Enhancement of oxygen reduction reaction performance of Pt nanomaterials by 1-dimensional structure and Au alloying

Abstract

The effect of aspect ratios and Au alloying on the ORR performance of carbon-supported Pt nanorods (NRs) is investigated. The ORR activity of PtAu NRs after durability tests shows excellent stability due to the modification of electronic structure and surface composition.

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Strategies for the improvement of the state-of-the-art carbon-supported Pt nanocatalysts (Pt/C) towards the oxygen reduction reaction (ORR) include morphology control, alloying of components, core/shell structures, or a combination of these.¹ Through novel and complicated processes and modification,² the mass- and specific-activities (MA and SA) of Pt-based catalysts can reach the target of the Department of Energy (DOE),^3,4 which is almost unattainable for Pt/C prepared by conventional methods. Generally, ORR enhancement stems from ensemble, ligand and geometric effects,^2,5 a third type of enhancement caused by a change in electronic structure is a feasible mechanism that can be applied to improve ORR performance.^6,7 However, little attention has been paid to elucidate the intrinsic ORR performance of Pt nanomaterials resulting only from electronic modification effects, which can be a good indicator for the further improvement of Pt-based electrocatalysts toward ORR. A term, the number of unoccupied d-state (h_Ts) extracted from X-ray absorption near edge spectroscopy (XANES) has been used to measure the d-band vacancies of Pt, which is related strongly to their ORR performance.⁸ It has been reported that the d-band vacancy of Pt/C has been modified due to morphological change, alloying and supports, to different extents.⁶

Therefore, in this study, we have demonstrated the effect of electronic structure modification of Pt on its ORR performance. By simply tuning the aspect ratio of Pt nanorods (NRs) through a one-step chemical reduction process, the d-band states of Pt can be modified, improving the ORR performance accordingly. To the best of our knowledge, this is the first time that the effect of aspect ratio on the ORR durability of Pt/C has been discussed. Besides, Au is alloyed into the Pt NRs to further promote the stability.

Carbon-supported Pt and PtAu NRs with a metal loading of about 50 wt% were prepared by the formic acid method (FAM).^6,7,9–12 H₂PtCl₆ (Alfa Aesar) was mixed with carbon black (Vulcan XC-72R), and then reduced by formic acid (98%) at room temperature for 72, 144 and 216 h. The as-deposited samples were then washed and subsequently dried at 340 K for 24 h. The as-prepared Pt NRs with an aspect ratio of 1.84, 2.34, and 3.75 and reaction for 72, 144, and 216 h were named as Pt-1, Pt-2, and Pt-3, respectively. For the preparation of PtAu NRs with a Pt/Au atomic ratio of 3, H₂PtCl₆ mixed with carbon black was reduced by formic acid at room temperature for 216 h and HAuCl₄ (Aldrich) was added and reduced by formic acid for another 48 h. The as-prepared PtAu NRs were named as PtAu. In addition, the Pt/C (46 wt%, TKK, Tanaka Kikinzoku Kogyo) catalyst was used for comparison.

X-Ray photoelectron spectroscopy (XPS) (Thermo VG Scientific Sigma Probe) using a monochromatic X-ray source (Al K_α) at a voltage of 20 kV and a current of 30 mA was executed to identify the surface chemical states of the catalysts. The morphologies of the catalysts were analyzed by high resolution transmission electron microscopy (HR-TEM) operated at a voltage of 200 kV.

The electrochemical measurements were conducted by a CHI611C potentiostat and a classical electrochemical cell with a three-electrode configuration, as reported previously.^6,7 All potentials in this study was referred to the normal hydrogen electrode (NHE). The catalysts were dispersed in 2-propanol, blended with diluted Nafion solution (5 wt%, DuPont), and then deposited onto the glassy carbon rotating disk electrode (RDE, area of 0.196 cm²). The metal loading on a RDE was about 0.04 mg cm⁻². The oxygen reduction current was gauged by linear sweep voltammetry (LSV) with a scan rate of 5 mV s⁻¹ and a rotational rate of 1600 rpm. The accelerated durability tests (ADT) were obtained in the potential range of 0.6 to 1.2 V with an applied scan rate of 50 mV s⁻¹ under O₂ atmosphere for 1000 cycles. The cyclic voltammograms (CV) were obtained from 0.0 to 1.2 V with a scan rate of 20 mV s⁻¹ under N₂ atmosphere. The electrochemical surface area (ECSA) was calculated by measuring the areas of H desorption between 0.05 and 0.4 V after the deduction of the double-layer region. The kinetic current density (I_k) was calculated based on the following equation:

where I, I_k, and I_d are the experimentally measured, mass-transport-free-kinetic and diffusion-limited current density, respectively. For each electrocatalyst, the MA and SA were obtained when I_k was normalized to the Pt loading and ECSA, respectively.

X-Ray absorption spectroscopy (XAS) of catalysts was obtained in transmission or fluorescence mode using the BL01C1 and 17C beamlines at National Synchrotron Radiation Research Center (NSRRC), Taiwan, as reported previously.^6,7 A Si monochromator was employed to select adequately the energy with a resolution ΔE/E better than 10⁻⁴ at Pt L_II (13 273 eV) and L_III-edges (11 564 eV). Based on the spectra, the fractional change in the number of d-band vacancies relative to the reference material (f_d) can be estimated:

h_Ts = (1 + f_d)h_Tr (3)

where ΔA₂ and ΔA₃ are expressed by:

ΔA₂ = A_2s − A_2r and ΔA₃ = A_3s − A_3r (4)

The terms A₂ and A₃, indicating the areas under the L_II and L_III absorption edges of the sample (s) and reference (r) material as well as the calculated h_Ts, were evaluated from band structure calculations.

Fig. 1(a)–(d) shows that the as-prepared Pt-1, Pt-2 Pt-3 and PtAu NRs are well-dispersed on the carbon support with an aspect ratio of 1.84 ± 0.08, 2.34 ± 0.05, 3.75 ± 0.11, and 3.97 ± 0.15, respectively. The aspect ratio distribution histograms of NRs are displayed in Fig. S1 in the ESI.† As Pt NRs are stretching out from the surface of the carbon support, O₂ diffusion towards the Pt surface might be improved.¹² Hence, the ORR performances for NRs with a higher aspect ratio may be expected to have enhanced activity.¹² After 1000 cycles of ADT, as shown in Fig. 1(e)–(h), different degrees of Pt aggregation, migration and carbon corrosion occur.¹³ It is worth mentioning that rod-like structures with a diameter of 5 and 4 nm and slight aggregation can be still observed for the Pt-3 and PtAu NRs displayed in Fig. 1(g) and (h). It seems that the NRs with a high aspect ratio and/or Au alloying have good stability, suggesting that elongated Pt and/or Au nanostructures may lessen the effect of dissolution, Ostwald ripening, and aggregation in acidic conditions.^14,15

Fig. 1 HRTEM micrographs of catalysts before and after ADT for Pt-1 (a) and (e), Pt-2 (b) and (f), Pt-3 (c) and (g), and PtAu (d) and (h), respectively.

Fig. 2(a) represents the LSV of as-prepared Pt, PtAu NRs and Pt/C. The j (experimentally measured current density normalized to the geometric surface area of RDE) of Pt/C, Pt-1, Pt-2, Pt-3 and PtAu are 2.2, 2.6, 2.8, 3.0 and 3.1 mA cm⁻² at 0.88 V, respectively, implying that the ORR performance of Pt nanomaterials is aspect ratio-dependent. It has been reported that the use of Pt nanostructure with high aspect ratio may reduce considerably the voltage loss, without the cathode suffering from slow O₂ diffusion.¹² Moreover, the stability of various catalysts is measured by ADT, which causes surface oxidation/reduction cycles of Pt and the formation of PtOH and PtO originating in the oxidation of water, resulting in the dissolution of Pt through the Pt²⁺ oxidation state.¹⁶ After 1000 cycles of ADT, the LSV results shown in Fig. 2(b) demonstrate clearly that PtAu still maintains a high j at 0.88 V of 1.8 mA cm⁻² with a decay of 41%, while others decrease to about 0.4–1 mA cm⁻² with a decay in j about 67–82% as listed in Table 1. Au clusters can have a stabilizing effect on Pt, suppressing Pt dissolution during the applied conditions, and further promoting the durability of the catalysts.^9,15 The XPS fitting results summarized in Table 1 elucidate that the surface chemical compositions of various catalysts are covered by Pt and Pt oxide and the Pt/PtO ratio gradually increases while the aspect ratio of NRs increases. The Pt/PtO ratio of PtAu is the highest owing to its high aspect ratio and Au alloying. In order to get insight into the reason of ORR enhancement for NRs, the h_Ts values were measured by XANES spectra at Pt L_III and L_II edges. The Pt electrons transfer from 2p_3/2 to 5d_5/2 at 11 564 V and the intensity of the white line depends on the degree of Pt oxidation.⁸ The h_Ts values are 0.3380, 0.3195, 0.3188, 0.3145 and 0.3108 for Pt/C, Pt-1, Pt-2, Pt-3 and PtAu, respectively. The h_Ts value is decreased obviously due to the formation of Pt NRs, suggesting the morphological change affects the d-band structure of Pt. Moreover, the h_Ts value is decreased with the higher aspect ratio of Pt and alloying with Au. In addition, among the various catalysts, the h_Ts value for PtAu is the lowest, implying that more electrons transfer to the d-state, the Pt–O⁻ binding is weaker and the ORR performance is promoted.⁸ Moreover, the CV curve shown in Fig. 2(c) suggests that the oxide formation and reduction peak around 0.75 V¹⁷ of PtAu is also insignificant, implying that it is more difficult for Pt–O⁻ to form, consistent with the XPS results.¹⁸ Besides, the calculated ECSA of NRs listed in Table 1 is smaller than that of Pt/C owing to the morphological effect. Fig. 2(d) illustrates the correlation between MA and h_Ts of Pt/C and various Pt and PtAu NRs before and after ADT. This shows that PtAu with lower h_Ts have lower unoccupied d-states, weaker Pt–O⁻ bonds, less Pt oxide formation and less pronounced white lines, leading to the promotion of ORR kinetics and stability.¹⁰ The relationship of SA and h_Ts compared in Fig. S2 in ESI† emphasizes the effect of morphology and alloying on the ORR stability of Pt. It is worth mentioning that the j of as-prepared PtAu NRs at a voltage of <0.85 is lower than that of Pt-2 and Pt-3, as shown in Fig. 2(a), due to the coverage of some Pt surface atoms by an Au cluster. However, surface Au can really promote the stability through the electronic-modification effect. As a result, the enhancement of ORR performance may be due to the morphological effect, the changes in numbers of unoccupied d-states and the surface alloying of Au. It is found that the higher aspect ratio and Au alloying modifying the d-band structure can promote the ORR activity and stability of Pt catalysts.

Fig. 2 LSV results obtained in O₂-saturated 0.5 M HClO₄ (a) before and (b) after 1000 cycles of ADT, (c) CV scans obtained in 0.5 M HClO₄ saturated with N₂, and (d) the comparisons of h_Ts and MA at 0.88 V before and after ADT for Pt/C and various NRs.

Table 1 The comparisons of j, MA, SA, decay rates at 0.88 V, ECSA, and surface compositions for Pt/C and various NRs

Sample	j088 (mA cm^−2)	Decay (%)	ESCA (m^2 gPt^−1)	MA088 (mA mg^−1)	SA088 (mA cm^−2)	Pt/PtO (at%)
Pt/C	2.2	81.8	67.5	83.1	0.12	70/30
Pt-1	2.6	80.0	41.4	126.2	0.30	75/25
Pt-2	2.8	78.6	36.2	143.0	0.40	79/21
Pt-3	3.0	66.7	34.0	174.3	0.51	80/20
PtAu	3.1	41.0	24.7	198.0	0.80	89/11

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Conclusions

In this study, Pt NRs with different aspect ratios and/or Au alloying have been prepared by FAM. The ADT results of various NRs suggest that the ORR performance of Pt nanomaterials is aspect ratio-dependent since Pt NRs stretch out from the surface of the carbon support, and O₂ diffusion toward the Pt surface might be improved. After 1000 cycles of ADT, PtAu NRs still retain their original activity of about 76%. The ORR enhancement of PtAu NRs stems from their high aspect ratio and Au alloying, leading to high Pt/PtO surface composition and low Pt unoccupied d-states, further promoting the stability of the ORR.

Acknowledgements

This work was supported by the Ministry of Science and Technology of Taiwan (no. 103-2221-E-008-039 and 103-2120-M-007-005) and Ministry of Economics Affairs, Taiwan (104-EC-17-A-22-0775).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01130b

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