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Article Cite This: ACS Appl. Energy Mater. 2019, 2, 2769−2778
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PtNi Nanoparticles Encapsulated in Few Carbon Layers as HighPerformance Catalysts for Oxygen Reduction Reaction Wenyue Li and Shouzhong Zou* Department of Chemistry, American University, 4400 Massachusetts Avenue NW, Washington D.C. 20016, United States
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ABSTRACT: PtNi alloys have been demonstrated to be one of the most promising catalysts for oxygen reduction reaction (ORR) in fuel cell applications due to their high catalytic activity and efficient utilization of Pt. However, improving the durability of such catalysts remains a significant challenge. Herein, we report on the formation of PtNi nanoparticles with an average diameter of around 10 nm embedded in few carbon layers (PtNi@C) and their ORR catalytic performance. The synthesis procedure entails the use of a solvothermal method to form 2-methylimidazole−Pt-Ni composites with a MOF-like structure (designated as MPN) followed by a thermal annealing treatment. The presence of Pt during the solvothermal process catalyzes the deposition of Ni, and the Ni in the MPN catalyzes the formation of well-graphitized carbon at a temperature as low as 400 °C. The PtNi@C catalyst exhibits an ORR mass activity (MA) of 0.84 A mgpt−1 and a specific activity (SA) of 1.54 mA cm−2 at 0.9 V (vs RHE), representing 6.5-fold and 8.4fold improvement over a commercial Pt/C catalyst. More significantly, the electrochemically active surface area of the PtNi@C catalyst shows little change after 5000 cycles of potential scans between 0.6 and 1.1 V in O2-saturated 0.1 M HClO4. This study demonstrates the feasibility of stabilizing Ptbased nanocatalysts through graphitic carbon encapsulation. KEYWORDS: oxygen reduction reaction, PtNi alloys, thin carbon layer encapsulation, metal organic frameworks, stability
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electrochemically active surface area (ECSA).3,4 To improve the durability, Huang’s group obtained a series of Pt3Ni octahedra surface-doped with other transition metals.17 Among these catalysts, the Mo-doped Pt3Ni exhibited the best ORR activity and durability, which were attributed to the formation of relatively strong Mo−Pt and Mo−Ni bonds that reduces the dissolution of Pt and Ni. Besides the structural engineering discussed above, stabilizing PtNi alloys with conductive and stable carbon films can also be a promising strategy to alleviate their catalytic activity degradation.18−20 The carbon layers can be a physical barrier for diffusion of the dissolved Pt or Ni away from the PtNi nanoparticles and the coalescence of the PtNi nanoparticles. They may also modify the electronic properties of the PtNi nanoparticles through PtNi−carbon interactions. It is also envisioned that decorating the carbon layer with selected functional groups may promote a desired reaction pathway, as has been recently demonstrated on permeable silicon oxide nanomembrane-encapsulated Pt thin films for carbon monoxide and methanol oxidation.21 However, the PtNi core-carbon shell hierarchical structure cannot be easily obtained, and as far as we know, only a few Ptbased alloys encapsulated by carbon layers have been reported so far.22−27 Therefore, developing a facile method to form this structure remains a challenge.
INTRODUCTION Pt and Pt-based nanoparticles supported on carbon black have been considered as the most effective catalysts for the kinetically sluggish oxygen reduction reaction (ORR), which is the cathode reaction for fuel cells and metal air batteries.1−4 Extensive efforts have been devoted to combining Pt with 3d transition metals such as Ni, Co, Fe, Cu, Ti, and Zn to form bior multimetallic alloys, which has been shown to be an effective way to decrease the Pt usage and improve the catalytic activity toward ORR.5−9 The enhanced activity for these Ptbased alloys results from the lattice compression and the modified electronic properties of Pt.10,11Among these Pt-based alloys, PtNi materials have shown the highest ORR activity so far. Unfortunately, most of these catalysts exhibited impressive initial ORR activity, but significant activity decay was often observed. Previous research mainly focused on controlling the morphology and structure of PtNi alloys to further increase the ORR activity12−16 and much less effort was spent on improving the durability of the catalysts, which is a critical issue for the practical applications of these catalysts. The catalytic degradation of these alloy catalysts is mainly caused by two factors. First, the leaching of Ni component in the strong acidic electrolyte at high potentials leads to the structure alteration, resulting in significant activity drops. Second, the continuous dissolution and redeposition processes of Pt together with the coalescence of nanoparticles through surface diffusion make these nanoparticles grow and change their morphology, resulting in a large decrease of the © 2019 American Chemical Society
Received: January 16, 2019 Accepted: March 6, 2019 Published: March 6, 2019 2769
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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ACS Applied Energy Materials
Figure 1. (a). Schematic illustration of the preparation of PtNi@C composites. (b) TEM, (c) HRTEM, and (d) HAADF images of MPN. (e, f) STEM/EDX elemental maps of Pt (e) and Ni (f) for MPN. (g, h) EDX spectra obtained from point 1 (g) and 2 (h) shown in (d). (II) chloride hexahydrate (NiCl2·6H2O, 98%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), and 2-methylimidazole (2-MIM, C4H6N2, 99%) were purchased from Sigma-Aldrich. Perchloric acid (HClO4, 70%, double distilled) was obtained from GFS Chemicals. Commercial J-M 20% Pt/C was purchased from FullCellStore. All reagents were used as received without further purification. Synthesis PtNi@C Composite. In a typical synthesis procedure, 15 mg of Pt(AcAc)2 (0.038 mmol) and 10 mg of NiCl2·6H2O (0.042 mmol) were dissolved in 15 mL of DMF solution under magnetic stirring at room temperature. Then, 60 mg of 2-MIM was added into the above mixture. After an additional 30 min of stirring, the solution was transferred into a 20 mL Teflon-lined stainless-steel autoclave, sealed, and heated at 150 °C for 5 h in an oven. The resulting precipitation was centrifuged, separated, and washed with ethanol and DI water successively. Then, the collected product was redispersed in DI water, frozen, and dried using a freeze-dryer. The obtained fluffy powder was loaded in a crucible, transferred into a tube furnace, and heated at different temperatures (300, 400, and 500 °C, designated as PtNi@C-1, PtNi@C-2, and PtNi@C-3, respectively) for 2 h in a 5% H2/Ar gas mixture. Finally, the products were cooled naturally to room temperature under the inert environment, and PtNi@C composites were obtained. In order to study the influence of Ni source on the structure and morphology of the PtNi@C, Ni(AcAc)2 was used to replace NiCl2 with other conditions unchanged. Synthesis of PtNi Nanoparticles without Encapsulation. The fabrication procedure was similar to that of PtNi@C, except that 2MIM was absent during the solvothermal process. Synthesis of Pt Nanoparticles. Pt nanoparticles were also synthesized according to the method described above, except for the
Herein, inspired by the success of using a metal organic framework (MOF) as carbon precursors to synthesize carbon− metal or carbon−metal oxide composites,28−30 we developed a straightforward approach to form carbon-layer-encapsulated PtNi alloy nanoparticles. In this approach, 2-methylimidazole (2-MIM) was used as an organic ligand and carbon source to first fabricate 2-MIM−Pt-Ni composites (designated as MPN) under solvothermal conditions. Then, a carbonization process at a relatively low temperature (400 °C) was introduced to obtain PtNi alloys encapsulated in a few carbon layers (PtNi@ C-2). Oxygen reduction reaction studies showed the PtNi@C2 has a much higher specific activity (SA) and mass activity (MA) than the commercial Pt/C catalyst. Moreover, benefiting from the protective carbon layers, the PtNi@C-2 catalyst also showed impressive electrochemical stability during an accelerated stability test (AST). The ECSA of PtNi@C-2 remained largely unaffected in 5000 AST potential cycles between 0.6 and 1.1 V (vs RHE) in O2-saturated 0.1 M HClO4 and only dropped by 17.7% after 15000 AST cycles. This study demonstrates the first step toward highly active and durable PtNi ORR catalysts through encapsulation in thin carbon layers.
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EXPERIMENTAL SECTION
Reagents and Chemicals. Platinum(II) acetylacetonate (Pt(AcAc)2, 97%), nickel(II) acetylacetonate (Ni(AcAc)2, 95%), nickel2770
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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ACS Applied Energy Materials absence of the Ni source and 2-methylimidazole or the Ni source in the solvothermal process. Catalyst Preparation. For the preparation of the catalyst, 3 mg of as-prepared PtNi@C, PtNi, or Pt nanoparticles were mixed with 12 mg of carbon black and dispersed in ethanol. The mixture was sonicated for 3 h, filtrated, and dried at 60 °C in an oven. The resulting powder was transferred into a tube furnace and annealed in Ar atmosphere at 185 °C for 10 h to remove any organic solvent absorbed on the catalysts. Characterization. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were carried out on JEOL-2100 and JEM-2100 FEG electron microscopes operating at 200 kV. X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex II diffractometer with Cu Kα radiation operated at 30 kV and 15 mA. Thermogravimetric analysis (TGA) was performed using a TGA Q500 instrument with a heating rate of 10 °C min−1 in air flow of 60 mL min−1. Nitrogen adsorption and desorption measurements were performed with an ASAP 2020 plus (Micromeritics) to obtain Brunauer−Emmett−Teller (BET) specific surface area and pore size distribution. Raman spectra were recorded using a Renishaw 2000 Raman microscope with a laser excitation at 633 nm. Fourier transform infrared (FTIR) spectra from KBr pellets of the samples were obtained with a Bruker ALPHA at a resolution of 2 cm−1. The atomic ratio of Pt to Ni in the PtNi@C catalysts was determined by an inductively coupled plasma optical emission spectrometer (Agilent 5100, ICP-OES). Surface chemical states of the samples were analyzed with X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALab220i-XL). The peaks were deconvoluted by the XPSPEAK41 program using mixed asymmetric Gaussian−Lorenzian functions and Shirley background. Electrochemical Measurements. A three-electrode glass cell with a glassy carbon rotating disk electrode (RDE, Pine, diameter, 5 mm) as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode was used to carry out the electrochemical measurements. All of the potentials were reported with respect to a homemade reversible hydrogen electrode (RHE). To prepare the catalyst ink, 2 mg of catalyst was dispersed in 1 mL of mixture of isopropanol (280 μL), water (700 μL), and Nafion (5%, 20 μL) through 30 min sonication. Then, 10 μL of the catalyst ink was cast on the RDE and dried under the ambient conditions. The electrode was then subjected to an electrochemical treatment by potential cycling between 0.02 and 1.1 V at 50 mV s−1 in N2-saturated 0.1 M HClO4 solution until stable voltammograms were obtained (50−100 cycles). The last recorded cycle was used to evaluate the HUPD-based electrochemically active surface area (ECSA). For the PtNi@C-2 sample, a CO stripping voltammogram was also used to confirm the HUPD ECSA. The irreversibly adsorbed CO layer was formed by immersing the electrode in CO-saturated 0.1 M HClO4 at 0.2 V for 5 min, followed by purging N2 into the solution for 15 min to remove the dissolved CO. The catalytic activity of the catalysts was measured by linear sweep voltammetry (LSV). This was performed in an O2-saturated electrolyte with a potential range between 0.05 and 1.1 V, a scan rate of 10 mV s−1, and a rotation speed of 1600 rpm. Stability measurements of PtNi@C-2 for 15000 cycles have been carried out by potential cycling between 0.6 and 1.1 V with a scan rate of 50 mV s−1 in O2-saturated 0.1 M HClO4 solution. CVs with 50 mV s−1 between 0.02 and 1.1 V in N2-saturated solution were recorded at every 5000 cycles. All of the presented electrochemical results are iR corrected.
MOF-like composite is designated as MPN. Although 2-MIM is one of the well-known organic ligands coordinating with metal ions (usually Co2+ and Zn2+) to form MOFs,31−33 formation of MOF with Ni2+, Pt2+, and 2-MIM under this solvothermal condition has not been reported before. To study the roles of the corresponding ions on the formation of MPN, several control experiments were performed, and the results are summarized in Figure S1. In the absence of Pt salt, no precipitation was formed after the solvothermal process, suggesting Pt ions are the key for the formation of MPN composite. This contention is further supported by the formation of similar albeit larger MOF-like composites in the absence of Ni salts (Figure S2). High angle annular dark field (HAADF) microscopy shows that each ball contains different numbers of embedded nanoparticles (Figure 1d), which are made of Pt and Ni as unveiled by the energy dispersive X-ray mapping (Figure 1e and f). Careful comparison of Figure 1d−f reveals that Pt and Ni also exist in areas where no apparent nanoparticles were observed, suggesting some Pt and Ni remain as ions in the MPN which was confirmed by XPS studies (vide infra). The energy dispersive X-ray (EDX) spectra acquired from two separated nanoparticles shown in Figure 1g and 1h confirm these particles are composed of Pt and Ni with a nominal Pt to Ni atomic ratio of 3 to 1. PtNi nanoparticles can also be formed using Ni(AcAc)2 instead of NiCl2 or in the absence of 2-MIM (Figures S3 and S4), suggesting their formation is insensitive to the Ni salt or the presence of 2-MIM. In addition, Pt nanoparticles were obtained under the same solvothermal conditions without adding Ni salts and 2-MIM, as shown in Figure S5. These observations agree with reports by others of the formation of PtNi nanocrystals under similar solvothermal conditions.34 Without 2-MIM, these particles are severely aggregated (Figures S4 and S5), suggesting 2-MIM interacts with the particles and serves as an inhibitor for particle aggregation. Infrared spectroscopy was employed to probe the interactions between 2-MIM and metals (Figure S6). Compared with the characteristic peaks of 2-MIM, the peaks at 1673, 1600, 1450, 1304, and 1115 cm−1 associated with the ring vibrations are all significantly red-shifted in MPN, suggesting strong interactions between 2-MIM and metal ions/particles. These peak shifts are analogous to several ZIF-based MOFs.35−37 The peaks in the pure 2-MIM spectrum located between 900 and 950 cm−1 together with the broad features around 1850 cm−1, assignable to the out-of-plane N−H bending mode38 and its overtone coupled with the N−H stretching,39 respectively, significantly diminished in the MPN spectrum, indicating direct coordination of nitrogen in 2-MIM to the metal ions. These spectral changes agree with the observation on ZIF-67, a MOF formed by Co ion and 2MIM.40 The new peak from MPN located at 455 cm−1, likely arising from the metal ion (Pt and/or Ni)-N (in 2-MIM) stretch that is similar to that observed on ZIF-67 and ZIF-8 formed by Co or Zn ions and 2-MIM,41 again is a manifestation of the presence of metal-N bonding in MPN. Although the IR spectrum of MPN shares some common features with ZIFs formed by 2-MIM and Co or Zn ions, there is no long-range order in MPN as revealed by the absence of sharp peaks in the powder X-ray diffraction pattern (vide infra). The nitrogen adsorption/desorption curve shown in Figure S7 yields a BET specific surface area of MPN of 344.6 m2 g−1 with a pore size distribution around 3.5 nm.
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RESULTS AND DISCUSSION The synthesis procedure of PtNi@C is illustrated in Figure 1a. A DMF solution containing 2-MIM and Pt and Ni salts first underwent a solvothermal process as described in the Experimental Section. The resulting precipitate was collected and dried using a freeze-dryer. Interestingly, as shown by the TEM image in Figure 1b, the precipitate has a ball-like structure of a diameter around 100 nm with embedded nanoparticles. The size of the particles is below 5 nm. This 2771
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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Figure 2. TEM and HRTEM images of PtNi@C samples annealed at different temperatures [(a, b) 300 °C; (c, d) 400 °C; and (e, f) 500 °C].
Figure 3. (a) XRD patterns, (b) Raman, (c) FTIR, and (d) XPS full spectra for different samples.
After thermal annealing in a reducing atmosphere, the fluffy yellow MPN powder turned into a black product. Figure 2 presents TEM micrograph images of products obtained after
annealing at different temperatures. After 2 h annealing at 300 °C (Figure 2a and 2b), the small PtNi nanoparticles buried in the amorphous MPN substrate were replaced with larger 2772
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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Figure 4. XPS spectra of (a) Pt 4f and (b) Ni 2p for different samples.
other two broad peaks at 2θ ∼ 15° and 33.5° are derived from the MPN substrate, which suggest an imperfect 2-MIM-based ZIF-like structure.46,48 Interestingly, although there are morphological differences between MPN and PtNi@C-1, as shown in the TEM images, their XRD patterns are nearly identical. This observation suggests that at this temperature, no new PtNi particles are formed and that the larger particles seen in Figure 2a and b may be a result of thermal coalescence of the small particles. Further increasing the annealing temperature to 400 and 500 °C, four diffraction peaks assignable to (111), (200), (220), and (311) planes of face-centered cubic (fcc) Pt group metals were observed.34 Compared with those of pure Pt nanoparticles (Figure S10), the diffraction peaks for both PtNi@C-2 and PtNi@C-3 samples are shifted to higher 2θ degrees, indicating that Ni is incorporated into the Pt lattice to form an alloy phase with lattice contraction.49 Accompanying these main peaks are a second set of peaks at higher reflection angles with a weaker intensity, suggesting the presence of at least two different PtNi alloy phases. Based on Vegard’s law, the dominant phase is Pt3Ni and the minor phase is PtNi3. The crystalline domain size estimated from the full width at half-maximum of the dominant (111) peaks using the Scherrer equation for PtNi@C-2 and PtNi@C-3 is about 4 to 6 nm. This value is smaller than the particle size observed in the TEM images, suggesting the particles are polycrystalline. Raman spectra from different samples are displayed in Figure 3b, and only PtNi@C-2 and PtNi@C-3 show a disorder-induced D band at about 1335 cm−1 and a graphitic G band at about 1608 cm−1 from the carbon layers.50 This observation agrees with the TEM results. The structure evolution for PtNi@C samples can also be demonstrated by the FTIR spectra displayed in Figure 3c. Vibration bands associated with the 2-MIM disappeared when the samples were annealed at 400 °C and above, resulting from the carbonization of 2-MIM.
particles, presumably caused by small particles fusing together. The ball-like MPN structure was also destroyed. When the annealing temperature was elevated to 400 °C, well-graphitized carbon-layer-encapsulated PtNi nanoparticles were obtained. As shown in Figure 2c and 2d, the average diameter of the PtNi nanoparticles is about 10 nm and the number of carbon layers is typically below eight. The lattice fringes with an interplane spacing of 0.22 nm exhibited in the inset of Figure 2d correspond to the PtNi {111} facet.42,43 The HAADF image of this product (PtNi@C-2) and the corresponding elemental mappings for Pt and Ni are shown in Figure S8, confirming both elements are well-dispersed in the final product. Further increasing the annealing temperature to 500 °C makes the PtNi nanocrystals grow significantly larger (Figure 2e and 2f), which will dramatically decrease the active surface area. Interestingly, when annealing the sample without Ni (Figure S2) at 400 °C, we only obtained Pt nanoparticles without well-graphitic carbon layer formation (Figure S9), suggesting Ni in MPN can catalyze the transformation of the amorphous substrate into graphitic carbon layers at relatively low temperatures. It has been reported by others that Ni can catalyze the conversion of amorphous to graphite carbon at a temperature as low as 730 K.44 Although carbon-supported or -confined Pt nanoparticles or Pt-based alloys have been reported by several other groups,18,23,24,26 as far as we know, few-carbon-layer-encapsulated PtNi nanoparticles have not been reported before. To further characterize the structures of MPN and PtNi@C samples, their XRD patterns were collected and shown in Figure 3a. Except for one broad diffraction peak center at 2θ ∼ 46.5°, there is no other characteristic peak assignable to PtNi nanocrystals for MPN. This observation is similar to some reported results from MOF-wrapped noble metal nanoparticles45−47and may result from the broadening effect of the small crystalline domain size of the PtNi particles. The 2773
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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Figure 5. (a) CV (N2-saturated 0.1 M HClO4 solution, scan rate: 50 mV s−1) and (b) LSV (O2-saturated 0.1 M HClO4 solution, scan rate: 10 mV s−1, rotation speed: 1600 rpm) curves for different samples. (c) CV and (d) LSV curves for Pt/C and PtNi@C-2 samples after the 1st and 5000th cycles. (e) ECSA, (f) MA, and (g) SA values of PtNi@C-2 compared with those of Pt/C catalyst.
signals are mainly from ionic species from the metal ion-2MIM network in MPN and PtNi@C-1. In agreement with the above discussion these results further support that the amorphous substrates are the products of strong interactions between 2-MIM, Pt, and Ni ions, presumably having a structrue similar to MOFs. With the annealling tempature increasing to 400 °C and above, the primary oxidation states of Pt and Ni change from cationic to metallic. In addition, the 4f7/2 peak for Pt(0) is located at 71.5 eV, which is higher than the reported value of 71.2 eV from Pt nanoparticles.52 The higher binding energy mainly arises from the electronic interactions between Pt and Ni and a possible minor contribution from the interactions of Pt with the carbon layers.53,54 To evaluate their catalytic performance toward ORR, different samples were deposited on a glassy carbon electrode and tested under N2- or O2-saturated 0.1 M HClO4. Figure 5a shows the CVs of Pt/C, MNP, and PtNi@C annealed at different temparatures. For MNP and PtNi@C-1, although both contain PtNi nanoparticles, no hydrogen adsorption/
XPS full spectra of different samples are shown in Figure 3d to unveil the presence of different elements in the samples. All of the spectra were corrected using the C1S signal located at 284.5 eV. The nitrogen atomic percentage of MPN is about 23.8% deriving from the nitrogen atoms of 2-MIM. After the samples were annealled at 400 °C, the nitrogen atomic percentage dropped significantly to 1.4% due to imidazole decomposition during the carbonization process. To identify the oxidation state of metals in MPN and PtNi@ C samples, Pt 4f and Ni 2P fine XPS spectra were deconvoluted and the results are displayed in Figure 4. In MPN and PtNi@C-1, Pt exists mainly in the oxidation state of +2 with a very small portion of Pt(0) 4f7/2 peak.51 The Ni 2p3/2 and Ni 2p1/2 peaks for both samples also mainly consisted of Nix+ 2p3/2 and Nix+ 2p1/2 (Figure 4b). These results are consistent with the TEM and XRD results discussed above where the metallic PtNi are buried in the metal ion-2-MIM network. XPS signals are mostly from the surface of materials with a typical detection depth below 10 nm, therefore the XPS 2774
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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Figure 6. (a) CV, (b) LSV curves, and (c) ECSA, MA, and SA percentage changes for PtNi@C-2 after activation and after 5000, 10000, and 15000 cycles. (d) TEM (inset: EDAX spectrum) and (e) HRTEM images of PtNi@C-2 after 15000 cycles.
S13, 65.5 m2 g−1Pt). The HUPD-based ECSA, MA, and SA values for PtNi@C-2 and commercial Pt/C are summarized in Figure 5e−g. PtNi@C-2 catalyst exhibits an MA of 0.84 A mg−1Pt and an SA of 1.54 mA cm−2 at 0.9 V, which is a 6.5-fold and 8.4-fold improvement over those of 20% commercial Pt/C catalyst (0.13 A mg−1Pt and 0.18 mA cm−2), respectively. The durability of Pt/C and PtNi@C-2 were evaluated through the accelerated stability test (AST) by applying a potential cycling between 0.6 and 1.1 V with a scan rate of 50 mV s−1 in an O2saturated 0.1 M HClO4 solution. As shown in Figure 5c and e, the ECSA of PtNi@C-2 (55.6 m2 g−1Pt) remains largely unchanged after 5000 cycles. In comparison, the ECSA of commerical Pt/C drops to 41.0 m2 g−1Pt. In addition, the PtNi nanoparticles prepared by the same solvothermal process without the carbon encapsulation also showed a significant reduction of ECSA from 33.2 m2 g−1Pt to 24.3 m2 g−1Pt (Figure S14), despite the fact that these particles are much larger. This observation strongly supports the assertion that the carbon layers significantly mitigate the ECSA degradation. At 0.9 V, the MA and SA of PtNi@C-2 are 0.51 A mg−1Pt and 0.90 mA cm−2, respectively, after 5000 cycles, which are still much better than those of the pristine Pt/C catalyst. Encouraged by the great ECSA stability of PtNi@C-2 catalyst, we prolonged the accelerated test further to 15000 cycles, and the results are exhibited in Figure 6a−c. The ECSA dropped by 17.7% to 45.6 m2 g−1Pt after 15000 cycles probably caused by the Ni leaching. Although the catalytic activity of PtNi@C-2 decreased singnificantly to an MA of 0.27 A mg−1Pt and a SA of 0.59 mA cm−2, they remain 2- and 3-fold better than that of the pristine commerical Pt/C catalyst. TEM and HRTEM images of PtNi@C-2 after 15000 cycles are displayed
desorption peaks were observed, suggesting most of the metallic Pt sites are concealed in the metal ion-2-MIM network, in agreement with the TEM and XPS results. With the annealing temperature increasing to 400 °C and above, the active sites become accessible as evident by the current peaks in the hydrogen adsoprtion/desorption as well as the surface oxidation/reduction regions. Positive-going ORR polarization curves of different samples are displayed in Figure 5b. The MPN and PtNi@C-1 are essentially inert toward ORR, consistent with the CV results. The small reduction current observed on these two samples is likely from the ORR on small PtNi nanopartilces like those observed in the TEM images. The reduction current from MPN is higher than that from PtNi@C-1, which is likely due to the loss of active sites from the coalescnece of PtNi particles upon heating, as shown in Figure 2a. PtNi@C-2 has the largest current density at 0.9 V (3.9 mA cm−2) and most positive half-wave potential among different samples, including PtNi and Pt nanoparticles prepared by the same solvothermal process (Figures 5b and S11). The ORR catalytic performances for PtNi@C-2 are normalized over the Pt loading and ECSA to yield the mass activity (MA) and specific activity (SA), respectively. From the ICP-OES measurements, the atomic ratio of Pt to Ni of PtNi@ C-2 is estimated as 1.2. The TGA results indicate the total metal content of PtNi@C-2 is about 85% (Figure S12). Based on these results, the Pt loading of PtNi@C-2 on the roating disk electrode is calculated as 2.7 μg. The ECSA estimated from the charge involved in the desorption of the underpotential-deposited (UPD) hydrogen for PtNi@C-2 is about 55.4 m2 g−1Pt, which is within the experimental error of that calculated from the charge of CO electro-oxidation (Figure 2775
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ACS Applied Energy Materials in Figure 6d and 6e. Compared with the pristine PtNi@C-2, the size of PtNi alloys decrease to 5−7 nm, probably resulting from the leaching of Ni during the long-term cycling. This assertion is further confirmed by the EDAX result that reveals the atomic ratio of Pt to Ni increased to about 1.6:1 from 1.2:1. The leaching of less noble metal in encapsulated Ptbased alloy nanoparticles has also been observed by others,27 presumably through the pores in the carbon layers.55 In this regard, it is interesting to note that a recent study of hydrogen evolution reaction on the SiOx membrane-protected Pt thin films revealed effective blocking of Cu2+ transport while facilitating transport of H+ through the SiOx.56 That work points to a potentially effective way to minimizing leaching of the less noble metal in Pt-based catalysts. Compared with other Pt and Pt-based alloys entrapped or wrapped in carbon layers reported in the recent literature, the PtNi@C-2 catalyst is among the best for ORR activity (Table S1). A couple of factors may contribute to the improved ORR activity of PtNi@C-2. First, the alloying of Pt with Ni changes the Pt d-band center position and optimizes the adsorption energy of oxygen-containing species, as has been demonstrated by many reports.57 Second, the possible interaction of the nitrogen-containing graphitic carbon with PtNi nanoparticles modifies the electronic structure of PtNi alloys that facilitates the ORR.58,59 Although the ORR activity of PtNi@C-2 decreases in the AST tests, which is likely caused by the Ni leaching, its ECSA only drops by 17.7% after 15000 cycles, suggesting that the Pt component in PtNi@C-2 is stable after being encapsulated by the carbon layers. The well-graphitic carbons that wrap around the PtNi particles improves the stability of the catalysts by diminishing the direct contact of nanoparticles with each other and providing a stronger support for the particles through increasing resistance to carbon corrosion at higher potentials. To alleviate the Ni leaching, formation of single crystal or an intermetallic PtNi core with optimized Pt/Ni ratio could be a good strategy, which has been demostrated in some reported results from bare particles.10,15,60,61 Studies along this line are currently underway in our laboratory.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. ORCID
Shouzhong Zou: 0000-0003-1952-8799 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Dr. Zhenyu Zhang from City University of Hong Kong for his help in XPS measurements. This work was partially supported by a startup fund from American University.
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REFERENCES
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CONCLUSIONS In summary, a facile solvothermal approach combined with subsequent mild thermal annealing for the creation of PtNi nanoparticles encapsulated in a few carbon layers has been developed. The results show that the Pt ions are the critical factor to construct the precursor and the Ni is the key promoter to form the few carbon layers at mildly elevated temperature. The PtNi@C-2 exhibits a much higher catalytic activity toward ORR than a commerical Pt/C catalyst, even after 15000 potential cycles between 0.6 and 1.1 V in O2saturated 0.1 M HClO4. The high activity and durability (in terms of ECSA at present) of PtNi@C-2 demonstrate that carbon encapsulation is a promising approach for improving the durability of high performance PtNi ORR catalysts.
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adsorption/desorption curves of MPN; EDX results of PtNi@C; XRD of Pt nanoparticles; LSV for ORR on Pt nanoparticles; TGA curves for MPN and PtNi@C; CO stripping voltammogram for PtNi@C-2; and an ORR activity comparison table (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00106. Photographs of samples prepared under different experimental conditions; TEM images of various control samples; FTIR spectra of 2-MIM and MPN; nitrogen 2776
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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PtNi Nanoparticles Encapsulated in Few Carbon Layers as HighPerformance Catalysts for Oxygen Reduction Reaction Wenyue Li and Shouzhong Zou* Department of Chemistry, American University, 4400 Massachusetts Avenue NW, Washington D.C. 20016, United States
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S Supporting Information *
ABSTRACT: PtNi alloys have been demonstrated to be one of the most promising catalysts for oxygen reduction reaction (ORR) in fuel cell applications due to their high catalytic activity and efficient utilization of Pt. However, improving the durability of such catalysts remains a significant challenge. Herein, we report on the formation of PtNi nanoparticles with an average diameter of around 10 nm embedded in few carbon layers (PtNi@C) and their ORR catalytic performance. The synthesis procedure entails the use of a solvothermal method to form 2-methylimidazole−Pt-Ni composites with a MOF-like structure (designated as MPN) followed by a thermal annealing treatment. The presence of Pt during the solvothermal process catalyzes the deposition of Ni, and the Ni in the MPN catalyzes the formation of well-graphitized carbon at a temperature as low as 400 °C. The PtNi@C catalyst exhibits an ORR mass activity (MA) of 0.84 A mgpt−1 and a specific activity (SA) of 1.54 mA cm−2 at 0.9 V (vs RHE), representing 6.5-fold and 8.4fold improvement over a commercial Pt/C catalyst. More significantly, the electrochemically active surface area of the PtNi@C catalyst shows little change after 5000 cycles of potential scans between 0.6 and 1.1 V in O2-saturated 0.1 M HClO4. This study demonstrates the feasibility of stabilizing Ptbased nanocatalysts through graphitic carbon encapsulation. KEYWORDS: oxygen reduction reaction, PtNi alloys, thin carbon layer encapsulation, metal organic frameworks, stability
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electrochemically active surface area (ECSA).3,4 To improve the durability, Huang’s group obtained a series of Pt3Ni octahedra surface-doped with other transition metals.17 Among these catalysts, the Mo-doped Pt3Ni exhibited the best ORR activity and durability, which were attributed to the formation of relatively strong Mo−Pt and Mo−Ni bonds that reduces the dissolution of Pt and Ni. Besides the structural engineering discussed above, stabilizing PtNi alloys with conductive and stable carbon films can also be a promising strategy to alleviate their catalytic activity degradation.18−20 The carbon layers can be a physical barrier for diffusion of the dissolved Pt or Ni away from the PtNi nanoparticles and the coalescence of the PtNi nanoparticles. They may also modify the electronic properties of the PtNi nanoparticles through PtNi−carbon interactions. It is also envisioned that decorating the carbon layer with selected functional groups may promote a desired reaction pathway, as has been recently demonstrated on permeable silicon oxide nanomembrane-encapsulated Pt thin films for carbon monoxide and methanol oxidation.21 However, the PtNi core-carbon shell hierarchical structure cannot be easily obtained, and as far as we know, only a few Ptbased alloys encapsulated by carbon layers have been reported so far.22−27 Therefore, developing a facile method to form this structure remains a challenge.
INTRODUCTION Pt and Pt-based nanoparticles supported on carbon black have been considered as the most effective catalysts for the kinetically sluggish oxygen reduction reaction (ORR), which is the cathode reaction for fuel cells and metal air batteries.1−4 Extensive efforts have been devoted to combining Pt with 3d transition metals such as Ni, Co, Fe, Cu, Ti, and Zn to form bior multimetallic alloys, which has been shown to be an effective way to decrease the Pt usage and improve the catalytic activity toward ORR.5−9 The enhanced activity for these Ptbased alloys results from the lattice compression and the modified electronic properties of Pt.10,11Among these Pt-based alloys, PtNi materials have shown the highest ORR activity so far. Unfortunately, most of these catalysts exhibited impressive initial ORR activity, but significant activity decay was often observed. Previous research mainly focused on controlling the morphology and structure of PtNi alloys to further increase the ORR activity12−16 and much less effort was spent on improving the durability of the catalysts, which is a critical issue for the practical applications of these catalysts. The catalytic degradation of these alloy catalysts is mainly caused by two factors. First, the leaching of Ni component in the strong acidic electrolyte at high potentials leads to the structure alteration, resulting in significant activity drops. Second, the continuous dissolution and redeposition processes of Pt together with the coalescence of nanoparticles through surface diffusion make these nanoparticles grow and change their morphology, resulting in a large decrease of the © 2019 American Chemical Society
Received: January 16, 2019 Accepted: March 6, 2019 Published: March 6, 2019 2769
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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ACS Applied Energy Materials
Figure 1. (a). Schematic illustration of the preparation of PtNi@C composites. (b) TEM, (c) HRTEM, and (d) HAADF images of MPN. (e, f) STEM/EDX elemental maps of Pt (e) and Ni (f) for MPN. (g, h) EDX spectra obtained from point 1 (g) and 2 (h) shown in (d). (II) chloride hexahydrate (NiCl2·6H2O, 98%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), and 2-methylimidazole (2-MIM, C4H6N2, 99%) were purchased from Sigma-Aldrich. Perchloric acid (HClO4, 70%, double distilled) was obtained from GFS Chemicals. Commercial J-M 20% Pt/C was purchased from FullCellStore. All reagents were used as received without further purification. Synthesis PtNi@C Composite. In a typical synthesis procedure, 15 mg of Pt(AcAc)2 (0.038 mmol) and 10 mg of NiCl2·6H2O (0.042 mmol) were dissolved in 15 mL of DMF solution under magnetic stirring at room temperature. Then, 60 mg of 2-MIM was added into the above mixture. After an additional 30 min of stirring, the solution was transferred into a 20 mL Teflon-lined stainless-steel autoclave, sealed, and heated at 150 °C for 5 h in an oven. The resulting precipitation was centrifuged, separated, and washed with ethanol and DI water successively. Then, the collected product was redispersed in DI water, frozen, and dried using a freeze-dryer. The obtained fluffy powder was loaded in a crucible, transferred into a tube furnace, and heated at different temperatures (300, 400, and 500 °C, designated as PtNi@C-1, PtNi@C-2, and PtNi@C-3, respectively) for 2 h in a 5% H2/Ar gas mixture. Finally, the products were cooled naturally to room temperature under the inert environment, and PtNi@C composites were obtained. In order to study the influence of Ni source on the structure and morphology of the PtNi@C, Ni(AcAc)2 was used to replace NiCl2 with other conditions unchanged. Synthesis of PtNi Nanoparticles without Encapsulation. The fabrication procedure was similar to that of PtNi@C, except that 2MIM was absent during the solvothermal process. Synthesis of Pt Nanoparticles. Pt nanoparticles were also synthesized according to the method described above, except for the
Herein, inspired by the success of using a metal organic framework (MOF) as carbon precursors to synthesize carbon− metal or carbon−metal oxide composites,28−30 we developed a straightforward approach to form carbon-layer-encapsulated PtNi alloy nanoparticles. In this approach, 2-methylimidazole (2-MIM) was used as an organic ligand and carbon source to first fabricate 2-MIM−Pt-Ni composites (designated as MPN) under solvothermal conditions. Then, a carbonization process at a relatively low temperature (400 °C) was introduced to obtain PtNi alloys encapsulated in a few carbon layers (PtNi@ C-2). Oxygen reduction reaction studies showed the PtNi@C2 has a much higher specific activity (SA) and mass activity (MA) than the commercial Pt/C catalyst. Moreover, benefiting from the protective carbon layers, the PtNi@C-2 catalyst also showed impressive electrochemical stability during an accelerated stability test (AST). The ECSA of PtNi@C-2 remained largely unaffected in 5000 AST potential cycles between 0.6 and 1.1 V (vs RHE) in O2-saturated 0.1 M HClO4 and only dropped by 17.7% after 15000 AST cycles. This study demonstrates the first step toward highly active and durable PtNi ORR catalysts through encapsulation in thin carbon layers.
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EXPERIMENTAL SECTION
Reagents and Chemicals. Platinum(II) acetylacetonate (Pt(AcAc)2, 97%), nickel(II) acetylacetonate (Ni(AcAc)2, 95%), nickel2770
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ACS Applied Energy Materials absence of the Ni source and 2-methylimidazole or the Ni source in the solvothermal process. Catalyst Preparation. For the preparation of the catalyst, 3 mg of as-prepared PtNi@C, PtNi, or Pt nanoparticles were mixed with 12 mg of carbon black and dispersed in ethanol. The mixture was sonicated for 3 h, filtrated, and dried at 60 °C in an oven. The resulting powder was transferred into a tube furnace and annealed in Ar atmosphere at 185 °C for 10 h to remove any organic solvent absorbed on the catalysts. Characterization. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were carried out on JEOL-2100 and JEM-2100 FEG electron microscopes operating at 200 kV. X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex II diffractometer with Cu Kα radiation operated at 30 kV and 15 mA. Thermogravimetric analysis (TGA) was performed using a TGA Q500 instrument with a heating rate of 10 °C min−1 in air flow of 60 mL min−1. Nitrogen adsorption and desorption measurements were performed with an ASAP 2020 plus (Micromeritics) to obtain Brunauer−Emmett−Teller (BET) specific surface area and pore size distribution. Raman spectra were recorded using a Renishaw 2000 Raman microscope with a laser excitation at 633 nm. Fourier transform infrared (FTIR) spectra from KBr pellets of the samples were obtained with a Bruker ALPHA at a resolution of 2 cm−1. The atomic ratio of Pt to Ni in the PtNi@C catalysts was determined by an inductively coupled plasma optical emission spectrometer (Agilent 5100, ICP-OES). Surface chemical states of the samples were analyzed with X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALab220i-XL). The peaks were deconvoluted by the XPSPEAK41 program using mixed asymmetric Gaussian−Lorenzian functions and Shirley background. Electrochemical Measurements. A three-electrode glass cell with a glassy carbon rotating disk electrode (RDE, Pine, diameter, 5 mm) as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode was used to carry out the electrochemical measurements. All of the potentials were reported with respect to a homemade reversible hydrogen electrode (RHE). To prepare the catalyst ink, 2 mg of catalyst was dispersed in 1 mL of mixture of isopropanol (280 μL), water (700 μL), and Nafion (5%, 20 μL) through 30 min sonication. Then, 10 μL of the catalyst ink was cast on the RDE and dried under the ambient conditions. The electrode was then subjected to an electrochemical treatment by potential cycling between 0.02 and 1.1 V at 50 mV s−1 in N2-saturated 0.1 M HClO4 solution until stable voltammograms were obtained (50−100 cycles). The last recorded cycle was used to evaluate the HUPD-based electrochemically active surface area (ECSA). For the PtNi@C-2 sample, a CO stripping voltammogram was also used to confirm the HUPD ECSA. The irreversibly adsorbed CO layer was formed by immersing the electrode in CO-saturated 0.1 M HClO4 at 0.2 V for 5 min, followed by purging N2 into the solution for 15 min to remove the dissolved CO. The catalytic activity of the catalysts was measured by linear sweep voltammetry (LSV). This was performed in an O2-saturated electrolyte with a potential range between 0.05 and 1.1 V, a scan rate of 10 mV s−1, and a rotation speed of 1600 rpm. Stability measurements of PtNi@C-2 for 15000 cycles have been carried out by potential cycling between 0.6 and 1.1 V with a scan rate of 50 mV s−1 in O2-saturated 0.1 M HClO4 solution. CVs with 50 mV s−1 between 0.02 and 1.1 V in N2-saturated solution were recorded at every 5000 cycles. All of the presented electrochemical results are iR corrected.
MOF-like composite is designated as MPN. Although 2-MIM is one of the well-known organic ligands coordinating with metal ions (usually Co2+ and Zn2+) to form MOFs,31−33 formation of MOF with Ni2+, Pt2+, and 2-MIM under this solvothermal condition has not been reported before. To study the roles of the corresponding ions on the formation of MPN, several control experiments were performed, and the results are summarized in Figure S1. In the absence of Pt salt, no precipitation was formed after the solvothermal process, suggesting Pt ions are the key for the formation of MPN composite. This contention is further supported by the formation of similar albeit larger MOF-like composites in the absence of Ni salts (Figure S2). High angle annular dark field (HAADF) microscopy shows that each ball contains different numbers of embedded nanoparticles (Figure 1d), which are made of Pt and Ni as unveiled by the energy dispersive X-ray mapping (Figure 1e and f). Careful comparison of Figure 1d−f reveals that Pt and Ni also exist in areas where no apparent nanoparticles were observed, suggesting some Pt and Ni remain as ions in the MPN which was confirmed by XPS studies (vide infra). The energy dispersive X-ray (EDX) spectra acquired from two separated nanoparticles shown in Figure 1g and 1h confirm these particles are composed of Pt and Ni with a nominal Pt to Ni atomic ratio of 3 to 1. PtNi nanoparticles can also be formed using Ni(AcAc)2 instead of NiCl2 or in the absence of 2-MIM (Figures S3 and S4), suggesting their formation is insensitive to the Ni salt or the presence of 2-MIM. In addition, Pt nanoparticles were obtained under the same solvothermal conditions without adding Ni salts and 2-MIM, as shown in Figure S5. These observations agree with reports by others of the formation of PtNi nanocrystals under similar solvothermal conditions.34 Without 2-MIM, these particles are severely aggregated (Figures S4 and S5), suggesting 2-MIM interacts with the particles and serves as an inhibitor for particle aggregation. Infrared spectroscopy was employed to probe the interactions between 2-MIM and metals (Figure S6). Compared with the characteristic peaks of 2-MIM, the peaks at 1673, 1600, 1450, 1304, and 1115 cm−1 associated with the ring vibrations are all significantly red-shifted in MPN, suggesting strong interactions between 2-MIM and metal ions/particles. These peak shifts are analogous to several ZIF-based MOFs.35−37 The peaks in the pure 2-MIM spectrum located between 900 and 950 cm−1 together with the broad features around 1850 cm−1, assignable to the out-of-plane N−H bending mode38 and its overtone coupled with the N−H stretching,39 respectively, significantly diminished in the MPN spectrum, indicating direct coordination of nitrogen in 2-MIM to the metal ions. These spectral changes agree with the observation on ZIF-67, a MOF formed by Co ion and 2MIM.40 The new peak from MPN located at 455 cm−1, likely arising from the metal ion (Pt and/or Ni)-N (in 2-MIM) stretch that is similar to that observed on ZIF-67 and ZIF-8 formed by Co or Zn ions and 2-MIM,41 again is a manifestation of the presence of metal-N bonding in MPN. Although the IR spectrum of MPN shares some common features with ZIFs formed by 2-MIM and Co or Zn ions, there is no long-range order in MPN as revealed by the absence of sharp peaks in the powder X-ray diffraction pattern (vide infra). The nitrogen adsorption/desorption curve shown in Figure S7 yields a BET specific surface area of MPN of 344.6 m2 g−1 with a pore size distribution around 3.5 nm.
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RESULTS AND DISCUSSION The synthesis procedure of PtNi@C is illustrated in Figure 1a. A DMF solution containing 2-MIM and Pt and Ni salts first underwent a solvothermal process as described in the Experimental Section. The resulting precipitate was collected and dried using a freeze-dryer. Interestingly, as shown by the TEM image in Figure 1b, the precipitate has a ball-like structure of a diameter around 100 nm with embedded nanoparticles. The size of the particles is below 5 nm. This 2771
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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Figure 2. TEM and HRTEM images of PtNi@C samples annealed at different temperatures [(a, b) 300 °C; (c, d) 400 °C; and (e, f) 500 °C].
Figure 3. (a) XRD patterns, (b) Raman, (c) FTIR, and (d) XPS full spectra for different samples.
After thermal annealing in a reducing atmosphere, the fluffy yellow MPN powder turned into a black product. Figure 2 presents TEM micrograph images of products obtained after
annealing at different temperatures. After 2 h annealing at 300 °C (Figure 2a and 2b), the small PtNi nanoparticles buried in the amorphous MPN substrate were replaced with larger 2772
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Figure 4. XPS spectra of (a) Pt 4f and (b) Ni 2p for different samples.
other two broad peaks at 2θ ∼ 15° and 33.5° are derived from the MPN substrate, which suggest an imperfect 2-MIM-based ZIF-like structure.46,48 Interestingly, although there are morphological differences between MPN and PtNi@C-1, as shown in the TEM images, their XRD patterns are nearly identical. This observation suggests that at this temperature, no new PtNi particles are formed and that the larger particles seen in Figure 2a and b may be a result of thermal coalescence of the small particles. Further increasing the annealing temperature to 400 and 500 °C, four diffraction peaks assignable to (111), (200), (220), and (311) planes of face-centered cubic (fcc) Pt group metals were observed.34 Compared with those of pure Pt nanoparticles (Figure S10), the diffraction peaks for both PtNi@C-2 and PtNi@C-3 samples are shifted to higher 2θ degrees, indicating that Ni is incorporated into the Pt lattice to form an alloy phase with lattice contraction.49 Accompanying these main peaks are a second set of peaks at higher reflection angles with a weaker intensity, suggesting the presence of at least two different PtNi alloy phases. Based on Vegard’s law, the dominant phase is Pt3Ni and the minor phase is PtNi3. The crystalline domain size estimated from the full width at half-maximum of the dominant (111) peaks using the Scherrer equation for PtNi@C-2 and PtNi@C-3 is about 4 to 6 nm. This value is smaller than the particle size observed in the TEM images, suggesting the particles are polycrystalline. Raman spectra from different samples are displayed in Figure 3b, and only PtNi@C-2 and PtNi@C-3 show a disorder-induced D band at about 1335 cm−1 and a graphitic G band at about 1608 cm−1 from the carbon layers.50 This observation agrees with the TEM results. The structure evolution for PtNi@C samples can also be demonstrated by the FTIR spectra displayed in Figure 3c. Vibration bands associated with the 2-MIM disappeared when the samples were annealed at 400 °C and above, resulting from the carbonization of 2-MIM.
particles, presumably caused by small particles fusing together. The ball-like MPN structure was also destroyed. When the annealing temperature was elevated to 400 °C, well-graphitized carbon-layer-encapsulated PtNi nanoparticles were obtained. As shown in Figure 2c and 2d, the average diameter of the PtNi nanoparticles is about 10 nm and the number of carbon layers is typically below eight. The lattice fringes with an interplane spacing of 0.22 nm exhibited in the inset of Figure 2d correspond to the PtNi {111} facet.42,43 The HAADF image of this product (PtNi@C-2) and the corresponding elemental mappings for Pt and Ni are shown in Figure S8, confirming both elements are well-dispersed in the final product. Further increasing the annealing temperature to 500 °C makes the PtNi nanocrystals grow significantly larger (Figure 2e and 2f), which will dramatically decrease the active surface area. Interestingly, when annealing the sample without Ni (Figure S2) at 400 °C, we only obtained Pt nanoparticles without well-graphitic carbon layer formation (Figure S9), suggesting Ni in MPN can catalyze the transformation of the amorphous substrate into graphitic carbon layers at relatively low temperatures. It has been reported by others that Ni can catalyze the conversion of amorphous to graphite carbon at a temperature as low as 730 K.44 Although carbon-supported or -confined Pt nanoparticles or Pt-based alloys have been reported by several other groups,18,23,24,26 as far as we know, few-carbon-layer-encapsulated PtNi nanoparticles have not been reported before. To further characterize the structures of MPN and PtNi@C samples, their XRD patterns were collected and shown in Figure 3a. Except for one broad diffraction peak center at 2θ ∼ 46.5°, there is no other characteristic peak assignable to PtNi nanocrystals for MPN. This observation is similar to some reported results from MOF-wrapped noble metal nanoparticles45−47and may result from the broadening effect of the small crystalline domain size of the PtNi particles. The 2773
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Figure 5. (a) CV (N2-saturated 0.1 M HClO4 solution, scan rate: 50 mV s−1) and (b) LSV (O2-saturated 0.1 M HClO4 solution, scan rate: 10 mV s−1, rotation speed: 1600 rpm) curves for different samples. (c) CV and (d) LSV curves for Pt/C and PtNi@C-2 samples after the 1st and 5000th cycles. (e) ECSA, (f) MA, and (g) SA values of PtNi@C-2 compared with those of Pt/C catalyst.
signals are mainly from ionic species from the metal ion-2MIM network in MPN and PtNi@C-1. In agreement with the above discussion these results further support that the amorphous substrates are the products of strong interactions between 2-MIM, Pt, and Ni ions, presumably having a structrue similar to MOFs. With the annealling tempature increasing to 400 °C and above, the primary oxidation states of Pt and Ni change from cationic to metallic. In addition, the 4f7/2 peak for Pt(0) is located at 71.5 eV, which is higher than the reported value of 71.2 eV from Pt nanoparticles.52 The higher binding energy mainly arises from the electronic interactions between Pt and Ni and a possible minor contribution from the interactions of Pt with the carbon layers.53,54 To evaluate their catalytic performance toward ORR, different samples were deposited on a glassy carbon electrode and tested under N2- or O2-saturated 0.1 M HClO4. Figure 5a shows the CVs of Pt/C, MNP, and PtNi@C annealed at different temparatures. For MNP and PtNi@C-1, although both contain PtNi nanoparticles, no hydrogen adsorption/
XPS full spectra of different samples are shown in Figure 3d to unveil the presence of different elements in the samples. All of the spectra were corrected using the C1S signal located at 284.5 eV. The nitrogen atomic percentage of MPN is about 23.8% deriving from the nitrogen atoms of 2-MIM. After the samples were annealled at 400 °C, the nitrogen atomic percentage dropped significantly to 1.4% due to imidazole decomposition during the carbonization process. To identify the oxidation state of metals in MPN and PtNi@ C samples, Pt 4f and Ni 2P fine XPS spectra were deconvoluted and the results are displayed in Figure 4. In MPN and PtNi@C-1, Pt exists mainly in the oxidation state of +2 with a very small portion of Pt(0) 4f7/2 peak.51 The Ni 2p3/2 and Ni 2p1/2 peaks for both samples also mainly consisted of Nix+ 2p3/2 and Nix+ 2p1/2 (Figure 4b). These results are consistent with the TEM and XRD results discussed above where the metallic PtNi are buried in the metal ion-2-MIM network. XPS signals are mostly from the surface of materials with a typical detection depth below 10 nm, therefore the XPS 2774
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Figure 6. (a) CV, (b) LSV curves, and (c) ECSA, MA, and SA percentage changes for PtNi@C-2 after activation and after 5000, 10000, and 15000 cycles. (d) TEM (inset: EDAX spectrum) and (e) HRTEM images of PtNi@C-2 after 15000 cycles.
S13, 65.5 m2 g−1Pt). The HUPD-based ECSA, MA, and SA values for PtNi@C-2 and commercial Pt/C are summarized in Figure 5e−g. PtNi@C-2 catalyst exhibits an MA of 0.84 A mg−1Pt and an SA of 1.54 mA cm−2 at 0.9 V, which is a 6.5-fold and 8.4-fold improvement over those of 20% commercial Pt/C catalyst (0.13 A mg−1Pt and 0.18 mA cm−2), respectively. The durability of Pt/C and PtNi@C-2 were evaluated through the accelerated stability test (AST) by applying a potential cycling between 0.6 and 1.1 V with a scan rate of 50 mV s−1 in an O2saturated 0.1 M HClO4 solution. As shown in Figure 5c and e, the ECSA of PtNi@C-2 (55.6 m2 g−1Pt) remains largely unchanged after 5000 cycles. In comparison, the ECSA of commerical Pt/C drops to 41.0 m2 g−1Pt. In addition, the PtNi nanoparticles prepared by the same solvothermal process without the carbon encapsulation also showed a significant reduction of ECSA from 33.2 m2 g−1Pt to 24.3 m2 g−1Pt (Figure S14), despite the fact that these particles are much larger. This observation strongly supports the assertion that the carbon layers significantly mitigate the ECSA degradation. At 0.9 V, the MA and SA of PtNi@C-2 are 0.51 A mg−1Pt and 0.90 mA cm−2, respectively, after 5000 cycles, which are still much better than those of the pristine Pt/C catalyst. Encouraged by the great ECSA stability of PtNi@C-2 catalyst, we prolonged the accelerated test further to 15000 cycles, and the results are exhibited in Figure 6a−c. The ECSA dropped by 17.7% to 45.6 m2 g−1Pt after 15000 cycles probably caused by the Ni leaching. Although the catalytic activity of PtNi@C-2 decreased singnificantly to an MA of 0.27 A mg−1Pt and a SA of 0.59 mA cm−2, they remain 2- and 3-fold better than that of the pristine commerical Pt/C catalyst. TEM and HRTEM images of PtNi@C-2 after 15000 cycles are displayed
desorption peaks were observed, suggesting most of the metallic Pt sites are concealed in the metal ion-2-MIM network, in agreement with the TEM and XPS results. With the annealing temperature increasing to 400 °C and above, the active sites become accessible as evident by the current peaks in the hydrogen adsoprtion/desorption as well as the surface oxidation/reduction regions. Positive-going ORR polarization curves of different samples are displayed in Figure 5b. The MPN and PtNi@C-1 are essentially inert toward ORR, consistent with the CV results. The small reduction current observed on these two samples is likely from the ORR on small PtNi nanopartilces like those observed in the TEM images. The reduction current from MPN is higher than that from PtNi@C-1, which is likely due to the loss of active sites from the coalescnece of PtNi particles upon heating, as shown in Figure 2a. PtNi@C-2 has the largest current density at 0.9 V (3.9 mA cm−2) and most positive half-wave potential among different samples, including PtNi and Pt nanoparticles prepared by the same solvothermal process (Figures 5b and S11). The ORR catalytic performances for PtNi@C-2 are normalized over the Pt loading and ECSA to yield the mass activity (MA) and specific activity (SA), respectively. From the ICP-OES measurements, the atomic ratio of Pt to Ni of PtNi@ C-2 is estimated as 1.2. The TGA results indicate the total metal content of PtNi@C-2 is about 85% (Figure S12). Based on these results, the Pt loading of PtNi@C-2 on the roating disk electrode is calculated as 2.7 μg. The ECSA estimated from the charge involved in the desorption of the underpotential-deposited (UPD) hydrogen for PtNi@C-2 is about 55.4 m2 g−1Pt, which is within the experimental error of that calculated from the charge of CO electro-oxidation (Figure 2775
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ACS Applied Energy Materials in Figure 6d and 6e. Compared with the pristine PtNi@C-2, the size of PtNi alloys decrease to 5−7 nm, probably resulting from the leaching of Ni during the long-term cycling. This assertion is further confirmed by the EDAX result that reveals the atomic ratio of Pt to Ni increased to about 1.6:1 from 1.2:1. The leaching of less noble metal in encapsulated Ptbased alloy nanoparticles has also been observed by others,27 presumably through the pores in the carbon layers.55 In this regard, it is interesting to note that a recent study of hydrogen evolution reaction on the SiOx membrane-protected Pt thin films revealed effective blocking of Cu2+ transport while facilitating transport of H+ through the SiOx.56 That work points to a potentially effective way to minimizing leaching of the less noble metal in Pt-based catalysts. Compared with other Pt and Pt-based alloys entrapped or wrapped in carbon layers reported in the recent literature, the PtNi@C-2 catalyst is among the best for ORR activity (Table S1). A couple of factors may contribute to the improved ORR activity of PtNi@C-2. First, the alloying of Pt with Ni changes the Pt d-band center position and optimizes the adsorption energy of oxygen-containing species, as has been demonstrated by many reports.57 Second, the possible interaction of the nitrogen-containing graphitic carbon with PtNi nanoparticles modifies the electronic structure of PtNi alloys that facilitates the ORR.58,59 Although the ORR activity of PtNi@C-2 decreases in the AST tests, which is likely caused by the Ni leaching, its ECSA only drops by 17.7% after 15000 cycles, suggesting that the Pt component in PtNi@C-2 is stable after being encapsulated by the carbon layers. The well-graphitic carbons that wrap around the PtNi particles improves the stability of the catalysts by diminishing the direct contact of nanoparticles with each other and providing a stronger support for the particles through increasing resistance to carbon corrosion at higher potentials. To alleviate the Ni leaching, formation of single crystal or an intermetallic PtNi core with optimized Pt/Ni ratio could be a good strategy, which has been demostrated in some reported results from bare particles.10,15,60,61 Studies along this line are currently underway in our laboratory.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. ORCID
Shouzhong Zou: 0000-0003-1952-8799 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Dr. Zhenyu Zhang from City University of Hong Kong for his help in XPS measurements. This work was partially supported by a startup fund from American University.
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REFERENCES
(1) Steele, B. C. H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 345−352. (2) Wang, Y. J.; Zhao, N. N.; Fang, B. Z.; Li, H.; Bi, X. T. T.; Wang, H. J. Carbon-Supported Pt-Based Alloy Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle Size, Shape, and Composition Manipulation and Their Impact to Activity. Chem. Rev. 2015, 115, 3433−3467. (3) Chen, A. C.; Holt-Hindle, P. Platinum-Based Nanostructured Materials: Synthesis, Properties, and Applications. Chem. Rev. 2010, 110, 3767−3804. (4) Shao, M. H.; Chang, Q. W.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594−3657. (5) Carpenter, M. K.; Moylan, T. E.; Kukreja, R. S.; Atwan, M. H.; Tessema, M. M. Solvothermal Synthesis of Platinum Alloy Nanoparticles for Oxygen Reduction Electrocatalysis. J. Am. Chem. Soc. 2012, 134, 8535−8542. (6) Jung, N.; Bhattacharjee, S.; Gautam, S.; Park, H. Y.; Ryu, J.; Chung, Y. H.; Lee, S. Y.; Jang, I.; Jang, J. H.; Park, S. H.; Chung, D. Y.; Sung, Y. E.; Chae, K. H.; Waghmare, U. V.; Lee, S. C.; Yoo, S. J. Organic-Inorganic Hybrid Ptco Nanoparticle with High Electrocatalytic Activity and Durability for Oxygen Reduction. NPG Asia Mater. 2016, 8, e237. (7) Shui, J. I.; Chen, C.; Li, J. C. M. Evolution of Nanoporous Pt-Fe Alloy Nanowires by Dealloying and Their Catalytic Property for Oxygen Reduction Reaction. Adv. Funct. Mater. 2011, 21, 3357−3362. (8) Kattel, S.; Duan, Z. Y.; Wang, G. F. Density Functional Theory Study of an Oxygen Reduction Reaction on a Pt3ti Alloy Electrocatalyst. J. Phys. Chem. C 2013, 117, 7107−7113. (9) Sode, A.; Li, W.; Yang, Y. G.; Wong, P. C.; Gyenge, E.; Mitchell, K. A. R.; Bizzotto, D. Electrochemical Formation of a Pt/Zn Alloy and Its Use as a Catalyst for Oxygen Reduction Reaction in Fuel Cells. J. Phys. Chem. B 2006, 110, 8715−8722. (10) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3ni(111) Via Increased Surface Site Availability. Science 2007, 315, 493−497. (11) Cui, C. H.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Compositional Segregation in Shaped Pt Alloy Nanoparticles and Their Structural Behaviour During Electrocatalysis. Nat. Mater. 2013, 12, 765−771. (12) Wang, X.; Choi, S. I.; Roling, L. T.; Luo, M.; Ma, C.; Zhang, L.; Chi, M. F.; Liu, J. Y.; Xie, Z. X.; Herron, J. A.; Mavrikakis, M.; Xia, Y. N. Palladium-Platinum Core-Shell Icosahedra with Substantially
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CONCLUSIONS In summary, a facile solvothermal approach combined with subsequent mild thermal annealing for the creation of PtNi nanoparticles encapsulated in a few carbon layers has been developed. The results show that the Pt ions are the critical factor to construct the precursor and the Ni is the key promoter to form the few carbon layers at mildly elevated temperature. The PtNi@C-2 exhibits a much higher catalytic activity toward ORR than a commerical Pt/C catalyst, even after 15000 potential cycles between 0.6 and 1.1 V in O2saturated 0.1 M HClO4. The high activity and durability (in terms of ECSA at present) of PtNi@C-2 demonstrate that carbon encapsulation is a promising approach for improving the durability of high performance PtNi ORR catalysts.
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adsorption/desorption curves of MPN; EDX results of PtNi@C; XRD of Pt nanoparticles; LSV for ORR on Pt nanoparticles; TGA curves for MPN and PtNi@C; CO stripping voltammogram for PtNi@C-2; and an ORR activity comparison table (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00106. Photographs of samples prepared under different experimental conditions; TEM images of various control samples; FTIR spectra of 2-MIM and MPN; nitrogen 2776
DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778
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DOI: 10.1021/acsaem.9b00106 ACS Appl. Energy Mater. 2019, 2, 2769−2778