Composition-driven shape evolution to Cu-rich PtCu octahedral alloy nanocrystals as superior bifunctional catalysts for methanol oxidation and oxygen reduction reaction

Chaozhong Li ab, Taiyang Liu a, Ting He b, Bing Ni b, Qiang Yuan *ab and Xun Wang *b
aDepartment of Chemistry, College of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou province 550025, P. R. China. E-mail: qyuan@gzu.edu.cn
bDepartment of Chemistry, Tsinghua University, Beijing 100084, P. R. China. E-mail: wangxun@mail.tsinghua.edu.cn

Received 28th December 2017 , Accepted 5th February 2018

First published on 6th February 2018


Synergetic effects between Pt and a cheap metal, downshift of the d-band center of Pt and the shape can boost the catalytic performance of Pt-based nanocrystals. Therefore, tailoring the shape and composition within the nanoscale is the key to designing a robust electrocatalyst in electrochemical energy conversion. Here, Cu-rich PtCu octahedral alloys achieved by a composition-driven shape evolution route have been used as outstanding bifunctional electrocatalysts for both methanol oxidation (MOR) and oxygen reduction reaction (ORR) in an acid medium. When benchmarked against commercial Pt black or Pt/C, for MOR, the specific activity/mass activity on Pt34.5Cu65.5 octahedra is 4.74/7.53 times higher than that on commercial Pt black; for ORR, the specific activity/mass activity on Pt34.5Cu65.5 octahedra is 7.7/4.2 times higher than that on commercial Pt/C. After a current–time test for 3600 s, the remaining mass activity on Pt34.5Cu65.5 octahedra is 35.5 times higher than that on commercial Pt black for MOR. After undergoing 5000 cycles for ORR, the remaining mass activity on Pt34.5Cu65.5 octahedra is 4.2 times higher than that on commercial Pt/C.


Recently, well-defined Pt-based nanocrystals have been paid great attention at the nanoscale because the geometrical shape of Pt-based nanocrystals can tailor their fundamental applications in catalysis through controlling surface active sites such as facets, edges and corners.1–5 For example, Yan and co-workers have shown facet-dependent electrocatalytic activity and durability of monodisperse sub-10 nm Pt–Pd tetrahedra and cubes toward methanol oxidation reaction (MOR).6 Yang and co-workers have shown a shape-dependent electrocatalytic activity of Pt3Ni nanocrystals toward oxygen reduction reaction (ORR) with an increasing order from cube, octahedron to icosahedron.7,8 Therefore, so far, the shape-controllable synthesis of well-defined Pt-based nanoalloys has been regarded as one of the most promising strategies to maximize their catalytic activity advantages.9–11

On the other hand, Pt-based nanocrystals are an irreplaceable part of the anode and cathode catalytic reactions in fuel cells that are considered as promising, clean and sustainable systems of energy generation. However, the sluggish kinetics, high cost, low activity and stability of the Pt-based nanocrystals tremendously obstruct their large-scale commercial application in fuel cells. Up to now, studies have shown that Pt alloying with earth-abundant elements (Cu, Ni, Co, Fe, Mn, and Pb) can not only reduce the cost but also boost the catalytic performance compared to the standard commercial Pt black or Pt/C.12–18 Because Pt alloying with a second element can offer three main advantages, that is the synergetic effect between elements, the strain effect induced by the lattice mismatch of different elements and the downshift of the d-band center of Pt,19–23 these would tune the surface catalytic capacity of Pt-based nanoalloys and be responsible for the high catalytic performance. Among these Pt-based nanoalloys, PtCu nanoalloys have been predicted to be promising bifunctional electrocatalysts toward both MOR in anode reaction and ORR in cathode reaction by employing DFT calculations24–26 because there are many merits of the nanosurface of the PtCu alloy such as strain effects triggered by stretching/compressing of the alloy lattice, ensemble effects on adsorbate binding and the obvious synergic effects. Such effects allow for improving the capacities to activate methanol, water, oxygen and the surface intermediates (such as CO˙ and OH˙), which leads to enhancement of the catalytic properties of MOR and ORR in an acid medium. Although many studies have shown the PtCu nanoalloys with enhanced catalytic properties on MOR27–36 or ORR37–41 in an acid medium, there are limited reports on high-performance bifunctional PtCu nanoalloys for both MOR and ORR applications in an acid medium. Therefore, the development of high-performance bifunctional PtCu nanoalloys toward MOR together with ORR is of significance in fundamental fuel cell alloy electrocatalysis.

Herein we introduce an effective one-pot approach to prepare Cu-rich PtCu octahedral alloy nanocrystals by varying the ratio of Cu(acac)2 and Pt(acac)2. To the best of our knowledge, this is the first example that Cu-rich PtCu octahedral alloy nanocrystals can be achieved by composition-driving of Pt and Cu precursors. The as-synthesized PtCu octahedral alloy nanocrystals exhibit very superior bifunctional performance toward MOR and ORR in an acid medium compared with commercial Pt black or Pt/C. For MOR, the mass activity/specific activity on PtCu octahedral alloy nanocrystals is 7.5/4.7 times higher than that on commercial Pt black. For ORR, the mass activity/specific activity on PtCu octahedral alloy nanocrystals is 4.2/7.7 times higher than that on commercial Pt black, respectively. Besides, the PtCu octahedral alloy nanocrystals display a much better stability than commercial Pt black or Pt/C for MOR or ORR.

Fig. 1A–F show the representative transmission electron microscopy (TEM) images of the as-synthesized PtCu nanocrystals collected by varying the molar ratio of Pt and Cu precursors under the same synthesis process conditions. As can be seen, when there are no Cu precursors in the synthesis system, the products consisted of uniform nanocubes with a size of around 3.2 nm (Fig. 1A and Fig. S1B, ESI). When the molar ratio of Pt[thin space (1/6-em)]:[thin space (1/6-em)]Cu was 3[thin space (1/6-em)]:[thin space (1/6-em)]1, the products consisted of lots of sub-5.0 nm small grains together with a few octahedral nanocrystals. With the amount of Cu precursors increasing, the octahedral nanocrystals were also increasing (Fig. 1B–D). When the molar ratio of Pt[thin space (1/6-em)]:[thin space (1/6-em)]Cu was increased to 1[thin space (1/6-em)]:[thin space (1/6-em)]2, the final products were almost entirely uniform octahedra (Fig. 1E and F and Fig. S1B and S2, ESI). The average size of PtCu2 was around 24.6 ± 0.6 nm. The composition of PtCu2 octahedra was Pt34.5Cu65.5 determined by using an inductively coupled plasma optical emission spectrometer (ICP-OES), which was in very good accordance with the feed ratio of metal precursors (Table S1). This implied that the metal precursors can be quantitatively converted on the basis of the feeding ratio in this system. Four peaks were shown in the X-ray diffraction (XRD) patterns (Fig. 2A) of these as-synthesized PtCu nanoparticles and can be indexed to (111), (200), (220) and (311) diffractions of a face-centered-cubic (fcc) structure. And these peaks were located between standard Pt peaks (JCPDS-65-2868) and Cu peaks (JCPDS-04-0836) that indicated the alloy formation of Pt and Cu. Furthermore, the peaks were shifted to a high angle with the increasing Cu content because of the incorporation of Cu atoms into the Pt fcc lattice.14,42,43 The alloy structure is further confirmed by the EDX line scanning (Fig. 2B) and elemental analysis mapping (Fig. 2C–F). Fig. 2B–F displayed the line-scan EDX spectra and elemental analysis mapping of the as-synthesized Pt34.5Cu65.5 octahedra. The signals of Pt and Cu distributed in the whole particles and the signal of Cu is obviously stronger than Pt, which meant that the Cu was rich in the as-synthesized Pt34.5Cu65.5 alloy octahedra. On the other hand, the results of X-ray photoelectron spectroscopy (XPS; Fig. 3) also confirmed that the surface of as-synthesized Pt34.5Cu65.5 alloy octahedra was Cu-rich, and the molar percentage of Cu[thin space (1/6-em)]:[thin space (1/6-em)]Pt was 60[thin space (1/6-em)]:[thin space (1/6-em)]40 in the surface. The binding energy of the Pt 4f7/2 and Pt 4f5/2 peaks (Fig. 3A) shifted to 71.0 eV and 74.3 eV compared to the standard peak of Pt (71.2 eV and 74.5 eV), and that of the Cu 2p3/2 an Cu 2p1/2 (Fig. 3B) shifted to 931.1 eV and 951.1 eV compared to the standard peak of Cu (931.6 eV and 951.6 eV), which demonstrated the strong electron effects between Pt and Cu and would improve the catalytic capacities of Pt34.5Cu65.5 alloy octahedra.44–46


image file: c7nr09669k-f1.tif
Fig. 1 TEM images of the as-synthesized Pt cubes (A), Pt73.3Cu26.7 (B), Pt63.9Cu36.1 (C), Pt52.0Cu48.0 (D), Pt34.5Cu65.5 octahedra (E) and HAADF-STEM images of Pt34.5Cu65.5 octahedra (F). The composition was determined by ICP-OES (Table S1).

image file: c7nr09669k-f2.tif
Fig. 2 XRD patterns of the as-synthesized Pt cubes, Pt73.3Cu26.7, Pt63.9Cu36.1, Pt52.0Cu48.0, and Pt34.5Cu65.5 octahedra (A), EDX line scanning profiles of a single Pt34.5Cu65.5 octahedron (B); HAADF-STEM images (C) and the corresponding elemental maps of (D) Pt (red), (E) Cu (yellow) and (F) overlap (Pt and Cu) of as-synthesized of Pt34.5Cu65.5 octahedra.

image file: c7nr09669k-f3.tif
Fig. 3 The XPS spectra of the as-synthesized Pt34.5Cu65.5 octahedra. (A) Pt 4f region and (B) Cu 2p region.

The electrocatalytic performance of octahedral Pt34.5Cu65.5 nanoalloys has been investigated in acid media toward methanol oxidation and oxygen reduction compared with Pt cubes and commercial Pt black or Pt/C. Fig. S3, ESI, shows the cyclic voltammograms (CVs) of these catalysts in 0.1 M H2SO4 solution at a scan rate of 50 mV s−1 at room temperature. The electrochemically active surface area (ECSA) of octahedral Pt34.5Cu65.5 nanoalloys, Pt cubes, commercial Pt black and Pt/C was 34.7, 16.1, 21.8 and 63.6 m2 gPt−1, respectively. The MOR was tested in 0.1 M H2SO4 + 0.5 M methanol solution with a scan rate of 50 mV s−1 (Fig. 4A–C). The specific activity/mass activity (J) on octahedral Pt34.5Cu65.5 nanoalloys, Pt cubes and commercial Pt black was 4.12 mA cm−2/1.43 mgPt−1, 0.87 mA cm−2/0.14 A mgPt−1 and 0.87 mA cm−2/0.19 A mgPt−1, respectively, which demonstrated that the specific activity/mass activity on Pt34.5Cu65.5 octahedra is 4.74/7.53 times higher than that on the commercial Pt black (Fig. 4A–C). The ORR polarization curves of octahedral Pt34.5Cu65.5 nanoalloys, Pt cubes and commercial Pt/C were obtained in an O2-saturated 0.1 M H2SO4 solution with a scan rate of 10 mV S−1 and a rotation rate of 1600 rpm at room temperature (Fig. 4D). The half-wave potential for octahedral Pt34.5Cu65.5 nanoalloys (0.94 V) was higher than that for Pt cubes (0.89 V) and commercial Pt/C (0.86 V), which implied that the octahedral Pt34.5Cu65.5 nanoalloys had a greatly reduced ORR overpotential. In order to compare the catalytic activity, we made use of the Koutecky–Levich equation to normalize the current density by ECSA and Pt loading at 0.9 V versus a reversible hydrogen electrode (RHE) (Fig. 4E). Observably, the octahedral Pt34.5Cu65.5 nanoalloys had the best catalytic activity among these catalysts, and the specific activity/mass activity on octahedral Pt34.5Cu65.5 nanoalloys, Pt cubes and commercial Pt/C was 1.70 mA cm−2/0.59 A mgPt−1, 0.75 mA cm−2/0.12 A mgPt−1 and 0.22 mA cm−2/0.14 A mgPt−1, respectively. The octahedral Pt34.5Cu65.5 nanoalloys showed 7.73 times and 4.21 times higher specific activity and mass activity than commercial Pt/C (Fig. 4E). The Tafel plots (Fig. 4F) also indicated that the octahedral Pt34.5Cu65.5 nanoalloys exhibited much higher activity than commercial Pt/C. Furthermore, according to the equation of ΔEact = kBT[thin space (1/6-em)]ln(SAPt34.5Cu65.5/SAPt/C),47,48Eact stands for the difference in activation energy, kB stands for Boltzmann's constant, T stands for temperature, SAPt34.5Cu65.5 and SAPt/C stand for the specific activity of octahedral Pt34.5Cu65.5 nanoalloys and commercial Pt/C in the equation, respectively) the activation energy for ORR on the octahedral Pt34.5Cu65.5 nanoalloys was reduced by 0.053 eV relative to commercial Pt/C, which meant that oxygen was more easily reduced by the octahedral Pt34.5Cu65.5 nanoalloys than the commercial Pt/C. The enhancement of catalytic performance of the octahedral Pt34.5Cu65.5 nanoalloys may be ascribed to the ensemble effect including the octahedral shape effect that exhibits eight {111} facets, twelve edges and six corners, composition effect that resulted in a synergic effect between Pt and Cu atoms and the downshift of the d-band center of Pt (Fig. 3A).24,42–46,49 Impressively, our Pt34.5Cu65.5 octahedron had a competitively high mass activity achieved in an acid medium in comparison with previously reported PtCu nanocrystals (Tables S2 and S3, ESI).


image file: c7nr09669k-f4.tif
Fig. 4 Cyclic voltammograms (specific activity (A) and mass activity (B)) of the as-synthesized Pt cubes, Pt34.5Cu65.5 octahedra and the commercial Pt black with a scan rate of 50 mV s−1 in 0.1 M H2SO4 + 0.5 M CH3OH solution at room temperature. (C) Histograms of MOR specific activity and mass activity. (D) Background-corrected ORR polarization curves. (E) Histograms of specific activity and mass activity of Pt cubes, Pt34.5Cu65.5 octahedra and the commercial Pt/C at 0.9 V recorded at room temperature in an O2-saturated 0.1 M H2SO4 solution with a scan rate of 10 mV s−1 and a rotation rate of 1600 rpm. (F) Specific activity Tafel plots for Pt34.5Cu65.5 octahedra and the commercial Pt/C.

The stability of the Pt34.5Cu65.5 octahedron toward MOR was tested using current–time curves recorded at 0.6 V for 3600 s (Fig. 5A), and the current density on Pt34.5Cu65.5 octahedra is much larger than those on Pt cubes and commercial Pt black over the entire time. After 3600 s, the remaining mass activity on Pt34.5Cu65.5 octahedra (0.21 A mgPt−1) was 35.5 times higher than that on commercial Pt black (0.0059 A mgPt−1), showing much higher stability of Pt34.5Cu65.5 octahedra than commercial Pt black. The LSV curves of CO oxidation also showed that Pt34.5Cu65.5 octahedra had a superior anti-CO poisoning ability compared to commercial Pt black (Fig. S4, ESI). The onset potential of Pt34.5Cu65.5 octahedra was obviously lower than that of commercial Pt black, and the peak potentials of Pt34.5Cu65.5 octahedra and commercial Pt black were 0.50 V and 0.65 V, respectively, which implied that it was easier to oxidize CO on Pt34.5Cu65.5 octahedra.50 Furthermore, the reactivated mass activity still remained 1.09 A mgPt−1 after the current–time test, which was still much higher than that of commercial Pt (0.19 A mgPt−1). Also, we performed the accelerated durability test for octahedral Pt34.5Cu65.5 nanoalloys and commercial Pt/C in an O2-saturated 0.1 M H2SO4 solution through linear potential scans with a potential range from 0.6 V to 1.0 V at 100 mV S−1. As can be seen, after 5000 cycles (Fig. 5C and D), the remaining mass activity of octahedral Pt34.5Cu65.5 nanoalloys was 0.42 A mgPt−1, which was 4.2 times higher than that of commercial Pt/C (0.10 A mgPt−1). These results indicated that the octahedral Pt34.5Cu65.5 nanoalloys had an excellent stability for both MOR and ORR. On the other hand, the TEM images showed that the Pt34.5Cu65.5 octahedra can also maintain the octahedral shape after the current–time test and accelerated durability test. However, some hollows emerged in the octahedra, which were caused by the electrochemical dealloying of Cu, and resulted in the decreasing of catalytic performance (Fig. S5, ESI). The EDS spectra showed that the atomic ratios of Pt[thin space (1/6-em)]:[thin space (1/6-em)]Cu were 74.0[thin space (1/6-em)]:[thin space (1/6-em)]26.0 and 93.9[thin space (1/6-em)]:[thin space (1/6-em)]6.1 after the current–time test (Fig. S6A, ESI) and accelerated durability test (Fig. S6B, ESI).


image file: c7nr09669k-f5.tif
Fig. 5 (A) Current–time curves of the Pt34.5Cu65.5 octahedra and the commercial Pt black recorded at 0.6 V for 3600 s in 0.1 M H2SO4 + 0.5 M CH3OH solution at room temperature, (B) mass activity comparison of MOR before and after the chronoamperometry test of Pt34.5Cu65.5 octahedra. (C) Background-corrected ORR polarization curves and (D) histograms of ORR mass activity summaries after 5000 cycles of an accelerated durability test between 0.6 and 1.1 V with a scan rate of 100 mV s−1.

Conclusions

In conclusion, we have demonstrated an example to prepare Cu-rich PtCu octahedral alloys based on composition-driven shape evolution. Owing to the ensemble effect including the octahedral shape, the synergic effect between Pt and Cu atoms and the downshift of the d-band center of Pt, the Pt34.5Cu65.5 octahedra exhibited much superior bifunctional performance including activity and stability toward MOR and ORR in acid medium benchmarked against commercial Pt or Pt/C. Thus, this work provides a novel synthetic strategy to develop a robust bifunctional PtCu electrocatalyst for both MOR and ORR in fuel cell applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21571038, 21361005, 91127040 and 21221062), the Open Fund of the Key Lab of Organic Optoelectronics & Molecular Engineering (Tsinghua University) and the State Key Laboratory of Physical Chemistry of Solid Surfaces (201520). We also thank Professor Zhi-You Zhou (Xiamen University) for help with ORR experiments and discussion.

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Footnote

Electronic supplementary information (ESI) available: Experimental details, TEM, HRTEM, CVs and tables. See DOI: 10.1039/c7nr09669k

This journal is © The Royal Society of Chemistry 2018