DOI: 10.1002/cssc.201402988
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Iridium Oxide Coatings with Templated Porosity as Highly Active Oxygen Evolution Catalysts: Structure-Activity Relationships Michael Bernicke,[a] Erik Ortel,[b] Tobias Reier,[a] Arno Bergmann,[a] Jorge Ferreira de Araujo,[a] Peter Strasser,[a] and Ralph Kraehnert*[a] Iridium oxide is the catalytic material with the highest stability in the oxygen evolution reaction (OER) performed under acidic conditions. However, its high cost and limited availability demand that IrO2 is utilized as efficiently as possible. We report the synthesis and OER performance of highly active mesoporous IrO2 catalysts with optimized surface area, intrinsic activity, and pore accessibility. Catalytic layers with controlled pore size were obtained by soft-templating with micelles formed from amphiphilic block copolymers poly(ethylene oxide)-b-poly(butadiene)-b-poly(ethylene oxide). A systematic study on the influence of the calcination temperature and film thickness on
the morphology, phase composition, accessible surface area, and OER activity reveals that the catalytic performance is controlled by at least two independent factors, that is, accessible surface area and intrinsic activity per accessible site. Catalysts with lower crystallinity show higher intrinsic activity. The catalyst surface area increases linearly with film thickness. As a result of the templated mesopores, the pore surface remains fully active and accessible even for thick IrO2 films. Even the most active multilayer catalyst does not show signs of transport limitations at current densities as high as 75 mA cm¢2.
Introduction The availability of fossil resources such as oil, coal, and natural gas is limited. A sustainable energy economy, therefore, requires alternative means to convert and store energy. Hydrogen is a promising candidate for chemical energy storage.[1] Its stored energy can be recovered as electrical energy in fuel cells.[2–4] Moreover, major chemical applications such as the Haber–Bosch process[5] or HCl production require sustainable hydrogen sources. Water electrolysis could enable sustainable hydrogen production if the required electricity is provided by, for example, photovoltaics,[6–8] hydroelectric power,[9] or wind power.[10, 11] Water electrolysis under acidic conditions requires the oxygen evolution reaction (OER) on the anode, whereas hydrogen evolution proceeds at the cathode. Active and stable catalysts with a high accessible surface area can minimize the overpotential of both reactions. Typical electrolyzers are limited by the OER because four electrons are needed to produce one molecule of oxygen and because the OER proceeds by a complex reaction mechanism.[12–14] [a] M. Bernicke, T. Reier, A. Bergmann, J. Ferreira de Araujo, Prof. Dr. P. Strasser, Dr. R. Kraehnert Department of Chemistry Technische Universitt Berlin Strasse des 17. Juni 124, 10623 Berlin (Germany) E-mail: [email protected] [b] Dr. E. Ortel Division 6.8 Surface Analysis and Interfacial Chemistry BAM Federal Institute for Materials Research and Testing Unter den Eichen 44–46, 12203 Berlin (Germany) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402988.
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The oxides of Ru and Ir show the lowest OER overpotential in acidic media.[13, 15, 16] Although ruthenium oxide shows a higher activity, it typically corrodes during OER potential cycles. Iridium oxide is, therefore, the best compromise for an active and stable OER catalyst.[13] However, limited abundance and competition with applications such as supercapacitors,[17, 18] stimulating neural electrodes,[19, 20] and microelectrodes for pH sensing[21, 22] require the most efficient utilization of IrO2 possible. Iridium oxide catalysts can be prepared in different ways. Johnson et al.[23] drop-cast a solution that contained iridium acetate and isopropanol onto a Ti cylinder, followed by heat treatment at 480 8C. Electrochemical testing in 0.1 m HClO4 indicated overpotentials of approximately 0.24 V vs. the reversible hydrogen electrode (RHE) at a current density of 1 mA cm¢2. Hu et al.[24] synthesized macroporous IrO2 utilizing colloidal SiO2 as the pore template. Electrochemical testing in 0.5 m H2SO4 indicated an overpotential of 0.25 V vs. RHE at a current density of 1 mA cm¢2. Kushner-Lenhoff[25] synthesized iridium oxide layers by the electrodeposition of organic precursors [Cp*Ir(H2O)3]2++ (Cp* = pentamethylcyclopentadienyl). OER yielded overpotentials of approximately 0.267 V at 0.5 mA cm¢2. A similar synthesis by Blakemore et al.[26] gave a catalyst with an overpotential of approximately 0.270 V at 0.5 mA cm¢2. In general, iridium oxide catalysts are suggested to be more active if the material crystallinity was low.[27] The introduction of porosity into the catalyst can decrease OER overpotentials, particularly at high current densities, by as much as 40 mV (to 0.35 V, measured at 100 mA cm¢2).[28]
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Full Papers The transport of evolved gases can be limited at high current densities. Zeradjanin et al.[29–31] studied the influence of the catalyst structure on the detachment of gas bubbles. Cracked surfaces, that is, surfaces with added transport pores, showed a higher frequency of bubble detachment in oxygen and chlorine evolution than crack-free samples. The addition of sufficiently large pores to OER catalysts could, therefore, provide a large and accessible active surface area and facilitate the transport of evolved gases. Figure 1. Influence of calcination temperature of micelle-templated iridium oxide films on a) film morphology, Moreover, quantitative studies of b) electrochemically accessible surface area, and c) OER performance. a) Top-view SEM images, b) CVs recorded in transport effects in gas evolution the ECSA range between 0.4 and 1.4 V at 50 mV s¢1, and c) CVs recorded in the OER range between 1.2 and 1.65 V ¢1 reactions would benefit from the at 6 mV s . All samples were coated on Ti cylinders and calcined for 5 min at the indicated temperature. All electrocatalytic data were recorded in 0.5 m H2SO4 electrolyte with rotating working electrode, RHE reference, and Pt availability of catalysts with a de- gauze counter electrode. fined and tunable pore size. This study utilizes evaporation-induced self assembly (EISA) [43] and exploits the potential peratures (see Figure S1 in the Supporting Information for of pore templating with polymer micelles to produce modelcomplete ECSA and OER data). type porous catalysts for the investigation of structure–activity SEM images of the sample calcined at 325 8C (Figure 1 a, relationships in gas evolution reactions and for the optimiza325 8C) show charging, which indicates that the polymer temtion of the performance of IrO2-based OER catalysts. Our replate is not fully removed at this temperature and still blocks cently developed synthesis based on pore templating with mithe pore system. Films treated at higher temperatures of 350, celles of amphiphilic block copolymers poly(ethylene oxide)-b375, 400, and 475 8C feature a fully developed system of locally poly(butadiene)-b-poly(ethylene oxide) (PEO-PB-PEO)[28, 32] is furordered mesopores (see, for example, small-angle X-ray scatterther improved to produce iridium oxide films with a controlled ing (SAXS) data for the sample calcined at 400 8C in Figure S4) pore size, film thickness, and crystallinity. The model systems with pore diameters of approximately (21 4) nm and a wall are used to study the influence of porosity and crystallinity on thickness of (11 1) nm (values derived from Figure 1 a, 350– the electrochemically accessible surface area (ECSA), OER per475 8C). The obtained mesoporosity agrees well with the pore formance, and gas transport. A controlled variation in the morphology observed typically for oxide films templated by thickness of the porous catalysts explores which parts of the micelles of the pore template PEO-PB-PEO (TiO2,[32] MgO,[36] catalyst can be utilized without transport limitations during ZnO,[37] Co3O4[37]). Moreover, SEM images of films calcined at high-current OER. The combined knowledge is used to design 550 and 625 8C indicate the beginning of the deformation of a multilayer IrO2 catalyst that shows the lowest overpotential the circular mesopore shape, which can be attributed to the reported so far for Ir-based OER catalysts. onset of sintering. The ECSA of each sample can be derived from the current response (normalized to the substrate geometric surface area) in the potential range of 0.4–1.4 V vs. RHE (Figure 1 b and FigResults and Discussion ure S1 b). Samples calcined at 325 8C show a very small surface Influence of calcination temperature on morphology, ECSA, area (Figure 1 b). This observation is in good agreement with and OER performance the SEM analysis in which significant charging was observed. If charging is caused by the incomplete removal of the pore The calcination temperature of micelle-templated oxide films template, then the pore system is still blocked and not electrocan strongly influence the precursor decomposition, template chemically accessible. For samples calcined at intermediate removal, film morphology, and surface area.[28] To relate the temperatures, a rapid increase in current density and accessimorphology of templated iridium oxide to its accessible surble surface is observed (Figure 1 b and Figure S1 b, 350 and face area and catalytic activity, dip-coated films were calcined 375 8C). Temperature treatment at 400 8C and above decreases at temperatures of 325, 350, 375, 400, 475, 550, and 625 8C and the ECSA progressively to yield the lowest ECSA values for analyzed by SEM, ECSA measurements, and OER testing. Topsamples heat-treated at 625 8C. view SEM images, cyclic voltammograms (CVs) in the ECSA The OER performance was measured on the same catalysts range,[33–35] and CVs recorded in the OER potential range are by CV in the potential range of 1.2–1.65 V. The current represented in Figure 1 for materials calcined at selected temChemSusChem 2015, 8, 1908 – 1915
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Full Papers sponse normalized to the geometrical surface area of the Ti cylinders is shown in Figure 1 C (see Figure S1 a for additional data). The OER activity depends strongly on the applied calcination temperature. All catalysts except for the sample calcined at 325 8C show significant OER activity. The OER activity increases up to 375 8C, whereas higher calcination temperatures lead to a progressive decrease in OER activity. Similar trends in ECSA and OER performance are also observed under alkaline conditions (Figure S5 a and b). The mesoporous catalyst calcined at 375 8C is approximately 20 times more active than a single-crystal Ir(11 0)[41] or an Ir-cylinder (bulk)[42] measured for reference under acidic conditions (Figure S2 c). The overpotential can be used as a measure of catalytic activity. Under standard conditions the thermodynamic potential of the OER amounts to 1.23 V. The respective overpotentials recorded at a geometric current density of 0.5 mA cm¢2 are listed in Table 1. The lowest overpotential (0.212 V vs. RHE) is ob-
Table 1. Influence of calcination temperature on OER overpotential (at 0.5 mA cm¢2) vs. RHE and comparison to IrO2 without templated mesopores (from Ref. [27]). Mesoporous IrO2 heat-treated at 375 8C (bold) shows the lowest overpotential. Tcalc [8C]
[email protected] mA cm¢2 [V] vs. RHE (Figure S2 b)
325 350 375 400 475 550 625
no significant activity 0.228 0.212 0.218 0.228 0.278 0.307
[email protected] mA cm¢2 [V] vs. RHE for untemplated IrO2[27] – 0.26 – – – 0.35 –
Faradaic efficiency To elucidate the faradaic efficiency, we investigated the production of H2O2 and dissolution of the catalyst, which are the most common side reactions of the OER. Titration with 0.001 mol L¢1 KMnO4 solution was performed on the fresh electrolyte and after 50 OER cycles with the most active catalyst (Tcalc = 375 8C) to test for H2O2. No significant differences were observed between the two electrolytes, which suggests that negligible amounts of H2O2 were formed. Moreover, inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of the used electrolyte did not detect dissolved Ir species within the limits of experimental accuracy. Both tests suggest the high Faradaic efficiency of the IrOx catalysts. To test further if residual carbon that could potentially remain after catalyst synthesis affects the OER current, additional tests were conducted by differential electrochemical mass spectrometry (DEMS) performed during OER. The formed product gases were measured by mass spectrometry (MS) which mainly consisted of O2 and CO2. The observed O2 concentration for the first applied CV amounts to 98.7 %, and the rest is CO2.The O2 selectivity further increases to 98.8 % for the second CV and 99.9 % for the tenth applied cycle (Figure S7). Hence, the current that results from the removal of residual carbon during the OER is rather small.
Relationship between OER activity, ECSA, and sample crystallinity
served for the sample calcined at 375 8C. Calcination at temperatures of 475 8C and above result in significantly increased overpotentials. Overpotentials obtained by Reier et al.[27] on catalysts prepared similarly without templated mesoporosity are also given in Table 1. This results in, for example, 32 and 72 mV higher overpotentials for samples calcined at 350 and 550 8C, respectively.
The relationship between the ECSA of the catalysts and the respective OER performance was analyzed by quantification of the respective electrochemical data. The total charge (obtained by the mean value of the integrated anodic and cathodic currents in the ECSA range as a measure of the catalyst total surface area), current density (normalized to the geometric electrode surface area recorded at 1.55 V during OER potential scans as a measure of OER activity), and the current density (at 1.55 V) normalized to the total charge (as an indicator of each catalysts intrinsic activity) plotted as a function of calcination temperature are shown in Figure 2.
Figure 2. ECSA, OER activity, and ECSA-normalized activity of mesoporous iridium oxide as a function of calcination temperature. a) ECSA as total charge obtained as a mean value of the integrated anodic and cathodic currents between 0.4 and 1.4 V vs. RHE, b) current density recorded at 1.55 V vs. RHE normalized to the geometric electrode surface area, and c) geometric current density at 1.55 V normalized to the ECSA charge as an indicator of each catalysts intrinsic activity.
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Full Papers The ECSA increases from 325 to 375 8C with increasing calcination temperature (Figure 2 a) and then rapidly decreases between 400 and 625 8C. The catalyst surface area measured by Kr physisorption shows a very similar behavior and a peak at the same calcination temperature of 375 8C (Figure S2 a). The observed OER activity also follows this trend (Figure 2 b). The OER activity increases rapidly with increasing calcination temperature to reach a maximum at 375 8C and declines steadily with further increasing temperature. For calcination temperatures higher than 325 8C, the measured OER activity is plotted against the respective ECSA in Figure 2 c. It can be seen that this surface-area-normalized activity stays almost constant for samples calcined between 350 and 475 8C but decreases steadily for higher calcination temperatures. It is, therefore, evident that at least two major factors contribute to the overall OER activity, that is, the accessible surface area as well as the intrinsic activity of each accessible site. To relate the observed ECSA and activity trends to the structural and compositional properties of the porous IrO2 catalysts, the respective samples were studied by XRD, selected area electron diffraction (SAED), and TEM. Diffractograms for the catalysts calcined at different temperatures, SAED images, and bright-field TEM images for representative samples calcined at 350, 475, and 625 8C are shown in Figure 3. After polishing, the Ti substrate shows only the expected reflections that correspond to metallic Ti (PDF 00-044-1294; Fig-
ure 3 a, Ti cyl). Heat treatment of the substrate under typical calcination conditions of this study (5 min, 550 8C) does not lead to a measurable formation of crystalline titanium oxides anatase or rutile (Figure 3 a, Ti cyl 550 8C). The coating of the substrates with mesoporous iridium oxide and calcination at temperatures between 350 and 475 8C does not produce any additional reflections (Figure 3 a, 350– 475 8C). Hence, the corresponding catalytic layers are X-ray amorphous: IrO2 has either not crystallized yet or the crystallites are too small to provide sufficiently intense diffraction signals. However, calcination at either 550 or 625 8C results in broad reflections at 2 q = 28.1 and 34.78 (Figure 3 a, 550 and 625 8C). The signals correspond well with the (11 0) and (1 0 1) reflections of crystalline IrO2. An estimate by the Scherrer equation provides a crystallite size of approximately 4 nm (550 8C). An increase of the calcination temperature to 625 8C increases the IrO2 crystallite size to approximately 5 nm. XRD analysis, therefore, suggests that calcination at 550 8C and higher temperatures forms crystalline IrO2, which is absent at lower calcination temperatures. The local crystallinity of mesopore walls was studied by electron diffraction analysis on IrO2 samples removed from the Ti substrate and placed on TEM grids. SAED images of samples calcined in air at 350, 475, and 625 8C are shown in Figure 3 b (see Figure S3 for additional calcination temperatures 375 and 550 8C). Films calcined at 350 and 375 8C show very broad dif-
Figure 3. Analysis of crystallinity (XRD, SAED) and morphology (TEM) for mesoporous iridium oxide calcined at different temperatures between 350 and 625 8C. a) Diffractograms of a polished Ti cylinder, the Ti cylinder calcined at 550 8C, and IrOx-coated Ti cylinders (350–625 8C). b) SAED analysis for IrOx samples calcined at 350, 475, and 625 8C. Indexing corresponds to IrO2 rutile (PDF 150870). c) Analysis of the pore structure of the samples by TEM.
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Full Papers fraction rings, which indicate the onset of crystallization. Samples calcined at 475 8C feature narrow diffraction rings. The respective hkl indices (Figure 3 b, 475 8C) correspond well with the lattice parameters of crystalline IrO2 rutile (PDF 150870). Calcination at 550 and 625 8C produces materials with sharp diffraction rings with clearly distinguishable diffraction spots indicative of high material crystallinity. SAED analysis of film samples removed from the substrate thus confirms the phase assignment as IrO2 rutile. Crystallinity increases with increasing calcination temperature, and significant amounts of crystalline iridium oxide are already present at 475 8C. TEM analysis of the same samples (Figure 3 c, Figure S3 b) confirms that calcination at 350, 375, and 475 8C yields spherical mesopores that originate from the pore template. However, the progressive crystallization at 625 8C leads to sintering, which results in the deformation of the initially spherical pore shape (Figure 3 c, 625 8C). The combined XRD, SAED, and TEM data thus suggest that catalyst films calcined between 350 and 475 8C are composed of iridium oxide with low crystallinity, whereas calcination at 550 or 625 8C produces a mesoporous well-crystallized IrO2 rutile phase with a slightly degraded pore structure. The morphology and phase composition (Figure 3) can now be related to the ECSA and activity data (Figure 2). The lowest studied calcination temperature (325 8C) is too low to decompose the template polymer. Hence pores are blocked and neither significant ECSA (Figure 2 a) nor OER activity (Figure 2 b) can be observed. Calcination at 350 or 375 8C removes the template polymer and forms an iridium oxide with very low crystallinity (Figure 3 b) but a highly accessible pore structure (Figure 2 a). The accessible pore structure consists of highly active sites (Figure 2 c), which results in optimal OER performance (Figure 2 b). Calcination at 400 or 475 8C produces a porous iridium oxide composed of sites with similar intrinsic activity (Figure 2 c). However, the ECSA decreases significantly with increasing calcination temperature (Figure 2 a) to result in a lower OER performance (Figure 2 b). A further increase in calcination temperature to 550 and 625 8C forms well-crystallized IrO2 (Figure 3 a and b), the pore structure of which is degraded by sintering (Figure 3 c). The sintering results in a further decrease of the ECSA (Figure 2 a). Moreover, the activity per accessible site decreases clearly with the increasing material crystallinity (Figure 2 c). Consequently, the overall OER performance decreases even further. The activity of pore-templated iridium oxide films is, therefore, determined by at least two major factors, that is, the accessible surface area, which reaches an optimum at 375 8C, and the intrinsic activity of the accessible sites, which is high between 350 and 475 8C. The best overall performance is thus obtained for samples calcined at 375 8C. Similar trends for the intrinsic OER activity were found by Reier et al. on untemplated iridium oxide thin-film catalysts.[27] They observed that the intrinsic OER activity remained independent of the calcination temperature between 250 and 350 8C but decreased with calcination at 450 8C and at higher temperatures. The High activity was assigned to an amorphous ChemSusChem 2015, 8, 1908 – 1915
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low-temperature iridium oxide, whereas crystalline high-temperature oxide was reported to be less active. However, they did not observe an increase in ECSA and geometric OER activity between 325 and 375 8C, which suggests that this effect is related to the removal of the pore template. Single-layer and multilayer dip coating The number of potentially active sites of a homogeneous catalytic coating is expected to scale linearly with the amount of coating (i.e., film thickness) and its specific surface area. By increasing the withdrawal rate of the Ti substrates, mesoporous single-layer iridium oxide coatings were prepared with high film thickness to increase the catalysts overall OER performance. However, the thickness of crack-free single layers of oxides that can be produced by EISA[43] is limited.[38] Therefore, thick films were produced by a new multilayer dip-coating procedure with intermediate calcination steps. The films were employed as scalable model systems with defined porosity to test if the transport of electrons to the active sites and/or the pore transport of produced oxygen gas to the outer film surface becomes limiting for thicker films at a higher current density. SEM cross-section images, ECSA analysis, and the OER performance are presented in Figure S6 a, b, and c for the respective films. Mesoporous crack-free catalyst films were obtained for all single-layer coatings up to a withdrawal rate during dip coating of 150 mm min¢1. (Note that the cracks visible in the images shown in Figure S6 a are a result of sample preparation for film thickness analysis and not the synthesis procedure.) The thickness of the produced mesoporous single-layer iridium oxide increases linearly with the increasing withdrawal rate of the substrate from 50 nm (10 mm min¢1) to 120 nm (50 mm min¢1), 170 nm (100 mm min¢1), and finally 225 nm (150 mm min¢1). A further increase in film thickness by faster substrate withdrawal resulted in cracking and peel-off of the films during calcination. However, significantly thicker films (480 nm) were produced by multilayer deposition (30 mm min¢1, four layers) with intermediate stabilization steps at 200 8C and a final calcination at 375 8C (Figure S6 a, multilayer). The ECSA recorded by CV in the potential window between 0.4 and 1.4 V shows that the geometrical current response increases linearly with the increasing film thickness (Figure S6 b). Moreover, the respective OER scans shift towards lower overpotentials with increasing film thickness (Figure S6 c). The influence of film thickness on the ECSA and OER activity was quantified by analysis of the respective electrocatalytic data. Plots of the obtained total ECSA charge versus film thickness are shown in Figure 4 a, and geometric OER current densities at 1.50, 1.53, and 1.55 V plotted versus the ECSA for each of the respective single-layer and multilayer films are shown in Figure 4 b. Clearly, the obtained ECSA scales linearly with film thickness for mesoporous IrO2 films between 50 and 480 nm, which indicates that for all films the complete film volume is accessible for ECSA analysis (Figure 4 a). Moreover, the geometric current density obtained at a given potential in OER measurements
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Figure 4. Influence of the thickness of mesoporous templated IrO2 films on ECSA and on OER performance for single layer (50–225 nm) and multilayer catalysts (four layers, 480 nm). a) ECSA as total charge obtained as a mean value of the integrated anodic and cathodic currents between 0.4 and 1.4 V vs. RHE plotted versus film thickness (from SEM). b) Current density recorded at 1.50, 1.53, and 1.55 V vs. RHE normalized to the geometric electrode surface area and plotted versus the ECSA. Layer thickness from cross-sectional SEM (Figure S6 a). ECSA and OER CVs are shown in Figures S6 b and S6 c.
scales linearly with the employed ECSA area (Figure 4 b). This analysis suggests that, independent of film thickness, each surface site contributes equally to the OER reaction, even for the thicker films and higher current densities. If any process other than the surface reaction, such as gas transport through the pore system or electron conduction to the active site, was the limiting step, a deviation from the linear behavior would be expected for thicker films at least at higher potentials and current densities. As this is not the case (Figure 4 b), transport limitations are absent in the case of iridium oxide catalyst films with templated mesopores at least up to 480 nm thickness, potentials of 1.55 V, and geometrical current densities as high as 75 mA cm¢2. To relate the achieved catalytic performance to values obtained in previous studies, the overpotentials can be compared at a given geometric current density (Table 2). Johnson et al.[23] prepared thick IrO2 layers by drop-casting and measured OER overpotentials of approximately 0.24 V vs. RHE at 1 mA cm¢2. Hu et al.[24] synthesized macroporous IrO2 with colloidal SiO2 by hard templating and measured an overpotential of approximately 0.25 V at 1 mA cm¢2. Nakagawa et al.[39] used an electroflocculation method to prepare 2 nm iridium oxide nanoparticles and reported an overpotential of 0.25 V at 0.5 mA cm¢2. Kushner-Lenhoff et al.[25] synthesized iridium oxide layers by electrodeposition from [Cp*Ir(H2O)3]2++ and reported an overpotential of 0.267 V at 0.5 mA cm¢2. A similar synthesis per-
formed by Blakemore et al.[26] yielded catalysts with overpotentials of approximately 0.270 V at 0.5 mA cm¢2. Previously, we achieved an overpotential of 0.220 V at 1 mA cm¢2 for thinner singlelayer IrO2.[28] The optimized catalyst in the present study (480 nm thick multilayer calcined at 375 8C) achieves an overpotential as low as 0.20 V at 1 mA cm¢2, which is significantly lower than values reported previously. A further increase in performance is likely if thicker films are obtained by further exploitation of the multilayer approach or dip coating in the capillary regime.[38]
Conclusions
A new approach for the synthesis of model-type oxygen evolution reaction (OER) catalysts with controlled thickness, pore size, and crystallinity is reported. In combination with the investigation of structure–activity relationships, the best OER catalyst based on iridium oxide reported so far in the literature is obtained. Mesopores introduced into the catalyst by templating with micelles of block copolymers enable a rapid transport of the produced oxygen at least up to current densities of 75 mA cm¢2. Thick mesoporous catalyst films are obtained by multilayer deposition. Even the most active catalyst did not show signs of limitation of electron transport, electrolyte access, or gas transport. The catalyst films are chemically and mechanically stable at current densities as high as 75 mA cm¢2. The investigation of structure–activity relationships revealed that the OER performance of mesoporous IrO2 is controlled by at least two independent factors, that is, the accessible surface area and the intrinsic activity per accessible site. Templating with polymer micelles produced very high electrochemically accessible surface areas (ECSA) with an optimum for samples calcined at 375 8C. At lower calcination temperatures, the pore system is still partially blocked by the remaining template. Higher calcination temperatures decrease the ECSA because of sintering and crystallite growth. Moreover, two different regimes of reactivity were identified. Calcination between 350 and 475 8C produces iridium oxide with a low crystallinity and consistently high activity. CalciTable 2. Comparison of OER overpotentials at a given current density with literature nation at higher temperatures induces progressive data. crystallization and decreases the OER activity. CalciMaterial Current density Overpotential Reference nation at 375 8C thus produces materials with the ¢2 [V] [mA cm ] highest ECSA and the highest reactivity. Multilayer mesoporous templated IrO2 (multilayer) 1 0.20 this work coatings calcined under such conditions provided 0.22 [28] mesoporous templated IrO2 (single layer) 1 optimal OER performance. 1 0.24 [23] IrO2 layer The developed films represent the first example of 1 0.25 [24] macroporous IrO2 0.5 0.25 [39] IrO2 nanoparticles a homogeneous and scalable model system for 0.5 0.267 [25] electrodeposited IrO2 layer transport phenomena in gas evolution reactions that electrodeposited IrO2 layer 0.5 0.27 [26] provides narrow pore size distribution, tunable pore
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Full Papers size, controlled layer thickness, and high activity. The concept enables the optimal utilization of the precious metal Ir. Moreover, the model system paves the way for the quantitative investigation of structure–activity relationships and transport properties in other gas evolution reactions. Further work will extend this concept to other reactions and catalysts with a lower content of noble metal.
Experimental Section Catalyst synthesis Mesoporous IrO2 films were synthesized on Ti cylinders. The substrates were polished to a mirror finish before film deposition using colloidal silica suspensions (Buehler, MasterMet 2, noncrystallizing colloidal silica suspension, 0.02 mm), then ultrasonicated in water, and rinsed three times in ethanol (VWR Chemicals, 99.98 % absolute). Dip-coating solutions for calcination studies were obtained by the addition of iridium(III) acetate (225 mg, Heraeus, 48.76 % Ir content) and a polymer template PEO-PB-PEO (45 mg, containing 18 700 g mol¢1 PEO and 10 000 g mol¢1 PB, from Polymer Service Merseburg GmbH)[32] to EtOH (1.5 mL). For thicker films and multilayers, a mixture of EtOH (1.3 mL) and H2O (0.1 mL) was used. The dip coating of single-layer catalysts was performed under a controlled atmosphere at a relative humidity of 40 % and a temperature of 25 8C at a withdrawal rate of 100 mm min¢1 in a dipcoater (Coater 5 AC, IdLabs Vesely). Films were calcined subsequently placing the samples in a hot muffle furnace at temperatures between 325 and 625 8C for 5 min. For the controlled variation of film thickness, the withdrawal rate of the substrates was adjusted between 10 and 150 mm min¢1 using the same calcination routine (5 min, 375 8C). Multilayer catalysts were obtained by the dip coating (30 mm min¢1) and heat treatment (30 min, 200 8C) of individual layers followed by a final calcination step (5 min, 375 8C).
To analyze the ECSA, the potential was swept between 0.4 and 1.4 V vs. RHE at a scan rate of 50 mV s¢1 The valence state of surface metal atoms will be changed by placing hydrous IrO2 in solution and by altering the potential. Furthermore, a reversible proton inclusion mechanism can take place as described by Trasatti et al.[33] [Eq. (1)]: IrOx ðOHÞy þd Hþ þd e¢ ! IrOx¢d ðOHÞyþd
ð1Þ
The ECSA of iridium oxide was then quantified by determining the mean value of the integrated anodic and cathodic scan of the resulting CV.[33–35]
DEMS The amount of formed gas products was assessed in separate DEMS experiments utilizing mesoporous templated IrO2 coated onto Ti cylinders. The DEMS apparatus consisted of an electrochemical flow cell (0.5 m H2SO4, flow rate: 5 mL s¢1) connected by a separation PTFE membrane to a PrismaTM quadrupole mass spectrometer (QMS 200, Pfeiffer-Vacuum) equipped with two turbomolecular pumps HiPace 80 that kept the MS chamber at 10¢6 mbar. The MS was calibrated with a reference gas CO2 in N2/ O2 (Linde, 75.04 % N2 ; 19.95 % O2 ; 5.01 % CO2). CV measurements were conducted in the OER regime by cycling the potential between 1.2 and 1.65 V vs. RHE (6 mV s¢1) and recording the ion current for O2 (mass 32) and CO2 (mass 44). The amount of evolved O2 and CO2 was calculated from the ion currents.
Acknowledgements
Catalyst characterization SEM images were obtained by using a JEOL 7401F instrument with an accelerating voltage of 10 kV. To determine the film thickness, a ceramic knife was used to scratch mesoporous iridium oxide films followed by SEM imaging of tilted Ti cylinders. SEM images were analyzed with ImageJ v1.43u[40] to derive film thickness and pore diameter. TEM and SAED images were obtained by using a FEI Tecnai G2 20 S-Twin at an accelerating voltage of 200 kV. The film samples were scraped off from the Ti cylinder and then collected on a TEM grid. XRD patterns were recorded by using a Bruker D8 Advance instrument using CuKa radiation, grazing incident for the incoming beam, and a Goebel mirror. The crystallite size was obtained by applying the Scherrer equation to the (11 0) reflection of the IrO2 phase.
Oxygen evolution and ECSA All electrocatalytic testing was performed by using a three-electrode disc setup using a RHE (Gaskatel, HydroFlex) as a reference and a Pt gauze (Chempur, 1024 mesh cm¢2, 0.06 mm wire diameter, 99.9 %) as a counter electrode. All potentials in this work are referenced to the reversible hydrogen electrode. Iridium oxide films coated on Ti cylinders were mounted on a rotating disk shaft and served as a working electrode (n = 1600 rpm) using 0.5 m H2SO4 as ChemSusChem 2015, 8, 1908 – 1915
the supporting electrolyte (Fixanal, Fluka Analytical) and a BioLogic SP-200 as potentiostat. The electrolyte solution was purged with nitrogen before the catalytic tests. The OER activity was investigated by CV in a potential window of 1.2–1.65 V vs. RHE with a scan rate of 6 mV s¢1. Impedance spectroscopy was measured to correct the Ohmic losses.
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The authors thank ZELMI (TU-Berlin) for TEM analysis. M.B. and R.K. appreciate generous funding from BMBF under contract FKZ 03K3009 and Einstein Stiftung Berlin (EJF-2011-95). Keywords: electrochemistry · iridium · structure–activity relationships · template synthesis · water splitting [1] R. Schlçgl, ChemSusChem 2010, 3, 209 – 222. [2] T. E. Springer, T. A. Zawodzinski, S. Gottesfeld, J. Electrochem. Soc. 1991, 138, 2334 – 2342. [3] K. D. Kreuer, J. Membr. Sci. 2001, 185, 29 – 39. [4] M. Winter, R. J. Brodd, Chem. Rev. 2004, 104, 4245 – 4269. [5] F. Haber, G. van Oordt, Z. Anorg. Allg. Chem. 1905, 44, 341 – 378. [6] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 1999, 285, 692 – 698. [7] A. J. Frank, N. Kopidakis, J. van de Lagemaat, Coord. Chem. Rev. 2004, 248, 1165 – 1179. [8] M. Grtzel, C. R. Chim. 2006, 9, 578 – 583. [9] R. Bakis, Energy Sources Part B 2007, 2, 259 – 266. [10] E. Koutroulis, K. Kalaitzakis, IEEE Trans. Ind. Electron. 2006, 53, 486 – 494. [11] A. Costa, A. Crespo, J. Navarro, G. Lizcano, H. Madsen, E. Feitosa, Renewable Sustainable Energy Rev. 2008, 12, 1725 – 1744. [12] A. GoÇi-Urtiaga, D. Presvytes, K. Scott, Int. J. Hydrogen Energy 2012, 37, 3358 – 3372.
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Full Papers [13] M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy 2013, 38, 4901 – 4934. [14] M. Y. Wang, Z. Wang, X. Z. Gong, Z. C. Guo, Renewable Sustainable Energy Rev. 2014, 29, 573 – 588. [15] M. Vukovic, D. Cukman, M. Milun, L. D. Atanasoska, R. T. Atanasoski, J. Electroanal. Chem. 1992, 330, 663 – 673. [16] R. Kçtz, S. Stucki, Electrochim. Acta 1986, 31, 1311 – 1316. [17] Y.-M. Chen, J.-H. Cai, Y.-S. Huang, K.-Y. Lee, D.-S. Tsai, K.-K. Tiong, Nanotechnology 2011, 22, 355708. [18] S. Chandrabose Raghu, M. Ulaganathan, T. M. Lim, M. S. Kazacos, J. Power Sources 2013, 238, 103 – 108. [19] I. S. Lee, J. M. Park, H. J. Son, J. C. Park, G. H. Lee, Y. H. Lee, F. Z. Ciu, Key Eng. Mater. 2005, 288 – 289, 307 – 310. [20] Y. Lu, Z. X. Cai, Y. L. Cao, H. X. Yang, Y. Y. Duan, Electrochem. Commun. 2008, 10, 778 – 782. [21] I. A. Ges, B. L. Ivanov, A. A. Werdich, F. J. Baudenbacher, Biosens. Bioelectron. 2007, 22, 1303 – 1310. [22] L. M. Kuo, K. N. Chen, Y. L. Chuang, S. C. Chao, ECS Solid State Lett. 2013, 2, P28 – P30. [23] B. Johnson, F. Girgsdies, G. Weinberg, D. Rosenthal, A. Knop-Gericke, R. Schlogl, T. Reier, P. Strasser, J. Phys. Chem. C 2013, 117, 25443 – 25450. [24] W. Hu, Y. Q. Wang, X. H. Hu, Y. Q. Zhou, S. L. Chen, J. Mater. Chem. 2012, 22, 6010 – 6016. [25] M. N. Kushner-Lenhoff, J. D. Blakemore, N. D. Schley, R. H. Crabtree, G. W. Brudvig, Dalton Trans. 2013, 42, 3617 – 3622. [26] J. D. Blakemore, N. D. Schley, G. W. Olack, C. D. Incarvito, G. W. Brudvig, R. H. Crabtree, Chem. Sci. 2011, 2, 94 – 98. [27] T. Reier, I. Weidinger, P. Hildebrandt, R. Kraehnert, P. Strasser, ECS Trans. 2013, 58, 39 – 51. [28] E. Ortel, T. Reier, P. Strasser, R. Kraehnert, Chem. Mater. 2011, 23, 3201 – 3209.
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Received: September 12, 2014 Revised: January 30, 2015 Published online on May 8, 2015
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Iridium Oxide Coatings with Templated Porosity as Highly Active Oxygen Evolution Catalysts: Structure-Activity Relationships Michael Bernicke,[a] Erik Ortel,[b] Tobias Reier,[a] Arno Bergmann,[a] Jorge Ferreira de Araujo,[a] Peter Strasser,[a] and Ralph Kraehnert*[a] Iridium oxide is the catalytic material with the highest stability in the oxygen evolution reaction (OER) performed under acidic conditions. However, its high cost and limited availability demand that IrO2 is utilized as efficiently as possible. We report the synthesis and OER performance of highly active mesoporous IrO2 catalysts with optimized surface area, intrinsic activity, and pore accessibility. Catalytic layers with controlled pore size were obtained by soft-templating with micelles formed from amphiphilic block copolymers poly(ethylene oxide)-b-poly(butadiene)-b-poly(ethylene oxide). A systematic study on the influence of the calcination temperature and film thickness on
the morphology, phase composition, accessible surface area, and OER activity reveals that the catalytic performance is controlled by at least two independent factors, that is, accessible surface area and intrinsic activity per accessible site. Catalysts with lower crystallinity show higher intrinsic activity. The catalyst surface area increases linearly with film thickness. As a result of the templated mesopores, the pore surface remains fully active and accessible even for thick IrO2 films. Even the most active multilayer catalyst does not show signs of transport limitations at current densities as high as 75 mA cm¢2.
Introduction The availability of fossil resources such as oil, coal, and natural gas is limited. A sustainable energy economy, therefore, requires alternative means to convert and store energy. Hydrogen is a promising candidate for chemical energy storage.[1] Its stored energy can be recovered as electrical energy in fuel cells.[2–4] Moreover, major chemical applications such as the Haber–Bosch process[5] or HCl production require sustainable hydrogen sources. Water electrolysis could enable sustainable hydrogen production if the required electricity is provided by, for example, photovoltaics,[6–8] hydroelectric power,[9] or wind power.[10, 11] Water electrolysis under acidic conditions requires the oxygen evolution reaction (OER) on the anode, whereas hydrogen evolution proceeds at the cathode. Active and stable catalysts with a high accessible surface area can minimize the overpotential of both reactions. Typical electrolyzers are limited by the OER because four electrons are needed to produce one molecule of oxygen and because the OER proceeds by a complex reaction mechanism.[12–14] [a] M. Bernicke, T. Reier, A. Bergmann, J. Ferreira de Araujo, Prof. Dr. P. Strasser, Dr. R. Kraehnert Department of Chemistry Technische Universitt Berlin Strasse des 17. Juni 124, 10623 Berlin (Germany) E-mail: [email protected] [b] Dr. E. Ortel Division 6.8 Surface Analysis and Interfacial Chemistry BAM Federal Institute for Materials Research and Testing Unter den Eichen 44–46, 12203 Berlin (Germany) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402988.
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The oxides of Ru and Ir show the lowest OER overpotential in acidic media.[13, 15, 16] Although ruthenium oxide shows a higher activity, it typically corrodes during OER potential cycles. Iridium oxide is, therefore, the best compromise for an active and stable OER catalyst.[13] However, limited abundance and competition with applications such as supercapacitors,[17, 18] stimulating neural electrodes,[19, 20] and microelectrodes for pH sensing[21, 22] require the most efficient utilization of IrO2 possible. Iridium oxide catalysts can be prepared in different ways. Johnson et al.[23] drop-cast a solution that contained iridium acetate and isopropanol onto a Ti cylinder, followed by heat treatment at 480 8C. Electrochemical testing in 0.1 m HClO4 indicated overpotentials of approximately 0.24 V vs. the reversible hydrogen electrode (RHE) at a current density of 1 mA cm¢2. Hu et al.[24] synthesized macroporous IrO2 utilizing colloidal SiO2 as the pore template. Electrochemical testing in 0.5 m H2SO4 indicated an overpotential of 0.25 V vs. RHE at a current density of 1 mA cm¢2. Kushner-Lenhoff[25] synthesized iridium oxide layers by the electrodeposition of organic precursors [Cp*Ir(H2O)3]2++ (Cp* = pentamethylcyclopentadienyl). OER yielded overpotentials of approximately 0.267 V at 0.5 mA cm¢2. A similar synthesis by Blakemore et al.[26] gave a catalyst with an overpotential of approximately 0.270 V at 0.5 mA cm¢2. In general, iridium oxide catalysts are suggested to be more active if the material crystallinity was low.[27] The introduction of porosity into the catalyst can decrease OER overpotentials, particularly at high current densities, by as much as 40 mV (to 0.35 V, measured at 100 mA cm¢2).[28]
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Full Papers The transport of evolved gases can be limited at high current densities. Zeradjanin et al.[29–31] studied the influence of the catalyst structure on the detachment of gas bubbles. Cracked surfaces, that is, surfaces with added transport pores, showed a higher frequency of bubble detachment in oxygen and chlorine evolution than crack-free samples. The addition of sufficiently large pores to OER catalysts could, therefore, provide a large and accessible active surface area and facilitate the transport of evolved gases. Figure 1. Influence of calcination temperature of micelle-templated iridium oxide films on a) film morphology, Moreover, quantitative studies of b) electrochemically accessible surface area, and c) OER performance. a) Top-view SEM images, b) CVs recorded in transport effects in gas evolution the ECSA range between 0.4 and 1.4 V at 50 mV s¢1, and c) CVs recorded in the OER range between 1.2 and 1.65 V ¢1 reactions would benefit from the at 6 mV s . All samples were coated on Ti cylinders and calcined for 5 min at the indicated temperature. All electrocatalytic data were recorded in 0.5 m H2SO4 electrolyte with rotating working electrode, RHE reference, and Pt availability of catalysts with a de- gauze counter electrode. fined and tunable pore size. This study utilizes evaporation-induced self assembly (EISA) [43] and exploits the potential peratures (see Figure S1 in the Supporting Information for of pore templating with polymer micelles to produce modelcomplete ECSA and OER data). type porous catalysts for the investigation of structure–activity SEM images of the sample calcined at 325 8C (Figure 1 a, relationships in gas evolution reactions and for the optimiza325 8C) show charging, which indicates that the polymer temtion of the performance of IrO2-based OER catalysts. Our replate is not fully removed at this temperature and still blocks cently developed synthesis based on pore templating with mithe pore system. Films treated at higher temperatures of 350, celles of amphiphilic block copolymers poly(ethylene oxide)-b375, 400, and 475 8C feature a fully developed system of locally poly(butadiene)-b-poly(ethylene oxide) (PEO-PB-PEO)[28, 32] is furordered mesopores (see, for example, small-angle X-ray scatterther improved to produce iridium oxide films with a controlled ing (SAXS) data for the sample calcined at 400 8C in Figure S4) pore size, film thickness, and crystallinity. The model systems with pore diameters of approximately (21 4) nm and a wall are used to study the influence of porosity and crystallinity on thickness of (11 1) nm (values derived from Figure 1 a, 350– the electrochemically accessible surface area (ECSA), OER per475 8C). The obtained mesoporosity agrees well with the pore formance, and gas transport. A controlled variation in the morphology observed typically for oxide films templated by thickness of the porous catalysts explores which parts of the micelles of the pore template PEO-PB-PEO (TiO2,[32] MgO,[36] catalyst can be utilized without transport limitations during ZnO,[37] Co3O4[37]). Moreover, SEM images of films calcined at high-current OER. The combined knowledge is used to design 550 and 625 8C indicate the beginning of the deformation of a multilayer IrO2 catalyst that shows the lowest overpotential the circular mesopore shape, which can be attributed to the reported so far for Ir-based OER catalysts. onset of sintering. The ECSA of each sample can be derived from the current response (normalized to the substrate geometric surface area) in the potential range of 0.4–1.4 V vs. RHE (Figure 1 b and FigResults and Discussion ure S1 b). Samples calcined at 325 8C show a very small surface Influence of calcination temperature on morphology, ECSA, area (Figure 1 b). This observation is in good agreement with and OER performance the SEM analysis in which significant charging was observed. If charging is caused by the incomplete removal of the pore The calcination temperature of micelle-templated oxide films template, then the pore system is still blocked and not electrocan strongly influence the precursor decomposition, template chemically accessible. For samples calcined at intermediate removal, film morphology, and surface area.[28] To relate the temperatures, a rapid increase in current density and accessimorphology of templated iridium oxide to its accessible surble surface is observed (Figure 1 b and Figure S1 b, 350 and face area and catalytic activity, dip-coated films were calcined 375 8C). Temperature treatment at 400 8C and above decreases at temperatures of 325, 350, 375, 400, 475, 550, and 625 8C and the ECSA progressively to yield the lowest ECSA values for analyzed by SEM, ECSA measurements, and OER testing. Topsamples heat-treated at 625 8C. view SEM images, cyclic voltammograms (CVs) in the ECSA The OER performance was measured on the same catalysts range,[33–35] and CVs recorded in the OER potential range are by CV in the potential range of 1.2–1.65 V. The current represented in Figure 1 for materials calcined at selected temChemSusChem 2015, 8, 1908 – 1915
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Full Papers sponse normalized to the geometrical surface area of the Ti cylinders is shown in Figure 1 C (see Figure S1 a for additional data). The OER activity depends strongly on the applied calcination temperature. All catalysts except for the sample calcined at 325 8C show significant OER activity. The OER activity increases up to 375 8C, whereas higher calcination temperatures lead to a progressive decrease in OER activity. Similar trends in ECSA and OER performance are also observed under alkaline conditions (Figure S5 a and b). The mesoporous catalyst calcined at 375 8C is approximately 20 times more active than a single-crystal Ir(11 0)[41] or an Ir-cylinder (bulk)[42] measured for reference under acidic conditions (Figure S2 c). The overpotential can be used as a measure of catalytic activity. Under standard conditions the thermodynamic potential of the OER amounts to 1.23 V. The respective overpotentials recorded at a geometric current density of 0.5 mA cm¢2 are listed in Table 1. The lowest overpotential (0.212 V vs. RHE) is ob-
Table 1. Influence of calcination temperature on OER overpotential (at 0.5 mA cm¢2) vs. RHE and comparison to IrO2 without templated mesopores (from Ref. [27]). Mesoporous IrO2 heat-treated at 375 8C (bold) shows the lowest overpotential. Tcalc [8C]
[email protected] mA cm¢2 [V] vs. RHE (Figure S2 b)
325 350 375 400 475 550 625
no significant activity 0.228 0.212 0.218 0.228 0.278 0.307
[email protected] mA cm¢2 [V] vs. RHE for untemplated IrO2[27] – 0.26 – – – 0.35 –
Faradaic efficiency To elucidate the faradaic efficiency, we investigated the production of H2O2 and dissolution of the catalyst, which are the most common side reactions of the OER. Titration with 0.001 mol L¢1 KMnO4 solution was performed on the fresh electrolyte and after 50 OER cycles with the most active catalyst (Tcalc = 375 8C) to test for H2O2. No significant differences were observed between the two electrolytes, which suggests that negligible amounts of H2O2 were formed. Moreover, inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of the used electrolyte did not detect dissolved Ir species within the limits of experimental accuracy. Both tests suggest the high Faradaic efficiency of the IrOx catalysts. To test further if residual carbon that could potentially remain after catalyst synthesis affects the OER current, additional tests were conducted by differential electrochemical mass spectrometry (DEMS) performed during OER. The formed product gases were measured by mass spectrometry (MS) which mainly consisted of O2 and CO2. The observed O2 concentration for the first applied CV amounts to 98.7 %, and the rest is CO2.The O2 selectivity further increases to 98.8 % for the second CV and 99.9 % for the tenth applied cycle (Figure S7). Hence, the current that results from the removal of residual carbon during the OER is rather small.
Relationship between OER activity, ECSA, and sample crystallinity
served for the sample calcined at 375 8C. Calcination at temperatures of 475 8C and above result in significantly increased overpotentials. Overpotentials obtained by Reier et al.[27] on catalysts prepared similarly without templated mesoporosity are also given in Table 1. This results in, for example, 32 and 72 mV higher overpotentials for samples calcined at 350 and 550 8C, respectively.
The relationship between the ECSA of the catalysts and the respective OER performance was analyzed by quantification of the respective electrochemical data. The total charge (obtained by the mean value of the integrated anodic and cathodic currents in the ECSA range as a measure of the catalyst total surface area), current density (normalized to the geometric electrode surface area recorded at 1.55 V during OER potential scans as a measure of OER activity), and the current density (at 1.55 V) normalized to the total charge (as an indicator of each catalysts intrinsic activity) plotted as a function of calcination temperature are shown in Figure 2.
Figure 2. ECSA, OER activity, and ECSA-normalized activity of mesoporous iridium oxide as a function of calcination temperature. a) ECSA as total charge obtained as a mean value of the integrated anodic and cathodic currents between 0.4 and 1.4 V vs. RHE, b) current density recorded at 1.55 V vs. RHE normalized to the geometric electrode surface area, and c) geometric current density at 1.55 V normalized to the ECSA charge as an indicator of each catalysts intrinsic activity.
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Full Papers The ECSA increases from 325 to 375 8C with increasing calcination temperature (Figure 2 a) and then rapidly decreases between 400 and 625 8C. The catalyst surface area measured by Kr physisorption shows a very similar behavior and a peak at the same calcination temperature of 375 8C (Figure S2 a). The observed OER activity also follows this trend (Figure 2 b). The OER activity increases rapidly with increasing calcination temperature to reach a maximum at 375 8C and declines steadily with further increasing temperature. For calcination temperatures higher than 325 8C, the measured OER activity is plotted against the respective ECSA in Figure 2 c. It can be seen that this surface-area-normalized activity stays almost constant for samples calcined between 350 and 475 8C but decreases steadily for higher calcination temperatures. It is, therefore, evident that at least two major factors contribute to the overall OER activity, that is, the accessible surface area as well as the intrinsic activity of each accessible site. To relate the observed ECSA and activity trends to the structural and compositional properties of the porous IrO2 catalysts, the respective samples were studied by XRD, selected area electron diffraction (SAED), and TEM. Diffractograms for the catalysts calcined at different temperatures, SAED images, and bright-field TEM images for representative samples calcined at 350, 475, and 625 8C are shown in Figure 3. After polishing, the Ti substrate shows only the expected reflections that correspond to metallic Ti (PDF 00-044-1294; Fig-
ure 3 a, Ti cyl). Heat treatment of the substrate under typical calcination conditions of this study (5 min, 550 8C) does not lead to a measurable formation of crystalline titanium oxides anatase or rutile (Figure 3 a, Ti cyl 550 8C). The coating of the substrates with mesoporous iridium oxide and calcination at temperatures between 350 and 475 8C does not produce any additional reflections (Figure 3 a, 350– 475 8C). Hence, the corresponding catalytic layers are X-ray amorphous: IrO2 has either not crystallized yet or the crystallites are too small to provide sufficiently intense diffraction signals. However, calcination at either 550 or 625 8C results in broad reflections at 2 q = 28.1 and 34.78 (Figure 3 a, 550 and 625 8C). The signals correspond well with the (11 0) and (1 0 1) reflections of crystalline IrO2. An estimate by the Scherrer equation provides a crystallite size of approximately 4 nm (550 8C). An increase of the calcination temperature to 625 8C increases the IrO2 crystallite size to approximately 5 nm. XRD analysis, therefore, suggests that calcination at 550 8C and higher temperatures forms crystalline IrO2, which is absent at lower calcination temperatures. The local crystallinity of mesopore walls was studied by electron diffraction analysis on IrO2 samples removed from the Ti substrate and placed on TEM grids. SAED images of samples calcined in air at 350, 475, and 625 8C are shown in Figure 3 b (see Figure S3 for additional calcination temperatures 375 and 550 8C). Films calcined at 350 and 375 8C show very broad dif-
Figure 3. Analysis of crystallinity (XRD, SAED) and morphology (TEM) for mesoporous iridium oxide calcined at different temperatures between 350 and 625 8C. a) Diffractograms of a polished Ti cylinder, the Ti cylinder calcined at 550 8C, and IrOx-coated Ti cylinders (350–625 8C). b) SAED analysis for IrOx samples calcined at 350, 475, and 625 8C. Indexing corresponds to IrO2 rutile (PDF 150870). c) Analysis of the pore structure of the samples by TEM.
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Full Papers fraction rings, which indicate the onset of crystallization. Samples calcined at 475 8C feature narrow diffraction rings. The respective hkl indices (Figure 3 b, 475 8C) correspond well with the lattice parameters of crystalline IrO2 rutile (PDF 150870). Calcination at 550 and 625 8C produces materials with sharp diffraction rings with clearly distinguishable diffraction spots indicative of high material crystallinity. SAED analysis of film samples removed from the substrate thus confirms the phase assignment as IrO2 rutile. Crystallinity increases with increasing calcination temperature, and significant amounts of crystalline iridium oxide are already present at 475 8C. TEM analysis of the same samples (Figure 3 c, Figure S3 b) confirms that calcination at 350, 375, and 475 8C yields spherical mesopores that originate from the pore template. However, the progressive crystallization at 625 8C leads to sintering, which results in the deformation of the initially spherical pore shape (Figure 3 c, 625 8C). The combined XRD, SAED, and TEM data thus suggest that catalyst films calcined between 350 and 475 8C are composed of iridium oxide with low crystallinity, whereas calcination at 550 or 625 8C produces a mesoporous well-crystallized IrO2 rutile phase with a slightly degraded pore structure. The morphology and phase composition (Figure 3) can now be related to the ECSA and activity data (Figure 2). The lowest studied calcination temperature (325 8C) is too low to decompose the template polymer. Hence pores are blocked and neither significant ECSA (Figure 2 a) nor OER activity (Figure 2 b) can be observed. Calcination at 350 or 375 8C removes the template polymer and forms an iridium oxide with very low crystallinity (Figure 3 b) but a highly accessible pore structure (Figure 2 a). The accessible pore structure consists of highly active sites (Figure 2 c), which results in optimal OER performance (Figure 2 b). Calcination at 400 or 475 8C produces a porous iridium oxide composed of sites with similar intrinsic activity (Figure 2 c). However, the ECSA decreases significantly with increasing calcination temperature (Figure 2 a) to result in a lower OER performance (Figure 2 b). A further increase in calcination temperature to 550 and 625 8C forms well-crystallized IrO2 (Figure 3 a and b), the pore structure of which is degraded by sintering (Figure 3 c). The sintering results in a further decrease of the ECSA (Figure 2 a). Moreover, the activity per accessible site decreases clearly with the increasing material crystallinity (Figure 2 c). Consequently, the overall OER performance decreases even further. The activity of pore-templated iridium oxide films is, therefore, determined by at least two major factors, that is, the accessible surface area, which reaches an optimum at 375 8C, and the intrinsic activity of the accessible sites, which is high between 350 and 475 8C. The best overall performance is thus obtained for samples calcined at 375 8C. Similar trends for the intrinsic OER activity were found by Reier et al. on untemplated iridium oxide thin-film catalysts.[27] They observed that the intrinsic OER activity remained independent of the calcination temperature between 250 and 350 8C but decreased with calcination at 450 8C and at higher temperatures. The High activity was assigned to an amorphous ChemSusChem 2015, 8, 1908 – 1915
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low-temperature iridium oxide, whereas crystalline high-temperature oxide was reported to be less active. However, they did not observe an increase in ECSA and geometric OER activity between 325 and 375 8C, which suggests that this effect is related to the removal of the pore template. Single-layer and multilayer dip coating The number of potentially active sites of a homogeneous catalytic coating is expected to scale linearly with the amount of coating (i.e., film thickness) and its specific surface area. By increasing the withdrawal rate of the Ti substrates, mesoporous single-layer iridium oxide coatings were prepared with high film thickness to increase the catalysts overall OER performance. However, the thickness of crack-free single layers of oxides that can be produced by EISA[43] is limited.[38] Therefore, thick films were produced by a new multilayer dip-coating procedure with intermediate calcination steps. The films were employed as scalable model systems with defined porosity to test if the transport of electrons to the active sites and/or the pore transport of produced oxygen gas to the outer film surface becomes limiting for thicker films at a higher current density. SEM cross-section images, ECSA analysis, and the OER performance are presented in Figure S6 a, b, and c for the respective films. Mesoporous crack-free catalyst films were obtained for all single-layer coatings up to a withdrawal rate during dip coating of 150 mm min¢1. (Note that the cracks visible in the images shown in Figure S6 a are a result of sample preparation for film thickness analysis and not the synthesis procedure.) The thickness of the produced mesoporous single-layer iridium oxide increases linearly with the increasing withdrawal rate of the substrate from 50 nm (10 mm min¢1) to 120 nm (50 mm min¢1), 170 nm (100 mm min¢1), and finally 225 nm (150 mm min¢1). A further increase in film thickness by faster substrate withdrawal resulted in cracking and peel-off of the films during calcination. However, significantly thicker films (480 nm) were produced by multilayer deposition (30 mm min¢1, four layers) with intermediate stabilization steps at 200 8C and a final calcination at 375 8C (Figure S6 a, multilayer). The ECSA recorded by CV in the potential window between 0.4 and 1.4 V shows that the geometrical current response increases linearly with the increasing film thickness (Figure S6 b). Moreover, the respective OER scans shift towards lower overpotentials with increasing film thickness (Figure S6 c). The influence of film thickness on the ECSA and OER activity was quantified by analysis of the respective electrocatalytic data. Plots of the obtained total ECSA charge versus film thickness are shown in Figure 4 a, and geometric OER current densities at 1.50, 1.53, and 1.55 V plotted versus the ECSA for each of the respective single-layer and multilayer films are shown in Figure 4 b. Clearly, the obtained ECSA scales linearly with film thickness for mesoporous IrO2 films between 50 and 480 nm, which indicates that for all films the complete film volume is accessible for ECSA analysis (Figure 4 a). Moreover, the geometric current density obtained at a given potential in OER measurements
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Figure 4. Influence of the thickness of mesoporous templated IrO2 films on ECSA and on OER performance for single layer (50–225 nm) and multilayer catalysts (four layers, 480 nm). a) ECSA as total charge obtained as a mean value of the integrated anodic and cathodic currents between 0.4 and 1.4 V vs. RHE plotted versus film thickness (from SEM). b) Current density recorded at 1.50, 1.53, and 1.55 V vs. RHE normalized to the geometric electrode surface area and plotted versus the ECSA. Layer thickness from cross-sectional SEM (Figure S6 a). ECSA and OER CVs are shown in Figures S6 b and S6 c.
scales linearly with the employed ECSA area (Figure 4 b). This analysis suggests that, independent of film thickness, each surface site contributes equally to the OER reaction, even for the thicker films and higher current densities. If any process other than the surface reaction, such as gas transport through the pore system or electron conduction to the active site, was the limiting step, a deviation from the linear behavior would be expected for thicker films at least at higher potentials and current densities. As this is not the case (Figure 4 b), transport limitations are absent in the case of iridium oxide catalyst films with templated mesopores at least up to 480 nm thickness, potentials of 1.55 V, and geometrical current densities as high as 75 mA cm¢2. To relate the achieved catalytic performance to values obtained in previous studies, the overpotentials can be compared at a given geometric current density (Table 2). Johnson et al.[23] prepared thick IrO2 layers by drop-casting and measured OER overpotentials of approximately 0.24 V vs. RHE at 1 mA cm¢2. Hu et al.[24] synthesized macroporous IrO2 with colloidal SiO2 by hard templating and measured an overpotential of approximately 0.25 V at 1 mA cm¢2. Nakagawa et al.[39] used an electroflocculation method to prepare 2 nm iridium oxide nanoparticles and reported an overpotential of 0.25 V at 0.5 mA cm¢2. Kushner-Lenhoff et al.[25] synthesized iridium oxide layers by electrodeposition from [Cp*Ir(H2O)3]2++ and reported an overpotential of 0.267 V at 0.5 mA cm¢2. A similar synthesis per-
formed by Blakemore et al.[26] yielded catalysts with overpotentials of approximately 0.270 V at 0.5 mA cm¢2. Previously, we achieved an overpotential of 0.220 V at 1 mA cm¢2 for thinner singlelayer IrO2.[28] The optimized catalyst in the present study (480 nm thick multilayer calcined at 375 8C) achieves an overpotential as low as 0.20 V at 1 mA cm¢2, which is significantly lower than values reported previously. A further increase in performance is likely if thicker films are obtained by further exploitation of the multilayer approach or dip coating in the capillary regime.[38]
Conclusions
A new approach for the synthesis of model-type oxygen evolution reaction (OER) catalysts with controlled thickness, pore size, and crystallinity is reported. In combination with the investigation of structure–activity relationships, the best OER catalyst based on iridium oxide reported so far in the literature is obtained. Mesopores introduced into the catalyst by templating with micelles of block copolymers enable a rapid transport of the produced oxygen at least up to current densities of 75 mA cm¢2. Thick mesoporous catalyst films are obtained by multilayer deposition. Even the most active catalyst did not show signs of limitation of electron transport, electrolyte access, or gas transport. The catalyst films are chemically and mechanically stable at current densities as high as 75 mA cm¢2. The investigation of structure–activity relationships revealed that the OER performance of mesoporous IrO2 is controlled by at least two independent factors, that is, the accessible surface area and the intrinsic activity per accessible site. Templating with polymer micelles produced very high electrochemically accessible surface areas (ECSA) with an optimum for samples calcined at 375 8C. At lower calcination temperatures, the pore system is still partially blocked by the remaining template. Higher calcination temperatures decrease the ECSA because of sintering and crystallite growth. Moreover, two different regimes of reactivity were identified. Calcination between 350 and 475 8C produces iridium oxide with a low crystallinity and consistently high activity. CalciTable 2. Comparison of OER overpotentials at a given current density with literature nation at higher temperatures induces progressive data. crystallization and decreases the OER activity. CalciMaterial Current density Overpotential Reference nation at 375 8C thus produces materials with the ¢2 [V] [mA cm ] highest ECSA and the highest reactivity. Multilayer mesoporous templated IrO2 (multilayer) 1 0.20 this work coatings calcined under such conditions provided 0.22 [28] mesoporous templated IrO2 (single layer) 1 optimal OER performance. 1 0.24 [23] IrO2 layer The developed films represent the first example of 1 0.25 [24] macroporous IrO2 0.5 0.25 [39] IrO2 nanoparticles a homogeneous and scalable model system for 0.5 0.267 [25] electrodeposited IrO2 layer transport phenomena in gas evolution reactions that electrodeposited IrO2 layer 0.5 0.27 [26] provides narrow pore size distribution, tunable pore
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Full Papers size, controlled layer thickness, and high activity. The concept enables the optimal utilization of the precious metal Ir. Moreover, the model system paves the way for the quantitative investigation of structure–activity relationships and transport properties in other gas evolution reactions. Further work will extend this concept to other reactions and catalysts with a lower content of noble metal.
Experimental Section Catalyst synthesis Mesoporous IrO2 films were synthesized on Ti cylinders. The substrates were polished to a mirror finish before film deposition using colloidal silica suspensions (Buehler, MasterMet 2, noncrystallizing colloidal silica suspension, 0.02 mm), then ultrasonicated in water, and rinsed three times in ethanol (VWR Chemicals, 99.98 % absolute). Dip-coating solutions for calcination studies were obtained by the addition of iridium(III) acetate (225 mg, Heraeus, 48.76 % Ir content) and a polymer template PEO-PB-PEO (45 mg, containing 18 700 g mol¢1 PEO and 10 000 g mol¢1 PB, from Polymer Service Merseburg GmbH)[32] to EtOH (1.5 mL). For thicker films and multilayers, a mixture of EtOH (1.3 mL) and H2O (0.1 mL) was used. The dip coating of single-layer catalysts was performed under a controlled atmosphere at a relative humidity of 40 % and a temperature of 25 8C at a withdrawal rate of 100 mm min¢1 in a dipcoater (Coater 5 AC, IdLabs Vesely). Films were calcined subsequently placing the samples in a hot muffle furnace at temperatures between 325 and 625 8C for 5 min. For the controlled variation of film thickness, the withdrawal rate of the substrates was adjusted between 10 and 150 mm min¢1 using the same calcination routine (5 min, 375 8C). Multilayer catalysts were obtained by the dip coating (30 mm min¢1) and heat treatment (30 min, 200 8C) of individual layers followed by a final calcination step (5 min, 375 8C).
To analyze the ECSA, the potential was swept between 0.4 and 1.4 V vs. RHE at a scan rate of 50 mV s¢1 The valence state of surface metal atoms will be changed by placing hydrous IrO2 in solution and by altering the potential. Furthermore, a reversible proton inclusion mechanism can take place as described by Trasatti et al.[33] [Eq. (1)]: IrOx ðOHÞy þd Hþ þd e¢ ! IrOx¢d ðOHÞyþd
ð1Þ
The ECSA of iridium oxide was then quantified by determining the mean value of the integrated anodic and cathodic scan of the resulting CV.[33–35]
DEMS The amount of formed gas products was assessed in separate DEMS experiments utilizing mesoporous templated IrO2 coated onto Ti cylinders. The DEMS apparatus consisted of an electrochemical flow cell (0.5 m H2SO4, flow rate: 5 mL s¢1) connected by a separation PTFE membrane to a PrismaTM quadrupole mass spectrometer (QMS 200, Pfeiffer-Vacuum) equipped with two turbomolecular pumps HiPace 80 that kept the MS chamber at 10¢6 mbar. The MS was calibrated with a reference gas CO2 in N2/ O2 (Linde, 75.04 % N2 ; 19.95 % O2 ; 5.01 % CO2). CV measurements were conducted in the OER regime by cycling the potential between 1.2 and 1.65 V vs. RHE (6 mV s¢1) and recording the ion current for O2 (mass 32) and CO2 (mass 44). The amount of evolved O2 and CO2 was calculated from the ion currents.
Acknowledgements
Catalyst characterization SEM images were obtained by using a JEOL 7401F instrument with an accelerating voltage of 10 kV. To determine the film thickness, a ceramic knife was used to scratch mesoporous iridium oxide films followed by SEM imaging of tilted Ti cylinders. SEM images were analyzed with ImageJ v1.43u[40] to derive film thickness and pore diameter. TEM and SAED images were obtained by using a FEI Tecnai G2 20 S-Twin at an accelerating voltage of 200 kV. The film samples were scraped off from the Ti cylinder and then collected on a TEM grid. XRD patterns were recorded by using a Bruker D8 Advance instrument using CuKa radiation, grazing incident for the incoming beam, and a Goebel mirror. The crystallite size was obtained by applying the Scherrer equation to the (11 0) reflection of the IrO2 phase.
Oxygen evolution and ECSA All electrocatalytic testing was performed by using a three-electrode disc setup using a RHE (Gaskatel, HydroFlex) as a reference and a Pt gauze (Chempur, 1024 mesh cm¢2, 0.06 mm wire diameter, 99.9 %) as a counter electrode. All potentials in this work are referenced to the reversible hydrogen electrode. Iridium oxide films coated on Ti cylinders were mounted on a rotating disk shaft and served as a working electrode (n = 1600 rpm) using 0.5 m H2SO4 as ChemSusChem 2015, 8, 1908 – 1915
the supporting electrolyte (Fixanal, Fluka Analytical) and a BioLogic SP-200 as potentiostat. The electrolyte solution was purged with nitrogen before the catalytic tests. The OER activity was investigated by CV in a potential window of 1.2–1.65 V vs. RHE with a scan rate of 6 mV s¢1. Impedance spectroscopy was measured to correct the Ohmic losses.
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Received: September 12, 2014 Revised: January 30, 2015 Published online on May 8, 2015
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