Cobalt Oxide Materials for Oxygen Evolution Catalysis via Single-Source Precursor Chemistry

Article abstract of DOI:10.1002/chem.201802632

The utilization of metal alkoxides as single-source precursors for (mixed-)oxide materials offers remarkable benefits, such as the possibility to precisely control the metal ratio in the resulting material, highly homogeneous distribution of the elements in the film, and the low temperatures required for film processing. Herein we report on the isolation and characterization of the bimetallic Co-Mo alkoxide [Co₃Mo₄O₁₀(OCH₃)₁₀(dmf)₄] (Co₃Mo₄; dmf=N,N-dimethylformamide), which was prepared by the anion metathesis reaction of the corresponding metal chlorides. The Co-Mo alkoxide was explored as a well-defined precursor of cobalt oxide catalysts for the oxygen evolution reaction (OER) in alkaline electrolyte MOH. The catalysts demonstrated excellent activity in the OER, manifested in low onset potentials and Tafel slopes and superb stability under the operating conditions both in alkaline and nearly neutral media. It was observed that the nature of the metal cation of the alkaline electrolyte MOH (M⁺=Li⁺, Na⁺, K⁺, Cs⁺) greatly affected the catalytic performance of the material. We propose that the positive effect of larger metal cations on the film activity in the OER could be explained by the higher hydration enthalpies of larger ions and enhanced mass transport within a larger interlayer space between the [CoO₂]^δ?_∞ sheets of the in situ formed binary oxides. It may be deduced that this trend is universal and may be extended to other types of metal oxides forming layered structures during the OER.

Full text of DOI:10.1002/chem.201802632

A Journal of
Accepted Article
Title: Cobalt oxide materials for the oxygen evolution catalysis via
single-source precursor chemistry
Authors: Denis A. Kuznetsov, Dmitry Konev, Sergey Sokolov, and Ivan
Fedyanin
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To be cited as: Chem. Eur. J. 10.1002/chem.201802632
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Cobalt oxide materials for the oxygen evolution catalysis via
single-source precursor chemistry
Denis A. Kuznetsov,*[a] Dmitry V. Konev,[a,b] Sergey A. Sokolov,[c,d] Ivan V. Fedyanin,[e]
Abstract: Utilization of the metal alkoxides as the single-source
precursors for the (mixed-)oxide materials offers remarkable benefits
such as possibility to precisely control the metal ratio in the resulting
material, highly homogeneous distribution of the elements in the film
and low temperatures required for films processing. Here we report
on the isolation and characterization of the bimetallic Co-Mo alkoxide,
electrolyzers, is a cherished target for the whole scientific society
as this potentially represents a long-awaited solution to a global
demand for a renunciation of exhaustible fossil fuels. Valuable
products (H2 - for the case of water reduction; CO, methanol,
hydrocarbons - for CO2 reduction) are produced during the
cathodic half-reaction, whereas an anodic process as a rule
[Co3Mo4O10(OCH3)10(dmf)4] (Co3Mo4) (dmf = N,N-dimethylformamide), releases no value-added O2 gas. The overall efficiency of such a
which was prepared through the anion metathesis reaction from the
corresponding metal chlorides. The latter was explored as a well-
defined precursor for the oxides explored as the catalysts for the
oxygen evolution reaction (OER). These catalysts demonstrate
excellent activity in OER that manifests in low onset potentials and
Tafel slopes and superb stability under operating conditions both in
alkaline and nearly neutral media. It was observed that the nature of
the metal cation of the alkaline electrolyte MOH (M+ = Li+, Na+, K+,
Cs+) greatly effects the catalytic performance of the material. We
propose that the positive effect of the larger metal cations on the film
activity in OER could be explained as a result of the higher hydration
enthalpies of larger ions and better mass transport within larger
interlayer space between [CoO2]δ-∞ sheets of the in situ formed binary
oxides. It may be deduced that this trend is universal and may be
device, however, is limited by the performance of both electrode
components, and for the particular case of water splitting, OER
overpotential limits to a considerable degree the overall efficiency
of the electrochemical cell[1,2]. Currently, the benchmark for the
OER catalysts are Ru and Ir oxides, however low abundance of
these metals precludes their large scale application. In this regard,
non-noble metal catalysts, particularly those based on Ni(Fe)[3]
[5,6]
and Co[4] oxo-hydroxides or perovskite oxides
which
demonstrate comparable or even superior performance
compared to RuO2 and IrO2, emerged as a promising option to
replace the noble metal oxides in the state-of-the-art devices.
Apart from tuning the intrinsic activity of the catalyst, which
is determined by its structure and composition, nanostructuring of
the material could greatly contribute to its overall performance.
extended to the other types of metal oxides forming layered structures. Increase of the specific surface area could be achieved, for
instance, by leaching or electrochemical dissolution of some of
the bulk constituents[7–11]
. Following this argumentation, a
synthetic protocol, which includes preparation of the multimetallic
material whose certain constituents are then removed thus
leaving porous, high surface area structure, could be devised[12,13]
Heterometallic alkoxides seem to be an excellent option to
accomplish this goal, as they are readily decomposable upon
hydrolytic or thermal treatment producing homogeneous
heterometallic oxides films whose element ratio is precisely
defined by the precursor stoichiometry. Moreover, the potential
for the tuning of the metal combinations is restricted only by
inherent thermodynamic instability of particular metal-oxygen
Introduction
.
Cost-effective conversion and storage of sunlight energy in form
of energy of chemical bonds, which could be accomplished either
directly - in the photoelectrocatalytic devices, or indirectly – in the
[a]
Dr. D. A. Kuznetsov, Dr. D. V. Konev
Institute of Problems of Chemical Physics, Russian Academy of
Sciences, Chernogolovka, Moscow region, 142432, Russian
Federation
bonds, that makes alkoxide-precursor-based approach
universal means for production of the oxide coatings[14–16]
particularly for the energy applications[13,17–22]
a
,
E-mail: kuznetsov.denis.alex@gmail.com
Dr. D. V. Konev
D. I. Mendeleev University of Chemical Technology of Russia,
Moscow, 125047, Russian Federation
.
[b]
[c]
[d]
[e]
Another factor contributing to the efficiency of the
electrolysis cell is the electrolyte species which, apart from the
buffering role, can affect the catalyst performance through the
structural tuning of the latter via cations or anions inclusion into
the catalyst structure or via cation/anion effect on the interfacial
water structure[23,24]. Dependence of the OER activity on the
electrolyte nature, particularly on the cation type[25–28] is frequently
observed. To explain this phenomenon, stabilization of the
surface-bound anionic oxygen intermediates through the non-
S. A. Sokolov
Department of Chemistry, M. V. Lomonosov Moscow State
University, Moscow, 119991, Russian Federation
S. A. Sokolov
Institute of Nanotechnology of Microelectronics, Russian Academy
of Sciences, Moscow, 119991, Russian Federation
Dr. I. V. Fedyanin
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences, Moscow, 119991, Russian Federation
Supporting information for this article is given via a link at the end of
the document.
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covalent interactions with counter-cations that might affect the
reactivity of the surface[27,28] is usually proposed. In this work,
based on Pourbaix diagrams of cobalt and results of the energy-
dispersive X-ray (EDX) spectroscopy, X-ray photoelectron
spectroscopy (XPS) demonstrating that alkaline metal
incorporates into the catalyst structure, we speculate that, in
addition to the electronic effects discussed above[27,28]
,
improvement of activity can also be due to the lower hydration
enthalpies of the larger alkaline cations and due to reduced mass
transfer limitations in the larger [CoO2]δ- interlayer space for
∞
bigger cations in the binary oxides M+ CoO2 in situ formed in
x
M+OH electrolyte. We may assume that this mechanism is also
applicable for the other types of metal oxides which tend to form
layered structure during OER.
Results and Discussion
Synthesis of the molecular precursor. Solid-state structure
of [Co3Mo4O10(OCH3)10(dmf)4]
Interest to molybdenum as a dopant for the cobalt- and nickel-
based materials for water oxidation catalysis had arisen as a
result of a number of reports describing high activity of the
catalysts containing molybdenum as a counterpart to Ni, Co,
Fe[13,29–33]. This urged us to investigate compounds comprising
these non-noble metals[13] in combination with molybdenum to
produce materials with high electrolcatalytic activity towards water
oxidation.
Complex [Co3Mo4O10(OCH3)10(dmf)4] (Co3Mo4) was
prepared in a single step in the reaction of joint alcoholysis of
CoCl2 and MoO2Cl2 in the presence of the stoichiometric amounts
of Mg(OCH3)2 at elevated temperature. As expected, thermal
treatment caused formation of the oxo-groups (through the
dialkylether elimination reaction[34,35]) that eventually led to the
isolation of a heptanuclear complex Co3Mo4 from the reaction
mixture.
Figure 1. X-ray molecular structure of [Co3Mo4O10(OCH3)10(dmf)4] (Co3Mo4) as
represented (a) by thermal ellipsoids and (b) by coordination polyhedra.
Hydrogen atoms on (a) are omitted for clarity. Ellipsoids on (a) are drawn at
30 % probability level. Color code: Co – green; Mo – brown; O – red; N – blue;
C – grey; H – white.
Thin film deposition and characterization
Planar centrosymmetric structure of Co3Mo4 comprised of
seven edge-sharing octahedra is typical for oxo-compounds,
particularly of cobalt and molybdenum[36–39]. Three cobalt(II) and
four molybdenum(VI) atoms are interconnected through the
network of μ2- and μ3-methoxo- and oxo-bridges giving rise to a
disc-like structure reminiscent to that of some bulk oxides
(including the polymeric CoOx(OH)y sheets deduced to be the
structural elements of CoPi OER catalysts developed by Nocera
et al.[38,40,41]). In this manuscript we will discuss the utilization of
Co3Mo4 as a precursor for the single-step deposition of oxides on
conducting substrates to catalyze OER.
Polycondensation of metal-organic compounds, particularly metal
alkoxides to form metal oxides could be performed under mild
reaction conditions via sol-gel process[15]. The first synthetic step
represents hydrolysis of the metal alkoxides to produce sol/gel of
hydrated metal oxides/hydroxides, which can then condense to
form extended M-O-M chain of the nanoparticulate or bulk oxide
phase through the dehydration or ether elimination reaction[16]
.
Such single-source precursor (SSP) based approach offers
notable advantages over conventional techniques, as the
molecular precursor can in principle ensure the atomic level
control over the product stoichiometry, elemental homogeneity in
the resulting oxide nanostructure which can be tuned to meet the
requirements of specific application. Overall, SSP approach has
a great potential for the synthesis of the inorganic energy-related
materials as it provides a tool for the precise control over material
composition and surface properties via the fine tuning of the
precursor structure and treatment conditions[42]. In this and
following section, we exemplify the advantages of the SSP-based
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Figure 2. (a) SEM images of (a) as-deposited Co3Mo4 film and (b) Cocat film. (c) Survey X-ray photoelectron (XPS) spectrum and (d) Co 2p region recorded for
Cocat film. Only the most intensive peaks corresponding to film constituents (Co, O, Na) are shown. Peaks corresponding to Sn, In are from ITO, and F is from
Nafion binder. Carbon peak was set at 284.8 eV. Films for SEM imaging were deposited on glassy carbon substrate, for XPS measurements – on ITO coated
glass substrate. Cocat films were obtained by cycling and galvanostatic treatment of pristine material (see Experimental details). Inset on (c) shows schematic
structure of the sodium salt of cobalt oxo-hydroxide [(CoOO)xNay].
approach by demonstrating utilization of the alkoxide complex
Co3Mo4 for the one-step preparation of the cobalt oxide-based
films characterized by the high catalytic activity and remarkable
stability in oxygen evolution (OER) reaction.
the electrochemically active surface area of a given material to
that of the atomically flat substrate). Interestingly, no notable
increase of the ECSA was observed in the course of cycling,
implying fast removal of Mo from the film (occurring presumably
once the electrode is immersed in electrolyte). The importance of
the presence of Mo in the precursor film can be highlighted by the
comparison of Co catalyst (Cocat) ECSA with that for various
electrodeposited and cast films for which the reported ECSA
Electrodes for catalytic activity evaluation were initially
prepared by drop-casting suspension of Co3Mo4 in
isopropanol/water (+ Nafion binder) mixture onto pretreated[43]
glassy carbon surface. Screening of the electrochemical
properties was performed in a conventional three-electrode setup
in one molar alkaline electrolyte following recently suggested
values are typically of the order of tens[1,45]
.
The Co 2p3/2 spectra of electrochemically treated film
showed spectral features characteristic for the compound
containing cobalt in the oxidation state Co3+. The overall line
shape or envelope in such Co 2p3/2 spectrum arises from multiplet
splitting peaks with principle line 780.0 eV, analogous to previous
XPS results[46,47]. Absence of a peak in a region ca. 6 eV higher
principle line, typical for multielectron excitation[46,48], suggests
Co3+ as the major oxidation state of cobalt in the sample,
confirming that Co3Mo4 converts mostly to cobalt oxo-hydroxide
CoOx(OH)1-x during electrochemical treatment.
benchmarking protocol[1,44,45]
.
Apart from electrochemical
investigation, films were investigated by means of scanning
electron microscopy (SEM), EDX and XPS measurements before
and after electrochemical treatment.
Co3Mo4 precursor is deposited as a relatively flat film with a
highly homogeneous distribution of elements and without any
elemental impurities as confirmed by EDX and XPS analysis (see
Figs. 2a, 2b). Before electrochemical testing, the film was
preconditioned by cycling and galvanostatic treatment in the
respective 1 M MOH electrolyte. As expected[13], such treatment
resulted in a complete removal of molybdenum from the film, that
lead to formation of the material characterized by the enhanced
surface area with estimated roughness factor 300±120 (ratio of
The assignment of Cocat as cobalt oxo-hydroxide is also
indirectly confirmed by CV measurements. 2 sets of peaks on CV
curves in a region 0.4 – 1.5 V vs. RHE (Fig. S3) correspond to
Co2+/Co3+ and Co3+/Co4+ transitions[49–51]. The oxidation at 1.1 V
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Figure 3. (a) Representative linear sweep voltammetry (LSV) curves and (b) derived Tafel plots recorded for Cocat in different 1 M MOH electrolytes (M = Li, Na,
K, Cs). The values of Tafel slopes are 59 ± 3, 51 ± 9, 47 ± 3, 46 ± 6 mV decade-1 for Li+, Na+, K+, Cs+-intercalated species, respectively. Tafel plots for RuO2 and
IrO2 (extracted from Ref. [45]) are given for comparison. (c) Galvanostatic curves recorded for Cocat in 0.1 M sodium borate buffer (NaBi, pH 9.2) and 1 M NaOH.
Electrodes were held at the constant current density of 10 mA cmgeo-2. Catalyst films for galvanostatic measurements were deposited on pyrolytic carbon electrodes.
(d) OER activity of Cocat as represented by Tafel slopes and values of potentials to achieve current densities of 1 and 10 mA cmgeo-2 as a function of the nature of
MOH electrolyte cation (M = Li, Na, K, Cs). Data points represent average values extracted from LSV curves, error bars are standard deviations from at least 3
independent measurements.
vs. RHE results in a notable increase of the capacitance current
assuming increase of the electrochemically active surface area
(ECSA) after this redox event which can be accounted for the
formation of the oxo-hydroxide phase comprised of oxide layers
more accessible for electrolyte compared to hydroxide phase.
According to SEM, Cocat films consist of 2 types of
landscapes: domains with amorphous porous structure and
regions comprised of the thin hexagonal plates (Fig. 2b, S1),
similar to the reported observations for cobalt oxo-hydroxide
species[46]. Our data also clearly demonstrate the role of
molybdenum in cobaltous films which was not explained in
literature reporting on the investigation of transition metals (Ni, Co,
Fe) molybdates for OER. Our study provides an evidence that
dissolution of molybdenum upon reaction with alkaline electrolyte
and potential cycling occurs, that results in formation of film with
high surface area, similar to the effect of chromium doping in Ni-
Fe oxo-hydroxides[7].
Effect of the alkaline metal cation and pH on OER activity
Electrocatalytic activity of Cocat in oxygen evolution reaction
(OER) was assessed through linear sweep voltammetry (LSV)
measurements, and was demonstrated to possess remarkable
pH and cation dependence. Figure 3 shows that OER activity of
Cocat strongly depends on the nature of M+ cation when tested in
1 M MOH electrolyte (M+ = Li+, Na+, K+, Cs+) where the activity
increases in a row Li+ < Na+ < K+≈ Cs+. Notably, the Tafel slopes
also scale with cation size: 59 ± 3, 51 ± 9, 47 ± 3, 46 ± 6 mV
decade-1 for Li+, Na+, K+, Cs+-intercalated species, respectively,
rendering the highest activity for K+ and Cs+-intercalated oxides,
reaching 10 mA cm-2 OER current density at the potential ca. 1.58
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V vs. RHE, which is on par with noble metal RuO2 and IrO2 oxides
(Fig. 3b, S5) and other state-of-the-art materials (Table S1).
The cation effect on activity was previously observed for
different reactions including OER[25–28,52–54], oxygen reduction
reaction (ORR)[23,55], CO reduction[56] where dependence of
activity on electrolyte nature is typically attributed to the difference
in binding energies of the electrolyte cations with surface oxygen
species and to the changes of the interfacial water structure. Here
we propose that the evolution of the catalyst structure triggered
by cation substitution may also be responsible for changes in
activity.
solution processed single-source precursor chemistry is an
appealing approach for the preparing of the multifunctional
coatings[17] for conducting/semiconducting materials on
a
commercial scale, due to simplicity of the deposition procedure,
its scalability and versatility of the potential precursors. In this
respect, high OER activity maintained even at nearly neutral pH,
is hugely beneficial as providing an opportunity for the use of Cocat
as the coating in photoelectrochemical cells utilizing natural water
resources.
From Pourbaix diagram of Co[49] and from our XPS and EDX
measurements, it clearly follows that under OER conditions,
cobalt exists in form of layered [CoOO]δ- species where counter-
cations are located in the interlayer space. The increase of cation
size results in a larger interlayer separation, which can benefit the
mass transport of the substrates and products (water and oxygen)
in the interlayer space, therefore effecting OER reaction kinetics.
Similarly, the positive effect of the larger interlayer distance on the
OER activity was recently demonstrated for model Mn-based
catalyst, where layers separation strongly correlated with catalytic
activity[53]. Additionally, it was shown for another class of layered
materials, MXenes, that the presence of cations in the interlayer
space leads to decrease of the diffusion coefficient of water in M+-
intercalated material[57] presumably due to the formation of stable
[M+(H2O)n] hydration complexes. Similarly, an analogous effect
might be deduced to be responsible for the decreased activity of
Cocat in alkaline solutions comprising smaller cations, which are
characterized with more negative hydration enthalpies[58] and
therefore form more stable hydrates. We should also note here
that inductive effect[59] from intercalated cations might also
influence the activity.
Conclusions
We have synthesized and fully characterized the novel bimetallic
compound [Co3Mo4O10(OCH3)10(dmf)4] (Co3Mo4) (dmf = N,N-
dimethylformamide). Well-defined molecular complex Co3Mo4
was drop-cast on conducting substrates to serve as a precursor
for the oxide films which were obtained in situ during cycling of
the electrodes in basic electrolytes at OER potentials.
Molybdenum was shown to be removed from the bimetallic films
providing high surface area films of cobalt oxo-hydroxides. These
electrodes demonstrate very high OER activity which is a pH and
cation-dependent. Both OER currents and Tafel slopes scale with
the type of cation of the alkaline electrolyte where the lowest
currents and highest Tafel slopes are characteristic for the films
investigated in MOH electrolytes with smaller M+. Increase of
activity upon transition from Li+ to Cs+ can be attributed to the
weaker binding of the larger cations to the surface oxygen species
and to a larger distance between [CoO2]δ- sheets of the catalyst
and higher enthalpies of formation [M(H2O)n]+ hydrates enhancing
water mobility in the interlayer space. The catalyst also
demonstrates excellent activity in nearly neutral electrolyte
(borate buffer, pH 9.2) making it a competing candidate for the
use in the photoelectrocatalytic devices.
Another notable feature of Cocat is a pH-dependence of
OER activity. Galvanostatic measurements represented on Fig.
3c clearly shows that OER activity of the film is substantially
higher when tested under high pHs, as manifested in lower
-
overpotentials required to achieve current density of 10 mA cmgeo
Experimental Section
2. Generally, the pH-dependent OER activity arises if the surface
deprotonation is involved in either rate-limiting step or a chemical
step preceeding the rate limiting one[60]. Such pH dependence is
observed for many highly active OER catalysts, such as Ni-Fe
oxo-hydroxides[61], certain RuO2 crystal orientations[62], some
Materials and reagents
All synthetic manipulations were carried out using standard Schlenk
techniques under an argon atmosphere. Methanol and acetonitrile were
dried upon refluxing with magnesium methoxide and CaH2, respectively.
Dimethylformamide (dmf) was distilled twice under reduced pressure.
Solvents were stored over molecular sieves (4 Å for dmf, 3 Å for methanol
and acetonitrile) under an argon atmosphere. Solution of Mg(OCH3)2 was
prepared by the reaction of magnesium turnings with methanol.
Commercially available MoO2Cl2 (Aldrich) was used as received.
Anhydrous CoCl2 was obtained by heating CoCl2·6H2O at 2500C in vacuo
for 12 h. Solutions for electrochemical measurement were prepared from
high purity LiOH, NaOH, KOH, CsOH.
perovskite oxides[63]
.
Despite being lower at less alkaline pH, the OER activity
and stability of Cocat in nearly neutral media (borate buffer, pH
9.2) is remarkable, as the gradual loss of activity upon cycling in
neutral media is a common phenomenon for many transition
metal oxides[64], whereas for Cocat currents remain stable even
after prolonged galvanostatic hold (fluctuations on E-t curve are
due to intensive O2 bubbles formation). Similarly high stability was
also observed when Cocat was tested in 1 M alkaline electrolytes,
-2
yielding 10 mA cmgeo OER current at 1.58 V vs. RHE under
Synthesis of [Co3Mo4O10(OCH3)10(dmf)4] (Co3Mo4) and X-ray structure
characterization
optimized conditions placing this material among the most active
and stable OER catalysts, which clearly outperforms other
reported molecular precursor-derived catalysts[21,22,65–67]
.
CoCl2 (1.0 mmol) solution in methanol (2 ml) and MoO2Cl2 (1.0 mmol)
solution in methanol (5 ml) were consequently added to a Schlenk flask.
To this mixture, Mg(OCH3) (2.0 mmol) in methanol (3 ml) was added.
Deep-red solution obtained was refluxed for 30 min and then left stirred
High catalytic activity and stability makes Cocat an attractive
option for the practical utilization in electrocatalytic devices, as the
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overnight at room temperature. Solution was decanted from pink
crystallinic precipitate and the latter was washed with methanol (2 + 2 ml).
To this wet pink residue (vacuum drying results in formation of unreactive
blue solid), acetonitrile (2 ml) and dmf (1 ml) were added producing bright
blue solution from which large pink prismatic crystals precipitated in a
course of 1 week (ca. 0.5 ml CH3OH should be added to this solution if
precipitation is not observed). Crystals were washed with methanol (2 + 2
ml) and gently dried affording Co3Mo4 in 5-10% yield. IR (cm−1): 2941 w,
2827 w, 1658 s, 1500 w, 1437 w, 1419 w, 1384 m, 1254 w, 1113 w, 1045
m, 1011 s, 931 s, 903 vs, 894 s, 681 vs. Calc. for C22H58O24N4Mo4Co3,
(%): C, 19.97; H, 4.42; N, 4.23. Found, (%): C, 20.52; H, 4.76; N, 4.19.
All electrochemical measurements were carried out at room temperature
using Autolab PGSTAT 302 N or Elins P-45X potentiostats controlled by
Nova or ES8 software, respectively, in a single-compartment cell using
conventional three-electrode setup. Co3Mo4-derived thin film-coated
glassy carbon (GC), pyrolytic carbon (PC) and ITO-coated glass
electrodes were prepared, as described below, and used as working
electrodes. Saturated silver chloride Ag/AgCl, KCl(sat.) [EAg/AgCl, KCl(sat.)
=
0.197 V vs. NHE] electrode was used as a reference electrode and a
platinum mesh was employed as a counter electrode. Potentials are
reported against RHE, recalculations were performed using the Nernst
equation: E (V vs. RHE) = E (V vs. Ag/AgCl, KCl (sat.)) + 0.197 + (0.059 ×
pH). Cyclic voltammetry (CV) and linear sweep voltammetry (LSV)
measurements were automatically corrected for IR drop using Nova or
ES8 software. Chronopotentiometric measurements were manually
corrected for IR drop. Electrical Impedance Spectroscopy (EIS) was used
to determine R value which was typically in the range 7–14 Ohm for 1 M
alkaline electrolyte solutions and 140-160 Ohm for 0.1 M sodium borate
solutions. Electrochemical capacitance was determined using CV
measurements in a potential range where there is a non-Faradaic current
response (on a plateau between Co2+/3+ and Co3+/4+ waves: 1.2 – 1.3 V vs
RHE).
X-ray diffraction data for Co3Mo4 were collected on a Bruker APEX DUO
diffractometer (λ(CuKα) = 1.54178 Å, 2θ < 141.97°, φ and ω scans with
0.6° scan step and 4 to 8 s per frame exposure). Pink crystals of
C22H58Co3Mo4N4O24 at 120(2) K are monoclinic, space group P21/c,
a = 8.0813(4), b = 14.6514(5), c = 18.4954(6) Å, β = 96.478(2)°,
V = 2175.92(15) Å3, Z = 2, dcalc = 2.020 g cm-3. The indexing revealed that
the structure was twinned. Frames were integrated by a narrow-frame
algorithm using SAINT software package[68], both twin domains were
included in the integration. Semi-empirical absorption correction was
applied with TWINABS[69] program using intensity data for equivalent
reflections. Intensities of 4206 independent reflections (Rint = 0.0918) out
of 46394 collected were used in structure solution and refinement. The
structure was solved by direct methods and refined by the full-matrix least-
squares technique against F2 in the anisotropic-isotropic approximation.
The positions of hydrogen atoms were calculated. Hydrogen atoms were
refined in riding model with Uiso(H) equal to 1.5 Ueq(C) and 1.2 Ueq(C) of
the connected methyl and other carbon atoms. The refinement converged
to R1 = 0.0534 (calculated for 3915 observed reflections with I>2σ(I)),
Electrode preparation
Glassy carbon (GC) electrodes (Sigradur® G) with a diameter of 3 mm or
pyrolytic carbon (PC) electrodes (Volta) with a diameter of 3 mm were
polished consequently with alpha alumina powders of decreasing size
suspended in distilled water. After each polishing step, the electrodes were
thoroughly rinsed with bidistilled water. Then the electrodes were
ultrasonicated consequently in bidistilled water and absolute ethanol for 10
seconds each, and dried with compressed air. Subsequently, the polished
wR2 = 0.1551 and GOF = 1.073. The refinement calculations was
[61]
GC electrodes were pretreated electrochemically as described in ref.[43]
.
performed with SHELX software package
. Atomic coordinates, bond
lengths and angles and thermal parameters have been deposited at the
Cambridge Crystallographic Data Center, CCDC number 1534944.
The bare working electrode underwent 20 CV cycles at a potential window
of 0.5 to 1.9 V vs. RHE at a scan rate of 100 mV s-1 in 1 M NaOH or KOH.
Finally, the pretreated electrode was rinsed with absolute ethanol and
dried with compressed air. No electrochemical pretreatment was
performed for the PC electrodes. Before deposition, Co3Mo4 (8 mg) was
suspended in 1 ml of H2O + isopropanol mixture (3:1 v/v) to which 20 μL
of Nafion (10 %) binder was added. Upon 30 min of sonication, 15 μL of
the Co3Mo4 suspension was drop-cast portionwise onto electrode surface
using microsyringe, and the solvent was allowed to evaporate in air. The
catalyst loadings were 1.70 mg cm-2 (0.36 mg cm-2 based on active mass
in form of cobalt(II) hydroxide Co(OH)2). Dried film was then covered with
drop of 1 M alkaline solution and was left to dry overnight in air. ITO-coated
glass (Lumtec, resistance 4-6 Ω sq-1, 0.7 mm thickness) sheets (2.5 ˟ 7.5
cm2) were cut into 2.5 ˟ 0.5 cm2 slides and were cleaned by consequent
ultrasonication in acetone, bidistilled water and finally in isopropanol for 5
min each. Electrodes were fabricated by drop casting the Co3Mo4
suspension as described previously[13]; catalyst loadings were the same
as for GC and PC electrodes. Before SEM/EDX and XPS measurements
electrodes were pretreated by cycling in 1 M alkaline solution until the
currents stabilize and were then held at 2 mA cm-2 for 60 min.
Instrumentation
The infrared spectra were measured on solid samples using a Perkin
Elmer Spectrum 100 Fourier Transform infrared spectrometer.
Electron microscopy images were recorded using either FEI Nova
NanoSEM 230 with a field emission source and adjustment of the
accelerating voltage up to 30 kV with a precision step of 0.1 kV coupled
with an energy dispersive x-ray detector Bruker XFlash 5010 or Hitachi
SU8000 field-emission scanning electron microscope equipped with
Oxford Instruments X-Max EDX detector. Images were acquired in a
secondary electron mode with accelerating voltages 2 - 15 kV.
X-ray photoelectron spectroscopy (XPS) of the as-deposited Co3Mo4 and
the catalyst films obtained upon preconditioning of the as-deposited
Co3Mo4 (treatment with corresponding base, potential cycling and 60 min
electrolysis in 1 M alkaline solution at 2 mA cm-2) were carried out using a
KRATOS AXIS ULTRA DLD spectrometer (Kratos Analytical Ltd., United
Kingdom). The probed area for each sample was 300 x 700 μm2, and the
probing depth was 1–2 nm. Monochromatic X-rays were generated by an
Al Kα source (1486.6 eV), energy resolution 0.6 eV. Binding energies of
XPS spectra were calibrated against the C 1s peak of the adventitious
carbon (284.8 eV) as an internal standard.
The mean values of Tafel slopes and potentials to achieve the current
density of 1 mA cm-2 or 10 mA cm-2 (Fig. 3d) represent average values
extracted from LSV curves (Fig. S4), error bars are standard deviations
from at least 3 independent measurements.
AFM images were obtained using an NTEGRA PRIMA instrument (NT-MDT,
Russia).
Acknowledgements
The work was supported by the Russian Foundation for Basic
Research (grant № 16-33-00754 mol_a). The authors are grateful
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10.1002/chem.201802632
Chemistry - A European Journal
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