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Page 2 of 13 Luo et al. Microstructures 2023;3:2023011 https://dx.doi.org/10.20517/microstructures.2022.41

[1,2]
portable electronic devices desires high-efficiency and high-capacity energy storage devices .
Supercapacitors (SCs) have attracted much interest due to their enhanced power density and long service
[3]
life . Standing in the intermediate zone of batteries and traditional capacitors, SCs could be generally
divided into two types. Electric double-layer capacitors (EDLCs) rely on the electrostatic attraction of ions
at the interfaces between electrode and electrolyte to complete charge storage, whereas pseudocapacitors
(PCs) take advantage of redox reactions during faradaic redox processes to store electric energies .
[4,5]
Importantly, developing SCs with stronger energy storage capacity inevitably demands the utilization of
better electrode materials .
[6]
Numerous transition metal nitrides (TMNs) such as Ni N , Co N , Fe N , VN and MoN have
[11]
[10]
[9]
[7]
[8]
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2
emerged as potential electrode materials for SCs by virtue of their distinctive electronic structure, stable
[12]
chemical resistance, remarkable electric conductivity, and flexible mechanical deformability . However,
most of the TMNs are synthesized by pyrolyzing the precursor under NH atmosphere, which leads to
3
massive waste of NH and causes immeasurable environmental pollution. Consequently, there is a desperate
3
need to develop a more convenient and green approach to prepare TMNs. Importantly, applying nontoxic
and environmentally friendly nitrogen sources is a priority. Nitrogenous organic small molecules, which are
easy to store and can produce NH under high temperatures, might serve as ideal substitutes for NH . For
3
3
example, Yang et al. converted vanadium-organic compounds (VAORCS) into vanadium nitride quantum
dots/nitrogen-doped hierarchical carbon nanocomposites (VNQD/NDHCs) by annealing the mixture of
VAORCS powder and melamine . Jin et al. mixed chloride salts of five different metals with urea by ball-
[13]
milling to form a metal-urea gel and obtained high-entropy metal nitride via calcining the gel . Inspired by
[14]
these previous reports, we chose urea as the nontoxic and cheap nitrogen source to prepare metal nitrides.
In addition, different from individual ones, bimetallic nanoparticles often exhibit higher catalytic activities,
richer redox sites, and better chemical stabilities [15-17] . Meanwhile, nickel and cobalt are chosen because they
have comparable atomic size and chemical valence state .
[18]
On the other hand, it is known to all that the most important factor affecting the performance of materials is
their morphology and structure. Compared to solid structures, hollow ones possess large inner voids,
reactive inner surfaces and indestructible structures . Hence, constructing hollow structures with low mass
[19]
transport resistance, rapid ion diffusion channels and high-volume electrical capacity stands out as an
efficient strategy to enhance SCs performance . Metal-small organic molecule complexes are ideal
[20]
precursors for hollow structures. Liu et al. coordinated Ni and Co with glycerol and then transformed the
2+
2+
[21]
solid complex into a yolk-shell structure via the hydrothermal method . Dong et al. synthesized hollow
carbon spheres by etching SiO template with HF . To avoid the use of a template and multifarious
[22]
2
synthesis steps, we designed a one-step strategy toward hollow structure by coordinating Ni and Co with
2+
2+
triethanolamine (TEOA) accompanied by the hydrolysis of metal alkoxide. Meanwhile, the nitrogenous
organic network could be pyrolyzed into N-doped carbon via calcine, which further enhances the electrical
conductivity and serves as strong support during long-term cycling .
[23]
Herein, we reported a hierarchical Ni N-Co N /nitrogen-doped carbon (NC) hollow nanoflower, which is
3
0.67
2
derived from annealing nickel/cobalt-TEOA complex (N C -TEOA) precursor with urea as nitrogen source.
1 2
The as-prepared Ni N-Co N /NC delivers larger specific surface area, superior energy storage capacity and
0.67
2
3
longer cycle lifespan. The Ni N-Co N /NC transformed from N C -TEOA sample shows an excellent
0.67
1
3
2
2
specific capacitance of 1582 F g at 1 A g and 83.79% capacitance retention after 5000 cycles. Furthermore,
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the assembled Ni N-Co N /NC//AC asymmetric device demonstrates a maximum energy density of
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0.67
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32.4 Wh kg and steady cycle performance of 95.8% after 5000 cycles.
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