Hydrovoltaic Electricity Generator with Hygroscopic Materials: A Review and New Perspective.

The global energy crisis caused by the overconsumption of nonrenewable fuels has prompted researchers to develop alternative strategies for producing electrical energy. In this review, a fascinating strategy that simply utilizes water, an abundant natural substance throughout the globe and even in air as moisture, as a power source is introduced. The concept of the hydrovoltaic electricity generator (HEG) proposed herein involves generating an electrical potential gradient by exposing the two ends of the HEG device to dissimilar physicochemical environments, which leads to the production of an electrical current through the active material. HEGs, with a large variety of viable active materials, have much potential for expansion toward diverse applications including permanent and/or emergency power sources. In this review, representative HEGs that generate electricity by the mechanisms of diffusion, streaming, and capacitance as case studies for building a fundamental understanding of the electricity generation process are discussed. In particular, by comparing the use and absence of hygroscopic materials, HEG mechanism studies to establish active material design principles are meticulously elucidated. The review with future perspectives on electrode design using conducting nanomaterials, considerations for high performance device construction, and potential impacts of the HEG technology in improving the livelihoods are reviewed.

Multifunctional Filter Membranes Based on Self-Assembled Core-Shell Biodegradable Nanofibers for Persistent Electrostatic Filtration through the Triboelectric Effect.

The massive production of polymer-based respiratory masks during the COVID-19 pandemic has rekindled the issue of environmental pollution from nonrecyclable plastic waste. To mitigate this problem, conventional filters should be redesigned with improved filtration performance over the entire operational life while also being naturally degradable at the end. Herein, we developed a functional and biodegradable polymeric filter membrane consisting of a polybutylene adipate terephthalate (PBAT) matrix blended with cetyltrimethylammonium bromide (CTAB) and montmorillonite (MMT) clay, whose surface properties have been modified through cation exchange reactions for good miscibility with PBAT in an organic solvent. Particularly, the spontaneous evolution of a partial core-shell structure (i.e., PBAT core encased by CTAB-MMT shell) during the electrospinning process amplified the triboelectric effect as well as the antibacterial/antiviral activity that was not observed in naive PBAT. Unlike the conventional face mask filter that relies on the electrostatic adsorption mechanism, which deteriorates over time and/or due to external environmental factors, the PBAT@CTAB-MMT nanofiber membrane (NFM)-based filter continuously retains electrostatic charges on the surface due to the triboelectric effect of CTAB-MMT. As a result, the PBAT@CTAB-MMT NFM-based filter showed high filtration efficiencies (98.3%, PM0.3) even at a low differential pressure of 40 Pa or less over its lifetime. Altogether, we not only propose an effective and practical solution to improve the performance of filter membranes while minimizing their environmental footprint but also provide valuable insight into the synergetic functionalities of organic-inorganic hybrid materials for applications beyond filter membranes.

Vertically and Horizontally Drawing Formation of Graphite Pencil Electrodes on Paper by Frictional Sliding for a Disposable and Foldable Electronic Device.

The objective of this study is to fabricate an electrode by frictional sliding caused by a rough paper surface. The pressure exerted during drawing induces adsorption of the graphite particles by the rough paper and simultaneously reduces the surface roughness of the paper electrode. Repetitive drawing in one-way direction reduced the roughness of the paper surface, decreasing the grain boundaries of graphite. This increases the electron pathway at the electrode, thus reducing the resistance to less than 50 Ω. At the same time, repetitive drawing could confirm that unstable errors caused by the hand could help converge within a certain margin of error. We quantified the relationship between pressure and resistance when drawing on the electrode using a pencil hardness tester. In addition, the electrodes formed by repeated drawing generated a new surface grain and boundary, parallel to the drawing direction, and changed the electrode characteristics with respect to the drawing direction. The grain boundary difference based on the drawing direction was measured via a heating test of the foldable device, a sound pressure level, and laser scattering vibrometer measurements of a linear speaker. The fabricated graphite electrodes can be used in disposable foldable paper electronics because they are prepared using inexpensive materials.