Lignin for hydrogen production: A comprehensive review of technologies, catalysts, and future perspectives

Lignin, the second most abundant natural aromatic polymer, remains a promising yet severely underutilized feedstock for sustainable green hydrogen production. As a low‑cost industrial byproduct generated at hundreds of millions of tons annually, lignin contains a hydrogen content of approximately 5–6 wt% and abundant reactive functional groups, enabling the co‑production of high‑purity hydrogen and value‑added aromatic compounds. This review provides a critical, systematic overview of state‑of‑the‑art lignin‑to‑hydrogen technologies, including electrochemical, photoelectrochemical, thermochemical, endogenous hydrogen, photocatalytic, and biological routes. Emphasis is placed on reaction mechanisms, catalyst design principles, performance benchmarks, and cross‑technology comparisons. Key challenges hindering industrial translation are rigorously analyzed, including feedstock heterogeneity, catalyst durability and cost, energy efficiency, product separation, and scalability. Among all pathways, photoelectrochemical coupling, supercritical water gasification, and methoxy‑directed endogenous hydrogen production are identified as the most promising, owing to their balanced energy utilization, feedstock compatibility, and economic potential. Future research priorities are proposed to advance lignin valorization toward industrial‑scale green hydrogen applications, focusing on non‑precious bifunctional catalysts, green pretreatment, and integrated biorefinery systems.