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Luo et al. Microstructures 2023;3:2023011 https://dx.doi.org/10.20517/microstructures.2022.41 Page 3 of 13

MATERIALS AND METHODS
Materials
Nickel chloride hexahydrate (NiCl ∙6H O), Cobalt chloride hexahydrate (CoCl ∙6H O), triethanolamine
2
2
2
2
(TEOA) and urea were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd and used
without further purification. The deionized water (DI water) involved in the experiment with an electrical
resistivity of 18.2 MΩ cm was prepared by ultrapure water polishing system.
-1
Sample preparation and characterization
Synthesis of heterogenous Ni3N-Co2N0.67/NC hollow nanoflowers
Firstly, 2 g TEOA was dissolved in 18 mL DI water under vigorous stirring. Subsequently, x mmol
NiCl ∙6H O and y mmol CoCl ∙6H O (x + y = 3, x:y = 0:3, 1:2, 1:1, 2:1, 3:0) were added into the above
2
2
2
2
mixture to form a homogenous solution. The solution was then transferred to a 50 mL Teflon-lined
stainless-steel autoclave, which was heated at 160 °C and kept for 12 h. The obtained product was washed
with DI water and ethanol three times, respectively. Finally, the product was collected by centrifugation and
vacuum dried in an oven for 12 h at 60 °C to obtain N C -TEOA hollow nanoflowers. The samples are
y
x
labeled as N C -TEOA, N C -TEOA, N C -TEOA, N C -TEOA and N C -TEOA.
1 1
3 0
0 3
1 2
2 1
The heterogenous Ni N-Co N /NC was synthesized by the following process. Initially, 50 mg N C -TEOA
1
2
2
0.67
3
and 500 mg urea (mass ratio: 1:10) were uniformly dispersed in the porcelain boats. Then the boat with urea
and N C -TEOA was placed upstream and downstream of the tube furnace, respectively. The furnace was
1
2
-1
heated to 400 °C at a heating rate of 2 °C min under N atmosphere and kept for 2 h. After natural cooling
2
down to room temperature, heterogenous Ni N-Co N /NC hollow nanoflowers were obtained.
0.67
2
3
Structure and morphological characterization
X-ray diffraction (XRD) was tested using Rigaku Ultimate IV powder X-ray diffractometer with Cu Kα
radiation (λ = 1.5418 Å) at a scanning speed of 5°/min. Scanning electron microscopy (SEM) was performed
on Zeiss sigma 300 scanning electron microscope. Transmission electron microscopy (TEM), high-
resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were
carried out on FEI Talos F200x transmission electron microscope. Brunauer-Emmett-Teller (BET) specific
surface areas and pore volumes were measured on ASAP 2460. X-ray photoelectron spectroscopy (XPS)
data was collected on Thermo Scientific K-Alpha using Al Ka X-ray as the excitation source
(hv = 1486.6 eV). Fourier transform infra-red (FTIR) tests were performed on a FTIR apparatus (Nicolet
MX-1E, USA).
Electrochemical characterization
The electrochemical measurements were tested by a three-electrode configuration in 1 M KOH electrolyte.
Platinum electrode and saturated calomel electrode were used as the counter electrode and reference
electrode, respectively. All the electrochemical performance was studied on a CHI760E electrochemical
workstation. The working electrode was fabricated by the following procedures. Active material
(Ni N-Co N /NC), polyvinylidene fluoride (PVDF), and acetylene black (mass ratio: 7.5:1:1.5) were
0.67
2
3
homogenously mixed and ground using 1-methyl-2-pyrrolidinone as solvent to form a slurry. The slurry
was then uniformly coated on 1 × 1 cm square area of a 1 × 5 cm nickel foam (NF). After vacuum drying in
2
2
an oven at 60 °C for 12 h, the slurry-coated NF area was pressed under 10 Mpa for 2 min. Finally, the NF
was soaked into 1 M KOH for 3 h for pre-activation. The mass load of the active material on the NF was
[24]
-1
around 2~3 mg. The specific capacitance (F g ) can be calculated from the following Eq. (1) :