Bilayer of Polyelectrolyte Films for Spontaneous Power Generation in Air Up to an Integrated 1,000-volt Output

Through the spontaneous adsorption of water molecules in air and induced diffusion of oppositely charged ions, one single heterogeneous moisture enabled electric generator unit can produce a high voltage of about 0.95 V.

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 Environmental-based power generation by converting natural resources into electricity has become a chronic issue due to global warming and increasing energy demand. In this regard, our group and others have designed diverse moisture enabled electric generators (MEGs) to convert the chemical potential energy of water molecules into electricity. The moisture enabled power generation provides a sustainable approach for energy conversion without environmental and regional restrictions. However, up to now, these MEGs can just deliver an open-circuit voltage of below 0.2 V under ambient conditions (25% RH, 25oC), because the single ionic species (positively charged H+) would limit the numbers and types of transferable carrier in these systems. Meanwhile, owing to lacking of scalable technology, pulsed electric output, complicated manufacture of a unit and vast size of integrated device, these adverse limitations lead to difficulties of large-scale integration.

Challenge 1: How to realize high voltage output of MEG under ambient condition?

 A popular saying goes, every coin has two sides. In many cell plasma membranes, lipid bilayer apparently possesses asymmetrical structure. The heterogeneous distribution in the bilayer of charged lipids could readily induce a transmembrane potential. Inspired by asymmetrical lipid bilayer with a considerable transmembrane potential, we design a heterogeneous moist-electric generator (HMEG) with bilayer of polyelectrolyte films (BPFs), which enables asymmetrical distribution and spontaneous diffusion of oppositely charged ions (negatively charged Cl- and positively charged H+) in HMEG under moist air for long time (Figure 1a). Such biomimetic BPF is constructed by the combination of polycation (i.e., polydiallyl dimethyl ammonium, PDDA) with polyanion (i.e., polystyrene sulfonic acid and polyvinyl alcohol, PSSA) film. Accordingly, the electricity generation process of HMEG could be proposed as follows: i) BPF adsorbs water molecules from moist air; ii) H+ and Cl- ions will dissociate from PSSA and PDDA layers, respectively; iii) Dual-ions (H+ and Cl-) will diffuse into the other layer driven by ions concentration difference, inducing the formation of electric potential and current flow. Therefore, dual-charge-carrier system enables to significantly increase the numbers and types of transferable charges in MEG and realize high output under low RH.

 By a simple casting and spraying strategy, the BPF with superior flexibility can be prepared in large area. The BPF as the electricity-generating layer is sandwiched between a pair of conductive carbon tape electrodes for a HMEG unit (Figure 1b). Due to dual-charge-carrier dissociation and diffusion in moist air, a HMEG unit produces a record voltage of about 0.95 V at a low RH of 25% and room temperature for a long term (Figure 1c), and delivers a high voltage of 1.38 V at RH of 85%. The areal maximum power density of HEMG approaches to 76 nW cm-2 and 5.52 mW cm-2 at 25% RH and 85% RH (25oC). Furthermore, when the HMEG is connected with optimal resistance of 20 MW, the output maximum volumetric power density can approach to 0.9 mW cm‒3.

Figure 1. a, Schematic illustration of moisture enabled electric generation in BPF. b, Scheme of a testing HMEG. c, The voltage output of a HMEG is sustained for 258 h under atmospheric environment of 15% ~ 30 % RH and 25 ± 5℃.

Challenge 2: How to complete large-scale integration of units and versatile application?

 Our group at Tsinghua University has a long-standing commitment to moisture enabled power generation and new method of ultra-fast mciro/nano laser fabrication. And the electricity-generating material of BPF features with large-area preparation, mechanical flexibility, arbitrarily tailorable ability. Thus, we develop a sequentially aligned stacking (SAS) strategy for large-scale integration of thousands of HMEG units by fast laser processing (Figure 2). Specifically, the top and bottom electrode arrays along with electricity-generating arrays are automatically machined by laser processing, then the top electrode is directly connected with bottom electrode in next unit, allowing diverse applications with flexible, folded and three-dimensionally deformable functions. The integrated device in large scale can be easily folded into a tiny bulk with several cubic centimeters by origami strategy, which concurrently delivers a hundreds-of-volts voltage and outstanding volumetric voltage up to 43 V cm-3. Moreover, integrated device enables to availably switch connection and controllably generate electricity via Miura-ori origami. The generated voltage is up to about 1,000 V by connecting 1,600 units in series in ambient condition (25% RH, 25oC). The integration of HMEGs achieves an approximately linear output, which could be contributed by the superiority of considerable voltage performance of units, simple device configuration, laser processing with high precision.

Figure 2. Schematic illustration of large-scale integration of HMEG units by sequentially aligned stacking strategy.

 Of importance, such integration device viably realizes versatile and practical applications, including driving a large electronic ink screen and 10 Watt lamp bulb. And we have an honor to collaborate with Dan Xie group at Tsinghua University, who are experts on field effect transistor (FET) devices, and Yanfeng Zhang group at Peking University, who have been researching the synthesis low dimensional nanomaterials. We successfully evaluate the role of HMEG as self-powered source. The HMEGs enables to supply gate voltage source to modulate the switching characteristics of MoS2 channel in a FET. The transfer characteristics of self-powered FET clearly reflect a large modulation of drain current with a high on-to-off current ratio of 105, demonstrating MoS2 FET worked as a typical n-type MOSFET (Metal-oxide-semiconductor is abbreviated as MOS). With above performance, we envisage that the unique HEMG could offer a versatile avenue to promote green and sustainable power generation.

For more information, please see our recent publication in Nature Nanotechnology:

Wang, H., Sun, Y., He, T. et al. Bilayer of polyelectrolyte films for spontaneous power generation in air up to an integrated 1,000 V output. Nat. Nanotechnol. (2021).

Haiyan Wang

Dr, Tsinghua University