Influence of wood anatomical and chemical traits on the properties of derived biochar for technological applications as cathode material in metal-air batteries

Biochar is defined as a solid material derived from the thermochemical degradation of biomass and has been proposed for many applications. A source of biomass for biochar production is represented by branch wood as forestry or fruit crop byproduct. Wood-derived biochar has been proposed as cathode material for metal-air batteries replacing other non-environmentally friendly or non-renewable materials. The electrocatalytic activity of the carbonaceous materials derived from wood depends on the structural and chemical characteristics of the raw materials, which can be preserved during the pyrolysis process.

In this study, we performed a characterisation of anatomical and chemical traits of wood from different softwood and hardwood species to identify suites of traits favoring the development of efficient air cathodes to be used in electrochemical energy conversion devices. Both softwood and hardwood species were considered, the first type being evolutionary younger and structurally simpler (relying on tracheids for both water flow and mechanical support) than hardwoods (having vessels and tracheids for water flow and fibres mainly for mechanical strength). Among the hardwoods, crop species including Vitis vinifera, Citrus, Pyrus, and Prunus species were analyzed considering the huge amount of wood waste produced worldwide from pruning activities. The different organization of the cell types in the various woods and quantitative traits (e.g. conduits lumen size, cell wall thickness, relative incidence and spatial distribution of the various cell types, density, etc.), the different ultrastructure and chemical composition of the cell walls (e.g. cellulose microfibrils arrangement, lignin content/type/distribution, etc.), and the occurrence of occlusions (e.g. tyloses and gums) are species-specific, vary within the tree architecture, with specific rules and trends (age-, size- and stress-related), and are responsible for the different properties of wood including porosity and mechanical strength. Therefore, a deep knowledge of the quantitative traits of the original wood material is crucial to achieving the desired final properties of the derived carbonaceous materials. Wood anatomy of the branch wood was analyzed through light-, epi-fluorescence, and scanning electron microscopy, as well as functional traits were quantified through digital image analysis. Chemical analyses were also performed to highlight the lignin content and elemental composition of the woody biomass. The woods were treated with pyrolysis cycles and impregnation of nitrogen-rich aqueous solutions, then cathodes for metal-air batteries were prepared. Polarization studies performed on wood-derived cathodes assembled in aluminum-air batteries have highlighted electrochemical performances sufficient for practical applications.

All analyzed parameters were integrated and elaborated through multivariate statistical methods to evaluate the relations among wood traits and the technological properties of the cathodes obtained from the biochar of the various species. The utilization of pruning residues for producing bio-inspired metal-air batteries thus represents a novel and sustainable way to valorize precious lignocellulosic byproducts in the framework of a circular economy.