# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 2 Current Understandings of Capillary Flow-Related Electricity Generation Mechanisms
### Upstream Proton Diffusion
This highlights the importance of distinguishing between genuine hydrovoltaic generation and artifacts arising from electrode reactions. To provide a more comprehensive overview of the progress in hydrovoltaic mechanisms, we have compiled a comparative table (Table 2) that summarizes key recent studies for each mechanism. This table highlights representative publications, their key findings, experimental conditions, and performance metrics. The intention is to give readers a clearer picture of the advancements and research focus within each mechanism. The inclusion of this comparative table complements the discussion of advantages and disadvantages provides a structured comparison of experimental outcomes. To provide a focused overview of the current research landscape for hydrovoltaic mechanisms, we analyzed publication trends based on article-type papers published in 2024, retrieved from the Scopus database (Figure 8). The data reveal that the majority of recent research in this field concentrates on streaming potential and ion concentration gradient mechanisms, each featuring over 70 publications. This trend reflects a continued interest in these well-established mechanisms, likely due to their extensive theoretical and practical background. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mechanisms** & **Advantages** & **Disadvantages** & **Objective** & **Key References** \\ \hline Streaming potential & Successfully explains electricity generation & Difficult to explain the electrode sealing issue & Understanding the role of material porosity in enhancing the streaming potential effect & [13; 40] \\ \hline Ion concentration gradient & Clear physical picture of ion movement electricity generation & Difficult to explain continuous electricity generation & Studying ion-diode-like structures to improve efficiency and continuity & [14; 61] \\ \hline Microbial electricity & Continuous electricity generation & Difficult to explain the source of the energy input & Evaluating the use of microbial biofilms for sustainable electricity generation & [17; 37] \\ \hline Ionovoltaic & Successfully explains the electrode sealing issue & Lack of direct evidence for ionovoltaic effect & Exploring potential ionovoltaic mechanisms and device structures & [20] \\ \hline Pseudo-streaming & Successfully explains the electrode sealing issue & Lack of direct evidence for pseudo-streaming effect & Providing theoretical insights into pseudo-streaming effects in porous materials & [19; 55] \\ \hline Evaporative potential & Successfully explains the electricity generation with traditionally viewed nonworking solution, ethanol & Lack of accurate mathematical description & Investigating the interaction between evaporation and surface properties to create electricity & [21] \\ \hline Upstream proton diffusion & Successfully explains the novel electricity generation opposite to the traditional view & Difficult to explain the concept of nanochannels & Studying the role of nanochannels in upstream proton diffusion mechanisms & [22] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of current understandings of capillary flow-related electricity generation mechanisms.