Engineering biomass-derived hard carbon for secondary batteries and supercapacitors. Are we there yet? A comprehensive review

Addressing the global energy demand requires innovative solutions, particularly in light of rising atmospheric CO₂ levels and the imperative to transition away from coal. Carbonaceous materials obtained from biomass waste streams (known as hard carbons) show considerable potential owing to their abundance and beneficial characteristics for energy storage devices. However, there is a lack of understanding of the complex and heterogeneous nature of biomass. This hinders industrialists from making well-informed decisions on material selection. This review explores the progression of waste biomass-derived hard carbon materials tailored as a sustainable alternative to conventional carbon sources. Additionally, the significance of usually disregarded qualitative analysis of biocarbon, including H/C and O/C ratio, and the structural heterogeneity of similar waste biomass feedstocks following different heat treatment methods are addressed. These compositional changes lead to variations in electrochemical performance. The correlation between material properties and electrochemical performance is established through a comprehensive analysis of synthesis methodologies and resulting structural modifications. Also, various recent publications report specific capacitance of biomass-based hard carbon prediction using Machine Learning (ML). However, this review identifies certain limitations in existing approaches. These limitations are also discussed in detail, contributing to new knowledge in the field. Finally, the challenges in storage mechanisms and perspectives on future research directions with an insight into ML influencing the performance of hard carbon-based electrodes are examined.