Bifunctional electrocatalysts of lanthanum-based perovskite oxide with Sb-doped SnO2 for oxygen reduction and evolution reactions

Journal of Power Sources 451 (2020) 227736

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Bifunctional electrocatalysts of lanthanum-based perovskite oxide with Sb-doped SnO2 for oxygen reduction and evolution reactions Naoko Fujiwara a, *, Tsukasa Nagai a, Tsutomu Ioroi a, Hajime Arai b, 1, Zempachi Ogumi b a

Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka, 563-8577, Japan b Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan

H I G H L I G H T S

� Carbon-free electrocatalyst (LCCMO þ ATO) active for both ORR and OER was proposed. � Gas-diffusion air electrode with the catalyst was evaluated in zinc-air batteries. � Its charge-discharge performance was similar to conventional carbon-based catalysts. � ATO support greatly improved durability during cycle test compared to carbon support. A R T I C L E I N F O

A B S T R A C T

Keywords: Oxygen reduction reaction Oxygen evolution reaction Perovskite oxide Antimony-doped tin oxide Rechargeable zinc-air battery

Bifunctional electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are developed with perovskite-type oxide catalyst and antimony-doped tin oxide (ATO) as an oxidationresistant conductive additive to increase the stability under an oxidizing atmosphere. In conventional carbonsupported catalysts, carbon plays an important role not only in increasing electric conductivity but also in catalyzing the first 2-electron reaction of the ORR. The ATO-mixed catalyst itself requires a perovskite-type oxide (ABO3) catalyst that is active in the first step of the ORR, such as La0.6Ca0.4MnO3 (LCMO), because of the poor ORR activity of ATO. On the other hand, OER performance strongly depends on the composition of the perovskite oxide and La0.6Ca0.4CoO3 (LCCO) shows high OER activity. Here, La0.6Ca0.4Co0.7Mn0.3O3 (LCCMO), in which Co and Mn are mixed at the B site of the perovskite-type oxide, mixed with ATO is proposed as a carbon-free bifunctional catalyst that shows activity for both the ORR and OER. The activity and durability of gasdiffusion air electrodes with the bifunctional catalysts are evaluated in zinc–air batteries, and the results sug­ gest that the catalyst with ATO achieved similar charge-discharge performance and a longer lifetime compared to the conventional catalyst with carbon.

1. Introduction

process in air batteries, respectively:

Bifunctional electrocatalysts that show activity in both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are essential for the direct conversion between electrical energy and chemicals in the practical application of reversible fuel cells and rechargeable metal-air batteries [1]. The four-electron reaction of oxy­ gen in an alkaline medium is shown in eq. (1), where the forward re­ action is the ORR and the reverse reaction is the OER in a fuel cell or the discharging process in air batteries and in electrolysis or the charging

O2 þ 2H2O þ 4 e → 4 OH

(1)

Although the reaction in eq. (1) theoretically proceeds at 1.23 V vs. RHE under standard conditions, there are large deviations from the theoretical potential (overpotentials), due to the sluggish kinetics of the ORR and OER. The development of efficient bifunctional electro­ catalysts for both the ORR and OER remains an important challenge, despite numerous studies [2–4]. Platinum group metals and their alloys are the most promising catalysts in acid and alkaline media with little

* Corresponding author. E-mail address: [email protected] (N. Fujiwara). 1 Present Address: School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama, 226-8502, Japan. https://doi.org/10.1016/j.jpowsour.2020.227736 Received 29 July 2019; Received in revised form 7 January 2020; Accepted 9 January 2020 Available online 16 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.

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Journal of Power Sources 451 (2020) 227736

consideration of cost and abundance [5,6]. Non-noble metal oxides including spinel-type oxides (AB2O4) [7–9], perovskite-type oxides (ABO3) [10–15] and other transition metal ox­ ides [16], are also powerful candidates as bifunctional catalysts with high ORR and OER activity and sufficient stability for use in alkaline medium. Perovskite-type oxides are among the most promising and have been extensively studied in the past few decades due to their high cat­ alytic activity and durability in an alkaline electrolyte. The properties of perovskite-type oxides (ABO3) can be controlled by selecting a wide variety of metal cations at both the A-site and B-site or by partially substituting A and B cations with other metals, and lanthanum-based – Mn, Fe, Ni), perovskite oxides, La1-xAxCo1-yByO3 (A ¼ Ca, Sr and B– have been reported to be the most promising bi-functional air electro­ catalysts so far [10,11,14,15]. Metal-oxide catalysts are usually mixed with carbon powders to compensate for their lack of electrical conductivity and to support catalyst particles without aggregation [9–15]. Although carbon is a su­ perior material with high electrical conductivity, light weight, low cost, hydrophobicity, large surface area, high porosity, and so on, it is theo­ retically oxidized into carbon dioxide at 0.207 V vs. RHE: C þ 2H2O → CO2 þ 4Hþ þ 4 e

durable bifunctional catalysts in an alkaline medium. Here, a few lanthanum-based perovskite oxides were examined as the ATO-mixed bi-functional electrocatalysts in terms of their application to air elec­ trodes in zinc-air batteries, and the activity and durability of the bat­ teries were evaluated by charge-discharge tests. 2. Experimental 2.1. Preparation and characterization of catalysts La(NO3)3⋅6H2O, Ca(NO3)2⋅4H2O, Co(NO3)2⋅6H2O and Mn (NO3)2⋅6H2O (Wako Pure Chemical Industries, Ltd.) were used as starting materials for preparing three perovskite-type oxides, La0.6Ca0.4CoO3 (LCCO), La0.6Ca0.4Co0.7Mn0.3O3 (LCCMO), and La0.6Ca0.4MnO3 (LCMO), by a conventional citrate method as described below. Stoichiometric amounts of those metal nitrates were dissolved in 2 mol dm 3 citric acid aqueous solution. The solution was then evapo­ rated on a hot plate heated at 80 � C. The resulting syrupy mixture was first heated at 200 � C for 2 h, and then calcined at 700 � C for 7 h in air. Carbon black (CB, Denkablack from Denka Co., Ltd.) and antimonydoped tin oxide (ATO, EP T-1S from Mitsubishi Materials Corp.) were used as conductive materials mixed with perovskite oxide catalysts. The perovskite oxides and conductive materials were mixed in an adequate ratio and used as electrocatalysts for the ORR and OER. 20 wt% Pt on carbon, (Pt/C, Johnson-Matthey HiSPEC® 3000) was used as a com­ mercial standard catalyst. The elemental compositions of the prepared perovskite oxides were quantitatively analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Jarrel-Ash IRIS Advantage). The crystal struc­ tures of the prepared oxide catalysts were identified by powder X-ray diffraction (XRD) analysis using Cu Kα radiation (RINT2200, Rigaku Co., Ltd.). The specific surface areas of the prepared oxide catalysts and conductive materials were determined from N2 adsorption isotherms by the Brunauer-Emmet-Teller (BET) method using BELSORP-mini II (MicrotracBEL Corp.). The electrical conductivities of the prepared oxide catalysts and conductive materials were measured by using a fourpoint-probe method with a powder resistivity measurement system (Mitsubishi Chemical Analytech, MCP-PD51). Each powder sample was put into a cylindrical measurement cell equipped with four electrodes at the bottom of the cell. The powder was compressed with a piston at the given pressure and then the conductivity was measured as a function of the loaded pressure.

(2)

There are concerns about the deterioration of conventional air electrodes with carbon supports due to carbon corrosion, since the po­ tential of the air electrode may increase above 1.5 V vs. RHE due to the large overvoltage for the OER [17,18]. In fact, air electrodes mainly degrade during the OER due to carbon corrosion resulting in a loss of electrochemically active surface area of the electrode, although elec­ trodes composed of a metal-oxide catalyst and graphitized carbon or carbon with an optimized specific surface area increase the stability against corrosion in highly concentrated alkaline solution [14,15,19, 20]. There is an urgent need for the development of bifunctional cata­ lysts that are highly active for both the ORR and OER, and also durable under high potential conditions during the OER in charging or electrolysis. Recently, considerable attention has been focused on noncarbon support materials as oxidation-resistant catalyst supports for air elec­ trodes in polymer electrolyte fuel cells (PEFCs) [21–24], polymer elec­ trolyte membrane water electrolyzers (PEMWEs) [25], unitized regenerative fuel cells (URFCs) [26,27], and rechargeable metal-air batteries [28,29] to increase the durability during long-term operation at higher temperature. Typical examples of noncarbon supports are conducting metal oxides, such as oxygen-deficient, substoichiometric titanium oxides (TinO2n-1, 4 < n < 10), and metal-doped oxides (e.g., Nb-doped titanium oxide, Nb-doped or Sb-doped tin oxide, and Sn-doped indium oxide). Platinum group metal catalysts deposited on a nano-sized sub-stoi­ chiometric titanium oxide (TiOx) showed greater stability than those on a carbon support after cycling between 1.0 V and 1.5 V vs. RHE in PEFCs [22]. TiOx-supported Pt or Ir catalysts also showed better performance and durability than unsupported Pt or Ir catalysts for both fuel cell and water electrolysis in URFCs [26]. Titanium-niobium oxides (TixNbyOz) mixed with Ti4O7 were proposed as precious metal- and carbon-free catalysts to demonstrate ORR activity and stability in H2SO4 aqueous solution [24]. Nb-, Sb- or Ta-doped tin oxides have also been used as catalyst substrates of Pt, IrOx, and Pt–IrO2 to improve the catalytic sta­ bility in PEFCs [23], PEMWEs [25], and URFCs [27]. These numerous studies have only addressed the application of conducting metal oxides as catalyst supports in acid media. Regarding the application to rechargeable metal-air batteries with an aqueous alkaline electrolyte, a spinel oxide of Co3O4 mixed with Ni powder [28] and NiCo2O4 coated on Ni powder [29] have been reported as carbon-free air electrodes. The main purpose of this study was to explore the potential use of antimony-doped tin oxide (ATO) as an oxidation-resistant conductive additive, as an alternative to a carbon, for the development of highly

2.2. Electrochemical measurements Aqueous suspensions containing the 4 mg of perovskite-type oxide, 0.76 mL of water, 0.24 mL of 2-propanol, and 4.6 μL of 5 wt% anionexchange resin solution (AS-4, Tokuyama Co., Ltd.) were ultrasoni­ cally dispersed and the 5.3 μL of the suspension was dropped onto the surface of glassy carbon (GC) disk electrodes (4 mm diameter) polished to a mirror finish with 0.05 μm alumina powder. The GC electrodes modified with 170 μg cm 2 of perovskite oxide were dried at 80 � C for 1 h and used as the working electrode. Linear sweep voltammetry (LSV) was conducted using a potentiostat (Model 7002C, ALS Co., Ltd) in O2saturated 1 mol dm 3 KOH aqueous solution at 10 mV s 1 and 25 � C with a conventional three-electrode arrangement of the working elec­ trode prepared as described above, a Pt spiral counter electrode, and a reversible hydrogen reference electrode (RHE). The working electrode was rotated at 2500 rpm by a rotating ring disk electrode setup (RRDE3A, ALS Co., Ltd.) during all measurements. ORR current was calculated as the difference between the currents at negative scan obtained in O2saturated solution and N2-saturated solution. Before the all measure­ ments, ORR and OER performance of LCMO þ ATO (1:3) was also evaluated with a glassy carbon rod as a counter electrode to confirm the presence or absence of the Pt contamination. Figs. S2(a) and (b) (Sup­ porting information) show the both data obtained using a Pt spiral 2

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Journal of Power Sources 451 (2020) 227736

electrode and a glassy carbon rod as a counter electrode are similar to each other and the contamination with Pt is negligible. 2.3. Preparation and performance test of air electrodes in Zn-air batteries The prepared catalysts, LCCMO mixed with conductive additives, were ultrasonically dispersed in water containing Triton X-100 (Kishida Chemical Co. Ltd.) followed by the addition of an appropriate amount of PTFE dispersion (D-210C, Daikin Industries, Ltd.), and blended at 80 � C to be a condensed catalyst ink. The catalyst ink was applied to a gasdiffusion layer made of hydrophobic fluorocarbon loaded on a carbon cloth (ELAT A6, BASF Fuel Cell, Inc.) using a spatula and then heated at 360 � C for 1 h under N2 flow to prepare air electrodes with a geometrical area of 2 cm2. The air electrode was mounted as a working electrode (WE) in an experimental cell with an anion exchange membrane (AEM A201, Tokuyama Co., Ltd.) at the interface between the liquid electro­ lyte and atmosphere, in a similar configuration as in a previous paper [30]. The cell was equipped with a zinc plate, a Hg/HgO electrode, and 4.0 mol dm 3 KOH aqueous solution as a counter electrode, a reference electrode, and electrolyte solution, respectively, and maintained at 25 � C. The air electrode was exposed to ambient air through multiple holes in the air electrode side of the zinc-air test cell. Polarization curves were obtained by monitoring potentials of the air electrode after applying each current density for 3 min using a galvanostat (Biologic, VSP) to the zinc-air battery. A charge-discharge cycle test of the zinc-air battery was performed by applying 4 mA cm 2 or 10 mA cm 2 for 1 h for each step.

Fig. 1. XRD patterns of the prepared perovskite oxide catalysts: La0.6Ca0.4CoO3 (a), La0.6Ca0.4Co0.7Mn0.3O3 (b), and La0.6Ca0.4MnO3 (c).

3. Results and discussion 3.1. Physical properties of the prepared perovskite-type oxides and conductive materials The amounts of La, Ca, Co, and Mn in the prepared perovskite-type oxides of LCCO, LCCMO, and LCMO were quantitatively analyzed by ICP-AES. The elemental compositions were calculated from their weight compositions of La, Ca, Co, and Mn under the assumption that the re­ sidual weight is that of O, as summarized in Table 1 after normalizing the B-site content (Mn þ Co) to be 1. Three kinds of perovskite-type oxides with a stoichiometric composition were successfully prepared in this study. Fig. 1 shows XRD patterns of LCCO, LCCMO, and LCMO corre­ sponding to the well-crystallized perovskite structure without any peaks resulting from crystalline impurities. The specific surface areas of the prepared oxide catalysts and conductive materials, determined by the BET method, were 16, 27, and 23 m2 g 1 for LCCO, LCCMO, and LCMO as shown in Table 1, and 63 and 42 m2 g 1 for CB and ATO, respectively. Fig. 2 compares the electrical conductivities of the prepared oxide catalysts and conductive materials as a function of the loaded pressure. The conductivity increases with an increase in the loaded pressure, since a higher pressure increases the interparticle contact of powders. The conductivities of the perovskite-type oxides strongly depend on their compositions, i.e., LCCO > LCCMO > LCMO under the same conditions. CB shows greater conductivity than the three kinds of perovskite-type oxides and plays an important role as a catalyst support providing electrical conductivity to the catalysts. Although the electric conduc­ tivity of ATO is an order of magnitude smaller than that of CB, it can also compensate for the lack of electrical conductivity when LCCMO or

Fig. 2. Electrical conductivity of a uni-axially pressed powder of LCCO, LCCMO, and LCMO catalysts and CB and ATO as a function of loaded pressure.

LCMO are applied as the catalyst. 3.2. ORR and OER performance of perovskite-type oxides in alkaline solution LSVs of LCCO and LCMO obtained in O2-saturated 1.0 mol dm 3 KOH solution are shown in Fig. 3 (a) and (b), respectively. According to Fig. 3 (a), LCCO alone shows poor ORR activity with a low onset po­ tential (0.76 V vs. RHE) and small reduction current. The addition of ATO to LCCO catalyst at a weight ratio of 1:1 or 1:3 had little effect on the onset potential (0.76 V vs. RHE) and the reduction current, but the addition of CB strongly increased the onset potential (0.86 V vs. RHE) and reduction current. These results show that ATO in the mixture with

Table 1 Weight and elemental composition, and BET surface area of the prepared perovskite oxides. Weight % La0.6Ca0.4CoO3 (LCCO) La0.6Ca0.4Co0.7Mn0.3O3 (LCCMO) La0.6Ca0.4MnO3 (LCMO)

Elemental Composition

BET surface area

La

Ca

Co

Mn

La

Ca

Co

Mn

O

m2 g

40 41 42

7.7 7.9 8.0

28 20 0

0 8.1 27

0.61 0.60 0.61

0.40 0.40 0.41

1.00 0.70 0

0 0.30 1.00

3.1 3.0 2.9

16 27 23

3

1

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Journal of Power Sources 451 (2020) 227736

Fig. 4. ORR performance of LCCO, LCMO, CB, ATO, and unmodified GC disk electrode in O2-saturated 1.0 mol dm 3 KOH solution at 10 mV s 1, 2500 rpm and 25 � C.

step (eq. (3)) and LCCO catalyzes the second step (eq. (4) or 5) in con­ ventional catalysts mixed with carbon, LCCO þ CB. In the catalyst (LCCO þ CB), CB plays a significant role in catalyzing the first 2-electron oxidation of oxygen (eq. (3)) rather than in providing electrical con­ ductivity. On the other hand, LCCO mixed with ATO did not show increased catalytic activity for ORR, since, unlike carbon, ATO cannot promote the first step of the ORR. In contrast, LCMO alone shows a higher onset potential than CB, which suggests that it may exhibit cat­ alytic activity for the first step of the ORR. This observation suggests that LCMO may catalyze both the first and second steps of the ORR and ATO acts only as a conductive additive in LCMO þ ATO catalyst. Electro­ chemical measurements with rotating ring-disk electrodes (RRDE) were also carried out and the results are shown in Fig. S1 and Table S1 (Supporting Information). From Table S1 (Supporting Information), electron transfer numbers in ORR process at 0.6 V vs RHE were 3.0 and 3.7, for LCCO and LCMO, respectively. This result can explain the different behavior of ORR performance obtained on LCCO and LCMO. Fig. 5(a) and (b) show LSVs of perovskite-type oxide catalysts mixed with ATO at a weight ratio of 1:3 corresponding to ORR and OER per­ formance, respectively, in O2-saturated 1.0 mol dm 3 KOH solution. LCCO þ ATO catalyst shows superior OER performance with a low onset potential (below 1.6 V vs. RHE) and large current density (25 mA cm 2 at 1.8 V vs. RHE), as shown in Fig. 5 (b). Although LCMO þ ATO showed good ORR activity as described above, its OER activity was much lower than that of LCCO þ ATO. The ORR performance of the commercial Pt/C catalyst was also shown in Fig. 5 (a). Although the LCMO þ ATO showed the best ORR performance in the perovskite-type oxide catalysts mixed with ATO studied in the present work, the ORR activity was much lower and still have larger overpotential than that of the Pt/C catalyst. In addition, the diffusion-limited current density was not observed for the perovskite-type oxide catalysts at the potential rage above 0.4 V vs RHE unlike the Pt/C catalyst, for the same reason. The superior OER per­ formance of LCCO compared to LCMO has been previously reported for a conventional air electrode mixed with carbon [11]. To solve this trade-off, LCCMO, in which Co and Mn were mixed at the B-site in a ratio of 7:3, was prepared as a bi-functional catalyst that was active for both the ORR and OER as shown in Fig. 5(a) and (b). The ORR and OER performance of LCCMO þ ATO are between those of LCCO þ ATO and LCMO þ ATO, suggesting that it has intermediate activity for both the ORR and OER.

Fig. 3. ORR performance of LCCO (a) and LCMO (b) with or without conductive additives in O2-saturated 1.0 mol dm 3 KOH solution at 10 mV s 1, 2500 rpm and 25 � C.

LCCO plays a less active role as an alternative to CB. However, the behavior of LCMO is quite different from that of LCCO, for which the addition of ATO increases the onset potential from 0.88 V to 0.92 V vs. RHE and the reduction current is almost equal to LCMO with CB when ATO is added at a weight ratio of 1:3, as shown in Fig. 3 (b). The ORR mechanism in alkaline solution can be explained by the following scheme (eqs. (3)–(5)), in which the ORR is considered to proceed via 2þ2-electron reduction. The first step in the ORR is O2 reduction to a hydrogen peroxide intermediate, HO2 (eq. (3)), which is followed by electrocatalytic reduction (eq. (4)) or disproportionation of HO2 (eq. (5)). The role of carbon mixed with perovskite oxide in the ORR has been studied intensively, and a synergetic 2þ2-electron transfer pathway has been proposed based on the catalytic activity of carbon in the initial 2-electron reduction of O2 [15,31,32]. A previous paper on the electrocatalytic activity of a LCCO-carbon composite electrode reported that O2 was first reduced at the more active carbon site to produce HO2 thorough a 2-electron pathway (eq. (3)), and the formed HO2 was then further reduced to OH at the adjacent LCCO site (eq. (4)) [31]: O2 þ H2O þ 2e → HO2 þ OH

(3)

HO2 þ H2O þ 2e → 3 OH

(4)

2 HO2 → 2 OH þ O2

(5)

Fig. 4 compares LSVs of perovskite-type oxide catalysts and conductive additives obtained in O2-saturated 1.0 mol dm 3 KOH so­ lution corresponding to ORR activities. The onset potentials of these materials were larger in the following order: LCMO ≫ CB ≫ ATO, LCCO, unmodified GC disk electrode, indicating that the onset potential of ATO is much lower than that of CB and ATO itself cannot assist ORR. If we consider the ORR mechanism described above, CB promotes the first

3.3. Performance test of air electrodes in Zn-air batteries Zinc-air batteries were prepared with gas-diffusion air electrodes made of LCCMO mixed with conductive additives as bi-functional cat­ alysts in the catalyst layer and their charge-discharge performance was 4

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Journal of Power Sources 451 (2020) 227736

Fig. 6. Charge-discharge performance of air electrodes at 25 � C. LCCMO þ CB (5:1 w/w, PTFE 10 wt%) and LCCMO þ ATO (1:2 w/w, PTFE 5 wt%).

Fig. 5. ORR (a) and OER (b) performance of LCCO, LCMO and LCCMO mixed with ATO (1:3 w/w), and Pt/C in O2-saturated 1.0 mol dm 3 KOH solution at 10 mV s 1, 2500 rpm and 25 � C.

then tested. Prior to the comparison test, the mixing ratio of LCCMO, the conductive additives and PTFE content was optimized. The performance optimization data were shown in Figs. S3 and S4 (Supporting informa­ tion). Fig. S3 shows the charge-discharge performance of air electrodes of LCCMO þ ATO (1:3 w/w) with PTFE contents of 2, 5, 10, and 15 wt%. Although the larger content of PTFE has a tendency for higher charge and discharge potential, PTFE 5 wt% was selected to minimize OER overpotential in the charge process in this work. The ratio of LCCMO to ATO was optimized in Fig. S4, suggesting that the LCCMO:ATO ¼ 1:2 w/ w showed the best reversibility for charge and discharge. The optimum ratios turned out to be 5:1 with PTFE 10% and 2:1 with PTFE 5% by weight for LCCMO þ CB and LCCMO þ ATO, respectively. Fig. 6 shows the charge-discharge performance of air electrodes of LCCMO with CB and ATO, where the potentials are plotted versus current density. Although the two electrodes showed similar performance, the LCCMO þ ATO electrode still showed worse reversibility with a higher charge potential and lower discharge potential than LCCMO þ CB. One possible explanation for why the LCCMO þ ATO electrode showed worse per­ formance than the LCCMO þ CB electrode in both the charge and discharge steps involves the lower conductivity of ATO compared to CB, as shown in Fig. 2. In addition, ATO’s lack of two-electron ORR activity may also be an important contributor to the poor discharge performance of the LCCMO þ ATO electrode. A charge-discharge cycle test of zinc-air batteries with the two kinds of air electrodes was conducted by applying a current density of 4 mA cm 2 for 30 cycles followed by a current density of 10 mA cm 2, for 1 h at each charge and discharge step. The cycle performance of the zinc-air batteries is shown in Fig. 7 as a plot of the cell voltage after the appli­ cation of a constant current density for 1 h versus the cycle number. For the zinc-air battery with LCCMO þ CB catalyst, the cell voltage rapidly increased in the charge step and decreased in the discharge step during

Fig. 7. Charge-discharge cycle performance of zinc-air batteries operated at 4 or 10 mA cm 2 and 25 � C. LCCMO þ CB (5:1 w/w, PTFE 10 wt%) and LCCMO þ ATO (1:2 w/w, PTFE 5 wt%).

30 cycles at 4 mA cm 2, suggesting that the air electrode underwent degradation due to carbon corrosion in the catalyst layer. Thus, in the air electrode with LCCMO þ CB catalyst during the charge step at 4 mA cm 2, CB was easily oxidized into CO2, which caused deterioration of the electrical contact with LCCMO catalyst particles. On the other hand, the cell voltage of the zinc-air battery with the LCCMO þ ATO catalyst was stable after 30 cycles at 4 mA cm 2 followed by 70 cycles at 10 mA cm 2, for a total of 100 cycles for 200 h, with little deterioration of the air electrode, although the initial reversibility was slightly worse with a higher charge and lower discharge voltage than the LCCMO þ CB catalyst. This result indicated that ATO mixed with LCCMO catalyst in the air electrode, as an alternative to CB, greatly improved the durability during the charge-discharge cycle test because of the high resistance of ATO to high potential, even above 0.8 V vs. Hg/HgO (1.7 V vs. RHE). 4. Conclusions Carbon-free bifunctional electrocatalysts that show activity in both the ORR and OER were successfully developed with lanthanum-based perovskite-type oxide mixed with ATO as an alternative to CB used as a conductive additive in conventional bifunctional catalysts to improve the durability at high potential during the OER. The ATO-mixed catalyst itself requires a perovskite-type oxide catalyst that is active in the first step of the ORR, such as LCMO, because of the poor ORR activity of ATO. On the other hand, OER performance strongly depends on the 5

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Journal of Power Sources 451 (2020) 227736

composition of the perovskite oxide and LCCO shows high OER activity. Hence, LCCMO þ ATO catalyst was considered to be a bifunctional catalyst that shows activity for both the ORR and OER. The activity and durability of gas-diffusion air electrodes with bifunctional catalysts were evaluated in zinc–air batteries. The results suggested that the catalyst with ATO achieved similar charge-discharge performance and a longer lifetime compared to the conventional cata­ lyst with carbon.

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Declaration of competing interest None. Acknowledgments This study was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Batteries” (RIS­ ING) project of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors are grateful to Tokuyama Corporation for providing anion-exchange resin solution and anionexchange membrane. The authors also thank Mr. Yasuzo Kousaka, Ms. Yumiko Hayashi, Ms. Akiko Nakamura, and Ms. Masami Shuuta for their kind support with the experiments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227736. References [1] K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992. [2] V. Neburchilov, H. Wang, J.J. Martin, W. Qu, A review on air cathodes for zinc-air fuel cells, J. Power Sources 195 (2010) 1271–1291. [3] Z.-L. Wang, D. Xu, J.-J. Xu, X.-B. Zhang, Oxygen electrocatalysts in metal-air batteries: from aqueous to nonaqueous electrolytes, Chem. Soc. Rev. 43 (2014) 7746–7786. [4] W.T. Hong, M. Risch, K.A. Stoerzinger, A. Grimaud, J. Suntivich, Y. Shao-Horn, Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis, Energy Environ. Sci. 8 (2015) 1404–1427. [5] J.K. Norskov, J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard, H. J� onsson, Origin of the overpotential for oxygen reduction at a fuelcell cathode, J. Phys. Chem. B 108 (2004) 17886–17892. [6] T. Reier, M. Oezaslan, P. Strasser, Electrocatalytic oxygen evolution reaction (OER) on Ru, Ir, and Pt catalysts: a comparative study of nanoparticles and bulk materials, ACS Catal. 2 (2012) 1765–1772. [7] E. Rios, J.-L. Gautier, G. Poillerat, P. Chartier, Mixed valency spinel oxides of transition metals and electrocatalysis: case of the MnxCo3-xO4 system, Electrochim. Acta 44 (1998) 1491–1497. [8] D.U. Lee, B.J. Kim, Z. Chen, One-pot synthesis of a mesoporous NiCo2O4 nanoplatelet and graphene hybrid and its oxygen reduction and evolution activities as an efficient bi-functional electrocatalyst, J. Mater. Chem. A 1 (2013) 4754–4762. [9] P.W. Menezes, A. Indra, N.R. Sahraie, A. Bergmann, P. Strasser, M. Driess, Cobaltmanganese-based spinels as multifunctional materials that unify catalytic water oxidation and oxygen reduction reactions, ChemSusChem 8 (2015) 164–171. [10] Y. Shimizu, K. Uemura, H. Matsuda, N. Miura, N. Yamazoe, Bi-functional oxygen electrode using large surface area La1-xCaxCoO3 for rechargeable metal-air battery, J. Electrochem. Soc. 137 (1990) 3430–3433.

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