A comprehensive review on nature-inspired redox systems based on humic acids: Bridging microbial electron transfer and high-performance supercapacitors

Humic acids (HAs) have attracted increasing attentions owing to their vibrant bioelectrochemical activities. Although recent research on HAs demonstrated their ability to promote electron transfer via quinone and phenolic moieties, which allow them to perform various functions in metal immobilization, microbial energy metabolism, and pollutant degradation. The underlying redox mechanisms are still unclear and occasionally reported as contradictory. According to this study, electron shuttling and metal ion-mediated internal electron “bridges” are probably essential to the functional roles of HAs in microbial systems. As natural redox mediators and electron conductors, HAs facilitate microbial metabolism and enhance redox efficiency through direct and indirect pathways. Furthermore, in bioelectrochemical systems, HAs serve as effective electrode modifiers or electron transfer enhancers, improving charge storage and transport efficiency. However, numerous unresolved queries remain regarding their structure–function interactions, synergies with conductive materials, and microscale electron transport behavior. Existing research often overlooks the structural and performance instability of HAs under different environmental conditions, leading to reduced predictability of its application effectiveness. Future research should explore the mechanisms underlying HAs’ role in microbial community succession and the dynamic changes in electron transfer pathways to provide innovative strategies for sustainable development.