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It is well known that some higher plants can synthesise ammonia or its derivatives directly from air and water at room temperature33,34. The ammonia produced by plants is normally directly used as fertiliser by the plants. To the best of our knowledge, there is no report on artificial synthesis of ammonia direct from air and water. It has been a dream for researchers who can imitate this natural process to synthesise ammonia under similar conditions. In this report , for the first time, we demonstrated that ammonia can be synthesised directly from air (instead of N2) and H2O (instead of H2) under a mild condition (room temperature, one atmosphere) with supplied electricity which can be obtained from renewable resources such as solar, wind or marine.
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Results
Proton conduction of mixed NH4+/H+ conducting Nafion 211 membrane
Ammonia is a base, the acidic Nafion membrane readily reacts with ammonia to form NH4+-form Nafion. In our experiments, for the first time, it was demonstrated that NH4+-form Nafion exhibits proton conduction through concentration cell measurements. A mixed NH4+/H+ conducting Nafion 211 membrane was used as solid electrolyte for electrochemical synthesis of ammonia. Conventional H+-form Nafion membrane will be converted to NH4+-form in the presence of ammonia. It is necessary to maintain the stability of the membrane electrolyte under the synthesis conditions35. It has been reported that the ionic conductivity of NH4+-form Nafion is very much dependent on the humidity and reached ~0.05 S/cm at 80°C with a relative humidity 100%35. The high ionic conductivity is believed due to NH4+ ions but proton conduction cannot be ruled out. It has been reported that some inorganic ammonium salts such as (NH4)3H(SeO4)2 exhibit proton conduction36,37. Therefore the NH4+-form Nafion may exhibit a certain level of proton conduction which can be used for continuous synthesis of ammonia.
The membrane electrode assembly (MEA) for synthesis of ammonia was fabricated by a process described in the experimental part. The H+-form Nafion 211 membrane was converted into NH4+-form Nafion through the reaction between 35 wt% ammonia aqueous solution and the H+-form membrane in the MEA. The MEA was then washed by de-ionised water for a week by pumping water through both sides of the cell until no ammonia can be detected at the outlets of the cell. A potential of 40 mV was applied to the MEA for 4 hours to activate the cell prior to concentration cell measurements. When wet H2 (ambient temperature humidification) was introduced to the anode, wet air (also ambient temperature humidification) was used at the cathode, an OCV of ~475 mV was obtained, indicating the membrane exhibit H+ or/and O2− conduction. When the air was replaced by 5%H2/Ar to form a hydrogen concentration cell, the OCV of the cell gradually decreased and stabilised at ~32.8 mV (Fig. 1A). The I–V curve of this hydrogen concentration cell is shown in Fig. 1B. A maximum current density of 7 mA cm−2 was observed indicating migration of protons through the membrane. The proton conduction of the thus treated membrane was thus demonstrated by a hydrogen concentration cell. The theoretical OCV of the wet H2/5%H2-Ar concentration cell is 38.47 mV estimated from the Nernst Equation. The proton transfer number is therefore ~85% assuming no leakage or crossover of gases. The other 15% ionic conductivity is possibly attributed to the NH4+ ions although NH4+ ions are proton carriers too. Therefore, the thus treated membrane is a mixed NH4+/H+ conductor and can be used for electrochemical synthesis of ammonia because it is chemically compatible with ammonia.
Figure 1
(A) The recorded potential change from a H2/air cell to a H2/5%H2-Ar concentration cell; (B) The I–V and power curves of the H2/5%H2-Ar concentration cell.
Thermodynamic evaluation on electrochemical synthesis of NH3 from H2/H2O and N2
In order to consider the potential for electrochemical synthesis of ammonia, a thermodynamic evaluation on synthesis of ammonia from H2 and N2 was carried- out38. In theory, the reaction is spontaneous at a temperature below ~175°C when the partial pressure of H2 and N2 is 1 bar (Fig. 2A). It is expected that ammonia would be produced as long the applied voltage is higher than the electrode over-potential. When water is used for electrochemical synthesis of ammonia, the positive standard Gibbs free energy change indicates the reaction is non-spontaneous and applied potential is required (Fig. 2A). The corresponding required voltage for electrochemical synthesis of NH3 from H2 or H2O is shown in Fig. 2B. At 25°C, a minimum voltage of 1.17 V is required for electrochemical synthesis of ammonia from liquid water and N2 at partial pressure of 1 bar.