Ni(II)-dimethylglyoxime derived Pd‒Ni‒[email protected] carbon hybrid nanocatalysts for oxygen reduction reaction

Applied Surface Science 479 (2019) 273–279

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full length article

Pd(II)/Ni(II)-dimethylglyoxime derived Pd‒Ni‒P@N-doped carbon hybrid nanocatalysts for oxygen reduction reaction

T

Xue Donga, Jingru Lia, Denghu Weib, Rui Lia, Konggang Qua, Lei Wanga, Shuling Xua, ⁎ ⁎ Wenjun Kanga, , Haibo Lia,c, a Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, PR China b School of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, PR China c Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

A R T I C LE I N FO

A B S T R A C T

Keywords: Pd‒Ni‒P hybrid nanocatalyst Oxygen reduction reaction Surface electronic structure Charge transfer Interfacial coupling interaction

Searching for an efficient non‑platinum electrocatalyst for oxygen reduction reaction (ORR) is of great significance for some energy conversion devices. In this work, Pd‒Ni‒P hybrid nanocatalysts loaded on nitrogendoped carbon supports (defined as Pd‒Ni‒P@NC) are prepared by simply reacting Pd(II)/Ni(II)-dimethylglyoxime with NaH2PO2 at 500 °C. Compared with Pd‒P@NC and Ni‒P@NC counterparts, the Pd‒Ni‒P@ NC exhibits a more positive onset potential and higher specific/mass activity towards ORR due to the interfacial coupling interaction between Pd3P0.8 and Ni2P. Moreover, it has a higher catalytic durability and better methanol tolerance than commercial Pt/C catalyst. The surface valence state analysis and electrochemistry characterization reveals that the enhanced electrocatalytic performance of Pd‒Ni‒P@NC is attributed to the modified Pd 3d electronic structure and improved charge transfer character, originating from the incorporation of Ni2P component. Our study provides an alternative non‑platinum electrocatalyst for ORR in fuel cell and metalair battery.

1. Introduction Oxygen reduction reaction (ORR) plays an important role in energy conversion devices such as fuel cell and metal-air battery. Metallic Pt shows an impressive electrocatalytic performance for ORR, however, its wide application is still hampered by the low abundance and high cost. Moreover, Pt elecrocatalyst usually suffers from poisoning in practical uses. Compared with widely used Pt, Pd-based catalyst has a less cost but a comparable catalytic activity, making it an alternative substitute [1,2]. To improve the electrocatalytic activity of Pd-based catalysts, a secondary nonprecious metal is introduced into Pd, and the Pd-metal (Pd‒M, M = Fe, Ni, Co, etc.) alloys usually exhibit an enhanced performance due to the strain and electronic structure alteration mechanism [3–7]. Recently, some non-metals such as sulfur (S) and boron (B) have been alloyed with Pd to investigate the catalytic performances [8–11]. It is reported that the doping of non-metals can weaken the adsorption of intermediates with nearly optimal binding energy during the ORR process. Similarly, the attempt of phosphorus (P) doping into palladium has also found successes in electrocatalytic application [12].

⁎

Up to now, Pd-based phosphides including amorphous and crystalline phases such as PdP2, Pd3P have been prepared by reacting Pd precursors with white phosphorus [13,14], sodium hypophosphite [15,16], and tri-n-octylphosphine [17,18]. Among previous reports, almost all studies focus on the electro-oxidation of small organic molecules, especially for formic acid [19–23]. However, few works are devoted to ORR electrocatalytic application [15,16,24–26]. Recent studies reveal that Pd-based bimetallic phosphide (i.e. Pd‒M‒P) exhibits a better electrocatalytic activity than its corresponding monometallic counterparts. The enhanced electrochemical property is generally attributed to the synergistic effects including the enhanced charge transfer and tuned electronic structure [27–30]. Moreover, to fully realize the catalytic activity of Pd-based phosphide, carbon materials such as Vulcan XC-72 and carbon nanotubes are usually utilized to support catalyst nanoparticles [15,21,31,32]. Herein, we demonstrate a facile method to synthesize Pd‒Ni‒P hybrid nanocatalysts loaded on nitrogen-doped carbon supports (defined as Pd‒Ni‒P@NC) by simply reacting Pd(II)/Ni(II)-dimethylglyoxime with NaH2PO2 at 500 °C. The Pd‒Ni‒P hybrid nanocatalysts are composed of Pd3P0.8 and Ni2P. It is noted that the in situ

Corresponding authors. E-mail addresses: [email protected] (W. Kang), [email protected] (H. Li).

https://doi.org/10.1016/j.apsusc.2019.02.071 Received 21 November 2018; Received in revised form 18 January 2019; Accepted 9 February 2019 Available online 11 February 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 479 (2019) 273–279

X. Dong, et al.

Fig. 1. Schematic illustration for the synthesis of Pd‒Ni‒P@NC by reacting Pd(II)/Ni(II)-dimethylglyoxime with NaH2PO2.

STA449.

formed nitrogen-doped carbon, derived from the pyrolysis of dimethylglyoxime ligands, can directly serves as catalyst supports to load Pd‒Ni‒P hybrid nanoparticles without introducing additional carbon supports. The incorporation of Ni2P component can modify Pd 3d electronic structure and improve the charge transfer feature, and then the obtained Pd‒Ni‒P@NC displays an enhanced electrocatalytic performance towards ORR.

2.4. Electrochemical measurements Electrochemical tests were conducted on a CHI832B electrochemical workstation (Shanghai Chenhua Co., China). The electrocatalytic performance towards ORR was evaluated in O2-saturated 0.1 M KOH solution with a three-electrode system including an Ag/AgCl (3.0 M NaCl) reference electrode, a Pt wire (d = 0.5 mm, L = 23 cm) counter electrode, and a glassy carbon working electrode (d = 4 mm). All potentials in the text were referenced to the reversible hydrogen electrode (RHE) without an IR-correction. The catalyst ink was prepared as follows: 2 mg catalyst was ultrasonically dispersed in a mixture solution of 800 μL water and 200 μL isopropanol to form a homogeneous ink. The working electrode was prepared by drop-casting 15 μL catalyst ink onto glassy carbon electrode. After drying at room temperature, the electrode surface was further covered with 2.0 μL 0.5% Nafion solution. For comparison, a commercial Pt/C (20 wt% Pt, Shanghai Hesen) catalyst was also loaded onto the working electrode with the Pt loading mass of 15 μg cm−2. Electrochemical impedance spectroscopy tests were conducted on an Autolab electrochemical workstation (Metrohm) with an amplitude of 5 mV in the frequency of 100 kHz to 50 mHz at 0.70 V (vs. RHE).

2. Experimental 2.1. Chemicals and materials The following chemicals were used without further purification: sodium hypophosphite (NaH2PO2), nickel sulfate hexahydrate (NiSO4·6H2O), and dimethylglyoxime (dmg) were purchased from Sinopharm Chemical Reagent Co., Ltd.; potassium tetrachloropalladate (II) (K2PdCl4) was supplied by Zhengzhou Alpha Chemical Co., Ltd. 2.2. Synthesis of Pd‒Ni‒P@NC hybrid nanocatalysts Pd‒Ni‒P@NC hybrid nanocatalysts were prepared in the following two steps. In the first step, Pd(II)/Ni(II)-dimethylglyoxime (PdNi-dmg) precursor was synthesized at room temperature. Typically, 150 mg NiSO4·6H2O and 20 mg K2PdCl4 were dissolved in 400 mL deionized water (solution A), and 146 mg dimethylglyoxime was dissolved in 12 mL ethanol followed by pH adjustment to 9–10 using 1.0 M NaOH solution (solution B). Then solution B was dropwise added into the solution A under constant stirring, and the mixed solution was further stirred for 30 min after finishing addition. The desired precipitates were separated by filtering and dried in an oven at 80 °C. In the second step, a certain amount of PdNi-dmg precursor and sodium hypophosphite with mole ratio of 1:5 were loaded into a porcelain boat and calcined at 500 °C for 1 h in N2-atmosphere. The collected product was ground and treated with 1.0 M HCl solution. The final black powder was fully washed with water and ethanol for several times, and then it was dried at 80 °C. The synthesis for Pd‒P@NC and Ni‒P@NC counterparts were similar with Pd‒Ni‒P@NC just by replacing PdNi-dmg precursor with Pd-dmg (obtained from 20 mg K2PdCl4 and 146 mg dmg) and Ni-dmg (obtained from 163 mg NiSO4·6H2O and 146 mg dmg), respectively. It should be pointed out that the pH of solution B should be adjusted to 2–3 using 1.0 M HCl for Pd-dmg synthesis.

3. Results and discussion 3.1. Structure and composition characterization Fig. 1 shows the schematic illustration for the synthesis of Pd‒Ni‒P@NC. In the process, sodium hypophosphite (NaH2PO2) is applied as the phosphorus source as it can produce PH3 in situ upon heating [22]. The generated PH3 further reacts with Pd(II) and Ni(II) to form Pd‒Ni‒P hybrid nanoparticles. It should be noted that the dimethylglyoxime ligands in PdNi-dmg directly transform into nitrogendoped carbon supports to load Pd‒Ni‒P catalyst following the pyrolysis process. For comparison, Ni‒P@NC and Pd‒P@NC catalysts are also prepared using a similar method. Their phase compositions were investigated by powder XRD. As shown in Fig. 2, the Pd‒P in Pd‒P@NC and the Ni‒P in Ni‒P@NC can be well indexed to the orthorhombic Pd3P0.8 (PDF# 3-0593) and hexagonal Ni2P (PDF# 3-0593), respectively. For the Pd‒Ni‒P@NC, its overall diffraction pattern is similar with that of P‒P@NC except the two peaks (marked with *) with low diffraction intensities. Careful comparison confirms that the two peaks correspond to the (111) and (201) planes of hexagonal Ni2P. It implies that the Pd‒Ni‒P is a hybrid catalyst, which is composed of Pd3P0.8 and Ni2P. The energy dispersive X-ray spectrometry (EDS) analysis in Fig. S1 also verifies the presence of Pd, Ni, and P in Pd‒Ni‒P@NC, and the atomic ratio of Pd:Ni:P is close to 3:4:3. Moreover, the broad peak at 2θ = ~25° comes from the carbon support. Due to the low carbonization temperature (500 °C) and incorporation of nitrogen heteroatom, the carbon support shows a low graphitization degree, which can be further confirmed by the intensity ratio of the D to G bands (ID/

2.3. Characterization Powder X-ray diffraction (XRD) measurements are performed by using a Rigaku SmartLab 9 X-ray diffractometer (Cu Kα, λ = 1.5418 Å). Raman spectrum was recorded on a MonoVista CRS500 Laser confocal Raman spectrometer (λex = 532 nm). X-ray photoelectron spectroscopy (XPS) study was carried out on a Thermo Scientific ESCLAB 250Xi spectrometer. Transmission electron microscopy (TEM) and element mapping analysis were conducted on a FEI Talos F200x (accelerating voltage: 200 kV). Thermogravimetric analysis was finished on a Netzsch 274

Applied Surface Science 479 (2019) 273–279

X. Dong, et al.

oxidized nickel(II) and pallidum(II) is attributed to the surface oxidation. For the P 2p spectrum (Fig. 3f), the peak at 132.9 eV corresponds to the oxidized P species (P‒O), and the peaks at 129.3 and 130.0 eV come from the P(0) 2p3/2 and P(0) 2p1/2 of Pd‒P or Ni‒P bond [36,37]. Compared with the red phosphorus (130.4 V), a negative shift for P(0) 2p occurs, implying a partial electron transfer from Pd(Ni) to P [35,38]. Similarly, the peak deconvolutions for C1s, N1 s, Pd 3d, Ni 2p, and P2p are also applied to Pd‒P@NC and Ni‒P@NC, and the corresponding results are shown in Fig. S3 and S4. The morphology of catalysts was characterized using a transmission electron microscope. Fig. 4a shows a low-magnification TEM image of Pd‒Ni‒P@NC. For the synchronous generation of both catalyst and support, the Pd‒Ni‒P nanoparticles with sizes of ~5–10 nm show a firm immobilization on carbon supports (Fig. 4b and c), which is expected to not only benefit electron transfer but also improve catalytic stability during the ORR process. To further confirm the phase composition of Pd‒Ni‒P@NC, high-resolution (HR) TEM technique is applied to study the microstructure. The HRTEM image in Fig. 4d shows two contacted nanoparticles. The well-defined lattice spacing of 0.23 nm is consistent with the (031) plane of Pd3P0.8, and the lattice spacing of 0.22 nm corresponds to the (111) plane of Ni2P. It is noted that the contact of Pd3P0.8 with Ni2P is favorable to realizing the interfacial coupling interaction [22], which may have an influence on the electrocatalysis for Pd‒Ni‒P hybrid catalyst [39]. In Fig. 4e, the lattice spacing of 0.34 nm can be indexed to the (002) plane of graphitic carbon. The HADDFSTEM and element mapping images imply a good distribution of Pd‒Ni‒P hybrid nanoparticles on nitrogen-doped carbon supports (Fig. 4f–k). The TEM characterizations for Pd‒P@NC and Ni‒P@NC show a similar morphology with Pd‒Ni‒P@NC, and they are shown in Fig. S5.

Fig. 2. XRD patterns of Ni‒P@NC, Pd‒P@NC, and Pd‒Ni‒P@NC.

IG = 0.91) in Raman spectrum (Fig. S2). XPS technique was employed to determine the surface element compositions and valence states (Fig. 3a). As shown in Fig. 3b, the high resolution C1s can be fitted to three peaks (284.2, 285.5, and 287.4 eV), corresponding to the C]C/C‒C, C]N, and C]O bonds [33]. The deconvolution of N1 s implies two kinds of nitrogen-containing species: pyridinic-type N (398.4 eV) and pyrrolic-type N (400.0 eV) (Fig. 3c) [34]. There are four peaks for high-resolution Pd 3d (Fig. 3d): the peaks at 335.3 and 340.6 eV can be indexed to 3d5/2 and 3d3/2 of Pd(0), respectively, and the peaks at 337.5 and 342.7 eV can be assigned to the 3d5/2 and 3d3/2 of Pd(II), respectively [35].Deconvolution of Ni 2p (Fig. 3e) confirms the presence of Ni(δ+) (0 < δ < 2, 853.0 eV) in Pd‒Ni‒P [22]. Moreover, peaks at 855.3 and 872.9 eV correspond to surface oxidation Ni(II) species of Ni 2p3/2 and Ni 2p1/2. The peaks at 861.4 and 880.3 eV belong to the satellite peaks of Ni(II) species. It is found that the bind energy of 2p3/2 of Ni(δ+) (853.0 eV) in Pd‒Ni‒P is slightly higher than that of metallic Ni (852.5–852.9 eV), implying a partial electron transfer from Ni to P(Pd). As there is no oxide phase of nickel or pallidum detected in XRD pattern (Fig. 2), the existence of

3.2. Electrocatalytic performance of Pd‒Ni‒P@NC In order to evaluate the electrocatalytic activity towards ORR, linear sweep voltammetry (LSV) measurements were performed in O2-saturated 0.10 M KOH. Fig. 5a shows the polarization curves of Pd‒Ni‒P@ NC at different rotation rates. The corresponding Koutecky–Levich

Fig. 3. (a) XPS survey spectrum of Pd‒Ni‒P@NC. Peak-fitting XPS spectra of (b) C1s, (c) N1s, (d) Pd 3d, (e) Ni 2p, and (f) P 2p for Pd‒Ni‒P@NC. 275

Applied Surface Science 479 (2019) 273–279

X. Dong, et al.

Fig. 4. (a–c) TEM, (d,e) HRTEM, (f) HADDF-STEM, and (g–k) element mapping images for Pd‒Ni‒P@NC; The HRTEM images in (d) and (e) correspond to the marked regions in (b) and (c), respectively.

(K–L) plots are obtained by fitting the inverse current density (1/J) versus the inverse of the square root of rotation rate (ω−1/2) (Fig. 5b), then the electron transfer number (n), an important evaluating parameter of ORR, can be calculated from the slope on the basis of K–L equation:

1 1 1 1 1 = + = + j JK JL JK Bω1/2

(1)

B = 0.62nFCO∗2 DO2/3 v−1/6 2

(2)

JK =

nFkCO∗2

respectively. The onset potential of Pd‒Ni‒P@NC just shows a negative shift by 10 mV compared with commercial Pt/C catalyst (0.98 V). It is found that the kinetic current density (JK) at 0.80 V (vs. RHE) for Pd‒Ni‒P@NC is 13.9 mA cmgeo−2. Although it is still below Pt/C catalyst (17.7 mA cmgeo−2), it has outperformed both Pd‒P@NC (10.5 mA cmgeo−2) and Ni‒P@NC (0.32 mA cmgeo−2) (Fig. 5d). Taking the Pd loading mass into account, the mass activity at 0.80 V (vs. RHE) for Pd‒Ni‒P@NC (316.3 mA mgPd−1) is 4.0 times of that of Pd‒P@NC (79.5 mA mgPd−1) (Fig. S7). What's more, the average electron transfer number (n) for Pd‒Ni‒P@NC is 3.6, being equal to Pt/ C catalyst, and it is slightly higher than that of Pd‒P@NC (3.5) and Ni‒P@NC (3.2) (Fig. S8). To gain insight into the ORR pathway for Pd‒Ni‒P@NC, rotating ring disk electrode (RRDE) was applied to monitor the H2O2 yield at catalyst-modified disk electrode. As the potential of Pt ring electrode is set to 1.47 V (vs. RHE), the generated H2O2 from disk electrode can be oxidated completely. As shown in Fig. 6a and b, both Pd‒Ni‒P@NC and Pt/C catalyst show a negligible ring current compared with the disk current, suggesting a low H2O2 yield. According to the following equation, the H2O2 yield and electron transfer number (n) can be determined:

(3)

where J, JK, and JL are the measured current density, kinetic current density, and the diffusion-limiting current density, respectively. ω is the electrode rotating rate, F represents the Faraday constant (F = 96,485 C mol−1), CO2∗ is the bulk concentration of O2 in 0.10 M KOH (1.2 × 10−6 mol cm−3), DO2 is the diffusion coefficient of O2 in 0.10 M KOH (1.9 × 10−5 cm2 s−1), ν is the kinematic viscosity of electrolyte (0.01 cm2 s−1). According to the K–L plots, the calculated n for Pd‒Ni‒P@NC is 3.45–4.07, implying a high efficient 4e− reduction mechanism. The polarization curves and K–L plots of Pd‒P@NC, Ni‒P@ NC, and commercial Pt/C catalyst are shown in Fig. S6. Fig. 5c displays the polarization curves of four catalysts at a rotation rate of 1600 rpm. The onset potentials (where the current density reaches 0.2 mA cm−2) for Pd‒Ni‒P@NC, Pd‒P@NC, and Ni‒P@NC are 0.97, 0.97, and 0.81 V,

H2 O2 % =

276

2IR / N ID + IR / N

(4)

Applied Surface Science 479 (2019) 273–279

X. Dong, et al.

Fig. 5. (a) Polarization curves of Pd‒Ni‒P@NC in O2-saturated 0.10 M KOH at different rotation rates, (b) Koutecky–Levich plots of Pd‒Ni‒P@NC at different potentials, (c) Comparison of polarization curves for all catalysts at a rotation rate of 1600 rpm in O2-saturated 0.10 M KOH, (d) Comparison of kinetic current density at 0.8 V (vs. RHE) for all catalysts.

n=

4ID ID + IR / N

3.3. Mechanism analysis of enhanced electrocatalytic performance for Pd‒Ni‒P@NC

(5)

where ID is the disk current, IR is the ring current, and N (0.44) is the current collection efficiency of Pt ring. From Fig. 6c, it is found that the H2O2 yield in the potential range of 0.90–0.15 V for Pd‒Ni‒P@NC is below 10%, being comparable to Pt/C catalyst (Fig. 6d). Besides, the determined electron transfer number (n) for Pd‒Ni‒P@NC is 3.83–3.97, well confirming a 4e− catalytic pathway, and this is also well consistent with the result obtained from K–L plots. Long-term stability is an important parameter for catalysts, so we investigate the electrochemical durability by cycling potentials in O2saturated 0.10 M KOH at a scanning rate of 100 mV s−1. Fig. 7a shows the ORR polarization curves of Pd‒Ni‒P@NC before and after 5000 cycles. It is observed that the half-wave potential of Pd‒Ni‒P@NC negatively shifts by 15 mV, which is much lower than that of commercial Pt/C catalyst (47 mV, Fig. 7b). Apart from the activity and stability, the catalytic selectivity for ORR is of great significance in fuel cells, especially for direct methanol fuel cells. As methanol molecule can penetrate Nafion membrane to the cathode, it may seriously influence the electrocatalytic activity of cathode catalyst, degrading the fuel cell's power efficiency [40]. To investigate the selectivity of catalysts, a methanol crossover test is carried out in O2-saturated 0.10 M KOH solution, and the corresponding current–time chromoamperometic response is recorded at a constant voltage of 0.62 V (vs. RHE). As shown in Fig. 7c, Pd‒Ni‒P@NC electrode keeps a stable amperometric response when 1.0 M methanol is introduced at 800 s. By contrast, the existence of methanol causes a dramatic current decay for commercial Pt/C catalyst (Fig. 7d). So it can be concluded that the Pd‒Ni‒P@NC exhibits a higher catalytic durability and better methanol tolerance than commercial Pt/C catalyst.

The enhanced catalytic performance for Pd‒Ni‒P@NC is attributed to the (I) Pd electronic structure alternation and (II) enhanced electron charge feature of catalyst. Compared with the Pd‒P@NC, the Pd(0) 3d5/2 and Pd(0) 3d3/2 of Pd‒Ni‒P@NC show a slightly negative shift by 0.4 eV (Fig. S9), demonstrating a rise of Pd 3d electron density. It comes from the electron transfer from Ni to Pd due to their slight electronegativity difference (Pd: 2.20; Ni: 1.91). Similar phenomena were also reported by Xing and Hu et al. in Ni2P‒Pd and CoP‒Pd hybrid systems [39,41,42]. The electron transfer can be further confirmed by the binding energy rise of Ni(δ+) 2p3/2 of Pd‒Ni‒P@NC (853.0 eV) compared with the Ni‒P@NC (852.6 eV) (Fig. 3e and S4c). So the presence of both Ni and P in Pd‒Ni‒P@NC hybrid catalyst has a synergistic effect to tailor Pd 3d electronic structure, leading to an optimal binding strength for oxygen-containing intermediates [9,43], and this improves the reaction kinetics for ORR on Pd‒Ni‒P@NC catalyst. Moreover, electrochemical impedance analysis was also carried out to study the enhanced ORR catalytic activity for Pd‒Ni‒P@NC. As depicted in Fig. S10, The Nyquist plot of Pd‒Ni‒P@NC exhibits a much smaller semicircle than that of Pd‒P@NC, indicating a lower charge transfer resistance in Pd‒Ni‒P@NC [44]. The small charge transfer resistance suggests a facile ORR kinetics for Pd‒Ni‒P@NC. 4. Conclusions In summary, Pd‒Ni‒P@NC hybrid nanocatalysts are prepared by facilely reacting Pd(II)/Ni(II)-dimethylglyoxime with NaH2PO2 at 500 °C. The Ni2P component in Pd‒Ni‒P@NC not only tunes Pd 3d electronic structure but also improves the charge transfer character, 277

Applied Surface Science 479 (2019) 273–279

X. Dong, et al.

Fig. 6. Polarization curves recorded on the RRDE for (a) Pd‒Ni‒P@NC and (b) Pt/C catalyst in O2-saturated 0.10 M KOH solution at a rotation rate of 1600 rpm, Calculated H2O2 yield and electron transfer numbers (n) for (c) Pd‒Ni‒P@NC and (d) Pt/C catalyst.

Fig. 7. Polarization curves of (a) Pd‒Ni‒P@NC and (b) Pt/C catalyst before and after 5000 cycles in O2-saturated 0.10 M KOH solution, Current-time chromoamperometic responses for (c) Pd‒Ni‒P@NC and (d) Pt/C catalyst at 0.62 V (vs. RHE) in O2-saturated 0.10 M KOH followed by adding 1.0 M methanol at 800 s. 278

Applied Surface Science 479 (2019) 273–279

X. Dong, et al.

which endows Pd‒Ni‒P@NC an enhanced electrocatalytic performance towards ORR compared with Pd‒P@NC and Ni‒P@NC counterparts. Moreover, it also has a higher catalytic durability and better methanol tolerance than commercial Pt/C catalyst. Our study provides an alternative non‑platinum electrocatalyst in fuel cell and metal-air battery field.

[18] G.H. Layan Savithra, R.H. Bowker, B.A. Carrillo, M.E. Bussell, S.L. Brock, Mesoporous matrix encapsulation for the synthesis of monodisperse Pd5P2 nanoparticle hydrodesulfurization catalysts, ACS Appl. Mater. Interfaces 5 (2013) 5403–5407. [19] J. Zhang, Y. Xu, B. Zhang, Facile synthesis of 3D Pd–P nanoparticle networks with enhanced electrocatalytic performance towards formic acid electrooxidation, Chem. Commun. 50 (2014) 13451–13453. [20] J.H. Byeon, Y.-W. Kim, Simple fabrication of a Pd–P film on a polymer membrane and its catalytic applications, ACS Appl. Mater. Interfaces 3 (2011) 2912–2918. [21] G. Yang, Y. Chen, Y. Zhou, Y. Tang, T. Lu, Preparation of carbon supported Pd–P catalyst with high content of element phosphorus and its electrocatalytic performance for formic acid oxidation, Electrochem. Commun. 12 (2010) 492–495. [22] X. Liang, B. Liu, J. Zhang, S. Lu, Z. Zhuang, Ternary Pd–Ni–P hybrid electrocatalysts derived from Pd–Ni core–shell nanoparticles with enhanced formic acid oxidation activity, Chem. Commun. 52 (2016) 11143–11146. [23] K.C. Poon, B. Khezri, Y. Li, R.D. Webster, H. Su, H. Sato, A highly active Pd–P nanoparticle electrocatalyst for enhanced formic acid oxidation synthesized via stepwise electroless deposition, Chem. Commun. 52 (2016) 3556–3559. [24] A.R.J. Kucernak, K.F. Fahy, V.N.N. Sundaram, Facile synthesis of palladium phosphide electrocatalysts and their activity for the hydrogen oxidation, hydrogen evolutions, oxygen reduction and formic acid oxidation reactions, Catal. Today 262 (2016) 48–56. [25] K.C. Poon, D.C.L. Tan, T.D.T. Vo, B. Khezri, H. Su, R.D. Webster, H. Sato, Newly developed stepwise electroless deposition enables a remarkably facile synthesis of highly active and stable amorphous Pd nanoparticle electrocatalysts for oxygen reduction reaction, J. Am. Chem. Soc. 136 (2014) 5217–5220. [26] S. Salomé, M.C. Oliveira, A.M. Ferraria, A.M.B. do Rego, A. Querejeta, F. Alcaide, P.L. Cabot, R. Rego, Synthesis and testing of new carbon-supported PdP catalysts for oxygen reduction reaction in polymer electrolyte fuel cells, J. Electroanal. Chem. 754 (2015) 8–21. [27] H. Xu, B. Yan, K. Zhang, C. Wang, J. Zhong, S. Li, P. Yang, Y. Du, Facile synthesis of Pd-Ru-P ternary nanoparticle networks with enhanced electrocatalytic performance for methanol oxidation, Int. J. Hydrog. Energy 42 (2017) 11229–11238. [28] X. Zhang, J. Li, Y. Sun, Z. Li, P. Liu, Q. Liu, L. Tang, J. Guo, N-doped reduced graphene oxide supported mixed Ni2PCoP realize efficient overall water electrolysis, Electrochim. Acta 282 (2018) 626–633. [29] R. Wang, Y. Ma, H. Wang, J. Key, S. Ji, Gas–liquid interface-mediated room-temperature synthesis of “clean” PdNiP alloy nanoparticle networks with high catalytic activity for ethanol oxidation, Chem. Commun. 50 (2014) 12877–12879. [30] Y. Kang, W. Wang, Y. Pu, J. Li, D. Chai, Z. Lei, An effective Pd-NiOx-P composite catalyst for glycerol electrooxidation: co-existed phosphorus and nickel oxide to enhance performance of Pd, Chem. Eng. J. 308 (2017) 419–427. [31] Y. Xie, W. Yu, J. Wang, Y. Wu, S. Niu, W. Guo, T. Lin, L. Shao, Pd-P nanoparticles supported on PxOy-incorporated carbon nanotubes for enhanced methanol oxidation in an alkaline medium, Phys. Chem. Chem. Phys. 19 (2017) 25214–25219. [32] M. Zhao, K. Abe, S.-i. Yamaura, Y. Yamamoto, N. Asao, Fabrication of Pd–Ni–P metallic glass nanoparticles and their application as highly durable catalysts in methanol electro-oxidation, Chem. Mater. 26 (2014) 1056–1061. [33] R. Zhang, X. Jing, Y. Chu, L. Wang, W. Kang, D. Wei, H. Li, S. Xiong, Nitrogen/ oxygen co-doped monolithic carbon electrodes derived from melamine foam for high-performance supercapacitors, J. Mater. Chem. A 6 (2018) 17730–17739. [34] H. Li, W. Kang, L. Wang, Q. Yue, S. Xu, H. Wang, J. Liu, Synthesis of three-dimensional flowerlike nitrogen-doped carbons by a copyrolysis route and the effect of nitrogen species on the electrocatalytic activity in oxygen reduction reaction, Carbon 54 (2013) 249–257. [35] Y. Liu, A.J. McCue, C. Miao, J. Feng, D. Li, J.A. Anderson, Palladium phosphide nanoparticles as highly selective catalysts for the selective hydrogenation of acetylene, J. Catal. 364 (2018) 406–414. [36] F. Luo, Q. Zhang, X. Yu, S. Xiao, Y. Ling, H. Hu, L. Guo, Z. Yang, L. Huang, W. Cai, H. Cheng, Palladium phosphide as stable and efficient electrocatalyst for overall water splitting, Angew. Chem. Int. Ed. 57 (2018) 14862–14867. [37] Z. Dan, F. Qin, T. Wada, S.-i. Yamaura, G. Xie, Y. Sugawara, I. Muto, A. Makino, N. Hara, Nanoporous palladium fabricated from an amorphous Pd42.5Cu30Ni7.5P20 precursor and its ethanol electro-oxidation performance, Electrochim. Acta 108 (2013) 512–519. [38] L. Chen, L. Lu, H. Zhu, Y. Chen, Y. Huang, Y. Li, L. Wang, Improved ethanol electrooxidation performance by shortening Pd–Ni active site distance in Pd–Ni–P nanocatalysts, Nat. Commun. 8 (2017) 14136. [39] L. Feng, J. Chang, K. Jiang, H. Xue, C. Liu, W.-B. Cai, W. Xing, J. Zhang, Nanostructured palladium catalyst poisoning depressed by cobalt phosphide in the electro-oxidation of formic acid for fuel cells, Nano Energy 30 (2016) 355–361. [40] C. Xu, Y. Liu, Q. Hao, H. Duan, Nanoporous PdNi alloys as highly active and methanol-tolerant electrocatalysts towards oxygen reduction reaction, J. Mater. Chem. A 1 (2013) 13542–13548. [41] J. Chang, L. Feng, C. Liu, W. Xing, X. Hu, An effective Pd–Ni2P/C anode catalyst for direct formic acid fuel cells, Angew. Chem. Int. Ed. 53 (2014) 122–126. [42] S. Wang, J. Chang, H. Xue, W. Xing, L. Feng, Catalytic stability study of a Pd-Ni2P/C catalyst for formic acid electrooxidation, ChemElectroChem 4 (2017) 1243–1249. [43] J. Liu, C.Q. Sun, W. Zhu, Origin of efficient oxygen reduction reaction on Pd monolayer supported on Pd-M (M = Ni, Fe) intermetallic alloy, Electrochim. Acta 282 (2018) 680–686. [44] F. Wang, H. Xue, Z. Tian, W. Xing, L. Feng, Fe2P as a novel efficient catalyst promoter in Pd/C system for formic acid electro-oxidation in fuel cells reaction, J. Power Sources 375 (2018) 37–42.

Acknowledgements The authors acknowledge the financial support from Shandong Provincial Natural Science Foundation (ZR2016BQ21, ZR2017QB010, ZR2017QB017), High Education Science and Technology Program of Shandong Province (J16LC03), Cultivation Fund of Liaocheng University (318011615). Graduate Education Quality Promotion Program (LCUYN1806, SDYY18186), and Students Innovation Fund (CXCY2018071). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.02.071. References [1] A. Mikolajczuk-Zychora, A. Borodzinski, P. Kedzierzawski, B. Mierzwa, M. Mazurkiewicz-Pawlicka, L. Stobinski, E. Ciecierska, A. Zimoch, M. Opałło, Highly active carbon supported Pd cathode catalysts for direct formic acid fuel cells, Appl. Surf. Sci. 388 (2016) 645–652. [2] E. Antolini, Palladium in fuel cell catalysis, Energy Environ. Sci. 2 (2009) 915–931. [3] Y. Tang, S. Cao, Y. Chen, T. Lu, Y. Zhou, L. Lu, J. Bao, Effect of Fe state on electrocatalytic activity of Pd–Fe/C catalyst for oxygen reduction, Appl. Surf. Sci. 256 (2010) 4196–4200. [4] Z. Liu, X. Yang, L. Cui, Z. Shi, B. Lu, X. Guo, J. Zhang, L. Xu, Y. Tang, Y. Xiang, Highperformance oxygen reduction electrocatalysis enabled by 3D PdNi nanocorals with hierarchical porosity, Part. Part. Syst. Charact. 35 (2018) 1700366. [5] C. He, J. Tao, P.K. Shen, Solid synthesis of ultrathin palladium and its alloys' nanosheets on rGO with high catalytic activity for oxygen reduction reaction, ACS Catal. 8 (2018) 910–919. [6] Y. Li, S. Lin, X. Ren, H. Mi, P. Zhang, L. Sun, L. Deng, Y. Gao, One-step rapid in-situ synthesis of nitrogen and sulfur co-doped three-dimensional honeycomb-ordered carbon supported PdNi nanoparticles as efficient electrocatalyst for oxygen reduction reaction in alkaline solution, Electrochim. Acta 253 (2017) 445–454. [7] M. Wang, W. Zhang, J. Wang, D. Wexler, S.D. Poynton, R.C.T. Slade, H. Liu, B. Winther-Jensen, R. Kerr, D. Shi, J. Chen, PdNi hollow nanoparticles for improved electrocatalytic oxygen reduction in alkaline environments, ACS Appl. Mater. Interfaces 5 (2013) 12708–12715. [8] C. Du, P. Li, F. Yang, G. Cheng, S. Chen, W. Luo, Monodisperse palladium sulfide as efficient electrocatalyst for oxygen reduction reaction, ACS Appl. Mater. Interfaces 10 (2018) 753–761. [9] T.T. Vo Doan, J. Wang, K.C. Poon, D.C.L. Tan, B. Khezri, R.D. Webster, R. D., H. Su, H. Sato, Theoretical modelling and facile synthesis of a highly active boron-doped palladium catalyst for the oxygen reduction reaction, Angew. Chem. Int. Ed. 55 (2016) 6842–6847. [10] J. Li, J. Chen, Q. Wang, W.-B. Cai, S. Chen, Controllable increase of boron content in B-Pd interstitial nanoalloy to boost the oxygen reduction activity of palladium, Chem. Mater. 29 (2017) 10060–10067. [11] Y. Jing, T. Heine, Two-dimensional Pd3P2S8 semiconductors as photocatalysts for the solar-driven oxygen evolution reaction: a theoretical investigation, J. Mater. Chem. A (46) (2018) 23495–23501. [12] K. Zhang, C. Wang, D. Bin, J. Wang, B. Yan, Y. Shiraishi, Y. Du, Fabrication of Pd/P nanoparticle networks with high activity for methanol oxidation, Catal. Sci. Technol. (16) (2016) 6441–6447. [13] T. Li, Y. Wang, Y. Tang, L. Xu, L. Si, G. Fu, D. Sun, Y. Tang, White phosphorus derived PdAu–P ternary alloy for efficient methanol electrooxidation, Catal. Sci. Technol. 7 (2017) 3355–3360. [14] S. Carenco, Y. Hu, I. Florea, O. Ersen, C. Boissière, C. Sanchez, N. Mézailles, Structural transitions at the nanoscale: the example of palladium phosphides synthesized from white phosphorus, Dalton Trans. 42 (2013) 12667–12674. [15] L. Cheng, Z. Zhang, W. Niu, G. Xu, L. Zhu, Carbon-supported Pd nanocatalyst modified by non-metal phosphorus for the oxygen reduction reaction, J. Power Sources 182 (2008) 91–94. [16] R. Rego, A.M. Ferraria, A.M. Botelho do Rego, M.C. Oliveira, Development of PdP nano electrocatalysts for oxygen reduction reaction, Electrochim. Acta 87 (2013) 73–81. [17] S. Carenco, D. Portehault, C. Boissière, N. Mézailles, C. Sanchez, 25th Anniversary Article: exploring nanoscaled matter from speciation to phase diagrams: metal phosphide nanoparticles as a case of study, Adv. Mater. 26 (2014) 371–390.

279