Biological hydrogen production from lignocellulosic biomass: Microbial pathways, integration and sustainability perspectives

Hydrogen plays a central role in emerging low-carbon energy systems, over 95% of current production relies on energy-intensive thermochemical processes such as steam methane reforming and coal gasification, which operate at high temperatures (~700–1400 °C) and pressures (up to ~70 bar) and generate substantial CO₂ emissions. From a stoichiometric perspective, steam methane reforming yields ~2.5–3.5 mol H₂ per mol CH₄, while coal gasification produces ~1.5–2.0 mol H₂ per mol carbon under practical conditions. In contrast, biological hydrogen production from lignocellulosic biomass offers a scalable and low-carbon alternative by converting renewable feedstocks under mild operating conditions while enabling waste valorisation. Dark fermentation (DF) enables rapid hydrogen generation but is thermodynamically constrained, with practical yields of ~1.8–2.8 mol H₂ per mol glucose. Photo-fermentation (PF) enhances hydrogen recovery by converting volatile fatty acids via nitrogenase, achieving ~2.5–4.2 mol H₂ per mol acetate, although its efficiency is limited by photon utilization and nitrogen regulation. Microbial electrolysis cells (MECs) further improve hydrogen recovery efficiency, achieving coulombic efficiencies of ~60–90% under optimized conditions. Integrated DF–PF–MEC systems emerge as the most effective strategy, achieving cumulative yields of ~5.0–6.2 mol H₂ per mol hexose through enhanced electron recovery and substrate utilization. Despite these advances, key bottlenecks persist, including pretreatment-induced inhibitors, limited light penetration and photonic efficiency, and electrode material cost and durability constraints. Techno-economic and life-cycle analyses indicate that significant reductions in greenhouse gas emissions are achievable when renewable energy inputs, optimized pretreatment, and process integration are implemented. Overall, integrated multi-stage biological systems offer a sustainable and energy-efficient alternative to conventional hydrogen production. Future progress will depend on advancing low-severity pretreatment strategies, improving photon-to-hydrogen conversion efficiency, developing cost-effective and durable catalytic materials, and optimizing system-level integration for scalable deployment. Biological hydrogen production pathways from lignocellulosic biomass. Graphical overview of lignocellulosic biomass conversion into biohydrogen via dark fermentation, photo-fermentation, biophotolysis, and microbial electrolysis cells (MEC), highlighting key process limitations, integration opportunities, and sustainability considerations toward future H₂ deployment.