###### Abstract  
The long-term CO\({}_{2}\) emissions of transformer substations require constant monitoring. In this study, we propose a piezoelectric vibration energy harvester designed for self-powered CO\({}_{2}\) monitoring of transformer substations. The proposed harvester comprises multiple slender piezoelectric cantilevers arranged in parallel, which results in a higher operational frequency and a significantly enhanced power output capability. Experimental investigations were conducted to assess the energy harvesting performance. The results show that the harvester can effectively capture the vibration energy, yielding an RMS power output of 2.99 mW, corresponding to the operational frequency of the transformer substation. Additionally, a wireless CO\({}_{2}\) sensor node was developed, demonstrating an operational mechanism for CO\({}_{2}\) monitoring. The capacitor takes approximately 1220 s to charge for the initial data measurement and transmission. The findings confirm that the energy harvester is capable of providing sufficient power to operate the sensor node for CO\({}_{2}\) monitoring in transformer substations. piezoelectric energy harvesting; multiple-layer cantilever; enhanced efficiency; wireless sensing; CO\({}_{2}\) monitoring +
Footnote †: journal: Applied sciences  
A non-resonance piezoelectric energy harvester was also proposed to achieve a robust performance at different frequency points [31]. In subsequent years, significant efforts were directed towards broadening the bandwidth of energy harvesters. Several innovative mechanisms were proposed, including Duffing oscillator-based nonlinearities [32; 33], segmental stiffness [34], internal resonance [35; 36; 37; 38], multimodal configurations [39], and frequency up-conversion mechanisms [40; 41]. Notably, Duffing-type piezoelectric energy harvesters (PEHs) incorporating permanent magnets have demonstrated large amplitude voltage outputs over a wide frequency range when subjected to large amplitude excitations [42; 43; 44; 45; 46; 47]. More recently, Duffing-type PEHs with multiple small, stable, and flexible potential barriers have been designed to optimize their performance under low-amplitude excitations [48; 49; 50; 51; 52]. The aforementioned PEHs were designed and optimized for general applications. In contrast, the mechanical vibrations in a transformer substation exhibit a constant operational frequency of 100 Hz in China and Europe, or 120 Hz in the US [14; 15; 16]. Slender piezoelectric cantilevers are typically used in these applications, because they can generate significant strain on their surface, effectively converting mechanical energy into electrical energy through the 1-3 electromechanical coupling effect. In contrast, thick beams and thick piezoelectric materials tend to experience large shear strains in the epoxy resin bonding layer, which significantly reduces the efficiency of the PEH. However, typical slender piezoelectric cantilevers exhibit low resonant frequencies [21; 22; 23; 24; 25; 26; 27; 28; 29], which limits their applicability in harvesting mechanical energy from transformer substations. Moreover, it is important to note that a single piezoelectric transducer has a limited power output due to the inherent strain and voltage constraints of its design. Increasing the number of piezoelectric transducers, particularly by employing multiple slender cantilevers, offers a promising solution to these limitations. In this research, we introduce a novel multiple-cantilever piezoelectric vibration energy harvester designed for self-powered CO\({}_{2}\) monitoring in transformer substations. The system features slender piezoelectric cantilevers arranged in parallel, with connectors at both the fixed and free ends, facilitating an S-shaped bending mode of vibration. This innovative design offers several advantages. The assembly of multiple cantilevers and the S-shaped bending mode significantly boost the resonant frequency of the PEH system, aligning it with the operational frequency of transformer substations.