Enhancing the Capacity and Stability by CoFe₂O₄ Modified g-C₃N₄ Composite for Lithium-Oxygen Batteries

Abstract

As society progresses, the task of developing new green energy brooks no delay. Li-O₂ batteries have high theoretical capacity, but are difficult to put into practical use due to problems such as high overvoltage, low charge-discharge efficiency, poor rate, and cycle performance. The development of high-efficiency catalysts to effectively solve the shortcomings of Li-O₂ batteries is of great significance to finding a solution for energy problems. Herein, we design CoFe₂O₄/g-C₃N₄ composites, and combine the advantages of the g-C₃N₄ material with the spinel-type metal oxide material. The flaky structure of g-C₃N₄ accelerates the transportation of oxygen and lithium ions and inhibits the accumulation of CoFe₂O₄ particles. The CoFe₂O₄ materials accelerate the decomposition of Li₂O₂ and reduce electrode polarization in the charge–discharge reaction. When CoFe₂O₄/g-C₃N₄ composites are used as catalysts in Li-O₂ batteries, the battery has a better discharge specific capacity of 9550 mA h g⁻¹ (catalyst mass), and the cycle stability of the battery has been improved, which is stable for 85 cycles.

1. Introduction

In the face of huge pressure from energy conservation and emission reduction advocates, it is necessary to replace traditional fuel vehicles with electric vehicles possessing high energy capacity and long range [1,2]. The theoretical energy density of Li-O₂ batteries is 3500 Wh kg⁻¹, much higher than other batteries. Li-O₂ batteries are expected to be used in electric vehicles and in large-scale productions [3,4,5]. However, so far, the actual specific energy of Li-O₂ batteries reported in literature is less than half the theoretical value. This is because during the charging and discharging processes, Li-O₂ batteries generate large polarization phenomena and high activation energy, leading to high energy loss [6]. One of the solutions to reduce energy loss for the Li-O₂ batteries is to develop new electrocatalysts with high activity. Metallic materials, such as platinum and iridium, or their alloys, have been shown to be the best electrode materials for oxygen reduction reaction. However, the high price of metallic materials hinders their commercial application [7,8]. Hence, more and more attention has been paid to the development of electrocatalysts prepared by some non-precious metals and non-metals [9].

Recently, spinel materials (such as Co₃O₄) with adjustable structure and stable chemical properties have attracted the attention of researchers [10,11]. However, compared with noble-metal-based catalysts, the catalytic performance of spinel material has a big gap. Studies have found that the structural stability and performance of the spinel material can be improved by replacing Co in Co₃O₄ with secondary metals such as Ni, Cu and Mn [12]. Unfortunately, when choosing this material as a catalyst, as the reaction begins, particles continue to aggregate, the number of active sites decreases, and cycle performance decreases. Choosing a suitable supporting substrate to form a stable structure can effectively reduce particle agglomeration and improve catalyst activity.

Graphite carbon nitride (g-C₃N₄) has a graphite-like planar phase, of which the nitrogen atoms have both three-fold coordination atoms and two-fold coordination atoms. They each also contain six nitrogen lone pairs of electrons [13,14,15]. This enables the g-C₃N₄-based catalyst to change the electronic structure and provides an ideal position for recombination [16,17]. Therefore, when employed as an ameliorative support for the nanoparticle surface, g-C₃N₄ plays a significant role in the catalysis process. It can not only restrain the migration of nanoparticles, but also acts as the second active-site supplier that imparts activity to the integral catalyst [18,19]. In addition, we know that rate capability, round-trip efficiency, and cycle life in Li-O₂ batteries are equally governed by parasitic reactions, which are now recognized to be caused by formation of the highly reactive singlet oxygen. Selected homogeneous catalyst approach to limit singlet oxygen release is a way to improve performance [20,21,22]. The uniformly dispersed particles supported on substrate are also beneficial to performance improvement due to more sufficient contact between the active constitutes and Li₂O₂ particles.

Taking these issues into account, we herein rationally design a scalable facile strategy for fabricating a CoFe₂O₄/g-C₃N₄ composite with the CoFe₂O₄ particles supported on the flaky g-C₃N₄. Thereinto, g-C₃N₄ not only provides rich catalytic sites, but also acts as a support for loading CoFe₂O₄ nanoparticles to restrain their aggregation. g-C₃N₄ and CoFe₂O₄ synergistically enhance the catalytic performance. The resultant CoFe₂O₄/g-C₃N₄ composite exhibits enhanced electrochemical performance of Li-O₂ batteries with respect to discharge capacity, voltage polarization, and cycling performance when compared with single CoFe₂O₄. This strategy may provide an efficient and versatile approach for designing spinel-based materials as efficient Li-O₂ batteries cathodes.

2. Materials and Methods

2.1. Synthesis

Preparation of g-C₃N₄: First, 4 g melamine was placed in a crucible with a cover. The crucible was annealed at 550 °C for 4 h in a muffle furnace, and the obtained product was grinded to obtain block C₃N₄. Next, 1 g C₃N₄ was dissolved in 35 mL hydrochloric acid and stirred for 30 min. The solution was then placed in a Teflon hydrothermal kettle at 110 °C for 300 min. Finally, the mixture was centrifuged, washed, and dried at 70 °C for 10 h.

Preparation of CoFe₂O₄/g-C₃N₄: First, 0.4 g g-C₃N₄ was dissolved in 30 mL ethylene glycol, then 0.0496 g cobalt nitrate and 0.138 g iron nitrate (n_Co:n_Fe = 1:2) were added and stirred vigorously for 30 min to achieve thorough mixing. The concentrated ammonia solution was added dropwise to the mixed solution to sustain the pH at 8, which was stirred for another 30 min. Next, the mixture was put in a 50 mL Teflon-lined autoclave and kept at the temperature of 160 °C for 20 h. The products were centrifuged, washed, and dried at 70 °C for 600 min. After grinding and annealing at 350 °C for 3 h, CoFe₂O₄/g-C₃N₄ composite was obtained.

Preparation of CoFe₂O₄: The procedure for the preparation of pure CoFe₂O₄ was the same as the preparation of CoFe₂O₄/g-C₃N_4, but without g-C₃N₄.

2.2. Material Characterization

The phase prepared in this experiment was tested with a D/MAX2500V X-ray diffractometer (XRD). The scan range was set to 10–90°, and the scan speed was 10° min⁻¹. For the observation and research of the microscopic morphology of the sample, the SU8020 field emission scanning electron microscope (SEM) produced by Japan’s JOEL company was used. The surface microstructure and preliminary quantitative analysis of the sample was tested and analyzed by the JEM-2100F field emission transmission electron microscope (TEM) and energy-dispersive spectrometer (EDS). A TriStar Ⅱ 3020 V1.03 specific surface area tester was employed and the specific surface area was calculated by the BET method from the adsorption isotherm of the sample with nitrogen gas. The component elements of the sample and the analysis of element valence were performed by ESCALAB250 X-ray photoelectron spectrometer (XPS).

2.3. Electrochemical Performance Test

2.3.1. ORR/OER Performance Test

The ORR/OER electrocatalytic performance test was carried out by an ATA-1B rotating disc electrode (RDE). Preparation of working electrode: A mixture of 10 mg CoFe₂O₄/g-C₃N₄ and 2 mg Vulcan XC-72 were added to 40 μL Nafion solution, and 2 mL isopropanol water with the given fraction (V_isopropanol:V_{Deionized water} = 1:5) was added into the solution. The ultrasonic treatment was then used to disperse the solution, then 5 μL was pipetted and mixed suspension added to the polished glassy carbon electrode surface. Finally, the electrode was dried by natural volatilization or low-temperature drying. Once the electrode was completely dried, it proceeded to the testing process. In the measurements, glassy carbon electrode (GCE) was used as the working electrode. Saturated calomel electrode (SCE) was used as the reference electrode. Graphite electrode was used as the auxiliary electrode.

For ORR/OER polarization curve testing, the scanning speed was set to 10 mV s⁻¹ and the rotation speed was 400~2000 rpm. The electrolyte was 0.1 M oxygen-saturated KOH solution. The ORR potential scanning interval was set between 0 and −0.6 V. The electrode rotation speed tested in OER was 1600 rpm, and the potential scanning interval was 0~1 V.

2.3.2. Battery Performance Test

Preparation of oxygen electrode: First, 15 mg catalyst and 30 mg KB were grinded and mixed for 1 h. Next, 83.3 mg PVDF (6 wt%) and 10 drops of NMP were added to the grinded powder to form a uniform slurry without obvious particles, which was then coated on carbon paper and dried at 90 °C for 10 h.

The electrochemical performance of batteries was tested by using 2032-type coin cell. Each cell was composed of a lithium metal anode, a glass fiber separator (Whatman grade GF/D), an electrolyte containing 1 M LiCF₃SO₃ in TEGDME, an oxygen cathode, and two pieces of nickel foam (1 mm thick) as the filler and the current collector. This was assembled in an argon-filled glove box (M. Braun). The oxygen cathodes were prepared by coating catalyst ink onto carbon paper homogenously. The catalyst mass loading of the oxygen cathode is about 0.5 ± 0.1 mg cm⁻².

For the evaluation of the battery discharge–charge performance and overvoltage performance test, voltage range was 2.2–4.5 V (vs. Li⁺/Li). The current density was 100 mA g⁻¹.

The voltage range for the battery cycle performance test was 2.0–4.5 V (vs. Li⁺/Li). The current density was 500 mA g⁻¹ and limited the capacity to 1000 mA h g⁻¹.

3. Results and Discussions

As shown in Scheme 1, the synthesis of CoFe₂O₄/g-C₃N₄ starts with a facile hydrothermal treatment of the solution containing cobalt nitrate, iron nitrate, and the prepared g-C₃N₄. The phase is studied by X-ray diffraction (XRD). In Figure 1a, g-C₃N₄ exhibits two characteristic diffraction peaks: A peak at 27.4° is formed by the stack of g-C₃N₄ rings, which is the (002) crystal plane [23]; another with relatively weak intensity is at 13.0°, which is a characteristic diffraction peak of Melamine substances and mainly refers to the in-plane nitrogen pores formed by the 3-s-triazine structure [24]. By comparing the two XRD patterns, before and after alkali treatment, the peak of the material does not change, which implies that the crystal structure of g-C₃N₄ remains unchanged. This confirms that carbon nitride possesses good chemical stability. For the XRD patterns of CoFe₂O₄/g-C₃N₄ composite, the peaks at 18.3°, 30.7°, 35.7°, 43.3°, 57.4°, and 63.2° are CoFe₂O₄ diffraction peaks. The peak at 2θ position of 27.7° correspond to the typical peak of g-C₃N₄, implying that the CoFe₂O₄/g-C₃N₄ material has been successfully synthesized.

The surface and internal morphology of materials are observed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Bulk g-C₃N₄ is composed of many layered nanosheets, which are stacked on each other (Figure S1). After alkali treatment, the g-C₃N₄ is stripped into flakes (Figure S2), which is conducive to the loading of spherical particles. Without g-C₃N₄, pure CoFe₂O₄ shows the particle morphology in Figure S3, while CoFe₂O₄/g-C₃N₄ exhibits stacked granular morphology connected with particles as shown in Figure 1b. TEM images indicate that CoFe₂O₄ nanoparticles are supported on the flaky g-C₃N₄ where the clear lattice fringes of 0.25 nm correspond well to the (311) plane of CoFe₂O₄, suggesting that these particles are CoFe₂O₄ (Figure 1c,d). Moreover, the element distribution C, N, O, Co, and Fe of the material is observed by energy-dispersive spectroscopic (EDS) (Figure 1e), indicating successfully prepared material of CoFe₂O₄/g-C₃N₄ composite and uniformly distributed CoFe₂O₄ nanoparticles on the flat g-C₃N₄. The specific surface area and pore volume of CoFe₂O₄/g-C₃N₄ are determined to be 244.1 m² g⁻¹ and 0.423 cm³ g⁻¹ by Braunauer–Emmett–Teller (BET) analysis (Figure S4), which is much larger than those of g-C₃N₄. Such a big increase for specific surface area and pore volume indicates that CoFe₂O₄ particles supported on the flaky g-C₃N₄ are effective for preventing g-C₃N₄ from stacking on each other. The increase in specific surface area and pore size can expose more active sites, increasing the ion migration rate during charging and discharging.

The elemental composition and chemical state of synthetic materials are analyzed by X-ray photoelectron spectroscopy (XPS). The full-spectrum scan result shows that the material contains C, N, O, Fe, and Co, which proves that the CoFe₂O₄/g-C₃N₄ material was successfully synthesized (Figure 2a). In the spectrum of C 1s (Figure 2b), two strong peaks are shown at 288.1 and 284.7 eV, which respectively belong to the sp² hybridized bounded carbon (C=N) and graphitic carbon (C-C) [25,26]. Figure 2c is the N 1s spectrum, which confirms that the CoFe₂O₄/g-C₃N₄ composite has three kinds of nitrogen. The 398.3 eV is the sp² hybrid nitrogen (N1), the 399.2 eV is the tertiary nitrogen (N2), and the 400.9 eV is the amino functional group (N3), respectively [27,28]. The high-resolution Co 2p_3/2 spectrum presents the signals of Co²⁺, Co³⁺, and satellite (Figure 2d), where the presence of Co³⁺, Co²⁺, and vibration satellite are respectively exhibited by the brown main peak (780.5 eV), the blue green peak at 782.2 eV, and the peak at 786.8 eV [29,30,31]. The Fe 2p_3/2 spectrum of the catalyst also exhibits Fe²⁺, Fe³⁺, and satellite peaks, which respectively appear at 710.7 eV, 712.2 eV, and 714.1 eV [31,32,33]. The above results indicate that two-electron pairs of Fe³⁺/Fe²⁺ and Co³⁺/Co²⁺ exist in the structure of CoFe₂O₄/g-C₃N₄ material. In addition, the O1s XPS peak at 530.2 eV is the intrinsic lattice, while another peak at 531.8 eV may be the adsorbed water [34,35].

The electrochemical catalytic activity of the sample was evaluated in 0.1M KOH solution by linear sweep voltammetry (LSV). In Figure 3a, the CoFe₂O₄/g-C₃N₄ displays an onset potential (E_onset) of 0.90 V and a half-wave potential (E_1/2) of 0.76 V, which are more positive than those of CoFe₂O₄ (0.67 V) and g-C₃N₄ (0.65 V) catalysts (Figure 3b), signifying the higher activity for ORR. Moreover, the CoFe₂O₄/g-C₃N₄ exhibits a larger diffusion-limited current, suggesting the material has strong mass transfer ability and fast electron transfer speed. The reaction kinetics of the material are calculated and analyzed by the LSV curve at different speeds, as shown in Figure 3c. The activity of all materials at different speeds is linearly related, and the calculated number of transferred electrons show Pt/C, CoFe₂O₄/g-C₃N₄, CoFe₂O₄, and g-C₃N₄ is 4.0, 3.8, 3.4, and 1.67, respectively. The ORR reaction of CoFe₂O₄/g-C₃N₄ composite is nearly a 4e^- process, which indicates a better ORR catalytic performance and is close to commercial Pt/C. Furthermore, the OER curves of various catalysts show the CoFe₂O₄/g-C₃N₄ catalyst has the limiting current density of 49.3 mA cm⁻², higher than CoFe₂O₄ (~18.8 mA cm⁻²), g-C₃N₄ (~1.93 mA cm⁻²), and Pt/C (~9.87 mA cm⁻²) (Figure 3d), indicating the CoFe₂O₄/g-C₃N₄ material has the best OER performance. This may be due to the synergies of g-C₃N₄ and CoFe₂O₄ which boost the performance.

The electrochemical catalytic ability of the material was evaluated by assembling 2032 button batteries. In Figure 4a, the CoFe₂O₄/g-C₃N₄ composite exhibits a discharge specific capacity of 9550 mA h g⁻¹, significantly higher than CoFe₂O₄ and XC-72. The overpotential of the CoFe₂O₄/g-C₃N₄ composite as exhibited in Figure 4b is 1.21 V, which is lower than the catalysts of CoFe₂O₄ (1.33 V) and XC-72 (1.87 V). These results indicate that when CoFe₂O₄/g-C₃N₄ composite is used as catalyst for Li-O₂ batteries, the degree of polarization during the discharge–charge process is relatively lower than pure CoFe₂O₄. Figure 4c,d show the cycle performance of CoFe₂O₄ and CoFe₂O₄/g-C₃N₄. The CoFe₂O₄/g-C₃N₄ composite can stably cycle 85 times, which is significantly higher than pure CoFe₂O₄ cathodes (16 times). In the battery cycle reaction, the flake structure of g-C₃N₄ can supply sufficient space to store discharge product Li₂O₂ and accelerate the transportation of O₂ and Li⁺; also, its larger specific surface area and rich N content provide more reactive sites for the discharge–charge reaction. In addition, g-C₃N₄ provides a stable support for restraining aggregation of CoFe₂O₄ nanoparticles due to its high chemical stability, which leads to an increase in the stability of the composite catalyst. Therefore, from the discussion, the synergistic effect between g-C₃N₄ and CoFe₂O₄ can effectively improve the electrocatalytic activity and stability of the catalyst in Li-O₂ batteries, which is much better than pure CoFe₂O₄ cathode. The capacity retention rate tests of Li-O₂ batteries of CoFe₂O₄/g-C₃N₄ material were carried out at four current densities (Figure 4e). Compared with the first discharge capacity, the corresponding capacity retention rates of the four current densities are 67.7%, 61.3%, 48.4%, and 42.3%, respectively (Figure 4f). All results prove that CoFe₂O₄/g-C₃N₄ composite can promote ORR/OER dynamics, thus improving the rate performance of the battery. Even at a high current, the battery keeps a good capacity retention rate.

To investigate the composition and morphology changes of CoFe₂O₄/g-C₃N₄ electrodes at different stages after the discharge–charge process, Figure 5a displays XRD spectrum of batteries at different states. Compared with the catalyst in the initial state, after discharging, the surface of carbon sheet clearly exhibits characteristic peaks of Li₂O₂ at 33°, 35°, and 58°, and neither Li₂O nor LiOH is detected. After charging, no diffraction peak of Li₂O₂ is observed, which indicates that the battery has a good reversibility. Figure 5b–d are the SEM images of the battery after different processes. From Figure 5c, the morphology of the Li₂O₂ after the battery being deeply discharged is clearly observed. The lithium peroxide is mainly distributed uniformly on the carbon sheet in the form of filaments. After the charging process (Figure 5d), Li₂O₂ disappeared completely, which is similar with the initial state (Figure 5b) and consistent with XRD results, indicating that the battery has good reversibility.

4. Conclusions

In short, the g-C₃N₄/CoFe₂O₄ material is synthesized by simple methods. The flake structure of the CoFe₂O₄/g-C₃N₄ catalyst accelerates the transportation of O₂ and Li⁺ and provides sufficient space to store the discharge product of Li₂O₂. Both g-C₃N₄ and CoFe₂O₄ can offer catalytic sites. In addition, g-C₃N₄ supplies a stable support for restraining the aggregation of CoFe₂O₄ nanoparticles. Therefore, the Li-O₂ batteries with such CoFe₂O₄ modified g-C₃N₄ composites as air cathodes deliver a discharge specific capacity of 9550 mA h g⁻¹, and the cycle stability has been enhanced more than pure CoFe₂O₄ cathodes. This strategy opens opportunities for rationally exploring different modified strategies on nanostructured electrocatalysts for diverse devices.