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Abstract Electron transfer efficiency in electroactive biofilm is the limiting factor for bioelectricity output of bioelectrochemical system. Here, carbon felt (CF) is coated with manganese dioxide (MnO 2) which acts as electron mediator in electroactive biofilm. A wrapping layer of conducting Poly 3,4-ethylenedioxythiophene is developed to protect the MnO 2 and enhance electron transfer efficiency of MnO 2 mediator. The hybrid bioanode (PEDOT/MnO 2 /CF bioanode) delivered the highest electron transfer efficiency (6.3 × 10−9 mol cm−2 s−1/2) and the highest capacitance of 4.78 F, much higher than bare CF bioanode (1.50 ± 0.04 × 10−9 mol cm−2 s−1/2 and 0.42 F). As a result, microbial fuel cells could produce a maximum power density of 1534 ± 13 mW m−2, approximately 57.7% higher than that with the bare carbon felt anode (972 ± 21 mW m−2). Possible mechanisms are proposed to help understanding the different function of the PEDOT and MnO 2 on the anodic layer. This study introduces an effective method for the fabrication of high performance anode. Graphical abstract Unlabelled Image Highlights • A MnO 2 layer acts as electron mediator in electrochemical biofilm. • The wrapping layer of PEDOT helps to enhance electron transfer efficiency. • Electron transfer efficiency improved by ~4 times. • PEDOT/MnO 2 /CF anode increases the maximum power density by 143%. [ABSTRACT FROM AUTHOR]

Inefficient extracellular electron transfer within bioanode leads to low current production by microbial fuel cell (MFC). In this study, magnetite was sprinkled in electroactive biofilm with the aid of magnet field. Magnetite located inside biofilm (interior-doped biofilm) facilitated the electron delivery of electroactive bacteria far away from electrode surface. Electron transfer efficiency was improved by 12% and 37% compared with that of biofilms with magnetite located at the interface of biofilm/electrode (surface-doped biofilm) and that without any magnetite (control biofilm) respectively. The output power density of MFC with interior-doped biofilm (764 ± 32 mW m −2 ) was greatly increased compared with that of MFCs with surface-doped biofilm (604 ± 22 mW m −2 ) and control biofilm (475 ± 12 mW m −2 ). SEM images showed that magnetite evenly distributed inside the interior-doped biofilm and penetrated the electroactive biofilm, which would facilitate extracellular electron transfer across electroactive biofilms and thus improving the bioelectricity production. [ABSTRACT FROM AUTHOR]

Revealing the pathways of electron transport within electrochemically active biofilms (EABs) is of great importance. Here, mixed-culture EABs are grown on three interdigitated microelectrode arrays (IDAs) with different gap widths of 10 μm, 20 μm and 50 μm and their conductances are determined. The maximum conductive current increases with the decrease of the gap width and charge transfer resistance obtained for the biofilm attaching on the electrode is 3–11 fold less than that for biofilm bridging gaps, suggesting high conductance of biofilm closer to the electrode surface. Results from Raman analysis show high abundance of multi-heme cytochromes in biofilm bridging 10 μm gap, leading to small resistance of biofilm growing on the IDA with 10 μm gap. In addition, the maximum conductive current of biofilm bridging gaps at the potential of −0.40 V vs. Ag/AgCl suggests that electron transfer in biofilm growing on IDAs is redox driven. All these results indicate the spatial heterogeneity of electron transport processes acting within biofilm. Biofilm conductance decreased with the increase of gap's width. Small resistance was found for biofilm adjacent to the electrode surface. Biofilm conductance was depended the abundance of multi-heme cytochrome. The process of electron transfer with EABs was spatial heterogeneous. Panpan Liu: Conducting a research and investigation process, writing the initial draft [ABSTRACT FROM AUTHOR]

Advancing the application of bioelectrochemical systems necessitates the comprehensive understanding of electron transfer process in electrode-associated biofilm. While in situ electrical measurement by using the split electrode can illustrate the electron conduction in biofilm, the accuracy and interpretation of the obtained data is still in debate. Here, conductance of Geobacter sulfurreducens biofilm is measured by using the split electrode as substrate concentration is changed. Biofilm conductance decreases with the current production as the consumption of substrate. In addition, the measured conductance increases with the area of drain electrode (13.8 ± 1.8 μS for 0.5 cm2 and 29.4 ± 4.2 μS for 1 cm2). These results suggest the measured conductance of biofilm is influenced by microbial metabolism. This effect will diminish when the measurement is performed by using electrochemical impedance spectroscopy or under non-turnover condition. Our results provide the insight on the electrochemical origin of biofilm conductance determined by the in situ electrical measurement. • Biofilm conductance decreased with current production as consumption of substrate. • Biofilm conductance increased with the area of drain electrode. • The measurement of biofilm conductance was influenced by microbial metabolism. • Biofilm conductance significantly decreased under non-turnover condition. [ABSTRACT FROM AUTHOR]

• The effect of gap width on biofilm conductivity was investigated. • Higher conductivity was observed for biofilm bridging on larger gaps. • Redox-driven EET was found for biofilms bridging gaps with different width. • Metabolic activity controls biofilm conductance. To understand electron transport in electrochemically active biofilms, it is necessary to elucidate the heterogeneous electron transport across the biofilm/electrode interface and in the interior of G. sulfurreducens biofilms bridging gaps of varying widths. The conductivity of Geobacter sulfurreducens biofilm bridging nonconductive gaps with widths of 5 µm, 10 µm, 20 µm and 50 µm is investigated. Results of electrochemical gating measurement show that biofilm conductivity peaks at the potential of −0.35 V vs. Ag/AgCl. The biofilm conductivity increases with gap width (10.4 ± 0.2 µS cm−1 in 5 µm gap, 13.3 ± 0.2 µS cm−1 in 10 µm gap, 16.7 ± 1.4 µS cm−1 in 20 µm gap and 41.8 ± 2.02 µS cm−1 in 50 µm gap). These results revealed that electron transfer in G. sulfurreducens biofilm is a redox-driven. In addition, higher biofilm conductivities and lower charge transfer resistances are observed in all gaps under a turnover condition than in those under a non-turnover condition. Our results offer insights into the spatial heterogeneity of biofilm structure and extracellular electron transfer in electrochemically active biofilms. [ABSTRACT FROM AUTHOR]

Originating from the notorious shuttle effect and intrinsically sluggish redox kinetics of polysulfides (especially under the conditions of high sulfur loadings and low electrolytes), the low capacity and sulfur utilization efficiency, unsatisfactory rate performance, and unremarkable cycle performance severely hinder the practical commercialization of lithium-sulfur batteries (LSBs). Furnishing rapid electron transfer and mass diffusion in virtue of interface modulation and nanostructure design is an effective strategy to enhance the activity of catalysts. Herein, a Mott-Schottky heterojunction composed of porous carbon (PC)-supported nickel (Ni) nanoparticles was fabricated on a functionalized carbon nanotube sponge (Ni/PC/FCNTS) via adjusting the geometric structure such as the particle size, aggregation state and pore volume (based on the different solvothermal reaction temperature of the Ni-MOF precursor and the high-temperature pyrolysis process). Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) demonstrated the charge redistribution and a self-generated built-in electric field accelerated electron transfer at the heterogeneous interface between Ni and PC. Meanwhile, the high pore volume of Ni/PC/FCNTS-170 provided a reaction space for rapid ion diffusion and sulfur redox process. Benefiting from the synergistic effect, the derived S@Ni/PC/FCNTS-170 cells manifested excellent rate performance with a capacity of 784.2 mAh g-1 at 2 C and a high areal capacity of 8.09 mAh cm-2 with a high sulfur loading of 17.6 mg cm-2 and low electrolyte/sulfur of 11.3 µL mg-1. [Display omitted] • A Ni/PC-170 Mott-Schottky heterojunction was controllably fabricated on a FCNTS. • Ni/PC-170 regulates the charge distribution and accelerates electron transfer. • Ni/PC-170 with high pore volume effectively promotes mass transfer capability. • S@Ni/PC/FCNTS-170 harvests superb rate performance and areal capacity. [ABSTRACT FROM AUTHOR]

• Conductivity is a comparable metric to characterize EET. • Nanowires and microbial aggregates have substantially different conductivity levels. • Several factors can cause a large variability in the measured conductivity. • Calculating the EET flux in microbial aggregates from conductivity measurement. Microbial extracellular electron transfer (EET) plays a crucial role in bioenergy production and resource recovery from wastewater. Interdisciplinary efforts have been made to unveil EET processes at various spatial scales, from nanowires to microbial aggregates. Electrical conductivity has been frequently measured as an indicator of EET efficiency. In this review, the conductivity of nanowires, biofilms, and granular sludge was summarized, and factors including subjects, measurement methods, and conducting conditions that affect the conductivity difference were discussed in detail. The high conductivity of nanowires does not necessarily result in efficient EET in microbial aggregates due to the existence of non-conductive substances and contact resistance. Improving the conductivity measurement of microbial aggregates is important because it enables the calculation of an EET flux from conductivity and a comparison of the flux with mass transfer coefficients. This review provides new insight into the significance, characterization, and optimization of EET in microbial aggregates during a wastewater treatment process. [ABSTRACT FROM AUTHOR]

Researchers believe that adding conductive mediums in electrochemically active biofilms (EABs) would improve the sensitivity of EAB-based biosensor for real-time water quality early-warning through facilitating the extracellular electron transfer (EET), which has been hardly evidenced mostly because naturally formed EABs employed in previous biosensor studies were recognized distinct and incapable of delivering comparable electrical signals. By preparing artificial EABs where Shewanella oneidensis MR-1 was encapsulated in sodium alginate (SA), this study solved how polypyrrole (PPy) as conductive medium would affect the sensitivity of EAB-based biosensor, as well as mass transfer of toxicant during this process. Different mass ratios (0.125:1, 0.25:1 and 1:1) of PPy over SA were tested, and the sensitivity promoted by 20%, 15% and 6%, respectively. Results indicated that a small amount of PPy addition (PPy: SA = 0.125: 1 in mass ratio) was more effective to increase the biosensor's sensitivity compared to larger amount of PPy employed in EAB. This was when improved conductivity introduced by PPy would dominate in affecting the sensitivity over contrarily weakened mass transfer in the meantime. • Artificial electroactive biofilms containing polypyrrole were prepared. • Polypyrrole increases conductivity as well as decreases mass transfer. • Small amount of polypyrrole better increased biosensor's sensitivity. [ABSTRACT FROM AUTHOR]