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

Abstract

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, La_0.2Sr_0.8Cu_0.4Co_0.6O_3−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.

1. 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 H₂O 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 O₂ molecule. In one pathway, two O_ad directly combine to form O₂. In the second pathway, O_ad reacts with H₂O or OH⁻ to form the intermediate product OOH_ad, which combines with H₂O or OH⁻ to form O₂ (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 RuO₂ and IrO₂ 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 ABO₃ structures, spinels with A’B’₂O₄ 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_0.6Sr_0.4CoO₃, LaNiO₃, and La_1−xCe_xNiO₃ 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_0.2Sr_0.8Cu_0.4Co_0.6O_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).

2. Experimental

2.1. Chemical Reagents

Sigma-Aldrich provides polyacrylonitrile (PAN). Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), copper nitrate pentahydrate (Cu(NO₃)₂·5H₂O), strontium nitrate (Sr(NO₃)₂), lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), N-dimethylformamide (DMF), and potassium hydroxide (KOH) were purchased from Zhiyuan Reagent (Tianjin, China). 2-Methylimidazole were provided from Aladdin (Beijing, China). Analytical grade chemicals that have not been further purified were used in this study.

2.2. 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%.

2.3. Synthesis of LSCC Nanofibers

The synthesized PAN/LSCC nanofibers were annealed in the furnace at the temperatures of 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, and 800 °C for 2 h with a heating rate of 1 °C min⁻¹ under air atmosphere, respectively. Correspondingly, the obtained products were denoted as LSCC-500 NFs, LSCC-550 NFs, LSCC-600 NFs, LSCC-650 NFs, LSCC-700 NFs, LSCC-750 NFs, and LSCC-800 NFs.

2.4. 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). Energy-dispersive X-ray (EDX, QUANTAX, Bruker, Beijing, China) spectroscopy in conjunction with scanning electron microscopy was used to analyze the composition of the samples.

2.5. 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 cm²), 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: ŋ = 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.

3. 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.

XRD patterns of LSCC-500 NFs, LSCC-550 NFs, LSCC-600 NFs, LSCC-650 NFs, LSCC-700 NFs, LSCC-750 NFs, and LSCC-500 NFs are shown in Figure 3. All diffraction peaks are consistent with those of La_0.2Sr_0.8Cu_0.4Co_0.6O_3−δ perovskite-type oxides (JCPDS No. 50-527). These diffraction patterns indicate that the LSCC crystal structure is not as well-formed at low calcination temperatures. Moreover, LSCC-750 NFs (calcined at 750 °C) also show the presence of impurities that are absent from the pure LSCC phase. These results indicate that the calcination temperature for preparing LSCC nanofibers should be lower than 750 °C. Increasing the calcination temperature to 750 °C leads to a corresponding increase in diffraction peak intensity. These XRD patterns show that the perovskite grain size increases with increasing calcination temperature. According to some research reports, smaller crystals contain more oxygen vacancies, leading to better OER performance [60,61]. As demonstrated by its XRD pattern, LSCC-650 NFs consist of a pure LSCC phase with a small grain size. Thus, LSCC-650 NFs are expected to have the highest oxygen vacancy concentration and the best OER performance.

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 Sr²⁺ [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 Cu²⁺ [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 Co²⁺, while the peaks at 778.3 and 794.2 eV are ascribed to Co³⁺ [68,69,70]. The O 1s spectrum shown in Figure 4f contains four characteristic peaks at 528.2, 529.4, 532.1, and 533.6 eV corresponding to lattice oxygen (O²⁻), adsorbed oxidative oxygen species (O⁻/O²⁻₂), adsorbed molecular oxygen and hydroxyl groups (O₂/OH⁻), and adsorbed water molecules (H₂O). Among them, O⁻/O²⁻₂ are active sites for OER in alkaline solutions, and the presence of these sites has a positive impact on oxygen transport [71,72,73].

The electrochemical OER activity of the prepared catalysts was measured in 6.0 M KOH. The polarization curves of these nanofibers are shown in Figure 5a. At a specific current density of j = 10 mA cm⁻², LSCC-650 NFs exhibit the lowest overpotential of 209 mV. This performance is superior to that of LSCC-500 NFs (217 mV), LSCC-550 NFs (260 mV), LSCC-600 NFs (214 mV), LSCC-700 NFs (242 mV), LSCC-750 NFs (303 mV), and LSCC-800 NFs (375 mV), as shown in Figure 5b. The reaction kinetic rates of these samples were also expressed by their Tafel slopes, as shown in Figure 5c. LSCC-650 NF show the lowest Tafel slope (48.7 mV dec⁻¹) compared to LSCC-500 NFs (68.1 mV dec⁻¹), LSCC-550 NFs (65.5 mV dec⁻¹), LSCC-600 NFs (67.7 mV dec⁻¹), LSCC-700 NFs (70.1 mV dec⁻¹), LSCC-750 NFs (121.4 mV dec⁻¹), and LSCC-800 NFs (132.4 mV dec⁻¹). 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.

4. 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.

Author Contributions

D.W.: conceptualization, data curation, and writing—original draft. Y.C.: software and formal analysis. Y.B.: investigation. C.Z.: writing—review & editing. M.Z.: conceptualization, supervision, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

All the relevant data are included in this published article.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Scheme

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

Figure 1. Scanning electron microscope (SEM) images of (a,b) LSCC-650; (c,d) TEM image of LSCC-650.

Figure 2. SEM images of (a) LSCC-500, (b) LSCC-550, (c) LSCC-600, (d) LSCC-700, (e) LSCC-750, and (f) LSCC-800.

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

Figure 4. High resolution XPS spectrum of (a) LSCC, (b) La 3d3/2, (c) Sr3/2, (d) Cu 2p3/2, (e) Co 2p3/2, and (f) O 1s in La0.2Sr0.8Cu0.4Co0.6O3−δ oxide.

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).

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/nano14010064/s1, Figure S1: OER reaction path. Blue line-acidic; red line-alkaline; Black Line-OOH_ad path; Green Line-2O_ads directly combine to produce O₂; Figure S2: EDX of the LSCC-500 nanofibers; Figure S3: EDX of the LSCC-550 nanofibers; Figure S4: EDX of the LSCC-600 nanofibers; Figure S5. EDX of the LSCC-650 nanofibers; Figure S6: EDX of the LSCC-700 nanofibers; Figure S7: EDX of the LSCC-750 nanofibers; Figure S8: EDX of the LSCC-800 nanofibers; Figure S9: SEM figure after 5000 cycles images of the LSCC-650 nanofibers; Table S1: Summary of OER overpotentials at 10 mA cm⁻² for perovskite oxides-based electrocatalysts obtained through composition engineering. References [38,39,40,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95] are cited in the Supplementary Materials.

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