Cyanide Radical Chemisorbed Pt Electrocatalyst for Enhanced

Accelerated durability testing (ADT) was performed by potential cycling between 0.6 and 1 V for 1000 cycles in an O2-saturated 0.1 M KOH solution at a...
1 downloads 9 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Cyanide Radical Chemisorbed Pt Electrocatalyst for Enhanced Methanol-Tolerant Oxygen Reduction Reactions Linfang Lu,† Renhong Li,*,†,‡ Kazuaki Fujiwara,† Xiaoqing Yan,‡ Hisayoshi Kobayashi,*,§ Wuzhong Yi,† and Jie Fan*,† †

Key Lab of Applied Chemistry of Zhejiang Province, and Department of Chemistry, Zhejiang University, 310027 Hangzhou, China Key Lab of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou, China § Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, 606-8585 Sakyo-ku, Kyoto, Japan ‡

S Supporting Information *

ABSTRACT: A new ·CN radical molecular surface chemisorption strategy was developed to modify Pt/C electrocatalysts in a green and facile manner via a photo-Fenton cyanation process. After the surface modification, the Pt/C electrocatalysts display better specific activity for ORR and a significantly enhanced methanol (MeOH)-tolerant property in both alkline and acidic media. The experimental observations and DFT calculations suggest that the unique electrochemical performance of CN−Pt/C might be derived from the covalent nature of the interaction between ·CN and Pt surface. In particular, it hinders the formation of detrimental OHad− K+(H2O)x, CN−K+(H2O)x, or CN−Na+(H2O)x clusters on the surface of PtNPs, which could block the access of oxygen molecules. Meanwhile, the adsorption of MeOH molecules becomes more difficult as compared to unmodified Pt in KOH solution on the basis of the DFT calculations, making CN−Pt/C an efficient cathode electrocatalyst to alleviate the MeOH crossover problem in alkaline DMFCs. Interestingly, experimental results show that the steric blocking effects of CN groups also improve MeOH-tolerant property CN−Pt/C catalysts operated in acid medium. The green nature and simplicity of the current strategy can facilitate their large-scale use to modify the Pt electrocatalysts with improved performance.

■

MeOH dehydrogenation.14 The Pt/C catalysts with surface nitridation,15 or coating of microporous carbon shells, show high ORR activity in the presence of MeOH by hindering the accessibility of MeOH molecules to Pt surface.16 However, a slight decrease of ORR reactivity is observed in the later case due to less exposed active sites. In general, these methods require complex synthetic procedures (e.g., pyrolysis, core− shell nanostructuring, and high-temperature alloying), which impose limitations on their practical use on a large scale. Herein, we report a simple ·CN radical chemisorption strategy to modify commercial Pt/C electrocatalyst using a green photo-Fenton cyanation process. After the surface modification, the electrocatalysts display a significantly enhanced MeOH-tolerant property in both alkaline and acidic media. Furthermore, in contrast to the previously reported cyanide anions (CN−) decorated Pt catalysts,17−19 which showed a 50-fold decrease of the ORR activity in alkaline solutions, ·CN chemisorbed Pt/C (hereafter denoted as CN−

INTRODUCTION Noble metal nanoparticles (MNPs), in particular PtNPs, are key catalysts for many important reactions.1−3 The catalytic properties of these MNPs can be improved by tuning their electronic properties and the interactions with reactants and products, which could be achieved by modifying their surface structure, composition, and metal-to-support interface.4−9 PtNPs have been widely used as cathode electrocatalysts for oxygen reduction reactions (ORRs) in direct methanol fuel cells (DMFCs). One of the major challenges for Pt-type cathode is the methanol (MeOH) crossover through the polymer electrolyte.10 The ORR and the MeOH oxidation reaction (MOR) occurring simultaneously at the cathodes not only reduce the cell voltage and fuel utilization but also increase the required oxygen stoichiometric ratio.11 This problem can be solved by new cathode electrocatalysts with both higher MeOH tolerance and higher activity for the ORR than bare PtNPs. Many strategies have been developed to inhibit the MeOH adsorption−dehydrogenation steps on Pt surface,12 while new insights into the pathways of MOR are of great importance.13 The alloying strategy is commonly used to suppress the MOR reaction on Pt surface by decreasing the probability of the existence of three neighboring Pt atoms that are required for © XXXX American Chemical Society

Received: March 23, 2016 Revised: May 13, 2016

A

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

electrodes, respectively. The potential measured against a SCE was converted to the potential versus the reversible hydrogen electrode (RHE) according to EvsRHE = EvsSCE + EθSCE + 0.0592 pH. Prior to the electrochemical experiments, the catalysts modified RDEs were subjected to continuous potential cycling (0−1 V vs RHE, 50 mV s −1) in 0.1 M aqueous KOH until CVs became reproducible. The CV measurements were performed until the saturation of N2 or O2. The flow of gas was controlled at 40 mL/min. The ORR measurements were performed in 0.1 M KOH solutions under flow of O2 using the catalysts modified RDE at a rotation rate of 900 rpm and a sweep rate of 10 mV s−1. For MeOH crossover study, a calculated portion of MeOH was directly added into the KOH solutions with O2 sparging. CVs and LSVs were then measured. The IT measurement was conducted similar to the ORR operation, but N2 instead of O2 was used at the initial stage. Accelerated durability testing (ADT) was performed by potential cycling between 0.6 and 1 V for 1000 cycles in an O2-saturated 0.1 M KOH solution at a scan rate of 50 mV s−1. The kinetic current densities (jk) associated with the intrinsic activity of the catalysts can be obtained by the following relation:

Pt/C) electrocatalysts show slightly decreased mass activity but enhanced specific activity. Obviously, the CN− anions surface modification process involves the “Elsner equation”, 4Pt + 8[CN]− + 2H2O + O2 → 4[Pt(CN)2]− + 4[OH]−, where hydroxide anions are inevitably formed on the Pt surface at the end of the reaction, while the present ·CN chemisorption strategy excluded the formation of negative charges because the direct radical addition, ·CN + Pt → PtCN, occurred instead. Therefore, the unique electrochemical performance of CN−Pt/C is suggested to be derived from the covalent nature of the interaction between ·CN and Pt surface,17,20−24 which in particular avoids the formation of detrimental OHad−K+(H2O)x as well as the CN−K+(H2O)x or CN−Na + (H 2O) x clusters on Pt catalyst surface. DFT calculations further show that the electronic interaction between guest molecules (e.g., O2 and MeOH) and ·CN chemisorbed Pt surface becomes rather different relative to unmodified Pt in KOH solution, making CN−Pt/C an efficient cathode electrocatalyst to alleviate the MeOH crossover problem in DMFCs.

■

EXPERIMENTAL SECTION Photocyanation Process. An amount of 40 mg Pt/C catalyst (20 wt %) bought from Alfa Aesar Co. Ltd. was mixed with 5 mL of MeCN solvent and 200 μL of H2O2 (30 wt %). The mixture was then stirred under UV light for merely 20 min (this photoreaction time is enough to generate sufficient ·CN radicals for subsequent Pt surface chemisorption) and subsequently stirred under dark conditions for several hours in order to rationally control the degree of CN modification on Pt surface; the solid product was finally obtained by rotary evaporation at 80 °C. Characterization. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-1230 operated at 80 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. All binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. X-band electron paramagnetic resonance (EPR) signals were recorded at ambient temperature on a Bruker EPR A-300 spectrometer. The settings for the EPR spectrometer were as follows: center field, 3511.39 G; sweep width, 100 G; microwave frequency, 9.86 G; modulation frequency, 100 kHz; power, 101 mW; conversion time, 10 ms. The spin trapping experiments were performed as follows: a calculated amount of ice-cooled 5,5dimethyl-1-pyrroline N-oxide (DMPO) solution (0.08 M) was added into the MeCN/H2O2 binary system. The resulting mixture was quickly transferred into a glass capillary tube and tested by EPR spectroscopy at room temperature with/without UV light irradiation. A 100 W Hg lamp (LOT Oriel) was used to provide UV light. Electrocatalysis. For commercial Pt/C or CN−Pt/C electrocatalysts, an amount of 3.5 mg of catalyst was dispersed in 1 mL of solution (the volume ratio of H2O/isopropanol/5% Nafion = 4:1:0.05) and sonicated for 1 h. An amount of 3 μL of the dispersion was then transferred onto the glass carbon rotating disk electrode (RDE) with a geometric area of 0.071 cm2 and then dried at room temperature so that the Pt loading is 29.7 μg/cm2 based on the geometric electrode area. Electrochemical measurements were carried out with a three electrode system on a CHI750e electrochemical workstation. A platinum wire and SCE were used as the counter and reference

1 1 1 = + j jk jd

where j is the measured current density, jk is the kinetic current density, and jd is the diffusion-limited current density, respectively. The eletrochemical surface area (ECSA) was calculated by Hupd (upd: underpotential deposition) using the following equation: ECSA =

QH 0.21[Pt]

where QH (mC) is the charge due to the hydrogen desorption in the hydrogen region (0.05−0.45 V) of the CVS, 0.21 mC cm−2 is the electrical charge associated with monolayer adsorption of hydrogen on Pt, and [Pt] is the loading of Pt on the working electrode. DFT Calculations. Adsorption of O2 and MeOH was evaluated with the Castep program.25 The PBE functional was used together with the ultrasoft core potentials.26,27 Among the three Pt layers, the lower two layers were fixed to the crystalline structure of Pt(111) surface, and the geometries of the top Pt layer, CN, KOH, and adsorbate were optimized. CN free surfaces were also examined for comparison.

■

RESULTS AND DISCUSSION Figure 1a shows the process of ·CN radical chemisorption. The generation of ·CN radical under UV irradiation was confirmed by in situ EPR technique, which will be shown later. It is expected that ·CN chemisorption strategy leads to a coverage of CN groups on the Pt surface. However, the traditional cyanation process involving the participation of CN− anions makes the Pt surface unavoidably covered with Na+ or K+ cations (Figure 1b), resulting in the feasible formation of detrimental OHad−K+(H2O)x as well as the CN−K+(H2O)x or CN−Na+(H2O)x clusters on Pt catalyst surface, especially in alkaline solution.17−19 Due to the mild reaction condition, we noticed that the microscopic morphology as well as the size distribution of PtNPs underwent small change after photo-Fenton cyanation B

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Schematic illustrations of (a) the ·CN radical chemisorption and (b) CN− anions surface modification processes.

(2.8−2.7 nm), as revealed by the TEM images (Figure 2), implying that only surface cyanation can be achieved.

Figure 3. (a) In situ EPR spectra of DMPO adducts recorded in MeCN/H2O2 binary system (inset shows the enlarged spectrum of the black line) and (b) normalized ·CN radical intensity of the H2O2 based photocyanation system (red line) and the benzaldehyde based photocyanation system (black line) as a function of reaction time. Figure 2. TEM images and size distribution of PtNPs on carbon supports (a) before and after photo-Fenton cyanation for (b) 10 h and (c) 20 h; scale bar = 20 nm.

·CN and original ·OH radicals were strengthened over extended photoreaction time. Importantly, the free radicals can exist in the reaction system for hours under dark conditions (Figure 3b, red line; herein, only the initial 10 min reaction is shown). This property is distinct from the case using benzaldehyde as the ROS initiator in which cyanation process ceased as soon as UV light was closed (Figure 3b, black line), demonstrating the superior performance of H2O2 reactants. Therefore, after turning off the UV light, PtNPs can still be chemisorbed with ·CN radicals. CN−Pt/C electrocatalysts with different ·CN coverage (CN−Pt/C-xh) were prepared by simply controlling the dark reaction time x, as introduced in the Supporting Information. The surface coverage of the ·CN and the electronic interaction between Pt and the radicals were determined by X-ray photoelectron spectroscopy (XPS). The detailed characterization information on the samples is given in Table S1 in the Supporting Information. Three typical samples, unmodified Pt/ C, CN−Pt/C-10h, and CN−Pt/C-20h were selected, and their XPS data are shown in Figure 4. The unmodified Pt has an amount of Pt(II) species (∼27.7%) most likely originating from the spontaneous oxygen capping effect, producing surface Pt(II)O species (Figure 4a, black line). However, as the photo-Fenton cyanation proceeded, the surface oxygen content became less and less (entries 1−5, Table S1), indicating that oxygen could be gradually replaced by CN groups. For the CN−Pt/C-10h sample, in addition to Pt(0) signal, Pt(I) dominates the oxidation state of Pt (Figure 4a, red line); the binding energies (BE) at 72.0 and 75.1 eV correspond to the Pt(I) 4f7/2 and Pt(I) 4f5/2 signals, respectively. It means that Pt(I)−CN species was formed on the surface of Pt, and its concentration was estimated to be approximately 23.1% (entry 4, Table S1). For the CN−Pt/C-20h sample (Figure 4a, blue

Moreover, the crystallinity shows no change after the chemisorption of CN on the surface of Pt NPs (Figure S5, Supporting Information). This feature quite differs from the case of the reaction with AuNPs, where complete cyanation totally changed the size and morphologies of the Au NPs.28 This difference is linked to their reactivities toward CN radicals.7 The radical-involved nature and the versatility of this photocyanation method were demonstrated by the in situ room-temperature EPR and spin-trapping technique. As shown in Figure 3a, a very weak EPR signal emerged in the ternary system containing DMPO spin-trapping reagent, H2O2, and MeCN solvent under dark conditions (black line), which is ascribed to the DMPO−•OOH adducts (inset shows its 6times enlarged hyperfine structure with αN = 13.5 G; αHβ = 12.6 G) most likely originating from the self-dissociation of H2O2 at room temperature. Notably, a rather prominent four-line signal appeared as soon as external UV light was onset (red line). We suggested that the broadened signals are essentially composed of dual but overlapped oxygen-centered radicals, that is, hydroxyl radicals (DMPO−•OH, αN = αHβ = 12.5 G) and peroxide radicals (DMPO−•OOH). In our previous work,28 it was revealed that the ·OH species plays an important role in cleaving the strong C−C bonds of MeCN into ·CN radicals. This rule still works here, since the similar six-line EPR signal (as indicated by the black dots) characteristic of the DMPO−•CN adduct (αN = 14.5 G; αHβ = 22.4 G) was also generated after 20 s of UV irradiation. Both of the intensities of C

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

this is the first time the ·CN radical adsorption on the surfaces PtNPs is achieved at room temperature. As an important constitution of DMFCs, alkaline DMFCs have attracted growing interest owing to their improved kinetics for both MOR and ORR reactions as well as recent advancements in anion-exchange membrane materials.11,31 In the present study, cyclic voltammetry (CV) was used to evaluate the electrochemical surface area (ECSA) of the CN− Pt/C nanocatalysts in KOH electrolyte. Figure 5a compares the

Figure 4. XPS spectra of (a) Pt 4f and (b) N 1s peaks of pristine Pt/C (black line (1)) and CN−Pt/C electrocatalysts where red (2) and blue lines (3) indicate CN−Pt/C-10h and CN−Pt/C-20h, respectively.

line), the Pt(II) signal increased significantly (the BEs at 73.1 and 76.8 eV correspond to the Pt(II) 4f7/2 and Pt(II) 4f5/2 signals, respectively), and the Pt(I)−CN plus Pt(II)(CN)2 concentration is about 52.1% (entry 5, Table S1). Thus, prolonged dark reaction apparently enabled the oxidation of Pt(I) into Pt(II) by free ·CN radicals; that is, one more CN group has grafted onto the same Pt atom, forming Pt(II)(CN)2 complex. The cyanation level could be estimated from the total concentration of Pt(I)−CN and Pt(II)(CN)2 species. It is concluded that 5.5%, 13.1%, 24.4%, and 52.1% of the surface Pt atoms have been cyanided after 1, 5, 10, and 20 h of photo-Fenton reaction, respectively. In addition, the surface nitrogen content analysis could also provide additional information on the cyanation levels (Figure 4b). According to the XPS analysis, the atomic ratio of N/Pt is calculated to be 6.2%, 17.5%, 29.3%, and 46.2% for CN−Pt/C-1h, CN−Pt/C5h, CN−Pt/C-10h, and CN−Pt/C-20h samples (entries 1−5, Table S1), respectively, which is in accord with the results calculated from the total Pt(CN)x species. The above results reveal that the density of surface CN groups can be easily tailored by the dark reaction time (Figure 4b). Compared to fast CN radical generation (Figure 3b), CN coverage gradually increases to 40% after a 20 h dark reaction, suggesting that the chemisorption is a much slower process. Thus, it is reasonable to use slow process (chemisorption) to rationally control the CN coverage of the Pt/C electrocatalyst. Additionally, it is interesting to note that the BE of N 1s in CN−Pt/C-20h sample shows a higher BE at 399.3 eV as compared to a lower BE at 398.4 eV for CN−Pt/C-10h. It implies that a strong electronic interaction may exist between the two CN groups chemisorbed on one Pt atom. The ·CN radical adsorption on metal surfaces has been achieved by dissociation of adsorbed dicyanogen in vacuum29 or high-temperature acetonitrile dissociation on hot Pt surface.30 To the best of our knowledge,

Figure 5. (a) Cyclic voltammetry curves of CN−Pt/C electrocatalysts with different cyanation levels under N2-saturated 0.1 M KOH electrolyte and (b) the surface CN coverage percentage (blue) and ECSA values (red) as a function of the dark reaction time.

CV curves on different Pt/C catalysts recorded in N2-purged KOH solution at a sweep rate of 50 mV/s. The ECSA of Pt/C catalysts was calculated by measuring the Coulombic charge for hydrogen desorption. It clearly shows that the ECSA values decrease as a function of CN coverage (Figure 5b), suggesting that ·CN radicals chemisorption could block the active sites on Pt surface. Notably, it is found that CN groups are remarkably stable between 0 and 1.2 V (vs RHE), which is in accord with previous ex situ and in situ measurements,32,33 so the CN−Pt/ C electrode can be considered as a stable chemically modified electrode. Consistent with the change of ECSA, ·CN radical chemisorbed Pt/C electrocatalysts show only a slightly decreased ORR activity with an increase of CN, which is completely different from a 50-fold decrease of the ORR activity observed for the CN− anions modified Pt catalyst.17−19 Figure 6a shows typical ORR polarization curves of clean Pt/C and CN−Pt/C electrodes obtained at room temperature in O2saturated 0.1 M KOH electrolyte using a rotating disk electrode (RDE) at 900 rpm and a potential sweep rate of 10 mV s−1. The corresponding linear sweep voltammetry (LSV) curves show well-defined diffusion limiting currents region below 0.8 V followed by a mixed kinetic−diffusion control region over a potential window of 0.8−1 V. The electrocatalytic activities of the different catalysts, as estimated from the half-wave potentials (E1/2), were maximized for Pt/C (0.865 V) and decreased successively for CN−Pt/C-1h, CN−Pt/C-10h, and D

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

covalent-bonding nature of ·CN radical and Pt surface. The covalent-bonding nature between ·CN radical and Pt surface avoids the formation of OHad−K+(H2O)x clusters on Pt surface, whereas the CN− anions carry a full negative electronic charge that needs surface adsorbed OHad−K+(H2O)x to balance the charge.34,35 In principle, the feasible formation of surface CN− K+(H2O)x or CN−Na+(H2O)x clusters in alkaline medium will block the O2 adsorption on CN− anion adsorbed Pt surfaces and will result in a drastic decrease of ORR activity.17 The enhanced intrinsic catalytic activity of the CN-Pt/C samples may be correlated with the changes in the adsorption capability to the oxygen reduction intermediates. Figure 5a shows that the current peak associated with the adsorption of oxygenate species (e.g., OH−) in the CV (0.7−0.8 V) obtained on the CN−Pt/C becomes weaker relative to the Pt/C catalyst, which may due to the chemisorption of CN on the surface of Pt. On the basis of this result, the desorption of oxygenate species on the CN−Pt/C surface may be easier than on the Pt/ C catalyst, leading to the enhanced intrinsic activity. More importantly, CN−Pt/C catalysts exhibit much enhanced MeOH-tolerant ability as compared with Pt/C. The polarization profiles of Pt/C and CN−Pt/C electrocatalysts recorded under O2-saturated KOH electrolyte with 0.1 M MeOH are shown in Figure 8. It is noted that the MOR signals

Figure 6. (a) Linear sweep voltammetry curves of CN−Pt/C electrocatalysts with different cyanation levels during ORR operation recorded under O2-saturated 0.1 M KOH in 900 rpm and (b) their corresponding mass activity at 0.85 V.

CN−Pt/C-20h with E1/2 values of 0.860, 0.858, and 0.850 V, respectively. For a better understanding of the ORR activities of the different catalysts, the kinetic current (jk) of each catalyst was obtained by constructing Koutecky−Levich plots for oxygen reduction with different catalysts. At 0.85 V, the mass activities for Pt/C, CN−Pt/C-1h, CN−Pt/C-10h, and CN−Pt/ C-20h were calculated to be 242.9, 219, 214.3, and 181 A·g−1pt, respectively (Figure 6b). Considered together with the observed E1/2 values, these data showed that the Pt/C with CN chemisorption displayed a slightly decreased electrocatalytic performance. Interestingly, as shown in Figure 7, the intrinsic ORR activity estimated from the specific activity was improved with increasing chemisorption of CN groups and then declined with more CN groups.

Figure 8. Linear sweep voltammetry curves of Pt/C and CN−Pt/C electrocatalysts with different cyanation levels during ORR operation in O2-saturated 0.1 M KOH electrolyte with 0.1 M MeOH.

around 0.7 V appeared for CN modified Pt/C regardless of the cyanation level, while observable mixed kinetic−diffusion control region for ORR has been well preserved. This result revealed that an appropriate concentration of MeOH can be tolerated during ORR operation after ·CN chemisorptions. In contrast, in the case of unmodified Pt/C (black line), its polarization profile indicates that MOR governed the electrocatalysis. The different responses demonstrated the much enhanced MeOH tolerance of CN−Pt/C as the ORR catalyst. Moreover, the CN−Pt/C-10h sample owns the best ORR activity as seen in mixed kinetic−diffusion control region between 0.8 and 1.0 V after adding 0.1 M MeOH, which is almost consistent with the polarization profile without MeOH as shown in Figure 6a. The sample also has the lowest MeOH oxidation response compared to CN−Pt/C-1h and CN−Pt/C20h. The current−time (IT) chronoamperometric response of Pt/ C and CN−Pt/C-10h upon adding 0.5 M MeOH was conducted in KOH solution. As shown in Figure 9, the ORR current was generated and gradually kept steady (∼2.5 mA) after sparging O2 for 400 s for Pt/C electrocatalysts with or

Figure 7. Specific activity of Pt/C and CN−Pt/C electrocatalysts with different cyanation levels at 0.85 V in 0.1 M KOH.

In previous reports, cyanide anions (CN−) decorated Pt catalysts showed a 50-fold decrease of the ORR activity in alkaline solutions.17−19 However, ·CN chemisorbed Pt/C electrocatalysts show slightly decreased mass activity but enhanced specific activity. The unusual electrocatalytic properties of the CN−Pt/C electrocatalysts can be ascribe to the E

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

addition of 0.3 M MeOH, the ORR peak of Pt/C thoroughly disappears, while it obviously exists for the CN−Pt/C-10h sample, indicating at least a 3-fold enhancement of MeOH tolerance after the CN radical chemisorption. Although the present CN−Pt/C catalysts only showed enhanced MeOHtolerant property and cannot completely prohibit the MeOH crossover problem, it is believed that the green photo-Fenton cyanation process introduced here can be readily extended to chemically modify other alloy electrocatalysts with controllable CN groups; thus the complete prohibition of MeOH crossover in DMFCs might be possible. In order to better understand the enhanced MeOH-tolerant property of CN−Pt/C electrocatalyst, DFT calculations were carried out to elucidate the electronic interactions between guest molecules (e.g., O2 and MeOH) and Pt surface with or without ·CN radical chemisorption. A linear on-top adsorption geometry is utilized in theoretical calculation based on previous study.35 Since the ·CN radical coverage is close to 0.3 in CN− Pt/C-10h sample, we applied an adsorption pattern that is identical to CN− anion adsorption system.20−22 The CN−Pt/ C-10h sample was selected because it has a similar ORR property but outperformed Pt/C catalyst in the anti-MeOH poisoning experiment. Therefore, the geometry of ·CN radical adlayer on Pt surface in CN−Pt/C-10h may adopt the routine topological structure with periodic six linearly chemisorbed CN groups constituting an inerratic hexagon on Pt atoms, leaving a free Pt atom inside (Scheme S1 in the Supporting Information). If this model is true, the optimal Pt(I)/Pt ratio should be at a percentage of ∼25% by taking both of the theoretical model and surface atom dispersion ratio into account.36 This ratio is in close agreement with the XPS results shown in Table S1 (∼23.1% for Pt(I)/Pt ratio and ∼29.3% for N/Pt ratio). In the case of Pt(CN)2 complex where overcyanation was obtained, the periodicity is interrupted by continuous ·CN radicals grafting on Pt atoms within hexagons. Unlike Pt−CN with CN “islands” regularly isolating on Pt(111) surface, the tight structure of Pt(CN)2 is supposed to prevent its successive cyanation and also the adsorption/ reduction of O2, resulting in its inferior ORR performance. A Pt36(CN)6 unit cell with six regularly adsorbed CN groups and six KOH molecules floating on the Pt36 surface (Figure 11) was employed for DFT calculations. The adsorption energies (EA’s) for O2 and MeOH are shown in Figure 12 for

Figure 9. Current−time response of Pt/C and CN−Pt/C-10h electrocatalysts in N2, O2, and MeOH (0.5 M) involved KOH electrolyte, respectively.

without CN chemisorption. Once a portion of MeOH (0.5 M equals the electrolyte solution) was purged into the reaction system, the current suddenly converted from negative to positive for Pt/C system, indicating that ORR was completely inhibited but MOR occurred instead. In contrast, in the case of CN−Pt/C-10h electrocatalyst, ORR current displayed an instant current jump to positive region but remained at the negative domain after the addition of MeOH, indicating its enhanced MeOH tolerance. To obtain more details of MeOH tolerance property, we compared the CV curves of unmodified Pt/C and CN−Pt/C10h electrocatalysts operated under different MeOH concentration (Figure 10a and Figure 10b). Well-defined ORR peak around 0.8 V is discerned for CN−Pt/C-10h sample with the addition of 0.1 M MeOH. However, the ORR peak of Pt/C becomes smaller and shifts to a lower potential. After the

Figure 10. Cyclic voltammetry curves of Pt/C and CN−Pt/C-10h electrocatalysts during ORR operation recorded under O2-saturated (a) 0.1 M and (b) 0.3 M KOH with different concentrations of MeOH.

Figure 11. Optimized structures for Pt36 unit cells without (a) and with (b) six KOH molecules. Pt slab consists of three layers, and only the atoms in the top layer are optimized as well as KOH and adsorbed molecules. F

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 12. Comparison of adsorption energies for O2 and MeOH molecules inside (termed as “In”) or outside (termed as “Out”) the KOH layer on the surface of different Pt36-based unit cells.

comparison (detailed information is shown in Figures S1−S4 and Table S2 in the Supporting Information). On the clean Pt surface with a KOH layer (labeled as “Pt36”), EA’s for O2 molecule outside or inside the KOH layer were estimated to be −225.6 and −595.2 kJ/mol, respectively. For Pt36(CN)6 cluster, the two EA’s were changed into −194.2 and −186.4 kJ/mol. Therefore, the O2 adsorption behavior does not change much on Pt surface with or without CN modification. However, the interactions between MeOH and Pt atoms became rather different after CN radical chemisorption. As shown in the right part of Figure 12, on clean Pt cluster, EA’s for MeOH molecule outside or inside the KOH layer were calculated to be −228.5 and −61.9 kJ/mol, respectively, whereas the same EA’s became rather weak on CN modified Pt surface whether MeOH molecules are outside (83.4 kJ/mol) or inside (126.5 kJ/mol) the KOH layer. It means that MeOH adsorption becomes very unstable on CN−Pt surface in alkaline solution. Therefore, it is concluded that ·CN radical chemisorbed Pt catalyst with neutral surface not only preserves the O2 adsorption ability but also hinders the adsorption of MeOH molecules. To investigate the generality of the ·CN radical chemisorbed Pt electrocatalyst, the electrochemical experiments were further conducted in acid medium. As shown in Figure 13, both the ECSA and mass activity of CN−Pt/C electrocatalysts decline in 0.1 M HClO4 with the increase of cyanation time, which is identical to the tendency in alkaline medium. Moreover, the specific activities of all the three chemisorbed electrocatalysts are better than Pt/C (Figure 14a), indicating the enhancement of the intrinsic activity. Importantly, the LSVs of Pt/C and CN−Pt/C electrocatalysts with different cyanation levels in O2-saturated 0.1 M HClO4 with 0.1 M MeOH show that the MeOH oxidation peak (∼0.7 V) of CN−Pt/C is lower than Pt/C, demonstrating the improved MeOH tolerance performance of CN−Pt/C even in acid medium (Figure 14b), indicating the steric blocking effect of CN groups on Pt surfaces. The catalysts’ stability was evaluated by the accelerated durability testing (ADT). Pt/C shows a degradation of 12 mV in its half-wave potential after 1000 potential cycles, whereas CN−Pt/C-10h declines by 10 mV, which indicates that the stability of the CN chemisorbed Pt electrocatalyst is slightly better than the commercial Pt/C (Figures S6 and S7, Supporting Information). The MeOH-tolerant property of the two catalysts after ADT was also investigated by LSV test (Figure 15). The ORR curve of CN−Pt/C after ADT was preserved well except for a mild peak observed around 0.7 V. However, the MOR occupied the electrocatalysis and ORR was

Figure 13. (a) CVs of CN−Pt/C electrocatalysts with different cyanation levels under N2-saturated 0.1 M HClO4 and (b) corresponding ECSA values. (c) LSVs of CN−Pt/C electrocatalysts with different cyanation levels under O2-saturated 0.1 M HClO4 in 900 rpm and (d) corresponding mass activity in 0.85 V.

Figure 14. (a) Specific activity of Pt/C and CN−Pt/C electrocatalysts with different cyanation levels at 0.85 V in 0.1 M HClO4. (b) LSVs of Pt/C and CN−Pt/C electrocatalysts with different cyanation levels in O2-saturated 0.1 M HClO4 with 0.1 M MeOH.

suppressed during the potential scan by Pt/C. The result indicates the MeOH-tolerant property of the CN radical chemisorbed Pt/C electrocatalyst still exists after the stability test.

■

CONCLUDING REMARKS In summary, we applied a ·CN radical chemisorption strategy to regulate the interactions between reactants and metal surface. In contrast to the normal CN− anions adsorbed Pt catalysts, the ·CN radical modified Pt/C electrocatalysts display good ORR performance in addition to a much enhanced G

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

■

ACKNOWLEDGMENTS We are grateful for financial support from the NSFC (Grants 21222307, 21373181, 21403197, 91545113, and 21503189), the Fundamental Research Funds for the Central Universities (Grant 2014XZZX003-02), Zhejiang Provincial Natural Science Foundation of China (Grant LY15B030009), and China Postdoctoral Science Foundation (Grants 2014M550333 and 2015T80636). Part of this work (the DFT calculations) was supported by the Grand-in-Aid (No. 23560934) from MEXT of Japan. The authors are also grateful to Prof. Jian-Qiang Wang from the Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, for the EXAFS analysis.

Figure 15. Linear sweep voltammetry curves of Pt/C after ADT and CN−Pt/C-10h after ADT during ORR operation in O2-saturated 0.1 M KOH electrolyte with 0.1 M MeOH.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02993. Table S1, the atomic ratio of Pt(x)/Pt and N/Pt of Pt/C and CN−Pt/C-xh samples based on the XPS data; Scheme S1, ball model of the (2√3 × 2√3)R30° structure adopted by the CN adlayer on Pt(111) surface; Figure S1, optimized structures of CH3OH on Pt366KOH. Outer (left) and lower (right) positions: Figure S2, optimized structures of CH3OH on Pt36(CN)6· 6KOH. Outer (left) and lower (right) positions; Figure S3, optimized structures of O2 on Pt36-6KOH. Outer (left) and lower (right) positions; Figure S4, optimized structures of O2 on Pt36(CN)6·6KOH. Outer (left) and lower (right) positions; Table S2, adsorption energy of O2 and CH3OH on modified Pt slab (PDF)

■

REFERENCES

(1) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299 (5613), 1688−1691. (2) Prieto, G.; Zecevic, J.; Friedrich, H.; de Jong, K. P.; de Jongh, P. E. Towards Stable Catalysts by Controlling Collective Properties of Supported Metal Nanoparticles. Nat. Mater. 2013, 12 (1), 34−39. (3) Friedrich, H.; de Jongh, P. E.; Verkleij, A. J.; de Jong, K. P. Electron Tomography for Heterogeneous Catalysts and Related Nanostructured Materials. Chem. Rev. 2009, 109 (5), 1613−1629. (4) Cao, A.; Veser, G. Exceptional High-Temperature Stability Through Distillation-Like Self-Stabilization in Bimetallic Nanoparticles. Nat. Mater. 2010, 9 (1), 75−81. (5) Cao, A.; Lu, R.; Veser, G. Stabilizing Metal Nanoparticles for Heterogeneous Catalysis. Phys. Chem. Chem. Phys. 2010, 12 (41), 13499−13510. (6) Lu, J. L.; Fu, B. S.; Kung, M. C.; Xiao, G. M.; Elam, J. W.; Kung, H. H.; Stair, P. C. Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition. Science 2012, 335 (6073), 1205−1208. (7) Schrinner, M.; Ballauff, M.; Talmon, Y.; Kauffmann, Y.; Thun, J.; Möller, M.; Breu, J. Single Nanocrystals of Platinum Prepared by Partial Dissolution of Au-Pt Nanoalloys. Science 2009, 323 (5914), 617−620. (8) Li, R. H.; Kobayashi, H.; Guo, J. F.; Fan, J. Visible-Light Induced High-Yielding Benzyl Alcohol-to-Benzaldehyde Transformation over Mesoporous Crystalline TiO2: A Self-Adjustable Photo-Oxidation System with Controllable Hole-Generation. J. Phys. Chem. C 2011, 115 (47), 23408−23416. (9) Li, R. H.; Kobayashi, H.; Guo, J. F.; Fan, J. Visible-Light-Driven Surface Reconstruction of Mesoporous TiO2: Toward Visible-Light Absorption and Enhanced Photocatalytic Activities. Chem. Commun. 2011, 47 (30), 8584−8586. (10) Corti, H. R.; Gonzalez, E. R. Direct Alcohol Fuel Cells: Materials, Performance, Durability and Applications; Springer Science & Business Media: New York, 2014. (11) Lei, M.; Wang, J.; Li, J. R.; Wang, Y. G.; Tang, H. L.; Wang, W. J. Emerging Methanol-Tolerant AlN Nanowire Oxygen Reduction Electrocatalyst for Alkaline Direct Methanol Fuel Cell. Sci. Rep. 2014, 4, 6013. (12) Batista, E. A.; Malpass, G. R. P.; Motheo, A. J.; Iwasita, T. New Mechanistic Aspects of Methanol Oxidation. J. Electroanal. Chem. 2004, 571 (2), 273−282. (13) Batista, E. A.; Malpass, G. R. P.; Motheo, A. J.; Iwasita, T. New Insight into the Pathways of Methanol Oxidation. Electrochem. Commun. 2003, 5 (10), 843−846. (14) Wang, W. M.; Huang, Q. H.; Liu, J. Y.; Zou, Z. Q.; Zhao, M. Y.; Vogel, W.; Yang, H. Surface and Structure Characteristics of CarbonSupported Pd3Pt1 Bimetallic Nanoparticles for Methanol-Tolerant Oxygen Reduction Reaction. J. Catal. 2009, 266 (1), 156−163. (15) Oh, J. G.; Kim, H. Synthesis and Characterization of PtNx/C as Methanol-Tolerant Oxygen Reduction Electrocatalysts for a Direct Methanol Fuel Cell. J. Power Sources 2008, 181 (1), 74−78.

MeOH-tolerant property in both alkaline and acidic solutions. The covalent-bonding nature between ·CN radical and Pt surface is expected to hinder the formation of OHad−K+(H2O)x and other relative detrimental clusters on Pt surface, leading to a slightly decreased mass activity but enhanced specific activity for ORR property of Pt/C operated in an alkaline electrolyte. On the basis of the DFT calculations, we also found that the adsorption of MeOH molecules becomes more difficult as compared to unmodified Pt in KOH solution, making CN−Pt/ C an efficient cathode electrocatalyst to alleviate the MeOH crossover problem. The green nature and simplicity of the current strategy can facilitate their pratical use on a large scale and may be extended to other important metal-catalyzed reactions, where the interactions with reactants and products are crucial.

■

Article

AUTHOR INFORMATION

Corresponding Authors

*R.L.: phone, +86-0571-8684-3611; e-mail, [email protected]. cn. *H.K.: phone, +81-75724-7580; e-mail, [email protected]. *J.F.: phone, +86-0571-8795-2338; e-mail:, [email protected]. Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.jpcc.6b02993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Sulphuric and Perchloric Media. Electrochim. Acta 1998, 44 (6), 943− 948. (33) Huerta, F. J.; Morallón, E.; Vazquez, J.; Aldaz, A. Voltammetric and Spectroscopic Characterization of Cyanide Adlayers on Pt (h, k, l) in an Acidic Medium. Surf. Sci. 1998, 396 (1), 400−410. (34) Kim, Y. G.; Yau, S. L.; Itaya, K. Direct Observation of Complexation of Alkali Cations on Cyanide-Modified Pt(111) by Scanning Tunneling Microscopy. J. Am. Chem. Soc. 1996, 118 (2), 393−400. (35) Daum, W.; Dederichs, F.; Muller, J. E. Covalent Bonding of CN to the Pt(111) Surface. Phys. Rev. Lett. 1998, 80 (4), 766−769. (36) According to the model, the optimal atomic ratio of Pt(I) against the total Pt species (Pt(I)/Pt) on the top layer of Pt(111) surface is calculated to approach 50%. In addition, for a particle with its size at the nanoscale (