3.1 Power generation and bioanode potential variation of the SAMFCS
Figure 1A show that the SAMFC-20Ω, SAMFC-1000Ω and SAMFC-OC generate a voltage of 0.093, 0.525 and 0.698V during nearly one month operation process. The SAMFC-20Ω generated a steady power of 0.432 mW, which is 61.7% higher than the SAMFC-1000Ω with 0.267 mW (Figure 1B). Moreover, the SAMFC acclimated under load of 20Ω could generated a maximum power density of 976.6 mW m-2 without power overshoot, while that acclimated under load of 1000Ω delivered a power of only 637 mW m-2 with an obvious power overshoot (Figure 1C).
The potentials of the SAMFC bioanodes during the whole acclimation process shown in Figure 2 display that, after running for about one week, the potentials of the SAMFC anodes change periodically with periodical refresh of the medium containing acetate. Once the medium is refreshed, the anode potential drops rapidly and then level off at a negative value due to the bioelectrochemical oxidation of acetate. Figure 2C displays that the anodes of the SAMFC-20Ω, SAMFC-1000 and SAMFC-OC could achieve a negative potential of -100, -450 and -480 mV, respectively. When the acetate is depleted, the anode potential increases and approaches the cathode potential (about 0 V) gradually. Figure 2B, 2D and 2E show that the SAMFCs fed with media containing both acetate and nitrate substrate display similar trend of anode potential variation to that fed with media containing only acetate substrate. However, a conspicuous potential platform appears in the curves of anodes fed with medium containing both acetate and nitrate (Figure 2D and 2E). Once the medium is refreshed, the anode potential of the SAMFC-20Ω, SAMFC-1000Ω and SAMFC-OC firstly drops from about 0V to negative and level off at -0.09, -0.369 and -0.386 V, respectively, for about 3.6 h. The evident platform of the potential curve might be a consequence of concurrent acetate oxidation and nitrate reduction at the anodes of the SAMFCs, which result in a mixed anode potential. Figure 2B, 2D and 2E reveal that the potential platform periodically exists in every cycle during denitrifers acclimatization. Subsequently, the anode potentials of the SAMFC-20Ω, SAMFC-1000Ω and SAMFC-OC drop again and level off at -0.11, -0.45 and -0.5V, respectively (Figure 2D), which are basically equal to that of the SAMFCs fed with media containing only acetate. This potential decline is believed to be caused by the depletion of nitrate. The nitrate concentration in a complete cycle at the 40th day is measured and shown in Figure 1G, it confirms that the nitrate is depleted in about 3 to 4 h in all the SAMFCs, which is well matched with the duration of the anode potential platform (Figure 2F). Thereby, the potential decline could be a signal of nitrate depletion and would be used for monitoring the nitrate concentration in wastewater. Finally, the anode potentials of SAMFC-20Ω increases gradually to positive potential values (Figure 2D and 2E) due to the depletion of acetate, which is similar to that of acclimation of exoelectrogens by feeding medium containing only acetate (Figure 2C).