An iron-doped cobalt phosphide nano-electrocatalyst derived from a metal-organic framework for efficient water splitting

Article abstract of DOI:10.1039/c9dt03619a

The development of hydrogen energy relies to a large extent on the electrocatalysts that are highly efficient and widely sourced. Although transition metal phosphides (TMPs) have made great achievements in reducing the overpotential of hydrogen evolution reaction (HER), improving oxygen evolution reaction (OER) performance that is relatively lagging in view of relatively large overpotentials and high kinetics energy barriers is yet to be achieved. Herein, we propose an extremely convenient and practical approach to prepare iron-doped cobalt phosphide nanoparticles (Fe-Co_xP NPs) via a one-step method by introducing an iron element in the in situ synthesis of a metal-organic framework (ZIF-67) and then subjecting to a phosphate treatment. The as-obtained Fe-Co_xP showed an excellent OER and acceptable HER activities. In particular, for OER, the optimized Fe-doped Co_xP (Fe_0.27Co_0.73P) exhibits an ultra-low overpotential of 251 mV at a current density of 10 mA cm^-2, a negligible electrocatalytic degradation after 3000 CV cycles, and time over 40 h-reliant current density stability. When employed as cathode and anode electrodes in water splitting, the current density of 10 mA cm^-2 can be achieved at a potential of 1.68 V. Our facile synthetic strategy and innovative ideas are undoubtedly beneficial to the design and construction of advanced water-splitting electrocatalysts.

Full text of DOI:10.1039/c9dt03619a

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An iron-doped cobalt phosphide nano-
electrocatalyst derived from a metal–organic
framework for efficient water splitting†
Cite this: Dalton Trans., 2019, 48,
16555
b
Can Lin,a Pengyan Wang,a Huihui Jin,a Jiahuan Zhao,a Ding Chen,a Suli Liu,
*
a
Chengtian Zhanga and Shichun Mu
*
The development of hydrogen energy relies to a large extent on the electrocatalysts that are highly
efficient and widely sourced. Although transition metal phosphides (TMPs) have made great achievements
in reducing the overpotential of hydrogen evolution reaction (HER), improving oxygen evolution reaction
(OER) performance that is relatively lagging in view of relatively large overpotentials and high kinetics
energy barriers is yet to be achieved. Herein, we propose an extremely convenient and practical approach
to prepare iron-doped cobalt phosphide nanoparticles (Fe–CoxP NPs) via a one-step method by intro-
ducing an iron element in the in situ synthesis of a metal–organic framework (ZIF-67) and then subjecting
to a phosphate treatment. The as-obtained Fe–CoxP showed an excellent OER and acceptable HER
activities. In particular, for OER, the optimized Fe-doped CoxP (Fe0.27Co0.73P) exhibits an ultra-low over-
potential of 251 mV at a current density of 10 mA cm−2, a negligible electrocatalytic degradation after
3000 CV cycles, and time over 40 h-reliant current density stability. When employed as cathode and
anode electrodes in water splitting, the current density of 10 mA cm−2 can be achieved at a potential of
1.68 V. Our facile synthetic strategy and innovative ideas are undoubtedly beneficial to the design and
construction of advanced water-splitting electrocatalysts.
Received 9th September 2019,
Accepted 6th October 2019
DOI: 10.1039/c9dt03619a
rsc.li/dalton
tion kinetics.4–7 Therefore, to reduce the high overpotentials of
the oxygen evolution reaction (OER) and hydrogen evolution
Introduction
A large amount of greenhouse gas emissions caused by the reaction (HER), numerous related studies have been devoted to
burning of fossil energy and by the exhaustion of non-renew- explore efficient water-splitting electrocatalysts.8–11 At present,
able energy have given rise great obstacles to the development Pt (Ir, Ru)-based electrocatalysts are believed to be the most
of human society.1 The development of a clean and sustain- efficient for HER (OER), while their comprehensive utilization
able energy, such as hydrogen energy, has become crucial.2,3 is severely restricted due to their expensive price, rare deposits
In recent years, the external application of voltage to promote and unsatisfactory stability during the electrolysis of water
the electrolysis of water molecules to produce hydrogen and reaction.12–14 Hence, the development of non-precious metal
oxygen is considered to be an ideal method of hydrogen pro- electrocatalysts that significantly reduce the overpotential of
duction in the future, which is environmentally friendly and OER and HER is an effective approach to alleviate the energy
easy to prepare on a large scale. However, the actual hydrogen stress.
(oxygen) evolution process requires a potential greater than the
Transition metal phosphides (TMPs), with strong electro-
theoretical value to overcome the kinetic hindrance, increasing catalytic activity and extensive resources, tend to exhibit out-
the energy consumption of hydrogen (oxygen) production, par- standing water-splitting properties in reported non-precious
ticularly the oxygen evolution process that exhibits slower reac- metal electrocatalysts.15–19 However, it has become difficult to
achieve these electrocatalysts due to their complex synthesis
procedures. Thus, simplifying the synthetic process and
aState Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, China.
E-mail: msc@whut.edu.cn
bKey Laboratory of Advanced Functional Materials of Nanjing,
Nanjing Xiaozhuang University, Nanjing 211171, China.
E-mail: niuniu_410@126.com
improving the electrocatalytic performance of electrocatalysts
are urgent. In addition, OER involving the transport of four-
electrons is more sluggish than HER involving the transport of
two-electrons, which makes OER determine the speed of the
electrolytic water, thereby resulting in large overpotentials and
high kinetic energy barriers.7 Therefore, reducing the over-
†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9dt03619a
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potential of OER to greatly improve the overall water-splitting
efficiency is actually feasible. However, the development of
facile synthesis processes to synthesize OER electrocatalysts
with a low overpotential is still a great challenge.
In this study, the precursors containing iron and cobalt
were synthesized via a simple doping strategy using a metal–
organic framework (ZIF-67) as the template, and then
phosphated to obtain iron-doped cobalt phosphide (Fe–CoxP).
Altering the ratio of iron salt to cobalt salt could achieve the
purpose of adjusting the doping degree of iron. The experi-
mental data showed that the optimal proportion of iron
doping could greatly improve the electrocatalytic activity of
cobalt phosphide, and the obtained Fe0.27Co0.73P showed
exceedingly good OER performance (η10 = 251 mV) and recog-
nized HER performance (η10 = 186 mV). When serving as the
cathode and anode electrodes in an electrolytic cell, the
Fe0.27Co0.73P electrodes (V10 = 1.68 V) demanded a much less
voltage than the CoP electrodes (V10 = 1.82 V).
Fig. 1 (a) SEM image of Fe0.27Co0.73 precursor. (b) SEM image of
Fe0.27Co0.73P. (c1) XRD pattern of Fe0.27Co0.73P. (c2) XRD patterns for a
slow scan of 1 degree per minute. (d) HRTEM image of Fe0.27Co0.73P.
(e–h) HAADF-STEM image and EDX element mapping images of
Fe0.27Co0.73P.
Experimental
Synthesis
The preparation of Fe-doped CoxP: formation of an organic–
ligand solution: 2.649 g of 2-methylimidazole (C4H6N2) was stacked nanoparticles (Fig. 1b). To investigate the reasons for
dispersed in deionized water (40 mL). Formation of a metal– such substantial changes in the morphology, the precursor
salt solution: iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O) and was calcined in N2 at the same temperature but without
cobalt chloride hexahydrate (CoCl2·6H2O) with varying feed adding phosphorus source (Fig. S2†). We could find large-
ratios of Fe/Co (1 : 4, 1 : 3, 2 : 3) were dispersed in deionized sized particle forms, indicating that the change in the mor-
water (20 mL). The total molar amount of metal salts was con- phology was due to the poor thermal stability of the precursors
stant: 4.03 mmol. Under agitation, the metal–salt solution was rather than due to the effect of the phosphating processes. The
transferred to the organic–ligand solution at one time. The organic ligand in the Fe0.27Co0.73 precursor decomposed easily
stirring process was continued for 30 minutes, and then, the during the calcination process,20 and the collapse of the
mixture was allowed to stand on a 35 °C water bath for 16 h. organic skeleton resulted in the final morphology with irregu-
The product was collected by centrifugation (the cleaning lar nanoparticles. Moreover, the XRD patterns of ZIF-67 and
agent used was ethanol), followed by vacuum drying. NaH2PO2 the Fe0.27Co0.73 precursor are shown in Fig. S3.† The XRD
and the FeCo precursor were placed upstream and downstream pattern of ZIF-67 is consistent with that of the one previously
of the intake port of a tube furnace (the mass ratio of the pre- reported,21 while the XRD patterns of the Fe0.27Co0.73 precursor
cursor to phosphorus source was 1 : 5), respectively. Fe-doped and ZIF-67 are basically the same. These indicate that iron
CoxP was obtained by phosphating the downstream precursor enters the lattice of ZIF-67 without destroying it.
with PH3 produced by the thermal decomposition of NaH2PO2
Fig. 1(c1) reveals the X-ray diffraction (XRD) pattern of
(2NaH2PO2 = PH3 + Na2HPO4). Using N2 as the atmosphere, Fe0.27Co0.73P, which is synthesized by controlling the ratio of
the heating procedure was employed to raise the temperature iron salt to cobalt salt to 1 : 3 in the raw material. A sequence
to 350 °C at a rate of 3 °C min−1 and kept it warm for 2 h. After of typical diffraction peaks at 2θ of 31.6°, 36.3°, 46.2°, 48.1°,
cooling by radiation, the solid Fe–CoxP was acquired. To inves- 48.4°, 56.0° and 56.7° are compatible with those of CoP
tigate the effect of iron doping, CoP was synthesized by the (JCPDS no. 29-0497), and the (011), (111), (112), (211), (202),
same method but without adding the iron salt.
(020) and (301) crystallographic planes match with these diffr-
action peaks, respectively. The diffraction peak at 40.8°
belongs to Co2P (JCPDS no. 54-0413), corresponding to the
(111) plane. Due to the low content of Co2P, the as-obtained
sample with CoP and Co2P was denoted as CoxP. No extra diffr-
action peaks of iron-based compounds were observed on ana-
Results and discussion
Structure and composition of Fe–CoxP electrocatalysts
Microscopic images acquired by scanning electron microscopy lyzing the XRD pattern, indicating that there were a few iron-
(SEM) demonstrate that the Fe0.27Co0.73 precursor mainly con- based compounds, or that most of the iron atoms entered the
tains nanosheets (Fig. 1a and S1†). After the phosphorization CoxP lattices.22 Furthermore, the XRD pattern with a slow scan
treatment, the nanosheets basically disappeared to form of 1° per minute for a specific peak near 31.7° reveals that the
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diffraction peak of Fe0.27Co0.73P slightly shifts to a large angle (785.8 and 801.5 eV) also appeared.23,24,26 The peaks (708.8,
compared to the undoped cobalt phosphide (as shown in 711.3, 714.1, 722.5 and 724.9 eV) of oxidized iron that majorly
Fig. 1(c2)), further demonstrating the substitutional doping of appeared due to the exposure to air exist in the Fe 2p spectra
Fe ions in the CoxP lattices. The inductively coupled plasma (Fig. 2c).27,28 Moreover, a small peak of Fe 2p at 706.0 eV could
atom emission spectrometry (ICP-AES) test demonstrated that be indexed to Fe–P bonding, while the peak at 706.0 eV for
the ratio of the two metal atoms (Fe : Co) in Fe–CoxP was Fe0.22Co0.78P with a lower proportion of iron added to the raw
0.27 : 0.73, which was basically in line with the initial Co/Fe material substantially disappeared (Fig. S8†). This could be
feeding ratio. In addition to Fe0.27Co0.73P, other iron-doped explained by the fact that after the entry of most of the iron
cobalt phosphides with different proportions of iron into the crystal lattice of cobalt phosphide, a small amount of
(Fe0.22Co0.78P, Fe0.44Co0.56P, CoP) were also investigated iron that failed to enter the crystal lattice was phosphatized.29
(Fig. S4 and S5†). The low-resolution TEM image is shown in The small amount of iron phosphide formed was advan-
Fig. S6.† It could be seen that Fe0.27Co0.73P was mainly com- tageous for charge re-distribution, and it synergistically pro-
posed of nanoparticles. The high-resolution transmission elec- moted the catalytic performance of the electrocatalyst.29,30 As
tron microscopy (HR-TEM) was used to reveal more infor- shown in Fig. 2d, the peaks (128.2 and 129.2 eV) representing
mation about the crystallographic structure of Fe0.27Co0.73P. P 2p3/2 and P 2p1/2 can be indexed to the metal phosphides.31
The interplanar spacing between the (200) facets of CoP and In addition, the peak (132.9 eV) of oxidized P species could be
the (111) facets of Co2P matched the 0.256 and 0.225 nm, as assigned to the air exposure.32 It can be seen that Fe0.27Co0.73
P
shown in Fig. 1d, respectively. These lattice spacings are mainly exists as Co–P bonding and M–O (M is Fe, Co and P)
slightly larger than that of 0.254 nm and 0.221 nm for bonding from the XPS analysis result, indicating that iron
undoped CoP and Co2P, respectively, which can be attributed enters the crystal lattice of cobalt phosphide mainly in the
to the Fe substitution. Due to the lamellar structure, the form of doping and thus iron-doped cobalt phosphides were
Fe0.27Co0.73 precursor had almost no channels.21 Therefore, as successfully prepared.
shown in Fig. S7,† Fe0.27Co0.73P exhibited a small BET surface
OER performance of electrocatalysts
area (3.40 m2 g−1), indicating no porous structure for it. On
the basis of the EDX test, there was an equidistribution of The OER performance of the pre-synthesized Fe0.27Co0.73
P
different elements in Fe0.27Co0.73P nanoparticles (Fig. 1e–h). coated on a Ni foam (Fe0.27Co0.73P/NF) was first investigated in
The X-ray photoelectron spectroscopy (XPS) test determined a 1.0 M KOH solution. As a reference, the electrocatalytic per-
the valence state of each element in Fe0.27Co0.73P. In addition formance of the commercial IrO2 and CoP coated on NF with
to observing the appearance of the expected elements (Fe, Co the same amount of load (3 mg cm−2) was also investigated
and P), other elements (C, N and O) also appear in the whole under the same conditions. IR compensation was applied to
spectrum of Fig. 2a, which can be derived from the carboniz- all of the initial polarization curves. Linear sweep voltammetry
ation of organic ligands and the surface oxidation when (LSV) curves (Fig. 3a) and OER performance histogram
exposed to air. As shown in Fig. 2b, the peaks (777.6 and 792.4 (Fig. 3b) show that the Fe0.27Co0.73P/NF electrode exhibits the
eV) representing Co 2p3/2 and Co 2p1/2 that can testify the exist- minimum overpotential (η10 = 251 mV) compared with that of
ence of Co–P bonding are found in the high-resolution spectra IrO2/NF (η10 = 293 mV), CoP/NF (η10 = 327 mV) and bare Ni
of Co 2p.23–25 In addition, peaks (780.6 and 796.8 eV) of oxi- foam (η10 = 417 mV). In addition, the overpotential of the elec-
dized cobalt generated by air oxidation and satellite peaks trocatalyst at different current densities was also an important
indicator for measuring the electrocatalytic performance. The
overpotentials required (η10 = 251 mV, η20 = 269 mV, and η50
=
292 mV) for Fe0.27Co0.73P/NF at different current densities are
shown in Fig. 3c. The distinguished OER performance was very
competitive with the recently reported transition metal-based
electrocatalysts, such as NiCo LDH,33 ZIF-8/ZIF-67,34 and Ni3N/
NF35 (Table S1†).
The Tafel curves were fitted in the low voltage region. As
shown in Fig. 3d, the Tafel slope values of Fe0.27Co0.73P/NF,
IrO2/NF, CoP/NF and bare Ni foam are 59.1, 71.4, 85.4 and
155.7 mV dec−1, respectively. In addition, the multi-step
current curve of Fe0.27Co0.73P/NF shown in Fig. S9† demon-
strates that the potential of each section remains almost stable
as the current density of the anode increases from 10 to
90 mA cm−2. When the current density decreased from 90 to
10 mA cm−2, the voltage at 10 mA cm−2 after a cycle remained
basically unchanged, indicating that Fe0.27Co0.73P/NF pos-
sessed splendid mass transport properties and satisfactory
mechanical robustness. Moreover, to assess the OER stability
Fig. 2 (a) XPS survey pattern of Fe0.27Co0.73P. High-resolution XPS
spectra of Co 2p (b), Fe 2p (c) and P 2p (d) regions for Fe0.27Co0.73P.
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Fig. 4 (a) HER polarization curves of Fe0.27Co0.73P/NF, CoP/NF and NF.
(b) Overpotentials at 10 mA cm−2. (c) Tafel plots. (d) Time-dependent
current density curve of Fe0.27Co0.73P/NF.
Fig. S14,† the degradation of the electrocatalytic performance
is almost negligible after 3000 CV cycles, revealing the out-
standing HER stability of Fe0.27Co0.73P.
Fig. 3 (a) OER polarization curves for Fe0.27Co0.73P/NF, CoP/NF, IrO2/
NF and NF. (b) Overpotentials at 10 mA cm−2. (c) The corresponding
overpotentials of Fe0.27Co0.73P when the current density is 10, 20,
50 mA cm−2. (d) Tafel plots. (e) Time-dependent current density curve of
Fe0.27Co0.73P/NF. (f) Polarization curves for Fe0.27Co0.73P/NF initial and
after 3000 CV cycles.
Overall water splitting of electrocatalysts
In view of the excellent electrocatalytic activities of
Fe0.27Co0.73P/NF toward OER and HER, Fe0.27Co0.73P/NF served
as cathode and anode simultaneously in the electrolytic cell to
test its water-splitting capacity. As shown in Fig. 5a and b,
Fe0.27Co0.73P/NF∥Fe0.27Co0.73P/NF only needs 1.68 V to attain
the current density of 10 mA cm−2, while CoP/NF∥CoP/NF, as
benchmark, requires as high as 1.82 V. As shown in Fig. 5c,
in comparison with the latter, the former exhibits a lower
Tafel slope. Moreover, chronoamperometry was performed to
evaluate the stability of Fe0.27Co0.73P/NF. The Fe0.27Co0.73P/
NF∥Fe0.27Co0.73P/NF system exhibited a high stability over
35 h, as shown in Fig. 5d.
of Fe0.27Co0.73P/NF, the weak attenuation of current density
was acceptable after 40 h in the time-reliant current density
stability test (as shown in Fig. 3e), and the degradation of the
electrocatalytic activity could be neglected after 3000 CV cycles
at a scan rate of 100 mV s−1 (as shown in Fig. 3f). The above
results indicated that Fe0.27Co0.73P/NF possessed an excellent
electrocatalytic stability in alkaline solutions. Besides, the OER
activities of Fe0.22Co0.78P/NF and Fe0.44Co0.56P/NF were also
explored. For various Fe doping proportions, optimized
Fe0.27Co0.73P/NF exhibited the best OER performance and
minimum Tafel slope compared with the other two electro-
catalysts, as shown in Fig. S10 and S11.†
HER performance of electrocatalysts
The same Fe0.27Co0.73P/NF working electrode was employed to
assess the HER electrocatalytic performance by applying the
iR-correction in 1.0
M KOH. For comparison, reference
materials, such as CoP/NF and Ni foam, were also tested for
their HER electrocatalytic performances in the identical con-
dition. As shown in Fig. 4a and b, the Fe0.27Co0.73P/NF elec-
trode exhibits the minimum overpotential (η10 = 186 mV) com-
pared with that of CoP/NF (η10 = 252 mV) and bare Ni foam
(η10 = 291 mV). Fig. 4c presents the Tafel slopes of 60.5, 113.4
and 131.0 mV dec−1 for Fe0.27Co0.73P/NF, CoP/NF and Ni foam,
respectively. The Fe0.27Co0.73P/NF electrode also shows the best
HER performance and minimum Tafel slope compared with
the other two electrocatalysts, as shown in Fig. S12 and S13.†
To assess the HER stability of Fe0.27Co0.73P/NF, the decay of
current density could be ignored after 40 h in the time-reliant
Fig. 5 (a) Polarization curves of an alkaline electrolyzer using
Fe0.27Co0.73P/NF∥Fe0.27Co0.73P/NF and CoP/NF∥CoP/NF as the two
electrodes. (b) Voltage at 10 mA cm−2
.
(c) Tafel curves.
(d) Chronoamperometric electrolysis at a constant voltage of 1.72 V for
current density stability test (Fig. 4d). Moreover, as shown in Fe0.27Co0.73P/NF∥Fe0.27Co0.73P/NF.
16558 | Dalton Trans., 2019, 48, 16555–16561
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the surface of the electrocatalyst, the decomposition of OH
under ionization, and the formation of active O on the active
sites used for oxygen evolution.36 Accelerating the decompo-
sition process could significantly speed-up the progress of the
electrocatalytic reaction. This could be achieved by a reason-
able combination of a variety of different electronegative metal
elements.36,37 Owing to the difference in the electronegativities
between Fe and Co, it was found that iron-doped cobalt phos-
phide (Fe–CoxP) exhibited a faster decomposition rate for OH−
and stronger adsorption and ability of bonding to the gener-
ated intermediate O than the undoped cobalt phosphide,
thereby allowing Fe0.27Co0.73P to possess a prominent electro-
catalytic activity for OER. To provide further insight into the
excellent OER performance of Fe0.27Co0.73P, the product after
the OER test was characterized by XPS. The result shows that
the Co–P and Fe–P bondings disappear and only the Co–O and
Fe–O bondings exist as shown in Fig. S15,† indicating that the
surface of Fe0.27Co0.73P was oxidized in the OER process and
then the active intermediates formed, which were beneficial to
promote the OER process.
Fe0.27Co0.73P/NF also displayed a preferable HER activity
and a Tafel slope smaller than the other ratios of Fe–CoxP/NF.
The Tafel value of Fe0.27Co0.73P/NF indicated that the HER
process of Fe0.27Co0.73P/NF followed Volmer–Heyrovsky mecha-
nism, which included the adsorption of H2O on the surface of
the electrocatalyst, the decomposition of H2O under ioniza-
tion, and the active H formed on the active sites that combined
with the H2O molecule for hydrogen evolution.38,39 Note that
Fe0.27Co0.73P/NF showed a lower Tafel slope value than that of
CoP/NF, indicating that the electrocatalytic kinetics of
Fe0.27Co0.73P/NF for HER was faster than that of CoP/NF, and
Fe doping was beneficial to improve the HER performance of
CoP. This could also be explained by the difference in electro-
negativities between Fe and Co, which was conducive in accel-
erating the decomposition rate of H2O and enhancing the
adsorption and bonding ability to the generated intermediate
H for HER. In total, Fe doping could enhance the adsorption
and bonding ability of O and H in the process of oxygen evol-
ution and hydrogen evolution and accelerate the decompo-
sition of OH in the OER and the decomposition of H2O in the
HER, thus greatly promoting the two half reactions for water
splitting.
Fig. 6 (a) Gas collection device of water splitting. (b, c, d, and e)
Photographs of hydrogen collected at different times. (f, g, h, and i)
Photographs of oxygen collected at different times. ( j) Photograph of
the electrolytic cell. (k) Photograph of H2 bubbles attached to Ni foam.
(l) Photograph of O2 bubbles attached to Ni foam. (m) Amount of H2/O2.
To test the faradaic efficiency of Fe0.27Co0.73P, hydrogen and
oxygen, produced by the electrolysis of water in an electrolytic
cell using the Fe0.27Co0.73P/NF electrodes, were collected via a
drainage method (Fig. 6a). Fig. 6b–e and f–i display the
volume of hydrogen and oxygen produced, respectively, at
different times. Fig. 6j shows the electrolytic cell device.
Hydrogen and oxygen bubbles attached to the Ni foam are
visible, as shown in Fig. 6k and l. According to the volume
ratio of hydrogen to oxygen, the calculated faradaic efficiency
was as high as 85% (Fig. 6m). The above data demonstrated
that Fe0.27Co0.73P was really an excellent water-splitting electro-
catalyst. The process through which the Fe0.27Co0.73P/NF elec-
trodes electrolyze water to produce hydrogen and oxygen is
shown in Video S1.† The process of collecting hydrogen and
oxygen by the drainage method is shown in Video S2.†
The effect of different iron doping amounts on the active
surface area of the electrocatalysis could be judged by the
double layer capacitance (Cdl). Fig. S16 and S17† show that the
calculated Cdl of Fe0.27Co0.73P/NF is larger than that of
Analysis of accelerated electrocatalytic kinetics of
electrocatalysts
The OER electrocatalytic performance of Fe0.27Co0.73P/NF sur- Fe0.22Co0.78P/NF and Fe0.44Co0.56P/NF, indicating that
passed the performance obtained for commercial IrO2/NF and Fe0.27Co0.73P/NF possesses larger active surface area than
CoP/NF, demonstrating that Fe0.27Co0.73P/NF possessed
a
Fe0.22Co0.78P/NF and Fe0.44Co0.56P/NF. Notice that a Cdl of
superior OER activity, and the iron doping could greatly 1.02 mF cm−2 for Fe0.27Co0.73P/NF was slightly smaller than
enhance the OER activity of CoP. Simultaneously, compared to Cdl of 1.23 mF cm−2 for CoP/NF, which could be interpreted by
the different proportions of Fe–CoxP/NF (Fe0.22Co0.78P/NF, the fact that in the OER process the amount of the active
Fe0.27Co0.73P/NF, Fe0.44Co0.56P/NF, and CoP/NF), Fe0.27Co0.73P/ species actually produced determines the real activity of the
NF with the smallest Tafel slope showed the fastest electro- actual reaction.40 The electronic transmission capabilities
catalytic kinetics for OER. As known universally, the process of of Fe–CoxP/NF could be reflected in the electrochemical
OER in alkaline solutions involved the adsorption of OH− on impedance spectroscopy (EIS). Fe0.27Co0.73P/NF displayed
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the minimum Rct compared to the other electrocatalysts
(Fig. S18†), indicating that Fe0.27Co0.73P/NF has faster reaction
kinetics and the decrease in charge transfer resistance contrib-
ute to the improvement in the electrocatalytic efficiency.
8 C. Tang, L. Gan, R. Zhang, W. Lu, X. Jiang, A. Asir, X. Sun,
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Conclusions
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To sum up, a facile and practical approach was proposed to
fabricate iron-doped cobalt phosphide (Fe0.27Co0.73P) based on 12 Y. Yang, L. Dang, M. Shearer, H. Sheng, W. Li, J. Chen,
a metal–organic framework. With an optimized Fe doping, the
obtained electrocatalyst coated on the 3D porous conductive
P. Xiao, Y. Zhang, R. Hamers and S. Jin, Adv. Energy Mater.,
2018, 8, 9.
Ni foam substrate (Fe0.27Co0.73P/NF) required very little over- 13 L. Xu, Q. Jiang, Z. Xiao, X. Li, J. Huo, S. Wang and L. Dai,
potentials toward OER (η10 = 251 mV) and HER (η10 = 186 mV), Angew. Chem., Int. Ed., 2016, 55, 5277–5281.
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Fe0.27Co0.73P/NF also showed an excellent long-term electro- Commun., 2014, 50, 11554–11557.
chemical stability during the OER and HER electrocatalytic 17 Z. Xing, Q. Liu, A. M. Asiri and X. Sun, Adv. Mater., 2014,
processes. Undoubtedly, our strategy for the design and con- 26, 5702–5707.
struction of bifunctional electrocatalysts would provide a gui- 18 C. Zhang, Z. Pu, I. Amiinu, Y. Zhao, J. Zhu, Y. Tang and
.
dance for the development of multifunctional electrocatalysts
with high electrocatalytic activity and stability.
S. Mu, Nanoscale, 2018, 10, 2902–2907.
19 Z. Pu, I. Amiinu, C. Zhang, M. Wang, Z. Kou and S. Mu,
Nanoscale, 2017, 9, 3555–3560.
20 W. T. Koo, S. Yu, S. J. Choi, J. S. Jang, J. Y. Cheong and
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8210.
Conflicts of interest
There are no conflicts of interest to declare.
21 M. Wang, J. Liu, C. Guo, X. Gao, C. Gong, Y. Wang, B. Liu,
X. Li, G. G. Gurzadyan and L. Sun, J. Mater. Chem. A, 2018,
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22 P. Wang, Z. Pu, Y. Li, L. Wu, Z. Tu, M. Jiang, Z. Kou,
I. Arniinu and S. Mu, ACS Appl. Mater. Interfaces, 2017, 9,
26001–26007.
Acknowledgements
This work was jointly supported by the Natural Science
Foundation of China (no. 51672204) and the National 23 J. Tian, Q. Liu, A. Asiri and X. Sun, J. Am. Chem. Soc., 2014,
Key Research and Development Program of China (no.
2016YFA0202603).
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24 Y. Ning, D. Ma, Y. Shen, F. Wang and X. Zhang,
Electrochim. Acta, 2018, 265, 19–31.
25 Y. L. Li, B. M. Jia, B. Y. Chen, Q. L. Liu, M. K. Cai,
Z. Q. Xue, Y. N. Fan, H. P. Wang, C. Y. Su and G. Q. Li,
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