Facilitated transport of ammonia in ultra-thin Prussian Blue membranes with potential-tuned selectivity

Abstract

Here we report on tunable ammonia transport through ultrathin Prussian Blue (PB) membranes facilitated with protonic carrier. Continuous gas-tight 15–50 nm Prussian Blue layers were prepared by cyclic electrodeposition on porous gold/anodic aluminum oxide supports. The membranes reveal fast ammonium transport with NH3 permeance exceeding 0.3 m3(STP) m-2 bar-1 h-1, ultimate ideal NH3/H2 selectivity attaining 40 and NH3/N2 selectivity over 100. In mixed-gas separation thermodynamically-favored sorption leads to additional NH3/H2 selectivity rise to ~70. Facilitation of NH3 transport occurs upon its conversion to NH4+ maintaining the electroactivity of the selective layer and reversibly tuned by electrochemical framework loading. Engagement of the protonation step is supported with Raman spectroscopy and close values of NH3 diffusion coefficients measured in gas permeation experiments and NH4+ diffusion coefficient determined by electrochemical impedance spectroscopy in liquid phase. The impact of water vapors partial pressure as backward protonic carrier on NH3 transport efficiency is exposed and discussed.