Nanofluidic diodes capable of generating electricity from ambient humidity for at least one month

Published in Materials
Nanofluidic diodes capable of generating electricity from ambient humidity for at least one month
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Background

Harvesting energy in moisture environment is a promising technique. Previous reported moisture-based power generators basically rely on the free hydrated ions diffusion in the functional materials driven by ion concentration gradient. This concentration gradient is induced by an asymmetrical moisture adsorption and desorption or a functional groups concentration gradient difference inside the materials. However, those devices usually face problems such as transient and low power output. Therefore, there is an urgent need to explore membrane materials with new sustained energy conversion mechanism giving rise to high output power density and long-term stability. The following challenges need to be addressed: (1) protons dissociated from water or ions should maintain stable directional migration; (2) the ion current in the humidity-sensitive materials should be continuously converted to the electronic current under the load resistance.

 

Design strategy

As shown in Fig. 1a, inspired by the form of the semiconductor PN junction in photovoltaic device, we propose a humidity-enabled electric generator (HEEG) device by adopting ionic diode–type PN junction in moist air. This diode-type PN junction exists at the interface of the carbon nanotubes (CNT) films and anodic aluminum oxide (AAO) membrane. As shown in Fig. 1b, 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. 

 Fig. 1. Ionic diode–type hybrid membrane device and the electric performance. a) Schematic diagram of solar photovoltaic power generation with PN junction, illustration of hydrovoltaic power generation principle inspired by photovoltaic effect. b) Schematic diagram of the working principle for HEEG, the darker the color, the greater the humidity.

 

Output performance

When the bottom electrode selects liquid metal, such a nanofluidic diode-based single unit can deliver a VOC of 1.1 V and an ISC of 7.7 μA under 93% RH, 25℃. The maximum short-circuit-current density is 11.3 μA·cm-2 when reducing the membrane size to 1 mm2, and the corresponding output power density is 1.3μW·cm-2 / 277μW·cm-3. Replacing the bottom electrode with a more active metal, such as Al, will further increase the VOC by 40% and the ISC by 390%. In longer-term monitoring, our fabricated membrane shows a stable VOC of 0.8-1.1V for at least a month, and ISC undergoes gradual attenuation from 1500nA to 100nA after 1 month. The degradation in current is possibly associated with either the loss of oxide groups of CNT in the ambient relative humidity or the gradual passivation of bottom electrode metal. 

 

Potential applications

A few HEEG devices can light up LED lights and electronic watches. Since different levels of humidity have a great influence on the performance output of the device, the HEEG can also be used in self-powered breathing monitoring scenarios. The high-performance and long-term electrical output endow the HEEG device with great development prospects in the Internet of things (IoT) field. Fig. 2 illustrates the potential applications of the HEEG devices.

 

Fig. 2. Application of HEEG devices. a) Combination of 18 devices in series and parallel connection can light up ten LEDs and power up a digital electronic watch directly under laboratory environment ~65%RH, 25℃. b) Self-powered breathing monitoring application and its signal display, the bottom is the test schematic diagram.

 

Advantages and outlook

The designed device holds the following six salient features: 

  • The built-in potential formed at the CNT/AAO interface facilitates the ionization and transportation of hydrated ions. And ordered AAO fluid channel lowers the transport resistance of the ion charge, thus further improving the output power. 
  • For inert metal bottom electrodes, there is no major redox reaction and the ion migration gives riseto electron transportation via charge adsorption. For active metal, electrodes participate in the partially reversible redox reactions. Similar to the electrode reactions of metal-air batteries. 
  • Water molecules continuously adsorb, ionize, and selectively transport due to the synergy between directed ion migration and redox reaction, guaranteeing a continuous flow of ion charge which boosts sustainable power output after an effective conversion between ion current and electron current. 
  • The strong hydrogen bonds and van der Waals forces formed at the CNT/AAO interface endow the hybrid membrane with excellent robustness in humid environment, and such robust structure enables steady-state ion transportation and guarantees the long-term stability. 
  • The raw materials have mature preparation technologies, good cost-effectiveness and wide sources, and the devices are suitable for large-scale manufacturing. At the same time, the product will not bring any environmental pollution and recycling problems during the manufacturing and working process. 
  • When stored in a climate-controlled facility with low humidity, ion production and transport are limited, so there is little self-discharge, ensuring a long shelf life unlike typical batteries.

 The integration of ionic diode-type hybrid membrane design into the nanofluid energy conversion system provides a new idea and method towards long-term stable power generation in an atmosphere moisture environment. Our work also promotes a better understanding of ion transport in confined nanospace and provides a universal effective way for the power supply of low-power IoT devices. For more details of this work, please see our recent article published in Nature Communications:

Yong Zhang, Tingting Yang*, Kedong Shang, Fengmei Guo, Yuanyuan Shang,  Shulong Chang, Licong Cui, Xulei Lu, Zhongbao Jiang, Jian Zhou,  Chunqiao Fu & Qi-Chang He*. Sustainable power generation for at least one month from ambient humidity using unique nanofluidic diode. Nature Communications 2022, 13:3484

 https://doi.org/10.1038/s41467-022-31067-z

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