Green Ammonia Electrosynthesis from Nitrate-Polluted Waters

Abstract

Ammonia (NH3) is the basis of all nitrogen fertilisers constituting one of the pillars of modern society, without which nearly half of the population would not be present on Earth.1 Moreover, due to its high energy content and density, it has recently emerged as a potential fuel on the route to decarbonisation.2 However, contemporary NH3 is mainly produced via the energy-intensive and fossil fuel-dependent Haber-Bosch process so to meet the future NH3 demand, the development of new green synthesis techniques is crucial.3 Conversely, due to its high solubility, nitrate (NO3 -) is considered one of the most widespread pollutants in groundwaters.4 Increasing NO3 - concentrations in water sources represent an environmental concern and a risk to human health, requiring the development of novel and efficient techniques to remove them5,6 To this end, electrochemical nitrate reduction reaction (E-NO3RR) has received considerable attention since, among other advantages, it permits the utilization of electricity from renewables. Besides, taking into account that molybdenum and sulphur play key roles in nitrogenase-based nitrogen fixation, molybdenum disulphide (MoS2) is expected to be active toward E-NO3RR, even if it has already demonstrated good catalytic activity toward hydrogen evolution reaction (HER, the main competitor of E-NO3RR).6 In this work, MoS2 has been tested for E-NO3RR in combination with K2SO4 0.1 M as electrolyte. Experiments have been performed employing a flow cell with a gas diffusion electrode (GDE), on which the catalyst is immobilized through air-brushing technique. The design of the experiment and surface response methodology (DoE/RSM) were used to gain further insight onto the influence of three operational variables (i.e. operating potential, catalyst loading and salt concentration in the electrolyte) on the Faradaic efficiency and NH3 yield of the NO3RR, as well as the possible interaction between those variables. Concretely a 3-factors Doehlert design with 3 replicates of the central point was employed. To select the proper range for every variable, screening experiments were first performed. Results reveal operational potential and salt concentration at the electrolyte as main parameter controlling the efficacy of the process Valve, H., et al. in Handbook of the Circular Economy (Edward Elgar Publishing, 2020).Smith, C., et al. Energy Environ. Sci. 13, 331–344 (2020).MacFarlane, D. R., et al. Joule 4, 1186–1205 (2020).Abascal, E., et al. Sci. Total Environ. 810, 152233 (2022).Garcia-Segura, S., et al. Appl. Catal. B Environ. 236, 546–568 (2018).Ingle, J., et al. J. Water Process Eng. 49, 103082 (2022).Zhang, L., et al. Adv. Mater. 30, 1800191 (2018).