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Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction
Accepted Manuscript Title: Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction Authors: Tongfei Li, Gengtao Fu, Jiahui Su, Yi Wang, Yinjie Lv, Xiuyong Zou, Xiaoshu Zhu, Lin Xu, Dongmei Sun, Yawen Tang PII: DOI: Reference:
S0013-4686(17)30308-0 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.044 EA 28909
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
11-1-2017 7-2-2017 8-2-2017
Please cite this article as: Tongfei Li, Gengtao Fu, Jiahui Su, Yi Wang, Yinjie Lv, Xiuyong Zou, Xiaoshu Zhu, Lin Xu, Dongmei Sun, Yawen Tang, Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction
Tongfei Li
a,‡
, Gengtao Fu
a,‡,
*, Jiahui Su a, Yi Wang a, Yinjie Lv a, Xiuyong Zou a, Xiaoshu Zhu a,b,*,
Lin Xu a, Dongmei Sun a, and Yawen Tang a,*
a
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Centre of
Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China b
Center for Analysis and Testing, Nanjing Normal University, Nanjing 210023, China
‡ T. F. Li and G. T. Fu contributed equally to this work. * Corresponding Authors: [email protected] (G. F); [email protected] (X. Z); [email protected] (Y. Tang)
1
Graphic Abstract Title: Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction
We develop a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst by a facile and novel phosphorus reduction method, and demonstrate the Au-P/C is a highly active and stable electrocatalyst for the ethanol oxidation reaction..
Research highlights
Au-P/C catalyst is synthesized by a facile and novel white-phosphorus reduce method.
AuP particles with ultrafine particle-size are uniformly dispersed on carbon support.
Au-P/C catalyst exhibits much higher content of P0 than reported metal/P catalysts.
Au-P/C catalysts show excellent catalytic properties for ethanol oxidation reaction. 2
Abstract: Herein, we develop a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst for the alkaline ethanol oxidation reaction (EOR). The Au-P/C catalysts with different Au/P ratio (i.e., AuP/C, Au3P2/C and Au4P3/C) can be obtained by a facile and novel hot-reflux method with white phosphorus (P4) as reductant and ethanol as solvent. The crystal structure, composition and particle-size of the Au-P/C catalysts are investigated by X-ray diffraction (XRD), Energy Dispersive Spectrometer (EDS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), etc. The results demonstrate that Au-P/C catalysts present an alloy phase with the high content of P, ultrafine particle-size and high dispersity on carbon support, which results in excellent electrocatalytic activity and stability towards the EOR compared with that of the free-phosphorus Au/C catalyst. In addition, among the various Au-P/C catalysts with different Au/P ratio, the AuP/C sample exhibits the best electrocatalytic performance in comparison with other Au3P2/C and Au4P3/C samples.
Keywords: white phosphorus, gold-phosphorus, alloy, electrocatalyst, ethanol oxidation reaction
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1. Introduction The ethanol oxidation reaction (EOR), as a critical electrocatalysis reaction, has attracted immense attention in the direct liquid fuel cells (DLFCs) because ethanol as a fuel possesses much lower toxicity and higher theoretical energy density (8.01 kW·h kg−1) in comparison to those of methanol (6.09 kW·h kg−1) and formic acid (1.740 kW·h kg−1) fuels [1-6]. Typically, the EOR is a multistep, twelve-electron process in alkaline medium: C2H5OH + 12OH− → 2CO2 + 9H2O + 12e−; E=1.14 V versus RHE (reversible hydrogen electrode). Without an active catalyst, the sluggish kinetics for this reaction make it energetically inefficient, which is why it is difficulty break the C-C bond completely at low potentials during the EOR [7, 8]. To date, noble metal nanomaterials including Pt, Pd and Au have been extensively investigated and reported to give excellent EOR activity in alkaline medium [2, 9-16], but high cost, limited availability and poor stability of noble metals have restricted their use for practical applications. Introducing a second metal (such as Cu, Ag, Pb, Ni, etc.) to form noble metal-based bimetallic catalysts has proven to be an efficient strategy to reduce noble metal loading and shown remarkable electrocatalytic activity for the EOR due to the bifunctional mechanism and/or electronic effect [1, 17-21]. Nevertheless, several critical issues during direct ethanol fuel cells (DEFCs) including the dissolution, corrosion of second metals, carbon supports and the poisoning of COads species would induces the catalyst degradation, which has made these bimetallic catalysts increasingly unattractive . More recently, emerging as a class of outstanding alternatives, the incorporation of metals with non-metal phosphorus (P) have attracted great attention as the various kinds of the electrocatalysts [22-39], owing to its abundant valence electrons, small atomic radius and excellent stability in both acid and alkaline mediums. A representative example is the use of a electro-codeposition method for 4
preparation the PtP alloy nanotube arrays which shows excellent catalytic activity and durability for the oxygen reduction reaction (ORR) [27]. Du et al. demonstrated a Na2HPO2 self-redox method for synthesis of Pd/P nanoparticles networks, and exhibited excellence electrocatalytic performance for the methanol oxidation reaction (MOR) [29]. Despite previous reports have shown the successful preparation of the noble-metal/P catalysts by electro-codeposition method and Na2HPO2 self-redox method [26-28, 32, 33, 40, 41], the P content of the reported materials were comparatively low. Therefore, the synthesis of the noble-metal/P catalysts with high P content is still a great challenge [34]. P4, white phosphorus, is the most widely employed starting precursor for all the chemistry pertaining to phosphorus derivatives. Very importantly, P4 can behave as a classical phosphine ligand via coordination of the lone pairs at phosphorus in well-designed metallic complexes[42, 43]. Considering such unique chemical property, the strong coordination reaction between P4 and a metal center (bearing labile ligands) would result in successive metal-insertions in P-P bonds to lead to the desired metal-phosphide with the high P content. Thus, in here we develop a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst by a facile and novel P4 reduction method, and demonstrate the Au-P/C is a highly active and stable electrocatalyst for the EOR. The physical characterizations indicate that Au-P/C catalyst with a Au/P atom ratio of 1:1 (named as AuP/C) presents an alloy phase with the high content of P, ultrafine particle-size and high dispersity on carbon support. Due to these afore-mentioned advantages, AuP/C catalyst was shown to provide excellent electrocatalytic activity and stability towards the EOR compared with those of the free-P Au/C catalyst. In addition, the prepared AuP/C catalyst also exhibited significantly improvements in respect of both activity and durability when benchmarked against other compositional Au-P/C 5
(Au3P2/C and Au4P3/C) catalysts.
2. Experimental section 2.1 Synthesis of the AuP/C catalyst Typically, 71.75 mg Vulcan XC-72 carbon resolved in the 50 ml anhydrous ethanol solution completely and then transferred to the round-bottomed three necked flack, then 2.5 ml of 0.0486 M chloroauric acid (HAuCl4) ethanol solution added, mixed with the XC-72 carbon together stirred for 30 mins, simultaneous N2 flowed constantly for 20 minutes to make the air completely removed. Subsequently, 15 ml of 16.0 mM white phosphorus (P4) ethanol solution were added, heating reflux for 2 h at 80 ℃. After the reaction, the samples were gathered by centrifugation at 10,000 rpm for 10 min and thoroughly washed with anhydrous ethanol three times to remove the redundant phosphorous compounds in the final solution, and then dried in a vacuum oven at 60 ℃ for 12 h, the Au-P nanoparticles supported on XC-72 carbon with a molar ratio of Au/P about 1:1 were obtained (named as AuP/C). For comparison, Au3P2/C and Au4P3/C catalysts were also synthesized in the same way, except for the use of different amount of P4. Au/C catalyst was prepared using the traditional NaBH4 reduction method (2.5 ml HAuCl4, 10 ml 100 mg NaBH4 aqueous solution). 2.3 Characterization XRD data were obtained on a Model D/max-rC X-ray diffractometer employing Kα radiation (λ=0.15406nm). TEM, high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) were obtained from a JEOL JEM-2100F transmission electron microscopy manipulated at an accelerating voltage of 200 kV. SEM images, Scanning transmission electron microscopy (STEM) 6
images and the EDS were acquired from a Hitachi S-4800 microscopy. XPS measurements were received on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al Kα radiator. The binding energy was calibrated by use of the C 1s peak energy of 284.6 eV. Fourier transform infrared (FT-IR) spectra were displayed in a Nicolet 520 SXFTIR spectrometer. 2.4 Electrochemical measurement All the electrochemical experiments were conducted by using a CHI 660 electrochemical analyzer (CH Instruments, Shanghai, Chenghua Co., Ltd) at 30 ± 1 ℃. A standard three-electrode system was performed for all electrochemical experiments, which composed of a Pt wire as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode, and a catalyst-modified glassy carbon electrode as the working electrode. An uniform distributed catalyst ink was obtained by ultrasonic the mixture of 2 mg of catalyst and 1 ml of distilled water for 20 min, and 6.0 µL of the resulting suspension was dripped on the pre-treated clean glassy carbon electrode (GCE, 3 mm diameter, 0.07 cm2) surface. Have been drying at room temperature, 3.0 µL of Nafion solution (5 wt%) was dropped on the modified electrode surface and make it dry once more. Therefore, the working electrode was acquired. The electrochemical measurement was performed in 0.5 M N2-saturated KOH solution with or without 1.0 M C2H5OH at a scan rate of 50 mV s-1. Chronoamperometry test was operated in an N2-saturated 0.5 M KOH + 1.0 M C2H5OH mixture solution for 3000 s at 0.20 V. The ECSA of the catalysts was calculated from the results obtained from CV conducted in a 0.5 M H2SO4 solution, by integrating the reduction charge of the Au oxide layer (Q) and assuming a value of 400 μC cm-2 for the reduction charge of a Au oxide monolayer on the Au surface, ECSA = Q/(400×m), where m is the loading of the catalyst on the glassy carbon electrode. 7
Results and Discussion Typically, the AuP/C sample with a Au/P atom ratio of 1:1 (named as AuP/C) was readily achieved by reducing HAuCl4 with P4 in the ethanol solution after reflux at 80 oC for 2 h, as illustrated in Figure 1a. During the synthesis process, the P4 serves as both reducing agent and dopant, Vulcan XC-72 carbon as the catalyst-support and ethanol as reaction solvent; ethanol was chosen as the reaction solvent because it not only can dissolve P4 fully but also has good safety (i.e., P4 will not self-ignite in ethanol solution). The free-P Au/C sample was also synthesized through similar steps to the AuP/C sample, except for the use of NaBH4 as reducing agent instead of P4. The experimental details are presented in the Experimental section. Figure 1b shows the XRD pattern of the AuP/C and Au/C samples. The XRD patterns of two samples displays a good crystalline structure of Au as evidenced by four sharp diffraction peaks corresponding to (111), (200), (220) and (311) crystal facets from the face-centered-cubic (fcc) Au phase (JCPDS 65-8601). The conspicuous diffraction peak located at 25.9° is attributed to the (002) peak of the Vulcan XC-72 carbon. It is noteworthy that all the diffraction peaks of the AuP/C sample are slightly shifted to lower 2θ values compared to that of the Au/C sample, particularly for the (111) and (200) peaks, indicating lattice contraction due to the partial substitution of Au by P. This result proves that as-prepared AuP/C sample is a AuP alloy-structure [34]. It well established that the width of the XRD peaks can reflect particle-size information, that is, the broader the diffraction peak is, the smaller the particle-size has [44]. The average particle-size for the AuP/C sample are 3.5 nm, as calculated from the (111) diffraction peak by the Debye–Scherrer equation [45, 46]. EDS of the AuP/C sample shown in Figure 1c indicates the coexistence of C, Au and P elements (Note: the difference of the EDS peak positions for Au and P can't be distinguished due to the similarity of the Au and P crystalline structures). The weight content 8
of Au-P in the AuP/C sample was calculated to be around 21.8%. Figure 1 The surface composition and electronic status of the AuP/C and Au/C samples were investigated by XPS. Based on the XPS survey scan spectra (Figure 2a), an obvious P2p signal peak for the AuP/C sample can be observed compared to that of the Au/C sample, further confirming P element has been introduced into Au successfully. The Au/P surface atomic ratio in AuP/C sample was measured to be around 1:1. The corresponding weight ratio of Au and P in the AuP/C sample is around 87:13, which is much higher than those of other noble-metal/P catalysts prepared with the self-redox method of Na2HPO2 [35] [22, 32, 47]. In consideration of the high content of P in the AuP/C sample, an excellent catalytic performance toward the electro-catalysis reaction is anticipated owing to the fact that (i) doping P species can effectively modify the metal atoms’ electronic state due to its abundant valence electrons [25, 27]; (ii) alloying noble-metals with P species can, to some extent, retard the dissolution and/or leaching of the metals from catalyst surface [27]. Figure 2b shows the high-resolution Au 4f spectra of these two samples. It shows that the binding energy values of the Au 4f peaks (4f7/2 = 83.40 eV, 4f5/2 = 87.05 eV) in the AuP/C sample are close to the standard values of bulk Au (4f7/2 = 83.80 eV, 4f5/2 = 87.45 eV) metals, demonstrating that the HAuCl4 precursor was successfully reduced in our synthesis. It is important to highlight that an apparent positive shift (0.4 eV) of the Au 4f binding energy for the AuP/C sample in comparison with that of the Au/C sample was also observed. The positive shift of the Au 4f binding energy for the AuP/C sample is probably ascribed to strong electronic interaction between Au and P [25, 27], which further gives strong proof for the formation of a AuP alloy.
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Figure 2 Figure 3a and 3b show typical SEM and TEM images for the AuP/C catalyst. As observed, AuP nanoparticles with ultrafine particle-size are uniformly dispersed on Vulcan XC-72 carbon. The histogram of particle-diameters was obtained from a statistic data of 100 particles in random regions (Figure 3c). The statistic result reveals that the AuP particles possess a relatively narrow size distribution with an average size of 3.7 nm, which is in good agreement with the above XRD analysis. Conversely, the free-P doped Au/C catalyst obtained from traditional NaBH4 reduction method reveals a large particle-size (5.6 nm) and a poor dispersion (Figure S1), due to the strong reducing activity of NaBH4. These observations indicate that AuP/C catalyst with ultrafine particle-size and uniform distribution can be synthesized via such novel phosphorus reduction method. More structural information was reflected by the high resolution TEM (HRTEM). Well-resolved interplanar spacings observed from the HRTEM image based on a single particle (Figure 3d) suggest that the AuP particles are dominated by the single crystalline structure, as further evidenced by the corresponding fast Fourier transform (FFT) with regularly and discretely dotted pattern (Figure 3e). The 6-fold rotational symmetry of the diffraction spots as well confirms that the AuP present {111} faces. A magnified HRTEM image (Figure 3f) and the corresponding profile of the lattice fringes (Figure 3g) taken from a single AuP particle marked by red square in Figure 2d show the lattice fringes with an inter-fringe distance of 0.245 nm, validating the AuP particles are indeed presented by {111} facets. Figure 3 For comparison, the Au3P2/C and Au4P3/C samples with different Au/P atom ratio were also prepared by the same phosphorus reduction method. The detailed characterizations of these two samples are 10
presented in Figures S2-4. The Au3P2/C and Au4P3/C samples exhibit the same alloy structure with the AuP/C sample, which is supported by the XRD and XPS patterns (Figure S3). TEM images (Figure S4) show that Au3P2 and Au4P3 particles are uniformly dispersed on the surface of the carbon support. The domain sizes for the Au3P2/C and Au4P3/C samples are 4.5 and 4.6 nm, respectively, slightly larger than that of the AuP/C catalyst. The results further indicate that the present phosphorus reduction method is high-efficiency for the preparation of highly dispersed Au-P alloy particles. Subsequently, we found that white phosphorus plays as an important role in the formation of the uniform carbon supported catalysts. This is supported by the FT-IR spectra, as shown in Figure 4. Notably, the clear characteristic peaks of phosphoric groups are detected at 950-1300 cm-1 for the AuP/C, Au3P2/C and Au4P3/C catalysts, while Au/C sample has no obvious peak in the corresponding range, implying the surface of the carbon supported Au-P catalysts possesses numerous phosphoric acid groups. The strong electrostatic repulsion interaction between the phosphate groups at Au–P surface may be responsible for the good dispersion of Au–P particles [48]. Figure 4 It is well accepted that nonstructural Au particles possess excellent catalytic performance towards the various electrocatalysis reactions including methanol oxidation [49, 50], ethanol oxidation [15, 16] and oxygen reduction [51] [52], which strongly depend on their structure, size, electronic status and dispersity. Considering the high content of P, ultrafine particle-size and excellent electronic structure as well as the high dispersity on carbon of the AuP particles, thus AuP/C catalyst is anticipated to demonstrate improved catalytic activity toward the electro-catalysis reaction. Herein, ethanol oxidation reaction (EOR) was selected as a model reaction to probe the potential electrocatalytic performance of the AuP/C catalyst. For a comparison, the as-prepared Au/C catalyst was used as 11
reference. Figure 5a shows the cyclic voltammogram (CV) curves of these two catalysts recorded in a N2-saturated 0.5 M H2SO4 solution. It can be seen that two catalysts exhibit typical oxidation peak and reduction peak of Au nanocrystal observed at 1.16 and 0.91 V. By integrating the reduction charge of the Au oxide species, the electrochemically active surface area (ECSA) of the AuP/C catalyst was calculated to be 18.2 m2 g-1, which is remarkably larger than that of the Au/C catalyst (6.1 m2 g-1). The higher ECSA can be rationally ascribed to the ultrafine particle-size and highly uniform dispersion of the AuP particles. The electrocatalytic performances of AuP/C versus Au/C catalysts toward the EOR were investigated in 0.5 M KOH + 1.0 M C2H5OH solution with a scanning rate of 50 mV s-1. As depicted in Figure 5b, the onset oxidation potential of the EOR on the AuP/C catalyst negatively shifts about 50 mV compared to that on Au/C catalyst, which is benefit for the enhancement of electrocatalytic activity at low potential [53]. The forward peak currents of two catalysts were normalized over both the ECSA and the loading of catalyst to give the specific activity and mass activity, respectively (Figure 5c). As observed, the specific activity of the AuP/C catalyst (3.53 mA cm-2) is about 2.63 times than that of the Au/C catalyst (1.34 mA cm-2); even the mass activity of the AuP/C catalyst (642.33 mA mg-1) is almost 7.83 fold as high as that of the Au/C catalyst (81.96 mA mg-1). The above results demonstrate that the AuP/C catalyst can be used as a promising electrocatalyst for the ethanol electro-oxidation. The excellent stability of a catalyst is great significant for the practical application in the fuel cells. To investigate the stability of the catalysts, chronoamperometry measurements were carried out in 0.5 M KOH + 1.0 M C2H5OH solution for 3000 s at a potential of 0.20 V. As shown in Figure 5d, it is obvious that as-prepared AuP/C catalyst displays a slower decay current density over time than that of the Au/C catalyst, suggesting a better stability during ethanol oxidation. Meanwhile, a much higher current density was 12
seen for the AuP/C catalyst versus the Au/C catalyst, further validating that the AuP/C catalyst is much more active for ethanol oxidation. Figure 5 It was also observed that Au-P/C catalyst demonstrates composition-dependent electrocatalytic activity towards the EOR (Figure S5 and Figure 6). We observed that the AuP/C catalyst exhibited the highest ECSA (Figure S5), mass activity and long-time stability (Figure 6) among the AuP/C, Au3P2/C and Au4P3/C three catalysts towards the EOR, which can be attributed to the high content of P and ultrafine particle-size. On the other hand, the Au3P2/C and Au4P3/C catalysts also show the better catalytic activities and stabilities compared to that of the Au/C catalyst (Figure S6). The results demonstrated that the superior electrocatalytic activity of the Au-P/C catalysts (including AuP/C, Au3P2/C and Au4P3/C) are mainly attributed not only to the ultrafine particle-sizes and highly uniform dispersity, but also to the electronic structure change of Au upon its synergistic interaction with P. The electronic structure change of Au was verified by the positive shift of XPS spectrum at Au 4f region (Figure 2b). It has been established that the positive shift of binding energy represents the downshift of d-band center [54, 55]. The d-band center theory demonstrated that adsorption energy of adsorbate is closely related to d-band center of metal: lowering of the d-band center can effectively weaken the adsorption strength between various adsorbates and metals surface, thus boosting the anti-poisoning capability and electro catalytic activity of the catalysts [56, 57]. Figure 6
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3. Conclusion In summary, we have designed and synthesized a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst by a highly efficient phosphorus reduction method. We found that the addition of P can effectually lead to an ultrafine particle-size, a homogeneous distribution of particles on carbon support and the electronic structure change of Au. Thanks to the aforementioned advantages, the as-prepared Au-P/C catalysts with different composition exhibit the better electrocatalytic activity and stability than that of the free-P doped Au/C catalyst towards the ethanol oxidation reaction, and may hold great promises in practical fuel cells. We believe that this work provides us a facile and effective approach for the design and synthesis of the P-doped electrocatalysts. Acknowledgements We acknowledge the financial supports from NSFC (No. 21576139, 21503111 and 21376122) and Natural Science Foundation of Jiangsu Higher Education Institutions of China (16KJB150020). We are also thanks for the help from National and Local Joint Engineering Research Center of Biomedical Functional Materials, and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Supporting Information TEM, EDS and other electrochemical measurements are available in Supporting Information.
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Captions
Figure 1. (a) Scheme for the synthesis of the Au-P/C catalysts; (b) XRD pattern of the AuP/C and Au/C samples; (c) EDS spectrum of the AuP/C sample.
Figure 2. (a) XPS survey scan spectra and (b) high-resolution Au 4f XPS spectra of the AuP/C and Au/C catalysts.
Figure 3. (a) SEM and (b) TEM images of the AuP/C catalyst; (c) Magnified TEM image and the inset is the corresponding particle-size histogram; (d) HRTEM image of a single AuP particle and (e) the corresponding FFT pattern; (f) Magnified HRTEM image taken from region marked by red square in (d) and (g) the corresponding profile of the lattice fringes with {111} facets.
Figure 4. FT-IR spectra of the as-prepared AuP/C, Au3P2/C, Au4P3/C and Au/C catalysts.
Figure 5. EOR performances of the AuP/C and Au/C catalysts: (a) CV curves of the catalysts in a N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV s−1; (b) ECSA and mass-normalized EOR CV curves of the catalysts recorded in 0.5 M KOH + 1.0 M C2H5OH solution at a scan rate of 50 mV s−1; (c) The specific activities and mass activities of the catalysts recorded at 0.20 V; (d) Chronoamperometry curves recorded in a N2-saturated 0.5 M KOH + 1.0 M C2H5OH solution for 3000 s at a potential of 0.20 V.
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Figure 6. EOR performances of the AuP/C, Au3P2/C and Au4P3/C catalysts in a N2-saturated 0.5 M KOH + 1.0 M C2H5OH solution at a scan rate of 50 mV s−1: (a) Mass-normalized CV curves; (b) Bar plots of the mass activity recorded at 0.20 V; (c) Chronoamperometry curves obtained at a potential of 0.20 V for 3000 s; (d) Bar plots of the mass activity for the three catalysts after the stability tests.
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S0013-4686(17)30308-0 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.044 EA 28909
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
11-1-2017 7-2-2017 8-2-2017
Please cite this article as: Tongfei Li, Gengtao Fu, Jiahui Su, Yi Wang, Yinjie Lv, Xiuyong Zou, Xiaoshu Zhu, Lin Xu, Dongmei Sun, Yawen Tang, Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction
Tongfei Li
a,‡
, Gengtao Fu
a,‡,
*, Jiahui Su a, Yi Wang a, Yinjie Lv a, Xiuyong Zou a, Xiaoshu Zhu a,b,*,
Lin Xu a, Dongmei Sun a, and Yawen Tang a,*
a
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Centre of
Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China b
Center for Analysis and Testing, Nanjing Normal University, Nanjing 210023, China
‡ T. F. Li and G. T. Fu contributed equally to this work. * Corresponding Authors: [email protected] (G. F); [email protected] (X. Z); [email protected] (Y. Tang)
1
Graphic Abstract Title: Carbon supported ultrafine gold phosphorus nanoparticles as highly efficient electrocatalyst for alkaline ethanol oxidation reaction
We develop a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst by a facile and novel phosphorus reduction method, and demonstrate the Au-P/C is a highly active and stable electrocatalyst for the ethanol oxidation reaction..
Research highlights
Au-P/C catalyst is synthesized by a facile and novel white-phosphorus reduce method.
AuP particles with ultrafine particle-size are uniformly dispersed on carbon support.
Au-P/C catalyst exhibits much higher content of P0 than reported metal/P catalysts.
Au-P/C catalysts show excellent catalytic properties for ethanol oxidation reaction. 2
Abstract: Herein, we develop a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst for the alkaline ethanol oxidation reaction (EOR). The Au-P/C catalysts with different Au/P ratio (i.e., AuP/C, Au3P2/C and Au4P3/C) can be obtained by a facile and novel hot-reflux method with white phosphorus (P4) as reductant and ethanol as solvent. The crystal structure, composition and particle-size of the Au-P/C catalysts are investigated by X-ray diffraction (XRD), Energy Dispersive Spectrometer (EDS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), etc. The results demonstrate that Au-P/C catalysts present an alloy phase with the high content of P, ultrafine particle-size and high dispersity on carbon support, which results in excellent electrocatalytic activity and stability towards the EOR compared with that of the free-phosphorus Au/C catalyst. In addition, among the various Au-P/C catalysts with different Au/P ratio, the AuP/C sample exhibits the best electrocatalytic performance in comparison with other Au3P2/C and Au4P3/C samples.
Keywords: white phosphorus, gold-phosphorus, alloy, electrocatalyst, ethanol oxidation reaction
3
1. Introduction The ethanol oxidation reaction (EOR), as a critical electrocatalysis reaction, has attracted immense attention in the direct liquid fuel cells (DLFCs) because ethanol as a fuel possesses much lower toxicity and higher theoretical energy density (8.01 kW·h kg−1) in comparison to those of methanol (6.09 kW·h kg−1) and formic acid (1.740 kW·h kg−1) fuels [1-6]. Typically, the EOR is a multistep, twelve-electron process in alkaline medium: C2H5OH + 12OH− → 2CO2 + 9H2O + 12e−; E=1.14 V versus RHE (reversible hydrogen electrode). Without an active catalyst, the sluggish kinetics for this reaction make it energetically inefficient, which is why it is difficulty break the C-C bond completely at low potentials during the EOR [7, 8]. To date, noble metal nanomaterials including Pt, Pd and Au have been extensively investigated and reported to give excellent EOR activity in alkaline medium [2, 9-16], but high cost, limited availability and poor stability of noble metals have restricted their use for practical applications. Introducing a second metal (such as Cu, Ag, Pb, Ni, etc.) to form noble metal-based bimetallic catalysts has proven to be an efficient strategy to reduce noble metal loading and shown remarkable electrocatalytic activity for the EOR due to the bifunctional mechanism and/or electronic effect [1, 17-21]. Nevertheless, several critical issues during direct ethanol fuel cells (DEFCs) including the dissolution, corrosion of second metals, carbon supports and the poisoning of COads species would induces the catalyst degradation, which has made these bimetallic catalysts increasingly unattractive . More recently, emerging as a class of outstanding alternatives, the incorporation of metals with non-metal phosphorus (P) have attracted great attention as the various kinds of the electrocatalysts [22-39], owing to its abundant valence electrons, small atomic radius and excellent stability in both acid and alkaline mediums. A representative example is the use of a electro-codeposition method for 4
preparation the PtP alloy nanotube arrays which shows excellent catalytic activity and durability for the oxygen reduction reaction (ORR) [27]. Du et al. demonstrated a Na2HPO2 self-redox method for synthesis of Pd/P nanoparticles networks, and exhibited excellence electrocatalytic performance for the methanol oxidation reaction (MOR) [29]. Despite previous reports have shown the successful preparation of the noble-metal/P catalysts by electro-codeposition method and Na2HPO2 self-redox method [26-28, 32, 33, 40, 41], the P content of the reported materials were comparatively low. Therefore, the synthesis of the noble-metal/P catalysts with high P content is still a great challenge [34]. P4, white phosphorus, is the most widely employed starting precursor for all the chemistry pertaining to phosphorus derivatives. Very importantly, P4 can behave as a classical phosphine ligand via coordination of the lone pairs at phosphorus in well-designed metallic complexes[42, 43]. Considering such unique chemical property, the strong coordination reaction between P4 and a metal center (bearing labile ligands) would result in successive metal-insertions in P-P bonds to lead to the desired metal-phosphide with the high P content. Thus, in here we develop a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst by a facile and novel P4 reduction method, and demonstrate the Au-P/C is a highly active and stable electrocatalyst for the EOR. The physical characterizations indicate that Au-P/C catalyst with a Au/P atom ratio of 1:1 (named as AuP/C) presents an alloy phase with the high content of P, ultrafine particle-size and high dispersity on carbon support. Due to these afore-mentioned advantages, AuP/C catalyst was shown to provide excellent electrocatalytic activity and stability towards the EOR compared with those of the free-P Au/C catalyst. In addition, the prepared AuP/C catalyst also exhibited significantly improvements in respect of both activity and durability when benchmarked against other compositional Au-P/C 5
(Au3P2/C and Au4P3/C) catalysts.
2. Experimental section 2.1 Synthesis of the AuP/C catalyst Typically, 71.75 mg Vulcan XC-72 carbon resolved in the 50 ml anhydrous ethanol solution completely and then transferred to the round-bottomed three necked flack, then 2.5 ml of 0.0486 M chloroauric acid (HAuCl4) ethanol solution added, mixed with the XC-72 carbon together stirred for 30 mins, simultaneous N2 flowed constantly for 20 minutes to make the air completely removed. Subsequently, 15 ml of 16.0 mM white phosphorus (P4) ethanol solution were added, heating reflux for 2 h at 80 ℃. After the reaction, the samples were gathered by centrifugation at 10,000 rpm for 10 min and thoroughly washed with anhydrous ethanol three times to remove the redundant phosphorous compounds in the final solution, and then dried in a vacuum oven at 60 ℃ for 12 h, the Au-P nanoparticles supported on XC-72 carbon with a molar ratio of Au/P about 1:1 were obtained (named as AuP/C). For comparison, Au3P2/C and Au4P3/C catalysts were also synthesized in the same way, except for the use of different amount of P4. Au/C catalyst was prepared using the traditional NaBH4 reduction method (2.5 ml HAuCl4, 10 ml 100 mg NaBH4 aqueous solution). 2.3 Characterization XRD data were obtained on a Model D/max-rC X-ray diffractometer employing Kα radiation (λ=0.15406nm). TEM, high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) were obtained from a JEOL JEM-2100F transmission electron microscopy manipulated at an accelerating voltage of 200 kV. SEM images, Scanning transmission electron microscopy (STEM) 6
images and the EDS were acquired from a Hitachi S-4800 microscopy. XPS measurements were received on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al Kα radiator. The binding energy was calibrated by use of the C 1s peak energy of 284.6 eV. Fourier transform infrared (FT-IR) spectra were displayed in a Nicolet 520 SXFTIR spectrometer. 2.4 Electrochemical measurement All the electrochemical experiments were conducted by using a CHI 660 electrochemical analyzer (CH Instruments, Shanghai, Chenghua Co., Ltd) at 30 ± 1 ℃. A standard three-electrode system was performed for all electrochemical experiments, which composed of a Pt wire as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode, and a catalyst-modified glassy carbon electrode as the working electrode. An uniform distributed catalyst ink was obtained by ultrasonic the mixture of 2 mg of catalyst and 1 ml of distilled water for 20 min, and 6.0 µL of the resulting suspension was dripped on the pre-treated clean glassy carbon electrode (GCE, 3 mm diameter, 0.07 cm2) surface. Have been drying at room temperature, 3.0 µL of Nafion solution (5 wt%) was dropped on the modified electrode surface and make it dry once more. Therefore, the working electrode was acquired. The electrochemical measurement was performed in 0.5 M N2-saturated KOH solution with or without 1.0 M C2H5OH at a scan rate of 50 mV s-1. Chronoamperometry test was operated in an N2-saturated 0.5 M KOH + 1.0 M C2H5OH mixture solution for 3000 s at 0.20 V. The ECSA of the catalysts was calculated from the results obtained from CV conducted in a 0.5 M H2SO4 solution, by integrating the reduction charge of the Au oxide layer (Q) and assuming a value of 400 μC cm-2 for the reduction charge of a Au oxide monolayer on the Au surface, ECSA = Q/(400×m), where m is the loading of the catalyst on the glassy carbon electrode. 7
Results and Discussion Typically, the AuP/C sample with a Au/P atom ratio of 1:1 (named as AuP/C) was readily achieved by reducing HAuCl4 with P4 in the ethanol solution after reflux at 80 oC for 2 h, as illustrated in Figure 1a. During the synthesis process, the P4 serves as both reducing agent and dopant, Vulcan XC-72 carbon as the catalyst-support and ethanol as reaction solvent; ethanol was chosen as the reaction solvent because it not only can dissolve P4 fully but also has good safety (i.e., P4 will not self-ignite in ethanol solution). The free-P Au/C sample was also synthesized through similar steps to the AuP/C sample, except for the use of NaBH4 as reducing agent instead of P4. The experimental details are presented in the Experimental section. Figure 1b shows the XRD pattern of the AuP/C and Au/C samples. The XRD patterns of two samples displays a good crystalline structure of Au as evidenced by four sharp diffraction peaks corresponding to (111), (200), (220) and (311) crystal facets from the face-centered-cubic (fcc) Au phase (JCPDS 65-8601). The conspicuous diffraction peak located at 25.9° is attributed to the (002) peak of the Vulcan XC-72 carbon. It is noteworthy that all the diffraction peaks of the AuP/C sample are slightly shifted to lower 2θ values compared to that of the Au/C sample, particularly for the (111) and (200) peaks, indicating lattice contraction due to the partial substitution of Au by P. This result proves that as-prepared AuP/C sample is a AuP alloy-structure [34]. It well established that the width of the XRD peaks can reflect particle-size information, that is, the broader the diffraction peak is, the smaller the particle-size has [44]. The average particle-size for the AuP/C sample are 3.5 nm, as calculated from the (111) diffraction peak by the Debye–Scherrer equation [45, 46]. EDS of the AuP/C sample shown in Figure 1c indicates the coexistence of C, Au and P elements (Note: the difference of the EDS peak positions for Au and P can't be distinguished due to the similarity of the Au and P crystalline structures). The weight content 8
of Au-P in the AuP/C sample was calculated to be around 21.8%. Figure 1 The surface composition and electronic status of the AuP/C and Au/C samples were investigated by XPS. Based on the XPS survey scan spectra (Figure 2a), an obvious P2p signal peak for the AuP/C sample can be observed compared to that of the Au/C sample, further confirming P element has been introduced into Au successfully. The Au/P surface atomic ratio in AuP/C sample was measured to be around 1:1. The corresponding weight ratio of Au and P in the AuP/C sample is around 87:13, which is much higher than those of other noble-metal/P catalysts prepared with the self-redox method of Na2HPO2 [35] [22, 32, 47]. In consideration of the high content of P in the AuP/C sample, an excellent catalytic performance toward the electro-catalysis reaction is anticipated owing to the fact that (i) doping P species can effectively modify the metal atoms’ electronic state due to its abundant valence electrons [25, 27]; (ii) alloying noble-metals with P species can, to some extent, retard the dissolution and/or leaching of the metals from catalyst surface [27]. Figure 2b shows the high-resolution Au 4f spectra of these two samples. It shows that the binding energy values of the Au 4f peaks (4f7/2 = 83.40 eV, 4f5/2 = 87.05 eV) in the AuP/C sample are close to the standard values of bulk Au (4f7/2 = 83.80 eV, 4f5/2 = 87.45 eV) metals, demonstrating that the HAuCl4 precursor was successfully reduced in our synthesis. It is important to highlight that an apparent positive shift (0.4 eV) of the Au 4f binding energy for the AuP/C sample in comparison with that of the Au/C sample was also observed. The positive shift of the Au 4f binding energy for the AuP/C sample is probably ascribed to strong electronic interaction between Au and P [25, 27], which further gives strong proof for the formation of a AuP alloy.
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Figure 2 Figure 3a and 3b show typical SEM and TEM images for the AuP/C catalyst. As observed, AuP nanoparticles with ultrafine particle-size are uniformly dispersed on Vulcan XC-72 carbon. The histogram of particle-diameters was obtained from a statistic data of 100 particles in random regions (Figure 3c). The statistic result reveals that the AuP particles possess a relatively narrow size distribution with an average size of 3.7 nm, which is in good agreement with the above XRD analysis. Conversely, the free-P doped Au/C catalyst obtained from traditional NaBH4 reduction method reveals a large particle-size (5.6 nm) and a poor dispersion (Figure S1), due to the strong reducing activity of NaBH4. These observations indicate that AuP/C catalyst with ultrafine particle-size and uniform distribution can be synthesized via such novel phosphorus reduction method. More structural information was reflected by the high resolution TEM (HRTEM). Well-resolved interplanar spacings observed from the HRTEM image based on a single particle (Figure 3d) suggest that the AuP particles are dominated by the single crystalline structure, as further evidenced by the corresponding fast Fourier transform (FFT) with regularly and discretely dotted pattern (Figure 3e). The 6-fold rotational symmetry of the diffraction spots as well confirms that the AuP present {111} faces. A magnified HRTEM image (Figure 3f) and the corresponding profile of the lattice fringes (Figure 3g) taken from a single AuP particle marked by red square in Figure 2d show the lattice fringes with an inter-fringe distance of 0.245 nm, validating the AuP particles are indeed presented by {111} facets. Figure 3 For comparison, the Au3P2/C and Au4P3/C samples with different Au/P atom ratio were also prepared by the same phosphorus reduction method. The detailed characterizations of these two samples are 10
presented in Figures S2-4. The Au3P2/C and Au4P3/C samples exhibit the same alloy structure with the AuP/C sample, which is supported by the XRD and XPS patterns (Figure S3). TEM images (Figure S4) show that Au3P2 and Au4P3 particles are uniformly dispersed on the surface of the carbon support. The domain sizes for the Au3P2/C and Au4P3/C samples are 4.5 and 4.6 nm, respectively, slightly larger than that of the AuP/C catalyst. The results further indicate that the present phosphorus reduction method is high-efficiency for the preparation of highly dispersed Au-P alloy particles. Subsequently, we found that white phosphorus plays as an important role in the formation of the uniform carbon supported catalysts. This is supported by the FT-IR spectra, as shown in Figure 4. Notably, the clear characteristic peaks of phosphoric groups are detected at 950-1300 cm-1 for the AuP/C, Au3P2/C and Au4P3/C catalysts, while Au/C sample has no obvious peak in the corresponding range, implying the surface of the carbon supported Au-P catalysts possesses numerous phosphoric acid groups. The strong electrostatic repulsion interaction between the phosphate groups at Au–P surface may be responsible for the good dispersion of Au–P particles [48]. Figure 4 It is well accepted that nonstructural Au particles possess excellent catalytic performance towards the various electrocatalysis reactions including methanol oxidation [49, 50], ethanol oxidation [15, 16] and oxygen reduction [51] [52], which strongly depend on their structure, size, electronic status and dispersity. Considering the high content of P, ultrafine particle-size and excellent electronic structure as well as the high dispersity on carbon of the AuP particles, thus AuP/C catalyst is anticipated to demonstrate improved catalytic activity toward the electro-catalysis reaction. Herein, ethanol oxidation reaction (EOR) was selected as a model reaction to probe the potential electrocatalytic performance of the AuP/C catalyst. For a comparison, the as-prepared Au/C catalyst was used as 11
reference. Figure 5a shows the cyclic voltammogram (CV) curves of these two catalysts recorded in a N2-saturated 0.5 M H2SO4 solution. It can be seen that two catalysts exhibit typical oxidation peak and reduction peak of Au nanocrystal observed at 1.16 and 0.91 V. By integrating the reduction charge of the Au oxide species, the electrochemically active surface area (ECSA) of the AuP/C catalyst was calculated to be 18.2 m2 g-1, which is remarkably larger than that of the Au/C catalyst (6.1 m2 g-1). The higher ECSA can be rationally ascribed to the ultrafine particle-size and highly uniform dispersion of the AuP particles. The electrocatalytic performances of AuP/C versus Au/C catalysts toward the EOR were investigated in 0.5 M KOH + 1.0 M C2H5OH solution with a scanning rate of 50 mV s-1. As depicted in Figure 5b, the onset oxidation potential of the EOR on the AuP/C catalyst negatively shifts about 50 mV compared to that on Au/C catalyst, which is benefit for the enhancement of electrocatalytic activity at low potential [53]. The forward peak currents of two catalysts were normalized over both the ECSA and the loading of catalyst to give the specific activity and mass activity, respectively (Figure 5c). As observed, the specific activity of the AuP/C catalyst (3.53 mA cm-2) is about 2.63 times than that of the Au/C catalyst (1.34 mA cm-2); even the mass activity of the AuP/C catalyst (642.33 mA mg-1) is almost 7.83 fold as high as that of the Au/C catalyst (81.96 mA mg-1). The above results demonstrate that the AuP/C catalyst can be used as a promising electrocatalyst for the ethanol electro-oxidation. The excellent stability of a catalyst is great significant for the practical application in the fuel cells. To investigate the stability of the catalysts, chronoamperometry measurements were carried out in 0.5 M KOH + 1.0 M C2H5OH solution for 3000 s at a potential of 0.20 V. As shown in Figure 5d, it is obvious that as-prepared AuP/C catalyst displays a slower decay current density over time than that of the Au/C catalyst, suggesting a better stability during ethanol oxidation. Meanwhile, a much higher current density was 12
seen for the AuP/C catalyst versus the Au/C catalyst, further validating that the AuP/C catalyst is much more active for ethanol oxidation. Figure 5 It was also observed that Au-P/C catalyst demonstrates composition-dependent electrocatalytic activity towards the EOR (Figure S5 and Figure 6). We observed that the AuP/C catalyst exhibited the highest ECSA (Figure S5), mass activity and long-time stability (Figure 6) among the AuP/C, Au3P2/C and Au4P3/C three catalysts towards the EOR, which can be attributed to the high content of P and ultrafine particle-size. On the other hand, the Au3P2/C and Au4P3/C catalysts also show the better catalytic activities and stabilities compared to that of the Au/C catalyst (Figure S6). The results demonstrated that the superior electrocatalytic activity of the Au-P/C catalysts (including AuP/C, Au3P2/C and Au4P3/C) are mainly attributed not only to the ultrafine particle-sizes and highly uniform dispersity, but also to the electronic structure change of Au upon its synergistic interaction with P. The electronic structure change of Au was verified by the positive shift of XPS spectrum at Au 4f region (Figure 2b). It has been established that the positive shift of binding energy represents the downshift of d-band center [54, 55]. The d-band center theory demonstrated that adsorption energy of adsorbate is closely related to d-band center of metal: lowering of the d-band center can effectively weaken the adsorption strength between various adsorbates and metals surface, thus boosting the anti-poisoning capability and electro catalytic activity of the catalysts [56, 57]. Figure 6
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3. Conclusion In summary, we have designed and synthesized a new kind of carbon supported gold-phosphorus (Au-P/C) electrocatalyst by a highly efficient phosphorus reduction method. We found that the addition of P can effectually lead to an ultrafine particle-size, a homogeneous distribution of particles on carbon support and the electronic structure change of Au. Thanks to the aforementioned advantages, the as-prepared Au-P/C catalysts with different composition exhibit the better electrocatalytic activity and stability than that of the free-P doped Au/C catalyst towards the ethanol oxidation reaction, and may hold great promises in practical fuel cells. We believe that this work provides us a facile and effective approach for the design and synthesis of the P-doped electrocatalysts. Acknowledgements We acknowledge the financial supports from NSFC (No. 21576139, 21503111 and 21376122) and Natural Science Foundation of Jiangsu Higher Education Institutions of China (16KJB150020). We are also thanks for the help from National and Local Joint Engineering Research Center of Biomedical Functional Materials, and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Supporting Information TEM, EDS and other electrochemical measurements are available in Supporting Information.
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Captions
Figure 1. (a) Scheme for the synthesis of the Au-P/C catalysts; (b) XRD pattern of the AuP/C and Au/C samples; (c) EDS spectrum of the AuP/C sample.
Figure 2. (a) XPS survey scan spectra and (b) high-resolution Au 4f XPS spectra of the AuP/C and Au/C catalysts.
Figure 3. (a) SEM and (b) TEM images of the AuP/C catalyst; (c) Magnified TEM image and the inset is the corresponding particle-size histogram; (d) HRTEM image of a single AuP particle and (e) the corresponding FFT pattern; (f) Magnified HRTEM image taken from region marked by red square in (d) and (g) the corresponding profile of the lattice fringes with {111} facets.
Figure 4. FT-IR spectra of the as-prepared AuP/C, Au3P2/C, Au4P3/C and Au/C catalysts.
Figure 5. EOR performances of the AuP/C and Au/C catalysts: (a) CV curves of the catalysts in a N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV s−1; (b) ECSA and mass-normalized EOR CV curves of the catalysts recorded in 0.5 M KOH + 1.0 M C2H5OH solution at a scan rate of 50 mV s−1; (c) The specific activities and mass activities of the catalysts recorded at 0.20 V; (d) Chronoamperometry curves recorded in a N2-saturated 0.5 M KOH + 1.0 M C2H5OH solution for 3000 s at a potential of 0.20 V.
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Figure 6. EOR performances of the AuP/C, Au3P2/C and Au4P3/C catalysts in a N2-saturated 0.5 M KOH + 1.0 M C2H5OH solution at a scan rate of 50 mV s−1: (a) Mass-normalized CV curves; (b) Bar plots of the mass activity recorded at 0.20 V; (c) Chronoamperometry curves obtained at a potential of 0.20 V for 3000 s; (d) Bar plots of the mass activity for the three catalysts after the stability tests.
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Figures Figure 1
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