A Low-Temperature Molecular Precursor Approach to Copper-Based Nano-Sized Digenite Mineral for Efficient Electrocatalytic Oxygen Evolution Reaction

Article abstract of DOI:10.1002/asia.202000022

In the urge of designing noble metal-free and sustainable electrocatalysts for oxygen evolution reaction (OER), herein, a mineral Digenite Cu₉S₅ has been prepared from a molecular copper(I) precursor, [{(PyHS)₂Cu^I(PyHS)}₂](OTf)₂ (1), and utilized as an anode material in electrocatalytic OER for the first time. A hot injection of 1 yielded a pure phase and highly crystalline Cu₉S₅, which was then electrophoretically deposited (EPD) on a highly conducting nickel foam (NF) substrate. When assessed as an electrode for OER, the Cu₉S₅/NF displayed an overpotential of merely 298±3 mV at a current density of 10 mA cm^?2 in alkaline media. The overpotential recorded here supersedes the value obtained for the best reported Cu-based as well as the benchmark precious-metal-based RuO₂ and IrO₂ electrocatalysts. In addition, the choronoamperometric OER indicated the superior stability of Cu₉S₅/NF, rendering its suitability as the sustainable anode material for practical feasibility. The excellent catalytic activity of Cu₉S₅ can be attributed to the formation of a crystalline CuO overlayer on the conductive Cu₉S₅ that behaves as active species to facilitate OER. This study delivers a distinct molecular precursor approach to produce highly active copper-based catalysts that could be used as an efficient and durable OER electro(pre)catalysts relying on non-precious metals.

Full text of DOI:10.1002/asia.202000022

DOI: 10.1002/asia.202000022
Full Paper
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A Low-Temperature Molecular Precursor Approach to Copper-
Based Nano-Sized Digenite Mineral for Efficient Electrocatalytic
Oxygen Evolution Reaction
Biswarup Chakraborty+,[a] Shweta Kalra+,[a] Rodrigo Beltrán-Suito,[a] Chittaranjan Das,[b]
Tim Hellmann,[c] Prashanth W. Menezes,*[a] and Matthias Driess*[a]
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Abstract: In the urge of designing noble metal-free and
sustainable electrocatalysts for oxygen evolution reaction
(OER), herein, a mineral Digenite Cu9S5 has been prepared
from a molecular copper(I) precursor, [{(PyHS)2CuI(PyHS)}2]
(OTf)2 (1), and utilized as an anode material in electrocatalytic
OER for the first time. A hot injection of 1 yielded a pure
phase and highly crystalline Cu9S5, which was then electro-
phoretically deposited (EPD) on a highly conducting nickel
foam (NF) substrate. When assessed as an electrode for OER,
the Cu9S5/NF displayed an overpotential of merely 298�
3 mV at a current density of 10 mAcmÀ 2 in alkaline media.
The overpotential recorded here supersedes the value
obtained for the best reported Cu-based as well as the
benchmark precious-metal-based RuO2 and IrO2 electrocata-
lysts. In addition, the choronoamperometric OER indicated
the superior stability of Cu9S5/NF, rendering its suitability as
the sustainable anode material for practical feasibility. The
excellent catalytic activity of Cu9S5 can be attributed to the
formation of a crystalline CuO overlayer on the conductive
Cu9S5 that behaves as active species to facilitate OER. This
study delivers a distinct molecular precursor approach to
produce highly active copper-based catalysts that could be
used as an efficient and durable OER electro(pre)catalysts
relying on non-precious metals.
Introduction
store renewable electricity.[2] However, oxidation of water to
oxygen or OER is the most fascinating reaction occurs in natural
photosynthesis harvesting photon to convert solar energy to
chemical energy.[3] Although, by mimicking nature’s strategy,
artificial photosynthesis scheme has become a promising
approach for clean and sustainable energy production,[4] the
large thermodynamic barrier of the OER, and the complicated
reaction kinetics make this process potentially challenging.[5]
Thus, the design of an efficient catalyst for OER remains a
perdurable challenge and requires immense attention.[6] Cur-
rently, the oxides of Ir or Ru have been considered to be the
state-of-the-art electrocatalysts for OER, however, the exorbitant
cost and scarcity of these noble metals seriously limit their
large-scale practical applications.[7] A large number of transition
metal-based materials such as oxides,[8] chalcogenides,[9]
pnictides,[10] intermetallics,[11] phosphates,[12] phosphites[13] and
borophosphates[12,14] have been regarded as attractive electro-
catalysts for the OER and the further improvement of electro-
catalytic activity and durability through synthesis and design
methods is still a great challenge.[12–14,15]
Increasing consumption of fossil fuels by mankind leads to a
fast depletion of natural abundant fuels and therefore, an
urgent requirement of alternative resources to harvest energy
has become a global issue.[1] Spitting of water into oxygen (O2)
and subsequent production of hydrogen (H2), an alternative
non-carbonaceous fuel, is one of the promising approaches to
[a] Dr. B. Chakraborty,+ S. Kalra,+ R. Beltrán-Suito, Dr. P. W. Menezes,
Prof. Dr. M. Driess
Department of Chemistry
Metalorganics and Inorganic Materials
Technische Universität Berlin
Straße des 17 Juni 135, Sekr. C2, 10623 Berlin (Germany)
E-mail: prashanth.menezes@mailbox.tu-berlin.de
matthias.driess@tu-berlin.de
[b] Dr. C. Das
Karlsruhe Institute of Technology (KIT)
Institute for Applied Materials (IAM-ESS)
Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen (Ger-
many)
[c] T. Hellmann
Surface Science Division
Despite the low cost, environmentally benign, a high
electrical conductivity[16] and promising catalytic activity,[17] of
copper-based materials, only in a handful of cases, copper-
chalcogenides have been used for electrocatalytic OER
study.[17c,18] In this direction, Cu2S nanostructure was found to
be the best pure Cu-based anode material with considerably
good OER activity with a low overpotential of 336 mV (at
Department of Materials Science
Technical University Darmstadt
Otto-Berndt-Str. 3, 64287 Darmstadt (Germany)
[+] Authors contributed equally
Supporting information for this article is available on the WWW under
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non-commercial and no modifications or adaptations are made.
10 mAcmÀ 2).[18a]
A recent study has suggested that the
efficiency of CuS for OER could be enhanced by the incorpo-
ration of second transition metal ion into the structure.[19]
Similarly, some of the copper-chalcogenides (Cu2-xSe) -pnictides
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(Cu3N, Cu3P) and -oxides have also shown promising catalytic
OER activity along with long-term durability.[10c,20]
Within the various synthetic methods applied for the design
of OER electrocatalysts, the low-temperature molecular precur-
sor approach has recently shown enormous advantages such as
better atomic-level control over the stoichiometry, uniformly
presented study shows a promising scope to explore new
defined transition metal-based molecular structures to derive
the unexplored class of functional materials for electrocatalytic
applications beyond water splitting.
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distributed nano-sized particles, homogenous elemental dis- Results and Discussion
persion, and unique electronic and surface structure, over
traditional high-temperature techniques.[21] Taking advantage of
this approach, we recently demonstrated excellent electro-
catalytic water splitting performance of molecularly derived
FeSe2, FeP, CoP and NiPt3@NiS nanostructures starting from a
molecular complex supported by organic nitrogen donor
ligands.[9,10a,b,22] In this context, 2-mercaptopyridine, an organo-
sulfur compound, existing as an equilibrium of tautomeric
mixture of 2-pyridine-thiol (PySH) and 2-pyridine-thione
The molecular precursor complex, [{(PyHS)2CuI(PyHS)}2](OTf)2 (1),
was isolated as crystalline yellow powder from a mixture of
copper(II) triflate and 2-mercaptopyridine (with a molar ratio of
1:4) in dichloromethane (Scheme 1). The purity of the isolated
complex was verified by elemental analysis, which precisely
matched with the calculated values. The Fourier-transform
infrared spectrum (FT-IR) of the isolated complex depicts two
strong vibrations at 1127 and 1366 cmÀ 1 attributed to the
vibrations of ν(S=O) and ν(C–F), respectively, of the triflate
counter anion (Figure S1). The electrospray ionization (ESI) mass
spectrum of 1 (in tetrahydrofuran, positive ion mode) exhibited
major molecular ion peaks at m/z value of 284.96, 361.98 and
395.97 with an expected isotopic distribution calculated for the
molecular fragments, [{(PyHS)2Cu]+, [{(PyHS)(PyS)Cu(Py)]+ and
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(PyHS),[23] including
a
zwitterionic structure (PyH+SÀ )[24]
(Scheme 1, square box) has been utilized as versatile sulfur
donor ligand to isolate diverse transition metal complexes and
could be an attractive choice to produce metal sulfide
nanostructures.[25]
In this work, we report a simple single-step synthetic
protocol to the homodinuclear copper(I) complex, [{(PyHS)2CuI
(PyHS)}2](OTf)2 (1) with a square-planar {CuI2S2} core. Each
copper center in 1 adopts a distorted tetrahedral geometry
surrounded by four 2-pyridine-SH ligands and both copper
centers are connected via two bridging μ2-S donor atoms of
two terminal 2-pyridine-thione (PyHS) units. The {CuI2S2} core
complex has been utilized as an suitable low-temperature
molecular precursor to isolate the highly crystalline mineral
Digenite Cu9S5 as an efficient OER electro(pre)catalyst in an
alkaline electrolyte. The Cu9S5 nanostructures acquired through
the hot-injection of 1 have been deposited on highly conduct-
ing NF electrode substrate through the EPD technique without
affecting the chemical identity. The fabricated Cu9S5/NF electro-
catalyst requires an overpotential of only 298�3 mV to attain a
current density of 10 mAcmÀ 2, outperforming benchmark
copper-based and even noble metal-based OER catalysts. Apart
from low overpotential, the Cu9S5/NF has also been demon-
strated to be a stable catalyst on a longer run. Post-OER
analyses indicated the formation of crystalline CuO overlayer on
Cu9S5, which acts as an active phase for OER while the
conductive inner core accelerates the electron mobility. The
1
[(PyHS)3Cu]+, respectively (Figure S2). The H NMR spectrum of
1 in CD3CN revealed resonances of the aromatic protons at the
chemically indistinguishable pyridine ring of the 2-mercapto-
thiol ligand within the range of
7 to 12.5 ppm, which
unambiguously confirms the diamagnetic nature of the com-
plex (Figure S3). Moreover, the high-resolution Cu 2p X-ray
photoelectron (XPS) spectra obtained with 1 further confirmed
the presence of CuI while the valances of S 2p, N 1s, O 1s, and C
1s are also consistent with the structure determined from the
as-synthesized crystalline powders (Figure S4). Although
a
copper(I) complex, similar to 1, has been isolated previously
from
a
reaction mixture of copper(II) halide and 2-
mercaptopyridine,[26] the redox non-innocent behavior of 2-
mercaptopyridine, in-situ reduction of iron(III) to an isolable
mononuclear iron(II) complex, [FeII(PySH)4](ClO4)2, was reported
only recently.[25b]
Single-crystals grown from
a concentrated acetonitrile
solution of 1 belong to a monoclinic space group I2/m
(Table S1). X-ray structure determination confirmed the pres-
ence of two triflate counter ions along with a dinuclear copper-
sulphur cluster as dicationic entity, very similar to [CuI
Scheme 1. Synthesis of the dinuclear copper(I) complex 1, [{(PyHS)2CuI(PyS}2](OTf)2, using the 2-mercaptopyridine as supporting ligand. The square planar
{CuI2S2} core in 1 was used as a single-source precursor to prepare the crystalline Cu9S5 nanostructure of the Digenite mineral (right; Cu: purple; S: yellow).
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(C5H5NS)3]2X2 (X=Cl, Br, NO3)[26b,c] and [CuI2(dmpymtH)6]2
(dmpymtH=4,6-dimethylpyrimidine-2-thione).[26a] The dication
consists of an almost square-planar {CuI2S2} core where the
copper centers are distorted tetrahedral coordinated by two
terminal monodentate PyHS and two μ2-S-bridging PyHS
ligands (Figure 1). The Cu-S distances and SÀ CuÀ S angles are in
oleylamine (see Experimental Section).[9,10a,b,21,22] The crystalline
structure, composition and phase purity of the as-obtained
crystals of the particles was determined by powder X-ray
diffraction (PXRD) technique. The XRD analysis of the as-
prepared sample confirmed its high crystallinity and the
reflections obtained correspond to the pure mineral Digenite
Cu9S5 (JCPDS 47–1748) phase (Figure 2a). The solid-state crystal
structure of Cu9S5 belongs to a rhombohedral, R-3m with lattice
parameters a=3.930(1) Å and c=48.14(3) Å, respectively.[27] In
Cu9S5, the mixed-valence copper centers comprise three differ-
ent coordination geometries varying from trigonal planar to
near square pyramidal with CuÀ S distance of 2.285–2.880 Å
resulting in a three-dimensional network (Figure S5).[27] The
information on the atomic structure of Cu9S5 was provided by
the transmission electron microscopy (TEM), which contained
hexagon-shaped particles (Figure 2b). The high-resolution TEM
image clearly indicated the d(101) lattice fringes with an
interplanar distance of 0.33 nm (Figure 2c), further confirming
the purity of the Cu9S5 product.[28] Additionally, the selected
area electron diffraction (SAED) pattern of the particles
displayed well-defined diffraction rings corresponding to the
miller indices (101) and (1010) planes of Cu9S5 (Figure 2c, inset).
The morphological features of pristine Cu9S5 were investigated
by scanning electron microscopy (SEM) that exhibited a well-
defined morphology in the aggregated particles (Figure 2d) and
the subsequent elemental mapping of the particles confirmed
the homogeneous distribution of Cu and S (Figure 2e-2f). The
chemical composition of the particles was also evaluated by
energy-dispersive X-ray spectroscopy and inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analyses evi-
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°
the range of 2.293(1)-2.496(1) Å and 97.79(4) –123.42(5) ,
respectively, which are in good agreement with metric values
of previously reported related transition-metal sulfides
(Table S2).[25,26]
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Using the single-source precursor approach, a crystalline
°
black powder was isolated via hot-injection of 1 at 250 C in
Figure 1. Molecular structure of the dication of 1; thermal ellipsoid plot
(40% probability). Colour code; carbon: grey, nitrogen: blue, sulphur: yellow,
copper: red. All the hydrogen atoms of the protonated pyridine rings were
omitted for clarity. The dicationic unit of 1 is symmetry generated from the
half of its complete structure with a symmetry (’) À x+3/2, À y+1/2, À z+1/
2.
Figure 2. Structural characterization of as prepared CuS derived from molecular complex 1; (a) PXRD pattern displaying sharp reflections, corresponding to
Cu9S5 (JCPDS-47-1748), (b) TEM images of Cu9S5 showing hexagonal-shaped nanocrystals (c) High-resolution TEM image with atomic fringes associated with
the (101) lattice plane of Cu9S5, and SAED pattern exhibiting diffraction rings of Cu9S5 (inset), (d) SEM image of Cu9S5 particles with (e, f) EDX mapping
revealing a homogenous distribution of elements Cu (orange) and S (yellow) (oxygen shown in Figure S6).
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dencing the Cu:S ratio of ~1.80:1 (Figure S7, S8 and Table S3),
which is consistent with the atomic composition of Cu9S5.
XPS analysis was utilized to study the surface electronic
state and the composition of as-prepared Cu9S5. High-resolution
core-level Cu 2p XPS spectrum displayed two major peaks at
synthesized, electrodeposited and measured in a similar
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condition. Remarkably, the overpotential of Cu9S5/NF electrode
attained at 10 mAcmÀ 2 was only 298�3 mV, which was lower
than those of CuO/NF (340�4 mV), Cu/NF (390�3 mV), and
bare NF electrode (490�5 mV). Incidentally, the electrocatalytic
OER performance of Cu9S5/NF at higher current density (360�
4 mV at 100 mAcmÀ 2) was even better than CuO/NF (440�
4 mV), Cu/NF (550�3 mV), and noble metal-based RuO2/NF
(421�5 mV at 100 mAcmÀ 2) and IrO2/NF (419�3 mV at
100 mAcmÀ 2) electrocatalysts (Figure S13), making it one of the
superior OER catalysts relying on one of the most earth-
abundant first-row transition metals. Furthermore, the catalytic
activity achieved with Cu9S5/NF is much higher than the well-
established copper-oxide and/or hydroxides OER catalysts,
however, it is comparable to Cu3N, Cu3P, and Cu2Se
(Table S4).[10c,17e,18a,19a–c,31] Besides, the catalytic activity of the
presented catalysts is also almost comparable to highly-active
binding energy 932.1 and 952.2 eV, for 2p3/2 and 2p1/2
,
respectively. The binding energy values attained here clearly
indicated the presence of CuI with minor amounts of CuII, which
appeared due to surface oxidation of the materials and is
common in non-oxidic Cu-based materials (Figure S9a).[19a,29]
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Furthermore, the spin-orbit coupling spacing value (2p3/2–2p1/2
)
of 19.6 eV also supported the CuI valency of copper in Cu9S5
(Figure S9a).[29] The S 2p spectrum showed the existence of
doublet species with 2p3/2 binding energy located at 161.53 eV
and the 2p1/2 binding energy at 163.8 eV, consistent with the
presence of SIIÀ (Figure S9b).[29] The binding energies and the
chemical composition attained by XPS analysis are in accord-
ance with the earlier reports of Cu9S5 and Cu2S
materials.[17c,18a,b,d]
CoOx, NiOx, and Fe doped NiOx, NixFe1-x
O OER catalysts
(Table S4).[10a,32]
After a thorough microscopic and spectroscopic analysis,
the Cu9S5 powder was deposited on NF through the EPD
process.[11b,30] To ensure the chemical stability, the as-deposited
Cu9S5 on NF were further analyzed by microscopic and
analytical techniques, which demonstrated its unaltered stabil-
ity upon EPD (Figure S10–S12). The OER activities of the as-
synthesized Cu9S5/NF were investigated in a 1 M aqueous KOH
electrolyte using a three-electrode cell. The polarization curve
from the linear sweep voltammetry (LSV) displayed a continu-
ous increment in current density up to 500 mAcmÀ 2 at 1.7 V vs.
RHE (Figure 3a), which clearly revealed catalytic oxidation of
water. For comparison, CuO and Cu nanoparticles were also
The OER kinetics of Cu9S5/NF, CuO/NF and Cu/NF were
investigated by the Tafel slope. The Tafel plots (Figure 3b) of all
catalysts were calculated by their corresponding LSV polar-
ization curves. As observed in Figure 3b, the Tafel slope of
Cu9S5/NF electrode (67�3 mVdecÀ 1) is lower than those of
CuO/NF (92�4 mVdecÀ 1), and Cu/NF electrode (146�
3 mVdecÀ 1), indicating a more efficient electron transfer, more
favorable electrocatalytic reaction kinetics, and high catalytic
activity toward OER for Cu9S5/NF. The electron (or charge)
transfer capacity was further evaluated by electrochemical
impedance spectra (EIS) where Cu9S5/NF exhibited the lowest
charge-transfer resistance (RCT) than CuO/NF and Cu/NF imply-
Figure 3. Electrochemical OER activities of Cu9S5/NF, CuO/NF, and Cu/NF; (a) OER polarization curves from LSV, (b) Tafel plot, (c) Nyquist plot from EIS analysis
(inset: equivalent RC circuit) and (d) double-layer capacitance (Cdl) determination from CV scans in a non-Faradaic potential range. (The LSV and Tafel slopes
were recorded at a scan rate of 1 mVsÀ 1 in 1 M aqueous KOH solution).
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ing the smallest electron and charge transfer resistance and
thus fast electrode kinetics of Cu9S5/NF (Table S5). A better OER
activity of Cu9S5/NF was achieved due to low RCT, which
indicates a spontaneous electron transfer between the elec-
trode surface and electrolyte (Figure 2c).[33] To gain insights into
the intrinsic activity of the presented catalysts, we measured
the double-layer capacitances (Cdl), which could be associated
with electrocatalytic active surface areas (ECSAs). A Cdl of
1.51 mFcmÀ 2 was determined for Cu9S5/NF by collecting CVs in
a non-faradic region (Figure S14), which is higher than CuO/NF
(1.05 mFcmÀ 2) and Cu/NF (0.98 mFcmÀ 2) (Table S6, Fig-
ure 3d).[32f,34] From Cdl, ECSA was calculated and an ECSA of
0.89 cm2 was obtained for Cu9S5/NF, which is the best among
the other copper materials reported herein (Table S6). The ECSA
normalized current density obtained for Cu9S5 is also higher
compared to other copper catalysts used herein, proving a
superior intrinsic activity of the Cu9S5 catalyst (Figure S14d).
Eventually, the long-term durability of Cu9S5/NF catalyst was
assessed by a chronoamperometry (CA) measurement con-
ducted at 1.55 V (vs. RHE), which maintained a current density
above 10 mAcmÀ 2 (Figure S15) over 12 h indicating the inher-
ent stability of the catalyst on a longer run. Faradaic efficiency
of ~96% for OER was calculated by quantifying the evolved O2
(gas) during electrolysis at a constant current of 50 mAcmÀ 2
using Cu9S5/NF as a working electrode (Table S7).
To uncover the role of electrode substrate in electro-
catalysis, additionally, Cu9S5 was also deposited (via EPD) on
fluorine-doped tin oxide (FTO) substrate. Microscopic and
analytical studies confirmed the structural integrity of materials
deposited on FTO (Figure S16–S18). Under a similar electro-
chemical OER condition (in 1 M aqueous KOH electrolyte, three-
electrode set-up), the LSV of Cu9S5/FTO showed an over-
potential of 380�3 mV at a current density of 10 mAcmÀ 2
while an overpotential of 455�5 and 610�5 mV was required
for CuO/FTO and Cu/FTO to achieve the same current density
(Figure S19). Interestingly, the presented Cu-based materials
deposited on FTO followed a similar trend as that of NF in the
reaction of OER.
Recent reports on alkaline OER have suggested that under
the electrochemical condition the transition metal-based (non-
oxide) materials undergo transformation/corrosion forming an
in-situ generated metal-oxide/hydroxide/oxyhydroxide surface
overlayer, which is considered as the active species for water
oxidation.[10a,d,11,12–14,15e] To understand the structure of active
species generated during alkaline OER with Cu9S5, we con-
ducted a post electrochemical analysis of the (pre)catalyst using
multiple ex-situ microscopic and spectroscopic techniques. The
SEM images (Figure S20-21) and successive EDX elemental
mapping indicated a considerable amount of oxygen incorpo-
ration into the structure, in addition to the initial Cu and S of
Cu9S5 (Figure 4a-d). High-resolution TEM images of the material,
carefully isolated from the electrode surface, indicated the
presence of Cu9S5 with a thin crystalline overlayer (Figure S22).
This thin layer consisted of highly crystalline CuO (JCPDS 72-
629) as depicted by the atomic fringes of d(111) planes
(Figure 4e). However, atomic fringes at the core displayed d
(1010) planes, which belonged to the crystalline core of Cu9S5
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Figure 4. Microscopic characterization of Cu9S5 post electrochemical OER. (a)
SEM image and (b–d) EDX mapping of elements (Cu, S, and O), (e) high-
resolution TEM image after OER with atomic fringes of Cu9S5 (JCPDS 47-
1748) and CuO (JCPDS 72-629) and (f) SAED pattern exhibiting well-defined
diffraction rings Cu9S5 and CuO.
(JCPDS 47–1748),[27] and substantiated the formation of
CuO@Cu9S5 core-shell type structures.[18b,35] The SAED pattern
further affirmed this claim with well-defined rings of CuO and
Cu9S5 (Figure 4f). The elemental composition obtained from the
EDX measurement (Figure S23–S24) provided a Cu to S ratio
~1.6:1 and which is due to loss of S from the surface and
concomitant incorporation of O into the structure. The surface
leaching of S (~20%) in the electrolyte solution was also
confirmed by ICP-AES analysis (Table S8).
The surface chemical changes of Cu9S5 film post-OER were
determined by XPS. The comparison of the binding energy
values of Cu 2p, S 2p and spin-orbit splitting of Cu 2p (2p1/2
–
2p3/2), of the as-deposited Cu9S5 and post-OER films, confirmed
a surface structural change under prolonged electrochemical
condition. The core level Cu 2p spectra for the post-OER film
showed two prominent peaks at 933.3 eV and 953.6 eV for Cu
2p3/2 and Cu 2p1/2, respectively, indicating the formation of a
substantial amount of CuII on the surface compared to as-
deposited film (Figure 5a).[20b,29] Additionally, the presence of
two strong satellite peaks at 942.2 eV and 962.1 eV strongly
supports the formation of CuII on the surface. The S 2p core-
level spectra showed a broad and weak signal indicating a
negligible amount of sulfur on the surface (Figure 5b). However,
the O 1s spectrum after OER displayed a sharp peak at 529.8 eV,
suggesting the formation of CuO on the surface to act as an
active catalyst along with some amount of surface hydroxides
(Figure S25).[31a,36] The results attained from XPS are in good
Chem Asian J. 2020, 15, 1–9
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metal-based systems for widespread use in various applications,
especially for CO2 reduction, supercapacitors, and lithium-ion
batteries.
1
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5
6
Experimental Section
General considerations and instrumentation: Synthesis of molec-
ular precursor 1 and Cu9S5 manipulations were carried out under
dry oxygen-free nitrogen using standard Schlenk techniques.
Organic solvents (CH2Cl2, diethyl ether) were dried by standard
methods and freshly distilled prior to use. Commercially available
reagents; 2-mercaptopyridine, Cu(OTf)2 (98%) were purchased from
Sigma-Aldrich and used as received. The commercial RuO2 (99%),
IrO2 (99%), were purchased from Alfa Aesar. NF and FTO, resistivity
(8–12 Ω/sq) were obtained from Racemat BV and Sigma Aldrich
respectively. The solution NMR spectrum was recorded on Bruker
Spectrometers AV 500 with residual solvent signals as internal
reference (CD3CN). The following abbreviations were used to
describe peak patterns when appropriate: br=broad singlet, s=
singlet, d=doublet, t=triplet, dd=doublet of doublets, m=
multiplet. Fourier transform infrared (FTIR) spectra were recorded
on a Thermo Fisher Nicolet iS5 IR spectrometer (ATR-Diamond)
under inert conditions. High-resolution ESI-MS, APCI-MS were
measured on a Thermo Scientific LTQ orbitrap XL. Elemental
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Figure 5. High resolution (a) Cu 2p and (b) S 2p core-level XPS spectra
obtained for Cu9S5 as-deposited (black curves) and post OER films (red
curves). After OER, the Cu 2p indicated the oxidation of Cu to Cu+ to Cu2+
with typical satellite peaks and spin-orbit splitting distance while the
negligible amount of S was found on the surface illustrating the
concomitant incorporation of O into the structure (Figure S24) forming
CuO@Cu9S5
agreement with the conclusions achieved from SEM, elemental
mapping, EDX, ICP-AES, TEM, and SAED.
Overall, the higher catalytic activity of Cu9S5 for OER in
strongly alkaline conditions could be attributed to the forma-
tion of an in-situ crystalline CuO overlayer under applied
oxidation potentials that could form CuIIIO(OH) species to
facilitate OÀ O bond formation to catalyze the reaction of
OER,[10c,20,28,37] while Cu9S5 could act as a highly conducting core
structure.[20a,31a,37] This observation is in analogy to various Cu-
based heterogeneous catalysts applied for OER. Besides, the
high ECSA of Cu9S5 exposes more active sites for OER while a
low charge transfer resistance of Cu9S5 signifies better change
transport across electrode/electrolyte and vice versa. Finally, the
NF was found to be a good choice as an electrode substrate
due to its improved conducting nature, increased mass trans-
port and enhancing the dissipation of evolved O2 gas.
analyses were recorded in
a Thermo FlashEA 1112 Organic
elemental analyzer. Related details of crystallographic measurement
and the materials characterizations have been discussed in the
supporting information.
Preparation of [{(PyHS)2CuI(PyHS)}2](OTf)2 (1). To a well stirred
solution of Cu(OTf)2 (0.500 g, 1.38 mmol) in dichloromethane
(10 mL), was added a dichloromethane solution (15 mL) of 2-
mercaptopyridinethiol (0.615 g, 5.52 mmol). After stirring the
resultant yellow suspension for 8 h under inert atmosphere, solvent
was evaporated under vacuum. The residue was washed twice with
diethyl ether (2×20 mL), filtered and dried under vacuum. X-ray
quality crystals were obtained by keeping concentrated acetonitrile
°
solution of 1 at À 20 C overnight. Yield: 0.990 g (89%). Elemental
analysis calculated for C32H30Cu2F6N6O6S8 (1092.19 gmolÀ 1): C 35.26;
H 2.59; N 7.71; S 23.53; found: C 34.9, H 2.7, N 7.6, S 22.9%. IR (ATR,
diamond): ν=3183(br), 3056(m)-2981(m), 1571(s), 1499(s), 1441(s),
1366(s), 1269(m), 1222(s), 1127(s), 1028(s), 750(s), 636(s). ESI-MS
(positive ion mode, THF): m/z=284.96 [Cu(pySH)2], 395.97 [Cu
(pySH)3]. 1-H NMR (500 MHz, CD3CN): δ=12.5 (br, 6H), 7.88 (d, J=
4.6 Hz, 6H), 7.65 (m, 12H), 7.05 (t, J=5.6 Hz, 6H) ppm. CCDC
1975340 contains the supplementary crystallographic data for this
paper. This data is provided free of charge by The Cambridge
Crystallographic Data Centre.
Conclusion
A
facile low-temperature molecular precursor route was
presented to give the crystalline Cu9S5 nanostructure of the
Digenite mineral. The respective dicopper(I) precursor complex
1 is easily accessible through a simple synthetic protocol using
2-mercapto pyridine as supporting ligand and {Cu(OTf)2} as Cu
source The dinuclear CuI2S2 core in 1 is ideal for building the
structurally versatile Cu9S5 nano-sized particles, where mixed-
valence copper centers are present in three different coordina-
tion geometries. The distinct Cu9S5 nanostructure, when
deposited on electrode substrates, behaves as a potent anode
for performing OER in alkaline media displaying considerably
low overpotential. A systematic post-OER study revealed that
the as-prepared Cu9S5 is a pre-catalyst and under applied
potentials, forming an in-situ crystalline CuO overlayer that acts
as active sites for OER while the superior conductivity of the
Cu9S5 core promotes a facile charge transport between the
catalytic sites to the electrode surface. This facile synthetic
method can easily be modified and expanded to the design of
high-performance catalysts based on earth-abundant non-noble
Synthesis of Cu9S5. To a three-necked round bottom Schlenk flask
fitted with a temperature sensor and a condenser, 25 mL olelyl-
amine (Fisher Scientific) was added. The solvent was degassed by a
3-cycle freeze-pump method. The whole set-up was degassed using
vacuum followed by refilling with nitrogen three times and then
°
the flask was heated to 250 C. The precursor (0.500 g, 0.5 mmol)
°
was dissolved in 5 mL of dry oleylamine at 30 C in another flask.
°
The solution was transferred to the three-necked flask at 250 C by
injection under inert conditions. The reaction temperature was
maintained at 250 C for one more hour and then the mixture was
°
allowed to cool down to room temperature. The whole reaction
mixture was transferred into a centrifuge tube and centrifuged
along with an additional 20 mL ethanol at 9000 rpm to produce a
black solid. Washing with ethanol was repeated thrice more to
remove any excess ligand and oleylamine. The precipitate was then
washed with acetone and dried to store for further use. Yield=
0.055 g. The carbon content was less than 5%.
Chem Asian J. 2020, 15, 1–9
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Synthesis of Cu and CuO.[10c] Solid copper(II) acetate monohydrate
Cdl/Cs, where Cs can be defined as the specific capacitance of the
material per unit area under identical electrolyte conditions.[39] The
specific capacitance (Cs) value of 1.7 mFcmÀ 2 based on the
literature reported values of NF in 1 M KOH solution is considered
to calculate the ECSA. EIS was recorded at 1.51 V vs. RHE for
samples deposited on NF to obtain the Nyquist plots. The
amplitude of the sinusoidal wave was examined in a frequency
range of 100 kHz to 1 mHz. All impedance spectra were fitted using
an equivalent RC circuit model. The charge-transfer resistance (RCT)
was then obtained from the diameter of the semicircle in the
Nyquist plots.[32f]
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(300 mg) was heated to 300 C with a heating rate of 5 CminÀ 1 and
maintained the temperature for 2 hours in an atmosphere of
flowing H2 (slow, 2–3 LhÀ 1). The resulting black powder product
was washed with water and ethanol (3×50 mL) and dried overnight
°
°
°
at 60 C to obtain 100 mg (yield>90%) of phase pure Cu. Following
a very similar procedure, CuO (yield>95%) nanoparticles (NP) were
obtained whereas the heating was done in presence of O2 flow
Electrochemical measurements. Electrocatalytic OER was carried
out in a standard three-electrode setup consisting of a Hg/HgO as a
reference electrode (RE), a Pt wire (0.5 mm diameter ×230 mm
length; A-002234, BioLogic) as a counter electrode (CE) and the
catalyst-modified working electrode (WE) using an aqueous electro-
lyte (1 M KOH, Sigma Aldrich). A potentiostat (SP-200, BioLogic
Science Instruments) controlled by the EC-Lab v10.20 software
package was used for all the electrochemical measurements and
further data analyses. Two different substrates NF and FTO were
used electrode support. The investigated materials were deposited
electrophoretically by a well-established method on both, NF and
FTO.[12,14,15e,22,30a] A potential difference of À 10 V in a mixture of
iodine and acetone on a 1×1 cm2 area was applied. The electric
charge on the catalyst in acetone is insufficient for EPD, as minimal
amounts of free ions exist in acetone. When iodine is used as the
dispersant, it can react with acetone through the keto-enol
tautomerization to produce protons as per the following
equation.[38]
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Acknowledgements
Funded by the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation) under Germany's Excellence Strategy – EXC
2008/1 – 390540038. Gefördert durch die Deutsche Forschungsge-
meinschaft (DFG) im Rahmen der Exzellenzstrategie des Bundes
und der Länder – EXC 2008/1 – 390540038. Authors thank J.
Niklas Hausmann (TU Berlin) for TEM and the group of Prof.
Martin Lerch (TU Berlin) for PXRD measurements.
Keywords: Copper sulfides · Single-source precursor · Digenite ·
Electrocatalytic OER · Non-noble metal catalysis
Thus, formed protons are adsorbed on the surface of the
suspended particles by making them positively charged. The
applied electric field induces the positively charged particles to
migrate towards and deposit on the cathode. Typically, 30–40 mg
of the catalyst powder was suspended in 10 mL acetone and
sonicated for 1 h, then 3 mg of iodine was added and the
suspension sonicated for another 3 min. The EPD was conducted at
À 10 V for 2 min over a 1×1 cm2 surface area of the electrode and
thin uniform films were obtained. The sample loading on each NF
and FTO was ~0.6 and ~0.5 mgcmÀ 2, respectively. A similar
procedure was also followed to deposit other materials (Cu, CuO,
RuO2, and IrO2) on NF.
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LSV measurements were performed with an applied iR compensa-
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Manuscript received: January 6, 2020
Revised manuscript received: January 30, 2020
Accepted manuscript online: February 3, 2020
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Full Paper
FULL PAPER
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Dr. B. Chakraborty, S. Kalra, R.
Beltrán-Suito, Dr. C. Das, T.
Hellmann, Dr. P. W. Menezes*,
Prof. Dr. M. Driess*
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Copper as the topper. The Digenite
mineral Cu9S5 phase has been
efficient electrocatalytic alkaline
oxygen evolution reaction, displaying
competent catalytic activity and
stability.
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A Low-Temperature Molecular
Precursor Approach to Copper-
Based Nano-Sized Digenite
produced from a molecular CuI2S2
precursor complex and applied for
Mineral for Efficient Electrocata-
lytic Oxygen Evolution Reaction