Study on high-performance membraneless aluminum-based electrochemical cells

Abstract

Aluminum is recognized as a novel and attractive energy carrier and fuel material due to its virtues of high energy density, large-base reserve, high recyclability and environmental friendliness. Energy utilization of aluminum materials includes electricity, hydrogen and heat generation. Among them, conversion to electricity from aluminum-air cell acts as the most efficient method. Aluminum-air cell is a promising electrochemical energy conversion device, with theoretical volumetric capacity and energy density reaching 8 Ah cm^(-3) and 22 Wh cm^(-3), respectively.

Up till now, its commercial application is still limited mainly because of the inherent twin-challenge on the aluminum anode. Upon discharge, a passive layer is generated which covers the aluminum surface, deviating the anodic potential from its theoretical value and thus, declining the cell performance. Although this passive layer can be effectively disintegrated by strong alkaline electrolyte, concomitant aluminum corrosion will be aggravated severely, leading to a fierce hydrogen evolution rather than the expectant electricity generation. It poses a great challenge in promoting aluminum-air cell into commercial applications. This study proposes several innovative efforts to alleviate the above-mentioned issues and improve the cell performance.

For the first part of the research, optimizations on conventional membraneless alkaline aluminum-air cells were conducted. A prototype of water-activating aluminum-air cell was firstly introduced. The cell is storable and readily activated when in need, alleviating the severity of corrosion during standby. Then a cell operating system, working with a mechanically rechargeable cell, was designed to demonstrate a strategy for energy utilization from alkaline aluminum-air cells. Capable of generating electricity and collecting hydrogen at the same time, this newly-designed system overcame the low-efficiency problem during cell operation. Economic analysis affirmed its cost competitiveness.

In the second section, several concepts of membraneless microfluidic dual-electrolyte aluminum-air cells were proposed for cell performance improvement. To overcome the self-corrosion issue directly, a microfluidic aluminum-air cell working with KOH methanol-based anolyte was developed. A high specific capacity over 2500 mAh g^(-1)–Al was achieved. In addition, the reaction mechanisms of aluminum in KOH methanol-based anolyte, including discharge and corrosion reactions, were investigated for the first time. Following this, a concept of mixed-pH microfluidic aluminum-air cell was demonstrated to further increase the cell power output. Benefiting from the higher chemical kinetics in acidic catholyte, this dual-electrolyte system showed a higher open-circuit voltage and power density. Based on this experiment, a scale-up approach was developed to increase the capability of microfluidic aluminum-air cell. A comprehensive impedance study on a full aluminum-air cell was conducted for the first time and employed to analyze the impedance evolution during its scaling up process.

Lastly, as a preliminary investigation of aluminum cell with rechargeability, an aqueous aluminum-ion cell was developed. The influence of rutile type TiO2 on cell performance was investigated, which exhibited a higher capacity.

In general, this research focused on optimizations of membraneless aluminum-based electrochemical cells by proposing various improving approaches on different aspects and provided insights into the physical-electrochemical mechanisms related to performance improvement. Finally, directions for future research works were suggested.