Efficient solar-driven electrocatalytic nitrate-to-ammonia conversion by 2D ultrathin Fe single-atom catalysts

Abstract

The controllable design of single-atom electrocatalysts with high active site exposure density, enhanced mass/volume specific activity, and low mass transfer resistance holds tremendous potential for green ammonia synthesis involving the electrochemical nitrate reduction reaction (eNO₃RR). Here we report the synthesis of ultrathin two-dimensional electrocatalysts with the inclusion of iron (Fe) single-atom catalytic active sites (2D Fe-SACs) for the nitrate reduction reaction (NO₃RR). Our isotopic nuclear magnetic resonance (NMR) analyses revealed that 2D Fe-SACs exhibit remarkable performance, with a maximum faradaic efficiency of 95.4 ± 4.00% for the NO₃RR to NH₃ at an overpotential of −0.40 V versus the reversible hydrogen electrode (vs. RHE). Density functional theory (DFT) calculations suggest that the enhanced selectivity of 2D Fe-SACs to produce NH₃ is attributed to a low energy barrier of 0.31 eV associated with the oxidation of *NO to *NHO. Then, we assembled the catalyst in a two-electrode electrolyzer connected to an InGaP/GaAs/Ge triple-junction solar cell and achieved a solar-to-ammonia (STA) conversion efficiency of 4.35% and a maximum yield rate of 0.29 mmol h⁻¹ cm⁻² equivalent to 5.10 mg h⁻¹ cm⁻². These findings open new avenues for developing platinum group metal (PGM)-free single-atom catalysts (SACs) to realize the Haber-Bosch process using solar energy.