Investigating the Influences of the Adsorbed Species on Catalytic

Subscriber access provided by Eastern Michigan University | Bruce T. Halle Library

Article

Investigating the influences of the adsorbed species on catalytic activity for hydrogen oxidation reaction in alkaline electrolyte Siqi Lu, and Zhongbin Zhuang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Investigating the influences of the adsorbed species on catalytic activity for hydrogen oxidation reaction in alkaline electrolyte Siqi Lu, Zhongbin Zhuang* State Key Lab of Organic–Inorganic Composites and Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. ABSTRACT: Catalysts for hydrogen oxidation reaction (HOR) in alkaline electrolyte are important for anion exchange membrane fuel cells. Understanding the role of OH− during the HOR catalytic process in alkaline electrolyte is essential to design highly active HOR catalysts. Here, we attempt to isolate the influence of OH− by using surface controlled Pt based nanoparticles as the model catalysts. By comparing the HOR activity between PtNi nanoparticles and acid washed PtNi nanoparticles, which have almost the same hydrogen binding energies but much different OH binding energies, it was found that the HOR activity in alkaline electrolyte is not mainly controlled by the OH adsorption. Therefore, a bifunctional catalyst promoting OH adsorption may not useful for HOR in alkaline electrolyte. Tuning the hydrogen binding energy was found to be an efficient way to enhance the HOR activity and making Pt base alloy is a reasonable way to tuning the hydrogen binding energies.

INTRODUCTION With the increasing energy demands and environmental problems, fuel cells, which directly convert chemical energy to electricity, have gained more and more attentions.1,2 Hydrogen is a clean fuel and it can be used in fuel cells.2,3 The hydrogen fuel cells are based on two half-cell reactions: hydrogen oxidation reaction (HOR) at anode and oxygen reduction reaction (ORR) at cathode. The hydrogen fuel cells ， exemplified by proton exchange membrane fuel cells (PEMFCs), can produce as high as 1 W cm−2 of power density. However, the high price partially coming from Pt catalysts hindered their applications. In practice, 0.2~0.5 mg cm−2 of Pt catalysts are acquired for PEMFCs, in which ~20% on the anode side and ~80% on the cathode side.4 The recently developed anion exchange membrane fuel cells (AEMFCs) become a promising alternative to PEMFCs,5-9 because some non-precious metal catalysts displayed comparable ORR activity to Pt in alkaline electrolyte.10-12 However, the HOR kinetics on Pt is about 2 orders of magnitude slower in alkaline than in acid electrolytes,13-16 which results in a much higher Pt loading at the anode side for AEMFCs (ca. 0.4 mg cm−2) compared with that for PEMFCs (less than 0.05 mg cm−2).17 Thus, it is desired to develop an effective HOR catalyst for AEMFCs to lower the Pt loading at the anode side. Understanding the mechanism of the HOR (also its reverse reaction, hydrogen evolution reaction, HER) under basic condition is essential for developing high performance HOR/HER catalyst. In acidic environment, the HOR reaction undergoes Tafel-Volmer or HeyrovskyVolmer mechanism.16 H2 is absorbed on the surface of the

catalysts to form absorbed H (Had). Had then releases an electron and is desorbed as a proton. Therefore, the hydrogen binding energy (HBE) is considered as the descriptor for the HOR activity.18-20 In basic environment, however, the role of OH− in the catalytic process is still unclear. There are two possible mechanisms when Had reacts with the abundant OH− in electrolyte. One is that Had is desorbed as a proton (eq. 1), like in the acidic environment, and then the proton reacts with OH− to generate water (eq. 2).

→ + − + ∗

(1)

−

(2)

+ →

−

The other one is OH absorbed on the surface of the catalysts to generate the absorbed OH (OHad) (eq. 3), and then the Hab and OHab combine to form water (eq. 4).

− + ∗ →

(3)

+ → + ∗ + ∗ −

(4)

The different OH roles bring different catalysts design routes. If OH− is not adsorbed, the HOR activity is solely impacted by HBE, like the case in acid. Yan and coauthors21 have done a systematic study on the relationship of HBE of different metal catalysts and their HOR activity in base. A Sabatierian volcano shaped relationship was shown, indicating HBE is the main descriptor for HOR activity in base. Gasteiger14 and Zhuang22 et al. also considered HBE as the major influence on the HOR activities. On the contrary, if OH− is adsorbed on the surface of the catalysts, a bifunctional catalyst with both H and OH adsorption sites is desired to make the surface reaction happen. Markovic and co-authors23 studied a series of metal catalysts, and they found that the metal with stronger OH− adsorption is more beneficial for the HOR activity.

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

They further deposited Ni(OH)2, a OH−-adsorption promoter, on the surface of Pt, and an enhanced HOR activity than pristine Pt is achieved.24 However, all these studies are based on the comparison between catalysts with different compositions that always have different H and OH binding energies. Consequently, it is hard to isolate the contributions from H and OH. In this study, we attempt to understand HOR mechanism in basic condition by using the surface controlled PtNi alloy nanoparticles (NPs) as the model catalysts to isolate the contributions from H and OH. We synthesized PtNi and Acid-PtNi (PtNi washed with perchloric acid to move the surface Ni atoms) NPs. These two types of alloy NPs have similar Pt band structures so that they have similar HBE. Because Ni is more oxophilic than Pt, the existence of surface Ni can significantly promote the OH adsorption. X-ray photoelectron spectroscopy (XPS) and cyclic voltammograms (CV) studies showed that these two catalysts have similar HBE but different OH bindings, confirming they are good models to investigate the influence of OH adsorption in HOR. Similar HOR activity was observed for PtNi and Acid-PtNi, indicating that the adsorption of OH is not a key factor for HOR in alkaline electrolyte, suggesting the OH unadsorbed mechanism. Compared with Pt/C, the exchange current densities of PtNi/C and Acid-PtNi/C in HOR increase by a factor of 3.3 and 3.4, respectively. The enhanced HOR activities are attributed to the weakened HBE resulted from the alloy effect. Therefore, we prove that bifunctional catalyst with enhanced OH adsorption may not useful and HBE is the major influence factor for HOR activity. Making alloy is an efficient way to enhance the HOR activity.

EXPERIMENTAL SECTION Nanoparticle Synthesis. Pt and PtNi NPs were synthesized through a previously reported organic solution method.25-27 In a typical synthesis of Pt NPs, Pt(acac)2 (0.094 g, 0.24 mmol) and 1,2-hexadecanediol (0.388 g, 1.50 mmol) were mixed with 20 mL of diphenyl ether. The mixture was heated to 110 °C under N2 flow. The resulting solution was held at 110 °C for 2 min, during which 0.34 mL of oleylamine was injected by syringe. The mixture was then heated to 175 °C and kept at this temperature for 1 h. After the reaction, it was cooled to room temperature naturally. Ethanol (60 mL) was added and the product was collected by centrifuge (8000 rpm, 3 min). The supernatant was discarded and the precipitate was dispersed in 5 mL of chloroform in the presence of 0.02 mL of oleylamine and 0.02 mL of oleic acid. The NPs were precipitated again by addition of ethanol and followed by centrifugation. The final obtained NPs were redispersed in chloroform. In a typical synthesis of PtNi NPs, Ni(ac)2⋅4H2O (0.167 g, 0.67 mmol) was mixed with 20 mL of diphenyl ether in the presence of oleylamine (0.4 mL) and oleic acid (0.4 mL). 1,2-tetradecanediol (0.076 g, 0.33 mmol) was added, and the solution was kept at 80 °C for 30 min under N2 flow to remove traces of water. After a transparent solution formed, it was further heated to 200 °C, and then 1.5

Page 2 of 15

mL of dichlorobenzene with dissolved Pt(acac)2 (0.130 g, 0.33 mmol) was injected. The solution was maintained at this temperature for 1 h and then cooled to room temperature naturally. The PtNi NPs were collected by the same procedure to the Pt NPs. Loading Nanoparticles on Carbon Supports. High surface area carbon (Vulcan XC-72, Carbot Co.) was dispersed in chloroform with assistant of ultrasonic. After that, the as-prepared Pt or PtNi NPs dispersion were added drop-wise to carbon chloroform dispersion. The Pt/carbon weight ratio was controlled to ca. 1/4. The mixture was sonicated for 1 h and the NPs were loaded on carbon. The carbon supported NPs were collected by filtration. The resulting catalyst powder was further calcinated in air at 200 °C for 5 h to remove the surfactants used in the NPs synthesis process. These catalysts were named as Pt/C or PtNi/C. Acid Treatment of the PtNi/C Catalysts. The acid treatment followed the procedure reported in the previous literature.27 About 50 mg of the as-prepared PtNi/C catalyst was mixed with 100 mL of 0.1 M HClO4. After overnight stirring, the product was collected by suction filtration and washed several times by deionized water. This sample was named as “Acid- PtNi/C”. Physical Characterization. The X-ray powder diffraction (XRD) patterns were obtained on a Rigaku D/Max 2500 VB2+/PC X-Ray powder diffractometer equipped with Cu Kα radiation (λ=0.154 nm) operating at 40 kV and 40 mA. All of the diffraction data were collected in a 2θ range from 30° to 80° at a scanning rate of 10° min−1. The transmission electron microscopy (TEM) observation was performed using a JEOL JEM-1230 transmission electron microscope operated at an acceleration voltage of 100 kV. High resolution TEM (HRTEM) was performed on a JEM2100 transmission electron microscope equipped with a filed-emission gun operating at an accelerating voltage of 200 kV. The XPS spectra were recorded on Thermo Fisher ESCALAB 250Xi XPS system with a monochromatic Al Kα X-ray source. The binding energies derived from XPS measurements were calibrated to the C1s at 284.45 eV. The elemental analyses were done by a Thermo-Fisher ICAP 6300 Radial inductively coupled plasma optical emission spectroscopy (ICP-OES). Electrochemical Measurements. The electrochemical measurements were operated in a three-electrode system controlled by a potentiostat (V3, Princeton Applied Research). 0.1 M KOH solution was served as the electrolyte. A 5 mm diameter glassy carbon electrode (PINE instruments) with deposited catalyst layer was used as the working electrode. The catalyst layer was prepared by casting the catalyst ink on the glassy carbon electrode, which has already been polished to a mirror using 0.05 µm alumina. The ink solutions were prepared by dispersing the catalysts in isopropanol with 0.02 wt% of Nafion to achieve the concentration of 0.2 mgPt mL−1. The thinfilm electrodes were prepared by dropping the ink onto glassy carbon electrodes (three times in total, and the amount of each time is 3.5 µL, 3.5 µL and 3 µL, respective-


Page 3 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ly), and the final Pt loading is 10 µgPt cmdisk−2. A Pt wire was used as the counter electrode. A saturated calomel electrode (SCE) with double junctions was used as the reference electrode. All the potentials used in this work were converted to the reversible hydrogen electrode (RHE). The zero point of RHE was calibrated by the equilibrium potential of HOR/HER of a Pt based catalysts in H2 saturated electrolyte. And the relationship between the potential versus SCE and RHE can be described as follows E(RHE)=E(SCE) + 0.99 V

(5)

Solution resistances were corrected for the polarization curves and the R were measured by A.C. impedance spectroscopy from 1 Hz to 100 kHz at 0 V vs. RHE with a voltage perturbation of 10 mV. For CO-stripping voltammetry, the electrode potential was held at 0.1 V vs. RHE 10 min for the purpose of adsorbing CO on the surface of catalysts electrochemically. Then the electrolyte solution was flushed with Ar for 20 min in order to purge CO completely. Then the adsorbed CO was stripped by scanning between 0.01 and 1.0 V vs. RHE at a scan rate of 50 mV s−1.

The Pt and PtNi NPs were synthesized through organic solution approach reported previously with some modifycations.26,27 Pt NPs were prepared by reduction of Pt(acac)2 by 1,2-hexadecanediol in the presence of oleylamine. For PtNi alloy NPs, Pt(acac)2 and Ni(ac)2⋅4H2O were both added, and they were reduced by 1,2-tetradecanediol in the presence of oleic acid and oleylamine. Figure 2a and b show the TEM images of the as-obtained Pt and PtNi NPs, respectively. The average particle sizes for both samples are ∼5 nm. HRTEM (Figure 2c) image of the PtNi NPs indicates the lattice fringe with an interplanar spacing of 0.22 nm, which is consistent with the (111) plane of PtNi alloy in face-centered cubic (fcc) phase. After loading the NPs on high surface area carbon (Vulcan 72-XC), they were calcinated in air to remove the surfactants on the surface of the NPs. Figure 2d and e show the TEM images of the Pt and PtNi NPs after loading on carbon (denoted as Pt/C and PtNi/C), respectively. The NPs are evenly distributed on the surface of the carbon supports. To remove the surface Ni of the PtNi NPs, the PtNi/C was washed with 0.1 M HClO4 for 12 h. Figure 2f shows that the PtNi/C after acid treatment (denoted as Acid-PtNi/C), maintains its morphology.

RESULTS AND DISCUSSION Nanoparticle Synthesis and Physical Characterization. Three types of NPs with different components or surface structures were used for comparison: pristine Pt, PtNi alloy, and PtNi alloy after acid treatment (Acid-PtNi). Figure 1 illustrated the structure of these three types of the NPs. The pristine Pt was used as the benchmark. Alloying Pt with Ni, the band structure of Pt was adjusted by the Ni additive, due to the electronic and geometric effect.28 The d-band center is lowered, which results in weaker hydrogen adsorption.29-31 More importantly, the co-existence of Pt and Ni on the surface provides both H and OH adsorption sites. Ni is more oxophilic than Pt, so it can promote the OH adsorption than the Pt sites.32 After washing the PtNi by acid, the surface Ni can be removed, but the inside Ni are maintained. Thus, Acid-PtNi has similar Pt band structure with the unwashed PtNi NPs, but without the strong OH adsorption sites on the surface. This means that compared with the unwashed PtNi NPs, acid washed PtNi has similar HBE, but much weaker OH adsorption. We studied the HOR activity of these three model catalysts in base, to get some insights about the reaction mechanism.

Figure 1. Scheme of the Pt, PtNi and Acid-PtNi nanoparticles model catalysts.

Figure 2. (a, b) TEM images of Pt (a) and PtNi (b) NPs, respectively. Insets are the histograms of particle size distributions. (c) HRTEM image of a PtNi NP. (d-f) TEM images of Pt/C (d), PtNi/C (e) and Acid-PtNi/C (f), respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Pt/Ni ratios and metal loadings to carbon supports of the catalysts were determined by ICP-OES. The Pt loading of the Pt/C, PtNi/C and Acid-PtNi/C were 19.8 wt%, 20.2 wt% and 20.4 wt%, respectively. The Pt-Ni atomic ratio in PtNi/C was 1.3:1. However, the Pt-Ni atomic ratio increased to 1.9:1 for the Acid-PtNi/C due to the partially dissolved Ni in acid. It was considered that the Ni from near-surface layers were removed, leading to the formation of Pt-skeleton surfaces.25 The crystal structures of the catalysts were examined by X-ray powder diffractometer (XRD). Figure 3a shows the XRD pattern of Pt/C. The peaks can all be signed to the Pt in face-centered-cubic phase (JCPDS card No.04-0802). The XRD pattern of PtNi/C (Figure 3b) shows diffraction peaks in between of the standard pattern of Pt and Ni (JCPDS card No.04-0850), indicating the formation of alloy of Pt and Ni. Using Vegard’s rule, the Pt-Ni ratio of PtNi/C is roughly estimated to be Pt59Ni41, which is consistent with the ICP-OES results shown before. For AcidPtNi/C, the diffraction peaks are in between of that for Pt/C and PtNi/C, indicating that the Acid-PtNi/C is still Pt-Ni alloy but with less Ni content. This is because the surface Ni is easier to be washed out than Pt in acid, but the underneath Ni is stable under the acid wash condition.33 The Pt-Ni ratio of Acid-PtNi/C is increased to Pt66Ni34 based on the estimation by Vegard’s rule, which is also consistent with ICP-OES result. The particle size of each sample was also calculated using the Scherrer equation based on the (111) peak. The calculated particle sizes for Pt/C, PtNi/C and Acid-PtNi/C are 4.2 nm, 4.0 nm and 3.9 nm, respectively. The average particle sizes from XRD patterns are reasonably close to that measured from TEM images.

Figure 3. XRD patterns of Pt/C (a), PtNi/C (b) and AcidPtNi/C (c) catalysts. The standard diffraction pattern of Pt (JCPDS card No.04-0802) and Ni (JCPDS card No.04-0850) are shown beneath the plots.

XPS Studies. Alloying Pt with Ni results in two effects: electronic effect and additional Ni sites on the surface. Acid treatment can remove the surface Ni sites but maintain the electronic effect. In order to confirm the electronic states and surface components of the NPs catalysts,

Page 4 of 15

XPS analyses were conducted and presented in Figure 4. Figure 4a shows the high resolution Pt 4f core-level spectra of Pt/C. The two peaks are corresponding to the Pt 4f7/2 and Pt 4f5/2, respectively. Each peak can be further deconvoluted into two peaks. Table S1 summarizes the results of peak deconvolution. For Pt/C, the most intense doublet at 70.81 and 74.22 eV is characteristic of metallic Pt 4f7/2 and Pt 4f5/2, respectively. The second and weaker doublet at 71.46 and 75.05 eV could be assigned to oxidized Pt in the forms of PtO and Pt(OH)2.34,35 The Pt 4f peaks of PtNi/C have been shifted to higher binding energies by comparison with that of Pt/C. For PtNi/C, the deconvoluted doublet at 71.89 and 75.20 eV are assignable to Pt (0), and the doublet at 72.65 and 76.55 eV are assignable to Pt oxides. This peak shift is an indication of electron transfer from Pt d band, which leads to a weaker adsorption of hydrogen at the surface of catalyst.36,37 For the Acid-PtNi/C, the peak positions are similar to that of PtNi/C, indicating that it maintains the electronic effect after acid treatment.

Figure 4. (a) Pt 4f core-level XPS spectra of Pt/C, PtNi/C and Acid-PtNi/C catalysts, respectively. (b) Ni 2f core-level XPS spectra of PtNi/C and Acid-PtNi/C catalysts, respectively.

The OH binding sites are considered to be Ni, and the surface Ni condition was examined by the high resolution Ni 2p core-level spectra (Figure 4b). For the PtNi/C, the


Page 5 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peak at 853 eV is assigned to the metallic Ni 2p3/2.38 A peak assigned to the Ni2+ species is also shown at 856 eV with a shakeup satellite signal at 861 eV, indicating the surface oxidation. Similar results are obtained for the Ni 2p1/2 core-level peaks between binding energy of 870 and 885 eV. The existence of the oxidized species of Ni also confirmed that it can serve as the OH adsorption sites. In contrast, the Acid-PtNi/C sample only showed a weaker peak assigned to the metallic Ni, without the oxidized species. It confirms that the surface Ni had been removed during the acid wash process. The Ni inside the NPs was maintained in metallic form, which can tune the electronic structure of the Pt from the alloying effect; however, it is difficult to be oxidized and that does not serve as the OH adsorption sites.

a bifunctional catalyst. CO stripping experiments were also employed to characterize the OH binding of the catalysts (Figure 5c). Compared with the two catalysts without Ni on the surface (Pt/C and Acid-PtNi/C), a much early onset CO oxidation peak (ca. 0.25 V) was observed for PtNi/C, indicating stronger OH adsorption.40

Electrochemical Characterizations. The electrochemical studies of the three catalysts were performed in 0.1 M KOH electrolyte by using a standard three-electrode system. Catalysts deposited on glassy carbon electrode were using as the working electrode. CVs in Ar were used to investigate the adsorption and desorption behaviors of the Pt based catalysts. The whole range CVs are shown in Figure S1 represent the adsorption/desorption of H and OH of the catalysts. The short range CVs for either H or OH are shown in Figure 5a and b. Figure 5a shows the CVs between 0.01 and 0.5 V vs. reversible hydrogen electrode (RHE, the same hereafter), which is corresponding to the Hupd region of Pt. For Pt/C, there are two Hupd peaks at lower potential 0.28 V and higher potential 0.38 V corresponding to Pt (110) and (100), respectively.13,39 For both PtNi/C and Acid-PtNi/C, there is an additional peak at lower potential (∼0.04 V). In the previous studies, the potential of the Hupd peaks are corresponding to the HBE under the test conditions.14,15,37 The emergence of Hupd peaks for the PtNi/C and Acid-PtNi/C at lower potential indicating the existence of weaker hydrogen binding sites. The weakened hydrogen binding energies for the alloy are due to the electronic and geometry effect from the Ni additive, and this effect is maintained after the acid treatment. The electrochemical active surface areas (ECSAs) were obtained from integrated charge transfer from the Hupd region in the short range CVs shown in Figure 5a, and the ECSA values are shown in Table S2.13 For all of the three catalysts, the ECSAs are in range of 20∼26 m2 gPt−1, due to similar particle sizes. After the acid treatment of PtNi/C, the ECSA increased from 20.5 to 25.1 m2 gPt−1. This may come from that the removal of the surface Ni atoms caused more Pt atoms exposed. The CVs for the adsorption/desorption of OH are shown in Figure 5b. For the PtNi/C, negative shift for the onset of OHad adsorption (at ca. 0.45 V) compare to Pt/C (ca. 0.8 V) is observed, which is caused by the presence of surface Ni, a more oxophilic 3d element than Pt. However, after acid treatment of the PtNi/C, the OHad adsorption/desorption shifts back to the potential near the pristine Pt/C, due to the removal of surface Ni and much weaker OH adsorption for Pt than Ni. Thus the presence of Ni on the surface of the NPs can serve as the promoted OH adsorption sites, and the PtNi/C can be considered as

Figure 5. Cyclic voltammograms of Pt/C, PtNi/C and AcidPtNi/C catalysts in Ar-saturated 0.1 M KOH at a scan rate of 1 50 mV s− . (a) low potential range for H adsorption/desorption; (b) high potential range for OH adsorption/desorption; (c) CO stripping experiments that the catalysts were pre-absorbed CO at 0.1 V. The Pt loadings for all 2 samples are ca. 10 µgPt cmdisk− .



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Based on the characterizations shown before, it indicates that (1) the Pt-Ni alloy catalysts (both PtNi/C and Acid-PtNi/C) have similar HBEs, which are weaker than that of Pt/C. This is due to the alloy effect, and this effect was maintained for the NPs after the acid treatment. (2) The surface Ni can significantly promote the OH adsorption, and the PtNi/C has much stronger OH adsorption than the Acid-PtNi/C or Pt/C. Thus these three catalysts are ideal models to investigate the influence of the adsorbed species (H and OH) on the HOR/HER activities. Comparing the activities between Pt/C and Acid-PtNi/C, we can understand the role of adsorbed H, because they have much different hydrogen binding energies. Comparing the activities between PtNi/C and Acid-PtNi/C, we can understand the role of adsorbed OH, because they have similar HBEs but significantly different OH binding energies.

Page 6 of 15

Where α is the transfer coefficient, η is the overpotential. Figure 6b shows the ECSA normalized kinetic current density (logarithmic plot) vs. the potential and the Bulter-

HOR Activities. The electrocatalytic activities for HOR/HER in alkaline electrolyte were obtained using rotating disk electrode (RDE) measurements. Figure 6a shows the polarization curves of Pt/C, PtNi/C and AcidPtNi/C in 0.1 M KOH with saturated H2 at a rotating speed of 1600 rpm, respectively. The current densities increase faster against the applied potential for the alloy catalysts (both PtNi/C and Acid-PtNi/C) than that of the pristine Pt/C, indicating the higher HOR activity for the alloy catalysts. However, the polarization curves for PtNi/C and Acid-PtNi/C are similar, indicating the surface Ni does not play key roles in the HOR activity. The ECSA normalized exchange current densities (i0) were worked out in order to make a fair comparison on the catalytic activity. In order to compare the catalytic kinetic activities of the catalyst, the mass transfer of H2 at the HOR branch was corrected following KouteckyLevich equation (eq. 6) to get the kinetic current (ik).

+

(6)

Where i is the measured overall current, ik is the kinetic current, and id is the diffusion limited current. Meanwhile, id is determined by the calculated Nernstian diffusional overpotential (ηdiffusion) defined as (eq. 7) and shown in Figure 6a as the dash line.

1 " !

(7)

Where R is the universal gas constant, T is the temperature in Kelvin, F is the Faraday’s constant, il is the H2limiting current density and can be worked out by Levich equation.13,40 The previous literature shows that when the polarization curves are approaching the diffusionalcontrolled overpotential curve, there will be large errors to work out the kinetic activity of the catalysts.13 In our case, there is still a considerable distance from the polarization curves of PtNi/C and Acid-PtNi/C to the diffusional-controlled overpotential curve, so reliable activities for these two catalysts can be obtained. Then i0 of HOR/HER was obtained by fitting ik into Bulter-Volmer equation (eq. 8) '(

#$ #% & )*+

&',-.( + )*

.

Figure 6. (a) HOR/HER polarization curves of Pt/C, PtNi/C and Acid-PtNi/C catalysts in H2-saturated 0.1 M KOH at a 1 scan rate of 10 mV s− with a rotation speed of 1600 rpm. The 2 Pt loading for all samples is 10 µgPt cmdisk− . (b) HOR/HER Tafel plots of the specific current density of Pt/C, PtNi/C and Acid-PtNi/C catalysts in H2-saturated 0.1 M KOH. The lines indicate the Butler-Volmer fitting. (c) Micro-polarization region (−5 mV to 5 mV) of Pt/C, PtNi/C and Acid-PtNi/C catalysts in H2-saturated 0.1 M KOH. The lines indicate the linear fitting.

(8)


Page 7 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Volmer fitted curves. i0 can also be obtained from the micro-polarization region41 (Figure 6c) that is only several millivolts deviated from the equilibrium potential (i.e., from −5 mV to 5 mV) by equation 9 # #%

+

(9)

The obtained i0 for the three modal catalysts are shown in Table 1. Although the obtained Pt NPs show irregular shape, the i0 for Pt/C we obtained is similar to that of spherical Pt/C nanoparticles reported in the literature,13,22,41 indicating the facet effect is insignificant here. Compared to Pt/C, i0 of PtNi/C and Acid-PtNi/C shows higher HOR activities. Table 1. Summary of the HOR exchange current density and transfer coefficients (α α) of Pt/C, PtNi/C and Acid-PtNi/C catalysts.

Catalyst

i0

i0

Bulter-Volmer Fitting

MicroPolarization

α

(mA⋅cmPt−2)

(mA⋅cmPt−2)

Pt/C

0.52 ± 0.03a

0.48 ± 0.01

0.49 ± 0.04

PtNi/C

1.55 ± 0.05

0.30 ± 0.03

1.61 ± 0.04

AcidPtNi/C

1.89 ± 0.09

0.45 ± 0.05

1.69 ± 0.07

a: uncertainly are the standard deviation of at least three sets of experimental repeats. The comparison between PtNi/C and Acid-PtNi/C shows that Acid-PtNi/C has a slightly higher activity. This result implies that promoting OH adsorption has less impact on HOR/HER activities. We further tested whether the surface Ni influence the OH adsorption of the NPs during the catalytic processes. The adsorbed OH is considered to be a key intermediate for ORR, the other halfreaction in the fuel cells. The ORR activities of the PtNi/C and Acid-PtNi/C have been tested and shown in Figure S2. The Acid-PtNi/C has higher ORR activity than PtNi/C, and the specific activities are 0.98 ± 0.08 and 0.51 ± 0.11 mA cmPt−2 at 0.9 V for Acid-PtNi/C and PtNi/C, respectively. The previous studies show that Pt has slightly stronger OH adsorption than the peak position of the “volcano” plot.25,35,42 The additional Ni sites provide even stronger OH adsorption, which is unfavorable for ORR. Thus, PtNi/C is less active than Acid-PtNi/C in ORR. This result indicates that the catalysts by our surface controlled approach indeed adjust the OH adsorption of the catalysts, and further influence the reaction rate of ORR, in which the adsorbed OH plays as an intermediate. However, for HOR under basic conditions, the OH adsorption does not influence the reaction rate too much so that we believed that the adsorbed OH is not an intermediate for HOR. We suggested that HOR in alkaline undergoes the adsorption of H2 on the surface of the catalysts to form Had, followed by a charge transfer and release of a proton. (eq.1) The proton quickly combines with

OH− in the electrolyte to generate water (eq.2), and the adsorbed OH is not a rate determine species for HOR. The generation of Had can follow either Tafel or Heyrovsky steps,21 which is beyond our research. Based on this mechanism, we suggested that tuning the HBEs of the catalysts is an efficient route to improve the HOR activity in alkaline electrolyte. PtNi/C and Acid-PtNi/C both show improved HOR activities than the Pt/C, by a factor of 3.3 and 3.4, respectively. The enhanced HOR activity can be ascribed to the weakened hydrogen bindings, as shown in the CVs (Figure 5a). The XPS also shows the peak shifts of Pt-Ni alloy by comparison with that of Pt/C, which is consistent with the improved HOR activity. Some groups have also reported that the Pt alloys can improve the catalytic activity for HOR in base. Yan and co-authors43 synthesized Ptcoated Cu nanowires (NWs) by partial galvanic displacement of Cu NWs. Tuning by the Cu substrate, the exchange current density on Pt/Cu NWs was 3.5 times as that of pure Pt. Zhuang and co-authors22 used PtRu alloy as the HOR catalyst, the PtRu/C has an exchange current density twice as high as Pt/C. Wong and co-authors37 synthesized a number of crystalline ultrathin PtM alloy NWs (M=Fe, Co, Ru), about 2-fold enhancement of Pt NWs alone was achieved by using PtM alloy catalysts. In our experiment, a 3-time enhancement of the HOR activity is also achieved by PtNi alloys. When a foreign atom is introduced, the electronic state of Pt can be adjusted, and a better HBE could be achieved. We suggested that higher HOR activity could be achieved by using alloy that finetuning the HBEs.

CONCLUSION In summary, we studied the HOR mechanism in alkaline electrolyte by using surface controlled PtNi alloy NPs as the model catalysts. An OH− unadsorbed HOR mechanism is suggested, which is H2 adsorbed on the surface of the catalysts to form Had, followed by a charge transfer and release of a proton. And then the proton quickly combines with OH− in the electrolyte to generate water. The HBE of the catalysts is found as the major factor to the HOR activity while the adsorbed OH has less influence. Therefore, our study suggests that the synthesis of electrocatalysts with tuned HBE is a promising way to enhance the HOR activity.

ASSOCIATED CONTENT Supporting Information. Additional CVs, polarization curves, tables for XPS results and ECSAs of the catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected].

Notes The authors declare no competing financial interest.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACKNOWLEDGMENT We thank Dr. Wenchao Sheng from Columbia University for her helpful discussion. This work was financially supported by National Natural Science Foundation of China (No. 21671014), State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology (No. oic201503003) and the Fundamental Research Funds for the Central Universities (buctrc201522).

REFERENCES (1) Gasteiger, H. A.; Markovic, N. M. Science 2009, 324, 48-49. (2) Bockris, J. O. M. Int. J. Hydrogen Energy 2013, 38, 25792588. (3) Bockris, J. O. M. Science 1972, 176, 232-254. (4) Strong, A.; Thornberry, C.; Beattie, S.; Chen, R.; Coles, S. R. J. Fuel Cell Sci. Tech. 2015, 12, 064001. (5) Arges, C. G.; Ramani, V.; Pintauro, P. N. Electrochem. Soc. Interface 2010, 19, 31-35. (6) Lu, S.; Pan, J.; Huang, A.; Zhuang, L.; Lu, J. T. Proc. Natl. Acad. Sci. USA 2008, 105, 20611-20614. (7) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; Xu, T.; Zhuang, L. Energy Environ. Sci. 2014, 7, 3135-3191. (8) Zhuang, Z. B; Giles, S. A.; Zheng, J.; Jenness, G. R.; Caratzoulas, S.; Vlachos, D. G.; Yan, Y. S. Nat. Commun. 2016, 7, 10141. (9) Stezler, B. P.; Zhuang, Z. B.; Wittkopf, J. A.; Yan, Y. S. Nat. Nanotech. 2016, 11, 1020-1025. (10) Wu, G.; Zelenay, P. Acc. Chem. Res. 2013, 46, 1878-1889. (11) Sa, Y. J.; Park, C.; Jeong, H. Y.; Park, S.; Lee, Z.; Kim, K. T.; Park, G.; Joo, S. H. Angew. Chem., Int. Ed. 2014, 53, 4102-4106. (12) Chung, H. T.; Won, J. H.; Zelenay, P. Nat. Commun. 2013, 4, 1922. (13) Sheng, W. C.; Gasteiger, H. A.; Yang, S. H. J. Electrochem. Soc. 2010, 157, B1529-B1536. (14) Durst, J.; Siebel, A.; Simon, C.; Hasché, F.; Herranz, J.; Gasteiger, H. A. Energy Environ. Sci. 2014, 7, 2255-2260. (15) Sheng, W. C.; Zhuang, Z. B.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. S. Nat. Commun. 2015, 6, 5848. (16) Lu, S.; Zhuang. Z. B. Sci. China Mater. 2016, 59, 217-238. (17) Shao, M.; Chang, Q.; Dodelet, J.; Chenitz, R. Chem. Rev. 2016, 116, 3594-3657. (18) Trasatti, S. J. Electroanal. Chem. 1972, 39, 163-184. (19) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; U. Stimming. J. Electrochem. Soc. 2005, 152, J23-J26. (20) Vojvodic, A.; Nørskov, J. K. Natl. Sci. Rev. 2015, 2, 140149. (21) Sheng, W. C.; Myint, M.; Chen, J. G.; Yan, Y. S. Energy Environ. Sci. 2013, 6, 1509-1512. (22) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Energy Environ. Sci. 2015, 8, 177-181. (23) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; Van, D. V. D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Nat. Chem. 2013, 5, 300-306. (24) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Science 2011, 334, 1256-1260. (25) Wang, C.; Chi, M.; Li, D.; Strmcnik, D.; van der Vliet, D.; Wang, G.; Komanicky, V.; Chang, K.; Paulikas, A. P.; Tripkovic, D.; Pearson, J.; More, K. L.; Markovic, N. M.; Stamenkovic, V. R. J. Am. Chem. Soc. 2011, 133, 14396-14403.

Page 8 of 15

(26) Liu, Z.; Shamsuzzoha, M.; Ada, E. T.; Reichert, W. M.; Nikles, D. E. J. Power Sources 2007, 164, 472-480. (27) Ahrenstorf, K.; Albrecht, O.; Heller, H.; Kornowski, A.; Görlitz, D.; Weller, H. Small 2007, 3, 271-274. (28) Guo, S.; Zhang, S.; Sun, S. Angew. Chem., Int. Ed. 2013, 52, 8526-8544. (29) Xu, C.; Hou, J.; Pang, X.; Li, X.; Zhu, M.; Tang, B. Int. J. Hydrogen Energy 2012, 37, 10489-10498. (30) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810-815. (31) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493497. (32) Wang, C.; Chi, M.; Wang, G.; van der Vliet, D.; Li, D.; More, K.; Wang, H.; Schlueter, J. A.; Markovic, N. M.; Stamenkovic, V. R. Adv. Funct. Mater. 2011, 21, 147-152. (33) Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2006, 128, 8813-8819. (34) He, D.; Zhang, L.; He, D.; Zhou, G.; Lin, Y.; Deng, Z.; Hong, X.; Wu, Y.; Chen, C.; Li, Y. Nat. Commun. 2016, 7, 12362. (35) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Science 2014, 343, 1339-1343. (36) Diao, W.; Tengco, J. M. M.; Regalbuto, J. R.; Monnier, J. R. ACS Catal. 2015, 5, 5123-5134. (37) Scofield, M. E.; Zhou, Y.; Yue, S.; Wang, L.; Su, D.; Tong, X.; Vukmirovic, M. B.; Adzic, R. R.; Wong, S. S. ACS Catal. 2016, 6, 3895-3908. (38) Zou, L.; Fan, J.; Zhou, Y.; Wang, C.; Li, J.; Zou, Z.; Yang, H. Nano Res. 2015, 8, 2777-2788. (39) Gisbert, R.; García, G.; Koper, M. T. M. Electrochim. Acta 2010, 55, 7961-7968. (40) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Nat. Mater. 2012, 11, 550-557. (41) Zheng, J.; Yan, Y. S.; Xu, B. J. Electrochem. Soc. 2015, 162, F1470-F1481. (42) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. J. Phys. Chem. B 2004, 108, 17886-17892. (43) Alia, S. M.; Pivovar, B. S.; Yan, Y. S. J. Am. Chem. Soc. 2013, 135, 13473-13478.


Page 9 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Scheme of the Pt, PtNi and Acid-PtNi nanoparticles model catalysts. Figure 1 80x37mm (300 x 300 DPI)


Page 10 of 15

Page 11 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a, b) TEM images of Pt (a) and PtNi (b) NPs, re-spectively. Insets are the histograms of particle size distribu-tions. (c) HRTEM image of a PtNi NP. (d-f) TEM images of Pt/C (d), PtNi/C (e) and Acid-PtNi/C (f), respectively. Figure 2 77x116mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. XRD patterns of Pt/C (a), PtNi/C (b) and Acid-PtNi/C (c) catalysts. The standard diffraction pattern of Pt (JCPDS card No.04-0802) and Ni (JCPDS card No.04-0850) are shown beneath the plots. Figure 3 64x64mm (300 x 300 DPI)


Page 12 of 15

Page 13 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Pt 4f core-level XPS spectra of Pt/C, PtNi/C and Acid-PtNi/C catalysts, respectively. (b) Ni 2f core-level XPS spectra of PtNi/C and Acid-PtNi/C catalysts, respectively. Figure 4 64x132mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Cyclic voltammograms of Pt/C, PtNi/C and Acid-PtNi/C catalysts in Ar-saturated 0.1 M KOH at a scan rate of 50 mV×s-1. (a) low potential range for H adsorp-tion/desorption; (b) high potential range for OH adsorp-tion/desorption; (c) CO striping experiments that the cata-lysts were pre-absorbed CO at 0.1 V. The Pt loadings for all samples are ca. 10 mgPt×cmdisk-2. Figure 5 69x199mm (300 x 300 DPI)


Page 14 of 15

Page 15 of 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) HOR/HER polarization curves of Pt/C, PtNi/C and Acid-PtNi/C catalysts in H2-saturated 0.1 M KOH at a scan rate of 10 mV×s-1 with a rotation speed of 1600 rpm. The Pt loading for all samples is 10 mgPt×cmdisk-2. (b) HOR/HER Tafel plots of the specific current density of Pt/C, PtNi/C and Acid-PtNi/C catalysts in H2-saturated 0.1 M KOH. The lines indicate the Butler-Volmer fitting. (c) Micro-polarization region (-5 mV to 5 mV) of Pt/C, PtNi/C and Acid-PtNi/C catalysts in H2-saturated 0.1 M KOH. The lines indicate the linear fitting. Figure 6 69x199mm (300 x 300 DPI)


Investigating the Influences of the Adsorbed Species on Catalytic

Recommend Documents