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W. Zhu et al. / Journal of Alloys and Compounds 828 (2020) 154465
exposed metal centers [22e24]. Conductivity is another important
factor for an electrocatalyst. Conductive species such as graphene
and carbon nanotubes are added to catalysts in order to improve
their conductivity [25]. Till date, various FeCo-based catalysts have
been prepared and investigated for oxygen chemistry. For example,
composite materials derived from FeCo-metal organic frameworks
[26,27] and FeCo-based alloys encapsulated in carbon [28] have
been demonstrated to be effective catalysts for oxygen chemistry.
Though crystalline OER electrocatalysts have been extensively
investigated, amorphous electrocatalysts exhibit several advan-
tages [29e31]. Amorphous materials only bear a short-range order
with irregular boundaries and are composed of randomly arranged
atoms. This structural feature generates a large number of defects,
which act as catalytic active sites and facilitate the diffusion of ions
through the catalyst layer. For example, amorphous CoOx clusters
[32], amorphous NiOx decorated on NiSe2 [33], and amorphous
FeOx-based products [34,35] have been identified to be highly
efficient OER catalysts.
2.3. Characterization
The morphologies of the prepared samples were examined us-
ing scanning electron microscopy (SEM, REGULUS-8230), while
their crystal phase structures were investigated using X-ray pow-
der diffraction (XRD, MAP18XAHF). The Fourier transform infrared
(FT-IR) spectra of the samples were recorded on a Nexus 870
spectroscope. The sizes and microstructures of the samples were
examined using transmission electron microscopy (TEM, JEOL JEM
2100). The X-ray photoelectron spectroscopy (XPS) analysis of the
samples was carried out on a Thermo Scientific Ka photoelectron
spectroscope. The magnetic properties of the samples were eval-
uated using an MPMS XL-7 superconducting quantum interference
device (SQUID). The N2 adsorption-desorption behaviors of the
samples were investigated using an ASAP 2010 sorption analyzer.
The metal molar ratios of the samples were determined using a
Vista-MPX inductively coupled plasma optical emission spec-
trometer (ICP-OES).
In this study, we designed and prepared porous and amorphous
FeCo bimetal alloys for application as OER electrocatalysts in basic
solutions. We used NaBH4, which is a strong reducing agent, to
reduce Co(II)/Fe(III) species. Owing to the strong reducing ability of
NaBH4, the obtained metallic samples showed amorphous features,
which improved their catalytic activity. The obtained products
showed a large number of open pores on the surface, thus exposing
a large number of metal sites during the catalysis. The strong
synergistic effect between the cobalt and iron sites in these bimetal
samples improved the intrinsic catalytic activity of the metal cen-
ters. The bimetal samples showed high conductivity. The amor-
phous FeCo alloy with the Co:Fe molar ratio of 2:1 showed excellent
OER electrocatalytic activity with an overpotential of 290 mV at a
current density of 10 mA cmꢀ2. This alloy also showed excellent
stability, as revealed by the i-t test results.
2.4. Electrochemical measurements
Electrochemical tests were carried out at room temperature on
an CHI 650E electrochemical analyzer with a KOH aqueous solution
(1 mol Lꢀ1) as the electrolyte. A typical three-electrode system
consisting of a Hg/HgO electrode (1 mol Lꢀ1of KOH) as the reference
electrode, a glassy-carbon electrode (3 mm in diameter) as the
counter electrode, and the electrocatalyst-modified glassy-carbon
electrode (3 mm in diameter) as the working electrode was used.
The working electrode was prepared using a procedure reported
in our previous study [21,36]. The catalyst ink was prepared by
mixing 4 mg of the catalyst and 3.5 mg of acetylene black in a
mixture composed of 1 mL of ethanol and 15
under ultrasonication. Then, the catalyst ink (5
m
L of 5 wt% Nafion
mL) was carefully
loaded on the glassy-carbon electrode, which was then dried at
room temperature. The loading density of the catalysts on the
working electrode was determined to be ~0.28 mg cmꢀ2. In this
manuscript, all the potentials are reported with respect to
2. Experimental section
2.1. Reagents and chemicals
the
reversible
hydrogen
electrode
(RHE),
according
Nafion solution (5 wt%) was purchased from Alfa Aesar. All the
other chemical reagents used in this study were obtained from
Sinopharm Chemical Reagent Co., Ltd. The chemicals used in this
study were of analytical grade and were used as-received without
any further purification.
toEvs: RHE ¼ Evs: Hg=HgO þ EHQg=HgO þ 0:0591pH (in volts). EHQg=HgO is
the potential of the Hg/HgO electrode (1 M NaOH, 0.098 V).
The as-prepared electrodes were activated by 1000 cyclic vol-
tammetry (CV) cycles over the potential range of 0.1e0.9 V vs. Hg/
HgO. The linear sweep voltammetry (LSV) curves of the samples
were recorded at a scanning rate of 5 mVsꢀ1. The LSV curves were
not corrected with the iR potential drop. The Tafel slopes of the
samples were calculated directly from their polarization curves by
plotting their overpotential against log(j), where j is the current
density. The durability of the electrodes was tested by i-t method
and by obtaining the polarization curves before and after 3000 CV
cycles.
2.2. Synthesis of amorphous alloys
The synthesis was carried out as follows: the aqueous solutions
of CoCl2 (1 mL, 1 mol Lꢀ1) and FeCl3 (1 mL, 1 mol Lꢀ1) were added to
2 mL of an aqueous solution of Na3Co(NO2)6 (0.5 mol Lꢀ1). The
resulting mixture was stirred at room temperature for 8 h. A dark
red viscous liquid was obtained. A NaBH4 solution (100 mL,
2 mol Lꢀ1) was then added to the reaction mixture under vigorous
stirring. This resulted in the formation of a black product. At the
same time, a lot of gases were released. The black solid product
(amorphous alloy) showed strong magnetism and was adsorbed on
a magnetic sticker. After 0.5 h of reaction, the product was sepa-
rated using a magnet or by centrifugation and was then washed
with water and ethanol. The resulting sample was labeled as a-
Co2Fe (based on the molar ratio of Co:Fe) and was dried at 60 ꢁC.
For comparison, the Co:Fe molar ratios of 1:0, 5:1, 7:2, and 1:2
were also used by changing the concentrations of the CoCl2 and/or
FeCl3 aqueous solutions while keeping the total metal ion con-
centration and other conditions constant. The products obtained at
the Co:Fe molar ratios of 1:0, 5:1, 7:2, and 1:2 were labeled as a-Co,
a-Co5Fe, a-Co7Fe2, and a-CoFe2, respectively.
3. Results and discussion
3.1. Morphology and structure
The amorphous alloys were synthesized at room temperature.
NaBH4, which is a strong reducing agent with a standard electrode
potential of ꢀ1.24 V, was used to reduce the involved metal ions. Be
noted that the cobalt and iron species can catalyze the hydrolysis of
BHꢀ4 , thus generating hydrogen [37,38]. For the synthesis,
Na3Co(NO2)6 was used as a cobalt source. The NOꢀ2 ligands are
coordinated by the involved metal ions, thus modulating the for-
mation rate of the alloys.
The amorphous products were examined using XRD. The XRD
patterns of the products are shown in Fig. 1. The XRD patterns did