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Direct electricity generation from water flow/evaporation, coined hydrovoltaic effect, has recently attracted intense interest as a facile approach to harvest green energy from ubiquitous capillary water flow or evaporation. However, the current hydrovoltaic device is inferior in output power efficiency compared to other renewable energy devices. Slow water evaporation rate and inefficient charge collection at device electrodes are two fundamental drawbacks limiting energy output efficiency. Here, we report a bioinspired hierarchical porous fabric electrode that enables high water evaporation rate, efficient charge collection, and rapid charge transport in nanostructured silicon-based hydrovoltaic devices. Such an electrode can efficiently collect charges generated in nanostructured silicon as well as induce a prompt water evaporation rate. At room temperature, the device can generate an open-circuit voltage (Voc) of 550 mV and a short-current density (Jsc) of 22 μA·cm–2. It can output a power density over 10 μW·cm–2, which is 3 orders of magnitude larger than all those reported for analogous hydrovoltaic devices. Our results could supply an effective strategy for the development of high-performance hydrovoltaic devices through optimizing electrode structures.

Hydrovoltaic devices are proposed as an alternative way to directly generate electricity due to the ubiquity of water and its interaction with specific porous structures. At present, the output power density of the reported device is limited by its low current density arising from the low surface charge density and inferior charge transport capability of the active materials. In this work, an asymmetric structure consisting of positively charged conductive polyaniline (PANI) and negatively charged Ti3C2TXMXene is proposed to build a hydrovoltaic device to achieve high conductivity and surface charge density simultaneously. An extra polyvinyl alcohol layer is utilized between PANI and MXene to reserve the asymmetric structure and maintain a constant voltage output. As a result, a peak current density of 1.8 mA/cm2is achieved, which is 18 times higher than the previous peak current density of the device with an inert electrode. Our work of incorporating an asymmetric structure provides an alternative way to target high-efficiency hydrovoltaic devices with a large current density.

Extensive efforts have been made to collect energy from water to generate electricity. However, producing a high density of electrical power for small mobile electronics is challenging. Triboelectric nanogenerator (TENG), which can convert droplet water into electricity, provides sustainable electrical power for small electronics. In addition, hydrovoltaic device (HD) can harvest energy from water evaporation into electricity for small electronics. Herein, an integrated device aimed at collecting energy from both impinging water droplets and evaporation is proposed by combining a TENG with an HD. An architecture that comprises a fluorinated ethylene propylene (FEP) film as a triboelectric layer with an aluminum electrode is used to collect energy from impinging water droplets. A silicon-based HD with an asymmetrical structure where nanostructured silicon plus hierarchical nanofabric carbon electrode is used to harvest energy from vaporizing water droplets. Silver is used as a mutual electrode to integrate the TENG and the HD to generate electricity by fully using falling water droplet energy. The microstructure is built on the FEP film surface to enlarge the contact area between the droplets and FEP, greatly boosting the output of the TENG with an open-circuit voltage of 200 V and a short circuit current of 60 μA in pulsed mode, respectively. Meanwhile, the HD device yields a consistent open-circuit voltage of 550 mV and a short circuit current density of 30 μA/cm2. This integrated device provides a smart strategy to generate electricity by fully collecting energy from impinging water droplets and evaporation, paving an alternative way to efficiently harvest water energy.

Scavenging energy stored at the water/solid interface into electrical power by the natural water evaporation process provides a promising method to supply sustainable electricity for self-powered electronics. The main barriers constraining its applications are the limited materials availability and ambiguous underlying mechanisms, detrimental to the improvement of output power. Herein, we report a highly flexible and efficient evaporation-induced electricity generator (EIEG) that dexterously exploits the directional water capillary flow inside the silicon nanowires (SiNWs) mesh nanopores. Benefiting from the large surface/volume ratio and high surface potential of nanostructured SiNWs mesh film, an EIEG continuously delivers a high open-circuit voltage of ~1.5 V and a maximum power density of over 160 μW·cm−3, which surpasses the analogous flexible EIEGs. Moreover, the correlation between the output power and capillary flow direction, diffusion length, and velocity as well as the species and ionic strength of various liquids have been systematically explored to identify the mechanism underlying the power generation. This study not only provides an in-depth understanding of water/solid interactions but also spikes a green technique to fabricate flexible generators that tap energy from the copious water reservoir.

Moisture-triggered electricity generator (MEG) harvesting energy from the ubiquity of atmospheric moisture is one of the promising potential candidates for renewable power demand. However, MEG device performance is strongly dependent on the moisture concentration, which results in its large fluctuation of the electrical output. Here, a Janus heterojunction MEG device consisting of nanostructured silicon and hygroscopic polyelectrolyte incorporating hydrophilic carbon nanotube mesh is proposed to enable ambient moisture harvesting and continuous stable electrical output delivery. The nanostructured silicon with a large surface/volume ratio provides strong coupling interaction with water molecules for charge generation. A polyelectrolyte of polydiallyl dimethylammonium chloride (PDDA) can facilitate charge selective transporting and enhance the effectiveness of moisture-absorbing in an arid environment simultaneously. The conductive, porous, and hydrophilic carbon nanotube mesh allows water to be ripped through as well as the generated charges being collected timely. As such, any generated charge carriers in the Janus heterojunction can be efficiently swept toward their respective electrodes, because of the device asymmetric contact. A MEG device continuously delivers an open-circuit voltage of 1.0 V, short-circuit current density of 8.2 μA/cm2, and output power density of 2.2 μW/cm2under an ambient environment (60% relative humidity, 25 °C), which is a record value over the previously reported values. Furthermore, the infrared thermal measurements also reveal that the moisture-triggered electricity generation power is likely ascribed to surrounding thermal energy collected by the MEG device. Our results provide an insightful rationale for the design of device structure and understanding of the working mechanism of MEG, which is of great importance to promote the efficient electricity conversion induced by moisture in the atmosphere.