High current production ofShewanella oneidensiswith electrospun carbon nanofiber anodes is directly linked to biofilm formation

Abstract

We study the current production of Shewanella oneidensis MR-1 with electrospun carbon fiber anode materials and analyze the effect of the anode morphology on the micro-, meso- and macroscale. The materials feature fiber diameters in the range between 108 nm and 623 nm, resulting in distinct macropore sizes and surface roughness factors. A maximum current density of (255 ± 71) μA cm-2 was obtained with 286 nm fiber diameter material. Additionally, micro- and mesoporosity were introduced by CO2 and steam activation which did not improve the current production significantly. The current production is directly linked to the biofilm dry mass under anaerobic and micro-aerobic conditions. Our findings suggest that either the surface area or the pore size related to the fiber diameter determines the attractiveness of the anodes as habitat and therefore biofilm formation and current production. Reanalysis of previous works supports these findings.

Local Acidification Limits the Current Production and Biofilm Formation of Shewanella oneidensis MR-1 With Electrospun Anodes

“…A platinum mesh served as counter electrode. The bioelectrochemical reactor holds 1 L growth medium and six places for working electrodes (Erben et al, 2021b ). The anodes are mounted in holders that expose 2.25 cm 2 to the growth medium.…”

“…The current densities reported in the above publications are substantially lower than the current density achieved with an electrospun carbon nanofiber anode material (ES300), recently developed by our group, and are presumably limited by the anode materials. The superior current production of ES300 [(255 ± 71) μA cm −2 ] is directly linked to the biofilm growth on the anode (Erben et al, 2021b ). Analysis of the biofilm revealed that the biofilm fills only about 8 % of the material's pore space and that the available internal surface area is not completely covered with cells.…”

“…High concentrations of mediators enable higher contributions of mediated electron transfer to the total current production in addition to increased biofilm formation. Lower mediator concentrations (~1 μ M ) are secreted by MR-1 under micro-aerobic conditions (Erben et al, 2021b ). Micro-aerobic conditions enhance the current production at the cost of reduced product yield (Teravest et al, 2014 ) through enhanced the biofilm formation (Erben et al, 2021b ).…”

“…This value is further reduced by the electrode. The reduction by the electrospun electrode can be calculated from the electrode's porosity (0.95 %, Erben et al, 2021b ) using Equations (5) and (8) in Inoue et al ( 2016 ) to a value of about 0.9. The effective value of D rel in the biofilm/electrode composite of about 0.7 is given by multiplication of the biofilm's and electrode's relative diffusion coefficients.…”

“…However, the MR-1 biofilm structure differs greatly from the dense, 170 μm–370 μm thick (Renslow et al, 2010 ; Marcus et al, 2011 ), Geobacter spp. biofilms: the biofilm is less dense, fills only about 8 % of the pore space, and extends through the full 500 μm thick electrode (Erben et al, 2021b ). This leads to reduced gradients that result in a lower diffusive flux and current production.…”