Controlling Proton and Electron Transfer Rates to Enhance the Activity of an Oxygen Reduction Electrocatalyst

Article abstract of DOI:10.1002/anie.201806795

An electrochemical approach is developed that allows for the control of both proton and electron transfer rates in the O₂ reduction reaction (ORR). A dinuclear Cu ORR catalyst was prepared that can be covalently attached to thiol-based self-assembled monolayers (SAMs) on Au electrodes using azide–alkyne click chemistry. Using this architecture, the electron transfer rate to the catalyst is modulated by changing the length of the SAM, and the proton transfer rate to the catalyst is controlled with an appended lipid membrane modified with proton carriers. By tuning the relative rates of proton and electron transfer, the current density of the lipid-covered catalyst is enhanced without altering its core molecular structure. This electrochemical platform will help identify optimal thermodynamic and kinetic parameters for ORR catalysts and catalysts of other reactions that involve the transfer of both protons and electrons.

Full text of DOI:10.1002/anie.201806795

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Accepted Article
Title: Controlling Proton and Electron Transfer Rates Enhances the
Activity of an Oxygen Reduction Electrocatalyst
Authors: Rajendra Gautam, Yi Teng Lee, Gabriel L Herman, Cynthia M
Moreno, Edmund C. M. Tse, and Christopher J Barile
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201806795
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Controlling Proton and Electron Transfer Rates Enhances the
Activity of an Oxygen Reduction Electrocatalyst
Rajendra P. Gautam, Yi Teng Lee, Gabriel L. Herman, Cynthia M. Moreno, Edmund C. M. Tse, and
Christopher J. Barile*
Abstract: Reactions involving proton and electron transfer are
fundamental to many chemical and biological processes. Here, we
We first designed a ligand that supports an active ORR catalyst
develop an electrochemical approach that allows for the control of
that can be incorporated into an electrode architecture with
both proton and electron transfer rates in the O
2
reduction reaction
3
tunable proton and electron transfer kinetics. The ligand, N -
5
(
ORR). We prepared a dinuclear Cu ORR catalyst that can be
covalently attached to thiol-based self-assembled monolayers
SAMs) on Au electrodes using azide-alkyne click chemistry. Using
benzyl-N -(but-3-yn-1-yl)-1H-1,2,4-triazole-3,5-diamine
(BTA),
was synthesized in four steps from benzylamine and contains
three important features (Scheme 1). First, the BTA ligand
contains a diaminotriazole core which upon coordination to Cu
forms a highly active ORR catalyst. This core is inspired by
previous studies with a dinuclear Cu complex of 3,5-diamino-
(
this architecture, the electron transfer rate to the catalyst is
modulated by changing the length of the SAM, and the proton
transfer rate to the catalyst is controlled with an appended lipid
membrane modified with proton carriers. By tuning the relative rates
of proton and electron transfer, we enhance the current density of
1
,2,4-triazole, which in terms of overpotential is one of the most
[23-25]
active molecular Cu ORR catalysts known.
Second, BTA
contains an alkyne moiety, which allows it to undergo the azide-
the lipid-covered catalyst without altering its core molecular structure. alkyne click reaction with azide-terminated SAMs. Lastly, BTA
possesses a benzyl group, which enables lipid monolayers to
assemble on top of a BTA SAM.[ The hydrophobic tails of the
lipid monolayer are appended to the hydrophobic benzyl groups
of the BTA SAM via Van der Waals interactions.
We envision that this type of electrochemical platform will aid in
identifying the optimal thermodynamic and kinetic parameters not
only for ORR catalysts, but for catalysts of other reactions that
involve the transfer of both protons and electrons.
20]
Reactions involving the transfer of multiple protons and
electrons are instrumental in many renewable energy conversion
schemes and biological processes.[1-7] One of the most important
2
redox reactions that involves proton transfer is the O reduction
reaction (ORR), which occurs at the cathode of fuel cells and in
mitochondria present in all aerobic life.[8-11] Unfortunately, the
mechanistic details of the ORR are difficult to elucidate because
of the complex interplay between the thermodynamics and
kinetics of the multiple proton and electron transfer steps
involved.[
12]
In this work, we design an electrochemical platform that allows
for the control of the thermodynamics and kinetics of proton and
electron transfer to a Cu-based molecular ORR catalyst. It is
well known that the electrode potential dictates the
thermodynamics of electron transfer and that the pH of the bulk
solution controls the thermodynamics of proton transfer in many
metal-centered proton transfer reactions.[13-16] The kinetics of
electron transfer can be modulated to a molecular catalyst
through the use of self-assembled monolayers (SAMs) with
varying alkyl chain lengths.[17-19] However, there are few
methods to control the kinetics of proton transfer to a catalyst in
an unconvoluted manner. Gewirth and coworkers recently
developed proton-permeable lipid membranes to alter the proton
transfer kinetics to a catalyst without perturbing its molecular
Scheme 1. Synthesis of BTA with three important features highlighted along
with the structure of the dinuclear CuBTA complex.
Electrodes containing the CuBTA ORR catalyst with tunable
proton and electron transfer rates were constructed in three
steps (Figures 1 and S1). First, Au electrodes were modified
with SAMs of azide-terminated thiols with different alkyl chain
lengths. Next, the BTA ligand was covalently attached to the
SAM using azide-alkyne click chemistry and subsequently
immersed in a Cu2 solution to form the active dinuclear Cu
complex. Lastly, a lipid monolayer containing 1-dodecylboronic
acid (DBA) as a proton carrier was appended on top of the SAM
to complete the electrode architecture.
identity.[
20,21]
These membranes contain amphiphilic alkyl proton
[
22]
carriers that deliver protons via a flip-flop diffusion process.
Here, we develop an electrode architecture containing an ORR
catalyst that allows for both the control of proton transfer rates
through the use of lipid membranes and the control of electron
transfer rates through the use of a modular SAM scaffold that
takes advantage of azide-alkyne click chemistry. The click
chemistry approach enables us to attach a synthesized ORR
catalyst to a SAM surface. The length of the catalyst-modified
SAM can be facilely modified without changing the identity of the
catalyst. Together, this click platform provides a means to
control electron transfer kinetics to the catalyst. By altering the
amount of proton carrier in the lipid layer of the same platform,
proton transfer kinetics can also be tuned.
+
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Having established that the CuBTA complex catalyzes the ORR,
we next evaluated the ORR activity of CuBTA when the SAM is
covered by
a
monolayer of 1,2-dimyristoyl-sn-glycero-3-
phosphocholine (DMPC) lipid. A LSV of CuBTA in the presence
of lipid displays an onset potential of about -0.2 V and a peak
current density of about -250 A cm-2 (Figure 2, red line). The
negative shift in onset potential and reduction in current density
indicates that the ORR activity of CuBTA significantly decreases
upon lipid formation. This inhibition of catalytic activity arises
from impeded proton transport across the hydrophobic lipid
membrane as has been observed in other lipid-covered
electrodes[ and is also manifested by an about 50 mV negative
shift in the midpoint potential of the Cu(I)/Cu(II) couple (Figure
26]
Figure 1. Schematic of the fabrication of lipid-modified SAMs used to control
the electron and proton transfer rates to a molecular O reduction catalyst.
2
S2). O
the ORR by lipid-covered CuBTA is not limited by a lack of O
Figure S6). Results obtained from electrochemical impedance
2
diffusion through the lipid monolayer is fast, and hence
2
Electrochemical techniques were used to assess the chemical
structure of the electrodes at each fabrication stage. Electrodes
containing the CuBTA catalyst were first assembled using an
azide-terminated thiol containing 5 methylene groups (Figures 1
and S1, n = 5). The presence of Cu in the catalyst was
confirmed by a Cu(I)/Cu(II) couple in cyclic voltammetry (CV)
experiments (Figure S2). By integrating the charge of the Cu
couple in the CV, the amount of Cu catalyst on the surface was
determined to be 1.3 x 10-11 mol cm , a value that matches what
is expected for a full monolayer of Cu (see SI). A linear sweep
voltammogram (LSV) of the CuBTA assembly without a lipid
layer demonstrates that CuBTA is an active ORR catalyst that
(
spectroscopy demonstrate that the molecular length of the
system increases upon performing the click reaction and further
increases upon lipid formation, as expected (Figure S7). Upon
incorporating the DBA proton carrier in the lipid, the CuBTA
-2
catalytic current density increases to about -425 A cm , but the
onset potential does not change considerably (Figure 2, blue
line). This result demonstrates that the presence of the proton
carrier enhances the kinetics of the ORR without significantly
altering the thermodynamics of the reaction. The peak in the
LSV in the presence of lipid and proton carrier is due to
kinetically-limited proton transfer. DBA delivers protons across
-2
exhibits an onset potential of about 0 V vs. Ag/AgCl and an O
diffusion-limited peak current density of about -900 A cm-2 in
-sparged pH buffer (Figure 2, black line). Control
experiments in the absence of O (Figures 2 and 3, dashed
2
the lipid membrane via flip-flop diffusion in
controlled fashion as discussed previously.[21] Electrochemical
blocking experiments using Fe(CN) 3-
demonstrate that the ORR
a kinetically-
O
2
7
6
2
does not compromise the integrity of the lipid layer regardless of
whether the proton carrier is present (Figure S8).
lines), with Zn, or without performing the click chemistry do not
exhibit significant ORR activity, further demonstrating that Cu is
necessary to produce an active catalyst (Figures S3 and S4).
Further control experiments comparing the activity of Cu
complexes of 3,5-diamino,1,2,4-triazole and 1,2,3-triazole
indicate that the Cu-1,2,3-triazole complex is not a competent
ORR catalyst, demonstrating that the 1,2,3-triazole linker formed
from the azide-alkyne click chemistry is not contributing to the
ORR activity measured (Figure S5).
To modify the electron transfer kinetics to the CuBTA catalyst,
we changed the SAM to an azide-terminated thiol containing 11
methylene groups (Figures 1 and S1, n = 11). As measured
using Laviron analysis[27] (Figure S9), this longer-chained SAM,
which possesses a greater barrier for electron tunneling, exhibits
about 30 times slower electron transfer than the previously
described thiol containing 5 methylene groups ((1.2 ± 0.2 ) s-1
and (39 ± 3) s-1 for the C11 and C
reduction by CuBTA attached to the C11 SAM displays an
onset potential of about 0 V and a peak current density of about
275 A cm-2 (Figure 3, black line). The onset potentials for ORR
using C11 and C SAMs are similar because the active catalyst is
SAMs, respectively). A LSV of
5
O
2
-
5
the same in both cases, which means that the thermodynamics
for catalyzing the ORR do not change when altering the SAM.
However, the peak current density is significantly less for the C11
SAM due to the slower electron transfer rate. Also, the Tafel
slope of the LSV for the C11 SAM is significantly higher than that
of the C
are impeded in the C11 case (Figure S10). Upon covering the
11-linked catalyst with lipid, the current density decreases and
the onset potential shifts negative in a manner similar to what is
observed with the lipid-covered catalyst on C SAM (Figure 3,
red line). Incorporation of the proton carrier also enhances the
5
SAM, further indicating that electron transfer kinetics
C
Figure 2. Linear sweep voltammograms of O
catalyst using an azide-terminated thiol SAM containing 5 methylene groups
black line) covered by a DMPC lipid monolayer (red line) with 10 mol% DBA
proton carrier (blue line) at 10 mV/s in O -saturated pH 7 buffer. Dashed lines
are the corresponding voltammograms in N -saturated pH 7 buffer.
2
reduction by the CuBTA
(
5
2
2
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current density when the electron transfer rate to the catalyst is
slow. In contrast, with a C SAM in which relatively fast electron
5
transfer occurs, the proton carrier’s ability to enhance the ORR
activity is not as pronounced. Strikingly, the current density
catalytic current, but does not significantly alter the onset
potential for catalysis (Figure 3, blue line). In short, similar
changes in the ORR voltammetry occur using both the C11 and
C
5
SAMs, but the slower electron transfer rate for the C11 SAM
5
enhancements are similar for the C SAM with the maximum
decreases the overall kinetics of catalysis.
amount of proton carrier (10 mol%) and the C11 SAM with the
minimum amount of proton carrier (0.2 mol%) because the two
cases have similar kH+/ke- values (compare rightmost red point
and leftmost black point, Figure 4). These results demonstrate
that the interplay between proton and electron transfer rates
dictates the overall activity of the ORR catalyst. We note that
relative changes in the peak current density do not strictly reflect
changes in the catalytic rate due to small shifts in the position of
the peaks, but these effects are minimal.
Figure 3. Linear sweep voltammograms of O
catalyst using an azide-terminated thiol SAM containing 11 methylene groups
black line) covered by a DMPC lipid monolayer (red line) with 10 mol% DBA
proton carrier (blue line) at 10 mV/s in O -saturated pH 7 buffer. Dashed lines
are the corresponding voltammograms in N -saturated pH 7 buffer.
2
reduction by the CuBTA
(
2
2
To complement the modulation of electron transfer rates using
SAMs, we also varied the proton transfer rate to the CuBTA
catalyst by changing the concentration of proton carrier in the
lipid layer. The data in Figure S11 show that as the amount of
proton carrier in the lipid layer increases, the current
enhancement afforded by the lipid layer compared to the lipid-
only case correspondingly increases. The current enhancement
observed saturates when around 10 mol% of proton carrier is
added to the lipid membrane, which is the maximum amount of
DBA that can incorporate in the lipid during vesicle formation.[21]
Since proton transfer rates across this lipid membrane have
already been measured (Figure S12),[21] we calculated the ratio
of the proton and electron transfer rates to CuBTA (kH+/ke-). The
ORR current density enhancement by the proton carrier
increases as a function of kH+/ke- (Figure 4). In the absence of
the proton carrier, proton transfer to the catalyst is almost
entirely blocked by the hydrophobic lipid layer, and as a result,
2
Figure 4. O reduction current density enhancement by CuBTA imparted by
the incorporation of the DBA proton carrier in the lipid as a function of the ratio
of proton and electron transfer rates (kH+/ke-) using an azide-terminated thiol
SAM containing 5 (red points) and 11 (black points) methylene groups.
To the best of our knowledge, this work is the first example of an
electrochemical platform that allows for the quantitative control
of both the electron and proton transfer rates to a single
molecular catalyst without changing its identity. For the CuBTA
ORR catalyst studied here, tuning the relative rates of proton
and electron transfer enable the catalytic activity to be
substantially enhanced by (297 ± 73)% compared to the lipid-
only case. We envision that this sort of electrode scheme will
enable researchers to elucidate the kinetic parameters needed
for optimal catalysis, not only for the ORR, but for any reaction
involving the transfer of protons and electrons.
-
[28]
the 1 e reduction of O
2
to superoxide predominantly occurs.
With added proton carrier, kH+/ke- increases, which favors the 4
-
e
reduction of
O
2
2
to H O as evidenced by dye-based
spectroelectrochemical experiments quantifying the amount of
partially reduced oxygen species (Figure S13) and results in
increased current density (Figure S14). Therefore, the addition
of proton carrier changes the rate-determining step (RDS) for
the ORR. In particular, the RDS changes from electron transfer
with lipid in the absence of proton carrier to proton transfer in the
presence of proton carrier as discussed previously.[20] The use of
both the C11 and C SAMs with different electron transfer rates
5
allows for two orders of magnitude of kH+/ke- to be accessed. The
current enhancement using the C11 SAM is greater than the
Acknowledgements
Acknowledgement is made to the donors of The American
Chemical Society Petroleum Research Fund for support of this
research. We also thank Research and Innovation at the
University of Nevada, Reno for support of this research. We
acknowledge the Shared Instrumentation Laboratory in the
Department of Chemistry at the University of Nevada, Reno.
E.C.M.T. acknowledges the Croucher Foundation for start-up
funding.
enhancement measured on the C
5
SAM regardless of the
amount of proton carrier in the lipid layer (compare black points
to red points, Figure 4). The greater enhancement with the C11
SAM occurs because the accelerated proton transfer rate with
the proton carrier has a larger relative impact on the catalytic
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[
[
[
[
13] A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals
and Applications, 2nd ed., John Wiley & Sons, Inc. Hoboken, NJ, 2001.
14] J. L. Oberst, M. S. Thorum, A. A. Gewirth, J. Phys. Chem. C 2012,
Conflict of interest
The authors declare no conflict of interest.
116, 25257-25261.
15] R. Boulatov, J. P. Collman, I. M. Shiryaeva, C. J. Sunderland, J. Am.
Chem. Soc. 2002, 124, 11923-11935.
Keywords: oxygen reduction • self-assembled monolayer •
electrocatalyst • proton and electron transfer • voltammetry
16] C. C. L. McCrory, X. Ottenwaelder, T. D. P. Stack, C. E. D. Chidsey, J.
Phys. Chem. A 2007, 111, 12641-12650.
[
[
17] C. E. D. Chidsey, Science 1991, 251, 919.
18] J. P. Collman, N. K. Devaraj, T. P. A. Eberspacher, C. E. D. Chidsey,
Langmuir 2006, 22, 2457-2464.
REFERENCES
[
[
[
19] N. K. Devaraj, R. A. Decreau, W. Ebina, J. P. Collman, , C. E. D.
Chidsey, J. Phys. Chem. B 2006, 110, 15955-15962.
[
1]
2]
C. J. Chang, M. C. Y. Chang, N. H. Damrauer, D. G. Nocera, Biochim.
Biophys. Acta 2004, 1655, 13-28.
20] C. J. Barile, E. C. M. Tse, Y. Li, T. B. Sobyra, S. C. Zimmerman, A.
Hosseini, A. A. Gewirth, Nat. Mater. 2014, 13, 619-623.
[
D. R. Weinberg, C. J. Gagliardi, J. F. Hull, C. F. Murphy, C. A. Kent,
B. C. Westlake, A. Paul, D. H. Ess, D. G. McCafferty, T. J. Meyer,
Chem. Rev. 2012, 112, 4016-4093.
21] E. C. M. Tse, C. J. Barile, N. A. Kirchschlager, Y. Li, J. P. Gewargis,
S. C. Zimmerman, A. Hosseini, A. A. Gewirth, Nat. Mater. 2016, 15,
7
54-759.
[
[
3]
4]
J. M. Mayer, I. J. Rhile, Biochim. Biophys. Acta 2004, 1655, 51-58.
C. A. Farberow, A. Godinez-Garcia, G. Peng, J. F. Perez-Robles, O.
Solorza-Feria, M. Mavrikakis, ACS Catalysis 2013, 3, 1622-1632.
L. Trotochaud, S. L. Young, J. K. Ranney, S. W. Boettcher, J. Am.
Chem. Soc. 2014, 136, 6744-6753.
[
[
[
[
[
22] C. J. Barile, E. C. M. Tse, Y. Li, J. P. Gewargis, , N. A. Kirchschlage,
S. C. Zimmerman, A. A. Gewirth, Biophys. J. 2016, 110, 2451-2462.
23] M. S. Thorum, J. Yadav, A. A. Gewirth, Angew. Chem. Int. Edit. 2009,
[
[
5]
6]
48, 165-167.
24] M. A. Thorseth, C. E. Tornow, E. C. M. Tse, A. A. Gewirth, Coordin.
Chem. Rev. 2013, 257, 130-139.
C. M. Che, K. Lau, T. C. Lau, C. K. Poon, J. Am. Chem. Soc. 1990,
112, 5176-5181.
25] A. A. Gewirth, J. A. Varnell, A. M. DiAscro. Chem. Rev. 2018, 118,
[
[
7]
8]
K.-F. Chin, K.-Y. C.-M. Wong, J. Chem. Soc. Dalton 1993, 1, 197-198.
A. M. Pasqualeti, E. Padgett, D.-Y. Kuo, D. A. Muller, F. H. B. Lima, J. J.
Suntivich, Electrochem. Soc. 2017, 164, F645-F650.
2313-2339.
26] Y. Li, E. C. M. Tse, C. J. Barile, A. A. Gewirth, S. C. Zimmerman, J.
Am. Chem. Soc. 2015, 137, 14059-14062.
[
[
[
9]
J. Suntivich, H. A. Gasteiger, N. Yabuuchi, H. Nakanishi, J. B.
Goodenough, Y. Shao-Horn, Nat. Chem. 2011, 3, 546.
[
27] E. Laviron, J. Electroanal. Chem. Interf. Electrochem. 1974, 52, 355-
93.
3
10] J. A. Herron, J. Jiao, K. Hahn, G. Peng, R. R. Adzic, M. Mavrikakis,
Electrocatalysis 2012, 3, 192-202.
[
28] A. Hosseini, C. J. Barile, A. Devadoss, T. A. Eberspacher, R. A.
Decreau, J. P. Collman, J. Am. Chem. Soc. 2011, 133, 11100-11102.
11] D. Banham, F. Feng, K. Pei, S. Ye, V. J. Birss, Mater. Chem. A 2013, 1,
2812-2820.
[
[
12] A. A. Gewirth, M. S. Thorum, Inorg. Chem. 2010, 49, 3557-3566.
a]
b]
Rajendra P. Gautam, Yi Teng Lee, Gabriel L. Herman, Cynthia M.
Moreno, Prof. Dr. Christopher J. Barile
Department of Chemistry, University of Nevada, Reno, 1664 N. Virginia
St., Reno, NV 89557
E-mail: cbarile@unr.edu
[
Prof. Dr. Edmund C. M. Tse
Department of Chemistry, The University of Hong Kong, Pokfulam
Road, Hong Kong SAR
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