Mutual interaction of pyrolysis operating conditions and surface morphology for the electrochemical performance of biochar-modified screen-printed electrodes

The transition towards a low-emission economy requires advanced carbon-based materials for multiple applications. This study aimed to correlate the temperature of intermediate pyrolysis with surface morphology and the electrochemical performances of biochar from hazelnut shells (HZS) and spent coffee grounds (SCG), obtained as by-products in bio-oil production. For this process, the biochar from HZS and SCG were produced using a labscale screw-type reactor designed in-house and operated in a semi-continuous regime, under two pyrolysis temperatures (450 degrees C and 550 degrees C) and thermal post-treatment (TT) durations of 10 and 60 minutes, respectively. Physical-chemical characterization through Scanning Electron Microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) revealed distinct structural and electrochemical differences, unrevealing the fundamental importance of the feedstock selection. SEM analysis highlighted a more homogenous and open structure of HZS than SCG-based biochars. Electrochemical testing of biochar-modified screen-printed electrodes (BC-SPEs) demonstrated enhanced electron-transfer efficiency and diffusivity for HZS produced at 550 degrees C, with the HZS_550 variant yielding a 1.5-fold increase in the heterogeneous electron transfer rate constant (k0) and a 2-fold increase in diffusion coefficient (D0) compared to SCG-SPEs. Notably, HZS_550-SPEs showed enhanced sensitivity for both reversible and non-reversible redox probes, achieving a limit of detection (LOD) in the micromolar (mu M) range, halving the LOD of unmodified SPEs. These findings underscore that biochar's electron-transfer efficiency and texture are key factors driving its sensing performance. Crucially, these properties are governed by the formation of graphite-like sheet structures (GSSs), along with crystallinity and aromaticity, which develop from the condensation of amorphous carbon sheets during primary pyrolysis and are largely unaffected by TT.