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An Extremely Active Hydrogen Evolution Catalyst Electrochemically Generated from a Ruthenium-based Perovskite-Type Precursor Yuuki Sugawara, Keigo Kamata, and Takeo Yamaguchi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01525 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Energy Materials

An Extremely Active Hydrogen Evolution Catalyst Electrochemically Generated from a Rutheniumbased Perovskite-Type Precursor Yuuki Sugawara,† Keigo Kamata,‡,§ and Takeo Yamaguchi*,†,# †

Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, R1-17, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan ‡

Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan §

Precursory Research for Embryonic Science and Technology (PRESTO), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

#

Kanagawa Institute of Industrial Science and Technology (KISTEC), R1-17, 4259 Nagatsutacho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan

KEYWORDS: hydrogen evolution reaction, perovskites, electrocatalysis, SrRuO3, water splitting, energy conversion

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ABSTRACT: Water splitting requires highly active, easily prepared, and inexpensive electrocatalysts for hydrogen evolution reaction (HER). Herein, we report that Ru-based compounds, which are electrochemically generated from a simple perovskite oxide, SrRuO3, exhibit remarkably high HER activity in 1 M KOH solution, and the overpotential of HER is 0.11 V, representing the best performance among all of the reported perovskite-type catalysts. In addition, the activity surpasses that of the state-of-the-art Pt/C catalyst after repeated HER measurements. Hence, SrRuO3 is a promising electrochemical precursor for an efficient electrocatalyst for water splitting application.


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The present energy and environmental concerns regarding global warming and the depletion of fossil fuels has stimulated the demand for breakthroughs in the utilization of sustainable alternative energy sources. Because of its inexhaustibility, renewable energy sources, such as solar and wind energy resources, are highly promising for large-scale exploitation. However, it is difficult to effectively use such intermittent natural energy resources, because these natural energy resources are temporally and spatially detached from the energy-consuming sites. Therefore, converting these energy forms into chemical fuels that can be efficiently stored and transported is a promising approach. Hydrogen is a clean and carbon-free fuel and one of most promising candidates for future energy supply due to its high energy density and lack of emissions. Thus, hydrogen generation through photocatalytic and electrochemical water splitting is crucial technology. Photo-triggered hydrogen generation from water can be catalyzed using photoactive materials, such as TiO2.1-3 On the other hand, alkaline electrolyzers can convert abundant water into pure hydrogen via water splitting using the electricity generated from solar and wind energy resources and have received considerable attention as a clean and cost-effective mode for energy storage. The present alkaline electrolyzers need to be improved, because the cathodic hydrogen evolution reaction (HER) (2H2O + 2e− → H2 + 2OH−) is inefficient and has a large overpotential.4 Carbon-supported platinum (Pt/C) is well-known as a state-of-the-art electrocatalyst for efficient HER, but its unstable performance is still a source of major concern.56

In addition, the use of the costly precious metal prevents the transition of this technology to

widespread use. Therefore, researchers have been attempting to find HER catalysts that are more promising than Pt/C. For instance, ruthenium (Ru)-based compounds7-10 have been recently reported to exhibit excellent HER activities in alkaline media. Although the HER activities of these compounds are close to or even surpass that of Pt/C, their burdensome synthetic processes


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or their large amounts of Ru required for the HER limit their practical applications. Hence, costeffective electrocatalysts that are fabricated by easy procedures are highly desirable. Perovskite oxides with the general ABO3 structure, where the A and B sites are occupied by alkaline-earth or rare-earth metals and transition metals, respectively, have attracted wide interest because of their high electrocatalytic activities in alkaline media. In addition, the physical and chemical properties of perovskites can be easily modified by substituting and/or doping on the A and B sites and by introducing oxygen vacancies to their crystalline structures.11-13 Perovskites have been considered to be promising candidates for practical use due to their facile synthesis, low cost, and environmental-friendliness. Although many reports have previously presented perovskite-type catalysts for oxygen evolution reaction and oxygen reduction reaction,14-15 only a few studies have reported perovskites showing HER activity. For example, Xu et al. first found that Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and praseodymium-doped BSCF exhibit HER activity in alkaline media.16 More recently, several papers17-20 have also reported perovskite oxides that catalyze HER. However, their HER activities are still far below that of Pt/C. We recently reported a simple and efficient synthetic method for a wide variety of perovskites oxide nanoparticles with high surface areas.21-24 The SrMnO3,22 BaFeO3−δ,24 and BaRuO323 nanoperovskite catalysts synthesized by the sol-gel method using aspartic acid and malic acid exhibited much higher catalytic activities for liquid-phase aerobic oxidation of hydrocarbons and organic sulfides than those synthesized by the conventional solid-phase or polymerized complex methods.21 In this study, we report a Ru-based compound, which is electrochemically generated from a perovskite oxide precursor with high surface area and demonstrate its outstanding HER activity in alkaline media. We have chosen Sr as the A-site metal. SrRuO3 is well known to be active in oxygen evolution reaction (OER); e.g., Chang et


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al.25 investigated a thin SrRuO3 film on an electrode concerning its OER activity and stability. In addition, Kim et al. also examined SrRuO3 in OER, and they concluded that SrRuO3 is unstable in alkaline media due to dissolution of Sr and Ru.26 In contrast, few studies have paid attention to the HER activity of SrRuO3. We fabricate SrRuO3 via a facile malic acid-aided sol-gel method to induce its high specific surface area and then elucidate its electrochemical HER activity and structural stability. SrRuO3 was synthesized by the malic acid-aided sol-gel method using metal acetates (see details provided in the Supporting Information).22-23 The powder X-ray diffraction (XRD) pattern for SrRuO3 was good in agreement with that for the previously reported cubic SrRuO3 phase [space group Pm-3m] (Figure S1).23 The Brunauer-Emmett-Teller (BET) specific surface area of SrRuO3 was as high as 25 m2 g−1.23,

27

The scanning electron microscopy (SEM) image of

SrRuO3 in Figure 1a showed aggregates of the nanoparticles ranging from 20–50 nm, which is in reasonable agreement with the value calculated from the (110) diffraction lines using the Scherrer equation (d=23 nm). To improve the low electrical conductivity of typical metal oxides, carbon nanotubes (CNTs) were employed, and a perovskite/carbon composite was prepared by physical mixing as shown in Scheme 1 (see Supporting Information for more details). We observed the composite by SEM and high resolution transmitted electron microscopy (HR-TEM) as depicted in Figure S2, proving that SrRuO3 particles are tangled with CNTs. The HR-TEM image shown in Figure 1b indicated that the SrRuO3 nanoparticles in the composite with CNTs also have the particle size of ca. 50 nm, which was consistent with the SEM observations. The magnified image in Figure 1c shows the fringes with the d-spacing of 2.9 Å and 3.8 Å on the nanoparticle, and the fringes are associated with the (110) and (100) planes, respectively. Figure 1d shows the STEM-EDX images of the SrRuO3/CNT composite and demonstrates that the


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simple perovskite oxide contained the Sr, Ru and O elements with a Sr:Ru:O molar ratio of 20:17:63.

(a)

(b)

10 nm

200 nm

(c)

2.9 Å

(d)

O

50 nm

3.8 Å 2 nm

Sr

Ru

Sr:Ru:O=20:17:63mol

Figure 1. Structural and compositional analyses of SrRuO3: (a) SEM image of synthesized SrRuO3; (b) HR-TEM image of SrRuO3/CNT composite; (c) magnified HR-TEM image of (b); (d) STEM-EDX images of SrRuO3/CNT composite.


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Scheme 1. Schematic representation for the preparation of the SrRuO3/CNT composite.


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Sample electrodes for the electrochemical measurements were prepared as described in the Supporting Information. Figure 2a shows the iR-corrected HER polarization curves for SrRuO3/CNT, Ru-based simple oxide (RuO2)/CNT, CNT, and Pt/C in 1 M KOH. We evaluated the HER overpotential, which is defined as the potential difference between the theoretical and experimentally obtained values at the current density of 10 mA cm−2. The HER polarization curves showed that SrRuO3/CNT had the HER overpotential of 109±3.39 mV, which was smaller than that for RuO2 (161±24.4 mV). Pristine SrRuO3 without CNT exhibited negligible current density as shown in Figure S4, though SrRuO3 is a ferromagnetic material and shows metallic conductivity,28 indicating that mixing with CNT is necessary. Unmixed CNTs showed almost no current, which means that CNTs are not catalytically active and only played the role of an electrically conductive material. It is notable that the catalytic performance of SrRuO3/CNT was superior to that of every previously reported perovskite catalyst and most of the other stateof-the-art HER catalysts (see Table S1 and S2). Furthermore, the HER overpotential obtained from SrRuO3 was close to that of the benchmark Pt/C catalyst. The measured activity of Pt/C showed good agreement with the data reported in the literature (Table S2). Moreover, it is also noteworthy that SrRuO3 caused such a large HER current despite the extremely small amount of the loaded perovskite oxide on the electrode, i.e., 0.018 mg cm−2. We then calculated the HER mass activity of SrRuO3, Pt/C, and the previously reported perovskite catalysts as shown in Figure 2b. The mass activity of SrRuO3 at −0.15 V is 1.4 A mg−1, which is ~1.5 times higher than that of Pt/C (0.88 A mg−1) and remarkably superior to the other previously reported perovskite catalysts. The assessment of the mass activity indicated that SrRuO3 is highly cost effective for practical use despite its Ru component.


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(a)

(b)

(c)

Figure 2. (a) HER polarization curves of SrRuO3/CNT, RuO2/CNT, CNT, and Pt/C in N2saturated 1 M KOH. The graph displays the 10th cycle of the reverse scan sweeps between −0.5 and 0.1 V (RHE). (b) HER mass activities at −0.15 V of SrRuO3, Pt/C, and previously reported perovskite oxides. The zoomed bar chart (inset) shows the data of the previously reported perovskite oxides. (c) Complementary Tafel slopes of SrRuO3/CNT, RuO2/CNT, and Pt/C.

To elucidate the HER kinetics, Tafel slopes were calculated from the Tafel plots. Figure 2c depicts the Tafel plots for SrRuO3/CNT, RuO2/CNT, and Pt/C. The Tafel slope for SrRuO3 was 45.0±3.77 mV dec−1 which was smaller than that of RuO2 (67.3±7.13 mV dec−1) and was comparable to that of Pt/C, suggesting that SrRuO3 has higher activity than RuO2 and HER kinetics similar to those of Pt/C. In addition, the Tafel slope of SrRuO3 was nearly the highest one among the previously reported perovskite-type HER catalysts (see Table S1), demonstrating its superior HER kinetics. Moreover, the HER mechanism can be described as proceeding through three processes in alkaline conditions: the Volmer (H2O + e− → Hads + OH−), Heyrovsky (H2O + Hads + e− → H2 + OH−), and Tafel (Hads + Hads → H2) processes.29 The rate-determining


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step of HER can be assigned using the slope of the Tafel plot;30 i.e., the Tafel slopes of ca. 120, 40, and 30 mV dec−1 indicate the rate-determining steps for the Volmer, Heyrovsky, and Tafel processes, respectively. The Tafel slope value obtained from SrRuO3 was close to 40 mV dec−1, suggesting that HER catalyzed by SrRuO3 was controlled by the Heyrovsky process. We further evaluated the catalytic performance of SrRuO3 in comparison with commercial Pt/C by continuing the potential cycles for HER measurements, with the results shown in Figure 3. SrRuO3/CNT showed no significant current loss, and the overpotential in the HER curve at 10 mA cm−2 did not increase after 500 cycles, as shown in Figure 3. By contrast, the overpotential of Pt/C dramatically increased by ca. 35 mV after 100 cycles as shown in Figure 3. Figure S5 depicts a TEM image of the aged Pt/C, showing that the size of the Pt nanoparticles was increased after the potential cycles. This result was consistent with the report of Xu et al.,10 who observed the instability of commercial Pt/C in the beginning of the electrochemical aging in 1 M KOH solution under the HER process. The degradation phenomenon was explained as due to the potential in HER inducing migration and detachment of the Pt nanoparticles from the carbon supports.7 The aged SrRuO3/CNT after the potential cycles was characterized structurally and elementally by HR-TEM and STEM-EDX as depicted in Figure S6 (Supporting Information), and revealed that the residual catalyst particles contained the Sr, Ru and O elements with a Sr:Ru:O molar ratio of 0.2:57:43, indicating the dissolution of Sr from SrRuO3 particles and the generation of Ru-based compounds, such as metallic Ru and Ru (hydr)oxides. It showed good agreement with the previous report of Kim et al.26, which demonstrated the instability of SrRuO3 in alkaline media. Interestingly, the material retained excellent HER activity despite the instability of SrRuO3. To compare the morphology of the in-situ-generated Ru-based compounds with RuO2, we also observed RuO2/CNT by TEM as shown in Figure S7. The TEM images show


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aggregates consisting of nanoparticles with size of 10–20 nm, whereas the electrochemically generated Ru-based compounds consist of nano-sized clusters with size of 2–5 nm, as depicted in Figure S6, suggesting that the excellent HER activity and the durability of the current density were caused by the distinct morphology of the Ru-based compounds from RuO2. The remarkable catalytic performance was yielded by the in-situ generation from SrRuO3 by potential cycles on RDE. Our results demonstrated that the electrochemically generated Ru-based compounds are more efficient HER catalysts than Pt/C for long-term performance. Thus, SrRuO3 nanoparticle is highly promising as an electrochemical precursor for an alternative HER catalyst to Pt/C, paving the way for a new approach in the field of alkaline electrolyzers.

Figure 3. Change of the overpotential of SrRuO3/CNT and Pt/C after repeated potential cycles of HER measurement. The overpotential is defined as the potential difference from the theoretical value at the current density of 10 mA cm−2.

In conclusion, we have demonstrated that Ru-based compounds were electrochemically generated from a SrRuO3 precursor by potential cycles at HER region and exhibited an ultrahigh HER activityin alkaline media that is better than those of every previously reported perovskite-


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type HER catalyst. Furthermore, the HER activity of the in-situ generated Ru-based catalysts surpasses that of Pt/C after several HER potential cycles. This study can provide a new avenue for the development of high-performance electrocatalysts and contribute to the field of energy science and materials.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental procedures and supplementary figures and tables. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Yuuki Sugawara: 0000-0002-2696-1833 Keigo Kamata: 0000-0002-0624-8483 Takeo Yamaguchi: 0000-0001-9043-4408 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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Y.S. and T.Y. are grateful for continued support from Tokyo Institute of Technology. T.Y. acknowledges support from KISTEC. K.K. acknowledges financial support from PRESTO program (No. JPMJPR15S3) of the JST. REFERENCES (1) Mondal, I.; Pal, U., Synthesis of MOF templated Cu/CuO@TiO2 nanocomposites for synergistic hydrogen production. Phys. Chem. Chem. Phys. 2016, 18, 4780-4788. (2) Zhang, M.; Sun, R. Z.; Li, Y. J.; Shi, Q. M.; Xie, L. H.; Chen, J. S.; Xu, X. H.; Shi, H. X.; Zhao, W. R., High H2 Evolution from Quantum Cu(II) Nanodot-Doped Two-Dimensional Ultrathin TiO2 Nanosheets with Dominant Exposed {001} Facets for Reforming Glycerol with Multiple Electron Transport Pathways. J. Phys. Chem. C 2016, 120, 10746-10756. (3)

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TOC graphic


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Schematic representation for the preparation of the SrRuO3/CNT composite. 85x152mm (600 x 600 DPI)



Structural and compositional analyses of SrRuO3: (a) SEM image of synthesized SrRuO3; (b) HR-TEM image of SrRuO3/CNT composite; (c) magnified HR-TEM image of (b); (d) STEM-EDX images of SrRuO3/CNT composite. 73x64mm (300 x 300 DPI)


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(a) HER polarization curves of SrRuO3/CNT, RuO2/CNT, CNT, and Pt/C in N2-saturated 1 M KOH. (b) HER mass activities at −0.15 V of SrRuO3, Pt/C, and previously reported perovskite oxides. The zoomed bar chart (inset) shows the data of the previously reported perovskite oxides. (c) Complementary Tafel slopes of SrRuO3/CNT, RuO2/CNT, and Pt/C. 70x28mm (300 x 300 DPI)



Change of the overpotential of SrRuO3/CNT and Pt/C after repeated potential cycles of HER measurement. The overpotential is defined as the potential difference from the theoretical value at the current density of 10 mA cm−2. 83x78mm (600 x 600 DPI)


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An Extremely Active Hydrogen Evolution Catalyst ... - ACS Publications

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