Toward Sustainable and High‐Performing Energy Storage from Biomass Waste Through Hydrothermal Carbonization

The demand for valuable and sustainable chemicals and nonfossil fuels, with a carbon-neutral or zero-carbon footprint from zero-cost, abundant waste streams, has garnered significant interest in recent years. This has been driven by a global desire to transition to a circular economy, by reducing global reliance and eventually the need for fossil-based resources. Hydrothermal carbonization (HTC), a thermal conversion technology, is widely used to recover carbon and energy from waste, avoiding the energy-intensive drying process for high-moisture feedstock, and operating at lower temperatures than conventional processes. The main product, waste-derived hydrochar (WHC), has attracted growing interest in electrochemical energy storage (EES) devices (e.g., rechargeable batteries and supercapacitors) due to its straightforward process, favorable properties, high carbon conversion, and environmental benefits. Hence, this critical review will provide the reader with a better understanding of the principles, technical feasibility, and limitations of carbon conversion from waste through various conversion pathways. Moreover, it presents detailed state-of-the-art studies previously reported on WHC production using one-step direct carbonization (DC) and the two-step HTC followed by DC for electrochemical energy storage applications, with a focus on the role of WHC-based electrodes. Lastly, the challenges and prospects for developing WHC materials for EES applications are examined.