Facet-controlled morphology of cobalt disulfide towards enhanced oxygen reduction reaction

Article abstract of DOI:10.1039/c8ta06710d

Catalytic activity has a significant contribution from exposed facets which are prominently correlated with morphology. It has been witnessed that even a small refinement in morphology alters the catalytic activity over several folds which is a key towards catalyst design. The present study thereby explores the design of octahedral CoS₂ crystals with the aim of exposing highly active facets for the oxygen reduction reaction (ORR) which were carefully synthesized to incorporate carbon inherently. One-pot hydrothermal synthesis was employed using trisodium citrate and sodium thiosulfate. The resultant crystals of octahedral CoS₂ with exposed {111} and {220} planes were revealed by HR-TEM and XRD studies. Detailed structure-activity insight regarding ORR was obtained using rotating ring disk electrode and electrochemical quartz crystal microbalance (EQCM) analysis and facet controlled octahedral CoS₂ was shown to have the 50 fold increase in activity relative to other variants in an acidic medium and is comparable to a state-of-the-art Pt/C (20%) catalyst. The local catalytic activity of the CoS₂ catalyst was visualized by the redox-competition mode of scanning electrochemical microscopy (RC-SECM).

Full text of DOI:10.1039/c8ta06710d

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Facet-controlled morphology of cobalt disulfide
towards enhanced oxygen reduction reaction†
Cite this: DOI: 10.1039/c8ta06710d
*
Vikram Singh, Aarti Tiwari and Tharamani C. Nagaiah
Catalytic activity has a significant contribution from exposed facets which are prominently correlated with
morphology. It has been witnessed that even a small refinement in morphology alters the catalytic activity
over several folds which is a key towards catalyst design. The present study thereby explores the design of
octahedral CoS2 crystals with the aim of exposing highly active facets for the oxygen reduction reaction
(ORR) which were carefully synthesized to incorporate carbon inherently. One-pot hydrothermal
synthesis was employed using trisodium citrate and sodium thiosulfate. The resultant crystals of
octahedral CoS2 with exposed {111} and {220} planes were revealed by HR-TEM and XRD studies.
Detailed structure–activity insight regarding ORR was obtained using rotating ring disk electrode and
electrochemical quartz crystal microbalance (EQCM) analysis and facet controlled octahedral CoS2 was
shown to have the 50 fold increase in activity relative to other variants in an acidic medium and is
comparable to a state-of-the-art Pt/C (20%) catalyst. The local catalytic activity of the CoS2 catalyst was
visualized by the redox-competition mode of scanning electrochemical microscopy (RC-SECM).
Received 12th July 2018
Accepted 3rd September 2018
DOI: 10.1039/c8ta06710d
rsc.li/materials-a
In addition to precious metal catalysts, morphologically
controlled transition metal oxides6,13,14 with typically exposed
1 Introduction
Intense demand for sustainable energy drives1 the power sector
towards electrochemical devices2,3 which intrinsically requires
a highly active and cost-effective electrocatalyst.4 The design of
catalytic surfaces bound by discrete facets has potential to
remarkably enhance the obtainable activity of even a non-noble
material5 by over several fold by activating chemical bonds
within the reactants.6 This has also been demonstrated by the
huge impact on the activity of an essential oxygen reduction
reaction (ORR) observed upon morphologically tailoring Pt-
based catalysts7–10 from nanoparticles to a Pt multi-octahedron
because the latter possessed a high ratio of {111} : {100}
exposed facets.11,12 This ORR is of central importance since it lies
at the heart of various energy devices such as fuel cells, metal–air
batteries and air-breathing cathodes in industrial electro-
catalytic processes which have the ability to transform the energy
sector towards sustainability.2 One of the important applications
of ORR is the cathode reaction in energy devices. However, the
poor kinetics, high cost and scarcity of state-of-the-art Pt-based
catalysts are a severe bottle-neck for their implementation in
energy devices. Therefore, great efforts are required to develop
an active and low-cost Earth-abundant alternative which can
alleviate kinetic disabilities and unavoidable major investment
for catalysts in the energy devices.
surfaces have been explored with respect to ORR, showing
striking differences in catalytic ability.15 In this regard, Co3O4
oxide surface bound by discrete {111}, {100} and {110} planes
was found to exhibit an ORR activity trend of {111} > {100} >
{110} due to differences in the respective surface energies.15
Similarly, octahedral crystals of (MnCo)3O4 bounded by the
{011} facet led to a 30-fold increase in ORR activity when
compared with its anisotropic nanoparticles.16 However, tran-
sition metal chalcogenides17,18 shown to have promising activity
under relatively mild synthetic conditions and low cost when
compared with their oxide counterparts19 and may play a vital
role in transforming the energy sector towards clean electro-
chemical technologies.20 In this class of materials, cobalt
suldes (CoS)20,21 have gained signicant attention due to the
Earth abundance of cobalt and its potential redox-dependent
electrochemical properties.22 It has been observed that the
catalytic performance of cobalt sulde is strongly inuenced by
its shape, size, ratio between Co and S and most importantly,
the exposed facet.20 However, there is persistently a major gap in
correlating these aspects with ORR catalysis primarily because
synthesizing CoS with uniform stoichiometry and control over
the exposed facet remains a formidable challenge due to the
high oxophilic nature of cobalt and its complex stoichiometry.23
The latter thereby results in anisotropic cobalt suldes having
contribution from other forms such as Co9S8, Co3S4, CoS and
CoS2 rendering design of a morphology-controlled CoS17 with
one composition extremely difficult, which is highly desirable at
present considering its immense potential.
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar,
Punjab-140001, India. E-mail: tharamani@iitrpr.ac.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta06710d
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So far, a few reports have surfaced in the recent past wherein introduce morphological differences which were optimized
CoS nanoparticles supported over pure carbon or a heteroatom- with respect to both ORR activity and morphology. The opti-
doped carbon matrix have been explored.24–26 However, lately mized conditions for the synthesis of targeted CoS2 are illus-
the concept of facet-controlled CoS2 crystals was explored by trated in Scheme 1.
Higgins et al. marking the rst attempt towards relating struc-
ture with ORR24 activity and later Zhang et al. emphasized the
2.2 Effect of trisodium citrate concentration
concept of tuning nanostructures to substantially improve
Trisodium citrate as a so-mold plays a crucial role not only by
catalysis.21 Strikingly, all these CoS were employed as composite
controlling morphology but also the crystal size. To obtain the
catalysts, that is, supported over a conductive or activating
optimal concentration of so-mold, experiments were con-
matrix which makes them vulnerable to catalyst corrosion.27
ducted by varying its concentration from 10 to 30 mM while
This on the other hand introduces severe inherent functional
keeping the rest of the reaction parameters as 5 mM CoCl2 and
issues like poor distribution of the catalyst in the absence of
0.1 M sodium thiosulfate at a reaction temperature of 200 ꢀC for
catalyst–support interaction and eventual catalyst leaching
12 h. The effect of the so-mold concentration on the
upon prolonged usage. Therefore, it is desirable to design
morphology of the synthesized material was analyzed by scan-
a pure monophasic, distinctly oriented and catalytically active
ning electron microscopy (SEM) and eld emission scanning
facets of cobalt sulde using a simple and scalable synthetic
electron microscopy (FE-SEM). As depicted in Fig. 1, the
strategy employing cost-effective ingredients.
synthesized CoS2 material shown to have aggregated spheres in
Considering these challenges, this study aimed to design
the absence of so-mold (Fig. 1a). However, aer the addition of
cobalt sulde (CoS2) with a uniform composition while control-
so-mold (10 mM),
a well-dened uniform octahedral
ling exposed facets but an emphasis was laid on intrinsically
incorporating carbon to eliminate additional conductive support.
The concept was therefore designed to take both activity and
stability into consideration to obtain a holistic catalyst material
for effective ORR performance. A chemical binding approach was
explored using tri-sodium citrate (Cit.) as a so mold by a simple
one-pot hydrothermal synthesis. The complexation chemistry
enables the strong affinity of cobalt towards citrate to control
both the crystal structure and exposed facet. Whereas, sodium
thiosulfate acting as the sulfur source enabling a high degree of
control over ligand replacement while synthesizing highly
monophasic pure cobalt sulde (CoS2) crystals.
The morphological evolution following different synthetic
conditions was monitored by scanning electron microscopy
(SEM) imaging which was correlated with electrochemical ORR
activity and later in-depth analysis was performed by employing
electrochemical quartz crystal microbalance (EQCM) measure-
ments and micro-electrochemistry. Surprisingly, the morpho-
logical variation brought about a 50-fold increase in the ORR
electrocatalysis which compares well with the state-of-the-art
Pt/C catalyst. To the best of our knowledge, this is the rst
instance where the electrocatalytic activity of cobalt sulde
towards ORR has been explored with respect to facet-controlled
morphological evolution.
morphology was obtained as shown in Fig. 1b. However, further
increasing the so-mold concentration to 20 mM resulted in
aggregated structures (Fig. 1c) with inferior uniformity while
the 30 mM variant caused entrapment of these agglomerated
structures in a close network (Fig. 1d). These so-mold
concentration variants were subsequently analyzed by XRD as
depicted in Fig. 1e. Several prominent and sharp diffraction
peaks appeared at 2q ¼ 27.8, 32.2, 36.2, 39.8, 46.3, 54.9, 57.6,
60.2 and 62.7 which were attributed to (111), (200), (210), (211),
(220), (311), (222), (230), and (321) corresponding to JCPDS no.
41-1471 28 thereby conrming the successful formation of CoS2
(Fig. S1 ESI†). Interestingly, in the absence of so-mold several
additional peaks (2q ¼ 15.3, 23.2 and 26.3) in the XRD corre-
sponding to Co9S8 (JCPDS no. 65-1765)29 appeared and a few of
these reappeared when the concentration was drastically
increased to 30 mM. Only the 10 and 20 mM variants gave phase
pure CoS2 wherein the former gave highly crystalline octahedral
crystals. These results suggest that the right amount of so-
mold plays a crucial role in controlling the morphology and
phase purity of CoS2 crystals.
2.3 Effect of cobalt (Co2+) concentration
The effect of Co2+ concentration on the microstructure and
morphology was studied by varying the CoCl2 concentration
from 5 mM to 20 mM while keeping the so mold to the opti-
2 Results & discussion
2.1 Shape selective synthesis of cobalt sulde
ꢀ
mized 10 mM concentration with 0.1 M Na2S2O3 at 200 C for
12 h. At the 5 mM Co2+ concentration octahedral crystals
The design of cobalt sulde (CoS2) crystals was executed while appeared which tended to agglomerate upon increasing the
aiming for pure monophasic and a facet-controlled morphology concentration to 10 mM, the extent of which further increased
by a simple synthetic strategy. This was achieved by using cobalt at 20 mM with complete loss of octahedral structure (Fig. S2a
chloride, sodium thiosulfate (the sulfur source) and tri-sodium and d ESI†). This is well supported by XRD studies, wherein the
citrate as a so mold through the hydrothermal method. The crystallinity decreased upon increasing the Co2+ concentration
basic methodology involved in the synthesis of CoS2 was (Fig. S2e, ESI†) as suggested by less intense XRD peaks.
complexation followed by ligand exchange mediated precipita- However, it was observed that when the ratio of Co2+ : Cit. was
tion where the ligand replacement was performed at different increased to 1 : 4, several additional peaks appeared in the XRD
temperatures between 200 ꢀC to 260 ꢀC at various reaction time pattern corresponding to Co9S8 providing a limiting concen-
from 6 h to 20 h. These synthetic variables were found to tration ratio for phase purity and more importantly the crystal
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Scheme 1 Schematic representation of morphological variation of cobalt sulfide with various reaction parameters.
structure of desired CoS2. Therefore, 5 mM CoCl2 yields
uniformly distributed octahedral crystals whose morphology
deforms with increasing concentration probably due to
hindrance in optimal crystal growth indicating that a 1 : 2 ratio
of Co2+ : Cit. is essential to obtain a monophasic octahedral
structure.
2.4 Effect of reaction time
Reaction time amongst all the reaction variants lies at the heart
of designing facet-controlled morphology with monophasic
octahedral CoS2 exposing typical facets. The impact of reaction
time was closely studied to investigate the growth process of
CoS2 starting from 6 h to 20 h while retaining the optimized
Fig. 1 FE-SEM images of cobalt sulfide at various trisodium citrate (soft mold) concentrations: (a) 0 mM, (b) 10 mM, (c) 20 mM and (d) 30 mM, and
(e) the corresponding XRD pattern.
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1 : 2 ratio of Co2+ : Cit. at a xed 0.1 M Na2S2O3 concentration at 200 ꢀC CoS2 could not be obtained whereas at 220 ꢀC and above
200 ꢀC. The outcome when analyzed by XRD (Fig. 2a), SEM and a progressive overgrowth occurred over octahedral crystals
FE-SEM measurements as detailed schematically in Fig. 2b (Fig. 3b and d) and this became predominant with increasing
results in well-dened octahedral structure only when the temperature. This was also supported by XRD analysis wherein
reaction is carried out for 12 h. A wide variation in morphology a decrease in crystallinity was observed upon increasing the
(Fig. S3, ESI†) was observed starting from 6 h which mainly reaction temperature as depicted in Fig. 3e.
comprised agglomerated spherical and pyramidal structures
exhibiting poor crystallinity as supported by less intense CoS2
2.6 Growth mechanism of CoS2 crystals
XRD peaks (Fig. 2a and S4, ESI†). As the reaction progresses to
8 h, spherical structures predominate but octahedral structures
also begin to appear (Fig. S3, ESI†); however, aer 10 h, partially
growing facets predominate homogeneously which subse-
quently transform to octahedral structures at 12 h with high
crystallinity and distinctly exposed facets. On further prolong-
ing the reaction time to 16 h, octahedrons deform into pyra-
midal structures (Fig. S3e and f, ESI†) and nally transform to
amorphous CoS2 at 20 h. Notably, beyond 16 h loss in phase
purity occurs as conrmed by the appearance of additional XRD
peaks of Co9S8 and prominently increases with reaction time.
More importantly, the peak around 2q ¼ 26.2ꢀ (marked with
a blue dot; Fig. 2a and S4, ESI†) appeared reecting the presence
of carbon which originates from citrate decomposition.
Based on the aforementioned observations, reaction parame-
ters like temperature, time and concentration of both cobalt
and so-mold play a crucial role in dening the shape and
controlling the facet of the crystal structure. Briey, a two-step
process is quite likely in the present scenario where the initial
formation of a so-mold chelated metal complex, that is, cobalt
citrate (Co–Cit.) occurs rapidly at room temperature followed by
replacement of Cit. (so-mold) by sulde ligand which was
formed in situ by thermal decomposition of sodium thiosulfate
to yield octahedral cobalt sulde crystals as follows:
Co2+ + Cit. / Co ꢁ Cit.
Na2S2O3 + H2O / Na2SO4 + H2S
Co ꢁ Cit. + H2S / CoS2
(1)
(2)
(3)
(4)
2.5 Effect of temperature
The substantial effects of temperature variation on facet-
controlled CoS2 synthesis are two-fold; that is, by altering the
availability of competent sulde ligand and also by modulating
the surface energy directing crystal growth.
Co2+ + H2S / CoS1ꢁx
Comprehensively, Co2+ ions react with so-mold (Cit.)
To study the effect of temperature variation, the reaction was present in the slightly alkaline solution (pH 9) to form the
carried out in the range 200 ꢀC to 260 ꢀC for 12 h. Fig. 3a shows Co–Cit. complex (eqn (1)), whose structure and ease of forma-
that uniform and symmetrical octahedral crystals possessing tion is known to be strongly inuenced by pH and also the
high crystallinity were obtained at 200 ꢀC. However, below concentration ratio of Co2+ : Cit. The Co–Cit. complex has been
Fig. 2 (a) The XRD patterns of cobalt sulfide synthesized with various reaction times from 6 h to 20 h and (b) schematic representation of cobalt
sulfide morphology with respect to various reaction times.
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Fig. 3 FE-SEM images of cobalt sulfide at various hydrothermal treatment temperatures: (a) 200 ꢀC, (b) 220 ꢀC and SEM images at (c) 240 ꢀC and
(d) 260 ꢀC, with (e) corresponding XRD patterns. [CoS2 is denoted by *].
reported to attain a cyclohexane-like structure under slightly increments suppress nucleation and promote over-growth
alkaline conditions which we speculate to act as the so-mold forming large irregular crystals with uncontrolled morphology
for directing the morphology of the CoS2 catalyst.30 This is (Fig. S2a and d, ESI†). Although the concentrations of both Co2+
subsequently transformed to an inorganic CoS2 crystal having and Cit. are crucial for CoS2 formation the reaction kinetics are
octahedral morphology upon thermal decomposition of sodium inuenced by temperature and reaction time. Wherein the
thiosulfate generating sulde anion (eqn (2))31 which replaces reaction temperature governs Cit. replacement with sulde
Cit. (so-mold) as shown in eqn (3). Therefore, the availability anion, which optimally occurs at 200 ꢀC beyond which the
of sulde ions plays a crucial role in morphological evolution surface energy of all the planes, namely {111}, {200} and {220},
ꢀ
(Fig. 3a–d). At 200 C the availability of sulde ions is limited tends to increase leading to growth in all directions and even-
which provides sufficient time to substitute the citrate in the tual aggregate formation (Fig. 3c and d).34,35 Simultaneously,
Co–Cit. complex by sulde to give the desired facet-controlled a reaction time of 12 h was found to be appropriate below which
morphology. As the temperature is gradually increased to agglomeration and plate-like structures are predominant and
260 ꢀC, sodium thiosulfate decomposition also increases above which the morphology tends to disrupt increasingly
causing pronounced availability of sulde thereby leading to (Fig. 2b and S3, ESI†).
a random and rapid replacement of Cit. resulting in agglom-
To gain more insight into the formation of octahedral CoS2
eration of cobalt sulde nuclei instead of controlled crystal crystals, the sample obtained at 200 ꢀC for 12 h was examined by
growth. Therefore, under favorable conditions the cobalt citrate transmission electron microscopy (TEM), high resolution TEM
undergoes ligand replacement by sulde slowly but steadily in (HR-TEM), energy dispersive spectroscopy (EDS) and elemental
the cyclohexane arrangement of the cobalt citrate complex to dot mapping. As can be seen from the TEM and HR-TEM images
result in cobalt sulde nuclei which further grows along the (Fig. 4a, c and d, and S6, ESI†), the octahedral structure mostly
lowest energy facet {111} forming octahedral crystals. This is dominated with distinguishable fringes over both facet and
demonstrated by a prominent peak at 619 cmꢁ1 which is edges with a d-spacing of 0.32 nm (Fig. S6c, ESI†) and 0.20 nm
attributed to the Co–S–Co stretching frequency in the FT-IR (Fig. 4d) respectively. These were found to correspond with
spectra (Fig. S5, ESI†).22,32 Strikingly, the peaks observed at {111} and {220} planes when compared with the XRD pattern of
1615, 908 and 852 cmꢁ1 denote the existence of citrate moiety CoS2.24 This reects that growth of the octahedral crystal occurs
even aer multiple washing of the obtained product.33 The along the most stable {111} facet as also demonstrated by the
morphological evolution as determined by SEM and FE-SEM increase in the peak height along {111} in the XRD peak pattern
analysis shows spheres (Fig. 1a) in the absence of the so with time variation (Fig. S4, ESI†). The atomic concentration of
mold but sequentially tends to attain a facet-controlled octa- Co : S was found to be 1 : 2 from the EDS spectrum with
hedral morphology in the presence of 10 mM of the so-mold a carbon content of 7.2%, while the elemental dot mapping
beyond which over nucleation causes agglomeration (Fig. 1c displays an even distribution of cobalt, sulfur and carbon in the
and d). Likewise, 5 mM cobalt chloride was determined to be octahedral conned crystal (Fig. 4e–h and S7, ESI†). During the
optimal from 5 to 20 mM concentration range as further reaction process, the functional groups such as –CO, –COOH
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Fig. 4 (a) TEM and (b) SEM images of the octahedral CoS2 crystal with corresponding HR-TEM images (c) at the crystal corner (more towards the
facet as marked in red) and (d) at the edge plane (as marked in cyan) along with the corresponding SAED pattern (displayed inset); (e) super-
imposed elemental dot mapping illustrating uniform distribution of (f) carbon, (g) sulfur and (h) cobalt in the octahedral CoS2 crystal.
and –OH from citrate are expected to strongly interact with RHE, detailed in the Experimental section). A steep increase in
inorganic cations and metals upon coordination33 eventually the reduction current was observed in the oxygen saturated
leading to carbon incorporation during CoS2 synthesis. This is electrolyte whereas a feeble quasi reversible peak was observed
further supported by X-ray photoelectron spectroscopy (XPS). in the absence of O2 (Fig. S8, Inset, ESI†) indicating that the
The deconvoluted Co 2p XP spectra of the CoS2 octahedral CoS2 catalyst is active towards ORR. Furthermore, the kinetic
crystal illustrated in Fig. 5a shows two peaks centered at parameters and electrocatalytic ability of synthesized CoS2 with
778.5 eV and 793.4 eV corresponding to 2p3/2 and 2p1/2
,
various so-mold concentrations towards ORR were evaluated
respectively, with an energy separation of 15.1 eV 21 conrming using RDE and RRDE measurements. The polarization curves
that the cobalt is in a +2 oxidation state. The S 2p XP spectra represented in Fig. S9 (ESI†) show noticeable changes in both
represented in Fig. 5b show a doublet at 162.4 and 163.6 eV onset potential and oxygen reduction current density. The CoS2
corresponding to 2p1/2 and 2p3/2, respectively, which are catalyst synthesized with the 10 mM so-mold concentration
attributed to the presence of terminal and bridged –S– in the shows a more positive onset potential of 0.83 V; however, with
octahedral arrangement of the CoS2 crystal.24
an increase in the so-mold concentration the onset potential
The presence of carbon in the CoS2 crystal was conrmed by shied towards being more negative (0.60 V at 30 mM). Inter-
C 1s XP spectra (Fig. 5c) where the spectra were deconvoluted estingly, the ORR current density increases with a decrease in
into four components centered at 284.5, 285.2, 286.1 and the so-mold concentration. The plot of so-mold concentra-
288.6 eV corresponding to C]C, C–C, C–O and C]O respec- tion vs. ORR current density (at 0.0 V) clearly demonstrates that
tively (Fig. 5c).33,36 This conrms the presence of inherent 10 mM shows a better electrocatalytic activity towards ORR,
carbon in the CoS2 octahedral crystals which is further sup- following the order 10 mM > 20 mM > 30 mM (Fig. 5d and S9,
ported by CHNS analysis wherein the amount of carbon was ESI† and detailed in Table S1, ESI†). Correlating the obtained
found to be 7% (Table 6, ESI) and is consistent with EDS electrocatalytic activity with morphology clearly demonstrates
analysis.
that the octahedral CoS2 with the {111} exposed facet (10 mM)
shows the best activity and the activity degrades at higher
concentrations (20 and 30 mM). This is because the capping
effect is very strong which precludes self-assembly and also due
to the shortage of available space for appropriate growth of the
crystal resulting in aggregated structures with inferior unifor-
mity (Fig. 1c and d). On the other hand, the absence of so mold
results in microspheres due to the absence of complexation
which in turn lack discretely exposed facets resulting in poor
ORR activity.
2.7 Electrochemical studies
Besides morphology, the structure–electrocatalytic activity
relationship of the synthesized CoS2 catalyst under various
conditions was studied using cyclic voltammetry (CV), rotating
disk electrode (RDE) and rotating ring disk electrode (RRDE)
measurements. Preliminary experiments were performed in
both O2 saturated (attained by purging with O2 for 30 min
before measurement and above the solution during measure-
ment) and O2 free (purging with Ar) 0.1 M HClO4 electrolyte at
25 mV sꢁ1 in the potential window between 1.2 to ꢁ0.1 V (vs.
Subsequently, the effect of Co2+ concentration and the
reaction time on ORR activity was also studied and the best
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Fig. 5 Deconvoluted XP spectra of (a) Co 2p, (b) S 2p and (c) C 1s of the octahedral CoS2 crystal; scatter plot representing ORR current density (at
0.0 V) along with onset potential (at ꢁ0.1 mA cmꢁ2) extracted from RDE measurements (Fig. S9 and S11 (ESI†)) at various (d) soft-mold
concentrations and (e) reaction times; (f) RDE polarization curves for octahedral CoS2 and Pt/C in O2-saturated 0.1 M HClO4 at 1300 rpm and
5 mV sꢁ1 scan rate (inset: K–L plot). CE : Pt mesh, RE: Ag/AgCl/3 M KCl (potentials converted to RHE) .
activity was observed for a 1 : 2 ratio of Co2+ : Cit. (that is, 5 mM CoS2 and Pt/C (20%) with a diffusion limiting current density of
of CoCl2 and 10 mM of so-mold) at a reaction time of 12 h ꢁ11 mA cmꢁ2 and ꢁ7.5 mA cmꢁ2 (at 0.0 V) respectively (Fig. 5f)
(Fig. 5e, S10 and S11, detailed in Table S2, ESI†). The enhanced and a comparable kinetic current density (Fig. S13, ESI†)
activity is clearly witnessed from the more positive onset demonstrating the similar competence of CoS2 with Pt/C (20%).
potential (0.83 V) with a diffusion limited current density of The number of electrons transferred as evaluated from the
ꢁ11 mA cmꢁ2 (0.0 V vs. RHE) following the activity sequence Koutecky–Levich (K–L) plot (Fig. 5f inset, detailed in ESI†) was
12 h > 10 h > 8 h > 6 h > 16 h > 20 h as illustrated in Fig. 5e and found to be 3.8 for CoS2 at 0.2 V signifying the ability of octa-
S11† (detailed in Table S3, ESI†). Similarly, a decrease in ORR hedral CoS2 with respect to electrocatalytic conversion of oxygen
activity was observed with an increase in reaction temperature to water. Subsequently, the ORR pathway was probed by RRDE
and is in the order 200 > 220 > 240 > 260 ꢀC (Fig. S12, ESI,† measurements to monitor the formation and further conver-
detailed in Table S4, ESI†), which could be due to the defor- sion of the intermediate peroxide species during ORR. The
mation in the octahedral structure following overgrowth peroxide intermediate once generated on the disk electrode
resulting in probable masking of the ORR active sites along the during ORR was detected at the Pt-ring electrode by polarizing
highly reactive facet. It is noteworthy to mention that the ob- the ring at a constant potential of 1.4 V wherein the produced
tained activity was solely due to the CoS2 catalyst without the peroxide was converted to oxygen. The comparative RRDE
addition of an external carbon or related conductive additive. response at 1300 rpm obtained over both the facet-controlled
Although inorganic, the synthesized CoS2 was designed to CoS2 and the Pt/C (20%) catalyst (Fig. S14, ESI†) shows
incorporate carbon intrinsically during the synthetic protocol increased ring current in the early stage of the cathodic scan
on account of the citrate so-mold which is converted to carbon which indicates the formation of peroxide intermediate and the
upon high temperature treatment.33 Comparing the observed quantitative estimation of H2O2 was found to be 20% (at 0.7 V
ORR activity over CoS2, it gave ꢁ2.5 mA cmꢁ2 kinetic current n ¼ 3.2). As the potential becomes increasingly cathodic, the
density at 0.7 V which is 1.7 fold higher than CoS2-CG24 and 2.2 ring current tends to diminish (Fig. S14a, ESI†) resulting in
times higher than the Co1ꢁxS/RGO composite,25 both acclaimed negligible amounts of H2O2 (<2%, with n ¼ 3.9) at 0.1 V which
to possess the highest activity amongst non-precious metal conrms that produced peroxide was later converted to water
chalcogenides. Therefore, intrinsic carbon in addition to the suggesting a sequential 2 + 2 electron pathway (Fig. S14c†).
{111} facet bound CoS2 octahedral crystal synergistically
elevates the ORR activity in perchloric acid medium. The
competence of the designed octahedral CoS2 catalyst was then
assessed against the state-of-the-art Pt/C (20%) catalyst and the
obtained response shows comparable ORR onset potentials for
2.8 Morphology-dependent EQCM analysis
In-depth insight into the adsorption–desorption processes
occurring at the CoS2 catalyst surface during ORR was further
analyzed by electrochemical quartz crystal microbalance
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(EQCM) analysis in terms of mass change. The extent of O2 and which is consistent with RRDE measurements. Upon reversing
ORR intermediate adsorption is an indication of the availability the potential sweep from ꢁ0.1 V anodically caused a decrease in
of active sites and the catalyst's ability regarding the nal mass at the catalyst surface indicating subsequent desorption
conversion of oxygen to water. EQCM analysis was performed of the cathodic products. The adsorption ability of the catalyst
under similar conditions to RDE (detailed in the Experimental indirectly relates to its available active sites for ORR. The plot of
section). Since the reaction time-dependent studies towards the normalized mass with respect to reaction time (Fig. 6b) clearly
evolution of CoS2 morphology (Fig. 2b) elaborated discrete demonstrates that the maximum change in mass was observed
changes and the course of facet-controlled octahedral forma- for the sample synthesized at 12 h (octahedral crystal) which
tion, time variable samples were tested regarding structure– drastically deteriorated for the 16 h and 20 h samples, corre-
activity correlation. Initially, the optimal 12 h treated sample lating well with the observed ORR activity (Fig. 5e). Therefore,
was analyzed (Fig. 6a), displaying an initial increase in mass the extent of normalized oxygen adsorption indicates active site
during the cathodic sweep which indicates interaction with availability which hence ensures superior ORR activity. These
electrolyte followed by a further increase in mass of approxi- aspects, when correlated with the morphology, suggested that
mately 8 ng near the ORR onset potential (0.83 V) which the {111} facet-bound octahedral CoS2 exhibits pronounced
becomes maximum (approximately 25 ng) in the diffusion ORR activity (Table S5, ESI†).
limited region of ORR due to electroactive adsorption of O2 and
its reduction intermediates.37 Interestingly, the mass drops
2.9 SECM analysis
when the potential sweeps from 0.1 V cathodically, which could
The local electrocatalytic activity of the CoS2 catalyst with
be ascribed to the complete conversion of adsorbed interme-
respect to ORR was visualized by the redox competition mode of
diate (peroxide) to water resulting in the reduction of mass
scanning electrochemical microscopy (RC-SECM). SECM
Fig. 6 (a) Cyclic voltammogram with corresponding mass change from EQCM analysis for the octahedral CoS2 catalyst in O2-saturated 0.1 M
HClO4 at a scan rate of 5 mV sꢁ1; (b) bar diagram representing ORR current density (at 0.0 V) with corresponding mass change extracted from
EQCM analysis for cobalt sulfide at various reaction times from 6 h to 20 h [CE: Au coil, RE: Ag/AgCl/3 M KCl (potentials converted to RHE)]; (c)
schematic representation of the RC-SECM mode with its corresponding 3D image (d) representing the local ORR activity of the octahedral CoS2
catalyst (WE1) polarized at 0.2 V and the Pt-tip (WE2) at 0.4 V in 0.1 M HClO4 electrolyte. CE: Pt coil, RE: Ag/AgCl/3 M KCl (potentials converted to
RHE).
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Fig. 7 (a) Chronoamperometric measurement of octahedral CoS2 and Pt/C (20%), (b) LSV curve taken before and after chronoamperometric
measurement in oxygen saturated 0.1 M HClO4 solution. (c and d) SEM images showing retention in morphology of octahedral CoS2 after 1000
minutes of the stability test.
measurements were performed using a four electrode assembly state that the CoS2 sample possesses a minimal charge transfer
consisting of our sample of interest, the CoS2 catalyst over the resistance (Rct) of 95 U (0.32 V vs. Ag/AgCl) as compared to
GC plate as WE1 with Pt and Ag/AgCl/3 M KCl as the counter approximately 230 U reported by Peng et al.38 (Fig. S15, detailed
and reference electrode respectively (potential converted to in ESI†) signifying the fast electron transfer. This was further
RHE). The activity was probed using a Pt-ultramicroelectrode evidenced by a high electrochemical active surface area (ECSA)
(WE2) acting as the tip and the measurements were per- of 56.8 m2 gꢁ1 (Fig. S16, detailed in ESI†) in comparison with
formed in 0.1 M HClO4 electrolyte while polarizing WE1 at 0.2 V 58.9 m2 gꢁ1 for Pt/C (20%).39 This therefore quantitatively
and WE2 at 0.4 V, that is, in their respective diffusion limited proves the comparable activity of the designed CoS2 catalyst to
ORR potential region. During the scanning when the Pt-tip Pt/C (20%) proving the former to be an alternative high
passed over the active catalyst (CoS2) zone, a decrease in the performance non-precious catalyst for acidic fuel cells.
WE2 reduction current (higher reduction current at spot; WE1) Although active, a host of such active catalysts are predomi-
was observed. The decrease in reduction current at the tip could nantly limited by their durability under catalytic conditions.40
be explained on the basis of competition between the Pt-tip and This was further analyzed by chronoamperometric measure-
the CoS2 catalyst for the dissolved oxygen present in a gap of 10 ment over a span of 16 h for both Pt/C and CoS2 catalysts in
mM (Fig. 6c) and its reductive conversion to water. As can be which CoS2 was found to exhibit more than 98% retention
seen from Fig. 6d, a good contrast between the catalyst spot and (Fig. 7a inset and 7b) of activity as compared to 92% for the
unmodied GC (where no catalyst is present) was observed. The state-of-the-art catalyst. Additionally, morphological analysis
image colors change from blue-green through yellow to red aer the stability test from SEM images distinctly demonstrates
from lower to higher ORR activity, with the red color indicating no modication or deformation in the octahedral morphology
smaller reduction current at the Pt-tip and hence higher ORR (Fig. 7c and d), providing further evidence to explain the
activity for CoS2 than the darker (blue-green) region. Therefore, retained activity.
the uniform distribution of red color over the catalyst spot
indicates the presence of ORR active sites.
3 Conclusions
The observed high electrocatalytic activity of the octahedral
CoS2 crystal towards ORR is further supported by the electro- A simple one-pot hydrothermal synthesis was performed to
chemical impedance spectroscopy (EIS) measurements which achieve facet-controlled CoS2 via complexation followed by
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25 H. Wang, Y. Liang, Y. Li and H. Dai, Angew. Chem., Int. Ed.,
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