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Unraveling the composition-activity relationship of PtRu binary alloy for hydrogen oxidation reaction in alkaline media
Journal of Power Sources 412 (2019) 282–286
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Unraveling the composition-activity relationship of PteRu binary alloy for hydrogen oxidation reaction in alkaline media
T
Gongwei Wanga, Wenzheng Lia, Nian Wua, Bing Huanga, Li Xiaoa,∗, Juntao Lua, Lin Zhuanga,b,∗∗ a b
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China Institute for Advanced Studies, Wuhan University, Wuhan, 430072, China
H I GH L IG H T S
preparation of PteRu planar electrodes with various compositions. • High-throughput alloying of Pt and Ru. • Complete • Volcano-shape relationship between HOR activity and Ru atomic fraction.
A R T I C LE I N FO
A B S T R A C T
Keywords: Pt-Ru Hydrogen oxidation reaction Alkaline polymer electrolyte fuel cells Magnetron sputtering Volcano-shape relationship
The research and development of hydrogen oxidation reaction catalysts in alkaline media is a prerequisite for the commercialization of alkaline polymer electrolyte fuel cell. PteRu bimetallic catalyst was found to be more active towards HOR than Pt in alkaline, but the composition-activity relationship of PteRu alloys is still lacking. Herein, a series of PteRu alloy planar electrodes with 14 different compositions were prepared in a highthroughput fashion using an improved magnetron sputtering method. The atomic ratio of Pt was controlled from 6.7% to 99.7% within the samples. The corresponding electro-catalytic activities towards the HOR in alkaline were systematically measured by rotating disc electrode method. A volcano-shape relationship between the HOR exchange current density and the Ru atomic fraction was revealed, and maximum activity was observed at ca. 55% content of Ru. The enhancement in electro-catalytic activity is attributed to a reduction in the electronic charge density of Pt upon Ru doping.
1. Introduction The development of highly active hydrogen oxidation reaction (HOR) catalysts in alkaline media is crucial for the application of alkaline polymer electrolyte fuel cells (APEFCs) [1,2]. As one of the most vital electrochemical processes, HOR has been comprehensively investigated for decades [3,4], mostly in acidic media [5–7], but relatively limited in alkaline media [8–10]. Pt has been considered as the most efficient pure metal catalyst toward the HOR in acidic media [11,12], but its catalytic activity decreases significantly when the reaction environment changes from acid to alkaline [13,14]. Recently, researchers found that the HOR catalytic activity of Pt can be considerably improved, in both acidic and alkaline media, by cooperating with certain amount of Ru [15–17]. Strmcnik et al. reported that Pt0.1Ru0.9 and Pt0.5Ru0.5 alloys were more active toward the HOR than Pt in alkaline media, and the increased catalytic ∗
activities were ascribed to a bi-functional reaction mechanism, namely, surface hydroxyl species (OHad) functions in removing the hydrogen intermediate (Had) and thus enhances the kinetics of HOR [18]. On the contrary, Durst et al. claimed that the electron transfer is the rate determining step of HOR in both acidic and alkaline media [19]. S. John et al. investigated the nature of the HOR on carbon supported PteRu nano-alloys with two compositions (Pt0.8Ru0.2 and Pt0.2Ru0.8), and found that the exchange current density (i0) for the HOR kinetics on Ptrich Pt0.8Ru0.2 catalyst was nearly 3 times faster than on Pt nanocatalyst [20]. Wang et al. used PtRu/C as the HOR catalyst and boosted the peak power density of APEFC to 1.0 W cm−2, as compared to 0.6 W cm−2 by using Pt/C as the anode, and the enhancement in the HOR kinetics was attributed to the Ru-induced weakening of the Pt-Had interaction [21]. Li et al. demonstrated that the surface doped Ru enhanced the HOR kinetics on Pt/C and supported the bi-functional reaction mechanism in alkaline [22]. Schwämmlein et al. studied the HOR activity of Ru@Pt
Corresponding author. Corresponding author. College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China. E-mail addresses: [email protected] (L. Xiao), [email protected] (L. Zhuang).
∗∗
https://doi.org/10.1016/j.jpowsour.2018.11.026 Received 3 September 2018; Received in revised form 5 November 2018; Accepted 8 November 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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samples were prepared and denoted as Ax and Bx (x = 1, 2, 3, 4, 5, 6, 7), respectively. Pure Pt and Ru electrodes were prepared in the same way using a single metal target. The values of sputtering currents, voltage and time are provided in Table S1.
core-shell catalysts with varying the Pt shell thickness and concluded that the close vicinity of Ru and Pt promoted the HOR owing to a variation of the electronic structure of Pt [23]. The contribution of Ru to the Pt-catalyzed HOR is still under debate. Meanwhile, the relationship between the HOR catalytic activity and the PteRu composition is also unclear. Most published works only focused on 2–3 Pt/Ru ratios, which are insufficient to establish a systematic trend. In addition, the synthetic method varies in each publication, which may bring extra size and morphology effects to the catalyst performance other than the composition difference, making the parallel comparison of the PteRu catalysts in different works difficult [24–27]. Therefore, it is of interest and importance to design parallel and systematic experiments to reveal the HOR catalytic activity-composition relationship of PteRu catalysts. To achieve the above goal, planar model electrodes without obvious morphological difference would be an ideal choice. In this work, PteRu binary alloy planar electrodes with 14 different compositions were prepared by an improved combinatorial magnetron sputtering method. The atomic ratio of Pt was controlled from 6.7% to 99.7% within these samples. The composition-activity relationship of PteRu binary alloys towards HOR has thus been systematically studied, and a clear volcano relationship has been revealed for the first time.
2.2. Materials characterizations X-ray diffraction (XRD) data were collected at a scanning rate of 2°/ min on a Shimadzu XRD-6000 X-ray diffractometer using a Cu Kα radiation source operating at 40 kV and 30 mA (λ = 1.5406 Å). To avoid the interference of diffraction peaks from Au substrate, seven glass substrates were placed adjacent to the Au tips and used only for characterization. X-ray fluorescence (XRF) analysis was performed on a Shimadzu EDX-720 using Rh X-ray tube operating at 50 kV and 30 μA. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Scientific Escalab 250Xi with Al Kα as X-ray source. Atomic force microscopy (AFM) analysis was performed using a Bruker MultiMode 8 system. The AFM probes were SCANASYST-AIR probes (f0 = 70 kHz, k = 0.4 N/m), purchased from Bruker Company. 2.3. Electrochemical measurements The coated Au tips were embedded onto a standard rotating disk electrode (RDE) for electrochemical tests (Scheme 1). Electrochemical experiments were carried out in a three-electrode system on a CHI-660 potentiostat with a RDE system (Pine Research Instruments). Typically, hydrogen evolution/oxidation reaction (HER/HOR) measurements were carried out in H2-saturated 0.1 M KOH solutions. A sheet of graphite paper (Toray) was used as the counter electrode. The reference electrode was a reversible hydrogen electrode (RHE) in the same solution. HER/HOR polarization curves were recorded by scanning the potential from −0.1 V to 1.0 V at 10 mV/s and 2500 rpm (rotation per minute). All solutions were prepared using ultrapure water (18 MΩ cm), and all measurements were conducted at 25 °C.
2. Experimental section 2.1. Electrode preparation PteRu binary alloy planar electrodes were prepared by an improved combinatorial direct current (DC) magnetron sputtering. The sputtering chamber was evacuated to a base pressure of 4 × 10−4 Pa and then fed with high purity Ar gas at 50 sccm under 1 Pa chamber pressure during deposition process. The holder equipped with substrates was placed ca. 12 cm far from targets to prevent samples overheating. The temperature of holder could keep below 30 °C throughout sputtering. Pt (99.95%) and Ru (99.95%) metals were used as targets to co-sputter a bimetallic film. As shown in Scheme 1, the holder remains stationary and the deposited amount decreases linearly with increasing the sputtering distance between target and substrate. Thus PteRu alloys with various compositions along the line of two targets were deposited onto the substrate. The range of composition is dependent on the relative sputtering powers of two targets. For convenience of electrochemical test, seven interchangeable Au rotating disk electrode tips (ϕ = 5 mm) were polished beforehand with an alumina suspension (0.05 mm), and evenly arranged onto the holder (Scheme 1). Two batches of PteRu
3. Results and discussion Fig. 1a shows XRD patterns of the Pt, Ru, and PteRu electrodes. Well-defined peaks for each sample are observed between 39° and 45°, whereas the intensities of other peaks are negligibly small, indicating all metal films were grown along the most stable orientation, [111] for Pt and [001] for Ru. The crystallization of pure Pt is a face centered cubic (f.c.c.) structure, while that of Ru is a hexagonal close packed (h.c.p.) structure. Generally, Pt and Ru form a solid solution with Ru
Scheme 1. PteRu binary alloy planar electrodes prepared by self-improved combinatorial DC magnetron sputtering (a). The installation of as-prepared planar electrode (b). 283
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Fig. 1. (a) XRD patterns for Pt, Ru and PteRu binary alloys with different composition (A1 ∼ A7, B1 ∼ B7). (b) Normalized fitting diffraction peaks between 39° and 45° marked with dashed frame in (a). (c) Corresponding composition of each sample obtained by XRF analysis and XRD calculation specifically. (d) Roughness factor values of each sample obtained from AFM analysis. Inserts are the corresponding AFM images.
h.c.p structure: a = aRu − kh . c . p .⋅xPt = aRu − 2(rRu − rPt )⋅xPt
atoms replacing Pt atoms in the f.c.c. structure when Ru atomic fraction (xRu) is less than 0.7 [28]. With further increasing of Ru atomic fraction, an h.c.p. structure is formed instead where Ru atoms are replaced by Pt atoms. No diffraction peaks corresponding to segregated phases or impurities are detected in Fig. 1a, suggesting a complete alloying of Pt and Ru for all as-sputtered samples. Fig. 1b displays the normalized fitting diffraction peaks marked in Fig. 1a. With increasing of Ru atomic fraction, the peaks shift regularly from low diffraction angle to high diffraction angle, and the lattice structures change from f.c.c. to h.c.p., indicating the formation of solid solution between Pt and Ru. To calculate the composition of PteRu, a universal simplified formula derived from Vegard's law was adopted as follow:
⎛ rh − rd ⎞⋅x = ah − a ah ⎝ rh ⎠ ⎜
Where a, aPt and aRu are the lattice parameters of PteRu alloy, Pt and Ru, respectively. rPt and rRu are the atomic radiuses of Pt and Ru, respectively. a of each PteRu binary alloy was calculated from the values of peak position (2θmax) (See details in the Supporting Information). Eq. (2) is used when xRu is lower than ca. 0.7 (f.c.c. structure), and Eq. (3) is used when xRu is greater than ca. 0.7 (h.c.p. structure). kf.c.c. (0.156 Å) obtained from Eq. (2) is close to the value of k in the literature (0.124 Å) [29,30], indicating the rationality of Eq. (1). Table S2 reports the relative fractions of Pt (xPt) and Ru (xRu) calculated from Eq. (1). Meanwhile, the PteRu alloy compositions were also confirmed with XRF analysis (Table S3). Fig. 1c displays xPt and xRu obtained from both XRD and XRF results. The two sets of data are close to each other with a consistency of regularity, further confirming a complete alloying of Pt and Ru in the bulk of all nominal compositions. The surface composition of PteRu alloys, however, usually tends to undergo a surface segregation during annealing [31]. In this work, in order to avoid surface segregation, all PteRu binary alloys were prepared under a low sputtering temperature (< 30 °C) without further annealing. XPS analysis was conducted to analyze the surface composition of a typical PteRu sample, B1. The result shows that its surface composition (44.4 atom% Ru) is close to its bulk composition (55.8 atom% Ru by XRF analysis and 42.6 atom% Ru by XRD calculation), indicating a similar
⎟
(1)
where rd and rh are the atomic radiuses of the doping and the host element, respectively. a and ah are the lattice parameters of alloy and pure host metal, respectively. x is the atomic fraction of doping element. Eq. (1) can also be rewritten as the following forms (See details in the Supporting Information):
f.c.c structure: a = aPt − kf . c . c .⋅xRu = aPt − 2 2 (rPt − rRu )⋅xRu
(3)
(2) 284
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Fig. 2. (a) The polarization curves of HER/HOR on Pt, Ru and PteRu binary alloys after iR-correction in H2-saturated 0.1 M KOH collected at a sweep rate of 10 mV/s and a rotation rate of 2500 rpm (positivegoing sweep); Insert is the polarization curves of HER/HOR on Pt before (dashed black line) and after (solid green line) iR-correction. (b) The relationship between HER/HOR exchange current densities (i0) calculated from (a) and Ru atomic fraction analyzed by XRF.
Fig. 3. Ru 3d (a) and Pt 4f (b) electron spectra of Pt, Ru and PteRu B1.
4. Conclusions
composition in both surface and bulk of the samples. Typically, the electro-catalytic current of the catalysts need to be normalized by the electrochemical surface area (ESA) for further comparison of the intrinsic catalytic activity [32]. In this work, the ESA of Pt (0.34 cm2), obtained by CO-stripping method (Fig. S1), is close to its geometric area (0.20 cm2). For other samples, however, although quite a few reports attempted to assess ESAs of Ru or PteRu alloy by CO adsorption or Cu under-potential deposition (Cuupd) stripping voltammetry, un-negligible error would be caused by using the same theoretical charge (420 μC/cm2) of Pt in the calculation [33]. Therefore, AFM was used to determine the morphology and surface rough factor of each sample (Fig. 1d insert). The surface roughness values (Ra and Rq) of the as-deposited films are presented in Fig. 1d. All samples are flat and conformal, indicating the ESAs of all samples are closed to the geometric area, thus the electro-catalytic current of the samples was normalized by the geometric area in this work. A RDE method was employed to evaluate the HOR electro-catalytic performance. Fig. 2a shows the HER/HOR polarization curves for Pt, Ru and PteRu binary alloys after iR-correction in H2-saturated 0.1 M KOH electrolyte. The small-polarization regions between −10 mV and +10 mV were linearly fitted and the exchange current densities (i0, normalized to geometric area) were calculated from the slopes (Table S4). A volcano-shape relationship between electro-catalytic activities and Ru atomic fraction can clearly be observed from Fig. 2b. Maximum activity is observed at ca. 55% content of Ru, whose i0 is 2.4 times larger than that of Pt. In order to understand the nature of enhancing Pt HOR electrocatalytic activity by Ru doping, Pt, Ru and PteRu B1 were characterized by XPS (Fig. 3). For PteRu B1, the Pt 4f signals shifted positively (ca. 0.56 eV) with respect to pure Pt, which corresponds to a decrease in the electronic charge density on the platinum atoms. Meanwhile, the Ru 3d5/2 peak shifted by ca. 0.15 eV towards lower binding energy. These results manifest an electronic interaction between Pt and Ru. It is believed that the deficiency in electronic charge density on Pt sites causes a decreased adsorption of hydrogen, which facilitates the rate-limiting step of HOR, and thus improving the HOR activity [19,21].
In summary, a series of PteRu binary alloy planar electrodes with 14 different compositions were prepared in a high-throughput fashion. Complete alloying of Pt and Ru was obtained in all nominal composition based on comprehensive analyses using XRD, XRF, and XPS. A volcano-shape relationship between the HOR exchange current density (i0) and Ru atomic fraction (xRu) was revealed, and maximum activity was observed at ca. 55% content of Ru. The enhancement in HOR kinetics is attributed to a reduction in the electronic charge density of Pt due to Ru doping. This improved combinatorial sputtering method is expected to be widely applied to similar binary and ternary alloy systems for screening better catalysts. Acknowledgments This work was supported by the National Natural Science Foundation of China (21872108, 21633008, 21573167 and 91545205), Wuhan University Innovation Team (2042017kf0232), the National Key Research and Development Program (2016YFB0101203), the Fundamental Research Funds for the Central Universities (2014203020207). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.11.026. References [1] D.E. Ramaker, C. Roth, ChemElectroChem 2 (2015) 1582–1594. [2] S. Lu, J. Pan, A. Huang, L. Zhuang, J. Lu, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 20611–20614. [3] N.M. Marković, P.N. Ross, Surf. Sci. Rep. 45 (2002) 117–229. [4] M.R. Tarasevich, O.V. Korchagin, Russ. J. Electrochem. 49 (2013) 600–618. [5] H. Uchida, K. Izumi, M. Watanabe, J. Phys. Chem. B 110 (2006) 21924–21930. [6] S. Chen, A. Kucernak, J. Phys. Chem. B 108 (2004) 13984–13994.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Unraveling the composition-activity relationship of PteRu binary alloy for hydrogen oxidation reaction in alkaline media
T
Gongwei Wanga, Wenzheng Lia, Nian Wua, Bing Huanga, Li Xiaoa,∗, Juntao Lua, Lin Zhuanga,b,∗∗ a b
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China Institute for Advanced Studies, Wuhan University, Wuhan, 430072, China
H I GH L IG H T S
preparation of PteRu planar electrodes with various compositions. • High-throughput alloying of Pt and Ru. • Complete • Volcano-shape relationship between HOR activity and Ru atomic fraction.
A R T I C LE I N FO
A B S T R A C T
Keywords: Pt-Ru Hydrogen oxidation reaction Alkaline polymer electrolyte fuel cells Magnetron sputtering Volcano-shape relationship
The research and development of hydrogen oxidation reaction catalysts in alkaline media is a prerequisite for the commercialization of alkaline polymer electrolyte fuel cell. PteRu bimetallic catalyst was found to be more active towards HOR than Pt in alkaline, but the composition-activity relationship of PteRu alloys is still lacking. Herein, a series of PteRu alloy planar electrodes with 14 different compositions were prepared in a highthroughput fashion using an improved magnetron sputtering method. The atomic ratio of Pt was controlled from 6.7% to 99.7% within the samples. The corresponding electro-catalytic activities towards the HOR in alkaline were systematically measured by rotating disc electrode method. A volcano-shape relationship between the HOR exchange current density and the Ru atomic fraction was revealed, and maximum activity was observed at ca. 55% content of Ru. The enhancement in electro-catalytic activity is attributed to a reduction in the electronic charge density of Pt upon Ru doping.
1. Introduction The development of highly active hydrogen oxidation reaction (HOR) catalysts in alkaline media is crucial for the application of alkaline polymer electrolyte fuel cells (APEFCs) [1,2]. As one of the most vital electrochemical processes, HOR has been comprehensively investigated for decades [3,4], mostly in acidic media [5–7], but relatively limited in alkaline media [8–10]. Pt has been considered as the most efficient pure metal catalyst toward the HOR in acidic media [11,12], but its catalytic activity decreases significantly when the reaction environment changes from acid to alkaline [13,14]. Recently, researchers found that the HOR catalytic activity of Pt can be considerably improved, in both acidic and alkaline media, by cooperating with certain amount of Ru [15–17]. Strmcnik et al. reported that Pt0.1Ru0.9 and Pt0.5Ru0.5 alloys were more active toward the HOR than Pt in alkaline media, and the increased catalytic ∗
activities were ascribed to a bi-functional reaction mechanism, namely, surface hydroxyl species (OHad) functions in removing the hydrogen intermediate (Had) and thus enhances the kinetics of HOR [18]. On the contrary, Durst et al. claimed that the electron transfer is the rate determining step of HOR in both acidic and alkaline media [19]. S. John et al. investigated the nature of the HOR on carbon supported PteRu nano-alloys with two compositions (Pt0.8Ru0.2 and Pt0.2Ru0.8), and found that the exchange current density (i0) for the HOR kinetics on Ptrich Pt0.8Ru0.2 catalyst was nearly 3 times faster than on Pt nanocatalyst [20]. Wang et al. used PtRu/C as the HOR catalyst and boosted the peak power density of APEFC to 1.0 W cm−2, as compared to 0.6 W cm−2 by using Pt/C as the anode, and the enhancement in the HOR kinetics was attributed to the Ru-induced weakening of the Pt-Had interaction [21]. Li et al. demonstrated that the surface doped Ru enhanced the HOR kinetics on Pt/C and supported the bi-functional reaction mechanism in alkaline [22]. Schwämmlein et al. studied the HOR activity of Ru@Pt
Corresponding author. Corresponding author. College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China. E-mail addresses: [email protected] (L. Xiao), [email protected] (L. Zhuang).
∗∗
https://doi.org/10.1016/j.jpowsour.2018.11.026 Received 3 September 2018; Received in revised form 5 November 2018; Accepted 8 November 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
Journal of Power Sources 412 (2019) 282–286
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samples were prepared and denoted as Ax and Bx (x = 1, 2, 3, 4, 5, 6, 7), respectively. Pure Pt and Ru electrodes were prepared in the same way using a single metal target. The values of sputtering currents, voltage and time are provided in Table S1.
core-shell catalysts with varying the Pt shell thickness and concluded that the close vicinity of Ru and Pt promoted the HOR owing to a variation of the electronic structure of Pt [23]. The contribution of Ru to the Pt-catalyzed HOR is still under debate. Meanwhile, the relationship between the HOR catalytic activity and the PteRu composition is also unclear. Most published works only focused on 2–3 Pt/Ru ratios, which are insufficient to establish a systematic trend. In addition, the synthetic method varies in each publication, which may bring extra size and morphology effects to the catalyst performance other than the composition difference, making the parallel comparison of the PteRu catalysts in different works difficult [24–27]. Therefore, it is of interest and importance to design parallel and systematic experiments to reveal the HOR catalytic activity-composition relationship of PteRu catalysts. To achieve the above goal, planar model electrodes without obvious morphological difference would be an ideal choice. In this work, PteRu binary alloy planar electrodes with 14 different compositions were prepared by an improved combinatorial magnetron sputtering method. The atomic ratio of Pt was controlled from 6.7% to 99.7% within these samples. The composition-activity relationship of PteRu binary alloys towards HOR has thus been systematically studied, and a clear volcano relationship has been revealed for the first time.
2.2. Materials characterizations X-ray diffraction (XRD) data were collected at a scanning rate of 2°/ min on a Shimadzu XRD-6000 X-ray diffractometer using a Cu Kα radiation source operating at 40 kV and 30 mA (λ = 1.5406 Å). To avoid the interference of diffraction peaks from Au substrate, seven glass substrates were placed adjacent to the Au tips and used only for characterization. X-ray fluorescence (XRF) analysis was performed on a Shimadzu EDX-720 using Rh X-ray tube operating at 50 kV and 30 μA. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Scientific Escalab 250Xi with Al Kα as X-ray source. Atomic force microscopy (AFM) analysis was performed using a Bruker MultiMode 8 system. The AFM probes were SCANASYST-AIR probes (f0 = 70 kHz, k = 0.4 N/m), purchased from Bruker Company. 2.3. Electrochemical measurements The coated Au tips were embedded onto a standard rotating disk electrode (RDE) for electrochemical tests (Scheme 1). Electrochemical experiments were carried out in a three-electrode system on a CHI-660 potentiostat with a RDE system (Pine Research Instruments). Typically, hydrogen evolution/oxidation reaction (HER/HOR) measurements were carried out in H2-saturated 0.1 M KOH solutions. A sheet of graphite paper (Toray) was used as the counter electrode. The reference electrode was a reversible hydrogen electrode (RHE) in the same solution. HER/HOR polarization curves were recorded by scanning the potential from −0.1 V to 1.0 V at 10 mV/s and 2500 rpm (rotation per minute). All solutions were prepared using ultrapure water (18 MΩ cm), and all measurements were conducted at 25 °C.
2. Experimental section 2.1. Electrode preparation PteRu binary alloy planar electrodes were prepared by an improved combinatorial direct current (DC) magnetron sputtering. The sputtering chamber was evacuated to a base pressure of 4 × 10−4 Pa and then fed with high purity Ar gas at 50 sccm under 1 Pa chamber pressure during deposition process. The holder equipped with substrates was placed ca. 12 cm far from targets to prevent samples overheating. The temperature of holder could keep below 30 °C throughout sputtering. Pt (99.95%) and Ru (99.95%) metals were used as targets to co-sputter a bimetallic film. As shown in Scheme 1, the holder remains stationary and the deposited amount decreases linearly with increasing the sputtering distance between target and substrate. Thus PteRu alloys with various compositions along the line of two targets were deposited onto the substrate. The range of composition is dependent on the relative sputtering powers of two targets. For convenience of electrochemical test, seven interchangeable Au rotating disk electrode tips (ϕ = 5 mm) were polished beforehand with an alumina suspension (0.05 mm), and evenly arranged onto the holder (Scheme 1). Two batches of PteRu
3. Results and discussion Fig. 1a shows XRD patterns of the Pt, Ru, and PteRu electrodes. Well-defined peaks for each sample are observed between 39° and 45°, whereas the intensities of other peaks are negligibly small, indicating all metal films were grown along the most stable orientation, [111] for Pt and [001] for Ru. The crystallization of pure Pt is a face centered cubic (f.c.c.) structure, while that of Ru is a hexagonal close packed (h.c.p.) structure. Generally, Pt and Ru form a solid solution with Ru
Scheme 1. PteRu binary alloy planar electrodes prepared by self-improved combinatorial DC magnetron sputtering (a). The installation of as-prepared planar electrode (b). 283
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Fig. 1. (a) XRD patterns for Pt, Ru and PteRu binary alloys with different composition (A1 ∼ A7, B1 ∼ B7). (b) Normalized fitting diffraction peaks between 39° and 45° marked with dashed frame in (a). (c) Corresponding composition of each sample obtained by XRF analysis and XRD calculation specifically. (d) Roughness factor values of each sample obtained from AFM analysis. Inserts are the corresponding AFM images.
h.c.p structure: a = aRu − kh . c . p .⋅xPt = aRu − 2(rRu − rPt )⋅xPt
atoms replacing Pt atoms in the f.c.c. structure when Ru atomic fraction (xRu) is less than 0.7 [28]. With further increasing of Ru atomic fraction, an h.c.p. structure is formed instead where Ru atoms are replaced by Pt atoms. No diffraction peaks corresponding to segregated phases or impurities are detected in Fig. 1a, suggesting a complete alloying of Pt and Ru for all as-sputtered samples. Fig. 1b displays the normalized fitting diffraction peaks marked in Fig. 1a. With increasing of Ru atomic fraction, the peaks shift regularly from low diffraction angle to high diffraction angle, and the lattice structures change from f.c.c. to h.c.p., indicating the formation of solid solution between Pt and Ru. To calculate the composition of PteRu, a universal simplified formula derived from Vegard's law was adopted as follow:
⎛ rh − rd ⎞⋅x = ah − a ah ⎝ rh ⎠ ⎜
Where a, aPt and aRu are the lattice parameters of PteRu alloy, Pt and Ru, respectively. rPt and rRu are the atomic radiuses of Pt and Ru, respectively. a of each PteRu binary alloy was calculated from the values of peak position (2θmax) (See details in the Supporting Information). Eq. (2) is used when xRu is lower than ca. 0.7 (f.c.c. structure), and Eq. (3) is used when xRu is greater than ca. 0.7 (h.c.p. structure). kf.c.c. (0.156 Å) obtained from Eq. (2) is close to the value of k in the literature (0.124 Å) [29,30], indicating the rationality of Eq. (1). Table S2 reports the relative fractions of Pt (xPt) and Ru (xRu) calculated from Eq. (1). Meanwhile, the PteRu alloy compositions were also confirmed with XRF analysis (Table S3). Fig. 1c displays xPt and xRu obtained from both XRD and XRF results. The two sets of data are close to each other with a consistency of regularity, further confirming a complete alloying of Pt and Ru in the bulk of all nominal compositions. The surface composition of PteRu alloys, however, usually tends to undergo a surface segregation during annealing [31]. In this work, in order to avoid surface segregation, all PteRu binary alloys were prepared under a low sputtering temperature (< 30 °C) without further annealing. XPS analysis was conducted to analyze the surface composition of a typical PteRu sample, B1. The result shows that its surface composition (44.4 atom% Ru) is close to its bulk composition (55.8 atom% Ru by XRF analysis and 42.6 atom% Ru by XRD calculation), indicating a similar
⎟
(1)
where rd and rh are the atomic radiuses of the doping and the host element, respectively. a and ah are the lattice parameters of alloy and pure host metal, respectively. x is the atomic fraction of doping element. Eq. (1) can also be rewritten as the following forms (See details in the Supporting Information):
f.c.c structure: a = aPt − kf . c . c .⋅xRu = aPt − 2 2 (rPt − rRu )⋅xRu
(3)
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Fig. 2. (a) The polarization curves of HER/HOR on Pt, Ru and PteRu binary alloys after iR-correction in H2-saturated 0.1 M KOH collected at a sweep rate of 10 mV/s and a rotation rate of 2500 rpm (positivegoing sweep); Insert is the polarization curves of HER/HOR on Pt before (dashed black line) and after (solid green line) iR-correction. (b) The relationship between HER/HOR exchange current densities (i0) calculated from (a) and Ru atomic fraction analyzed by XRF.
Fig. 3. Ru 3d (a) and Pt 4f (b) electron spectra of Pt, Ru and PteRu B1.
4. Conclusions
composition in both surface and bulk of the samples. Typically, the electro-catalytic current of the catalysts need to be normalized by the electrochemical surface area (ESA) for further comparison of the intrinsic catalytic activity [32]. In this work, the ESA of Pt (0.34 cm2), obtained by CO-stripping method (Fig. S1), is close to its geometric area (0.20 cm2). For other samples, however, although quite a few reports attempted to assess ESAs of Ru or PteRu alloy by CO adsorption or Cu under-potential deposition (Cuupd) stripping voltammetry, un-negligible error would be caused by using the same theoretical charge (420 μC/cm2) of Pt in the calculation [33]. Therefore, AFM was used to determine the morphology and surface rough factor of each sample (Fig. 1d insert). The surface roughness values (Ra and Rq) of the as-deposited films are presented in Fig. 1d. All samples are flat and conformal, indicating the ESAs of all samples are closed to the geometric area, thus the electro-catalytic current of the samples was normalized by the geometric area in this work. A RDE method was employed to evaluate the HOR electro-catalytic performance. Fig. 2a shows the HER/HOR polarization curves for Pt, Ru and PteRu binary alloys after iR-correction in H2-saturated 0.1 M KOH electrolyte. The small-polarization regions between −10 mV and +10 mV were linearly fitted and the exchange current densities (i0, normalized to geometric area) were calculated from the slopes (Table S4). A volcano-shape relationship between electro-catalytic activities and Ru atomic fraction can clearly be observed from Fig. 2b. Maximum activity is observed at ca. 55% content of Ru, whose i0 is 2.4 times larger than that of Pt. In order to understand the nature of enhancing Pt HOR electrocatalytic activity by Ru doping, Pt, Ru and PteRu B1 were characterized by XPS (Fig. 3). For PteRu B1, the Pt 4f signals shifted positively (ca. 0.56 eV) with respect to pure Pt, which corresponds to a decrease in the electronic charge density on the platinum atoms. Meanwhile, the Ru 3d5/2 peak shifted by ca. 0.15 eV towards lower binding energy. These results manifest an electronic interaction between Pt and Ru. It is believed that the deficiency in electronic charge density on Pt sites causes a decreased adsorption of hydrogen, which facilitates the rate-limiting step of HOR, and thus improving the HOR activity [19,21].
In summary, a series of PteRu binary alloy planar electrodes with 14 different compositions were prepared in a high-throughput fashion. Complete alloying of Pt and Ru was obtained in all nominal composition based on comprehensive analyses using XRD, XRF, and XPS. A volcano-shape relationship between the HOR exchange current density (i0) and Ru atomic fraction (xRu) was revealed, and maximum activity was observed at ca. 55% content of Ru. The enhancement in HOR kinetics is attributed to a reduction in the electronic charge density of Pt due to Ru doping. This improved combinatorial sputtering method is expected to be widely applied to similar binary and ternary alloy systems for screening better catalysts. Acknowledgments This work was supported by the National Natural Science Foundation of China (21872108, 21633008, 21573167 and 91545205), Wuhan University Innovation Team (2042017kf0232), the National Key Research and Development Program (2016YFB0101203), the Fundamental Research Funds for the Central Universities (2014203020207). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2018.11.026. References [1] D.E. Ramaker, C. Roth, ChemElectroChem 2 (2015) 1582–1594. [2] S. Lu, J. Pan, A. Huang, L. Zhuang, J. Lu, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 20611–20614. [3] N.M. Marković, P.N. Ross, Surf. Sci. Rep. 45 (2002) 117–229. [4] M.R. Tarasevich, O.V. Korchagin, Russ. J. Electrochem. 49 (2013) 600–618. [5] H. Uchida, K. Izumi, M. Watanabe, J. Phys. Chem. B 110 (2006) 21924–21930. [6] S. Chen, A. Kucernak, J. Phys. Chem. B 108 (2004) 13984–13994.
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