Identification and elimination of false positives in electrochemical nitrogen reduction studies

Abstract

Ammonia is of emerging interest as a liquefied, renewable-energy-sourced energy carrier for global use in the future. Electrochemical reduction of N₂ (NRR) is widely recognised as an alternative to the traditional Haber-Bosch production process for ammonia. However, though the challenges of NRR experiments have become better understood, the reported rates are often too low to be convincing that reduction of the highly unreactive N₂ molecule has actually been achieved. This perspective critically reassesses a wide range of the NRR reports, describes experimental case studies of potential origins of false-positives, and presents an updated, simplified experimental protocol dealing with the recently emerging issues.

Fig. 1. Electroreductive synthesis of ammonia.

Pathways towards NH₃/NH₄⁺ through the electrochemical reduction of N₂, gaseous NO_x, and ionic NO_x compounds with standard redox potentials (vs. standard hydrogen electrode) and examples^–,– of some of the known electrocatalysts for each process.

Fig. 2. Bi and Au/C: NRR vs. NO_x reduction.

Comparison of the NH₃ formation rates for electrodes modified with a Bi (tested at −0.65 V vs. reversible hydrogen electrode, RHE) and b Au/C (tested at −0.30 V vs. RHE) reported in the literature (entries 97–98 in Supplementary Table 1) and measured in our laboratory in the presence of Ar, N₂ and different NO_x compounds using 0.5 M K₂SO₄ aqueous electrolyte solutions (for further details, see ref. ). All data are presented as an average ± standard deviation calculated for tests with three independent samples of each type.

Fig. 3. CN_x and VN: NRR vs. decomposition.

NH₃ formation during the electroreduction of a CN_x- and b VN-modified electrodes previously reported in the literature (entries 35 and 43 in Supplementary Table 1), and as measured in our laboratory at different stages of a continuous test. Electrolyte solutions and potentials (vs. reversible hydrogen electrode) used in tests are provided in corresponding panels; further experimental details can be found in Supplementary Figs. 1 and 2 and ref. (CVA = cyclic voltammetry, CA = chronoamperometry).

Fig. 4. Updated and simplified NRR experimental protocol.

Tests with a fixed and/or low volume of the N₂ gas are recommended, as is the quantification of NO_x at every key step. Demonstration of the NRR as a function of time and potential is critical, while studies aiming at thorough kinetic analysis should also investigate the effect of N₂ concentration (which can be either decreased or increased with respect to the experiments at 1 atm). More detailed explanation of each key stage has been reported previously by us^, and others^,.