Electrospun fibers for high performance anodes in microbial fuel cells: optimizingmaterials and architecture
A novel porous conducting nanofiber mat (PCNM) with nanostructured polyaniline (nanoPANi) on the fiber surface was successfully prepared by simple oxidative polymerization. The composite PCNM displayed a core/shell structure with highly rough surface. The thickness and the morphology of PANi laye...
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|Summary:||A novel porous conducting nanofiber mat (PCNM) with nanostructured polyaniline (nanoPANi) on the fiber surface was successfully prepared by simple oxidative polymerization. The composite PCNM displayed a core/shell structure with highly rough surface. The thickness and the morphology of PANi layer on the electrospun polyamide (PA) fiber surface could be controlled by varying aniline concentration and temperature. The combination of the advantages of electrospinning technique and nanostructured PANi, let the PA/PANi composite PCNM possess more than five good properties, i.e. high conductivity of 6.759 S·m-1, high specific surface area of 160 m2·g-1, good strength of 82.88 MPa for mat and 161.75 MPa for highly aligned belts, good thermal properties with 5% weight loss temperature up to 415 oC and excellent biocompatibility. In the PA/PANi composite PCNM, PANi is the only conducting component, its conductivity of 6.759 S·m-1 which is measured in dry-state, is not enough for electrode. Moreover, the conductivity decreases in neutral pH environment due to the de-doping of proton. However, the method of spontaneous growth of nanostructured PANi on electrospun fiber mats provides an effective method to produce porous electrically conducting electrospun fiber mats. The combination advantages of nanostructured PANi with the electrospun fiber mats, extends the applications of PANi and electrospun nanofibers, such as chemical- and bio-sensors, actuators, catalysis, electromagnetic shielding, corrosion protection, separation membranes, electro-optic devices, electrochromic devices, tissue engineering and many others. The electrical conductivity of electrospun PCNM with PANi as the only conducting component is too low for application of as anode in microbial fuel cells (MFCs). So, we turn to electrospun carbon fiber due to its high electrical conductivity and environmental stability. The current density is greatly dependent on the microorganism density of anode in MFCs. While the two-dimensional electrospun carbon fiber mat (2D-ECFM) which was prepared by normal electrospinning only allows growth of microorganism on the surface with thin layer owing to the low porosity and small pore size in the mat. With the concept of increasing the power density of MFCs by increase of microorganism density in the anode, two novel 3D electrospun carbon fiber mats, porous three-dimensional electrospun carbon fiber mat (3D-ECFM) and layered 3D-ECFM, were developed. The porous 3D-ECFM made by GE-spinning shows high specific surface area due to small fiber diameter of about 1μm, stable highly porous structure with high porosity of 99%, big pore size of around 5.8 μm in the mat and very low density of 18 kg·m-3. The porous 3D-ECFM anode is very suitable for microbial biofilms growth and generates very high geometric current density of 3.0 mA·cm-2, and super-high weight current density of 714 mA·g-1. The layered 3D-ECFM made by layer-by-layer electrospinning also shows high porosity of 98.5% which mainly come from the void-space between layers, and high specific area due to small electrospun carbon fibers on each layer. This layered design is suitable for layer-by-layer growth of biofilm and generates geometric current density of 2.0 mA·cm-2 and weight current density of 294 mA·g-1. Though the porosity and pore size in the mats are high enough for penetration single small microorganism, the tendency of biofilms formation makes the biofilm is unable to be grown in whole mat but only in the upper layer about several hundreds micrometers. Because the growth of biofilm is affected by multiple factors, e.g nutrition transfer, but they are greatly hindered by the biofilm formed in the upper layer. The current density of 3D-ECFM anode could be further improved by further increasing porosity and introducing large holes or channels in the mats for sufficient nutrition transportation to inside the mats. According to the results of above, the porosity and the pore size in the fiber mat are utmost important for the performance of anode in MFCs. With concept of curve or helix in fibers can lead to higher porosity in the fiber mat, a novel 3D porous architecture, nanospring, was designed for high performance anode structure in future MFC. Polymeric nanospring was prepared by bicomponent electrospinning. The reasons for the formation of polymeric nanosprings were investigated by coaxial electrospinning of bicomponent rigid i.e. Nomex® or polysulfonamide (PSA) (rigid) and flexible polymers i.e. thermoplastic polyurethane (TPU) (flexible). The results indicated that the nanospring formation is attributed to longitudinal compressive forces which are resulted from the different shrinkages of the rigid and flexible two polymer components and a good electrical conductivity of one of the polymer solutions in coaxial electrospinning system. The modified electrospinning i.e. off-centered electrospinning and side-by-side electrospinning are much more effective than the coaxial electrospinning for generating polymer spring or helical structures, because of the higher longitudinal compressive forces which derived from the lopsided elastic forces. The aligned nanofiber mat with high percent of nanospring shows higher elongation and higher storage modulus below transition glass temperature (Tg) compared to that with straight fibers. The nanospring or helical shape preserves much void-space in the mat. It would be a potential architecture for highly efficient anode in future MFCs.|