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iScience
Article
Theoretical study of the effect of coordination
environment on the activity of metal macrocyclic
complexes as electrocatalysts for oxygen reduction
Ziqi Tian, Yuan
Wang, Yanle Li, Ge
Yao, Qiuju Zhang,
Liang Chen
tianziqi@nimte.ac.cn (Z.T.)
liyanle@nimte.ac.cn (Y.L.)
chenliang@nimte.ac.cn (L.C.)
Highlights
Metal macrocyclic
complexes are potential
electrocatalysts for ORR
An understanding on
structure-property
relationship is gained
based on simulation
Various modifications are
considered to improve the
performance
Tian et al., iScience 25,104557
July 15, 2022 ª2022 The
Author(s).
https://doi.org/10.1016/
j.isci.2022.104557
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iScience
Article
Theoretical study of the effect of coordination
environment on the activity of metal macrocyclic
complexes as electrocatalysts for oxygen reduction
Ziqi Tian,
1,2,
*Yuan Wang,
1,2,3
Yanle Li,
1,2,
*Ge Yao,
4
Qiuju Zhang,
1,2
and Liang Chen
1,2,5,
*
SUMMARY
Transition metal macrocyclic complexes are appealing catalysts for electrochem-
ical oxygen reduction reaction (ORR). Here, we perform first-principles calcula-
tions to gain a comprehensive understanding on the structure-property relation-
ship of the metal macrocyclic complex systems. Various modifications of the
complexes are considered, including centered metal, axial ligand, coordination
atom, substituent, and macrocycles. Based on simulation, introduction of appro-
priate apical ligand can improve the performance of all the three metals, whereas
replacement of nitrogen with oxygen or carbon as the coordination atoms may
enhance the Ni-centered systems. The antiaromatic ring stabilizes the *OOH in-
termediate, whereas the macrocycle with reduced electron density inhibits the
binding with oxygen. By regulating the coordination environment, the overpo-
tential can be significantly reduced. This work may assist the rational design of
ORR catalysts and is of great significance for the future development of oxygen
reduction catalysts.
INTRODUCTION
Hydrogen energy provides an attractive way to decarbonize many economic sectors, including transport
and power generation (Brandon and Kurban, 2017;Nolan and Browne, 2020). Fuel cell systems play a
key role in hydrogen energy technology (Dodds et al., 2015;Akal et al., 2020). The hydrogen-powered ve-
hicles have been available in the market, whereas there are still many challenges that hampered its wide-
spread commercialization (Jacobson et al., 2005;Wilberforce et al., 2017). One of the most urgent issues to
be addressed is the development of efficient and economical cathode materials (Banham et al., 2015;Tahir
et al., 2022). In most fuel cells, oxygen reduction reaction (ORR) takes place at the cathode (Xiao et al.,
2021), in which oxygen is reduced to water via a four proton-electron transfer process:
O
2
+4e
+4H
+
/2H
2
O (1)
Because the generation of water requires transfer of multiple electrons, the reaction suffers from sluggish
kinetics and high overpotential (Qin et al., 2021). Currently, platinum-based materials are the most
commonly used commercial catalysts (Shao et al., 2018). But their large-scale application is limited by
the scarcity and the high price of the noble metal. Designing high-performance catalysts using base metal
is highly desired.
Metal macrocyclic complexes, such as heme and other metalloporphyrins, are pivotal components in
enzyme systems that perform as reactive centers to catalyze biochemical redox reactions with extremely
high activity (Li et al., 2021). Since cobalt phthalocyanine was employed as an ORR catalyst by Jasinski in
1964 (Jasinski, 1964), these complexes have attracted much attention in electrochemistry. Much effort
has been devoted to promoting the catalytic activity of metal macrocyclic complexes (Liu et al., 2016;
Zhang et al., 2017). Numerous systems containing metal macrocyclic complex subunits have been synthe-
sized. Especially, the Fe-centered and Co-centered systems are widely studied as the promising electro-
catalysts for ORR. For example, using Fe/Co-phthalocyanines as the functional subunits, Yang prepared
two-dimensional conjugated aromatic networks via a one-step ball milling of the solid-phase synthesis
(Yang et al., 2019). The materials display outstanding ORR mass activity, even beyond commercial Pt/C. Ci-
chocka et al. report a series of Zr-based metal organic frameworks (MOFs) in which functional
1
Ningbo Institute of Materials
Technology and Engineering,
Chinese Academy of
Sciences, Ningbo 315201,
Zhejiang, China
2
University of Chinese
Academy of Sciences, 100049
Beijing, China
3
Nano Science and
Technology Institute,
University of Science and
Technology of China, Suzhou
215123, China
4
School of Physics,
Collaborative Innovation
Center of Advanced
Microstructures, and National
Laboratory of Solid State
Microstructures, Nanjing
University, Nanjing 210093,
China
5
Lead contact
*Correspondence:
tianziqi@nimte.ac.cn (Z.T.),
liyanle@nimte.ac.cn (Y.L.),
chenliang@nimte.ac.cn (L.C.)
https://doi.org/10.1016/j.isci.
2022.104557
iScience 25, 104557, July 15, 2022 ª2022 The Author(s).
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
1
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metalloporphyrins are grafted as the electrochemical active sites for ORR (Cichocka et al., 2020). The stable
Zr-MOF platforms tailor the immobilization and packing of metal macrocyclic subunits, significantly
enhancing ORR activity. Yue et al. fabricated two-dimensional covalent organic frameworks (CO-Fs)
composed of metalloporphyrins and donor-acceptor dyads for electrochemical ORR (Yue et al., 2021).
The Co-centered COF exhibits the best performance among their studied materials.
Moreover, derived from the metal macrocyclic complexes, a series of single atom catalysts (SACs), also
known as M-N-C systems, have been investigated extensively and intensively in past ten years (Wang
et al., 2018;Li et al., 2020a,2020b;Peng et al., 2020;Zhao et al., 2021). As characterized by XANES and other
spectrums, their active sites have many features in common with the corresponding metal macrocyclic
complexes. These materials marry the advantages of enzyme and heterogeneous catalysts, exhibiting
outstanding performance on electrocatalysis systems, including ORR. For example, Chen et al. made a
highly reactive and stable Fe/N-doped porous carbon. The ORR performance outperformsthe commercial
Pt/C catalysts, with low overpotential, high kinetic current density, and outstanding stability (Chen et al.,
2017). Experiments and simulations demonstrated that the isolated Fe-N4 sites were crucial to deliver
the outstanding performance. Jiao et al. synthesized Fe-N-C SAC via pyrolysis of porphyrinic metal–
organic frameworks (Jiao et al., 2018). The materials possess high content of single-atom Fe-N4 sites, hi-
erarchical pores, oriented nano-channels and high conductivity, leading to ultimate ORR activity. Lin
et al. employed an open framework platform with a large number of chelating ligands to prepare a series
of SACs containing Fe-N5 site (Lin et al., 2019). By increasing the coordination number of the metal site, the
interaction with the key intermediate is modulated. Excellent ORR activity with a half-wave potential of
0.89 V and high stability are achieved. On the other hand, the active site structures in the heterogeneous
catalysts are quite difficult to be well-defined. The metal macrocyclic complexes can be seen as the model
systems of the active sites, based on which one can understand the effect of the coordination environment
on catalytic activity in depth.
Furthermore, theoretical investigations have also been performed to give insight into the catalytic mechanism
and structure-property relationship. Seo et al. compared the electronic structures of ferrous phthalocyanine
and its derivative that was modified with diphenylphenthioether substituent (Seo et al., 2014). The ORR activity
could be well regulated by the incorporation of functional groups. The relative position of the metal dz2-orbital
can be controlled by the incorporation of functional groups, leading to the tunableORR activity. Ni et al. compu-
tationally investigated the relationship between the aromaticity/antiaromaticity of the macrocycles and the
activity of transition metal centered complexes as ORR electrocatalysts (Ni et al., 2021). The antiaromatic macro-
cyclic ligand can enhance adsorption strength with oxygenated intermediate. Metal centers require matching
macrocycles to improve ORR activity. Xu et al. systematically screened numerous metal macrocyclic complexes
and graphene-based single-atom catalysts toward ORR and other electrocatalysis reactions, indicating that the
activity is highly correlated with the chemical environment of the metal center, including coordination number
and the electronegativity of the coordination atoms (Xu et al., 2018).
In this work, we take metalloporphyrins as the starting point to study the effects of different modifications
on the performance of ORR electrocatalysis. Porphyrins are typical macrocyclic ligands in coordination
chemistry, which can accommodate many transition metal ions to form macrocyclic complexes (Zhang
and Warren, 2021). The extended aromatic structure of the macrocycle can support a range of oxidation
states, stabilizing critical intermediates in the redox reactions. In addition, porphyrins provide a versatile
platform for functionalization, thus fine-tuning of the intrinsic properties is available. Here, the catalytic
pathways of a series of metalloporphyrins on ORR have been systematically studied by applying density
functional theory (DFT) calculation. Several modifications are considered to modulate the performance,
such as substituents, apical ligand, and elements coordinated with metal. Three more macrocycle ligands
are also included further. The theoretical results show that Fe and Co complexes bind with oxygenated
intermediates much more intensely than that containing Ni. By introducing proper apical ligand and modi-
fying the coordination atoms and the macrocyclic structure, the formation energies of these key interme-
diates are tunable. On the other hand, the effect of substituent is not significant. In particular, in this work,
the macrocyclic complexes are considered as the reaction centers in porous framewo rks and functional mo-
tifs in SACs, thus the freestanding molecule models are utilized. In experiment, the macrocyclic complexes
have also been supported on various substrates, such as carbon nanotubes (Zhang et al., 2009;Abarca
et al., 2019;Govan et al., 2020;Loyola et al., 2021a,2021b). Proper substrates may also play a critical
role in the catalysis process as discussed in previous theoretical studies (Orellana, 2011,2012).
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RESULTS AND DISCUSSION
As shown in Figure 1A, tetraphenylporphyrin (PP) is first taken as the ligand, because it is a common func-
tional motif in porous framework. The relatively large phenyl group can keep the planar structure. Three
iron-group elements are studied as the center cations, i.e., Fe, Co, and Ni. Although the ORR mechanism
isstillanopenquestionandseveralpossiblepathwayshavebeenproposedinliteratures(Shi and Zhang,
2007;Sun et al., 2011;Ramaswamy and Mukerjee, 2011;He et al., 2012;Orellana, 2013), in this work our dis-
cussion is mainly based on the four-step path that goes through *OOH, *O and *OH intermediates (Fig-
ure 1B). The asterisk denotes the active metal site. We assume that the relative energies of the key inter-
mediates are consistent with the energetics trend derived from this path even if the reaction follows
other mechanisms. From the free energy profiles in Figure 1C, one can see that the potential-determining
steps (PDSs) of Fe-PP and Co-PP systems are both the last step that *OH is reduced to water and desorbs
from the catalyst. At an external potential of 1.23 V, the energy changes of the PDSs are 0.92 eV and 0.54 eV
for Fe-centered and Co-centered complexes, respectively. To improve the activity of the whole ORR pro-
cess, the interaction between metal and the oxygenated intermediate should be weakened. In the contrast,
the PDS of Ni-PP-catalyzed path is the first step to form*OOH with an energy change of 0.95 eV, namely that
the binding to oxygen is too weak. The strength of the binding between metal and oxygenated species
follows a trend as: Fe > Co > Ni. In experiment, Fe-containing and Co -containing systems have been widely
studied as the candidates of electrocatalysts for ORR. Many investigations demonstrated that the ORR pro-
cess on Fe-centered catalysts is limited by desorption of *OH, whereas the cobalt based materials bind ox-
ygen weakly for efficient ORR (Zitolo et al., 2017;Pegis et al., 2019). Such inconsistency may be because of
inaccurate description of the solvation effect and other systematic errors. But it is still worthwhile to inves-
tigate how the interaction can be regulated by modifying the coordination environment.
To improve the ORR performance of metalloporphyrin, the adsorption strength of oxygenated intermedi-
ate is expected to be reasonably changed by modifying the coordination environment. In the three studied
complexes, the coordination numbers of the centered cations are 4, indicating that the metal is unsatu-
rated and able to coordinate with additional ligands in axial direction (the purple ligand in Figure 1A). In
biochemistry,theapicalligandplaysacriticalroleinthecatalysisprocess(Liu et al., 2017). Herein, we study
the influence of different apical ligands on the adsorption strength of oxygenated species on metallopor-
phyrins, including -Cl, -OH, and pyridine (py for short). The free energy profiles are depicted in Figure 2.
Generally, the binding capacity of oxygenated intermediate is weakened after Fe-PP and Co-PP coordi-
nating with the additional apical anion ligand. The less unsaturated Fe and Co centers exhibit weakened
affinity to the oxygenated species. For example, the formation energies of *OOH species increase by
0.50 and 0.20 eV on Cl-coordinated Fe- and Co-PP, respectively. Meanwhile, the energy changes of the
PDSs, namely desorption of *OH, decrease to 0.50 and 0.36 eV on Fe- and Co-complexes, related to
improved oxygen reduction performance. In experiment, it has been also reported that the ultimate
ORR performance is achieved by constructing a single atom catalyst with coordination number of five
(Han et al., 2018;Li et al., 2022). In contrast, Ni
2+
cation possesses d
8
electron configuration. The crystal field
Figure 1. Schematic representation of the studied system and free energy diagrams of three typical catalysts
(A) Structures of metal-centered tetraphenylporphyrin (PP).
(B) Side views of three intermediates (*OOH, *O and *OH) on the 4-electron ORR pathway. Color code: white, H; gray, C;
blue, N; red, O; pink, metal.
(C) Free energy diagrams of ORR processes catalyzed by Fe-, Co-, and Ni-PP complexes.
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splitting results in the unoccupied d
x2-y2
orbital much higher than the other four occupied d-orbitals. The
electron transfer to the unoccupied orbital is energetically unfavorable. After introducing anion as the axial
ligand, the center cation becomes a d
7
configuration that can accept one more electron and ligand to form
a stable state. As a result, the binding between Ni and oxygenated species is strengthened. Free energy
changes of *OOH formation decreases from 0.95 eV to 0.74 and 0.65 eV with additional -Cl and –OH li-
gands, respectively. Therefore, the overall oxygen reduction performance is improved by introducing addi-
tional ligands as well. Previously, Tasca et al. (Cao et al., 2013;Riquelme et al., 2018;Viera et al., 2020;Loy-
ola et al., 2021a,2021b;Govan et al., 2021;Oyarzun et al., 2021) systematically studied similar Fe-centered
and Co-centered complexes with axial ligands by combining experiments and theoretical calculation. They
also demonstrated that the additional coordination promotes the overall ORR performance. Yet, they sup-
posed that the additional pyridine-like ligands improve catalytic activity mainly by enhancing the binding
between metal and O
2
molecules. Therefore, the whole mechanism is still worth discussing in future. In
particular, the kinetic process of the reaction and the solvent effect should be included in in-depth mech-
anistic studies.
Moreover, the atomic charge and densityofstate(DOS)areanalyzedtogivemoreinsightoftheadditional
ligand effect. As shown in Figure 3A, as coordinating with the axial anion ligands, the partial charges of Fe
and Co become more positive, indicating electron depletion of the metal center. Fewer valence electrons
lower the Fermi level, making it more difficult to form new bonds with oxygen. The DOS in Figures 3Band
3C also illustrates that the energy of the unoccupied orbitals rises and the band gaps increase after Fe and
Co bind to -OH. On the other hand, although the binding to the axial anion also results in a more positive
partial charge on Ni cation, the electron structure changes as well. The orbital degeneracy is removed and
the band gap is apparently reduced, corresponding to the easier formation of new Ni-O bonds.
Figure 2. Free energy diagrams of ORR processes on three M-PPs with different axial ligands, i.e., Cl
,OH
and
pyridine (py)
(A) Fe-centered complexes.
(B) Co-centered complexes.
(C) Ni-centered complexes.
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Introducing substituent is another common strategy to modulate the properties of the macrocycle (Wan-
nakao et al., 2017;Abarca et al., 2019;Govan et al., 2020;Oyarzun et al., 2021). Here, we considered four
substituents on the pyrrole subunits, i.e., -F, -Cl, -Br, and -CH
3
(the green groups in Figure 1A). As in Fig-
ure S1, these free energy diagrams of ORR processes represent that the substituents have little impact on
the binding to the oxygenated intermediates. The energy change of each step is within 0.2 eV. Thus the
PDS on each metal remains unchanged, namely that the interaction between oxygen and Fe or Co is
too strong while that between oxygen and Ni is too weak. For Fe and Co centered systems, the best per-
formance can be achieved when the substituents are -Cl and -CH
3
, respectively, with energy changes of
PDSs as 0.82 and 0.37 eV at external potential of 1.23 eV. For Ni centered systems, the substituents cannot
improve ORR performance effectively. Although it is difficult to obtain a general rule from the computa-
tional results, one can finely modulate the interactions between metal and oxygenated species by intro-
ducing suitable substituents. To tailor the binding strength more effectively, other modification is
necessary.
The porphyrin molecule is composed of four pyrrole subunits, which may be replaced by other heterocy-
cles, such as furan or cyclopentadienyl group. In many prepared SACs, metal centers can bind with carbon
and oxygen beyond nitrogen. Thus one or two nitrogen atoms that coordinate with metal (the nitrogen
atoms in yellow circle in Figure 1A) are replaced by oxygen or carbon (Li et al., 2020a,2020b;Tang
et al., 2021). We regard six C/O-replaced PP molecules, marked as N3O, N2O2-cis, N2O2-trans, N3C,
N2C2-cis, and N2C2-trans. These labels represent the atoms coordinating with metal. There are two con-
figurations for the PP molecule in which two nitrogen atoms are replaced by oxygen/carbon atoms, i.e., the
cis and the trans configurations. The free energy profiles are shown in Figure S2,andDG(*OOH)s are
analyzed to evaluate the binding strengths between metal and oxygenated species. As plotted in
Figure 3. Property analysis of PP-coordinated complexes without/with additional ligands
(A) The atomic charges of metal centers.
(B) DOSs of Fe-, Co-, and Ni-PP systems.
(C) DOSs of Fe-, Co-, and Ni-PP systems with axial -OH ligands.
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Figures 4A and 4B, the replacement of nitrogen by either oxygen or carbon enhances the interaction of the
metal center with the oxygenated intermediate. Therefore, ORR is suppressed on Fe and Co centers;
whereas the Ni center catalyzes the reaction efficiently by replacing nitrogen atoms with oxygen or carbon.
From the DOSs of three Ni-centered complexes in Figure 4C, it can be found that the replacement of het-
eroatom completely changes the electronic structure of the complex. The highest occupied orbital is
mainly composed of the macrocycle’s orbital. The band gap is consequently narrowed, facilitating the re-
action with oxygen.
Beyond the porphyrin macrocycles, other expanded porphyrins have been synthesized (Jasat and Dolphin,
1997;Saito and Osuka, 2011). We studied three more porphyrin-like macrocyclic molecules as ligands,
labeled as L1, L2, and L3 in Figure 4A, respectively. As shown in Figures 4B–4D, L1 (tetrabenzoporphyrin)
possesses the same coordination configuration and macrocyclic structure as PP, thus the M-L1 systems
exhibit very similar performance to that of MPP systems, in consistent with aforementioned conclusion
that the substituents have little impact on the interaction of centered cation to oxygenated groups.
Although if the macrocycle L2 (corrole) is taken as the ligand, the affinities of all three cations to *OOH
are enhanced apparently. As a result, performance of Ni-L2 on ORR is improved but that of Fe-L2 and
Co-L2 are inhibited. DG(*OOH) of Ni-L2 system decreases from 0.95 eV to 0.71 eV at an external potential
of 1.23 V. Previous study demonstrated that the complexes with antiaromatic macrocycles significantly
enhance adsorption strengths, mainly because of the various redox activities of macrocyclic ligands with
different aromaticities (Ni et al., 2021). The antiaromatic ligand is more likely to accept electrons to become
a stable state. The redox activity of ligands further affects the activity of metal via d-pconjugation. In this
case, L2 is an antiaromatic macrocycle, thus our results are in consistent with Ni’s conclusion. On the other
Figure 4. Investigation on the heteroatom substituted PPs in which one or two nitrogen atoms are replaced
(A) The relationship between DG(*OOH) and oxygen-substituted PPs.
(B) The relationship between DG(*OOH) and carbon-substituted PPs.
(C) The DOSs of three Ni-centered macrocyclic complexes.
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hand, four acarbon atoms in porphyrin are replaced by nitrogen atoms to generate L3 (5,10,15,20-tetraa-
zaporphyrin), keeping the number of pelectrons as 18. Phthalocyanine—a widely studied complex—can
be seen as a derivative of L3. L3 coordination with metal results in the weakened interaction with *OOH
and *OH. For instance, at a potential of 1.23 eV, the energy change of the desorption step (*OH/H
2
O)
decreases from 0.92 eV to 0.73 eV on Fe-centered macrocycle, and from 0.54 eV to 0.42 eV on Co-centered
macrocycle, corresponding to the promoted ORR performance. In comparison, the electron structures of
PP and L3 are quite similar. Although the unsaturated anitrogen atoms insert into the large conjugate sys-
tem, reducing electron density of the macrocycle and withdrawing electrons from the center cation. Ac-
cording to Bader charge analysis, the partial charges of the three cations all become more positive after
replacing ligand PP with L3, i.e., from +1.20|e| to +1.21|e| for Fe, from +1.05|e| to +1.12|e| for Co and
from +0.94|e| to +1.02|e| for Ni. The low electron density of the reaction site makes it difficult for oxygen
to gain electrons and inhibits the ORR process.
We summarize the energy changes of PDSs (DG
PDS
)inFigure 6. By modifying the coordination environ-
ment, the activities of the metal macrocyclic systems are significantly altered. DG
PDS
canbereducedby
approximately 0.4 eV. In our studied systems, Co-PP-Cl possesses the best calculated performance that
DG
PDS
is 0.36 eV. Even for these Ni-centered complexes that are not normally considered as effective
ORR catalysts, relatively acceptable performance may also be achieved by changing the coordination el-
ements, such as Ni-N3O and Ni-N2O2-trans systems with the same DG
PDS
as 0.43 eV.
Finally, we can compare our calculation with existing experiments. Volcano or linear correlations between
various descriptors and the ORR activity have been widely reported, such as binding energy of oxygen
molecule (and other oxygen-containing species), M
III
/M
II
cation redox potential, number of d-electrons,
and the intermolecular hardness (Zagal and Koper, 2016;Kumar et al., 2020;Loyola et al., 2021a,2021b).
Here we plot the relationship of DG(*OOH) versus DG
PDS
in Figure 7, which also exhibits typical volcano
correlation. In this volcano plot, one can see that Fe-centered and Co-centered species are on the left
side corresponding to too strong metal-oxygen binding, whereas Ni-centered species on the right side
corresponding to weak binding. If the binding between metal and oxygen is too weak, the MObond
is more likely to break than the O-O bond, leading to the generation of H
2
O
2
. Numerous experiments
have illustrated that Fe-O binding is too strong for the regeneration of catalyst (Cao et al., 2013;Sun,
2019;Loyola et al., 2021a,2021b), whereas Co- and Ni-containing systems promote the H
2
O
2
production
Figure 5. Study on other macrocyclic ligands
(A) Structures of three studied macrocyclic ligands.
(B) Free energy profiles of ORR processes catalyzed by Fe-centered macrocycles.
(C) Free energy profiles of ORR processes catalyzed by Co-centered macrocycles.
(D) Free energy profiles of ORR processes catalyzed by Ni-centered macrocycles.
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(Riquelme, 2018;Jia and Yao, 2020;Wang et al., 2022). Therefore, our calculation may systematically over-
estimate the binding strength between oxygen and metal, probably because of the deviation of calculation
method and inaccurate consideration of solvent. The peak of the volcano plot should be between the
points corresponding to Fe-PP and Co-PP. We roughly revise the volcano plot by moving the right part
down, as shown in the gray points and line in Figure 7. Then one can still infer that these points between
or close to Fe-PP and Co-PP (in the shadow area of Figure 7) are promising electrocatalysts for ORR,
including not only the synthesized systems (Fe-PP-Py (Cao et al., 2013;Loyola et al., 2021a,2021b), Fe-
PP-OH (Wang et al., 2019)Co-PP-Py(Riquelme et al., 2018;Viera et al., 2020;Govan et al., 2021)and
substituted Fe-PP (Abarca et al., 2019;Govan et al., 2020;Oyarzun et al., 2021), but also some hypothesized
models, such as Co-N3O and Ni-N2O2-cis. We expect experimental chemists to prepare these structures
and test the performance in the future.
Conclusion
In this work, a series of iron-group metal centered macrocyclic complexes have been systematically studied
as potential electrocatalysts on ORR by using density functional theory (DFT). The binding between metal
and oxygenated intermediate can be effectively regulated by modifying the coord ination environments. By
designing suitable ligands, optimal performance on ORR can be achieved. Specifically, introducing the
anion as the axial ligand and replacing acarbon of macrocycle with nitrogen reduce the electrons on Fe
and Co center, lowering the energy change of *OH desorption and thus improving ORR activity. On the
other hand, replacing coordination sites as oxygen or carbon and changing the macrocycle to a nonaro-
matic system may increase the electrons on Ni center, enhancing the affinity to oxygenated intermediates
and ORR performance as well. This investigation not only provides guidance for the design of novel mate-
rials that contain porphyrin-like subunits as reaction centers for electrocatalysis ORR but also points to
design directions for the construction of high-performance heterogeneous single atom catalysts.
Figure 6. Summary of DG
PDS
’s of all our simulated
systems
Figure 7. The volcano correlation of calculated
DG(*OOH) versus DG
PDS
According to reported experiments and linear scaling
relationship, the points with more positive DG(*OOH)’s
than the average value of Fe-PP and Co-PP are
approximately moved down in gray. The area with
DG(*OOH) between Fe-PP and Co-PP is marked in
shadow.
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Limitations of the study
The coupling of various features should be further studied in future. Solvation effect was roughly consid-
ered in this work, thus needs to be included in the in-depth study. The detailed reaction mechanism for
the porphyrin system is still an open question. In most experiments, the binding between oxygenated spe-
cies and the Co center is too weak for 4e-ORR to take place, whereas our resultsmay overestimate the bind-
ing. Thus the simulation method could be further revised.
STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following:
dKEY RESOURCES TABLE
dRESOURCE AVAILABILITY
BLead contact
BMaterials availability
BData and code availability
dEXPERIMENTAL MODEL AND SUBJECT DETAILS
dMETHOD DETAILS
dQUANTIFICATION AND STATISTICAL ANALYSIS
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.isci.2022.104557.
ACKNOWLEDGMENT
This work was supported by the National Key Research and Development Program of China (No.
2018YFB0704300), the National Science Foundation of China (No. 52171022), Zhejiang Provincial Natural
Science Foundation of China (No. LXR22B030001), NingBo S&T Innovation 2025 Major Special Programme
(No: 2018B10016) and Fujian Institute of Innovation, Chinese Academy of Sciences; K. C. Wong Education
Foundation (GJTD-2019-13). This research used computational resources of the High-Performance
Computing Center of Collaborative Innovation Center of Advanced Microstructures, Nanjing University.
AUTHOR CONTRIBUTIONS
L.C. and Z.T.designed the project and wrotethe manuscript. Y.W., Z.T. and Y.L. carried outDFT calculations.G.Y.
and Q.Z. provided helpful suggestions. All authors discussed the results and commented on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: January 28, 2022
Revised: April 23, 2022
Accepted: June 2, 2022
Published: July 15, 2022
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STAR+METHODS
KEY RESOURCES TABLE
RESOURCE AVAILABILITY
Lead contact
Further information and requests for resources should be directed to and will be fulfilled by the lead con-
tact, Liang Chen (chenliang@nimte.ac.cn).
Materials availability
This study did not generate new unique reagents.
Data and code availability
All the optimized structures of the metal macrocyclic complexes have been uploaded as Data S1.zip.
Related to STAR Methods.
Other data reported in this paper will be shared by the lead contact upon request.
This paper does not report original code.
Any additional information required to reanalyze the data reported in this paper is available from the lead
contact upon request.
EXPERIMENTAL MODEL AND SUBJECT DETAILS
(Omitted) Our study does not use experimental models typical in the life sciences.
METHOD DETAILS
Spin polarized DFT calculation was performed by using the Vienna Ab initio Simulation Package (VASP)
software package (Kresse and Furthmuller, 1996). PBE functional was employed with PAW method to
describe the interaction between ions and electrons (Blochl, 1994;Kresse and Joubert, 1999). The cutoff
energy of the plane wave was set to be 450 eV. To consider the solvent effect, the adsorption energies
of *OH and *OOH species were subtracted by 0.50 and 0.25 eV, respectively, as suggested in literature
(Rossmeisl et al., 2005). Since the isolated metal macrocyclic complexes are taken as the model systems,
only the G-point is sampled. A vacuum layer of 15 A
˚in each direction was used to avoid the interaction be-
tween neighboring images under periodic boundary condition. All the structures have been fully relaxed.
The optimized coordinations are attached as Data S1.zip in the supplement materials. The convergence
criterion of the total energy and force was set to be 10
4
eV and 0.03 eV/A
˚, respectively.
The four-electron ORR pathway goes through the four elementary steps as (Rossmeisl et al., 2005;Fang and
Liu, 2010):
*+O
2
(g) + H
+
+e
/*OOH
*OOH + H
+
+e
/*O + H
2
O
REAGENT or RESOURCE SOURCE IDENTIFIER
Software
VASP 5.4.4 Hafner (2008) https://www.vasp.at
Adobe Photoshop CC2018 https://www.adobe.com/
VESTA Momma and Izumi (2011) http://jp-minerals.org/vesta/
Materials Studio BIOVIA, Dassault Syste` mes https://www.3ds.com/products-services/biovia/products/molecular-modeling-
simulation/biovia-materials-studio/
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12 iScience 25, 104557, July 15, 2022
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Article
*O + H
+
+e
/*OH
*OH + H
+
+e
/*+H
2
O
Based on the computational hydrogen electrode (CHE) model, the Gibbs free energy change (DG) is calcu-
lated as:
DG=DE+DE
ZPE
-TS+DG
U
where DE is the internal energy change directly calculated from DFT; DE
ZPE
and TS refer to the change of
zero-point energy and entropy for *OOH, *O, and *OH intermediates (Rossmeisl et al., 2007). Vibrational
frequency calculations were carried out to obtain the zero-point energy and entropy. DG
U
is deduced from
–neU, where n is the number of transferred electrons and U is the external electrode potential vs. reversible
hydrogen electrode (RHE) (Norskov et al., 2004;Hansen et al., 2008). In the following discussion, an external
potential of 1.23 V is considered.
QUANTIFICATION AND STATISTICAL ANALYSIS
(Omitted) Our study does not include quantification or statistical analysis.
ll
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