Advances in carbon-based mono- and bimetallic nano catalysts for MEC hydrogen production: design and techno-economic perspectives.

The transition toward sustainable energy has intensified interest in hydrogen as a clean energy carrier for decarbonization and long-term energy storage. Microbial electrolysis cells (MECs) offer a promising route for hydrogen production from wastewater and organic waste, enabling simultaneous waste treatment and energy recovery. However, their practical application is still limited by slow hydrogen evolution reaction kinetics, high electrode overpotentials, catalyst instability, and the high cost and susceptibility to poisoning of noble-metal cathodes. Carbon-based mono- and bimetallic catalysts have emerged as attractive alternatives because they combine high electrical conductivity, tunable surface chemistry, large surface area, low cost, and strong potential for microbial-electrode interaction. Incorporating single or dual metal species into carbon frameworks can enhance active-site density, improve electron transfer, optimize hydrogen adsorption energy, reduce overpotential, and strengthen catalytic durability. This review critically examines recent advances in the design, synthesis, characterization, and performance of carbon-supported mono- and bimetallic catalysts for MEC hydrogen production. Emphasis is placed on structure-activity relationships, metal-carbon interactions, catalytic mechanisms, electrochemical behavior, and microbial electrode communication. The effects of key influencing factors, including substrate concentration, pH, temperature, applied voltage, electrode potential, and reactor configuration, are also discussed in relation to hydrogen yield and system efficiency. In addition, techno-economic aspects, scalability challenges, catalyst stability, and future research directions are evaluated.