Recycling chicken eggshell membranes for high-capacity sodium battery anodes

Abstract

As a waste recycling process, pyrolytic eggshell membrane carbonization can produce carbonized eggshell membrane (CEM) materials for sodium battery anodes. The CEM contains a high interconnectivity porous structure of carbon fiber networks with a controllable surface area and nitrogen content. The electrochemical properties of CEM combined with this process have potential applications in providing a new type of sustainable resource for clean energy storage.

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New energy conversion and storage technologies have attracted considerable attention taking a long view of both sustainable development and environmental protection.^1,2 Among the candidates, lithium-ion batteries (LIBs) are playing an ever-growing role because of their high energy density and long cycle lifetime.^3,4 However, the expanding market for LIBs is confronted with the problem of the high cost and low abundance of lithium in the earth.^5,6 Because of the natural abundance of sodium resources and the similarity between sodium and lithium, sodium-ion batteries (NIBs) are gaining recognition as an intriguing alternatives for LIBs.^6–8

In fact, the development and practical applications of NIBs are restricted, since the poor kinetics of the Na⁺ de/insertion reaction and lack of appropriate electrode materials is caused by the larger ionic radius of Na⁺ (0.102 nm) than that of Li⁺ (0.076 nm). One typical example is commercial graphite in LIBs, which does not allow the insertion of sodium ions and is electrochemically irreversible.⁹ Other carbonaceous materials have been investigated later, such as petroleum cokes,¹⁰ soft carbons (small regions of ordered graphene)⁹ and hard (disordered) carbons.¹¹ It was found that the higher graphitization degree of carbon results in the decay of the sodium storage capacity,^12,13 and Na⁺ is more inclined to intercalate between the disordered layers of carbon; thus, the large interlayered disordered carbon appears to be more suitable for NIBs.^14–16 Although remarkable progress has been made to date, the capacity and cycling properties of carbonaceous materials need to be further improved in order to realize the practical application of NIBs.

It is well-known that the electrochemical performance of carbonaceous materials can be enhanced through surface functionalities (such as N, B and S), which provides higher conductivity and extra Li-ion storage sites in LIBs,^17–19 and these strategies can also be used for the development of carbonaceous anode materials for NIBs.^20–23 Yu et al. reported N-doped porous carbon fibres, which can deliver a capacity of 243 mA h g⁻¹ at a current density of 50 mA g⁻¹ after 100 cycles.²³ Cao et al. synthesized hollow carbon nanowires through the pyrolyzation of polyaniline, which can deliver a capacity of 206.3 mA h g⁻¹ at a current density of 50 mA g⁻¹ after 400 cycles.¹² Huang et al. investigated N-doped carbon nanofibers, which can deliver a capacity of 134.2 mA h g⁻¹ at a current density of 200 mA g⁻¹ after 200 cycles.²⁰ Compared with traditional preparation methods, one eco-friendly, low-cost and scalable way to synthesise nitrogen incorporated carbonaceous materials is carbonizing organic waste directly, and there has been many researchers making great progress.^24–28 Conceivably, nitrogen incorporated porous carbon obtained by this approach is especially promising because it offers a simple approach, which is free of additives and extraneous steps.

The eggshell membrane, which is located between the egg white and the inner surface of the eggshell is a valuable and usable waste. The composition of eggshell membrane, is comprised of proteins as the major constituents with a small amounts of carbohydrates and lipids.²⁹ In this study, we report for the first time, the recycling of carbonized eggshell membranes (CEM) as an anode material for NIBs directly. By annealing the eggshell membrane at different temperatures, CEM materials with different surface areas and nitrogen contents have been synthesized. The results show that the CEM-800 (annealing temperature is 800 °C) sample exhibits exceptional performance with a very high specific discharge capacity of 246 mA h g⁻¹ at 50 mA g⁻¹ after 200 cycles. Compared to previous methods, our approach possesses advantages such as: (1) simplicity of operation and easy pyrolytic eggshell membrane process, (2) no further complicated activation process^22,24,26,27 or the use of a template,²⁵ which requires more energy consumption, and (3) a considerably lower specific surface area with comparable capacity and cycling stability; thus, increasing the volumetric energy density and industrial applications value.

The as-prepared carbonized eggshell membrane is named CEM-600, CEM-700 and CEM-800, respectively, corresponding to the different annealing temperature. The X-ray diffraction (XRD) patterns of the three CEM samples are displayed in Fig. 1a. The broad and weak diffraction peak of the CEM samples appear around 2θ = ∼24° and ∼43°, corresponding to the (002) and (100) diffraction, which deviate from the graphitic carbon, indicating the amorphous nature or low degree of crystallinity of CEM samples. Because the position of the (002) diffraction peaks are smaller than that of graphite (26.38°), the corresponding calculated interlayer spacing is wider than that of graphite (i.e. 0.336 nm), and it is theoretically beneficial for the reversible storage of sodium considering the larger diameter of the sodium ion.¹³ Furthermore, the weak intensity of the (100) peak demonstrates the low crystalline structure of the CEM samples, which is consistent with the Raman spectra as shown in Fig. 1b. All of the three CEM samples have two featured Raman shifts, the G-band (ordered graphitic structure) peak at 1590 cm⁻¹ and the D-band (disordered portion) located at 1352 cm⁻¹. It is reported that the intensity ratio and positions of the D and G bands are sensitive to the microstructure of carbon materials, including defects, edges, disorder and carbon grain size. The intensity ratio of the D and G band and the wide bands reflect that the loss of long-range ordering of the graphene sheets in the CEM samples.

Fig. 1 (a) XRD patterns and (b) Raman spectra of CEM-600, CEM-700, and CEM-800.

The information on the functional groups of the CEM samples was measured by Fourier transform infrared spectroscopy (FTIR) (Fig. 2). As can be seen, the bands located within the range of 3250–3550 cm⁻¹ are assigned to N–H and O–H stretching vibrations. The bands at 1646 and 1465 cm⁻¹ is related to C=N and the stretching vibration of C–NO, respectively. Moreover, the band at 1070 cm⁻¹ (C–N bonding) and 880 cm⁻¹ (N–H bonding) also confirm the existence of nitrogen in the three CEM samples. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to identify the chemical features at the top layers of the CEM samples, which mainly include C 1s, N 1s and O 1s (Fig. 3a, high-resolution N 1s core level peaks of the three CEM samples are shown in Fig. 3b and c). It is reported that nitrogen-containing carbon materials would exhibit more electrochemically active sites and better conductivities compared with that of nitrogen-free carbon materials.^17,30–32 There are four nitrogen species, which exist in the CEM samples, including pyridinic nitrogen (N-6 at 398.0 ± 0.2 eV), pyrrolic nitrogen (N-5 at 399.7 ± 0.2 eV), quaternary nitrogen (N-Q at 400.8 ± 0.2 eV) and oxidized pyridinic nitrogen (N-X at 402.5 ± 0.2 eV) as illustrated in Fig. 3e.^33,34 Among them, N-Q lead to the decrease of conjugated electrons in the carbon layers, resulting in a poorer electrical conductivity. On the other hand, the N-5, N-6 and N-X species could enhance the electric conductivity of the carbon materials and provide more active sites, which are beneficial for the storage of sodium ions. The percentage of each component is shown in Table 1, and it is apparent that the percentage of more active nitrogen species at the edge of the carbon layers (N-5, N-6, and N-X) in the CEM-800 sample is the highest among the three. Table S1† presents the XPS and combustion elemental analysis results for the CEM samples, listing the weight percentages of the main elements (C, O, N, H) and a small amount of S. Thermal gravimetric analysis (TGA) in Fig. S1† reveals the weight loss of the eggshell membrane during the pyrolytic process. It is obvious that the weight of the final products decreased with the increase in the pyrolysis temperature, which indicates a more complete pyrolysis reaction at higher temperatures. In addition, the result is basically consistent with the elemental composition data shown in Table S1,† from which we can see that the C content in the CEM-700 and CEM-800 samples is higher than that of the CEM-600 sample. Moreover, the loss of nitrogen may be because of the pyrolysis of the nitrogen functional groups at higher temperatures.

Fig. 2 FTIR spectra of CEM-600, CEM-700, and CEM-800.

Fig. 3 (a) XPS spectra of CEM-600, CEM-700, and CEM-800. Detailed XPS information of N 1s orbital of (b) CEM-600, (c) CEM-700, (d) CEM-800. (e) Four types of nitrogen species in the nitrogen containing carbon materials.

Table 1 Approximate distribution of N functional groups obtained by fitting the N 1s core level XPS spectra

Functional groups	N-6 [%]	N-5 [%]	N-Q [%]	N-X [%]
CEM-600	25.58	26.35	32.68	15.39
CEM-700	32.37	26.48	30.46	10.69
CEM-800	36.78	28.55	23.92	10.75

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The microstructure of the CEM-800 sample is shown in Fig. 4a. The morphologies of CEM-600 and CEM-700 (shown in Fig. S2†) are similar to that of CEM-800. It can be seen that the CEM-800 is composed of interlaced carbon fiber networks with a diameter of about several micrometers, and appears as macroporous structures in the surface. Such type of high interconnectivity and porous structure is important for the application in batteries, by providing a robust structure, stability and large electrode–electrolyte contact area upon cycling. Observation from the transmission electron microscopy (TEM) of the CEM-800 sample reveals an amorphous structure with some mesopores at the edge of the carbon fibers (Fig. 4b). The specific Brunauer–Emmett–Teller (BET) surface area of the CEM-600, CEM-700 and CEM-800 sample is 1.35, 1.71 and 28.11 m² g⁻¹, respectively. The large difference between the CEM samples may be because of the different annealing temperatures, Moreover, it shows that there is no big difference between the specific surface area of the CEM-600 and CEM-700 sample. While the surface area of these two samples are far less than that of the CEM-800 sample, which may reduce the contact area between the electrolyte and active electrode materials and limit the transportation of Li⁺.³⁵

Fig. 4 (a) SEM and (b) TEM images of the CEM-800 sample.

The Na⁺ insertion–extraction behavior of CEM-600, CEM-700 and CEM-800 electrodes was investigated by cyclic voltammetry (CV) and galvanostatic discharge–charge cycling (Li⁺ insertion–extraction behavior of CEM electrodes was also conducted for comparison, CV curves in Fig. S3,† charge–discharge capacity in Fig. S4† and rate capacity in Fig. S5†). Fig. 5a–c illustrates the first two charge–discharge curves of the three CEM samples at a current density of 50 mA g⁻¹. It was found that the first discharge capacity of CEM-600, CEM-700 and CEM-800 is 452.2, 446.8 and 720 mA h g⁻¹, respectively, and the corresponding charge capacity is 212.6, 216.1 and 327 mA h g⁻¹, corresponding to coulombic efficiencies of 47%, 48.37%, and 45.42%, respectively. The large irreversible capacity loss may originate from electrolyte decomposition, the formation of the solid electrolyte interphase (SEI) layer at the surface of CEM, and irreversible sodium absorption onto special positions.^13,20

Fig. 5 Electrochemical characterization and battery performance of the CEM as an anode material for Na-ion batteries. (a)–(c) First two charge–discharge profiles of the three CEM electrodes between 0.001 and 3.0 V at a current density of 50 mA g⁻¹. (d) CV curves of the CEM-800 electrode between 0.001 and 3.0 V at a potential sweep rate of 0.1 mV s⁻¹.

In the first CV cycle (Fig. 5d), the sharp reduction peak at 0.26 V during the cathodic process is likely assigned to the propylene carbonate (PC) decomposition and the formation of a SEI layer, as compared to the lithium and sodium system.^12,13,36 In addition, the clear sodium insertion peak observed at lower voltages near 0 V is similar for lithium insertion in carbonaceous materials. In the second and the following CV curves, the reduction peak at 0.26 V in the first cycle disappeared, which can be attributed to the isolation of the anode from the electrolyte because of the dense SEI layer formed on the surface of the anode in the first discharge. A pronounced reduction peak is observed near 0.01 V, analogous to the CV curves of the CEM electrode in the lithium ion insertion in other carbonaceous materials for LIBs. Obviously, these subsequent CV curves almost overlap, indicating that the CEM electrode offers good stability towards sodium ion insertion–extraction.

Fig. 6a shows a comparison of the cycling performance of the CEM electrodes at a current density of 50 mA g⁻¹. The CEM-600, CEM-700 and CEM-800 electrodes maintain reversible discharge capacities of 150, 175 and 246 mA h g⁻¹ after 200 cycles, respectively. The capacity decay mainly happens in the initial cycles, which possibly stems from the SEI film stabilization and irreversible sodium ion insertion. Although the initial coulombic efficiency of the CEM electrodes was not high as mentioned above, after the initial cycle the coulombic efficiencies of the three CEM electrodes reached and maintained about 100% beyond 5 cycles. The results demonstrate that the annealing temperatures have less impact on the cycling stability, while they have significant influence on the specific capacity of the CEM electrodes. In addition this phenomenon may result from the higher utilization of the active electrode material in the CEM-800 sample with a larger specific surface area, which can provide more contact area between the active material and electrolyte. Furthermore, the rate performance of the CEM electrodes was also studied as illustrated in Fig. 6b. Obviously, the CEM-800 electrode exhibited the best rate capability among the samples, which delivered the reversible capacities of 260, 237, 200, 110 and 77 mA h g⁻¹ at the current rates of 50, 100, 200, 500 and 1000 mA g⁻¹, respectively. Moreover, the capacity could recover to 220 mA h g⁻¹ when the current density decreased to 50 mA g⁻¹. Moreover, it is apparent that the CEM-800 sample maintains the highest reversible capacity and superior rate capability. It is believed that the interconnected carbon fiber networks play a critical role in enhancing the cycling stability of the CEM samples. While for the specific capacity and rate capacity of the CEM sample, the more critical factors may be the higher active nitrogen species and higher specific area, which could increase the conductivity and provide a larger contact area between the CEM samples and electrolyte during cycling. In addition, it should be noted that the most reported carbon electrodes with large specific surface areas are not practically feasible because the large surface means a low tap density, depressed volumetric energy density and higher potential safety hazard.^37,38 Although the surface areas of the CEM samples in our work are small, the relative electrochemical performance of the CEM samples is good, and this is conducive to the practical application.

Fig. 6 (a) Cycling performance of the CEM-600, CEM-700 and CEM-800 electrodes at a current density of 50 mA g⁻¹. (b) Charge–discharge capacities of the CEM-600, CEM-700 and CEM-800 electrodes as a function of discharge rate (50–1000 mA g⁻¹). (c) Impedance spectra of the CEM-600, CEM-700 and CEM-800 electrodes in fresh cells.

For comparison, impedance analysis was carried out for fresh cells at an open circuit voltage (OCV), as shown in Fig. 6c. A semicircle (the high- and middle-frequency regions) and a straight line (the low-frequency region) for each electrode, referring to contact resistance and the diffusion of sodium ion in the carbon electrode, respectively, can be obviously observed. The results indicate that the contact impedance of the CEM-700 and CEM-800 samples are smaller than that of the CEM-600 sample, which might be attributed to the improved electrical conductivity when higher active nitrogen species are introduced. Combined with the cycling performance and rate capacity of the three CEM samples shown above, the superior cycling performance of the CEM samples anode originate from their favorable interconnected structure and active nitrogen, which naturally exist. Compared with the other two samples, the CEM-800 sample shows a high capacity and superior rate capability, which can be because of the larger specific surface area and higher active nitrogen species. Therefore, 800 °C should be the optimal carbonization temperature here.

Conclusions

In summary, we have demonstrated that the recycled natural eggshell membrane can be used as a promising anode material for a sodium-based battery. It was found that all of the CEM samples posses a uniform microporous structure and an interwoven macroporous carbon fiber networks with micropores at the edge of the carbon fibers, while the CEM-800 sample with highest BET surface and active nitrogen content exhibited the best electrochemical performance such as storage capacity and cyclability in the NaClO₄/PC electrolyte, which was about 246 mA h g⁻¹ after 200 cycles at current of 50 mA g⁻¹. Given that egg consumption reaches hundreds of billions on a global scale, the promising battery data herein show how a part of the waste from one of the most popular foods, eggshell membranes, can be a sustainable resource for clean energy storage.

Acknowledgements

This work was supported by the National Natural Science Fund of China (no. 91022033, 21201158), the 973 Project of China (no. 2011CB935901), Anhui Provincial Natural Science Foundation (1208085QE101) and the Fundamental Research Funds for the Central Universities (no. WK 2340000027).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07995g

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