Electrochemical Promotion of Oxygen Reduction on Gold with

ARTICLE pubs.acs.org/JPCC

Electrochemical Promotion of Oxygen Reduction on Gold with Aluminum Phosphate Overlayer Yejun Park, Seunghoon Nam, Yuhong Oh, Hongsik Choi, Jungjin Park, and Byungwoo Park* Department of Materials Science and Engineering, and Research Institute of Advanced Materials, Seoul National University, Seoul 151-744, Korea

bS Supporting Information ABSTRACT: The activities of Au electrodes with an AlPO4 overlayer were examined for oxygen-reduction reactions in alkaline media. Oxygen molecules on gold catalysts are mainly reduced by a two-electron path, forming hydrogen peroxide with half efficiency. On the AlPO4 overlayer deposited Au, larger current densities corresponding to a nearly four-electron path were recorded within the potential range of approximately 0.7
1.0 V, which were correlated with the decomposition (disproportionation) of hydrogen peroxide. This enhancement was attributed to the electronic interactions and changed activities of the intermediate state, as confirmed by X-ray photoelectron spectroscopy and the voltammetric profiles of hydrogen peroxide, respectively.

’ INTRODUCTION Proton-exchange-membrane fuel cells (PEMFCs) based on the well-developed Nafion membrane have advantageous characteristics.1
3 Platinum-based catalysts show high catalytic activities, but the cost of Pt prohibits commercialization of fuel cells. However, no alternative materials have outperformed Pt in an acidic environment. Alkaline fuel cells (AFCs) or anion-exchange-membrane fuel cells (AEMFCs) are one of the most developed fuel cell technologies and have some advantages, such as feasible activities for nonplatinum metals, lower anode fuel crossover, tolerance of CO poisoning, etc.4
8 Therefore, recently many studies have been performed on nonplatinum catalysts in alkaline media, such as Au and Ag,8
10 including the development of anion-exchange-membranes for AEMFCs11 and hybrid fuel cells designed with an acidic anode and alkaline cathode.12 To operate fuel cells at low temperature, an electrocatalytic material is essential to promote the electrochemical reactions and generate sufficient electrical power. Especially, the activity of the electrocatalyst for the oxygen-reduction reaction (ORR) is important because oxygen (or air) is available in the atmosphere and a unique chemical source for the reduction reaction, as compared to anode fuels, such as hydrogen, small organic molecules, etc.6,13 The ORR pathways have been typically considered to involve three different paths, viz. the two-electron path (eq 1), four-electron serial path (eq 1 f eq 2), and four-electron direct path (eq 3), according to Anastasijevic et al.’s model.14 The potential values are converted to a reversible hydrogen electrode (RHE), considering the dependence of the equilibrium potential on the pH (at pH 13):15 O2 þ H2 O þ 2e
T HO2
þ OH
E ¼ 0:72 V vs RHE r 2011 American Chemical Society

ð1Þ

HO2
þ H2 O þ 2e
T 3OH
E ¼ 1:74 V vs RHE

ð2Þ

O2 þ 2H2 O þ 4e
T 4OH
E ¼ 1:23 V vs RHE ð3Þ Hydrogen peroxide is the main reaction intermediate for the ORR, which inevitably occurs on Au catalysts,16 and its further reaction is an essential factor for the complete reduction of oxygen. However, the reduction of hydrogen peroxide to the hydroxyl ion (eq 2) rarely occurs on Au catalysts except for the (100) facet.17 In acidic media, hydrogen peroxide diffuses away with the protonation of HO2
to H2O2 without any further reactions.16 In an alkaline environment, the decomposition of hydrogen peroxide is another possible reaction path. The reduction (eq 2) and oxidation (eq 1) reactions of hydrogen peroxide occur simultaneously, and the sum of these two reactions is described by the decomposition of hydrogen peroxide:15 2HO2
T O2 þ 2OH


ð4Þ

The decomposition of hydrogen peroxide in alkaline media produces oxygen and the hydroxyl ion. The hydroxyl ion is the completely reduced form of oxygen, and the oxygen regenerated in the vicinity of the electrodes can take part in the ORR by iteration, increasing the reaction efficiency.18 Gold has a higher oxidation potential (Au3þ þ 3e
T Au, E = 1.498 V) than Pt (Pt2þ þ 2e
T Pt, E = 1.188 V)15 and has the advantage of improved stability for cathode electrodes.19,20 The Received: January 12, 2011 Revised: February 21, 2011 Published: March 17, 2011 7092

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The Journal of Physical Chemistry C activities of Au catalysts for oxygen reduction are enhanced by combining them with several oxide materials, such as defective MgO21,22 and nonstoichiometric SnOx.23,24 The anodic defects of oxide matrices affect the catalytic activities of gold with the transfer of electrons.21
24 The sharing and polarization of the electron density cause changes in the electronic structure of Au, allowing the catalytic activity of Au to be modified, which is known as an electronic effect on catalysts.25 A metal phosphate was selected as the overlayer material because it offers resistance to dissolution under the corrosive conditions of the fuel cell cathode, while providing oxygen permeability owing to its porous nature.26 The nanoporous structures of amorphous (or poor crystalline) metal phosphates allow the suitable transfer of protons, hydroxyl ions, water, methanol, and oxygen.27
30 Various metal phosphates have been investigated as solid-state electrolytes, including amorphous aluminum phosphate.31 The structural diversity and easy hydration of aluminum phosphate derivatives32,33 enable the characteristics of the phosphate layer to be more highly controlled. Therefore, the AlPO4 overlayer permits both the access and release of the reactants, intermediates, and products, depending on the permeability of each chemical substance. In our previous work, the enhanced oxygen reduction activities of Au/AlPO4 nanocomposites synthesized under various sputtering conditions were investigated.34 The simultaneous deposition of Au and AlPO4 changed both the valence band structure of AlPO4 and the grain size of Au. In this article, the deposition of aluminum phosphate is followed by the deposition of Au to form a simplified Au/AlPO4 bilayer structure. The catalytic activities of Au for the ORR can be enhanced by these controlled AlPO4 overlayers in an alkaline environment. The cause and effect of the modified electrode kinetics for the ORR are investigated in terms of the behaviors of the intermediates and electronic interactions, concurrently confirming the negligible changes in the surface area and facets of Au.

’ EXPERIMENTAL PROCEDURE Gold was deposited on indium
tin oxide (ITO) coated glass (Samsung Corning) by rf magnetron sputtering before the deposition of the AlPO4 overlayer. The deposition was performed at room temperature (RT) under an Ar atmosphere at 25 mtorr for both Au and AlPO4. The deposition time of AlPO4 was controlled in the range of 15
180 min for the purpose of obtaining AlPO4 overlayers with various thicknesses. Platinum electrodes were also prepared to compare their ORR activities.27 The surface morphology was observed by field-emission scanning electron microscopy (FE-SEM, SU70: Hitachi). X-ray photoelectron spectroscopy (XPS, Sigma Probe: Thermo VG Scientific) was used to analyze the surface chemical states with monochromatic Al KR radiation (1486.6 eV). The binding energy was calibrated by setting the C 1s peak at 285.0 eV.35 As the amount of phosphate on Au was increased, the photoelectrons of Au 4f became more attenuated in the overlayer matrices, and their intensities were gradually diminished in the XPS spectra. The attenuation of the photoelectron intensity is based on the formula I = I0(e
d/λ), where d is the thickness of the matrices which the photoelectrons traverse and λ is the attenuation length.36 The thicknesses of the AlPO4 matrices were roughly estimated from the attenuation of the intensity, and values of ∼0.7, ∼1.4, ∼2.7, ∼5.4, and ∼8.1 nm were obtained with different deposition times of 15, 30, 60, 120, and 180 min,

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Figure 1. Plan-view images of (a) ITO substrate and (b) Au deposited on ITO. After the deposition of the Au layer, AlPO4 layers are sequentially deposited on Au with thicknesses of approximately (c) 0.7 nm and (d) 2.7 nm.

respectively. The attenuation length depends on the kinetic energy of the photoelectrons and the density of the matrices.36 The difference in the kinetic energy of the Si 2p and Al 2p photoelectrons induces a difference of only 1.6% in the inelastic mean-free path,37 and the densities of SiO2 and AlPO4 are similar. The thickness of AlPO4 is roughly estimated from the attenuation length of SiO2,36 even though the attenuation length depends on the nanostructural changes and coverage of the overlayer matrices.38,39 The electrochemical measurements were made with an electrochemical analyzer (CHI 604A: CH Instrumental Inc.) using a conventional three-electrode configuration at RT. The Au- or AlPO4-deposited Au electrodes, Pt wire, and Hg/HgO electrode served as the working, counter, and reference electrodes, respectively. All of the potentials are presented versus RHE in this article. The electrochemical surface area (ESA) can be determined from the electric charge for the desorption peak of lead, using the underpotential deposition (UPD) of lead(II) on a gold surface40 with 1 mM lead acetate (Pb(CH3CO2)2 3 3H2O; Aldrich) dissolved in 0.1 M NaOH solution. The hydrogen peroxide activities were also examined in 1.2 mM hydrogen peroxide (3 wt % in H2O; Sigma-Aldrich) containing 0.1 M NaOH solution. These two solutions were adjusted to pH 13 and purged with nitrogen before each measurement. The ORR activities were examined with cyclic voltammetry in an oxygensaturated 0.1 M NaOH solution. The oxygen flow was fixed at 30 sccm during the ORR measurement. All of the cyclic voltammetry measurements were performed at a scan rate of 50 mV/s.

’ RESULTS AND DISCUSSION The FE-SEM images in Figure 1 show the surface morphologies of the gold electrodes and AlPO4-deposited gold electrodes. The Au layer was deposited on the ITO substrate at RT, and its morphology follows that of the substrate due to the limited coarsening process of Au. Then, AlPO4 overlayers were deposited on the Au electrodes. As the amount of phosphate is increased, AlPO4 covers the gold and gradually fills the gaps in it. Using the adsorption and desorption of lead on the gold surface in the UPD region, the ESA values and surface facets of gold can be determined. The reactions in the underpotential 7093

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Figure 2. (Color) Voltammetric profile of Au and AlPO4 overlayer deposited Au electrodes with 1 mM Pb(CH3COO)2 dissolved in 0.1 M NaOH solution at a scan rate of 50 mV/s. The main desorption peaks corresponding to each gold surface facet are marked.40

Figure 3. (Color) Oxygen-reduction polarization curves on the electrodes with AlPO4 overlayer deposition of approximately (a) 0.7
2.7 nm and (b) 2.7
8.1 nm, in oxygen-saturated 0.1 M NaOH solution at a scan rate of 50 mV/s. The calculated current densities corresponding to the two-electron and four-electron paths are marked.

region are specific to the surface and typically occur up to a monolayer compared to those in the overpotential region.25,40 Therefore, underpotential methods are suitable for observing the surface characteristics. The widely used hydrogen-desorption method is also based on the underpotential reactions of

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Figure 4. (Color) Voltammetric profiles of Au and AlPO4 overlayer deposited Au electrodes with 1.2 mM hydrogen peroxide in 0.1 M NaOH solution at a scan rate of 50 mV/s. The average points of the forward and backward scans are marked with arrows.

hydrogen on the Pt surface.41,42 The electrodes are examined in the Pb2þ containing solution, as shown in Figure 2. The charge of lead desorption is proportional to the surface area of gold, and the ESA values that were determined are 1.37, 1.54, and 1.52 (cm2 per cm2 sample) for Au, ∼2.7 nm AlPO4 on Au, and ∼8.1 nm AlPO4 on Au, respectively. The peak potentials of lead adsorption and desorption exhibit distinct signatures with respect to the orientation of the gold surface. The main Pb2þ desorption peaks of the Au (111), (110), and (100) facets are 0.435, 0.56, and 0.475 V, respectively.40 The Au(100) facets rarely exist and the Au(111) and Au(110) facets exist in certain proportions. The surface areas of the Au(111) facets are 0.46, 0.45, and 0.45 (cm2 per cm2), and those of the Au(110) facets are 0.91, 1.09, and 1.06 (cm2 per cm2) for Au, ∼2.7 nm AlPO4 on Au, and ∼8.1 nm AlPO4 on Au, respectively. The surface areas and surface facets of the electrodes are not significantly changed after the deposition of the AlPO4 overlayer. In order to compare the ORR activities of the Au electrodes and AlPO4-deposited Au electrodes, the voltammetric profiles of the electrodes in oxygen-saturated 0.1 M NaOH solution are summarized in Figure 3. By increasing the amount of phosphate overlayer, the shape of the voltammetric profile is changed and the ORR is promoted within the potential region of approximately 0.7
1.0 V. The current densities are nearly doubled on the AlPO4-deposited Au electrodes, as shown in Figure 3a. This indicates that the almost complete reduction of oxygen occurs within a certain potential range with the proper amount of AlPO4 overlayer, considering that Au electrodes typically show only half efficiency. As seen on the reverse sweep, the oxidation peak at ∼0.95 V is indicative of the oxidation of the residual HO2
and O2
(O2 þ e
T O2
), which are the intermediates of incomplete oxygen reduction.43,44 The increase in the efficiency of the ORR decreases the oxidation currents of the residual intermediates. The limiting current density iL can be calculated from the formula iL =
DnFc*/δ,45 where the O2 diffusion coefficient D = 2.6  10
5 cm2/s,46 the number of electrons involved n = 4, the Faraday constant F = 96 485 C/mol, the O2 concentration c* = 1.22 mM,17 and the diffusion layer thickness δ = 200 μm without any convection47 in the oxygen-saturated 0.1 M NaOH. Based on 7094

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and hydrogen peroxide on the electrodes, thereby changing the catalytic activities of the electrodes.

’ CONCLUSIONS The surface properties of the Au layer were conserved before and after the deposition of the nanoscale AlPO4 overlayer, in terms of the electrochemical surface area and facet orientation. However, the catalytic activities of Au were varied with the amount of the AlPO4 overlayer. The potential region of the enhanced ORR (approximately 0.7
1.0 V) is associated with the reaction path including the decomposition (disproportionation) of hydrogen peroxide. These enhancements were attributed to the electronic effects on Au and the changed behaviors of the intermediate hydrogen peroxide. ’ ASSOCIATED CONTENT Figure 5. (Color) XPS spectra of the Au 4f region for the Au electrode and the AlPO4 overlayer deposited Au electrodes with thicknesses in the range from 0.7 to 8.1 nm. The dashed lines are from the standard sample of Au.

these values, the limiting current density iL is approximately
0.61 mA/cm2 for the four-electron process, which is consistent with the experimental ORR data in Figure 3. When the AlPO4 overlayer becomes sufficiently thick, the oxygen accessibility is small, so that the equivalent effect of the extended diffusion layer is observed. As a result, the overall ORR current density is gradually diminished once the AlPO4 overlayer becomes thicker than ∼2.7 nm (Figure 3b). To verify the changes in the activities of the intermediates, the voltammetric profile of the electrodes with hydrogen peroxide in 0.1 M NaOH solution is presented in Figure 4. The net current is zero at the counterbalance point between the reduction (eq 2) and oxidation (eq 1) of hydrogen peroxide. The potentials at zero current gradually shift to positive potential, indicating the relative promotion of the reduction of hydrogen peroxide, which is associated with the tendency of the potential shift of the ORR peak current, as can be seen in Figure 3a. Hydrogen peroxide tends to be oxidized to oxygen at potentials above 0.72 V and reduced to hydroxyl ions at potentials below 1.74 V at pH 13.15 Hydrogen peroxide is doubly unstable in this potential range (0.72
1.74 V) and can chemically decompose into oxygen and hydroxyl ions (eq 4), which is considered as the electrochemical catalysis of the chemical reaction. In the course of the ORR, hydrogen peroxide is generated as an intermediate below the onset potential (∼1.0 V). Therefore, the decomposition of hydrogen peroxide can occur within the range of approximately 0.72
1.0 V, which nearly corresponds to the potential region of enhanced ORR efficiency in Figure 3. The repeating reaction loop including the decomposition of hydrogen peroxide contributes to the enhanced efficiency of the ORR. Figure 5 shows the XPS spectra of the Au 4f region as a function of the growth time of the AlPO4 overlayer on the Au electrodes.48,49 The shift of Au 4f toward a lower binding energy is observed with increasing amounts of the AlPO4 overlayers, which in the first interpretation indicates the negatively charged Au. The observations by XPS are consistent with the activities of oxygen and hydrogen peroxide, as seen in Figures 3 and 4. The electron transfer to Au can induce alterations in the electronic structure of Au, and this can affect the adsorption state of oxygen

bS

Supporting Information. Additional characterizations (XPS of Al 2s and P 2p). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]; phone þ82-2-880-8319; fax þ82-2-885-9671.

’ ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea, through the Ministry of Education, Science and Technology (MEST, 2010-0024382), the Korean Government (2010-0029065), and the World Class University (WCU, R312008-000-10075-0). ’ REFERENCES (1) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (2) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5. (3) Mauritz, K. A.; Moore, R. B. Chem. Rev. 2004, 104, 4535. (4) Spendelow, J. S.; Goodpaster, J. D.; Kenis, P. J. A.; Wieckowski, A. J. Phys. Chem. B 2006, 110, 9545. (5) Matsuoka, K.; Miyazaki, K.; Iriyama, Y.; Kikuchi, K.; Abe, T.; Ogumi, Z. J. Phys. Chem. C 2007, 111, 3171. (6) Bunazawa, H.; Yamazaki, Y. J. Power Sources 2008, 182, 48. (7) Varcoe, J. R.; Slade, R. C. T. Fuel Cells 2005, 5, 187. (8) Varcoe, J. R.; Slade, R. C. T.; Wright, G. L.; Chen, Y. J. Phys. Chem. B 2006, 110, 21041. (9) Hernandez, J.; Solla-Gullon, J.; Herrero, E.; Aldaz, A.; Feliu, J. M. J. Phys. Chem. C 2007, 111, 14078. (10) Blizanac, B. B.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2006, 110, 4735. (11) Varcoe, J. R. Phys. Chem. Chem. Phys. 2007, 9, 1479. (12) Unlu, M.; Zhou, J.; Kohl, P. A. J. Phys. Chem. C 2009, 113, 11416. (13) Gonzalez, M. J.; Hable, C. T.; Wrighton, M. S. J. Phys. Chem. B 1998, 102, 9881. (14) Anastasijevic, N. A.; Vesovic, V.; Adzic, R. R. J. Electroanal. Chem. 1987, 229, 305. (15) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions: National Association of Corrosion Engineers: Houston, TX, 1974. (16) Shao, M. H.; Adzic, R. R. J. Phys. Chem. B 2005, 109, 16563. (17) Markovic, N. M.; Adzic, R. R.; Vesovic, V. B. J. Electroanal. Chem. 1984, 165, 121. 7095

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