Two-dimensional ultrathin nanosheets over mackinawite FeS for efficient electrochemical N2 reduction

Abstract

Electrocatalytic N2 reduction reaction (NRR) has been considered as a promising and alternative strategy for the synthesis of NH3, which will contribute to the goal of carbon neutrality and sustainability. However, this process often suffers from the barrier for N2 activation and competitive reactions, resulting in poor NH3 yield and low Faraday efficiency (FE). Here, we report a two-dimensional (2D) ultrathin FeS nanosheets with high conductivity through a facile and scalable method under mild condition. The synthesized FeS catalysts can be used as the work electrode in the electrochemical NRR cell with N2-saturated Na2SO4 electrolyte. Such a catalyst shows a NH3 yield of 9.0 μg·h−1·mg−1 (corresponding to 1.47 × 10−4 μmol·s−1·cm−2) and a high FE of 12.4%, which significantly outperformed the other most NRR catalysts. The high catalytic performance of FeS can be attributed to the 2D mackinawite structure, which provides a new insight to explore low-cost and high-performance Fe-based electrocatalysts, as well as accelerates the practical application of the NRR.

1. Introduction

Ammonia (NH3) has been regarded as one of the most important raw productions for carbon neutrality and sustainable development in chemical enterprises, which plays a crucial role in human life. Owing to the significant advances in high energy density, low liquefaction temperature, and easy storage and transportation, NH3 has been emerging as an efficient carbon-free energy platform for manufacturing clean energy materials [1–4]. Currently, Haber–Bosch process is mainly used to produce industrial-scale NH3, however, it is heavily limited to the extreme reaction conditions, such as high temperature (300–600°C) and high pressure (150–200 bar) [5–8]. Besides, this process will consume excessive energy and cause irreversible environmental problems, which goes against the original intention of human beings for environment sustainable development. Therefore, it is of great necessary that sustainable approaches should be developed for NH3 production under milder conditions, which not only meets the need of green and sustainable development, but also contributes to the goal climate change.

Electrocatalytic nitrogen (N2) reduction reaction (NRR) has been investigated as a promising and replaceable strategy for production of NH3, due to the properties of controllable reaction, less energy consumption, and zero carbon emission [9]. To date, great efforts have been paid to develop assorted electrocatalysts for NRR. Over the past few decades, noble metal-based materials such as Pt [10], Rh [11], and Ru [12] have been proved that they perform high NRR activity and electrochemical stability, but the cost issue seriously restrain large-scale application. In addition, recently, intensive studies have been focused on other electrocatalysts based on non-precious metal compounds [5–6,13–17]. For example, many studies have paid more investigation on transition metal compounds including sulfides [18], carbides [19], nitrides [20], and carbonitrides [21] by density functional theory (DFT) to model reaction pathway, providing the guidance to predict favorable NRR. However, the reported Faraday efficiency (FE) and NH3 production rates were still comparatively low. Having these aspects in mind, it is still a great and urgent need to develop low-coast and highly-efficient non-precious NRR electrocatalysts.

It is well known that iron (Fe) is the cheapest and earth-abundant non-precious metals, meanwhile, it is the key element of nitrogenase enzymes in natural nitrogenase system, suggesting that Fe plays an important role in electrocatalytic NRR. Nowadays, Fe-based catalysts have aroused much attention as the great potential candidates for the electrocatalytic NRR [9,22–26]. For example, Wang et al. [27] have fabricated the Fe2O3 nanoparticles catalysts by using glycerine as solvent with a subsequent calcination process, which exhibited an NH3 yield...