Electrochemical Ammonia Synthesis Via Nitrogen Reduction Under Mild Conditions

Abstract

Ammonia is produced commercially via the Haber-Bosch (HB) process, which produced 144 million tonnes of ammonia 2014, approximately 88% of which is used in the fertilizer industry [1,2]. However the HB process requires very high temperatures and pressures and as a result it is only be done in large plants which consume large amounts of power and generate large quantities of CO2 from the production of the hydrogen used in the HB process[1,2]. It is estimated that the HB process accounts for nearly 1% of annual global power consumption [3]. Due to the sheer scale of the HB process, distribution from the point of production to the point of use becomes an additional carbon and energy burden. Alternative methods of manufacturing ammonia that enable small scale, distributed generation at the point of use using renewable energy and sustainable feedstocks could have a great impact on global CO2 emissions. Electrochemical synthesis of ammonia at low temperatures and pressures is one promising solution that has recently begun to gain more attention as a viable method of sustainable, on-site generation of NH3. Several electrochemical approaches to NH3 synthesis have been reported in the literature with varying success, included high temperature proton-conducting ceramic electrolytes, molten hydroxides, PEM membrane and AEM membrane systems and more [4,5,6]. Most low temperature and low pressure production rates range between 10-12 and 10-8 mol NH3 cm-2 s-1 with Faradaic efficiencies that are typically very low on the order of 5% due to the competing hydrogen evolution reaction (HER) [4,5,6]. Cathode catalysts that enable increased NH3 production rates while suppressing the HER are critical to realizing the benefits of sustainable electrochemical synthesis of ammonia. As a result catalysts that adsorb nitrogen more strongly than hydrogen must be investigated for an efficient electrochemical synthesis. By utilizing alkaline media, non-platinum group metals (PGMs) may become feasible catalysts, thus lowering costs and use of very limited global supply of PGM. The electrochemical synthesis of ammonia was run under mild temperatures and pressures in alkaline media according to the following reactions: (1) (2) where reactions (1) and (2) take place at the cathode and anode of the electrochemical cell, respectively. The overall reaction leads to the synthesis of ammonia with a theoretical cell voltage of 0.059 V, according to: (3) Humidified nitrogen is flowed over a Pt/Ir electrode where it is reduced to ammonia (Eq. 1), and hydroxide ions are transported through a gel electrolyte to the anode where they are oxidized hydrogen to produce water (Eq. 2). The major challenge associated with electrochemical production of ammonia is the competing HER which can occur at the cathode. In order to mitigate this issue we hypothesize that using a polymer based gel electrolyte to control the amount of water present in the system could allow us to limit the HER, thus increasing the efficiency of the reaction. We believe that by using this process we will be able to examine different catalysts for nitrogen reduction in order to find a catalyst where the HER is less dominant [7].