Sulfur-Doped Graphene-Supported Nickel-Core Palladium-Shell

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Sulfur-doped graphene-supported nickel-core palladium-shell nanoparticles as efficient oxygen reduction and methanol oxidation electrocatalyst Dimitrios K Perivoliotis, Yuta Sato, Kazu Suenaga, and Nikos Tagmatarchis ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00631 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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ACS Applied Energy Materials

Sulfur-doped Graphene-supported Nickel-Core Palladium-Shell Nanoparticles as Efficient Oxygen Reduction and Methanol Oxidation Electrocatalyst Dimitrios K. Perivoliotis#, Yuta Sato¶, Kazu Suenaga¶ and Nikos Tagmatarchis*# #

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece.

¶

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan.

Corresponding Author * E-mail: [email protected] (N. Tagmatarchis)

ACS Paragon Plus Environment

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Abstract

The design of novel platinum-free highly efficient electrocatalysts for the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) has been regarded as the core challenge toward the development of commercially available fuel cell devices. In this contribution, a facile strategy was applied to prepare surfactant-free nickel-core palladium-shell nanoparticles, abbreviated as Pd@NiNPs, uniformly distributed on sulfur-doped graphene (SG). The Pd@NiNPs/SG hybrid material was realized by employing a modified polyol method for the insitu preparation of NiNPs/SG, followed by deposition of a Pd shell through the galvanic replacement method and complementary characterized by Raman and IR spectroscopy, STEM/EELS, SEM and EDS as well as XRD and TGA. The Pd-to-Ni molar ratio was optimized in view of the hybrid’s electrocatalytic performance. Interestingly, the Pd@NiNPs/SG hybrid was proved to be a highly efficient and stable electrocatalyst towards ORR and MOR. It exhibited comparable initial ORR performance with the benchmark Pd/C catalyst and importantly greater stability, as after 2,000 potential cycles it possesses 36% and 67% higher diffusion-limited and kinetic current density, respectively, having a minimal loss of its initial activity. Further investigations on the reaction kinetics showed a 4-electron direct reduction of oxygen to water for the Pd@NiNPs/SG as well as Tafel slopes of -48/-116 mV dec-1, values much close to those proposed for the polycrystalline platinum. In addition, Pd@NiNPs/SG revealed enhanced MOR specific activity by 82% over Pd/C (2.26 mA cm-2 vs. 1.24 mA cm-2) and by far better antipoisoning abilities. Overall, Pd@NiNPs/SG hybrid is a highly efficient and low cost electrocatalyst, revealing huge potential for application in high-performance energy conversion devices.


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Keywords: sulfur-doped graphene, core-shell nanoparticles, hybrid electrocatalysts, oxygen reduction reaction, methanol oxidation reaction 1.

Introduction The world’s rapidly growing demand for energy along with the depletion of fossil fuel

reserves render the exploration of new environmentally friendly energy sources an urgent issue. Fuel cells, based on potentially renewable fuels, such as hydrogen and methanol, have attracted considerable attention as the most promising power sources for portable electronic devices and transportation vehicles due to their high energy conversion efficiency.1,2 The most important cathodic reaction involved in fuel cells is the oxygen reduction reaction (ORR), which can be proceeded through different pathways. However, ORR is kinetically challenging as characterized by sluggish kinetics and high over-potential, thus significantly limiting the performance of fuel cells. Hence, it is absolutely necessary to incorporate catalysts in the fuel cell device to improve activity. Besides ORR, methanol oxidation reaction (MOR) is another critical reaction in energy conversion applications, and especially in direct methanol fuel cells. MOR is also characterized by slow kinetics, involving the production of various intermediates (i.e. CO, formic acid and formaldehyde), and thus requires active catalytic sites not only for the methanol adsorption and oxidation but also for the oxidation and desorption of the adsorbed intermediates. Currently, platinum and its alloys are commonly employed to catalyze both ORR and MOR. Recent efforts are focused on replacing Pt as cathode electrocatalysts owed to their prohibitive high cost, low abundance and limited stability/durability in the harsh reaction conditions. In addition, when considering MOR, another drawback of using Pt is the poisoning effects caused by the adsorption of the carbonaceous reaction intermediates on the catalyst surface. In this direction, the development of novel, stable enough, low cost, Pt-free catalysts with improved


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electrocatalytic activity is a key challenge for the sustainable commercialization of the fuel cell technology.3-5 Graphene, owed to its remarkable physical and chemical properties such as large surface area, excellent electrical conductivity, and good chemical stability has received significant research interest as a novel platform for the construction of highly efficient electrocatalysts.6 In the same context, graphene heteroatom doping is regarded as an effective way to govern the physicochemical properties of graphene, resulting in enhanced electrocatalytic activities.7,8 Generally speaking, in the case of ORR, the origin of the enhanced catalytic activities of graphene-doped materials is attributed to the electronegativity difference between carbon and the doping element, which polarizes the adjacent carbon atoms in the graphene lattice, hence, facilitating oxygen adsorption and dissolution.7 Although nitrogen-doped graphene has been thoroughly explored as both metal-free electrocatalyst and metal nanoparticles (NPs) supporting material,9-14 the electrocatalytic properties screening of other heteroatom-doped graphene is scarce. Particularly focusing on sulfur-doped graphene (SG), with sulfur being more electron rich than carbon, an n-type doping effect in graphene analogous to that of nitrogen-doped is provided.15 However, unlike other n-type dopants, the difference in electronegativity between sulfur and carbon as compared to nitrogen and carbon is small, implying that a different mechanism for improved electrocatalytic activity is prevalent in SG.16 Briefly, sulfur dopant modifies graphene’s electronic structure by inducing a non-uniform spin density distribution, which derives from the mismatch of the outermost orbitals of sulfur and carbon atoms, being responsible, along with the charge density, for the SG intrinsic electrocatalytic activity.16,17 Therefore, SG is considered as efficient metal-free electrocatalyst, while electrochemical investigations showed improved activity along with extreme long-term stability and


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selectivity.18-21 These findings clearly indicate that there is ample room for significant advances when enabling SG as catalyst component en route to the next generation of fuel cell hybrid electrocatalysts.22 Exploring new hybrid electrocatalysts with enhanced ORR and MOR activity and stability for replacing Pt is a prerequisite for the sustainable commercialization of fuel cell devices. In this context, Pd-based nanocatalysts have been employed, since Pd presents comparable catalytic activities with Pt, albeit with lower cost.23,24 Importantly, strategies employing noble metals alloying with transition metals (Fe, Ni, Co, etc.) have also been applied both to reduce the amount of the noble metal used and enhance the electrocatalytic activity by providing a modified electronic structure, hence, offering more active sites to the reactants, according to the “alloying effect” route.25 Moreover, core-shell NPs are currently of immense interest since they not only save precious metals but also present enhanced catalytic performance mainly ascribed to the surface segregation of the active metals, according to the “geometric effect”. Needless to mention, by coating precious metal catalysts as a monolayer on the surface of non-precious metal-based cores, highly efficient and cost effective electrocatalysts can be produced.26,27 In this frame, during last years, a series of carbon black supported metal NPs based on Pd and transition metals have revealed great potential in catalyzing the reduction of oxygen2833

as well as the methanol electro-oxidation,34 depending on their composition, morphology and

size. To the best of our knowledge, hybrids based on SG and PdM (M: transition metal) NPs with core-shell structure as ORR and MOR electrocatalysts have yet to be reported. The present work goes beyond the current state-of-the-art, by employing SG as a novel platform for immobilizing nickel-core palladium-shell NPs, abbreviated as Pd@NiNPs, aiming at the development of highly efficient and low Pd loading ORR and MOR hybrid electrocatalysts.


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Herein, we succeeded on preparing the Pd@NiNPs/SG hybrid material, outperforming in terms of ORR and MOR activity and importantly stability not only the reference materials employed but also the benchmark Pd/C catalyst. The exceptional performance of Pd@NiNPs/SG was attributed to the (a) inherit ORR activity of SG, (b) homogeneous Pd@NiNPs dispersion onto the SG surface, (c) synergistic effect between Pd/NiNPs and SG, (iv) core-shell nanostructure morphology of Pd@NiNPs, and (v) absence of any capping agent (surfactant) for the immobilization of Pd@NiNPs onto SG. 2.

Experimental 2.1. Materials All chemicals and solvents were purchased from Aldrich and used without further

purification unless otherwise stated. 2.2.Preparation of Pd@NiNPs/SG For the SG synthesis, 40 mg of graphene oxide (GO) were dispersed in 60 mL diethylene glycol methyl ether, and ~700 mg of Lawesson’s reagent were added to the solution and sonicated for 1 h. The reaction mixture was refluxed at 185 oC for 48 h under nitrogen atmosphere and then allowed to cool at room temperature. The resulting suspension was washed with methanol and vacuum dried. For a typical synthesis of NiNPs/SG, 8 mg of SG were ultrasonically dispersed in 15 mL of ethylene glycol to form a black homogeneous dispersion. Subsequently, 8.5 mg of nickel acetate tetrahydrate Ni(OAc)2·4H2O dissolved in 10 mL of ethylene glycol were mixed with the as-prepared polyol solution, degassed and stirred for 30 min. Then, 6.5 mg NaBH4 and 340 µL 1M NaOH dissolved in 4 mL ethylene glycol were added gradually and the reaction mixture was stirred under nitrogen for 60 min. The resulting suspension was washed with ethanol and vacuum dried. The designed NiNPs loading was 20% wt.


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Next, the Pd@NiNPs/SG electrocatalyst was prepared via the galvanic replacement method. Typically, 6.5 mg NiNPs/SG and the appropriate amount of potassium tetrachloropalladate (K2PdCl4) were dissolved in 1 mL distilled water. The resulting solution was uniformly dispersed by sonication for 10 min, and then vigorously stirred for 24 h at room temperature. The resulting suspension was centrifuged with distilled water and vacuum dried. The designed Pd-to-Ni molar ratio was 1:4 and 1:10, yielding Pd@NiNPs (1:4)/SG and Pd@NiNPs (1:10)/SG hybrids, respectively. For comparison, PdNPs supported on SG and GO (denoted as PdNPs/SG and PdNPs/GO) were also obtained directly via a modified polyol method (for further details see Supporting Information). 2.3. Physical characterization Scanning Electron Microscope (SEM) imaging and Energy Dispersive X-ray Spectroscopy (EDS) were performed using a FE-SEM (model JSM-7610F) equipped with an EDAX (X-ACT, Oxford instrument). Scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) were performed using a JEOL JEM-2100F microscope equipped with a Gatan Quantum electron spectrometer at an electron accelerating voltage of 60 kV. EELS chemical maps of Pd and Ni were obtained by measuring their M4,5 and L2,3 edges, respectively, at each point of a scanned area. The crystalline phase of the Pd@NiNPs/SG and PdNPs/SG was identified by X-ray diffraction (XRD) using D8 Advance X-ray diffraction (Bruker axs company, Germany) equipped with Cu-KR radiation (λ) 1.5406 (Å). Infrared (IR) spectra were acquired on a Fourier Transform IR spectrometer (Equinox 55 from Bruker Optics) equipped with a single reflection diamond ATR accessory (DuraSamp1IR II by SensIR Technologies). Raman measurements were performed with a Renishaw confocal spectrometer at 514 nm. The thermogravimetric analysis (TGA) was carried out using a TGA


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Q500 V20.2 Build 27 instrument by TA under a nitrogen inert atmosphere. UV-Vis spectra were recorded on a Perkin-Elmer (Lambda 19) UV–Vis spectrophotometer. 2.4. Catalyst preparation and deposition onto the working electrode To prepare the catalyst ink, 4.0 mg of the hybrid catalytic powder were dispersed in a mixture of solvents (1 mL) containing water, isopropanol, and 5% Nafion (v/v/v=4:1:0.02) and sonicated for 30 min. The working electrode was first cleaned through polishing by 6, 3 and 1 mm diamond pastes, rinsed with deionized water, and sonicated in double-distilled water. Then, 3 µL aliquots of the catalyst ink were casted on the electrode surface and dried at room temperature. 2.5. Electrochemical testing All the electrochemical measurements were carried out in a standard three-compartment electrochemical cell using a rotating disk electrode (RDE) setup from Metrohm Autolab connected to an EG&G Princeton Applied Research potensiostat/galvanostat (Model PARSTATR 2273A). As counter electrode, a platinum wire was used and as reference an Hg/HgO (0.1 M KOH) electrode was placed into Luggin capillary. The working electrode was a RDE with glassy carbon (GC) disk (geometric surface area: 0.071 cm2). The ORR measurements were realized at room temperature in O2-saturated 0.1M KOH aqueous solution. Linear sweep voltammetry (LSV) measurements on RDE of different catalysts were conducted at different rotation rates recorded with a scan rate of 5 mV s−1. Accelerated durability tests were performed by cycling the electrode potential under oxygen saturation between -0.8 V and 0.2 V (vs. Hg/HgO) at a scan rate of 200 mV s

−1

. Chronoamperometric

measurements for (Pd/NiNPs)/SG, PdNPs/SG and Pd/C samples were probed at -0.45 V (vs.


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Hg/HgO) at a rotation rate of 1600 rpm for 10,000 s. The kinetic current densities (jk) were calculated using the Koutecky–Levich (K-L) equation: 1/j = 1/jd + 1/jk (1) where j and jd are the experimentally measured and the diffusion-limited current density, respectively. The number of electrons transferred in the reduction of one O2 molecule (n) can be determined by modifying the K–L equation as follows: 1/j = 1/jd + 1/jk = 1/Bω1/2 +1/jk

(2)

where ω is the angular velocity and B is K-L slope given by the following equation: B = 0.20nFC0D02/3ν-1/6 (3) Here, n is the electron transfer number, F is the Faraday constant (F = 96485 C/mol), D0 is the diffusion coefficient of O2 (D0 =1.9 ×10-5 cm2 s-1), ν is the kinematic viscosity of the solution (ν =0.01 cm2 s-1) and C0 is the concentration of dissolved O2 in the solution (C0= 1.2 × 10-6 mol cm3

). The constant of 0.2 is adopted when the rotation speed is expressed in rpm. Tafel plots

(Potential vs. log(jk) ) were calculated in the mixed kinetic–diffusion region from the following equation: jk = j/(jd – j) (4) at a single electrode rotation rate (ω = 1600 rpm). The activity of the catalysts towards the MOR was measured in a mixture of aqueous 0.5 M KOH and 0.5 M CH3OH solution, by cycling the electrode potential under nitrogen saturation between -0.85 V and 0.15 V (vs. Hg/HgO) at a scan rate of 10 mV s −1. Cyclic voltammetry tests without presence of methanol were also performed at the rage of -1.0 to 0.0 V (vs. Hg/HgO) under nitrogen saturation at a scan rate of 25 mV s−1. 3.

Results and discussion


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3.1.

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Preparation

The preparation of Pd@NiNPs/SG hybrid material is illustrated in Figure 1. First, SG was realized upon treatment of GO with Lawesson’s reagent as both sulfur source and reducing agent. Next, a modified polyol method was employed for the in-situ preparation of NiNPs/SG hybrid, followed by the under potential deposition of a Pd shell through the galvanic replacement method. Interestingly, this method is a feasible way to ensure the formation of the core-shell structure, avoiding the separate nucleation of the two metals. In our case, the spontaneous Ni replacement by Pd occurred according to equation (5): NiNPs/SG + PdCl42-→ Pd@NiNPs/SG + Ni2++ 4Cl- (5) This is a thermodynamically favorable reaction since the standard potential of the Ni2+/Ni couple is -0.257 V vs. SHE, lower than that of the PdCl42-/Pd couple (+0.620 V vs. SHE). The successful Ni replacement was confirmed and quantified by measuring the UV-Vis absorption of the released Ni2+ species in the supernatant solution after the Pd@NiNPs/SG synthesis (Supporting Information, Figure S1). Based on these data the Pd-to-Ni mass ratio was calculated to be 21:79 and 10:90, for the Pd@NiNPs (1:4)/SG and Pd@NiNPs (1:10)/SG hybrids, respectively. Last but not least, the incorporated sulfur species on the graphene framework serve as anchoring sites for Ni-ion nucleation and subsequent NPs growth, stabilizing the in-situ formed NPs without using any additional capping agents or surfactants, which normally impair the hybrid’s catalytic activity.6


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Figure 1. Illustrative scheme for the preparation of Pd@NiNPs/SG hybrid. 3.2.

Characterization

Direct proof for the formation of SG was given by ATR-IR and Raman spectroscopy. In more detail, although the IR spectrum of GO is governed by bands centred at 1720 cm−1 attributed to carbonyl stretching vibrations of the carboxylic acid units, and at 1220 and 1050 cm−1 due to ether moieties, after treatment with the Lawesson’s reagent, not only those bands were faded, but also a new and relatively broad band was appeared at around 1080 cm−1 owed to C-S (Figure 2A). Notably, the stretching vibration mode for C=C was shifted from 1580 cm−1 in GO to 1640 cm−1 in SG, as a result of the partial restoration of the sp2 conjugated carbon network. Raman analysis further confirmed the accomplishment of S-doping and the simultaneous reduction of GO forming SG. Graphene-based materials show two characteristic Raman bands, namely, the D-band at ~1350 cm−1 related to the presence of structural defects in sp2-hybridized carbon systems and the G-band at ~1600 cm−1 assigned to the in-plane vibration of the sp2 C–C bonds. The relative intensity of the D/G ratio is widely used to estimate the disorder degree in graphene-based materials. Sulfur doping and reduction can induce drastic changes to position and intensity of the two Raman modes. In general, the G-band is sensitive to chemical doping and commonly used to identify the doping type of graphene. In our case, the Gband of SG was found at 1591 cm-1, down-shifted by ~9 cm−1 and stiffened as compared to that of GO (Figure 2B), which is a typical behavior of graphene n-type substitutional doping.35 Besides, the D/G intensity ratio for SG (1.12) was calculated higher than that of GO (0.75), suggesting an increased defect density in graphene sheets due to the doping procedure, further proving the successful formation of graphene S-doping.36 Last, EDS and thermogravimetric


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analysis (TGA) provided additional proofs for the sucessful SG preparation (Supporting Information, Figure S2).

Figure 2. (A) ATR-IR, and (B) Raman spectra for SG and GO. The morphology and elemental composition of as-synthesized Pd@NiNPs/SG was investigated by SEM imaging and EDS analysis and compared with those owed to PdNPs/SG and PdNPs/GO employed as reference hybrid materials. Images of Pd@NiNPs (1:4)/SG and PdNPs/SG (Figure 3A and Supporting Information, Figure S3A-C, respectively) revealed a dense and uniform NPs distribution onto the SG surface, while the absence of sulfur dopant in GO resulted in PdNPs/GO hybrids, characterized by the presence of large agglomerates and a sparse distribution of individual PdNPs on graphene sheets (Supporting Information, Figure S3D-F). Hence, S-doping enhances the particle nucleation on graphene surface, resulting in a more uniform and dense NPs dispersion onto SG. Indeed, sulfur possesses high affinity to interact with metal NPs in general, thus, effectively stabilizing them on the SG substrate.22 Regarding the chemical composition of Pd@NiNPs (1:4)/SG, EDS analysis confirmed the existence of carbon, oxygen, sulfur, nickel and palladium elements (Figure 3B). Furthermore, to get meaningful insight on the Pd@NiNPs (1:4)/SG structure, scanning transmission electron microscopy (STEM) imaging along with energy electron loss microscopy (EELS) chemical mapping was performed


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(Figure 3C-D). Annular dark-field (ADF) STEM images confirmed the uniform distribution of Pd@NiNPs with an average size of 3-4 nm onto SG surface. Electron energy loss spectroscopy (EELS) chemical mapping revealed a core-shell structure Pd@NiNPs with a Ni-rich core (~2 nm) and a Pd-rich shell (~1-2 nm). STEM images and EELS chemical mapping for Pd@NiNPs (1:10)/SG (Supporting Information, Figure S4) were also obtained for comparison reasons. The mean NP size was deduced to be around 3 nm, while EELS chemical mapping revealed the existence of both Ni and Pd. However, the presence of Ni in Pd@NiNPs (1:10)/SG was dominant, indicating that the 1:10 Pd-to-Ni molar ratio was not enough, when considering a well-defined core-shell structure. Hence, based on these observations, the 1:4 molar ratio was the more favorable one. Figure 3E shows the XRD patterns of binary core-shell Pd@NiNPs (1:4)/SG and monometallic PdNPs/SG hybrids. Both materials present a broad peak at 2θ = ~25ο assigned to SG, further proving the effective GO reduction by the Lawesson’s reagent.37 While PdNPs/SG pattern presents the stardard diffraction peaks of Pd, in the case of Pd@NiNPs (1:4)/SG characteristic diffraction peaks for both metals can be observed,38 demonstrating the succesful formation of bimetallic phases of Ni and Pd. In more depth, the more dominant peaks appeared at 2θ = ~40.0o and ~44.0o correspond to the [111] crystalographic planes of the Pd and Ni, respectively, indicating the preferential orientation of [111] facet in Pd@NiNPs (1:4)/SG. Ιt worths also mentioning that no peaks related to metal oxides were detected. Last, the Raman spectra of PdNPs/SG and Pd@NiNPs (1:4)/SG (Supporting Information, Figure S5) was found to be almost similar with that of SG in terms of the shapes and positions of the Raman peaks, a trend that has been also reported in the literature.38


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Figure 3. (A) Representative SEM image, (B) EDS spectra obtained on SiO2–Si substrate, (C) ADF-STEM image, (D) EELS chemical mapping for Pd@NiNPs (1:4)/SG, and (E) XRD diagram for Pd@NiNPs (1:4)/SG as compared with that of PdNPs/SG. EELS chemical maps for Pd and Ni were obtained by measuring their M4,5 and L2,3 edges, respectively, at each point of the scanned area. 3.3.

Oxygen Reduction Reaction (ORR)

Linear sweep voltammetry (LSV) on rotating disk electrode (RDE) was employed to measure the ORR activity of Pd@NiNPs (1:4)/SG in an O2-saturated 0.1 M KOH aqueous solution and compare it with that of Pd@NiNPs (1:10)/SG, PdNPs/SG, PdNPs/GO, SG and commercially available palladium on carbon black (Pd/C, 20 wt % Pd). Figure 4A shows ORR polarization curves obtained at a rotation rate of 1600 rpm, where in all cases, a well-defined


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plateau, ascribed to the diffusion-limited potential region was recorded, following the mixed kinetic-diffusion control region. The kinetic current density (jk) for each material was determined at both -60 mV vs. Hg/HgO and half-wave potential (E1/2) based on the K-L equation. Electrocatalyst

stability

studies

involving

continuous

cyclic

potential

sweeps

and

chronoamperometric tests were performed. The LSVs recorded after 2,000 cycles, the corresponding diffusion-limited and kinetic current density as well as their chronoamperometric response are shown in Figure 4B-E and Supporting Information, Figure S6A-B. More detailed investigations on the ORR reaction mechanism were carried out by altering the rotation rate of RDE. Figure 5A, C show the ORR polarization curves at different rotation rates for Pd@NiNPs/SG and PdNPs/SG, revealing that the diffusion-limited current density increased with increasing rotation speed. The corresponding K-L plots, depicting the inverse current density (j-1) as a function of the inverse of the square root of the rotation speed (ω-1/2) at different potential values were constructed (Figure 5B, D). Regarding the SG electrocatalytic performance, it was demonstrated that the sulfurdoping process results in significant improvement in GO intrinsic ORR activity, which normally presents a substantial reduction process in the presence of oxygen (Figure 4A). Indeed, the oxygen reduction on the SG surface commences at more positive potential ca. -165 mV (vs. Hg/HgO), as compared to that owed to GO, followed by a sharper increase in the reduction current density. The diffusion-limited as well as the kinetic current density at E1/2 for SG were found to be 2.20 and 1.90 mA cm-2, approximately 4 and 10 times greater, respectively, compared to those of GO. These findings are mainly ascribed to the improvement in electrical conductivity arising from the partial restoration of the sp2 carbon network and the distortion of graphene’s electronic structure due to the presence of sulfur as dopant in SG.16 Furthermore, K-L


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analysis for the SG substrate (Supporting Information, Figure S7A, B) revealed that the twoelectron pathway was the most dominant, since the electron transfer number (n) was calculated to be between 1.9 and 2.1 at various potentials. This is also consistent with the observation that after the plateau the current increased, implying that hydrogen peroxide reduction concurred in the global ORR.39 Notably, the same trend was observed in the case of GO, indicating that the oxygen reduction on GO also proceeds by the two-electron pathway, however, presenting by far lower ORR activities.


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Figure 4. LSV ORR polarization curves obtained at a rotation speed of 1600 rpm for (A) all materials, and (B) Pd@NiNPs (1:4)/SG electrocatalyst, as compared to those due to Pd@NiNPs (1:10)/SG, PdNPs/SG and Pd/C, before and after 2,000 potential cycles; (C, D) corresponding diffusion-limited and kinetic current density values; (E) chronoamperometric response for ORR for Pd@NiNPs (1:4)/SG as compared to that of Pd@NiNPs (1:4)/SG, PdNPs/SG and Pd/C, in O2-


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saturated 0.1 M KOH solution at -0.45 V (vs. Hg/HgO) at a rotation rate of 1600 rpm for 10,000 sec. All measurements were conducted in O2-saturated 0.1 M KOH electrolyte and the corresponding LSV were recorded at a scan rate of 5 mV sec-1. In all graphs the current densities are normalized to the geometric electrode area. The Pd@NiNPs (1:4)/SG electrocatalyst exhibited comparable initial performance with the one owed to the benchmark Pd/C catalyst and clearly improved activity compared with that due to PdNPs/SG and PdNPs/GO (Figure 4A). Indeed, the ORR half-wave potential of the core-shell hybrid was determined to be -0.097 V (vs. Hg/HgO), being 18 and 79 mV more positive than that of PdNPs/SG and PdNPs/GO, respectively. Furthermore, the diffusion-limited current density for Pd@NiNPs (1:4)/SG reached 5.10 mA cm-2, while the kinetic current density measured at -60 mV vs. Hg/HgO was found to be 1.70 mA cm-2. Importantly, these values are increased by ~10% when compared with those of PdNPs/SG and dramatically improved when compared with those of PdNPs/GO. To study the effect of different ratios of nickel-to-palladium on the catalytic activity, the ORR performance of Pd@NiNPs (1:4)/SG was further compared with that of Pd@NiNPs (1:10)/SG. The results demonstrated a more positive half-wave potential by 14 mV as well as greater diffusion-limited and kinetic current density (at -60 mV vs. Hg/HgO) values by 12% and 38%, respectively, for the Pd@NiNPs (1:4)/SG catalyst. Overall, the initial ORR performance was in the order of Pd@NiNPs (1:4)/SG ≈ Pd/C > Pd@NiNPs (1:10)/SG ≈ PdNPs/SG > PdNPs/GO, highlighting that the core-shell Pd@NiNPs structure as well as the use of SG as support leads to electrocatalysts with similar ORR activity with the commercial ones, albeit with significantly reduced Pd loading. It can be also concluded that the Pd-to-Ni ratio plays a critical role on hybrid’s ORR activity and the optimum ratio was found to be 1:4. Moreover, the latter result was also supported by the ADF-STEM/EELS assays. Next, the electron-transfer number for both


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Pd@NiNPs (1:4)/SG, Pd@NiNPs (1:10)/SG and PdNPs/SG was calculated as ~4.0, indicating that the oxygen reduction was governed by a four-electron pathway, where O2 is directly reduced to H2O, without involving H2O2 formation as intermediate (Figure 5A-F). Generally, the fourelectron route is considered as more efficient and it is highly preferred in the fuel cell technology. On the other hand, the two-electron and the four-electron pathways coexist in the case of PdNPs/GO (Supporting Information, Figure S7C, D).


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Figure 5. ORR polarization curves at different rotation rates (400–3600 rpm), and the corresponding K-L plots for Pd@NiNPs (1:4)/SG (A-B) as compared to those of Pd@NiNPs (1:10)/SG and PdNPs/SG (C-D and E-F, respectively). All measurements were conducted in O2saturated 0.1 M KOH electrolyte and the corresponding LSV were recorded at a scan rate of 5 mV sec-1. In all graphs the current densities are normalized to the geometric electrode area.


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Durability is another critical parameter to evaluate the ORR performance of catalysts, especially when considering the application in real fuel cell devices. Figure 4B-D demonstrates that the Pd@NiNPs (1:4)/SG hybrid is by far more stable than the reference electrocatalysts, outperforming after 2,000 cycles of repetitive potential sweeps not only PdNPs/SG and PdNPs/GO, but also the commercial Pd/C catalyst. Needless to say, the core-shell based hybrid material presents ~14 mV more positive half-wave potential as well as ~36% and 67% greater diffusionlimited and kinetic current density values, respectively, than those registered for Pd/C. Interestingly, after cycling, less than 2 % and 10 % decrease in diffusion-limited and kinetic current density (at -60 mV vs. Hg/HgO) was recorded, respectively, for the Pd@NiNPs (1:4)/SG electrocatalyst, while the corresponding values for the commercial Pd/C were reduced more than 30 % and 55 %. These observations are consistent with the chronoamperometric measurements as, after 10,000 sec, Pd@NiNPs (1:4)/SG exhibited greater stability over PdNPs/SG and commercial Pd/C (Figure 4E). Impressively, the Pd@NiNPs (1:4)/SG hybrid preserves the 86% of its initial activity while the current loss for PdNPs/SG and Pd/C under the same conditions was as high as 37.2% and 29.7%, respectively. The long-term performance of Pd@NiNPs (1:10)/SG catalyst was also investigated and the results (Figure 4B-D) clearly demonstrated that after 2,000 cycles, the Pd@NiNPs (1:4)/SG continues to outperform its 1:10 counterpart. Needless to say, although its initial diffusion-limited current density remained almost unchanged, a significant half-wave potential negative shift of 24 mV followed by a ~55% decrease in its initial kinetic current density value (at -60 mV vs. Hg/HgO) was observed. Consequently, after potential cycling, the Pd@NiNPs (1:4)/SG possesses 25 mV more positive half-wave potential, increased diffusion-limited current density by ~14 % and more importantly 3.3 times greater kinetic


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current density than those of Pd@NiNPs (1:10)/SG, confirming that the optimum Pd-to-Ni ratio is 1:4. At this point, it should be highlighted that the ORR in aqueous alkaline media is a complex electrochemical reaction, in which various intermediates are involved (O, OH, O2−, HO2−), resulting in numerous possible mechanisms.40 To get meaningful insight into the ORR mechanism, the mass-transfer corrected Tafel plots (Figure 6), for Pd@NiNPs (1:4)/SG, Pd@NiNPs (1:10)/SG, PdNPs/SG, PdNPs/GO and Pd/C catalysts, were calculated. In general, the larger Tafel slope, the lower the catalytic activity is, as the overpotential increases faster with the current density.41 Two regions with distinct slopes were clearly distinguished, namely, in the low current density region, the slope value for both Pd@NiNPs/SG hybrids (i.e. Pd:Ni ratio 1:4 and 1:10) was -48 mV dec-1, and at high current densities, the slope value was approximately -116 mV dec-1. These values are very close to the corresponding values registered for the commercial catalyst (i.e. -47 and -112 mV dec-1, respectively). In general, the transition in Tafel slope is not associated with a change of the reaction mechanism, but it is rather ascribed to the potentialdependent formation of surface oxides that inhibit the adsorption of the oxygen molecules. In more depth, a value close to -120 mV at the high overpotential range indicates that the rate determining step is the transfer of the first electron to the oxygen molecule, while a Tafel slope around -60 mV at the low overpotential range suggests that the presence of surface oxides limits the ORR.12 Regarding PdNPs/GO, the calculated Tafel slopes were found to be -91 and -178 mV dec-1 at the low and high current densities, respectively, significantly higher as compared to those of PdNPs/SG and attributed to a change in the rate-determining step. In such cases, the adsorption of molecular oxygen as the rate determining step has been proposed.42 All in all, the higher Tafel slopes values for SG based hybrids confirm the major role of S-doping on the ORR activity of


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the hybrid electrocatalysts. Finally, it worth mentioning that after 2,000 potential cycles, only a slight increase in Pd@NiNPs (1:4)/SG Tafel slopes was observed (-56/ -153 mV dec-1), implying no change on ORR mechanism after durability tests.

Figure 6. Tafel plot for the Pd@NiNPs (1:4)/SG hybrid electrocatalyst as compared to that due to Pd@NiNPs (1:10)/SG, PdNPs/SG, PdNPs/GO and Pd/C. Data derived from Figure 4A. Importantly, the reasoning for the improved ORR activity and stability of Pd@NiNPs (1:4)/SG is attributed to the following reasons. First, the core-shell nanostructure with a Pd-rich shell could not only reduce the noble metal consumption but also takes advantage of the strain and ligand effects, which promote the electrochemical activity and stability of Pd@NiNPs (1:4)/SG hybrid.32,43 At the same time, the inherit electrocatalytic activities of SG benefit the hybrid’s performance, since SG not only acts as substrate for metal NPs but also contributes to the total hybrid’s ORR activities. Next, the good affinity of metal NPs with sulfur atoms implies enhanced interactions between them and SG substrate, enabling the oxygen absorption and dissolution on the metal NPs surface.22 Furthermore, the homogeneous dispersion of Pd@NiNPs


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is a critical factor as the presence of agglomerates may significantly reduce the electrochemical active surface area, leading to poor ORR activities.44 Last but not least, the absence of any capping agent during the synthesis of the hybrid material further improves the catalytic activity by providing more active sites for the ORR on the NP surface.36 Table 1 summarizes the ORR parameters for all electrocatalysts before and after 2,000 cycles. Table 1. ORR electrochemical parameters for Pd@NiNPs (1:4)/SG as compared to Pd@NiNPs (1:10)/SG, PdNPs/SG, PdNPs/GO, SG, GO, and Pd/C.

Onset potential Catalyst

Halfwave potential

Diffusionlimited current density a

[mV vs. [mV vs. Hg/HgO] Hg/HgO] [mA cm-2]

Kinetic current density b

Kinetic current density c

Tafel slopes

[mA cm-2]

[mA cm-2]

[mV dec1 ]

Electron transfer number

Pd@NiNPs (1:4)/SG

+20

-97

5.10

4.70

1.70

-48 / -116

3.9 4.0

Pd@NiNPs (1:4)/SG after c

+18

-110

5.00

4.60

1.50

-56 / -153

-

Pd@NiNPs (1:10)/SG

+10

-111

4.45

4.05

1.05

-48 / -115

3.9 4.1

-135

4.40

3.95

0.45

-46/ -135

-

Pd@NiNPs (1:10)/SG after -5

–

–

d

PdNPs/SG

+44

-115

4.60

4.50

1.60

-66 / -167

4.0 4.1

PdNPs/SG after d

+28

-125

3.95

3.85

1.20

-60 / -178

-

PdNPs/GO

+10

-176

3.00

2.80

0.40

-91 / -178

2.9 3.2

PdNPs/GO after d -15

-207

2.80

2.70

0.15

-

-

SG

-275

2.20

1.90

-

-58 / -102

1.9 2.1

-165

–

–

–


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GO

-227

-395

0.60

0.20

-

-

-

Pd/C

+13

-93

5.40

5.15

2.00

-47 / -112

3.8 4.2

+17

-124

3.70

3.60

0.90

-62 / -137

-

Pd/C after

c

–

a

at 1600 rpm rotation rate (at ~0.50 V vs. Hg/HgO); b calculated at half-wave potential (E1/2) by K-L equation; c calculated at -60 mV vs. Hg/HgO by K-L equation; d after 2,000 potential cycles.

An overview of the recently developed Pd-based ORR electrocatalysts is provided in Table 2. Briefly, different carbon-based materials have been employed as substrates for PdM (M=Fe, Ni, Cu) NPs as ORR electrocatalysts. In this context, carbon black supported PdNi32 and PdCuM (M=Mo, Ni)28 exhibited high current density values (5.6 and 5.2 mA cm-2, respectively), along with extreme durability after cycling. In another example, PdCu NPs with different Pd-to-Cu ratios

30,41

were tested and the results for the optimum system30 suggested a jd value of 4.6 mA

cm-2 and sufficient stability. In a recent work, PdNi-based ternary NPs on nitrogen doped graphene33 possessed a diffusion-limited current density value of 3.5 – 3.8 mA cm-2 as well as a ~5% loss in its initial activity after 5,000 potential cycles. Based on those reports and data, the currently prepared and examined Pd@NiNPs (1:4)/SG hybrid can be classified among the top rated electrocatalysts towards ORR.

Table 2. Comparison of the ORR performance of Pd@NiNPs (1:4)/SG with that of various recently reported Pd-based electrocatalysts.

Catalytic System

ORR performance in alkaline environment

Ref.

Cu3Pd NPs (5.3 nm) supported on graphene

Half-wave potential: -0.15 V vs. SCE Diffusion-limited current density: 3.0 mA cm-2 at 1600 rpm Electron transfer number: 4.0

41


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Durability: after 3,000 cycles, 8.8% loss in initial current density PdCu NPs (6.8 nm) supported on graphene oxide

Half-wave potential: -0.15 V vs. SCE Diffusion-limited current density: 4.6 mA cm-2 at 1600 rpm Electron transfer number: 3.9 - 4.0 30 Durability: after 1,000 cycles, no changes in initial current density

PdCuM (M=Mo, Ni) ternary metal NPs (~5 nm) supported on carbon black

Half-wave potential: 0.87 V vs. RHE Diffusion-limited current density: 5.6 mA cm-2 at 1600 rpm Electron transfer number: 4.0 28 Durability: after 10,000 cycles, no obvious change in its initial activity

FePd3 NPs (

Sulfur-Doped Graphene-Supported Nickel-Core Palladium-Shell

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