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Cite This: ACS Appl. Energy Mater. 2019, 2, 8930−8938

Cobalt Amide Imidate Imidazolate Frameworks as Highly Active
Oxygen Evolution Model Materials
Alberto Bucci,† Suvendu Sekhar Mondal,† Vlad Martin-Diaconescu,† Alexandr Shafir,§
and Julio Lloret-Fillol*,†,‡
†

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Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avinguda Països Catalans,
16, 43007 Tarragona, Spain
‡
Catalan Institution for Research and Advanced Studies (ICREA), Lluïs Companys, 23, 08010 Barcelona, Spain
§
Institute of Advanced Chemistry of Catalonia IQAC−CSIC, c/Jordi Girona 18-26, 08034 Barcelona, Spain
S
* Supporting Information

ABSTRACT: Two imidazolate-based Co-MOFs, IFP-5 and IFP8 (imidazolate framework Potsdam), with a different peripheral
group −R (−Me and −OMe, respectively) have been synthesized
by a solvothermal method and tested toward the oxygen evolution
reaction (OER). Remarkably, IFP-8 presents much lower
overpotentials (319 mV at 10 mA/cm2 and 490 mV at 500
mA/cm2) than IFP-5 toward OER, as confirmed by online gas
chromatography measurements (Faradaic yield of O2 > 99%).
Moreover, the system is extraordinarily stable during 120 h, even
when used as a catalyst toward the overall water splitting reaction
without any sign of fatigue. An integrated ex situ spectroscopic
study, based on powder X-ray diffraction, extended X-ray
absorption fine structure, and attenuated total reflection, allows
the identification of the active species and the factors that
determine the catalytic activity. Indeed, it was found that the performances are highly affected by the nature of the −R group,
because this small change strongly influences the conversion of the initial metal organic framework to the active species. As a
consequence, the remarkable activity of IFP-8 can be ascribed to the formation of Co(O)OH phase with a particle size of a few
nanometers (3−10 nm) during the electrocatalytic oxygen evolution.
KEYWORDS: metal−organic framework (MOF), imidazolate framework Potsdam (IFP), oxygen evolution reaction, electrocatalysis,
cobalt

■

INTRODUCTION
The oxygen evolution reaction (OER) is the ideal process for
accessing a diverse number of renewable fuels and chemicals,1,2
but its applicability is hampered by the difficulty to obtain
stable and efficient catalysts.3,4 Cobalt-based OER catalysts,
both molecular5 and heterogeneous,6−10 have attracted
enormous interest because of their relatively high activities
and the metal’s abundance in the earth crust. Homogeneous
molecular catalysts can be finely tuned to improve performances and allow for mechanistic studies, but the poor stability
and recyclability of the reported systems hinder their
applications.11 In contrast, heterogeneous catalysts are more
stable and cost-effective,12 although the formation of the active
centers on solid catalytic surfaces is difficult to control, identify,
and quantity, making mechanistic studies very challenging.13
Besides the well-investigated applications such as adsorption,14
storage,15 separation,16 and conventional catalysis,17−20 metal−
organic frameworks (MOFs) are emerging materials for
OER.21 Among Co-based MOFs, ZIF-67 (ZIF = zeolitic
imidazolate framework) is one of the benchmarks for oxygen
© 2019 American Chemical Society

evolution under several different working conditions. Indeed, it
was demonstrated to be able to perform 21 TONs with a TOF
of 0.035 s−1 under photocatalytic conditions, whereas if
supported onto FTO (fluorine-doped tin oxide), ZIF-67
operates under a wide range of pH, requiring 1.73 V vs
NHE to give a stable current of 1 mA/cm2 (at pH 9).22
Nevertheless, the performances are quite far from those
required for applications, because the poor conductivity of the
framework generally hampers the direct use of MOFs in
electrocatalysis.23−31 Alternatively, MOFs can be subjected to
framework degradation either by thermal decomposition,32−35
chemical oxidation,36 or in situ electrochemical aging,37,38 in
order to afford carbon/metal or metal oxide composites with
improved performances.39−42 As an example, An et al.
demonstrated that thermal decomposition of ZIF-67 affords
ultrathin Co3O4 nanomesh, rich in oxygen defects, with
Received: October 7, 2019
Accepted: November 19, 2019
Published: November 19, 2019
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remarkable performances at pH 14 (η = 370 mV at 10 mA/cm2
and a Tafel slope of 74 mV/dec),43 whereas Wang and coworkers improved the performances of Zn0.2Co0.8OOH via
chemical oxidation of ZIF-67 in the presence of (NH4)2S2O8,
lowering η10 to only 235 mV.36
A clear advantage of those approaches consists in the
possibility to control the formation of the new phases by
changing the reaction condition. However, when the MOF
undergoes electrochemical aging, the system becomes much
more dynamic, and the identification of the active species is
uncertain and sometimes controversial.37,38,44 For example, Ai
and co-workers proposed the conversion of UTSA-16 to CoOx
active phase under OER conditions,38 whereas Zhang et al.
suggested that electrochemical aging in O2 saturated electrolyte induces the exfoliation of a 3D pillared-layer structure to
achieve a 2D amorphous ultrathin phase, excluding the
possibility to form CoOx because of its much lower activity.37
Moreover, Fischer et al. proposed that the active phase of ZIF9 consists of a nitrogen-coordinated cobalt oxyhydroxide
N4COOH species.44
In order to understand how to control the transformation of
the MOF to the active catalytic phase, we used two model
systems named IFP (imidazolate framework Potsdam),45,46
based on penta-coordinated Co secondary building units
(SBU’s) connected by multidentate amido-imidate-imidazolate
linkers. The models differ in the nature of the substituent at
the imidazole C2 position, which can be either a −Me (IFP-5)
or an −OMe (IFP-8) group. We have studied the OER activity
of IFP-5 and IFP-8 under basic conditions (pH 14 and 13),
demonstrating their high catalytic activity. More interestingly,
the ex situ X-ray absorption spectra (XAS), powder X-ray
diffraction (PXRD), and attenuated total reflection (ATR)
experiments gave insight into how the transformation of the
framework is able to tune the formation of the active species,
which leads to excellent catalytic performances.

Figure 1. Top: general synthetic strategy of IFP frameworks,
highlighting the in situ ligand hydrolysis. Bottom: 1D hexagonal
channel structure of IFP, with an enlargement of the asymmetric unit
of IFPs, clarifying the Co2+ coordination environment. The label R
stands for −Me and −OMe in the case of IFP-5 and IFP-8,
respectively.

The Co K-edge of the extended X-ray absorption fine
structure (EXAFS) spectra of IFPs show pre-edge 1s → 3d
transitions centered around 7709.5 eV and rising edges at
7721.2 eV. These features are 0.4 and 1.5 eV lower in energy
than the mixed Co2+/Co3+ centers in the Co3O4 reference,
which is consistent with the Co2+ oxidation state for the IFP
series. Moreover, EXAFS analysis (Figure S2) shows a 5coordinated metal center having two first coordination sphere
N/O scattering shells, with 2 N/O atoms at 2.00 Å and 3 N/O
atoms at 2.11 Å. On the other hand, ZIF-67 has an intense preedge at 7709.9 eV that can be rationalized in terms of increased
p−d mixing due to its tetrahedral geometry. Conversely to
what is observed for the IFP model, ZIF-67 is better described
by a single shell of N/O scattering atoms at 2.00 Å.
Putting together the pieces of information obtained, the
chemical environment of the Co2+ ion in the IFP series is
constituted by five donor atoms of three ligands to form a
distorted trigonal-bipyramidal geometry (Figure 1, down). In
this arrangement, the imidate N4 and O2 and the amide O1
reside in equatorial positions, with the two imidazolate N
atoms (N1 and N2) occupying the axial positions. The
multidentate imidazolate-based ligands, produced by the in situ
hydration of the dicyano precursor, combine with Co2+ ions to
form the neutral microporous imidazolate MOF with 1-D
hexagonal channels. The Co2+ SBUs at the IFP structure and
the bridging ligands act as 3-connected topological species
forming a net with a uninodal topology, named etb. The
functional groups at the imidazolic C2 position protrude into
the channel opening, tuning the pore aperture (4.2−1.7 Å),
polarity, and functionality of the channel walls (Figure 1). As a
consequence, the specific surface area of IFP is much higher for
IFP-5 (666.2 m2g−1) than for IFP-8 (45.8 m2g−1) based on the
narrowing the pore aperture window of the IFP structure
(Figure S3). The scanning electron microscopy (SEM)
analysis of IFP samples (Figure S4) revealed that IFP-5

■

RESULT AND DISCUSSION
Synthesis and Characterization. The synthesis of Coamide-imidate-imidazolate frameworks [Co(C5H3N4O2)-R]
(with R = −Me45 and −OMe46) were previously reported
by Holdt et al. The synthesis of the MOFs proceeds by the
reaction of 2-substituted-4,5-dicyanoimidazole under solvothermal conditions in the presence of Co(NO3)2·6H2O (see
the Supporting Information (SI) for details). During the
reaction, the partial hydrolysis of the ligand affords the
imidazolate-4-amide-5-imidate (C5H3N4O2), which is the real
linker of the IFP (Figure 1, top).
For comparison, the canonical ZIF-67 was synthesized
according to the reported literature procedure.47 The
experimental PXRD patterns, with the simulated ones, as
well as XAS of the as synthesized IFPs are shown in Figure 2.
The experimental diffraction patterns perfectly match the
simulated ones, showing characteristic peaks at 7.4°, 9.9°,
15.9°, 17.3°, 23.3°, and 30.2°. The sharpness of the diffraction
peaks indicates that the newly obtained IFP-5 is highly
crystalline, whereas IFP-8 is less so.
Furthermore, the ATR spectrum (Figure S1) clearly shows
the presence of characteristic bands around 1560 and 1660
cm−1, which are due to the formation of amide and imide
groups. Furthermore, between 3320 and 3330 cm−1, a broad
amide-imidate N−H band with a considerably fine structure
was observed.
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Figure 2. PXRD pattern (a) and Co K-edge X-ray absorption near edge structure (XANES) profiles (b) of IFP-5 (blue), IFP-8 (red), and ZIF-67
(magenta). Co3O4 (black) was used as a reference in the XANES.

Figure 3. (a) Polarization curves and (b) long-term chronopotentiometry (CP) at J = 10 mA/cm2 for IFP-5 (blue), IFP-8 (red), and ZIF-67
(magenta); (c) LSV of IFP-8 before (black) and after (blue) the CP; (d) comparison in the overpotential at 10 mA/cm2 with the most common
MOFs in the literature at pH 14; (e) evaluation of the nominal TOF for the material studied calculated at η = 400 mV and the catalyst loading
taking account for the leaching (vide infra) and (f) performances of IFP materials at pH 13 (0.1 M KOH). For all the experiments, the current
collector used was a 1 × 1 cm2 geometric area of NF.

obvious pronounced changes after 24 and 72 h at pH 13
(Figure S5), but some modifications took place when the pH
was raised to 14. The new samples, labeled IFP-Xbt (where X
identifies the corresponding IFP number and bt stands for
“basic treatment”) were characterized via PXRD and ATR

displays hexagonal nanostructures, whereas IFP-8 forms
spherical morphologies.
In order to investigate the chemical stability of the assynthesized IFP materials, these were subjected to aging under
very basic conditions. The final PXRD patterns revealed no
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Figure 4. (a) Overall water splitting LSV (in red, HER region and in blue OER region). The potentials at 10 mA/cm2 are highlighted in the
picture, whereas thermodynamic potential for HER and OER are marked as E0 in the figure. (b) Represents a CP experiment carried out at 10 mA/
cm2 during 120 h for the overall water splitting reaction (inset shows the H2/O2 ratio recorded during the measurement). All the measurements
were performed at pH 14 by KOH, supporting IFP-8 onto NF.

this trend, since IFP-8 has the highest capacitance in the entire
potential window investigated (Figure S15 and Table S1). This
is an indication that IFP-8 exhibits a higher electrochemical
surface area (ECSA) with respect to IFP-5 and, consequently, a
higher number of active sites exposed. Indeed, the ECSA
measured from the double layer capacitance is 11.2 and 2.7
cm2 for IFP-8 and IFP-5, respectively (see SI for calculation
details).
Remarkably, chronopotentiometry (CP) at 10 mA/cm2
(Figure 3b) shows a slight decrease in the overpotential for
both IPFs at the beginning of the CP, but then it remains
stable during the successive 13 h, clearly indicating that an
activation process occurs. Indeed, η10 of IFP-8 is 319 mV after
the activation phase, and the η50 became as low as 390 mV
(Figure 3c). Furthermore, a remarkable and sustained current
of 480 mA/cm2 was observed at an overpotential as low as 490
mV. Such stability is uncommon for simply deposited MOF on
the electrode material so this prompts us to push the stability
tests toward a very long CP run during 180 h at 10 mA/cm2
(Figure S16). For comparison, the bioinspired cobalt-citrate
MOF, UTSA-16, needs 408 mV of overpotential to deliver a
stable current of 10 mA/cm2 during 7 h before deactivation,38
whereas the Co2(OH)2BDC nanosheets recently reported by
Pang et al., exhibit much lower η10 (273 mV) and η50 of ca. 360
mV, but such outstanding activity at pH 14 is maintained for
only 3 h, before that current drops up to 60% of its initial value
and η50 increases up to 410 mV (as estimated by LSV showed
therein).30 In general, the performances of IFPs are much
better than the majority of Co-based MOFs and even with
respect to many pure inorganic materials (Figure 3d and Table
S4).25,49
Nevertheless, we performed an additional long-term
chronopotentiometry experiment (4 h at 20 mA/cm2) aimed
at monitoring the gas evolution by means of online gaschromatography (see SI for calculation details). A quantitative
Faradaic efficiency (>99%) was found during the entire
experiment for both IFP-5 and IFP-8 (Figure S16), which
ensures that the current measured is due to O2 evolution.
According to the measures of the inductively coupled plasma
optical emission spectrometry (ICP-OES), IFP-8 also has the
highest nominal TOF value (0.53 s−1 at η = 0.4 V) over IFP-5
and ZIF-67 (Figure 3e). However, it should be noted that
these TOF values are an underestimation of the real
performances because they are calculated under the
assumption that all the catalytic centers equally participate in
catalysis.50 However, even the nominal TOF value reported for

spectroscopy (Figures S6, S7). The broadening of the PXRD
peaks clearly suggests the partial loss of crystallinity (Figures
S6a, S7a). In the ATR spectrum, a very sharp stretching at
3630 cm−1 appears (Figures S6b, S7b) consistent with the
formation of hydroxyl groups, indicating the occurrence of
hydrolysis. Interestingly, the intensity of the signals at ca. 2800
and 1100 cm−1 that are attributable to CH3−O and C−O−C
stretching, respectively,48 decreases in IFP-8bt, whereas the
intensities of all peaks are maintained for IFP-5 after the basic
treatment, suggesting different lability of the corresponding R
group toward hydrolysis.46
Along with the molecular precursors, we also synthesized the
corresponding cobalt oxides Co3O4-X (when X is 5 or 8, stands
for IFP-5 and IFP-8, respectively; whereas, when X is 67 or R,
stands for the calcinated ZIF-67 and the reference Co3O4,
respectively). The synthetic procedure to obtain Co3O4 spinels
involves a simple thermal treatment at 700 °C of the
corresponding IFP under N2 atmosphere (Figure S8), followed
by acid etching in 0.5 M H2SO4 for 2 h, that affords Co3O4
phase as demonstrated by PXRD, ATR, XAS (Figure S9), SEM
(Figure S10), and TEM (Figure S11 and S12) analysis, with an
average particle size of 46 nm.
Electrocatalytic OER, Ex Situ Characterization of
Catalytic Materials. As an initial step, we compared the
electrochemical performances at pH 14 of IFPs and ZIF-67
(ZIF-67 has been used as a reference under exactly the same
reaction conditions) by drop casting 200 μg of the
corresponding material onto the nickel foam (NF).
Remarkably, IFPs exhibit better performances than the wellknown ZIF-67 benchmark. IFP-8 is the most active electrocatalyst (Figure 3a−d), with a low overpotential at 10 mA/cm2
(η10) of 319 and 257 mV at the onset, respectively (Figure
S13). Also, the Tafel slope of 64 mV/dec is particularly low
(Figure S14). The η10 for the other tested catalysts are 340 and
360 mV, whereas the onsets are 273 and 269 mV for IFP-5 and
ZIF-67, respectively (Figure S13). The values of the Tafel
slope (Figure S14) are slightly higher (ca. 70 mV/dec for
both). Such values are considerably lower than previously
reported cobalt-based MOF benchmarks, as those reported by
and Pang and Qu.30,31
In order to rationalize this remarkable activity, electrochemical impedance spectroscopy (EIS) in the potential range
of 0.9−1.8 V vs RHE has been performed. The resistance
associated with OER increases from IFP-8 (1.9 Ω at 1.6 V vs
RHE) to IFP-5 (2.6 Ω at 1.6 V vs RHE), whereas the values of
the pseudocapacitance for the different catalysts inversely fit
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Figure 5. Characterization of IFP materials plated on FTO. Panels (a) and (b) offer a comparison between PXRD pattern and ATR before (black)
and after (red) OER for IFP-5 (upper) and IFP-8 (lower), respectively, whereas TEM images (scale bars = 50 nm) are depicted for IFP-8 before
(c) and after (d) OER. (e) Co K-edge XANES profiles for the IFP-8 measured as a pellet (red line) and supported in FTO before (dark red line)
and after catalysis (blue line) versus the Co3O4 reference (black line). Fourier transformed EXAFS data and corresponding fits within the inset
showing k-space spectra of IFP-8 supported in FTO after catalysis (f).

splitting catalyst. HER tests (Figure S18) were performed in a
three-electrode cell with the same configuration as for OER,
but this time, the electrolyte solution was continuously
degassed by N2 flow, in order to remove O2. Furthermore,
the potential at the working electrode was varied between 0.9
V to −0.5 V vs RHE at a scan rate of 1 mV/s. The η10 of 227
mV is quite lower if compared to other Co-based
catalysts10,55−58 and competitive with the most recent Cobased hybrid systems.59−61 As indicated in Figure 4, the overall
water splitting reaction can be performed at 10 mA/cm2 by
applying ca. 1.89 V. A long CP experiment highlights the
extreme durability of IFP-8 under total water splitting
conditions (for 120 h at 10 mA/cm2), as the activity remains
rather constant for the entire duration of the experiment
without suffering apparent fatigue (Figure 4b). The overall
water splitting is confirmed by the online measured H2/O2
ratio of ca. 1.9, with quantitative faraday efficiency during the
first 3 h of reaction (Figure S19). In total, after 120 h of
electrolysis, minimum TONs of 26 000 and 13 000 are

IFP-8 largely exceeds the most common benchmarks reported
so far; for example, that of Co-MOF nanosheet array directly
grown on NF (0.18 s−1 at η = 0.4 V) recently reported by Qu
et al.31
Since the chemical stability tests revealed that IFPs at pH 13
do not suffer of partial hydrolysis, the same measurements
were performed at this lower pH (Figure 3f). Remarkably, the
activity of IFPs is competitive even under these conditions, and
indeed, η10 are 360 and 410 mV for IFP-8 and IFP-5,
respectively, which are comparable with similar single Cobased MOF MCF-12 nanosheets recently reported by Zhang
et al. (η10 = 310 mV)37 and largely exceeding the performances
of other benchmarks as MAF-X27-OH (η10 = 461 mV),25
MOF(Fe1−Co3)550N (η10 = 390 mV),51 including inorganic
materials.52−54 Interestingly, at this pH, the η10 drifts after 12 h
of long-term chronopotentiometry experiments (15 h at 10
mA/cm2), approaching 500 and 680 mV (Figure S17).
The remarkably low overpotentials at 10 mA/cm2 prompt us
to study IFP-8 as a bifunctional catalyst for both OER and
hydrogen evolution reaction (HER) and therefore as a water
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Figure 6. Comparison of the CP activation (blue line) of IFP-5 (a) and IFP-8 (b) with the formation of the catalytic active species (red dots) and
the disappearance of the organic framework (black dots). The values of the ATR areas of the organic region are normalized to the initial values
calculated at t = 0. For those experiments, IFPs were supported onto the FTO electrode and employed in 1 M KOH solution.

particular, in the diffraction pattern, only the main peak at 10°
is visible, whereas in the infrared spectrum, all the features
from the framework (1000−1700 cm−1) appear, but with
reduced intensity. In the TEM images, it is apparent that the
surface remains intact with minor degradation and roughness,
suggesting that the initial Co distribution could be maintained
or undergoes minor aggregation (Figure S22). These
indications, together with minor metal leaching detected by
ICP-OES measures (accounting for only 5% of the total metal
deposited), point toward the retention of the original structure
of IFP-5.
A different scenario can be depicted for IFP-8@FTO
because the attenuation of the PXRD pattern, the disappearance of ATR signals in the organic region, and the metal
leaching of ca. 35% clearly indicate that the framework
undergoes major degradation. Moreover, TEM images reveal
the formation of amorphous and very small particles (3−10
nm) embedded in a thin and plain surface (Figure 5c,d and
Figure S23).
However, all the samples clearly show a new dominant phase
in the 400−650 cm−1 region of the ATR, corresponding to
Co−O stretching (Figure 5).38,62−65 Such a broad band was
proposed to be a convolution of LCoO (Co4+), LCoOH
(Co3+), and LCoOOH (Co3+) species.44,66,67 In order to probe
the chemical nature of the new phase, XAS measurements were
performed. The spectrum of IFP-8 after catalysis exhibits a
rising edge comparable to that of Co3O4 but with a clear shift
to higher energy, having a pre-edge at 7710.4 eV and a rising
edge at 7725.6 eV. Such a shift, together with the structure of
the XANES and the Fourier transformed EXAFS spectrum are
compatible with the previously reported Co-Pi or CoO(OH).68−70 Indeed, in the FT data, two dominant peaks can
be observed, corresponding to (Co−O at 1.90 Å) and Co−Co
scattering paths with distance of 2.85 Å (Figure 5e,f). Those
signals, which are symptomatic of an octahedral environment,
correspond to a single first shell of 6 N/O scattering atoms,
whereas the number of scattering atoms (N) for the latter shell
averages to 4.3 ± 0.4, which is lower than the 6 expected for
CoO(OH). Subsequently, in analogy with the previously
reported EXAFS analysis by Nocera et al., the model requires
the inclusion of four more contributions: Co−O (3.43 Å),
Co−Co (4.94 Å), multiple scattering Co−Co−Co (5.77 Å), as
intralayer paths and, finally, also the interlayer scattering shell
Co−O (3.72 Å). Among them, the features at R(Å) at 4.94
and 5.77 Å are still in agreement with the CoO(OH) and bulk
Co-Pi spectra and definitely rule out the presence of Co3O4
that does not have the path at 4.94 Å. However, it is important
to notice that the coordination numbers for the paths at 4.94

calculated for H2 and O2, respectively, considering all the
deposited material over the electrode.
In order to disclose the origin of the difference in the activity
among those catalysts and the mechanism of activation, we
performed several experiments. First of all, IFPs were
compared to their related calcinated derivatives Co3O4-X
samples and Co3O4-R, the latter prepared independently via
solution combustion synthesis (see SI for further details). IFP8 exhibits much higher performances than both Co3O4-8 (η10
= 380 mV) or the Co3O4 reference (η10 = 390 mV), and the
same trend is observed for IFP-5 (Figure S20). The difference
in the catalytic activity between IFPs and Co3O4-X is even
more evident when TOF values are compared (Figure S20c),
suggesting that the active species of the two families should be
quite different in nature. In other experiments, the comparison
was focused between IFP and IFP-bt, finding nearly identical
activity (Figure S20). More interestingly, the ECSA of the
materials treated with base and IFP-8 are very close (ca. 12
cm2), and it is much higher than that of IFP-5 (ca. 2 cm2), in
agreement with the fact that the loss of crystallinity might
induce higher exposure of the active sites. This suggests that
the pH-induced degradation of the MOF might be one of the
factors that drives the formation of the active species.37
Encouraged by these results, PXRD, ATR, and EXAFS
experiments were performed ex situ before and after the
catalysis, by plating the materials onto FTO as the electrode
material (IFP@FTO).
The electrochemical studies of IFPs@FTO were carried out
analogously to the characterization in NF. Initial LSVs confirm
the reactivity order found in NF (Figure S21a,b). At the same
time, during the 5 h CP experiments performed at 5 mA/cm2,
the reduction of the potential and the increased number of
exposed active sites (as indicated by the integrated charge of
Co redox features and by ECSA, Table S5) indicate that the
catalysts undergo an activation process, although with different
rate with respect to that observed in NF. Nevertheless, the
main kinetic descriptors for the intrinsic activity, that is, TOF
and ECSA-normalized current density, decrease. In particular,
TOF values (at η = 300 mV) were observed to almost half
from 0.29 s−1 to 0.16 s−1 for IFP-8 and reduce from 0.35 s−1 to
0.25 s−1 for IFP-5. The decrease of the TOF value might be
symptomatic for the loss of molecular behavior in favor of an
aggregation process, which would likely be stronger in the case
of IFP-8 than for IFP-5 because of the higher drop of the TOF.
In order to better describe the activation process, after the 5
h CP, the samples were investigated spectroscopically. The
PXRD pattern and the ATR spectrum of IFP-5@FTO
presented important attenuation of the signals (Figure 5). In
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and 5.77 are 4.3 and 8.6, instead of 6 and 12, respectively. Such
a loss indicates that the forming Co(O)OH exhibits significant
lattice imperfections, in agreement with the traces of
degradation observed in the TEM.
The kinetic of the degradation of the framework and the
formation of CoOOH species was further studied by collecting
ATR spectra at different times during the CP experiment
(Figure 6).
Figure 6 suggests a correlation between the increment of the
catalytic activity during time (the η5 lowers), the formation of
active species (Co−O bond signal), and the framework
degradation (Figure S24). Moreover, full evolution/activation
of IFP-8 occurs in only 3 h; however, for IFP-5, the η5
stabilizes after 5 h, although the organic framework is still
largely present. Furthermore, the ATR signal for the Co−O
stretching of IFP-5 is 3 times lower in intensity than for IFP-8
(Figure S25).
A plausible explanation for different activation rates relies on
the robustness of the organic linker, which should be imparted
by the −R group for two main aspects. The first is that the
−Me moiety from IFP-5 infers hydrophobic character to the
MOF pores, repelling water and preventing the structural
degradation.71 As a second reason, the degradation might
depend on the different rate in the hydrolysis of the −R group
in the C2 position of the imidazolium moiety, which is
expected to be easier for the −OMe group than for the −Me
one.46 An indirect proof for this was already found for the basic
treated samples (Figure S7, vide supra). Once the hydrolysis
occurs, the successive oxidative imidazolium ring-opening
mechanism might be the most favored event under those
conditions even at room temperature, as well described by
Elabd et al.46,72
In this sense, the −R group modulates the extent of
conversion of the MOF into the active species. For IFP-8, the
fast and complete degradation of the framework leads to a
higher amount of the active species, and discrete particles
became visible in TEM. During electrochemical tests, IFP-8
exhibited higher capacitance over IFP-5, indicating that the
same active species forms more active sites (Figure S25c) with
respect to its −Me analogue.

■

Article

ASSOCIATED CONTENT

S
* Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsaem.9b01977.
Linear sweep voltammetry of IFP-8 supported on nickel
foam 5 mV/s up to 500 mV overpotential (MP4)
Physical methods; synthetic procedures; electrochemical
details and calculations, including the Faradaic efficiencies; further characterization by PXRD, SEM, and
EXAFS; linear sweep voltammetry experiments; calculation of the Tafel Slope; long-term stability tests; ex situ
ATR experiments; and tables summarizing the results of
the fits for EIS, ECSA, and EXAFS (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: jlloret@iciq.es.
ORCID

Vlad Martin-Diaconescu: 0000-0002-7575-2237
Julio Lloret-Fillol: 0000-0002-4240-9512
Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
We thank the ICIQ Foundation, the European Research
Foundation for project H2020-ERC-2015-CoG (J.L.-F.),
H2020-FETPROACT-01-2016 A-LEAF (J.L.-F./A.B.), MINECO (CTQ2016-80038-R; J.L-F. and CTQ2017-86936-P; A.S.)
and AGAUR 2017-SGR-1647 (J.L.-F.). S.S.M. thanks
COFUND for postdoctoral scholarship. We would like to
thank the support of the SAMBA beamline at the SOLEIL
synchrotron (France) for their support and time for data
acquisition. We thank to Prof. H.-J Holdt (University of
Potsdam) for ligand precursor supplying.

■

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