The unending demand for portable, flexible, and even wearable electronic devices that have an aesthetic appeal and unique functionality stimulates the development of advanced power sources that have excellent electrochemical performance and, more importantly, shape versatility. The challenges in the fabrication of next-generation flexible power sources mainly arise from their limited form factors, which prevent their facile integration into differently shaped electronic devices, and from the lack of reliable electrochemical materials that exhibit optimized attributes and suitable processability. This review describes the technological innovations and challenges associated with flexible energy storage and conversion systems such as lithium-ion batteries and supercapacitors, along with an overview of the progress in flexible proton exchange membrane fuel cells (PEMFCs) and solar cells. In particular, recently highlighted cable-type flexible batteries with extreme omni-directional flexibility are comprehensively discussed.

Porous silicon (PS) with a micro-nano-hybrid structure has been successfully fabricated with an electrochemical etching process. The micropores consist of one-dimensional tunnels, which vary from ca. 1 to <mml:math display="inline"><mml:mrow><mml:mn>1.5</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>1.5μm in pore diameter and extend up to <mml:math display="inline"><mml:mrow><mml:mn>15</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">μ</mml:mi><mml:mi mathvariant="normal">m</mml:mi></mml:mrow></mml:math>15μm in depth. The walls of these micropores are covered with a nanoporous structure that consists of small spherical particles, the feature size of which is of the order of tens of nanometers or smaller. The as-prepared PS structures show cathodic/anodic peaks for lithiation/delithiation during cyclic voltammetry with minimal destruction of either the micropore or nanopore of its wall even after 50 cycles. Furthermore, the peak current, the cumulative charge increase, and the electrochemical impedance for electrode reactions consistently decreases with the surface area of the tunnel wall, indicating that processes at the tunnel wall govern the overall lithiation/delithiation reactions. A hybrid porous structure consisting of microtunnels with nanostructured surface layers appears to provide a viable and practical way to utilize silicon for anode materials in rechargeable microbatteries.

We present faceted and non-faceted crystal cupric oxide (CuO) nanoribbons synthesized by different processes for glucose-sensing applications. The faceted and non-faceted CuO nanoribbons are grown via hydrothermal and microwave heating processes, respectively. Their growth mechanisms are considered as surface energy kinetics supported by a chemical simulation process. Both methods follow a redox precipitation mode for the formation of the nanostructures. Both faceted and non-faceted CuO nanostructures are revealed to exhibit different morphologies, but they have a single crystal structure. In a non-enzymatic glucose sensor, the amperometric oxidation currents of both faceted and non-faceted CuO electrodes increase proportionally as the glucose concentration increases gradually from 0.05 to 3.5 mM. However, the faceted CuO exhibits higher crystallinity with much more surface-active sites as faceted crystal planes compared to the non-faceted ones. Therefore, the glucose sensitivity of a faceted CuO electrode is superior to that of a non-faceted CuO one. The electrochemical glucose detection of the faceted CuO electrode shows a minimum concentration of 58 μM and a specific current sensing of 412 μA mM⁻¹ cm⁻², whereas the non-faceted CuO electrode shows low sensitivities of 71 μM and 356 μA mM⁻¹ cm⁻².