CoPc- and CoPcF16-Modified Ag Nanoparticles as Novel Catalysts

ARTICLE pubs.acs.org/JPCC

CoPc- and CoPcF16-Modified Ag Nanoparticles as Novel Catalysts with Tunable Oxygen Reduction Activity in Alkaline Media Junsong Guo,† Haixia Li,† Hui He,† Deryn Chu,‡ and Rongrong Chen*,† † ‡

Richard G. Lugar Center for Renewable Energy, Indiana UniversityPurdue University, Indianapolis, Indiana 46202, United States U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States ABSTRACT: A new class of non-Pt “hybrid” catalysts is reported that is intended as a lower cost alternative that meets the performance and durability requirements of the oxygen cathode in fuel cells with alkaline electrolytes. These novel hybrid catalysts are cobalt (Co) phthalocyanine on silver (Ag) nanoparticles on a carbon (C) support. For O2 reduction, a catalyst with cobalt phthalocyanine (CoPc) on Ag nanoparticles on carbon (CoPc@Ag/C) or the fully fluorinated cobalt phthalocyanine (CoPcF16) on Ag nanoparticles on carbon (CoPcF16@Ag/C) showed lower overpotentials and higher rate constants at a given potential than did CoPc and CoPcF16 on carbon (CoPc/C and CoPcF16/C) and Ag nanoparticles on carbon (Ag/C). The oxygen reduction reaction (ORR) activity of the cobalt-phthalocyanine-modified Ag nanocatalysts is tuned by adjusting the composition of the metalated phthalocyanine molecules adsorbed on Ag. X-ray photoelectron spectroscopy (XPS) and electrochemical characterizations indicate that the ORR on these new catalysts occurs almost entirely by a 4-electron pathway and shows 50 mV lower overpotential in the half-wave potential for CoPc@Ag/C and 82 mV for CoPcF16@Ag/C as compared to that found when using Ag/C as the catalyst. The current densities are higher and Tafel slopes are lower for ORR on CoPc@Ag/C and CoPcF16@Ag/C than on Ag/C catalysts in the high overpotential region. This indicates higher rate constants at any given potential are obtained with CoPc@Ag/C and CoPcF16@Ag/C than with Ag/C as the catalyst.

1. INTRODUCTION In recent years, there has been a growing interest in oxygen reduction reactions (ORRs) in alkaline media due to the development of anion exchange membrane fuel cells (AEMFCs).18 As compared to proton exchange membrane fuel cells (PEMFCs), one of the key advantages of the AEMFCs is the possibility of using non-Pt electrocatalysts for ORR. Several non-Pt catalysts, including Ag,9,10 Au,1113 Pd,14,15 cobalt and manganese oxide,1619 porphyrins,20 and phthalocyanines,21 have been extensively studied. Among the various catalytic systems recently studied, Ag, relatively inexpensive and abundant, is considered a top candidate for replacing Pt as a cathode catalyst for AEMFC applications. Ag has a relatively high electrocatalytic activity for oxygen reduction via an approximated 4-electron ORR pathway.22 However, the improvement of ORR kinetics on Ag nanoparticles in an alkaline media remains a desirable goal: for example, the ORR overpotentials on Ag catalysts are more than 100 mV higher than that on Pt catalysts under the same test conditions.2224 Alkalinebased fuel cells or metalair batteries that utilize Ag-based catalysts for ORR are often limited to low current density applications. Prior to the current study, high current densities had not been achievable on Ag-based electrodes. Adsorbed organic compounds on metallic substrates have recently attracted considerable attention with respect to the controlled functionalization of surfaces in the nanoscale.2527 Planar metal complexes such as M(II)porphyrins (MP) and r 2011 American Chemical Society

M(II)phthalocyanines (MPc) are of particularly interest due to their unique physical and chemical properties.20,21,2733 The metal centers of MP or MPc molecules often possess no axial ligands and represent coordinatively unsaturated sites with potential catalytic functionality; for example, CoPc and FePc molecules are known to have good electrocatalytic activities for oxygen reduction.21 Electronic and geometric properties of Co tetraphenylporphyrin (CoTPP) layers on Ag (111) were studied by Auw€arter et al. using photoelectron diffraction (PED), near-edge X-ray absorption fine-structure (NEXAFS) measurements, and DFT calculations.31 This fundamental work indicated that (1) the central Co atom of the Co-TPP resides predominantly above the fcc and hcp hollow sites of the Ag (111) substrate; and (2) the interaction of the CoTPP with the Ag (111) substrate can induce modifications of the CoTPP molecular configuration, such as a distorted macrocycle with a shifted position of the Co metal center. By using similar experimental techniques and DFT calculations, Baran et al. investigated the interaction of a number of phthalocyanine molecules (SnPc, PbPc, and CoPc) with the Ag (111) surface.33 Each of the phthalocyanine molecules (SnPc, PbPc, and CoPc) was found to donate a charge to the silver surface, and that back-donation from Received: December 22, 2010 Revised: March 21, 2011 Published: April 07, 2011 8494

dx.doi.org/10.1021/jp112197f | J. Phys. Chem. C 2011, 115, 8494–8502

The Journal of Physical Chemistry C Ag to the metal atom Co, Sn, or Pb was only significant for CoPc. The adsorbed MP or MPcs molecules were found to induce a local restructuring process of the metallic Ag substrate, which could alter Ag’s functionality and the morphology of the adsorbed MP or MPc molecules.25,3335 By combining electrochemical measurements with theoretical DFT calculations, we have conducted a series of fundamental studies to understand key factors that control ORR activity and stability on CoPc and FePc model electrodes.21,36 Adsorption energy of O2, OH, and HOOH on CoPc or FePc molecules plays a key role in determining their ORR activity and stability. The substitution of hydrogen atoms by other atoms in the ligand, such as fluorine, is also found to influence the ORR activity.37 Furthermore, when macrocyclic molecules are adsorbed on the surface of underlying substrate atoms, the electronic and geometric properties of the adsorbed macrocyclic compound molecules and the substrate atoms could be altered significantly,2535 which renders the ORR catalytic activities tunable through the selection of the adsorbed molecules and the substrate atoms. The present work studies the ORR activities of CoPc- and CoPcF16-modified Ag nanoparticles supported on carbon (CoPc@Ag/C and CoPcF16@Ag/C). These hybrid catalysts, CoPc@Ag/C and CoPcF16@Ag/C, were found to have more than 50 mV favorable O2 reduction potentials than the Ag/C catalysts. The ORR activity of the CoPc- and CoPcF16-modified Ag nanocatalysts is tunable through the adjustment of the adsorbed organic molecules.

2. EXPERIMENT SECTION 2.1. Computational Methods. DFT calculations were performed with the Material Studio DMol3 module from Accelrys (Materials Studio 4.1, DMol3, Accelrys Inc., San Diego, CA). The generalized gradient approximation of Perdew and Wang38 was used for exchange and correlation, and the double numeric basis with polarization functions was used as the automatic basis set. Density functional semicore pseudopotentials, generated by fitting all electron relativistic DFT results, were used to reduce the computation cost. Spin-unrestricted wave functions and Fermi orbital occupations were used in our calculations. The energy convergence for geometry optimization was set to 1 105 eV. Symmetry was not imposed on any calculations. 2.2. Catalyst Preparation. 200 mg of 60 wt % silver loadings on carbon black was prepared by a citrate-protecting method as follows. 1066.2 mg of sodium citric (Alfa Aesar) and 666.0 mg of NaOH (JT Baker) were mixed to prepare 111.0 mL of 50 mM sodium citrate solution, and then 111.0 mL of 10 mM AgNO3 was added. Later, 150 mL of 7.4 mM NaBH4 (Strem chemicals) solution was added dropwise, while vigorously stirring, to obtain a yellowish-brown Ag colloid. Next, 80 mg of Vulcan XC-72 carbon black (Cabot Corp.) was dispersed into the above Ag colloid. After the suspension was stirred for 12 h, the black suspension was filtered, washed, and dried in a vacuum oven at 80 °C for 12 h, and then the 60 wt Ag/C catalyst sample was obtained. CoPc- or CoPcF16-modified Ag/C and carbon-supported CoPc and CoPcF16 catalysts were prepared by mixing 15 wt % cobalt phthalocyanine (Alfa Aesar) or 15 wt % cobalt hexadecafluoro phthalocyanine (Aldrich) with 85 wt % (60% wt % Ag/C) or 85 wt % Vulcan XC-72 carbon black uniformly in ethanol by ultrasonic stirring. After drying, the obtained samples were denoted as CoPc@Ag/C, CoPcF16@Ag/C, CoPc/C, and CoPcF16/C respectively. The CoPc and CoPcF16 are

ARTICLE

not water-soluble, but have some degree of solubility in ethanol, and are quite soluble in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). Both the CoPc@Ag/C and the CoPcF16@Ag/C catalysts were prepared using more than 10 times excessive amounts of CoPc or CoPcF16 to modify the surfaces of Ag nanoparticles. The surface coverage by the CoPc or CoPcF16 molecules on the Ag nanoparticles is not considered as a variable in this work. 2.3. Physical Characterization. X-ray photoelectron spectroscopy (XPS) was recorded by a Kratos Ultra DLD imaging spectrometer (UK) using an Al KR radiation (1486.6 eV). The binding energies were calibrated relative to C (1s) peak from carbon composition of samples at 284.8 eV. 2.4. Electrochemical Characterization. Electrochemical activities of the catalysts were measured by a setup consisting of a computer-controlled Pine potentiostat, a radiometer speed control unit from Pine instruments company (MSRX speed control), and a rotating ring disk electrode (RRDE, glassy carbon with a diameter of 5.5 mm as the disk and with platinum as the ring). Catalyst ink was prepared by ultrasonically mixing 2.0 mg of catalyst samples with 10 μL of the Nafion solution (5%), 1 mL of ethanol, and 1 mL of deionized water. Next, 40 μL of the prepared catalyst ink was dropped on the surface of the glassy carbon to form a working electrode. The electrochemical measurements were conducted in an argon or oxygen-purged 0.1 M NaOH solution using a standard three-electrode cell with a Pt wire serving as the counter electrode and a Hg/HgO/0.1 M OH electrode (0.163 V vs NHE)39 as the reference electrode. H2O2 production in O2-saturated 0.1 M NaOH electrolytes was monitored in a RRDE configuration using a polycrystalline Pt ring biased at 0.3 V vs Hg/HgO/0.1 M OH. The ring current (Iring) was recorded simultaneously with the disk current (Idisk). Collection efficiency (N) of the ring electrode was calibrated by a K3Fe(CN)6 redox reaction in an Ar-saturated 0.1 M NaOH solution. The value of the collection efficiency (N = Iring/Idisk) determined is 0.41 for the Pt/C electrode. The fractional yields of H2O2 in the ORR were calculated from the RRDE experiments as XH2O2 = (2Iring/N)/(Idisk þ Iring/N). 2.5. Oxygen Electrode Characterization. Catalyst performance of Ag/C, CoPc/C, CoPcF16/C, CoPc@Ag/C, CoPcF16@Ag/C, and Pt/C catalysts was further characterized on oxygen gas diffusion electrodes (GDE) in a cell containing 6.0 M of a NaOH solution saturated with oxygen. A catalyst ink was prepared by ultrasonically mixing 3 mg of the catalyst, 200 μL of ethanol, and 44.5 μL of the Nafion solution (5 wt %). The ink was pipetted on a 1.61 cm2 gas diffusion layer (GDL LT 1200-W, E-TEK) to prepare the oxygen electrode. The oxygen electrode was assembled into a cell with 0.71 cm2 active surface area in the working electrode. A carbon sheet was used as the counter electrode, and a Hg/HgO/6.0 M OH electrode (0.050 V vs NHE)40 was used as the reference electrode. A 6.0 M NaOH solution was adopted as the electrolyte instead of the 0.1 M NaOH solution to decrease the influence of the IR drop. Polarization curves were recorded galvanostatically with a stepwise increasing current of 2 mA s1 at room temperature.

3. RESULTS AND DISCUSSION 3.1. Physical Characterization. Using the DFT method implemented in Dmol3, the optimized structure of a CoPc and CoPcF16 molecule was obtained through structural optimization calculations and is shown in Figure 1. When the peripherical 8495

dx.doi.org/10.1021/jp112197f |J. Phys. Chem. C 2011, 115, 8494–8502

The Journal of Physical Chemistry C

ARTICLE

Figure 1. DFT-optimized CoPc and CoPcF16 molecular and electronic structures.

Table 1. Summary of DFT Calculation Results O2 adsorption energy (eV) sample

DMN (Å)

Mulliken charge

end-on A

end-on B

CoPc

1.930

0.214

0.402

0.377

CoPcF16

1.933

0.230

0.406

0.467

hydrogen atoms of the benzene rings of the CoPc (Figure 1a) were substituted with fluorine atoms of the CoPcF16 (Figure 1b), the CoN bond distance was increased, and the charges on the central Co became much more positive. After obtaining the optimized molecular structures of CoPc and CoPcF16, we calculated the adsorption energy for O2 molecule on the CoPc and CoPcF16 molecules using the same method described in our previous papers.21,36 In Table 1, we give the predicted adsorption energy for the most stable (the lowest energy) adsorption configuration of O2 molecule on the CoPc and CoPcF16, respectively. The adsorption energy of O2 molecule on the CoPcF16 molecule was 0.467 eV, which is 0.065 eV lower than that on the CoPc molecule. Higher O2 adsorption energy should result in more positive ORR onset and half-wave potentials as confirmed by the reported results.21,37 Fluorine atoms induce a higher electron affinity to the entire CoPc molecule, leading to a much more active system. Figure 2a shows the XPS narrow scan spectra of Co2p3/2 core level for Ag/C, CoPc@Ag/C, CoPcF16@Ag/C, CoPc/C, and CoPcF16/C catalysts. A peak at 780.9 eV for CoPc/C and at

781.4 eV for CoPcF16/C was observed and can be attributed to Co2þ.41 The binding energy of Co2p3/2 positively shifted 0.5 eV for the CoPcF16 catalyst, which agrees with the DFT calculation results (Figure 1c and d): more positive charges on the central Co of CoPcF16 tend to increase the Co2þ peak energy in the XPS. For the CoPc@Ag/C and CoPcF16@Ag/C, the peak positions of Co2p3/2 of Co2þ did not shift as compared to those observed for the CoPc/ C and the CoPcF16/C catalysts. However, another new peak at 779.3 eV appeared clearly for the CoPc@Ag/C and the CoPcF16@Ag/C catalysts, which can be attributed to the Co2p3/2 of Co041 and indicates that the electronic properties of the metal centers (Co) of the adsorbed CoPc or CoPcF16 molecules are affected by the underlying Ag substrate atoms. From the XPS narrow scan spectral of Ag3d5/2 in Figure 2b, two peaks located at 368.8 and 368.1 eV were distinguished by deconvolution, which can be attributed to Ag3d5/2 of Ag0 and Agþ, respectively. The observed binding energy shift of the 3d peak toward the negative direction for Ag metal versus Ag2O agrees well with what was reported.42 The contents of Ag2O on the silver surface can be calculated from the areas of the two peaks at 368.8 and 368.1 eV, which are 10.36%, 11.56%, and 12.78% for Ag/C, CoPc@Ag/C, and CoPcF16@Ag/C, respectively. The XPS results imply that electron transfer occurs between Ag and Co2þ in CoPc@Ag/C and CoPcF16@Ag/C catalysts, which could be due to back electron donation from Ag to the Co metal atom, as was reported by Baran et al.33 The electron transfer between the adsorbed organic molecules and the Ag substrate can be tuned by changing the ligand groups of the adsorbed molecules. 8496



ARTICLE

Figure 2. XPS narrow scan spectra of the Co 2p3/2 core level (a) and Ag 3d5/2 (b) for Ag/C, CoPc@Ag/C, CoPcF16@Ag/C, CoPc/C, and CoPcF16/C catalysts.

3.2. Electrochemical Characterization. Figure 3 shows the cyclic voltammetry (CV) of CoPc/C, CoPcF16/C, Ag/C, CoPc@Ag/C, and CoPcF16@Ag/C catalysts in argon-saturated 0.1 M NaOH solutions. Several redox peaks at 0.6 and 0.3 V in Figure 3a are observed due to the redox reaction of Co2þ/Coþ in CoPc and CoPcF16, respectively. The positive shift of redox potential of Co2þ/Coþ in CoPcF16 is because of the electrondrawing effect of fluorine.37 From the CV curves of CoPc@Ag/C and CoPcF16@Ag/C shown in Figure 3b, the redox reation peaks of Co2þ/Coþ are also observed, which overlap with the CV curves of Ag/C, and there are no obvious changes for redox reaction of Co2þ/Coþ in CoPc@Ag/C and CoPcF16@Ag/C. The three anodic peaks and one cathodic peak at over 0.0 V are attributed to the redox reactions of silver as discussed in our previous study.22 Comparing the anodic and cathodic peaks obtained on Ag/C catalyst, the intensity of the peaks was much stronger when CoPc and CoPcF16 molecules were adsorbed on the surface of silver, especially for the anodic peaks at about 0.340 and 0.450 V, which implies silver to be oxidized easily when the silver surface was modified with CoPc and CoPcF16 molecules. This can be also proven by the relatively high ratio of Ag2O/Ag on the surface of CoPc@Ag/C and CoPcF16@Ag/C as shown in the XPS results (Figure 2). Figure 4 shows the ORR polarization curves obtained on CoPc/C, CoPcF16/C, Ag/C, CoPc@Ag/C, CoPcF16@Ag/C, and Pt/C (50 wt %, Alfa Aesar), with a rotation rate of 2500 rpm in a 0.1 M NaOH solution saturated with oxygen, and the results for oxygen reduction were summarized in Table 2. The half-wave potential, E1/2 (the potential corresponding to 50% of the peak

current), for CoPc@Ag/C or CoPcF16@Ag/C is 50 mV, or 80 mV more positive as compared to the E1/2 observed for the Ag/C. Apparently, Ag-nanoparticles modified with CoPc or CoPcF16 macromolecules are more active and more likely to interact with O2 than the Ag/C catalysts. The adsorbed CoPcF16 molecules on the Ag/C catalysts have more significant impacts for positively shifting the E1/2 than do the adsorbed CoPc molecules. By further tuning the geometry and electronic structures of the adsorbed Co-macrocyclic molecules on the Ag-nanoparticle surfaces, future improvements of E1/2 will be feasible. While the half-wave potential E1/2 for the ORR on the CoPc@Ag/C or CoPcF16@Ag/C catalysts is 5080 mV more favorable than that on Ag/C catalysts, the observed limiting currents on the CoPc@Ag/C or CoPcF16@Ag/C catalysts are as high as what was observed on the Ag/C catalyst, and almost twice as high as that of the CoPc/C and CoPcF16/C catalysts (Figure 4). The electrochemical reduction of O2 is a multielectron reaction that has two main possible pathways: one involving the transfer of 2 electrons to produce H2O2, and the other involving a 4-electron pathway to produce water. The limiting currents at different rotation rates in Figure 5 can be used to construct the Levich plots43 (Figure 6). The number of electrons transferred for the ORR on Ag/C, CoPc@Ag/C, CoPcF16@Ag/C, CoPc/C, and CoPcF16/C catalysts was calculated as 3.85, 3.93, 3.97, 2.07, and 2.05, respectively. Although the ORR on either the CoPc/C or the CoPcF16/C catalysts was mainly going through the 2-electron pathway, both the CoPc@Ag/C and the CoPcF16@Ag/C catalysts show a close 4-electron transfer, 8497



ARTICLE

Figure 3. Comparison of cyclic voltammetries of different catalysts in Ar-purged 0.1 M NaOH solution at the fifth cycle: (a) CoPc/C and CoPcF16/C; (b) Ag/C, CoPc@Ag/C, and CoPcF16@Ag/C.

which is comparable to that of the Ag/C catalysts that carry the ORR via a 4-electron pathway in alkaline solutions.22 By using the rotating ring disk electrode (RRDE) measurements, the formation of H2O2 during the ORR process can be monitored, and the ORR pathways on the electrodes prepared with various catalysts can be verified. The inset of Figure 4 gives the H2O2 yields on the six studied catalysts. On both Ag/C and Pt/C electrodes, no significant solution phase H2O2 was detected, and thus the H2O2 yield was negligible, which supports a 4-electron pathway to produce water. For the CoPc/C and the CoPcF16/C electrodes, a significant ring current was detected starting at the ORR onset potential of the disk electrode, and up to 50% and 30% of the H2O2 yield was measured on the CoPc and the CoPcF16 electrodes, respectively. This result indicates

that H2O2 is a main product for the ORR catalyzed by the CoPc/ C or the CoPcF16/C catalysts. For the CoPc@Ag/C and the CoPcF16@Ag/C catalysts, the disk limiting currents are as high as those on the Ag/C catalyst, but the ring currents are slightly higher than those on the Ag/C catalyst and much lower than those on the CoPc/C and the CoPcF16/C catalysts. The observed ring currents on either the CoPc@Ag/C or the CoPcF16@Ag/C catalyst may be due to the unoptimized preparation method that the CoPc or CoPcF16 molecules are not fully adsorbed on the silver surface and the ORR could occur on the CoPc/C or the CoPcF16/C catalysts to produce H2O2 as the final product. However, only less than 10% of the H2O2 yields at the potentials lower than 0.5 V vs Hg/HgO were observed on either the CoPc@Ag/C or the CoPcF16@Ag/C catalyst, which indicates 8498



ARTICLE

that the ORRs occur mainly on the hybrid CoPc or CoPcF16modified Ag catalysts. These RRDE results also confirm the results calculated from the Levich equation that the ORR electron exchange number is close to 4 electrons for the ORR on the Ag/C, the CoPc@Ag/C, or the CoPcF16@Ag/C catalysts, but about 2 electrons for the ORR on the CoPc/C or the CoPcF16/C. Figure 7 presents the mass-corrected Tafel plots of Ik (mA cm2) versus the electrode potential E (vs Hg/HgO) for the ORR on the electrodes prepared with various catalysts in an O2-saturated 0.1 M NaOH solution. These Tafel curves were derived from the polarization curves of Figure 5 with a rotation rate of 2500 rpm. The Tafel plot slopes (listed in Table 2) at the lower overpotential region (where E > 0 V vs Hg/HgO) for Pt/C and Ag/C catalysts are 58 and 59 mV dec1, respectively, which is close to 60 mV dec1 and indicates that the first electron

Figure 4. RRDE measurements of oxygen reduction reactions with a rotation rate of 2500 rpm in oxygen-purged 0.1 M NaOH. Collection efficiency N = 0.41; ring potential Er = 0.3 V vs Hg/HgO; scan rate 20 mV s1.

transfer is the rate-determining step at the low overpotentials.43 At the higher overpotential region (where E < 100 mV vs Hg/ HgO), the Tafel slope for Ag/C is 133 mV dec1, which is much higher than that of Pt/C (116 mV dec1) and accounts for the lower activity of the Ag/C catalysts. After modification of the Ag/ C catalysts with either the CoPc or the CoPcF16 molecules, the Tafel slopes for the CoPc@Ag/C and the CoPcF16@Ag/C catalysts at the higher overpotential regions drop significantly to 101 and 95 mV dec1, respectively. At an alkaline fuel cell cathode working potential (0.100 V vs Hg/HgO, equivalent to an overpotential of 0.320 V), the ORR kinetic current on the CoPc@Ag/C and the CoPcF16@Ag/C is 1.46 and 2.73 mA cm2, which is about 3.2 times higher than that of the Ag/C electrode (0.85 mA cm2). In a pure oxygen and a 0.1 M NaOH electrolyte environment, the CoPc@Ag/C and the CoPcF16@Ag/C catalysts display better performance than the Ag/C, the CoPc/C, or the CoPcF16/C catalysts and are almost as good as Pt/C catalysts. The performance of the oxygen cathode prepared with various catalysts was also tested in a half-cell using a gas diffusion electrode (GDE) in an oxygen-saturated 6.0 M NaOH solution as the electrolyte. The iE curves were recorded point-by-point with the increasing current. The performance of the oxygen cathode was highly dependent on the catalyst used. Figure 8 shows the polarization curves for oxygen reduction on the Ag/C, CoPc/C, CoPcF16/C, CoPc@Ag/C, CoPcF16@Ag/C, and Pt/C cathodes. The polarization of the CoPc@Ag/C and the CoPcF16@Ag/C cathodes is significantly lower than that of the Ag/C, CoPc/C, and CoPcF16/C electrodes in both low current density and high current density regions. At the high current density region, the polarization of the CoPcF16@Ag/C electrode is almost the same as what was observed on the Pt/C electrode. In Figure 8, the performance of Ag/C at low current (0 150 mA cm2) is worse than the performances of CoPc/C and CoPcF16/C, which may appear inconsistent with the results obtained from the RRDE measurements (Figure 4). However, a closer examination of the data easily dispels any concern on consistency: The oxygen gas concentration in a GDE is much higher than that in the 0.1 M NaOH solutions for a RRDE. The iE polarizations observed with the GDE at low current densities can be attributed to the kinetic current, while the polarizations at low current densities with the RRDE in Figure 4 include both kinetic and mass-diffusion currents. Tafel plots (shown in Figure 7) are used to correlate the results obtained from the GDE at low current density. Because the catalyst loadings on the GDE were 11 times higher than those on the RDE, the crossover current value (below which the Ag/C performance is better than that of the hybrid catalysts) for the GDE results corresponding to the crossover current of 1.4 or 2.4

Table 2. Electrochemical Parameters for Oxygen Reduction Estimated from Polarization Curves Tafel plot slopes (mV dec1) electrode

E1/2/V

Ilim/mA (@ 0.5 V, 2500)

low η

high η 78

CoPc/C

0.168

0.87

N/A

CoPcF16/C

0.152

0.93

N/A

70

Ag/C

0.214

1.56

59

133

CoPc@Ag/C CoPcF16@Ag/C

0.164 0.132

1.58 1.61

55 55

101 95

Pt/C

0.109

1.63

58

116

8499



ARTICLE

Figure 5. Oxygen reduction polarization curves at 400, 900, 1600, and 2500 rpm in O2-purged 0.1 M NaOH. Scan rate: 20 mV s1.

mA cm2 in the RDE data can be estimated by simply multiply the RDE crossover current by 11, which gives 15.4 and 26.4 mA cm2, respectively; the performance of the Ag/C on the GDE would show a better performance than the CoPcF16 or

CoPc only when the ORR reduction current is lower than 15.4 or 26.4 mA cm2. From iE polarization curves shown in the inseted figure of Figure 8, the performance of Ag/C indeed is better than that of CoPcF16/C or CoPc/C when ORR reduction 8500



Figure 6. Levich plots of O2 reduction on Ag/C, CoPc@Ag/C, CoPcF16@Ag/C, CoPc/C, CoPcF16/C, and Pt/C catalysts.

ARTICLE

Figure 8. Polarization curves of oxygen electrodes with Ag/C, CoPc@Ag/C, CoPcF16@Ag/C, CoPc/C, CoPcF16/C, and Pt/C at room temperature. Alkaline solution: 6.0 M NaOH. Scan rate: 2 mA s1. Oxygen flow rate: 100 sccm. Catalyst loading: 1.86 mg cm2.

Figure 7. Tafel plots of the ORRs on the Ag/C, CoPc@Ag/C, CoPcF16@Ag/C, CoPc/C, CoPcF16/C, and Pt/C catalysts derived from Figure 5.

have clearly demonstrated the plausibility of electronic modifications caused by the adsorption of CoPc or CoPcF16 molecules onto the Ag nanoparticle surfaces. The electronic modifications of CoPc or CoPcF16 on Ag surfaces are also confirmed by the changes of Ag characteristic peaks at 0.340 and 0.450 V in cyclic voltammetry (Figure 3). Because of stronger electronic modifications of the adsorbed CoPcF16 than those of the adsorbed CoPc on the Ag surfaces, a 32 mV lower overpotential in the E1/2 is observed for CoPcF16@Ag/C than for CoPc@Ag/C catalyst. The current densities are also higher, and Tafel slopes are lower for ORR on the CoPcF16@Ag/C than on the CoPc@Ag/C catalysts. The ORR activities are found to be tunable with the electronic modifications originated by the adsorption of CoPc or CoPcF16 molecules onto the Ag nanoparticle surfaces. The electronic modifications of Ag nanoparticle surfaces by the adsorbed CoPc or CoPcF16 molecules play an important role for ORR. By optimizing the composition and morphology of adsorbed macrocyclic molecules on the metallic nanoparticle surfaces, we expect to further improve the ORR activities on these new “hybrid” catalysts.

current is less than 6.4 or 19.9 mA cm2, respectively, which are consistent with the above predictions. There are two mechanisms that lead to ORR via a 4-electron pathway in alkaline solutions:4448 one is a direct 4-electron pathway with O2 directly reduced to water; another is a 2 2-electron pathway with peroxide as intermediates, which are further decomposed to oxygen or reduced to water with an overall reaction of 4-electron transferring. Because Ag is a highly efficient peroxide decomposer,49 oxygen reduction on the CoPc@Ag/C and CoPcF16@Ag/C catalysts is possible to proceed via a 2 2-electron pathway with oxygen reduced to peroxide on the CoPc or CoPcF16 molecules, and peroxide is subsequently reduced or decomposed on silver catalysts. However, the kinetic currents observed on the CoPc@Ag/C and CoPcF16@Ag/C catalysts, as shown in Figure 7, are more than twice higher than those on CoPc/C, CoPcF16/C, or Ag/C separately at the same electrode potential when electrode potentials are higher than 0.15 V. Both the DFT and the XPS results discussed above

4. CONCLUSION In this Article, a new class of hybrid catalysts with Co-based phthalocyanines on Ag nanoparticles on carbon was found to have much higher oxygen reaction activity in alkaline media than the Ag catalysts on carbon and Co-based phthalocyanines on carbon used separately. The high activity of CoPc on Ag nanoparticles originates from the strong interaction of cobalt ions with silver surface indicated by the DFT simulation and XPS results. The electrocatalytic activity toward oxygen reduction can be modified by adsorbing different derivatives of CoPc, such as CoPcF16. Electrochemical characterizations demonstrated that the ORR took place on the new catalysts at an approximate 4-electron pathway with the half-wave potential, E1/2, positively shifted about 50 mV for CoPc@Ag/C and 82 mV for CoPcF16@Ag/C as compared to the Ag/C catalysts. The Tafel slopes for ORR on CoPc@Ag/C and CoPcF16@Ag/C were lower than those on the Ag/C catalysts in the high overpotential region, which indicates their superior current density and greater rate 8501


The Journal of Physical Chemistry C constant than the Ag/C. Besides CoPc and their derivatives, other Co-organic molecules containing CoN2 or CoN4 structures and the like, such as cobalt prophyrins and their derivates, may have similar catalytic improvement effect for ORR when they are adsorbed on the surface of silver nanoparticles. Optimization of the composition and morphology of this new class of hybrid catalysts is expected to further improve its already fairly high ORR catalytic activity.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: (317) 274-4280. Fax: (317) 274-0789. E-mail: rochen@ iupui.edu.

’ ACKNOWLEDGMENT This work was supported by the U.S. Army Research Lab (Grant No. W911NF-07-2-0036). Dr. Guofeng Wang supervised Dr. H. He to conduct DFT simulations of CoPc and CoPcF16. The detailed DFT results will be reported soon. ’ REFERENCES (1) Roche, I.; Chainet, E.; Chatenet, M.; Vondrak, J. J. Appl. Electrochem. 2008, 38, 1195. (2) Meng, H.; Jaouen, F.; Proietti, E.; Lefevre, M.; Dodelet, J. Electrochem. Commun. 2009, 11, 1986. (3) Olson, T.; Pylypenko, S.; Atanassov, P. J. Phys. Chem. C 2010, 114, 5049. (4) Jin, W.; Du, H.; Zheng, S.; Xu, H.; Zhang, Y. J. Phys. Chem. B 2010, 114, 6542. (5) Wang, C.; Daimon, H.; Sun, S. Nano Lett. 2009, 9, 1493. (6) Lee, Y.; Loew, A.; Sun, S. Chem. Mater. 2010, 22, 755. (7) Chen, Z.; Higgins, D.; Tao, H.; Hsu, R.; Chen, Z. J. Phys. Chem. C 2009, 113, 21008. (8) Li, B.; Prakash, J. Electrochem. Commun. 2009, 11, 1162. (9) Chatenet, M.; Bultel, L.; Aurousseau, M.; Durand, R.; Andolfatto, F. J. Appl. Electrochem. 2002, 32, 1131. (10) Bunazawa, H.; Yamazaki, Y. J. Power Sources 2008, 182, 48. (11) Shao, M. H.; Adzic, R. J. Phys. Chem. B 2005, 109, 16563. (12) Lin, H.; Tang, W.; Shwarsctein, A.; Mcfarland, E. J. Electrochem. Soc. 2008, 155, B200. (13) Tang, W.; Lin, H.; Shwarsctein, A.; Stucky, G.; Mcfarland, R. J. Phys. Chem. C 2008, 112, 10515. (14) Jiang, L.; Hsu, A.; Chu, D.; Chen, R. J. Electrochem. Soc. 2009, 156, B643. (15) Yang, Y.; Zhou, Y.; Cha, C. Electrochim. Acta 1995, 40, 2579. (16) Wang, Y.; Zhang, D.; Liu, H. J. Power Sources 2010, 195, 3135. (17) Sun, W.; Hsu, A.; Chen, R. J. Power Sources 2011, 196, 44914498. (18) Cheng, F.; Su, Y.; Liang, J.; Tao, Z.; Chen, J. Chem. Mater. 2010, 22, 898. (19) Xiao, W.; Wang, D.; Lou, X. J. Phys. Chem. C 2010, 114, 1694. (20) Gojkovic, S.; Gupta, S.; Savinell, R. J. Electroanal. Chem. 1999, 462, 63. (21) Chen, R.; Li, H.; Chu, D.; Wang, G. J. Phys. Chem. C 2009, 113, 20689. (22) Guo, J.; Hsu, A.; Chu, D.; Chen, R. J. Phys. Chem. C 2010, 114, 4324. (23) Lima, F.; Zhang, J.; Shao, M.; Sasaki, K.; Vukmirovic, M.; Ticianelli, E.; Adzic, R. J. Phys. Chem. C 2007, 111, 404. (24) Chatenet, M.; Bultel, L.; Aurousseau, M.; Durand, R.; Andolfatto, F. J. Appl. Electrochem. 2002, 32, 1131. (25) Barth, J. V. Rev. Phys. Chem. 2007, 58, 375.

ARTICLE

(26) Lin, N.; Stepanow, S.; Ruben, M.; Barth, J. V. Top. Curr. Chem. 2008, 128, 150. (27) Bai, Y.; Buchner, F.; Kellner, I.; Schmid, M.; et al. New J. Phys. 2009, 11, 125004. (28) Gottfried, J. M.; Marbach, H. Z. Phys. Chem 2009, 223, 53. (29) Auw€arter, W.; Schiffrin, A.; Weber-Bargioni, A.; Pennec, Y.; Riemann, A.; Barth, J. V. Int. J. Nanotechnol. 2008, 5, 1171. (30) Yoshimoto, S.; Itaya, K. J. Porphyrins Phthalocyanines 2007, 11, 313. (31) Auw€arter, W.; Seufert, K.; Klappenberger, F.; Reichert, J.; Weber-Bargioni, A.; Verdini, A.; Cvetko, D.; Dell’Angela, M.; Floreano, L.; Cossaro, A.; Bavdek, G.; Morgante, A.; Seitsonen, A. P.; Barth, J. V. Phys. Rev. B 2010, 81, 245403. (32) Flechtner, K.; Kretschmann, A.; Steinruck, H.; Gottfried, J. J. Am. Chem. Soc. 2007, 129, 12110. (33) Baran, J. D.; Larsson, J. A.; Woolley, R. A. J.; Cong, Y.; Moriarty, P. J.; Cafolla, A. A.; Schulte, K.; Dhanak, V. R. Phys. Rev. B 2010, 81, 075413. (34) Toader, M.; Gopakumar, G.; Abdel-Hafiez, M.; Hietschold, M. J. Phys. Chem. C 2010, 114, 3537. (35) Lukasczyk, T.; Flechtner, K.; Merte, L.; Jux, N.; Maier, F.; Gottfried, J.; Steinruck, H. J. Phys. Chem. C 2007, 111, 3090. (36) Wang, G.; Ramesh, N.; Hsu, A.; Chu, D.; Chen, R. Mol. Simul. 2008, 34, 1051. (37) Arihara, K.; Mao, L.; Liddell, P.; Ochoa, E.; Moore, A.; Imase, T.; Zhang, D.; Sotomura, T.; Ohsaka, T. J. Electrochem. Soc. 2004, 151, A2047. (38) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (39) Nickel, R.; Zhu, W.; Payne, R.; Cahela, D.; Tatarchuk, B. J. Power Sources 2006, 161, 1217. (40) Fang, H.; Liu, C.; Liu, C.; Li, F.; Liu, M.; Cheng, H. Chem. Mater. 2004, 16, 5744. (41) Ladouceur, M.; Lalande, G.; Guay, D.; Dodelet, J. J. Electrochem. Soc. 1993, 140, 1974. (42) Weaver, J.; Hoflund, G. J. Phys. Chem. 1994, 98, 8519. (43) Bard, A.; Faulken, L. Electrochemical Methods: Fandamental and Applications, 2nd ed.; Wiley: New York, 2001. (44) Wang, T.; Kaempgen, M.; Nopphawan, P.; Wee, G.; Mhaisalkar, S.; Srinivasan, M. J. Power Sources 2010, 195, 4350. (45) Meng, H.; Shen, P. Electrochem. Commun. 2006, 8, 588. (46) Olson, T.; Pylypenko, S.; Fulghum, J.; Atanassov, P. J. J. Electrochem. Soc. 2010, 157, B54. (47) Roche, I.; Chainet, E.; Chatenet, M.; Vondrak, J. J. Appl. Electrochem. 2008, 38, 1195. (48) Liu, Z.; Li, Z.; Qin, H.; Liu, B. J. Power Sources, doi: 10.1016/ j. jpowsour.2011.01.102. (49) Campbell, F.; Belding, S.; Baron, R.; Xiao, L.; Compton, R. J. Phys. Chem. C 2009, 113, 9053.

’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published on April 7, 2011, with an error to Figure 2 and Section 3.1. The corrected version was reposted April 13, 2011.

8502


CoPc- and CoPcF16-Modified Ag Nanoparticles as Novel Catalysts

Recommend Documents