Ultrathin Amorphous Nickel Doped Cobalt Phosphates with Highly Ordered Mesoporous Structures as Efficient Electrocatalyst for Oxygen Evolution Reaction

Article abstract of DOI:10.1002/smll.201906766

Herein, the facile preparation of ultrathin (≈3.8 nm in thickness) 2D cobalt phosphate (CoPi) nanoflakes through an oil-phase method is reported. The obtained nanoflakes are composed of highly ordered mesoporous (≈3.74 nm in diameter) structure and exhibit an amorphous nature. Attractively, when doped with nickel, such 2D mesoporous Ni-doped CoPi nanoflakes display decent electrocatalytic performances in terms of intrinsic activity, and low kinetic barrier toward the oxygen evolution reaction (OER). Particularly, the optimized 10 at% Ni-doped CoPi nanoflakes (denoted as Ni10-CoPi) deliver a low overpotential at 10 mA cm^?2 (320 mV), small Tafel slope (44.5 mV dec^?1), and high stability for OER in 1.0 m KOH solution, which is comparable to the state-of-the-art RuO₂ tested in the same condition (overpotential: 327 mV at 10 mA cm^?2, Tafel slope: 73.7 mV dec^?1). The robust framework coupled with good OER performance enables the 2D mesoporous Ni10-CoPi nanoflakes to be a promising material for energy conversion applications.

Full text of DOI:10.1002/smll.201906766

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Ultrathin Amorphous Nickel Doped Cobalt Phosphates
with Highly Ordered Mesoporous Structures as Efficient
Electrocatalyst for Oxygen Evolution Reaction
Lan Yang, Hao Ren, Qinghua Liang, Khang Ngoc Dinh, Raksha Dangol, and Qingyu Yan*
and
composition
usually
undergo
Herein, the facile preparation of ultrathin (≈3.8 nm in thickness) 2D cobalt
phosphate (CoPi) nanoflakes through an oil-phase method is reported. The
obtained nanoflakes are composed of highly ordered mesoporous (≈3.74 nm
in diameter) structure and exhibit an amorphous nature. Attractively, when
doped with nickel, such 2D mesoporous Ni-doped CoPi nanoflakes display
decent electrocatalytic performances in terms of intrinsic activity, and low
kinetic barrier toward the oxygen evolution reaction (OER). Particularly, the
optimized 10 at% Ni-doped CoPi nanoflakes (denoted as Ni10-CoPi) deliver a
low overpotential at 10 mA cm−2 (320 mV), small Tafel slope (44.5 mV dec−1),
and high stability for OER in 1.0 m KOH solution, which is comparable
to the state-of-the-art RuO2 tested in the same condition (overpotential:
327 mV at 10 mA cm−2, Tafel slope: 73.7 mV dec−1). The robust frame-
work coupled with good OER performance enables the 2D mesoporous
Ni10-CoPi nanoflakes to be a promising material for energy conversion
applications.
obvious evolution under strong oxidation
condition.[3c,7] It has been reported that the
phosphate group plays an important role
in facilitating adsorption and stabilizing
catalytic centers,[8] however transition
metal phosphates are rarely explored as
electrocatalysts for OER. Among the few
reports on phosphate materials as OER
catalyst, their performances are still far
from satisfactory.[8c,9] As known, the elec-
trocatalytic performance depends on many
factors, such as the number of accessible
active sites, mass transport efficiency,
and the ability to resist corrosion in elec-
trolytes, which are related to the internal
compositional properties as well as the
external structure and morphology.[3c,10]
Hence, there are multiple effective
strategies to optimize the OER perfor-
mance of transition metal phosphates.
Owing to the depletion of fossil fuel and increasing concern on
global warming, there is an urgent desire for sustainable and
clean energy resources. Hydrogen is deemed as one of the most
promising candidate, which could be produced from water
electrolysis.[1] The main challenge is that the anodic reaction of
water splitting–oxygen evolution reaction (OER) exhibits slug-
gish kinetics and high energy barrier (overpotential) because it
First, cation doping can improve the intrinsic activity. Mixed
metal cations show more favorable adsorption/desorption
of oxygen-containing intermediates than that of the single-
metal counterparts.[8b,c] The transition metal Fe, Co, Ni and
their mixed compounds are excellent catalysts in alkaline OER
reactions, and different combinations of these three elements
can significantly change the properties of the catalysts. It was
reported that moderate amount of Fe and Ni doping in cobalt-
based catalysts yielded higher OER activity and lower overpo-
tential, which are associated with reduction of the adsorption
energy on the catalyst surface, and Fe, Ni substitutions were
revealed to facilitate surface reconstruction into active Co oxy-
hydroxides under OER conditions.[11] Due to the similar atomic
structure of Co and Ni subgroup elements, Ni doping in CoPi
is beneficial to keep the original morphology and structure.
Moreover, reducing the thickness is another effective way since
it can accelerate the mass transport.[12] Mass transport is an
important parameter to describe the transfer rate of electrocat-
alysts to the electrolyte. Faster mass transport leads to higher
electrocatalysts utilization, which means better performance of
the electrocatalysts for the same mass loading.[13] Furthermore,
a highly ordered porous structure such as mesopores can offer
higher surface area, which means more active sites are exposed
to electrolyte.[14] The increased effectively active surface area
facilitates the adsorption/desorption process and yields better
electrocatalytic activity. Besides, the design of amorphous struc-
ture has merits such as short-range structure ordering and high

involves four electron–electron process and O O bond forma-
tion. Highly active electrocatalysts are needed to fasten reaction
kinetics and lower that overpotential. However, the benchmark
OER catalysts so far are still based on the expensive and low-
abundance noble metals such as IrO2 and RuO2.[2]
The past decade has witnessed extraordinary progress on
overcoming the bottleneck of OER electrocatalysis by transition
metal phosphides,[3] dichalcogenides,[4] nitrides,[5] and thiophos-
phate materials.[6] Nevertheless, the real active sites of the above
materials remain controversial because their phase, structure,
L. Yang, Dr. H. Ren, Dr. Q. Liang, K. N. Dinh, R. Dangol, Prof. Q. Y. Yan
School of Materials Science and Engineering
Nanyang Technological University
Singapore 639798, Singapore
E-mail: alexyan@ntu.edu.sg
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201906766.
DOI: 10.1002/smll.201906766
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2p XPS spectrum (Figure 1b), the spin–
orbit doublets of Co 2p3/2 and Co 2p1/2
are located at binding energy of 797.4 and
781.64 eV, respectively. The spin energy
interval is around 16.76 eV, suggesting the
existence of Co2+.[17] In the corresponding
Ni 2p profile (Figure 1c), the main two
strong peaks observed at the binding energy
of 856.23 and 873.88 eV can be attributed
to 2p3/2 and 2p1/2 from the Ni2+ species,
respectively.[18] The P 2p (Figure 1d) and O
1s (Figure S1b, Supporting Information)
XPS spectra show the feature peaks at 133.7
and 531.2 eV, respectively, which matches
the characteristic peaks of (PO4)3−.[19] The P
2p and C 1s (Figure S1c, Supporting Infor-
mation) spectra revealed the existence of

P C bonding with the corresponding fea-
ture peak showing at 132.7 and 285.2 eV,
respectively.[20]
To characterize the morphology of the
obtained materials, transmission electron
microscopy (TEM) and scanning electron
microscopy (SEM) were carried out. The
low-magnification TEM images clearly
reveal the nanoflakes morphology of
Ni10-CoPi with a lateral size of >800 nm
(Figure 2a). The low and high-magnifica-
tion SEM images in Figure S2 (Supporting
Figure 1. a) XRD patterns of CoPi (in black), Ni5-CoPi (in red), Ni10-CoPi (in blue), and Ni15-
CoPi (in green). b–d) XPS spectrums of CoPi and Ni10-CoPi in Co 2p region, Ni 2p region, and
P2p region.
density of active sites density.[15] Compared to crystallized mate-
rials, the disordered structures of amorphous electrocatalysts
offer more active sites for highly efficient water oxidation.
In consideration of the above merits, herein, a facile oil-
phase method is employed to realize the compositional, mor-
phological, and lattice structural engineering of transition metal
phosphates. Particularly, the amorphous 10 at% Ni-doped CoPi
nanoflakes (denoted as Ni10-CoPi) composed of highly ordered
mesoporous structure were prepared as an efficient and elec-
trostable OER electrocatalyst. The obtained mesoporous Ni10-
CoPi nanoflakes show an ultrathin thickness of ≈3.8 nm and an
average pore diameter of ≈3.74 nm. During OER process, the
as-prepared Ni10-CoPi electrodes show a decent activity with
low overpotential of 320 mV at 10 mA cm−2 and small Tafel
slope of 44.5 mV dec−1.
We prepared the mesoporous CoPi nanoflakes with dif-
ferent Ni doping content of 0%, 5%, 10%, and 15% based on
a modified oil-phase method (see the Experimental Section for
details).[16] The Co/Ni atomic ratio of the as-obtained CoPi, Ni5-
CoPi, Ni10-CoPi, and Ni15-CoPi was confirmed by individu-
ally coupled plasma-optical emission spectrometry (ICP-OES),
which is close to that was used in the corresponding synthesis
(Table S1, Supporting Information).
Information) also confirmed such 2D layered nanoflakes
structure. Furthermore, the high-resolution TEM (HRTEM)
images in Figure 2b,c show that there are many channels
on the nanoflakes. The distance between the centers of two
neighboring channels (defines as Dcenter) is measured to be
≈3.74 nm (inset of Figure 2c), indicating the mesoporous
structure of the obtained material. The HRTEM images of
CoPi, Ni5-CoPi, and Ni15-CoPi show the same mesoporous
morphology and very similar pore diameters of ≈3.42, ≈3.4,
and ≈3.75 nm, respectively (Figure S3, Supporting Informa-
tion). The selected area electron diffraction (SAED) pattern fur-
ther confirms the amorphous nature of the material (inset of
Figure 2b), which agrees well with the XRD result above. The
low-angle XRD in Figure 2d shows three resolved diffraction
peaks. According to Bragg’s law (2d sin θ = nλ), the d values
for the three peaks are calculated to be 1.87, 1.26, and 0.94 nm.
1
1
The three d values are are /2, /3, 1/4 of the Dcenter, thus indi-
cating that the three peaks can be accordingly indexed as (200),
(300), and (400) reflections and proves that the mesopores are
highly ordered distributing along the [100] direction. The low-
angle XRD patterns of Co-Pi, Ni5-CoPi, and Ni15-CoPi nano-
flakes (Figure S4, Supporting Information) also show the same
correlation between the d values and Dcenter, indicating that Ni-
doping does not affect the structure and morphology of the syn-
thesized materials. It is worth noting that the thickness of the
flake measured by atomic force microscopy (AFM) (≈3.8 nm)
is very close to d and Dcenter (Figure S5, Supporting Informa-
tion), indicating that the nanoflake may contain only one layer
of the mesoporous channels. The ultrathin nature of the mate-
rials is conducive to mass transport and charge transfer, which
X-ray diffraction (XRD) patterns of the synthesized CoPi,
Ni5-CoPi, Ni10-CoPi, and Ni15-CoPi (Figure 1a) show no sharp
diffraction peak, indicating the amorphous nature of the tested
samples. To investigate the chemical bonds and oxidation states
of the obtained materials, X-ray photoelectron spectroscopy
(XPS) was performed on CoPi and Ni10-CoPi (Figure 1b–d;
Figure S1, Supporting Information). In the high-resolution Co
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Figure 2. a) TEM image, b,c) HRTEM images, and d) low-angle XRD pattern of Ni10-CoPi. The inset in (b) is the SAED pattern of Ni10-CoPi. The inset
in (c) is the enlarged TEM image, showing the distance between neighboring channels is 3.74 nm.
leads to high electrocatalytic performance toward water oxida-
tion. The Brunauer–Emmett–Teller (BET) surface area obtained
from N2 adsorption–desorption isotherms is 35 m2 g−1, and the
pore size distribution calculated by the Barrett–Joyner–Halenda
(BJH) method shows a narrow peak with a mean value of
4.5 nm (Figure S6, Supporting Information). The mesoporous
structures provide a large surface area and expose more active
sites. Besides, mesoporous structure has the advantages of reg-
ular channel structure and narrow pore size distribution, which
is conducive to the adsorption and desorption process during
OER. Moreover, the energy-dispersive X-ray spectroscopy (EDX)
elemental mapping confirmed the presence of P, Co, and Ni
and their uniform distribution overlapping the Ni10-CoPi
nanoflakes, which is consistent with the XPS results (Figure S7,
Supporting information).
To study the electrocatalytic performance of Ni10-CoPi, we
tested the Ni10-CoPi nanoflakes for OER in the alkaline solu-
tion of 1.0 m KOH. A three-electrode system was used with a
glassy-carbon electrode coated with catalyst as the working
electrode, carbon rod, and Hg/HgO as the contact and refer-
ence electrode, respectively. The OER activities of CoPi, Ni5-
CoPi, Ni15-CoPi, and commercial RuO2 were also evaluated for
comparison.
In the iR-compensated polarization curves obtained by
steady-state linear sweep voltammetry (LSV), it can be seen
that the Ni10-CoPi electrode exhibits an onset overpotential of
272 mV (Figure 3a), which is obviously lower than those of
CoPi (302 mV), Ni5-CoPi (286 mV), and Ni15-CoPi (279 mV).
The polarization curves without iR-correction (Figure S8, Sup-
porting Information) show an onset overpotential of 273 mV
for Ni10-CoPi, which is lower than those of CoPi (303 mV),
Ni10-CoPi (287 mV), and Ni15-CoPi (280 mV). Moreover,
Tafel slopes obtained from Tafel equation indicate that Ni10-
CoPi electrode has faster OER kinetics with a smaller Tafel
slope of 44.5 mV dec−1. The Tafel slope of CoPi, Ni5-CoPi,
and Ni15-CoPi is 68.3, 55.1, and 51.2 mV dec−1, respectively
(Figure 3b). Tafel slope of CoPi is more than 60 mV dec−1,
indicating that the discharge of OH−1 is the rate-determining
step, while the Tafel slope of Ni-doped CoPi is in the range
of 40–60 mV dec−1, implying that the adsorption of OH−1 on
the catalysts is the rate-determining step. This revealed that
Ni doping moves the rate-limiting step to the end of the mul-
tielectron transfer reaction and speed up the OER kinetic
process, which is very important for improving the catalytic
activity. Besides, the performance of Ni10-CoPi also surpasses
the tested OER electrocatalyst RuO2, which requires 327 mV
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Figure 3. a) iR-corrected OER polarization curves of CoPi, Ni5-CoPi, Ni10-CoPi, and Ni15-CoPi in 1.0 m KOH at scan rate of 2 mV s−1 and b) their cor-
responding Tafel slope. c) The overpotentials of above samples at the current density of 10 mA cm−2. d) The chronoamperometric measurements of
Ni10-CoPi electrode under static overpotential (η = 330 mV) for 20 h.
to drive a current density of 10 mA cm−2 and the corre-
sponding Tafel slope is 73.7 mV dec−1 (Figure S9, Supporting
Information). The respective η10, i.e., required overpotential to
realize the current density of 10 mA cm−2, for the optimized
Ni10-CoPi nanoflakes is 320 mV, which is smaller than that of
CoPi (354 mV), Ni5-CoPi (335 mV), and Ni15-CoPi (325 mV)
(Figure 3c). To investigate the effect of mesoporous structure
on OER, we also synthesized Ni10-CoPi nanoflakes without
mesoporous structures by improving the reaction temperature
(see the Experimental Section) and tested the OER performance
under same condition (Figure S10, Supporting Information).
The TEM images (Figure S10a–c, Supporting Information)
show that there is no as obvious channels as those on Ni10-
CoPi with mesoporous structure, indicating that too high
reaction temperature is not conducive to the formation of this
morphology. In the OER polarization curve (Figure S10d, Sup-
porting Information), Ni10-CoPi without mesoporous structure
exhibits the performance nowhere near as good as Ni10-CoPi
with mesoporous structure, showing an overpotential of
350 mV at 10 mA cm−2. This contrast further proved that
mesoporous structure plays an important role in OER pro-
cess by providing large surface area and making more active
sites exposed. More importantly, the chronoamperometry test
performed on carbon paper shows that a stable current den-
sity of 20 mA cm−2 was maintained for 20 h with ignorable
change less than 5%, demonstrating the excellent durability
of the electrode (Figure 3d). The XRD characterization shows
that the Ni10-CoPi electrode still maintains the amorphous
structure after 20 h stability test (Figure S11a, Supporting
Information), indicating the structural stability during the
OER process. For TEM analysis of Ni10-CoPi after stability
test, the as-prepared electrode was dispersed into ethanol by
vigorous sonication and then dropped onto the copper grid.
TEM characterizations show that the mesoporous structure
can be still maintained after 20 h OER testing (Figure S11b,c,
Supporting Information). On the other hand, it is undeniable
that the morphology of the Ni10-CoPi electrode was mostly
damaged, which can be attributed to the formation of oxyhy-
droxide under highly oxidizing OER condition. To specify, post
mortem XPS was carried out. As shown in Ni 2p XPS profile
(Figure S12a, Supporting Information), the two separate peaks
at the binding energy of 855.15 and 861.83 eV correspond to
the Ni2+ and Ni3+ states, respectively.[21] The corresponding
peaks of Co2+ and Co3+ in Co 2p spectrum (Figure S12b, Sup-
porting Information) appear at 781.50 and 779.80 eV, respec-
tively.[22] Noteworthy, the atomic ratio of Ni3+/Ni2+ and Co3+/
Co2+ are all 5:4, indicating that the doped Ni were equally
oxidized as Co during OER and played as important role as
Co in the electrocatalytic process. Figure S12c (Supporting
Information) shows the loss of P after OER. In contrast, the
O content increases after 20 h OER, as revealed by the O 1s
spectrum (Figure S12d, Supporting Information). The atomic
ratio of P/Co is 0.84:1.69 after OER test from XPS analysis,
comparing to the ratio of 16.14:64.4 before OER test. More-
over, P spectrum shifted to higher binding energy, indicating

the oxidation of P in the P C bonding. Furthermore, the two
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Figure 4. a) The charging current density differences (Δj = janodic−jcathodic) plotted against scan rates of the as-prepared CoPi, Ni5-CoPi, Ni10-CoPi, and
Ni15-CoPi electrodes. b) EIS spectrum measured at 340 mV overpotential of CoPi, Ni5-CoPi, Ni10-CoPi, and Ni15-CoPi.
corresponding peaks in the O 1s profile indicate that the ratio
of OH− (531.3 eV) and O2− (533.2 eV) is 2:1, which supports
the oxyhydroxide formation during OER.[23]
(ΙΙ) in the synthesized materials. Earlier studies have revealed
the importance of the preoxidation of Co (ΙΙ) for OER in Co-
based catalysts, where Co (ΙΙ) is inclined to be oxidized to Co
(ΙΙΙ) or higher oxidation state, which is believed to be a crit-
ical step to generate active oxyhydroxide sites for OER.[11b,25]
To study this behavior, the CVs of the four samples were
explored. We found that the oxyhydroxide formation here dis-
plays different pseudocapacitive behaviors depending on the
presence or not of Ni. Figure 5 reveals the first and second
cycles of the CV profiles. For both CoPi (Figure 5a) and Ni10-
CoPi (Figure 5c), the first cycle displays a larger pseudoca-
pacitive charge corresponding to the peak at 1.29 and 1.27 V,
respectively. For Ni10-CoPi, significant changes are observed
between the first and second cycles in the oxidation peak and
the pseudocapacitive charges from 1.27 to 1.12 V, indicating
a change in surface chemistry, while no such obvious con-
trast observed on CoPi. Thus, a more thorough reconstruction
may happen on the surface of Ni10-CoPi and then lowered
the overpotential. The CV profiles exhibit negligible changes
during the subsequent cycles (Figure S16, Supporting Infor-
mation), indicating the surface reconstruction might be irre-
versible evolving oxyhydroxide serving as stable active sites for
OER. Similar changes can also be observed on Ni5-CoPi from
1.24 to 1.11 V (Figure 5b) and Ni15-CoPi from 1.31 to 1.23 V
(Figure 5d), while the change on Ni15-CoPi is not as huge as
Ni10-CoPi, indicating that surface reconstruction almost satu-
rated with 10% of Ni substituting Co.
In conclusion, the amorphous Ni10-CoPi nanoflakes
composed of highly ordered mesoporous structures were
prepared by a facile oil-phase method and demonstrated
for the application of OER. The amorphous structure and
mesoporous morphology provide high active sites density
and large exposed surface area. Ni doping improves the
synergistic work and facilitates the surface reconstruction
evolving into active oxyhydroxide sites under OER. The
materials displayed its decent OER performance, delivering
a durable current density of 10 mA cm−2 at 320 mV. Its reac-
tion kinetics is comparable to noble metal based electro-
catalyst such as Ir/RuOx with a gentle Tafel slope of only
44.5 mV dec−1. The mesoporous Ni10-CoPi shows promise
as a low-cost alternative due to its abundance as compared
to Ru and Ir.
The electrochemically effective surface area (ECSA) was
estimated to better understand the electrocatalytic proper-
ties of the Ni10-CoPi catalyst toward OER by conducting
cyclic voltammetry (CV) at various scan rate (Figure S13,
Supporting Information). The linear fit results in Figure 4a
reveal that the Ni10-CoPi has a double-layer capacitance (Cdl)
value of 262.8 µF cm−2, which is comparable with that of
CoPi (277.7 µF cm−2), Ni5-CoPi (259.2 µF cm−2), and Ni15-
CoPi (317.2 µF cm−2). Based on the Cdl values, the symbolic
ECSA was calculated as well.[24]The Ni10-CoPi nanoflakes dis-
play an ECSA of 6.57 mECSA2, which is comparable with that
of CoPi (6.94 mECSA2), Ni5-CoPi (6.48 mECSA2), and Ni15-CoPi
(7.93 mECSA2). Furthermore, the specific activity from the
normalization of ECSA were also calculated to elucidate the
origin of the superior performance of Ni10-CoPi (Figure S14,
Supporting Information). Among the four electrodes, the
Ni10-CoPi sample shows the highest specific activity of
−2
0.91 mA cmECSA at the 380 mV OER overpotential, which
outperforms those of CoPi (0.17 mA cmECSA−2), Ni5-CoPi
(0.37 mA cmECSA−2), and Ni15-CoPi (0.41 mA cmECSA−2). The
higher specific activity indicates that Ni10-CoPi has a higher
intrinsic activity, which contributes to the better electrochem-
ical performance. On the other hand, RuO2 tested within the
same potential window and test condition exhibits a huge Cdl
value of 3 mF cm−2 (Figure S15, Supporting Information), and
yields a specific activity of only 0.09 mA cmECSA−2, which fur-
ther stands out the superior intrinsic activity of Ni10-CoPi.
We also tested the electrochemical impedance spectroscopy
(EIS) of the four samples (Figure 4b) to investigate the charge
transfer ability of the materials. The charge transfer resistance
(Rct) of the Ni10-CoPi electrode is only 38.59 Ω, which is much
smaller than those of CoPi (190.5 Ω), Ni5-CoPi (92.17 Ω), and
Ni15-CoPi (44.25 Ω). The smaller Rct indicates its faster charge
transfer kinetics of the optimized Ni10-CoPi electrode. Com-
paring with CoPi, the smaller Rct of Ni-doped CoPi indicates
that the introduction of Ni enhances the electroconductivity of
the cobalt phosphate materials.
To investigate the origin of the enhanced OER performance
of Ni10-CoPi, attention was driven to the preoxidation of Co
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Figure 5. a–d) Pseudocapacitive behavior in the first and second cycles of CoPi, Ni5-CoPi, Ni10-CoPi, and Ni15-CoPi during CV cycling.
dried in air. Synthesis of the amorphus 10 at% Ni doped CoPi nanoflakes
without mesoporous structure is similar except heating ODE to 320 °C.
Experimental Section
Materials: Cobalt (II) chloride hexahydrate (CoCl2·6H2O, 98%), nickel
(II) chloride hexahydrate (NiCl2·6H2O, 98%), sodium oleate (NaOA,
≥82% fatty acids (as oleic acid) basis powder), 1-octadecene (ODE,
technical grade, 90%), tetradecylphosphonic acid (TDPA, 97%), and
Nafion117 solution (5%, in a mixture of lower aliphatic alcohols and
water) were purchased from Sigma-Aldrich. Isopropyl alcohol (IPA,
technical grade, 99%), ethanol absolute (99.9%), hexane (99.9%), and
methanol (99.9%) were purchased from Aik Moh. All chemicals and
materials were used without further purification.
Synthesis of Co(OA)2 and Ni-Doped Co(OA)2 Precursor: The synthesis
of Co(OA)2 and Ni-doped Co(OA)2 precursor was followed by thermal
decomposition. According to a typical method,[26] different molar ratio
of Co/Ni solution (1:0, 95:5, 90:10, 85:15 for the synthesis of Co(OA)2
and 5%, 10%, 15% Ni-doped Co(OA)2, respectively) were obtained
by adding CoCl2·6H2O, NiCl2·6H2O, and 20 mmol NaOA in a mixture
solution of 16 mL ethanol absolute, 12 mL deionized (DI) water, and
28 mL hexane. The total amount of metal cations (Co2+and Ni2+) were
kept at 10 mmol. Then the solution was heated at 70 °C for 4 hours
with vigorous stirring. The obtained products were firstly dissolved into
hexane and then washed with DI water for three times and subsequently
washed with methanol for three times. The pure brown Co(OA)2 and
Ni-doped Co(OA)2 precursors were then dried at 80 °C in a vacuum
oven overnight.
Characterizations: The morphology of the samples was characterized
with the TEM system (JEOL 2010 UHR) operating at 200 kV, and field
emission scanning electron microscopy (FESEM, JEOL, JSM-7600F).
X-ray diffraction was performed to characterize the samples on a Bruker
D8 Advance powder XRD at the 2θ range of 10° to 80° with Cu Kα
(λ = 1.5406 Å) radiation. The scanning rate is 2° min−1 with a step size of
0.01°. Low-angle XRD was tested by Panalytical X’Pert Pro at the 2θ range
of 3° to 10° with Cu Kα (λ = 1.5406 Å) radiation. XPS was conducted on
an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific)
using monochromatic Al Kα X-ray beam (1486.6 eV). The voltage step
size was 1 eV for survey spectra and 0.1 eV for high-resolution scan. The
EDX mapping was tested on a TEM (JEOL 2100HR). The composition
of the synthesized materials was confirmed by the PerkinElmer Optima
8000 ICP-OES. AFM Asylum Research Cypher S was used to confirm the
thickness of the obtained materials. The BET surface areas were obtained
from N2 adsorption–desorption isotherms (ASAP Tri-star II 3020).
Electrochemical Measurement: For OER testing, 2.5 mg of the catalyst
materials was dispersed in 0.5 mL of solution containing 485 µL of IPA and
15 µL of 5 wt% Nafion followed by ultrasonication for 30 min. Then, 5 µL of
the prepared suspension was pipetted onto a prepolished glassy carbon (GC)
disk electrode (3 mm in diameter), followed by drying at room temperature
for 5 min. Electrochemical measurements were performed in a typical three-
electrode system on an electrochemical station (Solartron). A graphite rod
and Hg/HgO served as the counter electrode and the reference electrode,
respectively. The catalyst-loaded GC electrode was used as the working
electrode. All measurements were carried out in 1.0 m KOH electrolyte
under room temperature. CV was performed within a potential window of
0.8–1.8 V (vs RHE) at a scan rate of 100 mV s−1. LSV was conducted at a
scan rate of 2 mV s−1. In all measurements, Hg/HgO RE was calibrated:
E(RHE) = E(Hg/HgO) + 0.059 pH + 0.098. All LSV curves were corrected
with 90% iR-compensation. By plotting overpotential η versus logarithm of
current density from polarization curves, Tafel slopes were obtained.
Synthesis of Amorphous CoPi and Ni-Doped CoPi Nanoflakes with
Highly Ordered Mesoporous Structure: The synthesis is based on a slightly
modified oil-phase method.[16] To start, 5 mL of ODE was heated to
300 °C under Ar protection in a three-neck flask. Next, 0.9 mmol Co(OA)2
precursor and 0.6 mmol TDPA were added into the hot oil and kept
heating at 300 °C for 1.5 h. Then cooled to room temperature naturally.
Ni-doped CoPi was conducted with similar procedure by replacing
Co(OA)2 with Ni-doped Co(OA)2. The obtained products were washed
by hexane and IPA for 5 to 6 times and collected by centrifuging and
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EIS measurements were carried out at a potential of 1.57 V (vs RHE)
in the frequency range from 100 kHz to 0.1 Hz in 1.0 m KOH solution.
ECSA was estimated from the electrochemical double-layer capacitance
(Cdl) of the materials. The Cdl was determined by a simple CV method.
The CV was conducted in a potential window of −0.1 to 0 V (vs Hg/
HgO reference electrode) at various scan rate of 10, 20, 40, 60, 80, and
100 mV s−1. Then by plotting capacitive current difference Δj = ja − jc
at −0.05 V against scan rates, an obtained linear slope is equivalent to
S. Li, Q. Yan, Nano Energy 2018, 47, 257; c) Q. Liang, L. Zhong,
C. Du, Y. Luo, J. Zhao, Y. Zheng, J. Xu, J. Ma, C. Liu, S. Li, Q. Yan,
ACS Nano 2019, 13, 7975.
[7] a) Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2015,
44, 2060; b) P. F. Liu, X. Li, S. Yang, M. Y. Zu, P. Liu, B. Zhang,
L. R. Zheng, H. Zhao, H. G. Yang, ACS Energy Lett. 2017, 2, 2257;
c) Y. Jiang, Y.-P. Deng, R. Liang, J. Fu, D. Luo, G. Liu, J. Li, Z. Zhang,
Y. Hu, Z. Chen, Adv. Energy Mater. 2019, 9, 1900911.
[8] a) K. Jin, J. Park, J. Lee, K. D. Yang, G. K. Pradhan, U. Sim, D. Jeong,
H. L. Jang, S. Park, D. Kim, N.-E. Sung, S. H. Kim, S. Han,
K. T. Nam, J. Am. Chem. Soc. 2014, 136, 7435; b) Y. Li, C. Zhao,
Chem. Mater. 2016, 28, 5659; c) Y. Zhan, M. Lu, S. Yang, C. Xu,
Z. Liu, J. Y. Lee, ChemCatChem 2016, 8, 372; d) H. Kim, J. Park,
I. Park, K. Jin, S. E. Jerng, S. H. Kim, K. T. Nam, K. Kang, Nat.
Commun. 2015, 6, 8253.
twice of the double-layer capacitance (Cdl) value. ECSAs were obtained
−2
from Cdl values divided by 40 µF cm−2 cmECSA
.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
[9] a) M. Pramanik, C. Li, M. Imura, V. Malgras, Y.-M. Kang,
Y. Yamauchi, Small 2016, 12, 1709; b) Y. Zhou, H. C. Zeng, Small
2018, 14, 1704403.
Acknowledgements
This study was financially supported by Singapore MOE AcRF Tier 1 Grant
2016-T1-002-065, and Tier 2 under Grant Nos. 2017-T2-2-069 and 2018-T2-
01-010. The authors greatly thank the Facility for Analysis, Characterization,
Testing and Simulation (FACTS) of Nanyang Technological University,
Singapore, for using their TEM, SEM, and XRD equipment.
[10] Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov,
T. F. Jaramillo, Science 2017, 355, eaad4998.
[11] a) D. Friebel, M. W. Louie, M. Bajdich, K. E. Sanwald, Y. Cai,
A. M. Wise, M.-J. Cheng, D. Sokaras, T.-C. Weng, R. Alonso-Mori,
R. C. Davis, J. R. Bargar, J. K. Nørskov, A. Nilsson, A. T. Bell, J. Am.
Chem. Soc. 2015, 137, 1305; b) T. Wu, S. Sun, J. Song, S. Xi, Y. Du,
B. Chen, W. A. Sasangka, H. Liao, C. L. Gan, G. G. Scherer, L. Zeng,
H. Wang, H. Li, A. Grimaud, Z. J. Xu, Nat. Catal. 2019, 2, 763.
[12] C. Tan, X. Cao, X.-J. Wu, Q. He, J. Yang, X. Zhang, J. Chen, W. Zhao,
S. Han, G.-H. Nam, M. Sindoro, H. Zhang, Chem. Rev. 2017, 117,
6225.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
[13] L. Su, H. Du, C. Tang, K. Nan, J. Wu, C. M. Li, J. Colloid Interface Sci.
2018, 528, 36.
amorphous mesoporous, cobalt phosphate, electrocatalysts, nickel
doping, oxygen evolution reaction
[14] Y. Wan, H. Yang, D. Zhao, Acc. Chem. Res. 2006, 39, 423.
[15] a) W. Liu, H. Liu, L. Dang, H. Zhang, X. Wu, B. Yang, Z. Li, X. Zhang,
L. Lei, S. Jin, Adv. Funct. Mater. 2017, 27, 1603904; b) M. W. Kanan,
D. G. Nocera, Science 2008, 321, 1072; c) R. D. L. Smith,
M. S. Prévot, R. D. Fagan, S. Trudel, C. P. Berlinguette, J. Am. Chem.
Soc. 2013, 135, 11580.
Received: November 20, 2019
Revised: December 29, 2019
Published online:
[16] D. Yang, Z. Lu, X. Rui, X. Huang, H. Li, J. Zhu, W. Zhang,
Y. M. Lam, H. H. Hng, H. Zhang, Q. Yan, Angew. Chem., Int. Ed.
2014, 53, 9352.
[17] F. B. Noronha, M. Schmal, B. Moraweck, P. Delichère, M. Brun,
F. Villain, R. Fréty, J. Phys. Chem. B 2000, 104, 5478.
[18] a) J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang,
L. Zhi, F. Wei, Adv. Funct. Mater. 2012, 22, 2632; b) Y. Zhang,
W. Sun, X. Rui, B. Li, H. T. Tan, G. Guo, S. Madhavi, Y. Zong,
Q. Yan, Small 2015, 11, 3694.
[19] Y. Yin, P. Wu, H. Zhang, C. Cai, Electrochem. Commun. 2012,
18, 1.
[1] a) S. Chu, A. Majumdar, Nature 2012, 488, 294; b) T. R. Cook,
D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets,
D. G. Nocera, Chem. Rev. 2010, 110, 6474; c) J. A. Turner, Science
2004, 305, 972; d) Y.-P. Deng, Y. Jiang, D. Luo, J. Fu, R. Liang,
S. Cheng, Z. Bai, Y. Liu, W. Lei, L. Yang, J. Zhu, Z. Chen, ACS Energy
Lett. 2017, 2, 2706.
[2] a) R. Frydendal, E. A. Paoli, B. P. Knudsen, B. Wickman,
P. Malacrida, I. E. L. Stephens, I. Chorkendorff, ChemElectroChem
2014, 1, 2075; b) Y. Lee, J. Suntivich, K. J. May, E. E. Perry, Y. Shao-
Horn, J. Phys. Chem. Lett. 2012, 3, 399; c) T. Reier, M. Oezaslan,
P. Strasser, ACS Catal. 2012, 2, 1765; d) M. Vukovic´, J. Appl. Electro-
chem. 1987, 17, 737.
[20] P. Zhu, Z. Yang, H. Zhang, J. Yu, Z. Zhang, J. Cai, C. Li, J. Alloys
Compd. 2018, 745, 164.
[3] a) C. G. Read, J. F. Callejas, C. F. Holder, R. E. Schaak, ACS Appl.
Mater. Interfaces 2016, 8, 12798; b) J. Masud, S. Umapathi,
[21] A. P. Grosvenor, M. C. Biesinger, R. S. C. Smart, N. S. McIntyre,
Surf. Sci. 2006, 600, 1771.
N. Ashokaan, M. Nath, J. Mater. Chem.
c) N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu, H. M. Chen,
Chem. Soc. Rev. 2017, 46, 337.
A 2016, 4, 9750;
[22] C. Yan, Y. Zhu, Z. Fang, C. Lv, X. Zhou, G. Chen, G. Yu, Adv. Energy
Mater. 2018, 8, 1800762.
[23] C. S. Lim, C. K. Chua, Z. Sofer, K. Klímová, C. Boothroyd,
M. Pumera, J. Mater. Chem. A 2015, 3, 11920.
[24] a) D. Voiry, M. Chhowalla, Y. Gogotsi, N. A. Kotov, Y. Li,
R. M. Penner, R. E. Schaak, P. S. Weiss, ACS Nano 2018, 12, 9635;
b) C. C. L. McCrory, S. Jung, J. C. Peters, T. F. Jaramillo, J. Am.
Chem. Soc. 2013, 135, 16977.
[25] a) M. W. Kanan, J. Yano, Y. Surendranath, M. Dinca˘, V. K. Yachandra,
D. G. Nocera, J. Am. Chem. Soc. 2010, 132, 13692; b) B. S. Yeo,
A. T. Bell, J. Am. Chem. Soc. 2011, 133, 5587.
[4] a) J. Wu, M. Liu, K. Chatterjee, K. P. Hackenberg, J. Shen, X. Zou,
Y. Yan, J. Gu, Y. Yang, J. Lou, P. M. Ajayan, Adv. Mater. Interfaces
2016, 3, 1500669; b) X. Duan, J. Xu, Z. Wei, J. Ma, S. Guo, H. Liu,
S. Dou, Small Methods 2017, 1, 1700156.
[5] a) J. Xie, Y. Xie, Chem. - Eur. J. 2016, 22, 3588; b) A. K. Tareen,
G. S. Priyanga, K. Khan, E. Pervaiz, T. Thomas, M. Yang, ChemSu-
sChem 2019, 12, 3941; c) M.-S. Balogun, Y. Huang, W. Qiu, H. Yang,
H. Ji, Y. Tong, Mater. Today 2017, 20, 425.
[6] a) P. Liu, R. N. Ali, J. Li, G. Hu, X. Zhu, Y. Lu, B. Xiang, Appl. Surf.
Sci. 2019, 484, 54; b) Q. Liang, L. Zhong, C. Du, Y. Luo, Y. Zheng,
[26] D. Ge, J. Wu, G. Qu, Y. Deng, H. Geng, J. Zheng, Y. Pan, H. Gu,
Dalton Trans. 2016, 45, 13509.
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