I. Introduction
In recent years, billions of energy-autonomous wireless sensor nodes are being mounted to collect information for home automation, building monitoring and environmental monitoring [1]-[3]. For example, if the early damage of long-span bridges or dams is not detected and continues to develop and accumulate to a certain extent, the entire structure may suffer a sudden failure, which is a disaster to people’s lives and property [4]. Therefore, using the wireless sensor nodes to monitor the health status is of significance to extend the lifetime and ensure the safety of infrastructures. In these cases, the wireless sensor nodes are required to work for many years without any maintenance, setting a drastic requirement on the power sources and the power consumption [1], [2]. On the one hand, batteries with higher energy density and miniatrized energy harvesters, which convert ambient solar energy, vibration energy and wind energy into electricity, have been developed as the power sources of these nodes [5]-[9]. On the other hand, ultra-low power sensors based on new mechanisms attract increasing attention [3], [9].