Bioelectrochemical systems (BESs) are novel systems that utilize biological reactions coupled with electrochemical reactions for organics removal [1], electricity production [1,2] or product/nutrients recovery [2]. Specific attention was dedicated to the electrodes materials improvements. The anode material selected is usually based on three-dimensional conductive carbonaceous materials in order to respond to the necessary features that the anode needs for the optimum oxidation reaction. Particularly, the anode has to be: i) electrically conductive, ii) biologically friendly to accommodate bacteria, iii) mechanically durable and corrosion-free, iv) low cost.

The main problem is instead related with the cathode. In fact, at neutral working pH, the cathode suffers of tremendous losses mainly due to activation overpotentials and low kinetics. Enzymatic based cathode have been showed to have the lowest overpotentials [3] but it has also been showed the enzyme are not very durable under “clean” or “polluted” conditions. Metal-free catalysts based on carbonaceous materials have been also used for the oxygen reduction reaction (ORR) in neutral media. Those carbonaceous materials possess high surface area, electronic conductivity, mechanical strength and durability over time and consequently can be considered suitable for microbial fuel cell (MFC) application. We have showed previously that the addition of an iron-based catalyst in an air-breathing gas diffusional electrode increased significantly the performance compare to activated carbon (AC) cathode [4]. In that study, iron-aminoantipyrine (Fe-AAPyr) was used as cathode catalyst and the catalyst was prepared using sacrificial support method with FeNO₃ as a metal source and aminoantipyrine as a nitrogen-rich precursor. The advantage of Fe-AAPyr compared to AC cathode was 50% [4]. We have also successfully tried Fe-AAPyr in double chamber MFC [5] and in ceramic-based MFC [6].

In this work, eight novel catalysts have been synthesized, characterized chemically and morphologically and the electrochemical performances have been studied in clean condition and in a working MFC. The novelty of the catalysts was in the low-cost organic precursors utilized during the preparation. The organic precursors used were named: niclosamide, ricobendazole, guanosine, succinylsulfathiazole, sulfacetamide, quinine, sulfadiazine and pyrazinamide.

SEM images showed clear 3-D structure typical from the sacrificial support method utilized. XPS showed 2- 3% of atomic nitrogen with the distribution of different types of nitrogen species typical for M-N-C obtained by SSM. Pyridinic nitrogen and nitrogen coordinated to the metal, which have been shown to be important species for ORR [7] are detected in significant amounts.

The catalysts have been embedded into a mixture of AC, carbon black (CB) and PTFE and pressed on a stainless steel mesh. The cathode configuration was an air breathing gas diffusion electrode. The new catalysts have been compared with Pt and AC used as a control. The cathode was inserted on a lateral hole of a single chamber MFC. Cathode polarization curves were run in phosphate buffer. Results showed that six catalysts (Fe-Ricobenzadole, Fe-Niclosamide, Fe-Pyrazinamide, Fe-Guanosine Fe-Succinylsulfathiazole and Fe-Sulfacetamide) outperformed compared to Pt and all outperformed compared to AC. Similar trend was achieved when the cathodes were inserted in working MFCs (Figure 1). Actually three catalysts (Fe-Ricobenzadole, Fe-Niclosamide and Fe-Pyrazinamide) had the highest power density output that was measured between 202 and 209 μWcm^-2 (Figure 1). Correlations between surface properties and performance showed a linear relationship between the power achieved and the amount of pyridinic nitrogen, nitrogen coordinated to metal and pyrrolic nitrogen. The positive role of N_x-Fe and pyridinic nitrogen for ORR in acidic and alkaline conditions has been shown before [7]. In contrast to the observations in this report, in an acidic environment, pyrrolic N causes partial reduction of oxygen to hydrogen peroxide thereby reducing an overall activity. 

References

 [1] X. Wang, C. Santoro, P. Cristiani, G. Squadrito, Y. Lei, A. G. Agrios, U. Pasaogullari, B. Li. J. Electrochem. Soc. 160 (7), G117 (2013).

[2] I. Gajda, J. Greenman, C. Melhuish, C. Santoro, B. Li, P. Cristiani, I. Ieropoulos. Sustainable Energies Technologies and Assessments 7, 187 (2014).

[3] C. Santoro, F. Soavi, A. Serov, C. Arbizzani, P. Atanassov. Biosens. Bioelectron. 78, 229 (2016).

[4] C. Santoro, A. Serov, C.W. Narvaez Villarrubia, S. Stariha, S. Babanova, K. Artyushkova, A.J. Schuler, P. Atanassov. Sci. Rep. 5, 16596 (2015).

[5] C. Santoro, A. Serov, C.W. Narvaez Villarrubia, S. Stariha, S. Babanova, A.J. Schuler, K. Artyushkova, P. Atanassov. ChemSusChem 8(5), 828 (2015).

[6] C. Santoro, K. Artyushkova, I. Gajda, S. Babanova, A. Serov, P. Atanassov, J. Greenman, I. Ieropoulos, A. Colombo, S. Trasatti, P. Cristiani. Int. J. Hydrogen Energy 40(42), 14706 (2015)

[7]. K. Artyushkova, A. Serov, S. Rojas-Carbonell, P. Atanassov, J. Phys. Chem. C 119(46), 25917 (2015).