# Progress of Capillary Flow-Related Hydrovoltaic Technology: Mechanisms and Device Applications
## 1 Introduction  
Carbon emissions present a significant threat to global civilization. Without substantial efforts to mitigate them, many Sustainable Development Goals (SDGs) that are already experiencing setbacks, such as SDG 13 (Climate Action) and SDG 7 (Affordable and Clean Energy), will fail to meet the 2030 targets [1, 2]. Among many innovative technologies aimed at achieving multiple SDGs, an innovative water energy, hydrovoltaic technology, holds great potential. This technology can greatly contribute to various goals by not only promoting sustainable energy (SDG 7) but also enhancing water security (SDG 6), reducing poverty (SDG 1), and improving the quality of life in rural areas (SDG 11). The interlinkages between these goals underscore the importance of a holistic approach to sustainability [3]. However, action is needed to translate lab results into real-world applications to fully realize these benefits [4, 5]. Traditionally, when people talk about water energy, we first think about water dams. In a water dam, the mechanical energy of water drives turbines, thereby generating electricity. The emerging hydrovoltaic effect focuses on ubiquitous properties of water, such as water evaporation and moisture absorption. This novel technology leverages the inherent physicaland chemical properties of water to generate energy, representing a significant paradigm shift from traditional hydroelectric dams. In a broad sense, the hydrovoltaic effect, encompasses various subsets within this flourishing research field. For example, twistron generators in water [6], droplet generators [7], triboelectricity-related nanogenators [8; 9], and osmotic energy generators [10; 11; 12]. Each utilizes a unique mechanism to harvest water energy, demonstrating the diversity and innovative vitality in this field. In this review, we aim to focus on the capillary flow-related hydrovoltaic effect, an emerging and flourishing field in recent years. The capillary flow phenomenon is prevalent in the natural environment. In short, it is the material surface tension in a confined space that drives the movement of water. Capillary flow is widely observed in water evaporation and moisture absorption-induced water transport in absorptive materials. Therefore, the capillary flow-related hydrovoltaic effect, i.e., water evaporation and moisture-induced electricity, features a commonly found energy source, providing broad applicability. Pioneering findings on water evaporation-induced electricity [13] and moisture-induced electricity [14] have only been discovered within the past decade. Since then, significant progress has been made; key voltage and current parameters have increased by more than 100-fold [15]. However, the mechanism of electricity generation in this capillary flow-related hydrovoltaic effect is still under active debate. Quite a few mechanisms have been proposed since the discovery of this fascinating phenomenon, such as traditional theories like streaming potential [13] and ion concentration gradient flow [16], more recent concepts of microbial electricity [17; 18], pseudo-streaming [19], ionovoltaic effect [20], evaporating potential [21], and the latest theory of upstream proton diffusion [22]. With the emergence of the capillary flow-related hydrovoltaic technology field, researchers worldwide have published many excellent reviews. Among them, some are great tutorials, providing readers, especially newcomers to this field, with a general sense of the field [23; 24; 25]. Others offer in-depth insights, covering topics from device physics to material chemistry, thereby presenting readers with a comprehensive picture of the field [26; 27]. Moreover, some reviews specifically focus on moisture-induced electricity [28; 29], while others emphasize on water evaporation-induced electricity [30; 31]. However, a review focused on electricity generation mechanisms is rare. Therefore, in this review, we aim to focus specifically on various mechanisms of capillary flow-related electricity generation.