Non-planar dielectrics derived thermal and electrostatic field inhomogeneity for boosted weather-adaptive energy harvesting

ABSTRACT Weather-adaptive energy harvesting of omnipresent waste heat and rain droplets, though promising in the field of environmental energy sustainability, is still far from practice due to its low electrical output owing to dielectric structure irrationality and unscalability. Here we present atypical upcycling of ambient heat and raindrop energy via an all-in-one non-planar energy harvester, simultaneously increasing solar pyroelectricity and droplet-based triboelectricity by two-fold, in contrast to conventional counterparts. The delivered non-planar dielectric with high transmittance confines the solar irradiance onto a focal hotspot, offering transverse thermal field propagation towards boosted inhomogeneous polarization with a generated power density of 6.1 mW m−2 at 0.2 sun. Moreover, the enlarged lateral surface area of curved architecture promotes droplet spreading/separation, thus travelling the electrostatic field towards increased triboelectricity. These enhanced pyroelectric and triboelectric outputs, upgraded with advanced manufacturing, demonstrate applicability in adaptive sustainable energy harvesting on sunny, cloudy, night, and rainy days. Our findings highlight a facile yet efficient strategy, not only for weather-adaptive environmental energy recovery but also in providing key insights for spatial thermal/electrostatic field manipulation in thermoelectrics and ferroelectrics.


Table of Contents
Note S1. Discussion on non-planar dielectrics for weather-adaptive energy harvesting ..  Outdoor test of scalable prototype for weather-adaptive energy harvesting  Note S1. Discussion on non-planar dielectrics for weather-adaptive energy harvesting From the point of view of the Second Law of Thermodynamics, the total entropy of an isolated system can never decrease over time, so the disordered, decentralized, and distributed energy forms are widespread around the surroundings (Fig. 1a). In this context, the temporal temperature change (dT/dt), a ubiquitous thermal energy resource of greater abundance which arises from any non-static illuminations due to wind, cloud cover etc., is a cornucopia of energy sources with immense potential in the field of green energy to meet the climate target and sustainable development goals (SDG) [1-3]. In specific, there is commonly weather/seasondependent low-light irradiance, and rainfall fluctuates with wind speed and humidity changes [4][5][6]. Unfortunately, these non-static, weather-dependent, low-grade heat variations are of equal significance as the static temperature gradient while are far-flung and agelong negligence.
More importantly, in terms of energy sustainability and carbon neutrality, the challenges are "how can we harness this energy that is all around us, but not yet utilized for much of anything" [7,8]. For instance, how to engineer the configuration and develop sustainable approaches from the material, device, and system levels, as well as nanoscale to macroscale views, to capture environmental underexplored energies into cost-effective electricity via an adaptive manner?
Traditionally, the planar film configuration was utilized for environmental energy harvesting owing to bottom-up layer-by-layer structure design and facile fabrication procedure. However, the planar structure with a low degree of freedom is not a priority to meet high-entropy energy harvesting of diffused, disordered, decentralized, and distributed energy sources due to its high demand for structure adaptiveness and device efficacy. Explicitly, conventional planar pyroelectrics capture the solar heat homogenously, limited by simultaneous and uniform thermal field propagation across the entire device (Fig. S1) [6,9,10]. These approaches with spatiotemporally coupled thermodynamic processes restrain the heat variation and temperature 5 gradient (ΔT), thus resulting in limited pyroelectric output [11,12]. Meanwhile, traditional rain droplet-based electricity generators mainly utilize flat triboelectric dielectrics with a tilting angle via electrostatic induction and triboelectrification during the temporal liquid-solid contact/separation process along the sliding direction [13][14][15][16][17][18]. Thereby, one of the facing challenges in the domain of environmental energy harvesting is how to enhance the temporal heat variation and droplet spreading change adaptively via a facile configuration. According to pyroelectric fundamentals, an intense dT/dt from large ΔT contributes to creating a larger dipole moment shift (PS), thus leading to higher electrostatic intensity induced surface charge density [11]. Also, the contact electrification mechanism suggests that the triboelectric output can be enhanced by promoting the temporal droplet spreading area change (dS/dt) [16]. Therefore, it is urged to develop non-planar dielectrics with a high degree of freedom for flexible manipulation of thermal and electric field propagation, aiming to boost the power output.
Herein, we first focused on how the facile, in-plane, macroscopic heat modulation contributes to upcycle and rationalizes the inexhaustible but unusable, low-light solar irradiance into inplane heat propagation towards enhanced PS change and pyroelectric output without tailoring materials properties [19,20], altering pyroelectric coefficients [21], or applying electric fields [22]. We solely redistributed the incident light onto the PVDF-based film via a non-planar dielectric to trigger inhomogeneous heat propagation from the hotspot to non-irradiation areas along the transverse direction, thus achieving intense dT/dt and PS changes (Figs 1b and 2d and 2e). Second, we analyzed how the non-planar dielectric promote droplet spreading area change on the enlarged, curved surface for high triboelectric output from experiments and simulations.
Our findings verified the non-planar dielectric is capable of increasing the output by 1-2 folds for pyroelectricity and droplet-based triboelectricity (Fig. 1c). 6 Note S2. Experimental section

Device fabrication and characterization
A ferroelectric poly(vinylidene difluoride) (PVDF) (Fils Co., Ltd.) thin film with a dimension of 16 mm (diameter) × 80 μm (thickness) is utilized for solar heat harvesting. The top and bottom sides of the PVDF thin film were deposited with carbon nanotubes (CNT, Xianfeng Nano) electrodes using evaporation coating methods, and details can be found in the previous work [6,23]. These CNT solar absorbers also offer desirable solar-to-heat conversion efficiency.

Pyroelectric and triboelectric measurements
The electrical measurement of the pyroelectric device was conducted in a semi-closed moisture-

Finite element analysis of droplet spreading/separation
The droplet spreading/separation dynamics on the surface of the TEH unit were performed by the finite element analysis method using COMSOL Multiphysics coupled with Laminar Flow and Level Set modules. The measured TEH and droplet dimensions, as well as other parameters selected from the COMSOL materials library, were utilized in the simulation (Table S2).

Outdoor test of scalable weather-adaptive energy harvester
The outdoor test was conducted at the location of 22°36 '27 For sunny/cloudy/night measurements, the sunlight irradiation together with wind/humiditydriven heat convection facilitates the hotspot temperature variation on the PVDF/CNT surface.
In rainy conditions, the falling rain droplet impinges the PTFE conical array to induce triboelectricity. The electrical signal of pyroelectric and triboelectric devices was recorded using a 6514 electrometer. The PVDF surface temperature was monitored at the local hotspot by a temperature datalogger (SSN-61, YUWESE). The in-situ solar irradiation was measured using a solar power meter datalogger (1333R, TES). The ambient temperature, ambient relative humidity (RH), and wind speed were recorded by a hot-wire anemometer (1341, TES). The simultaneous outdoor test of a PVDF-based device and solar cell (polycrystalline silicon, TELESKY) with an identical area of 30 mm × 30 mm, was performed at the location of 1°17'56'' N, 103°46'19'' E, the grass ground behind E3-03-01, Multidisciplinary Lab, Kent Ridge campus, National University of Singapore, in the time duration from 08.00 PM (31 December 2022) to 9 8.00 PM (1 January 2023). In specific, the solar flux, voltage output of solar cell, ambient temperature, and PVDF temperature were recorded concurrently by using a four-channel datalogger (HD35EDLW wireless data loggers, DeltaOHM), and the generated voltage of pyroelectric PVDF-based device was acquired by using an electrometer (6514, Keithley) incorporated with LabView software. All data were collected simultaneously and connected to a laptop for processing. 10

Note S3. Load measurement of PEH and TEH units
The load measurement of planar and non-planar PEH units was conducted at a solar intensity of 0.2 sun with a heating/cooling (light on/off) time duration ratio of 0.5. A tunable load resistor (ZX79, Fuyang Precision) was serially connected with a PEH unit was used to vary the matched resistance and load voltage (load resistance was switched from 0.1, 0.5, 1, 5, 10, 25, 50, to 75 GΩ). The corresponding voltage signal was recorded using a 6514 electrometer under ten periodic heating/cooling cycles for each load measurement. Based upon Kirchhoff's voltage law and Ohm's law, the power ( ) was extracted by time integration of voltage under corresponding load impedance (R) [26] where the initial time is , the integration time was selected from the recorded timedependent voltage signals. The calculated results using equation S1 are the average power in the time duration , and not the peak power. Similarly, for load measurement of the planar TEH unit, the load resistance was switched from 0.01, 0.1, 0.5, 1, 5, to 10 GΩ. Moreover, for load measurement of the non-planar TEH unit, the load resistance was switched at 0.001, 0.01, 0.1, 1, 10, and 100 (unit: GΩ). The power density of TEH units is then calculated from formula S1.
Apart from the average power density , the peak power density ( ) is also an evaluation matrix for directly determining the electrical output of energy harvesters. Typically, the is dependent on the maximum current, voltage profiles, internal resistance, and load impedance.
Specifically, the is achieved as long as the internal resistance of the device equals the matched load resistance [26], as given by where and represent the peak-to-peak current and voltage calculated from timedependent signals at a specific time, respectively.     (dS/dt), where the spreading dS/dt is calculated from the wetting arc (w) and DSVspread, the spreading factor (β) is defined as the ratio between w and water droplet diameter at initial state [16,28,29].  This work † where ϕ stands for the diameter of circular PEH units, the power density was estimated from the formula S1, and the output/area was calculated from the average power density at 1 sun illumination. ‡ The power density was estimated from formula S2 and Fig. S7e. The apex angle of the non-planar sample is around 90° (PR = 0.5, corresponding θ = 45°), and the wettability (contact angle) of the PTFE surface is around 118° (Fig. S8c). The dropping height of deionized water droplet (20 μl) was fixed at 20 cm and the dropping frequency was 1 Hz (Note S2). The Weber number (We) of impacting droplets in the experimental conditions was estimated to be 180.8 from ρv 2 L/γ, where ρ is the density, L is the characteristic length or droplet diameter, v is the impact velocity, and γ is the surface tension of droplets [14].