Elementary kinetics of nitrogen electroreduction on Fe surfaces

Abstract

Electrochemical ammonia synthesis could provide a sustainable and efficient alternative to the energy intensive Haber-Bosch process. Development of an active and selective N2 electroreduction catalyst requires mechanism determination to aid in connecting the catalyst composition and structure to performance. Density functional theory (DFT) calculations are used to examine the elementary step energetics of associative N2 reduction mechanisms on two low index Fe surfaces. Interfacial water molecules in the Heyrovsky-like mechanism help lower some of the elementary activation barriers. Electrode potential dependent barriers show that cathodic potentials below −1.5 V-RHE (reversible hydrogen electrode) are necessary to give a significant rate of N2 electroreduction. DFT barriers suggest a larger overpotential than expected based on elementary reaction free energies. Linear Brønsted-Evans-Polanyi relationships do not hold across N–H formation steps on these surfaces, further confirming that explicit barriers should be considered in DFT studies of the nitrogen reduction reaction.

Does a Thermoneutral Electrocatalyst Correspond to the Apex of a Volcano Plot for a Simple Two‐Electron Process?

“…Volcano plots are av aluable tool to screen electrode materials for electrocatalytic processes and may contribute to the identification of sustainable electrode materials on the way toward afuture energy scenario.However,the numerous assumptions made in theoretical electrocatalysis inherently limit the applicability of the current methods. [45] Ther ecent literature particularly encourages scientists to reflect critically on the underlying assumptions of the volcano method in electrocatalysis,aswell as to expand the volcano approach by including overpotential and/or kinetics in the analysis.While in the past years major progress in ap roper description of electrocatalytic kinetics by first-principles computations has been made, [46][47][48][49][50][51][52][53] the main problem still refers to the fact that ab initio kinetic studies are computationally demanding. Most of these works refer to amodel system, whereas it is unclear if the findings obtained hold true in ah omologous series of materials on which the screening of electrodes is based.…”

“…The recent literature particularly encourages scientists to reflect critically on the underlying assumptions of the volcano method in electrocatalysis, as well as to expand the volcano approach by including overpotential and/or kinetics in the analysis. While in the past years major progress in a proper description of electrocatalytic kinetics by first‐principles computations has been made, the main problem still refers to the fact that ab initio kinetic studies are computationally demanding. Most of these works refer to a model system, whereas it is unclear if the findings obtained hold true in a homologous series of materials on which the screening of electrodes is based.…”

Understanding Potential-dependent Competition Between Electrocatalytic Dinitrogen and Proton Reduction Reactions

“…Here, we attempt to quantify the effect of potential-dependent surface coverage on the NRR activity and unveil the origin of premature decrease of NRR activity. We note that while many theoretical studies investigated the details of NRR mechanisms on catalysts such as Ru 25,26 , Fe 27 , transition metal nitrides [28][29][30] and late transition metal surfaces 31 , these studies did not investigate the potential-dependent behavior of NRR discussed above, and used neutral state density functional theory (DFT) calculations with the computational hydrogen electrode (CHE) model 32 . In the CHE model, due to the constant charge constraint, the workfunction (or chemical potential) of the system changes from reactants to transition states (or final states) and fractional charge transfer is not allowed.…”

Theoretical insights into the electroreduction mechanism of N2 to NH3 from an improved Au(111)/H2O interface model

“…These above theoretical studies assumed that activation barriers of reaction pathways is related with reaction free energies on different transition metal surfaces and only considered various elementary step thermodynamics, whereas the potential-dependent kinetic barriers that predicting catalytic activity were not further calculated. Most recently, Janik et al calculated explicitly the potential-dependent activation barriers for elementary electroreduction reactions included in associative distal and alternating pathways during N 2 electroreduction using their previously developed method, [47][48][49] and concluded that the alternating mechanism by direct surface hydrogenation may be more favorable on late transition metals due to smaller barriers at 0 V (vs. RHE), 50,51 in which N 2 electroreduction into N 2 H species is rate determining step of overall reaction. However, despite various theoretical efforts, N 2 electroreduction mechanisms are still not systematically understood and mechanistic inconsistencies remain exist.…”