Po-Chun Hsu1
Duke University1
Looking up to the sky, there are the most powerful renewable heat source and cold sink: The sun and the deep space. In the past decade, both solar heating and radiative cooling have shown immense promise in reducing the demand on fossil fuel-based heating and cooling. For many applications involving varying thermal environments, such as building HVAC and personal thermal management, it is essential to have both heating and cooling functionalities in a dynamic and tunable way. However, such tuning involves ultrawideband (from UV to mid-IR) photonic engineering, and the required properties are the opposite for solar and thermal radiation frequency regimes. Moreover, it is preferable that the tuning is electrical and does not have moving part. <br/>In this talk, I will introduce our recent progress of adaptive solar heating and radiative cooling (A-SHARC) device. We developed two key components to accomplish the A-SHARC: (i) an ultra-wideband transparent conductor and (ii) a reversible electrochemical reaction for tuning the device solar absorptivity and mid-IR emissivity. We showed that monolayer graphene and gold microgrid composite can serve as the ultra-wideband transparent conductor with high transmittance (T<sub>UV-Vis</sub> = 85.63%, T<sub>near-IR</sub> =87.85%, and T<sub>mid-IR</sub> = 84.87%) and low sheet resistance (R<sub>s</sub> = 22.4 ohm/sq). Enabled by the transparent conductor, we perform metal electrodeposition to vary the emissivity between 0.12 and 0.94 (Δε = 0.82). By manipulating the interfacial chemistry, we can control the metal morphology to become near-percolating nanoparticle clusters that work as a plasmonic selective absorber for solar heating at low emissivity. When cooling is needed, reversing the bias can dissolve the metal nanoparticles back to visibly transparent state, exposing the underlying mirror for solar reflectivity and electrolyte for high thermal emissivity. The optimal solar absorptivity (α) and thermal emissivity (ε) of solar heating and radiative cooling mode are (α, ε) = (0.60, 0.20), and (0.33, 0.94), respectively. The dual-band solar and mid-IR electrochromic device can bring vast opportunities for applications in heat management, camouflage, display, and building energy efficiency.<br/>Reference: ACS Energy Lett. 2021, 6, 3906−3915. doi.org/10.1021/acsenergylett.1c01486