Phonon-Engineered Hard-Carbon Nanoflorets Achieving Rapid and Efficient Solar-Thermal Based Water Evaporation and Space-Heating
Ananya Sah, Sumit Sharma, Sandip K. Saha, Chandramouli Subramaniam
Abstract
Generation and utilization of green heat produced from solar energy demand broadband absorbers with the elusive combination of strong phonon-driven photon thermalization and, contrastingly, weak phonon-lattice thermal conductivity. Here, we report a new class of porous, nanostructured hard-carbon florets (NCFs) consisting of isotropically assembled conical microcavities for greater light entrapment and efficient broad-band absorption (95% over 250–2500 nm). Resembling marigolds, the NCF exhibits short-range graphitic order that promotes instantaneous and efficient solar-thermal conversion (η STC = 87%) while exhibiting long-range intrinsic disorder providing low thermal conductivity (1.5 W m –1 K –1 ) to minimize thermal loss (13%). Solution processable NCF coatings on arbitrarily substrates (filter paper, terracotta, Cu and Al tubes) generate surface temperature of 400 ± 2 K and exhibit high thermal effusance (519 W s 0.5 m –2 K –1 ) to achieve highest combination of (a) rate of solar-driven interfacial water evaporation ( R w = 5.4 kg m –2 h –1, 2 sun), (b) solar-vapor conversion efficiency (η SVC = 186%), and (c) η STC (87%) among known materials. Such robust performance is retained for beyond 30 days of continuous operation and under different solar power (1 sun to 5 sun). Furthermore, active space heating (outlet air temperature = 346 ± 3 K) using NCF coatings is demonstrated.