iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 3 3 9 iTSSe TSS FEATURE 7 Polyether ether ketone (PEEK) has promising potential to be a MEET material because of its chemistry and its excellent mechanical, thermal, and electrical properties. Despite its high cost, PEEK’s resistance to chemical degradation makes it an excellent solution for MEET applications, including in high-temperature environments. Additionally, PEEK’s water absorption and desorption properties make it a suitable material for long-term power generation as its time to saturation in water is considerably long[14]. Such an attribute is crucial because it allows the generation of a DC current for long periods of time, unlike the carbon-based materials that are currently used and can only generate DC current for short intervals (i.e., a fewminutes), beforepolarity reversal or saturation take place. COLD SPRAY FOR MEET DEVICES Current proposed methods to build MEET devices use multiple chemical processes or a combination of chemical and electrical processes that can be hard to scale up for large volume production. It is believed that thermal spray technology, in particular, cold spray (CS), is a simpler process for producing MEET devices that can be adapted to large-scale, large volume manufacturing. It is also deemed a greener process as it uses air or nitrogen as process gases and has shown promise in regard to powder recycling[15]. Demonstration of the ability of producing MEET devices by CS was performed successfully at the uOttawa Cold Spray Laboratory. For this first demonstration, PEEK was cold sprayed on aluminum, with the CS layer acting as the as they are bound to large molecules. Subsequently, the protons migrate from the oxygen-rich (and thus proton-rich) side to the low-concentration side through a diffusion mechanism, giving rise to an electrical current[13]. The hydration as well as the diffusion mechanism are shown in Fig. 2. MATERIALS FOR MEET DEVICES Design variations exist that allow evaporation to enhance the process by constantly removing water molecules from the film, thus preventing dynamic balance from occurring and allowing direct-current same-sign voltage generation[13]. Carbon-based materials are predominantly used in the manufacturing of MEET devices. Graphenebased films rich in oxygen-containing functional groups, like hydroxyl and carboxyl, or doped with nucleophilic elements, have been used for harvesting electrical power from moist air and water droplets. Another way to establish a hydrogen ion gradient within the film is to fabricate devices that are asymmetric. The graphene oxide film in this case does not contain a gradient of oxygen-rich groups through its thickness. It rather uses a moisture-insulating side (like glass, paper, and polyethylene terephthalate), only allowing water molecules to infiltrate the film from one side[13]. Such a configuration ensures that the side exposed to moisture always contains a higher concentration of hydrogen ions, thus allowing the diffusionmechanismto take place fromthe moisture permeable side to the moisture insulating side[13]. Ion migration causes a net flow of positive charge (current) which translates into a potential across the electrodes. Fig. 2 — Schematic of a graphene-oxide-based MEET device[14].
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