealloying by potential hold at 1.2 V voltammetry is faster, the ORR is measured after two time duraHold I 3.4 0.3 – 0.68 0.07 – 20 2 – 10 2 – tions: 15 min (hold I) and 2 h (hold II). Although after Hold II 2.4 0.2 2.0 0.2 0.77 0.08 0.44 0.05 32 3 22 2 80 1 89 1 15 min the specific activity is rather high, that is, [a] Specific activity; [b] Mass activity; [c] Electrochemically active surface area. 3.4 mA cmPt2, it decreases to 2.4 mA cmPt2 after 2 h.
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CHEMCATCHEM FULL PAPERS After 2 h, the ECSA increases by more than 50 % but decreases again to the original value of 22 m2 gPt1 after the degradation, in parallel with a slight decrease in the specific activity. In addition to the electrochemical measurements, the percentage of Cu leached during the dealloying and degradation processes is determined from the inductively coupled plasma mass spectrometry analysis of the electrolyte at each stage. The specific activity decrease can be directly correlated with the amount of Cu removed from the catalyst. Only the specific activity for the potential hold after 2 h (80 % Cu leached) is higher, which reaches the value after dealloying with cycling, at which significantly less Cu has leached (56 %). Although it is not straightforward to conclude absolute effects from a catalyst with such a broad size distribution as in our study, certain peculiar trends in the relation between structure and activity can still be derived. The improved electrocatalytic activity of Pt alloys for the ORR compared to that of plain Pt is well known and has been studied extensively by several research groups.[1–13] Specific activities of 1.9–2.5 mA cmPt2 and mass activities of approximately 0.4–0.6 A mgPt1 at 0.9 V in this study for dealloyed PtCu/C catalysts are well in line with the typical improvement of a factor 2–4 reported in the literature for Pt catalysts of comparable size.[52] This effect of the alloying material has been attributed predominately to the lowering of Pt d-band states (ligand effect[7]) and/or compressed lattice (strain effect[6]) on the Pt shell of core–shell structures formed after dealloying. However, the specific activity after 15 min of potential hold is significantly higher, as Cu is only partially leached at this stage (9 %), which suggests that an additional ligand and/or strain effect of Cu in the near- or subsurface region may contribute at this stage. However, it is clear that this structure is not sufficiently stable, as the specific activity already decreases after 2 h of potential hold. Another aspect of this study is that the structural changes of 10–20 nm spherical particles to faceted particles with lowindex planes containing a thin Pt shell have no measurable effect on the specific activity, whereas work on single crystal alloy surfaces clearly shows a huge improvement.[53] This observation is also in contrast to studies on octahedral and cubic Pt and Pt alloy nanoparticles with similar size and composition, which expose different ratios of (111) and (1 0 0) facets and thus demonstrate different activities.[32–34] This could be due to the fact that the Cu composition in the subsurface layers, which is the crucial parameter for the activity,[54] is not the same for the faceted structure formed in situ through dealloying. The most appealing result is the increase in the ESCA by 50 % after dealloying by potential hold for 2 h, in which percolated nanoparticles above 20 nm are formed. The area of the pores is thus accessible for the reaction and contributes by increasing the mass activity by more than 20 % compared to a cycled electrode. Despite losing significantly more Cu than during any other pretreatment, the specific activity still remains relatively high. A similar effect has been described by Erlebacher et al., who explained the nanoconfinement effect with the increased attempt frequencies and thus improved kinetics.[24, 26] Because the analysis of a porous electrode network 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemcatchem.org is challenging, considering the parallel effect of the remaining alloy material, a quantitative description of the extent of activity within the pores and mass transport limitations depending on the pore aspect ratio is still pending.
Conclusions Herein, we demonstrate the dependence of the degradation pathways on identical locations of a highly active PtCu catalyst on the type of electrode potential perturbation. We observe (1) the change to faceted particles from spherical particles of 10–20 nm in size only during potential cycling, not during constant potential condition; (2) the surface roughening of larger particles upon potential cycling (> 50 nm); (3) porosity formation on all particles larger than 20 nm under constant potential conditions, but much less porosity under extended potential cycling; (4) no particle detachment from the support and carbon corrosion irrespective of the potential protocol. The high specific and mass activity of the catalyst decreases only slightly on performing an accelerated aging test, mainly owing to additional Cu leaching. Hence, the retained high activity originates, in addition to the particle size and the remaining strain and ligand effects, from the roughness owing to porosity formation (nanoconfinement effect). The faceting of particles during potential cycling, however, play only a limited role in improving the activity.
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