Journal
ACS CATALYSIS
Volume 10, Issue 24, Pages 14592-14603Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.0c04864
Keywords
electrochemical N-2 reduction; renewable ammonia synthesis; gas-diffusion electrode; density functional theory; electrolyte effects
Categories
Funding
- Office of Science of the U.S. Department of Energy [DE-AC0205CH11231]
- SHyNE Resource [NSF ECCS-1542205]
- IIN
- Northwestern's MRSEC program [NSF DMR-1720139]
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The electrochemical reduction of N-2 to produce NH3 at ambient conditions is an effective and sustainable route to store and carry hydrogen, balance the nitrogen cycle, and provide means to produce on-demand fertilizers. The efficient electrosynthesis of NH3 is challenging because of the lower activation of N-2 and higher activity toward the hydrogen evolution reaction (HER). Here, we propose theory-guided activity descriptors to identify an efficient N-2 reduction reaction (NRR) catalyst, followed by its implementation in a flow-through gas diffusion electrode (GDE) to quantify the effects of pH, cation identity, H2O saturation, and N-2 concentration on the kinetics of the NRR. The identified Cu catalyst with dominant (111) facets electrodeposited on a carbon paper provides optimal active sites to obtain maximum NH3 faradaic efficiency (FE) of 18 +/- 3% at-0.3 V vs RHE and the maximum NH3 current density of 0.25 +/- 0.03 mA cm(-2) (0.86 nmol.cm(-2).s(-1)) at-0.5 V vs RHE in alkaline medium. The electrolyte pH mostly affects the HER by pH-induced binding of *H and reorganization of H2O, which favor the NRR at an optimal pH of 13.5. Increasing the size of monovalent cations stabilizes NRR intermediates and increases the NH3 current density from Li+ to K+. However, increasing the size of the cation from K+ to Rb+ reduces the FE of NRR, which is due to a direct reduction of H2O in the solvation shell of larger cations to produce H-2. Another strategy to improve NH3 FE is to reduce the H2O saturation on the catalyst, which can be achieved by sparging the reactant gas directly through the GDE. Increasing the N-2(g) flow rate not only increases the gas-liquid mass transfer coefficient but also reduces the H2O saturation in the pores of the GDE, which primarily suppresses the HER. The fixed potential DFT calculations reveal an associative distal mechanism for the NRR over Cu(111), where the hydrogenation of *N-2 is the rate-limiting step. This finding also corroborates with the measured reaction order with respect to N-2.
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