In recent decades, the growing demands for energy saving and accompanying heat mitigation concerns, together with the vital goal for carbon neutrality, have drawn human attention to the zero‐energy‐consumption cooling technique. Recent breakthroughs in passive daytime radiative cooling (PDRC) might be a potent approach to combat the energy crisis and environmental challenges by directly dissipating ambient heat from the Earth to the cold outer space instead of only moving the heat across the Earth's surface. Despite significant progress in cooling mechanisms, materials design, and application exploration, PDRC faces potential functionalization, durability, and commercialization challenges. Herein, emerging materials and rational strategies for PDRC devices are reviewed. First, the fundamental physics and thermodynamic concepts of PDRC are examined, followed by a discussion on several categories of PDRC devices developed to date according to their implementation mechanism and material properties. Emerging strategies for performance enhancement and specific functions of PDRC are discussed in detail. Potential applications and possible directions for designing next‐generation high‐efficiency PDRC are also discussed. It is hoped that this review will contribute to exciting advances in PDRC and aid its potential applications in various fields.

This review reports the recent progress and future prospects of wettability gradient surfaces (WGSs), particularly focusing on the governing principles, fabrication methods, classification, characterization, and applications. While transforming the inherent wettability into artificial wettability via bioinspiration, topographic micro/nanostructures are produced with changed surface energy, resulting in new droplet wetting regimes and droplet dynamic regimes. WGSs have been mainly classified in dry and wet surfaces, depending on the apparent surface states. Wettability gradient has long been documented as a surface phenomenon inducing the droplet mobility in the direction of decreasing wettability. However, it is herein critically emphasized that the wettability gradient does not always result in droplet mobility. Indeed, the sticky and slippery dynamic regimes exist in WGSs, prohibiting or allowing the droplet mobility, respectively. Lastly, the stringent bottlenecks encountered by WGSs are highlighted along with solution-oriented recommendations, and furthermore, phase change materials are strongly anticipated as a new class in WGSs. In all, WGSs intend to open up new technological insights for applications, encompassing water harvesting, droplet and bubble manipulation, controllable microfluidic systems, and condensation heat transfer, among others.

Quasi-solid thermocells show great potential to save power terminals from periodic charging but still face the grand challenge of low thermoelectric efficiency. Despite many efforts devoted to improve thermopower, few studies have been reported to address the trade-off between thermopower and ionic conductivity encountered by n-type quasi-solid thermocells. Herein, a directional freeze-thawing method is developed to fabricate high-performance n-type quasi-solid thermocells with hierarchically anisotropic networks, enabling the decoupling of thermopower and ionic conductivity. The n-type thermopower is up to 0.74 mV/K, and the ionic conductivity is independently improved to be about 9.3 S/m. Thus, the output power density reaches ∼200 mW/m2, which is the same level among the quasi-solid n-type thermocells. Meanwhile, benefiting from the crystalline domains and alignment structures of the solid network, the thermocells achieve the strength of ∼380 kPa and an elongation at break of ∼320%. Moreover, the thermocells work stably when being pressed, bent, and stretched in practical uses. We believe this work not only demonstrates a particularly important example for fabricating high-performance n-type quasi-solid thermocells but also inspires the development of thermocell devices to achieve large-scale low-grade heat harvesting in wearable systems.