Stepless IR Chromism in Ti3 C2 Tx MXene Tuned by Interlayer Water Molecules.

Real-time control over infrared (IR) radiation of objects is highly desired in a variety of areas such as personal thermal regulation and IR camouflage. This requires the dynamic modulation of IR emissivity in a stepless manner over a wide range (>50%), which remains a daunting challenge. Here, an emissivity modulation phenomenon is reported in stacked 2D Ti3 C2 Tx MXene nanosheets, from 12% to 68% as the intercalation/discharging of water molecules within the interlayers. The intercalation of water molecules dynamically changes the electronic properties and the complex permittivity in IR frequencies of Ti3 C2 Tx . This emissivity modulation is a stepless and reversible process without the assistance of any external energy input. Further, intercalating cellulose nanofibers into the Ti3 C2 Tx interlayers makes this dynamic process highly repeatable. Last, a sweat-responsive adaptive textile that can improve thermal comfort of human body under changes in metabolic rates and environmental conditions is demonstrated, showing great potential of this mechanism in passive on-demand radiation modulation.

Shear-Enhanced Ion Rejection during Seawater Freezing.

Ion rejection during seawater freezing is the basis for freeze desalination. A high ion rejection rate is desired for improving the performance of freeze desalination. In this work, we propose a method to enhance the ion rejection rate through external shear, which is demonstrated through molecular dynamics (MD) simulations and experiments. MD simulations show that the ion rejection rate increases with an increasing shear rate. This is attributed to the disruption of the hydration bonds between ions and water molecules in the hydration shell caused by the shear. Consequently, the mobility of ions is increased, and the energy barrier is reduced at the ice-water interface such that ions have a greater chance of diffusing into the aqueous solution, leading to an enhanced ion rejection rate. The MD results in this work are qualitatively confirmed by experiments and provide insights into the enhancement of the ion rejection rate through external parameters.

A Janus Textile Capable of Radiative Subambient Cooling and Warming for Multi-Scenario Personal Thermal Management.

Textiles with radiative cooling/warming capabilities provide a green and effective solution to personal thermal comfort in different climate scenarios. However, developing multiple-mode textiles for wearing in changing climates with large temperature variation remains a challenge. Here a Janus textile is reported, comprising a polyethersulfone (PES)-Al2 O3 cooling layer optically coupled with a Ti3 C2 Tx warming layer, which can realize sub-ambient radiative cooling, solar warming, and active Joule heating. Owing to the intrinsically high refractive index of PES and the rational design of the fiber topology, the nanocomposite PES textile features a record high solar reflectance of 0.97. Accompanied by an infrared (IR) emittance of 0.91 in the atmospheric window, sub-ambient cooling of 0.5-2.5 °C is achieved near noontime in humid summer under ≈1000 W m-2 solar irradiation in Hong Kong. The simulated skin covered with the textile is ≈10 °C cooler than that with white cotton. The Ti3 C2 Tx layer provides a high solar-thermal efficiency of ≈80% and a Joule heating flux of 66 W m-2 at 2 V and 15 °C due to its excellent spectral selectivity and electrical conductivity. The switchable multiple working modes enable effective and adaptive personal thermal management in changing environments.

Reversible bipolar thermopower of ionic thermoelectric polymer composite for cyclic energy generation.

The giant thermopower of ionic thermoelectric materials has attracted great attention for waste-heat recovery technologies. However, generating cyclic power by ionic thermoelectric modules remains challenging, since the ions cannot travel across the electrode interface. Here, we reported a reversible bipolar thermopower (+20.2 mV K-1 to -10.2 mV K-1) of the same composite by manipulating the interactions of ions and electrodes. Meanwhile, a promising ionic thermoelectric generator was proposed to achieve cyclic power generation under a constant heat course only by switching the external electrodes that can effectively realize the alternating dominated thermodiffusion of cations and anions. It eliminates the necessity to change the thermal contact between material and heat, nor does it require re-establish the temperature differences, which can favor improving the efficiency of the ionic thermoelectrics. Furthermore, the developed micro-thermal sensors demonstrated high sensitivity and responsivity in light detecting, presenting innovative impacts on exploring next-generation ionic thermoelectric devices.

Uncooled self-powered hemispherical biomimetic pit organ for mid- to long-infrared imaging.

Infrared vision is highly desirable for applications in multifarious fields. Of the few species with this visual capability, snakes have exceptional infrared perception with the assistance of pit organs. Inspired by the pit organ design we present here a hemispherical biomimetic infrared imaging device. The devices use high-density ionic thermoelectric polymer nanowire arrays that serve as the sensing nerve cells. The individual nanowires exhibit notable voltage response to temperature variation in test objects. An infrared sensor array with 625 pixels on the hemispherical substrate is successfully demonstrated with an ultrawide field of view up to 135°. The device can image body temperature objects without a cooling system and external power supply. This work opens up opportunities for the design and fabrication of bioinspired infrared imaging devices based on emerging ionic thermoelectric materials.

All-weather thermochromic windows for synchronous solar and thermal radiation regulation.

Adaptive control of solar and thermal radiation through windows is of pivotal importance for building energy saving. However, such synchronous passive regulations are challenging to be integrated into one thermochromic window. Here, we develop a solar and thermal regulatory (STR) window by integrating poly(N-isopropylacrylamide) (pNIPAm) and silver nanowires (AgNWs) into pNIPAm/AgNW composites. A hitherto unexplored mechanism, originating from the temperature-triggered water capture and release due to pNIPAm phase transition, is exploited to achieve simultaneous regulations of solar transmission and thermal emission. The STR window shows excellent solar modulation (58.4%) and thermal modulation (57.1%) and demonstrates effective regulation of indoor temperatures during both daytime and nighttime. Compared to other thermochromic technologies, the STR window reduces heat loss in cold environment while promotes heat dissipation in hot conditions, achieving efficient energy saving in all weathers. This dual solar and thermal regulation mechanism may provide unidentified insights into the advancement of smart window technology.

Selectively tuning ionic thermopower in all-solid-state flexible polymer composites for thermal sensing.

There has been increasing interest in the emerging ionic thermoelectric materials with huge ionic thermopower. However, it's challenging to selectively tune the thermopower of all-solid-state polymer materials because the transportation of ions in all-solid-state polymers is much more complex than those of liquid-dominated gels. Herein, this work provides all-solid-state polymer materials with a wide tunable thermopower range (+20~-6 mV K-1), which is different from previously reported gels. Moreover, the mechanism of p-n conversion in all-solid-state ionic thermoelectric polymer material at the atomic scale was presented based on the analysis of Eastman entropy changes by molecular dynamics simulation, which provides a general strategy for tuning ionic thermopower and is beneficial to understand the fundamental mechanism of the p-n conversion. Furthermore, a self-powered ionic thermoelectric thermal sensor fabricated by the developed p- and n-type polymers demonstrated high sensitivity and durability, extending the application of ionic thermoelectric materials.

A Solution-Processed Inorganic Emitter with High Spectral Selectivity for Efficient Subambient Radiative Cooling in Hot Humid Climates.

Daytime radiative cooling provides an eco-friendly solution to space cooling with zero energy consumption. Despite significant advances, most state-of-the-art radiative coolers show broadband infrared emission with low spectral selectivity, which limits their cooling temperatures, especially in hot humid regions. Here, an all-inorganic narrowband emitter comprising a solution-derived SiOx Ny layer sandwiched between a reflective substrate and a self-assembly monolayer of SiO2 microspheres is reported. It shows a high and diffusive solar reflectance (96.4%) and strong infrared-selective emittance (94.6%) with superior spectral selectivity (1.46). Remarkable subambient cooling of up to 5 °C in autumn and 2.5 °C in summer are achieved under high humidity without any solar shading or convection cover at noontime in a subtropical coastal city, Hong Kong. Owing to the all-inorganic hydrophobic structure, the emitter shows outstanding resistance to ultraviolet and water in long-term durability tests. The scalable-solution-based fabrication renders this stable high-performance emitter promising for large-scale deployment in various climates.

Giant and bidirectionally tunable thermopower in nonaqueous ionogels enabled by selective ion doping.

Ionic thermoelectrics show great potential in thermal sensing owing to their ultrahigh thermopower, low cost, and ease in production. However, the lack of effective n-type ionic thermoelectric materials seriously hinders their applications. Here, we report giant and bidirectionally tunable thermopowers within an ultrawide range from −15 to +17 mV K−1 in solid ionic liquid­based ionogels. Particularly, a record high negative thermopower of −15 mV K−1 is achieved in the ternary ionogel, rendering it among the best n-type ionic thermoelectric materials under the same condition. A thermopower regulation strategy through ion doping to selectively induce ion aggregates to enhance ion-ion interactions is proposed. These selective ion interactions are found to be decisive in modulating the sign and magnitude of the thermopower in the ionogels. A prototype wearable device integrated with 12 p-n pairs is demonstrated with a total thermopower of 0.358 V K−1, showing promise for ultrasensitive thermal detection.

2D Ti3 C2 Tx MXenes: Visible Black but Infrared White Materials.

Black inorganic materials with low infrared absorption/emission (or IR white) are rare in nature but highly desired in numerous areas, such as solar-thermal energy harvesting, multispectral camouflage, thermal insulation, and anti-counterfeiting. Due to the lack of spectral selectivity in intrinsic materials, such counter-intuitive properties are generally realized by constructing complicated subwavelength metamaterials with costly nanofabrication techniques. Here, the intrinsically low mid-IR emissivity (down to 10%) of the 2D Ti3 C2 Tx MXene is reported. Associated with a high solar absorptance (up to 90%), it embraces the best spectral selectivity among the reported intrinsic black solar-absorbing materials. Its appealing potential in several of the aforementioned areas is experimentally demonstrated. First-principles calculations reveal that the IR emissivity of MXene relies on both the nanoflake orientations and terminal groups, indicating great tunability. The calculations also suggest more potential low-emissivity MXenes including Ti2 CTx , Nb2 CTx , and V2 CTx . This work opens the avenue to further exploration of a family of intrinsically low-emissivity materials with over 70 members.