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Page 4 of 13 Luo et al. Microstructures 2023;3:2023011 https://dx.doi.org/10.20517/microstructures.2022.41

where I, Δt, m, ΔV indicate the applied current (A), discharge time (s), the mass load of the active material
(g) and the working potential (V), respectively.

In the Ni N-Co N /NC//AC asymmetric supercapacitor system, active carbon (AC) was applied as the
2
3
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negative electrode material, which was prepared using the same steps as the positive electrode. The mass of
[25]
AC can be computed based on the charge balance equation showing as follows :

where m , m , C , C , V , V represent the mass (g), specific capacitance (F g ) and operating voltage window
-1
-
+
+
-
+
-
(V) of the positive and negative electrode, respectively.
-1
In addition, the energy density (Wh kg ) and power density (W kg ) at different current densities were
-1
calculated from the following Eqs. :
[26]

where C, ΔV, Δt are the specific capacitance (F g ), working potential (V) and discharge time (s) of the
-1
device, respectively.

RESULTS AND DISCUSSION
As schemed in Figure 1, the complete synthesis route of Ni N-Co N /NC includes two steps. In the first
0.67
2
3
hydrothermal process, TEOA could serve as an ideal solvent and more importantly as a ligand for metal
complexes . To avoid inhibition of the complex growth kinetics and nonuniform dispersion of the
[27]
reaction system due to strong viscosity of TEOA, DI water was chosen as the only cosolvent. Initially, TEOA
molecules react with Ni (II) and Co (II) to form Ni Co -TEOA complex at low temperatures. With the
y
x
temperature rising, this metal alkoxide begins to hydrolyze, leading to the hollow porous nanoflower
structure . In the subsequent thermal treatment, carbon skeleton is pyrolyzed at 400 °C and transformed to
[21]
N-doped carbon owing to the existence of N center atom of TEOA. Simultaneously, urea decomposes into
NH which further reacts with Ni (II) and Co (II), producing Ni N and Co N eventually. Additionally,
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2
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various mass ratios of Ni to Co (0:3, 1:2, 1:1, 2:1, 3:0) were used to investigate the impact on morphology
2+
2+
and electrochemical performance of the complex and the best one was chosen to further derivative to the
final product.
XRD patterns of N C -TEOA, N C -TEOA, N C -TEOA, N C -TEOA and N C -TEOA are presented in
3
1
0
2
1
0
3
1
1
2
Supplementary Figure 1. As expected, all the precursors show no sharp and distinct peaks, exhibiting salient
amorphous characteristics . The wide peak bulge at 15°-25° is assigned to the (002) crystal face of carbon,
[28]
[29]
indicating the successful formation of nitrogen-doped carbon after calcining [Figure 2A] . The peaks at
38.9°, 42.1°, 44.5°, 58.5°, 70.6° and 78.4° could be attributed to the (110), (002), (111), (112), (300) and (113)