Abstract

1. The pursuit of energy-efficient cooling technologies has led to the development of adsorption-driven chillers (ADCs) that utilize water as a benign cooling agent. The article introduces a nanoscale metal-organic framework, CAU-23, with exceptional properties for water adsorption and stability, making it ideal for ultra-low temperature-driven cooling applications.
2. CAU-23 demonstrates a water adsorption capacity of 0.37 gH2O/gsorbent at a relative pressure of P/P0 = 0.3 and remarkable cycling stability of over 5000 cycles. Its unique crystal structure, determined through single crystal electron diffraction, contributes to its hydrophilicity and low driving temperature of 60°C.
3. The material's performance is further validated through Monte Carlo simulations that reveal the atomic-level water adsorption mechanism, highlighting its potential for efficient energy use and environmental sustainability.

Research Background

1. Industry Problems: Traditional cooling systems rely on harmful refrigerants and consume a significant portion of global energy. The International Energy Agency reports that air conditioning units account for 10% of global energy use, with numbers expected to triple in the next 30 years.
2. Existing Solutions: While adsorption-driven chillers have been in use for over a century, they have been limited by the lack of efficient materials that can operate at low driving temperatures.
3. Innovative Approach: The authors present CAU-23, a MOF with a unique structure that allows for lower driving temperatures and high water adsorption capacity, addressing the need for more efficient and environmentally friendly cooling technologies.

Experimental Details

1. Green Synthesis: CAU-23 was synthesized through a reaction between AlCl3, NaAlO2, and Na2TDC, yielding a white powder with an 84% yield based on H2TDC.
2. Structure Determination: Due to the nano-scale particle size, traditional single crystal X-ray diffraction was not feasible. Instead, continuous rotation electron diffraction (cRED) was used to determine the crystal structure.
3. Water Sorption Behavior: Isotherms were recorded at varying temperatures, revealing an S-shaped curve and high water uptake capacity at low relative pressures.
4. Stability Tests: CAU-23 underwent 5000 cycles of water sorption to test its stability and performance under continuous operation.

Analysis and Testing

1. Thermogravimetric Analysis (TGA): Showed the thermal stability of CAU-23 up to 400°C.
2. Infrared Spectroscopy and Elemental Analysis: Confirmed the composition of CAU-23.
3. Nitrogen Sorption Measurements: Determined a BET area of 1250 m²/g, aligning with theoretical calculations.
4. Powder X-ray Diffraction (PXRD): Used to monitor structural changes during water sorption and stability tests.
5. Water Sorption: The CAU-23 material demonstrates exceptional water adsorption capacity, with its water adsorption isotherm showing a steep rise at 25-30°C and around 30% humidity, reaching 0.375 gH2O/g.
6. Grand Canonical Monte Carlo (GCMC) Simulations: Simulated the water adsorption isotherm at 25°C, providing insights into the adsorption mechanism.

Summary

1. CAU-23 stands out as a breakthrough material for ADCs, offering a combination of high water adsorption capacity, low driving temperature, and proven stability over 5000 cycles.
2. Its performance surpasses that of other materials, such as Al-fum, which despite similar theoretical COP values, exhibits less favorable water sorption isotherm characteristics.
3. The green synthesis of CAU-23 and its potential for scalable production offer a promising outlook for industrial applications in energy-efficient cooling technologies.

 

A metal–organic framework for efficient water-based ultra-low-temperature-driven cooling
Authors: Hongyi Xu, Hans-Jörg Bart, Christoph Janiak, Stefan Henninger, Guillaume Maurin, Xiaodong Zou & Norbert Stock 
DOI：10.1038/s41467-019-10960-0
Links：https://www.nature.com/articles/s41467-019-10960-0#citeas