The ubiquitous and spontaneous phase transitions between liquid and gaseous water contain substantial energy that can be harvested by emerging hydrovoltaic technologies, including evaporation‐induced generators (EIGs) and moisture‐induced generators (MIGs). Featuring the virtues of ubiquity, spontaneity, and direct current output, this emerging technology broadens the technical feasibility of harvesting energy from the natural water cycle, enabling self‐powered and sustainable electronics. Elaborated materials synthesis and innovative device design have been proposed to facilitate efficient water energy harvesting. However, some critical challenges in gaining insight into the mechanisms, power improvements, and practical applications are imperative to be overcome for this nascent yet impactful field. Herein, the evolution and advances of EIGs and MIGs are comprehensively summarized. The basic principles of phase transitions between liquid and gaseous water and the mechanisms of power generation are clarified. Then the current materials systems and strategies to further enhance the device performance are reviewed and thoroughly evaluated. The exemplary applications assembled in EIGs and MIGs are also outlined. Finally, the main challenges and future orientations of this burgeoning technology are presented.

Electricity Generation from Phase Transitions between Liquid and Gaseous Water

Heteroatom-doped porous carbon materials with distinctive surface properties and capacitive behavior have been accepted as promising candidates for supercapacitor electrodes. Currently, the researches mainly focus on developing facile synthetic method and unveiling the structure-activity relationship to further elevate their capacitive performance. Here, the B, N co-doped porous carbon sheet (BN-PCS) is constructed by one-pot pyrolysis of agar in KCl/KHCO3 molten salt system. In this process, the urea acts as directing agent to guide the formation of 2D sheet morphology, and the decomposition of KHCO3 and boric acid creates rich micro- and mesopores in the carbon framework. The specific capacitance of optimized BN-PCS reaches 361.1 F g-1 at a current density of 0.5 A g-1 in an aqueous KOH electrolyte. Impressively, the fabricated symmetrical supercapacitor affords a maximum energy density of 43.5 Wh kg-1 at the power density of 375.0 W kg-1 in 1.0 mol L-1 TEABF4 /AN electrolyte. It also achieves excellent long-term stability with capacitance retention of 91.1% and Columbic efficiency of 100% over 10 000 cycles. This study indicates one-pot molten salt method is effective in engineering advanced carbon materials for high-performance energy storage devices.

Unveiling Superior Capacitive Behaviors of One-Pot Molten Salt-Engineered B, N Co-Doped Porous Carbon Sheets.

Self-chargeable supercapacitor made with MXene-bacterial cellulose nanofiber composite for wearable devices.

Carbon materials play an important role in supercapacitor applications due to their diverse structures, rich surface morphology, excellent chemical stability and highly active surfaces. Currently, there are many reports on...

Molten salt technique for synthesis of carbon-based materials for supercapacitor

Sustainable moisture energy

The water utilization is accompanied with the development of human beings. Whereas, the gaseous moisture is usually regarded as underexploited resources. The advances of highly-efficient hygroscopic materials endow the atmospheric water harvesting an intriguing solution to convert moisture into clean water. The discovery of hygroelectricity, which refers to the charge build-up at material surface dependent on humidity, and the following moisture enabled electric generation (MEG) realizes the energy conversion and directly output electricity. Much progress has been made since then to optimize the MEG performance, pushing forward the applications of MEG into practical level. Herein, the evolvement and development of MEG are systematically summarized in a chronological order. The optimization strategies of MEG are discussed and comprehensively evaluated. Then, the latest applications of MEG are presented, including high-performance powering units and self-powered devices. In the end, a perspective on the future development of MEG is given for inspiring more researchers into this promising area. This article is protected by copyright. All rights reserved.

Moisture Enabled Electricity from Hygroscopic Materials: A New Type of Clean Energy.

Pitch-based carbon materials: a review of their structural design, preparation and applications in energy storage

Hierarchically porous carbons (HPCs) have attracted much attention because of their potential application in carbon-based supercapacitors. Here, we list a facile and green method to obtain HPCs by annealing a coal tar pitch with sodium bicarbonate. The produced gases and sodium carbonate template can guide the formation of carbon products with a multiscale pore structure. Further activation by KOH endows HPC with a surface area of 2851.7 m2 g–1 and a total pore volume of 2.4 cm3 g–1. Accordingly, such an HPC electrode achieves a specific capacitance of 321.5 F g–1 (0.5 A g–1) and retains 230.0 F g–1 (100.0 A g–1). Moreover, the HPC-based symmetric supercapacitor exhibits superior cyclic stability (1.1% capacity decay after 30,000 cycles) in an aqueous electrolyte. In the TEABF4/acetonitrile electrolyte, the assembled device displays a high energy density of 53.0 Wh kg–1 and still retains 77.9% even when the power density is increased from 750.0 to 15,000.0 W kg–1. We believe that the salt-assisted synthesis strategy for HPC paves a green way to developing high-performance supercapacitors. 

Dual-Role Salt-Assisted Construction of Hierarchically Porous Carbon for Supercapacitors with High Energy and Power Density

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Papers

Moisture Enabled Electricity from Hygroscopic Materials: A New Type of Clean Energy.

Pitch-based carbon materials: a review of their structural design, preparation and applications in energy storage

Dual-Role Salt-Assisted Construction of Hierarchically Porous Carbon for Supercapacitors with High Energy and Power Density