Supplementary information
Competitive Photoelectrochemical Methanol and Water Oxidation with Hematite Electrodes
Benjamin Klahr#, Sixto Gimenez††*, Omid Zandi‡‡, Francisco Fabregat-Santiago††, Thomas Hamann‡‡*
#
Department of Chemistry, Northwestern University, 2145 Sheridan Road Evanston, Illinois 60208
††
Photovoltaics and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, 12071 Castelló,
Spain
‡‡
Department of Chemistry, Michigan State University, East Lansing, MI 48824-1322
*Email: [email protected], [email protected]
Oxygen Measurements
Figure S1. Theoretical oxygen produced under 2 sun illumination (lines) and measured O2 concentration (shapes)
for a hematite electrode in contact with H2O (solid line and circles), 0.2M CH3OH (dotted line and downward
pointing triangles), 2M CH3OH (short dashed line and squares), 5M CH3OH (double dotted dashed line and
diamonds), 10M CH3OH (long dashed line and upward pointing triangles), and CH3OH (sigle dotted line and
hexagons). Applied potentials for O2 concentration measurement was 0.75 V vs Ag/AgCl.
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Electrochemical Impedance Spectroscopy
a)
b)
Figure S2. Nyquist plots of impedance data gathered under 1 sun illumination for a H2O (red circles) and 5M
CH3OH in H2O (green diamonds) electrolytes at 0.66 V vs Ag/AgCl. B) Nyquist plot in CH3OH measured at 1.06
V vs Ag/AgCl.
a)
b)
Figure S3. a) Rct,ss for H2O (red circles) and 5M CH3OH (green diamonds) electrolytes as well as Rct for CH3OH
(blue hexagons) fit from IS data measured under 1 sun illumination. b) Rtrap for the same electrolytes.
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Table S1. Summary of the Mott-Schottky analysis from impedance spectroscopy measurements carried out in the
dark including the flat band potential (EFB) and the donor density (ND) of hematite phototoelectrodes in different
solution media.
EFB vs Ag/AgCl (mV)
ND (·1018 cm-3)
H2O
137
5.3
5M CH3OH
143
5.3
CH3OH
330
5.3
Cyclic voltammetry
a)
b)
c)
Figure S4. The first (solid blue line) and second (dashed purple line) scan of a CV measured in the dark of a
hematite electrode in contact with (a) H2O (b) 5M CH3OH in H2O (c) and CH3OH.
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a)
b)
c)
d)
Figure S5. Calculated capacitance of the first scan of the CVs measured in the dark for a hematite electrode in
contact with (a) H2O (b) 5M CH3OH in H2O (c) and CH3OH. (d) Magnified image of (c). The capacitance was
calculated by C=J/scan rate. Scan rates were 50 mV/s (red solid line), 100 mV/s (orange long line), 200 mV/s
(yellow short line), 500 mV s (green shorter line) and 1000 mV/s (blue dotted line).
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Figure S6. Current density, J, for the first scan of the CVs measured in the dark for a hematite electrode in
contact with H2O (red), 0.2 M CH3OH (orange), 2 M CH3OH (yellow), 5M CH3OH (green), and 10 M CH3OH
(teal) measured at 200 mV/s).
Figure S7. Ctrap values fit from model shown in figure 2b (orange triangles) and Ctrap values fit from model shown
in figure 2c (yellow squares). Capacitance calculated of the first scan of CV experiments in CH3OH solution
measured at 1000 mV/s . C=J/scan rate.
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Mott-Schottky, J-V and measurements in MeCN
a)
b)
c)
Figure S8. (a) Mott-Schottky plot of a hematite electrode in contact with anhydrous MeCN. (b) and (c) J-V and
the first cathodic peak in the CV, measured in anhydrous MeCN (dotted red), and after the addition of 0.2
(dashed orange), 0.4 (dash-dotted green) and 1% V (solid cyan) of H2O.
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Chopped illumination versus constant illumination J-V curves
a)
b)
c)
Figure S9. Chopped light (grey line) and steady state (colored line) J-V measured for a hematite electrode in
contact with (a) H2O (b) 5M CH3OH in H2O and (c) CH3OH.
Potentiostatic current transients
a)
b)
Figure S10. (a) anodic and (b) cathodic transients measured for a hematite electrode in contact with H2O at
applied potentials of 0.35 (solid teal line), 0.45 (dotted blue line) and 0.55 (dashed purple line) V vs Ag/AgCl.
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a)
b)
Figure S11. (a) anodic and (b) cathodic transients measured for a hematite electrode in contact with 5M CH3OH
in H2O at applied potentials of 0.35 (solid teal line), 0.45 (dotted blue line) and 0.55 (dashed purple line) V vs
Ag/AgCl.
a)
b)
Figure S12. (a) anodic and (b) cathodic transients measured for a hematite electrode in contact CH3OH at applied
potentials of 0.55 (solid teal line), 0.65 (dotted blue line) and 0.75 (dashed purple line) V vs Ag/AgCl.
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a)
b)
Figure S13. a) anodic and (b) cathodic transients in H2O (red solid line), CH3OH in H2O (orange dotted) and
MeOH (yellow dashed line) measured within 5 mV of 0.79 V vs ECB. c) cathodic transient measured within 5 mV
of 0.94 V vs ECB. d) Lifetimes of transients fit by a single exponential decay for H2O (red circles), 5M CH3OH
(orange triangles), and CH3OH (yellow squares).
Experiments in N2 and O2 saturation conditions
a)
b)
Figure S14: (a) J-V curves and (b) Ctrap for an hematite electrode tested under N2 and O2 saturation conditions.
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Physical model
Figure S15. a) Full equivalent circuit used to conceptualize hematite-liquid junctions. Simplifications of
this model lead to the equivalent circuits showed in Figure 4. The formal potential for methanol oxidation is
0.02 V vs. RHE (Electrochim. Acta, 2002, 47, 3663––3674), compared to 1.23 V for water. The flat band potentials
are shown above in Table SI1.
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