Challenges and prospects in artificial nitrogen cycle for energy decarbonization

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dynamic management for the whole integrated process.
Electrocatalytic synthesis.Electrocatalytic synthesis of ammonia (N 2 + 3H 2 O → 2NH 3 + 3/2O 2 ) can be directly driven by electric power from renewable energy.Notably, N 2 reduction pathway in electrolysis is an associative route that enables ammonia synthesis under mild conditions, which is different from common dissociative route in thermocatalysis.The reported NH 3 synthesis rates are generally in the range of 10 −13 -10 −9 mol cm −2 s −1 , and far away from the feasible target of 10 −7 mol cm −2 s −1 in industry [4].The main challenges of electrocatalytic synthesis of ammonia are low ammonia productivity and low Faraday efficiency, due to poor dissolution and diffusion of N 2 , weak N 2 activation, and competitive hydrogen evolution reaction [5,6].To overcome these obstacles, the rational design of multi-active sites and the optimization of the interface of catalyst/electrolyte and electrode/catalyst are promising ways to improve electrocatalytic synthesis of ammonia.
Photocatalytic synthesis.The photocatalytic process for ammonia synthesis is driven by solar energy for activation of N 2 and H 2 O.In the photocatalytic process, charge carriers are generated after light absorption.The electron-hole pairs are formed on photoactive sites, and ammonia is produced via combination of the photoactivated N 2 and protons.The technical bottlenecks, in photocatalytic synthesis of ammonia, are low utilization efficiency of solar energy and low productivity of ammonia.It is important to increase efficiency of the separation and migration of photo-induced electrons and the N 2 activation.The ideal photocatalysts are supposed to be compatible with proper band gap, highly efficient separation of electrons and holes, superior activation ability of H 2 O and N 2 .Creation of vacancy sites, heteroatom doping and composite construction may be effective strategies to improve photocatalytic performance for ammonia synthesis.
Ammonia decomposition.Ammonia can be decomposed into hydrogen for further utilization, in support of "hydrogen economy" [7].The ammonia decomposition and synthesis are strongly correlated according to the principle of microscopic reversibility.However, they are not necessarily the same cases because of varied Nbinding energies with catalysts under different reaction conditions.Commercially, the ammonia decomposition is carried out by Ni-based catalysts under high temperatures (>800°C), resulting in high energy consumption and poor safety.Despite numerous efforts, the most challenge is still the precise design of lowtemperature catalysts [1].The guidance for improving catalysts is as follows: (1) precise control of particle size/shape of active metals (e.g., Ru and Ni) to maximize the number of active sites; (2) incorporating second metal for enhanced synergistic catalysis via structure stabilization and electronic effects; (3) introducing electronic and structural promoters/supports for activity promotion.Given that the obtained hydrogen from ammonia can be utilized in hydrogen refueling station, proton exchange membrane or solid oxide fuel cells (SOFCs), integrating ammonia decomposition with these systems to choose/design rational catalysts with matched operating temperatures might be the potential chance in the research area.
Direct ammonia fuel cells.Ammonia can be directly fed into fuel cells to generate power with superior ammonia-to-electricity conversions [8].The NH 3 -SOFC is a feasible fuel cell operating at high temperatures (500-1000°C).Nevertheless, the performances of NH 3 -SOFCs were lower than hydrogen-fed SOFCs.The exploration of suitable anode catalysts that can catalyze both thermal NH 3 decomposition and NH 3 /H 2 electro-oxidation is the main challenge.The anode degradation and deactivation (coarsening and poisoning of Ni species) are major concerns as well.To tackle these issues, two potential strategies were proposed: (1) in-situ construction of multifunctional region at anode, which can separate thermal cracking of NH 3 and electro-oxidation of H 2 on different catalyst layers with complementary interoperations; (2) functionalize and stabilize the anode catalysts at three phase boundaries to achieve superior thermal-electro catalysis.Recently, low-temperature direct ammonia alkaline exchange membrane fuel cells (NH 3 -AEMFCs) have attracted much attention due to their fast start-up and mobile superiority [9].Nevertheless, the development of NH 3 -AEMFCs is at an earlier stage because of the more complex and sluggish NH 3 oxidation, which requires the further development of low Pt-based and non-noble catalysts.
Ammonia combustion.Ammonia combustion can directly generate heat and power in gas furnaces, turbines and engines [10].The most advantage is that it can be carried out on existing power-generation infrastructure with simple modification of equipment.The large-scale application is limited by the narrow flammability, low laminar burning velocity and activity in direct ignition and flame stability.Notably, the ammonia combustion would result in the formation of harmful NO x .Motivated by tackling these challenges, directional control strategies for combustion and tail-gas treatment are critical for improving the ammonia utilization and controlling NO x emissions: (1) blending ammonia with hydrogen for combustion with production of harmless dinitrogen and water; (2) integrating ammonia combustion and selective catalytic reduction to achieve net-zero NO x emission; (3) direct catalytic combustion of ammonia by using rational catalysts.
Energy decarbonization is vital to diminishing CO 2 emissions.To promote artificial nitrogen cycle for energy decarbonization, we focus on the catalytic processes of ammonia synthesis and ammonia utilization.Although the thermocatalytic, electrocatalytic and photocatalytic synthesis of ammonia can store renewable energies, the low compatibility of catalytic systems or the low ammonia yield rate significantly hinders the efficient storage of renewable energies.The decomposition, fuel cells and combustion of ammonia provide multiple methods to release the renewable energies, yet exploration of the low-temperature catalysts and reduction of side reactions are still challenging.This perspective proposes that the catalytic systems, routes and catalysts of artificial nitrogen cycle should be rationally designed for energy decarbonization.We would like to emphasize that artificial nitrogen cycle will offer a sustainable approach to realize complete decarbonization of energy system.

Figure 1
Figure 1 Schematic for ammonia synthesis and utilization via artificial nitrogen cycle for energy decarbonization.The map element in the figure was modified with permission from Ref. [1].Copyright 2022 American Chemical Society.