- Email: [email protected]
The exceptional performance of polyhedral porous carbon embedded nitrogen-doped carbon networks as cathode catalyst in microbial fuel cells
Journal of Power Sources 442 (2019) 227229
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
The exceptional performance of polyhedral porous carbon embedded nitrogen-doped carbon networks as cathode catalyst in microbial fuel cells Rui Yang a, b, c, Kexun Li a, b, c, *, Cuicui Lv a, b, c, **, Benqiang Cen a, b, c, Bolong Liang a, b, c a
College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, China MOE Key Laboratory of Pollution Processes and Environmental Criteria, Nankai University, Tianjin, 300071, China c Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution, Tianjin, 300071, 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
� PPC/NC composites were synthesized via a simple pyrolysis reaction. � Polyhedral porous carbon was well embedded in nitrogen-doped carbon networks. � The 3D stacked structure strongly contributed to catalytically active sites. � The MPD of PPC/NC-treated MFC was 2401 mW m 2, 3.3 times higher than the control.
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyhedral porous carbon Polypyrrole Nitrogen-doped carbon networks Oxygen reduction reaction Microbial fuel cells
The ZIF-8/polypyrrole fabricated polyhedral porous carbon embedded nitrogen-doped carbon networks complex (PPC/NC) is successfully prepared via a carbonization process followed by an acid washing. The PPC/NC with special hybrid structure presents a significant exchange current density (37.113 � 10 4 A cm 2) and a higher open circuit potential (0.310 V) compared with the control. The maximum power density of PPC/NC modified activated carbon air-cathode microbial fuel cells is up to 2401 � 0.7 mW m 2, 3.3 times as high as that of the control (725 � 0.5 mW m 2). Besides, the PPC/NC expresses a desired four-electron process towards ORR com parable to commercial Pt/C catalyst. The PPC/NC displays polyhedral porous carbon well embedded in inter connected carbon networks hierarchical structure, in which the carbon networks with the highly conductive hollow tubular structure acts as the central skeleton of the entire conductive system, thereby facilitating electron diffusion between polyhedral carbon particles. This exceptional 3D stacked structure enriches catalytically active sites and contributes significantly to the oxygen reduction reaction as the obtained complex possesses large content of nitrogen and hierarchical carbon structure. PPC/NC is considered to be a prospective catalyst to enhance the capability of the microbial fuel cells.
* Corresponding author. College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, China. ** Corresponding author. College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, China. E-mail addresses: [email protected] (K. Li), [email protected] (C. Lv). https://doi.org/10.1016/j.jpowsour.2019.227229 Received 19 July 2019; Received in revised form 2 September 2019; Accepted 28 September 2019 Available online 1 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
1. Introduction
material, AC-PPC/NC manifests large power generation capacity and excellent electrocatalytic activity.
The microbial fuel cells (MFCs) can convert the chemical energy contained in sewage into electricity through microbes, demonstrating an enormous potential in new energy exploitation [1–3]. The oxygen reduction reaction (ORR) of the cathode in air-cathode MFCs is the primary factor influencing the electrochemical activity [4,5]. Pt-based catalysts have been proved to exhibit ORR activity as an effective metal catalyst, while high cost and relatively weak electrochemical property in neutral and alkaline conditions hinder them from applica tion in MFCs [6–8]. Cheap activated carbon (AC) is a generally utilized cathode material to substitute expensive Pt, while its ORR activity is unsatisfactory. Hence, more attempts need to be devoted to exploring fresh catalysts to further enhance its catalytic activity and ORR effi ciency [9,10]. Recently, heteroatom doped metal-free carbon materials have been used as effective electrocatalysts due to their excellent role in enhancing the catalytic ability of AC, as well as their low cost and excellent elec trical conductivity. Defects caused by heteroatoms in the host material add its active sites favoring ORR, which is affected by the nitrogendoping ratio and nitrogen bond types [11,12]. For example, nitrogen-doped carbon nanotubes (N-CNTs) prepared by chemical vapor deposition (CVD) demonstrated high catalytic efficiency, which was associated with their high nitrogen content [13]. Li et al. reported that the multi-walled N-CNTs synthesized using a CVD technology displayed a comparable ORR activity to commercial Pt/C [14]. Nitrogen-containing polymers, such as polypyrrole (PPy), polyani line, etc., have been commonly employed as precursors to derive het eroatom nitrogen doped carbon, Among them, the PPy attracts increasing attention due to its merits such as simple synthesis process, low cost, high conductivity (1–104 S cm 1) [15]. CNTs coated with PPy and Pt particles prepared by Oh et al. were applied to the battery at a current density of 1.71 mA cm 2. The catalytic activity of Fe/N-CNTs synthesized by Liu et al. using PPy as a precursor was greatly advanced comparable to commercial Pt/C catalyst [16]. Liu et al. pre pared N-CNTs by pyrolysis of PPy and loaded them on Pt, and the ORR activity of the obtained catalyst can compete with Pt/C [17]. Thus, PPy has been thought to be a favorable precursor to obtain nitrogen-doped carbon (NC) catalyst. Besides, Metal-organic frameworks (MOFs), a type of organicinorganic hybrid crystalline materials that supplies a wide variety of central metals and ligands, have been turned out to be outstanding precursors to prepare nitrogen-doped carbon materials with high elec trocatalysis activity, ascribed to their unique polyhedral frameworks, high specific surface area and pore structure [18,19]. A polyhedron porous carbon (PPC) derived from ZIF-8 was known as a perfect elec trode material with large specific surface area [20]. However, its con ductivity and porosity were low [21]. Through the synergy between PPy and ZIF-8, ZIF-8/PPy complex was converted to a complex carbon ma terial composed of polyhedron porous carbon embedded in nitrogen-doped carbon networks structure by carbonization process. It was anticipated to span the electrode materials with high electro chemical activity as the high conductive hollow tubular structure in the composite material acted as the central skeleton of the entire conductive system, facilitating electron diffusion and enhancing the conductivity [22]. This study presented an initial attempt to employ ZIF-8/PPy as precursors to prepare a complex carbon material composed of poly hedron porous carbon embedded in nitrogen-doped carbon networks structure (PPC/NC), which was expected to be an effective cathode catalyst to improve the electrochemical activity of AC. It was also the first time that the PPC/NC modified activated carbon as an air-cathode material (AC-PPC/NC) for MFCs. The gained PPC/NC might offer a polyhedron-embedded carbon networks hierarchical porous architec ture and express an advanced specific surface area. This 3D stacked structure adds catalytically active sites. When applied as MFCs cathode
2. Experiment methods 2.1. Preparation of ZIF-8/PPy PPy nanowire was firstly synthesized and its synthesis process was consulted in Fig. S1 (supporting information). 0.15 g PPy was dissolved in 500 mL methanol under underultrasound. Then 4.21 g Zn (NO3)2⋅6H2O was added and stirred for 1 h. Simultaneously, 8.40 g 2methylimidazole was dissolved in 500 mL methanol, and then added to the above solution and continued stirring for 1.5 h. After standing for 48 h, it was centrifuged and washed 5 times with methanol, and then dried at 70 � C for 10 h to collect ZIF-8/PPy. Besides, ZIF-8 was prepared by a similar process without the addition of PPy as shown in Fig. S2. 2.2. Preparation of PPC/NC Fig. 1a expressed the synthetic process of PPC/NC. First, Zn (NO3)2⋅6H2O was added to a methanol solution containing uniformly dispersed PPy nanowires. Free Zn2þ was adsorbed on the nanowires acting as a nucleation center due to electrostatic action [23]. ZIF-8 polyhedron was grown after the slow addition of 2-methylimidazole followed by standing for 48 h to form ZIF-8/PPy precursor. ZIF-8/PPy was kept at 800 � C for 2 h at a rate of 5 � C min 1 under a nitrogen atmosphere. The residual Zn was washed away with 1 mol L 1 of dilute hydrochloric acid. The obtained carbon material composed of polyhedron porous carbon embedded in nitrogen-doped carbon net works and was expressed as PPC/NC. For comparison, the same pro cedure was conducted to convert ZIF-8 and PPy to PPC and NC, respectively. 2.3. Air-cathode production All air cathodes were formed by rolling press method according to the existing literature [24]. Air cathodes included a catalyst layer, a stainless steel mesh, and a gas diffusion layer. The reproducibility of all data was gained with three parallel groups. The gas diffusion layer was generated by rolling carbon black and polytetrafluoroethylene (PTFE) with a mass ratio of 3:7 and then placed in the muffle furnace (340 � C) for 25 min. The catalyst layer was generated by rolling the activated carbon (AC) doped with different catalysts (NC, PPC, PPC/NC) and PTFE with a mass ratio of 6:1. The specific preparation method was as follows: 0.70 g activated carbon and 0.30 g catalyst and 5 mL ethanol were evenly dispersed under ultrasonication, then 0.167 g PTFE was added and stirred for 30 min to obtain the catalytic layer composed of the AC doped with catalyst. The samples with different catalysts were named as AC-NC, AC-PPC, AC-PPC/NC, respectively. Bare AC was used as a con trol. Each cathode contained a loading of AC-PPC/NC (or AC-NC, AC-PPC, bare AC) that was 0.143 g cm 2. Then all air cathodes were dried at 30 � C for 12 h. 2.4. MFCs fabrication Single-chamber MFCs had an inner volume of 28 mL plexiglass cy lindrical chamber [25], which had an electrode spacing of 4 cm and a diameter of 3 cm. Both electrodes had the projected 7 cm2 area. Tita nium wires were applied to connect the anode the cathode to transfer the electrons. Anodes were carbon felt which was immersed in acetone for 12 h and then washed thoroughly with distilled water. The chambers were filled in by domestic wastewater mixed with 12.5 mL L 1 trace minerals, 50 mmol L 1 phosphate buffered saline (PBS, 10.305 g L 1 Na2HPO4⋅12H2O, 0.13 g L 1 KCl, 0.31 g L 1 NH4Cl, 3.321 g L 1 NaH2 PO4⋅2H2O [26], pH ¼ 7) and 5 mL L 1 vitamins. After two weeks, the medium was changed to PBS and acetate solution (2 g L 1). The culture 2
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 1. (a) Schematic diagram of the synthetic path of PPC/NC; (b) TGA curves of PPy, ZIF-8, ZIF-8/PPy and PPC/NC; (c) XRD patterns of NC, PPC and PPC/NC.
temperature was hold at 30 � C and the voltage was recorded by the data acquisition card (MorpheusElectronic Co. Ltd, Beijing, China).
changing the external resistance in the range of 70–9000 Ω. Each resistor was maintained for 15 min to get a precise voltage by a potentiostat (VersaSTAT 3, Princeton Applied Research, USA). A three-electrode mode was adopted, where Ag/AgCl and platinum sheet (1 cm2) were utilized as reference electrode and counter electrode, respectively. Linear sweep voltammetry (LSV) was tested from open circuit potential (OCP) to 0.3 V at a scan rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) was tested from 100 kHz to 10 mHz at the open cir cuit potential. Exchange current density (io) was tested via scanning from the overpotential scope (η) of 0–0.1 V at a rate of 1 mV s 1 io was obtained by the Tafel equation, which was expressed as log i ¼ log io βnFη/2.303RT [24]. Cyclic Voltammetry (CV) curves were tested between 0.8 and 1.4 V (vs. Ag/AgCl) with the scanning rate of 50 mV s 1 in O2 and air saturated PBS, respectively [27]. Rotating disk electrode (RRDE-3 A, ALS, Japan) was adopted to test the ORR capability of the different catalysts. 5 mg catalyst was dispersed ultrasonically into 1 mL 0.05 wt% Nafion solution and 5 mL ethanol to form a catalyst ink. About 2 μL of catalyst ink was dropped on a glassy carbon electrode and dried. ORR was examined in the O2-saturated PBS by linear scanning from 0.3 to 1.0 V (vs. Ag/AgCl) at a scan rate of 5 mV s 1 with different rotation rates (625, 900, 1225, 1600, 2000, 2500 and 3000 rpm) [28].
2.5. Characterization and electrochemical analysis Scanning electron microscopy (SEM, S-3500 N, Hitachi) and field emission transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer (EDS) were utilized to research the structure and morphology of the sample. Brunauer - Emmett - Teller (BET) was performed to obtain the specific surface area and pore structure by an adsorption meter (ASAP, 2020/TRISTAR 3000, MICROMERITICS). Thermogravimetric analysis (TGA) was performed on a synchronous thermal analyzer (TGA/DSC1) at a heating rate of 5 � C min 1 under a nitrogen atmosphere. X-ray diffraction (XRD) was tested by a D/max-2500 X-ray generator with Cu Kα radiation to forecast the crystallinity of the catalyst with a diffraction 2-theta from 10 to 90� . Raman spectroscopy was performed on Microscopic confocal Raman spectrometer (SR-500I-A). X-ray photoelectron spectroscopy (XPS, KAepna, Thermo Fisher Scientific Inc, USA) was conducted to survey the content of the elements and their types of the catalysts on an XPS spectrometer (K-Aepna, Thermo Fisher Scientific, USA). Power density curves and polarization curves were gained by 3
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
3. Results and discussion
surface area and pore volume [40]. These features added the catalyti cally active sites and reduced the barriers in the diffusion of ions and electrons, resulting in a decrease in Rct and total resistance. Raman spectra of NC, PPC and PPC/NC were displayed in Fig. 3b. All materials displayed two distinct peaks at 1344 and 1590 cm 1, which were D-band and G-band, respectively. The D-band was generated owing to the breathing pattern of k-point phonons of A1g symmetry, which was correlative with disorder of sp3 carbon atom, yet the G-band was related to the conjugated structure of sp2 carbon regions [41]. IG/ID (intensity ratio) presented the graphitization level of carbon material [42]. The IG/ID values of NC, PPC and PPC/NC were 0.98, 0.99 and 1.01, respectively, demonstrating the embedding of NC into PPC upgraded the graphitization degree of the PPC/NC complex. A high graphitization degree reportedly suggested an advantageous electric conductivity [43]. Thence, PPC/NC might have a superior electrical conductivity compared to PPC. The XPS spectra of PPC/NC, PPC, and NC were expressed in Fig. 4a. Table 1 illustrated the contents of C, N and O in all samples. The N content in PPC/NC was 12.65%, which was far higher than CX1000@polypyrrole (3.55%) [44]. The C1s spectra of materials were resolved into two peaks at 284.6 (C–C) and 286.6 (C–O/C–N) in Fig. S11. The N1s spectra (Fig. 4(b–d)) of different materials were resolved into three peaks around 398.3 eV, 401.0 eV, 402.6 eV, which ascribed to pyridinic-N (N1), graphitic-N (N2) and valley-N (N3), respectively [45]. PPC/NC contained the most N1 and N2 content, accounting for almost 79.69%, as shown in Table 1 and Fig. 4(e and f). The graphitic N reportedly raised the limiting current density and pyridinic N height ened the onset potential of the ORR, yet pyrrole and oxidized N hardly advanced the electrocatalytically ability of nitrogen-doped material [46]. Therefore we might draw conclusion and confirm that the high content of pyridinic and graphitic N in the PPC/NC enhanced superior ability toward ORR.
3.1. Physicochemical characterization Fig. 1b compared the TGA results of the PPy, ZIF-8, ZIF-8/PPy and PPC/NC. PPy was thermally degraded in three steps. The first step was to evaporate water up to 100 � C, afterwards the counterions was degraded (100–310 � C) and last the polymer backbone was degraded (310–460 � C) [29,30]. ZIF-8/PPy was almost stable below 400 � C and begun to lose weight as the temperature sustained to rise. When the temperature was 800 � C, ZIF-8/PPy and ZIF-8 had evident weight loss and the tendency was close, which was consistent with the large ratio of ZIF-8 in ZIF-8/PPy complex. PPC/NC was stable at around 800 � C, so 800 � C was selected as the calcination temperature. At this time, ZIF-8 was changed into polyhedron porous carbon (PPC) and PPy was also completely degraded to NC, while ZIF-8/PPy complex was converted to PPC/NC. The XRD results were illustrated in Fig. 1c. All samples possessed two diffraction peaks near 26� (broad peak) and 45� (weak peak), which were indexed to the (002) and (100) crystal planes of carbon, respec tively. The broad (002) peak demonstrated that its internal structure was shapeless and disordered [31,32], which verified that PPy and ZIF-8/PPy had been transformed to NC and PPC/NC respectively. All peaks with high diffraction intensity did not conformed to the simulated ZIF-8 [33], indicating that ZIF-8 had been completely converted to carbon. The absence of diffraction peaks of Zn and other impurities also certificated that Zn impurities had been completely removed and a metal-free carbon material had been synthesized [34]. The SEM and TEM results were displayed in Fig. 2. PPC exhibited a polyhedron configuration with approximately 80 nm particles in Fig. 2a. TEM result indicated in Fig. 2b gave further sight into the polygon shape with obvious defects at the edges of the PPC. Fig. 2c indicated that the linear NC were entangled to form a honeycomb network structure, whose surface was covered with rough and slightly convex spots owing to the formation of nanocrystalline particles embedded therein. Fig. 2d presented its amorphous structure with a lot of edge defects [35] and illustrated the porous hollow structure. Fig. 2e and Fig. S3 illustrated the SEM analysis of PPC/NC. The PPC/NC obviously presented that poly hedral porous carbon derived from ZIF-8 had been embedded in the nitrogen-doped carbon network. This polyhedral-embedded carbon network hierarchical structure was exhibited clearly in the Fig. 2f and high-angle annular dark-field scanning TEM (HAADF-STEM, Fig. 2g). The intersecting carbon networks penetrated the inside of the poly hedron as bridges for electron transfer [36,37]. The PPC having a size of 500 nm was embedded in intertwined carbon network to compose a PPC-embedded hierarchical porous carbon networks structure. The combination of ZIF-8 and PPy promoted the growth of ZIF-8, which increased the average size of PPC. This special layered structure increased the surface area in contact with the electrolyte and also facilitated electron diffusion, which reduced resistance and improved ORR efficiency [38]. The element mappings (Fig. 2(h and i)) expressed the uniform distribution and existence of C, N, O in the PPC/NC. The energy-dispersive X-ray spectroscopy (EDS, Fig. S4 and Table S1, sup porting information) spectrum had illustrated atomic and mass ratio of the C, N, O. Fig. 3a displayed N2 adsorption-desorption isotherms. All samples were type IV adsorption isotherm and the hysteresis loop was exhibited at higher relative pressure (0.9–1.0), which expressed that they con tained mesopores and macropores [39]. As shown in Table S2, PPC/NC possessed a high specific surface area of 342.3 m2 g 1 with a pore vol ume of 0.43 cm3 g 1, which were bigger than those of PPC (225.1 m2 g 1 and 0.42 cm3 g 1) and NC (129.0 m2 g 1 and 0.12 cm3 g 1). Barrett-Joyner-Halenda (BJH) pore size distribution (Fig. S5) demon strated micropores structure of NC and hierarchical porous structure of PPC/NC and PPC with a small scope centered at 2 nm, a broad scope centered around 50 nm and some macropores, which raised its specific
3.2. Performance of MFCs The power density curves and the polarization curves were assessed after one month of constant temperature culture to form a mature anode biofilm and a stable potential. The power densities and voltages of all MFCs were displayed in Fig. 5a. The catalyst-modified AC air cathode MFCs all expressed higher power densities than that of the control (bare AC), which had a power density of 725 � 0.5 mW m 2. AC-PPC/NC possessed a maximum power density (MPD) of 2401 � 0.7 mW m 2, which was 3.3 times higher than control, and the power densities from AC-PPC and AC-NC were 1802 and 1601 mW m 2, respectively, which were identical as the order of open circuit voltage. The power densities of AC-NC and AC-PPC did not exceed the MPD of AC-PPC/NC with a polyhedron-embedded carbon networks hierarchical porous structure. Furthermore, the power density of AC-PPC/NC was 2 times higher than that of the Pt/C cathode (1209 � 15 mW m 2, Fig. S7), and the cost of AC-PPC/NC was much lower than Pt/C (Table S3). These results rep resented that the MFCs equipped with AC-PPC/NC can replace Pt/C to generate more electric power [47]. The respective polarization curves of cathode and anode were indi cated in Fig. 5b. It was clear that all the anode potentials had the same trend while the cathode potentials of the modified AC were noticeable better than the control, which demonstrated that the cathode was the principal part affecting the superior properties of MFCs and the modified AC expressed powerful ORR catalytic activity. In previous paper, the MPD of MFCs equipped with polypyrrole/ mercapto-2-sulfonate film cathode was 574.9 mW m 2 [48]. Nitrogen-doped toner derived from polypyrrole as a precursor outputed MPD of 934.7 � 5 mW m 2 [49]. The MPD of MOF-Cu3 (BTC)2 modified air-cathode MFC was 1772 � 15 mW m 2 [50]. The Metal organic framework-derived Co3O4/NiCo2O4 double-shelled nanocage exhibited advanced MPD of 1810 mW m 2 [51]. Yet all these catalysts showed much lower MPD than the PPC/NC catalyst (2401 � 0.7 mW m 2). 4
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 2. SEM images of (a) PPC, (c) NC and (e) PPC/NC. TEM images of (b) PPC, (d) NC and (f) PPC/NC. (g) HAADF-STEM image and (h–i) element mappings of the PPC/NC.
5
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 3. (a) N2 adsorption-desorption isotherms and (b) Raman spectrum of all samples.
Fig. 4. (a) XPS survey spectrum; the N1s spectra of PPC/NC (b), PPC (c) and NC (d), respectively; Elemental content (e) and the atomic percentages of all nitrogen functionalities (f) of all samples. 6
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Table 1 Elemental analysis of the PPC/NC, PPC and NC by XPS spectrum. Sample PPC/NC PPC NC
C (%)
O (%)
N (%)
C–C
C–O/C–N
Total C
Total O
Total N
Pyridinic-N
Graphitic-N
Valley-N
59.71 60.08 61.83
40.29 39.92 38.17
85.32 87.60 88.46
2.03 3.88 3.24
12.65 8.52 8.30
37.78 31.31 46.41
41.86 68.69 28.04
20.36 – 25.55
Fig. 5. (a) Polarization and power density curves; (b) Separate polarization curves of cathode and anode in operational process.
3.3. Electrochemical analysis
different materials in air-cathode. The equivalent circuit mode was demonstrated in Fig. S9, which contained an external resistance be tween the counter electrode and air-cathode surface (Ro), a diffusion resistance that was the electronic resistance of the current collector and contact resistances (Rd) and a charge transfer resistance (Rct). The Nyquist plot was demonstrated in Fig. 6b, and the calculated resistances were displayed in Table 2. Compared with the control, the air cathode resistances of all modi fied ACs were significantly reduced, which indicated an increase in conductivity and power output. The total AC-PPC/NC resistance (6.481 Ω) was much lower than commercial Pt/C electrodes (16.4 Ω) [53]. The Rct of all the air-cathodes demonstrated the order: AC-PPC/NC (1.104 Ω) < AC-PPC (3.577 Ω) < AC-NC (3.797 Ω) < bare AC (4.599 Ω). It expressed that the PPC/NC could reduce the Rct obviously. The lower Rct was advantageous to accelerate electronic transportation [26]. The diffusion resistances (Rd) of bare AC, AC-NC, AC-PPC, AC-PPC/NC were 5.237 Ω, 2.217 Ω, 0.667 Ω, 0.568 Ω, respectively. The Rd of bare AC was about 10 times larger than the AC-PPC/NC. The results were probably due to the superior pore structure of PPC/NC. It had been reported that large pore size and pore volume could greatly accelerate ion transport, resulting in a decrease in Rd [54]. In addition, the unique polyhedron-embedded carbon networks hierarchical porous structure
In order to ensure the optimal doping ratio of different catalysts in AC, we compared their LSVs performance, as shown in Fig. S8 (a-c). It was proved that at the end of the scanning potential, AC-PPC/NC, AC-PPC and AC-NC expressed the highest current density when doped into AC at 30% mass ratio, where the current density of AC-PPC/NC was 32.92 mA cm 2, which was 205% larger than the control (10.81 mA cm 2), representing that PPC/NC modification sped up the ORR significantly. LSVs of different air cathodes with doping ratio of 30% were exhibited in Fig. 6a. At the potential of 0.1 V, the current density of the air cathodes were arrayed in the order of AC-PPC/NC (12.45 mA cm 2) > AC-PPC (8.44 mA cm 2) > AC-NC (5.94 mA cm 2) > bare AC (4.64 mA cm 2). The current density of AC-PPC/NC was three times larger than the con trol, and was far superior to Pt/C (6.71 mA cm 2, Fig. S8d), representing that PPC/NC could efficiently substitute Pt as ORR catalyst. Besides, the onset potential of air-cathodes were in the sequence of AC-PPC/NC (0.310 V) > AC-PPC (0.287 V) > AC-NC (0.270 V) > Bare AC (0.194 V). High onset potential corresponded to a higher ORR catalytic activity, which indicated a beneficial modification of the obtained catalysts [52]. Electrochemical impedance spectroscopy (EIS) by mimicking the equivalent circuit was employed to analyze the different resistances of
Fig. 6. LSV curves (a) and Nyquist plots of EIS (b) of bare AC and different catalysts modified AC. Lines labeled ‘Cal’ were fitting data from the equivalent circuit. 7
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Table 2 Fitting data of different cathodes based on the equivalent circuit. R0 (Ω) Rd (Ω) Rct (Ω) Rt (Ω)
Table 3 Exchange current density (io) calculated from the Tafel plots.
bare AC
AC-NC
AC-PPC
AC-PPC/NC
The air-cathode
Linear fitting equation (R2)
10
8.274 5.237 4.599 18.110
4.352 2.217 3.797 10.366
4.500 0.667 3.577 8.744
4.809 0.568 1.104 6.481
bare AC NC PPC PPC/NC
y y y y
11.001 18.201 21.008 37.113
shortened the ion diffusion distance and played a considerable role in catalytic activity [55]. Therefore, PPC/NC would be a potential catalyst for higher power density in MFCs.
¼¼¼¼-
2.92548 þ 1.71485x 2.74090 þ 4.78394x 2.67762 þ 4.55569x 2.43304 þ 2.64225x
(0.998) (0.997) (0.997) (0.995)
4
io (A⋅cm
2
)
corresponding K-L plots of PPC/NC and bare AC were illustrated in Fig. 7 (c and d) and Fig. S10 (a-b), respectively. The electron transfer number (n) of PPC/NC and bare AC were calculated to be 3.95 and 0.83 at 0.6 V, respectively, demonstrating that PPC/NC catalyzed ORR via an effective four-electron path superior to Pt/C (n ¼ 3.6) [57]. Fig. 8a expressed the CV curves of all samples in O2 saturated PBS. The CV curves (taking the 60th cycles) of AC and PPC/NC displayed an ORR reduction peak at about 0.25 V (vs. Ag/AgCl), where the PPC/NC had the highest current density of 0.65 mA cm 2 superior to PPC (0.32 mA cm 2) and NC (0.30 mA cm 2), suggesting the PPC/NC had higher active ORR behavior [35]. The CV curves (Fig. S11a) of AC and PPC/NC in air saturated PBS were smooth and smaller peak emerged, which meant ORR occurred gently due to relative low oxygen concen tration. Galvanostatic charge/discharge curves still confirmed that the good stability of PPC/NC (Fig. 8b and Fig. S11b), meaning that PPC/NC possessed superior ORR catalytic activity and stability. Comprehensive physicochemical characterization and electro chemical analysis, compared with other metal-containing catalysts of various morphologies, such as nanoparticles [58], polyhedral nanocage [51], urchin-like spinel [59], irregular shape [60], ortho hexagon [61],
3.4. Catalytic kinetics of ORR The exchange current density (io) was obtained by the Tafel equation for research of catalytic kinetics. Fig. 7a compared the Tafel diagram of all air cathodes. The linear region (R2 > 0.99) in the scope of 60–80 mV was illustrated in Fig. 7b. The data for the linear fitting was shown in Table 3. The arrangement sequence of io was as follows: AC-PPC/NC (37.113 � 10 4 A cm 2) > AC-PPC (21.008 � 10 4 A cm 2) > AC-NC 4 2 (18.201 � 10 A cm ) > bare AC (11.001 � 10 4 A cm 2), which was equal to the sequence of EIS and LSV. A higher io represented less resistance and stronger ORR activity. The io of AC-PPC/NC was near 3.4 times bigger than bare AC, which demonstrated the increase the utili zation of electrons and finally devoted to enhance MPD output [3]. The RDE test characterized the ORR kinetic mechanism. The electron transfer number (n) of the bare AC and PPC/NC in the ORR was obtained by the Koutecky-Levic (K-L) equation [56]. The voltammograms and
Fig. 7. (a) Tafel plots of AC cathodes with different catalysts. The potential window ranges from open circuit potential to the overpotential of 100 mV. Scan rate: 1 mV s 1; (b) Linear fitting of from 60 mV to 80 mV; (c) Rotating-disk voltammograms of the PPC/NC in O2-saturated PBS electrolyte at different rotation rates and (d) Corresponding Kouteckye-Levich plot. 8
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 8. (a) Cyclic voltammetric curves of different catalysts in O2-saturated PBS (50 mM); (b) Galvanostatic charge/discharge curves of PPC/NC at 50 mM PBS. Table 4 Comparison of electrochemical performance of electrocatalysts with different morphologies. Electrocatalyst
Morphology and structure
OCP (V)
The current density (mA cm 2)
PPC/NC PPC NC NiCo2S4/AC [57] Co3O4/NiCo2O4 DSNC [33] NiCo2O4 [58] N and P dual-doped carbon [59] Nano-Co3O4 [60] Fe3O4 [61]
Polyhedron-embedded- networks Polyhedron Carbon networks Nanoparticles Polyhedron nanocage Urchin-like spinel Irregular shape Ortho hexagon Nanosphere
0.310 0.287 0.270 0.245 0.252 0.236 0.343 0.245 0.270
12.45 8.44 5.94 7.13 7.86 8.34 6.94 8.90 6.87
0.1v
nanosphere [62], as shown in Table 4, PPC/NC complex with the special structure of polyhedron porous carbon-embedded carbon networks exhibited the best electrochemical capability. On the one hand, the carbon networks with the highly conductive hollow tubular structure acted as the central skeleton of the entire conductive system, which promoted electron diffusion and significantly reduced Rt. On the other hand, ZIF-8 derived polyhedral porous carbon structure offered poly hedral frameworks, high specific surface area and pore structure. PPC/NC derived from ZIF-8/PPy complex as precursor was referred as an ideal cathode catalyst with excellent structure superiority. Metal-free carbon composites with such 3D stacked structure enriched the catalytic active sites and possessed large content of nitrogen, which prompted the occurrence of ORR, thereby increasing the MPD of MFCs.
4
Rt (Ω)
10
io (A⋅cm
6.48 8.74 10.37 11.14 11.00 14.60 9.83 10.24 12.15
37.113 21.008 18.201 17.383 19.700 25.486 17.270 18.865 18.710
2
)
MPD (mW m 2)
0.3 V 32.92 24.43 20.59 16.50 16.92 19.01 23.64 18.00 17.23
2401 1802 1601 2000 1801 1730 1603 1500 1430
Acknowledgements This work was supported by the Natural Science Foundation of Tianjin (17JCYBJC23300) and National Key R&D Program of China (No. 2016YFC 0400704 and No. 2016YFC0401407). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227229. References [1] P. Pandey, et al., Recent advances in the use of different substrates in microbial fuel cells toward wastewater treatment and simultaneous energy recovery, Appl. Energy 168 (2016) 706–723. [2] S. Calabrese Barton, J. Gallaway, P. Atanassov, Enzymatic biofuel cells for implantable and microscale devices, Chem. Rev. 104 (10) (2004) 4867–4886. [3] N. Yang, et al., Complete nitrogen removal and electricity production in Thaueradominated air-cathode single chambered microbial fuel cell, Chem. Eng. J. 356 (2019) 506–515. [4] W. Xia, et al., Earth-abundant nanomaterials for oxygen reduction, Angew Chem. Int. Ed. Engl. 55 (8) (2016) 2650–2676. [5] J. Qi, et al., Surface dealloyed PtCo nanoparticles supported on carbon nanotube: facile synthesis and promising application for anion exchange membrane direct crude glycerol fuel cell, Green Chem. 15 (5) (2013) 1133. [6] Z. Wang, C. Cao, Y. Zheng, et al., Abiotic oxygen reduction reaction catalysts used in microbial fuel cells, ChemElectroChem 1 (11) (2014) 1813–1821. [7] E. Antolini, Composite materials for polymer electrolyte membrane microbial fuel cells, Biosens. Bioelectron. 69 (2015) 54–70. [8] C. Santoro, et al., Double-chamber microbial fuel cell with a non-platinum-group metal Fe-N-C cathode catalyst, ChemSusChem 8 (5) (2015) 828–834. [9] X. Chen, et al., Multi-component nanoporous platinum–ruthenium–copper–osmium–iridium alloy with enhanced electrocatalytic activity towards methanol oxidation and oxygen reduction, J. Power Sources 273 (2015) 324–332. [10] X. Zhang, D. Pant, F. Zhang, et al., Long-term performance of chemically and physically modified activated carbons in air cathodes of microbial fuel cells, ChemElectroChem 1 (11) (2014) 1859–1866. [11] C.R. Raj, et al., Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. 4 (29) (2016) 11156–11178.
4. Conclusion Herein, a special carbon hybridization complex (PPC/NC) was ob tained through carbonization of ZIF-8/PPy precursor. After the carbonization, The ZIF-8 derived polyhedral porous carbon and poly pyrrole (PPy) derived carbon networks were crosslinked tightly and the polyhedral-embedded carbon networks were well prepared. The mi crobial fuel cells equipped with AC-PPC/NC presented a maximum power density of 2401 � 0.7 mW m 2, which was superior to commer cial Pt/C. Furthermore, AC-PPC/NC presented a more excellent ex change current density (37.113 � 10 4 A cm 2) and a better open circuit potential (0.310 V) compared with the control. Furthermore, PPC/NC presented a four-electron transfer pathway (n ¼ 3.95) toward the ORR. The improvement of the ORR was mainly due to the ZIF-8-derived polyhedral embedded PPy-derived carbon networks hierarchical struc ture and the present of a large amount of graphite nitrogen and pyr idinium nitrogen, as well as the advanced specific surface area. The PPC/NC with 3D stacked structure could be expected to replace Pt as an extremely prospective electrode material in the practical application of MFCs.
9
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
[12] L. Zhao, et al., Sustainable nitrogen-doped carbonaceous materials from biomass derivatives, Carbon 48 (13) (2010) 3778–3787. [13] Z. Chen, D. Higgins, Z. Chen, Nitrogen doped carbon nanotubes and their impact on the oxygen reduction reaction in fuel cells, Carbon 48 (11) (2010) 3057–3065. [14] H. Li, H. Liu, Z. Jong, et al., Nitrogen-doped carbon nanotubes with high activity for oxygen reduction in alkaline media, Int. J. Hydrogen Energy 36 (3) (2011) 2258–2265. [15] A. Zhao, J. Masa, M. Muhler, et al., N-doped carbon synthesized from N-containing polymers as metal-free catalysts for the oxygen reduction under alkaline conditions, Electrochim. Acta 98 (2013) 139–145. [16] B. Liu, W. Dai, Z.X. Liang, et al., Fe/N/C carbon nanotubes with high nitrogen content as effective non-precious catalyst for oxygen reduction reaction in alkaline medium, Int. J. Hydrogen Energy 42 (9) (2017) 5908–5915. [17] Q. Liu, Z. Pu, C. Tang, et al., N-doped carbon nanotubes from functional tubular polypyrrole: a highly efficient electrocatalyst for oxygen reduction reaction, Electrochem. Commun. 36 (2013) 57–61. [18] Q.L. Zhu, J. Li, Q. Xu, Immobilizing metal nanoparticles to metal–organic frameworks with size and location control for optimizing catalytic performance, J. Am. Chem. Soc. 135 (28) (2013) 10210–10213. [19] Y. Huang, et al., Bimetallic alloy nanocrystals encapsulated in ZIF-8 for synergistic catalysis of ethylene oxidative degradation, Chem. Commun. 50 (70) (2014) 10115–10117. [20] Y. Liu, et al., Metal-organic framework-derived porous carbon polyhedra for highly efficient capacitive deionization, Chem. Commun. 51 (60) (2015) 12020–12023. [21] J. Tang, R.R. Salunkhe, J. Liu, et al., Thermal conversion of core–shell metal–organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon, J. Am. Chem. Soc. 137 (4) (2015) 1572–1580. [22] Y. He, Y. Bai, X. Yang, et al., Holey graphene/polypyrrole nanoparticle hybrid aerogels with three-dimensional hierarchical porous structure for high performance supercapacitor, J. Power Sources 317 (2016) 10–18. [23] V. Chandra, K.S. Kim, Highly selective adsorption of Hg2þ by a polypyrrolereduced graphene oxide composite, Chem. Commun. 47 (13) (2011) 3942–3944. [24] H. Dong, et al., A novel structure of scalable air-cathode without Nafion and Pt by rolling activated carbon and PTFE as catalyst layer in microbial fuel cells, Water Res. 46 (17) (2012) 5777–5787. [25] H. Liu, B.E. Logan, Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane, Environ. Sci. Technol. 38 (14) (2004) 4040–4046. [26] L. Pu, et al., Silver electrodeposition on the activated carbon air cathode for performance improvement in microbial fuel cells, J. Power Sources 268 (2014) 476–481. [27] H. Xia, et al., 2D MOF nanoflake-assembled spherical microstructures for enhanced supercapacitor and electrocatalysis performances, Nano-Micro Lett. 9 (4) (2017) 43. [28] F.W.T. Goh, et al., Ag nanoparticle-modified MnO2 nanorods catalyst for use as an air electrode in zinc–air battery, Electrochim. Acta 114 (2013) 598–604. [29] P. Wang, et al., A novel electroactive PPy/HKUST-1 composite film-coated electrode for the selective recovery of lithium ions with low concentrations in aqueous solutions, Electrochim. Acta 306 (2019) 35–44. [30] G. Nystrom, et al., A nanocellulose polypyrrole composite based on microfibrillated cellulose from wood, J. Phys. Chem. B 114 (12) (2010) 4178–4182. [31] Z.-Y. Sui, et al., A highly nitrogen-doped porous graphene – an anode material for lithium ion batteries, J. Mater. Chem. 3 (35) (2015) 18229–18237. [32] R. Zheng, et al., Heteroatom-doped carbon nanorods with improved electrocatalytic activity toward oxygen reduction in an acidic medium, Carbon 69 (2014) 132–141. [33] M. Jiang, et al., Hierarchically porous N-doped carbon derived from ZIF-8 nanocomposites for electrochemical applications, Electrochim. Acta 196 (2016) 699–707. [34] S. Gadipelli, Z.X. Guo, Tuning of ZIF-derived carbon with high activity, nitrogen functionality, and yield - a case for superior CO2 capture, ChemSusChem 8 (12) (2015) 2123–2132. [35] Z.Y. Sui, X. Li, Z.Y. Sun, et al., Nitrogen-doped and nanostructured carbons with high surface area for enhanced oxygen reduction reaction, Carbon 126 (2018) 111–118. [36] Y. Liu, et al., Supercapacitor with high cycling stability through electrochemical deposition of metal-organic frameworks/polypyrrole positive electrode, Dalton Trans. 47 (38) (2018) 13472–13478. [37] X. Xu, et al., Three-Dimensional networked metal-organic frameworks with conductive polypyrrole tubes for flexible supercapacitors, ACS Appl. Mater. Interfaces 9 (44) (2017) 38737–38744.
[38] X. Xu, et al., Rational design and fabrication of graphene/carbon nanotubes hybrid sponge for high-performance capacitive deionization, J. Mater. Chem. 3 (25) (2015) 13418–13425. [39] H. Li, et al., The capacitive deionization behaviour of a carbon nanotube and reduced graphene oxide composite, J. Mater. Chem. 1 (21) (2013) 6335. [40] Y. Yang, et al., In situ synthesis of zeolitic imidazolate frameworks/carbon nanotube composites with enhanced CO2 adsorption, Dalton Trans. 43 (19) (2014) 7028–7036. [41] W. Chaikittisilp, et al., Nanoporous carbons through direct carbonization of a zeolitic imidazolate framework for supercapacitor electrodes, Chem. Commun. 48 (58) (2012) 7259–7261. [42] L.-F. Chen, et al., Flexible all-solid-state high-power supercapacitor fabricated with nitrogen-doped carbon nanofiber electrode material derived from bacterial cellulose, Energy Environ. Sci. 6 (11) (2013) 3331. [43] X. Wen, et al., Three-dimensional graphene-based hierarchically porous carbon composites prepared by a dual-template strategy for capacitive deionization, J. Mater. Chem. 1 (39) (2013) 12334. [44] M. Sevilla, et al., Polypyrrole-derived mesoporous nitrogen-doped carbons with intrinsic catalytic activity in the oxygen reduction reaction, RSC Adv. 3 (25) (2013) 9904. [45] J. Zheng, et al., High content of pyridinic- and pyrrolic-nitrogen-modified carbon nanotubes derived from blood biomass for the electrocatalysis of oxygen reduction reaction in alkaline medium, Electrochim. Acta 168 (2015) 386–393. [46] J. Yin, et al., Enhancement of electrocatalytic activity for oxygen reduction reaction in alkaline and acid media from electrospun nitrogen-doped carbon nanofibers by surface modification, RSC Adv. 3 (36) (2013) 15655. [47] F. Zhang, et al., Power generation using an activated carbon and metal mesh cathode in a microbial fuel cell, Electrochem. Commun. 11 (11) (2009) 2177–2179. [48] X. Zhang, et al., Inorganic/organic mesostructure directed synthesis of wire/ ribbon-like polypyrrole nanostructures, Chem. Commun. (16) (2004) 1852–1853. [49] X. Yuan, et al., Use of polypyrrole in catalysts for low temperature fuel cells, Energy Environ. Sci. 6 (4) (2013) 1105. [50] P. Tian, et al., Porous metal-organic framework Cu3(BTC)2 as catalyst used in aircathode for high performance of microbial fuel cell, Bioresour. Technol. 244 (Pt 1) (2017) 206–212. [51] S. Zhang, et al., Metal organic framework-derived Co3O4/NiCo2O4 double-shelled nanocage modified activated carbon air-cathode for improving power generation in microbial fuel cell, J. Power Sources 396 (2018) 355–362. [52] W. Yang, et al., Low-cost Fe–N–C catalyst derived from Fe (III)-chitosan hydrogel to enhance power production in microbial fuel cells, Chem. Eng. J. 380 (2020) 122522. [53] Q. Wen, et al., MnO2–graphene hybrid as an alternative cathodic catalyst to platinum in microbial fuel cells, J. Power Sources 216 (2012) 187–191. [54] H. Jiang, J. Ma, C. Li, Hierarchical porous NiCo2O4 nanowires for high-rate supercapacitors, Chem. Commun. 48 (37) (2012) 4465–4467. [55] Z. Jiang, et al., LDH nanocages synthesized with MOF templates and their high performance as supercapacitors, Nanoscale 5 (23) (2013) 11770–11775. [56] M.H.M.T. Assumpç~ ao, et al., A comparative study of the electrogeneration of hydrogen peroxide using Vulcan and Printex carbon supports, Carbon 49 (8) (2011) 2842–2851. [57] C.O. Ania, et al., New copper/GO based material as an efficient oxygen reduction catalyst in an alkaline medium: the role of unique Cu/rGO architecture, Appl. Catal. B Environ. 163 (2015) 424–435. [58] L. Pu, et al., Carbon-supported binary transition metal chalcogenide used as cathode catalyst for oxygen reduction in microbial fuel cell, Int. J. Hydrogen Energy 42 (20) (2017) 14253–14263. [59] B. Ge, et al., The performance of nano urchin-like NiCo2O4 modified activated carbon as air cathode for microbial fuel cell, J. Power Sources 303 (2016) 325–332. [60] B. Liang, et al., Nitrogen and phosphorus dual-doped carbon derived from chitosan: an excellent cathode catalyst in microbial fuel cell, Chem. Eng. J. 358 (2019) 1002–1011. [61] B. Ge, et al., The addition of ortho-hexagon nano spinel Co3O4 to improve the performance of activated carbon air cathode microbial fuel cell, Bioresour. Technol. 195 (2015) 180–187. [62] Z. Fu, et al., The performance and mechanism of modified activated carbon air cathode by non-stoichiometric nano Fe3O4 in the microbial fuel cell, Biosens. Bioelectron. 74 (2015) 989–995.
10
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
The exceptional performance of polyhedral porous carbon embedded nitrogen-doped carbon networks as cathode catalyst in microbial fuel cells Rui Yang a, b, c, Kexun Li a, b, c, *, Cuicui Lv a, b, c, **, Benqiang Cen a, b, c, Bolong Liang a, b, c a
College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, China MOE Key Laboratory of Pollution Processes and Environmental Criteria, Nankai University, Tianjin, 300071, China c Tianjin Key Laboratory of Environmental Technology for Complex Trans-Media Pollution, Tianjin, 300071, 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
� PPC/NC composites were synthesized via a simple pyrolysis reaction. � Polyhedral porous carbon was well embedded in nitrogen-doped carbon networks. � The 3D stacked structure strongly contributed to catalytically active sites. � The MPD of PPC/NC-treated MFC was 2401 mW m 2, 3.3 times higher than the control.
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyhedral porous carbon Polypyrrole Nitrogen-doped carbon networks Oxygen reduction reaction Microbial fuel cells
The ZIF-8/polypyrrole fabricated polyhedral porous carbon embedded nitrogen-doped carbon networks complex (PPC/NC) is successfully prepared via a carbonization process followed by an acid washing. The PPC/NC with special hybrid structure presents a significant exchange current density (37.113 � 10 4 A cm 2) and a higher open circuit potential (0.310 V) compared with the control. The maximum power density of PPC/NC modified activated carbon air-cathode microbial fuel cells is up to 2401 � 0.7 mW m 2, 3.3 times as high as that of the control (725 � 0.5 mW m 2). Besides, the PPC/NC expresses a desired four-electron process towards ORR com parable to commercial Pt/C catalyst. The PPC/NC displays polyhedral porous carbon well embedded in inter connected carbon networks hierarchical structure, in which the carbon networks with the highly conductive hollow tubular structure acts as the central skeleton of the entire conductive system, thereby facilitating electron diffusion between polyhedral carbon particles. This exceptional 3D stacked structure enriches catalytically active sites and contributes significantly to the oxygen reduction reaction as the obtained complex possesses large content of nitrogen and hierarchical carbon structure. PPC/NC is considered to be a prospective catalyst to enhance the capability of the microbial fuel cells.
* Corresponding author. College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, China. ** Corresponding author. College of Environmental Science and Engineering, Nankai University, Tianjin, 300071, China. E-mail addresses: [email protected] (K. Li), [email protected] (C. Lv). https://doi.org/10.1016/j.jpowsour.2019.227229 Received 19 July 2019; Received in revised form 2 September 2019; Accepted 28 September 2019 Available online 1 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
1. Introduction
material, AC-PPC/NC manifests large power generation capacity and excellent electrocatalytic activity.
The microbial fuel cells (MFCs) can convert the chemical energy contained in sewage into electricity through microbes, demonstrating an enormous potential in new energy exploitation [1–3]. The oxygen reduction reaction (ORR) of the cathode in air-cathode MFCs is the primary factor influencing the electrochemical activity [4,5]. Pt-based catalysts have been proved to exhibit ORR activity as an effective metal catalyst, while high cost and relatively weak electrochemical property in neutral and alkaline conditions hinder them from applica tion in MFCs [6–8]. Cheap activated carbon (AC) is a generally utilized cathode material to substitute expensive Pt, while its ORR activity is unsatisfactory. Hence, more attempts need to be devoted to exploring fresh catalysts to further enhance its catalytic activity and ORR effi ciency [9,10]. Recently, heteroatom doped metal-free carbon materials have been used as effective electrocatalysts due to their excellent role in enhancing the catalytic ability of AC, as well as their low cost and excellent elec trical conductivity. Defects caused by heteroatoms in the host material add its active sites favoring ORR, which is affected by the nitrogendoping ratio and nitrogen bond types [11,12]. For example, nitrogen-doped carbon nanotubes (N-CNTs) prepared by chemical vapor deposition (CVD) demonstrated high catalytic efficiency, which was associated with their high nitrogen content [13]. Li et al. reported that the multi-walled N-CNTs synthesized using a CVD technology displayed a comparable ORR activity to commercial Pt/C [14]. Nitrogen-containing polymers, such as polypyrrole (PPy), polyani line, etc., have been commonly employed as precursors to derive het eroatom nitrogen doped carbon, Among them, the PPy attracts increasing attention due to its merits such as simple synthesis process, low cost, high conductivity (1–104 S cm 1) [15]. CNTs coated with PPy and Pt particles prepared by Oh et al. were applied to the battery at a current density of 1.71 mA cm 2. The catalytic activity of Fe/N-CNTs synthesized by Liu et al. using PPy as a precursor was greatly advanced comparable to commercial Pt/C catalyst [16]. Liu et al. pre pared N-CNTs by pyrolysis of PPy and loaded them on Pt, and the ORR activity of the obtained catalyst can compete with Pt/C [17]. Thus, PPy has been thought to be a favorable precursor to obtain nitrogen-doped carbon (NC) catalyst. Besides, Metal-organic frameworks (MOFs), a type of organicinorganic hybrid crystalline materials that supplies a wide variety of central metals and ligands, have been turned out to be outstanding precursors to prepare nitrogen-doped carbon materials with high elec trocatalysis activity, ascribed to their unique polyhedral frameworks, high specific surface area and pore structure [18,19]. A polyhedron porous carbon (PPC) derived from ZIF-8 was known as a perfect elec trode material with large specific surface area [20]. However, its con ductivity and porosity were low [21]. Through the synergy between PPy and ZIF-8, ZIF-8/PPy complex was converted to a complex carbon ma terial composed of polyhedron porous carbon embedded in nitrogen-doped carbon networks structure by carbonization process. It was anticipated to span the electrode materials with high electro chemical activity as the high conductive hollow tubular structure in the composite material acted as the central skeleton of the entire conductive system, facilitating electron diffusion and enhancing the conductivity [22]. This study presented an initial attempt to employ ZIF-8/PPy as precursors to prepare a complex carbon material composed of poly hedron porous carbon embedded in nitrogen-doped carbon networks structure (PPC/NC), which was expected to be an effective cathode catalyst to improve the electrochemical activity of AC. It was also the first time that the PPC/NC modified activated carbon as an air-cathode material (AC-PPC/NC) for MFCs. The gained PPC/NC might offer a polyhedron-embedded carbon networks hierarchical porous architec ture and express an advanced specific surface area. This 3D stacked structure adds catalytically active sites. When applied as MFCs cathode
2. Experiment methods 2.1. Preparation of ZIF-8/PPy PPy nanowire was firstly synthesized and its synthesis process was consulted in Fig. S1 (supporting information). 0.15 g PPy was dissolved in 500 mL methanol under underultrasound. Then 4.21 g Zn (NO3)2⋅6H2O was added and stirred for 1 h. Simultaneously, 8.40 g 2methylimidazole was dissolved in 500 mL methanol, and then added to the above solution and continued stirring for 1.5 h. After standing for 48 h, it was centrifuged and washed 5 times with methanol, and then dried at 70 � C for 10 h to collect ZIF-8/PPy. Besides, ZIF-8 was prepared by a similar process without the addition of PPy as shown in Fig. S2. 2.2. Preparation of PPC/NC Fig. 1a expressed the synthetic process of PPC/NC. First, Zn (NO3)2⋅6H2O was added to a methanol solution containing uniformly dispersed PPy nanowires. Free Zn2þ was adsorbed on the nanowires acting as a nucleation center due to electrostatic action [23]. ZIF-8 polyhedron was grown after the slow addition of 2-methylimidazole followed by standing for 48 h to form ZIF-8/PPy precursor. ZIF-8/PPy was kept at 800 � C for 2 h at a rate of 5 � C min 1 under a nitrogen atmosphere. The residual Zn was washed away with 1 mol L 1 of dilute hydrochloric acid. The obtained carbon material composed of polyhedron porous carbon embedded in nitrogen-doped carbon net works and was expressed as PPC/NC. For comparison, the same pro cedure was conducted to convert ZIF-8 and PPy to PPC and NC, respectively. 2.3. Air-cathode production All air cathodes were formed by rolling press method according to the existing literature [24]. Air cathodes included a catalyst layer, a stainless steel mesh, and a gas diffusion layer. The reproducibility of all data was gained with three parallel groups. The gas diffusion layer was generated by rolling carbon black and polytetrafluoroethylene (PTFE) with a mass ratio of 3:7 and then placed in the muffle furnace (340 � C) for 25 min. The catalyst layer was generated by rolling the activated carbon (AC) doped with different catalysts (NC, PPC, PPC/NC) and PTFE with a mass ratio of 6:1. The specific preparation method was as follows: 0.70 g activated carbon and 0.30 g catalyst and 5 mL ethanol were evenly dispersed under ultrasonication, then 0.167 g PTFE was added and stirred for 30 min to obtain the catalytic layer composed of the AC doped with catalyst. The samples with different catalysts were named as AC-NC, AC-PPC, AC-PPC/NC, respectively. Bare AC was used as a con trol. Each cathode contained a loading of AC-PPC/NC (or AC-NC, AC-PPC, bare AC) that was 0.143 g cm 2. Then all air cathodes were dried at 30 � C for 12 h. 2.4. MFCs fabrication Single-chamber MFCs had an inner volume of 28 mL plexiglass cy lindrical chamber [25], which had an electrode spacing of 4 cm and a diameter of 3 cm. Both electrodes had the projected 7 cm2 area. Tita nium wires were applied to connect the anode the cathode to transfer the electrons. Anodes were carbon felt which was immersed in acetone for 12 h and then washed thoroughly with distilled water. The chambers were filled in by domestic wastewater mixed with 12.5 mL L 1 trace minerals, 50 mmol L 1 phosphate buffered saline (PBS, 10.305 g L 1 Na2HPO4⋅12H2O, 0.13 g L 1 KCl, 0.31 g L 1 NH4Cl, 3.321 g L 1 NaH2 PO4⋅2H2O [26], pH ¼ 7) and 5 mL L 1 vitamins. After two weeks, the medium was changed to PBS and acetate solution (2 g L 1). The culture 2
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 1. (a) Schematic diagram of the synthetic path of PPC/NC; (b) TGA curves of PPy, ZIF-8, ZIF-8/PPy and PPC/NC; (c) XRD patterns of NC, PPC and PPC/NC.
temperature was hold at 30 � C and the voltage was recorded by the data acquisition card (MorpheusElectronic Co. Ltd, Beijing, China).
changing the external resistance in the range of 70–9000 Ω. Each resistor was maintained for 15 min to get a precise voltage by a potentiostat (VersaSTAT 3, Princeton Applied Research, USA). A three-electrode mode was adopted, where Ag/AgCl and platinum sheet (1 cm2) were utilized as reference electrode and counter electrode, respectively. Linear sweep voltammetry (LSV) was tested from open circuit potential (OCP) to 0.3 V at a scan rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) was tested from 100 kHz to 10 mHz at the open cir cuit potential. Exchange current density (io) was tested via scanning from the overpotential scope (η) of 0–0.1 V at a rate of 1 mV s 1 io was obtained by the Tafel equation, which was expressed as log i ¼ log io βnFη/2.303RT [24]. Cyclic Voltammetry (CV) curves were tested between 0.8 and 1.4 V (vs. Ag/AgCl) with the scanning rate of 50 mV s 1 in O2 and air saturated PBS, respectively [27]. Rotating disk electrode (RRDE-3 A, ALS, Japan) was adopted to test the ORR capability of the different catalysts. 5 mg catalyst was dispersed ultrasonically into 1 mL 0.05 wt% Nafion solution and 5 mL ethanol to form a catalyst ink. About 2 μL of catalyst ink was dropped on a glassy carbon electrode and dried. ORR was examined in the O2-saturated PBS by linear scanning from 0.3 to 1.0 V (vs. Ag/AgCl) at a scan rate of 5 mV s 1 with different rotation rates (625, 900, 1225, 1600, 2000, 2500 and 3000 rpm) [28].
2.5. Characterization and electrochemical analysis Scanning electron microscopy (SEM, S-3500 N, Hitachi) and field emission transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer (EDS) were utilized to research the structure and morphology of the sample. Brunauer - Emmett - Teller (BET) was performed to obtain the specific surface area and pore structure by an adsorption meter (ASAP, 2020/TRISTAR 3000, MICROMERITICS). Thermogravimetric analysis (TGA) was performed on a synchronous thermal analyzer (TGA/DSC1) at a heating rate of 5 � C min 1 under a nitrogen atmosphere. X-ray diffraction (XRD) was tested by a D/max-2500 X-ray generator with Cu Kα radiation to forecast the crystallinity of the catalyst with a diffraction 2-theta from 10 to 90� . Raman spectroscopy was performed on Microscopic confocal Raman spectrometer (SR-500I-A). X-ray photoelectron spectroscopy (XPS, KAepna, Thermo Fisher Scientific Inc, USA) was conducted to survey the content of the elements and their types of the catalysts on an XPS spectrometer (K-Aepna, Thermo Fisher Scientific, USA). Power density curves and polarization curves were gained by 3
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
3. Results and discussion
surface area and pore volume [40]. These features added the catalyti cally active sites and reduced the barriers in the diffusion of ions and electrons, resulting in a decrease in Rct and total resistance. Raman spectra of NC, PPC and PPC/NC were displayed in Fig. 3b. All materials displayed two distinct peaks at 1344 and 1590 cm 1, which were D-band and G-band, respectively. The D-band was generated owing to the breathing pattern of k-point phonons of A1g symmetry, which was correlative with disorder of sp3 carbon atom, yet the G-band was related to the conjugated structure of sp2 carbon regions [41]. IG/ID (intensity ratio) presented the graphitization level of carbon material [42]. The IG/ID values of NC, PPC and PPC/NC were 0.98, 0.99 and 1.01, respectively, demonstrating the embedding of NC into PPC upgraded the graphitization degree of the PPC/NC complex. A high graphitization degree reportedly suggested an advantageous electric conductivity [43]. Thence, PPC/NC might have a superior electrical conductivity compared to PPC. The XPS spectra of PPC/NC, PPC, and NC were expressed in Fig. 4a. Table 1 illustrated the contents of C, N and O in all samples. The N content in PPC/NC was 12.65%, which was far higher than CX1000@polypyrrole (3.55%) [44]. The C1s spectra of materials were resolved into two peaks at 284.6 (C–C) and 286.6 (C–O/C–N) in Fig. S11. The N1s spectra (Fig. 4(b–d)) of different materials were resolved into three peaks around 398.3 eV, 401.0 eV, 402.6 eV, which ascribed to pyridinic-N (N1), graphitic-N (N2) and valley-N (N3), respectively [45]. PPC/NC contained the most N1 and N2 content, accounting for almost 79.69%, as shown in Table 1 and Fig. 4(e and f). The graphitic N reportedly raised the limiting current density and pyridinic N height ened the onset potential of the ORR, yet pyrrole and oxidized N hardly advanced the electrocatalytically ability of nitrogen-doped material [46]. Therefore we might draw conclusion and confirm that the high content of pyridinic and graphitic N in the PPC/NC enhanced superior ability toward ORR.
3.1. Physicochemical characterization Fig. 1b compared the TGA results of the PPy, ZIF-8, ZIF-8/PPy and PPC/NC. PPy was thermally degraded in three steps. The first step was to evaporate water up to 100 � C, afterwards the counterions was degraded (100–310 � C) and last the polymer backbone was degraded (310–460 � C) [29,30]. ZIF-8/PPy was almost stable below 400 � C and begun to lose weight as the temperature sustained to rise. When the temperature was 800 � C, ZIF-8/PPy and ZIF-8 had evident weight loss and the tendency was close, which was consistent with the large ratio of ZIF-8 in ZIF-8/PPy complex. PPC/NC was stable at around 800 � C, so 800 � C was selected as the calcination temperature. At this time, ZIF-8 was changed into polyhedron porous carbon (PPC) and PPy was also completely degraded to NC, while ZIF-8/PPy complex was converted to PPC/NC. The XRD results were illustrated in Fig. 1c. All samples possessed two diffraction peaks near 26� (broad peak) and 45� (weak peak), which were indexed to the (002) and (100) crystal planes of carbon, respec tively. The broad (002) peak demonstrated that its internal structure was shapeless and disordered [31,32], which verified that PPy and ZIF-8/PPy had been transformed to NC and PPC/NC respectively. All peaks with high diffraction intensity did not conformed to the simulated ZIF-8 [33], indicating that ZIF-8 had been completely converted to carbon. The absence of diffraction peaks of Zn and other impurities also certificated that Zn impurities had been completely removed and a metal-free carbon material had been synthesized [34]. The SEM and TEM results were displayed in Fig. 2. PPC exhibited a polyhedron configuration with approximately 80 nm particles in Fig. 2a. TEM result indicated in Fig. 2b gave further sight into the polygon shape with obvious defects at the edges of the PPC. Fig. 2c indicated that the linear NC were entangled to form a honeycomb network structure, whose surface was covered with rough and slightly convex spots owing to the formation of nanocrystalline particles embedded therein. Fig. 2d presented its amorphous structure with a lot of edge defects [35] and illustrated the porous hollow structure. Fig. 2e and Fig. S3 illustrated the SEM analysis of PPC/NC. The PPC/NC obviously presented that poly hedral porous carbon derived from ZIF-8 had been embedded in the nitrogen-doped carbon network. This polyhedral-embedded carbon network hierarchical structure was exhibited clearly in the Fig. 2f and high-angle annular dark-field scanning TEM (HAADF-STEM, Fig. 2g). The intersecting carbon networks penetrated the inside of the poly hedron as bridges for electron transfer [36,37]. The PPC having a size of 500 nm was embedded in intertwined carbon network to compose a PPC-embedded hierarchical porous carbon networks structure. The combination of ZIF-8 and PPy promoted the growth of ZIF-8, which increased the average size of PPC. This special layered structure increased the surface area in contact with the electrolyte and also facilitated electron diffusion, which reduced resistance and improved ORR efficiency [38]. The element mappings (Fig. 2(h and i)) expressed the uniform distribution and existence of C, N, O in the PPC/NC. The energy-dispersive X-ray spectroscopy (EDS, Fig. S4 and Table S1, sup porting information) spectrum had illustrated atomic and mass ratio of the C, N, O. Fig. 3a displayed N2 adsorption-desorption isotherms. All samples were type IV adsorption isotherm and the hysteresis loop was exhibited at higher relative pressure (0.9–1.0), which expressed that they con tained mesopores and macropores [39]. As shown in Table S2, PPC/NC possessed a high specific surface area of 342.3 m2 g 1 with a pore vol ume of 0.43 cm3 g 1, which were bigger than those of PPC (225.1 m2 g 1 and 0.42 cm3 g 1) and NC (129.0 m2 g 1 and 0.12 cm3 g 1). Barrett-Joyner-Halenda (BJH) pore size distribution (Fig. S5) demon strated micropores structure of NC and hierarchical porous structure of PPC/NC and PPC with a small scope centered at 2 nm, a broad scope centered around 50 nm and some macropores, which raised its specific
3.2. Performance of MFCs The power density curves and the polarization curves were assessed after one month of constant temperature culture to form a mature anode biofilm and a stable potential. The power densities and voltages of all MFCs were displayed in Fig. 5a. The catalyst-modified AC air cathode MFCs all expressed higher power densities than that of the control (bare AC), which had a power density of 725 � 0.5 mW m 2. AC-PPC/NC possessed a maximum power density (MPD) of 2401 � 0.7 mW m 2, which was 3.3 times higher than control, and the power densities from AC-PPC and AC-NC were 1802 and 1601 mW m 2, respectively, which were identical as the order of open circuit voltage. The power densities of AC-NC and AC-PPC did not exceed the MPD of AC-PPC/NC with a polyhedron-embedded carbon networks hierarchical porous structure. Furthermore, the power density of AC-PPC/NC was 2 times higher than that of the Pt/C cathode (1209 � 15 mW m 2, Fig. S7), and the cost of AC-PPC/NC was much lower than Pt/C (Table S3). These results rep resented that the MFCs equipped with AC-PPC/NC can replace Pt/C to generate more electric power [47]. The respective polarization curves of cathode and anode were indi cated in Fig. 5b. It was clear that all the anode potentials had the same trend while the cathode potentials of the modified AC were noticeable better than the control, which demonstrated that the cathode was the principal part affecting the superior properties of MFCs and the modified AC expressed powerful ORR catalytic activity. In previous paper, the MPD of MFCs equipped with polypyrrole/ mercapto-2-sulfonate film cathode was 574.9 mW m 2 [48]. Nitrogen-doped toner derived from polypyrrole as a precursor outputed MPD of 934.7 � 5 mW m 2 [49]. The MPD of MOF-Cu3 (BTC)2 modified air-cathode MFC was 1772 � 15 mW m 2 [50]. The Metal organic framework-derived Co3O4/NiCo2O4 double-shelled nanocage exhibited advanced MPD of 1810 mW m 2 [51]. Yet all these catalysts showed much lower MPD than the PPC/NC catalyst (2401 � 0.7 mW m 2). 4
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 2. SEM images of (a) PPC, (c) NC and (e) PPC/NC. TEM images of (b) PPC, (d) NC and (f) PPC/NC. (g) HAADF-STEM image and (h–i) element mappings of the PPC/NC.
5
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 3. (a) N2 adsorption-desorption isotherms and (b) Raman spectrum of all samples.
Fig. 4. (a) XPS survey spectrum; the N1s spectra of PPC/NC (b), PPC (c) and NC (d), respectively; Elemental content (e) and the atomic percentages of all nitrogen functionalities (f) of all samples. 6
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Table 1 Elemental analysis of the PPC/NC, PPC and NC by XPS spectrum. Sample PPC/NC PPC NC
C (%)
O (%)
N (%)
C–C
C–O/C–N
Total C
Total O
Total N
Pyridinic-N
Graphitic-N
Valley-N
59.71 60.08 61.83
40.29 39.92 38.17
85.32 87.60 88.46
2.03 3.88 3.24
12.65 8.52 8.30
37.78 31.31 46.41
41.86 68.69 28.04
20.36 – 25.55
Fig. 5. (a) Polarization and power density curves; (b) Separate polarization curves of cathode and anode in operational process.
3.3. Electrochemical analysis
different materials in air-cathode. The equivalent circuit mode was demonstrated in Fig. S9, which contained an external resistance be tween the counter electrode and air-cathode surface (Ro), a diffusion resistance that was the electronic resistance of the current collector and contact resistances (Rd) and a charge transfer resistance (Rct). The Nyquist plot was demonstrated in Fig. 6b, and the calculated resistances were displayed in Table 2. Compared with the control, the air cathode resistances of all modi fied ACs were significantly reduced, which indicated an increase in conductivity and power output. The total AC-PPC/NC resistance (6.481 Ω) was much lower than commercial Pt/C electrodes (16.4 Ω) [53]. The Rct of all the air-cathodes demonstrated the order: AC-PPC/NC (1.104 Ω) < AC-PPC (3.577 Ω) < AC-NC (3.797 Ω) < bare AC (4.599 Ω). It expressed that the PPC/NC could reduce the Rct obviously. The lower Rct was advantageous to accelerate electronic transportation [26]. The diffusion resistances (Rd) of bare AC, AC-NC, AC-PPC, AC-PPC/NC were 5.237 Ω, 2.217 Ω, 0.667 Ω, 0.568 Ω, respectively. The Rd of bare AC was about 10 times larger than the AC-PPC/NC. The results were probably due to the superior pore structure of PPC/NC. It had been reported that large pore size and pore volume could greatly accelerate ion transport, resulting in a decrease in Rd [54]. In addition, the unique polyhedron-embedded carbon networks hierarchical porous structure
In order to ensure the optimal doping ratio of different catalysts in AC, we compared their LSVs performance, as shown in Fig. S8 (a-c). It was proved that at the end of the scanning potential, AC-PPC/NC, AC-PPC and AC-NC expressed the highest current density when doped into AC at 30% mass ratio, where the current density of AC-PPC/NC was 32.92 mA cm 2, which was 205% larger than the control (10.81 mA cm 2), representing that PPC/NC modification sped up the ORR significantly. LSVs of different air cathodes with doping ratio of 30% were exhibited in Fig. 6a. At the potential of 0.1 V, the current density of the air cathodes were arrayed in the order of AC-PPC/NC (12.45 mA cm 2) > AC-PPC (8.44 mA cm 2) > AC-NC (5.94 mA cm 2) > bare AC (4.64 mA cm 2). The current density of AC-PPC/NC was three times larger than the con trol, and was far superior to Pt/C (6.71 mA cm 2, Fig. S8d), representing that PPC/NC could efficiently substitute Pt as ORR catalyst. Besides, the onset potential of air-cathodes were in the sequence of AC-PPC/NC (0.310 V) > AC-PPC (0.287 V) > AC-NC (0.270 V) > Bare AC (0.194 V). High onset potential corresponded to a higher ORR catalytic activity, which indicated a beneficial modification of the obtained catalysts [52]. Electrochemical impedance spectroscopy (EIS) by mimicking the equivalent circuit was employed to analyze the different resistances of
Fig. 6. LSV curves (a) and Nyquist plots of EIS (b) of bare AC and different catalysts modified AC. Lines labeled ‘Cal’ were fitting data from the equivalent circuit. 7
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Table 2 Fitting data of different cathodes based on the equivalent circuit. R0 (Ω) Rd (Ω) Rct (Ω) Rt (Ω)
Table 3 Exchange current density (io) calculated from the Tafel plots.
bare AC
AC-NC
AC-PPC
AC-PPC/NC
The air-cathode
Linear fitting equation (R2)
10
8.274 5.237 4.599 18.110
4.352 2.217 3.797 10.366
4.500 0.667 3.577 8.744
4.809 0.568 1.104 6.481
bare AC NC PPC PPC/NC
y y y y
11.001 18.201 21.008 37.113
shortened the ion diffusion distance and played a considerable role in catalytic activity [55]. Therefore, PPC/NC would be a potential catalyst for higher power density in MFCs.
¼¼¼¼-
2.92548 þ 1.71485x 2.74090 þ 4.78394x 2.67762 þ 4.55569x 2.43304 þ 2.64225x
(0.998) (0.997) (0.997) (0.995)
4
io (A⋅cm
2
)
corresponding K-L plots of PPC/NC and bare AC were illustrated in Fig. 7 (c and d) and Fig. S10 (a-b), respectively. The electron transfer number (n) of PPC/NC and bare AC were calculated to be 3.95 and 0.83 at 0.6 V, respectively, demonstrating that PPC/NC catalyzed ORR via an effective four-electron path superior to Pt/C (n ¼ 3.6) [57]. Fig. 8a expressed the CV curves of all samples in O2 saturated PBS. The CV curves (taking the 60th cycles) of AC and PPC/NC displayed an ORR reduction peak at about 0.25 V (vs. Ag/AgCl), where the PPC/NC had the highest current density of 0.65 mA cm 2 superior to PPC (0.32 mA cm 2) and NC (0.30 mA cm 2), suggesting the PPC/NC had higher active ORR behavior [35]. The CV curves (Fig. S11a) of AC and PPC/NC in air saturated PBS were smooth and smaller peak emerged, which meant ORR occurred gently due to relative low oxygen concen tration. Galvanostatic charge/discharge curves still confirmed that the good stability of PPC/NC (Fig. 8b and Fig. S11b), meaning that PPC/NC possessed superior ORR catalytic activity and stability. Comprehensive physicochemical characterization and electro chemical analysis, compared with other metal-containing catalysts of various morphologies, such as nanoparticles [58], polyhedral nanocage [51], urchin-like spinel [59], irregular shape [60], ortho hexagon [61],
3.4. Catalytic kinetics of ORR The exchange current density (io) was obtained by the Tafel equation for research of catalytic kinetics. Fig. 7a compared the Tafel diagram of all air cathodes. The linear region (R2 > 0.99) in the scope of 60–80 mV was illustrated in Fig. 7b. The data for the linear fitting was shown in Table 3. The arrangement sequence of io was as follows: AC-PPC/NC (37.113 � 10 4 A cm 2) > AC-PPC (21.008 � 10 4 A cm 2) > AC-NC 4 2 (18.201 � 10 A cm ) > bare AC (11.001 � 10 4 A cm 2), which was equal to the sequence of EIS and LSV. A higher io represented less resistance and stronger ORR activity. The io of AC-PPC/NC was near 3.4 times bigger than bare AC, which demonstrated the increase the utili zation of electrons and finally devoted to enhance MPD output [3]. The RDE test characterized the ORR kinetic mechanism. The electron transfer number (n) of the bare AC and PPC/NC in the ORR was obtained by the Koutecky-Levic (K-L) equation [56]. The voltammograms and
Fig. 7. (a) Tafel plots of AC cathodes with different catalysts. The potential window ranges from open circuit potential to the overpotential of 100 mV. Scan rate: 1 mV s 1; (b) Linear fitting of from 60 mV to 80 mV; (c) Rotating-disk voltammograms of the PPC/NC in O2-saturated PBS electrolyte at different rotation rates and (d) Corresponding Kouteckye-Levich plot. 8
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
Fig. 8. (a) Cyclic voltammetric curves of different catalysts in O2-saturated PBS (50 mM); (b) Galvanostatic charge/discharge curves of PPC/NC at 50 mM PBS. Table 4 Comparison of electrochemical performance of electrocatalysts with different morphologies. Electrocatalyst
Morphology and structure
OCP (V)
The current density (mA cm 2)
PPC/NC PPC NC NiCo2S4/AC [57] Co3O4/NiCo2O4 DSNC [33] NiCo2O4 [58] N and P dual-doped carbon [59] Nano-Co3O4 [60] Fe3O4 [61]
Polyhedron-embedded- networks Polyhedron Carbon networks Nanoparticles Polyhedron nanocage Urchin-like spinel Irregular shape Ortho hexagon Nanosphere
0.310 0.287 0.270 0.245 0.252 0.236 0.343 0.245 0.270
12.45 8.44 5.94 7.13 7.86 8.34 6.94 8.90 6.87
0.1v
nanosphere [62], as shown in Table 4, PPC/NC complex with the special structure of polyhedron porous carbon-embedded carbon networks exhibited the best electrochemical capability. On the one hand, the carbon networks with the highly conductive hollow tubular structure acted as the central skeleton of the entire conductive system, which promoted electron diffusion and significantly reduced Rt. On the other hand, ZIF-8 derived polyhedral porous carbon structure offered poly hedral frameworks, high specific surface area and pore structure. PPC/NC derived from ZIF-8/PPy complex as precursor was referred as an ideal cathode catalyst with excellent structure superiority. Metal-free carbon composites with such 3D stacked structure enriched the catalytic active sites and possessed large content of nitrogen, which prompted the occurrence of ORR, thereby increasing the MPD of MFCs.
4
Rt (Ω)
10
io (A⋅cm
6.48 8.74 10.37 11.14 11.00 14.60 9.83 10.24 12.15
37.113 21.008 18.201 17.383 19.700 25.486 17.270 18.865 18.710
2
)
MPD (mW m 2)
0.3 V 32.92 24.43 20.59 16.50 16.92 19.01 23.64 18.00 17.23
2401 1802 1601 2000 1801 1730 1603 1500 1430
Acknowledgements This work was supported by the Natural Science Foundation of Tianjin (17JCYBJC23300) and National Key R&D Program of China (No. 2016YFC 0400704 and No. 2016YFC0401407). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227229. References [1] P. Pandey, et al., Recent advances in the use of different substrates in microbial fuel cells toward wastewater treatment and simultaneous energy recovery, Appl. Energy 168 (2016) 706–723. [2] S. Calabrese Barton, J. Gallaway, P. Atanassov, Enzymatic biofuel cells for implantable and microscale devices, Chem. Rev. 104 (10) (2004) 4867–4886. [3] N. Yang, et al., Complete nitrogen removal and electricity production in Thaueradominated air-cathode single chambered microbial fuel cell, Chem. Eng. J. 356 (2019) 506–515. [4] W. Xia, et al., Earth-abundant nanomaterials for oxygen reduction, Angew Chem. Int. Ed. Engl. 55 (8) (2016) 2650–2676. [5] J. Qi, et al., Surface dealloyed PtCo nanoparticles supported on carbon nanotube: facile synthesis and promising application for anion exchange membrane direct crude glycerol fuel cell, Green Chem. 15 (5) (2013) 1133. [6] Z. Wang, C. Cao, Y. Zheng, et al., Abiotic oxygen reduction reaction catalysts used in microbial fuel cells, ChemElectroChem 1 (11) (2014) 1813–1821. [7] E. Antolini, Composite materials for polymer electrolyte membrane microbial fuel cells, Biosens. Bioelectron. 69 (2015) 54–70. [8] C. Santoro, et al., Double-chamber microbial fuel cell with a non-platinum-group metal Fe-N-C cathode catalyst, ChemSusChem 8 (5) (2015) 828–834. [9] X. Chen, et al., Multi-component nanoporous platinum–ruthenium–copper–osmium–iridium alloy with enhanced electrocatalytic activity towards methanol oxidation and oxygen reduction, J. Power Sources 273 (2015) 324–332. [10] X. Zhang, D. Pant, F. Zhang, et al., Long-term performance of chemically and physically modified activated carbons in air cathodes of microbial fuel cells, ChemElectroChem 1 (11) (2014) 1859–1866. [11] C.R. Raj, et al., Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. 4 (29) (2016) 11156–11178.
4. Conclusion Herein, a special carbon hybridization complex (PPC/NC) was ob tained through carbonization of ZIF-8/PPy precursor. After the carbonization, The ZIF-8 derived polyhedral porous carbon and poly pyrrole (PPy) derived carbon networks were crosslinked tightly and the polyhedral-embedded carbon networks were well prepared. The mi crobial fuel cells equipped with AC-PPC/NC presented a maximum power density of 2401 � 0.7 mW m 2, which was superior to commer cial Pt/C. Furthermore, AC-PPC/NC presented a more excellent ex change current density (37.113 � 10 4 A cm 2) and a better open circuit potential (0.310 V) compared with the control. Furthermore, PPC/NC presented a four-electron transfer pathway (n ¼ 3.95) toward the ORR. The improvement of the ORR was mainly due to the ZIF-8-derived polyhedral embedded PPy-derived carbon networks hierarchical struc ture and the present of a large amount of graphite nitrogen and pyr idinium nitrogen, as well as the advanced specific surface area. The PPC/NC with 3D stacked structure could be expected to replace Pt as an extremely prospective electrode material in the practical application of MFCs.
9
R. Yang et al.
Journal of Power Sources 442 (2019) 227229
[12] L. Zhao, et al., Sustainable nitrogen-doped carbonaceous materials from biomass derivatives, Carbon 48 (13) (2010) 3778–3787. [13] Z. Chen, D. Higgins, Z. Chen, Nitrogen doped carbon nanotubes and their impact on the oxygen reduction reaction in fuel cells, Carbon 48 (11) (2010) 3057–3065. [14] H. Li, H. Liu, Z. Jong, et al., Nitrogen-doped carbon nanotubes with high activity for oxygen reduction in alkaline media, Int. J. Hydrogen Energy 36 (3) (2011) 2258–2265. [15] A. Zhao, J. Masa, M. Muhler, et al., N-doped carbon synthesized from N-containing polymers as metal-free catalysts for the oxygen reduction under alkaline conditions, Electrochim. Acta 98 (2013) 139–145. [16] B. Liu, W. Dai, Z.X. Liang, et al., Fe/N/C carbon nanotubes with high nitrogen content as effective non-precious catalyst for oxygen reduction reaction in alkaline medium, Int. J. Hydrogen Energy 42 (9) (2017) 5908–5915. [17] Q. Liu, Z. Pu, C. Tang, et al., N-doped carbon nanotubes from functional tubular polypyrrole: a highly efficient electrocatalyst for oxygen reduction reaction, Electrochem. Commun. 36 (2013) 57–61. [18] Q.L. Zhu, J. Li, Q. Xu, Immobilizing metal nanoparticles to metal–organic frameworks with size and location control for optimizing catalytic performance, J. Am. Chem. Soc. 135 (28) (2013) 10210–10213. [19] Y. Huang, et al., Bimetallic alloy nanocrystals encapsulated in ZIF-8 for synergistic catalysis of ethylene oxidative degradation, Chem. Commun. 50 (70) (2014) 10115–10117. [20] Y. Liu, et al., Metal-organic framework-derived porous carbon polyhedra for highly efficient capacitive deionization, Chem. Commun. 51 (60) (2015) 12020–12023. [21] J. Tang, R.R. Salunkhe, J. Liu, et al., Thermal conversion of core–shell metal–organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon, J. Am. Chem. Soc. 137 (4) (2015) 1572–1580. [22] Y. He, Y. Bai, X. Yang, et al., Holey graphene/polypyrrole nanoparticle hybrid aerogels with three-dimensional hierarchical porous structure for high performance supercapacitor, J. Power Sources 317 (2016) 10–18. [23] V. Chandra, K.S. Kim, Highly selective adsorption of Hg2þ by a polypyrrolereduced graphene oxide composite, Chem. Commun. 47 (13) (2011) 3942–3944. [24] H. Dong, et al., A novel structure of scalable air-cathode without Nafion and Pt by rolling activated carbon and PTFE as catalyst layer in microbial fuel cells, Water Res. 46 (17) (2012) 5777–5787. [25] H. Liu, B.E. Logan, Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane, Environ. Sci. Technol. 38 (14) (2004) 4040–4046. [26] L. Pu, et al., Silver electrodeposition on the activated carbon air cathode for performance improvement in microbial fuel cells, J. Power Sources 268 (2014) 476–481. [27] H. Xia, et al., 2D MOF nanoflake-assembled spherical microstructures for enhanced supercapacitor and electrocatalysis performances, Nano-Micro Lett. 9 (4) (2017) 43. [28] F.W.T. Goh, et al., Ag nanoparticle-modified MnO2 nanorods catalyst for use as an air electrode in zinc–air battery, Electrochim. Acta 114 (2013) 598–604. [29] P. Wang, et al., A novel electroactive PPy/HKUST-1 composite film-coated electrode for the selective recovery of lithium ions with low concentrations in aqueous solutions, Electrochim. Acta 306 (2019) 35–44. [30] G. Nystrom, et al., A nanocellulose polypyrrole composite based on microfibrillated cellulose from wood, J. Phys. Chem. B 114 (12) (2010) 4178–4182. [31] Z.-Y. Sui, et al., A highly nitrogen-doped porous graphene – an anode material for lithium ion batteries, J. Mater. Chem. 3 (35) (2015) 18229–18237. [32] R. Zheng, et al., Heteroatom-doped carbon nanorods with improved electrocatalytic activity toward oxygen reduction in an acidic medium, Carbon 69 (2014) 132–141. [33] M. Jiang, et al., Hierarchically porous N-doped carbon derived from ZIF-8 nanocomposites for electrochemical applications, Electrochim. Acta 196 (2016) 699–707. [34] S. Gadipelli, Z.X. Guo, Tuning of ZIF-derived carbon with high activity, nitrogen functionality, and yield - a case for superior CO2 capture, ChemSusChem 8 (12) (2015) 2123–2132. [35] Z.Y. Sui, X. Li, Z.Y. Sun, et al., Nitrogen-doped and nanostructured carbons with high surface area for enhanced oxygen reduction reaction, Carbon 126 (2018) 111–118. [36] Y. Liu, et al., Supercapacitor with high cycling stability through electrochemical deposition of metal-organic frameworks/polypyrrole positive electrode, Dalton Trans. 47 (38) (2018) 13472–13478. [37] X. Xu, et al., Three-Dimensional networked metal-organic frameworks with conductive polypyrrole tubes for flexible supercapacitors, ACS Appl. Mater. Interfaces 9 (44) (2017) 38737–38744.
[38] X. Xu, et al., Rational design and fabrication of graphene/carbon nanotubes hybrid sponge for high-performance capacitive deionization, J. Mater. Chem. 3 (25) (2015) 13418–13425. [39] H. Li, et al., The capacitive deionization behaviour of a carbon nanotube and reduced graphene oxide composite, J. Mater. Chem. 1 (21) (2013) 6335. [40] Y. Yang, et al., In situ synthesis of zeolitic imidazolate frameworks/carbon nanotube composites with enhanced CO2 adsorption, Dalton Trans. 43 (19) (2014) 7028–7036. [41] W. Chaikittisilp, et al., Nanoporous carbons through direct carbonization of a zeolitic imidazolate framework for supercapacitor electrodes, Chem. Commun. 48 (58) (2012) 7259–7261. [42] L.-F. Chen, et al., Flexible all-solid-state high-power supercapacitor fabricated with nitrogen-doped carbon nanofiber electrode material derived from bacterial cellulose, Energy Environ. Sci. 6 (11) (2013) 3331. [43] X. Wen, et al., Three-dimensional graphene-based hierarchically porous carbon composites prepared by a dual-template strategy for capacitive deionization, J. Mater. Chem. 1 (39) (2013) 12334. [44] M. Sevilla, et al., Polypyrrole-derived mesoporous nitrogen-doped carbons with intrinsic catalytic activity in the oxygen reduction reaction, RSC Adv. 3 (25) (2013) 9904. [45] J. Zheng, et al., High content of pyridinic- and pyrrolic-nitrogen-modified carbon nanotubes derived from blood biomass for the electrocatalysis of oxygen reduction reaction in alkaline medium, Electrochim. Acta 168 (2015) 386–393. [46] J. Yin, et al., Enhancement of electrocatalytic activity for oxygen reduction reaction in alkaline and acid media from electrospun nitrogen-doped carbon nanofibers by surface modification, RSC Adv. 3 (36) (2013) 15655. [47] F. Zhang, et al., Power generation using an activated carbon and metal mesh cathode in a microbial fuel cell, Electrochem. Commun. 11 (11) (2009) 2177–2179. [48] X. Zhang, et al., Inorganic/organic mesostructure directed synthesis of wire/ ribbon-like polypyrrole nanostructures, Chem. Commun. (16) (2004) 1852–1853. [49] X. Yuan, et al., Use of polypyrrole in catalysts for low temperature fuel cells, Energy Environ. Sci. 6 (4) (2013) 1105. [50] P. Tian, et al., Porous metal-organic framework Cu3(BTC)2 as catalyst used in aircathode for high performance of microbial fuel cell, Bioresour. Technol. 244 (Pt 1) (2017) 206–212. [51] S. Zhang, et al., Metal organic framework-derived Co3O4/NiCo2O4 double-shelled nanocage modified activated carbon air-cathode for improving power generation in microbial fuel cell, J. Power Sources 396 (2018) 355–362. [52] W. Yang, et al., Low-cost Fe–N–C catalyst derived from Fe (III)-chitosan hydrogel to enhance power production in microbial fuel cells, Chem. Eng. J. 380 (2020) 122522. [53] Q. Wen, et al., MnO2–graphene hybrid as an alternative cathodic catalyst to platinum in microbial fuel cells, J. Power Sources 216 (2012) 187–191. [54] H. Jiang, J. Ma, C. Li, Hierarchical porous NiCo2O4 nanowires for high-rate supercapacitors, Chem. Commun. 48 (37) (2012) 4465–4467. [55] Z. Jiang, et al., LDH nanocages synthesized with MOF templates and their high performance as supercapacitors, Nanoscale 5 (23) (2013) 11770–11775. [56] M.H.M.T. Assumpç~ ao, et al., A comparative study of the electrogeneration of hydrogen peroxide using Vulcan and Printex carbon supports, Carbon 49 (8) (2011) 2842–2851. [57] C.O. Ania, et al., New copper/GO based material as an efficient oxygen reduction catalyst in an alkaline medium: the role of unique Cu/rGO architecture, Appl. Catal. B Environ. 163 (2015) 424–435. [58] L. Pu, et al., Carbon-supported binary transition metal chalcogenide used as cathode catalyst for oxygen reduction in microbial fuel cell, Int. J. Hydrogen Energy 42 (20) (2017) 14253–14263. [59] B. Ge, et al., The performance of nano urchin-like NiCo2O4 modified activated carbon as air cathode for microbial fuel cell, J. Power Sources 303 (2016) 325–332. [60] B. Liang, et al., Nitrogen and phosphorus dual-doped carbon derived from chitosan: an excellent cathode catalyst in microbial fuel cell, Chem. Eng. J. 358 (2019) 1002–1011. [61] B. Ge, et al., The addition of ortho-hexagon nano spinel Co3O4 to improve the performance of activated carbon air cathode microbial fuel cell, Bioresour. Technol. 195 (2015) 180–187. [62] Z. Fu, et al., The performance and mechanism of modified activated carbon air cathode by non-stoichiometric nano Fe3O4 in the microbial fuel cell, Biosens. Bioelectron. 74 (2015) 989–995.
10