Background and Summary BACKGROUND OF THE INVENTION

A. Field of the Invention

In general present invention concerns an energy-efficient ammonia production from air and water using electrocatalysts with limited faradaic efficiency.

B. Description of the Related Art

Ammonia is an industrial large volume chemical {Ore, I. et al. MINERAL COMMODITY SUMMARIES 2019. (2019)). It is used in fertilizers and many chemical products and materials, ( Philibert , C. Renewable energy for industry: From green energy to green materials and fuels. Int. Energy Agency (2017) doi: 10.1111/j.l 365-2990.2010.01130 and CXP Group. Business Applications Trends in 2017 and 2018. (2018)) and it pops up as a candidate green energy vector ( Giir , T. M. Review of electrical energy storage technologies, materials and systems: Challenges and prospects for large-scale grid storage. Energy Environ. Sci. 11, 2696-2767 (2018)). Today, the industrial production of ammonia is dominated by the Haber-Bosch process departing from natural gas or other fossil fuel (Howarth, R. W. Coastal nitrogen pollution: A review of sources and trends globally and regionally. Harmful Algae 8, 14-20 (2008) and Rafiqul, I., Weber, C., Lehmann, B. & Voss, A. Energy efficiency improvements in ammonia production - Perspectives and uncertainties. Energy 30, 2487-2504 (2005)). This process is responsible for about 1.6 % of the global C02 emissions (Zhou, F. et al. Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 10, 2516-2520 (2017)). Electrochemical ammonia production from water and nitrogen gas using renewable electricity is a potential solution to reduce the C02 footprint of ammonia production. Electrocatalysts with steadily increasing faradaic efficiency are being reported, but there seems to be a trade-off between ammonia selectivity and catalytic activity (Zhou, F. et al. Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 10, 2516-2520 (2017); Wang, M. et al. Over 56.55% Faradaic efficiency of ambient ammonia synthesis enabled by positively shifting the reaction potential. Nat. Commun. 10, 1-8 (2019) and Song, Y. et al. A physical catalyst for the electrolysis of nitrogen to ammonia. Sci. Adv. 4, el700336 (2018)). Hydrogen gas is the main by-product (Wang, M. et al. Over 56.55% Faradaic efficiency of ambient ammonia synthesis enabled by positively shifting the reaction potential. Nat. Commun. 10, 1-8 (2019) and Garagounis, Vourros, Stoukides, Dasopoulos & Stoukides. Electrochemical Synthesis of Ammonia: Recent Efforts and Future Outlook. Membranes (Basel). 9, 112 (2019)).

While liquid ammonia for its high energy density has the potential to be a chemical energy vector of the future and it can play a supportive role in the hydrogen economy, to make this possible, ammonia production needs to be decarbonized, and green ammonia needs to be produced with renewable energy instead of natural gas or other fossil energy source.

Electrochemical reduction of nitrogen gas using hydrogen gas is an option, but using water as source of hydrogen atoms is even more appealing. State-of-the-art electrocatalysts for ammonia synthesis from nitrogen gas and water produce lots of hydrogen by-product.

Present invention provides a system that demonstrates that low ammonia selectivity of electrocatalysts does have not to be an obstacle to energy-efficient ammonia production. The SECAM process (Solar Electrochemical AMmonia synthesis) integrates nitrogen gas production from air, electrocatalytic ammonia synthesis, reaction product separation and hydrogen recycling with an overall energy efficiency similar to the Haber-Bosch process departing from grey hydrogen. In present invention the electrochemical ammonia synthesis process is powered with photovoltaics and take advantage of the day-night cycle for converting the excess hydrogen by-product produced during the day to make additional ammonia at night. In a particular embodiment of present invention the process is operated using electrocatalysts for energy-efficient production of green ammonia.

Present invention provides an electrochemical ammonia production process that copes with the cyclic nature of renewable electricity production by using the hydrogen by-product for bridging the dark periods.

The invented process brings ammonia a step closer to becoming a green fuel.

SUMMARY OF THE INVENTION In accordance with the purpose of the invention, as embodied and broadly described herein, the invention is broadly drawn to energy-efficient ammonia production from air and water.

The present invention solves the problems of energy-efficient ammonia production from air and water using electrocatalysts with limited faradaic efficiency by a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ and further characterized in that it has two modes of operation: (A) energy intensive production of ammonia out of nitrogen gas and water whereby to make air suitable for ammonia production, whereby O ₂ is removed by reaction with H ₂ and whereby air is used as a source of nitrogen according to equation N ₂ +3 H ₂ →2 NH ₃ E ₀ = 0.057 V, and (B) an energy extensive production of ammonia out of an N ₂ /H ₂ gas mixture according to equation N ₂ +3 H ₂ O→2 NH ₃ +3/2 O ₂ E ₀ = -1.172 V

The present invention solves the problems of energy-efficient ammonia production from air and water using electrocatalysts with limited faradaic efficiency of 20-50%, preferably 20-60 20- 50%, yet more preferably 20-30 % by a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ and further characterized in that it has two modes of operation: (A) energy intensive production of ammonia out of nitrogen gas and water whereby to make air suitable for ammonia production, whereby O ₂ is removed by reaction with H ₂ and whereby air is used as a source of nitrogen according to equation N2+3 H ₂ O→2 NH ₃ +3/2 O ₂ E ₀ = -1. 17 2 V, and (B) an energy extensive production of ammonia out of an N ₂ /H ₂ gas mixture according to equation N ₂ +3 H ₂ →2 NH ₃ E ₀ = 0.057 V

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

According to the present invention there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂

According to the present invention there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ further characterized in that it has two modes of operation: (A) energy intensive production of ammonia out of nitrogen gas and water whereby to make air suitable for ammonia production, whereby

O ₂ is removed by reaction with H ₂ and whereby air is used as a source of nitrogen according to equation N ₂ +3 H ₂ O→2 NH ₃ +3/2 O ₂ E ₀ = -1.172 V, and (B) an energy extensive production of ammonia out of an N ₂ /H ₂ gas mixture according to equation N ₂ +3 H ₂ →2 NH ₃ E ₀ = 0.057 V.

Preferred embodiments of said detection method are as defined in the annexed dependent claims 2 to 23.

According to the present invention yet there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ and whereby O ₂ is removed by reaction with thin a fuel cell, generating electricity, or in a burner, generating heat.

According to the present invention yet there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ further characterized in that it has two modes of operation: (A) energy intensive production of ammonia out of nitrogen gas and water whereby to make air suitable for ammonia production, whereby O ₂ is removed by reaction with H ₂ and whereby air is used as a source of nitrogen according to equation N ₂ +3 H ₂ O→2 NH ₃ +3/2 O ₂ E ₀ = -1.172 V, and (B) an energy extensive production of ammonia out of an N ₂ /H ₂ gas mixture according to equation N ₂ +3 H ₂ →2 NH ₃ E ₀ = 0.057 V and whereby O ₂ is removed by reaction with H ₂ in a fuel cell, generating electricity, or in a burner, generating heat.

According to the present invention there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂

It is practical to have O ₂ removed by a reaction with H ₂ in a fuel cell, generating electricity, or in a burner, generating heat and consequently the gas containing already some water from the reaction of O ₂ with H ₂ , is sent through a humidifier where additional water vapour is added.

It is most practical to have O ₂ removed by reaction with H ₂ in a fuel cell, generating electricity, or in a burner, generating heat and consequently the gas containing already some water from the reaction of O ₂ with H ₂ , is sent through a humidifier where additional water vapour is added and consequently the hydrated nitrogen gas is fed to the electrochemical cell, where ammonia is formed on the cathode.

According to the present invention there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ further characterized that when operated with an electrocatalysts with limited faradaic efficiency of 20- 30 % and excess H ₂ gas is produced.

According to the present invention there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ further characterized in that it has two modes of operation: (A) energy intensive production of ammonia out of nitrogen gas and water whereby to make air suitable for ammonia production, whereby 0 ₂ is removed by reaction with H ₂ and whereby air is used as a source of nitrogen according to equation N ₂ +3 H ₂ O→2 NH ₃ +3/2 O ₂ E ₀ = -1.172 V, and (B) an energy extensive production of ammonia out of an N ₂ /H ₂ gas mixture according to equation N ₂ +3 H ₂ →2 NH ₂ E ₀ = 0.057 V further characterized that when operated with an electrocatalysts with limited faradaic efficiency of 20-30 % and excess H ₂ gas is produced.

According to the present invention there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ further characterized that the electrocatalyst should have a faradaic efficiency for ammonia production of 84 - 86%, preferably 84 - 85 %, more preferably 85% so that when mode A is run such as to produce exactly the amount of hydrogen needed to eliminate the O ₂ from the intake air.

According to the present invention there is provided a process for ammonia production from air and water, characterised in that the process uses hydrogen gas in (i) a reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) in performing electrocatalytic ammonia synthesis in an electrochemical cell using H ₂ further characterized in that it has two modes of operation: (A) energy intensive production of ammonia out of nitrogen gas and water whereby to make air suitable for ammonia production, whereby

O ₂ is removed by reaction with H ₂ and whereby air is used as a source of nitrogen according to equation N ₂ +3 H ₂ O→2 NH ₃ +3/2 O ₂ E ₀ = -1.172 V, and (B) an energy extensive production of ammonia out of an N ₂ /H ₂ gas mixture according to equation N ₂ +3 H ₂ →2 NH ₃ E ₀ =0.057 V further characterized that the electrocatalyst should have a faradaic efficiency for ammonia production of at least 85 % so that when mode A is run such as to produce exactly the amount of hydrogen needed to eliminate the O ₂ from the intake air.

The process described above may be embodied as that the outlet gas of the electrochemical cell composed of NH ₃ , H ₂ and unreacted N ₂ , the NH ₃ is condensed out of the gas stream, resulting in a residual stream of N ₂ and H _2. In this process the residual stream of N ₂ and H ₂ can be refed to the inlet of the process to have O ₂ removal out of inlet air and part is stored in a tank as feed for Mode B . The process described above may also be embodied as that water is the source of H-atoms and air is the source of N-atoms.

The process described above may also be embodied as that the molar ratio of N ₂ /H ₂ in the process is fixed at 1/3 by tuning the air and water intake of the process.

The process described above may also be embodied as that the H ₂ /N ₂ gas mixture is stoichiometrically converted to ammonia in Mode B and stored temporarily, together with nitrogen.

The process described above may also be embodied as that the H ₂ /N ₂ gas mixture is sent to the anode of the electrochemical cell, where the hydrogen oxidation reaction (HOR) takes place (mode B) and consequently the remaining gas is sent to the cathode, where ammonia and hydrogen gas are formed. In Mode B ammonia is produced until all N ₂ and H ₂ gas is converted by recycling.

The process described above may also be embodied as that the mode A and mode B steps of the process make use of the same electrochemical cell or that the ammonia production in Mode B consumes less than 30 % of the electric power required for Mode A.

The process described above may also be embodied as that electrochemical cell is energized via photovoltaics.

The process described above may also be embodied as that the anode and or cathode is electrocatalytic.

The process described above may also be embodied as that the mode of operation is sequences of operation of mode A and mode B so that when a large amount of energy is available, for example at noon, mode A is executed and when the energy supply is limited, for example at night or on clouded days, mode B is executed or that at least two SECAM reactors run the process in parallel and operated in mode A or B to optimize the ammonia productivity according to the availability of energy.

The process described above may also be embodied as that the condensation of ammonia is carried out under a pressure above atmospheric pressure or whereby the condensation of ammonia is carried under a pressure in the range of 1 MPa to 2 MPa, preferably a pressure of 1.5 MPa to 1.7 MPa and most preferably a pressure of 1.55 MPa to 1.65 MPa or whereby the ammonia is condensed at a temperature in the range of 18 °C - 22 °C.

The process described above may also be embodied as that the separation of ammonia out of the gas stream carried at atmospheric pressure and the ammonia is recovered by an extraction.

Detailed Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof.

Several documents are cited throughout the text of this specification. Each of the documents herein (including any manufacturer’s specifications, instructions etc.) are hereby incorporated by reference; however, there is no admission that any document cited is indeed prior art of the present invention.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to the devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B .

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.

It is intended that the specification and examples be considered as exemplary only.

Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are part of the description and are a further description and are in addition to the preferred embodiments of the present invention.

Each of the claims set out a particular embodiment of the invention.

The following terms are provided solely to aid in the understanding of the invention.

The Haber-Bosch process for ammonia production is one of the oldest industrial catalytic processes ( Licht , S. et al. Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nano scale Fe203. Science (80-. ). 345, 637-640 (2014)). The first ammonia plant went on stream in 1913 ( Zapp , K.-H. et al. Ammonia, 1. Introduction. Ullmann’s Encycl. Ind. Chem. 263-285 (2012) doi:10.1002/14356007.a02). In the Haber-Bosch process N ₂ gas is reduced to NH ₃ using H ₂ gas (Eq.1): N ₂ +3 H ₂ →2 NH ₃ E ₀ = 0.057 V (1)

Elevated temperatures are needed to activate the iron-based catalyst. The reaction kinetics are peculiar in that the chemisorption of nitrogen gas molecules on the catalyst surface is limiting the reaction rate (Emmett, P. H. & Brunauer, S. The Adsorption of Nitrogen by Iron Synthetic Ammonia Catalysts. J. Am. Chem. Soc. 1738, 34-41 (1934) and Kobayashi, Y., Kitano, M., Kawamura, S., Yokoyama, T. & Hosono, H. Kinetic evidence : the rate -determining step for ammonia synthesis over electride-supported Ru. Catal. Sci. Technol. 7, 47-50 (2016)). The H ₂ for the Haber-Bosch process is typically produced by methane steam reforming. CO ₂ emission of the Haber-Bosch process amounts up to 1.9 ton per ton of ammonia produced (Rafiqul, L, Weber, C., Lehmann, B. & Voss, A. Energy efficiency improvements in ammonia production - Perspectives and uncertainties. Energy 30, 2487-2504 (2005)). In 2017, ammonia production worldwide was responsible for ca. 420 Mt CO ₂ (Zhou, F. et al. Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 10, 2516-2520 (2017)).

The use of water and electricity instead of hydrogen out of methane is an alternative pathway for ammonia synthesis (Eq. 2) (Chen, S. et al. Electrocatalytic Synthesis of Ammonia at Room Temperature and Atmospheric Pressure from Water and Nitrogen on a Carbon-Nanotube- Based Electrocatalyst. Angew. Chemie - Int. Ed. 56, 2699-2703 (2017) and Guo, X., Du, H., Qu, F. & Li, J. Recent progress in electrocatalytic nitrogen reduction. J. Mater. Chem. A 7, 3531-3543 (2019)). N ₂ +3 H ₂ O→2 NH ₃ +3/2 O ₂ E ₀ = -1.172 V (2)

Different types of electrocatalysts performing this reaction have been reported, but they exhibit two shortcomings: low activity and low ammonia selectivity. Currently, the highest reported Faradaic efficiencies of electrochemical ammonia synthesis out of N ₂ and H ₂ O are in the range of 60 % (Zhou, F. et al. Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids. Energy Environ. Sci. 10, 2516-2520 (2017); . Wang, M. et al. Over 56.55% Faradaic efficiency of ambient ammonia synthesis enabled by positively shifting the reaction potential. Nat. Commun. 10, 1-8 (2019), Hao, Y. C. et al. Promoting nitrogen electroreduction to ammonia with bismuth nanocrystals and potassium cations in water. Nat. Catal. 2, (2019)). One exception, which uses a fundamentally different process with Li-cycling, achieves a Faradaic efficiency of 88.5 % at a temperature of 450 °C (McEnaney, J. M. et al. Ammonia synthesis from N2 and H20 using a lithium cycling electrification strategy at atmospheric pressure. Energy Environ. Sci. 10, 1621-1630 (2017)).

Common electrochemical ammonia synthesis has hydrogen gas as the main by-product, consuming a significant part of the invested electric energy.

The invented SECAM process (Solar Electrochemical AMmonia synthesis) uses this hydrogen gas for two purposes: (i) reaction with oxygen of air to prepare nitrogen gas and water to be fed to the ammonia synthesis reactor, and (ii) performing electrocatalytic ammonia synthesis using

H ₂ .

The SECAM process has two modes of operation: energy intensive production of ammonia out of nitrogen gas and water according to Eq.1 (Mode A, Fig. 1A), and an energy extensive production of ammonia out of an N ₂ /H ₂ gas mixture according to Eq.2 (Mode B, Fig. 1B). The half reaction N ₂ + 6 H ₂ O + 6 e- → 2 NH ₃ + 6 OH- in alkaline environment, or N2 + 6 H ⁺ + 6 e- → 2 NH ₃ in acidic environment takes place in both Mode A and Mode B and is essential for the electrochemical production of ammonia.

In mode A, air is used as a source of nitrogen. To make air suitable for ammonia production, O ₂ is removed by reaction with H ₂ . This can be done in a fuel cell, generating electricity, or in a burner, generating heat. Alternatively, well-established technology, such as pressure swing adsorption, membrane separation or cryogenic distillation can be used to produce N ₂ .

Next, the gas containing already some water from the reaction of O ₂ with H ₂ , is sent through a humidifier where additional water vapour is added. Alternatively, water can be injected directly into the reactor. After these two steps, the hydrated nitrogen gas is fed to the electrochemical cell, where ammonia is formed on the cathode. The hydrogen evolution reaction (HER) is competing with ammonia synthesis.

When mode A is run such as to produce exactly the amount of hydrogen needed to eliminate the O ₂ from the intake air, the electrocatalyst should have a faradaic efficiency for ammonia production of 85 %. State-of-the-art electrocatalysts have lower faradaic efficiency (' Garagounis , Vourros, Stoukides, Dasopoulos & Stoukides. Electrochemical Synthesis of Ammonia: Recent Efforts and Future Outlook. Membranes (Basel). 9, 112 (2019)), and excess H ₂ gas is produced.

The resulting outlet gas of the cathode compartment is composed of NH ₃ , H ₂ and unreacted N ₂ . NH ₃ is condensed out of the gas stream, resulting in a residual stream of N2 and H ₂ . Part of this stream serves the O ₂ removal out of inlet air; part is stored in a tank as feed for Mode B. Water is the source of H-atoms and air is the source of N-atoms. The molar ratio of N ₂ /H ₂ in the process is fixed at 1/3 by tuning the air and water intake of the process.

This gas mixture is stoichiometrically converted to ammonia in Mode B and stored temporarily, together with nitrogen.

In Mode B (Fig. 2B), the H2/N2 gas mixture is sent to the anode of the electrochemical cell, where the hydrogen oxidation reaction (HOR) takes place. Next, the remaining gas is sent to the cathode, where ammonia and hydrogen gas are formed. In Mode B ammonia is produced until all N2 and H2 gas is converted by recycling.

Modes A and B make use of the same electrochemical cell. Ammonia production in Mode B consumes less than 20 % of the electric power required for Mode A.

Operation of SECAM according to Mode A and B is dependent of the availability of solar energy. When a large amount of energy is available, for example at noon, mode A is executed. When the energy supply is limited, for example at night or on clouded days, mode B is executed. Several SECAM reactors can run in parallel and operated either in Mode A or B to optimize the ammonia productivity according to the availability of energy.

To facilitate the condensation of ammonia, the processes are operated at a pressure of 1.6 MPa. At this pressure. An additional benefit of the increased pressure is a positive effect on the reaction rate by the first order kinetics (Zhang, Zhao, Shi, Waterhouse and Zhang, Photocatalytic ammonia synthesis: recent progress and future. EnergyChem. 1, 2 (2019)). The increased pressure entails an additional energy consumption, and materials cost for making the reactor pressure resistant. Alternatively, the process can be run at atmospheric pressure if the produced ammonia is recovered by an extraction with water.

Table 1: Comparison of the performance of SECAM, implemented in Leuven, Belgium (solar energy = 158 kWh/m2.year) with a FE of 10, 50 and 85 %, in the Atacama desert, Chile (353 kWh/m2.y) with a FE of 22.5, 50 and 85 %. Average energy consumption and average daily operation time of mode A and mode B are reported.

The energy performance of a SECAM processes was simulated for two different geographic locations (Table 1). The specifications of the process parameters are given in SI. In Leuven, Belgium, a solar panel with an efficiency of 15 % produces on average 430 Wh/m2.day with a maximal power delivery of 150 W/m2. This means mode A needs 150 W peak capacity (electrochemical reactor + compressor) for every square meter of solar panel. The produced solar electricity is sufficient to operate in mode A for 2.9 h/day on average. During these 2.9 hours, a reactor with a FE of 10 % produces enough N2/H2 gas for mode B to run for 21.1 hours. Together, this completes a day cycle of 24 hours. Therefore, the minimal FE is 10 % in Belgium. The highest average solar irradiation is encountered in the Atacama desert in Chile. There, a solar panel with 15 % efficiency produces 967 Wh/m2.day. Analogously, the minimal FE is 22.5 % in Chile. The energy needed for ammonia production and the share of operation Modes A and B of SECAM processes with different Faradaic efficiencies at the two locations are presented in Table 1.

The energy consumption of SECAM ammonia synthesis is plotted against the Faradaic efficiency of the electrocatalysts is plotted in Fig. 3. It is clear the energy consumption starts to increase rapidly at FE’s below 20 %. However, at FE’s above 20 %, the curve flattens. Compared to an electrocatalyst with a FE of 85 %, a 2.8 fold decrease to a FE of 30 % results in only a 37 % increase in energy consumption. In the past, previous studies were mainly focused on obtaining very high FE’s. The ARPA-E (Advanced Research Projects Agency- Energy) determined a minimal FE of 90 % for the process to be economically feasible (ARPA- E. Renewable energy to fuels through utilization of energy -dense liquids (REFUEL)). However, with SECAM, a minimal FE of 20-30 % is sufficient. Therefore, the main focus of future research should be on how to increase the current density, which is still far too low for commercial ammonia production. The following table provides a list of electrocatalysts with a sufficient faradaic efficiency:

Despite the large pressures and temperatures required for the Haber-Bosch process, it is surprisingly efficient at a very large scale (Chen, Crooks, Seefeldt, Bren, Morris, Darensbourg et al. Beyond fossil fuel-driven nitrogen transformation. Science (80-.). 360, 6391 (2018)). The traditional natural gas-based Haber-Bosch process is reported to have an energy consumption ranging from 0.58 MJ/mol to 0.81 MJ/mol, depending on the source (Rafiqul, Weber, Lehmann & Voss. Energy efficiency improvements in ammonia production perspectives and uncertainties. Energy. 30, 2487-2504 (2005), Renner, Greenlee, Herring & Ayers. Electrochemical synthesis of ammonia: A low pressure, low temperature approach. Electrochem. Soc. 24, 51-57 (2002) and Giddey, Badwal & Kulkarni. Review of electrochemical ammonia production technologies and materials. Int. J. Hydrogen Energy. 38, 14576-14594 (2013)). SECAM uses similar amounts of energy, ranging from 0.56 MJ/mol for a FE of 85 % to 0.92 MJ/mol for a FE of 20 %. As an additional advantage, SECAM allows for efficient decentralized production. The Haber-Bosch process is only cost efficient at a very large scale and requires distribution infrastructure. This infrastructure is absent in less developed parts of the world (Gallowway & Cowling. Reactive nitrogen and the world: 200 years of change. AMBIO A J. Hum. Environ. 31, 64-71 (2002)). With an efficient delocalized ammonia production at farms, transport costs for fertilizers can also be almost eliminated (Jewess & Crabtree. Electrocatalytic nitrogen fixation for distributed fertilizer production ?. ACS Sustain Chem Eng. 4, 5855-5858 (2016)). The process discussed in this paper has no intrinsic C02 footprint and can be operated with fluctuating power source, unlike the Haber- Bosch process.

Present invention shows how to execute efficient electrochemical ammonia production that is competitive with the Haber-Bosch process in terms of energy consumption. In contrast to the Haber-Bosch process, the discussed process is equally efficient at a small scale and allows a delocalised ammonia production. The process only requires a faradaic efficiency of 20-30 % to operate efficiently. With this work, we hope to stimulate future research to prioritise increasing the current density instead of trying to obtain very high faradaic efficiencies at a low current density.

EXAMPLES

To be able to demonstrate the efficiency of the process performance of SECAM, several assumptions were made:

For both Mode A and Mode B

The combined FE of the NRR and HER is 100 %. The formation of other by-products at the cathode, besides H ₂ and NH ₃ , is neglected.

The ambient temperature is 20 °C

The temperature of the system is assumed to be at 40 °C due to heat development in the H ₂ -bumer and energy losses. The maximal amount of vapour present in the gas phase is independent of the gas composition. The maximal vapour content (relative humidity = 100 %) is taken to be 8.83 x 10 ^-3 mol H ₂ O/mol gas ( Engineering ToolBox, Compressed Air and Water Content, https://www.engineeringtoolbox.com/water-content-compressed- air- d_1275.html, (accessed 20 December 2019)).

The condenser removes all the ammonia out of the gas phase

The faradaic efficiency is independent of the composition of the feed gas of the reactor The overpotential is independent on the gas composition

The reactor operates in an alkaline environment. However, the same conclusions apply for an acidic environment.

The intake air only consists out of N ₂ and O ₂ . Other compounds, such as Ar, are neglected. In reality, it might be necessary to add a purge stream during operation in Mode B.

For Mode A

Incoming air is dry

The reactor has a constant power supply of 150 W.

The stoichiometrically limiting reagents in the reactor is H ₂ O. Half of the H ₂ O entering the reactor is consumed. The rest is recycled.

The oxygen present in the incoming air is removed by burning H ₂ . Energy is not recuperated in a FC.

Only ammonia condenses in the condenser. Water stays in the gas phase For Mode B

The local temperature rise in the electrochemical cell is large enough to avoid the condensation of water due to the reduced amount of gas.

20 % of the hydrogen coming into the reactor is converted at the anode.

The stream coming from the condenser has 8.83 x 10 ^-3 mol H ₂ O/mol gas (. Engineering ToolBox, Compressed Air and Water Content, https://www.engineeringtoolbox.com/water-content-compressed- air-d_1275.html, (accessed 20 December 2019)). The excess H ₂ O is condensed, resulting in a small amount of H ₂ O present in the produced ammonia.

Mass balance

To calculate the molar flow rate of the different streams, the following set of equations is solved. The indices correspond to the numbered flows in fig 4 - 7. Mode A

H2 burner

Humidifier

Electrochemical cell

Condensation

Stream splitter

Maximal humidity constraint

Mode B

Electrochemical cell

Condenser

Mixing streams

Maximal humidity constraint Ṅ _H2O,5 = 0.00883 (Ṅ _N2,5 +Ṅ _H2,5 ) En er gy analysis

To be able to analyse the energy consumption of the SECAM process, the cell potential needs to be determined. This was done with the following equations and the constants provided hereunder. The Gibbs free energy of formation is withdrawn from Lange’s Handbook of Chemistry (J. a. Dean, Lange’s Handb. Chem., 1979, 6, 31-56).

Mode A

Mode B

The set of equations of the mass balance and energy analysis are combined and solved in Matlab. The result is shown in Fig. 4- 7 for a FE of 10, 22.5, 50 and 85 % and an energy consumption of 150 W in Mode A.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Ref. 1 = Engineering ToolBox, Horsepower required to Compress Air, https://www.engineeringtoolbox.com/horsepower-compressed-air -d_1363.html, (accessed 28 November 2019)

Ref. 2 = Engineering ToolBox, Pressure Energy, https://www.engineeringtoolbox.com/pressure-energy-d_1822.ht ml, (accessed 28 November 2019)

Ref. 3 = L. I. Krishtalik, BBA - Bioenerg., 1986, 849, 162-171 Ref. 4 = X. Guo, H. Du, F. Qu and J. Li, J. Mater. Chem. A, 2019, 7, 3531-3543

Table 2 Drawing Description

BRIEF DESCRIPTION OF THE DRAWINGS AND TABLES

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG.1: provides a schematic overview of the half reactions occurring in the electrochemical cell, for mode A and mode B.

FIG. 2: provides a schematic overview of a process that we call the SECAM process for electrochemical ammonia production. (A) Energy intensive production of ammonia from water and air. (B) Energy extensive production of ammonia from an N2/H2 gas mixture. A is shown electrochemical reactor [1] with the electrocatalysts, cathode [1a] and anode [1b] that when energized with DC transfers H ₂ O and N ₂ present in the reactor into NH ₃ , H ₂ and O ₂ . In Mode A, when H ₂ , N ₂ and NH ₃ is fed into a condenser the NH ₃ can be separate. Eventually H ₂ and N ₂ is temporarily stored. H ₂ and N ₂ can be consequently fed with air into a burner where O ₂ + 2 H ₂ is transformed in to 2H ₂ O, where after H ₂ , N ₂ , and H ₂ O is fed into an humidifier. In Mode B H ₂ and N ₂ form the temporarily storage tank is fed with H ₂ and N ₂ from the condenser into the electrochemical reactor [1] with the electrocatalysts, cathode [1a] and anode [1b] that when energized with DC transfers H ₂ and N ₂ into NH ₃ . Eventually H ₂ and N ₂ from the electrochemical reactor [1] is fed back into the electrochemical reactor [1] and H ₂ , N ₂ and NH ₃ is guided into the condenser to remove NEb.

FIG. 3 is a graphic that shows the average energy consumption of SECAM per mole of ammonia produced, as a function of the faradaic efficiency of the catalyst (solid line), compared to the energy consumption of the natural gas-based Haber-Bosch process, (Rafiqul, I., Weber, C., Lehmann, B. & Voss, A. Energy efficiency improvements in ammonia production - Perspectives and uncertainties. ( Energy 30, 2487-2504 (2005), Renner, J. N., Greenlee, L. F., Herring, A. M. & Ayers, K. E. Electrochemical synthesis of ammonia: A low pressure, low temperature approach. Electrochem. Soc. Interface 24, 51-57 (2015) and Giddey, S., Badwal, S. P. S. & Kulkarni, A. Review of electrochemical ammonia production technologies and materials. Int. J. Hydrogen Energy 38, 14576-14594 (2013))

FIG. 4: is a schematic quantitative representation of a SECAM process with FE = 10 % with (top) Mode A, operating at 150 W, and (bottom) the accompanying Mode B FIG. 5: is a schematic quantitative representation of a SECAM process with FE = 22.5 % with (top) Mode A, operating at 150 W, and (bottom) the accompanying Mode B FIG. 6: is a schematic quantitative representation of a SECAM process with FE = 50 % with

(top) Mode A, operating at 150 W, and (bottom) the accompanying Mode B

FIG. 7: is a schematic quantitative representation of a SECAM process with FE = 85 % with

(top) Mode A, operating at 150 W. Operation in mode B is not required for full conversion to ammonia.

Table 1 provides a comparison of the performance of our invented SECAM, (solar energy = 158 kWh/m2.year) with a FE of 10, 50 and 85 %, in the Atacama desert, Chile (353 kWh/m2.y) with a FE of 22.5, 50 and 85 %. Average energy consumption and average daily operation time of mode A and mode B are reported.

Table 2: provides values used for the energy cost of air- and water compression and standard- and overpotential of the different half reactions relevant in the electrochemical cell.

*Li et al. (S. J. Li, D. Bao, M. M. Shi, B. R. Wulan, J. M. Yan and Q. Jiang, Adv. Mater., , D01:10.1002/adma.201700001 ) and Wang et al. (M. Wang, S. Liu, T. Qian, J. Liu, J. Zhou, H. Ji, J. Xiong, J. Zhong and C. Yan, Nat. Commun., 2019, 10, 1-8 reported an overpotential of 0.259 V and 0.227 V respectively for the NRR)

**Zhang et al. (B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. Garcia-Melchor, L. Han, J. Xu, M. Liu, L. Zheng, F. P. G. De Arquer, C. T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, A. Janmohamed, H. L. Xin, H. Yang, A. Vojvodic and E. H. Sargent, Science (80-. )., 2016, 352, 333-337) and Zhao et al. ( S . Zhao, Y. Wang, J. Dong, C. T. He, H. Yin, P. An, K. Zhao, X. Zhang, C. Gao, L. Zhang, J. Lv, J. Wang, J. Zhang, A. M. Khattak, N. A. Khan, Z. Wei, J. Zhang, S. Liu, H. Zhao and Z. Tang, Nat. Energy, 2016, 1, 1-10 ) reported an overpotential of 0.191 V and 0.189 V respectively for the OER at 10 mA/cm2

***Zhuang et al. (Z. Zhuang, S. A. Giles, J. Zheng, G. R. Jenness, S. Caratzoulas, D. G. Vlachos and Y. Yan, Nat. Commun., 2016, 7, 1-8) reported a current density of 2.33 mA/cm2 at an overpotential of 0.05 V for the HOR.

Symbols in this application

ΔG Change in Gibbs free energy of the halfreaction [kJ/mol]

E ⁰ Standard potential of the halfreaction [V]

E _cell Cell potential [V]

F Faraday constant (96485 C/mol) FE Faradaic efficiency of the electrochemical cell towards the NRR n Number of electrons needed for the halfreaction Ṅ _i,j Molar flow rate of compound i in stream j [mol/h] η _a Overpotential at the anode [V] η _c Overpotential at the cathode [V]

Pi Partial pressure of compound i [Pa]

R Ideal gas constant (8.314 J/mol.K) r _Burn Rate of the reaction of H ₂ with O ₂ for the removal of O ₂ out of the air [mol/h] r _HER Rate of the hydrogen evolution reaction (HER) in the electrochemical cell [mol/h] r _NRR Rate of the nitrogen reduction reaction (NRR) in the electrochemical cell [mol/h]