A Hybrid Material Combined Copper Oxide with Graphene for an Oxygen Reduction Reaction in an Alkaline Medium

In this work, an electrode material based on CuO nanoparticles (NPs)/graphene (G) is developed for ORR in alkaline medium. According to the characterization of scanning electron microscope and transmission electron microscope, CuO NPs are uniformly distributed on the wrinkled G sheets. The X-ray diffraction test reveals that the phase of CuO is monoclinic. The CuO/G hybrid electrode exhibits a positive onset potential (0.8 V), high cathodic current density (3.79 × 10−5 mA/cm2) and high electron transfer number (four-electron from O2 to H2O) for ORR in alkaline media. Compared with commercial Pt/C electrocatalyst, the CuO/G electrode also shows superior fuel durability. The high electrocatalytic activity and durability are attribute to the strong coupling between CuO NPs and G nanosheets.


Instrumentation and Measurements
The products were characterized by powder X-ray diffraction (XRD, Cu Ka irradiation; λ = 0.154 nm) with a SIEMENS D5000 X-ray diffractometer. The crystallite sizes of copper oxide can be calculated by the Scherrer's formula: Where D is the average crystalline size, K is the shape factor being 0.890, λ is the wavelength of Xray being 0.154 nm for Cu Kα radiation, β is the full width at half maximum of the peak, θ is the diffraction angle of the peak.
The morphology of the synthesized samples was tested by scanning electron microscopy (SEM, JEOL JSM-6701F electron microscope operating at 5 KV). Transmission electron microscopy (TEM) images were examined by a Philips Tecnai 20U-TWIN transmission electron microscope with linear resolution of 0.14 nm and dot resolution of 0.19 nm. Raman spectra tests were conducted by a TriVista TM 555CRS Raman spectrometer at 785 nm. X-ray photoelectron spectroscopy (XPS) data was collected by an ESCALABMKII X-ray photoelectron spectrometer (VG Scienta, USA) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The pressure in the chamber during the measurements was kept at 1×10 −7 Pa. The analyzer was operated at a pass energy of 50 eV for high resolution scans and at a pass energy of 100 eV for survey scans. The binding energy of the C 1s peak at 284.6 eV was taken as a reference for the binding energy calibration. A background subtraction and peak fitting were deconvolved using the XPS peak fitting software (XPSPEAK41 by Prof. R. W. M. Kwok).

Electrode preparation and electrochemical tests
5 mg of the prepared catalyst powder was dispersed in the mixture of 450 µL of deionized water and 50 µL of Nafion (5 wt% solution alcohols, DuPont). The mixture was fully sonicated to form a homogeneous ink. Then 5 µL of the ink was dropped onto a glassy carbon (GC) electrode of 3 mm in diameter and fully dried. Cyclic voltammetry measurements were performed using a CHI 760E electrochemical workstation (CH Instrument, USA) by conventional three-electrode cell. The coated glass carbon (GC) electrode is employed as the working electrode, graphite as the counter-electrode, and a saturated calomel electrode (Hg/Hg 2 Cl 2 ) (SCE) as the reference electrode.
Before the ORR tests, cyclic voltammetry (CV) tests were performed from 0.2 to -0.8 V at 5 mV/s in Ar-saturated electrolyte to clean the electrode surface. 20 cycles were carried out to stabilize the current-potential signal. Thereafter, the electrolyte was saturated with oxygen before the start of every experiment by bubbling O 2 at least 30 min, which was maintained over the electrolyte in order to ensure its continued O 2 saturation during the recording. The working electrode was cycled at least 20 cycles before data were recorded at a scan rate of 5 mV/s from 0.2 to -0.8 V vs. Hg/Hg 2 Cl 2 in O 2 -saturated 0.1 mol/L KOH electrolytes.
The Tafel tests were also conducted at a sweeping rate of 5 mV/s. Rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) tests were performed using a RRDE-3A electrode at the same sweeping rate. For RRDE tests, the working electrode was a glassy carbon disk (5.61 mm in diameter) and a platinum ring leading to a collection efficiency of the ring disk electrode. The RRDE tests were performed at 1600 rpm in O 2 -saturated solution. The Pt ring electrode was polarized at -0.3 V vs.
Hg/Hg 2 Cl 2 for oxidizing the hydrogen peroxide ion during oxygen reduction at the modified GC disk electrode. All the experiments were carried out in 0.1 mol/L KOH solution at room temperature.
The Tafel tests were also conducted at a sweeping rate of 5 mV/s. the exchange current density was derived from the mass-transport correction using Eq. (2): Where E represents the tested electrode potential, E 0 is the thermodynamics electrode potential, F is the Faraday constant, R is the ideal gas constant, T is the thermodynamic temperature, i d is the measured current density, and i 0 is the exchange current density.
Rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) tests were performed using a Where I k is the kinetic current and w is the angular velocity (w = 2πN, N is the linear rotation speed). For RRDE test, the ORR percentage of peroxide species and electron transfer numbers with respect to total ORR products on our samples were calculated from the following equations: