Anodic Considerations for Power Optimization of Aluminum-Air Batteries

Aluminum-air batteries are a promising energy storage and conversion technology due to their high theoretical energy density. While this is where they have a competitive advantage, power output represents a challenge. The literature has focused on maximizing aluminum utilization, but power must be optimized in conjunction with energy density for practical use cases. At high rates of discharge, cell performance is largely dictated by ohmic losses at the anode associated with the aluminum product layer. Turnover of this passivating layer entails conversion of insoluble aluminum hydroxide into soluble aluminate ions. In this investigation, aluminum-electrolyte interactions were first studied in a three-electrode cell. Optimal discharge conditions were determined then applied in a full-cell configuration. Pulsing protocols were then explored as a way to improve extended discharge behavior. Electrochemical impedance spectroscopy, polarization curves, and galvanostatic discharge were used as the primary experimental techniques along with scanning electron microscopy. Finally, a semiempirical numerical model was created to capture experimentally observed resistance associated with the aluminum product layer. Electrolyte alkalinity and temperature were found to significantly influence ohmic behavior. Inclusion of a corrosion-inhibiting sodium stannate electrolyte additive also decreased film impedance, but this benefit was contingent on aluminum achieving a state of critical tin deposition. Higher electrolyte flow rates in the full cell improved peak power by increasing product turnover at limiting current densities. A performance discrepancy was observed between the operating point indicated by polarization curve and that obtained during extended discharge, and this was attributed to the transience of tin deposition. Cathodic pulsing of aluminum potential was able to promote tin deposition and bypass the startup period necessary to achieve polarization-curve-representative performance. On-off cycling was also employed to improve discharge longevity by limiting product accumulation. The developed numerical model presents a plausible explanation for experimentally observed ohmic behavior through explicit consideration of product formation and dissolution. Management of the product film is critical for power optimization of aluminum-air batteries, and this investigation provides insight toward the realization of this goal.