Charge Trapping‐Based Electricity Generator (CTEG): An Ultrarobust and High Efficiency Nanogenerator for Energy Harvesting from Water Droplets

Abstract

and droplet-based electricity generator (DEG). [9] However, inherent flaws exist in current approaches. Reverse electrowetting energy harvesting devices always need external voltages. [1] Triboelectric nanogenerator (TENG), [10,11] which was first invented in 2012 by Wang and coworkers, [12,13] has provided a passive energy harvesting approach. But the performance of TENG is limited by the low density and poor stability of surface charges on tribo-layers. High surface charge density could only be achieved in vacuum environment [14] or by utilizing external pumping or excitation sources. [11,15] The droplet energy harvesting efficiency of the conventional TENG was only 0.01%. [5] Recently, Z. K. Wang and coworkers have reported a water dropbased electric generator, DEG, [9] showing significantly enhanced energy harvesting efficiency to 2.2%. Nevertheless, the energy harvesting efficiency of DEG is still limited by the density and stability of charges generated by triboelectrification during drop impact. The maximum surface charge density of DEG displayed around 0.184 mC m −2 (49.8 nC for 2.7 cm 2). [9] The surface charges in DEG were superior stability compared to the conventional TENG, although the charge density still degraded in a harsh environment with 100% humidity. Moreover, the efficiency greatly dropped with increasing salt Strategies toward harvesting energy from water movements are proposed in recent years. Reverse electrowetting allows high efficiency energy generation, but requires external electric field. Triboelectric nanogenerators, as passive energy harvesting devices, are limited by the unstable and low density of tribo-charges. Here, a charge trapping-based electricity generator (CTEG) is proposed for passive energy harvesting from water droplets with high efficiency. The hydrophobic fluoropolymer films utilized in CTEG are pre-charged by a homogeneous electrowetting-assisted charge injection (h-EWCI) method, allowing an ultrahigh negative charge density of 1.8 mC m −2. By utilizing a dedicated designed circuit to connect the bottom electrode and top electrode of a Pt wire, instantaneous currents beyond 2 mA, power density above 160 W m −2 , and energy harvesting efficiency over 11% are achieved from continuously falling water droplets. CTEG devices show excellent robustness for energy harvesting from water drops, without appreciable degradation for intermittent testing during 100 days. These results exceed previously reported values by far. The approach is not only applicable for energy harvesting from water droplets or wave-like oscillatory fluid motion, but also opens up avenues toward other applications requiring passive electric responses, such as diverse sensors and wearable devices.

Electrowetting‐Assisted Generation of Ultrastable High Charge Densities in Composite Silicon Oxide–Fluoropolymer Electret Samples for Electric Nanogenerators

“…The samples were charged using the h‐EWCI process, as developed earlier. [ 13 ] In short, an adhesive tape with a 1 cm 2 opening was applied to the sample surface as a mask to define the surface area to‐be‐charged. Subsequently, a large puddle of aqueous NaOH solution (1 × 10 −3 m ), wider than the opening in the mask, was deposited on the sample and a charging voltage U C was applied for charging periods τ c of 2 min to 4 h between the silicon wafer and a Pt wire immersed into the puddle.…”

“…When a spreading droplet touches the wire, a current is generated with a peak value that is proportional to the surface potential and inversely proportional to the load resistance (bottom‐left inset Figure 1f). [ 13,14 ] The surface potential U S can thus be assessed by multiplying the load resistance with the peak value of the generated current, U S = I peak R L , as illustrated in Figure 1f. As expected, | U S | is found to increase with increasing | U C |.…”

“…In search of clean and renewable energy sources, so‐called (tribo)electric nanogenerators that convert mechanical energy into electrical energy have recently attracted substantial attention. [ 1–16 ] The efficiency of triboelectric nanogenerators is, however, limited by the relatively small [ 1,6–9,11–14 ] and often unstable [ 5,7,8,15 ] and poorly controllable [ 5,15,17–19 ] surface charge density generated by triboelectrification. Achieving large and stable charge densities by controlled injection (or ejection) of charge carriers into a material by various methods [ 20 ] was studied before in the context of electrets: (quasi‐)permanently charged materials that are ubiquitous because of their use in the majority of (consumer) microphones, [ 20–22 ] as nowadays commonly used in smartphones.…”

“…[ 42 ] As a modification of the process, homogeneous EWCI (h‐EWCI) that suppresses the contact line singularity of the electric field and involves a dielectrically stable oxide layer underneath a hydrophobic fluoropolymer film, subsequently enabled charge densities of the order of −1 mC m −2 and higher that were homogeneous over cm 2 areas (h‐EWCI), stable over ≥100 days under ambient conditions, and showed no appreciable degradation under the impact of (highly saline) droplets. [ 13,14 ] (High stability could only be achieved for negative charging polarity. [ 42 ] ) Notwithstanding these substantial improvements compared to conventional triboelectric nanogenerators as well as corona‐based charging of electrets, the mechanisms behind the h‐EWCI process, the origin of the long‐term stability, as well as the parameters to optimize it even further have so far remained elusive.…”

Electrowetting‐Controlled Dropwise Condensation with Patterned Electrodes: Physical Principles, Modeling, and Application Perspectives

“…[ 48 ] In the presence of electric fields, this effect is even more pronounced and can even be exploited to generate well‐controlled charge densities and charge patterns. [ 49,50 ] Therefore, the development of reliable hydrophobic fluoropolymer coatings that remain stable throughout the life time of various types of devices has been a long standing challenge in applied EW research. One recommendation from these investigations has been to avoid water as an operating fluid whenever possible.…”