Due to the world’s dependence on ammonia production and the necessity to drastically reduce our greenhouse gas emissions, tremendous efforts have been dedicated towards the synthesis of “green” ammonia. Many pathways are currently investigated, such as water splitting to supply H2, and revisiting the Haber-Bosch process itself. One attractive avenue towards this goal is to develop robust and cheap electrocatalysts that can mediate N2 to NH3 conversion under mild conditions. Direct electrosynthesis of ammonia, coupled with renewable energies, could compete with the current state-of-the-art Haber-Bosch process. Our group has recently reported the first Fe-based electrocatalyst that can convert N2 to NH3 at -2.1 V vs Fc+ with a 5.5 TON. To improve on this initial result, we have targeted modifying the ligand scaffolds in a way that alters the redox potential without altering the primary coordination sphere of the metal. In this context, we will present our latest data, which suggests that the incorporation of positively charged ammonium groups in our trisphosphine-borane ligand backbone drastically affects the redox potential of the Fe complexes without decreasing N2 activation. The catalytic activity of these new Fe complexes for N2 fixation, as well as their electrochemical properties, will be presented. Notably, we will compare the selectivity and efficiency to the neutral trisphosphine-borane parent system. This allows us to correlate the redox potential of a given complex to its catalytic activity for N2 fixation and helps us develop new catalyst designs towards improved efficiency. Finally, we will present our preliminary data on new catalyst structures containing crown ethers. The incorporation of crown ether moieties enables installation of alkali and alkali earth metal cations such as Na+, K+, and Ca2+, depending on the crown ether size. This synthetic achievement not only tunes the redox potential but also alters the N2 activation step.

Development of Fe-based electrocatalysts for N2 fixation

ACS Spring 2022

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