Alternating Current Etching Method for Fabrication of High-performance Aluminum Electrolytic Capacitors

We optimized a multistage electrochemical etching method for fabricating high-performance aluminum electrolytic capacitors using alternating current (AC) to maximize electrostatic capacitance while preserving the mechanical properties of high-purity aluminum foil.The etching process was systematically investigated under various AC frequencies, current densities, durations, solution compositions, and aluminum ion concentrations.Pretreatment and chemical modification with phosphoric acid (H 3 PO 4 ) provided a highly uniform corrosion pit distribution, preventing localized over-dissolution.The optimal etching conditions were identified as a solution containing 350 mL/L HCl, 15 mL/L HNO 3 , 15 mL/L H 3 PO 4 , and 5 g/L Al 3+ , a temperature of 35 °C, a current density of 0.6 A/cm 2 , and a four-stage frequency-stepping profile of 30-30-25-20 Hz for a total etching time of 120 s.Under these conditions, the 50-μm-thick aluminum foil (99.99% purity) achieved a peak specific capacitance of 510.2 μF/cm 2 , indicating a >20% improvement over conventional single-stage methods.Microstructural analysis revealed a highly dense pit network with an average pit diameter of 180-240 nm, a nucleation density of 4.2 × 10 9 pits/cm 2 , an average surface roughness of 1.68 μm, and an exceptional surface enlargement factor of approximately 96.5.Lattice orientations of [220], [311], [200], and [422] in this order contributed to this surface expansion.Process reliability was confirmed by a low capacitance coefficient of variation (<1.2%), whereas the etched foil retained over 75% of its pristine tensile strength.Electrical evaluations demonstrated an exceptionally low equivalent series resistance of 0.12 Ω at 120 Hz, a superior frequency response (68% capacitance retention at 1 kHz), and a robust stability under accelerated life testing (85 °C at a rated voltage of 6.3 V for 1000 h) with less than 4.5% degradation.These results show that this multistage AC etching method directly supports the miniaturization, signal stability, and ruggedization demands of next-generation intelligent sensor technologies, particularly for localized power filtering in automotive modules and low-power wireless networks.