Increased activity in hydrogen evolution electrocatalysis for partial anionic substitution in cobalt oxysulfide nanoparticles

Article abstract of DOI:10.1039/c5ta08706f

The functionality of an electrocatalyst depends sensitively on the surface chemistry. In the case of transition-metal compounds, both the transition metal cation and the anion must be controlled to maximize the electrocatalytic activity. This realization has driven many efforts devoted to engineering the cation chemistry, producing many state-of-the-art electrocatalysts. Motivated by the critical role the cation plays in electrocatalysis, we seek to understand whether a similar effect can be achieved with the anion. Herein, we present a study on the effect of the anion substitution on the hydrogen evolution reaction (HER) electrocatalysis on cobalt oxysulfide nanoparticles. To control the sulfur substitution, we use ammonium sulfide to introduce sulfur to the cobalt oxide nanoparticles at low temperature without inducing secondary phase formation. We find that a lightly doped oxysulfide catalyst, which has the composition CoO_xS_0.18, exhibits a metastable, distorted S-substituted CoO phase and is 2-3 times more active for the HER than either end-member of the substitution series. Our first-principles calculations attribute the HER enhancement to the stronger surface H adsorption which is maximally favorable at a relatively low doping level. Our work provides a protocol for synthesizing metastable mixed-anion materials and reveals the critical role of the anion on the surface physiochemical properties and the HER electrocatalysis.

Full text of DOI:10.1039/c5ta08706f

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Increased Activity in Hydrogen Evolution Electrocatalysis for Par-
tial Anionic Substitution in Cobalt Oxysulfide Nanoparticles†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Andrew Nelson,a § Kevin E. Fritz,a § Shreyas Honrao,a b Richard G. Hennig,b a Richard D. Robinson,a ‡
and Jin Suntivicha c ‡
www.rsc.org/
The functionality of an electrocatalyst depends sensitively on the such as the surface-adsorbate interaction27, 28 to find the most
2, 11
surface chemistry. In the case of transition-metal compounds, electrocatalytically active surface.1,
Thus far, approaches
both the transition metal cation and the anion must be controlled based on cationic substitutions have been highly successful in
to maximize the electrocatalytic activity. This realization has controlling the surface-adsorbate interaction for engineering
driven many efforts devoted to engineering the cation chemistry, the electrocatalytic activities.25, 29, 30 These investigations have
producing many state-of-the-art electrocatalysts. Motivated by produced many mixed-cation electrocatalysts that are consid-
the critical role the cation plays in electrocatalysis, we seek to ered the state-of-the-art.11, 13, 25, 29, 31, 32
understand whether a similar effect can be achieved with the
Considering these positive results from cationic substitu-
anion. Herein, we present a study on the effect of the anion tion, a natural question is whether a similar substitution of the
substitution on the hydrogen evolution reaction (HER) anion can also benefit the activity of the transition-metal-
electrocatalysis on cobalt oxysulfide nanoparticles. To control the containing electrocatalysts. Transition metal compounds with
sulfur substitution, we use ammonium sulfide to introduce sulfur mixed anions have been explored for batteries,33 transparent
to the cobalt oxide nanoparticles at low temperature without conducting oxides,34 photocatalysts,35, 36 and electrocatalysts.
inducing secondary phase formation. We find that a lightly doped In electrocatalysis, many works using mixed anionic com-
39
oxysulfide catalyst, which has the composition CoOxS0.18, exhibits pounds such as MoS2-MoSe2,37 MoOx-MoS2,38,
sulfidized
a metastable, distorted S-substituted CoO phase and is 2-3 times MoP,40 phosphidized CoS2,41 selenized Ni2P,42 or amorphous
more active for the HER than either end-member of the Co-O-S43 have established that mixed anions can lead to supe-
substitution series. Our first-principles calculations attribute the rior performance. Still, it is not clear how the anion substitu-
HER enhancement to the stronger surface H adsorption which is tion affects the physicochemical and electrocatalytic proper-
maximally favorable at a relatively low doping level. Our work ties. This is partly because many of these studies have focused
provides a protocol for synthesizing metastable mixed-anion
materials and reveals the critical role of the anion on the surface
physiochemical properties and the HER electrocatalysis.
One of the largest factors limiting the commercialization of air-
breathing electrochemical energy storage technologies lies in
the requirement of rare, precious-metal catalysts.1-10 Transi-
tion-metal-containing compounds such as oxides,10-15 chalco-
genides,16-21 and pnictides22-24 exhibit high activities and are
attractive alternatives to rare, precious metals. The pursuit of
these transition-metal-containing electrocatalysts has driven
many efforts11, 25, 26 in controlling physicochemical properties
Figure 1. Schematic of the anionic substitution in CoOx nanoparticles
as a testbed for HER electrocatalysis (a). Results from this work indi-
cate that the maximum HER activity occurs at low doping levels (e.g.,
CoOxS0.18), while at higher doping levels, activity decreases (b). The
HER activity is normalized to the surface areas of the catalysts.
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on compounds at the extremes of each compositional range or 200°C heat-treatment under N2 for 1 hour. These annealed
have used materials with poorly-defined morphologies, such NPs serve as a control to explore the phase stability of Co-
as porous films or polydisperse nanoparticles.18, 44, 45 In view of oxysulfides following the anion substitution.
the extensive literature on transition metal chalcogenides17, 21,
To assess the stoichiometry of the Co-oxysulfide particles,
27, 46 and phosphides,22, 23, 47 knowledge of the role of the anion we apply inductively coupled plasma-optical emission spec-
in controlling these physicochemical properties can help to trometry (ICP-OES). We find the approximate stoichiometries
rationalize future design strategies for optimizing electrocata- for the (non-annealed) doped samples to be CoOxS0.18
,
lytic activity. CoOxS1.03, and CoOxS1.27 for the (NH4)2S:Co molar ratios of 3:10,
In this work, we synthesize partially anion-exchanged na- 1:1, and 3:1, respectively. Annealing the sample did not signifi-
noparticles (NPs) to determine how the anion substitution can cantly alter its stoichiometry (see Supporting Information).
affect electrocatalytic activity, using cobalt oxysulfide, CoOxSy, This elemental analysis confirms that significant amounts of S,
as a model system (Fig. 1a). We focus on the hydrogen evolu- in direct proportion to the amount of (NH4)2S used, are incor-
tion reaction electrocatalysis (HER, 2H2O + 2e−
↔
H2 + 2OH− in porated into the NPs. As observed previously,66 under our
alkaline), which has been reported to be active on cobalt sul- conditions the resulting S:Co ratio in the NPs will not exceed
48-50
fides43,
and other cobalt chalcogenides.51 We show that the stoichiometry of Co3S4, even with a large excess of (NH4)2S.
The CoOx NPs obtained after oxidation are hollow and pol-
the HER activity depends sensitively on the anion chemistry,
which can subsequently be optimized during catalyst synthe- ycrystalline, with a diameter of 13.0±1.5 nm (Fig. 2b). The
sis. Specifically, we demonstrate that there is an optimum most dilutely doped oxysulfide NPs (CoOxS0.18), created from
composition for the HER electrocatalysis at relatively small the (NH4)2S:Co ratio of 3:10, retain a morphology virtually
levels of sulfide doping (Fig. 1b). We attribute the increased identical to the initial pure oxide NPs (diameter 12.9±1.4 nm)
HER activity after anion exchange to the stronger hydrogen
(Fig. 2c). Annealing the most dilutely doped NPs does not
adsorption energy following the substitution of S2- in place of change the morphology (Fig. S1). For higher degrees of anion-
O2- in Co-oxysulfide, effectively reducing the intermediate en- exchange doping, the diameter increases (CoOxS1.03, 15.0±1.7
ergy during the HER.
nm, Fig. 2d; CoOxS1.27, 14.5±1.6 nm, Fig. 2e). This increase is
To carry out the partial substitution of the anions, we use a consistent with the previous report for cobalt oxide NPs fully
protocol based on organic-phase colloidal synthesis. Colloidal anion-exchanged into cobalt sulfide NPs.66 The increase in the
NP synthesis offers many advantages, such as controlled nu- particle size follows the growth of the cavity within the NPs as
cleation and growth in solution, thereby producing particles a result of the nanoscale Kirkendall effect.67-69
with well-defined, highly-uniform size and shape.52-55 The
The structural changes in the sulfidized CoOxSy NPs are
monodispersity that is afforded by colloidal synthesis offers an assessed using selected area electron diffraction (SAED) (Fig.
unprecedented opportunity to establish and optimize struc- 2f, and Fig. S1), showing that the CoOx and CoOxS0.18 NPs ex-
57
Furthermore, the nanoscale hibit a rock-salt structure with broad reflections from the
ture-activity relationships.56,
nature of the produced materials can allow for non- (111), (200), and (220) planes. Close examination of the rota-
conventional transformation kinetics, resulting in a metastable tionally averaged SAED patterns shows that the crystal struc-
structure that is considered unobtainable according to the tures for these NPs agrees well with the reference CoO struc-
phase diagram.58 A disadvantage of colloidal synthesis is the ture; however, the CoOxS0.18 SAED pattern exhibits a small shift
requirement of a surfactant to act as a stabilizer and particle to lower scattering angles, indicating an expansion of the unit
growth regulator.59-61 This requires a surface ligand removal cell (vertical dashed line in Fig. 2f). Lattice expansion could be
step such as high-temperature annealing,57 replacement of caused by the larger S anion occupying the O2- lattice. Based
long-chain ligands with more volatile or soluble ones,62, 63 or, on the magnitude of the shift, however, the calculated in-
as used in this work, ligand displacement using highly reactive crease in unit cell size is, at most, 0.02 Å relative to the refer-
inorganic species.61, 64, 65
ence structure for CoO. Interestingly, this shift is an order of
Our method to create the anion-substituted series is based magnitude less than what one might expect based on the ex-
on our previous finding that ammonium sulfide, (NH4)2S, can trapolation of the Co-O/Co-S bond lengths from the Shannon
insert S into oxides under relatively mild conditions.66 In this radii70 (see Supporting Information). We therefore anticipate
work, we achieve partial substitution by limiting the amount of that the S concentration at the surface may be higher than in
added (NH4)2S to control the diffusion of an isovalent anionic the bulk.
dopant, S2−, into organosoluble CoOx NPs, thereby producing S-
To develop an intuition for this shift, we include the SAED
substituted CoOx-ySy NPs (see schematic of this evolution Fig. pattern for an annealed Co oxysulfide sample that was pre-
67
2a). Briefly, we first oxidize cobalt NPs into CoOx and then pared in the same way as CoOxS0.18 (using a 3:10 (NH4)2S:Co
sulfidize them into CoOxSy NPs. The oxygen in the CoOx is ani- molar ratio, Fig. 2f, blue dashed line). ICP-OES for this sample
on-exchanged for sulfur through the addition of (NH4)2S dis- revealed a composition of CoOxS0.17, similar to that of the un-
solved in oleylamine to the CoOx NP solution at 100°C under N2 annealed NPs. As shown in Fig. 2f, the lattice spacing for the
atmosphere. We explore the nominal molar ratios of (NH4)2S heat-treated CoOxS0.17 sample is more similar to the CoO ref-
to Co at 3:10 (dilute doping), 1:1, and 3:1 (excess sulfur). We erence than the unannealed CoOxS0.18 NPs. We interpret this
also investigate the effect of annealing by re-suspending the finding as an indication of phase separation in the annealed
NPs following re-isolation in an organic solvent followed by a samples (CoOxS0.17) into CoO and either the Co9S8 or the Co3S4
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se Co oxysulfide NPs leads to a significant solid-state recon-
struction within the NPs, which results in a phase separation
into a structurally distorted oxide and sulfide (see Supporting In-
formation). We note that this phase separation occurs without sig-
nificantly changing the compositions. The annealed Co oxysul-
fides have stoichiometries of CoOxS0.88 and CoOxS1.26 for the
(NH4)2S:Co molar ratios 1:1 and 1:3, respectively, similar to
those of the unannealed Co oxysulfides
(CoOxS1.03 and
CoOxS1.27). These annealed, higher sulfur-content NPs show a
much less pronounced expansion in size relative to the starting
NPs compared to their non-annealed analogues (see Fig. S1a-
h
).
To better understand the substitution mechanism of S2- for
O2-, we perform density-functional theory (DFT) calculations to
study the energetics of the Co oxysulfide system (see Support-
ing Information). We find that although it is thermodynamical-
ly favorable for S2- to reside in place of O2-, the migration ener-
gy barrier for S2- diffusion through CoO is significant (2.63 eV,
as compared to 0.18 eV for Co), preventing rapid movement
into the CoOx NPs.66 These observations support the possibility
that S2- substitutes in place of O2- through under-coordinated
surface and defect sites, as has been observed for sulfidized
MoP.40 Our calculations of the formation energies of different
phases in the Co-O-S system suggest that mixtures of CoO and
Co-sulfide phases (Co9S8 and Co3S4) are significantly lower in
energy than the rocksalt CoOxSy structures. This DFT finding is
consistent with the observed phase separation seen by SAED.
We therefore propose that the unannealed Co oxysulfides are
a metastable phase characterized by the substitution of sulfur
onto the under-coordinated anion sites. With more introduced
sulfide, the driving force for sulfidization becomes large
enough that Co rapidly diffuses outward,66 forming an amor-
phous phase. Upon annealing, phase separation occurs, allow-
ing the NPs to form Co-sulfide domains (Co9S8 and Co3S4). At
the lowest (NH4)2S:Co ratio, the resulting grains of Co-sulfide
are likely too small to be observed.
Figure 2. Schematic of the synthesis of cobalt oxysulfides via partial
and “full” anion exchange (a). Transmission electron microscopy
characterization of structural transformations of CoOxSy nanoparti-
cles during anion exchange (a) of O2− with S2− (b-e, scale bar 25 nm)
and the rotationally averaged SAED patterns (f). The particle mor-
phology and crystal structure of the CoOx particles (b) are retained
at low S2− doping levels (c), but at higher doping levels the particles
swell and become less crystalline (d,e). Reference lines are given in
(f) for CoO (JCPDS 00-048-1719); the dashed vertical line identifies
the (220) reflection, at which the shifts are most apparent.
To understand how the anionic substitution affects the
electrochemical properties, we conducted cyclic voltammetry
(CV) experiments in Ar-saturated 0.1M KOH. Fig. 3 shows the
CV results of the NPs examined using a thin-film electrode
prepared from a composite of the NPs, acetylene black, and
K+-Nafion®.71 To ensure that the electrochemical response of
these NPs is intrinsic to the surface chemistry, we treated the
NPs with 0.1 M KOH in isopropanol under sonication for 30
minutes to strip off the ligands.64, 65 To ensure ligand removal,
we compared the CV result of our CoOx NPs to thin-film elec-
trodes prepared in the same manner using nanopowders of
known stoichiometry (CoO and Co3O4). The CoO reference
showed a similar electrochemical response to our CoOx NPs
(see Fig. S2), confirming the surface chemistry of our CoOx
NPs. We point out that the CV of our CoOx NPs is distinct from
that of the Co3O4 reference, supporting our conclusion that the
CoOx catalyst has a surface CoO crystal structure.
phase. In contrast, we believe the data indicates the unan-
nealed CoOxS0.18 sample has sulfur occupying the anion sites in
the rock salt structure. We did not, however, observe reflec-
tions for the Co9S8 or Co3S4 phases. We hypothesize that these
sulfides must have domain sizes too small to give observable
reflections or that they form amorphous phases during the
phase separation process (see Supporting Information). As we
will show later in our first-principles calculations, the two-
phase nature is the energetically favourable phase in the Co
oxysulfide material.
We further increase sulfur substitution by using higher
(NH4)2S:Co ratios (1:1 and 1:3) to increase the driving force for
S incorporation. At these high S concentrations, the particles
(CoOxS1.03 and CoOxS1.27) become partially crystalline or amor-
phous (Fig. 2 d,e). We observe weak peaks that could corre-
spond to smaller grains of CoO in the CoOxS1.03 samples (ap-
proximately 15 Å in size, compared to about 30 Å for CoOx and
dilutely doped samples as estimated by the Scherrer equation;
see Table S1). With more sulfur addition, the primary rocksalt
CoO reflections become very weak in CoOxS1.27. Annealing the-
Our CV results capture the evolution of the surface chemis-
try of the Co oxysulfide particles throughout the anion substi-
tution process. Notably, although CoOx and CoOxS0.18 share the
same crystal structure (Fig. 2f), their surface chemistries as
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coefficients are significantly larger than the reported Tafel
slopes for metals76, 77 and other sulfide catalysts.16, 17, 51 Future
work in understanding this rather high Tafel slope is essential
to pinpoint the microscopic mechanism for the observed en-
hancement in the HER.
To understand the origin of the HER activity enhancement,
which was experimentally observed to be most effective at low
S concentration, we apply the activity descriptor approach
pioneered by Parsons, Gerischer, and Norskov.26, 78 In this ap-
proach, the H adsorption energy estimates the surface’s ability
to form the HER intermediate.79 We calculate the H adsorption
energy on the CoO(100) surface, which is charge neutral and
lowest in energy, as a function of the surface S composition
using DFT (see Supporting Information, Fig. S4). In this frame-
work, the most active HER electrocatalyst should have the H
adsorption energy at a value that minimizes the energy differ-
ence between the reactant, the intermediate (H*), and the
product. Generally, this value is found to be around the H ad-
sorption energy on Pt.26, 79, 80
We observe that H* prefers to bind to a Co ion rather than
Figure 3. Cyclic voltammograms of carbon-supported thin-film
an O or S ion. Further analysis shows that H* develops a partial
electrocatalysts containing (samples not subjected to annealing)
CoOx (a), CoOxS0.18 (b), CoOxS1.03 (c), and CoOxS1.27 (d) nanoparticle negative charge, explaining its tendency to sit above a positive-
electrocatalysts in Ar-saturated 0.1 M KOH at 50 mV/s scan rate.
The grey dashed background CV in (b)-(d) shows a comparison to
the CoOx starting material.
ly charged Co ion. As we substitute more S onto the surface,
the H* absorption energy first decreases, indicating stronger
bonding between H* and Co. Crucially, at an average of only 1
revealed by the CV measurement are distinct. The
starting in 8 O2- ions substituted with S2- on a CoO(100) surface 2×2
CoOx material shows a broad, shallow redox peak between unit cell, the adsorption energy of H approaches that of
~1.0 and ~1.3 V vs. RHE, which is consistent with the Co2+/Co3+ Pt(111), which we use as a benchmark for an “ideal” HER cata-
redox couple in literature (Fig. 3a).31, 72, 73 With the inclusion of lyst (Fig. 5, blue plot). Also plotted are the experimentally de-
more S2-, a reversible peak in the region of 1.1 V vs. RHE grows termined values for HER current at 375 mV, as a function of
(
Fig. 3b-d). This observation is consistent with previous reports S:Co ratio (Fig. 5, red plot). As more O is displaced, the H bind-
on Co sulfides, which have suggested that this redox couple ing energy returns to the initially high value. Assuming that the
can be attributed to the reaction(s) CoS + OH−
CoS-OH + thermodynamic formation of the adsorbed H intermediate
H2O + e− and/or CoS-OH + OH− CoSO + H2O + e−.74, 75 This limits the HER kinetics,27,
↔
79-81
↔
our DFT calculations broadly
assignment is consistent with the growth of the ~1.1 V vs. RHE agree with our experimental HER results. Specifically, the re-
peak CoOxS1.03 sample with the addition of S2-, but we caution duction in the H adsorption energy following the partial S2-
that future work remains to be done to verify the nature of exchange increases HER activity initially; however, as more
this redox couple. Interestingly, the peak height decreases for
the CoOxS1.27 sample. At the moment, it is unclear what causes
the peak height to decrease.
We characterize the HER activity on these partially-anion-
substituted materials in an H2-saturated 0.1 M KOH electrolyte
at 10 mV/s (Fig. 4a). The polarization curves show that the
addition of sulfur improves the HER activity beyond that of
CoOx. The improvement in the HER activity is most pro-
nounced for the most dilute doping (CoOxS0.18). In comparison
to the starting CoOx material: the specific HER activity of
CoOxS0.18 is ~3x more than the CoOx starting material at -375
mV (vs. RHE). Further addition of S2- does not sustain this in-
crease in activity; the observed HER activity returns to a level
close to that of CoOx for samples with additional S2- beyond
CoOxS0.18. These results are summarized in the corresponding
Tafel plots (Fig. 4b and Fig. S3). Interestingly, the CoOx,
CoOxS1.03, and CoOxS1.27 samples exhibit a large Tafel slope of
160-170 mV/decade, which corresponds to the phenomeno-
logical transfer coefficient (α) of ~1/3, while the CoOxS0.18
sample shows a Tafel slope of 200 mV/decade. These transfer
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Acknowledgements
This work was supported in part by the National Science Foun-
dation (NSF) under award numbers CHE-1152922, CMMI-
1344562, CHE-1507753, DMR-1149036, and ACI-1440547. This
work made use of the Cornell Center for Materials Research
Shared Facilities, which are supported through the NSF MRSEC
program (DMR-1120296). This research used computational
resources provided by the Texas Advanced Computing Center
under Contract No. TG-DMR050028N and the Extreme Science
and Engineering Discovery Environment (XSEDE), which is sup-
ported by National Science Foundation grant number ACI-
1053575. We also thank Gregg McElwee of the Cornell Geo-
chemistry Laboratory for analytical measurements.
Figure 5. Calculated DFT adsorption energies of hydrogen and their
correlation with experimental results for the activity of CoOxSy
nanoparticle electrocatalysts (error bars give the standard devia-
tion based on three independent measurements). At low doping
levels (e.g., CoOxS0.18) the experimentally-determined HER specific
activity of the particles substantially increases (red curve) while at
higher doping levels, activity decreases. This activity change is a
result of S2- strengthening the H adsorption energy on the CoO
surface (blue curve), bringing it closer to the H adsorption energy
on Pt(111) (blue dashed curve) at low S2- doping levels.
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sulfur is incorporated, the formation of the H* intermediate
becomes more energetically intensive and consequently im-
pedes the HER kinetics. We emphasize not just that isovalent
anionic dopants may be used to tune the hydrogen adsorption
free energy to optimize electrocatalytic activity but also that
the composition of the doped electrocatalyst must be precisely
tuned in order to exploit the surface’s ability to facilitate an
electrochemical reaction. To emphasize the importance of the
anion control, we show through our HER measurement on
sulfur-rich CoS2 nanoparticles that the HER electrocatalysis on
CoS2 is also not as high our Co-oxysulfide nanoparticles (see
Fig. S7-S8). Recent results from others also reach a similar con-
clusion as ours, highlighting the need for an anion control .41, 42
In summary, we report the critical role of an isovalent ani-
onic substitution in controlling the electrocatalytic activity of a
transition-metal-containing electrocatalyst. As shown in the Co
oxysulfide system, anionic substitution can positively affect the
HER activity; however, the substitution process must be tightly
controlled to obtain the desired enhancement. In our cobalt
oxysulfide nanoparticles, we accomplish this control by engi-
neering the anion-exchange driving force by adjusting the
amount of the highly reactive S2- precursor. Using this proto-
col, we produce a series of cobalt oxysulfide nanoparticles
with different amounts of S2-, which allow us to establish the
trend between the HER activity and the sulfide content. We
find a small amount of S is best for enhancing the HER; further
sulfur addition disrupts the H* intermediate energy and also
alters the phase of the oxysulfides. We attribute the activity
enhancement at low S2- concentration to a ‘dopant’ effect,
whereby the presence of the S2- in the surface layer modifies
the hydrogen adsorption on the oxysulfide electrocatalyst. Our
results highlight an opportunity to control the anionic compo-
sition as a strategy for not just clarifying the role of the anions
but also for further increasing the activity of electrocatalysts.
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