Electrochemically Activated Iridium Oxide Black as Promising

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Electrochemically Activated Iridium Oxide Black as Promising Electrocatalyst Having High Activity and Stability for Oxygen Evolution Reaction Shin-Ae Park, Kyu-Su Kim, and Yong-Tae Kim ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00368 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Energy Letters

Electrochemically Activated Iridium Oxide Black as Promising Electrocatalyst Having High Activity and Stability for Oxygen Evolution Reaction Shin-Ae Parka, Kyu-Su Kima and Yong-Tae Kim*a a

Department of Energy System, Pusan National University, Busan 609-735, Republic of Korea

ABSTRACT

In this study, we report a promising approach to drastically enhance the activity of Iridium retaining the high stability nature; electrochemically activated Ir oxide black (A-IrOx-B) prepared by the selective etching of less noble component from the binary oxide. Binary IrOs oxide black (IrOsOx-B) were formed by using a solvent-free method, and then an electrochemical activation was performed by the selective etching of Os oxide in order to obtain the activated Ir oxide black possessing high surface area. It was interestingly revealed that the OER activity of A-IrOx-B surpassed that of IrOx black, and approached that of state of the art surface segregated IrRu oxide (S-IrRuOx-B). Most notably, the OER activity of S-IrRuOx-B decreased after stability testing because of the dissolution of Ru, while the activity of A-IrOx-B changed little and eventually surpassed the activity of S-IrRuOx-B.

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ACS Energy Letters

The use of hydrogen as an energy carrier is an essential element for the utilization of intermittent renewable energy sources in the hydrogen economy.1-3 Among the various hydrogen production technologies,4, 5 the electrolysis or photoelectrolysis of water, in which hydrogen is obtained from the direct splitting of water using electrical energy or a combination of sunlight and electricity, is widely recognized to be the optimal solution.6, 7 However, the most serious hurdle for the commercialization of electrolyzers or solar water splitting devices is the low conversion efficiency due to the huge overpotential of the oxygen evolution reaction (OER) at the anode, which is a counter-reaction of the hydrogen evolution reaction (HER) at the cathode.8-10 Generally, Ir oxides and their binary or ternary mixed metal oxides are considered to be the best electrocatalysts for OER in acidic media, in which it is beneficial for the operating current density to be typically over 3 times higher than in alkaline medium.11 In recent, the surface-segregated IrRu12 and the core-shell IrNi binary oxides13, 14 exhibited improved activity and stability for OER.15-23 However, since the dissolution of less noble metal in all of the Ir binary oxides is still the most serious problem in the long term operation,24 it is a promising approach to develop how to enhance the OER activity with the minimization of stability loss of sole Ir oxide. In this study, we report electrochemically activated Ir oxide black (A-IrOx-B) that can overcome an inherent instability while show a similar or better activity than binary oxide catalysts. The A-IrOx-B were obtained through applying an electrochemical selective etching process to IrOs binary oxide black, and surpassed the state-of-the-art surface-segregated IrRu binary oxides in terms of both activity and stability in OER conditions. Until now, activity and stability were widely considered to have an inverse relationship; however, in this study, an anomalous relationship between the activity and stability of A-IrOx-B is present.


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IrOsOx black was prepared via a solvent-free method for use as a precursor to obtain the surface etched IrOx catalyst, and segregated IrRuOx(S-IrRuOx), Ir, Ru, and Os balck were also synthesized via the same method. The morphology and structure of the prepared catalysts were characterized by TEM and XRD. The morphology of the catalysts (Ir, Os, Ru, IrOsOx, and IrRuOx) was observed using TEM as shown in Figure 1. In the TEM images, the average particle size of the catalysts is approximately 10–50 nm, and smaller particles with sizes of approximately 5–10 nm were also observed, similar to the observations in the previous study.25 Since the average particle sizes of the different catalysts were similar, the effect of particle size on the OER activity was believed to be negligible. The crystal structures of the catalysts were examined by measuring the XRD patterns. Figure 1(f) presents the XRD patterns of Ir, Os, Ru, IrOsOx, and surface segregated IrRuOx. Ir, Os, Ru, and IrOsOx were successfully synthesized as a single phase. The XRD pattern of Ir matched well with that of a typical face-centered cubic structure ((111), (200), (220), and (311) facets); the XRD patterns of Os, Ru, and IrOsOx matched well with that of a typical hexagonal structure ((100), (002), (101), (102), (110), (103), and (112) facets) with no additional peaks.26 The XRD pattern of the surface segregated IrRu is consistent with both face-centered cubic and hexagonal structures. The heating process in a H2/Ar (5 %) atmosphere was thought to cause the segregation of Ir on the IrRu surface, resulting in the face-centered cubic and hexagonal structures. The X-ray diffraction patterns showed mostly broad peaks, indicating the similar crystallite sizes among the catalysts,13 which was in agreement with the TEM results and further supported the belief that the effect of particle size on the OER activity is negligible.


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ACS Energy Letters

Figure 2(a) shows the linear sweep voltammetry (LSV) curves for the etching process of IrOs. Its oxidation as OsO42- started at about 1.0 V, and subsequently dissolution of OsO4 occurred.27 Compared with the dealloying process of the IrOs thin film in our previous report,28 the etching time for IrOsOx powder was very short. Because the amount of low coordinate sites is much greater in the powder catalyst than in the thin film catalyst, it is thought that the selective etching of Os from the powder occurs much more readily than from the thin film. It is plausible that almost all the Os was dissolved into liquid phase, resulting in the formation of surface activated IrOx, because in the LSV curve for A-IrOx-B, the OsO4 oxidation peak was hardly observed after the etching process. As shown in the Figure 2(a) inset, the ICP-MS results confirmed that the composition was changed from IrOs (1:1) to IrOs (97:3) after etching of Os oxide, which implies that most of the Os was eliminated during the etching process. Figure 3 and Figure S1 show TEM images and FETEM-EDS line profile from before and after the electrochemically activating process(selective etching of Os oxide) for IrOsOx black, confirming the IrOx with high surface area after the etching process of the IrOsOx black. In Figure 3(a), the average particle size of IrOsOx was approximately 10–50 nm. Figure 3(b), however, demonstrates porous particles with sizes of 5–10 nm after the Os oxide etching process. In Figure 3(c) and Figure 3(d), most Os were removed by selective etching process. The particles before etching exhibit flat intensity, while after selective etching of the low density of edge and high density of center of the particles demonstrate the deformation of the particle morphology. To assess the unique behavior observed in the IrOsOx alloys and optimize the composition of IrOsOx for use as an OER catalyst, IrOsOx alloys with different Os contents in the precursor alloy IrOsOx(3:1, 1:1, and 1:3) were electrochemically etched. The OER activity of


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the catalysts after the selective etching process is shown in Figure 2(b) and Figure S2. The OER activity of A-IrOx-B(3:1) was significantly lower than that of A-IrOx-B(1:1). This reveals that the low content of Os in A-IrOx-B(3:1) caused insufficient formation of the fully etched shape. The OER activity of A-IrOx-B(1:3) was also lower than that of A-IrOx-B(1:1). In this case, the excessive amount of Os leads to the collapse of the structure. Therefore, atomic composition of A-IrOx-B(1:1) was confirmed to be optimal for the A-IrOx-B system. Figure 4(a) shows the CV curves of electrochemically activated Ir oxide black (A-IrOxB) and Ir. In the CV curve of Ir, underpotential hydrogen deposition (Hupd) and stripping peaks between 0.05–0.4 V reveal typical hydrogen surface electrochemistry.13 On the other hand, in the CV curve of A-IrOx-B, unlike in that of pure Ir, an oxidative wave between 0.6–1.2 V demonstrated the high surface area of the Ir oxide formed through selective etching of Os oxide. The CV area of this catalyst was higher than that of the nanoporous Ir thin film in our previous report.28 As shown in Figure 4(b), the electrochemically active surface areas (ECSAs) of the catalysts were calculated using a previously reported method.29 The ECSA of A-IrOx-B (52.5 cm2) was higher than that of Ir (37.3 cm2), indicating that the surface area of A-IrOx-B increased during the selective etching of Os oxide, resulting in the formation of the electrochemically activated catalyst. The severe dissolution of Ru nanoparticles due to their low stability at the operational potential of the OER is a well known phenomenon.24 Markovic et al. reported the formation of surface segregated IrRu, which showed high activity and stability, using a H2 atmosphere. However, to fundamentally eliminate problems of dissolution, we prepared porous Ir black (AIrOx-B) through the electrochemical etching of IrOsOx. The OER activity of A-IrOx-B was compared with that of S-IrRuOx-B and Ir using a RDE technique.


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ACS Energy Letters

Figure 5(a) shows the OER activity of Os, Ru, S-IrRuOx-B, A-IrOx-B, and Ir. The dissolution of Os and Ru began at 1.0 V and 1.4 V, respectively. The calculated charge of the dissolution of Ru to Ru8+ ions (0.696 C) in the LSV curve was higher than the theoretical charge (0.255 C) of the initial Ru metal loaded on the electrode surface. This indicated that the oxygen evolution charge was added to the dissolution charge of Ru in the LSV curve, which implies that the dissolution and oxygen evolution reactions are simultaneous. In contrast, calculated charge of dissolution for Os8+ ions (0.158 C) was similar to the theoretical charge (0.170 C) of the initial Os metal loaded on the electrode surface. It was confirmed that Os was almost completely dissolved in the potential range of oxygen evolution, as reported previously.28, 30 Hence, Os was used as a porogen material in the etching process to successfully obtain electrochemically activated IrOx black.28 The OER activity of the catalysts followed the order S-IrRuOx-B (1.548 V) < A-IrOx-B (1.554 V) < Ir (1.589 V), vs RHE at 10 mA cm-2geo. The OER activity of A-IrOxB was significantly higher than that of Ir, and furthermore approached that of the state-of-the-art surface segregated IrRu.12 Compared with recently reported OER catalysts, the activity of AIrOx-B (1.554 V) was higher than that of IrO2 (1.67 V) at 10 mA cm-2geo. As the overpotential for A-IrOx-B was 0.283 V, lower than that of surface segregated Ir0.5Ru0.5O2 (0.295 V) at 5 mA cm2

geo,

the superior activity of A-IrOx-B was demonstrated.12 The enhanced OER activity of A-

IrOx-B was caused by the increased surface area through selective etching of Os oxide. Since OER stability is considered to be a highly significant factor for the activity in the design of catalysts, short term stability tests of the catalysts were carried out. Figure S3 presents the chronopotentiometry (CP) measurements at 1 mA cm-2geo for 30 min to test the stability of the catalysts. It is interesting that the initial potential of surface segregated IrRuOx black(SIrRuOx-B) was lower than that of A-IrOx-B; however, the potential of S-IrRuOx-B increased and


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after 10 min was greater than that of A-IrOx-B. This resulted from the continual dissolution of surface segregated IrRu, which indicates its low stability, whereas A-IrOx-B showed stable OER activity with little increase of its potential. Figure 5(b) shows the OER curves of the catalysts after the stability test. It is notable that the OER activity of S-IrRuOx-B decreased significantly after stability test. On the other hand, the OER activity for the A-IrOx-B catalyst, which is composed of only Ir metal, remained stable. The OER activity and CV curves of each catalyst are shown individually in Figure S4 and Figure S5. A comparison of the concentrations of dissolved noble metal ions after polarization at a constant current of 10 mA cm-2geo and the OER activity of the catalysts after the stability test are shown in Figure 5(c) and Figure S6. The overpotential of A-IrOx-B is lower than that of S-IrRuOx-B after the stability test, indicating its remarkable OER activity. The concentration of dissolved metal ions for A-IrOx-B, S-IrRuOx-B, and Ir were 0.056 ppb, 0.259 ppb, and 0.037 ppb, respectively. The metal ion concentration for A-IrOx-B was lower than that for S-IrRu-B, which implies the exceptional activity and stability properties of this catalyst, which do not follow the generally well known inverse relationship between activity and stability.30 This can be explained by the high surface roughness and the large number of active sites of A-IrOx-B, as well as the absence of the severe dissolution caused by Ru. The OER activity, specific activity, mass activity, and the dissolution rate before and after the stability test are presented in Table 1. The specific activity of S-IrRuOx-B was markedly decreased after the stability test, because of the decreased Ru content due to dissolution. The mass activity of S-IrRuOx-B also decreased after the stability test, demonstrating the loss of Ru. Although in the literature up to now, OER activity and stability have been reported to have an inverse relationship,30 herein, we have obtained the exceptional result that


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ACS Energy Letters

both the OER activity and the stability of A-IrOx-B are higher than those of S-IrRuOx-B after the stability test. The activity of A-IrOx-B was enhanced through control of its extrinsic activity by increasing its surface area through selective etching of Os oxide. IrRu had high intrinsic activity but low stability. To solve this problem, S-IrRuOx-B was initially surface segregated, but it is hard to completely prevent the dissolution of Ru. Hence, in the harsh conditions of the OER, such as its high operational potential and acidic environment, it is desirable to use catalysts composed of Ir for stability. In addition, the A-IrOx-B catalyst increased its surface area. In conclusion, the electrochemically activated A-IrOx-B catalyst formed by selective etching of Os oxide was not restricted by the traditional anticorrelation between activity and stability. In this study, we demonstrate that powder-type selective etched IrOsOx for PEM electrolyzers, showed outstanding activity and stability. Ir, Ru, Os, IrOsOx, and IrRuOx black were synthesized using a solvent-free synthesis method, and electrochemically activated Ir oxide with a high surface area was produced by the electrochemical etching of Os oxide for IrOs. The OER activity of electrochemically activated Ir oxide black(A-IrOx-B) surpassed that of Ir, and furthermore approached that of state of the art surface segregated IrRu black(S-IrRuOx-B). Most notably, the OER activity of S-IrRuOx-B, which underwent Ru dissolution, was decreased after stability test, while the activity of A-IrOx-B changed little, resulting in its activity surpassing that of S-IrRuOx-B. In conclusion, we suggest that the electrochemically activated Ir oxide black is highly active and stable, and that this catalyst is not restricted by the traditional inverse relationship between OER activity and stability.


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Figure 1. TEM micrographs of (a) Ir, (b) Os, (c) Ru, (d) IrOsOx, (e) IrRuOx, and (f) X-ray diffraction patterns of Ir, Os, Ru, IrOsOx and IrRuOx.


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ACS Energy Letters

Figure 2. (a) Current-potential curves for the dissolution of Os oxide for IrOsOx and the oxygen evolution reaction curve for A-IrOx-B in an Ar-purged 0.1 M HClO4 solution at 10 mV s-1 and (inset) the atomic ratios of IrOsOx and A-IrOx-B. (b) Oxygen evolution reaction curves for AIrOx-B(3:1, 1:1, and 1:3) in an Ar-purged 0.1 M HClO4 solution at 10 mV s-1.


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Figure 3. TEM micrographs of (a) before and (b) after the selective etching process for IrOs oxide black(IrOsOx-B). FETEM –EDS line profile of Ir and Os in (c) IrOsOx-B and (d) A-IrOxB. The red lines in the inset indicate the directions of the EDS line profile.


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ACS Energy Letters

Figure 4. (a) Cyclic voltammetry curves of A-IrOx-B and Ir in an Ar-purged 0.1 M HClO4 solution at 50 mV s-1. (b) Double-layer capacitance measurements for determining the electrochemically active surface area of Ir from voltammetry. The cathodic (red open circle) and anodic (blue open square) charging currents, measured at 0.4 V vs RHE, are plotted as a function of scan rate. The double-layer capacitance of the system was calculated as the average of the absolute values of the slopes of the linear fits to the data. The ECSA of the catalyst was then calculated from the double layer capacitance.


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Figure 5. Oxygen evolution polarization curves of Os, Ru, A-IrOx-B, S-IrRuOx-B, and Ir (a) before the stability test at 1600 rpm in an Ar-purged 0.1 M HClO4 solution at 10 mV s-1. (b) Oxygen evolution polarization curves of Os, Ru, A-IrOx-B, S-IrRuOx-B, and Ir after the chronopotentiometric measurements at a current density of 1 mA cm-2 for 30 min. (c) Activity/stability relationships of the OER for A-IrOx-B, S-IrRuOx-B, and Ir represented as the overpotential at 10 mA cm-2geo and concentration of noble metal ions in solution after extended polarization at 10 mA cm-2geo.


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ACS Energy Letters

Table 1. Activity, stability, and dissolution rate of A-IrOx-B, S-IrRuOx-B and Ir. Activity

Mass activity

-2

/ V @10 mA cm

-1 (geo)

Specific activity -2

/ Ag @ η=0.25V

/ mA cm @ η=0.25V

Before CP

0.324

25.026

0.0183

After CP

0.323

21.384

0.0220

Before CP

0.308

27.449

0.0264

Dissolution rate -1 / ppb h

0.112

A-IrOx-B

S-IrRuOx-B After CP

0.332

14.492

0.0159

Before CP

0.379

7.492

0.0148

After CP

0.380

5.957

0.0137

Ir 0.042 Ru 0.476

0.074

Ir


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental section, TEM micrographs of catalysts, OER curve normalized by ECSA, Chronopotentiometry curve, Cyclic voltammetry, Oxygen evolution polarization curves for catalysts before and after stability test and Activity/Stability relationship graph.(PDF) AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Research Foundation (NRF) of Korea grant (2015R1A2A1A10056156, 2017R1A4A1015533). ABBREVIATIONS A-IrOx-B, electrochemically activated Ir oxide black; IrOsOx-B, IrOs oxide black; S-IrRuOx-B, surface segregated IrRu oxide. REFERENCES (1) Dresselhause, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337.


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ACS Energy Letters

(2) Schlapbach, L.; Zuttel, A. Hydrogen-Storage Materials for Mobile Aplications. Nature 2001, 414, 353-358. (3) Steele, B. C. H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 354-352. (4) Gray, H. B. Powering the Planet with Solar Fuel. Nat. Chem. 2009, 1, (1), 7-7. (5) Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, (5892), 1072-1075. (6) Lu, Y.-C.; Xu, Z.; Gasteiger, H. A.; Chen, S.; Hamad-Schifferli, K.; Shao-Horn, Y. Platinum-gold Nanoparticles: A Highly Active Bifunctional Electrocatalyst for Rechargeable Lithium-air Batteries. J. Am. Chem. Soc. 2010, 132, (35), 12170-12171. (7) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, (7179), 652657. (8) Pletcher, D.; Walsh, F. C.; Industrial Electrochemistry; Springer Netherlands: Dordrecht, 1993. (9) Marshall, A.; Borresen, B.; Hagen, G.; Tsypkin, M.; Tunold, R. Hydrogen Production by Advanced Proton Exchange Membrane (PEM) Water Electrolysers-Reduced Energy Consumption by Improved Electrocatalysis. Energy 2007, 32, (4), 431-436. (10) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, (11), 6474-6502. (11) Katsounaros, I.; Cherevko, S.; Zeradjanin, A. R.; Mayrhofer, K. J. J. Oxygen Electrochemistry as a Cornerstone for Sustainable Energy Conversion. Angew. Chem., Int. Ed. 2014, 53, (1), 102-121. (12) Danilovic, N.; Subbaraman, R.; Chang, K. C.; Chang, S. H.; Kang, Y.; Snyder, J.; Paulikas, A. P.; Strmcnik, D.; Kim, Y. T.; Myers, D., et al. Using Surface Segregation To Design Stable Ru-Ir Oxides for the Oxygen Evolution Reaction in Acidic Environments. Angew. Chem., Int. Ed. 2014, 53, (51), 14016-14021. (13) Nong, H. N.; Gan, L.; Willinger, E.; Teschner, D.; Strasser, P. IrOx Core-Shell Nanocatalysts for Cost- and Energy-Efficient Electrochemical Water Splitting. Chem. Sci. 2014, 5, (8), 2955-2963. (14) Reier, T.; Pawolek, Z.; Cherevko, S.; Bruns, M.; Jones, T.; Teschner, D.; Selve, S.; Bergmann, A.; Nong, H. N.; Schlogl, R., et al. Molecular Insight in Structure and Activity of Highly Efficient, Low-Ir Ir–Ni Oxide Catalysts for Electrochemical Water Splitting (OER). J. Am. Chem. Soc. 2014, 137, (40), 13031-13040. (15) Feng, J.; Lv, F.; Zhang, W.; Li, P.; Wang, K.; Yang, C.; Wang, B.; Yang, Y.; Zhou, J.; Lin, F., et al. Iridium-Based Multimetallic Porous Hollow Nanocrystals for Efficient OverallWater-Splitting Catalysis. Adv. Mater. 2017, 1703798. (16) Zhang, T.; Li, S.-C.; Zhu, W.; Zhang, Z.-P.; Gu, J.; Zhang, Y.-W. Shape-Tunable Pt-Ir Alloy Nanocatalysts with High Performance in Oxygen Electrode Reactions. Nanoscale 2017, 9, (3), 1154-1165. (17) Nong, H. N.; Oh, H.-S.; Reier, T.; Willinger, E.; Willinger, M.-G.; Petkov, V.; Teschner, D.; Strasser, P. Oxide-Supported IrNiOx Core–Shell Particles as Efficient, Cost-Effective, and Stable Catalysts for Electrochemical Water Splitting. Angew. Chem., Int. Ed. 2015, 54, (10), 2975-2979.


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Electrochemically Activated Iridium Oxide Black as Promising

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