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Dealloyed PtCu catalyst as an efficient electrocatalyst in oxygen reduction reaction
Current Applied Physics 15 (2015) 993e999
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Dealloyed PtCu catalyst as an efficient electrocatalyst in oxygen reduction reaction Yeonsun Sohn a, Jin Hoo Park a, Pil Kim a, *, Ji Bong Joo b, ** a
School of Chemical Engineering, School of Semiconductor and Chemical Engineering, Nanomaterials Processing Research Center, Chonbuk National University, JeonJu, Jeonbuk, Republic of Korea b Low Carbon Process Lab, Korea Institute of Energy Research, Deajeon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 23 May 2015 Accepted 29 May 2015 Available online 30 May 2015
Pt-transition metal alloy catalysts with an active Pt surface have exceptional properties for use in oxygen electro-reduction reactions in fuel cells. Herein, we report the simple synthesis of dealloyed PtCu catalysts and their catalytic performance in oxygen reduction. The dealloyed PtCu catalysts consisted of a Ptenriched shell with a PteCu alloy core and were synthesized through a chemical co-reduction process followed by thermal annealing and chemical dealloying. During synthesis, thermal annealing leads to a high degree of formation of PtCu alloy particles (e.g., PtCu or PtCu3), and chemical dealloying causes selective dissolution of unstable Cu species from the surface layers of the PtCu alloy particles, resulting in a PtCu alloy@Pt-enriched surface coreeshell configuration. Our PtCu3/C catalyst exhibits a great improvement in the oxygen reduction reaction with a mass activity of 0.501 A/mgPt, which is 2.24 times greater than that of a commercial Pt catalyst. In this article, the synthesis details, characteristics and performance improvements in ORR of chemically dealloyed PtCu catalysts are systemically explained. © 2015 Published by Elsevier B.V.
Keywords: PtCu catalysts Chemical dealloying Coreeshell Oxygen reduction PEMFC
1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have attracted a tremendous amount of attention as potential sources of power for vehicles and for residential use due to their desirable characteristics, including a high energy conversion efficiency and zero carbon emissions [1e3]. Over the past decades, extensive efforts have been made to solve challenges not only from the perspective of the fundamental science, but also in terms of the economic issues relevant to commercialization [4e9]. One of major barriers to commercialization is the high cost of the Pt catalyst that is generally used for both the anode and cathode electrodes in fuel cells, in contrast to the relatively low activity that has been reported so far when employed in practical systems. Although significant progress has been made in research and development of the technology to decrease the Pt loading for both electrodes, recent economic calculations suggest that a further reduction by at least factor of 4 must be realized if fuel cells are to become viable for vehicles [10e12].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Kim), [email protected] (J.B. Joo). http://dx.doi.org/10.1016/j.cap.2015.05.013 1567-1739/© 2015 Published by Elsevier B.V.
An oxygen reduction reaction (ORR) that occurs on the cathode in fuel cells is exceptionally sluggish, and state-of-the-art Pt catalysts must be employed with large quantities on the cathode. Over the past decades, tremendous efforts have focused on lowering the cost and improving the ORR activity by alloying Pt with non-noble metals (i.e., Pt-M alloys, where M ¼ Fe, Co, Ni, etc.), and it was eventually found that these materials have 2e4 times higher ORR performance than standard Pt catalysts [13e15]. The improvement in ORR activity over the Pt-M alloy catalysts can be attributed to a lattice contract that can facilitate the adsorption of molecular oxygen, the formation of an active Pt-skin layer through the dissolution of the transition metal atom in an acidic solution during oxygen reduction, and d-band occupancy effects [16e18]. Recently, several research groups have reported on different PtM catalyst systems that have active coreeshell particles with active Pt-rich shells and Pt-M or M cores. Strasser and co-workers developed the concept through the selective dealloying of transition metals for pre-alloyed Pt-M catalysts and observed exceptional ORR performance when using dealloyed PteCu catalysts [19e22]. They first synthesized PteCu precursors by mixing a Cu precursor chemical with a commercial Pt catalyst, followed by annealing under a reducing environment. The PteCu precursors were applied as electrode catalysts either in a half cell or in single cells and were
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electrochemically dealloyed to provide a highly active Pt-rich shell with a PteCu alloy core [20,22]. Adzic and co-workers developed another strategy to synthesize coreeshell electrocatalysts through Cu underpotential deposition (UPD) on noble metal particle with a subsequent galvanic replacement of the Cu layers by Pt4þ ions [23e26]. This method leads to the formation of Pt monolayers on other noble metal surfaces, such as Au, Pd and Ru, resulting in an increase in ORR activity by multiple times, based on the per unit mass of Pt. Manthiram et al. also synthesized a series of carbon-supported PteCu coreeshell electrocatalysts with Cu or PteCu alloy core and Pt shell with a galvanic displacement of Cu by Pt4þ using prefabricated Cu/C and demonstrated a significant improvement in ORR activity relative to a commercial Pt catalyst [27]. Prior studies and our later discussions have indicated that the formation of an active Pt-rich shell on the surface of either Pt-M alloys or other metals is essential to fabricating highly-active ORR electrocatalysts. Although a variety of synthetic strategies were demonstrated to synthesize active coreeshell electrocatalysts over the last couple of years, it is still necessary to find an easy, simple, and facile methodology to do so. In this paper, we report a simple synthetic approach to prepare PteCu alloy electrocatalysts with Ptrich surfaces. We first synthesized the PteCu catalysts with a different composition between Pt and Cu through a chemical reduction and transformed active coreeshell type particle with a Pt-rich shell via simple annealing followed by chemical dealloying. The chemically dealloyed PteCu catalysts showed desirable characteristics such as the Pt-rich shell with a PtCu alloyed core resulting in a significant performance improvement in ORR. Our current work reports a simple process to synthesize Pt-metal alloy catalysts with a Pt-rich surface, which could provide one good examples to reduce the costs of fuel cells and to improve the ability for commercialization.
2. Experimental details 2.1. Catalyst preparation A bimetallic PtCu/C catalyst was prepared through a conventional chemical reduction using sodium borohydride as a reducing agent. Briefly, a precursor solution was first prepared by dissolving a specific amount of H2PtCl6 and CuCl2 in D.I. water (150 mL). Commercial carbon black (Vulcan XC-72R, Cabot) was then well dispersed into the above solution, and the mixture was sonicated to achieve a homogeneous state. The aqueous NaBH4 solution (0.211 M) was then added dropwise with vigorous stirring, and the mixture was further stirred for 1 h to ensure that all metal ions had been completely reduced. The resulting black precipitates were filtered, washed with copious amounts of D. I. water, and dried to obtain PtCux/C catalysts (x is the relative molar ratio of Cu to Pt precursors that was employed, x ¼ 1, 2, and 3). In order to increase the degree of alloying, we conducted heattreatment under a reducing atmosphere at 750 C. In order to make reducing conditions, a mixed stream of H2 (5 mL/min) and N2 (45 mL/min) was provided. The heat-treated PtCu catalysts are denoted as PtCux/C-HT (Heat treated PtCux/C catalysts). The heattreated samples were dealloyed with a chemical method using an acid chemical (H2SO4). The catalysts were then treated with 1.5 M H2SO4 for 3 h under vigorous stirring. The samples were then washed several times with copious amounts of de-ionized water, isolated via filtration, and dried for further characterization and electrochemical experiments. The chemically dealloyed samples are denoted as PtCux/C-AT (Acid treated PtCux/C catalysts).
2.2. Characterization and electrochemical measurements The crystalline properties of the samples were investigated via X-ray diffraction (XRD, Rigaku D/MAX 2500) using Cu Ka radiation (l ¼ 1.54056 Å) at 50 kV and 30 mA. The microscopic morphology of the samples was observed via transmission electron microscopy (TEM, JEOL 2010). A Pt gauze was used as a counter electrode, and Ag/AgCl (Cl saturated) was used as a reference electrode. Homogenous catalyst ink was prepared by mixing a specific amount of catalyst with deionized water, IPA, and Nafion solution with ultrasonication. A working electrode was prepared by coating the above catalyst ink on the surface of the rotating disk electrode (RDE). Cyclic voltamograms of the samples were then reordered in the potential range from 0.05 to 1.2 V (versus RHE) at a scan rate of 100 mV/s in N2 saturated 0.1 M HClO4 solution. The electrochemical active surface area (ECSA) was evaluated using hydrogen adsorption/desorption charge, assuming that the charge for the hydrogen monolayer desorption was of 0.21 mC/cm2. The oxygen reduction reaction (ORR) activity of the samples was evaluated by performing linear sweep voltammetry (LSV) in the potential range from 0.8 to 0.215 V (versus Ag/AgCl) at a scan rate of 10 mV/s in the O2 saturated 0.1 M HClO4 solution under a certain rotating speed. The kinetic current at 0.9 V was calculated by using the equation ik ¼ (iL i)/(iLei), where ik, iL and i represent the kinetic current, the limiting current and the measured current, respectively. 3. Results and discussion 3.1. Physical properties of PtCux/C-HT and PtCux/C-AT catalysts We used ICP-AES techniques to monitor the changes in the metal composition during thermal annealing followed by acid treatment. Table 1 compares the metal content and the atomic metal ratio of PtCux/C-HT and PtCux/C-AT catalysts prepared in this study. All of the PtCux/C-HT catalysts showed ca. 18e19 wt% total metal content with different atomic metal ratios between the Pt and Cu from 1:0.84 to 1:4.65. It should be noted that the atomic metal ratio that was measured matched well the theoretical values even though there was a small deviation, indicating that PtCux catalysts were well synthesized with the target compositions. After the acid treatment, the total metal content decreased and the metal ratio of Pt to Cu, relative to that of the PtCux/C-HT catalysts for each catalyst, changed dramatically for the PtCux/C-AT catalysts because the unstable Cu species were selectively dissolved out from the PtCu alloy particles. While the PtCu/C-AT catalyst lost 1 wt% of Cu content, PtCu3/C-AT and PtCu5/C-AT showed 6.91 and 10.05 wt% loss of Cu content respectively. In terms of the atomic metal ratio of Pt to Cu, the acid-treated PtCux/C-AT catalysts showed similar ratio values in the range of 1:0.53 to 1: 0.76 relative to those of PtCux/CHT (1:0.84 to 1:4.65). This indicates that a certain amount of stable Cu species can be left on the PtCu alloy particle after the acid treatment (Table 2). Fig. 1 compares the crystalline properties of PtCux/C catalysts that were prepared by co-reduction, followed by heat-treatment and sequential acid treatment. Fig. 1(a) showed that the asreduced PtCux/C catalysts had several broad peaks, indicating that small particles were formed on the carbon support due to NaBH4 reduction. The PtCu/C catalyst showed peaks of 2q ¼ 40.1, 47.8 and 68.5, which could be attributed to (006), (404), and (048) planes, respectively. In addition, small peaks related to the CuO phase can also be observed at 2q ¼ 35.7 and 38.7, which can be attributed to CuO(-111) and CuO(111) respectively. As the relative ratio of Cu to Pt increases, the peak intensity of CuO continuously increases,
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Table 1 ICP-AES analysis results indicating metal contents and total metal contents and atomic metal ratio between Pt and Cu on PtCux/C catalysts employed in this works. Sample
Metal contents (wt %)
PtCu/C-HT PtCu3/C-HT PtCu5/C-HT PtCu/C-AT PtCu3/C-AT PtCu5/C-AT
Pt
Cu
14.8 10.1 7.46 14.8 10.12 7.2
4.03 9.43 11.3 3.01 2.52 1.25
Table 2 Mass activity and specific activity values at 0.9 V (versus RHE) of the catalysts employed in this work. The values are obtained by using kinetic current equation from ORR polarization curves. Sample (@ 0.9 V)
Mass activity (A/mg)
Specific activity (uA/cm2)
Pt/C commercial PtCu/C-AT PtCu3/C-AT PtCu5/C-AT
0.223 0.200 0.501 0.407
387.7 879.5 1073.5 921.8
indicating that more CuO particles are generated during coreduction. As Fig. 1(b) shows, the PtCu/C-HT catalyst exhibited several strong diffraction peaks at 2q ¼ 20.5, 38.9, 40.9, 41.7, 47.9, 52.8 and 69.6 which can be attributed to PtCu(021), PtCu(205), PtCu(006), PtCu(111), PtCu(004), PtCu(241) and PtCu(048), respectively, indicating the formation of a PtCu alloy between Pt and Cu due to the heat treatment under a reducing environment. As the relative ratio of Cu to Pt increases, the resulting PtCu3/C-HT and PtCu5/C-HT shows differences in their XRD patterns, indicating that different crystalline structures were formed when compared to the PtCu/CHT catalyst. PtCu3/C-HT and PtCu5/C-HT have main peaks at 2q ¼ 24, 34.1, 42.3, 49.3 and 72.3 that are indexed as the PtCu3(100), PtCu3(110), PtCu3(111), PtCu3(200) and PtCu3(220) planes, respectively. These correspond to a face-centered cubic (fcc) PtCu3 alloy structure. As the amount of the Cu precursor increases above certain points, the final metal particles were observed to prefer forming PtCu3 rather than PtCu or any other compositions through the heat-treatment. Fig. 1(c) shows the XRD patterns of a commercial Pt catalyst and dealloyed PtCux/C catalysts that were acid-treated by using a concentrated sulfuric acid solution. As is well known, a Pt/Ccommercial catalyst showed typical characteristic peaks of fcc Pt for (111), (220) and (200) planes at 39.9, 46.7 and 68 , respectively. The PtCu/C-AT catalyst, however, showed several sharp peaks related to PtCu(021), PtCu(205), PtCu(006), PtCu(111), PtCu(004), PtCu(241) and PtCu(048) ca. at 2q ¼ 20.5, 38.9, 40.9, 41.7,47.9, 52.8 and 69.6 , respectively. The XRD pattern of the dealloyed PtCu/C-AT catalysts is similar to that of the heat-treated PtCu/C-HT catalyst, indicating that chemical dealloying is not sufficient under the acidic conditions employed in this study. The PtCu/C-AT catalyst showed an almost identical XRD pattern after the acid treatment, while the PtCu3/C-AT and PtCu5/C-AT catalysts have different patterns in comparison to the original ones. The PtCu3/C-AT and PtCu5/C-AT catalysts showed peaks at ca. 2q ¼ 41, 47.6 and 70 that correspond to the (111), (200) and (222) planes, indicating a face centered cubic phase that was identical to that of pure Pt catalyst, and the peaks become much broader than in the original sample, meaning that the average size of the metal particles decreased. In particular, the characteristic peaks of PtCu3/C-AT and PtCu5/C-AT catalysts for the (111) plane dramatically shift to higher angles. This shift in angles is consistent with the existence of a Pt-metal alloy [6,28]. The (220)
Total metal content on carbon (wt%)
Atomic metal ratio (Pt:Cu)
18.83 19.53 18.76 17.81 12.64 8.45
1:0.84 1:2.86 1:4.65 1:0.62 1:0.76 1:0.53
peak shift becomes more significant through heat treatment followed by acid treatment. The (220) lattice parameters for commercial Pt/C, PtCu3/C-AT and PtCu5/C-AT catalysts were calculated to be of about 3.904, 3.788 and 3.797, respectively, based on the XRD results, indicating that the Pt lattice was deformed due to the small size of the Cu atoms. Dealloying by using either chemical or electrochemical methods is well known to induce perturbations in the Pt surface with other non-precious metal-depleted interiors. Strasser and co-workers also reported that electrochemically dealloyed PtCu3 thin films have a Pt-enriched surface with perturbed thin layers that cover the Cu-depleted interior [21]. In our work, PtCu3/C-AT and PtCu5/C-AT showed the same fcc structure when compared with Pt catalyst while the Pt lattices were deformed in the XRD results, indicating the existence of a Pt-rich PtCu alloy structure. Although it is hard to determine form the XRD data whether our dealloyed PtCu metal particles had homogeneous or coreeshell structures, it should have a Pt-rich shell with a PteCu alloy core, as discussed in previous references and in the later parts of this paper [22,27,29]. We used TEM to investigate the morphological changes of the PtCux/C catalysts after heat treatment and acid treatment (Fig. 2). All of the catalysts showed that metal nanoparticles were dispersed on the carbon particles. As shown in Fig. 2(a), the PtCu/C-HT catalyst showed a broad range of metal particles on carbon supports when compared to the other two catalysts (PtCu3/C-HT and PtCu5/ C-HT). As the relative ratio of Cu to Pt increases during synthesis and heat treatment, it seems that the size of the PteCu nanoparticles becomes smaller [Fig. 2(a), (c) and (e)]. These results suggest that a greater amount of Cu contents might induce a greater formation of CuO and Cu(OH)2 on carbon supports during synthesis and heat treatment, inhibiting metal sintering and suppressing the formation of the big particles. After the dealloying process with the acid treatment, the morphologies of the PtCu/C-AT catalyst did not change, even though the overall diameter of the particle on the carbon support slightly decreased [Fig. 2(b)]. In particular, the PteCu particles that were supported still show a dense state, indicating that dealloying did not sufficiently occur, which is consistent with the results obtained from the ICP-AES and XRD experiments, respectively [Table 1 and Fig. 1(b)]. As shown in Fig. 2(c), the PtCu3/C-HT catalysts showed a relatively higher metal dispersion with a small size for the metal nanoparticles on the carbon supports as compared to PtCu/C-HT although a couple big metal particles were observed. The morphology of the supported metal nanoparticles of the PtCu3/CHT sample changed after the chemical dealloying process had been applied [Fig. 2(d)]. The most obvious change observed in the TEM images was the weakened contrast for some metal particles of the PtCu3/C-AT catalyst while original metal particles for the PtCu3/CHT samples seemed to be dense and dark. This indicates that some portion of the heat-treated PteCu metal particles had been dissolved and that the original dense particles became porous. As shown at a high magnification (Fig. 2(d) inset), the acid-treated metal particles can be easily seen to have a white pin-hole
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consists of Pt-rich shell on PtCu core after dealloying process (Fig. S1 electronic supplementary material, ESM). PtCu3/C-HT sample showed dense morphology (Fig. S1a inset) and strong Cu intensity in whole region of particle (Fig. S1a) indicating that exposed surface should be Cu-rich states. However, PtCu3/C-AT sample displays a lot of pores in the particle (Fig. S1b inset) indicating, that the unstable metal (Cu) should be dissolved out, resulting in a porous structure. As shown in line profiling results in Figure S1b, although Cu signal can be observed on the core position, signal intensity of Pt is much stronger than that of Cu in whole region indicating that exposed surface must be Pt-rich layer resulted from preferential Cu dissolution in shell region of the particle through dealloying. As shown in the above ICP-AES, XRD, TEM and line-scan profile results, our PtCux/C catalysts first have alloy-type PtCu and PtCu3 particles after heat treatment. Since the amount of Cu used in this work is excessive when compared to the amount of Pt used, the exposed active Pt site might be limited. Even though some portion of the exposed active site might have a higher ORR activity, it will be even enhanced by generating an active Pt-rich shell with a Pt-M alloy core. In our observations, the PtCu3/C-AT and PtCu5/C-AT catalysts still maintain a Pt-M alloy, even after acid treatment. During acid treatment, dissolvable Cu species on the surface layer of the PtCu3 alloy particle can be preferentially etched out, resulting in a porous morphology, and the core portion should consist of a stable alloy phase. We could therefore presume that our PtCu3/C-AT and PtCu5/C-AT should have coreeshell type particles with a Pt-rich shell and a PtCu core since many research groups have reported similar observations thus far with a high electrocatalytic activity that will be discussed in the next section. 3.2. Oxygen reduction reaction performance of PtCux/C-HT and PtCux/C-AT catalysts
Fig. 1. XRD patterns of (a) as-reduced PtCux/C samples, (b) heat-treated PtCux/C-HT samples prepared at 750 C under reduction atmosphere and (c) acid treated PtCux/CAT samples prepared with 1.5 M H2SO4 for 3 h under ambient conditions.
indicating, that the non-precious metal (Cu) should have dissolved out, resulting in a porous Pt-rich shell. PtCu5/C-HT catalysts showed a similar trend as that of the PtCu3/C-HT catalysts. The heat-treated PtCu5/C-HT catalysts show well-dispersed dense metal particles on carbon supports, and the dealloyed PtCu5/C-AT catalysts have partially etched particles with porous structures. We also investigated the position of each metal on supported metal particles by line-scan profiling to confirm that our catalyst
The characteristics of the metal surface of the PtCu electrocatalysts are investigated by conducting an electrochemical analysis. As shown in 1st cycle of cyclic voltammograms (CVs), while the PtCu/C-HT catalyst showed a typical CV of the Pt-based catalyst, the other two catalysts exhibit different CVs (Fig. 3(a)). The PtCu/CHT catalysts have typical Hþ ion electro-adsorption/desorption, a double-layer charging current, Pt oxide formation, and an oxygen reduction region. However, the PtCu3/C-HT and PtCu5/C-HT exhibit strong peaks at 0.32 V and 0.28 V in an anodic and cathodic scan, respectively, indicating that electrochemical Cu oxidation/reduction occurred [22]. As the Cu content in the PteCu alloy catalysts increases, the peak current related to Cu oxidation/reduction increases because a greater amount of unstable Cu species in the PtCu alloy particle can be dissolved out and are reducible. The dissolution of Cu is well known to occur continuously by repeating cyclic voltammetry experiment, in which it is usually referred as electrochemical dealloying [19,20,22]. In our system, we also observed similar phenomena where the dissolution of Cu species from PteCu catalysts became saturated after 20 cycles, showing stable Pt-like CV profiles (Fig. 3(b)). Fig. 3(c) shows the CVs of commercial Pt/C and chemically dealloyed PtCux/C-AT catalysts. Commercial Pt/C catalysts exhibit typical CV data of polycrystalline Pt. Even though PtCux/C-AT catalysts have a little high current value in the potential range of Pt oxide formation, all of the PtCux/C-AT catalysts also showed a similar shape of CV indicating that an active surface is most likely a Pt-rich layer [12,27]. While the PtCu/C-AT catalyst shows an identical CV shape to that of PtCu/C-HT, indicating little dealloying on the PtCu particle, the PtCu3/C-AT and PtCu5/C-AT clearly exhibit different shapes as those in PtCu3/C-HT and PtCu5/C-HT meaning that acid treatment leads to the complete dissolution of the Cu shell
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Fig. 2. TEM images of the PtCux/C catalysts by heat treatment at 750 C under reduction atmosphere followed by acid treatment with 1.5 M H2SO4 for 3 h: (a) PtCu/C-HT (b) PtCu/CAT (c) PtCu3/C-HT, (d) PtCu3/C-AT (e) PtCu5/C-HT and (f) PtCu5/C-AT respectively. All scale bars are 50 nm.
region, resulting in a Cu-free shell with a PteCu alloy core. As shown in Fig. 3(d), although there is a minor current decrease in the potential range of oxide formation as compared to 1st cycle (Fig. 3(c)), CV shape of PtCux/C-AT catalysts is well maintained even after 20 cycles indicating that electrochemically unstable Cu species is negligible and major portion of catalyst surface is Pt-rich surface. In order to calculate electrochemically active surface area by using 20 cycles CV results, the hydrogen desorption charge (Q) was used according to following formula: EAS (cm2/g) ¼ Q (mC)/ [210 (mC/cm2) x M (g)], where M is the catalyst loading on the electrode. The values calculated for the EAS of the Pt/C-commercial,
PtCu/C-AT, PtCu3/C-AT, and PtCu5/C-AT are 57.5, 22.8, 46.7 and 46.1 m2/g, respectively. In the ORR region, such as in 1.1e0.7 V cathodic sweeps, PtCu3/C-AT and PtCu5/C-AT catalysts exhibit a lower overpotential on the oxide reduction than commercial Pt/C catalysts [21,27]. It should be noted that the ORR kinetics of the PteCu alloy catalysts were dramatically improved through heattreatment followed by chemical dealloying. The catalytic activity of the prepared PtCu catalysts in terms of ORR was evaluated by conducting linear sweep voltammetry with the use of a rotating disk electrode (RDE) in an O2 saturated HClO4 environment. Fig. 4(a) shows the ORR polarization curve and the
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Fig. 3. Cyclic voltammograms of heat-treated PtCux/C-HT in (a) 1st cycle and (b) 20th cycles, and acid treated PtCux/C-AT samples in (c) 1st cycle and (d) 20th cycles, respectively. The CVs were recorded in N2 saturated 0.1 M HClO4 condition with scan rate of 100 mV/s and the loading amount of Pt on the glassy disk electrode was 15 mg Pt per cm2.
ORR onset potential of the catalysts that were employed in this work. The experiment features Tafel (ca. > 0.95 V), mixed kineticdiffusion (0.65e0.95 V) and diffusion-limited regions (ca. < 0.65 V), which are typically observed in ORR polarization curves over Pt-based electrodes. We compared the ORR onset potential of each catalyst to evaluate the catalytic activity, and the order is as follows: PtCu3/C-AT PtCu5/C-AT > PtCu/CAT commercial Pt/C. Among the different catalysts, PtCu3/C-AT exhibits the highest activity in terms of ORR onset potential. The ORR activity was also evaluated by calculating the kinetic current at 0.9 V, assuming a negligible resistance in the Nafion film. The mass-normalized (mass activity) and the EAS-normalized (specific activity) kinetic current were applied for a precise comparison of the ORR activity of each catalyst. As shown in Fig. 4(b), PtCu3/C-AT and PtCu5/C-AT have a higher activity than commercial Pt/C and PtCu/C-AT in terms of a mass activity at 0.9 V. The PtCu/CAT catalyst showed a similar mass activity as commercial Pt/C, although the latter showed a larger EAS value, indicating a high intrinsic activity of the PtCu alloy catalysts. By considering the EAS values, the PtCux/C-AT catalysts had a remarkably higher specific activity than commercial Pt/C catalyst. The kinetic currents based on the Pt mass and EAS of the PtCu3/C-AT, which is the most active catalyst, are of 0.501 A/mgPt and 1073 mA/cm2 at 0.9 V, respectively. These represent a 2.24- and 2.7-fold increase in activity relative to those of commercial Pt/C (0.223 A/mgPt and 387.7 mA/cm2, respectively). These large improvements in the dealloyed PtCu catalysts could be attributed to the formation of an active coreeshell structure. As the above results of the characterization confirm, the supported PtCu catalysts prepared by heat treatment followed by chemical dealloying consist of Pt-enriched surfaces and PtCu alloy cores, which are analogous to the Pt-Skin on a Pt-transition
metal alloy core structure. The Pt-enriched surface (e.g. Pt-skin) on the Pt-metal alloy core is well known to have a modified electronic structure and a shorter interatomic distance that can weaken the adsorption of oxygenated species on the surface of the Pt atom [12,22,27]. This makes the surface activity of the Pt-Metal alloy catalysts to be higher than that of pure Pt catalyst in ORR. Similarly, the Pt surface of our PtCu3/C-AT catalyst should have a favorable state for adsorption/desorption of oxygenated species, leading to an improvement in ORR activity due to the existence of the PtCu alloy core. 4. Conclusions We demonstrated that active PteCu alloy electrocatalysts with a Pt-rich shell and a PtCu alloy core could be prepared by using a simple synthetic approach. First, PteCu catalysts were synthesized through the chemical reduction of Pt and Cu precursors and were transformed into active PtCu alloy catalysts with a Pt-rich shell and a PteCu alloy core via thermal annealing followed by chemical dealloying. The dealloyed PtCu catalysts showed favorable PtCualloy@Pt-enriched surface coreeshell features that are well known as active Pt surface for ORR. In particular, the PtCu3/C-AT catalysts are the most active catalyst among the catalysts employed in this work and exhibited a great mass activity of 0.501 A/mgPt. This represents a 2.24-fold improvement in activity relative to that of commercial Pt catalysts and also exceeds the activity target of the fuel cell catalysts (0.44 A/mgPt). The dramatic improvement of our dealloyed PtCu catalysts should be mainly attributed to the formation of active Pt surfaces on the Pt-metal alloy, resulting in a modified electronic structure and a reduced interatomic distance that can be favorable to the adsorption/desorption of oxygenated
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Resources Development program (No. 20134030200330) of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.cap.2015.05.013. References
Fig. 4. (a) Polarization curve for the oxygen reduction reaction and (b) comparison of mass and specific activity at 0.9 V of the catalysts employed in this work. Volatmmograms were obtained in the O2 saturated 0.1 M HClO4 solution with scan rate of 10 mV/s by constant rotation of 1600 rpm. Loading amount of Pt on the disk electrode was 15 mg Pt per cm2.
species on the surface of Pt atoms leading to an improvement in ORR activity. Our simple process to synthesize Pt-metal alloy catalysts should provide a good opportunity to reduce the amount of Pt in the fuel cells and should be helpful for commercialization. Acknowledgements This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20133030011320) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20133030011320) and by the Human
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Dealloyed PtCu catalyst as an efficient electrocatalyst in oxygen reduction reaction Yeonsun Sohn a, Jin Hoo Park a, Pil Kim a, *, Ji Bong Joo b, ** a
School of Chemical Engineering, School of Semiconductor and Chemical Engineering, Nanomaterials Processing Research Center, Chonbuk National University, JeonJu, Jeonbuk, Republic of Korea b Low Carbon Process Lab, Korea Institute of Energy Research, Deajeon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 23 May 2015 Accepted 29 May 2015 Available online 30 May 2015
Pt-transition metal alloy catalysts with an active Pt surface have exceptional properties for use in oxygen electro-reduction reactions in fuel cells. Herein, we report the simple synthesis of dealloyed PtCu catalysts and their catalytic performance in oxygen reduction. The dealloyed PtCu catalysts consisted of a Ptenriched shell with a PteCu alloy core and were synthesized through a chemical co-reduction process followed by thermal annealing and chemical dealloying. During synthesis, thermal annealing leads to a high degree of formation of PtCu alloy particles (e.g., PtCu or PtCu3), and chemical dealloying causes selective dissolution of unstable Cu species from the surface layers of the PtCu alloy particles, resulting in a PtCu alloy@Pt-enriched surface coreeshell configuration. Our PtCu3/C catalyst exhibits a great improvement in the oxygen reduction reaction with a mass activity of 0.501 A/mgPt, which is 2.24 times greater than that of a commercial Pt catalyst. In this article, the synthesis details, characteristics and performance improvements in ORR of chemically dealloyed PtCu catalysts are systemically explained. © 2015 Published by Elsevier B.V.
Keywords: PtCu catalysts Chemical dealloying Coreeshell Oxygen reduction PEMFC
1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have attracted a tremendous amount of attention as potential sources of power for vehicles and for residential use due to their desirable characteristics, including a high energy conversion efficiency and zero carbon emissions [1e3]. Over the past decades, extensive efforts have been made to solve challenges not only from the perspective of the fundamental science, but also in terms of the economic issues relevant to commercialization [4e9]. One of major barriers to commercialization is the high cost of the Pt catalyst that is generally used for both the anode and cathode electrodes in fuel cells, in contrast to the relatively low activity that has been reported so far when employed in practical systems. Although significant progress has been made in research and development of the technology to decrease the Pt loading for both electrodes, recent economic calculations suggest that a further reduction by at least factor of 4 must be realized if fuel cells are to become viable for vehicles [10e12].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Kim), [email protected] (J.B. Joo). http://dx.doi.org/10.1016/j.cap.2015.05.013 1567-1739/© 2015 Published by Elsevier B.V.
An oxygen reduction reaction (ORR) that occurs on the cathode in fuel cells is exceptionally sluggish, and state-of-the-art Pt catalysts must be employed with large quantities on the cathode. Over the past decades, tremendous efforts have focused on lowering the cost and improving the ORR activity by alloying Pt with non-noble metals (i.e., Pt-M alloys, where M ¼ Fe, Co, Ni, etc.), and it was eventually found that these materials have 2e4 times higher ORR performance than standard Pt catalysts [13e15]. The improvement in ORR activity over the Pt-M alloy catalysts can be attributed to a lattice contract that can facilitate the adsorption of molecular oxygen, the formation of an active Pt-skin layer through the dissolution of the transition metal atom in an acidic solution during oxygen reduction, and d-band occupancy effects [16e18]. Recently, several research groups have reported on different PtM catalyst systems that have active coreeshell particles with active Pt-rich shells and Pt-M or M cores. Strasser and co-workers developed the concept through the selective dealloying of transition metals for pre-alloyed Pt-M catalysts and observed exceptional ORR performance when using dealloyed PteCu catalysts [19e22]. They first synthesized PteCu precursors by mixing a Cu precursor chemical with a commercial Pt catalyst, followed by annealing under a reducing environment. The PteCu precursors were applied as electrode catalysts either in a half cell or in single cells and were
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electrochemically dealloyed to provide a highly active Pt-rich shell with a PteCu alloy core [20,22]. Adzic and co-workers developed another strategy to synthesize coreeshell electrocatalysts through Cu underpotential deposition (UPD) on noble metal particle with a subsequent galvanic replacement of the Cu layers by Pt4þ ions [23e26]. This method leads to the formation of Pt monolayers on other noble metal surfaces, such as Au, Pd and Ru, resulting in an increase in ORR activity by multiple times, based on the per unit mass of Pt. Manthiram et al. also synthesized a series of carbon-supported PteCu coreeshell electrocatalysts with Cu or PteCu alloy core and Pt shell with a galvanic displacement of Cu by Pt4þ using prefabricated Cu/C and demonstrated a significant improvement in ORR activity relative to a commercial Pt catalyst [27]. Prior studies and our later discussions have indicated that the formation of an active Pt-rich shell on the surface of either Pt-M alloys or other metals is essential to fabricating highly-active ORR electrocatalysts. Although a variety of synthetic strategies were demonstrated to synthesize active coreeshell electrocatalysts over the last couple of years, it is still necessary to find an easy, simple, and facile methodology to do so. In this paper, we report a simple synthetic approach to prepare PteCu alloy electrocatalysts with Ptrich surfaces. We first synthesized the PteCu catalysts with a different composition between Pt and Cu through a chemical reduction and transformed active coreeshell type particle with a Pt-rich shell via simple annealing followed by chemical dealloying. The chemically dealloyed PteCu catalysts showed desirable characteristics such as the Pt-rich shell with a PtCu alloyed core resulting in a significant performance improvement in ORR. Our current work reports a simple process to synthesize Pt-metal alloy catalysts with a Pt-rich surface, which could provide one good examples to reduce the costs of fuel cells and to improve the ability for commercialization.
2. Experimental details 2.1. Catalyst preparation A bimetallic PtCu/C catalyst was prepared through a conventional chemical reduction using sodium borohydride as a reducing agent. Briefly, a precursor solution was first prepared by dissolving a specific amount of H2PtCl6 and CuCl2 in D.I. water (150 mL). Commercial carbon black (Vulcan XC-72R, Cabot) was then well dispersed into the above solution, and the mixture was sonicated to achieve a homogeneous state. The aqueous NaBH4 solution (0.211 M) was then added dropwise with vigorous stirring, and the mixture was further stirred for 1 h to ensure that all metal ions had been completely reduced. The resulting black precipitates were filtered, washed with copious amounts of D. I. water, and dried to obtain PtCux/C catalysts (x is the relative molar ratio of Cu to Pt precursors that was employed, x ¼ 1, 2, and 3). In order to increase the degree of alloying, we conducted heattreatment under a reducing atmosphere at 750 C. In order to make reducing conditions, a mixed stream of H2 (5 mL/min) and N2 (45 mL/min) was provided. The heat-treated PtCu catalysts are denoted as PtCux/C-HT (Heat treated PtCux/C catalysts). The heattreated samples were dealloyed with a chemical method using an acid chemical (H2SO4). The catalysts were then treated with 1.5 M H2SO4 for 3 h under vigorous stirring. The samples were then washed several times with copious amounts of de-ionized water, isolated via filtration, and dried for further characterization and electrochemical experiments. The chemically dealloyed samples are denoted as PtCux/C-AT (Acid treated PtCux/C catalysts).
2.2. Characterization and electrochemical measurements The crystalline properties of the samples were investigated via X-ray diffraction (XRD, Rigaku D/MAX 2500) using Cu Ka radiation (l ¼ 1.54056 Å) at 50 kV and 30 mA. The microscopic morphology of the samples was observed via transmission electron microscopy (TEM, JEOL 2010). A Pt gauze was used as a counter electrode, and Ag/AgCl (Cl saturated) was used as a reference electrode. Homogenous catalyst ink was prepared by mixing a specific amount of catalyst with deionized water, IPA, and Nafion solution with ultrasonication. A working electrode was prepared by coating the above catalyst ink on the surface of the rotating disk electrode (RDE). Cyclic voltamograms of the samples were then reordered in the potential range from 0.05 to 1.2 V (versus RHE) at a scan rate of 100 mV/s in N2 saturated 0.1 M HClO4 solution. The electrochemical active surface area (ECSA) was evaluated using hydrogen adsorption/desorption charge, assuming that the charge for the hydrogen monolayer desorption was of 0.21 mC/cm2. The oxygen reduction reaction (ORR) activity of the samples was evaluated by performing linear sweep voltammetry (LSV) in the potential range from 0.8 to 0.215 V (versus Ag/AgCl) at a scan rate of 10 mV/s in the O2 saturated 0.1 M HClO4 solution under a certain rotating speed. The kinetic current at 0.9 V was calculated by using the equation ik ¼ (iL i)/(iLei), where ik, iL and i represent the kinetic current, the limiting current and the measured current, respectively. 3. Results and discussion 3.1. Physical properties of PtCux/C-HT and PtCux/C-AT catalysts We used ICP-AES techniques to monitor the changes in the metal composition during thermal annealing followed by acid treatment. Table 1 compares the metal content and the atomic metal ratio of PtCux/C-HT and PtCux/C-AT catalysts prepared in this study. All of the PtCux/C-HT catalysts showed ca. 18e19 wt% total metal content with different atomic metal ratios between the Pt and Cu from 1:0.84 to 1:4.65. It should be noted that the atomic metal ratio that was measured matched well the theoretical values even though there was a small deviation, indicating that PtCux catalysts were well synthesized with the target compositions. After the acid treatment, the total metal content decreased and the metal ratio of Pt to Cu, relative to that of the PtCux/C-HT catalysts for each catalyst, changed dramatically for the PtCux/C-AT catalysts because the unstable Cu species were selectively dissolved out from the PtCu alloy particles. While the PtCu/C-AT catalyst lost 1 wt% of Cu content, PtCu3/C-AT and PtCu5/C-AT showed 6.91 and 10.05 wt% loss of Cu content respectively. In terms of the atomic metal ratio of Pt to Cu, the acid-treated PtCux/C-AT catalysts showed similar ratio values in the range of 1:0.53 to 1: 0.76 relative to those of PtCux/CHT (1:0.84 to 1:4.65). This indicates that a certain amount of stable Cu species can be left on the PtCu alloy particle after the acid treatment (Table 2). Fig. 1 compares the crystalline properties of PtCux/C catalysts that were prepared by co-reduction, followed by heat-treatment and sequential acid treatment. Fig. 1(a) showed that the asreduced PtCux/C catalysts had several broad peaks, indicating that small particles were formed on the carbon support due to NaBH4 reduction. The PtCu/C catalyst showed peaks of 2q ¼ 40.1, 47.8 and 68.5, which could be attributed to (006), (404), and (048) planes, respectively. In addition, small peaks related to the CuO phase can also be observed at 2q ¼ 35.7 and 38.7, which can be attributed to CuO(-111) and CuO(111) respectively. As the relative ratio of Cu to Pt increases, the peak intensity of CuO continuously increases,
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Table 1 ICP-AES analysis results indicating metal contents and total metal contents and atomic metal ratio between Pt and Cu on PtCux/C catalysts employed in this works. Sample
Metal contents (wt %)
PtCu/C-HT PtCu3/C-HT PtCu5/C-HT PtCu/C-AT PtCu3/C-AT PtCu5/C-AT
Pt
Cu
14.8 10.1 7.46 14.8 10.12 7.2
4.03 9.43 11.3 3.01 2.52 1.25
Table 2 Mass activity and specific activity values at 0.9 V (versus RHE) of the catalysts employed in this work. The values are obtained by using kinetic current equation from ORR polarization curves. Sample (@ 0.9 V)
Mass activity (A/mg)
Specific activity (uA/cm2)
Pt/C commercial PtCu/C-AT PtCu3/C-AT PtCu5/C-AT
0.223 0.200 0.501 0.407
387.7 879.5 1073.5 921.8
indicating that more CuO particles are generated during coreduction. As Fig. 1(b) shows, the PtCu/C-HT catalyst exhibited several strong diffraction peaks at 2q ¼ 20.5, 38.9, 40.9, 41.7, 47.9, 52.8 and 69.6 which can be attributed to PtCu(021), PtCu(205), PtCu(006), PtCu(111), PtCu(004), PtCu(241) and PtCu(048), respectively, indicating the formation of a PtCu alloy between Pt and Cu due to the heat treatment under a reducing environment. As the relative ratio of Cu to Pt increases, the resulting PtCu3/C-HT and PtCu5/C-HT shows differences in their XRD patterns, indicating that different crystalline structures were formed when compared to the PtCu/CHT catalyst. PtCu3/C-HT and PtCu5/C-HT have main peaks at 2q ¼ 24, 34.1, 42.3, 49.3 and 72.3 that are indexed as the PtCu3(100), PtCu3(110), PtCu3(111), PtCu3(200) and PtCu3(220) planes, respectively. These correspond to a face-centered cubic (fcc) PtCu3 alloy structure. As the amount of the Cu precursor increases above certain points, the final metal particles were observed to prefer forming PtCu3 rather than PtCu or any other compositions through the heat-treatment. Fig. 1(c) shows the XRD patterns of a commercial Pt catalyst and dealloyed PtCux/C catalysts that were acid-treated by using a concentrated sulfuric acid solution. As is well known, a Pt/Ccommercial catalyst showed typical characteristic peaks of fcc Pt for (111), (220) and (200) planes at 39.9, 46.7 and 68 , respectively. The PtCu/C-AT catalyst, however, showed several sharp peaks related to PtCu(021), PtCu(205), PtCu(006), PtCu(111), PtCu(004), PtCu(241) and PtCu(048) ca. at 2q ¼ 20.5, 38.9, 40.9, 41.7,47.9, 52.8 and 69.6 , respectively. The XRD pattern of the dealloyed PtCu/C-AT catalysts is similar to that of the heat-treated PtCu/C-HT catalyst, indicating that chemical dealloying is not sufficient under the acidic conditions employed in this study. The PtCu/C-AT catalyst showed an almost identical XRD pattern after the acid treatment, while the PtCu3/C-AT and PtCu5/C-AT catalysts have different patterns in comparison to the original ones. The PtCu3/C-AT and PtCu5/C-AT catalysts showed peaks at ca. 2q ¼ 41, 47.6 and 70 that correspond to the (111), (200) and (222) planes, indicating a face centered cubic phase that was identical to that of pure Pt catalyst, and the peaks become much broader than in the original sample, meaning that the average size of the metal particles decreased. In particular, the characteristic peaks of PtCu3/C-AT and PtCu5/C-AT catalysts for the (111) plane dramatically shift to higher angles. This shift in angles is consistent with the existence of a Pt-metal alloy [6,28]. The (220)
Total metal content on carbon (wt%)
Atomic metal ratio (Pt:Cu)
18.83 19.53 18.76 17.81 12.64 8.45
1:0.84 1:2.86 1:4.65 1:0.62 1:0.76 1:0.53
peak shift becomes more significant through heat treatment followed by acid treatment. The (220) lattice parameters for commercial Pt/C, PtCu3/C-AT and PtCu5/C-AT catalysts were calculated to be of about 3.904, 3.788 and 3.797, respectively, based on the XRD results, indicating that the Pt lattice was deformed due to the small size of the Cu atoms. Dealloying by using either chemical or electrochemical methods is well known to induce perturbations in the Pt surface with other non-precious metal-depleted interiors. Strasser and co-workers also reported that electrochemically dealloyed PtCu3 thin films have a Pt-enriched surface with perturbed thin layers that cover the Cu-depleted interior [21]. In our work, PtCu3/C-AT and PtCu5/C-AT showed the same fcc structure when compared with Pt catalyst while the Pt lattices were deformed in the XRD results, indicating the existence of a Pt-rich PtCu alloy structure. Although it is hard to determine form the XRD data whether our dealloyed PtCu metal particles had homogeneous or coreeshell structures, it should have a Pt-rich shell with a PteCu alloy core, as discussed in previous references and in the later parts of this paper [22,27,29]. We used TEM to investigate the morphological changes of the PtCux/C catalysts after heat treatment and acid treatment (Fig. 2). All of the catalysts showed that metal nanoparticles were dispersed on the carbon particles. As shown in Fig. 2(a), the PtCu/C-HT catalyst showed a broad range of metal particles on carbon supports when compared to the other two catalysts (PtCu3/C-HT and PtCu5/ C-HT). As the relative ratio of Cu to Pt increases during synthesis and heat treatment, it seems that the size of the PteCu nanoparticles becomes smaller [Fig. 2(a), (c) and (e)]. These results suggest that a greater amount of Cu contents might induce a greater formation of CuO and Cu(OH)2 on carbon supports during synthesis and heat treatment, inhibiting metal sintering and suppressing the formation of the big particles. After the dealloying process with the acid treatment, the morphologies of the PtCu/C-AT catalyst did not change, even though the overall diameter of the particle on the carbon support slightly decreased [Fig. 2(b)]. In particular, the PteCu particles that were supported still show a dense state, indicating that dealloying did not sufficiently occur, which is consistent with the results obtained from the ICP-AES and XRD experiments, respectively [Table 1 and Fig. 1(b)]. As shown in Fig. 2(c), the PtCu3/C-HT catalysts showed a relatively higher metal dispersion with a small size for the metal nanoparticles on the carbon supports as compared to PtCu/C-HT although a couple big metal particles were observed. The morphology of the supported metal nanoparticles of the PtCu3/CHT sample changed after the chemical dealloying process had been applied [Fig. 2(d)]. The most obvious change observed in the TEM images was the weakened contrast for some metal particles of the PtCu3/C-AT catalyst while original metal particles for the PtCu3/CHT samples seemed to be dense and dark. This indicates that some portion of the heat-treated PteCu metal particles had been dissolved and that the original dense particles became porous. As shown at a high magnification (Fig. 2(d) inset), the acid-treated metal particles can be easily seen to have a white pin-hole
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consists of Pt-rich shell on PtCu core after dealloying process (Fig. S1 electronic supplementary material, ESM). PtCu3/C-HT sample showed dense morphology (Fig. S1a inset) and strong Cu intensity in whole region of particle (Fig. S1a) indicating that exposed surface should be Cu-rich states. However, PtCu3/C-AT sample displays a lot of pores in the particle (Fig. S1b inset) indicating, that the unstable metal (Cu) should be dissolved out, resulting in a porous structure. As shown in line profiling results in Figure S1b, although Cu signal can be observed on the core position, signal intensity of Pt is much stronger than that of Cu in whole region indicating that exposed surface must be Pt-rich layer resulted from preferential Cu dissolution in shell region of the particle through dealloying. As shown in the above ICP-AES, XRD, TEM and line-scan profile results, our PtCux/C catalysts first have alloy-type PtCu and PtCu3 particles after heat treatment. Since the amount of Cu used in this work is excessive when compared to the amount of Pt used, the exposed active Pt site might be limited. Even though some portion of the exposed active site might have a higher ORR activity, it will be even enhanced by generating an active Pt-rich shell with a Pt-M alloy core. In our observations, the PtCu3/C-AT and PtCu5/C-AT catalysts still maintain a Pt-M alloy, even after acid treatment. During acid treatment, dissolvable Cu species on the surface layer of the PtCu3 alloy particle can be preferentially etched out, resulting in a porous morphology, and the core portion should consist of a stable alloy phase. We could therefore presume that our PtCu3/C-AT and PtCu5/C-AT should have coreeshell type particles with a Pt-rich shell and a PtCu core since many research groups have reported similar observations thus far with a high electrocatalytic activity that will be discussed in the next section. 3.2. Oxygen reduction reaction performance of PtCux/C-HT and PtCux/C-AT catalysts
Fig. 1. XRD patterns of (a) as-reduced PtCux/C samples, (b) heat-treated PtCux/C-HT samples prepared at 750 C under reduction atmosphere and (c) acid treated PtCux/CAT samples prepared with 1.5 M H2SO4 for 3 h under ambient conditions.
indicating, that the non-precious metal (Cu) should have dissolved out, resulting in a porous Pt-rich shell. PtCu5/C-HT catalysts showed a similar trend as that of the PtCu3/C-HT catalysts. The heat-treated PtCu5/C-HT catalysts show well-dispersed dense metal particles on carbon supports, and the dealloyed PtCu5/C-AT catalysts have partially etched particles with porous structures. We also investigated the position of each metal on supported metal particles by line-scan profiling to confirm that our catalyst
The characteristics of the metal surface of the PtCu electrocatalysts are investigated by conducting an electrochemical analysis. As shown in 1st cycle of cyclic voltammograms (CVs), while the PtCu/C-HT catalyst showed a typical CV of the Pt-based catalyst, the other two catalysts exhibit different CVs (Fig. 3(a)). The PtCu/CHT catalysts have typical Hþ ion electro-adsorption/desorption, a double-layer charging current, Pt oxide formation, and an oxygen reduction region. However, the PtCu3/C-HT and PtCu5/C-HT exhibit strong peaks at 0.32 V and 0.28 V in an anodic and cathodic scan, respectively, indicating that electrochemical Cu oxidation/reduction occurred [22]. As the Cu content in the PteCu alloy catalysts increases, the peak current related to Cu oxidation/reduction increases because a greater amount of unstable Cu species in the PtCu alloy particle can be dissolved out and are reducible. The dissolution of Cu is well known to occur continuously by repeating cyclic voltammetry experiment, in which it is usually referred as electrochemical dealloying [19,20,22]. In our system, we also observed similar phenomena where the dissolution of Cu species from PteCu catalysts became saturated after 20 cycles, showing stable Pt-like CV profiles (Fig. 3(b)). Fig. 3(c) shows the CVs of commercial Pt/C and chemically dealloyed PtCux/C-AT catalysts. Commercial Pt/C catalysts exhibit typical CV data of polycrystalline Pt. Even though PtCux/C-AT catalysts have a little high current value in the potential range of Pt oxide formation, all of the PtCux/C-AT catalysts also showed a similar shape of CV indicating that an active surface is most likely a Pt-rich layer [12,27]. While the PtCu/C-AT catalyst shows an identical CV shape to that of PtCu/C-HT, indicating little dealloying on the PtCu particle, the PtCu3/C-AT and PtCu5/C-AT clearly exhibit different shapes as those in PtCu3/C-HT and PtCu5/C-HT meaning that acid treatment leads to the complete dissolution of the Cu shell
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Fig. 2. TEM images of the PtCux/C catalysts by heat treatment at 750 C under reduction atmosphere followed by acid treatment with 1.5 M H2SO4 for 3 h: (a) PtCu/C-HT (b) PtCu/CAT (c) PtCu3/C-HT, (d) PtCu3/C-AT (e) PtCu5/C-HT and (f) PtCu5/C-AT respectively. All scale bars are 50 nm.
region, resulting in a Cu-free shell with a PteCu alloy core. As shown in Fig. 3(d), although there is a minor current decrease in the potential range of oxide formation as compared to 1st cycle (Fig. 3(c)), CV shape of PtCux/C-AT catalysts is well maintained even after 20 cycles indicating that electrochemically unstable Cu species is negligible and major portion of catalyst surface is Pt-rich surface. In order to calculate electrochemically active surface area by using 20 cycles CV results, the hydrogen desorption charge (Q) was used according to following formula: EAS (cm2/g) ¼ Q (mC)/ [210 (mC/cm2) x M (g)], where M is the catalyst loading on the electrode. The values calculated for the EAS of the Pt/C-commercial,
PtCu/C-AT, PtCu3/C-AT, and PtCu5/C-AT are 57.5, 22.8, 46.7 and 46.1 m2/g, respectively. In the ORR region, such as in 1.1e0.7 V cathodic sweeps, PtCu3/C-AT and PtCu5/C-AT catalysts exhibit a lower overpotential on the oxide reduction than commercial Pt/C catalysts [21,27]. It should be noted that the ORR kinetics of the PteCu alloy catalysts were dramatically improved through heattreatment followed by chemical dealloying. The catalytic activity of the prepared PtCu catalysts in terms of ORR was evaluated by conducting linear sweep voltammetry with the use of a rotating disk electrode (RDE) in an O2 saturated HClO4 environment. Fig. 4(a) shows the ORR polarization curve and the
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Fig. 3. Cyclic voltammograms of heat-treated PtCux/C-HT in (a) 1st cycle and (b) 20th cycles, and acid treated PtCux/C-AT samples in (c) 1st cycle and (d) 20th cycles, respectively. The CVs were recorded in N2 saturated 0.1 M HClO4 condition with scan rate of 100 mV/s and the loading amount of Pt on the glassy disk electrode was 15 mg Pt per cm2.
ORR onset potential of the catalysts that were employed in this work. The experiment features Tafel (ca. > 0.95 V), mixed kineticdiffusion (0.65e0.95 V) and diffusion-limited regions (ca. < 0.65 V), which are typically observed in ORR polarization curves over Pt-based electrodes. We compared the ORR onset potential of each catalyst to evaluate the catalytic activity, and the order is as follows: PtCu3/C-AT PtCu5/C-AT > PtCu/CAT commercial Pt/C. Among the different catalysts, PtCu3/C-AT exhibits the highest activity in terms of ORR onset potential. The ORR activity was also evaluated by calculating the kinetic current at 0.9 V, assuming a negligible resistance in the Nafion film. The mass-normalized (mass activity) and the EAS-normalized (specific activity) kinetic current were applied for a precise comparison of the ORR activity of each catalyst. As shown in Fig. 4(b), PtCu3/C-AT and PtCu5/C-AT have a higher activity than commercial Pt/C and PtCu/C-AT in terms of a mass activity at 0.9 V. The PtCu/CAT catalyst showed a similar mass activity as commercial Pt/C, although the latter showed a larger EAS value, indicating a high intrinsic activity of the PtCu alloy catalysts. By considering the EAS values, the PtCux/C-AT catalysts had a remarkably higher specific activity than commercial Pt/C catalyst. The kinetic currents based on the Pt mass and EAS of the PtCu3/C-AT, which is the most active catalyst, are of 0.501 A/mgPt and 1073 mA/cm2 at 0.9 V, respectively. These represent a 2.24- and 2.7-fold increase in activity relative to those of commercial Pt/C (0.223 A/mgPt and 387.7 mA/cm2, respectively). These large improvements in the dealloyed PtCu catalysts could be attributed to the formation of an active coreeshell structure. As the above results of the characterization confirm, the supported PtCu catalysts prepared by heat treatment followed by chemical dealloying consist of Pt-enriched surfaces and PtCu alloy cores, which are analogous to the Pt-Skin on a Pt-transition
metal alloy core structure. The Pt-enriched surface (e.g. Pt-skin) on the Pt-metal alloy core is well known to have a modified electronic structure and a shorter interatomic distance that can weaken the adsorption of oxygenated species on the surface of the Pt atom [12,22,27]. This makes the surface activity of the Pt-Metal alloy catalysts to be higher than that of pure Pt catalyst in ORR. Similarly, the Pt surface of our PtCu3/C-AT catalyst should have a favorable state for adsorption/desorption of oxygenated species, leading to an improvement in ORR activity due to the existence of the PtCu alloy core. 4. Conclusions We demonstrated that active PteCu alloy electrocatalysts with a Pt-rich shell and a PtCu alloy core could be prepared by using a simple synthetic approach. First, PteCu catalysts were synthesized through the chemical reduction of Pt and Cu precursors and were transformed into active PtCu alloy catalysts with a Pt-rich shell and a PteCu alloy core via thermal annealing followed by chemical dealloying. The dealloyed PtCu catalysts showed favorable PtCualloy@Pt-enriched surface coreeshell features that are well known as active Pt surface for ORR. In particular, the PtCu3/C-AT catalysts are the most active catalyst among the catalysts employed in this work and exhibited a great mass activity of 0.501 A/mgPt. This represents a 2.24-fold improvement in activity relative to that of commercial Pt catalysts and also exceeds the activity target of the fuel cell catalysts (0.44 A/mgPt). The dramatic improvement of our dealloyed PtCu catalysts should be mainly attributed to the formation of active Pt surfaces on the Pt-metal alloy, resulting in a modified electronic structure and a reduced interatomic distance that can be favorable to the adsorption/desorption of oxygenated
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Resources Development program (No. 20134030200330) of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.cap.2015.05.013. References
Fig. 4. (a) Polarization curve for the oxygen reduction reaction and (b) comparison of mass and specific activity at 0.9 V of the catalysts employed in this work. Volatmmograms were obtained in the O2 saturated 0.1 M HClO4 solution with scan rate of 10 mV/s by constant rotation of 1600 rpm. Loading amount of Pt on the disk electrode was 15 mg Pt per cm2.
species on the surface of Pt atoms leading to an improvement in ORR activity. Our simple process to synthesize Pt-metal alloy catalysts should provide a good opportunity to reduce the amount of Pt in the fuel cells and should be helpful for commercialization. Acknowledgements This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20133030011320) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20133030011320) and by the Human
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