An Active-Site Sulfonate Group Creates a Fast Water Oxidation Electrocatalyst That Exhibits High Activity in Acid

Article abstract of DOI:10.1002/anie.202008896

The storage of solar energy in chemical bonds will depend on pH-universal catalysts that are not only impervious to acid, but actually thrive in it. Whereas other homogeneous water oxidation catalysts are less active in acid, we report a catalyst that maintained high electrocatalytic turnover frequency at pH values as low as 1.1 and 0.43 (k_cat=1501±608 s^?1 and 831±254 s^?1, respectively). Moreover, current densities, related to catalytic reaction rates, ranged from 15 to 50 mA cm^?2 mM^?1 comparable to those reported for state-of-the-art heterogeneous catalysts and 30 to 100 times greater than those measured for two prominent literature homogeneous catalysts at pH 1.1 and 0.43. The catalyst also exhibited excellent durability when a chemical oxidant was used (Ce^IV, 7400 turnovers, TOF 0.88 s^?1). Preliminary computational studies suggest that the unusual active-site sulfonate group acts a proton relay even in strong acid, as intended.

Full text of DOI:10.1002/anie.202008896

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: An Active-Site Sulfonate Group Creates a Fast Water Oxidation
Electrocatalyst That Exhibits High Activity in Acid
Authors: Aaron G. Nash, Colton J. Breyer, Brett D. Vincenzini, Gregory
I. Elliott, Jens Niklas, Oleg G. Poluektov, Arnold L. Rheingold,
Diane K. Smith, Djamaladdin G. Musaev, and Douglas
Grotjahn
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10.1002/anie.202008896
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COMMUNICATION
An Active-Site Sulfonate Group Creates a Fast Water Oxidation
Electrocatalyst That Exhibits High Activity in Acid
Aaron G. Nash,[a] Colton J. Breyer,[a],# Brett D. Vincenzini,[a],# Gregory I. Elliott,[a] Jens Niklas,[b] Oleg G.
Poluektov,[b] Arnold L. Rheingold,[c] Diane K. Smith,[a] Djamaladdin G. Musaev,[d] Douglas B.
Grotjahn[a],*
[a]
Dr. A. G. Nash, C. J. Breyer, B. D. Vincenzini, Dr. G. I. Elliott, Prof. D. K. Smith, Prof. D. B. Grotjahn
Department of Chemistry and Biochemistry
San Diego State University
5500 Campanile Drive, San Diego, CA 92182-1030, U.S.
E-mail: dbgrotjahn@sdsu.edu
[b]
[c]
Dr. J. Niklas, Dr. O. G. Poluektov
Solar Energy Conversion Group
Argonne National Laboratory
9700 S. Cass Ave., Lemont, IL 60439, U.S.
Prof. A. L. Rheingold
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093, U.S.
Prof. D. G. Musaev
Emerson Center for Scientific Computation
Emory University, Atlanta, GA 30322, U.S.
These authors contributed equally.
[d]
#
Supporting information for this article is given via a link at the end of the document.
Abstract: The storage of solar energy in chemical bonds will depend
on pH-universal catalysts that are not only impervious to acid, but
actually thrive in it. Whereas other homogeneous water oxidation
catalysts are less active in acid, here we report a catalyst that
maintains high electrocatalytic turnover frequency at pH values as low
as 1.1 and 0.43 (kcat = 1501 ± 608 s-1 and 831 ± 254 s-1, respectively).
Moreover, current densities, related to catalytic reaction rates, ranged
from 15 to 50 mA cm-2 mM-1 comparable to those reported for state-
of-the-art heterogeneous catalysts and 30 to 100 times greater than
those measured here for two prominent literature homogeneous
catalysts at pH 1.1 and 0.43. The catalyst also exhibits excellent
durability when a chemical oxidant is used (CeIV, 7400 turnovers, TOF
0.88 s-1). Preliminary computational studies suggest that the unusual
active-site sulfonate group acts a proton relay even in strong acid, as
intended.
first coordination sphere can facilitate proton transfer during
catalysis.[5]
We were intrigued by Thummel’s improvement on the iconic
system[6] [(terpy)Ru(NN)(H2O)]2+ (1a), best illustrated by replacing
NN ligand bipy and water with phenanthroline-2-carboxylate, as
in 2[7] (Fig. 1). Catalyst 2 reached ~700 turnover number (TON, a
measure of catalyst durability, which was ~2 times more than for
1a) when driven with ceric ammonium nitrate (CAN), and the
carboxylate was shown to de-coordinate at least at the Ru(II) state
and allow binding of water.
O
2
L
O
OH2
N
N
N
N
N
N
Ru
L
Ru
N
N
Ru
N
N
O
O
O
X
O
N
N
4
The search for molecular water oxidation catalysts (WOCs)
that are stable and active, particularly under acidic conditions, is
a highly active area of science and technology, and is motivated
by the tremendous potential of solar energy storage in renewable
fuels.[1] Successful operation of a WOC at low pH is very
significant because low pH values aid proton reduction[2] that is
needed to complete a water splitting process. Thus far, few
molecular catalysts have been able to maintain high performance
at low pH.[1] One avenue to improved stability and reactivity of
molecular WOC that has been explored in recent years is the
inclusion of anionic pendent bases to aid proton transfer away
from the WOC active site (cf. OEC of Photosystem II).[3] Indeed,
the presence of an anionic group in the second coordination
sphere of the catalyst can play a decisive role as a proton relay
during O-O bond formation and improve stability of the catalyst.[4]
It has also been found that dianionic phosphonate groups in the
a L =
b L =
N
N
a
b
1
X = C
X = N
2
X
O
H
O
O
S
N
O
O
O
O
S
H
N
N
N
Ru
N
N
Ru
N
N
N
N
= [Ru]+
= [Ru]-OH2
3-OH2
3
Figure 1. Known species 1, 2, and 4, and novel sulfonate 3 and aquo form 3-
OH2 reported here. Triflate (CF3SO3 ) is the counterion for cationic species
-
studied here (1a, 3, 3-OH2).[8]
1
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However, the basicity of a carboxylate would be compromised
at sufficiently acidic pH (< 2-3).[9a] We therefore considered
sulfonate as a pendant base, hypothesizing that it would remain
unprotonated even at low pH, and would be able to act as an
effective proton relay under highly oxidizing conditions realistically
faced by a WOC during prolonged operation.
Inspired by this expectation, here we report a WOC with
sulfonate moiety in the active site that retains its very high
electrocatalytic activity in acid even at pH as low as 0.43. We are
unaware of another molecular WOC with this property.[1,2] To our
knowledge, the literature contains one WOC bearing a sulfonate
in the active site, Cp*Ir(κ2-N,O-PySO3)Cl, that was 3 times faster
than a carboxy analog.[10a] WOC sulfonates remote from the
active site have been reported to interact with Ce4+ and increase
TON by 50%,[10b] and to increase water solubility[10c,10d] or to
promote supramolecular assembly,[10e-10g] but the effect of a
sulfonate in the first coordination sphere of a WOC remains
underexplored. Thus we prepared sulfonate analog 3 (Fig. 1).
Catalyst synthesis and structure. The previously
unreported phenanthroline-2-sulfonic acid was prepared in 68%
Ru(bda)(isoquinoline)2 (~8000 TON),[13] 2 (~700 TON),[7] and 1a
(178 or 390 TON).[6b,6c] At initial concentration of 39.5 μM, 3
achieved >95% yield of O2 within 1 h; at 5 μM, the yield was 72%
after 25 h with 7400 TON. Initial rate studies of concentration in
[3-OH2]0 (5.0-39.5 μM, [CAN]0 = 0.2 M) led to the rate law kobs
=
k[3-OH2][CAN], with k = 0.88 s-1, ~6 times faster than 2 (0.15 s-1
under the same conditions) and ~45 times faster than for 1a
(Figures S4-S8). That 3-OH2 was ~45 times faster than 1a and
achieved more than 18 times as many turnovers motivated us to
investigate electrocatalytic activity, which permits exploration of
catalytic performance over a range of pH values.
Electrochemical studies began with cyclic voltammetry (CV)
on glassy carbon (GC) or boron-doped diamond (BDD) at pH
values between 7 and 0.43. The RuII/III couple is reversible at pH
7, with E1/2 = 0.66 V (Table S4b; referenced to Ag/AgCl, as are all
potentials in this paper). However, at pH 1.1 and 0.43 (0.1 M and
0.5 M HNO3 [14]), respectively, E1/2 shifts to 0.91 V. From pH ~1.85
to 12, the potential of the RuII/III couple is sensitive to pH, with a
slope of 54.2 mV per pH unit, as shown by a Pourbaix diagram
(Fig. S26).
yield
by
nucleophilic
aromatic
substitution
of
2-
As for electrocatalysis by 3, Fig. 3 shows peak catalytic
currents on BDD electrodes that are 500-1473 times greater
(Table S4b) than the forward current of the II/III couple, regardless
of whether neutral or acidic solutions are used (left part of Figure
3). Large catalytic currents are also observed on GC as shown in
Figure S20 and Table S4a, although BDD electrodes offer the
advantage of exceptionally low background currents (Fig. S21)
and greater stability at these positive potentials.[15] With either
electrode, excellent performance was maintained at pH 1.1 and
0.43, as discussed more below; for other molecular WOC,
electrocatalytic performance in acid usually suffers.[1] Regardless
of pH, significant formation of bubbles at the electrode was
observed; experiments described in pp. S23-S27 and S38
suggest that the formation of bubbles may explain the observation
of a peak for the catalytic current rather than the expected plateau
by decreasing the effective area of the electrode (Figures S17,
S18, Table S5). Given this, we believe our kcat values may be
underestimates. Furthermore, the current densities in Fig. 3
translate to 25 to 50 mA cm-2 per mM,[16] which are comparable to
values for state-of-the-art heterogenous water oxidation catalysts
under acidic conditions.[2]
We focus first on catalysis at pH 7. KNO3 was used to maintain
Itotal at 0.50 M. Scanning from 0 → +1.8 → -1.0 → 0 V at 5 mV s-
1, we measured icat at ~1.74 V. As concentration of phosphate
buffer was decreased while ionic strength was maintained,
catalytic performance measured with BDD electrodes improved
significantly: phosphate buffer concentration of 0.0294 M the
highest kcat value was seen (2168 ± 540 s-1; 95% confidence
intervals are cited in this paper). At this buffer concentration the
return wave of the RuII/III couple was shifted by ~50 mV, which
likely indicates that oxidation activity of the catalyst at potentials
> 1.4 V is overcoming the buffer, and that the pH value at the
electrode is no longer 7 (Fig. S26). In contrast, in 0.235 M
chlorophenanthroline with sodium sulfite in hot water-ethanol
using a modification of a procedure from 1920.[11] A solution of the
sodium salt of the sulfonic acid was then reacted with the solvento
species [(terpy)RuL3](OTf)2 (L = solvent), formed by reacting 2
equiv of AgOTf with (terpy)RuCl2(dmso), in water-ethanol at 95o
C for 24 h, affording 3 after a simple workup in 80% yield. The
powdery product gave satisfactory CHN analyses when one water
molecule was included in the formula; IR analysis of the solid
(ATF) showed two sharp bands at 3547.4 and 3463.3 cm-1,
consistent with coordinated, not adventitious, water. The
presence of an outer-sphere trifluoromethanesulfonate
counterion precluded using IR spectroscopy to determine whether
the ligand sulfonate was coordinated or free. Calculations (below)
suggest that coordination of water is energetically favorable,
leading to formulation of the isolated sample as aquo complex 3-
OH2, not as structure 3. Nonetheless, when an aqueous solution
of 3 / 3-OH2 is analyzed by mass spectrometry (also see below),
a cluster of ions from 3 (not 3-OH2) is seen; moreover,
crystallization from MeOH led to 3 as analyzed by X-ray diffraction
(Fig. 2). The Ru-O bond is 0.034(2) Å longer than in 2, and
perhaps to compensate the Ru-Nphen bonds are each about 0.02
Å shorter, but clearly, the structures are more similar than different
(Fig. S3 in Supporting Information).
phosphate buffer,
3 exhibited significantly lower turnover
frequency (kcat = 243 s-1). Phosphate ions may play an inhibitory
role, possibly competing with water for the catalyst active site at
sufficiently high concentrations, a phenomenon observed by
Meyer’s group for 1b.[9b]
Figure 2. Molecular structure of the cation of 3.[12]
Remarkably, excellent electrocatalysis was maintained at pH
1.1 (kcat = 1501 ± 608 s-1 on BDD) as well as at pH 0.43 (kcat =831
± 254 s-1 on BDD). For 3 we found that nitric and triflic acids at
Catalyst performance. Exposure of 3 to CAN ([CAN]0 = 0.20
M) led to as much as 7400 TON, which compares favorably with
other Ru complexes such as the prominent catalyst
2
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Figure 3. (left) Comparison of 3 at pH 7, 1.1, and 0.43. Individual runs and their kcat values are shown here, whereas in the manuscript text, averages and standard
deviations for kcat values are given for replicate runs that are all shown in SI. (right) Comparison of 3, 1a, and 4a in 0.1 M HNO3 + 0.4 M KNO3 (pH 1.1). Background-
subtracted CVs were obtained under conditions leading to a maximum current at ~1.75 V. Backgrounds are dashed lines (shown at left only); inset shows II/III
couples. Conditions: 3 mm boron-doped diamond electrode, 0.125 mM concentration of catalysts, I = 0.50 M, ν = 5 mV s-1. The comparison of 3 at three different
pH values (left) shows high peak catalytic current at the lower pH values, which is unlike the behavior of other molecular WOC in the literature. (right) The comparison
of three catalysts, all at pH 1.1 and at the same concentration of 0.125 mM, shows the significant benefit of the sulfonate group, raising icat / ip, the ratio of the peak
catalytic current to the forward current of the II/III couple, from values of 31 and 9 for 1a and 4a, respectively, to 958 ± 293 for 3 (see Table S4).
pH 1.1 gave almost the same results (1350 and 1147 s-1, GC
electrode, Fig. S19c), strongly suggesting that O atom transfer
from nitrate or assistance of nitrate as base is not responsible, as
previously documented by the Berlinguette group for analogs of
1a.[17] Bulk electrolysis on BDD at 1.8 V at pH 1.1 (Fig. S28)
showed that the initial current was above 50 mA cm-2 mM-1, similar
to our CV data. Over the first hour the current remained above 18
mA cm-2 mM-1 cm-2 mM-1. Over the next 5 h the current dropped
to ~7 mA cm-2 mM-1. However, when a second electrolysis on the
same solution is run for 3 h, the current from 1 to 3 h is 85-90%
of the value for the first run, showing significant stability.
Identification of catalytic intermediates by experiment.
From pH ~1.85 to 12, the potential of the RuII/III couple is sensitive
to pH, with a slope of 54.2 mV per pH unit. That behavior and the
pH independence of the II/III couple below pH 1.85 are essentially
identical to the behaviour of dicationic aquo complex 1a (Fig.
S27),[6a] which leads us to assign the electroactive species as
aquo complex ([RuII]-H2O)+ (= 3-OH2), being oxidized to either
([RuIII]-H2O)2+ below pH 1.85, or to ([RuIII]-OH)+ at higher pH
values. The fact that the pH-dependent changes of the II/III couple
for 1a and 3-OH2 are identical, even though 1a and 3-OH2 start
with a 2+ and 1+ charge, respectively, is rather remarkable, and
highlights the importance of the sulfonate group in enhancing the
catalysis, a kinetic behavior. Neither CV, DPV nor square wave
voltammetry (SWV) show evidence of a discrete redox event
before the catalytic wave, so we are unable to ascertain the
identity of more highly oxidized species by these methods.
as the conjugate acid ([Ru]O+H+)2+ is seen, computed to bear the
added proton on the sulfonate, not the Ru-O unit; the species is
computed as a possible precursor to ([Ru]O)+. Finally, a fourth
new entity with significantly higher mass and 1+ charge is fully
+
consistent with a CeIII(NO3)3 adduct, [[Ru]O + CeIII(NO3)3
]
(examples of metal-oxo-lanthanide species are known[18]).
Changing from H2O, D2O, or 18OH2 led to the expected changes
(or lack of changes) in mass, within 0.003 Dalton of calculated
values. Over a period of 1-2 h, most of these oxidized species
disappeared, leaving mostly [RuII]+ = 3, possibly because of
catalyst turnover. We also treated 3 in H2O with oxygen atom
transfer reagent 2-iodoso-1-t-butylsulfonylbenzene that has been
used to form metal oxo complexes,[19] and saw the ion clusters for
([Ru]O)+ and unreacted [RuII]+, but not ions for the other species
mentioned above.
To seek additional information on oxidized intermediates,
resonance Raman spectroscopy with excitation at 532 nm was
performed on solutions of 3 (~2 mM) in either H2O, D2O, or 18OH2
before and after addition of 10 equiv of CAN, as in the mass
spectral experiments. Analysis of Figures S32-S34 concludes
(Table S6) that when 18OH2 is used instead of H2O or D2O, three
peaks at 871.2, 832.4 (equally intense), and 808.7 (less intense)
shift as expected for a Ru-O harmonic oscillator. The peaks seen
in H2O compare with 732 cm-1 for a [RuIII]-oxyl with strong σ-donor
trans to the oxyl,[20a] and 780-833 cm-1 for RuIV oxo species[20b-20d]
.
Shimoyama et al. ascribe their lower value to the [RuIII]-oxyl
formulation,[20a] but one cannot rule out the effect of the strongly
donating N-heterocyclic carbene.
Additional evidence for catalytic intermediates was sought
using high accuracy (±0.0001 Dalton) mass spectrometry.
Mixtures of 3 and CAN (10 equiv) freshly formed in either H2O,
D2O, or 18OH2 showed four new identifiable species, along with
some remaining [RuII]+ = 3 (Figures S29-S31). An observed
cation with 2+ charge corresponding to [RuIII]2+ is reasonably
assigned to be formed from ([RuIII]-H2O)2+ by loss of water in the
vacuum of the spectrometer. A 1+ species with one O added to
starting material could be [RuIV]O+, or the redox tautomer
([RuIII]O.)+ bearing an oxyl ligand. A second 2+ species assigned
Complex 3 was found to be EPR silent at 9 K, but upon
treatment with 1 to 25 equiv of CAN two features appear at g⊥ =
2.4 (more intense) and gll = 1.73 (less intense). These data,
though preliminary, are in some agreement with those reported
by Shimoyama et al. for their [RuIII]-oxyl (Figure S38).[20a] We
cannot rule out the presence of an EPR-silent RuIV species.[20e]
Computational studies. Computations shed light on the
experimental findings above. In these calculations we used
density functional theory (DFT, for more details see caption of Fig.
3
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4, as well as Supporting Information) starting with cationic
complex 3 with counter anion OTf. Reaction of 3 and H2O with
decoordination of the Ru-OSO2 bond and coordination of water
molecule to the Ru(II)-center exothermically gives 3-OH2, with
ΔH/ΔG = -17.3/-5.4 kcal/mol but may require an energy barrier
which was not identified in this study. In the resulting 3-OH2
complex (see Fig. 4) the calculated Ru-OH2 bond distance is
2.195 Å, and the water molecule is hydrogen bonded to SO3 with
two O2SO---H bonds of 1.761 and 1.840 Å. As could be expected,
this H-bonding weakens the O–H bonds, which is manifested in
the change of O–H bond stretches: the calculated (unscaled) O–
H bond stretches are 3450 and 3546 cm-1, smaller than 3800 and
3902 cm-1 for a free water molecule, and in excellent agreement
with experimental values of 3463.3 and 3547.4 cm-1 reported
above. As mentioned above, these computational data enabled
us to formulate the isolated sample in experiments as aqua
complex 3-OH2.
Starting with the optimized structure of 3-OH2, we set out to
elucidate the mechanism of water oxidation, calculating a series
of two sequential 1-electron oxidation and deprotonation
processes (i.e. ETPT, see Fig. 4 and SI for more details). Triflate
ion is present in all computed structures but for consistency is not
included in the formula names below. Oxidation by Ce(IV) was
modeled using the Ce(IV)/Ce(III) reduction energy of 128.8
kcal/mol.
spin of the complex ([RuIII]-OH)+ is delocalized between the Ru-
center and OH-ligand as 0.66 |e| and 0.32 |e|, respectively.
Further calculations suggest that oxidation with another
equivalent of CAN converts ([RuIII]-OH)+ in another facile ETPT
(ΔG° = −9.1 kcal/mol) to ([Ru]O)+. This complex has a triplet
electronic ground state with a Ru–O bond distance of 1.789 Å.
Spin density analyses show that the Ru-center bears 1.06 |e|
unpaired spins, while 0.90 |e| spin resides on the O-center. The
calculated Ru–O vibrational frequency of 818.0 cm-1 is in good
agreement with our experimental results.
Conclusions. Electrocatalytic water oxidation by novel
sulfonate-bearing complex 3 is exceptional. The structures of 3
and analogs 1a,b without sulfonate are very similar, as are the pH
dependences of the RuII/RuIII couples. In striking contrast,
catalysis by 3 is ~45 times faster than by 1a,b using Ce(IV) at pH
1. Most remarkably, 3 is fast at pH 1.1 and 0.43, kcat =1501 ± 608
and 831 ± 254 s-1, respectively. Low concentrations of phosphate
buffer at pH 7 boost kcat to 2168 ± 540 s-1, whereas higher
concentrations lower kcat to about 300 s-1. Some of the fastest
molecular water oxidation electrocatalysts with pendent
carboxylates or phosphonates give rates 8000-10,000 s-1 ,[4] but
these data were obtained at pH 7 or higher, where the pendent
base remains active. In contrast, sulfonate complex 3 appears to
be unique in that it maintains high activity under acidic conditions,
that are more compatible with water splitting systems in which
tandem proton reduction is favored by acidic pH. Moreover, the
CV current densities in Fig. 3 translate 25 to 50 mA cm-2 per mM,
and are far greater than currents exhibited by two prominent
literature homogeneous catalysts (1a and 4a) at pH 0.43 and 1.1.
Further mechanistic and synthetic studies to determine the
operative role of the sulfonate group in 3 and related species are
ongoing. We conclude here by emphasizing that the pH
dependences of II/III couples of 3 and 1a without a pendent
sulfonate are almost identical and are both consistent with
presence of a coordinated water, strongly suggesting that the
hemilability of the sulfonate group alone is not responsible for
enhanced catalysis. Instead, the sulfonate anion may act as either
a pendent base (Figure 4) or possibly through a noncovalent
charge effect.[21] Clearly, active-site sulfonate groups deserve
further investigation for a variety of oxidation catalysts.
Acknowledgements
Figure 4. Calculated structures of important intermediates formed after two
sequential oxidation/deprotonation (ETPT) steps starting with 3-OH2. Bond
distances are given in Å. After the RuII starting material undergoes loss of one
electron and one proton, ([RuIII]OH)+ is formed with shorter Ru-O and H---O
bonds. After further electron and proton loss, the spin distribution of ([Ru]O)+ is
computed to be Ru 1.06 |e|, O 0.90 |e|, consistent with formulation as a triplet
species. (see SI for details).
We thank Justin Lee and Prof. Andy Borovik for essential
assistance with preliminary EPR experiments. Research at SDSU
was supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Award DE-
SC0018310. The EPR work at Argonne National Laboratory is
supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences under contract number DE-AC02-
06CH11357. D.G.M. acknowledges the support from the U.S.
Department of Energy, Office of Basic Energy Sciences, Solar
Photochemistry Program (DE-FG02-07ER-15906).
We established first that 1-electron oxidation of 3-OH2 by
Ce(IV) is slightly exergonic (ΔG° = −2.2 kcal/mol) and leads to
formation of the Ru(III)-aqua complex ([RuIII]-OH2)2+. The Ru(III)
aquo complex releases one proton (to water solvent, protonation
energy of solvent, 265.0 kcal/mol, was utilized) (ΔG° = −11.2
kcal/mol) and transforms to the Ru(III)-hydroxyl complex ([RuIII]-
OH)+. The Ru-O bond distance contracts from 2.195 Å in 3-OH2
to 1.939 Å in ([RuIII]-OH2)2+. Furthermore, in ([RuIII]-OH)+, the OH-
ligand is strongly H-bonded to the SO3-unit with a bond distance
of 1.680 Å between the H-atom and the closest sulfonate oxygen.
Spin density analyses show that a total of one unpaired alpha-
Keywords: homogeneous catalysis • electrochemistry •
sulfonates • water oxidation • ruthenium
4
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[16] Only relatively low concentrations of 3 (0.125 mM) can be achieved in
aqueous solvents with buffer, hence we normalize current density on a
mM basis to compare various catalysts. We note that in the absence of
buffer ions, e.g. in Raman and EPR studies, 1 to 2 mM concentrations
can be achieved.
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10.1002/anie.202008896
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What a wave! Other molecular water oxidation catalysts suffer in acid, producing (much) lower catalytic current than at pH 7, a sign
of reduced catalytic rate. Here a sulfonate group is in the active site creates a catalyst that is at least 30 times faster, using a variety
of measures. The electrocatalytic current density produced reaches 50 mA cm-2 per mM, 30 to 100 times higher than literature
catalysts tested.
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