Transport And Electricity-generation Characteristics Of Microfluidic Microbial Fuel Cells Based On The Regulation Of Biofilm Distribution

Microbial fuel cells?MFCs?represent a novel green-technology that harness the metabolism of bacteria to convert the organic matters to bio-electricity.Recently,the scale of MFCs has been downsized to form the microsized microbial fuel cells or microfluidic MFCs?MMFCs?.It has been successfully developed as niche power sources and on-line analysis technology.Similar to other microfluidic devices,MMFC has become one of the research spots with the respect of environmental detection,microorganism analysis and power supply.Present MMFCs are basically scaled down the large MFCs and retarded by the insufficient biofilm formation,considerable operation cost,high internal resistance and low power density per capital cost.Meanwhile,the mechanisms of dynamic fluid flow and mass transport in MMFCs are not fully illustrated especially the biofilm formation under the laminar flow in micro-environments.In response to these limitations from the aspect of biofilm-distribution optimization,tremendous experimental studies on characteristics of mass transport and electricity-generation have been carried out according to the theory of engineering thermol physics.The research mainly includes electrode design and cell-structure construction,and details are specifically listed as follows:?1?A graphite-based MMFC is assembled,and the effects of operational parameters are studied including reactant concentrations and volumetric flow rates.The biofilm distribution along flow direction is visualized;?2?MMFCs with different channel structures are constructed.In order to ease the issue of diffusion zone,a MMFC with diverging channel?MMFC-D?is proposed.The effects of channel geometries on biofilm distribution,anodic resistance and power generation are specifically investigated;In order to decrease the thickness of the anodic boundary layer,a MMFC with multiple anolyte inlets?MMFC-MI?is constructed and compared with typical MMFC with one anolyte inlet?MMFC-OI?in biofilm distribution,cell internal resistance and power-generation capacity.The mechanism of multiple inlets is illustrated by controlling the states of extra inlets;?3?A new three-dimensional bio-anode and air-breathing MMFC are proposed.In the view of enhancement of bacterial adhesion on the electrode,a nitrogen-doped graphene aerogel?N-GA?-based bio-anode is synthesized and studied on the sides of material chemistry and bio-chemistry.A MFC equipped with the N-GA electrode is investigated on the bacterial distribution,electron transfer on the anode and power generation;In order to simplify the flow pattern,a single-fluid controlled MMFC with air-breathing cathode is constructed.The oxygen reduction reaction?ORR?catalyst is studied from the aspects of physicochemical and electrochemical characteristics.An air-breathing MMFC with three-dimensional anode is assembled and the property of power generation is tested under continuous-flow and batch modes.The major achievements are listed as follows:1)A graphene-based Y-type MMFC is constructed.It is found the cell performance is initially increased,and then decreased with the increasing of anolyte concentration and volumetric flow rates;a maximum power density of 618±4 mWm^-2 is obtained at the fuel concentration of 1500 mgL^-1?represented by chemical oxygen demand,COD?and at the volumetric flow rate of 10 mL h-1.The biofilm morphologies are visualized by optical microscope.It is found that the thickness of biofilm is gradually decreased along the flow direction.Namely,the thickness of biofilm is higher than those in fully developing zones;2)Three MMFCs with different channel geometries are constructed,respectively are converging channel?MMFC-C?,straight channel?MMFC-S?and diverging channel.Among them,the most densely-packed biofilm and lowest charge transfer resistance are observed in the case of MMFC-D due to the ease of unfavorable mixing zone.MMFC-D delivers the highest areal power density of 2447.7±38.9 mWm^-2,which is 5.29 and 1.24 folds of MMFC-C(462.7±17.5 mWm^-2)and MMFC-S(1980.1±27.5 mWm^-2);3)A MMFC with multiple anodic inlets is proposed.The biofilm distribution in the case of MMFC-MI is more densely packed than that of MMFC-OI especially near three equally-spaced inlets.The maximum power density of MMFC-MI under closed extra inlets is 85.6% of that of opened extra inlets.It indicates the contribution of extra inlets mainly lies in the bacteria-aggregation process rather than the mass transfer enhancement after inoculation;4)A nitrogen doped graphene aerogel bioanode is constructed.The special structure and N-functional groups of N-GA enable the well-distribution of bacterial cells on the electrode.At the same time,nitrogen element could lower the charge transfer resistance between biofilm and electrode.N-GA based MFC delivers a maximum power density of 225 ± 12 Wm^-3?proportional to chamber volume?and 750 ± 40 Wm^-3?proportional to electrode volume?;5)A single-fluid controlled MMFC with air-breathing cathode is proposed.At the cathode side,a nitrogen-doped graphene-activated carbon aerogel?AC@N-GA?catalyst is sythesized and served as the ORR catalyst.AC@N-GA exhibits a superior oxygen reduction property.The number of electron transfer during the ORR process is estimated to 3.92,and the generation rate of H₂O₂ is 4.5%.This microfluidic MFC delivers the highest volumetric power density of 1181.4 ±135.6 Wm^-3 at continuous-flow mode,and 690.2±62.3 Wm^-3 at batch mode.The value of volumetric power density is 10-folds higher than the most-recent reported value.