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Human activity recognition widely exists in applications such as sports management and activity classification.The current human activity recognition applications are mainly divided into three types: camera-based, wearable device-based, and Wi-Fi awareness-based.Among them, the camera-based human activity recognition application has the risk of privacy leakage, and the wearable device-based human activity recognition application has problems such as short battery life and poor accuracy.Human activity recognition based on Wi-Fi sensing generally uses Wi-Fi network cards or software-defined radio devices to identify the rules of channel state information changes, so as to infer user activity.It does not have the problems of privacy leakage and short battery life.But Wi-Fi network cards need to rely on computers and software-defined radio platforms are expensive, which greatly limit the application scenarios of Wi-Fi sensing.Aiming at the above problems, a human activity recognition system based on the low-cost IoT chip ESP32 was proposed.Specifically, the Hampel filter and Gaussian filter were used to preprocess the channel state information obtained by ESP32.Then, the principal component analysis and discrete wavelet transform were utilized to reduce the dimension of the data.Finally, the K-nearest neighbor (KNN) algorithm was applied to classify data.The experimental results show that the system can achieve a recognition accuracy which close to the current mainstream Wi-Fi perception system (Intel 5300 network card) when only two ESP32 nodes are deployed, and the average accuracy rate for the six activities is 98.6%.

Palladium is often used as the preferred material for hydrogen isotope separation, but pure palladium tends to be pulverized due to internal stress after repeated hydrogen absorption and desorption cycles. The pulverized palladium particles will increase hydrogen flow resistance, and even block the whole separation system in serious cases, thus affecting the separation efficiency and stability. In order to find suitable materials for hydrogen isotope separation, Pd/Al2O3 composite with 40.1% (mass fraction) palladium content was prepared by impregnation-reduction method using porous alumina as carrier and PdCl2 solution as palladium precursor. X-ray diffraction (XRD), BET surface area measurement, transmission electron microscope (TEM), and energy dispersive X-ray spectroscopy (EDS) techniques were used to study the phase, specific surface area and pore size distribution, morphology and composition of the material. The kinetics and thermodynamic properties of hydrogen absorption and desorption, hydrogen absorption and desorption cycling properties and anti-pulverization properties were tested by the volumetric absorption/desorption apparatus. Phase analysis results show that the alumina carrier exists in amorphous structure, the supported palladium particles have good crystallinity, and the precursor palladium is fully reduced without introducing other impurities. The results of specific surface area and pore size analysis show that most of the pores in the alumina carrier are mesoporous, the supported palladium exists in the pores of the Al2O3 carrier, and the Al2O3 carrier can bear a higher amount of palladium. The morphology analysis show that the palladium particles are well dispersed and the size range is from 8 nm to 50 nm. The hydrogen storage performance tests show that compared with the pure palladium, the maximum hydrogen capacity of Pd/Al2O3 composites is about 95% of the pure palladium, the plateau pressures of absorption and desorption increase by 21% and 12% separately, and the slope of absorption/desorption plateau increases. The hydrogen absorption rate is 5 times higher than that of pure palladium. It is speculated that the large specific surface area of nanocrystalline palladium particles increases the contact area with hydrogen, and the grain boundaries of nanocrystalline palladium particles provide more diffusion channels and reduce diffusion distance for hydrogen atoms. After 2 000 cycles of thermo induced hydrogen absorption and desorption, the hydrogen uptake capacity and plateau pressure of Pd/Al2O3 composite remain unchanged, and the hydrogen absorption rate increases slightly. No palladium particle pulverization or alumina ball rupture is observed. Pd/Al2O3 composite exhibits excellent anti-pulverization performance and has the potential of application in hydrogen isotope separation.