C Electrodes in Alkaline

Article pubs.acs.org/JPCC

Tuning the Electrochemical Interface of Ag/C Electrodes in Alkaline Media with Metallophthalocyanine Molecules Junsong Guo,† Jie Zhou,† Deryn Chu,‡ and Rongrong Chen*,† †

Richard G. Lugar Center for Renewable Energy, Indiana University−Purdue University, Indianapolis, Indiana 46202, United States U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States

‡

S Supporting Information *

ABSTRACT: Transition metal phthalocyanine (MPc) molecules were shown to be able to modify the electrochemical interface of the Ag catalyst in the alkaline electrolyte. Depending on the binding strength of the MPc (M = Fe, Co, Ni, and Mn) molecules with the OH− species, the electrochemically active sites of the Ag electrode surfaces can be altered accordingly. A structural model for illustrating the electrochemical interface of the Ag/C catalysts modified by the MPc molecules in alkaline media is proposed, which can be used to elucidate the ORR kinetics observed on various MPc@Ag/C catalysts in AEMFCs. The optimal CoPc−OH and Ag− CoPc interactions resulted in the highest kinetic current density and power density (536 mW cm−2 at 50 °C) observed with the AEMFCs using CoPc modified Ag/C cathode catalysts in comparison with the Ag/C and other MPc (M = Fe, Ni, and Mn) modified Ag/C catalysts . ORR kinetics. In a recent study,21 we discovered that the OH−binding effect on the cathode catalysts plays an important role in determining the ORR kinetics and OH− transport resistance in AEMFCs. In that study, we examined the ORR activity on various carbon-supported metallophthalocyanine (MPc/C, M = Fe, Co, Ni, and Mn) catalysts in both 0.1 M NaOH solutions and in AEMFCs. Density functional theory (DFT) calculations were used to analyze the adsorption energy of O2, OH, H2O2, and H2O species on various MPc molecules, and the results indicated that the adsorption of OH on the MPc molecules was energetically more favorable than the adsorption of O2 . The adsorption energy of OH on the MPc molecules depended on the transition metal in the MPC and followed an order of FePc > MnPc > CoPc > NiPc. A balance of O2 and OH adsorption on the MPc catalysts was desirable for achieving high ORR activity in the AEMFCs. However, our further systematic DFT calculations predicted that none of the “known” single MPc catalysts seems to have the desired balance of O2 and OH adsorption strength for AEMFCs. For example, FeN 4 complexes tend to have a too strong OH adsorption strength, while CoN4 complexes have an O2 adsorption that is too weak.22 Bifunctional or multifunctional catalysts are likely the better choices for ORR in alkaline media23−28 and, in particular, in AEMFCs. The combination of the relatively high ORR activity and low material cost of the Ag/C catalyst makes it an attractive alternative to Pt/C as a cathode catalyst for AEMFC

1. INTRODUCTION Anion exchange membrane fuel cells (AEMFCs) are a relatively new development as a low-cost alternative to the state-of-theart proton exchange membrane fuel cells (PEMFCs).1−20 By using polymeric anion exchange membranes instead of liquid alkaline electrolytes, the AEMFCs not only offer the advantages of alkaline fuel cells (AFCs) but also eliminate the carbonate precipitates problems faced by AFCs. Many less expensive materials, such as carbon-supported transition metals (Pd/C, Ag/C, Co/C, and Ni/C), transition-metal macrocyles (M−N− C), metal oxides (MnO2/C), and multifunctional materials (metallic alloys, metallic MnO2, or macrocycle treated metals), have been studied as electrocatalysts to replace platinum catalysts for oxygen reduction reactions (ORRs) in alkaline media.4−26 The ORR in alkaline media occurs via either a twoelectron (2e−) pathway O2 + H 2O + 2e− → HO2− + OH−, E0 (1)

= −0.065 V vs SHE −

or a direct four-electron (4e ) pathway O2 + 2H 2O + 4e− → 4OH−, E0 = 0.40 V vs SHE

(2)

Regardless of the ORR pathway, the OH− ions are produced on the cathode catalysts in alkaline media, which is one of the key differences from the ORR in a PEMFC that produces water at the cathode. Removing OH− species efficiently from the cathode catalyst surface in the AEMFC is essential to preventing the active sites of the catalysts being blocked by the OH− adsorbates, which could hinder O2 adsorption and © XXXX American Chemical Society

Received: October 28, 2012 Revised: January 25, 2013

A

dx.doi.org/10.1021/jp310655y | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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and an Hg/HgO/0.1 M OH− electrode (0.163 V vs NHE) as the reference electrode. The potentials of the electrodes were converted to RHE. 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). 2.3. Anion-Exchange Membrane Fuel Cell Characterizations. The ORR electrocatalytic performances of Ag/C, MnPc@Ag/C, FePc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C catalysts were further characterized in the AEMFCs. A Tokuyama A901 membrane and an A4 ionomer were used as the polymeric electrolytes for preparing the membrane electrode assemblies (MEAs). Commercial carbon-supported Pt/C (50 wt % Pt, Alfa Aesar) was used as the anode catalyst. The catalyst inks for preparing the anode and the cathode consisted of a 16 mg catalyst sample, 2 mL of ethanol, and an A4 ionomer with an 80:20 catalyst/ionomer weight ratio. After homogenizing by sonication, the ink was airbrushed onto a 6.25 cm2 Tokuyama A901 membrane. The MEA was sandwiched between two sheets of Toray Carbon Paper (TGP-H-090) to form an AEMFC with an active electrode area of 4.5 cm2. A fuel cell testing system (Scribner Associates Model 850e) was used for controlling the cell temperature, humidity, H2 and O2 flow rate, and back-pressures. The temperature of the fuel cell was maintained with a tolerance of ±0.2 °C. The fuel cells were operated at 50 °C and 20 psi back-pressure for both the H2 and O2 gases, with a flow rate of 200 sccm. The electrochemical impedance of the AEMFC was tested using an 8-channel Solartron 1470 cell tester. Impedance spectra were recorded by superimposing a 10 mV AC signal at the different cell voltages in the potentiostatic mode with frequencies ranging from 100 K to 0.1 Hz. The electrochemical impedance spectra were simulated by the ZSimpWin software. 2.4. DFT Computational Methods. DFT calculations were performed with the Material Studio DMol3 module from Accelrys (Materials Studio 5.5, DMol3, Accelrys Inc., San Diego, CA, U.S.A.). The generalized gradient approximation (GGA) corrected Perdew−Burke−Ernzerhof (PBE) 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 × 10−5 eV. Symmetry was not imposed on any calculations.

applications.29,30 However, the AEMFCs with the Ag/C cathodes suffer large initial voltage losses, about 100−150 mV larger than those with the Pt/C electrodes.4,12 To reduce the ORR overpotentials on the Ag/C cathode, new approaches to form bifunctional catalysts, including the use of Ag alloys (with other transition metals)23,24 and the addition of metal oxides25 or MPc molecules,28 have been investigated by several authors. Through rotating disk electrode (RDE) studies in alkaline solutions, we observed that, compared to pure Ag/C catalysts, carbon-supported Ag nanoparticles modified with either cobalt phthalocyanine (CoPc) or the fully fluorinated cobalt phthalocyanine (CoPcF16) had 50−80 mV lower overpotentials than those of the pure Ag/C catalysts, and much lower Tafel slopes in the high overpotential region.27 In the present work, we aim to optimize this type of bifunctional catalysts for the enhanced ORR activity by understanding the roles of MPc molecules (M = Fe, Co, Ni, and Mn) for tuning the electrochemical interface between the Ag catalyst and the alkaline electrolyte.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Physical Characterization. A total of 1 g of Ag/C catalyst with a 40 wt % silver loading on an XC-72 Vulcan carbon was prepared by a citrateprotecting method: 3554 mg sodium citric (Alfa Aesar) and 2220 mg NaOH (JT Baker) were mixed to prepare 370.0 mL of a 50 mM sodium citrate solution, and 370 mL of 10 mM AgNO3 was added. Then, a 500 mL 7.4 mM NaBH4 (Strem chemicals) solution was added dropwise, while vigorously stirring, to obtain a yellowish-brown Ag colloid. Next, 600 mg of Vulcan XC-72 carbon black (Cabot Corp.) was dispersed into the Ag colloid. After the suspension was magnetic bar stirred for 12 h, the black suspension was filtered, washed, and dried at room temperature to obtain the 40 wt Ag/C catalyst sample. MnPc, FePc, CoPc, and NiPc-modified Ag/Cs were prepared by adsorbing 6.32 mg of MnPc, FePc, CoPc, and NiPc, respectively, on 200 mg 40 wt % Ag/C in dimethylformamide (DMF) by magnetic bar stirring. After washing with ethanol and drying at room temperature, the obtained samples were denoted as MnPc@Ag/C, FePc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C, respectively. Carbon-supported MnPc, FePc, CoPc, and NiPc were prepared using a similar method as described above by mixing 3 wt % MnPc, FePc, CoPc, or NiPc with 97 wt % Vulcan XC-72 carbon black in DMF by magnetic bar stirring. After washing and drying, the obtained catalysts were designated as MnPc/C, FePc/C, CoPc/ C, and NiPc/C, respectively. The morphologies of the catalysts were studied using a Tecnai G2 TWIN/BioTWIN transmission electron microscope (FEI Company) operated at 80 kV. 2.2. 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 (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). The catalyst ink was prepared by ultrasonically mixing 2.0 mg of the catalyst samples with 10 μL of the Nafion solution (5%), 1 mL of ethanol, and 1 mL of deionized water. Then, 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 oxygensaturated 0.1 M NaOH solution using a standard threeelectrode cell with an Au foil serving as the counter electrode

3. RESULTS 3.1. Electrochemical Characterizations of Various Ag/ C Catalysts in 0.1 M NaOH Solutions. Various MPc modified Ag/C catalysts (denoted as MPc@Ag/C, where M = Fe, Co, Ni, and Mn) were prepared using the methods described in the Experimental Section. The TEM images of the prepared catalysts are shown in Figure S1 of the Supporting Information. The adsorption of MPc on the Ag/C catalysts was found not to affect the morphology of the silver nanoparticles that had the mean particle size around 15 nm. The typical cyclic voltammetry (CV) curves obtained on the MPc@Ag/C and Ag/C electrodes in Ar-saturated 0.1 M NaOH solutions are shown in Figure 1a. For the comparison, the baseline CVs on the MPc/C electrodes in Ar-saturated 0.1 M NaOH solutions were shown in Figure S2. In Figure 1a, one can see that there B



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observed on the MPc@Ag/C catalysts (Figure 1a). The charges for silver oxidation reactions on the MPc@Ag/C catalysts were plotted by subtracting the background current on the corresponding MPc/C electrodes and are shown in Figure 1b. One can see clearly that the effects of MPc molecules for increasing active sites of the Ag nanoparticles in 0.1 M NaOH solutions follow this trend: FePc@Ag/C > MnPc@Ag/C > CoPc@Ag/C ≫ NiPc@Ag/C ∼ Ag/C. Figure 2 shows the ORR polarization curves obtained on a rotating ring disk electrode (RRDE) with various MPc@Ag/C and Ag/C catalysts at a rotation rate of 2500 rpm in O2saturated 0.1 M NaOH solutions at room temperature. The mass transport-corrected Tafel plots of kinetic current Ik (mA cm−2 gAg−1) versus the electrode potential E (vs RHE) are plotted in Figure 3. The half-wave potentials and Tafel slopes for ORRs on varying MPc@Ag/C and Ag/C catalysts are also summarized in Table 1. The half wave potentials for the ORR on the FePc@Ag/C, CoPc@Ag/C, MnPc@Ag/C, and NiPc@ Ag/C catalysts were found to shift positively about 178, 64, 32, and 24 mV, respectively, as compared with that observed on the Ag/C catalysts. The diffusion limited currents on the FePc@ Ag/C, MnPc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C catalysts were as high as what was observed on the Ag/C catalyst and much higher than those observed on MnPc/C, CoPc/C, and NiPc/C catalysts. Koutecky−Levich plots (not shown) obtained from the RDE polarization curves on the FePc@ Ag/C, MnPc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C catalysts at various rotation rates indicate that oxygen is being reduced by a 4e− pathway, which are consistent with the RRDE results that show negligible H2O2 formation on the MPc@/Ag/C catalysts. The amounts of H2O2 formation on either FePc/C or Ag/C catalysts were also negligible, while significant amounts of H2O2 were detected on the NiPc/C and CoPc/C catalysts. Except for the FePc/C catalysts, other MPc/C (M = Co, Ni, and Mn) catalysts were found to be less active toward the ORR than the Ag/C catalysts. The ORR onset potentials and halfwave potentials overlapped for both the FePc/C and the FePc@Ag/C catalysts and were ∼200 mV more positive than those on the Ag/C catalyst. From Table 1, one can see that the Tafel slope for the CoPc@Ag/C, MnPc@Ag/C, NiPc@Ag/C, and Ag/C catalysts is around 60 mV decade−1 at the low overpotentials and varies between 120 and 150 mV decade−1 at the high overpotentials. It seems that the adsorption of CoPc, MnPc, or NiPc molecules on the Ag/C catalysts does not change the ORR kinetics of Ag/ C catalysts. However, much lower Tafel slopes were observed on the FePc@Ag/C catalysts in both the low and high overpotentials, which suggest that the ORR kinetics on the FePc@Ag/C catalysts are likely different from the Ag/C and other MPc@Ag/C catalysts. The kinetic currents obtained on the FePc@Ag/C, CoPc@Ag/C, MnPc@Ag/C, and NiPc@Ag/ C catalysts are about 67.85, 3.09, 1.53, and 1.37 times higher, respectively, than what was observed on the Ag/C catalyst (35.46 mA cm−2 g−1) at 0.68 V (vs RHE). Varying the MPc molecules resulted in various degrees of improvements in ORR activity of Ag/C catalysts. In 0.1 M NaOH solutions, the ORR activity on various Ag-based catalysts follows the order of FePc@Ag/C > CoPc@Ag/C > MnPc@Ag/C > NiPc@Ag/C > Ag/C, which is similar as the enhanced Ag-oxidation charges by varying MPc molecules (Figure 1b). 3.2. Electrochemical Characterizations of Various Ag/ C Catalysts in AEMFCs. The performance of the Ag/C, MnPc@Ag/C, FePc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C

Figure 1. Comparison of cyclic voltammetries of different catalysts in an Ar-purged 0.1 M NaOH solution at the fifth cycle: (a) Ag/C, MnPc@Ag/C, FePc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C and (b) the accumulated charges for silver oxidation.

are three anodic peaks and a cathodic peak shown on all the five catalysts at potentials higher than 0.8 V (vs RHE), which can be attributed to the redox reactions of Ag nanoparticles in the NaOH solutions.30 Compared to the anodic peaks and the cathodic peak obtained on the Ag/C catalyst without the MPc modifications, the peak currents on the FePc@Ag/C, MnPc@ Ag/C, and CoPc@Ag/C catalysts were significantly higher, especially for the anodic peaks at about 1.2 and 1.4 V. The anodic peaks for the silver oxidation on the NiPc@Ag/C catalyst were very similar to those observed on Ag/C catalyst: only a slightly increasing current of the anodic peak at 1.35 V was observed on the NiPc@Ag/C as compared to the Ag/C catalyst. Although the Ag metal loadings in all the MPc@Ag/C samples were the same, the observations of the increased Ag peak currents in both the anodic and cathodic peaks were similar to those we previously observed with increasing Ag contents in the Ag/C catalyst,30 which suggests that the MPc molecules affect the structures and compositions of electrochemical interface of Ag nanoparticles in the electrolyte, resulting in more active sites available for the oxidization of the silver nanoparticles. The baseline CV curves (S2) on the carbon supported MPc catalysts (MPc/C) show no redox peaks for the potential region 1.0−1.2 V (vs RHE), and the oxygen evolution reaction on the MPc/C catalysts starts around 1.2 V. The highest anodic currents at E = 1.5 V vs RHE observed on the MPc/C catalysts were more than an order smaller than those C



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Figure 2. Comparison of RRDE measurements of oxygen reduction reactions on (a) MnPc/C and MnPc@Ag/C, (b) FePc/C and FePc@Ag/C, (c) CoPc/C and CoPc@Ag/C, and (d) NiPc/C and NiPc@Ag/C with Ag/C and XC-72 catalysts with a rotation rate of 2500 rpm in oxygen-saturated 0.1 M NaOH solutions. Collection efficiency N = 0.41; ring potential Er = 0.3 V vs Hg/HgO; scan rate = 20 mV s−1.

cathode catalysts was also tested in AEMFCs that were assembled with the identical anodes, membranes (Tokuyamma A901), and ionomers (Tokuyamma A4), but varying cathode catalysts with the same catalyst loadings. The cell voltage and power density versus current density curves obtained with various MPc@Ag/C and Ag/C cathode catalysts are shown in

Figure 4. The current densities at cell voltages of 0.8 and 0.6 V and peak power density obtained with AEMFCs containing various MPc@Ag/C and Ag/C cathode catalysts are also listed in Table 2. In Figure 4, one can see that there were two distinct discharge regions observed in the polarization curve for the Ag/ C cathode. When the cell voltage was above 0.60 V, the cell D



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Table 2. Comparison of Discharge Current Density and Peak Power Density of AEMFCs with Various Cathode Catalysts

i (mA cm−2) at 0.8 V i (mA cm−2) at 0.6 V peak power (mW cm−2)

Table 1. Electrochemical Parameters for Oxygen Reduction Estimated from Polarization Curves Tafel slopes (mV dec−1) Ag/C MnPc@Ag/C FePc@Ag/C CoPc@Ag/C NiPc@Ag/C

E1/2 (V) Ilim/mA (@0.426 V, 2500 rpm) low η high η 0.677 0.709 0.855 0.741 0.701

1.452 1.474 1.500 1.510 1.438

61 56 33 71 74

MnPc@ Ag/C

FePc@ Ag/C

CoPc@ Ag/C

NiPc@ Ag/C

39.1

38.4

24.1

121.1

67.2

265.6

420.4

287.1

768.2

391.5

356.5

360.6

338.5

536.6

344.0

in comparison with that observed on the Ag/C cathode. However, larger cell voltage losses at high discharge current densities were observed on the AEMFCs with the FePc@Ag/C cathodes. The high ORR activity observed on the RRDE (Figure 2) with the FePc@Ag/C catalysts in 0.1 M NaOH solutions could not be confirmed in the AEMFCs (Figure 4), which is similar as what were observed on the FePc/C catalysts.21 As shown in Table 2, the current density at 0.8 V with the AEMFCs containing various cathode catalysts follows the order: CoPc@Ag/C > NiPc@Ag/C > Ag/C > MnPc@Ag/ C > FePc@Ag/C. To understand the polarization behaviors of various cathodes in the AEMFCs, electrochemical impedance spectroscopy (EIS) was performed by discharging AEMFCs at various cell voltages, including 0.9, 0.8, 0.7, 0.6, 0.5, and 0.4 V, respectively. By using the same test protocol, we were able to distinguish the effects of OH− transports on the polarizations of electrodes of AEMFCs that contained various amounts of functional groups in the membranes20 or ionomers.19 Combining the EIS and DFT calculations, the OH− binding effects on the performance of AEMFCs with various MPc/C catalysts were also successfully disclosed.21 The impedance spectra of the AEMFCs with various Ag/C-based cathode catalysts were recorded and shown in Figure 5. Depending on whether the mass transport was a limited factor of electrode polarizations, the impedance spectra could be fitted by using the equivalent circuits shown in either Figure S3a or S3b,21 where Ri is the fuel cell internal resistance and Ra,ct and Rc,ct are the charge transfer resistances at the anode and cathode, respectively. The resistance of mass transport is designated as Rd,mass, which includes the resistance of water transport (Rd,H2O), O2 transport (Rd,O2), and OH− transport (Rd,OH−) from the cathode to anode through the membrane. The constant phase elements CPEa, CPEc, and CPEd were used in the capacitance simulation due to the nature of the porous electrodes. The simulation results for various cathodes at various cell voltages are listed in Table S1 of the Supporting Information. At high cell voltages or low discharge current density, the Rc,ct is significantly larger than other parameters, such as Ra,ct or Rd,mass, and can be considered as the key factor that limits the performance of an AEMFC. Rc,ct declined with the dropping cell voltages or increasing discharge current density and depended on the type of MPc molecules on the Ag/C electrodes. The values of the Rc,ct obtained on various Ag/Cbased catalysts as the function of the cell voltages are plotted in Figure 6a. At 0.8 V, the Rc,ct is the largest on the FePc@Ag/C cathodes and the smallest on the CoPc@Ag/C cathodes. When the cell voltages were below 0.5 V by increasing discharge current density, the Rc,ct became as large as the Rd,mass (Figure 6b). Interestingly, the Rd,mass for the Ag/C and FePc@Ag/C cathodes declined significantly when the cell voltages dropped

Figure 3. Tafel plots of the ORRs on the Ag/C, MnPc@Ag/C, FePc@ Ag/C, CoPc@Ag/C, and NiPc@Ag/C catalysts.

electrode

Ag/C

150 145 84 120 142

Figure 4. Comparison of polarization and power density curves of anion exchange membrane fuel cells with Ag/C, MnPc@Ag/C, FePc@ Ag/C, CoPc@Ag/C, and NiPc@Ag/C as cathode catalysts. Anode: 50 wt % Pt/C, flow rate 200 sccm H2, 100% humidity. Cathode: various silver catalysts, flow rate 200 sccm O2, 100% humidity. Test temperature: 50 °C; back pressure: 20 psi.

voltages fell sharply with increasing discharge currents, and then there was a voltage plateau around 0.6 V. When discharge current density was increased, the cell voltages dropped below 0.5 V, but not as sharp as that observed in the high cell voltage region. The CoPc@Ag/C cathodes showed the highest cell performance with current density of 121 mA cm−2 at 0.8 V and 768 mA cm−2 at 0.6 V (Table 2). The performance of the MnPc@Ag/C and NiPc@Ag/C cathodes was slightly improved E



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Figure 5. Electrochemical impedance spectral of anion exchange membrane fuel cells with Ag/C, MnPc@Ag/C, FePc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C cathode catalysts at (a) 0.9, (b) 0.8, (c) 0.7, (d) 0.6, (e) 0.5, and (f) 0.4 V. Test temperature: 50 °C; back pressure: 20 psi.

cathodes. Again, the Rd,mass was also found to be the largest on the FePc@Ag/C cathodes and the smallest on the CoPc@Ag/ C cathodes within the fuel cell working potentials of 0.9 to 0.1 V versus RHE. Both the high Rc,ct and high Rd,mass obtained with

from 0.8 to 0.5 V although discharge current density increased more than an order. Further increasing discharge current density and reducing cell voltage to 0.4 V, the Rd,mass seems to increase slightly from that observed at 0.5 V for all the tested F



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Fe, Co, Ni, and Mn) adsorbed on the top layer of Ag(111) was calculated and listed in Table 3, while the adsorption energy of Table 3. Calculated Adsorption Energy (in Units of eV) of Different MPcs on Ag(111) by DFT Ag(111)

MnPc

FePc

CoPc

NiPc

−2.09

−1.72

−1.35

−0.63

the hydroxide on Ag(111) and MPc molecules is listed in Table 4. From the DFT calculation results, one can see that the Table 4. Calculated OH Adsorption Energy on MnPc, FePc, CoPc, and NiPc by DFT OH

MnPc

FePc

CoPc

NiPc

−2.93

−3.41

−2.36

−1.33

adsorption of the MPc molecule on the Ag(111) surface was energetically favorable. The MPc−OH interactions (1.3−3.4 eV) are stronger than the Ag-MPc interactions (0.6−2.1 eV), and much stronger than the noncovalent interactions (0.1−0.15 eV) of cations and OHad species. The adsorption energy for an OH− on a CoPc molecule is −2.36 eV, which is 1 eV weaker than that on the FePc molecule, but 1 eV stronger than on the NiPc molecule. Based on the calculated adsorption energy of the hydroxide ion on the four different MPc molecules, the hydroxide binding strength follows the order of FePc > MnPc > CoPc > NiPc, which is consistent with the enhanced Agoxidation charges (Figure 1b) and the ORR activity observed on these Ag-based catalysts in 0.1 M NaOH solutions (Figure 2).

4. DISCUSSION 4.1. ORR Activity on the Ag/C Catalyst in Alkaline Media. In our earlier work, ORR activities on Ag/C catalysts with four different metal loadings but similar particle sizes (∼15 nm) have been studied using RDE and RRDE in 0.1 M NaOH solutions.30 Ag/C catalytic activities for the ORRs depended on the formation of a surface monolayer of Ag2O films, which was a key parameter that can be used to estimate the ORR activities on the Ag/C catalysts. By increasing the Ag loading onto the carbon supports, the formation of Ag2O films increased linearly and the ORR onset-potential shifted positively about 62 mV as Ag loading increases from 10 to 60 wt %. However, Ag/C catalysts had 50−100 mV higher over potentials than the Pt/C catalysts in the 0.1 M NaOH solutions. The performance of the Ag/C cathode in the AEMFC is shown in Figure 4 and Table 2, which shows that the current densities produced in the AEMFCs (∼40 mA mg−1 cm−2) at 0.8 V (vs RHE) are almost an order of magnitude higher than that from an RDE test (5 mA mg−1 cm−2). The higher O2 concentration (20 psi) and higher operation temperatures (50 °C) in the AEMFCs (as compared to the RDE test) are responsible for the higher rates of O2 reduction. The peak power density of ∼356 mW cm−2 with a current density of 860 mA cm−2 at 50 °C was achieved in the AEMFCs with Ag/C cathodes, which was one of the highest AEMFC performances reported for Ag/C cathodes so far. EIS analysis results (Table S1) indicate that the cell internal resistances on the Ag/C cathodes were around 0.1 Ω cm2, which is an order of magnitude smaller than what was reported4 and did not change significantly with increased current densities. In Table S1, the charge transport resistance, Rc,ct,

Figure 6. Plots of (a) cathode charge transfer resistance and (b) mass transfer resistance of AEMFCs obtained with the Ag/C, MnPc@Ag/C, FePc@Ag/C, CoPc@Ag/C, and NiPc@Ag/C cathode catalysts at cell voltages of 0.9, 0.8, 0.7, 0.6, 0.5, and 0.4 V, respectively.

the AEMFCs containing the FePc@Ag/C cathode resulted in the large cell voltage losses with increasing discharge current densities. 3.3. DFT Simulations of MPc−Ag and MPc−OH Interactions. To understand the roles of MPc molecules in tuning electrochemical interfaces of the Ag/C catalysts in alkaline media, density functional theory (DFT) calculations were performed to study the interactions between the MPc molecule and the Ag atoms on the Ag(111) surface. The Ag(111) surface was calculated with a periodic rectangular slab model containing three layers of slabs. The MPc molecule was placed on the surface of the Ag(111) surface and separated by a vacuum space of 20 Å. The silver atoms of the third layer underneath the slab were fixed. All the atoms of the MPc molecule and the Ag atoms of the upper two slabs were fully relaxed during the geometry optimization. The adsorption energy (Ead) is defined as Ead = EMPc@Ag(111) − EMPc − EAg(111)

(3)

Here the EMPc@Ag(111), EMPc, and EAg(111) stand for the total energy of the MPc on the Ag(111) with a relaxed adsorption system, a free MPc molecule, and the Ag(111) surface, respectively. The adsorption energy of the four MPcs (M = G



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AEMFCs, it is the key to reduce OH− binding effects on the Ag/C surface, especially at the high cell voltage region. 4.2. Roles of MPc Molecules in Enhancing the ORR Activity of Ag/C Catalysts. Among the various pure metallic electrocatalysts, such as Pt, Pd, and Ag, it was recognized that the O2 adsorption on Ag surfaces is relatively weak and so the O−O bond cannot be broken on Ag as efficiently as on Pt.38,39 Because the O2 adsorption on the FePc/C catalysts was found to be stronger than that on Pt,36 the O2 adsorption on the FePc/C catalysts are by far stronger than the O2 adsorption on the Ag surfaces. As a result, for the FePc@Ag/C catalysts, O2 adsorption on the FePc molecules is preferred over its adsorption on the Ag surface, and as a result, the ORR activity observed on the FePc@Ag/C is similar to that on the FePc/C, which is demonstrated by the RRDE polarization curves shown in Figure 2b. The O2 adsorption on either CoPc or NiPc molecules was predicted to be much weaker than that on the FePc molecules21 and are comparable to that on the Ag surface. Because the electrochemical active sites on the Ag surfaces are significantly larger than those on the MPc molecules (∼15 Å per molecule), the ORR tends to occur on the Ag surface rather than on the CoPc or NiPc molecules, which is supported by the RRDE measurement results (Figure 2). Unlike the CoPc/C or NiPc/C catalysts that reduced O2 via the 2e− pathway,21,33 the ORR on the CoPc@Ag/C and NiPc@Ag/C catalysts were found to proceed via the 4e− pathway. The Tafel plots (Figure 3) obtained on the CoPc@ Ag/C, MnPc@Ag/C, and NiPc@Ag/C catalysts are quite similar to that on the Ag/C, but different from those for the CoPc/C, MnPc/C, and NiPc/C catalysts.21 Therefore, one can reasonably assume that the active sites for the O2 reduction on the CoPc@Ag/C and NiPc@Ag/C catalysts are primarily on the Ag surfaces rather than on the MPc molecules. One exception to the observation and conclusion is the FePc@Ag/C catalyst; unlike the other MPc@Ag/C catalysts tested, the FePc@Ag/C catalyst showed similar ORR activity as that on the FePc/C, but much lower Tafel slopes than those observed on the Ag/C catalysts, which suggest that the active sites for the O2 reduction on the FePc@Ag/C catalysts are primarily on the FePc molecules rather than on the Ag surfaces. It appears that the nature of the MPc molecules and their interactions with silver surfaces determines their roles in ORR. The optimal ORR catalyst should exhibit not just O−O bond-breaking capability, but also the electro-reduction of the oxygenated intermediates. For the ORR in alkaline media, the reaction products, such as OH− species, need to be removed efficiently from the catalyst surface to prevent the active sites from being blocked. Our DFT calculations indicate that the OH adsorption on either Co- or Fe- macrocylic molecules is more energetically favorable than the O2 adsorption on the same molecules.21,22,34−37 For example, the O2 adsorption energy of 1.16 eV on the FePc was predicted by the DFT calculations, while the OH adsorption energy was 3.41 eV.21 The stronger OH− adsorption onto the FePc molecules resulted in the poor stability of ORR on the FePc/C electrode observed in the RDE stability tests and the poor performance in AEMFCs.2136, Similarly, the performance of the FePc@Ag/C cathode in AEMFCs was poorer than the Ag/C cathodes (Figure 4b). The high ORR activity observed on either the FePc/C or FePc@Ag/C by a RDE in O2 saturated 0.1 M NaOH solutions could not be duplicated in the AEMFCs, which supports our hypothesis that the OH− binding effect plays a crucial role in determining the ORR activities of the

on the Ag/C cathode was 1.37 Ω cm2 at the cell voltage of 0.8 V and then reduced almost an order of magnitude to 0.18 Ω cm2 when the cell voltages dropped to 0.50 V. The mass transport resistance, Rd,mass, was also much higher at 0.8 V (0.69 Ω cm2) than that at 0.5 V (0.11 Ω cm2). Both Rc,ct and Rd,mass decrease with lower cell voltages or higher current densities, similar to what was observed with the AEMFCs using membranes with various ionic exchange capacities20 or various MPc/C cathode catalysts.21 The OH− binding effect, especially at high cell voltages, is identified as a key factor controlling the values of Rc,ct and Rd,mass, and so the electrode polarizations in the AEMFCs. Similar to that on Pt surfaces, the rate-determining step (rds) for the ORR on Ag surfaces can be assumed to be the first charge-transfer step31,32 and described as O2,ad + e− → ·O2,ad−

The rate of the ORR on an Ag surface can be expressed ⎛ −γrθad ⎞ ⎛ −βFE ⎞ ⎟ exp⎜ ⎟ i = nFkcO2(1 − θad)x exp⎜ ⎝ RT ⎠ ⎝ RT ⎠

(4) 31

as (5)

where n is the number of electrons, k is the chemical rate constant, cO2 is the concentration of O2 in the alkaline media, θ is the degree of the total surface coverage by all adsorbed species, x is either 1 or 2, depending on the number of sites required by the adsorbates, β and γ are the symmetry factors, r is the parameter for the rate of change of free energy of the adsorption of the reacting intermediates, and E is the applied potential. By increasing either cO2 or temperature T, the rate of the ORR should increase accordingly, which correlates well with the observations of higher kinetic current densities in AEMFCs than in RDE. Equation 5 also indicates that the value of θad is an important factor affecting the rates of ORR. In an AEMFC, the cathode is less than 10 μm apart from the anode since a thin anionexchange-membrane was used to prepare the MEA. The potential of the cathode is always more positive than that of the anode. At high cell voltages, a strong positive electrical field that is favorable to attracting the OH− species is presented at the cathode. The OH− species produced by the ORR can be adsorbed on the Ag surface due to the positively charged Ag surfaces in the AEMFCs. The OH− binding effect could be quite detrimental in that it would cause blockage of the active catalytic sites needed for O2 adsorption and ORR, resulting in substantial voltage losses at the cathode during AEMFC operation.21 Therefore, at high cell voltages, the OHad species are considered as the dominating adsorbates on the Ag surface and determine the value of θad. The steady-state polarization curve obtained in AEMFCs using the Ag/C cathodes agrees well with the OH adsorption isotherm on the Ag single crystal surfaces reported by Blizanac et al.32 As shown in Figure 4, the cell voltages of AEMFCs using the Ag/C cathode catalyst dropped sharply with increasing current densities, then a voltage plateau (0.60−0.50 V) was observed. When the cell voltages continued to drop to 0.1 V, the discharge current density increased substantially due to reduced OH− adsorption and reduced coverage factor θOH. It is quite evident that the rate of ORR on Ag/C cathodes increases when the OH coverage θOH is reduced, which releases electrochemical active sites for O2 reduction and results in enhanced ORR kinetics by lowing the Rc,ct and Rd,mass, as shown in Figure 6. To further enhance the ORR activity on the Ag/C cathodes in the H



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Figure 7. Schematic illustration of the active sites on the positively charged: (a) unmodified Ag surface having a high OH− coverage and (b) MPc molecules modified Ag(111) surface with a reduced OH− coverage.

catalysts in the AEMFCs. For the catalysts, such as FePc/C or FePc@Ag/C, with the strong OH− adsorption capacity, the active sites of FePc in the cathode of the AEMFC tend to be quickly blocked by the OH− adsorbates, especially at the high cell voltages. Consequently, the observed ORR activities on FePc/C or FePc@Ag/C catalysts in the AEMFCs were mainly attributed to either the carbon supports21 or the Ag/C supports (Figure 4). The FePc molecules could not serve as the active sites for the O2 reduction in the AEMFCs, but could strongly adsorb OH− species that were produced on the Ag/C or carbon electrodes. As the result, the Rd,mass on the FePc@Ag/C catalysts at the cell voltage of 0.4 V was the higher than other Ag/C or MPc@Ag/C catalysts (Figure 6b). The other MPc@Ag/C (M = Mn, Co, and Ni) catalysts and the Ag/C catalysts showed comparable ORR activities in both the RDE measurements and in the AEMFCs. Because the active sites for the O2 reduction on these MPc@Ag/C catalysts are identified to be on the Ag surface, the enhanced ORR activity on the MPc@Ag/C in comparison to the Ag/C can be attributed to the modification of Ag electrode by the MPc molecules. The DFT calculation results (Table 3) indicate that the adsorption energy of a MPc molecule on the Ag(111) surface follows an order of MnPc-Ag > CoPc-Ag > NiPc-Ag, which correlates well with the Ag oxidation charges (Figure 1b). The strong interactions of MnPc or CoPc molecules with the Ag surfaces result in significant increases of Ag oxidation

charges and more electrochemically active sites for the O2 reduction. On the other hand, the weak NiPc−Ag interaction leads to a slight increase in the active sites toward the Ag oxidation. The ORR kinetic currents (Figure 2) were found to follow the order of the Ag oxidation charges: CoPc@Ag/C > MnPc@Ag/C > NiPc@Ag/C > Ag/C. The fact that the adsorption of MPc molecules on the Ag/C catalysts increased the Ag oxidation charges and enhanced ORR kinetic currents implies that the MPc molecules, especially CoPc, play a role in destabilizing or reducing the θOH on the Ag/C electrode in alkaline media. 4.3. Structural Model for the Electrochemical Interfaces of MPc@Ag/C Electrodes in Alkaline Media. A structural model for describing the roles of the MPc molecules in tuning the electrochemical interface of Ag(111) in alkaline media is illustrated in Figure 7. Similar as what was proposed by Lucas et al.,31 the active sites on an unmodified positively charged Ag surface tend to have a high OH− coverage, as shown in Figure 7a. The OH− coverage (θOH) on the Ag single crystal surfaces depended on the electrode potentials: the θOH was 30−50% at potentials above 0.7 V versus RHE, but decreased steeply with decreasing potentials in alkaline solutions. A θOH plateau was observed at potentials of 0.4 to 0.7 V versus RHE, and then the θOH became quite small at the potential around 0.1 V versus RHE.32 The presence of adsorption of OH− species (OHad) on the positively charged I



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Ag(111) surface in a KOH electrolyte could be stabilized by the hydrated K+ cations through a noncovalent interaction forming a compact double layer structure.31 When the MPc molecules are adsorbed on the Ag surface, as shown in Figure 7b, the adsorption of OHad on the Ag surface can be destabilized due to the strong interaction between the MPc molecules and Ag surface (MPc−Ag), as shown in Table 3, and the strong interaction between the MPc molecules and OH− species (MPc−OH) (Table 4). Both the MPc−Ag and MPc−OH interactions were estimated by the DFT calculations, which show that the MPc−OH interactions (1.3−3.4 eV) are stronger than the Ag−MPc interactions (0.6−2.1 eV) and much stronger than the noncovalent interaction forming a compact double layer structure between the K+ cations and hydrated OH− species.31 Therefore, the structures and compositions of electrochemical interface of the Ag/C electrode in alkaline media are influenced by the adsorbed MPc molecules. As the result, the rates for electrochemical reactions, such as the Ag oxidation reactions and the O2 reduction reaction, on the MPcmodified Ag/C electrodes could be varied accordingly. Based on the DFT calculations, the OH− adsorption on the FePc molecules was found to be too strong that resulted in the higher cathode charge transfer resistance Rc,ct and the mass (OH−) transport resistance Rd,mass on the FePc@Ag/C than on the Ag/C cathode in the AEMFCs (Figure 6). On the other hand, the estimated Rc,ct at high voltages and the Rd,mass at low voltages were the lowest on the CoPc@Ag/C catalysts compared to the other MPc@Ag/C catalysts. The kinetic current of the CoPc@Ag/C in the RDE at 0.8 V is 10 mA mg−1 cm−2 at room temperature, twice that of the Ag/C catalysts (Figure 3). Consistent with the RDE measurements, the current densities observed on the CoPc@Ag/C cathodes in the AEMFC are also 2−3 times higher than that observed on the Ag/C cathodes (Table 2). The combination of increased active sites on the Ag surfaces by the CoPc modification and the optimal OH− adsorption strength on the CoPc molecules seems to be the key factors leading to the highest ORR kinetics observed on the CoPc@Ag/C catalysts in AEMFCs.

5. CONCLUSIONS The effects of various transition-metal phthalocyanine (MPc, M = Fe, Co, Ni, and Mn) molecules adsorbed on Ag/C catalysts were studied by cyclic voltammetry in Ar-saturated 0.1 M NaOH solutions and by DFT calculations. The MPc molecules were found to alter the electrochemical interfaces of the Ag/C electrodes in the 0.1 M NaOH solutions due to the interaction between the MPc molecules and the OHad species. The intensity of the MPc−OH interactions followed the order of FePc−OH > MnPc−OH > CoPc−OH > NiPc−OH. Based on the reported interface structures of Ag(111) in 0.1 M KOH solutions and our DFT calculation results, a structural model to describe the roles of MPc molecules in modifying the electrochemical interfaces of Ag/C electrodes in alkaline media is proposed and can be used to interpret the various oxygen reduction activities observed from both the various RDE tests of MPc@Ag/C catalysts in 0.1 M NaOH solutions and the AEMFCs tests. The conclusions derived from this work are summarized as follows: 1. The MPc molecules can destabilize the OHad adsorption on the Ag/C surfaces and result in an increase in the electrochemically active sites of the Ag surface. The ability of the MPc molecules to increase the number of

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Ag electrochemically active sites depends on the strength of the MPc−OH interactions, which follows the order of FePc > MnPc > CoPc > NiPc. 2. Among the various MPc moledules, CoPc molecules were shown to have the largest effect on improving ORR activity in both the 0.1 M NaOH solutions and the AEMFCs, while FePc molecules showed the highest ORR activity in the 0.1 M NaOH solutions but the poorest performance in the AEMFCs. The strong FePc− OH interactions may have caused both the loss of the active sites of the FePc molecules and the loss of the function of the FePc for tuning the Ag/C surface at the high cell voltages of an AEMFC. Among four tested MPc-modified Ag/C catalysts, the NiPc−OH interaction was the weakest. Therefore, the NiPc@Ag/C showed only slightly improved ORR activity in the 0.1 M NaOH solutions and in the AEMFCs. 3. Comparing the ORR kinetics obtained with the RDE in 0.1 M NaOH solutions and in AEMFCs with various MPc@Ag/C (M = Co, Ni, and Mn) and Ag/C cathode catalysts, it was derived that the active sites for the O2 reduction were primary on the Ag surfaces, while the MPc molecules destabilized the OH− adsorption on the Ag surface due to the rather strong MPc−OH − interactions. Among the MPc@Ag/C catalysts, the CoPc@Ag/C catalysts seem to have the optimal CoPc−Ag and CoPc−OH interactions that resulted in its best performance in the AEMFCs. 4. Besides transition metal−phthalocyanines and their derivatives, other transition metal−organic molecules containing M−N2 or M−N4 structures and the like, such as metal prophyrins and their derivatives, may have similar effects on modifying the active sites on the electrode surfaces. The new roles of macrocylic molecules for tuning electrochemical interfaces of electrodes can be useful for better-designed, highperformance “multi-functional” cathode catalysts for fuel cell applications. Further optimizing the composition and morphology of this unique catalyst system is expected to continue improving activities toward O2 reduction in AEMFCs.

ASSOCIATED CONTENT

S Supporting Information *

Additional supporting data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1 317 274 4280. Fax: +1 317 274 0789. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Army Research Lab (W911NF-10-2-0075). REFERENCES

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C Electrodes in Alkaline

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