The emerging wearable electronics integrated into textiles are posing new challenges both in materials and micro-fabrication strategies to produce textile-based energy storage and power source micro-devices. In this regard, inkjet printing (IJP) offers unique features for rapid prototyping for various thin-film (2D) devices. However, all-inkjet-printed capacitors were very rarely reported in the literature. In this work, we formulated a stable Ti3C2 MXene aqueous ink for inkjet printing current-collector-free electrodes on TPU-coated cotton fabric, together with an innovative inkjet-printable and UV-curable solvent-based electrolyte precursor. The electrolyte was inkjet-printed on the electrode’s surface, and after UV polymerization, a thin and soft gel polymer electrolyte (GPE) was obtained, resulting in an all-inkjet-printed symmetrical capacitor (a-IJPSC). The highest ionic conductivity (0.60 mS/cm) was achieved with 10 wt.% of acrylamide content, and the capacitance retention was investigated both at rest (flat) and under bending conditions. The flat a-IJPSC textile-based device showed the areal capacitance of 0.89 mF/cm2 averaged on 2k cycles. Finally, an array of a-IJPSCs were demonstrated to be feasible as both a textile-based energy storage and micro-power source unit able to power a blue LED for several seconds.

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Phosphating is one of the most common conversion coatings for steels and other ferrous alloys. Owing it to the process simplicity and unique morphology of the coatings, this treatment has cemented itself in a wide variety of applications, ranging from painting to corrosion resistance and cold working operations. In recent years, the need for more sustainable technologies has however driven the research for new alternatives to phosphating. One of the main drawbacks of this process is in fact the formation of a sludge byproduct composed mainly of phosphates of heavy metals, which must be continuously filtered off from the deposition tanks and disposed properly. These operations take a big toll on the environmental burden of the process and are the reason why an alternative will eventually become a necessity. One possible solution to this problem is to study conversion coatings with a different chemical nature. Formulations based on vanadium [1] and zirconium [2] have already been proposed as an alternative to phosphating, showing good results in terms of corrosion protection and coating adhesion; however it’s still unclear whether their application at the industrial scale would have a lower environmental impact than phosphating. A completely opposite solution is instead to keep the same, well-known chemistry of phosphating baths and change the method of application. In electrolytic zinc phosphating the coating process is carried out in an electrochemical cell, where the conditions required for phosphate precipitation are promoted by the application of a cathodic potential. In this scenario, substrate dissolution is prevented thanks to the cathodic polarization applied and the formation of the sludge byproduct is completely avoided. [3] By applying a cathodic polarization, hydrogen evolution is promoted at the substrate-electrolyte interface, leading to a local increase in pH, which triggers zinc phosphate precipitation. Therefore, the deposition rate in electrolytic phosphating is related to the speed at which the ideal pH conditions are created. In principle, it’s therefore possible to increase the current density to enhance the deposition rate, similarly to common electroplating systems. In practice, a current density too high, would lead to accumulation of hydrogen bubbles on the substrate surface and growth of dendritic metallic zinc, which would drastically reduce the quality of the final coating. [4] To overcome these limitations, in this work we demonstrate the use of pulsed current deposition as a mean to achieve higher coating weights in electrolytic zinc phosphating on mild steel substrates. The high current density applied during the impulse promotes the formation of the proper pH conditions for phosphate precipitation to occur, while the off-time allows for hydrogen bubbles to leave the surface, improving the overall coating quality. To highlight the effects of the impulse parameters on the final coatings, surface characterization is carried out with multiple techniques, such as SEM imaging, EDS mapping, XRD and GDOES. Moreover, corrosion resistance of the zinc phosphate coatings is evaluated with both destructive and non-destructive techniques in a NaCl 3.5% environment. Bibliography [1] M. Motamedi and M. M. Attar, “Nanostructured vanadium-based conversion treatment of mild steel substrate: Formation process: Via noise measurement, surface analysis and anti-corrosion behavior,” RSC Adv., vol. 6, no. 50, pp. 44732–44741, May 2016. [2] A. Ghanbari and M. M. Attar, “Surface free energy characterization and adhesion performance of mild steel treated based on zirconium conversion coating: A comparative study,” Surf. Coatings Technol., vol. 246, pp. 26–33, May 2014. [3] S. Jegannathan, T. S. N. Sankara Narayanan, K. Ravichandran, and S. Rajeswari, “Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests,” Electrochim. Acta, vol. 51, no. 2, pp. 247–256, Oct. 2005. [4] C. Kavitha, T. S. N. N. Sankara Narayanan, K. Ravichandran, I. S. Park, and M. H. Lee, “Deposition of zinc–zinc phosphate composite coatings on steel by cathodic electrochemical treatment,” J. Coatings Technol. Res., vol. 11, no. 3, pp. 431–442, May 2014.

Flexible electronic devices based on smart fabrics are generating a vast amount of interest and increasing efforts are being devoted to their development. In the next future, smart textiles will integrate diverse functions such as energy harvesting, human health monitoring and more. As a consequence, the development of flexible energy storage as a mean to power wearable devices has become ever more urgent. Among the manufacturing techniques for flexible batteries and capacitors, inkjet printing has cemented itself as one of the most popular, mainly thanks to its versatility and simplicity. In this scenario, a great effort is being devoted towards research and development of innovative, more sustainable materials that satisfy the requirements for both printability and energy storage capabilities. [1] Prussian blue analogues (PBA) are a class of materials that have been studied extensively as cathodic materials for a wide variety of energy storage devices, such as Li-ion batteries, Na-ion batteries, Zn-ion batteries, and supercapacitors. [2] Their open framework structure allows reversible intercalation of hydrated cations, making them perfectly compatible with aqueous electrolytes. Thanks to this feature, along with their agile synthetic route, low cost and benign nature of their precursors, PBA are regarded as one of the most promising materials for future energy storage devices. [3] Despite their increasing popularity, inkjet printing of PBA for flexible energy storage devices has not been reported yet. In this work, inkjet printing of PBA is proposed as a viable technique to manufacture flexible asymmetric supercapacitors with a Ti3C2 MXene anode. MXenes are a new class of 2D materials which have gained a lot of interest due to their metallic conductivity and exceptional water processability. While their use in symmetric capacitors has already been demonstrated, these devices tend to operate in a small potential window (≈ 500 mV) as to prevent unwanted MXene oxidation, which limits their specific power. [4] By coupling the MXene anode with a PBA cathode, it is therefore possible to extend the potential window of the device and consequently the power that can be delivered. Prussian blue analogues powders are synthesized by the traditional co-precipitation route. Printable, water-based inks containing the PBA particles are then used to manufacture flexible electrodes on various substrates. Electrochemical characterization is carried out in custom, 3D printed, three electrodes cells, allowing for non-destructive testing of the printed electrodes. Finally, the electrochemical properties of the capacitors will be tested in aqueous electrolytes as well as with semi-solid electrolytes at rest and under mechanical deformation. Bibliography [1] K. H. Choi, D. B. Ahn, and S. Y. Lee, “Current Status and Challenges in Printed Batteries: Toward Form Factor-Free, Monolithic Integrated Power Sources,” ACS Energy Lett., vol. 3, no. 1, pp. 220–236, Jan. 2018. [2] G. Du and H. Pang, “Recent advancements in Prussian blue analogues: Preparation and application in batteries,” Energy Storage Mater., vol. 36, pp. 387–408, Apr. 2021. [3] B. Wang et al., “Prussian Blue Analogs for Rechargeable Batteries,” iScience, vol. 3, pp. 110–133, May 2018. [4] D. Wen et al., “Inkjet Printing Transparent and Conductive MXene (Ti3C2Tx) Films: A Strategy for Flexible Energy Storage Devices,” ACS Appl. Mater. Interfaces, vol. 13, no. 15, pp. 17766–17780, Apr. 2021.

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