An outlook on printed microsupercapacitors: Technology status, remaining challenges, and opportunities

Abstract

The concept of the Internet of Things is dramatically changing the way society interacts with physical spaces and portable technologies. For the last couple of years, intensive research has been devoted on the design of several flexible and even wearable devices, such as displays and health-care sensors. Further developments on these new technologies are heavily conditioned by the lack of compatible energy storage/conversion units. Contrary to lithium-ion batteries, supercapacitors can be easily miniaturized and integrated on flexible/wearable technologies without losing their electrochemical performance. In this review, some of the most recent developments on the design and printing of light, flexible, and thin microsupercapcitors along with promising and further practical applications are presented.

Introduction

Modern technology and electronics are currently undergoing a transition from bulk and rigid configurations toward flexible/wearable designs [1, 2, 3, 4]. Flexible displays, chemical sensors, and health-care devices are quickly becoming a part of every day's life [1,3]. These devices demand the development of ultrathin, lightweight, and flexible energy storage devices [1,2,5,6]. Owing to their ubiquitous presence in modern society, lithium-ion batteries would likely be the ideal energy storage device for future flexible applications. However, state-of-the-art thin-film lithium-ion batteries (LIBs) still suffer from several technical disadvantages, as their energy per volume tends to rapidly decrease in the micrometer scale [7]. It is also quite challenging to integrate these devices within electronic circuits, severely conditioning the miniaturization of devices [5]. Moreover, LIBs are also plagued with short life cycles, safety concerns regarding the use of lithium, and flammable electrolytes [5]. Current energy storage technologies are still lagging other technical and circuitry components. Thus, the fast-paced development of emerging flexible/wearable technologies is driving a worldwide demand for new and improved energy storage devices [1,6,8]. In this context, microsupercapacitors (MSCs) are highly desired as power sources for flexible devices as they present an enhanced power density (>10,000 W kg⁻¹), fast charge discharge rates, optimal cyclability, long shelf life, and capability of direct on-chip integration [1,5,7,9, 10•, 11, 12••]. MSCs are also considered suitable devices to complement (or even replace) microbatteries in applications requiring transient high peak power pulse [5,13,14]. Recent developments in materials synthesis and fabrication processes resulted in MSC exhibiting energy densities similar to those of thin-film batteries (10⁻³ and 10⁻² Wh.cm⁻³) [12]. However, to be fully integrated into wearable and flexible technologies, supercapacitors need to be flexible enough to undergo large mechanical deformations, without compromising the device performance [15]. Commercially available vertical sandwich supercapacitors do not present the most suitable configuration for devices meant to be bent or even rolled up, due to the presence of rigid components and the constant risk of harmful electrolyte leakage [8,14]. In this context, printed MSCs are being proposed as the best option for powering flexible/wearable electronics [1,6,8]. Thus, several printing methodologies are currently being considered for MSC fabrication, as briefly summarized in Figure 1. Printing methods allow an easy fabrication of devices, while being compatible with emergent materials for energy storage/conversion applications (Figure 1) [10,12,16].

When deposited onto a flexible substrate, interdigitated in-plane MSCs usually exhibit optimal mechanical properties, while keeping their electrochemical performance and compatibility with the geometries of integrated microfabrication processes [5,8,14]. As expected, these technological shifts raise questions regarding suitable active materials, electrolyte formulations, substrate type, geometries, and, of course, fabrication methods [1,6,14]. Herein, an overview on the achievements and on-going shortcomings on the development of energy storage/conversion for flexible/wearable technologies will be provided. For the sake of simplicity, the term MSCs will refer (unless stated otherwise) to planar printed microsupercapacitors.