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The active site for the water oxidising anodic iridium oxide probed through in situ Raman spectroscopy

Zoran Pavlovic , Chinmoy Ranjan *, Maurice van Gastel and Robert Schlögl
Department of Heterogeneous Catalysis, Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, Muelheim and Ruhr, 45470, Germany. E-mail: chinmoy.ranjan@cec.mpg.de

Received 25th August 2017 , Accepted 9th October 2017

First published on 9th October 2017


The structure of anodic iridium oxide (IrOx) under water oxidation was explored using in situ Raman spectroscopy and theoretical calculations. Isotopic substitution experiments and theoretical calculations confirmed formation of an Ir[double bond, length as m-dash]O species during oxygen evolution reaction.


The oxygen electrode remains a central challenge in implementing a hydrogen-based economy.1,2 IrOx remains one of the most important catalysts for water oxidation reaction.3 Dimensionally stable anodes (DSA) are composed of primarily IrO2.4–8 These materials require being treated through activation protocols before they can be implemented. IrOx materials were shown to be very active by Mallouk et al.9 In the alkaline media, Mallouk et al. proposed that these materials are unstable and form molecular species that show oxygen activity.10 In fact, the activity shown by these materials in alkaline electrolytes was much higher than in acidic or neutral electrolytes. We have recently characterized the active IrOx material through ex situ Raman spectroscopy on anodic IrOx grown on an iridium foil.11 Attempts to understand the mechanism of OER on IrOx has encouraged operando studies using XPS and XAS.12,13 These investigations focused on an Ir-centered approach to understanding the nature of the active catalyst. XPS studies observed an additional Ir 4f peak at higher binding energy (+0.7 eV) during OER and suggested an Ir5+ species forming during the catalytic event.12 Minguzzi et al. suggested the formation of both Ir3+ and Ir5+ during OER. But both these studies do not compare the shifts to standard Ir-containing compounds. High oxidation states (+5 or greater) are only known for fluorine-containing molecules amongst the stable Ir compounds or in IrOx molecular species isolated within a noble gas matrix. Reier et al. and Pfeiffer et al. observed an additional feature at 529 eV in the O-Kedge NEXAFS spectra of highly OER active Ir–NiOx and amorphous IrOx respectively. Using DFT calculations, they have suggested this to be an electrophilic oxygen species present in these materials.14–16 By the nature of their inquiry, these studies cannot clarify the exact chemical nature of the IrOx active site which is possible through vibrational spectroscopy. Mo et al. carried out in situ Raman spectroscopy on IrOx/Au system but essentially used XAS data for interpretation of results.17 Frei and coworkers have investigated IrOx clusters as OER catalysts using rapid scan FTIR under visible light.18 They reported detection of a transient Ir–OOH species with O–O vibration being detected at 830 cm−1. This work goes beyond work reported by Pavlovic et al. and uses in situ Raman spectroscopy to understand the behavior of these materials under conditions of OER.11 Specifically we focus on active site that forms in situ. We suggest a Ir[double bond, length as m-dash]O species, with five coordinated square pyramidal Ir as the active site for water oxidation and suggest a mechanistic pathway for product formation and replenishment of the active site.

Cyclic voltammogram of IrOx materials in different electrolytes is well known (Fig. S1, ESI). All the potentials reported in this paper are referenced to the reversible hydrogen electrode (RHE). The material was electroprecipitated at 1.5 V onto a Au substrate as shown by Mallouk et al.9 In our previous publication we have shown that the anodic Iridium oxide is essentially a hydrous material with OH and H2O groups playing important role in its overall structure.11 Experiments of IrOx/Au were conducted in an in situ Raman cell, using Biologic VSP-300 potentiostat. Raman spectra were collected with Ocean Optics QE65 pro spectrometer using 785 nm Laser source. The Laser intensity was maintained at 500 mW per 0.1 mm2 at the sample (collection time of 10 s). The potential was scanned from 0.4–1.8 V, at steps of 0.1 V.

Results of the in situ Raman experiment are shown in Fig. 1. More details of the experiments are provided in the ESI. Peaks at 225 and 324 cm−1 originate from the Au surface. The peaks from 450–780 cm−1 originate from iridium oxygen vibrations, where peak assignments have been used from Pavlovic et al.11 Electrochemically, four regions (R) in the IrOx CV can be clearly identified. R1(E < 0.4 V), R2 (0.4 < E < 0.7 V), R3 (0.7 < E < 1.2 V) and R4 (E < 1.4 V). Whereas R1 corresponds to a colorless compound rich in Ir3+, R2 represents the region of first oxidation wave from Ir3+↔ Ir4+. At the beginning of the experiment at OCP, there is a certain peak structure (465, 527, 600, 702 cm−1), which can be attributed to the Ir–O–Ir stretch vibrations of μ-oxo type from the basal plane of edge sharing IrO6 octahedra.11 As soon as the potential of 0.4 V is applied, a sharp peak at 520 cm−1 is formed. This is the most dominant peak δ in region R1. Region R1 has no ε peak which appears in R2. The ε peak loses intensity in R3. R4 shows significant drift of peaks γ → ε towards higher frequencies. At potentials 1.3 V and higher, AuOx is formed (Fig. S4, ESI). Thus peaks in R4 have substantial AuOx contribution to them. The final peaks observed at OCP in the treated materials are composed of both IrOx and AuOx peaks. The peak η at 771 cm−1 is seen in R4. This peak disappears as soon as the applied potential is switched off. This peak does not exist in experiments done on a bare Au surface and thus must be assigned to IrOx material under conditions of water oxidation. Isotopic substitution using D2O and H2O18 as synthesis and reaction media were also carried out. Results have been tabulated in Table 1. Fig. 2 shows effects of isotopic substitution on η. In the experiment that has 50[thin space (1/6-em)]:[thin space (1/6-em)]50 (H2O[thin space (1/6-em)]:[thin space (1/6-em)]H2O18), only two peaks of nearly equal intensity were visible. Thus this peak could be assigned to Ir[double bond, length as m-dash]O vibration (as opposed to any other species that contained O–O, or OH vibration). If O movement is the primary oscillator in this vibration, then one can expect a shift of image file: c7cc05669a-t1.tif upon O18 substitution. Deuterium substitution resulted in almost no shift of the peak (Δf ∼ 5 cm−1). For Ir–O–OH containing groups, the shift reported by Frei et al. due to deuteration was 30 cm−1.18 Small shifts due to deuteration are possible through altered hydrogen bonding contributions from the solvent. Vibrations where the motion of the hydrogen atom explicitly contribute to the vibrational mode show significant alteration of frequency upon deuteration due to the large mass change image file: c7cc05669a-t2.tif. Fig. S5 (ESI) shows the results from various experiments with combinations of H2O, D2O, and H2O18 containing electrolytes. Fig. 2, shows η peak at OER potentials in variously labeled electrolytes. Fig. S5(d) (ESI) shows the results of experiments when material was synthesized in H2O18 and tested in H2O.16 Peaks γ to ε begin to split into two sets of peaks in the beginning of R3 indicating that material exchanges oxygens with the electrolyte even at potentials as low as 0.7 V. Even the position of η peak only shows the contribution from O16 isotope.


image file: c7cc05669a-f1.tif
Fig. 1 (a) In situ Raman spectroscopy of IrOx/Au system in 0.1 M NaOH. The peaks labeled by greek letters originate from Ir–O stretch vibrations. Rest of the peaks can be assigned to Au substrate (ESI, Section 2). η peak is only visible during the process of OER and disappears with removal of applied potential. Peak assignments (in greek letters) have been borrowed from Pavlovic et al.11 (b) CV of IrOx/Au in 0.1 M NaOH (c) Average currents from in situ experiments in various electrochemical media.
Table 1 Peaks obtained in the Raman spectra of IrOx/Au labeled at 0.8 V. The peaks tend to move with applied potential and thus their exact location can be described only at a certain potential. The Ir–O stretching assignments have been borrowed from Pavolovic et al.11
Peak labels H2O D2O H2O18 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (H2O[thin space (1/6-em)]:[thin space (1/6-em)]H2O18) H2O18 tested in H2O
a Collected at 1.7 V.
γ 476 475 423 454, 482 430, 458, 480
δ 527 554 476 541, 482 554
ε 595 602 554 605, 552 608
ηa 771 767 712 722, 771



image file: c7cc05669a-f2.tif
Fig. 2 Raman spectra of the η peak in various isotopically labeled electrolytes. The peaks are background corrected and normalized (details in Experimental section).

Theoretical calculations using density functional theory were carried out to better understand the IrOx system (Fig. S6–S8, ESI). Specifically, calculations were performed to understand the identity of the Ir[double bond, length as m-dash]O species. A trimeric IrO6 unit was used to construct a model system for calculation. It was found that an Ir–O unit on the central Ir atom resulted in the formation of a square pyramidal central Ir atom. [Ir4+–Ir4+–Ir4+] trimeric IrOx cluster was used for computation (Fig. 3). The Ir–O vibration was computed to be at 829 cm−1. Furthermore it was found that the vibrations could be significantly modulated through hydrogen bonding near the “Ir–O” e.g. the Ir–O vibration was reduced to 766 cm−1 from 829 cm−1 by placing a H2O molecule near the Ir–O moiety. Besides, the relative intensity of this vibration was found to be very strong compared to the other vibrations within this molecule. We believe at the optimum potential some of the [IrO6]n species turn into the OER active Ir[double bond, length as m-dash]O species that removes any further oxidative strain from the material. Even though present in much less quantity, the extremely high Raman activity of the Ir–O stretch vibration makes it possible for the active species to be detected within a matrix of [IrO6]n containing material. The 771 cm−1 peak can also be detected in acidic media but has a much smaller intensity (the OER activity of the material in acidic and neutral media is much less than alkaline media (Fig. S2, ESI)). This is likely because the material is highly condensed in the acidic and neutral media and not many exposed active sites containing Ir[double bond, length as m-dash]O species form, which gets reflected in the overall activity of the material. Ir–OOH species (Fig. S7, ESI) were also computed. The frequency of vibration of O–O stretch was 715 cm−1, besides the O–O stretch Raman intensity was much smaller. In light of our experimental and computational investigations we assign the observed 771 cm−1 vibration to Ir[double bond, length as m-dash]O stretch of a square pyramidal Ir site. The frontier orbitals of IrOx were calculated and have been visualized in Fig. 3. The SOMO (Fig. 3(b)) shows that Ir–O unit is the most reactive part of the molecule with a major part of the molecular orbital localised in this region. The frontier orbitals are anti-bonding in nature between Ir-d and O-p with significant localization on O atom making it a suitable target for reaction. We believe this oxygen may be responsible for making the important O–O bond during the formation of molecular O2 from water (Fig. 4). M–O species responsible for oxidizing water are common in literature.19 In case of Mn-containing Photosystem II catalyst, the O–O bond formation either happens through a nucleophilic attack at the Mn–O center (Mn5+–O, Mn4+–O or Mn4+–oxyl) species or by an oxyl–oxyl radical coupling mechanism.20–22 But the exact nature of the species is debatable. It has been suggested in the literature that an electrophilic oxygen at the active site is essential to make reactions like O–O bond formation and C–H bond activation, possible.16,23,24 Wang et al. and Concepcion and coworkers suggested an O–O bond formation using nucleophilic attack on Ru[double bond, length as m-dash]O site.25,26 Using DFT based theory, Mai et al. suggested that C–H activation essentially consists of two parts, an Fe4+[double bond, length as m-dash]O species which was a precursor to Fe3+–O˙ oxyl species that carried out the hydrogen atom transfer.23 Similarly, using DFT calculations Yang et al. have argued that Ru5+[double bond, length as m-dash]O can act as a precursor to Ru4+–O˙ oxyl species which carries out water O–O bond formation. For the IrOx cluster shown in Fig. 3, the total Löwdin charge at the oxygen atom amounts to only −0.12 (as compared to −0.35 for the bridging oxo-s) and the Löwdin spin population at the central Ir and oxo amounts to 47% and 29%, respectively. Semantically, although this charge distribution would be compatible with a metal-oxyl (O˙) species, the spin distribution rather indicates a much lower spin population at the oxygen (29%) than expected for a radical species (∼100%). This is the result of a somewhat covalent Ir–O π-interaction that involves the spin carrying dxz orbital (t2g in octahedral symmetry) at Ir (ESI). We believe what we see in our experiment, is the structure that is precursor to the reactive oxyl species as proposed by Mai and Yang et al.23,24 The observation of a reactive oxyl radical species under experimental conditions remains difficult, specifically at room temperature. Herlihy et al. observed an oxyl species on TiOx system using in situ ultrafast infrared spectroscopy.27 Based on our data, we claim an Ir–O species that can in principle act as a precursor to a highly electrophilic oxyl species that reacts with water (or dissociated water) to form an O–O bond.


image file: c7cc05669a-f3.tif
Fig. 3 Theoretical calculations were carried out on IrOx species containing with “Ir–O” moiety. The calculations resulted in an Ir–O species that was essentially square pyramidal at the Ir center. (Top) Ir4+ containing species with the central atom having the Ir–O bond. The oxygen atom has been labeled with a O* (Top). (bottom) SOMO of the species.

image file: c7cc05669a-f4.tif
Fig. 4 Proposed active species responsible for water oxidation in IrOx materials (alkaline media). Suggested mechanism of oxidation of water and regeneration of the active site is shown. The oxo species is likely to go through an oxyl type species before the O–O bond is formed.

The scaffold enabling the OER is derived from a highly hydrous [IrO6]n species (of the type IrOx(OH)y(H2O)z), where each Ir atom is octahedrally coordinated by oxygens. Whereas the colour and vibrational spectroscopy of most of this material can be traced using ex situ spectroscopy, the formation of an active a square pyramidal “Ir–O” species is detectable only through in situ spectroscopy under applied potential. We propose that this Ir–O species is the active site within the molecule which is open to attack from H2O molecules leading to formation of O2 through formation of an O–O bond as shown in Fig. 4. The OER chemistry is likely to proceed primarily in the ligand sphere.

The authors thank BMWi for supporting this work through grant 03ESP106D (EKOLYSER). Open Access funding provided by the Max Planck Society.

Conflicts of interest

There are no conflicts to declare.

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Footnote

Electronic supplementary information (ESI) available: Experimental details, cyclic voltammograms, Raman spectra in various electrochemical media and computational details. See DOI: 10.1039/c7cc05669a

This journal is © The Royal Society of Chemistry 2017