Activation of Biomass-Derived Char into Nanoporous Hydrochar: A Comprehensive Review of the Activation Methods

The UN Sustainable Development Goals (SDGs 7 and 13) have accelerated the search for sustainable carbon materials to address energy and environmental challenges. Hydrothermal carbonization (HTC) effectively converts wet biomass into hydrochar without energy-intensive drying. However, raw hydrochar’s low surface area and limited porosity restrict its performance in adsorption, catalysis, and energy storage, necessitating activation to produce nanoporous hydrochar. This review critically evaluates physical (steam, CO2), chemical (KOH, H3PO4, ZnCl2), metal-based (Fe-based), and emerging green (H2O2, carbonate-based) activation strategies, emphasizing pore formation mechanisms, parameters, post-treatments, and their influence on material properties. Comparative analysis shows that KOH activation consistently yields the highest surface areas and microporosity, while H3PO4 and ZnCl2 generate hierarchical pore structures at lower temperatures. Physical activation offers environmentally benign alternatives, whereas metal-assisted and green oxidative methods introduce additional functionalities. The review identifies key challenges regarding scalability, chemical recovery, wastewater management, pore control, and techno-economic feasibility. To enable industrial deployment, future research must prioritize scalable, closed-loop recovery methods, machine learning optimization, and pilot-scale demonstrations.