Gels for Flexible Electronics and Energy Devices

A special issue of Gels (ISSN 2310-2861). This special issue belongs to the section "Gel Applications".

Deadline for manuscript submissions: closed (31 March 2024) | Viewed by 16134

Special Issue Editors


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Guest Editor
State Key Laboratory of Organic Electronics and Information Displays (SKLOEID), Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications, Nanjing 210023, China
Interests: flexible electronics; printed electronics; organic light-emitting devices; organic lasers; organic semiconductors
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Guest Editor
Institute of Advanced Materials and Flexible Electronics (IAMFE), School of Chemistry and Materials Science, Nanjing University of Information Science and Technology, Nanjing 210044, China
Interests: printed electronics; flexible electrochemical energy storage; MXene; hydrogels; 3D printing
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

It is our great pleasure to invite you to contribute to this Special Issue of Gels on “Gels for Flexible Electronics and Energy Devices”.

The field of flexible electronics has been undergoing booming development, showing promise to revolutionalize healthcare, the internet of things, and everyday electronics, hallmarked by products such as electronic skins, soft robots, and human-machine interfaces. Gels, or more specifically, hydrogels, undoubtedly, play a key role as materials for such applications due to their high biocompatibility, mechanical compliances, widely tunable properties, and interesting charge transport behaviors. These characteristics make gels good sensing units for flexible and stretchable pressure/strain sensors, electrolytes and separators that enable flexible electrochemical energy storage devices such as supercapacitors and batteries, active materials in novel flexible generators that harvest energy from ambient environments, to name a few. The exponential growth in publications indicates the popularity of these lines of studies, thus we see it as an opportune moment to launch a special issue that focuses on these new applications (electronic, sensing, energy storage and harvesting) of gels. Both research and review works that are related to this topic are welcome. We look forward to seeing your valuable contributions soon!

Prof. Dr. Wen-Yong Lai
Prof. Dr. Yi-Zhou Zhang
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Gels is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • hydrogels
  • flexible sensors
  • strain/pressure sensors
  • gel electrolytes
  • flexible batteries
  • flexible supercapacitors
  • energy harvesting

Published Papers (10 papers)

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Research

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17 pages, 4950 KiB  
Article
Electrochemical Performance of Symmetric Solid-State Supercapacitors Based on Carbon Xerogel Electrodes and Solid Polymer Electrolytes
by Boryana Karamanova, Emiliya Mladenova, Minju Thomas, Natalia Rey-Raap, Ana Arenillas, Francesco Lufrano and Antonia Stoyanova
Gels 2023, 9(12), 983; https://doi.org/10.3390/gels9120983 - 15 Dec 2023
Viewed by 1040
Abstract
For the development and optimization of solid-state symmetrical supercapacitors, herein, we propose using carbon-based electrodes and sodium- and lithium-form Aquivion electrolyte membranes, which serve as the separator and electrolyte. Carbon xerogels, synthesized using microwave-assisted sol-gel methodology, with designed and controlled properties were obtained [...] Read more.
For the development and optimization of solid-state symmetrical supercapacitors, herein, we propose using carbon-based electrodes and sodium- and lithium-form Aquivion electrolyte membranes, which serve as the separator and electrolyte. Carbon xerogels, synthesized using microwave-assisted sol-gel methodology, with designed and controlled properties were obtained as electrode materials. Commercial activated carbon (YP-50F, “Kuraray Europe” GmbH) was used as the active material for comparison. Notably, the developed solid-state symmetrical supercapacitors provide sufficiently high specific capacitances of 105–110 F g−1 at 0.2 A g−1, along with an energy density of 4.5 Wh kg−1 at 300 W kg−1, and a voltage window of 0–1.2 V in aqueous environments, also demonstrating an excellent cycling stability for up to 10,000 charge/discharge cycles. These results can demonstrate the potential applications of carbon xerogel as the active electrode material and cation exchange membrane as the electrolyte in the development of solid-state supercapacitor devices. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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15 pages, 4263 KiB  
Article
Flexible Polymer-Ionic Liquid Films for Supercapacitor Applications
by Christo Novakov, Radostina Kalinova, Svetlana Veleva, Filip Ublekov, Ivaylo Dimitrov and Antonia Stoyanova
Gels 2023, 9(4), 338; https://doi.org/10.3390/gels9040338 - 16 Apr 2023
Cited by 3 | Viewed by 1202
Abstract
Mechanically and thermally stable novel gel polymer electrolytes (GPEs) have been prepared and applied in supercapacitor cells. Quasi-solid and flexible films were prepared by solution casting technique and formulated by immobilization of ionic liquids (ILs) differing in their aggregate state. A crosslinking agent [...] Read more.
Mechanically and thermally stable novel gel polymer electrolytes (GPEs) have been prepared and applied in supercapacitor cells. Quasi-solid and flexible films were prepared by solution casting technique and formulated by immobilization of ionic liquids (ILs) differing in their aggregate state. A crosslinking agent and a radical initiator were added to further stabilize them. The physicochemical characteristics of the obtained crosslinked films show that the realized cross-linked structure contributes to their improved mechanical and thermal stability, as well as an order of magnitude higher conductivity than that of the non-crosslinked ones. The obtained GPEs were electrochemically tested as separator in symmetric and hybrid supercapacitor cells and showed good and stable performance in the investigated systems. The crosslinked film is suitable for use as both separator and electrolyte and is promising for the development of high-temperature solid-state supercapacitors with improved capacitance characteristics. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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20 pages, 22243 KiB  
Article
Conducting ITO Nanoparticle-Based Aerogels—Nonaqueous One-Pot Synthesis vs. Particle Assembly Routes
by Samira Sang Bastian, Felix Rechberger, Sabrina Zellmer, Markus Niederberger and Georg Garnweitner
Gels 2023, 9(4), 272; https://doi.org/10.3390/gels9040272 - 25 Mar 2023
Viewed by 1201
Abstract
Indium tin oxide (ITO) aerogels offer a combination of high surface area, porosity and conductive properties and could therefore be a promising material for electrodes in the fields of batteries, solar cells and fuel cells, as well as for optoelectronic applications. In this [...] Read more.
Indium tin oxide (ITO) aerogels offer a combination of high surface area, porosity and conductive properties and could therefore be a promising material for electrodes in the fields of batteries, solar cells and fuel cells, as well as for optoelectronic applications. In this study, ITO aerogels were synthesized via two different approaches, followed by critical point drying (CPD) with liquid CO2. During the nonaqueous one-pot sol–gel synthesis in benzylamine (BnNH2), the ITO nanoparticles arranged to form a gel, which could be directly processed into an aerogel via solvent exchange, followed by CPD. Alternatively, for the analogous nonaqueous sol–gel synthesis in benzyl alcohol (BnOH), ITO nanoparticles were obtained and assembled into macroscopic aerogels with centimeter dimensions by controlled destabilization of a concentrated dispersion and CPD. As-synthesized ITO aerogels showed low electrical conductivities, but an improvement of two to three orders of magnitude was achieved by annealing, resulting in an electrical resistivity of 64.5–1.6 kΩ·cm. Annealing in a N2 atmosphere led to an even lower resistivity of 0.2–0.6 kΩ·cm. Concurrently, the BET surface area decreased from 106.2 to 55.6 m2/g with increasing annealing temperature. In essence, both synthesis strategies resulted in aerogels with attractive properties, showing great potential for many applications in energy storage and for optoelectronic devices. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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9 pages, 5214 KiB  
Communication
Transparent Gelation of Ionic Liquids Trapped in Silicone Microcup Structures under Scanning Electron Microscopy
by Kaede Iwasaki and Masayuki Okoshi
Gels 2023, 9(3), 179; https://doi.org/10.3390/gels9030179 - 24 Feb 2023
Cited by 1 | Viewed by 982
Abstract
It is expected that ionic liquids will be used in the future as electrolytes for electric double layer capacitors, but currently microencapsulation with a conductive or porous shell is required for their fabrication. Here, we succeeded in fabricating a transparently gelled ionic liquid [...] Read more.
It is expected that ionic liquids will be used in the future as electrolytes for electric double layer capacitors, but currently microencapsulation with a conductive or porous shell is required for their fabrication. Here, we succeeded in fabricating a transparently gelled ionic liquid trapped in hemispherical silicone microcup structures just by observing with a scanning electron microscope (SEM), which allows the microencapsulation process to be eliminated and electrical contacts to be formed directly. To see the gelation, small amounts of ionic liquid were exposed to the SEM electron beam on flat aluminum, silicon, silica glass, and silicone rubber. The ionic liquid gelled on all the plates, and a color change to brown was observed on all the plates except for silicone rubber. This change might be caused by reflected and/or secondary electrons from the plates producing isolated carbon. Silicone rubber could remove the isolated carbon due to the large amount of oxygen inside it. Fourier transform infrared spectroscopy revealed that the gelled ionic liquid included a large amount of the original ionic liquid. Moreover, the transparent, flat gelled ionic liquid could also be made into three-layer structures on silicone rubber. Consequently, the present transparent gelation is suitable for silicone rubber-based microdevices. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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20 pages, 5911 KiB  
Article
Efficient Tuning of the Opto-Electronic Properties of Sol–Gel-Synthesized Al-Doped Titania Nanoparticles for Perovskite Solar Cells and Functional Textiles
by Qana A. Alsulami, Zafar Arshad, Mumtaz Ali and S. Wageh
Gels 2023, 9(2), 101; https://doi.org/10.3390/gels9020101 - 24 Jan 2023
Cited by 6 | Viewed by 1799
Abstract
The efficient electron transport layer (ETL) plays a critical role in the performance of perovskites solar cells (PSCs). Ideally, an unobstructed network with smooth channels for electron flow is required, which is lacking in the pristine TiO2-based ETL. As a potential [...] Read more.
The efficient electron transport layer (ETL) plays a critical role in the performance of perovskites solar cells (PSCs). Ideally, an unobstructed network with smooth channels for electron flow is required, which is lacking in the pristine TiO2-based ETL. As a potential solution, here we tuned the structure of TiO2 via optimized heteroatom doping of Al. Different concentrations (1, 2, and 3 wt%) of Al were doped in TiO2 and were successfully applied as an ETL in PSC using spin coating. A significant difference in the structural, opto-electronic, chemical, and electrical characteristics was observed in Al-doped TiO2 structures. The opto-electronic properties revealed that Al doping shifted the absorption spectra toward the visible range. Pure titania possesses a bandgap of 3.38 eV; however, after 1, 2, and 3% Al doping, the bandgap was linearly reduced to 3.29, 3.25, and 3.18 eV, respectively. In addition, higher light transmission was observed for Al-doped TiO2, which was due to the scattering effects of the interconnected porous morphology of doped-TiO2. Al-doped titania shows higher thermal stability and a 28% lower weight loss and can be operated at higher temperatures compared to undoped titania (weight loss 30%) due to the formation of stable states after Al doping. In addition, Al-doped TiO2 showed significantly high conductivity, which provides smooth paths for electron transport. Thanks to the effective tuning of band structure and morphology of Al-doped TiO2, a significant improvement in current densities, fill factor, and efficiency was observed in PSCs. The combined effect of better Jsc and FF renders higher efficiencies in Al-doped TiO2, as 1, 2, and 3% Al-doped TiO2 showed 12.5, 14.1, and 13.6% efficiency, respectively. Compared to undoped TiO2 with an efficiency of 10.3%, the optimized 2% Al doping increased the efficiency up to 14.1%. In addition, Al-doped TiO2 also showed improvements in antibacterial effects, required for photoactive textiles. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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13 pages, 3819 KiB  
Article
Mechanically Interlocked Hydrogel–Elastomer Strain Sensor with Robust Interface and Enhanced Water—Retention Capacity
by Wenyu Zhao, Zhuofan Lin, Xiaopu Wang, Ziya Wang and Zhenglong Sun
Gels 2022, 8(10), 625; https://doi.org/10.3390/gels8100625 - 30 Sep 2022
Cited by 2 | Viewed by 2092
Abstract
Hydrogels are stretchable ion conductors that can be used as strain sensors by transmitting strain-dependent electrical signals. However, hydrogels are susceptible to dehydration in the air, leading to a loss of flexibility and functions. Here, a simple and general strategy for encapsulating hydrogel [...] Read more.
Hydrogels are stretchable ion conductors that can be used as strain sensors by transmitting strain-dependent electrical signals. However, hydrogels are susceptible to dehydration in the air, leading to a loss of flexibility and functions. Here, a simple and general strategy for encapsulating hydrogel with hydrophobic elastomer is proposed to realize excellent water-retention capacity. Elastomers, such as polydimethylsiloxanes (PDMS), whose hydrophobicity and dense crosslinking network can act as a barrier against water evaporation (lost 4.6 wt.% ± 0.57 in 24 h, 28 °C, and ≈30% humidity). To achieve strong adhesion between the hydrogel and elastomer, a porous structured thermoplastic polyurethane (TPU) is used at the hydrogel-elastomer interface to interlock the hydrogel and bond the elastomer simultaneously (the maximum interfacial toughness is over 1200 J/m2). In addition, a PDMS encapsulated ionic hydrogel strain sensor is proposed, demonstrating an excellent water-retention ability, superior mechanical performance, highly linear sensitivity (gauge factor = 2.21, at 100% strain), and robust interface. Various human motions were monitored, proving the effectiveness and practicability of the hydrogel-elastomer hybrid. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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13 pages, 3411 KiB  
Article
Investigation on In Situ Carbon-Coated ZnFe2O4 as Advanced Anode Material for Li-Ion Batteries
by Mir Waqas Alam, Amal BaQais, Mohammed M. Rahman, Muhammad Aamir, Alaaedeen Abuzir, Shehla Mushtaq, Muhammad Nasir Amin and Muhammad Shuaib Khan
Gels 2022, 8(5), 305; https://doi.org/10.3390/gels8050305 - 16 May 2022
Cited by 1 | Viewed by 2079
Abstract
ZnFe2O4 as an anode that is believed to attractive. Due to its large theoretical capacity, this electrode is ideal for Lithium-ion batteries. However, the performance of ZnFe2O4 while charging and discharging is limited by its volume growth. [...] Read more.
ZnFe2O4 as an anode that is believed to attractive. Due to its large theoretical capacity, this electrode is ideal for Lithium-ion batteries. However, the performance of ZnFe2O4 while charging and discharging is limited by its volume growth. In the present study, carbon-coated ZnFe2O4 is synthesized by the sol–gel method. Carbon is coated on the spherical surface of ZnFe2O4 by in situ coating. In situ carbon coating alleviates volume expansion during electrochemical performance and Lithium-ion mobility is accelerated, and electron transit is accelerated; thus, carbon-coated ZnFe2O4 show good electrochemical performance. After 50 cycles at a current density of 0.1 A·g−1, the battery had a discharge capacity of 1312 mAh·g−1 and a capacity of roughly 1220 mAh·g−1. The performance of carbon-coated ZnFe2O4 as an improved anode is electrochemically used for Li-ion energy storage applications. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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Review

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24 pages, 5683 KiB  
Review
Three-Dimensional Printing of Hydrogels for Flexible Sensors: A Review
by Suhail Ayoub Khan, Hamza Ahmad, Guoyin Zhu, Huan Pang and Yizhou Zhang
Gels 2024, 10(3), 187; https://doi.org/10.3390/gels10030187 - 08 Mar 2024
Viewed by 1004
Abstract
The remarkable flexibility and heightened sensitivity of flexible sensors have drawn significant attention, setting them apart from traditional sensor technology. Within this domain, hydrogels—3D crosslinked networks of hydrophilic polymers—emerge as a leading material for the new generation of flexible sensors, thanks to their [...] Read more.
The remarkable flexibility and heightened sensitivity of flexible sensors have drawn significant attention, setting them apart from traditional sensor technology. Within this domain, hydrogels—3D crosslinked networks of hydrophilic polymers—emerge as a leading material for the new generation of flexible sensors, thanks to their unique material properties. These include structural versatility, which imparts traits like adhesiveness and self-healing capabilities. Traditional templating-based methods fall short of tailor-made applications in crafting flexible sensors. In contrast, 3D printing technology stands out with its superior fabrication precision, cost-effectiveness, and satisfactory production efficiency, making it a more suitable approach than templating-based strategies. This review spotlights the latest hydrogel-based flexible sensors developed through 3D printing. It begins by categorizing hydrogels and outlining various 3D-printing techniques. It then focuses on a range of flexible sensors—including those for strain, pressure, pH, temperature, and biosensors—detailing their fabrication methods and applications. Furthermore, it explores the sensing mechanisms and concludes with an analysis of existing challenges and prospects for future research breakthroughs in this field. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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24 pages, 5028 KiB  
Review
Progress of Research on Conductive Hydrogels in Flexible Wearable Sensors
by Juan Cao, Bo Wu, Ping Yuan, Yeqi Liu and Cheng Hu
Gels 2024, 10(2), 144; https://doi.org/10.3390/gels10020144 - 14 Feb 2024
Viewed by 1690
Abstract
Conductive hydrogels, characterized by their excellent conductivity and flexibility, have attracted widespread attention and research in the field of flexible wearable sensors. This paper reviews the application progress, related challenges, and future prospects of conductive hydrogels in flexible wearable sensors. Initially, the basic [...] Read more.
Conductive hydrogels, characterized by their excellent conductivity and flexibility, have attracted widespread attention and research in the field of flexible wearable sensors. This paper reviews the application progress, related challenges, and future prospects of conductive hydrogels in flexible wearable sensors. Initially, the basic properties and classifications of conductive hydrogels are introduced. Subsequently, this paper discusses in detail the specific applications of conductive hydrogels in different sensor applications, such as motion detection, medical diagnostics, electronic skin, and human–computer interactions. Finally, the application prospects and challenges are summarized. Overall, the exceptional performance and multifunctionality of conductive hydrogels make them one of the most important materials for future wearable technologies. However, further research and innovation are needed to overcome the challenges faced and to realize the wider application of conductive hydrogels in flexible sensors. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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28 pages, 2738 KiB  
Review
High-Performing Conductive Hydrogels for Wearable Applications
by Hossein Omidian and Sumana Dey Chowdhury
Gels 2023, 9(7), 549; https://doi.org/10.3390/gels9070549 - 06 Jul 2023
Cited by 3 | Viewed by 2050
Abstract
Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human–machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical [...] Read more.
Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human–machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity. Full article
(This article belongs to the Special Issue Gels for Flexible Electronics and Energy Devices)
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