Synthesis, Structure, and Electrochemical Stability of Ir-Decorated

Dec 21, 2015 - ABSTRACT: One major challenge of proton-exchange membrane fuel cells, water electrolyzers, and unitized regenerative fuel cells is to ...
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Synthesis, Structure, and Electrochemical Stability of Ir-Decorated RuO2 Nanoparticles and Pt Nanorods as Oxygen Catalysts Christian Gutsche, Christoph J. Moeller, Martin Knipper,* Holger Borchert, Juergen Parisi, and Thorsten Plaggenborg University of Oldenburg, Department of Physics, Energy and Semiconductor Research Laboratory, Carl-von-Ossietzky Strasse 9-11, 26129 Oldenburg, Germany ABSTRACT: One major challenge of proton-exchange membrane fuel cells, water electrolyzers, and unitized regenerative fuel cells is to increase the oxygen catalysts’ stability by the systematic preparation of inhomogeneous catalyst surfaces via decoration with cocatalysts. Iridium (Ir) nanodots with a typical diameter of 2 nm were colloid chemically deposited on ruthenium oxide (RuO2) nanoparticles as oxygen evolution catalyst and on platinum (Pt) nanorods to yield bifunctional oxygen catalysts. The stepwise synthesis allows a higher control of the composite nanoparticles’ morphology and thus their activity and stability. The stability toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was studied with cyclic voltammetry. Structural characterization was carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) spectroscopy before and after the stability protocols. Ir nanodots as well as Irdecorated RuO2 nanoparticles were found to be less stable toward the OER stability protocol than pure RuO2 nanoparticles. Bifunctional catalysts made of Ir deposited on Pt nanorods showed different behavior than the individual components. Concerning the OER, the stability decreased compared to unsupported Ir (monometallic OER catalyst), whereas improved stability was observed for the ORR in comparison to pure Pt nanorods (monometallic ORR catalyst).

1. INTRODUCTION The transition toward a sustainable energy system implies the challenge of matching the volatile energy production by wind and solar power plants with the energy demand. One approach enabling short- and long-term energy storage as well as clean mobility is the production and consumption of hydrogen.1,2 One technology for hydrogen production is the protonexchange membrane water electrolyzer (PEMWE).3,4 Usually, iridium (Ir) and ruthenium (Ru) mixed oxides are used as catalysts for the oxygen evolution reaction (OER) in the PEMWE because of the harsh conditions. 5−7 Another technology combining hydrogen production and consumption in one device is the unitized regenerative fuel cell (URFC).5,8−12 The OER catalyst during charging is mostly IrO2. As catalyst for the oxygen reduction reaction (ORR), during discharging platinum (Pt) is commonly used.6,13−17 The mentioned catalysts are also used in other energy storage systems utilizing oxygen reactions like, for example, the vanadium air redox flow battery.18,19 To lower the costs of the catalysts, the use of nanoparticles is an object of catalyst research because the higher surface to volume ratio enables massively increased mass specific activities of the catalysts. Still, the need to increase the nanocatalysts’ stability is one important task.3,6,7,14,20−28 Moreover, introducing composite materials can be applied to enhance the properties of the initial pure, single phase catalyst. A new © XXXX American Chemical Society

approach to increase the activity as well as the stability is to prepare systematically inhomogeneous catalyst surfaces via nanosegregation of mixed metals or mixed oxides or via decoration of the catalyst with a cocatalyst.27 The latter approach offers a higher degree of control over properties such as the shape and the structure, and thus the stability and activity, than does a simultaneous synthesis. A better dispersion of composite catalysts can be achieved using colloid−chemical synthesis instead of physical mixing.22 The aim of this study is to prepare noble metal based composite nanocatalysts for ORR and OER via a colloid− chemical process and compare their stability with the initial single-phase catalysts, namely, Ir nanodots are deposited on RuO2 OER catalysts and on Pt nanorods to yield Ir−Pt based bifunctional ORR−OER catalysts. RuO2 is generally recognized as one of the OER catalysts with the highest activity.7,29,30 Unfortunately, RuO2 is oxidized to RuO4 under OER conditions in acidic media that dissolves into the electrolyte.6,7,29,31−33 IrO2 does not show the high catalytic activity of RuO2 but rather a higher stability.3,22,23,34,35 To increase both the activity and stability of noble metal based OER catalysts, studies on Ru−Ir mixed oxides were Received: November 23, 2015 Revised: December 18, 2015

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The Journal of Physical Chemistry C conducted.6,7,22,33−35 Koetz et al. observed a significantly higher stability and an inhibition of RuO4 production on sputtered Ru−Ir mixed oxides under OER conditions,33 whereas there is only a small loss of activity. This behavior can already be observed for small amounts of Ir. It is suggested that a common electronic d-band is formed that allows the electron transfer from Ru sites to Ir sites, thus mitigating Ru oxidation. Danilovic et al. showed that the bulk and the nanoparticle forms of nanosegregated Ru−Ir mixed oxides possess a similar activity but a 3−4 times higher stability compared to those of homogeneous Ru−Ir mixed oxides.34 They observed a surface enrichment of Ir in the nanosegregated samples and explained the higher stability with the higher stability of surface Ir compared to that of Ru.34 Audichon et al. prepared IrxRu1−xO2 nanoparticles and tested the performance and stability in an electrolyzer cell. Ru0.9Ir0.1O2 had a smaller overpotential and a higher stability compared to those of pure RuO2.36 The aim of our study is to decorate RuO2 nanoparticles with Ir nanodots that are oxidized to IrO2 later on at least at the surface and to compare the stability of both single nanoparticle catalysts and the composite catalyst. The advantage of this stepwise approach is the possibility to control more easily the morphology and thus activity and stability of both nanoparticle species. For example, bifunctional ORR−OER catalysts are used in URFCs.6,9,11,22,37−51 Because of the poor stability of RuO2, binary bifunctional catalysts are commonly based on Pt for ORR and IrO2 for OER. The best round-trip efficiency of URFCs was commonly achieved using low amounts of Ir or IrO2.6,22,43,47 In the literature it is reported that physical mixing of both catalysts does not lead to a good dispersion of the catalysts.22,51 The colloid−chemical synthesis of bifunctional catalysts for ORR and OER leads to a better dispersion and offers the possibility of synergistic effects, so mostly a better round trip efficiency of the URFC is reported using the colloid−chemically prepared catalysts instead of the mixed ones. Also in this research field, stability studies of the catalysts are rare. Kong et al. prepared Ir in the presence of IrO2 and mixed it with Pt nanoparticles.38,39 After activation cycles that oxidized the Ir surface they carried out accelerated potential cycling tests for ORR and OER separately. They apparently compared each one single measurement of mixed Pt/IrO2 particles with mixed Pt/Ir−IrO2 particles. In the latter case, the stability of the IrO2 did not change, whereas the Pt stability increased. Papazisi et al. prepared PtxIr1−xO2 mixed oxide nanoparticles and studied the degradation of the OER currents during 180 cycles between OER and hydrogen evolution potentials. The OER currents of the pure IrO2 decreased to 82% of the initial value, whereas it decreased to 86% for Ir0.8Pt0.2O2 and 60% for Ir0.2Pt0.8O2.43 There are no studies known to the authors that compare the stability of Pt−Ir-based nanoparticles with single Pt- and Ir-based catalysts under URFC conditions (meaning under simulated charge−discharge cycles that include ORR and OER potentials). It is known that such a treatment is harsher than single ORR or OER protocols.18,52 This is explained by the degrading effect of repeated build up and reduction of an (hydr)oxide layer on the Pt surface.18 In this study, we used Pt nanorods that are known to exhibit an excellent stability compared to that of Pt nanoparticles under ORR cycling tests53−55 and decorated them with Ir nanodots. The stepwise synthesis allows a higher control over the morphology of the nanoparticles and thus their activity and

stability. Then, the OER and ORR stability of the unsupported Ir nanodots and pure Pt nanorods are studied in comparison to those of Ir-decorated Pt nanorods.

2. EXPERIMENTAL METHODS 2.1. Synthesis of Pt Nanorods. The synthesis of the Pt nanorods is based on the synthesis of Sun et al.53−55 H2PtCl6· 6H2O (32 mg) and formic acid (1 mL) were simultaneously added to 20 mL of ultrapure water at room temperature. Within 1 day, turbidity of the solution due to black precipitates could be observed along with decoloration of the formerly yellow solution due to the consumption of Pt ions. The synthesis turned out to be robust against changes in the concentrations of the chemicals. After the synthesis, the dispersion was filled in a centrifuge tube and diluted with isopropanol. The centrifugation was usually carried out for 5 min at around 5000g. The supernatant was decanted, and the nanoparticles were redispersed in isopropanol by ultrasonication for 5 min. The centrifugation/redispergation procedure was carried out two times. 2.2. Synthesis of Ir Nanodots. The polyol process-based synthesis of Ir nanodots was adapted from Kong et al.38,39 The synthesis was carried out without support particles as well as in the presence of previously synthesized Pt nanorods and commercially available RuO2 nanoparticles (Johnson Matthey) as support. Changes in the concentration of the chemicals did not alter the morphology of the nanodots. Ethylene glycol, IrCl3·nH2O (58 wt % Ir, Johnson Matthey), isopropanol, 0.5 M NaOH if applicable, and support particles if needed were filled into a flask and stirred under Ar atmosphere for several minutes. Then, the flask was immersed into an oil bath and heated to 140 °C under Ar flow. Three minutes after the immersion, the liquid lost the yellow color accompanied by a turbidity, indicating the consumption of Ir ions as well as the formation of nanoparticles. During the syntheses using support particles, this is harder to observe but can easily be controlled after centrifugation. Afterward, the dispersion was filled into a centrifuge tube and diluted with isopropanol. The centrifugation was carried out for at least 5 min at around 5000g. The supernatant was decanted, and the nanoparticles were ultrasonically redispersed in isopropanol for 5 min. The centrifugation/redispersion procedure was carried out two times. Unsupported Ir nanodots were synthesized with varying Ir salt concentrations (20, 40, 60, and 80 mg of IrCl3·n H2O). Ethylene glycol (10 mL) served as solvent. The immersion time of the flask in the oil bath was 10 min. For the synthesis of Ir nanodots in the presence of RuO2 nanoparticles, 20 mL of ethylene glycol with 12.5 mM NaOH was used as solvent, and 20 mg of RuO2 was added. The amount of Ir salt was varied between 0 and 100 mg. The part of Ir nanodots that did not deposit on the support particles could be reduced to a negligible concentration compared to the RuO2-supported Ir nanodots by centrifugating, decanting, and redispersing six times. The Ir decoration of Pt nanorods was carried out in a manner similar to that of the synthesis of pure Ir nanodots. Pt nanorods (1 mL) were dispersed in isopropanol, and 8 mg of Ir salt was added to 9 mL of ethylene glycol. For comparison, a synthesis without Ir salt was also conducted. The cleaning procedure was carried out five times. 2.3. Structural Characterization. Structural characterization was carried out with powder X-ray diffractometry B

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nanoparticles,18 an OER stability protocol was also carried out simulating charge−stop cycles: • Activation protocol (six low-potential characterization cycles, ten wide-potential characterization cycles, six lowpotential characterization cycles) • OER−ORR stability protocol (2400 OER−ORR degradation cycles in groups of 50 to 200 cycles, interrupted by two wide-potential characterization cycles and six lowpotential characterization cycles, respectively) • Or: OER stability protocol (2400 OER degradation cycles in groups of 50 to 200 cycles, interrupted by two wide-potential characterization cycles and six lowpotential characterization cycles, respectively) For stability analysis, the last OER and ORR characterization cycles were always used. To enable TEM measurements on particles after the stability protocol, a measurement cell with a 1 × 1 in. glassy-carbon working electrode prepared with nanoparticles, an MMSE, and a Pt coil counter electrode were used. iR compensation was applied. After electrochemical stability protocols, the electrode coated with particles was ultrasonicated in ethanol to redisperse the particles. The dispersion was then dropped onto TEM grids.

(XRD), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray (EDX) spectroscopy. XRD data were collected using a PANalytical X’Pert Pro diffractometer operating with Cu Kα radiation in Bragg−Brentano θ−θ geometry using an automatic divergence slit. Samples were prepared by dropping the dispersion on low-background silicon sample holders. Reference data were taken from the database of the International Centre for Diffraction Data (ICDD). The crystallite size was determined via the Scherrer equation56,57 using the simplifying assumption of monodisperse, quasispherical particles (K = 0.9) without inhomogeneous lattice strain in the case of Ir nanodots and RuO2. A linear background was subtracted prior to further analysis. A peak broadening due to the instrument was corrected by subtracting 0.08° from the full width at half-maximum. TEM images and EDX spectroscopic data were collected on a Jeol 2010F system with an acceleration voltage of 200 kV. To determine the Ir/Pt and Ir/Ru atomic ratio via EDX sum, spectra of 10 agglomerates were collected, and the ratios were averaged. 2.4. Electrochemical Characterization. For electrochemical measurements, selected dispersions were sonicated for 15 min, and an aliquot was dropped onto a previously polished glassy-carbon electrode (7.1 mm2 surface area) and air-dried in the vent hood. The cyclic voltammograms (CVs) were recorded in an undivided three-electrode setup with 2 M H2SO4 electrolyte and a Pt counter electrode. Potentials were measured against a mercury/mercury sulfate electrode (MMSE), and a CH Instruments potentiostat was used. All measurements were carried out at room temperature. The solution was purged with N2 for 15 min before the measurements. Comparing measurements on RuO2 nanoparticles, Irdecorated RuO2 (Ir@RuO2) nanoparticles, and Ir nanodots comprised an activation protocol followed by an OER stability protocol that simulates the start and stop of the water electrolyzer’s operation: • Activation protocol (6 low-potential characterization cycles, 10 wide-potential characterization cycles) • OER stability protocol (2000 OER degradation cycles in groups of 50−200 cycles interrupted by 2 wide-potential characterization cycles, respectively) The potential regimes and scan rates are shown in Table 1. For the stability analysis, the last characterization prior to the degradation cycles was always used.

3. RESULTS AND DISCUSSION 3.1. Ir Nanodots. 3.1.1. Structural Characterization. Unsupported Ir nanodots were synthesized with varying Ir salt concentrations (20, 40, 60, and 80 mg of IrCl3·nH2O). As expected, the XRD patterns (Figure 1) show only peaks for Ir

Figure 1. XRD patterns of Ir nanodots synthesized with (a) 20, (b) 40, (c) 60, and (d) 80 mg of Ir salt. All patterns are normalized to the 111 peak. The line pattern shows the reference pattern for Ir (PDF no. 03065-9327, ICCD, [2009]).

Table 1. Potential Regimes and Scan Rates of the CVs Used in the Electrochemical Protocols type of CV low-potential characterization wide-potential characterization OER degradation OER−ORR degradation

potential regime (V vs MMSE)

scan rate (mV/s)

−0.65...0.2

100

−0.65...0.95

100

0.35...0.85 −0.2...0.85

500 500

(Powder Diffraction File (PDF) 03-065-9327, ICDD, [2009]). In the patterns b−d, there is a small peak shift for the 111, 220, and 311 peak of 0.1−0.3° to higher angles that can be explained by compressive strain and superposition with the other Ir peaks. In the case of pattern a, the shift for these three peaks ranges from −0.6° (311 peak) to 0.4° (111 peak), probably because of more superposition of the broader peaks. The width of the peaks indicates small crystalline domains. Assuming monodisperse, quasi-spherical particles (K = 0.9) without inhomogeneous lattice strain, the lower angle half of the 111 peak and the 220 peak were analyzed using the Scherrer equation. Accordingly, the mean diameter of the crystalline domains is 2 nm except for the synthesis with the smallest Ir salt concentration (Table 2). In the latter case, the mean diameter

The comparison of measurements of Pt nanorods, Irdecorated Pt (Ir@Pt) nanorods, and Ir nanodots also consisted of an activation protocol followed by an OER−ORR stability protocol simulating multiple changes between charge and discharge mode of the URFC. Because it is known that cycling between ORR and OER potentials is very degrading for Pt C

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The Journal of Physical Chemistry C Table 2. Mean Value and Standard Deviation of the Ir Crystallite Diameter for Syntheses with Varying Ir Salt Concentration Determined via Scherrer Equation from XRD Measurements amount of Ir salt used in synthesis (mg) 20 40 60 80

mean crystallite diameter (nm) 1.5 1.9 2.0 2.0

± ± ± ±

0.1 0.2 0.3 0.1

is 1.5 nm. This deviation can be explained by the shorter growth time of the nanodots due to lower Ir salt concentration. In accordance with the results from XRD analysis, the TEM images (Figure 2) of all samples show single-crystalline nanodots with a typical diameter in the range of 2 nm. Kong’s synthesis also resulted in particles with a size in this range.38 3.2. Ir-Decorated RuO2 Nanoparticles. 3.2.1. Structural Characterization. For the synthesis of Ir nanodots in the presence of RuO2 nanoparticles, 20 mg of RuO2 were used, and the amount of Ir salt was varied between 0 and 100 mg. Figure 3 shows TEM images of Ir nanodots being well-dispersed on the bigger RuO2 nanoparticles. EDX measurements yield an Ir/ Ru atomic ratio of (10 ± 4)% for the RuO2 sample treated with 100 mg of Ir salt. Figure 4 shows the XRD patterns of RuO2 nanoparticles as bought and after syntheses with 0, 20, 40, 60, and 80 mg of Ir salt and those of Ir nanodots without RuO2 support. The RuO2 peak positions of the as-bought sample are shifted to higher values compared to the calculated positions (PDF no. 03-065-2824, ICDD, [2009]). The shift averaged over the 110, 101, 211, 220, and 002 peak is (0.32 ± 0.08)°. This shift might be explained by a homogeneous compressive strain of the nanoparticles. The synthesis procedure conducted without Ir salt does not change the RuO2 peak positions compared to those of the as-bought sample. The positions of the mentioned peaks shift to smaller angles after Ir synthesis. The shift does not depend on the amount of added Ir salt and is (0.08 ± 0.04)°. This shift might be explained by a relaxation of the RuO2 lattice due to interaction with the Ir nanodots or, more probably, by enrichment of IrO2 in the RuO2 particles and overlap of the very similar XRD patterns. The mean crystallite diameters estimated via the Scherrer equation from the 110 and 101 peaks for the different samples are shown in Table 3. Comparing the peak widths of as-bought RuO2 nanoparticles and RuO2 nanoparticles after the synthesis conducted with 100 mg of Ir salt shows an apparent decrease of the mean crystallite size from (30.1 ± 0.4) to (18.4 ± 0.4) nm.

Figure 3. (a and b) TEM images of as-bought RuO2 nanoparticles. Panel b shows crystalline RuO2 nanoparticles. (c and d) TEM images of the Ir@RuO2 nanoparticles from the synthesis using 100 mg of Ir. The marked area in c is shown with higher magnification in d. The Ir nanodots are well-dispersed on the RuO2 support particles.

An alternative explanation for the peak broadening is an increase in inhomogeneous lattice spacings that might originate from additive inhomogeneous strain due to interaction with the Ir nanodots or by enrichment of IrO2 in the RuO2 particles. The TEM images in Figure 3 show crystalline RuO 2 nanoparticles with a mean diameter in the range of the mentioned values In the XRD patterns of the samples prepared with 80 and 100 mg of Ir salt, a broad feature evolves around 40°. According to the XRD patterns of the pure Ir nanodot samples, this feature is attributed to the Ir main peak. Compared to the unsupported Ir sample, the Ir main peak is broadened. The TEM images in Figure 3 show Ir nanodots with a typical diameter in the range of 2 nm deposited on the RuO2. Therefore, this peak broadening might be explained by higher lattice strain of the Ir nanodots caused by interaction with the RuO2 support.

Figure 2. Exemplary TEM images of Ir nanodots. The areas marked in a and b are shown with a higher magnification in b and c, respectively. The TEM images show agglomerates of crystalline Ir nanodots with a typical diameter in the range of 2 nm. D

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characterization CVs of the stability protocol (after the activation procedure) normalized to the peak current. The CV of the RuO2 nanoparticles shows the typical RuIII/RuIV redox couple at −0.1 V vs MMSE and the RuIV/RuVI redox couple at 0.6 V vs MMSE and HUPD features below −0.3 V vs MMSE.6,35,36,58 The CV of the oxidized Ir nanodots shows no distinct features. Probably, the redox features are broadened because of the small size of the nanodots. In the Ir@RuO2 CV, a very weak feature can be observed at 0.3 V vs MMSE that might be attributed to the IrIII/IrIV redox couple.35,43 The onset potential for the OER is nearly the same for all samples; only that of the Ir@RuO2 CV is a little bit shifted to higher potentials. To evaluate the stability of the catalysts with respect to the OER, we analyzed the decrease of the OER peak current in the CVs. To calculate the OER stability, the OER peak currents of the characterization CVs within the stability protocol were divided by the initial OER peak current. The mean values and standard deviation of four measurements were calculated for the stability diagrams. Figure 6 shows the stability of the three

Figure 4. (a) XRD patterns of as-bought RuO2 nanoparticles. RuO2 nanoparticles after the synthesis conducted (b) without Ir salt and with (c) 20, (d) 40, (e) 60, (f) 80, and (g) 100 mg of Ir salt and (h) Ir nanodots prepared without RuO2 nanoparticles. The patterns a−g are normalized to the 110 peak. Selected RuO2 peaks are labeled in the figure. The line patterns show the reference patterns of RuO2 (black, PDF no. 03-065-2824, ICDD, [2009]) and Ir (light blue, PDF no. 03065-9327, ICDD, [2009]).

Table 3. Mean Value and Standard Deviation of the RuO2 Crystallite Diameter For Different Samples Estimated from the 110 and 101 XRD Peaks sample RuO2 RuO2 RuO2 RuO2 RuO2 RuO2 RuO2

as bought + 0 mg of Ir salt + 20 mg of Ir salt + 40 mg of Ir salt + 60 mg of Ir salt + 80 mg of Ir salt + 100 mg of Ir salt

mean diameter of RuO2 crystallites (nm) 30.1 28.5 23.9 24.0 21.5 18.9 18.4

± ± ± ± ± ± ±

0.4 0.4 3.5 1.2 4.7 5.4 0.4

Figure 6. OER stability of RuO2 (blue dashed trace and squares), Ir (black trace and triangles), and Ir-decorated RuO2 (red trace and circles) defined as the OER peak current in a characterization CV after the given amount of degradation cycles normalized to the initial OER peak current. The lines serve as guide to the eye. Each curve shows the mean and standard deviation of four measurements. RuO2 is the most stable catalyst, followed by Ir@RuO2 and Ir nanodots.

3.2.2. Electrochemical Characterization. In the first lowpotential CVs of the activation protocol, the Ir hydrogen under potential deposition (HUPD) features can be observed on the Ir and the Ir@RuO2 (from the synthesis with 100 mg of Ir salt) samples. These features vanish within a few wide-potential cycles because of oxidation of Ir at least at the nanodots’ surface in agreement with the literature.40,58 Figure 5 shows the initial

catalysts as a function of the number of degradation cycles carried out within the stability protocol. After 2000 cycles, the OER peak current has decreased to around 60, 50, and 40% of the initial value for RuO2 nanoparticles, RuO2 nanoparticles decorated with Ir nanodots, and pure Ir nanodots, respectively. Although the material-specific stability of IrO2 is higher than that of RuO26,7,22 the Ir nanodots degrade faster than the RuO2. We attribute this to a size effect: As shown for fuel cell catalysts, smaller nanoparticles are less stable than bigger ones because of their high surface energy.20,59,60 The decoration of RuO2 with Ir nanodots does not lead to a stability increase. TEM images of the Ir-decorated RuO2 nanoparticles after 2000 cycles of the stability protocol do not show a significant alteration of the shape or size of the RuO2 nanoparticles (Figure 7). Moreover, there are still Ir nanodots on the RuO2 support. Accordingly, EDX measurements yield an Ir/Ru atomic ratio of (7 ± 3)% after 2000 cycles (initial value: (10 ± 4)%). 3.3. Ir-Decorated Pt Nanorods. 3.3.1. Structural Characterization. The Pt nanorod synthesis proved to be robust against changes in the concentration of the different chemicals.

Figure 5. Initial characterization CVs of the stability protocol of Ir nanodots (black solid trace), RuO2 nanoparticles (blue dashed trace), and Ir-decorated RuO2 nanoparticles (red solid trace). The currents are normalized to the peak current. The inset offers a detailled view of the low-potential regime of the CVs. E

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Figure 7. TEM images of Ir-decorated RuO2 nanoparticles after 2000 cycles of the stability protocol. The marked areas in a and b are shown with higher magnification in b and c, respectively. No significant change in the morphology of the RuO2 nanoparticles can be observed. Ir nanodots are still present.

ORR activity. Regarding the Ir@Pt sample, Figure 10 shows that the composite catalyst has pronounced peaks in both the OER and ORR region. Thus, a bifunctional catalyst was obtained. Interestingly, the low-potential CV of Ir@Pt at the end of the activation protocol shows no significant change compared to the CV of Pt. The wide-potential CV shows a broadening of the ORR peak and a negative shift of the currents at low-potentials compared to the CV of pure Pt nanorods. We explain this by the higher amount of O2 that is produced at OER potentials and that can consequently be reduced at lower potentials. Figure 11 shows the OER stability of pure Ir nanodots compared to Ir@Pt during the OER stability protocol and the OER−ORR stability protocol as well as the ORR stability of pure Pt nanorods compared to Ir@Pt during these protocols. The OER stability is defined as the OER peak current IOER in the wide-potential characterization CV after a certain number of degradation cycles normalized to the initial OER current IOER,0. The mean value and standard deviation of four measurements are shown. The ORR stability of the Pt is defined as the hydrogen desorption charge after a certain number of degradation cycles QHdes normalized to the initial value QHdes,0. This charge is correlated with the electrochemically active surface area of the Pt and calculated by integration of the hydrogen desorption currents corrected by the double layer currents.61 The low-potential characterization CVs serve as a basis for these values to ensure that there is no influence of the OER currents on the hydrogen related features. The figure shows the mean value and standard deviation of four measurements. The OER stability decreases strongly during the first hundreds of cycles. Three of four data sets show a decrease of the OER current to roughly 40% after 2000 cycles, whereas it is below 20% in the case of Ir@Pt after 2000 cycles of the OER−ORR stability protocol. In the case of the Ir@Pt samples, there is a small positive offset in the OER stability originating from the OER currents resulting from the Pt support. We explain the difference in the OER stability by the strong Pt degradation when experiencing potential cycling between ORR and OER potentials. This degradation is explained by the repeated build up and reduction of a (hydr)oxide layer on the Pt.18,52 Comparing the OER stability after 200 cycles of the OER−ORR protocol with the results from Papazisi’s study, our unsupported Ir nanodots are as stable as their pure IrO2, whereas our Ir decorated on Pt degrades faster than both their Ir0.8Pt0.2O2 and Ir0.2Pt0.8O2 (decrease of OER current to 47% compared to 86 and 60%).43

As expected, the XRD pattern (Figure 8) shows the Pt peaks to be broadened because of the small crystallite size. The positions

Figure 8. XRD patterns of (a) Ir nanodots prepared without Pt, (b) Pt nanorods, and (c) Pt nanorods decorated with Ir nanodots. The patterns b and c are normalized to the 111 peak. The line patterns show the reference patterns of Pt (black, PDF no. 03-065-2868, ICDD, [2009]) and Ir (light blue, PDF no. 03-065-9327, ICDD, [2009]). The hkl values in the figure are identical for both Ir and Pt.

of the Pt 111, 200, 220, and 311 peaks are shifted by (0.3 ± 0.1)° toward higher angles. This can be explained by a homogeneous compressive strain compared to bulk material. The peak position does not change after Ir decoration. Because the lattice structure of Pt and Ir is very similar and the amount of Ir is low compared to the amount of Pt, no significant changes in the XRD patterns can be observed. The TEM images show agglomerates of crystalline Pt nanorods (Figure 9). Ir nanodots were successfully deposited on the Pt nanorods (Figure 9). The nanorods’ morphology is stable during Ir decoration. The Ir/Pt atomic ratio determined from EDX spectroscopy is (6 ± 3)%. 3.3.2. Electrochemical Characterization. In the first lowpotential CVs of the activation protocol, the Ir HUPD features can be observed on the Ir and the Ir@Pt samples. These features vanish within a few wide-potential cycles because of oxidation of Ir at least at the nanodots’ surface. This is in agreement with the literature.51,58 Figure 10 shows CVs of Ir nanodots, Pt nanorods, and Ir-decorated Pt nanorods after the activation protocol, i.e., after the electrochemical oxidation of the Ir nanodots to IrO2. The CVs of the Pt nanorods and of the (oxidized) Ir nanodots show the typical features:58 Pt exhibits ORR activity but only weak OER activity, whereas the Ir nanodots are a good OER catalyst but show no pronounced F

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Figure 9. TEM images of Pt nanorods (a−c) before and (d−f) after Ir decoration. The marked areas in a, b, d, and e are shown with higher magnification in b, c, e, and f, respectively.

Figure 10. CVs of Pt nanorods (black solid trace), Ir nanodots (blue dashed trace) and Ir@Pt (red solid trace) including OER potentials. Inset: CVs in the lower potential regime used for the determination of the active surface area of the Pt. The CVs of Pt and Ir are as expected. The lower potential CV of the Ir@Pt sample shows no significant changes compared to the Pt nanorod sample. The wide-potential CV of Ir@Pt shows a broadening of the ORR peak and a negative shift of the currents in the lower potential regime.

Figure 11. (a) OER stability and (b) ORR stability of the Ir-decorated Pt nanorods (black) compared to that of the unsupported Ir nanodots (blue) and the pure Pt nanorods (red) as a function of the number of cycles of the OER−ORR stability protocol (crosses) and the OER stability protocol (circles). Lines serve as guide to the eye. The OER− ORR stability protocol is more degrading for Pt than the OER stability protocol. Ir-decorated on Pt degrades faster during the OER−ORR protocol because of Pt degradation.

The OER stability of Kong’s particles determined from the peak current densities after 2000 OER cycles was the same in both cases with 87 ± 5% (Pt/Ir-IrO2) and 85 ± 5% (Pt/IrO2) and was significantly higher than the OER stability of both the unsupported Ir nanodots and the Pt supported ones (roughly 40% after 2000 OER cycles).38 We assume the higher OER stability of Kong’s particles to be due to the contribution of the bigger and thus more stable IrO2 support particles. The results of the ORR stability measurements of pure Pt and Ir@Pt during the OER and OER−ORR stability protocol presented in Figure 11 show that in accordance with the literature the stability is massively decreased when not only OER but also ORR potentials are applied during the protocol.18,52 Whereas there is no significant difference in the stability values of pure

and Ir-decorated Pt throughout the OER protocol, the stability is slightly higher in the OER−ORR stability protocol, when Pt is decorated with Ir. We assume that this difference is due to a partial coverage of the Pt surface by Ir at the beginning. The loss of Ir within the stability protocol leads to an uncovering of formerly blocked Pt surface. The difference in the stability behavior can be explained by an initial blocking of 10% of the Pt surface by Ir. The TEM images of Ir@Pt samples after 400 cycles of the OER−ORR stability protocol support this G

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Figure 12. TEM images of Ir-decorated Pt nanorods after 400 cycles of the stability protocol. The marked areas in a and b are shown at a higher magnification in b and c, respectively. The nanorod structure is conserved. Ir nanodots cannot be observed.

Figure 13. (a) Low-potential (inset) and wide-potential CVs of an Ir@Pt sample at the beginning of the stability protocol, after 400 cycles, and at the end. (b) Ratio of the OER peak current and the second hydrogen desorption peak current of the low-potential CV corrected by the double layer current as a function of the number of stability cycles. Lines serve as guide to the eye. The ratio is in the range of 2 for Pt nanorods. For Ir-decorated Pt nanorods, the ratio is much higher at the beginning of the stability protocol and decreases quickly but is still higher at the end of the stability protocol than that of the Pt sample. This indicates the existence of small amounts of Ir on the Pt even at the end of the protocol.

explanation. In the TEM images, no Ir nanodots could be observed, whereas the nanorod morphology is conserved (Figure 12). EDX measurements of this sample show that the Ir/Pt atomic ratio is below the detection limit after 400 cycles of the OER−ORR stability protocol (compared to (6 ± 3)% initially). Figure 13 shows a CV of an Ir@Pt sample before, during, and at the end of the stability protocol. At the end, the CV looks similar to a CV of pure Pt. To study quantitatively if there are Ir-related OER currents observable in the last CV, we defined a ratio of the OER peak current and the second hydrogen desorption peak current at around −0.4 V vs MMSE. The latter is related to the active Pt surface area and thus the ORR. The second hydrogen desorption peak current was taken from the low-potential CVs and corrected by the double layer current. The figure shows the mean and standard deviation of four measurements. As shown in Figure 13, this ratio is in the range of 2 throughout the whole stability protocol for a pure Pt nanorod sample. In the case of the Ir-decorated Pt nanorods, the ratio is much higher initially because of the Ir loading that leads to much higher OER currents. Within the stability protocol, this ratio decreases quickly, implying a stronger decrease of the OER current than of the ORR-related hydrogen desorption current. Interestingly, the ratio does not reach the values of pure Pt even at the end of the stability protocol. Therefore, we suggest that there is still a small amount of Ir left on the Pt nanorods even after 2400 cycles. This amount is too small to be detectable with EDX spectroscopy, which measures integral over the sample’s volume. Because the Ir was deposited on the support particles, the surface ratio of Ir to Pt is higher

than the volume ratio. Because cyclic voltammetry is a surfacesensitive method, it is capable of detecting Ir (oxides) even below the EDX detection limit.

4. CONCLUSIONS The synthesis of Ir nanodots with a size of 2 nm without support as well as those deposited on RuO2 nanoparticles and Pt nanorods could successfully be conducted. The Ir nanodots are well-dispersed on the RuO2 oxygen evolution catalysts. A shift of the RuO2 XRD peaks is explained by compressive strain and is reduced by Ir synthesis because of interaction with the Ir decoration or by enrichment of IrO2 in the RuO2. In the case of Ir@RuO2, peak analysis with the Scherrer equation leads to an apparent decrease of the mean crystallite diameter after Ir decoration. The underlying peak broadening might also be explained by an increase in lattice inhomogeneities due to interaction with the Ir nanodots or due to enrichment of IrO2 in the RuO2 particles. The Ir peak of the Ir@RuO2 sample is broadened because of interaction with the RuO2. The stability studies yield a decrease of the OER currents of RuO2, Ir@RuO2, and Ir to 60, 50, and 40% of the initial value after 2000 cycles of the OER stability protocol. The higher material-specific stability of IrO2 seems to be overcompensated by size effects. The stability of RuO2 could not be increased by the decoration with Ir nanodots. Still, Ir nanodots are observable in TEM and EDX measurements after 2000 cycles. Ir nanodots could successfully be deposited on Pt nanorods to yield bifunctional OER−ORR catalysts. The nanorods’ morphology is stable during Ir synthesis. To investigate the H

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stability of the bifunctional Ir@Pt OER−ORR catalyst toward simulated charge−stop and charge−discharge operation of an URFC, OER stability protocols as well as OER−ORR stability protocols were conducted. The OER currents of unsupported Ir decrease to 40% of the initial value after 2000 cycles of both stability protocols. The degradation of Ir deposited on Pt is the same in the case of the OER stability protocol, but in the case of the OER−ORR stability protocol, the OER current is less than 20% of the initial value after 2000 cycles. This is due to the rapid Pt degradation when experiencing alternating ORR and OER potentials. The active surface area of Pt with and without Ir decoration is still more than 90% of the initial value after 2000 cycles of the OER stability protocol. In the case of the OER−ORR stability protocol, the pure Pt nanorods’ active surface area degrades to less than 50%, whereas it is in the range of 60% when the Pt is decorated with Ir. This can be explained by the initial blockage of Pt surface by Ir and degradation of Ir during the stability protocol and, consequently, uncovering of formerly blocked Pt surface. Although no Ir is observable in the Ir@Pt sample after 400 cycles of the OER−ORR stability protocol in EDX and TEM measurements, the analysis of CVs indicated the presence of small amounts of Ir left even at the end of the stability protocol. Further research is required for a deeper understanding of the interaction of the parts of composite catalysts at the surface and to enhance further the stability of composite oxygen catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.G. thanks the Stiftung der Metallindustrie im Nord-Westen and the Reiner Lemoine Stiftung for financial support via scholarships. We gratefully acknowledge funding by EWE AG Oldenburg. Furthermore, we thank M. Macke for technical support and E. Rhiel and U. Friedrich for support with TEM measurements. Funding of the HR-TEM by the DFG (INST 184/106-1 FUGG) is acknowledged.

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