1
Electronic Supplementary Information
Water oxidation catalysis – role of redox and structural dynamics
in biological photosynthesis and inorganic manganese oxides
Ivelina Zaharievaa*, Diego González-Floresa, Baraa Asfaria, Chiara Pasquinia,
Mohammad Reza Mohammadia, Katharina Klingana, Ivo Zizakb, Stefan Loosa, Petko
Cherneva, Holger Daua*
a Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin
b Helmholtz-Zentrum-Berlin, Institute for Nanometre Optics and Technology, Albert-Einstein-
Straße 15, 12489 Berlin
*corresponding authors: [email protected], [email protected]
Electronic Supplementary Material (ESI) for Energy & Environmental Science.This journal is © The Royal Society of Chemistry 2016
2
Materials and Methods
Freeze-quench X-ray absorption experiment. Mn oxides deposited on thin glassy carbon
were frozen after 3 min exposure to the selected potential (in 0.1 M phosphate buffer adjusted
to pH 7). The electrolyte-exposed samples were frozen inside of an adapted electrochemical
cell, which subsequently served as a sample holder in the low-temperature XAS measurements;
during the freezing the voltage between the working and counter electrodes was kept constant.
For details on this freeze-quench, quasi-in situ approach 1, see Fig. S1.
EXAFS Fourier-transforms and EXAFS simulations. After transition from an energy to a
wave-vector scale (k-scale) and weighting by k3, the Fourier transforms shown in Fig. 2 in the
main text and Fig. S5b were calculated for k ranging from 2.6 to 12.2 Å-1 (same k-range for
PSII and all oxide films). Cosine windows covering 10% at the low-k and high-k side of the
spectra were applied before calculation of the Fourier transforms. The absolute value of the
Fourier-transform amplitudes is shown.
Simulations of the k3-weighted EXAFS spectra of the catalytically active Mn-only oxide
(MnCat) and the inactive Mn oxide were performed as described in Refs.2, 3 using the k-range
from 2 to 14 Å-1. In order to minimize the number of free parameters and to improve the
significance of the simulation results, a joint fit approach was used where the interatomic
distances were kept the same for all spectra of one data set (the same oxide type at various
potentials). In addition, the Debye-Waller parameters for all oxygen shells were kept the same
for all potentials. The Debye-Waller parameters for all manganese shells were fixed to 0.063
Å. Also, the sum of the EXAFS coordination numbers in the two oxygen shells was fixed to 6.
The EXAFS coordination numbers for the Mn-Mn vectors were varied freely. For the
catalytically active oxide, the short Mn-Mn distance (~ 2.9 Å) was simulated with two separate
shells. For the inactive material, only one shell was used because splitting of this distance into
two shells did not result in significant improvement of the fit quality, but in an increased
number of fit parameters and thus increased uncertainty in the parameter values. This
observation suggests that the short Mn-Mn distances are more uniform in the inactive Mn
oxide, as expected from the better ordered structure. The simulation results are presented in
Table S1. The used amplitude reduction factor (S02) was 0.7. For calculation of the Fourier-
filtered error (described in Ref.4), the range from 1 to 3.5 Å on the reduced distance scale was
used.
Time-resolved in situ X-ray absorption measurements. For an overview scheme of the time-
resolved in situ experiment, see Fig. S9. The measurements were performed at 20 °C at the
BESSY II synchrotron radiation source at beamline KMC-3. A silicon (111) double-crystal
monochromator was used for selecting a fixed X-ray excitation energy (6553.3 eV).
Manganese oxide deposited on thin glassy carbon was attached as a window to the wall of a
home-made Teflon cell filled with 0.1 M phosphate buffer (adjusted to pH 7) and placed in the
path of the X-ray beam. The size of the area illuminated by the X-ray beam was approximately
11 x 3 mm. All X-ray absorption signals were collected in fluorescence mode. The excited Mn
X-ray fluorescence passed through the glassy carbon and then through a Cr foil (10 μm)
shielding against scattered X-rays. The fluorescence was monitored perpendicular to the
3
incident beam by a scintillation detector (19.6 cm2 active area, 51BMI/2E1-YAP-Neg, Scionix;
shielded by 2 μm Al foil against visible light). The detector consisted of a scintillating crystal
(YAP) converting X-ray photons into visible light (~50% efficiency) detected by a fast
photomultiplier operated at 1.1 kV. The current signal from the photomultiplier passed through
a 1 MΩ resistor for current-to-voltage conversion and was fed into a low-noise amplifier
(Stanford Research Systems, model SR560; 10 Hz low-pass filtering, 6 dB/oct; amplification
factor of 20). The amplified signal was finally recorded with a time resolution of 10 ms by the
potentiostat (Biologic SP-300) that also operated the electrochemical cell and recorded the
signal from an ionization chamber monitoring the intensity of the incoming X-ray beam. Short-
range Mn K-edge absorption spectra of the oxide film on the glassy carbon electrode were
recorded immediately before and after each time-resolved experiment, in order to facilitate
normalization of time-resolved data and to ascertain that there was no significant film
dissolution. In separate X-ray scans, absorption spectra of KMnO4 powder were measured for
precise energy calibration, setting the maximum of the pre-edge absorption peak of the KMnO4
to 6543.3 eV.
4
Figure S1. Freeze-quench (quasi-in situ) approach for investigation of Mn(Ca) oxide film by
low-temperature X-ray absorption spectroscopy (XAS). (A) Sample cell made of transparent
PVC (33 mm x 19 mm x 2 mm) with a window (14 mm x 14 mm) and two holes drilled for
insertion of a platinum wire, which can serve as a counter electrode (CE). The window is closed
by the working electrode (WE) consisting of a glassy carbon sheet (thickness of 100 µm)
supported by Kapton® tape. Copper tape was connected to the glassy carbon, facilitating
electric contact by using crocodile clamps (not shown). (B) The Mn(Ca) oxide were
electrodeposited on the working electrode as described in the Materials and Methods section
of the main article. After electrodeposition of the Mn(Ca) oxide films, ~250 μl of Mn-free
electrolyte (0.1 M potassium phosphate, pH 7.0) were filled in the cell and a mercury sulfate
REF
WE: Glassy carbon
PVC frame
copper tapeconnected
to WE
CE: Pt wire
WE: Glassy carbon
A
CE: Pt wire
A B
Electrolytecovering oxidefilm and fillingcavity(Pi, pH 7)
Oxide film deposited on WE
V
XAS sample holder before EC Operation as 3-electrode electrochemical cell
C D
Oxide film and electrolyte1st frozen in liquid N2 (l)2nd WE/CE disconnected by cutting
under liquid nitrogen3rd XAS measurement at 20 K
A
V
WE: ITO on PET
CE: Pt wire
Electrolyte(Pi, pH 7)Mn oxide film
deposited on WE
Operation as 2-electrode cell during freezing Cell for XAS measurements
5
reference electrode (REF, Hg/Hg2SO4/K2SO4 +650 mV vs. NHE) was immersed into the
electrolyte as shown in the scheme. Then the desired potential was applied for 2 minutes and
the voltage, VWE/CE, between the working electrode and the counter electrode was measured.
(C) The reference electrode was removed and VWE/CE was applied between the working
electrode and counter electrode (meaning operation in a two-electrode configuration). (D) After
1 minute the whole sample cell was quickly frozen by spilling liquid nitrogen over the cell
assembly. Only after the whole cell had been frozen, the electrode clamps were detached and
the copper tape was cut off (under liquid nitrogen). Finally, the platinum wire was removed
and the samples were stored in liquid nitrogen until use in the XAS measurements.
6
Figure S2. Cyclic voltammograms (CVs) of catalytically active Mn oxide (MnCat, red line)
and inactive Mn oxide (Inactive, blue line). Scan rate 20 mV/s; the second CV of a series is
shown.
Figure S3. Calibration line relating Mn K-edge energy position and the formal Mn oxidation
state. Edge energies of the commercial Mn compounds MnIIO, -MnIII2O3 and -MnIVO2 were
used for calibration. Using additional Mn oxide references results in a very similar calibration
curve (see ref. 2). The linear regression line results in the following calibration equation for
estimating the mean Mn oxidation state: zox = (Eedge – 6538.4 eV)/3.8. All edge energies were
determined using an ‘integral method’, which is clearly less sensitive to edge-shape changes
than other methods.4
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
0.9 1 1.1 1.2 1.3 1.4
Cu
rren
t d
ensi
ty [
mA
/cm
2]
Potential vs. NHE [V]
ITO-coated glass electrode
MnCat
Inactive
6545
6546
6547
6548
6549
6550
6551
6552
6553
6554
6555
1.5 2.0 2.5 3.0 3.5 4.0 4.5
Ed
ge
en
erg
y [
eV
]
Mn oxidation state
7
Figure S4. k3-weighted EXAFS spectra (top) and their Fourier transforms (bottom) of the
catalytically active Mn oxide (MnCat, left) and the inactive oxide (right) equilibrated at
different potentials (vs. NHE, pH 7). Black lines represent experimental data, red lines –
simulated spectra. The simulation parameters are listed in Table S1.
-70
-60
-50
-40
-30
-20
-10
0
10
20
2 3 4 5 6 7 8 9 10 11 12 13 14
Inactive
0.45 V
0.70 V
0.85 V
1.15 V
1.35 V
1.45 V
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
2 3 4 5 6 7 8 9 10 11 12 13 14
0.45 V
0.85 V
0.95 V
1.15 V
1.20 V
1.35 V
1.45 V
MnCat
0.70 V
k [Å-1] k [Å-1]
k3(k
)[Å
-3]
k3(k
)[Å
-3]
-35
-30
-25
-20
-15
-10
-5
0
5
10
0 1 2 3 4 5 6 7
MnCat
0.45 V
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.35 V
1.45 V-25
-20
-15
-10
-5
0
5
10
0 1 2 3 4 5 6 7
Inactive Mn oxide
0.45 V
0.70 V
0.85 V
1.15 V
1.35 V
1.45 V
Reduced distance [Å]Reduced distance [Å]
FT
of
EX
AF
S
FT
of
EX
AF
S
8
Table S1. Simulation parameters for the EXAFS spectra of the catalytically active Mn-only
oxide (MnCat) and of the inactive Mn oxide. The interatomic distances, R, for each shell are
shown in bold (in Å) and were kept the same at all potentials. The rest of the values show the
coordination numbers at the respective potential. The errors representing 68% confidence
intervals of the respective fit parameters are shown in parentheses. For the MnCat, the Debye-
Waller parameter () for the two oxygen shells was 0.065 0.001 Å (at all potentials) and the
filtered R-factor was 10.9%. For the inactive Mn oxide, the Debye-Waller parameter () for
the two oxygen shells was 0.059 0.001 Å and the filtered R-factor 11.4%. The asterisks
indicate that the sum of the EXAFS coordination numbers of the two oxygen shells was forced
to be equal to six; the error is indicated only for one of the two coordination numbers. The
Debye-Waller parameters for all manganese shells were fixed to 0.063 Å.
Mn-O Mn-O Mn-Mn Mn-Mn Mn-Mn Mn-Mn Mn-Mn
catalytically active MnCat
all 1.90 (0.001) 2.28 (0.01) 2.85 (0.01) 2.99
(0.01) 3.16 (0.01) 3.42 (0.01) 3.81 (0.02)
0.45 V 4.7 (0.2)* 1.3* 2.6 (0.4) 1.7 (0.6) 1.1 (0.5) 0.7 (0.4) 0.4 (0.5)
0.70 V 5.0 (0.2)* 1.0* 3.0 (0.4) 2.8 (0.7) 2.0 (0.6) 1.3 (0.4) 0.2 (0.5)
0.85 V 5.3 (0.2)* 0.7* 3.6 (0.4) 3.3 (0.7) 2.8 (0.7) 2.0 (0.5) 0.4 (0.5)
0.95 V 5.4 (0.2)* 0.6* 3.6 (0.4) 3.0 (0.7) 2.6 (0.6) 1.7 (0.4) 0.5 (0.5)
1.15 V 5.4 (0.2)* 0.6* 3.5 (0.4) 2.9 (0.7) 2.3 (0.7) 2.0 (0.4) 0.9 (0.5)
1.20 V 5.5 (0.2)* 0.5* 3.7 (0.4) 3.1 (0.7) 2.7 (0.7) 2.3 (0.5) 0.4 (0.5)
1.35 V 5.4 (0.2)* 0.6* 3.7 (0.4) 3.0 (0.7) 2.3 (0.7) 1.9 (0.4) 0.5 (0.5)
1.45 V 5.6 (0.2)* 0.4* 4.1 (0.4) 3.1 (0.7) 2.2 (0.7) 1.8 (0.4) 0.6 (0.5)
inactive Mn oxide
all 1.90 (0.001) 2.28 (0.01) 2.87 (0.002) 3.14 (0.01) 3.49 (0.01) 3.78 (0.01)
0.45 V 5.0 (0.2)* 1.0* 4.6 (0.2) 0.7 (0.3) 1.7 (0.4) 0.4 (0.5)
0.70 V 5.3 (0.2)* 0.7* 4.8 (0.2) 0.9 (0.3) 1.7 (0.4) 0.7 (0.5)
0.85 V 5.5 (0.2)* 0.5* 4.9 (0.2) 0.9 (0.3) 1.3 (0.4) 0.1 (0.5)
1.15 V 5.7 (0.2)* 0.3* 4.9 (0.3) 1.5 (0.3) 1.5 (0.4) 1.3 (0.5)
1.35 V 5.7 (0.2)* 0.3* 4.6 (0.2) 0.8 (0.3) 1.6 (0.4) 0.9 (0.5)
1.45 V 5.8 (0.2)* 0.2* 4.8 (0.2) 0.7 (0.3) 1.4 (0.4) 1.1 (0.5)
9
Figure S5. Averaged MnO distances calculated from the simulation results presented in Table
S1. Mn oxidation state calculated according to the bond-valence sum rules5 is +3.48 for the
MnCat and +3.61 for the inactive Mn oxide at 0.45 V and +3.86 for the MnCat and +3.95 for
the inactive Mn oxide at 1.45 V.
1.90
1.92
1.94
1.96
1.98
2.00
0.4 0.6 0.8 1 1.2 1.4
Mn
O
bo
nd
le
ng
th [
Å]
Potential vs. NHE [V]
MnCat
Inactive Mn oxide
10
Figure S6. Fourier-isolated EXAFS oscillations corresponding to the MnMn peak (Fourier
isolation from 2.1 Å to 2.9 Å on the reduced distance scale) of the data for the MnCat shown
in Fig. S4. For clarity, only the positive amplitudes of the EXAFS oscillations are shown. In
panel A, the experimental data is presented. In panel B, the simulated oscillations with fixed
Debye-Waller parameter (using the simulation approach presented in Fig. S4 and Table S1) are
shown. Panel C shows the results from simulations in which (i) the coordination numbers for
the two short Mn-Mn distances (2.85 Å and 2.99 Å) were fixed to the values obtained at 0.45 V
and (ii) the Debye-Waller factors for these two shells were freely varied. Only four potentials
are shown for clarity. The experimental data is better modeled with fixed Debye-Waller
parameter (in B), indicating that the observed potential-induced increase in the amplitude of
the second peak is a result rather from an increase in the coordination number than from a
decreased inhomogeneity in the length of the shortest Mn-Mn distances.
0
5
10
15
20
25
30
35
40
2 3 4 5 6 7 8 9 10 11 12 13 14
EX
AF
S a
mp
litu
de
[a
.u.]
Wevenumber [Å-1]
Simulation: fixed coordination number (N)C
0
5
10
15
20
25
30
35
40
2 3 4 5 6 7 8 9 10 11 12 13 14
EX
AF
S a
mp
litu
de
[a
.u.]
Wevenumber [Å-1]
Simulation: fixed Debye-Waller factor (σ)B
0
5
10
15
20
25
30
35
40
2 3 4 5 6 7 8 9 10 11 12 13 14
EX
AF
S a
mp
litu
de [
a.u
.]
Wevenumber [Å-1]
Experimental0.45 V
0.70 V
0.95 V
1.35 V
A
11
Figure S7. XANES spectra (a) and Fourier transformed EXAFS spectra (b) of the MnCa
oxides equilibrated at the indicated potentials (vs. NHE, pH 7). The inset in (a) show the
extracted pre-edge features. For each FT peak, the corresponding structural motif is
schematically shown (Mn, magenta; O, red). In the MnCa oxide, the third FT-peak also
contains contributions from Mn-Ca distances.
0
1
6535 6540 6545 6550 6555 6560 6565
No
rma
lize
d f
luo
res
ce
nc
e
X-ray energy [eV]
0.45 V
0.70 V
0.85 V
0.95 V
1.20 V
1.35 V
1.45 V
0
0.05
6535 6540 6545
3.3 3.8
a
0
2
4
6
8
10
12
0 1 2 3 4 5 6 7
FT
of
EX
AF
S
Reduced distance [Å]
0.45 V0.70 V0.85 V0.95 V1.20 V1.35 V1.45 V
1
2
3
45 6
b
12
Figure S8. Absorption spectra of (a) the catalytically active Mn-only oxide (MnCat), (b) of
the inactive Mn oxide and (c) the absorption 370 nm of the catalytically active Mn-only oxide
(MnCat, red circles) and the inactive film (blue circles) as a function of the applied potential
(vs. NHE at pH 7). The catalyst films were deposited on optically transparent ITO-covered
glass slides and equilibrated at the indicated potentials (vs. NHE, pH 7) for 2 min before
measuring the UV-vis spectra.
0.1
0.2
0.4 0.6 0.8 1.0 1.2 1.4
Ab
so
rban
ce [
OD
]
Potential vs NHE [V]
MnCat
Inactive film
0
0.1
0.2
0.3
300 400 500 600 700 800 900
Ab
so
rba
nc
e [
OD
]
Wavelength [nm]
0.45V
0.70V
0.95V
1.10V
1.20V
1.35V
1.40V
1.45V
0
0.1
0.2
0.3
300 400 500 600 700 800 900
Ab
so
rba
nc
e [
OD
]
Wavelength [nm]
0.45 V
0.70 V
0.95 V
1.10 V
1.20 V
1.35 V
1.40 V
1.45 V
a
b
c
13
Figure S9. Estimation of the initial rate of Mn reduction from the UV-vis time courses of Fig. 4
in (a) active oxide (MnCat) and (b) inactive oxide. To obtain a rough estimate, the time period
for an absorption change corresponding to I/I = 0.5 10-3 was determined graphically. Values
of about 20 ms and 3000 ms are obtained for active and inactive oxide, respectively. (In this
estimation, it is assumed that equal I/I values correspond to roughly similar changes in the
average Mn oxidation state, as suggested by comparison of Fig. S8c with Fig. 3.)
Tra
ns
mit
ed
lig
ht
ins
en
tity
, I(
t)
a
2,347
2,348
2,349
2,350
2,382
2,383
2,384
2,385
2,386
9,9 10 10,1 10,2
Tra
ns
mis
sio
n [
a.u
.]
Time [s]
MnCat
Inactive
1.4
V
OC
P
100 ms
I/I = 0.5 10-3
~20 m
s
I/I = 0.5 10-3
~3000 ms
b
14
Figure S10. Time course of current and open-circuit potential (OCP) measured in parallel to
the UV-vis experiment of Figure S9. Top panels: Time courses of the current for stepping the
electrode working conditions from OCP top application of 1.4 V vs NHE (pH 7). Bottom
panels: Time courses of the electrode potential for switching between application of 1.4 V (vs.
NHE, pH 7 and OCP) conditions. The right panels show the respective time course in form of
a double-logarithmic plot.
0,001
0,01
0,1
1
0,01 0,1 1 10
Cu
rre
nt
de
ns
ity [
mA
/cm
2]
Time [s]
MnCat
Inactive
at 1.4 V
1,2
1,25
1,3
1,35
1,4
0,01 0,1 1 10
Po
ten
tia
l v
s N
HE
[V
]
Time [s]
MnCat
Inactive
at OCP conditions
15
Figure S11. Time-resolved in situ X-ray absorption experiment. (a) Illustration of the
experimental concept. Panel-a shows XANES spectra collected in freeze-quench experiments
for the MnCat equilibrated at different electrode potentials. The X-ray absorption at 6553.3 eV
(arrow) decreases (see inset) because the edge position shifts with increasingly positive
potentials to higher energies, as a result of Mn oxidation. This absorption change at 6553.3 eV
was traced in the time-resolved X-ray experiments. (b) Scheme of the experimental setup for
time-resolved X-ray absorption measurements. Monochromatic synchrotron radiation (6553.3
eV) passes through an ionization chamber (IC, for recording the variations in the incoming
beam intensity) and a photoshutter (PS) before hitting the Mn oxide film (Mn). The Mn oxide
film deposited on a thin, X-ray-transparent glassy carbon (GC) electrode serves as the working
electrode in a 3-electrode electrochemical setup, with reference electrode (RE) and a platinum
mesh serving as counter electrode (CE). The cell is filled with 0.1 M phosphate buffer (pH 7).
The excited Mn X-ray fluorescence passes through the glassy carbon and then through a 10 μm
chromium foil (Cr), which absorbs a major fraction of the scattered X-ray photons. The
fluorescence is monitored perpendicular to the incident beam by a scintillation detector
consisting of a scintillating crystal and a fast photomultiplier operated at 1.1 kV. The signal
from the photomultiplier passes through a 1 MΩ resistor for current-to-voltage conversion and
is fed into an amplifier. The amplified signal is finally recorded by the same potentiostat device
that also operates the electrochemical cell and records the IC signal. (c) Mn oxidation-state
changes of catalytically active Mn oxide (red) and inactive oxide (blue) induced by stepping
the electrode potential between 1.4 V and 0.6 V and tracked by recording the K/ X-ray
fluorescence intensity for excitation at 6553.3 eV (same data as in Fig. 5a, but with extended
time window shown and estimated Mn oxidation states indicated). The Mn oxidation state
estimates were derived from rapid XAS scans before and after the voltage-jump experiments;
the corresponding numbers are less precise than in Figure 3.
0
1
6535 6540 6545 6550 6555 6560 6565
No
rma
lize
d f
luo
res
ce
nc
e
X-ray energy [eV]
0.45 V
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.35 V
1.45 V
0.5
0.8
0.4 0.9 1.4Potential vs NHE [V]
Synchrotron
radiation
6553.3 eV
MnGC
RE
CE
PS
Scintillationdetector
with photo-multiplier
Cr
Amplifier
IC
Potentiostat
Electrochemical
cell
3.4
3.5
3.6
3.7
3.8
3.9
4.0
0.55
0.6
0.65
0.7
0.75
0.8
8 10 12 14 16 18 20 22 24 26 28 30
No
rmalize
d X
-ray f
luo
rescen
ce
Time [s]
MnCat
Inactive
0.6 V 1.4 V 0.6 V
oxidation reduction
Mn
ox
ida
tion
sta
te
a b
c
16
Figure S12. Bode plots for electrochemical impedance spectra obtained at different potentials
(vs. NHE, pH 7) for the catalytically active Mn-only oxide (MnCat, in a and b) and the inactive
Mn oxide (c, d). The experimental data are presented by points, the simulation results according
to the model in Fig. S14 are indicated by solid lines.
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0.1 1 10 100 1000 10000
Ph
ase []
Frequency [Hz]
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.25 V
1.35 V
1.40 V
10
100
1000
10000
0.1 1 10 100 1000 10000
Z [
]
Frequency [Hz]
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.25 V
1.35 V
1.40 V
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0.1 1 10 100 1000 10000
Ph
ase []
Frequency [Hz]
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.25 V
1.35 V
1.40 V
10
100
1000
10000
100000
0.1 1 10 100 1000 10000
Z [
]
Frequency [Hz]
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.25 V
1.35 V
1.40 V
a
b
c
d
17
Figure S13. Nyquist plots for electrochemical impedance spectra obtained at different
potentials for the catalytically active Mn-only oxide (MnCat, in a, b, and c) and the inactive
Mn oxide (d, e f). In the different plots, different scales are used. The experimental data are
presented by points; the simulations according to the model in Fig. S14, by solid lines. The
simulations parameters are shown in Fig. S14.
-2000
-1500
-1000
-500
0
0 500 1000 1500 2000
Im(Z
) [
]
Re(Z) []
-30000
-25000
-20000
-15000
-10000
-5000
0
0 5000 10000 15000 20000 25000 30000
Im(Z
) [
]
Re(Z) []
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.25 V
1.35 V
1.40 V
-500
-400
-300
-200
-100
0
0 100 200 300 400 500
Im(Z
) [
]
Re(Z) []
-2000
-1500
-1000
-500
0
0 500 1000 1500 2000
Im(Z
) [
]
Re(Z) []
-30000
-25000
-20000
-15000
-10000
-5000
0
0 5000 10000 15000 20000 25000 30000
Im(Z
) [
]
Re(Z) []
0.70 V
0.85 V
0.95 V
1.15 V
1.20 V
1.25 V
1.35 V
1.40 V
-500
-400
-300
-200
-100
0
0 100 200 300 400 500
Im(Z
) [
]
Re(Z) []
a
b
c
d
e
f
18
Figure S14. Parameters for the simulation of the impedance spectra according the equivalent
circuit shown bottom right. ROhm – summed Ohmic resistance of electrolyte and ITO electrode;
Cdl – double-layer capacitance, modelled as a constant-phase element; Cox – capacitance
describing oxidation state changes of the catalyst film (in other contexts also denoted as
pseudo-capacitance), modelled as a constant phase element; Rox – resistance describing the
oxidation state changes of the catalyst film; Rcat – catalytic resistance of the oxide (in other
contexts also denoted as charge-transfer resistance). The values for the Ohmic resistance were
fixed during the simulations (46.5 for the MnCat and 53 for the inactive Mn oxide). The
double layer capacitance (Cdl) and the oxidation capacitance (Cox) were calculated according
to the given equation (following the Brug conversion rules6), where α is the phase parameter
of the constant phase element (with values between 0.75 and 1 in all simulations) and Q is the
capacitance parameter of the constant phase element in units of Fs(α-1). The formal uncertainty
ranges for the fit parameters were below 4%.
ROhm Rox Rcat
Cdl
Cox
1
10
100
1000
10000
100000
0.6 0.8 1.0 1.2 1.4
Ro
x[
]
Potential vs. NHE [V]
RoxMnCat
Inactive Mn oxide
10
100
1000
10000
100000
1000000
0.6 0.8 1.0 1.2 1.4
Rcat[
]
Potential vs. NHE [V]
Rcat
0.001
0.01
0.1
1
10
0.6 0.8 1.0 1.2 1.4
cd
l, c
ox
[mF
]
Potential vs. NHE [V]
Cox
Cdl
19
References
1. M. Risch, F. Ringleb, M. Kohlhoff, P. Bogdanoff, P. Chernev, I. Zaharieva and H. Dau,
Energy Environ. Sci., 2015, 8, 661-674.
2. I. Zaharieva, P. Chernev, M. Risch, K. Klingan, M. Kohlhoff, A. Fischer and H. Dau,
Energy Environ. Sci., 2012, 5, 7081-7089.
3. M. Wiechen, I. Zaharieva, H. Dau and P. Kurz, Chem. Sci., 2012, 3, 2330-2339.
4. H. Dau, P. Liebisch and M. Haumann, Anal. Bioanal. Chem., 2003, 376, 562-583.
5. I. D. Brown and D. Altermatt, Acta Crystallographica Section B, 1985, 41, 244-247.
6. G. J. Brug, A. L. G. van den Eeden, M. Sluyters-Rehbach and J. H. Sluyters, J.
Elelectroanal. Chem., 1984, 176, 275-295.
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