Synthesis of Fe-doped octahedral Pt₃Ni nanocrystals with high electro-catalytic activity and stability towards oxygen reduction reaction

Abstract

Octahedral Pt₃Ni nanocrystals doped with Fe (Pt₃Ni-Fe) are synthesized through solvothermal methods in N,N-dimethylformamide solvent at 210 °C. By manipulating the amount of combinational capping agents, the size of the octahedral Pt₃Ni-Fe nanocrystals can be controlled between 7 and 19 nm. It is found that one of the capping agents, n-octadecylphosphonic acid, is critical in obtaining the octahedral morphology. A volcano-shaped relation between oxygen reduction reaction (ORR) activity and nanocrystal size is determined for the carbon-supported octahedral Pt₃Ni-Fe, with the highest ORR activity observed from the 13 nm nanocrystal. It is also found that Pt₃Ni-Fe electro-catalysts show enhanced stability. This work demonstrates that Fe-doping can enhance the electro-catalytic performance of octahedral Pt₃Ni.

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Pt-based alloy materials have gained enormous attention during the last few decades, due to their applications in electro-catalysis, petrochemical catalysis, photochemistry, and other energy conversion systems.^1–6 It has been confirmed that Pt-based alloys show enhanced electro-catalytic performance compared with traditional Pt catalysts in proton exchange membrane fuel cell (PEMFC), especially towards oxygen reduction reaction (ORR) at the cathode.^7–13 Systematic studies have been carried out for bimetallic core–shell-structured or alloy nanocatalysts consisting of Pt and late transition metals, e.g. Ni, Co, Cu, Fe, and Mn etc.^14–20 Large amount of studies have revealed that modified surface electronic properties and surface atomic arrangement account for their drastically enhanced ORR activities.^21–24

Electro-catalysis of highly-active Pt₃Ni single crystal was studied by Stamenkovic and coworkers, and it was found that Pt₃Ni(111) facet shows the highest activity among low-index facets with a 10-time enhancement in terms of ORR activities compared with Pt(111) surface.²⁵ The enhancement of activity is attributed to the modified surface electronic structure induced by the surface compositional profiles enriched with Pt, as well as the atomic arrangement.^25,26 For nano-sized electro-catalysts, it is known that their morphologies determine atomic arrangements on exposed facets, and thus affect their catalytic activities.^27,28 Pt-based bimetallic alloy nanocatalysts have been synthesized into various morphologies to demonstrate the distinguished activities induced by specific exposed facets, such as cuboctahedral, icosahedral, cubic nanocrystals (NCs), etc.^29–36 It has been reported by several groups that Pt₃Ni alloy NCs with octahedral morphology display particularly high specific and mass activities in ORR.^33–35

For Pt-based alloy electro-catalyst, it is generally considered that they suffer severe dissolution of transition metal element at high potential region.^37–39 Although the initial dissolution is beneficial to their ORR activities due to the formation of Pt-skin structure, the continuing dissolution is harmful to the electro-catalytic performance, which could be caused by the destruction of the Pt-skin as well as the ripening of NCs.^40–43 As a result, the stability issue of Pt-based alloys limits their applications. So far, very few efforts have been made to solve the stability issue of bimetallic alloys. Wang and coworkers prepared intermetallic Pt–Co alloy nanocatalysts supported on carbon black through high-temperature treatment, achieved 200% increase in mass activity, and more importantly, obtained minimal loss of activity after 5000 potential cycles.¹¹ However, for Pt–Ni bimetallic alloys, the stability issue remains a problem.

In this work, aiming to improve both the ORR activity and stability of Pt-based alloys, we synthesized octahedral Pt₃Ni NCs doped with Fe atoms, with sizes controllable between 7 and 19 nm. It is found that the doped Fe atoms not only enhances the specific activity on the exposed (111) facets towards ORR, but also improves the stability of the Pt–Ni NCs at high potentials.

We utilized a simple solvothermal approach to synthesize the NCs. A combination of oleylamine (OAm), oleic acid (OAc), and n-octadecylphosphonic acid (ODPA) was used as capping agent. Pt(acac)₂, Ni(acac)₂, and Fe(acac)₃ were used as metal precursors, and N,N-dimethylformamide (DMF) was used as solvent, complexing and reducing agent, to prepare the Fe-doped octahedral Pt₃Ni NCs (denoted as Pt₃Ni-Fe). In earlier reports, Fe(acac)₂ and Fe(CO)₅ were commonly employed as precursors of iron in solvothermal syntheses carried out below 180 °C because they can be easily reduced or thermally decomposed.^44,45 However, it was found in our work that, in order to obtain the octahedral morphology at temperatures above 200 °C, Fe(acac)₃ needs to be used as the precursor of iron. Interestingly, when using Fe(acac)₂ as the starting chemical, no octahedral NCs can be obtained even when only trace amount of Fe(acac)₂ was added, as shown in Fig. S1.†

The high resolution TEM image in Fig. 1 shows the atomic lattices of a 7 nm NC, in which the 1.96 and 2.24 Å correspond to the d-spacings of (200) and (111) crystallographic planes, lower than the d-spacings of (200) and (111) planes of pure Pt, indicating that there is a shrink of lattice induced by the incorporation of Ni and Fe with smaller atomic radius. Fig. 1(c) shows the fast Fourier transfer (FFT) graph obtained from the NC in Fig. 1(b), consistent with the FFT of face-centered cubic (FCC) crystal structure taken from 〈110〉 zone axis. Fig. 1(d) shows the schematic of an octahedron with similar 2D projection with NC in Fig. 1(b). The EDX spectrum (Fig. S2†), obtained from a single octahedral NC with a composition Pt/Ni/Fe = 71/24/5, is consistent with the bulk composition measured from ICP-OES (68/26/6), indicating that the composition is uniform for all NCs.

Fig. 1 (a) TEM and (b) HRTEM images of octahedral Pt₃Ni-Fe NCs. (c) FFT of the single NC shown in (b) along 〈110〉 zone axis. (d) Schematic of an octahedral NC with the same 2D projection as (b).

By manipulating the ratio of OAm/OAc, octahedral Pt₃Ni-Fe NCs with different sizes can be synthesized. It was found that, when increasing the OAm/OAc ratio in the starting solution, alloy NCs maintain the octahedral morphology but show a larger size. It is possibly due to the fact that OAm can form a stable complex with metal atoms, meanwhile, it serves as an alternative reducing agent.⁴⁶ Combining the above two effects, the existence of more OAm molecules leads to the formation of lower number of nuclei and thus larger NC size.⁴⁷ In this work, octahedral NCs with sizes between 7 and 19 nm were synthesized with very similar Pt/Ni/Fe ratios, as shown in Fig. 2. It can be seen that the 7 and 19 nm octahedral NCs show narrower size distributions while the 13 nm NCs show a wider distribution. It is worth noting that for the 7 nm octahedral Pt₃Ni-Fe NCs, the TEM image shows that they attach to a soft material with stripes which are the residues of unwashed ODPA. The NCs would otherwise severely aggregate if the ODPA molecules were fully removed. Acetone can be used to effectively remove the ODPA molecules under ultrasonication. It should also be noted that although the existence of ODPA is critical in achieving the octahedral morphology, neither shape nor size of NCs is sensitive to the amount of ODPA. To achieve a fair comparison in electrochemical study, we also synthesized octahedral Pt₃Ni NCs with a size of 10 nm as shown in Fig. S3.† It is generally considered that low temperature is beneficial to the shape formation of NCs, but this work demonstrates that the shape-controlled synthesis can also be achieved at temperatures above 200 °C in DMF solvent.³⁵

Fig. 2 TEM images of octahedral NCs with size of (a and b) 19 nm, (d and e) 13 nm, and (g and h) 7 nm. (c, f and i) Schematics showing 2D projections of respective NCs in (b, e and h).

To further explore the formation mechanism of the octahedral NCs, we studied the influence of ODPA which serves as one of the capping agents. It can be indicated from Fig. S4† that, with the addition of ODPA, the morphology of the NCs transformed from cuboctahedron to octahedron, meaning that the (100) facets disappear with the existence of ODPA, which indicates that ODPA molecules prefer to bind to (111) facets, and thus stabilize (111) facets. However, when using ODPA in the absence of OAm/OAc, most NCs obtained show cuboctahedral morphology, which implies that, with only ODPA, the growth in 〈100〉 direction can also be suppressed to some extent. It suggests that the formation of octahedral morphology is complicated, which may involve the interactions among OAm, OAc and ODPA molecules. It is possible that the binding of these capping molecules to (100)/(111) facets can be affected by other species. It should also be noted that the addition of ODPA affects the composition of the final NCs. With the increase of initial ODPA concentration, less Ni atoms can be included into the NCs, confirmed by both EDX and ICP. It is speculated that ODPA can form more stable complexes with Ni than with other metallic species. As a result, it is more difficult to fully reduce the Ni into atoms, and thus lower the Ni/Pt ratio in the final NCs compared with the ratio of the starting chemicals.⁴⁸

To prevent the aggregation of the capping-agent-removed octahedral NCs and utilize them as a practical and stable electro-catalyst, the NCs were loaded onto the high-surface-area carbon black (Vulcan XC-72, CB). Fig. S5(a) and (b)† show the 7 and 19 nm octahedral NCs supported on commercial CB. The NCs are uniformly dispersed on CB particles. To eliminate the residues of capping agents binding on NC surface, and make the NCs more approachable by electrolyte and reactants, a standard thermal treatment procedure within 260–280 °C was employed for all catalysts. The temperatures were cautiously selected because the morphology of the NCs have to maintain octahedral morphology during the thermal treatment at the presence of oxygen, whereas the organic residues should be fully oxidized and leave the surface in the temperature window. Generally, the catalysts were heated in air for 2 hours, followed with a flow of H₂–N₂ (1/9) mixed gas for another 2 hours at the same temperature. Fig. S5(c) and (d)† show the morphology of NCs after thermal treatment, indicating that the treated NCs can remain octahedral with sharp edges.

To evaluate the electro-catalytic ORR activities of the carbon-supported octahedral NCs, the catalysts were loaded onto glassy carbon electrode (GCE). The masses of loaded metals (including all metallic elements) were similar for all catalysts around 16 μg cm², with the exact loading mass depending on the quality of catalyst films.

In this work, the electrode was prepared by a two-step approach. The carbon-supported electro-catalysts form the first layer, followed by a Nafion® film formed by drying 10 μL of 100-time-diluted Nafion® solution on top. The electrochemical surface area (ECSA) was determined by integration of charges induced by hydrogen adsorption/desorption (between 0.05 and 0.40 V vs. RHE) in cyclic voltammograms (CV) curves in Fig. S6.† Table S1† lists the ECSAs of electro-catalysts with octahedral NCs of different sizes. It can be inferred that with the size increase of octahedral NC from 7 to 19 nm, ECSA does not decrease in an expected trend. The theoretical ECSAs calculated for the 7 and 19 nm octahedral NCs with the same composition are 70.1 and 30.2 m² g⁻¹ respectively, both of which are higher than the measured values, especially for the 7 nm octahedral NCs. Hydrogen adsorption/desorption behaviour is sensitive to Pt atoms but affected by Ni or Fe atoms, so the ECSAs determined in this method can only be used as a rough estimation of Pt sites on the surface.⁴⁹ As a result, the low ECSAs of the catalysts indicate that the surface composition is different from the bulk composition. XPS results (Fig. S7†) of 7 nm octahedral NCs reveal that Pt/Ni/Fe ratio on the surface is approximately 17/42/41, which deviates significantly from the bulk composition (68/26/6). It implies that the surface of the 7 nm NC is rich in Ni and Fe, leading to the low ECSA value. Among the three metallic precursors, the reduction barrier is the lowest for Pt(acac)₂, so it is the first element to be reduced, forming the main component of the seeds and auto-catalyzing the reduction of Ni and Fe precursors. This is consistent with the finding from Cui et al.³⁵ The measured ECSAs for octahedral Pt₃Ni and commercial Pt/C catalysts purchased from Johnson Matthey (JM) are 20.1 and 68.8 m² g_Pt⁻¹, respectively.

All electro-catalysts were electrochemically activated with cyclic potential scans between 0 and 1.2 V. The CV curves are usually stabilized within approximately 50 cycles, after which the ORR catalytic activities are quantitatively measured. The effects of NC size and Fe-doping on the ORR activity were studied. The electrode was linearly polarized between 0.1 and 1.0 V at the scanning rate of 20 mV s⁻¹ and the rotating rate of 1600 rpm in O₂-saturated 0.1 M HClO₄ electrolyte at room temperature. Linear scanning voltammetry (polarization curve) was recorded after the voltammogram was stabilized, as shown in Fig. 3. The 7 nm Pt₃Ni-Fe shows higher specific and mass activities than Pt₃Ni, indicating that the inclusion of Fe enhances the catalytic activity of the octahedral NCs.

Fig. 3 (a) LSVs of different electro-catalysts in 0.1 M HClO₄ saturated with oxygen. (b) Mass-transport-corrected specific and mass activities at 0.9 V. (Red column: specific activity; black column: mass activity.)

With first-principle calculation, the density of states (DOS) in d-band is calculated for Pt₃Ni and Pt₃Ni-Fe surfaces. The Pt₃Ni-Fe surface model is constructed by replacing one Pt atom with Fe on the top atomic layer of Pt₃Ni surface. It can be inferred that DOS profile in d-band (Fig. S8†) is higher at energies close to the Fermi level, but is relatively lower at energies far below. It means that there are more active electrons when Fe atom is doped at the top surface of Pt₃Ni, leading to faster electron transfer and thus higher ORR activity.²

In addition, it is surprising to find that for octahedral Pt₃Ni-Fe electro-catalysts, specific and mass activities do not show monotonic behaviour with the size of NCs. When the size increases from 7 to 13 nm, there is a 50% enhancement in specific activity, from 0.85 to 1.28 mA cm_Pt⁻², and almost a two-fold enhancement in mass activity, from 0.19 to 0.37 A mg_Pt⁻¹, at 0.9 V (vs. RHE). However, when the size of the octahedral NCs increases from 13 to 19 nm, the specific activity decreases again, to 0.92 mA cm⁻², and the mass activity falls to 0.19 A mg⁻¹. For comparison, the commercial JM Pt/C catalyst exhibits a specific activity of 0.17 mA cm⁻² and a mass activity of 0.13 A mg⁻¹, consistent with previously reported results.² It can be concluded that the 13 nm octahedral Pt₃Ni-Fe NC shows the highest electro-catalytic activity, approximately 8-time enhancement in specific activity and 3-time improvement in mass activity compared with commercial catalysts. It is generally agreed that the electro-catalytic activity will increase with the size for unsupported NCs.⁵⁰ For the octahedral Pt₃Ni-Fe NCs supported on CB in this work, the synergism between NCs and support, affected by the size of NCs, will also influence the electro-catalytic activities.⁵¹ In this work, the mass ratio of NC/CB is the same for all catalysts, implying that the dispersion of catalysts on CB with larger NC size is lower, as shown in Fig. S9.† The low dispersion of NCs on CB induces the enhanced electrical double layer capacitance, as well as the low synergism between NCs and the support, which leads to the detriment to electron transfer among oxygen molecule, NC surface and the support.^51–53 As a result, the NCs supported on CB show the volcano-shaped behaviour with the increase of NC size.

The durability test was carried out to evaluate the stability of the electro-catalysts. A total of 16 000 potential cycles were scanned between 0.6 and 1.1 V at the scan rate of 50 mV s⁻¹ for each sample, and the ECSAs, specific and mass activities were determined every 4000 cycles. The last CV and polarization curves after 16 000 potential scans were compared with initial curves, as shown in Fig. 4. JM Pt/C catalyst loses about 40% of its initial ECSA, and loses 40% and 36% of the specific activity and mass activity, respectively. However, the 13 nm octahedral Pt₃Ni-Fe retains approximately 95% of the starting ECSA, and loses only 25% of both the specific and mass activities. The durability of 10 nm Pt₃Ni/C catalysts was also tested as comparison (Fig. S10†). The result implies that the stability of octahedral Pt₃Ni with Fe-doping is similar to the undoped NC. For nanoscale electro-catalyst, it is common that the size and shape are difficult to retain after long time exposure to high potentials. In this work, it can be seen that the NCs will remain octahedral morphology with sharp edges and almost the same size as the initial 19 nm Pt₃Ni-Fe/C catalyst (Fig. S11†). The Pt/Ni/Fe ratio, determined by EDX, is 79/17/4 after the durability test, compared with the initial composition 71/24/5, indicating that there is little dissolution of Ni and Fe at high potentials. The durability test reveals that the octahedral Pt₃Ni-Fe/C not only enhances the electro-catalytic activities but also improves the stability.

Fig. 4 (a) CVs and (b) LSVs of JM Pt/C before and after the 16 000 CV potential cycles. (c and d) for 13 nm octahedral Pt₃Ni-Fe/C catalysts. (Solid: initial curve, dashed: final curve after durability test.)

Conclusions

By using combination of OAc, OAm, and ODPA as capping agents with appropriate ratios, we have synthesized size-controllable Fe-doped octahedral Pt₃Ni-Fe NCs, from 7 to 19 nm in size. After being loaded onto CB support, the octahedral Pt₃Ni-Fe/C catalysts display a volcano-shaped relationship between electro-catalytic activity and the NC size, with the highest activities observed from the 13 nm Pt₃Ni-Fe/C, as high as 8-time and 3-time enhancement in specific and mass activities respectively compared with the state-of-the-art JM Pt/C catalyst. The high activities of Pt₃Ni-Fe/C can be attributed to the modification of distribution of DOS in d-band induced by the inclusion of iron atoms on NC surface. The octahedral Pt₃Ni-Fe/C catalysts also display impressive stabilities in catalytic activity, size, morphology and composition after accelerated durability test, implying the possibility of utilizing them as practical cathode electro-catalysts in fuel cells. The structure–activity relationships provide a new approach of designing ternary alloy fuel cell catalysts with high activity and stability.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, TEM images, element analysis, electrochemical results, first principal calculation results mentioned in the main text. See DOI: 10.1039/c3ra46299d

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