Development of novel processes for electrochemical synthesis of ammonia

Abstract

Electrochemical synthesis of ammonia, an important nitrogen-compound precursor and fertilizer, is one promising nitrogen fixation technology in that energy consumption, if matured, can be decreased compared to the conventional Haber-Bosch process even while avoiding carbon emissions. In most cases, however, the synthesis rate and faradaic efficiencies were intolerably low

indubitably, the competing hydrogen evolution from protons or water molecules is much more favorable compared to nitrogen reduction, whose sluggish nature comes from the strong triple bond of di-nitrogen. An innovative breakthrough is therefore absolutely necessary to best achieve high selectivity toward nitrogen reduction and at the same time, to avoid electron-stealing hydrogen evolution, thereby leading to the mass production of ammonia.

This research, therefore, explored the development of novel processes for highly efficient electro-synthesis of ammonia from water and nitrogen. First, 2-propanol was used as an electrolyte medium, and its effectiveness in the electrochemical reduction of nitrogen to ammonia under ambient temperature and pressure was evaluated by galvanostatic electrolysis. Ammonia synthesis and faradaic efficiency using a mixture of 2-propanol/water (9:1, v/v) surpassed those carried out using solely water. The optimal condition, in terms of the $H_2SO_4$ concentration and applied current density, was 10 mM and 0.5 mA $cm^{-2}$, respectively, for which an ammonia synthesis rate and faradaic efficiency of 1.54 × $10^{-11} mol s^{-1} cm^{-2}$ and 0.89 % were observed, respectively. This study revealed that the use of 2-propanol, with better nitrogen solubility and inhibition of hydrogen evolution, offered an effective way to improve the electrochemical synthesis of ammonia under ambient temperature and pressure conditions.

As another ambient temperature ambient pressure approach, a novel electrolysis cell was developed for efficient ammonia electro-synthesis using ethylenediamine (EDA) as a cathodic solvent. In this particular electrochemical system, the anodic compartment was filled with 0.05 M $H_2SO_4$ aqueous solution, while the cathodic compartment was filled with 0.1 M LiCl/EDA. The two chambers were divided by a cation exchange membrane called the CMX. Due to the cathodic stability of EDA even in a negative potential region, the faradaic efficiency for the $NH_3$ synthesis was observed to be quite high at 17.2 % with a corresponding synthesis rate of 3.58 × $10^{-11} mol s^{-1} cm^{-2}$. Along with the results obtained using 2-propanol as a cathodic solvent, this study demonstrated that the use of an appropriate solvent provided a way for efficient ammonia electro-synthesis under ambient conditions.

In the next study, a molten alkali-metal chloride (LiCl-KCl-CsCl) was chosen as a promising electrolyte due to its high ionic conductivity, thermodynamic stability, wide electrochemical window, and low vapor pressure, and particular attention was paid to accelerate the sluggish nitrogen reduction to nitride ion in this electrolyte. Various 3d metals (titanium, iron, cobalt, and nickel) were investigated as potential cathode materials. Electrochemical performances of the electrodes were measured using linear sweep voltammetry (LSV) and chronoamperometry (CA). The highest activity in nitrogen reduction was observed with cobalt, followed by nickel, iron, and titanium, in terms of an effective nitrogen reduction current. This performance order seems to be caused by the synergetic effect between the electrical resistivity and wettability of the material. In the following study, nano-$Fe_2O_3$ and $CoFe_2O_4$ were suspended in the molten chloride, and their catalytic activity in ammonia synthesis was evaluated by potentiostatic electrolysis. The presence of a nanoparticle suspension led to improved ammonia production with a synthesis rate of 1.78 × $10^{-10}$ and 3.00 × $10^{-10} mol s^{-1} cm^{-2}$ for $CoFe_2O_4$ and $Fe_2O_3$, which are 102 % and 240 % higher than that without a nano-catalyst, respectively. The nanoparticles were speculated to trigger both electrochemical nitrogen reduction and the chemical reaction between nitrogen and hydrogen that was produced by the water electro-reduction on the cathode. The use of nano-catalysts in the form of a suspension turned out to offer an innovative way to overcome the sluggish nature of nitrogen reduction in the molten chloride electrolyte.

In the following study, a novel lithium-mediated process comprised of three successive processes consisting of (i) lithium deposition

(ii) nitridation

and (iii) ammonia formation was proposed using the principle of a rechargeable Li-air battery. A proof-of-concept study revealed that ammonia was successfully synthesized starting from nitrogen and water yielding a maximum faradaic efficiency and synthesis rate of 49.93 % and 1.88× $10^{-9} mol s^{-1} cm^{-2}$, respectively. As an approach to improve more the performance of the Li-mediated approach, cesium perchlorate was used as an additive in the cathodic electrolyte of a Li deposition cell, and their effect on the morphology of the deposited Li as well as on the $NH_3$ synthesis was investigated. $NH_3$ synthesis was improved in the presence of 0.03 M $Cs^+$ with a maximum faradaic efficiency of about 82.3%. The promising faradaic efficiency reported in the Li-mediated approach, while retaining a comparable synthesis rate, presents a new type of nitrogen fixation strategy for a next-generation energy storage system.