High performance all-solid-state flexible supercapacitor for wearable storage device application

Highlights

• Pregnant woman cloth is used firstly as the flexible substrate.

• The flexible all-solid-state ASC device delivers high energy density.

• The flexible all-solid-state supercapacitor is integrated with solar battery as power pack.

• The power pack has self-sustaining high performance.

• The power pack can monitor the human physiological signals continuously.

Abstract

Flexible power packs combining a flexible photovoltaic part with a wearable all-solid-state supercapacitor as the self-sustaining energy system to power wearable device have attracted great interest due to the increasing demands for green energy and the tendency for multi-functionalization in electronics industry. To meet this energy requirement, we report an asymmetric all-solid-state supercapacitor, then integrate with commercial flexible solar cells to develop a self-sustaining power pack. In view of comfort for wearable electronics, cotton-textile radiation-proof clothes commonly used for pregnant woman cloth (PWC) are selected as the flexible substrate to construct wearable energy storage devices, which have the properties of flexible, green, renewable, breathable and excellent conductivity. Experimental tests demonstrate that the wearable asymmetric supercapacitors with high power density and relatively large energy density, fast charge/discharge capability, light-weight, excellent reliability and flexibility can enable the solar energy captured from the environment to afford a continuous and stable output of electric power and diminish the solar energy fluctuations. The supercapacitor is assembled with the Co–Ni layered double hydroxides (Co-Ni LDH) nanosheets as the positive electrode and the FeOOH as the negative electrode. Furthermore, the fabricated self-sustaining power pack as the energy source can continuously power the press sensor for monitoring the human physiological signals regardless of the sunlight fluctuation, demonstrating its potential usage in future wearable and portable electronic devices.

Introduction

The rapid fossil energy consumption has resulted in a series of serious problems such as large greenhouse-gas emissions and environmental pollution [1], [2], [3]. Finding a sustainable and clean energy supply has become one of the most important scientific and technological challenges. Due to the development of less expensive and more efficient photovoltaic cells, solar energy has been harnessed and widely used as an alternative energy source. However, due to the fluctuation of light intensity and the diurnal cycle, the output of a solar cell can be non-continuous and unstable. Therefore, there is an ever-increasing urgent need to combine solar cells with energy storage devices (serving as a buffer) to mitigate the power fluctuations and to allow operation of the cell and storage device as a reliable source of energy. Among these emerging storage battery, supercapacitors can be regarded as an ideal candidate for storing the solar energy because of their fast charge-discharge rate, power delivery, long cycle life and good operational safety [4], [5], [6].

Recently, much effort has been devoted to the flexible supercapacitors [7], [8], [9] because of their potential applications in wearable electronic devices, such as power packs, roll-up displays, electronic paper, wearable devices, mobile phones and computers [10], [11], [12], [13], [14], [15]. But the relatively low energy density of flexible supercapacitors limits their wide application. This problem can be effectively solved by high performance pseudocapacitive materials [16], [17], [18], especially for the mostly investigated Co-Ni LDH as the cathode materials and FeOOH as the anode materials [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]. In addition, in the most reported wearable flexible supercapacitor, carbon-based paper-like materials are used as the substrates [37], [38], [39]. However, these substrates as the wearable device are uncomfortable, somewhat expensive, and a relatively complicated fabrication process which may impede their practical applications in wearable energy storage devices. Pregnant woman cloth (PWC), a source of flexible, comfortable, cotton-textile radiation-proof clothes, is an excellent wearable platform for constructing flexible energy storage devices. The PWC owns high conductivity (0.04 Ω cm⁻¹), large specific surface area and cost-effective textile substrate, which is prepared by a metal layer (copper and nickel) coating on cotton. It possesses a low cost, high mechanical strength, good chemical resistance and reliable quality.

Recently, many attempts have been devoted to developing safe, lightweight power packs by integrating of the solar cell and supercapacitor [40], [41]. However, a streamlined manufacturing process for integrating a flexible energy harvesting unit with a flexible energy storage powering pulse unit has not been achieved due to the lack of effective packaging technology. In this study, facile asymmetric flexible supercapacitors are assembled with nano-structured Co-Ni LDH/PWC and FeOOH/PWC electrodes by one-step electrochemical deposition strategy on the substrate of PWC, which is “green”, renewable, breathable clothing, and these all-solid-state ASCs can be severed as a good buffer to realize a stable power output with great cycling stability (87.38% capacitance retention after 3000 cycle). More significantly, the assembled self-sustaining power pack combining the flexible ASCs with a flexible solar cell can continuously monitor the pulse signal with or without sunlight, which could early diagnose of diseases such heart disease at anytime and anywhere.