Three-dimensional (3D) printing technology has been recognized as an emerging advanced fabrication technology in both industry and academia. Direct ink writing (DIW), a type of 3D printing technology, can build 3D structures through the deposition of custom-made inks, printing devices with complex architectures, excellent mechanical properties and enhanced functionalities. DIW can greatly facilitate the fabrication of miniaturized or flexible electronic components. These components are potentially useful for their applications in advanced wearable devices. This article highlights recent advancements in 3D direct ink written electronic components with an emphasis on their potential applications for wearable devices. The relationship among ink formulations, DIW techniques and printed devices is highlighted. In particular, the DIW-assisted fabrication of key components in wearable electronics, including power generation (nanogenerators), energy storage (e.g. lithium ion batteries) and energy consuming products (e.g. strain sensors) are reviewed in terms of performance metrics and fabrication strategies. Optimized ink preparations, evolving DIW techniques, and device designs can work synergistically to enhance the development of printed advanced wearable devices.
Although direct ink writing (DIW) is a versatile 3D printing technique, progress in DIW has been constrained by the stringent rheological requirements for printable conductive nanocomposites, particularly at smaller length scales. In this work, we overcome these challenges using an aqueous nanocomposite ink with polydimethylsiloxane (PDMS) submicrobeads and an electrochemically-derived graphene oxide (EGO) nanofiller. This nanocomposite ink possesses a thixotropic, self-supporting viscoelasticity. It can be easily extruded through very small nozzle openings (as small as 50 µm) allowing for the highest resolution PDMS DIW reported to date. With a mild thermal annealing, the DIW-printed device exhibits low resistivity (1660 Ω•cm) at a low percolation threshold of EGO (0.83 vol%) owing to the unique nanocomposite structure of graphene-wrapped elastomeric beads. The nanocomposite ink was used to print wearable, macro-scale strain sensing patches, as well as remarkably small, micron-scale pressure sensors. The large-scale strain sensors have excellent performance over a large working range (up to 40% strain), with high gauge factor (20.3), and fast responsivity (83 ms) while the micron-scale pressure sensors demonstrated high pressure sensitivity (0.31 kPa -1 ) and operating range (0.248-500 kPa). Ultrahigh resolution, multimaterial layer-by-layer deposition allows the engineering of microscale features into the devices, features which can be used to tune the piezoresistive mechanism and degree of piezoresistivity.
Since the discovery of graphene in 2004, [2] research interest for ultrathin 2D materials has grown exponentially across various fields. [3,4] As an emerging class of 2D materials, transition metal carbon/nitrides (MXenes) with a general structure of M n+1 X n T x (n = 1, 2, or 3), where M is a transition metal (e.g., Ti, V, Nb, and Mo), X represents C and/or N, and T x symbolizes the surface termination groups (such as O, F, or OH), have garnered tremendous research interest since their discovery in 2011. [5,6] MXenes can be obtained from ceramic MAX phases by removing the A atom (generally Al) through selective etching methods. [7] 2D MXenes are semimetallic materials and feature excellent properties including high electronic conductivities, excellent surface activities, and good hydrophilicity. These fascinating properties make MXenes attractive for a variety of potential applications including energy storage, solar cells, and catalysis. [8][9][10][11] Since the first report by Seh et al. in 2016, [12] MXenes have received significant attention as promising candidates for use in the electrocatalytic hydrogen evolution reaction (HER). [13][14][15] In particular, recent theoretical studies suggest that 2D-layered materials have attracted increasing attention as low-cost supports for developing active catalysts for the hydrogen evolution reaction (HER). In addition, atomically thin Ti 3 C 2 T x (MXene) nanosheets have surface termination groups (T x : F, O, and OH), which are active sites for effective functionalization. In this work, heteroatom (boron)-doped Ti 3 C 2 T x (MXene) nanosheets are developed as an efficient solid support to host ultrasmall ruthenium (Ru) nanoparticles for electrocatalytic HER. The quantummechanical first-principles calculations and electrochemical tests reveal that the B-doping onto 2D MXene nanosheets can largely improve the intermediate H* adsorption kinetics and reduce the charge-transfer resistance toward the HER, leading to increased reactivity of active sites and favorable electrode kinetics. Importantly, the newly designed electrocatalyst based on Ru nanoparticles supported on B-doped MXene (Ru@B-Ti 3 C 2 T x ) nanosheets shows a remarkable catalytic activity with low overpotentials of 62.9 and 276.9 mV to drive 10 and 100 mA cm −2 , respectively, for the HER, while exhibiting excellent cycling stabilities. Moreover, according to the theoretical calculations, Ru@B-Ti 3 C 2 T x exhibits a near-zero value of Gibbs free energy (ΔG H* = 0.002 eV) for the HER. This work introduces a facile strategy to functionalize MXene for use as a solid support for efficient electrocatalysts.
Although graphene oxide (GO) has shown enduring popularity in the research community, its synthesis remains cost prohibitive for many of its demonstrated applications. While significant progress has been made on developing an electrochemical route to GO, existing methods have key limitations regarding their cost and scalability. To overcome these challenges, we employ a combination of commercially available fused-deposition-modeling-based 3D printing and highly robust boron-doped diamond with a wide electrochemical potential window to fabricate a scalable packed-bed electrochemical reactor for GO production. The scalability of the reactor along the vertical and lateral dimensions was systematically demonstrated to facilitate its eventual industrial application. Our current reactor is cost-effective and capable of producing electrochemically derived GO (EGO) on a multiple-gram scale. By oxidizing flake graphite directly in an 11.6 M sulfuric acid electrolyte, the production of EGO was streamlined to a one-step electrochemical reaction, followed by a simple water-wash purification. Almost all of the converted graphite oxide can be recovered, and the final mass yield is typically 155% of the starting graphite material. The as-produced EGO is dispersible in water and other polar organic solvents (e.g., ethanol and dimethylformamide) and can be exfoliated down to predominantly single-layered GO. Through a detailed study of the product intermediates, the graphite was found to first form a stage III or higher graphite intercalation compound, followed by electrochemical oxidation proceeding from the top of the packed graphite bed down. The EGO can be easily deoxygenated with low-temperature thermal annealing (<200°C) to produce thermally converted EGO with significantly enhanced conductivity, and its promising application as a conductive nanofiller in lithium-ion battery cathodes was demonstrated. The simplicity, cost-effectiveness, and unique EGO properties make our current method a viable contender for large-scale synthesis of GO.
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