Smart Windows: Instant Darkness at a Touch

Glazing largely determines how much sunlight and heat enters buildings – and thus how much energy is required for heating, cooling and lighting. A newly developed electrochromic smart glass uses an organometallic layer that precisely changes its optical properties when voltage is applied. The color change ranges from almost colorless to green to deep brown and takes place in a matter of seconds. At the same time, the coating remains stable over numerous switching cycles. The question is whether such materials can make the step from laboratory scale to large-scale facade application and thus become an important building block for energy-efficient buildings.

Windows not only determine the view and daylight, but also a large part of the energy flow in buildings. Around 40 percent of final energy consumption worldwide comes from buildings, around half of which comes from heating, cooling and lighting. A significant proportion of this energy is lost through glazing because it only passively allows sunlight and heat radiation to pass through. For several years now, researchers have been working on electrochromic glass whose light transmission and absorption can be actively controlled with applied voltage. Laboratory prototypes such as a dual-band electrochromic window with a nanowire structure show that visible radiation and near-infrared can be modulated separately, thereby significantly increasing the energy efficiency of buildings. In order for such systems to move beyond the laboratory into facades, they must switch quickly, remain color-stable and function reliably over tens of thousands of cycles.

A key challenge is to find materials that are simultaneously porous enough for rapid ion transport, chemically stable and capable of being processed in thin, transparent layers. Particularly promising here are metal-organic framework compounds, or MOFs for short, which are made up of metal nodes and organic ligands and have a finely adjustable pore system. Such networks have already been used to selectively filter carbon dioxide from moist exhaust gases and can in principle also be used as an active layer in electrochemical components. A tailor-made metal-organic framework compound of the type Ni-IRMOF-74 now forms the core of a smart window in which thin, transparent films are applied to conductive glass and the optical properties can be specifically changed using migrating ions.

Electrochromic windows: principle and previous approaches

Electrochromic glass is based on the fact that when a voltage is applied, ions are embedded in a thin active layer or released again, thus changing the optical properties of the material. Classic systems use transition metal oxides such as tungsten oxide, which change color when small cations are incorporated and allow less light to pass through. Newer approaches couple multiple electrochromic materials to separately control visible light and near-infrared radiation. In the trade journal Nano-Micro Letters, for example, a dual-band electrochromic window was presented that can modulate the transmittance in both the visible range and the near-infrared range by more than 70 percent and only shows a loss of a few percent in capacity even after 10,000 switching cycles. Such concepts prove that active coatings can, in principle, remain permanently stable as long as ion transport and mechanical stresses in the material are kept in balance.

• High optical contrast between transparent and darkened states
• Fast switching times in the range of a few seconds
• Low operating voltages in the single-digit volt range
• Stable performance over tens of thousands of electrochemical cycles
• Scalable coating processes for large glass surfaces

In addition to classic inorganic layers, hybrid and porous materials are increasingly coming into focus because they accelerate ion transport and thus enable shorter switching times. Porous nanostructures increase the internal surface area and provide additional binding sites for charged particles, but can also create mechanical weak points if the framework deforms significantly with each insertion and removal of ions. Organometallic networks offer an interesting intermediate position here: They combine the defined crystal structure of inorganic solids with the chemical variability of organic ligands. This allows channels and cavities to be designed so that ions can diffuse as easily as possible without destroying the grid. The system now being examined based on Ni-IRMOF-74 follows exactly this approach.

Metal-organic framework compound Ni-IRMOF-74 in the smart window

The system now presented is the first to use a metal-organic framework compound of the type Ni-IRMOF-74 as an electrochromically active layer. In this structure, nickel centers are linked via biphenyl-based dicarboxylate ligands to form a porous network whose pore diameter and crystal structure favor ion transport. Xueying Fan’s research team applied thin Ni-IRMOF-74 films to fluorine-doped tin oxide, the surface of which was previously functionalized with 4-mercaptobenzoic acid to ensure stable bonding of the film. In ACS Energy Letters, the researchers report that they can produce homogeneous layers just a few micrometers thick that remain transparent and still have high electrochemical activity. The metal-organic framework compound Ni-IRMOF-74 acts as a defined storage space for charges and ions.

To make it a smart window, the researchers combined the Ni-IRMOF-74 film with a gel electrolyte based on lithium perchlorate. A sandwich made of conductive glass, an organometallic layer and an ion-conducting polymer is created between two transparent electrodes. If a low voltage of around 0.8 volts is applied, lithium ions migrate from the electrolyte into the cavities of the framework structure and change their electronic state; The previously almost colorless glass then appears green and absorbs some of the visible light. If the voltage is increased to around 1.6 volts, the incorporation of ions increases, the coating takes on a dark red to brown color and the electrochromic glass only lets a little light through. If the polarity of the voltage is reversed, the process reverses and the window becomes transparent again.

Switching speed, color change and cycle stability

What is crucial for practical use is how quickly and how often such a system can be switched without losing its properties. The researchers first determined the electrochromic properties of the pure Ni-IRMOF-74 layer in the laboratory sample. The optimized version of the film switched between transparent and colored states with response times of approximately 1.9 seconds for coloring and 2.0 seconds for lightening. At around 331 square centimeters per coulomb, the color change efficiency was well above the values ​​of many established materials, which means that even small amounts of charge make a big difference in light transmission. In repeated cycles, the film still retained around 95.7 percent of its original optical modulation after 4,500 switching operations, which suggests a robust insertion and removal of the ions in the metal-organic framework.

In the next step, the team integrated the coating into a complete electrochromic setup with two glass panes and an enclosed gel electrolyte. This combination resulted in a somewhat slower but still practical dynamic for the smart window: the transmission changed within around 2.3 seconds when darkening and reached the clear state again after around 7.9 seconds when brightening. Here too, the function was maintained for at least 1200 switching cycles, with around 85 percent of the initial contrast between bright and dark states still being measured. In both color modes, the coating not only changes the subjective impression of brightness, but also influences how much thermal radiation enters the room, which is important for future coupling to air conditioning and lighting control systems.

Perspectives for energy efficiency of buildings

The combination of fast response, high color change efficiency and good cycle stability that has now been demonstrated makes clear, but still early, progress towards electrochromic MOF windows that are suitable for everyday use. These are still relatively small test areas in the laboratory, and aspects such as large-area coating, long-term stability under real weather conditions or integration into existing facade systems are still open. However, simulations for other electrochromic concepts show that adaptive glazing can reduce a building’s heating and cooling requirements by up to 20 percent if they react specifically to solar radiation and the outside climate. In the future, smart windows based on Ni-IRMOF-74 or related organometallic networks could not only partially replace blinds and sun protection films, but also be integrated into building controls that coordinate lighting, temperature and energy requirements in real time and thus increase the energy efficiency of buildings.

ACS Energy Letters, Biphenyl Dicarboxylic-Based Ni-IRMOF-74 Film for Fast-Switching and High-Stability Electrochromism; doi:10.1021/acsenergylett.4c00492

Nano-Micro Letters, An Efficient and Flexible Bifunctional Dual-Band Electrochromic Device Integrating with Energy Storage; doi:10.1007/s40820-024-01604-0