One-Dimensional La0.2Sr0.8Cu0.4Co0.6O3−δ Nanostructures for Efficient Oxygen Evolution Reaction

Producing oxygen and hydrogen via the electrolysis of water has the advantages of a simple operation, high efficiency, and environmental friendliness, making it the most promising hydrogen production method. In this study, La0.2Sr0.8Cu0.4Co0.6O3−δ (LSCC) nanofibers were prepared by electrospinning to utilize non-noble perovskite oxides instead of noble metal catalysts for the oxygen evolution reaction, and the performance and electrochemical properties of LSCC nanofibers synthesized at different firing temperatures were evaluated. In an alkaline environment (pH = 14, 6 M KOH), the nanofibers calcined at 650 °C showed an overpotential of 209 mV at a current density of 10 mA cm−2 as well as good long-term stability. Therefore, the prepared LSCC-650 NF catalyst shows excellent potential for electrocatalytic oxygen evolution.


Introduction
Metal-air batteries and fuel cells have been widely investigated to address global environmental and energy issues [1][2][3][4][5].However, while these energy conversion and storage systems are environmentally friendly and have high energy density, their commercialization has been limited by the slow kinetics of the oxygen evolution reaction (OER) [6][7][8].The oxygen evolution reaction is a semi-anodic reaction that occurs during water electrolysis.OER involves a four-electron transfer process, which is characterized by its high energy requirements, slow chemical kinetics, and complex mechanisms.The OER process involves a series of sequential reactions in an alkaline/neutral electrolyte: first, OH − or H 2 O adsorbed on an active site is oxidized with an electron to form OH ad .Second, the removal of a proton and electron from OH ad forms O ad .Finally, O ad is transformed by one of two potential pathways to form a detached O 2 molecule.In one pathway, two O ad directly combine to form O 2 .In the second pathway, O ad reacts with H 2 O or OH − to form the intermediate product OOH ad , which combines with H 2 O or OH − to form O 2 (Figure S1) [9][10][11].Numerous research studies have focused on exploring synthetic materials for the preparation of suitable OER catalysts with the goal of improving the performance of water electrolysis electrodes.Two crucial factors that significantly contribute to improved electrode performance are stability and drivability.Within the realm of electrochemistry, catalysts play an indispensable role in facilitating desired chemical reactions at the surface of the electrode.Catalysts have the ability to increase reaction rates, reduce energy barriers, and enhance the efficiency of electrochemical processes.Consequently, the development of catalysts that are both efficient and stable holds immense significance.
Many synthetic materials such as RuO 2 and IrO 2 have been evaluated as OER catalysts due to their exceptional performance across various pH environments [12][13][14].However, noble metal-based catalysts have the disadvantages of resource scarcity and a high cost.Therefore, a growing body of research has focused on the evaluation of a diverse range of non-noble metal oxides as catalysts.In particular, perovskite materials with ABO 3 structures, spinels with A'B' 2 O 4 structures, and composite oxide materials with layered structures all exhibit excellent performance as electrocatalysts [15][16][17][18][19][20][21][22][23][24][25][26][27][28].In addition to metal oxides, metal sulfides composed of sulfur groups, non-oxide catalysts, and metal coordination compounds have also shown excellent promise in the field of electrocatalysis [29][30][31][32][33][34][35][36][37].These materials offer high catalytic activity, good electrical conductivity, and excellent chemical stability, which are essential factors for improving the performance of electrodes.Transition metal-based oxides have the advantages of a low cost, an easy synthesis, and good environmental protection.In addition, these oxides are good candidates for electrocatalytic OER because they are highly stable in alkaline solutions and have good electrical conductivity.Moreover, because transition metal oxidation states are relatively active and these oxides have relatively free coordination environments, various oxide catalysts can be obtained through rich combinations of transition metals, including oxides with perovskite structures.Thus, metal oxide catalysts exhibit a wide range of properties and performance.Perovskite-based electrocatalysts are considered to be highly promising for OER due to their excellent chemical, physical, and catalytic properties (such as high ionic conductivity and rapid oxidation exchange).Moreover, these materials are environmentally friendly and have low-cost synthesis procedures.For example, La 0.6 Sr 0.4 CoO 3 , LaNiO 3 , and La 1−x Ce x NiO 3 have been evaluated as OER electrocatalysts [38][39][40].
Conventional strategies such as sol-gel, hydrothermal, co-precipitation, and electrospinning methods can be used to synthesize perovskite oxide nanomaterials [41][42][43][44][45][46][47][48][49][50][51][52].Compared with other methods, electrospun nanofibers show great potential in OER applications due to their excellent inherent properties and obvious advantages.For instance, electro spun nanofibers have unique physical and chemical properties, including ultra-long length, good electrical conductivity, excellent stability, and good mechanical strength.Moreover, electrospinning can be used to build a very rich pore structure, which can promote rapid mass and electron transfer [53].The electrocatalytic performance of electrospun nanofibers can be improved by the addition of active substances during the electrospinning process to produce a layered structure [49,54].Due to the controllability and unique structures of these nanofibers, electrocatalytic activity can be further enhanced by other strategies such as heteroatom doping and surface modification [55,56].Finally, electrospun nanofibers can be converted into carbon nanofibers (CNFs) or non-carbon nanofibers as well as derivatives rich in nanoparticles by appropriately controlling the atmosphere and annealing temperature.This can also significantly enhance electrocatalytic performance.Because of these advantages, electrospun nanomaterials have been widely evaluated as OER electrocatalysts [57][58][59].One main focus in the field of OER electrocatalysts is the synthesis of nanomaterials with large specific surface areas.This is because oxygen molecules are mainly adsorbed and deionized on the surface and interface of the catalyst during the OER reaction.Thus, high-surface-area and easily modified electrospun nanofibers show excellent promise as OER electrocatalysts.
In this study, La 0.2 Sr 0.8 Cu 0.4 Co 0.6 O 3−δ (LSCC) nanofibers (NFs) with a perovskite structure and large specific surface area were synthesized by electrospinning.The morphology and electrocatalytic performance of the LSCC NFs calcined at different temperatures (500-800 • C) were studied.LSCC-650 NFs, which were obtained at a calcination temperature of 650 • C, show a lower overpotential and Tafel slope than other perovskite materials (Table S1).

Synthesis of PAN/LSCC Nanofibers
A 0.2 g amount of 2-methylimidazole and 0.2 g of PAN powder were added to 4 mL DMF.This solution was stirred for 12 h at room temperature using a magnetic stirrer to ensure even mixing.Next, 86.60 mg of lanthanum nitrate, 169.31 mg of strontium nitrate, 174.62 mg of cobalt nitrate, and 96.64 mg of copper nitrate pentahydrate were mixed in a molar ratio of 0.2:0.8:0.4:0.6 and dispersed in the DMF solution.This mixed precursor solution was loaded into a syringe for electrospinning.The electrospinning process can be divided into three main steps: charge generation, charge acceleration, and fiber curing.First, a high voltage is generated by a charge-generating device, such as a high-voltage generator.The high voltage ionizes the molecules in the material, releasing positive and negative charges.The charged material is then ejected through a nozzle while being subjected to an electric field.The positive and negative charges are accelerated in an electric field, forming slender fibers.Finally, these fibers solidify rapidly in the air to form fiber structures with specific properties.After 12 h of high-pressure spraying, we can obtain a thick circular fiber overlap on the receiver.Electrospinning was performed with a working voltage of 6 kV, an acquisition distance of 15 cm, and a humidity of 20-30%.

Materials Characterization
The crystal structure of prepared materials was determined by X-ray diffraction (XRD, Rigaku D/max2600, Tokyo, Japan).The atomic structure of catalysts was observed using transmission electron microscopy (TEM, FEI, Tecnai TF20, Tokyo, Japan).The surface chemical properties of the composite were determined by X-ray photoelectron spectroscopy (XPS, Kratos, AXIS SUPRA+, Tokyo, Japan).The morphologies of the samples were observed by scanning electron microscopy (SEM, SU70, Hitachi, Tokyo, Japan).Energydispersive X-ray (EDX, QUANTAX, Bruker, Beijing, China) spectroscopy in conjunction with scanning electron microscopy was used to analyze the composition of the samples.

Electrochemical Measurements
Functional electrodes were prepared using 15 mg of LSCC, PVDF, and acetylene black in a mass ratio of 8:1:1.PVDF was used as the binder, while acetylene black was used as the conductor.This mixture was added to a small amount of N-methyl pyrrolidone solvent (NMP), and the resulting slurry was vigorously stirred and mixed, and subsequently ultrasonicated for 2 h to obtain a uniform solution.After ultrasonication, the mixture was evenly coated on carbon paper (1 × 1 cm 2 ), which was then dried under a vacuum at 50 • C for 12 h.The resulting sample contained an active catalyst loading of about 4 mg cm −2 .
Electrochemical tests were performed in a standardized VMP-300 electrochemical apparatus using a three-electrode system.The reaction environment for all electrochemical studies was 6 M KOH.A platinum plate was used as the counter electrode, an Ag/AgCl electrode was used as the reference electrode, and the catalyst sample was used as the working electrode.All experimental results were evaluated in terms of the reversible hydrogen electrode (RHE): E RHE = E SCE + 0.059 pH + 0.196 V (pH = 14), overpotential: in a standardized VMP-300 electrochemical he reaction environment for all electrochemias used as the counter electrode, an Ag/AgCl de, and the catalyst sample was used as the were evaluated in terms of the reversible hy-H + 0.196 V (pH = 14), overpotential: ŋ = ERHE med for 50 cycles.Good activity and accurate ate of 50 mV s −1 .Linear sweep voltammetry mV s −1 .The overpotential (η), the Tafel slope ing the equation η = b log (j) + a. Electrochemrmed at a voltage of 500 mV in the frequency raday current regions were evaluated by CV ECSA).The double-layer capacitance of each = E RHE − 1.23 V. Cyclic voltammetry (CV) was performed for 50 cycles.Good activity and accurate CV curves were obtained with a scanning rate of 50 mV s −1 .Linear sweep voltammetry (LSV) was performed at a scanning rate of 5 mV s −1 .The overpotential (η), the Tafel slope (b), and current density (j) were evaluated using the equation η = b log (j) + a. Electrochemical impedance spectroscopy (EIS) was performed at a voltage of 500 mV in the frequency range from 0.01 Hz to 100 kHz.The non-Faraday current regions were evaluated by CV to obtain the electrochemical surface area (ECSA).The double-layer capacitance of each sample was determined by obtaining a CV curve at a special scanning rate.Scanning rates of from 20 to 100 mV s −1 with an interval of 20 mV s −1 were evaluated in this study.Electrocatalytic stability testing was performed at a current density of 10 mA cm −2 , and stability was evaluated by chronopotentiometry after 48 h.Cyclic stability was evaluated by conducting 5000 consecutive cyclic tests at 100 mV s −1 .All potentials were compensated and corrected with 95% IR.

Results and Discussion
The steps used to prepare the LSCC multi-particle nanochain structure are shown in Scheme 1. First, nanofibers were prepared by electrospinning.The electrospun nanofibers were then annealed in air (500-800 • C) to obtain LSCC NFs.The formation of nanofibers was promoted in the strong oxidizing environment during calcination.The nanofiber structure consisted of well-dispersed nanoparticles forming a smooth surface.The diameter and shape of the LSCC nanofibers calcined at 500-800 • C were evaluated by SEM and TEM, as shown in Figure 1.Low-and high-magnification SEM images of LSCC-650 NFs (prepared by calcining at 650 • C) are shown in Figure 1a,b, respectively.A TEM image showing the microstructure of LSCC-650 NFs is displayed in Figure 1c.The LSCC-650 NF sample is composed of nanoparticles and shows many pores, which are marked by red circles.This high porosity provides a larger surface area for the reaction.The presence of pores on the nanofiber wall is due to the release of decomposition gases during calcination.The high-resolution TEM image shown in Figure 1d shows a lattice stripe spacing of approximately 0.283 and 0.304 nm, corresponding to the (110) and (211) lattice planes of LSCC-650 NF, respectively.After calcination at different temperatures, varying morphologies were obtained (Figure 2a-f).SEM images show that the obtained nanofibers generally exhibit diameters in the range of 160-220 nm, and the diameter slightly decreases with increasing calcination temperature from 500 to 800 • C.Moreover, the surface particle size increases with increasing calcination temperature.Calcination at temperatures higher or lower than 650 • C still results in the formation of some small pores, but these pores are not as obvious as those on the LSCC-650 NF surface.As a result, residual carbon forms and impedes particle growth, causing the LSCC NFs prepared at other calcination temperatures to have a more compact appearance.EDX analysis was used to confirm the presence of carbon residue in the samples (Supporting Information).As shown in Figures S2-S8, the carbon content of the samples decreases with increasing calcination temperature.
After calcination at 450 • C, LSCC-500 NFs had a carbon content of 81.02%.However, when the calcination temperature was raised to 650 • C, the carbon content of LSCC-650 NFs decreased to 19.70%.This suggests that reducing the carbon content is conducive to enhancing the catalytic performance of the perovskite component of the samples, leading to improved OER activity.The surface chemical properties of LSCC-650 NFs were evaluated by XPS.The survey spectrum of LSCC-650 NFs is shown in Figure 4a, and the high-resolution La 3d, Sr 3d, Co 2p, Cu 2p, and O 1s spectra are shown in Figure 4b-f.As displayed in Figure 4b, the La 3d spectrum exhibits four prominent bands.The peaks at 832.5 and 835.8 eV correspond to La 3d 5/2 , while those at 849.0 and 852.5 eV correspond to La 3d 3/2 .These bands reveal that the La in LSCC-650 NFs is mostly in a metallic state [62][63][64].As shown in Figure 4c, the Sr 3d spectrum shows two peaks at 132.2 eV and 135.5 eV corresponding to the Sr-O bond (Sr-O 3d 5/2 and Sr-O 3d 3/2 , respectively).Moreover, a peak at 133.8 eV corresponds to Sr 2+ [65,66].The Cu 2p spectrum shown in Figure 4d shows three main peaks.The peaks at 935.1 and 954.5 eV correspond to Cu + , while the peak at 963.6 eV is assigned to Cu 2+ [67].The deconvoluted Co 2p spectrum (Figure 4e) displays four prominent bands and two satellite peaks.The main peaks at 780.0 and 796.8 eV are ascribed to Co 2+ , while the peaks at 778.   .These results demonstrate that the OER performance of LSCC NFs declines as the calcination temperature is raised above 650 • C.This is because the size and specific surface area of the perovskite crystals increases with increasing calcination temperature, removing the advantageous properties of the nanoparticles.LSCC-650 NF show intact nanofibers and a well-retained structure after the reaction, which is one of the reasons for its high OER performance (Figure S9).Thus, due to its hierarchical structure, the OER activity of the prepared LSCC-650 NFs compares favorably with other reported perovskite OER electrocatalysts (Table S1).The C dl values of the prepared catalysts were calculated to evaluate their ECSAs.The CV curves of the LSCC NFs were obtained at different scanning rates, and the activity of the bilayer capacitor was calculated.The scanning potential was set to 0-0.1 V vs. RHE.As shown in Figure 5d, LSCC-650 NFs (1.48 mF cm −2 ) are superior compared to the other samples, proving that the LSCC-650 NF electrode has the most OER active sites.The long-term OER stability of LSCC-650 NFs was evaluated in a cycling test, as shown in Figure 5e.After every 1000 cycles, OER activity only slightly declined.After 5000 cycles, a decline of just 0.86% can be observed, confirming the excellent OER stability of this catalyst.This excellent stability demonstrates that, even after electrolysis, the LSCC-650 NF electrocatalyst can still maintain its nanofiber structure and electrocatalytic properties.Therefore, LSCC-650 NFs are suitable as a functional OER electrocatalyst.

Conclusions
In summary, LSCC NFs were prepared by a simple and easily scalable electrospinning technique followed by calcination.The obtained LSCC NFs showed good OER electrochemical performance, with LSCC-650 NFs exhibiting a low overpotential of just 209 mV at 100 mA cm −2 .After 48 h of cycling (5000 cycles), only a slight change in overpotential was observed, demonstrating the good electrochemical stability of LSCC-650 NFs.Overall, the prepared catalysts exhibit good durability and performance due to their nanofiber structure.Moreover, these catalysts were synthesized using low-cost non-noble metals.Therefore, they show excellent promise for use as OER electrodes in water-splitting applications.In future work, the LSCC content of the catalysts will be enhanced to further improve their electrochemical performance.

Scheme 1 .
Scheme 1. Schematic illustration of the synthesis processes of LSCC NFs.

Figure 3 .
Figure 3.The XRD patterns of the LSCC nanofibers produced in different proportions.

Figure 5 .
Figure 5. Electrocatalytic OER performance of the LSCC nanofibers produced in different temperatures.(a) LSV curves of the prepared electrodes in 6.0 M KOH.(b) For comparison, the corresponding overpotential of the synthesized catalysts realized a current density of 10 mA cm −2 .(c) Comparison of the Tafel plots of synthesized catalysts.(d) The capacitive current plots as a function of the scan rate.(e) LSV curves of LSCC-650 NFs before and after 5000 cycles.(f) EIS pattern of the LSCC produced in different proportions researched at a fixed potential of 0.5 V (vs.SCE).