4.6 Supplementary Information
4.6.5 In situ investigation near the onset of iridium’s OER activity
4.6 Supplementary Information
To precondition and activate the Ir films on Nafion®117, we always performed a sequence of 35 CVs between 0.1 V vs. SHE and 1.6 V vs. SHE prior to all other measure-ments. Subsequently, we recorded a scan of the O K-edge, an XPS survey and the Ir 4f, C 1s, and O 1s core levels atEocto capture the initial state of the Ir electrode surface. Fi-nally, we applied OER-relevant potentials, monitored the corresponding current den-sities of the WE and oxygen QMS traces and recorded NEXAFS and XPS to observe changes in the electronic structure of the iridium electrodes. In the following, we will show three examples of typical experiment results and how the oxygen evolution rate and current density are related with the presence of OI− species on the Ir electrode surface.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0
1
2 CV100 mV s-1
current density / mA cm-2
potential / V vs. SHE
Figure S4.6.12: Cyclic voltammogram of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23894) recorded in the three-electrode cell with the indicated potentials vs. SHE applied (p=0.5 Pa, 0.1 M H2SO4).
0 5 10 15 20 25
01 23 45 6
Eoc,2
Eoc,1 1.6 V 1.7 V 1.8 V 1.9 V
current density / mA cm-2
time / min
1.65 V CA
oxygen QMS signal QMS
1 2 3 4 5
0.7 0.8 0.9 1.0 1.1
lin. regression R2=0.999
QMS oxygen ion current / nA
current density / mA cm-2 1.6 V
1.65 V 1.7 V
1.8 V
1.9 V
Figure S4.6.13: (left) Chronoamperometry (bottom) and oxygen QMS signal (top) and (right) linear correlation between current density and evolved oxygen of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23894) recorded in the three-electrode cell with the indicated potentials vs. SHE applied (p=0.5 Pa, 0.1 M H2SO4).
4.6 Supplementary Information
Figures S4.6.12 and S4.6.13 show the CV and the subsequently recorded CA of sample 23894, a Nafion®117 that was sputter-coated for 180 s with metallic Ir. The CV mainly shows the oxidation/reduction signal of Nafion®117 at ≈0.6 V vs. SHE, a slight indication of the Ir-oxidation signals at 1 V vs. SHE and 1.4 V vs. SHE and the OER onset at around 1.5 V vs. SHE. When OER-relevant potentials are applied, the current density in the CA increases stepwise with each potential increase. A concomitant stepwise increase is observed in the oxygen evolution rate as mirrored in the the oxygen QMS signal. We observe a linear relation between the current density and the oxygen evolution activity. When the potential is turned off, the oxygen signal immediately drops back to its original value.
525 530 535 540 545 550
1.9 V 1.8 V 1.7 V 1.65 V 1.6 V
TEY intensity / arb. unit
excitation energy / eV Eoc,1
O K-edge
Figure S4.6.14: O K-edges of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23894), consecutively recorded (bottom to top) in the three-electrode cell with the indicated potentials vs. SHE applied (ring current=70 mA, p=0.5 Pa, 0.1 M H2SO4).
Figure S4.6.14 displays the O K-edges recorded at consecutively applied potentials.
At the pressure established during this experiment, the O K-edge still shows minor resonances of gas-phase water. Nevertheless, these resonances do not severely influ-ence the spectra. In this representation, we observe nearly no changes in the spectra in dependence of the applied potential. However, when we zoom into the region of inter-est at low excitation energies, we do observe clear changes depending on the applied potential (see Figure S4.6.15). To quantify the observed changes, we used the spectra calculated16for OI− and OII−(shown in Figure S4.6.27) to fit the low excitation energy region of the O K-edge. We obtain good agreement between the measured spectra and the resulting fit envelope (see Figure S4.6.15).
In a next step, we wanted to identify the relation between the observed oxygen species and the oxygen evolution activity of the electrode. For this purpose, we plot-ted the relative concentration of OI−and OII−against the current density recorded with the potentiostat and the oxygen ion current registered by QMS, respectively (see
Eoc,1 calc. O
calc. O
calc. OI- + calc. O
II-TEY intensity / arb. unit
1.6 V
528 529 530
1.7 V
528 529 530
1.8 V
528 529 530
1.9 V
excitation energy / eV
1.65 V
Figure S4.6.15: Zoomed and fitted low excitation energy regions of O K-edges of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23894) consecutively recorded (left to right, top to bottom) in the three-electrode cell with the indicated potentials vs. SHE applied (ring current=70 mA, p=0.5 Pa, 0.1 M H2SO4).
ure S4.6.16). The determined error values originate from the fluctuations in measured current densities (x-error) and the uncertainties in peak height determination (y-error).
We observe a linear relationship between the OI−-species and both the current density measured with the potentiostat and the oxygen ion current registered by QMS (R2 -values of 0.94 and 0.95). In contrast, we observe only a loose dependence of the OII−
concentration on current density and ion current (R2-values of 0.66 and 0.67).
1 2 3 4 5
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
lin. regression O
I-R2=0.94
current density / mA cm-2 lin. regression O
II-R2=0.66
conc. (OI- ) / conc. (OI- )max
conc. (OII- ) / conc. (OII- )max
0.7 0.8 0.9 1.0 1.1
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
lin. regression O
I-R2=0.95
QMS oxygen ion current / nA
conc. (OI- ) / conc. (OI- )max
conc. (OII- ) / conc. (OII- )max
lin. regression O
II-R2=0.67
Figure S4.6.16: Normalized OI− and OII− concentrations over (left) current den-sity and (right) QMS oxygen ion current of Ir-coated Nafion®117 (180 s Ir sput-tered, sample 23894) at consecutively applied potentials between 1.6 V vs. SHE and 1.9 V vs. SHE.
4.6 Supplementary Information
To confirm the results obtained with sample 23894, we repeated the experiments with sample 23895, which was also a 180 s Ir-sputtered Nafion®117 from the same batch of sample. Figures S4.6.17 to S4.6.21 show the same features and trends as ob-served in the previous experiment: The CV in Figure S4.6.17 counts with the oxidation waves of iridium oxides at 1 V vs. SHE and 1.4 V vs. SHE and the additional reversible oxidation/reduction feature of Nafion®117 at ≈0.6 V vs. SHE. The CA and the QMS oxygen ion current in Figure S4.6.18 show a linear increase with increasing potential applied to the Ir WE. While the overview spectrum of the O K-edge in Figure S4.6.19 does not show marked changes during the experiment, a zoom in the low excitation energy region and the corresponding fits in Figure S4.6.20 shows a clear increase of OI−
concentration with increasing potential.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0
1
2 CV
100 mV s-1
current density / mA cm-2
potential / V vs. SHE
Figure S4.6.17: Cyclic voltammogram of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23895) recorded in the three-electrode cell with the indicated potentials vs. SHE applied (p=0.3 Pa, 0.1 M H2SO4).
0 5 10 15 20 25 30 35 40
01 23 45 6
Eoc,2 Eoc,1 1.6 V 1.65 V 1.7 V 1.75 V 1.8 V 1.9 V 2 V current density / mA cm-2
time / min
1.85 V CA
oxygen QMS signal QMS
1 2 3 4 5 6
1 2
3 2 V
1.65 V1.7 V1.75 V1.8 V1.85 V 1.9 V
lin. regression R2=0.997
QMS oxygen ion current / nA
current density / mA cm-2 1.6 V
Figure S4.6.18: (left) Chronoamperometry (bottom) and oxygen QMS signal (top) and (right) linear correlation between current density and evolved oxygen of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23895) recorded in the three-electrode cell with the indicated potentials vs. SHE applied (p=0.3 Pa, 0.1 M H2SO4).
525 530 535 540 545 550 2 V1.9 V
1.85 V 1.8 V 1.75 V 1.7 V 1.65 V 1.6 V
TEY intensity / arb. unit
excitation energy / eV Eoc,1
O K-edge
Figure S4.6.19: O K-edges of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23895), consecutively recorded (bottom to top) in the three-electrode cell with the indicated potentials vs. SHE applied (ring current=60 mA, p=0.3 Pa, 0.1 M H2SO4).
Eoc,1 calc. O
calc. O
calc. OI- + calc. O
1.6 V 1.65 V
1.7 V
TEY intensity / arb. unit
1.75 V 1.8 V
528 529 530
1.85 V
528 529 530
1.9 V
excitation energy / eV
528 529 530
2 V
Figure S4.6.20: Zoomed and fitted low excitation energy regions of O K-edges of Ir-coated Nafion®117 (180 s Ir sputtered, sample 23895), consecutively recorded (left to right, top to bottom) in the three-electrode cell with the indicated potentials vs. SHE applied (ring current=60 mA, p=0.3 Pa, 0.1 M H2SO4).
4.6 Supplementary Information
A quantification of the relative OI− and OII− concentrations and their plots against the current density measured with the potentiostat and the oxygen ion current deter-mined by QMS in Figure S4.6.21 confirm the linear dependence of oxygen evolution activity and OI−concentration.
1 2 3 4 5 6
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
conc. (OII- ) / conc. (OII- )max
R2=0.85
current density / mA cm-2 lin. regression O
I-R2=0.95
lin. regression OII- I-I- conc. (O) / conc. (O)max
1.0 1.5 2.0 2.5 3.0
0.0 0.2 0.4 0.6 0.8 1.0
lin. regression O
I-R2=0.96
conc. (OII- ) / conc. (OII- )max
QMS oxygen ion current / nA
0.0 0.2 0.4 0.6 0.8 1.0
lin. regression OII- I-I- conc. (O) / conc. (O)max R2=0.87
Figure S4.6.21: Normalized OI− and OII− concentrations over (left) current den-sity and (right) QMS oxygen ion current of Ir-coated Nafion®117 (180 s Ir sput-tered, sample 23895) at consecutively applied potentials between 1.6 V vs. SHE and 2 V vs. SHE.
In a final in situ investigation, we tested the stability of the OI− species and alter-natively switched on and off the potential applied to the Ir WE. We first confirmed the similar behavior of the Ir-coated Nafion®117 (sample 23898, 60 s Ir sputtered) in cyclic voltammetry and obtained a similar CV shape as for the previous samples (see Figure S4.6.22 (left)).
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0
1
CV100 mV s-1
current density / mA cm-2
potential / V vs. SHE
0 10 20 30 40 50 60 70 80 90 100
01 23 45 6
current density / mA cm-2
time / min
1.6 V 1.7 V1 Eoc,2 1.9 V
Eoc,1 Eoc,4
CA oxygen QMS signal
1.7 V2 Eoc,3 QMS
Figure S4.6.22: (left) Cyclic voltammogram and (right) chronoamperometry (bot-tom) and oxygen QMS signal (top) of Ir-coated Nafion®117 (60 s Ir sputtered, sam-ple 23898) recorded in the three-electrode cell with the indicated potentials vs. SHE applied (p=0.45 Pa, 0.1 M H2SO4).
We then applied OER-relevant potentials of 1.6 V vs. SHE, 1.7 V vs. SHE, and 1.9 V vs. SHE and turned off the potential in between. The resulting current densities and QMS oxygen ion currents are shown in Figure S4.6.22 (right). At 1.9 V vs. SHE the current density and the corresponding oxygen ion current increase sharply for a short period of time, in which the electrode possibly reaches a highly active state. Due to the short time period, it was not possible to record the corresponding O K-edge. The measurement at 1.9 V vs. SHE was recorded from 80 min onwards.
In the overview spectra of the O K-edge, we can observe already in this represen-tation that the 529 eV species is switched on and off with the applied potential (see Figure S4.6.23). This observation becomes even clearer when considering the zoomed in and fitted low excitation energy region in Figure S4.6.24.
525 530 535 540 545 550
1.9 V 1.7 V2 1.7 V1 Eoc,4 Eoc,3 Eoc,2
TEY intensity / arb. unit
excitation energy / eV Eoc,1
1.6 V O K-edge
Figure S4.6.23: O K-edges of Ir-coated Nafion®117 (60 s Ir sputtered, sample 23898), consecutively recorded (bottom to top) in the three-electrode cell with the indicated potentials vs. SHE applied (ring current=13 mA, p=0.45 Pa, 0.1 M H2SO4).
Eoc,1 calc. O
calc. O
calc. OI- + calc. O
II-TEY intensity / arb. unit
1.6 V 1.7 V1 Eoc,2
528 529 530
1.7 V2
528 529 530
Eoc,3
excitation energy / eV
528 529 530
1.9 V
528 529 530
Eoc,4
Figure S4.6.24: Zoomed and fitted low excitation energy regions of O K-edges of Ir-coated Nafion®117 (60 s Ir sputtered, sample 23898), consecutively recorded (left to right, top to bottom) in the three-electrode cell with the indicated potentials vs. SHE applied (ring current=13 mA, p=0.45 Pa, 0.1 M H2SO4).
4.6 Supplementary Information
Figures S4.6.25 and S4.6.26 show the corresponding Ir 4f and O 1s spectra of sam-ple 23898 recorded at the different applied potentials. The major change observed in the Ir 4f spectrum occurs at the first application of an OER-relevant potential of 1.7 V vs. SHE. We observe slightly more intensity at higher binding energy, suggest-ing a slight surface oxidation, which is in line with the observation of the increassuggest-ing contribution of OI− and OII− at this applied potential (see Figure S4.6.24). Subsequent potential cycles of turning the applied potential on and off have nearly no impact on the shape of the Ir 4f spectrum. In the O 1s spectra, complementary to the O K-edge, we observe an increased intensity at lower binding energies of 529 eV, where the OI− are located, while the OER proceeds. Since this sample was measured at a low storage ring current of 13 mA, the signal-to-noise ration of these spectra is rather poor and we con-centrated our interpretation on the O K-edge. Nevertheless, the O 1s spectra confirm the trends observed in the O K-edge.
72 70 68 66 64 62 60 58 56
norm. XPS intensity
binding energy / eV Ir 4f
450 eV KE E 1.7 Voc,11
Eoc,2 1.7 V2 Eoc,3 1.9 V Eoc,4
72 70 68 66 64 62 60 58 56
norm. XPS intensity
binding energy / eV Ir 4f
450 eV KE E 1.7 Voc,11
1.7 V1 Eoc,1
Figure S4.6.25: Ir 4f signals of Ir-coated Nafion®117 (60 s Ir sputtered, sample 23898) recorded in the three-electrode cell with the indicated potentials vs. SHE applied (p=0.45 Pa, 0.1 M H2SO4).
536 534 532 530 528
Eoc,1 1.7 V1 Eoc,2
norm. XPS intensity
binding energy / eV
1.7 V2 Eoc,3 1.9 V Eoc,4 O 1s450 eV KE
536 534 532 530 528
norm. XPS intensity
binding energy / eV Eoc,1 1.7 V1 1.7 V1 Eoc,1 O 1s450 eV KE
Figure S4.6.26: O 1s signals of Ir-coated Nafion®117 (60 s Ir sputtered, sample 23898) recorded in the three-electrode cell with the indicated potentials vs. SHE applied (p=0.45 Pa, 0.1 M H2SO4).