Ammonium crossover as a function of membrane type and operating conditions in flow cells for ammonia synthesis and water treatment applications

Abstract

Electrochemical flow cells are promising designs for both ammonium () electrosynthesis from dinitrogen and removal/recovery from wastewater. The crossover is undesirable for electrosynthesis but is favourable for removal. The crossover is investigated herein under different current densities, concentrations, and feed locations using cation‐exchange (Nafion N112, N350) and anion‐exchange (Sustainion X37‐50) membranes and microporous diaphragms (Celgard 3400, 3500, and 5550). For Nafion N112, the crossover from catholyte to anolyte decreases with higher concentrations from 81.9 ± 4.7% at 1 ppm to 10.7 ± 0.7% at 3400 ppm. At a constant concentration, increasing the current density leads to more intense electrolyte pH polarization, which leads to volatilization in favour of recovery up to 78.1 ± 1.1% at a cathode superficial current density of −10 A m−2. When comparing the recovery efficiency, the cathode‐ and symmetric fed operations were outperformed by the anode‐fed mode for 3400 ppm due to the equilibrium that buffers the pH change. For Celgard diaphragms, modest crossover (<5%) was only demonstrated at low current densities (≤−1 A m−2), but the separation was compromised by the bulk electrolyte transport through micropores and electrolysis‐induced pH polarization, highlighting future needs to develop and rigorously verify separators toward electrosynthesis.