Highly efficient hydrogen evolution from water electrolysis using nanocrystalline transition metal phosphide catalysts

Article abstract of DOI:10.1039/c8ra07195k

Nanocrystalline transition metal phosphides (CoP and Ni₂P) were successfully synthesized by a simple calcination method by using transition metal hydroxides and NaH₂PO₂ as raw materials. Their catalytic activities for the hydrogen evolution from water electrolysis were evaluated with silicotungstic acid as an electron-coupled-proton buffer, whereby hydrogen and oxygen could be produced separately. It was found that the CoP sample showed higher catalytic activity (32 mmol min^?1 g^?1) and good stability (12 h) as compared to the Ni₂P sample, and its catalytic activity could rival that of the commercial Pt/C catalyst. The electrochemical results revealed that CoP had high cathodic current and small charge transfer resistance, which further suggested it was indeed an efficient noble metal-free catalyst for hydrogen evolution from water electrolysis.

Full text of DOI:10.1039/c8ra07195k

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PAPER
Highly efficient hydrogen evolution from water
electrolysis using nanocrystalline transition metal
phosphide catalysts†
Cite this: RSC Adv., 2018, 8, 39291
*
Weiming Wu, Xiao-Yuan Wu,
Sa-Sa Wang
and Can-Zhong Lu
Nanocrystalline transition metal phosphides (CoP and Ni2P) were successfully synthesized by a simple
calcination method by using transition metal hydroxides and NaH2PO2 as raw materials. Their catalytic
activities for the hydrogen evolution from water electrolysis were evaluated with silicotungstic acid as an
electron-coupled-proton buffer, whereby hydrogen and oxygen could be produced separately. It was
found that the CoP sample showed higher catalytic activity (32 mmol minꢀ1 ꢀ1) and good stability (12 h)
g
Received 30th August 2018
Accepted 18th October 2018
as compared to the Ni2P sample, and its catalytic activity could rival that of the commercial Pt/C catalyst.
The electrochemical results revealed that CoP had high cathodic current and small charge transfer
resistance, which further suggested it was indeed an efficient noble metal-free catalyst for hydrogen
evolution from water electrolysis.
DOI: 10.1039/c8ra07195k
rsc.li/rsc-advances
Silicotungstic acid can accept two electrons to from a 2e-
reduced species in water. Furthermore, its 2e-reduced species
1. Introduction
can reduce H+ ions, which come from the oxidation of water
(2H2O ¼ 4H+ + O2), into H2 gas directly in the present of cata-
lysts (such as Pt/C). This may not only separate H2 and O2 gases,
but also give the catalysts freedom from the limitation of the
low work surfaces of electrodes. However, nobel metal Pt is
oen used as a catalyst for the hydrogen evolution in these
studies. For real wide-spread application, catalysts made from
elementally abundant and less expensive materials are urgently
required.
Hydrogen is considered as an ideal clean green fuel.1,2 Its large-
scale application can solve the energy and environment issues
at a global level. Water electrolysis is an important method for
hydrogen production on a large scale. Therefore, this tech-
nology has received considerable attention in the eld of
chemistry. Generally, water and energy (in form of electricity)
are the only required inputs for the electrolysis of water.3,4 The
separation of hydrogen and oxygen is an important process in
this technology, because these gases will diffuse in electrolysis
devices and can give rise to hazardous O2/H2 mixtures. The
proton exchange membrane electrolysis is the most mature
technology used for mitigating the gas crossover, but it still
a challenge to prevent such gas crossover.5,6
If hydrogen and oxygen are produced separately in the
electrolyzer systems, this annoying problem will be rooted out.
Cronin group has introduced a concept of the electron-coupled
proton buffer (ECPB), which can decouple electrolytic H2 and O2
production, producing these gases separately.7–9 In the reported
systems, silicotungstic acid is one attractive material as
a candidate of the ECPBs for the electrolysis of water due to its
high solubility and reversible redox ability in water.9
Earth-abundant transition metal phosphides have received
special attention in recent years, because these phosphide
nanocrystals can efficiently split water into H2 by utilizing
electricity.10–21 Nevertheless, very few reports describe the H2
evolution from water electrolysis using these phosphide nano-
materials in the ECPBs systems. Herein, nanocrystalline tran-
sition metal phosphides (CoP and Ni2P) were successfully
synthesized by a simple calcination method, and their catalytic
activities for the hydrogen evolution from water electrolysis
were evaluated with silicotungstic acid as an ECPB. The highly
efficient catalytic activity of CoP nanocrystal was conrmed via
various electrochemical techniques. Our results may allow us to
highlight the promising application of transition metal phos-
phides in the highly efficient hydrogen evolution from water
electrolysis.
CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian
Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
E-mail: czlu@irsm.ac.cn; Fax: +86 591 63173355; Tel: +86 591 63173355
2. Results and discussion
2.1. Preparation and characterization
† Electronic supplementary information (ESI) available: Electrochemical
reduction of H4SiW12O40 by a two-compartment home-made H-cell by using
a classical two-electrode conguration; production of hydrogen and oxygen
during the durability experiment for the CoP sample; XRD patterns for the CoP
sample before and aer the catalytic test. See DOI: 10.1039/c8ra07195k
The crystal structures and morphologies of the as-prepared
transition metal phosphides have been characterized by X-ray
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diffraction (XRD) and transmission electron microscope (TEM) 786.1 and 802.9 eV are observed for the Co 2p in the CoP
techniques, respectively. Fig. 1a shows the XRD pattern of the sample.22 The additional peaks at 781.6 and 797.7 eV in the
CoP sample. All diffraction peaks of the pattern are well indexed high-resolution XPS spectrum of Co 2p and the broad peak at
to orthorhombic cobalt phosphide (CoP, JCPDS card no. 29- 133.5 eV in the high-resolution XPS spectrum of P 2p may come
0497). The main peaks at 2q values about 31.6, 32.0, 35.3, 36.3, from the cobalt phosphate formed on the surface of CoP due to
36.7, 45.1, 46.2, 48.1, 48.4, 52.3, 56.0, 56.4 and 56.8ꢁ can be the exposure of the sample to air.12,22,23 These results suggest
assigned to the diffraction peaks of the (011), (002), (200), (111), that nanocrystalline transition metal phosphides can be ob-
(102), (210), (112), (211), (202), (103), (020), (212) and (301) tained in this work by a simple calcination method via using
planes, respectively. Furthermore, as shown in Fig. 1d, the transition metal hydroxides and NaH2PO2 as raw materials.
nickel phosphide sample is mainly composed of hexagonal Ni2P
(JCPDS card no. 03-0953), and contains a small amount of
hexagonal Ni5P4 (JCPDS card no. 18-0883). The TEM images of
the as-prepared samples are showed in Fig. 1b and e, respec-
tively. It is found that the cobalt phosphide sample consists of
CoP nanoparticles with a diameter of 10–20 nm, while the
particle size of the Ni2P sample ranges from 20 to 30 nm.
Moreover, HRTEM images show clear lattice fringes for the as-
prepared samples. Note that the d ¼ 0.189 and 0.283 nm of
lattice fringes in Fig. 1c match those of the (211) and (011)
crystallographic planes of CoP, respectively. And the lattice
spacing of d ¼ 0.221 nm in Fig. 1f corresponds to the (111) plane
of Ni2P. In addition, the morphology of the CoP sample has
been conrmed by scanning electron microscope (SEM) (see
Fig. S1†). Surface chemical compositions of the as-prepared
sample have also been studied by X-ray photoelectron spec-
troscopy (XPS). Fig. S2† shows the high-resolution XPS spectra
of Co 2p and P 2p for the CoP sample. The peaks at 778.6, 793.6,
129.3 and 130.1 eV can be attributed to the Co 2p3/2, Co 2p1/2, P
2p3/2 and P 2p1/2 in CoP, respectively.12,22 Two satellite peaks at
2.2. Catalytic performance for H2 evolution from water
electrolysis
The catalytic activities of the as-prepared transition metal
phosphides for the hydrogen evolution from water electrolysis
have been evaluated with silicotungstic acid as an ECPB,
whereby hydrogen and oxygen can be produced separately. In
the present system, H4SiW12O40 is rstly reduced to H6SiW12O40
in a two-compartment home-made H-cell by using a classical
two-electrode conguration (see Fig. S3†), and water are
oxidized to molecular oxygen at the same time. Then, the 2e-
reduced silicotungstic acid will reduce H+ ions to molecular
hydrogen in water with the obtained samples as the catalysts.
Fig. 2 shows the hydrogen yield as a function of reaction time in
the presence of the as-prepared catalysts. It is found that the as-
prepared samples show excellent activities for the H2 evolution
from water electrolysis, and the catalytic activity of the CoP
sample is much higher than that of the Ni2P sample. For the
CoP sample, the initial rate of the H2 evolution is 32
mmol minꢀ1 gꢀ1, corresponding to a turnover frequency of
2.9 minꢀ1. In theory, 74.6 mL (3.3 mmol) of H2 can be obtained,
because the 2e-reduced silicotungstic acid can provide
6.7 mmol of electrons to take part in the hydrogen evolution.
However, only a half of the hydrogen yield (ꢂ37 mL) is collected
in the present system. This can be ascribed to the low reducing
ability of the 1e-reduced silicotungstic acid (E (H5SiW12O40/
H4SiW12O40) ¼ 0.01 V vs. RHE).9 As a part of this work, the
catalytic activity of the commercial Pt/C (5 wt%, Aladdin Co.)
has been investigated under same conditions (see Fig. 2). Note
Fig. 1 XRD patterns, TEM images and high-resolution TEM images of
the as-prepared samples. (a)–(c) for the CoP sample and (d)–(f) for the Fig. 2 Hydrogen yield as a function of reaction time in the presence of
Ni2P sample.
the as-prepared catalysts.
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that the catalytic activity the CoP sample can rival that of the aer the reaction have also been studied by XRD, TEM and XPS
commercial Pt/C catalyst. These results may highlight the (see Fig. S5–S7†). There are no obvious changes in the crystal
promising application of earth-abundant transition metal structure, morphology and surface chemical compositions aer
phosphides in the highly efficient hydrogen evolution from the reaction for the CoP sample. Furthermore, the Co2+
water electrolysis.
concentration measured by inductively coupled plasma-atomic
emission spectrometry (ICP-AES) is only 40 ppm aer the
reaction. The Co2+ ions in the present system may come from
the cobalt phosphate formed on the surface of the CoP particles
due to the exposure of the sample to air.10,12,13,17,22,23 These
results suggest that the CoP catalyst exhibits good stability in
the present system.
2.3. Stability test
It is widely regarded that the stability of a catalyst is a very
important factor for its practical applications. Therefore, the
stability of the CoP sample for the hydrogen evolution from
water electrolysis has been carried out. For the sake of
simplicity, the CoP sample is directly added into the one
compartment of the two-compartment H-cell containing 1e-
reduced silicotungstic acid (H5SiW12O40). Aer a potential of
8.3 V is set between the working electrodes, the evolving H2 and
O2 gas are captured in two 100 mL measuring cylinder lled
with water, respectively. As shown in Fig. 3 and S4,† H2 and O2
evolutions proceed continuously in a molar ratio of H2/O2 of 2.1
(ꢂ49 mL hꢀ1 for H2 and ꢂ23 mL hꢀ1 for O2), almost equal to the
theoretical value of 2 for overall water splitting. Moreover, it can
be seen that the production rates of hydrogen and oxygen do not
obviously decrease aer 12 h with a faradaic efficiency of nearly
100%. The low rate of the H2 evolution for the initial phase can
be attributed to the introduction of air during the addition of
the CoP sample. The crystal structure, morphology and surface
chemical compositions of the as-prepared sample before and
2.4. Roles of ECPB and catalyst
Production rate of H2 in the two-electrode system have been
investigated under different conditions (see Table 1). It is found
that the production of H2 is negligible in the present of
H4SiW12O40. However, aer reducing H4SiW12O40 to
H5SiW12O40 and adding the CoP sample, H2 can be detected
with a rate of 48 mL hꢀ1. Control experiments exhibit only a half
of H2 is obtained without the ECPB, and the adding of the
catalyst can't improve the production of H2 (see entry 3 and 4 in
Table 1). These results conrm H4SiW12O40 and CoP are the
efficient ECPB and catalyst, respectively. Furthermore, H2
evolution from such a system is no longer directly coupled to
the rate of water oxidation, and the activity of the catalyst don't
limit by the electrode's work surface.
2.5. Electrochemical properties of transition metal
phosphides
Electrochemical measurements have been conducted in
a typical three-electrode cell to get further insight into the
hydrogen evolution reaction process of transition metal phos-
phides. Fig. 4a shows the polarization curves of the as-prepared
samples loaded on glassy carbon electrodes, which are
measured in the N2-saturated 0.5 M H2SO4 with a scan rate of
100 mV sꢀ1. The Pt/C electrode prepared with a similar content
has also been examined for comparison. The overpotential
required for different electrocatalysts to produce a cathodic
current density of 10 mA cmꢀ2 (h10) is usually compared in the
literature to assess their catalytic activities, and the lower
overpotentials are helpful for the hydrogen evolution from
water electrolysis.11–14 The h10 of the CoP and Ni2P sample
shown in Fig. 4a are 65 and 150 mV, respectively. Although the
h10 of the CoP sample is higher than that of the Pt/C sample (h10
Fig. 3 Production rates of hydrogen and oxygen during the durability
experiment for the CoP sample.
Table 1 Production rate of H2 in the two-electrode system under different conditionsa
Entry
Componentsb
Current (mA)
Production rate of H2 (mL hꢀ1
)
1
2
3
4
H4SiW12O40
H4SiW12O40 + CoP
H3PO4
100
100
50
No detected
48
26
28
H3PO4 + CoP
50
a
Conditions: a two-compartment H-cell separated by a piece of 0.18 mm-thick Naon N-117 membrane; platinum mesh working electrode (1 ꢃ 1
cm2), 0.9 mL of H3PO4 and 60 mL of H2O in the le compartment; carbon felt working electrode (3 ꢃ 2 cm2) and various components in the right
b
compartment; Ar atmosphere; reaction time, 4 h. Dosages: H4SiW12O40, 10 mmol; H3PO4, 0.9 mL (13.3 mmol); CoP, 50 mg.
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compared to the Ni2P sample, and its catalytic activity could
rival that of the commercial Pt/C catalyst. The stability tests
demonstrated that the production rate of hydrogen did not
obviously decrease aer 12 h. And the analysis results of the
XRD, TEM, XPS and ICP showed that the crystal structure,
morphology and surface chemical compositions of the as-
prepared CoP sample had no obvious change. These results
revealed that it exhibited good stability in the present system.
The electrochemical results indicated that CoP had high
cathodic current and small charge transfer resistance, which
further suggested it was indeed a noble metal-free catalyst for
efficient hydrogen evolution. Our results may allow us to high-
light the promising application of transition metal phosphides
in the highly efficient hydrogen evolution from water
electrolysis.
Fig. 4 (a) Polarization curves, (b) Nyquist plots for different samples
and (c) polarization curves for the CoP sample before and after 1000
cycles.
4. Experimental section
4.1. Preparation
5 mmol of transition metal chlorides (NiCl2$6H2O or CoCl2-
$6H2O, Sinopharm Chemical Reagent Co. (SCRC)) was dissolved
in 40 mL of deionized water. Then, 20 mL of 1 M NaOH aqueous
solution (SCRC) was dropped under stirring (200 rpm). Transi-
tion metal hydroxides were collected and were washed with
deionized water two times aer 30 min of stirring, and then
¼ 28 mV), the CoP sample shows a high cathodic current in the
range of ꢀ0.1 to ꢀ0.2 V vs. RHE. It only needs ꢂ130 mV to afford
a current density of 100 mA cmꢀ2, which is lower than that of
the Pt/C sample (h100 > 200 mV). This may explain the efficient
catalytic activity of the CoP sample at a low redox potential
(E(H6SiW12O40/H5SiW12O40) ¼ ꢀ0.49 V vs. RHE), which can rival
that of the commercial Pt/C catalyst.
ꢁ
were dried in air at 60 C overnight.
50 mg of the obtained hydroxides and 250 mg of NaH2PO2
(Aladdin Co.) were mixed well in an agate mortar. These
mixtures ꢀw1 ere heated to 300 ꢁC with a ramping rate of
Furthermore, the charge transfer rate has been studied by
electrochemical impedance spectroscopy (EIS) and the expected
semicircular Nyquist plots for the transition metal phosphides
(Fig. 4b), with a signicantly decreased diameter for the CoP
sample, have been obtained. It is generally accepted that a small
diameter means a low impedance and gives rise to fast charge
transfer kinetics.18,24 These results imply that the CoP sample
indeed is an efficient catalyst for the H2 evolution from water
electrolysis in this work. The polarization curves that are
measured for the CoP sample before and aer 50 cyclic vol-
tammetry cycles at a scan rate of 100 mV sꢀ1 are shown in
Fig. 4c. At the end of the cycling experiment, a signicant
improvement of current density has occurred, which can be
attributed to the elimination of the oxidation layer on the CoP
sample. While increasing the cycling times to 1000, the curve
exhibits negligible loss in current density compared to that of
the 50th scan. This further suggests the excellent stability of the
CoP sample for the H2 evolution from water electrolysis in acid
condition.
ꢁ
ꢁ
2 C min , and kept at 300 C for 60 min. This process was
carried out in Ar ow (50 mL minꢀ1) all through. Aer naturally
cooled to room temperature under Ar atmosphere, the as-
prepared phosphides were soaked in 1 M HCl aqueous solu-
tion (SCRC) for 36 h to remove any unstable species. Then, the
transition metal phosphides were washed with deionized water
and absolute ethyl alcohol three times, and dried in vacuum at
ꢁ
60 C for 1 h.
4.2. Characterization
XRD patterns were collected on a Miniex II X-ray diffractometer
(Rigaku Co.) with Cu Ka radiation. The data were recorded in a 2q
range of 25–70ꢁ. TEM images were obtained using a FEI Tecnai 20
transmission electron microscope at an accelerating voltage of
200 kV. Samples for TEM imaging were prepared by placing
a drop of the sample ethanol suspension on a copper grid coated
with carbon lm and dried in the atmosphere. SEM image was
obtained on a JEOL JSM-7500F eld emission scanning electron
microscope at an accelerating voltage of 5 kV. XPS measurement
3. Conclusions
Nanocrystalline transition metal phosphides (CoP and Ni2P) was carried out by using a Escalab 250Xi X-ray photoelectron
with a diameter of 10–30 nm were successfully synthesized by spectrometer (Thermo Fisher scientic Co.) equipped with Al Ka
a simple calcination method via using transition metal X-ray source. The binding energy calibration of the spectra was
hydroxides and NaH2PO2 as raw materials. They were used as referred to C 1s peak located at 284.6 eV for the analysis. Elec-
the catalysts for the hydrogen evolution from water electrolysis trochemical experiments were taken on a CHI660e workstation
with silicotungstic acid as an ECPB, whereby hydrogen and (CH Instruments, Inc.). The electrochemical measurements were
oxygen could be produced separately. It was found that the CoP performed in a typical three electrode cell, using Pt wire and
sample showed higher catalytic activity (32 mmol minꢀ1 gꢀ1) as saturated Ag/AgCl electrode as counter electrode and reference
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electrode, respectively. Glassy carbon electrode (3 mm in diam- the National Natural Science Foundation of China (21521061
eter) coated with the as-prepared samples were used as the and 21701169), the Natural Science Foundation of Fujian
working electrodes. In the fabrication of the working electrode, Province (2006L2005), and the China Postdoctoral Science
the catalyst ink was prepared by dispersing 5.0 mg of the catalyst Foundation (2015M570561).
into a mixed solvent containing 0.98 mL of deionized water and
20 mL of 5 wt% Naon solution (D521, Alfa Aesar Co.), and then
the mixture was sonicated for 30 min to form a homogeneous
Notes and references
ink. Aer that, 5 mL of the catalyst ink was loaded onto glassy
carbon electrode and dried at room temperature. The electrolyte
was a 0.5 M H2SO4 aqueous solution (SCRC) and was purged with
nitrogen gas for 30 min prior to the measurements. In all
measurements, the saturated Ag/AgCl reference electrode was
calibrated with respect to reversible hydrogen electrode (RHE). In
0.5 M H2SO4, E(RHE) ¼ E(Ag/AgCl) + 0.198 V.
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4.3. H2 evolution from water electrolysis
The electrochemical reduction of silicotungstic acid
(H4SiW12O40) was carried out by a modied method:8,9 10 mmol
of H4SiW12O40 (SCRC) and 60 mL of deionized water were added
into one compartment of a two-compartment H-cell, and 0.9 mL
H3PO4 (13.3 mmol, SCRC) and 60 mL of deionized water were
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argon atmosphere throughout the experiment.
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deionized water in a 50 mL three-necked round-bottomed ask.
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Acknowledgements
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the Key Program of Frontier Science, CAS (QYZDJ-SSW-SLH033),
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