Enzyme-Inspired Iron Porphyrins for Improved Electrocatalytic Oxygen Reduction and Evolution Reactions

Article abstract of DOI:10.1002/anie.202015478

Nature uses Fe porphyrin sites for the oxygen reduction reaction (ORR). Synthetic Fe porphyrins have been extensively studied as ORR catalysts, but activity improvement is required. On the other hand, Fe porphyrins have been rarely shown to be efficient for the oxygen evolution reaction (OER). We herein report an enzyme-inspired Fe porphyrin 1 as an efficient catalyst for both ORR and OER. Complex 1, which bears a tethered imidazole for Fe binding, beats imidazole-free analogue 2, with an anodic shift of ORR half-wave potential by 160 mV and a decrease of OER overpotential by 150 mV to get the benchmark current density at 10 mA cm^?2. Theoretical studies suggested that hydroxide attack to a formal Fe^V=O form the O?O bond. The axial imidazole can prevent the formation of trans HO-Fe^V=O, which is less effective to form O?O bond with hydroxide. As a practical demonstration, we assembled rechargeable Zn-air battery with 1, which shows equal performance to that with Pt/Ir-based materials.

Full text of DOI:10.1002/anie.202015478

Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 7576–7581
International Edition:
German Edition:
doi.org/10.1002/anie.202015478
doi.org/10.1002/ange.202015478
Electrocatalytic Oxygen Reduction
Enzyme-Inspired Iron Porphyrins for Improved Electrocatalytic
Oxygen Reduction and Evolution Reactions
+
+
Lisi Xie , Xue-Peng Zhang , Bin Zhao, Ping Li, Jing Qi, Xinai Guo, Bin Wang, Haitao Lei,
Wei Zhang, Ulf-Peter Apfel, and Rui Cao*
Abstract: Nature uses Fe porphyrin sites for the oxygen
reduction reaction (ORR). Synthetic Fe porphyrins have been
extensively studied as ORR catalysts, but activity improvement
is required. On the other hand, Fe porphyrins have been rarely
shown to be efficient for the oxygen evolution reaction (OER).
We herein report an enzyme-inspired Fe porphyrin 1 as an
efficient catalyst for both ORR and OER. Complex 1, which
bears a tethered imidazole for Fe binding, beats imidazole-free
analogue 2, with an anodic shift of ORR half-wave potential by
< 0.80 V
(RHE).
versus
reversible
hydrogen
electrode
[
10,11,14,25,26]
Thus, ORR activity improvement of
molecular Fe porphyrins is required.
In the structure of CcOs, there is a histidine imidazole
group binding to the axial position of Fe (Figure 1a).
Synthetic modeling studies from Collman
demonstrated that this axial imidazole can increase electron
density on Fe through a so-called “push effect”. This effect is
[
19]
[27]
and Dey
favorable for O binding and more importantly can assist the
2
1
60 mVand a decrease of OER overpotential by 150 mV to get
OÀO bond cleavage by increasing pK of oxygen atoms of the
a
À2
the benchmark current density at 10 mAcm . Theoretical
studies suggested that hydroxide attack to a formal Fe =O form
resulted Fe-O adduct, which leads to both improved activity
2
V
and selectivity for O reduction to water. Recently, by using
2
the OÀO bond. The axial imidazole can prevent the formation
Fe phthalocyanines and Co corroles, electron-donating axial
ligands have also been reported to improve ORR activity by
notably shifting ORR half-wave potentials to the anodic
V
of trans HO-Fe =O, which is less effective to form OÀO bond
with hydroxide. As a practical demonstration, we assembled
rechargeable Zn-air battery with 1, which shows equal
performance to that with Pt/Ir-based materials.
[
28]
[29]
[30]
direction by Cho,
Zagal,
and us.
Therefore, results
from natural and artificial examples suggested that proper
axial ligands on Fe porphyrins will promote electrocatalytic
ORR performance in terms of both activity and selectivity.
N
ature uses Fe porphyrin heme sites to activate and reduce
[
1,2]
O .
Specifically, O reduction to water, as mediated by
2
2
cytochrome c oxidases (CcOs), is a vital process in respira-
[
1,3]
tion. Inspired by nature, many synthetic Fe porphyrins and
other related metal macrocycles have been designed and
[
4–18]
investigated as catalysts for O reduction.
From these
2
studies, fundamental knowledge to boost catalyst perfor-
mance is learned. For example, efficient electron transfer to
Fe porphyrin sites and proper hydrogen bonding interactions
with Fe-O adducts can facilitate the 4e reduction of O to
2
2
[
11,19–24]
water.
However, despite these achievements in improv-
ing selectivity, Fe porphyrins in general showed moderate
electrocatalytic ORR activity with half-wave potential
[
+]
[+]
[
*] L. S. Xie, Dr. X.-P. Zhang, B. Zhao, P. Li, Dr. J. Qi, X. A. Guo,
Dr. B. Wang, Dr. H. T. Lei, Prof. Dr. W. Zhang, Prof. Dr. R. Cao
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of
Education, School of Chemistry and Chemical Engineering, Shaanxi
Normal University
Xi’an 710119 (China)
E-mail: ruicao@snnu.edu.cn
Prof. Dr. U.-P. Apfel
Ruhr-Universitꢀt Bochum, Fakultꢀt fꢁr Chemie und Biochemie,
Anorganische Chemie I
Universitꢀtsstrasse 150, 44801 Bochum (Germany)
and
Fraunhofer UMSICHT
Osterfelder Strasse 3, 46047 Oberhausen (Germany)
Figure 1. a) Diagram showing the electronic “push effect” of the trans
axial histidine imidazole group in CcOs. b) Molecular structure of Fe
porphyrins 1 and 2 used in this work. c) UV/Vis spectra of 1, 2, and 2
added with one equivalent of methylimidazole (Melm) in THF. d) X-
band EPR data of 1 and 2 in THF at 100 K.
+
[
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202015478.
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III
On the other hand, Fe porphyrins have been rarely shown
LS:HS Fe of 1 increased (Figure S15), confirming the
[
31,32]
to be efficient for OER. Metal porphyrins, including Co,
equilibrium between binding and unbinding of OTf anions
on Fe. All these results demonstrate that tethered imidazole
in 1 binds with Fe.
[
33–35]
[36]
Ni,
and Cu, have been reported to be able to electro-
catalyze water oxidation to evolve O with low overpotentials.
2
Unlike these metal porphyrins, Fe porphyrin analogues are
much less efficient for OER electrocatalysis, although molec-
ular Fe complexes of other N-based ligands have been shown
Carbon nanotubes (CNTs) were used as supporting
materials for electrocatalysis (Figure S16). Fe porphyrins
were loaded on CNTs through physical adsorption to give 1/
CNT and 2/CNT (details for preparation were described in
Supporting Information). The resulted hybrids were charac-
terized by scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM), showing no aggregated
particles (Figures 2a,b, and S17–19). The successful loading of
Fe porphyrins on CNTs was confirmed by energy-dispersive
X-ray (EDX) elemental mappings (Figures 2c and S19), line-
scan imaging analyses (Figures S20, S21), and infrared (Fig-
ure 2d) and X-ray photon electron spectroscopy (XPS, Fig-
ure S22). In XPS, the spectrum of 1/CNT in the Fe 2p region
showed two peaks at 711.3 and 724.6 eV with two satellites,
which could be assigned to Fe 2p and Fe 2p , respec-
[
37–43]
to be highly active for water oxidation.
water oxidation to O is the reverse reaction of O reduction,
Considering that
2
2
we propose that the catalytic OER feature of Fe porphyrins
can be significantly improved if their structures can be
properly designed.
Based on these considerations, we herein report an
enzyme-inspired Fe porphyrin 1 (Figure 1b) as an efficient
electrocatalyst for both ORR and OER. By tethering an
imidazole group for Fe binding, the resulted coordination
structure of Fe in 1 resembles that in CcOs. Importantly,
1
represents an unparalleled example of bifunctional Fe
porphyrins for efficient ORR and OER. Control experiments
using analogue 2, which lacks the axial imidazole ligand,
further underlines the critical role of tethered imidazole in
boosting ORR and OER. To demonstrate the practical use,
we assembled Zn-air battery with 1, which shows equal
performance to that with Pt/Ir-based materials. Note that only
few molecular complexes can catalyze both ORR and OER
under the same conditions with high efficiency and durabil-
3
/2
1/2
[
53,54]
tively.
For 2/CNT, these two signals were at 711.6 and
724.9 eV, respectively. As compared to 2/CNT, the small but
significant shift of 0.3 eV to the lower energy direction
observed for Fe signals of 1/CNT is consistent with the axial
imidazole binding, which led to increased electron density on
Fe in 1. All these results confirm the loading of intact
molecules of 1 on CNTs.
[
44]
ity,
a feature required for rechargeable Zn-air batter-
By precisely modifying molecular structures, this
Electrocatalytic ORR was evaluated in 0.1 M KOH
solutions. The cyclic voltammogram (CV) of 1/CNT in N2-
[
45–50]
ies.
work underlines unique benefits and potential applications of
molecular electrocatalysis in new energy technologies.
and O -saturated electrolytes confirmed its ORR activity
2
(Figure S23), and showed that its performance is superior to
that of 2/CNTand CNTs under the same conditions. Note that
all potentials reported in this work are referenced to RHE.
The ORR activity was also examined by rotating ring-disk
electrode (RRDE) measurements. The linear sweep voltam-
mogram (LSV) of 1/CNT displays an ORR wave with an
onset potential Eonset = 930 mV (measured at current density
Fe porphyrins 1 and 2 were synthesized (detailed synthetic
procedures were descried in the Supporting Information).
The identity and purity of triflate (OTf) salts of 1 and 2 were
proved by high-resolution mass spectrometry (Figures S9,
S10) and by elemental analysis. In the structure of 1, the
binding of tethered imidazole on Fe was confirmed. As shown
in Figure 1c, UV/Vis spectra of 1 and 2 display different Soret
bands, indicating their dissimilar coordination structures.
Importantly, with the addition of one equivalent of methyl-
imidazole, the UV/Vis spectrum of 2 became almost identical
to that of 1. On the other hand, we can detach tethered
+
imidazole from Fe in 1 by adding excess AgOTf, as Ag ions
can competitively bind with imidazoles. The resulted UV/Vis
spectrum of 1 turned to be similar to that of 2 (Figure S11).
Note that addition of excess NaOTf caused negligible changes
to the UV/Vis spectrum of 1 (Figure S12). Moreover, the
electron paramagnetic resonance (EPR) spectrum of
III
1
showed both high-spin (HS) and low-spin (LS) Fe states,
III
while 2 showed only HS Fe state (Figure 1d). We propose
that for the Fe ion of 1, in addition to the axial imidazole
III
group, the OTf counter anion may bind and unbind with Fe,
leading to an equilibrium between six-coordinate LS and five-
III
[51,52]
coordinate HS Fe states.
Similar to UV/Vis studies, with
addition of one equivalent of methylimidazole, the EPR
spectrum of 2 became identical to that of 1 (Figure S13),
suggesting the binding of imidazole on Fe. When excess
III
AgOTf was added to 1, the LS Fe signals disappeared, which
was consistent with the detaching of the tethered imidazole
from Fe (Figure S14). With the addition of NaOTf, the ratio of
Figure 2. a) SEM, b) TEM, and c) EDX elemental mapping images of
1/CNT. d) Infrared spectra of CNT, 1, and 1/CNT.
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À2
0
.10 mAcm ) and a half-wave potential E1/2 = 840 mV (Fig-
active for OER and can reach the benchmark current density
of 10 mAcm at 500 mVoverpotential, which is superior to 2/
À2
ure 3a), representing state-of-the-art catalytic ORR activity
as compared to reported Fe porphyrins and other Fe-based
molecular catalysts (Table S1). For 2/CNT, these values are
Eonset = 850 mV and E1/2 = 680 mV. This large 160-mV anodic
shift of ORR half-wave potential highlights the critical role of
tethered imidazole group on boosting ORR electrocatalysis.
The ORR turnover frequency (TOF) with 1/CNT was
CNT (h = 650 mV). Unmodified CNTs displayed poor
1
0
activity for OER under the same conditions (Figure S29).
By normalizing OER activity with the electrochemical sur-
[
62]
face area (ECSA, Figure S30), 1/CNT is still much more
active than 2/CNT (Figure S31). Because 1 and 2 have almost
identical structures except that 1 bears a tethered imidazole
for Fe binding, this large activity difference observed between
these two complexes underlines the critical role of the axial
imidazole in boosting OER electrocatalysis. It is worth noting
that the OER performance of 1 is superior to that obtained
with other molecular catalysts functioning under similar
conditions (Table S2).
À1
estimated to be 0.49 s at 0.8 V per Fe site. This value is
much larger than that of 2/CNT (0.062 s at 0.8 V). Compar-
À1
ison with previously reported molecular catalysts under
similar conditions also demonstrates the high ORR activity
[
23,25,29,55–59]
of 1/CNT.
Note that unmodified CNTs have very
poor electrocatalytic ORR activity (Figure 3a).
Next, the number (n) of electrons transferred per O2
molecule with 1/CNT was determined to be 3.97 based on
RRDE results, while it was 3.84 with 2/CNT (Figure 3b).
Similar n values were obtained by using Koutecky–Levich
We can estimate the OER TOF per Fe site of 1/CNT to be
À1
0.54 s at h = 570 mV, which is larger than that of 2/CNT
À1
(0.17 s at h = 570 mV) and other molecular catalysts func-
[
63–65]
tioning under similar conditions.
The Tafel slope of 1/
[60,61]
(
K–L) analyses (Figures S24, S25).
The durability of 1/
CNT was 84 mV/dec (Figure S32), while it was 110 mV/dec
for 2/CNT, implying more favourable catalytic kinetics with 1/
CNT. Next, we examined the stability of 1/CNT for OER by
bulk electrolysis with an applied overpotential of h = 490 mV.
As shown in Figure 3e, during 8 h electrolysis, the OER
current maintained stable. The slight (< 7%) current
decrease is due to pH decrease of the solution (from 13.00
to 12.86). Moreover, the accumulated charge during electrol-
ysis displayed a linear dependence on time (Figure S33).
After OER electrolysis, 1/CNT was applied to ORR test,
showing no loss of activity (Figure S34). This result indicates
the stability of 1 during OER. UV/Vis and XPS spectra of 1/
CNT further confirmed that the molecular structure of 1 kept
unchanged after OER electrolysis (Figures S35 and S36).
Importantly, the after-electrolysis 1/CNT was rinsed with
tetrahydrofuran to remove molecules of 1. The resulted
material displayed no signals of Fe in XPS (Figure S37), and
showed similar currents as unmodified CNTs in CV measure-
ments (Figure S38). These results together suggested the
CNT was then evaluated, showing almost no loss of electro-
catalytic ORR activity after 1000 CV cycles (Figure 3c). In
prolonged electrolysis, the catalytic ORR current with 1/CNT
dropped only by < 6% after 10 h (Figure S26). UV/Vis
(
Figure S27) and XPS (Figure S28) spectra of 1/CNT before
and after durability tests are almost identical, confirming its
high stability during ORR electrocatalysis.
In addition to ORR, electrocatalytic OER was also
evaluated in 0.1 M KOH. As shown in Figure 3d, 1/CNT is
absence of any heterogeneous FeO in the after-electrolysis 1/
x
CNT and thus suggested the high stability of 1 for OER
catalysis. The Faradaic efficiency for O generation with 1/
2
CNT was determined to be > 98% (Figure 3 f).
In order to address the effect of axial ligand, we
investigated OER mechanisms with 1 and 2 using density
functional theory (DFT) calculations (Figure 4, computa-
tional details are described in the Supporting Information).
III
The ground state of Fe -OH of 1 is calculated to be doublet
III
with an unpaired electron locating at Fe. From Fe -OH, a 1e
oxidation followed by a deprotonation of Fe-bound hydroxyl
IV
take place, giving a formal Fe =O species. Importantly, the
III
FeÀO bond length decreases from 1.83 ꢀ in Fe -OH to
IV
IV
1
.66 ꢀ in Fe =O (Figure 4a). In Fe =O, two unpaired
electrons are each located at Fe and O atoms, and thus it is
III
better described as Fe -OC radical. Direct hydroxide nucle-
IV
ophilic attack to this Fe =O has a calculated free energy
Figure 3. a) RRDE data of 1/CNT, 2/CNT, and unmodified CNT with
a rotation rate of 1600 rpm. b) ORR n values with 1/CNT an 2/CNT as
determined by RRDE. c) LSV of 1/CNT before and after 1000 CV cycles.
d) OER CVs of 1/CNT and 2/CNT. e) OER electrolysis with 1/CNT at
À1
barrier of 38.6 kcalmol
indicating that the formal Fe =O is not likely to undergo
for the OÀO bond formation,
IV
IV
this OÀO bond formation. Further 1e oxidation of Fe =O
V
+
generates formal [Fe =O] intermediate. DFT studies sug-
h=490 mV. f) Observed and theoretical amounts of O produced
2
during electrolysis with 1/CNT at h=490 mV.
gested that porphyrin ligand (denoted as L) is redox-non-
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With high electrocatalytic activity for both ORR and
OER, we assembled a Zn-air battery using 1/CNT (Fig-
ure 5a). The resulted battery operates stably and displays
a high open-circuit potential of 1.45 V (Figure 5b). For
comparison, the battery assembled with 2/CNT shows
a lower open-circuit potential of 1.40 V (Figure S39).
As shown in Figure 5c, at a constant current density of
À2
2
7
0 mAcm , the specific capacity of battery with 1/CNT is
À1
85.9 mAhg , corresponding to 95.8% of the theoretical
capacity of 820 mAhg for a Zn-air battery. Note that the
specific capacity of Pt/Ir-based battery (assembled using Pt/C-
Ir/C) is 757.7 mAhg , and it is 696.4 mAhg for battery with
/CNT (Figure S40). Moreover, the battery with 1/CNT
À1
À1
À1
2
displays smaller charge/discharge polarization voltages as
compared to those with Pt/C-Ir/C (Figure 5d) and with 2/
CNT (Figure S41). The maximum power density of battery
À2
with 1/CNT is 132.9 mWcm , which is larger than that with
À2
À2
Pt/C-Ir/C (61.6 mWcm ) and with 2/CNT (110.8 mWcm ).
As shown in Figure 5e, the battery with 1/CNT shows
Figure 4. a) DFT-calculated OER pathway with 1. b) Optimized struc-
À2
a charge/discharge voltage gap of 0.89 V at 2 mAcm ,
V
+
ture of formal [Fe =O] of 1 and its transition state for the OÀO bond
which is smaller than that with Pt/C-Ir/C (0.91 V) and with
V
V
formation, noted as TSO-O@Fe , and also the structure of HO-Fe =O of 2
with a trans hydroxide ion on Fe. Dianionic porphyrin ligands are
omitted for clarity. Note: Bond lengths in purple underline, spin
density distributions in green sphere, unpaired electron in half red
arrow, and atomic charge of oxo in italic blue.
À2
2/CNT (0.99 V, Figure S42). Furthermore, at 10 mAcm , the
charge/discharge voltage gap of 1/CNT is only 1.10 V (Fig-
ure S43). The durability of the battery with 1/CNT is also
examined, showing high stability with almost no degradation
throughout 100 cycling tests (Figure 5 f). This result is signifi-
cant to show an unparalleled Zn-air battery assembled with
molecular Fe porphyrins, which operates with equal perfor-
mance to batteries assembled with Pt/Ir-based materials.
V
+
+
III
innocent. Thus, [Fe =O] is better described as CL -Fe -OC
V
+
diradical. Hydroxide attack to [Fe =O] has a much smaller
free energy barrier of 13.3 kcalmol and has an exothermic
energy of 8.7 kcalmol . Hence, the formal [Fe =O] species
is suggested to be the key intermediate to trigger OÀO bond
formation.
À1
À1
V
+
V
Unlike 1, the formal Fe =O of 2 will bind a hydroxide ion
V
at its unoccupied trans axial site to afford HO-Fe =O species
(
Figure 4b). The calculated exothermic energy is 19.4 kcal
À1
mol , indicating that the coordination of hydroxide is
favoured. Because of the strong electron-pushing effect of
À
V
negatively charged OH , the terminal O atom of HO-Fe =O
in 2 has an atomic charge of À0.486, while this value is only
V
+
À0.404 for the O atom of [Fe =O] in 1. This increased
V
electron density on the terminal O atom of HO-Fe =O in 2
makes its OÀO bond formation via nucleophilic attack
unfavourable. Based on these results, we propose that the
tethered imidazole group in 1 is likely to protect the trans site
V
V
of Fe =O by preventing the formation of HO-Fe =O, which is
less effective to undergo nucleophilic attack to form OÀO
III
bond. It is worth noting that Fe complexes of tetraamido
macrocyclic ligands (TAML) have been shown to be able to
[
39–41]
catalyze water oxidation with high rates.
Comparing with
Fe porphyrins, Fe TAMLs have stable square planar Fe-N4
coordination structures without trans axial ligands due to the
much stronger s-donating ability of tetraanionic TAML
ligands. As a consequence, in the corresponding formal
Figure 5. a) Schematic diagram of a Zn-air battery. b) The open-circuit
V
plot of a battery with 1/CNT. c) Discharge capacity data of batteries
with 1/CNT and Pt/C-Ir/C at a constant current density of 20 mAcm
Fe = O states, the Fe ions adopt a square pyramidal
À2
.
[
66,67]
coordination geometry without trans axial ligands.
This
d) Charge/discharge data and their corresponding peak power density
structure is suggested to be critical for Fe TAMLs to undergo
data of batteries with 1/CNT and Pt/C-Ir/C. e) Charge/discharge test of
efficient OÀO bond formation via nucleophilic attack with
À2
battery with 1/CNT at current density 2 mAcm . f) Long cycling test
water or hydroxide.
(20 min per cycle).
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In conclusion, we report an enzyme-inspired Fe porphyrin
as an efficient catalyst for both ORR and OER in alkaline
solutions. With a tethered imidazole for Fe binding, 1 signifi-
cantly beats its imidazole-free analogue 2 for electrocatalytic
ORR and OER. For ORR, this axial imidazole can increase
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V
+
to allow hydroxide attack to [Fe =O] for the OÀO bond
V
+
formation by protecting the trans axial site of [Fe =O] . As
a practical demonstration, Zn-air battery assembled with this
Fe porphyrin shows equal performance to batteries with Pt/Ir-
based materials. This work represents the first example of
molecular porphyrin-based Zn-air battery and underlines
unique benefits and promising applications of molecular
electrocatalysis in new energy technologies.
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molecular electrocatalysis · oxygen evolution · oxygen reduction
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Manuscript received: November 19, 2020
Revised manuscript received: January 14, 2021
Accepted manuscript online: January 18, 2021
Version of record online: February 26, 2021
Angew. Chem. Int. Ed. 2021, 60, 7576 –7581
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www.angewandte.org
7581