Oxygen reduction activity of a copper complex of 3,5-diamino-1,2,4-triazole supported on carbon black

Article abstract of DOI:10.1002/anie.200803554

(Chemical Equation Presented) Reduction deduction: Precipitation of an insoluble copper triazole complex onto a carbon black support leads to the formation of an efficient catalyst for the fourelectron reduction of O ₂ to H₂O. The ox

Full text of DOI:10.1002/anie.200803554

Angewandte
Chemie
DOI: 10.1002/anie.200803554
Electrocatalysis
Oxygen Reduction Activity of a Copper Complex of 3,5-Diamino-1,2,4-
triazole Supported on Carbon Black**
Matthew S. Thorum, Jessica Yadav, and Andrew A. Gewirth*
Electrocatalysis of the oxygen reduction reaction (ORR) is
currently of widespread interest because of its application in
fuel-cell cathodes. Slow reaction kinetics significantly impact
the efficiency of fuel cells and, even with Pt catalysts, the
onset of the ORR occurs at approximately 1.0 V, which is well
below the reversible potential for oxygen reduction of 1.23 V
versus the reversible hydrogen electrode (RHE). Multi-
copper oxidases (exemplified by laccase) activate oxygen at a
site containing three Cu atoms with spacings of approximately
The efficacy of laccase led us to wonder if Cu coordination
complexes or polymers composed of Cu coordinated with
II
bridging azole-type ligands, such as the 3,5-diamino-1,2,4-
[
14]
triazole (Hdartz) ligand (Scheme 1), and with other weakly
[
1]
3
.5 ꢀ and exhibit remarkable ORR electroactivity at poten-
[
2,3]
tials approaching 1.2 V (versus RHE).
Given the high cost
Scheme 1. 3,5-Diamino-1,2,4-triazole (Hdatrz) ligands bridging two
copper centers. Typical Cu···Cu spacing is 3.5 ꢀ. Counterions and
ligated water are omitted.
and limited supply of Pt, a copper-containing complex
adsorbed on an electrode surface that exhibited this level of
reactivity would be highly desirable.
There are few reports of synthetic Cu complexes that
exhibit significant ORR activity. Several mononuclear Cu
complexes have been investigated by adsorption onto graph-
coordinated ligands (such as water or sulfate) might provide
stability in addition to multicopper sites that could potentially
[4–9]
II
ite electrodes.
Cu complexes with phenanthroline ligands,
bind and activate O (as these have Cu···Cu spacings similar to
2
[
6]
which were popularized by Zhang and Anson, are the best-
studied to date. McCrory et al. have investigated the Cu
complexes of a variety of substituted phenathrolines, the best
of which demonstrated an ORR onset of about 0.68 V (RHE)
at pH 4.8, and concluded that further increases in activity
laccase). The combination of aqueous solutions of CuSO and
4
a variety of substituted pyrazoles and triazoles led to the
precipitation of insoluble compounds. The lack of solubility
was encouraging from a stability standpoint, but made them
incompatible with the techniques previously used to prepare
electrodes for electrochemical characterization which require
[
7]
were unlikely by using this system. Attempts that used
putative multicopper complexes include a water-soluble
[4–13]
the use of solution phases.
II
catalyst formed by the coordination of two Cu ions in a
Herein, we report a simple method for the evaluation of
the electrocatalytic activity of insoluble coordination com-
pounds by direct precipitation onto carbon black (Vulcan XC-
72). Utilization of a carbon black support allows the use of
electrode fabrication methods and characterization techni-
ques developed for the analysis of carbon-supported Pt
catalysts and could also facilitate the transition to practical
[10]
hexaazamacrocyclic ligand, and electrodes modified with
II
solution-cast polymers containing Cu ions, including a
[
11]
II
copper–poly(histidine) complex,
and a polymeric Cu
[
12]
oxalato complex. However, the demonstration of polynu-
clearity when adsorbed on an electrode in any of these
systems is lacking. A cytochrome c oxidase model compound
with a Cu–Fe site has also been investigated by adsorption
onto graphite, but the ORR onset is quite low at about 0.2 V
versus RHE and the complex that contains only Fe has similar
[
1,15]
application in a fuel cell.
We have applied this method to
the evaluation of the ORR activity of the insoluble multi-
copper complex formed with 3,5-diamino-1,2,4-triazole
(Hdatrz).
[
13]
activity.
Figure 1 shows the characterization of the [Cu(Hdatrz)]
complex supported on Vulcan and a blank sample of
unmodified Vulcan using a rotating ring-disk electrode
(RRDE) obtained in an oxygen-saturated electrolyte at
pH 7. The disk potential was swept to determine the potential
dependence of the ORR and the ring potential was held
constant at 1.2 V to observe the oxidation of any generated
[
*] M. S. Thorum, J. Yadav, Prof. A. A. Gewirth
Department of Chemistry
University of Illinois at Urbana-Champaign
600 S. Mathews Ave., Urbana, IL 61801 (USA)
Fax: (+1)217-244-3186
E-mail: agewirth@illinois.edu
[
**] This work was supported by the DOE (DE-FG02-87ER46260). M.S.T.
acknowledges a Snyder Fellowship in Chemistry. Magnetic sus-
ceptibility data were kindly provided by Amber C. McConnell,
William W. Shum, and Prof. Joel S. Miller (University of Utah) who
were supported by U.S. DOE Basic Energy Sciences (grant no. DE
FG 03-93ER45504).
[15]
peroxide intermediate. The collection efficiency of the ring
under the conditions of the experiment was calculated to be
0
.04 from the ring-to-disk current ratio obtained for the two-
electron reduction of O by the unmodified Vulcan. This was
2
used to calculate the fraction of peroxide intermediates
[15]
formed with a method described elsewhere.
The onset of O2
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803554.
reduction occurs at a disk potential of 0.73 V (versus RHE)
Angew. Chem. Int. Ed. 2009, 48, 165 –167
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
165

Communications
equivalent to a À30 mV per pH unit change using a pH-
independent reference. This is half the value of À60 mV per
pH unit predicted by the Nernst equation for a reaction
+
involving the transfer of one H ion per electron and implies
that the rate-determining step involves the transfer of two
+
electrons and one H ion. This is consistent with either the
II
reduction of two Cu centers and the concomitant protona-
À
2À
tion of a bridging OH or O ligand or with the reduction of
À
O to a hydroperoxo (HOO ) intermediate.
2
The observed ORR onset of 0.86 V (versus RHE) at
pH 13 represents a reduction of the ORR overpotential to
0
.37 V. To the best of our knowledge, this makes the
[
Cu(Hdatrz)] complex the most efficient synthetic copper-
based ORR electrocatalyst reported to date and the first to be
evaluated at this pH value. The 0.73 V onset at pH 7 is higher
than the 0.67 V onset reported for a Cu complex with a
hexaazamacrocycle which was previously the highest onset
Figure 1. Reduction of O at a rotating Pt ring/glassy carbon disk
electrode supporting [Cu(Hdatrz)]/Vulcan (solid line) or unmodified
Vulcan XC-72 (dashed line) at 1600 rpm in NaClO (0.1m) and
4
2
Britton–Robinson buffer (0.04m, pH 7.0) saturated with O . Disk
2
[10]
current density (jdisk) is shown on the lower half and ring current (Iring
)
reported at this or any pH. At more acidic pH values the
onset is comparable to the best mononuclear Cu phenanthro-
is shown on the upper half of the y-axis. The disk potential was
À1
scanned at 5 mVs ; the ring potential was constant at 1.2 V.
[7]
line complexes.
At pH 7 the catalyst shows excellent cycle-to-cycle
stability; 24 hour stability tests showed a decay of approx-
imately 4% in limiting current (see the Supporting Informa-
tion). At pH 1 and pH 13 there was a significant decline in
activity upon repeated cycling of the applied potential, which
is likely to arise from the instability of the complex at these
pH values.
and the reduction current becomes diffusion-limited at
approximately 0.35 V. At more negative values, the ring
current is less than 1 mA, which corresponds to less than 5%
peroxide intermediate generation or almost complete four-
electron reduction of O to water. At more positive potentials,
the ring current increases until 50% peroxide intermediate
formation is observed at approximately 0.7 V, which corre-
The two reported crystal structures containing Cu and the
hdatrz ligand are consistent with the bridging coordination
mode illustrated in Scheme 1 and consistent with the approx-
imate 1:1 ratio between Cu and Hdatrz shown by elemental
2
sponds to an average of three electrons per O molecule
2
[
14]
(
more detailed analyses, including Koutecky-Levich and Tafel
analysis.
In order to characterize the adsorbed catalyst
plots are given in the Supporting Information).
complex, magnetic susceptibility measurements were under-
The catalyst is sufficiently stable to allow the evaluation of
ORR activity over a wide range of pH values (Figure 2). The
onset potential was found to increase linearly at a rate of
taken as shown in Figure 3. The expected value of cT for an
1
À1
S = = system is 0.374 emuKmol , which is independent of
2
the temperature and would indicate the presence of normal
spin-only mononuclear Cu sites. The values of cT for the
crystalline complexes of Cu with Hdatrz increase with
temperature, which is consistent with an antiferromagnetic
3
0 mV per pH unit of the electrolyte (Figure 2), which is
II
interaction between Cu ions with a singlet ground state and a
Figure 2. Potentials at which the onset of O reduction occurs at
2
electrodes modified with [Cu(Hdatrz)] in electrolytes of varying pH and
a linear fit ((31Æ4) mV per pH unit slope and (0.49Æ0.03) V y-
intercept). Electrodes were rotated at 1600 rpm in either HClO
4
(
0.1m), NaClO (0.1m), and Britton-Robinson buffers (0.04m, pH 4, 7,
4
or 10) or NaOH (0.1n) saturated with O . Potential scanned at
Figure 3. Temperature dependence of the cT value of the [Cu(Hdatrz)]/
Vulcan composite. Data are shown per mole of Cu and corrected for
the significant diamagnetism of the Vulcan substrate.
2
À1
5
mVs . Onset potential chosen as the potential at which the current
À2
density reaches À5 mAcm
.
1
66
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 165 –167

Angewandte
Chemie
[
14]
thermally populated triplet state. For the adsorbed [Cu-
Hdatrz)] complex, we observe the expected low cT value at
calibrated to the RHE scale after each experiment by saturating the
cell with H and measuring the open circuit potential at the Pt ring
(
2
electrode. All potentials are reported versus RHE. Britton–Robinson
buffers were used to control the pH at intermediate values and were
prepared to contain CH COOH (0.04m), H PO (0.04m), H BO4
low temperature. This value increases with temperature to a
peak value of 0.372 emuKmol at 83 K, which implies that
most of the Cu spins are effectively unpaired at this temper-
ature. However, as the temperature is raised further there is a
subsequent decrease in the cT value followed by a gradual
À1
3
3
4
3
(
(
0.04m), and NaClO (0.1m) and the pH was adjusted with NaOH
0.1n).
4
Magnetic characterization: The 5 to 300 K magnetic susceptibil-
À1
increase to a value of 0.283 emuKmol at 295 K. This
ity, c(T), was measured on a Quantum Design MPMS-5XL magneto-
meter. In order to correct for the diamagnetism of the carbon support,
the susceptibility of unmodified Vulcan XC-72 was also measured
across the entire temperature range and then subtracted.
complicated behavior indicates that the complex is distinct
from those previously crystallized, but indicates that spin
pairing between Cu centers occurs and that closely spaced Cu
centers must be present.
Received: July 21, 2008
The development of simple methods to adsorb insoluble
Cu complexes onto carbon black opens the possibility of
developing other catalysts that are more closely aligned to the
three Cu active site of laccase. It also allows the use of ex situ
methods to characterize and obtain structural information
about the on-electrode catalyst. The observed ORR activity
of the [Cu(Hdatrz)] complex is very high for synthetic copper-
based electrocatalysts and, given the demonstrated activity of
laccase, we believe large increases in activity may be realized
upon the development of improved model complexes.
Published online: November 26, 2008
Keywords: copper · electrocatalysis · energy conversion ·
.
oxygen · reduction
[1] H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Appl.
Catal. B 2005, 56, 9.
[
[
[
2] N. Mano, V. Soukharev, A. Heller, J. Phys. Chem. B 2006, 110,
1180.
3] C. F. Blanford, R. S. Heath, F. A. Armstrong, Chem. Commun.
006, 1710.
4] J. Zhang, F. C. Anson, J. Electroanal. Chem. 1993, 348, 81.
1
2
Experimental Section
[5] V. L. N. Dias, E. N. Fernandes, L. M. S. da Silva, E. P. Marques, J.
Zhang, A. L. B. Marques, J. Power Sources 2005, 142, 10.
[6] J. Zhang, F. C. Anson, J. Electroanal. Chem. 1992, 341, 323.
[7] C. C. L. McCrory, X. Ottenwaelder, T. D. P. Stack, C. E. D.
Chidsey, J. Phys. Chem. A 2007, 111, 12641.
[8] C. X. Cai, K. H. Xue, X. Y. Xu, Q. H. Luo, J. Appl. Electrochem.
1997, 27, 793.
[9] M. Wang, X. Xu, J. Gao, N. Jia, Y. Cheng, Russ. J. Electrochem.
2006, 42, 878.
[10] K. Slowinski, Z. Kublik, R. Bilewicz, M. Pietraszkiewicz, J.
Chem. Soc., Chem. Commun. 1994, 1087
[11] Y. C. Weng, F. R. F. Fan, A. J. Bard, J. Am. Chem. Soc. 2005, 127,
17576.
Preparation of [Cu(hdatrz)]-modified Vulcan XC-72: Vulcan XC-72
(
1.00 g, Cabot), Cu(SO ) ·5H O (0.200 g, 0.801 mmol, Aldrich,
4 2 2
9
9.995%), and water (20 mL, Milli-Q UV Plus, 18.2 MW) were
combined and sonicated to form a viscous suspension. A solution of
,5-diamino-1,2,4-triazole (hdatrz, 0.159 g, 1.60 mmol, Aldrich, 98%)
3
in water (10 mL) was then added dropwise with stirring. The mixture
was stirred for 18 h, the solids were collected by suction filtration,
dried in vacuo for 3 h at 908C, and pulverized with a mortar and
pestle. Elemental analysis (%) found: C 85.2, H 0.14, N 5.36, Cu 3.76.
Electrochemical characterization: A catalyst ink containing
À1
[
(
Cu(Hdatrz)] (1.0 mgmL
)
on Vulcan and Nafion solution
À1
4 mLmL 5%, Aldrich) was prepared in water and sonicated for
1
0 min. A 20 mL drop was applied to a glassy carbon disk electrode
[12] J. Kim, A. A. Gewirth, Bull. Korean Chem. Soc. 2007, 28, 1322.
[13] H. Shin, D.-H. Lee, C. Kang, K. D. Karlin, Electrochim. Acta
2003, 48, 4077.
[14] E. Aznar, S. Ferrer, J. Borrꢁs, F. Lloret, M. Liu-Gonzꢁlez, H.
Rodrꢂguez-Prieto, S. Garcꢂa-Granda, Eur. J. Inorg. Chem. 2006,
5115.
2
(0.196 cm ) with a Pt ring (Pine Instruments, glassy carbon polished
with alumina (0.05 m) and sonicated in water prior to electrode
assembly) and dried under a stream of Ar. The Ar flow was adjusted
by trial and error until a uniform catalyst distribution across the
electrode surface was obtained. Electrochemical measurements were
performed using a CHI 760C bipotentiostat (CH Instruments) using
with a Pt gauze counter electrode and a “no-leak” Ag/AgCl reference
electrode (Cypress Systems), which was separated from the working
electrode by a Luggin capillary. The reference electrode was
[15] U. A. Paulus, T. J. Schmidt, H. A. Gasteiger, R. J. Behm, J.
Electroanal. Chem. 2001, 495, 134.
Angew. Chem. Int. Ed. 2009, 48, 165 –167
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
167