Air-breathing bio-cathodes based on electro-active biochar from pyrolysis of Giant Cane stalks

Abstract

An innovative low-tech solution to fabricate electro-active biochar (e-biochar) electrodes for bioelectrochemical systems (BES) is proposed. Ligno-cellulosic stalks of Giant Cane (Arundo Donax L.) were subjected to pyrolysis treatment at 900 °C for 1 h. The material kept its original hollow cylindrical shape, rigid morphology and porous texture, as confirmed by 3DX-ray micro-computed tomography. These characteristics are suitable for its use at the air-water interface in BES, as air-breathing bio-cathodes. BET (Brunauer-Emmett-Teller) specific surface area was equal to 114 ± 4 m 2 g-1 , with more than 95% of pores in the microporosity range (pore diameter < 1 nm). Surface electrocatalytic activity was sufficient to sustain oxygen reduction reaction at pH 7, in terms of both onset potential (-0.02 V vs Ag/AgCl) and reduction limiting current density (1 A m-2). Electrical resistivity measurements confirmed sufficient conductivity (8.9×10 −3 ± 1×10 −4  m) of the material and Raman spectroscopy allowed to estimate a graphitization degree in relation to the ID/IG, equal to 2.26. In parallel, the e-biochar were tested as airexposed bio-cathodes in BES, coupled to carbon cloth bio-anodes. After inoculation with wastewater from swine-farming, current densities were generated in the range of 100-150 mA m-2 , along more than 2 2 months of operation, under sodium acetate feeding. Confocal laser scanning imaging revealed consistent biofilm formation on the water-side surface of the cathodes, while a nearly-complete absence of it at the air-side. These e-biochar electrodes might open innovative perspectives to scale-up BES for different applications. Here, consistent salts depositions on the material after 70 days of exposure to the wastewater, suggest that e-biochar biocathodes might serve to recycle nutrients to agricultural soils, through mineralsenriched biochar. 1. INTRODUCTION Real scale application of bio-electrochemical systems (BES) has been facing the need to find an optimal balance between processes efficiencies and costs. Many researches are addressing to develop low-cost and environmentally-compatible materials to fabricate electrodes for large scale applications, such as wastewater treatment, soil bioremediation, etc. [1-3]. To date, the most competitive materials are based on carbon and include graphite-based rods, fiber brushes and granules, carbon-fiber cloths, carbon paper sheets, carbon felt and reticulated vitreous carbon [1,2,4]. They are selected due to their strong biocompatibility and inert properties at room temperature. The use of such materials as air-breathing cathodes has been coupled to the fabrication of microporous layers, made of activated carbon, pressed on carbon cloths (or other current collector) in the presence of polymeric binders (Nafion, PTFE, etc.) [5,6]. High surface area favors the cathodic oxygen reduction reaction (ORR) that needs the simultaneous presence of solid, liquid and gaseous phases. Microporous layers also act as gas diffusion layer at air-water interface, because the p...

Biochar-Terracotta Conductive Composites: New Design for Bioelectrochemical Systems

“…The produced power density is estimated in 40 mW m −2 , normalized to the projected cathode surface. The same power density was achieved in MFCs from previous tests with lignocellulosic separators having an internal resistance of around 60-90 Ω (Marzorati et al, 2018). Unequivocally higher performance (2,250 ± 21 mW m −2 ) were noticed with most optimized MFCs using other types of electrolytes and an external circuit connecting electrodes with a resistance of 1000 Ω (Daud et al, 2020).…”

“…On the other hand, it can be remarked that the biochar from giant cane has a relatively low resistivity value (4 Ωcm), which slightly differs from the value (2.8 Ωcm) of resistivity of carbon cloth commonly used in microbial fuel cells. This conductive biochar was recently tested as an air-cathode electrode of microbial fuel cells targeted to nutrients' recovery from wastewaters (Marzorati et al, 2018). In those experiments, the charge transfer between the electrodes was catalyzed by bacteria settled in the biochar.…”

“…A suitable combination of these parameters allows the achievement of biochar characterized by high conductivity, porosity, and ions conductivity (Giudicianni et al, 2014). Nitrogen and other macro-micro-nutrients were already recovered from wastewater in single-chamber air-breathing microbial fuel cell systems, entrapped in the porosity of biochar-based cathodes produced by the pyrolysis of giant cane (Marzorati et al, 2018). The mechanical fragility of biochar-based MFCs was an underlined issue, in that case, precluding its possible scaling-up.…”

Editorial: Microbial Synthesis, Gas-Fermentation and Bioelectroconversion of CO2 and Other Gaseous Streams

“…A more stable current density was observed with GDE vs. submerged experiments, which significantly varied with pH and respective CO 2 solubility. An interesting synergy between METs and syngas fermentation is that the bio-char resulting from biomass pyrolysis can have interesting properties for the fabrication of bio-electrodes, such as electrical conductivity and a high surface area for microbial biofilm growth (Marzorati et al, 2018;Prado et al, 2019).…”

Granular activated carbon in capacitive microbial fuel cells

“…Van Eerten-Jansen et al, 2013) and Total Nitrogen (TN) (Molenaar et al, 2018) content, which relate to the total amount of biomass. The spatial distribution of a biofilm is commonly obtained by microscopy techniques such as scanning electron microscopy (SEM) (Marzorati et al, 2018;Saba et al, 2017;Zakaria et al, 2018) or confocal laser scanning microscopy (CLSM) (Saba et al, 2017;Sun et al, 2016). However, they are destructive (e.g.…”