Toward understanding of a mechanism for electrical current generation from ionic water flow using metal nanofilms

“Hydrovoltaic” technologies convert energy from flowing water to electricity via a mechanism that primarily relies on ion adsorption and desorption at water-solid interfaces. Recently, we found metal/metal-oxide nanofilms that generate electrical current from aqueous flow of alternating salinity gradients. Atom probe tomography revealed the 10 nm thick nanofilms are composed of a thermal oxide overlayer about 2-4 nm thick and a lower layer of metal about 6-8 nm thick. Experiments suggested the following design rules for the nanofilms: (i) the metal oxide needs to be redox-active, containing several metal-oxidation states, and (ii) there is an optimal thickness for the nanofilms, comparable to electron mean-free path. For example, 10 nm thick Fe:FeOx films with an alternating flow of sea water and de-ionized water produce current-densities of several microA cm^-2 at the flow rate of a few cm s^-1, where iron oxide (FeOx) has iron in both the Fe²⁺ and Fe³⁺ oxidation states. In this poster, we present simulation and theoretical approaches to understand and design the connections between microscopic variables and device-level observables.