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Jan 19, 2012 - ... Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions ... and §Electrochemical Energy Laboratory, Massachusetts Instit...
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Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions Youngmin Lee,†,§,∥,⊥ Jin Suntivich,‡,§,∥ Kevin J. May,†,§ Erin E. Perry,‡,§ and Yang Shao-Horn*,†,‡,§ †

Department of Mechanical Engineering, ‡Department of Materials Science and Engineering, and §Electrochemical Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: The activities of the oxygen evolution reaction (OER) on iridium-oxide- and ruthenium-oxide-based catalysts are among the highest known to date. However, the OER activities of thermodynamically stable rutile iridium oxide (r-IrO2) and rutile iridium oxide (r-RuO2), normalized to catalyst mass or true surface area are not well-defined. Here we report a synthesis of rIrO2 and r-RuO2 nanoparticles (NPs) of ∼6 nm, and examine their OER activities in acid and alkaline solutions. Both r-IrO2 and r-RuO2 NPs were highly active for OER, with r-RuO2 exhibiting up to 10 A/goxide at 1.48 V versus reversible hydrogen electrode. When comparing the two, r-RuO2 NPs were found to have slightly higher intrinsic and mass OER activities than r-IrO2 in both acid and basic solutions. Interestingly, these oxide NPs showed higher stability under OER conditions than commercial Ru/C and Ir/C catalysts. Our study shows that these r-RuO2 and r-IrO2 NPs can serve as a benchmark in the development of active OER catalysts for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications. SECTION: Energy Conversion and Storage

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particle sizes,14−16 different crystal structures (rutile18 or amorphous 15 ), and different degrees of hydration (as IrO2·xH2O23,24 or RuO2·xH2O25). Interestingly, recent density functional studies have shown that rutile RuO2 (r-RuO2) may exhibit higher intrinsic OER activity than rutile IrO2 (r-IrO2).12 However, it is not apparent whether this holds true for these oxide NPs. To assist with the quantitative comparison in the OER activities of these two catalysts at the nanoscale, we report a synthesis of r-IrO2 and r-RuO2 NPs of ∼6 nm and a systematic study of their OER activities (normalized to either mass or surface area) in both acid and basic solutions. In this study, r-IrO2 and r-RuO2 NPs were prepared by first synthesizing metal NPs in an oleylamine-mediated solution, followed by thermal oxidation of the NPs in O2 atmosphere. Xray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) results showed high crystallinity of the synthesized r-IrO2 and r-RuO2 NPs. Thin films of r-IrO2 and r-RuO2 NPs with low roughness factor (∼10 cm2oxide/cm2disk), similar to those used extensively to evaluate the activities of oxygen reduction reaction (ORR) in the fuel cell electrocatalysis,26,27 were used for OER measurements. The intrinsic and the mass activities of r-IrO2 and r-RuO2 NPs in acid (pH 1) and alkaline (pH 13) solutions were quantified and compared.

he efficiency of hydrogen fuel generation from photoelectrochemical water-splitting1−3 and electrolysis4−6 is severely limited by the sluggish kinetics of the oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e− in acid, 4OH− → O2 + 2H2O + 4e− in alkaline). Finding an effective electrocatalyst can lower the overpotential needed to sustain an appreciable current, and is therefore an avenue to improve the efficiency of fuel generation technologies.6−8 In the search for highly active and cost-effective OER electrocatalysts, the OER activities of many catalysts have been screened on an absolute current basis,7 where electrodes can have different catalyst mass loadings and catalysts can have different true surface area per unit mass. This makes direct comparison of catalysts challenging; true-surface-area- and mass-normalized OER activities are required to establish an activity descriptor from volcano trends9−12 and project actual device performance,13 respectively. Iridium oxide (IrO2) and ruthenium oxide (RuO2) catalysts are among the most active reported to date.7 Due to their high cost and low elemental abundance, IrO2-based and RuO2-based nanoparticles (NPs) with high surface area-to-mass ratios (similar to that of Pt NPs for fuel cell applications13) have been widely studied. For example, iridium oxide NPs have been synthesized in aqueous phase14−17 or using dimethyl sulfoxide (DMSO)−metal adducts,18 and ruthenium oxide NPs can be made via chemical vapor deposition,19 electrochemical deposition,20 and polyol methods.21,22 However, the OER activities of these iridium oxide and ruthenium oxide NPs are often not comparable, which can be attributed to different © 2012 American Chemical Society

Received: December 15, 2011 Accepted: January 16, 2012 Published: January 19, 2012 399

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Scheme 1. Schematic Illustration of IrO2 and RuO2 NP Synthesis Route

Both r-IrO2 and r-RuO2 NPs were prepared by a two-step procedure, as shown in Scheme 1. IrCl4·xH2O or RuCl3·xH2O precursor was dissolved in a solution of 1,2,3,4-tetrahydronaphthalene (tetralin) and oleylamine (10 mgprecursor/mLtotal‑solvent) at 60 °C. Reducing agent, tetrabutylammonium borohydride (TBABH), was injected into the mixture at the same temperature, then the mixture was heated to 200 °C. The reaction solvent was kept at a 1:1 mixture of oleylamine and tetralin, where only weakly binding oleylamine was used as a stabilizing agent for facile removal in the next step. Monodispersed Ir NPs of 5 (±1) nm and Ru NPs of 4 (±1) nm (Figure 1A,B) were obtained after 19 and 11 h of reaction

Figure 2. XRD patterns of Ir NPs (space group: Fm3̅m) and Ru NPs (P63/mmc), r-IrO2 NPs (P42/mnm), and r-RuO2 NPs (P42/mnm).

NPs had uniform sizes, with an average particle size of 7 (±2) nm and 6 (±2) nm, respectively (Figures 1C−F and S1 (Supporting Information)). Fast Fourier Transform (FFT) analyses of HRTEM images of r-IrO2 and r-RuO2 NPs further confirm the rutile structure and high crystallinity. Thermogravimetric analyses showed that the surface of r-IrO2 and r-RuO2 NPs was surfactant-free as stabilizing agents on the surfaces of Ir and Ru NPs were removed with the annealing treatment (Figure S2). We now describe the OER activity measurement of r-IrO2 and r-RuO2 NPs in both acid and alkaline solutions. Cyclic voltammograms (CVs) were collected from a thin-film of rIrO2 (Figure 3A,B) and r-RuO2 NPs (Figure 3C,D) supported on a glassy carbon electrode (GCE) at a loading of 0.05 mgox cm−2disk in O2-saturated 0.1 M KOH (pH ∼ 13) and 0.1 M HClO4 (pH ∼ 1). The onset of oxidation currents of both rIrO2 and r-RuO2 NPs in both acid and base occurs at ∼1.4 V versus reversible hydrogen electrode (RHE), which is assigned to the OER current. To ensure that the observed oxidation current resulted from the OER, we used a rotating ring disk electrode (RRDE) to detect the amount of evolving O2 gas, which was reduced at the Pt ring electrode at 0.4 V vs RHE (Figure S3). Considering the ring collection efficiency of 21% calibrated to the [Fe(CN)6]3−/[Fe(CN)6]4− couple, the observed ring ORR current (19%) suggests that the generated oxygen can account for nearly all of the oxidation current. To extract the intrinsic and mass OER activities of r-IrO2 and r-RuO2 NPs, ohmic drop (iR) and capacitive current corrections were applied to the measured OER currents (see Experimental Section and Figure S4). No transport correction was applied because no significant difference between the OER polarizations measured at 100 and at 1600 rpm was observed. The capacitance-corrected and iR-corrected OER currents were then normalized to the true surface area of the catalyst, which was estimated from particle size analysis from TEM images (see

Figure 1. TEM images of (A) ∼5 nm Ir NPs, (B) ∼4 nm Ru NPs, (C) ∼7 nm r-IrO2 NPs, and (D) ∼6 nm r-RuO2 NPs obtained from annealing metal NPs (A) and (B) in pure O2. HRTEM images of (E) IrO2 NPs and (F) RuO2 NPs with FFTs in the inset indexed to the rutile structure with space group P4 2 /mnm. Lattice fringes corresponding to the (101) and (110) planes in the Miller indices were pronounced in both oxides.

in Ar, respectively. These Ir and Ru NPs were annealed subsequently at 500 °C for 20 h in pure O2. XRD analysis of Ir and Ru NPs before and after the annealing revealed the formation of single-crystalline, r-IrO2 and r-RuO2 NPs (space group P42/mnm), as shown in Figure 2. Low- and highresolution TEM images showed that the r-IrO2 and r-RuO2 400

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Figure 3. CVs of r-IrO2 and r-RuO2 NPs supported on GCE (0.05 mgox cm−2disk) in O2-saturated (A-B) 0.1 M KOH and (C-D) 0.1 M HClO4 at 1600 rpm at 10 mV s−1 at room temperature. Sweep directions are shown by arrows.

μA cm−2ox at η = 0.25 V28). r-RuO2 NPs of this study, however, show a lower specific activity than that of the RuO2-based catalyst prepared from a thermal treatment of RuCl3·xH2Ocoated Ti substrate reported by Burke et al.29 (ca. 40 μA cm−2ox) in 1 M H2SO4. While the reason for this discrepancy requires further investigation, the difference may be attributable to the presence of some amorphous RuO2-based species in the latter study; recently, amorphous RuO2 has been reported to be more active than r-RuO225 The capacitance-corrected, iR-corrected OER current was normalized to the catalyst loading to give mass activities of rIrO2 and r-RuO2 NPs, as shown in Figure 4A,B. The mass activities of r-IrO2 and r-RuO2 NPs are higher in 0.1 M HClO4 than in 0.1 M KOH, and the mass activities of r-IrO2 NPs are slightly lower than r-RuO2 NPs in both acid and basic solutions, owing to comparable particle sizes and the higher specific activity of r-RuO2. The mass activities of r-IrO2 NPs in this work (3 A g−1ox at overpotential η = 0.25 V in acid) are in good agreement with those of IrO2 NPs reported by Rasten et al.23 (1−2 nm in a Nafion-based membrane electrode assembly, ∼5 A g−1ox) and reported by Marshall et al.6 (prepared via thermal decomposition of iridium salt on Ti support in 0.5 M H2SO4, ∼2 A g−1ox) at the same overpotential. Similarly, our OER mass activities of r-RuO2 (11 A g−1ox at η = 0.25 V in acid) are comparable to those of RuO2 prepared via the thermal decomposition of a catalyst ink containing RuO2 supported

Supporting Information and Figure S2), to give the specific activity, is. Figure 4C,D shows the specific OER activities of r-IrO2 and r-RuO2 NPs from CV measurements, respectively. The r-IrO2 OER specific activity was consistent with what we previously reported in our earlier study.11 It is worth mentioning that the specific OER activity of r-IrO2 NPs was found to be independent of NP loading or the presence of carbon support or Nafion on GCE (Figure S5), and matched the activity that was extracted from steady-state galvanostatic measurements (Figure S6). We observe that the specific OER activities in acid are higher than in base for both r-IrO2 and r-RuO2 NPs. Interestingly, the specific OER activities of r-IrO2 NPs are slightly lower than rRuO2 at low overpotentials in both acid and alkaline electrolytes. Quantitatively, the intrinsic OER activities of rIrO2 NPs are ∼4 μA cm−2ox in acid and ∼2 μA cm−2ox in alkaline (consistent with our previous report11), while r-RuO2 NPs show OER activities of ∼10 μA cm−2ox in acid and ∼3 μA cm−2ox in alkaline at η = 0.25 V (1.48 V vs RHE). The higher OER activity of r-RuO2 is in agreement with a computational study by Rossmeisl et al.,12 owing to its optimum bonding strength (neither too weak nor too strong) to the OER intermediate species from a thermodynamics standpoint over rIrO2. In comparison to the literature, our r-IrO2 NPs exhibit a comparable specific OER activity to the extrapolated value of a porous IrO2 film supported on Ti in 0.5 M H2SO4 value of (∼ 2 401

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Figure 4. Capacitance-corrected and iR-corrected OER specific activities, is, and mass activities, im, of (A,B) r-IrO2 and (C,D) r-RuO2 NPs as a function of potential using estimated surface area from TEM images (see Experimental Section) and catalyst loadings (0.05 mgox cm−2disk) in 0.1 M KOH or 0.1 M HClO4. Error bars represent standard deviations from at least three independent repeated measurements.

on SnO2 on Ti substrate in 0.5 M H2SO4 (ca. 19 A g−1RuO2 at η = 0.25 V).30 The OER activities of r-IrO2 and r-RuO2 NPs were compared against commercial Ir and Ru NPs supported on carbon, which are often used for OER activity benchmarking.31,32 We found that the OER mass activity of Ir/C (40% Premetek, davg ∼ 4 nm) was 9 A g−1Ir at 1.48 V vs RHE in 0.1 M KOH, which was comparable to the recent work of Gorlin and Jaramillo.31 Although the mass activity of Ir/C was higher than that of r-IrO2 (Figure S7), the activity of Ir/C gradually degrades over subsequent CV cycles, unlike r-IrO2 NPs, which have stable activity during CV cycling over the course of a few hours (Figure S7). The commercial Ru/C catalyst (40% Premetek) degraded much faster than Ir/C and r-RuO2 (Figure S8), where very high oxidation currents with an onset at 1.3 V vs RHE were found in the first cycle but faded significantly in the subsequent cycle. This is in agreement with studies on Ru black electrodes by Burke and O’Meara.33 As the surfaces of Ir and Ru NPs supported on carbon are largely oxidized at the OER potentials,34−37 the activity degradation of Ir/C and Ru/C for OER can be attributed to the instability of surface oxides on the surfaces of Ir and Ru NPs and carbon support, which is in line with the finding that both surfaces corrode significantly above the OER potential.33,38 In summary, we report the quantification of true-surfacearea-normalized (“intrinsic or specific activity”) or massnormalized (“mass activity”) of r-IrO2 and r-RuO2 NPs of ∼6

nm in acid and basic solutions. These findings can serve as an OER activity benchmark for the development of highly active and stable oxide catalysts.

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EXPERIMENTAL SECTION Chemicals. The following chemicals were used as received: Iridium(IV) chloride hydrate (IrCl4·xH2O, 99.99%, SigmaAldrich), ruthenium(III) chloride hydrate (RuCl3·xH2O, 99.98%, Sigma-Aldrich), TBABH (Alpha Aesar), oleylamine (70%, Sigma-Aldrich), 1,2,3,4-tetrahydronaphthalene (tetralin, 99%, Sigma-Aldrich), chloroform (99%, Sigma-Aldrich). Synthesis of IrO2 and RuO2 NPs. A solution of 0.6 mmol of IrCl4·xH2O, 10 mL of tetralin and 10 mL of oleylamine was prepared at room temperature (20 °C) in a four-neck flask. The mixture was heated to 60 °C under Ar atmosphere (Airgas 99.999%) and kept at that temperature for 20 min to dissolve the precursor homogeneously into the solution with continuous stirring. TBABH (2.4 mmol) in 2 mL of chloroform was quickly injected into the above solution and left for 20 min. The reaction mixture was further heated up to 200 °C and kept at that temperature for 19 h. The solution was cooled down to room temperature, and Ir NPs were precipitated by ethanol addition and collected by centrifugation. The product was redispersed in hexane and separated by ethanol addition and centrifugation. This procedure was repeated twice. The final product was dispersed in hexane. In the synthesis of RuO2, A 402

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solution of 1.0 mmol of RuCl3·xH2O, 10 mL of tetralin, and 10 mL of oleylamine was prepared at room temperature (20 °C) in a four-neck flask. The rest of the procedure is same as that of the above Ir NP synthesis, except that 3 mmol of TBABH was used and the reaction at 200 °C was kept for 11 h. The metal NPs was transferred to a porcelain boat and was annealed in a tube furnace at 500 °C for 20 h under O2 atmosphere (Airgas, 99.994%). Structural Characterization. Powder XRD patterns from NPs drop-cast on a glass slide were obtained on a PANalytical X’pert Pro with Cu Kα radiation (λ = 1.5418 Å). Samples for TEM analysis were prepared by depositing and drying a single drop of diluted NP dispersion in hexane on amorphous carboncoated copper grids under ambient conditions. TEM images of NPs were obtained on a JEOL 2010 (200 kV) and a JEOL 2010F (200 kV), from which particle sizes and true surface area were estimated. Electrochemical Measurements. All electrochemical measurements were conducted in a three-electrode glass cell (Pine Instrument) and using a rotator (Pine) to which the thin-film RDE working electrodes were attached; the potential was controlled using a VoltaLab PST050 potentiostat. 0.1 M KOH was prepared from Milli-Q water (18 MΩ cm) and KOH pellets (99.99% purity, Sigma-Aldrich) and 0.1 M HClO4 was prepared from the Milli-Q water and HClO4 solution (GFS Chemicals, Veritas double-distilled). The thin-film electrode was prepared from a slow drying of a catalyst ink prepared in 70 wt % isopropanol (Sigma-Aldrich, >99.5%) in Milli-Q water, dropcast on a GCE (Pine Instruments). All measurements were conducted at 10 mV s−1 under O2 (99.994%, Airgas) saturation at room temperature to ensure that the thermodynamic potential of the OER is at 1.23 V vs RHE. A saturated calomel electrode (SCE) reference electrode (Pine Instruments) was calibrated in the same electrolyte by measuring hydrogen oxidation/evolution currents on a Pt-RDE and defining the potential of zero current as RHE. All the potentials in this study were referenced to the RHE potential scale and correspond to the applied potentials, Eapplied, unless they are stated to be iRcorrected potentials, denoted as E − iR, where i is the current and face R is the uncompensated ohmic electrolyte resistance (∼45 Ω for 0.1 M KOH and ∼37 Ω for 0.1 M HClO4) measured via high-frequency AC impedance in the same electrolyte under O2-saturation. The accuracy of our surface area estimation method has been reported in our previous work, where we found that the oxide catalyst’s surface areas determined from electron micrograph and BET measurement agree within a factor of 2−3.11,26 This is similar to the case of platinum NPs, where it was found that the methods of surface area estimation using TEM and electrochemical surface area analysis are within a factor of 2−3 as well.39 Combining with the uncertainty in the thin-film loading (up to an additional factor of 2 reflected by the error bars in the voltages in Figure 4), we note that the experimental uncertainty in this study can influence the accuracy in the intrinsic or specific activities of rIrO2 and r-RuO2 NPs. This should be taken into account when considering the slightly higher specific activities of RuO2 versus IrO2 NPs observed in this study.

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This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.

Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The electron microscopy portion of this work made use of the Shared Experimental Facilities supported by the MRSEC Program of the National Science Foundation under award number DMR-0819762. The synthesis and the catalytic components were supported by Eni S.p.A under the Eni-MIT Alliance Solar Frontiers. J. S. was supported in part by the Chesonis Foundation Fellowship, K.J.M. was supported in part by a Natural Sciences and Engineering Research Council of Canada postgraduate scholarship, and E.E.P. was supported by the John Reed scholarship from the Undergraduate Research Opportunities Program. S. Chen is acknowledged for her assistance with high-resolution TEM measurements.

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ASSOCIATED CONTENT

S Supporting Information *

Details about particle size analysis, capacitance measurement, thermogravimetry, RRDE, and OER electrochemical methods. 403

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