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When H2 and N2 were used for electrochemical synthesis of ammonia, at the oxidation electrode, hydrogen loses electrons while protons are formed.
The formed protons will transfer though the H+/NH4+-form Nafion to the other side to react with N2 to electrochemically form NH3;
The overall reaction is:
However, the reaction between proton and N2 depends on both thermodynamics and kinetics. Considering the over-potential on both electrodes, if the ‘net' potential difference between the two electrodes is above the value displayed in Fig. 2B, then thermodynamically reaction (3) should happen. However, a lot of reactions are under kinetic control particularly at low temperatures. The Faraday efficiency of reaction (3) is shown in Fig. 3B. It is about 2% when 0.2 V voltage was applied while decreased to less than 1% when higher voltage was applied. When air was used at the oxidation electrode or water at the reduction electrode, the Faraday efficiency for ammonia formation was both less than 1% (Figs. 4B and and5B)5B) which means only a small portion of supplied electricity was converted into ammonia. The Faraday efficiency also increased when the current across the cell is relatively low indicating higher voltage may also facilitate reaction (2) (Fig. 5B) when water instead of hydrogen was flowed at the cathode. More efficient catalysts at the N2/air side are required. The protons at the reduction electrode not only react with N2 to form NH3, but can receive electrons to form H2 as well;
When 0.2 V is applied, only 2% applied electricity was converted to ammonia while the other 98% was converted to H2. The H2 flow rate at the reduction electrode is proportional to the current across the cell. When higher voltage was applied, the current across the cell also increased (Fig. 3A). Most of the protons were converted to H2 again because the dwelling time of protons on the Pt/C electrode will be shorter thus the overall Faraday efficiency for ammonia formation decreased (Fig. 3B).
When N2 at the reduction electrode was replaced by air, besides reaction (4), another important reaction is between protons and oxygen in the air;
This is also the cathode reaction for a H2/O2 fuel cell. This reaction indicates that, under certain conditions, a small amount of ammonia may be formed when air was used as oxidant in hydrogen fuel cells. It has been reported that ammonia can passivate the oxygen reduction reaction (5) at the cathode of a proton exchange membrane fuel cell40. On the other hand, the passivation of ammonia on Pt/C catalysts for reaction (5) may supress the formation of H2O, which may favour the competitive reaction between protons and N2 in air to form ammonia. From this point of view, if a suitable catalyst is identified to suppress the formation of H2O according to reaction (5) while in favour of reaction (3), air can be directly used as nitrogen sources for electrochemical synthesis of ammonia.
Production of hydrogen through electrolysis for electrochemical energy storage has been widely investigated. Water can be used for direct electrochemical synthesis of ammonia. Then the reaction at the oxidation electrode is:
The formed protons will transfer through the proton-conducting membrane, react with N2 in air to form ammonia while O2 is formed at the oxidation electrode.
The overall reaction is:
While ammonia is produced at the air side, O2 is also produced at the water side which can be used for other applications such as oxyfuel combustion.
It should be noted that this is just a starting point to directly synthesis ammonia from air and water at room temperature although theoretically Pt is not among the best catalysts for ammonia synthesis32. In the future, other low cost ammonia synthesis catalysts such as Co3Mo3N and Ni2Mo3N41 can be used to replace Pt for selective ammonia synthesis under mild conditions. The acidity of the H+/NH4+-form Nafion membrane would be much weaker than the H+-form Nafion allowing selection of a large range of catalysts for ammonia synthesis. This is a low temperature, low pressure process with flexibility in scale and location. This technology will break the link between ammonia industry and fossil fuels. Considering climate change and the depletion of fossil fuels used for synthesis of ammonia by conventional method, this is a renewable and sustainable chemical synthesis process for future.