Searching for the Rules of Electrochemical Nitrogen Fixation

Tuning the Acid Hardness Nature of Cu Catalyst for Selective Nitrate-to-Ammonia Electroreduction

Electrocatalytic nitrate reduction reaction (NO3RR) in alkaline electrolyte presents a sustainable pathway for energy storage and green ammonia (NH3) synthesis. However, it remains challenging to obtain high activity and selectivity due to the limited protonation and/or desorption processes of key intermediates. Herein, we propose a strategy to regulate the acid hardness nature of Cu catalyst by introducing appropriate modifier. Using density functional theory calculations, we firstly identified that the BaO-modified Cu showed optimal Gibbs free energies for key NO3RR steps, including the protonation of *NO and the desorption of *NH3. Experimentally, the BaO-modified Cu catalyst exhibited 97.3 % Faradaic efficiency (FE) for NH3 with a yield rate of 356.9 mmol h-1 gcat -1. It could also maintain high activity across a wide range of applied potentials and nitrate substrate concentrations. Detailed experimental and theoretical studies revealed that the Ba species could modulate the local electronic states of Cu, enhance the electron transfer rate, and optimize the adsorption/protonation/desorption processes of the N-containing intermediates, leading to the excellent catalytic performance for NO3 --to-NH3.

A carbon cathode for lithium mediated electrochemical ammonia synthesis

To introduce the potential for tuneability of the cathode in lithium mediated ammonia synthesis, we report a carbon cathode which produces ammonia at a faradaic efficiency of 37%. This provides a basis to optimise properties of carbon electrodes to achieve high current densities and faradaic efficiencies.

Electrochemical Ammonia Synthesis: The Energy Efficiency Challenge

We discuss the challenges associated with achieving high energy efficiency in electrochemical ammonia synthesis at near-ambient conditions. The current Li-mediated process has a theoretical maximum energy efficiency of ∼28%, since Li deposition gives rise to a very large effective overpotential. As a starting point toward finding electrocatalysts with lower effective overpotentials, we show that one reason why Li and alkaline earth metals work as N2 reduction electrocatalysts at ambient conditions is that the thermal elemental processes, N2 dissociation and NH3 desorption, are both facile at room temperature for these metals. Many transition metals, which have less negative reduction potentials and thus lower effective overpotentials, can dissociate N2 at these conditions but they all bind NH3 too strongly. Strategies to circumvent this problem are discussed, as are the other requirements for a good N2 reduction electrocatalyst.

Techno-economic assessment of different small-scale electrochemical NH3 production plants

Electrochemical ammonia synthesis via the nitrogen reduction reaction (NRR) has been poised as one of the promising technologies for the sustainable production of green ammonia. In this work, we developed extensive process models of fully integrated electrochemical NH3 production plants at small scale (91 tonnes per day), including their techno-economic assessments, for (Li-)mediated, direct and indirect NRR pathways at ambient and elevated temperatures, which were compared with electrified and steam-methane reforming (SMR) Haber-Bosch processes. The levelized cost of ammonia (LCOA) of aqueous NRR at ambient conditions only becomes comparable with SMR Haber-Bosch at very optimistic electrolyzer performance parameters (FE > 80% at j ≥ 0.3 A cm-2) and electricity prices (<$0.024 per kW h). Both high temperature NRR and Li-mediated NRR are not economically comparable within the tested variable ranges. High temperature NRR is very capital intensive due the requirement of a heat exchanger network, more auxiliary equipment and an additional water electrolyzer (considering the indirect route). For Li-mediated NRR, the high lithium plating potentials, ohmic losses and the requirement for H2, limits its commercial competitiveness with SMR Haber-Bosch. This incentivises the search for materials beyond lithium.

Electrochemical Nitrogen Reduction: The Energetic Distance to Lithium

Energy-efficient electrochemical reduction of nitrogen to ammonia could help in mitigating climate change. Today, only Li- and recently Ca-mediated systems can perform the reaction. These materials have a large intrinsic energy loss due to the need to electroplate the metal. In this work, we present a series of calculated energetics, formation energies, and binding energies as fundamental features to calculate the energetic distance between Li and Ca and potential new electrochemical nitrogen reduction systems. The featured energetic distance increases with the standard potential. However, dimensionality reduction using principal component analysis provides an encouraging picture; Li and Ca are not exceptional in this feature space, and other materials should be able to carry out the reaction. However, it becomes more challenging the more positive the plating potential is.

What Metals Should Be Used to Mediate Electrosynthesis of Ammonia from Nitrogen and Hydrogen from a Thermodynamic Standpoint?

Recently, metal-mediated electrochemical conversion of nitrogen and hydrogen to ammonia (M-eNRRs) has been attracting intense research attention as a potential route for ammonia synthesis under ambient conditions. However, which metals should be used to mediate M-eNRRs remains unanswered. This work provides an extensive comparison of the energy consumption in the classical Haber Bosch (H-B) process and the M-eNRRs. The results indicate that when employing lithium and calcium, metals popularly used to mediate the M-eNRRs, the energy consumption is more than 10 times greater than that of the H-B process even assuming a 100% Faradaic efficiency and zero overpotentials. Only electrosynthesis with a cell voltage not exceeding 0.38 V might have the potential to rival the H-B process from an energetic perspective. A further analysis of other metals in the periodic table reveals that only some heavy metals, including In, Tl, Co, Ni, Ga, Mo, Sn, Pb, Fe, W, Ge, Re, Bi, Cu, Po, Tc, Ru, Rh, Ag, Hg, Pd, Ir, Pt, and Au, can potentially consume less energy than that of the H-B process purely from a thermodynamic standpoint, but whether they can activate N2 under ambient conditions is yet to be explored. This work shows the importance of performing thermodynamic analysis for the development of an innovative strategy to synthesize ammonia with the ultimate goal of replacing the H-B process on a large scale.