Current thermochemical methods for hydrogen (H 2 ) production, such as coal gasifi cation and steam reforming, inevitably

produce anthropogenic CO 2 . In contrast, biomass-derived H 2 off ers a carbon–neutral pathway. Despite its high energy density,

lignin—accounting for ~ 30% of lignocellulosic biomass—has been underutilized. This study presents an alkaline thermal

treatment (ATT) process that effi ciently converts lignin into high-purity H 2 while minimizing CO 2 emissions by sequestering

carbon as solid carbonate, potentially rendering the process carbon negative. Compared to conventional gasifi cation, the ATT

process operates at 170–450 °C lower temperature. Under optimized conditions, lignin-ATT produced 116.02 mmol H 2 /g

lignin (2.6 L H 2 /g lignin), the highest H 2 yield reported for lignocellulosic biomass, far exceeding cellulose-ATT (39.07 mmol

H 2 /g cellulose), with a maximum H 2 purity of 94.75%. Importantly, this study confi rms that stoichiometric NaOH addition

enables H 2 production reaching nearly 97% of the theoretical maximum yield at signifi cantly lower temperatures than steam

gasifi cation. The reactivity of diff erent alkaline hydroxides is also compared. This study also demonstrates a sustainable

approach using black liquor, real waste lignin, and a NaOH recycling system. This strategy reduces the production costs and

generates CaCO₃ as a valuable by-product.