Cobalt Oxide and Cobalt‐Graphitic Carbon Core–Shell Based Catalysts with Remarkably High Oxygen Reduction Reaction Activity

The vital role of ethylenediaminetetraacetic acid on the structure and the oxygen reduction reaction activity of the non‐precious‐metal‐based pyrolyzed catalyst is reported and elaborated. The resultant catalyst can overtake the performance of commercial Pt/C catalyst in an alkaline medium.


Characterizations
Powder X-ray diffraction (XRD, Rigaku Smartlab 3kW) was performed to identify the constituent phases of the synthesized catalysts using filtered Cu-Kα radiation (λ =1.5418Å) operating at a tube voltage of 40kV and a current of 200 mA. The diffraction patterns were obtained using a step scan at 2θ = 10-75° with a step size of 0.02°.The morphology of the CoOx/Co@GC-NC sample was obtained using a field emission scanning electron microscope (FE-SEM, HITACHI-S4800). The transmission electron microscopy (TEM) was conducted at 200 kV with FEI Tecnai G2T20 electron microscope and the corresponding EDX mappings were obtained using an FEI Tecnai G2F30 S-TWIN field emission transmission electron microscope equipped with EDAX operated at 300 kV. The chemical compositions and surface element states were probed by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe spectrometer equipped with an Al-Kα X-ray source). The Raman spectra were collected using an HR800 UV Raman microspectrometer (JOBIN YVON, France) with the green line of an argon laser as the excitation source. The specific surface area and pore size distribution were measured using N2-adsorption on a BELSOR-MAX instrument.The elemental analysis results were acquired using a Vario EL elemental analyzer.

Electrodes preparation and electrochemical characterizations
For accurate comparison, analogous mass loading of catalyst was used in all electrochemical measurements performed on a CHI 760E Bipotentiostat. Electrochemical characterizations were conducted in a standard thee-electrode electrochemical cell with glassy carbon as the working electrode substrate, Pt wire as the counter electrode and Ag|AgCl (3.5 M KCl) as the reference electrode. Working electrode ink was prepared by dispersing 20 mg of each catalyst sample into a mixture of 100 μL of 5 wt % Nafion solution and 1 mL of ethanol followed by sonication for at least 1 hour. A 5 μL of ink aliquot was drop-casted onto glassy carbon substrate (5 mm diameter, 0.196 cm 2 area) and dried at room temperature inside an upside-down glass jar, leading to an approximate catalyst loading of 0.464 mg cm -2 .The commercial 20wt. % Pt/C catalyst (Johnson Matthey Corp.) was used as a comparison. Except for Pt/C case which used a quarter loading only of 5 mg, the working electrodes were prepared following the identical process which led to an approximate catalyst loading of 0.116 mgtotal cm -2 .
For approximately 30 min prior to the start of each test till the end of test, O2 or N2 was continuously bubbled through 0.1 M KOH aqueous electrolyte solution. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were carried out using a rotating disk electrode (RDE). LSVs were performed at different rotation speeds (400, 800, 1200, 1600, 2000 and 2500 rpm) at a scan rate of 5 mV s -1 from 0.2 V to -0.6 V versus Ag|AgCl (3.5 M KCl) in O2-saturated 0.1M KOH.Notably, the LSVs of the commercial 20 wt % Pt/C catalyst were recorded in the positive scan (−0.6 to 0.2 V) to avoid performance loss caused by anion adsorption. For CV, the scan rate was kept at 10 mV s -1 with potential from 0.1 V to -0.9 V versus Ag|AgCl (3.5 M KCl) in a N2 or O2 atmosphere.
Ag|AgCl (3.5 M KCl) was used as the reference electrode in all electrochemical measurements, which was calibrated to reversible hydrogenelectrode (RHE). The calibration was performed in the high purity hydrogen saturated0.1 M KOH solution with a platinum rotating disk electrode (0.126cm 2 , Pine Research Instrumentation) as the working electrode. CV was cycled at a scan rate of 1mVs -1 from -0.8V to -1.15 V and the cross-over point at which the hydrogen evolution current changes into the hydrogen oxidation currents was taken as the thermodynamic (zero) potential for the hydrogen electrode reactions. The kinetic parameters such as the electron transfer number were determined using Koutecky-Levich (K-L) equation: Where J is the measured current density, JL is the diffusion-limited current density, Jk is the mass-transport-corrected kinetic ORR current density, n is the electron transfer number, F is the Faraday constant, C 0 is the saturated concentration of oxygen in 0.1 M KOH, ω is the rotating rate (rad s −1 ), DO2 is the diffusion coefficient of oxygen, ν is the kinetic viscosity of the solution and k is the rate constant of ORR.
Rotating ring-disk electrode (RRDE) measurements were performed using Pt ring electrode to obtain the electron transfer number (n) and peroxide yield (X, percentage of HO2 − relative to total products). For each measurement, the working (disk) electrode was scanned cathodically at 1600 rpm using a scan rate of 5 mV s -1 from 0.2 V to -0.6 V whereas the ring potential was set at 0.5 V versus Ag|AgCl (3.5 M KCl) to induce complete peroxide (OH2 -) decomposition. The electron transfer number (n) and peroxide yield (X, % HO2 -) were calculated as follows: Where Id is the disk current, Ir is the ring current and N is the current collection efficiency of the Pt ring (0.422 according to earlier measurements).            [3] Co3O4/N-rmGO 0.  [14] Co3O4 OC/RGO 0.4 ----0.14V vs Hg/HgO Sci. Rep. 2013, 3, 2300. [15]