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

[34]
NiCo O nanocrystals . Compared with borides and oxides, transition metal selenide nanocrystals, which
2
4
have been reported, can potentially enhance further the OER catalytic performance because of their high
density of active sites and excellent intrinsic catalytic activities. Especially, nickel selenide nanocrystals with
[35]
unique electron vacancies and special hybrid orbitals possess expectantly activity in OER electrocatalysis .
Also, the formation of nickel hydroxide or selenium oxide plays an important role in the OER process.
Zheng et al. synthesized a series of Ni Se (0.5 ≤ x ≤ 1) nanocrystals with different compositions as
x
[36]
potential OER electrocatalysts . The analysis reveals that Ni Se nanocrystalline exhibits better OER
0.5
catalytic activity than counterparts and noble metals. Up to now, although many multiphase selenide
nanocrystals with excellent properties have been explored, the understanding of determining phase-
dependent properties is still limited. In addition, the ORR performances are relatively unsatisfactory. To
address ORR activity issue, one promising approach is incorporating composite with splendid ORR
materials with high intrinsic activity, such as Co/NC , Co porphyrin , FeNi /NC , FeSo-yCNSs-
[38]
[39]
[37]
3
A , Fe -N-C , and so on. For these catalysts, transition metal-N species is a key factor for
[41]
[40]
X
SA
obtaining high ORR activity. More importantly, atomically dispersed Fe-N species are considered to
X
be the most active platinum group metal-free ORR catalysts. Transition metal phthalocyanines, as typical
M-N (x = 2, 4) moieties, have been widely investigated due to their presented exceptional ORR
X
activities. Among different phthalocyanine catalysts based on 3d transition metals (Co, Ni, Cu, and Mn),
FePc-based catalysts with Fe-N as the characteristic structure exhibited the best catalytic activity .
[42]
4
In this work, we report a FePc π-π conjugation interaction on Se@NiFe composites (FePc/Se@NiFe) via a
facile hydrothermal procedure for the purpose of improving the ORR/OER activities and ZAB performance.
As verified by physical characterization evidence, the dual-functional catalyst is composed of Fe-N species
4
and Se, Fe O , and Ni Se crystal phases. In addition, we found that the FePc not only provides a carbon
3
4
4
3
source to increase the conductivity of the catalyst, but also introduces defects to modify the electronic
structure of the catalyst. Combining the electrochemical measurements results, the metallic Fe-N ligand
4
species are responsible for ORR, while Ni Se crystal phases are the best OER active sites. The resultant
4
3
FePc/Se@NiFe material shows remarkable dual-functional oxygen catalytic activity and ZABs performance
indicators, outperforming that of benchmark Pt/C + RuO in alkaline media. These results suggest that the
2
FePc/Se@NiFe may be a potential candidate for practical energy technologies in alkaline medium, and it
also helps to understand the precise ORR and OER active sites.
EXPERIMENTAL
Chemicals and reagents
Nickel(II) nitrate hexahydrate (Ni(NO ) ·6H O, ≥ 99%), iron(III) nitrate nonahydrate [Fe(NO ) ·9H O,
3 2
2
2
3 3
≥ 99%], ammonium fluoride (NH F, 99.99%), urea [CO(NH ) , AR], iron phthalocyanine (FePc, 98%), and
4
2 2
ruthenium(IV) oxide (RuO , 99.9%) were purchased from Macklin Reagent Biochemical Co., Ltd (Shanghai,
2
China). Ethanol absolute (C H OH, 99.9%) and hydrazinehydrate diamidhydrate (N H ·H O, 36%~38%)
2
4
2
5
2
were obtained from Sinopharm Chemical Reagent Co., Ltd. Nafion (5%) and Pt/C (20%) were distributed
from DuPont and Johnson Matthey companies (USA). ultrapure water (18 MΩ·cm ) was produced from
-1
Millipore system for all experiments. All the above chemical reagents were directly used.
Materials synthesis
Synthesis of the NiFe-LDH, Ni-LDH, and Fe-LDH precursor: NiFe-LDH was prepared by hydrothermal
reaction. Specifically, Ni(NO ) ·6H O (0.67 mmol), Fe(NO ) ·9H O (0.33 mmol), NH F (4.0 mmol), and urea
2
3 2
4
3 3
2
(6.0 mmol) were separately dissolved in 60 mL of deionized water (18.2 MΩ·cm). The solution was stirred
vigorously for 30 min to obtain a light blue solution, and then transferred to an autoclave lined with 100 mL
Teflon and subjected to hydrothermal reaction at 120 °C for 12 h. Subsequently, the precursor was gained
by filtration and washing with deionized water and dried under vacuum at 60 °C. Finally, the yellow powder