Triangular trinuclear metal-N4 complexes with high electrocatalytic activity for oxygen reduction

Article abstract of DOI:10.1021/ja203776f

A new class of macrocyclic metal-N₄ complexes [MN ₄]_n (M = Co and Fe) were designed and synthesized based on a triangular ligand. Their unique triangular trinuclear structure provides a high density of active sites and facilitates the reduction of dioxygen via a four-electron pathway. Among them, a [CoN₄]₃/C catalyst (20 wt %) exhibits high catalytic activity and long-time stability for the oxygen reduction reaction (ORR) in alkaline conditions, superior to the commercial Pt/C catalyst. Such structurally well-defined [MN₄] _n complexes provide a platform for a new generation of nonprecious metal catalysts (NPMCs) for fuel cell applications.

Full text of DOI:10.1021/ja203776f

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Triangular Trinuclear Metal-N4 Complexes with High Electrocatalytic
Activity for Oxygen Reduction
Ruili Liu,†,§ Christian von Malotki,† Lena Arnold,† Nobuyoshi Koshino,‡ Hideyuki Higashimura,‡
Martin Baumgarten,*,† and Klaus M€ullen*,†
†Max-Planck-Institut f€ur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany
‡Tsukuba Laboratory, Sumitomo Chemical Co., Ltd., 6 Kitahara, Tsukuba, Ibaraki 300-3294, Japan
S Supporting Information
b
ABSTRACT: A new class of macrocyclic metal-N4 complexes
[MN4]n (M = Co and Fe) were designed and synthesized
based on a triangular ligand. Their unique triangular trinuclear
structure provides a high density of active sites and facilitates
the reduction of dioxygen via a four-electron pathway. Among
them, a [CoN4]3/C catalyst (20 wt %) exhibits high catalytic
activity and long-time stability for the oxygen reduction
reaction (ORR) in alkaline conditions, superior to the com-
mercial Pt/C catalyst. Such structurally well-defined [MN4]n
complexes provide a platform for a new generation of non-
precious metal catalysts (NPMCs) for fuel cell applications.
ue to the potential energy crisis and the demand for
environmental protection, fuel cells have attracted consid-
D
erable interest in recent years as efficient and clean energy
converters.1,2 In present fuel cells, Pt nanoparticles supported
on carbon black (Pt/C) remain the best cathode catalyst for the
oxygen reduction reaction (ORR), which is the crucial reaction
deciding the cell performance.3,4 However, the high cost, the
time dependent drift, and the carbon monoxide deactivation of
the Pt-based catalysts are still big obstacles for the commercia-
lization of fuel cells.5 Searching for efficient, less expensive, and
more abundant nonprecious metal catalysts (NPMCs) for ORRs
therefore is an important issue from practical and theoretical
points on fuel cell techniques.
Figure 1. Synthesis of triangular trinuclear metal-N4 complexes
[MN4]3 (M = Co(II) and Fe(II)). The complexes [MN4]3 were
prepared from 8 or 10. Reaction conditions: (a) DDQ in DMF/
Toluene, microwave: 2 h/140 °C/200 W; (b) Metal acetate in DMF,
microwave: 4 h/190 °C/200 W.
in high catalytic activity and selectivity.16 Indeed, [CoN4]3 with all
three centers of the chelating ligand occupied by Co(II) ions was
shown to catalyze an efficient four-electron ORR process with a
much higher electrocatalytic activity and better long-term operation
stability than those of the commercially available Pt/C catalyst in
alkaline electrolytes. More important, our trinuclear metal-N4 com-
plexes [MN4]n possess structurally defined catalytic sites for ORRs,
which provides a platform for a new generation of NPMCs
and presents an important advancement in the development
of fuel cells.
Macrocyclic metal-N4 complexes6ꢀ12 as NPMCs have been
developed for over five decades as a promising alternative to Pt
catalysts for ORRs. And enormous efforts have been devoted to
the study of various macrocyclic metal-N4 complexes, including a
single macrocycle,6 cofacial macrocycle-based dimers,8ꢀ12 and
their derivatives with functional groups13ꢀ15 that contain four
nitrogen donors (i.e., metal-N4 chelates). Up to now, however,
very few complexes qualify as a catalyst for ORRs due to the low
selectivity and poor stability.10ꢀ12
Herein, we present a new class of structurally well-defined
macrocyclic metal-N4 complexes as NPMCs for oxygen reduc-
tion. In particular, novel trinuclear metal-N4 complexes [MN4]n
(M = Co (II) and Fe (II)) were synthesized from a novel
triangular aromatic macrocycle as a ligand, in which three [16]-
annulene like N4-macrocycles were condensed in one conjugated
plane via a central hexaazatrinaphthylene (Figure 1). The unique
features including large planar conjugation and an unprecedented
high density of active sites prompt the ꢀOꢀOꢀ cleavage and result
The synthesis of the ligand 1,4,7,10,13,16-tris-[20-(400-octyl-
phenyl-2H-pyrrole-20-ylidenemetyhl)-1H-pyrrole-50-yl]-
5,6,11,12,17,18-hexaazatrinaphthylenes (10) started from the
condensation of 2,3-diamino-1,4-dibromobenzene (1) with hex-
aketocyclohexane (2) to afford the hexabromohexaazatrinaph-
thalene core (3) (see Supporting Information).17 Subsequent
Suzuki coupling of 3 with 2-boronic acid of N-protected pyrrole
(4) led to the protected hexapyrrolo-hexaazatrinaphthalene (5)
in almost quantitative yield, which was thermally deprotected to
Received: April 24, 2011
Published: June 14, 2011
r
2011 American Chemical Society
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Figure 2. Cyclic voltammograms of [CoN4]3/C on a glassy-carbon
RDE electrode at a scan rate of 100 mV/s in Ar-saturated and
O2-saturated aqueous solution of 0.1 M KOH. A catalyst loading of
25.45 μg/cm2 was used for all RDE voltammograms.
Figure 3. (a) Rotating-disk electrode (RDE) linear sweep voltammo-
grams of [CoN4]3/C supported on a GC electrode in 0.1 M KOH
saturated with O2 at different rotation rates; (b) Koutechy-Levich plot of
produce hexapyrrolo-hexaazatrinaphthalene (6). 1,4,7,10,13,16-
Tris-[20,200-(4000-octylphenylmethylene)-bis-1H-pyrrole-50,500-yl]-5,-
6,11,12,17,18-hexaazatrinaphthalene (8) was achieved via con-
densation of 6 with the p-n-octyl-benzaldehyde (7). And the
dehydrogenation of 8 to the fully conjugated macrocycle 10
was then performed with 2,3-dichloro-5,6-dicyanobenzoquinone
(DDQ) (9) in toluene/DMF by microwave irradiation (Figure 1).
The metal complexation was executed via the acetate methods
for porphyrins, where CoII- and FeII-acetates were added in stoi-
chiometric amounts to the DMF solution of 8 or 10 (Figure 1)
(see Supporting Information).18 Density functional theory (DFT)
calculations of the ligand and of the metal complex indicate fully
planar structures for both, with a CoꢀCo distance of 6.6 Å (Figure
S4a,c,d). All [MN4]n complexes tested in this report were supported
on carbon black, denoted as [MN4]n/C (20 wt % [MN4]n; see
Supporting Information). Different from the adsorption on the sur-
face of a pyrolytic graphite electrode,6,9,14,15,19utilization of a carbon
black support will facilitate the future practical application in fuel cells.
Figure 2 shows the cyclic voltammogram (CV) curves of the
[CoN4]3/C in Ar-saturated (dotted curve) and O2-saturated
(solid curve) aqueous solution of 0.1 M KOH at a scan rate of
100 mV/s, respectively. As compared to no cathodic peak in the
Ar-saturated solution, a significant enhancement of the cathodic
peak at ꢀ0.26 V in the O2-saturated solution indicates the elec-
trocatalytic activity of [CoN4]3/C for the ORR.
To gain further insight into the role of [CoN4]3/C during the
ORR electrochemical process, the reaction kinetics was studied
by rotating disk voltammetry. Figure 3a presents the voltam-
metric profiles of [CoN4]3/C in the O2-saturated 0.1 M KOH
electrolyte, where the current density enhances with increasing
rotation rates (from 400 to 3600 rpm). The onset potential of
[CoN4]3/C for the ORR is approximately at ꢀ0.14 V, close to
that (ꢀ0.13 V) obtained from CV measurements (Figure 2).
Figure 3b depicts the corresponding KouteckyꢀLevich plots
(vs) at various electrode potentials. The data exhibit good
linearity, and the slopes remain approximately constant over
the potential range from ꢀ0.275 to ꢀ0.450 V, suggesting similar
electron transfer numbers for oxygen reduction at different
electrode potentials. The linearity and parallelism of the plots
are usually taken as an indication of first-order reaction kinetics
with respect to the concentration of dissolved O2.
J
ꢀ1 vs wꢀ1/2 at different electrode potentials. Symbols are experimental
data obtained from (A), and lines are linear regressions. (c) ORR
polarization curves for [MN4]3/C catalysts (M = Co and Fe) and Pt/C
(20 wt %) in 0.1 M KOH saturated with O2; (d) ORR polarization
curves for [CoN4]3/C, [CoN4]1.45/C, and 10/C. In (c) and (d), the
electrode rotation rate is 1600 rpm and the scan rate is 10 mV/s. A
catalyst loading of 25.45 μg/cm2 was used for all RDE voltammograms,
and the actual complex content in the catalyst was 20 wt %. Current
densities were normalized in reference to the geometric area of the RDE
(0.0707 cm2).
on Vulcan XC-72R). The same amount of catalysts by mass (25.45
μg/cm2) was loaded onto a glass carbon (GC) rotating disk
electrode (RDE). A characteristic set of polarization curves for
the ORR on [CoN4]3/C, [FeN4]3/C, and Pt/C are displayed in
Figure 3c. [CoN4]3/C shows the same onset potential of ꢀ
0.14 V as [FeN4]3/C, close to that of commercial Pt/C (ꢀ0.11
V). By using the KouteckyꢀLevich equations,20,21 the number of
electrons transferred (n) and kinetic limiting current density (JK)
were calculated and summarized in Table 1. Based on the analysis of
the onset potential, n and JK, for a given ligand 10, Co(II) ions as the
central metal ions in the [MN4]n complexes exhibit better catalytic
activities than Fe(II), which is different from the order of the MN4
phthalocyanines.22 Remarkably, the [CoN4]3/C catalyst shows the
highest selectivity and the highest mass activity of 9.63 mA/cm2
at ꢀ0.35 V (Table 1), which is more than two times higher than that
(4.44 mA/cm2) of the commercial Pt/C catalyst (20 wt % platinum
on Vulcan XC-72R) (Figure 3c). To the best of our knowledge, this
is the first time such an excellent electrochemical performance in the
ORR with transition metal N4-chelate molecules has been observed.
As mentioned above, the nature of active sites for the ORR
holds the key to the development of NPMCs. In comparison with
the recently reported high performance NPMCs based on the
mixture of the individual nitrogen-containing precursors and
transition metal salts,23,24 our structurally well-defined trinuclear
complexes can provide more information about the relationship
between molecular structure and the catalytic activity. Therefore,
we further investigated the effect of different numbers of Co(II)
ions on the catalytic activity for the ORR with metal-free ligand
10, [CoN4]1.45, and [CoN4]3, as depicted in Figure 3d. First,
[CoN4]3/C and [CoN4]1.45/C give the same onset potential
We benchmarked the electrocatalytic properties of [CoN4]3/C
and [FeN4]3/C against commercial Pt/C (20 wt % platinum
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Table 1. Summary of the Kinetic Parameters for Oxygen
Reduction at [CoN4]3/C, [CoN4]1.45/C, 10/C, [FeN4]3/C,
and the Reference Pt/C on GC Electrodes
planar conjugated macrocycle as a ligand. After coordination with
three Co ions, the [CoN4]3 complex supported on carbon black
as ORR catalysts reveals much better electrochemical activity and
long-term stability than the commercially available Pt/C catalyst
in alkaline electrolytes. Thus, our work leads to efficient, non-
precious, and stable metal catalysts as alternatives of Pt in fuel
cells and makes an important step toward a future “electromo-
bility”. Furthermore, the well-defined structures of these planar
trinuclear metal-N4 complexes provide more information about
the nature of active sites, which will facilitate the design of a more
magnificent structure with higher catalytic activity.
Onset
potential
(V)b
JK
na
(mA/cm2) c
Durability
[CoN4]3/C
[CoN4]1.45/C
10/C
3.7
3.3
1.8
2.9
3.9
ꢀ0.14
ꢀ0.14
ꢀ0.20
ꢀ0.15
ꢀ0.11
9.63
7.00
3.00
4.80
4.44
84.0%
73.5%
73.5%
27.2%
52.5%
[FeN4]3/C
Pt/C
a The electrons transferred number (n) per O2 molecule was calculated
from eqs 1 and 2 in the Supporting Information. b The onset potentials
were determined from rotating-disk voltammograms. c The kinetic
current density at ꢀ0.35 V was derived from the KouteckyꢀLevich plots.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis of ligands and metal
b
complexes, DFT calculations, structural characterizations, and
electrochemical measurements. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
baumgart@mpip-mainz.mpg.de; muellen@mpip-mainz.mpg.de
Present Addresses
§School of Environmental and Chemical Engineering, Shanghai
University, Shangda Road 99, Shanghai 200444, P. R. China.
’ ACKNOWLEDGMENT
We thank Dr. H. Norouzi-Arazi and Dr. D. Wu for valuable
discussions and Dr. M. Wagner for NMR technical support. We
are grateful for the financial support by Sumitomo Chemical.
Figure 4. Currentꢀtime (iꢀt) chronoamperometric response of
[CoN4]3/C and Pt/C modified GC electrode at ꢀ0.26 V in O2-saturated
aqueous solution of 0.1 M KOH at a rotation rate of 1600 rpm.
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