A lateral microfluidic cell for imaging electrodeposited zinc near the shorting condition

The morphology evolution of zinc electrodeposited from alkaline ZnO/KOH is imaged in situ using a microfluidic cell. Working and counter electrodes are in a lateral configuration, separated by a flow channel with a height of 90 μm, resulting in quasi-two-dimensional zinc layers. At a flow rate of 0.3 cm/s, zinc packing in the channel is highest at a current density just above the transition from porous to dense zinc, i≈170 mA/ cm². When deposited, compact zinc is approximately 3 times as dense as porous zinc, as determined by image analysis of the layer. The dense mode invariably leads to ramifications and critical growth, causing cell shorting. Greater zinc packing is possible at a flow rate of 3.1 cm/s, although flow rates of this order are impractical for flow-assisted zinc batteries. Ramified zinc tips approach a kinetically limited rate, independent of electrolyte flow rate. Therefore, increased flow rate cannot control critical growth once it begins. Increased flow rate results in a higher density of ramified tips at equivalent cell potential. The zinc deposition reaction has a Tafel slope of 130 mV below 10 mA/ cm² and 50 mV in the second Tafel region >10 mA/ cm². The second Tafel region is relevant to zinc secondary batteries.