Significantly boosted oxygen electrocatalysis with cooperation between cobalt and iron porphyrins

Article abstract of DOI:10.1039/d1dt00441g

Developing electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of great importance. Herein, Co tetrakis(pentafluorophenyl)porphyrin (Co-P) and Fe chloride tetrakis(pentafluorophenyl)porphyrin (Fe-P) were loaded on carbon nanotubes (CNTs) for combining the electrocatalytic advantages of both Co-P and Fe-P. The resultant (Co-P)_0.5(Fe-P)_0.5@CNT composite displayed significantly boosted activity for the selective four-electron ORR with a half-wave potential of 0.80 Vversusreversible hydrogen electrode (RHE) and for the OER with a potential of 1.65 VversusRHE to obtain 10 mA cm^?2current density in 0.1 M KOH. A Zn-air battery assembled from (Co-P)_0.5(Fe-P)_0.5@CNT exhibited a small charge-discharge voltage gap of 0.74 V at 2 mA cm^?2, a high power density of 174.5 mW cm^?2and a good rechargeable stability (>120 cycles).

Full text of DOI:10.1039/d1dt00441g

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Significantly boosted oxygen electrocatalysis with
cooperation between cobalt and iron porphyrins†
Cite this: Dalton Trans., 2021, 50,
5120
Haitao Lei, Qingxin Zhang, Yabo Wang, Yimei Gao, Yanzhi Wang,
Received 9th February 2021,
Accepted 23rd March 2021
Zuozhong Liang,
Wei Zhang * and Rui Cao
*
DOI: 10.1039/d1dt00441g
rsc.li/dalton
Developing electrocatalysts for the oxygen reduction reaction complexes, metal porphyrins have attracted increasing interest
(ORR) and the oxygen evolution reaction (OER) is of great impor- as ORR catalysts. As inspired by cytochrome c oxidases (CcOs),
tance. Herein, Co tetrakis(pentafluorophenyl)porphyrin (Co–P) which have heme active sites for the selective 4e ORR, a variety
and Fe chloride tetrakis(pentafluorophenyl)porphyrin (Fe–P) were of Fe and Co porphyrins have been synthesized and studied as
loaded on carbon nanotubes (CNTs) for combining the electro- catalysts for the ORR.21,31–33,44–49 In general, Fe porphyrins
catalytic advantages of both Co–P and Fe–P. The resultant (Co– usually show high selectivity for the 4e ORR although they
P)0.5(Fe–P)0.5@CNT composite displayed significantly boosted require high overpotentials to achieve considerable activities.
activity for the selective four-electron ORR with a half-wave On the other hand, Co porphyrins typically catalyze the 2e
potential of 0.80 V versus reversible hydrogen electrode (RHE) and ORR with much higher activities at relatively much smaller
for the OER with a potential of 1.65 V versus RHE to obtain 10 mA overpotentials.32,40 Theoretical calculations indicated that the
cm−2 current density in 0.1 M KOH. A Zn–air battery assembled poor 4e ORR activity of Co porphyrins was due to the difficulty
from (Co–P)0.5(Fe–P)0.5@CNT exhibited a small charge–discharge in transferring its 3d electrons to the π* orbital of the O2
voltage gap of 0.74 V at 2 mA cm−2, a high power density of adducts to weaken the O–O bond.50
174.5 mW cm−2 and a good rechargeable stability (>120 cycles).
In order to achieve the electrocatalytic ORR with high
activity and selectivity, we herein report a simple strategy by
combining the advantages of both Fe and Co porphyrins. Fe
and Co complexes of tetrakis(pentafluorophenyl)porphyrin
(denoted as Fe–P and Co–P, respectively) were loaded on CNTs
through simple adsorption to form (Co–P)x(Fe–P)y@CNT (x + y
= 1) hybrids. The strong electron-withdrawing pentafluorophe-
nyl substituents will make porphyrins easier to be reduced,
which will usually decrease the overpotentials for catalytic
reduction reactions. We show that (Co–P)0.5(Fe–P)0.5@CNTs
display high activity and selectivity for the 4e ORR, which is a
result of having both Co–P and Fe–P on CNTs. As a practical
demonstration, a Zn–air battery assembled using (Co–P)0.5(Fe–
P)0.5@CNT showed a comparable performance to that with Pt/
C. This work provides a simple strategy to significantly boost
the ORR activity and selectivity by combining two metal por-
phyrins, which is valuable to be extended to other catalyst
systems.
The reaction of tetrakis(pentafluorophenyl)porphyrin with
Co and Fe salts in dimethylformamide afforded Co–P and Fe–
P, respectively.51 High quality crystals of Co–P (Fig. S2†) and
Fe–P (Fig. S3†) were obtained, and their structures were charac-
terized by the single crystal X-ray diffraction method. The
purity of the bulk samples of Co–P and Fe–P was confirmed by
high-resolution mass spectrometry (HRMS, Fig. S4 and S5,†
respectively) and UV-vis spectroscopy (Fig. S6†).
Globally increasing energy demands and environmental con-
cerns have prompted extensive and intensive research studies
on clean energy conversion and storage systems.1–4 The
electrocatalytic ORR and OER play important roles in energy
conversion and storage devices.5–10 In particular, the ORR is a
key process involved in fuel cells and/or metal–air
batteries.11–16 Pt/C, as the most active four-electron ORR cata-
lyst, is usually used in fuel cells.17 However, the high cost and
low natural abundance of Pt, and also the deactivation and
thus the unsatisfied stability of Pt/C in an electrocatalytic
process have forced researchers to find highly efficient and
robust ORR electrocatalysts made from Earth-abundant tran-
sition metal elements.18–29
In the past decade, extensive efforts have been made to
make
ORR
electrocatalysts
from
transition
metal
macrocyclic complexes, such as porphyrins,21,23,27,30–33
phthalocyanines,34–39 and corroles.19,20,22,40–43 Among these
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: zw@snnu.edu.cn, ruicao@snnu.edu.cn
†Electronic supplementary information (ESI) available: Experimental details of
syntheses, characterization and electrochemical studies and Fig. S1–S12. See
DOI: 10.1039/d1dt00441g
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Fig. 1 (a) SEM image of unmodified CNTs. (b) SEM image, (c)
HAADF-STEM image, and (d–f) the corresponding elemental mapping
images of (Co–P)0.5(Fe–P)0.5@CNT.
After loading Co–P and Fe–P on CNTs through physical
adsorption (Fig. 1a and b), the resultant hybrids were analyzed
by scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM), which showed the absence of aggre-
gated particles of metal porphyrins. The high-angle annular
dark-field transmission electron microscopy (HAADF-STEM)
image of (Co–P)0.5(Fe–P)0.5@CNTs (Fig. 1c) and the corres-
ponding energy dispersive X-ray (EDX) elemental mapping
images of N (Fig. 1d), Fe (Fig. 1e) and Co (Fig. 1f) clearly con-
firmed the adsorption and uniform distribution of metal por-
phyrins on the surfaces of CNTs. The EDX analysis indicated a
Fig. 2 (a) CVs, (b) RDE polarization data (at a rotation rate of 1600 rpm)
and (c) Tafel slopes of CNTs, Co–P@CNT, Fe–P@CNT, (Co–P)0.5(Fe–
P)0.5@CNT and 20% Pt/C in 0.1 M KOH solution. RDE polarization data of
Co–P@CNT (d), Fe–P@CNT (e), and (Co–P)0.5(Fe–P)0.5@CNT (f) at
different rotation rates. Inset: K–L plot of j−1 versus ω−1
.
Co/Fe elemental ratio close to 1 : 1 in the (Co–P)0.5(Fe– 0.86 V. The Tafel slope of (Co–P)0.5(Fe–P)0.5@CNT (70.1 mV per
P)0.5@CNT (Fig. S7†). This ratio was also confirmed by induc- decade) is smaller than that of Co–P@CNT (83.6 mV per
tively coupled plasma mass spectrometry (ICP-MS, Table S1†). decade) and Fe–P@CNT (98.2 mV per decade), indicating good
In addition, the X-ray photoelectron spectroscopy (XPS) ana- kinetics during the ORR (Fig. 2c). In RDE measurements, the n
lysis of (Co–P)0.5(Fe–P)0.5@CNT showed signals attributed to N value was calculated from the K–L analysis, giving n = 3.33 for
1s (398.7 eV), Co 2p (796.8 and 780.6 eV) and Fe 2p (724.2 and Co–P@CNT, 3.98 for Fe–P@CNT, and 3.95 for (Co–P)0.5(Fe–
710.8 eV), further confirming the presence of Co–P and Fe–P P)0.5@CNT (Fig. 2d–f).
on CNTs (Fig. S8†).
In addition, the n value was also evaluated by using RRDE
Electrocatalytic ORR studies were carried out in 0.1 M KOH measurements. In RRDE, the H2O2 yielded during the catalytic
solution. As shown in Fig. 2a, the cyclic voltammograms (CVs) ORR was calculated to be 73.2% for CNTs, 36.5% for Co–
of Co–P@CNT and Fe–P@CNT showed catalytic ORR currents P@CNT, 5.9% for Fe–P@CNT, 5.1% for (Co–P)0.5(Fe–
with peak potentials at 0.71 and 0.64 V versus RHE (all poten- P)0.5@CNT, and 0.5% for Pt/C (Fig. 3a). Based on these results,
tials reported in this work are with reference to RHE unless the average n values were evaluated, giving n = 2.54 for CNTs, n
otherwise noted), respectively. Notably, the CV of (Co–P)0.5(Fe– = 3.27 for Co–P@CNT, n = 3.88 for Fe–P@CNT, n = 3.89 for
P)0.5@CNT showed a catalytic ORR wave with a peak potential (Co–P)0.5(Fe–P)0.5@CNT, and n = 3.99 for Pt/C (Fig. 3b). These
of 0.70 V, which was almost identical to the activity of Co– results were consistent with those obtained from the K–L ana-
P@CNT. Rotating disk electrode (RDE) and rotating ring-disk lysis. Furthermore, we changed the ratio of Co–P and Fe–P
electrode (RRDE) measurements were then carried out in O2- loaded on CNTs to make (Co–P)0.3(Fe–P)0.7@CNT and (Co–
saturated 0.1 M KOH aqueous solution. As shown in Fig. 2b, P)0.7(Fe–P)0.3@CNT. Under the same conditions, these two
the linear sweep voltammetry (LSV) of (Co–P)0.5(Fe–P)0.5@CNT composites displayed relatively poorer ORR activity and 4e
displayed an ORR wave with the half-wave potential E1/2 = 0.80 selectivity than (Co–P)0.5(Fe–P)0.5@CNT (Fig. S9–11†). By com-
V, which is very close to that of Co–P@CNT (0.81 V) and is ano- paring with Co porphyrin and Fe porphyrin alone when loaded
dically shifted as compared to unmodified CNTs (0.70 V) and on carbon substrates, (Co–P)0.5(Fe–P)0.5@CNT shows the best
Fe–P@CNT (0.73 V). Being used as a reference, commercial Pt/ ORR performance (Table S2†). The durability tests are shown
C (20 wt%) displayed ORR activity with a half-wave potential of in Fig. 3c. After 10 h of controlled potential electrolysis, the
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Fig. 3 (a) RRDE measurements and (b) n values for CNTs, Co–P@CNT,
Fe–P@CNT, (Co–P)0.5(Fe–P)0.5@CNT and 20% Pt/C measured in O2-
saturated 0.1 M KOH solution at 1600 rpm with RRDE. (c) Controlled
potential electrolysis of GC electrodes coated with (Co–P)0.5(Fe–
P)0.5@CNT and Pt/C. (d) LSVs of (Co–P)0.5(Fe–P)0.5@CNT tested in O2-
saturated 0.1 M KOH electrolyte at 1600 rpm with a scan rate of 5 mV
s−1
.
Fig. 4 (a) Diagram of the Zn–air battery. (b) Charge and discharge
polarization data, (c) discharge data at 20 mA cm−2, (d) open circuit
plots, (e) galvanostatic discharge–charge cycle data at 2 mA cm−2 of
(Co–P)0.5(Fe–P)0.5@CNT and 20% Pt/C. (f) The long-time cycling of a
rechargeable Zn–air battery using (Co–P)0.5(Fe–P)0.5@CNT at 2 mA
electrocatalytic ORR currents with Pt/C and (Co–P)0.5(Fe–
P)0.5@CNT decreased by 17% and 6%, respectively.
Moreover, the hybrid (Co–P)0.5(Fe–P)0.5@CNT was also
active for the electrocatalytic OER by reaching 10 mA cm−2
current density at an overpotential of 420 mV in 0.1 M KOH
(Fig. 3d). As a practical demonstration, this bifunctional elec-
trocatalyst was used to assemble a rechargeable Zn–air battery
(Fig. 4a). As shown in Fig. 4b, the battery with (Co–P)0.5(Fe–
P)0.5@CNT displayed a higher discharge current than that with
Pt/C. The peak power density of the battery with (Co–P)0.5(Fe–
P)0.5@CNT is 174.5 mW cm−2, which is larger than that with
Pt/C (75.8 mW cm−2). The specific capacity was tested at a dis-
charge current density of 20 mA cm−2 and was normalized to
the mass of Zn (Fig. 4c). The specific capacity of the battery
cm−2
.
sites and the generated partially reduced O2 species can be
further reduced at the Fe–P sites, achieving both high activity
and selectivity for the 4e ORR. This work provides a new strat-
egy by combining different molecular catalysts to achieve
cascade reactions, which is valuable to be explored in other
electrocatalytic processes.
with (Co–P)0.5(Fe–P)0.5@CNT was found to be 752.2 mA h g−1
,
Conflicts of interest
which is higher than that with Pt/C (720.5 mA h g−1). As
shown in Fig. 4d, the open circuit voltage is 1.49 V for (Co–
P)0.5(Fe–P)0.5@CNT, whereas it is 1.45 V for Pt/C. Moreover, the
battery with (Co–P)0.5(Fe–P)0.5@CNT exhibited good cyclic
stability and its initial voltage gap during charging–dischar-
ging was 0.74 V, which is smaller than that with Pt/C (0.98 V),
There are no conflicts to declare.
Acknowledgements
(Co–P)0.3(Fe–P)0.7@CNT (0.80 V), and (Co–P)0.7(Fe–P)0.3@CNT We are grateful for support from the Fok Ying-Tong Education
(0.82 V) at 2 mA cm−2 (Fig. 4e and S12†). The durability is Foundation for Outstanding Young Teachers in University, the
shown in Fig. 4f, indicating that the Zn–air battery with (Co– National Natural Science Foundation of China (Grants
P)0.5(Fe–P)0.5@CNT displayed a stable performance over 40 h.
In conclusion, we reported the electrocatalytic ORR features Science Foundation (2018M631120 and 2019T120877), the
of a hybrid with both Co–P and Fe–P loaded on CNTs. The Shaanxi
Province Postdoctoral Science Foundation
21773146, 21808138 and 21902099), the China Postdoctoral
resultant (Co–P)0.5(Fe–P)0.5@CNT displayed high 4e ORR (2018BSHEDZZ107), the open fund of State Key Laboratory of
activity and selectivity with a half-wave potential of 0.80 V, Structural Chemistry, Fundamental Research Funds for the
which is superior to Co–P and Fe–P alone when loaded on Central Universities (GK202103045 and GK202103050), and
CNTs. It is suggested that O2 may be first reduced at the Co–P Research Funds of Shaanxi Normal University.
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