Article pubs.acs.org/JPCC
Suitability of Simplified (Ir,Ti)Ox Films for Characterization during Electrocatalytic Oxygen Evolution Reaction B. Johnson,* F. Girgsdies, G. Weinberg, D. Rosenthal, A. Knop-Gericke, and R. Schlögl Fritz-Haber-Institut, Berlin, Germany
T. Reier and P. Strasser Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Berlin Germany ABSTRACT: Simplified IrOx electrodes lacking typical mud crack structure have been produced on polycrystalline Ti cylinders with spin-coating using an iridium acetate solution and are compared to thicker samples in terms of stability, composition, and suitability as a model system. The spin-coating process forms smooth, thin islands of IrOx with limited cracking and decreases the surface-to-bulk ratio to allow a more intimate study of the growth, composition, and stability of the layer without the complications of the mud crack morphology. XPS and XRD measurements show a resulting (Ir,Ti)Ox surface (x near 2) with OH− groups and H2O. Cyclic voltammetry measurements indicate the expected high catalytic activity for the oxygen evolution reaction as well as a dry IrOx phase resulting from the thermal manufacturing process, although evidence of hydrous phases are found in XPS. Both films required only small overpotentials for the oxygen evolution reaction, with the spin-coated sample showing a slightly lower activity. CO temperature desorption spectroscopy analysis showed CO → CO2 oxidation and, in combination with XPS, an unstable surface. The oxidation of CO was not due to the TiOx, and the absence of any evidence of an Ir-suboxide phase indicates the presence of near-surface active species present after synthesis or an active surface termination.
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INTRODUCTION
Generally, the surface of such electrodes possesses the wellknown mud crack film morphology2,6,7 obtained by painting or pipetting an Ir-containing solution onto substrates. Here we modify the method of film preparation by making layers with smoother islands of IrOx tens of nanometers thick deposited with spin-coating using an iridium acetate (IrOAc) solution. Thick layers (micrometer range) with dimensions similar to standard mud crack films but lacking the ubiquitous cracks and fissures were also synthesized as a comparison although most of the emphasis in this study is on the characterization of the spincoated samples. Like the mud crack films, both films here also show a compressed powder nanostructure in SEM.8 The synthesis procedure of the spin-coated films aims at investigating the stability of samples lacking the mud crack morphology and moves toward simplification of the surface to obtain results which can be more easily and accurately interpreted than those from the mud crack samples. That is, we attempt to bridge the gap between model systems and the more complex systems found in the literature. Eventually, we hope to obtain results for samples that can be described approximately by a simple layer system, as this will be of great value when comparing experimental results with theoretical models which still remain comparatively simple. However, this
Electrocatalytic water splitting represents the possibility of a boundless source of CO2-free energy if the power needed to drive the reaction can be harnessed from renewable energy sources. In order to increase the efficiency of the entire process, an essential step is to improve the oxygen evolution reaction (OER) at the electrode/electrolyte interface which constitutes the anodic half-cell reaction in water splitting and is the origin of the main efficiency losses. This can only be achieved through a better understanding of the reaction itself, the key to which is an increased knowledge of electron transport through the electrode layer in addition to the electron transfer at the catalytically active surface. For this we must understand the composition of the entire electrode layer including the interplay between substrate and top layer (electrode). In this study we investigate the structure of a simplified IrOx thin film electrode and focus primarily on the substrate/ electrode interface, the composition of the interface-near region, and the efficacy of surface characterization on such systems. IrO2 and RuO2 are the most common transition metal oxide electrodes for the electrocatalytic oxygen evolution reaction in acidic environments. Both are often found in various mixed forms with TiO2, which is often used as a carrier material.1,2 While the overpotential needed for the OER is known to be higher for IrO2 than for RuO2, the IrO2 films show higher stability and longer lifetimes.1,3−5 © 2013 American Chemical Society
Received: May 16, 2013 Revised: October 28, 2013 Published: November 21, 2013 25443
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Electrochemical measurements were performed in a threeelectrode setup with a Luggin capillary using a mercury/ mercury sulfate reference and a platinum mesh counter electrode. All potentials in this work are given with reference to the reversible hydrogen electrode against which the mercury/mercury sulfate electrode was calibrated. The 0.1 M HClO4 electrolyte was prepared from 70% perchloric acid (Sigma-Aldrich, 99.999%) using deionized water (18 Mohm cm at room temperature). The working electrodes were embedded into a custom-made PEEK sample holder and connected to a rotating disk electrode setup (Pine Research Instrumentation) with the potential controlled by an SP-200 potentiostat (BioLogic). In the electrochemical protocol the electrode was immersed at 1.00 V into the nitrogen degassed electrolyte followed by an impedance measurement applied for ohmic resistance determination. Subsequently, 53 potential scans into the OER potential region corrected for the electrolyte’s ohmic drop were performed starting from 1.00 V. A scan rate of 6 mV/s was applied for the first and the last scan; all intermediate scans were conducted at 200 mV/s. Thereafter, 10 chronoamperometric potential steps in the OER potential region were measured holding at each potential for 5 min before being increased 15 mV for the following step. At each step impedance spectroscopy was applied for ohmic resistance determination. Furthermore, cyclic voltammetry was measured between 0.4 and 1.4 V at 500, 50, and 20 mV/s in a freshly nitrogendegassed electrolyte for electrochemical surface characterization. The OER scans were measured with an electrode rotation rate of 1600 rpm, and the cyclic voltammetry was measured at 0 rpm.
is a difficult task because although lateral homogeneity is desired for more accurate interpretation of experimental results, the catalytic activity of oxide films depends on the presence of defect structures and, thus, the lack of surface homogeneity. Creating an adequate system will be an iterative process, and emphasis has, therefore, also been placed on assessing the efficacy of our characterization methods and not only the interpretation of the results themselves.
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SAMPLE PREPARATION The thick samples were prepared on planar polycrystalline Ti cylinders polished to a final roughness of 0.02 μm with a diamond suspension followed by a silica suspension. 12.52 mg of IrOAc (Heraeus) was dissolved in 2 mL of isopropanol, and seven layers were pipetted onto the Ti cylinder using 40 μL of the solution per layer. After every cycle the sample was dried at 80 °C for 10 min, calcined at 480 °C for 10 min, and then cooled for 15 min. After the final layer was applied the entire sample was tempered in a tube furnace by ramping up to 480 °C at 2 °C/min and dwelling for 60 min under a constant flux of synthetic air. The spin-coated samples were prepared on similar Ti cylinders using a solution of 40 mg/mL IrOAc (Chempur) in dry ethanol. The solution was dripped onto the spinning Ti substrate in a spin-coating device rotating at 3000 rpm. After deposition, the samples were calcined in a tube furnace under O2 (30 nmol/min)/Ar (120 nmol/min) by ramping up to 480 °C at 2 °C/min and dwelling for 60 min. The prepared samples were transported in air before analysis.
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EXPERIMENTAL SECTION The samples were characterized with X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature desorption spectroscopy (TDS), cyclic voltammetry (CV), and scanning electron microscopy (SEM). The XPS/TDS chamber with a base pressure of ∼8 × 10−10 mbar was equipped with a Phoibos 150 MCD-9 hemispherical electron analyzer from Specs; measurements were performed using the Al Kα anode and calibrated with Au 4f at 84.0 eV. The TDS measurements were made using a Prisma quadrupole mass spectrometer (QMS 200) from Balzers housed in the same chamber so that the XPS analysis could be performed before and after the TDS measurements without further disturbance of the sample. During heating for TDS the sample was positioned approximately 1 mm away from the opening of the mass spectrometer, and CO was always adsorbed after cooling to 85 K with liquid nitrogen prior to heating. Quantitative XPS results were achieved by normalizing the fitted XP peaks with the number of scans, electron inelastic mean free path, ionization cross sections (taken from ref 9), and the transmission function of the electron analyzer. X-ray diffraction data were measured in grazing incidence using a Cu Kα source, a Göbel mirror, and a scintillation counter as detector in a D8 Advance X-ray diffractometer (Bruker AXS). The angle of incidence was 1° with 2θ in the range 20°−85°, the step size was 0.03°, and the measuring time was 40 s per step. SEM images were made in secondary electron mode with a Hitachi S4800 and a JEOL 7401F, and EDX spectra were taken using EDAX Genesis Software (version 6.10) and a Sapphire detector.
RESULTS AND DISCUSSION SEM images seen in Figures 1 and 2 show the different regions and morphology found on the spin-coated samples along with EDX spectra. The surface morphology of the spin-coated sample (Figure 1a) is a result of the spin-coating process and an uneven distribution of the precursor solution. The measurements contained in the following concentrate on the central region (marked “center” in Figure 1a) where the most even distribution of Ir and Ti is found. The dark and lighter regions in Figure 1a correspond to different surface compositions, and EDX spectra corresponding to several labeled points can be found in Figure 2c. The dark regions (Figure 1b) contain mainly TiOx (EDX spectrum 01) and particles about 100 nm in size. The lighter regions (Figure 1c) have smaller grains about 15 nm in size and contain an increased amount of Ir as seen in EDX spectrum 02. The increased brightness is due to the smaller particle sizes found in the iridium-rich regions. The microroughness in these regions leads to a smaller electron escape depth and more scattering which produces a greater number of secondary electrons, and the regions appear brighter in secondary electron mode. The backscattered portion of the signal is suppressed due to the low voltages used. This is exemplified again in the darker, upper right portion of Figure 1c which has a lower Ir content. Figure 2a shows a detail of the region marked “center” in Figure 1a. This region contains both Ir and Ti and, correspondingly, has a brightness in Figure 1a between those of the other two regions already discussed. Looking at the three marked points in Figure 2b and the corresponding EDX spectra 03, 04, and 05 in Figure 2c, three regions can be differentiated with differing stoichiometries. It is important, however, to note 25444
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structure. However, the peak positions of the latter deviate slightly from the Ti metal reference data. A clear splitting of the 002 reflection at 37.8°/38.3° 2θ (circled) indicates overlapping signals from two Ti-like phases with slightly differing lattice parameters. Based on the comparison with reference data, the smaller fraction can be assigned to Ti metal, while the larger fraction can be explained by a suboxide, TixO. Because of the strong overlap between most peaks of the two phases, the lattice parameters cannot be determined precisely, and a more detailed assignment of the suboxide phase (such as Ti6O, Ti3O, Ti2O) is not possible. These suboxides are derived from the Ti metal hcp structure by insertion of interstitial oxygen atoms, leading to superstructures of the simple Ti cell due to the ordering of the oxygen. For the fit, however, the small contribution of the oxygen atoms was neglected, and the simple Ti unit cell was used. The much broader peaks of rutile-type IrO2 are less conspicuous because they overlap mostly with other peaks. Nevertheless, the presence of IrO2 can be safely deduced from the fit of the pattern. All identified phases exhibit minor but noticeable deviations of the relative intensities compared to their ideal values, indicating some degree of preferred orientation of the crystallites. While the deviations can be successfully accounted for in the fit empirically, the very convoluted nature of the pattern does not allow a more detailed physical interpretation of the underlying orientation effects. Furthermore, some degree of cross-doping between TiOx and IrOx cannot be completely ruled out. Apart from the four phases described above, some small unexplained features in the XRD pattern (small peak at 26.2°, broad peak at 47°, shoulder at 68.5°) indicate the possibility of at least one other, unidentified phase. However, there is no indication that this is an Ir-suboxide. The XRD pattern of the thick sample (not shown) is dominated by the broad peaks of IrO2, superimposed with the small but distinctly sharp peaks of Ti metal. The lack of TiO2 is supported by XPS and is likely the result of the thicker IrOx layer shielding the underlying Ti substrate from oxidation. XPS survey scans of the spin-coated layers before and after TDS in Figure 4 show the sample surface to be composed of Ir, Ti, O, and C. XPS confirms the complete oxidation of the surface of the Ti cylinder as no signal corresponding to Ti metal was measured. Part of the Ti signal originates from the regions of thin IrOx coverage (point 05 in Figure 2b), although the [Ir]/[Ti] ratios of the samples’ central regions determined with quantitative XPS between 0.4 and 0.6 indicate there may also be other contributions. These ratios show a large amount of Ti although consideration of the deposition method would mean the top layers of the sample consist mainly of IrOx. Some small cracks are visible in the IrOx layer (Figures 1 and 2) although another possibility is Ti diffusion into the IrOx layer during tempering.1,2 This mixing could be the result of diffusion of Ti into the IrOx, whereby domains of pure TiOx are formed, or possibly a mixed (Ir,Ti)Ox phase, although no direct evidence of the latter has been observed. Another possibility is that the oxidation of the Ti metal causes the TiOx to break or “mushroom” into the surface IrOx leading to an intrusion-like region of TiOx and could be the cause of some of the observed film cracking. The O 1s core level from the spin-coated sample (Figure 5) shows mainly oxygen which we associate with the oxide phases (530 eV) although noticeable contributions are also made from hydroxides (531.8 eV) and H2O (533.1 eV).6,10,11 There is also
Figure 1. Scanning electron microscope images: (a) Top view of a section of the spin-coated sample on a Ti crystal with iridium acetate solution and subsequent annealing. Several different regions are visible with differing morphologies and surface compositions. (b) Detail of dark area from (a) showing a region of predominantly TiOx with larger particles. (c) Detail of lighter area from (a) showing a region with increased Ir content and finer particles. The region marked “center” contains a mixture of Ti- and Ir-rich regions and is the focus of much of the study.
that all regions contain both Ir and Ti meaning that the IrOx indeed covers the entire “center” region, albeit with differing thicknesses. In these regions, the trends from Figure 1 are continued with larger grain sizes corresponding to regions containing predominantly TiOx and the small grain sizes associated with increased Ir content. Figure 2b shows that the latter regions contain some cracking of the Ir layer although this is limited as evident from Figure 2a. Interestingly, the layers shown here are more inhomogeneous than those found in SEM cross sections of reference IrOx layers on square Si wafers with otherwise identical preparation. Especially in the regions similar to point 05 in Figure 2b the spin-coated layers become extremely thin while the layers on Si were 17−19 nm thick, crack-free, and homogeneous. As a reference, an SEM image of the thick IrO2 sample is shown in Figure 2d. This may be compared to Figure 2b to see clearly the similarities in the compressed powder nanostructure, or nanoporosity, of the two materials. The higher magnification chosen in Figure 2d is meant to emphasize this morphology. Quantitative XPS results on the central region of the spincoated samples show a layer composition of (Ir,Ti)Ox + H2O + OH. The ratio [Ti + Ir]/Oox is between 0.48 and 0.53 in all samples, Oox at 530 eV binding energy being the portion of the O 1s peak attributable to oxygen bound in an oxide.6,10,11 Thus, the surface composition of the (Ir,Ti)Ox phase is mainly (Ir,Ti)O2 as evidenced by the XPS and XRD results. There are also some hydrous regions in the layer, as will be discussed further below. The XRD pattern of the spin-coated sample in Figure 3 shows clear reflections from rutile TiO2 and a Ti metal-like hcp 25445
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Figure 2. Scanning electron microscope images: (a) Detail of region marked “center” from Figure 1a). (b) Area marked “center” at higher magnification showing regions of differing composition. (c) EDX spectra at 5 kV excitation energy showing the compositions of several corresponding points marked in Figure 1. (d) Sample with a thick IrO2 film made by pipetting the iridium acetate solution onto the polycrystalline Ti substrate and subsequent annealing. The nanostructure of the two films in (b) and (d) are seen to be similar.
Figure 4. XPS survey scan of the spin-coated (Ir,Ti)Ox sample before and after TDS showing peaks corresponding to Ir, Ti, O, and C.
Figure 3. XRD pattern of the spin-coated IrOx sample. Upper panel: full pattern fit with circled 002 reflections from TixO and Ti. Lower panel: calculated patterns of the phases used to fit the measurement.
after TDS. The ratios of the O 1s constituent peaks Oox:OH−:H2O are 13:4:2 before TDS and 18:4:2 after TDS. Moving to the Ir 4f peak of the spin-coated sample, the layers contained only a small amount of IrOx reduced to Ir metal by heating during TDS which can be seen in the slight broadening of the Ir 4f peak toward metallic Ir at lower binding energies in Figure 6.12 This is accompanied by a change in the [Ir]/[Oox] ratio from 0.17 to 0.20, the increase in Ir supplying further support for the emergence of a reduced/metallic phase. Despite the simple appearance of the peak, fitting was not possible without the inclusion of components at higher binding energies and requires further investigation before a discussion of these components can take place. In contrast to the spin-coated samples, the surface Ir of the thick sample showed a much
a very small contribution just below 535 eV. The hydroxide and water contributions are slightly reduced in intensity and shifted during TDS due to changes in and/or removal of individual subspecies. The persistence of the signals after heating means these are not only adsorbed surface species but rather are largely bound into the electrode layer as regions of hydrous rutile phases. Although the oxide peak at 530 eV appears to grow after TDS, this is most probably due to a slightly different sample position seeing as the [Ir + Ti]/[Oox] ratios also grow. Desorption of surface adsorbate layers could also play a roll as evidenced by, for example, the decrease in C 1s peak intensity 25446
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fully stable with respect to heating directly after the initial calcination and is in agreement with the thermodynamic stability found in ref 13. Here thermodynamical stability is predicted up to 570 K at the O2 partial pressure of ∼10−10 mbar found in our vacuum chamber. Also, the samples may be degraded by the CO oxidation reaction itself. No change was seen in the form of the Ti 2p spectrum of the spin-coated sample after TDS and indicates a stable TiOx phase. Al Kα radiation was used for an initial valence band study before and after TDS. There was no indication in the spincoated samples of the emergence of the Ir 5d (or Ti 3d) states out of the O 2p states, confirming the reduced Ir made only a small contribution to the sample surface (compare Figure 6).14,15 In contrast, the emergence of the Ir 5d states in the thick samples supports the increased reduction seen in the Ir 4f core level (not shown). The stability and catalytic activity were also analyzed with TDS, which gives an indication of the number of active sites and their stability on the sample surface through the CO to CO2 oxidation reaction. These sites are responsible for the oxidation of the CO which is commonly used as a probe in electroreduction of molecular oxygen on metals and metal oxides.16,17 Although it cannot be proven whether water will only adsorb on these sites, the sites observed during TDS are real and provide a lower bound for active sites during OER. Figure 7 shows a TD spectrum from the spin-coated sample performed after the initial XPS analysis and the adsorption of 1L of CO at 85 K. Some catalytic activity is seen (CO → CO2 oxidation) as well as the release of H2O (not shown). The release of water corresponds to the loss of intensity of the OH− and H2O peaks in the O 1s XPS spectrum. Unfortunately, there is little in the way of TD spectra on IrO2 films in the literature. Takasu et al.18 show TD spectra on different mixtures of RuO2, IrO2, and TiO2 on Ti metal substrates after the adsorption of CO. Keeping in mind that the method of measuring the CO leaving the sample was different than in our study, the IrO2/Ti samples show no CO desorption while the TiO2/Ti shows only one desorption peak at low temperature, apparently due to CO bound weakly to the sample surface. In this study we obtain similar results for CO, with contributions to the small observed amount of CO desorption possibly coming from CO adsorption sites provided by the TiOx phases. In fact, the amount of detected CO and CO2 was rather independent of the amount of adsorbed CO (up to 10 langmuirs), with the first TDS runs always producing the highest yields of both CO and
Figure 5. O1 s from the spin-coated sample before and after TDS. TDS resulted in a slight reduction in intensity and shift of the OH− and H2O species and growth of the [Ir + Ti]/[Oox] peak area ratios.
Figure 6. Ir 4f from the spin-coated sample before and after TDS showing only a slight reduction of the IrOx to metallic Ir.
larger reduction after TDS (heating to 550 K) through a shift in the Ir 4f peak toward metallic Ir and an increase of the [Ir]/ [Oox] ratio by a factor of 2. This shows that the samples are not
Figure 7. TDS on the spin-coated sample after adsorption of 1 langmuir of CO at 85 K: (a) run 1 after initial XPS analysis and (b) run 2 after second XPS analysis with an inset showing the entire CO2 desorption. Inset: enlarged y-axes showing the full CO desorption peak. The difference in the runs is evident and reflects an unstable sample surface. 25447
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Figure 8. Cyclic voltammograms of (a) the spin-coated IrOx film and TiOx reference sample and (b) the spin-coated and thick IrOx films recorded with 50 mV/s at room temperature in 0.1 M HClO4.
(capacitive current) and a surface chemical redox reaction (faradaic current) according to:19,20
CO2. However, a CO-saturated surface was reached quickly in every case. TDS run 2, also with 1 langmuir of CO adsorbed at 85 K, shows similar CO2 production at temperatures just above 100 K. The y-axes of runs 1 and 2 have been set equal with the full CO desorption peak shown in the inset. However, in contrast to run 1, the CO2 production in run 2 falls off almost completely when moving to higher temperatures. Run 1 had a total CO2 production of 2.7 × 1015 molecules/cm2 while run 2 had just 8.3 × 1013 molecules/cm2. Correspondingly, in run 2 almost all of the CO desorbs precipitously at 100 K inhibiting any further oxidation and shows the CO was bound more weakly to the electrode surface in this run. The total CO desorption of both runs was similar and the sum of desorbed [CO + CO2] was, therefore, much higher in run 1 (∼1015) than run 2 (∼1014). The number of adsorption sites on the electrode surface was reduced by an order of magnitude after run 1. The H2O leaving the sample in run 2 (not shown) was also drastically reduced. The lower CO oxidation seen in run 2 compared to run 1 shows that the TiO2, present in large quantities in the bulk, is not responsible for the production of CO2 in the spin-coated sample. Because we find no evidence of Ir-suboxides and IrO2 is not itself active here,18 the CO oxidation reaction must have been driven by near-surface active species left after synthesis or by an active IrOx surface termination, the responsible mechanism being deteriorated after the first TDS run. This is supported by our TDS experiments on TiO2 reference samples without IrOx which resulted in no CO2 production. Finally, one sees in run 1 that at about 500 K the sample begins to dissociate, whereas this is not so clear for run 2 although there are indications of the beginnings of the process above 500 K. Thus, we find further strong evidence of sample degradation during TDS which confirms the observations of the Ir 4f peak. In contrast to the spin-coated films, the TDS investigation of the thick samples was unusable as even after repeated heating the signal from outgassing was still approximately 4 orders of magnitude larger than the signal from CO2 oxidized after CO adsorption. The surface redox chemistry and the available active surface area of the oxide layers were further studied with cyclic voltammetry (CV). The observed voltammetric response of IrOx films (see Figure 8) is generally composed of charging of the electrical double layer at the electrode/electrolyte interface
IrOx (OH)y + δ H+ + δ e− ⇄ IrOx − δ (OH)y + δ
(1)
The integrated anodic charge q* in a fixed potential window is therefore proportional to the active oxide surface area and can be used to normalize the measured currents in order to obtain surface charge specific current densities (surface charge specific catalytic activities).7,21 Because the spin-coated IrOx film contains significant amounts of TiOx (see XPS and XRD results), it must be clarified whether the TiOx makes a significant contribution to the anodic charge q*, possibly distorting the IrOx surface charge determination. For this, a TiOx film free of IrOx (referred to hereafter as the “reference sample”), treated mechanically and thermally the same as the IrOx, was compared with the spin-coated IrOx film using CV (see Figure 8). The integrated anodic charge of the reference sample is approximately 10 times smaller than the integrated anodic charge of the spin-coated IrOx film, limiting the systematic experimental error of the IrOx surface charge determination to approximately 10%. The charge based method revealed a surface area for the thick sample 50 times greater than for the spin-coated film. The rather large difference in surface area is explained by the known morphology of iridium oxide electrodes which resembles a powder compressed onto a solid surface. Because of this, these electrodes offer a surface roughness up to a factor 1000 larger than a smooth surface, whereby every electrically contacted oxide grain accessible to the electrolyte should contribute to the electrochemically determined surface area.8 SEM images in Figure 2 demonstrate the compressed powder morphology to be present for both samples here. Considering the different IrOx loadings applied here (the thicker film contains approximately 62 times more material than the spincoated film), the compressed powder morphology explains the observed difference in measured active oxide surface area. Looking at the CV in Figure 8a, the IrOx-free reference sample offers approximately 1 order of magnitude lower geometric current densities and a rather flat CV compared to the spin-coated IrOx film. Therefore, the voltammetric response of the spin-coated IrOx film is largely dominated by IrOx, although TiOx was detected near the surface (see XPS and XRD). CVs for the spin-coated and thick films are compared in Figure 8b. Here, the comparison cannot be adequately made on the basis of geometric current densities since the surface area of 25448
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Figure 9. Quasi-stationary potential scans of the TiOx reference sample, thick and spin-coated IrOx films recorded with 6 mV/s and 1600 rpm rotating disk speed in 0.1 M HClO4 at room temperature: (a) geometric and (b) specific current densities.
the thick film is approximately 50−60 times greater than the surface area of the spin-coated film and would result in vastly different values. Therefore, currents were normalized by the surface charge q* to obtain comparable surface charge specific current densities. Both films exhibit a rather featureless and similar CV (see Figure 8b) which is indicative of predominantly anhydrous IrOx20,21 with only small amounts of a hydrous phase in accordance with the XPS results and the high temperature annealing during synthesis. Two broad peaks are apparent for the thick film caused by the stepwise oxidation of Ir to higher oxidation states.7 In contrast, the spin-coated IrOx film reveals only one broad peak located at potentials between the peaks of the thick IrOx film. Consistent with literature, the former is typical for thick IrOx films7 while the latter has been observed for IrOx films prepared by thermal oxidation of iridium films.22 Moreover, a series of voltage scan rates revealed only one peak for the spin-coated sample at all rates (not shown); hence, the single peak is not an artifact stemming from time resolution problems. We highlight that the absence of a second redox transition for the spin-coated sample is possibly connected to the higher stability of this sample observed in XPS after the first TDS run. The electrocatalytic OER activity of both films was investigated by slow and quasi-stationary linear potential scans, shown in Figure 9, based on (a) the geometric and (b) the surface charge specific current densities. Figure 9a shows a negligible OER activity for the reference sample, ruling out any significant OER activity of TiOx in the applied potential window. The spin-coated IrOx film shows distinctly higher OER overpotentials at the same geometric current density compared to the thick IrOx film which appears reasonable due to the large difference in surface area. A meaningful intrinsic OER activity comparison can be made using surface charge specific current densities (eliminating the influence of different surface areas; see Figure 9b). Although the actual overpotential difference at the same current density now becomes small, the spin-coated IrOx film still maintains the need for a higher overpotential pointing to a slight intrinsic difference in the electrocatalytic OER performance of both films. Nevertheless, both films offer excellent OER activity common in IrO2 electrodes.22,23 Considering the XRD and XPS results, the main compositional difference between both films are the TiOx phases found in the spin-coated IrOx film and are expected to be responsible for the observed intrinsic difference in electrocatalytic OER activity. A TiO2 interlayer between substrate and catalyst raises
the actual OER overpotential due to its rather poor electrical conduction properties.24 Although this may explain the observed higher overpotential for the spin-coated IrOx film, it may also be a result of the differences in each film found with TDS and XPS.
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CONCLUSION/OUTLOOK
As a first step toward bridging the gap between model and real catalytic systems, simplified IrOx layers were produced via spin coating on polycrystalline Ti cylinders from a precursor of iridium acetate in ethanol. The layers lacked the mud crack morphology of standard transition metal oxide electrodes, albeit with a compressed powder nanostructure, and allowed for an intimate study of the substrate/electrode interface. The layered system was determined to be (Ir,Ti)Ox/TiO2/Ti-suboxide/Timetal with x near 2. It could not be determined whether a mixed surface (Ir,Ti)Ox phase exists or rather separate pure oxide domains. The spin-coated layers showed improved structural and electronic stability during heating compared to thicker films although degradation was still observed. The CO → CO2 oxidation process differed in successive TDS runs due to the degradation of the CO oxidation mechanism (near surface active species or an active surface termination) after the initial TDS run. The form of the curves in CV was typical of dry oxide layers, although XPS showed the existence of a minor amount of hydrous phase. Also the spin-coated IrOx layer offered the expected excellent electrocatalytic OER activity but required a slightly larger overpotential to reach the same surface charge specific OER current density as the thick sample. Typical difficulties using surface sensitive measurements such as signal averaging over lateral variations could not be overcome, and the extreme instability during heating and outgassing of the thick samples during TDS ruled them out as ideal systems for such studies. Instead, thin samples are needed, and tools with higher spatial resolution such as a synchrotron would be advantageous. Further studies will be aimed at in-situ stability studies at lower temperatures while identifying more specifically the structure of the layer stack and the phases of its components. With the latter we are specifically aiming at tackling the divide between real samples and theoretical models. 25449
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AUTHOR INFORMATION
Corresponding Author
*Tel 0049 30 8413 4523, e-mail
[email protected] (B.J.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Ralph Kraehnert at the TU Berlin for access to the electron microscope and the Cluster of Excellence in Catalysis UNICAT. Financial support provided by the German Federal Ministry of Education and Research under the promotional reference number 03SF0433A (MEOKATS) as well as the German Research Society (DFG) project SPP 1613.
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