Metal-Organic Framework-Based and Humidity-Tolerant Quartz Crystal Microbalance Sensor for Difluoromethane Refrigerant Leak Detection

The need for low-cost refrigerant leak detection sensors is crucial for the rapid adoption of next-generation refrigerants. A metal-organic framework (MOF)-based quartz crystal microbalance (QCM) sensor was used for the detection of difluoromethane (R32). Initially, five well known MOFs, MIL-101, HKUST-1, MOF-74, UiO-66, and ZIF-67, were tested to evaluate their response to R32. Among the MOFs tested, only ZIF-67 shows a stable response to R32 in high humidity conditions and is used for the sensor. A thin layer of ZIF-67 is coated on a QCM, which exhibits a quick response and recovery time of 3.5 and 1 s, respectively, to 25% lower flammability limit (LFL) of R32, which is under the 30 s time limit set by the UL 60335-2-40 standard. A cost-effective integrated prototype with a ZIF-67-coated QCM sensor is built and tested with results still showing a linear response with increasing R32 concentrations. A calibration curve is developed for the sensor to predict refrigerant concentration from the sensor response, temperature, and humidity, and a validation study is conducted, with results showing a median relative error below 6%. These results show that the ZIF-67-coated QCM sensor can be used for reliable and fast detection of R32 refrigerant leaks.