Effect of surface roughness, porosity and roughened micro-pillar structures on the early formation of microbial...

The early formation of electroactive biofilms was investigated with gold electrodes inoculated with Geobacter sulfurreducens. Biofilms were formed under an applied potential of 0.1 V/SCE, with a single batch of acetate 10 mM, on flat gold electrodes with different random surface roughness. Roughness with arithmetical mean height (S_a) ranging from 0.5 to 6.7 μm decreased the initial latency time, and increased the current density by a factor of 2.7 to 6.7 with respect to nano-rough electrodes (S_a = 4.5 nm). The current density increased linearly with S_a up to 14.0 A·m^-2 for S_a of 6.7 μm. This linear relationship remained valid for porous gold. In this case, the biofilm rapidly formed a uniform layer over the pores, so porosity impacted the current only by modifying the roughness of the upper surface. The current density thus reached 14.8 ± 1.1 A·m^-2 with S_a of 7.6 μm (7 times higher than the nano-rough electrodes). Arrays of 500-μm-high micro-pillars were roughened following the same protocol. In this case, roughening resulted in a modest gain around 1.3-fold. A numerical model showed that the modest enhancement was due to ion transport not being sufficient to mitigate the local acidification of the structure bottom.