Supporting Information. Molecule-level g-c3n4 Coordinated Transition Metals as a New Class of Electrocatalysts for Oxygen Electrode Reactions

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1 Supporting Information Molecule-level g-c3n4 Coordinated Transition Metals as a New Class of Electrocatalysts for Oxygen Electrode Reactions Yao Zheng 1, Yan Jiao 1, Yihan Zhu 2, Qiran Cai 3, Anthony Vasileff 1, Lu Hua Li 3, Yu Han 2, Ying Chen 3, Shi-Zhang Qiao 1 * 1 School of Chemical Engineering, University of Adelaide, Adelaide, SA 5005, Australia 2 King Abdullah University of Science and Technology, Advanced Membranes and Porous Materials Center, Physical Sciences and Engineering Division, Imaging and Characterization Core Lab, Thuwal , Saudi Arabia 3 Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia *Corresponding to: s.qiao@adelaide.edu.au S1

2 Part I: Experimental Section 1.1 Synthesis of Materials Pristine multi-walled carbon nanotubes (CNT, Sigma-Aldrich) were sonicated in a 3:1 v/v solution of sulfuric acid (98%) and nitric acid (70%) at room temperature for 24 h to introduce hydrophilic functional groups on the surface. After repeatedly washing, the treated CNT was stirred with a 5 M nitric acid solution for 24 h to eliminate metal impurities. For the synthesis of Co-C3N4/CNT, based on theoretical predictions, one tri-s-triazine (C6N8) can coordinate one Co atom; that means that two g-c3n4 moieties can coordinate one Co atom. Therefore, it was expected that 3 mol of g-c3n4 precursor, dicyandiamide (DCDA), can coordinate 1 mol of Co. Based on this assumption, 25 mg of treated CNT was dispersed in 100 ml water, then 50 mg (~0.6 mmol) of DCDA and 23 mg of CoCl2 6H2O (~0.1 mmol) were added and stirred overnight at 50 o C. Note here that the amount of DCDA was doubled to ensure coordination efficiency and to also account for melamine sublimation during the polymerisation of g-c3n4 from DCDA. The mixture was then lyophilised and the collection was annealed at 600 o C in N2. After annealing, DCDA was converted to g-c3n4 via a series of polycondensation reactions while the CoCl2 was thermally reduced to Co atoms embedded within the g-c3n4 matrix. The powder was subsequently washed with 1 M HCl twice and then with water several times to remove uncoordinated cobalt oxides. As control samples, C3N4/CNT was prepared by the same procedure as Co-C3N4/CNT but without the addition of metal salt; cobalt oxide supported on CNT (CoOx/CNT) was prepared using the same procedure as Co- C3N4/CNT but without the addition of DCDA. The cobalt-nitrogen complex supported on CNT (Co- N/CNT) was prepared by an annealing step at 900 o C in N2 after washing the Co-C3N4/CNT. 1.2 Physical and Chemical Characterization The NEXAFS measurements were carried out in an ultrahigh vacuum chamber (~10 10 mbar) of the undulator soft X-ray spectroscopy beamline at the Australian Synchrotron. The samples were dispersed in deionised water and then deposited and dried on gold plates. In all NEXAFS scans, 50 mev energy steps were used. The raw NEXAFS data was normalised to the photoelectron current of the photon beam, measured on an Au grid. The EXAFS spectra were recorded in transmission mode using an ionisation chamber filled with a mixture of Ar, N2 and He at the XAS beamline at the Australian Synchrotron. The photon energy was controlled using Si(111) double-crystal monochromators operating at the peak of the rocking curve, and higher harmonics were rejected using X-ray mirrors. The data was normalised and analysed using Athena and Artemis software. The background subtraction and normalisation procedures were carried out using standard routines with default parameters determined by the Athena software. S2

3 Low-resolution TEM images were collected on on a JEM-2100 microscope operating at 200 kv. High resolution TEM images were acquired on a FEI Titan ST microscope and STEM images were acquired on a Cs-corrected FEI Titan G ST microscope equipped with a HAADF detector, both of which were operated at 300 kv. XPS measurements were performed on a lab-based Kratos Axis ULTRA X- ray Photoelectron Spectrometer with a 165 mm hemispherical electron energy analyser and Monochromatic Al Kα X-ray radiation (1,486.6 ev) at 225 W (15 kv, 15 ma). 1.3 Electrochemical Measurements The as-prepared catalyst powder was first dispersed in distilled water (Milli-Q) containing 0.05 wt.% of Nafion by ultrasonication. 40 μl of the aqueous catalyst dispersion (2.0 mg/ml) was then deposited onto a glassy carbon rotating disk electrode (RDE, cm 2, Pine Research Instrumentation) serving as a working electrode. The working electrode was rotated at 1,600 rpm for both ORR and OER tests. The reference electrode was an Ag/AgCl in 4 M KCl solution and the counter electrode was a graphite rod. All potentials were referenced to the reversible hydrogen electrode (RHE) and all polarisation curves were corrected for the ir contribution within the cell. The electrolytes for the ORR and OER tests were O2 saturated 0.1 M KOH and N2 saturated 1 M KOH solutions respectively. Rotating ring-disk electrode (RRDE) voltammogram measurements for the calculation of the ORR electron transfer number were conducted on an RRDE configuration with a 320 μm gap Pt ring electrode. The ring potential was constant at +0.5 V for oxidising any HO2 - intermediate. The electron transfer number (n) was determined as follows: n 4 I d Id I / N where Id is the disk current, Ir is the ring current and N is the current collection efficiency of the Pt ring which was determined to be All data was recorded using a CHI 760 D bipotentiostat. r S3

4 Part II: Computation Section 2.1 Methods and Models All calculations in this work were performed using plane-wave density functional theory (DFT) employing periodic boundary conditions as implemented in the VASP code. 1-4 The electronic exchange-correlation energy was modelled using the Perdew-Burke-Ernzerhof (PBE) functional within the generalised gradient approximation (GGA). 5,6 The projector augmented wave (PAW) method was used to describe the ionic cores. 7,8 For the plane-wave expansion, a 550 ev kinetic energy cut-off was used after testing a series of different cut-off energies. A Monkhorst-Pack k-point grid was used to sample the Brillouin zone. The convergence criterion for the electronic structure iteration was set to be 10-4 ev, and that for geometry optimisations was set to be 0.01 ev/å on force. A Gaussian smearing of 0.2 ev was applied during the geometry optimisation and for the total energy computations. To better describe the dispersion interaction within water adsorption systems, vdw correction was considered by adopting the Grimme s D2 scheme. 9 The optimised lattice parameter for g-c3n4 was 7.13 Å. The constructed supercell for the free energy calculation contained a 2 2 unit cells with 20 Å separation between two layers in the z direction. 2.2 Free Energy Calculation First, the binding energy (Eb) of a series of transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn) with that of g- C3N4 ligand was evaluated as: Eb = - (EM-C3N4 Eg-C3N4 EM) where EM-C3N4 is the total energy of the transition metal embedded in g-c3n4, Eg-C3N4 is the energy for the g-c3n4 and EM is the energy for the isolated metal atom. Then, the ORR and OER reaction pathway on the strongest M-C3N4 (M = Co, Fe, or Ni) models were calculated. The computational hydrogen electrode was utilised to obtain free energies for each state as done in refs 10 and 11. In alkaline media, the four electron ORR pathway could be summarised by the 12, 13 following elementary steps: O2 + * O2* O2* + H2O + e OOH* + OH OOH* + e O* + OH O* + H2O + e OH* + OH S4

5 OH* + e * + OH The OER pathway could be summarised as: * + OH OH* + e OH* + OH O* + H2O + e O* + OH OOH* + e OOH* + OH * + O2 + H2O + e The investigated sites include the embedded metal atom itself, a pyridinic nitrogen atom at the edge of g-c3n4 and that in the vacancy of g-c3n4 without coordinating a metal atom. The entropy and zero point energy were obtained by frequency calculation at 300K. The table below shows the adopted value. The gas phase values were acquired as: 14 TS (ev) ZPE (ev) *OOH *OH *O H 2 O H Construction of Dual Volcano Plot Onset overpotential was used to describe the activity of M-C3N4 for the ORR and OER on the dual volcano plot, which is defined as: ηorr = MAX [GOOH* (0), GO* (0) GOOH* (0), GOH* (0) GO* (0), -GOH* (0) ] 0.22 ev, and ηoer = MAX [GOH* (0), GO* (0) GOH* (0), GOOH* (0) GO* (0), -GOOH (0) ] ev where MAX indicates the maximum value among the four in the bracket, GOOH* (0), GOH* (0), GO* (0) are the free energies of the corresponding state at equilibrium potential (η = 0, setting the initial and final state as the reference energy level), 0.22 ev is the correction obtained from ref. 13. GOOH* (0), GOH* (0), GO* (0) are obtained from adsorption energies (Ead) for OOH*, OH*, and O* as: GOOH* (0) = Ead(OOH*) + EOOH + G(OH-) G(O2) G(H2Ol) eV + (ZPE T S) OOH* S5

6 GOH* (0) = Ead(OH*) + EOH G(OH-) 0.455eV + (ZPE T S) OH* GO* (0) = Ead(O*) + EO + 2 G(OH-) eV G(H2Ol) G(O2) + (ZPE T S) O* where EOOH, EOH, and EO are the energies of standalone OOH, OH, and O whose values are ev, ev, and ev respectively. Based on the linear relationship between Ead(OOH*), Ead(OH*), Ead(O*): Ead(OOH*) = Ead(OH*) eV Ead(O*) = Ead(OH*) eV The equations describing the ORR process for the dual volcano plot are therefore: η = -GOH* (0) 0.220eV (GOH* (0) < eV) η = GOH* (0) eV (GOH* (0) > eV) The equations describing the OER process for the dual volcano plot are therefore: η = GOH* (0) eV (GOH* (0) < eV) η = GOH* (0) eV (-1.762eV < GOH* (0) < eV) η = GOH* (0) 0.964eV (GOH* (0) > eV) S6

7 Part III: Supplementary Results a b Cycle: E=0.00 ev Above edge: E=0.30 ev Above middle: E=0.77 ev Above nitrogen: E=2.50 Figure S1. (a) Illustration of the starting position for Co adsorption on g-c3n4. (b) Top and side views of Co coordinated in different sites of the g-c3n4 matrix after geometry optimisation. Green, blue, and red represent: C, N, and Co atoms respectively. Bottom values indicate the relative binding energies of different atomic configurations. The reference energy level was set to be that of the Cycle configuration in the top left panel. S7

8 a 2 ORR 0.1 M KOH b 2 U= 0 V vs NHE (0.83 V vs RHE) U= 0.75 V vs NHE (1.58 V vs RHE) U= 0.46 V vs NHE (1.23 V vs RHE) 1 1 Free energy (ev) 0-1 Free energy (ev) U= 0 V vs NHE (0.77 V vs RHE) U= 0.06 V vs NHE (0.83 V vs RHE) U= 0.46 V vs NHE (1.23 V vs RHE) Reaction pathway -2 OER 1 M KOH Reaction pathway Figure S2. Free energy diagram of (a) ORR and (b) OER on the Co-C3N4 model under different potentials in alkaline media. Composition of reaction intermediates are the same as marked in Figure 1a,b. a b c Co2p O1s N1s C1s Intensity (a.u.) after wash before wash 200 nm Binding energy (ev) Figure S3. TEM images of (a) the fresh Co-C3N4/CNT catalyst and (b) after acid washing. (c) XPS survey spectra of fresh Co-C3N4/CNT catalyst and after acid washing. S8

9 a b Co-N x Normalized intensity (a.u.) Normalized intensity (a.u.) Co-N x pure g-c 3 Co-C 3 /CNT Photon energy (ev) Photon energy (ev) Figure S4. (a) N K-edge NEXAFS spectra of Co-C3N4/CNT and pure g-c3n4 reference sample. (b) Co L-edge NEXAFS spectrum of Co-C3N4/CNT. Normalized Absorbance (A.U.) ev ev ev CoO Co 3 O 4 Co-C 3 /CNT Energy (ev) Figure S5. XANES spectra of Co-C3N4/CNT and cobalt oxides references. Table S1. Fitting parameters of Co-C3N4/CNT EXAFS spectra including coordination number (N), interatomic distance (R), Debye-Waller factor (σ 2 ), and energy shift (E0). Path N R (Å) σ 2 (Å 2 ) E 0 (ev) Co-N Co-C S9

10 a 0.0 b 4 3 Current (ma) Co-C 3 /CNT (disk) Co-C 3 /CNT (ring) Pt/C (disk) Pt/C (ring) E vs. RHE (V) Electron transfer 2 1 Co-C 3 /CNT Pt/C E vs. RHE (V) Figure S6. (a) Disk (solid curves) and (b) corresponding ring (dashed curves) currents of Co-C3N4/CNT and Pt/C electrocatalysts on RRDE with a consistent rotation of 1,600 rpm in O2-saturated 0.1 M KOH solutions. Calculated electron transfer numbers of two electrocatalysts under different potentials. a 1.0 Before wash After wash b 10 8 Before wash After wash Current (ma) Current (ma) E vs. RHE (V) E vs. RHE (V) Figure S7. CV curves of before and after washed Co-C3N4/CNT samples under (a) ORR and (b) OER zones in N2-satuated alkaline media. Arrows indicate cobalt reduction/oxidation redox peaks on the sample before washing. S10

11 a 0 b After wash Before wash After wash Before wash -2-2 j (ma cm ) -2 j (ma cm ) E vs. RHE (V) E vs. RHE (V) Figure S8. (a) ORR and (b) OER polarization curves on Co/CNT samples before and after acid washing. It is clear that without g-c3n4 coordination, Co/CNT lost most active sites after washing. Dashed curves are on Co-C3N4/CNT samples for comparison. Figure S9. Electron density differences on various 3d transition metals coordinated to the g-c3n4 framework, and Fe-N-C for comparison. Surface value for panel a is 0.05 e/a3 and for the bottom panel b is e/a3. S11

12 Figure S10. Free energy diagram of (a) ORR on different transition metal embedded g-c3n4 models at an overpotential of 0.4 V; and (b) OER on Fe and Ni g-c3n4 models at an overpotential of 0.35 V in alkaline media. Different intermediates are the same as marked in Figure 1a,b. (c,d) Top and side views of atomic configurations for ORR/OER intermediate states on Fe-C3N4 and Ni-C3N4 models. Color codes are the same as marked in Figure 1d-e. S12

13 E F Fe-C 3 Density of States Co-C 3 Ni-C Energy (ev) Figure S11. D-band DOS of different transition metal embedded g-c3n4 models. EF indicates the fermi level. S13

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