Scientific Mapping of Mixed-Culture Dark Fermentation Research for Biohydrogen Production: A Bibliometric Study

Biohydrogen production via dark fermentation with mixed microbial cultures is a promising strategy for sustainable waste valorization and renewable energy generation. This study provides a comprehensive analysis of advancements and emerging trends in mixed-culture dark fermentation for biohydrogen production over the past two decades through a detailed bibliometric approach. A structured search query was developed, and relevant publications published between 2005 and 2025 were retrieved from the Web of Science Core Collection database. Bibliometric analysis and network visualization were conducted using the Bibliometrix package (RStudio) and VOSviewer software. The results reveal substantial scientific progress in this field, with an average annual growth rate of approximately 25.48%. Contributions originated from 83 countries, with China, India, and Brazil ranking among the leading contributors. In total, 1,177 institutions and 3,231 authors participated in this research domain, indicating the development of a broad and increasingly interconnected global network. The University of São Paulo was identified as the most productive institution. A strong and statistically significant positive correlation ( r = 0.9295; p < 0.05) was observed between publication volume and citation impact, reflecting both quantitative expansion and increasing scholarly influence. Keyword analysis further indicates that current research primarily emphasizes process optimization, waste valorization strategies, and the integration of dark fermentation into sustainable and hybrid bioenergy systems. Overall, this study offers a structured overview that facilitates the identification of key research trends, influential contributors, and leading institutions in mixed-culture dark fermentation for biohydrogen production.