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).