Abstract
A key challenge to realizing practical electrochemical N2 reduction reaction (NRR) is the decrease in the NRR activity before reaching the mass-transfer limit as overpotential increases. While the hydrogen evolution reaction (HER) has been suggested to be responsible for this phenomenon, the mechanistic origin has not been clearly explained. Herein, we investigate the potential-dependent competition between NRR and HER using the constant electrode potential model and microkinetic modeling. We find that the H coverage and N2 coverage crossover leads to the premature decrease of NRR activity. The coverage crossover originates from the larger charge transfer in H+ adsorption than N2 adsorption. The larger charge transfer in H+ adsorption, which potentially leads to the coverage crossover, is a general phenomenon seen in various heterogeneous catalysts, posing a fundamental challenge to realize practical electrochemical NRR. We suggest several strategies to overcome the challenge based on the present understandings.
Highlights
A key challenge to realizing practical electrochemical N2 reduction reaction (NRR) is the decrease in the NRR activity before reaching the mass-transfer limit as overpotential increases
In the NRR measurements on Fe@N4, the maximum NH3 yield rate is obtained at U = 0, −0.05 V vs. reversible hydrogen electrode (RHE) in 0.1 M KOH13,14 and U = −0.40 V in 0.1 M phosphate-buffered saline (PBS)[23]
We find that reaction energy obtained by the constant electrode potential (CEP) model is different from that obtained by the computational hydrogen electrode (CHE) model at the same potential (Supplementary Fig. 11)
Summary
A key challenge to realizing practical electrochemical N2 reduction reaction (NRR) is the decrease in the NRR activity before reaching the mass-transfer limit as overpotential increases. The larger charge transfer in H+ adsorption, which potentially leads to the coverage crossover, is a general phenomenon seen in various heterogeneous catalysts, posing a fundamental challenge to realize practical electrochemical NRR. Potential-dependent measurements often showed that the NRR activity (NH3 yield rate) begins to decrease even at a low overpotential region[7]. ~200 mV, decreased at large overpotentials before reaching the mass-transfer limit It results in the NRR current much smaller than the expected mass-transfer-limited values considering saturated N2 concentration in aqueous solution at ambient conditions, i.e., ~1 mM7
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