Inside-out dual-doping effects on tubular catalysts

Address correspondence to Debin Kong, [email protected]; Bin Wang, [email protected]; Linjie Zhi, [email protected]

Electronic Supplementary Material

Inside-out dual-doping effects on tubular catalysts: Structural andchemical variation for advanced oxygen reduction performance Yang Gao1,2,3, Debin Kong1 (), Jiaxu Liang1, Daliang Han2, Bin Wang1 (), Quan-Hong Yang2, and Linjie Zhi1,2,3 ()

1 CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience

and Technology, Beijing 100190, China 2 School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China 3 University of Chinese Academy of Sciences, Beijing 100049, China Supporting information to https://doi.org/10.1007/s12274-021-3484-y

Experimental section

Sample preparation

A coaxial electro-spinneret was used in this study. For the preparation of the pristine films of N,P-CST, the core solution was composed of 0.52 g of polyvinylpyrrolidone (PVP, Mw =1,300,000, Alfa), 0.8 g of triphenylphosphine (TPP, 99%, Alfa), and 4 mL of N,N-dimethylformamide (DMF, AR, Beijing Chemical Works). To obtain the shell solution, 1.0 g of polyacrylonitrile (PAN, Mw = 150,000, Sigma) and 0.83 g of dicyandiamide (DCDA, 99%, Aladdin) were dissolved in 12.5 mL of DMF. A flat Al foil covered with non-dust cloth was used as a collector and put about 15 cm away from the nozzle tip. A commercial high-voltage source (SL50P60, Spellman High Voltage Electronics Corporation) was used for electrospinning, and a voltage of 16 kV was applied to the solution to start the spinning process. For the preparation of the pristine films of N,S-CST and N,B-CST, the only difference was the replacement of TPP by dibenzyl disulfide and triphenylborane in the core solution, respectively. With regard to N-CST, no additional reagent was used except for PVP in the core solution. The pristine films of CSTs were placed in a ceramic boat, heated to 550 °C (2 °C min–1), and maintained for 4 h in a tube furnace. The temperature in the furnace was further raised to 1,000 °C at a ramp rate of 5 °C min–1 and kept for 1 h under Ar flow. After that, the furnace was cooled down to room temperature naturally.

Sample characterization

The scanning electron microscopy (Hitachi S4800) and field emission transmission electron microscopy (FEI Tecnai G2 F20 U-TWIN) were used to characterize the morphologies of the samples. Raman spectra were collected by a Renishaw inVia Raman microscope with a laser wavelength of 514.5 nm. X-ray diffraction (XRD) patterns were obtained using Cu Kα radiation (Rigaku D/max-TTRIII). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB250Xi apparatus with an Al Kα X-ray source. Nitrogen adsorption/desorption isotherms were measured at 77 K with an ASAP 2020 physisorption analyzer. The Brunauer-Emmett-Teller (BET) method and the non-local density functional theory (NLDFT) pore model were used to calculate the specific surface area (SSA) and pore size distribution, respectively.

Electrochemical characterization

All electrocatalytic measurements were performed in a three-electrode cell using a rotating disk electrode (RDE, PINE Research Instrumentation) with a Bio-logic VMP potentiostat workstation at room temperature. A Pt foil (1 cm2) was used as the counter electrode, and an Ag/AgCl (Saturated KCl) electrode was selected as the reference electrode. All potentials in this study were referred to RHE, E(RHE) = E(Ag/AgCl) + 0.059× pH + 0.199. All of the potentials were 95% iR corrected.

To prepare the working electrode, 2.5 mg catalysts were dispersed in 0.52 mL solution containing 0.3 mL ethanol, 0.2 mL water, and 20 μL 5 wt.% Nafion solution, followed by ultrasonication for 60 min. Then, 10 μL catalyst ink was pipetted onto the glassy carbon (GC) electrode (0.196 cm2) and dried at room temperature. The catalyst loading for the CSTs and commercial Pt/C (20 wt.%, Johnson Matthey) on the GC electrode was ~0.25 mg cm–2.

The electrochemical experiments were carried out in an O2- or N2-saturated 0.1 M KOH electrolyte, and O2 or N2 was bubbled for 30 min prior to the test and maintained in the headspace of the electrolyte throughout the testing process. Cyclic voltammetry (CV) curves were recorded at a sweep rate of 50 mV s–1, until a stabilized current was obtained. RDE measurements were carried out by linear sweep voltammetry (LSV) at a scan rate of 10 mV s–1 and various rotating speeds from 400 to 2500 rpm. The Koutecky-Levich (K-L) equation was used for determining the electron transfer number (n):

1 1 1B 1/2 (S1)

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where jk is the kinetic current density and ω is the electrode rotating rate. B is determined from the slope of the K-L plots according to the Levich equation as given below:

B 0.2 F O2 O22/3 1/6 (S2)

where n represents the transferred electron number per oxygen molecule. F is Faraday constant (F = 96485 C mol–1). DO2 is the diffusion coefficient of O2 in 0.1 M KOH (DO2 = 1.9 × 10–5 cm2 s–1). is the kinetic viscosity ( = 0.01 cm2 s–1). CO2 is the bulk concentration of O2 (CO2 = 1.2 × 10–6 mol cm–3). The constant 0.2 is adopted when the rotation speed is expressed in rpm.

Computational methods

All the calculations were conducted using the Vienna ab initio simulation package (VASP) [S1, 2] based on density functional theory (DFT) [S3] and the projector-augmented wave (PAW) [S4, 5] method. The electronic exchange and correlation effects were described by the generalized-gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional [S6]. To ensure the convergence for the total energy, a plane-wave cutoff energy of 450 eV with Fermi-level smearing of 0.05 eV was used. The Brillouin zone was sampled with Monkhorst-Pack k-point meshes of 4×1×1. The spin-polarization and van der Waals corrections [S7] were used in all the calculations. Besides, geometry optimizations were performed until the force of the system converges to 0.02 eV/Å and the energies were converged within 10–5 eV. All the nanoribbons were built as three-dimensional periodic structures, and vacuum layers were set around 16 and 20 Å in the y- and z-directions, respectively.

The ORR can take place through a two-step two-electron pathway or a direct four-electron process. Previous works have demonstrated that the ORR pathway proceeding on doped graphene is through the four-electron mechanism, [S8] and the associative mechanism of ORR with a four-electron transfer process was considered here. According to the associative mechanism, the four-electron reaction of ORR in an acid environment goes through the following elementary steps:

2 (S3)

2 (S4)

(S5)

2 (S6)

where * represents an active site on the surface of graphene nanoribbons, (l) and (g) refer to liquid and gas phases, respectively, and HOO*, HO*, and O* stand for the adsorbed intermediates.

For the ORR process in an alkaline medium, the elementary steps are as follows:

2 2 (S7)

(S8)

2 (S9)

(S10)

To obtain the rate determining step of ORR on active sites for different model catalysts, the reaction free energy change of each step was calculated using Density Functional Theory. The reaction free energy change G of each ORR step is defined as the difference between the free energies of the initial and final states and is given by the expression: [S9]

∆ ∆ ∆ZPE ∆ ∆ ∆ pH (S11)

where E is the reaction energy of reactant and product molecules adsorbed on the surface of catalysts, ZPE and TS are the zero-point energy difference and the entropy change between the adsorbed state and the free state, respectively, and T is the temperature (300 K in this work). ΔGU is included to consider the effect of a bias on all states involving an electron in the electrode and can be determined using ΔGU = ‒eU, where e is the elementary charge and U is the potential versus the standard hydrogen electrode (SHE) at the electrode. ΔGpH is the free energy change of H+ according to the following equation:

∆ pH B ln 10 (S12)

where kB is Boltzmann's constant, and pH = 13. At the equilibrium potential such as U = 0.461 V at pH 13, the overpotential for ORR can be determined by the following

method:

ORR max ∆ 1, ∆ 2, ∆ 3, ∆ 4 (S13)

ORR ORR / (S14) where ΔG1, ΔG2, ΔG3, and ΔG4 are the reaction free energy changes of Reactions (S6)-(S9), respectively.

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Figure S1 SEM images of N,P-CST, N,S-CST, N,B-CST, and N-CST.

Figure S2 HRTEM images of N,S-CST (a), N,B-CST (b), and N-CST (c).

Figure S3 XPS survey spectra of N,P-CST, N,S-CST, N,B-CST, and N-CST.

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Figure S4 HAADF-STEM images of N,P-CST (a), N,S-CST (b), and N,B-CST (c). EDS line scans of N,P-CST (d), N,S-CST (e), and N,B-CST (f) along the radial direction of tube walls. The open edge and/or the split parts of these tubes were selected to check the element distribution inner the tubes.

Figure S5 P 2p XPS spectrum of N,P-CST.

Figure S6 S 2p XPS spectrum of N,S-CST.

Figure S7 B 1s XPS spectrum of N,B-CST.

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Figure S8 (a) CV curves of N,S-CST in an O2- or N2-saturated 0.1 M KOH solution. (b) LSVs curves of N,S-CST for ORR at different rotating speeds. (c) K-L plots of N,S-CST at different potentials.

Figure S9 (a) CV curves of N,B-CST in an O2- or N2-saturated 0.1 M KOH solution. (b) LSVs curves of N,B-CST for ORR at different rotating speeds. (c) K-L plots of N,B-CST at different potentials.

Figure S10 (a) CV curves of N-CST in an O2- or N2-saturated 0.1 M KOH solution. (b) LSVs curves of N-CST for ORR at different rotating speeds. (c) K-L plots of N-CST at different potentials.

Figure S11 CV curves of Pt/C before and after the addition of 3 M methanol solution.

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Figure S12 Molecular models for N,P co-doped graphene nanoribbons with zigzag edges. (a) Z-N1P4, (b) Z-N1Pc, (c) Z-N4P1-O, and (d) Z-NcP1-O. Symbols a, a#, and A denote the reaction sites considered in the calculations.

Figure S13 Molecular models for N,P co-doped graphene nanoribbons with armchair edges. (a) A-N1Pb, (b) A-N1Pc, (c) A-N2Pa-O, (d) A-N2Pb, (e) A-N2Pc, (f) A-N3Pa-O, and (g) A-N3Pb. Symbols 1, a, a#, and A denote the reaction sites considered in the calculations.

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Figure S14 (a) Scaling relationship between GHO* and GHOO*. (b) Scaling relationship between GHO* and GO*.

Figure S15 Atomic spin density distributions of the most active structure and the pristine graphene nanoribbon with zigzag edges.

Figure S16 Schematic free energy profiles for the ORR pathway on pristine graphene nanoribbon with zigzag edges in an alkaline medium.

Table S1 Material parameters for different materials.

Sample C at% O at% N at% P at% S at% B at% ID/IG SBET (m2 g-1) N,P-CST 92.22 5.32 2.35 0.11 - - 0.90 684 N,S-CST 91.33 5.18 3.13 - 0.36 - 0.90 361 N,B-CST 83.22 7.98 5.38 - - 3.42 0.97 213 N-CST 91.52 5.64 2.84 - - - 0.89 414

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Table S2 Comparison of the ORR activities of the recently reported dual-doped carbon-based electrocatalysts.

Catalyst Electrolyte (KOH) Mass loading (mg cm−2) ORR Tafel slope

(mV dec−1) ORR E1/2 (V vs. RHE) Ref.

N,P-CST 0.1 M 0.25 52 0.81 This work NSCNT-3 0.1 M 0.41 67 0.81 [S10]

N, S@CM-1000 0.1 M - 104 0.76 [S11] N–S-DPCs 0.1 M 0.10 - 0.80 [S12]

N,S-hcs-900 0.1 M - - 0.81 [S13] NPCF-1 0.1 M 0.2 69 0.78 [S14]

N,P-GFs-TPA-TPP 0.1 M 0.36 81 0.80 [S15] N/S-2DPC-60 0.1 M 0.25 - 0.75 [S16]

SNG 0.1 M 0.128 45 0.72 [S17] N,P-CGHNs 0.1 M 0.3 - 0.82 [S18]

Table S3 The computed free energy changes on different active sites at N,P co-doped graphene nanoribbons with zigzag edges. The computed Gibbs adsorption free energy values (Unit: eV) of O*, HO*, and HOO* are denoted as GO*, GHO*, and GHOO*, respectively. G1, G2, G3, and G4 are reaction free energy values in each ORR step at U = 0.461 V in an alkaline medium. ORR is the calculated overpotential of ORR (Unit: V).

GO* GHOO* GHO* G1 G2 G3 G4 ORR Z-N1P4-a 1.904 3.557 0.410 -0.133 -0.423 -0.264 0.820 0.820

Z-N1P4-a# 2.327 3.606 0.413 -0.084 -0.049 -0.684 0.817 0.817 Z-N1P4-A 1.905 3.575 0.406 -0.115 -0.441 -0.269 0.824 0.824

Z-N4P1-O-a 1.912 3.973 0.766 0.283 -0.831 0.084 0.464 0.464 Z-N4P1-O-a# 2.222 3.798 0.623 0.108 -0.346 -0.369 0.607 0.607 Z-N4P1-O-A 1.913 3.960 0.760 0.270 -0.817 0.077 0.470 0.470

Z-N1Pc-a 1.195 3.208 -0.054 -0.482 -0.783 -0.019 1.284 1.284 Z-N1Pc-a# 1.401 3.784 0.468 0.094 -1.154 0.298 0.762 0.762 Z-N1Pc-A 1.252 3.829 0.542 0.139 -1.346 0.520 0.688 0.688

Z-NcP1-O-a 2.099 4.020 0.801 0.330 -0.690 -0.068 0.429 0.429 Z-NcP1-O-a# 2.052 4.189 1.117 0.499 -0.907 0.294 0.113 0.499 Z-NcP1-O-A 1.792 4.231 1.028 0.541 -1.209 0.466 0.202 0.541

Table S4 The computed free energy changes on different active sites at N,P co-doped graphene nanoribbons with armchair edges. The computed Gibbs adsorption free energy values (Unit: eV) of O*, HO*, and HOO* are denoted as GO*, GHO*, and GHOO*, respectively. G1, G2, G3, and G4 are reaction free energy values in each ORR step at U = 0.461 V in an alkaline medium. ORR is the calculated overpotential of ORR (Unit: V).

GO* GHOO* GHO* G1 G2 G3 G4 ORR A-N1Pb-a# 1.205 4.410 1.205 0.720 -1.656 0.911 0.025 0.911 A-N1Pb-A 1.523 3.691 0.507 0.001 -0.938 0.213 0.723 0.723 A-N1Pc-a 1.058 3.870 0.734 0.180 -1.581 0.905 0.496 0.905

A-N1Pc-a# 1.212 3.869 0.644 0.179 -1.426 0.662 0.586 0.662 A-N1Pc-A 1.215 4.251 1.116 0.561 -1.806 1.130 0.114 1.130

A-N2Pa-O-1 0.033 3.680 0.266 -0.010 -2.418 1.464 0.964 1.464 A-N2Pa-O-a# 2.005 4.223 1.049 0.533 -0.988 0.274 0.181 0.533 A-N2Pa-O-A 2.006 4.467 1.370 0.777 -1.231 0.593 -0.140 0.777

A-N2Pb-1 -0.624 2.895 -0.522 -0.795 -2.289 1.332 1.752 1.752 A-N2Pb-a -0.662 2.817 -0.480 -0.873 -2.250 1.412 1.710 1.710

A-N2Pb-a# 0.153 3.008 -0.212 -0.682 -1.625 0.866 1.442 1.442 A-N2Pb-A 0.915 2.981 -0.213 -0.709 -0.835 0.102 1.443 1.443 A-N2Pc-1 -0.888 2.483 -0.764 -1.207 -2.141 1.354 1.994 1.994 A-N2Pc-a 0.328 3.417 0.246 -0.273 -1.859 1.148 0.984 1.148

A-N2Pc-a# 0.677 3.040 -0.143 -0.650 -1.133 0.411 1.373 1.373 A-N2Pc-A 0.678 3.508 0.329 -0.182 -1.600 0.881 0.901 0.901

A-N3Pa-O-1 1.705 3.810 0.353 0.120 -0.875 -0.122 0.877 0.877 A-N3Pa-O-a# 1.805 4.585 1.166 0.895 -1.550 0.591 0.064 0.895 A-N3Pa-O-A 1.806 4.222 1.078 0.532 -1.186 0.502 0.152 0.532

A-N3Pb-1 -0.183 3.340 -0.087 -0.350 -2.293 1.325 1.317 1.325 A-N3Pb-a -0.830 2.285 -0.937 -1.405 -1.885 1.123 2.167 2.167

A-N3Pb-a# 0.743 3.516 0.355 -0.174 -1.543 0.841 0.875 0.875 A-N3Pb-A 1.057 2.995 -0.210 -0.695 -0.708 -0.037 1.440 1.440

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Inside-out dual-doping effects on tubular catalysts