Supplementary Information Nanosheets Co 3 O 4 Interleaved with Graphene for Highly Efficient Oxygen Reduction Taiwo Odedairo, Xuecheng Yan, Jun Ma, Yalong Jiao, Xiangdong Yao, Aijun Du, Zhonghua Zhu,* School of Chemical Engineering, The University of Queensland, St Lucia Brisbane Australia. Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan Campus, Australia. School of Engineering, University of South Australia, Mawson Lakes, SA, Australia. School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane Australia. * Corresponding author. Fax: +61 73365 4199. E-mail address: z.zhu@uq.edu.au (Zhonghua Zhu)
MATERIALS AND METHODS Materials Graphite powder (99.9995%) was obtained from Alfa-Aesar Co. Ltd and the commercial carbon-supported Pt/C electrocatalyst (20 wt.% Pt Vulcan XC-72) was obtained from Premetek. Sodium nitrate (NaNO 3 ), sulfuric acid (H 2 SO 4, 98%), potassium permanganate (KMNO 4 ), urea (NH 2 CONH 2 ) and cobalt nitrate (Co(NO 3 ) 2. 6H 2 O) were purchased from Aldrich Chemical. Hydrochloric acid (HCl, 36%) and hydrogen peroxide (H 2 O 2, 30%) were purchased from Ajax Finechem. Oxidation of Graphite 5 g of graphite powder was placed into a mixture containing 80 ml of concentrated sulfuric acid and 3.5 g of NaNO 3. The resulting mixture was stirred in an ice bath for 5 h. After the powder was homogeneously dispersed in the solution, KMnO 4 (20 g) was slowly added over 1 h to the solution in an ice bath at 273 K. The solution was removed from the bath after 2 h, and was further stirred for 4 days to produce brown-coloured viscous slurry. Then, 500 ml of deionized water was added to the mixture and maintained at 368 K. After 1 h, the mixture was poured into 600 ml deionized water containing 20% H 2 O 2, resulting in a brilliant yellow colour. This produced graphene oxide (GO) was collected by filtration and washed several times with a total of 3 L of 10% HCl solution followed by 5 L of deionized water to remove sulphate ions. Graphene oxide slurry was then used as below. S-1
Synthesis of Reduced Graphene Oxide (G) and Loosely Connected Spherical Co 3 O 4 Nanoparticles/Graphene Composite. For comparison, reduced GO (G) was synthesized by treating GO under microwave argonplasma at 750 W for 5 sec. The nitrogen-doped G was synthesized by annealing the powder in 100% NH 3 at 500 C for 3 h at 5 C/min. Co 3 O 4 nanoparticle/graphene composites (Co-P/G) were synthesized for comparison purposes, by impregnating the N-doped G with Co 3 O 4 nanoparticles, followed by treating the dried slurry under microwave argon-plasma at 750 W for 5 sec. Electrochemical Measurements Rotating Disk Electrode (RDE) Measurement. The working electrode was scanned cathodically at a rate of 10 mv/s with varying rotating speed from 400 to 2500 rpm. Koutecky-Levich plots (J -1 vs ω -1/2 ) were analyzed at various electrode potentials. The slopes of their best linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky-Levich equation: 1 J 1 J L 1 + J K 1 = Bw 1 + 2 1 J = (1) K 2 / 3 1/ 6 0 D0 v ; B= 0.62nFC J K = nfkc 0 (2) where J is the measured current density, J k and J L are the kinetic and diffusion-limiting current densities, ω is the angular velocity, n is transferred electron number, F is the Faraday constant (F = 96485 C/mol), D 0 is the diffusion coefficient of O 2 in 0.1 M KOH (1.9 x 10-5 cm 2 /s), C 0 is the bulk concentration of O 2 (1.2 x 10-6 mol/cm 3 ), v is the kinetic viscosity of the electrolyte (0.01 cm 2 /s), and k is the electron-transfer rate constant. The constant 0.2 is adopted when the rotation speed is expressed in rpm. S-2
Rotating Ring-Disk Electrode (RRDE) Measurement. The transferred electron number per oxygen molecule (n) and % HO 2 - during ORR can also be calculated from Eq. (3) based on RRDE measurements, n I d 4I + I d = (3) r / N HO 2 = 200I I + I d r r / / N N (4) where I d is disk current, I r is ring current, and N is current collection efficiency of the Pt ring. N was determined to be 0.43 from the reduction of K 3 Fe[CN] 6. The measured potentials versus the Ag/AgCl reference electrode were converted to the reversible hydrogen electrode (RHE) scale via the Nernst equation: E RHE = E Ag/AgCl + 0.059pH + E Ag/AgCl (5) where E RHE is the converted potential versus RHE, E Ag/AgCl is the experimental potential measured against the Ag/AgCl reference electrode, and E Ag/AgCl is the standard potential of Ag/AgCl at 25 C (0.197 V). The electrochemical measurements were carried out in 0.1 M KOH (ph = 13) at room temperature; therefore, E RHE = E Ag/AgCl + 0.964 V. RDE and RRDE measurements were performed using a CHI Electrochemical Station (Model 760E) in a standard three-electrode cell. A 4.0 mm diameter glassy carbon disk (disk geometric area 0.126 cm 2 ) was used as the working electrode; a Pt wire and Ag/AgCl (in saturated KCl solution) were used as the counter and reference electrodes, respectively. Before use, the electrode was polished to mirror flat with alumina powder. The ink formulation (Co-S/G and Co-P/G) is 1 mg of catalyst in a 2.3:1 water-ethanol mixture (1 ml) containing 20 µl of Nafion solution (5 wt%). The ink was shaken and sonicated in bath S-3
sonicator for 1 h. 10 µl of catalyst ink was loaded on the electrode surface resulting in a catalyst loading of ~0.08 mg/cm 2. The loading of catalyst is 32 µg Pt cm -2 for the commercial Pt/C (20 wt% Pt on Vulcan XC-72, Premetek). Prior to testing, the electrolyte of 0.1 M KOH was saturated with O 2 for at least 30 min, and O 2 purging was kept during electrochemical experiments. Density Funtional Theory (DFT) Calculations DFT calculations were computed by the plane-wave basis Vienna Ab-initio Simulation Packages (VASP) code 1,2 implementing the Perdew-Burke-Erzonhoff (PBE) exchange correlation functional. 3 A damped van der Waals correction is also incorporated based on Grimme s scheme 4 to better describe the non-bonding interaction between graphene and Co 3 O 4 layer. The cut-off energy for plane waves is set to be 500 ev and the vacuum space is at least 18 Å, which is large enough to avoid the interaction between periodical images. A Morkhost pack mesh of K-points (5 3 1) and (9 7 1) is used to sample the two dimensional Brillouin zone respectively for optimizing the geometry and for calculating the charge density. The convergence of tolerance force on each atom during structure relaxation was set to 0.01 ev/å. The interface adhesion energy (E ad ) reported in the study was calculated as follows: E ad = E E E (6) comp graphene Co 3 O 4 where E comp, E graphene and Ec o3o4 are the total energy of the relaxed Co 3 O 4 /graphene composite, pure graphene sheet (or N-doped graphene sheet) and Co 3 O 4 nanosheet, respectively. S-4
RESULTS a G b Co-S/G-3 c Co-S/G-2 d Co-S/G-1 e T-Co-S f Co-S Figure S1. Nitrogen adsorption/desorption analysis. (a) G, (b) Co-S/G-3, (c) Co-S/G-2, (d) Co-S/G-1, (e) T-Co-S and (f) Co-S. S-5
a G b Co-S/G-3 c Co-S/G-2 d Co-S/G-1 Figure S2. The pore size distribution. (a) G, (b) Co-S/G-3, (c) Co-S/G-2 and (d) Co-S/G-1. S-6
Figure S3. XRD pattern of treated Co 3 O 4 nanosheets (T-Co-S). S-7
a b Figure S4. (a) FESEM and (b) HRTEM micrographs of reduced graphene oxide (G). Scale bar: 100 nm (a) and 100 nm (b). The inset in (b) is the SAED pattern. S-8
a G b T-Co-S T-Co-S G d c T-Co-S G Figure S5. (a c) TEM micrographs of Co-S/G-2, and (d) SAED pattern of Co-S/G-2. Scale bar, (a) 200 nm, (b) 100 nm, (c) 50 nm and (d) 2 1/nm. S-9
Figure S6. Raman spectra of G, Co-P/G, Co-S/G-3, Co-S/G-2, Co-S/G-1 and T-Co-S (Region for the Co 3 O 4 peaks are marked by rectangular box). S-10
a T-Co-S b Co-S/G-2 Co-S/G-1 Co-S/G-3 c Co-P/G Figure S7. XPS spectra of (a) T-Co-S, (b) Co 3 O 4 nanosheets/graphene composites and (c) Co 3 O 4 nanoparticles/graphene composite (Co-P/G). S-11
a b Figure S8. N1s XPS spectra of (a) Co-S/G-3 and (b) Co-P/G. S-12
Figure S9. TEM micrographs of Co-P/G. Scale bar, 50 nm S-13
Figure S10. CV curves of Co-P/G, Co-S/G-1 and Co-S/G-2 at 100 mv/s in O 2 -staurated 0.1 M KOH solution. S-14
Figure S11. CV curves of commercial Pt/C and Co-S/G-3 at 100 mv/s in O 2 - staurated 0.1 M KOH solution. S-15
Figure S12. Linear sweep voltammograms of the Co-S/G-1, Co-P/G-2, Co-P/G and commercial Pt/C in O 2 -staurated 0.1 M KOH solution at 1600 rpm with a scan rate of 10 mv/s. S-16
Figure S13. Linear sweep voltammograms of the Co-S/G-3 and commercial Pt/C in O 2 -staurated 0.1 M KOH solution at 1600 rpm with a scan rate of 10 mv/s. S-17
a Co-S/G-3 b Co-S/G-3 n = ~4 c d Pt/C Pt/C n = ~4 Figure S14. (a and b) Linear sweep voltammograms of Co-S/G-3 and (c and d) commercial Pt/C at different rotation speed and corresponding Koutecky-Levich plots. The scan rate is at 10 mv/s in O 2 - staurated 0.1 M KOH solution. S-18
Figure S15. Percentage of peroxide and electron transfer number of Co-S/G-3 and commercial Pt/C at various potentials based on the RRDE data. S-19
Figure S16. LSVs of commercial Pt/C measured during cycling durability in O 2 - saturated (cycles: 5000; potential range 0.6 1.0 V (vs RHE) at 50 mv/s). S-20
Co 3 O 4 /graphene Figure S17. DFT calculation studies of Co-S/G-3. The side (left) and top (right) views of the optimized Co 3 O 4 nanosheets/graphene composite interface. Brown, blue, red and green balls represent C, Co, O and N atoms, respectively. S-21
Table S1. Characteristics of the Electrocatalysts Product Abbreviations Ratios of Co oxide /graphene (Co/G) a BET surface area (m 2 g -1 ) b Total pore volume (cm 3 g -1 ) c Micropore volume (cm 3 g -1 ) d Cobalt content (wt. %) 1. Co-S - 4.94 0.02 - - 2. T-Co-S - 66.2 0.10 0.02-3. G - 597.7 3.12 0.23-4. Co-P/G 1:22 - - - 4.32 5. Co-S/G-1 1:6 158.0 0.80 0.06 15.03 6. Co-S/G-2 1:10 230.8 1.24 0.09 9.46 7. Co-S/G-3 1:23 296.9 1.60 0.11 4.12 a BET specific surface area calculated within the relative pressure range of 0.05 0.20. b Total pore volume at P/P 0 = 0.99. c Micropore volume, obtained by applying the Dubinin-Radushkevich method to N 2 adsorption isotherm. d Cobalt content was determine by ICP-OES analysis. S-22
Table S2. XPS for Surface Composition of Composites Sample Surface composition (at. %) C 1s O 1s Co 2p N 1s Co-P/G 88.75 7.86 1.50 1.89 Co-S/G-1 88.29 7.48 1.96 2.27 Co-S/G-2 89.73 6.30 1.38 2.59 Co-S/G-3 88.68 5.53 0.93 4.86 S-23
Table S3. Summary of Reported Current Densities for Non-Precious Metal and Pt-Based Electrocatalysts Catalyst Catalyst loading (mg cm -2 ) Current density (ma cm -2 ) Rotation rate (r.p.m.) Reference Co-S/G-3 0.08 ~ 5.0 1600 This work Co-S/G-3 0.32 ~5.8 1600 This work Co-S/G-3 0.32 ~4.3 900 This work Co-S/G-3 0.64 ~6.6 1600 This work 20 wt% Pt/C 32 µg Pt cm -2 ~ 5.6 1600 This work 20 wt% Pt/C 32 µg Pt cm -2 ~ 4.4 900 This work 20 wt% Pt/C 16 µg Pt cm -2 ~ 5.5 1600 This work Co 3 O 4 nanoparticles/ N-doped graphene 0.17 ~ 5.0 1600 (5) N-Fe-CNT/CNP 1.0 ~3.5 900 (6) 20 wt% Pt/C 60 µg Pt cm -2 ~3.4 900 (6) 20 wt% Pt/C 20.4 µg Pt cm -2 ~4.7 1600 (7) 20 wt% Pt/C 60 µg Pt cm -2 ~5.5 1600 (8) 20 wt% Pt/C 57 µg Pt cm -2 ~5.0 1500 (9) 20 wt% Pt/C 84.8 µg Pt cm -2 ~3.7 1500 (10) 40 wt% Pt/C 259.6 µg Pt cm -2 ~6.5 1600 (11) Co 3 O 4 /Co 2 MnO 4 0.10 ~5.0 1600 (12) S-24
Table S4. Summary of Reported Half-Wave Potential (E 1/2 ) at 1600 rpm for Non-Precious Metal and Pt-Based Electrocatalysts Catalyst Catalyst loading Half-wave potential (E 1/2 vs RHE) Electrolyte Fe-phthalocyanine Unknown ~0.611 V 0.1 M KOH (13) Mn 3 O 4 /rgo 0.085 mg/cm 2 ~0.70 V 0.1 M KOH (14) Reference MnOx/Ketjblack 0.085 mg/cm 2 ~0.70 V 0.1 M KOH (15) Fe 3 O 4 /N-graphene 0.01 mg/cm 2 ~0.59 V 0.1 M KOH (16) FeCo-PDAP 0.5 mg/cm 2 ~0.76 V PBS (ph = 7) (17) Mn 3 O 4 Film ~0.75 V 0.1 M KOH (18) CoFe-N-C 0.5 mg/cm 2 ~0.78 V 0.1 M KOH (19) Fe/Fe 3 C-Melamine 0.286 mg/cm 2 ~0.76 V 0.1 M KOH (20) N-doped carbon 0.255 mg/cm 2 ~0.69 V 0.1 M KOH (21) P-doped carbon 0.79 mg/cm 2 ~0.72 V 0.1 M KOH (22) 20 wt% Pt/C 0.102 mg/cm 2 ~0.83 V 0.1 M KOH (7) 20 wt% Pt/C 0.30 mg/cm 2 ~0.83 V 0.1 M KOH (8) 20 wt% Pt/C 0.28 mg/cm 2 ~0.83 V 0.1 M KOH (9) 40 wt% Pt/C 0.65 mg/cm 2 ~0.83 V 0.1 M KOH (11) 20 wt% Pt/C 0.14 mg/cm 2 ~0.81 V 0.1 M KOH (12) 20 wt% Pt/C 0.122 mg/cm 2 ~0.78 V 0.1 M KOH (23) 20 wt% Pt/C 0.102 mg/cm 2 ~0.78 V 0.1 M KOH (24) Co-S/G-3 0.08 mg/cm 2 ~0.82 V 0.1 M KOH This work Co-S/G-3 0.32 mg/cm 2 ~0.832 V 0.1 M KOH This work S-25
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