ABSTRACT
The development of high density energy storage systems is of great scientific and technological importance. While in past years lithium-ion batteries (LIBs) have captured the most attention, even the highest energy density achievable for LIBs is insufficient to satisfy the requirements of full electric vehicles and modern electronic devices with highly integrated functions. Metal-air semi-fuel cells, which can theoretically offer 5-9-fold increases in energy density compared with LIBs at a lower cost, have recently received renewed interest (Cheng and Chen, 2012;Wang et al., 2013; Zhong et al., 2012; Zu and Li, 2011). Of different metal anodes, aluminum has the highest volumetric electrochemical equivalent (8.04Ah cm−3, compared with 2.06 for lithium, 5.85 for zinc, and 3.83 for magnesium), and therefore, it is particularly worth studying for portable or mobile applications (Li and Bjerrum, 2002). Aluminum-air cells usually operate with an electrolyte flowing inmicrochannels.The submillimeter tomillimeter characteristic dimensions of the electrolyte channels can greatly enhance the cell performance by providing a reduced internal resistance and rapid species transport. However, to date, aluminum-air technologies havemet very limited commercial success. The competing processes of surface passivation and self-corrosion that occur on aluminum during discharge, pose a great challenge in developing appropriate anode alloys and electrolytes (Abdel-Gaber et al., 2008; Beck and Rüetschi, 2000; Ferrando, 2004; Tang et al., 2004). An alternative strategy that can increase the fuel efficiency of aluminum is to incorporate the hydrogen production into an alkaline aluminum-air cell (Zhuk et al., 2006). To some extent, this strategy loosens the restrictions on both anode alloys and electrolytes since it treats the gas-evolving parasitic corrosion of aluminum as a route for hydrogen production, instead of a worthless but detrimental reaction that needs to be suppressed. Meanwhile, however, it raises another critical issue of hydrogen management inside the cell, which requires a thorough understanding on the intricate physico-electrochemical processes associated with two-phase flow in the micro-fluidic cell channel.
