624
D. Ni et al. / Journal of Alloys and Compounds 737 (2018) 623e629
Electrochemical capacitor, also called supercapacitor offers
3. Characterization
manifold excellent characteristics including high power density,
fast charging and discharging capability, superior cycle lifetime,
high reliability and security [30,31], which makes it to be one of the
hottest members among various energy storage devices [32,33]. As
known to all, the electrode material is the critical factor for the
eventual properties of a device. Many excellent works on electrode
The powders were examined by X-ray diffraction (XRD,
Bruker D8 Advance) using Cu K
a
(l
¼ 1.5406 Å) radiation. X-ray
photoelectron spectroscopy (XPS) analyses were carried out on an
X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi). The
morphology and microstructure of the samples were observed by
field-emission scanning electron microscopy (FESEM) (Nova
NanoSEM 450) and high resolution transmission electron micro-
scopy (HRTEM) (JEM-2100F).
materials have been reported, such as MoO
36], polypyrrole(PPy) [32], PPy/V [37], PPy/MoO3 [38], PPy/
MoS [33]and so on. To the best of our knowledge, only hydro-
3 3 2
[34], CoCO [35], MoS
[
2 5
O
2
thermal synthesis method has been used for preparing SnSe elec-
trode material for supercapacitors [39] and the synthesis of SnSe by
the microwave-assisted synthesis method has never been reported
until now.
In this work, we used the microwave-assisted synthesis method
to prepare SnSe electrode material for supercapacitors. We suc-
cessfully synthesized SnSe particles using SnCl as the tin source
2
and trioctylphosphine (TOP)-Se as the selenide source with the
help of borane-tert-butylamine complex (BTBC) and 1,3-dimethyl-
3.1. Electrochemical measurements
In a three-electrode system, the as-prepared SnSe particles
coated on nickel foam, a platinum sheet, and a saturated calomel
electrode (SCE) were used as the working electrode, counter elec-
trode, and reference electrode, respectively. The working electrode
was composed of active SnSe particles, conductive material (acet-
ylene black, 20 wt %) and binder [poly(tetrafluoroethylene) (PTFE),
10 wt %]. The mixture was first coated onto the surface of a piece of
3
,4,5,6-terahydro- 2(1H)-pyrimidinone (DMPU) according to ref.
ꢁ
[39] which utilized a hydrothermal synthesis method for preparing
nickel foam sheet (2 cm ꢀ 1 cm) and then dried at 50 C under
SnSe. The role of each reagent is as follows: (1) DMPU acts as a
solvent and a weak oxidizing agent. (2) BTBC plays the role of a
reducing agent and also a morphology inhibiting reagent. (3) TOP is
a solvent for Se powder and a morphology inhibiting reagent that
binds with BTBC to realize selective growth.
For comparison, we prepared three different samples by mi-
crowave heating for 5, 10 and 15 min, respectively. And the specific
capacitance, rate capabilities and cycling stabilities of the samples
were analyzed.
vacuum for 12 h.
The electrochemical performance measurements were carried
out at room temperature by the electrochemical workstation (CHI
627C). The cyclic voltammogram curves (CVs) were measured at
different scan rates (5, 10, 20, 50 and 100 mV/s). The galvanostatic
charge-discharge (GCD) curves were tested at various current
densities of 1, 2, 5, 10 and 20 A/g, respectively. The specific ca-
pacitances (C
GCD curves via the following equation: C
where I, t, V and m denote discharge current (A), the discharge
s
) of the electrode materials were calculated from
s
¼ I ꢀ t/( V ꢀ m),
D
D
D
D
2
. Experimental
.1. Raw materials
Anhydrous SnCl
time (s), potential window (V) and practical active mass (g),
respectively.
Electrochemical impedance spectroscopy (EIS) tests were car-
ried out in the frequency range from 100 kHz to 0.01 Hz at open
circuit potential using Bio-logic VMP3.
2
2
and Se powder were purchased from Sino-
pharm Chemical Reagent Co., Ltd. borane-tert-butylamine complex
BTBC), 1,3-dimethyl-3,4,5,6-terahydro-2(1H)-pyrimidinone (DM-
(
4. Results and discussion
PU) and trioctylphosphine (TOP) were obtained from J&K Scientific.
All the reagents were used directly without further purification.
The reaction scheme for the synthesis of SnSe is shown in Fig. 1.
Fig. 2 shows the XRD patterns of the samples S-1, S-2 and S-3. All
the characteristic peaks except that marked by “*” in the XRD
patterns of the S-1, S-2 and S-3 can be well indexed to the ortho-
rhombic SnSe (JCPDS No. 89-0232). Meanwhile, impurity phase Sn
(JCPDS No. 04-0673) is detected in the S-1 and S-2 (the intensity of
the Sn diffraction peaks marked by “*” in the S-1 is stronger than
that in the S-2) but not in the S-3. Thus, the reaction is more
completely as the reaction time prolongs, and pure crystalline SnSe
is successfully synthesized when the microwave heating time is
15 min.
2.2. Preparation of tin source
0
2
.12 g of SnCl and 1.5 g of BTBC were dissolved in 30 mL DMPU
under mild stirring for 30 min at room temperature.
2.3. Preparation of selenide source (TOP-Se)
0
.632 g of Se powder was added into 20 mL TOP and stirred for
3
0 min at room temperature.
Moreover, the three samples were further characterized via
XPS analysis (Fig. 3). The XPS spectra confirm the presence of the
Sn and Se elements for all the three samples. As shown in Fig. 3,
the Se 3d peaks of the S-1(d), S-2(e), S-3(f) are located at
53.3 eV(Se 3d3/2)and 54.3 eV (Se 3d5/2); however, the Sn 3d peaks
of the S-1 and S-2 are much different from that of the S-3. The Sn
3d3/2 peak of the S-1(a) and S-2(b) are resolved into two individual
peaks by using a curve-fitting procedure (marked as 487.0 and
485.6 eV for the S-1 and 486.6 and 485.1 eV for the S-2) due to the
co-existence of SnSe and Sn; whereas the Sn 3d3/2 peak (494.8 eV)
of the S-3(c) is consistent with the previous reports of pure SnSe
[40,41]. This further confirms that the S-1 and S-2 samples contain
impurity Sn besides SnSe and that the S-3 is pure SnSe. The for-
mation mechanism of the SnSe can be described by the chemical
reactions as follows:
2
.4. Synthesis of SnSe particles
1.5 mL of TOP-Se was added to the tin source and stirred for
2
0 min at room temperature, and then the solution was poured into
a microwave reactor. The reactor was placed in the household
microwave oven (Galanz, P70F23P-G5(SO)). The microwave-
assisted reaction was lasted for 5, 10 and 15 min, respectively,
with a medium-low fire and then naturally cooled to room tem-
perature, and the corresponding samples were called S-1, S-2 and
S-3, respectively.
The products were collected and washed with heptane and
absolute ethanol in sequence for several times, then separated by
centrifugation at 4000 rpm for 5 min and finally dried in vacuum at
3
33 K for 8 h for further characterization and analysis.