Characterization of the particulate anode of a laboratory flow Zn–air fuel cell

A tapered-end flow Zn–air fuel cell (ZAFC), mechanically refuelable with Zn microspheres, was employed to study the effect of aging of KOH electrolyte on the Zn anode. A complete description of the architecture of the adopted cell is reported. The electrochemical characterization of the ZAFC was performed by long-term current discharge tests in galvanostatic mode. An insightful investigation on the particulate Zn anode consisting of spheres of diameter 0.4 mm was performed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and Raman spectroscopy in order to characterize the crystallographic structure, surface morphology, and chemical nature of residual metal and solid corrosion products. Electrochemical impedance spectroscopy (EIS) allowed to obtain information on the charge-transfer mechanism of zinc anode reaction and on the thickness, compactness, and blocking features of the passive film as a function of the aging of the electrolyte. The results of our analysis revealed the formation of a passive layer of zinc consisting of a white and porous film of ZnO precipitate (type I) and a light-gray to black compact film (type II). The failure of the particulate anode was chiefly caused by the increase in zincate concentration in the electrolyte, but it was enhanced by the nonuniform spatial current distribution due to the instability of the passive film at high pH.