Ultrahigh current density and fatigue stability in flexible energy harvester by designing delivery paths

Highlights

• A novel co-chained structure is proposed, providing a potential solution to enhance current density and heat dissipation in flexible composites.

• Delivery paths constructed by co-chained structure is proved to improve current output and heat dissipation by experiment and finite element simulation.

• Ultrahigh current density (4.7 μA/cm²) and thermal conductivity (0.31 W/(m∙K)) are obtained in piezocomposites with co-chained structure.

Abstract

Flexible piezoelectric energy harvesters that efficiently convert mechanical energy into electrical energy have been extensively studied due to their great application potential in low-power wearable electronics and self-powered sensors. However, the low current output and poor thermal fatigue resistance severely restrict their practical applications. Here, we propose a new strategy to simultaneously improve the current density and thermal conductivity of the flexible piezocomposites (PCs) by designing delivery paths. High-quality (Ba_0·85Ca_0.15) (Ti_0·90Zr_0.10)O₃/copper nanorods/polydimethylsiloxane (BCZT/Cu NRs/PDMS) PCs with a novel co-chained structure prepared using a well-suited technique of dielectrophoresis were reported for the first time. The BCZT particles and Cu NRs aligned in same chains throughout PDMS matrix results in an efficient delivery path both for induced charge transfer and heat dissipation, thus leading to ultrahigh current density (4.7 μA/cm²) and thermal conductivity (0.31 W/(m · K)). Further, a faster charging speed and a dramatically fatigue stability were realized in the co-chained PCs. Our work is expected to provide a potential solution for simultaneously enhancing current density and heat dissipation for a variety of composites in the field of flexible energy harvesting.

Introduction

The emergence and rapid development of low-power and miniaturized wearable electronics has stimulated researchers' enthusiasm for flexible piezocomposites (PCs) in the field of energy harvesting [[1], [2], [3]]. Among, piezo-/matrix composites fabricated by dispersing piezoelectric particles in polymer, have been proven to achieve high voltage output while maintaining the flexibility of the materials, and also have advantages of low cost and simple preparation [[4], [5], [6], [7], [8]]. Therefore, they are widely used in flexible sensor field such as pressure sensing, health monitor and limb motion sensing [[9], [10], [11]]. However, previous results show an unsatisfactory power output caused by poor current density. Thus, it's hard to directly supply a sustainable power for a normal operation of devices, which is not conducive to the development of miniaturization and integration of flexible electronics.

To achieve high current density in PCs, the selection of piezoelectric fillers is decisive undoubtedly. Lead zirconate titanate (PZT) and PZT-based systems with excellent piezoelectric properties have been extensively used to fabricate PCs [2,12,13]. However, the toxicity of lead has raised environmental concerns during their preparation, process, and even disposal. For maintaining sustainable development, a great deal of effort has been put into the research and development of lead-free piezoelectric materials [[14], [15], [16], [17]]. In recent years, lead-free piezoelectric material (Ba_0·85Ca_0.15) (Ti_0·90Zr_0.10)O₃ (BCZT) is considered as a strong candidate for preparing PCs due to their high piezoelectricity (d₃₃∼620 pC/N) [18]. Wu et al. [19] demonstrated a BCZT composite generator producing open-circuit voltage (V_OC) of 3 V, and short-circuit current (I_SC) of 85 nA. Zhang et al. [20] reported a similar device using BCTZ with a three-dimensional ceramic skeleton that can generate a high V_OC of 25 V, and I_SC density of 550 nA/cm². While the reported results are encouraging, current density (mainly in nA/cm² scale) of the BCZT-based PCs still needs a further improvement for practical applications.

Another representative approach to obtain high current density is introducing a conductive phase, such as carbon nanotubes [[21], [22], [23]], copper nanorods (Cu NRs) [24], or silver nanowires [[25], [26], [27]] into PCs. The conductive phase not only plays a role of dispersing fillers and reinforcing stress transfer in the PCs, but acts as a conductive agent [21,24,28]. The establishment of conductive cross-linked network can quickly transfer induced charge generated by piezoelectric phase to the surface of the material, thereby significantly enhances the current output [23,27,29]. However, an excessive addition of the conductor above the percolation threshold will cause an abnormal increase in leakage current [30,31], resulting ineffective poling of the piezoelectric phase [[32], [33], [34], [35], [36]], which extremely restricts the power output of the piezo-/conductor/matrix PCs.

Additionally, small yet powerful flexible energy harvesters will generate intense heat during severe vibration and deformation that may cause device failure; thus, there is a need to address thermal dissipation in the energy harvester designs. It has been proved that in an isotropic composite system, the thermal conductivity depends on the overall characteristics of the materials [37]. However, the current organic materials used in flexible PCs usually have a very low thermal conductivity. For example, the thermal conductivity of polydimethylsiloxane (PDMS) is only 0.16 W/(m · K) [38]. Therefore, there is an urgent need to increase the thermal conductivity of flexible PCs for electronic devices.

In this work, we provide a novel insight to simultaneously improve the current density and thermal conductivity by designing delivery paths in PCs. We firstly choose the BCZT particles with extremely high d₃₃ as the piezoelectric phase, the highly conductive Cu NRs as the conductive phase, and polydimethylsiloxane (PDMS) with preeminent flexibility and chemical resistance as the organic matrix. Then, a well-suited technique of dielectrophoresis (DEP) is employed to control the arrangement of BCZT particles and Cu NRs, forming the co-chained structure (BCZT and Cu NRs aligned in same chains throughout PDMS matrix). An ultrahigh current density of 4.7 μA/cm² and thermal conductivity of 0.31 W/(m · K) were obtained owing to the efficient delivery path established by the co-chained structure as shown in Fig. 1 [7,8,24,26,27,[39], [40], [41]]. The induced charge generated by BCZT can be efficiently transferred; meanwhile, the co-chained structure can also act as thermally conductive paths.