Boosting the oxygen evolution reaction performance of CoS2 microspheres by subtle ionic liquid modification

Article abstract of DOI:10.1039/c8cc05352a

Hierarchical CoS₂ microspheres were prepared and modified with a cationic ionic liquid (IL). The composite shows enhanced oxygen evolution reaction (OER) performances attributed to the fact that the OER equilibrium is driven forward by the IL, according to Le Chatelier's principle, and by the electrostatic affinity generated at the CoS₂/electrolyte interface.

Full text of DOI:10.1039/c8cc05352a

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Boosting the Oxygen Evolution Reaction Performance of CoS2
Microspheres by Subtle Ionic Liquid Modification
Siyuan Ji, #a,b Tongtong Li, #a Zhi-Da Gao,a Yan-Yan Song*a and Jing-Juan Xu*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Hierarchical CoS2 microspheres were prepared and modified with phosphate (Co-Pi) or borate (Co-Bi).6 The Yu group designed a
a cationic ionic liquid (IL). The composite shows enhanced oxygen series of water oxidation catalysts based on the modified
evolution reaction (OER) performances attributed to the fact CoSe2/diethylenetriamine nanobelt composite.7-9 Song and Hu
that OER equilibrium is driven forward by the IL, according to Le synthesize the NiCo/CoCo layered double hydroxides (LDHs) as
Chatelier's principle, and by the electrostatic affinity generated at OER catalyst by liquid phase exfoliation.10 Noticeably, the
the CoS2/electrolyte interface.
common feature of these highly active OER catalysts is their
unique ultrathin lamellar structure allowing for a high surface
area which effectively accelerates the mass and charge
transfer during the OER process.
The rising concerns about the resources exhaustion and
environmental pollution issues require the expansion of
renewable energy sources to replace traditional fossil fuels.
Among lots of energy storage technologies and conversion
systems, the H2 production from water splitting has triggered
considerable research interests.1 However, the two
fundamental reactions of water splitting, namely the hydrogen
evolution reaction (HER) and the oxygen evolution reaction
(OER), always require the use of catalysts to achieve a
sufficiently fast kinetics in practical photolysis or electrolysis
systems. In particular, the OER at the anode suffers from a
complex and sluggish four-electron oxidation process.2 The
associated high overpotential significantly reduces the overall
water splitting efficiency. To date, some of noble metal oxides
like RuO2 or IrO2 exhibited excellent catalytic activity for the
OER. Nevertheless, the high cost and scarcity of these
materials severely limit their practical applications. Therefore,
finding earth-abundant, low-cost and high efficiency
electrocatalysts is one of the key steps to realize the hydrogen
economy via a water electrolytic system.
Apart from nanostructuring the active sites in size, shape
and composition, tuning the solid/liquid (catalyst/electrolyte)
interface also plays a key role in improving the performance of
an electrocatalyst.11 Thanks to their good ionic conductivity,
wide electrochemical window and suitable thermal/chemical
stability, ionic liquids (ILs) have already been widely applied in
electrochemistry.12 However, ILs have been frequently applied
in heterogeneous catalysis, their application in an
electrocatalytic system to improve its catalytic performance is
rare.13 Recently, catalysts modified with hydrophobic ILs
demonstrated an improved ORR activity (O2 + 4H+ + 4e-
→
2H2O) in an acidic medium.13-16 According to Le Chatelier's
principle, the improved performance was attributed to the
high oxygen solubility, proton conductivity and hydrophobicity
of the specific IL, which allowed to drive forward the ORR.14
This phenomenon has inspired us to reasonably hypothesize
that such a local change in reaction microenvironment might
also be able to enhance OER (4OH-
→
O2 + 2H2O + 4e-) activity
Over the past years, cobalt-based non-noble materials,
such as metal oxides, phosphates, perovskite, chalcogenides,
hydro(oxy)oxides and even cobalt ions have been reported to
show a remarkable activity for water oxidation.3-5 For example,
of the catalyst in an alkali media. Moreover, since H2O is one of
the products, the wettability of the surface would influence
the chemical equilibrium.
Herein, we synthesize self-assembled three dimensional (3D)
Kanan et al. reported on highly active OER catalysts fabricated CoS2 microspheres with well-defined flower-like morphology
via the electrodeposition of Co2+ salts in the presence of
as a primary model OER catalyst. Taking advantage from the
large surface area of interconnected thin petals, as- prepared
CoS2 microspheres exhibit an inherent high OER activity in
alkaline conditions with a current density of 10 mA cm-2 at an
overpotential of 400 mV. When further modifying the
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Scheme 1. Schematic illustration describing the synthesis
mechanism of the 3D flower-like CoS2 microspheres.
CoS2 with an hydrophobic IL ([MTBD][NTf2]), the resulted
catalysts display a surprisingly improved OER performance. A
current density of 10 mA cm-2 is achieved at the small
overpotential of 310 mV, testifying the great potential of the IL
modification to promote the overall water splitting efficiency.
CoS2 microspheres were prepared by a facile solvothermal
method using diethylenetriamine (DETA) as a molecular
template (see the ESI). The formation mechanism of CoS2
flower-like structures is illustrated in Scheme 1. Under
solvothermal conditions, DETA with a linear configuration, was
Figure 1. (a,b) SEM images of CoS2 microspheres at different
magnifications. (c) TEM image and (d) HR-TEM image of the
CoS2 microsphere. (e) XRD pattern and (f) Raman spectra of
the CoS2 microspheres.
protonated via the reaction with water (step
CoSO4 reacts with Na2SO3 at high temperature to form CoS2
Ⅰ
).17 Meanwhile,
which agrees with HR-TEM results. The Raman spectrum in
Figure 1f shows four peaks located at 289,316, 394 and 416
+
cm-1, corresponding to the characteristic CoS2 active modes of
(step
protonated DETA coordinate with S atoms from CoS2, leading
to the anisotropic growth of CoS2 nanosheets (step
Ⅱ ). Then, positively charged [NH4 ] groups of the
20
Eg, Tg(1), Ag and Tg(2), respectively.19,
The other peaks
Ⅲ
); finally, observed in the Raman spectrum of CoS2 are ascribed to cobalt
these nanosheets self-assemble to form the petals of flower- oxides due to the unavoidable oxidation of CoS2 in air.21 The
FT-IR spectrum (Figure S2) of the as-prepared CoS2
like CoS2 microsphere by the Ostwald ripening (step
Ⅳ
).17, 18
microspheres shows the vibration bands of -CH2-, -NH2, C-N,
and –NH, demonstrating the presence of DETA in the final
product.
Figure 1a-b shows scanning electron microscopy (SEM) images
of the as-prepared CoS2 microspheres. Clearly, the
microspheres present a uniform 3D flower-like structure with a
To investigate the composition of the as-prepared
microspheres, we performed EDS elemental mapping (Figure
2a and Figure S3) and X-ray photoelectron spectroscopy (XPS)
analysis (Figure 2b-d). Figure 2a shows that both S and Co
elements are uniformly distributed in the microspheres.
Moreover, N is observed in the outline region of the spheres.
The XPS survey spectrum (Figure 2b) confirms the presence of
Co, S, C, N and O elements; the related surface atomic
concentrations are summarized in Table S1. The C and N
elements are ascribed to the DETA template in the flower-like
architecture. This is also evidenced by the binding energy of
the N 1s peak at 400.0 eV (Figure S4), which corresponds to
diameter ranging from 15 to 20 ꢀm (Figure 1a); each sphere is
made of smooth nanosheets which connect with each other
like petals in a flower. The petals thickness is lower than 100
nm (Figure S1a, S1b and S1c). Transmission electron
microscopy (TEM) images in Figure 1c, S1d and S1e confirm
the flower-like morphology made of a multitude of thin
nanosheets. This flower-like hierarchical architecture allows
for a fast penetration of the electrolyte, which is crucial for
improving the mass transfer and the electrical contact
between the electrodes and the electrolyte. Additionally, no
obvious cracks or collapses are observed on these nanosheets,
implying that the microspheres are very compact and steady.
High-resolution TEM (HR-TEM) images (Figure 1d, S1f) reveal
that the nanosheets contain two distinct lattice fringes of 2.77
Å and 2.26 Å, assigned to the (200) and (211) planes of the
cubic CoS2 phase, respectively. The Brunauer–Emmett–Teller
(BET) surface area of CoS2 microspheres was measured as
8.214 m2 g-1. The crystalline structure of the as prepared
flower-like microspheres was examined by powder X-ray
+
the protonated amine (-NH4 ).17 The O element is derived from
cobalt oxide surface layer on CoS2 formed by the oxidation in
air. In Figure 2c, the Co 2p doublets present two obvious
shake-up satellite peaks. Both Co 2p1/2 and Co 2p3/2 show
multiplex splitting with the high binding energy component
corresponding to Co2+ and the low one corresponding to Co3+
of cobalt oxide.22 The high resolution S 2p spectrum (Fig. 2d)
was fitted with a doublet (S 2p1/2 and S 2p3/2) in the range of
160–165 eV; the peak at 167.6 eV is attributed to the binding
energy of S-O bond.21 The XPS results indicate a slight surface
oxidation, which is consistent with Raman analysis,
diffraction (XRD, Figure 1e). The peaks at 2θ of ~9° and ~18°
correspond to (001’) and (002’) of the typical layered structure
formed by DETA.17 Other diffraction peaks are ascribed to the
cubic cattierite- structure phase of CoS2 (PDF no. 41-1471),
2 | J. Name., 2012, 00, 1-3
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Figure 3. (a) OER polarization curves of the bare GC electrode,
a commercial 20 wt% Pt/C, pure CoS2 microspheres and IL-
modified CoS2 microspheres in 0.1 M KOH at 1600 rpm. (b)
Tafel plots derived from (a). (c) OER polarization curves of IL-
modified CoS2 microspheres before and after the continuous
CV test. The inset in (c) shows the chronoamperometric
responses of IL modified CoS2 under a static overpotential of
310 mV. (d) Schematic illustration of the oxidation of water
molecules on the catalysts surface in an alkali medium.
Figure 2. (a) EDS elemental mapping of the as-prepared
microspheres. (b) XPS survey spectra of the microspheres.
High-resolution XPS spectra of (c) Co 2p and (d) S 2p.
demonstrating the formation of CoS2 as shown in Figure 1e.
The cobalt oxide surface layer might protect the CoS2 from
further oxidation.23, 24
The electrochemical activity of CoS2 microsphere was
investigated by acquiring linear scan voltammograms (LSVs) at
a scan rate of 5 mV s-1 in 0.1 M KOH (Figure S6). The
The solubility of O2 is quite low in both water (1.26 mM for 0.1
M KOH) and in the ionic liquid (2.28 mM). The loading amount
microsphere prepared by using 2 mmol Na2SO3 and VDETA/VDI
=
1:1 result in a better OER performance in a wide range of
potentials. The optimized catalyst achieves a current density of
10 mA cm-2 at 1.63 V. This potential is far smaller than what
was observed with the other samples. The IL [MTBD][NTf2] was
prepared following the protocol described in a previous report
and was verified by nuclear magnetic resonance (NMR) (Figure
S5).12 The IL is immobilized at the surface of the CoS2 petals
during the evaporation process thanks to its extremely low
vapor pressure. The hydrophobicity of the IL prevents it from
being washed away by the electrolyte (more details are given
in the ESI).13, 16, 25 Different volumes of IL were casted on CoS2
microsphere to achieve different degrees of pore filling (α)
defined according to a previous study.13 In this work, α was
varied from 5 to 55% (samples are named CoS2-IL-α, as
detailed in the ESI). For comparison, the OER activity of the
bare glassy carbon (GC), a commercial Pt/C catalyst (20 wt%)
of IL on the electrode surface is very small (~0.0145 ꢀL). During
OER process, the IL film is quickly saturated with O2 and then
the O2 is evolved as bubbles thrown off from the catalyst.15, 27-
29. Moreover, Igaseous O2 increases very quickly especially at high
potential,28 suggesting that the influence from the slightly
enhanced O2 concentration in the IL film could be ignored. On
the other side, the IL improves the ionic conductivity and
thanks to its perfluorinated side chains (NTf2) also provides for
a hydrophobic environment. As H2O is one of the products of
the OER, the hydrophobic environment is likely to drive water
out, which could result in the significant improvement the
catalytical OER activity.14, 26 The OER kinetics of the different
catalysts was estimated by comparing the derived Tafel plots,
as displayed in Figure 3b. The Tafel slope of the bare CoS2
microspheres is 81.7 mV dec−1, which is obviously lower with
respect to 20 wt% Pt/C (244.3 mV dec−1) and close to that
obtained with the commercial RuO2 (63.4 mV dec−1) testifying
the favourable OER kinetics of CoS2 microspheres. Noticeably,
and
a commercial RuO2 were also characterized. The
polarization curve of CoS2-IL-45 shows a much earlier onset
potential and higher catalytic current in a wide potential range
with respect to the bare CoS2 without the IL (Figure 3a).
Remarkably, the CoS2-IL-45 catalyst could achieve the current
the CoS2-IL composites show
a larger Tafel slope. This
phenomenon might be due to the fact that the IL addition
modifies the pathway of the charge transfer between the
catalyst surface and the electrolyte. To assess the OER stability
of the CoS2-IL-45 catalyst, a continuous potential cycling was
performed between 0.3 and 0.8 V vs. Ag/AgCl at a sweep rate
of 100 mV s-1. Compared with the initial curve, after 1000 CV
cycles, the CoS2-IL-45 only shows 8 mV increase in η to achieve
the current density of 10 mA cm−2 (Figure 3c). This negligible
degradation suggests a remarkable stability under harsh OER
conditions. Additionally, the chronoamperometric curve (inset
in Figure 3c) further demonstrates the high operational
stability of the CoS2-IL-45 catalyst in alkaline media.
density of 10 mA cm−2 at a small overpotential η
∼
0.310 V. This
value is much smaller than for the commercial RuO2 catalyst (η
of 0.353 V measured in the current study) and comparable to
∼
the performance of the best reported noble-metal-free OER
catalysts (Table S2). It is noticeable that all the CoS2-IL-α
composites exhibit an improved OER activity with respect to
pure CoS2 microspheres (Figure S7). In the OER process, the
catalysis current is related to both the dissolved oxygen and
undissolved oxygen (gas bubbles).28
IOER = Idissolved O2 + I gaseous O2
(1)
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Since the OER process involves a complex and sluggish four-
electron oxidation steps, it is important to better clarify the
OER boosting mechanism induced by the CoS2-IL systems. The
difference in the OER performance is known to be influenced
by the charge transfer resistance (RCT) because of the
electrochemical reactions involved in the oxygen evolution.
The RCT values were comparatively measured using
electrochemical impedance spectroscopy (EIS). As shown by
the Nyquist plots in Fig. S8, a smaller RCT is achieved for CoS2-IL
composites with respect to pure CoS2; the IL modification thus
allows for an increased OER rate. As shown in Figure 3d, the
generally accepted elementary steps of the OER are (i) the
4
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the thermodynamic barrier to form intermediate OOHad
species by attracting OH- anion via electrostatic affinity. 16 J. Snyder, K. Livi and J. Erlebacher, Adv. Funct. Mater., 2013, 23,
Furthermore, the improved ionic conductivity and hydrophobic
environment induced by the IL can provide a favourable 17 M.-R. Gao, W.-T. Yao, H.-B. Yao and S.-H. Yu, J. Am. Chem. Soc.,
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aforementioned. The IL can thus synergistically boost the OER Appl. Mater. Interfaces, 2016, 8, 2562-2572.
performance of CoS2 microspheres to achieve an excellent OER 19 S. G. Lyapin, A. N. Utyuzh, A. E. Petrova, A. P. Novikov, T. A.
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like CoS2 microspheres by using a simple amine-assisted
hydrothermal strategy. The unique hierarchical configuration
of CoS2 microspheres provides for a high density of active
surface sites and facilitates the mass and charge transfer
between the catalyst and the electrolyte. When the CoS2
electrocatalyst is modified by the [MTBD][NTf2] IL addition, a
hydrophobic and cationic IL, the OER activity is surprisingly
improved. This work thus provides an alternative and reliable
route to improve the OER activity of catalysts.
This work was supported by the National Natural Science
Foundation of China (No. 21775016) and the Fundamental
Research Funds for the Central Universities (N170506005,
N170502003, N170908001).
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Conflicts of interest
There are no conflicts to declare.
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4 | J. Name., 2012, 00, 1-3
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