Enhancing Role of Nickel in the NickelâPalladium Bilayer for

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On the Enhancing Role of Nickel in Nickel-Palladium Bilayer for Electrocatalytic Oxidation of Ethanol in Alkaline Media Julie Anne Del Rosario, Joey Duran Ocon, Hongrae Jeon, Youngmi Yi, Jae Kwang Lee, and Jaeyoung Lee J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 Sep 2014 Downloaded from http://pubs.acs.org on September 8, 2014

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The Journal of Physical Chemistry

On the Enhancing Role of Nickel in Nickel-Palladium Bilayer for Electrocatalytic Oxidation of Ethanol in Alkaline Media Julie Anne D. del Rosario,†,§ Joey D. Ocon,†,§ Hongrae Jeon,† Youngmi Yi, ‡ Jae Kwang Lee,‡ and Jaeyoung Lee*,†,‡ †

Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Technology, ‡Ertl Center for Electrochemistry and Catalysis, RISE, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea § Laboratory of Electrochemical Engineering (LEE), Department of Chemical Engineering, University of the Philippines Diliman, Quezon City 1101, Philippines ABSTRACT Direct ethanol fuel cells (DEFCs) have been widely studied because of their potential as highenergy density and low-toxicity power source of the future. Suitable catalysts for the anode reaction, however, are necessary to fully utilize the advantages of DEFCs. In this paper, we fabricated nickel (Ni)-palladium (Pd) bimetallic catalysts with a bilayer structure, using sputtering deposition on a titanium (Ti) foil substrate, and investigated the activity and stability of the catalysts towards ethanol electro-oxidation in alkaline media. Our results suggest that while Pd is the active component and Ni has negligible activity towards ethanol oxidation, Nimodified Pd (NiPd/Ti) provides the best activity in comparison to PdNi/Ti and the monometallic catalysts. In fact, optimizing the Ni amount could lead to a highly active and stable bimetallic electrocatalyst because of Ni’s ability to increase the active surface area of the Pd layer, provide hydroxyl species to replenish the active sites, and act as a protective layer to the Pd. Overall, these results provide a better understanding on the role of Ni in bimetallic catalysts, especially in a bilayer configuration, to allow the use of an ethanol oxidation reaction (EOR)-active electrocatalyst with a much lower Pd content. KEYWORDS: bilayer catalyst, sputtering deposition, direct ethanol fuel cells, synergistic effect

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1. Introduction Biomass energy has increasingly become a vital part in the global renewable energy mix. In particular, biofuels – fuels generated from biomass– have increased their popularity due to rising oil prices and the need for energy security. Bioethanol is a readily available and clean fuel from plant-based feedstocks, leading to lower combustion emissions than gasoline and diesel as it only releases the same amount of carbon dioxide as plants take during growth. While majority of the worldwide production of bioethanol are currently used as gasoline additive, direct ethanol fuel cells (DEFCs) are receiving widespread attention because of their high thermodynamic efficiency, high volumetric energy density, and low toxicity.1-7 The economic and environmental benefits of DEFCs, however, can only be fully exploited once kinetic constraints in the ethanol oxidation reaction (EOR) are solved. For decades, researchers have focused on finding the right catalyst that could easily break the strong carbon-carbon bond in ethanol and could effectively drive the 12-electron transfer needed to complete the oxidation. In acidic media, platinum (Pt)-based catalysts provide the highest activity, and fuel efficiency for EOR.1,2 Pt, however, is easily deactivated due to poisoning from carbon monoxide-like intermediates.3 Studies on EOR kinetics in alkaline media have intensified lately because of the rekindled interest in alkaline fuel cells (AFC). The focus on AFCs these days is explained by the faster EOR kinetics in alkaline than in acidic media, in addition to the possible use of non-Pt catalysts, e.g. palladium (Pd).2,4,5 In alkaline media, Pd is more active and much less prone to poisoning as compared to Pt because of Pd’s oxyphilic nature.4-7 Metal oxides such as CeO2, NiO, Co3O4, and Mn3O4 are found to significantly enhance the activity of Pd/C during electro-oxidation.8 In fact, Pd-NiO has been reported to have the best activity in EOR.8 Many results have already proven the co-


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catalytic effect of Ni and its species in the promotion of EOR on Pd. Alloys of Pd-Ni also have better activity than pure Pd, with a maximum current density achieved by adding 10 wt. % Ni.9 Beyond this concentration, Ni shows an adverse effect to the activity due to surface passivation by NiO species. In the case of Pd-Ni/C, Ni has been shown to enhance the catalytic activity by adsorbing additional hydroxyl (OH-) species that refreshes Pd active sites.10 In addition, Ni assists Pd by oxidizing CO-like adsorbed species on the surface, leading to higher catalytic activity and stronger tolerance from catalyst poisoning.11 The direct role of Ni in EOR, however, is still vague because of the negligible activity of pure Ni.12,13 In the search for new catalysts, bilayer structures are valuable model systems for studying the relation between the structure and functional properties of bimetallic surfaces. Interestingly, the order of metal deposition is found to be significant in predicting the structure/reactivity properties of the bilayers.14,15 As an example, the CO-adsorption and oxidation properties of Ptcovered Ru(0001) single crystal electrodes vary from that of Pt(111) decorated with Ru nanoparticles.14 While most of the studies started with Pt (hkl) surfaces that were modified with other metals, subsequent investigations were made with the more expensive metal deposited on the less expensive metal (e.g. spontaneous deposition of Pt on Ru surfaces, which considerably lowers the Pt loading).16 The unique design of the latter, termed as inverted spontaneous deposition, provides a bimetallic surface with similar properties as pure Pt but with slightly different electronic and geometric structure. In this study, we assembled Ni-Pd bilayers and evaluated their activities for ethanol oxidation in alkaline media. The first type involves the deposition of Ni on Pd surface, while the other one incorporates Pd on the Ni surface. The performances in EOR of the fabricated catalysts were examined and compared with those of the mono-metals of Pd and Ni to find the best


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configuration of the Pd-Ni bilayer. Finally, after selecting the better bilayer configuration, we optimized the thickness of Ni to produce the best activity of Ni-Pd bilayer in EOR.

2. Experimental Method Pd and Ni were deposited on Ti foil (0.127 mm thick, 99.7% metals basis, Aldrich) by direct current (DC) sputtering deposition in a plasma sputter coater (EMITECH SC7640). The coater chamber was filled with argon gas and maintained at room temperature with a bleeding gas pressure below 0.5 bar. Ion energy was fixed at 1.0 keV, which provided plasma current of ca. 20 mA during Pd deposition and ca. 30 mA during Ni deposition. Sputtering rates were estimated to be 3.2 nm min-1 and 1 nm min-1 for Pd and for Ni, respectively, using cross-sectional images in scanning electron microscopy (SEM, JEOL JSM 5200). To compare the activities of the monometals and the bilayers with different configuration, the sputtering times were set at 5 min for Pd and 10 min for Ni. For the optimization of Ni content, however, the sputtering time of Pd was fixed at 5 min while Ni deposition time was varied. All samples were named with the top layer as first metal, bottom layer as second metal, and the substrate as the last. The bottom layer refers to the metal deposited first, and the top layer refers to the metal used to modify the surface and deposited on top of the first metal. For example, NiPd/Ti denotes that Pd is first deposited on Ti foil, and then Ni is finally deposited on top of Pd. Catalytic activities were then assessed by chronoamperometry (CA) and cyclic voltammetry (CV) in 1.0 M potassium hydroxide (KOH) solutions, with and without the presence of 1.0 M ethanol (C2H5OH). Electrolyte solutions were prepared from ACS reagent grade KOH pellets (Sigma-Aldrich), ethanol (Sigma-Aldrich) and ultra-pure water (Millipore Milli-Q water, 18.2 MΩ cm-1). Electrochemical tests were performed with continuous nitrogen


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bubbling, hence, the electrolyte solutions can be assumed to be completely free of oxygen. For all voltammetry experiments, Pt wire and Hg/HgO (1.0 M NaOH) were used as counter and reference electrodes, respectively. The bilayer catalysts, which were used as working electrodes, were mounted on a Teflon cup electrode holder. Current densities were then calculated from the measured current and geometrical surface area of the samples, which was fixed for all samples (1 cm2). Moreover, the surface chemical composition was studied using X-ray photoelectron spectroscopy (XPS, VG Multilab2000) with an Mg Kα X-ray source (1253.6 eV) at a base pressure of 2 x 10-9 Torr. Pd/Ti, NiPd/Ti and PdNi/Ti samples were first subjected to three potential sweeps between -0.8 V to 0.3 V in 1.0 M KOH and were dried before undergoing XPS analysis.

3. Results and Discussion Ni-Pd bilayer structures were fabricated on Ti foil using sputtering deposition. As mentioned in the experimental section, the sputtering rates were estimated from the SEM images of the sputtered metals on flat silicon wafer (see Figure S1). The catalytic performances of NiPd/Ti and PdNi/Ti were compared with those of the mono-metals on Ti. Figure 1 shows the cyclic voltammograms of the Ni-Pd bilayer electrodes in 1.0 M KOH at the potential range between -0.9 V and 0.4 V, with a scan rate of 50 mV s-1. For comparison, the cyclic voltammograms of Ti foil, Ni/Ti, and Pd/Ti taken in the same conditions are also presented. The hydrogenadsorption/desorption regions of Pd/Ti and NiPd/Ti clearly appear between -0.9 V and -0.5 V. The pairs of palladium oxide formation and reduction peaks are also apparent starting at -0.2 V in the positive-going sweep and, at -0.12 V in the negative-going sweep, respectively. On the other hand, PdNi/Ti gives a distinct curve with a suppressed H-adsorption/desorption and Pd-O


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reduction peaks, while Ti foil and Ni/Ti do not have redox peaks in these potential ranges. The area under the current-voltage curve of the Pd-O reduction peak in the CV when divided by the scan rate indicates the charge value for the reduction of the Pd-O on the electrodes, providing information on the electrochemical active surface area (ECSA) of the layered catalyst. Figure 2 shows the typical cyclic voltammograms of the electro-oxidation of ethanol in basic conditions with two obvious peaks, one on the anodic scan (positive-going sweep) and the other on the cathodic scan (negative-going sweep).9, 10, 17 For EOR on the Pd/Ti electrode (Figure 2 (b)), the anodic current starts to increase at about -0.52 V, due to oxidation of chemisorbed decomposition products of the adsorbed ethanol molecule, and peaks at about -0.19 V.9,17 At higher potentials, oxidation of surface Pd occurs, which blocks the adsorption of reactive species and causes a decrease in current. On the reverse scan, the oxidized Pd surface is reduced recovering the lost oxidation current. The cathodic peak, however, is sometimes associated with the carbonaceous species that are not completely oxidized during the anodic scan.10 The current starts to decrease when the potential becomes too negative. To investigate the electrochemical activity and stability of the catalysts, we performed chronoamperometry (CA) using Pd/Ti and the bilayers. The experiments were carried out at -0.2 V for 600 seconds in 1.0 M KOH + 1.0 M C2H5OH solution. In Figure 3, the current variation in EOR is plotted against time. For all three catalysts, the current densities dropped rapidly at first and then became relatively stable. The current decay is possibly due to poisoning of the intermediate species on the active surface of the catalyst.6 Compared to NiPd/Ti, Pd/Ti shows faster activity decrease. After 10 min, the activity of NiPd/Ti remains three times higher in comparison to that of Pd/Ti. It is believed that the high stability of the NiPd/Ti catalyst was due


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to the upper layer of Ni that acted as a protective layer, shielding Pd from poisoning. Further proof for this hypothesis is explained in the XPS analysis below. The electrocatalytic characteristics of EOR presented in Figure 2 and 3 show a strong correlation between ECSA and the activity. NiPd/Ti, which showed the highest ECSA, has also the highest activity among the electrocatalysts. The higher current density of NiPd/Ti in comparison to Pd/Ti directly indicates that Ni on the surface of Pd enhances the catalytic activity of Pd. On the other hand, PdNi/Ti showed the lowest activity in comparison with Pd/Ti and NiPd/Ti even though the Pd surface is highly exposed. These results, however, are counterintuitive since Pd is obviously the active material and Ni has negligible activity, as seen in Figures 1b and 1c. To prove that the Ti foil substrate does not affect the results above, similar activities were also observed when using gold (Au) quartz crystal as substrate for the bimetallic and monometallic catalysts (see Figure S2 and Figure S3). NiPd/Au and Pd/Au showed very high activity as compared to the PdNi/Au and the mono-metals. While Au itself shows catalytic activity towards EOR at higher potentials, Ti is inactive in EOR and thus, succeeding investigations were performed using various layered Ni-Pd catalysts deposited on Ti substrate. To explain the seemingly counter-intuitive electrocatalytic activity of the Ni-Pd bilayers, we investigated the surface chemical states of Ni and Pd on the electrocatalysts by XPS. Considering the thickness of the Pd and Ni layers in relation to the sampling depth in XPS, we expected that the metal deposited on the topmost layer would be the dominant species on the surface of the samples. The X-ray photoelectron spectra of Pd/Ti, PdNi/Ti, and NiPd/Ti in the Pd 3d and Ni 2p3/2 regions are shown in Figure 4. The XPS analysis was taken after voltammetric cycling in 1.0 M KOH. As presented in Table 1, the atomic percentages of the elements prove


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that samples were prepared as desired. Pd and O are the dominant species on the surface of Pd/Ti and PdNi/Ti, while Ni and O are the principal elements on the surface of NiPd/Ti. Presence of XPS peaks from the bottom metal layer denotes that during cyclic voltammetry, the metal underneath the top layer is exposed to the electrolyte and also serves as active surface area for the electro-oxidation of ethanol. The similarity in shape of the voltammograms of NiPd/Ti and Pd/Ti provides further proof that indeed the bottom layer surface is accessible to the electrolyte. Looking at the binding energies, it is evident that Ni on the surface exists in its oxide forms. Ni 2p3/2 peaks correspond mainly to the oxide, hydroxide, and oxyhydroxide species, which are in good agreement with previous studies, although there is a possibility that Ni2O3 (Ni3+, 855.8 eV) also exists in this region.18,19 Conversely, the Pd 3d3/2 peaks are characteristic peaks of metallic Pd (Pd, 335.1 eV) and PdO (Pd2+, 337.0 eV).20, 21 It is good to note also that the relative amounts of metallic Pd and PdO in PdNi/Ti and NiPd/Ti vary, possibly resulting from the different propensities of Pd and Ni to be oxidized in the KOH solution during the voltammetric cycling before the XPS analysis. In PdNi/Ti, there is an enrichment of PdO, whereas in NiPd/Ti, metallic Pd is more dominant. This also serves as further proof that Ni acts as a protective layer for the EOR-active metallic Pd in the NiPd/Ti, in contrast to the PdNi/Ti having more oxide than metallic Pd. Based on the results of electrochemical and XPS analyses, we hypothesized a possible explanation for the contrasting catalytic activities of PdNi/Ti and NiPd/Ti. In PdNi/Ti, while the EOR-active Pd layer is on the upper layer, the surface of the second layer is covered primarily with the Ni oxides thereby increasing the electrical resistance and decreasing the catalytic activity significantly. On the other hand, when Ni is the top layer, the electrolyte can still access the EOR active sites on Pd via the crevices in the Ni layer, as seen in the XPS spectra where Pd


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3d peaks were observed. Moreover, the surface of the EOR-active Pd layer is in contact with Ni in the interface, creating positive synergy between the two metals, in agreement with numerous literatures on alloys of Pd and Ni for ethanol oxidation.9, 10, 22 Upon confirming that NiPd/Ti gives the better bilayer configuration, the amount of Ni in the upper layer was optimized by changing the sputtering time (0–20 min) while the Pd thickness was fixed. In the presence of ethanol (Figure 5a), the activity is highest when Ni is deposited for 10 min. Increasing the deposition time of Ni up to 10 min linearly increased the activity of the catalyst in EOR as more Ni on the surface provides plenty of Pd-Ni contact surfaces, resulting in enhanced electrocatalytic oxidation activity. Beyond the optimal amount of Ni, however, excess Ni on the surface fully covers the cracks and pores. Therefore, the electrolyte cannot reach the EOR-active Pd surfaces anymore and catalytic activity goes down. The effect of Ni is clearly observed when the peak current density from EOR and the coulombic charge, calculated from the Pd oxide reduction peaks of the CV in 1.0 M KOH, are compared (Figure 5b). Both the peak current density and the coulombic charge increased when Ni was added on top of Pd. Since the coulombic charge provides information on the ECSA, the increase in the coulombic charge suggests that Ni roughens the surface of Pd, or that the synergistic effect between Pd and Ni modifies Pd’s electronic state to become a more active electrocatalyst. The reasons for the highly improved activity of NiPd/Ti that were discussed above are easily understood by looking at the bilayer structure in Scheme 1. Pd + CH CH OH ↔ Pd − CH CH OH 1 Pd − CH CH OH + 3OH → Pd − CH CO + 3H O + 3e 2 Pd − CH CO + Pd − OH → Pd − CH COOH + Pd 3 Pd − CH COOH + OH → Pd + CH COO + H O 4


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Written above is the accepted mechanism for EOR on Pd, with the third step as the ratedetermining step for this reaction.10 In the anodic scan, ethanol and OH- adsorption starts at lower potentials. Although there are still doubts with regard to this mechanism, it has been widely accepted that OH− ions are first chemisorbed at the onset of oxide formation, and are then transformed into higher valence oxides at higher potentials. Moreover, the Ni(OH)2/Ni reduction occurs within this potential range (-0.76 V to -0.66 V vs. Hg/HgO).13 In this work, we believe that aside from increasing the roughness of the Pd layer, Ni also helps in ethanol oxidation by supplying hydroxyl species at the low potential range, as evidenced by the increase in current at the OH- adsorption region as the Ni amount increases (see Figure S4). The adsorbed hydroxides, in turn, replenish the Pd active sites by stripping reaction intermediates, thereby accelerating further the EOR kinetics.

4. Conclusion In this paper, we present two types of Ni-Pd bilayer according to the order of metal deposition and summarize their electrochemical activity in the oxidation of ethanol in alkaline condition. Knowing that Pd is active in EOR while Ni has negligible activity, the bilayer configuration significantly affects the reactivity towards EOR. Interestingly, NiPd/Ti provided the best catalytic activity among various configurations, having higher current density and stability than both Pd/Ti and PdNi/Ti. As pointed in Scheme 1, the deposition of Ni on Pd layer, which contains both metallic Pd and Pd oxide, produces a thin Ni oxide layer with pores and cracks that allow electrolyte access to the Pd surface. The presence of Ni considerably improved the EOR kinetics, both in terms of activity and stability, for three main reasons: 1) during deposition, Ni roughens the Pd surface to further increase the Pd active surface area, 2) in EOR, the Ni layer


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acts as a protective layer of the EOR-active Pd layer against poisoning intermediates, and 3) in the Ni-Pd contact surfaces, Ni provides Pd surface the essential hydroxyl species that can replenish the active sites. These results provide better understanding on the role of Ni in bimetallic catalysts, especially in a bilayer configuration, and hopefully lead towards highly active and stable EOR catalysts with much lower noble metal content.

Supporting Information: SEM images of Pd and Ni on silicon wafer, CVs of the bimetallic and monometallic catalysts in Au quartz crystal substrate in 1.0 M KOH, CVs of the bimetallic and monometallic catalysts in Au quartz crystal substrate in 1.0 M KOH and 1.0 M C2H5OH, CVs of NiPd/Ti samples at different deposition times in 1.0 M KOH. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *Tel: +82-62-715-2440. Fax: +82-62-715-2434. E-mail: [email protected] Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2011-013-D00030).


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Figure 1. Cyclic voltammograms of a) Ti foil, b) Pd/Ti, c) Ni/Ti, d) PdNi/Ti, and e) NiPd/Ti electrodes in N2-purged 1.0 M KOH using a scan rate of 50 mV s-1. The current density was calculated based on the geometric surface area of the substrate. The schematic configuration of each catalyst is shown beside the voltammograms.


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Figure 2. Cyclic voltammograms of a) NiPd/Ti, b) Pd/Ti, and c) PdNi/Ti electrodes in N2purged 1.0 M KOH + 1.0 M C2H5OH using a scan rate of 10 mV s-1. The current density was calculated based on the geometric surface area of the substrate.


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Figure 3. Chronoamperometry (CA) in 1.0M KOH + 1.0 M C2H5OH using a) NiPd/Ti, b) Pd/Ti, and c) PdNi/Ti electrodes at an applied voltage of -0.2 V for 10 min. The current density was calculated based on the geometric surface area of the substrate.


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Figure 4. (a) Ni 2p3/2 and (b) Pd 3d XPS spectra of i) Pd/Ti, ii) PdNi/Ti, and iii) NiPd/Ti after three potential cycles between -0.8 V to 0.3 V in 1.0 M KOH.


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Figure 5. (a) Cyclic voltammograms of NiPd/Ti samples with different Ni deposition time (rate: 1 nm min-1) in 1.0M KOH + 1.0 M C2H5OH (scan rate: 10 mV s-1) and (b) the anodic peak current density versus Ni deposition time, with the corresponding coulombic charge calculated from the Pd oxide reduction peak. The current density was calculated based on the geometric surface area of the substrate.


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Table 1. Atomic percentages of Pd, Ni, and O on Ni-Pd/Ti from XPS analysis Sample

Pd 3d

Ni 2p

O 1s

Pd/Ti

23.31

-

35.23

NiPd/Ti

1.03

14.32

35.74

PdNi/Ti

11.31

1.37

39.71


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Scheme 1. A schematic illustration of the NiPd/Ti electrocatalyst. The Pd surface is exposed to the electrolyte through the crevices on the thin Ni layer. Along with the increased surface area of Pd accesible to the electrolyte, the high number of interfacial boundaries between Ni and Pd considerably enhances the activity of the NiPd/Ti electrocatalysts for EOR.


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On the Enhancing Role of Nickel in Nickel-Palladium Bilayer for Electrocatalytic Oxidation of Ethanol in Alkaline Media Julie Anne D. del Rosario,†,§ Joey D. Ocon,†,§ Hongrae Jeon,† Youngmi Yi, ‡ Jae Kwang Lee,‡ and Jaeyoung Lee*,†,‡ †

Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Technology, ‡Ertl Center for Electrochemistry and Catalysis, RISE, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea § Laboratory of Electrochemical Engineering (LEE), Department of Chemical Engineering, University of the Philippines Diliman, Quezon City 1101, Philippines

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Enhancing Role of Nickel in the NickelâPalladium Bilayer for

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