Analysis of primary and secondary current distributions in a wedge-type aluminum-air cell

The primary and secondary current distributions near the leading edges of the cathode and anode of a wedge-type aluminum-air cell design were analyzed. Numerical calculations were accomplished by using a finite difference method and introducing an overlapping two-grid system technique. The calculations indicate that the current distributions on the cathode and anode at distances from the edges greater than 2 times the cell gap are uniform. In the edge region, the wedge angle between 0 and 10° has a negligible effect on the current distribution. High current densities at the cathode edge, which are detrimental to cathode life, are reduced by kinetic effects and by oversizing the cathode itself. The latter also favors cell performance but adds to the cell costs. An effectiveness factor is introduced which demonstrates the effectiveness of cathode oversize and the sensitivity to kinetics as represented by the Wagner number. The calculations indicate that only marginal performance gains can be expected when the cathode extends beyond the anode a distance greater than that of 1.5 times the amode-cathode gap.Nomenclature A1, A2, A3 anode curves 1, 2, 3 - b slope ofi vs curve at mean value of _mean (A cm^–2V^–1) - C1, C2, C3 cathode curves 1, 2, 3 - D distance between electrodes (cm) - i_s local current density on electrode (A cm^–2) - I^* dimensionless local current value defined by Equation 9 - N dimensionless effectiveness factor defined by Equation 10 - V_met constant potential of electrode (V) - W dimensionless Wagner number defined by Equation 7 - X dimensionless cathode oversize defined asx/D - x position parallel to anode with origin at the anode apex (cm) - y position perpendicular to electrode surface (cm) - K conductivity of electrolyte (ohm cm^–1) - surface overpotential (V) - potential in solution phase (V) - ₀ potential in solution adjacent to electrode surface (V)