## 3 Results and Discussion
### Characterization of the Biologically Activated Electrodes  
The PLA-GO electrodes were biologically activated following the method described in Section 2.1. In this set of tests, we seek to characterize the electrochemical performance and durability of the electrodes. To ensure that all the electrodes were activated under identical conditions (i.e., inoculum, pH, conductivity, etc. ), they were cultured within the same single-chamber cell. The cell was polarized at a potential of 0.1 V vs. Ag/AgCl and fed with sodium acetate as a carbon source. The parameters of pH, conductivity and dissolved O\({}_{2}\) are given in Table S1 in the Supporting Information. The electrodes needed three cycles (21 days) to reach a stable current, after which the experimental phase started. Figure 8 shows the averaged current density obtained for the electrodes with depths of pattern of 0.2, 0.6 and 1.4. The largest averaged current density was measured in the electrode with a depth pattern of 1.4 mm, followed by 0.6 and 0.2, a result that is consistent with the abiotic characterization experiments, where the performance of the electrode seems to improve with the depth of the pattern. Another important aspect shown in Figure 8 is that the current produced by all three electrodes decreased with time, although at different rates: the 0.2 electrode suffered a 56% current reduction between cycles 3 and 9 and decayed by 35% and 37% in the 0.6 and 1.4 electrodes, respectively. In addition, from cycle 5 onwards, the physical integrity of all the electrodes started to collapse, with particles detached from the electrode being clearly discernible by the bare eye in the bottom of the reactor. This led us to suspect that the decline in current could be, at least partially, explained by the physical disintegration of the electrode. Samples collected from the sediments settled in the bottom of the cell, formed the fresh  
Figure 7: Electrochemical impedance results for the ED-PLA electrode. (**A**) EIS and (**B**) electric equivalent circuit. PLA filament and, from the GO employed for the electrodeposition, were analyzed through thermogravimetry to investigate the degradation pattern. The derivative weight loss of sediments was relatively similar to that of the GO (Figure 9), showing peaks at temperatures of around 200 \({}^{\circ}\)C and 460 \({}^{\circ}\)C. In contrast, the weight loss profile of the PLA filament was very different from that of the GO or the sediment, showing an abrupt drop in weight between 200 \({}^{\circ}\)C and 500 \({}^{\circ}\)C.  
These results seem to indicate that the degradation of the electrode can be mainly explained by the detachment of the electrodeposited GO. However, this contrasts with the visual inspection of the electrode at the end of the experiments, which showed evident signs of matrix deterioration. Therefore, we are encouraged to hypothesize that the PLA is suffering from biochemical degradation caused most probably by the proliferation of a biofilm on its surface. Although the possible degradation of the electrode apparently starts almost at the beginning of the experiment, given the decreasing point currents of the three electrodes, they seem to have a lifetime of about 56 days (not counting the 21 days of inoculation) with the optimum performance point at 35 days. This result would be thus highlighting the limited applicability of PLA to the fabrication of electrodes in bioelectrochemical systems due to its biodegradability. Figure 8: Current variation of the different patterns measured along the cycles. Figure 9: Thermogravimetric analyses for the PLA-ED electrodes after 56 days of biotic operation. (**A**) Weight loss against temperature and (**B**) weight loss derivative. Note that in (**B**), cell sediments and commercial GO are referred to in the left axis, and PLA/carbon black is referred to in the right axis. Cyclic voltammetry tests were conducted at the end of the different batch cycles to monitor biofilm development in time (Figure 10).