(Invited) Plasma-Assisted Nitrogen Fixation By Water: Development and Evaluation of Hybrid Membrane Based Plasma-Electrochemical Reactor

Abstract

The worldwide energy crisis and environmental issues have greatly driven the current research on exploring and efficiently utilizing the environmentally-friendly and sustainable energy sources1. Most sustainable sources such as solar and wind energy are in principle able to meet the global energy demand. Nevertheless, they are intermittent and require new concepts of conversion and storage of electricity. Chemical feedstock, i.e. storing energy in form of the binding energy of molecules, is an economically feasible option for long term (seasonal) storage. However, the main challenge is how to address the problem of developing an effective process for converting electrical energy into molecules of high energy for chemical feedstock.In this context, nitrogen fixation is unquestionably one of the most important chemical conversion process since it converts atmospheric nitrogen (low energy molecule) into molecules of high energy (e.g. NH3, NO)2,3.However, contemporary chemical industry for nitrogen fixation impose great concerns about the environmental sustainability in terms of immense energy consumption and burdened emissions profile. Nevertheless, plasma-technology that can be directly powered by renewable electricity, has been receiving renewed attention as an alternative “green” approach for N2 activation4 which is one of the fundamental requirement for NO or NH3 synthesis.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 performance5. In this work, we will present a unique solution to the aforementioned limitations by employing a hybrid type reactor consisting of a plasma reactor and solid state water electrolysers with oxygen ion6 or proton conducting membranes. Unlike conventional plasma catalysis that requires the co-activation of reactants, in the proposed alternative approach, electrolysers provide reacting species on catalysts with a controllable manner while a radiofrequency plasma is used to increase the reactivity of nitrogen7. Such spatial separation of nitrogen dissociation and catalytic formation of the target molecules provides truely independent parameters to optimise the nitrogen fixation process. One aided benefit of the proposed approach is that both technologies, i.e. water electrolyser and plasma activation, utilize base molecules (N2 and H2O) and can be directly powered by renewable electricity. Such a scheme may be a stepping stone to zero carbon footprint processes. Moreover, the advantages of proposed approach will be also compared to conventional plasma catalysis or pure plasma processes.