Bimetallic NiPd Nanoparticles Incorporated Ordered Mesoporous

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Bimetallic NiPd Nanoparticles Incorporated Ordered Mesoporous Carbon as Highly Efficient Electrocatalyst For Hydrogen Production via Overall Urea Electrolysis Muthuchamy Nallal, Sanha Jang, Ji Chan Park, Sungkyun Park, and Kang Hyun Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03275 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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ACS Sustainable Chemistry & Engineering

Bimetallic NiPd Nanoparticles Incorporated Ordered Mesoporous Carbon as Highly Efficient Electrocatalyst For Hydrogen Production via Overall Urea Electrolysis† Nallal Muthuchamy1, Sanha Jang1, Ji Chan Park2, Sungkyun Park3, Kang Hyun Park1* 1

Department of Chemistry, Pusan National University, 2 Busandaehak-ro, 63 beon-gil,

Geumjeong-gu, Busan 46241, Korea 2

Clean Fuel Laboratory, Korea Institute of Energy Research, 152 Gajeong-Ro, Daejeon 34129,

Korea 3

Department of Physics, Pusan National University, 2 Busandaehak-ro, 63 beon-gil, Geumjeong-

gu, Busan 46241, Korea *

Corresponding Author: Kang Hyun Park, Email: [email protected], Phone: +82-51-510-

2238.

ABSTRACT Efficient catalysts for energy conversation from wastewater and energy storage are still existing. The effective hydrogen energy production through lower energy consumption is considering as a promising approach to access the world’s clean energy demand. Herein, an extraordinary bifunctional electrocatalytic activity on hydrogen energy production via urea electro-oxidation reaction in alkaline electrolyte is evaluated by the highly dispersed tiny nickelpalladium

bimetallic

nanoparticles

incorporated

ordered

mesoporous

carbon

(Ni(10%)Pd(10%)/OMC). The systematic electrocatalytic studies on Ni(10%)Pd(10%)/OMC bifunctional electrocatalyst shows a very low required overpotentials (1.346 V and -0.117 V vs RHE in 1 M

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KOH + 0.33 M urea) to achieve urea electro-oxidation and hydrogen evolution reactions at a respective electrocatalytic current density of 30 mA cm-2, which is comparatively very lower overpotential than the oxygen evolution reaction (1.585 V vs RHE) in the water electrolysis system. Moreover, there is no remarkable electrocatalytic activity losses even after 5000 voltammetric cycles on urea electro-oxidation and hydrogen evolution reactions. The Ni(10%)Pd(10%)/OMC bifunctional electrocatalyst exhibits highly efficient and faster reaction kinetics on urea electrolysis than the other three individual electrocatalysts and reasonable performance with the benchmark Pt20%@C and IrO@C catalysts. Notably, the presence of nonprecious nickel metal promotes good electron density at palladium active sites of Ni(10%)Pd(10%)/OMC, which in turn facilitates efficient electrocatalytic reduction reactions to enhance the rate of overall urea electrolysis. Overall, the superior bifunctional ability of Ni(10%)Pd(10%)/OMC could be an appropriate energy efficient electrocatalyst to produce clean energy from wastewater treatment and fuel cell applications. KEYWORDS: hydrogen evolution reaction; bifunctional catalyst; urea electro-oxidation; mesoporous carbon; Ni-Pd bimetal

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INTRODUCTION In recent decades, the global scientific society is considering that the advanced researches such as (i) switching from a fuel to a hydrogen (H2) economy for efficient energy storage and (ii) wastewater treatment could alleviate many new technology developments for future energy and environmental applications.1,2 Among the various energy-storing electrolysis techniques newly developed, urea electrolysis (direct urea fuel cell (DUFC)) is remarkably intriguing, in that it allows simultaneous urea rich wastewater treatment by degrading the urea in it and a clean, environmentally sustainable mass H2 energy production.3 Especially, the utilization of urea electro-oxidation reaction (UOR) can potentially replace oxygen evolution reaction (OER) in ordinary water splitting with great thermodynamic potential shift from 1.23 V (OER) to a mere 0.37 V (UOR) vs. reversible hydrogen electrode (RHE). Urea an ideal, promising H2 and carbon dioxide (CO2) storage medium due to its high density (16.9 MJ L-1, 10.1 weight % of H2), relatively nontoxic, cheap, stable, safety and an easy for transportation and storage. However, great challenge lies in the complexity of UOR, which undergoes slugged kinetics owing to a six-electron (6 e-) transfer process with consisting gas evolution steps, and hence highly effective and affordable electrocatalysts are absolutely needed for this great energy conversion model.4 Although noble metals such as Pt, Rh and Pt-Ir have been examined for efficient UOR,5 but systems that aim of low-cost, high-performance is still urgent for alternative electrocatalysts. Nanotechnology and its pre-defined superstructures play a widespread and significant role in the field of energy, environmental, material science and healthcare applications.6–8 Notably, there has been a huge attention in enlightening the properties of bimetallic and multimetallic nanoparticles since they show superior electrical, optical, magnetic biological, chemical and catalytic properties than the monometallic nanoparticles.9–12 Among the metallic nanocomposites,

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bimetallic nanoparticles are leading in many potential applications such as energy production and portable power storages due to their unique structural properties.13 Apart from the various bimetallic nanostructures, nickel and/or palladium based bimetallic system with the other transition metal elements (Ni/Fe, Pd/Fe, Ni/Si, Ni/Ag, Co@Pd, Pd@Au, Pd@Pt, Pd@Cu, Pd83Ni17 HNS, Pt47Ni53 NPHs) shows good fascinating properties.9,14–18 From the recent researches, it has been bound that the catalysts containing either nickel or palladium system have attracted tremendous attention in various potential applications due to their easy availability, good catalytic activity, fast reaction rate, low cost, highly stable and reproducibility.15 Moreover, the most of bimetallic nanoparticles eventually have core-shell, alloy or mixture of single metal nanoparticles and it can be tunable the structures and surface compositions by individual monometallic source. However, the unique and intrinsic properties of bimetallic nanoparticles are depending upon the range of following main factors like morphological size, shape, nanostructure, and the composition of particles. Recently, the choice of the catalyst support has great influence in respective applications. Meanwhile, several experimental studies indicated that the carbon and its derivatives (such as graphene, reduced graphene, graphene oxide, graphite, carbon nanotubes and porous carbon materials) has been considered as a perfect support material and good stabilizer for various metal nanoparticles due to their excellency in hydrothermal stability, conductivity, surface area, porosity and functionalization chemistry.19–21 These notable great interests in carbon support materials could not only an efficient substrate for loading of nanoparticles, but also to prevent agglomeration of nanoparticles and provide the interaction between the substrate and nanoparticles22,23. Among the various carbon supports, the ordered mesoporous carbon (OMC) was founded as a high influenced catalyst support for fuel cell and battery applications due to its large pore volume, ideal

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pore-size distribution, high surface area, highly electrical conductivity nature and good charge carrying properties.24–26 It is also noted that the surface chemistry, particularly surface functionalization (e.g. polymerization, amine, -COOH, -SH, heteroatom) of catalyst support materials has superior impacts,27–29 unfortunately highly functionalization could been a reason for that the mass and ohmic transfer losses of catalyst supports and also attributed to lower specific surface area, higher agglomeration of nanoparticles and decreased electrical conductivity.30 However, there is still a lack of researches in the field of the facial synthetic route for effective bimetallic nanoparticles loading on suitable high-performance catalyst supports and sustainable fuel cell applications31–34. Herein, we demonstrate a facile strategy for preparation of nickel-palladium (equal percentage of weight (%wt) loaded) bimetallic nanoparticles incorporated on ordered mesoporous carbon support (Ni(10%)Pd(10%)/OMC) with controlled nanostructures via following major steps (Scheme 1); (i) infiltrated NiPd bimetallic salt formation on OMC through co-infiltration of Ni and Pd salts and (ii) simultaneous thermal reduction and alloying of infiltrated NiPd bimetallic salt to form Ni(10%)Pd(10%)/OMC. The resultant of Ni(10%)Pd(10%)/OMC electrocatalyst possess a well ordered mesoporous structure with highly dispersed tiny NiPd bimetallic nanoparticles (~2 nm), high-surface area (862.4 m2 g-1), a narrow pore size distribution (5.3 nm) and larger pore volume (0.91 cm3 g-1). As a result, Ni(10%)Pd(10%)/OMC electrocatalyst can be used as an efficient bifunctional electrocatalyst for H2 energy production (90-95% energy efficiency) at low overpotential via overall urea electro-oxidation reaction for the first time. Moreover, highly stable electrocatalytic system of bimetallic NiPd nanoparticles, sufficient electron conducting pathway from OMC, higher active sites, high-surface area of Ni(10%)Pd(10%)/OMC exhibiting superior bifunctional (cathode and anode) electrocatalytic activity towards overall urea electrolyzer along

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with long-term stability and superior to already reported bifunctional electrodes (Table 1). Scheme 1. Table 1.

EXPERIMENTAL Synthesis of Ni (10%)Pd (10%) /OMC Electrocatalyst All chemicals and reagents were used as such received without any further purification. Initially, ordered mesoporous carbon (OMC) were prepared by mesoporous silica hard template (SBA-15) method and detailed explanation were provided in supporting information -1 (S1). To prepare a Ni(10%)Pd(10%)/OMC, 310 mg and 156 mg of nickel(II) nitrate hexahydrate and palladium(II) nitrate dihydrate were dissolved in 0.3 mL of distilled water. The mixed solution was dropped on OMC powder (0.5 g) and co-infiltrated by grinding the mixture in a mortar for several minutes under ambient condition. Then, the mixed powders were placed in a 30 mL polypropylene bottle and aged at 60 °C in an oven. After aging for 24 h, the sample was cooled in an ambient atmosphere and transferred to an alumina boat in a tube-type furnace. Finally, Ni(10%)Pd(10%)-incorporated OMC powder was slowly heated at a ramping rate of 2.7 °C∙min-1 up to 350 °C under a H2 flow of 200 mL∙min-1. The sample was thermally treated at 350 °C for 4 h under the continuous H2 flow. Finally, the as synthesized Ni(10%)Pd(10%)/OMC electrocatalyst was collected. Ni(5%)Pd(15%)/OMC and Ni(15%)Pd(5%)/OMC electrocatalysts were prepared as similar like Ni(10%)Pd(10%)/OMC synthesis but only varied the mass of Ni and Pd precursors, respectively. Other two electrocatalysts namely Ni (10%) /OMC and Pd (10%) /OMC were also prepared followed by same procedure in the absence of palladium(II) nitrate dihydrate (Ni (10%) /OMC) and nickel(II) nitrate hexahydrate (Pd (10%) /OMC), respectively. Characterizations High power powder X-ray diffraction (XRD) analysis were performed on Rigaku D/MAX6


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2500, 18 kW. High resolution transmission electron microscopy (TEM) analysis was conducted by a Tecnai TF30 ST and a Titan Double Cs corrected TEM (Titan cubed G2 60-300). Energydispersive X-ray spectroscopy (EDS) elemental mapping data were characterized using a higher efficiency detection system (Super-X detector). N2-sorption isotherms were measured at 77 K with a Tristar II 3020 surface area analyser. Before measurement, the samples were degassed at 300 °C for 4 h under N2 flow. X-ray photoelectron spectroscopy (XPS) was conducted on scanning X-ray microscope (ESCALab250 instrument, Thermo, Waltham, MA, USA). Inductive coupled plasma (ICP)-optical emission spectrometry(OES)/Mass spectrometry (MS) were performed on ThermoFischer 6300/Agilent ICP-MS 7900. The all electrochemical performances such as cyclic voltammetry (CV), liner sweep voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were performed at a CHI700E electrochemical workstation (CH Instruments, Austin, TX, USA) and using a one compartment three-electrode electrochemical cell at room temperature (~25 ºC). Preparation of Ni(10%)Pd(10%)/OMC Electrode and Electrochemical Measurements The working electrode was fabricated by a simple drop-coating the slurry mixture of 5 mg asprepared Ni(10%)Pd(10%)/OMC well dispersed in the volume ratio of 5 wt% of Nafion, deionized water and 2-propanol is 1:7:7. Subsequently, 7 µL of the electrocatalyst slurry were drop-coated on the pre-cleaned glassy carbon electrode surface (area = 0.1963 cm2 , diameter = 5 mm) and then the solvents evaporated at 80 ºC. Instead of glassy carbon electrode, the carbon cloth electrode (CE, area = 1cm x 1cm) was used in two-electrode set up. A graphite rod electrode (surface area = 10.3 cm2; diameter = 3.05 mm and height = 10 mm) and Ag/AgCl were used as the counter and reference electrodes, respectively. The mass loading of active electrocatalyst was about 0.6857 mg cm-2. The electrochemical cell arrangement and the method of potential conversion from Ag/AgCl

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to RHE are the same as that is reported elsewhere.35 CV and LSV experiments were carried out in N2-saturated 1 M KOH with and without 0.33 M urea at scan rates = 10 mV s-1, and 5 mV s-1 , respectively. CA and EIS analysis were performed on a staircase applied cell voltages and respective OCV vs RHE with a frequency range between 100 KHz-10 Hz in N2-saturated 1 M KOH.

RESULTS AND DISCUSSION Active Ni(10%)Pd(10%) bimetal nanoparticles were easily obtained by co-infiltration of nickel nitrate hydrate and palladium nitrate hydrate salts in OMC as support and subsequent hydrothermal decomposition under a H2 gas flow. The typical physicochemical characteristics of Ni(10%)Pd(10%)/OMC; Ni(10%)/OMC; Pd(10%)/OMC and OMC were characterized by X-ray diffraction (XRD) analysis; transmission electron microscopy (TEM), high-resolution TEM (HRTEM); X-ray photoemission spectroscopy(XPS) and N2 adsorption/desorption isotherms. To investigate the crystalline nature of Ni(10%)Pd(10%)/OMC; Ni(10%)/OMC and Pd(10%)/OMC electrocatalysts were characterized by XRD (Figure 1a). The diffraction broad peaks were observed at 24.5º, 24.8º and 24.7º corresponding to (002) plane, representing the graphitic structure of carbon in Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC and Pd(10%)/OMC, respecftively.36,37 The XRD pattern of Ni(10%)Pd(10%)/OMC resembles two main characteristic peaks of face-centered cubic (fcc) crystalline structures of bimetallic NiPd alloy were clearly shown and matched well with standard JCPDS No: 01-072-2515. Also, the size of bimetallic NiPd alloy nanoparticles was calculated as small as 1.6 nm using the Scherrer equation. Meantime, for Ni(10%)/OMC and Pd(10%)/OMC electrocatalysts, three main crystalline peaks of fcc were observed and matched well with JCPDS No: 04-0850 (for Ni) and JCPDS No: 46-1043 (for Pd), also, particle size of Ni and Pd nanoparticle

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on OMC was determined to be 1.5 nm and 2.8 nm, respectively. The size and distribution of the embedded NiPd alloy nanoparticles on OMC (Ni(10%)Pd(10%)/OMC) was obtained by TEM and HRTEM. Figure 1b and Figure 1c shows that the uniform, highly dispersed and very tiny bimetallic NiPd nanoparticles was successfully loaded on OMC support. Furthermore, the histogram graph of corresponding loaded bimetallic NiPd nanoparticles clearly shows that the narrowest size distribution of NiPd nanoparticles with the average size of 2.17 nm (Figure S1). The parallel lattice fringe with measured spacing distance of 0.22 nm in Figure 1d is corresponds to the (111) plane of high crystalline NiPd nanoparticles. Figure 1e-i shows the high-angle annular dark-field (HAADF) image and the corresponding elemental mappings of Ni(10%)Pd(10%)/OMC, indicating that the 10 wt% of both Ni and Pd elements containing bimetallic nanoparticles were uniformly incorporated throughout on OMC. Before that TEM analysis were conducted to identify the porous and morphological characteristics of OMC, as a result the highly ordered porous structure of the OMC is clearly visible in Figure S2. Figure S3 shows the TEM and HAADF images of the Ni(10%)/OMC and Pd(10%)/OMC, the open-structured nanopores of OMC are homogeneously filled with well dispersed of Ni (Figure S3a-b) or Pd (Figure S3c-d) throughout the OMC with large surface area. Figure 1.

In Figure 1j-k are shown the isotherms of adsorption/desorption of nitrogen and pore size distributions of Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC, Pd(10%)/OMC and OMC. The detailed structural parameters, such as pore volume, pore diameter, surface area and the loading amount of metals were summarized in Table S1. The determined porous structure in prepared Ni(10%)Pd(10%)/OMC and Pd(10%)/OMC exhibits a type IV adsorption–desorption hysteresis at a higher relative pressure as similar like OMC, which indicated that the ordered mesoporous structure was not disarranged even after Ni, Pd, and NiPd bimetallic nanoparticles incorporation in the pores of OMC. From the 9



N2 adsorption -desorption isotherms, as prepared OMC has a Brunauer−Emmett−Teller (BET) surface area of 1434 m2 g-1; pore volume of 1.45 cm3 g-1 and the calculated average pore size of 5.3 nm from the adsorption branches of N2 isotherms by using the Barrett–Joyner-Halenda (BJH) method. BET surface area and pore volume of Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC and Pd(10%)/OMC were also determined as 862 m2 g−1 , and 0.91 cm3 g-1; 1243 m2 g−1 and 1.17 cm3 g-1; 1078 m2 g−1 and 1.26 cm3 g-1, respectively. Meanwhile, the calculated pore size of Ni(10%)Pd(10%)/OMC and Pd(10%)/OMC was found to be 5.3 nm and 5.2 nm, respectively, which was almost the same as that of the OMC. Additionally, the wt% of alloyed bimetallic NiPd loading on Ni(10%)Pd(10%)/OMC was 19.03 wt% (Ni: 10 wt%, Pd: 9.03 wt%), as determined by ICP-OES/MS, which is a well match with the nominal metal nanoparticle loading value of 20 wt%. XPS were performed furtherly to investigate the nature of chemical compositions, oxidation states of the Ni(10%)Pd(10%)/OMC electrocatalyst. The low resolution XPS survey spectrum of Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC; Pd(10%)/OMC and OMC illustrated in Figure S4, and the two additional peaks except O1s and C1s peaks were clearly found in Ni(10%)Pd(10%)/OMC electrocatalyst with peaks centered at 335.5 eV and 856 eV, which are corresponding to Pd and Ni, respectively. The high resolution XPS (HR-XPS) spectra of detailed Ni 2p displayed in Figure 2a reveal two low energy bands (Ni 2p3/2 = 855.9 eV; Ni 2p3/2 satellite = 860.8 eV) for Ni(10%)Pd(10%)/OMC. Additionally, the coexistence of Ni+2 (855.99 eV) and Ni+3 (858.2 eV) valences suggesting a random occupancy of Ni cations in the Ni(10%)Pd(10%)/OMC structure.38 Moreover, metallic Ni0 (853.59 eV) peak was clearly found in Ni(10%)Pd(10%)/OMC. Figure 2b shows the deconvoluted Pd 3d spectrum with the availability of two chemical states. The peaks of Pd 3d5/2 and Pd 3d3/2 were located at 335.5 eV and 340.58 eV, respectively, are assigned to zero valent form of Pd nanoparticles in Ni(10%)Pd(10%)/OMC electrocatalyst (Figure 2b). Four other

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peaks with small intensities at 336.9 eV; 338.3 eV; 342.2 eV and 343.7 eV are related to NiPd and Pd oxide species in 3d5/2 and 3d3/2, respectively. Figure 2c shows the deconvoluted C1s XPS spectrum into various peaks which are related to the binding energies of sp2–C (284.3 eV); sp3–C (285.3 eV); C–O (286.4 eV); C=O (287.5 eV); O=C–O (288.7 eV) and π–π* (289.9 eV), respectively. This is interesting that the co-infiltration of bimetallic reduction method allows the OMC to reduce the bimetal NiPd ions into the tiny bimetallic NiPd nanoparticles that are well embedded in Ni(10%)Pd(10%)/OMC electrocatalyst. As shown in Figure 2d, the high resolution O1s spectra of OMC and Ni(10%)Pd(10%)/OMC were deconvoluted into three peaks as follows:39 peaks at 531.4 eV and 531.3eV (C=O group); peaks at 533 eV and 533.3 eV (C–O); peaks at 536.5 eV and 5.38.5 eV (chemisorbed oxygen and water), respectively. The peak ratio of the area of C=O to the area of C–O increased 2.3 times after loading of bimetallic NiPd nanoparticles to OMC, which is from the formation of metal oxide (Pd oxide species). Moreover, the higher concentration of C=O holds more active sites and would be a reason for the enhanced catalytic activity.40 Figure 2.

To verify the electrocatalytic behavior of Ni(10%)Pd(10%)/OMC, CV, LSV, CA and EIS experiments were performed in a three electrode alkaline (1 M KOH) electrolyte. Figure S5a shows the CV responses of Ni(15%)Pd(5%)/OMC, Ni(10%)Pd(10%)/OMC, Ni(5%)Pd(15%)/OMC Ni(10%)/OMC, Pd(10%)/OMC and OMC elctrocatalysts in N2-saturated blank 1 M KOH electrolyte at a scan rate of 10 mV s-1. The CV curve of Ni(15%)Pd(5%)/OMC, Ni(10%)Pd(10%)/OMC, Ni(5%)Pd(15%)/OMC (Figure S5a) clearly exhibits the presence of Ni and Pd. The Ni(10%)/OMC shows only a pair of redox peaks centered at 1.4 V/1.26 V vs RHE, corresponding to the reversible transformation between Ni2+ and Ni3+. Moreover, a broad peak obtained in the potential range of 1.23 to 1.27 on Pd(10%)/OMC, Ni(10%)Pd(10%)/OMC and Ni(5%)Pd(15%)/OMC polarization curves due to the water activation at Pd active sites.41 However, the highly electroactive Ni(10%)Pd(10%)/OMC 11



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electrode shows a pair of broader and a much larger redox peaks than the other two different wt% loaded NiPd electrocatalyst. Maybe the reason is that the equally mass loaded Ni(10%)Pd(10%)/OMC electrode possesses the bimetallic redox couples of surface rich NiPd alloyed nanoparticles in the electrocatalyst. Thus, it can be a benefit to produce two types of electroactive centers for redox reactions.42 Particularly, Ni(10%)Pd(10%)/OMC electrocatalyst exhibits 2.5 and 1.3 times higher current response than the those 15 wt% of Ni and 15 wt% of Pd loaded bimetallic NiPd electrocatalyst, which is indicate that the equal mass of Ni(10%) and Pd(10%) loading on OMC leads the higher electrocatalytic property duo to the presence of the NiPd phases. Also, the obtained results clearly indicating that an appropriate wt% of Ni and Pd on OMC also important to the excellent electrochemical performances of electrocatalysts. Figure S5b shows the CV plot of fabricated electrodes in N2-saturated 1 M KOH electrolyte with the addition of 0.33 M urea. Compared to the CV curves of Ni(15%)Pd(5%)/OMC, Ni(10%)Pd(10%)/OMC, Ni(5%)Pd(15%)/OMC and Ni(10%)/OMC in N2-saturated 1 M KOH electrolyte, an intense increase in anodic current densities was shown in the presence of 0.33 M urea with an onset potential (Eonset) = 1.335 V, 1.33 V, 1.34 V and 1.344 V vs RHE, respectively. Unfortunately, Pd(10%)/OMC and bare OMC electrocatalysts did not exhibits any prominent oxidation current towards UOR. According to the reports in previous literature, the Eonset of UOR is consistent with the potential of NiOOH species formation, and the newly formed Ni3+ species in NiOOH are the important active sites for UOR.43 In the case of bimetallic NiPd nanoparticle loaded electrocatalysts, the presence of urea the high current density with redox cycles occur in the positive potential range corresponding to the conversation of NiPd(OH)2 to NiPdOOH as shown in Eqs. (1) and (2). 𝑁𝑖𝑃𝑑(𝑂𝐻)2 + 𝑂𝐻 − → +𝐻2 𝑂 + 𝑒 − 12


(1)

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6𝑁𝑖𝑃𝑑𝑂𝑂𝐻 + 𝐶𝑂(𝑁𝐻2 )2 + 6 𝑂𝐻 − → 6 𝑁𝑖𝑃𝑑(𝑂𝐻)2 + 𝑁2 + 5𝐻2 𝑂 + 𝐶𝑂2 + 6𝑒 −

(2)

From the results, we clearly noted that the optimized wt% (Ni: 10 wt%, Pd: 10 wt%) loaded bimetallic NiPd nanoparticle on OMC electrocatalyst facilitated the high electroactive sites than the individual 10 wt% (Ni(10%)/OMC and Pd(10%)/OMC) catalysts or higher wt% loaded (Ni(15%)Pd(5%)/OMC and Ni(5%)Pd(15%)/OMC) bimetallic NiPd based electrocatalysts. Moreover, in the case of equally wt% loaded Ni(10%)Pd(10%)/OMC, the surface is rich in both Ni and Pd active sites, which support the superior electroactivity towards UOR electrolysis. The electrocatalytic UOR activities of Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC, Pd(10%)/OMC, and OMC electrocatalysts were evaluated and compared with the benchmark IrO@C electrocatalyst in the presence of 0.33 M urea in N2-saturated 1 M KOH electrolyte solution at a scan rate of 5 mV s-1 are shown in Figure 3a. The Eonset and electrocatalytic current density are the most important parameters for the electrolysis studies. The LSV polarization curve of Ni(10%)Pd(10%)/OMC exhibits much superior current density among all other three prepared electrocatalysts with on very smaller overpotential at 30 mA cm-2 (ƞ30) of 1.346 V vs RHE, which is much lower than that of Ni(10%)/OMC (1.373 V), Pd(10%)/OMC (1.63 V), and almost nearer to that of benchmark IrO@C electrocatalyst (1.33 V) (Figure S6). Additionally, the UOR kinetics of prepared electrocatalysts were also compared using their Tafel plots (Figure 3b). Relatively, Ni(10%)Pd(10%)/OMC shows the Tafel slope values of 31 mV dec-1 for UOR, which is much smaller compared to Ni(10%)/OMC (36 mV dec-1), Pd(10%)/OMC (59 mV dec-1) and OMC (64 mV dec-1) and little higher than IrO@C electrocatalyst (25 mV dec1

) (Figure S6). It suggested that the smaller Tafel slope value of Ni(10%)Pd(10%)/OMC electrocatalyst

leads a promising and faster catalytic kinetics on UOR. The influence of urea concentrations on Ni(10%)Pd(10%)/OMC electrocatalyst in N2-saturated 1 M KOH electrolyte was performed and displayed in Figure 3c.

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Notably, while increasing the concentration of urea from 0 M to 0.50 M the anodic peak current density also increases linearly and reached its maximum current density at 0.40 M of urea concentration. It may be the reason of good coverage of urea and its intermediates on the Ni(10%)Pd(10%)/OMC electrocatalyst surface from the high concentration urea solution. Durability is another important criterion for UOR electrocatalyst. Additionally, the electrocatalytic OER (oxygen evolution reaction as anode) experiments were also performed on Ni(10%)Pd(10%)/OMC electrocatalyst in N2-saturated 1 M KOH alkaline medium, moreover, it demands a very much larger overpotential around 1.585 V vs RHE at ƞ30. At the same time, for electrocatalytic UOR performance by Ni(10%)Pd(10%)/OMC system exhibits the overpotential of a mere 1.346 V vs RHE required to reach the current density of 30 mA cm-2 (ƞ30), which is 236 mV lower than the overpotential of OER in simple alkaline medium. This very low overpotential of Ni(10%)Pd(10%)/OMC electrocatalyst for UOR is much better than other yet reported electrocatalysts in literatures (Table 1), highlighting the great effectiveness of our Ni(10%)Pd(10%)/OMC electrocatalyst as anode of UOR. These remarkable higher catalytic activities such as high current density, good conductivity, low overpotential, low Tafel value and good durability of Ni(10%)Pd(10%)/OMC electrocatalyst deliver much improved UOR performance, may be due to following main reasons; (i) the presence of Ni3+ cationic active species at NiOOH makes the effortless e- hopping process, which may stimulate to rise e- transfer process of electrocatalytic reactions; (ii) OMC holds a very high surface area, pore volume and pore diameters, which may expose the higher interfacial electroactive sites and ion/e- transfer rates for UOR. Figure 3.

In order to confirm our expectation of bifunctional electrocatalytic activity, the as-prepared electrocatalysts were also investigated for HER measurements in with and without 0.33 M urea

14


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solution in N2-saturated 1 M KOH electrolyte. As shown in Figure 4a, the LSV polarization curve of the Ni(10%)Pd(10%)/OMC electrocatalyst exhibits the high catalytic activity with very low overpotential of -0.117 V vs RHE at 30 mA cm-2 (ƞ30), which is comparatively very low overpotential than the Ni(10%)/OMC, Pd(10%)/OMC and OMC electrocatalysts. Meanwhile, commercial Pt20%@C electrocatalyst (20 wt% of Pt loaded carbon) exhibits the overpotential of 0.07 V at ƞ30vs RHE. It indicates that the synergistic effects of Ni(10%)Pd(10%)/OMC electrocatalyst has significant roles in the case of higher electrochemical HER performance. The corresponding Tafel slopes of these electrocatalysts are derived from the LSV polarization curves (Figure 4b), to be 29 mV dec-1, 37 mV dec-1, 65 mV dec-1, 76 mV dec-1 and 210 mV dec-1 for Pt20%@C, Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC, Pd(10%)/OMC and OMC electrocatalysts, respectively. In addition, the lower overpotential (ƞ30 = -0.117 vs RHE) and Tafel slope (37 mV dec-1) values corresponding to Ni(10%)Pd(10%)/OMC electrocatalyst (Figure 4c) suggesting the highly efficient and fast kinetics of HER reaction. Based on well-known mechanisms for electrocatalytic HER activity, the Tafel slope value of 37 mV dec-1 for the Ni(10%)Pd(10%)/OMC electrocatalyst implies a Volmer-Heyrosky mechanism.44 Furthermore, the another important criterion, the good satisfied durability of Ni(10%)Pd(10%)/OMC as an HER electrocatalyst has been experimentally confirmed in Figure 4d. Notably, even after 5000 cycle of CV performances, a negligible (2.8 mV at ƞ30) decay of its HER activity is observed from the LSV polarization curves of before and after 5000 cycles (Figure 4d). Other than the excellent HER activity with 0.33 M urea solution in N2-saturated 1 M KOH electrolyte, the Ni(10%)Pd(10%)/OMC electrocatalyst also performed in blank N2-saturated 1 M KOH electrolyte. Fortunately, there is no big changes of cathodic current density was observed in Figure 4d, suggesting that as prepared Ni(10%)Pd(10%)/OMC electrocatalyst has superior catalytic activity in both overall water and urea electrolysis applications.

15



Figure 4.

Another important factor is the electrochemical active surface area (ECSA), which is a high influence in the rate of both electrochemical UOR and HER applications, were determined by the proportion calculation of double-layer capacitance (Cdl) in the potential window of charging/discharging of an electrical double - layer of as prepared electrocatalysts (Figure 5a-c). Results shown in Figure 5a, the current density of charge/discharge showed the trend OMC < Pd(10%)/OMC < Ni(10%)/OMC < Ni(10%)Pd(10%)/OMC. To specifically note, the charge/discharge response at Ni(10%)Pd(10%)/OMC electrocatalyst is largest among the other three electrocatalysts. Also Ni(10%)Pd(10%)/OMC electrocatalyst shows good linear response of current density while increasing the scan rates (Figure 5b). The result shows that the Cdl of 35.1 mF cm-2 for Ni(10%)Pd(10%)/OMC electrocatalyst (Figure 5c). Additionally, the conductivity and the electrocatalytic kinetics of the as prepared electrocatalysts were evaluated by the EIS (Figure 5d) test at respective overpotentials with a frequency range between 100 KHz-10 Hz in N2-saturated 1 M KOH (Figure 5d). The equivalent electric circuits in Figure 5d(insert) could be applied to fit the experimental data with the theoretical ones. The identical charge transfer resistance (Rct) – constant phase element (CPE) network was connected in serious to the solution resistance (Rs). In this network, Rct and CPE were connected in parallel.

Ni(10%)Pd(10%)/OMC electrocatalyst

possesses a smaller Rct of 4.6 Ω than that of Ni(10%)/OMC (13.2 Ω), Pd(10%)/OMC (38.5 Ω) and OMC (115 Ω), which suggests the fast electrocatalytic kinetics, high charge transfer rate and good conductivity for Ni(10%)Pd(10%)/OMC electrocatalys. Figure 5.

Figure 6.

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Benefiting from the highly bifunctional electrocatalytic activity of Ni(10%)Pd(10%)/OMC electrocatalyst towards both the HER (cathode) and UOR (anode), we have performed the overall urea electrolysis system in two-electrode set up using carbon cloth electrode as a substrate. Figure 6a shows the overall urea polarization curves of precious IrO@C/CE//Pt(20%)@C/CE electrocatalyst and non-precious Ni(10%)Pd(10%)/OMC/CE//Ni(10%)Pd(10%)/OMC/CE electrocatalyst based overall urea. The Ni(10%)Pd(10%)/OMC/CE//Ni(10%)Pd(10%)/OMC/CE cell requires 1.35 V for UOR and -0.118 V for HER to afford the 30 mA cm-2 current density, but, in the case of benchmark IrO@C/CE//Pt(20%)@C/CE cell needs low voltages of 1.33 V for UOR and -0.07 V for HER to reach

the

30

mA

cm-2

current

density.

It

confirms

that,

our

bifunctional

Ni(10%)Pd(10%)/OMC/CE//Ni(10%)Pd(10%)/OMC/CE cell requires only 20 mV (for UOR) and 47 mV (for HER) extra potential than the benchmark electrodes to obtain 30 mA cm-2 current density, which is very close to the benchmark electrocatalysts. Moreover, our bifunctional electrocatalyst shows the high electrocatalytic activity than the already reported electrocatalysts, especially bifunctional electrocatalysts (Table 1). Additionally, the applied voltage-step analysis was performed to find out the amount of current density generated from water electrolysis and urea electrolysis at different applied cell voltages. Figure 6b shows the different applied voltage vs current density responses of Ni(10%)Pd(10%)/OMC electrode in 1 M KOH electrolyte with and without urea. The significantly higher current densities were observed for urea electrolysis (1 M KOH + 0.33 M urea electrolyte) than the water electrolysis (1 M KOH electrolyte). Also, Figure 6b revealed that the obtained higher current density for water electrolysis at 1.6 V contributes to the total current efficiency. The energy efficiency is another important parameter for urea electrolysis. It can be directly evaluated for the percentage of current efficiency (ƞ) occurred on UOR using the equations provided in SI-2. Figure 6b (insert) shows the efficiency (%) of

17



Ni(10%)Pd(10%)/OMC

electrocatalyst

at

different

applied

cell

Page 18 of 35

voltages

in

UOR

and

Ni(10%)Pd(10%)/OMC electrocatalyst possess the above 90% of energy efficiencies in all applied cell voltages from 1.35 to 1.6 V. Additionally, the Faradic efficiency for H2 generation was measured at the current density of 30 mA cm-2,. The results implying a high Faradic efficiency (96-98%) at different operating periods (Figure S7). Additionally, HRTEM, EDX and XRD analysis on Ni(10%)Pd(10%)/OMC electrocatalyst after 5000 cycles were characterized and shows in Figure 7ac. Moreover, HRTEM and EDX images (Figure 7a-c) clearly confirmed that the stable morphology, uniform size distribution of bimetallic NiPd nanoparticles on OMC. Stable crystalline nature of bimetallic NiPd nanoparticles was also confirmed by XRD analysis (Figure 7c(insert)). The main reasons why Ni(10%)Pd(10%)/OMC electrocatalyst contribute to the highly stable, higher current density and energy efficiency at low cell voltage of total urea electrolysis is that (i) the OMC can successfully protect our electrocatalyst from the reaction gas products such as CO2, N2 and H2O to diffuse back into the electrolyte; (ii) high conductive nature of OMC also strongly attributed for ionic and electronic transfer; (iii) the stability and activity of Ni nanoparticles were improved by the introduction of Pd nanoparticles; (iv) Ni(10%)Pd(10%) bimetallic nanoparticles can improve the high electrical conductivity, maintain the lower charge-transfer resistance of Ni(10%)Pd(10%)/OMC electrocatalyst and also thus enhancing the overpotential of the overall urea electrolysis. Figure 7.

CONCLUSIONS In summary, highly electroactive Ni(10%)Pd(10%)/OMC catalyst was synthesized through a simple co-infiltration and reduction method, exhibits extremely high bifunctional electrocatalytic activity in overall urea electrolysis with very low overpotentials of -0.117 V (HER) and 1.346 V

18


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(UOR) vs RHE at 30 mA cm-2. Additionally, the bifunctionality of Ni(10%)Pd(10%)/OMC was proven by

the

corresponding

two-electrode

urea

electrolyzer

system

applying

fabricated

Ni(10%)Pd(10%)/OMC based electrodes as both cathode and anode. This system needs very low cell voltage of -0.118 (HER) and 1.35 V (UOR) to afford the current density of 30 mA cm-2. Furthermore, the production of hydrogen energy from urea mixed water using UOR as the anode reaction is barely reported yet. And this unique and remarkable bifunctional electrocatalyst offers us the highest energy conversion efficiency via replacing traditional oxygen evolution reaction with thermodynamically favorable UOR, at the same time which is also capable of sewage treatment. Moreover, this study may open new avenues to explore the use of high surface area containing ordered mesoporous carbon as a suitable matrix for higher loading of bimetallic nanoparticles toward its wide catalytic applications for clean energy production.

SUPPORTING INFORMATION Preparation of OMC, energy efficiency calculation. The size distribution histogram of bimettalic Ni(10%)Pd(10%) nanoparticles on OMC, TEM of OMC and TEM-HAADF images of Ni(10%)/OMC, TEM-HAADF images of Pd(10%)/OMC. XPS spectrum of Ni(10%)Pd(10%)/OMC and OMC (Survey scan). CV plots of Ni(15%)Pd(5%)/OMC, Ni(10%)Pd(10%)/OMC, Ni(5%)Pd(15%)/OMC Ni(10%)/OMC, Pd(10%)/OMC and OMC electrodes in 1 M KOH electrolyte without and with 0.33 M urea at a scan rate of 10 mV s-1. Overpotentials for UOR at 30 mA cm−2 vs RHE and Tafel slopes for the corresponding electrocatalysts. Table S1. Metal content, surface area, pore volume, and pore diameter of OMC, Ni10%/OMC, Pd10%/OMC, and Ni10%Pd10%/OMC. ACKNOWLEDGMENTS This work is ﬁnancially supported by Basic Science Research Program through the

19



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Contents: Scheme 1. The schematic illustration for preparation of bimetallic Ni(10%)Pd(10%)/OMC electrocatalyst. Figure 1. (a) XRD spectra of Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC and Pd(10%)/OMC. (b) Lowresolution TEM image, (c-d) HRTEM images, (e) HAADF TEM image, (f-i) scanning TEM images with elemental mapping of Ni(10%)Pd(10%)/OMC. The bars represent 20 nm (b, e, f, d, h, i), 2 nm (c, d), respectively. (j) N2 adsorption/desorption isotherms and (k) pore size distribution diagrams of OMC, Ni(10%)/OMC, Pd(10%)/OMC, and Ni(10%)Pd(10%)/OMC. Figure 2. Deconvoluted XPS spectra of (a) Ni 2p, (b) Pd 3d, (c) C 1s energy bands of Ni(10%)Pd(10%)/OMC and (d) O 1s energy bands of OMC; Pd(10%)/OMC; Ni(10%)/OMC and Ni(10%)Pd(10%)/OMC. Figure 3. UOR electrocatalytic properties of IrO@C, Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC, Pd(10%)/OMC and OMC. (a) LSV plots in N2-saturated 1 M KOH with 0.33 M urea at scan rate = 5 mV s-1; (b) respective Tafel plots derived from (a); (c) LSV plots of Ni(10%)Pd(10%)/OMC in different concentration of urea electrolyte; and (d) durability and OER analysis of Ni(10%)Pd(10%)/OMC performed without urea for OER and with 0.33 M urea (for UOR) in N2saturated 1 M KOH at scan rate = 5 mV s-1. Figure 4. HER electrocatalytic properties of Pt(20%)@C, Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC, Pd(10%)/OMC and OMC. (a) LSV plots in N2-saturated 1 M KOH with 0.33 M urea at scan rate = 5 mV s-1; (b) respective Tafel plots derived from (a); (c) Overpotentials at 30 mA cm-2 vs RHE and LSV plots and Tafel slope values for the corresponding electrocatalysts; and (d) durability and HER analysis of Ni(10%)Pd(10%)/OMC performed without and with 0.33 M urea in N2-saturated 1 M KOH at scan rate = 5 mV s-1. Figure 5. (a) CV of Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC, Pd(10%)/OMC and OMC at scan rate = 5 mV s-1 in 1 M KOH. (b) CV for Ni(10%)Pd(10%)/OMC at various scan rates between 2 to 14 mV s-1, (c) linearly fitted curve of corresponding scan rate vs capacitive currents. (d) EIS spectra of Ni(10%)Pd(10%)/OMC, Ni(10%)/OMC, Pd(10%)/OMC and OMC at respective OCV vs RHE with a frequency range between 100 KHz-10 Hz in N2-saturated 1 M KOH. Insert: The equivalent electric circuit. Figure 6. (a) LSV plots for overall urea electrolysis system of Ni(10%)Pd(10%)/OMC/CE//Ni(10%)Pd(10%)/OMC/CE and IrO@C/CE//Pt(20%)@C/CE in N2-saturated 1 M KOH with 0.33 M urea at scan rate = 5 mV s-1; (b) CA response on a staircase applied cell voltages for Ni(10%)Pd(10%)/OMC. Insert shows the applied potential and current efficiency relationship for Ni(10%)Pd(10%)/OMC in urea electrolysis. Figure 7. (a,b) HRTEM images, (c) EDX and XRD spectra (insert) of Ni(10%)Pd(10%)/OMC electrocatalyst after stability test.

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Figures and Tables

Scheme 1.

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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Table 1. Comparisons of bifunctional (HER, UOR and/or OER) performances of Ni(10%)Pd(10%)/OMC electrocatalyst with already reported bifunctional electrodes. Catalyst

Potential (V vs RHE) Anodic applications

Cathodic application

Current density

Mass loading

(mA cm-2)

(mg cm-2)

Refence

UOR

OER

HER

Ni(10%)Pd(10%)/OMC

1.346

1.585

0.117

30

0.6857

This work

NiCo2S4 electrode

1.45

1.47

0.181

10

-

45

1.53

0.266

50

NS/CC

Ni(OH)2 //FNi3S2/Ni F (NF-20)

1.36

1.38

-

50

-

46

MnO2/MnCo2O4

1.33

-

0.2

10

1.27

47

pa-NiFe NS/NIF

1.362

1.463

-

30

-

48

1.35

1.93

-

50

-

49

P-NiMo4N5@Ni-1

-

1.527

0.118

100, 10

2.5

10

Fe‐Ni@NC‐powder

-

1.6

0.409

10

0.5

50

Fe‐Ni@NC‐CNTs

-

1.517

0.202

Fe‐Ni@C

-

1.61

0.356

1.37

1.55

0.011

10

6

51

1.42

1.72

0.053

100

LDH

Ni2P NF/CC

NF/NiMoO-Ar

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For Table of Contents Use Only

Synopsis An ultra-efficient bifunctional electrocatalyst for hydrogen evolution and urea electro-oxidation reactions was successfully synthesized based on highly dispersed tiny nickel-palladium bimetallic nanoparticles incorporated ordered mesoporous carbon.

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Bimetallic NiPd Nanoparticles Incorporated Ordered Mesoporous

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