Engineering electrochemical cells for ambient ammonia synthesis via nitrogen reduction

Abstract

Ammonia is an important chemical in agriculture, pharmaceutical and textile industries, which is industrially produced via a Haber-Bosch process that converts atmospheric nitrogen to ammonia by a chemical reaction with hydrogen using metal catalysts at high temperatures and pressures. However, this energy-consuming process will annually consume more than 1 % global fossil fuels to create the reaction condition of high temperatures (673 - 773 K) and high pressures (20 - 30 MPa), leading to 1.6 % CO2 emission per year. In response to energy and environmental consequences, an electrochemical ammonia production technology via nitrogen reduction reaction under ambient conditions has been recently developed and extensively investigated, which is capable of converting nitrogen and water to ammonia and oxygen (N2 + H2O → NH3 + O2) by utilizing renewable power generated from wind or solar energy. Typically, an electrochemical cell for ammonia synthesis consists of three key components: an anode for oxygen evolution, an ion exchange membrane for charge-carrier migration, a cathode for nitrogen reduction, all of which are saturated with liquid electrolyte. However, highly stable triple bonds of nitrogen molecules lead to high activation energy for nitrogen dissociation, thus causing a poor ammonia yield rate (typically < 3.0 × 10-10 mol cm-2 s-1) in electrochemical cells, far away from the target proposed by U.S. Department of Energy (> 10-6 mol cm-2 s-1). The primary objective of this thesis is to advance electrochemical cells for efficient ammonia synthesis via nitrogen reduction reaction, including the nanostructure design of electrocatalysts, the composition optimization of electrolytes, rational design and construction of porous electrodes, and their performance evaluations in electrochemical cells. Firstly, two-dimensional tin oxide (SnO2) nanosheets exposing oriented crystalline (101) facets are designed and prepared. Density functional theory calculation shows that (101) facets of SnO2 are preferrable for nitrogen adsorption and ammonia desorption. In addition, the presence of oxygen vacancies and surface uncoordinated atoms also facilitates nitrogen dissociation by strengthening Sn-N bonds. Resultantly, using the nanostructured SnO2 as the cathode in an electrochemical H-type cell enables an ammonia yield rate of 6.24 × 10-10 mol cm-2 s-1 and a Faradaic efficiency of 11.33%, exceeding most of recently reported performances achieved by using metal-based electrocatalysts. Secondly, the effect of the electrolyte composition containing various anions on nitrogen reduction and ammonia synthesis is investigated. It is found that a more positive onset potential of -0.2 V (vs. RHE) for nitrogen reduction is achieved in the electrolyte containing 1.0 M hydroxide ions (OH-), compared to those containing chloride ions (Cl-) with an onset potential of -0.45 V or thiocyanate ions (SCN-) with an onset potential of -0.65 V. The current densities attributed to nitrogen reduction reaction (jNRR) are measured at given potentials by linear scan voltammetry technique. The jNRR achieved at pH≥13 shows a two- or three-fold increase compared to those at 11≤pH≤12. The performance enhancement is attributed to the appearance of hydronium ions (H3O+) at pH≥13, which facilitates the dissociation of N2 (*N2 + H3O+ + e- → *N2H + H2O). While at pH≥12, the dissociation of N2 can be triggered only by water dissociation (*N2 + H2O + e- → *N2H + OH-), which requires a more negative onset potential of - 0.27 V (vs. RHE), as compared to -0.20 V at pH≥13. Thirdly, a binder-free porous composite electrode (bismuth nanoparticles@nickel foam) is prepared via a displacement reaction (2Bi3+ + 3Ni → 3Ni2+ + 2Bi). The synergistic effect of bismuth and nickel facilitates the nitrogen reduction to ammonia, in which bismuth provides active sites for nitrogen adsorption (* + N2 → *N2) and nickel facilitates subsequent nitrogen dissociation for ammonia synthesis (*N2 + 6H+ + 6e- → 2NH3) by supplying sufficient protons via water dissociation. It is demonstrated that using the as-prepared electrode in an electrochemical H-type cell displays an ammonia yield rate of 8.77 × 10-11 mol cm-2 s-1 and a Faradaic efficiency of 6.3 %. Lastly, an electrochemical flow cell, which can be scalable for practical application, is designed and fabricated for ammonia synthesis. Radically different from conventional electrochemical cells, this flow cell has the layer-by-layer architecture similar to that of fuel cells. Such a design retains the unique advantages of the fuel cell architecture, while increases the flexibility of component design and system optimization, creating plenty of room for performance improvement and system cost reduction. Experimentally, this flow cell configuration exhibits an ammonia yield rate of 1.61 × 10-10 mol cm-2 s-1, which is much higher than that achieved by a conventional one (1.20 × 10-10 mol cm-2 s-1).