A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions: Nano-Co3O4-Deposited La0.5Sr0.5MnO3 via Infiltration

For rechargeable metal–air batteries, which are a promising energy storage device for renewable and sustainable energy technologies, the development of cost-effective electrocatalysts with effective bifunctional activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been a challenging task. To realize highly effective ORR and OER electrocatalysts, we present a hybrid catalyst, Co3O4-infiltrated La0.5Sr0.5MnO3-δ (LSM@Co3O4), synthesized using an electrospray and infiltration technique. This study expands the scope of the infiltration technique by depositing ~18 nm nanoparticles on unprecedented ~70 nm nano-scaffolds. The hybrid LSM@Co3O4 catalyst exhibits high catalytic activities for both ORR and OER (~7 times, ~1.5 times, and ~1.6 times higher than LSM, Co3O4, and IrO2, respectively) in terms of onset potential and limiting current density. Moreover, with the LSM@Co3O4, the number of electrons transferred reaches four, indicating that the catalyst is effective in the reduction reaction of O2 via a direct four-electron pathway. The study demonstrates that hybrid catalysts are a promising approach for oxygen electrocatalysts for renewable and sustainable energy devices.


Introduction
Renewable and sustainable energy storage and conversion systems such as fuel cells, electrolysis cells, and secondary batteries have attracted a great deal of attention in recent years to address the environmental challenges we face today [1][2][3][4]. Specifically, metal-air batteries are considered to be promising systems due to their extremely high theoretical energy density [4][5][6][7][8][9]. Metal-air batteries consist of a pure metal (e.g., Li, Na, Zn, Al) anode and an external cathode of ambient air with an aqueous or aprotic electrolyte. To be suitable for rechargeable batteries, the oxygen electrocatalysts should exhibit effective catalytic activity, not only for the oxygen reduction reaction (ORR), but also for the oxygen evolution reaction (OER)-that is, they should exhibit bifunctionality [10][11][12][13][14][15]. However, due to the sluggish reaction kinetics of the ORR and OER (which respectively correspond to the discharge and charge processes), the development of highly efficient and cost-effective oxygen electrocatalysts is considered to be an essential challenge. Although Pt, IrO 2 , RuO 2 , and their alloys and composites show superior activities for ORR or OER, they suffer from the drawbacks of poor stability, scarcity, and high cost.
Hybrid catalysts composed of more than two materials have been suggested as an approach to secure the desired bifunctionality. For example, Cu-nanoparticle-loaded Co 3 O 4 microspheres, Co 3 O 4 -graphene composite, LaNiO 3 -Pt/C core-corona structure catalyst, and Co 3 O 4 nanocrystals grown on graphene have been demonstrated [8,17,[27][28][29][30][31]. Based on earlier studies, hybrid catalysts (A@B) show enhanced ORR/OER activity compared to the individual catalysts (A or B) and/or their physical mixtures (A + B) due to the synergistic effects between the constituent materials [17,27,32]. Therefore, in this study, a hybrid catalyst consisting of La 0.5 Sr 0.5 MnO 3-δ (LSM) and Co 3 O 4 was designed to achieve high catalytic activities for both ORR and OER.
Nanosizing fabrications were employed to maximize the performances of each constituent material by endowing enlarged reaction sites and shortened ion-/charge-transport path [33][34][35]. Among the various fabrication techniques, electrospray and infiltration techniques have been used due to their simplicity, effectiveness, and reproducibility. In addition, the stoichiometric ratios can be precisely controlled because the fabrication techniques rely on the precursor mixing at the molecular level.
LSM nanoparticles were prepared through electrospray, and Co 3 O 4 was deposited on the LSM surface through infiltration to synthesize the hybrid catalyst (LSM@Co 3 O 4 ). We discovered that 20 wt.% Co 3 O 4 -infiltrated LSM exhibited a comparable catalytic activity to benchmark bifunctional oxygen electrocatalysts. Furthermore, because the calculated number of transferred electrons is four, it indicates that O 2 follows a direct reduction pathway with less thermodynamic reaction potential. Figure 1a illustrates the electrospray fabrication of La 0.5 Sr 0.5 MnO 3-δ (LSM) with the precursor solution including La-, Sr-, and Mn nitrates, PVP, and DMF. A syringe needle tip and aluminum foil collector were connected to the anode and cathode of a high-voltage power supply, respectively. After a heat treatment at 850 • C, X-ray diffraction (XRD) was conducted in the range of 20 • < 2θ < 60 • to identify the crystal structure of the prepared samples. As shown in Figure 1b, LSM was successfully synthesized with a singlephase structure of simple perovskite without any significant impurities. The scanning electron microscopy (SEM) images in Figure 1c,d illustrate that spherical LSM nanoparticles 60-100 nm in size were formed and organically connected.

Structural and Morphological Characterization
Employing the infiltration technique for these nanoparticle scaffolds is a new approach and a unique attempt. Typically, micro-scale pore/particles have been used as the scaffold for infiltration technique and the nanoparticles deposited on the scaffold surface have a size of 60 nm or more. However, the nano-sized LSM scaffold has low interactive force with the precursor solution due to the reduced contact area. To overcome this problem and increase the wettability, ethanol was used as the solvent in this study due to its low boiling point, low surface tension (22.3 mN m −1 at 20 • C), and good solubility of cobalt nitrate. The contact angle of the precursor solution was measured to check the wettability by dropping 3 µL of the precursor solution on the polished LSM pellet. The cobalt precursor solution had a low contact angle of 15.8 • (inset in Figure 2a), which indicates good wettability, and the use of ethanol facilitated the deposition of precursors on the surface of LSM nanoparticles. Table 1 summarizes the abbreviations for the samples.

Abbreviations Chemical Composition
LSM@Co3O4-20 20 wt.% Co3O4-infiltrated LSM LSM@Co3O4- 25 25 wt.% Co3O4-infiltrated LSM To obtain an effective Co3O4 electrocatalyst, the infiltrated powder was sintered at 600 °C and the sintered powder is denoted as LSM@Co3O4. The XRD pattern of LSM@Co3O4 (Figure 2b) shows the phase of LSM and Co3O4 without any detectable impurity phases, indicating the successful synthesis of Co3O4. The broad peak due to Co3O4 can be attributed to the nano size of the deposited particles. The high-magnification SEM images of LSM@Co3O4 in Figure 2c,d confirm that the smaller nanoparticles (~18 nm) of Co3O4 were uniformly deposited on the entire LSM nano-scaffold (~70 nm). Furthermore, LSM@Co3O4 retained the organically connected microstructures, which is effective for the transfer of electrons.

Abbreviations
Chemical LSM@Co3O4-20 20 wt.% Co3O4-infiltrated LSM LSM@Co3O4- 25 25 wt.% Co3O4-infiltrated LSM To obtain an effective Co3O4 electrocatalyst, the infiltrated powder was sintered at 600 °C and the sintered powder is denoted as LSM@Co3O4. The XRD pattern of LSM@Co3O4 ( Figure 2b) shows the phase of LSM and Co3O4 without any detectable impurity phases, indicating the successful synthesis of Co3O4. The broad peak due to Co3O4 can be attributed to the nano size of the deposited particles. The high-magnification SEM images of LSM@Co3O4 in Figure 2c,d confirm that the smaller nanoparticles (~18 nm) of Co3O4 were uniformly deposited on the entire LSM nano-scaffold (~70 nm). Furthermore, LSM@Co3O4 retained the organically connected microstructures, which is effective for the transfer of electrons.    (Figure 2b) shows the phase of LSM and Co 3 O 4 without any detectable impurity phases, indicating the successful synthesis of Co 3 O 4 . The broad peak due to Co 3 O 4 can be attributed to the nano size of the deposited particles. The high-magnification SEM images of LSM@Co 3 O 4 in Figure 2c,d confirm that the smaller nanoparticles (~18 nm) of Co 3 O 4 were uniformly deposited on the entire LSM nano-scaffold (~70 nm). Further-more, LSM@Co 3 O 4 retained the organically connected microstructures, which is effective for the transfer of electrons.

Rotating Ring-Disk Electrode (RRDE) Test
With different loading amounts of Co 3 (Figure 3a). LSM has a nano-sized microstructure, but its OER current density showed a low value of 1.85 mA cm −2 at 1.70 V vs.
reversible hydrogen electrode (RHE). After conducting Co 3 O 4 infiltration, the current density was significantly enhanced to 13.01 mA cm

Four-Electron Pathway
As shown in Figure 4a,b, the electrochemical reduction of O2 can occur via two pathways, either a direct 4-electron pathway or a 2+2-electron pathway with peroxide intermediates. For oxygen electrocatalysis, the 4-electron pathway is most desirable because the direct pathway requires less thermodynamic reaction potential ( Table 2). The 2+2-elec-

Four-Electron Pathway
As shown in Figure 4a,b, the electrochemical reduction of O 2 can occur via two pathways, either a direct 4-electron pathway or a 2+2-electron pathway with peroxide intermediates. For oxygen electrocatalysis, the 4-electron pathway is most desirable because the direct pathway requires less thermodynamic reaction potential ( Table 2). The 2+2-electron pathway is preferred in the industry for H 2 O 2 production rather than for ORR catalysis. where Id represents the disk current, Ir indicates the ring current, and N is the current collection efficiency of the Pt ring. N was experimentally determined to be 0.41 from the reduction of K3Fe[CN]6. The measured H2O2 yield was below 3% for LSM, LSM@Co3O4-20, and LSM@Co3O4-25, which is comparable to that for Pt/C and much lower than that for Co3O4 (Figure 4c). The number of electrons transferred for LSM@Co3O4-20 was calculated to be 3.96-4.00 (Figure 4d), which is higher than that for LSM (3.90-3.96) and LSM@Co3O4-25 (3.90-3.97). These results confirm that LSM, LSM@Co3O4-20, and LSM@Co3O4-25 followed a 4-electron reduction pathway rather than the 2+2-electron pathway. Especially, LSM@Co3O4-20 formed a lower quantity of peroxides than Pt/C, allowing the ORR to proceed through an almost ideal 4-electron reduction pathway.

X-Ray Photoelectron Spectroscopy (XPS) Anaylsis
Based on the RRDE results, LSM@Co3O4 exhibited good catalytic activity for both ORR and OER, comparable to the benchmark catalysts (Pt/C, Co3O4, and IrO2). Since the catalytic activities of LSM@Co3O4-20 and LSM@Co3O4-25 were similar and the n-value of LSM@Co3O4-20 was closer to four, we selected LSM@Co3O4-20 as a representative sample of LSM@Co3O4 for X-ray photoelectron spectroscopy (XPS) characterization.
XPS was performed to analyze the surface electronic state of the catalysts (i.e., LSM, Co3O4, and LSM@Co3O4-20) for O 1s, Co 2p, and Mn 2p. The XPS results were calibrated with the binding energy (BE) of the C 1s peak at 284.3 eV. In Figure 5a-c, the O 1s spectrum can be deconvoluted into three main peaks of lattice oxygen (Olattice, red), surface-adsorbed oxygen species (Oad, green), and adsorbed molecular water (H2Oad, blue). The relative contents of the oxygen species were calculated from the relative area of the three peaks, and   (Figure 4c). The number of electrons transferred for LSM@Co 3 O 4 -20 was calculated to be 3.96-4.00 (Figure 4d), which is higher than that for LSM (3.90-3.96) and LSM@Co 3 O 4 -25 (3.90-3.97). These results confirm that LSM, LSM@Co 3 O 4 -20, and LSM@Co 3 O 4 -25 followed a 4-electron reduction pathway rather than the 2+2-electron pathway. Especially, LSM@Co 3 O 4 -20 formed a lower quantity of peroxides than Pt/C, allowing the ORR to proceed through an almost ideal 4-electron reduction pathway.  Figure 5a-c, the O 1s spectrum can be deconvoluted into three main peaks of lattice oxygen (O lattice , red), surface-adsorbed oxygen species (O ad , green), and adsorbed molecular water (H 2 O ad , blue). The relative contents of the oxygen species were calculated from the relative area of the three peaks, and are listed in Table 3 According to previous studies, Co 3+ cations at the surface play a significant role in the OER due to their unique electronic state, favorable as both an electron donor and an electron acceptor for O 2 and electron capturing. The Co 3+ /Co 2+ ratio was 0.45 for Co 3 O 4 and 0.68 for LSM@Co 3 O 4 -20, respectively. The highly concentrated Co 3+ of LSM@Co 3 O 4 -20 indicates the high number of donor-acceptor reduction sites, supporting the enhanced OER catalytic activity. As presented in Figure 5f,g, the Mn 3+ /Mn 4+ ratio was reduced from 0.83 for LSM to 0.77 for LSM@Co 3 O 4 -20, indicating ligand effects between LSM and the infiltrated Co 3 O 4 layer. The ligand effect, a type of interfacial effect that leads to synergistic effects on the catalytic activity, refers to the electron transfer derived by the different electronic configurations of two adjacent materials. It has good agreement with the XPS results for Co 2p, in which LSM@Co 3 O 4 -20 had a higher Co 3+ concentration compared to Co 3 O 4 . Based on these results, we speculate that the electrons in the LSM layer were transferred to the infiltrated Co 3 O 4 layer by ligand effects, resulting in an electron-rich state on the surface. Correspondingly, the bond strength between the metal oxide and oxygen species at the surface of an electrocatalyst can be altered and the catalytic activity for ORR and OER can be tuned.

X-ray Photoelectron Spectroscopy (XPS) Anaylsis
sults for Co 2p, in which LSM@Co3O4-20 had a higher Co 3+ concentration compared to Co3O4. Based on these results, we speculate that the electrons in the LSM layer were transferred to the infiltrated Co3O4 layer by ligand effects, resulting in an electron-rich state on the surface. Correspondingly, the bond strength between the metal oxide and oxygen species at the surface of an electrocatalyst can be altered and the catalytic activity for ORR and OER can be tuned.

Conclusions
In this study, we presented a hybrid catalyst as a bifunctional catalyst for both ORR and OER. To synthesize the hybrid catalyst, 18 nm Co 3 O 4 nanoparticles were uniformly deposited over the entire surface of a~70 nm LSM nano-scaffold through simple electrospray and infiltration. The hybrid catalysts exhibited comparable OER and ORR performances compared to the benchmarked commercial catalysts. In particular, the hybrid catalysts followed a four-electron pathway, meaning a very effective path to the ORR. Due to differences in the electronic configurations of the ions in Co 3 O 4 and LSM, the concentration of Co 3+ and electron-rich phases were increased in the hybrid catalyst, contributing to the enhancement in the catalytic activity of ORR and OER.

Materials and Methods
La 0.5 Sr 0.5 MnO 3-δ nanoparticles were synthesized via electrospray using a precursor solution consisting of La(NO 3 ) 3 ·6H 2 O (99+%, Sigma Aldrich Co.), Sr(NO 3 ) 2 (99+%, Sigma Aldrich Co.), Mn(NO 3 ) 2 (98%, Sigma Aldrich Co.), poly-vinylpyrrolidone (PVP, avg. molecular weight~1,300,000 by LS, Sigma-Aldrich Co.), and N,N-dimethylformamide (DMF, Alfa Aesar Co.). The precursor solution was prepared by mixing the metal nitrates in DMF solvent with continuous stirring. After adding the PVP to the resulting solution, it was stirred for 12 h at 60 • C. The as-prepared LSM/PVP solution was filled into a plastic syringe with a capillary tip (D = 0.5 mm), electrically connected to a high-voltage power supply from Korea switching Co. The needle tip and aluminum collector were connected to the anode and cathode of the power supply, respectively, and were placed horizontally (Figure 2a). The applied voltage was 15 kV, and the distance between the tip and aluminum foil collector was 14 cm. The collected precursor was peeled off and heated at 850 • C for 4 h at a rate of 2 • C min −1 in air.
For the infiltration process, the precursor solution was prepared by dissolving Co(NO 3 ) 2 ·6H 2 O (98+ %, Sigma Aldrich Co.) in ethanol, which was penetrated into the LSM powder (prepared via electrospray). The resultant product was calcinated at 450 • C for 15 min in air. These penetration-calcination steps were repeated until the loading amount reached 20 and 25 wt.%. The loading amounts of Co 3 O 4 was chosen with the consideration of interfacial effects, investigated in a previous study [12]. The samples with 20 and 25 wt.% Co 3 O 4 are designated hereafter as LSM@Co 3 O 4 -20 and LSM@Co 3 O 4 -25, respectively. The infiltrated LSM was sintered at 600 • C for 4 h at a ramping rate of 2 • C min −1 in ambient air to obtain Co 3 O 4 . The loading amount was calculated using Equation (3): Loading amount of Co 3 O 4 (wt.%) = (W LSM@Co3O4 − W LSM )/ W LSM × 100 where W LSM is the weight of the LSM before infiltration and W LSM@Co3O4 is the weight of LSM@Co 3 O 4 after sintering at 600 • C. X-ray diffraction (XRD, Bruker D8 Advance) was performed in the range of 20 • < 2θ < 60 • for structural analyses. Scanning electron microscopy (SEM) (Nova SEM) was conducted to observe the microstructures of LSM and Co 3 O 4 @LSM. X-ray photoelectron spectroscopy (XPS) data were acquired with ESCALAB 250XI (Thermo Fisher Scientific) with a monochromated Al-Kα (ultraviolet He1, He2) X-ray source. To evaluate the electrocatalytic activity for ORR and OER, rotating ring-disk electrode (RRDE) experiments were performed with the prepared ink composed of 20 mg of sample, 0.9 mL of solvent (ethanol: isopropyl alcohol = 1:1), and 0.1 mL of 5 wt.% Nafion solution (Aldrich, 274704). The well-dispersed catalyst ink (5 mL) was spread on the pre-polished glassy carbon (GC) disk electrode (0.1256 cm 2 ). The electrochemical performances with an RRDE-3A rotating disk electrode system were recorded with a Biologic VMP3. A Pt wire, Hg/HgO electrode filled with 1 M NaOH, and 0.1 M KOH solution saturated with oxygen were respectively used as the counter electrode, reference electrode, and electrolyte. To estimate the amount of peroxide from the disk electrode during the ORR, +0.4 V was applied to the ring electrode.
To convert the potential values from vs. Hg/HgO to vs. the reversible hydrogen electrode (RHE), the difference was measured in a cell using Pt wires for the working and counter electrodes under H 2 -saturated electrolyte 0.1 M KOH with an Hg/HgO reference electrode. The open-circuit potential was −0.89 V at 1 mV s -1 and the relationship between Hg/HgO and RHE was represented as: Funding: Support from the Welch Foundation, grant F-1254, is gratefully acknowledged.
Institutional Review Board Statement: Not applicable.