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Biochars are an exciting class of environmental materials with a wide range of applications, including carbon storage and sequestration, soil enhancement, and pollution remediation. However, the limited knowledge of their molecular structures and compositions and the lack of comprehensive understanding of the relationship between these structures and biochars' diverse functionality, is hindering advancements in their development. In this work, we further advance the approach, first introduced by Wood et al. (2023), to constructing biochar molecular models; and now include control of microporosity (pores < 2 nm size) within the developed models. We construct biochar models representative of woody biochars which are experimentally produced at 600 -- 650 oC highest heating temperatures. Our models reproduce experimental H/C and O/C atomic ratios, percentage aromatic carbon, true density, cumulative porosity, and pore size distribution. The development of microporous biochar molecular models allows us to identify the importance of chemical structures involved in the assembly of biochar materials, and describe the relationship between these structures and obtained micropores. To facilitate other researchers integrating our approach into their work, we detail the steps taken, including the tests and reasons for each decision, in the construction of the biochar models. Furthermore, we share our developed molecular models in a format that can be easily integrated into other group's work in the form of molecular dynamics simulations.

Biochars have been attracting renewed attention as economical and environmentally friendly carbon sequestration materials with a diverse range of applications. However, experimental developments may be limited by the lack of molecular-level knowledge of the key interactions driving these applications. Molecular modelling techniques, such as molecular dynamics simulations, offer a systematic and reproducible alternative and yield atomistic insights into physicochemical processes, allowing the identification of adsorption mechanisms and, through this, informing and guiding experimental development. In this work, on the basis of the critical assessment of the analytical techniques for characterisation of biochars and collation of a large volume of experimental data, we develop molecular models of three woody biochar materials, representative of those produced under low-, medium-, and high-temperature treatments. We characterise these models, validating them against experimental data, and share them with the research community. Furthermore, we detail our iterative approach to the design of these biochar models, discuss what we have learned about the relationship between biochar composition and its morphology, and finally share all of the building blocks used to create these biochar models. With this work, we hope to speed up the uptake of molecular dynamics simulations for the study and development of biochar materials and, to this end, we distribute our easy-to-use surface-exposed biochar models ready for the adsorption studies.

Biochars are black carbonaceous solids produced through biomass pyrolysis under conditions of little or no oxygen. Whilst their properties are well studied, and their applications numerous, the underlying molecular structures within biochars still need to be defined. This raises a substantial barrier to the molecular modelling of biochars and has limited computational study of these materials, despite the advantages of such techniques. In this work, we critically assess the analytical techniques used to characterise biochars and use this information to gain molecular-level insights into biochars' molecular compositions and nanostructures. We focus on properties present at the nanoscale and which provide atomic-resolution insights into the molecular structures within these materials. Our goal is to create a holistic understanding of biochars' chemical, physical and molecular properties and to lay the foundation for future work focused on developing realistic molecular models of these materials.