Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency

The performance of a microbial fuel cell (MFC) depends on a complex system of parameters. Apart from technical variables like the anode or fuel cell design, it is mainly the paths and mechanisms of the bioelectrochemical energy conversion that decisively determine the MFC power and energy output. Here, the electron transfer from the microbial cell to the fuel cell anode, as a process that links microbiology and electrochemistry, represents a key factor that defines the theoretical limits of the energy conversion. The determination of the energy efficiency of the electron transfer reactions, based on the biological standard potentials of the involved redox species in combination with the known paths (and stoichiometry) of the underlying microbial metabolism, is an important instrument for this discussion. Against the sometimes confusing classifications of MFCs in literature it is demonstrated that the anodic electron transfer is always based on one and the same background: the exploitation of the necessity of every living cell to dispose the electrons liberated during oxidative substrate degradation.     

Towards an engineering-oriented strategy for building microbial anodes for microbial fuel cells

The objective of the work was to give some first insight into an engineering-oriented approach to MFC design by focusing on anode optimisation. The effect of various parameters was firstly investigated in half cell set-ups under well-controlled conditions. Microbial anodes were formed from soil leachate under polarisation at -0.2 V vs. SCE with different concentrations of substrate, salt and buffer. It was shown that non-turnover CV could be used to assess the electroactive maturity of the anodes during polarisation. This first phase resulted in the definition of a set of optimal parameter values. In the second phase, an optimal anode was formed in a half-cell under the defined optimal conditions. A numerical approach was then developed to calculate the theoretical maximum power that the anode could provide in an ideal MFC. The concept of "ideal MFC" introduced here allowed the theoretical maximum power to be calculated on the sole basis of the kinetic characteristics of the anode. Finally, a MFC designed in the aim of approaching such ideal conditions generated stable power densities of 6.0 W m(-2), which were among the highest values reported so far. The discrepancy between the theoretical maximum (8.9 W m(-2)) and the experimental results pointed out some limit due to the source of inoculum and suggested possible paths to improvement.     

A novel mediator-polymer-modified anode for microbial fuel cells

A high-performance anode system based on a combination of mediator-polymer-modified graphite felt and bacteria capable of reducing extracellular materials shows significant potential for practical use in microbial fuel cells (MFCs).     

Polyaniline-MXene-coated carbon cloth as an anode for microbial fuel cells

Journal of Solid State Electrochemistry - Anodes play an important role in the extracellular electron transfer (EET) process in microbial fuel cells (MFCs). Herein, polyaniline modified...     

Decorating anode with bamboo-like nitrogen-doped carbon nanotubes for microbial fuel cells

Anode electrodes play a key role in generating electricity from microbial fuel cells (MFCs) because they directly affect microbial activities. This communication reports the preparation of nitrogen-doped carbon nanotubes with a bamboo-like nanostructure (Bamboo-NCNTs) by catalytic pyrolysis of ethylene diamine and application of the Bamboo-NCNTs as anode-modifying materials in MFCs. The Bamboo-NCNTs significantly improved performance of an MFC in current production and power output, and reduced internal resistance of the anode compared with conventional CNTs-modified and unmodified anodes. The improved performance could be attributed to the increased active sites induced by multiple joint structures and enhanced biocompatibility originated from nitrogen dopant.     

Bioenergetics and extracellular electron transfer in microbial fuel cells and microbial corrosion

Bioenergetics can be used to analyze the theoretical voltage output of a microbial fuel cell (MFC) and the thermodynamic driving force in microbiologically influenced corrosion (MIC). MFCs involve both inward and outward extracellular electron transfer (EET), whereas only inward EET is behind EET–MIC caused by an electroactive biofilm's harvest of energy from a metal. EET is often rate-limiting, and it is an important process in microbial energy metabolism. EET is critical to the understanding of MFCs and EET–MIC bioelectrochemical processes. Many advances have been made in the past decade on EET by MFC and MIC researchers. Gene manipulations have been used to improve EET in MFCs, leading to enhanced energy output. They have also been used to elucidate the EET processes for better understanding of EET–MIC, which aids in MIC analysis and decision-making of biocide treatment and its efficacy assessment. Researchers are starting to integrate EET knowledge from both fields.     

表面改性炭材料在微生物燃料电池中的应用(英)

研究了在空气阴极微生物燃料电池中修饰方法如硝酸处理和硝酸-氨水酸碱等对XC-72R作为阴极氧还原催化剂催化活性的影响,并且使用傅里叶变换红外光谱(FTIR)、Boehm滴定法和X射线光电子能谱(谱(XPS)等手段对催化剂进行了表征.FTIR测试证明硝酸处理可引入含氧基团氨水处理可引入含氮基团.另外,还测试了含有不同表面官能团的XC-72R对氧还原的活性,并将其作为阴极催化剂用在MFC中,测试了电池性能.实验表明,经酸碱修饰的XC-72R作为空气阴极MFC的催化剂具有很好的应用前景.     

Macroporous composite capacitive bioanode applied in microbial fuel cells

Interfacial electron transfer between electroactive biofilm and the electrode was crucial step for microbial fuel cells(MFCs).A three-dimensional multilayer porous sponge coating with nitrogen-doped carbon nanotube/polyaniline/manganese dioxide(S/N-CNT/PANI/MnO2)electrode has been developed for MFC anode.Here,the S/N-CNT/PANI/MnO2 anode can function as a biocapacitor,able to store electrons generated from the degradation of organic substrate under the open circuit state and release the accumulated electrons upon requirement.Thus,the mismatching of the production and demand of the electricity can be overcome.Comparing with the sponge/nitrogen-doped carbon nanotube(S/N-CNT)bioanode,S/N-CNT/PANI/MnO2 capacitive bioanode displays a strong interaction with the microbial biofilm,advancing the electron transfer from exoelectrogens to the bioanode.The maximum power density of MFC with S/N-CNT/PANI/MnO2 capacitive bioanode is 1019.5 mW/m^2,which is 2.2 and5.8 times as much as that of S/N-CNT/MnO2 bioanode and S/N-CNT bioanode(470.7 mW/m^2 and176.6 mW/m^2),respectively.During the chronoamperometric experiment with 60 min of charging and 20 min of discharging,the S/N-CNT/PANI/MnO2 capacitive bioanode was able to store 10743.9 C/m^2,whereas the S/N-CNT anode was only able to store 3323.4 C/m^2.With a capacitive bioanode,it is possible to use the MFC simultaneously for production and storage of electricity.     

Recovering platinum from wastewater by charring biofilm of microbial fuel cells (MFCs)

Reduction in and recovery of precious metals are research hotspots in the environmental engineering field. In this study, we investigated the transformation and distribution of platinum in microbial fuel cells (MFCs) and demonstrated a feasible approach to recover platinum (Pt) from wastewater with less than 16.88?mg/L platinum through charring biofilms in MFCs and generate Pt/C catalyst. The optimal reaction condition was identified, and charred biofilms were analyzed via SEM-EDS, XRD and XPS. Results showed that less than 10% of Pt was in MFC effluents, and less than 0.5% was in the cathode chamber when the influent concentration was below 16.88?mg/L. Close to 40% of Pt could be recovered. The recovery efficiency could be higher should the reactions run longer. SEM-EDS and XRD results indicated that the metallic form Pt⁰ is one of the reduction products in MFCs. XPS results induced that Pt (IV) was reduced to Pt (II) and Pt⁰.     

Electrospun carbon fiber mat with layered architecture for anode in microbial fuel cells

Layered carbon fiber mats have been prepared by layer-by-layer (LBL) electrospinning of polyacrylonitrile onto thin natural cellulose paper and subsequent carbonization. The layered carbon fiber mat has been proved to be a promising microbial fuel cell anode for high density layered biofilm propagation and high bioelectrocatalytic anodic current density.     

阴极催化剂对微生物燃料电池性能的影响

以不同载量的MnO_2/rGO和Pt/C修饰阴极电极构建了生物阴极型双室微生物燃料电池(MFC),考察了不同阴极催化剂修饰MFC对其产电性能以及老龄垃圾渗滤液主要污染物去除效果的影响。结果表明,以MnO_2/rGO修饰MFC阴极电极材料,能显著提高MFC产电性能及对老龄垃圾渗滤液中污染物去除效果;输出电压为372 mV,功率密度为194 mW/m~3(是未经催化剂修饰MFC的两倍),内阻为264Ω,化学需氧量(COD)和氨氮(NH_3-N)去除率分别为58.68%和76.64%。当MnO_2/rGO载量为.0 mg/cm~2时,MFC性能与负载Pt/C的MFC性能接近,但构建成本却明显降低。     

Analysis of different approaches for evaluation of surface energy of microbial cells by contact angle goniometry

Microbial adhesion on solid substrate is important in various fields of science. Mineral-microbe interactions alter the surface chemistry of the minerals and the adhesion of the bacterial cells to mineral surface is a prerequisite in several biobeneficiation processes. Apart from the surface charge and hydrophobic or hydrophilic character of the bacterial cells, the surface energy is a very important parameter influencing their adhesion on solid surfaces. There were many thermodynamic approaches in the literature to evaluate the cells surface energy. Although contact angle measurements with different liquids with known surface tension forms the basis in the calculation of the value of surface energy of solids, the results are different depending on the approach followed. In the present study, the surface energy of 140 bacterial and seven yeast cell surfaces has been studied following Fowkes, Equation of state, Geometric mean and Lifshitz-van der Waals acid-base (LW-AB) approaches. Two independent issues were addressed separately in our analysis. At first, the surface energy and the different components of the surface energy for microbial cells surface are examined. Secondly, the different approaches are evaluated for their internal consistency, similarities and dissimilarities. The Lifshitz-van der Waals component of surface energy for most of the microbial cells is realised to be approximately 40 mJ/m2 +/-10%. Equation of state and Geometric mean approaches do not possess any internal consistency and yield different results. The internal consistency of the LW-AB approach could be checked only by varying the apolar liquid and it evaluates coherent surface energy parameters by doing so. The electron-donor surface energy component remains exactly the same with the change of apolar liquid. This parameter could differentiate between the Gram-positive and Gram-negative bacterial cells. Gram-negative bacterial cells having higher electron-donor parameter had lower nitrogen, oxygen and phosphorous content on their cell surfaces. Among the four approaches, LW-AB was found to give the most consistent results. This approach provides more detailed information about the microbial cell surface and the electron-donor parameter differentiates different type of cell surfaces.     

Biosensor nanostructures based on dual-chamber microbial fuel cells for rapid determination of biochemical oxygen demand and microbial community analysis

Journal of Solid State Electrochemistry - A low-cost dual-chamber microbial fuel cell (MFC) was constructed as a biosensor for the rapid determination of biochemical oxygen demand (BOD) in domestic...     

GldA overexpressing-engineered E. coli as superior electrocatalyst for microbial fuel cells

Faster electron transfer between bacteria and electrodes in microbial fuel cells can significantly improve the power density of MFCs for practical applications. A recombinant Escherichia coli (E. coli) strain overexpressing glycerol dehydrogenase (GldA) was engineered as the MFC biocatalyst instead of the natural bacteria. Efficient mediators were produced in the fuel cell with this engineered E. coli resulting in lower polarization and much higher power density than with natural E. coli and E. coli with electro-evolved mediators. For the first time, we demonstrate that engineering E. coli by introduction of appropriate oxidoreductase via gene manipulation can greatly improve the rate of electron transfer. This work provides an efficient and economic approach to biologically engineering bacteria for improving MFC performance.     

阳极底物对微生物燃料电池处理秸秆水解物性能的影响

以双室微生物燃料电池为反应器,铁氰化钾为阴极液,研究污水处理厂活性污泥菌液和玉米秸秆水解液对MFC的产电性能的影响。结果表明,随着阳极中活性污泥菌液体积（1.5、3.0、4.5、6.0 mL）增加,MFC的产电量逐渐增加,当活性污泥的体积增加至7.5 mL时,产电量开始呈下降趋势;玉米秸秆水解液在底物中的浓度为0、10、15、20、30、40 g/L时,电池的稳定电压分别为54、157、248、208、170、146 mV。当阳极活性污泥菌液体积为6 mL、玉米秸秆水解液浓度为15 g/L时,微生物燃料电池的产电性能最佳,此时MFC的功率密度为54.6 mW/m²,内阻为496 Ω。同时,循环伏安曲线（C-V）和交流阻抗曲线（EIS）测试可知,MFC的电极过程由电荷传递和扩散过程共同控制,反应过程受电子传递控制。     

阳极改性对单室微生物燃料电池性能的影响

构建了老龄垃圾渗滤液为底物的空气阴极型单室微生物燃料电池,以考察阳极不同改性方式对微生物燃料电池产电性能和对老龄垃圾渗滤液处理效果的影响。结果表明,碳毡阳极经过热处理、浓硝酸、酸性重铬酸钾、混酸的改性后,电池的最大输出功率密度分别提高了104%、241%、51%、181%,COD的去除率变化不大,但氨氮去除率分别增加了22.2%、21.8%、2.3%、47.3%。垃圾渗滤液pH值升高、电导率呈下降趋势。     

Hierarchical micro/nano structures of carbon composites as anodes for microbial fuel cells

We demonstrate the utility of hierarchical micro/nano structures of electrically conductive carbon composites as anodes for microbial fuel cells (MFCs). To construct the hierarchical structures, carbon nanotubes (CNTs) were directly grown on micro-porous graphite felts at high densities. Using the CNT-modified felts as anodes, power outputs from MFCs were increased ～7 fold compared to those with bare graphite-felt anodes. We also show that this power improvement is sustainable even in MFCs operated with naturally occurring microbial communities. These results suggest the wide utility of the hierarchical micro/nano structures of conductive carbon composites for bio-electrochemical processes.     

Enhance cathodic capacitance to eliminate power overshoot in microbial fuel cells

Journal of Solid State Electrochemistry - Power overshoot that presents in the power curves of microbial fuel cells (MFCs) is one of the main reasons for the deterioration of MFC performance over...     

Oxygen reduction kinetics on graphite cathodes in sediment microbial fuel cells

Sediment microbial fuel cells (SMFCs) have been used as renewable power sources for sensors in fresh and ocean waters. Organic compounds at the anode drive anodic reactions, while oxygen drives cathodic reactions. An understanding of oxygen reduction kinetics and the factors that determine graphite cathode performance is needed to predict cathodic current and potential losses, and eventually to estimate the power production of SMFCs. Our goals were to (1) experimentally quantify the dependence of oxygen reduction kinetics on temperature, electrode potential, and dissolved oxygen concentration for the graphite cathodes of SMFCs and (2) develop a mechanistic model. To accomplish this, we monitored current on polarized cathodes in river and ocean SMFCs. We found that (1) after oxygen reduction is initiated, the current density is linearly dependent on polarization potential for both SMFC types; (2) current density magnitude increases linearly with temperature in river SMFCs but remains constant with temperature in ocean SMFCs; (3) the standard heterogeneous rate constant controls the current density temperature dependence; (4) river and ocean SMFC graphite cathodes have large potential losses, estimated by the model to be 470 mV and 614 mV, respectively; and (5) the electrochemical potential available at the cathode is the primary factor controlling reduction kinetic rates. The mechanistic model based on thermodynamic and electrochemical principles successfully fit and predicted the data. The data, experimental system, and model can be used in future studies to guide SMFC design and deployment, assess SMFC current production, test cathode material performance, and predict cathode contamination.     

Direct electrochemistry and electrocatalytic mechanism of evolved Escherichia coli cells in microbial fuel cells

E. coli cells evolved under electrochemical tension in a microbial fuel cell possess direct electrochemical behavior due to the excretion of hydroquinone derivatives through a highly permeable outer membrane, and their catalyzed fuel cell demonstrates excellent performance.