High-Performance A-Site Deﬁcient Perovskite Electrocatalyst for Rechargeable Zn–Air Battery

: Zinc–air batteries are one of the most excellent of the next generation energy devices. How-ever, their application is greatly hampered by the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of air electrode. It is of great importance to develop good oxygen electrocatalysts with long durability as well as low cost. Here, A-site deﬁcient (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 perovskites have been studied as potential OER electrocatalysts prepared by EDTA–citrate acid complexing method. OER electrocatalytic performance of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 was also evaluated. (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 electrocatalysts exhibited good OER activities in 0.1 M KOH with onset potential and Tafel slope of 1.50 V and 87 mV dec − 1 , similar to that of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 (BSCF-5582). Assembled rechargeable Zn–air batteries exhibited good discharge potential and charge potential with high stability, respectively. Overall, all results illustrated that (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 is an excellent OER electrocatalyst for zinc–air batteries. Additionally, this work opens a good way to synthesize highly efﬁcient electrocatalysts from A-site deﬁcient perovskites.


Introduction
Rechargeable zinc-air battery has been paid much more attention due to its excellent energy density, safety, and economic costs [1][2][3][4]. OER is restricted due to slow kinetics as well as high overpotential [5]. Until now, IrO 2 and RuO 2 have been regarded as efficient commercial OER electrocatalysts. However, the high cost and scarcity greatly restricted the wide commercialization of rechargeable zinc-air batteries [6]. Hence, it is urgent to develop non-platinum substances to replace RuO 2 and IrO 2 electrocatalysts. Among what has been mentioned above, perovskite electrocatalysts have been considered practicable electrocatalysts because of low cost, high electronic conductivity, and electrocatalytic activity [7][8][9][10][11][12].
There have been many reports regarding tuning A-site deficient perovskites by yielding positive effects on the electrocatalytic activity toward OER [19][20][21][22][23][24][25][26][27][28]. Zhu YL et al. [19] reported that La 0.95 FeO 3-delta (L 0.95 F) demonstrated the highest OER activity due to surface oxygen vacancies, highlighting the importance of cation deficiency in perovskites by enhancing OER activity. Moreover, Wu XY et al. [20] reported A-site deficient BSCF nanofibers (300 nm diameter) prepared by electrospinning and found OER potential of optimized BSCF, which was stable after long-time tests. It was also reported that it had a great potential in the field of aqueous and flexible zinc-air batteries. However, the universality of deficient effects and mechanisms on ABO 3 perovskite were ambiguous and needed to be clarified.
In this paper, A-site deficient (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 perovskites with OER performance under alkaline condition in the application of rechargeable zinc-air batteries were studied in detail. (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 delivered good OER activity and stability. Assembled initial Zn-air battery by (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 exhibited good cycling stability. This work sheds light on a facile method to prepare (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 perovskite electrocatalyst and enhance its potential application of rechargeable zinc-air battery. Figure 1 illustrates the XRD patterns of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 sample sintered at 850 • C for 3 h. The sample showed perovskite structure, which was similar to that of published results regarding Sm 0.5 Sr 0.5 CoO 3 . The inverted triangles in the XRD pattern referred to different crystal planes of perovskite structure [29]. ICP-OES test results can indicate that Co was 25.4290 wt%, Pt was 7.2015 wt%, Sm was 22.2669 wt%, Sr was 19.7917 wt%, and O was 25.3109 wt%. Figure 2a shows SEM images of the catalyst for (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 sample. Nanoparticles were columnar and the average size was around 150-250 nm. TEM images for (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 catalysts are also shown in Figure 2b. The large size was probably due to the sintering and aggregation during heat treatment. As shown in the HADDF images ( Figure 3), homogeneity features were found in (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 , which suggested the distributing of particle sizes in TEM. Sm, Sr, Co, Pt, and O elements were overlapped well and distributed homogeneously within the structure according to the element mapping result. Figure 2c,d demonstrate that the SAED pattern of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 gave Debye-Scherrer rings, confirming the crystalline nature. The d-spacing of 0.297 nm was indexed to the (110) plane of ABO 3 perovskites, as shown in Figure 2c. The selected-area electron diffraction (SAED) pattern of nanoparticles is shown in Figure 2d, imparting a crystalline nature.      The electrocatalytic behavior of perovskites was strongly relied on the valence state of transition metal as well as O anion ordering on catalysts' surface [30][31][32][33]. Figure 4a shows the survey scan of the catalysts for (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 electrocatalysts, indicating the presence of Sm, Sr, Co, and O elements. Moreover, some unknown elements? were also detected, which might be due to the contamination of tests. Deconvoluted Co 2p and Pt 4f, O1s spectra of the electrocatalyst are shown in Figure 4b,c. Upon deconvolution, Co2p 3/2 (about 780.0 eV) and Co2p 1/2 (about 795.2 eV) peaks corresponding to Co 3+ were observed. Besides, O 1s spectra (Figure 4c) was deconvoluted into two peaks [31]. The first one was a highly oxidative oxygen species (530.4 eV for O 2 2− /O − ) and the second one was hydroxyl group or the surface-adsorbed oxygen (531.4 eV for OH − or O 2 ) [34]. From what has been mentioned in the paper, good OER activity in an alkaline solution might be attributed to O 2 2− /O − on the surface of catalysts [17,35]. The electrocatalytic behavior of perovskites was strongly relied on the valence state of transition metal as well as O anion ordering on catalysts' surface [30][31][32][33]. Figure 4a shows the survey scan of the catalysts for (SmSr)0.95Co0.9Pt0.1O3 electrocatalysts, indicating the presence of Sm, Sr, Co, and O elements. Moreover, some unknown elements ? were also detected, which might be due to the contamination of tests. Deconvoluted Co 2p and Pt 4f, O1s spectra of the electrocatalyst are shown in Figure 4b,c. Upon deconvolution, Co2p3/2 (about 780.0 eV) and Co2p1/2 (about 795.2 eV) peaks corresponding to Co 3+ were observed. Besides, O 1s spectra (Figure 4c) was deconvoluted into two peaks [31]. The first one was a highly oxidative oxygen species (530.4 eV for O2 2− /O − ) and the second one was hydroxyl group or the surface-adsorbed oxygen (531.4 eV for OH − or O2) [34]. From what has been mentioned in the paper, good OER activity in an alkaline solution might be attributed to O2 2− /O − on the surface of catalysts [17,35].   Figure 5a shows OER activity of (SmSr)0.95Co0.9Pt0.1O3 electrodes under O2-0.1 M KOH at 5 mV/s at 1600 rpm. (SmSr)0.95Co0.9Pt0.1O3 showed good OER activity with onset potential of ~1.58 V, which was similar to that of BSCF (1.61 V). The maximum current  Figure 5a shows OER activity of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 electrodes under O 2 -0.1 M KOH at 5 mV/s at 1600 rpm. (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 showed good OER activity with onset potential of~1.58 V, which was similar to that of BSCF (1.61 V). The maximum current density measured at 2.0 V was 27 mA cm −2 for (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 , which was similar to that of BSCF (26.7 mA cm −2 ). Table 1 lists the OER activity of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 with BSCF perovskite-based catalysts in 0.1 M KOH. The results showed its great potential as a highly efficient OER electrocatalyst. Nitrogen adsorption/desorption isotherm patterns of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 and BSCF-5582 are also shown in Figure 6.        [14]. Figure 5c further compares the mass activity of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 and BSCF catalysts. Mass activity for (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 at (η = 0.77 V) was 66 A g −1 , while for BSCF was 65 A g −1 . According to the results, the electrochemical activity of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 catalysts was also comparable with that of BSCF. The BET surface area of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 sample is shown in Figure 6. Relatively lower Tafel slope might indicate higher increase of current density, lower overpotential, faster reaction coefficient, which can indicate the relatively higher electrochemical performance.

Cell Performance
The practical applicability of (SmSr)0.95Co0.9Pt0.1O3 electrocatalyst was tested in aqueous rechargeable Zn-air battery. Figure 9 shows the stability of Zn-air battery, which was evaluated by charging for 10 min and discharging for 10 min over repeated cycles at 5 mA cm −2 for nearly 100 h. Initially, it showed the discharge potential and charge potential of 1.10 V and 2.02V. Therefore, it could be calculated that the voltage gap (Δη)

Cell Performance
The practical applicability of (SmSr)0.95Co0.9Pt0.1O3 electrocatalyst was tested in aqueous rechargeable Zn-air battery. Figure 9 shows the stability of Zn-air battery, which was evaluated by charging for 10 min and discharging for 10 min over repeated cycles at 5 mA cm −2 for nearly 100 h. Initially, it showed the discharge potential and charge

Cell Performance
The practical applicability of (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 electrocatalyst was tested in aqueous rechargeable Zn-air battery. Figure 9 shows the stability of Zn-air battery, which was evaluated by charging for 10 min and discharging for 10 min over repeated cycles at 5 mA cm −2 for nearly 100 h. Initially, it showed the discharge potential and charge potential of 1.10 V and 2.02V. Therefore, it could be calculated that the voltage gap (∆η) was 0.92 V. After 100 h, voltage gap increased to 1.0 V, respectively. Such a phenomenon was probably due to irreversible Zn plating-stripping process. It can be seen that (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 -Zn-air battery had potential recharge ability. Moreover, (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 is cheaper than commercial IrO 2 , which shows its high economic potential.  7) were mixed in distilled water with calculated molar ratios of total metal ions/citric acid/EDTA(1:1.5:1). Sol was prepared after mixing and stirring at 80 • C for several hours, and then gel was put into an oven for 12 h at 200 • C to form precursor. Then, the precursor was calcined in air for 3 h at 850 • C. BSCF-5582 powders were prepared as mentioned in our previous paper [37].

Catalysts Characterization
XRD (Bruker, D8 Advances) was used to test the prepared samples' phase in the range of 5-80 • (2θ). N 2 adsorption/desorption isotherms were tested by Micromeritics TriStar II instrument at P/P0 from 0.05 to 0.35. The samples were analyzed by XPS (Thermo Fisher company, ESCALAB 250Xi instrument). SEM from ZEISS was used to characterize the prepared powders' morphology. TEM and EDS were conducted by Titan G2 60-300 microscope.

Electrochemical Tests
(SmSr) 0.95 Co 0.9 Pt 0.1 O 3 (4 mg) was mixed with 1 mL of an ethanol-Nafion mixture (with ethanol:Nafion = 9:1) to form suspension. Pt wire was used as a counter electrode, and Hg/HgO electrode was used as a reference electrode. Suspension (10 µL) was dipped into glassy carbon rotating disk electrode (GC-RDE, 0.196 cm 2 , Pine Research Instrumentation, USA). The catalyst loading was 0.2 mg/cm 2 . CV measurements [36] were tested as mentioned in the procedure, which was the same as our previous published paper [37]. The flow rate of oxygen supply was 10 mL/min. Before starting the electrochemical test, oxygen was flowed for 30 min to fill the reaction tank with saturated oxygen. CV was conducted at a 5 mV/s between −1 and 1 V for 5 cycles to obtain stable data. LSV was conducted at a 5 mV/s between 0 and 1 V. Tafel plots were tested at 1 mV/s with rotating speed of 1600 rpm. Chronopotentiometry was conducted at different current densities.

Battery Assembly and Test
Catalyst inks were prepared as mentioned above and then it was distributed uniformly on the carbon cloth with Ni-foam (current collector). Then, carbon cloth (cathode) and polished zinc plate (anode) were assembled in rechargeable Zn air battery by 6M KOH including 0.2 M ZnCl 2 . Aqueous Zn-air battery tests were carried out by a LAND CT2001A testing device. Charge and discharge data were obtained at 5 mA cm −2 .

Conclusions
In conclusion, A-site efficient (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 oxygen electrocatalysts were successfully prepared by EDTA-citrate complexing sol-gel approach and adopted as a potential OER electrode in Zn-air battery. Optimized (SmSr) 0.95 Co 0.9 Pt 0.1 O 3 electrocatalysts showed a higher OER intrinsic activity and durability, which was comparable with that of BSCF.