Highly Active and Stable Iridium Pyrochlores for ... - ACS Publications

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Highly Active and Stable Iridium Pyrochlores for Oxygen Evolution Reaction Dmitry Lebedev,† Mauro Povia,‡ Kay Waltar,‡,∥ Paula M. Abdala,§ Ivano E. Castelli,⊥ Emiliana Fabbri,‡ Maria V. Blanco,¶ Alexey Fedorov,†,§ Christophe Copéret,*,† Nicola Marzari,*,⊥ and Thomas J. Schmidt*,‡,† †

Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, CH-8093 Zürich, Switzerland Electrochemistry Laboratory, Paul Scherrer Institute, CH-5232, Villigen PSI, Switzerland § Department of Mechanical and Process Engineering, ETH Zürich, Leonhardstrasse 21, CH-8092, Zürich, Switzerland ⊥ Theory and Simulation of Materials (THEOS) and National Centre for Computational Design and Discovery of Novel Materials (MARVEL), École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ¶ European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France ‡

S Supporting Information *

ABSTRACT: Proton exchange membrane water electrolysis (PEMWE) is a promising technology for electricity-to-fuel conversion which allows for direct production of hydrogen from water. One of the key problems limiting widespread implementation of PEMWE into energy systems is the sluggish kinetics of the anodic process: the oxygen evolution reaction (OER). Additionally, state-of-the-art OER materials contain large amounts of low abundant noble metals (Ru, Ir), and therefore, development of low-cost, highly active and stable OER catalysts remains an important challenge. We developed a synthetic approach to the iridium pyrochlores−complex oxides of iridium with reduced content of the noble metal as compared to IrO2. The materials were synthesized from molten sodium nitrate (Adams fusion method) at moderate temperatures (500−575 °C) and consist of highly crystalline iridium pyrochlore nanoparticles with surface areas of up to 40 m2 g−1, which is a significant improvement compared to the traditional high temperature solid-state synthesis. Electrochemical measurements in acidic media showed that yttrium and bismuth pyrochlore catalysts possess high OER activity approaching the activity of state-of-the-art IrO2 nanoparticles. High intrinsic activities and stability behavior of yttrium iridium catalysts were correlated with the formation of the highly active IrOx surface layer due to leaching of the Y3+ cations into the electrolyte solution, revealed both experimentally and computationally using density functional theory calculations.

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INTRODUCTION Energy storage has become a significant challenge due to the continuous growth of electricity production from intermittent renewable energy sources.1,2 Proton exchange membrane water electrolysis (PEMWE) with a solid proton conductive electrolyte is a promising technology for direct conversion of electrical energy into H2 at pressures up to 300 bar enabling H2 downstream usage without or only with small further mechanical compression.3−7 Moreover, PEMWE takes place in acidic environment and thus benefits from fast kinetics of the cathodic hydrogen evolution reaction and high-voltage efficiencies at high current densities. However, harsh acidic conditions necessitate the use of highly durable anode materials for the oxygen evolution reaction (OER); the pool of such catalysts is primarily limited to noble metal oxides, for instance, RuO2, IrO2, and their solid solutions.8,9 The high cost and low abundance of the latter catalysts hamper wide implementation of the PEMWE technology, making the development of active, © 2017 American Chemical Society

stable, and inexpensive OER catalysts a particularly important objective. To reduce the noble metal content in the OER catalysts, noble metal oxides can be dispersed on the high surface area conductive supports.10−12 Another promising strategy is the synthesis of mixed or complex oxides (containing more than one metal) where noble metals are partially replaced by earth-abundant elements providing a compromise between activity, stability, and cost of the OER catalysts.13−17 Noble metal pyrochlores, particularly Ru and Ir-based, are active and stable OER catalysts for water electrolysis.18,19 Pyrochlores are a family of complex oxides with a general formula A2B2O6.5+x, where A is typically Bi, Pb, Ae (alkaline earth metal), Ln (rare earth metal), and B could be Ti, Sn, Nb, Ir, Ru, Os.20 The highly versatile and flexible structure of Received: February 22, 2017 Revised: May 30, 2017 Published: May 31, 2017 5182

DOI: 10.1021/acs.chemmater.7b00766 Chem. Mater. 2017, 29, 5182−5191

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Chemistry of Materials

Contrary to the hydrothermal synthesis, the Adams fusion method does not require use of pressurized vessels and strong unstable oxidizing agents.

pyrochlores (Figure 1) allows for a control of their properties that were exploited in a broad range of applications including

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EXPERIMENTAL SECTION

Chemicals. IrCl3·xH2O (99.9%-Ir) was purchased from Strem Chemicals (the water content was calculated based on the Certificate of Analysis provided by Strem Chemicals); BiONO3 (99.999%) and Y(NO3)3·6H2O (99.9%) were purchased from ABCR. Pb(NO3)2 (>99%), IrO 2 reference XAS catalyst (99.9%), and NaNO 3 (>99.5%) were purchased from Sigma-Aldrich. Bi(NO3)3·xH2O (99.999%) and Nafion perfluorinated resin solution (5 wt %, Nafion 117) were purchased from Alpha Aesar. Milli-Q water and HClO4 (60%, Kanto Chemical Co., Inc.) or H2SO4 (96% by Merck KGaA) were used for electrolyte preparation. Materials Preparation. (A, A′)2Ir2O6.5+x pyrochlore materials (A, A′ = Bi, Pb, Y) were synthesized via a modified Adams fusion method according to the following procedure. Synthesis of Yttrium Pyrochlore (Y−Ir): Representative Procedure. IrCl3·xH2O (0.212 g, 0.6 mmol), Y(NO3)3·6H2O (0.276 g, 0.72 mmol, 1.2 equiv), and NaNO3 (10 g) were dissolved in 100 mL of deionized H2O, stirred for 10 min with sonication, and then dried under about 15 mbar to give a black powder. The powder was transferred into a porcelain crucible and heated in a muffle furnace at 550 °C for 2 h (heating program: 2 °C min−1 to 150 °C, 30 min at 150 °C, ramp to the target reaction temperature for 2 h). Thusobtained sample was washed with deionized water and dried at 150 °C overnight under about 2 mbar to give the pyrochlore as a black powder (see Supporting Information and Figure S1 for more synthesis details). Synthesis of Bismuth, Lead, and Mixed Pyrochlores. Other pyrochlore materials (denoted as Bi−Ir, Pb−Ir, BiY−Ir, BiPb−Ir, and YPb−Ir) were synthesized analogously from IrCl3·xH2O and BiONO3, Pb(NO3)2, Y(NO3)3·6H2O or their mixtures. Bi(NO3)3·xH2O and BiONO3 can be used interchangeably. Synthetic conditions were optimized to prepare phase-pure pyrochlore materials; in cases where a mixture of phases formed, the procedure was optimized to minimize the amount of impurity (details are given in the Supporting Information). Addition of 10−20% excess of Bi or Y precursor was necessary to suppress the formation of IrO2 (Table 1); the use of excess of Pb(NO3)2 did not allow to completely avoid the formation of iridium oxide; thus, lead pyrochlores contain minor amount of IrO2 detectable only by transmission electron microscopy (TEM) imaging (see further). Synthesis of IrO2 Materials. IrO2 reference for XAS was prepared by the Adams fusion method at 500 °C (see the Supporting Information for the detailed procedure). IrO2 reference OER catalysts (IrO2−150 and IrO2−30 with surface area indicated in the name of the material) were also prepared by the Adams fusion method following a recently published procedure.34 Characterization Methods. Nitrogen adsorption/desorption experiments were performed on a BELsorp-mini II, and the specific surface area of the samples was determined using Brunauer−Emmett− Teller (BET) analysis.35 Powder XRD experiments were performed on

Figure 1. Structures of iridium pyrochlore and IrO2 (rutile).

OER catalysis for Li−O2 batteries,21 photocatalytic water splitting,22 and complex magnetic materials.23 A promising approach to achieve high mass activity and reduced catalyst cost with more efficient catalyst utilization is the development of OER materials with increased surface area, for example, consisting of small particles. In addition, stability of the catalyst under the reaction conditions is also a key factor that determines the lifetime of the electrolyzer. The requirement for anode stability in a PEMWE is set by the high electrochemical potential (>1.5 V), low pH (0−1), and the reaction temperature (∼80 °C). Similarly to iridium oxide, which features the best stability under OER conditions compared to the other oxides (including RuO2),24 iridium pyrochlores have highest stability among other pyrochlore materials.25,26 However, no simple synthetic method yields Ir pyrochlores as high surface area materials.18,19,26−30 Indeed, conventional solid-state synthesis requires high temperatures to prepare phase pure materials (typically ≥800 °C), which produce catalysts with low surface area (≤2 m2 g−1).18 Few recent reports describe the preparation of high surface area iridium pyrochlores (up to 80 m2 g−1, 46 m2 g−1 for Bi2Ir2O7) using hydrothermal conditions27,31 in the presence of strong oxidizing agents (H2O2, Na2O2).19,27 Overall, the state-of-theart synthetic methods illustrate the rather limited options to prepare iridium pyrochlores with high surface area. Synthesis of nanoparticles from molten sodium nitrate (Adams fusion method) is a powerful tool for preparation of noble metal oxides with high surface area upon tuning the reaction conditions.32−34 Thus, we explored this approach for the synthesis of complex iridium oxides, in particular bismuth, lead, and yttrium pyrochlores, as these compositions provide active OER catalysts.18 We report a simple, general, and gramscalable approach that yields highly OER active and stable iridium pyrochlores with surface areas of up to 40 m2 g−1.

Table 1. Synthesis Conditions, Surface Area, Unit Cell Parameters, XAS, XPS, and Conductivity Data for Synthesized Pyrochlores XPS composition, molM%a pyrochlore

A/A′/Ir nominal ratio

Bi−Ir Y−Ir Pb−Ir BiY−Ir BiPb−Ir YPb−Ir

1.2:1 1.2:1 1:1 1.1:1.1:2 1.1:1:2 1.1:1:2

a

synthesis temperature and time

surface area, m2 g−1

unit cell parameter a, Å

XAS edge energy, eV

°C, °C, °C, °C, °C, °C,

30 21 24 28 38 11

10.489(1) 10.190(5) 10.381(4) 10.330(3) 10.457(2) 10.251(3)

11 217.6(2) 11 217.3(2) 11 217.6(2) 11 217.4(2) 11 217.3(2) 11 217.4(2)

500 550 500 550 500 575

1h 2h 1h 1.5 h 1h 2h

A position Bi 61.3(9) Y 58(3) Pb 51.5(6) Bi 28.1(8); Y 28.7(6) Bi 26.0(5); Pb 29(1) Y 33.8(2); Pb 26.6(4)

Ir 38.7(8) 42(3) 48.5(6) 43(1) 45(2) 39.6(1)

conductivity, S cm−1 2(1) 0.10(2) 4(1) 0.5(1) 5(1) 1.1(1)

molM% refers to mole percent of element with respect to total metal content (A (+A′) + Ir = 100%). 5183


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Chemistry of Materials a STOE Padi Diffractometer in Debye−Scherrer Mode (2θ) with a Dectris Mythen 1K area detector using Cu Kα1 radiation. Synchrotron X-ray total scattering measurements were performed at the ID31 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using 0.15609 Å radiation; the details about measurement, Rietveld refinement, and the pair distribution function (PDF) data analysis are given in the Supporting Information. Transmission electron microscopy was done on Philips CM12 (100 kV) equipped with CCD detector and Tecnai F30 FEI (300 kV). X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG ESCALAB 220iXL spectrometer (Thermo Fischer Scientific) equipped with an Al Kα monochromatic source (15 kV/ 150 W, 500 μm beam diameter) and a magnetic lens system. The binding energies of the acquired spectra were referenced to the C 1s line at 284.6 eV. Background subtraction was performed according to Shirley,36 and the atomic sensitivity factors (ASF) of Scofield were applied to estimate the atomic composition.37 Electrical conductivities of pyrochlores were determined based on four-point DC measurements of powder disks under 0.625 MPa pressure with thicknesses in the range 50−400 μm. Specific electrical conductivity was calculated using Ohm’s law and averaged for different pellet thicknesses to give the reported values. Elemental analysis of the electrolyte solutions was performed after the stability measurements using inductively coupled plasma optical emission spectroscopy (ICP-OES, Varian−Agilent Technologies Inc., VISTA Pro AX). X-ray absorption (XAS) spectra (Ir LIII edge) were collected at the SuperXAS beamline of the Swiss Light Source (SLS) (PSI, Villigen, Switzerland). The beamline energy was calibrated with Pt reference foil to the Pt LIII-edge position at 11564 eV. X-ray absorption data were analyzed using the Demeter program package,38,39 which included energy calibration (based on the simultaneously measured Pt reference foil), background subtraction, and edge step normalization. To obtain extended X-ray absorption fine structure (EXAFS) data, the resulting spectra were converted to the photoelectron wave vector k (in units Å−1) by assigning the photoelectron energy origin E0, corresponding to k = 0, to the first inflection point. The resulting χ(k) functions were weighted with k2 to compensate for the dampening of the XAFS amplitude with increasing k. These χ(k) functions were Fourier transformed over 3−12 Å−1 for Bi−Ir and Pb− Ir, and over 3−14 Å−1 for Y−Ir, BiY−Ir, BiPb−Ir, and YPb−Ir. The theoretical model used for the EXAFS fitting was generated from Bi2Ir2O7 and Y2Ir2O7 pyrochlore structures using the FEFF6.2 library. Details of the EXAFS spectra fitting approach can be found in the Supporting Information. Electrochemical Measurements. The electrochemical oxygen evolution activity of the prepared pyrochlore materials was evaluated in a standard single-compartment three-electrode cell using an RDE setup (Pine Instruments, USA) and a BioLogic VMP-300 potentiostat. All glassware was vigorously cleaned in a solution of 98% H2SO4 and 30% H2O2 and then boiled in Milli-Q water several times before use.40 Catalyst ink to prepare thin-film RDEs suspensions40 was prepared using 10 mg of the pyrochlore catalyst, 1.0 mL of Milli-Q H2O, 4.0 mL of isopropyl alcohol, and 20 μL of 5 wt % Nafion solution. The ink suspensions were sonicated for 30 min and spin coated on freshly polished glassy carbon electrode (twice using an aliquot of 5 μL with intermediate drying) resulting in the overall loading of 102 μg cm−2. A piece of a platinum mesh served as the counter electrode, and a Hg/ HgSO4 electrode served as the reference electrode, all potentials reported herein refer to the RHE scale. The 0.1 M HClO4 or 1M H2SO4 electrolytes were saturated with synthetic air during all measurements. Cyclic voltammograms (CVs) were recorded in the potential range of 1.0−1.4 V at 50 mV s−1, mass-normalized surface charge was obtained by integration of the CVs with normalization to the amount of the catalyst used (20 μg). Polarization curves were obtained from the steady-state chronoamperometric measurements: the potential was gradually stepped from 1.2−1.6 V while holding for 1 min at each potential. Average of the last 10 s of the current for each potential step and solution ohmic drop corrected potentials were used for Tafel plot construction. Electrochemical impedance spectroscopy measurements were recorded in the range of 15 kHz to 1 Hz with an

amplitude of 10 mV. To test the electrochemical stability of the catalysts, the RDE setup has been tilted about 15° compared to its standard vertical configuration keeping a rotation speed of 2900 rpm for the whole stability test. The use of the rotational drying method (50 rpm) to deposit the catalysts on the glassy carbons results in uniform electrodes,41 while the fast rotation of the titled RDE setup allows removal of most of the gas bubbles formed when the electrode is polarized in the OER regime. The use of both strategies allows a more reliable evaluation of the intrinsic electrochemical stability of the catalysts. During the stability measurements, the potential was switched between 1.0 and 1.6 V with 10 s holding at each value (overall 500 cycles). Reported current density values were obtained at 1.6 V after every 100 cycles. All measurements were repeated at least three times to ensure reproducibility. Reported current densities were either normalized to geometrical surface area of electrode (mA cm−2geom.), surface area of the electrode catalyst (mA cm−2oxide), or the amount of the iridium in the sample (determined from XPS measurements, A g−1Ir). Density Functional Theory Calculations. Stability of the pyrochlore materials was additionally studied by calculating the phase and Pourbaix diagrams using density functional theory (DFT).42,43 Details of the calculations are given in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis and Characterization. Highly crystalline pyrochlore nanoparticles were synthesized via a modified Adams fusion method from molten sodium nitrate using iridium chloride and metal nitrates as precursors. According to BET analysis of the N2 adsorption/desorption isotherms, the prepared materials have surface areas in the range from 11−38 m2 g−1 (Table 1) with a slight correlation with the synthesis temperature (higher temperature−lower surface area). Synthesis of the iridium pyrochlores with these surface areas is a significant improvement compared to conventional high temperature solid-state synthesis, which typically requires much higher temperatures (≥800 °C) and yields catalysts with surface area ≤2 m2 g−1.18 Powder XRD studies of synthesized materials (Figure 2) indicate that all catalysts crystallized in the F-centered cubic unit cell (Table 1). The unit cell parameters of the synthesized pyrochlores are slightly larger (by ∼1%) than those reported in

Figure 2. Powder XRD patterns of the pyrochlores arranged with the increase of the unit cell parameter from top to bottom with Miller indexes shown for the pyrochlore peaks. ◊ and ∗ mark minor amount of impurities: phase of the Bi3Ru3O11 type (peaks 28.07, 31.09, 35.08° 2Theta) and Y2O3 (peaks 33.69, 48.55° 2Theta) for Bi−Ir and Y−Ir, correspondingly. 5184


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Chemistry of Materials Table 2. Mass-Normalized Surface Charge, Tafel Slope, and Electrochemical Activity of Pyrochlores pyrochlore or iridium oxide Bi−Ir Y−Ir Pb−Ir BiY−Ir BiPb−Ir YPb−Ir IrO2-150 IrO2-30

surface charge, C/goxide 50 49 27 22 34 16

(3) (3) (1) (1) (2) (1)

Tafel slope, mV dec−1

E at 10 A g−1Ir, V

J at 1.525 V, A g−1Ir

J at 1.525 V, mA cm−2oxide

45(1) 50(2) 65(1) 40(1) 42(1) 41(1) 44(2) 45(2)

1.503(15) 1.492(7) 1.533(18) 1.507(4) 1.514(10) 1.518(5) 1.497(3) 1.544(3)

26(13) 34(14) 9(4) 18(4) 13(3) 13(1) 45(9) 3.9(4)

0.028(14) 0.082(35) 0.015(8) 0.028(7) 0.013(3) 0.047(4) 0.025(5) 0.012(1)

the literature (10.3256(1) Å for Bi2Ir2O7,44 10.26450(4) Å for Pb2Ir2O6.5,44 and 10.10580(7) Å for Y2Ir2O745), which might result from the smaller particle size of the materials prepared by Adams method. The unit cell parameters of mixed pyrochlores (containing several metals in the A position) lie in between those of the two extreme pyrochlores indicating that solid solutions were indeed formed in all three cases (Table 1). Powder XRD of yttrium iridium pyrochlore was additionally measured at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using 0.15609 Å radiation. Rietveld refinement and pair distribution function (PDF) analysis of the diffraction data confirm formation of Fd3̅m pyrochlore structure (Figures S2−S4, Table S1). Rietveld analysis was performed using a model with A position fully occupied by Y and B position occupied by both Y and Ir (with the total occupancy fixed to 1). The refinement yielded partial occupancies of 0.915(1) for Ir and 0.084(1) for Y in line with the nominal composition taken for the synthesis and the XPS composition given in Table 2 (see discussion further). The pyrochlore unit cell parameter was found to be 10.183(3) Å, and Y2O3 content was refined to 5.7(2) wt % (Table S1). TEM studies indicate formation of 10−200 nm nanoparticles mostly of cubic shape with the broad size distribution (Figure 3), consistent with the measured surface area. High-resolution TEM (HRTEM) images of pyrochlores show formation of highly crystalline materials with the pyrochlore cubic unit cell structure fully consistent with the powder XRD diffraction data (Figure 3g,h). The TEM images of lead containing materials (Figure 3c,e,f) show the presence of rod-shape particles, which indicates formation of minor amounts of IrO2 in these specific samples, based on HRTEM studies (Figure S5), and our previous results.34 Other bulk analyses (in particular powder XRD studies) of the lead pyrochlores do not indicate formation of the separate iridium oxide phase, which implies that the amount of the impurity is low. To evaluate the oxidation state and the local structural environment of iridium, the synthesized pyrochlore materials were investigated by X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) at the Ir−LIII edge. The XANES data (Figure 4a) indicate that all the materials have similar oxidation state of iridium with the absorption edge energy (inflection point, maximum of the first derivative) being about 11 217.4 eV (see Table 1). Similar energy of 11 217.3(1) eV was found for IrO2 prepared at 500 °C by the same Adams fusion method (synthesis procedure is given in the Supporting Information) and for commercial IrO2 obtained from Sigma-Aldrich (Figure S6). For bismuth iridium pyrochlore, Sardar at el. reported the minimum of the second derivative of Ir−LIII absorption spectrum at 11 220.75 eV27 and based on their calibration iridium oxidation state was found to be +4.5. Our Bi−Ir data

Figure 3. TEM images of (a) Bi−Ir, (b) Y−Ir, (c) Pb−Ir, (d) BiY−Ir, (e) BiPb−Ir, (f) YPb−Ir, scale bars are 100 nm; HRTEM images of (g) Pb−Ir and (h) Y−Ir, scale bars are 5 nm, Fourier transformed images are shown as insets.

suggest the minimum of the second derivative at slightly lower value of 11 220.1(2) eV, which would correspond to the iridium oxidation state of about +4, consistent with the measurements of IrO2 samples. The Fourier transformed EXAFS spectra of the pyrochlores in R-space (FT-EXAFS, Figure 4b) are similar to each other and mainly consist of the peaks corresponding to the first and second scattering shells; there are no major peaks beyond 4 Å. The truncation of the scattering shell is most probably caused by the relatively small particle size and possibly some longrange disorder. To gain more information regarding the local coordination environment of iridium, the EXAFS data were fitted using the scattering paths from the six oxygen atoms at ∼2 Å, six Ir, and six A (Bi, Pb, or Y) atoms at ∼3.6 Å and six oxygen atoms at ∼3.8 Å. Details of the fitting models and the results of the fits are given in the Supporting Information (Table S2 and Figures S7 and S8). The local coordination environment of iridium in Bi−Ir, Y−Ir, and BiY−Ir samples corresponds well to the crystal structures of nonoxygen deficient Bi2Ir2O7 and Y2Ir2O7 5185


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iridium coordination environment with the paths calculated based on Bi2Ir2O7 structure. Such structural deviation observed by EXAFS typically cannot be seen by powder XRD since the supercell reflections (e.g., 002, 024: F4̅3m for Pb2Ir2O6.5 compared to Fd3m ̅ for Bi2Ir2O7 and Y2Ir2O7) are usually very weak to be detected. Formation of the symmetrical iridium environment in lead samples, in particular Pb−Ir, might be due to the structural disorder caused by partial intermixing between Ir and Pb in A and B positions, which can possibly be accompanied by partial oxidation of Pb2+ to Pb4+. To capture the mixed occupancy of A position in (A, A′)2Ir2O6.5+x pyrochlores, EXAFS data of BiY−Ir and YPb−Ir were modeled using two separate paths for Bi/Pb and Y at ∼3.6 Å (each with coordination number 3), which improved the fit compared to the model with only one A cation used (Table S2). The EXAFS of Y−Ir material was further studied using the Wavelet transform analysis.46 This analysis provides a correlation of R and k-spaces and ultimately allows for the distinction between different atoms positioned at similar distances from the scattering center: in the case of Y−Ir pyrochlore for distinction of Y and Ir located at 3.57 Å (see Figure S3 for PDF data and Figures S7 and S8 and Table S2 for EXAFS fits). According to the 2D map (Figure 4c), the path at about 3.5 Å corresponding to Ir−Ir and Ir−Y (Table S2) scatterings separates into two distinct spots located at about 4 Å−1 and 15 Å−1. The intensity at higher k-values was attributed to Ir−Ir scattering (since atoms having high atomic numbers are more efficient backscatterers at high k-values),46 while the spot at about 4 Å−1 was attributed to Ir−Y scattering. Therefore, the Wavelet analysis together with EXAFS fitting, Rietveld, and PDF diffraction analyses shows the formation of Y2Ir2O7 pyrochlore structure in case of Y−Ir sample. Electrical conductivities of the pyrochlore materials are given in Table 1. The highest resistivity values were found for yttrium-containing materials (especially Y−Ir) in agreement with previous reports, which showed that Bi and Pb pyrochlores are metallic conductors,47,48 while Y2Ir2O7 shows insulating behavior.49 The surface composition and metal oxidation states of the pyrochlores were studied by XPS. The details of XPS analysis and survey spectra can be found in the Supporting Information (Figures S9−10). In general, we found a slight enrichment of the near-surface composition of pyrochlores with the A metal (see Table 1) with the metal ratios being close to A(+A′)/Ir = 60/40; similar enrichment was previously reported for the Bi2Ir2O7.44 In our case, however, the higher content of A elements can be explained by higher nominal composition (excess) of metal nitrates taken for the pyrochlore synthesis. Normalized Ir 4f spectra of the pyrochlores (Figure 5) show strongly asymmetric shape for Bi−Ir, Pb−Ir, and BiPb−Ir and are more symmetrical for yttrium containing samples (especially Y−Ir). Strongly asymmetric line shapes of the Ir 4f XPS are well-known for the metallic IrO2 and were rationalized in terms of the 5d conduction electron screening and presence of shakeup satellites above the main Ir 4f line.50,51 Therefore, the metallic nature of the Bi and Pb pyrochlores results in the similar conduction electron screening effects and thus causes asymmetric Ir 4f XPS line shapes. At the same time, less conductive yttrium pyrochlore shows almost symmetrical Ir 4f spectrum consistent with previous reports.49 Electron screening effects also result in the asymmetry of the Bi 4f, Pb 4f, and Y 3d (slightly) spectra for the conductive pyrochlores

Figure 4. (a) Ir−LIII edge XANES, (b) FT-EXAFS (R space with kweight 2) spectra of pyrochlores, and (c) Wavelet transform analysis of Y−Ir pyrochlore.

pyrochlores (slightly distorted octahedron [IrO6] with Ir−O distance of 2.00−2.02 Å). Interestingly, modeling of EXAFS of lead containing samples using the paths generated from oxygen deficient Pb2Ir2O6.5 structure with strongly unsymmetrical Ir environment (3 oxygens at ∼1.82 Å and 3 oxygens at ∼2.18 Å) did not converge to the reasonable fits. FT-EXAFS of these samples clearly show only one Ir−O scattering feature at 2.00−2.02 Å, similar to Bi2Ir2O7 and Y2Ir2O7 pyrochlores (note that the shoulder around 1−1.2 Å is an artifact of the Fourier transform and does not correspond to any physical scattering event). Therefore, EXAFS data of Pb−Ir, BiPb−Ir, and YPb−Ir samples were fitted using a structural model with symmetrical 5186


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prepared via a hydrothermal route (45 mV dec−1, acidic environment).27 A Tafel slope of 65(7) mV dec−1 (basic conditions) was reported for Bi2Ir2O7 synthesized by a solidstate method.29 While relatively low Tafel slopes were reported for Pb2Ir2O6.5 in basic conditions (38(2) mV dec−129 and 30 mV dec−126), our data suggest the Tafel slope for Pb−Ir pyrochlore in 0.1 M HClO4 of 65(1) mV dec−1, which is the highest among the prepared pyrochlore materials. These data suggest that most likely the OER proceeds through a similar mechanism on Bi, Y, and mixed pyrochlores, while lead pyrochlore could possibly show a different catalytic pathway. However, because of the Pb−Ir degradation under the catalytic conditions revealed by stability tests (see further), larger Tafel slope can result from presence of other parallel reactions (e.g., dissolution process−corrosion). In addition to the Tafel slope analysis, the activity of the synthesized pyrochlores was estimated as the current density (in A g−1Ir and mA cm−2oxide) at 1.525 V versus RHE (Table 2 and Figure 7). The highest mass activities (current normalized

Figure 5. Ir 4f XPS of the pyrochlore materials.

(Figures S9−10). Bi, Pb, Y, and Ir peak energies are consistent with BiIII, PbII (and possibly PbIV since their energies are close), YIII, and IrIV; obtained energies are similar to previously reported XPS data of pyrochlores.27,44,49 Electrochemical Activity Studies. The obtained pyrochlore samples were tested for the oxygen evolution reaction activity in 0.1 M HClO4. Prior to the chronoamperometric measurements, cyclic voltammograms (CVs) of the catalysts were recorded in the range from 1.0−1.4 V (Figure S11), and the mass-normalized surface charge was obtained by integration of the CVs (Table 2). The surface charge of the catalyst can be tentatively correlated with the number of active sites present on its surface;52 Bi−Ir and Y−Ir pyrochlore samples having the largest surface charge show in fact the highest catalytic activity among studied materials (see further). The corresponding Tafel plots of the catalysts obtained from chronoamperometric measurements are shown in Figure 6, and the calculated Tafel slopes are given in Table 2. All pyrochlore materials except Pb−Ir show similar Tafel slopes of 40−50 mV dec−1. These values are slightly lower than commonly reported values for IrO2 in acidic environment (ca. 60 mV dec−1),53 however, close to the values reported by Marshall for IrO2 (between 42 to 48 mV dec−1)54 and Sardar et al. for Bi2Ir2O7 Figure 7. OER activities of the pyrochlores and IrO2 samples: (a) mass- and (b) oxide surface-normalized current densities at 1.525 V versus RHE.

per amount of iridium on the electrode, A g−1Ir) were found for Y−Ir, Bi−Ir, and BiY−Ir samples, with the values approaching the most active IrO2 with high surface area of 150 m2 g−1, disclosed by us recently.34 Comparison of the intrinsic activities of the pyrochlores (current normalized per surface area of the catalyst, mA cm−2oxide) shows that yttrium containing samples and Bi−Ir are the most OER active pyrochlores with intrinsic activities surpassing that of IrO2. Therefore, synthesis of Bi and Y pyrochlores with high surface area should lead to superior OER catalysts. Observed improvement of the intrinsic OER activity of pyrochlores compared to IrO2 may result from the involvement of Y or Bi surface sites in the OER mechanism: either directly as intermediates on the catalytic pathway,55 or indirectly by changing the adsorption energies of oxygenated species on Ir

Figure 6. OER Tafel plots showing the steady-state current density recorded after 1 min in 0.1 M HClO4. 5187


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Chemistry of Materials sites due to presence of Bi/Y in the second coordination shell.56 In both cases, this involvement would allow overcoming the fundamental volcano limitation and decrease the OER overpotential.57 Another possible explanation for the high intrinsic activities of pyrochlores originates from predicted thermodynamic instabilities of these materials in aqueous media, which can result in the leaching of A cations in solution and formation of the active IrOx surface layer, similar to observed behavior of SrIrO3 perovskite in acidic media (see the discussion further).17 To compare OER activities of our samples with the previously reported data for pyrochlores in acidic media, Bi− Ir was tested under identical experimental conditions with respect to electrolyte and catalyst loading (in 1 M H2SO4 with loading of 402 μg cm−2) as reported by Sardar.27 A current density of 1.1 mA cm−2geom. was found at 1.525 V, which is slightly higher than about 1 mA cm−2geom. (estimated from CV with sweep rate of 1 mV s−1), reported for Bi2Ir2O7 prepared via a hydrothermal route.27 This material was reported to have a surface area of 46 m2 g−1, which is about 1.5-times higher than the surface area of our Bi−Ir pyrochlore, meaning that the Adams fusion method leads to the pyrochlores with higher intrinsic activities compared to the hydrothermal route. Electrochemical Stability and DFT Studies. The stability of pyrochlore samples was evaluated by stepping the electrode potential between 1.00 V (a potential close to the open circuit value, that is, where no OER current is observed) and 1.60 V versus RHE (where an appreciable OER current density >10 A g−1Ir is observed). Such stability tests are intended to simulate the start/stop behavior of a PEM electrolyzer. Shown in Figure 8 are the current densities at 1.6 V versus RHE over the course

prepared by solid state synthesis.44 Formation of such symmetrical structure atypical for lead iridium pyrochlore may result in the instability of Pb−Ir, leading to the degradation of the sample under OER conditions. Contrary to the Pb−Ir sample, all yttrium-containing pyrochlores (especially Y−Ir) show an initial increase in the OER activity followed by the slight decrease after 100−200 cycles. Mixed BiPb−Ir pyrochlore was also found to increase the activity with the potential cycling reaching a plateau after about 300 cycles. To understand the high activities of yttrium pyrochlores and observed stability behavior, the phase and Pourbaix diagrams of the synthesized materials were calculated using DFT. Details and the results of calculations are given in the Supporting Information (Figures S13 and S14). The general trend observed in the Pourbaix diagrams is the thermodynamic instability of the yttrium pyrochlores in aqueous solutions. In the acidic environment, these pyrochlores were found to leach Y3+ in the solution, leaving either IrO2 (in case of Y−Ir) or remaining pyrochlore (in case of mixed BiY−Ir and YPb−Ir) as the solid phase. Such leaching process will lead to the formation of IrOx surface layer, which is responsible for high intrinsic activities and the activity increase during the initial 100−200 cycles, similar to recently reported behavior of SrIrO 3 perovskite in acidic media (Sr2+ leaching during OER measurements lead to formation of IrOx/SrIrO3 and increased activity).17 A similar mechanism can be responsible for the increase of the OER activity of BiPb−Ir sample: leaching of Pb2+ in the solution was predicted by Pourbaix diagram calculations (Figure S14). To experimentally establish the leaching of Y3+ associated with the formation of IrOx surface layer for yttrium-containing pyrochlores, we have analyzed the electrolyte composition after the stability measurements of Y−Ir sample and performed CV measurements of the Bi−Ir and Y−Ir samples during the stability tests. ICP-OES analysis of the electrolyte after the stability test of Y−Ir clearly shows the presence of yttrium and absence of iridium in the solution. We found that after 500 cycles, 150 mL of the electrolyte contain 0.032 ± 0.01 ppm of yttrium, which corresponds to a loss of 4.8 ± 1.5 μg (75 ± 24% of the initial yttrium content in 20 μg of the catalyst on the electrode). The CVs of Bi−Ir and Y−Ir samples (Figure S12) recorded every 100 cycles of the stability tests show no significant changes in the surface charge for the Bi−Ir pyrochlore, while the Y−Ir sample significantly develops the surface charge during the initial 100−200 cycles (increase of the electrochemical capacitance). We correlate the observed increase in the surface charge to the formation of a fresh surface during the OER stability measurement, which, together with the observed Y3+ leaching, is consistent with the formation of IrOx surface layer for Y−Ir pyrochlore. Overall, the electrochemical stability data (increase of the current and surface charge during the first 100−200 cycles), elemental analysis, and calculated Pourbaix diagrams unambiguously show leaching of Y3+ into the electrolyte solution from the surface of yttrium pyrochlores and therefore indicate in situ formation of a highly OER active IrOx layer. Following our recent thermodynamic argumentation58 and experimental finding for the high surface area IrO2,34 the remaining IrOx phase in the pyrochlore samples is most likely amorphous and consists of Ir−oxy−hydroxide responsible for the high OER activity.

Figure 8. Stability measurements for the pyrochlore samples.

of 500 potential step cycles for the pyrochlore samples. In general, all pyrochlores show good OER stability maintaining high geometrical current densities, the most pronounced degradation is observed for Pb−Ir sample (ca. 25% activity lost after 500 cycles). Such degradation can possibly be correlated with the disordered nature of the lead pyrochlore, revealed by EXAFS modeling. As discussed previously, Pb−Ir was found to crystallize in the symmetrical pyrochlore structure (i.e., with symmetrical Pb environment), contrary to the structural data reported for polycrystalline Pb2Ir2O6.5 samples 5188


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Funding

CONCLUSIONS We report the synthesis, characterization, and electrochemical studies of iridium pyrochlore OER catalysts. Preparation of the materials from molten sodium nitrate (Adams fusion method) at moderate temperatures (500−575 °C) yields highly crystalline pyrochlore nanoparticles with surface areas up to 40 m2 g−1, which is a considerable improvement compared to the traditional high temperature solid-state synthesis. Adams fusion method is a quick, facile, safe, and easy gram-scalable method, which does not require use of pressurized vessels and strong unstable oxidizing agents (as compared to hydrothermal method used for pyrochlore synthesis). Therefore, we propose the Adams fusion method as a general approach for synthesis of the complex oxides of noble metals in the form of nanoparticles. The synthesized pyrochlore materials were characterized by a combination of methods including XRD, TEM, N2 adsorption/ desorption, XPS, and XAS, which showed the formation of highly crystalline cubic shape nanoparticles of desired composition and structure. Electrochemical measurements in acidic media showed that among synthesized materials, yttrium and bismuth pyrochlores possess highest OER activity approaching activity of the state-of-the-art IrO2 nanoparticles. High intrinsic activities and the activity increase during the stability measurements for yttrium pyrochlores were correlated with the Y3+ leaching from the pyrochlore structure and thus formation of highly active IrOx surface layer, supported by ICPOES, electrochemical capacitance measurements, and modeling of the Pourbaix diagrams of the catalysts. Overall, this study shows that OER catalysts with pyrochlore structure improve the specific activity of Ir-based catalysts, which offers the possibility to develop low cost, active, and stable anodes for PEMWE.

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The authors would like to acknowledge the Competence Center Energy and Mobility (CCEM-CH, Project Renerg2), the Commission for Technology and Innovation Switzerland, and the Swiss Competence Center for Energy Research (SCCER) Heat and Electricity Storage as well as the Swiss Federal Office of Energy and Swiss Electric Research for their financial support. Additionally, we are grateful to Swiss National Science Foundation for their support through NCCR Marvel. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank ScopeM (ETH Zürich) for the use of their electron microscopy facilities, European Synchrotron Radiation Facility for provision of beam time at ID31 beamline, Paul Scherrer Institute for the provision of beam time at the SuperXAS beamline of the Swiss Light Source SLS, and Dr. G. Siddiqui for performing XAS measurements and extensive help with the XAS data processing and analysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00766. Experimental procedures; details of synchrotron powder XRD measurements, Rietveld refinement, and PDF analysis; HRTEM images, XPS and electrochemical data; details of the synchrotron XAS measurements and EXAFS data analysis; details and results of DFT phase and Pourbaix diagrams calculations (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: nicola.marzari@epfl.ch. *E-mail: [email protected]. ORCID

Dmitry Lebedev: 0000-0002-1866-9234 Ivano E. Castelli: 0000-0001-5880-5045 Alexey Fedorov: 0000-0001-9814-6726 Christophe Copéret: 0000-0001-9660-3890 Present Address ∥

Surface Physics Laboratory, Department of Physics, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland. 5189


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