Yerba Mate pyrolyzed waste as a catalytic material for oxygen reduction reaction in bioelectrochemical systems

BERNASSANI FLORENCIA; RODRIGUEZ SEBASTIÁN; FUCHS, JULIO S.; BONELLI PABLO; FIGUEREDO FEDERICO; CORTÓN EDUARDO

Viña del Mar

Congreso; SAM- CONAMET 2023/IBEROMAT 2023; 2023

SOCHIM, Sociedad Chilena de Metalurgia y Materiales

Carbon materials have gained popularity as a replacement for noble metals in bioelectrochemical systems due to their exceptional physical and electrochemical properties. The production and implementation of biochar, obtained from lignocellulosic biomass waste, is currently a topic of interest in the bioelectrochemical field because it follows the principles of circular economy, sustainability, recycling and environmental friendliness, while retaining some properties of other conventional carbons1.Argentina is one of largest producers of Yerba Mate (Ilex paraguariensis), which branches (leaves and twigs) are used as a popular herbal tea-like beverage. During the manufacturing process, a percentage of twigs are removed to create a more palatable product2. As a result, massive amounts of lignocellulosic waste are discarded. Through proper carbonization, the biochar obtained from Yerba Mate waste can be useful as a material for bioelectrochemical systems. The goal of this work is to investigate pyrolyzed material obtained from Yerba Mate lignocellulosic waste from physicochemical, electrochemical, and ecotoxicological perspectives. Further research was conducted to determine if the biochar had the potential to catalyze the oxygen reduction reaction (ORR), a process that takes place in the cathodes of biofuel cells.In a tubular electric furnace, Yerba Mate twigs were pyrolyzed at 500°C and chemically activated with KOH at 700°C (under N2 atmosphere). The obtained biochar was studied employing morphological and chemical characterization techniques such as surface electron micrography (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (DRX). SEM micrographs of biochar showed rough and microporous (pore diameter around 2 nm) structures retaining the naturally organized hierarchical structure of plant tissues. Both DRX and XPS obtained with biochar showed the graphitic C signals of typical carbon structure. However, XPS studies also identified the presence of N, O, and Fe.Glassy Carbon Electrodes (GCE) were modified by drop casting with biochar- or Vulcan (XC-72)-based inks. The cyclic voltammetry (CV) obtained with biochar in presence of potassium ferro/ferricyanide redox couple revealed the typical oxidation and reduction peaks but with a significant non-faradaic current. Additionally, biochar exhibited higher peak current than those observed with Vulcan or bare GCE (Ic= 31 ± 7 μA; Ic= 11 ± 2 μA; Ic= 11.5 ± 0.8 μA, respectively). This result reflects the effect of the microporous structure of biochar since the electrochemical surface area is increased. CV and lineal sweep voltammetry with rotatory disk electrode (RDE-LSV) were performed to study the ORR. C/Pt 10% was used as a control. The results obtained by CV and RDE-LSV revealed that biochar can be used as a catalyst material for ORR providing an alternative metal noble-free. According to RDE-LSV studies, biochar and Vulcan exhibit similar overpotential and catalytic activity (Table 1). The potential toxicity of the materials was assessed employing the ISO 11348-3:2007 ecotoxicity bioassay based on Vibrio fischeri, a bioluminescent microorganism. These findings encourage further research into composite synthesis taking advantage of microporous biochar surfaces and the incorporation of these low-cost, non-harmful materials into bioelectrochemical systems such as biocells.Table 1. Electrochemical parameters obtained in RDE-LSV studies of ORR activity.Eonset (V vs Ag/AgCl)Ilim at 1600 rpm (μA) C/Pt 10% 0.1 ± 0.09 140 ± 7 Biochar 0.03 ± 0.02 116 ± 22 Vulcan XC-72 0.02 ± 0.01 118 ± 13