# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 2 Current Understandings of Capillary Flow-Related Electricity Generation Mechanisms
### Upstream Proton Diffusion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mechanism** & **Year** & **Key findings** & **Experimental Conditions** & **Performance Metrics** \\ \hline Streaming potential & 2017, [13] & Water evaporation from carbon black sheets & Carnebach sheets (1.0 \(\times\) 2.5 cm\({}^{2}\)) with & Voltage output up to 1 V, stable short-circuit current \\ Streaming potential & 2017, [13] & Streaming potential due to quality-driven & MWCNT electrodes, exposed to ambient & around 150 nA \\  & & water flow is key & humidity (33.5-46.76) & \\ \hline Ion concentration gradient & 2015, [14] & Gradient COF provides power from moisture & Graphene oxide film with gradient oxygen & \\  & with energy conversion efficiency of up to 62\% & content under RT = 70\%, & Voltage output \(\approx\) 35 mV, power density \(\approx\) 42 mV/m\({}^{2}\) \\ \hline Microbial electricity & 2020, [17] & Protein nanowires from Geokhardt & Thin film of protein nanowires (\(\overline{\
u}\) unit thick) & \\  & & valurtonous generate continuous power from geokhardt electrodes under ambient & Voltage output \(\approx\) 05 V, current density \(\approx\) 17 \(\mu\)A/cm\({}^{2}\) & \\  & & ambient moisture for at least 23 h & humidity (45.5-85.76) & \\ \hline Ionovoltaic & 2020, [20] & Water infiltration-induced electricity generation & Process CuO nanowire film (PCND) & Voltage output \(\approx\) 0.45 V, current \(\approx\) 0.23 nA for 40 min \\  & & in CoO nanowire film due to ion-surface interaction & & \\ \hline Pseudo-streaming & 2022, [55] & TICCTs: Mouse-based transportation driven & M/cme-coated cotton fabrics, applied & Voltage output of 0.68 V, current of 120 nA, power \\  & & high pseudo-streaming current due to fast & NaCl solution & density of 30.9 mV/cm\({}^{2}\) \\ \hline \multirow{2}{*}{Evaporative potential} & \multirow{2}{*}{202, [21]} & Ethanol evaporation from carbon nanostructures & \multirow{2}{*}{Proous carbon nanostructured film, ethanol } & \multirow{2}{*}{Voltage output up to 1 V, continuous electricity } \\  & & generates available electricity due to charge & evaporation, ambient conditions & generation for 12+ h. \\ \hline Upstream proton diffusion & 2024, [22] & Proton movement against water flow generates electricity in M/cm/VA & M/cme-PVA composite film, 5 uL DI & Voltage output \(\approx\) 400 mV for 300 min, \\  & & sustained by capillary infiltration & water droplet & peak current \(\approx\) 4 jA \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Representative literature for each hydrovolatical mechanism. In contrast, fewer publications were found for newer mechanisms, such as microbial electricity, ionovoltaic, pseudo-streaming, evaporating potential, and upstream proton diffusion. These mechanisms are still emerging in the field and face challenges related to experimental validation and theoretical complexity, which could explain their limited representation in recent publications. These findings emphasize the substantial opportunity for advancing research in these underexplored mechanisms, especially given their promising potential in sustainable energy applications. To conclude this subsection and suggest future directions, it is evident that a common drawback of current mechanism explanations is the lack of accurate mathematical description and practical physical understanding. Therefore, based on previous excellent works and our own understanding, we propose focusing more on a hybrid model that combines elements of a capacitor and a streaming current source. This hybrid model helps explain both the capacitive behavior and the continuous current generation observed in capillary flow-related photovoltaic systems. In this model, the capacitor component represents the electric double layer (EDL) that forms at the interface between the liquid and the porous solid material. This EDL acts as a charge storage layer, which explains the observed capacitive characteristics, such as the open-circuit voltage in photovoltaic systems.