Alfalfa leaf-derived porous heteroatom-doped carbon materials as

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9766-9773

pubs.acs.org/journal/ascecg

Alfalfa Leaf-Derived Porous Heteroatom-Doped Carbon Materials as Efficient Cathodic Catalysts in Microbial Fuel Cells Lifang Deng,†,§,⊥ Yong Yuan,*,‡ Yuyuan Zhang,∥ Yazhuo Wang,§,⊥ Yong Chen,§,⊥ Haoran Yuan,*,§,⊥,∥ and Ying Chen† †

School of Materials and Energy, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, China ‡ School of Environmental Science and Engineering, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, China § Guangzhou Institute of Energy Conversion, Key Laboratory of Renewable Energy, Chinese Academy of Sciences, No. 2 Nengyuan Road, Guangzhou 510640, China ∥ College of Materials Science and Energy Engineering, Foshan University, No. 18 Jiangwan Yi road, Foshan 528000, China ⊥ Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, No. 2 Nengyuan Road, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Heteroatom-doped lamellar-structured carbon with a high surface area synthesized from alfalfa leaves is utilized as a cathode catalyst in this study to improve the power output of microbial fuel cells (MFCs). Different chemical activation agents are used to treat alfalfa leaf-derived carbon (ALC). It is found that chemical activation agents substantially affect the catalytic activities of the alfalfa leaf-derived carbon materials in the power output of MFCs and the oxygen reduction reaction (ORR). ALC materials activated by KOH (ALC-K) exhibit the best electrochemical activity compared with those of materials activated by FeCl3 (ALC-Fe) or ZnCl2 (ALC-Zn). A high limiting current density and excellent long-term stability can be seen with ALC-K as the cathode catalyst, which gives superior results to those of Pt/C. Moreover, a maximum power density of approximately 1328.9 mW/m2 is obtained from an MFC equipped with an ALC-K cathode, offering performance characteristics comparable to those of a Pt/C cathode as well. This work demonstrates a new method for the production of inexpensive nanostructured carbon materials derived from natural resources that exhibit high performance in MFCs. KEYWORDS: Alfalfa leaf-derived carbon, Oxygen reduction reaction, Chemical activation, Microbial fuel cell

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INTRODUCTION Microbial fuel cells (MFCs) have been used as clean alternative energy sources that can simultaneously achieve organic and inorganic biodegradation and electricity generation.1,2 In MFCs, oxygen is the most commonly used terminal electron acceptor and is suitable for MFC applications because of its abundant availability in the environment and its ability to produce a high voltage.3,4 However, as the efficiency and performance of fuel cells are severely affected by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR), extra catalysts are required.5 Precious metals such as platinum are commonly used as ORR catalysts because of their excellent ORR catalytic activity, but their high cost and scarcity hinder their large-scale application in MFCs. Therefore, great effort has been put forth to explore efficient and low-cost candidates for the cathode ORR catalyst of MFCs.6,7 The use of nanocarbon-based materials (e.g., carbon nanotubes, nanofibers, graphene, ordered mesoporous graphitic © 2017 American Chemical Society

arrays and nanosheet carbons) doped with heteroatoms (e.g., N, S or P) has been considered an efficient way to prepare metal-free electrocatalysts for ORR because of their good ORR electrocatalytic activities, long-term operational stabilities, excellent electrical conductivities, high tolerances to fuel poisoning, and relatively low costs.8−10 However, the fabrication and modification of such nanocarbons usually require toxic chemicals, intricate reaction processes, and specialized precursors and equipment. Natural biomass is an inexpensive and suitable carbon raw material that is available in high qualities and quantities for the synthesis of valuable carbon materials and may serve as a single precursor for both carbon and heteroatoms during the synthesis processes, thus eliminating the need for multiple hazardous chemicals. Received: May 19, 2017 Revised: September 6, 2017 Published: September 14, 2017 9766

DOI: 10.1021/acssuschemeng.7b01585 ACS Sustainable Chem. Eng. 2017, 5, 9766−9773

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic procedure for the preparation of alfalfa carbon catalysts. SEM and TEM images of ALC (a and e), ALC-K (b and f), ALC-Fe (c and g) and ALC-Zn (d and h).

performance in MFCs after fabricating the as-prepared materials on the surface of the cathodes.

Heteroatom-doped carbon catalysts synthesized directly from natural biomass at relatively low cost but in sufficient quantities have been demonstrated to be efficient ORR catalysts with potential applications as cathodes in MFCs. For example, Zhou et al. demonstrated that carbon nanoparticle-coated, massderived porous biocarbon materials could be synthesized by a combination of hydrothermal treatment and pyrolysis carbonization, resulting in high ORR activity and offering better performance characteristics than those of Pt/C cathodes in MFCs.11 Yuan et al. reported that chitin-derived porous structured carbon sheets with high nitrogen content could serve as effective metal-free electrocatalysts for ORR with excellent activity, modified kinetics and admirable stability in MFCs.12 Yang et al. reported that bamboo charcoal obtained by carbonizing bamboo branches exhibited high performance for the ORR, and the maximum power density from an MFC with a bamboo charcoal cathode was similar to that of Pt/C in MFCs.13 Nevertheless, their ORR activity is still not as good as that of Pt/C catalysts, especially without activation, thereby making them less competitive for use in MFCs. Alfalfa is a plant in the genus Medicago, which can be found all over the world, for example, in China, Asia Minor, Iran, Transcaucasia and Eurasia, and is usually grown for fodder and as a forage grass.14 Alfalfa is capable of producing large quantities of biomass by absorbing large amounts of nutrients and may provide the majority of the nitrogen fertilizer requirements for maize production for two years after the alfalfa stand is plowed down.15,16 As reported by Bourquin et al., alfalfa leaves are high in crude protein and low in cell wall density, with relatively high N, S and P contents, which suggested the possibility of converting this material into metalfree carbon catalysts for ORR.17,18 Herein, we converted alfalfa leaves into carbon and compared the electrocatalytic activities of alfalfa-leaf carbon after activation by different agents (ZnCl2, KOH and FeCl3). The intrinsic properties of these materials were analyzed in terms of structure, morphology and surface composition. Then, we measured their electrochemical

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EXPERIMENTAL SECTION

Preparation and Characterization of Alfalfa Leaf-Derived Activated Carbon. Alfalfa was obtained from a ranch in northwest China. First, alfalfa leaves were semicarbonized at 250 °C for approximately 2 h under a N2 atmosphere in a furnace; then, the semicarbonized alfalfa was physically mixed with KOH, FeCl3 or ZnCl2 with an activating agent/carbon ratio of 3/1 (g/g), respectively, followed by carbonization at 900 °C in a furnace for another 2 h in a N2 atmosphere. For comparison, the alfalfa leaves were also directly carbonized without any activation at 900 °C. The materials obtained were labeled ALC, ALC-K, ALC-Fe and ALC-Zn. After cooling, the materials were immersed in a 3 mol/L HCl solution and stirred magnetically for approximately 24 h before being extensively washed with water, filtered and dried at 95 °C. Finally, they were milled until the particle size was between 100 and 150 mesh. Structure Characterization. The Brunauer−Emmett−Teller (BET) method was used to measure the specific surface area of the alfalfa leaf-derived catalysts. A sorptometer (model 1800, Carlo Erba Instruments, Italy) was used to measure the nitrogen (N2) adsorption isotherm at 77 K. Field-emission scanning electron microscopy (FESEM, model S-4800) and high-resolution transmission electron microscopy (HRTEM, JEM-2100F) were used to reveal the morphology of the alfalfa leaf-derived catalysts. A LabRAM HR800 Aramis confocal microscope Raman spectrometer system (HORIBA JY, France) was used to record the Raman spectra. An X-ray photoelectron spectrometer (Thermo Fisher Scientific ESCALAB 250 spectrometer) was used to perform X-ray photoelectron spectroscopy (XPS). Configuration and Operation of MFC. As previously described, single-chamber MFCs composed of poly(tetrafluoroethylene) (with an inner volume of 28 mL) were constructed.19 The anodes were graphite fiber brushes, and the cathodes were carbon cloth with a waterproof catalytic layer coating on one side. All MFCs were started up with Pt/ C as the cathode catalyst and were inoculated with the solution from another MFC that had been run for more than 2 months. Then, the Pt/C cathode was replaced by the as-prepared metal-free carbon cathodes. 9767


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ACS Sustainable Chemistry & Engineering The MFCs were fed with 1 g/L sodium acetate solution medium in 50 mM phosphate buffer solution (PBS). The PBS (1 L) contained 4.58 g of Na2HPO4, 2.45 g of NaH2PO4·H2O, 0.31 g of NH4Cl, 0.13 g of KCl, 10 mL of mineral solution and 10 mL of vitamin solution. The anode and cathode were connected by a 1000 Ω external resistance throughout the experiment except when measuring the polarization and power density curves. The MFCs were placed in a 30 °C incubator. When stable voltage outputs were achieved, the external resistance was varied from 10 000 Ω to 50 Ω to obtain the power density curves, and the individual electrode potentials were measured versus a saturated calomel electrode (SCE). All tests were performed three times, and the mean values are presented. Electrochemical Measurement. A computer-controlled CHI660D electrochemical workstation with a conventional threeelectrode cell was used to conduct the electrochemical measurements. The measurement was conducted in a 0.1 mol/L phosphate buffer solution (PBS, pH = 7.0) at room temperature, with a saturated calomel electrode (SCE) as the reference, a platinum (Pt) wire electrode as the counter electrode and a glassy carbon (GC) electrode (5.0 mm) loaded with as-prepared catalysts as the working electrode. An aerobic (or anaerobic) environment was created by bubbling O2 (or N2) through the PBS solution for 30 min before each measurement. The GC was polished with 0.3 and 0.05 mm alumina slurries in turn and then washed with water and ethanol successively. The alfalfa leaf-derived catalysts or Pt/C catalysts (1.0 mg) were mixed with Nafion solution (5% Nafion, 10 μL) and ethanol (1.0 mL) to make catalyst inks. Then, 20 μL of the as-prepared ink was loaded on the GC electrodes to create the working electrodes. The scope of the ORR measurements was −0.6 to +0.6 V with a scan rate of 10 mV/s, and during the cyclic voltammetry (CV) experiments, a flow of O2 was maintained throughout the electrolyte solution to achieve continuous O2 saturation. The linear sweep voltammogram (LSV) of the modified GC electrode was conducted at rotation speeds from 500 to 2000 rpm in 0.1 mol/L PBS electrolyte. Koutecky−Levich (K-L) plots were used to investigate the LSV curves at various electrode potentials.

Dehydration of the carbon precursor occurs easily, which leads to charring and aromatization because ZnCl2 is intercalated into the carbon matrix via impregnation. As the activation temperature increases to values higher than 1003 K (the boiling point of ZnCl2), carbon atoms interact with the Zn species, resulting in widened interlayer spacing of the carbon and creating pores in the carbon matrix at the same time.21 With similar characteristics, FeCl3 may act analogously to ZnCl2.22 The morphologies of ALC, ALC-K, ALC-Fe and ALCZn were also characterized by TEM. As shown in Figure 1e,f,g,h, clear microporous structures and irregular particles are observed in all the active carbon materials, and the pores in ALC-K seem more pronounced than those in other samples. Figure 1f confirms that ALC-K shows nanoscale pores with multilayer characteristics, implying that this sample can present a high specific surface area. In addition, the high-resolution TEM image (HRTEM) in Figure 1 also shows the highly curved graphitic layers in ALC-K. The bending carbon structure may help oxygen molecules gain easier access to the electrocatalytic sites, which is favorable for the ORR process.23 In general, chemical activation is capable of transforming nonporous, low-surface-area carbon materials into porous materials with high surface areas. The previous results demonstrated that different activation agents were capable of creating activated carbon with high specific surface areas.20−22 As detected by the N2 adsorption−desorption isotherm method (Figure 2a), a steep increase can be seen at low relative

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RESULTS AND DISCUSSION Figure 1 illustrates the synthesis strategy of the as-prepared catalysts derived from alfalfa leaves. Two heat treatment steps were included. In a typical synthesis, alfalfa leaves were semicarbonized at 250 °C for 2 h in a furnace, and the resulting semicarbonized materials were then annealed at 900 °C in the presence of chemical agents. The resulting carbon materials from the alfalfa leaves were washed and collected for further study. The surface composition and morphological characteristics of the as-prepared catalysts were observed by SEM and TEM. As shown by the SEM images in Figure 1, the ALC had a flat structure with a smooth surface, implying a low specific surface area. However, lamellar structures appeared after chemical activation by KOH (ALC-K), FeCl3 (ALC-Fe) or ZnCl2 (ALC-Zn), which might contribute to an increase in the surface area of the as-prepared materials. −OK functional groups were formed in the presence of KOH and the carbon precursor, inducing the cross-linking oxidation of carbon atoms in the adjacent lamellas with the help of potassium and oxygen bonds. A slightly wrinkled or folded structure was formed when the lamellas of the crystallite in normal form were disturbed. Then, the potassium salts present in the carbon particles were undocked by washing with water. Meanwhile, interlayer voids were created because the formed lamellas could not return to their original state.20 Therefore, porous carbon with high surface area was produced because the lamellas remained separated. However, ZnCl2 not only generates a porous material but also enhances the carbon content in carbon materials by eliminating hydrogen and oxygen atoms as water.

Figure 2. (a) N2 adsorption−desorption isotherms of the as-prepared catalysts; (b) pore size distribution of the as-prepared catalysts; (c) Raman spectra of the as-prepared materials; (d) quantitative EDS element mapping of C, O, N and P in ALC-K.

pressures for the isotherm of ALC, which suggests dominant microporous characteristics. However, the adsorption branch resembles that of a type-I isotherm in the IUPAC classification. After chemical activation, the weak hysteresis loop in ALC-Zn suggests a combined I/IV-type isotherm and indicates that these materials are primarily mesoporous. ALC-K and ALC-Fe show typical type-IV adsorption−desorption isotherms with large hysteresis loops, suggesting the presence of mesoporous slit-shaped pores.22 The specific surface areas and the porosity of ALC, ALC-K, ALC-Fe and ALC-Zn are shown in Table 1. ALC-K exhibited the highest surface area (approximately 883.67 m2/g), followed by those of ALC-Fe (773.32 m2/g) 9768


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ACS Sustainable Chemistry & Engineering Table 1. Oxygen Reduction Reaction Activities of the As-Prepared Catalysts in 0.1 mol/L PBS Catalysts

Specific surface area (m2/g)

ORR peak potential (V vs SCE)

Onset potential (V vs SCE)

Half-wave potential (V vs SCE)

Current density at −0.6 V (mA/cm2)

Pt/C ALC ALC-K ALC-Fe ALC-Zn

148.13 883.68 773.32 517.87

0.10 −0.05 0.08 0.08 0.07

0.22 0.15 0.21 0.19 0.18

−0.04 −0.18 −0.03 −0.12 −0.17

6.00 3.14 5.80 5.05 3.05

and ALC-Zn (517.87 m2/g), which were much higher than that of ALC (148.13 m2/g). More surface catalytic site exposure to oxygen molecules should be provided by the relatively higher surface area of ALC-K, and superior ORR activity is thus expected. Furthermore, the pore size distributions of ALC, ALC-K, ALC-Fe and ALC-Zn are shown in Figure 2b, showing that all the carbon materials had similar pore sizes. Raman spectroscopy is capable of providing accurate information about the extent of defects, ordered structures, and graphitic nature of carbon materials.24 Figure 2c depicts the Raman spectra of ALC, ALC-K, ALC-Fe and ALC-Zn. Two obvious peaks at ∼1588 and ∼1335 cm−1 are visible, corresponding to the G and D bands, respectively. The D band is indicative of disordered graphite structure, corresponding to sp3 defect sites, while the G band shows the existence of crystalline graphitic carbon due to the sp2-hybridized carbon layer.25 The D and G band ratio (ID/IG) is usually used to identify the extent of disorder within the carbon materials.26 As shown in Figure 2, the value of the ID/IG ratios for ALC-K is approximately 1.12, which is significantly higher than that of ALC (1.02), ALC-Fe (1.06) and ALC-Zn (1.02). The higher ID/IG value after KOH activation is likely a result of the presence of more surface defects in the ALC-K structure, possibly introduced by heteroatom doping.27 Moreover, the ID/ IG values of the as-prepared materials were lower than those of recently reported carbonized natural biomass materials,28,29 indicating a higher degree of graphite and a lower amount of disorder in the structure of the as-prepared materials. XPS is a surface analysis technique that can offer valuable information about the nature of functional groups. Herein, XPS was also used to further reveal the chemical structures of alfalfa leaf-derived carbon catalysts. The surveys of ALC-K, ALC-Fe and ALC-Zn are shown in Figure 3a, showing the presence of C, N, and O and trace P, S and Fe. The energy-dispersive X-ray (EDX) maps presented in Figure 2d verify the distribution of these elements on the resulting ALC-K. As shown by the XPS data in Table S1, the following atomic percentages were obtained for ALC-K: C (79.41%), O (13.52%), N (2.41%), P (0.34%), S (0.39%) and Fe (0.57%), respectively. The detailed structural properties of these elements were revealed by highresolution XPS (Figure 3, S1 and S3). As shown in Figure S1, the dominant peaks at 284.1 and 284.6 eV can be attributed to the CC and CH bond, respectively, whereas the peak at 285.2 eV can be attributed to CC.30−32 In the O 1s spectra, three peaks were observed and were identified as OC bonds (531.1−531.8 eV), OH bonds (532.1−532.8 eV) and OC bonds (533.1−533.5 eV).33,34 The N 1s spectra in Figure 3 shows that four types of N functionalities, including pyridinic (∼398.2 eV), pyrrolic (∼399.5 eV), graphitic (∼400.8 eV) and oxidized (∼402.2 eV) nitrogen, were doped on the alfalfa leafderived carbon materials.35,36 As the chemical activation process may have profound effects on the transformation of the N groups, the N groups in the alfalfa leaf-derived carbon

Figure 3. (a) Full-scan XPS of ALC-K, ALC-Fe and ALC-Zn; (b, c and d) the XPS N 1s spectra of ALC-K, ALC-Fe and ALC-Zn.

materials prepared with different activation agents were detected (Figure S2). The total N (2.41%), pyridinic-N (0.47%) and graphitic-N (0.83%) percentages in ALC-K were higher than those in ALC-Fe (2.11) and ALC-Zn (2.00), but the oxidized-N percentage in ALC-Zn was much higher than that in ALC-K, implying that more pyridinic-N and graphitic-N were synthesized during KOH activation. According to Table S1, the total P and S percentages were similar in ALC-K, ALCFe and ALC-Zn, indicating that the chemical activation step has limited influence on the S and P functionalities. However, the Fe percentage in ALC-Fe increased slightly due to residual ferric salt. In addition, typical P 2p, S 2p and Fe 2p XPS spectra of ALC-K are presented in Figure S3. The peaks at 132.5, 133.5 and 134.9 eV in the high-resolution spectra of P 2p are assigned to PC and PO groups. These P functional groups reportedly contribute to the high ORR electrocatalytic activity of carbon catalysts.36,37 The peaks located at 163.7, 164.9, 167.7 and 169.0 eV in the high-resolution spectra of S 2p are attributed to CS, conjugated −CS− and oxidized sulfur bonds, respectively. Because the CS bonds lie predominantly at the edges, they might have a strong influence on the catalytic process for ORR.38,39 As shown in Figure S3g,h, there is trace N-coordinated Fe observed in the Fe 2p spectra of ALC-K and ALC-Fe.40 To investigate the ORR activity of the as-prepared carbon materials, CV scans of the catalysts in 0.1 M PBS were performed. As shown in Figure S4, the CV of the alfalfa leafderived carbon materials exhibited a typical pseudocapacitive behavior under a N2-saturated electrolyte. In contrast, all the materials showed well-defined cathodic peaks when the electrolyte solutions were bubbled with O2, which was regarded 9769


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specific surface area and high electrical conductivity are highly desirable properties for an ideal ORR catalyst because they can offer efficient pathways for mass transfer and electron conduction.45,46 Meanwhile, higher graphitic and pyridinic nitrogen probably contributed to the higher catalytic activity of metal-free carbon catalysts toward the ORR because graphiticN and pyridinic-N species are believed to greatly increase the limiting current density and convert the ORR reaction mechanism from a 2e−-dominated process to a 4e−-dominated process, respectively.11 The reason is that doping with electronegative graphitic-N reduces the electron density on the adjacent C atoms, which favors the adsorption of O2.47,48 Either highly positive spin density or highly positive atomic charge density at the adjacent carbon atom can be induced by pyridinic-N in N-doped carbon catalysts, which severely weakens the OO bond via the bonding between the oxygen and carbon atom and favors the ORR via a 4e−-dominated process.49 The resulting ALC-K had higher surface area and higher pyridinic- and graphitic-N content than those of ALC-Fe and ALC-Zn, which guaranteed higher ORR catalytic activity of ALC-K compared with those of ALC-Fe and ALC-Zn (Figure S6). In addition, the ID/IG value of ALC-K was also higher than those of ALC-Fe and ALC-Zn, indicating higher amounts of surface/edge defects in ALC-K (Figure S6). The presence of numerous surface/edge defects has been considered to create more catalytic sites on carbon materials for the ORR.50,51 In addition, the length and angle changes of the CP bond may have a positive effect on the defect-induced active sites, which are also responsible for O2 adsorption. Similar to N doping, the asymmetric spin density in adjacent carbon atoms may also be enhanced by the available electron pair in phosphorus, which could eventually promote ORR activity.52 Note that the KOH activation of alfalfa leaf-derived carbon resulted in higher levels of pyridinic-N, graphitic-N and CP functionalities than those resulting from FeCl3 (ALC-Fe) or ZnCl2 (ALC-Zn) activation. These results demonstrate that the catalytic activities could be affected severely by the chemical activation agents. Porous carbon materials with large surface area, an abundance of N functionalities, and high CP and CS functionalities might furnish the possibility of acquiring very high-performance catalytic activity for ORR. The KOH activation increased the surface area, active N-species content, PC content and porosity in ALC-K, which provided many more active sites for oxygen adsorption and reduction. The resulting carbon materials were evaluated as catalysts to replace Pt/C in air-cathode single-chamber MFCs. As shown in Figure 5a, the maximum power density from the MFC with an ALC-K cathode was 1328.9 mW/m2 in the first week, which was comparable to that from the MFC with a Pt/C catalyst (1337.7 mW/m2) and higher than that produced by the ALCFe (1260.1 mW/m2) and ALC-Zn (1099.3 mW/m2) cathodes. The individual cathode and anode curves are shown in Figure 5b, verifying that the differences in power densities mainly result from the use of different cathodes. There were large variations in the cathode potentials for different MFCs, whereas the anode potentials were similar among all MFCs. Notably, MFCs are usually operated with a near-neutral electrolyte that is favorable for bacterial growth. Therefore, it is desirable to develop metal-free carbon catalysts that can work well under neutral conditions. The ORR is known to be initiated by adsorption and subsequent superoxide formation via single electron transfer.35,53 This initial electron transfer is very facile in alkaline conditions, whereas it is very difficult in neutral

as representing the electrocatalytic reduction of O2 at the electrodes. The CV of ALC-K demonstrates a clearly enhanced ORR current compared with that of ALC. Moreover, ALC-K exhibited a peak potential at 0.07 V vs SCE, which was more positive and had higher peak intensities than those of ALC-Fe and ALC-Zn but was only mildly less active than that of the Pt/ C (approximately 0.10 V vs SCE) (Figure S4). To further explore the electrocatalytic performance of the alfalfa leaf-derived carbon materials, rotating disk electrodes (RDEs) were used while scanning the scope from −0.6 to +0.6 V (vs SCE) in O2-saturated 0.1 M PBS solutions. As shown in Figure 4a, ALC-K, ALC-Fe, ALC-Zn and ALC samples were

Figure 4. (a) LSV curves of the as-prepared catalysts in O2-saturated 0.1 mol/L PBS at 1600 rpm; (b) RDE curves of ALC-K at different rotation rates; (c) calculated ORR electron transfer numbers for ALC, ALC-K, ALC-Fe, ALC-Zn and Pt/C at different potentials; (d) K-L plots at −0.2 V for ALC, ALC-K, ALC-Fe, ALC-Zn and Pt/C.

found to have onset potentials of 0.21, 0.19, 0.18 and 0.15 V vs SCE. The onset potential of ALC-K was comparable to that of Pt/C (0.22 V vs SCE, only 10 mV less active). Interestingly, ALC-K showed a half-wave potential of −0.03 V (vs SCE), which was slightly more positive than that of Pt/C (−0.04 V vs SCE). According to Figure 5b and S5, the limiting current density of the as-prepared materials increases as the rotation speed increases, implying that the diffusion distance is inversely associated with the rotation speed.14,41 The corresponding Koutecky−Levich (K-L) plots at different electrode potentials showed good linearity with parallelism, which was considered an indication of first-order reaction kinetics for the ORR.42 On the basis of the K-L equations, the kinetic parameters (i.e., electron transfer number (n) and kinetic current density (jk)) were analyzed.43 The electron transfer number calculated using the K-L equations was approximately 3.85−3.93 for ALC-K, demonstrating that a four-electron transfer pathway occurred during the ORR process. As shown in Figure 4c,d, the jk and n values at −0.2 V were 11.36, 5.02 and 2.24 mA/cm2 and 3.86, 3.56 and 2.94 for ALC-K, ALC-Fe and ALC-Zn, respectively. The higher jk and n of ALC-K further confirmed its superior ORR activity. It is well-known that the adsorption and reduction of O2 in the ORR process are interfacial reactions.44 Therefore, a large 9770


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from poisoning and deactivation under most conditions, which allows them to offer better long-term operation stability than that of commercially available Pt/C. Moreover, the O species of the metal-free carbon catalysts can benefit their stability because the active centers for ORR can be protected by O H groups, which potentially improves the proton-tolerance ability during long-term operation.63 The slight loss of ORR activity for the metal-free carbon cathode might be due to the clogging of micropores by biofilms or salt precipitates in the MFCs.61,63 These results demonstrated that the as-prepared carbon-material cathodes were more durable than the Pt/C cathode over time, confirming that low-cost carbon-material catalysts produced via a simple preparation technique can exhibit excellent performance in power generation and stability for microbial energy harvesting. Meanwhile, KOH chemical activation can be considered a useful pretreatment for the preparation of carbon-material catalysts from alfalfa leaves.

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Figure 5. (a) Power densities and (b) electrode potentials of MFCs with different cathodes as a function of current density after the first week; (c) the maximum power densities of ALC, ALC-K, ALC-Fe, ALC-Zn and Pt/C after the 1st, 6th and 12th weeks of operation.

CONCLUSIONS In summary, N-, P- and S-doped lamellar-structured carbon derived from alfalfa leaves could be synthesized via a facile pyrolysis carbonization method associated with chemical activation. The resulting carbon material had a high BET surface area and excellent electrochemical activity for ORR in neutral solutions. The high activity of the alfalfa leaf-derived carbon may be due to its large surface area, its lamellar structures, and the presence of abundant heteroatoms. The ALC-K obtained by using KOH as the chemical agent showed the highest surface area, greatest enrichment of active N and P species, and highest porosity among all activated carbon materials, providing the highest number of active sites for oxygen adsorption and reduction. Specifically, the MFC with an ALC-K cathode displayed a maximum power density of 1328.9 mW/m2, which is comparable to that of Pt/C (1337.7 mW/ m2). This study offers a new method for the synthesis of biomass-derived electrocatalysts as MFC cathodes with remarkable electrochemical activities and durability.

conditions. As a result, catalysts with large specific surface areas and good electronic properties are highly desirable to overcome the spin restrictions of oxygen molecules and successfully facilitate superoxide formation in neutral conditions.35 In this regard, ALC-K is able to adsorb oxygen molecules and overcome the spin restrictions of oxygen molecules to efficiently facilitate the initial inner-sphere electron transfer in neutral electrolytes, which is also likely attributable to its high surface area, N content and surface/edge defects. The longevity of such catalysts was confirmed by a long-term stability test, and approximately 90 full discharge−charge cycles were conducted in the MFCs over 12 weeks. As shown in Figure 5c, after operating for 6 weeks, the maximum power densities decreased by approximately 8.3%, 19.5% and 26.7% in the MFCs with ALC-K, ALC-Fe and ALC-Zn cathodes, respectively. However, the MFC with the Pt/C cathode decreased dramatically by approximately 51.0% (∼654.9 mW/ m2), which is consistent with the findings of Xia et al.42 After operating for 12 weeks, the maximum power densities of the MFCs with Pt/C, ALC-K, ALC-Fe and ALC-Zn cathodes further decreased by 29.4%, 2.5%, 2.7% and 3.1%, respectively (Figure 5c). The maximum output voltage of ALC-K decreased to 89.8% of the original maximum (from 0.59 to 0.53 V), which was higher than those of the MFCs with Pt/C (72.5%), ALCFe (85.5%) and ALC-Zn (85.2%) cathodes (Figure S7). The inevitably formed cathodic biofilms are considered to significantly impact the performance of the cathodes by clogging the micropores and inhibiting mass transport in the MFCs.54,55 No significant difference in the structure and thickness were observed to be caused by the cathodic biofilms on either Pt/C or ALC-K cathodes (Figure S8), demonstrating that ALC-K did not inhibit the growth of cathodic biofilm. Therefore, biofilm development might not have been the primary reason for the deterioration of the Pt/C cathodes, which was consistent with previous findings.56,57 Numerous previous studies have also reported that metal-free carbon cathodes are much stable than Pt/C cathodes during long-term operation in MFCs.42,54 The significant loss of ORR activity for the Pt/C cathode might be due to the susceptibility of the platinum to poisoning and deactivation, as well as salt precipitation,58−62 whereas metal-free carbon catalysts are free

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01585. Chemical and physical properties of the as-prepared carbon materials, figures related to XPS of the carbon materials, CV and LSV curves of the resulting catalysts, long-term stability tests of the MFCs, and electrode potentials vs current densities of MFCs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Yuan). *E-mail: [email protected] (H. Yuan). ORCID

Haoran Yuan: 0000-0003-1757-4695 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (51406207), the 9771


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Guangdong Natural Science Funds for Distinguished Young Scholars (2014A030306033), the National science and technology support (2015BAL04B02), the Guangzhou university-industry collaborative innovation major projects (2016201604030077), Cooperation Project of Industry-University-Research in Guangdong Province (2015B090904009), and Youth Innovation Promotion Association CAS (2014320).

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Alfalfa leaf-derived porous heteroatom-doped carbon materials as

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