Oxygen Deficient LaMn0.75Co0.25O3−δ Nanofibers as an Efficient

10 hours ago - The rational design of efficient and durable oxygen evolution reaction (OER) is important for energy conversion and storage devices. He...
0 downloads 0 Views 3MB Size
Download PDF
Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Oxygen Deficient LaMn0.75Co0.25O3−δ Nanofibers as an Efficient Electrocatalyst for Oxygen Evolution Reaction and Zinc−Air Batteries Juanjuan Bian,†,‡ Zhipeng Li,§ Nianwu Li,†,‡ and Chunwen Sun*,†,‡ †

CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China ‡ School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China § School of Materials Science and Technology Beijing, Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing 100083, P. R. China

Downloaded by BETHEL UNIV at 14:14:20:518 on June 05, 2019 from https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b01034.

S Supporting Information *

ABSTRACT: The rational design of efficient and durable oxygen evolution reaction (OER) is important for energy conversion and storage devices. Here, we develop a two-step calcination method to prepare cobalt nanoparticles uniformly dispersed on perovskite oxide nanofibers and to tune oxygen vacancies in perovskite LaMn0.75Co0.25O3−δ nanofibers. The obtained product shows enhanced activity toward OER. In particular, the oxygen deficient LMCO-2 catalyst prepared by a two-step calcination shows excellent OER performance that is 27.5 times that of the LMO catalyst and is comparable to that of the commercial RuO2 catalyst. It also demonstates good stability because of its novel structure, abundant oxygen vacancies, and larger number of metal ions with a high oxidation state. As an air electrode for a flexible zinc−air battery, the cell with the LMCO-2 catalyst delivers a higher power density of 35 mW cm−2 and excellent cycling stability for 70 h. Moreover, the cell exhibits excellent flexibility under different bending conditions. overpotential and enhance the catalytic activities.12 It is worth noting that doping at the B site with reducible transition metal cations and subsequent exsolution of reducible cations can produce an oxygen vacancy.14 Recent results indicate that metal or alloy nanoparticles can be exsolved from perovskite oxides by annealing under a reducing atmosphere, and these nanoparticles show high catalytic activity for SOFCs.15−19 However, there have been few reports of the effects of the exsolved nanoparticles from oxides on the OER properties. Perovskite LaMnO3 shows outstanding catalytic activity toward ORR, which is ascribed to the polyvalent Mn ions that can provide a moderate interaction between the surface of the catalyst and intermediate species, but LaMnO3 exhibits poor catalytic activity for OER.1,20 Here, we propose a two-step calcination method to prepare uniformly dispersed nanoparticles pinned on oxide nanofibers and to tune oxygen vacancies and change the cation configuration in perovskite LaMn0.75Co0.25O3−δ nanofibers. To improve the activity toward OER, we first doped the reducible Co at the B site of perovskite LaMnO3 and subsequently reduced the perovskite under 5% H2−Ar to enable Co nanoparticles to segregate on the surface of perovskite nanofibers and generate more oxygen

1. INTRODUCTION Zinc−air batteries (ZABs) that have a high theoretical energy density and are inexpensive have become one of the most promising energy storage technologies. However, the catalysts used in ZABs often exhibit sluggish kinetic processes, which limit their practical applications.1−3 The precious metal electrocatalysts have high catalytic performance but are limited due to the prohibitive costs.4,5 Therefore, it is highly desirable to develop efficient and cost-effective non-noble electrocatalysts. Low-cost transition metal oxides have been widely studied as catalysts for various applications due to their tunable properties by means of oxygen vacancies and various valence states of transition metals. Previous studies have shown that oxygen vacancies can effectively modulate the electronic structures and catalytic performance of transition metal oxides.6,7 Perovskite-type oxides (ABO3) with unique crystal structures, novel electronic structures, and abundant defects are one kind of efficient and important material for solid oxide fuel cells (SOFCs), water splitting, and rechargeable metal−air battery applications.7−11 Nevertheless, perovskite oxides commonly show large initial overpotentials as catalysts for oxygen evolution reaction (OER).12,13 Doping at the A or B site can greatly modulate the chemical and electronic properties, which can lead to a decrease in the OER © XXXX American Chemical Society

Received: April 9, 2019

A

DOI: 10.1021/acs.inorgchem.9b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

iR correction. EiR‑corrected = E − iR, where i is the current and R is the ohmic resistance of the electrolyte solution. 2.4. Flexible Zinc−Air Battery Tests. Typically, a polished zinc foil was used as the anode and the electrolyte was composed of PVA (98−99% hydrolyzed, medium molecular weight) gel and an 18 M KOH solution with a volume ratio of 1:1. The air cathode was made by coating the mixture ink consisting of the catalyst, Super P, and a 5% Nafion solution (mass ratio of 3:6:1) on a nickel foam with a loading of 1 mg cm−2 of the mixture. The other side of the nickel foam was coated with an air diffusion layer consisting of activated carbon and PTFE. The gel flexible ZAB was tested at 2 mA cm−2 for long-term cycling performance, and the stability of ZAB was tested at 2, 5, and 10 mA cm−2.

vacancies simultaneously. Then, we recalcined the product mentioned above under an Ar atmosphere to produce more oxygen vacancies by decreasing the oxygen pressure.21 Via the doping of 25 mol % Co in LaMnO3, the OER performance of LaMn 0.75 Co 0.25 O 3 is greatly enhanced. LaMnO 3 and LaMn0.75Co0.25O3 are hereafter termed LMO and LMCO, respectively. Sample LaMn0.75Co0.25O3 annealed at 830 °C under a reducing H2/Ar atmosphere is denoted as LMCO-1, while the sample recalcined at 700 °C under an Ar atmosphere is denoted as LMCO-2. X-ray photoelectron spectroscopy (XPS) results indicate that the polyvalent metal ions were generated and oxygen vacancies formed in LMCO-1. In addition, more oxygen vacancies and high TM oxidation states formed in LMCO-2 NFs, which is confirmed by electron energy loss spectroscopy (EELS). As an air electrode for a flexible zinc−air battery, the cell with an LMCO-2 catalyst delivers a power density of 35 mW cm−2 with excellent cycling stability for 70 h. The cell also exhibits excellent flexibility under different bending conditions.

3. RESULTS AND DISCUSSION X-ray diffraction (XRD) was used to determine the crystallinity of the products. As shown in Figure 1a, the patterns of LMO

2. EXPERIMENTAL SECTION 2.1. Nanofiber Synthesis. In a typical synthesis, La(NO3)3·6H2O (0.3248 g, 99.9%), Mn(NO3)2·6H2O (0.1883 g, 99%), and poly(vinylpolypyrrolidone) (0.5 g, MW 1,300,000 powder) were added to N,N-dimethylformanide (3 mL, 99%) while being vigorously magnetically stirred for 2 h until a homogeneous solution was formed. Then, the obtained solution was loaded into a syringe. The feed rate of the solution was kept at 0.2 mL h−1. The distance between the collector and the needle tip was 20 cm, and the applied voltage was fixed to 20.0 kV. The as-spun nanofibers were dried at 80 °C for 6 h and then calcined at 900 °C for 2 h. The preparation of LMCO NFs was similar to that of LMO NFs except that 0.1883 g of Mn(NO3)2· 6H2O was replaced with 0.1412 g of Mn(NO3)2·6H2O and 0.0545 g of Co(NO3)2·6H2O (99.9%). The synthesis of LNMO-1 NFs was based on the synthesized products mentioned above (LMCO). The LNMO NFs were reduced at 830 °C under a 5% H2/Ar atmosphere for 10 min to produce LMCO-1 NFs. Then LMCO-1 NFs were recalcined at 700 °C for 3 h under an Ar atmosphere to obtain LMCO-2 NFs. 2.2. Characterization. Powder X-ray diffraction (XRD) analysis was characterized on a PANalytical X’Pert powder diffractometer with Cu Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA) in a 2θ range of 10− 80°. The morphology was characterized by transmission electron microscopy (TEM) (Tecnai G2 F30) and field-emission scanning electron microscopy (FESEM) (SU8020). Raman spectra were used to examine oxygen structure information by employing a LABRAM HR Raman spectrometer with Ar+ (532 nm) laser excitation. XPS was performed on a Thermo Scientific Escalab 250Xi instrument with Mg Kα radiation. The XPS data were fitted with XPSPEAK software. EELS was performed on a Tecnai G2 F30 instrument. The L3/L2 intensity ratios were extracted from EELS by integrating the remaining intensity in the given window width.22 The Brunauer− Emmett−Teller (BET) test was performed on an ASAP 2460 instrument. Thermogravimetric (TG) analysis was performed on a MEITTLER TGA/DSC 1 SF/1382 instrument from room temperature to 830 °C under a 5% H2/Ar atmosphere and 700 °C under an Ar atmosphere for LMCO-1 and LMCO-2 samples, respectively. 2.3. Electrode Preparation and Electrochemical Measurements. Electrocatalyst inks were prepared by dispersing 2.25 mg of catalyst/Super P (99%+) (1:1, mass ratio) in 1 mL of alcohol (A.R.) and 0.5 mL of 0.5 wt % Nafion solution by sonication treatment. Then, 20 μL of ink was dropped on a polished glassy-carbon (GC) rotating disk electrode. The loading of the catalysts is 0.153 mg cm−2. The electrochemical measurements were performed in a 0.1 M KOH electrolyte with a three-electrode configuration to test the OER and ORR performances of the catalysts. All of the curves were recorded by

Figure 1. (a) XRD patterns of LMO NFs, LMCO NFs, LMCO-1 NFs, and LMCO-2 NFs. The marked diffraction peaks at 44° in LMCO-1 and LMCO-2 NFs are assigned to metallic Co. (b) Raman spectra of LMCO-1 NFs and LMCO-2 NFs. (c) SEM image of an LMCO NF. (d) SEM image of an LMCO-2 NF.

NFs and LMCO NFs exhibit a hexagonal structure in space group R3m. The main peaks at 22.88°, 32.39°, 39.98°, 46.67°, and 57.91° of the LMO NF and LMCO NF samples can be indexed to the (101), (110), (021), (202), and (122) reflections, respectively, of the LaMnO3 phase. For LMCO-1 NFs, the phase structrue is well maintained except that all of the main peaks shift to smaller angles, indicating larger lattice parameters caused by oxygen loss. The marked diffraction peak at 44° in LMCO-1 NFs is attributed to metallic Co,23 which indicates that in situ exsolution of Co nanoparticles occurs on the surface of nanofibers after reduction at 830 °C under a 5% H2/Ar atmosphere. After recalcination in Ar at 700 °C for 3 h, the peaks further shift to smaller angles, which means more oxygen vacancies form. Furthermore, the lattice parameters of these samples were calculated, as shown in Table S1. It is obvious that LMCO-2 possesses a higher c/a ratio compared with that of LMCO-1, which means the volume increases.24 Moreover, the 25−28° peaks of the LMCO-1 sample in the XRD pattern are attributed to a small amount of Co−La alloy, which was generated due to the reaction of the surfaceB

DOI: 10.1021/acs.inorgchem.9b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

HRTEM image of an LMCO-1 NF. The marked interplanar spacing in Figure 2b is ∼0.290 nm, which corresponds to the (011) lattice planes of LMCO-1 NFs. Panels c−g of Figure 2 show the mappings of La, Mn, Co, and O elements. The results confirm that the segregated small nanoparticles are Co nanoparticles, which are uniformly distributed on the surface of an LMCO-1 NF (Figure 2f). Figure S2 shows the corresponding elemental mappings of La, Mn, Co, and O of the unreduced LMCO NF. The La, Mn, Co, and O elements show a homogeneous distribution. After being annealed at 700 °C, the Co nanoparticles still exist, as shown in Figure 2h. Figure 2i shows the HRTEM image of an LMCO-2 NF. The marked interplanar spacing in Figure 2i is ∼0.415 nm, which corresponds to the (102) lattice planes of an LMCO-2 NF. Figure S3 shows a TEM image of LMCO NFs. The surface of the nanofiber is smooth without nanoparticle segregation as observed in Figure 1c. All of this experimental evidence indicates that Co nanoparticles are exsolved from the perovskite after reduction in a 5% H2/Ar atmosphere. The Brunauer−Emmett−Teller (BET) surface areas of LMCO-1 NFs and LMCO-2 NFs are 7.3314 and 9.5047 m2 g−1, respectively. The pore volumes of LMCO-1 NFs and LMCO-2 NFs are 0.0119 and 0.0159 cm3 g−1, respectively. To determine the optimal doping amount of Co in LMO and the annealing condition in an Ar atmosphere, we evaluate the OER activities of various samples prepared under different conditions, which are shown in Figure S4. Cyclic voltammetry (CV) measurements were performed first in a N2-saturated 0.1 M KOH solution to activate the catalysts before OER testing. Then liner sweep voltammograms (LSVs) were tested in O2saturated 0.1 M KOH. Figure S4a shows LSV curves of the LMCO NF catalysts with different Co doping amounts tested at 1600 rpm for OER performance characterization. Figure S4b shows LSV curves of the LMCO-2 catalysts prepared under different annealing conditions. It can be seen that the optimal doping content of Co is 25% and the prominent annealing condition is at 700 °C for 3 h. In Figure 3a, the overpotential of the LMCO-2 NFs catalyst is greatly reduced at 10 mA cm−2

exsolved Co nanoparticles with the LaMnO3 nanofibers. These weak peaks become weaker in LMCO-2 compared with those of LMCO-1. In addition, Raman spectra were further used to verify the formation of an oxygen vacancy, as shown in Figure 1b. The peaks at around 490 and 610 cm−1 are ascribed to the oxygen vacancies.21,25,26 It can be seen that the LMCO-2 NF sample has more oxygen vacancies after recalcination, which are beneficial for the enhancement of OER performance. Panels c and d of Figure 1 show scanning electron microscopy (SEM) images of LMCO NF and LMCO-2 NF, respectively. Small nanoparticles were observed on the surface of LMCO-2 NF, while the surface of LMCO NF is smooth. Compared with the LMCO-1 nanofiber (Figure S1), the LMCO-2 NF shows a slight shrinkage in diameter of ∼300 nm and the segregated Co nanoparticles have an average size of ∼50 nm. The characteristics of one-dimensional (1D) nanofibers with particles on their surface play an important role in enhancing the OER properties because the 1D structure is favorable for the transport of electron.27,28 Transmission electron microscopy (TEM) was used to further confirm the exsolution of Co nanoparticles in LMCO-1 NFs and characterize the microstructure of the obtained products. From Figure 2a, the exsolution of the Co nanoparticle is clearly observed. Figure 2b shows the

Figure 3. (a) LSV curves of the various catalysts at 1600 rpm for OER performance. (b) Tafel plots of the OER curves in panel a. (c) Double-layer capacitance (CDL) of LMCO, LMCO-1, and LMCO-2 catalysts. (d) Chronamperometric response at a constant potential of 1.68 V (vs RHE) vs time of the LMCO-2 catalyst.

Figure 2. (a) TEM image of an LMCO-1 NF. (b) HRTEM image of an LMCO-1 NF. (c−g) Elemental mappings of La, Mn, Co, and O of an LMCO-1 NF. (h) TEM image of an LMCO-2 NF. (i) HRTEM image of an LMCO-2 NF. C

DOI: 10.1021/acs.inorgchem.9b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry compared with those of other catalysts, and the onset potential of the LMCO-2 NFs catalyst is the lowest among all of the samples, which can be attributed to more oxygen vacancies and higher metal oxidation states in the LMCO-2 NFs catalyst. The oxygen vacancies could enhance electron and oxygen ion transport and thus reduce the OER overpotential and enhance the OER dynamics.6,12 From Table S2, one can see that the activities of LMCO NFs, LMCO-1 NFs, and LMCO-2 NFs catalysts are increased by 1.1-, 4.4-, and 27.5-fold, respectively, compared to that of the LMO NFs catalyst. Figure S5 shows a comparison of the OER performance of commercial RuO2 and LMCO-2 catalysts, which indicates that the LMCO-2 catalyst possesses excellent OER performance comparable to that of commercial RuO2. The Tafel slope of the LMCO-1 catalyst is smaller than those of LMCO and LMO catalysts, which means that its dynamics property is enhanced after reduction. In comparison, the LMCO-2 catalyst possesses the lowest Tafel slope, which is related to the abundant oxygen vacancies (Figure 3b). In addition, electrocatalytic activity is also related to the electrochemically active surface (ECSA) of the catalysts. The OER efficiency can be improved by increasing the ECSA. The ECSA is calculated from the double-layer capacitance (CDL). Figure 3c shows that the CDL of the LMCO-2 catalyst is larger than those of LMCO and LMCO-1 catalysts. The ECSAs of various catalysts are shown in Figures S6−S8. The LMCO-2 catalyst exhibits the largest ECSA of 7.20 cm−2, which means the LMCO-2 catalyst has more electrochemical active sites and is favorable for improving the OER performance. Furthermore, we normalized the OER performance of the catalysts by the Brunauer−Emmett−Teller (BET) surface areas and ECSA. As shown in Figure S9, it can be seen that the LMCO-2 catalyst possesses the optimal OER performance in two cases, which confirms the enhanced OER performance did indeed result from the larger number of oxygen vacancies in the LMCO-2 catalyst. We also tested the stability of the LMCO-2 catalyst using a chronoamperometry measurement at a constant potential of 1.68 V (vs RHE), which is shown in Figure 3d. The current density shows little decay for 30000 s, indicating that the LMCO-2 catalyst has an excellent stability. It may be attributed to the stable structure because the exsolved Co nanoparticles are pinned into the oxide surface, which may prevent the oxide nanoparticle from coarsening during testing.14 The stabilities of LMCO and LMCO-1 catalysts are shown in Figure S10. The current density of the LMCO catalyst is reduced to 82% of the initial value after it had been tested for 24000 s, while the current density of the LMCO-1 catalyst decays slightly, which is due to the stable structure of the LMCO-1 catalyst as mentioned above. We also tested the ORR performance of these catalysts, as shown in Figure S11. The ORR performance of the LMCO-2 catalyst is close to that of the LMO catalyst and better than that of the LMCO-1 catalyst. Figure S12 shows the stabilities of LMCO-2, LMCO-1, and LMCO catalysts determined by the accelerated durability test (ADT). LMCO-2 and LMCO-1 catalysts show no decay after 3000 cycles, while the LMCO catalyst shows a slight deterioration. Therefore, one can conclude that the oxygen deficient LMCO-2 catalyst possesses outstanding OER and ORR performances compared to those of other catalysts. The surface compositions and oxidation state information about ions of the LMCO-1 sample are studied by XPS. Figure 4a shows the survey XPS spectrum of the LMCO-1 sample,

Figure 4. XPS spectra of the LMCO-1 sample: (a) survey spectrum, (b) Mn 2p3/2 spectrum, (c) Co 2p3/2 spectrum, and (d) O 1s spectrum.

which includes La, Co, Mn, and O peaks. In Figure 4b, the XPS Mn 2p3/2 spectrum splits into three peaks of Mn2+ (640.7 eV), Mn3+ (641.9 eV), and Mn4+ (643.4 eV),29,30 indicating that polyvalent Mn ions coexist after annealing and the Mn ions with high oxidation states are beneficial for OER. As shown in Figure 4c, the Co 2p3/2 spectrum could be deconvoluted into two peaks located at 778.3 and 780.1 eV corresponding to metallic Co and Co2+, respectively.23,31 Figure 4c indicates that metallic Co indeed emerges after reduction in a H2/Ar atmosphere, which is favorable for electron transfer and thus beneficial for OER.32 Apart from the effect of metal ions on OER properties, the electrocatalytic activity is also related with the active oxygen species.23,33 Figure 4d shows that various oxygen species exist in the LMCO-1 sample with two typical peaks of O1 at 529.5 eV and O2 at 531.5 eV. The O1 peak is typically ascribed to transition metal−oxygen bonds, whereas the O2 peak is assigned to oxygen defect sites with low oxygen coordination, further demonstrating the existence of oxygen vacancies.34−36 The increased level of hybridization of TM−O bonds with the transition metal oxidation state increasing can improve the OER performance.37 The oxygen vacancy sites can also facilitate the transfer of electrons and accelerate the desorption of oxygen.31 From these results, we can conclude that both polyvalent metal ions generated and oxygen vacancies formed in the LMCO-1 sample are beneficial for OER. To clarify the reason for the enhanced OER performance of LMCO-2 compared with that of the LMCO-1 sample, EELS was further used to gain insight into the local electronic state and coordination environment of metal ions in LMCO-2. We acquired the low-loss spectra of transition metal (TM) O-K and TM-L2,3 edges of LMCO-1 and LMCO-2 samples because these edges are related to metal−oxygen bonding and the information about the oxidation state and symmetry of the coordination environment of 3d TM ions.37 As shown in Figure 5a, the intensity of the prepeak of O-K decreases, indicating the formation of oxygen vacancies and an increase in the TM oxidation state.22,38 This phenomenon can be explained by the lattice oxygen in LMCO-1 being released to form O2, thus generating oxygen vacancies.39 In addition, from D

DOI: 10.1021/acs.inorgchem.9b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

LMCO-2 catalyst. The flexible battery exhibits stable cycling performance at 2 mA cm−2 for 70 h with little change in potential. Figure 6b shows discharging polarization curves of the flexible zinc−air batteries with LMCO-1 and LMCO-2, respectively. The battery with the LMCO-2 catalyst can run up to 70 mA cm−2, which is higher than that of the battery with the LMCO-1 catalyst, and delivers a higher power density of 35 mW cm−2. We further studied its stability at different current densities, as shown in Figure 6c. The cell can still work steadily without obvious polarization. We also tested the flexibility of a zinc−air battery under different bending conditions. As shown in Figure 6d, the charge and discharge curves show no obvious polarization under repeated bending between 90° and 0°, indicating that the zinc−air batteries have a good flexibility performance, which demonstrated that they can be used in many folding situations. The performances of a flexible battery with the LMCO-1 cathode are shown in Figure S14. Figure S14a shows discharging and charging polarization curves of the flexible ZABs with the LMCO-1 catalyst. Figure S14b shows the long-term cycling performance of the flexible ZABs with the LMCO-1 catalyst. One can see that the flexible ZAB with the LMCO-1 cathode can also cycle steadily for 70 h with little voltage change at 2 mA cm−2. In addition, the ZAB with the LMCO-1 cathode also exhibits good flexibility, as shown in panels c and d of Figure S14. The favorable cycling performances of ZABs with the LMCO-1 cathode and LMCO2 cathode are attributed to the excellent OER performance and good stability of these catalysts. Table S3 shows a comparison of the cycling performance of flexible ZABs with various catalysts. One can see that this flexible ZAB with the LMCO-2 catalyst shows excellent cycling performance and less polarization, which is attributed to the excellent electrochemical performance of the air electrode.

Figure 5. Representative EELS spectra of LMCO-1 and LMCO-2 samples. (a) EELS spectra of the O-K edge. (b) Overall EELS spectra of Mn-L and Co-L edges.

Figure 5b, Co-L edges show an obvious shift and the Co L3/L2 ratio decreases, which indicates that the valence of Co increases in LMCO-2, while the L3/L2 ratio of Mn increases, which suggests that the valence of Mn decreases. A high TM oxidation state can reduce the gap between oxygen 2p and transition metal 3d states, thus enhancing the covalency of TM−O bonds.37 From Co-L2,3 and O-K edges, we can conclude that the oxidation state of Co increases and the Co− O covalency changes. High-valence Co ions have a low-energy 3d state, which indicates an enhancement of Co−O covalency. Therefore, the increased Co−O covalency in LMCO-2 leads to an OER activity that is higher than that of LMCO-1.40 Both oxygen vacancies and high-valence transition metal ions incorporated into metal oxides are favorable for enhancement of the OER performance,41 and oxygen vacancies and high TM oxidation states in LMCO-2 NFs can significantly enhance the OER performance. Furthermore, we analyzed quantitatively the oxygen vacancies through the thermogravimetric (TG) analysis test. Figure S13 shows TG results for LMCO-1 and LMCO-2 samples. One can see that the molar amounts of oxygen vacancies in LMCO-1 and LMCO-2 catalysts are calculated to be 0.105 and 0.205, respectively. The feasibility of LMCO-2 NFs as a cathode catalyst for flexible zinc−air batteries was examined. Figure 6a displays the long-term cycling performance of a zinc−air battery with the

4. CONCLUSIONS In summary, we developed a method to boost the OER performance of perovskite oxide catalysts by preparing uniformly dispersed Co nanoparticles on oxide nanofibers and tuning oxygen vacancies in perovskite LaMn0.75Co0.25O3−δ nanofibers. The obtained oxygen deficient LMCO-2 NFs catalyst possesses a high OER performance due to the highly active Co nanoparticles, unique nanostructure, and abundant oxygen vacancies. The LMCO-2 catalyst possesses an excellent OER performance, which is ∼27.5 times that of the LMO catalyst and is comparable to that of the commercial RuO2 catalyst. As an air electrode for a flexible zinc−air battery, the cell with the LMCO-2 catalyst delivers a power density of 35 mW cm−2 with excellent cycling stability for 70 h. The cell also exhibits an excellent flexibility under different bending conditions. This work provides an effective strategy for improving the electrocatalytic activity of perovskite oxides.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01034.

Figure 6. (a) Long-term cycling performance of the flexible ZABs with the LMCO-2 catalyst. (b) Discharging polarization curves of the flexible ZABs with the LMCO-1 and LMCO-2 catalysts. (c) Cycling performance of the flexible ZABs with the LMCO-2 catalyst at different current densities. (d) Discharge/charge cycling curves of the flexible ZABs with the LMCO-2 cathode under different folded conditions between 0° and 90°.

Lattice parameters of the four samples and a comparison of the current densities for four catalysts, SEM images, STEM image and elemental mappings, TEM images, electrochemical results, TG results, and performance of flexible zinc−air batteries with LMCO-1 catalysts (PDF) E

DOI: 10.1021/acs.inorgchem.9b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

■

(13) Zhu, Y. P.; Chen, G.; Zhong, Y. J.; Chen, Y. B.; Ma, N. N.; Zhou, W.; Shao, Z. P. A Surface-Modified Antiperovskite as an Electrocatalyst for Water Oxidation. Nat. Commun. 2018, 9, 2326. (14) Zhu, T. L.; Troiani, H. E.; Mogni, L. V.; Han, M. F.; Barnett, S. A. Ni-Substituted Sr(Ti, Fe)O3 SOFC Anodes: Achieving High Performance via Metal Alloy Nanoparticle Exsolution. Joule 2018, 2, 478. (15) Du, Z. H.; Zhao, H. L.; Yi, S.; Xia, Q.; Gong, Y.; Zhang, Y.; Cheng, X.; Li, Y.; Gu, L.; Swierczek, K. High Performance Anode Material Sr2FeMo0.65Ni0.35O6‑δ with in Situ Exsolved Nanoparticle Catalyst. ACS Nano 2016, 10, 8660−8669. (16) Sun, Y. F.; Li, J. H.; Cui, L.; Hua, B.; Cui, S. H.; Li, J.; Luo, J. L. A-Site Deficiency Facilitated in Situ Growth of Bimetallic Ni−Fe Nano-Alloys: a Novel Coking-Tolerant Fuel Cell Anode Catalyst. Nanoscale 2015, 7, 11173−11181. (17) Li, Y. F.; Zhang, W. Q.; Zheng, Y.; Chen, J.; Yu, B.; Chen, Y.; Liu, M. L. Controlling Cation Segregation in Perovskite Based Electrodes for High Electrocatalytic Activity and Durability. Chem. Soc. Rev. 2017, 46, 6345−6378. (18) Neagu, D.; Tsekouras, G.; Miller, D. N.; Ménard, H.; Irvine, J. T. S. In Situ Growth of Nanoparticles through Control of NonStoichiometry. Nat. Chem. 2013, 5, 916−923. (19) Ma, Z. H.; Sun, C. W.; Ma, C.; Wu, H.; Zhan, Z. L.; Chen, L. Q. A Highly Active Ni-doped La0.6Sr0.4FeO3‑δ Symmetrical Electrode for Hydrocarbon-Fueled Solid Oxide Fuel Cells. Chin. J. Catal. 2016, 37, 1347−1353. (20) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Design Principles for OxygenReduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal−Air Batteries. Nat. Chem. 2011, 3, 546−550. (21) Zhao, R. Q.; Jin, K. J.; Xu, Z. T.; Guo, H. H.; Wang, L.; Ge, C.; Lu, H. B.; Yang, G. Z. The Oxygen Vacancy Effect on the Magnetic Property of the LaMnO3‑δ Thin Films. Appl. Phys. Lett. 2013, 102, 122402. (22) Tan, H. Y.; Verbeeck, J.; Abakumov, A.; Van Tendeloo, G. Oxidation State and Chemical Shift Investigation in Transition Metal Oxides by EELS. Ultramicroscopy 2012, 116, 24−33. (23) Sun, Y. F.; Li, J. H.; Zhang, Y. Q.; Hua, B.; Luo, J. L. Bifunctional Catalyst of Core−Shell Nanoparticles Socketed on Oxygen-Deficient Layered Perovskite for Soot Combustion: In Situ Observation of Synergistic Dual Active Sites. ACS Catal. 2016, 6, 2710−2714. (24) Fell, C. R.; Qian, D.; Carroll, K. J.; Chi, Mi. F.; Jones, J. L.; Meng, Y. S. Correlation Between Oxygen Vacancy, Microstrain, and Cation Distribution in Lithium-Excess Layered Oxides During the First Electrochemical Cycle. Chem. Mater. 2013, 25, 1621−1629. (25) Dubey, A.; Sathe, V. G.; Rawat, R. Signature of Jahn−Teller Distortion and Oxygen Stoichiometry in Raman Spectra of Epitaxial LaMnO3+δ Thin Films. J. Appl. Phys. 2008, 104, 113530. (26) Guo, M.; Lu, J. Q.; Wu, Y. N.; Wang, Y. J.; Luo, M. F. UV and Visible Raman Studies of Oxygen Vacancies in Rare-Earth-Doped Ceria. Langmuir 2011, 27, 3872−3877. (27) Xu, J. J.; Xu, D.; Wang, Z. L.; Wang, H. G.; Zhang, L. L.; Zhang, X. B. Synthesis of Perovskite-Based Porous La0.75Sr0.25MnO3 Nanotubes as a Highly Efficient Electrocatalyst for Rechargeable LithiumOxygen Batteries. Angew. Chem., Int. Ed. 2013, 52, 3887−3890. (28) Zhang, X. L.; Gong, Y. D.; Li, S. H.; Sun, C. W. Porous Perovskite La0.6Sr0.4Co0.8Mn0.2O3 Nanofibers Loaded with RuO2 Nanosheets as an Efficient and Durable Bifunctional Catalyst for Rechargeable Li-O2 Batteries. ACS Catal. 2017, 7, 7737−7747. (29) Tang, T.; Jiang, W. J.; Niu, S.; Liu, N.; Luo, H.; Chen, Y. Y.; Jin, S. F.; Gao, F.; Wan, L. J.; Hu, J. S. Electronic and Morphological Dual Modulation of Cobalt Carbonate Hydroxides by Mn Doping toward Highly Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. J. Am. Chem. Soc. 2017, 139, 8320−8328. (30) Sun, Y. F.; Zhang, Y. Q.; Chen, J.; Li, J. H.; Zhu, Y. T.; Zeng, Y. M.; Amirkhiz, B. S.; Li, J.; Hua, B.; Luo, J. L. New Opportunity for in Situ Exsolution of Metallic Nanoparticles on Perovskite Parent. Nano Lett. 2016, 16, 5303−5309.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-82854648. Fax: +86-10-82854648. Email: [email protected]. ORCID

Nianwu Li: 0000-0001-9679-7699 Chunwen Sun: 0000-0002-3610-9396 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (51672029, 51372271, 51602025, and 21703010), the National Key R&D Project from the Ministry of Science and Technology, China (2016YFA0202702), and the Fundamental Research Funds for Central Universities (FRF-TP-17-043A1).

■

REFERENCES

(1) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385. (2) Li, Y.; Dai, H. J. Recent Advances in Zinc-Air Batteries. Chem. Soc. Rev. 2014, 43, 5257−5275. (3) Sun, C. W.; Li, F.; Ma, C.; Wang, Y.; Ren, Y.; Yang, W.; Ma, Z.; Li, J.; Chen, Y.; Kim, Y.; Chen, L. Q. Graphene−Co3O4 Nanocomposite as an Efficient Bifunctional Catalyst for Lithium−Air Batteries. J. Mater. Chem. A 2014, 2, 7188−7196. (4) Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martinez, J.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3, 1159−1165. (5) Fabbri, E.; Habereder, A.; Waltar, K.; Kötz, R.; Schmidt, T. J. Developments and Perspectives of Oxide-Based Catalysts for the Oxygen Evolution Reaction. Catal. Sci. Technol. 2014, 4, 3800−3821. (6) Liu, D. L.; Wang, C. H.; Yu, Y. F.; Zhao, B. H.; Wang, W. C.; Du, Y. H.; Zhang, B. Understanding the Nature of Ammonia Treatment to Synthesize Oxygen Vacancy-Enriched Transition Metal Oxides. Chem 2019, 5, 376−389. (7) Tong, Y.; Guo, Y. Q.; Chen, P. Z.; Liu, H. F.; Zhang, M. X.; Zhang, L. D.; Yan, W. S.; Chu, W. S.; Wu, C. Z.; Xie, Y. Spin-State Regulation of Perovskite Cobaltite to Realize Enhanced Oxygen Evolution Activity. Chem 2017, 3, 812−821. (8) Jin, C.; Cao, X. C.; Zhang, L. Y.; Zhang, C.; Yang, R. Z. Preparation and Electrochemical Properties of Urchin-Like La0.8Sr0.2MnO3 Perovskite Oxide as a Bifunctional Catalyst for Oxygen Reduction and Oxygen Evolution Reaction. J. Power Sources 2013, 241, 225−230. (9) Yan, L. T.; Lin, Y.; Yu, X.; Xu, W. C.; Salas, T.; Smallidge, H.; Zhou, M.; Luo, H. M. La0.8Sr0.2MnO3-Based Perovskite Nanoparticles with the A-Site Deficiency as High Performance Bifunctional Oxygen Catalyst in Alkaline Solution. ACS Appl. Mater. Interfaces 2017, 9, 23820−23827. (10) Sun, C. W.; Hui, R.; Roller, J. Cathode Materials for Solid Oxide Fuel Cells: A review. J. Solid State Electrochem. 2010, 14, 1125− 1144. (11) Bian, J. J.; Su, R.; Yao, Y.; Wang, J.; Zhou, J. G.; Li, F.; Wang, Z. L.; Sun, C. W. Mg Doped Perovskite LaNiO3 Nanofibers as Efficient Bi-Functional Catalyst for Rechargeable Zinc-air Batteries. ACS Appl. Energy Mater. 2019, 2, 923−931. (12) Kim, J.; Yin, X.; Tsao, K. C.; Fang, S. H.; Yang, H. Ca2Mn2O5 as Oxygen-Deficient Perovskite Electrocatalyst for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 14646−14649. F

DOI: 10.1021/acs.inorgchem.9b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (31) Fu, Y.; Yu, H. Y.; Jiang, C.; Zhang, T. H.; Zhan, R.; Li, X. W.; Li, J. F.; Tian, J. H.; Yang, R. Z. NiCo Alloy Nanoparticles Decorated on N-Doped Carbon Nanofibers as Highly Active and Durable Oxygen Electrocatalyst. Adv. Funct. Mater. 2018, 28, 1705094. (32) Zheng, X.; Zhang, B.; De Luna, P.; Liang, Y.; Comin, R.; Voznyy, O.; Han, L.; García de Arquer, F.; Liu, M.; Dinh, C.; Regier, T.; Dynes, J.; He, S.; Xin, L.; Peng, H.; Prendergast, D.; Du, X.; Sargent, E. Theory-Driven Design of High-Valence Metal Sites for Water Oxidation Confirmed Using in Situ Soft X-ray Absorption. Nat. Chem. 2018, 10, 149−154. (33) Tong, Y.; Wu, J. C.; Chen, P. Z.; Liu, H. F.; Chu, W. S.; Wu, C.Z.; Xie, Y. Vibronic Superexchange in Double Perovskite Electrocatalyst for Efficient Electrocatalytic Oxygen Evolution. J. Am. Chem. Soc. 2018, 140, 11165−11169. (34) Choudhury, T.; Saied, S. O.; Sullivan, J. L.; Abbot, A. M. Reduction of Oxides of Iron, Cobalt, Titanium and Niobium by LowEnergy Ion Bombardment. J. Phys. D: Appl. Phys. 1989, 22, 1185− 1195. (35) Jimenez, V. M.; Fernandez, A.; Espinos, J. P.; Gonzalez-Elipe, A. R. The State of the Oxygen at the Surface of Polycrystalline Cobalt Oxide. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 61−71. (36) Su, Y. Z.; Xu, Q. Z.; Chen, G. F.; Cheng, H.; Li, N.; Liu, Z. Q. One dimensionally spinel NiCo2O4 nanowire arrays: facile synthesis, water oxidation, and magnetic properties. Electrochim. Acta 2015, 174, 1216−1224. (37) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y. L.; Risch, M.; Hong, W. T.; Zhou, J. G.; Shao-Horn, Y. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun. 2013, 4, 2439. (38) Yan, P. F.; Zheng, J. M.; Zhang, J. G.; Wang, C. M. Atomic Resolution Structural and Chemical Imaging Revealing the Sequential Migration of Ni, Co, and Mn upon the Battery Cycling of Layered Cathode. Nano Lett. 2017, 17, 3946−3951. (39) Lei, F. C.; Sun, Y. F.; Liu, K. T.; Gao, S.; Liang, L.; Pan, B. C.; Xie, Y. Oxygen Vacancies Confined in Ultrathin Indium Oxide Porous Sheets for Promoted Visible-Light Water Splitting. J. Am. Chem. Soc. 2014, 136, 6826−6829. (40) Zhou, Y.; Sun, S. N.; Song, J. J.; Xi, S. B.; Chen, B.; Du, Y. H.; Fisher, A. C.; Cheng, F. Y.; Wang, X.; Zhang, H.; Xu, Z. C. Enlarged Co-O Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction. Adv. Mater. 2018, 30, 1802912. (41) Zhu, Y.; Zhou, W.; Zhong, Y.; Bu, Y.; Chen, X.; Zhong, Q.; Liu, M.; Shao, Z. A Perovskite Nanorod as Bifunctional Electrocatalyst for Overall Water Splitting. Adv. Energy Mater. 2017, 7, 1602122.

G

DOI: 10.1021/acs.inorgchem.9b01034 Inorg. Chem. XXXX, XXX, XXX−XXX