Anion engineering of hierarchical Co-A (A = O, Se, P) hexagrams for efficient electrocatalytic oxygen evolution reaction

Article abstract of DOI:10.1016/j.cclet.2021.04.051

Water electrolysis is considered to be an effective and promising technology to make high-purity H₂, however, the relationship between anion species and catalytic performance of electrocatalysts is still not completely clear. Herein, we report an anion engineering strategy to tune electrocatalytic water oxidation activity for Co-based materials. Novel hierarchical Co-based oxide/selenide/phosphide (Co-A, A = O, Se, P) hexagrams have been chosen as model materials. Electrochemical results and theoretical calculations reveal that the electron configuration, the electrical conductivity, and the oxidation potential of Co element in Co-A hexagrams could be moderated by the substitution of P atoms, which leads to the superior OER performance. Particularly, Co-P hexagram displays a low overpotential (η = 269 mV) at j = 10 mA/cm² for the oxygen evolution reaction (OER) compared to Co-O hexagram (η = 399 mV) and Co-Se hexagram (η = 347 mV). This work is of great importance in understanding coordination atoms (O, Se and P) induced electrocatalytic properties of hierarchical Co-based materials.

Full text of DOI:10.1016/j.cclet.2021.04.051

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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Anion engineering of hierarchical Co-A (A = O, Se, P) hexagrams for
efficient electrocatalytic oxygen evolution reaction
Zuozhong Lianga, Chenxi Yang b, Wei Zhanga, Haoquan Zhenga,∗, Rui Caoa,∗
a Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University,
Xi’an 710119, China
b Sinopec Beijing Research Institute of Chemical Industry, Beijing 100013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Water electrolysis is considered to be an effective and promising technology to make high-purity H2,
however, the relationship between anion species and catalytic performance of electrocatalysts is still not
completely clear. Herein, we report an anion engineering strategy to tune electrocatalytic water oxidation
activity for Co-based materials. Novel hierarchical Co-based oxide/selenide/phosphide (Co-A, A = O, Se, P)
hexagrams have been chosen as model materials. Electrochemical results and theoretical calculations re-
veal that the electron configuration, the electrical conductivity, and the oxidation potential of Co element
in Co-A hexagrams could be moderated by the substitution of P atoms, which leads to the superior OER
performance. Particularly, Co-P hexagram displays a low overpotential (η = 269 mV) at j = 10 mA/cm2
for the oxygen evolution reaction (OER) compared to Co-O hexagram (η = 399 mV) and Co-Se hexa-
gram (η = 347 mV). This work is of great importance in understanding coordination atoms (O, Se and P)
induced electrocatalytic properties of hierarchical Co-based materials.
Received 22 December 2020
Revised 23 March 2021
Accepted 27 April 2021
Available online xxx
Keywords:
OER
Anion engineering
CoP
CoSe2
Co3O4
© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia
Medica, Chinese Academy of Medical Sciences.
With the increasing serious energy and environmental issues,
water splitting has been considered as a promising technology to
make H2 energy [1]. Water splitting involves two half-reactions:
H2 evolution reaction (HER) and O2 evolution reaction (OER). Com-
pared to the HER, the effectiveness of OER is hindered due to
sluggish kinetics. At present, precious metal oxides (i.e., IrO2, and
RuO2) are effective electrocatalysts for OER [2]. However, the low
reserve and high price dramatically limit their widespread ap-
plication [3]. Therefore, researchers focus on the development of
transition-metal-based materials [4–10]. Cobalt (Co)-based mate-
rials were regarded as efficient water oxidation catalysts owing
to their rich content, low price, and excellent performance [11–
13]. Novel methodologies have been proposed to develop highly
efficient Co-based catalysts [14–16]. Noted that hierarchical struc-
tures have advantages of high permeability, high conductivity and
rich active sites, which are benefit for electrocatalytic OER [17–19].
However, electrocatalytic activities of these Co-based hierarchical
materials are normally limited by their compositions.
perovskites [27], Co chalcogenides [28], Co selenides [29], Co phos-
phides [30,31], Co phosphates [32], and Co phosphosulfides [33],
etc. For example, Hua et al. reported a hybrid CoP/Co2P material,
which exhibited efficient overall water splitting activity with a cell
voltage of 1.57 V at j = 10 mA/cm2 [34]. Wang et al. found that
the introduction of S2− into the Co-based hydroxide could tune the
electronic and surface structure and lead to enhanced OER perfor-
mance [35]. It is usually believed that Co atom is the real active
site for OER and the anions such as S and P could modify the elec-
tronic structure of metal Co [36,37]. Though great contributions
have been dedicated to developing advanced OER materials, re-
vealing the relationship between anion species and electrocatalytic
activities still remains a great challenge. Furthermore, comprehen-
sive investigation into electronic structures and catalytic properties
of hierarchical Co-based materials with the same morphology and
size is still lacking.
Herein, we synthesized Co-A (A = O, Se, P) hexagrams from
the same hierarchical Co(OH)2/Co(OH)F precursor. The oxida-
tion/selenation/phosphorization processes do not change the mor-
phology and size of the precursor, which is important to exclude
their influence. In addition, hierarchical structures not only can
help the diffusion process but also expose more active sites. Elec-
trochemical measurements show that Co-P hexagram exhibits well
OER performance with an overpotential of 269 mV, outperforming
Co-based catalysts used for OER mainly includes: Co-N-C ma-
terials [20–22], Co nitrides [23], Co oxides/hydroxides [24–26], Co
∗
Corresponding authors.
E-mail addresses: zhenghaoquan@snnu.edu.cn (H. Zheng), ruicao@snnu.edu.cn
(R. Cao).
https://doi.org/10.1016/j.cclet.2021.04.051
1001-8417/© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Please cite this article as: Z. Liang, C. Yang, W. Zhang et al., Anion engineering of hierarchical Co-A (A = O, Se, P) hexagrams for efficient
electrocatalytic oxygen evolution reaction, Chinese Chemical Letters, https://doi.org/10.1016/j.cclet.2021.04.051

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Fig. 1. SEM image (a) and structural illustration (b) of Co precursor, and (c-e) crys-
tal structures of Co-A (A = O, Se, P) hexagrams.
Co-O (399 mV) and Co-Se (347 mV) at j = 10 mA/cm2 in 1.0 mol/L
KOH. By combining the theoretical calculation, the P substitution
could modulate the electronic structure of Co.
The Co precursor was first synthesized using hydrothermal syn-
thesis. The powder X-ray diffraction (PXRD) pattern demonstrates
that the obtained material is a mixture of Co(OH)2 (JCPDS No.
30-0443) and Co(OH)F (JCPDS No. 50-0827) (Fig. S1 in Support-
ing information). Scanning electron microscope (SEM) image shows
that Co precursor has a novel hexagram-like morphology with six
branches surrounded on a hexagonal core (Fig. 1). In particular,
each branch consists of uniform nanorods with a width of ~200 nm
and a length ranging from 1 μm to 3 μm (Fig. 1a). During the
formation process, hexagonal Co(OH)2 core first forms, and then
Co(OH)F nanorod grows along hexagon edges due to matched unit
cell parameters. The specific growth mechanism has been studied
in details in our previous work [38]. Therefore, Co precursor is con-
structed with a two-dimensional (2D) Co(OH)2 as the internal core
and one-dimensional (1D) Co(OH)F nanorods as external branches
(Fig. 1b).
Fig. 2. TEM and HRTEM images of Co(OH)2/Co(OH)F (a-c), Co3O4 (d–f), CoSe2 (g–i),
and CoP (j–l) hexagrams.
tance of 0.25 nm, which is indexed to the (111) plane of CoP
(Fig. 2j–l). Co-O hexagrams exhibit a Brunauer-Emmett-Teller sur-
face area of 17.3 m2/g and an average pore size of ~ 25 nm (Fig.
S4 in Supporting information). These catalysts have similar surface
area due to the same morphology.
X-ray photoelectron spectroscopy (XPS) spectra measurements
of Co-A (A = O, Se, P) hexagrams were performed to confirm va-
lence states of surface elements (Fig. S5 in Supporting informa-
tion). Peaks appeared at 779.7 and 781.3 eV can be indexed to the
main photoelectron lines from Co3+ and Co2+ ions in Co3O4, re-
spectively (Fig. 3a) [39,40]. Peaks appeared at 784.7 and 789.7 eV
are attributed to their satellite shake-up peaks [41,42]. In the XPS
spectrum of O 1s of Co3O4, three peaks were observed (Fig. S5b).
The peak appeared at 529.9 eV is attributed to the lattice oxygen,
while the peak appeared at 531.8 eV is ascribed to oxygen vacan-
cies [16]. Another peak appeared at 533.4 eV corresponds to hy-
droxyl species of surface-adsorbed water molecules. For Co-Se hex-
agram, the peak of Co 2p3/2 appeared at 778.4 eV is corresponded
to Co2+ cations in Co-Se hexagrams (Fig. S5c). Peaks of Se 3d at
54.2 and 55.2 eV are also in good agreement with CoSe2 (Fig. S5d)
[43]. Co 2p XPS spectrum of Co-P hexagrams shows a peak ap-
peared at 778.5 eV indexing to the Co 2p3/2 of Co species in Co-
P hexagrams (Fig. S5e). Peaks appeared at 782.4 and 786.7 eV are
well indexed to the Co 2p3/2 of oxidized Co species. The peaks for P
2p at 129.8 and 131.4 eV are well matched with the P3− (Fig. S5f).
Co-A (A = O, Se, P) hexagrams were prepared by the oxida-
tion/selenation/phosphorization process at the same temperature
(350 °C). Crystal structures of Co-A (A = O, Se, P) hexagrams are
shown in Figs. 1c–e. Characteristic peaks in PXRD patterns of Co-A
(A = O, Se, P) hexagrams indicate that these materials are Co3O4
(JCPDS No. 43-1003), CoSe2 (JCPDS No. 09-0234) and CoP (JCPDS
No. 29-0497), respectively (Fig. S2 in Supporting information). Co-
O hexagrams were obtained by heating precursors at the muffle
furnace under air. Herein, the structure of Co-O hexagrams (Co3O4)
is different from previous reported CoO hexagrams, which were
obtained by heating the precursor under Ar [15]. Co-Se and Co-P
hexagrams were prepared by heating Se powder and NaH2PO2·H2O
at the upstream of pipe furnace under Ar flow, respectively. The
unique and novel hexagram-like morphology of Co precursor re-
mains after the oxidation, selenation or phosphorization process,
which is important to further discuss their electrochemical proper-
ties (Fig. S3 in Supporting information).
Transmission electron microscope (TEM) images of Co precur-
sor and Co-A (A = O, Se, P) hexagrams are displayed in Fig. 2.
TEM and high-resolution TEM (HRTEM) images of Co precursor
demonstrate that the crystal lattice has a space of 0.26 nm, which
is attributed to the (011) face of Co(OH)F (Figs. 2a–c). The Co3O4
branch shows perfect crystal lattice with a spacing of 0.29 nm
corresponding to the (200) face of Co3O4 (Figs. 2d–f). The crys-
tal lattice with a d-spacing of 0.24 nm, which is assigned to the
(211) facet of CoSe2, confirming the formation of the CoSe2 phase
(Figs. 2g–i). The HRTEM image of Co-P shows an interplanar dis-
3−
The predominate peak appeared at 135.2 eV is assigned to PO4
[44]. XPS results indicate that Co-A (A = O, Se, P) hexagrams with
the similar morphology and size possess different chemical envi-
ronments, which may further affect their electrocatalysis.
Electrochemical
cyclic
voltammetry
(CV)
data
of
Co(OH)2/Co(OH)F (Co-OH) and Co-A (A = O, Se, P) hexagrams
were measured in 1.0 mol/L KOH solution at 0.05 V/s (Fig. 3a).
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Fig. 3. CV data (a), Tafel plots (b), EIS (c), and controlled potential electrolysis (d)
of Co-A (A = O, Se, P) hexagrams.
Fig. 4. TDOS and PDOS calculated for the Co-O (a), Co-Se (b), and Co-P (c). Black
dashed horizontal line represents the Fermi level.
Co-P hexagrams show the smallest overpotential (η) of 269 mV
compared to Co-OH (η = 413 mV), Co-O (η = 399 mV) and Co-Se
hexagrams (η = 347 mV) at j = 10 mA/cm2, demonstrating the
excellent OER performance of Co-P hexagrams. Co-P hexagrams
exhibit good OER performance compared to other reported Co-
based catalysts (Table S1 in Supporting information). In addition,
Co-P hexagrams show an obvious CoII/CoIII couple at ~1.12 V (vs.
reversible hydrogen electrode, RHE) compared to Co-Se and Co-O
hexagrams (Fig. S6 in Supporting information). This is because
Co atoms in Co-P hexagrams are easily oxidized due to relatively
larger number of valence electron as confirmed with the theoreti-
cal calculation. Considering the full oxidation of CoIII/CoIV event at
~1.46 V (vs. RHE) and CoIV/CoIII event at ~1.44 V (vs. RHE) in the
cathodic return scan for Co-O hexagrams, a −CoV=O formal oxida-
tion state might be formed, which involves the cleavage/formation
of O−O [17]. In contrast, Co-P and Co-Se hexagrams start to oxidize
water with the occurring of CoIII/CoIV event (Fig. S6). Therefore,
Co species in Co-P and Co-Se hexagrams are more active than that
in Co-O hexagrams. Linear sweep voltammetry (LSV) data further
confirm this result (Fig. S7 in Supporting information). Herein,
the OER performance of Co-O hexagrams (Co3O4, η = 399 mV) is
much lower than that of previous reported CoO hexagrams (CoO,
η = 269 mV). This is because CoO hexagrams have abundant
oxygen defects and long-term ordering when pyrolyzed at critical
temperature (400 °C) under Ar [15]. Tafel plots of Co-A (A = O, Se,
P) hexagrams were utilized to study their OER kinetics (Fig. 3b).
Co-P hexagrams have the smallest Tafel slope (61.3 mV/dec)
compared to Co-OH precursor (74.2 mV/dec), Co-O hexagrams
(70.6 mV/dec) and Co-Se hexagrams (66.4 mV/dec), indicating
the rapid OER rates. These similar Tafel values demonstrated that
Co-A hexagrams had the same rate-limiting step for OER process.
The improved catalytic activity of Co-P may be attributed to the
enhanced charge and mass transfer. Electrochemical impedance
spectroscopy (EIS) results indicate that Co-P hexagrams have the
smallest charge-transfer resistance compared to Co-O and Co-Se
hexagrams (Fig. 3c). Therefore, the P substitution increases the
electrical conductivity compared to O and Se anions.
fore, Co-P hexagrams expose more active sites. The controlled po-
tential electrolysis (CPE) of Co-A (A = O, Se, P) hexagrams is tested
at 1.63 V (vs. RHE) with the indium tin oxide (ITO) electrode. Co-P
hexagrams show well stability at j = ~10 mA/cm2 during the CPE
of 10 h compared to Co-Se and Co-O hexagrams (Fig. 3d).
Redox peaks of CV current-potential responses for Co-P hexa-
grams appeared at ~1.05 and ~1.50 V (vs. RHE) are attributed to the
oxidation of CoII/CoIII and CoIII/CoIV, respectively (Fig. S7) [17]. Lin-
ear i-ν1/2 responses of Co-A (A = O, Se, P) hexagrams indicate their
diffusion-controlled kinetics in OER (Figs. S10–S12 in Supporting
information). The linear correlation of potentials (E1 and E2) vs. pH
proves the process of proton-coupled electron transfer (Figs. S10d–
f). For Co-Se and Co-O hexagrams, similar linear correlations were
also observed (Figs. S11 and S12 in Supporting information).
XPS results of Co 2p, O 1s, Se 3d, and P 2p for Co-A (A = O, Se,
P) hexagrams after CPE are shown in Fig. S13 (Supporting informa-
tion). Main XPS peaks of Co 2p3/2 of Co3O4 are almost unchanged,
while the appearance of a shoulder peak appeared at 530.2 eV of
O 1 s indicates that there are surface Co oxide species, i.e., CoOOH,
after CPE (Figs. S13a and b) [17]. The main peaks of Co 2p of CoSe2
still remain after CPE (Fig. S13c), while a broad peak of SeOx at
60.4 eV is observed after electrolysis (Fig. S10d) [43]. For Co-P hex-
agram, an additional peak at 780.5 eV ascribed to Co oxide ap-
pears (Fig. S13e) [46,47]. Oxidized species were observed in Co-A
(A = O, Se, P) hexagrams, indicating that they all experienced ox-
idation processes during the OER. HRTEM images after OER test
further confirm these results (Fig. S14 in Supporting information).
Therefore, the effects of O, Se and P on the intrinsic structures of
Co and the OER activity cannot be ignored.
To further understand electronic structures, we calculated total
density of states (TDOS) and projected DOS (PDOS) of Co-A (A = O,
Se, P) are shown in Fig. 4.
The existence of intrinsic band gap and the mergence of Fermi
levels into the conduction band indicate that both Co-O and Co-
Se undergo a transition from semiconducting to metallic state. The
Co-P displays a zero-band gap, i.e., a metallic nature, which is ben-
eficial for the electron transfer process. Furthermore, a strong hy-
bridization throughout a wide energy range is found between Co-d
and O-p/Se-p states, showing a strong covalency of the bonding in
Co-O/Co-Se; while P-p state in Co-P only has an insignificant influ-
ence on the overall DOS, which retains the metallic property of Co.
Therefore, the Co-P has the rapid charge-transfer kinetic compared
To further understand the improved water oxidation activity,
we measured the electrochemical double-layer capacitance (Cdl) to
calculate the electrochemical surface area (Fig. S8 in Supporting in-
formation) [45]. The Cdl of Co-P hexagrams (6.8 mF/cm2) is much
larger than that of Co-Se (2.4 mF/cm2) and Co-O hexagrams (1.0
mF/cm2), respectively (Fig. S9 in Supporting information). There-
3

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to Co-O/Co-Se, which agrees well with the experimental charge-
transfer resistance (Fig. 3c).
University Association for Science and Technology in Shaanxi,
China (No. 20200602).
In addition, a Bader analysis of the charge density was per-
formed for understanding the interplay between Co cation and an-
ions (O, Se, and P). The obtained valence electron number of Co is
7.92, 8.64, and 8.83 for Co-O, Co-Se, and Co-P, respectively. Thus, it
can be inferred that the valence electron donated by Co in Co-P is
less than that in Co-O or Co-Se. Thus, the interaction between Co
and P is weaker than the interactions between Co and O, or be-
tween Co and Se, which is in accordance with the DOS analyses. In
addition, It has been reported that the formation of adsorbed ∗OOH
species is the rate-limiting step of OER [48]. Therefore, the interac-
tion between adsorbed ∗OOH species and 3d orbital of transition
metal determines the resulting OER activity. The theoretical cal-
culation demonstrates that the increased 3d orbital electron den-
sity of Co-based materials favors the formation of adsorbed ∗OOH
species, thus enhancing OER activity [49]. Herein, the metal Co in
Co-P has the largest valence electron number (8.83) compared to
Co-O (7.92) and Co-Se (8.64). Therefore, the Co-P is beneficial for
the formation of adsorbed ∗OOH species and exhibits the best OER
performance. Theoretical calculation demonstrated that the intro-
duction of P could tune the electronic structure of Co compared to
O and Se and then improve the charge transfer and lead to the eas-
ier generation of higher average oxidation state of Co species dur-
ing OER process. Therefore, Co cation in Co-P hexagram is easier
to be oxidized experimentally, when performing CV measurements
(Fig. S6).
Appendix A. Supplementary data
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.cclet.2021.04.051.
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In conclusion, hierarchical Co-A hexagrams (A = O, Se, P) with
the same morphology and size were prepared by oxidation, selena-
tion, and phosphorization of Co precursors, respectively. Co-P hex-
agrams display the highest OER efficiency compared to Co-O and
Co-Se hexagrams. The introduction of O, Se and P could effectively
change the intrinsic electronic structure of Co and then acceler-
ate the charge transfer. Furthermore, the P institution leads to the
easier generation of active Co species during OER process, which
might be ascribed to the metallic nature of Co-P hexagrams and
the weak interaction between Co and P. This work offers a funda-
mental understanding of anions induced electrocatalytic properties
of hierarchical Co-based materials.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
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influence the work reported in this paper.
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