Journal of the American Chemical Society
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carbon plate as a working electrode and with a Clark elecꢀ
The Supporting Information is available free of charge via
Experimental and additions DFT results.
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trode placed at the headspace of the electrochemical cell for
measurement of the generated dioxygen in situ (Figure
S33). An applied potential Eapp = 1.25 V for 20 minutes
yielded 0.34 Coulombs and 0.21 µmols of O2, corresponding
to a Faradaic efficiency of 24.5%, similar to the calculated
value based on RDDE experiments. The TONs are > 5300
and are the largest ever reported for first row transition
metalꢀbased molecular catalysts (See Table S4 in the SI).17
The low Faradaic efficiency is then likely due to graphene
oxidation in parallel to water oxidation reaction in basic
solutions.7 Nevertheless, the molecular catalyst remains
intact after catalysis as evidenced by CV and XAS spectrosꢀ
copy. Indeed, Figure 1C shows that the species after bulk
electrolysis are identical to those obtained before catalysis.
More important, Figs 1Cꢀ1D show that no traces of CuO are
revealed by XANES or EXAFS spectra, suggesting that the
molecular G-22ꢀ active catalyst is robust. This observation is
extremely important since most of the molecular catalysts
reported so far degrade during the catalytic process yielding
the corresponding oxides. This is particularly acute with
WOCs based on first row transition metals.3 Figure 3D
shows the catalytic Tafel plots for 12ꢀ, 22ꢀ, G-12ꢀ and G-22ꢀ. It
is interesting to observe that the pyrene functionalization of
the tetraamide ligand and anchoring to the graphene supꢀ
port has two beneficial effects: decrease the overpotential
(η) for catalytic water oxidation by about 200 mV and inꢀ
creases the TOFmax by about two orders of magnitude.
AUTHOR INFORMATION
Corresponding Author
*victor.batista@yale.edu
*allobet@iciq.cat
Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
MINECO, FEDER, ALBA, APS and US DOE. Additional info
can be found in the Supporting information.
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87. Related examples in the literature also show that for the pyrene
radical cation the absorption at 420 nm either disappear or suffer a
very severe loss of intensity.
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2130. It is important to realize here that the oxidation potential of
naphthalene is 700 mV lower than that of benzene. Thus even if the
πꢀdelocalization of the pyrene moiety over the phenyl ring of the
tetraamdie ligand is small, it will have a very strong impact into the
ligand based redox potential.
In the homogeneous phase, the role of the pyrene group is
to stabilize the aromatic ring of the tetraamide moiety via πꢀ
delocalization leading to a drastic reduction of the overpoꢀ
tential (η) necessary for catalysis. In the heterogenized G-12ꢀ
–a complex without a pyrene functionality– πꢀdelocalization
is provided by graphene. Interestingly, G-22ꢀ exploits the
benefit of having both the pyrene moiety and stacking interꢀ
actions with graphene and end up being the best catalyst.
The larger TOFmax of G-22ꢀ when compared to G-12ꢀ sugꢀ
gests that the resulting extended πꢀdelocalization due to
pyreneꢀgraphene interactions enhances the ET from the
catalyst to the graphene electrode, supporting ET as the rds
of the catalytic process.
In conclusion, we have found an extremely rugged and efꢀ
ficient molecular WOC based on Cu, a firstꢀrow transition
metal complex that is efficient both in the homogeneous
phase and heterogenized on graphene electrodes. Imꢀ
portantly, we demonstrated that the molecular catalyst
remains intact under catalytic turnover when immobilized
on graphene exhibiting no sign of decomposition or forꢀ
mation of CuO during or after catalysis. Furthermore, we
found that the pyrene functionality not only acts as a very
robust anchoring unit but also facilitates the electrocatalytic
oxidation of water to dioxygen both from a thermodynamic
and kinetic perspective. Finally, G-12ꢀ and G-22ꢀ are oxidaꢀ
tively robust hybrid materials with exceptional catalytic
performance for water oxidation, rendering them as excelꢀ
lent electroanode candidates for direct solar waterꢀsplitting
devices.
(14) All redox potentials describe in this paper are referred to the
NHE reference electrode.
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ChemSusChem, 2016, 9, 3361−3369.
(17) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Chem.
Rev. 2015, 115, 12974–13005.
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