Electrochemical ammonia compression

Abstract

Every year a large amount of ammonia is mechanically compressed with a low efficiency (∼65%) as an environmentally friendly chemical for fuel storage, transportation, food preservation and air conditioning. Electrochemical compression with a high efficiency (93%) offers an opportunity for substantial energy savings. But ammonia is never considered electrochemically compressible because of the known decomposition at high applied potentials. The concept of using a carrier gas to co-compress ammonia was proposed but never experimentally validated. Here for the first time, we realized electrochemical compression of ammonia by using a proton exchange membrane as an NH₄⁺ conductor and hydrogen as a carrier to avoid decomposition. The ammonia transfer mechanism and kinetics in a Nafion membrane was investigated and verified using electro-analytical methods, and the continuous electrochemical ammonia compression at a constant voltage of 200 mV with a stable current density for 7 hours has been demonstrated. NH₄⁺ transports in the Nafion membrane by hopping and vehicular mechanisms with a high NH₄⁺ conductivity of 4 × 10⁻² S cm⁻¹ at 50% RH and 70 °C, and a constant transfer ratio of 2 between NH₃ and H₂ ensure a stable and high compression rate.