Overview of emerging catalytic materials for electrochemical greenammonia synthesisand process

1 INTRODUCTION Increasing energy demands over global warming led to finding alternative sustainable technologies for the next generation. In this regard, scientists and engineers are driving toward advanced energy conversion and storage systems (ECSs) including proton-anion-exchange membrane fuel cells, 1 , 2 direct methanol fuel cells (DMFCs), 3 solid oxide fuel cells, 4 water splitting, 5 solid-state batteries and metal-air batteries, 6 - 9 sodium-ion and sulfur batteries, 10 , 11 and supercapacitors (SCs). 12 Among all, the electrochemical nitrogen reduction reaction (ENRR) has received great attention owing to its potential application in the ammonia-based industry. 13 - 17 Currently, NH 3 is the most versatile zero-carbon molecule due to its exceptional properties such as high volumetric H 2 content and energy density compared with methylcyclohexane, methanol, and lithium-ion batteries (LIBs).

18 - 20 In addition, the storage and transport of NH 3 are easy because of their desirable conditions such as liquid phase, atmospheric temperature, and boiling point at −33°C. For the next decades, NH 3 can play a significant role in day-to-day life, especially in transportation and agriculture. 21 Globally, as we know, NH 3 is the largest chemical produced after sulfuric acid, ethylene, sodium hydroxide, and propylene. The formation of NH 3 in liquid or gas form is advantageous and important for preservative foods, medical research, and the sterilized electronics industry.

The key players in the NH 3 industry are Yara, Eurochem, Qatar Fertilizer Company, Nutrien, BASF SE, CF Industries Holdings Inc., OCI nitrogen, and Saudi Basic Industries Corporation (SABIC) among others. Similarly, the present NH 3 production reaches 200 million tons and by 2050 is expected to increase by 40% globally. 22 , 23 Nevertheless, at 300 ppm, NH 3 exposure causes a risk to human health so we need to build infrastructure at every scale for the process of NH 3 . Recently, the concept of green-ammonia is rising toward the next sustainable future by using renewable energy sources.

24 For securing the ammonia resources, three major countries South Korea, Japan, and Germany have gone toward green ammonia and other key energy patrons are certain to survey. Figure 1 shows the production of green ammonia through electrocatalysis, storage and transportation, and corresponding utilization in various applications. Figure 1 Open in figure viewer PowerPoint Electrochemical green-ammonia synthesis and its corresponding storage, transportation, and H 2 decomposition for the utilization of different sectors. NH 3 is widely used in various industries including pharmaceutical, fertilizer production, dye, synthetic fiber, food and agricultural, textile industries, and nitric acid.

25 , 26 Most of the ammonia conversion processes have been based on Haber–Bosch (H–B) and azotobacter processes. The biological nitrogen fixation was performed through nitrogenase enzymes, but this method was limited by its ultralow reaction rate. Most of the industrial NH 3 production uses H−B technology. However, this process (H−B) under harsh conditions consumes a large number of fossil fuels (e.g., 300 million tons of CO 2 annually).

22 , 27 In this regard, a sustainable and economically viable process is required for the production of NH 3 . Today several approaches are proposed, for instance, plasma technology, biochemical photocatalytic ENRR, photo-electrocatalysis, electrochemical, and chemical looping. 23 , 28 - 33 Among these methods, the ENRR process using heterogeneous catalysts received great attention due to their excellent properties and opened a new avenue for the carbon-free NH 3 production from N 2 and H 2 O directly. 34 Here are two major challenges for ENRR: (i) high energy is needed for splitting the nonpolar N 2 triple bond and (ii) the other one is dominance of hydrogen evolution reaction (HER) in the electrochemical system, which results in low Faradaic efficiency (FE).

From the above points, it is needed to optimize the whole electrochemical system including the electrolyte/solvent, reactor configuration such as reaction condition, and the selectivity of adjustment of catalysts to achieve low overpotential as well as limit the HER activity. 35 Based on the above points, a series of electrocatalysts is proposed for ammonia synthesis. The catalysts mainly consist of MXenes, 36 , 37 metal–organic frameworks (MOFs), 38 , 39 metal-free electrocatalysts (B-doped graphene, black phosphorus), 40 noble (Pt, Pd, Rh, Ir, Ru, and Au), 41 - 43 and nonnoble (Ni, Co, Fe, Ti, etc.) metal-based electrocatalysts, and single-metal-atom catalysts. 44 - 46 Typically, there are two possible methods for improving the catalyst activity of ENRR: (1) expending the intrinsic activity, which means controlling the electronic structure of catalysts (e.g., doping a new atom into the catalyst) and (2) defect engineering, which is motivated by the crystal structure and generating porous structures by integral and superficial activity, such as size and shape.

Further, the catalysts have increased surface area and utilization of active sites. The purpose of this review is to understand the rising green NH 3 production by electrochemical synthesis. In this review article, we present the emerging electrocatalysts such as SACs, MXenes, and MOF-derived catalysts. First, we discuss the value of NH 3 as an emerging clean energy carrier for future mobility and other industries.

Second, the principles and reaction mechanism of ENRR are analyzed, and then we introduce electrochemical cell configuration systems involved in the nitrogen reduction reaction. Third, we analyze the key parameters such as NH 3 yield rate, FE, and stability under several electrolyte mediums explored via in situ techniques including chronoamperometry (CA), linear sweep voltammetry (LSV), and electrochemical surface area analysis by cyclic voltammetry (CV). To this end, we present emerging electrocatalysts such as MXenes, MOFs, and SACs toward ENRR and provide some technical challenges and future research directions. The advantages and technical targets of NH 3 utilization are described in Table 1 .

Table 1. Ammonia utilization and technical targets with different methods. Ammonia utilization Methods NH 3 decomposition Electrolysis of NH 3 Direct NH 3 fuel cells NH 3 combustion Energy utilization form Providing carbon-free hydrogen Providing carbon-free hydrogen Providing electricity Providing power (heat energy) Advantages Carbon-free H 2 Available commercialized catalysts Compatible with H 2 fuel cells Carbon-free H 2 Mild reaction conditions Suitable for small-scale application Direct use of NH 3 Reduction of the loss of energy efficiency by cracking NH 3 High octane number of NH 3 Direct use of NH 3 Suitable for hybrid systems Challenges Insufficiently high H 2 production rate High operation temperatures for full conversion of NH 3 ( T 500°C) NH 3 slip Low energy efficiency Control anode overpotentials Production of NO x NH 3 slip Catalyst surface Control anode overpotentials Production of NO x NH 3 slip High operation temperatures High content NH 3 At low temperatures insufficient combustion efficiency Production of NO x NH 3 slip R&D task Inexpensive catalysts with high durability H 2 purification technology compatible with NH 3 decomposition Nonnoble metal anodes with lower overpotentials Catalysts durability Electrolysis of pure (liquid) NH 3 Nonnoble electrocatalysts Alternative electrolytes Materials and techniques for NH 3 removal Catalysts with full conversion and high selectivity at low temperature Catalytic technology for NH 3 Source : Reproduced with permission: Copyright 2021, Wiley-VCH. 67