System-level integration of hydrogen economy and circular economy: A bibliometric analysis of techno-economic and infrastructure-oriented research

The transition toward a hydrogen-based energy system is increasingly framed within circular economy principles to address sustainability, resource efficiency, and system-wide integration challenges. However, the rapid expansion of hydrogen economy research has produced a fragmented knowledge landscape, particularly across techno-economic, infrastructure, and system integration perspectives. This study aims to consolidate and structure this expanding body of knowledge through a comprehensive bibliometric analysis of hydrogen economy research embedded within circular economy frameworks. Bibliographic data retrieved from the Scopus database (1984-2026) were analyzed using an integrated bibliometric approach, combining co-authorship, co-citation, and author keyword co-occurrence analyses implemented through VOSviewer and R. Publication and citation dynamics, geographical and institutional contributions, source impacts, and thematic evolution were systematically examined to capture both the structural foundations and temporal development of the research field. The findings reveal a pronounced acceleration in publications and citations in recent years, marking a maturation phase driven primarily by highly cited system-level reviews, techno-economic syntheses, and strategic hydrogen roadmaps. The intellectual structure of the field is characterized by interdisciplinary clusters linking hydrogen production technologies, infrastructure development, economic feasibility, and sustainability assessment. Keyword evolution further indicates a clear shift from component-level technological studies toward integrated hydrogen systems aligned with circular economy and energy transition frameworks, with emerging themes emphasizing infrastructure scalability, value chain integration, and system-level coordination. By elucidating the structural and thematic transformation of hydrogen economy research, this study provides a coherent system-level reference framework to support future engineering-oriented investigations, infrastructure planning, and policy-relevant decision-making in the transition toward sustainable hydrogen-based energy systems.