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Published on 05 September 2017. Downloaded by State University of New York at Binghamton on 27/10/2017 07:26:03.
Cite this: Chem. Commun., 2017,
53, 10556
Received 25th June 2017,
Accepted 5th September 2017
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Nickel hexacyanoferrate/carbon composite as a
high-rate and long-life cathode material for
aqueous hybrid energy storage†
Dapeng Zhang,a Junshu Zhang,a Zengxu Yang,a Xiaochuan Ren,b Hongzhi Mao,a
Xianfeng Yang,c Jian Yang *a and Yitai Qianad
DOI: 10.1039/c7cc04914e
A nickel hexacyanoferrate (NiHCF)/carbon composite is prepared
to realize reduced structure vacancies and enhanced conductivity
simultaneously. The resultant composite as a cathode material
exhibits good capacity retentions both for rate capability (93% of
that at 0.1 A g1 for 2 A g1) and cycle stability (94% after 900 cycles
at 0.5 A g1). This feature is also kept in an aqueous hybrid
energy storage device, after coupling with rGO as the anode. After
5000 cycles at 2 A g1, 94% of the initial capacity is preserved,
exhibiting extraordinary stability at high rates.
Lithium-ion batteries (LIBs) as a reliable energy storage device,
have succeeded in many applications, such as consumer
electronics, electric vehicles, smart grids, etc. In spite of this,
the safety concerns, limited lithium resources and low power
density cast serious shadows on the future of LIBs. Thus,
rechargeable energy storage devices using aqueous solutions
as the electrolyte and sodium ions as the charge carriers, have
been developed to address these issues.1,2 In this context, an
aqueous electrolyte offers high ionic conductivity to reduce
electric resistance, and large high heat capacity to increase
the safety. Na-ions as the charge carriers effectively lower the
cost. Moreover, the assembly can be conducted in air, thus
avoiding complicated operations in an inert atmosphere.
However, electrode materials that can be used in this case are
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shandong University,
Jinan, 250100, P. R. China. E-mail:
Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of
Science and Technology, Wuhan 430074, P. R. China
Analytical and Testing Centre, South China University of Technology,
Guangzhou 510640, P. R. China
Hefei National Laboratory for Physical Science at Microscale, Department of
Chemistry, University of Science and Technology of China, Hefei, 230026,
P. R. China
† Electronic supplementary information (ESI) available: Experimental section;
element mapping of eNiHCF/C; CV curve of carbon particles; rate performance of
rGO; galvanostatic discharge/charge profiles of rGO and eNiHCF/C//rGO; comparison of capacity retention of eNiHCF/C//rGO with the reported studies;
element analysis result of eNiHCF/C. See DOI: 10.1039/c7cc04914e
10556 | Chem. Commun., 2017, 53, 10556--10559
quite limited, due to the narrow potential window of water. The
big ionic diameter and heavy mass of Na+ ions slow down their
transfer in electrode materials. So, the selection of electrode
materials becomes a great challenge.
Prussian blue (PB) and its analogues (PBAs) have been
regarded as one of the promising electrode candidates in
aqueous energy storage devices, due to their open framework
for Li+/Na+ transportation, robust skeleton upon cycling, and
ease-of-manufacture. More importantly, the redox potentials of
PB/PBAs locate within the potential window stable to water.
So, PB/PBAs have been extensively explored for rechargeable
aqueous batteries and supercapacitors.3,4 However, many structure vacancies and poor electron conductivity deteriorate the
electrochemical performances of PB/PBAs, particularly in terms
of high rates and long-term cycling.5,6 So, either the synthetic
kinetics is carefully tailored to reduce the vacancies,5 or carbon
materials are coupled to enhance the electron conductivity.6
Although both of these techniques have been demonstrated to be
successful individually, the realization of them simultaneously for
PB/PBAs has not been reported before.
On the other hand, another challenge is how to select a
counter electrode that would not compromise the performance
of PB/PBAs. Most of the reports took metallic Zn as the anode
material, and PB/PBAs as the cathode materials.7–9 This could
be attributed to the high capacity and low redox potential of
metallic Zn. But the performances of these full cells are not
satisfactory, probably due to poor stability of Zn, side reactions
between Zn2+ and PBAs, and so on.9 So, Kim et al. replaced
metallic Zn with disodium naphthalenediimide (SNDI) as the
anode, resulting in a capacity retention of 88% after 100 cycles.10
Li et al. employed mRGO, a typical capacitor material, as the
anode, to pair with CuHCF.3 The rate capability of this aqueous
Na-ion supercapacitor (ASSC) was greatly improved, but still
limited by the side of CuHCF.
Here, a nickel hexacyanoferrate/carbon composite (eNiHCF/
C) was synthesized via a simple reaction between NiCl2 and
K3Fe(CN)6 in the presence of ethylene diamine (en) and carbon
particles (see the ESI†), to realize reduced structure vacancies
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Fig. 1 (a) XRD patterns and (b–d) TEM images of eNiHCF/C, eNiHCF and
and enhanced conductivity simultaneously. Meanwhile, two
controls, eNiHCF obtained without carbon particles, and NiHCF
obtained without en molecules and carbon particles, were prepared to disclose the influences of en and carbon particles. Fig. 1a
shows the XRD patterns of eNiHCF/C, eNiHCF and NiHCF, where
all the diffraction peaks are similar to each other. This result
implies that they share the same crystal structure as that reported
for Prussian blue.11 However, the peaks of NiHCF are broader than
those of eNiHCF/C and eNiHCF, indicating the small size and
high-density defects. This phenomenon could be correlated with
en molecules in the syntheses of eNiHCF/C and eNiHCF, which
coordinate with Ni2+ ions in the solution, and reduce the reaction
rate. Such a case would inhibit the formation of structure defects
and leave more materials for crystal growth. Fig. 1b–d displays
TEM images of NiHCF, eNiHCF and eNiHCF/C. All three samples
consist of irregular particles. In order to clarify the location of
carbon powders and eNiHCF, elemental mapping was conducted.
As illustrated in Fig. S1 (ESI†), eNiHCF and carbon powders are
well mixed in eNiHCF/C. The good contact between them offers
efficient pathways for electron transportation.6 The content of
carbon powders in eNiHCF/C could be roughly estimated by
elemental analysis. As listed in Table S1 (ESI†), the contents of
C, N and H in eNiHCF/C are 35.1 wt%, 15.2 wt% and 2.8 wt%.
After the carbon content from NiHCF is deduced on the basis of
the nitrogen content, the content of carbon powders in eNiHCF/C
could be obtained as B22.1 wt%. Then, associated with the metal
contents from flame atomic absorption spectra (Table S2, ESI†),
the formula of eNiHCF/C, eNiHCF, and NiHCF could be written
as K0.249Ni[Fe(CN)6]0.878U0.12, K0.3Ni[Fe(CN)6]0.867U0.133 and
K0.238Ni[Fe(CN)6]0.8U0.2. These results confirm less structure
vacancies in eNiHCF and eNiHCF/C, suggesting the influence
of en molecules on NiHCF nanoparticles. Fig. S2 (ESI†) shows
the TGA curves of eNiHCF/C, eNiHCF, and NiHCF, where the
weight loss below 200 1C could be attributed to the removal of
water.12 So, the water content is 17.95% for eNiHCF/C, 26.58%
for eNiHCF and 28.31% for NiHCF. Fig. S3 (ESI†) gives the
FTIR spectra of eNiHCF/C, eNiHCF, and NiHCF, where the
This journal is © The Royal Society of Chemistry 2017
characteristic peaks of en molecules at 2850 cm1 and 2963 cm1
are absent. This indicates that en molecules are efficiently removed
from the product.
The electrochemical properties of NiHCF, eNiHCF and
eNiHCF/C are evaluated in a typical three-electrode setup, with
Pt mesh as the counter electrode and a saturated Ag/AgCl
electrode as the reference electrode. Fig. 2a shows their cycle
voltammetry (CV) curves at 5 mV s1. All of them present one pair
of redox peaks within a potential window of 0–1 V (vs. Ag/AgCl),
which likely comes from the redox couple of FeII/FeIII.13,14 Fig. 2b
presents the galvanostatic charge/discharge profiles of NiHCF,
eNiHCF and eNiHCF/C at 0.1 A g1. The highly symmetric shapes
indicate the high reversibility and fast kinetics for all of them. Their
reversible capacities are 52.8 mA h g1 for NiHCF, 52.5 mA h g1
for eNiHCF, and 46.4 mA h g1 for eNiHCF/C. The low capacity of
eNiHCF/C, with regard to those of eNiHCF and NiHCF, could be
attributed to carbon powders that are inactive within this potential
window (Fig. S4, ESI†). So, the overall capacity of eNiHCF/C is
decreased. Fig. 2c gives the rate performances of NiHCF, eNiHCF
and eNiHCF/C. It is found that eNiHCF/C only degrades a little as
the current density increases. Even at 2 A g1, 93.8% of the initial
capacity could be maintained, indicating extraordinarily fast
kinetics. In contrast, the capacity retentions of eNiHCF and NiHCF
at the same rate are B74.9% and 69.2%, much lower than that of
eNiHCF/C. Meanwhile, it is noted that NiHCF is worse than
eNiHCF, which is likely due to the high-density defects caused by
fast nucleation.5 Fig. 2d illustrates the long-term cycling stability of
NiHCF, eNiHCF and eNiHCF/C. After 900 cycles at 0.5 A g1,
eNiHCF/C exhibits a capacity retention of 94.5%, higher than
eNiHCF (81%) and NiHCF (75.6%). These results confirm that
the good contact between NiHCF and C, together with less
structure vacancies is vital to rate capability and to cycling stability,
which could also be obtained by another series of comparisons
from eNiHCF/C, NiHCF/C, and NiHCF. Here, NiHCF/C was synthesized by a similar protocol to eNiHCF/C, just without en molecules
in the recipe. As shown in Fig. S5 (ESI†), NiHCF/C is better than
Fig. 2 (a) CV curves, (b) charge–discharge profiles, (c) rate performances
and (d) cycle performances of eNiHCF/C, eNiHCF and NiHCF.
Chem. Commun., 2017, 53, 10556--10559 | 10557
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NiHCF, but worse than eNiHCF/C in rate capability and cycling
stability. These results agree well with the above conclusions,
confirming the importance of en and carbon powders in the
synthesis. It is also noted that the performances of eNiHCF/C
are better than the reported studies (Table S3, ESI†).
The improved kinetics in eNiHCF/C is also supported by EIS
spectra measured at 0.38 V (vs. Ag/AgCl). As shown in Fig. 3a, all
three spectra consist of a depressed semicircle, followed by a
diffusion drift. In the spectra, the semicircle of eNiHCF/C
exhibits the smallest diameter (1.31 O), reflecting its lowest
charge-transfer resistance. Meanwhile, the Warburg coefficient
(s) of eNiHCF/C derived from its diffusion drift, corresponding
to the slope of Zre vs. o1/2 in Fig. 3b, is also the smallest. This
indicates the highest Na-ion diffusion coefficient for eNiHCF/C
on the basis of the following equation.15
R2 T 2
2A2 n4 F 4 C2 s2
where R stands for gas constant, T for absolute temperature,
A for electrode area, n for the number of transferred electrons
per formula with cycling, F for Faraday’s constant, and C for the
concentration of sodium ions in the electrode. These results
demonstrate the fact that eNiHCF has the largest Na-ion
diffusion coefficient (D) and the smallest charge-transfer resistance (Rct), both of which greatly enhance the kinetics. To get
more information about the nature of this fast kinetics, CV
curves of eNiHCF/C at different scan rates are measured, as
shown in Fig. 3c. The electrode polarization gradually enlarges
with an increase in scan rate, a sign of a typical diffusioncontrolled process. Then, the peak current (ip) is plotted against
the square of the scan rate (v1/2) (Fig. 3d). The linear relationship
between them confirms the diffusion-controlled nature of this
redox reaction.
The excellent performances of eNiHCF/C arise from its good
contact with carbon, and low-density defects. Both of them are
Fig. 3 (a) Nyquist plots and (b) Zre vs. o1/2 of eNiHCF/C, eNiHCF and
NiHCF. (c) Cyclic voltammograms of eNiHCF/C at different scan rates.
(d) Peak current (ip) vs. square of scan rate (v1/2) of eNiHCF/C.
10558 | Chem. Commun., 2017, 53, 10556--10559
closely related to en molecules in the synthesis. En molecules
could strongly coordinate with Ni2+, greatly reducing free Ni2+
ions in the solution and then slowing the surface deposition on
carbon particles. The slow deposition allows Ni2+ ions to stay
at thermo-dynamically favourable sites, thereby alleviating the
structure distortion. So, the electrochemical performances of
eNiHCF/C are improved. Then, different molar ratios of en/Ni2+
were screened. The resultant product is labelled as eNiHCF1/C,
eNiHCF2/C, or eNiHCF3/C, produced by the reaction with the
molar ratio of en/Ni2+ at 1, 2, or 3. NiHCF/C is also included for
this comparison, because it presents the case with the ratio at 0.
As illustrated in Fig. S6 (ESI†), the capacity retention both
in rate and cycling performances increases at a high ratio of
The excellent rate capability and cycling stability of eNiHCF/
C inspired us to couple it with reduced graphene oxide (rGO)
for rechargeable energy storage. In this prototype, Na+ ions are
intercalated/deintercalated in eNiHCF/C, but adsorbed/
desorbed on rGO. In spite of different mechanisms, both
eNiHCF/C and rGO show excellent rate capability and close
Na+ capacity (Fig. S7, ESI†), which makes them well matched
within this hybrid energy storage device. Fig. 4a shows the CV
curves of eNiHCF/C and rGO at a scan rate of 10 mV s1 in 1 M
Na2SO4. The CV curve of rGO presents a nearly rectangular
shape, a typical feature of capacitors.16 That of eNiHCF/C is
consistent with what is observed in Fig. 2a. After they are
coupled with rGO, there is one redox pair in the CV curve
(the middle of Fig. 4a), B0.6 V for the cathodic peak and
B1.0 V for the anodic peak.
Fig. S8 (ESI†) shows the galvanostatic discharge/charge
profiles of eNiHCF/C//rGO at different rates. There are apparent
plateaus in these profiles, consistent with the electrochemical
behaviour of eNiHCF/C. The small polarization, as evidenced by
the discharge/charge profiles, implies the good reversibility.
This feature is preserved at high rates, confirming the good match
of eNiHCF/C with rGO. Fig. 4b illustrates the rate performances of
Fig. 4 (a) CV curves of rGO, eNiHCF/C and eNiHCF/C//rGO in 1 M
Na2SO4. (b) Rate performance of eNiHCF/C//rGO. (c) Long-term cycling
and coulombic efficiency of eNiHCF/C//rGO at 2 A g1.
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eNiHCF/C//rGO. Even at B5 A g1, the specific capacity estimated
based on the cathode materials is maintained at B40 mA h g1,
almost 100% of that at 0.1 A g1. These data are much better than
the reported studies of PBAs in aqueous batteries and supercapacitors (Fig. S9, ESI†). ZnHCF//Zn at 1.2 A g1 exhibited only
49% of the capacity at 60 mA g1.7 The capacity retention was
improved to 62% for NiHCF//Zn at 1 A g1, as compared to that at
0.1 A g1.9 A similar result was also reported for MnHCF//Fe3O4/
rGO that exhibited a capacitance retention of B64% at 4 A g1,
relative to that at 0.5 A g1.17 The recent record of the capacity
retention was 81% for CuHCF//Zn tested at 0.6 A g1.8 The low
coulombic efficiency at small rates might be related to the side
reactions on electrode materials, such as an oxygen evolution
reaction on the cathode and a hydrogen evolution reaction on
the anode. As the rate increases, the overpotentials also increase,
thereby greatly inhibiting the side reactions. So, the coulombic
efficiency approaches 100% as a result. The excellent performance
of eNiHCF/C//rGO is also demonstrated by a cycling test. After 5000
cycles at 2 A g1, the capacity retention is kept at B94% (Fig. 4c),
confirming the good stability. This result also outperforms the
reported studies on PBAs. NiHCF//Zn showed a capacity retention
of 81% after 1000 cycles at 500 mA g1.9 These data were promoted
to 90% by InHCF//NaTi2(PO4)3, but this cell was only tested for
200 cycles at 78 mA g1.18 A similar result was also obtained for
MnHCF//mrGO, which gave a capacitance retention of about 89%
after 4000 cycles.3 The high rate capability and good cycling stability
in Fig. 4 indicate the promising potential of this device assembled
by rGO and eNiHCF/C.
In summary, an eNiHCF/C nanocomposite is successfully
synthesized using a simple process at room temperature, using
en molecules as a ligand. It is believed that en molecules
could effectively reduce the concentration of free Ni2+ ions,
thus benefiting the surface deposition on carbon powders and
inhibiting the formation of structure defects. The results greatly
enhance the charge-transfer kinetics and the reversibility of Na+
intercalation/deintercalation. So, eNiHCF/C exhibits excellent
electrochemical performances. With Pt as the counter electrode,
eNiHCF/C shows a capacity retention of 94% after 900 cycles at
0.5 A g1. At 2 A g1, the capacity could be kept at 93% of that at
0.1 A g1. Similar results are also obtained in eNiHCF/C//rGO,
where the good match between them makes the stability still
astonishing. This device could keep 94% of the initial capacity
This journal is © The Royal Society of Chemistry 2017
after 5000 cycles at 2 A g1, or almost 100% of that at 0.1 A g1
when tested at 5 A g1. The high ionic conductivity, low safety risk,
and good cycling stability and rate capability make eNiHCF/C//rGO
quite promising as a new hybrid energy storage device.
This work was supported by the National Natural Science
Foundation of China (No. 21471090 and 61527809), Key
Research and Development Programs of Shandong Province
(2017GGX40101), and Taishan Scholarship in Shandong Provinces (No. ts201511004).
Conflicts of interest
There are no conflicts to declare.
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