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Figure 4
(A) Current density of an Air Pt | Nafion 211 | Pt, H2 cell under different applied voltages. Cathode was supplied with air, anode was supplied with H2. (B) The ammonia formation rate at air and H2 sides, total ammonia formation rate and Faraday efficiency. ...
Synthesis of ammonia directly from H2O and air
Water is the most abundant source for hydrogen. It would be a better choice if we can directly synthesise ammonia from air and water bypassing the hydrogen production stage. Therefore H2 at the anode was replaced by water. At 1.2 V, the current density of the cell was 47 mA cm−2, lower than that for the H2/N2 cell, possibly due to the high electrode polarisation at the water side (Fig. 5A). The ammonia formation rates increase at higher cell voltage (Figs. 5B). The ammonia formation rate is slightly lower than that when H2 was fed at the anode. The highest ammonia formation rate was observed at low applied voltage when H2 was used at anode (Figs. 3B & 4B); however, when water was supplied at the anode, the ammonia formation rate increased against applied voltage (Fig. 5B). When a dc voltage was applied to the cell, hydrogen was pumped to the cathode through transfer of protons in the electrolyte membrane. At high applied voltage, the high hydrogen flow rate may limit the lifetime of hydrogen species at the electrode/electrolyte/gas interfaces therefore the ammonia formation rate is relatively lower (Fig. 5C). In conclusion, for the first time, this experiment clearly indicates that ammonia can be directly synthesised from air and water at room temperature and one atmosphere.
Figure 5
(A) Current density of an Air, Pt | Nafion 211 | Pt, H2O cell under different applied voltages. Cathode was supplied with air, anode was supplied with H2O. (B) The ammonia formation rate at air and H2O sides, total ammonia formation rate and Faraday efficiency. ...
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Discussion
In most reports, H2 and N2 were commonly used as precursors for electrochemical synthesis of ammonia while H2 production and N2 separation are essential4. H2 production can be bypassed if H2O was used as a precursor; however, the reaction between H2O and N2 to form ammonia is thermodynamically non-spontaneous under normally pressure (Fig. 2B); however, this can be achieved through electrochemical process because the applied voltage provides extra driving force. Although there are a few reports on electrochemical synthesis of ammonia from N2 and H2O at 570°C15 or 300°C22,23, the formed ammonia tends to decompose to N2 and H2 because thermodynamically the decompositiontemperature of ammonia is around 175°C (Fig. 2A). Therefore, a synthesis temperature below 175°C is required in order to avoid decomposition of formed ammonia.
In order to demonstrate that the produced ammonia is from the electrochemical process, the maximum amount of dissolved ammonia has been estimated. It has been reported that the maximum H2O uptake of Nafion 117 membrane was 20.64 vol% at room temperature, investigated by small angle neutron scattering technology39. It is assumed that Nafion 211 membrane would exhibit similar behaviour. The maximum dissolved ammonia in the absorbed water in the used Nafion membrane was estimated to be 8.02 × 10−6 mol (please see supplement information) which is smaller than the generated ammonia in the first experiment, from H2 and N2 while applied at 0.2 V for 1 hour (1.13 × 10−5 mol, Fig. 3C). This value is also higher than the formed ammonia from possible decomposition of NH4+-form Nafion (9.14 × 10−6 mol). The total ammonia from decomposition of NH4+-form Nafion and dissolved NH3 in absorbed water is 1.72 × 10−5 mol which is significantly smaller than the total measured ammonia 7.15 × 10−5 mol (Figs. 3C, ,4C4C and and5C).5C). It should be noted that the NH4+-form Nafion membrane was washed by de-ionisedwater until no ammonia was detected in outlets of the cell. Considering the amounts of generated ammonia in experiments, it is clear that the collected ammonia cannot be from the dissolved ammonia from absorbed water in or the decomposition of ammonium Nafion membrane.