Do you BET on routine? The reliability of N2 physisorption for the quantitative assessment of biochar's surface area

Abstract
A large specific surface area is one of the structural characteristics which makes biochar a promising material for novel applications in agriculture and environmental management. However, the high complexity and heterogeneity of biochar’s physical and chemical structure can render routine surface area measurements unreliable. In this study, N₂ and CO₂ characterization of twelve biochars from three feedstocks with production temperatures ranging from 400 °C to 900 °C were used to evaluate materials with varying structural properties. The results indicate that the frequently reported peak in the surface area of biochars around 650 °C is an artefact of N₂ measurements and not confirmed by CO₂ analysis. Contradicting results indicate an influence of the structural rigidity of biochar on N₂ measurements due to pore deformation in certain biochars. Pore non-specific calculation models like the Brunauer-Emmett-Teller method do not allow for adjustments to these changes. Instead, the use of a pore specific model and the exclusion of pores smaller than 1.47 nm was found to achieve more representative results. The proposed calculation was validated on an external dataset to highlight the applicability of the method. Our results provide novel insights for understanding the structural evolution of biochar related to production temperature.