Low-cost pyrolysis of biomass-derived nitrogen-doped porous carbon: chlorella vulgaris replaces melamine as a nitrogen source

doi:10.21203/rs.3.rs-3865787/v1

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Research Article

Low-cost pyrolysis of biomass-derived nitrogen-doped porous carbon: chlorella vulgaris replaces melamine as a nitrogen source

https://doi.org/10.21203/rs.3.rs-3865787/v1

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published 31 Mar, 2024

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Porous carbon generated from biomass has a rich pore structure, is inexpensive, and has a lot of promise for use as a carbon material for energy storage devices. In this work, nitrogen-doped porous carbon was prepared by co-pyrolysis using bagasse as the precursor and chlorella as the nitrogen source. The thermal weight loss experiments showed that the pyrolysis temperatures of bagasse and chlorella overlap, which created the possibility for the synthesis of nitrogen-rich biochar. The optimum sample (ZBC@C-5) possessed a surface area of 1508 m²g^-1 with abundant nitrogen-containing functional groups. ZBC@C-5 in the three-electrode system exhibited 244.1F/g at 0.5A/g, which was extremely close to ZBC@M made with melamine as the nitrogen source. This provides new opportunities for the use of low-cost nitrogen sources. Furthermore, the devices exhibit better voltage retention (39%) and capacitance retention (96.3%). The goal of this research is to find a low cost, and effective method for creating nitrogen-doped porous carbon materials with better electrochemical performance for highly valuable applications using bagasse and chlorella.

biomass

pyrolysis

nitrogen doping

porous carbons

supercapacitor

With the development of modern agriculture, the environmental pollution and resource waste caused by the massive accumulation of agricultural waste are becoming increasingly prominent, so it is crucial to actively advance the comprehensive utilization of agricultural waste to safeguard the environment and encourage sustainable agricultural development(Mujtaba et al., 2023). The two primary traditional ways for treating agricultural waste are (1) incineration, in which straw is combined with coal as fuel to limit the emission of flue gas pollutants(Ma et al., 2022), and (2) pyrolysis, in which biochar or oil is produced by pyrolysis and utilized as a biofuel or adsorbent(Paul Nayagam & Prasanna, 2022; X. Zhang et al., 2021). However, there are still problems of environmental pollution and alkali metal corrosion incinerator after incineration, which does not fully realize the green conversion of agricultural waste. Pyrolysis plays a crucial role in maximizing the utilization of biomass for energy production and enhancing the value-added utilization of agricultural waste (Hakeem et al., 2022).

The pyrolysis process produces pyrolytic tar and pyrolytic carbon, which can be transformed into hydrogen, methane and other gases through catalytic reforming, while pyrolytic carbon is mostly used as an adsorbent to purify air and water(Freitas et al., 2019; Xu et al., 2023). With the implementation of "carbon neutrality", research is turning to carbon capture, carbon fixation, and energy storage devices(Saravanakumar et al., 2023). The biomass fuel formed by pyrolysis of tar still releases carbon dioxide and other gases through combustion. Compared with pyrolysis tar, pyrolysis charcoal not only immobilizes carbon in agricultural waste, but also has a rich pore structure, which has a very high potential for application as a material for energy storage devices. Thanks to the excellent performance of biomass-derived porous carbon in carbon fixation and energy storage, more and more researchers are applying pyrolytic carbon to supercapacitor electrode materials(Khedulkar et al., 2023).

Numerous research has demonstrated that adding N atoms to carbon-based materials may increase the capacitance of supercapacitors even further. According to Wang et al., the addition of melamine to the chemical activation process can enrich the material's functional groups that contain nitrogen and significantly increase the material capacitance (Y. Wang et al., 2016). Jiang et al. investigated the electrical properties of the materials under different doping ratios of melamine (Jiang et al., 2020). Wang et al. synthesized nitrogen-doped lignin-derived graded porous carbons using alkaline lignin and melamine as raw materials (S. Wang et al., 2022). In addition, some researchers have attempted to prepare nitrogen-doped porous carbon by self-doping with nitrogen-rich biomass. By using a straightforward carbonization technique, Jin et al. produced porous carbon generated from xanthophyll that had a significant specific surface area with high heteroatom content. (Jin et al., 2022). By employing soybean pulp as the raw material, Ding et al. showed that the supercapacitor electrode active material produced from soymilk as a raw material has the potential for mass production (Y. Ding et al., 2021). However, soymilk does not have a rich pore structure, which very much limits the application of the material. Furthermore, most non-edible biomasses are low in nitrogen, but finding nitrogen-rich biomass with excellent structure is accidental. Most importantly, the extensive use of nitrogen-containing chemicals (e.g., melamine) will greatly raise the cost of preparation, which defeats the purpose of utilizing solid waste. Therefore, it is essential to find a low-cost nitrogen source and process method for the production of biomass-derived nitrogen-doped porous carbon.

Bagasse is a common agricultural waste and is considered an ideal activated carbon precursor due to its rich fiber structure. Chlorella is aquatic biomass with higher lipid and protein content compared to lignocellulosic biomass (Mustapha et al., 2021). Moreover, chlorella has a quick growth cycle, a large dispersion, and produces a lot of oil. It is also frequently used in pyrolysis to create nitrogenous compounds with a high added value (Huang et al., 2022). Although chlorella has a high nitrogen content, it does not have a good pore structure, which limits its application in activated carbon. As a consequence of their investigation into the impact of nitrogen conversion during the co-pyrolysis of algae and lignocellulosic biomass, Chen et al. discovered that the amount of nitrogen in fixed carbon was significantly enhanced (W. Chen et al., 2018). Inspired by the above study, this work will use bagasse as a precursor to create nitrogen-doped porous carbons and chlorella as a source of nitrogen. Although KOH has excellent pore-making ability, it will cause serious corrosion to the equipment, which is not conducive to green production(Singh et al., 2023). ZnCl₂ can create wide micropores and small mesopores during activation, which promotes the migration of electrolyte ions (Martin et al., 1996). Moreover, Su et al. found that ZnCl₂ as a sacrificial additive, could play the role of a N-bank during the thermal conversion of nitrogen-containing carbon precursors to nitrogen-doped carbon, which can sequester nitrogen in nitrogen-containing precursors and significantly inhibit nitrogen loss (Su et al., 2021). Therefore, ZnCl₂ will be selected as an activator in this work because of its good pore-forming ability and nitrogen fixation function.

In this work, nitrogen-doped porous carbon was prepared by a two-step pyrolysis method. First, co-pyrolysis of chlorella and bagasse at different doping ratios was performed to enrich more nitrogen in the biochar products. The pyrolysis product biochar was activated with ZnCl₂ as an activator and nitrogen-fixing agent. The electrochemical performance of each sample was evaluated in three-electrode system. In particular, to verify that chlorella is an effective nitrogen source, melamine was introduced as another nitrogen source as a reference indicator. It was found that the electrical performance of the carbon materials prepared by chlorella and bagasse in the appropriate mixing ratio was comparable to those prepared from melamine and bagasse. Compared with melamine, chlorella is a green and low-cost nitrogen source. This work demonstrates that the nitrogen-containing functional groups can be effectively doped by slow co-pyrolysis, and provide a green, and effective synthetic strategy for the preparation of nitrogen-doped porous carbon derived from biomass.

2.1 Preparation of porous carbon materials

Preparation of biochar: The raw materials used were bagasse and chlorella. The ultimate and proximate analyses of bagasse and chlorella are shown in Table 1. The raw materials were first cleaned with deionized water to get rid of contaminants, and they were then dried for 48 hours in an oven at 105°C. With a grinder, the material was thoroughly ground to a particle size of 0.2 mm. The bagasse and chlorella were weighed out at 5:5, 3:7, and 7:3, respectively, and 10 g of the mixture was uniformly distributed. Slow pyrolysis mixture heated from 30°C to 400°C using a vertical tube furnace and held for one hour (N₂ atmosphere, 3 ℃/min). The biochar prepared by pyrolysis was named BC@C-x (x denoted the mass of doped chlorella), for example, BC@C-5 denoted 10 g of the mixture doped with 5 g of chlorella. To ensure that an inert atmosphere was maintained in the quartz tubes, N₂ was continuously supplied for ten minutes before the experiment.

The activator utilized was ZnCl₂. 3g of ZnCl₂ and 1g of biochar were thoroughly mixed. The combination was then transferred to an onyx mortar and thoroughly mixed. The mixture was added to a quartz hopper and then moved to an activation tube furnace. In order to create porous carbons, it was first heated at a heating rate of 3°C/min to the activation temperature of 500°C for 1.5 hours. The furnace was then brought down to room temperature spontaneously in a N₂ atmosphere. Ultimately, samples of porous carbon were produced by acid washing and drying. The porous carbons of different biochar (BC@C-3, BC@C-5, BC@C-7) activated by ZnCl₂ were named as ZBC@C-3, ZBC@C-5, ZBC@C-7, respectively.

To further demonstrate the effect of chlorella doped as a nitrogen source on the functional groups and capacitive properties of the resulting porous carbon, the following samples were prepared: (1) The porous carbon prepared from bagasse was designated as ZBC. The preparation method was as follows: the product of pyrolysis of sugarcane bagasse at 400°C is activated with ZnCl2 at a mass ratio of 1:3 for 1.5 h at 500°C. (2) porous carbon prepared from chlorella under the same conditions was designated as ZCC; (3) the porous carbon prepared with melamine and bagasse was designated as ZBC@M. The nitrogen content in 5g of chlorella was known by elemental analysis, and the biochar was prepared by homogeneously mixing the mass of melamine with 5g of bagasse under the same nitrogen stoichiometry and pyrolyzing at 400℃ (Supporting information 2.1). The resulting biochar was activated with ZnCl₂ at a mass ratio of 1:3 at 500℃ for 1.5h. The pyrolysis and activation steps of the above experiments were in N₂ atmosphere with a heating rate of 3°C/min.

Table 1

Ultimate and proximate analysis of bagasse and chlorella.
Samples	Ultimate analyses (wt.% dry basis)					Proximate analyses (wt.% dry basis)
Samples	C	H	O^a	N	S	VM^b	FC^c	Ash
Bagasse	46.1	5.526	45.244	0.83	/	82.88	14.82	2.3
Chlorella	49.62	6.639	28.911	9.14	0.5	80.78	14.03	5.19

^a Calculated by difference: O = 100-C-H-N-S-Ash.

^b VM, Volatile matters.

^c FC, fixed carbon. Calculated by difference: FC = 100-VM-Ash.

2.2 Characterization

The surface morphology and pore structure of materials were observed by scanning electron microscopy (SEM, Merlin, Zeiss), and transmission electron microscopy (TEM, JEM 1400 Plus, JEOL). The crystal structures were characterized by X-ray diffraction (XRD, Ultima Ⅳ, Rigaku). The graphitization degrees were determined by Laser Confocal Micro Raman Spectroscopy (HJY LabRAM Aramis, Horiba Jobin Yvon). The surface area, and pore size distribution of the porous carbons were measured by automatic specific surface and porosity analyzer (Micromeritics ASAP2460, USA). X-ray photoelectron spectroscopy (XPS, Axis Supra+, Kratos) technique was used to detect surface functional groups. (Supporting information 2.2 lists the specific parameters)

2.3 Thermogravimetric analysis (TGA)

To speculate on the reaction mechanism of bagasse and chlorella during co-pyrolysis, bagasse, chlorella and their mixtures in different proportions were pyrolyzed using a thermogravimetric analyzer and their weight loss data were recorded. The samples were heated from 100 to 400°C at a heating rate of 3°C/min. The pyrolysis process was carried out under N₂ environment with a flowing rate of 80 ml/min.

2.4 Electrochemical measurements

The preparation process and test parameters of the poles in the Supporting information 2.3. The specific capacitance of the electrode materials were counter by Eq. (1):

$$\begin{array}{c}C=\frac{I\varDelta t}{m\varDelta V}\#\left(1\right)\end{array}$$

In two-electrode system, the specific capacitance, energy density and power density were calculated by the following.

$$\begin{array}{c}C=\frac{2I\varDelta t}{m\varDelta V}\#\left(2\right)\end{array}$$

$$\begin{array}{c}E=\frac{C}{8}\times \varDelta {V}^{2}\times \frac{1}{3.6}\#\left(3\right)\end{array}$$

$$\begin{array}{c}P=\frac{E}{\varDelta t}\times 3600\#\left(4\right)\end{array}$$

Where I represents the charge-discharge current (A), Δt represents discharge time (s), m represents the loading mass (g) of the active material in the working electrode, and ΔV represents the potential window (V) of the charge–discharge excluding voltage drop.

3.1 Co-pyrolysis process of bagasse and chlorella and the potential mechanism

Co-pyrolysis is a pyrolysis process that combines different species and has an important influence on product distribution. In order to prevent pore blockage caused by the introduction of nitrogen atoms, nitrogen-rich biochar was prepared by slow co-pyrolysis prior to activation. To further investigate the interactions occurring between bagasse and chlorella during the pyrolysis of biochar preparation, the co-pyrolysis behavior of bagasse and chlorella was analyzed by the thermal weight loss method.

As exhibited in Fig. 1(a), there are two primary processes involved in the pyrolysis of bagasse. In the first stage (150–350°C) the mass started to decrease significantly, mainly due to the pyrolysis of hemicellulose and cellulose, with a maximum weight loss peak (2.83%/min) when 314°C occurs, which was mainly attributed to the devolatilization fraction of cellulose (D. Chen et al., 2022). The typical pyrolysis temperature range of cellulose devolatilization fraction was 300–400°C. The second stage (350–400°C) was mainly due to the continued pyrolysis of lignin (Zheng et al., 2023). The pyrolysis process of chlorella was divided into two main stages. Internal water evaporated during the first stage (50–213°C), and low molecular weight components were volatilized. The dehydroxylation, depolymerization, and cleavage of proteins, lipids, and carbohydrates were thought to occur in the second stage (213–400°C) (C. Chen et al., 2022). Bagasse and chlorella have similar main reaction temperature ranges, which raises the possibility that these mixtures could interact.

Figure 1(b) depicted the thermal weight loss process of the mixture of bagasse and chlorella. The maximum weight loss peaks during pyrolysis of the three mixtures with different mixing ratios all appeared near 325°C, and with the increase of the mixing ratio of chlorella, the peak weight loss peak values steadily dropped. Compared with the thermal weight loss curves of bagasse and chlorella, the thermal weight loss peaks of the mixtures with different mixing ratios all showed different degrees of variation, indicating that there was indeed an interaction between bagasse and chlorella in the co-pyrolysis process. Figure 1(c) shown the compositional changes and potential mechanisms of bagasse and chlorella during pyrolysis. The pyrolysis of cellulose and hemicellulose, the main components of bagasse during pyrolysis, produced mainly hydroxyacetaldehyde, 5-hydroxymethylfurfural, and levoglucan (Mullen & Boateng, 2011). Proteins in chlorella undergo peptide bond breaking and the amino acids formed undergo deamination, dehydration, and decarboxylation reactions to produce NO_x and N₂O or their precursors (amines, amides, and nitrile) (Che et al., 2022). The main nitrogen-containing products in biochar are cyano-N, heterocyclic-N, and amine-N. This suggests that the enrichment of nitrogen in biochar could be attributed to the interaction between bagasse and the pyrolysis products of chlorella. Specifically, the nucleophilic groups released by the peptide bond breakage of proteins in chlorella react with the electrophilic aerobic species (hydroxy acetaldehyde, acetaldehyde, and furfural) released from lignocellulose during bagasse pyrolysis. This results in the formation of more stable nitrogen-containing species, which are then distributed into the coke (Cheng et al., 2020),(Wei et al., 2018).

3.2 Material characterization

3.2.1 Morphology and structural analysis

The XRD pattern of the porous carbons were shown in Fig. 2(a), from which a broad diffraction peak centered at 2θ = 25.6° was observed and was identified as the (002) crystal plane of the graphite lattice (D. Wang et al., 2016). The presence of 002 crystal planes indicates that the produced material has parallel graphite layer stacking. A sharp and narrow peak near 2θ = 26.5° indicates the presence of graphitization(Mohammed et al., 2019a). It was noteworthy that the peak intensity gradually decreased with the increase in the percentage of chlorella doping. The low intensity and large width of the peaks indicated that the prepared porous carbon is in a highly amorphous state. The XRD results indicated that the prepared porous carbon has the amorphous carbon of graphite crystals (Nanaji et al., 2019).

As shown in Fig. 2(b), in the Raman spectrograms of all samples, two prominent peaks appeared at 1350 cm^− 1 and 1582 cm^− 1, respectively, which are called D-band and G-band (Song et al., 2019). The chaotic structure of carbon is represented by the D-band, whereas the ordered arrangement of carbon atoms is represented by the G-band. The relative intensity ratio (I_D/I_G) of measures the level of atomic ordering in the carbon material (Mohammed et al., 2019b). As seen in the Raman spectrograms, the I_D/I_G values of samples ZBC@C-3, ZBC@C-5, and ZBC@C-7 were 1.007, 0.9812, and 1.007 increased compared to those of ZBC (0.7276), implying that the degree of atomic ordering in the prepared porous carbon materials was intensified due to the introduction of nitrogen atoms. In addition, a broader weak peak (2D band) appears around 2860 cm-1, indicating that the material has a three-dimensional graphene structure (Zickler et al., 2006).

As shown in Fig. 3 and Table 2, the adsorption-desorption curve of ZBC exhibited a type I isotherm, reflecting the microporous filling phenomenon on the microporous material (J. Ding et al., 2015). The other samples exhibited type IV isothermal curves. The adsorption of N₂ increased significantly with an increase before P/P₀ = 0.3, showing the existence of micropores (Braghiroli et al., 2015). Hysteresis loop in isotherms due to capillary condensation of mesopores. Large number of micropores (≤ 2 nm) can be seen in the pore size distribution diagram. Thanks to the abundance of micropores and mesopores, the efficiency of ion storage and transport is greatly increased. (Xia et al., 2008). However, the specific surface area decreases with increasing chlorella doping rate. Combined with the SEM results, this may be due to the lamellar structure of chlorella wrapping the natural pipe structure of bagasse, resulting in the inability of the activator to penetrate deeply into the internal structure. It was simple to observe that the specific capacitance was connected to pore size distribution and nitrogen-containing functional groups l rather than a linear relationship with the specific surface area when taken along with the findings of the performance of electrical characteristics in section 3.3.

Table 2

Textural parameters of porous carbons.
Samples	S_BET (m²g^− 1)	S_mic (m²g^− 1)	V_total (cm³g^− 1)	V_micro (cm³g^− 1)	V_mic/V_total	D_average (nm)
ZBC	1039.458	902.798	0.428	0.346	80.84	1.647
ZCC	1144.589	522.444	0.574	0.218	37.98	2.007
ZBC@C-3	1698.2	524.135	0.899	0.225	25.03	2.119
ZBC@C-5	1508.097	1166.781	0.663	0.456	68.78	1.76
ZBC@C-7	1137.004	953.483	0.509	0.368	72.30	1.789
ZBC@M	1777.754	950.076	0.84	0.387	46.07	1.89

The SEM and TEM images of the samples were shown in Fig. 4. Fig.S1(a) shown the void structure of ZBC after activation, but the natural pipe structure of bagasse was seriously damaged. Fig.S1(c) shown the structure of ZCC after activation, and it can be seen that the activated chlorella exhibits a blocky structure and did not have a good pore structure. (a) and (b) shown that ZBC@C-5 still maintained the pipe structure after activation, and there were holes formed by ZnCl₂ etching on the pipes. Chlorella fragments can also be seen clinging to the pipe surface, which added to the possibility of a nitrogen-rich structure. (c) and (d) shown the destruction of the pipe structure of ZBC@C-7 after activation, which may be caused by the interaction between bagasse and chlorella during pyrolysis. (e) and (f) shown the unfolding of the ZBC@C-3 pipeline structure after activation, which exposed more space for ZnCl₂ activation and explains the relatively large specific surface area of ZBC@C-3. In the TEM images, (g) shown the dense graded porous structure of ZBC@C-5, (h) shown the dispersed graded porous structure of ZBC@C-7, while (i) shown the onion-like macroporous structure of ZBC@C-3 without the graded porous structure, which corresponded to the results of N₂ adsorption-desorption experiments.

3.2.2 Differences in nitrogen-containing functional groups of materials when using chlorella and melamine

Detection of surface functional groups of samples by XPS technique. As shown in Fig. 5(a), the primary constituents of the sample may be identified as C, N, O, and the binding energies of the C1s, N1s, and O1s peaks were observed to be 281.2 eV, 397.3 eV, and 528.8 eV, respectively. As shown in Fig. 5 (b), it can be seen that the N1s spectrum of ZBC can be divided into pyridine-N (398.5 ± 0.3 eV) and pyrrole-N (400.5 ± 0.3 eV). However, the waveform was chaotic and unstable, demonstrating that N content was low. The N1s XPS spectra can be divided into pyridine-N (398.5 ± 0.3 eV), protein-N (399.8 ± 0.3 eV), pyrrole-N (400.5 ± 0.3 eV), and graphite-N (400.9 eV) after doping with chlorella (W. Chen et al., 2017; Figueiredo & Pereira, 2010). The presence of protein nitrogen was most likely due to the presence of proteins that are not completely decomposed under low-temperature pyrolysis. To confirm the reasonableness of the N1s XPS spectra of ZBC@C-x samples, doped melamine was used as the nitrogen source for comparison. The N1s XPS spectra of ZBC@M sample could be classified as pyridine-N (398.5 ± 0.3 eV), pyrrole-N (400.5 ± 0.3 eV), and graphite-N (400.9 eV). The above results indicated that doped chlorella as a nitrogen source can indeed produce abundant nitrogen-containing functional groups on the porous carbon surface. It has been suggested that pyridine-N and pyrrole-N are considered active sites that can effectively improve pseudocapacitor (Czerw et al., 2001; Deng et al., 2015), and graphite nitrogen can promote rapid electron transfer and enhance electrical conductivity (Hao et al., 2013). Figure 5(c) analyzed the composition and content of nitrogen in different samples, and it can be seen that the nitrogen content of ZBC@C-5 and ZBC@M were very close to each other, which further confirmed that chlorella was an effective nitrogen source.

3.3 Electrochemical performance

3.3.1 Comparison of electrochemical properties of materials in three electrode systems

The results of the characterization showed that the produced samples have a substantial specific surface area, a suitable pore distribution, and a wealth of nitrogen-containing functional groups, making them potentially effective as an electrode material for supercapacitors. The electrochemical properties of the prepared porous carbons were evaluated in a three-electrode system. The migration rate of electrolyte ions in the material can be seen from the form of the CV curve. As shown in Fig. 6(a), the CV curves exhibited a trapezoid-like structure, which is actually a deformation of the rectangular-like structure, showing that the material had great ion transport capabilities and EDLC qualities. The capacitance value of the material increases as the mask covered by the CV curve grows larger (Uppugalla & Srinivasan, 2019). The CV curve regions of ZBC@C-x samples are all bigger than those of ZBC, which indicated that doping chlorella as a nitrogen source can indeed effectively improve the specific capacitance. It was also found that the CV curves of ZBC@C-5 and ZBC@M were very close in shape, which indicated that the specific capacitance of ZBC@C-5 and ZBC@M were approximately the same. Figure 6 (b) shown the constant current charge/discharge experiment of all the samples. It can be observed that the GCD curves shown a triangular and symmetric shape. Meanwhile, the curve does not show a perfectly straight line during the charging/discharging process, which is mainly due to the pseudo-capacitance generated by the functional groups on the surface of the material (Lu et al., 2015). The specific capacitances of samples ZBC, ZCC, ZBC@C-3, ZBC@C-5, ZBC@C-7 were 195.7 F/g, 225.5 F/g, 233 F/g, 244.1 F/g, 224F/g, respectively. This shown that the doping of different proportions of chlorella can improve the specific capacitance of the material to different degrees. However, with the addition of the doping proportion of chlorella, the specific capacitance of the prepared nitrogen-doped porous carbon showed a trend of first increase and then decrease, which can be presumed to be caused by the underdeveloped specific surface area from the characterization results in section 3.2. One of the samples, ZBC@C-5, shown the best nitrogen doping effect, with an increase of 48.4 F/g over the capacitance. It was noteworthy that the specific capacitance exhibited by ZBC@M (247.8 F/g) was very close to that of ZBC@C-5 (244.1 F/g), and it was also evident from the GCD curves that the GCD curves of ZBC@C-5 and ZBC@M almost overlap. It illustrated that sample ZBC@C-5 prepared by using chlorella as the nitrogen source and sample ZBC@M prepared by using melamine as the nitrogen source had very close effects on the electrical properties when the mixing ratio of bagasse and chlorella was 5:5. Figure 6(c) shown the multiplicative performance of porous carbon, and it can be seen that the capacitance value of ZBC@C-5 was the highest among all ZBC@C-x and was very close to the capacitance retention of ZBC@M, which may be attributed to the suitable ratio of bagasse and chlorella. Charge transfer was further evaluated by EIS tests in the frequency range of 10^− 2 HZ to 10⁵HZ, the results were shown in Fig. 6(d). In the low frequency, it was observed that all materials shown almost vertical straight lines, indicating good EDLC behavior of the materials (Chang et al., 2014). Two features were observed in the high frequency region: (1) circular arcs, indicating the charge transfer resistance R_ct, related to the semicircular radius, and (2) the Nyquist curve's intersection with the Z' axis, which represents the solution resistance Rs and was mostly correlated with the resistance of the electrolyte and electrode (S. Zhang & Pan, 2015). As can be seen from the figure, although the R_ct values of ZBC, ZBC@C-3, and ZBC@C-7 were smaller than those of ZBC@C-5 and ZBC@M, the circular segments of ZBC@C-5 and ZBC@M were extremely small, indicating that the charge transfer resistance was quite small, which was the point of interest.

3.3.2 Device Performance

To further investigate the application of the obtained carbon, ZBC@-5 was assembled into supercapacitors and the devices were tested. Figure 7 (a) The CV curves of the display device do not undergo serious deformation at different scan rates; Fig. 7 (b) The GCD curves exhibiting a maximum capacitance of 156 F/g (1A/g) for the device; Fig. 7 (c) demonstrates that the device exhibits a small solution resistance (0.58 Ω) and a small transfer resistance; Fig. 7 (d) shows that the device exhibits an energy density of 5.4 Wh/kg at a power density of 250 W/kg; The leakage current in the I-t curve of Fig. 7 (e) stabilizes as the constant voltage time increases, and the leakage current of 76 µA. is obtained after 6000 seconds of testing. Based on the leakage current measurements, the device reduced from 1V to 0.39V after 1h with 39% voltage retention; Fig. 7 (f) The display device exhibits 96.3% capacity retention and 99.4% Coulomb efficiency in 10,000 cycle life tests. The test results showed that the feasibility of applying the material to supercapacitors.

In conclusion, the advantages of both the excellent pipeline structure of bagasse and the high nitrogen content of chlorella were effectively combined by slow co-pyrolysis. While the specific surface area of ZBC@C-5 is only 1508 m²g^− 1, pyrolysis and activation reveal that the surface of porous carbon contains a range of nitrogen-containing functional groups. Electrochemical test results shown that ZBC@C-5 delivers a specific capacitance of 244.1F/g at 0.5A/g. Compared to the specific capacitance exhibited by the porous carbon (ZBC) prepared from bagasse, ZBC@C-5 was improved by 48.4 F/g. The sample ZBC@M was prepared by selecting melamine as nitrogen source. ZBC@C-5 and ZBC@M showed very close electrochemical properties, moreover, devices assembled from ZBC@C-5 exhibit better voltage retention (39%) and capacitance retention (96.3%). These results suggest that the nitrogen-rich biochar produced by slow co-pyrolysis of chlorella and bagasse in the proper amounts is successfully doped with nitrogen-containing functional groups, laying the groundwork for the production of porous carbon that is doped with nitrogen. The activated nitrogen-doped porous carbon, in particular, exhibits excellent electrochemical characteristics, offering a green, low cost and effective method to achieve high value-added use of bagasse and chlorella by pyrolysis.

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Ethical Approval

Not applicable.

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Consent to Publish

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Funding

This work was supported by the National Natural Science Foundation of China (22373035), the Key Technologies Research and Development Program of Guangzhou(202206010122) and the CAS Key Laboratory of Renewable Energy，Guangzhou Institute of Energy Conversion (E229kf0201).

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

Changxing Lu: Investigation, Methodology, Writing - original draft, Jing Yang: Investigation, Methodology, Writing - review & editing. Zhaosheng Yu: Conceptualization, Funding acquisition, Conceptualization, review & editing. Xikui Zhang: Investigation, Methodology, review & editing. Meirong Li: Investigation, review. Zigan Huang, Investigation, review & editing. Xiaoqian Ma: Writing - review & editing, Resources, Supervision.

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Low-cost pyrolysis of biomass-derived nitrogen-doped porous carbon: chlorella vulgaris replaces melamine as a nitrogen source

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Journal Publication

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Abstract

Figures

1. Introduction

2. Material and methods

2.1 Preparation of porous carbon materials

2.2 Characterization

2.3 Thermogravimetric analysis (TGA)

2.4 Electrochemical measurements

3. Results and discussion

3.1 Co-pyrolysis process of bagasse and chlorella and the potential mechanism

3.2 Material characterization

3.2.1 Morphology and structural analysis

3.2.2 Differences in nitrogen-containing functional groups of materials when using chlorella and melamine

3.3 Electrochemical performance

3.3.1 Comparison of electrochemical properties of materials in three electrode systems

3.3.2 Device Performance

4. Conclusions

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1