Thermal-Gradient-Driven Hydrovoltaic Nanogenerators: Opportunities and Challenges for Advanced Energy Conversion

The global energy crisis necessitates innovative approaches to harness ambient low-grade energy. Hydrovoltaic technology, which generates electricity from water- nanomaterial interaction, is promising but limited by low power density and environmental intermittency. While previous reviews have focused on materials and isolated mechanisms, this review pioneers a comprehensive analysis of a strategic solution: the active integration of thermal gradients to govern and enhance hydrovoltaic phenomena. We systematically dissect the fundamental synergistic mechanisms-such as thermal-osmosis and evaporation-driven potential-that underpin this coupling. The review critically evaluates key advancements in material innovations (e.g., phase-engineered 2D materials, functionalized carbon, hydrogels) and structural designs (e.g., 3D porous and asymmetric architectures) essential for optimizing thermal-hydrovoltaic performance. Furthermore, we chart the expanded application landscape, from wearable electronics and self-powered sensors to dual-purpose electricity and freshwater production systems. Finally, we confront persistent challenges in material stability, scalability, and system integration, outlining a forward-looking research roadmap focused on multi-physics modeling and intelligent power management. This work also identifies the underexplored potential of high thermal gradients as a critical future direction. By transcending single-source energy harvesting, integrated thermal-gradient-driven hydrovoltaics represents a transformative paradigm for sustainable, distributed power generation.