Mapping Hydrogen Research Frontiers: A Multi-Query Bibliometric Analysis of Electrochemical and Biotechnological Pathways

Hydrogen production technologies are undergoing rapid diversification, driven by the dual imperative of decarbonization and resource circularity. While conventional water electrolysis, particularly PEM and alkaline systems, represents a mature and scalable solution for centralized hydrogen generation, biologically mediated pathways such as microbial electrolysis cells (MECs), dark fermentation, and anaerobic digestion are gaining visibility as decentralized, low-energy alternatives. This review presents a bibliometric analysis of hydrogen research from 2021 to 2026, based on three multi-query strategies that retrieved 6017 works in MQ1, 7551 works in MQ2, and 1930 works in MQ3. The year 2026 is included in the dataset because Scopus indexes articles already accepted and released in early access, assigning them their forthcoming official publication year. Keyword co-occurrence mapping using VOSviewer highlights thematic clusters and disciplinary shifts. The results reveal a strong dominance of electrochemical research, with biohydrogen production emerging as a distinct but less mature frontier rooted in biotechnology and environmental science. MECs, in particular, occupy a transitional zone between electrochemical and biological paradigms, offering multifunctional platforms for simultaneous waste valorization and hydrogen generation. However, their low Technology Readiness Levels (TRLs) and unresolved engineering challenges limit their current scalability. The comparative analysis of bibliometric queries underscores the importance of integrating electrochemical and biotechnological approaches to build a resilient and context-adaptive hydrogen economy. This study provides a structured overview of the evolving knowledge landscape and identifies key directions for future interdisciplinary research and innovation.