Ni2P hollow microspheres for electrocatalytic oxygen evolution and reduction reactions

Article abstract of DOI:10.1039/c8cy00211h

H₂ generated by solar-driven water splitting is a clean and environmentally benign fuel and is an ideal alternative to replace fossil fuels, whose uses have caused a series of energy and environmental issues. The synthesis of Ni₂P and its catalytic properties foroxygen evolution reduction (OER) and oxygen reduction reaction (ORR) was reported. The as-prepared Ni₂P material has a hollow microsphere structure, which has a very high surface-to-volume ratio and is beneficial for fast charge transfer and mass diffusion. These features are useful for electrocatalysis. The high activities and stabilities of this Ni2P material for both OER and ORR were confirmed, representing a rare example of bifunctional OER and ORR catalysts among Ni phosphides. Results showed that NI₂P can catalyze water oxidation, achieving a 10 mA cm^-2 current density at a 280 mV overpotential and can catalyze the selective four-electron reduction of O₂ to H₂O at an onset potential of 0.92 V, making it one of the most active metal phosphide catalysts for OER and ORR.

Full text of DOI:10.1039/c8cy00211h

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Ni2P hollow microspheres for electrocatalytic oxygen evolution and
reduction reactions
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Haitao Lei,a Mingxing Chen,b Zuozhong Liang,a Chengyu Liu,b Wei Zhang,a and Rui Cao*ab
www.rsc.org/
Ni2P hollow microspheres were synthesized and examined as an catalysts, only a few of them are active for O2 reduction.31,32
electrocatalyst for both O2 evolution and reduction reactions. This On the other hand, metal phosphides have been studied as
hollow microsphere structure, assembled by ultrathin nanosheets, OER catalysts.33,34 During electrocatalysis, the outmost spheres
has a high surface-to-volume ratio and is beneficial for fast charge of metal phosphides will be converted in situ to metal
transfer and mass diffusion. This Ni2P material is efficient for oxides/hydroxides, which perform better catalytic activity than
water oxidation by getting a 10 mA cm−2 current at 280 mV their corresponding initial phosphides and are real OER
overpotential. It is also effective in catalyzing the reduction catalysts. It is likely that roughening and micro-structural
of O2 with an onset potential of 0.92 V, representing a rare reconstruction during the electrolysis process give more active
example of Ni phosphides as O2 reduction catalysts.
sites. In addition, the phosphate, which is converted from
phosphide during surface oxidation, may adjust coordination
modes of the metal ion during redox switching processes to
facilitate the OER process. The interior metal phosphides,
which have better electronic conductivity than metal oxides,
can be maintained and can facilitate fast electron transfer
between the surface catalytic sites and the electrode. Based
on the above factors, we expect that Ni phosphides will be
effective in catalyzing both OER and ORR. However, until very
recently, Dai and co-workers reported the first example of Ni
phosphide acting as a bifunctional catalyst for OER and ORR.35
Herein, we report the synthesis of Ni2P and its catalytic
properties for OER and ORR. The as-prepared Ni2P material has
a hollow microsphere structure, which has a very high surface-
to-volume ratio and is beneficial for fast charge transfer and
mass diffusion. These features are useful for electrocatalysis.
The high activities and stabilities of this Ni2P material for both
OER and ORR were confirmed, representing a rare example of
bifunctional OER and ORR catalysts among Ni phosphides.
The Ni2P hollow microsphere was synthesized via a facile
two-step strategy, including hydrothermal synthesis of Ni(OH)2
followed by phosphorization with NaH2PO2 under argon. In a
typical synthesis, Ni(NO3)2 and oleylamine were dissolved in
ethanol and were heated at 180 °C for 12 h.36 The blue-green
Ni(OH)2 precipitate that formed was collected, washed, and
was then subjected to phosphorization to give Ni2P black
powders. The powder X-ray diffraction (XRD) patterns of the
Ni(OH)2 precursor and Ni2P are shown in Fig. 1a. XRD results
suggested the formation of pure α-Ni(OH)2 (JCPDS 380715).
For Ni2P, its characteristic diffraction peaks at 40.6°, 44.5°,
47.3°, 54.1°, 54.9°, 66.2°, 72.6° and 74.7° were identified (pure
Ni2P, JCPDS 030953), corresponding to the crystal face (111),
(201), (210), (300), (211), (310), (311) and (400), respectively.
The disappearance of Ni(OH)2 peaks and the strong peaks of
Hydrogen generated by solar-driven water splitting is a clean
and environmentally benign fuel and is an ideal alternative to
replace fossil fuels, whose uses have caused a series of energy
and environmental issues.1-3 The oxygen evolution reaction
(OER) and oxygen reduction reaction (ORR) are the half
reactions in the generation (via water splitting) and the use
(via fuel cell) of H2, respectively.4-6 However, these two
reactions are sluggish and thus limit the overall progress to
realize a hydrogen-based society.1 Developing efficient OER
and ORR catalysts have attracted extensive attention.
Materials of noble metals are active for OER (i.e., RuO2 and
IrO2) and ORR (i.e., Pt),7-9 but their uses are restricted due to
the high cost and low natural abundance of noble metal
elements. Recently, materials of earth-abundant transition
metal elements, including Mn,10,11 Fe,12,13 Co,14-19 Ni,20-23 and
Cu,24,25 have been shown to be active for OER and ORR.
However, Ni-based materials that are active for both OER and
ORR have seldom been reported.
Transition metal phosphides have high electrical
conductivity26,27 and are widely studied as electrocatalysts for
water reduction to evolve H2.28-30 In addition to the metallic
property, metal phosphides are beneficial for water reduction
due to the strong electron-donating ability of phosphorus. This
feature makes metal center electron-rich to assist the binding
and reduction of protons. Although considerable progress has
been made by using metal phosphides as water reduction
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Ni2P at 40.6° and 47.3° in XRD indicate the complete Ni(OH)2 and Ni2P (> 230 m2 g−1) were obtained, confirming that
conversion of Ni(OH)2 to Ni2P under the synthetic conditions. the high surface area of the hollow structure was reserved
Energy-dispersive X-ray spectroscopy (EDX) gave an elemental after phosphorization.
Ni/P atomic ratio of 2:1, which is consistent with the Ni2P
formulation (Fig. S1). This Ni/P atomic ratio was further
confirmed by inductively coupled plasma mass spectrometry
(ICP-MS). For comparison, the EDX analysis of the Ni(OH)2
precursor was provided in Fig. S2.
The X-ray photoelectron spectroscopy (XPS) spectrum of
Ni2P shows the characteristic peaks of Ni and P (Fig. 1b). The
narrow scan spectrum of Ni 2p displays peaks centered at
853.1 and 870.4 eV, which can be assigned to the Ni 2p3/2 and
Ni 2p1/2 of metallic Ni2P, respectively (Fig. 1c).28,30,37 The peaks
at 856.4 and 874.2 eV can be ascribed to NiOx species arisen
from surface oxidation of Ni2P in air. In the narrow scan
spectrum of P 2p (Fig. 1d), an intensive peak can be fitted into
two peaks at 129.3 and 130.5 eV, which are assigned to the P
2p3/2 and 2p1/2 peaks in Ni2P, respectively.28,30,37 The peak at
133.6 eV is attributed to oxidized P species. Importantly, peaks
at 853.1 for Ni 2p3/2 and 129.3 eV for P 2p3/2 are identical to
the binding energy values of Ni and P in Ni2P reported
previously.28,30,37
Fig. 2 (a, b) SEM, (c) TEM, (d) high-magnification TEM, and (e, f)
elemental mapping images of the Ni2P microsphere.
First, we investigated the electrocatalytic OER activity of
Ni2P in 1.0 M KOH. The catalyst was drop-casted onto a carbon
cloth electrode with a loading of 0.20 mg cm−2. Fig. 3a shows
the linear sweep voltammetry (LSV) of Ni2P. Significantly, in
successive scans, the OER onset (current density j = 5 mA cm−2)
potential decreased and the catalytic current increased,
indicating the formation of active catalytic species on the
surface of Ni2P microspheres during LSV scans. An irreversible
anodic peak at 1.28 V vs reversible hydrogen electrode (RHE,
all potentials are versus RHE unless otherwise stated) was
identified, which was assigned to the NiIII/NiII redox
progress.20,38-41 No further activity enhancement was observed
after 20 scans, suggesting the complete oxidation of surface
Ni2P to active NiOx. Fig. S5 shows the cyclic voltammetry (CV)
of Ni2P after 20 LSV scans, which is consistent with the data
Fig. 1 (a, b) XRD patterns and XPS survey spectra of Ni2P and
Ni(OH)2. (c, d) XPS narrow scan spectra of Ni 2p and P 2p of Ni2P.
The structure and morphology of the Ni(OH)2 precursor from LSV. Fig. 3b presents the LSV of Ni2P after 30 and 500
and Ni2P were studied by scanning electron microscopy (SEM) scans, showing their almost the same behaviors. This result
and transmission electron microscopy (TEM). As shown in Fig. suggests that the activated material is stable for catalytic
S3, Ni(OH)2 precursor has a hollow microsphere structure with water oxidation.
a typical diameter of 2.1 μm. After phosphorization, the hollow
For comparison, LSV of electrode without catalyst or with
microsphere structure can be maintained (Fig. 2a and 2b). The Ni(OH)2 precursor are provided in Fig. 3c. The current density
high-magnification TEM images of the resulted Ni2P confirmed of 10 mA cm−2, an approximate value expected for reaching a
the hollow structure and showed that the microsphere is built 12.3% solar-to-hydrogen efficiency,42 can be achieved at 280-
up by numerous ultrathin nanosheets (Fig. 2c and 2d). In mV overpotential for Ni2P, while it is 390 mV for the Ni(OH)2
addition, the homogeneous dispersion of Ni, P and O elements precursor. Comparison of Ni2P with other metal phosphides
was confirmed by elemental mapping analysis (Fig. 2e, 2f and shows that it is among the most active metal phosphide OER
Fig. S4). These results are significant to show (1) the successful catalysts (Table S1). The Tafel slope of Ni2P material was 115
conversion of Ni(OH)2 to Ni2P and (2) the maintenance of the mV per decade (Fig. S6). The durability of this catalyst material
hollow microsphere structure after the phosphorization was confirmed by chronopotentiometry (Fig. 3d). Constant
process. Similar Brunauer-Emmett-Teller (BET) surface areas of overpotentials are required for maintaining current densities
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at 5, 10 and 20 mA cm−2 during 10-h measurements, which is (current density j = 80 μA cm−2) and half-wave potentials of
supportive of the catalyst stability for water oxidation. During 0.92 V and 0.81 V, respectively. The use of carbon black as a
electrolysis, numerous O2 gas bubbles were generated on the support for Ni2P was aimed to prevent the agglomeration of
electrode surface. By using a gastight electrochemical cell, the Ni2P microspheres during electrocatalysis and also to facilitate
amount of O2 formed on the headspace can be determined by the diffusion of O2 to the catalyst.40,41 More importantly, this
gas chromatography, which gave a Faradaic efficiency > 93%.
activity is one of the best reported for metal phosphide ORR
Electrochemically active surface area (ECSA) analysis catalysts (Table S2). We also compared the ORR activity of Ni2P
showed similar double layer capacitance (Cdl) values for Ni2P with the Ni(OH)2 precursor. In order to compare their intrinsic
(41 μF) and Ni(OH)2 (44 μF) (Fig. S7). The ECSA of Ni2P and activities, we loaded pure Ni2P and Ni(OH)2 without carbon
Ni(OH)2 can be calculated to be 1.52 and 1.63 cm2, respectively black on the electrode and measured their LSV for ORR under
(see ESI for details). Electrochemical impedance spectroscopy the same conditions. As shown in Fig. S11, Ni(OH)2 is much less
(EIS) measurements showed that Ni2P has a smaller resistance active than Ni2P to catalyze ORR.
and thus a higher charge transfer ability than Ni(OH)2 does
(Fig. S8). This result is consistent with the metallic property of
Ni2P. On the basis of these results, it is concluded that the
better OER performance of Ni2P as compared with Ni(OH)2 is
largely due to its better electrical conductivity.
Fig. 3 (a) LSV scans of Ni2P, showing the anodic activation of Ni2P.
(b) LSV scans of Ni2P after 30 and 500 scans, showing almost
identical behaviors. (c) LSV scans of electrode loaded without Fig. 4 (a) LSV scans of Ni2P, carbon black, Ni2P/C and Pt/C in an O2-
catalyst or with Ni2P or Ni(OH)2. (d) Chronopotentiometry plots of saturated 0.1 M KOH at 1600 rpm. (b) The n values of O2 reduction
Ni2P to get current densities of 5, 10 and 20 mA cm−2. Conditions: with Ni2P, carbon black, Ni2P/C and Pt/C. (c) RDE measurements of
1.0 M KOH; 5 mV s−1 scan rate.
Ni2P/C at various rotation rates. (d) K-L plots of Ni2P/C. (e)
Controlled potential electrolysis of Ni2P/C and Pt/C at 0.6 V in O2-
saturated 0.1 M KOH solutions. (f) Chronoamperometric responses
of Ni2P/C and Pt/C upon the introduction of methanol.
The catalyst material after electrolysis was analyzed. The
XPS narrow scan spectra of Ni 2p and P 2p are displayed in Fig.
S9. Significant differences between the XPS spectra before and
after electrolysis can be identified. The disappearance of Ni 2p
The number of electrons (n) transferred for O2 reduction
peaks at 853.1 and 870.4 eV and P 2p peak at 129.3 eV was obtained by RDE and rotating ring-disk electrode (RRDE)
indicates the surface oxidation of Ni2P. Importantly, the measurements. In RRDE, the disk electrode was scanned from
comparison of the grazing-incidence XRD patterns of the 1.1 V to 0.3 V with a rate of 10 mV s−1, while the ring potential
catalyst loaded on an indium tin oxide (ITO) electrode before was held at 0.5 V. The measured H2O2 yields are 17.5% (Ni2P),
and after electrolysis confirmed the presence of Ni2P in the 33.4% (carbon black), and 8.7% (Ni2P/C) over the potential
catalyst bulk phase (Fig. S10). Based on these results, we can range of 0.45-0.75 V. The n value of 3.8 was calculated for
conclude that the outmost spheres of Ni2P are oxidized to NiOx Ni2P/C (Fig. 4b). The same n value of 3.8 was also calculated
during electrocatalysis, which is the real OER catalyst.
from the RDE Koutecky-Levich (K-L) method for Ni2P/C (Fig. 4c
Next, we evaluated the ORR activity of Ni2P in an O2- and 4d). The linearity and near parallelism of the fitting lines
saturated 0.1 M KOH solution. Catalyst materials were drop- suggest a first-order reaction kinetics on the concentration of
casted onto a rotating-disk electrode (RDE). As shown in Fig. dissolved O2 and similar electron transfer numbers for ORR at
4a, Ni2P and carbon black alone showed poor ORR activities. different potentials from 0.35 V to 0.65 V.
However, the hybrid material of Ni2P and carbon black (Ni2P/C)
The stability of Ni2P for ORR was then studied. Controlled
exhibited a pronounced catalytic ORR current with the onset potential electrolysis at 0.6 V showed that Ni2P/C had only a
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Table of Contents
The Ni2P hollow microsphere is synthesized via a facile two-step strategy and is shown to
be active bifunctional electrocatalyst for oxygen evolution and reduction reactions.