Waste‑to‑hydrogen technologies: Advances in catalytic, thermochemical, and biochemical conversion pathways for a circular hydrogen economy

The evolution towards a circular hydrogen economy requires the deployment of sophisticated technologies capable of transforming various waste streams into high-purity hydrogen while minimizing environmental impacts. This review presents a comprehensive evaluation of recent advancements in catalytic, thermochemical, and biochemical methodologies, highlighting their operational efficacy, techno-economic viability, and environmental sustainability. Catalytic methodologies, including nanostructured, photocatalytic, and electrocatalytic systems, have achieved hydrogen production rates of 100–250 mL H₂ g⁻¹ h⁻¹ with Faradaic efficiencies of 80–90 %. However, obstacles such as catalyst deactivation and scalability issues persist. Thermochemical methodologies, encompassing pyrolysis, gasification, and plasma-assisted reforming, generate syngas comprising 20–55 vol% H₂ with energy demands of 0.6–0.8 mol H₂ kWh⁻¹; however, they necessitate trade-offs between capital intensity and operational expenses. Biochemical techniques, such as dark fermentation (DF), photofermentation (PF), and microbial electrolysis cells (MECs), exhibit yields of 2–6 mol H₂ mol⁻¹ substrate under moderate conditions, with potential for co-product valorization, yet constrained by sluggish kinetics and pretreatment requirements. Comparative life-cycle assessment (LCA) and techno-economic analysis (TEA) suggest that hybrid systems amalgamating thermochemical and biochemical pathways can achieve costs as low as 1.8–2.5 USD kg⁻¹ H₂ and lifecycle emissions below 2 kg CO₂ kg⁻¹ H₂, thereby surpassing single-process configurations.