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Supplementary Information

Activation of surface oxygen sites on an iridium-based model catalyst for the oxygen evolution reaction

Alexis GRIMAUD,1,2* Arnaud DEMORTIERE,2,3 Matthieu SAUBANERE,1,2,4 Walid DACHRAOUI,2,3 Martial

DUCHAMP,5 Marie-Liesse DOUBLET2,4 and Jean-Marie TARASCON1,2,6,7

1. Chimie du Solide et de l’Energie, UMR 8260, Collège de France, 75231 Paris Cedex 05, France 2. Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 80039 Amiens Cedex, France 3. Laboratoire de Réactivité et Chimie des Solides, UMR CNRS 7314, 33 Rue Saint Leu, 80039 Amiens Cedex, France 4. Institut Charles Gerhardt, CNRS UMR 5253, Université Montpellier, Place E. Bataillon, 34095 Montpellier, France 5. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons (ER-C) and Peter Grünberg Institute (PGI), Forschungszentrum Jülich, 52428 Jülich, Germany 6. ALISTORE-European Research Institute, FR CNRS 3104, 80039 Amiens, France 7. Sorbonne Universités – UPMC Univ Paris 06, 75005 Paris, France

Corresponding author: [email protected]

Activation of surface oxygen sites on aniridium-based model catalyst for the oxygenevolution reaction

SUPPLEMENTARY INFORMATIONARTICLE NUMBER: 16189 | DOI: 10.1038/NENERGY.2016.189

NATURE ENERGY | www.nature.com/natureenergy 1

http://dx.doi.org/10.1038/nenergy.2016.189

http://www.nature.com/natureenergy


Supplementary Methods

X-ray absorption spectroscopy

XPS measurements were carried out with a Kratos Axis Ultra spectrometer, using a focused

monochromatized Al Ka radiation (hn=1486.6eV). The XPS spectrometer was directly connected

through a transfer chamber to an argon dry box, in order to avoid moisture/air exposure of the

samples. For the Ag3d5/2 line, the full width at half maximum (FWHM) was 0.58eV under the

recording conditions. The analysed area of the samples was 300*700 µm². Peaks were recorded with

a constant pass energy of 20 eV, and 160 eV for the survey spectra. The pressure in the analysis

chamber was around 5.10-9 mbar. Short acquisition time spectra were recorded before and after

each normal experiment to check that the samples did not suffer from degradation during the

measurements. The binding energy scale was calibrated from the hydrocarbon contamination using

the C1s peak at 285.0 eV. Core peaks were analysed using a nonlinear Shirley-type background1. The

peak positions and areas were optimized by a weighted least-squared fitting method using 70%

Gaussian, 30 % Lorentzian lineshapes. Quantification was performed on the basis of Scofield’s

relative sensitivity factors2.

Elastic constants calculation

The elastic constants are calculated based on the equilibrium structure as proposed in Vasp3 and

analyzed within the ElAM code.4 Essentially, the strains are applied to each independent direction,

data are then collected from the fully relaxed structure under each strain and the nine elastic

constants are extracted. From the calculated elastic constants of the crystal, it is possible to extract

the bulk structure parameters: bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio

within the Voigt–Reuss–Hill approximation.5


SUPPLEMENTARY INFORMATIONDOI: 10.1038/NENERGY.2016.189




Supplementary Discussion

Determination of Ir5+/Ir4+ redox couple in Figure 1.

In the literature, McCalla et al6 reported Li2IrO3 phase that shows a Ir5+/Ir4+ mostly cationic redox at

3.4 V vs Li+/Li when cycled in organic solvent against lithium metallic. In aqueous media, Lyons et al7

reported the extensive study of hydrous IrOx surfaces for which a redox associated with Ir5+/Ir4+

couple can be measured at pH 14 in 1 M NaOH at 0.2 V vs. SCE, corresponding to 3.5 V vs. Li+/Li after

rescaling.

Calculation of oxygen release in Supplementary Figure 11.

6.18905 10-8 mol O2(g) is produced during the oxidation, corresponding to 2.47562 10-7 mol of e-

following the reaction: 2 O2- + 4e- O2(g)

With a charge of 1.5386 C passed during the oxidation, corresponding to 1.5943 10-5 mol of e-, only

1.55 % of the electron passed during the charged produces oxygen.

In total, the CO2 formation (1.53 10-7 mol) accounts for 2% of the total charge if assuming a 2e-

oxidation process from carbonate groups. This demonstrates that the majority of the charge is

indeed related to the bulk delithiation.

One can then conclude that only the surface of the La2LiIrO6 sample degasses, and that a cationic

Ir5+/Ir6+ redox triggered by the delithiation is at play in the bulk, consistent with the XRD patterns and

the DFT calculation in Figure S1 showing the stability of the phase upon oxidation.

The CO2 found upon charge at a potential above 4.5 V come from the decomposition of the

carbonate electrolyte via a nucleophilic attack of the carbonate on the oxidized surface oxygen,

breaking the carbonates to form CO2. An average ratio of 1:2 O2:CO2 is found at a potential higher

than 4.5 V. Therefore, we postulate that the direct pairing of surface oxidized oxygen is less favorable

that the nucleophilic attack of carbonates on these surface oxidized oxygen that are electrophilic.






Supplementary Table 1: Structural parameters measured and obtained by DFT for the pristine and

the charged sample at C/20 as shown in Figure S1.

XRD La2LiIrO6 XRD La2Li1-xIrO6 S.G. P 21/c P 21/c a (Å) 5.5588(5) 5.5633 b (Å) 5.6215(2) 5.5785 c (Å) 7.8586(9) 7.7460 (°) 90.18(2) 90.88

Supplementary Table 2: Relaxed cell parameters for La2LiIrO6 and La2IrO6 as a function of the Ueff

parameter for DFT+U calculations performed with and without spin-orbit interactions. These

calculations show almost no variation of the parameters depending on the Ueff parameter and the

use of spin orbit interactions.

La2LiIrO6

La2IrO6






Supplementary Table 3: Atomic percentage obtained from XPS results for the pristine La2LiIrO6

sample compared to La2LiIrO6 after oxidation at 1.65 V vs. RHE for 10min and after 25 cycles in H2SO4

solution at pH 1.

O 1s C 1s Ir 4f La 4d Li 1s F 1s S 2p Pristine 44.44 36.85 3.86 11.75 3.1 - - 1.65 V 19.73 30.98 2.38 0.54 - 43.23 2.47 25 cycles 20.07 31.22 2.67 0.34 - 42.72 2.41

Supplementary Table 4: Ir concentration detected by ICP after testing in H2SO4 solution at pH 1, as

well as the percentage of dissolved Ir and the corresponding thickness of La2LiIrO6 from where Ir is

dissolved. (Electrodes were introduced at 1.2 V vs. RHE, the loading is 500g, the volume of

electrolyte is 50mL and the particle size is 460 nm as estimated from SEM images and the BET surface

area assuming spherical particles, the glassware was cleaned in H2SO4 for 10min followed by boiling

in deionized water for 20min in between each measurements.

[Ir] ppb %Ir dissolved ngIr/cm² Thickness (nm) 1.6 V for 20 min 124 ±3 3.7 729 9.7 1.8 V for 20 min 1363 ±9 40.6 8018 76.7

5 mA/cm² for 20 min 148 ±5 4.4 871 10.8 5 mA/cm² for 1 hour 111 ±2 3.3 653 9.1 25 cycles up to 1.7 V 147 ±3 4.4 865 10.8






Supplementary Figure 1: a) Voltage profile for La2LiIrO6 and La2LiRuO6 in LP30 at C/10. Note that the

reversibility is very limited and only ≈ 0.05-0.1 Li+ can be reinserted in reduction. However, a redox

potential of about 4.4 V vs Li+/Li can be observed. b) XRD patterns for the pristine La2LiIrO6, the

chemically oxidized with NO2BF4 and the electrochemically charged at C/20. No modifications are

observed after oxidation. The peak at 44 ° observed for the chemically and electrochemically oxidized

samples corresponds to the sample holder. This demonstrates the stability of the perovskite

structure upon oxidation. Structures obtained by DFT calculation for the c) pristine La2LiIrO6 and d)

oxidized La2IrO6, showing no strong modifications of the structure after Li+ removal. The lattice

parameters obtained are given in Table S1.






Supplementary Figure 2: Density of States (DOS) in arbitrary units of the lithiated La2LiIrO6 and

delithiated La2IrO6 compounds and Fukui functions (f-) computed within the DFT+U framework with

Ueff = U – J = 0, 2 and 4 eV and J = 1eV. As shown on this picture, neither the DOS nor the Fukui

function are significantly affected by the change in the Ueff parameter, except for the lithiated

La2LiIrO6 phase computed at Ueff = 0eV which shows that the orbitals involved in the oxidation

process is a mixture of more than one « t2g »-like metallic orbital, in full agreement with the over-

delocalization of the electrons expected from the self-interaction error of the DFT formalism and

with the subsequent absence of gap in the electronic structure. The activation of the oxygen

electronic levels in further oxidation process from the delithiated La2IrO6 phase is fully confirmed,

irrespectively of the Ueff value.






Supplementary Figure 3: Cyclic voltammetry at 10 mV/s in 0.5 M H2SO4 for La2LiRuO6. It clearly

demonstrates the non-reversibility of the oxidation process, most probably arising from RuO4-

solubility alike encountered for RuO2 under the same conditions.






Supplementary Figure 4: Cycling behavior for La2LiIrO6 on carbon paper support with a loading of

500g for 50 cycles in 0.5 M H2SO4 at 10 mV/s in between 1.1 and 1.7 V vs. RHE. XRD patterns for the

cycled sample compared to the pristine sample. No clear modification is observed after cycling. The

peak at around 26° corresponds to the carbon gas diffusion layer used as a support.






Supplementary Figure 5: First cycle for La2LiIrO6 in 0.1 M H2SO4 at 10 mV/s in between 0.5 and 1.7 V

vs. RHE compared with a first cycle in between 0.5 and 1.4 V vs. RHE. An oxidation is clearly observed

starting at around 1.45 V vs. RHE, corresponding to ≈ 1.4 V vs. NHE and 4.5 V vs. Li+/Li.






Supplementary Figure 6: First cycle for La2LiIrO6 in O2-saturated 0.1 M KOH (pH 13) at 10 mV/s

compared to a cycle recorded under the same conditions after activation process of 10 cycles in 0.1

M H2SO4 solution. This demonstrates that the surface activation process occurs when the surface can

be oxidized, and that the new surface created after oxidation becomes active at pH 13, alike IrO2. It

then becomes clear that the key parameter governing the OER activity for this type of phase is the

creation of an hydrated surface through its oxidation.






Supplementary Figure 7: HRTEM micrograph of La2LiIrO6 after 5 cycles in 0.1 M KOH showing the

crystalline surface and no formation of IrO2 nano particles. Nevertheless, an electrochemical grinding

can be seen and the particle size is largely decreased after OER measurement in 0.1 M KOH.

Interestingly, this decrease of the particle size after OER measurement doesn’t go with an increase of

the OER activity. This further demonstrates that the amount of surface active sites is not primordial

for the OER activity to increase, but the nature of these sites and more specifically the oxidation and

hydration of the surface is required for the OER to proceed on such surface.






Supplementary Figure 8: OER Tafel plots comparing the surface specific activity (top figure) and mass

activity normalized to the mass of platinum group metal (bottom figure) La2LiIrO6 measured in this

work with different transition metal oxides catalysts measured under acidic conditions in the

literature (RuO2 sputtered,8 IrO2 film,9 Ru0.7Ni0.3O2-,10 Ru0.33Ni0.67O2-x film,11 IrO2 and RuO2

nanoparticles12 and 0.1 x 106 u RuO2 NP13 and Ba2NdIrO614) as well as with the perovskite SrIrO3 film

tested in alkaline conditions.15






Supplementary Figure 9: Particle size distribution for the IrO2 nanoparticles obtained from HRTEM

images and observed on the La2LiIrO6 surface after a) 25 and b) 50 cycles.






Supplementary Figure 10: High resolution image for the 50 cycles sample with FFT image for two

different areas. The HRTEM image of 50 cycles sample exhibits IrO2 nanocrystals localized at the

surface and the reconstructed bulk crystal. The FFT image of the blue area (Figure S10) reveals the

spots of the initial lattice (blue rectangular) but mostly reveals additional spots of substructure (red

rectangular), which are associated with the super-cell. The schematic in the Figure S10 shows the

original cell (blue) and the super-cell (red) after the diffusion of iridium atoms from the bulk structure

to the surface. The HRTEM simulation is based on this super-cell. On the other side, the green zone

FFT image shows a similar pattern except for two spots (red arrows) that are related to the presence

of IrO2 nanocrystals.






Supplementary Figure 11: DEMS data obtained for La2LiIrO6 with a charge at C/10 in EC:DMC (1:1)

1M LiPF6 on the upper panel and the corresponding gas evolution with O2 and CO2 detection. The

spark of CO2 observed at the beginning of the charge is characteristic when using carbon with the

active material.16 At a potential above ≈ 4.5 V Vs Li+/Li, O2 starts to be detected. Note that at that

potential a part of the charge is also coming from the electrolyte decomposition and traces of CO2

are constantly observed.






Supplementary Figure 12: STEM-HAADF image and STEM-EDX mapping (La, Ir, O) of the pristine

La2LiIrO6 showing the perfect cationic distribution from the bulk to the surface.






Supplementary Figure 13: STEM-HAADF image and STEM-EDX mapping (La, Ir, O) of La2LiIrO6 after 50

cycles in 0.1 M H2SO4 showing the surface which is rich in Ir and depleting in La. This is explained by

the oxygen loss and Ir migration occurring during the oxidation/delithiation.






Supplementary Figure 14: XPS results for La2LiIrO6 with, from top to bottom, the Ir 4f spectra

collected for the pristine sample and the sample oxidized at 1.65 V vs RHE for 10min at cycled 25

times in H2SO4 solution at pH 1.The shift to lower binding energy observed for the samples after OER

measurements demonstrate the reduction of Ir5+ to lower oxidation state as observed by XAS in

Figure in the manuscript. XPS results coupled with TEM observations demonstrate the oxidation

process occuring on the surface of La2LiIrO6 when cycled in acidic conditions where the surface is

found depleted in Li+ following the delithiation process but also from La as also observed by TEM.

Moreover, as shown by XAS, Ir is reduced under these oxidative conditions following the surface

oxygen loss and the surface reconstruction process.






0 2500 5000 7500 100000.0

0.2

0.4

0.6

0.8

1.0

Capa

city

(C)

Time (s)

Supplementary Figure 15: Top figure: evolution with cycling of the capacity measured during 50

cycles for La2LiIrO6 in 0.1 M H2SO4. Bottom figure: evolution of the OER potential measured for 20

minutes at 5 mA/cm² applied and of the OER current measured for 20 minutes at 1.6 V vs. RHE in

H2SO4 solution at pH 1 vs. capacity. For these three measurements, the total capacity of the

electrode is 8.3 mC. Even though a rotation speed of 1600rpm was used, large bubbles were formed

during the measurements. The drops observed for the potential and the current are due to bubbles

forming on the surface of the electrode that lead to an increase of the overpotential (or a decrease

of the OER current).






Supplementary Figure 16: A series of high-resolution TEM images simulated using the Dr. Probe

software for different Ir occupancies. The schematic of La2LiIrO6 structure shows red, green and blue

atomic columns for which specific Ir occupancies were used (0%, 50% and 100%) with different

combinations. To get a proper comparison, the simulations were made with the same HRTEM

conditions, i.e. a fs defocus value of 20 nm and a Th thickness value of 5.43 nm.






Supplementary Figure 17: Experimental HRTEM image with dark patches. The schematic of La2LiIrO6

structure showing hexagonal pattern of oxygen atomic columns for which the occupancy was

changed. Simulations were made with an Ir atomic occupancy combination of 0%-50%-100% as

determined Fig. S16 and an oxygen occupancy of 100% and 0%, respectively (fs = 20 nm and Th =

5.43 nm).

Even though the simulated images cannot fully reproduce the experimental image, this study shows

that oxygen vacancies can form dark patterns around Ir deficient sites. However, more remains to be

done to fully assign these patterns to the formation of oxygen vacancies and cautious should be

taken at this point of the study.






Supplementary Figure 18: Computed bulk modulus (BM, Young modulus (YM for La2LiIrO6, La2IrO6

and IrO2, demonstrating a greater bulk modulus for IrO2 than for the perovskite and explaining the

deformation observed in delithiated La2IrO6 after oxidation and formation of IrO2 nanoparticles.






Supplementary Figure 19: Additional HRTEM image showing the dislocations formed on the 50 cycles

sample (white circles).






Supplementary Figure 20: Cyclic voltammetry for La2LiIrO6 in 0.5 M H2SO4 with different Li2SO4

concentration (0, 10, 50, 100 and 500 mM). No substantial difference in the OER current is observed,

demonstrating that Li+ is loss from the surface and doesn’t further participate to the mechanism.

Instead, the surface is protonated, forming an hydrated surface in acidic media that is primordial for

the OER to proceed.

1.2 1.4 1.60

50

100

150

200

250

300

i (A/

g)

E - iR (V vs. RHE)

0 to 500 mM Li2SO4






Supplementary References

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3. Le Page, Y. & Saxe, P. Symmetry-general least-squares extraction of elastic data for strained materials from ab initio calculations of stress. Phys. Rev. B 65, 104104 (2002).

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8. Frydendal, R. et al. Benchmarking the Stability of Oxygen Evolution Reaction Catalysts: The Importance of Monitoring Mass Losses. ChemElectroChem 1, 2075–2081 (2014).

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12. Lee, Y., Suntivich, J., May, K. J., Perry, E. E. & Shao-horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. (2012).

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