Electrochemical synthesis of ammonia in molten salts

Journal of Energy Chemistry 43 (2020) 195–207

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Electrochemical synthesis of ammonia in molten salts Jiarong Yang, Wei Weng, Wei Xiao∗ School of Resource and Environmental Sciences, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan 430072, Hubei, China

a r t i c l e

i n f o

Article history: Received 3 August 2019 Revised 30 August 2019 Accepted 3 September 2019 Available online 11 September 2019 Keywords: Electrochemical ammonia synthesis Molten salt electrolysis N2 reduction Hydrogen source

a b s t r a c t Ammonia is important feedstock for both fertilizer production and carbon-free liquid fuel. Many techniques for ammonia formation have been developed, hoping to replace the industrial energy-intensive Haber–Bosch route. Electrochemical synthesis of ammonia in molten salts is one promising alternative method due to the strong solubility of N3− ions, a wide potential window of molten salt electrolytes and tunable electrode reactions. Generally, electrochemical synthesis of ammonia in molten salts begins with the electro-cleavage of N2 /hydrogen sources on electrode surfaces, followed by diffusion of N3− /H+ containing ions towards each other for NH3 formation. Therefore, the hydrogen sources and molten salt composition will greatly affect the reactions on electrodes and ions diffusion in electrolytes, being critical factors determining the faradaic efficiency and formation rate for ammonia synthesis. This report summarizes the selection criteria for hydrogen sources, the reaction characteristics in various molten salt systems, and the preliminary explorations on the scaling-up synthesis of ammonia in molten salt. The formation rate and faradaic efficiency for ammonia synthesis are discussed in detail based on different hydrogen sources, various molten salt systems, changed electrolysis conditions as well as diverse catalysts. Electrochemical synthesis of ammonia might be further enhanced by optimizing the molten salt composition, using electrocatalysts with well-defined composition and microstructure, and innovation of novel reaction mechanism. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Jiarong Yang is pursuing her master degree in Environmental Engineering from Wuhan University. Her research interests include preparation of functional materials by electrolysis and pyrolysis in molten salts.

Wei Weng received his B.S. and Ph.D. degrees in Metallurgical Engineering from Wuhan University of Science and Technology in 2012 and University of Science and Technology Beijing in 2018, respectively. Then, he worked as a postdoctor in Wuhan University. His research interests include rare metals extraction and functional materials preparation by molten salt electrolysis.

∗

Corresponding author. E-mail address: 0 0 030 [email protected] (W. Xiao).

Wei Xiao obtained B.S. in Chemistry (2002) and Ph.D. in Physical Chemistry (2007) in Wuhan University, China. After three-and-half-year stay in National University of Singapore, Nanyang Technological University and the University of Nottingham as a post-doctoral research fellow, he joined Wuhan University as an associate professor in 2011 and was promoted to a full professor in 2017. He was listed as one of the Top 1% Highly Cited Chinese Authors in Royal Chemistry Society Journals (in 2014 and 2015). He is a Fellow of the Royal Society of Chemistry (FRSC) since 2017. His research focuses on molten salt chemistry towards energy and environmental sustainability.

1. Introduction Ammonia with an annual production exceeding 150 million tons plays a critical role in both agricultural and industrial fields. Nearly 80% of the synthesized ammonia is utilized as the fertilizer feedstock, supporting the world’s growing population [1]. Ammonia is also a potential carbon-free green energy source for substituting the carbonaceous fuels because of the high hydrogen content (17.6 wt%) [2,3]. Moreover, transportation and storage of ammonia is extremely convenient because ammonia is easy to be liquefied, therefore the usage of ammonia is safe. Due to the above

https://doi.org/10.1016/j.jechem.2019.09.006 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

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merits, facile synthesis of ammonia with high efficiency has long been the dream of human beings. Currently, Haber–Bosch process is the dominant route for ammonia synthesis in industry. Although being adopted for more than one hundred years [4], this process still faces many challenges from the beginning of its birth. Firstly, the N≡N bond energy is high up to 962 kJ mol−1 , therefore high reaction temperature (30 0–50 0 °C) is essential to the cleavage of the strong N≡N bond for the formation of ammonia, leading to high energy consumption [4–6]. Secondly, an extremely high-pressure is the perquisite for realizing an acceptable ammonia formation rate. The reaction for ammonia formation (N2 +3H2 = 2NH3 , H400 ◦ C = −104.3 kJ mol−1 ) is exothermic thermodynamically. Therefore, although high temperature can promote the cleavage of N≡N bond, low conversion rate is the concomitant consequence. To overcome such a dilemma, the reaction pressure exerted in industry is as high as 15–30 MPa [7,8], which extraordinarily raises the equipment cost and decreases the work safety. Besides, ammonia can damage the skins, eyes, and tissues of human beings. Inhaling excessive ammonia can even lead to death. Therefore, high-pressure synthesis of ammonia, which remarkably increases the risk of ammonia leakage, is always a threat for health of workers in industry. Thirdly, catalysts are indispensable in this process, which usually suffer from deactivation due to sintering in such high reaction temperatures. Specially, precious metals, such as Ru-based catalysts, are adopted to gain high conversion and selectivity in some cases, further increasing the overall cost. Even under such extreme conditions, the conversion ratio for ammonia formation is still as low as 10%–15% [4]. Finally, the H2 feedstock for ammonia synthesis is produced from fossil fuels, which results in approximately 300 million tons of CO2 emission annually [9,10], compromising the competitiveness of this process in terms of environmental protection. Obviously, new technologies under mild conditions as well as readily available feedstocks for ammonia synthesis are in urgent need. Electrochemical synthesis of ammonia is one of the most investigated alternative technologies due to the following merits [11,12]: (i) instead of H2 , easier accessible H2 O (H+ in the electrolytes) can be used as the hydrogen source for ammonia formation, free of extra energy input for feedstocks preparation. (ii) Electrochemical synthesis can be conducted in the atmospheric pressure, contributing to a facile synthesis process. (iii) Instead of fossil fuels, electricity which can be derived from sustainable energy such as solar energy can be used for electrochemical synthesis of ammonia [13], therefore promising a green process from the perspective of the whole process [14]. For the electrochemical synthesis of ammonia, the electrolyte plays a key role in charge of transporting the N/H-containing ions to the electrode surfaces, in which ammonia synthesis takes place. Based on different electrolytes, the ammonia synthesis by electrochemical routes can be generally divided into three categories, which means aqueous solution electrolyte, solid oxide electrolyte and molten salt electrolyte [15,16]. Although electrochemical synthesis of ammonia in aqueous solution can be conducted at low temperatures (generally at room temperature), the slow reaction rate and low faradaic efficiency are the insurmountable challenges, which are caused by the intrinsic insufficient solubility of N2 and inevitable side reactions (mainly H2 evolution) in aqueous solution electrolyte [17]. By using the solid oxide electrolytes, the side reaction of H2 evolution can be greatly restrained, and therefore contributing to a high faradaic efficiency. However, the sluggish diffusion of reactive ions in solid oxide electrolytes makes the ammonia formation rate unfavorable [15,16]. Compared with the above two electrolytes, molten salt electrolyte possesses many unique advantages. The liquid molten salts show a strong capability to dissolve the reactants and transfer the N/H-containing ions to the electrode surfaces [18], contributing to a very high ammonia

Table 1. Comparison for electrochemical synthesis of ammonia in various electrolytes. Electrolytes

Advantages

Disadvantages

Aqueous solution

Low temperature (<100 °C) High faradaic efficiency High reaction rate Low temperature (<500 °C) High faradaic efficiency

Slow reaction rate Low faradaic efficiency Corrosive medium Low stability of electrode materials

[17]

High Temperature (>800 °C) Low reaction rate (Sluggish ion diffusion)

[18,19]

Molten salt electrolytes

Solid oxide electrolytes

Ref.

[15,16]

formation rate [19], as shown in Fig. 1(a). In addition, benefiting from the absence of H2 O (especially in chloride molten salts), side reaction (H2 evolution) can be largely retarded in molten salts, resulting in greatly enhanced faradaic efficiency for ammonia formation (Fig. 1(a)). Moreover, the wide electrochemical window of molten salt (Fig. 1(b)) makes the electrode reactions for ammonia formation easily tunable in a very broad potential range [20,21]. Furthermore, the reaction temperature in molten salt (as low as 200 °C in some cases [22], as shown in Fig. 1(c)) can be substantially decreased when compared with the traditional Haber-Bosch process (30 0–50 0 °C), therefore the deactivation of catalysts can be retarded. The advantages and disadvantages for ammonia synthesis in the three electrolytes are presented in Table 1. The reaction rate and faradaic efficiency are two of the most important factors for electrochemical ammonia formation, which are highly related to the electrode reactions and diffusion of N/H-containing reactants in molten salts. Taking the ammonia formation from N2 and H2 as an example, soluble N/H-containing ions (such as N3− and H+ ) are firstly generated by N2 reduction on cathode and H2 oxidation on anode respectively, which then diffuse toward each other to form gaseous NH3 (N3− +3H+ =NH3 ). Variation of both hydrogen sources and molten salt composition will change the electrode reactions and dissolution/diffusion of N/H-containing reactants in molten salt, therefore affecting the reaction rate and faradaic efficiency for ammonia formation. In this paper, the selection criteria of different hydrogen sources for electrochemical ammonia formation in molten salts are firstly discussed. Then, reaction characteristics including reaction mechanisms, ions diffusion and side reactions in various molten salt electrolytes are reviewed. Finally, comparison of various routes for ammonia formation rate and preliminary explorations for scaling-up ammonia synthesis in molten salt are also conducted. 2. Selection of hydrogen sources Hydrogen and nitrogen sources are indispensable for ammonia formation. The cheap N2 can be effectively and economically acquired by cryogenic air separation, therefore being selected as the nitrogen source in almost all electrochemical ammonia synthesis systems. Although H2 is still widely chosen as the hydrogen source, seeking for other cheap substitutes such as H2 O, CH4 , HCl and H2 S never stops because preparation of H2 is very energy-consuming and environmentally unfriendly [16,32]. Understanding the reaction characteristics of these potential substitutes is beneficial for selecting the most appropriate hydrogen source in future researches. 2.1. Thermodynamics considerations The theoretical decomposition voltages for ammonia formation are shown in Fig. 2(a), which are calculated by HSC 6.0 software based on the following reactions:

N2 (g ) + 3H2 (g )= 2NH3 (g )

(1)

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Fig. 1. (a) Examples of different electrochemical ammonia synthesis systems with experimental studies and results [17,22–25]; (b) electrochemical window of aqueous electrolytes, molten salt electrolytes and solid electrolyte (taking CeO2 as an example) at different temperatures (calculated by the HSC6.0 software); (c) the electrolysis temperature ranges for different electrolytes: aqueous electrolytes [26–29], molten salt electrolytes [17,22,25,30–33], solid state electrolytes [34–39].

Fig. 2. (a, b) Theoretical decomposition voltages (Eθ ) of reactions for formations of ammonia and byproducts; (c) Gibbs free energy changes (Gθ ) of related side reactions; (d) Faradaic efficiency for ammonia formation from various hydrogen sources: H2 [17], H2 O [32], CH4 [40], HCl (∗ the ammonia converted to ammonium chloride) [41], H2 S (∗∗ calculated based on nickel sulfides) [42].

N2 (g ) + 3H2 O(g ) = 2NH3 (g ) + 3/2O2 (g )

(2)

N2 (g ) + 3/2CH4 (g ) = 2NH3 (g ) + 3/2C

(3)

N2 (g ) + 6HCl(g ) = 2NH3 (g ) + 3Cl2 (g )

(4)

N2 (g ) + 3H2 S(g ) = 2NH3 (g ) + 3S

(5)

A lower decomposition voltage means easier formation of ammonia. For various hydrogen sources, the decomposition voltage in the commonly used temperature range of 20 0–50 0 °C in molten salts obeys the order of H2 < CH4 < H2 S < HCl < H2 O (Fig. 2(a)), therefore the theoretical energy input for ammonia synthesis from H2 (reaction 1) is the lowest among the above-mentioned hydrogen sources. It should be noted that decomposition voltages for ammonia formation from CH4 and H2 S are very close to that from

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H2 , implying that CH4 and H2 S are the most desirable substitutes of H2 for ammonia formation from the viewpoint of thermodynamics. The decomposition voltage for ammonia formation from H2 O ranks the highest, followed by HCl, meaning that H2 O and HCl are the most undesirable choice under thermodynamics considerations. Thermodynamically, energy input for ammonia formation is the lowest when using H2 as the hydrogen source. CH4 and H2 S are the most appealing substitutes of H2 for ammonia formation. The energy input for ammonia formation is the highest when utilizing H2 O as the hydrogen source. 2.2. Taking side reactions into considerations In addition to thermodynamics, side reactions in real electrochemical systems should also be taken into considerations during the selection of hydrogen sources for ammonia formation. According to reaction (1), no byproducts generate using H2 as the hydrogen source, indicating that H2 is the ideal hydrogen source no matter in considerations of thermodynamics or side reactions. However, the hydrogen gas required for ammonia synthesis comes from steam methane reforming of natural gas or fossil fuels by energy and carbon-intensive processes [16], resulting in more than 300 million tons of CO2 emissions per year [43]. Therefore, researches seeking for other easily accessible hydrogen sources are flourishing. CH4 is chosen as the hydrogen source for ammonia formation by some researchers because of the following merits: (i) 76% H2 for industrially ammonia synthesis is derived from steam methane (CH4 ) reforming, therefore using CH4 as the hydrogen source can avoid the steam reforming process. (ii) Thermodynamically, the theoretical potential required for the synthesis of ammonia from CH4 is the closest to that from H2 (as shown in Fig. 2(a)), and even lower when at temperatures exceeding 550 °C. However, some detrimental disadvantages impede the applicability for ammonia formation from CH4 . For example, instead of NH3 formation, electrochemical conversion of CH4 to H2 and C (reaction 6) is easier, as revealed by lower decomposition voltages in Fig. 2(b) for the latter case. Therefore, in addition to NH3 , H2 is also massively produced by reaction (6) during electrolysis of CH4 in molten salts [40]. Besides, unfavorable C is deposited (reaction 6) at the same time, leading to deactivation of catalysts or blocking of pores in the gas-diffusion electrode.

From the viewpoint of thermodynamics (Fig. 2(a)), H2 O is the most difficult candidate for electrochemical synthesis of ammonia in molten salt. Even so, H2 O is still more investigated than other hydrogen sources (CH4 , H2 S and HCl) to substitute H2 for ammonia synthesis because of the following reasons: (i) H2 O is a clean and abundant hydrogen source. (ii) No detrimental byproducts generate. The only disadvantage when using H2 O as hydrogen source is that the concomitant H2 evolution side reaction (as shown in Fig. 2(b)) compromises the faradaic efficiency and yield rate for ammonia formation. Even so, H2 O is still the most desirable hydrogen source to substitute H2 for ammonia formation. In summary, the energy input for ammonia formation among various hydrogen source obeys the order of H2 < CH4 < H2 S < HCl < H2 O thermodynamically. Formation of ammonia using H2 as hydrogen source is the easiest, but preparation of H2 leads to huge energy consumption and CO2 emission. Compared with H2 , side reactions usually take place during electrochemical synthesis of ammonia from other hydrogen sources, leading to much lower faradaic efficiencies (Fig. 2(d)). H2 O is the most desirable candidate to substitute H2 for synthesis of ammonia due to the merits of being easily accessible, environmentally friendly and free of hazardous byproducts. 3. Ammonia formation in various molten salt electrolytes After nitrogen and hydrogen sources are converted to N/Hcontaining ions in the electrodes/molten salt interfaces during electrolysis, swift diffusion away of these intermediates from the electrode surfaces is beneficial for the refreshment of reactive sites on electrodes, promising continuous and stable operation. In addition, electrochemical conversions of N2 and hydrogen sources to N/H-containing ions take place separately on cathode and anode, respectively, which means that ammonia formation can only occur after the successful contact of these intermediate ions via diffusion toward each other in molten salts. Therefore, the molten salt electrolytes play an important role in enhancing the electrochemical formation of ammonia. The molten salt electrolytes used for electrochemical synthesis of ammonia can be categorized to molten chloride salts, molten hydroxide salts and composite electrolyte (molten carbonates stored in porous solid oxide electrolyte) [15]. The reaction mechanism, ammonia yield rate and the faradaic efficiency show huge difference in various molten salt systems due to diverse physicochemical properties.

CH4 (g ) = C + 2H2 (g )

(6)

3.1. Ammonia synthesis in chloride molten salts

xM + S = Mx S

(7)

HCl (g ) + NH3 (g ) = NH4 Cl(g )

(8)

H2 O(g ) = H2 (g ) + 1/2O2 (g )

(9)

In 1994, Ito and co-workers reported an electrochemical method for the surface nitriding of titanium in the molten chloride electrolytes [44]. When Li3 N was dissolved in a LiCl–KCl molten electrolyte as a nitride ion source at 450 °C, the nitridation of titanium surface was achieved with the formation of TiN. From then on, electrochemical reduction of nitrogen gas in LiCl–KCl eutectic melt was carried out [45].

H2 S and HCl, which are usually contaminants in industry processes, are also considered as hydrogen sources for ammonia formation, realizing both the disposal of industrial pollutants and waste reuse [41,42]. Disappointingly, no appropriate anode materials exist for H2 S electrolysis in molten salt because the anodic byproduct S will convert most metallic/carbonaceous anodes to insulating metal/carbon sulfides (as shown in Fig. 2(c), taking Ni, Mo, Al and C as examples.), impeding the continuous operation. HCl is also unsuitable as hydrogen source because of the following reasons: (i) the spontaneous formation of NH4 Cl between HCl and NH3 in the temperature range of 200∼350 °C (reaction 8, Fig. 2(c)) makes the collection of ammonia extremely difficult. (ii) Both the HCl and the anodic gaseous byproduct Cl2 (reaction 4) is very corrosive, making the operation safety and set-up cost unacceptable.

3.1.1. The critical role of Li3 N Generally, N2 is electrochemically reduced to N3− on the cathode surface. Swift transferring away of N3− from the cathode to react with anodic H+ is the key for continuous and fast formation of ammonia. Therefore, seeking molten salts which can rapidly dissolve N3− is very important. Exhilaratingly, the solubility of Li3 N in LiCl–KCl–CsCl molten salt is high up to 0.5 wt% even at a low temperature of 300 °C [45,46], therefore shuttling of N3− from cathode surface to react with anodic H+ can be easily realized. Murakami et al. systematically investigated the influence of Li3 N on the faradaic efficiency and formation rate for ammonia synthesis in KCl–LiCl–CsCl molten

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Fig. 4. Polarization curves (a) and chronoamperometry curves (b) on various metal cathodes for N2 reduction in chloride molten salt (scan rate: 0.5 mV s−1 , cathodic potential: 0.2 V) [51] (Copyright 2015, Elsevier).

Fig. 3. (a) Faradaic efficiencies for ammonia synthesis with/without adding Li3 N into the molten chloride salts. (b) Illustration for ammonia formation from N2 and H2 in chloride molten salts containing soluble Li3 N [17] (Copyright 2003, American Chemical Society). (c) Illustration for cathodic reactions, diffusion of N3− ions in molten salt and side reactions on anode surface [30,32] (Copyright 1998, Elsevier).

salt [32]. In the absence of Li3 N, the faradaic efficiency and formation rate of ammonia are only 0.14% and 1.78 × 10−10 mol s−1 cm−2 , respectively [47]. After addition of 0.5 wt% Li3 N, the faradaic efficiency and ammonia formation rate are 23% and 2.0 × 10−8 mol s−1 cm−2 , respectively (as shown in Fig. 3(a)), which are 163 and 111 times higher than that without dissolved Li3 N. The current efficiency can be further increased to ∼72% by optimizing the hydrogen source and electrode materials [17], which will be discussed in the following part. The above results undoubtedly indicate that the conversion of N2 /N3− couple is greatly enhanced by the dissolved Li3 N in the molten salt [48,49], as illustrated in Fig. 3(b). Resultantly, the rate-determining step of ammonia formation in molten chloride was changed from the cleavage of N2 to the dissolution or diffusion of hydrogen source in the molten salt [50]. 3.1.2. Influence of hydrogen source and electrode materials Absolutely different anodic reactions are found for various hydrogen sources in the KCl–LiCl–CsCl–0.5 wt% Li3 N molten salt, greatly affecting the faradaic efficiency and ammonia formation rate. When H2 is used as the hydrogen source, the current efficiency is high up to 72%. The loss of current efficiency (28%) is attributed to the reoxidation of N3− to N2 in the anodic region (reaction 10), as illustrated in Fig. 3(c). H2 O is another widely investigated hydrogen source for ammonia formation in the KCl– LiCl–CsCl–0.5 wt% Li3 N molten salt due to the following merits: (i) H2 O is much easier accessible than H2 , promising a low-cost and environmentally friendly process. (ii) Unlike the H2 which needs electrooxidation in advance in the anodic region, H+ in H2 O can directly combine with dissolved N3− to form NH3 by reaction (11), further promoting the ammonia formation. For example, when replacing the H2 with H2 O in the same molten salt system, the ammonia formation rate is increased from 3.33 × 10−9 mol s−1 cm−2 to 2.0 × 10−8 mol s−1 cm−2 . However, the faradaic efficiency decreases substantially (only 23%) after using H2 O as hydrogen source. As shown in Fig. 2(b), the theoretical voltage for electrosplitting H2 O to H2 and O2 is lower than that for electrochemical synthesis of ammonia from H2 O and N2 . Therefore, the concomitant electro-splitting of H2 O is a serious competing reaction when using H2 O as hydrogen source for ammonia synthesis, which is

the main reason for current efficiency loss. Generation of anodic CO2 (reaction 12) on carbonaceous anodes is another reason leading to the current efficiency loss for ammonia synthesis from H2 O and N2 in chloride molten salt systems [32]. Moreover, the formation of hydroxide ions from water and oxide ions (reaction 13) will further decrease the current efficiency when using H2 O as hydrogen source [30]. Other hydrogen source such as CH4 [40], H2 S [42] and HCl [41] are also explored, but shows negligible current efficiency and ammonia formation rate due to detrimental side reactions discussed in the former part.

2N3− = N2 (g ) + 6e−

(10)

3H2 O(g ) + 2N3− = 2NH3 (g ) + 3O2−

(11)

C + 2O2− = CO2 (g ) + 4e−

(12)

H2 O(g )+ O2− = 2OH−

(13)

The electrode material is another factor affecting the current efficiency and ammonia formation rate. Kim et al. reported the effect of four cathode materials (Ti, Fe, Co, Ni) on the electrochemical nitrogen reduction in chloride molten salt electrolytes (LiCl–KCl–CsCl). Both the linear sweep voltammetry (LSV) and chronoamperometry (CA) results reveal that the electrochemical activity of these metals for N2 reduction is in the order of Co > Ni > Fe > Ti (Fig. 4(a, b)) [51]. Murakami and co-workers also found that the ammonia formation rate on the Al plate cathode is 4 times larger than that on a porous Ni cathode when other conditions are the same [17,50]. The activity difference for N2 reduction is likely attributed to the diverse electrical resistivity and wettability of various metals in molten salts [51]. A lower electrical resistance means a lower voltage drop during electrochemical synthesis of ammonia, benefiting to ammonia synthesis with a high formation rate. For electrodes possessing a good wettability with the molten salts, the N2 molecules can diffuse across the meniscus layer on the electrode-molten salt interface very smoothly, facilitating the contact of N2 molecules with the electrode surface. In addition to cathode materials, anode material also affects the electrochemical formation of ammonia in molten salts. For example, Murakami and co-workers compared the faradaic efficiency and ammonia formation rate on glassy carbon rod and boron-doped diamond anodes in LiCl–KCl–CsCl–0.5% Li3 N molten salt, using N2 /H2 O as feedstocks and porous Ni as cathode, respectively [30,32]. Compared with the boron-doped diamond counterpart, the glassy carbon rod exhibits a higher ammonia formation rate. The highest faradaic efficiency for ammonia synthesis using the boron-doped diamond can reach to 80%, but slowly decrease to 10% in the end [30]. Conclusively, the soluble Li3 N in chloride molten salt is critical for enhancing the N2 /N3− conversion, promising the ammonia formation with high rate and current efficiency. When different hydrogen sources are used, the anodic side reaction shows a

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huge difference, leading to completely different characteristics for ammonia formation. In addition, the electrochemical synthesis of ammonia in molten chloride salts is also influenced by electrode materials. It should be mentioned that the high cost and operational complexity limit the scalable utilization of Li3 N in practical application, hence it is necessary to explore economical and efficient intermediates or catalysts as alternatives. 3.2. Ammonia synthesis in hydroxide molten salt Compared with the molten chloride salts, the hydroxide molten salt can work in a much lower temperature (∼200 °C) [22] and is less corrosive [22,52]. The above merits promise the facile preparation of ammonia in hydroxide molten salts. However, unlike the chloride molten salts which is free of hydrogen-containing ions, OH− is the bulk component in hydroxide molten salts, therefore the concomitant H2 evolution reaction can occur very easily, leading to compromised faradaic efficiency for ammonia formation. Besides, hydroxide molten salts show negligible capability to dissolve Li3 N than that of molten chlorides, therefore N2 /N3− conversion in the former case is much more difficult, meaning a sluggish ammonia formation rate in hydroxide molten salt. Due to the absence of soluble Li3 N in hydroxide molten salt, extra catalysts are usually added to promote the N2 /N3− conversion. In addition, H2 evolution from OH− reduction (2OH− + 2e− = H2 + 2O2− ) and N3− formation from N2 cleavage (N2 + 6e− = 2N3− ) during electrolysis are two competing reactions on cathode. Therefore, research in hydroxide molten salts mainly focus on improving the activity of catalysts, exploring new mechanism for N2 /N3− conversion and optimizing electrolysis conditions for restraining H2 evolution, all aiming to increase ammonia formation rate and faradaic efficiency [53]. 3.2.1. Mechanisms for NH3 formation on catalyst surface The volcano plot of Fig. 5(a) reveals the activity comparison between various metals for ammonia formation. Mo and Fe on the top of the volcano diagrams possess the highest activity for ammonia formation among the numerous non-precious metal catalysts. The early transition metals such as Y, Ti and Zr on the left area of the volcano plot are more favorable to bind Nad , but the oxidation of the metal surface still limits their application [54]. Therefore, Fe-based catalysts are the most investigated candidates for ammonia synthesis in both industry and experimental research [5]. The cleavage of N2 on the hematite surface for ammonia formation is uncovered to undergo an associative pathway by the DFT (density functional theory) calculation [55]. Generally, two possible mechanisms, which mean dissociative and associative pathways as illustrated in Fig. 5(b), are suggested for ammonia formation on the heterogenous surface of catalysts [56,57]. The associative pathway is further divided into the distal pathway and the alternating pathway depending on whether two N atoms are simultaneously hydrogenated, as illustrated in Fig. 5(b) [57]. The Gibbs free energy change for each reaction steps was calculated by DFT method according to the following equations [55]:

G = E + DZPE–T S (without proton transfer)

(14)

G = E + DZPE–T S –eU (with proton transfer)

(15)

where E, ZPE, T, S, U are the internal energy change, zero point energy change, temperature, entropic energy change, and applied bias voltage, respectively. Based on the calculation results, the most difficult step is determined to be the transfer of the first proton to adsorbed nitrogen for formation of ∗ NNH intermediate, which requires an applied bias voltage of −1.84 V vs. NHE on

Fig. 5. (a) The volcano plot about the dissociative (solid lines) and associative (dashed lines) mechanisms on both flat (black) and stepped (red) transition metal surfaces [54] (Copyright 2012, Royal Society of Chemistry); (b) two reduction mechanisms for NRR to form NH3 on heterogeneous catalysts [57] (Copyright 2017, Elsevier).

Fig. 6. Free energy of intermediates relative to the total one of the reactants on a Fe–O3 –Fe– (a, c) and Fe–Fe–O3 – (b, d) surface calculated by MT. Nguyen et al. A,B and the number from 0 to 9 represent the reactant, product and intermediates, solid lines represent the energy favorable reaction paths [55] (Copyright 2015, Royal Society of Chemistry). The specific meanings of intermediates for 1–9 can be found in Ref. [55].

the Fe–O3 –Fe– surface or −1.14 V vs. NHE on Fe–Fe–O3 – surface, respectively (Fig. 6). Such a value (−1.14 V vs. NHE) is very close to the experimental bias voltage of 1.2 V [22], validating the reliability of results obtained by DFT calculations. Conclusively, the associative pathway of N2 for ammonia formation is suggested as

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Fig. 7. (a) Apparatus used by Licht et al. for the electrochemical ammonia synthesis in molten hydroxide electrolyte [57] (Copyright 2017, Elsevier); (b) electrolysis efficiency of ammonia synthesis from air/N2 and H2 O with the suspend of nano- or micron-sized Fe2 O3 [22] (Copyright 2014, American Association for the Advancement of Science); (c) yield rate and faradaic efficiency of ammonia production with the catalyst of Fe2 O3 /AC [52] (Copyright 2017, Royal Society of Chemistry); (d) ammonia formation rate varies with current density in Licht‘s works [22,58].

follows [55]:

∗ + N2 → ∗ N2 → ∗NNH → ∗NHNH → ∗NHNH2 (∗NH + NH3 ) → ∗NH2 + NH3 → ∗NH3 + NH3 → ∗ + 2NH3 where ∗ is a surface reactive site. Licht and co-workers further verified that metallic Fe is the real active phase catalyzing the ammonia formation in molten hydroxide electrolyte according to the following electrode reactions [22].

Cathode : Fe2 O3 +3H2 O(g ) + 6e− = 2Fe + 6OH−

(16)

Chemical : 2Fe + 3H2 O(g )+N2 (g ) = 2NH3 (g ) + Fe2 O3

(17)

Anode : 6OH− = 3/2O2 (g ) + 3H2 O(g ) + 6e−

(18)

Net : N2 (g ) + 3H2 O(g ) = 2NH3 (g ) + 3/2O2 (g )

(19)

Using Fe2 O3 nanoparticles as catalysts and H2 as hydrogen source, no ammonia is detected when no current is applied, implying that ammonia formation from the adsorbed H2 and N2 on the Fe2 O3 surface is ruled out. Instead, ammonia evolution is obviously enhanced when the electrolysis current is applied. The ammonia formation rate is much lower in the absence of Fe2 O3 catalysts when the same current is applied. The above phenomena imply that the electrons are transferred from the electrode surface to Fe2 O3 nanoparticles, with metallic Fe generated by reaction (14). Subsequently, ammonia is formed on the surface of catalysts by chemical reaction (15), with Fe2 O3 regenerated at the same time. 3.2.2. Catalysts modifications and electrolysis condition optimization Both the particle size and the surface composition of Fe2 O3 catalysts can significantly affect the ammonia formation. Licht and co-workers found that micro Fe2 O3 particles suspended in the molten salts show negligible current efficiency for ammonia formation (Fig. 7(a, b)) because of the blocking of active sites caused by descending of Fe2 O3 particles to the bottom of molten salts [52]. Instead, the nano Fe2 O3 particles can keep colloidal

in molten salt with fully exposure of active sites, therefore contributing to extraordinarily increase of current efficiency to ∼35%. The influence of surface composition on catalysts on ammonia formation is investigated by introducing activated carbon (AC) to the surface of Fe2 O3 (Fig. 7(c)). No ammonia was detected without catalysts or with only AC added, and the coulombic efficiency also remains very low for ammonia synthesis over bare Fe2 O3 powder. Conversely, the highest coulombic efficiency of 13.7% and the maximum ammonia formation rate of 8.27 × 10−9 mol s−1 cm−2 is achieved at 250 °C with a suspension of Fe2 O3 /AC catalyst in the molten hydroxide electrolyte. This was explained by the fact that the AC on the surface of Fe2 O3 catalyst can inhibit hydrogen evolution and improve reduction of iron oxide to promote ammonia synthesis. The competing reactions between ammonia formation and H2 evolution can be affected or even reversed by the electrolysis conditions, therefore greatly influencing the current efficiency for ammonia formation. When the current density is increased from 2 mA cm−2 to 200 mA cm−2 , the ammonia production rate is enhanced from 2 × 10−9 mol s−1 cm−2 to 1.0 × 10−8 mol s−1 cm−2 correspondingly, as shown in Fig. 7(d) [58]. However, the synchronized increase in electrolysis voltage also triggers enhanced H2 evolution at the same time, leading to obvious current efficiency decrease for ammonia formation (from 35% at 2 mA cm−2 to 1.45% at 200 mA cm−2 , Fig. 7(d)). Therefore, small current density is beneficial to ammonia formation with a high current efficiency at the expense of decreasing ammonia formation rate while large current efficiency can contribute to high ammonia formation rate at the expense of current efficiency loss. For example, the current efficiency can be adjusted to be as high as 71% for ammonia formation using an extremely low current density of 0.7 mA cm−2 , but with extremely low ammonia formation rate [58]. 3.2.3. Mechanism innovation for NH3 formation Compared with the chloride molten salt, the absence of soluble Li3 N in molten hydroxide restricts the enhancement of ammonia formation (Fig. 1(b)). Such a dilemma may be solved by introducing electrodeposition of metallic lithium (Li) during

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Fig. 8. (a) Schematic cross-section of the electrolysis cell and (b) the three steps of ammonia synthesis from N2 and H2 O using a lithium cycling electrification strategy [59] (Copyright 2017, Royal Society of Chemistry).

the electrochemical formation of ammonia from N2 and H2 O in hydroxide molten salt at ambient pressure [59], as illustrated in Fig. 8(a). The demonstrated process consists of LiOH electrolysis for Li deposition (reaction 20), chemical reaction between Li and N2 for Li3 N formation (reaction 21), and the exothermic release of NH3 from reaction between Li3 N and H2 O to regenerate LiOH (reaction 22), with anodic O2 simultaneously released (reaction 23). Consequently, the net reaction is shown in reaction (24), meaning that N2 and H2 O are converted to cathodic NH3 and anodic O2 , respectively. Such a cycling strategy (Fig. 8(b)) can circumvent the hydrogen evolution side reaction, enabling NH3 production with a coulombic efficiency high up to 88.5%. However, further optimization and better understanding of the reaction mechanisms remain to be further investigated.

Cathode : LiOH + e− = Li + OH−

(20)

Chemical reaction : 6Li + N2 (g ) = 2Li3 N

(21)

Chemical reaction : Li3 N + 3H2 O(g ) = 3LiOH + NH3 (g )

(22)

Anode : 4OH− = O2 (g ) + 4e− +2H2 O(g )

(23)

Overall : 2N2 (g ) + 6H2 O(g ) = 4NH3 (g )+ 3O2 (g )

(24)

In summary, ammonia synthesis in the hydroxide molten salts can be conducted in very low temperatures (<300 °C), promising facile preparation conditions. The ammonia formation routes, catalyst modifications and mechanism innovations are explored in detail. However, the ammonia formation rate and current efficiency still need to be greatly improved in the hydroxide molten salts. 3.3. Ammonia synthesis in the composite electrolytes Solid oxide electrolytes possess unique merits that electrolytes in liquid state are short of [60]. Solid oxide electrolytes are free of electrolyte leakage risk, therefore being of high safety. In addition, transportation and storage of solid oxide electrolytes are much more convenient than that of electrolytes in liquid state. Besides, the shape of solid oxide electrolytes can be custom-made, satisfying the demand for the design of electrolysis cell with special structure. However, the working temperatures of solid oxide electrolytes are very high, mostly exceeding 600 °C (Fig. 1(c)), leading to a high energy consumption for ammonia synthesis. What is worse, ions diffusion in solid oxide electrolytes is much slower than that in liquid-state electrolytes, leading to very sluggish reaction dynamics for ammonia formation. The composite electrolytes which consist of solid oxide electrolytes and molten salt show potential for overcoming the above shortages. As illustrated in Fig. 9(a–c), using the porous solid oxide

electrolyte as container (Fig. 9(a)), the liquid-state molten salt can be encapsulated inside the solid oxide electrolyte (Fig. 9(b)), combining the advantages of both solid oxide electrolytes (such as high safety) and molten salt electrolyte (such as superior ions transportation ability). After electrolysis for 2 h, the morphology and electrode-interfaces suffer from negligible changes (Fig. 9(c)), showing a high stability of the composite electrolyte. Besides, the solid oxide electrolyte can act as a membrane to selectively conduct oxygen ions (Fig. 9(d)) or proton (Fig. 9(e)), which means that anodic gas (such as O2 ) and cathodic gas (such as NH3 or H2 ) can be in-situ separated, facilitating the instantaneous collection of electrolysis products. The most adopted solid oxide electrolyte in the composite electrolyte is LiAlO2 and ceria-based oxides. Using the LiAlO2 containing 50 wt% alkaline metal ternary carbonate molten salt (Li2 CO3 –Na2 CO3 –K2 CO3 , LiNaKCO3 ) as electrolyte, Amar and coworkers investigated the influence of various catalysts on the ammonia formation rate and faradaic efficiency [31,64,65], revealing that Co3 Mo3 N is the best candidate among three investigated catalysts (CoFe2 O4 , Fe3 Mo3 N and Co3 Mo3 N). Using the Co3 Mo3 N as catalysts and LiAlO2 -50 wt% LiNaKCO3 as composite electrolyte, the maximum faradaic efficiency (3.83%) for ammonia formation can be obtained at 450 °C and 0.4 V, however, with the ammonia formation rate being only 0.75 × 10−10 mol s−1 cm−2 . Although the ammonia formation rate can be further increased to 3.27 × 10−10 mol s−1 cm−2 by increasing the voltage to 0.8 V, the faradaic efficiency suffers from obvious loss due to enhanced H2 evolution reaction at high voltages. Ceria-based oxides containing LiNaKCO3 molten salt is another widely-investigated composite electrolyte system for ammonia formation [25,33,66]. Unlike the LiAlO2 solid oxide electrolyte, ceria possesses the ability to selectively conduct O2− (as illustrated in Fig. 9(d)), which can be greatly promoted by doping other metallic ions into ceria. Gd, Ca, Sm and Sr are the mostly investigated elements for preparation of single doped ceria composite electrolytes (SDC-carbonate), such as Ce0.8 Sm0.2 O2−δ –(Li/Na/K)2 CO3 . Compared with single doped ceria, co-doped ceria, such as the Ca and Gd co-doped ceria (CGDC), shows better performance in ionic conductivity [61,62]. Another factor affecting the performance for ammonia formation in the SDC-carbonate system is the usage of catalysts. When the La0.6 Sr0.4 Fe0.8 Cu0.2 O3− δ catalysts was introduced into the Ce0.8 Sm0.2 O2− δ (Li/Na/K)2 CO3 (70:30 wt%) composite electrolyte, the electrochemical formation rate of ammonia from wet nitrogen (N2 and H2 O) can be increased by an order of magnitude to 5.39 × 10−9 mol s−1 cm−2 at 450 °C and 0.8 V [67], revealing the important role of catalysts in the SDC-carbonate system. Various catalysts are investigated in detail for the SDC-carbonate system, including Pr0.6 Ba0.4 Fe0.8 Cu0.2 O3−δ (PBFCu) [25], La0.8 Cs0.2 Fe0.8 Ni0.2 O3−δ (LCFN) [33], Sm0.6 Ba0.4 Fe0.8 Cu0.2 O3−δ

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203

Fig. 9. (a) The catalyst of LSCrF after calcinations in air at 1300 °C [61] (Copyright 2017, Elsevier); A Cross-sectional view of the composite electrolyte (CFO–CGDC) and electrodes (CoFe2 O4 ) before (b) and after (c) electrolysis [62] (Copyright 2014, Elsevier); schematic diagram of the ammonia synthesis in composite electrolytes in the (d) oxygen ion or (e) proton conducting reactor cell [63] (Copyright 2017, Elsevier).

Table 2. Experimental materials and results for ammonia formation in different molten salt electrolytes. Electrolyte

Reactants

Cathode

Anode

T (°C)

QE (%)

Rate (×10−9 mol s−1 cm−1 )

LiCl–KCl–CsCl (0.5%Li3 N)

N2 /H2

Ni

400

72

N2 /H2 O

Porous Ni Al plate Porous Ni

300

23 10–80

[17] [50] [32] [30]

N2 /H2 O

Porous Ni

Glassy carbon rod Baron-doped diamond Li–Al alloy

3.33 16.70 20 5.80

327

0.17

0.30

[47]

N2 /H2 O

Monel alloy

Ni

200

Ni Stainless steel Co3 Mo3 N–Ag (LSFCu)–SDC PBFCu–CGO LSCrF–CGDC

Ni Ag–Pd Ni-SDC PBFCu–CGO SSCO–CGDC

35 71 9.3 4.91 3.0 7.5 4.8 3.87

2.40 16 0.65 8.27 0.33 5.39 0.18 0.40

[22] [58] [71] [52] [31] [67] [25] [70]

LiCl–KCl–CsCl (Nano-Fe2 O3 ) NaOH–KOH (Nano-Fe2 O3 )

NaOH–KOH (Fe2 O3 /AC) LiAlO2 –(LiNaK–CO3 ) SDC–(LiNaK–CO3 ) CGO–(LiNaK–CO3 ) CGDC–(LiNaK–CO3 )

N2 /H2 N2 /H2 O

210 250 450 400 375

Ref.

Notes: LiNaK–CO3 ((Li/Na/K)2 CO3 ); SDC (Ce0.8 Sm0.2 O2− δ ); CGO (Ce0.8 Gd0.2 O2− δ ); CGDC (Ce0.8 Gd0.18 Ca0.02 O3− δ ); LSFCu (La0.6 Sr0.4 Fe0.8 Cu0.2 O3− δ ); PBFCu (Pr0.6 Ba0.4 Fe0.8 Cu0.2 O3− δ ); LSCrF (La0.75 Sr0.25 Cr0.5 Fe0.5 O3− δ ); SSCO (Sm0.5 Sr0.5 CoO3− δ ). QE means the faradaic efficiency.

(SBFCu) [66], CoFe2 O4 [62], La0.6 Sr0.4 Fe0.8 Cu0.2 O3−δ (LSFCu) [68], La0.6 Sr0.4 FeO3−δ (LSF) [69], La0.75 Sr0.25 Cr0.5 Fe0.5 O3−δ (LSCrF) [70], and La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF) [61]. Among these catalysts, PBFCu and LSCrF are the two desirable candidates, which promises a maximum ammonia formation rate of 1.83 × 10−10 mol s−1 cm−2 and 4 × 10−10 mol s−1 cm−2 , respectively, correspondingly with the faradaic efficiencies being 4.8% and 3.87% [25,70]. Three different electrolyte systems which means chloride molten salt, hydroxide molten salt and composite electrolyte are discussed. The ammonia formation rate and faradaic efficiency are compared in different molten salt electrolytes for ammonia formation, as shown in Table 2. Benefiting from a high solubility of Li3 N, both the N3− diffusion and electrode reactions are very swift in chloride molten salts, contributing to appealing values for both the ammonia formation rate and faradaic efficiency. Hydroxide molten sat electrolyte can work in much lower temperatures, however, the severe H2 evolution side reaction in the OH− -containing hydroxide molten salts leads to very low faradaic efficiency and ammonia formation rate. Composite electrolyte integrates the merits of solid oxide electrolyte and molten salt electrolyte, but the performance

for ammonia formation still needs to be substantially improved when compared with the other two electrolyte types. Further development for electrochemical synthesis of ammonia in molten salt are expected according to the following aspects. First, high ammonia formation rate can be realized with the aid of soluble Li3 N in molten chlorides. However, Li3 N is unstable in air and explosive, leading to knotty problems in storage and transportation. Therefore, exploration of metal nitrides with high stability and solubility in molten salts to replace the Li3 N is very promising. Secondly, catalysts with high activity for N2 -splitting while possessing negligible activity for H2 production should be designed. The side reaction of H2 evolution is still a huge problem for ammonia synthesis in molten salt, especially when a H2 O hydrogen source is used. Currently, catalysts designs are mainly concentrated on increasing the activity for electro-splitting of N2 , ignoring the intrinsic activity of catalysts for H2 production. To obtain both high ammonia formation rate and high current efficiency, catalysts with high activity for N2 -splitting and low activity for H2 evolution at the same time is desired. Thirdly, the low ion conductivity of solid electrolyte is a main reason for the

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Fig. 10. (a) Schematic of the chemical looping process for ammonia synthesis; (b) comparison of chemical looping and thermo-catalytic processes in ammonia synthesis rate [75] (Copyright 2018, Springer Nature).

low ammonia formation rate when the composite electrolyte is adopted. Therefore, solid electrolyte with higher ion conductivity should be explored in the future.

restricted. [79]. In Chen’s chemical looping route, activation of N2 and subsequent hydrogenation processes are separately fulfilled on the metals/compounds and alkaline/alkaline earth hydrides, respectively (Fig. 10(a)), contributing to the energy of hydrogenation steps less dependent on the active sites for N2 activation. Therefore, the relationship between activation of N2 and binding of −NHx are weakened. Such an elaborately designed route breaks the scaling relations, contributing to remarkable increase in ammonia formation rates by an order of magnitude when compared with conventional thermo-catalytic process (Fig. 10(b)). The general comparison between the molten salt electrolysis strategy, chemical looping method and traditional Haber–Bosch process are presented in Table 3. Both molten salt electrolysis and chemical looping processes produce ammonia at moderate temperature and ambient pressure, thereby cutting away the energy consumed by the high pressure. Furthermore, the chemical looping routes use metal hydride-metal imide pairs to break the scaling relation and demonstrate excellent formation rates. However, the sectional and periodic exposure of the catalysts to nitrogen and hydrogen also complicate the operation and increase the application costs. Another practical question is the direct utilization of hydrogen gas, which is still derived from expensive and intensive production process. When water is used as the hydrogen source, high temperature is required to obtain an appropriate production rate. In the molten salt electrolysis process, the ammonia synthesis from water and nitrogen has been extensively investigated and achieved some positive results. Besides, the electrochemical ammonia formation rate in molten salts is in the order of 10−9 to 10−8 mol s−1 cm−2 , roughly close to the industrial level (10−7 mol s−1 cm−2 ) [79]. However, the faradaic efficiency and ammonia production rate need to be substantially improved in the future.

4. Comparison with other processes and scaling-up exploration

4.2. Preliminary explorations on scaling-up synthesis of ammonia

4.1. Comparison with thermochemical processes

Scaling-up synthesis of ammonia by molten salt electrolysis is highly possible. Compared with other emerging routes for ammonia synthesis, the relatively high formation rate and faradaic efficiency during electrochemical synthesis of ammonia from molten salt systems make its industrial application economically feasible [19,21]. In addition, the merits of high ion conductivity, strong dissolving capacity, physical and chemical stability for molten salt are also beneficial to the practical scaling-up process. Moreover, the technical maturity of molten salt electrolysis, such as the successful industrial production of primary aluminum for more than 100 million tons annually by molten salt electrolysis [81], also makes the scaling-up of electrochemical synthesis of ammonia in molten salt promising. Ammonia formation rate and energy consumption are the most critical factors in industrial production activities. As illustrated in Table 1, the ammonia formation rate in molten chlorides outperforms that in both hydroxide molten salt and composite electrolyte. Therefore, scaling-up explorations on electrochemical synthesis of ammonia from H2 O and N2 are mainly conducted in chloride molten salt electrolyte [82,83]. The key factor to decrease the energy consumption for ammonia formation by molten salt electrolysis is to make the cell voltage as low as possible. Constant current electrolysis is commonly implemented in industrial molten salt electrolyzer, therefore the energy consumption for electrochemical synthesis of ammonia in molten salt is largely determined by the cell voltage. The

Although almost all ammonia in industry is produced by the Haber–Bosch process, endeavors struggling to explore alternative routes never stop because of the harsh conditions (30 0–50 0 °C and 15–30 MPa) and serious CO2 emissions for the Haber–Bosch process (1.9 metric tons of CO2 per metric ton of NH3 produced). Almost 15% of the energy consumption was caused by the sophisticated high-pressure operations [72]. Among various investigated alternatives [73], the electrochemical and chemical looping processes at ambient pressure are one of the most promising processes [74]. Inspired by the ideas of alkali and alkaline earth metal hydrides in hydrogen storage, Chen and colleagues designed a novel chemical looping process for ammonia formation [74–78]. As illustrated in Fig. 10(a), the chemical looping is carried out in two steps. The alkaline earth metal hydrides (AHx in Fig. 10(a)) first react with nitrogen gas to form the alkaline earth metal amide (A2/ x NH in Fig. 10(a)), followed by reaction of the metal amide with hydrogen gas to regenerate the original metal and release ammonia to complete the cycle. To promote ammonia formation under ambient temperature and pressure, catalysts with strong activation to N2 (Eact ) and weak binding to −NHx (E) are desired. However, a linear relationship between Eact and E (kinetic scaling relations) makes the independently optimization of Eact and E highly Table 3. Comparison of different ammonia synthesis processes. Processes

Advantages

Disadvantages

Ref.

Haber–Bosch Chemical looping Molten salt electrolysis

Mature technique Break scaling relation High production rate Ambient pressure Clean energy Ideal hydrogen source (H2 O) Ambient pressure

High energy consumption Low efficiency High pressure Process complexity High temperature (use H2 O) Limited faradaic efficiency Unsatisfied production rate

[7,8,80] [75–78] [22–25]

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Fig. 11. (a) The advanced-type electrolytic cell as proposed by Ito [84] (Copyright 2012, The Electrochemical Society); (b) schematic explaining the required voltage decrease of the advanced-type electrolytic cell [82] (Copyright 2016, Royal Society of Chemistry).

theoretical cell voltage needed for ammonia synthesis form water and nitrogen gas (N2 + 3H2 O = 2NH3 + 3/2O2 ) is 1.17 V at 327 °C (the commonly used temperature in molten chloride). However, the inevitable polarizations and side reactions make the actually needed minimum cell voltage as high as 2.0 V [82], leading to increase in energy consumption [84]. Ito and co-workers designed an advanced-type reactor consisting of two cells with a hydrogen permeable metal membrane between them (Fig. 11(a)) [82,83], reducing the required electrolytic potential below 1.5 V. According to the cathodic and anodic reactions as shown in Fig. 11(b), the required voltage for electrolysis is calculated to be 2.0 V. When a hydrogen permeable metal membrane was used to separate the cells into molten chloride electrolyte and molten hydroxide electrolyte, the cathodic reactions on the left side was actually converted to the reaction of water electrolysis (Fig. 11(c)), the voltage caused by the spontaneous ammonia synthesis reaction (3H2 O + 2N3− = 2NH3 + 3O2− ) is recovered, contributing to great decrease in overall cell voltage. Such a pioneering work makes the scaling-up synthesis of ammonia with a low energy consumption possible. Certainly, this only stays in concept and needs to be proved in the future. A 10 A-scale experiment for electrochemical ammonia synthesis in molten salt is successfully verified by the R&D (Fig. 12(a)) [85], further validating the feasibility of scaling-up for ammonia synthesis by molten salt electrolysis. The comparison of two ammonia synthesis processes is shown in Fig. 12(b). The energy

consumption for ammonia synthesis by molten salt electrolysis is 34.5 GJ/t-NH3 , which is comparable to the value for the conventional Haber–Bosch process (30–35 GJ/t-NH3 ) [83,85]. It should be mentioned that extra steps are needed for production of H2 used in the conventional Haber–Bosch process, leading to unfavorable CO2 emissions. For the ammonia synthesis by molten salt electrolysis in R&D’s experiment, the easily available and environmentally friendly H2 O is adopted as the hydrogen source, therefore contributing to a short process and the reduction of CO2 emissions. 5. Conclusions and perspectives In this review, the hydrogen source selection, the ammonia formation in various molten salt systems, and the preliminary explorations in scaling-up electrochemical synthesis of ammonia in molten salt are discussed in detail. H2 O is the ideal hydrogen source to replace H2 for electrochemical formation of ammonia in molten salts. Chloride molten salts containing soluble Li3 N can contribute to both high formation rate and desirable faradaic efficiency for ammonia synthesis. Preliminary exploration on a 10 A scale trial demonstrates comparable energy consumption for ammonia synthesis with that of industrial Haber–Bosch process. Although the electrode reactions, molten salt compositions and the catalyst optimizations have been extensively investigated, the side reactions such as concomitant H2 evolution on cathode are still very hard to be avoided, leading to undesirable faradaic

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Fig. 12. (a) Photos of a 10 A-scale equipment for the further R&D experiments; (b) the energy consumption of the two routes for ammonia synthesis from water and nitrogen [83] (Copyright 2018, Electrochemical Society of Japan).

efficiency and the ammonia formation rate. In addition, the reaction mechanism for cleavage of N2 on the three phase interfaces (electrode-molten salt-gaseous N2 ) should be better understood in the future. It is presumed that electro-catalysts with welldefined composition and microstructure are the key for further enhancing the ammonia formation rate and faradaic efficiency by restraining the side reactions on electrode. Specially, the amount of electrochemically synthesized ammonia is usually very small that it cannot be firmly attributed to the electrochemical nitrogen fixation rather than contaminants from nitrogen-containing compounds such as nitrates, nitrogen oxides, and nitrides et al. Therefore, methods such as quantitative isotope measurements for more accurate quantitation of electrochemically synthesized ammonia is very essential in future experiments [86]. Moreover, the operando spectroscopy and microscopy techniques including X-ray diffraction, mass spectroscopy, nuclear magnetic resonance (NMR) et al. are essential to better elucidating the reaction mechanism for N2 cleavage on the electrode surface in molten salts. Acknowledgments The authors acknowledge the funding support from the National Natural Science Foundation of China (51722404, 51674177, 51804221 and 91845113), the National Key R&D Program of China (2018YFE0201703) and the China Postdoctoral Science Foundation (2018M642906 and 2019T120684). References [1] J.H. Montoya, C. Tsai, A. Vojvodic, J.K. Norskov, ChemSusChem 8 (2015) 2180–2186. [2] A.F.S. Molouk, J. Yang, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, J. Power Sources 305 (2016) 72–79. [3] C.H. Christensen, T. Johannessen, R.Z. Sørensen, J.K. Nørskov, Catal. Today 111 (2006) 140–144. [4] H. Liu, Chinese J. Catal. 35 (2014) 1619–1640. [5] X.F. Li, Q.K. Li, J. Cheng, L. Liu, Q. Yan, Y. Wu, X.H. Zhang, Z.Y. Wang, Q. Qiu, Y. Luo, J. Am. Chem. Soc. 138 (2016) 8706–8709.

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