Colors of hydrogen: A scientometric and topic modeling assessment of hydrogen energy research

Hydrogen is emerging as a cornerstone of the global energy transition. This study presents a scientometric and topic-modeling analysis of 5506 Web of Science publications (1991–2024) organized by color-coded production pathways classified by carbon intensity. Green hydrogen dominates the research landscape, accounting for 4960 publications, or 90.08% of the total dataset, while blue hydrogen represents the second-largest category with 271 publications. Rising contributions from chemistry, materials science, and environmental disciplines indicate growing interdisciplinarity. Collaboration-network mapping and thematic evolution reveal distinct trajectories across green, blue, gray, and turquoise hydrogen. Green hydrogen research emphasizes renewable-energy integration, electrolyzer optimization, and techno-economic efficiency, whereas blue hydrogen focuses on carbon capture performance and life-cycle trade-offs. Gray hydrogen remains centered on emission mitigation and process benchmarking, while turquoise hydrogen has emerged as a frontier topic related to methane pyrolysis and carbon valorization. Country-level collaboration mapping shows that research activity is concentrated among a few industrialized countries, particularly China, South Korea, the United States, Germany, and England, reflecting persistent geopolitical and technological asymmetries. LDA topic modeling identified four latent themes for each hydrogen pathway, highlighting electrolysis and optimization in green hydrogen, techno-economic and life-cycle assessment in blue hydrogen, emission reduction in gray hydrogen, and methane pyrolysis and catalysis in turquoise hydrogen. By integrating topic analysis evidence with techno-economic, life-cycle, system-integration, and policy/market perspectives, this study provides a holistic understanding of the evolving hydrogen research ecosystem. These insights support Sustainable Development Goals 7 and 13 for a cleaner and more equitable energy future.