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Page 4 of 16 Li et al. Energy Mater 2023;3:300021 https://dx.doi.org/10.20517/energymater.2023.09

was obtained and named NiFe-LDH. Ni-LDH and Fe-LDH were prepared in the same way as NiFe-LDH,
except that Fe(NO ) ·9H O and Ni(NO ) ·6H O were not added, respectively.
3 2
3 3
2
2
Synthesis of the Se@NiFe: First, 79 mg of NiFe-LDH was dispersed in 57 mL of deionized water with the aid
of ultrasonication (labeled as A solution). 79 mg of selenium powder was dispersed into 3 mL of hydrazine
hydrate solution (labeled as B solution). Then, A and B solutions were mixed and stirred for 4 h. Finally, the
above mixed solution was reacted in a 100 mL Teflon-lined autoclave at 100 °C for 6 h. The product
Se@NiFe was achieved after washing with deionized water and vacuum drying at 60 °C.

Synthesis of the FePc/Se@NiFe: The synthesis process for FePc/Se@NiFe was the same as the above-
mentioned one. Specifically, 10 mg of FePc was dispersed into 10 mL of absolute ethanol and ultrasonically
treated for 30 min (labeled as A solution); 79 mg of selenium powder was dispersed into 3 mL of hydrazine
hydrate solution (labeled as B solution); 79 mg of NiFe-LDH was ultrasonically dispersed in 47 mL of
deionized water (labeled as C solution). Next, the three solutions of A, B and C were stirred for another 4 h.
The resulting mixture was then poured into a 100 mL Teflon-lined autoclave and reacted at 100 °C for 6 h.
The final FePc/Se@NiFe was obtained by washing with deionized water and drying under vacuum at 60 °C.
The synthesis of FePc/Se@Ni was by converting the NiFe-LDH precursor to Ni-LDH and FePc/Se@Fe uses
Fe-LDH as the precursor.

Material characterizations
X-ray powder diffraction (XRD) was performed on a Bruker D8 Advance diffractometer with Cu Kα
radiation (λ = 0.154178 nm) at room temperature. The X-ray photoelectron spectroscopy (K-Alpha 1063)
was employed to detect the element compositions and valence states of catalysts, and all the peaks were
corrected by the standard of C 1s line at 284.8 eV. The carbon defects and graphitization degree were
measured by Raman spectroscopy (InVia). The specific surface area (SSA) and pore volume of samples were
tested with N adsorption and desorption isotherms by Brunauer-Emmett-Teller (BET) (Micromeritics
2
ASAP 2020). The structural morphologies of catalysts were observed by employing a high-resolution
transmission electron microscope (HRTEM, FEI Tecnai F20) and field-emission scanning electron
microscope (FESEM, Zeiss sigma 300). The relevant elemental distributions were presented by using
energy-dispersive X-ray spectroscopy (EDX).

Electrochemical measurements
The electrochemical performance of the prepared catalyst was tested on CHI660E by a traditional three-
electrode system. The electrolyte solution was 0.1 M KOH (ORR) or 1.0 M KOH (OER), Ag/AgCl
(saturated KCl solution) was used as a reference electrode, and a platinum sheet was used as an auxiliary
electrode. The ORR test was a glassy carbon electrode with a catalyst mass loading of 0.75 mg cm and the
-2
-2
OER test was a carbon paper electrode loaded with 0.75 mg cm catalyst. All cyclic voltammetry (CV) tests
were operated at a scan rate of 50 mV·s , and linear sweep voltammetry (LSV) tests were executed at a scan
-1
-1
rate of 5 mV·s . Double-layer capacitance (C ) data were acquired by CV at different scan rates of 2, 4, 6, 8,
dl
and 10 mV·s in the absence of faradaic current. Tafel slope was determined by the following equation:
-1
η = b log(j) + a, where η, b and j are the overpotential, Tafel slope, the current density, respectively. The
electrochemically active surface area (ECSA) could be calculated by ECSA = S × C /C equation, where C
s
dl
s
values were 0.04 mF/cm in alkaline conditions. Accelerated durability test (ADT) was conducted between
2
-0.3~-0.8 V and 0.2~0.7 V at 100 mV s in 0.1 M KOH (O -saturated) and 1.0 M KOH electrolytes (O -
-1
2
2
saturated), and then measured the LSV curve. The relative current of the I-T curve was also a means of
assessing stability. The electron transfer number (n) of catalysts was figured below in the following
Koutechy-Levich (K-L) equation: