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The architectural design of electrodes offers new opportunities for next-generation electrochemical energy storage devices (EESDs) by increasing surface area, thickness, and active materials mass loading while maintaining good ion diffusion through optimized electrode tortuosity. However, conventional thick electrodes increase ion diffusion length and cause larger ion concentration gradients, limiting reaction kinetics. We demonstrate a strategy for building interpenetrated structures that shortens ion diffusion length and reduces ion concentration inhomogeneity. This free-standing device structure also avoids short-circuiting without needing a separator. The feature size and number of interpenetrated units can be adjusted during printing to balance surface area and ion diffusion. Starting with a 3D-printed interpenetrated polymer substrate, we metallize it to make it conductive. This substrate has two individually addressable electrodes, allowing selective electrodeposition of energy storage materials. Using a Zn//MnO2 battery as a model system, the interpenetrated device outperforms conventional separate electrode configurations, improving volumetric energy density by 221% and exhibiting a higher capacity retention rate of 49% compared to 35% at temperatures from 20 to 0 °C. Our study introduces a new EESD architecture applicable to Li-ion, Na-ion batteries, supercapacitors, etc.

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Flexible electronic skin with features that include sensing, processing, and responding to stimuli have transformed human-robot interactions. However, more advanced capabilities, such as human-like self-protection modalities with a sense of pain, sign of injury, and healing, are more challenging. Herein, a novel, flexible, and robust diffusive memristor based on a copolymer of chlorotrifluoroethylene and vinylidene fluoride (FK-800) as an artificial nociceptor (pain sensor) is reported. Devices composed of Ag/FK-800/Pt have outstanding switching endurance >106  cycles, orders of magnitude higher than any other two-terminal polymer/organic memristors in literature (typically 102 -103 cycles). In situ conductive atomic force microscopy is employed to dynamically switch individual filaments, which demonstrates that conductive filaments correlate with polymer grain boundaries and FK-800 has superior morphological stability under repeated switching cycles. It is hypothesized that the high thermal stability and high elasticity of FK-800 contribute to the stability under local Joule heating associated with electrical switching. To mimic biological nociceptors, four signature nociceptive characteristics are demonstrated: threshold triggering, no adaptation, relaxation, and sensitization. Lastly, by integrating a triboelectric generator (artificial mechanoreceptor), memristor (artificial nociceptor), and light emitting diode (artificial bruise), the first bioinspired injury response system capable of sensing pain, showing signs of injury, and healing, is demonstrated.

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Medical implants are routinely tracked and monitored using different techniques, such as MRI, X-ray, and ultrasound. Due to the need for ionizing radiation, the two former methods pose a significant risk to tissue. Ultrasound imaging, however, is non-invasive and presents no known risk to human tissue. Aerogels are an emerging material with great potential in biomedical implants. While qualitative observation of ultrasound images by experts can already provide a lot of information about the implants and the surrounding structures, this paper describes the development and study of two simple B-Mode image analysis techniques based on attenuation measurements and echogenicity comparisons, which can further enhance the study of the biological tissues and implants, especially of different types of biocompatible aerogels.

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In this work, we demonstrate the use of direct ink writing (DIW) technology to create 3D catalytic electrodes for electrochemical applications. Hybrid MoS2/graphene aerogels are made by mixing commercially available MoS2 and graphene oxide powders into a thixotropic, high concentration, viscous ink. A porous 3D structure of 2D graphene sheets and MoS2 particles was created after post treatment by freeze-drying and reducing graphene oxide through annealing. The composition and morphology of the samples were fully characterized through XPS, BET, and SEM/EDS. The resulting 3D printed MoS2/graphene aerogel electrodes had a remarkable electrochemically active surface area (>1700 cm2) and were able to achieve currents over 100 mA in acidic media. Notably, the catalytic activity of the MoS2/graphene aerogel electrodes was maintained with minimal loss in surface area compared to the non-3D printed electrodes, suggesting that DIW can be a viable method of producing durable electrodes with a high surface area for water splitting. This demonstrates that 3D printing a MoS2/graphene 3D porous network directly using our approach not only improves electrolyte dispersion and facilitates catalyst utilization but also provides multidimensional electron transport channels for improving electronic conductivity.

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Electrochemical reactors utilizing flow-through electrodes (FTEs) provide an attractive path toward the efficient utilization of electrical energy, but their commercial viability and ultimate adoption hinge on attaining high currents to drive productivity and cost competitiveness. Conventional FTEs composed of random, porous media provide limited opportunity for architectural control and engineering of microscale transport. Alternatively, the design freedom engendered by additively manufacturing FTEs yields additional opportunities to further drive performance via flow engineering. Through experiment and validated continuum computation we analyze the mass transfer in three-dimensional (3D)-printed porous FTEs with periodic lattice structures and show that, in contrast to conventional electrodes, the mesoscopic length scales in 3D-printed electrodes lead to an increase in the mass correlation exponent as inertial flow effects dominate. The inertially enhanced mass transport yields mass transfer coefficients that exceed previously reported 3D-printed FTEs by 10 to 100 times, bringing 3D-printed FTE performance on par with conventional materials.

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Previous works from our laboratory have firmly established that aerogels are a suitable substrate to elicit accelerated neurite extension. On non-conducting aerogels, in the presence of an externally-applied DC bias, neurons extended neurites which were preferentially aligned towards the anode. In this investigation, we sought to determine whether electrically-conductive carbon aerogels elicited a more robust alignment of neurites toward the anode than non-conductive aerogels due to the capacity of conductive aerogels to sustain a current, thereby providing a direct interface between neurons and the external electrical stimulus. To determine if this was the case, we plated PC12 neuronal cells on electrically conductive carbon aerolges derived from acetic acid-catalized resorcinol formaldehyde aerogels (ARF-CA) and subjected them to an external electric field. The voltages applied at the electrodes of the custom-built electro-stimulation chamber were 0 V, 15 V, and 30 V. For each voltage, the directionality and length of the neurites extended by PC12 cells were determined and compared to those observed when PC12 cells were plated on non-conductive aerogels subjected to the same voltage. The results show that the directionality of neurite extension was similar between conductive and non-conductive aerogels. A higher neurite length difference was observed on conductive aerogels with increasing voltage, 43% and 106% for 0-15 V and 0-30 V respectively, compared to non-conductive aerogels, 12% and 20%. These findings indicate that conductive carbon aerogels have a greater potential as scaffolds for nerve regeneration than non-conductive ones.

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Maintaining fast charging capability at low temperatures represents a significant challenge for supercapacitors. The performance of conventional porous carbon electrodes often deteriorates quickly with the decrease of temperature due to sluggish ion and charge transport. Here we fabricate a 3D-printed multiscale porous carbon aerogel (3D-MCA) via a unique combination of chemical methods and the direct ink writing technique. 3D-MCA has an open porous structure with a large surface area of ∼1750 m2 g-1. At -70 °C, the symmetric device achieves outstanding capacitance of 148.6 F g-1 at 5 mV s-1. Significantly, it retains a capacitance of 71.4 F g-1 at a high scan rate of 200 mV s-1, which is 6.5 times higher than the non-3D printed MCA. These values rank among the best results reported for low temperature supercapacitors. These impressive results highlight the essential role of open porous structures for preserving capacitive performance at ultralow temperatures.

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The performance of pseudocapacitive electrodes at fast charging rates are typically limited by the slow kinetics of Faradaic reactions and sluggish ion diffusion in the bulk structure. This is particularly problematic for thick electrodes and electrodes highly loaded with active materials. Here, a surface-functionalized 3D-printed graphene aerogel (SF-3D GA) is presented that achieves not only a benchmark areal capacitance of 2195 mF cm-2 at a high current density of 100 mA cm-2 but also an ultrahigh intrinsic capacitance of 309.1 µF cm-2 even at a high mass loading of 12.8 mg cm-2 . Importantly, the kinetic analysis reveals that the capacitance of SF-3D GA electrode is primarily (93.3%) contributed from fast kinetic processes. This is because the 3D-printed electrode has an open structure that ensures excellent coverage of functional groups on carbon surface and facilitates the ion accessibility of these surface functional groups even at high current densities and large mass loading/electrode thickness. An asymmetric device assembled with SF-3D GA as anode and 3D-printed GA decorated with MnO2 as cathode achieves a remarkable energy density of 0.65 mWh cm-2 at an ultrahigh power density of 164.5 mW cm-2 , outperforming carbon-based supercapacitors operated at the same power density.

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In recent years, 3D printing has led to a disruptive manufacturing revolution that allows complex architected materials and structures to be created by directly joining sequential layers into designed 3D components. However, customized feedstocks for specific 3D printing techniques and applications are limited or nonexistent, which greatly impedes the production of desired structural or functional materials. Colloids, with their stable biphasic nature, have tremendous potential to satisfy the requirements of various 3D printing methods owing to their tunable electrical, optical, mechanical, and rheological properties. This enables materials delivery and assembly across the multiple length scales required for multifunctionality. Here, a state-of-the-art review on advanced colloidal processing strategies for 3D printing of organic, ceramic, metallic, and carbonaceous materials is provided. It is believed that the concomitant innovations in colloid design and 3D printing will provide numerous possibilities for the fabrication of new constructs unobtainable using traditional methods, which will significantly broaden their applications.

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Boron carbide (B4C) is one of the hardest materials in existence. However, this attractive property also limits its machineability into complex shapes for high wear, high hardness, and lightweight material applications such as armors. To overcome this challenge, negative additive manufacturing (AM) is employed to produce complex geometries of boron carbides at various length scales. Negative AM first involves gelcasting a suspension into a 3D-printed plastic mold. The mold is then dissolved away, leaving behind a green body as a negative copy. Resorcinol-formaldehyde (RF) is used as a novel gelling agent because unlike traditional hydrogels, there is little to no shrinkage, which allows for extremely complex molds to be used. Furthermore, this gelling agent can be pyrolyzed to leave behind ~50 wt% carbon, which is a highly effective sintering aid for B4C. Due to this highly homogenous distribution of in situ carbon within the B4C matrix, less than 2% porosity can be achieved after sintering. This protocol highlights in detail the methodology for creating near fully dense boron carbide parts with highly complex geometries.

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Monolithic nanoporous metals, derived from dealloying, have a unique bicontinuous solid/void structure that provides both large surface area and high electrical conductivity, making them ideal candidates for various energy applications. However, many of these applications would greatly benefit from the integration of an engineered hierarchical macroporous network structure that increases and directs mass transport. We report on 3D (three-dimensional)-printed hierarchical nanoporous gold (3DP-hnp-Au) with engineered nonrandom macroarchitectures by combining 3D printing and dealloying. The material exhibits three distinct structural length scales ranging from the digitally controlled macroporous network structure (10 to 1000 µm) to the nanoscale pore/ligament morphology (30 to 500 nm) controlled by dealloying. Supercapacitance, pressure drop, and catalysis measurements reveal that the 3D hierarchical nature of our printed nanoporous metals markedly improves mass transport and reaction rates for both liquids and gases. Our approach can be applied to a variety of alloy systems and has the potential to revolutionize the design of (electro-)chemical plants by changing the scaling relations between volume and catalyst surface area.

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This corrects the article DOI: 10.1038/nmat4795.

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Three-dimensional (3D) carbon-based materials are emerging as promising electrode candidates for energy storage devices. In comparison to the 1D and 2D structures, 3D morphology offers new opportunities in rational design and synthesis of novel architectures tailor-made for promoting electrochemical performance. The capability of building hierarchical porous structures with 3D configuration can significantly advance the performance of energy storage devices by simultaneously enhancing the ion-accessible surface area and ion diffusion. This feature article presents an overview of recent progress in design, synthesis and implementation of 3D carbon-based materials as electrodes for electrochemical capacitors. Synthesis methodologies of four types of 3D carbon-based electrodes: 3D exfoliated carbon structures, 3D graphene scaffolds, 3D hierarchical porous carbon foams, as well as 3D architectures with periodic pores derived from direct ink writing, are thoroughly discussed and highlighted with selected experimental works. Finally, key opportunities and challenges in which different 3D carbons can significantly impact the energy storage and conversion communities will be provided.

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Low-density metal foams have many potential applications in electronics, energy storage, catalytic supports, fuel cells, sensors, and medical devices. Here, we report a new method for fabricating ultralight, conductive silver aerogel monoliths with predictable densities using silver nanowires. Silver nanowire building blocks were prepared by polyol synthesis and purified by selective precipitation. Silver aerogels were produced by freeze-casting nanowire aqueous suspensions followed by thermal sintering to weld the nanowire junctions. As-prepared silver aerogels have unique anisotropic microporous structures, with density precisely controlled by the nanowire concentration, down to 4.8 mg/cm3 and an electrical conductivity up to 51 000 S/m. Mechanical studies show that silver nanowire aerogels exhibit "elastic stiffening" behavior with a Young's modulus up to 16 800 Pa.

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Here we report the first example of a class of additively manufactured carbon fiber reinforced composite (AMCFRC) materials which have been achieved through the use of a latent thermal cured aromatic thermoset resin system, through an adaptation of direct ink writing (DIW) 3D-printing technology. We have developed a means of printing high performance thermoset carbon fiber composites, which allow the fiber component of a resin and carbon fiber fluid to be aligned in three dimensions via controlled micro-extrusion and subsequently cured into complex geometries. Characterization of our composite systems clearly show that we achieved a high order of fiber alignment within the composite microstructure, which in turn allows these materials to outperform equivalently filled randomly oriented carbon fiber and polymer composites. Furthermore, our AM carbon fiber composite systems exhibit highly orthotropic mechanical and electrical responses as a direct result of the alignment of carbon fiber bundles in the microscale which we predict will ultimately lead to the design of truly tailorable carbon fiber/polymer hybrid materials having locally programmable complex electrical, thermal and mechanical response.

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Organic-inorganic halide perovskite materials have emerged as attractive alternatives to conventional solar cell building blocks. Their high light absorption coefficients and long diffusion lengths suggest high power conversion efficiencies, and indeed perovskite-based single bandgap and tandem solar cell designs have yielded impressive performances. One approach to further enhance solar spectrum utilization is the graded bandgap, but this has not been previously achieved for perovskites. In this study, we demonstrate graded bandgap perovskite solar cells with steady-state conversion efficiencies averaging 18.4%, with a best of 21.7%, all without reflective coatings. An analysis of the experimental data yields high fill factors of ∼75% and high short-circuit current densities up to 42.1 mA cm-2. The cells are based on an architecture of two perovskite layers (CH3NH3SnI3 and CH3NH3PbI3-xBrx), incorporating GaN, monolayer hexagonal boron nitride, and graphene aerogel.

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A robust, millimeter-sized low-density Cu foam with ∼90% (v/v) porosity, ∼30 nm thick walls, and ∼1 µm diameter spherical pores is prepared by the slip-casting of metal-coated polymer core-shell particles followed by a thermal removal of the polymer. In this paper, we report our key findings that enable the development of the low-density Cu foams. First, we need to synthesize polystyrene (PS) particles coated with a very thin Cu layer (in the range of tens of nanometers). A simple reduction in the amount of Cu deposited onto the PS was not sufficient to form such a low-density Cu foams due to issues related to foam collapse and densification upon the subsequent polymer removal step. Precise control over the morphology of the Cu coating on the particles is essential for the synthesis of a lower density of foams. Second, improving the dispersion of PS-Cu particles in a suspension used for the casting as well as careful optimization of a baking condition minimize the formation of irregular large voids, leading to Cu foams with a more uniform packing and a better connectivity of neighboring Cu hollow shells. Finally, we analyzed mechanical properties of the Cu foams with a depth-sensing indentation test. The uniform Cu foams show a significant improvement in mechanical properties (∼1.5× modulus and ∼3× hardness) compared to those of uncontrolled foam samples with a similar foam density but irregular large voids. Higher surface areas and a good electric conductivity of the Cu foams present a great potential to future applications.

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Graphene is an atomically thin, two-dimensional (2D) carbon material that offers a unique combination of low density, exceptional mechanical properties, thermal stability, large surface area, and excellent electrical conductivity. Recent progress has resulted in macro-assemblies of graphene, such as bulk graphene aerogels for a variety of applications. However, these three-dimensional (3D) graphenes exhibit physicochemical property attenuation compared to their 2D building blocks because of one-fold composition and tortuous, stochastic porous networks. These limitations can be offset by developing a graphene composite material with an engineered porous architecture. Here, we report the fabrication of 3D periodic graphene composite aerogel microlattices for supercapacitor applications, via a 3D printing technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g(-1)) and power densities (>4 kW·kg(-1)) that equal or exceed those of reported devices made with electrodes 10-100 times thinner. This work provides an example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrable energy storage devices optimized for a broad range of applications.

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Correction for 'Nanoscale structure and superhydrophobicity of sp(2)-bonded boron nitride aerogels' by Thang Pham et al., Nanoscale, 2015, 7, 10449-10458.