Modeling of Electrocatalytic Dinitrogen Reduction on Microstructured Electrodes

Abstract

will benefit from a model that includes both the electrochemical NRR and HER kinetics.We aim to contribute a model for electrochemical N 2 reduction with the following factors taken into consideration. As both NRR and HER are proton-coupled electron transfer (PCET) reactions that consumes protons, [7,16,17] the underlying HER and NRR mechanisms are indeed intertwined. A microkinetic approach on the electrode should be considered and the model should not only manage the fate of reducing electrons but also tackle the protonation of reduced species. As N 2 exhibits extremely weak Brønsted basicity, we posit that the initial NRR steps include an electron transfer (ET) followed by a proton transfer (PT) (Figure 1A). On the other hand, the mechanism of HER should include a Volmer step followed by a Tafel or Heyrovský step ( Figure 1B). [18][19][20] Additionally, we are reluctant to preemptively define the rate-determining steps (RDSs) in both NRR and HER, although it is commonly argued that at least some NRR catalysts undergo a rate-determining ET process. [7,[21][22][23] Such a practice may help to illustrate the importance of proton availability on the reaction selectivity, which has been evident in the case of electrochemical CO 2 reduction. [24] Last, the developed microkinetic model should include electrode morphology and be capable to provide guidance for the development of nano-or microstructured electrodes. As N 2 exhibits limited solubility and there may be limited sources of proton in electrolyte, the NRR selectivity could be modulated by the mass transport of redox species in nano-or microstructured electrodes, analogous to the reports in electrochemical CO 2 reduction. [25,26] Here we report a model of electrochemical N 2 reduction that satisfies the abovementioned requirements. As a proofof-principle, the model was numerically simulated with finite element method under an aqueous electrolyte of 0.1 m nonoxidizing strong acid. A database of results was constructed, by simulating the model under a library of different materials' NRR and HER catalytic activities represented by the corresponding exchange current densities. The numerical simulations yield results consistent with expectations from experimental reports. [23,27] The results also imply that the PT step in NRR process can be dominant under certain combinations of NRR and HER exchange current densities and mass transport conditions, although in most cases the ET step is the One major challenge of electrocatalytic dinitrogen (N 2 ) reduction is the competition between nitrogen reduction reaction (NRR) and hydrogen evolution reaction (HER). While experimental development of selective NRR catalysts is indispensable, the aim is to derive insights of catalyst design by developing a model that includes both the kinetics of competing reactions on the electrode and the mass transport of reactants in solution. The developed microkinetic model is reported here and it is applied to evaluate NRR selectivity on planar and microstructured electrodes. As a proof-of...

Enhancing electrocatalytic N2 reduction via tailoring the electric double layers

“…ÃN À 2 , and the proton transfer (PT) step, ÃN À 2 + H + ! *N 2 H. 44 The detailed derivations of the theoretical treatment of the two chemical reactions (ET and PT) are presented in Appendix S1, where we also introduce a phenomenological parameter for each of the two reactions to characterize the (quantum-) chemistry of the process, and thus yielding the local kinetic rate of the NRR process per unit surface area as:…”

“…Â 10 À6 mA/cm 2 that is three orders of magnitudes lower than the corresponding HER case. Moreover, we cannot ignore the role of proton transfer (PT) process in the overall reaction, 44 and thus, a small value of i 0,2,NRR (10 À10 mA/cm 2 ) with respect to the i 0,1,NRR is considered.…”

“…Therefore, to further improve the NRR performance, a deep theoretical understanding of EDL formation and its effect on the NRR process are urgently required, which require the development of new theoretical methods. Recently, a continuum model for electrochemical N 2 reduction that includes both the NRR and the HER kinetics coupled to diffusive mass transport of solution species has been reported, 44 but without accounting for the steric hindrance of the ions and the effect of EDLs. Compared to the Poisson–Nernst–Planck (PNP) equations, the modified PNP (MPNP) equations include the effect of volume exclusion due to the solvated size of solution species, thus, providing a better description of EDLs in concentrated electrocatalytic systems 45 …”

“…In this article, we develop a theoretical framework to study the reaction‐transport process of the NRR system in an acidic aqueous electrolyte. The calculation is implemented by combining MPNP equations with a microkinetic model that features both the electrochemical NRR and HER kinetics 44 . The MPNP equations solve the migration, diffusion, and reaction processes self‐consistently by accounting for steric effects which are ignored in PNP equations 46 .…”

“…The calculation is implemented by combining MPNP equations with a microkinetic model that features both the electrochemical NRR and HER kinetics. 44 The MPNP equations solve the migration, diffusion, and reaction processes self-consistently by accounting for steric effects which are ignored in PNP equations. 46 The comparison of MPNP and PNP calculations highlights the necessity of involving the steric effects in our study for the reaction-transport process.…”