Formation of conformal NiO underlayer on carbon for strong metal-support interactions effects on electrocatalytic performance of supported Pd nanoparticles

Journal Pre-proofs Perspective Article Formation of Conformal NiO Underlayer on Carbon for Strong Metal-support Interactions Effects on Electrocatalytic Performance of Supported Pd Nanoparticles Wenjuan Shi, Ah-Hyeon Park, Byeong Jun Cha, Hyun-Uk Park, Young-Dok Kim, Young-Uk Kwon PII: DOI: Reference:

S0169-4332(19)33171-X https://doi.org/10.1016/j.apsusc.2019.144355 APSUSC 144355

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

5 September 2019 9 October 2019 9 October 2019

Please cite this article as: W. Shi, A-H. Park, B.J. Cha, H-U. Park, Y-D. Kim, Y-U. Kwon, Formation of Conformal NiO Underlayer on Carbon for Strong Metal-support Interactions Effects on Electrocatalytic Performance of Supported Pd Nanoparticles, Applied Surface Science (2019), doi: https://doi.org/10.1016/ j.apsusc.2019.144355

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Formation of Conformal NiO Underlayer on Carbon for Strong Metal-support Interactions Effects on Electrocatalytic Performance of Supported Pd Nanoparticles

Wenjuan Shia, Ah-Hyeon Parka, Byeong Jun Chaa, Hyun-Uk Parka, Young-Dok Kima and Young-Uk Kwonab*

a

b

Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea

School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China

Graphical abstract:

1

Highlights:

1. The effects of NiO underlayer on carbon support to the electrocatalysis by Pd nanoparticles are explored. 2. The sonochemical method could produce conformal NiO thin coating on the entire surface of carbon. 3. The electrocatalytic performance of Pd on NiO/C support is 3-5 times enhanced from that of Pd on carbon support for formic acid oxidation and oxygen reduction reaction. 4. The enhancement can be explained by the electronic interaction between Pd and NiO and the bifunctional mechanism enabled by NiO surface.

*

Corresponding author. Tel.: +82 31 290 7070; fax: +82 31 290 7075

E-mail address: [email protected]

2

Abstract Enhancing the performance of noble metal electrocatalysts is regarded as an urgent issue for the commercialization of fuel cells in the near future. In this study, we demonstrate that the electrocatalytic performance of Pd nanoparticles (NPs) supported on carbon can be enhanced by introducing NiO coating on carbon. We further demonstrate that a uniform thin layer of NiO coating on carbon support can be readily achieved by using a sonochemical reaction (ultrasound-assisted polyol synthesis, UPS) method. NiO-modified carbon (NiO/C) supports were synthesized by two different methods, atomic layer deposition (ALD) and UPS methods, and Pd NPs were formed on them. Compared with Pd NPs on carbon support, Pd NPs on NiO/C supports showed 3-5 fold enhanced electrocatalytic performance for formic acid oxidation and oxygen reduction reactions. Between the two NiO/C supports, the one synthesized by UPS method outperforms the one by ALD by up to 2 times. Detailed structural analysis data show that the NiO/C synthesized by UPS is composed of uniform and continuous thin (0.42 nm) NiO coating on the external surface of carbon while the NiO/C by ALD has thicker (1.13 nm) NiO islands to form patch-like coating on the carbon surface. The enhanced electrocatalytic performance of Pd NPs on NiO/C supports can be explained by the change of the electronic structure of strong metal-support interaction between Pd and NiO and the bifunctional mechanism enabled by the NiO surface around Pd NPs. 3

Keywords: Strong Metal-support Interaction, Electrocatalysis, Palladium, Nickel Oxide Underlayer, Fuel Cells

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1. Introduction

Fuel cells, especially those operate at relatively low temperatures such as polymer-electrolyte membrane fuel cell, require noble metals such as Pt and Pd as electrocatalysts [1-3]. For fuel cells to be economically competitive, the material efficiency of noble metal electrocatalysts needs to be improved considerably. Toward this end, a great deal of efforts has been made, mostly focusing on how to design noble metal nanoparticles (NPs) such as controlling their shape and size, and compositions when a second element is to be introduced to form alloy or core-shell NPs [1, 4-7]. When such function-optimized NPs are to be implemented in the electrodes, they are typically supported on carbon to connect the NPs electronically and keep the NPs well dispersed. The use of carbon support in electrocatalysis requires additional factors such as inter-NP separation and the interaction between NPs and the carbon support to be taken into consideration [8]. At the same time, the NP-support interaction provides an additional dimension to explore for enhancing the performance of noble metal electrocatalysts. Strong metal-support interaction (SMSI) effect has been well-known for decades and has been widely used in heterogeneous catalysis [9-13]. According to the related literature, metal NPs supported on metal oxides such as TiO2, CeO2 and SnO2 often show significantly high catalytic activity when compared with unsupported metal NPs, 5

which is explained as due to the electronic interaction between the metal NPs and the metal oxide support altering the electronic structure of the metal NPs [14, 15]. This system has been widely investigated, there are some review papers on related subject [9, 10]. On the contrary, unlike the advances in utilizing SMSI effects in heterogeneous catalysis, utilization of SMSI in electrocatalysis is limited, probably because the supports in electrocatalysis need to be conductive which requirement is not readily satisfied with most of metal oxides. Indeed, most of the cases in which SMSI is utilized in electrocatalysis have used conductive metal oxides. For instance, He et al. reported high electrocatalytic activity of Fe2O3/Pd in oxygen evolution reaction (OER) [16]. Zheng et al. synthesized Ti0.7W0.3O2 into rutile structure and used it as a support for Pt NPs to synthesize highly active electrocatalyst for oxygen reduction reaction (ORR) [17]. Luo et al. also used SnO2-SiO2 composite as a support for Pt NPs for ORR. An alternative approach to utilize SMSI in electrocatalysis is to use a mixture between non-conductive metal oxide and carbon as a support [18]. Saleem et al. showed that CeO2-CNT composite, synthesized by impregnation cerium nitrate onto CNT, could be used as a good support for Pd NPs to be used for formic acid fuel cells [19]. A mixture between TiO2 and CNT, synthesized by depositing Pt on TiO2/CNT by metal-organic chemical vapor deposition, was shown to be a good support for Pt NPs for ORR [20]. Pt-CeO2/C electrocatalyst, composed of Pt NPs on CeO2/C composite support, with enhanced methanol oxidation reaction from that of Pt/C also is a representative example of this approach [21].

6

Of these two approaches to utilize SMSI in electrocatalysis, the one of compositing metal oxides with carbon is more appealing than the one using conductive metal oxide in that it is more diverse in the choice of metal oxide and more suitable for large scale production. However, as far as the examples in the literature are concerned, this approach appears to be limited by the method to synthesize such composites. Ideally, metal oxides in such composite support are required to form a uniform thin coating on carbon. Because the carbon materials are porous and have complicated textures, depositing thin layer of metal oxide on the surface of carbon is not an easy task at all. In the present work, we synthesized NiO-modified carbon supports (NiO/C) by using two different methods, atomic-layer deposition (ALD) and ultrasound-assisted polyol synthesis (UPS) method, and formed Pd NPs on them to obtain Pd/NiO/C catalysts. Both Pd/NiO/C catalysts show significantly enhanced electrocatalytic properties from that of Pd NPs on carbon (Pd/C) as the anode for formic acid oxidation reaction (FAOR) and the cathode for ORR, demonstrating that NiO underlayer can induce SMSI effect. In addition, Pd/NiO/C catalyst obtained through the UPS method showed higher electrocatalytic performance than the other. Analysis data on the NiO/C supports and Pd/NiO/C catalysts show that the structure and distribution of NiO on carbon are greatly different between the two methods, by which the different electrocatalytic performance between the two Pd/NiO/C catalysts can be explained.

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2. Experimental section

2.1. Chemicals

Palladium(II) acetylacetonate (Pd(acac)2 99%), nickel(II) acetylacetonate (Ni(acac)2, 95%), bis(cyclopentadienyl) nickel (Ni(Cp)2), perchloric acid (70%), potassium hydroxide, palladium black (Pd-black, 99.95%, 40-60 m2 g-1), and a Nafion solution (5 wt%) were purchased from Aldrich. Ketjen Black (carbon support) was donated from Samsung Advanced Institute of Technology. Carbon nanotubes (CNT, multi-walled, >98% carbon basis, O.D. × L 6-13 nm × 2.5-20 μm) were purchased from Aldrich. Ethylene glycol (99.9%) and ethanol (99.9%) were purchased from Samchun Pure Chemical Co. Nitrogen gas (96.7%) and O2 gas (1:1) were purchased from the Deokyang Co. Filter papers were purchased from Whatman. Ultrapure water (Genesys, 18 MΩ cm) was used throughout the work.

2.2 Synthesis

NiO-modified carbon supports NiO-modified carbon supports (NiO/C) were synthesized by depositing NiO on a carbon (Ketjen Black) by ALD and UPS methods. In case of using ALD, carbon was 8

placed in a reactor and Ni(Cp)2 (metal precursor) and O2 gas (oxidizing agent) were alternately reacted with it at 150oC. Each ALD cycle consists of 200 sec of Ni(Cp)2 pulse, 30 sec of Ni(Cp)2 exposure at 300 mtorr, two repetitive N2 purging and evacuation processes, 10 sec of O2 pulse, 30 sec of O2 exposure at 1.8 torr, and additional two repetitive N2 purging and evacuation processes. During the cycle, the temperature of the Ni(Cp)2 container was maintained at 60 oC and for every N2 purging and evacuation step, N2 of pressure 10 torr was purged for 30 sec and then the reactor was evacuated for 60 sec to have under 10 mtorr of base pressure. This cycle was repeated for 50 times. The NiO/C substrate so-obtained will be denoted as NiO/C_A. The procedure to synthesize NiO/C by UPS method was as follows: 27 mg of Ni(acac)2 and 40 mg of carbon support were places in a three-necked flask containing ethylene glycol (30 ml). Pure Ar was bubbled through the solution for 15 min to remove oxygen before the addition of the reagents. A 500 W ultrasound (Sonic and Materials, VC-500, 20 kHz with a 13 mm solid probe) with 30% amplitude was applied for 3 h under an Ar-environment at room temperature. The obtained blackish slurry was filtered, washed with ethanol, and then dried under a vacuum for 12 h at room temperature. The support obtained by UPS method will be denoted as NiO/C_U.

Synthesis of supported Pd nanoparticle catalysts Pd NPs on NiO/C supports were formed following the procedure used to synthesize NiO/C_U described above except that the reagents were 30.7 mg of Pd(acac)2 and 9

0.03 g NiO/C support (NiO/C_A or NiO/C_U), instead of Ni(acac)2 and carbon. The catalysts so-obtained will be named as Pd/NiO/C_A and Pd/NiO/C_U to indicate the type of NiO/C support used. In addition, Pd/C catalyst in which carbon instead of NiO/C was used as the support was synthesized in the same procedure as a reference. The compositions of the catalysts analyzed by ICP-AES are shown in Table S1.

2.3 Characterization

X-ray diffraction (XRD) patterns were recorded with a powder X-ray diffractometer (DC/Max 2000, Rigaku, Cu Kα, (λ = 1.54056 Å)), in the two-theta range of 10 ~ 90 degree, at a scan speed of 5 degree min-1. For transmission electron microscopic (TEM) studies, a small amount of a catalyst was well dispersed by sonication in ethanol, and then several drops of the solution were deposited onto carbon-coated gold grids, followed by drying under ambient conditions. High-resolution TEM (HRTEM) images were obtained on JE-3011 (JEOL) operating at 300 kV for the information on the size and size distribution of NPs. Energy dispersive X-ray spectroscopy (EDS) line profiling were recorded on a scanning TEM (JEOL, JEM-2100F) for the elemental distribution. In order to obtain overall compositions of catalysts, 0.1 g of each catalyst was dissolved in aqua regia at ~120 °C for 3 h and then the resultant solution was diluted with distilled water. The carbon support was separated from the solution by a syringe-filter. The analyses were conducted by 10

inductively coupled plasma-atomic emission spectroscopy (ICP-AES, OPTIMA 4300DV Perkin Elmer). The reported compositions are averaged values of three independent measurements on each catalyst. Fourier transform infrared spectroscopy (FTIR) was performed using a Bruker IFS-66/S. Raman spectra were measured at room temperature using a confocal Raman spectroscopy at 532 nm (WITEC alpha300). X-ray photoelectron spectroscopy (XPS) was conducted with a monochromatic Al Kα source (1486.6 eV, ESCA 2000, VG Microtech.) for the survey spectra. For high-resolution XPS (HR-XPS) to observe Ni species, Mg Kα source (1253.6 eV, CHA, PHOIBOS-HAS 3500, SPECS) was used with the pass energy 30 eV/scan and the scan number of 30 at the energy step of 0.2 eV.

2.4 Fabrication of electrodes and electrocatalytic analysis

A homogeneous dispersion of 5 mg of a catalyst in 2.5 g of distilled water was prepared with an ultrasound. 5 μL aliquots of the solution were dropped separately onto the surfaces of glassy carbon rotating disc electrodes (RDE, diameter: 3 mm). After the electrodes had been air-dried at ambient temperature, a Nafion solution (0.05 wt%, 5 μL) was placed onto each of the electrodes. Three-electrode electrochemical cells were assembled using a RDE coated with catalyst as the working electrode, a Pt net as the counter electrode, and an Ag/AgCl electrode as the reference electrode, and the electrocatalytic activity was by using a potentiostat 11

(Ivium Compactstat, Ivium technology). Cyclic voltammograms (CVs) were measured in a 0.1 M HClO4 aqueous solution at a scan rate of 50 mV s–1 under a N2 environment in the potential range of -0.25 – 1.0 V. Electrocatalytic activity for FAOR was investigated by running CVs in a 0.1 M HClO4 containing 0.5 M formic acid at a scan rate of 50 mV s–1 in the potential range of -0.25 – 1.0 V. ORR activity was measured by running linear sweep voltammetry (LSV) in an oxygen-saturated 0.1 M KOH solution with the electrode rotated at 1600 rpm. For CO-stripping measurements, a monolayer coating of CO on the electrode was achieved by bubbling CO into the electrolyte for 25 min. After purging the solution with N2 for 10 min to remove free CO, CV was performed in the potential range of 0 – 1.0 V. All tests were conducted at room temperature.

3. Results and discussion

In this work, we synthesized two types of NiO/C support, one by using ALD and the other UPS methods, and formed Pd NPs on them to see the effects of NiO underlayers on the electrocatalytic performance of the Pd NPs. We will discuss on the NiO/C supports first and proceed to Pd/NiO/C electrocatalysts. In order to characterize the Ni-species in NiO/C supports, we analyzed them by a number of different analysis methods. The amounts of NiO deposited were estimated 12

by the residual masses after decomposition in air up to 800C on a TGA, which are 23 wt % in NiO/C_A and 11 wt % in NiO/C_U (Fig. S1). A direct proof for the presence of NiO in them was obtained from their Raman spectra. As shown in Fig. 1, the Raman spectra on NiO/C supports show peaks at 560 and 1062 cm-1 that can be assigned to the first and second order phonon scattering modes of NiO, respectively, along with the D (at 1340 cm-1) and G (at 1584 cm-1) bands of carbon [22, 23]. The IR spectra also show the Ni-O vibrational absorption peak at 567 cm-1 along with the peaks of carbonate and/or C=O group (at 1434, 1036 and 882 cm-1), C-H group (1629 cm-1) and O-H group (3431 cm-1) of carbon [24, 25]. On the contrary, the XRD patterns show peaks of carbon only suggesting that the NiO in both NiO/C supports is amorphous (Fig. S2). In the HR-XPS, both NiO/C supports show two weak peaks at 874 and 856 eV (Fig. S3), which are assigned to the Ni 2p1/2 and Ni 2p3/2 peaks of Ni2+, respectively [26]. Pd NPs were formed on both NiO/C supports by using the UPS method. The XRD patterns of so-obtained Pd/NiO/C samples show peaks of Pd in the face-centered cubic (fcc) structure (2 = 40.6, 46.2, 67.8 and 81.4°) and carbon (2 = ~23.8°) (Fig. 2) [27]. Amorphous NiO does not appear in the XRD. The peak positions of Pd NPs in Pd/NiO/C samples are the same as the Pd NPs in Pd/C reference with the lattice parameter a = 0.3893 nm. The fact that the peak positions do not change indicates that the Pd NPs in Pd/NiO/C samples are pure Pd or the content of Ni in Pd-Ni alloy is very small [27]. If a part of NiO is reduced and reacted with Pd to form Pd-Ni alloy 13

during the formation of Pd NPs, the Pd peaks of Pd/NiO/C samples would be shifted to higher angles. The crystallite sizes of the Pd NPs are estimated to be 7.9, 7.4 and 8.1 nm for Pd/NiO/C_A, Pd/NiO/C_U and Pd/C catalysts by the Scherrer equation on the (111) peak at 2 = 40.6. The IR spectra of Pd/NiO/C catalysts are essentially the same as those of Pd/C except that they show an additional peak at 567 cm-1 from NiO (Fig. S4) [24]. TEM images on Pd/C and Pd/NiO/C catalysts are shown in Fig. 3. While Pd/C and Pd/NiO/C_U samples show well-dispersed Pd NPs dispersed on the supports, the Pd NPs in Pd/NiO/C_A sample are heavily aggregated and only a few places show well-dispersed Pd NPs (Fig. S5). The Pd NPs are formed into triangular or round shape. Based on our previous study in which Pd NPs were synthesized by UPS method, the triangular particles are mostly single crystalline while the round ones are composed of multiple numbers of differently oriented Pd crystallites without any specific crystallographic relationship among them [27, 28]. The particle sizes, obtained by averaging over 250 NPs, are 8.7, 8.6, and 9.6 nm for Pd/NiO/C_A, Pd/NiO/C_U and Pd/C catalysts, respectively. In case of Pd/NiO/C_A, the NPs in the dispersed region (Fig. S5) were measured for this calculation. Because this value is close to the estimated from the XRD peak width, we believe that the particles in the aggregated regions are similar to those in the dispersed regions for their size and shape. Based on these, it appears that the presence of NiO underlayer and the method to form it does not influence the size and shape of Pd NPs formed on them [29]. 14

However, the different extents of dispersion of Pd NPs depending on the nature of support is striking. The reason for the aggregation seen in Pd/NiO/C_A sample is not clear at present. We believe that the different surface states of the support (carbon vs. NiO) influence the nucleation mechanism of Pd NPs. That is, the hydrophilic NiO is more favorable for Pd nuclei to form than the hydrophobic carbon [30]. Consequently, the Pd NPs grown on NiO surface are smaller by ~10% in diameter ( 30% in volume) than those on carbon. The aggregation seen in Pd/NiO/C_A is likely to be a result of the inhomogeneous surface of NiO/C_A patched with NiO on carbon, as is found in the HR-TEM images (below), and preferential deposition of Pd NPs on the NiO-modified region. In the HR-TEM images on Pd/NiO/C catalysts focused on the surface regions (Fig. 4) show an interesting contrast between Pd/NiO/C _U and Pd/NiO/C_A. In Pd/NiO/C_U catalyst, there is a thin layer of a thickness of ~0.5 nm covering the entire surface of carbon as highlighted with two parallel lines in the image (Fig. 4a). On the contrary, Pd/NiO/C_A catalyst shows a thicker (1.13 nm) layer on some parts of carbon surface (also highlighted with two lines) while the other parts do not have such a layer at all (Fig. 4b). The same images without the highlighting lines are shown in Fig. S6 for comparison. We confirmed that such a difference between the two catalysts persists on all over the samples by inspecting the other regions of the samples. Some of such images are shown in Fig. S7. In all the images, the thicknesses of the layers do not agree with the repeating distance of carbon layers, suggesting a 15

different material from carbon. Analyzing for the composition of the layer material on Pd/NiO/C catalyst would be very difficult if not impossible because the layer is very thin and the surface of carbon is irregular. Because of these problems, we synthesized NiO on CNT surface by UPS to take advantage of the well-defined smooth surface of CNT and analyzed so-obtained NiO/CNT by TEM and EDS line-scan profile for Ni across the CNT (Fig. S8). The TEM image shows many small NPs on the CNT surface and the EDS line scan shows two strong peaks of Ni on both ends of the scan corresponding to the surface of the CNT, clearly showing the formation of NiO layer on the surface of CNT. These results suggest that our UPS method is capable forming NiO NP layers on carbon supports. Previously, we observed that (Mn,Co)3O4 NPs and (Ni,Fe)O(OH) NPs could be formed and densely deposited on carbon supports by UPS reactions [29, 31]. Based on these, we believe that the thin layer observed in the TEM of Pd/NiO/C_U catalyst is NiO. The NiO NPs in this sample cover almost entire surface of carbon. The HR-TEM images in Fig. 4 also show Pd NPs. The lattice fringe patterns show spacings of 0.23 nm, which correspond to the (111) plane of the fcc lattice of Pd. The XPS of the three catalysts are compared in Fig. 5. In the Pd 3d XPS, all samples show Pd 3d5/2 and 3d3/2 peaks. Deconvolution of the peaks show that the majority is Pd0 with minor Pd2+ which may be from the surfaces of the Pd NPs. Each of them has almost the same BE in the two Pd/NiO/C catalysts with those in 16

Pd/NiO/C_A slightly upshifted by 0.01 eV from the corresponding ones in Pd/NiO/C_U (BE(Pd/NiO/C_A)/BE(Pd/NiO/C_U) (eV) = 341.18/341.17 for 3d3/2; = 335.72/335.71 for 3d5/2) [32, 33]. On the contrary, those of Pd/C (3d3/2 at 334.85 eV and 3d5/2 at 340.30 eV) are about 1 eV lower than those of Pd/NiO/C catalysts. Evidently, the interaction with NiO induces upshifts of Pd 3d BEs. In the literature of SMSI, such upshifts of BEs of noble metal electrocatalysts are often used as the evidence of SMSI. Of the different explanations proposed to explain SMSI, we think that the one involving the orbital interactions between O2- ions of NiO and Pd atoms can be applies to our case [34-37]. According to the d-band center theory, the upshift of BE of core-electrons of a metallic catalyst means the lowering of the d-band center, which is a signature of the rise of the Fermi level (EF) energy, which, in turn, is desirable

in

enhancing

the

electrocatalytic

performance

of

noble

metal

electrocatalysts. In Fig. 5b, the Ni 2p XPS of Pd/NiO/C_U catalysts and NiO/C_U support are compared where the two peaks are assigned to Ni2+ of NiO. The comparison between the Ni 2p XPS of Pd/NiO/C_A catalysts and NiO/C_A support shows the same results and, thus, are not shown. The BEs of the peaks in Pd/NiO/C_U (857.43 eV for 2p3/2; 875.11 eV for 2p1/2) are upshifted from the corresponding ones of NiO/C_U (856.44 eV for 2p3/2; 874.10 eV for 2p1/2). From the large upshifts of both Pd and Ni peaks in the XPS, it is clear that both Pd/NiO/C catalyst have SMSI effects [26]. The electrochemical behaviors of the three catalysts were studied. Fig. 6a shows 17

CVs in a 0.1 HClO4 solution. At low potentials, between 0.03 to and 0.2 V (vs. RHE), the peaks are related to hydrogen adsorption/desorption on Pd NPs. The small peaks between 0.9 and 1.1 V during the anodic sweep are due to the transformation of Pd into its oxide. Its reverse reaction is observed as strong peaks between 0.70 and 0.8 V in the cathodic sweep [38, 39]. The electrochemical active surface areas (ECSAs), estimated from the amounts of charge for this reaction [40], are 56.4, 44.7 and 28.2 m2g-1 for Pd/NiO/C-U, Pd/NiO/C_A and Pd/C, respectively. Pd/NiO/C catalysts have significantly increased ECSAs from that of Pd/C. Probably, this is because of the smaller particle sizes in the former group than the latter. The altered electronic structure of Pd in Pd/NiO/C catalysts also may contribute to the increase of ECSA. Between the two Pd/NiO/C catalysts, it seems that the extent of dispersion of Pd NPs affects ECSA so that Pd/NiO/C_U with Pd NPs well-dispersed shows larger ECSA than Pd/NiO/C_A with most of Pd NPs aggregated. The electrocatalytic performance of Pd/C, Pd/NiO/C_U and Pd/NiO/C_A catalysts for FAOR was tested in 0.1 M HClO4 and 0.5 M HCOOH solution. The oxidation of formic acid on Pd is known to follow a dual pathway. Anodic oxidation pathway is through the dehydrogenation of the formic acid molecule and CO2 accumulation [41]: HCOOH → CO2 + 2H+ + 2e-

(1)

In the cathodic oxidation pathway, through the dehydration of formic acid, strongly adsorbed carbon monoxide (CO) is formed as a reaction intermediate [41]: HCOOH → COads + H2O → CO2 + 2H+ + 2e- (2) 18

In the CV curves for FAOR in Fig. 6b, the current density of the anodic oxidation peak of Pd/NiO/C_U catalyst is higher than that of the cathodic peak, suggesting that the formic acid oxidation is mainly through the direct electron transfer pathway (eq. 1). On the contrary, the other catalysts show a cathodic oxidation peak comparable to or higher than the anodic peak, indicating higher contribution of the intermediate pathway than in Pd/NiO/C_U. The peak positions of Pd/NiO/C_U catalyst are lower than the other two, indicating more favorable kinetics [42, 43]. The mass-normalized current density for the anodic reaction on Pd/NiO/C_U is calculated to be 2.88 A mgPd-1, which is 1.27-fold higher than that of Pd/NiO/C_A (2.27 A mgPd-1) and 4.43-fold higher than Pd/C (0.65 A mgPd-1). Evidently, the electrocatalytic performance of Pd/NiO/C_U is significantly better than Pd/C and is also better than Pd/NiO/C_A. The CO-stripping measurement data (Fig. 6c) show that the peaks of CO stripping on Pd/NiO/C catalysts are lower than that on Pd/C, suggesting weaker CO adsorption strength on the former. The enhancement of Pd/NiO/C catalysts can be explained as that it has much more active sites and oxygen sources for the oxidation of CO-like intermediates at lower potentials than the others [44]. Probably, NiO around Pd NPs is capable of providing rich active OH species; the oxygenated species generated from NiO facilitate the oxidative removal of the adsorbed intermediates on Pd surface, thus improving the performance of Pd for formic acid oxidation [30]. In addition, the altered electronic structure of Pd through SMSI with NiO, evidenced by the XPS data, makes Pd NPs less susceptible to the poisoning effect of the intermediates, such as CO and favors C-H cleavage at low potential [45, 46]. 19

The catalysts were tested for durability by running CV cycles in 0.1 HClO4 + 0.5 M HCOOH solution (Fig. 6d). After 1000 cycles, the peak current densities of formic acid oxidation on Pd/NiO/C catalysts are much higher than Pd/C, indicating that Pd/NiO/C catalysts are more active and stable than Pd/C. In order to evaluate the inherent stability of the electrocatalyst without the effect of the decrease in the concentration of formic acid during the cycling, we also performed the durability test by transferring Pd/NiO/C_U electrode to a new electrolyte solution after every 100 cycles. The initial current density is recovered each replenishment of electrolyte until 1000 cycles (inset of Fig. 6d). Therefore, we can conclude that the inherent activity of Pd/NiO/C_U catalyst remains intact during the cycling test. The catalysts were also tested for ORR in O2-saturated 0.1 M KOH solution. The LSV curves for ORR (in Fig. 6e) show that Pd/NiO/C catalysts have much higher onset potentials and half-wave potentials than Pd/C, indicating that the introduction of NiO layer to the Pd catalyst significantly reduces the ORR overpotential. The reduction in overpotential is believed mainly due to the electron-donation from NiO to Pd, which favors the reduction of O2 [47]. Between the two Pd/NiO/C catalysts, Pd/NiO/C_U shows better performance than Pd/NiO/C_A. Compared with Pd/C for ORR mass activity (MA) and specific activity (SA), both Pd/NiO/C catalysts show significantly enhanced performance (Fig. 6f). Evidently, the presence of NiO underlayer exerts positive effects in enhancing the electrocatalysis of Pd, which is probably through the SMSI mechanism as the XPS and TEM data indicate. The 20

enhancement factor for MA (SA) with respect to Pd/C is 2.9 (2.3) in Pd/NiO/C_A and 4.6 (4.1) in Pd/NiO/C_U. Between the two Pd/NiO/C catalysts, Pd/NiO/C_U outperforms Pd/NiO/C_A by 1.6 times for MA and 1.8 times for SA. Probably, the higher degree of dispersion of Pd NPs in the former also contribute to the enhancement.

4. Conclusions

We report a composite support for Pd catalyst in the electrochemical FAOR and ORR. Two different NiO/C were prepared by ALD and UPS methods, and used as supports for Pd NPs to form Pd/NiO/C catalysts. NiO/C synthesized by UPS method is characterized by a thin NiO coating on the entire surface of carbon whereas NiO/C synthesized by ALD method patch-like thick NiO coatings on the external surface of carbon. The introduction of NiO underlayer brings in significant enhancement of the electrocatalytic performance of Pd NPs for FAOR and ORR. Moreover, depending on the structure of NiO underlayer the electrocatalytic performance can be further enhanced. Based on our results, uniform thin NiO coating on carbon is better than patch-like thick NiO coating. The enhancement of electrocatalytic activity and stability of Pd/NiO/C can be explained by the change of the electronic structure of Pd NPs through interaction with NiO and also by the bifunctional effect of NiO surface around Pd NPs. This study demonstrates the promising application of composite 21

NiO/C as a support for Pd catalyst for the oxidation of small organic molecules in low temperature fuel cells. In addition, the structure of NiO/C_U and its superior performance as a support suggests that the sonochemical reaction method in this study can be applied in many related fields where uniform coatings of NiO and probably other metal oxides as well are required.

Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF-2019R1F1A1059485). References

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Figure Captions

Fig. 1. (a) Raman spectrum and (b) IR spectrum of NiO/C_U and NiO/C_A supports. The shadows highlight the regions where NiO peaks appear. Fig. 2. XRD patterns of Pd/C, Pd/NiO/C_U and Pd/NiO/C_A catalysts. Fig. 3. TEM images and corresponding histograms of particle size distribution of (a, d) Pd/C, (b, e) Pd/NiO/C_U and (c, f) Pd/NiO/C_A catalysts. Fig. 4. HR-TEM images of (a) Pd/NiO/C_U and (b) Pd/NiO/C_A catalysts. NiO layers on carbon are highlighted by two parallel lines. The lattice fringes of Pd NPs are also marked. Fig. 5. (a) XPS spectra of Pd 3d: Pd/C, Pd/NiO/C_U and Pd/NiO/C_A catalysts. (b) XPS spectra of Ni 2p: NiO/C_U and Pd/NiO/C_U catalyst. Fig. 6. Electrochemical data on Pd/C and Pd/NiO/C catalysts: (a) CVs in 0.1 M HClO4, (b) CVs of formic acid oxidation, (c) CO-stripping, (d) stability test results for FAOR. (The saw-like curve in the upper part is the result on Pd/NiO/C_U with the initial electrolyte condition is restored after every 100 cycles.) (e) ORR polarisation curves and (f) histogram of mass and specific activity for ORR at 0.85 V.

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Figures

Fig. 1. (a) Raman spectrum and (b) IR spectrum of NiO/C_U and NiO/C_A supports. The shadows highlight the regions where NiO peaks appear.

Pd (JCPDS no. 87-0643)

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Fig. 2. XRD patterns of Pd/C, Pd/NiO/C_U and Pd/NiO/C_A catalysts. 31

Fig. 3. TEM images and corresponding histograms of particle size distribution of (a, d) Pd/C, (b, e) Pd/NiO/C_U and (c, f) Pd/NiO/C_A catalysts.

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Fig. 4. HR-TEM images of (a) Pd/NiO/C_U and (b) Pd/NiO/C_A catalysts. NiO layers on carbon are highlighted by two parallel lines. The lattice fringes of Pd NPs are also marked.

Fig. 5. (a) XPS spectra of Pd 3d: Pd/C, Pd/NiO/C_U and Pd/NiO/C_A catalysts. (b) XPS spectra of Ni 2p: NiO/C_U and Pd/NiO/C_U catalyst.

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Fig. 6. Electrochemical data on Pd/C and Pd/NiO/C catalysts: (a) CVs in 0.1 M HClO4, (b) CVs of formic acid oxidation, (c) CO-stripping, (d) stability test results for FAOR. (The saw-like curve in the upper part is the result on Pd/NiO/C_U with the initial electrolyte condition is restored after every 100 cycles.) (e) ORR polarisation curves and (f) histogram of mass and specific activity for ORR at 0.85 V.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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