Surface molecular engineering of axial-exchanged Fe(III)Cl- and Mn(III)Cl-porphyrins towards enhanced electrocatalytic ORRs and OERs

Article abstract of DOI:10.1016/j.ica.2020.119584

Herein, pyrene-pyridine (Pyr-Py) molecule was applied as the axial exchanged ligand to bridge Fe(III) and Mn(III)porphyrin immobilized on rGO. These axially exchanged metalloporphyrin functionalized nanocomposites revealed enhanced electrochemically catal

Full text of DOI:10.1016/j.ica.2020.119584

InorganicaChimicaActa507(2020)119584
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Inorganica Chimica Acta
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Research paper
Surface molecular engineering of axial-exchanged Fe(III)Cl- and Mn(III)Cl-
porphyrins towards enhanced electrocatalytic ORRs and OERs
⁎
Isaac Kwaku Attatsia,b, Weihua Zhua,b, , Xu Lianga,⁎
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
b School of Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Herein, pyrene-pyridine (Pyr-Py) molecule was applied as the axial exchanged ligand to bridge Fe(III) and Mn
(III)porphyrin immobilized on rGO. These axially exchanged metalloporphyrin functionalized nanocomposites
revealed enhanced electrochemically catalyzed oxygen reductions and evolutions that demonstrated the surface
molecular engineering through axial ligand exchange is an effective strategy to enhance the catalytic efficiency.
Noncovalent immobilization
Metalloporphyrin
Electrocatalysis
Oxygen reduction
Oxygen evolution
1. Introduction
(III) porphyrin in an ordered manner for enhanced oxygen reduction
[14,28]. Thus, surface molecular engineering of axial-exchanged me-
The upsurge in global warming owing to increasing demand and
usage of fossil fuels has being a matter of concern to environmental
engineers [1,2]. This has necessitated a search for clean, cheap, and
highly efficient energy systems which serve as alternative to fossil fuels
[3,4]. Electrochemical oxygen reduction reactions (ORR) and oxygen
evolution reactions (OER) play important role in many energy con-
version systems, such as fuel cells, metal-air batteries [5–7] and elec-
trocatalysis [8,9]. The major challenge associated with these reactions
is their sluggish kinetics at cathodes of fuel cells [10,11]. Owing to this
challenge, molecular electrocatalysts with advances in research are
developing new ways to eliminate the sluggish kinetics of ORR in other
to make fuel cell technology well appreciated [12]. Traditionally, noble
metal Pt is used in energy conversion devices as a form of replacement
to fossil fuels but due to high cost [9,13], instability, scarcity [14] and
ease of methanol poisoning arising from Pt, its use could not be sus-
tained [15,16]. After series of research, molecular catalysts from earth-
abundant 3d transition metals such as Mn [17,18] and (Co/Fe) [19–21]
showed to be a better alternative to Pt based catalyst, such as me-
talloporphyrin [22–24]. Recently, Wang and co-workers reported on
metal porphyrin intercalated rGO nanocomposite as efficient electro-
catalyst for oxygen reduction [25]. Previously, Zuo reported the or-
ientation mode of Co(II)porphyrins for electrocatalytic dioxygen re-
duction [26], and Marianov’s group discussed the covalent and/or non-
covalent immobilized Mn(III)Cl-TPP towards electrocatalytic water
oxidation and oxygen reductions [27]. On the other hand, Wei and
Chlistunoff focused on the bio-inspired axial ligand coordinated iron
talloporphyrinoids became one of the interesting topics towards en-
hanced ORR activity and stability compared to both non-axially linked
metalloporphyrin and Pt/C catalyst in aqueous media especially. In-
sight from these studies reveals the enhanced electrocatalytic perfor-
mance of metalloporphyrins especially when axially coordinated. It
should be pointed here, when pyrene-pyridine hybrid (Pyr-Py) was
used as the key linker, both axial exchanged behaviors and surface
order assembling could be facilely included within one system. Thus,
the current investigations will provide useful information on how
functionalized linkers work for surface molecular engineering towards
enhanced catalytic behaviors (Scheme 1).
2. Results and discussions
2.1. Spectroscopic investigations
Mn(III)Cl-tetraphenylporphyrin [27] 2 and pyrene-pyridine [29]
(Pyr-Py) were prepared according to literature procedures (Scheme S1,
see ESI), and Fe(III)Cl-tetraphenylporphyrin was commercially pur-
chased. Upon introduction of 0.4 µM Pyr-Py solution to 1 and 2, Pyr-Py
characterization peaks surfaced in addition to a decrease in both Soret
and Q absorption bands of 1 and 2 (Fig. 1a and b) [30,31]. When pure
pyridine was added, new bands related to metalloporphyrins appeared
[28], a behaviour which wasn’t seen using Pyr-Py (Fig. 1c and d).
Despite this occurrence, decrease in absorption bands of metallopor-
phyrins appeared during introduction of either pyridine or Pyr-Py. It is
⁎ Corresponding authors at: School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China.
E-mail addresses: sayman@ujs.edu.cn (W. Zhu), liangxu@ujs.edu.cn (X. Liang).
https://doi.org/10.1016/j.ica.2020.119584
Received 22 December 2019; Received in revised form 5 March 2020; Accepted 5 March 2020
Availableonline13March2020
0020-1693/©2020PublishedbyElsevierB.V.

I.K. Attatsi, et al.
InorganicaChimicaActa507(2020)119584
Scheme 1. Molecular structures of Fe(III)Cl-TPP (left), Mn(III)Cl-TPP (middle) and Pyr-Py (right).
Fig. 1. UV–vis spectra of 1 and 2 upon addition of 0.4 µM Pyr-Py (a and b) and 12.35 M pure pyridine (c and d) in CH2Cl2.
worthy to note that as equivalent ratios of Pyr-Py increases its ab-
sorption spectra begin to appear clearly. This is consistent with the
work of snitkoff et al., where the imidazole peaks increase as the ratio
of the linker increases [31]. On the other hand, axial exchange could
also be confirmed when UV–vis spectra of rGO composite materials are
measured. As shown in Fig. S1, the appearance of 1 and 2 together with
our Pyr-Py in our composite materials showed little or no change in
both Soret and Q-band wavelength but a minimal decrease in absorp-
tion intensity which is consistent with results obtained in solution de-
void of rGO. This observation could arise from coordination interaction.
Another observation is the rise in baseline of the composite materials
which is attributed to the presence of rGO in the nanocomposites. At the
same measuring conditions, spectroscopic evidence to support direct
interaction between metalloporphyrin 1 and 2 to rGO is the appearance
of respective metalloporphyrin peaks at both the Soret and Q regions
accompanied with a rise in absorption bassline (Fig. S1, see ESI).
Also, XRD patterns of rGO, 1/rGO and 1/rGO/Pyr-Py showed broad
peaks around 25° which is a characteristic feature of rGO as shown in
Fig. 3a and c. This diffraction angle is indexed to graphitic sheet of
reduced GO related to [0 0 2] diffraction of graphene sheet [34], be-
sides XRD measurement didn’t show any assigned metallic peaks. In
addition, these rGO supported nanocomposites were also characterized
by FT-IR spectroscopy (Fig. 3b and d). Composite materials of 1 showed
the characteristic peaks of tetraphenylporphyrin itself, for example, a
broad band at ca. 3052 cm−1 is assigned as the CeH bond stretching in
phenyl rings of 1. Peaks at 1594 cm−1 (C]C stretching vibrations of
phenyl rings), 1444 cm−1 (C]C), 1337 cm−1 (CeN), 1071 cm−1 (C]C
stretching vibrations of pyrrole ring), 1001 cm−1 (CeC vibrations of
pyrrole ring), 808 cm−1 (out of plane bending of outer CeH moiety),
701 cm−1 (CeN) and 661 cm−1, that indicated the presence of 1 in the
nanocomposite 1/rGO/Pyr-Py [35,36]. Similarly, FT-IR spectra of 2/
rGO/Pyr-Py composite also showed the characteristic peaks of 2 and
Pyr-Py (Fig. 3d).
2.2. Structural characterizations
2.3. Electrocatalytic oxygen reduction
Morphological characterization of rGO and its supported nano-
composites using SEM showed the good exfoliated structure (Fig. 2),
besides, no significant aggregation or decomposition were observed
upon introducing metalloporphyrin 1 and 2 [32,33] as well as Pyr-Py.
Herein, the electrochemically catalyzed oxygen reductions of 1/
rGO/Pyr-Py and 1/rGO nanocomposites were firstly investigated in the
acid media. As shown in Fig. 4, the cyclic voltammetry data of 1/rGO/
2

I.K. Attatsi, et al.
InorganicaChimicaActa507(2020)119584
Fig. 2. SEM images of (a) rGO, (b) rGO/Pyr-Py, (c) 1/rGO, (d) 1/rGO/Pyr-Py, (e) 2/rGO and (f) 2/rGO/Pyr-Py.
Fig. 3. (a) XRD patterns of rGO, 1/rGO and 1/rGO/Pyr-Py, (b) FT-IR spectra of Pyr-Py, 1 and 1/rGO/Pyr-Py nanocomposites, (c) XRD patterns of 2/rGO and 2/
rGO/Pyr-Py, and (d) FT-IR spectra of Pyr-Py, 2 and 2/rGO/Pyr-Py.
Pyr-Py has a more positive onset potential and halve-wave potentials
compared with that of 1/rGO. Wei and co-workers reported the im-
portant role of a more positive onset and halve-wave potentials which is
employed to evaluate the electrocatalytic activity of materials [14]. In
addition, similar trends of enhanced electrochemically catalyzed be-
haviors were also observed in basic and neutral media where the onset
potentials, halve-wave potentials and current densities of 1/rGO/Pyr-
Py are enhanced than 1/rGO (Fig. 4c–f) owing to the presence of the
axially coordinated Pyr-Py. The enhanced catalytic activity brought
about by the Pyr-Py was further investigated using 2 as the catalyst to
produce 2/rGO/Py-Py.
clearly confirmed. Liang et al., tested the state-of-the-art Pt/C catalyst in
KOH and reported a halve-wave value at 0.87 V (versus RHE) [37]. Cao
et al., in KOH also found the halve wave potential of Pt/C at 0.88 V [38]
which is consistent with Liang’s work. These results compared to our
material (2/rGO/Pyr-Py) showed a better electrocatalytic performance
(0.90 V versus RHE, halve-wave potential) than state-of-the-art Pt/C in
same media under our experimental condition. The summary of CV
data of 1 and 2 with and without Pyr-Py on rGO towards ORR in all
media is presented in Table 1. It is important to state that the reversible
redox wave displayed at E1/2 = 0.65 and 0.43 vs RHE is attributed to
the iron porphyrins reversible FeIII/FeII couple which is responsible for
the electrocatalysis. On the other hand, a plot of current vs scan rates
(10–500 mV s−1) for 1/rGO/Pyr-Py, 1/rGO, 2/rGO/Pyr-Py and 2/rGO
In addition, when 2/rGO/Pyr-Py and 2/rGO were compared
(Fig. 5), the positive effect brought up by our Pyr-Py ligand were also
3

I.K. Attatsi, et al.
InorganicaChimicaActa507(2020)119584
Fig. 4. CVs of GC electrodes coated with 1/rGO/Pyr-Py and 1/rGO in 0.5 M H2SO4 (a and b), 1.0 M KOH (c and d) and 0.1 M PBS (e-f) aqueous solutions under N2
and O2. Conditions: 50 mV s−1 scan rate at 25 °C.
in 0.1 M KOH were examined (Fig. S2, see ESI). A linear relationship
between peak current and scan rate indicates the immobilized species
are present. On the other hand, redox events of the catalyst materials
appeared more clearer as scan rates increased illustrating more active
sites are exposed. This further indicates the transition from Fe+3/Fe+2
and Mn3+/Mn2+ increases with increasing scan rates thereby exposing
more active sites for catalysis.
On the other hand, kinetic activities derived from LSV for our ma-
terials in KOH are presented in Fig. 6. The Tafel slopes indicate higher
kinetic rates with respect to Pyr-Py composites in the order 1/rGO
(116 mV/dec) < 1/rGO/Pyr-Py (93 mV/dec) and 2/rGO (110 mV/
dec) < 2/rGO/Pyr-Py (93 mV/dec) as shown in Fig. 6a′ and b′ re-
spectively. The low Tafel slopes shown by our two target materials
further elaborates the effectiveness of these materials to catalyze
oxygen reduction reactions.
Electrochemical tests were performed using our target materials (1/
rGO/Pyr-Py and 2/rGO/Pyr-Py) in both acidic and basic environment
to stability. Linear sweep voltammetry responses before and after 300
cycles revealed our target materials (metalloporphyrin/rGO/Pyr-Py)
showing some levels of catalytic fading in both media (Fig. 7a–d). This
fading especially in acidic medium, could be linked to protonation of
aromatic nitrogen in pyridine that makes the complexation reaction a
little unfavorable for longer cycles. Notably, the stability test showed 1/
rGO/Pyr-Py materials better than 2/rGO/Pyr-Py (Fig. 7a–d). The su-
perior stability exhibited by 1/rGO/Pyr-Py illustrates a stronger co-
ordination property between iron porphyrin and Pyr-Py moiety
compared to that of manganese porphyrin. This property gives a more
advantage of 1/rGO/Pyr-Py over 2/rGO/Pyr-Py.
2.4. Oxygen evolution reaction
Electrocatalytic OER properties of our composite materials were
examined in 1.0 M KOH and LSV data shown in Fig. 8a and b. From the
data, materials with Pyr-Py exhibited enhanced current density com-
pared to the materials directly on rGO suggesting surpassing kinetics of
these materials (metalloporphyrins/rGO/Pyr-Py). At j = 1.0 mA/cm2
(1.481 V), 1/rGO/Pyr-Py displayed an overpotential of 251 mV while
1/rGO at the same current density recorded overpotential of 335 mV
exhibiting a difference of 84 mV. Lower overpotential exhibited by 1/
rGO/Pyr-Py indicates its ability to evolve oxygen more than 1/rGO.
Also, at j = 10 mA/cm2, 1/rGO/Pyr-Py showed the lowest over-
potential (521 mV) which is 109 mV lower than 1/rGO whose over-
potential was 630 mV. The OER activities of 2/rGO/Pyr-Py and 2/rGO
were also monitored at j = 1.0 and 10 mA/cm2 and the results reported
in Table 2. In all aspects 2/rGO/Pyr-Py showed lower overpotential
compared to 2/rGO as expected. This is another avenue to praise the
catalytic enhancement effect of our Pyr-Py linker. To gain insight into
kinetics of electrode reaction, we performed Tafel analysis using our
LSV measurement (Fig. 8a′ and b′). The analysis revealed a slope in the
order 1/rGO/Pyr-Py < 1/rGO < rGO/Pyr-Py < rGO and 2/rGO/
Pyr-Py < 2/rGO < rGO/Pyr-Py < rGO similar to that of ORRs.
Marianov and co-workers in their experiment indicated a positive Tafel
4

I.K. Attatsi, et al.
InorganicaChimicaActa507(2020)119584
Fig. 5. CVs of GC electrodes coated with 2/rGO/Pyr-Py and 2/rGO in 0.5 M H2SO4 (a and b), 1.0 M KOH (c and d) and 0.1 M PBS (e-f) aqueous solutions under N2
and O2. Conditions: 50 mV s−1 scan rate at 25 °C.
target materials towards catalysis.
Table 1
Comparison between ORR activities of composite materials in aqueous media.
3. Conclusion
Composite
materials
Aqueous
media
Onset-
Half-wave
Current
potential (V vs potential (V vs
RHE)
density (mA/
RHE)
cm2)
In this work, a pyrene-pyridine hybrid (Pyr-Py) was employed as an
axial ligand to bridge Fe(III)Cl-TPP and Mn(III)Cl-TPP through non-
covalent immobilization on rGO. Comparatively, significantly enhanced
electrocatalytic performance towards ORR and OER through axial li-
gand exchanging strategy was exhibited. Therefore, we are hopeful this
strategy of integrating molecular catalysts on Pry-Py through axial li-
gation via rGO could enhance new applications in electrocatalysis to-
wards ORR and OER.
1/rGO/Pyr-Py
1/rGO
H2SO4
KOH
PBS
H2SO4
KOH
PBS
H2SO4
KOH
PBS
H2SO4
KOH
PBS
+0.535
+0.859
+0.689
+0.528
+0.837
+0.659
+0.345
+1.101
+0.579
+0.273
+1.082
+0.556
+0.401
+0.779
+0.574
+0.352
+0.740
+0.506
+0.202
+1.001
+0.453
+0.100
+0.991
+0.430
−7.314
−2.127
−2.431
−4.021
−1.089
−1.978
−4.699
−2.368
−3.808
−1.003
−0.663
−0.637
2/rGO/Pyr-Py
2/rGO
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
slope of above 230 mV/dec with their noncovalent immobilized MnTPP
via carbon cloth [28]. Comparison of our Tafel slope for both 1/rGO/
Pyr-Py and 2/rGO/Pyr-Py (216 mV/dec) to their work indicates a
better kinetics of our material than their work. We have therefore once
again demonstrated the enhanced electrocatalytic performance of our
Acknowledgements
This work was financially supported by the National Natural Science
5

I.K. Attatsi, et al.
InorganicaChimicaActa507(2020)119584
Fig. 6. LSV curves for 1/rGO/Pyr-Py, 1/rGO (a) and 2/rGO/Pyr-Py, 2/rGO (b) at 50 mV/s in 1.0 M KOH O2 saturated solution at 25 °C with their corresponding
Tafel Plots (a′ and b′).
Fig. 7. LSV of GC coated with 1/rGO/Pyr-Py (a) and 2/rGO/Pyr-Py (b) before and after 300 cycles in 0.5 M H2SO4 (a and b) and 1.0 M KOH (c and d) at 50 mV s−1
scan rates.
Foundation of China (21701058), the Natural Science Foundation of
Jiangsu province (BK20160499), the State Key Laboratory of
Coordination Chemistry (SKLCC1817), the Key Laboratory of
Functional Inorganic Material Chemistry (Heilongjiang University) of
Ministry of Education, the China post-doc foundation (2018 M642183),
the Lanzhou High Talent Innovation and Entrepreneurship Project
(2018-RC-105) and the Jiangsu University (17JDG035).
6

I.K. Attatsi, et al.
InorganicaChimicaActa507(2020)119584
Fig. 8. LSV curves for 1/rGO/Pyr-Py, 1/rGO (a) and 2/rGO/Pyr-Py, 2/rGO (b) at 50 mV/s in 1.0 M KOH O2 saturated solution at 25 °C with their corresponding
Tafel Plots (a′ and b′).
Table 2
Comparison between OER activities of composite materials of 1 and 2 in KOH.
Composite materials
@ j = 1.0 (E/V)
Overpotential/ mV
@ j = 10 (E/V)
Overpotential/mV
Tafel slope/mV dec−1
1/rGO/Pyr-Py
1/rGO
2/rGO/Pyr-Py
2/rGO
1.481
1.565
1.665
1.712
251
335
435
482
1.751
1.860
1.978
2.165
521
630
748
935
216
280
216
325
The OER overpotentials were calculated using the formula overpotential (η) = ERHE − 1.23 V.
Author contributions
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whole work was supervised and manuscript edited by Prof. Dr. Xu Liang
and Prof. Dr. Weihua Zhu. The manuscript was well discussed by all
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119584.
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