Smart materials for flexible electronics and devices: hydrogel.

In recent years, flexible conductive materials have attracted considerable attention for their potential use in flexible energy storage devices, touch panels, sensors, memristors, and other applications. The outstanding flexibility, electricity, and tunable mechanical properties of hydrogels make them ideal conductive materials for flexible electronic devices. Various synthetic strategies have been developed to produce conductive and environmentally friendly hydrogels for high-performance flexible electronics. In this review, we discuss the state-of-the-art applications of hydrogels in flexible electronics, such as energy storage, touch panels, memristor devices, and sensors like temperature, gas, humidity, chemical, strain, and textile sensors, and the latest synthesis methods of hydrogels. Describe the process of fabricating sensors as well. Finally, we discussed the challenges and future research avenues for flexible and portable electronic devices based on hydrogels.

Ultra-thin proton conducting carrier layers for scalable integration of atomically thin 2D materials with proton exchange polymers for next-generation PEMs.

Scalable approaches for synthesis and integration of proton selective atomically thin 2D materials with proton conducting polymers can enable next-generation proton exchange membranes (PEMs) with minimal crossover of reactants or undesired species while maintaining adequately high proton conductance for practical applications. Here, we systematically investigate facile and scalable approaches to interface monolayer graphene synthesized via scalable chemical vapor deposition (CVD) on Cu foil with the most widely used proton exchange polymer Nafion 211 (N211, ∼25 µm thick film) via (i) spin-coating a ∼700 nm thin Nafion carrier layer to transfer graphene (spin + scoop), (ii) casting a Nafion film and cold pressing (cold press), and (iii) hot pressing (hot press) while minimizing micron-scale defects to <0.3% area. Interfacing CVD graphene on Cu with N211 via cold press or hot press and subsequent removal of Cu via etching results in ∼50% lower areal proton conductance compared to membranes fabricated via the spin + scoop method. Notably, the areal proton conductance can be recovered by soaking the hot and cold press membranes in 0.1 M HCl, without significant damage to graphene. We rationalize our finding by the significantly smaller reservoir for cation uptake from Cu etching for the ∼700 nm thin carrier Nafion layer used for spin + scoop transfer compared to the ∼25 µm thick N211 film for hot and cold pressing. Finally, we demonstrate performance in H2 fuel cells with power densities of ∼0.23 W cm-2 and up to ∼41-54% reduction in H2 crossover for the N211|G|N211 sandwich membranes compared to the control N211|N211 indicating potential for our approach in enabling advanced PEMs for fuel cells, redox-flow batteries, isotope separations and beyond.

Kinetic Control of Angstrom-Scale Porosity in 2D Lattices for Direct Scalable Synthesis of Atomically Thin Proton Exchange Membranes.

Angstrom-scale pores introduced into atomically thin 2D materials offer transformative advances for proton exchange membranes in several energy applications. Here, we show that facile kinetic control of scalable chemical vapor deposition (CVD) can allow for direct formation of angstrom-scale proton-selective pores in monolayer graphene with significant hindrance to even small, hydrated ions (K+ diameter ∼6.6 Å) and gas molecules (H2 kinetic diameter ∼2.9 Å). We demonstrate centimeter-scale Nafion|Graphene|Nafion membranes with proton conductance ∼3.3-3.8 S cm-2 (graphene ∼12.7-24.6 S cm-2) and H+/K+ selectivity ∼6.2-44.2 with liquid electrolytes. The same membranes show proton conductance ∼4.6-4.8 S cm-2 (graphene ∼39.9-57.5 S cm-2) and extremely low H2 crossover ∼1.7 × 10-1 - 2.2 × 10-1 mA cm-2 (∼0.4 V, ∼25 °C) with H2 gas feed. We rationalize our findings via a resistance-based transport model and introduce a stacking approach that leverages combinatorial effects of interdefect distance and interlayer transport to allow for Nafion|Graphene|Graphene|Nafion membranes with H+/K+ selectivity ∼86.1 (at 1 M) and record low H2 crossover current density ∼2.5 × 10-2 mA cm-2, up to ∼90% lower than state-of-the-art ionomer Nafion membranes ∼2.7 × 10-1 mA cm-2 under identical conditions, while still maintaining proton conductance ∼4.2 S cm-2 (graphene stack ∼20.8 S cm-2) comparable to that for Nafion of ∼5.2 S cm-2. Our experimental insights enable functional atomically thin high flux proton exchange membranes with minimal crossover.

Subatomic species transport through atomically thin membranes: Present and future applications.

Atomically thin two-dimensional materials present opportunities for selective transport of subatomic species. The pristine lattice of monolayer graphene and hexagonal boron nitride, although impermeable to helium atoms, allows for transmission of electrons and permits transport of thermal protons and its isotopes. We discuss advances in selective subatomic species transport through atomically thin membranes and their potential for transformative advances in energy storage and conversion, isotope separations, in situ electron microscopy and spectroscopy, and future electronic applications. We outline technological challenges and opportunities for these applications and discuss early adoption in imaging and spectroscopy that are starting to become commercially available, as well as emerging applications in the nuclear industry and future application potential in grid storage, clean/green transportation, environmental remediation, and others.

Ionic Conductance through Graphene: Assessing Its Applicability as a Proton Selective Membrane.

Inspired by recent reports on possible proton conductance through graphene, we have investigated the behavior of pristine graphene and defect engineered graphene membranes for ionic conductance and selectivity with the goal of evaluating a possibility of its application as a proton selective membrane. The averaged conductance for pristine chemical vapor deposited (CVD) graphene at pH1 is ∼4 mS/cm2 but varies strongly due to contributions from the unavoidable defects in our CVD graphene. From the variations in the conductance with electrolyte strength and pH, we can conclude that pristine graphene is fairly selective and the conductance is mainly due to protons. Engineering of the defects with ion beam (He+, Ga+) irradiation and plasma (N2 and H2) treatment showed improved areal conductance with high proton selectivity mostly for He-ion beam and H2 plasma treatments, which agrees with primarily vacancy-free type of defects produced in these cases confirmed by Raman analysis.

Simple and Versatile Detection of Viruses Using Anodized Alumina Membranes.

A simple sensor for viral particles based on ionic conductivity through anodized alumina membranes was demonstrated using MS2 bacteriophage as an example. A facile two-point measuring scheme is geared toward realization using a computer's sound card input/output capabilities suitable for a fast and inexpensive point of care testing. The lowest detection concentration down to ~7 pfu/mL and a large dynamic range up to ~2000 pfu/mL were obtained due to physical optimization that included proper length and diameter for the pores, removing the oxide layer at the electrode, as well as the chemical optimization of covalent binding of antibodies to the pore's walls.