Na4Ni3P4O15-Ni(OH)2 core-shell nanoparticles as hybrid electrocatalysts for the oxygen evolution reaction in alkaline electrolytes

Article abstract of DOI:10.1039/d0dt01205j

There is wide interest in developing efficient, robust and low-cost electrode materials for the electrolysis of water to produce clean hydrogen fuel. It is especially important to improve the performance and durability of electrocatalysts for the OER. Here we have shown that the transformation of nanoparticle (n-NNP) and crystalline (c-NNP) forms of mixed phosphate Na₄Ni₃(PO₄)₂P₂O₇ in highly alkaline solutions occurs along various routes and provokes the generation of 2D Ni(OH)₂ nanosheets or stable core(phosphate)-shell(Ni(OH)₂) particles, respectively. In both cases, in the carbon matrix (through chemical and electrochemical conversion of phosphate in situ during electrolysis in a 6 M KOH or NaOH solution) stable OER electrocatalysts with low overpotentials of 250-290 mV at a current density of 10 mA cm^-2 were obtained. The best candidate for the OER process is core-shell particles, which maintain overpotentials of around 250 mV in 6 M KOH for more than 3 days. The activity enhancement can be attributed to the formation of abundant NiOOH nanoparticles on the shell surface due to improved lattice matching. This report discusses future prospects for the creation of core-shell particles to reduce the overpotential of durable electrocatalysts for the OER.

Full text of DOI:10.1039/d0dt01205j

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DOI: 10.1039/D0DT01205J
ARTICLE
Nanoparticles Na4Ni3Р4О15-Ni(OH)2 Core-Shell as hybrid
electrocatalyst for the oxygen evolution reaction in alkaline
electrolyte
Received 00th January 20xx,
Accepted 00th January 20xx
Chen Mu,a Denys S. Butenko,b Ievgen V. Odynets,a,c Іgor V. Zatovsky,*a Junzhi Li,b Wei Han,b,d and
Nickolai I. Klyui*a,e
DOI: 10.1039/x0xx00000x
There are wide interests in developing efficient, robust and low-cost electrode materials for electrolysis of water to produce
clean hydrogen fuel. It is especially important to improve the performance and durability of electrocatalysts for OER. Here
we showed that the transformation of nanoparticles (n-NNP) and crystalline (c-NNP) forms of mixed phosphate
Na4Ni3(PO4)2P2O7 in highly alkaline solutions occurs along various routes and provokes generation of 2D-nanosheets Ni(OH)2
or stable particles of core(phosphate)-shell(Ni(OH)2), respectively. In both cases, in carbon matrix (through chemical and
electrochemical conversion of phosphate in situ during electrolysis in 6 M KOH or NaOH solution) stable OER electrocatalysts
with low overpotentials of 250-290 mV at a current density of 10 mA·cm-2 were obtained. The best candidate for OER process
is core-shell particles, which maintain overpotentials of around 250 mV in 6 M KOH more than 3 days. The activity
enhancement can be attributed to formation of abundant system of NiOOH nanoparticles on the shell surface due to
improved lattice matching. The report discusses further prospects for the creation of core-shell particles to reduce the
overpotential of durable electrocatalysts for OER.
overpotential values are taken as reference electrocatalysts for
OER.4 However, the high cost and low abundance of these noble
metals in the soil are unfavorable factor for their widespread
Introduction
It is widely known that hydrogen is an environmentally friendly
and highly efficient “ideal” fuel. Among the various advanced
technologies, hydrogen production by electrolysis is one of the
most attractive trends.1 At the same time, during the process of
water oxidation significant overpotentials are required for the
oxygen and hydrogen evolution reaction, OER and HER
respectively, which leads to additional energy losses. Thus,
intensive research is carried out to develop a variety of
electrocatalysts that can shift the water splitting potentials
closer to the thermodynamic limits.1-3 First of all, there is
demand to fabricate an effective electrocatalysts for OER, since
this reaction is the “bottleneck” of water splitting and involves
four successive processes.2,3 Currently, RuO2 or IrO2 with low
application. Therefore, over the past decades, a large number
of borides,5 nitrides,6,7 oxides or hydroxides,8-16 phosphides17-21
and chalcogenides18,22 of transition metals have been proposed
as effective and economically competitive electrocatalysts for
OER process.3 In particular, good catalytic activity was found for
various complex oxides with different types of crystal lattice
(spinel’s, perovskites, etc.).23-30 It has been shown that
amorphous phosphate films31-34 and crystalline phosphates of
d-metals31-42 demonstrate a very high catalytic efficiency in the
reaction of gaseous oxygen production.
In recent years, interest in phosphate electrocatalysts for water
splitting has increased significantly.43 Remarkable catalytic
features have been found in d-metal phosphates with different
composition and structure, such as FePO4,44 Сo3(PO4)2,45
Co3(PO4)2·8H2O,46 (Co0.5Ni0.5)3(PO4)2,41 Ni2P2O7·8H2O,47 Co2-
xNixP2O7,48 LiNi0.75Fe0.25PO4,49 Li(Ni,Fe)PO4,35 AFePO4 (A – Na,
a.
College of Physics, Jilin University, Changchun 130012, P. R. China. E-mail:
zvigo@yandex.ru, klyuini@ukr.net
Sino-Russian International Joint Laboratory for Clean Energy and Energy
b.
38
K),50 NaCoPO4,51 Na2СoP2O7 and etc. Meanwhile in recent
Conversion Technology, College of Physics, Jilin University, Changchun 130012, P.
R. China.
years, several new phosphates, for instance Ni2P4O12,52
(NiFe)PO4(OH)53 and NaCo(PO3)354 were included into the group
of effective electrocatalysts.
c. Department of Chemistry, Pavla Tychyna Uman State Pedagogical Universiyty, 2
str. Sadova, Uman 20300, Ukraine.
d.
International Center of Future Science, Jilin University, Changchun City 130012, P.
It should be noted that it is possible to draw a clear parallel
dependency between a lot of high voltage cathodes for Li or Na
batteries55,56 and electrocatalysts for OER process based on
R. China.
e.
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41 pr. Nauki,
03028 Kyiv, Ukraine.
†Electronic
Supplementary
Information
(ESI)
available:
Additional
photomicrographs, SEM image and elemental mapping, XRD powder data, TEM phosphate materials. A significant number of phosphate
images and TEM of high-resolution, XPS spectra, additional electrochemical
measurements, section “Electrochemical measurements”, section “Crystallization
of the Na4Ni3PO15 melt”, section “Investigation of crystallization process from Na-
Ni-P-O self-fluxes”, and Table S1-S3. See DOI: 10.1039/x0xx00000x
structures (frames) is capable to stabilize d-metals in various
oxidative states, which is a perfect precondition for the
presence of catalytic properties for oxidative-reduction
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transformations. Thus, in35 the process of electrochemical OER, Na4Ni3P4O15-Ni(OH)2/СС electrode showed highly efficient
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tuning of olivine-type Li(Ni,Fe)PO4 phosphates (widely- electrocatalytic characteristics with an ovDeOrpI:o1t0e.1n0t3i9a/lDo0fD2T50012m05VJ
researched family of cathode materials in lithium ion batteries) at 10 mA·cm-2 and excellent stability (at least 70 hours with
is described. In the study of this material in order to effectively continuous electrolysis). Further prospects and ways of creating
increase the efficiency of using LiMPO4 for OER in aqueous effective catalysts for core-shell nanoparticles were also
solution (generating a current density of 10 mA·cm-2 at an analyzed and discussed.
overpotential of only 0.27 V in 0.1 M KOH), the strategy of
partial extraction of lithium ions from phosphate in the organic
electrolyte was successfully implemented. Another example is
Experimental
NaCo(PO3)3. In 2018, it was shown the possibility of using this
Synthesis of nanoparticles and crystal form of Na4Ni3P4O15
compound as a cathode material for sodium ion batteries.57 And
All used reagents (NaH2PO4·2H2O, Na2HPO4·12H2O, NH4H2PO4,
in the next year it was found that this phosphate shows
NiO, Ni(NO3)2·6H2O and NH3·H2O 25 % aqueous solution) have
electrocatalytic properties for OER with very small
analytical grade. The anhydrous powders of NaPO3 and Na4P2O7
overpotential of 340 mV and high mass activity of 532 A·g-1.54
were prepared by dehydration of NaH2PO4·2H2O and
Thus, in general, we can assume that phosphate cathode
Na2HPO4·12H2O at 450°C and 900°C (in a platinum cup),
materials are potential candidates for OER catalysts. However,
respectively.
there is a significant difference between non-water high voltage
Synthesis nanoparticles of Na4Ni3P4O15: 3.989 g Na4P2O7, 3.452
ion batteries and water solutions for electrolysis. Under the
g NH4H2PO4 and 13.082 g Ni(NO3)2·6H2O were dissolved in 50 ml
action of aggressive water electrolytes, especially in the case of
distilled water with stirring to make homogenous solution.
Aqueous ammonia was added to the prepared solution to value
the pH about 7.0. The solution was then evaporated at 180C
and the resulting powder was calcined at 600C for 2 h in air
alkaline media, d-metal phosphates can partially or completely
transform into other forms, which leads to changes of catalytic
activity. Unfortunately, only a small number of works devoted
to phosphate electrocatalysts analyzes this issue. Thus, for Co-
atmosphere. As-prepared of Na4Ni3P4O15 (n-NNP) is a rounded
42
58
contained compounds Co2(PO4)3 and Na4Co3P4O15 in situ
transformation into forms of cobalt oxide/hydroxide was found,
resulting in a general decrease in catalytic activity. Instead, for
nickel-phosphate films, the formation of nickel oxyhydroxide
(NiOOH) in 1M KOH during water oxidation proceeded only on
the surface of phosphate.31 Obtained hybrid composite shows
unique catalytic properties: overpotential 220 and 290 mV at a
current density of 10 and 500 mA·cm-2 (1M KOH), respectively,
as well as 332 mV at a high current density of 1600 mA·cm-2 (5M
KOH). The authors argue that Ni(Fe)OOH serves as the main
catalyst for OER, and phosphate as a co-catalyst for water
adsorption.31 Also, remarkable electrocatalytic performances
were observed in case of joint use of phosphates and
phosphides (core-shell structure Ni12P5-Ni3(PO4)2) in alkaline
media59 or in the occurrence of phosphate co-catalysts in situ
for NiCoP nanosystems.19 In all cases, the presence of a
composite material (phosphate-oxide/hydroxide or phosphide-
phosphate-oxide/hydroxide) contributes to decrease in the
oxygen overpotential values and high catalytic stability over
time.19,31,59,60 Thus, the application of phosphorus-containing
materials for production of hybrid catalysts in situ is a very
attractive idea.60-65 Structural type diversity of d-metal
phosphates determines their different stability in alkaline
medium. At the same time, a systematic study of this class of
compounds have not been carried out yet.
conglomerates of nanoparticles with diameter to 0.5 mm in size
of light yellow color (see ESI †, Fig. S1a).
For preparation of crystal form Na4Ni3P4O15 (c-NNP): 10.64 g
Na4P2O7, 12.24 g NaPO3 and 4.04 g NiO were mixed and
grounded in an agate mortar. The mixture was melted in
platinum crucible at 1000C and kept in isothermal conditions
for 5 hours. Then homogeneous flux was cooled at a rate 20·h-1
down to 700С. The crystals were washed from the rest of the
melt components with hot distilled water, followed by drying at
120C in an air atmosphere oven for 1h. The resulting needle-
like crystals (up to 1 cm long) have a yellow-orange color and
form splices (see ESI †, Fig. S1b).
Material characterization
Powdered X-Ray diffraction (XRD) patterns were recorded at
room temperature using a Shimadzu XRD-6000 X-ray powder
diffractometer (Cu radiation with λ CuKα1 = 1.54056 Å, λ CuKα2
=
1.54439 Å and graphite counter monochromator). Data has
been collected from 5 to 70° in step scan mode 2θ (step 0.02°
and account time of 2 s per point). Transmission XRD pattern
was measured on Rigaku MicroMax-007HF (with CuKα
radiation). The reference patterns of the crystalline compounds
from the PDF2 database were used to identify the samples. The
microstructure, morphology and element compositions of the
samples were analyzed using scanning electron microscopy
(SEM, Magellan 400, recorded at 10 kV) and transmission
electron microscopy (TEM, JEOL JSM 2010F, recorded at 200
kV). The FTIR-spectra were obtained in range 400–4000 cm-1 at
a room temperature using Shimadzu IRAffinity-1S spectrometer
(samples were pressed into discs with KBr). The surface
chemical states of the catalysts were analyzed using X-ray
photoelectron spectroscopy (XPS) measurements performed on
a VG ESCALAB MK II electron spectrometer.
Complex phosphate Na4Ni3P4O15 is promising material for Na-
ion batteries66 and supercapacitor.67 In this work we
investigated mixed phosphate Na4Ni3(РО4)2Р2О7 as potential
candidate for electrocatalytic water splitting. Here we describe
synthesis features of nanoparticles and crystals for this
phosphate, which are converted in various ways in an alkaline
medium. Both final forms (Ni(OH)2 nanosheets and
Na4Ni3P4O15-Ni(OH)2 particles with a core-shell structure) can
be used as excellent electrocatalysts for OER. As a catalyst for
2 | J. Name., 2012, 00, 1-3
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Fabrication of electrodes
ARTICLE
hexamine)67 as fuel have not been added. Subsequently, the
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DOI: 10.1039/D0DT01205J
For the fabrication of electrodes, the crystals of c-NNP were initial mixture of reagents after evaporation of water was
ground. Crystalline form of Na4Ni3P4O15 were manually milled in calcined at 600С (similar to67). The general scheme of n-NNP
an agate mortar, then in a planetary mill (alcohol media; obtaining corresponded to the reaction:
grinding time 1 h; milling speed of 565 rpm; ZrО2 balls with
diameter of 10 mm; balls to powder ratio = 10:1 wt.). As a result,
Na4P2O7 + 3Ni(NO3)2 + 2NH4H2PO4 + 2.5O2 → Na4Ni3P4O15
8NO2 + 6H2O (1)
+
powder with a size less than 10 µm was obtained (Fig. S2, ESI †). In case of c-NNP, synthesis was carried out as described in,68 but
To manufacture of working electrode the powder of n-NNP or only a mixture of phosphates was obtained (see: Crystallization
c-NNP was mixed together with conductive carbon black of the Na4Ni3PO15 melt and Fig. S4, ESI †). Therefore, we
(“Super P”) and polyvinylidenefluoride (PVDF) in a weight ratio conducted a comprehensive study of self-flux in Na–Ni–Р–O
of 85:10:5, respectively. Then the cream-like slurry was system and found the condition for obtaining single crystals of
prepared by adding N-methylpyrolidone (NMP), pasted onto a Na4Ni3P4O15 without impurities (see: Investigation of
carbon fiber (HCP331N, 1×1 cm2 working area) and finally dried crystallization process from Na-Ni-P-O self-fluxes and Table S1,
at 100°C for 24 h in a vacuum oven. SEM image and elemental ESI †). Although the crystallization of Na4Ni3(РО4)2Р2О7 in the
mapping of electrodes show mosaic distribution of Na4Ni3P4O15 self-fluxes can be represented by a fairly simple equation:
particle agglomerations, monotonically surrounded by carbon
4NaPO3 + 3NiO → Na4Ni3(РО4)2Р2О7
(2),
material (Fig. S3, ESI †). IrO2 (Aladdin Industrial Corp, Shanghai, but in self-fluxes with an initial Na/P ratio equal to 1.0 this
China) electrode prepared by the similar way was used as a compound is not formed. Only when initial Na/P ratios ranging
benchmark. The fabricated electrodes were hereinafter from 1.3 to 1.5 can be obtained purely of Na4Ni3P4O15. A similar
referred to as n-NNP-С/CC, c-NNP-С/CC and IrO2-C/CC.
situation was observed for many of self-fluxes type MI–Me–Р–
O (MI – alkali metals, Me – polyvalent metals).69-71
The phase purity of the obtained samples was measured by the
XRD characterizations, as shown in Fig. 1a. The positions of the
diffraction peaks of n-NNP and c-NNP fully coincide with the
PDF card #01-087-0977 which belongs to target product
Chemical stability test
The test was performed in 6M solutions of sodium or potassium
hydroxide. Powder n-NNP or c-NNP (≈ 0.2 g) was immersed into
alkali media and left from 1 to 30 days with periodical stirring.
Afterward precipitate was filtered, washed by distilled water,
dried in air and analyzed (according to XRD, and FTIR-spectra
data).
Na4Ni3P4O15. Both diffraction patterns show
a
good
preservation of the ratios between the intensities of most part
reflexes, example indexed to the [001], [420] + [510], [040] +
[620], [222] + [402] crystal planes (Fig. 1a). However, reflexes
Electrochemical testing
Electrochemical measurements were carried out on an with double indices [110] + [200], [020] + [310] and [002] + [312]
electrochemical analyzer/workstation CH Instruments Model of c-NNP are significantly more intense (Fig. 1a). This fact
760E (China). A typical three-electrode set-up was used with a indicates that the texture of needle-like crystals Na4Ni3P4O15
Hg/HgO electrode as a reference electrode and a platinum plate (perpendicular to the с-axis of the orthorhombic cell) is present
as
a counter electrode. Impedance spectroscopy was even after a very long grinding of c-NNP.
performed in the frequency range from 105 to 10-2 Hz with the Typical SEM images for n-NNP are represented in Fig. 1b,c.
amplitude of 5 mV at the open circuit potential. Multiple cyclic Spherical agglomerates n-NNP is polydisperse and consists of
voltammetry (CV) scans were performed for 20 cycles at the smaller particles (Fig. 1b). At higher magnification, we can see
scan rate of 10 mV·s-1 until stable work cell. The linear sweep that the sample consists of nanoparticles less than 1 µm with
voltammetry (LSV) curves were collected at the scan rate of 1 grain-like structure (Fig. 1c). A similar morphology was also
mV·s-1 with iR correction (90%). Chronopotentiometry was described for nano-Na4Ni3P4O15 in.67 For both samples, the
carried out under a constant current density of 10 mA·cm-2 for results of EDX and elemental mapping confirm composition and
20 h or more. Calculations for electrochemical measurements homogeneous distribution of elements (Fig. S5, ESI †).
are presented in detail in the section "Electrochemical According to the image of SEM, the sample c-NNP milled in the
measurements" ESI †.
planetary mill is fragments of single crystals with a size less than
10 µm (Fig. S2, ESI †).
In the FTIR spectra of both samples n-NNP and c-NNP, an
identical set of absorption peaks is observed as combination of
the vibration modes of the PO4 and P2O7 groups (Fig. 1d). The
following frequency regions of the vibration modes of the PO4
Results and discussion
With a purpose to obtain nano (n-NNP) and crystal (c-NNP) form
of Na4Ni3P4O15 we used the results of study given in works.67,68
But our attempts to synthesize the high-purity form of complex
phosphate, as described in these papers, were unsuccessful.
Therefore, in both cases, the methods were improved as a
result of additional studies.
For the synthesis of n-NNP (according to67) the initial reagents
Na4P2O7, Ni(NO3)2·6H2O and NH4H2PO4 were used. However,
the components were taken in proportion to obtain a finite ratio
of Na:Ni:Р as 4:3:4. Moreover, organic reagents (urea, glycine or
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at 879.2 and 861.3 eV (Fig. 1f), which indicates the presence of
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trivalent nickel.73 The presence of d-metals with different
DOI: 10.1039/D0DT01205J
oxidation states on the sample surface, in general, is typical
situation for Na4МII P4O15 compounds.58,67 This factor should be
3
favorable for passing electro-catalytic transformations on the
material surface. The position of Na 1s peak located at 1071.3
eV (Fig. 1g) and almost equal to 1073.3 eV which given for
Na4Ni3P4O15 in.67 The singlet peak in the P 1p spectrum at 133.8
eV (Fig. 1h) confirms that phosphorus exists in oxidation state
5+ to form P-O bond.32,36 Only three peaks with centers at 530.8,
531.8 and 535.4 eV can be distinguished in the O 1s spectrum
(Fig. 1i). Peaks with values 530.8 and 531.8 eV should be
considered as derivatives of different types of P-O bonds.32,36
The presence of weak peak at 535.4 eV possibly related to a
small amount of physically adsorbed water or other oxygen
molecules groups at the surface.74
As noted in the introduction,42,58 complex phosphates under the
action of an alkaline media can decompose, which is
accompanied by leaching of phosphate anions. Thus, a
preliminary test of chemical behavior of phosphate in an
alkaline solution allows partial modeling of the conversion
processes, which may shed light on the dependencies between
the composition and structure of phosphate electrocatalysts in
OER conditions. Post-treatment analysis (samples was hold in
6M KOH or NaOH solution) indicates that the decomposition
process of n-NNP and c-NNP samples is different. After
immersion in an alkaline solution, both samples have changed
their initial color from yellow to light green, which is caused by
chemical conversion.
In cases of n-NNP, there is a rapid and complete decomposition (less
than 1h) of the original sample and formation of Ni(OH)2. This
interaction can be represented by the next equation:
Na4Ni3Р4О15 + 8NaOH → 3Ni(OH)2↓ + 4Na3PO4 + H2O (3)
The formation of pure nickel (II) hydroxide is confirmed by results of
XRD and FTIR analysis. On XRD profile can be clearly seen only a few
wide and strong diffraction reflections from Ni(OH)2 (Fig. 6a, ESI †).
In addition, broadening of the reflection indicates weak crystalline
state of compound. This result is consistent by TEM images (Fig.
Fig. 1 (a) The results XRD for c-NNP and n-NNP. (b,c) Typical SEM images
at various magnification of n-NNP. (d) FTIR spectra of n-NNP and c-NNP. S6b,c, ESI †): Ni(OH)2 looks like 2D nanosheets, scattered or bounded
(e) overall XPS spectrum for c-NNP and high-resolution XPS (f) Ni 2p, (g)
Na 1s, (h) P 2p, (i) O 1s.
into conglomerates. The high-resolution TEM image shown in Fig.
S6d ESI † confirms that the Ni(OH)2 have lattice fringes with
interplanar distances of 0.47, and 0.23 nm, corresponding to the [001]
and [101] lattice planes of Ni(OH)2 (PDF # 00-014-0117). In addition,
and P2O7 groups can be distinguished in the spectra: the bands
in the range of 650–400 cm-1 belong to δ (PO) vibrations; the
EDX analysis and mapping also indicate complete destruction of n-
intense sharp peak at 720 cm-1 corresponds to asymmetric
NNP sample with residual amount of sodium and phosphorus less
stretching of the P–O–P bonds, which indicates the presence of
than 0.25% wt. (Fig. S7 ESI †).
the P2O7 group; the bands in the region of 850–1250 cm-1 are
assigned as νas, νs (PO3) and νas (POP) stretching vibrations. This
For c-NNP samples, the interaction scheme (3) is also implemented,
but the final result is different. According to the XRD data, the
phosphate phase is partially retained even after 30 days treatment
in an alkaline medium. The XRD profile simultaneously includes wide
set of absorption peaks is characteristic for an isostructural
complex phosphates of general composition Na4МII (PO4)2P2O7
3
(MII – Fe, Mn, Co, Ni).
reflections from Ni(OH)2 and sharp c-NNP peaks (Fig. S8a, ESI †). The
According to full XPS survey, the main signals of O 1s, P 2p, Na
formation of the composite Ni(OH)2 and c-NNP also confirmed by the
1s and Ni 2p elements were detected (Fig. 1e). In the high-
results of FTIR spectroscopy (Fig. S8b, ESI †): absorption bands of
resolution Ni 2p spectrum there is a spin-orbit splitting on two
pristine compound in the region of 1200-880 cm-1 are still present
energy states Ni 2p3/2 (856.2 eV) and Ni 2p1/2 (873.8 eV) with an
(vibration modes of PO4/P2O7 groups) while a new intense peak at
energy gap of 17.6 eV (Fig. 1f). The value of 17.6 eV is close to
the 3643 cm-1 have been appeared (vibration mode of O-H bond from
17.4 eV, which should be considered as standard for the
Ni(OH)2). TEM studies have clarified this question. These images
splitting of Ni2+.72 Each of Ni2+ signal is accompanied by satellites
clearly show that the c-NNP particles are coated with a shell of
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Ni(OH)2, which apparently protect the core from further destruction
by an alkaline solution (Fig. S8с,d, ESI †). Thus, we can state the fact
of formation of a stable Na4Ni3Р4О15-Ni(OH)2 core-shell structure.
We assume that the differences in the destruction between n-NNP
and c-NNP in an alkaline medium are due to the microstructure and
peculiarity of the crystal structure of Na4Ni3P4O15. The crystalline
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framework of Na4MII P4O15 (MII - divalent 3d-metals) is classified as a
3
layered type of structure, in which fragments from infinite parallel
2D sheets of MII P2O13 are linked by bridges of P2O7 groups (to 3D
3
framework) and separated by layers of sodium atoms.68,69
Accordingly, this type of crystalline packing suggests the presence of
anisotropy of the growth of the Na4Ni3P4O15 crystal. As noted above,
the appearance of a stable texture perpendicular to the c axis occurs
for c-NNP (Fig. 1a). The presence of texture indicates anisotropic
properties of c-NNP microcrystals. Therefore, the rate of destruction
of various faces of the Na4Ni3P4O15 crystal in an alkaline medium will
be different. Respectively, the formation of 2D Ni(OH)2 films on the Fig. 2 Multiple CV of n-NNP-C/CC and c-NNP-C/CC c-NNP electrodes: (a) 1st
cycle in 6 M KOH and NaOH solutions; (b) 1st, 10th, 20th and 40th cycles for n-
c-NNP surface is slower and, as a result, the Na4Ni3P4O15 microcrystal
NNP-C/CC in 6 M KOH; (c) 1st, 10th, 20th and 40th cycles for c-NNP-C/CC in 6 M
is wrapped with a shell, which prevents the final destruction. In
KOH; (d) 1st, 10th, 20th and 40th cycles for c-NNP-C/CC in 6 M NaOH. In all case
contrast, n-NNP material consists of nanoparticle conglomerates
(without texture) and its destruction in an alkaline solution occurs
faster and quantitatively.
scan rate 10 mV·s-1.
Initial CV tests show that behavior of electrodes based on n-NNP and
c-NNP is significantly diverse. For all samples within the potential
window from 1.1 to 1.85 V (V vs. RHE), two sections can be
distinguished on CV curves of the first cycle (Fig. 2a). The first of them
in the range 1.2 to 1.5 V corresponds to the nickel-reversible redox
transitions Ni2+Ni3+Ni4+.35,36,38 The second section is located
above 1.5 V, where the OER passes (gas bubbles are observed on the
surface of the electrodes). As can be seen from Fig. 2b, for an
electrode n-NNP-C/CC in 6M KOH already in the first cycle at a
voltage of 1.85 V, a current density of more than 140 mV·cm-2 is
achieved, and as a consequence there is an extremely intense release
of oxygen bubbles, which gradually destroys the surface of the
electrode. Upon further cyclization, small particles of the catalytic
mass are broken off from the surface of the electrode, which is
accompanied by a rapid decrease in catalytic activity.
Already after the 40th CV cycle, practically the entire electrode mass
is separated from the surface of the CC collector, and the maximum
current density does not exceed 35 mV·cm-2 (Fig. 2b). A similar
situation is observed in the case of the 6M NaOH electrolyte (Fig. S9a,
ESI †). Accordingly, to stabilize the electrodes n-NNP-C/CC for further
studies, CV cycling was performed in the range from 0.85 to 1.45 V
(Fig. S9b,c, ESI †), which ensured their integrity. The inverse situation
is observed in the case of c-NNP-C/CC (Fig. 2c,d). Already in the first
CV cycle, current densities of 55 and 80 mV·cm-2 for electrolytes
based on 6 M NaOH and KOH were reached at a voltage of about 1.85
V. Moreover, with further cyclization, the maximum current density
did not decrease, but rather increased (Fig. 2c,d). However, no visual
signs of electrode destruction were observed. Additionally, in section
1.1-1.85 V it is clearly seen that as the number of cycles increases,
the centers of the nickel oxidation/reduction peaks gradually shift to
the left and right, respectively. Moreover, on the CV curves for KOH
electrolyte, there is a splitting of the nickel oxidation peak (Fig. 2c),
which indicates the simultaneous presence of different nickel-
containing phases.
In addition, we tested n-NNP and c-NNP samples in 1 M and 0.1 M
alkaline solutions (test details and results are presented in section
"Chemical stability test in solutions of hydroxides" ESI †). As noted
above, noticeable destruction of the n-NNP material in an alkaline
solution (including 1M and 0.1 M concentrations) occurs. The rate of
Ni(OH)2 nanosheets formation on the surface of the n-NNP form
slows down as alkali concentration in solution decreases. Accordingly,
tuning of the core-shell ratio can be carried out by selecting the
concentration of the alkaline solution and controlling the exposure
time. It should also be noted that we did not observe a noticeable
difference in the interaction of phosphates with NaOH and KOH in
the same concentration.
In general, the result of the conversion of n-NNP and c-NNP in an
alkaline medium is the appearance of Ni(OH)2 2D nanosheets, that
can be a favorable factor for the electrocatalytic OER process.8,10,75
To date, a significant portion of industrial electrolyzers for water
splitting uses electrolytes with a high alkaline concentration (20-
30%).76 Thus, for study of OER activity as a part of our electrode
developing program, we chose a typical three-electrode cell in 6M
NaOH and KOH solutions. As shown in the previous tests, complete
(n-NNP) or partial (c-NNP) conversion to the Ni(OH)2-form should be
expected in an alkaline medium. Accordingly, for fabricated
electrodes, there must be a stage of in situ transformation of initial
material to a stabilized state form. The CV method was used to
identify the features of this process. Based on previous CV tests,
recorded at different scan speed and in variable intervals within
potentials, we found stabilization conditions for electrodes. Overall,
for OER activity evaluation and comparison of results was proposed
testing algorithm which includes
a
series of successive
measurements of cyclic voltammetry (CV), impedance spectroscopy
(AC), linear sweep voltammetry (LSV) and chronopotentiometry (CP).
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Such a striking difference in the behavior of n-NNP and c-NNP substrate, accompanied by the electrolyte coloration in black, begins
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DOI: 10.1039/D0DT01205J
material can be explained in terms of the kinetics of the quite intensively. Accordingly, it is to be expected that the
decomposition of various forms of phosphate in an alkaline solution.
As noted above, the interaction of phosphate nanoparticles with
alkali occurs rapidly with complete conversion to Ni(OH)2. It can be
predicted that the rapid occurrence of large quantities of highly
active catalytic forms of Ni(OH)2 and NiOOH in the middle of a carbon
matrix saturated by alkaline solution results in extremely intense
oxygen evolution at a voltage rise that does not have time to diffuse
to the electrode surface. As a result, a considerable pressure of gas
occurs in the bulk of electrode, which leads to the destruction of its
base. Instead, for c-NNP-C/CC generation of Ni(OH)2 is slow, allowing
the electrode to stabilize over time.
After electrode stabilization, impedance spectroscopy (AC imp)
measurements were performed, which are presented in Fig. 3a,b.
The impedance spectra correlate well with the equivalent circuit
shown in Fig. S10, ESI †. The Nyquist plots for all samples are similar
in shape and interpreted in Table S2, ESI †. All values of Rs (including
electrolyte and interface resistance) are in the range of 1 to 1.8 Ω·cm-
2, indicating good catalytic converter contact with a favorable factor
for charge transfer. In addition, small values of Rct (determined by
the diameter of the semicircle) within 4.0-6.9 Ω·cm-2 correspond to a
faster charge transfer of the electrolyte catalyst. These factors are a
prerequisite for good catalytic activity.
A subsequent LSV measurement at a scan rate of 1 mV·s-1,
comparison with IrO2 catalyst and CC current collector is shown in Fig.
3c,d. In the case of n-NNP-C/CC, the LSV curves present an extremely
intense peak centered at about 1.4 V, which should be associated
with the conversion of Ni(OH)2 to NiOOH form. For c-NNP-C/CC, this
peak is also presented, but it shifted to the lower-voltage region and
located at 1.37 and 1.28 V for KOH and NaOH electrolytes,
respectively. This indicates differences in catalytic forms for n-NNP
and c-NNP. In LSV areas above 1.5 V, there is a rapid increase in
current, which leads to the intensive release of oxygen as a result of
the OER. In the vast majority of data presented in the literature
regarding to the measurement of the electrocatalytic characteristics
in an alkaline medium, the results are given generally for 1 M KOH or
NaOH solutions. Therefore, for a better understanding of the
correlation of the obtained values of overpotential in comparison
with those obtained earlier, we conditionally assumed a pH of 14 for
6 M alkalis solutions. Under this condition, against the RHE scale for
the LSV 6 M KOH curves at current densities of 100 and 200 mV·cm-
2, the overpotential of oxygen evolution are 290 and 330 mV for n-
NNP-C/CC and 340 and 380 mV for с-NNP-C/CC. These values are
even better than the corresponding result for IrO2-C/CC (360 mV of
overpotential at 100 mA·cm-2 and 400 mV at 200 mA·cm-2). In a 6 M
NaOH solution, the overpotential values are higher: 360 mV at 100
mA·cm-2 and 410 mV at 200 mA·cm-2 (n-NNP-C/CC), 380 mV at 100
mA·cm-2 and 440 mV at 200 mA·cm-2 (IrO2-C/CC) and 480 mV at 100
mA·cm-2 and 560 mV at 200 mA·cm-2 (c-NNP-C/CC). The CC base of
the electrode is generally characterized by low OER conversion
activity (Fig. 3c).
Fig. 3 Electrochemical measurements for the OER: (a) and (b) AC impedance
spectroscopy in 6 M KOH and NaOH, respectively; (c) and (d) polarization
curves before the long-term stability test for n-NNP-C/CC, c-NNP-C/CC,
IrO2/CC and CC at a scan rate of 1 mV·s-1 in 6 M KOH and NaOH, respectively;
(e) time dependencies of potential under a constant current density of 10
mA·cm-2 for n-NNP-C/CC and c-NNP-C/CC in 6 M KOH and NaOH; (f) LSV curves
of c-NNP-C/CC, c-NNP-C/CC at a scan rate of 1 mV·s-1 after the long-term
stability test in 6M KOH and NaOH; (g) long-time CP test at a current density
of 10 mA·cm-2 for n-NNP-C/CC electrode in 6 M KOH (after stabilization of
electrode for 10 hours at a current density of 10 mA·cm-2).
interface of the current collector-electrode material may be impaired,
which may interfere with obtaining reliable data during long-term
testing. Therefore, at the next stage, CP was performed at a current
density of 10 mV·cm-2 for 20 h (Fig. 3e). During the first 10 minutes
of the test, there is an increase in the voltage, which gradually
decreases in the next 5-7 hours and indicates the final stabilization
for all electrodes. An interesting fact is that during this time, the
initial values for the overpotential of c-NNP-C/CC and n-NNP-C/CC in
6M KOH are reversed, indicating that there is transformation for the
c-NNP material (Fig. 3e). After 10 hours of CP test, there is a very
Before the CP test, we additionally conducted electrochemical tests
of the CC electrode in 6 M alkalis (Fig. S11, ESI †). The results show
that the CC current collector is sufficiently stable at a current density
of 10 mV·cm-2, however, at 20 mV·cm-2, the destruction of the CC
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slight increase in voltage (just a few percent). The final values CP test (Fig. 4b) confirms noticeable degradation of phosphate
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DOI: 10.1039/D0DT01205J
overpotential of oxygen evolution after 20 hours were about 250 mV phases in process of water splitting
for c-NNP-C/CC, 260 mV for n-NNP-C/CC (6M KOH), and 280 mV for
c-NNP-C/CC, 290 mV for n-NNP-C/CC (6M NaOH).
The results of repeated LSV test after CP process are shown in Fig. 3f.
For n-NNP-C/CC electrodes in both electrolytes, a peak about at 1.4
V, whose center is slightly shifted to the higher voltage region
(approximately 0.03 V), is presented. Another situation is observed
for the c-NNP-C/CC form where the peaks are split. In the case of KOH
solution, there are two intense peaks approximately at 1.36 and 1.39
V, and in NaOH solution, the difference increases up to 0.1 V (at 1.28
and 1.38 V). This picture generally confirms the significant difference
in electrochemical behavior of n-NNP and c-NNP materials.
It is widely accepted that OER consists of four consecutive stages,
according to the next basic Equations:2,13,38
Mcat + H2O → Mcat-OHad + e
Mcat-OHad → Mcat-Oad + e
+ H+
Mcat-Oad + H2O → Mcat-OOHad + e
Mcat-OOHad → Mcat + O2 + e
+ H+
̅
+ H+
(4)
(5)
(6)
̅
̅
+ H+
̅
(7)
In our case, the active catalytic sites are atoms of nickel. These atoms
are subject to cyclic transformation of oxidation states: after water
adsorption on the active site (4), transitional forms of Ni-O (5) and
Ni-OOH (6) subsequently arise, and oxygen is released at the final
stage (7). This mechanism is most probable for phosphate35-38 and
oxide/hydroxide catalysts.8-16
The fundamental kinetic mechanism of OER, which occurs on the
surfaces of the electrocatalysts, is usually described by Tafel plots.
Tafel plots are typically determined from the straight-line portion of
the LSV in the region of onset of OER conversion. However, in our
case, this area significantly overlaps with the broad peaks of
electrochemical Ni2+Ni3+ transitions (Fig. 3c,d,f), which does not
allow a correct calculation. Accordingly, for these calculations, we
have performed an additional study of the sample with the best
characteristics (c-NNP sample in 6 M KOH solution). A thin layer of
this electrode material was deposited on a titanium mesh and after
electrode stabilization electrochemical tests were performed (details
and descriptions of these studies are given in Fig. S12). The calculated
Tafel plot for c-NNP-C/Ti was evaluated as 74.2 mV·dec-1, which
shows good catalytic activity against the standard IrO2-C/CC
electrode with Tafel equal to 55.7 mV·dec-1 in 6M KOH (Fig. S12, ESI
†).
For a deeper understanding of the features in situ transformation
features of n-NNP and c-NNP forms in alkaline electrolytes after long-
time electrochemical treatment, we examined the electrodes by
diverse methods. EDX analysis and elemental mapping show that for
n-NNP-C/CC (Fig. S13, ESI †) and c-NNP-C/CC (Fig. 4a) in comparison
with initial substance the elemental composition is conversed in
different ways. In case of n-NNP-C/CC, phosphate is completely
destroyed: nickel and oxygen are identified on the electrode surface,
but phosphorus, sodium and potassium are present in very low
quantities, only as an impurity (Fig. S13, ESI †). Whereas, for c-NNP-
C/CC only a slight decreasing of content was observed for
phosphorus, lesser for sodium, while for nickel and oxygen remained
practically unchanged (Fig. 4a). Moreover, the particles shape and
their total distribution on the surface of the electrode have not
changed. Comparison of FTIR spectra of c-NNP-C/CC before and after
Fig. 4 The results of the study of c-NNP-C/CC electrode after the long-term
stability test for OER in 6 M KOH: (a) SEM image, EDX analysis and elemental
mapping; (b) FTIR spectra; (c) transmission XRD profile; (d) overall XPS
spectrum and high-resolution XPS (e) Ni 2p, (f) Na 1s, (g) P 2p, (h) O 1s.
(similar changes in the FTIR spectrum were observed for c-NNP
material after chemical treatment in alkaline solution, Fig. S8b, ESI
†). On transmission XRD profile of c-NNP-C/CC measured after CP
test only one type of phase can be clearly distinguished, this phase is
initial phosphate (Fig. 4c). This suggests the presence of an
amorphous phase on the surface of phosphate. As described above,
the interaction of Nа4Ni3Р4О15 with an alkaline solution leads to the
formation of nickel hydroxide. At the same time, it is well known that
the nickel hydroxide is transformed to nickel oxyhydroxide (NiOOH)
in the conditions of the OER process.11-15
This transformation pathway is consistent with results of XPS
spectroscopy. As it should be expected in the XPS spectra signals of
O 1s and Ni 2p elements of the n-NNP-C/CC electrode after the CP
test were detected, whereas the spectrum of P 2p and Na 1s are
absent (Fig. S14a,c,d, ESI †). The Ni 2p core-level XPS spectra are
displayed in Fig. S14b, in which Ni 2p3/2 (856.0 eV) and Ni 2p1/2 (873.7
eV) with an energy gap of 17.7 eV were assigned to Ni2+ in Ni(OH)2
(Fig. S14b, ESI †).72,75 Two more intense peaks at 861.9 eV and 880.7
eV correspond to Ni3 + in NiOOH.73 Moreover, the O1s high-resolution
spectra (Fig. S14e, ESI †) showed only two peaks at 531.2 eV and
532.1 eV due to the presence Ni–O and –OH groups, respectively.77,78
Accordingly, the transformation of n-NNP to Ni(OH)2 occurs in an
alkaline solution, but an additional oxidative condition of the OOR
process causes a partial conversion of Ni(OH)2 → NiOOH.
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It should be noted that XPS spectra of the c-NNP-C/CC electrode after
long-term CP measurement (Fig. 4d-h) can be only partially be
considered as a superposition XPS spectra of c-NNP and Ni(OH)2 +
NiOOH (n-NNP-C/CC after CP). In the high-resolution XPS spectra, the
positions of the peaks for Na 1s at 1071.7 eV (Fig. 4f) and P 2p at
133.6 eV (Fig. 4g) are very close to the values obtained for pure c-
NNP (Fig. 1). This fact clearly indicates a partial conservation of the
initial c-NNP phosphate. At the same time, changes are occurring in
the high-resolution spectra of Ni 2p and O 1s. Thus, a certain shift is
observed both for the more intense peaks at 855.9 eV and 873.4 eV
(Ni 2p3/2 and Ni 2p1/2 with an energy gap of 17.5 eV) related to Ni2+,
and less intense at 861.1 eV and 879.9 eV due to the presence of Ni3+
(Fig. 4e). Moreover, in the O 1s spectrum, in addition to the peaks at
531.2 eV and 532.2 eV (corresponding to Ni–O and –OH for Ni(OH)2
+ NiOOH, Fig. S14e, ESI †), and peak of 529.4 eV are observed (Fig.
4h), which indicates the presence of additional types of Ni–O bonds.
New types of Ni-O bonds and peaks shift in the high resolution
spectra of Ni 2p indicate to the emergence of an additional binding
interface for c-NNP material and Ni(OH)2 + NiOOH coverings. This can
provide a stable core-shell system (nanoparticles Na4Ni3Р4О15-
Ni(OH)2 core-shell).
Visualization of transformed c-NNP particles after long-term CP was
performed using TEM. Fig. 5 shows TEM images at various
magnifications. It can be seen that the surface of the n-NNP material
is covered with a dense shell composed of small particles (Fig. 5a). At
a larger magnification (Fig. 5b,c), it becomes visible that the shell
consists of nanoparticles with a diameter of 30 to 50 nm, which form
of branched "antennas" on the phosphate core. According to the
high-resolution TEM results (Fig. 5d), the nanoparticles layer consists
of nickel oxyhydroxide (NiOОH has priority lattice fringes with
interplanar distances of 0.688 and 0.344 nm, corresponding to the
[111] and [222] lattice planes of NiOОH, # 96-901-2320).
Summarizing obtained results, we can propose the following
conversion scheme for с-NNP material: (i) the formation of a Ni(OH)2
layer on the phosphate surface is a consequence of the interaction
of c-NNP and alkaline electrolyte to generation of Na4Ni3Р4О15-
Ni(OH)2 (core-shell) particles; (ii) under the conditions of the OOR
process, partial oxidation of nickel hydroxide to NiOOH occurs, which
is formed on the surface of Ni(OH)2 as nanoparticles with a diameter
range of 30–50 nm; (iii) NiOOH catalyst nanoparticles grow on the
surface like bunches of grapes and form an additional layer on the
surface that protects the core from further degradation. Thus, it is
possible to assume the appearance of successive stable bound
interfaces: c-NNP (core) - Ni(OH)2 film (first shell) - NiOOH
nanoparticles (second shell). According to XRD data, the core of such
system is c-NNP (Fig. 4b), but XPS (Fig. 4d-h) and HR-TEM (Fig. 5d)
results clearly indicate transformations in the surface layers.
Combination of these results suggests that Ni(OH)2 and NiOOH shells
have both short-range order and long-range disordered structure. A
similar situation was also previously observed after the OER process
for other hybrid systems, as an example of NiFe-OH-PO4.53
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DOI: 10.1039/D0DT01205J
Fig. 5 The results TEM for c-NNP-C/CC electrode after the long-term stability
test for OER in 6 M KOH: (a) image c-NNP; (b) and (c) nanoparticles on surface
of с-NNP at various magnifications; (d) high-resolution TEM of spherical
nanoparticles.
additional maximum long-time CP test (more than 110 hours in 6 M
KOH, Fig. 3g). It turned out that after stabilization, this electrode can
provide an overpotential of 250 mV for OER at a current density of
10 mA·cm-2. During continuous electrolysis, this value slowly
increases at 2.7, 7.4, and 13.8% in the 1st, 2nd, and 3rd days,
respectively (Fig. 3g). After 80 hours of CP testing, the overpotential
reaches a value of about 300 mV and the electrolyte becomes dark.
Subsequently, intense evolution of oxygen bubbles continues to be
observed, but particles begin to crumble from the electrode surface
as result of carbon base destruction of the c-NNP-C/CC electrode.
Over the next 30 hours, the carbon base continues to collapse, which
is accompanied by an acceleration of overpotential growth (Fig. 3g).
It should be also noted that overpotential values of OER for c-NNP-
C/CC (and n-NNP-C/CC) electrodes are lower by about 30 mV in 6 M
KOH than 6 M NaOH electrolyte (Fig. 3e). This effect is due to the fact
that the conductivity of 6 M electrolyte KOH is almost twice higher in
comparison as 6 M NaOH solution. Destruction energy of the
hydration shell of K+ ions is lower in comparison with shell Na+ ions.
Therefore, in our case, it should be expected a lower overpotential
values of OER in the 6M KOH electrolyte. Moreover, overpotential
values of oxygen evolution reaction for reference electrode IrO2/CC
were about 290 mV (6M KOH) and 330 mV (6M NaOH) at a current
density of 10 mA·cm-2 (Fig. 3c,d).
For the core-shell particles considered here overpotentials of OER
are lower than many reported values for phosphates (Table S3, ESI
†). Analyzing these results, we have evaluated following criteria: (i)
stability and ways of transformation of the initial composition in
highly alkaline solutions at OER condition; (ii) values of OER
overpotential; (iii) electrocatalyst stability. Unfortunately, many
reports do not discuss phosphate transformation after OER tests
(Table S3, ESI †). Although, in recent years, a number of works
describe transformation of phosphates in alkaline electrolytes after
OER tests, but in most of them this question has not been discussed
(Table S3, ESI †). A significant surface area of different nanoforms
These facts indicate that a certain time is required for the formation
of a stable electrolytic shell on the surface of c-NNP. Therefore, we
previously provide stabilization of c-NNP-C/CC electrode for 10 hours
at a current density of 10 mA·cm-2 and then conduct an
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promotes the rapid leaching of phosphate anions and the formation complex phosphates compounds with increased sVtiaebwiAlirttyicleinOnlainne
DOI: 10.1039/D0DT01205J
of d-metal hydroxides, which under OER conditions partially or alkaline medium.
completely transform to oxide-hydroxides. Phosphate anions can
only dope oxide-hydroxides, as was observed for Ni2P4O12/CC52 and
Co-Fe-Pyro/GCE79 electrodes. Partial preservation of the initial
phosphate is possible only in the case of hybrid composites, in which
the surface of the phosphate is already covered with a protective
shell of hydroxides (as for the NiFe-OH-PO4/NF53 electrode), or dense
shell will be grown as a result of interaction with an alkaline solution.
We assumed that in all cases, an oxide/hydroxide layer is formed on
the surface of the phosphate with conditional composition MOx(OH)y
(M – d-metal or metals) in an alkaline medium. Accordingly, for
phosphate conversion products presented in Table S3, ESI †,
following series of intervals can be created (values of OER
overpotential at a current density of 10 mA·cm-2): FeNiOx(OH)y (200-
300 mV)31,35,49,53,77  NiCoOx(OH)y (240-280 mv)41,48  NiOx(OH)y (250-
290 mV)47,52  CoFeOx(OH)y (260-290 mV)79  CoOx(OH)y (290-370
mV).45,46,54,58,80,81 This general dependence has a good correlation
with the results obtained for oxide/hydroxides. For example, the
presence of iron in FeNiOx(OH)y increases the activity of Ni cations
for OER.11-15 A significant range of the obtained OER values for
MOx(OH)y determine not only the composition, but also the
morphology of the resulting nanostructure. This question should be
considered separately, taking into account the type of initial
phosphate morphology and the peculiarities of its transformation.
In general, stability of electrodes for OER is simultaneously affected
Conclusions
In summary, it was shown that usage of various forms of mixed
phosphate Na4Ni3P4O15 (nanoparticles or crystals) allows generation
of different types of nanosystems in alkaline solutions. The complete
destruction of phosphate nanoparticles leads to appearance of 2D-
nanosheets Ni(OH)2, while core-shell particles form from crystals.
During electrolysis of water under OER conditions, a partial
transformation of Ni(OH)2 to NiОOH is realized for phosphate-carbon
composites in situ. The resulting systems are excellent
electrocatalysts for splitting water (requires an overpotential in rang
250-290 mV for OER at a current density of 10 mA·cm-2 in 6 M KOH
or NaOH electrolytes). Such a design is suitable to provide the low
OER overvoltage value (250 mV at a current density of 10 mAcm-2)
with maximum long-term electrochemical stability (at least 70 hours)
in 6 M KOH. Result analysis presented herein provides a new idea for
the manufacture of highly efficient and highly durable phosphate
based catalysts.
Conflicts of interest
There are no conflicts to declare.
by
a significant number of factors: the morphology of
electrocatalysts, material of electrode (current collector), presence
of composite, which contributes to quickest possible electron
transfer between catalyst and current collector. As can be seen from
Table S3, ESI †, in comparison with other phosphate catalysts,
Na4Ni3Р4О15-Ni(OH)2 particles display excellent catalytic activity
toward OER, and are highly stable and durable, but СС base of
fabricated electrodes are gradually destroyed in a strongly alkaline
electrolyte under OER conditions. As noted above, fast formation of
Na4Ni3Р4О15-Ni(OH)2 structure occurs in a highly alkaline medium or
can be gradual when 0.1 M alkali solutions are used. This fact
indicates additional capabilities. For example, an electrode can be
first treated with an alkaline solution to form core-shell particles, and
then used to water splitting in neutral or near-neutral
electrolytes.29,30,60-62 Such an idea may become as basis to number of
future researches.
Acknowledgements
This work was supported by the national long-term project [no.
WQ20142200205] of “Thousand Talents Plan of Bureau of Foreign
Experts Affairs” of People’s Republic of China and National Natural
Science Foundation of China (NSFC Grant No.21571080), Natural
Science Foundation of Jilin province (No. 20170101193JC).
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3
4
5
6
7
8
9
Thus, stable M-phosphate-MOx(OH)y (core-shell) particles can be
basis to creating a new family of stable OER electrocatalysts. It should
be expected that the use of multimetal phosphates (contain various
combinations of d-metals) will allow
a further decrease in
overvoltage. At the same time, methods for creating core-shell
particles require further improvement. These particles can be
preliminarily prepared using various synthetic approaches or formed
in situ. In the case of in situ, with purpose to preserve the phosphate
core and formation of
a
reliable core-shell connection,
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transformation process of phosphate should be unhurried. Several
ways can be suggested for this goal: (i) the use of ground crystalline
phosphates (as was successfully demonstrated here); (ii) doping
phosphate by ion, which increase its stability in an alkaline solution
(for example, such as Ti, Zr or Hf); (iii) search among a wide variety of
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