title: Paper-based Microfluidics for Electrochemical Applications
Creators: name: Shen, Liuliu
Publisher: name: UNSPECIFIED
Year: 2020

description: Paper-based microfluidic platforms enable miniaturization and integration of various reaction processes and technologies into on-chip devices, aiming at fabricating a broad range of low cost custom products of our daily life. This dissertation focuses on applying the paper-based microfluidic concept to construct portable and disposable electrochemical devices with simple structure and affordable price. For three examples, their structural features, working principles and electrochemical performance are systematically investigated and discussed. The examples are: 1) a paper-based microfluidic electrochemical sensor for detection of heavy metal ions, 2) a paper-based microfluidic formate fuel cell, 3) a paper-based microfluidic high-performing aluminum-air battery. The microfluidic electrochemical sensor is based on a microfluidic paper channel combined with a three-dimensional (3D) configuration, and the three electrodes are all pristine graphite foil without any modifier or catalyst. This low-cost, simple and portable sensor exhibits a high sensitivity toward trace detection of heavy metal ions in aqueous solution with detection limits down to 1.2 µg/L for Cd2+ and 1.8 µg/L for Pb2+. Detections of Cd2+ and Pb2+ in more sophisticated environments (e.g., mineral water or solution containing interfering ions) are also carried. The 3D configuration is found to be the key factor for achieving high detection performance. The paper-based microfluidic sensor is highly robust and the measurements on a single device result to be highly reproducible. The paper-based microfluidic formate fuel cell is designed as a single-use and environmentally friendly power source for portable and disposable electronics, such as sensing devices. The key structural parameters that impact the overall performance of the fuel cell were systematically studied, including the fuel crossover, cell resistance, concentration of the redox reactants, catalyst loading, and the microfluidic paper channel properties. After optimization, an open circuit voltage of 0.86 V and a maximum power density of 7.1 mW/cm2 can be achieved on a single cell. By correlating the cell performance with the electrolyte flow rate within different paper channels, for the first time it is unraveled that the textual properties of the paper largely influence the cell performance through mass transfer and depletion effect. To achieve a higher energy and power density on miniaturized power device, paper-based microfluidic concept is further introduced to fabricate high-performing aluminum-air batteries. The unique microfluidic configuration reduces or even eliminates the major drawbacks of conventional aluminum-air batteries including battery self-discharge, product-induced electrode passivation, expensive air electrode and auxiliary fluid control system. The paper-based microfluidic Al-air battery exhibits impressive electrochemical performance in specific capacity (2750 Ah/kg) and energy density (2900 Wh/kg), both of which are superior among the reported miniaturized power sources. The paper-based microfluidic Al-air battery can be assembled into a user-friendly pouch cell configuration by simple lamination process, showing great potential toward real application. This dissertation shows that paper-based microfluidics is an attractive concept to build low-cost, green, portable and user-friendly electrochemical devices with various functionalities. Aside from designing and fabricating high-performing paper-based microfluidic electrochemical devices, systematical investigations are also made to understand how the device structure (e.g., electrode modification and arrangement) and microfluidic flow behavior (e.g., flow rate and diffusion rate) influence the overall performance, which might have implications for the construction of other paper-based microfluidic electrochemical devices.