CATHODES IN AIR-CATHODE MICROBIAL FUEL CELLS

Giulia Massaglia

_{, Valentina Margaria}

_{, Adriano Sacco}

_{, Micaela Castellino}

_{, , Marzia Quaglio}

_,

b _{DISAT, Politecnico di Torino, Corso Duca Degli Abruzzi 24, 10129, Torino, Italy}

Abstract –This work aims to optimize the performances of Air Cathode Single Chamber Microbial Fuel Cells (SCMFCs). To reach this goal, the attention is focused on the development of N-doped carbon nanofibers (N-CNFs) as catalyst layer at the cathode, leading thus to provide the direct oxygen reduction reaction (ORR). Therefore, the catalyst layer in SCMFCs must ensure a number of electrons as close as possible to the ideal value of 4. N-CNFs, thanks to their content of nitrogen defects together with their high surface area, are among the most promising catalyst layer at the cathode. N-CNFs are prepared by using electrospinning technique, starting from a polymeric solution containing polyacrylonitrile. The spun nanofibers were stabilized at 280 °C in air and thermally treated at 900 °C under inert atmosphere for 1 hour. The good electrochemical behavior and typical properties of the nanostructures permitted to apply N-CNFs as catalyst layer in SCMFs.

Index Terms – N-doped Carbon Nanofibers, Electrospinning, Single Chamber Microbial Fuel Cells, polyacrylonitrile.

I. NOMENCLATURE

Polyacrylonitrile (PAN); N-doped Carbon nanofibers (N-CNFs); Platinum/carbon (Pt/C); Single Chamber Microbial Fuel Cells (SCMFCs); N-N dimethylformamide (DMF); commercial carbon paper (CP); X-ray photoelectron spectroscopy (XPS); Rotating Ring Disk Electrode (RRDE);

II. INTRODUCTION

Nowadays the presence in water of various compounds,

effects on its quality and on human health [1]. The development of strategies to clean water is one of the main humanitarian targets of the future. The SCMFCs can represent a new method able to monitor the water quality, obtaining real time in situ measurements and with good operational stability [2]. SCMFC is a bio-electrochemical system able to directly transduce the chemical energy into electrical energy by the action of microorganisms (exoelectrogens), which are able to oxidize the organic matter (so called fuel). In this configuration, the oxygen is the terminal electron acceptor. The catalyst layer must ensure a number of electrons as close as possible to the ideal value of 4, leading thus to ensure the direct oxygen reduction reaction (ORR) and avoid the intermediate reduction reaction, which releases H2O2 that is toxic for microorganisms. Platinum is currently the best performing catalyst, however it is expensive and not-abundant, then its application must be limited. In order to develop a nanostructured material with a high surface area, able to increase the number of catalytic sites exposed to oxygen, carbon nanofiber mat is investigated [3]. Indeed, N-CNFs, thanks to their content of graphitic, pyrrolic and pyridinic nitrogen defects together with their high surface area, are among the most promising catalyst layer at the cathode [3]. N-CNFs are prepared by using electrospinning technique, starting from a polymeric solution containing PAN, chosen thanks to its high yield of carbonization together with its high content of nitrogen. As spun these nanofibers were stabilized at 280 °C in air and thermally treated until 900°C under inert atmosphere for 1h to obtain N-CNFs. XPS confirmed a high content of graphitic nitrogen and a proper

ORR performances. Good electrochemical properties of the samples were established by Rotating Ring Disk Electrode (RRDE). The calculated electron transfer number is equal to 3.9 for N-CNFs, which is quite similar to the ideal value of 4, obtained with Pt/C catalyst used as reference materials. The good electrochemical behavior and typical properties of the nanostructures permitted to apply N-CNFs as catalyst layer in SCMFs. In order to evaluate the performances of the nanostructured material in SCMFCs, N-CNFs or Pt/C catalysts were deposited on carbon paper (CP).

III. EXPERIMENTAL SECTIONS

A. Material and Methods

PAN (average molecular weight Mw=150,000kDa) and DMF (assay 99.8%) were purchased from Sigma Aldrich. Samples were prepared by electrospinning technique, starting from polymeric solutions containing 12wt% PAN in DMF. As spun these nanofibers were stabilized at 280 °C in air and thermally treated at 900°C under inert atmosphere for 1h to obtain conductive N-doped carbon nanofibers. XPS is performed in order to establish the nitrogen defects content and its position related to the nearby carbon atoms in the main chains of N-CNFs. N- CNFs are characterized by a high content of graphitic nitrogen and by an enough amount of pyridinic and pyrrolic nitrogen, as shown in Fig.1a).

Fig. 1. a) High-resolution N1s spectra for N-CNFs; b) Comparison of electron transfer number (left axis) and peroxide hydrogen (right axis) evaluated from RRDE measurements of N-CNFs (purple line and dash line) compared with the commercial catalyst based on Pt/C (black line and dash line)

Moreover, this sample is characterized by a high electrical conductivity, close to 0.5 S/cm. All these properties of N-CNFs, such as high conductivity, high content of graphitic nitrogen and an enough amount of pyridinic and pyrrolic nitrogen, enhance their electrocatalytic behaviour for ORR, as also confirmed by several work in the literature [3].The RRDE technique allows validating the catalytic pathways of the proposed materials by using 4-electrodes measurements. RRDE measurements were performed by cathodically scanning the disk electrode in the range -0.3 ÷ -0.8 V vs Ag/AgCl with fixed scan rate (5 mV/s) and rotating speed (2500 RPM); the ring electrode was maintained at 0.2 V. In particular, the currents of disk and ring are measured; the disk current is related to the four-electron ORR of the analysed materials, while the current of the ring electrode is associated to the two electron ORR peroxide species [4]. The percentage of H2O-% and the electron transfer number were calculated with the following equation (1):

HO2−% = 200 x IR N ⁄ ID+ IR⁄_N n = 4 x ID ID+ IR⁄_N (1) The Fig.1b). shows the results for N_CNFs, compared with the Pt/C, which ensures a number of electron equal to 3.96. It is possible to notice that N-CNFs provides a number of electron transfer close to 3.9, indicating that this catalyst proceeds the direct ORR through an efficient 4- electron pathways similar to that of Pt/C electrode.

Thanks to their good electrochemical properties and their morphological charcteristics, N-CNFs, developed as catalyst layer on the cathode, optimize the SCMFs overall performances. Indeed, the maximum power density reached with N-CNFs (close to 150 mW/m2_{) is two order} of magnitude higher than the one obtained with Pt catalyst layer (equal to 4 mW/m2_{), as sketched in Fig.2.}

Fig.2. Comparison of power density trend of SCMFCs with N-CNFs as catalyst layer (left axis) and power density trend reached by SCMFCs with Pt/C catalyst layer, defined as reference cathode electrode (right axis)

IV. CONCLUSION

In this work, we demonstrate the optimization of SCMFCs obtained by designing N-CNFs as catalyst layer on the cathode electrode. Indeed, N-CNFs showed good electrical conductivity and exhibited noticeable features for their application as an ORR catalyst, thanks to its high amount of graphitic-N together with the content of pyridinic-N, as confirmed by XPS. Their good electrochemical properties, added to all intrinsic properties of the nanostructures, permitted to develop this sample as catalyst layer on cathode electrode in SCMFCs. We designed the cathode electrode by applying N-CNFs on commercial carbon based electrode (CP), compared with Pt/C deposited on CP, used as reference material. The obtained results confirm the improvement of overall device performance with N-CNFs as catalyst, leading to reach a maximum power density two order of magnitude higher than the one obtained with Pt/C layer.

REFERENCES

[1] Schwarzenbach R et al. The challenge of micropollutants in aquatic systems. Science 2006; 313: 1072-1077

[2] Di Lorenzo et al., A small-scale air-cathode microbial fuel cell for on-line monitoring of water quality. Biosensors and Bioelectronics 2014; 62:182-188

[3] Yang D.S et al. Preparation of Nitrogen-Doped Porous Carbon Nanofibers and the Effect of Porosity, Electrical Conductivity, and Nitrogen content on their Oxygen Reduction Reaction Performance. ChemcatChem. 2014, 6: 1236-1244 [4] Orazem M., Tribollet B. Electrochemical impedance spectroscpoy. Hoboken, New Jersey: Wiley, 2008

December 12-15, 2017, Naples, Italy

EFC17069

DEMCOPEM-2MW COGENERATIVE PEM FUEL CELL UNIT FOR HYDROGEN