# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 2 Current Understandings of Capillary Flow-Related Electricity Generation Mechanisms
### Evaporating Potential  
Following the pioneering work [13] in the field, [PERSON]'s group has delved deeper to elucidate the exact mechanism of electricity generation. In their recent study, they proposed a new mechanism called \"evaporating potential\" [21; 57; 58]. As the name suggests, this  
Figure 5: Pseudo-streaming mechanism. A combination of the evaporation-induced diffusion leading to potential and the ionovoltaic effect from water absorption on the material leading to electron/hole dissociation. Reproduced from ref. [55] with permission from the Royal Society of Chemistry, copyright 2022.  
mechanism attributes electricity generation to the evaporation of the working solution. Specifically, when a water droplet is placed onto one end of a modified porous carbon or semiconducting material film, it infiltrates the film via capillary action. Since the film is not sealed and is open to the ambient environment, the water evaporates into the air, creating a dynamic solid-liquid-gas three-phase contact interface.  
Different from the \"ionovoltaic\" or \"pseudo-streaming\" mechanisms discussed previously, the evaporating potential mechanism suggests that it is the evaporation at the three-phase interface that creates the potential difference, thus generating the output voltage. [PERSON]'s group first demonstrated a device using ethanol as the working solution to obtain more than 1 V open-circuit voltage [21]. Later, they successfully enhanced the voltage to as high as 5 V using a semiconducting material and water as the working solution [57].  
We note that the evaporating potential mechanism is fundamentally different from the streaming potential mechanism. In the streaming potential mechanism, electricity is generated by the movement of the electric double layer caused by pressure-driven flow. In contrast, the evaporating potential mechanism suggests water molecules become charged by their interaction with the device material. This interaction leads to either electron donation or acceptance, depending on the surface functional groups of the material. As these charged water molecules evaporate, they induce charge movement within the material, thus creating a voltage potential across the film (Figure 6).  
The evaporating potential mechanism successfully explains the puzzle discussed in the streaming potential mechanism--specifically, why electricity is generated even when the film electrode is sealed, blocking the solution flow. Despite being promising, challenges remain. Although first-principle simulations provide theoretical support, there is no direct evidence confirming that evaporated water molecules are really charged. Can we obtain direct evidence that the evaporated water molecules are charged? Additionally, although a mathematical equation has been proposed, its practical meaning is limited, resembling more of a linear regression of data fitting rather than a physically meaningful model. Therefore, further study is needed to develop a comprehensive understanding of evaporation-induced electricity. Advanced characterization techniques and studies are essential to provide direct evidence to complement this mechanism.  
Figure 6: Evaporating potential mechanism. Evaporation causes water molecules to become charged and dissipate into the ambient environment, leading to oppositely charged material. Reproduced from ref. [21] with permission from Elsevier, copyright 2022.