This is the accepted version of the following article: Liu J., Luo Z., Li J., Yu X., Llorca J., Nasiou D., Arbiol J., Meyns M., Cabot A.. Graphene-supported palladium phosphide PdP 2 nanocrystals for ethanol electrooxidation. Applied Catalysis B: Environmental, (2019). 242. : 258 - . 10.1016/j.apcatb.2018.09.105, which has been published in final form at https://dx.doi.org/10.1016/j.apcatb.2018.09.105 © https://dx.doi.org/10.1016/j.apcatb.2018.09.105. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Accepted Manuscript Title: Graphene-supported palladium phosphide PdP2 nanocrystals for ethanol electrooxidation Authors: Junfeng Liu, Zhishan Luo, Junshan Li, Xiaoting Yu, Jordi Llorca, Déspina Nasiou, Jordi Arbiol, Michaela Meyns, Andreu Cabot PII: S0926-3373(18)30948-2 DOI: https://doi.org/10.1016/j.apcatb.2018.09.105 Reference: APCATB 17088 To appear in: Applied Catalysis B: Environmental Received date: 15-6-2018 Revised date: 22-9-2018 Accepted date: 30-9-2018 Please cite this article as: Liu J, Luo Z, Li J, Yu X, Llorca J, Nasiou D, Arbiol J, Meyns M, Cabot A, Graphene-supported palladium phosphide PdP2 nanocrystals for ethanol electrooxidation, Applied Catalysis B: Environmental (2018), https://doi.org/10.1016/j.apcatb.2018.09.105 This is a PDF file of an unedited manuscript that has been accepted for publication. 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Graphene-supported palladium phosphide PdP2 nanocrystals for ethanol electrooxidation Junfeng Liu,a Zhishan Luo,a Junshan Li,a Xiaoting Yu,a Jordi Llorca,b Déspina Nasiou,c Jordi Arbiol, c,d Michaela Meyns,a,* Andreu Cabot a,d,* PT a Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930 Barcelona, Spain RI b Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research SC Center in Multiscale Science and Engineering, Universitat Politecnica de Catalunya, Eduard Maristany 10-14, 08019 Barcelona, Spain U c N Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain A M d ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Catalonia, Spain * Corresponding Author D TE michaela.meyns@awi.de, acabot@irec.cat EP CC A 1 Graphical abstract PT RI SC U N A M D Highlights  A new procedure was developed to produce single-phase PdP2 crystals in the nanometer size TE region.  PdP2 nanocrystals were applied as electrocatalysts for ethanol oxidation reaction for the first time. EP  PdP2 nanocrystals supported on reduced graphene oxide showed excellent performances, comparable to the best Pt- and Pd- based catalysts.  PdP2 and PdP2/rGO nanocrystal-based catalysts demonstrated improved stability when CC compared with Pd and Pd/rGO. A ABSTRACT We present a procedure to produce single-phase PdP2 nanocrystals (NCs). The approach involves the reaction of palladium(II) acetylacetonate and hexamethylphosphoroustriamide to nucleate defective Pd5P2 nanoparticles that subsequently, with further phosphorous incorporation, 2 crystallize into PdP2. The synthesized PdP2 NCs were supported on reduced graphene oxide (rGO) and applied as electrocatalysts for ethanol oxidation. The activity of PdP2 toward the ethanol oxidation reaction (EOR) was over a threefold higher than that of Pd NCs prepared under similar conditions. Even better performance was obtained from PdP2 NCs supported on rGO, which showed current densities up to 51.4 mA cm-2 and mass activities of 1.60 A mg-1Pd, that is 4.8 and 15 times higher than Pd NCs. Besides, PdP2 NCs and PdP2/rGO catalysts showed improved PT stability during EOR than Pd NCs and Pd/rGO. RI Keywords: Palladium diphosphide; colloidal synthesis; electrocatalysis; ethanol oxidation SC U 1. Introduction N Direct liquid fuel cells are electrochemical devices able to directly convert chemical energy A stored within liquid fuels into electricity. Among the possible liquids, ethanol has a high energy M density (8.01 KW h kg-1) [1,2], low toxicity, a relatively high boiling point for safe storage and transportation, and the potential to be bio-sourced in large quantities from the fermentation of D biomass. Owing to these advantages, direct ethanol fuel cells (DEFCs) have become one of the most appealing alternatives to electrochemical batteries and conventional fossil fuel-based TE combustion engines in numerous fields of application. However, the high price and limited performance of current fuel oxidation electrocatalysts is limiting their deployment. These EP limitations, extensive to most fuel cell technologies, are particularly severe in DEFCs since the complete ethanol oxidation reaction (EOR) involves the release of 12 electrons per molecule and CC the cleavage of a C-C bond. Extensive research on electrocatalysts for DEFCs has been carried out on acidic conditions, A which allow taking advantage of convenient proton exchange membranes for the device fabrication. However, the slow reaction kinetics and low stability of electrocatalysts in these conditions are very tough barriers to be overcome [3,4]. As an alternative, effort has been moved toward developing electrocatalysts for alkaline-type DEFCs. The enhanced kinetics of the catalytic 3 reaction in alkaline solution increases the efficiency in the use of precious metals and even permits the use of less expensive catalysts. Pt and Pt-based alloys have been extensively studied as the most active catalysts for DEFCs. Alternatively, Pd is a more abundant and less demanded element, what currently translates in significantly lower costs. Pd [5] and Pd-based electrocatalysts, such as PdNi [6,7], Pd2Ru [8], PdCu PT [9-11], PdAu [12] and Pd2Sn [4,13], have demonstrated electrocatalytic activities in the electrooxidation of different types of liquid fuels and particularly ethanol comparable to those of RI Pt-based catalysts. Pd and Pd-based materials are also excellent catalysts for the Suzuki coupling reaction [14-16] and oxygen reduction reaction [17]. An especially interesting case is that of Pd- SC based phosphides, which have shown excellent activity toward hydrogen evolution, oxygen reduction, formic acid oxidation and hydrodesulfurization [18,19]. Additionally, palladium U phosphides are characterized with high electrical conductivities and show improved corrosion N resistance with respect to Pd. However, surprisingly, palladium phosphides have not been investigated as electrocatalysts for the EOR. A M Actually, few attempts have been done to produce palladium phosphide NCs at all. Most attempts to produce palladium phosphides were based on the reaction of premade Pd with a D phosphorus precursor such as trioctylphosphine (TOP) or white phosphorous at relatively high temperatures [18,20-22]. Following this strategy, quasi amorphous palladium phosphide particles TE with irregular shapes and broad size distributions were obtained [21,22]. The synthesis of crystalline palladium phosphide NCs have proved to be very challenging. Single-phase Pd5P2 EP crystallites with a few hundred nanometers were prepared using Pd(acac)2 as metal precursor and TOP as phosphorus precursor [19]. Using the same strategy, Pd5P2 nanoparticles encapsulated CC within mesoporous silica were also prepared [19]. Herein, we report a procedure to produce single-phase colloidal PdP2 NCs. Compared with A previous synthesis strategies based on the reaction of premade metallic Pd with a phosphorous source, our protocol involved the nucleation of Pd5P2 and their subsequent crystallization and growth into PdP2 NCs with the incorporation of additional P. After ligand exchange, PdP2 NCs were uniformly deposited on reduced graphene oxide sheets and tested as electrocatalyts for the EOR. 4 2. Experimental 2.1. Chemicals PT Hexamethylphosphoroustriamide (HMPT, 97 %), and palladium(II) acatylacetonate (Pd(acac)2, Pd 34.7 wt%) were purchased from Alfa Aesar. Oleylamine (OAm, approximate C18 content 80- RI 90 %) was purchased from ACROS Organics. TOP (97 %), methylamine hydrochloride (MAHC, 98 %), ammonium thiocyanate (NH4SCN, 99 %), ammonium chloride (NH4Cl, 99.5 %), potassium SC hydroxide (KOH, 85 %), Nafion (5 wt% in a mixture of low aliphatic alcohols and water), graphite, potassium permanganate (KMnO4, 99 %), sodium nitrate (NaNO3, 99 %), sulfuric acid (H2SO4, U 95-98 %), hydrochloric acid (HCl, 37 %), hydrogen peroxide (H2O2, 30 wt% in H2O) were obtained from Sigma-Aldrich. Chloroform, ethanol and acetone were of analytical grade and N obtained from various sources. MilliQ water was obtained by using a PURELAB flex from ELGA. A All chemicals were used as received without further purification. M 2.2. Synthesis of PdP2 NCs D Caution: Because this procedure involves the use and high-temperature decomposition of HMPT TE that can liberate toxic and flammable phosphine gas or toxic phosphorus oxides, the reaction should only be carried out by appropriately trained personnel using rigorously air-free conditions EP and the Schlenk line should be connected with gas absorption solutions. In a typical synthesis, 60.9 mg (0.2 mmol) of Pd(acac)2, 67.5 mg (1 mmol) of MAHC or 53.5 CC mg (1 mmol) ammonium chloride, and 10 mL of OAm were mixed in a 50 mL three-neck flask. The reaction system was purged with argon at 120 °C for 1 h. Then 0.1 mL (0.57 mmol) of HMPT A was injected into the solution and the temperature was further increased to 300 °C in 20 min. During the heating up, the solution became black at around 250 °C. The flask was maintained at 300 °C for 1 h before the solution was allowed to cool to room temperature. The isolation of the resultant black precipitate was achieved with excess ethanol followed by centrifugation at 5000 5 rpm (3200 g). Purification was achieved by multiple dispersion/precipitation steps using chloroform and ethanol. Finally the powder was suspended in 5 mL of chloroform in a vial. 2.3. Synthesis of Pd NCs Reference Pd NCs were prepared in a similar way as PdP2 NCs, i.e. by mixing the same amounts PT of Pd(acac)2, MAHC and OAm and at the same condition as in the synthesis of PdP2 NCs, but without injection of HMPT. The reaction was kept at 300 °C for 1 h and then washed following RI the same procedures as in the synthesis of PdP2 NCs. SC 2.4. Synthesis of rGO U Graphene oxide was produced by a modified Hummers method as originally presented by Kovtyukhova [23]. Briefly, in a 500 mL flask, 2.0 g of graphite, 1.5 g of NaNO3 and 96 mL of N concentrated H2SO4 were vigorously stirred in ice-water bath. Then, 9.0 g of KMnO4 was slowly A added to the flask over 30 min. Stirring was continued for 1.5 h in the ice-water bath and then M followed by another 2 h at 35 °C. Afterward, 280 mL of H2O was added into the mixture. During the process, the solution was heated to 98 °C and kept at this temperature for 2 h. 5 mL of 30 wt% H2O2 were added to the solution after the temperature was decreased to 60 °C. To remove ionic D impurities, the obtained solution was washed by vacuum filtration and rinsing with aqueous 3 wt% TE HCl for 3 times, followed by H2O for another 3 times. The resulting product was dried under vacuum at 60 °C and then calcined in a tube furnace at 500 °C for 2 h in flowing argon. As a result, EP the reduced graphene oxide (rGO) was obtained. CC 2.5. Ligand exchange The ligand exchange procedure used here was similar to that previously reported by Fafarman A et al. [24]. In a typical procedure, 0.5 mL of 130 mM NH4SCN in acetone was added to 1 mL of a dispersion of as-synthesized PdP2 NCs in chloroform (10 mg in 1 mL). The resulted solution was agitated for 1 min; a complete flocculation was observed within seconds. The slurry was centrifuged at 3000 rpm (1200 g) for 1 min and the supernatant discarded. Another 1 mL of acetone was added to the precipitate; the mixture was sonicated for 1 min and followed by centrifuge at 6 3000 rpm for 1 min. The washing steps were repeated another time and the obtained precipitate was dried at room temperature. 2.6. Catalyst preparation The catalyst ink for electrochemical measurements was prepared by mixing 5 mg of PdP2 NCs PT and 5 mg of rGO in 5 mL of deionized water. The mixture was sonicated for 1 h to form a homogeneous catalyst ink. For comparison, catalysts of Pd, Pd/C, Pd/rGO and PdP2 were also RI prepared through dissolving 10 mg of as-synthesized Pd, Pd and rGO (1:1 weight ratio) or PdP2 NCs in 5 mL of deionized water by sonication for 1 h. SC The working electrode was prepared using a glassy carbon electrode (GCE, 5 mm in diameter) U which was previously polished using diamond paper and 0.05 μm alumina slurry sequentially, followed by ultrasonication in ethanol first and water later, for 1 min each. The cleaned GCE was N dried under argon flow at room temperature. The electrode was modified by drop casting 10 μL of A the catalyst ink and posterior drying at room temperature. To immobilize the catalysts on the M electrode, 10 μL of a 0.5 wt% Nafion solution was dropped on the surface of the electrode. 2.7. Electrochemical measurements D TE Electrochemical measurements were conducted at room temperature on a BioLogic electrochemical workstation using a standard three-electrode cell with the modified GCE as EP working electrode, a Pt mesh as counter electrode and Hg/HgO (1 M KOH) as reference electrode. All the cyclic voltammetry (CV) measurements were conducted at room temperature. The CC electrolyte solutions were purged with argon for 30 min before use. The background was obtained by CV measurement in 0.5 M KOH solution from -1.0 to 0.3 V vs. Hg/HgO at 50 mV s-1. Afterward, for EOR measurement, the working electrodes were tested in 0.5 M KOH with 0.5 M A ethanol solution under the same conditions. The chronoamperometry measurements were conducted at -0.1 V vs. Hg/HgO for 10000s in the electrolyte of 0.5 M KOH and 0.5 M ethanol in order to evaluate the stability of the catalysts. 2.8. Characterization 7 Transmission electron microscopy (TEM) characterization was carried out using a ZEISS LIBRA 120, operating at 120 kV and a JEOL 1011 operating at 100 kV. Carbon-coated TEM grids from Ted-Pella were used as substrates. High-resolution TEM (HRTEM) studies were conducted using a field emission gun FEITM Tecnai F20 microscope at 200 kV with a point-to-point resolution of 0.19 nm. Annular dark-field (HAADF) STEM was combined with electron energy loss spectroscopy (EELS) in the Tecnai F20, by using a GATAN QUANTUM filter. Scanning electron PT microscopy (SEM) analyses were done in a ZEISS Auriga microscope with an energy dispersive X-ray spectroscopy (EDS) detector operating at 20 kV. Powder X-ray diffraction (XRD) patterns RI were collected directly from the as-synthesized NCs dropped on Si (501) substrate on a Bruker- AXS D8 Advanced X-ray diffractometer with Ni-filtered (2 μm thickness) Cu K radiation (λ = SC 1.5406 Å) operating at 40 kV and 40 mA. A LynxEye linear position-sensitive detector was used in reflection geometry. X-ray photoelectron spectroscopy (XPS) was done on a SPECS system U equipped with an Al anode XR50 source operating at 150 mWand a Phoibos 150 MCD-9 detector. N The pressure in the analysis chamber was below 10-7 Pa. The area analyzed was about 2 mm × 2 mm. The pass energy of the hemispherical analyzer was set at 25 eV and the energy step was set A at 0.1 eV. Data processing was performed with the CasaXPS program (Casa Software Ltd., UK). M Binding energy values were centered using the C 1s peak at 284.8 eV. Fourier transform infrared spectroscopy (FTIR) was performed on an Alpha Bruker FTIR spectroscopy with a platinum D attenuated total reflectance (ATR) single reflection module. FTIR spectra data were recorded from TE 400 cm−1 to 4000 cm−1. EP 3. Results and discussion CC 3.1. NC synthesis A PdP2 NCs were produced within OAm from the reaction of Pd(acac)2 with HMPT in the presence of MAHC (see details in the experimental section). All elements were introduced in the reaction flask at a low temperature and the mixture was heated up to 300 °C. During the heating up process, the solution became black at around 250 °C, indicating the precursor reaction at this temperature. 8 Figure 1a shows a TEM micrograph of the NCs obtained after 1 h reaction at 300 °C. NCs showed elongated, tear drop-like geometry and had an average size of its longer axis of 12  2 nm. XRD patterns showed the NCs to have a PdP2 monoclinic phase, with I2/c space group (Figure 1b). No additional XRD peak corresponding to secondary crystal phases was detected. HRTEM analysis confirmed the monoclinic phase of the NCs and allowed determining the lattice parameters: a = 6.207 Å, b = 5.857 Å and c = 5.874 Å (Figure 2a). Annular dark field scanning PT TEM (ADF-STEM) and STEM-EELS elemental composition maps revealed all NCs to contain Pd and P and to have both elements uniformly distributed throughout each NC (Figure 2b). SEM- RI EDS analysis showed the NCs to have an atomic ratio of P/Pd = 2.0 (Figure S1). SC a b Counts U Intensity (a.u.) 0 10 20 Size (nm) N A PdP2 M 100 nm 20 30 40 50 60 70 80 2 (degree) D TE Figure 1. a) Representative TEM micrograph of PdP2 NCs. Inset shows their size (long axis) distribution histogram. b) XRD pattern of PdP2 NCs including the JCPDS 77-1421 reference. Inset shows a unit cell of the PdP2 monoclinic phase (Pd in blue and P in pink). EP CC A 9 PT RI SC Figure 2. a) HRTEM image of several PdP2 NCs and power spectrum fitting with the PdP2 phase U of one of them. b) ADF-STEM image of several PdP2 NCs and EELS compositional maps for Pd (blue) and P (green). N A To gain inside of the NC growth mechanism, aliquots were extracted at different reaction times. M Spherical nanoparticles with a highly defective/disordered Pd5P2 phase, as deduced from the XRD band at about 2= 40°, and an average size of 7 nm were recovered from aliquots extracted after D 1 min reaction at 300 °C (Figure S2 and S3). As the reaction time increased, additional P was progressively incorporated into the Pd5P2 nanoparticles, which crystallized into elongated PdP2 TE NCs (Figure S2 and S3). EP To determine the role of MAHC in the synthesis, the reaction was performed in the absence of this compound. In these conditions, before adding the P precursor, the solution changed to black CC at 120 °C, during the heating up process. XRD analysis of the recovered solid showed that it consisted of Pd (Figure S4). We conclude that in the absence of MAHC, the Pd precursor is reduced by OAm at low temperature, resulting in the formation of Pd NCs. Once HMPT was A injected in the solution containing the reduced Pd precursor, Pd NCs reacted with P to produce very small Pd-P nanoparticles, which after reaction at 300 °C for 1 h resulted in a mixture of small Pd5P2 and PdP2 NCs (Figure S5). 10 In the presence of 0.2 mmol of MAHC, the precursors reacted at higher temperature, about 250 °C, indicating that the presence of MAHC prevented the reduction of the Pd precursor by OAm. After reacting at 300 °C for 1 h, teardrop shaped PdP2 NCs with some spherical Pd5P2 particles were obtained. With larger amounts of MAHC, the shape of the NCs became uniform and less spherical Pd-P quasi-amorphous particles were observed in the products (Figure S6). This last point was followed by the decrease of the XRD band at about 2= 40° assigned to a PT disordered/defective Pd5P2 phase (Figure S7), which was observed to decrease with the amount of MAHC introduced. This band completely disappeared when 1 mmol MAHC was present in the RI reaction system. At this point, pure phase PdP2 NCs were obtained. SC To elucidate the role of chlorine and the amine group of MAHC, the synthesis was carried out in the presence of NH4Cl instead of MAHC. In these conditions, a similar reaction temperature U with respect to the presence of MAHC, as observed from the color change of the solution, and a N similar product was obtained (Figure S8). This experimental result pointed toward Cl- ions having a main role on the stabilization of the Pd precursor in solution, preventing its reduction with OAm A and yielding pure phase PdP2 when reacting with the P source. This result also demonstrates that M ammonium chloride could be used instead of MAHC for the synthesis of PdP2 NCs. However, all the following results were obtained using PdP2 NCs produced in the presence of MAHC. D To determine the role of the P precursor, HMPT was replaced by TOP. Using TOP, no NCs were TE obtained at 300 °C, which we associated to the much lower reactivity of TOP when compared with HMPT. When reacting at 350 °C for 1 h, irregular and large-size NCs were produced (Figure S9). EP XRD and EDS showed these NCs to consist of a mixture of Pd5P2 and PdP2 phases (Figure S10). In this case, the produced NCs were irregular in shape and highly polydisperse in size. CC Figure 3 shows the Pd 3d and the P 2p XPS spectra of the PdP2 NCs. Two doublets were fitted to the Pd 3d states. The main Pd component, accounting for 64 % of the total Pd detected, was A found at a Pd 3d5/2 binding energy 335.9 eV. This binding energy was slightly above that of Pd(0) in pure Pd (335.1-335.5 eV) [25,26], which is consistent with the chemical environment of Pd within a Pd phosphide. A more oxidized component, accounting for 36% of the Pd detected, was 11 found at a Pd 3d5/2 binding energy of 337.7 eV. This component was assigned to Pd(II) species at the surface of the air-exposed NCs that had undergone oxidation [27]. Two P chemical states were also identified from XPS analysis of the P 2p electronic states. A main P 2p3/2 peak at 130.3 eV matched well with P in a metal phosphide environment. This component accounted for 64 % of the detected P. The second component, accounting for 36 % of PT the P detected, was found at a higher binding energy, P 2p3/2 peak at 134.1 eV, and it was assigned to a phosphate environment. We believe this phosphate to result from the partial oxidation of the RI PdP2 surface due to air exposure [28-30]. SC The atomic ratio of Pd and P at the NC surface detected by XPS was approximately 1, which points toward a Pd enriched surface compared to the PdP2 core. U a Pd 3d N b P 2p A Intensity (a.u.) Intensity (a.u.) M D TE 348 344 340 336 332 140 135 130 125 Binding energy (eV) Binding energy (eV) EP Figure 3. XPS spectra of PdP2 NCs in the Pd 3p (a) and P 2p (b) regions. CC 3.2. NC surface treatment and catalyst preparation A FTIR spectra of as-produced PdP2 NCs displayed peaks at 2920 and 2852 cm-1 attributed to the C-H stretching vibration of the alkyl group of OAm (Figure 4). Additionally, peaks attributed to the bending vibration of C-H in alkyl and alkenyl part of OAm, at 1457 and 968 cm-1, and to the bending vibrations of N-H and C-N, at 1551 and 1059 cm-1, were also identified. These features were slightly shifted compared with pure OAm, consistently with its binding to the NC surface. 12 While the presence of OAm at the NC surface rendered the NCs colloidally stable in non-polar solvents and limited their growth, OAm might also block catalytic sites and hinder charge transfer. Thus, it had to be removed prior to NC catalytic application. OAm was displaced from the NC surface using a short inorganic ligand, ammonium thiocyanate, following the procedure reported by Fafarman et al. (see experimental section for details) [24]. PT FTIR spectra of the NCs after ligand displacement with ammonium thiocyanate showed the presence of peaks at 2045 and 1596 cm-1, assigned to the C≡N antisymmetric and asymmetric RI vibration, and a broad peak at ca. 1400 cm-1 corresponding to the N-H stretching vibration of the amino group of ammonium thiocyanate (NH4SCN). SC U PdP2 after ligand exchange NH4SCN N A Intensity (a.u.) PdP2 M OAm D TE 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) EP Figure 4. FTIR of OAm, NH4SCN, as-synthesized PdP2 NCs and PdP2 after ligand exchange by CC NH4SCN. A Electrocatalysts were prepared by supporting the colloidal PdP2 NCs onto reduced graphene oxide (rGO). Graphene is considered an ideal electrocatalyst support owing to its excellent electric conductivity, large surface area, superior mechanical flexibility and chemical stability [31]. The use of graphene as the support material not only improves the electrical conductivity of the nanocomposite catalyst, but also increases the dispersion of the active components, which helps to 13 prevent their aggregation during the catalytic reaction. Graphene oxide (GO) sheets were prepared here by a modified Hummers method as presented by Kovtyukhova [23], and then reduced to rGO at high temperature (Figure S11). rGO exhibited a rippled and crumpled morphology and paper- like structure with single or few layers (Figure 5), offering a large surface area to support the PdP2 NCs. PdP2 NCs were supported by mixing the proper amount of rGO and PdP2 NCs in water (see experimental section for details). As shown in Figure 5, PdP2 NCs were homogeneously PT distributed on the rGO sheet after solvent removal. The nanocomposite was drop-casted onto GCE and then dried at room temperature. On top of it, Nafion was deposited to immobilize the catalysts RI on the electrode (see details in the experimental section). SC a b U N A M 200 nm 500 nm D Figure 5. TEM images of as-prepared rGO (a) and PdP2/rGO (b) catalyst. TE 3.3. Cyclic voltammetry in base solution EP CV of PdP2/rGO, unsupported PdP2, 12  3 nm Pd (Figure S12) and Pd/rGO electrocatalysts was initially investigated in a 0.5 M KOH aqueous solution (Figure 6a). The same total amount CC of catalyst was used to produce each of the tested electrodes, which translated into significantly lower amounts of Pd in the PdP2 and especially on the PdP2/rGO catalysts. A All catalysts showed similar coulombic features, but with small differences in relevant regions of the voltammograms. The peaks in the region between -0.6 V and -0.8 V vs. Hg/HgO were attributed to the adsorption (cathodic scan) and desorption (anodic scan) of hydrogen. In the anodic scan, the PdP2 catalyst showed a positive shift of peak potentials compared to Pd, while in the 14 cathodic scan, the PdP2 showed a slight negative shift of the peak potential. The distinct anodic and cathodic behaviors of PdP2 and Pd catalysts were related to their different electronic structure involving partial electron donation from Pd to P sites, consistent with XPS results [32]. The peaks in the anodic scan region from -0.4 to -0.2 V vs. Hg/HgO were attributed to the formation of palladium oxide on the NC surface. Subsequently, a PdO reduction peak appeared at PT about -0.2 V vs. Hg/HgO during the negative scans. The electrochemically active surface area (ECSA) of the catalysts was estimated from the coulombic charge for the reduction of PdO, i.e. RI from the area over the voltammetry curve in the PdO reduction peak region [13]: SC Q ( C  cm 2 ) (1) ECSA  QPdO ( C  cm )  Pd loading(mg  cm 2 ) 10 2 Pd U where QPdO = 405 μC cm-2 is the charge value given for the reduction of a PdO monolayer, the N coulombic charge Q is calculated by integrating the area of the PdO reduction peak, and Pdloading A is the Pd mass on the working electrode. M ECSA values obtained for Pd, Pd/rGO, PdP2, and PdP2/rGO catalysts were 49.4 m2 g-1, 63.8 m2 g-1, 97.9 m2 g-1, and 105.1 m2 g-1, respectively. The Pd utilization effectiveness was estimated D taking into account that the active surface area for full utilization of 1 g of Pd would be 448 m2 TE [13,33]. Thus, the Pd utilization efficiencies of PdP2, PdP2/rGO, Pd/rGO and Pd were 21.8 %, 23.4 %, 14.2 % and 11.0 %, respectively (Figure 6e). EP Besides slight differences related to the dissimilar geometry and size of PdP 2 and Pd NCs, the larger ECSA and Pd utilization efficiency of PdP2 over Pd demonstrated PdP2 to have a highly Pd- CC rich surface, as observed from XPS measurements. The oxidation of PdP2 beyond the surface monolayer could also partially explain the higher ECSA obtained for PdP2. Besides, the slightly A improved Pd utilization efficiency of PdP2/rGO over unsupported PdP2 might be attributed to the high surface area of rGO which allowed a better dispersion of the PdP2 NCs. 3.4. Cyclic voltammetry in ethanol solution 15 The EOR electrocatalytic activities of Pd, Pd/rGO, PdP2 and PdP2/rGO catalysts were investigated in a 0.5 M KOH and 0.5 M ethanol aqueous solution (Figure 6b). CVs from all catalysts showed the two well defined anodic peaks associated with the ethanol oxidation in alkaline media, one in the forward and one in the reverse scan. The oxidation peak in the forward scan was associated with the oxidation of freshly chemisorbed species coming from ethanol adsorption. The oxidation peak in the reverse scan was related to the removal of carbonaceous PT species that were not completely oxidized in the forward scan. RI The ethanol oxidation process may take place through two parallel pathways [8]: SC CH3CH2OH→CH3CH2OH(ad)→CH3CHO→CH3COOH (2) U CH3CH2OH→CH3CH2OH(ad)→CO(ad), CHx(ad)→CO2 (3) N In the path schematized in equation 2 (C2 pathway), the ethanol is first oxidized to acetaldehyde A and subsequently to acetic acid or acetate in alkaline solution. The C-C bond is not broken during M this pathway. In the path of equation 3 (C1 pathway), the C-C bond is broken and intermediate species such as CO and CHx fragments are produced, which can be further oxidized to CO2 or D carbonate in alkaline solution. TE During the positive scan, to reach a current density of 1 mA cm-2, overpotentials of -0.48 V and -0.45 V vs. Hg/HgO were required for PdP2/rGO and PdP2, respectively, which was less than the EP overpotential required for Pd/rGO (-0.43 V vs. Hg/HgO) and Pd NCs (-0.41 V vs. Hg/HgO), indicating a lower activation energy for ethanol oxidation on PdP2. The forward oxidation current CC peaked at around -0.12 V vs. Hg/HgO for Pd, -0.09 V for PdP2, -0.07 V for Pd/rGO and -0.04 V for PdP2/rGO (Table 1). Higher potentials resulted in a decrease of current density associated with the oxidation of the Pd surface. In the forward scan, peak current densities for the EOR in Pd, A Pd/rGO, PdP2 and PdP2/rGO were 10.6, 16.9, 22.1 and 51.4 mA cm-2, respectively (Table 1). When the values were normalized to reflect the amount of Pd, the mass peak current density of PdP2 and PdP2/rGO were 0.344 and 1.60 A mg-1Pd, respectively, which is ca. 3.3 and 15 times higher than that of Pd (0.104 A mg-1Pd), and 1.03 and 4.8 times higher than that of Pd/rGO (0.332 A mg-1Pd) 16 (Figure 6f). Overall, PdP2/rGO showed very high specific and mass activities toward the EOR, comparable to the best Pt- and Pd-based catalysts reported to date (Figure 7). In terms of loading, while bare rGO showed no activity toward EOR (Figure S13), with the increasing amount of PdP2 on rGO from 25% to 75%, the catalyst showed increasing specific activity and decreasing mass activity. A 50 wt% loading of PdP2 on PdP2/rGO was considered the PT best compromise and was the composition selected to carry on all measurements. RI The ratio of the forward (JF) and reverse (JR) peak current densities was calculated to get insight of the reaction pathway: the higher the JF/JR ratio, the more efficient oxidation of ethanol and the SC less accumulation of carbonaceous species on the catalyst [4]. As calculated from figure 6b, the JF/JR ratios for Pd, Pd/rGO, PdP2 and PdP2/rGO were 0.82 and 1.00 to 0.99 and 1.22, respectively U (Table 1). The increased ratio suggested more efficient ethanol oxidation process for PdP2 and especially PdP2/rGO than Pd. N A To evaluate the stability of the catalysts, chronoamperometry (CA) measurements were carried M out in a solution of 0.5 M KOH + 0.5 M ethanol with a constant applied potential of -0.1 V vs. Hg/HgO. As displayed in figure 6c, initial rapid decreases in current density were observed for all D the catalysts, followed by a sluggish decrease until a pseudo-steady state. The sharp initial decrease was ascribed to the adsorption and accumulation of strongly adsorbed reaction intermediates on TE the surface of free active sites. Subsequently, the adsorption of ethanol depends on the liberation of the active sites resulting from the oxidation of ethanol or the intermediates that form during the EP initial state. After 10000 s activity, the current obtained from PdP2 (0.90 mA cm-2) and PdP2/rGO (1.56 mA cm-2) was much higher than that of Pd (0.15 mA cm-2) and Pd/rGO (0.57 mA cm-2). CC When comparing by mass activities, PdP2/rGO (48.5 mA mg-1Pd) and PdP2 (14.0 mA mg-1Pd) showed about 32 and 9.5 times higher values than Pd (1.47 mA mg-1Pd), and 4.3 and 1.3 times higher values than Pd/rGO (Figure 6g). Notice that no morphological or structural change of the A NCs was observed to take place during EOR (Figure S14). 17 1.0 a 0.5 b 50 2 PdP2/rGO J (mA cm ) -2 J (mA cm-2) 0.0 40 1 ) Pd/rGO -2 -0.5 30 J (mA cm 0 PdP2/rGO -0.50 -0.45 -0.40 -0.35 PdP2 -1.0 E vs. Hg/HgO (V) Pd 20 Pd/rGO -1.5 PdP2 PT 10 Pd -2.0 0 -2.5 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 RI E vs. Hg/HgO (V) E vs. Hg/HgO (V) c 40 2.0 d -0.30 SC -1 Pd dec 1.5 mV J (mA cm ) -2 -0.35 176 30 E vs. Hg/HgO (V) 1.0 Pd/rGO ) -2 -1 0.5 -0.40 U ec J (mA cm Vd 20 17 1m 0.0 PdP2 9000 9500 10000 Time (s) -0.45 10 PdP2/rGO PdP2 N -0.50 133 mV d ec -1 PdP2/rGO A Pd/rGO Pd -1 155 mV dec 0 -0.55 M 0 2000 4000 6000 8000 10000 -0.4 -0.2 0.0 0.2 Time (s) Log(J) e f 50 g 25 50 Specific activity D Mass activity 1.5 20 40 Pd efficiency (%) 40 J (mA mg-1Pd) TE ) ) 15 Pd 30 -2 1.0 J (mA cm 30 -1 J (A mg 10 20 20 0.5 EP 5 10 10 0 0 0.0 0 Pd GO PdP 2/rGO 2 Pd O P2 O Pd O P2 O Pd /r Pd /rG Pd P/ rG /rG Pd P2/rG P Pd 2 Pd Pd Pd CC Figure 6. a) Cyclic voltammetry measurements of Pd, Pd/rGO, PdP2 and PdP2/rGO in a 0.5 M A KOH solution and b) in a 0.5 M KOH + 0.5 M ethanol solution. c) Chronoamperometric measurements of Pd, Pd/rGO, PdP2 and PdP2/rGO in a 0.5 M KOH + 0.5 M ethanol solution at - 0.1 V vs. Hg/HgO. d) Tafel plot of the Pd, Pd/rGO, PdP2 and PdP2/rGO. e) Comparison of Pd utilization efficiencies. f) Comparison of specific activity and mass activity of the Pd, Pd/rGO, PdP2 and PdP2/rGO. g) Mass activity after chronoamperometric measurement for 10000 s. 18 Table 1. Current density and potential at the forward and reverse peak and ratio of the peak current density between the forward and reverse oxidation peaks JF/JR. Forward Scan Reverse Scan Catalysts JF/JR EF (V vs. Hg/HgO) JF (mA cm-2) ER (V vs. Hg/HgO) JR (mA cm-2) Pd -0.12 10.6 -0.194 12.9 0.82 Pd/rGO -0.07 16.9 -0.185 16.9 1.00 PT PdP2 -0.09 22.1 -0.192 22.3 0.99 PdP2/rGO -0.04 51.4 -0.178 42.1 1.22 RI SC U 80 Pd NWAs PdSn-SnO2/C N Specific activity (mA cm-2) A 60 PdAg3 PdP2/rGO M Pd2Sn/C PdAg 40 Pd3Ag PdCo/CFC D Pt9RhFe3/C PtRu0.5Sn0.5/rGO Pd59Ni41/C TE 20 PdFe-Fe2O3/MWNTs PtSnRh/C Pt68Cu32 PdCu2/C Pd3Pb PtRh/rGO EP PdCo/NPNCs 0 0.0 0.5 1.0 1.5 2.0 2.5 -1 Mass activity (A mg Pd/Pt ) CC Figure 7. Specific and mass activities of the PdP2/rGO electrocatalysts tested in the present work A compared with reported best performing catalysts: PtRh/rGO [34], Pt9RhFe3/C [35], PtRu0.5Sn0.5 [36], Pd2Sn/C [4], Pd59Ni41/C [37], Pd3Pb [38], PdCo/NPNCs [39], PdCu2/C [40], PdFe- Fe2O3/MWNTs [41], Pt68Cu32 [42], Pd NWAs [43], PtCo/CFC [44], PdAg3, PdAg and Pd3Ag [45], PtCu [46], PtSnRh/C [47] and PdSn-SnO2/C [48]. Abbreviations: reduced graphene oxide (rGO), 19 N-doped porous carbon nanocapsules (NPNCs), multiwalled carbon nanotubes (MWNTs), nanowire arrays (NWAs) and carbon fiber cloth (CFC). Figure 6d shows a linear region of the Tafel plots obtained from the CV curves in the range from -0.5 to -0.3 V vs. Hg/HgO. As the potential further increased above -0.2 V, the Tafel plot became curved, indicating a change in the factor limiting the reaction rate. As shown in figure 6d, PT Pd NCs and Pd/rGO showed a Tafel slope of 176 and 171 mV dec-1, well above that of PdP2 and PdP2/rGO, with Tafel slopes of 155 and 133 mV dec-1, respectively. These values were close to RI the theoretical value for the EOR as calculated by Liang (120 mV dec-1) [49]. The lower Tafel slopes indicated that the charge-transfer kinetics of the EOR on the PdP2 and especially PdP2/rGO SC catalysts were faster than that on Pd in the alkaline medium, which can be ascribed to the positive effect of P on the electronic state of Pd, the enhanced electron conductivity of rGO and the uniform U dispersion of PdP2 NCs on rGO. 4. Conclusions N A M In summary, single-phase PdP2 NCs were produced in the presence of chlorine ions and using a highly active P precursor, HMPT. PdP2 NCs showed high mass activity and long-term stability D toward the EOR in alkaline media. The enhanced properties were attributed to the effect of P on the electronic structure of Pd and the Pd-rich surface of the produced PdP2 NCs. The activity and TE stability of the PdP2-based catalyst were further improved by supporting PdP2 NCs onto rGO, taking advantage of an enhanced electrical conductivity and a high dispersion of the PdP2 NCs. EP Additionally, the slower current decay over time combined with the higher measured activity for PdP2 and PdP2/rGO electrocatalysts when compared with Pd and Pd/rGO, indicated a higher CC tolerance to the carbonaceous species formed. Notes A The authors declare no competing financial interest. Acknowledgements 20 D. Nasiou and J. Arbiol acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ANAPHASE (ENE2017-85087-C3). ICN2 acknowledges support from the Severo Ochoa Programme (SEV-2013-0295) and is funded by the CERCA Programme / Generalitat de Catalunya. J. Llorca is a Serra Hunter Fellow and is grateful to ICREA Academia program and grants MINECO/FEDER ENE2015-63969-R and GC 2017 SGR 128. J. Liu, J. Li and X. Yu thank the China Scholarship Council for scholarship support. M. Meyns acknowledges PT a Juan de la Cierva formación grant by the Spanish MINECO. RI Appendix A. Supplementary data SC Supplementary material related to this article can be found, in the online version. U References N [1] S.T. Nguyen, H.M. Law, H.T. Nguyen, N. Kristian, S. Wang, S.H. Chan, X. Wang, A Enhancement effect of Ag for Pd/C towards the ethanol electro-oxidation in alkaline media, Appl. M Catal. B Environ. 91 (2009) 507-515. [2] W. Hong, J. Wang, E. Wang, Facile synthesis of highly active PdAu nanowire networks as D self-supported electrocatalyst for ethanol electrooxidation, ACS Appl. Mater. Inter. 6 (2014) 9481- TE 9487. EP [3] M. D. Obradovic, Z. M. Stancic, U. C. Lacnjevac, V. V. Radmilovic, A. Gavrilovic- Wohlmuther, V. R. Radmilovic, S. L. 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