ABSTRACT
Smart sensors with self-healing capabilities have recently aroused increasing interest in applications in soft electronics. However, challenges remain in balancing the sensors' self-healing and compatibility between their sensing and substrate layers. This study evaluated several self-healing polymer substrates and graphene ink-based strain-sensing coatings. The optimum electrochemically exfoliated graphene (e-graphene)/silver nanoparticle-coated tannic acid (TA)/superabsorbent polymer/graphene oxide (GO) blended poly(vinyl alcohol) polymer composites exhibited improvements of 47.1 and 39.2%, respectively, for the healing efficiency in a substrate crack area and in the graphene-based sensing layer due to conductive layer adhesion. While TA was found to improve healing efficiency on the coating surface by forming hydrogen bonds between the sensing and polymer layers, GO healed the polymer surface due to its ability to form bonds in the polymer matrix. The superabsorbent polymer was found to absorb excess water in e-graphene dispersion due to its host-guest interaction, while also reducing the coating thickness.
ABSTRACT
Electrochemically exfoliated graphene (e-G) thin films on Nafion membranes exhibit a selective barrier effect against undesirable fuel crossover. This approach combines the high proton conductivity of state-of-the-art Nafion and the ability of e-G layers to effectively block the transport of methanol and hydrogen. Nafion membranes are coated with aqueous dispersions of e-G on the anode side, making use of a facile and scalable spray process. Scanning transmission electron microscopy and electron energy-loss spectroscopy confirm the formation of a dense percolated graphene flake network, which acts as a diffusion barrier. The maximum power density in direct methanol fuel cell (DMFC) operation with e-G-coated Nafion N115 is 3.9 times higher than that of the Nafion N115 reference (39 vs 10 mW cm-2@0.3 V) at a 5M methanol feed concentration. This suggests the application of e-G-coated Nafion membranes for portable DMFCs, where the use of highly concentrated methanol is desirable.
ABSTRACT
Two-dimensional (2D) materials, such as graphene, are seen as potential candidates for fabricating electronic devices and circuits on flexible substrates. Inks or dispersions of 2D materials can be deposited on flexible substrates by large-scale coating techniques, such as inkjet printing and spray coating. One of the main issues in coating processes is nonuniform deposition of inks, which may lead to large variations of properties across the substrates. Here, we investigate the role of surface morphology on the performance of graphene ink deposited on different paper substrates with specific top coatings. Substrates with good wetting properties result in reproducible thin films and electrical properties with low sheet resistance. The correct choice of surface morphology enables high-performance films without postdeposition annealing or treatment. Scanning terahertz time-domain spectroscopy (THz-TDS) is introduced to evaluate both the uniformity and the local conductivity of graphene inks on paper. A paper-based strain gauge is demonstrated and a variable resistor acts as an on-off switch for operating an LED. Customized surfaces can thus help in unleashing the full potential of ink-based 2D materials.