Edge‐hosted Atomic Co−N4 Sites on Hierarchical Porous Carbon for Highly Selective Two‐electron Oxygen Reduction Reaction

Abstract Not only high efficiency but also high selectivity of the electrocatalysts is crucial for high‐performance, low‐cost, and sustainable energy storage applications. Herein, we systematically investigate the edge effect of carbon‐supported single‐atom catalysts (SACs) on oxygen reduction reaction (ORR) pathways (two‐electron (2 e−) or four‐electron (4 e−)) and conclude that the 2 e−‐ORR proceeding over the edge‐hosted atomic Co−N4 sites is more favorable than the basal‐plane‐hosted ones. As such, we have successfully synthesized and tuned Co‐SACs with different edge‐to‐bulk ratios. The as‐prepared edge‐rich Co−N/HPC catalyst exhibits excellent 2 e−‐ORR performance with a remarkable selectivity of ≈95 % in a wide potential range. Furthermore, we also find that oxygen functional groups could saturate the graphitic carbon edges under the ORR operation and further promote electrocatalytic performance. These findings on the structure–property relationship in SACs offer a promising direction for large‐scale and low‐cost electrochemical H2O2 production via the 2 e−‐ORR.

3 characterized by X-ray diffraction (Shimadzu, XRD-6100) using a high-intensity Cu Kα radiation source (λ=1.54 Å) and operating at a voltage of 40 kV and a current of 30 mA. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo ESCALB 250XI X-ray photoelectron spectrometer. Co K-edge X-ray absorption spectra were collected on the hard X-ray beamline at the Australian Synchrotron (Melbourne, Australia). The extended X-ray absorption fine structure (EXAFS) analyses were conducted via the ATHENA module. Fourier transform of the k 2 -weighted EXAFS oscillations was used to evaluate the contribution of each bond to the Fourier transform peak.
Electrochemical measurements: All electrochemical measurements were performed on a CHI 760E electrochemical workstation (CH Instrument) coupled with a rotatingring disc electrode (RRDE) in a three-electrode cell. A graphite rod and an Ag/AgCl electrode with saturated KCl were used as the counter and reference electrodes, respectively. To prepare the working electrode, 2 mg of catalyst powder was dispersed into 1.0 mL of 1:1 water/ethanol solution and 20 μL of Nafion solution (10 wt.%), followed by ultrasonic treatment. Then, the catalyst ink was cast on the surface of RRDE with a catalyst loading of ∼0.1 mg cm −2 . All the potentials were calibrated to the reversible hydrogen electrode (RHE) where Id is the disk current; Ir is the ring current; N is the current collection efficiency of the Pt ring with a value of 0.3769, which was determined by the [Fe(CN)6] 3− / 4− redox couple ( Figure S19).
The kinetic current density (jk) was calculated to the equation: where jm indicates the measured total current density and jL is the diffusion-limiting current density. The value of jL is usually determined by the highest steady current measured in the entire potential range.
The turnover frequency (TOF) was calculated based on the following equation: where IH2O2 is the current for H2O2 production; MCo is the molar mass of cobalt (58.933 g mol −1 ); n is the number of transferred electrons based on RRDE results; F is the Faraday constant (96485 C mol −1 ); Mcat is the mass of catalyst on the electrode; ωCo is the mass ratio of Co in the catalyst.
Tafel slopes were calculated from the Tafel equation: where η is the potential, b is the Tafel slope, j is the current density, and j0 is the exchange current density.
where n is the electron transfer number (2 for the dioxygen reduction into H2O2); F is the Faraday constant (96485.3 C mol −1 ); is the consumed quantity of electric charge (C).

Theoretical calculations
The density functional theory (DFT) calculations were performed in the Vienna ab initio simulation packages (VASP), [2] using the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) parameterization. [3] According to previous reports, [4] the Co-N4 unit-embedded monolayer graphene structures were adopted for simulation. A vacuum space of 20 Å was set to eliminate possible interaction between the periodic images. The energy cut-off of the plane wave basis was set as 520 eV. The semi-empirical dispersion corrected DFT-D3 scheme proposed by Grimme was used to describe the van der Waals interactions. [5] Spinpolarization was applied for all the calculations. During the geometry optimization, the energy change criterion was set to 10 −5 eV, and the maximum force was 0.03 eV The formation energy (Eform) of the atomic Co-N4 moiety is defined as: where ECo@gra Egra, and ECo represent the total energies of single atom configuration, graphene substrate, and Co atom in the bulk phase. According to this definition, the more negative formation energy indicates the thermodynamically more favorable to stabilize atomic Co-N4 moiety.