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Figure 2
(A) The Gibbs free energy change for electrochemical synthesis of ammonia from N2 and H2, N2 and H2O (gaseous or liquid) at pressure of 1 bar; (B) The minimum applied voltage required for electrochemical synthesis of ammonia from N2 and H2 at pressure ...
Synthesis of ammonia from H2 and N2
Ammonia was first synthesised from conventional precursors, H2 and N2 using the electrochemical cell. As shown in Fig. 3A, an initial current of 58 mA cm−2 was observed even at an applied voltage of 0.2 V. The current decreases after 3 minutes, possibly due to the reaction between produced ammonia and the membrane. Instead of decreasing, the current gradually increases when 0.4V is applied indicating the reaction has completed. A current density of 390 mA cm−2 has been achieved at room temperature when 1.2 V is applied (Fig. 3A). The ammonia formation rates at different applied voltage are shown in Fig. 3B. The highest ammonia formation rate of 3.1 × 10−5 mol m−2 s−1 was observed at 0.2 V which is about three orders of magnitude higher than the previously reported value reported by Kordali et al30 and is comparable to those reported by Liu et al31. As shown in Fig. 3C, the formed ammonia increased with time. The amount of generated ammonia was 1.13 × 10−5 mol after applying 0.2 V for 1 hour (Fig. 3C) which is higher than the estimated maximum amounts of ammonia that could be generated from the decomposition of the NH4-form of the membrane (9.14 × 10−6 mol) if all the current arose from NH4+ transport which is unlikely, or estimated dissolved ammonia (8.02 × 10−6 mol) (please see supplementary information). This result demonstrates that the generated ammonia is from the electrosynthesis process.
Figure 3
(A) Current density of a N2, Pt | Nafion 211 | Pt, H2 cell under different applied voltages. Cathode was supplied with N2, anode was supplied with H2. (B) The ammonia formation rate at N2 and H2 sides, total ammonia formation rate and Faraday efficiency. ...
The higher ammonia formation rate at lower voltage may be due to the lower hydrogen ion supply at the cathode which gives more time for formation of ammonia according to reaction (2). Between 0.6 and 1.2 V, the formed ammonia slightly increased at higher voltage (Fig. 3B). Although ammonia was mainly observed at the N2 side, a small amount of ammonia was also observed at the H2 side when the formation rate was relatively high. One of the possible reasons is that, ammonia is very soluble in water, at higher formation rate, some of the formed ammonia at N2 side may in situ dissolve in water, diffuse to the H2 side then brought out by the flowing H2. On the other hand, there could be some cross-over effects too which is common in electrochemical cells based on polymer electrolytes.
Synthesis of ammonia from H2 and air
Air contains 78% N2 therefore it would be better to synthesise ammonia directly from air without the separation process. When N2 at the cathode was replaced by air, a stable current density of 142 mA cm−2 was observed when the cell voltage was 0.2 V (Fig. 4A). At 1.0 V, the current density stabilised at 537 mA cm−2. This indicates that the membrane is fairly stable. Comparing to the H2/N2 cell, the current densities are higher for the H2/air cell, possibly due to the extra driving force from O2 in air. Interestingly, ammonia was also produced on the air side with a small amount at the H2 side when the formation rate is relatively high (Fig. 4B). At the same cell voltage, the ammonia formation rates are also higher than those for H2/N2 cell. The formed ammonia increased against time (Fig. 4C). This experiment indicates that ammonia can be synthesised directly from air without gas separation. This is consistent with thermodynamic evaluation concluding reaction (3) is spontaneous at a temperature below 175°C when the partial pressure of N2 is at 1 bar although it is slightly lower in air (Fig. 3A). Another parallel reaction at the cathode is formation of water between proton and O2 in air; however, if a selective catalyst for ammonia synthesis is used, the reaction can be kinetically in favour of ammonia formation. Comparing to conventional Haber-Bosch process, due to different catalysts used along with various synthesis conditions, the oxygen poisoning on ammonia synthesis catalysts is not an issue.