Metallated azo-naphthalene diimide based redox-active porous organic polymer as an efficient water oxidation electrocatalyst

Article abstract of DOI:10.1039/c8ta06588h

The importance of developing a stable, efficient and low-cost catalyst for the oxygen evolution reaction (OER) is at its pinnacle due to the reaction's poor reaction kinetics. Porous organic polymers (POPs) have recently emerged as a versatile platform fo

Full text of DOI:10.1039/c8ta06588h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Metallated Azo-Naphthalene Diimide based Redox-Active Porous
Organic Polymer as Efficient Water Oxidation Electrocatalyst
Sajad Ahmad Bhat,#a Chayanika Das #a and Tapas Kumar Maji*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The importance of developing a stable, efficient and low-cost catalyst for oxygen evolution reaction (OER) is at the
pinnacle due to its poor reaction kinetics. Porous organic polymers (POPs) have recently emerged as a versatile platform
for the development of conventional heterogeneous catalysts, but their role in developing electrocatalysts for water
splitting is underexplored. Herein, we report synthesis and characterization of an azo-functionalized POP (NDI-PG) in
aqueous solution, under mild conditions, via diazo coupling of redox active naphthalene diimide (NDI) based diamine with
phloroglucinol. The azo nitrogen from the NDI diamine and one of the adjacent phenolic oxygen in the POP are utilized to
coordinate CoII (CoII-NDI-PG) and was exploited as an OER electrocatalyst. The CoII-NDI-PG with metal loading as low as 8%
exhibited the best electrocatalytic activity toward water oxidation in alkaline solution with an overpotential of 340 mV.
The catalyst additionally showed to uplift the reaction kinetics with a low Tafel slope of 68 mV/decade. This report also
compares the electrocatalytic activity of a CoII coordinated POP containing benzidine (CoII-BD-PG) in place of NDI diamine.
NDI, being redox active planar moiety unlike benzidine, exhibits better π-electron delocalization facilitating an
uninterrupted charge transfer which was beneficial for the electrocatalysis.
www.rsc.org/
stability.6–8 Metal oxide catalysts like RuO2 and IrO2 have
emerged as the promising electrocatalysts and have been
chosen as state of art OER electrocatalysts.9–14 However, their
Introduction
Electrochemical water splitting to produce hydrogen and
oxygen has been envisioned as an effective and more
preferable means of fuel source for renewable energy
production in fuel cells and regenerative fuel cells.1,2 The
electrochemical water splitting involves oxidation of water to
oxygen, known as oxygen evolution reaction (OER),3 which is
also an important half-cell reaction for more contemporary
energy applications like metal-air batteries, besides fuel cells.
The oxygen evolution at the anode is kinetically listless and
thermodynamically intensive reaction involving the breaking of
four O−H bonds with the generation of four electrons and
subsequent formation of the O=O double bond.4,5 Ideally, the
process should require a potential of 1.23 V, calibrated on a
reversible hydrogen electrode (RHE) reference scale.
Practically, the oxidation occurs at higher potentials even with
the catalytic assistance, thus high catalytic efficiency is
required for OER to proceed at low overpotentials. Recently,
considerable research efforts have been put forward for the
design and synthesis of high performance heterogeneous
oxygen evolution anode materials (catalysts) in order to drive
this reaction at lowest possible overpotentials with higher
high cost and scarcity put a barrier in commercializing them in
large-scale clean and renewable energy technologies. It is
therefore highly desirable to search for a noble metal free,
more efficient and cost-effective OER electrocatalysts.
Recently, significant research efforts have been directed to
develop highly efficient electrocatalysts based on earth-
abundant first-row transition metals like Mn, Co, Ni, Fe and
Cu.7,15–19 Among these the cobalt oxides/hydroxides, molecular
Co catalysts, nanoparticles, layered structures and Co
coordinated macromolecular systems have been under the
limelight of research.20–25
Porous organic polymers (POPs) are a unique class of
organic polymers linked by strong covalent bonds with
inherent porosity. They have emerged as multifunctional
materials with outstanding properties and a wide range of
applications in various fields such as gas storage and
separation, chemical encapsulation, energy conversion and
storage, sensing and heterogeneous catalysis.26–37,60 POPs
involved in heterogeneous catalysis are designed and
synthesized either through coupling reactions between
metallated building blocks or post-synthetic metallation to the
active Lewis basic donor sites in POPs or by the encapsulation
of well-defined metal nanoclusters or nanoparticles within the
pores of POP.38 Such POPs functionalized with robust catalytic
active sites tailored through heteroatom incorporation have
emerged as promising next-generation OER catalysts.39–42 The
intrinsic porosity and presence of catalytic active centres
a. Molecular Materials Laboratory, Chemistry and Physics of Materials Unit,
School of Advanced Materials (SAMat), Jawaharlal Nehru Centre for
Advanced Scientific Research, Jakkur, Bangalore, India, *E-mail:
tmaji@jncasr.ac.in
# Contributed equally
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within the POPs have been discerned to improve exposure of min at 140 ± 5 °C under microwave irradiation. The resulting
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DOI: 10.1039/C8TA06588H
active sites as well as facilitate electron and mass mixture was filtered and the residue was washed with copious
transport.39,43 However, these materials are synthesized either amounts of water, acetone and then chloroform. NDI-DA was
through Schiff base condensation or metal catalysed coupling isolated as a brown powder (1.55 g, 93%) and was used
1
reactions or pyrolysis methods, raising a concern regarding without further purification. H-NMR (Fig. S1) (400 MHz: [D6]-
their cost, and stability against reaction conditions involved.44– DMSO) δ = 8.75 (4H, s, H4), 7.01 (4H, d, J = 8.5 Hz, H3) 6.66
48
(4H, d, J = 8.5 Hz, H2) 5.29 (4H, s, H1).
Naphthalene diimides (NDI) are planar, chemically robust
molecules with excellent electron accepting and charge Synthesis of NDI-PG Polymer:
transport properties, thus NDI based materials are promising
0.53 g (1.2 mmol) of naphthalenediimide diamine (NDI-DA)
candidates for optoelectronics, electrochromic and molecular
was taken in a flat-bottomed 250 mL flask and charged with
devices, and also showed applications in sensing and
100 mL of distilled water and 1.4 mL of 35% hydrochloric acid.
battery.49–52 Thus, in the polymer design, redox active NDI can
The mixture was stirred for 15-20 minutes at 0-5 °C and 30 mL
act as a good charge carrier mobilizer with excellent thermal
of aqueous solution of sodium nitrite (0.17 g, 2.45 mmol) was
and oxidative stability. These characteristics justify its
added dropwise. The solution was stirred for further 30
candidature in designing an electrocatalyst. Azo linked POPs
minutes to make amino groups to convert completely to
possessing an azo group (–N=N–), a versatile ligand studied in
diazonium salts. The solution was neutralized with a dilute
coordination chemistry, have been found to show high CO2
solution of sodium carbonate. The mixture was then added in
adsorption capacity and catalytic activity for the conversion of
portions to an aqueous solution of trihydroxybenzene (0.1 g,
CO2 to value added chemicals.53–56 Having other donor
0.8 mmol) and Na2CO3 (0.25 g, 2.38 mmol) at 0-5 °C. During the
addition, the colour of the solution changed to brick red and
was further stirred overnight. The solid sample was separated
atoms/sites in the vicinity of azo group (–N=N–) embedded in
POPs can help to incorporate various metal ions through
chelation and thus act as a heterogeneous support. In this
from the mixture by filtration and was washed with methanol,
work, we have adopted an approach to incorporate a redox
THF, DMF, water and further purification of the sample was
active NDI-diamine into a o-hydroxyazobenzene POP through
carried out by soxhlet extraction with water, THF, acetone and
DMF. The product was dried in vacuum at 100 °C and isolated
as brick red solid (yield: 0.52 g).
diazo coupling, so that the integration of azo and phenolic –OH
in the redox active polymer can be exploited for chelating a
catalytically active metal centre (CoII). The CoII CoII-NDI-PG
( )
coordinated polymer acts as a noble metal free heterogeneous
catalyst with high activity and great stability for OER in
aqueous solution.
Preparation of CoII -NDI-PG Polymer:
50 mg of assynthesized NDI-PG polymer (in 10 mL of ethanol)
was treated with a calculated amount of Co(OAc)2·4H2O (50
mg, 25 mg, 12.5 mg) dissolved in 10 mL of ethanol and 0.5 mL
of triethylamine. After refluxing for 10 hours, the mixture was
filtered and washed with copious amount of methanol and
dried for overnight under vacuum at 100 °C. The 10%, 8% and
7% cobalt loadings were obtained as realized through ICP
analysis. The highest cobalt loading was obtained on using 50
mg of Co(OAc)2·4H2O and loading of the cobalt content was
found to be remain constant even after treatment with more
than 50 mg of Co(OAc)2·4H2O.
Experimental Section
Materials and methods
All the reagents employed were commercially available and
were used as provided without further purification. Cobalt
acetate
naphthalenetetracarboxylic
diamine,
tetrahydrate
[Co(OAc)2·4H2O],
anhydride,
benzidine
1,4,5,8-
paraphenylene
and
Electrochemical Characterizations of NDI-DA, NDI-PG and CoII-NDI-
PG, BD-PG and CoII-BD-PG
tetrabutylammoniumhexafluorophosphate
(TBAPF6)
was
Electrochemical characterization of NDI-DA, NDI-PG and CoII-
NDI-PG were done in a three-electrode cell configuration with
Pt-mesh as the counter electrode (CE), Ag/Ag+ as reference
electrode (RE) and mirror polished glassy carbon electrode
(GCE) as working electrode (WE). The electrolytic solution for
recording CV of NDI-DA contained 1 mM of NDI-DA and 100
mM of TBAPF6 in DMSO and the CV was recorded in between a
potential window of 0 to -2.5 V with respect to Ag/Ag+. For
recording CV of NDI-PG and CoII-NDI-PG the WE was a mirror
polished GCE coated with an ink of the respective samples. The
electrolytic solution was 100 mM TBAPF6 in acetonitrile. The
CV was recorded in the potential range of 0 to -2 V with
respect to Ag/Ag+.
obtained from Sigma Aldrich Chemicals. Phloroglucinol was
purchased from Spectrochem. All other reagents and solvents
were purchased from commercial sources and used as
supplied. Naphthalenediimide diamine (NDI-DA) was
synthesised following the literature procedure.57
Synthesis of naphthalenediimide diamine (NDI-DA):
1,4,5,8-naphthalenetetracarboxylic dianhydride (0.34 g, 1.3
mmol) and 1,4-diaminobenzene (0.34 g, 3.2 mmol) were
suspended in 12 mL of dry DMF, in a 30 mL microwave vial and
sonicated for 30 min. Triethylamine (0.86 mL, 65 mmol) was
added and the reaction mixture was heated to reflux for 15
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Figure 1: (a, b) Structures of NDI-PG and CoII
-
NDI-PG respectively; c) Solid-state 13C-NMR of NDI-PG, d) FESEM and e) TEM of NDI-PG (inset showing magnified TEM
f) N2 adsorption isotherm of NDI-PG at 77K, the inset shows the corresponding pore size distribution.
image), respectively showing their cotton like fluffy morphology
,
Preparation of ink: For all the samples the ink was prepared by polymer. The FT-IR spectrum (Fig. S3 and S4) of the polymer
dispersing 2 mg of respective samples in 1 ml of 49:49:2 of shows a peak around 1408 cm-1 which is attributed to the
water, isopropanol and nafion. These inks were also used for asymmetric vibration of the N=N bond.53 The peak
electrocatalytic studies.
corresponding to the C=O stretch (1715 cm-1) is slightly shifted
Electrocatalysis: All cyclic voltammetry (CV), linear sweep from the NDI-DA. The presence of phenolic-OH groups within
voltammetry (LSV) and chronoamperometry (CA) for the the framework can be easily confirmed from the presence of
evaluation of electrocatalytic activity of NDI-PG,
CoII-NDI-PGs, broad peak around 3400 cm-1 in FT-IR spectrum. The
BD-PG and CoII-BD-PG were also recorded in three-electrode insolubility of as prepared NDI-PG polymer in common organic
cell configuration where mirror polished GCE coated with solvents, depicts the hypercrosslinked structure and hence
respective sample inks were used as WE, Pt-mesh and Ag/AgCl may be expected to be highly stable. The thermogravimetric
were used as CE and RE, respectively. 0.1 M KOH solution was analysis of NDI-PG polymer indicated its stability up to 400 °C
used as the electrolytic solution.
under N2, while the initial loss below 100 °C is due to adsorbed
moisture (Fig. S5). The broadness in the powder X-ray
Results and Discussions
The polymer is synthesized via a diazo-coupling reaction
between naphthalenediimide diamine (NDI-DA) and
phloroglucinol (PG). The formation of red NDI-PG (Fig 1a)
polymer was characterized by the solid-state cross-polarization
magic angle spinning (CP/MAS) 13C-NMR and fourier-transform
infrared (FT-IR) spectroscopy. 13C-NMR spectrum (Fig. 1c, S2),
exhibits a signal at 162 ppm, corresponding to the carbonyl
carbon from the imide ring of NDI unit and is consistent with
the reported values in the literature for the related NDI based
polymers.58,59 The peaks in the range 125-141 ppm are
ascribed to the aromatic carbons in the framework of the
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hysteresis at high pressures and low adsorption at low
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pressures suggests that the polymer possesses micropores and
mesopores. Using Brunauer–Emmett–Teller (BET) model in the
pressure range of P/P0 = 0.05–0.3, the apparent surface area
(SABET) was calculated as 93 m2 g-1. Pore size distribution
analysis by non-linear density functional theory (NLDFT)
method (inset of Fig. 1f) suggests significant microporosity
centred at 1.4 nm. Smaller degree of mesoporosity is also
observed at 2.9 nm and 4.7 nm, which can be attributed to the
interparticulate void spaces formed by the assembly of fluffy
cotton like particles. After encapsulating CoII, no significant
change in porosity is observed and the surface area of 8 % CoII-
NDI-PG is around 88.4 m2g-1
(Fig. S13). Similar characteristics
are also reflected in pore size distribution analysis (Fig. S13
inset). As azo bridged porous organic polymers have been
found to show promising CO2 adsorption capacity due to their
high CO2-phillic nature, we also looked into the CO2 gas storage
of NDI-PG. Under cryogenic conditions (195 K) (Fig. S14), NDI-
PG displayed typical Type I CO2 uptake profile, further
confirming the microporous nature of NDI-PG polymer.
Figure 2: The deconvulated X-ray photoelectron spectroscopy spectra of a) C 1s
core level, b) N 1s core level and c) O 1s of NDI-PG and d) C 1s core level, e) N 1s
core level, f) O 1s and g) two broad sets of signals corresponding to 2p3/2 (782.8
eV) and 2p1/2 (798.6 eV) core levels of cobalt of 8% CoII-NDI-PG.
diffraction (PXRD) of NDI-PG (Fig. S6) denotes the absence of
periodicity in the polymer structure, usually observed for
POPs.57 Cobalt metal ions were encapsulated into the NDI-PG
polymer by refluxing a known amount of NDI-PG in ethanolic
cobalt acetate (Co(OAc)2.4H2O) solution in presence of
XPS spectra of NDI-PG confirmed the presence of C, N and
O elements in the porous polymer. The C 1s core-level XPS
spectrum of NDI-PG showed three peaks:
∼
286.95 eV
(originating from the carbonyl groups),
∼285.91 eV
triethylamine and named it CoII-NDI-PG
(Fig. 1b). Inductively
(originating from the C-N units), and
from aromatic benzene type units) (Fig. 2a). The N 1s core
level showed two peaks at 400.6 eV (imide moieties) and
401.6 eV (azo units) (Fig. 2b). The deconvoluted oxygen O 1s
∼284.73 eV (originating
coupled plasma-optical emission spectroscopy (ICP-OES)
analysis suggested a Co loading of 8 % and we denote it as 8%
CoII-NDI-PG. All the studies were done with 8% CoII-NDI-PG
until and unless specified. The PXRD pattern of the cobalt
∼
∼
peak revealed two peaks at
∼ 531.2 eV indicating carbonyl
loaded NDI-PG (
CoII-NDI-PG) (Fig. S7 and S8) is also devoid of
groups and
∼ 534.5 eV indicating quinone groups (Fig. 2c).
Upon cobalt impregnation (8% CoII-NDI-PG), besides the C, N
and O signals (Fig. 2d, e, f), two broad sets of signals
corresponding to 2p3/2 (782.8 eV) and 2p1/2 (798.6 eV) core
levels of cobalt were also observed, which further support the
presence of cobalt metal ions within the porous network (Fig.
2g).
any peaks confirming the retention of amorphous nature of
the NDI-PG, as well as the absence of any undesired starting
metal salt or metallic cobalt moieties. The cobalt ions within
the porous polymer were also confirmed through solid state
UV-vis spectroscopy, ICP-OES analysis, energy dispersive X-ray
spectroscopy (EDS) and X-ray photoelectron spectroscopy
(XPS) spectra. The NDI-PG polymer exhibited strong and broad
absorption band in the solid-state UV/vis spectrum in the
range from 300-640 nm (Fig. S9). Upon complexation with CoII,
absorption band further broadens to 700 nm (Fig. S9), which
probably happens due to d-d transition60 and there is also
slight blue shift in the absorption maxima with respect to NDI-
PG polymer. The EDS of CoII-NDI-PG shows presence of Co (Fig.
S10). The morphology of NDI-PG and CoII-NDI-PG were
observed using field emission scanning electron microscopy
(FESEM) and transmission electron microscopy (TEM). The
FESEM and TEM images of NDI-PG shows typical fluffy cotton
like morphology (Fig. 1d,e) and the morphology remained
unchanged on coordination with cobalt acetate (Fig S11).
Transmission electron microscopy (TEM) of CoII-NDI-PG (Fig.
S12) also, does not show presence of any free metal
nanoparticles or metal oxide particles and supports a very
homogenous cobalt loading. The textural property of the NDI-
PG was investigated via a N2 adsorption desorption study at 77
K. The N2 adsorption desorption isotherm of NDI-PG obtained
is of type IV which indicates micro and mesoporous
characteristics of the porous material (Fig. 1f). The apparent
With all the above structural information, we proceeded
further to study their electrochemical properties and
application. Firstly, CV of NDI-DA was recorded where GCE was
used as the working electrode (WE), Pt-mesh and Ag/Ag+ as
the counter electrode (CE) and the reference electrode (RE),
respectively. The CV (Fig. S15) shows two reduction peaks at -
0.97 V and -1.39 V. The first electron transfer at -0.97 V
.
corresponds to the formation of the radical anion (NDI -) and
the second electron transfer supports the formation of dianion
(NDI2-).61 The two reduction peaks of NDI-DA also attest to its
electron accepting nature and justifies our hypothesis to
include NDI in the framework. To verify the presence and the
significance of NDI in the POP, CV of the NDI-PG was studied in
a similar three-electrode cell setup with TBAPF6 in acetonitrile
as the electrolyte where POP ink (ink preparation in
experimental section) drop casted on the shiny GCE was used
as the WE. The CV of NDI-PG (Fig. 3a) also shows two
reduction peaks at similar positions confirming the presence of
NDI moiety in the POP. The reduction peaks are reversible in
both the cases as expected from NDI.61 The CV of 8% CoII-NDI-
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PG
(Fig. 3b) was also recorded in the same potential window loading (12 %) and also with Vulcan carbon as a conductive
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and we observed the characteristic reduction peaks of NDI,
support.47 The fact that the metallatedDOPOI: 1P0s.10c3o9u/lCd8TbAe06u58se8Hd
without any Vulcan carbon indicates the intrinsic conductivity
of the POPs. Thus, acknowledging our hypothesis to include a
redox-active π-conjugated unit in the network. Apparently, it is
due to the π-conjugation throughout the network based on
NDI and PG units that permits an uninterrupted charge
transfer and hence the use of conducting support could be
avoided, unlike the other reported electrocatalysts.47
To explicate the role of NDI-DA into the electrocatalytic
activity of CoII-NDI-PG catalyst, we have also synthesized and
characterized (Fig. S17) an analogous polymer as reported in
the literature53 by replacing NDI-DA with benzidine (BD), we
call it BD-PG
BD-PG
(
Fig. 4a) and the CoII coordinated BD-PG as CoII-
(
Fig. 4b) (with an optimum Co loading of 7%), and
checked the OER activity. At first CV of BD-PG and 7% CoII-BD-
PG in 0.1 M KOH was recorded and found that their activities
are poorer than that of 8% CoII-BD-PG
of 7% CoII
mV to reach a current density of 1 mAcm-2
much higher than that of 8% CoII
NDI-PG catalyst. The
obtained results in terms of lower overpotential, higher
current in case of 8% CoII NDI-PG over 7% CoII
BD-PG Fig. 4e
and a higher Tafel slope of 81 mV/decade for 7% CoII
BD-PG
Fig. 4f) indicates that incorporation of rigid redox active NDI-
(
Fig. 4c). From the LSV
BD-PG, the catalyst requires an overpotential of 470
Fig. 4d) which is
-
(
-
-
-
(
)
-
(
DA unit with good π-cloud delocalization within the porous
polymer largely enhances the electrochemical performance of
CoII
-
NDI-PG
.
Figure 3: a) CV of NDI-PG, b) CV of 8% CoII-NDI-PG at 50 mV/s, c,d) CV and LSV of
NDI-PG and 8% CoII-NDI-PG in 0.1 M KOH, respectively, at 1600 rpm and 50mV/s.
The respective insets show CoII/III oxidation peaks. e) LSVs of NDI-PG and 10%,
8% and 7% CoII-NDI-PG at 1600 rpm and 50mV/s, f) the corresponding Tafel
slopes of 10%, 8% and 7% CoII-NDI-PG.
Next, we were interested to have an insight into the
mechanistic pathway of the forward process. The appearance
of a reversible redox peak (insets Figure 3c and 3d) at
1.15/1.09 V for all the CoII-NDI-PGs is attributed to the
formation of CoIII/CoII couple. This signifies a single electron
transfer process as discussed in the literature.47 However, OER
is a multistep four electron complex process, one of which is
the rate-determining step (RDS).62 The electron transfers may
occur prior to the RDS (np), at the RDS (nq) or after the RDS (nr).
For a better cognizance, comparing experimentally calculated
Tafel slopes and that from the Butler-Volmer equation is
inevitable.63 To calculate the Tafel slopes of CoII-NDI-PGs, we
confirming the existence of NDI units intact in the metallated
POP. The electrocatalytic activity of NDI-PG, and, CoII-NDI-PG
polymers toward OER in 0.1 M KOH was evaluated by CV and
LSV in a typical three-electrode system. The WE was rotated at
1600 rotation per minute (rpm) for all the experiments. The
potentials for the OER polarization curves are reported on the
RHE scale. The CV of 8% CoII-NDI-PG showed a clear onset of
an intense anodic wave at around 1.59 V at an overpotential of
360 mV, with a marked difference in OER activity compared to
that of plain NDI-PG polymer (Fig. 3c). The LSV curves of the
8% CoII-NDI-PG exhibit an overpotential of 340 mV to generate
an anodic current density of 1 mAcm-2, whereas 410 mV of
overpotential is required to generate a current density of 10
mAcm-2 (Fig. 3d), comparable to the OER activities of RuO2 and
IrO2 (Fig S16). To compare the effect of different Co-loading in
the POP on the electrocatalytic activity and to report the
optimum Co loading, 7% and 10% CoII-NDI-PG were also
synthesized and studied as OER electrocatalysts. As can be
seen from the Fig. 3e, 7% and 10% CoII-NDI-PG also shows the
same overpotential but with decreased overall current
densities. Thus 8% CoII loading is optimum to get the best
activity. A sample loading of 8 μg (without any Vulcan carbon)
accounted for a current density of ~35 mAcm-2 in case of 8%
CoII-NDI-PG. This is comparable to the reported cobalt
modified covalent organic framework with a higher cobalt
plotted the overpotential (휂) against the log of current density
and the slopes were found to be 68, 68 and 81 mV/decade for
10%, 8% and 7% CoII-NDI-PG, respectively (Fig. 3f). The Butler-
Volmer equation for the oxidation process is given by equation
1:
where, itotal = total current, i0 = exchange current density, 훽f
=
transfer co-efficient for oxidation reaction. When no electrons
are transferred at the RDS and only one electron is transferred
after the RDS (i.e., np=3, nq=0 and nr=1) the Tafel slope is given
by:
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The Tafel slope of 68 mV/decade for CoII-NDI-PG is comparable transfer of the electrons prior to the RDS and one electron
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to the theoretical value of 60 mV/decade, which unfolds the
Figure 4: a,b) Structures of BD-PG and CoII-BD-PG, c) CVs showing the difference in catalytic activity of 8% CoII-NDI-PG, 7% CoII-BD-PG
CoII-BD-PG, e) LSVs showing the difference in catalytic activity of 8% CoII-NDI-PG, 7% CoII-BD-PG NDI-PG and BD-PG, f) Tafel plots of 8% CoII-NDI-PG and 7% CoII-
NDI-PG
, NDI-PG and BD-PG, d) LSV of 7
%
,
.
transfer after RDS hence the rate of the electrochemical processes at higher overpotential region. Since the catalytic
oxidation process is entitled to be faster. This higher kinetics of activities of the polymers were initiated at overpotentials
the catalyst for OER can be attributed to the overriding below 500 mV, we were interested to compare their
production of surface adsorbed species at an early stage of the polarization resistances at an overpotential of 500 mV (Fig.
OER multistep process.64 The porous nature of the material 5d). The Rp for 7% CoII-BD-PG at
ensuring an easy accessibility of the reactant to the active sites the highest, signifying the evolution process to be less facile.
=500 mV was found to be
and the presence of NDI facilitating the charge transfer, may
be synergistically responsible for this kinetically rich oxidation.
The stability of the 8% CoII
method at an overpotential of 0.4 V (Fig. 6a) for 24 h. A
-NDI-PG was tested by using CA
Electrochemical impedance spectroscopy (EIS) was periodic rise and stabilization of the current density is
performed to have a better perception of the charge transfer observed with approximately 10% decrease in the overall
processes. Impedance spectra of all the samples were current density, which elucidates the stability of the catalyst.
recorded at various overpotentials (Fig. S18) and fitted to the During the course of electrocatalysis, the oxygen bubbles
equivalent circuit shown in Fig. 5a, where RE is the electrolyte formed at the electrode surface blocks the active sites. Hence,
resistance, Rp is polarization resistance, Rs is the resistance as these insulating bubbles starts growing the current density
from surface groups formed during the multi electron transfer starts decreasing and gets stabilized and after a certain time
processes.65 In fact, Rp and Rs together represents all the interval the bubbles get ruptured clearing the active site
electroadsorption
respectively. Rfilm signifies the resistance of the polymer film. oxidation rate exhibits a faster kinetics, this rise and fall of the
At
=0, fitted Rfilm (Fig. 5b) value is lowest for 8% CoII-NDI-PG current density is distinguishable in the CA curve unlike many
signifying better conductivity of the polymer film than the rest of the previously reported electrocatalysts. Additionally, the
of the polymers studied in context (Table S1). Since at =0, no LSV curves recorded before and after 500 cyclic voltammetric
faradaic processes take place, the polarization resistances of oxidation cycles differ only slightly with respect to catalytic
all the polymers are high (Table S1). Impedance spectra of 8% onset potentials and slopes in the catalytic regions (Fig. 6b
and
electrodesorption
processes, blockage, resulting in shooting-up of current density. Since the


)
CoII-NDI-PG as a function of increasing applied potential is describing the cyclibility of the catalyst. Absence of cobalt ions
shown in Fig 5c, where we notice the decrease in Rp with in the post catalytic solutions before and after 500 oxidation
increasing overpotential, signifying the obvious facile faradaic cycles, as observed via ICP-OES
6 | J. Name., 2012, 00, 1-3
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Figure 5: a) Equivalent circuit used for fitting impedance data; impedance
Figure 6: a) Chronoamperometric curve showing stability of 8% CoII-NDI-PG for
24 h, b) LSV of 8% CoII-NDI-PG showing the cycling stability after 500 cycles, c)
current-scan rate plot for calculating roughness factor before and after 500
cycles, d) photograph showing generation of oxygen bubbles at Carbon paper
anode.
recorded-b) at 0 mV overpotential for 10%, 8%, 7% CoII-NDI-PG NDI-PG, 7% CoII-
,
BD-PG and BD-PG; c) for 8% CoII-NDI-PG at varying overpotentials and d) at an
overpotential of 500 mV for the OER active polymers for 10%, 8%, 7% CoII-NDI-
PG and 7% CoII-BD-PG.
analysis indicates negligible cobalt leaching from the porous
polymer during electrocatalysis.
density was 1 mAcm-2) when generation of oxygen starts
adequately.
The roughness factor (RF) and the turn over frequency
(TOF) are the other two aspects which testify the efficiency of
an electrocatalyst.66 RF, defined as the ratio of the
electrochemically active surface area (EASA) to the
geometrical area, directly influences the electrocatalytic
activity of the electrocatalyst. EASA is calculated from the ratio
of the double layer capacitance (DLC), which is the slope of
current-scan rate (Fig. 6c) plot derived from the CV recorded in
the DLC potential region, of the 8% CoII-NDI-PG coated surface
and that of the bare GCE (Fig. S19).66 We calculated the RFs of
the GCE coated with 8% CoII-NDI-PG both before and after 500
oxidation cycles, from the CV recorded in the double layer
capacitance region (Fig. S19, S20), to be 11.09 and 11. The
negligible change in the RF values enlightened the stability of
the material. TOF is referred to the rate of product evolved per
electrochemically active surface sites and is calculated (see SI,
Fig S21) to be 0.33 s at 1.64 V which is comparably higher than
the other reported TOFs.47 Fig. 6d shows a photograph
showing the oxygen bubbles formed at the electrode surface.
The faradaic efficiency (ε) of 8% CoII-NDI-PG was done using
rotating ring disk electrode (RRDE) where the platinum ring
was maintained at a constant potential of 1.2 V with respect to
Conclusions
The present study introduced a new strategy whereby a
redox active POP was synthesized by simple diazo coupling
reaction of NDI-DA and PG in aqueous medium. The Azo
nitrogens and the phenolic -OH of PG are exploited to
coordinate CoII and have been explored as an efficient noble
metal free electrocatalyst for the oxygen evolution reaction
(without any Vulcan carbon as conducting support) in an
alkaline media (pH 13.6). A catalytic current density of 1
mAcm-2 can be achieved at an overpotential of 410 mV. The
fast electron transfer kinetics of the oxidation was explained
by the low Tafel slope of 68 mV/decade. The electrocatalyst
showed 90 % retention in activity for 24 h and almost no loss
in activity after 500 LSVs. The efficiency of the electrocatalyst
was reflected in TOF of 0.33 s. The design of including a redox
active spacer in the POP structure which can coordinate with a
metal centre was beneficial for the electrocatalytic activity.
Thus, this work paved the way for developing noble metal free
POPs with redox active spacer for efficient OER
electrocatalysis.
RHE and the current was ramped from 0.5 to 15 mAcm-2
(Fig
S22).47,66 The estimated ε is 93% at 1.5 V (where the current
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
SAB acknowledges National Postdoctoral fellowship (DST
project No. PDF/2016/000762). CD and TKM is grateful to the
SERB DST (Project No. MR-2015/001019) and TKM is also
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33 K. Venkata Rao, R. Haldar, T. K. Maji and S. J. George, Phys.
thankful to DST (Project No. TRC-DST/C.14.10/16-2724,
JNCASR), Govt. of India and JNCASR for financial support.
View Article Online
Chem. Chem. Phys., 2016, 18, 156.
34 S. Roy, A. Bandyopadhyay, M. Das, P. DP.ORI:a1y0,.1S0.39K/.CP8aTtAi0a6n5d88TH.
K. Maji, J. Mater. Chem. A, 2018, 6, 5587.
35 Y. Xu, S. Jin, H. Xu, A. Nagai and D. Jiang, Chem. Soc. Rev.,
2013, 42, 8012.
Notes and references
36 W. Wang, M. Zhou and D. Yuan, J. Mater. Chem. A, 2017,
5
,
1
N. Coutard, N. Kaeffer and V. Artero, Chem. Commun., 2016,
52, 13728.
1334.
37 B. Bonillo, S. Sprick and A. I. Cooper, Chem Matter, 2016, 28
,
2
I. Roger, M. A. Shipman and M. D. Symes, Nat. Rev. Chem.,
3469
.
2017, 1, 0003.
38 P. Kaur, J. T. Hupp and S. T. Nguyen, ACS Catal., 2011,
1
, 819.
3
4
R. Eisenberg and H. B. Gray, Inorg. Chem., 2008, 47, 1697.
M. D. Kärkäs, O. Verho, E. V. Johnston and B. Åkermark,
Chem. Rev., 2014, 114, 11863.
S. Berardi, S. Drouet, L. Francàs, C. Gimbert-Suriñach, M.
Guttentag, C. Richmond, T. Stoll and A. Llobet, Chem. Soc.
Rev., 2014, 43, 7501.
39 Z.-S. Wu, L. Chen, J. Liu, K. Parvez, H. Liang, J. Shu, H.
Sachdev, R. Graf, X. Feng and K. Müllen, Adv. Mater., 2014,
26, 1450.
40 Z. Xiang, Y. Xue, D. Cao, L. Huang, J.-F. Chen and L. Dai,
Angew. Chem. Int. Ed. 2014, 53, 2433.
5
41 Q. Lin, X. Bu, A. Kong, C. Mao, F. Bu and P. Feng, Adv. Mater.,
2015, 27, 3431.
6
J. Li, R. Güttinger, R. Moré, F. Song, W. Wan and G. R. Patzke,
Chem. Soc. Rev., 2017, 46, 6124.
42 G. Lu, Y. Zhu, K. Xu, Y. Jin, Z. J. Ren, Z. Liu and W. Zhang,
7
8
L. Han, S. Dong and E. Wang, Adv. Mater., 2016, 28, 9266.
J. D. Blakemore, R. H. Crabtree and G. W. Brudvig, Chem.
Rev., 2015, 115, 12974.
C. Jackson, G. T. Smith, M. Markiewicz, D. W. Inwood, A. S.
Leach, P. S. Whalley, A. R. Kucernak, A. E. Russell, D. Kramer
and P. B. J. Levecque, J. Electroanal. Chem., 2018, 819, 163.
Nanoscale, 2015, 7, 18271.
43 Q. Zuo, G. Cheng and W. Luo, Dalt. Trans., 2017, 46, 9344.
44 S. Nandi, S. K. Singh, D. Mullangi, R. Illathvalappil, L. George,
C. P. Vinod, S. Kurungot and R. Vaidhyanathan, Adv. Energy
9
Mater., 2016, 6, 1601189.
45 D. Mullangi, V. Dhavale, S. Shalini, S. Nandi, S. Collins, T.
10 M. Rana, M. Chhetri, B. Loukya, P. K. Patil, R. Datta and U. K.
Woo, S. Kurungot and R. Vaidhyanathan, Adv. Energy Mater.,
Gautam, ACS Appl. Mater. Interfaces, 2015, 7, 4998.
2016, 6, 1600110.
11 M. Chhetri, M. Rana, B. Loukya, P. K. Patil, R. Datta and U. K.
46 H. Jia, Y. Yao, Y. Gao, D. Lu and P. Du, Chem. Commun., 2016,
52, 13483.
Gautam, Adv. Mater., 2015, 27, 4430.
12 T. Reier, M. Oezaslan and P. Strasser, ACS Catal., 2012, 2,
47 H. B. Aiyappa, J. Thote, D. B. Shinde, R. Banerjee and S.
Kurungot, Chem. Mater., 2016, 28, 4375.
1765.
13 T. Audichon, T. W. Napporn, C. Canaff, C. Morais, C.
48 H. Jia, Z. Sun, D. Jiang and P. Du, Chem. Mater., 2015, 27
,
Comminges and K. B. Kokoh, J. Phys. Chem. C, 2016, 120
2562.
,
4586.
49 C. R. Wade, M. Li and M. Dinca, Angew. Chem. Int. Ed., 2013,
52, 13377.
14 E. Antolini, ACS Catal., 2014, 4, 1426.
15 J. Nai, Y. Lu, L. Yu, X. Wang and X. W. D. Lou, Adv. Mater.,
2017, 29, 1703870.
50 N. Sikdar, K. Jayaramulu, V. Kiran, K. V. Rao, S. Sampath, S. J.
George and T. K. Maji, Chem. - A Eur. J., 2015, 21, 11701.
51 M. Al Kobaisi, S. V. Bhosale, K. Latham, A. M. Raynor and S.
V. Bhosale, Chem. Rev., 2016, 116, 11685.
16 Y. Yan, B. Zhao, S. C. Yi and X. Wang, J. Mater. Chem. A, 2016,
4
, 13005.
17 M. D. Kärkäs and B. Åkermark, Dalt. Trans., 2016, 45, 14421.
52 F. Xu, H. Wang, J. Lin, X. Luo, S. Cao and H. Yang, J. Mater.
18 E. Fabbri, A. Habereder, K. Waltar, R. Kötz and T. J. Schmidt,
Chem. A, 2016, 4, 11491.
Catal. Sci. Technol., 2014, 4, 3800.
53 G. Ji, Z. Yang, H. Zhang, Y. Zhao, B. Yu, Z. Ma and Z. Liu,
Angew. Chemie Int. Ed., 2016, 55, 9685.
19 X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, 44, 5148.
20 P. Manna, J. Debgupta, S. Bose and S. K. Das, Angew. Chemie
Int. Ed., 2016, 55, 2425.
54 Z. Yang, H. Zhang, B. Yu, Y. Zhao, Z. Ma, G. Ji, B. Han and Z.
Liu, Chem. Commun., 2015, 51, 11576.
55 P. Arab, M. G. Rabbani, A. K. Sekizkardes, T. İslamoğlu and H.
M. El-Kaderi, Chem. Mater., 2014, 26, 1385.
56 J. Lu and J. Zhang, J. Mater. Chem. A, 2014, 2, 13831.
21 J. Wang, W. Cui, Q. Liu, Z. Xing, A. M. Asiri and X. Sun, Adv.
Mater., 2016, 28, 215.
22 D. K. Dogutan, R. McGuire and D. G. Nocera, J. Am. Chem.
Soc., 2011, 133, 9178.
23 A. Zhao, J. Masa, W. Xia, A. Maljusch, M.-G. Willinger, G.
Clavel, K. Xie, R. Schlögl, W. Schuhmann and M. Muhler, J.
Am. Chem. Soc., 2014, 136, 7551.
24 L. Wu, Q. Li, C. H. Wu, H. Zhu, A. Mendoza-Garcia, B. Shen, J.
Guo and S. Sun, J. Am. Chem. Soc., 2015, 137, 7071.
25 F. Song and X. Hu, J. Am. Chem. Soc., 2014, 136, 16481.
26 K. V. Rao, S. Mohapatra, C. Kulkarni, T. K. Maji and S. J.
George, J. Mater. Chem., 2011, 21, 12958.
57 S. P. Black, D. M. Wood, F. B. Schwarz, T. K. Ronson, J. J.
Holstein, A. R. Stefankiewicz, C. A. Schalley, J. K. M. Sanders
and J. R. Nitschke, Chem. Sci., 2016, 7, 2614.
58 M. E. Bhosale and K. Krishnamoorthy, Chem. Mater., 2015,
27, 2121.
59 T. B. Schon, A. J. Tilley, E. L. Kynaston and D. S. Seferos, ACS
Appl. Mater. Interfaces, 2017, z,15631.
60 D. R. McMillin, R. A. Holwerda and H. B. Gray, Proc. Natl.
Acad. Sci. U. S. A., 1974, 71, 1339.
27 K. V. Rao, S. Mohapatra, T. K. Maji and S. J. George, Chem. - A
Eur. J., 2012, 18, 4505.
61 H.-M. Wang and S.-H. Hsiao, J. Mater. Chem. C, 2014,
2,
1553.
28 Y.-L. Wong, J. M. Tobin, Z. Xu and F. Vilela, J. Mater. Chem. A,
62 G. Mattioli, P. Giannozzi, A. Amore Bonapasta and L. Guidoni,
J. Am. Chem. Soc., 2013, 135, 15353.
2016, 4, 18677.
29 P. Peng, Z. Zhou, J. Guo and Z. Xiang, ACS Energy Lett., 2017,
, 1308.
63 S. Fletcher, J. Solid State Electrochem., 2009, 13, 537–549.
64 T. Shinagawa, A. T. Garcia-Esparza and K. Takanabe, Sci. Rep.,
2
30 K. V. Rao, R. Haldar, C. Kulkarni, T. K. Maji and S. J. George,
Chem. Mater., 2012, 24, 969.
2015, 5, 13801.
65 R. L. Doyle and M. E. G. Lyons, Phys. Chem. Chem. Phys.,
2013, 15, 5224.
31 V. M. Suresh, S. Bonakala, S. Roy, S. Balasubramanian and T.
K. Maji, J. Phys. Chem. C, 2014, 118, 24369.
32 V. M. Suresh, A. Bandyopadhyay, S. Roy, S. K. Pati and T. K.
Maji, Chem. - A Eur. J., 2015, 21, 10799.
66 C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, J.
Am. Chem. Soc., 2013, 135, 16977.
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