Electrochemical measurements

A standard three-electrode cell was used for the electrochemical tests. A saturated calomel electrode (SCE) was used as a reference electrode, a Pt plate was used as a counter electrode and the catalyst film coated rotating ring-disk electrode (RRDE, 4 mm in diameter) was used as the working electrode. To prepare the catalyst ink, 5 mg of catalyst was ultrasonically dispersed in a solution of 50 μL Nafion (5 wt %) and 950 μL ethanol. The catalyst ink was cast onto a glassy carbon electrode. The catalyst loadings on RRDE were 0.4 mg cm⁻² for metal-free catalysts and 20 μg_Pt cm⁻² for commercial Pt/C catalyst. 0.1 M KOH solution and 0.1 M HClO₄ were used as alkaline and acidic media, respectively, for ORR measurement. The potentials are presented with respect to the reversible hydrogen electrode (RHE). The conversion factors for SCE to RHE are 0.998 V in 0.1 M KOH solution and 0.304 V in 0.1 M HClO₄, and were acquired by measuring the voltage between the SCE and a Pt-black-coated Pt wire that was immersed in the same electrolyte saturated with H₂. To saturate the electrolyte with Ar or O₂, the electrolyte was bubbled with Ar or O₂ for 20 min prior to each experiment. To ensure O₂ saturation during the linear sweep voltammetry measurement, O₂ was flowed through the electrolyte. Cyclic voltammetry experiments were conducted with a scan rate of 50 mV s⁻¹ between 0 and 1.2 V vs. RHE at room temperature. The catalytic activities were determined using linear sweep voltammetry (LSV) from 0 to 1.2 V at a scan rate of 5 mV s⁻¹ at different rotation rates, while the ring electrode was held at 1.3 V vs. RHE. The current densities were normalized to the geometry area of the glassy carbon disk in the LSV curves in this paper. The H₂O₂ collection coefficient at the ring in RRDE experiments was 0.37, as measured using a Fe(CN)₆^4-/3- redox couple. All the ORR currents presented in the figures are Faradaic currents, i.e. after correction for the capacitive current. The RRDE measurements were also carried out to determine the electron transfer number of ORR. The following equations were used to calculate n (the apparent number of electrons transferred during ORR) and %H₂O₂ (the percentage of H₂O₂ released during ORR),

where I_D is the Faradaic current at the disk, I_R the Faradaic current at the ring and N is the H₂O₂ collection coefficient at the ring. The apparent number of electrons transferred for ORR on the electrodes was also determined by the Koutechy–Levich equation given blow:

where j is measured current density, j_k is kinetic current density, j_L is diffusion limited current density, ω is electrode rotation rate, F is Faraday constant (96,485 C mol⁻¹), C₀ is bulk concentration of O₂ (1.2 × 10⁻³ mol L⁻¹ for both 0.1 M KOH solution and 0.1 M HClO₄ solution), D₀ is diffusion coefficient of O₂ (1.9 × 10⁻⁵ cm² s⁻¹ for 0.1 M KOH solution and 1.93 × 10⁻⁵ cm² s⁻¹ for 0.1 M HClO₄ solution) and ν is kinetic viscosity of the electrolyte (0.01 cm² s⁻¹ for both 0.1 M KOH solution and 0.1 M HClO₄ solution)⁵⁰.

Tafel slope was calculated according to Tafel equation:

Where E is the applied potential in the LSV test, a is a constant, b is the Tafel slop and j_k is the kinetic current density.

The stability tests were conducted by continuous cyclic voltammetry between 0.6 and 1.2 V (vs. RHE) in O₂-saturated 0.1 M KOH or HClO₄ solution with scan rate of 100 mV s⁻¹ for 5000 cycles. The stability was determined by measuring the change of LSVs before and after the cyclic voltammetry. Chronoamperometric (CA) measurements were also used to judge the stability of the catalysts with the potential holding at 0.7 V (vs. RHE) in O₂-saturated 0.1 M KOH electrolyte and 0.65 V (vs. RHE) in O₂-saturated 0.1 M HClO₄ electrolyte at a rotating rate of 400 rpm.

Methanol tolerance experiments were developed with CA measurements at 0.8 V (vs. RHE)/0.65 V (vs. RHE) in O₂-saturated 0.1 M solution at rotating rate of 1600 rpm. A volume of 4 mL methanol was injected into the KOH/HClO₄ solution at ca. 400 s.

Electrochemical impedance spectroscopy (EIS) was used to study the charge transfer resistance of the catalysts. EIS tests were conducted in O₂-saturated 0.1 M KOH with the rotating rate of 1600 rpm.

All the electrochemical tests were carried out at ambient temperature.