# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 2 Current Understandings of Capillary Flow-Related Electricity Generation Mechanisms
### Ionovoltaic  
Among the various materials for evaporation-induced electricity, semiconducting materials have successfully produced devices that generate electricity [38; 52]. Initially, streaming potential mechanisms were used to explain the phenomenon, which can account for some instances. However, the streaming potential model requires the channel material to be insulating, a condition not met by semiconducting materials.  
In 2019, a group of Korean researchers utilized porous ZnO thin films to build an evaporation-induced electricity device [53]. They later confirmed the viability of semiconducting materials for this purpose by employing porous CuO to fabricate similar devices [20]. Notably, these devices generated an open-circuit voltage of 0.45 V and a short-circuit current of 0.23 \(\mu\)A using a NaCl solution (Figure 4a). These results cannot be explained by the streaming potential mechanism due to the high conductivity of the solution. Therefore, the researchers proposed a new mechanism, termed the ionovoltaic effect.  
The ionovoltaic effect involves the interaction between ions and the material's surface, leading to a potential asymmetry between wet and dry regions. Briefly, when water infiltrates the porous thin film, hydrated ions (H\({}^{+}\) in this case) from the water adsorb onto the surface of the CuO nanowires. This adsorption consequently changes the carrier concentration on the nanowire surface, creating a potential difference between wet and dry areas of the CuO thin film that drives electricity generation (Figure 4b,c).  
The ionovoltaic mechanism effectively explains phenomena observed in the electricity output measurements [54]. Its theory of contact line movement is novel and opens new possibilities in this field. It successfully explains why NaCl can drive evaporation-induced electricity, which cannot be accounted for by the streaming potential theory. In essence, a high ion concentration in the infiltrating solution leads to a high potential difference between wet and dry areas, resulting in high voltage, as demonstrated by experiments. However, the key to this mechanism is the formation of hole and electron traps in the wet and dry regions, respectively. Currently, there is no direct observation supporting this argument. Additionally, since the potential difference driving the electricity output depends on the contact line movement from wet to dry areas, it is challenging to explain the steady, continuous electricity output. This boundary between wet and dry areas is supposed to disappear when the water infiltrates the entire film, thus diminishing the electricity output.  
Figure 4: Ionovoltaic effect mechanism. (**a**) A water droplet is placed on one end of the porous CuO, and then electric current is observed from the dry to the wet end. (**b**,**c**) Overall picture of the ionovoltaic mechanism based on wet/dry area movement and hole/electron trap theories. Reproduced from ref. [20] with permission from the Royal Society of Chemistry, copyright 2020.  
Therefore, despite the promising aspects of the ionovoltaic mechanism, several challenges and disadvantages remain. The current mechanism description explains observed phenomena in most instances, but future work should focus on developing accurate quantitative analyses to further refine and perfect this theory, particularly in verifying the formation of hole and electron traps resulting from contact line movement.