Electronic Supplementary Material

Biaxial strained dual-phase palladium-copper bimetal boosts formic acid electrooxidation

Jiarun Geng§, Zhuo Zhu§, Youxuan Ni, Haixia Li, Fangyi Cheng, Fujun Li (), and Jun Chen

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin 300071, China § Jiarun Geng and Zhuo Zhu contributed equally to this work.

Supporting information to https://doi.org/10.1007/s12274-021-3471-3

Experimental

Chemicals and Materials. Sodium tetrachloropalladate (II) (Na2PdCl4, 99%), copper (II) acetate dihydrate (Cu(CH3COO)2∙2H2O, 99%), and cetyltrimethylammonium bromide (CTAB, 99%) were purchased from Sigma-Aldrich. Oleylamine (OAm, 80~90%) and carbon black (Vulcan XC-72) were purchased from Macklin. Absolute ethanol (99.9%), cyclohexane (99%), and N,N- dimethylformamide (DMF, 99%), isopropanol were provided by Tianjin Concord Co., Ltd. The commercial Pd/C (5 wt% Pd) was purchased from Alfa-Aesar. All the materials were used without further purification. Synthesis of DP-PdCu and various PdCu/C catalysts. In the synthesis of DP-PdCu nanoparticles, 16.5 mg of Na2PdCl4, 35.5 mg of Cu(CH3COO)2∙2H2O, and 50.0 mg of CTAB were added into 16.5 mL of oleylamine. After 15 min vigorous stirring, it was heated at 160 oC for 30 min under Ar atmosphere, followed by heated at 250 oC for another 40 min. The product was precipitated by centrifugation, and washed by cyclohexane and absolute ethanol for several times. The synthetic procedure for FCC-PdCu and BCC-PdCu was similar to that of DP-PdCu, but without CTAB for FCC-PdCu and adding 100.0 mg of CTAB for BCC-PdCu. The as-synthesized PdCu nanoparticles were dispersed in 10.0 mL of cyclohexane for further use. In the synthesis of PdCu/C catalysts, 10.0 mL of the abovementioned PdCu suspension and 45.0 mg of Vulcan XC-72 were mixed in 30.0 mL of DMF and sonicated for 30 min. The products were collected by centrifugation at 9000 rpm for 5 min and washed with absolute ethanol for three times. Finally, the PdCu/C catalysts were dried at 50 oC in a vacuum oven for 12 h. Characterization. The phase structure was identified by a Rigaku SmartLab X-ray diffraction (XRD, Cu Kα radiation, λ = 1.5406 Å). The morphologies and crystal features were analyzed by high resolution transmission electron microscopy (HRTEM, FEI, Talos F200X G2, AEMC) and aberration-corrected scanning transmission electron microscopy (STEM, Titan Cubed Themis G2 300). The compositions of PdCu nanoparticles were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer Optima 8300). X-ray photoelectron spectroscopy (XPS) measurements were employed on a PerkinElmer PHI 1600 ESCA system. Electrochemical measurements. The working electrode was prepared by dropping catalyst ink onto a glassy carbon electrode (GC, 4 mm in diameter, 0.126 cm2 in area). Before use, the GC electrode was polished by aluminum oxide powders with size down to from 1.0 μm, 0.3 μm, and 0.05 μm, followed by washing with ultrapure water (ULUPURE, 18.2 MΩ cm) and ethanol for several times. 5.0 mg of PdCu/C catalyst was dispersed in a solution of 950 μL of isopropanol and 50 μL of Nafion solution (Alfa Aesar, 5 wt%) for 15 min sonication, 5.0 μL of which was carefully dropped on the clean GC electrode (Pd loading is 12.4 μg cm-2). The working electrode was dried naturally at room temperature. The electrochemical measurements were employed on a CHI 760E (CH Instruments, Inc., Shanghai) electrochemical workstation in a three-electrode system. An Ag/AgCl (1.0 M KCl) electrode and Pt foil (1 cm × 1 cm) served as the reference and counter electrode, respectively. All measurements were performed at room temperature. The PdCu/C catalysts coated GC electrodes were -1 cleaned by CV measurements in a solution of Ar-saturated 0.5 M H2SO4 for 50 cycles at 50 mV s . Formic acid oxidation -1 measurements were carried out in Ar-saturated 0.5 M H2SO4 + 0.5 M HCOOH solution from −0.2 to 1.0 V (vs. Ag/AgCl) at 10 mV s . The chronoamperometric (CA) measurements were conducted at 0.5 V (vs. Ag/AgCl) for 1000 s. The CO stripping measurements are performed with the following procedures. Firstly, the working electrode was held at -0.1 V (vs. Ag/AgCl) for 10 min under a flow of CO (10% CO, 90% N2). Then, Ar was bubbled into the electrolyte for 15 min to remove the remaining CO. The CO stripping curves were collected at -0.2 to 1.0 V (vs. Ag/AgCl) at 10 mV s-1 for 2 continuous cycles. Quantitative analyses of phase components. The phase ratios of FCC and BCC on PdCu nanoparticles were quantitatively calculated by reference intensity ratio (RIR) method [1]. Ix and IAl2O3 are the diffraction intensity of phase X and corundum (Al2O3, standard materials), respectively, which are captured from XRD pattern for a mass ratio of 1:1 of X and Al2O3. Wx and WAl2O3 are the

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weight fraction of phase X and Al2O3, respectively. Kx represents the reference intensity ratio and can be obtained from the standard PDF cards.

IWxx ==KK x IWAl23 O Al 23 O

IIFCC BCC Therefore, KKFCC==11.01(PDF#01-- 071 7854), BCC = IIAl23 O Al23 O For a specific sample composed of FCC and BCC phase,

IIFCC BCC WWFCC== FCC BCC FCC IIKFCC++ BCC BCC IIK BCC FCC BCC

FCC KFCC KBCC = KBCC Computational details. DFT calculations were performed by using the Vienna ab initio Simulation Package (VASP) with the projector augment wave method [2-4]. The generalized gradient approximation of the Perdew-Burke-Ernzerhof functional was applied to describe the exchange-correlation effect [5]. The cutoff energy was set as 400 eV. The convergence criteria of energy and force is set as 1 × 10-4 eV and 0.02 eV Å-1, respectively. A 1 × 1 surface unit cell was used for PdCu (100), PdCu (110) and PdCu3 (200) surface. A five-layer slab with a vacuum layer larger than 15 Å was used. The Brillouin zone was sampled with 10 × 10 × 1, 10 × 7 × 1, and 8 × 8 × 1 K points for PdCu (100), PdCu (110) and PdCu3 (200), respectively. All atoms were fully relaxed under stress or not.

Figure S1 TEM image (a) and size distribution (b) of DP-PdCu nanoparticles.

Figure S2 HAADF-STEM images of the DP-PdCu nanoparticles.

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Figure S3 STEM-EDS mappings (a) and EDS spectrum (b) of DP-PdCu nanoparticles. Mo element comes from the carbon-coating TEM grids.

Figure S4 (a) Atomic-resolution HRTEM image of DP-PdCu nanoparticles. (b) Intensity profile recorded along the yellow arrow in (a). Scale bar: 1 nm.

Figure S5 Simulated atomic model around the dual-phase interface without strain.

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Figure S6 (a) Aberration-corrected HAADF-STEM image of DP-PdCu along [001]BCC zone axis. (b) Integrated pixel intensity profile recorded along the yellow arrow in (a). (c) Aberration-corrected HAADF-STEM image of DP-PdCu along [110]BCC zone axis. (c1)-(c3) Integrated pixel intensity profiles recorded along the corresponding yellow arrows in (c).

Figure S7 HRTEM images of the surface defects in DP-PdCu nanoparticles. (a) (b) Twin boundaries. (c) (d) Dislocations.

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Figure S8 HRTEM image (a) and FFT pattern (b) of 0-PdCu.

Figure S9 HRTEM image (a) and FFT pattern (b) of 100-PdCu.

Figure S10 TEM images and size distributions of (a) (b) 0-PdCu, (c) (d) 50-PdCu, (e) (f) 100-PdCu, and (g) (h) 150-PdCu.

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Figure S11 TGA curve of DP-PdCu/C catalysts.

Figure S12 TEM images of Vulcan XC-72 supported various PdCu nanoparticles.

-1 Figure S13 CV curves recorded in the solution of Ar-saturated 0.5 M H2SO4 at 10 mV s .

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-1 Figure S14 CV curves recorded in the solution of Ar-saturated 0.5 M H2SO4 + 0.5 M HCOOH at 10 mV s . The current densities are normalized by ECSA.

-1 Figure S15 CO stripping curves tested in the solution of Ar-saturated 0.5 M H2SO4 at 10 mV s .

Fig. S16 Pd 3d (a) and Cu 2p (b) XPS spectra of DP-PdCu nanoparticles. The standard XPS positions are marked as the red dashed line.

Table S1 Atomic ratios of Pd/Cu in 0-PdCu, DP-PdCu, and 100-PdCu examined by ICP-OES measurements. Pd/Cu-1 PdCu-2 PdCu-3 Average 0-PdCu 1/1.05 1/1.09 1/1.01 1/1.05 DP-PdCu 1/1.10 1/1.15 1/1.07 1/1.10 100-PdCu 1/1.08 1/1.17 1/1.13 1/1.12

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Table S2 Quantification of strains at dual-phase interface along different projections analyzed by the line intensity profiles (+: tension, −: compression).

Theoretical Observed lattice spacing of Strain Zone Axes Projections lattice spacing of FCC {020}/{200} plane (relative to FCC PdCu) FCC {200} plane

[100]BCC 1.95 Å +5.4% 1.85 Å [010]BCC 1.93 Å +4.3% Observed lattice spacing of Theoretical Strain [001]BCC BCC {110}/ {110} plane lattice spacing of BCC{110} plane (relative to BCC PdCu)

[100]BCC 1.98 Å −4.8% 2.08 Å [010]BCC 2.01 Å −3.4%

Theoretical Observed lattice spacing of Strain lattice spacing of FCC {200}/{020} plane (relative to FCC PdCu) FCC {200} plane

[001]BCC 1.76 Å −4.8% 1.85 Å [ 110 ]BCC 1.95 Å +5.3% Theoretical Observed lattice spacing of Strain [110]BCC lattice spacing of BCC {100}plane (relative to BCC PdCu) BCC {100} plane

[001]BCC 3.09 Å 2.95 Å +4.6% Theoretical Observed lattice spacing of Strain lattice spacing of BCC {110} plane (relative to BCC PdCu) BCC {110} plane

[ 110 ]BCC 1.98 Å 2.08 Å −4.3%

Table S3 Comparison of electrocatalytic performance of Pd-based nanocatalysts. Formic Acid Oxidation Materials Ref. Jm Js Testing condition

0.5 M H2SO4 DP-PdCu 0.55 1.94 0.5 M HCOOH This Work 10 mV s-1

0.5 M H2SO4 core-shell 0.502 4.93 0.5 M HCOOH 1 CuPd@Pd tetrahedra 50 mV s-1

0.5 M H2SO4 hollow palladium/ ~0.1 N/A 0.25 M HCOOH 2 platinum nanocubes 50 mV s-1

0.5 M H2SO4 hyperbranched PdRu nanospine assemblies 1.10 3.23 0.25 M HCOOH 3 50 mV s-1

0.1 M HClO4 Pd-Ru Nanoparticle 0.612 3.22 0.25 M HCOOH 4 50 mV s-1

0.1 M HClO4 Pd nanosheets 0.634 N/A 0.2 M HCOOH 5 50 mV s-1 -1 -2 Jm: mass activity, A mgPd ; Js: specific activity, mA cmPd

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References [1] Chen, Y. F.; Yang, Y. F.; Fu, G. T.; Xu, L.; Sun, D. M.; Lee, J. M.; Tang, Y. W. Core-shell CuPd@Pd tetrahedra with concave structures and Pd-enriched surface boost formic acid oxidation. J. Mater. Chem. A 2018, 6, 10632-10638. [2] Huang, X. Q.; Zhang, H. H.; Guo, C. Y.; Zhou, Z. Y.; Zheng, N. F. Simplifying the creation of hollow metallic nanostructures: one-pot synthesis of hollow palladium/platinum single-crystalline nanocubes. Angew. Chem. Int. Ed. 2009, 48, 4808-4812. [3] Wang, H. J.; Li, Y. H.; Li, C. J.; Wang, Z. Q.; Xu, Y.; Li, X. N.; Xue, H. R.; Wang, L. Hyperbranched PdRu nanospine assemblies: an efficient electrocatalyst for formic acid oxidation. J. Mater. Chem. A 2018, 6, 17514-17518. [4] Wu, D. S.; Cao, M. N.; Shen, M.; Cao, R. Sub-5 nm Pd-Ru nanoparticle alloys as efficient catalysts for formic acid electrooxidation. ChemCatChem 2014, 6, 1731-1736. [5] Zhang, Y.; Wang, M. S.; Zhu, E. B.; Zheng, Y. B.; Huang, Y.; Huang, X. Q. Seedless growth of palladium nanocrystals with tunable structures: from tetrahedra to nanosheets. Nano Lett. 2015, 15, 7519-7525. [6] Qiu, Y.; Xin, L.; Li, Y.; McCrum, I. T.; Guo, F.; Ma, T., Ren, Y.; Liu, Q.; Zhou, L.; Gu, S.; Janik, M. J.; Li, W. BCC-phased PdCu alloy as a highly active electrocatalyst for hydrogen oxidation in alkaline electrolytes. J. Am. Chem. Soc. 2018, 140, 16580-16588. [7] Kresse, G.; Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169-11186. [8] Kresse, G.; Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15-50. [9] Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979. [10] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

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