Crossover, volatilization, and adsorption of ammonium ions in a proton-exchange membrane electrolyzer in relation to electrochemical ammonia production

Abstract

Electrochemical ammonia (NH3) production from dinitrogen (N2) could be an environmentally sustainable alternative to the traditional manufacturing process (Haber-Bosch). However, a flow electrolysis cell that could elevate the ammonia production to a scale of practical relevance has not been demonstrated yet for this emerging technology. Besides N2, NH3 can be electrochemically produced from nitrogen-containing compounds such as nitrogen oxides (NOx), nitrite (NO2–), and nitrate (NO3–). Product crossover in those processes could significantly interfere with accurate performance evaluation, which has received limited attention. Herein, the commonly used Nafion N112 membrane was found ineffective at preventing the undesired crossover of NH3 or ammonium (NH4+) ions in a flow cell even at the 1 ppm concentration level, with a normalized flux as high as 247 µmol m-2h−1 in Na2SO4 at −0.1 mA cm−2. Severe NH4+ adsorption in Nafion was also observed, particularly when no supporting electrolyte is present. Moreover, the interactions between Nafion and NH4+ depend on the supporting electrolyte type, pH changes induced by ion exchange and applied current during continuous electrolysis. Increasing the membrane thickness altered the rate of NH4+ losses, however, the NH4+ crossover and volatilization cannot be fully mitigated. Lastly, the cation exchange between membrane-adsorbed NH4+ and Na+ from the electrolyte is proposed as a membrane-cleaning procedure to reduce false positives in electrochemical NH3/NH4+ production studies.