A broad review of metal/metal oxide based integrated thermal, biochemical and electrochemical systems for waste into hydrogen conversion

Abstract

Global efforts to turn waste into useful fuels have accelerated due to the growing demand for clean and sustainable energy, with hydrogen emerging as a vital energy carrier. Metals and metal oxides have shown remarkable promise as catalysts and functional elements in a variety of Waste-to-Hydrogen (WtH) pathways among the materials studied. The functions and mechanisms of metals and metal oxides in the three main processes of hydrogen generation via thermal, biochemical, and electrochemical conversion are critically examined in this review. Metal oxides aid in cracking, reforming, and tar reduction in thermal pathways like pyrolysis and gasification, increasing hydrogen yields and lowering carbon emissions. Metal oxide additives enhance microbial activity and electron transfer efficiency, which promotes the production of biohydrogen in biochemical processes, especially anaerobic digestion and dark fermentation. Metal and metal oxide-based catalysts are essential for enhancing electrocatalytic activity, stability, and overall system efficiency in electrochemical systems, such as water electrolysis and microbial electrolysis cells. Recent developments in hybrid catalyst systems, process integration techniques, and nanostructured and doped metal oxides are also covered in this review. The three WtH approaches are compared, with important considerations like economic viability, scalability, environmental impact, and energy efficiency assessed. Additionally, the incorporation of WtH technologies with carbon capture and circular economy principles is explored. By addressing current challenges and identifying future research directions, this review underscores the transformative potential of metal/metal oxides in advancing sustainable hydrogen production from waste resources.