**1. Introduction**

Industrial activity in parts of the world often has several direct and indirect adverse environmental consequences. Carbon dioxide (CO2), a natural greenhouse gas (GHG) that helps keep the globe warm, is out of control and triggering a climate crisis. This anthropogenic emission endangers human health, agriculture, natural ecosystems and atmospheric stability [1]. As reported in the Intergovernmental Panel on Climate Change (IPCC) Climate Change Mitigation Report (2014), CO2 is the main contributor to GHG emissions, which have 76% (including 11% of forests and land use) of the overall share, while CH4 and N2O accounted for 16% and 6%, respectively [2]. According to National Oceanic and Atmospheric Administration (NOAA), global ambient CO2 concentrations rose from 280 ppm to 407.4 ppm in 2018, setting a new high for the last 800,000 years [3,4].

A growing number of international reports illustrates the health and environmental effects of GHG emissions [5–7]. The ultimate goal is to maintain GHG concentrations at a point where the dangerous effect of climate change can be prevented. In 1997, the convention was supplemented and updated by the Kyoto Protocol [8]. Unlike the United

**Citation:** Hasan, M.H.; Mahlia, T.M.I.; Mofijur, M.; Rizwanul Fattah, I.M.; Handayani, F.; Ong, H.C.; Silitonga, A.S. A Comprehensive Review on the Recent Development of Ammonia as a Renewable Energy Carrier. *Energies* **2021**, *14*, 3732. https://doi.org/10.3390/en14133732

Academic Editor: Rob J.M. Bastiaans

Received: 30 April 2021 Accepted: 16 June 2021 Published: 22 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Nations Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol binds parties from a developed country to reduce GHG emissions [9]. Recently, under the terms of the Paris Agreement of 2015, the United Nations (UN) has committed itself to a long-term target to keep temperatures below 2 ◦C, compared to pre-industrial levels, to avoid the worst consequences of global warming [10,11]. Figure 1 shows the top 10 countries on CO2 emissions worldwide and their goal in the Paris Agreement.

In the last decades, the world has put great effort and investment into developing renewable energy to decarbonise the economy. The global pandemic disruption caused by COVID-19 teaches humanity that reducing the proportion of fossil fuels in human activity will substantially increase the regeneration of the Earth's atmosphere due to a reduction in air pollution. For example, in China, restricting the activity of people caused by the COVID-19 pandemic resulted in a 25% and 50% reduction in CO2 and NOX emissions, respectively [12,13]. However, the problem with these renewable sources of energy is that they are highly weather-dependent. Therefore, it is necessary to have an affordable and efficient method for storing energy. Several prevailing storage technologies, such as chemical, mechanical, electrical and thermal energy storages, can be applied from small to large-scale applications. Prominent storage solutions, such as batteries, provide durability and technological sophistication but are far too expensive and cannot provide the power needed for seasonal and grid-scale demand. Consequently, hydrogen has been the best candidate since the 1970s, positioning this as both energy storage and a clean energy source [14]. However, storage and delivery issues associated with H2 remain an obstacle to its implementation [15]. To store and distribute H2 efficiently, other indirect storage media such as ammonia (NH3), with its proven transportability and high flexibility, is required [16].

**Figure 1.** Top 10 global CO2 emitters and the target in the Paris Agreement [17].

In some countries, ammonia has become a part of their energy roadmap. For example, Japan's Cross-Ministerial Strategic Innovation Program (SIP) attempts to show ammonia, hydrides and hydrogen as essential elements of the nation's hydrogen energy system [18]. The Japanese Ministry of Economy, Trade and Industry (METI) described ammonia in conjunction with "the concept of importing renewable energy produced in other countries" [19]. In the United States, The Advanced Research Projects Agency-Energy (ARPA-E) announced a cumulative grant of \$32.7 million for 16 projects, 13 of which focus on ammonia [16]. Governments in New Zealand and Australia have announced federal grants to support the development of ammonia plants driven by renewable energy. More recently, Australia has awarded Yara and ENGIE AU\$995,000 for their solar ammonia project, Yara Pilbara, from the Australian Renewable Energy Agency (ARENA). As for New Zealand, Ballance-Agri Nutrients and Hiringa Energy received NZ\$19.9 million from

the Provincial Growth Fund for their wind-fed ammonia plant in Kapuni [20]. Similarly to Japan, the country is also looking into the possibility of exporting renewable energy in the form of ammonia. In addition, Toyota and Commonwealth Scientific and Industrial Research Organization (CSIRO) designed the first ammonia fuel car tested in Australia in 2018 [21].

Today, with a global production rate of more than 176 million metric tonnes of ammonia, the chemical is being used as fertiliser and a building block in the manufacture of many products [22,23]. With 28.5% of global production, China is known as the main producer of ammonia [24]. The uses of ammonia as an intermediate for the production of fertilisers account for over 80% of the total production of ammonia [25]. Other applications include fibre and plastics, pharmaceuticals, mining and metallurgy, pulp and paper, refrigeration and explosives [26]. Other than that, ammonia has also being recently proposed to be used in automotive applications for NOx emission control (DeNOx) technologies [27]. Furthermore, ammonia has also been researched as a source of energy for fuel cells, transport, industry and power generation [28].

Unfortunately, the current industrial ammonia synthesis method is complicated, energy-intensive and heavily dependent on hydrocarbon. The Haber–Bosch process that is currently used to synthesise ammonia is responsible for almost 11% of global industrial CO2 emissions [29,30]. In addition, the nature of renewable energy sources, which are irregular, requires turnkey systems that can be instantly switched on and off [31]. Thus, the challenge for the global deployment of ammonia as energy storage is, therefore, the simpler and more efficient production of ammonia from abundant sources, such as ambient air and water, with a ready to go system, which can be driven by intermittent energy sources.

The number of research studies on renewable ammonia, its innovation on the production route and its performances as a fuel has increased substantially in recent years, as shown by a growing number of scientific papers and review papers (Figure 2a–c). There has been a significant increase in the numbers of scientific articles related to ammonia, renewable energy, energy storage or energy carriers in recent years. For innovative approaches of ammonia synthesis, electrochemistry gained great attention in recent years due to the direct conversion of electricity into ammonia. Of all the devices capable of converting ammonia into energy, the Internal Combustion (IC) engine appears to receive the most development effort, noting that use for other applications is less explored.

**Figure 2.** Recent publications of (**a**) ammonia as a renewable energy carrier, (**b**) innovations in ammonia production, (**c**) ammonia as a fuel in Scopus databases.

This study comprehensively reviews all aspects of ammonia as energy storage, including innovative approaches for converting renewable energy into ammonia and devices that

convert ammonia into energy. The comparison of the recent review article of ammonia as renewable energy and this work is given in Table 1.


