# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 2 Current Understandings of Capillary Flow-Related Electricity Generation Mechanisms
### Microbial Electricity  
Evaporation-induced electricity requires a water reservoir to maintain the evaporation water flow. In comparison, moisture-induced electricity only requires ambient humidity, thus providing versatility advantage. However, its electricity output features an electric current pulse due to the ion concentration gradient mechanism. Once the ion concentration gradient is erased by ion movement from high to low concentration, the external electric current ceases naturally. Therefore, achieving continuous electricity output for moisture-induced electricity warrants further study. In 2020, continuous electricity output from ambient humidity was realized by employing protein nanowires produced from the microbe _Geobacter sulfurreducens_[17]. This thin-film device maintained a continuous open-circuit voltage of 0.5 V for over two months, representing a groundbreaking achievement for moisture-induced electricity (Figure 3a,b). The proposed mechanism is similar to the ion concentration gradient, suggesting that the nanowires maintain a moisture gradient, thus enabling continuous electricity (Figure 3c,d). Following this advancement, the same research group demonstrated improved device performance and practical applications based on the microbe-produced protein nanowires [49; 50; 51]. For example, [PERSON] et al. demonstrated that microbial biofilms could produce a power density of approximately 1 \(\upmu\)W/cm\({}^{2}\), comparable to that achieved by microbial fuel cells, providing a viable and sustainable approach for biofilm-based electricity generation [50]. This finding opens new avenues for scalable biofilm-based devices in practical applications. Recently, [PERSON] et al. introduced the concept of the Air-Gen effect, where nanoporous materials facilitate sustainable energy harvesting from air humidity. The work showed that such an effect could be leveraged in various biological materials, including microbial biofilms, by allowing dynamic adsorption and desorption at the porous interface, resulting in a sustained charging gradient [51]. This generic effect demonstrates that the structure of microbial biofilms can potentially be optimized to enhance energy output for microbial electricity generation. Then, a logical question arises: where does this energy come from? Although the exact mechanism remains elusive, we can conclude that the energy source in evaporation-induced electricity is latent heat driving the water phase change from liquid to vapor, while the energy source in conventional moisture-induced electricity is from the chemical potential originating from the material's chemical heterogeneity. However, in the case of protein nanowires, environmental humidity does not change, and the moisture within the nanowire thin film should saturate within a short period of time. Then, where is the energy potential driving this continuous electricity output? This device seems to suggest a perpetual motion machine coming into reality, which would violate the second law of thermodynamics. In 2022, a similar device demonstrated evaporation-induced electricity. Instead of using protein nanowires produced from _Geobacter sulfureducens_, the researchers directly utilized the microbe to form a biofilm-based device. The key output parameters, such as voltage and current, were comparable to those of the protein nanowire-based device [37]. This similarity naturally raises the question of whether the electro-active microbe itself, rather than the protein nanowire, is driving the electricity output. After all, this type of microbe is indeed responsive to electrical stimuli [18]. Be that as it may, the authors who prepared the protein nanowire-based device argued that the microbes were filtered out during the device preparation process. Therefore, to elucidate the mechanism behind microbial moisture-induced electricity, future research should specifically study whether the protein nanowires or the electro-active microbes play the crucial role. Figure 3: Microbial electricity mechanism.