# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 4 Outlook and Conclusions  
In this review, we explore the progress in capillary flow-related hydrovoltaic technology. Rather than covering comprehensive perspectives of the entire field, we focus on the underlying mechanisms and diverse device applications. We began by examining the mechanisms that explain electricity generation from water evaporation and moisture, including conventional explanations like streaming potential, ion concentration gradients, and microbial electricity, as well as more recent concepts like the ionovoltaic effect, evaporating potential, and upstream proton diffusion. Each mechanism offers unique insights into the complexities of this emerging field. However, significant challenges remain, particularly in developing accurate mathematical models and fully understanding the practical physical processes involved. Despite the significant theoretical progress, it is essential to validate these models through experimental studies. Future work should involve conducting controlled experiments to measure the performance of hydrovoltaic devices under different environmental conditions, such as variations in humidity, temperature, and water flow rates. These experimental validations will be crucial in confirming the proposed theories and optimizing device performance, thereby advancing the development of practical hydrovoltaic applications. We also highlighted innovative applications based on capillary flow-related hydrovoltaic technology, primarily including energy harvesting systems and self-powered sensing units. These applications demonstrate the versatility and potential of capillary flow-related hydrovoltaic technology, particularly in harnessing water evaporation and environmental moisture as sustainable energy sources. With future efforts in device durability and scalability, we envision promising development for this technology to become commercially viable. Looking forward, future efforts should prioritize refining the theoretical models for the existing mechanisms, to accurately explain and predict device performance under various conditions. Advanced characterization is urgently needed to provide direct evidence for recent novel mechanisms like ionovoltaics, pseudo-streaming, and evaporative potential. The proposed ion transport at the solute front interface is especially intriguing in terms of physical chemistry. We expect thrilling discoveries in this area. Furthermore, the development of hybrid systems integrating solar power induces more complexity [67]. Elucidating the intertwined working mechanisms of thermal energy and flow-induced mechanical energy is thus a pressing need. Moreover, future research should focus on the use of advanced characterization techniques to validate the proposed mechanisms for hydrovoltaic energy generation. Techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), and electrochemical impedance spectroscopy (EIS) can provide valuable insights into the interactions occurring at the solid-liquid interface, the nature of the electric double layer, and ion movement under different environmental conditions. By employing these methods, researchers can gain direct evidence of the physical and chemical processes, thereby enhancing the credibility and robustness of the proposed models. Overall, capillary flow-related photovoltaic technology holds significant promise in achieving multiple sustainable development goals. Concerted efforts from global researchers are urgently needed to advance this field to maturity, overcome current challenges, and realize the full potential of water evaporation- and moisture-induced electricity as sustainable power and water generation. Conceptualization, W.Z., R.T.L. and Y.H. ; writing--review and editing, W.Z., R.T.L. and Y.H. ; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.