Monolayer Growth Front of Precious Metals through Insulating

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Monolayer Growth-Front of Precious Metals through Insulating Mesoporous Membranes Nicholas Linck, Alex Peek, and Bruce J. Hinds ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03135 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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ACS Applied Materials & Interfaces

Monolayer Growth-Front of Precious Metals through Insulating Mesoporous Membranes Nicholas Linck§‡, Alex Peek†‡, and Bruce J Hinds*,† § Department of Chemical and Materials Engr., University of Kentucky, Kentucky 40506, USA † Department of Material Science and Engineering, University of Washington, Washington 98105 Energy, Monolayer, Mesoporous, Electrocatalysis, Membrane

Monolayers of precious metals are deposited within the pores of insulating mesoporous anodized aluminum oxide (AAO) membranes via a new electrochemical underpotential Cu deposition growth front mechanism, followed by spontaneous galvanic replacement of copper by platinum or iridium as demonstrated by XPS, ICP-OES, conductivity, and current analysis. Applications include fuel cells, hydrogen storage, flow batteries, and electrocatalytic conversions.

1. Introduction Engineering a source of clean, renewable and storable energy remains a key challenge confronting the world today. Hydrogen fuel generation from renewable sources is a leading candidate to replace the current fossil fuel based energy industry.1, 2 One of the most attractive processes for the generation of hydrogen fuels is proton exchange membrane (PEM) water

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electrolysis. PEM devices are lightweight, compact, and operate at low temperatures with high current densities.3 The primary hurdle faced by these industries is the prohibitively high cost of the precious metal catalytic electrodes in the device.4 The conventional cathode material, platinum, offers excellent catalytic activity for the hydrogen evolution reaction. Iridium, ruthenium, or their oxides are often chosen as the anode for the oxygen evolution reaction.5, 6 Platinum (among other precious metals) is also commonly used for numerous catalytic applications, including the electrooxidation of methanol in direct methanol fuel cells enabling biofuel production and conversion to electrical energy.7,

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Pt is also industrially used as a

hydrogenation catalyst for unsaturated hydrocarbons,9 the decomposition of hydrogen peroxide and complete combustion of exhaust hydrocarbons (CxHy) in automobile catalytic converters.10-12 The state of the art technique uses nanoparticles (~2 nm) of the precious metal on mesoporous supports, such as high surface area carbon or zeolite materials.13-16 This practice leaves only 2530% of the atoms on the surface and thus available for catalysis with 70-75% of atoms unused. The ideal situation would be to use a monolayer of catalyst on a highly porous support material, making all atoms in the device available for catalysis and take advantage of efficient mass transport flowing through a membrane geometry. The method for underpotential deposition (UPD) has previously been used to plate a monolayer of platinum. This was achieved by first depositing a monolayer of Cu atoms at a less negative voltage than bulk deposition.17-20 The copper monolayer can then be spontaneously replaced by platinum (or iridium) atoms due to their difference in reduction potential.18, 21 This method for monolayer plating has been extensively studied on expensive conductive substrates, such as single crystal Au[III],22 nanoporous gold,23 and conductive Au or Pd nanoparticles shells.21,

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More recently, chemical modification of conductive substrates followed by


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monolayer deposition has been investigated. Notably, Pt monolayers have been deposited onto self-assembled monolayers over Au substrates and diazonium-modified CNT mattes;25,

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however, a key limitation remains: the monolayer can only deposit in the immediate vicinity of the conducting electrode. Needed is a UPD method on inexpensive and corrosion resistant materials, preferably with efficient mass transport as is seen across membranes. In this work, a monolayer of platinum or iridium is plated within insulating porous monoliths of anodic aluminum oxide (AAO) membranes. Importantly, a Cu monolayer growth front allows a coating over mesoporous insulating materials. The membrane geometry offers key advantages for hydrogen generation: robust support, high surface area for electrochemistry, and flow-driven reagent replenishment to the catalytic centers. 2. Experimental Section 2.1 Membrane Fabrication. Anopore aluminum oxide membranes (Anodisc) of 0.02 µm pore size and 13mm or 25 mm diameter were obtained from Whatman. A 10 nm thick layer of Au was thermally evaporated on the membranes at 5*10-6 mBar in a Cressington 308R coating system.

Coated membranes were mounted in polycarbonate housings. Cu tape

established the electrical connection to the working electrode and remained physically isolated from the electrolyte. 2.2 SEM Microscopy. Cross section SEM micrographs of the membranes were collected with a FEI Sirion XL 30 in ultra-high resolution mode. Samples were cleaved, then sputtered with 1nm of Au to prevent charging. The micrographs were used to measure the surface area of the AAO membranes.


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2.3 Copper Underpotential Deposition: Electrochemistry was performed with an eDAQ Potentiostat using an Ag/AgCl (1 M KCl) reference electrode and a platinum counter electrode. All voltages are referenced vs 1M Ag/AgCl. The membranes were soaked in 10 mL of 5 mM CuSO4·5H2O (Sigma) for 2 hours to allow Cu2+ cations to fill pores of the membrane. Next, the membrane was set up in a three electrode cell as the working electrode. The electrolyte solution consisted of 50 mL of 5 mM CuSO4·5H2O and 0.1 M H2SO4 (Sigma) and was degassed for 30 minutes prior to experiments. Underpotential deposition was carried out by applying 5 mV (versus Ag/AgCl) to the sample in the potentiostat setup for 8 minutes. The membrane was immediately placed in a 1 mM solution of K2Cl4Pt (Sigma-Aldrich) (or IrCl3·xH2O for iridium plating) and held there for 10 min to monotonically replace Cu atoms with precious metal: Cu + Pt → Cu + Pt OR 3Cu + 2Ir → 3Cu + 2 Ir . The membrane was moved to 10 mL deionized water and left there for 10 min to remove any Pt2+ or Ir3+ cations that could have physisorbed to the surface. 2.4 X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer. Control membranes were coated with a 10 nm thick layer of gold and then soaked in K2PtCl4 (Sigma) or IrCl3·xH2O (Sigma) solution for 10 minutes, followed by a 10 minute rinse in DI water. 2.5 Inductively coupled plasma optical emission spectroscopy: (ICP-OES) was carried out using a Varian Vista Pro ICP-OES. After monolayer coating, membranes were placed in 10% HNO3 and heated to 100°C for 3 hours to digest the metal surface sample. Calibration curve was constructed using 5, 1, 0.5, 0.1, and 0.05 ppm standard solutions of the analyte.


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2.6 Water Electrolysis on Precious metal monolayer: Pt or Ir monolayer coated AAO membranes (13mm diameter) were connected in the same electrochemical cell as above with degassed 0.1M H2SO4 electrolyte. Again, control membranes were coated with a 10 nm thick layer of gold and then soaked in K2PtCl4 (IrCl3·xH2O) solution for 10 minutes, followed by a 10 minute rinse in DI water. 3. Results and Discussion The underpotential deposition of a copper monolayer followed by spontaneous redox replacement is the primary approach to plate platinum or iridium monolayers onto an insulating mesoporous membrane support. Starting with a deposited electrode on one side of the membrane, a Cu monolayer growth front mechanism through the pores would be required to achieve the goal as diagrammed (Scheme 1). Scheme 1. Electrochemical Underpotential deposition of Cu monolayer onto insulating AAO and spontaneous replacement of Cu with precious metal. (i)

Au Film

(ii)

(iii)

(v)

(iv)

0

Cu Atom 0

Pt 2+ Atom Cu Ion 2+

Pt Ion

(i) AAO pore with evaporated gold film. (ii) Novel growth front of Cu monolayer via Underpotential Deposition at 5 mV. (iii) Cu monolayer formed on insulating pore. (iv) Cu spontaneously replaced with Pt (or Ir). (v) Catalytic Pt (or Ir) monolayer formed.


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The cross-sectional SEM image of AAO membranes show 200nm diameter parallel pore channels which branch into 20nm pores near the top surface (Figure 1).

(a)

(b) 1 µm

Copper Growth Front

Figure 1. (a) Cu monolayer grows from the conductive Au across the mesoporous, insulating AAO under constant voltage conditions. (b) SEM cross section of AAO membrane. The majority of the membrane consists of 200nm diameter channels. The 200nm channels branch into 20nm pores near the Au surface.

The general method of proving the UPD monolayer growth mechanism is: 1) increase in conductivity across the insulating membrane, 2) quantitative chemical analysis of dissolved precious metal to show the amount corresponds to a monolayer over the entire membrane area, 3) XPS observation of a monolayer on the top surface of the membrane face that is opposite the growth electrode and 4) electro-catalytic activity of the membrane composite in water spiltting experiments. AAO Membranes, with only one side coated with evaporated Au were used as the working electrode in an electrochemical cell. The electrolyte solution, containing Cu2+ ions and H2SO4, was adapted from literature methods for Cu UPD on Au substrates.17-19 A cyclic voltammogram (Figure 2) shows the Cu UPD plating voltage, bulk Cu anodic deposition peak, and bulk Cu cathodic dissolution peaks at A’, B’, and C’ respectively.


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(a)

(b) -2

-2

C’

B’

Current (μA*cm )

Time(s) Current (μA*cm )

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A’

Potential (V)

Figure 2. (a) Cyclic voltammogram of AAO plated with 10nm Au in 0.1 M H2SO4 and 5 mM CuSO4. A’) Cu UPD deposition voltage. B’) Cu bulk deposition peak. C’) Bulk Cu oxidation peak. (b) Chronoamperogram at 5 mV versus Ag/AgCl shows current limiting as Cu monolayer covers AAO surface.

UPD was carried out by applying 5mV (versus Ag/AgCl) to the sample for 8 minutes to deposit a small, fixed number of monolayers. The current values decrease to a solution background current of 3µA because the area of un-plated surface diminishes rapidly, thereby demonstrating a self-limiting plating reaction. The charge assigned to Cu UPD is 121 µC*cm-2, normalized to the total membrane area. For comparison, Cu UPD charge densities on highly ordered [111] Au substrates is 350 uC*cm-2.19, 27, 28 In practice it is best to confirm integrated charges roughly near 120uC/cm2 (normalized to entire AAO area) to insure UPD conditions. However we have found significant variability in commercial AAO membrane area with variations in total membrane thickness and ‘skin layer’ of 20nm pore network over 200nm monolith, which accounts for about half of the calculated area. After deposition, the membrane was immediately placed in a solution of Pt2+ (or Ir3+) ions and held there for 10 minutes to monotonically replace Cu0 atoms with precious metal. Finally, the membrane was rinsed in 10 mL deionized water for 10 min to remove any Pt2+ or Ir3+ cations that could have physisorbed onto the surface. Stoichiometric Pt exchange on Cu UPD monolayers has been shown to be quantitative within 10s and examining literature diffusion coefficient for the PtCl42- gives a characteristic diffusion length of 1mm, far in excess of the 60um membrane thickness for complete exchange in the pores. 29, 30


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After replacement of the copper monolayer with platinum, the trans-membrane resistance was found to be 390 kΩ (average over 5 samples) across the membrane, which was insulating prior to the UPD treatment. This indicated that a metallic layer had indeed grown across the insulating mesoporous insulator. Another confirmation of a thin layer growing within the pore, instead of bulk electroplating blocking the pore entrance, is to measure pore size via a pressure driven water flow rate. A nominal 20nm diameter AAO membranes initially sputtered with ~5nm Au and mounted in a liquid flow setup with the hydrodynamic pressure was recorded at set a flow rate using syringe pump. Membrane pore size calculations via the Hagen-Poiseuille equation and known porosity gave 16.3 nm diameter before and 16.0 nm after Cu UDP plating (±0.3% uncertainty for the ratio) consistent with monolayer growth and conclusively disproving a bulk plating mechanism that would block pores. To show that this conductive layer was consistent with a monolayer across a known surface area, the Pt or Ir in the membranes was dissolved in 10 mL of 10% nitric acid and quantitatively analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). The closest packed Cu monolayer density on [111] Au (2.27*10-9 mol*cm-2) was used to estimate of expected Pt and Ir concentration in the ICP-OES solution.

[19,27,28]

The crystallographic structure of the Cu monolayer and absorbed

anions over an insulator is unknown but the difference between closest packed hcp [111] and the more open [100] is about 30%. The surface area of the membrane was estimated via SEM micrographs and a simple model: a 47 µm thick array of 200nm cylindrical channels in series with a 1 µm thick array of 20nm cylindrical channels. The results of ICP measured concentrations and calculated areas are summarized in (Table 1). Table 1. ICP-OES Quantification of precious metal monolayer coverage on AAO membranes


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ICP-OES Results

Au Surface Only (Theoretical)

Full Membrane (theoretical)

Full Membrane (Experimental)

Surface Area (m2)

8.72 x 10-5

2.78 x 10-2

2.78 x 10-2

Pt Concentration (µM)

1.58 x 10-2

5.03

5.53 ± 0.4

Ir Concentration (µM)

1.05 x 10-2

3.36

6.67 ± 3.0

ICP-OES analysis shows that the dissolved AAO membrane contained an average concentration of 5.53±0.4 µM of Pt. Based on the calculated estimate, the sample would only be 1.98*10-2 µM if the monolayer was only plated on the conductive face of the membrane. Coating the entire surface of the membrane, including the conductive face, the pores, and the insulating face would result in a 6.33 µM sample solution. Since the actual concentration of the digested sample is close to the estimates of full membrane coverage this measurement supports monolayer coverage of Pt on the AAO membrane. Similarly, Ir solution dissolved from the membrane (6.67±3.0 µM) was consistent with a monolayer over the entire surface of the membrane (4.22 µM), rather than simply plating on the gold face (1.32*10-2 µM). Quantifying the amount of Pt or Ir plated on the sample still leaves the possibility of bulk plating on the conductive Au surface that gives a coincidental amount consistent with monolayer coverage. To prove that Pt coated the entire AAO sample, the top surface on the opposite side of the membrane to the Au working electrode was analyzed with the highly surface sensitive XPS analysis. Pt 4d peaks (secondary) with binding energies between 350 and 315 eV were used to determine Pt presence since the stronger primary 4f peak overlapped with Al 2p binding energy. The results of the analysis are shown in (Figure 3).


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(a) X-ray Source

Photoelectron Detector

(b)

(c) Iridium Monolayer Control Membrane

Iridium Monolayer Control Membrane 10000

6000 4000 2000

3000

Ir 4f7/2

2500

Ir 4f5/2

2000 1500 1000 500

0 340

320

Binding Energy (eV)

300

0 70

65

60

Intensity (CPS)

8000

Intensity (CPS)

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Binding Energy (eV)

Figure 3. (a) XPS experiment schematic: Photoelectrons are collected from non-Au coated face. (b) XPS analysis for 350-315 eV region shows small amounts of Pt. (c) XPS spectrum for AAO membrane plated with iridium monolayer shows characteristic binding energies for Ir 4f orbitals. 2+ 2+ Control membranes were Au sputtered, soaked in Pt or Ir and rinsed in DI water.

XPS analysis shows faint Pt 4d doublet peaks, proving that small amounts of Pt are present on the insulating face of the membrane. These small peaks are to be expected since the amount of Pt associated with a monolayer is low and the smaller secondary peaks had to be analyzed. The control sample was exposed to the same solutions and rinsing methods but without using a potentiostat for the UPD of Cu. The XPS data from only the top surface, demonstrates that the Pt coated the entire sample and not just the Au working electrode. The coating of AAO pore channels is inferred since growth would have to initiate at the working electrode on the other side of the membrane. This observation requires a Cu monolayer growth front mechanism, since the copper would not be able to plate on the insulating face of the membrane without first nucleating on the conductive face and growing across the pores of the membrane.

This Cu is then


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galvanically replaced with Pt in a 1:1 ratio. Similar to the Pt samples, Ir coated membranes were also characterized using XPS to determine presence of Ir on the surface opposite the gold coated face. This time, the primary (strongest signal) XPS peaks of Ir 4f between 60-70 eV were used to determine Ir presence, as they do not overlap with Al 2p peaks as in the Pt case. As shown in Figure 3b, Ir is clearly detected in the sample against the control. This confirms a Cu monolayer growth front which is subsequently replaced by Ir. Line-scan SEM/EDX of cross-sections were performed but the monolayer signal is expected to be near signal/noise level and thus inconclusive. Additionally, the phenomena of monolayer growth fronts is expected on planar surfaces but is a challenge to measure via XPS on ~100um length scales between electrodes with similar UPD processes as well of much less area for resistance measurements compared to porous monoliths in a trans-membrane geometry. After demonstrating that a monolayer of platinum could be plated onto an insulating support, the catalytic activity of the membranes was tested using electrochemical water electrolysis via cyclic voltammetry as shown in (Figure 4).


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(a) Control Membrane 60 40 Current (mA) 20 0 1 2 -20 0 Potential (V) -40

-1

-2.5

-1.5

Current (mA) st

40

1 Scan

20

50 thScan 100 Scan

0 -0.5 -20

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th

0.5

1.5

2.5

Potential (V)

-40 -60 -80 -100

(b) Control Membrane

-1

-2.5

40 Current (mA) 20 0 1 2 -20 0 Potential -40

-1.5

40

Current (mA)

st

1 thScan 50 thScan 100 Scan

20 0 -0.5 -20

0.5

1.5

2.5

Potential (V)

-40 -60 -80

Figure 4. (a) Water electrolysis cyclic voltammagram on AAO plated with Pt monolayer in deareated 0.1M H2SO4. Attenuation of current with increasing scan number likely due to delamination caused by generation of gas bubbles. Inset is Control membrane with only Au film. (b) Water electrolysis cyclic voltammogram on AAO plated with Iridium monolayer in deaerated 0.1M H2SO4.

Samples were subjected to 100 cycles between -1.0 V and 2.0 V in 0.1 M H2SO4. Control membranes showed the expected gold Au voltammogram with the characteristic reduction of AuO at 0.7 V. The membrane plated with a platinum monolayer showed a large increase in water splitting rate versus the control membrane. After monolayer plating, the H2 (at -1 V) evolution current increased from 38 mA to 90 mA and the O2 (at +2 V) evolution current increased from 26 mA to 37 mA. Smaller currents in the later scans is likely due to platinum delamination due to the mechanical stress of gas bubble formation. The membrane with the platinum monolayer shows a large peak around 0 V due to absorbed hydrogen oxidation. H2 gas evolved at negative overpotentials remains trapped in the membrane pores and significantly enhances the 0 V peak.


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This unusual effect is not observed on platinum electrode geometries such as wires or foils because generated gas bubbles will dissolve and diffuse away from the electrode quickly, where in the membrane geometry the gas is trapped in the pores at our experimental time scales. The exact size and length of the bubbles is difficult to estimate due to H2 mass transport into solution that makes the peak unobservable for planar substrates. The cathodic peak at around 0.2V corresponds to a monolayer of hydrogen on electrically connected Pt and can be used to calculate electro-active surface area. Using literature values for polycrystalline Pt surface atom density (1.3*1015 sites/cm2), Charge integration (9.6mC) yields a surface area of 46cm2. By comparison, the initially deposited electrode has a surface area of just 0.87 cm2. This detection of 53 times greater platinum surface area, compared to the initially sputtered electrode area, supports the primary hypothesis of the paper and indicates deposition of electrically connected Pt down the pore via a growth front. There is a factor of 6 loss in electrically connected area compared to geometrically estimated total AAO area, presumably due to surface migration during Pt/Cu exchange, aging or anodic portion of hydrolysis scan. A similar series of catalytic experiments were performed for the Ir monolayer plating as shown in Figure 3b.

The catalytic activity of the membranes was tested in 0.1M H2SO4 using

electrochemical water electrolysis with the same cyclic voltammetry parameters as the platinum monolayer. The membrane plated with an iridium monolayer showed a large increase in water splitting rate versus the control membrane (Au). After monolayer plating, the H2 (at -1 V) evolution current increases from 38 mA to 70 mA. During the positive sweep rate scan between 1.1-1.3V, the Ir monolayer film is converted to IrO2 and the O2 (at +2 V) evolution current rises from 13 mA to 35 mA. Again we see a large oxidation peak around 0 V due to absorbed


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hydrogen trapped in the membrane pores. The membrane demonstrates significant electrochemical robustness after 100 scans. We have for the first time succeeded to plate a precious metal monolayer onto an insulating support using the underpotential deposition of copper, followed by the redox replacement by precious metal. Importantly shown here is that the underpotential plating of Cu can occur on a material other than precious metal conductors, which have a subtle effect to change the free energy of the first monolayer with respect to bulk Cu. In this case, the Lewis acidic nature of Al and free oxygen/OH at the surface appear to stabilize the growth of initial monolayer with respect to the bulk plating of Cu in AAO pores. The observation of growth on opposite surface requires a novel copper monolayer growth front plating mechanism. Mesoporous membranes are an important catalytic platform since by geometry they have efficient mass transport across the surface of the catalyst, while minimizing the quantity of expensive catalyst to the ideal monolayer limit. Applications for these membranes include membrane fuel cells, hydrogen production through water splitting, flow battery energy storage, and electrocatalytic conversions.

Corresponding Author Professor Bruce Hinds. [email protected]. University of Washington, Department of Materials Science and Engineering. Author Contributions The manuscript was written through contributions of all authors. ‡These authors contributed equally.


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ACKNOWLEDGMENT N.L. and A.P. contributed equally to this work. Funding support by DOE EPSCoR (DE-FG0207ER46375) and NSF CBET (1460922) Bibliography 1. Schlapbach, L.; Züttel, A., Hydrogen-Storage Materials for Mobile Applications. NATURE 2001, 414, 353-358. 2. Züttel, A., Hydrogen Storage Methods. NATURWISSENSCHAFTEN 2004, 91, 157-172. 3. Porembsky, V. I.; Fateev, V. N.; Grigoriev, S.; Porembsky, V.; Fateev, V., Pure Hydrogen Production by PEM Electrolysis for Hydrogen Energy. INT J HYDROGEN ENERG 2006, 31, 171-175. 4. Mbemba, N.; Grigoriev, S. A.; Fateev, V. N.; Aukauloo, A.; Millet, P.; Etiévant, C., Electrochemical Performances of PEM Water Electrolysis Cells and Perspectives. INT J HYDROGEN ENERG 2011, 36, 4134-4142. 5. Fritz, D. L.; Merge, J.; Stolten, D.; Carmo, M.; Mergel, J., A Comprehensive Review on PEM Water Electrolysis. INT J HYDROGEN ENERG 2013, 38, 4901-4934. 6. Zhang, H.; Ma, X.; Shao, Z.; Baker, R. T.; Song, S.; Yi, B., Electrochemical Investigation of Electrocatalysts for the Oxygen Evolution Reaction in PEM Water Electrolyzers. INT J HYDROGEN ENERG 2008, 33, 4955-4961. 7. Srinivasan, S.; Antonucci, V., DMFCs: From Fundamental Aspects to Technology Development. FUEL CELLS 2001, 1, 133-161. 8. Song, C. J.; Zhang, L.; Zhang, J. J.; Wang, H. J.; Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P., A Review of Anode Catalysis in the Direct Methanol Fuel Cell. J POWER SOURCES 2006, 155, 95-110. 9. Basu, S.; Bhaduri, S.; Lahiri, G. K.; Maity, P., Superior Performance of a Nanostructured Platinum Catalyst in Water: Hydrogenations of Alkenes, Aldehydes and Nitroaromatics. ADV SYNTH CATAL 2007, 349, 1955-1962. 10. Locke, B. R.; Mededovic, S.; Locke, B., Platinum Catalysed Decomposition of Hydrogen Peroxide in Aqueous-Phase Pulsed Corona Electrical Discharge. APPL CATAL B-ENVIRON 2006, 67, 149-159. 11. Kwon, K.; You, D. J.; Pak, C.; Chang, H.; Joo, S. H.; Kim, J. M., Preparation of High Loading Pt Nanoparticles on Ordered Mesoporous Carbon with a Controlled Pt Size and Its Effects on Oxygen Reduction and Methanol Oxidation Reactions. ELECTROCHIM ACTA 2009, 54, 5746-5753. 12. Blakely, D. W.; Somorjai, G. A., Mechanism of Catalysis of Hydrocarbon Reactions by Platinum Surfaces. NATURE 1975, 258, 580-583. 13. Markovic, N. M.; Ross, P. N.; Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., H2 and CO Electrooxidation on Well-Characterized Pt, Ru, and Pt-Ru. 1. Rotating Disk Electrode Studies of the Pure Gases Including Temperature Effects. J PHYS CHEM-US 1995, 99, 82908301. 14. Qiu, H.; Hou, L., Enhanced Electrocatalytic Performance of Pt Monolayer on Nanoporous PdCu Alloy for Oxygen Reduction. J POWER SOURCES 2012, 216, 28-32. 15. Millet, P.; Fateev, V. N.; Grigoriev, S. A., Evaluation of Carbon-Supported Pt and Pd Nanoparticles for the Hydrogen Evolution Reaction in PEM Water Electrolysers. J POWER SOURCES 2008, 177, 281-285.


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16. Mojovic, Z.; Cvjeticanin, N.; Tesic, Z., Electrochemical Water Splitting on Zeolite Supported Platinum Clusters. J NEW MAT ELECTR SYS 2004, 7, 213-220. 17. Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Yee, D.; Sorensen, L. B., Electrochemical Deposition of Copper on a Gold Electrode in Sulfuric Acid: Resolution of the Interfacial Structure. PHYS REV LETT 1995, 75, 4472-4475. 18. Wang, J. X.; Adzic, R. R.; Brankovic, S. R.; Adžić, R. R., Metal Monolayer Deposition by Replacement of Metal Adlayers on Electrode Surfaces. SURF SCI 2001, 474, L173-L179. 19. Abruna, H. D.; Herrero, E.; Abruña, H. D., Underpotential Deposition of Mercury on Au(111): Electrochemical Studies and Comparison with Structural Investigations. LANGMUIR 1997, 13, 4446-4453. 20. Menon, V. P.; Martin, C. R.; Brumlik, C. J.; Menon, V. P.; Martin, C. R., Template Synthesis of Metal Microtubule Ensembles Utilizing Chemical, Electrochemical, and Vacuum Deposition Techniques. J MATER RES 1994, 9, 1174-1183. 21. Hu, Y.; Liu, X.; Deng, W.; Wang, X.; Yu, Y., The Study of Pt@Au Electrocatalyst Based on Cu Underpotential Deposition and Pt Redox Replacement. ELECTROCHIM ACTA 2009, 54, 3092-3097. 22. Tomita, E.; Kuwabara, T.; Yagi, M.; Tomita, E.; Kuwabara, T., Remarkably High Activity of Electrodeposited IrO2 Film for Electrocatalytic Water Oxidation. J ELECTROANAL CHEM 2005, 579, 83-88. 23. Wang, R. Y.; Wang, C.; Cai, W. B.; Ding, Y., Ultralow-Platinum-Loading HighPerformance Nanoporous Electrocatalysts with Nanoengineered Surface Structures. Advanced Materials 2010, 22, 1845-+. 24. Fagerson, I. S., Thermal Degradation of Carbohydrates; a Review. J AGR FOOD CHEM 1969, 17, 747-750. 25. Uosaki, K.; Qu, D., Formation of Continuous Platinum Layer on Top of an Organic Monolayer by Electrochemical Deposition Followed by Electroless Deposition. J ELECTROANAL CHEM 2011, 662, 80-86. 26. Zhan, X.; Hinds, B. J.; Su, X., Pt Monolayer Deposition onto Carbon Nanotube Mattes with High Electrochemical Activity. J MATER CHEM 2012, 22, 7979-7984. 27. Kanazawa, K. K.; Gordon, J. G.; Ashley, K.; Richer, J.; Borges, G. L.; Kanazawa, K.; Gordon, J. G.; Ashley, K.; Richer, J., An In-situ Electrochemical Quartz Crystal Microbalance Study of the Underpotential Deposition of Copper on Au(111) Electrodes. J ELECTROANAL CHEM 1994, 364, 281-284. 28. Hanson, K. J.; Green, M. P., Copper Adlayer Formation on Au(111) From Sulfuric Acid Electrolyte. J VAC SCI TECHNOL A 1992, 10, 3012-3018. 29. Podlovchenko, B. I.; Zhumaev, U. E.; Maksimov, Y. M., Galvanic Displacement of Copper Adatoms on Platinum in PtCl42- Solutions. Journal of Electroanalytical Chemistry 2011, 651, 30-37. 30. Gollas, B.; Elliott, J. M.; Bartlett, P. N., Electrodeposition and Properties of Nanostructured Platinum Films Studied by Quartz Crystal Impedance Measurements at 10 MHz. Electrochimica Acta 2000, 45, 3711-3724. Table of contents graphic


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Cu Growth Front


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Monolayer Growth Front of Precious Metals through Insulating

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