Ti3C2Tx MXene-Reduced Graphene Oxide Composite Electrodes for Stretchable Supercapacitors.

The development of stretchable electronics requires the invention of compatible high-performance power sources, such as stretchable supercapacitors and batteries. In this work, two-dimensional (2D) titanium carbide (Ti3C2Tx) MXene is being explored for flexible and printed energy storage devices by fabrication of a robust, stretchable high-performance supercapacitor with reduced graphene oxide (RGO) to create a composite electrode. The Ti3C2Tx/RGO composite electrode combines the superior electrochemical and mechanical properties of Ti3C2Tx and the mechanical robustness of RGO resulting from strong nanosheet interactions, larger nanoflake size, and mechanical flexibility. It is found that the Ti3C2Tx/RGO composite electrodes with 50 wt % RGO incorporated prove to mitigate cracks generated under large strains. The composite electrodes exhibit a large capacitance of 49 mF/cm2 (∼490 F/cm3 and ∼140 F/g) and good electrochemical and mechanical stability when subjected to cyclic uniaxial (300%) or biaxial (200% × 200%) strains. The as-assembled symmetric supercapacitor demonstrates a specific capacitance of 18.6 mF/cm2 (∼90 F/cm3 and ∼29 F/g) and a stretchability of up to 300%. The developed approach offers an alternative strategy to fabricate stretchable MXene-based energy storage devices and can be extended to other members of the large MXene family.

Bacterially driven cadmium sulfide precipitation on porous membranes: Toward platforms for photocatalytic applications.

The emerging field of biofabrication capitalizes on nature's ability to create materials with a wide range of well-defined physical and electronic properties. Particularly, there is a current push to utilize programmed, self-organization of living cells for material fabrication. However, much research is still necessary at the interface of synthetic biology and materials engineering to make biofabrication a viable technique to develop functional devices. Here, the authors exploit the ability of Escherichia coli to contribute to material fabrication by designing and optimizing growth platforms to direct inorganic nanoparticle (NP) synthesis, specifically cadmium sulfide (CdS) NPs, onto porous polycarbonate membranes. Additionally, current, nonbiological, chemical synthesis methods for CdS NPs are typically energy intensive and use high concentrations of hazardous cadmium precursors. Using biosynthesis methods through microorganisms could potentially alleviate these issues by precipitating NPs with less energy and lower concentrations of toxic precursors. The authors adopted extracellular precipitation strategies to form CdS NPs on the membranes as bacterial/membrane composites and characterized them by spectroscopic and imaging methods, including energy dispersive spectroscopy, and scanning and transmission electron microscopy. This method allowed us to control the localization of NP precipitation throughout the layered bacterial/membrane composite, by varying the timing of the cadmium precursor addition. Additionally, the authors demonstrated the photodegradation of methyl orange using the CdS functionalized porous membranes, thus confirming the photocatalytic properties of these composites for eventual translation to device development. If combined with the genetically programmed self-organization of cells, this approach promises to directly pattern CdS nanostructures on solid supports.

Programmable assembly of pressure sensors using pattern-forming bacteria.

Biological systems can generate microstructured materials that combine organic and inorganic components and possess diverse physical and chemical properties. However, these natural processes in materials fabrication are not readily programmable. Here, we use a synthetic-biology approach to assemble patterned materials. We demonstrate programmable fabrication of three-dimensional (3D) materials by printing engineered self-patterning bacteria on permeable membranes that serve as a structural scaffold. Application of gold nanoparticles to the colonies creates hybrid organic-inorganic dome structures. The dynamics of the dome structures' response to pressure is determined by their geometry (colony size, dome height, and pattern), which is easily modified by varying the properties of the membrane (e.g., pore size and hydrophobicity). We generate resettable pressure sensors that process signals in response to varying pressure intensity and duration.

Stretchable and high-performance supercapacitors with crumpled graphene papers.

Fabrication of unconventional energy storage devices with high stretchability and performance is challenging, but critical to practical operations of fully power-independent stretchable electronics. While supercapacitors represent a promising candidate for unconventional energy-storage devices, existing stretchable supercapacitors are limited by their low stretchability, complicated fabrication process, and high cost. Here, we report a simple and low-cost method to fabricate extremely stretchable and high-performance electrodes for supercapacitors based on new crumpled-graphene papers. Electrolyte-mediated-graphene paper bonded on a compliant substrate can be crumpled into self-organized patterns by harnessing mechanical instabilities in the graphene paper. As the substrate is stretched, the crumpled patterns unfold, maintaining high reliability of the graphene paper under multiple cycles of large deformation. Supercapacitor electrodes based on the crumpled graphene papers exhibit a unique combination of high stretchability (e.g., linear strain ~300%, areal strain ~800%), high electrochemical performance (e.g., specific capacitance ~196 F g(-1)), and high reliability (e.g., over 1000 stretch/relax cycles). An all-solid-state supercapacitor capable of large deformation is further fabricated to demonstrate practical applications of the crumpled-graphene-paper electrodes. Our method and design open a wide range of opportunities for manufacturing future energy-storage devices with desired deformability together with high performance.