# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 2 Current Understandings of Capillary Flow-Related Electricity Generation Mechanisms
### Upstream Proton Diffusion  
A recent study reports a surprising discovery for capillary flow-generated electricity, leading to a new mechanism called upstream proton diffusion. The device is made of MXene/poly(vinyl alcohol) (PVA) film (MPCF) featuring densely packed nanochannels, which theoretically should hinder water infiltration and permeation. According to the streaming potential theory, when a water droplet is placed onto one end of the MPCF, the material's functional groups should hydrolyze, releasing hydrated protons that move to the other electrode, driven by the capillary water flow. Therefore, the external current should flow from the water transport end electrode to the starting electrode, termed the downstream direction of the current (Figure 7a). However, the researchers observed the opposite: the external circuit flowed in the upstream direction, from the droplet placement electrode to the end electrode. They also recorded a steady open-circuit voltage of 400 mV for nearly an hour with a 5 \(\upmu\)L water droplet. The researchers attributed the upstream current flow direction to the slow speed of water flow within the MPCF, which they referred to as \"sluggish water flow\". Unlike fast water flow that drags hydrated protons and induces downstream current flow, the densely packed film results in slow water flow. Hence, dissociated protons from the material's functional groups concentrate near the droplet placement electrode, rather than flowing to the end electrode with the water. This concentration of protons induces an electron current in the external circuit, leading to the upstream current (Figure 7b). In the upstream mechanism, the water permeation speed within the MPCF must be slow to allow protons to concentrate near the electrode and sustain electricity generation. If the film is loosely packed, the mechanism reverts to the conventional streaming potential mechanism driven by fast water flow. Indeed, this newly discovered phenomenon opens new possibilities for the hydro-voltaic effect. However, challenges remain. Firstly, the concept of \"nanochannel\" is still elusive and awaits direct evidence. Since its first mention in porous carbon [13], nanochannels have been referenced in the classic streaming potential model. However, to date, no direct evidence, apart from molecular simulations, proves that water molecules actually transport through nanochannels. Given the hydraulic resistance against water flow induced  
Figure 7: Upstream proton diffusion. (**a**) Traditional streaming potential mechanism. (**b**) Upstream proton diffusion causes the external electric current to move in an opposite direction. A in the figure represents the electric current (amperes) measured during the process. Reproduced from ref. [59] with permission from Nature Publishing Group, copyright 2024.  
by nanometer-scale confinement, it is practically impossible for water to flow into such narrow channels solely by capillary action [40]. Previously, this kind of superfluid flow was always induced by external pressure with sophisticated instruments [10; 11]. Secondly, there is still no mathematical description of this phenomenon. Nevertheless, the newly proposed upstreaming mechanism highlights the extreme complexity behind capillary flow-related electricity, inviting more researchers from various fields to challenge current understanding and explore new opportunities for sustainable electronics. In Table 1, we summarize these mechanisms in terms of their advantages and disadvantages for a clear understanding. It is also worth noting that some reported hydrovoltaic effects may actually be due to differences in the standard electrode potential of the materials used, as pointed out by [PERSON] et al. [60]. Their work suggests that the hydrovoltaic effect and contributions from active metal or impure carbon electrodes are often confused, implying that certain reported devices function more like primary batteries rather than utilizing true hydrovoltaic mechanisms.