Synthesis of ammonia directly from wet nitrogen using a redox stable

Ammonia was directly synthesised from wet nitrogen at intermediate temperature (375-425 °C) based on the oxygen-ion conduction of the Ce 0.8 Gd 0.18 Ca 0.02 O 2- (cid:303) -(Li/Na/K) 2 CO 3 ) composite electrolyte. A redox stable perovskite-based catalyst, La 0.75 Sr 0.25 Cr 0.5 Fe 0.5 O 3- (cid:303) (LSCrF), was synthesised via a combined EDTA-citrate complexing sol-gel process to be used as a component of the La 0.75 Sr 0.25 Cr 0.5 Fe 0.5 O 3- (cid:303) - Ce 0.8 Gd 0.18 Ca 0.02 O 2- (cid:303) composite cathode for ammonia synthesis. Ammonia formation was studied at 375, 400 and 425 °C and the maximum ammonia formation rate of 4.0×10 -10 mol s -1 cm -2 with corresponding Faradaic efficiency of 3.87 % was observed at 375 ºC when applied voltage was 1.4 V. This is much higher than the 7.0  10 -11 mol s -1 cm -2 at 1.4V and 400 °C when Cr-free Sr-doped LaFeO 3-  , La 0.6 Sr 0.4 FeO 3-  was used as the catalysts for electrochemical synthesis of ammonia, indicating LSCrF is potentially a better catalyst. Ammonia was successfully synthesised using a redox stable cathode with higher formation rates at reduced temperature. Introduction of Cr 3+ ions at the B-site of doped LaFeO 3 improves both chemical stability and catalytic activity for ammonia synthesis.


Introduction
Ammonia is the second most produced chemical in large quantity in the world. It is not only an end product but also an important intermediate in the manufacture many chemicals including; urea, nitric acid, ammonium nitrate, ammonium sulphate and ammonium phosphate. 1 In 2011, approximately 136 million metric tons of ammonia was produced of which ~ 80 % is consumed in fertiliser industry. 1,2 Currently, ammonia is produced on large-scale via the Haber-Bosch process which is developed in the early 1900s. This process suffers from many drawbacks including; the low ammonia conversion (10-15%), high energy consumption, operating at high temperature (~ 500 °C) and high pressure (150-300 bar) and severe environmental pollution (CO 2 emission). 1 Therefore, to avoid the Haber's process thermodynamic limitations (limited conversion) and to reduce the CO 2 emission, alternative ammonia synthesis approaches have been proposed. In 1996, Panagos et al. 3 proposed a model process using a solid state proton conductor to overcome the thermodynamic constraints of the traditional ammonia synthesis process. In 1998, Marnellos and Stoukides 4 confirmed the first experimental ammonia synthesis from its constituents (H 2 and N 2 ) at atmospheric pressure using an electrochemical cell based on proton-conducting electrolyte SrCe 0.95 Yb 0.05 O 3-(SCYb). 4 In the literature, several solid state proton conductors have been used as electrolytes for synthesising ammonia electrochemically. [5][6][7][8][9][10] In these mentioned reports, pure H 2 was used as source of the required protons + (H ) for ammonia synthesis. However, there are some problems associated with using H 2 as one of the precursors including; the production, purification, storage and transportation of hydrogen. 11,12 On other hand, water can be used as an ideal proton source. In 2009, Skodra and Stoukides 13 reported the synthesis of ammonia for first time directly from H 2 O and N 2 without the need for hydrogen production stage. In that study, either solid oxide protonic or oxygen ion conductors was used as an electrolyte and Ru-based catalyst was used as a working electrode (cathode). Ammonia was produced under atmospheric pressure with a maximum formation rate of ~ 4 × 10 -13 mol s -1 cm -2 at 650 ºC at 2 V. Recently, ammonia has been synthesised directly from H 2 O and N 2 using an electrolytic cell based on CoFe 2 O 4 -CGDC composite as a cathode and doped ceria-carbonate composite as 2 O  conducting electrolyte 14 . In that study, the ammonia production rate of 6.5 ×10 -11 mol s -1 cm -2 was obtained at 400 ºC and 1.6 V.
The principle electrochemical synthesis of ammonia from water and N 2 based on oxide-ion 2 (O )  conducting electrolytes can be written as follows, 13 At the cathode, At the anode, the transported oxygen ions through the electrolyte will combine to form oxygen gas; The overall reaction will be; Under the circumstance, if wet nitrogen is fed in the cathode in a two chamber cell, when a dc voltage is applied to the cell, ammonia will be produced at the cathode, oxygen at the anode.
However, water splitting reaction is another completing reaction at the cathode, The produced  2 O ions will transport to the anode, releasing oxygen according to reaction (2). The overall reaction for water splitting reaction is, Therefore the ammonia synthesis and water splitting processes are competing with each other. Both ammonia and hydrogen can be produced at the cathode, depending on the catalytic activity of the cathode catalysts. Very important, the produced hydrogen at the cathode may further reaction with the catalysts, causing degradation of catalytic activity.
Therefore an ideal ammonia synthesis catalyst should be redox stable which can sustain the highly reducing atmosphere in the presence of hydrogen at high temperatures.
The perovskite-based oxides are of interest owing to their ease of synthesis, low manufacture cost, high thermal stability and good catalytic activity. 15 These oxides have been used as electrodes in many applications such as solid oxide fuel cells (SOFCs), [16][17][18][19] solid oxide steam electrolysis cells (SOECs) [20][21][22] and electrochemical synthesis of ammonia. 23

Materials Characterisation
X-ray diffraction (XRD) data were collected at room temperature using a Panalytical X'Pert Pro diffractometer with Ni-filtered CuK radiation ( =1.5405 Å), using 40 kV and 40 mA, fitted with a X'Celerator detector. Absolute scans were recorded in the 2 range 5-100º, with a step size of 0.0167º.
The microstructures of the prepared catalyst and the cross-sectional area of the single cell were examined using a Hitachi SU6600 Scanning Electron Microscope (SEM).
Thermogravimetry and differential scanning calorimetry (TGA/DSC) analyses were performed using a Stanton Redcroft STA/TGH series STA 1500, operating through a Rheometric Scientific system interface controlled by the software RSI Orchestrator. The thermal behaviour of the perovskite based cathode (LSCrF) was investigated in N 2 atmosphere from room temperature to 500 ºC with a heating/cooling rate of 10 ºC/min.

Fabrication of the single cell for ammonia synthesis
A tri-layer single cell was fabricated by a cost-effective, one-step, dry-pressing method. The composite anode was prepared by mixing in a mortar SSCo, CGDC and a pore former

XRD, SEM and thermal analysis
The XRD patterns of LSCrF powder calcined in air at different temperatures is shown in Fig. . As can be seen, a single-phase perovskite oxide of LSCrF was obtained when the corresponding ash was fired at 1300 °C for 2 h (Fig. c). Below 1300°C, a small amount of second phase SrCrO 4 (JCPDS card no 35-734) was detected 32 . The crystallite size of LSCrF is about 46.75 nm, estimated from Sherrer's formula. In order to investigate the compatibility between the CGDC and the perovskite oxide (LSCrF), the composite cathode (LSCrF-CGDC) was fired in air at 700 °C which is the sintering temperature for the single cell. As can be seen from Fig. , the XRD pattern of LSCrF-CGDC (Fig. c) displays only the corresponding peaks for CGDC (Fig. a) and LSCrF (Fig. b), no extra peaks were detected indicating that LSCrF is chemically compatible with CGDC at the single cell sintering temperature.
The SEM micrograph of the LSCrF powder calcined in air at 1300 ºC for 2 h is shown in Fig. a. As can be seen, the microstructure of LSCrF powder morphology is characterised by sphere-type particles with a slight agglomeration. The thermal behaviour of LSCrF cathode was investigated under N 2 , as the cathode is exposed to this atmosphere during the ammonia synthesis. The TGA-DSC curves of LSCrF catalyst in N 2 atmosphere from room temperature up to 500 °C are shown in Fig. . As can be seen, a slight (~ 0.26 %) weight gain was observed which is due to the buoyancy effect of air.
The DSC curve shows no obvious thermal effects, indicating that there are no first order phase transitions. Sample decomposition or reaction between this perovskite-based cathode and N 2 in the measured temperature range is unlikely. This suggests that LSCrF cathode is thermally stable in N 2 within the measured temperature range. represent the polarisation resistance while CPE is a constant phase element. It can be also seen from Fig. a, with increasing the cell operating temperature, the Rs which is mainly related to the ohmic resistance of the electrolyte decreased significantly due to the melting of (Li,Na,K) 2 CO 3 carbonates. In addition, the total polarisation resistance, Rp (R1 + R2) decreased significantly with increasing the operating temperature due to enhanced catalytic activity of the electrode at evaluated temperatures. to 425 ºC. The ammonia production rates dropped significantly as the operating temperature increased from 375 to 425 ºC. Furthermore, the maximum ammonia formation rate was 4.0×10 -10 mol s -1 cm -2 at 375 ºC at current density of 2.99 mA/cm 2 . The corresponding Faradaic efficiency was 3.87 %. 26 This low efficiency indicates that there is more than one process over the cathode surface and that the competitive hydrogen evolution reaction (HER) is the dominant one. 33,34 This decrease in the ammonia formation rate with temperature, although the electrolyte ionic conductivity increases with temperature could be due the ammonia decomposition which becomes predominant at high temperature. 35,36 Therefore the ammonia synthesis at higher temperature was not carried out. This experiment confirms that low temperature will benefit ammonia formation during electrochemical synthesis process.
Therefore we fix the operating temperature to 375 °C in the following study. O ions is also higher. The two opposite effects will determine the current density. Therefore lowest current was observed at an applied voltage of 1.2 V (Fig. 8). The normalised final current density of the cell are 4.13, 2.88. 3.39 and 3.32 mA/cm 2 respectively. The corresponding resistances are 0.29, 0.49, 0.47 and 0.54 k cm 2 respectively. The total resistance tends to increase at higher applied voltages, indirectly confirming the blocking effect. This phenomenon was also observed in electrochemical synthesis of ammonia when a H + /Li + /NH 4 + mixed conducting membrane was used as the electrolyte. 38 The ammonia formation rates and corresponding Faradaic efficiency of the cell at 375 °C with different applied voltages are shown in Fig. 9. Significant increase in ammonia formation rate was observed when applied voltage increased from 1.2 to 1.4 V. When the electrolytic cell operated at a voltage higher than 1.4 V, the ammonia formation rate dropped significantly which could be due to the competitive adsorption between the N 2 and H 2 over the cathode surface 33,34 . The maximum ammonia formation rate was 4.0×10 -10 mol s -1 cm -2 at 1.4 V. An ammonia formation rate of 7.010 -11 mol s -1 cm -2 was observed for Cr-free Srdoped LaFeO 3- , La 0.6 Sr 0.4 FeO 3- at 1.4 V and 400 °C. 28 The ammonia formation rate is also higher than the 5 × 10 -11 and 1.2310 -10 mol s -1 cm -2 at 1.4V and 400 °C when La 0.6 Sr 0.4 Fe 0.8 Cu 0.2 O 3-and La0.8Cs0.2Fe0.8Ni0.2O3- was used as the cathode catalyst respectively for electrochemical synthesis of ammonia indicating LSCrF is potentially a better catalyst. 26,40 The low ammonia production rate with the low current efficiencies (< 4 %) mean that there was more than one process occurring over the cathode surface and the hydrogen evolution is the dominant one 8,34 . Introduction of Cr 3+ ions at the B-site in doped LaFeO 3 not only improve the stability in a reducing atmosphere, but also enhance the catalytic activity for ammonia synthesis. Cr 3+ ions are potential promoter for ammonia synthesis catalysts based on Fe-containing perovskite oxides.