Sustainable power generation for at least one month from ambient humidity using unique nanofluidic diode

ARTICLE https://doi.org/10.1038/s41467-022-31067-z OPEN Sustainable power generation for at least one month from ambient humidity using unique nanofluidic diode 1 ✉, Kedong Shang 1, Xulei Lu 1, Fengmei Guo 1, Zhongbao Jiang 2, Yuanyuan Shang 1, Jian Zhou 2, 1, Chunqiao Fu1 & 1234567890():,; Yong Zhang 1, Tingting Yang Shulong Chang 2, Licong Cui Qi-Chang He 1,3 ✉ The continuous energy-harvesting in moisture environment is attractive for the development of clean energy source. Controlling the transport of ionized mobile charge in intelligent nanoporous membrane systems is a promising strategy to develop the moisture-enabled electric generator. However, existing designs still suffer from low output power density. Moreover, these devices can only produce short-term (mostly a few seconds or a few hours, rarely for a few days) voltage and current output in the ambient environment. Here, we show an ionic diode–type hybrid membrane capable of continuously generating energy in the ambient environment. The built-in electric field of the nanofluidic diode-type PN junction helps the selective ions separation and the steady-state one-way ion charge transfer. This directional ion migration is further converted to electron transportation at the surface of electrodes via oxidation-reduction reaction and charge adsorption, thus resulting in a continuous voltage and current with high energy conversion efficiency. 1 Tribology Research Institute, School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, PR China. 2 Key Laboratory of Material Physics, Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, PR China. 3 MSME, Univ Gustave Eiffel, CNRS UMR 8208, F-77454 Marne-la-Vallée, France. ✉email: ; NATURE COMMUNICATIONS | (2022)13:3484 | https://doi.org/10.1038/s41467-022-31067-z | www.nature.com/naturecommunications 1 ARTICLE I NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31067-z n recent years, water-based power generators have become a promising power generation technology due to the abundance, cleanliness and sustainability of water. Numerous waterpowered generators using pure liquid water and aqueous solution as energy source have emerged1–7, relying on streaming potential8–13, dragging potential14–16, waving potential17–20 and triboelectric potential1,20–22 etc. However, these devices require continuous or periodic water supplementation, which limits their installation location for practical use. As one form of water, moisture is abundantly present in air. The ubiquity of atmospheric moisture makes the development of moisture-based energy-harvesting technologies promising for solving the energy problem of low-power electronics and Internet of things (IoTs) devices23–26. In one early strategy, water vapor adsorption on porous carbon film with a nonhomogeneous vertical distribution of carboxy groups can generate a concentration gradient of the released H+ ions27–29. But after a short duration of power output (~600 s)29, the device voltage and current collapse since ionized mobile charge diffusion gradually reaches equilibrium, terminating the generation of electricity. Desirably, the power output should not be a transient phenomenon. However, a continuous power output relying on ambient atmospheric moisture remains challenging. Some innovative chemical and structural designs have been proposed to satisfy requirements for continuous electric output. A typical strategy is based on nanofluidic devices, because confined nanospace and capillaries are sensitive to external stimuli30–32 and can interact with water through many unique phenomena such as electric double layer coupling. For example, a power generator using protein nanowires film adopts the process of continuous exchange of water molecules at the solid interfaces to build a self-maintained moisture gradient33. Under the moisture gradient, nanowires with a high density of nanometre-scale pores and surface functional groups facilitate ionization and charge transfer for continuous electric output. Indeed, the open-circuit voltage (VOC) and short-circuit current (ISC) of around 0.5 V and 250 nA are generated. Significantly, the devices maintain a continuous VOC of 0.4–0.6 V for more than 2 months and a continuous current for at least 20 h before selfrecharging. Transpiration-driven electrokinetic power generator adopting a hydrological cycle with the surrounding air is another example34. The incorporation of CaCl2 to collect water vapor from the surrounding environment is crucial to acquiring a stable water supply to form the wet side of a carbon film. The water evaporation facilitates capillary flow from the wet to dry side, which induces a pseudo-streaming current. Meanwhile, a vertical setup causes the gradient distribution of CaCl2 content by gravity. Thus, two asymmetries, i.e., protons and Ca2+ ions, of the conductive nanoporous carbon surfaces are established, driving a continuous electrical output for at least 10 days. The devices exhibit maximum VOC (0.74 V), ISC (22.5 µA) and electric power (2.02 µW) when the film size is 3 cm × 9 cm × 0.12 mm. In addition to common strategies such as chemical modification and microstructure control, some new materials have also been introduced into the field of moisture-based power generation. Polyelectrolyte, which releases free ions (such as protons) under moisture, has been explored as one type of efficient moist-electric generating material. When one side of the polyelectrolyte membrane is under constant moisture feeding, protons gradually migrate to the other side under the proton concentration gradient, offering a maximum VOC (0.8 V), ISC density (100 µA • cm−2)35. However, for long-term measurement, the electric output drops back to zero after 2 days. Bilayer of polyelectrolyte film with heterogeneous distribution of charged mobile ions in moist air can extend the working time of one single device to at least 250 h with VOC of 0.95 V under 25% RH36. Large-scale integration of abundant generator units is even able to offer a VOC of more than 1000 V. However, the generated current output during 150 h 2 shows an obvious decrement (40 nA at the beginning, 6 nA after 6 h, and 2 nA after 150 h). In regard to daily electronic appliances, the life span of several months to several years is the threshold, which requires longer voltage and current output to meet applications in various fields. However, the performance summary of the existing humidityenabled electric generator (HEEG) is shown in Supplementary Table 1, and simultaneous continuous voltage/current output for more than one month has not been realized. Therefore, obtaining membrane materials with new sustained energy conversion mechanism giving rise to high output power density and longterm stability is an urgent need. The following challenges need to be addressed: (1) protons dissociated from water or of ions should maintain stable directio (...truncated)