Extracellular Electron Transfer Efficiency Research Articles

Enhancing microbial fuel cell performance with free-standing three-dimensional N-doped porous carbon anodes

Microbial fuel cells (MFCs) utilize microorganisms as catalysts to transform the chemical energy to electrical power. However, the practical application of MFCs is currently impeded by low power output, which is caused by insufficient microbial colonization on the anode, slow extracellular electron transfer at the microbe-anode interface, and low oxygen reduction reaction catalytic efficiency at the cathode. Herein, a three-dimensional (3D) hierarchical porous anode of Carbonization-CTS-MF/rGOtempreture (CCMF/rGOT) by pyrolyzing crosslinked composite of melamine formaldehyde resin (MF), chitosan (CTS) and graphene oxide (GO). The macroporous anode provides a biocompatible surface for exoelectrogens and 3D electron transfer pathways. In addition, an appropriate balance of graphitic-N and pyrrolic-N content optimizes both conductivity and the number of active sites, thereby synergistically enhancing extracellular electron transfer (EET) efficiency. Therefore, the power density of CCMF/rGO1000 anode is 11.28 W/m3, which surpasses the CCMF1000 anode without 3D reduced graphene oxide (rGO) conductive network (10.04 W/m3), and traditional carbon cloth anode (4.36 W/m3). Moreover, the anode runs stably for more than 200 days with a maximum operating voltage of 0.69 V, which achieves chemical oxygen demand (COD) removal rate of 95.4 %. This work provides a promising strategy to prepare a free-standing 3D anode with enriched exoelectrogens and enhanced EET to improve the MFCs performance.

Bifunctional electrode materials: Enhancing microbial fuel cell efficiency with 3D hierarchical porous Fe3O4/Fe-N-C structures

The rational development of high-performance anode and cathode electrodes for microbial fuel cells (MFCs) is crucial for enhancing MFC performance. However, complex synthesis methods and single-performance electrode materials hinder their large-scale implementation. Here, three-dimensional hierarchical porous (3DHP) Fe3O4/Fe-N-C composites were prepared via the hard template method. Notably, Fe3O4/Fe-N-C-0.04-600 demonstrated uniformly dispersed Fe3O4 nanoparticles and abundant Fe-Nx and pyridinic nitrogen, showing excellent catalytic performance for oxygen reduction reaction (ORR) with a half-wave potential (E1/2) of 0.74 V (vs. RHE), surpassing Pt/C (0.66 V vs. RHE). Moreover, Fe3O4/Fe-N-C-0.04-600 demonstrated favorable biocompatibility as an anode material, enhancing anode biomass and extracellular electron transfer efficiency. Sequencing results confirmed its promotion of electrophilic microorganisms in the anode biofilm. MFCs employing Fe3O4/Fe-N-C-0.04-600 as both anode and cathode materials achieved a maximum power density of 831.8 ± 27.7 mW m−2, enduring operation for 38 days. This study presents a novel approach for rational MFC design, emphasizing bifunctional materials capable of serving as anode materials for microorganism growth and as cathode catalysts for ORR catalysis.

Mechanistic insights into Fe3O4-mediated inhibition of H2S gas production in sludge anaerobic digestion

The addition of iron-based conductive materials has been extensively validated as a highly effective approach to augment methane generation from anaerobic digestion (AD) process. In this work, it was additionally discovered that Fe3O4 notably suppressed the production of hazardous H2S gas during sludge AD. As the addition of Fe3O4 increased from 0 to 20 g/L, the accumulative H2S yields decreased by 89.2 % while the content of element sulfur and acid volatile sulfide (AVS) respectively increased by 55.0 % and 30.4 %. Mechanism analyses showed that the added Fe3O4 facilitated sludge conductive capacity, and boosted the efficiency of extracellular electron transfer, which accelerated the bioprocess of sulfide oxidation. Although Fe3O4 can chemically oxidize sulfide to elemental sulfur, microbial oxidation plays a major role in reducing H2S accumulation. Moreover, the released iron ions reacted with soluble sulfide, which promoted the chemical equilibrium of sulfide species from H2S to metal sulfide. Microbial analysis showed that some SRBs (i.e., Desulfomicrobium and Defluviicoccus) and SOB (i.e., Sulfuritalea) changed into keystone taxa (i.e., connectors and module hubs) in the reactor with Fe3O4 addition, showing that the functions of sulfate reduction and sulfur oxidation may play important roles in Fe3O4-present system. Fe3O4 presence also increased the content of functional genes encoding sulfide quinone reductase and flavocytochrome c sulfidedehydrogenase (e.g., Sqr and Fcc) that could oxidize sulfide to sulfur. The impact of other iron-based conductive material (i.e., zero-valent iron) was also verified, and the results showed that it could also significantly reduce H2S production. These findings provide new insights into the effect of iron-based conductive materials on anaerobic process, especially sulfur conversion.

3D Bioprinting of Engineered Living Materials with Extracellular Electron Transfer Capability for Water Purification.

Attention is widely drawn to the extracellular electron transfer (EET) process of electroactive bacteria (EAB) for water purification, but its efficacy is often hindered in complex environmental matrices. In this study, the engineered living materials with EET capability (e-ELMs) were for the first time created with customized geometric configurations for pollutant removal using three-dimensional (3D) bioprinting platform. By combining EAB and tailored viscoelastic matrix, a biocompatible and tunable electroactive bioink for 3D bioprinting was initially developed with tuned rheological properties, enabling meticulous manipulation of microbial spatial arrangement and density. e-ELMs with different spatial microstructures were then designed and constructed by adjusting the filament diameter and orientation during the 3D printing process. Simulations of diffusion and fluid dynamics collectively showcase internal mass transfer rates and EET efficiency of e-ELMs with different spatial microstructures, contributing to the outstanding decontamination performances. Our research propels 3D bioprinting technology into the environmental realm, enabling the creation of intricately designed e-ELMs and providing promising routes to address the emerging water pollution concerns.

Boosting bioelectricity generation in bioelectrochemical systems with nitrogen-doped three-dimensional graphene aerogel anode

Bioelectricity generation by electrochemically active bacteria has become particularly appealing due to its vast potential in energy production, pollution treatment, and biosynthesis. However, developing high-performance anodes for bioelectricity generation remains a significant challenge. In this study, a highly efficient three-dimensional nitrogen-doped macroporous graphene aerogel anode with a nitrogen content of approximately 4.38 ± 0.50 at% was fabricated using hydrothermal method. The anode was successfully implemented in bioelectrochemical systems inoculated with Shewanella oneidensis MR-1, resulting in a significantly higher anodic current density (1.0 A/m2) compared to the control one. This enhancement was attributed to the greater biocapacity and improved extracellular electron transfer efficiency of the anode. Additionally, the N-doped aerogel anode demonstrated excellent performance in mixed-culture inoculated bioelectrochemical systems, achieving a high power density of 4.2 ± 0.2 W/m², one of the highest reported for three-dimensional carbon-based bioelectrochemical systems to date. Such improvements are likely due to the good biocompatibility of the N-doped aerogel anode, increased extracellular electron transfer efficiency at the bacteria/anode interface, and selectively enrichment of electroactive Geobacter soli within the NGA anode. Furthermore, based on gene-level Picrust2 prediction results, N-doping significantly upregulated the conductive pili-related genes of Geobacter in the three-dimensional anode, increasing the physical connection channels of bacteria, and thus strengthening the extracellular electron transfer process in Geobacter.

Enhanced electroactive bacteria enrichment and facilitated extracellular electron transfer in microbial fuel cells via polydopamine coated graphene aerogel anode

The structure and surface physicochemical properties of anode play a crucial role in microbial fuel cells (MFCs). To enhance the enrichment of exoelectrogen and facilitate extracellular electron transfer (EET), a three-dimensional macroporous graphene aerogel with polydopamine coating was successfully introduced to modify carbon brush (PGA/CB). The three-dimensional graphene aerogel (GA) with micrometer pores improved the space utilization efficiency of microorganisms. Polydopamine (PDA) coating enhanced the physicochemical properties of the electrode surface by introducing abundant functional groups and nitrogen-containing active sites. MFCs equipped with PGA/CB anodes (PGA/CB-MFCs) demonstrated superior power generation compared to GA/CB-MFCs and CB-MFCs (MFCs with GA/CB and CB anodes respectively), including a 23.0 % and 30.1 % reduction in start-up time, and an increase in maximum power density by 2.43 and 1.24 times respectively. The higher bioelectrochemical activity exhibited by the biofilm of PGA/CB anode and the promoted riboflavin secretion by PGA modification imply the enhanced EET efficiency. 16S rRNA high-throughput sequence analysis of the biofilms revealed successful enrichment of Geobacter on PGA/CB anodes. These findings not only validate the positive impact of the synergistic effects between GA and PDA in promoting EET and improving MFC performance but also provide valuable insights for electrode design in other bioelectrochemical systems.

Accelerating cell division of Shewanella oneidensis to promote extracellular electron transfer rate for efficient pollution treatment

The slow rate of extracellular electron transfer (EET) of electroactive microorganisms (EAMs) remains a predominate bottleneck that restricts practical applications of bio-electrochemical systems. Cell division has significant effects on cell cycle, morphology, growth and metabolism. However, the relation between cell division and the EET rate of Shewanella oneidensis has not been established. Here, we employed modular engineering strategy to accelerate DNA replication in the C period and divisome formation in the D period of cell cycle, which decreased cellular volume and enhanced the EET efficiency. Assembly of the C and D period modules further decreased the cell volume by 82.0 % and enhanced power density by 3.12-fold. Electrophysiological and transcriptomic analyses synergistically revealed that the programmed cell volume decrease facilitated lactate uptake and cellular metabolism due to the increased specific surface area (SSA), which consequently reinforced intracellular electron generation. Moreover, the reduced cell size facilitated electroactive biofilm formation. Finally, programmed increase in riboflavin biosynthesis and transport further strengthened indirect EET and boosted output power density to 1537.8 ± 116.9 mW m−2, 21.1-fold of that of the WT. The engineered strains exhibited superior abilities for Cr6+ reduction and azo dyes degradation. This study shed light on the underlying mechanism how reduced cell size impacts electrophysiology of EAMs, and indicated accelerating cell division is a promising avenue to increase the EET of EAMs for efficient environmental pollution treatment.

Efficient Enhancement of Extracellular Electron Transfer in Shewanella oneidensis MR-1 via CRISPR-Mediated Transposase Technology.

Electroactive bacteria, exemplified by Shewanella oneidensis MR-1, have garnered significant attention due to their unique extracellular electron-transfer (EET) capabilities, which are crucial for energy recovery and pollutant conversion. However, the practical application of MR-1 is constrained by its EET efficiency, a key limiting factor, due to the complexity of research methodologies and the challenges associated with the practical use of gene editing tools. To address this challenge, a novel gene integration system, INTEGRATE, was developed, utilizing CRISPR-mediated transposase technologies for precise genomic insertion within the S. oneidensis MR-1 genome. This system facilitated the insertion of extensive gene segments at different sites of the Shewanella genome with an efficiency approaching 100%. The inserted cargo genes could be kept stable on the genome after continuous cultivation. The enhancement of the organism's EET efficiency was realized through two primary strategies: the integration of the phenazine-1-carboxylic acid synthesis gene cluster to augment EET efficiency and the targeted disruption of the SO3350 gene to promote anodic biofilm development. Collectively, our findings highlight the potential of utilizing the INTEGRATE system for strategic genomic alterations, presenting a synergistic approach to augment the functionality of electroactive bacteria within bioelectrochemical systems.

Shewanella oneidensis MR-1 coupled with biomass-derived carbon dots for promoted bioelectrochemical CO2 reduction: Mechanism elucidation of intensified energy metabolism

Bioelectrochemical CO2 reduction reaction (CO2RR) has been touted as one of the promising strategies to convert CO2 to high-value chemicals via the efficient microbial electrosynthesis (MES). However, the sluggish electron transfer efficiency between microbes and cathodes hampers the CO2 reduction efficiency. Herein, biomass-derived carbon dots (CDs) with high electrical conductivity were coupled with Shewanella oneidensis (S. oneidensis) MR-1 to construct a nanomaterial-bacterial biohybrid system to promote CO2-to-formate formation. Specifically, the corncob-derived CDs (cc-CDs) were highly-biocompatible with S. oneidensis MR-1, with such a constructed biohybrid system showing a marked formate titer of 0.50 mmol/L. Upon doping with Fe element, the resultant Fe-cc-CDs exhibited a considerably boosted formate titer of 1.22 mmol/L, which was 7.7 times higher than the pure S. oneidensis MR-1 system. Such significant CO2-to-formate conversion enhancement was contributed by the internalized CDs in S. oneidensis MR-1 stimulating extracellular electron transfer efficiency, therefore accelerating energy metabolism for formate bioelectrosynthesis. The intensified energy metabolism in S. oneidensis MR-1 was corroborated by the increased concentrations of ATP, NADH and overexpression of energy-related genes. This work demonstrates an intensified energy metabolism exemplification for CO2 reduction, which can be expanded to a host of different nanomaterials-internalized MES systems for bioelectrochemical implications.

Enhancing Extracellular Electron Transfer of a 3D-Printed Shewanella Bioanode with Riboflavin-Modified Carbon Black Bioink.

3D printing of a living bioanode holds the potential for the rapid and efficient production of bioelectrochemistry systems. However, the ink (such as sodium alginate, SA) that formed the matrix of the 3D-printed bioanode may hinder extracellular electron transfer (EET) between the microorganism and conductive materials. Here, we proposed a biomimetic design of a 3D-printed Shewanella bioanode, wherein riboflavin (RF) was modified on carbon black (CB) to serve as a redox substance for microbial EET. By introducing the medicated EET pathways, the 3D-printed bioanode obtained a maximum power density of 252 ± 12 mW/m2, which was 1.7 and 60.5 times higher than those of SA-CB (92 ± 10 mW/m2) and a bare carbon cloth anode (3.8 ± 0.4 mW/m2). Adding RF reduced the charge-transfer resistance of a 3D-printed bioanode by 75% (189.5 ± 18.7 vs 47.3 ± 7.8 Ω), indicating a significant acceleration in the EET efficiency within the bioanode. This work provided a fundamental and instrumental concept for constructing a 3D-printed bioanode.

Fe3C-modified porous carbon as anode electrocatalyst for improving extracellular electron transfer and removing Cr(VI) in microbial fuel cells

The anode, as a host site for electricigens, is regarded as an important determinant of the power generation of microbial fuel cells (MFCs), which is confined by the sluggish extracellular electron transfer (EET) at the electricigens/anode interface. In this study, a novel spindle-shaped material, Fe-modified porous carbon (Fe3C/PC), was rationally designed to boost the efficiency of EET. As expected, the MFCs equipped with the as-synthesized Fe3C/PC@CF anode exhibited excellent electrocatalytic activity for charge transfer and achieved much higher power density (1425 mW m−2). Ultimately, mature MFCs would be used to treat real effluent and Cr(VI) sewage. The excellent performance of MFCs was due to the active surface area provided by the porous carbon in Fe3C/PC@CF, which was conducive to electricigens adhesion, and the Fe3C doping provided abundant catalytic active sites for the anode. This work provides a new concept for the design of high performance anode electrocatalysts for MFCs and wastewater treatment.

Widespread extracellular electron transfer pathways for charging microbial cytochrome OmcS nanowires via periplasmic cytochromes PpcABCDE

Extracellular electron transfer (EET) via microbial nanowires drives globally-important environmental processes and biotechnological applications for bioenergy, bioremediation, and bioelectronics. Due to highly-redundant and complex EET pathways, it is unclear how microbes wire electrons rapidly (>106 s−1) from the inner-membrane through outer-surface nanowires directly to an external environment despite a crowded periplasm and slow (<105 s−1) electron diffusion among periplasmic cytochromes. Here, we show that Geobacter sulfurreducens periplasmic cytochromes PpcABCDE inject electrons directly into OmcS nanowires by binding transiently with differing efficiencies, with the least-abundant cytochrome (PpcC) showing the highest efficiency. Remarkably, this defined nanowire-charging pathway is evolutionarily conserved in phylogenetically-diverse bacteria capable of EET. OmcS heme reduction potentials are within 200 mV of each other, with a midpoint 82 mV-higher than reported previously. This could explain efficient EET over micrometres at ultrafast (<200 fs) rates with negligible energy loss. Engineering this minimal nanowire-charging pathway may yield microbial chassis with improved performance.

Electric field coupled with Fe3O4 modified biochar to enhance anaerobic degradation of phenolic compounds in coal gasification wastewater (CGW)

Anaerobic treatment of coal gasification wastewater is usually limited by the inhibition of high concentration of phenolics. In this study, the Fe3O4 loaded biochar (FeBC) coupled with electric field (E-FB) was used to enhance anaerobic phenolics degradation. E-FB significantly accelerated anaerobic removal of phenol, p-cresol and 3,5-xylenol with methane production improved by 84.2 % compared to the control, where FeBC contributed most. E-FB improved anodic oxidation capacity, adsorption-biodegradation and Fe (III) reduction process, which resulted from the synergistic effects between electric field and FeBC. E-FB significantly enhanced the microbial activity and extracellular electron transfer (EET) in the suspended sludge with the maximum electron transfer system activity (132.9 %), cytochrome C (7.2 nmol/g MLSS) and adenosine triphosphate contents (87.8 nmol/ g MLSS) obtained. Compared with FB, E-FB stimulated the secretion of proteins and humic acid-like substances in extracellular polymeric substance, which could contribute to higher EET efficiency. More abundant phenol degrading bacteria, dissimilatory iron reducing bacteria, electroactive bacteria and potential direct interspecies electron transfer (DIET) partners (e.g., Geobacter and Methanobacterium) were enriched in E-FB especially on the anode. The benzoyl-CoA pathway on the anode and CO2-reduction methanogenesis (possibly via DIET) in the suspended sludge and anode were improved in E-FB.

Exogenous electric field as a biochemical driving factor for extracellular electron transfer: Increasing power output of microbial fuel cell

Although microbial fuel cells (MFCs) offer a promising avenue for clean power production, they are hindered by inefficient extracellular electron transfer (EET), thereby resulting in a low power output. This study focused on augmenting MFC power by boosting the EET rate using an exogenous electric field (EEF). By leveraging the dual benefits of microbial electrolysis cells (MECs) in organic waste treatment and energy recovery, we developed an MFC–MEC system that utilizes the EEF from MEC as a connecting link. Two configurations, EEF in the same direction (SD-MFC–MEC) and EEF in the reverse direction (RD-MFC–MEC), were used to examine the EET kinetic rate at the MFC anode in the EEF environment using various electrochemical methods for quantitative evaluation. Our findings revealed that EEF significantly enhanced MFC anode biofilm formation and electrochemical activity, leading to a prominent improvement in electrode reaction rates. The EET rate constants in the SD-MFC–MEC and RD-MFC–MEC were more than double those in the control R-MFC, with power density increases of 117.8% and 108.4%, respectively. The formation of electrochemically active sites within the biofilm induced by EEF has emerged as a crucial factor in amplifying the EET rate, and consequently, the MFC power output. Additionally, the variation in EET rate constants between the SD-MFC–MEC and RD-MFC–MEC was minimal, thereby indicating the negligible impact of the EEF orientation on the EET efficiency. This study provides a novel method for quantitatively examining EET rates, opens up new avenues for further facilitating microbe-to-electrode electron transfer, and promotes the application of MFC–MEC systems in energy recovery, CO2 emission reduction, and waste treatment.

CNFs@MnO2 nanofiber as anode material for improving the extracellular electron transfer of microbial fuel cells

The relatively high microbial loading, high extracellular electron transfer efficiency, and good electrocatalytic activity at the anode interface of microbial fuel cells enhance their power generation efficiency.

Electrostatic self-assembled nickel cobalt sulfide/Ti3C2 Mxene as a bio-capacitive anode for specific attracting sulfur-cycling bacteria and regulating extracellular electron transfer efficiency

Microbial fuel cells have been proposed as one of the sustainable and potential technologies for treating contaminants and synchronously producing green energy. However, the poor performance of the anode is a major obstacle in the practical application of MFCs. To address this challenge, the rational design of anode materials with enhanced biocatalytic and biocompatible properties has been considered as an effective approach to improve extracellular electron transfer (EET) processes and general MFC performance. Herein, in light of the interaction between the transition metal and exoelectrogens, nickel-cobalt sulfide/Ti3C2 Mxene (NiCo2S4/Mxene) is prepared by electrostatic assembly, which combined positively charged sea-urchin-like NiCo2S4 with negatively charged Mxene. This not only prevents the stacking of NiCo2S4 and Mxene but also accelerates the EET process by harnessing their biocompatibility and biocatalyst. As expected, the MFC with NiCo2S4/Mxene harvested higher electricity energy (9.66 ± 0.28 W/m3), significantly exceeded those of plain carbon felt (CF, 1.68 ± 0.13 W/m3), Mxene (4.32 ± 0.20 W/m3) and NiCo2S4 (5.41 ± 0.22 W/m3). Moreover, the microbial community analysis further revealed that the sulfide composite attracts a higher percentage of sulfur-cycling exoelectrogens (Desulfuromonas (24.25%) and Marinobacterium (15.75%)), motivating associated metabolic pathways with more efficient electricity generation processes. Such commendable performances are mainly attributed to the synergistic effect between NiCo2S4 and Mxene, which confers the bioanode with good biocompatibility and biocatalyst for biofilm adhesion and colonization, as well as providing abundant transition metals and sulfide active sites for the EET process. Our finding provides a facile routine for developing sulfide with appropriate modification as anode material for favorable performance of MFC, providing insights for sustainable and cleaner energy production in the future.

All roads lead to Rome: Cyclic di-GMP differentially regulates extracellular electron transfer in &lt;i&gt;Geobacter&lt;/i&gt; biofilms

&lt;p&gt;Microbial extracellular electron transfer (EET) in dissimilatory metal-reducing microorganisms (DMRMs) is a widespread biological process and is involved in biogeochemical cycling of a variety of elements on the planet of Earth. However, the regulatory networks controlling such important process have been under-investigated. Here, we reported that the intracellular messenger bis-(3��-5��) cyclic dimeric guanosine monophosphate (c-di-GMP) signaling network controls EET in &lt;i&gt;Geobacter sulfurreducens&lt;/i&gt;. The low and high levels of c-di-GMP both improved EET in &lt;i&gt;G. sulfurreducens&lt;/i&gt; electrode-respiring biofilms by differentially regulating the expression of EET-associated genes. In particular, we found that a low c-di-GMP level reduced the formation of the anode biofilm but enhanced EET by upregulating the transcription of all known nanowire genes (i.e., &lt;i&gt;pilA&lt;/i&gt;, &lt;i&gt;omcS&lt;/i&gt;, &lt;i&gt;omcZ&lt;/i&gt; and &lt;i&gt;omcE&lt;/i&gt;). Upregulated &lt;i&gt;omcZ&lt;/i&gt; transcription was further determined to play a decisive role in improving EET. Given that c-di-GMP is present in diverse DMRMs, this study substantially expands our understanding of the regulatory role of c-di-GMP signaling and the varied strategies for efficient EET employed by DMRMs. In addition to be fundamentally significant to understand microbe-mineral and microbe-microbe interactions driven by EET, it is also instructive to develop effective engineered microbial systems for practical applications.&lt;/p&gt;

Enhancing microbial extracellular electron transfer efficiency through increased configuration entropy of bioanodes

Microbial fuel cells (MFCs) have garnered significant attention in the field of bioelectrochemistry as a clean energy source, with a focus on increasing their power density. This study reports the successful application of a spinel-type high-entropy oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)Fe2O4 as an anode material for microbial fuel cells. The samples were prepared through ball milling and sintering at 900 °C. Electrochemical tests demonstrated that the high-entropy oxide significantly enhanced the electrochemical performance and biocompatibility of the anode. The MFCs loaded with (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)Fe2O4 exhibited a power density of up to 3.43 W/m2, which is significantly higher than that of a simple mixture of six metals (2.61 W/m2) and pure Fe2O3 (2.64 W/m2). This outstanding performance results from the disorder induced by the high-entropy composition. This disorder facilitates improved coordination among the primary constituent oxides, maximizing their advantages. Simultaneously, it minimizes the reduction in active sites caused by uneven distribution. High-throughput sequencing results revealed an enrichment of Geobacter in the anodic biofilm, reaching a content as high as 52.51% after domestication. This study elucidates the contribution of entropy to biocatalysis from an entropy perspective and provides a new strategy for developing high-performance materials for microbial fuel cells.

Resorufin retainment by extracellular DNA ensures efficient extracellular electron transfer of Geobacter biofilm

Electrochemically active bacteria (EAB) have promising applications in renewable energy recovery, biofuel production, environmental remediation, and wastewater treatment. Resazurin (RZ) is an effective exogenous electron mediator to facilitate extracellular electron transfer (EET) in EAB, but it is not fully understood that how it catalyzes EET within thick electroactive biofilm. Using Geobacter sulfurreducens as a model EAB, this study investigated the effect and mechanisms of RZ-mediated electron transfer in anode biofilm in a bioelectrochemical system. It was found that the amendment of RZ substantially enhanced current generation of anode biofilm, and the facilitating effect of RZ remained even after fresh electrolyte replacement without RZ replenishment. As an intermediate product of RZ, resorufin (RR) was the actual electron shuttle retained within the entire anode biofilm, responsible for the observed stable electron shuttling efficiency. The retention of RR in anode biofilm might be the consequences of insoluble RR deposition and RR binding with extracellular DNA (eDNA) through intercalative interactions. This study provided in vivo and in vitro evidences for the first time that eDNA provided binding sites for RR in Geobacter biofilm, and proposed a novel electron shuttling mechanism of phenoxazine catalyzing efficient EET of electroactive biofilm.

Improving EET kinetics and energy conversion efficiency of 3D hydrophilic composite hydrogel through 2D direction biofilm growth

The limited power output of microbial fuel cells (MFCs) has impeded their widespread adoption application. Thoughtful selection of a suitable anode material not only enhances extracellular electron transfer (EET) efficiency but also stimulates microbial adhesion and proliferation. Herein, a three-dimensional conductive polymer hydrogel MXene/(polypyrrole)-(polyacrylamide) (MPP) is proposed, which can enhance the power generation performance of the bioanode by promoting the enrichment of microorganisms in the two-dimensional direction. The integration of hydrogel molecular chains within the layers of MXene effectively inhibits the self-aggregation of MXene layers, thereby facilitating the formation of ion transport channels for expedited charge storage. As a result, the maximum power density increases from 3.12 W/m3 (CF) to 6.71 W/m3 (MPP-25), the protein content of MPP-25 bioanode (59.66 ± 2.39 mg/cm2) is 3.34 times higher than that of CF (17.85 ± 1.53 mg/cm2), and the exchange current density value increases from 0.06 mA/cm2 (CF) to 0.26 mA/cm2. Conductive hydrogels prepared by MXene as a modifier are a promising bioelectrocatalyst for high-performance MFCs.