Unraveling the Active Sites on Mesoporous CuFe2O4@N-Carbon Catalysts with Abundant Oxygen Vacancies and M-N-C Content for Boosted Nitrogen Reduction Toward Electrosynthesis of Ammonia.

Transition metal centers dispersed over nitrogen-doped carbon (M-NC) supports have been widely explored for electrocatalytic reactions; however, sparsely reported for electrochemical nitrogen reduction reaction (ENRR). Particularly, the single-atom catalysts (SACs) have shown reasonable ammonia yield rate and faradaic efficiency (FE), but their complex synthesis and low durability for long-term electrocatalysis runs restrict their use on a larger scale. Importantly, the catalytic active sites in metal nanostructured-based M-NC catalysts toward enhanced N2 adsorption and activation are still not clear as they are highly challenging to reveal. A few studies have predicted that the surface oxygen vacancies (Ovac) favor an enhanced ENRR performance. Herein, a strategy using tailored M-NC content and Ovac in a single catalyst for enhanced ammonia electrosynthesis is devised. A mesoporous bimetallic spinel oxide (CuFe2O4) supported over N-doped carbon (CuFe2O4@NC) derived from Prussian blue analog (PBA) via controlled pyrolysis possess is found to show boosted ENRR activity. Moreover, operando NH3 formation over the catalyst is observed using four electrode set up. This approach enables rapid evaluation ofelectrocatalytic efficacy and avoids false positive results. The rotating disc electrode results reveal that mass transport in acidic media and surface absorption in alkline media primarily regulate ENRR over CuFe2O4@NC electrocatalyst.

Mechanistic insights into the electrolyte effects on the electrochemical nitrogen reduction reaction using copper hexacyanoferrate/f-MWCNT nano-composites.

Developing an efficient, selective, and stable electrocatalysis system for the electrocatalytic N2 reduction reaction (ENRR) is a promising strategy for the green and sustainable production of ammonia. The activity, selectivity, and stability of various electrocatalysts in different electrolyte solvents, mainly acidic and alkaline electrolytes, are commonly compared in the literature. However, a mechanistic insight into the effect of these electrolytes on ENRR activity is lacking. Herein we demonstrate that the acidity or alkalinity of the electrolyte is a key factor in determining the rate-limiting step and, by extension, the ENRR performance of an electrochemical setup for the electroproduction of ammonia. Our results from ex situ X-ray photoelectron, Raman, and FTIR spectroscopy analysis of the fresh and spent Cu-hexacyanoferrate Prussian blue analogue-decorated functionalized carbon nanotube (CuFe PBA/f-CNT) catalyst reveal that NH4+-species are more strongly adsorbed on the catalyst surface during the ENRR in acidic than in alkaline electrolytes. The results of our detailed rotating ring-disc electrode voltammetry studies suggest that the ENRR over CuFe PBA/f-CNT is mostly controlled by surface adsorption in an acidic electrolyte and by mass transport in an alkaline electrolyte. In situ Raman spectroscopy confirms this finding and shows that the leaching of Fe(CN)6 species from the CuFe PBA/f-CNT composite in an alkaline electrolyte greatly affects the ENRR performance. We believe that the work presented herein offers a new insight into the mechanistic aspects of the ENRR in different electrolyte systems and hence can prove very valuable for the development of effective ENRR electrode/electrolyte systems for practical applications.