Active MoS2-based electrode for green ammonia synthesis

Abstract

Nitrogen electro-reduction under mild conditions is one promising alternative approach of the energy-consuming Haber-Bosch process for the artificial ammonia synthesis. One critical aspect to unlocking this technology is to discover the catalysts with high selectivity and efficiency. In this work, the N2-to-NH3 conversion on the functional MoS2 is fully investigated by density functional theory calculations since the layered MoS2 provides the ideal platform for the elaborating copies of the nitrogenase found in nature, wherein the functionalization is achieved via basal-adsorption, basal-substitution or edge-substitution of transition metal elements. Our results reveal that the edge-functionalization is a feasible strategy for the activity promotion; however, the basal-adsorption and basal-substitution separately suffer from the electrochemical instability and the NRR inefficiency. Specifically, MoS2 functionalized via edge W-substitution exhibits an exceptional activity. The energetically favored reaction pathway is through the distal pathway and a limiting potential is less than 0.20 V. Overall, this work escalates the rational design of the high-effective catalysts for nitrogen fixation and provides the explanation why the predicated catalyst have a good performance, paving the guidance for the experiments.

1. Introduction

Artificial ammonia synthesis is of pressing concern for fertilizer production and hydrogen storage [1e4]. In industry, the N2-to-NH3 conversion is achieved through the Haber-Bosch reaction, which is highly energy consumption due to the inert N≡N triple bond (941 kJ mol−1) [5e9]. Such conventional method cannot satisfy the pursuit of energy sustainability. Thus, developing a green and clean technology to boost the nitrogen reduction reaction (NRR) is of paramount importance. In the direction, the ammonia synthesis via electrocatalysis under the mild conditions has triggered enormous investigations, wherein one critical hotpot is to discover the catalysts with high efficiency [10e15].

The marvelous nature provides an easy way to achieve the nitrogen fixation. For instance, Azotobacter endowed with nitrogenase enzymes enables the conversion from the atmospheric nitrogen to ammonia. As discovered, the nitrogenase is a two-component system composed of the FeMo protein and the electron-transfer Fe protein. The FeMo protein contains the FeMo cofactor which provides the active site for the binding of reactant and reduction reactions [16]. Inspired by the nature, counterfeiting well-designed nitrogenase structure with inorganic structures made of similar elements and analogous configurations is an innovative method to design the catalysts for nitrogen fixations. Amongst varied materials, molybdenum dichalcogenide MoS2 with layered configuration provides ideal platform for the elaborating copies of active site of natural nitrogenase [17e19]. Therefore, there are great enthusiasms focused on MoS2 in the application of NRR electrocatalysis.