Substituent position effect of Co porphyrin on oxygen electrocatalysis

Article abstract of DOI:10.1016/j.cclet.2021.02.032

Substituent effect of metal porphyrin molecular catalysts plays a crucial role in determining the catalytic activity of oxygen electrocatalysis. Herein, substituent position effect of Co porphyrins on oxygen electrocatalysis, including the oxygen reductio

Full text of DOI:10.1016/j.cclet.2021.02.032

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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Substituent position effect of Co porphyrin on oxygen electrocatalysis
Haoyuan Lv, Hongbo Guo, Kai Guo, Haitao Lei, Wei Zhang, Haoquan Zheng,
*
*
Zuozhong Liang , Rui 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
A R T I C L E I N F O
A B S T R A C T
Article history:
Substituent effect of metal porphyrin molecular catalysts plays a crucial role in determining the catalytic
activity of oxygen electrocatalysis. Herein, substituent position effect of Co porphyrins on oxygen
electrocatalysis, including the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER),
was investigated. Two Co porphyrins, namely 2,4,6-OMe-CoP and 3,4,5-OMe-CoP, were selected as the
research objects. The ORR and OER performance was evaluated by drop-coating molecular catalysts on
carbon nanotubes (CNTs). The resulted 3,4,5-OMe-CoP/CNT exhibited high bifunctional electrocatalytic
activities and better long-term stability for both ORR and OER than 2,4,6-OMe-CoP/CNT. Furthermore,
when applied in the Zn-air battery, 3,4,5-OMe-CoP/CNT exhibited comparable performance to that with
precious metal-based materials. The enhanced catalytic activity may be attributed to the improved
charge transfer rate, mass transfer and hydrophilicity. This work provides an effective strategy to further
enhance catalytic activity by introducing substituent position effect, which is of great importance for
developing more efficient energy-related electrocatalysts.
Received 29 December 2020
Received in revised form 7 February 2021
Accepted 17 February 2021
Available online xxx
Keywords:
Oxygen electrocatalysis
Oxygen reduction reaction
Oxygen evolution reaction
Co porphyrin
Substituent effect
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Electrocatalytic oxygen reduction reaction (ORR) and oxygen
evolution reaction (OER), as important reactions in several new
energy conversion technologies, such as fuel cells, metal-air
batteries, and water electrolysis devices, have attracted increasing
interests [1–12]. Efficient electrocatalysts for ORR and OER are
highly required due to the very slow kinetics of the two reactions
[13–24]. At present, Pt-based materials [25] and Ru-based oxides
[26] exhibited excellent catalytic activity for ORR and OER,
respectively. However, the low reserve and high price of these
precious metals limit their large-scale applications [27]. Therefore,
it is important to design low-cost and efficient catalysts for both
ORR and OER.
Inspired from Fe porphyrin of heme in nature, porphyrin-based
molecular catalysts have attracted great attention recently for
oxygen electrocatalysis [28–36]. The catalytic activity of metal
porphyrins can be finely tuned through regulating their structures
[37]. Furthermore, the clear and stable molecular structures of
metal porphyrins are beneficial for the study of catalytic reaction
mechanisms [38–41]. In addition, the resulted structure-function
relationships will further guide the design and development of
more efficient molecular catalysts [42]. Particularly, the meso-
substituent effects have been widely investigated to regulate the
catalytic activity of metal porphyrins through tuning electronic
structure of metal centers [43]. It is suggested that electron-
donating substituents [44], such as -OCH3 (-OMe), can increase the
electron density of metal centers and thus increases the binding
and electron transfer with O2 [45], which is crucial for determining
the activity of ORR. Therefore, fine-tuning the meso-substituent
structure of metal porphyrins is an appealing strategy to improve
the ORR activity.
Herein, we designed two Co porphyrin molecular catalysts with
three -OMe groups at 2,4,6- and 3,4,5-positions of meso-phenyl
substituents, named 2,4,6-OMe-CoP and 3,4,5-OMe-CoP (Figs. 1a
and b). Molecular structures of these two porphyrins were
characterized with nuclear magnetic resonance (NMR), mass
spectra (MS), UV–vis, Fourier transform infrared spectrometer
(FTIR), and single crystal X-ray diffraction method. Coporphyrin
catalysts were drop-coated on carbon nanotubes (CNTs) for oxygen
electrocatalysis. The catalytic activity, selectivity, and stability of
these two molecular catalysts were evaluated. The 3,4,5-OMe-CoP
exhibited better ORR performance than 2,4,6-OMe-CoP likely due
to its enhanced mass transfer, improved charge transfer and
hydrophilicity. The OER performance of molecular catalysts and
Zn-air battery assembled with 3,4,5-OMe-CoP were further
evaluated. This work provides a better understanding of the
* Corresponding authors.
E-mail addresses: liangzuozhong@snnu.edu.cn (Z. Liang), ruicao@snnu.edu.cn
(R. Cao).
https://doi.org/10.1016/j.cclet.2021.02.032
1001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article as: H. Lv, H. Guo, K. Guo et al., Substituent position effect of Co porphyrin on oxygen electrocatalysis, Chin. Chem. Lett.,
https://doi.org/10.1016/j.cclet.2021.02.032

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Fig. 1. Molecular structures of (a) 2,4,6-OMe-CoP and (b) 3,4,5-OMe-CoP. (c) Thermal ellipsoid plot of single crystal X-ray structure of 3,4,5-OMe-CoP (50 % probability). (d)
SEM image and (e, f) TEM images of 3,4,5-OMe-CoP/CNT.XPS spectra of 3,4,5-OMe-CoP/CNT: (g) Co 2p, (h) O 1s and (i) N 1s.
substituent position effect of -OMe groups for Co porphyrin
molecular catalysts.
out. From the full survey spectrum, obvious peaks of Co, O, N and C
were observed, demonstrating the successful loading of Co
porphyrins (Fig. S9 in Supporting information). The high resolution
XPS spectrum of Co 2p confirms that the valence state of Co is CoII
(Fig. 1g). From the high resolution XPS spectra of O 1s and N 1s,
only one kind of peak was observed in the composite 3,4,5-OMe-
CoP/CNT, indicating that 3,4,5-OMe-CoP has been adsorbed on
CNTs (Figs. 1h and i).
Electrocatalytic activities of 3,4,5-OMe-CoP/CNT and 2,4,6-
OMe-CoP/CNT for ORR were evaluated with rotating disk electrode
(RDE) and rotating ring-disk electrode (RRDE) in 0.1 mol/L KOH.
The linear sweep voltammetry (LSV) data show that 3,4,5-OMe-
CoP/CNT exhibits a half-wave potential E1/2 of 0.80 V vs. reversible
hydrogen electrode(RHE), which is larger than that of 2,4,6-OMe-
CoP/CNT with an E1/2 of 0.77 V vs. RHE (Fig. 2a). It is worth noting
that both 2,4,6-OMe-CoP/CNT and 3,4,5-OMe-CoP/CNT exhibited
much better ORR performance than CNTs (E1/2 = 0.70 V vs. RHE),
indicating that 2,4,6-OMe-CoP and 3,4,5-OMe-CoP is the real active
site for ORR. In contrast, commercial Pt/C (20 wt%) exhibited an
E1/2 of 0.86 V vs. RHE. Tafel slopes for CNTs, 2,4,6-OMe-CoP/CNT,
3,4,5-OMe-CoP/CNT and Pt/C are 73, 58, 49 and 70 mV/dec,
respectively, suggesting fast kinetics and rapid mass transfer of
these catalysts (Fig. 2b).
First, 2,4,6-OMe-CoPand 3,4,5-OMe-CoP were synthesized and
characterized with high-resolution MS, NMR, FTIR, and UV–vis
spectra (Fig. 1 and S1–S5 in Supporting information) [46]. UV–vis
spectra of 2,4,6-OMe-CoP and 3,4,5-OMe-CoP showed Soret and Q
bands of Co porphyrins (Fig. S6 in Supporting information),
indicating the integrity of the porphyrin structure [47]. Under N2
conditions, cyclic voltammetry (CV) data of 2,4,6-OMe-CoP and
3,4,5-OMe-CoP were tested in dimethylformamide (Fig. S7 in
Supporting information). For 2,4,6-OMe-CoP, there is a reversible
redox wave and an irreversible redox wave at –1.50 and –2.32 V vs.
ferrocene, respectively, corresponding to the CoII/CoI and CoI/Co0
redox couples. In contrast, 3,4,5-OMe-CoP has two reversible redox
waves at –1.36 and –2.52 V vs. ferrocene, corresponding to the
CoII/CoI and CoI/Co0 redox couples, respectively. These peaks of
3,4,5-OMe-CoP exhibit anodic shift by about 200 mV comparing to
those of 2,4,6-OMe-CoP. The crystal structure of 3,4,5-OMe-CoP
was obtained with single crystal X-ray diffraction (Fig. 1c). The Co
ion is coordinated by the four N atoms of the porphyrin unit, which
define an equatorial plane. The 3,4,5-OMe-CoP crystallized in
triclinic space group Pı with Z = 2 (Table S1 in Supporting
information).
To evaluate the catalytic activity of oxygen electrocatalysis, Co
porphyrin molecular catalysts were drop-coated on CNTs, named
3,4,5-OMe-CoP/CNT. Scanning electron microscope (SEM) and
transmission electron microscope (TEM) images of CNT and 3,4,5-
OMe-CoP/CNT were obtained (Figs. 1d and f, Fig. S8 in Supporting
information). CNT has very good dispersion with a diameter of
ꢀ15 nm (Fig. S8). The 3,4,5-OMe-CoP/CNT still shows good
dispersion (Figs. 1d and f). No obvious aggregated particles
were observed on the surface of CNTs, indicating the uniform
distribution of Co porphyrins.
Subsequently, the selectivity of ORR was evaluated by measur-
ing the number of electrons transferred with RRDE (Fig. S10 in
Supporting information). According to the current density
detected at the disk electrode and the ring electrode, the n value
was determined to be 2.37 for 2,4,6-OMe-CoP/CNT, 2.33 for 3,4,5-
OMe-CoP/CNT, 2.31 for CNTs and 3.91 for Pt/C (Fig. 2c). These
results demonstrated that the ORR of CNTs, 2,4,6-OMe-CoP/CNT
and 3,4,5-OMe-CoP/CNT follows 2e reduction process with the
production of H2O2. The values of n were further confirmed by
using Koutecky-Levich (K-L) equations (Fig. S11 in Supporting
information). In contrast, Pt/C follows 4e reduction process with
the production of H2O. Moreover, we carried out the stability test of
two molecular catalysts with controlled potential electrolysis. The
In order to verify the central metal valence state of Co porphyrin
and the successful loading of molecular catalysts, X-ray photoelec-
tron spectroscopy (XPS) test of 3,4,5-OMe-CoP/CNT was carried
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CNT has an overpotential
h
of 482 mV to reach j =10 mA/cm2, which
is smaller than that of 2,4,6-OMe-CoP/CNT (h =500 mV) (Fig. 2e). To
obtain the electrochemical surface area (ECSA), the double layer
capacitance (Cdl) was calculated first by performing CV data in the
Faraday zone with different scan rates (20, 40, 60, 80, 100 and
120 mV/s) (Figs. S12a and b in Supporting information). The results
demonstrated that the Cdl of 2,4,6-OMe-CoP/CNT is 16 mF, which is
larger than that of 3,4,5-OMe-CoP/CNT (4 mF) (Fig. S12c in
Supporting information). The following Eq. 1 can be applied to
calculate the ECSA.
ECSA ¼ Cdl=Cs
ð1Þ
Herein, the specific capacitance Cs for Co-based materials is
2.75 mF/cm2 [48]. Therefore, the ECSA of 3,4,5-OMe-CoP/CNT and
2,4,6-OMe-CoP/CNT is 1.5 and 5.8 cm2, respectively. The normal-
ized LSV data demonstrated that 3,4,5-OMe-CoP/CNT has higher
intrinsic activity that 2,4,6-OMe-CoP/CNT (Fig. S12d in Supporting
information). The turnover frequency (TOF) could be calculated
with the Eq. (2):
JS
4nF
TOF ¼
ð2Þ
Herein, J is the current density at a given potential, S is the surface
area of electrode, F is the faraday constant (96485.3 s A/mol), and n
is the number of active sites. The TOF of 3,4,5-OMe-CoP/CNT and
2,4,6-OMe-CoP/CNT under the overpotential of 550 mV is 20.4 and
12.7 sꢁ1, respectively. In addition, 3,4,5-OMe-CoP/CNT displays
smaller Tafel slope with a value of 81 mV/dec as compared to the
value of 90 mV/dec for 2,4,6-OMe-CoP/CNT, 98 mV/dec for CNTs
and 67 mV/dec for RuO2 (Fig. 2f). Therefore, 3,4,5-OMe-CoP/CNT
exhibits better OER performance than 2,4,6-OMe-CoP/CNT.
To understand catalytic activity for 3,4,5-OMe-CoP/CNT, the
electron transfer efficiency, mass transfer rate and hydrophilicity
were investigated (Fig. 3). The [Fe(CN)6]3ꢁ redox process of 2,4,6-
OMe-CoP/CNT and 3,4,5-OMe-CoP/CNT coated glassy carbon
electrode at different scan rates (0.01ꢁ0.42 V/s) were carried out
in 0.1 mol/L KCl (Figs. 3a and b). The relationship between the
Fig. 2. (a) LSV data, (b) Tafel slopes and (c) electron transfer number of CNT, 2,4,6-
OMe-CoP/CNT, 3,4,5-OMe-CoP/CNT and commercial Pt/C for ORR. (d) Controlled
potential electrolysis of 2,4,6-OMe-CoP/CNT and 3,4,5-OMe-CoP/CNT measured at
0.30 V (vs. RHE) in O2-saturated 0.1 mol/L KOH solution with RDE at 1600 rpm. (e)
LSV data and (f) Tafel slopes of CNT, 2,4,6-OMe-CoP/CNT, 3,4,5-OMe-CoP/CNT and
commercial RuO2 for OER measured in 1.0 mol/L KOH with glassy carbon electrode.
relative current of 2,4,6-OMe-CoP/CNT decreased by 31%, while
3,4,5-OMe-CoP/CNT only dropped by 6% after running for 12 h
under the applied potential of 0.3 V vs. RHE (Fig. 2d).
The electrocatalytic activity of OER was studied with glassy
carbon electrode in 1.0 mol/L KOH solution. The 3,4,5-OMe-CoP/
Fig. 3. The [Fe(CN)6]3ꢁ redox results of (a) 2,4,6-OMe-CoP/CNT and (b) 3,4,5-OMe-CoP/CNT at different scanning rates (0.01-0.42 V/s) at 0.1 mol/L KCl. (c) The relationship
between the position of redox peaks and the logarithm of scan rates. RDE measurements for ORR at RDE electrode loaded with (d) 2,4,6-OMe-CoP/CNTand (e) 3,4,5-OMe-CoP/
CNTat different rotation rates (0, 400, 625, 900,1225,1600 and 2025 rpm). (f) Tafel plots based on the kinetic controlled current density. Contact angle test of (g) CNT, (h) 2,4,6-
OMe-CoP/CNT and (i) 3,4,5-OMe-CoP/CNT.
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position of redox peaks and the logarithm of scan rates was
constructed (Fig. 3c). The electron transfer efficiency of the catalyst
surface was compared by calculating the surface electron transfer
rate constant (ks). The ks value of 3,4,5-OMe-CoP/CNT (0.116 sꢁ1) is
almost three times larger than that of 2,4,6-OMe-CoP/CNT
(0.043 sꢁ1), demonstrating that 3,4,5-OMe-CoP/CNT has better
charge transport ability compared to 2,4,6-OMe-CoP/CNT.
In order to further compare the mass transfer rate of these two
catalysts, LSV data of 2,4,6-OMe-CoP/CNT and 3,4,5-OMe-CoP/CNT
were measured at different rotation speeds with RDE (Figs. 3d and
e). The kinetic controlled currents at different potentials were
made into Tafel diagrams, and then Tafel slopes excluding mass
transfer resistance were obtained (Fig. S11). Kinetic controlled
Tafel slope of 2,4,6-OMe-CoP/CNT is 72 mV/dec, while the value of
3,4,5-OMe-CoP/CNT is 45 mV/dec (Fig. 3f). This result indicates
charge/discharge voltage gap is 0.8 V, which is smaller than that
of Pt/C + RuO2 (0.87 V) and larger than that of CNT (1.01 V) under
the same condition, indicating that the 3,4,5-OMe-CoP/CNT
exhibits the best charge and discharge performance.
In summary, we investigated substituent position effect of Co
porphyrin molecular catalysts on both ORR and OER. The 3,4,5-
OMe-CoP/CNT exhibited an E1/2 of 0.80 V vs. RHE for ORR in
0.1 mol/L KOH and an overpotential of 482 mV (at j =10 mA/cm2)
for OER measured in 1.0 mol/L KOH, which is superior than that of
2,4,6-OMe-CoP/CNT with an E1/2 of 0.77 V vs. RHE for ORR and an
overpotential of 500 mV (at j =10 mA/cm2) for OER. The enhanced
ORR/OER performance of 3,4,5-OMe-CoP/CNT may be attributed to
the fast charge transfer, enhanced mass transfer and hydrophilici-
ty. The Zn-air battery constructed with 3,4,5-OMe-CoP/CNT
exhibited comparable performance with precious metal-based
material (Pt/C + RuO2). This work provides new ideas for the design
of molecular catalysts with different substituent positions, and has
new inspiration for the design of high-performance catalysts for
clean energy conversion technology.
that 3,4,5-OMe-CoP/CNT has
a much higher mass transfer
efficiency than that of 2,4,6-OMe-CoP/CNT. In other words,
3,4,5-OMe-CoP/CNT has very good mass transfer ability.
We further studied the hydrophilicity of these two catalysts and
CNTs. Contacting angles of CNTs (Fig. 3g), 2,4,6-OMe-CoP/CNT
(Fig. 3h) and 3,4,5-OMe-CoP/CNT (Fig. 3i) were 137.5ꢂ, 132.3ꢂ and
127.2ꢂ, respectively. This result indicates that 3,4,5-OMe-CoP/CNT
has relatively good hydrophilic property.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Based on the good electrocatalytic ORR and OER performance of
3,4,5-OMe-CoP/CNT, a rechargeable Zn-air battery was assembled
using this catalyst. The polished Zn foil was used as the anode, and
the catalyst-dropped carbon cloth/gas diffusion layer was used as
the cathode (Fig. 4a). The electrolyte was 6.0 mol/L KOH with
0.2 mol/L Zn acetate, to ensure the reversible reactions when
charging. Fig. 4b shows the charge and discharge electrochemical
polarization data and the corresponding power density data. The
3,4,5-OMe-CoP/CNT has the largest power density with a value of
144.8 mW/cm2 compared to that of Pt/C + RuO2 (137.1 mW/cm2)
and CNT (106.3 mW/cm2). Fig. 4c displays the discharge charac-
teristic curve of CNT, 3,4,5-OMe-CoP/CNT and Pt/C + RuO2 at
j =20 mA/cm2. The specific capacitance of 3,4,5-OMe-CoP/CNT is
634.02 mAh/g, which is larger than that of CNT (583.55 mA h/g)
and smaller than that of Pt/C + RuO2 (668.21 mAh/g). Fig. 4d shows
the charge/discharge cycle data of CNT, 3,4,5-OMe-CoP/CNT and
Acknowledgments
We are grateful for support from National Natural Science
Foundation of China (Nos. 21808138 and 21773146), Fok Ying-Tong
Education Foundation for Outstanding Young Teachers in Univer-
sity, Fundamental Research Funds for the Central Universities (Nos.
GK202103029 and GK202103045), Young Talent fund of University
Association for Science and Technology in Shaanxi, China, China
Postdoctoral Science Foundation (No. 2019T120877), and Research
Funds of Shaanxi Normal University.
Appendix A. Supplementary data
Pt/C + RuO2 measured at
j
=2 mA/cm2. The discharge/charge
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
online version, at doi:https://doi.org/10.1016/j.cclet.2021.02.032.
voltage is 1.19 V and 1.99 V, respectively, for Zn-air battery
assembled with 3,4,5-OMe-CoP/CNT. Therefore, the resulted
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