Effects of Heat Treatment on the Physicochemical Properties and Electrochemical Behavior of Biochars for...

The present work reports the synthesis and the physicochemical characterization of biochar from the organic wastes of nopal (Opuntia Leucotricha), coffee grounds (Coffea arabica) and Ataulfo mango seeds (Mangifera indica) as alternative electrocatalyst supports to Vulcan XC-72 carbon black. The biochars were prepared using pyrolysis from organic wastes collected at three temperatures, 600, 750 and 900 °C, under two atmospheres, N₂ and H₂. The synthesized biochars were characterized using Raman spectroscopy and scanning electron microscopy (SEM) to obtain insights into their chemical structure and morphological nature, respectively, as a function of temperature and pyrolysis atmosphere. A N₂ adsorption/desorption technique, two-point conductivity measurements and cyclic voltammetry (CV) were conducted to evaluate the specific surface area (SSA), electrical conductivity and double-layer capacitance, respectively, of all the biochars to estimate their physical properties as a possible alternative carbon support. The results indicated that the mango biochar demonstrated the highest properties among all the biochars, such as an electrical conductivity of 8.3 S/cm^-1 at 900 °C in N₂, a specific surface area of 829 m²/g at 600 °C in H₂ and a capacitance of ~300 mF/g at 900 °C in N₂. The nopal and coffee biochars exhibited excellent specific surface areas, up to 767 m²/g at 600 °C in N₂ and 699 m²/g at 750 °C in H₂, respectively; nonetheless, their electrical conductivity and capacitance were limited. Therefore, the mango biochar at 900 °C in N₂ was considered a suitable alternative carbon material for electrocatalyst support. Additionally, it was possible to determine that the electrical conductivity and capacitance increased as a function of the pyrolysis temperature, while the specific surface area decreased for some biochars as the pyrolysis temperature increased. Overall, it is possible to conclude that heat treatment at a high temperature of 900 °C enhanced the biochar properties toward electrocatalyst support applications.