Impact of multivalent ions on osmotic power generation in a bipolar conical pore: a numerical analysis based on modified electrokinetic models

Abstract

Energy harvesting based on a salinity gradient is established to be a promising technique for power generation in microfluidic systems used in healthcare and other devices. We have demon- strated through numerical analysis that a mixed electrolyte of mono- and multivalent salts in a bipolar pore can augment the output voltage and hence, the energy conversion efficiency. In presence of multivalent ions the short-range ion-ion correlation manifests and the electric double layer characteristics can no longer be analyzed through mean-field based electrokinetic models. We have adopted a modified continuum based model to incorporate the correlations among finite-sized ions. The electrochemical potential of ionic species are modified by including the volume exclusion effect of finite-sized ions. Modification of the electrolyte viscosity and hence, ionic diffusivity due to the suspension of finite-sized ions is accounted through two particle hydrodynamic interactions. Based on our numerical solutions for multivalent counterions we have established experimentally observed phenomena such as, overscreening of surface charge and formation of ionic layers near the charged surface. Output voltage as well as conversion efficiency augment in the bipolar pore, which further amplifies when a valence asymmetric salt is mixed with the brine solution. As a result, a bipolar configuration with a multivalent electrolyte can generate an appreciable maximum osmotic power as high as 116 pW under a 800-fold concentration ratio. An ion selectivity of the pore by creating overlapping electric double layer in a geometrically asymmetric pore can attenuate the output voltage. We have illustrated the dependency of the output voltage and maximum power generation on the electrokinetic parameters in a bipolar conical pore.