Nanoscopic Combination of Edge and Flat Planes in the Active Site for Oxygen Reduction and Evolution

Article abstract of DOI:10.1002/ejic.201900761

An iron-containing carbonaceous flat plane was enriched with edges by substrate fine etching and metal oxide loading, resulting in substantial enhancement of the catalytic activity for the oxygen reduction and evolution, which was fundamentally shown using the highly oriented pyrolytic graphite basal plane as the substrate. The evolution close to the standard electrode potential was elucidated by a mechanism involving the nanoscopic edge-flat plane combination.

Full text of DOI:10.1002/ejic.201900761

DOI: 10.1002/ejic.201900761
Communication
Bifunctional Catalyst
Nanoscopic Combination of Edge and Flat Planes in the Active
Site for Oxygen Reduction and Evolution
Jun Maruyama*[a] Shohei Maruyama,[a] Tomoko Fukuhara,[a] Yuko Takao,[b] and
Kohei Miyazaki[c]
Abstract: An iron-containing carbonaceous flat plane was en- damentally shown using the highly oriented pyrolytic graphite
riched with edges by substrate fine etching and metal oxide basal plane as the substrate. The evolution close to the stan-
loading, resulting in substantial enhancement of the catalytic dard electrode potential was elucidated by a mechanism involv-
activity for the oxygen reduction and evolution, which was fun- ing the nanoscopic edge-flat plane combination.
Introduction
One of the earliest studies of the Zn-air batteries used
carbon materials as the bifunctional catalyst.[5] The advantages
of the carbon materials are a high electron conductivity, chemi-
cal stability, and feasibility to form various structures based on
various types of carbons, such as fullerene, carbon nanotubes,
and graphene, which are also applicable to design the surface
structure. The doping of N, P, and S atoms, and metal ions is
also possible, imparting the function as catalysts. In particular,
it has long been known that the metal ion coordinated by N
and embedded in the carbon surface with a planar configura-
tion (metal–Nx unit) functions as the active site for the ORR.[6]
Quite recently, the application as the OER catalyst was re-
ported;[7–9] however, the microscopic three-dimensional surface
structure was not considered.
The electrochemical oxygen reduction (ORR) and evolution
(OER) reactions are essential reactions involved in major energy
conversion devices between chemical and electrical energies,
such as fuel cells, water electrolysis, and metal-air batteries.
Both ORR and OER are involved as the discharge and charge
reactions, respectively, in secondary metal-air batteries, typically
exemplified by Zn-air batteries, which have recently attracted
much attention due to their safety, low production cost, and
high energy-density.[1] An alkaline electrolyte is typically used[2]
and the ORR and OER are expressed as follows.
This study thus highlights the effect of the surface structure
on both the ORR and OER by exploiting the potential of the
carbon materials to form various structures. We used the basal
plane of the highly-oriented pyrolytic graphite and generated
nanoscopic etched structures on it. The basal plane is assumed
to be an almost ideal flat surface suitable for fundamental in-
vestigations and for enhanced durability for the OER as the sub-
strate for the metal-Nx unit by imparting a planar configuration
and reducing the structural disorder.[10,11] We found a substan-
tial enhancement effect of the nanoscopic combination of the
basal and edge planes on both reactions, as well as on the
onset of the OER at a very low η, which was clarified and con-
firmed for the first time in this study using an O2 detecting
technique.
Both are slow reactions and cause a high overpotential (η)
leading to the low energy efficiency of the Zn-air battery. Bi-
functional catalysts for both reactions with a high activity and
low cost are therefore required to enhance the reactions and
have been studied for decades.[3] There are a wide variety of
materials that show the bifunctional catalysis; i.e., the transition
metal oxides, such as MnOx and Co3O4, La1–xSrxMnO3, NiCo2O4,
carbon materials, and their composites with metal oxides.[4]
Most of the studies have focused on the development of new
materials by introducing additional elements and changing
their composition; however, little is known about the funda-
mental effect of the nanoscopic surface structure on the bifunc-
tional catalysis.
[a] Research Division of Environmental Technology, Osaka Research Institute
of Industrial Science and Technology
Results and Discussion
1-6-50, Morinomiya, Joto-ku, Osaka 536-8553, Japan
E-mail: maruyama@omtri.or.jp
The surface etching of the HOPG basal plane was enabled by
coating with a carbonaceous thin film derived from cobalt
phthalocyanine (CoPc) and thermal oxidation based on our re-
cent study.[12] Co oxide nanoparticles were generated during
the thermal oxidation and functioned as a catalyst for the fine
carbon surface etching, which were easily removed by an acid
treatment afterwards. The Fe–Nx unit was imparted by coating
https://orist.jp/
[b] Research Division of Organic Materials,
Osaka Research Institute of Industrial Science and Technology
1-6-50, Morinomiya, Joto-ku, Osaka 536-8553, Japan
[c] Graduate School of Global Environmental Studies, Kyoto University
Kyoto Daigaku Katsura, Nishikyo-ku, 615-8510 Kyoto, Japan
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900761.
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the FePc-derived carbonaceous thin film and treating it with an (CFePc) on the basal plane of the HOPG with a step structure,
acid to remove the soluble Fe species. MnOx was deposited to which was retained due to the sufficient thinness of the film.
form another type of edge at the periphery of the deposited Small bright dots in the AFM images were caused by the Fe
particle. The low-η OER was then observed for the combination aggregates, which were generated by the FePc decomposition
of the Fe–Nx unit on the basal and edge planes and the MnOx and present below the carbonaceous thin film blocking the acid
edge, which also showed a synergetic effect for the ORR as well. access. Particulate or sheet-like MnOx was generated by the
treatment C.
The surface etching (treatment A, denoted by “–e”), FePc-
derived thin film coating (treatment B, denoted by “–CFePc”),
The local structures around Fe and Mn were investigated by
and MnOx loading (treatment C, denoted by “–Mn”) were car- measurements of the X-ray absorption fine structures. Figure 2
ried out in this order and all of the eight combinations of execu- shows the Fe and Mn K-edge X-absorption near edge fine struc-
tion and non-execution of each treatment were investigated as ture (XANES) spectra for HOPG-e-CFePc-Mn. In spite of the in-
an ORR-OER bifunctional electrode. Table 1 lists the serious of sufficient peak intensity of the Fe 2p XPS spectrum, a clear ab-
HOPG electrodes and their surface compositions measured by sorption peak was obtained for the Fe K-edge spectrum meas-
X-ray photoelectron spectroscopy.
ured at a low X-ray incident angle and a fluorescence mode.
The experimentally obtained Fe K-edge XANES spectrum well
fit the simulated curve including those for the Fe–N4 and
Fe–N2 models. The Fe–N2 structure was previously proposed[13]
and probably present at the end of the basal plane, i.e., at the
immediate vicinity of the edge plane.[14] Further detailed exper-
imental results to show the Fe–N2 structure are being obtained
and will be reported in another paper.
Table 1. Combination of HOPG surface treatment displayed in label and sur-
face composition.[a,b]
Surface concentration [atom %]
C
O
N
Mn
HOPG
HOPG-Mn
98.74
94.97
96.92
92.66
98.13
96.87
96.16
94.35
1.26
4.08
1.98
5.76
1.77
2.40
3.21
4.11
0.95
1.33
0.52
1.18
HOPG-CFePc
HOPG-CFePc-Mn
HOPG-e
HOPG-e-Mn
HOPG-e-CFePc
HOPG-e-CFePc-Mn
0.99
0.23
0.10
0.21
0.61
0.33
[a] The XPS spectra are shown in Figure S2. [b] Although Fe 2p peaks were
detected, their intensities were low and insufficient for reliable determination
of the surface concentration.
Figure 1 shows typical atomic force microscopy (AFM) im-
ages of the electrode surface. The generation of nano- to sub
micrometer streaky and hole-like concaves was observed for
the etched surface. Almost no change in the Raman spectrum
after the etching (Figure S1) indicated that the accompanying
edge plane generation occurred only in the vicinity of the sur-
face and the D band intensity was below the detection limit.
Treatment B formed a FePc-derived carbonaceous flat film
Figure 2. (a) Fe and (b) Mn K-edge XANES spectra for HOPG-e-CFePc-Mn. The
simulated Fe K-edge XANES curve was obtained by the weighted addition of
those for FePc, Fe foil, and those calculated by using FEFF8.2 for the five and
three-atom models consisting of a Fe atom surrounded by four nitrogen at-
oms in a square-planar coordination (Fe–N4 model) and two nitrogen atoms
in an isosceles right triangle configuration (Fe–N2 model). The simulated Mn
K-edge XANES curve was obtained by the weighted addition of those for
MnOOH and Mn(CH3CO2)2.
The experimentally obtained Mn K-edge XANES spectrum
well fit the curve obtained by the weighted addition of the
spectra for MnOOH and Mn(CH3CO2)2, indicating that the ox-
Figure 1. AFM images of HOPG-CFePc, HOPG-CFePc-Mn, HOPG-e-CFePc, and
HOPG-e-CFePc-Mn. AFM images of all the samples are shown in Figure S3.
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ides generated through oxidation of Mn(CH3CO2)2 adsorbed on also observed at HOPG-CFePc-Mn and the OER current in the
the HOPG surface by KMnO4 was MnOOH.
low η region was enhanced by the etching (HOPG-e-CFePc-Mn).
The essential role of Fe was indicated by the lower activities for
the ORR and OER at the HOPG coated with the metal-free-Pc-
derived carbonaceous thin film than those containing Fe (Fig-
ure S4).
The relationships between the ORR current and electrode
potential for these series of electrodes are shown in Figure 3a.
The CFePc on the HOPG substantially decreased η to enhance
the catalytic activity for the ORR. The MnOOH loaded on it fur-
ther enhanced the peak current, which was attributed to the
synergetic effect of the CFePc and MnOOH because almost no
enhancement was observed at HOPG-Mn. The effect was ex-
plained by the catalytic effect of MnOOH for the electrochemi-
cal reduction or the decomposition of the ORR intermediate,
The low-η OER has been reported;[19–22] however, it has not
been fully clarified whether the current was caused by the OER,
or by electric double-layer charging and oxidation of the elec-
trode itself; i.e., the carbon surface oxidation and the MnOOH
oxidation and dissolution.[23] We thus attempted to electro-
chemically detect the evolved O2 by placing a Pt wire over the
HOPG-CFePc-Mn electrode for the reduction of O2 diffused from
the electrode surface (Figure S5a, Figure S5b). In this combined-
electrode system, the low-η OER phenomenon was reproduced
(Figure S5c). A reduction current at the Pt wire was observed
in accordance with the OER current (Figure S5d). Because the
potential setting of the Pt wire (0 V) and the electrolyte concen-
tration excluded the possibilities of the reduction of the oxid-
ized carbon species and the Mn dissolution in the low-η OER
region, respectively, the reduction current was related to the O2
evolved at the HOPG-CFePc-Mn electrode. This result indicated
that the oxidation current observed in the low-η region in Fig-
ure 3b was caused by the OER, which was confirmed for the
first time. It should be noted here that this study was focused
on the clear demonstration of the effect of the surface structure
using the HOPG basal plane as an almost ideal flat surface that
was suitable for the fundamental study in spite of the low spe-
cific surface area leading to the low current density.
–
HO2 , generated on the CFePc to increase the overall number
of electrons involved in the ORR per O2 molecule.[15] The effect
of the surface etching on the ORR enhancement was observed,
in particular, in the low potential region. The enhancement was
–
attributed to the capture and reduction of HO2 at the edge
plane,[16] implying the significant effect of the surface structure
on the ORR.
In the series of electrodes examined in this study, the low-η
OER was observed for HOPG-e-CFePc, HOPG-CFePc-Mn, HOPG-
e-CFePc-Mn, and not for the other electrodes. The requirement
for the low-η OER occurrence was therefore the combinations
of the CFePc coating–surface etching or CFePc–MnOOH. The
nearly equivalent OER current at HOPG-e-CFePc and HOPG-
CFePc-Mn suggested a similar OER mechanism. The structure in
common for the two combinations was the presence of the flat
CFePc plane and the edge. The latter was derived from the step
of the HOPG surface or the MnOOH particle loaded on the
CFePc thin film.
Quite recently, a two-dimensional lattice-like π-conjugated
structure parallel to the graphene plane was found in CFePc
using transmission electron microscopy, measurement of the N
K-edge X-ray absorption near-edge structure (XANES), and its
simulation using the density functional theory (DFT).[24] The ex-
perimental N K-XANES characteristics were well explained by
the simulation based on the FePc fused dimeric model. It would
be reasonable to assume that a part of the structure was
chipped in CFePc at the edge of the HOPG surface to generate
the Fe–N2 unit based on the Fe K-XANES spectrum. We thus
formulated a model to represent the structure close to the edge
by the chipped FePc dimer with the Fe–N2 unit, which was
superimposed on the FePc dimer representing the flat plane of
the lower part of the step (Figure 4, C1). The energy profile
Figure 3. Relationships between currents and electrode potential for (a) ORR
and (b) OER in O2-saturated 1 mol dm–3 KOH at 25 °C. The potential scan rate
was 10 mV s–1. The sign of the reduction (cathodic) current was taken as
negative. The reference electrode was Hg/HgO/1 mol dm–3 KOH.
Figure 3b shows the relationships between the OER current
and the electrode potential. The OER current was clearly ob-
served at HOPG-Mn, in particular, in the high potential region,
which was in agreement with previous reports.[17,18] It should
be noted that the onset of the OER was 0.3 V for HOPG-e-CFePc,
indicating the occurrence of the OER at a significantly low η,
because the standard electrode potential in a solution of pH = of the OER was then evaluated based on the DFT calculation
14 is 0.401 V vs. standard hydrogen electrode, i.e., 0.28 V vs. (Supporting information) using the model and a newly postu-
Hg/HgO/1 mol dm–3 KOH. This low-η OER phenomenon was lated mechanism to explicate the low-η OER phenomenon.
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Figure 4. (a) Energy profiles obtained by DFT calculations based on conventional reaction pathway involving planar active site (b, blue line) and reaction
pathway involving combination of edge and planar active sites (c, red line). The number near the intermediate structure model indicates the corresponding
reaction step in the energy profile.
A favorable energy profile was obtained with a lower η for mechanism involving the three-dimensional active site. The
the reaction pathway involving both the Fe–N4 and the Fe–N2 combination of the edge and flat planes thus showed the po-
units than for the conventional pathway involving only the pla- tential to improve the Zn-air battery efficiency and the stability,
nar active site (Figure 4). Because the combination of the CFePc in particular, by a lower voltage for the OER.
flat plane and the edge of the loaded MnOOH was the similar
situation to the combination of the flat and edge plane on
Acknowledgments
HOPG-e-CFePc, HOPG-CFePc-Mn was anticipated to enable the
low-η OER, although the detailed structure of the edge is un-
clear at present and its investigation would require further ex-
tensive studies.
A part of this study was conducted in the project “Advanced
Research Program for Energy and Environmental Technologies,”
undertaken by New Energy and Industrial Technology Develop-
ment Organization, Japan. This study was also supported by JST
CREST (Grant No. JPMJCR18R3) and the Network Joint Research
Center for Materials and Devices (No. 20194045). The XAFS
measurements were performed with the approvals of SPring-8
(Proposal No. 2016A1027).
Conclusions
The nanoscopic combination of the edge and flat planes at the
active site for the ORR and OER was generated on the HOPG
surface by the surface etching of the basal plane, the FePc-
derived carbonaceous thin film coating, and the MnOOH depo-
sition, which substantially enhanced both the ORR and OER.
The XANES spectrum showed the Fe–N4 and Fe–N2 units on the
step-enriched surface; the latter structure of which was as-
sumed to be present at the edge. The low-η OER was experi-
mentally confirmed by the evolved O2 detection and theoreti-
cally supported by the DFT calculations based on the new
Keywords: Heterogeneous catalysis · Energy conversion ·
Nanostructures · Density functional calculations · Reaction
mechanisms
[1] Y. Li, H. Dai, Chem. Soc. Rev. 2014, 43, 5257–5275.
[2] Z.-L. Wang, D. Xu, J.-J. Xu, X.-B. Zhang, Chem. Soc. Rev. 2014, 43, 7746–
7786.
Eur. J. Inorg. Chem. 2019, 4117–4121
www.eurjic.org
4120
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[3] K. Kinoshita, Electrochemical Oxygen Technology, John Wiley & Sons, New
York, 1992.
[4] D. U. Lee, P. Xu, Z. P. Cano, A. G. Kashkooli, M. G. Park, Z. Chen, J. Mater.
[15]
[16]
[17]
Y. Miyahara, K. Miyazaki, T. Fukutsuka, T. Abe, J. Electrochem. Soc. 2014,
161, F694–F697.
J. Maruyama, M. Yamamoto, T. Hasegawa, S. Iwasaki, Z. Siroma, A. Mine-
shige, Electrochim. Acta 2013, 90, 366–374.
M. M. Najafpour, G. Renger, M. Hołyńska, A. N. Moghaddam, E.-M. Aro,
R. Carpentier, H. Nishihara, J. J. Eaton-Rye, J.-R. Shen, S. I. Allakhverdiev,
Chem. Rev. 2016, 116, 2886–2936.
Y. Gorlin, B. Lassalle-Kaiser, J. D. Benck, S. Gul, S. M. Webb, V. K. Yachan-
dra, J. Yano, T. F. Jaramillo, J. Am. Chem. Soc. 2013, 135, 8525–8534.
Y. Gorlin, T. F. Jaramillo, J. Am. Chem. Soc. 2010, 132, 13612–13614.
J. T. Mefford, X. Rong, A. M. Abakumov, W. G. Hardin, S. Dai, A. M. Kolpak,
Chem. A 2016, 4, 7107–7134.
[5] A. J. Appleby, G. Crepy, G. Feuillade, Power Sources 1977, 6, 549–568.
[6] M. Shao, Q. Chang, J.-P. Dodelet, R. Chenitz, Chem. Rev. 2016, 116, 3594–
3657.
[7] H. Fei, J. Dong, Y. Feng, C. S. Allen, C. Wan, B. Volosskiy, M. Li, Z. Zhao,
Y. Wang, H. Sun, P. An, W. Chen, Z. Guo, C. Lee, D. Chen, I. Shakir, M. Liu,
T. Hu, Y. Li, A. I. Kirkland, X. Duan, Y. Huang, Nat. Catal. 2018, 1, 63–72.
[8] J. Wang, F. Ciucci, Small 2017, 13, 1604103.
[18]
[19]
[20]
[9] S. Ibraheem, S. Chen, J. Li, Q. Wang, Z. Wei, J. Mater. Chem. A 2019, 7,
9497–9502.
K. P. Johnston, K. J. Stevenson, Nat. Commun. 2016, 7, 11053.
[10] J. Maruyama, J. Okamura, K. Miyazaki, Y. Uchimoto, I. Abe, J. Phys. Chem.
C 2008, 112, 2784–2790.
[11] C. Domínguez, F. J. Pérez-Alonso, M. A. Salam, S. A. Al-Thabaiti, M. A.
Peña, F. J. García-García, L. Barrioa, S. Rojas, Appl. Catal. B 2016, 183, 185–
196.
[12] J. Maruyama, S. Maruyama, T. Fukuhara, K. Hanafusa, J. Phys. Chem. C
2017, 121, 24425–24433.
[13] M. Lefèvre, J. P. Dodelet, P. Bertrand, J. Phys. Chem. B 2002, 106, 8705–
8713.
[21]
T. Takeguchi, T. Yamanaka, H. Takahashi, H. Watanabe, T. Kuroki, H. Naka-
nishi, Y. Orikasa, Y. Uchimoto, H. Takano, N. Ohguri, M. Matsuda, T. Mu-
rota, K. Uosaki, W. Ueda, J. Am. Chem. Soc. 2013, 135, 11125–11130.
J. Zhang, Z. Zhao, Z. Xia, L. Dai, Nat. Nanotechnol. 2015, 10, 444–452.
W. Wang, J. Luo, S. Chen, Chem. Commun. 2017, 53, 11556–11559.
[22]
[23]
[24]
J. Maruyama, T. Amano, S. Inoue, Y. Muramatsu, N. Yoshizawa, E. M. Gul-
likson, Chem. Commun. 2018, 54, 8995–8998.
[14] Y. Wang, D. Adekoya, J. Sun, T. Tang, H. Qiu, L. Xu, S. Zhang, Y. Hou, Adv.
Funct. Mater. 2019, 29, 1807485.
Received: July 10, 2019
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