Designing Binary RuâSn Oxides with Optimized Performances for the

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Designing binary Ru-Sn oxides with optimized performances for the air electrode of rechargeable zinc-air batteries Ting-Hsuan You, and Chi-Chang Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18948 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Materials & Interfaces

Designing binary Ru-Sn oxides with optimized performances for the air electrode of rechargeable zinc-air batteries Ting-Hsuan You, Chi-Chang Hu* Laboratory of Electrochemistry & Advanced Materials, Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu 30013, Taiwan *: Corresponding author KEYWORDS: Ru-Sn oxide, oxygen evolution, oxygen reduction, bifunctional catalyst, rechargeable Zn-air batteries.

ABSTRACT

Due to the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), binary ruthenium-tin oxides synthesized by a hydrothermal method with post annealing at 450oC for 2 h are firstly proposed as bifunctional catalysts for these two reactions on the air electrode of rechargeable zinc-air batteries. The binary Ru-Sn oxides in various compositions show the typical oxide solid solution in the rutile phase. Among all binary Ru-Sn oxides, RuSn73 (70 at.% RuO2 and 30 at.% SnO2) and RuSn37 (30 at.% RuO2 and 70 at.% SnO2) show the highest catalytic activities towards the OER and ORR, respectively.

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Consequently, a novel design of the air electrode consisting of a RuSn37 coating on the carbon paper and a Ti mesh coated with RuSn73 (denoted as RuSn(37-C|73-Ti)) is proposed to possess the optimal charge-discharge performances. A unique cell employing such an air electrode has been demonstrated to exhibit a very low charge-discharge cell voltage gap of 0.75 V at 10 mA cm–2. This cell with a peak power density of 120 mW cm-2 at the current density of 235 mA cm-2 also shows an outstanding charge-discharge stability over 80 h. This cell also exhibits an exceptionally high charge rate capability at 150 mA cm-2 with a low charging voltage of 2.0 V.

1. INTRODUCTION The most promising clean power sources of electric vehicles (EVs) are the polymer electrolyte membrane fuel cell (PEMFC) and metal–air batteries because of their high energy density although Li-ion batteries have been regarded as the major energy source in portable devices and EVs.1-2 Due to the limitation in the energy density of Li-ion batteries, more and more researches focus on the development of so-called post Li-ion batteries, such as Li-S batteries and metal-air batteries in order to meet the energy density requirement of EVs and other applications.3-8 Moreover, the aqueous electrolyte-based rechargeable metal-air batteries have additional advantages of good safety, easy handle, and low cost. Hence, rechargeable zinc-air batteries are widely investigated in recent years because of low cost and high safety in comparison with the other metal-air batteries, high theoretical energy density (1086 Wh kg-1), and environmental friendliness.9-10 Consequently, they show promising application potentials in many fields and systems.8-9, 11-12 Three main issues in developing rechargeable zinc-air batteries need to be solved: (1) Zn ion solubility and Zn dendrites formation (poor cycle stability), (2) large charge-discharge cell


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voltage gaps (low energy efficiency), and (3) formation of carbonate salts due to CO2 capture from the air, reducing the hydrophobicity of the gas diffusion layer.13-14 The sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) results in the large charge-discharge cell voltage gaps. Hence, the electrocatalysts for both the OER and ORR play an important role in reducing the cell voltage gap between charge and discharge processes.15-16 For the primary zinc-air battery, several oxides such as MnO2, Co3O4, etc. have been reported to reduce the ORR overpotential,17-22 which may be poor catalysts for the OER. Nowadays, researchers are looking for the bifunctional catalysts to simultaneously promote the activities of both the ORR and OER which occur in the discharge and charge processes.23-27 It is well-known that RuO2-based oxides are the key component of the dimensionally stable anodes (DSA®) which have been applied to the chlor-alkali industry and the catalyst for oxygen evolution in water electrolysis.28-29 To the best of our knowledge, RuO2 and IrO2 have been considered to be the best OER catalysts comparing with the other materials,30-31 not only the highly catalytic activity but also the excellent stability. Unfortunately, RuO2 shows relatively worse performance in the ORR in comparison with the OER.32 On the other hand, introduction of Ti and Sn oxides in the rutile phases has been found to further promote the OER activity and stability of RuO2 and IrO2.33 Accordingly, the atomic scale mixing of transition metal oxides in the same rutile structure into RuO2 is expected to be a good idea to enhance the catalytic activity of RuO2 for the ORR without considering the fact that the cost of RuO2 and IrO2 catalysts can be significantly reduced by the introduction of TiO2 and SnO2. Consequently, RuO2-based oxides in the rutile phase may be developed to be a desirable bifunctional catalyst for the air electrode of rechargeable Zn-air batteries.


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Based on the cost and functionality considerations, SnO2 has been applied as the electrode materials in gas sensors,34-35 solar cells,36-37 and lithium-ion batteries.38-39 Very recently, SnO2 has benefitted in the Li-O2 batteries as the catalyst to induce the ORR stability,40 supporting our idea that combination of RuO2 and SnO2 is good for developing bifunctional electrocatalysts. In this study, SnO2 is introduced into RuO2 to form a solid solution of the rutile phase with the bifunctional catalytic activities for the OER and ORR although adding carbon in various forms into transition metal oxides to enhance the conductivity is usually the solution for improving the ORR. However, the serious oxidation on the carbon material during the charging process has been found to reduce the charge-discharge performances of resultant cells.12 The (Ru-Sn)O2 solid solutions with metallic conductivity and excellent O2-evolving stability are desirable bifunctional catalysts of the air electrode in rechargeable Zn-air batteries. To the best of our knowledge, this work is the first article investigating binary (Ru-Sn)O2 solid solutions as a bifunctional catalyst, which meets the requirements of good conductivity and stability without adding carbons in assembling rechargeable zinc-air batteries. An additional advantage of adding SnO2 is to reduce the RuO2 content from the cost consideration.

2. EXPERIMENTAL SECTION 2.1. Synthesis of binary (Ru-Sn)O2 catalysts The binary (Ru-Sn)O2 catalysts were synthesized by a hydrothermal method from the precursor solutions of a constant total metallic ion concentration. In preparing the precursor solutions, the molar ratios of metallic ions between RuCl3·xH2O (Alfa Aesar) and SnCl4·5H2O (Alfa Aesar) were carefully controlled to be 7/3, 6/4, 5/5, and 3/7, respectively. These precursors were dissolved in 100 mL deionized water and stirred with a magnetic bar for 1 h. The precursor


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solution was transferred to a Teflon-lined autoclave with a stainless steel shell, which was sealed and heated to 180 oC for 5 h in the oven. The binary Ru-Sn oxides were collected by centrifugation and washed with deionized water several times until pH close to 7 in order to remove impurities. The products were dried in the vacuum oven at 45oC overnight. Finally, the dried powders were calcined at 450 oC for 2 h and these as-prepared sample powders were denoted as RuSn37, RuSn55, RuSn64, RuSn73, and RuO2 when 30, 50, 60, 70, and 100 at.% RuCl3·xH2O were added in the precursor solutions. 2.2. Materials characterization X-ray diffraction patterns were examined by an X-ray diffractometer (CuKα, Ultima IV, Rigaku) at an angular speed of (2θ) 1o min-1. The surface morphology and energy dispersive spectroscopy (EDS) of binary (Ru-Sn)O2 catalysts were conducted in a field emission scanning electron microscope (FE-SEM) (HITACHI, SU8010). The specific surface area of catalysts was calculated from the Brunauer-Emmett-Teller (BET) method using the adsorption data from the N2 adsorption/desorption isotherms (Micromeritics Instrument Co., ASAP 2020 Physisorption Analyzer) in the relative pressure range between 0.02 and 0.2. 2.3. Electrochemical measurements The catalytic properties were evaluated via the rotating ring disk electrode (RRDE) voltammetry by an electrochemical analyzer (CH Instruments 730D coupled with a Pine Research Instrumentation, USA) under O2 or N2 atmospheres in alkaline solution. In preparing the catalyst ink, 0.5 mg catalyst powders were dispersed in 192 µL ethanol and 8 µL 5wt.% Nafion® (E.I. du Pont de Nemours & Co., USA). The RRDE was polished with Al2O3 slurry and cleaned via ultrasonication for 30 min before dropping catalysts ink onto the glassy carbon disk


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electrode (surface area of 0.247 cm-2). The mass loading on the disk electrode is carefully controlled to be 0.1 mg cm-2. The linear sweep voltammograms and polarization curves were conducted in 0.1 M KOH with saturated-O2 or N2 atmospheres at room temperature. An Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V vs. RHE at 25oC) and a large piece of platinum gauze were employed as the reference and counter electrodes, respectively. Note that all electrode potentials measured in 0.1 M KOH under the three-electrode mode have been converted into the reversible hydrogen electrode (RHE) scale using E(RHE) = E(Ag/AgCl) + 0.970 V. 2.4. The full cell test The ORR electrode included a catalyst layer (on the electrolyte side) and a gas-diffusion layer (on the air side) on a 25BC carbon paper. The catalyst paste was a homogeneous mixture containing 0.1 g (Ru-Sn)O2 powders, 0.25 g ethylene glycol, and 0.035 g Nafion®. This paste was coated onto the carbon paper to form an air electrode sheet with an exposed surface area of 10 mm × 10 mm for the electrocatalysts by means of the doctor-blade method without any pressing. Finally, the air electrodes were dried in an oven at 85oC for 24 h. We applied the same paste preparation and coating methods in preparing the RuO2-/Pt/C-coated air electrodes for measuring their LSV polarization curves in the full cell test. The total mass loading of all catalyst pastes on the air electrodes was fixed to be 20±2 mg cm-2. For the oxide-coated Ti mesh electrodes, prior to the dip-coating process, commercial Ti meshes (35 × 25 mm) were pretreated and cleaned using a 6 M HCl solution at 90 oC for 30 min. After the pretreatment process, the Ti meshes were rinsed with DI water. These meshes were dipped in an aqueous solution containing 3 mg mL-1 of RuO2⋅xH2O without any binder or


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additives and then dried at 85 oC for 2 h. After drying, the Ti meshes were calcined at 250 oC for 2 h to form an extremely thin RuO2 film to improve the adhesion of (Ru-Sn)O2.41 Although RuO2 is a typical electrocatalyst for the OER,42 the aforementioned interlayer of the RuO2 film exhibited negligible activity in the OER. The electrocatalyst paste was a homogeneous mixture containing 0.15 g (Ru-Sn)O2 powders, 0.2 g ethylene glycol, and 0.35 g Nafion®, which was brushed onto the surface of 35 × 25 mm RuO2/Ti meshes. Finally, the electrodes were dried in a vacuumed oven at 85 oC for 24 h. The full cell comprised an air electrode and a zinc foil. There are two types of air electrodes in this work. The first type is the 25BC carbon paper coated with a catalyst layer without any current collector. The second type consists of the above catalyst-coated carbon paper and oxidecoated Ti mesh electrodes according to our previous design.43 For example, an air electrode consisting of a carbon paper coated with RuSn37 and a Ti mesh coated with RuSn73 is denoted as 37-C|73-Ti. The LSV polarization curves of all air electrodes were measured by a CH Instrument 1128C in 0.1 M KOH under ambient temperature and air. All the discharge-charge cycling performance was carried out by the same electrochemical analyzer in a solution of 6 M KOH and 0.2 M zinc acetate under the ambient air and temperature.

3. RESULTS AND DISCUSSION 3.1. Materials characterization Figure 1 compares the X-ray diffraction patterns of Ru-Sn oxides in various Ru/Sn ratios before and after calcination at 450oC. The standard patterns of SnO2 and RuO2 in the rutile structure with three high-intensity peaks located at 26.6o, 33.9o, 51.8o, and 28.1o, 35.1o, 54.3o, respectively, are shown here for comparisons. From patterns 1-4 in Figure 1a, the samples from


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the hydrothermal process clearly show poor crystallinity although crystallites in the subnanometer scale should be formed.44 These samples after adopting the calcination treatment reveal improved crystallinity from their well-defined diffractions peaks from a comparison of all XRD patterns in Figure 1a. Note that (110), (101) and (211) peaks of all binary (Ru-Sn)O2 catalysts are located between the peaks corresponding to pure RuO2 and SnO2. Accordingly, the (211) peaks of various (Ru-Sn)O2 catalysts are carefully measured and compared (see Figure 1b). Clearly, this peak is gradually shifted from 52.03o to 53.67o when the Ru content in (RuSn)O2 is increased from 30 to 70 at.%. In addition, the other detectable peaks, e.g., facets (110) and (101), are also slightly shifted to a higher angle with introducing a higher Ru content. These results indicate the successful formation of (Ru-Sn)O2 solid solutions in the rutile phase.45-46 Hence, the electrochemical catalytic activities of RuO2 for the OER and ORR are possibly altered by the atomic scale mixing of Ru and Sn in the (Ru-Sn)O2 solid solutions.

Figure 1. (a) XRD patterns of (Ru-Sn)O2 (1-4) before and (5-8) after calcination at 450 oC for 2 h with the Ru content of (1,5) 30, (2,6) 50, (3,7) 60, and (4,8) 70 at.%. (b) The detailed XRD


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diffraction patterns with 2θ from 50o to 56o for (5) RuSn37, (6) RuSn55, (7) RuSn64, and (8) RuSn73.


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Figure 2. The SEM surface morphologies of (a) RuSn37, (b) RuSn55, (c) RuSn64, (d) RuSn73 and (e) the Ru-Sn contents of (Ru-Sn)O2. The surface morphologies of all (Ru-Sn)O2 powders with annealing at 450 oC are shown in Figures 2a-2d. From an examination of these SEM images, the particles are of the similar granular shape although aggregates of primary particles are clearly visible. In addition, the EDS results in Figure 2e (also see the EDS results shown in Figure S1 in the Supporting Information) confirm that the composition of resultant samples is very close to that of their corresponding precursor solutions, revealing the successful synthesis of (Ru-Sn)O2 by the mild hydrothermal method with the calcination step.


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Figure 3. The N2 adsorption-desorption isotherms of (a) RuSn37, (b) RuSn55, (c) RuSn64, and (d) RuSn73. Since both the OER and ORR are heterogeneous reactions, the specific surface area of (RuSn)O2 catalysts should be important and all samples prepared in this work have been examined by the N2 adsorption-desorption isotherms (see Figure 3) in order to deduce the specific surface area. Clearly, the N2 adsorption-desorption isotherms of all samples show the typical type IV curves with a hysteresis loop.47 The presence of a hysteresis loop indicates the mesoporous nature of resultant oxides, probably due to aggregation of primary nanoparticles by the calcination treatment. The specific surface areas of all (Ru-Sn)O2 catalysts were estimated by the BET method, which are equal to 67, 70, 72, and 68 m2 g-1 for RuSn37, RuSn55, RuSn64, and RuSn73, respectively. These results indicate the very similar specific surface area of all binary oxides prepared in this work. 3.2.Electrochemical characteristics The voltammetric charges evaluated by CV in the N2-purged solution has been confirmed to be an activity index of the OER in developing DSA,42 and typical CV curves of all binary (RuSn)O2 measured in N2-purged and O2-purged 0.1 M KOH are shown in Figure 4. From the CV curves measured in the N2-purged solution, the voltammetric charges of RuSn73, RuSn64, RuSn55, and RuSn37 are equal to 17.94, 17.89, 8.50 and 1.78 mC cm-2 (also see Supporting Information to know how to obtain these charges), indicating that the OER activity of (Ru-Sn)O2 is generally increased with the Ru content. Note that the intrinsic catalytic activity of (Ru-Sn)O2 for the OER can be indexed as the ratio between voltammetric charge and specific surface area obtained from the BET method. Accordingly, the order of oxides with respect to decreasing the


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intrinsic activity of the OER is: RuSn73 ≈ RuSn64 > RuSn55 > RuSn37, revealing that Ru is the active species catalyzing the OER.

Figure 4. Cyclic voltammograms of (a) RuSn37, (b) RuSn55, (c) RuSn64, and (d) RuSn73 on the RRDE at 2500 rpm in (1) N2-purged and (2) O2-purged 0.1 M KOH. From the CV curves measured in the O2-purged solution, an irreversible reduction is visible on both positive and negative sweeps when the electrode potentials are more negative than ca. 800 mV (vs. RHE), revealing the ORR activity of (Ru-Sn)O2. Since the voltammetric currents in this ORR region includes the faradaic current of oxygen reduction as well as the redox currents of surface oxyruthenium species (e.g., Ru(III)/Ru(II) and Ru(IV)/Ru(III)) of (Ru-Sn)O2,42 the


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voltammetric current density cannot be considered an index of the ORR activity. In addition, SnO2 is not redox-active in this potential region but it obviously exhibits the catalytic activity to the ORR. Hence, the voltammetric charges of (Ru-Sn)O2 (mainly contributed from the redox transitions of RuO2) cannot be considered the index of the ORR activity, either.

Figure 5. (a) The RRDE voltammograms and (b) the corresponding mean electron transfer number of (1) Pt/C, (2) RuO2, (3) RuSn73, (4) RuSn64, (5) RuSn55, and (6) RuSn37 in the O2-saturated 0.1 M KOH solution at 10 mV s-1 and 1600 rpm. The ORR activities of (Ru-Sn)O2 catalysts have to be evaluated by the RRDE voltammograms where the ring and disk current densities were measured against the disk electrode potential since the oxygen solubility is low. Figure 5a shows the typical RRDE


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voltammograms at 1600 rpm for four binary oxides prepared in this work. The mean electron transfer number (n) of the ORR can be estimated on the basis of the RRDE voltammograms according to the following equation:48-49

n=

4I D I I D +  R   N

(1)

where ID, IR, and N represent the disk current, ring current, and the current collection efficiency of the electrode (0.37 ± 0.02), respectively. The dependence of n on the disk potential (vs. RHE) for all catalysts prepared in this work is shown in Figure 5b. Note that the onset potential of the ORR on RuO2 and binary (Ru-Sn)O2 is slightly shifted negatively with decreasing the Ru content meanwhile the disk current between 800 and 400 mV (vs. RHE) generally increases with the Ru content. These results suggest that Ru species should be the main catalytic species for the ORR although the ORR activity of RuO2-based materials is worse than that of Pt/C. The above idea is also supported by the higher n value (3.9) of RuO2 in comparison with all binary oxides. On the other hand, the ORR currents on the disk electrode for all oxide catalysts are significantly enhanced at potentials negative to ca. 400 mV. These results suggest a high activation overpotential of the ORR on SnO2 (also see Figure S2). Moreover, the mean electron transfer number of the ORR from 600 to 150 mV significantly decreases with decreasing the Ru content for Ru-enriched oxides. This phenomenon has been independently confirmed 3 times in this work. Fortunately, an obvious increase in the mean electron transfer number of the ORR has been found for RuSn37 which is slightly higher than RuSn73 (RuSn73, n = 3.78; RuSn37, n = 3.8) although their compositions are significantly different. From all the above results and discussion, RuSn37 exhibits an acceptable ORR activity and will be verified in


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the polarization test of oxide-coated carbon paper electrodes since the oxygen diffusion rate usually determines the discharge performance of resultant cells in the high current density range.

Figure 6. The LSV curves of (a) ORR and (b) OER on the 25BC carbon paper coated with (1) RuSn37, (2) RuSn73, (3) RuO2, and (4) Pt/C in 0.1 M KOH with ambient air. Insets in (a) and (b) show the corresponding Tafel plots. The LSV polarization curves of the OER and ORR on the 25BC carbon paper coated with (Ru-Sn)O2, RuO2 and Pt/C catalysts were measured to confirm their bifunctional catalytic activities for the rechargeable Zn-air batteries since Pt/C and RuO2 are the best catalysts for the ORR and OER, respectively. The results of the ORR are shown in Figure 6a where the current differences among all the i-E curves are not large in the whole potential range although the Pt/C electrode shows the highest ORR current density in the whole potential range of investigation. Since the double-layer and pseudocapacitive currents of various air electrodes probably cloak the ORR current in judging the onset potential, the significant difference in the onset potential of the ORR among various catalysts on the air electrodes can be confirmed by the quasi-steady-state


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cell voltages of full cells obtained in the low current density region (see Figure 8a). For example, the difference in the ORR overpotential between Pt/C and RuSn37 at 2 mA cm-2 is about 90 mV, which is consistent with the RRDE results that Pt/C possesses a much higher onset potential of the ORR than RuSn37. In fact, the onset potential of the ORR is determined by the Ru content for the RuO2-based catalysts, also consistent with the ORR results evaluated by the RRDE voltammetry (Figure 5a). These results suggest that the ORR rate at high current densities is mainly determined by the oxygen diffusion rate in the gas-diffusion layer since the differences in the onset potential and current density of the ORR are visible in the low current density range (< 10 mA cm-2, see inset in Figure 6a). Clearly, the commercial Pt/C-coated electrode shows the highest catalytic activity at highly positive potentials, meanwhile the ORR activity and Tafel slope are significantly affected by the catalyst type and composition of (RuSn)O2. The above oxygen diffusion drawback can be overcome by the usage of the 10AA carbon paper because of a much higher gas permeation rate.50 Unfortunately, the thin 10AA carbon paper during the long-term charge-discharge-cycling test showed the electrolyte leakage issue. In addition, because of the poor mechanical strength, this carbon paper is very easy to be broken by the coating machine in preparing the air electrode. Accordingly, there should be a compromise between gas diffusion rate and mechanical strength in selecting the gas diffusion layer. From Figure 6b, the order of catalysts with respect to decreasing the OER activities is: RuSn73 > RuO2 > RuSn37 > Pt/C. In addition, at 1700 mV, the oxygen-evolving current density of RuSn73 is about 5 times of RuSn 37 and is higher than the other catalysts. All these results confirm that introduction of suitable amount of SnO2 into RuO2 can enhance the OER activity. The Tafel plots in the inset of Figure 6b supports the conclusion that RuSn73 exhibits a much


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higher oxygen-evolving activity than RuSn37, consistent with the CV results obtained in Figure 4. 3.3. Evaluating the bifunctional catalytic activity of Ru-Sn oxides In order to verify the bifunctional catalytic characteristics of RuO2-based materials, the full cell consisting of a Zn foil and an air electrode consisting of a 25BC carbon paper coated with Pt/C, RuSn73, and RuSn37 were subjected to various charge-discharge tests. Figure 7 shows the typical galvanodynamic (1 mA s-1) discharge and charge curves of the above three rechargeable Zn-air batteries. Note that the cell voltage (Ucell) under the open-circuit state for the cell containing the Pt/C-coated electrode (i.e., Ucell = 1.42 V at i = 0, denoted as UOC) is significantly higher than that of the other two cells, revealing the unique catalytic activity of Pt/C for the ORR. In addition, this cell exhibits the best discharge performance (see curves 1 and 4), which exhibits a maximum power density of 117 mW cm-2 at ca. 175 mA cm-2, revealing the importance of the electrocatalyst. Fortunately, the cell employing RuSn37 as the catalyst shows a comparable power density of 108 mW cm-2 at 200 mA cm-2 (see curves 2 and 5). From a comparison of curves 2 and 3, the discharge performance on curve 3 in the low current density range (< 60 mA cm-2) is the same or even slightly better than that on curve 2, suggesting the very similar ORR activity of two binary Ru-Sn oxides. However, at high current densities, the cell containing RuSn73 only shows a maximum power density of 93 mW cm-2. A similar result is obtained for the cell containing pure RuO2. The above relatively worse discharge performance of the cells employing the RuO2-enriched catalysts is not attributable the electric conductivity issue since the conductivity of RuO2 and RuSn73 is much higher than RuSn37. Accordingly, SnO2enriched oxides are believed to exhibit an improved ORR activity in the high current density range.


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For the charge curves, the cell voltage of the battery using the Pt/C catalyst is sharply increased to 2.2 V when the applied current density is lower than 15 mA cm-2, reasonably attributed to the poor OER activity of Pt/C. Accordingly, Pt/C is not a good bifunctional catalyst for rechargeable Zn-air batteries. For the cells using binary oxides, the cell voltages increase gradually with the current density and reach 2.2 V at 75 and 120 mA cm-2 for the batteries using RuSn37 and RuSn73 catalysts, respectively. Clearly, the catalytic activity of oxygen evolution on RuSn73 is significantly higher than that on RuSn37, consistent with its outstanding OER performance shown in Figures 4 and 6b. More importantly, the above results demonstrate the high-charging-rate capability of our Zn-air battery using the (Ru-Sn)O2-based air electrode.


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Figure 7. The charge-discharge curves and corresponding power density profiles of full cells in 6 M KOH + 0.2 M zinc acetate under the ambient condition, where the air electrode is 25BC carbon papers coated with (1,4,7) Pt/C (2,5,8), RuSn37, and (3,6,9) RuSn73. For detailed comparisons of the bifunctional catalytic activities of Ru-Sn oxides, quasisteady-state charge and discharge performances of the Zn-air batteries employing 25BC carbon papers coated with RuSn37, RuSn73, RuO2, and Pt/C are evaluated (Figures 8a and 8b). From Figure 8a, the open-circuit cell voltage (i.e., Ucell at i = 0) of the cell containing the Pt/C-coated electrode (ca. 1.4 V) is significantly higher than the other three cells, again, revealing the unique catalytic activity of Pt/C for the ORR. In addition, UOC gradually decreases with decreasing the Ru content, consistent with the onset potential data of the ORR obtained in the RRDE study. When current density is equal to 2 mA cm-2, the above trend is still visible, indicating that RuO2 shows the catalytic activity of the ORR in the low ORR current density region. On the other hand, when the current densities are equal to/above 10 mA cm-2, the quasi-steady state cell voltage of the cell employing the RuSn37 catalyst is higher than those of the cells using the Ruenriched catalysts. This phenomenon becomes more obvious with increasing the discharging current density (e.g., at 50 mA cm-2, Ucell = 1.08, 1.06, 0.93, and 0.90 V for the cells using Pt/C, RuSn37, RuSn73, and RuO2, respectively). The above results support the finding that SnO2enriched oxides exhibit an improved ORR activity in the high current density range. From Figure 8b for the charging process, the cell voltage is sharply increased from UOC to ca. 1.85 ∼ 2.0 V when 2 mA cm-2 is applied although the cell voltage of the battery using the Pt/C catalyst gradually increases at this charging current density, probably due to the large double-layer charging current of Pt/C. The very poor OER activity of the Pt/C electrode can be observed when the charging current density is equal to/above 10 mA cm-2, meanwhile its cell


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voltage seems to not reach a stable value during every 10-min charging process. Due to the very high charging cell voltage, the Pt/C electrode cannot serve as a rechargeable air electrode. For the cells using the RuO2-based air electrodes, the cell voltages easily reach their stable values and gradually increase with the charging current density. In addition, the stable Ucell reaches 2.0 V at a high current density of 50 mA cm-2 when the cell employs the RuSn73 air electrode, which is obviously lower than the other batteries using the air electrodes coated with RuSn37, Pt/C or even RuO2. All the above results reveal the low OER overpotential of RuSn73. From the charge-discharge data mentioned above, RuSn37 and RuSn73 are the appropriate ORR and OER catalysts because of their high discharge and low charge cell voltages. Accordingly, an electrode combined both catalysts is believed to be a promising air electrode for the rechargeable Zn-air batteries.


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Figure 8. The quasi-steady-state (a) discharge and (b) charge performances at 0, 2, 10, 20, 30, 40, 50 mA cm-2 as well as (c,d) charge-discharge cycling stability of zinc-air batteries at 10 mA cm-2 for 100 cycles in 6 M KOH + 0.2 M zinc acetate under the ambient condition. The air electrode is 25BC carbon paper coated with (1) RuSn73, (2) RuSn37, (3) Pt/C, and (4) RuO2 catalysts. Another important index evaluating the performance of air electrodes is the charge-discharge cycling stability in the full cell testing and typical results for the above 4 cells at 10 mA cm-2 under the ambient air condition for 100 cycles are shown in Figures 8c and 8d. As mentioned before, the discharge cell voltage depends on not only catalysts but also the gas permeation rate of the gas-diffusion layer; hence, Ucell of various cells during the initial few cycles is very similar (see the first 6 cycles in Figure 8c). The cell employing the Pt/C-coated electrode shows a little


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higher cell voltage during the first discharge process in comparison with the other cells because the commercial Pt/C catalyst exhibits a high ORR activity. However, the large OER overpotential of Pt/C in the charging process may cause aggregation of Pt clusters,51 since the cell voltage in the charging step is obviously high (> 2.3 V in the first charging process). This causes the gradual decrease in the discharge Ucell because the ORR overpotential of Pt/C increases with the charge-discharge cycles, indicating the poor cycling stability. After 100 cycles of charge-discharge steps (Figure 8d), the discharge and charge Ucell of this cell are equal to ca. 1.0 and 2.65 V, respectively, revealing the low energy efficiency of this rechargeable battery. On the other hand, the charge and discharge Ucell of the other three cells using the (Ru-Sn)O2-/RuO2coated electrodes are very stable during the above 100-cycle test (also see Figure S3). Especially, the charge Ucell of the cell with the RuSn73-coated electrode maintains at 1.9 V, revealing the good stability and great catalytic activity of both binary (Ru-Sn)O2 and RuO2 for the OER. From all the above results and discussion, the RuSn73-based carbon electrode possesses a comparable ORR activity to the RuSn37-coated one in the discharging process but a much higher OER activity during the charging process, revealing its excellent bifunctional catalytic activity. 3.4.The charge-discharge performances of full cells with the air electrode containing two catalysts According to the results of the full cells with their air electrodes consisting of a carbon paper coated with one catalyst, RuSn73 can serve as a good bifunctional catalyst for rechargeable zincair batteries. However, by considering the high cost of Ru-enriched oxides and the excellent ORR activity of RuSn37, an air electrode combining RuSn73 and RuSn37 for the OER and ORR, respectively is desirable for the rechargeable zinc-air battery application. This idea can be


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demonstrated by the air electrode assembled with a 25BC carbon paper coated with the good ORR catalyst (e.g., RuSn37) and a Ti mesh coated with the prior OER catalyst (i.e., RuSn73) according to the design reported in our previous work.43 This unique RuSn37/C-RuSn73/Tiassembled air electrode (denoted as 37-C|73-Ti) is believed to exhibit an acceptable low charge cell voltage and the highest discharge cell voltages with additional advantages of decreasing the cost of Ru and Ti mesh acting as a current collector although this electrode design need the additional catalyst on the Ti mesh (i.e., double amounts of catalysts). Figure 9 compares the charge-discharge and power performances of several rechargeable Zn-air batteries using the special design of air electrodes consisting of diverse binary (Ru-Sn)O2 catalysts. Note that from the discharge curves, the order of the air electrode with respect to decreasing the UOCP is: 73-C|73-Ti (1.35 V) > 37-C|73-Ti ≈ 64-C|64-Ti (1.29 V) > 73-C|37-Ti (1.18 V) > 55-C|55-Ti (1.16 V), confirming the catalytic activity of Ru-enriched oxides for the ORR in the low current density region. Interestingly, at Ucell = 0.4 V, the discharge curve of the battery using 37-C|73-Ti could reach 290 mA cm-2 which is larger than the other design, indicating the catalytic activity of Sn-enriched oxides for the ORR in the high current density region. In addition, the power density of this cell reaches a maximum at 121 mW cm-2 when the current density is 235 mA cm-2 although the cell with the 73-C|73-Ti electrode shows a maximum power of 118 mW cm-2 at 200 mA cm-2. The performances of the above two cells are much better than the other systems (e.g., the cells with 73-C|37-Ti and 55-C|55-Ti electrodes provide maximum powers of 79 mW cm-2 and 59 mW cm-2 at 141 and 109 mA cm-2, respectively.


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Figure 9. The charge-discharge curves and corresponding power density profiles of full cells in 6 M KOH + 0.2 M zinc acetate under the ambient condition. The air electrodes are 37C|73-Ti, 55-C|55-Ti, 64-C|64-Ti, 73-C|37-Ti, and 73-C|73-Ti. For the charge curves, when the cell voltage reaches 2.1V, the order of the air electrode with respect to decreasing the charging current density is: 37-C|73-Ti (159 mA cm-2) > 73-C|73-Ti (116 mA cm-2) > 64-C|64-Ti (83 mA cm-2) > 73-C|37-Ti (68 mA cm-2) > 55-C|55-Ti (54 mA cm2

). However, the charging current density of the cell using the 73-C|73-Ti electrode can even

reach 250 mA cm-2 at Ucell = 2.4 V, resenting the very high charging rate capability of this rechargeable Zn-air battery. Moreover, from a comparison of Figures 7 and 9, the air electrodes consisting the carbon paper-coated ORR catalyst and Ti mesh-coated OER catalyst could reduce the charging and discharging overpotentials and enlarge the charging and discharging current densities, revealing the unique performance of our electrode design. In addition, from the


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discharge and charge curves of cell using the 37-C|73-Ti electrode in Figure 9, RuSn37 on carbon paper and RuSn73 on Ti mesh mainly provide the catalytic activities of the ORR and OER, respectively. This result indicates that our air electrode design assembling two catalysts can reach their synergistic performance. Also, introduction of suitable Sn contents into RuO2 significantly improves the ORR and OER activities at the same time although Sn-enriched (RuSn)O2 catalysts are preferable because of the cost consideration.

Figure 10. Repeated discharge-charge curves of a zinc-air battery employing the 37-C|73-Ti air electrode in 6 M KOH + 0.2 M zinc acetate under the ambient air condition at 10 mA cm-2 for 80 h (an hour per step).


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From a comparison of the cells using 37-C|73-Ti and 73-C|73-Ti electrodes with our previous work employing the similar design consisting of an α-MnO2+XC72 ORR-coated carbon paper and a Fe0.1Ni0.9Co2O4-coated Ti mesh,43 the maximum power density of 88mW cm-2 for the latter cell is much lower than those obtained in this study, revealing the excellent catalytic activity of (Ru-Sn)O2. From all the above results and discussion, binary Ru-Sn oxides can serve as bifunctional catalysts and have been successfully assembled into exceptional air electrodes, providing much higher maximum power densities and very high charging rate capabilities for rechargeable Zn-air batteries. In order to demonstrate the practical usage of our electrode, a long-term discharge-charge test for the Zn-air battery employing the 37-C|73-Ti air electrode was conducted in 6 M KOH + 0.2 M zinc acetate under the ambient air condition at 10 mA cm-2 for 80 h (an hour per step, see Figure 10). Clearly, this cell with the 37-C|73-Ti air electrode exhibits excellent chargedischarge stability. The initial charge-discharge cell voltage gap is 0.74 V, where the cell voltages of charge and discharge are equal to ca. 1.94 and 1.20 V, respectively. Moreover, after the 80-h charge-discharge test, the cell voltage gap between charge and discharge slightly increases to 0.83 V. These evidences reveal the unique bifunctional air electrode designed in this work for rechargeable zinc-air batteries under an air supply condition. Table 1 compares the Znair battery status and cell voltage gap for various bifunctional electrocatalysts having been reported in the literature, clearly demonstrating the very outstanding performance of our catalysts and electrode design.


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Table 1. Performances of rechargeable zinc-air batteries using various bifunctional catalysts reported in the literature.

Ucell (V) Ucell Gap Charge (V)

Catalyst

Ucell (V) Discharge

RuSn73

1.20

1.91

RuSn (37-C|73-Ti)

1.20

RuO2@C

Condition

Atm.

Ref.

0.71

10 mA cm-2, 10 min per cycle, 100 cycles

Air

This work

1.94

0.74


Air

This work

1.05

2.05

1


Air

51

RuO2-coated MCNA

1.25

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