Review of electrochemical ammonia production technologies and materials

Ammonia, being a good source of hydrogen, has the potential to play a significant role in a future hydrogen economy. The hydrogen content in liquid ammonia is 17.6 wt% compared with 12.5 wt% in methanol. Although a large percentage of ammonia, produced globally, is currently used in fertiliser production, it has been used as a fuel for transport vehicles and for space heating. Ammonia is an excellent energy storage media with infrastructure for its transportation and distribution already in place in many countries. Ammonia is produced at present through the well known Haber–Bosch process which is known to be very energy and capital intensive. In search for more efficient and economical process and in view of the potential ammonia production growth forecast, a number of new processes are under development. Amongst these, the electrochemical routes have the potential to substantially reduce the energy input (by more than 20%), simplify the reactor design and reduce the complexity and cost of balance of plant when compared to the conventional ammonia production route. Several electrochemical routes based on liquid, molten salt, solid or composite electrolytes consisting of a molten salt and a solid phase are currently under investigation. In this paper these electrochemical methods of ammonia synthesis have been reviewed with a discussion on materials of construction, operating temperature and pressure regimes, major technical challenges and materials issues.     

Technoeconomic analysis for hydrogen and carbon Co-Production via catalytic pyrolysis of methane

Catalytic Methane Pyrolysis (CMP) is an innovative method to convert gaseous methane into valuable H₂ and carbon products. The catalytic approach to methane pyrolysis has the potential to decrease the required operating temperature for methane decomposition from >1000 °C to under 700 °C. In this work, a novel inexpensive catalyst is discussed that displays low operating temperatures, while still maintaining high reactivity and long proven lifetimes. The kinetics associated with the catalyst's performance are modeled and a correlation was developed for use with practical simulation tools. A techno-economic assessment was conducted applying experimentally determined kinetics for the CMP reaction with the specific catalyst. Two process concepts that utilize CMP using the novel catalyst are presented in this work. Optimizations were considered in these processes and the CO₂ emissions and cost of hydrogen production of the two optimized cases, CMP with H₂ combustion (CMP-H₂) and CMP with CH₄ Combustion (CMP-CH₄), are compared to that of the current industrial standard for hydrogen production, Steam Methane Reforming with carbon capture and sequestration (SMR-CCS). Both of the proposed concepts convert methane into gaseous hydrogen and valuable carbon products, graphitic carbon to carbon Nano fibers. The carbon price was treated as a variable to determine the sensitivity of hydrogen production cost to the carbon price. The analysis indicates that cost of hydrogen production is highly dependent on the recovery and sale of carbon byproducts. Based on Aspen modeling of these two concepts for large scale hydrogen production (216 tons/day), the cost of hydrogen production, without considering carbon sales, was estimated to be $<3.25/kg, assuming a natural gas price of $7/MMBTU and conservative catalyst cost of $8/kg. Assuming 100% recovery of carbon, the price can be reduced to $0/kg by selling the carbon at <$1/kg. A market assessment suggests that values of graphitic carbon and carbon fibers range from ~$10/kg and ~$25–113/kg, respectively. The cost of H₂ production via conventional SMR is ~$2.2/kg when accounting for the cost of CO₂ sequestration. The proposed processes produce a maximum of 0–2 kg CO₂/kg H₂ in contrast to the 10 kg CO₂/kg H₂ produced via conventional SMR-CCS. The process displays an enormous potential for competitive economics accompanied by reduced greenhouse gas emissions.     

Production of ammonia as potential hydrogen carrier: Review on thermochemical and electrochemical processes

Hydrogen (H₂) is a secondary energy source (energy carrier) which has advantages of high cleanliness and efficiency, leading to its potential utilization in the future energy system. However, H₂ suffers a great challenge in its storage because of low volumetric energy density. Among the available technologies and media for H₂ storage, ammonia (NH₃) is considered very promising due to its characteristics of high hydrogen density, excellent storage, high stability, and matured technology and infrastructure. Currently, NH₃ is massively produced through Haber-Bosch process conducted at high pressure and temperature. Therefore, large energy is consumed to synthesize NH₃. There are several other alternative technologies for NH₃ synthesis, including thermochemical, electrochemical, photochemical, and plasma-assisted processes. This paper reviews mainly both thermochemical and electrochemical NH₃ production technologies, including their updates and challenges, and also by considering both technological feasibility and applicability. In addition, several projects and efforts carried out by several countries to utilize NH₃ as potential fuel in the energy system are also overviewed. Furthermore, technological analysis, challenges, and recommendations are also provided with the objective of evaluating the potential adoption of NH₃ in the future energy system.     

A new synthesis route of ammonia production through hydrolysis of metal – Nitrides

Recently ammonia has emerged as a potential hydrogen storage material because it contains 17.8 wt% hydrogen. Here, we propose a new synthesis route of ammonia production using hydrolysis of nitrides, which is based on the conversion technique using thermal energy, solar heat or exhaust heat to form NH₃ directly. Lithium metal has been tested as a starting material for the above purpose. We present the detailed results on room temperature nitridation of lithium metal, it is found that the nitridation properties are strongly affected by the surface state of lithium metal. The ammonia synthesis via hydrolysis of lithium nitride succeeds and it is strongly dependent on the reaction rate and temperature.     

Estimating global production and supply costs for green hydrogen and hydrogen-based green energy commodities

Green energy commodities are expected to be central in decarbonising the global energy system. Such green energy commodities could be hydrogen or other hydrogen-based energy commodities produced from renewable energy sources (RES) such as solar or wind energy. We quantify the production cost and potentials of hydrogen and hydrogen-based energy commodities ammonia, methane, methanol, gasoline, diesel and kerosene in 113 countries. Moreover, we evaluate total supply costs to Germany, considering both pipeline-based and maritime transport. We determine production costs by optimising the investment and operation of commodity production from dedicated RES based on country-level RES potentials and country-specific weighted average costs of capital. Analysing the geographic distribution of production and supply costs, we find that production costs dominate the supply cost composition for liquid or easily liquefiable commodities, while transport costs dominate for gaseous commodities. In the case of Germany, importing green ammonia could be more cost-efficient than domestic production from locally produced or imported hydrogen. Green ammonia could be supplied to Germany from many regions worldwide at below the cost of domestic production, with costs ranging from 624 to 874 $/t NH₃ and Norway being the cheapest supplier. Ammonia production using imported hydrogen from Spain could be cost-effective if a pan-European hydrogen pipeline grid based on repurposed natural gas pipelines exists.     

Performance of direct ammonia fuel cell with PtIr/C,PtRu/C,and Pt/C as anode electrocatalysts under mild conditions

While ammonia (NH₃) is an attractive alternative to pure hydrogen, its direct use in fuel cells is fraught with difficulties. A direct ammonia fuel cell (DAFC) with PtIr/C (Pt:Ir = 1:1), PtRu/C (Pt:Ru = 1:1), and Pt/C anode electrocatalyst was investigated at 25 °C and 100 kPa inlet gas pressure. Due to the synergistic and electronic effects of the PtIr alloy, their open-circuit voltages (OCVs) were rated as PtIr/C > PtRu/C > Pt/C, with the DAFC with PtIr/C anode achieving the highest OCV of 0.50 V and peak power density (PPD) of maximum 1.68 mW cm^?2. Meanwhile, an online Fourier transform infrared (FTIR) spectrometer detected an increase in ammonia permeation in the cathode exhaust gas, indicating a possibility of fuel permeation and cathode electrocatalyst degradation. The degradation of DAFC efficiency with rising cycle numbers may be due to ammonia cross-over and poisoning over the surface of the electrocatalyst.     

Recent progress towards solar energy integration into low-pressure green ammonia production technologies

Large progress has been made in the last decades to reduce the carbon footprint of ammonia, which is an essential commodity of the food, chemical and energy industry. Apart from alternative routes for green feedstock production, such as hydrogen via electrolysis and nitrogen via solar thermochemical methods, alternatives are explored to replace the Haber-Bosch process. The present article reviews four promising mild condition ammonia production methods: solid state synthesis, molten salt synthesis, thermochemical looping and photocatalytic routes. Contrary to the Haber-Bosch method, which requires high pressures of 200–400 bar, they operate at low-pressures, furthermore such routes open the possibility for direct ammonia production from H₂O and N₂ without the intermediate hydrogen production step. These advantages allow easier renewable energy integration; however, R&D activities are needed for scaling-up. An analysis is given on renewable energy integration with focus on solar resources both in the form of electricity and heat.