Cobalt Ferrite bearing Nitrogen Doped Reduced. Graphene Oxide Layers Spatially Separated with. Electrocatalyst

Transcription

1 Supporting Information Cobalt Ferrite bearing Nitrogen Doped Reduced Graphene Oxide Layers Spatially Separated with Microporous Carbon as Efficient Oxygen Reduction Electrocatalyst Varchaswal Kashyap,, Santosh K. Singh, and Sreekumar Kurungot *, Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 41108, India. Academy of Scientific and Innovative Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi , India. S-1

2 Catalyst Preparation: For the preparation of the final catalyst, CF/N-rGO was annealed at 150 o C for 12 h. The annealed sample was mixed well with Vulcan carbon in the ratio of 2:8 (CF/NrGO-150: Vulcan) by using a mortar and pestle. This catalyst is designated as CF/N-rGO-150- Vulcan and is the best catalyst among the series in terms of the electrochemical performance. The catalyst slurry was prepared by dispersing 10 mg/ml of active material in IPA:water (3:2) solution followed by the addition of 40 μl of 5% Nafion as a binder and the solution was ultrasonicated for 1 h. 10 μl of the obtained slurry was drop coated over the Glassy carbon electrode and was used as the working electrode (WE) for all the electrochemical analyses. Similar procedure was followed for the slurry preparation and catalyst coating for the other control catalysts. In case of CF/N-rGO-150, CF/N-rGO-Vulcan (20% CF/N-rGO + 80 % Vulcan carbon), CF/Vulcan* and CF-vulcan (20% cobalt ferrite + 80% Vulcan carbon), 10 mg of the catalysts were dispersed in the 1 ml of IPA: Water (3:2) following 40 μl of 5% Nafion as a binder. As prepared slurry was used for the RRDE and RDE studies. Kouetcky-Levich (K-L) Plot: The K-L plot study discloses the diffusion and kinetic limitation of the catalyst. The equation 1 represents the relation between the kinetic and the limiting current density. The Linear Sweep Voltamogram (LSV) of the catalyst includes contribution from diffusion-limiting (j L ) and kinetic (j K ) current density. Equation 2 represents the more elaborated form of Equation 1. = + (1) S-2

3 =1/0.62nFC o * D o 2/3 ν -1/6 ω 1/2 +1/nFkC o * (2) where j is disk electrode current density, n is number of electron transferred during ORR, C o is the concentration of oxygen (1.22 x 10-6 mol/cm 3 ), D o is the diffusion coefficient of oxygen (1.93 x 10-5 cm 2 /s), ν is the kinematic viscosity of the electrolyte (0.01 cm 2 /s), ω is the angular rotation of the electrode in radians/sec ( = 2 /60) and k is the rate constant. K-L plot gives the linear relation between 1/j and 1/ω 1/2. From the intercept of the plot, kinetic current density (j K ) can be calculated. Rotating Ring-Disk Electrode (RRDE) Measurement: The catalysts inks were coated over the RRDE electrode and the electrode was dried under an IR lamp. During the analysis, the ring potential was kept at a constant value of 0.60 V vs Hg/HgO and the disk electrode was scanned at a scan rate of 10 mv/s. For the calculation of H 2 O 2 % and the number of electron transfer Equation 3 and 4 were, respectively, used. = / / (3) S-3

4 = / (4) where I d is the disk current, I r is the ring current and N is the collection efficiency of the ring electrode. The collection efficieny of the ring was determind to be 0.37 from K 3 Fe[CN] 6. Figure S1: (a), (b) and (c) represent the particle size histograms of CF nanopartilces prepared under the water contents of 0, 50 and 66%, respectively, in ethanol. S-4

5 Figure S2.(a) Cyclic voltamogram recorded in oxygen saturated 0.1 M KOH at a scan rate of 50 mv/s by using the cobalt ferrite nanoparticles synthesized at different compositions of water and ethanol in the solvothermal process (the corresponding water % is indicated inside the bracket); in oxygen saturated condition faradic process overcomes capacitance whereas in N 2 saturation no contribution from the faradic reaction is evident. (b) LSVs of cobalt ferrite nanoparticles at 10 mv scan rate and 1600 rpm in oxygen saturated 0.1 M KOH solution. (c) TGA profile of CF/N- to rgo recorded under air media indicating a residue content of 35%, which is corresponding the S-5

6 amount of CF accommodated by N-rGO. (d) X-ray diffractograms of the unsupported CF nanoparticles synthesized under various water to ethanol ratios by the solvothermal method. (e) X-ray diffratogram of CF (50%) after 600 o C annealing for 4 hours. Figure S3. (a) BET isotherms of the different samples; (b) O 1s XPS spectra of CF and CF/N- represent the rgo-150 which were synthesized with 1:1 water:ethanol ratio; (c) and (d) deconvoluted 2p spectra of Co and Fe in CF. S-6

7 Figure S4. EDAX of CF synthesized at a water to ethanol ratio of 1:1 in the presence (a) and absence (b) of N-rGO. S-7

8 Figure S5. (a) SEM image of CF/N-rGO-150-Vulcan. (b), (c), (d), (e) and (f) are the elemental mapping of CF/N-rGO-150-Vulcan. S-8

9 Figure S6. (a) and (b) represent the LSVs of the catalysts recorded in O 2 saturated 0.1 M KOH solution with an electrode rotation speed of 1600 rpm. The LSVs of CF/N-rGO, CF/N-rGO-150 and CF/N-rGO-600 clearly indicate that the 150 o C annealed sample has better ORR activity. (c) Represents the Nyquist plots of CF/N-rGO-150 and the composite with Vulcan carbon. The spectra show better conductivity, charge transfer and mass diffusion in the Vulcan mixed composite. (d) Voltage vs Current (V-I) plots showing linear relation for the different catalysts; CF/N-rGO-150 (black line) shows high resistance compared to the other catalysts. S-9

10 Figure S7. CF/N-rGO in the absence (a) and presence (b) of an external magnetic field. Figure S8.(a) Comparison of the mass activity of the various CF based catalysts with the commercial 20 wt.% PtC catalyst at V (vs Hg/HgO); (b) polarization plots obtained from the ZABs based on CF/N-rGO-150-Vulcan and 20 wt.% PtC as the air electrodes and 1 M KOH as the electrolyte. S-10

11 Table 1. A comparison of the air catalysts explored for the cathode in zinc air batteries. Air Catalyst Core-Shell Fe-Cu Nanoparticle Co 3 O 4 with carbon black Battery Type Performance of the Battery Mechanically rechargeable Flexible rechargeable 1.2 V at 25 ma/cm 2 current density 1.2 V at 20 μm Electrolyte Battery Power Density Discharge capacity Reference 6 M KOH 212 mw/cm mw 460 mah g -1 2 CoO/N-CNT Rechargeable 1.2 V at 50 6 M KOH mah 3 ma/cm 2 mw/cm 2 g -1 N-CNT Primary - 6 M KOH mw/cm 2 Co 3 O 4 decorated nanofiber Rechargeable 1.21 v at 2 ma/cm 2 6 M KOH 125 mw/cm M KOH mah 6 6 M KOH 70 CMO/N-rGO Rechargeable 1.1 V at 20 ma/cm 2 g -1 G-PMF Primary 1.15 at mah 7 ma/cm 2 mw/cm 2 g -1 at cut off 0.9 V Co 3 O 4 - SP/NGr-24 Cu-Pt Nanocage CF/N-rGO- 150-Vulcan Mechanically rechargeable 1.35 at 10 ma/cm 2 6 M KOH 190 mw/cm 2 Primary ~ 1.3 V at 20 6 M KOH ~250 ma/cm 2 mw/cm 2 Primary 1 V at 30 6 M KOH, 155 ma/cm 2 1 M KOH mw/cm 2, 55 mw/cm mah 8 g mah 9 g mah This work g -1 S-11

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