(Invited) Plasma Activated Electrocatalysis for Nitrogen Fixation
Authors: Tsampas Michail , Sharma Rakesh , Welzel Stefan , van de Sanden Richard 
Published: July 2021

With increasing global interest in renewable energy technology in the backdrop of climate change, storage of electrical energy has become particularly relevant [1]. However, due to their intermittent nature and the harvesting in the form of electricity, their direct introduction into the value chain of e.g. chemical industry remains challenging. Therefore, technologies based on renewable electricity that can transform base molecules (i.e. H2O, N2, CO2) into energy rich ones (e.g. H2, NH3) or chemical feedstock (e.g. NO, CO) is of paramount importance [2].
Among the three base molecules, N2 is by far the least reactive since the N≡N triple bond is very strong and difficult to activate due to the absence of a permanent dipole [3]. Consequently, even with the best available catalysts, a substantial energy input is required to activate N2. At industrial level nitrogen fixation is realized via ammonia and nitric oxide synthesis [4,5]. Up to now solutions were mainly sought on material axis, however recent theoretical studies have revealed that there are intrinsic limitations of catalysis (i.e. scaling relationships) which keep the processes far from the optimum performance [6].
In this contribution, we will present a unique (all electric) solution to the aforementioned limitations. We employ solid oxide electrolysis cells (SOECs) [7] that allow us to provide reacting species on catalysts with a controllable manner while a radiofrequency plasma [8] is used to increase the reactivity of nitrogen. By employing oxygen ion or proton conducting SOECs we can produce either the ammonia or nitric oxide by supressing hydrogen or oxygen evolution respectively [2]. The spatial separation of nitrogen dissociation and catalytic formation of the target molecules provides true independent parameters to optimise the electrocatalytic reactions. In both cases concentration of products is orders of magnitude higher than equilibrium at the same operation conditions (in the absence of plasma) while very high selectivity to nitrogen fixation is observed. Our results reveal new opportunities for sustainable nitrogen fixation.
References
[1] T.M. Gur, Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage, Energy Environ. Sci., 11 (2018) 2696-2767.
[2] H. Patel et al, Plasma-activated electrolysis for cogeneration of nitric oxide and hydrogen from water and nitrogen, ACS Energy Lett. 5 (2019) 2091-2095
[3] C.J.M. van der Ham et al, Challenges in reduction of dinitrogen by proton and electron transfer, Chem. Soc. Rev., 43 (2014) 5183–5191.
[4] D.R. MacFarlane et al, A roadamp to ammonia economy Joule, 4 (2020) 1186-1205.
[5] A. Boagerts and E.C Neyts, Plasma technology: An emerging technology for energy storage, ACS Energy Lett., 3 (4) (2018) 1013-1027.
[6] P. Mehta et al, Catalysis Enabled by plasma activation of strong chemical bonds: A review, ACS Energy Lett., 4 (2019) 1115-1133.
[7] M.N. Tsampas et al, Applications of yttria stabilized zirconia (YSZ) in catalysis, Catal. Sci. Technol., 5 (2015) 4884-4900
[8] V. Guerra et al, Dissociation mechanisms in nitrogen discharges, Plasma Sources Sci. Technol. 12 (2003) 576-581

Figure 1