C Catalysts for Enhancing Oxygen Reduction via

Research Article www.acsami.org

Bifunctional Ag/Fe/N/C Catalysts for Enhancing Oxygen Reduction via Cathodic Biofilm Inhibition in Microbial Fuel Cells Ying Dai,†,‡ Yingzi Chan,†,§ Baojiang Jiang,† Lei Wang,† Jinlong Zou,*,†,§ Kai Pan,† and Honggang Fu*,† †

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China ‡ School of Civil Engineering, Heilongjiang Institute of Technology, Harbin 150050, China § Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, College of Heilongjiang Province, Heilongjiang University, Harbin 150080, China S Supporting Information *

ABSTRACT: Limitation of the oxygen reduction reaction (ORR) in single-chamber microbial fuel cells (SC-MFCs) is considered an important hurdle in achieving their practical application. The cathodic catalysts faced with a liquid phase are easily primed with the electrolyte, which provides more surface area for bacterial overgrowth, resulting in the difficulty in transporting protons to active sites. Ag/Fe/N/C composites prepared from Ag and Fe-chelated melamine are used as antibacterial ORR catalysts for SC-MFCs. The structure− activity correlations for Ag/Fe/N/C are investigated by tuning the carbonization temperature (600−900 °C) to clarify how the active-constituents of Ag/Fe and N-species influence the antibacterial and ORR activities. A maximum power density of 1791 mW m−2 is obtained by Ag/Fe/N/C (630 °C), which is far higher than that of Pt/C (1192 mW m−2), only having a decline of 16.14% after 90 days of running. The Fe-bonded N and the cooperation of pyridinic N and pyrrolic N in Ag/Fe/N/C contribute equally to the highly catalytic activity toward ORR. The ·OH or O2− species originating from the catalysis of O2 can suppress the biofilm growth on Ag/Fe/N/C cathodes. The synergistic effects between the Ag/Fe heterojunction and N-species substantially contribute to the high power output and Coulombic efficiency of Ag/Fe/N/C catalysts. These new antibacterial ORR catalysts show promise for application in MFCs. KEYWORDS: biofouling, microbial fuel cells, nitrogen doping, oxygen reduction reaction, stability

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INTRODUCTION Microbial fuel cells (MFCs) can convert chemical energy to electricity using biochemical pathways during the wastewater treatment process.1−4 In MFCs, electricity-generating microbes are able to catalyze the oxidation of fuels (organic matter in wastewater) in anodes to provide the electrons and protons for the reduction of the electron acceptor in cathodes.5−7 Oxygen (O2) is usually used as the terminal electron acceptor for MFCs (especially for single chamber MFCs, i.e., SC-MFCs) because of its high reduction potential (1.229 V) and ready availability.8−11 The efficiency of the oxygen reduction reaction (ORR, via the two-electron (2e−) or four-electron (4e−) pathway) in an air cathode (electrode surface) plays a critical role in both energy recovery and power output.8 However, because of the high energy barrier of activation of ORR, an efficient catalyst is needed to reduce the overpotential to achieve the ORR via the 4e− pathway in MFCs.10,11 Pt-based materials are usually used as the ORR catalysts, therefore hindering the practical application of MFCs.12,13 Alternative ORR catalysts, including activated carbon (AC), carbon black, © 2016 American Chemical Society

graphitic carbon (GC), nitrogen (N)-doped carbon nanotubes and graphenes, transition metals/GC, metal oxides/GC, and metal carbides/GC, have been explored.11,13−17 Currently, Fe or Co species/GC composites have shown excellent performance for ORR and can be considered as one of the most likely alternative catalysts to replace Pt/C.17−21 The GC supported Fe or Co species have excellent electron donor capacity through weakening the O−O bonds and providing convenient pathways for electron transport, which therefore enhance the ORR activity and power density.17,21 Graphene oxide (GO)-based Co/N/rGO(NH3) cathode catalysts using polyethylenimine and Co(NO3)2·6H2O as precursors have been successfully prepared by heating first in argon and then in ammonia.22 The ORR pathways for Co/N/rGO(NH3) are dominated by the 4e− reduction of O2 to H2O, having higher stability and performance than that of the commercial Pt/C Received: November 27, 2015 Accepted: March 3, 2016 Published: March 3, 2016 6992

DOI: 10.1021/acsami.5b11561 ACS Appl. Mater. Interfaces 2016, 8, 6992−7002

Research Article

ACS Applied Materials & Interfaces (20%) catalyst.22 Fe/N/C composites can be obtained by pyrolyzing a mixture of melamine foam, carbon black (ketjenblack) and ferrous chloride (FeCl2) at 800 °C under argon atmosphere, which exhibit higher power density and stability than that of Pt/C as a cathode in MFCs.23 N-enriched Fe/Fe3C@C nanorods (core−shell structure) have been prepared by pyrolyzing a mixture of cyanamid and FeCl3 at 750 °C, which generate higher ORR activity than that of Pt/C in double-chamber MFCs.24 Melamine has been used as both carbon and nitrogen sources for preparing nitrogen-doped Fe/ Fe3C/partly graphitized carbon (Fe/Fe3C/NPGC) catalysts at 640 and 650 °C.25 The highest Coulombic efficiency (30%) and power density (1323 mW m−2) of SC-MFCs are obtained by the Fe/Fe3C/NPGC (650 °C) cathode because of its efficiently active components (Fe3C and Fe−Nx species) and low charge transfer resistance (Rct), which have advantages in “capturing” and “consuming” electrons for ORR.25 For these excellent catalysts, what factors will limit the power output of MFCs from being further improved? This may be a problem faced by most of the catalysts. In a previous study, we found that biofilms over the air cathodes with Fe/Fe3C/NPGC (640 and 650 °C) and Fe/NPGC (660 and 700 °C) are inevitably formed, originating from the anode heterotrophic bacteria in SC-MFCs.25 The overgrowth of microorganisms on the cathodes will suppress the conductivity, electron transportation and entire ORR performance of the catalysts.18,25 The coated biomass (weight) on the cathodes is not only associated with the ORR activity (stability) of catalysts, but also with the power output (electricity) of SC-MFCs.18,25 The lower the cathodic biomass of the attached biofilms, the higher the generated electricity output.18 Silver nanoparticles (AgNPs) based materials with reduction capacity, which are toxic to most of the natural and environmental bacterial communities, are widely used as the antibacterial agents.18,26,27 Recently, AgNPs/ graphite plates (12 mA) and AgNPs/AC (1080 mW m−2) have been utilized as the antibacterial ORR electrocatalysts in MFCs; no toxic effects on anodic microorganisms have been found by inhibiting the cathodic biofilm overgrowth.18,26,27 In our previous study, Ag/Fe3O4/GC as ORR catalyst has been prepared from the carbonization of waste pomelo skin at 1000 °C.18 The MFCs performance (1712 mW m−2) of Ag/Fe3O4/ GC with 1 h of acid washing is the best, attributing to the proper ratio of Ag/Fe and the exposure of abundant oxygencontaining functional groups in the skeleton.18 Therefore, it can be deduced that if the Ag/Fe-species/C is assisted by the structurally bonded N (Fe-bonded N, pyridinic N, and pyrrolic N), the electrical conductivity and catalytic activity may be further improved by the synergistic effects between O-groups and N-species. Moreover, the required carbonization temperature may be significantly lowered. In this study, we intend to prepare the N-doped Ag/Fe/C (Ag/Fe/N/C) catalysts at relatively low temperatures (620− 900 °C) by using Ag and Fe-chelated melamine as the carbon and nitrogen sources, aiming to improve the catalytic activity for ORR and lower the biofouling, overpotentials, and internal resistance of SC-MFCs. The Ag/Fe/N/C composites termed as bifunctional catalysts are expected to provide both antibacterial and ORR activities. The cointroduction of Ag, Fe-species and N-species can promote the catalytic status and distribution of active sites, which may facilitate the desirable 4e− ORR process by strengthening the proton (H+) and OH− transport abilities. The long-time running performance of Ag/Fe/N/C as the cathode catalysts in SC-MFCs is investigated to clarify the

relationship between active-constituents and electrocatalytic properties. The critical mechanisms in determining the power output are also expected to be further clarified to better understand the relationship between antibacterial reaction and ORR.

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EXPERIMENTAL SECTION

Synthesis of Ag/Fe/N/C Composites. An in situ, simultaneous doping/reduction method was used to prepare the Ag/Fe/N/C composites.25 First, 2 g of melamine, 0.2 g of Fe3+ (Fe(NO3)3), and 30 mL of deionized water were mixed and continuously stirred at 100 °C for 3 h. After complete dissolution, 0.1 g of Ag+ (AgNO3) was added to the mixture, which was vigorously stirred at 100 °C to completely remove the water. In the preparation procedure, the melamine-Fe3+ chelated complex was first obtained and then Ag+ was coordinated (or trapped) into the structure to form a complex with C/Fe3+/Ag+ of 1:0.1:0.05 (mass ratio). During the evaporation (polymerization), a homogeneous paste similar to the polymer gel was gradually formed. In a tubular furnace, the resulting mixture was carbonized at 620, 630, 640, 650, 660, 700, 800, and 900 °C (holding at the final temperature for 4 h), with a heating rate of 3 °C min−1, under highly pure N2 flow (50−60 mL min−1), followed by naturally cooling to room temperature (25 °C) under N2 flow. The obtained samples were ground into powder, which were marked as Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900). The samples were conserved in N2-filled polyethylene tubes for future use. Electrode Preparation and Operation Condition for MFCs. A graphite fiber brush, which was properly pretreated according to the previously reported methods,17,25 was used as the anode of SC-MFCs. The gas diffusion layer (GDL) of the cathode was prepared by rolling carbon black and PTFE (60 wt %) with a mass ratio of 7:3 onto a stainless steel mesh and then sintering at 340 °C for 30 min to form a hydrophobic and breathable fibrosis layer.17 The catalysts mixed with PTFE (mass ratio of 2:1) were uniformly rolled onto the opposite side of the GDL and then dried at 80 °C in an oven.25 Performances of the cathodes made with Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) were compared with the commercial Pt/C catalyst (10 wt %) in SC-MFCs. The SC-MFCs used in this study were the same as those in the previous study.17,25 All of the reactors were inoculated with the effluent from the well-run SC-MFCs. The simulated wastewater (electrolyte) consisted of glucose (1 g L−1) and phosphate buffered solution (PBS, pH = 7.4).25 All of the SC-MFCs were placed in a large glass box, with a constant temperature of 30 °C. In each cycle, the electrolytes in SC-MFCs were replaced when the voltage output was less than 50 mV (approximately). To achieve statistical soundness, at least three reactors were parallel operated. Material Characterization and Electrochemical Analysis. Xray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses were conducted for the catalysts according to the methods reported in our previous studies.18,25 The ion concentration (Ag+) was measured using a PerkinElmer Optima 5300DV Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, Waltham, MA). Electrochemical tests were conducted on an electrochemical workstation (BAS100B, Germany) with a typical three-electrode testing system. The cathodes with catalysts were tested as the working electrode (approximately 7 cm2), equipped with a 1 cm2 platinum plate counter electrode and an Ag/AgCl reference electrode (+0.195 V vs standard hydrogen electrode, 3.0 M KCl). Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) tests were performed in 50 mM PBS solution in the potential range from −0.3 to +0.4 V (1 mV s−1) and −0.8 to +0.3 V (50 mV s−1), respectively. A glassy carbon electrode with a diameter of 0.4 cm (0.126 cm2) was used for CV tests. A rotating disk (glassy carbon) electrode (diameter of 0.4 cm and area of 0.126 cm2) was used to detect the pathway of ORR (RDE tests). Then, 5 mg of catalysts was added to the mixture of 100 μL of ethanol and 50 μL of Nafion (5 wt % solution) under ultrasound conditions (30 min). Next, 5 μL of catalyst ink was dropped onto the electrodes and dried in the air. RDE 6993


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of intermediate oxygen binding species on Ag (111).29 It is no doubt that the release of Ag+ from the crystalline core of AgNPs may result in the function loss of these catalysts. It is expected that AgNPs are tightly embedded in the carbon structure, which may substantially improve the stability of AgNPs under the exposure of oxygen. The peaks at 44.5° and 64.6° correspond to the typical diffraction peaks of α-Fe with a body-centered cubic structure,30 which means that the α-Fe in Ag/Fe/N/C exists in the form of metallic nanoparticles. For Ag/Fe/N/C900, a new diffraction peak is detected at 35.7°, corresponding to the (311) plane of Fe3O4 with the face-centered cubic structure (JCPDS, No. 89-6466).17,31,32 However, whether the metallic nanoparticles (Ag and α-Fe) exert a specific stability remains an elusive question. The following study will help both to advance ORR applications of Ag/Fe and to clarify their antimicrobial behavior in SC-MFCs. The N2 adsorption−desorption isotherms and pore size distribution curves for Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) are shown in (Figure S1). It can be seen that all of the samples show typical type-IV isotherms with a sharp capillary condensation step and H3 type hysteresis loops. The pore size distributions for these Ag/Fe/N/C samples, which mainly consisted of macropores, are very wide (Figure S1). Ag/Fe/N/C-660 has the highest SBET (32.46 m2 g−1), which is slightly higher than those of the others (Table S1). The porous structures of these six samples are slightly different from each other, leading to a slight difference in their SBET and pore volume. This implies that the differences in the exposure of catalytic active sites and the transport of the ORRrelevant species (such as O2, e−, H2O, H+, and OH−) are minimal for Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900).33 As shown in Figure 2, the surface structure of Ag/Fe/N/C-x (x = 620−800) is similar to each other. The surface pores are derived from the decomposition of polymers (such as C3N4) originating from the thermal polymerization of melamine before 700 °C. As shown in Figure 2h, the short rods with diameters between approximately 50 and 200 nm are randomly stacked with each other. The morphology evolution of Ag/Fe/ N/C gradually occurs as temperature increases and is correspondingly accompanied by the increase of the crystallization of active components (Ag and Fe) and the graphitization of the C skeleton.17,18,25 As shown in the XPS survey spectra (Figure 3a), the dominant (narrow) C 1s peaks are located at approximately 284.6 eV, along with Ag 3d peaks at approximately 370 eV, N 1s peaks at approximately 398 eV, O 1s peaks at approximately 530 eV and Fe 2p peaks at approximately 710 eV for the Ag/ Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) catalysts, suggesting that Ag, Fe, and N are successfully incorporated into the structure of Ag/Fe/N/C catalysts. The weight percentages (wt %) of the atoms (Fe, O, C, Ag and N) in Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) are shown in Table S2. The corresponding XPS peaks of C 1s and O 1s are shown in Figure S2, and the detailed analyses are provided in Supporting Information. In Figure 3b, the peaks observed at approximately 368.2 and 374.2 eV are ascribed to Ag 3d5/2 and Ag 3d3/2, respectively, with 6.0 eV splitting between these two bands that is ascribed to the metallic Ag phase,34,35 indicating the successful incorporation of AgNPs into the skeleton of Ag/Fe/N/C catalysts. The binding energy of Ag 3d5/2 shifts slightly to higher binding energy when comparing with the standard

tests were carried out in an oxygen-saturated PBS (50 mM) under continuous aeration conditions.18,28 The rotation rates were adjusted from 225 to 2025 rpm by using a modulated speed rotator (BAS Inc., Japan). The average number of transferred electrons (n) was calculated using the Koutecky−Levich (K−L) equation.18,28 Electrochemical impedance spectroscopy (EIS) was conducted after 90 days of operation at the open circuit voltage (VOC) over a frequency range from 105 to 10−2 Hz, with a sinusoidal perturbation of 10 mV amplitude. ZSimpWin 3.10 software (Echem, Lufkin, TX) was used to fit the EIS data to calculate the Rct. The voltage output of MFCs (30 °C), with an external circuit resistance of 1000 Ω, was recorded by using a data acquisition system (PISO-813, ICP-DAS, Taiwan). The measurements of polarization, power density, Coulombic efficiency (CE) and chemical oxygen demand (COD) of effluents were conducted according to the methods in our previous studies.17,25

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RESULTS AND DISCUSSION Composition and Structural Characterization of Ag/ Fe/N/C Composites. XRD patterns of the Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) are shown in Figure 1. As shown in Figure 1a, the obvious peaks at 2θ of 38.1, 44.3, 64.4, and 77.3° are attributed to the (111), (200), (220), and (311) planes of face-centered cubic crystals of AgNPs, respectively.29 It is reported that oxygen adsorption is energetically favored on the surface and subsurface (facecentered cubic) sites of Ag(111) because of the easy formation

Figure 1. (a) XRD patterns of Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) and (b) the local-zoom patterns of Ag/Fe/N/ C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900). 6994


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Figure 2. SEM images of Ag/Fe/N/C-x, x = 620 (a), 630 (b), 640 (c), 650 (d), 660 (e), 700 (f), 800 (g), and 900 (h).

2p3/2 ferric state (approximately 711.4 eV), the Fe 2p1/2 Fe−N (approximately 720.2 eV), and the Fe 2p1/2 ferric state (approximately 724.3 eV).25 It is reported that N can be coordinated with Fe during heating, enlightening the formation of Fe−N species, which can act as the efficiently active sites for ORR, in the Ag/Fe/N/C composites.37,39,42,43 Furthermore, the presence of ferric state species also verifies the formation of Fe3+-Nx (Fe−N bonds),37,43 consistent with the N 1s analyses. Note that as temperature increases, the percentage contents of Fe−N bonds correspondingly decrease (Table 1), suggesting that the contribution of the Fe−Nx active-sites (active centers) may decline. These results demonstrate that the encapsulation of Fe-species into the carbon structure should be helpful in accelerating the electron transfer rate,40,41 because they are believed to play crucial roles in promoting ORR catalytic performance. ORR Activity Comparison among Different Catalysts. LSV scan tests for Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800 ,and 900) and Pt/C are conducted to analyze the catalytic activity of ORR. As shown in Figure 4a, Ag/Fe/N/C630 has higher current density than other cathodes, suggesting that efficient N doping and the proper crystallization of metallic components are beneficial for ORR on the three-phase boundary.37 Ag/Fe/N/C-x (x = 630, 640, 650, and 660) cathodes have higher ORR catalytic activity than that of Pt/C. The synergistic effects between Ag/Fe and N-species, which play an important role in improving the catalytic performance for ORR, are favorable for attracting oxygen onto the active sites in the N-doped GC skeleton of Ag/Fe/N/C-x (x = 630, 640, 650, and 660). The crystallinity of α-Fe in Ag/Fe/N/C620 is the lowest, leading to the low content of Ag/Fe conjunction (active sites) in the structure. Therefore, Ag/Fe/ N/C-620 possesses weak synergistic effects between Ag/Fe and N-species, which correspondingly results in the lowest activity in ORR. The relatively low activity of Ag/Fe/N/C-x (x = 700, 800, and 900) may be attributed to the volatilization of Nspecies in them, which results in the loss of active sites and functional groups.25 CV curves for Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) and Pt/C are shown in Figure 4b. The Pt/C cathode has higher current density (−4.71 mA cm−2) than the Pt-free cathodes. This result is not consistent with the LSV trends. However, note that the CV tests are performed on a glassy carbon electrode, while the LSV tests are conducted with

binding energy of pure metallic Ag (the peaks of Ag 3d5/2 and Ag 3d3/2 are located at approximately 368 and 374.2 eV, respectively),34,35 thereby leading to the decrease of electron density around Ag and the weakening of the shielding effect. This binding energy shift of Ag is mainly attributed to the electron transfer from metallic Ag to Fe crystals,35 which indirectly verifies the existence of an Ag/Fe heterojunction. As shown in Figure 3c, the shapes of the N 1s spectra of Ag/ Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) are similar to each other. The N species in Ag/Fe/N/C-630 (selected as the representative sample) can be decomposed into three components originating from Fe-bonded N (398.3 eV), pyridinic N (399.4 eV) and pyrrolic N (400.9 eV; Figure 3d). It is consistent with the result that the N peak at 398.3 eV may correspond to the presence of the Fe-bonded N.36 The percentage contents of different types of N in the total N are provided in Table 1. As shown in Table 1, the total N in Ag/ Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) decreases from 24.05 to 2.85% as heating temperature increases. From comparisons of the ratio of N species, the percentage contents of Fe-bonded N is the maximum in all of the samples, being generally considered as the efficiently active components for ORR in N-doped carbon materials.37,38 Furthermore, the pyridinic N and pyrrolic N, which were previously identified to be the active sites, may also play important roles for ORR.37,38 Most importantly, it is worth emphasizing that the pyridinic species are known to stabilize the singlet dioxygen by forming a stable adduct between molecular oxygen and pyridinic N.39 The formation of such a stable adduct has an oxygen chemisorption geometry for adsorbing molecular oxygen.39 The pyridinic N has an extra electron lone pair, which influences the conjugation of the N lone pair electrons on the N and hexagonal C p-system, and thus creates active sites for ORR.40 Therefore, the presence of N species (Fe-bonded N, pyridinic N, and pyrrolic N) in Ag/ Fe/N/C may energetically contribute to the highly catalytic activity toward ORR. Asymmetric bands of lower energy (Fe 2p3/2) and higher energy (Fe 2p1/2), which originate from the spin orbital splitting of Fe 2p, can be observed in Figure 3e.41 As shown in Figure 3f and Figure S3, Fe 2p peaks for Ag/Fe/N/C-x (x = 620, 630 and 640) can be decomposed into five components originating from metallic iron (α-Fe, approximately 706.9 eV), the Fe 2p3/2 Fe−N bond (approximately 709.9 eV), the Fe 6995


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Figure 3. (a) XPS spectra of (b) Ag 3d and (c) N 1s binding energy regions for Ag/Fe/N/C, and high-resolution spectra of N 1s (d) for Ag/Fe/N/ C-630; (e) Fe 2p binding energy regions for Ag/Fe/N/C and (f) high-resolution XPS of Fe 2p spectra for Ag/Fe/N/C-630. As shown in (f): (1) Fe 2p3/2 → Fe0 (α-Fe), (2) Fe 2p3/2 → Fe−N bond, (3) Fe 2p3/2 → Fe3+, (4) Fe 2p1/2 → Fe−N bond, and (5) Fe 2p1/2 → Fe3+.

a real cathode. The different testing conditions may result in the different performances of Pt/C. Moreover, by using the rolling method, the aggregation of Pt/C (mixed with PTFE) is more serious than that of others. The Ag/Fe/N/C-630 cathode has the highest current density (−3.34 mA cm−2) among the eight Ag/Fe/N/C cathodes, suggesting that it may have better electrocatalytic activity than others. The performance (activity) of Ag/Fe/N/C-900 is the worst. Generally, the graphitization degree of Ag/Fe/N/C-900 should be higher than that of others, which may be favorable to conductivity and electron transport efficiency. However, the low number of active sites and functional groups may substantially inhibit its activity.25 Furthermore, as reported in the previous study, the cointroduction (or conjunction) of Fe3O4 and AgNPs to partly

Table 1. Percentage Contents (wt %) of Various Chemical States of N in Fe-Species/NPGC-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) samples

Fe-bonded N

pyridinic N

pyrrolic N

total N

Ag/Fe/N/C-620 Ag/Fe/N/C-630 Ag/Fe/N/C-640 Ag/Fe/N/C-650 Ag/Fe/N/C-660 Ag/Fe/N/C-700 Ag/Fe/N/C-800 Ag/Fe/N/C-900

45.42 49.27 45.20 42.23 42.02 41.25 40.88 40.64

25.23 29.34 33.26 32.25 31.73 30.00 26.14 34.32

29.35 21.39 21.54 25.52 26.25 28.75 32.98 25.03

24.05 18.94 16.04 6.98 6.02 6.03 3.20 2.85

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of cathodic ORR. The PMAX of the Ag/Fe/N/C-x (x = 630, 640, 650, 660, 700, and 800) catalysts are higher than that of Pt/C, which is consistent with the LSV results (Figure 4a) and voltage outputs (Figures S4 and S5). It has been reported that biofouling may cause the loss of Pt/C catalysts from the cathode, thus resulting in poor power output.44 Note that Ag/ Fe/N/C-700 and Ag/Fe/N/C-650 have far higher SBET, but their catalytic performances are slightly lower than that of Ag/ Fe/N/C-630, maybe due to their relatively low doping N species contents.44 The stability and durability of the catalysts, which are tested after 90 days running (Figure 5c,d), are important when considering their practical application in SCMFCs. The highest power density (1502 mW m−2, 0.78 V) is still obtained by the Ag/Fe/N/C-630 cathode (decline of 16.14%), while that (989 mW m−2, 0.71 V) of Pt/C has decreased by 17.03% in the final cycle, indicating the better stability of Ag/Fe/N/C-630 compared with Pt/C (Table 2). Nyquist curves and electrochemical impedance fitting results for the nine cathodes are shown in Figure 5e and (Table S3). The Ag/Fe/N/C-x (x = 630, 640, 650, 660, 700, and 800) cathodes have lower resistances than that of Pt/C. The Ag/Fe/ N/C-630 cathode has the lowest Rct (5.46 ± 0.1 Ω), while the Ag/Fe/N/C-700 cathode has the lowest ohmic resistance (Ro, 8.72 ± 0.4 Ω) after 90 days of running (Pt/C, Rct = 11.56 ± 0.3 Ω and Ro = 11.58 ± 0.4 Ω). The low Rct of Ag/Fe/N/C-630 can be attributed to its excellent antibacterial property and enhanced ORR kinetics. It is also deduced that the uniform dispersion of Ag and Fe particles (or the Ag/Fe heterojunction) is beneficial for the adsorption and activation of O2,18 thus leading to the improved ORR performance of Ag/Fe/N/C-630. The low Ro of Ag/Fe/N/C-700 (including Ag/Fe/N/C-800) is mainly attributed to the high conductivity of its skeletons, which are partly retrogressed to graphite by the simultaneous incorporation of Fe and N into the C framework.25,37,43 As shown in Figure 5f, SC-MFCs with the Ag/Fe/N/C-630 cathode show the highest average CEs of 32.70% (COD removal rate of 94.50%), while the Ag/Fe/N/C-700 cathode has the highest COD removal rate of 95.57% (CEs of 32.06%). These results are consistent with the EIS results in Figure 5e. Determination of Main Active Constituents in Ag/Fe/ N/C. As shown in Figure 6 and (Figure S6), the obtained power density (the highest) and CEs of the Ag/Fe/N/C-630 cathode are indispensably related to its components, which can be considered as the main active-constituents. It can be seen from (Figure S6a) that except the effects of AgNPs and Fe (Ag/Fe heterojunction), the efficient electrocatalytic property of Ag/ Fe/N/C-630 is mainly attributed to the synergistic effects between oxygen-containing functional groups (O-groups) and Fe−N species that act as effective catalytic sites for ORR,15,45,46 as well as the proper molecular rearrangements of C−N moieties (a small amount of N substitution). Moreover, the hierarchically porous structure of Ag/Fe/N/C-630 can also provide sufficient active sites; as a result, the absorption and reduction of O2 molecules can be effectively improved.47 However, note that the C−N species (i.e., pyridinic N and pyrrolic N) in Ag/Fe/N/C-630 also function as the important active-constituents of ORR,25 and may be identical to (or slightly lower than) those of the combination of O-groups and Fe−N species. Note that the minimum decline (4.34%) in PMAX (1478 mW m−2) is obtained by Ag/Fe/N/C-640 (Figurea 5 and Figure 6a and Figure S6b), which may be attributed to its better capacity for inhibition of biofilm growth on the cathode. Furthermore, its higher stability can also be attributed to the

Figure 4. LSV (a) and CV (b) curves of Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) and Pt/C in PBS medium.

graphitized carbon may inhibit the ORR activity of Ag/Fe/N/ C-900.18 Performances of MFCs with Different Cathodes. To further compare the ORR activities of Ag/Fe/N/C, we used these composites as the catalysts in SC-MFCs cathodes. The voltage outputs of SC-MFCs operated for more than 90 days at a fixed resistance of 1000 Ω are shown inFigure S4. The maximum voltage outputs of SC-MFCs with the nine cathodes in each cycle are shown in Figure S5. Ag/Fe/N/C-630 (Figure S4b) has the highest voltage output (0.65 ± 0.03 mV), which is far higher than that of Pt/C (0.55 ± 0.05 mV) and almost has no fluctuation during the long duration of operation. The cooperation of AgNPs and Fe-species (such as the metallic Fe and Fe−N species) in Ag/Fe/N/C-630 can be considered as the most efficient active-species for ORR activity. Note that the voltage outputs of SC-MFCs with Ag/Fe/N/C cathodes (except for Ag/Fe/N/C-x, x = 620 and 900) are higher than that of Pt/C. All of the above results suggest that the ORR activity of Ag/Fe/N/C catalysts may mainly be attributed to the combination of the metallic Ag/Fe heterojunction and Fe− N species. As shown in Figure 5a and Table 2, the maximum power density (PMAX) and VOC of the Ag/Fe/N/C-630 cathode in the initial cycle are 1791 mW m−2 (the highest) and 0.79 V, respectively. By using the same anodes (Figure 5b), the power outputs of the SC-MFCs are mainly related to the kinetic rates 6997


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Figure 5. Power density and the corresponding electrode potentials of SC-MFCs (vs Ag/AgCl) with Ag/Fe/N/C and Pt/C cathodes, as a function of current density, during (a and b) the initial cycle and (c and d) the last cycle; (e) Nyquist curves for the nine cathodes in SMFCs (inset picture shows the equivalent circuit (EC) of the electrochemical interface; Ro represents ohmic resistance, Rct is charge-transfer resistance, Zw is Warburg impedance, and C is double-layer capacitance); (f) COD removal rate and CEs of SC-MFCs with the nine cathodes during each cycle.

Table 2. PMAX, VOC, and CE for MFCs with Different Cathodes PMAX (mW m−2) cathodes Ag/Fe/N/C-620 Ag/Fe/N/C-630 Ag/Fe/N/C-640 Ag/Fe/N/C-650 Ag/Fe/N/C-660 Ag/Fe/N/C-700 Ag/Fe/N/C-800 Ag/Fe/N/C-900 Pt/C

initial cycle 171 1791 1545 1659 1624 1705 1377 91 1192

± ± ± ± ± ± ± ± ±

8 21 12 21 11 10 17 19 14

VOC (mV)

final cycle 174 1502 1478 1215 1284 1421 1265 82 989

± ± ± ± ± ± ± ± ±

9 9 13 18 21 15 13 13 11

initial cycle 629 791 845 833 827 803 766 257 817

presence of the C−N covalent bonds15,45,46 and the improved chemical stability of metallic Ag (main role for stability) and Fe in Ag/Fe/N/C-640. As shown in Figure 5a−5d, Figure 6 and (Figure S6), the stability and durability of the Ag/Fe/N/C-x (x

± ± ± ± ± ± ± ± ±

4 2 4 3 2 4 6 7 4

CE (%) final cycle 465 780 801 741 762 783 762 255 712

± ± ± ± ± ± ± ± ±

3 4 3 1 3 3 2 3 2

initial cycle 19.01 32.05 32.00 33.11 29.17 31.07 28.99 18.02 21.01

± ± ± ± ± ± ± ± ±

1.9 1.1 0.9 1.3 2.1 1.6 1.0 1.9 1.4

final cycle 16.94 32.71 33.23 30.41 28.15 32.42 28.39 19.02 18.53

± ± ± ± ± ± ± ± ±

1.4 1.5 1.3 1.1 1.8 1.1 1.6 1.1 1.7

= 630, 640, 650, 660, 700, and 800) catalysts are higher than that of Pt/C, suggesting that the Ag/Fe and N incorporated sites are supposed to actively participate in the oxygen reduction and microbial inhibition. Moreover, the catalytic 6998


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Hence, the bacterial toxicity of Ag+ released from AgNPs can be excluded in this study. It was previously found that the AgNPs toxicity to bacteria also occurs through the interactions with reactive oxygen species and that the oxygen stress plays an important role in bacterial inhibition.48−51 In SC-MFCs, the oxygen diffusion rate, electrolyte characteristics and light may be the factors influencing the reactivity of AgNPs, which in turn influence the aquatic reactive oxygen species and the toxicity of AgNPs.51 The electron−hole pairs may exist on the AgNPs surface due to the plasma resonance of AgNPs, and the electrons may react with oxygen diffusing from air, generating the hydroxyl radical (·OH) and O2− species.52,53 Furthermore, the generation of H2O2 from the 2e− reduction of O2 (O2 + 2H+ + 2e− → H2O2) and ·OH radicals from the transformation of H2O2 (H2O2 → 2· OH) may also be conducted.46,52 The ·OH or O2− species can suppress the synthesis of intracellular adenosine triphosphate (ATP) and reduce the plasma membrane potential, thereby causing lipid oxidation and finally leading to cell death.46,48−51 The deduced mechanism for reducing the biofouling on the Ag/Fe/N/C cathodes is shown in Figure 7. As shown in Figure

Figure 6. Relationship between the N-species of Ag/Fe/N/C-x and power density of SC-MFCs (a); the relationship between the components (Fe, O, C and Ag) of Ag/Fe/N/C-x (x = 620, 630, 640, 650, 660, 700, 800, and 900) and CEs of SC-MFCs during the initial and last cycles (b). Figure 7. Mechanisms for bacterial toxicity and ORR pathways on the Ag/Fe/N/C cathodes.

activities of these Ag/Fe/N/C catalysts are also attributed to the increased positive charges on the surrounding C atoms due to the high electronegativity of both O and N atoms.37,43 Relationship between Antibacterial Activity and Durable Power Output. As shown in (Figure S7), the biofilms (heterotrophic bacteria) that formed on the eight cathodes (Ag/Fe/N/C catalysts) are thinner than that of the Pt/C cathode, which may lead to lower impedance (Rct) for proton transfer (Table S3). The inhibition of microorganisms overgrowth on the cathodes is substantially enhanced by the introduction of AgNPs into the C structure, which correspondingly promotes the electron transportation and ORR performance.18,26,28 It is generally recognized that the contact of Ag+ and mercapto groups of enzymes can damage the cell respiration and ion transport across a cell membrane, resulting in a depress in the replication ability of bacteria.18,48−50 The amounts of Ag+ leached from the Ag/Fe/N/C composites are shown in Table S4. By embedding in the C skeleton, the release of Ag+ from Ag/Fe/N/C composites into water is strictly restrained (Table S4), especially when the catalysts are mixed with PTFE in the cathodes, which can dramatically reduce the secondary pollution due to excessive Ag+. Note that if the antibacterial components (Ag+) are released from these cathodes, they may also affect the electricity-generating microorganisms on the anode (no membrane). However, the running (even in the initial cycle, Figure S4) results indicate no toxic effects of the leached Ag+ on anode microorganisms.

8, the n values of Ag/Fe/N/C-630 (3.360), Ag/Fe/N/C-700 (2.871) and Pt/C (4.027) are calculated from the slopes of the K−L plots. Pt/C shows a better performance in the RDE tests (rotating disk electrode) but not in the MFCs power generation (real cathode). This is attributed to the aggregation of Pt/C, biofouling, and the inactivation of Pt/C poisoned by sulfur compounds (SO2, SO4−, or H2S/HS−) that originate from the microbial metabolism of sulfur reducing bacteria.54 The RDE results verify that the ORR proceeds predominantly via the 4e− reaction pathway and that the Ag/Fe/N/C catalysts possess a certain proportion of the 2e− oxygen reduction process with the generation of H2O2 (·OH), which is favorable for eliminating the cathodic biofouling,18 consistent with the above analyses. Furthermore, although the well-crystallized Fe in Ag/Fe/N/ C-x (x = 700, 800, and 900) delivers sluggish ORR activity due to the intrinsic low activity of metallic Fe,41,43 the Fe incorporation (acted as the graphitized catalysts and Fe−Nx species) in the C skeleton has a positive effect on the improvement of the catalytic behavior. Moreover, it is remarkably reported that to transport OH− (4e− reaction pathway, O2 + 2H2O + 2e− → 4OH−, not dominant) from the active sites to the anodic electrolyte, the presence of an OH− concentration gradient is necessary.33 The embedded Ag in the C skeleton of Ag/Fe/N/C-x (x = 630, 640, 650, 660, 700, and 6999


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Figure 8. RDE tests of (a) Ag/Fe/N/C-630, (b) Ag/Fe/N/C-700 and (c) Pt/C at different rotation rates; (d) Koutecky−Levich analysis of different catalysts.

800) can effectively suppress Ag+ release and maintain longterm antibacterial and ORR activities (stabilities) via the smooth transportation of OH− (thin biofilm layer).33 Therefore, the synergistic effects between Ag/Fe (independent or heterojunction), O-groups and N-species (Fe−Nx, pyridinic N and graphitic N) are indispensable to the high ORR activity of Ag/Fe/N/C catalysts.

atures; percentage contents (wt %) of Fe, O, C, Ag, and N in Ag/Fe/N/C, electrochemical impedance fitting results of different cathodes at the final cycle; content or concentration of Ag+ leached from the Ag/Fe/N/C); N2 adsorption/desorption isotherms and pore size distributions curves for Ag/Fe/N/C, C 1s and O 1s binding energy regions for Ag/Fe/N/C; high-resolution XPS of Fe 2p spectra for Ag/Fe/N/C-620 and Ag/Fe/N/C-640; voltage output of SC-MFCs, maximum voltage outputs; relationship between the components of Ag/Fe/N/C; power density and CEs, and photos (biofilms) of Ag/Fe/ N/C and Pt/C cathodes after 120 days of running. (PDF)

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CONCLUSIONS The efficient Ag/Fe/N/C catalysts exhibit high ORR activity and excellent electrochemical and antibacterial stabilities in SCMFCs. The promising ORR performances of the prepared Ag/ Fe/N/C catalysts are strongly associated with their homogeneous distribution of numerous metals/nitrogen active sites. Moreover, the introduction of Ag can eliminate the blocking of proton/OH− transport channels by inhibiting biofilm growth, which correspondingly contributes to a lower overpotential in SC-MFCs. After 90 days of running, the Ag/Fe/N/C-630 cathode obtains the highest power density (1502 mW m−2) and has higher performance and better stability than those (989 mW m−2) of Pt/C. With the synergistic effects of Ag/Fe, Ogroups, and N-species, the ORR and antibacterial activities for these catalysts are energetically obtained. This work presents an attractive toolbox for designing tunable catalysts with Ag doping, presenting enhanced antibacterial and ORR properties for broad applications such as MFCs and other renewable energy systems.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 451 8660 8549. E-mail: [email protected]. *Tel: (+86)451 8660 9115. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support by Key Program Projects of the National Natural Science Foundation of China (21031001), National Natural Science Foundation of China (51578218, 51108162, 20971040, 51210105014, 21001042, 91122018), Natural Science Foundation of Heilongjiang Province (B201411, QC2015009), Postdoctoral Science Foundation of Heilongjiang Province (LBH-Q14137), Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708029), and Excellent Young Teachers Fund of Heilongjiang University and Hundred Young Talents in Heilongjiang University.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11561. Additional details are available, including the textural properties of Ag/Fe/N/C obtained at different temper7000


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