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Nitrogen and phosphorus dual-doped carbon derived from chitosan: An excellent cathode catalyst in microbial fuel cell
Accepted Manuscript Nitrogen and phosphorus dual-doped carbon derived from chitosan: an excellent cathode catalyst in microbial fuel cell Bolong Liang, Kexun Li, Yi Liu, Xiaowen Kang PII: DOI: Reference:
S1385-8947(18)31934-X https://doi.org/10.1016/j.cej.2018.09.217 CEJ 20056
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
5 July 2018 25 September 2018 29 September 2018
Please cite this article as: B. Liang, K. Li, Y. Liu, X. Kang, Nitrogen and phosphorus dual-doped carbon derived from chitosan: an excellent cathode catalyst in microbial fuel cell, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.09.217
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Nitrogen and phosphorus dual-doped carbon derived from chitosan: an excellent cathode catalyst in microbial fuel cell Bolong Liang a, b, Kexun Li a, b*, Yi Liu a, b, Xiaowen Kang c a The College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China b MOE Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation Pollution Control, Nankai University, Tianjin 300071, China c Energy Research Institute (ERI), National Development and Reform Commission (NDRC)Chinese Academy of macroeconomic Research (CAMR), Beijing, 100824, China
Abstract Carbon derived from chitosan, followed by phosphoric acid activation during thermal treatment to obtain N and P dual-doped catalyst, is studied as the catalyst for air-cathode in a microbial fuel cell (MFC). The maximum power density (MPD) of 1603.6 ± 80 mW·m-2 is achieved, which is 5 times as high as that of the control (322.4 ± 16 mW·m-2) when N, P-codoped carbon was calcined at 850 ℃. The better performance is due to a higher open circuit potential, lower total resistance and higher exchange current density. In addition, the optimized catalyst possessed the largest
Corresponding author: Kexun Li: Tel. :+86-22-23501117; Fax: +86-22-23495200 E-mail:
[email protected].
specific surface area of 982.18 m2·g-1, so it could transfer more oxygen and supplied added active sites. And the existence of graphitic nitrogen, phosphorus and high content of carbon oxygen bonds promoted the cathode performance in MFC. In brief, the N and P dual-doped carbon from chitosan was potentially low-cost catalyst for the high performance of MFC. Keywords: microbial fuel cell, carbonization, phosphorylation, oxygen reduction reaction 1. Introduction Microbial fuel cell (MFC), a new energy recovery technology, is fabricated to degrade organic and inorganic substances and generate electricity in waste water simultaneously by utilizing current-producing bacteria[1]. The single-chambered MFC with the same sewage immersing the cathode and anode gradually gets attention because of its simple structure, low cost and high efficiency[2, 3]. Noteworthily, the air-cathode catalyst has been considered as the crucial factor to improve the activity of oxygen reduction reaction (ORR)[4] in single-chambered MFC, because better ORR activity of catalyst means better performance for the MFC. Platinum (Pt) as cathode catalyst has a good performance of ORR while its steep cost determines that it can’t be applied practically[5]. Hence, it is necessary to seek a kind of low cost and efficient cathode catalyst. Currently, activated carbon materials have been studied extensively due to the minimal cost, high surface area and excellent electrochemical properties. Based on these advantages various kinds of modified carbon materials[6,
7] emerge rapidly to further promote the performance of ORR. Recently, doping heteroatoms (N and P) in carbon-based materials such as activated carbon[8], carbon nanotube[9], and graphene[10], has been proved to improve electrochemical reactivity of ORR significantly[11]. It is proposed that doping carbon with two or more selected heteroatoms could further improve the catalytic activity of metal-free carbon catalysts owing to the synergistic effects between the heteroatoms[12]. The N, P-codoped hierarchical porous foams exhibited superior ORR activity close to Pt/C [13]. Polydopamine-derived N, P-codoped micro porous carbon spheres synthesized through the self-polymerization of dopamine induced by the phosphonic species showed efficient performance towards electrocatalytic oxygen reduction[14]. Therefore, carbon modified by N and P is regarded as a promising cathode material[15-19]. Meanwhile, carbon obtained from different precursors such as chitosan[3], cellulose[16] and bamboo charcoal[20] has been reported to introduce heteroatoms in many prior researches. What is noteworthy is that chitosan (2–amino-2–deoxy–D-glucose) is naturally rich of nitrogen due to the presence of amino. Hence, chitosan can be chosen as the precursor to prepare Ndoped carbon materials retrenching the procedure to introduce nitrogen. Furthermore, carbon could successfully carry heterocyclic phosphorus atom activated by phosphoric acid, which substantially accelerated the activity of ORR. Such a facile, scalable and high-yield synthesis of N, P-codoped carbon material would play an important role to serve as the efficient cost-effective electrocatalysts.
Although modified chitosan has been investigated much on electrochemical properties, few studies were reported concerning N, P-codoped carbon derived from chitosan used as air-cathode catalyst in MFC up to now. In this study, we took chitosan as the precursor and developed a low-cost, metal-free and highly effective method to synthesize the air-cathode catalyst. Carbon made by chitosan was heated to 800 ℃, 850 ℃, 900 ℃, 950 ℃ for optimization at the attendance of phosphoric acid, respectively. It was proved to be a superior catalyst with admirable power density. In addition, the performances of the prepared carbon material were investigated and a series of material characterization was analyzed for the mechanism such as BrunauerEmmett-Teller, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. 2. Experimental 2.1 chemicals Chitosan ( (C6H11NO4)n, Lanji Technology development Co. Ltd, Shanghai, China), Phosphoric acid (H3PO4, Ailian electronic technology Co. Ltd, Tianjin, China),
Carbon
black
(Jinqiushi
Chemical
Co.
Ltd,
Tianjin,
China),
Polytetrafluoroethylene (PTFE, 60wt%, Hesen, Electrical Co, Ltd, Shanghai, China), Sodium acetate (CH3COONa, Meryer chemical technology Co. Ltd, Shanghai, China), Ammonium chloride (NH4Cl, Meryer chemical technology Co. Ltd, Shanghai, China), Sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O, Ailan chemical technology Co. Ltd, Shanghai, China), Disodium hydrogen phosphate
(Na2HPO4, Meryer chemical technology Co. Ltd, Shanghai, China). All the reagents above were analytical and used without further purification. Nitrogen (N2, 99.99%, Huanyu gas Co. Ltd, Tianjin, China). 2.2 Fabrication of catalyst Chitosan was heated to 350 ℃ at the rate of 5 ℃ min-1 from room temperature in a tube furnace, and it kept the temperature for 3 h at the presence of N2 with a rate of 300 mL·min-1[21]. The pretreated carbon was prepared for next step at this stage. The pretreated carbon above was soaked with 0.5 M H3PO4. Then it was dried in the oven at 80 ℃ for 12 h. The dried sample was calcined afterwards in the tube furnace at 800 ℃, 850 ℃, 900 ℃, 950 ℃ under the same condition of carbonization, respectively.
The
supplementary
material
supplied
the
details
about
the
phosphorylation. The as-prepared samples were appointed as SP1, SP2, SP3, SP4 from low to high temperature, respectively. Besides, commercial activated carbon (AC), with abundant surface area, was served as a contrast without N and P. The prepared carbon treated at 850 ℃ without phosphorylation, named SP0, was arranged as the control with inherent nitrogen. And the prepared carbon activated with KOH at 850 ℃ for enough surface area, named SP5, was observed to explore the difference between N, P dual-doping and pure N doping of carbon materials. 2.3 preparations of cathode The cathode was fabricated by rolling-press method[22]. It consisted of three parts: conductive gas diffusion layer (CGDL), stainless steel mesh (SSL) and catalyst layer
(CL). The hydrophobic CGDL was made by mixing carbon black and a proper amount of ethanol in a 100mL beaker with ultrasonication until the mixture was almost dried. Then the polytetrafluoroethylene (PTFE) was added to the beaker with carbon black and PTFE’s ratio of 3:7. When the mixture was like a paste, it was rolled on the SSL which was immersed in ethanol for a several minutes in advance. In the end, the paste was heated at 340 ℃ for 20 min to sinter and turn to hard. The CL was established with the prepared carbon and PTFE at a mass ratio of 6:1. It was rolled on the other side of the SSL which was cut into a little circular piece with a diameter of 3 cm. Finally, the cathode was put into biochemical incubator for 6 h. Moreover, the anode preparation process was detailed in the supplementary. 2.4 Assembling and cultivation of MFC The assembling single-chamber MFC with 28 mL inner volumes was designed to be cylindrical internally and cuboid externally. The distance of cathode and anode was set 4 cm. The effective area of the electrodes was about 7 cm2 (diameter 3 cm). The cell containing the electrodes was put together by titanium wires, silicon gasket, sealing washer, rubber plug, bolt and nut. After assembly, it was checked for leakage. At the beginning of cultivation, half of the domestic wastewater (JinGu sewage treatment plant, Tianjin, China) used as inoculated substrate and half of the nutrient solution consisted of 50 mM phosphate buffer solution (PBS, pH=7) and 2 g·L-1 sodium acetate were injected into the cell together. Contents of PBS were described in the supplemental material. At the first week, 2/3 of the MFC fluid was replaced with
1/3 of the wastewater and 1/3 of the nutrient every day. Then 1/2 of the MFC was updated by the nutrient solution and sodium acetate every two days for about two months. During the cultivation, external resistance (1000 Ω) was linked to the MFC, with a data acquisition system (Morpheus Electronic Co.Ltd, Beijing, China) to record the voltages each minute. All the MFC devices was kept at 30 ℃ in a thermostatically controlled incubator. 2.5 Material characterization and electrochemical analyses To obtain the morphological characteristic of the samples, the scanning electron microscope (SEM, S-3500N, Hitachi) equipped with an energy-dispersive X-ray spectrometer (EDS) and field emission transmission electron microscope (FE-TEM, Tecnai G2 F20 S-Twin) were employed. To get specific surface areas and pore structure, Brunauer-Emmett-Teller (BET), a means of adsorption and desorption, was shown on the condition of nitrogen with an adsorption meter (ASAP 2460 / Tristar 3000, MICROMERITICS). And the pore size distribution was completed by the method of Horvath-Kawazoe (HK) and BarrettJoyner-Halenda (BJH). X-ray photoelectron spectroscopy (XPS), a method of analyzing elemental content and state, was used to investigate surface electrochemical characteristics with a XPS spectrometer (ESCALAB 250 XI, ThermoFisher Scientific, USA) scanning from 1200 eV to 0 eV. Fourier transform infrared spectroscopy and Raman spectroscopy were also
employed to analyze the mechanism whose parameters were outlined in supplementary. The performance of oxygen reduction reaction (ORR) of cathode was detected by linear sweep voltammetry (LSV) using a potentiostat (Versa STAT 3, Princeton Applied Research, USA) with a three-electrode system. The three-electrode system included the reference electrode using Ag/AgCl electrode (0.195 V versus a standard hydrogen electrode, Aidahengsheng Electrode Co. Ltd., Tianjin, China), the counter electrode using platinum sheet (1 cm2), and the working electrode using the aforementioned cathode. The scanning voltages were set from OCP to -0.3 V at a rate of 0.1 mV·s-1. Electrochemical impedance spectroscopy (EIS) was introduced to measure the resistance of cathode by imposing sine wave additionally with the frequency changed from 100 kHz to 10 mHz under OCP. According to the spectra, Nyquist plots could be fitted for further interpretation. Another important electrochemical index, exchange current density (i0, A·cm-2), was calculated through Tafel curves, with the overpotential range of 0~100 mV. The Butler-Volmer equation was applied below[22]. i Fcathode lg( ) i0 2.303RT (1)
In the equation, η is served as the cathodic overpotential (V), R is as the ideal gas constant (8.31 J·mol-1·K-1), T is for the absolute temperature (K), β is the symmetry factor (a constant), F is the Faraday’s constant (96485·C·mol-1), i is the current
density (mA·m-2) and i0 is the exchange current density. The power density and polarization curves were obtained by using the resistance box, multimeter and reference electrode (Ag/AgCl). Before the testing, the MFCs were placed for at least 6 h under the open circuit potential (OCP) without the peripheral resistance. The total voltage, cathode voltage and anode voltage could be achieved by changing the resistance box from 9000 Ω to 70 Ω, and once the resister was adjusted, 10 minutes were necessary for a stabilization. 3. Results and discussion 3.1 The morphological and physical characterization of cathode materials The morphological characterization of the cathode catalysts by SEM and TEM was revealed in Fig. 1. As seen from Fig. 1a and b, there were more wrinkles on the surface of SP2 than SP0 caused by dual doping[12]. The anomalous surface could disturb the distribution of uniform charge density and spin density which lead to better ORR performance[23]. For understanding the material of N, P dual-doped carbon clearly, the EDS images of SP2 were mapped for C, N and P in Fig. 1c. Besides, it could be seen clearly that more pores appeared in SP2 than in SP0 according to Fig. 1d and e, which was also proved by BET test results. The nitrogen adsorption and desorption isotherms were laid out in Fig. S1, and the BET surface area and pore volume were shown in Table 1. If the nitrogen adsorption and desorption isotherms bent sharply at low relative pressure (P/P0 < 0.4), it implied the materials existed micropore. And if the isotherms showed a hysteresis loop at high
relative pressure (P/P0 = 0.4~1.0), it revealed the existence of mesoporous[24]. Judging from Fig. S1a, the adsorption and desorption curves belonged to type-I isotherms, and there was no apparent hysteresis loop, which could indicate the existence of mesoporous structure[25], just as the result of SP5 in Fig. S1b. According to the above, N, P-codoped carbon materials were identified as micropore structure. Besides, pretreated carbon was certified as no-pore structure, which was the main reason to cause unstable OCP and huge Rct. In addition, the specific surface area of SP2 was 982.18 m2·g-1, the largest one in all dual-doped samples, which was composed of 306.32 m2·g-1 of mesoporous area and 675.86 m2·g-1 of micropore area. Its surface area was 78.1 times bigger than SP0 (12.26 m2·g-1). Meanwhile, it was higher than that of SP1 (140.08 m2·g-1), SP3 (891.44 m2·g-1), SP4 (889.07 m2·g-1) for 601%, 10.18% and 10.47%, respectively. Compared to other samples, SP2, possessing the much more mesoporous area and similar micropore area, demonstrated the most excellent performance of ORR in all N, P-codoped samples. What’s more, the BET surface area of SP2 was much larger than that of other air-cathode materials, like carbon nanotube[26], carbon nanofiber[4], and graphitic biochar[27], demonstrating the fantastic advantage of SP2. Amazingly, SP5 obtained the highest special surface area (2295 m2·g-1), 134% increase compared to SP2. On the other hand, it had a lower current density than SP2, probably because the micropore area of SP2 was 1.8 times as big as SP5 under the effect of phosphoric acid. In previous studies, the mesopore could deliver oxygen effectively, and the micropore supplied most
active sites for electrons on the air cathode[28, 29]. AC also had a larger specific surface area according to Table 1, while it showed poorer electrochemical activity, which was due to the absence of phosphorus. In brief, both micropore and mesopore had a remarkable capacity, and appropriate micropore and mesopore area could enhance the activity of ORR. Additionally, the pore volume of SP2 was also the highest among N, P-codoped samples, which further indicated that appropriate temperature created more pore. Phosphoric acid could’t be disintegrated adequately at the lower temperature so there was no enough gas going out to extend the pore volume, while too high temperature might make the pores collapse[30]. Other materials heated at a much higher temperature also got a slightly smaller total pore volume[31, 32]. The micropore volume / total pore volume ratios of all samples were calculated to be all above 50%, which further supported the view of the micropore structure. Moreover, all samples had almost identical distribution of pore size (most 1.18 nm) except SP0 which had slight pore volume. To sum up, it was inferred that phosphoric acid could help enlarge the surface area of carbon derived from chitosan which was possibly caused by the gas from pyrolysis of the phosphorus acid or the steam of dehydration. The enlarged surface could serve as the active sites and promote the electron transfer during oxygen reduction reaction. In order to further explore the catalytic mechanism of N and P dual-doped carbon, the methods of structural characterization were employed to analyze the difference of
SP2 and SP0. Firstly, FTIR illustrated the present surface groups and changes of chemical structure of the phosphorylated and unmodified samples at different temperatures. The spectra of different samples were shown in Fig. S2a, and a narrower spectral range (1600~600 cm-1) covered the most absorption wavelength was drawn in Fig. S2b. The corresponding functional groups were summed in Table S1. Depending on Fig. S2a, the spectra of all five samples presented the similar absorption peak in a broader spectral range (4000~400 cm-1), and the detailed analysis was described in the supplementary materials. In short, SP2 had more phosphorous groups, including –HN–P, –C–N–P and C–O–P, which might be the decisive factor that SP2 obtained the highest ORR activity. On the other hand, it was carried out to investigate the difference between N, P dual-doping and pure N doping, and it turned out that N and P dual-doping could further promote the electron transfer[33]. It was concluded that the significant decrease of Rd and Rct in SP2 compared to SP0 was probably due to the presence nitrogen and phosphorus groups. Secondly, XPS was utilized to analyze the presence and types of elements in the SP2. The element contents were shown in Table 2, including SP0 and SP2, and the detailed chemical compositions of different elements were plotted in Table 3. The survey spectra evidenced the contents of C1s (285.07 eV), O1s (532.43 eV), N1s (400.95 eV) and P2p (133.57 eV). After phosphorylation at high temperature, the elemental composition had a great difference. Just as the following, carbon level had a drop from 87.07% to 77.67%, which probably was caused by the introduction
of phosphorus, overflow of carbon dioxide, and the oxygen content raising from acid groups at the higher temperature. Simultaneously, the nitrogen level was almost invariable and phosphorus atom was introduced. Thus, it could be inferred that the high contents of nitrogen, phosphorus and residual oxygen-containing functional groups promoted the activity of ORR[11]. The C1s spectra, shown in Fig. 2b, was divided into four carbon contributions, consisting of sp2-hybridization (C=C, 284.5 eV), sp3-hybridization (C-C, 285.0 eV), C-N bonds (286.2 eV) and C-O bonds (286.6 eV)[9, 25]. Detailed contributions could be found in Table 3, and there was a decrease in percent of sp2-hybridization, which might result from the attack of phosphorus atom, while the sp3-hybridization had a slight change due to stable structure of carbon-carbon bonds. The introduction of phosphorus heterocyclic atom affected the change of olefinic bonds. Meanwhile, C-N bond percentages were basically unchanged for they were hard to be destroyed at high temperature. Amazingly, the percentage of C-O bonds was significantly 6.88 times higher than that of the control, and the added part was probably from the phosphorus oxygen bonds in phosphoric acid. The N1s spectrum in Fig. 2c was deconvoluted into four spin-orbit doublets, including pyridinic-N (N1, 398.3 eV), pyrrolic-N (N2, 399.2 eV), graphitic-N (N3, 401.2 eV) and valley-N (N4, 402.5 eV)[25, 34]. It was widely known that pyridinicN and graphitic-N were beneficial for electron transfer to enhance catalytic activity[15]. As shown in Table 3, SP2 possessed more N1 and N3, accounting for
roughly 61.43%, while SP0 was only almost 23.9%. The content of pyridinic-N in two samples was almost the same while pyrolyzing at 850 ℃[34]. Besides, the content of graphitic -N in SP2 was much higher than that in SP0, as observed in Table 3. That was to say that the obvious increase of graphitic -N percentage was caused by the phosphorization during annealing process. Considering the small current exchange density of SP0, it was inferred that pyrrolic-N and valley-N were ineffective ORR catalytic sites[35]. The P2ps peak of SP2 could be deconvoluted into two components demonstrated in Fig. 2d, and the two resolved peaks at different binding energy were severally assigned as the bond of C-N-P (P1, 132.9 eV) and the bond of C-O-P (P2, 134.5 eV)[36]. It was obvious that the intensities of the two peaks had a difference, which could be concretely observed from Table 3. The content of P2 was 6.8 times more than that of P1, which was on account of the easy introduction of phosphorus oxygen bond from acid during annealing[16]. In addition, there was no P2p spectrum in SP0 without activization by ortho-phosphoric acid confirming that there was a strong correlation between the excellent activity of ORR and the doping of heterocyclic phosphorus atom. From the above, both FTIR and XPS testified the high content of N and P functional groups. And it was shown in Table 3 that SP2 obtained more carbon oxygen bonds after high-temperature phosphating. The oxygen-containing functional groups could enhance the reaction kinetics and supply more catalytic
sites[37]. Especially, the C-O-P bonding was deemed to enhance electron poverty which was favourable for the ORR[7]. In addition, the C-N bonds were claimed to be charge shift bonds in protonated and methylated ammonium compounds, which were thought to lessen Rct. The pyridinic-N could provide a lone electron pair and contribute p-electron to the graphitic π-system[38], and the graphitic-N promised for facilitating the 4-electron transfer [17, 39]. In addition, the synergistic effect of N and P doping which could supply more electrons to the π-conjugated system of carbon promoted electron transfer even to improve the activity of ORR. In sum, the dual-doping made more defects and active sites for increasing the electrochemical activity[11]. It further showed that N and P co-modification was better than single modification under the contrast of SP2 and SP0. Raman spectroscopy was used to further confirm the structure of carbon materials, and the spectroscopy of different cathode materials were shown in Fig 3. It revealed the degree of graphitization of carbon materials by measuring the ratio of intensity of D-band approximately at 1595 cm-1 to intensity of G-band almost at 1350 cm-1 (ID / IG)[19]. The high ratio of ID/IG indicated that a highly disordered carbon or surface defects were present[40]. The ratios of intensities separately were 0.875, 0.846, 0.887, 0.909, 0.910 and 0.830 for SP0, SP1, SP2, SP3, SP4 and SP5, respectively. The ratios of phosphorylated samples were slightly bigger than the control (SP0) as a result of the introduction of defects made by phosphorus atoms[33] and structural collapse caused by the thermal annealing process[31]. The
surface defects caused by N, P-codoped were found to create more active sites[41] than SP0 and SP5. The results demonstrated that the doping phosphorus atoms successfully contributed to the domination of the amorphous carbon. And it was easily recognizable that SP1 with the poorer activity had the lower ratio illustrating the higher degree of graphitization for the fewer defects and active sites compared to SP2. In a word, the doping of phosphorus changed the structure of carbon materials and formed more surface defects which were consistent with the above, while the N and P dual-doping strengthened the performance of ORR because of the faster electron transfer and the more active sites. 3.2 Electrochemical analyses of cathode LSV curves were used to measure the activity of ORR, which were presented in Fig. 4. As displayed in the curves, the current densities of N, P-codoped samples were prominently higher than the control (SP0), which could further demonstrate that the synergy of N and P made a profound influence on the ORR catalysis process. In Fig. 4a, the OCP above was displayed in descending order: SP3 (0.366 V), SP2 (0.343 V), SP4 (0.321 V), SP1 (0.279 V), and the current density at the end of the scanning voltage were all over 16.00 mA·cm-2, higher than that of pure N-doping[8] and pure P-doping[31]. The OCP in Fig. 4b was shown descendingly as follows: SP0 (0.315 V), SP5 (0.219V), AC (0.187 V), while the current density of SP2 (23.64 mA·cm-2) was 3.4 times as high as that of SP0 (7.04 mA·cm-2). The current density of N, Pcodoped sample SP2 was also higher than that of other N-doped carbon[42] and P-
doped carbon[32] individually. The OCP of N doped only or N, P-codoped samples were all higher than most air cathode materials including the common activated carbon AC in this work (0.187 V), which meant doping heteroatoms could enhance the OCP. Obviously, the SP3 got the highest potentials, while it didn’t achieve the highest current density, probably because the treatment temperature was already too high and limited the performance of ORR, which was further analyzed later. That was to say, the higher treated temperature didn’t always work. The current densities of SP2 were much higher than that of other N or P-doped carbon like graphene[10, 39], carbon nanotube[34] and activated carbon[8, 31] at the same scanning voltage. In short, SP2 manifested the excellent ORR catalytic activity with N, P-codoping, indicating that the synergy between N and P was much better than the individual dopant. EIS was employed to verify the extraordinary performance of ORR, which was shown in Fig. 5 and Fig. S3. The equivalent circuit was adopted consisting of three types of dominant resistances: an ohmic resistance (Ro), a diffusion resistance (Rd), a charge transfer (Rct)[18]. The calculated resistances of different cathodes were listed with detail in Table 4, and the total resistance Rt was exhibited in the ascending order: SP2 (9.8301 Ω), SP1 (11.0689 Ω), SP3 (12.572 Ω), SP5 (12.8488), SP4 (13.168 Ω), AC (15.759), SP0 (17.05 Ω). The N, P-codoped samples possessed the lower total resistance than the others, and SP2 showed the minimum total resistance which was consistent with the result of LSV. It was found that Rt of doping N and P samples was
much lower than that of activated carbon treated with different acid[8]. More specifically, the Ro of the samples were all smaller than AC (11.853 Ω), and the value of N-doped sample (SP5) was the second largest. In addition, the Rd of SP2 was the lowest among the samples and the Rct of it was only inferior to that of SP5. This result was probably connected with the structural characteristics. All in all, carbon modified N and P made a positive effect to decrease the resistance and the suitable temperature was of equal importance for the activity of ORR. Exchange current density (i0, A·cm-2), which represented the rate of electron transfer between electrolyte and electrode, was one of the important indexes for measuring the kinetics of ORR. Tafel curves and fitted lines were presented in Fig. 6, and linear fitting equations were exhibited in Table 5. Based on Eq. (1), i0 was calculated and it decreased as following order: SP5 (19.01×10-4 A·cm-2), SP2 (17.27×10-4 A·cm-2), SP3 (15.65×10-4 A·cm-2), SP4 (15.44×10-4 A·cm-2), SP1 (10.24×10-4 A·cm-2), AC (5.63×10-4 A·cm-2), SP0 (2.36×10-4 A·cm-2). According to the results, each N, P-codoped sample had a higher i0 than the control, standing for the better performance of ORR. In detail, the i0 of SP5 was increased 7.05 times compared to the control, while SP2 was increased for 6.32 times. It might be associated with the high specific surface area for more oxygen which was further discussed later. What’s more, the i0 of N, P-codoping was much higher than SP0 and AC in this work, which implied that the synergy of N and P was better than free modification and monoatomic modification. SP2, the best one in dual-doped samples,
was also better than other materials used as air-cathode catalyst in MFC, like activated carbon treated with HNO3 (7.52×10-4 A·cm-2, 130% higher) and activated carbon treated with H3PO4 (15.27×10-4 A·cm-2, 13.1% higher)[8]. In a nutshell, N, P-codoped carbon achieved a much higher kinetic activity than the contribution of nonheteroatom or mono-dopant. 3.3 The performance of MFC The power density was regarded as the main factor to weigh the capacity of electricity production of the MFC. To obtain stable voltage, all MFCs were tested after four weeks. And the curves were drawn in Fig. 7. From the power density curves in Fig. 7a, the optimized processing temperature (850 ℃ ) was found from SP2’s larger maximum power density (MPD) than any others. SP2 held the largest MPD of 1603.6 ± 80 mW·m-2 among all of them. Besides, SP1, SP3 and SP4 showed the MPD of 1003.2 ± 50 mW·m-2, 1488.2 ± 74 mW·m-2, 1308.5 ± 66 mW·m-2, respectively. Fig. 7c showed that the MPD of N, P-codoped catalyst was much higher than that of individual dopant and no dopant. The MPD of SP2 was a 11.7% increase compared to SP5 (1423.0 ± 43 mW·m-2), a 125% increase compared to AC (705.0 ± 34 mW·m-2), and 5 times as high as the control SP0 (322.4 ± 16 mW·m-2). The MPD was exactly corresponding with the results of LSV and Tafel curves, and it stated obviously that SP2 doped by N and P for synergy produced higher power density than SP0, SP5 and AC, which probably changed the chemical structure of carbon under the participation of phosphorous at high temperature. Comparing with other carbon materials, the MPD
of SP2 was higher than that of N-doped carbon aerogel[43], phosphatized activated carbon heated at 400 ℃[32], and N, P-codoped activated carbon heated at 800 ℃ in this work for 65.8%, 46.3%, and 25.5%, respectively. Moreover, the polarization curves changed almost the same. In Fig. 7b, the SP2 had the highest total potential and the cathode potential, with others following in the order: SP3, SP4, SP1, which further manifested the performance of cathode catalysts. The cathode voltages decreased with the same order as the polarization curves, while the cathode voltage of SP4 minished slower than others[44], which might cause of higher phosphorus content. And the anode voltages varied similarly to those of cathode. From Fig. 7d, SP2 showed the highest cathode potential, while it had a larger decline than SP5. According to the above, higher potential suggested a better catalyst, so the trend was achieved: SP2 > SP5 > AC > SP0. In short, high temperature broke the original structure of carbon, and introduced a phosphorus atom-containing heterocyclic group which was conducive for the performance of MFC. The stable performance was another important parameter for MFC. To verify the long-term stability of the catalysts, the voltages recorded by data acquisition system were plotted during the continuous cultivation for two months. The nutrient in the MFCs were renewed every two days. As shown in Fig. S4, SP2 showed a better performance than any others in this work. The stable voltage of SP2 were also higher than that of activated carbon[45] and N-doped carbon[43] air cathode MFCs. It proved that the N, P-dual-doped carbon remained active for a long time without
attenuating. 4. Conclusions In summary, we developed a low-cost, metal-free and highly effective aircathode catalyst. The N, P-codoped catalyst treated at 850 ℃ showed the maximum power density of 1603.6 ± 80 mW·m-2, 5 times as high as the control. The specific surface area was notably expanded in the value of 982.18 m2·g-1. Due to the synergy of N and P, electron transfer efficiency was accelerated, resulting in a smaller Rct, larger exchange current density, and much higher OCP. Consequently, N, Pcodoped carbon derived from chitosan was a promising catalyst for its simple technics, high yields and excellent cost-effectiveness. The strategy of N and P codoping and the method of high-temperature phosphorization are worth considering for higher power generation. Acknowledgments This work was supported by the project of the National Science Foundation of Tianjin (17JCYBJC23300) and National Key R&D Program of China (Grant No.2016YFC0400704). Reference [1] Y. Ahn, M.C. Hatzell, F. Zhang, B.E. Logan, Different electrode configurations to optimize performance of multi-electrode microbial fuel cells for generating power or treating domestic wastewater, Journal of Power Sources 249 (2014) 440-445. [2] C. Gao, A. Wang, W.M. Wu, Y. Yin, Y.G. Zhao, Enrichment of anodic biofilm inoculated with anaerobic or aerobic sludge in single chambered air-cathode microbial fuel cells, Bioresource technology 167 (2014) 124-132.
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Fig. 1 Optical photographs of (a) SEM image of SP0; (b) SEM image of SP2; (c) SEM image of SP2 and the corresponding elemental mapping images of C, N, and P; (d) TEM image of SP0; (e) TEM image of SP2.
C1s (a)
(b)
2
sp 3 sp C-O C-N
Intensity
Intensity
O1s
N1s P2p
1400 1200 1000
800
600
400
200
300
0
295
(c)
Binding energy / eV
275
395
C-O-P C-N-P
Intensity
Intensity
400
280
(d)
N1 N2 N3 N4
405
285
Binding energy / eV
Binding energy / eV
410
290
390
145
140
135
130
Binding energy / eV
Fig. 2 High resolution of fitted XPS: (a) sum XPS spectra of SP2; (b)the C1s spectra of SP2; (c) the N1s spectra of SP2; (d) the P2p spectra of SP2.
125
120
14000 12000
SP0 SP1 SP2 SP3 SP4 SP5
Intensity
10000 8000 6000 4000 2000 0 0
500 1000 1500 2000 2500 3000 3500 4000
Raman shift/cm
-1
Fig. 3 Raman spectrum of different air cathodes
0
0
(a)
-5
-10
Current density/mA cm
-2
-2
-5
Current density/mA cm
(b)
SP1 SP2 SP3 SP4
-15
-20
-10
SP0 SP2 SP5 AC
-15
-20
-25
-25 -0.3
-0.2
-0.1
0.0
0.1
0.2
Potential/V vs.Ag/AgCl
0.3
0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
Potential/V vs.Ag/AgCl
Fig. 4 LSV curves: test of different air cathodes.
0.3
0.4
2.5
1.6 1.4
(a)
SP1 SP2 SP3 SP4
1.2
2.0
SP0-CAL SP2-CAL SP5-CAL AC-CAL
1.5
Z''/Ω
1.0
Z''/Ω
SP0 SP2 SP5 AC
(b)
SP1-CAL SP2-CAL SP3-CAL SP4-CAL
0.8
1.0
0.6 0.5
0.4 0.2
0.0 0
1
2
3
Z'/Ω
4
5
0
2
4
Z'/Ω
Fig. 5 Nyquist plots of EIS: revealing the resistance in different air cathodes.
6
8
Fig. 6 Tafel plots of different cathodes by sweeping the over potential from 0 mV to 100 mV at 1 mv/s; and inset: linear fitting of over potential from 60 mV to 80 mV.
SP3-P SP3-V
SP4-P SP4-V
300
1800
200
1600
(a)
1400
600
1200
500
1000
400
800
300
600 400
200
200
100
Potential/mV (vs. Ag/AgCl)
700
SP2-P SP2-V
-2
Potential/mV (vs. Ag/AgCl)
800
SP1-P SP1-V
Power density/mW m
900
(c)
SP0-P SP0-V
SP2-P SP2-V
SP5-P SP5-V
AC-P AC-V
-200 -300 -400 -500
1600 1400
600
1200
500
1000 800 600
300
400
200
200
100
0 0
1
2
3
Current density/A m
4
-2
1
2
3
4
Current density/A m 300
1800
400
(b)
0
700
SP4-C SP4-A
-100
5
4
-2
SP3-C SP3-A
0
200
-2
800
3
Power density/mW m Potential/mV (vs. Ag/AgCl)
Potential/mV (vs. Ag/AgCl)
900
2
1
Current density/A m
SP2-C SP2-A
100
0 0
SP1-C SP1-A
SP0-C SP0-A
(d)
SP2-C SP2-A
-2
5
SP5-C SP5-A
AC-C AC-A
4
5
100 0 -100 -200 -300 -400 -500 -600 0
5
1
2
3
Current density/A m
-2
Fig. 7 The performance of MFCs. (a) and (c): polarization and power density curves; (b) and (d): cathodes and anodes potential curves. Table 1 Porous structure parameters of different cathodes. SP0
SP1
SP2
SP3
SP4
SP5
AC
BET Surface Area(m2/g)
12.26
140.08
982.18
891.44
889.07
2295.01
2190.25
Micropore Area(m2/g)
3.48
126.00
675.86
738.71
749.61
368.74
1319.54
Mesopore Area(m2/g)
8.77
14.09
306.32
152.73
139.46
1926.27
870.71
Total pore volume(cm3/g)
0.010
0.059
0.416
0.372
0.369
1.209
0.942
Micropore volume(cm3/g)
0.002
0.052
0.269
0.299
0.305
0.146
0.516
Average Pore Size(nm)
3.15
0.69
1.69
1.67
1.66
1.90
1.72
Table 2 Results of ultimate analysis by XPS of SP0 and SP2. Component
Elemental Content (at. %)
C1s
O1s
N1s
P2p
SP0
87.07
8.72
4.21
-
SP2
77.67
16.96
3.03
2.35
Table 3 The distribution of functional groups obtained from the deconvolution of C1s, N1s, and P2p peaks analyses C1s (%)
Sample
N1s (%)
sp2
sp3
C-N
C-O
SP0
78.91
4.68
14.03
2.39
21.64 69.91
SP2
66.37
3.57
11.22
18.83
13.21 11.23
N1
N2
P2p (%)
N3
N4
P1
P2
2.26
6.30
-
6 48.22 27.34
11.3
88.7
Table 4 Fitting resistance of different cathodes based on the equivalent circuit. SP0
SP1
SP2
SP3
SP4
SP5
AC
R0 (Ω)
9.268
5.622
7.263
9.109
7.97
10.272
11.853
Rd (Ω)
2.938
0.5729
0.4784
0.801
1.035
0.597
1.012
Rct (Ω)
4.844
4.874
2.089
2.662
4.163
1.979
2.894
Rt (Ω)
17.05
11.0689
9.8301
12.572
13.168
12.848
15.759
Table 5 Exchange current density calculated from the Tafel plots. The air cathode
Linear fitting equation(R2)
10-4i0(Acm-1)
SP0
y=-3.6270+4.3721x (0.9991)
2.36
SP1
y=-2.9910+4.7430x (0.9979)
10.21
SP2
y=-2.7626+4.6875x (0.9977)
17.27
SP3
y=-2.8070+4.9765x (0.9968)
15.60
SP4
y=-2.8373+4.8734x (0.9973)
14.54
SP5
y=-2.7211+4.4864x (0.9965)
19.01
AC
y=-3.2494+4.7302x (0.9984)
5.63
HIGHLIGHTS ● A N and P dual-doped carbon derived from chitosan used as cathode catalyst.
● The MPD was 1603.6 ± 80 mW·m-2, 5 times as high as the control. ● The phosphating increased the OCP, surface area, and transformation of graphiticN. ● Co-doped carbon increased the content of C-O and the active sites caused by defects.
S1385-8947(18)31934-X https://doi.org/10.1016/j.cej.2018.09.217 CEJ 20056
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
5 July 2018 25 September 2018 29 September 2018
Please cite this article as: B. Liang, K. Li, Y. Liu, X. Kang, Nitrogen and phosphorus dual-doped carbon derived from chitosan: an excellent cathode catalyst in microbial fuel cell, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.09.217
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Nitrogen and phosphorus dual-doped carbon derived from chitosan: an excellent cathode catalyst in microbial fuel cell Bolong Liang a, b, Kexun Li a, b*, Yi Liu a, b, Xiaowen Kang c a The College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China b MOE Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation Pollution Control, Nankai University, Tianjin 300071, China c Energy Research Institute (ERI), National Development and Reform Commission (NDRC)Chinese Academy of macroeconomic Research (CAMR), Beijing, 100824, China
Abstract Carbon derived from chitosan, followed by phosphoric acid activation during thermal treatment to obtain N and P dual-doped catalyst, is studied as the catalyst for air-cathode in a microbial fuel cell (MFC). The maximum power density (MPD) of 1603.6 ± 80 mW·m-2 is achieved, which is 5 times as high as that of the control (322.4 ± 16 mW·m-2) when N, P-codoped carbon was calcined at 850 ℃. The better performance is due to a higher open circuit potential, lower total resistance and higher exchange current density. In addition, the optimized catalyst possessed the largest
Corresponding author: Kexun Li: Tel. :+86-22-23501117; Fax: +86-22-23495200 E-mail:
[email protected].
specific surface area of 982.18 m2·g-1, so it could transfer more oxygen and supplied added active sites. And the existence of graphitic nitrogen, phosphorus and high content of carbon oxygen bonds promoted the cathode performance in MFC. In brief, the N and P dual-doped carbon from chitosan was potentially low-cost catalyst for the high performance of MFC. Keywords: microbial fuel cell, carbonization, phosphorylation, oxygen reduction reaction 1. Introduction Microbial fuel cell (MFC), a new energy recovery technology, is fabricated to degrade organic and inorganic substances and generate electricity in waste water simultaneously by utilizing current-producing bacteria[1]. The single-chambered MFC with the same sewage immersing the cathode and anode gradually gets attention because of its simple structure, low cost and high efficiency[2, 3]. Noteworthily, the air-cathode catalyst has been considered as the crucial factor to improve the activity of oxygen reduction reaction (ORR)[4] in single-chambered MFC, because better ORR activity of catalyst means better performance for the MFC. Platinum (Pt) as cathode catalyst has a good performance of ORR while its steep cost determines that it can’t be applied practically[5]. Hence, it is necessary to seek a kind of low cost and efficient cathode catalyst. Currently, activated carbon materials have been studied extensively due to the minimal cost, high surface area and excellent electrochemical properties. Based on these advantages various kinds of modified carbon materials[6,
7] emerge rapidly to further promote the performance of ORR. Recently, doping heteroatoms (N and P) in carbon-based materials such as activated carbon[8], carbon nanotube[9], and graphene[10], has been proved to improve electrochemical reactivity of ORR significantly[11]. It is proposed that doping carbon with two or more selected heteroatoms could further improve the catalytic activity of metal-free carbon catalysts owing to the synergistic effects between the heteroatoms[12]. The N, P-codoped hierarchical porous foams exhibited superior ORR activity close to Pt/C [13]. Polydopamine-derived N, P-codoped micro porous carbon spheres synthesized through the self-polymerization of dopamine induced by the phosphonic species showed efficient performance towards electrocatalytic oxygen reduction[14]. Therefore, carbon modified by N and P is regarded as a promising cathode material[15-19]. Meanwhile, carbon obtained from different precursors such as chitosan[3], cellulose[16] and bamboo charcoal[20] has been reported to introduce heteroatoms in many prior researches. What is noteworthy is that chitosan (2–amino-2–deoxy–D-glucose) is naturally rich of nitrogen due to the presence of amino. Hence, chitosan can be chosen as the precursor to prepare Ndoped carbon materials retrenching the procedure to introduce nitrogen. Furthermore, carbon could successfully carry heterocyclic phosphorus atom activated by phosphoric acid, which substantially accelerated the activity of ORR. Such a facile, scalable and high-yield synthesis of N, P-codoped carbon material would play an important role to serve as the efficient cost-effective electrocatalysts.
Although modified chitosan has been investigated much on electrochemical properties, few studies were reported concerning N, P-codoped carbon derived from chitosan used as air-cathode catalyst in MFC up to now. In this study, we took chitosan as the precursor and developed a low-cost, metal-free and highly effective method to synthesize the air-cathode catalyst. Carbon made by chitosan was heated to 800 ℃, 850 ℃, 900 ℃, 950 ℃ for optimization at the attendance of phosphoric acid, respectively. It was proved to be a superior catalyst with admirable power density. In addition, the performances of the prepared carbon material were investigated and a series of material characterization was analyzed for the mechanism such as BrunauerEmmett-Teller, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. 2. Experimental 2.1 chemicals Chitosan ( (C6H11NO4)n, Lanji Technology development Co. Ltd, Shanghai, China), Phosphoric acid (H3PO4, Ailian electronic technology Co. Ltd, Tianjin, China),
Carbon
black
(Jinqiushi
Chemical
Co.
Ltd,
Tianjin,
China),
Polytetrafluoroethylene (PTFE, 60wt%, Hesen, Electrical Co, Ltd, Shanghai, China), Sodium acetate (CH3COONa, Meryer chemical technology Co. Ltd, Shanghai, China), Ammonium chloride (NH4Cl, Meryer chemical technology Co. Ltd, Shanghai, China), Sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O, Ailan chemical technology Co. Ltd, Shanghai, China), Disodium hydrogen phosphate
(Na2HPO4, Meryer chemical technology Co. Ltd, Shanghai, China). All the reagents above were analytical and used without further purification. Nitrogen (N2, 99.99%, Huanyu gas Co. Ltd, Tianjin, China). 2.2 Fabrication of catalyst Chitosan was heated to 350 ℃ at the rate of 5 ℃ min-1 from room temperature in a tube furnace, and it kept the temperature for 3 h at the presence of N2 with a rate of 300 mL·min-1[21]. The pretreated carbon was prepared for next step at this stage. The pretreated carbon above was soaked with 0.5 M H3PO4. Then it was dried in the oven at 80 ℃ for 12 h. The dried sample was calcined afterwards in the tube furnace at 800 ℃, 850 ℃, 900 ℃, 950 ℃ under the same condition of carbonization, respectively.
The
supplementary
material
supplied
the
details
about
the
phosphorylation. The as-prepared samples were appointed as SP1, SP2, SP3, SP4 from low to high temperature, respectively. Besides, commercial activated carbon (AC), with abundant surface area, was served as a contrast without N and P. The prepared carbon treated at 850 ℃ without phosphorylation, named SP0, was arranged as the control with inherent nitrogen. And the prepared carbon activated with KOH at 850 ℃ for enough surface area, named SP5, was observed to explore the difference between N, P dual-doping and pure N doping of carbon materials. 2.3 preparations of cathode The cathode was fabricated by rolling-press method[22]. It consisted of three parts: conductive gas diffusion layer (CGDL), stainless steel mesh (SSL) and catalyst layer
(CL). The hydrophobic CGDL was made by mixing carbon black and a proper amount of ethanol in a 100mL beaker with ultrasonication until the mixture was almost dried. Then the polytetrafluoroethylene (PTFE) was added to the beaker with carbon black and PTFE’s ratio of 3:7. When the mixture was like a paste, it was rolled on the SSL which was immersed in ethanol for a several minutes in advance. In the end, the paste was heated at 340 ℃ for 20 min to sinter and turn to hard. The CL was established with the prepared carbon and PTFE at a mass ratio of 6:1. It was rolled on the other side of the SSL which was cut into a little circular piece with a diameter of 3 cm. Finally, the cathode was put into biochemical incubator for 6 h. Moreover, the anode preparation process was detailed in the supplementary. 2.4 Assembling and cultivation of MFC The assembling single-chamber MFC with 28 mL inner volumes was designed to be cylindrical internally and cuboid externally. The distance of cathode and anode was set 4 cm. The effective area of the electrodes was about 7 cm2 (diameter 3 cm). The cell containing the electrodes was put together by titanium wires, silicon gasket, sealing washer, rubber plug, bolt and nut. After assembly, it was checked for leakage. At the beginning of cultivation, half of the domestic wastewater (JinGu sewage treatment plant, Tianjin, China) used as inoculated substrate and half of the nutrient solution consisted of 50 mM phosphate buffer solution (PBS, pH=7) and 2 g·L-1 sodium acetate were injected into the cell together. Contents of PBS were described in the supplemental material. At the first week, 2/3 of the MFC fluid was replaced with
1/3 of the wastewater and 1/3 of the nutrient every day. Then 1/2 of the MFC was updated by the nutrient solution and sodium acetate every two days for about two months. During the cultivation, external resistance (1000 Ω) was linked to the MFC, with a data acquisition system (Morpheus Electronic Co.Ltd, Beijing, China) to record the voltages each minute. All the MFC devices was kept at 30 ℃ in a thermostatically controlled incubator. 2.5 Material characterization and electrochemical analyses To obtain the morphological characteristic of the samples, the scanning electron microscope (SEM, S-3500N, Hitachi) equipped with an energy-dispersive X-ray spectrometer (EDS) and field emission transmission electron microscope (FE-TEM, Tecnai G2 F20 S-Twin) were employed. To get specific surface areas and pore structure, Brunauer-Emmett-Teller (BET), a means of adsorption and desorption, was shown on the condition of nitrogen with an adsorption meter (ASAP 2460 / Tristar 3000, MICROMERITICS). And the pore size distribution was completed by the method of Horvath-Kawazoe (HK) and BarrettJoyner-Halenda (BJH). X-ray photoelectron spectroscopy (XPS), a method of analyzing elemental content and state, was used to investigate surface electrochemical characteristics with a XPS spectrometer (ESCALAB 250 XI, ThermoFisher Scientific, USA) scanning from 1200 eV to 0 eV. Fourier transform infrared spectroscopy and Raman spectroscopy were also
employed to analyze the mechanism whose parameters were outlined in supplementary. The performance of oxygen reduction reaction (ORR) of cathode was detected by linear sweep voltammetry (LSV) using a potentiostat (Versa STAT 3, Princeton Applied Research, USA) with a three-electrode system. The three-electrode system included the reference electrode using Ag/AgCl electrode (0.195 V versus a standard hydrogen electrode, Aidahengsheng Electrode Co. Ltd., Tianjin, China), the counter electrode using platinum sheet (1 cm2), and the working electrode using the aforementioned cathode. The scanning voltages were set from OCP to -0.3 V at a rate of 0.1 mV·s-1. Electrochemical impedance spectroscopy (EIS) was introduced to measure the resistance of cathode by imposing sine wave additionally with the frequency changed from 100 kHz to 10 mHz under OCP. According to the spectra, Nyquist plots could be fitted for further interpretation. Another important electrochemical index, exchange current density (i0, A·cm-2), was calculated through Tafel curves, with the overpotential range of 0~100 mV. The Butler-Volmer equation was applied below[22]. i Fcathode lg( ) i0 2.303RT (1)
In the equation, η is served as the cathodic overpotential (V), R is as the ideal gas constant (8.31 J·mol-1·K-1), T is for the absolute temperature (K), β is the symmetry factor (a constant), F is the Faraday’s constant (96485·C·mol-1), i is the current
density (mA·m-2) and i0 is the exchange current density. The power density and polarization curves were obtained by using the resistance box, multimeter and reference electrode (Ag/AgCl). Before the testing, the MFCs were placed for at least 6 h under the open circuit potential (OCP) without the peripheral resistance. The total voltage, cathode voltage and anode voltage could be achieved by changing the resistance box from 9000 Ω to 70 Ω, and once the resister was adjusted, 10 minutes were necessary for a stabilization. 3. Results and discussion 3.1 The morphological and physical characterization of cathode materials The morphological characterization of the cathode catalysts by SEM and TEM was revealed in Fig. 1. As seen from Fig. 1a and b, there were more wrinkles on the surface of SP2 than SP0 caused by dual doping[12]. The anomalous surface could disturb the distribution of uniform charge density and spin density which lead to better ORR performance[23]. For understanding the material of N, P dual-doped carbon clearly, the EDS images of SP2 were mapped for C, N and P in Fig. 1c. Besides, it could be seen clearly that more pores appeared in SP2 than in SP0 according to Fig. 1d and e, which was also proved by BET test results. The nitrogen adsorption and desorption isotherms were laid out in Fig. S1, and the BET surface area and pore volume were shown in Table 1. If the nitrogen adsorption and desorption isotherms bent sharply at low relative pressure (P/P0 < 0.4), it implied the materials existed micropore. And if the isotherms showed a hysteresis loop at high
relative pressure (P/P0 = 0.4~1.0), it revealed the existence of mesoporous[24]. Judging from Fig. S1a, the adsorption and desorption curves belonged to type-I isotherms, and there was no apparent hysteresis loop, which could indicate the existence of mesoporous structure[25], just as the result of SP5 in Fig. S1b. According to the above, N, P-codoped carbon materials were identified as micropore structure. Besides, pretreated carbon was certified as no-pore structure, which was the main reason to cause unstable OCP and huge Rct. In addition, the specific surface area of SP2 was 982.18 m2·g-1, the largest one in all dual-doped samples, which was composed of 306.32 m2·g-1 of mesoporous area and 675.86 m2·g-1 of micropore area. Its surface area was 78.1 times bigger than SP0 (12.26 m2·g-1). Meanwhile, it was higher than that of SP1 (140.08 m2·g-1), SP3 (891.44 m2·g-1), SP4 (889.07 m2·g-1) for 601%, 10.18% and 10.47%, respectively. Compared to other samples, SP2, possessing the much more mesoporous area and similar micropore area, demonstrated the most excellent performance of ORR in all N, P-codoped samples. What’s more, the BET surface area of SP2 was much larger than that of other air-cathode materials, like carbon nanotube[26], carbon nanofiber[4], and graphitic biochar[27], demonstrating the fantastic advantage of SP2. Amazingly, SP5 obtained the highest special surface area (2295 m2·g-1), 134% increase compared to SP2. On the other hand, it had a lower current density than SP2, probably because the micropore area of SP2 was 1.8 times as big as SP5 under the effect of phosphoric acid. In previous studies, the mesopore could deliver oxygen effectively, and the micropore supplied most
active sites for electrons on the air cathode[28, 29]. AC also had a larger specific surface area according to Table 1, while it showed poorer electrochemical activity, which was due to the absence of phosphorus. In brief, both micropore and mesopore had a remarkable capacity, and appropriate micropore and mesopore area could enhance the activity of ORR. Additionally, the pore volume of SP2 was also the highest among N, P-codoped samples, which further indicated that appropriate temperature created more pore. Phosphoric acid could’t be disintegrated adequately at the lower temperature so there was no enough gas going out to extend the pore volume, while too high temperature might make the pores collapse[30]. Other materials heated at a much higher temperature also got a slightly smaller total pore volume[31, 32]. The micropore volume / total pore volume ratios of all samples were calculated to be all above 50%, which further supported the view of the micropore structure. Moreover, all samples had almost identical distribution of pore size (most 1.18 nm) except SP0 which had slight pore volume. To sum up, it was inferred that phosphoric acid could help enlarge the surface area of carbon derived from chitosan which was possibly caused by the gas from pyrolysis of the phosphorus acid or the steam of dehydration. The enlarged surface could serve as the active sites and promote the electron transfer during oxygen reduction reaction. In order to further explore the catalytic mechanism of N and P dual-doped carbon, the methods of structural characterization were employed to analyze the difference of
SP2 and SP0. Firstly, FTIR illustrated the present surface groups and changes of chemical structure of the phosphorylated and unmodified samples at different temperatures. The spectra of different samples were shown in Fig. S2a, and a narrower spectral range (1600~600 cm-1) covered the most absorption wavelength was drawn in Fig. S2b. The corresponding functional groups were summed in Table S1. Depending on Fig. S2a, the spectra of all five samples presented the similar absorption peak in a broader spectral range (4000~400 cm-1), and the detailed analysis was described in the supplementary materials. In short, SP2 had more phosphorous groups, including –HN–P, –C–N–P and C–O–P, which might be the decisive factor that SP2 obtained the highest ORR activity. On the other hand, it was carried out to investigate the difference between N, P dual-doping and pure N doping, and it turned out that N and P dual-doping could further promote the electron transfer[33]. It was concluded that the significant decrease of Rd and Rct in SP2 compared to SP0 was probably due to the presence nitrogen and phosphorus groups. Secondly, XPS was utilized to analyze the presence and types of elements in the SP2. The element contents were shown in Table 2, including SP0 and SP2, and the detailed chemical compositions of different elements were plotted in Table 3. The survey spectra evidenced the contents of C1s (285.07 eV), O1s (532.43 eV), N1s (400.95 eV) and P2p (133.57 eV). After phosphorylation at high temperature, the elemental composition had a great difference. Just as the following, carbon level had a drop from 87.07% to 77.67%, which probably was caused by the introduction
of phosphorus, overflow of carbon dioxide, and the oxygen content raising from acid groups at the higher temperature. Simultaneously, the nitrogen level was almost invariable and phosphorus atom was introduced. Thus, it could be inferred that the high contents of nitrogen, phosphorus and residual oxygen-containing functional groups promoted the activity of ORR[11]. The C1s spectra, shown in Fig. 2b, was divided into four carbon contributions, consisting of sp2-hybridization (C=C, 284.5 eV), sp3-hybridization (C-C, 285.0 eV), C-N bonds (286.2 eV) and C-O bonds (286.6 eV)[9, 25]. Detailed contributions could be found in Table 3, and there was a decrease in percent of sp2-hybridization, which might result from the attack of phosphorus atom, while the sp3-hybridization had a slight change due to stable structure of carbon-carbon bonds. The introduction of phosphorus heterocyclic atom affected the change of olefinic bonds. Meanwhile, C-N bond percentages were basically unchanged for they were hard to be destroyed at high temperature. Amazingly, the percentage of C-O bonds was significantly 6.88 times higher than that of the control, and the added part was probably from the phosphorus oxygen bonds in phosphoric acid. The N1s spectrum in Fig. 2c was deconvoluted into four spin-orbit doublets, including pyridinic-N (N1, 398.3 eV), pyrrolic-N (N2, 399.2 eV), graphitic-N (N3, 401.2 eV) and valley-N (N4, 402.5 eV)[25, 34]. It was widely known that pyridinicN and graphitic-N were beneficial for electron transfer to enhance catalytic activity[15]. As shown in Table 3, SP2 possessed more N1 and N3, accounting for
roughly 61.43%, while SP0 was only almost 23.9%. The content of pyridinic-N in two samples was almost the same while pyrolyzing at 850 ℃[34]. Besides, the content of graphitic -N in SP2 was much higher than that in SP0, as observed in Table 3. That was to say that the obvious increase of graphitic -N percentage was caused by the phosphorization during annealing process. Considering the small current exchange density of SP0, it was inferred that pyrrolic-N and valley-N were ineffective ORR catalytic sites[35]. The P2ps peak of SP2 could be deconvoluted into two components demonstrated in Fig. 2d, and the two resolved peaks at different binding energy were severally assigned as the bond of C-N-P (P1, 132.9 eV) and the bond of C-O-P (P2, 134.5 eV)[36]. It was obvious that the intensities of the two peaks had a difference, which could be concretely observed from Table 3. The content of P2 was 6.8 times more than that of P1, which was on account of the easy introduction of phosphorus oxygen bond from acid during annealing[16]. In addition, there was no P2p spectrum in SP0 without activization by ortho-phosphoric acid confirming that there was a strong correlation between the excellent activity of ORR and the doping of heterocyclic phosphorus atom. From the above, both FTIR and XPS testified the high content of N and P functional groups. And it was shown in Table 3 that SP2 obtained more carbon oxygen bonds after high-temperature phosphating. The oxygen-containing functional groups could enhance the reaction kinetics and supply more catalytic
sites[37]. Especially, the C-O-P bonding was deemed to enhance electron poverty which was favourable for the ORR[7]. In addition, the C-N bonds were claimed to be charge shift bonds in protonated and methylated ammonium compounds, which were thought to lessen Rct. The pyridinic-N could provide a lone electron pair and contribute p-electron to the graphitic π-system[38], and the graphitic-N promised for facilitating the 4-electron transfer [17, 39]. In addition, the synergistic effect of N and P doping which could supply more electrons to the π-conjugated system of carbon promoted electron transfer even to improve the activity of ORR. In sum, the dual-doping made more defects and active sites for increasing the electrochemical activity[11]. It further showed that N and P co-modification was better than single modification under the contrast of SP2 and SP0. Raman spectroscopy was used to further confirm the structure of carbon materials, and the spectroscopy of different cathode materials were shown in Fig 3. It revealed the degree of graphitization of carbon materials by measuring the ratio of intensity of D-band approximately at 1595 cm-1 to intensity of G-band almost at 1350 cm-1 (ID / IG)[19]. The high ratio of ID/IG indicated that a highly disordered carbon or surface defects were present[40]. The ratios of intensities separately were 0.875, 0.846, 0.887, 0.909, 0.910 and 0.830 for SP0, SP1, SP2, SP3, SP4 and SP5, respectively. The ratios of phosphorylated samples were slightly bigger than the control (SP0) as a result of the introduction of defects made by phosphorus atoms[33] and structural collapse caused by the thermal annealing process[31]. The
surface defects caused by N, P-codoped were found to create more active sites[41] than SP0 and SP5. The results demonstrated that the doping phosphorus atoms successfully contributed to the domination of the amorphous carbon. And it was easily recognizable that SP1 with the poorer activity had the lower ratio illustrating the higher degree of graphitization for the fewer defects and active sites compared to SP2. In a word, the doping of phosphorus changed the structure of carbon materials and formed more surface defects which were consistent with the above, while the N and P dual-doping strengthened the performance of ORR because of the faster electron transfer and the more active sites. 3.2 Electrochemical analyses of cathode LSV curves were used to measure the activity of ORR, which were presented in Fig. 4. As displayed in the curves, the current densities of N, P-codoped samples were prominently higher than the control (SP0), which could further demonstrate that the synergy of N and P made a profound influence on the ORR catalysis process. In Fig. 4a, the OCP above was displayed in descending order: SP3 (0.366 V), SP2 (0.343 V), SP4 (0.321 V), SP1 (0.279 V), and the current density at the end of the scanning voltage were all over 16.00 mA·cm-2, higher than that of pure N-doping[8] and pure P-doping[31]. The OCP in Fig. 4b was shown descendingly as follows: SP0 (0.315 V), SP5 (0.219V), AC (0.187 V), while the current density of SP2 (23.64 mA·cm-2) was 3.4 times as high as that of SP0 (7.04 mA·cm-2). The current density of N, Pcodoped sample SP2 was also higher than that of other N-doped carbon[42] and P-
doped carbon[32] individually. The OCP of N doped only or N, P-codoped samples were all higher than most air cathode materials including the common activated carbon AC in this work (0.187 V), which meant doping heteroatoms could enhance the OCP. Obviously, the SP3 got the highest potentials, while it didn’t achieve the highest current density, probably because the treatment temperature was already too high and limited the performance of ORR, which was further analyzed later. That was to say, the higher treated temperature didn’t always work. The current densities of SP2 were much higher than that of other N or P-doped carbon like graphene[10, 39], carbon nanotube[34] and activated carbon[8, 31] at the same scanning voltage. In short, SP2 manifested the excellent ORR catalytic activity with N, P-codoping, indicating that the synergy between N and P was much better than the individual dopant. EIS was employed to verify the extraordinary performance of ORR, which was shown in Fig. 5 and Fig. S3. The equivalent circuit was adopted consisting of three types of dominant resistances: an ohmic resistance (Ro), a diffusion resistance (Rd), a charge transfer (Rct)[18]. The calculated resistances of different cathodes were listed with detail in Table 4, and the total resistance Rt was exhibited in the ascending order: SP2 (9.8301 Ω), SP1 (11.0689 Ω), SP3 (12.572 Ω), SP5 (12.8488), SP4 (13.168 Ω), AC (15.759), SP0 (17.05 Ω). The N, P-codoped samples possessed the lower total resistance than the others, and SP2 showed the minimum total resistance which was consistent with the result of LSV. It was found that Rt of doping N and P samples was
much lower than that of activated carbon treated with different acid[8]. More specifically, the Ro of the samples were all smaller than AC (11.853 Ω), and the value of N-doped sample (SP5) was the second largest. In addition, the Rd of SP2 was the lowest among the samples and the Rct of it was only inferior to that of SP5. This result was probably connected with the structural characteristics. All in all, carbon modified N and P made a positive effect to decrease the resistance and the suitable temperature was of equal importance for the activity of ORR. Exchange current density (i0, A·cm-2), which represented the rate of electron transfer between electrolyte and electrode, was one of the important indexes for measuring the kinetics of ORR. Tafel curves and fitted lines were presented in Fig. 6, and linear fitting equations were exhibited in Table 5. Based on Eq. (1), i0 was calculated and it decreased as following order: SP5 (19.01×10-4 A·cm-2), SP2 (17.27×10-4 A·cm-2), SP3 (15.65×10-4 A·cm-2), SP4 (15.44×10-4 A·cm-2), SP1 (10.24×10-4 A·cm-2), AC (5.63×10-4 A·cm-2), SP0 (2.36×10-4 A·cm-2). According to the results, each N, P-codoped sample had a higher i0 than the control, standing for the better performance of ORR. In detail, the i0 of SP5 was increased 7.05 times compared to the control, while SP2 was increased for 6.32 times. It might be associated with the high specific surface area for more oxygen which was further discussed later. What’s more, the i0 of N, P-codoping was much higher than SP0 and AC in this work, which implied that the synergy of N and P was better than free modification and monoatomic modification. SP2, the best one in dual-doped samples,
was also better than other materials used as air-cathode catalyst in MFC, like activated carbon treated with HNO3 (7.52×10-4 A·cm-2, 130% higher) and activated carbon treated with H3PO4 (15.27×10-4 A·cm-2, 13.1% higher)[8]. In a nutshell, N, P-codoped carbon achieved a much higher kinetic activity than the contribution of nonheteroatom or mono-dopant. 3.3 The performance of MFC The power density was regarded as the main factor to weigh the capacity of electricity production of the MFC. To obtain stable voltage, all MFCs were tested after four weeks. And the curves were drawn in Fig. 7. From the power density curves in Fig. 7a, the optimized processing temperature (850 ℃ ) was found from SP2’s larger maximum power density (MPD) than any others. SP2 held the largest MPD of 1603.6 ± 80 mW·m-2 among all of them. Besides, SP1, SP3 and SP4 showed the MPD of 1003.2 ± 50 mW·m-2, 1488.2 ± 74 mW·m-2, 1308.5 ± 66 mW·m-2, respectively. Fig. 7c showed that the MPD of N, P-codoped catalyst was much higher than that of individual dopant and no dopant. The MPD of SP2 was a 11.7% increase compared to SP5 (1423.0 ± 43 mW·m-2), a 125% increase compared to AC (705.0 ± 34 mW·m-2), and 5 times as high as the control SP0 (322.4 ± 16 mW·m-2). The MPD was exactly corresponding with the results of LSV and Tafel curves, and it stated obviously that SP2 doped by N and P for synergy produced higher power density than SP0, SP5 and AC, which probably changed the chemical structure of carbon under the participation of phosphorous at high temperature. Comparing with other carbon materials, the MPD
of SP2 was higher than that of N-doped carbon aerogel[43], phosphatized activated carbon heated at 400 ℃[32], and N, P-codoped activated carbon heated at 800 ℃ in this work for 65.8%, 46.3%, and 25.5%, respectively. Moreover, the polarization curves changed almost the same. In Fig. 7b, the SP2 had the highest total potential and the cathode potential, with others following in the order: SP3, SP4, SP1, which further manifested the performance of cathode catalysts. The cathode voltages decreased with the same order as the polarization curves, while the cathode voltage of SP4 minished slower than others[44], which might cause of higher phosphorus content. And the anode voltages varied similarly to those of cathode. From Fig. 7d, SP2 showed the highest cathode potential, while it had a larger decline than SP5. According to the above, higher potential suggested a better catalyst, so the trend was achieved: SP2 > SP5 > AC > SP0. In short, high temperature broke the original structure of carbon, and introduced a phosphorus atom-containing heterocyclic group which was conducive for the performance of MFC. The stable performance was another important parameter for MFC. To verify the long-term stability of the catalysts, the voltages recorded by data acquisition system were plotted during the continuous cultivation for two months. The nutrient in the MFCs were renewed every two days. As shown in Fig. S4, SP2 showed a better performance than any others in this work. The stable voltage of SP2 were also higher than that of activated carbon[45] and N-doped carbon[43] air cathode MFCs. It proved that the N, P-dual-doped carbon remained active for a long time without
attenuating. 4. Conclusions In summary, we developed a low-cost, metal-free and highly effective aircathode catalyst. The N, P-codoped catalyst treated at 850 ℃ showed the maximum power density of 1603.6 ± 80 mW·m-2, 5 times as high as the control. The specific surface area was notably expanded in the value of 982.18 m2·g-1. Due to the synergy of N and P, electron transfer efficiency was accelerated, resulting in a smaller Rct, larger exchange current density, and much higher OCP. Consequently, N, Pcodoped carbon derived from chitosan was a promising catalyst for its simple technics, high yields and excellent cost-effectiveness. The strategy of N and P codoping and the method of high-temperature phosphorization are worth considering for higher power generation. Acknowledgments This work was supported by the project of the National Science Foundation of Tianjin (17JCYBJC23300) and National Key R&D Program of China (Grant No.2016YFC0400704). Reference [1] Y. Ahn, M.C. Hatzell, F. Zhang, B.E. Logan, Different electrode configurations to optimize performance of multi-electrode microbial fuel cells for generating power or treating domestic wastewater, Journal of Power Sources 249 (2014) 440-445. [2] C. Gao, A. Wang, W.M. Wu, Y. Yin, Y.G. Zhao, Enrichment of anodic biofilm inoculated with anaerobic or aerobic sludge in single chambered air-cathode microbial fuel cells, Bioresource technology 167 (2014) 124-132.
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Fig. 1 Optical photographs of (a) SEM image of SP0; (b) SEM image of SP2; (c) SEM image of SP2 and the corresponding elemental mapping images of C, N, and P; (d) TEM image of SP0; (e) TEM image of SP2.
C1s (a)
(b)
2
sp 3 sp C-O C-N
Intensity
Intensity
O1s
N1s P2p
1400 1200 1000
800
600
400
200
300
0
295
(c)
Binding energy / eV
275
395
C-O-P C-N-P
Intensity
Intensity
400
280
(d)
N1 N2 N3 N4
405
285
Binding energy / eV
Binding energy / eV
410
290
390
145
140
135
130
Binding energy / eV
Fig. 2 High resolution of fitted XPS: (a) sum XPS spectra of SP2; (b)the C1s spectra of SP2; (c) the N1s spectra of SP2; (d) the P2p spectra of SP2.
125
120
14000 12000
SP0 SP1 SP2 SP3 SP4 SP5
Intensity
10000 8000 6000 4000 2000 0 0
500 1000 1500 2000 2500 3000 3500 4000
Raman shift/cm
-1
Fig. 3 Raman spectrum of different air cathodes
0
0
(a)
-5
-10
Current density/mA cm
-2
-2
-5
Current density/mA cm
(b)
SP1 SP2 SP3 SP4
-15
-20
-10
SP0 SP2 SP5 AC
-15
-20
-25
-25 -0.3
-0.2
-0.1
0.0
0.1
0.2
Potential/V vs.Ag/AgCl
0.3
0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
Potential/V vs.Ag/AgCl
Fig. 4 LSV curves: test of different air cathodes.
0.3
0.4
2.5
1.6 1.4
(a)
SP1 SP2 SP3 SP4
1.2
2.0
SP0-CAL SP2-CAL SP5-CAL AC-CAL
1.5
Z''/Ω
1.0
Z''/Ω
SP0 SP2 SP5 AC
(b)
SP1-CAL SP2-CAL SP3-CAL SP4-CAL
0.8
1.0
0.6 0.5
0.4 0.2
0.0 0
1
2
3
Z'/Ω
4
5
0
2
4
Z'/Ω
Fig. 5 Nyquist plots of EIS: revealing the resistance in different air cathodes.
6
8
Fig. 6 Tafel plots of different cathodes by sweeping the over potential from 0 mV to 100 mV at 1 mv/s; and inset: linear fitting of over potential from 60 mV to 80 mV.
SP3-P SP3-V
SP4-P SP4-V
300
1800
200
1600
(a)
1400
600
1200
500
1000
400
800
300
600 400
200
200
100
Potential/mV (vs. Ag/AgCl)
700
SP2-P SP2-V
-2
Potential/mV (vs. Ag/AgCl)
800
SP1-P SP1-V
Power density/mW m
900
(c)
SP0-P SP0-V
SP2-P SP2-V
SP5-P SP5-V
AC-P AC-V
-200 -300 -400 -500
1600 1400
600
1200
500
1000 800 600
300
400
200
200
100
0 0
1
2
3
Current density/A m
4
-2
1
2
3
4
Current density/A m 300
1800
400
(b)
0
700
SP4-C SP4-A
-100
5
4
-2
SP3-C SP3-A
0
200
-2
800
3
Power density/mW m Potential/mV (vs. Ag/AgCl)
Potential/mV (vs. Ag/AgCl)
900
2
1
Current density/A m
SP2-C SP2-A
100
0 0
SP1-C SP1-A
SP0-C SP0-A
(d)
SP2-C SP2-A
-2
5
SP5-C SP5-A
AC-C AC-A
4
5
100 0 -100 -200 -300 -400 -500 -600 0
5
1
2
3
Current density/A m
-2
Fig. 7 The performance of MFCs. (a) and (c): polarization and power density curves; (b) and (d): cathodes and anodes potential curves. Table 1 Porous structure parameters of different cathodes. SP0
SP1
SP2
SP3
SP4
SP5
AC
BET Surface Area(m2/g)
12.26
140.08
982.18
891.44
889.07
2295.01
2190.25
Micropore Area(m2/g)
3.48
126.00
675.86
738.71
749.61
368.74
1319.54
Mesopore Area(m2/g)
8.77
14.09
306.32
152.73
139.46
1926.27
870.71
Total pore volume(cm3/g)
0.010
0.059
0.416
0.372
0.369
1.209
0.942
Micropore volume(cm3/g)
0.002
0.052
0.269
0.299
0.305
0.146
0.516
Average Pore Size(nm)
3.15
0.69
1.69
1.67
1.66
1.90
1.72
Table 2 Results of ultimate analysis by XPS of SP0 and SP2. Component
Elemental Content (at. %)
C1s
O1s
N1s
P2p
SP0
87.07
8.72
4.21
-
SP2
77.67
16.96
3.03
2.35
Table 3 The distribution of functional groups obtained from the deconvolution of C1s, N1s, and P2p peaks analyses C1s (%)
Sample
N1s (%)
sp2
sp3
C-N
C-O
SP0
78.91
4.68
14.03
2.39
21.64 69.91
SP2
66.37
3.57
11.22
18.83
13.21 11.23
N1
N2
P2p (%)
N3
N4
P1
P2
2.26
6.30
-
6 48.22 27.34
11.3
88.7
Table 4 Fitting resistance of different cathodes based on the equivalent circuit. SP0
SP1
SP2
SP3
SP4
SP5
AC
R0 (Ω)
9.268
5.622
7.263
9.109
7.97
10.272
11.853
Rd (Ω)
2.938
0.5729
0.4784
0.801
1.035
0.597
1.012
Rct (Ω)
4.844
4.874
2.089
2.662
4.163
1.979
2.894
Rt (Ω)
17.05
11.0689
9.8301
12.572
13.168
12.848
15.759
Table 5 Exchange current density calculated from the Tafel plots. The air cathode
Linear fitting equation(R2)
10-4i0(Acm-1)
SP0
y=-3.6270+4.3721x (0.9991)
2.36
SP1
y=-2.9910+4.7430x (0.9979)
10.21
SP2
y=-2.7626+4.6875x (0.9977)
17.27
SP3
y=-2.8070+4.9765x (0.9968)
15.60
SP4
y=-2.8373+4.8734x (0.9973)
14.54
SP5
y=-2.7211+4.4864x (0.9965)
19.01
AC
y=-3.2494+4.7302x (0.9984)
5.63
HIGHLIGHTS ● A N and P dual-doped carbon derived from chitosan used as cathode catalyst.
● The MPD was 1603.6 ± 80 mW·m-2, 5 times as high as the control. ● The phosphating increased the OCP, surface area, and transformation of graphiticN. ● Co-doped carbon increased the content of C-O and the active sites caused by defects.