Xabier García-Casas

The development of organic ferroelectric materials with scalable, simplified fabrication routes remains a key challenge for next-generation energy-harvesting technologies. Croconic acid (CA) emerges as a distinctive candidate, exhibiting a proton transfer-driven polarization mechanism that enables large spontaneous polarization through collective hydrogen-bond rearrangements. Herein, polycrystalline CA thin films were vacuum-sublimated onto plasma-treated flexible substrates and stabilized by in situ adamantane-based plasma-polymer encapsulation, ensuring ambient stability for over one year. The CA films exhibit well-defined, robust ferroelectric domain structures and localized low coercive fields at the nanoscale. The films exhibit both piezoelectric and pyroelectric responses to mechanical and thermal stimuli, respectively. Besides, structured CA within dielectric polymeric layers enhances macroscopic device performance, reaching a maximum output power density of 37 µW/m2. Crucially, despite the convention that high structural crystallinity is imperative for sustaining ferro-, piezo-, and pyroelectricity, our polycrystalline CA thin films demonstrate an exceptional room-temperature pyroelectricity that has not been explored in CA-based devices so far. It exhibits a pyroelectric coefficient of ~10 µC/m2K, which underscores a robust functional response that directly competes with established benchmarks in both organic and inorganic pyroelectric material classes. This room-temperature, all-vacuum strategy enables scalable integration of lead-free organic ferroelectrics into flexible multisource energy harvesters.

This paper introduces a novel, non-invasive, and ultra-fast experimental procedure for the characterization of electrostatic energy harvesters. The method builds on a new analytical solution derived from a piecewise expansion of the generator’s effective time-varying capacitance. The formulation is general and applies to both electret-based and electret-free electrostatic nanogenerators (ENGs), providing a unified inter­pretation of their voltage and charge dynamics. Unlike the standard power output characterization, which typically relies on diodes and active circuitry that inevitably load and perturb the harvester and require long data acquisitions under periodic excitation to ensure convergence, our method extracts the intrinsic properties of the device from a short measurement of a single ENG event. The test setup is extremely simple, re­­qui­­ring only an oscilloscope and no external excitation source. This fast and non-invasive operation is particularly relevant in the current trend of device miniaturization, where ENGs become smaller and the parasitic contributions of the measurement setup grow comparable to the contribution of the device. The methodology has been validated through detailed elec­trical simulations and experimental evaluation on different triboelectric nanogenerators (TENGs) for water droplet energy harvesting applications (WD-TENGs), demonstrating accurate parameter extraction and reliable performance predictions. The accompanying characterization tool C-ENGiNE was used to automate and reproduce the full workflow during validation.

Triboelectric nanogenerators (TENGs) are major drivers in on-site power generation for smart devices, enable self-powered sensors, and introduce novel catalytic processes. Here, we demonstrate adamantane plasma polymers as versatile dual-polarity triboelectric surfaces capable of exhibiting either tribopositive or tribonegative behavior through simple control of the plasma deposition configuration, without additives or post-functionalization. By modifying the substrate orientation relative to the plasma, thin-film polymers with distinct dielectric, mechanical, secondary electron emission, and triboelectric properties are obtained. The high conformality, stability, and processability of the polymers enable their direct implementation into solid-solid, solid-liquid, and hybrid piezo-triboelectric configurations. Additional surface texturization through buckling provides voltage and current outputs as high as 90 Vpeak-to-peak and 0.6 µA cm-2 for a 2.8 µm (tribonegative) vs. 400 nm (tribopositive) combination with a maximum mean power density of 2.1 µW cm-2. A maximum peak power of 200 µW is generated from salty droplets in drop-TENG configuration employing a 500 nm-thick tribopositive adamantane polymer. These layers sustain more than 105 cycles in solid-solid nanogenerators and 104 droplet impacts in solid-liquid devices. Combined with an environmentally friendly and industrially scalable synthesis route, these results position adamantane plasma polymers as a robust and versatile platform for next-generation triboelectric energy-harvesting materials.

We report on the development of a novel flexible piezo/pyro-electric nanogenerator (PPNG) that combines a uniform film of poly(vinylidene fluoride) (PVDF) infiltrated into vertically supported SiO2 nanowires (NWs) to enhance both piezoelectric and pyroelectric energy harvesting capabilities. The synthetic procedure involves a low-temperature, multi-step approach, including the soft-template formation of SiO2 NWs on a flexible substrate, followed by the infiltration of a PVDF thin film (TF). The plasma-enabled fabrication of SiO2 NWs facilitated vertical alignment and precise control over the surface microstructure, density, and thickness of the confined nanostructures. These strategic structural systems promote the development of the most favorable electroactive β- and γ-phases in the PVDF matrix. Notably, electrical poling plays a major role in aligning the random dipoles of the PVDF macromolecular chain in a more ordered fashion to nucleate the amplified electroactive phases. The electrostatic interaction between PVDF and SiO2 NWs, which helps to facilitate the electroactive phase, is confirmed by the surface potential distribution. As a proof-of-concept, the fabricated PPNG exhibited a significant improvement in the instantaneous piezoelectric output power density (P), with a ∼9-fold amplification relative to its bare PVDF TF counterpart. In addition, PPNG has demonstrated excellent stability and durability. Analogously, the pyroelectric coefficient (p) demonstrated a 4-fold superior performance compared to the reference PVDF TF-based PPNG. Thus, the engineered system of SiO2 NWs@PVDF comprising PPNG offers a promising pathway toward multisource energy harvesting capabilities through efficient energy transduction at mechanical excitation frequencies of 10–12 Hz and across a temperature difference of 9–22 K.

Triboelectric nanogenerators (TENGs) have emerged as potential energy sources, as they are capable of harvesting energy from low-frequency mechanical actions such as biological movements, moving parts of machines, mild wind, rain droplets, and others. However, periodic mechanical motion can have a detrimental effect on the triboelectric materials that constitute a TENG device. This study introduces a self-healable triboelectric layer consisting of an Ecoflex-coated self-healable polydimethylsiloxane (SH-PDMS) polymer that can autonomously repair mechanical injury at room temperature and regain its functionality. Different compositions of bis(3-aminopropyl)-terminated PDMS and 1,3,5-triformylbenzene were used to synthesize SH-PDMS films to determine the optimum healing time. The SH-PDMS films contain reversible imine bonds that break when the material is damaged and are subsequently restored by an autonomous healing process. However, the inherent stickiness of the SH-PDMS surface itself renders the material unsuitable for application in TENGs despite its attractive self-healing capability. We show that spin-coating a thin layer (≈32 μm) of Ecoflex on top of the SH-PDMS eliminates the stickiness issue while retaining the functionality of a triboelectric material. TENGs based on Ecoflex/SH-PDMS and nylon 6 films show excellent output and fatigue performance. Even after incisions were introduced at several locations in the Ecoflex/SH-PDMS film, the TENG spontaneously attained its original output performance after a period of 24 h of healing. This study presents a viable approach to enhancing the longevity of TENGs to harvest energy from continuous mechanical actions, paving the way for durable, self-healable mechanical energy harvesters.

Within these studies, the effect of surface topography for hydrophobic coatings was studied both numerically and experimentally. Chemically modified polyurethane coating was patterned by application of a laser beam. A set of patterns with variously distant linear peaks and grooves was obtained. The cross section of the pattern showed that the edges of the peaks and grooves were not sharp, instead forming a rounded, rectangle-like shape. For such surfaces, experimental studies were performed, and in particular the static contact angle (SCA), contact angle hysteresis (CAH), and roll-off angle (ROA) were measured. Profilometry was used to create a numerical representation of the surface. Finite volume method was then applied to simulate the behavior of the water droplets. The model developed herewith enabled us to reproduce the experimental results with good accuracy. Based on the verified model, the calculation was extended to study the behavior of the water droplet on the simulated patterns, both spiked and rectangular. These two cases, despite a similar SCA of the water droplet, have shown extremely different ROA. Thus, more detailed studies were dedicated to other geometrical features of such topography, such as the size and distance of the surface elements. Based on the results obtained herewith, the future design of superhydrophobic and/or icephobic topography is discussed.

The growth of TiO₂and ZnO thin films is studied by means of coarse‐grained kinetic Monte Carlo simulations under conditions typically encountered in plasma‐enhanced chemical vapor deposition experiments. The basis of our approach is known to work well to simulate the growth of amorphous materials using cubic grids and is extended here to reproduce not only the morphological characteristics and scaling properties of amorphous TiO₂but also the growth of polycrystalline ZnO with a good approximation, including the evolution of the film texture during growth and its dependence on experimental conditions. The results of the simulations have been compared with available experimental data obtained by X‐ray diffraction, analysis of the texture coefficients, atomic force microscopy, and scanning electron microscopy.