In the U.S., most buildings need heating and air-conditioning to maintain the temperature all year around. This energy consumption is drastically increased in the hot summer and cold winter. According to the statistical study, nearly 30% of the energy consumption and 10% of the greenhouse gas come from buildings. Looking for diverse approaches to maintain a comfortable building temperature becomes a key point to creating a sustainable and carbon-negative future. Instead of controlling the indoor temperature in conventional ways, we keep wondering: can we treat the building like the human body? Like the emergency blanket, a single thin layer of metal coating will give the human a warmer feeling due to the suppressed thermal radiation. However, such a product only helps the body trap heat in cold weather and will not have any benefit in releasing heat in hot summer. If the function is tunable, which means switching between heating (trapping heat inside) and cooling (releasing heat out), it would be possible to maintain a comfortable temperature just by wearing this. By the same token, if we could develop a switchable thin layer coating for buildings, which allows buildings to change their optical properties according to the weather, the indoor temperature would be effectively and passively maintained all year round.
According to the law of blackbody radiation, every object on earth at room temperature will radiate its heat into the deep universe. The temperature of the deep universe is only 3K and therefore offers a significant cooling source for the object. Meanwhile, the amplitude of such a radiative heat transfer process is linearly dependent on the thermal emissivity of the object’s surface. Tuning the surface thermal emissivity can control the heat loss of the building on demand. Ideally, thermal emissivity should be increased to one in the hot summer to help the building release more heat and, in winter, the emissivity should be decreased to near zero to suppress the heat loss. If the emissivity can be tuned electrically, the indoor temperature of the building can be controlled on demand.
With this idea, we chose electrochromism as an approach to tune its optical property. However, most of the conventional electrochromism works in the visible and near-infrared wavelength regions. The mid-infrared regions have not yet been utilized, which means that the ability to save energy has not been brought into full play. Thus, our goal is to develop a mid-infrared electrochromic device that can continuously and reversibly vary its thermal emissivity for all-season radiative thermoregulation. One of the main challenges is to fabricate an ultra-wideband transparent conductive electrode. This is because the electrochromic device works like a battery and we will use the top electrode materials’ properties change to vary the thermal emissivity, so the top electrode must be transparent to the mid-infrared thermal radiation. Initially, it is hard to find the appropriate materials that are conductive and transparent in the mid-infrared wavelength regime. None of the commercial transparent conductive materials like ITO, FTO, and silver nanowires work in this long-wavelength regime. Then, graphene came to our eye. Due to its unique energy band structure, it is both conductive and transparent in the mid-infrared. To improve its mechanical strength and long-range conductivity, we also combined it with polyethylene film and gold microgrid. The final electrode shows a very impressive optical and electrical figure of merit as a transparent electrode.
Another key challenge is to figure out a decent redox reaction for reversible metal electrodeposition. This requires an in-depth physical understanding of interfacial electrochemistry. Inspired by Dr. Michael D. McGehee from the University of Colorado, Boulder, we designed an aqueous copper-based electrolyte, which shows impressive cycle life and non-volatile electrodeposition. These outstanding performance benefit from the rational design of the redox reaction by removing the Br-/Br3- in the conventional DMSO-based electrolyte. In the traditional reversible electrodeposition device, the DMSO-based electrolyte is the main character. However, it contains Br-/Br3- and will etch away the deposited metal. Therefore, the non-volatile electrodeposition definitely improves the energy efficiency of the device because we do not need to supply the bias to maintain the deposited state.
With the above-mentioned design, this aqueous electrochromic system realizes the state-of-the-art tuning contrast of the thermal emissivity (Δε = 0.85) without a significant degradation after 2500 cycles, thanks to the Pt-modification on the monolayer graphene and optimized aqueous electrolyte. In the experiment, we found that the copper deposition would be more uniform and reversible if we modify the graphene with 2 nm thick Pt. This is very exciting because it may point out a pathway to improve the electrodeposition of different kinds of metals. Hence, we have to understand the fundamental physics behind this. We collaborated with Dr. Venkatasubramanian Viswanathan from Carnegie Mellon University to help us do theoretical calculations to better understand the physical reasons. Density functional theory simulations show that Pt has a high affinity for deposited Cu atoms. This results in uniform electrodeposition and low overpotential.
It is worth saying, during this process, the most memorable thing for me is that we encountered COVID-19, which caused a lot of difficulties. During the pandemic, the closure of the laboratory made it more difficult for us to do experiments. Fortunately, with limited resources, we were able to finish our project with satisfying performance and mechanistic understanding. At the same time, our collaborator, Dr. Venkatasubramanian Viswanathan of Carnegie Mellon University also gave us a lot of help in theoretical calculations, which gave us confidence and courage in the most difficult times.
Given the enhanced electrodeposition performance, we went further to see how much energy it can save for the buildings in real applications. At the system level, building energy simulations show that an average of up to 43.1 MBtu of HVAC energy can be saved by using electrochromic devices as dynamic building envelopes. This research develops durable and safe electrochromic devices and demonstrates the energy-saving capability of applying such devices to building envelopes.
Our flexible electrochromic thermal emissivity tuning system is compatible with a wide range of buildings (historic and new construction) and offers a non-destructive way to enhance the sustainability of the buildings. It pushed forward the approach of radiative thermoregulation to dynamic. The optical and electrochemical designs of the systems show how advances in photonics and electrochemistry can be applied in thermal and building engineering.
What is the next step? One limitation is that some of the materials are still expensive and complicated to fabricate, such as monolayer graphene and gold microgrid, which significantly impedes the scalability of the device. Moreover, for the ideal energy saving, solar modulation should be considered to achieve daytime sub-ambient radiative cooling in the hot weather and low-emissive solar heating in the winter.
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