Exploiting Geometric Flexibility of Coordination Complexes for Selective Cobalt-Nickel Membrane Separations

Conventional membrane separation mechanisms based on size, charge, and valency cannot effectively differentiate between nearly identical transition metal ions such as cobalt, nickel, and manganese. To circumvent these fundamental limitations, we develop a cobalt-selective membrane that facilitates ion transport through specific ion-ligand coordination geometry, an underutilized separation mechanism in membranes. First, we fabricate and characterize a phosphonate-functionalized polyelectrolyte multilayer membrane designed to provide a sterically constrained coordination environment. Next, we evaluate ion flux and membrane selectivity in multisalt diffusion cell transport experiments, achieving Co2+/Ni2+ selectivity of ∼2.3, Mn2+/Co2+ of 1.9, and Zn2+/Co2+ of 17.3, and reversing selectivity expectations of the classical Irving-Williams stability series (Mn2+ < Co2+ < Ni2+ > Zn2+). Through isothermal titration calorimetry and density functional theory simulations, we demonstrate that the thermodynamically favorable binding of Co2+ over Ni2+ drives a coordinative "hopping" mechanism. Transport is governed by the ion's ability to partially dehydrate and accommodate a distorted pseudo-octahedral geometry imposed by the rigid ligand matrix. These results establish coordination geometry as a promising driving force for the design of high-precision, ion-selective materials for critical mineral recovery.