Heterocyclic-Additive-Activated Dinuclear Dysprosium Electrocatalysts for Heterogeneous Water Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.1c00209

Heterogeneous catalysis based on air-stable lanthanide complexes is relatively rare, especially for electrochemical water oxidation and reduction. Therefore, it is highly desired to investigate the synergy caused by cocatalysts on the lanthanide complex family for heterogeneous catalysis because of their structural diversity, air/moisture insensitivity, and easy preparation under an air atmosphere. Two mononuclear and three dinuclear dysprosium complexes containing a series of Schiff-base ligands have been demonstrated as robust electrocatalysts for triggering heterogeneous water oxidation in alkaline solution, in which the complex [Dy2(hmb)2(OAc)4]·MeCN(3) was revealed to have the best activity toward heterogeneous water oxidation among all five complexes in the present study. The molecular activation of dysprosium complexes has also been investigated with a series of N-containing heterocyclic additives [i.e., 4-(dimethylamino)pyridine (DMAP), bis(triphenylphosphine)iminium chloride ([PPN]Cl), indole, and quinoline]. In particular, the corresponding overpotential was effectively enhanced by 211 mV (at a current density of 10 mA cm-2) with the assistance of DMAP. On the basis of electrochemical and ex situ/in situ spectroscopic investigations, the best catalyst, DMAP-complex 3 on a carbon paper electrode, was confirmed with well-maintained molecular identity during heterogeneous water oxidation free of forming any dysprosium oxide and/or undesired products.

Full text of DOI:10.1021/acs.inorgchem.1c00209

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Heterocyclic-Additive-Activated Dinuclear Dysprosium
Electrocatalysts for Heterogeneous Water Oxidation
Ching-Wei Tung,† Cheng-Han Tso,† Bo-Yi Chen, Hang Chu, Cheng-Hung Hou, Hsiao-Chien Chen,
Mu-Chieh Chang, Jing-Jong Shyue, Po-Heng Lin,* and Hao Ming Chen*
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ABSTRACT: Heterogeneous catalysis based on air-stable lantha-
nide complexes is relatively rare, especially for electrochemical
water oxidation and reduction. Therefore, it is highly desired to
investigate the synergy caused by cocatalysts on the lanthanide
complex family for heterogeneous catalysis because of their
structural diversity, air/moisture insensitivity, and easy preparation
under an air atmosphere. Two mononuclear and three dinuclear
dysprosium complexes containing a series of Schiff-base ligands
have been demonstrated as robust electrocatalysts for triggering
heterogeneous water oxidation in alkaline solution, in which the
complex [Dy2(hmb)2(OAc)4]·MeCN(3) was revealed to have the
best activity toward heterogeneous water oxidation among all five
complexes in the present study. The molecular activation of dysprosium complexes has also been investigated with a series of N-
containing heterocyclic additives [i.e., 4-(dimethylamino)pyridine (DMAP), bis(triphenylphosphine)iminium chloride ([PPN]Cl),
indole, and quinoline]. In particular, the corresponding overpotential was effectively enhanced by 211 mV (at a current density of 10
mA cm−2) with the assistance of DMAP. On the basis of electrochemical and ex situ/in situ spectroscopic investigations, the best
catalyst, DMAP−complex 3 on a carbon paper electrode, was confirmed with well-maintained molecular identity during
heterogeneous water oxidation free of forming any dysprosium oxide and/or undesired products.
metal centers during OER and further suppress intramolecular
ligand degradation from the multielectron transformation.12,13
Numerous multinuclear complexes, such as [Ir(pyalc)-
INTRODUCTION
■
Inspired by natural enzymes in photosystem II, employing
synthetic molecular catalysts to drive artificial photosynthesis
has been investigated for decades.1−3 The solar/electrical
energy from renewable sources can be effectively stored via
(photo)electrochemical hydrogen evolution or carbon dioxide
(CO2) reduction, and these processes are inevitably coupled
with a four-electron water oxidation (2H2O → O2 + 4H+ +
4e−). However, this half-reaction is regarded as a kinetic
bottleneck for the above-mentioned artificial photosynthesis
because it requires a significant amount of energy to trigger the
multielectron-transfer processes.4,5 Among the wide-range
library of heterogeneous catalysts, a molecular catalyst is
known for its well-defined structure, tunable design of ligand,
and highly uniform active site, which can provide an
opportunity to study the fundamental catalysis at the atomic
level. Accordingly, numerous molecular catalysts based on
noble metals, such as Ru6 and Ir,7 and earth-abundant
transition metals, such as Mn,8 Co,9 Fe,10 and Ni,11 have
been demonstrated to yield remarkably high activities toward
oxygen evolution reaction (OER). Among them, multinuclear
molecular complexes can offer an interesting role in this
multielectron-transfer process because of their high flexibility
of molecular configuration, which could afford high-valent
( H 2 O ) 2 ( μ - O ) ] 2 , [ F e I I 4 F e I I I ( μ 3 - O ) ( μ - L ) 6 ] 3 +
,
[Mn12O12(L)16(H2O)4], [(bpy)Cu(μ-OH)]2(OAc)2, and di-
meric ruthenium catalysts, have been investigated, which
allows us to bridge the concepts extracted by their mechanisms
behind active intermediates and intrinsic catalytic activities in
order to achieve efficient molecular catalysts.7,8,14,15 A series of
molecular complexes with high turnover-frequency (TOF)
values for homogeneous catalysis have been revealed.
However, most of them are suffering from an inadequate
current and/or an unacceptable overpotential,16 which plagues
their practical applications at a large scale. Furthermore, metal
complexes commonly require the usage of excess oxidants,
specific electrolytes, and organic-solvent/water mixtures to
generate active intermediates (i.e., high-valent species) in
Special Issue: Heterogeneous Interfaces through the
Lens of Inorganic Chemistry
Received: January 22, 2021
https://doi.org/10.1021/acs.inorgchem.1c00209
© XXXX American Chemical Society
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homogeneous catalytic systems. The aforementioned limita-
tions significantly circumscribe the usage of molecular
complexes for triggering water splitting on an industrial scale.
For this reason, incorporating molecular complexes into
practical catalysis should be considered to be operated in
aqueous mixtures and heterogeneous conditions.
EXPERIMENTAL SECTION
■
General Materials and Methods. All chemicals were purchased
from Strem, Sigma. or Alfa Aesar without further purification. Three
dinuclear complexes, [Dy2(hmb)2(OAc)4]·MeCN (3·MeCN),
[Dy2(hmi)2(OAc)2(MeOH)2]·H2O (4·H2O), and [Dy2(hb)2(μ-OA-
c)2(OAc)2(H2O)2]·DMF·H2O (5·DMF·H2O), and their ligands were
prepared as described in the literature.29
Another strategy is to introduce molecular cocatalysts, such
as pyridine base additives, that have been characteristic for
enhancing the catalytic activity and product selectivity in many
homogeneous catalytic systems.17,18 For example, cocatalysts,
such as 4-(dimethylamino)pyridine (DMAP) and bis-
(triphenylphosphine)iminium chloride ([PPN]Cl), in combi-
nation with different metal complexes for promoting catalysis
have been revealed.19,20 The principles to improve heteroge-
neous catalysis have also been adopted by a polycrystalline
metal electrode in electrochemical heterogeneous catalysis,
which further facilitated the product formation occurring in the
heterogeneous case.21,22 This observation clearly confirms the
fact that organic additives on film electrodes can be expected
to significantly effect their catalytic activity in heterogeneous
catalysis. However, the polycrystalline metal system in the
literature was shown to have difficulty determining their active
sites as well as actual TOF values. Furthermore, mechanistic
studies on such thin-film systems are very challenging because
the outermost atoms may display a behavior significantly
different from that of their subsurface region (i.e., bulk
region).23 One possible solution to overcoming these problems
is operating molecular complexes on a heterogeneous support
to further clarify the correlation between the cocatalyst
behavior and catalytic enhancement. On the other hand, the
stability of the molecular complex is another critical issue for
OER because the molecular complexes may have a high affinity
for decomposing and further forming the corresponding metal
oxides in aqueous solution under anodic potential.24 This
implies that the only solution for developing such a system is
to realize a robust molecular complex in heterogeneous
catalytic conditions.
Recently, the usage of rare-earth/lanthanide complexes as
catalysts in both organic reactions25,26 and polymerization
processes27,28 has attracted increasing attention. In previous
work, lanthanide-based complexes containing Schiff-base
ligands have been utilized for triggering CO2/epoxide
copolymerization, in which all dinuclear dysprosium complexes
are air-insensitive molecules and highly active in such a
homogeneous catalytic system.29 Indeed, heterogeneous
catalysis based on air-stable lanthanide complexes is relative
rare, especially for electrochemical water oxidation and
reduction. Therefore, it is highly desired to investigate the
synergy caused by cocatalysts on the lanthanide complex family
for heterogeneous catalysis because of their structural diversity,
air/moisture insensitivity, and easy preparation under an air
atmosphere. Herein, two mononuclear and three dinuclear
dysprosium complexes with a series of Schiff-base ligands have
been used for heterogeneous oxygen evolution. Remarkably, a
correlation between the organic additives and OER over-
potential has been revealed, in which the [Dy2(hmb)2(OAc)4]·
MeCN complex can display the best performance toward
heterogeneous water oxidation. We also noted that this
heterogenized molecular catalyst could retain its molecular
nature for long-term electrochemical catalysis, as confirmed by
in situ Raman spectroscopy.
Synthesis of H-hdni. 3,5-Dinitro-2-hydroxybenzaldehyde30 (20
mmol, 4.24 g) and isonicotinohydrazide (20 mmol, 2.74 g) were
dissolved in 300 mL of methanol in a 500 mL round-bottom flask and
refluxed for 18 h, yielding a white precipitate. The product was then
filtered, washed with methanol and ether, and dried with a vacuum
system to obtain 5.62 g of H-hdni (yield: 85%). 1H NMR (400 MHz,
DMSO-d6): δ 8.84 (d, 2H), 8.81 (d, 2H), 7.95 (d, 2H).
Synthesis of H-hdmob. 3-Methoxy-5-nitrobenzaldehyde31 (20
mmol, 3.94g) and benzohydrazide (20 mmol, 2.72 g) were dissolved
in 300 mL of methanol in a 500 mL round-bottom flask and refluxed
for 18 h, yielding a pale-yellow precipitate. The product was then
filtered, washed with methanol, and dried with a vacuum system to
obtain 5.67 g of H-hdmob (yield: 90%). 1H NMR (400 MHz,
DMSO-d6): δ 8.84 (d, 2H), 8.81 (d, 2H), 7.95 (d, 2H).
Synthesis of Complexes 1 and 2. Synthesis of [Dy(hmnob)-
(H2O)2(NO3)2]·MeCN·H2O (1·MeCN·H2O). To a solution of Dy-
(NO3)3·5H2O (0.25 mmol, 0.1096 g) in methanol (15 mL) was
added a solution of H-hdmob (0.25 mmol 0.0828 g) and pyridine
(0.020 mL, 0.25 mmol) in methanol (15 mL). Bright-yellow crystals
of the mononuclear complex 1·MeCN·H2O yielded from the resulting
bright-yellow solution in 33.7% (0.0296 g) yield after 2 weeks. IR
(KBr, cm−1): 3344 (br), 2266 (m), 1603 (s), 1568 (s), 1505 (w),
1484 (m), 1455 (m), 1391 (w), 1300 (s), 1260 (s), 1251 (m), 1187
(w), 1117 (m), 1097 (w), 1026 (m), 971 (m), 896 (m), 845 (m), 815
(m), 781 (w), 744 (s), 717 (s), 660 (w). Anal. Calcd for 1·MeOH·
OH−: C, 29.3; H, 3.18; N, 12.06. Found: C, 29.28; H, 3.15; N, 12.05.
CCDC 2056360.
Synthesis of [Dy(hdni)2(MeOH)2]·MeOH·OH− (2·MeOH·OH−). To
a solution of Dy(NO3)3·5H2O (0.25 mmol, 0.1096 g) in methanol
(15 mL) was added a solution of H-hdni (0.25 mmol 0.0828 g) and
pyridine (0.020 mL, 0.25 mmol) in methanol (15 mL). The resulting
yellow solution yielded yellow crystals of the mononuclear complex 2·
2MeOH in 24.5% (0.0232 g) yield after 2 weeks. IR (KBr, cm−1):
3749 (s), 2988 (s), 2869 (s), 2283 (w), 1613 (m), 1560 (m), 1489
(w), 1412 (w), 1394 (m), 1305 (s), 1233 (m), 1186 (w), 1143 (w),
1102 (w), 1071 (m), 1105 (m), 925 (w), 907 (w), 840 (m), 811 (w),
787 (w), 746 (m), 717 (s), 687 (w). Anal. Calcd for 2·MeOH·OH−:
C, 31.67; H, 3.65; N, 7.35. Found: C, 31.50; H, 3.65; N, 7.30. CCDC
2056361.
Crystallographic Studies. Single crystals of complexes 1 and 2
suitable for X-ray diffraction measurements were mounted on a
Bruker D8 VENTURE diffractometer. The unit cell was determined
using the Bruker SMART APEX 3 software suite to employ graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å), and the intensity
data were collected with ω scans. Data collection and reduction were
performed with the CrysAlisPro software, and the absorptions were
corrected by the SCALE3 ABSPACK multiscan method. The space
group determination was based on the symmetry and the systematic
absences displayed in the diffraction pattern. The structure was solved
and refined with the Olex2 1.2-ac21 package. Anisotropic thermal
parameters were used for all non-H atoms, and fixed isotropic
parameters were used for H atoms.
Electrode Fabrication. Complexes 3−5 (9.296 × 10−2 mmol each)
were dispersed in 5 mL of tetrahydrofuran, and complexes 1 and 2
(1.859 × 10−1 mmol each) were dispersed in 5 mL of ethanol. The
mixture was treated by ultrasound for at least 30 min, and then 20 μL
of the suspension was dropped onto carbon paper (C.P.) within an
area of 0.5 × 0.5 cm, followed by 20 μL of a 0.1 wt % Nafion solution
after the previous layer was dried under ambient air. The 0.1 wt %
Nafion solution was diluted from a commercial Nafion perfluorinated
resin solution (5 wt %, Sigma) by ethanol. Finally, the mole density of
the Dy ion on the electrode was 2.975 × 10−3 mmol cm−2. For
B
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cocatalyst experiments, 1.859 × 10−1 mmol of cocatalyst was dissolved
in 5 mL of tetrahydrofuran, and then 1 mL of a cocatalyst solution
was mixed with 1 mL of a didysprosium suspension. The mixture was
treated by ultrasound for at least 30 min, and then 40 μL of the
suspension was dropped onto C.P. within an area of 0.5 cm × 0.5 cm,
followed by 20 μL of a 0.1 wt % Nafion solution after the previous
layer was dried under ambient air. The mole density of the Dy ion on
the electrode was the same as that in the experiments without
cocatalyst.
Structural Characterization. The morphology and elemental
mapping of the specimen were performed by employing a JEM-
6700F (JEOL) scanning electron microscope at 10 kV. Chemical
composition analysis of the specimen was performed using a JEOL
JEM-2100F field-emission scanning electron microscope equipped
with an energy-dispersive spectroscopy (EDS) probe.
Electrochemical Measurements. All electrochemical measure-
ments were carried out on a Bio-Logic VSP-128 potentiostat at
room temperature. Pt wire and saturated calomel electrode (SCE)
with a saturated KCl filling solution were used as the counter and
reference electrodes, respectively. The N2-saturated 1.0 M KOH
electrolyte was prepared by bubbling N2 (99.999%) for at least 30
min. The potential versus reversible hydrogen electrode (RHE) was
calculated as follows:
Rotating-Ring-Disk-Electrode (RRDE) Measurements. A total of
9.296 × 10−2 mmol of complex 3 in 5 mL of tetrahydrofuran was
dropped onto the carbon disk (area: 0.2472 cm2) of a RRDE tip
(E7R9, Pine Research), followed by 5 μL of a 1% Nafion solution
(diluted by ethanol) after the previous layer was dried under ambient
air. The speed of the RRDE was controlled by a modulated speed
rotator (Pine Research) at 1600 rpm, and LSV was performed at a
scan rate of 100 mV s−1.
Structural Characterization of Complex/C.P. Electrode. The
morphology of the complex/C.P. electrodes was conducted by
employing a JEM-6700F (JEOL) scanning electron microscope at 15
kV. Chemical composition analysis and elemental mapping of the
complex/C.P. specimen were performed using energy-dispersive X-ray
analysis by a scanning electron microscope equipped with an EDS
probe. X-ray photoelectron spectroscopy (XPS) analysis was done
with a PHI 5000 Versa Probe (ULVAC-PHI, Japan) system, using
monochromatic Al Kα. In order to avoid surface potential buildup
during measurement, all spectra were acquired, while the sample
surface was neutralized by e− and Ar+ beams with an acceleration
voltage of 10 V. In situ Raman spectra were recorded by a UniNano
UNIDRON-A Raman microscopy system, employing a diode laser at
532 nm.
RESULTS AND DISCUSSION
ERHE = Emeasure + ESCE + 0.05916pH − iRu
(1)
■
Synthesis and Characterization. The obtained 1·MeCN·
The TOF values were calculated by assuming that every Fe atom was
involved in the catalysis.
H2O crystallizes in the triclinic P1
̅
space group. A partially
labeled X-ray structure of complex 1 is shown in Figure 1, and
the X-ray information (including cell parameters) is given in
Tables S1−S3. Selected bond distances and angles are given in
Table S4. The DyIII ion of the mononuclear complex adopts a
nine-coordinate spherical triangular dodecahedron as deter-
mined by SHAPE 2.1, where one ligand, two nitrate molecules,
and two coordinated H2O molecules occupy the coordination
sites. The polydentate Schiff-base ligand coordinates to the Dy
center via two O atoms (O1 and O2) and one N atom (N2).
Charge-balance considerations for the molecule indicate that
the ligand must have one negative charge resulting from one
deprotonated O2 atom and one double bond between C7 and
O1. In comparison with the other reported mononuclear
complex, [Dy(hni)(NO3)2(DMF)2]·DMF [H-hni = 2-(hy-
droxyl-3-methoxy-5-nitrophenyl)methylene(isonictino)-
hydrazine],32 it is notable that the phenyl group of hmnob−
affected the coordination environments of DyIII. The two
nitrate anions are on the same axis in complex 1 but vertical in
the complex reported in the literature. Complex 2 crystallized
in the monoclinic I121/a1 space group (Tables S5−S7). The
structure is composed of an eight-coordinated spherical
JA
4Fn
TOF =
(2)
The pH value of the 1 M KOH electrolyte used in this work was
14.00, and the uncompensated resistance (Ru) was measured using
potentiostatic electrochemical impedance spectroscopy (EIS). C.P.
was used as the working electrode for linear-sweep voltammetry
(LSV) and cyclic voltammetry (CV). Fabrication of the electrode was
mentioned previously. LSV and CV were performed at a scan rate of
100 mV s−1. The current densities were normalized by the working
electrode geometric surface area of 0.25 cm2.
Impedance measurements were performed for photoinduced water
oxidation with a frequency range of 1.0−100 kHz. The EIS spectra
were fitted by the EC-Lab software, and the fitting model is given in
Scheme 1.
Scheme 1. Fitting Model of the EIS Spectra
Figure 1. Molecular structures of complexes 1 (left) and 2 (right). Color code: Dy, yellow; O, red; N, blue; C, gray. H atoms were omitted for
clarity.
C
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Scheme 2. Structures of Complexes 1−5 with a Series of Schiff-Base Ligands
Figure 2. SEM image and corresponding elemental mapping of complex 3 on a C.P. electrode.
triangular dodecahedron DyIII ion, where two ligands and two
coordinated MeOH solvent molecules (O13 and O14) occupy
the coordination sites. The ligands coordinated with DyIII in
the same coordination modes as complex 1 as the keto form of
hmnob−. The figures of packing arrangements and the
asymmetric units for complexes 1 and 2 are also shown in
Figures S1 and S2. Furthermore, because of the necessity for
surface texture. Elemental mapping with EDS, as illustrated in
Figure 2 (right), verified the obvious presence of dysprosium
on the C.P. electrode.
Heterogeneous Water Oxidation Performance. The
electrochemical behaviors of a series of dysprosium complexes
on C.P. electrodes for both water reduction and oxidation were
investigated with LSV in a 1.0 M KOH solution. As shown in
Figure 3a, the presence of a reductive current from ∼0.1 to
∼−0.4 V (vs RHE) was related to the reduction of DyIII in two
steps (DyIII + e− → DyII and DyII + 2e− → Dy0). This
phenomenon indicated that these dysprosium complexes are
unable to maintain their metal sites in the ligand-coordinated
environment under cathodic potential in an alkaline electro-
lyte. Besides, as for the cathodic scan, the activities of all
catalysts under identical conditions revealed that the loading of
dysprosium complexes would have a smaller current response
−
charge balance, the OH ion was also present within the
lattice structure. Three dinuclear dysprosium complexes, 3−5,
were also synthesized for further catalytic studies (Scheme 2)
in order to compare the catalytic activities between mono- and
dinuclear systems. The scanning electron microscopy (SEM)
image and corresponding elemental mapping of complex 3 on
a C.P. electrode are shown in Figure 2 (left) as an example,
which revealed that the molecular catalyst was well-distributed,
loading onto the carbon support because of the presence of the
D
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Figure 3. Electrochemical water oxidation properties of dysprosium molecular complexes onto C.P. (a) HER and (b) OER polarization curves of
complexes 1−5/C.P. in 1.0 M KOH at a scan rate of 20 mV s−1 with iR compensation. (c) RRDE measurement of a 3 catalyst in O2-saturated 1.0
M KOH (rotation speed of 1600 rpm). (d) Potential versus time data on complex 3/C.P. in 1.0 M KOH for 2 h at a constant current density of 10
mA cm−2 (vs RHE), in which no significant decay was observed.
and a larger onset potential than those of a bare C.P. electrode.
This phenomenon suggests that the formation of dysprosi-
um(0) species from heterogeneous conditions has no
significant activity for triggering hydrogen evolution reaction
(HER; i.e., water reduction). In contrast, as for the anodic
scan, we were able to clarify the remarkably electrocatalytic
OER activity for complexes 1−5 on C.P. compared to that of a
bare C.P. electrode. Among all complexes, complex 3/C.P. was
revealed to exhibit the lowest overpotential (η) at a current
density of 10 mA cm−2 (the current response necessary for
about 10% efficient solar-to-fuel conversion)33 compared to
those of the mononuclear complexes of 1/C.P. and 2/C.P.
Furthermore, the other two dinuclear dysprosium complexes,
4/C.P. and 5/C.P., were observed to have better activities than
those of mononuclear complexes, which indicated an
interesting interplay as a consequence of adjacent dysprosium
(vide infra). Accordingly, significant cathodic shifts of 813 and
817 mV can be achieved in the overpotential compared with
complexes 1/C.P. (η = 908 mV) and 2/C.P. (η = 935 mV),
respectively, because of the contribution from the adjacent
dysprosium.
The combination of molecular complexes and a heteroge-
neous catalytic system could maximize atomic utilization;
moreover, the amount of metallic waste from electrode
fabrication and catalytic reaction could be suppressed. That
is, the loading amount of complex 3/C.P. was also optimized
to achieve an optimal condition with atomic-scale precision
(∼3.0 μmol cm−2; Figure S3). The Tafel slopes for dinuclear
complexes (259 mV dec−1 for complex 3, 282 mV dec−1 for
complex 4, and 301 mV dec−1 for complex 5) are much smaller
than those of mononuclear complexes (363 mV dec−1 for
complex 1 and 354 mV dec−1 for complex 2), suggesting that
superior kinetics of water oxidation could have arisen from
complexes with a dinuclear oxygen-bridged nature (Figure S4).
Besides, the SEM images confirm that the lower activity of
complexes 1 and 2 is not due to the distribution of catalysts on
carbon supports (Figure S5). To further confirm the
production of oxygen gas, a RRDE investigation was carried
out, as shown in Figure 3c. A cathodic current was detected on
the ring electrode at +0.4 V, together with a rise of the OER
current on the disk electrode, demonstrating that once the
oxygen gas was formed on the disk electrode, oxygen reduction
took place on the ring electrode. Moreover, a negligible current
density on the ring electrode (at +1.5 V) suggested that OER
occurred through a four-electron pathway (4OH− → O2 +
2H2O + 4e−). The above RRDE analysis clarified that the
bubbles generated from complex 3/C.P. were exactly oxygen
gas, and the main current response was attributed to
electrochemical water oxidation. The robust stability was also
validated in a chronopotentiometry measurement, where only
a slight decay in the applied potential for achieving 10 mA
cm−2 was observed after working for 2 h (Figure 3d). This
manifests that the complex of 3/C.P. can act as a practical and
robust electrocatalyst for heterogeneous oxygen evolution.
Consequently, the chemical robustness of the present
dysprosium molecular electrocatalyst in an alkaline environ-
ment could be appropriately utilized to further reveal the
synergic effects of cocatalysts toward OER electrocatalysis.
Molecular-Cocatalyst-Assisted Water Oxidation. In-
spired by the previously reported OER activity as a result of
different metal−N coordination environments of single-atom
electrocatalysts, we postulated that the character of N-
containing heterocyclic additives may affect the OER behaviors
of molecular complexes under heterogeneous catalytic
E
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Figure 4. (a) Molecular structures of N-containing heterocyclic additives, including DMAP, indole, quinoline, and [PPN]Cl. (b) OER polarization
curves and (c) Tafel slopes (overpotential vs log current) of complex 3/C.P. in 1.0 M KOH with different cocatalysts. (d) TOF values versus
overpotential of complex 3/C.P. and cocatalyst−complex 3/C.P. (e) Nyquist plots of complex 3/C.P. and cocatalyst−complex 3/C.P. measured in
1.0 M KOH at applied potentials of 1.2 and 1.65 V.
conditions.34 A series of N-containing heterocyclic additives
(DMAP, indole, quinoline, and [PPN]Cl) were introduced to
alter OER electrocatalysis of complex 3/C.P. as the cocatalyst
(Figure 4a), in which the as-prepared electrodes consist of a
mixture of dysprosium complexes, Nafion membranes, and
heterocyclic additive cocatalysts. The corresponding OER
catalytic nature of these functionalized electrodes was clarified
by LSV in a 1.0 M KOH solution (Figure 4b). Impressively,
the presence of N-containing heterocyclic additives showed an
evident difference on their overpotential from that of the
original complex 3/C.P. Except for the addition of [PPN]Cl,
the incorporation of pyridine- and indole-based cocatalysts
into complex 3/C.P. demonstrated remarkably enhanced
activity toward water oxidation. The addition of DMAP
resulted in the best performance (η = 531 mV) among this
series, which could shift the overpotential cathodically at a
current density of 10 mA cm−2 by 211 mV compared with
complex 3/C.P. (η = 742 mV). Comparing the activity among
all catalysts under identical conditions indicated that quino-
line−complex 3/C.P. (η = 653 mV) and indole−complex 3/
C.P. (η = 710 mV) were poorly active, which indicated that the
different σ-donating characters of heterocyclic additives could
significantly affect the local environment (i.e., isostructurality)
around the catalytic center.35,36 Among these functionalized
electrodes, only [PPN]Cl−complex 3/C.P. presented a clear
decline in the resulting activity, which may be due to the bulky
cations of PPN that can suppress the formation of reaction
intermediates from the DyIII metal center for OER. Notably,
the functionalized electrode formed in the presence of
pyridine-based additives exhibited a significant enhancement
in the overpotential, especially for the case of DMAP
activation. Accordingly, the influence of the N atom around
metal centers toward OER was confirmed by preparing the
electrodes with various ratios of Dy/DMAP (1:1, 1:2, and 2:1).
As shown in Figure 4b, the overpotential changed slightly with
an increasing amount of DMAP but dramatically increased
with a 2:1 ratio of the Dy site and DMAP. The dependence of
the DMAP ratio indicates that the addition of a cocatalyst plays
a central role in these heterogenized molecular catalysts. The
Tafel slopes were calculated to understand the electrocatalytic
OER kinetics, as shown in Figure 4c. In line with the
polarization curves, DMAP−complex 3/C.P. endows the
smallest Tafel slope (164 mV dec−1), much lower than those
of quinoline−complex 3/C.P. (209 mV dec−1), indole−
complex 3/C.P. (235 mV dec−1), and [PPN]Cl−complex 3/
C.P (323 mV dec−1). These results manifested that the OER
kinetics of complex 3/C.P. was essentially improved by
introducing N-containing heterocyclic additives. To mechanis-
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Figure 5. (a) XPS spectra of complex 3/C.P. (top) and DMAP−complex 3/C.P. taken after 2 h of heterogeneous water oxidation (bottom) at a
current density of 10 mA cm−2. (b) Schematic diagram of an in situ Raman apparatus applied to a customized liquid cell. (c) Raman spectra of
DMAP−complex 3/C.P. versus control potentials in 1.0 M KOH.
tically investigate the interplay between the cocatalyst and
complex 3/C.P., the TOF of the dysprosium complex was
estimated to evaluate the intrinsic activity of the series of
cocatalyst−complex 3/C.P., which referred to the activity per
metal site in a heterogeneous reaction (Figure 4d). DMAP−
complex 3/C.P. could exhibit excellent electrocatalytic activity;
for example, the overpotential required to achieve a TOF of 10
s−1 was only 541 mV, which was much lower than those of
quinoline−complex 3/C.P. (667 mV), indole−complex 3/C.P.
(726 mV), [PPN]Cl−complex 3/C.P (820 mV), and complex
3/C.P. (755 mV). The TOF value of DMAP−complex 3/C.P.
and corresponding overpotential were presented along with the
values of some molecular catalysts for heterogeneous catalysis,
such as dinuclear iridium complexes (with 100 equiv of
NaIO4) and mononuclear transition-metal complexes, as
shown in Table S8. This clearly shows that the TOF value
of DMAP−complex 3/C.P. is comparable with that of
[Ir(pyalc)(H2O)2(μ-O)]2 (TOF = 7.9 s−1 at η = 520 mV)7
and relatively high compared to that of Fe(TAMLS)H2O
(TOF = 0.081 s−1 at η = 750 mV).37 Additionally, prolonged
testing of DMAP−complex 3/C.P. at a constant current
density of 10 mA cm−2 was performed to reveal a steady
applied potential for 2 h without noticeable decay (Figure S6).
SEM images also confirmed the robustness of DMAP−
complex 3 on carbon supports after OER in heterogeneous
conditions (Figure S7). Furthermore, impedance measurement
was performed to investigate the interfacial properties of
electrodes at various voltages near the OER region. The
semicircular regions in the Nyquist impedance plots and these
curves were fitted by using a proposed equivalent circuit
(Table S9), which included the charge-transfer resistance (Rct),
double-layer capacitance (Cdi), and solution resistance (Rs). As
shown in Figure 4e, the addition of DMAP to complex 3/C.P.
was unable to substantially alter the feature of the semicircular
region before OER onset (+1.2 V). Once a potential beyond
the OER onset was applied, Rct of DMAP−complex 3/C.P.
(3.5 Ω) decreased significantly to 12.6 Ω. This observation
indicated that the addition of DMAP can effectively enhance
the charge-transfer processes at the catalyst/electrolyte inter-
face during OER. In addition, the increase in Cdi of DMAP−
complex 3/C.P. at +1.65 V (1.4 mF cm−2) reflected its
effective electrochemical active surface area. The above results
revealed that the remarkable enhancement in OER kinetics can
be attributed to the incorporation of N-containing heterocyclic
additives into the heterogenized molecular electrode. More-
over, we validated the evidence that the ratio between organic
additives and active sites could be accurately manipulated in
heterogeneous electrochemical systems, which allowed us to
design a more efficient electrocatalytic system. Operating
molecular complexes on heterogeneous supports offered
available information to further understand how to enhance
the activity with the assistance of cocatalysts in practical
electrochemical conditions. This system evidently is able to
combine the organic molecules and metal complexes that
contain well-defined active sites at the atomic scale for
triggering heterogeneous catalysis.
It is paramount to clarify that the ligand-coordinated
complexes are robust and able to remain steady during OER
in heterogeneous conditions. According to the XPS spectra
(Figures 5a and S8 and S9), it is shown that complex 3 was
able to robustly maintain the original molecular structure on
the C.P. electrode (153 and 156 cm−1 for 4d5/2 and 4d3/2
,
respectively), indicating that the metal centers are still
coordinated by a Schiff-base ligand after heterogeneous
electrocatalysis at a current density of 10 mA cm−2 for 2 h.
To further realize the robustness of complex 3 for acting as a
molecular catalyst during electrocatalysis, in situ Raman
spectra of DMAP−complex 3/C.P. were performed as
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illustrated in Figure 5b.38,39 Indeed, the results revealed that
the peaks of complex 3 were evidently observed and steady at
applied anodic potentials, while no other features (374, 464,
and 585 cm−1) as a result of metal oxide formation40 were
shown in electrochemical oxidation (Figures 5c and S10).
Consequently, we can conclude that its molecular identity was
maintained during heterogeneous water oxidation without
forming any dysprosium oxide and/or undesired products.
Hao Ming Chen − Department of Chemistry, National
Taiwan University, Taipei 106, Taiwan; orcid.org/0000-
0002-7480-9940; Email: haomingchen@ntu.edu.tw
Authors
Ching-Wei Tung − Department of Chemistry, National
Taiwan University, Taipei 106, Taiwan
Cheng-Han Tso − Department of Chemistry, National
Taiwan University, Taipei 106, Taiwan
Bo-Yi Chen − Department of Chemistry, National Chung
CONCLUSION
■
Hsing University, Taichung 402, Taiwan
In summary, a simple and tunable system in atomic scale for
heterogeneous water oxidation with the synergy of an
additional molecular cocatalyst has been realized. A series of
N-containing heterocyclic additives were successfully em-
ployed to enhance the OER activity of the dysprosium
complex. Even though the OER overpotentials of our rare-
earth complexes are relatively higher than those of other noble-
or transition-metal catalysts, it still offers the possibility of
studying fundamental aspects of heterogeneous catalytic
reaction, especially for how the different σ-donating characters
of organic additives influence the activity of heterogenized
molecular complexes. In particular, selecting DMAP as a
cocatalyst was revealed to significantly improve the onset
potential of complex 3/C.P. for triggering the water oxidation
reaction. The molecular complex was robust after periods of
OER testing in heterogeneous conditions, as confirmed by XPS
and in situ Raman spectroscopy. These results not only
provided the first demonstration of applying rare-earth
molecular complexes for OER, the strategy of the combination
of N-containing heterocyclic additives, and a heterogenized
molecular electrode but also established a prototype for
scalably tuning the OER catalyst at the atomic scale and
offered fundamental knowledge for future investigations of
OER in heterogeneous conditions.
Hang Chu − Department of Chemistry, National Taiwan
University, Taipei 106, Taiwan
Cheng-Hung Hou − Research Center for Applied Sciences,
Academia Sinica, Taipei 106, Taiwan; orcid.org/0000-
0002-5150-7106
Hsiao-Chien Chen − Center for Reliability Science and
Technologies, Chang Gung University, Taoyuan 333, Taiwan
Mu-Chieh Chang − Department of Chemistry, National
Taiwan University, Taipei 106, Taiwan; orcid.org/0000-
0002-7270-0319
Jing-Jong Shyue − Research Center for Applied Sciences,
Academia Sinica, Taipei 106, Taiwan; orcid.org/0000-
0002-8508-659X
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00209
Author Contributions
†The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. These authors contributed equally to this
work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
ASSOCIATED CONTENT
■
■
sı
We acknowledge support from the Ministry of Science and
Technology, Taiwan (Contracts MOST 108-2628-M-002-004-
RSP and MOST 109-2113-M-005-019).
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00209.
Packing arrangements, polarization curves, Tafel slopes,
SEM images and corresponding elemental mapping,
potential versus time data, and XPS and Raman spectra
(Figures S1−S10) and crystallographic data, selected
angles and bond lengths, calculation results, camparison
of TOF values, and Rs, Rct, and Cdi values fit from the
EIS data, and (Tables S1−S9) (PDF)
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Corresponding Authors
Po-Heng Lin − Department of Chemistry, National Chung
Hsing University, Taichung 402, Taiwan; orcid.org/
0000-0002-0893-0806; Email: poheng@
dragon.nchu.edu.tw
H
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