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Nitrogen and phosphorus co-doped carbon networks derived from shrimp shells as an efficient oxygen reduction catalyst for microbial fuel cells
Journal of Power Sources 446 (2020) 227356
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Nitrogen and phosphorus co-doped carbon networks derived from shrimp shells as an efficient oxygen reduction catalyst for microbial fuel cells Feng-Yi Zheng a, b, Ruisong Li a, b, Shiyu Ge b, Wen-Rong Xu a, c, Yucang Zhang a, b, * a
Key Laboratory of Advanced Materials of Tropical Island Resources of Ministry of Education, Hainan University, Haikou, 570228, China College of Chemical Engineering and Technology, Hainan University, Haikou, 570228, China c Department of Chemistry, College of Science, Hainan University, Haikou, 570228, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Shrimp shells as nitrogen source were directly used to prepare ORR catalysts. � Removing CaCO3 and in-situ pore in shrimp shells promoted the active sites. � Hydrothermal process with phosphoric acid facilitated similar spherical carbon. � N, P co-doped carbon exhibited good ORR property, stability and application in MFC.
A R T I C L E I N F O
A B S T R A C T
Keywords: Shrimp shells N P co-doped carbon Oxygen reduction reaction Stability Microbial fuel cell
The waste shrimp shells (SS) were used as both carbon and nitrogen sources to directly synthesize N and P codoped carbon networks with abundant mesopores and high specific surface area by simple acid pretreatment and carbonization. Using exogenous phosphorus as dopant, the prepared catalyst (PA-SS 900) favorably possessed a high oxygen reduction reaction (ORR) activity regarding half-wave potential (0.82 V) and limiting current (4.47 mA cm 2), which approached those of commercial 20 wt% Pt/C. For practical application in microbial fuel cell (MFC), the N, P co-doped PA-SS 900 achieved a maximum power density (MPD) of 802 mW m 2 and an open circuit voltage (OCV) of 653 mV, which also were close to that (892 mW m 2 and 752 mV) based on 20% Pt/C as cathode catalyst. Remarkably, the synthetic catalyst had a better long-term stability than that of 20% Pt/C in alkaline medium. These results demonstrated that N, P co-doped PA-SS 900 was an accessible and efficient ORR catalyst in air-cathode MFC for relatively desirable energy generation and wastewater treatment. Further, the direct utilization of waste shrimp shells replacing chitin or chitosan in energy conversion is worthy of in-depth study due to the advantages of simple process, environmental friendliness and availability.
1. Introduction As a device with the dual functions of electricity generation and sewage purification [1], microbial fuel cells (MFCs) have been
considered as a potential technology for the applications of energy conversion [2] and ecological remediation [3]. Lots of practical cases [4–6] suggest that the cathode determines the reaction kinetics and the running cost for achieving high conversion from biomass energy to
* Corresponding author. Key Laboratory of Advanced Materials of Tropical Island Resources of Ministry of Education, Hainan University, Haikou, 570228, China. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227356 Received 17 July 2019; Received in revised form 16 October 2019; Accepted 25 October 2019 Available online 31 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 446 (2020) 227356
electrical energy in a whole device. Especially, Air-cathode MFC (oxy gen as the electron acceptor in cathode) has been identified to be the most feasible configuration for large-scale practical promotion [7] due to its simple structure, high efficiency, non-limiting availability of ox ygen from air and low cost [8–10]. However, the sluggish kinetics for oxygen reduction reaction (ORR) extremely limit the output power of MFC, which demands efficient and low-cost electrocatalysts [11]. Pt-based catalysts have been verified to have prominent ORR perfor mances, but the high cost and poor durability for methanol or alkaline environment hinder their commercial applications in MFCs. Currently, various substitutable ORR catalysts have been explored to reduce manufacturing cost, including transition metal composites [12,13], non-noble metal oxides [14,15] and heteroatom doped carbon-based composites [16,17]. Among these materials, the heteroatom doped carbon-based composites have been extensively researched due to their availability, renewability, low cost and simple manufacturing process, which also stimulates the rapid development of various modified carbon materials to further improve their ORR performances [18–20]. An incorporation of nitrogen into carbon-based materials including activated carbon derived from biomass [21], carbon nanotube [22] and graphene [23,24] is an efficient pattern to improve the electrocatalytic property for ORR. Furthermore, introducing two or more heteroatoms (such as B, S and P) to the N-doped carbon materials could promote higher catalytic activity because of the synergistic effects resulted from the electronic and surface polarities among heteroatoms [25]. Particu larly, the N, P co-doped carbon materials have excellent ORR perfor mances. For instance, N and P co-doped carbon derived from chitosan showed good power density, which was 5 times as high as that of the control material [26]. Phytic acid doping polyaniline could prepare N, P co-doped activated carbon as ORR catalyst, which possessed a large specific surface area of 649.3 m2 g 1, a low ORR over-potential and twice as high open circuit voltage as pristine activated carbon does [27]. There is no doubt that N, P co-doped carbon composite is a promising alternative to Pt-based materials. Nevertheless, extensive researches have showed that it is difficult to incorporate nitrogen dopant into carbon-based materials by high-temperature pyrolysis at ammonia at mosphere [28] or immersion in ammonium salt solution [29]. At this point, the natural biomass is regarded as the best precursor to prepare N, P co-doped carbon materials owing to its abundant reserves, inherently special structure and diversified heteroatoms [30–32]. Especially, chitin, a second largest biomass resource with abundant nitrogen in world, has gained wide attention in polymer material field [33,34]. Its derivative chitosan also can be used as the nitrogen source to synthetize N-doped carbon materials for ORR [35]. In addition, the heteroatom phosphorus can successfully be loaded onto the as-prepared N-doped carbon by immersing in phosphoric acid to afford efficient and low-cost N, P co-doped carbon materials. It is well known that almost all commercial chitin and chitosan are extracted from crustaceans. And the traditional extraction process is complex, energy-intensive and polluting, which includes demineraliza tion with dilute acid, deproteinization with dilute alkali and depig mentation with strong oxidant [36,37]. Herein, we report a simple and low-cost approach to prepare N, P co-doped carbon catalysts with large specific surface area using waste shrimp shells as raw materials. Firstly, shrimp shells were soaked in dilute hydrochloric acid solution, which could remove calcium carbonate (CaCO3) and simultaneously form porous structure in situ to increase the specific surface area. Subse quently, the treated shrimp shells were carbonized via hydrothermal process in phosphoric acid solution and pyrolysis at 800 � C, 900 � C and 1000 � C, respectively, to afford N, P co-doped carbon catalysts. The morphology, compositions and specific surface area of the catalysts were characterized by SEM, Raman and XPS, and BET, respectively. Mean while, their catalytic performances for ORR and practical application in MFCs were investigated to evaluate their activity.
2. Experimental 2.1. Chemicals and materials Shrimp shells were collected from Haikou seafood processing factory (Hainan, China). Phosphoric acid (H3PO4, 80%) and hydrochloric acid (HCl, 37%) were purchased from West Long Science Co., Ltd. Poly tetrafluoroethylene (PTFE, 60 wt%), Nafion solution (5 wt%), carbon black, carbon cloth, graphite fiber brushes and 20 wt% Pt/C were pur chased from Hesen Electrical Co., Ltd. Sodium acetate (CH3COONa), ammonium chloride (NH4Cl), sodium dihydrogen phosphate dihydrate (NaH2PO4⋅2H2O), disodium hydrogen phosphate (Na2HPO4) and po tassium hydroxide (KOH, electronic grade) were obtained from Aladdin Co., Ltd. All the reagents and chemicals above were used as received. Nitrogen and Oxygen (N2 and O2, 99.999%) were purchased from Jin hou gas Co., Ltd. Sludge bacterial species were taken from Dongpo Lake of Hainan University. 2.2. Catalyst preparation After washing and drying in oven at 60 � C for 24 h, the shrimp shells (20 g) were soaked in a 400 mL of 7 wt% HCl aqueous solution with vigorous stirring at room temperature for 24 h to remove the calcium carbonate. Then filtrating and washing with deionized water, the solid was dried at 60 � C to constant weight and crushed to 20 mesh to afford shrimp shell flakes (defined as SS). A mixture of the as-prepared SS (2.0 g) and phosphoric acid (PA) solution (10 wt%, 80 mL) with ultra sonic treatment for 10 min was added into a Teflon lined stainless autoclave and kept at 160 � C for 24 h, and naturally cooled to room temperature. The resulting solid carbon material was defined as PA-SS. Meanwhile, the control material (DW-SS) was synthetized with deion ized water (DW) replacing phosphoric acid solution during hydrother mal process. Finally, the PA-SS was annealed in a quartz tube under nitrogen atmosphere at different carbonization temperature of 800 � C, 900 � C and 1000 � C with a heating rate of 5 � C min 1 for 2 h. The collected products were denoted as PA-SS 800, PA-SS 900 and PA-SS 1000, respectively. As control, the DW-SS was annealed at 900 � C in a similar process to afford DW-SS 900. 2.3. Material characterization Scanning electron microscopy (SEM) was conducted using a field emission microscope (Gemini SEM 300/VP, Germany) operated at 20 kV and equipped with an energy-dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) was performed on an X-ray diffractometer (Bruker D8 Advance, Germany) operated at 30 kV and 100 mA with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were accom plished on an X-ray diffractometer (ESCALAB 250 Xi, USA) equipped with a monochromatic Al Kα source and all bonding energies could be used as a reference for the peak of C 1s hydrocarbons at 284.6 eV. The special surface area was calculated by Brunauer-Emmett-Teller method [38] according to nitrogen adsorption-desorption isotherms (V-Sorb 2800 TP, China). Raman spectra were employed to analyze the degree of graphitization of catalyst materials on a Raman microscope with He/Ne Laser excitation at 532 nm (HORIB Jobin Yvon, France). 2.4. Electrochemical characterization All the ORR measurements were performed at a three-electrode system with 0.1 M KOH solution as electrolyte on a CHI 760E electro chemical workstation (Shanghai Chenhua Co., Ltd). Ag/AgCl (25 � C, saturated with KCl solution) and spiral Pt wire (Phychemi Co., Ltd, Hong Kong) were used as the reference electrode and counter electrode, respectively. All potentials could be converted to a reversible hydrogen electrode (RHE) according to Text S1. Prior to electrochemical mea surements, the electrolyte was aerated with N2 for 30 min and then 2
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cyclic voltammetry (CV) was carried out for 50 circles to activate cat alysts. To prepare catalyst ink, 10 mg of catalyst was dispersed in a mixture solution of 1000 μL deionized water, 500 μL ethanol and 100 μL Nafion solution with sonication for 1 h. For CV tests, 10 μL of catalyst ink (0.32 mg cm 2 of catalyst loading) was loaded onto glassy carbon elec trode (Φ ¼ 5 mm) and measured at a scanning voltage rate of 5 mV/s from 0.2 V to 1.2 V. As for linear scanning voltammetry (LSV) tests, 10 μL of catalyst ink was coated on a rotating disk electrode (RDE, Φ ¼ 5 mm) equipped with a speed controller (Pine Co., Ltd, USA) at a scanning rate of 5 mV/s from 1.2 V to 0.2 V. Electron transfer number (n) was estimated according to the resulting RDE voltammetry and Koutecky–Levich (K-L) equations (1) and (2) [39]. 1/j ¼ 1/jk þ1/kω1/2 k ¼ 0.2
1/2 1/6 C0* nFAD2/3 0 ω ν
(1)
Scheme 1. Schematic route for the synthesis of N, P co-doped carbon networks derived from shrimp shells.
(2)
Where j and jk are the testing current density and kinetic limiting current density at a specific potential, respectively, n is the number of electrons, D0 is the diffusion coefficient of the analyte (1.9 � 10 5 cm2 s 1 in O2), F is the Faraday constant (96485C/mol), A represents the geometric area of the electrode (0.19625 cm2), ν represents the kinematic viscosity of the electrolyte (0.01 cm2 s 1), and C0* represents the solubility of oxy gen in the electrolyte (1.2 � 10 6 mol cm 3 in 0.1 M KOH aqueous solution). To accurately calculate the n and H2O2 yield during ORR measure ments, 12.6 μL of catalyst ink was placed on a rotating disk-ring elec trode (RDEE, Φ ¼ 5.61 mm) with a speed controller. The n and H2O2 yield were calculated by equations (3) and (4) [40]. n ¼ 4 � Id /(Id þ Ir/N)
(3)
H2O2% ¼ 200 � Ir/(Id � N þ Ir)
(4)
electrode (Ag/AgCl, Tianjin Aidao Co., Ltd) and multimeter detector (15Bþ, Fluke Co., Ltd, Shanghai). Prior to measurement, the MFCs were placed at least 4 h under the open circuit potential (OCP). A series of total voltage, cathode voltage and anode voltage was determined by changing the resistance box from 5000 Ω to 25 Ω, and a waiting time of 20 min was necessary for each resistance adjustment. The power density was calculated by Ohm’s law. To detect the running voltage, external resistance (1000 Ω) was linked to MFCs, and a data acquisition system (MPS-010602, Morpheus Electronic Co., Ltd, China) was used to record the voltage at per minute in an incubator of 37 � C. Once the running voltage of MFC dropped below 80 mV, the fresh nutrient solution was injected into MFC modules again, and repeated until the devices could reach a steady cycle. 3. Results and discussion
Where Id and Ir represent the measured disk current and ring current, respectively, N refers the collection efficiency for Pt ring and is taken as 37% in this study according to the Pine Co., Ltd.
As shown in Scheme 1, a series of N, P co-doped carbon catalysts with large specific surface area are prepared using waste shrimp shells as raw materials via acid pretreatment and subsequent heat treatments. Shrimp shells replacing chitin and chitosan as precursor not only can avoid the complex extraction process of chitin but also remain protein to enhance the nitrogen content. The detailed procedure to prepare PA-SS and DWSS catalysts was described in experimental section. A simple acid pre treatment can remove CaCO3 to destroy the dense frame and meanwhile forms porous structure. The residual protein and chitin can be directly used as carbon and nitrogen sources due to their intrinsic high nitrogen content. Notably, phosphoric acid plays a vital role during the synthesis process. Firstly, phosphoric acid was served as solvent in hydrothermal process to change carbon matrix structure. Secondly, the phosphoric acid also could be regarded as exogenous phosphorus source to improve catalytic activity resulted from the synergistic effect of N and P. Besides, carbonization temperature can greatly affect the graphitization degree, efficient active sites, and carbon network structures which allow more active sites to be exposed in carbon materials. Hence, the resulting catalysts PA-SS x (x represents the pyrolysis temperature) were obtained by pyrolysis at different temperatures. For comparison, using deionized water to replace phosphoric acid as the solvent in hydrothermal process combined pyrolysis process to afford nitrogen doped carbon material.
2.5. Fabrication and operation of MFCs Single-chamber MFCs with air as electron acceptor in cathode were used to investigate the practically catalytic performances including power density, polarization curves and running voltage for SS-based materials. The air-cathodes were prepared by a coating method using Nafion solution, PTFE and carbon black as binder, diffusion layer and current collector, respectively, according to the previous report [41]. The specific preparation process of air-cathodes was as follows: a uni form mixture of carbon black (25 mg) and PTFE (300 μL) was loaded on one side of carbon cloth (4 cm � 4 cm), and then annealed at 370 � C for 25 min with a heating rate of 5 � C min 1 to be the conducting layer. On the same side of carbon cloth, the desired 60 wt% PTFE solution was coated on the conducting layer and annealed at 370 � C for 12 min, and repeated three times to obtain the diffusion layer. On the other side of carbon cloth, the uniform catalyst ink consisting of catalyst (60 mg), deionized water (50 μL), Nafion solution (400 μL) and isopropanol (200 μL) was loaded and stood at room temperature for 24 h as the catalytic layer. Graphite fiber brushes with a calcination treatment at 450 � C for 30 min were used as the anodes. All the anodes of MFCs were inoculated with sludge bacterial species, which were collected from lakebed mud and operated for over 6 months. In order to fabricate a complete productivity system, the nutrient solution containing acetate (1 g L 1), vitamin solution (5 mL L 1), trace element solution (12.5 mL L 1) in 1 L of phosphate buffer solution (PBS, 50 mM) was added to MFC modules to keep the metabolism of bacteria. The specific recipes of vitamin solution, trace element solution and PBS were pro vided in Table S1. The power density and polarization curves were measured by the testing system equipped with a resistance box (5000 Ω–25 Ω), reference
3.1. Materials characterization The changing morphological characterization of the cathode cata lysts derived from shrimp shells was revealed by SEM investigation during preparation process. The surface morphology of shrimp shells displayed obvious changes before and after dilute acid pretreatment. Abundant filamentary portions and fillers in original shrimp shells could be observed (Fig. 1a), which corresponded to chitin filament, protein and CaCO3, respectively [42]. In order to remain protein and remove CaCO3, the shrimp shells were pretreated with a facile dilute acid, and a 3
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Journal of Power Sources 446 (2020) 227356
Fig. 1. SEM images of (a) Original shrimp shells, (b) DW-SS, (c) PA-SS, (d) SS, (e) DW–SS 900 and (f) PA-SS 900. TEM images of (g) and (h) PA-SS 900. Element mapping images of (i) N and (j) P in PA-SS 900.
temperature, the small molecular substances were volatilized to expose the chitin filaments, and the carbon skeletons and porous structures of chitin were still retained (Fig. 1d). In addition, the PA-SS 900 (efficient cathode catalyst) was synthetized by hydrothermal method with phos phoric acid as solvent and subsequent pyrolysis at 900 � C. As seen from Fig. 1c and f, the introduction of phosphoric acid effectively promoted the formation of more small molecules, which resulted in a fluffy and compact structure (Fig. 1e) during hydrothermal process. Interestingly, the spongy structure was transformed into similar spherical carbon networks (Fig. 1f) with higher specific surface area after calcination at 900 � C, which could coincide with the BET results. The detailed micro structure of the PA-SS 900 catalyst was further studied by TEM. Fig. 1g showed well-defined porous structure with alternating light and dark. High-resolution transmission microscope image in Fig. 1h also revealed its porous structure. Furthermore, Fig. 1i and j investigated the N and P elements could homogeneously distribute in the carbon matrix for the PA-SS 900 sample. The specific surface area and pore sizes of DW-SS 900 and PA-SS 900 were measured by BET tests. Fig. 2a showed their nitrogen adsorption and desorption isotherm curves. The isothermal adsorption lines dis played the characteristics of type IV according to the IUPAC classifica tion, suggesting the as-prepared cathode catalysts were mesoporous materials. The specific surface area of DW-SS 900 and PA-SS 900 were 299.29 m2 g 1 and 447.10 m2 g 1, respectively. Obviously, phosphoric acid as solvent during hydrothermal process could improve the pore pattern due to the generation and volatilization of more small molecules. Further, as shown in Fig. 2b, the average pore size of DW-SS 900 and PASS 900 were 13.69 nm and 8.07 nm, respectively. These pore size dis tributions further verified the formation of mesoporous materials, which could allow bulkier substances to access catalytic active sites and pro mote good mass transport [44], naturally stimulating the better ORR activity.
Fig. 2. (a) Nitrogen adsorption/desorption isotherms and (b) pore-size distri bution curves of DW-SS 900 and PA-SS 900.
large of porous structures appeared in SS (Fig. 1b), which was conduc tive to enhance the specific surface area of subsequent cathode mate rials. Further, hydrothermal process using deionized water as solvent and pyrolysis at 900 � C provided DW-SS 900. It could be seen that there were numerous wrinkles on the surface of DW-SS (Fig. 1c), which was related to the absorbing bio-oil and liquefied products with small mo lecular weight during hydrothermal process [43]. After calcined at high 4
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Journal of Power Sources 446 (2020) 227356
to its low ID/IG ratio. These results demonstrated that doping phos phorus atoms successfully contributed to the predominance of crystal defect to strengthen ORR performance [47]. The XRD patterns of cath ode catalysts were plotted in Fig. 3b. Without other obvious peaks appearing, the SS showed a sharp peak at 19.2� and a shoulder peak at 26.3� , which were the characteristic peaks of chitin, suggesting that acid pretreatment could completely remove CaCO3. The peaks at approxi mately 26.5� and 43.6� corresponding to the 002 and 100 plane of graphitic carbon could be found in all the cathode catalysts, which indicated the presence of graphitic structures. The increasing peak strength signified higher graphitization degree when the calcination temperature changed from 800 � C to 1000 � C for PA-SS materials. Comparing with the intensity of DW-SS 900 at 26.5� and 43.6� , PA-SS 900 showed a slight decrease, which might declare that PA-SS 900 possessed a lower degree of graphitization than that of DW-SS 900. These phenomena coincided with the Raman results. Finally, the heteroatoms of nitrogen (N) and phosphorus (P) doping to SS were confirmed using XPS measurements. The peaks of C 1s (285.1 eV), O 1s (532.4 eV), N 1s (401.0 eV) and P 2p (133.6 eV) in XPS survey spectra distinctly demonstrated the dominant presence of C, O, N and P in DW-SS and PA-SS cathode catalysts (Fig. 4a), and the element contents were recorded in Table 1. It could be seen that doping P not only facilitated the retention of nitrogen in SS, but also could increase the content of oxygen, which can promote the adsorption of O2 to decrease the ORR overpotential [48]. These results also could be confirmed by energy dispersive X-ray spectroscopy (EDS) analysis. (shown in Fig. S1 and Table S2). However, the higher calcination
Fig. 3. (a) Raman spectra and (b) XRD patterns of the SS, DW-SS 900, PA-SS 800, PA-SS 900 and PA-SS 1000.
To understand the crystal structure of DW-SS 900 and PA-SS 900, the XRD and Raman tests were carried out as shown in Fig. 3. In the Raman spectra of Fig. 3a, all the cathode catalysts exhibited typical D and G bands corresponding to ~1330 cm 1 and ~1596 cm 1, which repre sented disordered and graphitic phases in carbon, respectively. The graphitization degree of carbon materials could be evaluated by measuring the intensity ratio of D-band to G-band (ID/IG) [45]. The ID/IG value of DW-SS 900, PA-SS 800, PA-SS 900 and PA-SS 1000 were 0.87, 0.93, 0.97 and 0.98, respectively. The cathode catalysts with phosphoric acid as hydrothermal solvent (PA-SS series) had slightly higher ID/IG ratio than that of DW-SS, which was caused by the incorporation of phosphorus atom defects and structural collapse during thermal annealing process [46], indicating that they had more active sites. Inversely, the DW-SS 900 held a higher graphitization degree according
Table 1 The detailed distribution of various catalysts for C, N, O and P based on XPS survey. Samples DW-SS 900 PA-SS 800 PA-SS 900 PA-SS 1000
Elemental content (at. %) C
O
N
P
90.21 81.97 86.93 87.45
7.68 12.39 9.24 10.01
2.11 3.71 2.53 1.49
1.93 1.30 1.05
Fig. 4. XPS survey spectra of DW-SS and PA-SS cathode catalysts, C 1s XPS spectra of (b) DW-SS 900 and (c) PA-SS 900, N 1s XPS spectra of (d) DW-SS 900 and (e) PA-SS 900, (f) P 2p XPS spectrum of PA-SS 900. 5
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Table 2 The detailed distribution of various patterns based on the deconvolution of C1s, N1s, and P2p peaks analyses. Samples
C1s (%)
DW-SS 900 PA-SS 800 PA-SS 900 PA-SS 1000 a b c d
N1s (%) a
O¼C–O
C–O
C–N
C¼C
N1
14.78 11.94 18.23 6.87
5.52 10.43 5.12 9.16
16.62 21.85 20.55 29.41
63.07 31.63 56.10 54.56
10.95 15.15 13.66 12.56
P2p (%) N2
b
33.63 28.24 26.24 29.41
N3
c
44.86 42.90 49.02 46.40
N4d
P–O
P–C
10.55 13.71 10.68 11.78
39.34 31.79 21.44
60.66 68.21 78.56
Pyridinic-N. Pyrrolic-N. Graphitic-N. Oxidized-N.
measurements were conducted in a three electrodes system with 0.1 M KOH solution as electrolyte. The ORR characteristic parameters of different cathode catalysts were summarized in Table 3. Fig. 5a showed the CV curves of PA-SS 800, PA-SS 900, PA-SS 1000 and DW-SS 900 in O2 or N2 saturated KOH solution. Compared with N2 atmosphere (black line in Fig. 5a), a distinct ORR peak at 0.78 V (vs. RHE) could be observed for PA-SS 900 in the presence of oxygen (red line in Fig. 5a), indicating that the as-prepared PA-SS 900 catalyst had good ORR ac tivity. After O2 saturated electrolyte for 30 min, all the cathode catalysts emerged obvious ORR peaks. However, in the DW-SS 900 plot, the ox ygen reduction peak appeared at 0.72 V (vs. RHE), which was lower than that of PA- SS 900 (0.78 V vs. RHE) and PA-SS 1000 (0.78 V vs. RHE). Remarkably, doping phosphorus atoms could promote the Ep (voltages corresponding to the oxygen reduction peaks) shifting towards positive direction. In addition, the PA-SS 900 had the highest current density among all cathode catalysts. These results suggested that the N and P cofunctionalized PA-SS 900 had the lowest overpotential toward ORR in alkaline media. In fact, with 0.1 M KOH solution (pH ¼ 13) as electrolyte for ORR, all the surface groups were deprotonated to lead to a reactive dependence on the high concentration of hydroxyl groups, which can specifically adsorb onto the active sites such as pyridine nitrogen [55]. Thus, a surface-confined redox-mediated process predominated the re action to yield a 4e pathway at high potential, and a parallel hydrogen peroxide process was induced by the outer-sphere electron transfer mechanism at low potential [56,57]. Further, Fig. 5b exhibited the LSV curves of DW-SS, PA-SS and 20% Pt/C in O2 saturated KOH solution at 1600 revolutions per minute (rpm). The P, N co-doped PA-SS 900 presented more positive Eonset and higher limiting current density at 0.2 V (vs. RHE) than those of DW-SS 900, PA-SS 800 and PA-SS 1000, which were well assigned with the CV consequences. Meanwhile, the PA-SS 900 showed an Eonset of 0.96 V and E1/2 of 0.82 V, which were slightly worse than those of 20% Pt/C (1.03 V and 0.91 V), but it was still a potential catalyst due to its low cost and stability. Meanwhile, the catalyst is comparable to most of the re ported carbon materials based chitin or chitosan (Table S3). Combining with the previous structure and elementary characteristic analysis, the conspicuous CV and LSV performances of PA-SS 900 might result from its similar spherical carbon networks and large specific surface area, which could fabricate multidimensional electron transfer routes and expose more active sites. Moreover, the doping phosphorus and inherent nitrogen source from SS also incorporated the various defect sites into the graphitic framework. RDE measurements with different rotation rates were employed to research the electron transfer pathway in O2-saturated 0.1 M KOH electrolyte. The LSV curves and corresponding K-L plots of PA-SS 900 and DW-SS 900 were showed in Fig. 5c and d. Those of PA-SS 800 and PA-SS 1000 were displayed in Figs. S4a and 4c. The current density raised with increasing rotation speed due to the shortened diffusion distance at high velocity. Based on the K-L plots (J 1 with respect to ω 1/ 2 in Figs. S4b and 4d, and inset of Fig. 5c and d), the electron transfer number (n) was calculated to be 2.87–3.33, 3.45–3.62, 3.36–3.52 and 3.10–3.25 at a potential range of 0.20–0.60 V (vs. RHE), corresponding
Table 3 The ORR performance parameters of different cathode catalysts measured in 0.1 M KOH solution. Samples
E p (V vs. RHE)
E onset (V vs. RHE)
E 1/2 (V vs. RHE)
Diffusion limiting current at 0.2 V (mA cm 2)
PA-SS 800 PA-SS 900 PA-SS 1000 DW-SS 900 20% Pt/C
<0.72
0.87
0.74
3.36
0.78
0.96
0.82
4.47
0.78
0.96
0.82
3.86
0.72
0.93
0.79
3.86
0.82
1.03
0.91
5.01
temperature would reduce the active sites of O, N and P. The highresolution XPS spectra of C 1s, N 1s and P 2p were analysed to obtain the detailed distribution of various elements. The spectra of DW-SS 900 and PA-SS 900 were shown in Fig. 4b–f and the detailed elemental contents were calculated in Table 2. The spectra of PA-SS 800 and PA-SS 1000 were plotted in Figs. S2 and S3. As shown in Fig. 4b–c, the carbon – C, C–N, C–O patterns of DW-SS 900 and PA-SS 900 were composed of C– – C–O, corresponding to 284.6 eV, 285.6 eV, 286.2 eV and and O– – C occupied a major contri 288.5 eV, respectively [49]. Graphitic C– bution to the C1s peak of both DW-SS 900 and PA-SS 900, which confirmed their high graphitization. The percentage of sp2-hybridization – C) of PA-SS 900 decreased slightly (Table 2), which was related to (C– the more crystal defects of phosphorus doping. The N 1s spectra of DW-SS 900 and PA-SS 900 were displayed in Fig. 4d and e. There were four types of nitrogen contribution containing pyridinic-N (398.3~399.5 eV, N1), pyrrolic-N (400.1~400.9 eV, N2), graphitic-N (401.2~402.0 eV, N3) and oxidized-N (403.4~410.0 eV, N4) [50], and the atomic percent of each pattern was calculated in Table 2. Numerous researches have investigated that pyridinic-N and graphitic-N were conducive to electron transfer to enhance catalytic activity [51,52]. Comparing with DW-SS 900, PA-SS 800 and PA-SS 1000, the PA-SS 900 held a higher amount of pyridinic-N and graphitic-N, suggesting that PA-SS 900 had higher ORR performances. Besides, the high content of graphitic-N in PA-SS 900 might be caused by doping phosphorus and annealing process [53]. The high-resolution P 2p spectrum of PA-SS 900 was shown in Fig. 4f. Two bands appeared at 131.8 eV and 133.6 eV corresponding to P–C and P–O. Both of them could cause the charge delocalization of carbon adjacent to heteroatom with higher electro negativity to form electron poverty, which was generally beneficial for ORR [54]. However, the oxygen-containing functional groups including – C–O, C–O and P–O gradually lost as the calcination temperature O– increased. 3.2. Electrochemical characterization of cathode catalysts To evaluate the ORR activity of the as-prepared cathode catalysts, the cyclic voltammetry (CV) and linear sweep voltammetry (LSV) 6
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Fig. 5. (a) CV curves in O2 (red) or N2-saturated (black) 0.1 M KOH, (b) LSV curves at 1600 rpm, LSV curves of (c) DW-SS 900 and (d) PA-SS 900 at different rotation from 400 to 2250 rpm, (e) Electron transfer number derived from LSV curves at different rotation speeds, (f) Plots of H2O2 yield (up) and n value (down) against electrode potential of PA-SS 800, PA-SS 900, PA-SS 1000, and compared with that of Pt/C in O2-saturated 0.1 M KOH, (g) Tafel plot, (h) the i-t chronoamperometric curves of PA-SS 900 and 20% Pt/C catalysts at 0.7 (vs. RHE) in O2-saturated 0.1 M KOH electrolyte, (i) the tolerate to 3 M methanol. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
to PA-SS 800, PA-SS 900, PA-SS 1000 and DW-SS 900, respectively (Fig. 5e). These values were further validated by rotating ring-disk electrode (RRDE) measurements (shown in Fig. S5). According to the measured ring current and disk current, the electron transfer number was calculated to be 3.38–3.68 for PA-SS 900 at a potential range of 0.8–0.2 V (vs. RHE) in Fig. 5f. These results were markedly consistent with the K-L results, indicating a four-electron pathway dominant for PA-SS 900 in alkaline medium. Uniformly, the RRDE measurements also declared that the PA-SS 800 had a great potential to generate H2O2 with a yield beyond 53.5% at potential range of 0.8–0.2 V (vs. RHE). Thus, these results suggested that PA-SS 900 had a high ORR catalytic effi ciency in alkaline medium. The ORR activity of cathode catalysts was further recorded by Tafel plots as Fig. 5g. Notably, the Tafel slope of PA-SS 900 (96 mV/decade) was smaller than those of PA-SS 800 (135 mV/decade), PA-SS 1000 (100 mV/decade) and DW-SS 900 (111 mV/decade), which was comparable to 92 mV/decade of 20% Pt–C catalyst. The smaller Tafel slope meant the higher intrinsic catalytic activity for ORR, suggesting a faster reac tion kinetic in alkaline media for PA-SS 900. Certainly, the durable test of PA-SS 900 with high ORR performances was essential for its practical application. An i-t chronoamperometric result was revealed in Fig. 5h. The relative current of both PA-SS 900 and 20% Pt/C gradually reduced at 0.7 V (vs. RHE) in O2-saturated
0.1 M KOH electrolyte over time. Fortunately, the PA-SS 900 (90.34%) showed a slower attenuation rate than 20% Pt/C (85.40%), supporting a better long-term stability of PA-SS 900 over the 20% Pt/C in an alkaline electrolyte after operating 12000s. Besides, the PA-SS 900 was further subjected to test the possible poison effect in the presence of methanol. It could be seen that after the 3 M methanol addition at 300s, 20% Pt/C showed a sharp decrease, but the PA-SS 900 kept excellent resistance (Fig. 5i). Such high selectivity of the PA-SS 900 toward ORR and remarkably good tolerance to methanol can be attributed to the much lower ORR potential than that required for oxidation of the methanol. 3.3. MFCs performance and analysis for cathode catalysts In order to determine the practical ORR performance, a series of PASS 800, PA-SS 900 and PA-SS 1000 fabricated single-chamber air-cath ode MFCs were inoculated with bacteria on carbon brush. All the MFCs were operated for approximately 5 days to acclimate microorganism. As a comparison, 20% Pt/C fabricated MFC was also run at the same con dition. Fig. 6 showed the running behaviour of voltage variation with time after domestication. A similar cyclic process that the output voltage promptly increased, then reached a stably durable platform and ended up with attenuation could be observed. Once the fresh nutrient solution was added into MFCs, the periodic process about 20–30 h would be 7
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OCV reduce of 13.16% compared to those of the commercial 20% Pt/C (892 mW m 2 and 752 mV). The MPD exactly agreed with the results of LSV and Tafel curves. Surprisingly, the control DW-SS 900 derived from SS without exogenous heteroatom doping also exhibited favourable properties, which was probably attributed to the inherent source of ni trogen from chitin and protein in SS [58]. Further, the doping phos phorus probably changed the chemical structure of carbon after high-temperature calcination, thus the PA-SS 900 showed better ORR activity than other catalysts and closer performances to 20% Pt/C. Fig. 7b illustrated the potential behaviour of cathode and anode following with the external resistance range from 5000 Ω to 25 Ω. There was a sharp decline about 391 mV in cathode curves. Meanwhile, a slight rise approximately 131 mV could be observed in anode curves for all catalysts. This result revealed that the cathode presented the domi nant function of MFCs. Besides, among DW-SS and PA-SS based cathode catalysts, PA-SS 900 had the highest total potential and cathode po tential, further manifesting the excellent catalytic performance.
Fig. 6. The plots of out-put voltage variation with time after complete domestication.
4. Conclusions
repeated. In details, when the last cyclic behaviour kept stable in Fig. 6, the output voltage of PA-SS 900 (503 � 5 mV) was higher than that of PA-SS 800 (421 � 3 mV), PA-SS 1000 (456 � 3 mV) and DW-SS 900 (446 � 8 mV), only 3.26% lower than Pt-MFC (521 � 10 mV), suggesting that PA-SS 900 held a high-efficient ORR catalytic activity and even could compete with Pt/C catalyst in practical application. Besides, the power density was considered as an important factor to assess the capacity of electricity production of MFCs in practical appli cation. Therefore, when all MFCs could reach a stable maximum voltage platform after one month of cyclic culture, they were tested to obtain power density curves and polarization curves, as shown in Fig. 7. It could be observed from Fig. 7a that the PA-SS 900 fabricated MFC represented the highest maximum power density (MPD) of 802 mW m 2 and open circuit voltage (OCV) of 653 mV among all experimental cathode catalysts of PA-SS 800 (686 mW m 2 and 586 mV), PA-SS 1000 (759 mW m 2 and 610 mV) and DW-SS 900 (615 mW m 2 and 685 mV). Of course, the PA-SS 900 possessed a slight MPD decrease of 10.09% and
In this work, the waste shrimp shells were directly converted into a nitrogen and phosphorus co-doped mesoporous carbon framework catalyst with favourable ORR performances by an available, low-cost and efficient synthesis. Prior to carbonization, a simple dilute acid pre-treatment could remove minerals to obtain chitin wrapped with protein from waste shrimp shells, while the resulting in-situ porous frameworks increased the specific surface area. As a metal-free catalyst, the SS prepared control cathode catalyst also exhibited excellent MFC property due to crystal defects derived from inherent nitrogen in chitin and protein. After doping extraneous phosphorus, more crystal defects and higher specific surface area were incorporated into PA-SS 900, which presented a more-positive oxygen reduction peak at 0.78 V, halfwave potential of 0.82 V (vs. RHE) and limiting current of 4.47 mA cm 2. These data were superior to the control sample DW-SS 900 and approaching commercial catalyst 20% Pt/C. Besides, it
Fig. 7. (a) Polarization and power density curves, (b) cathode and anode polarization curves in MFCs for PA-SS 800, PA-SS 900, PA-SS 1000, DW-SS 900 and 20% Pt/C. 8
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showed an excellent long-term stability with more than 90.34% reten tion of I/I0 compared with 85.40% of 20% Pt/C in alkaline media after running 3 h. For practical application in MFCs, the PA-SS 900 revealed a running voltage of 503 mV, maximum power density of 802 mW m 2 and open circuit voltage of 653 mV, which were preferable than those of DW-SS 900 and comparable to 20% Pt/C (521 mV, 892 mW m 2 and 752 mV). The simple and efficient acid pretreatment and carbonization process provided carbon catalysts with excellent performance for fuel cells, which was significant to the development of eco-friendly catalyst and would open up a new way for the utilization of shellfishery waste.
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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of the manuscript entitled “Nitrogen and phosphorus co-doped carbon networks derived from shrimp shells as an efficient oxygen reduction cata lyst for microbial fuel cells”. We also confirm that this work reported herein has not been previ ously submitted to Journal of Power Sources and is not under consideration for publication elsewhere in any medium. All authors have read and approve this version of the article, and due care has been taken to ensure the integrity of the work. Acknowledgements This work is financially supported by National Natural Science Foundation of China (grant number 51762013), National Natural Sci ence Foundation of China (21978059), High-level Innovation and Entrepreneurship Talents Project of Hainan Province (Qiong Talent Office [2014] 8) and Hainan Provincial Postgraduate Innovation Research Project (Hys2018-74). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227356. References [1] K.P. Katuri, C.M. Werner, R.J. Jimenez-Sandoval, W. Chen, S. Jeon, B.E. Logan, Z. Lai, G.L. Amy, P.E. Saikaly, Environ. Sci. Technol. 48 (2014) 12833–12841. [2] H.-S. Lee, P. Parameswaran, A. Kato-Marcus, C.I. Torres, B.E. Rittmann, Water Res. 42 (2008) 1501–1510. [3] K.C. Wrighton, P. Agbo, F. Warnecke, K.A. Weber, E.L. Brodie, T.Z. DeSantis, P. Hugenholtz, G.L. Andersen, J.D. Coates, ISME J. 2 (2008) 1146. [4] A. Ter Heijne, D.P.B.T.B. Strik, H.V.M. Hamelers, C.J.N. Buisman, Environ. Sci. Technol. 44 (2010) 7151–7156. [5] C. Santoro, A. Serov, R. Gokhale, S. Rojas-Carbonell, L. Stariha, J. Gordon, K. Artyushkova, P. Atanassov, Appl. Catal. B Environ. 205 (2017) 24–33. [6] S. You, X. Gong, W. Wang, D. Qi, X. Wang, X. Chen, N. Ren, Adv. Energy Mater. 6 (2016) 1501497. [7] K.-Y. Kim, W. Yang, P.J. Evans, B.E. Logan, Bioresour. Technol. 221 (2016) 96–101. [8] Y. Liu, Y. Zhao, K. Li, Z. Wang, P. Tian, D. Liu, T. Yang, J. Wang, J. Power Sources 378 (2018) 1–9. [9] N. Yang, G. Zhan, D. Li, X. Wang, X. He, H. Liu, Chem. Eng. J. 356 (2019) 06–515. [10] R. Rudra, V. Kumar, N. Pramanik, P.P. Kundu, J. Power Sources 341 (2017) 285–293. [11] H. Tang, S. Cai, S. Xie, Z. Wang, Y. Tong, M. Pan, X. Lu, Adv. Sci. 3 (2016) 1500265.
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Nitrogen and phosphorus co-doped carbon networks derived from shrimp shells as an efficient oxygen reduction catalyst for microbial fuel cells Feng-Yi Zheng a, b, Ruisong Li a, b, Shiyu Ge b, Wen-Rong Xu a, c, Yucang Zhang a, b, * a
Key Laboratory of Advanced Materials of Tropical Island Resources of Ministry of Education, Hainan University, Haikou, 570228, China College of Chemical Engineering and Technology, Hainan University, Haikou, 570228, China c Department of Chemistry, College of Science, Hainan University, Haikou, 570228, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Shrimp shells as nitrogen source were directly used to prepare ORR catalysts. � Removing CaCO3 and in-situ pore in shrimp shells promoted the active sites. � Hydrothermal process with phosphoric acid facilitated similar spherical carbon. � N, P co-doped carbon exhibited good ORR property, stability and application in MFC.
A R T I C L E I N F O
A B S T R A C T
Keywords: Shrimp shells N P co-doped carbon Oxygen reduction reaction Stability Microbial fuel cell
The waste shrimp shells (SS) were used as both carbon and nitrogen sources to directly synthesize N and P codoped carbon networks with abundant mesopores and high specific surface area by simple acid pretreatment and carbonization. Using exogenous phosphorus as dopant, the prepared catalyst (PA-SS 900) favorably possessed a high oxygen reduction reaction (ORR) activity regarding half-wave potential (0.82 V) and limiting current (4.47 mA cm 2), which approached those of commercial 20 wt% Pt/C. For practical application in microbial fuel cell (MFC), the N, P co-doped PA-SS 900 achieved a maximum power density (MPD) of 802 mW m 2 and an open circuit voltage (OCV) of 653 mV, which also were close to that (892 mW m 2 and 752 mV) based on 20% Pt/C as cathode catalyst. Remarkably, the synthetic catalyst had a better long-term stability than that of 20% Pt/C in alkaline medium. These results demonstrated that N, P co-doped PA-SS 900 was an accessible and efficient ORR catalyst in air-cathode MFC for relatively desirable energy generation and wastewater treatment. Further, the direct utilization of waste shrimp shells replacing chitin or chitosan in energy conversion is worthy of in-depth study due to the advantages of simple process, environmental friendliness and availability.
1. Introduction As a device with the dual functions of electricity generation and sewage purification [1], microbial fuel cells (MFCs) have been
considered as a potential technology for the applications of energy conversion [2] and ecological remediation [3]. Lots of practical cases [4–6] suggest that the cathode determines the reaction kinetics and the running cost for achieving high conversion from biomass energy to
* Corresponding author. Key Laboratory of Advanced Materials of Tropical Island Resources of Ministry of Education, Hainan University, Haikou, 570228, China. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227356 Received 17 July 2019; Received in revised form 16 October 2019; Accepted 25 October 2019 Available online 31 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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electrical energy in a whole device. Especially, Air-cathode MFC (oxy gen as the electron acceptor in cathode) has been identified to be the most feasible configuration for large-scale practical promotion [7] due to its simple structure, high efficiency, non-limiting availability of ox ygen from air and low cost [8–10]. However, the sluggish kinetics for oxygen reduction reaction (ORR) extremely limit the output power of MFC, which demands efficient and low-cost electrocatalysts [11]. Pt-based catalysts have been verified to have prominent ORR perfor mances, but the high cost and poor durability for methanol or alkaline environment hinder their commercial applications in MFCs. Currently, various substitutable ORR catalysts have been explored to reduce manufacturing cost, including transition metal composites [12,13], non-noble metal oxides [14,15] and heteroatom doped carbon-based composites [16,17]. Among these materials, the heteroatom doped carbon-based composites have been extensively researched due to their availability, renewability, low cost and simple manufacturing process, which also stimulates the rapid development of various modified carbon materials to further improve their ORR performances [18–20]. An incorporation of nitrogen into carbon-based materials including activated carbon derived from biomass [21], carbon nanotube [22] and graphene [23,24] is an efficient pattern to improve the electrocatalytic property for ORR. Furthermore, introducing two or more heteroatoms (such as B, S and P) to the N-doped carbon materials could promote higher catalytic activity because of the synergistic effects resulted from the electronic and surface polarities among heteroatoms [25]. Particu larly, the N, P co-doped carbon materials have excellent ORR perfor mances. For instance, N and P co-doped carbon derived from chitosan showed good power density, which was 5 times as high as that of the control material [26]. Phytic acid doping polyaniline could prepare N, P co-doped activated carbon as ORR catalyst, which possessed a large specific surface area of 649.3 m2 g 1, a low ORR over-potential and twice as high open circuit voltage as pristine activated carbon does [27]. There is no doubt that N, P co-doped carbon composite is a promising alternative to Pt-based materials. Nevertheless, extensive researches have showed that it is difficult to incorporate nitrogen dopant into carbon-based materials by high-temperature pyrolysis at ammonia at mosphere [28] or immersion in ammonium salt solution [29]. At this point, the natural biomass is regarded as the best precursor to prepare N, P co-doped carbon materials owing to its abundant reserves, inherently special structure and diversified heteroatoms [30–32]. Especially, chitin, a second largest biomass resource with abundant nitrogen in world, has gained wide attention in polymer material field [33,34]. Its derivative chitosan also can be used as the nitrogen source to synthetize N-doped carbon materials for ORR [35]. In addition, the heteroatom phosphorus can successfully be loaded onto the as-prepared N-doped carbon by immersing in phosphoric acid to afford efficient and low-cost N, P co-doped carbon materials. It is well known that almost all commercial chitin and chitosan are extracted from crustaceans. And the traditional extraction process is complex, energy-intensive and polluting, which includes demineraliza tion with dilute acid, deproteinization with dilute alkali and depig mentation with strong oxidant [36,37]. Herein, we report a simple and low-cost approach to prepare N, P co-doped carbon catalysts with large specific surface area using waste shrimp shells as raw materials. Firstly, shrimp shells were soaked in dilute hydrochloric acid solution, which could remove calcium carbonate (CaCO3) and simultaneously form porous structure in situ to increase the specific surface area. Subse quently, the treated shrimp shells were carbonized via hydrothermal process in phosphoric acid solution and pyrolysis at 800 � C, 900 � C and 1000 � C, respectively, to afford N, P co-doped carbon catalysts. The morphology, compositions and specific surface area of the catalysts were characterized by SEM, Raman and XPS, and BET, respectively. Mean while, their catalytic performances for ORR and practical application in MFCs were investigated to evaluate their activity.
2. Experimental 2.1. Chemicals and materials Shrimp shells were collected from Haikou seafood processing factory (Hainan, China). Phosphoric acid (H3PO4, 80%) and hydrochloric acid (HCl, 37%) were purchased from West Long Science Co., Ltd. Poly tetrafluoroethylene (PTFE, 60 wt%), Nafion solution (5 wt%), carbon black, carbon cloth, graphite fiber brushes and 20 wt% Pt/C were pur chased from Hesen Electrical Co., Ltd. Sodium acetate (CH3COONa), ammonium chloride (NH4Cl), sodium dihydrogen phosphate dihydrate (NaH2PO4⋅2H2O), disodium hydrogen phosphate (Na2HPO4) and po tassium hydroxide (KOH, electronic grade) were obtained from Aladdin Co., Ltd. All the reagents and chemicals above were used as received. Nitrogen and Oxygen (N2 and O2, 99.999%) were purchased from Jin hou gas Co., Ltd. Sludge bacterial species were taken from Dongpo Lake of Hainan University. 2.2. Catalyst preparation After washing and drying in oven at 60 � C for 24 h, the shrimp shells (20 g) were soaked in a 400 mL of 7 wt% HCl aqueous solution with vigorous stirring at room temperature for 24 h to remove the calcium carbonate. Then filtrating and washing with deionized water, the solid was dried at 60 � C to constant weight and crushed to 20 mesh to afford shrimp shell flakes (defined as SS). A mixture of the as-prepared SS (2.0 g) and phosphoric acid (PA) solution (10 wt%, 80 mL) with ultra sonic treatment for 10 min was added into a Teflon lined stainless autoclave and kept at 160 � C for 24 h, and naturally cooled to room temperature. The resulting solid carbon material was defined as PA-SS. Meanwhile, the control material (DW-SS) was synthetized with deion ized water (DW) replacing phosphoric acid solution during hydrother mal process. Finally, the PA-SS was annealed in a quartz tube under nitrogen atmosphere at different carbonization temperature of 800 � C, 900 � C and 1000 � C with a heating rate of 5 � C min 1 for 2 h. The collected products were denoted as PA-SS 800, PA-SS 900 and PA-SS 1000, respectively. As control, the DW-SS was annealed at 900 � C in a similar process to afford DW-SS 900. 2.3. Material characterization Scanning electron microscopy (SEM) was conducted using a field emission microscope (Gemini SEM 300/VP, Germany) operated at 20 kV and equipped with an energy-dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) was performed on an X-ray diffractometer (Bruker D8 Advance, Germany) operated at 30 kV and 100 mA with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were accom plished on an X-ray diffractometer (ESCALAB 250 Xi, USA) equipped with a monochromatic Al Kα source and all bonding energies could be used as a reference for the peak of C 1s hydrocarbons at 284.6 eV. The special surface area was calculated by Brunauer-Emmett-Teller method [38] according to nitrogen adsorption-desorption isotherms (V-Sorb 2800 TP, China). Raman spectra were employed to analyze the degree of graphitization of catalyst materials on a Raman microscope with He/Ne Laser excitation at 532 nm (HORIB Jobin Yvon, France). 2.4. Electrochemical characterization All the ORR measurements were performed at a three-electrode system with 0.1 M KOH solution as electrolyte on a CHI 760E electro chemical workstation (Shanghai Chenhua Co., Ltd). Ag/AgCl (25 � C, saturated with KCl solution) and spiral Pt wire (Phychemi Co., Ltd, Hong Kong) were used as the reference electrode and counter electrode, respectively. All potentials could be converted to a reversible hydrogen electrode (RHE) according to Text S1. Prior to electrochemical mea surements, the electrolyte was aerated with N2 for 30 min and then 2
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cyclic voltammetry (CV) was carried out for 50 circles to activate cat alysts. To prepare catalyst ink, 10 mg of catalyst was dispersed in a mixture solution of 1000 μL deionized water, 500 μL ethanol and 100 μL Nafion solution with sonication for 1 h. For CV tests, 10 μL of catalyst ink (0.32 mg cm 2 of catalyst loading) was loaded onto glassy carbon elec trode (Φ ¼ 5 mm) and measured at a scanning voltage rate of 5 mV/s from 0.2 V to 1.2 V. As for linear scanning voltammetry (LSV) tests, 10 μL of catalyst ink was coated on a rotating disk electrode (RDE, Φ ¼ 5 mm) equipped with a speed controller (Pine Co., Ltd, USA) at a scanning rate of 5 mV/s from 1.2 V to 0.2 V. Electron transfer number (n) was estimated according to the resulting RDE voltammetry and Koutecky–Levich (K-L) equations (1) and (2) [39]. 1/j ¼ 1/jk þ1/kω1/2 k ¼ 0.2
1/2 1/6 C0* nFAD2/3 0 ω ν
(1)
Scheme 1. Schematic route for the synthesis of N, P co-doped carbon networks derived from shrimp shells.
(2)
Where j and jk are the testing current density and kinetic limiting current density at a specific potential, respectively, n is the number of electrons, D0 is the diffusion coefficient of the analyte (1.9 � 10 5 cm2 s 1 in O2), F is the Faraday constant (96485C/mol), A represents the geometric area of the electrode (0.19625 cm2), ν represents the kinematic viscosity of the electrolyte (0.01 cm2 s 1), and C0* represents the solubility of oxy gen in the electrolyte (1.2 � 10 6 mol cm 3 in 0.1 M KOH aqueous solution). To accurately calculate the n and H2O2 yield during ORR measure ments, 12.6 μL of catalyst ink was placed on a rotating disk-ring elec trode (RDEE, Φ ¼ 5.61 mm) with a speed controller. The n and H2O2 yield were calculated by equations (3) and (4) [40]. n ¼ 4 � Id /(Id þ Ir/N)
(3)
H2O2% ¼ 200 � Ir/(Id � N þ Ir)
(4)
electrode (Ag/AgCl, Tianjin Aidao Co., Ltd) and multimeter detector (15Bþ, Fluke Co., Ltd, Shanghai). Prior to measurement, the MFCs were placed at least 4 h under the open circuit potential (OCP). A series of total voltage, cathode voltage and anode voltage was determined by changing the resistance box from 5000 Ω to 25 Ω, and a waiting time of 20 min was necessary for each resistance adjustment. The power density was calculated by Ohm’s law. To detect the running voltage, external resistance (1000 Ω) was linked to MFCs, and a data acquisition system (MPS-010602, Morpheus Electronic Co., Ltd, China) was used to record the voltage at per minute in an incubator of 37 � C. Once the running voltage of MFC dropped below 80 mV, the fresh nutrient solution was injected into MFC modules again, and repeated until the devices could reach a steady cycle. 3. Results and discussion
Where Id and Ir represent the measured disk current and ring current, respectively, N refers the collection efficiency for Pt ring and is taken as 37% in this study according to the Pine Co., Ltd.
As shown in Scheme 1, a series of N, P co-doped carbon catalysts with large specific surface area are prepared using waste shrimp shells as raw materials via acid pretreatment and subsequent heat treatments. Shrimp shells replacing chitin and chitosan as precursor not only can avoid the complex extraction process of chitin but also remain protein to enhance the nitrogen content. The detailed procedure to prepare PA-SS and DWSS catalysts was described in experimental section. A simple acid pre treatment can remove CaCO3 to destroy the dense frame and meanwhile forms porous structure. The residual protein and chitin can be directly used as carbon and nitrogen sources due to their intrinsic high nitrogen content. Notably, phosphoric acid plays a vital role during the synthesis process. Firstly, phosphoric acid was served as solvent in hydrothermal process to change carbon matrix structure. Secondly, the phosphoric acid also could be regarded as exogenous phosphorus source to improve catalytic activity resulted from the synergistic effect of N and P. Besides, carbonization temperature can greatly affect the graphitization degree, efficient active sites, and carbon network structures which allow more active sites to be exposed in carbon materials. Hence, the resulting catalysts PA-SS x (x represents the pyrolysis temperature) were obtained by pyrolysis at different temperatures. For comparison, using deionized water to replace phosphoric acid as the solvent in hydrothermal process combined pyrolysis process to afford nitrogen doped carbon material.
2.5. Fabrication and operation of MFCs Single-chamber MFCs with air as electron acceptor in cathode were used to investigate the practically catalytic performances including power density, polarization curves and running voltage for SS-based materials. The air-cathodes were prepared by a coating method using Nafion solution, PTFE and carbon black as binder, diffusion layer and current collector, respectively, according to the previous report [41]. The specific preparation process of air-cathodes was as follows: a uni form mixture of carbon black (25 mg) and PTFE (300 μL) was loaded on one side of carbon cloth (4 cm � 4 cm), and then annealed at 370 � C for 25 min with a heating rate of 5 � C min 1 to be the conducting layer. On the same side of carbon cloth, the desired 60 wt% PTFE solution was coated on the conducting layer and annealed at 370 � C for 12 min, and repeated three times to obtain the diffusion layer. On the other side of carbon cloth, the uniform catalyst ink consisting of catalyst (60 mg), deionized water (50 μL), Nafion solution (400 μL) and isopropanol (200 μL) was loaded and stood at room temperature for 24 h as the catalytic layer. Graphite fiber brushes with a calcination treatment at 450 � C for 30 min were used as the anodes. All the anodes of MFCs were inoculated with sludge bacterial species, which were collected from lakebed mud and operated for over 6 months. In order to fabricate a complete productivity system, the nutrient solution containing acetate (1 g L 1), vitamin solution (5 mL L 1), trace element solution (12.5 mL L 1) in 1 L of phosphate buffer solution (PBS, 50 mM) was added to MFC modules to keep the metabolism of bacteria. The specific recipes of vitamin solution, trace element solution and PBS were pro vided in Table S1. The power density and polarization curves were measured by the testing system equipped with a resistance box (5000 Ω–25 Ω), reference
3.1. Materials characterization The changing morphological characterization of the cathode cata lysts derived from shrimp shells was revealed by SEM investigation during preparation process. The surface morphology of shrimp shells displayed obvious changes before and after dilute acid pretreatment. Abundant filamentary portions and fillers in original shrimp shells could be observed (Fig. 1a), which corresponded to chitin filament, protein and CaCO3, respectively [42]. In order to remain protein and remove CaCO3, the shrimp shells were pretreated with a facile dilute acid, and a 3
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Fig. 1. SEM images of (a) Original shrimp shells, (b) DW-SS, (c) PA-SS, (d) SS, (e) DW–SS 900 and (f) PA-SS 900. TEM images of (g) and (h) PA-SS 900. Element mapping images of (i) N and (j) P in PA-SS 900.
temperature, the small molecular substances were volatilized to expose the chitin filaments, and the carbon skeletons and porous structures of chitin were still retained (Fig. 1d). In addition, the PA-SS 900 (efficient cathode catalyst) was synthetized by hydrothermal method with phos phoric acid as solvent and subsequent pyrolysis at 900 � C. As seen from Fig. 1c and f, the introduction of phosphoric acid effectively promoted the formation of more small molecules, which resulted in a fluffy and compact structure (Fig. 1e) during hydrothermal process. Interestingly, the spongy structure was transformed into similar spherical carbon networks (Fig. 1f) with higher specific surface area after calcination at 900 � C, which could coincide with the BET results. The detailed micro structure of the PA-SS 900 catalyst was further studied by TEM. Fig. 1g showed well-defined porous structure with alternating light and dark. High-resolution transmission microscope image in Fig. 1h also revealed its porous structure. Furthermore, Fig. 1i and j investigated the N and P elements could homogeneously distribute in the carbon matrix for the PA-SS 900 sample. The specific surface area and pore sizes of DW-SS 900 and PA-SS 900 were measured by BET tests. Fig. 2a showed their nitrogen adsorption and desorption isotherm curves. The isothermal adsorption lines dis played the characteristics of type IV according to the IUPAC classifica tion, suggesting the as-prepared cathode catalysts were mesoporous materials. The specific surface area of DW-SS 900 and PA-SS 900 were 299.29 m2 g 1 and 447.10 m2 g 1, respectively. Obviously, phosphoric acid as solvent during hydrothermal process could improve the pore pattern due to the generation and volatilization of more small molecules. Further, as shown in Fig. 2b, the average pore size of DW-SS 900 and PASS 900 were 13.69 nm and 8.07 nm, respectively. These pore size dis tributions further verified the formation of mesoporous materials, which could allow bulkier substances to access catalytic active sites and pro mote good mass transport [44], naturally stimulating the better ORR activity.
Fig. 2. (a) Nitrogen adsorption/desorption isotherms and (b) pore-size distri bution curves of DW-SS 900 and PA-SS 900.
large of porous structures appeared in SS (Fig. 1b), which was conduc tive to enhance the specific surface area of subsequent cathode mate rials. Further, hydrothermal process using deionized water as solvent and pyrolysis at 900 � C provided DW-SS 900. It could be seen that there were numerous wrinkles on the surface of DW-SS (Fig. 1c), which was related to the absorbing bio-oil and liquefied products with small mo lecular weight during hydrothermal process [43]. After calcined at high 4
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to its low ID/IG ratio. These results demonstrated that doping phos phorus atoms successfully contributed to the predominance of crystal defect to strengthen ORR performance [47]. The XRD patterns of cath ode catalysts were plotted in Fig. 3b. Without other obvious peaks appearing, the SS showed a sharp peak at 19.2� and a shoulder peak at 26.3� , which were the characteristic peaks of chitin, suggesting that acid pretreatment could completely remove CaCO3. The peaks at approxi mately 26.5� and 43.6� corresponding to the 002 and 100 plane of graphitic carbon could be found in all the cathode catalysts, which indicated the presence of graphitic structures. The increasing peak strength signified higher graphitization degree when the calcination temperature changed from 800 � C to 1000 � C for PA-SS materials. Comparing with the intensity of DW-SS 900 at 26.5� and 43.6� , PA-SS 900 showed a slight decrease, which might declare that PA-SS 900 possessed a lower degree of graphitization than that of DW-SS 900. These phenomena coincided with the Raman results. Finally, the heteroatoms of nitrogen (N) and phosphorus (P) doping to SS were confirmed using XPS measurements. The peaks of C 1s (285.1 eV), O 1s (532.4 eV), N 1s (401.0 eV) and P 2p (133.6 eV) in XPS survey spectra distinctly demonstrated the dominant presence of C, O, N and P in DW-SS and PA-SS cathode catalysts (Fig. 4a), and the element contents were recorded in Table 1. It could be seen that doping P not only facilitated the retention of nitrogen in SS, but also could increase the content of oxygen, which can promote the adsorption of O2 to decrease the ORR overpotential [48]. These results also could be confirmed by energy dispersive X-ray spectroscopy (EDS) analysis. (shown in Fig. S1 and Table S2). However, the higher calcination
Fig. 3. (a) Raman spectra and (b) XRD patterns of the SS, DW-SS 900, PA-SS 800, PA-SS 900 and PA-SS 1000.
To understand the crystal structure of DW-SS 900 and PA-SS 900, the XRD and Raman tests were carried out as shown in Fig. 3. In the Raman spectra of Fig. 3a, all the cathode catalysts exhibited typical D and G bands corresponding to ~1330 cm 1 and ~1596 cm 1, which repre sented disordered and graphitic phases in carbon, respectively. The graphitization degree of carbon materials could be evaluated by measuring the intensity ratio of D-band to G-band (ID/IG) [45]. The ID/IG value of DW-SS 900, PA-SS 800, PA-SS 900 and PA-SS 1000 were 0.87, 0.93, 0.97 and 0.98, respectively. The cathode catalysts with phosphoric acid as hydrothermal solvent (PA-SS series) had slightly higher ID/IG ratio than that of DW-SS, which was caused by the incorporation of phosphorus atom defects and structural collapse during thermal annealing process [46], indicating that they had more active sites. Inversely, the DW-SS 900 held a higher graphitization degree according
Table 1 The detailed distribution of various catalysts for C, N, O and P based on XPS survey. Samples DW-SS 900 PA-SS 800 PA-SS 900 PA-SS 1000
Elemental content (at. %) C
O
N
P
90.21 81.97 86.93 87.45
7.68 12.39 9.24 10.01
2.11 3.71 2.53 1.49
1.93 1.30 1.05
Fig. 4. XPS survey spectra of DW-SS and PA-SS cathode catalysts, C 1s XPS spectra of (b) DW-SS 900 and (c) PA-SS 900, N 1s XPS spectra of (d) DW-SS 900 and (e) PA-SS 900, (f) P 2p XPS spectrum of PA-SS 900. 5
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Table 2 The detailed distribution of various patterns based on the deconvolution of C1s, N1s, and P2p peaks analyses. Samples
C1s (%)
DW-SS 900 PA-SS 800 PA-SS 900 PA-SS 1000 a b c d
N1s (%) a
O¼C–O
C–O
C–N
C¼C
N1
14.78 11.94 18.23 6.87
5.52 10.43 5.12 9.16
16.62 21.85 20.55 29.41
63.07 31.63 56.10 54.56
10.95 15.15 13.66 12.56
P2p (%) N2
b
33.63 28.24 26.24 29.41
N3
c
44.86 42.90 49.02 46.40
N4d
P–O
P–C
10.55 13.71 10.68 11.78
39.34 31.79 21.44
60.66 68.21 78.56
Pyridinic-N. Pyrrolic-N. Graphitic-N. Oxidized-N.
measurements were conducted in a three electrodes system with 0.1 M KOH solution as electrolyte. The ORR characteristic parameters of different cathode catalysts were summarized in Table 3. Fig. 5a showed the CV curves of PA-SS 800, PA-SS 900, PA-SS 1000 and DW-SS 900 in O2 or N2 saturated KOH solution. Compared with N2 atmosphere (black line in Fig. 5a), a distinct ORR peak at 0.78 V (vs. RHE) could be observed for PA-SS 900 in the presence of oxygen (red line in Fig. 5a), indicating that the as-prepared PA-SS 900 catalyst had good ORR ac tivity. After O2 saturated electrolyte for 30 min, all the cathode catalysts emerged obvious ORR peaks. However, in the DW-SS 900 plot, the ox ygen reduction peak appeared at 0.72 V (vs. RHE), which was lower than that of PA- SS 900 (0.78 V vs. RHE) and PA-SS 1000 (0.78 V vs. RHE). Remarkably, doping phosphorus atoms could promote the Ep (voltages corresponding to the oxygen reduction peaks) shifting towards positive direction. In addition, the PA-SS 900 had the highest current density among all cathode catalysts. These results suggested that the N and P cofunctionalized PA-SS 900 had the lowest overpotential toward ORR in alkaline media. In fact, with 0.1 M KOH solution (pH ¼ 13) as electrolyte for ORR, all the surface groups were deprotonated to lead to a reactive dependence on the high concentration of hydroxyl groups, which can specifically adsorb onto the active sites such as pyridine nitrogen [55]. Thus, a surface-confined redox-mediated process predominated the re action to yield a 4e pathway at high potential, and a parallel hydrogen peroxide process was induced by the outer-sphere electron transfer mechanism at low potential [56,57]. Further, Fig. 5b exhibited the LSV curves of DW-SS, PA-SS and 20% Pt/C in O2 saturated KOH solution at 1600 revolutions per minute (rpm). The P, N co-doped PA-SS 900 presented more positive Eonset and higher limiting current density at 0.2 V (vs. RHE) than those of DW-SS 900, PA-SS 800 and PA-SS 1000, which were well assigned with the CV consequences. Meanwhile, the PA-SS 900 showed an Eonset of 0.96 V and E1/2 of 0.82 V, which were slightly worse than those of 20% Pt/C (1.03 V and 0.91 V), but it was still a potential catalyst due to its low cost and stability. Meanwhile, the catalyst is comparable to most of the re ported carbon materials based chitin or chitosan (Table S3). Combining with the previous structure and elementary characteristic analysis, the conspicuous CV and LSV performances of PA-SS 900 might result from its similar spherical carbon networks and large specific surface area, which could fabricate multidimensional electron transfer routes and expose more active sites. Moreover, the doping phosphorus and inherent nitrogen source from SS also incorporated the various defect sites into the graphitic framework. RDE measurements with different rotation rates were employed to research the electron transfer pathway in O2-saturated 0.1 M KOH electrolyte. The LSV curves and corresponding K-L plots of PA-SS 900 and DW-SS 900 were showed in Fig. 5c and d. Those of PA-SS 800 and PA-SS 1000 were displayed in Figs. S4a and 4c. The current density raised with increasing rotation speed due to the shortened diffusion distance at high velocity. Based on the K-L plots (J 1 with respect to ω 1/ 2 in Figs. S4b and 4d, and inset of Fig. 5c and d), the electron transfer number (n) was calculated to be 2.87–3.33, 3.45–3.62, 3.36–3.52 and 3.10–3.25 at a potential range of 0.20–0.60 V (vs. RHE), corresponding
Table 3 The ORR performance parameters of different cathode catalysts measured in 0.1 M KOH solution. Samples
E p (V vs. RHE)
E onset (V vs. RHE)
E 1/2 (V vs. RHE)
Diffusion limiting current at 0.2 V (mA cm 2)
PA-SS 800 PA-SS 900 PA-SS 1000 DW-SS 900 20% Pt/C
<0.72
0.87
0.74
3.36
0.78
0.96
0.82
4.47
0.78
0.96
0.82
3.86
0.72
0.93
0.79
3.86
0.82
1.03
0.91
5.01
temperature would reduce the active sites of O, N and P. The highresolution XPS spectra of C 1s, N 1s and P 2p were analysed to obtain the detailed distribution of various elements. The spectra of DW-SS 900 and PA-SS 900 were shown in Fig. 4b–f and the detailed elemental contents were calculated in Table 2. The spectra of PA-SS 800 and PA-SS 1000 were plotted in Figs. S2 and S3. As shown in Fig. 4b–c, the carbon – C, C–N, C–O patterns of DW-SS 900 and PA-SS 900 were composed of C– – C–O, corresponding to 284.6 eV, 285.6 eV, 286.2 eV and and O– – C occupied a major contri 288.5 eV, respectively [49]. Graphitic C– bution to the C1s peak of both DW-SS 900 and PA-SS 900, which confirmed their high graphitization. The percentage of sp2-hybridization – C) of PA-SS 900 decreased slightly (Table 2), which was related to (C– the more crystal defects of phosphorus doping. The N 1s spectra of DW-SS 900 and PA-SS 900 were displayed in Fig. 4d and e. There were four types of nitrogen contribution containing pyridinic-N (398.3~399.5 eV, N1), pyrrolic-N (400.1~400.9 eV, N2), graphitic-N (401.2~402.0 eV, N3) and oxidized-N (403.4~410.0 eV, N4) [50], and the atomic percent of each pattern was calculated in Table 2. Numerous researches have investigated that pyridinic-N and graphitic-N were conducive to electron transfer to enhance catalytic activity [51,52]. Comparing with DW-SS 900, PA-SS 800 and PA-SS 1000, the PA-SS 900 held a higher amount of pyridinic-N and graphitic-N, suggesting that PA-SS 900 had higher ORR performances. Besides, the high content of graphitic-N in PA-SS 900 might be caused by doping phosphorus and annealing process [53]. The high-resolution P 2p spectrum of PA-SS 900 was shown in Fig. 4f. Two bands appeared at 131.8 eV and 133.6 eV corresponding to P–C and P–O. Both of them could cause the charge delocalization of carbon adjacent to heteroatom with higher electro negativity to form electron poverty, which was generally beneficial for ORR [54]. However, the oxygen-containing functional groups including – C–O, C–O and P–O gradually lost as the calcination temperature O– increased. 3.2. Electrochemical characterization of cathode catalysts To evaluate the ORR activity of the as-prepared cathode catalysts, the cyclic voltammetry (CV) and linear sweep voltammetry (LSV) 6
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Fig. 5. (a) CV curves in O2 (red) or N2-saturated (black) 0.1 M KOH, (b) LSV curves at 1600 rpm, LSV curves of (c) DW-SS 900 and (d) PA-SS 900 at different rotation from 400 to 2250 rpm, (e) Electron transfer number derived from LSV curves at different rotation speeds, (f) Plots of H2O2 yield (up) and n value (down) against electrode potential of PA-SS 800, PA-SS 900, PA-SS 1000, and compared with that of Pt/C in O2-saturated 0.1 M KOH, (g) Tafel plot, (h) the i-t chronoamperometric curves of PA-SS 900 and 20% Pt/C catalysts at 0.7 (vs. RHE) in O2-saturated 0.1 M KOH electrolyte, (i) the tolerate to 3 M methanol. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
to PA-SS 800, PA-SS 900, PA-SS 1000 and DW-SS 900, respectively (Fig. 5e). These values were further validated by rotating ring-disk electrode (RRDE) measurements (shown in Fig. S5). According to the measured ring current and disk current, the electron transfer number was calculated to be 3.38–3.68 for PA-SS 900 at a potential range of 0.8–0.2 V (vs. RHE) in Fig. 5f. These results were markedly consistent with the K-L results, indicating a four-electron pathway dominant for PA-SS 900 in alkaline medium. Uniformly, the RRDE measurements also declared that the PA-SS 800 had a great potential to generate H2O2 with a yield beyond 53.5% at potential range of 0.8–0.2 V (vs. RHE). Thus, these results suggested that PA-SS 900 had a high ORR catalytic effi ciency in alkaline medium. The ORR activity of cathode catalysts was further recorded by Tafel plots as Fig. 5g. Notably, the Tafel slope of PA-SS 900 (96 mV/decade) was smaller than those of PA-SS 800 (135 mV/decade), PA-SS 1000 (100 mV/decade) and DW-SS 900 (111 mV/decade), which was comparable to 92 mV/decade of 20% Pt–C catalyst. The smaller Tafel slope meant the higher intrinsic catalytic activity for ORR, suggesting a faster reac tion kinetic in alkaline media for PA-SS 900. Certainly, the durable test of PA-SS 900 with high ORR performances was essential for its practical application. An i-t chronoamperometric result was revealed in Fig. 5h. The relative current of both PA-SS 900 and 20% Pt/C gradually reduced at 0.7 V (vs. RHE) in O2-saturated
0.1 M KOH electrolyte over time. Fortunately, the PA-SS 900 (90.34%) showed a slower attenuation rate than 20% Pt/C (85.40%), supporting a better long-term stability of PA-SS 900 over the 20% Pt/C in an alkaline electrolyte after operating 12000s. Besides, the PA-SS 900 was further subjected to test the possible poison effect in the presence of methanol. It could be seen that after the 3 M methanol addition at 300s, 20% Pt/C showed a sharp decrease, but the PA-SS 900 kept excellent resistance (Fig. 5i). Such high selectivity of the PA-SS 900 toward ORR and remarkably good tolerance to methanol can be attributed to the much lower ORR potential than that required for oxidation of the methanol. 3.3. MFCs performance and analysis for cathode catalysts In order to determine the practical ORR performance, a series of PASS 800, PA-SS 900 and PA-SS 1000 fabricated single-chamber air-cath ode MFCs were inoculated with bacteria on carbon brush. All the MFCs were operated for approximately 5 days to acclimate microorganism. As a comparison, 20% Pt/C fabricated MFC was also run at the same con dition. Fig. 6 showed the running behaviour of voltage variation with time after domestication. A similar cyclic process that the output voltage promptly increased, then reached a stably durable platform and ended up with attenuation could be observed. Once the fresh nutrient solution was added into MFCs, the periodic process about 20–30 h would be 7
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Journal of Power Sources 446 (2020) 227356
OCV reduce of 13.16% compared to those of the commercial 20% Pt/C (892 mW m 2 and 752 mV). The MPD exactly agreed with the results of LSV and Tafel curves. Surprisingly, the control DW-SS 900 derived from SS without exogenous heteroatom doping also exhibited favourable properties, which was probably attributed to the inherent source of ni trogen from chitin and protein in SS [58]. Further, the doping phos phorus probably changed the chemical structure of carbon after high-temperature calcination, thus the PA-SS 900 showed better ORR activity than other catalysts and closer performances to 20% Pt/C. Fig. 7b illustrated the potential behaviour of cathode and anode following with the external resistance range from 5000 Ω to 25 Ω. There was a sharp decline about 391 mV in cathode curves. Meanwhile, a slight rise approximately 131 mV could be observed in anode curves for all catalysts. This result revealed that the cathode presented the domi nant function of MFCs. Besides, among DW-SS and PA-SS based cathode catalysts, PA-SS 900 had the highest total potential and cathode po tential, further manifesting the excellent catalytic performance.
Fig. 6. The plots of out-put voltage variation with time after complete domestication.
4. Conclusions
repeated. In details, when the last cyclic behaviour kept stable in Fig. 6, the output voltage of PA-SS 900 (503 � 5 mV) was higher than that of PA-SS 800 (421 � 3 mV), PA-SS 1000 (456 � 3 mV) and DW-SS 900 (446 � 8 mV), only 3.26% lower than Pt-MFC (521 � 10 mV), suggesting that PA-SS 900 held a high-efficient ORR catalytic activity and even could compete with Pt/C catalyst in practical application. Besides, the power density was considered as an important factor to assess the capacity of electricity production of MFCs in practical appli cation. Therefore, when all MFCs could reach a stable maximum voltage platform after one month of cyclic culture, they were tested to obtain power density curves and polarization curves, as shown in Fig. 7. It could be observed from Fig. 7a that the PA-SS 900 fabricated MFC represented the highest maximum power density (MPD) of 802 mW m 2 and open circuit voltage (OCV) of 653 mV among all experimental cathode catalysts of PA-SS 800 (686 mW m 2 and 586 mV), PA-SS 1000 (759 mW m 2 and 610 mV) and DW-SS 900 (615 mW m 2 and 685 mV). Of course, the PA-SS 900 possessed a slight MPD decrease of 10.09% and
In this work, the waste shrimp shells were directly converted into a nitrogen and phosphorus co-doped mesoporous carbon framework catalyst with favourable ORR performances by an available, low-cost and efficient synthesis. Prior to carbonization, a simple dilute acid pre-treatment could remove minerals to obtain chitin wrapped with protein from waste shrimp shells, while the resulting in-situ porous frameworks increased the specific surface area. As a metal-free catalyst, the SS prepared control cathode catalyst also exhibited excellent MFC property due to crystal defects derived from inherent nitrogen in chitin and protein. After doping extraneous phosphorus, more crystal defects and higher specific surface area were incorporated into PA-SS 900, which presented a more-positive oxygen reduction peak at 0.78 V, halfwave potential of 0.82 V (vs. RHE) and limiting current of 4.47 mA cm 2. These data were superior to the control sample DW-SS 900 and approaching commercial catalyst 20% Pt/C. Besides, it
Fig. 7. (a) Polarization and power density curves, (b) cathode and anode polarization curves in MFCs for PA-SS 800, PA-SS 900, PA-SS 1000, DW-SS 900 and 20% Pt/C. 8
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showed an excellent long-term stability with more than 90.34% reten tion of I/I0 compared with 85.40% of 20% Pt/C in alkaline media after running 3 h. For practical application in MFCs, the PA-SS 900 revealed a running voltage of 503 mV, maximum power density of 802 mW m 2 and open circuit voltage of 653 mV, which were preferable than those of DW-SS 900 and comparable to 20% Pt/C (521 mV, 892 mW m 2 and 752 mV). The simple and efficient acid pretreatment and carbonization process provided carbon catalysts with excellent performance for fuel cells, which was significant to the development of eco-friendly catalyst and would open up a new way for the utilization of shellfishery waste.
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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of the manuscript entitled “Nitrogen and phosphorus co-doped carbon networks derived from shrimp shells as an efficient oxygen reduction cata lyst for microbial fuel cells”. We also confirm that this work reported herein has not been previ ously submitted to Journal of Power Sources and is not under consideration for publication elsewhere in any medium. All authors have read and approve this version of the article, and due care has been taken to ensure the integrity of the work. Acknowledgements This work is financially supported by National Natural Science Foundation of China (grant number 51762013), National Natural Sci ence Foundation of China (21978059), High-level Innovation and Entrepreneurship Talents Project of Hainan Province (Qiong Talent Office [2014] 8) and Hainan Provincial Postgraduate Innovation Research Project (Hys2018-74). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227356. References [1] K.P. Katuri, C.M. Werner, R.J. Jimenez-Sandoval, W. Chen, S. Jeon, B.E. Logan, Z. Lai, G.L. Amy, P.E. Saikaly, Environ. Sci. Technol. 48 (2014) 12833–12841. [2] H.-S. Lee, P. Parameswaran, A. Kato-Marcus, C.I. Torres, B.E. Rittmann, Water Res. 42 (2008) 1501–1510. [3] K.C. Wrighton, P. Agbo, F. Warnecke, K.A. Weber, E.L. Brodie, T.Z. DeSantis, P. Hugenholtz, G.L. Andersen, J.D. Coates, ISME J. 2 (2008) 1146. [4] A. Ter Heijne, D.P.B.T.B. Strik, H.V.M. Hamelers, C.J.N. Buisman, Environ. Sci. Technol. 44 (2010) 7151–7156. [5] C. Santoro, A. Serov, R. Gokhale, S. Rojas-Carbonell, L. Stariha, J. Gordon, K. Artyushkova, P. Atanassov, Appl. Catal. B Environ. 205 (2017) 24–33. [6] S. You, X. Gong, W. Wang, D. Qi, X. Wang, X. Chen, N. Ren, Adv. Energy Mater. 6 (2016) 1501497. [7] K.-Y. Kim, W. Yang, P.J. Evans, B.E. Logan, Bioresour. Technol. 221 (2016) 96–101. [8] Y. Liu, Y. Zhao, K. Li, Z. Wang, P. Tian, D. Liu, T. Yang, J. Wang, J. Power Sources 378 (2018) 1–9. [9] N. Yang, G. Zhan, D. Li, X. Wang, X. He, H. Liu, Chem. Eng. J. 356 (2019) 06–515. [10] R. Rudra, V. Kumar, N. Pramanik, P.P. Kundu, J. Power Sources 341 (2017) 285–293. [11] H. Tang, S. Cai, S. Xie, Z. Wang, Y. Tong, M. Pan, X. Lu, Adv. Sci. 3 (2016) 1500265.
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