Highly Efficient Electrocatalysis and Mechanistic Investigation of

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Highly efficient electrocatalysis and mechanistic investigation of intermediate IrOx(OH)y nanoparticle films for water oxidation Debraj Chandra, Daisuke Takama, Takeshi Masaki, Tsubasa Sato, Naoto Abe, Takanari Togashi, Masato Kurihara, Kenji Saito, Tatsuto Yui, and Masayuki Yagi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00621 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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Highly efficient electrocatalysis and mechanistic investigation of intermediate IrOx(OH)y nanoparticle films for water oxidation

Debraj Chandra1)*, Daisuke Takama1), Takeshi Masaki1), Tsubasa Sato1), Naoto Abe1), Takanari Togashi2), Masato Kurihara2), Kenji Saito1), Tatsuto Yui1) and Masayuki Yagi1)*

1)

Department of Materials Science and Technology, Faculty of Engineering, Niigata

University, 8050 Ikarashi-2, Niigata, 950-2181, Japan.

2)

Department of Material and Biological Chemistry, Faculty of Science, Yamagata

University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan.

*Author to whom correspondence should be addressed. E-mail: [email protected]; Tel & Fax: +81-25-262-6790 E-mail: [email protected]

Submitted to ACS Catalysis as Full paper

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Abstract A new transparent IrOx film on fluorine-doped tin oxide (FTO) electrodes were achieved from a homogeneous precursor complex solution by employing a facile spin-coat technique. The composition of the nanostructure and crystallinity of the IrOx film is tunable by a simple annealing treatment of a compact complex layer, which is responsible for their significantly different electrocatalytic performances for water oxidation. Transmission electron microscope (TEM) observation showed uniformly dispersed small IrOx nanoparticles of ca. 2-5 nm dimension for the film annealed at 300 ºC and the nanoparticles gradually agglomerated to form relatively large particles at higher temperatures (400 and 500 ºC). The IrOx films prepared at different annealing temperature are characterized by Raman spectroscopic data to reveal intermediate IrOx(OH)y nanoparticles with two oxygen binding motifs: terminal hydroxo and bridging oxo at 300 and 350 ºC annealing, via amorphous IrOx at 400 ºC, transforming ultimately to crystalline IrO2 nanoparticles at 500 ºC. The cyclic voltammetry suggests that the intrinsic activity of catalytic Ir-sites in intermediate IrOx(OH)y nanoparticles formed at 300 °C annealing is higher compared with amorphous and crystalline IrOx nanoparticles. Electrochemical impedance data showed that the charge transfer resistance (Rct = 232 Ω) for the IrOx(OH)y film annealed at 300 °C is lower relative to films annealed at higher temperatures. This is ascribable to the facilitated electron transfer in grain boundaries between smaller IrOx particles to lead the efficient electron transport in the film. The high intrinsic activity of catalytic Ir-sites and efficient electron transport are responsible for the high electrocatalytic performance observed for the intermediate IrOx(OH)y film annealed at 300 °C; it provides the lowest overpotential (η) of 0.24 V and Tafel slope of 42 mV dec-1 for water oxidation at neutral pH, which are 2


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comparable with amorphous IrOx·nH2O nanoparticle films (40-50 mV dec-1) reported as one of the most efficient electrocatalysts so far.

Keywords artificial photosynthesis, water oxidation, electrocatalysis, iridium oxide, nanostructure engineering.

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Introduction Widespread attention has recently been paid to artificial photosynthesis as promising energy-providing systems for the future to produce green and storable hydrogen fuel.1-3 The efficiency of hydrogen generation from photoelectrochemical4-7 and electrochemical8-11 water splitting is severely limited by the sluggish kinetics of oxidation of water. Therefore, development of efficient electrocatalysts for water oxidation is an avenue to improve competence of a fuel generation technology.2,8,12 In general, metal oxides of ruthenium (RuO2), iridium (IrO2), cobalt (Co3O4), rhodium (Rh2O3) and manganese (Mn2O3) were proclaimed to have catalytic activity for water oxidation.13-17 Although RuO2 and IrO2 are very efficient electrocatalysts14,15,

RuO2

easily converts to unstable RuO4.18 Iridium oxide (IrOx) catalysts have been widely studied for water oxidation in both electrochemical19-31 and photochemical systems32-34 owing to its actuation over a broad pH range, low resistivity and superior chemical/thermal stability. The vapor-phase techniques including chemical vapor deposition and pulsed-laser deposition,35-37 as well as thermal decomposition of an iridium salt

38-40

generally have been employed to prepare crystalline IrO2 thin films in numerous studies. Nevertheless, the electrocatalytic activities of IrOx films are often not comparable, which can be attributed to different particle sizes,20-22,41 crystallinity20,25 and degree of hydration.42,43 The electrocatalytic activity of crystalline IrO2 is reported to be lower compared to that of amorphous one.43-46 However, the key factor(s) to develop efficient IrOx catalyst films are still unclear because the electrocatalytic properties of different IrOx films on the same platform has not been compared due to the lack of fabrication approach to control the composition of nanostructure and crystallinity of the IrOx film 4


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without significantly altering their textural properties. So far, amorphous IrOx nanoparticles has been extensively utilized as among the finest catalysts for water oxidation.12,19-24,32-34,41 Electrocatalyst films composed of amorphous IrOx nanoparticles have been fabricated by particle assembly21-23 and anodic deposition26,47,48 employing aqueous phase synthesis of colloidal nanoparticle solutions with or without capping ligands. For example, our group demonstrated that 50-100 nm diameter citrate-stabilized IrO2 nanoparticles are spontaneously adsorbed on an ITO electrode.21,22 Murray et. al. reported formation of a mesoporous IrOx film of 2 nm diameter nanoparticles on a glassy carbon electrode by employing an electroflocculation technique.26 Compared to the preformed colloidal IrOx nanoparticle solutions, a homogeneous molecule-based Ir-precursor solution could be effective to provide more stably adherent coating layers of IrOx on various substrates.19 Recently, Mallouk et. al. reported in-situ formation of hydrous IrOx·nH2O nanoparticle films on different electrode surfaces by anodic deposition from a homogeneous solution of [Ir(OH)6]2complexes to give one of the most efficient catalysts for electrochemical water oxidation so far.19 However, the present techniques of either spontaneous adsorption, electroflocculation or electrodeposition are only available for fixed composition of amorphous IrOx nanoparticle films. In this context, a blue layer of amorphous IrOx as a highly efficient electrocatalyst for water oxidation was also reported by Brudvig et. al. from different organometallic Ir-complexes by anodic deposition.49-51 Fukuzumi et. al. demonstrated efficient catalysis by the formation of insoluble nanoparticles composed of Ir(OH)3 and carbonaceous residues from a series of homogeneous mononuclear Ir-complexes in a chemical water oxidation system.12 Herein, we report a new transparent IrOx film for highly efficient 5


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electrocatalytic water oxidation comparable to the existing benchmarks.19,24,26,51 We employed facile spin-coating of a homogenous solution of an Ir precursor complex to provide a compacted complex layer followed by simple annealing at different temperature (Scheme 1a). This fabrication technique provides stably adherent and uniform IrOx thin films with the sequentially controlled composition of nanostructures and crystallinity by changing annealing temperature without significantly altering their textural properties. The electrocatalytic performance of the IrOx film dramatically depended on annealing temperature is characterized to give the significantly high performance at 300 ºC annealing, and its mechanistic insights is provided to gain better understanding of electrocatalysis of the IrOx nanoparticle film.

Experimental section Materials Potassium hexachloroiridate(IV) (K2IrCl6) and sodium hydroxide (NaOH) were obtained from Wako Chemical Co. The F-doped tin oxide (FTO) glass substrate was obtained from Asahi Glass Co. All the chemicals of analytical grade were used as received. All the solutions were prepared with Millipore water.

Synthesis of nanoparticle IrOx electrodes In a typical synthesis, 2 mL of a 0.1 M aqueous NaOH solution was slowly added to 0.10 g (0.21 mmol) of K2IrCl6 under vigorous stirring at 50 ºC. The initial dark brown solution gradually changed to light amber in color due to hydrolysis of the Ir-precursor. After stirring for 15 min at 50 ºC, the resultant homogeneous solution (pH ≈ 7) was stored in a refrigerator at 2 ºC and used for spin-coating. (For prolong time 6


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exposure in room temperature, the solution gradually turns bluish after 2h and finally started producing black precipitation after 6h.) Spin-coating was conducted on a FTO substrate with a spinning rate of 3000 rpm. Before spin-coating, the FTO substrate was cleaned up by a UV-ozone treatment (SEN LIGHTS Co. Photo Surface Processor PL16-110) for 15 min and the coated area was fixed to be 0.8 × 1.25 cm2. The as-coated film was dried at 80 ºC for 15 min, and annealed at 150 ºC for 20 min. After repeating the procedure twice, the film was annealed at 200-500 ºC (1 ºC min-1) in air and maintained at these temperatures for 4 h. After cool down to room temperature the IrOx-coated substrate was washed with copious amount of water and dried in air. In some cases, the FTO substrate was pre-annealed at 500 ºC for 4 h before IrO2 film coating to evaluate the influence of the annealed substrate at high temperature on the electrochemical data.

Structural characterization The nanostructures and the crystalline phase were characterized by transmission electron microscopy (TEM; JEOL, JEM 2100F, operated at 200 kV) and powder

X-ray

diffraction

(XRD;

Rigaku

MiniFlexII

diffractometer)

using

monochromated Cu Kα (λ = 1.54 Å) radiation, respectively. The TEM images and XRD data were taken of powders scratched off from the annealed films on the glass substrate. The surface morphology and film thickness were observed by scanning electron microscopy (SEM; JEOL, JSM-6500F). UV-visible spectra were recorded on a diode array spectrophotometer (Shimadzu, Multi-Spec-1500). Dynamic light scattering (DLS) measurement was carried out using a zeta-potential and particle size analyzer (Photal Otsuka Electronics, ELSZ-2N). Raman spectra were recorded using a Raman 7


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spectrophotometer (a Horiba-Jobin-Yvon LabRAM HR) using 532 nm excitation and silicon standard wavenumber (520.7 cm-1). The XPS spectra were recorded using a JEOL JPS-9000 and calibrated by Au4f7/2 peak, appeared at 83.0 eV. For analysis of the iridium amount, as-prepared IrOx films on the FTO electrode was dissolved in an aqua regia and the iridium amount in the dissolved solution was measured using an inductively coupled plasma mass (ICP-MS) spectrometer (Yokogawa, HP4500).

Electrochemical measurements Electrochemical measurements were carried out using an electrochemical analyzer (Hokutodenkou HZ-7000). A two-compartment electrochemical cell separated by a Nafion membrane was used. A three-electrode-type system has been employed by using an IrOx-coated FTO electrode as working electrode and an Ag/AgCl reference electrode in one compartment and a Pt wire counter electrode in the other compartment. The potentials were expressed as the values versus Ag/AgCl unless otherwise noted. An aqueous 0.1 M phosphate solution (pH = 6.7) was used as an electrolyte in both compartments of the electrochemical cell, which was sealed and saturated with Ar gas prior to the electrochemical measurement. The cyclic voltammograms (CV) were recorded at a scan rate of 50 mV s-1 between the ranges of 0 V and 1.6 V at 25 ºC. Tafel plots were derived using a linear sweep voltammogram (LSV) measured at a scan rate of 0.5 mV s-1 between the ranges of 0.8 V and 1.1 V. Electrochemical impedance spectra were measured at an applied potential of 0.94 V in 0.1 M phosphate solution of pH = 6.7 in a frequency range of 10 mHz to 20 kHz (amplitude of 10 mV) using the electrochemical analyzer. To detect H2 and O2 gasses evolved during electrocatalysis, the reaction cell was purged by Ar gas for 1 h prior to electrocatalysis to remove 8


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residual air. The electrolyte in the working compartment was 5 mL and the headspace volume was 87.3 mL. Electrocatalytic water oxidation was conducted at 1.2 V of applied potential for 1 h, and the current was recorded during the course of electrolysis by the electrochemical analyzer. The amounts of H2 and O2 evolved during electrocatalysis were analyzed in the gas phases (headspace regions) of the counter and working electrode compartments, respectively on a gas chromatograph (Shimadzu, GC-8A with TCD detector and molecular sieve 5A column and Ar carrier gas).

Results and discussion Preparation and physicochemical characterizations UV-visible absorption spectral change in basic hydrolysis of a K2IrCl6 solution is shown in Fig. 1. The spectrum of the K2IrCl6 solution with absorption maximums at λmax = 425 and 490 nm20 changed immediately after hydrolysis, exhibiting absorption maximums at λmax = 329 nm (close to monomeric hydroxyiridate complexes19,20,52) and λmax = 388 nm probably for a partially hydrolyzed [Ir(OH)x(OH2)yClz]n- complex species (Fig. 1a). The loss of visible bands (λmax = 425 and 490 nm) after hydrolysis suggests the possibility of spontaneous reduction of metal center from IrIV to IrIII.53,54 (OH- is a poor donor ligand and act as reducing agents in an aqueous solutions to destabilize the high oxidation state.) Recently, Mallouk et.al. also showed that complete alkaline hydrolysis of K2IrCl6 at pH 13 produced a reduced hydroxyiridate complex with majority (ca. 70 %) of IrIII.52 The absorption bands at 329 nm and 388 nm remained after aging of the solution for 1 day at 2 ºC (Fig. 1b), indicating uncompleted hydrolysis with [Ir(OH)x(OH2)yClz]n- complex species remaining stably under the conditions employed. However, during aging in room temperature the initial transparent solution 9


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gradually turned bluish as indicated by appearance of an absorption band at 563 nm at 6h (Fig. 1c) due to formation of colloidal IrOx·nH2O nanoparticles21 and finally produced black precipitation after prolonged aging time. The solution is stable for at least several weeks when aged at 2 ºC. Absence of any preformed colloidal and/or nanoparticle species in the precursor complex solution before spin-coating was also verified by the dynamic light scattering (DLS) measurement, which is consistent with the UV-visible absorption spectral observation. The films were deposited over FTO electrodes from this precursor solution by simple spin-coating followed by annealing at 200-500 ºC. The composition of the precursor solution with partially hydrolyzed complexes is in principle represented as (NavKw)[Ir(OH)x(OH2)yClz]. Elemental analysis of the IrOx film being washed thoroughly with water was conducted using an ICP-MS technique to examine the composition of the film. The signals for Na, K and Cl are comparable levels with a blank solution as well as the film prepared from a completely hydrolyzed precursor solution using a highly basic solution (1 M aqueous NaOH), confirming the film composition of IrOxHy. The films deposited over FTO electrodes is highly uniform and transparent as shown in the Fig. S1. The film thickness could be easily modulated by repeated coating. From the cross-sectional SEM image (Fig. 2a) of a representative IrOx film annealed at 300 ºC, the film thickness was measured to be ca. 70-80 nm. The SEM image of a tilted IrOx film annealed at 300 ºC (Fig. 2b) shows that the flawless and smooth IrOx layer can be extended over several micrometer without any crack formation. The higher magnification SEM image (Fig. 2c) exhibits a uniform coating layer of densely packed small IrOx nanoparticles embedded over the FTO surface and the nanoparticles being 10


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well-connected to each other, which is in contrast with no observable IrOx nanoparticles in a compact complex layer before annealing (Fig. S2). The TEM images of the IrOx films annealed at 300-500 ºC are shown in Fig. 3. Uniformly dispersed small IrOx nanoparticles of dimension ca. 2-5 nm were observed over a large domain for the film annealed at 300 ºC. With the increase of annealing temperatures (400 and 500 ºC) nanoparticles gradually agglomerated to form large particles during progressive crystallization. The HRTEM images of the corresponding IrOx nanoparticle samples shown in the inset of Fig. 3 revealed the presence of the completely amorphous structure at 300 and 400 ºC and transformation into crystalline lattice fringes at 500 ºC, which are consistent with their respective wide angle XRD patterns (Fig. S3). The wide-angle XRD patterns also revealed a completely amorphous phase of oxide framework up to 400 °C, which are crystallized to tetragonal phase of the IrO2 (JCPDS: 150870) after being annealed at 450 and 500 °C. Average crystallite sizes, estimated from Scherrer equation using [101] reflection and assuming Scherrer constant as 0.94, are about 4.3 and 4.6 nm at 450 and 500 °C, respectively, which indicates progressive growth of the nanocrystals through crystallization at higher temperatures. The Raman spectra of the films as-prepared and annealed at 200-500 ºC are shown in Fig. 4. The Raman signals for the as-prepared film are observed at 168, 290, 315 and 335 cm-1 (assigned to Ir-OH moieties of the [Ir(OH)x(OH2)yClz]n- complex species in its compact layer), which is identical with the Raman spectra of a control sample of monomeric hydroxyiridate complex produced by complete alkaline hydrolysis of K2IrCl6 at pH 13.19,52 The peak at 315 cm-1 shifted to 320 cm-1 at 200 ºC of the annealing temperature, corresponding to condensation of the complexes in its 11


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compact layer during heat treatments at 200 ºC. As the annealing temperature increased to 300 and 350 ºC, the peaks further shifted to 325 cm-1 with a broad intense band appearing in the vicinity of 516 cm-1, which is agrees with the bridging-oxo binding modes of amorphous IrOx.55,56 These two different oxygen binding motifs (hydroxo and bridging-oxo) in the films annealed at 300 and 350 ºC were also supported by XPS measurement (Fig. S4). Raman spectra of the films annealed at 300 and 350 °C did not change when they are immersed in water over 1 day due to its insolubility in water, in contrast to the high water-solubility of either the as-made film or the film annealed at 200 °C. These results show that the films annealed at 300 and 350 ºC are no longer composed of the condensed complex layer, and the intermediate IrOx film containing partial hydroxo moieties (denoted as IrOx(OH)y film) is formed. The ratio of the peak area for Ir-OH signals (300 and 325 cm-1) versus bridging-oxo (~516 cm-1) for amorphous IrOx decreased from 0.53 to 0.22 with an annealing temperature increase from 300 to 350 ºC, showing that the content fraction of the Ir-OH signals decreases at 350 ºC. The Ir-OH signals completely disappeared at 400 °C, and the broad intense band at 516 cm-1 and weak bands at 302 and 713 cm-1 were provided, which are a sign of formation of amorphous IrOx based on the literature.55,56 The characteristic bands of crystalline IrO2 at 559, 729 and 746 cm-1 which are assigned as Eg, B2g and A1g phonon bands, respectively, started to appear at 450 °C, being prominent at 500 °C.57-59 Sequential formation mechanism of the IrOx film is illustrated in Scheme 1b. The precursor complex undergoes gradual condensation during annealing and nanoparticles formation are initiated through shrinkage of the compact complex layer. Further condensation gradually produces intermediate IrOx(OH)y nanoparticles at 300 and 350 ºC. The IrOx(OH)y nanoparticles completely changes to amorphous IrOx at 400 ºC, being 12


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finally transformed into crystalline IrO2 nanoparticles above 450 ºC.

Electrocatalytic properties The cyclic voltammogram (CV) for the IrOx-coated electrodes in the potential window of 0 to 0.9 V shows the broad anodic peak at 0.3 V and 0.55 V (Fig. 5A), assigned to oxidation of IrIII/IrIV and IrIV/IrV redox pairs of the IrOx film, respectively according to literature.19,22 The capacitive current is negligible compared with the Ir redox-based current in this potential window because the CV data extended to low potential of -0.5 V (Fig. S5) showed the anodic current decreasing to nearly zero with the negative scan to -0.5 V after the redox response at -0.03 V that could be assigned to a IrII/IrIII redox pair. This is consistent with very thin thickness of ca. 70-80 nm for the present IrOx film. Unfortunately, it is difficult to reliably analyze the amount of electroactive Ir-sites because of the fused IrIII/IrIV and IrIV/IrV redox pairs in Fig. 5A. Nevertheless, the relative amounts (Γea) of electroactive Ir-sites were estimated from the entire anodic current area from 0.09 to 0.80 V. Interestingly, Γea are variable by a factor of 0.55~1.6 among the 300 ~ 500 ºC-annealing films in spite of the same coverage (Γcov = 3.8 ± 0.6 × 10-8 mol cm-2) of Ir atoms as measured by an ICP-MS technique (Table 1). The increase of Γea with increasing temperature from 300 ºC to at 350 ºC could be possibly due to less isolated nanoparticles during progressive particle formation, with Γea gradually decreasing at higher temperatures due to large crystalline particle formation. In Fig. 5B, CV in the wide potential range of 0 to 1.6 V concomitantly exhibited anodic current due to water oxidation over 0.9 V. The onset potentials for the catalytic current are 0.88 and 0.89 V (pH 6.7) for the IrOx-coated electrode annealed at 13


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300 and 350 °C, respectively, corresponding to the significantly low overpotentials (η) of 0.24 and 0.25 V respectively, which are comparable with those (0.20-0.25) for one of the most efficient IrOx-based catalysts.19,20,26 The highest catalytic current density (Icat) as 12.4 mA cm-2 at 1.6 V was observed at 300 °C annealing, which significantly decreased at higher temperatures (Table 1). This result shows that the annealing temperature conditions give a huge impact over the observed electrocatalytic performances of the IrOx-coated electrode. The relative intrinsic activities (Acat = Icat / Γea) of Ir-sites for electrocatalytic water oxidation were defined as Icat normalized by Γea (Table 1). Acat for 300 °C is 2.6 - 4.3 times higher than those for the other annealing temperatures. The substantially high Acat at 300 °C indicates that intrinsic catalytic activity of the IrOx(OH)y nanoparticles is significantly higher compared with amorphous IrOx and crystalline IrO2 nanoparticles when taking characterization by the Raman spectral data into account (vide supra). We are considering that the Ir-site binding OH might be a possible catalytic site being responsible for the higher intrinsic activity of IrOx(OH)y. This is consistent with the recent report, which demonstrated highly enhanced water oxidation catalysis by insoluble nanoparticles composed of Ir(OH)3 and carbonaceous residues in a chemical water oxidation system12. However, we do not have any information for the catalytic site on IrOx(OH)y. Acat at 500 °C is 1.4 times higher than that at 400 °C, suggesting that the intrinsic activity of Ir-sites in the crystalline IrO2 film is higher than that in the amorphous IrOx film. However, the observed Icat value at 500 °C is the lowest of the films employed. Electrochemical impedance measurement was carried out to evaluate the electron transport and its influence on electrocatalytic properties for IrOx-coated

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electrodes annealed at 300-500 °C. Nyquist plots of the electrodes gave a semicircle in a high frequency region and subsequent flat plots in a low frequency region at each annealing temperature, and the diameter of the semicircles increased with the annealing temperature (Fig. 6). Nyquist plots were analyzed in a high frequency region over 30 mHz (100 and 50 mHz for 300 and 350 °C, respectively) using the equivalent circuit including solution phase resistance (Rsol), charge transfer resistance (Rct), and constant phase element (CPE) (see inset of Fig. 6) because analysis of the flat plots in a low frequency region (reflecting relatively slow kinetic process) circuits is complicated. The given parameters of the elements of the equivalent circuits are summarized in Table 2. Rsol (139 Ω) was significantly high at 500 °C annealing compared with Rsol (47 ~ 78 Ω) at 300 ~ 450 °C. This is attributed to the increased resistance of the FTO substrate over 500 °C because Rsol includes the electrode resistance in addition to that of the electrolyte solution. The Rct value increased from 232 to 1219 Ω with an increase of annealing temperature from 300 to 500 °C. The low Rct at 300 °C is ascribable to efficient electron transport in the film due to the facilitated electron transfer in grain boundaries between smaller particles and the high intrinsic catalytic activity. (Water oxidation catalysis proceeds at 0.94 V of an applied potential for impedance measurement.) The CPE constant (TCPE / F), reflecting electric double layer capacitance, increased with temperature from 300 to 400 °C and then decreased above 450 °C. The electric double layer capacitance should decrease with an increase of the IrOx particle size in principle because the interface with an electrolyte solution is reduced. However, TCPE (3.3 mF) at 400 °C was higher than that (1.1 mF) at 300 °C despite a large particle size at 400 °C. This could be explained by the reduced potential at the electric double layer due to the

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efficient electron transport and high intrinsic catalytic activity of IrOx(OH)y nanoparticles formed at 300 °C. As the increased resistance of the FTO substrate annealed at 500 °C was indicated by the impedance measurement, its influence on the electrochemical data including Icat was examined by measuring CVs of the IrOx-coated electrodes that were prepared using the FTO substrates pre-annealed at 500°C for 4h. The electrochemical data analyzed from the CV data (Figure S6A and B) are summarized in Table 1. The η values are comparable or little higher than those in the normal preparation (without the pre-annealing treatment). Γea increased to 1.9 at 350°C annealing and then decreased with the annealing temperature to be 0.37 at 500 °C annealing. The tendency of annealing temperature-dependent Γea is similar to that in the normal preparation. Although Icat became lower by a factor of 1.3 - 1.7 compared with those in the normal preparation, Icat decreased with the annealing temperature similarly to the case in the normal preparation. Consequently, Acat decreased with the annealing temperature increase from 300 to 400 °C and then increased at above temperatures, being consistent with temperature-dependent Γea for the case in the normal preparation. This indicates that Acat still reflects the intrinsic activity of Ir-sites for electrocatalytic water oxidation even though the resistance of the FTO substrate is increased by high temperature annealing. Tafel plots of the IrOx-coated electrodes based on water oxidation were measured to characterize the IrOx electrocatalyst, as shown in Fig. 7,19,42,47 and given Tafel slope are listed in Table 1. Tafel plots depended on annealing temperature; the lowest Tafel slope was given as 42 mV dec-1 at 300 °C and it gradually increased at

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higher temperatures (Table 1). This is consistent with the lowered Icat for water oxidation at elevated annealing temperature for the CV measurement. Tafel slopes for IrOx-coated electrodes (300-400 °C) are comparable with those (40-50 mV dec-1) of amorphous IrOx·nH2O nanoparticle films reported previously as one of the most efficient electrocatalysts19 and distinctly lower than other reports (60-130 mV dec-1) of nanostructured IrOx.38,39,47 Tafel slopes increased by the pre-annealing treatment by 2 – 33 mV dec-1 due to the increased resistance of the FTO substrate. Electrocatalysis was also conducted under potentiostatic conditions to evaluate the water oxidation performance of the IrOx-coated electrodes annealed at different temperatures. The current-time profiles during electrocatalysis are displayed in Fig. 8. After an initial spike in current density (related to the capacitance component at the electrode-liquid interface) an anodic current due to electrocatalytic water oxidation was observed for respective IrOx electrodes. The superior electrocatalytic performance of the IrOx(OH)y-coated electrode annealed at 300 °C is reflected from the highest charge amount passed (5.96 C) and the amount (nO2) of O2 evolved (15.2 µmol, 98% Faradaic efficiency (F.E.O2)). The nO2 value was 2.4 and 3.7 times higher than those of the amorphous IrOx electrode annealed at 400 °C (6.4 µmol, 96% F.E.O2) and crystalline IrO2 electrode annealed at 500 °C (4.1 µmol, 96% F.E.O2), respectively (Table 3). The high observed electrocatalytic current for the electrode annealed at 300 °C is resulted from the high intrinsic catalytic activity of Ir-sites in the IrOx(OH)y nanoparticles (signified by Acat in CV measurement) and efficient electron transport in the film due to the small nanoparticles (2-5 nm diameter) (signified by Rct in electrochemical impedance measurement). The observed electrocatalytic current for the 500 °C-annealed

17


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electrode is lower than that for the 400 °C-annealed electrode in spite of higher Acat of the former film, which is explained by insufficient electron transport through the electron transfer in grain boundaries between relatively larger crystalline IrO2 particles in the former film. The stability of IrOx-coated electrodes annealed at 300 ~500 °C was examined by the extended electrocatalysis over 8 h (Fig. S7). After 8 h electrocatalysis the catalytic currents steadily declined with 60% decrease for the 300 °C-annealed electrode, which is a little lower than those (72% and 75% decrease) of the electrodes annealed at 400 and 500 °C, respectively. Consequently, the preserved catalytic current after 8 h electrocatalysis for the 300 °C-annealed electrode was 2.6 and 3.7 times higher compared to those of the 400 and 500 °C-annealed electrodes, respectively. The SEM image of the 300 ºC-annealed electrode after 8 h electrocatalysis displays no observable change of the catalyst nanoparticle layer over the FTO surface (Fig. S8). However, the Raman spectroscopic data (Fig. S9) shows that the ratio of the peak area for Ir-OH versus bridging-oxo signals decreased from 0.53 to 0.38 after electrocatalysis, suggesting the decreased content fraction of the Ir-OH moieties. This is in agreement with IrOx(OH)y nanoparticles being important for the high electrocatalytic performance at 300 °C-annealed electrodes. Few of previous reports mentioned the stability of related IrOx-based electrodes with relatively high coverages in acidic and basic electrolyte solutions as well as a solid polymer electrolyte.19,28,29,45 We can compare the stability of the present IrOx-coated electrodes only with the previous self-assembled IrOx colloid nanoparticle electrode that works stably under the similar conditions.21 In potentiostatic electrocatalysis at 1.3 V vs.

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Ag/AgCl (1.8 V vs. RHE) in 0.1 M KNO3 solutions (pH 5.3) using the self-assembled IrOx colloid nanoparticle (0.2 ~ 1 x 10-8 mol cm-2 coverage) electrode, the catalytic current due to water oxidation declined with a 33.4% decrease for 1 h, which is higher compared with that (22.7% decrease) observed during 1 h catalysis for the present IrOx-coated electrode annealed at 300 ºC (Figure 8). Moreover, the catalytic current throughout 1 h catalysis for the 300 ºC-annealed electrode is ca. 2 times higher than that for the self-assembled IrOx colloid nanoparticle electrode. This indicates the higher stability of the present 300 ºC-annealed IrOx-coated electrode.

Conclusions A homogeneous Ir precursor complex solution was spin-coated over a FTO electrode and then annealed at 300 °C to yield a stably adherent and uniform IrOx layer of densely packed of 2-5 nm nanoparticles. This fabrication technique makes it possible to sequentially control composition of nanostructures and crystallinity by changing annealing temperature without significantly altering their textural properties to understand mechanism and key factors for electrocatalytic water oxidation. Raman spectroscopic data suggested that an intermediate IrOx(OH)y nanoparticles with two dispersed terminal-hydroxo and bridging-oxo motifs are formed at 300 and 350 ºC annealing, via amorphous IrOx at 400 ºC, transforming ultimately to crystalline IrO2 nanoparticles at 500 ºC. The IrOx-coated electrode annealed at 300 ºC works efficiently for electrocatalytic water oxidation compared with amorphous IrOx (400 ºC) and crystalline IrO2 (500 ºC) annealed at higher temperatures in terms of the intrinsic catalytic activity of the IrOx(OH)y nanoparticles and efficient electron transport in the film due to the smaller nanoparticles (2-5 nm diameter). The IrOx-coated electrode 19


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annealed at 300 °C is available as a benchmark electrocatalyst for water oxidation and expected to be widely applicable to artificial photosynthesis technology because the highly active film is easily prepared without any complicated procedure.

Supporting information Photographs of films, SEM images, Wide-angle XRD patterns, XPS spectrum, Long time range electrocatalysis data. This materials are available free of charge via the Internet http://pubs.acs.org.

Competing interests The authors declare that they have no competing interests.

Acknowledgment This work was supported by the JST PRESTO program, JSPS KAKENHI Grant Number 24107003, 24350028. DC thanks JSPS for providing postdoctoral fellowship.

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Tables Table 1. Electrochemical properties of IrOx-coated electrodes annealed at different temperatures as measured in a 0.1 M phosphate solution (pH = 6.7). Annealing

Icat

Γeaa

temperature

-2

[mA cm ]

[°C] pre-annealingd

a

Acatb

pre-annealingd

pre-annealingd

ηc

Tafel slope

[V]

[mV dec-1]

pre-annealingd

pre-annealingd

300

1

1

12.4

7.3

1

1

0.24

0.25

42

50

350

1.6

1.9

7.8

5.7

0.39

0.42

0.25

0.27

49

51

400

1.4

1.6

4.0

2.8

0.23

0.24

0.31

0.35

51

84

450

0.75

1.3

3.4

2.5

0.37

0.26

0.35

0.38

58

85

500

0.55

0.37

2.2

1.7

0.32

0.63

0.43

0.46

77

91

Γea (relative amount of electroactive Ir) was calculated from integrated anodic current between 0.09 and 0.80 V of CV and expressed

as relative values to the value at 300 °C; b Acat (relative intrinsic catalytic activity of Ir-sites) was defined as Icat normalized by Γea and expressed as relative values to the value at 300 °C; c η is an overpotential for water oxidation and calculated from the difference between the applied potential and the theoretical potential (0.636 V vs Ag/AgCl at pH = 6.7, corresponding to 1.23 V vs RHE) for water oxidation; d the FTO substrate was pre-annealed at 500 °C for 4 h before IrO2 film coating.

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Table 2. Parameters of the elements of Rsol, Rct and CPE in the equivalent circuit (inset of Fig. 6) given by fitting to impedance data at 0.94 V vs Ag/AgCl of IrOx-coated electrodes annealed at different temperatures.a)

Annealing

Rsol / Ω

Rct / Ω

temperature / °C

a)

CPE TCPEb)

pc)

/ mF

300

47

232

1.1

0.83

350

63

253

2.7

0.82

400

78

362

3.3

0.89

450

65

595

2.5

0.88

500

139

1219

1.6

0.82

Given by fitting to the impedance data in a frequency region of 30 mHz to 20 kHz

(100 mHz to 20 kHz for 300 °C, and 50 mHz to 20 kHz for 350 °C), constant, c) p is CPE exponent of non-ideality.

27


b)

TCPE is CPE

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Table 3. Summary of data on electrocatalytic water oxidation at 1.2 V vs Ag/AgCl for 1h in a 0.1 M phosphate solution (pH = 6.7) using IrOx-coated electrodes annealed at different temperatures. Annealing

Charge / C

nO2 / µmol

F.E.O2a (%)

nH2b / µmol

F.E.H2c (%)

200

0.05

0.1

57

0.2

74

300

5.96

15.2

98

28.3

92

350

3.93

9.9

98

18.8

92

400

2.56

6.4

96

12.5

94

450

2.16

5.4

97

10.5

94

500

1.66

4.1

96

7.9

92

temperature / °C

a

F.E.O2 is Faradaic efficiency for O2 evolution; b nH2 is the amount of H2 evolved in the

Pt counter electrode compartment during the electrocatalysis; efficiency for H2 evolution.

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c

F.E.H2 is Faradaic

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Figure Captions

Scheme 1. (a) Formation route of IrOx nanoparticle films: spin-coating of a precursor complex solution to provide a compacted complex layer followed by annealing at different temperature. IrOx(OH)y nanoparticles are formed at 300 and 350 ºC (b) Conceptual image illustration for the sequentially changed composition of nanostructure and crystallinity of the film with increased annealing temperature (see text).

Fig. 1. UV-visible absorption spectra in hydrolysis of K2IrCl6 precursor solutions for (a) immediate solution after hydrolysis and after aging for (b) 24h at 2 °C and (c) 6h at room temperature. Dotted line shows UV-visible spectra of 0.1 mM aqueous K2IrCl6 solution.

Fig. 2. SEM images of IrOx-coated films annealed at 300 °C for (a) cross-sectional and (b and c) tilted (60°) view. Interface between the IrOx layer and FTO substrate is defined with white dotted line in (c).

Fig. 3. TEM images of IrOx-coated films annealed at (a) 300 °C, (b) 400 °C and (c) 500 °C. HRTEM image of the corresponding samples are shown in the inset. The latticing in the HRTEM images in (a) and (b) is a matrix artifact.

Fig. 4. Raman spectra of IrOx-coated films being as-prepared and annealed at 200-500 °C. Gray line shows the Raman spectra of a monomeric hydroxyiridate complex produced by complete alkaline hydrolysis of K2IrCl6 at pH 13. 29


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Fig. 5. Cyclic voltammograms of IrOx-coated electrodes annealed at 300-500 °C as measured in a 0.1 M phosphate solution (pH = 6.7) in potential windows (A) between 0 and 0.9 V and (B) between 0 and 1.6 V vs Ag/AgCl.

Fig. 6. Nyquist plots of IrOx-coated electrodes annealed at 300-500 °C as measured in a 0.1 M phosphate solution (pH = 6.7) at 0.94 V vs Ag/AgCl. The solid lines show the simulated spectra by fitting to the impedance data in a frequency region of 30 mHz to 20 kHz (100 mHz to 20 kHz for 300 °C, and 50 mHz to 20 kHz for 350 °C) using the equivalent circuits (inset). Rct, Rsol and CPE are charge transfer resistance, solution phase resistance and constant phase element, respectively.

Fig. 7. Tafel plots of IrOx-coated electrodes annealed at 300-500 °C for electrocatalytic water oxidation as measured in a 0.1 M phosphate solution at pH = 6.7. η is an overpotential for water oxidation and calculated from the difference between the applied potential and the theoretical potential (0.636 V vs Ag/AgCl at pH = 6.7, corresponding to 1.23 V vs RHE) for water oxidation.

Fig. 8.

Current-time profiles during electrocatalysis using IrOx-coated electrodes

annealed at 300-500 °C as measured in a 0.1 M phosphate solution (pH = 6.7) at 1.2 V vs Ag/AgCl.

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Scheme 1 Chandra et al.

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Fig. 1 Chandra et al.

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Highly Efficient Electrocatalysis and Mechanistic Investigation of

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