Boosting Power Density of Microbial Fuel Cells with 3D Nitrogen‐Doped Graphene Aerogel Electrode

A 3D nitrogen‐doped graphene aerogel (N‐GA) as an anode material for microbial fuel cells (MFCs) is reported. Electron microscopy images reveal that the N‐GA possesses hierarchical porous structure that allows efficient diffusion of both bacterial cells and electron mediators in the interior space of 3D electrode, and thus, the colonization of bacterial communities. Electrochemical impedance spectroscopic measurements further show that nitrogen doping considerably reduces the charge transfer resistance and internal resistance of GA, which helps to enhance the MFC power density. Importantly, the dual‐chamber milliliter‐scale MFC with N‐GA anode yields an outstanding volumetric power density of 225 ± 12 W m−3 normalized to the total volume of the anodic chamber (750 ± 40 W m−3 normalized to the volume of the anode). These power densities are the highest values report for milliliter‐scale MFCs with similar chamber size (25 mL) under the similar measurement conditions. The 3D N‐GA electrode shows great promise for improving the power generation of MFC devices.

Histogram compares the ECSA of the three bio-anodes.
To estimate the electrochemically accessible surface area (ECSA) of the bio-anodes, we have carried out the cyclic voltammetry (CV) in 5 mM potassium ferricyanide aqueous solution containing 0.1 M LiClO 4 as the supporting electrolyte. The ECSAs were evaluated using the following equation: [2] i p = (2.69×10 5 )n 3/2 AD 0 where i p (in A) stands for the peak current, n the number of electrons transferred in the following balanced equation (Equation S2, which we assumed 1 for this case), A (in cm 2 ) the ECSA (not normalized to total mass), D 0 the diffusion coefficient of Fe(CN) 6 3-(0.7×10 -5 cm 2 s -1 in aqueous solution [3] ), C o * the bulk concentration of Fe(CN) 6 3-(5×10 -6 mol mL -1 ), and v (in V s -1 ) the scan rate.
The ECSA can be evaluated from the slope of i p vs. v 1/2 plot.

4-[Equation S2
] To obtain the slope, CV curves of the three bio-anodes were collected at small scan rates (2-   To show the amine groups imbibe positive charge, we measured the zeta-potentials of N-GA at various pH values and compared them with GA ( Figure S5). Zeta-potential is a measurement on surface potential that largely depends on the surface charge. [4] The up-shifted zeta-potential of N-GA at pH=7 clearly revealed that surface of N-GA is more positive than that of GA, owing to the presence of positively charged amine groups, which is consistent with previous reports. [4][5]    considerably higher slope than GA, indicating N-GA is more electrically conductive than GA. Figure S7b shows the cyclic voltammograms collected in a 3 M KOH aqueous electrolyte. It has been reported that the slope of cyclic voltammograms near two ends (highlighted by the two dashed boxes) is directly related to the electrical conductivity. [6] N-GA has a steeper slope than GA, suggesting a higher electrical conductivity.  The rotating ring disk electrode (RRDE) measurement was performed to evaluate N-GA's oxygen reduction reaction (ORR) activity as a MFC cathode. Figure S9a shows the RRDE voltammograms collected at various scan rates. Cathodic currents at all scan rates started to emerge at approximately +0.88 V (vs. reversible hydrogen electrode, RHE). This on-set potential is only slightly lower than that of 10 wt% Pt/C (+0.94 vs. RHE). [7] The ring current is about 10 times lower than the disk current ( Figure S9b), indicating peroxide species are rarely generated in the process.
The number of transferred electrons (n) can be estimated based on the following equation: [8] = ( ) where I R is the ring current, I D the disk current and N the collection efficiency (37%). [9] At the high rotating speed (1600 rpm), n increases drastically from 0 to 3.45 in the potential range from 1.0 to 0.82 V ( Figure S9c). The maximum total number of transferred electrons is 3.75, suggesting a fourelectron pathway dominates the reduction process. The yield of peroxide anion [%(HO 2 -)] can be determined by the following equation: [10] %( ) = 200 × / (

/ )
A low peroxide yield (13.3%) is obtained in the potential range of 0.1 V~0.2 V. It again verifies a four-electron pathway that leads to the formation of hydroxide ions is the major process. The catalytic performance of N-GA for ORR is comparable to some other carbon-based catalysts including carbon-supported Fe-N electrocatalyst, [11] and graphitized multi-walled carbon nanotubes. [12]