Journal of The Electrochemical Society, 165 (9) H481-H487 (2018)
H483
Figure 4. Linear sweep voltammograms of 0.5 mM of complexes in the pres-
ence of 160 mM AcOH/DMSO containing 0.1 M [Bu4N]PF6 using glassy
carbon electrode. Control experiment using AcOH alone (160 mM, black) was
also displayed.
Figure 3. Cyclic voltammograms of 0.5 mM of cobalt(II) porphyrin com-
plexes in 0.1 M [Bu4N]PF6/DMSO at scan rate of 0.1 V/s using glassy carbon
working electrode at room temperature.
AcOH. Except ligand of 2, which showed some catalytic activity, all
others showed no catalytic activity at all (Fig. 5a). Thus, an increase in
catalytic current is due to the catalytic cycles that produce molecular
hydrogen. It is also apparent that, acidification didn’t affect Co2+/1+
reduction feature of all complexes, indicating that protonation of Co0
presumably yields cobalt(II)-hydride as a reactive intermidate.30 In
principle, the Co(II)-H intermediate formed upon protonation of Co0,
can readily generate H2 under acidic conditions. Supported by control
experiments, the enhancement on current peak is not due to a direct
reduction of acetic acid.
To assess faradaic efficiency and TOF of H2 evolution, controlled
potential electrolysis of 0.5 mM solution of cobalt complexes was
conducted at −1.6 V vs NHE for 11 h in the presence of 160 mM
AcOH in 10 mL of DMSO (Fig. 6) and quantified H2 produced by
GC. Although an increase in charge with time is clearly observed
for all of the complexes, the amount of charge accumulation per
second varies depending on the inherent activity of catalyst toward
proton reduction. Significant increment was noticed for complex 3,
followed by 2 and 5, when the electrolysis was conducted for 11 h.
Moreover, CPE experiments of blank acetic acid show insignificant
catalytic activity under the same experimental conditions. Following
CPE experiment, the headspace gas mixture was analyzed by GC
(Fig. 7) and the amount of H2 generated was quantified (Fig. 8) us-
ing calibration curve of standard H2. The TOF, FE, and TON are
deduced from CPE experiment at a potential of −1.6 V vs NHE,
in which charge ranging from 17.8 to 101 C was passed during
electrolysis.
Electrochemical and catalytic study.—Electrochemical studies
showed that the redox potentials of cobalt(II) porphyrins with the
substituent on each of the meso-phenyl ring, are highly tuneable. As
expected, the potentials of the first Co2+/1+ and the second Co1+/0
redox couples both changed systematically with the electronic na-
ture of the substituents. The potentials were measured with reference
to Ag/AgCl and converted to NHE by adding 0.21 V to the mea-
sured potentials.32,33 The CV measurement using a glassy carbon
working electrode in DMSO and 0.1 M tetra-n-butylammonium hex-
afluorophosphate, [Bu4N]PF6 as the supporting electrolyte showed
electrochemically reversible redox processes at potentials (vs. NHE)
ranging from −0.27 to −0.59 for Co2+/1+ and −1.34 to −1.58 V for
The complex with the EW -COOMe substituents exhibits the least
negative Co2+/1+ and Co1+/0 reduction potentials (−0.27 and −1.34 vs.
NHE). The congener with the ED -OH group has shifted to the most
negative potential (−0.59 V and −1.58 V vs. NHE), revealing sub-
stantial electronic communication through the porphyrin ligand to the
metal center.
Upon addition of AcOH to the solution of cobalt complexes in
DMSO, catalytic current dramatically enhanced at the potential close
to the Co1+/0 redox couple, with onset potentials varying from −0.955
to −1.38 V vs NHE. For complex 5, however, the catalytic peak
appeared at a potential much less negative than that of Co1+/0 pair
measured in the absence of AcOH, which is consistent with an electron
deficient metal center resulted from the protonation of amino group
to -NH3+. As can be seen from linear sweep voltammetry (LSV) in
Figure 4, the magnitudes of catalytic current enhancement and onset
potential vary with the most positive shift and highest catalytic current
enhancement being for complex 3 with terminal sulfonates, which
suggests an efficient proton delivery at favorable applied potential. It
is about 320 mV less negative than the parent molecule and 420 mV
positive shift when compared to complex 6 (the most negative onset
potential) as determined from CV and LSV.
Scanning to more negative potentials in the presence of excess
AcOH resulted in the sharp increase in current, indicative of electro-
catalytic proton reduction and H2 generation as also confirmed from
GC chromatograms. Such subsequent sharp rise in current at lower
onset potential in the presence of excess protons (Fig. 5 and SI 27-30)
highly depends on the acidity and electronic properties of the func-
tional groups. This implies that substitutions in fact do adjust the po-
tential at which catalysis arises (onset potential). Control experiments
employing cobalt acetate in the presence of 160 mM AcOH showed
no catalytic activity at the potential ranges where catalyst operates.
Moreover, catalytic activities of porphyrinic ligands of 2, 3, 5 and 6,
without the cobalt(II) ion, were examined in the presence of 160 mM
Comparison of catalytic performance parameters.—Analysis of
the headspace by gas chromatography following electrolysis for
around 11 h at −1.6 V (vs NHE) obtained parameters of the cat-
alytic performance such as onset overpotential (Eovp), FE, TON, and
TOF. Large diversity on the catalytic performance were observed by
tuning the substituents on the porphyrin core with faradaic efficien-
cies (FE) ranging from 44 to 99.4%, TONs from 1.5 to 104, TOFs
from 0.23 to 9.12 h−1, and Eovp from 25 to 445 mV (Table I). The
trends of these parameters are consistent with catalytic current en-
hancements and onset potential features observed in CV, LSV and
DPV experiments under acidic conditions. Therefore, such a signif-
icant variation reveals an important role of the functional groups on
tuning the catalytic activity.
Even though it is anticipated that molecules with strong electron-
donating groups have more negative redox potentials due to the more
electron-rich metal center and porphyrin core and are expected to be
more active toward H2 generation, the −1.6 V (vs NHE) of applied
potential in this study wasn’t negative enough to produce a large
amount of H2 in the cases of complexes 6 and 7. Moreover, for most
of the earth-abundant transition metal molecular catalysts reported to
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