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The electrocatalytic ammonia synthesis using water (along with nitrogen) as a hydrogen source is proposed as an alternative green and clean technology to the energy-intensive and CO2-emitting process (Haber-Bosch) for ammonia production. Besides, a selective electrocatalyst for ammonia synthesis versus the competing hydrogen evolution remains elusive. This study aims to investigate the electrocatalytic activity of non-noble metal Co and Fe-free perovskite oxide-based composite cathode (La0.75Sr0.25Cr0.5Mn0.5O3-δ-Ce0.8Gd0.18Ca0.02O2-δ) towards ammonia synthesis from H2O and N2. The electrocatalyst was synthesized via a sol-gel process and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ammonia was successfully with a maximum formation rate of 2.5 × 10-10mol s-1cm-2and Faradaic efficiency of 0.52% at 400 oC and applied voltage of 1.4 V. The results demonstrated that the proposed non-noble metal-based electrocatalyst is a promising material for the carbon-free ammonia synthesis process.

A La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ -Ce 0.8 Gd 0.18 Ca 0.02 O 2−δ composite cathode was used to investigate the electrochemical synthesis of ammonia from wet nitrogen. Wet nitrogen was flown through a dual chamber reactor under atmospheric pressure leading to the successful synthesis of ammonia. Ammonia was synthesised at a rate of 1.5 × 10 −10 mol s −1 cm −2 at 400 °C when applying a dc voltage of 1.4 V, which is the highest reported to date. This rate is twice that of the observed ammonia formation rate (7 × 10 −11 mol s −1 cm −2 ) when Co-free cathode, La 0.6 Sr 0.4 FeO 3−δ -Ce 0.8 Gd 0.18 Ca 0.02 O 2−δ was used as the cathode catalyst. A higher catalytic activity for ammonia synthesis may be obtained when using a catalyst with high oxygen vacancies, with the introduction of oxygen vacancies at the cathode being a good strategy to improve the catalytic activity of ammonia synthesis.

Ammonia was directly synthesised from wet air at 400 °C at atmospheric pressure. A new perovskite Sm0.6Ba0.4Fe0.8Cu0.2O3−δ was used as the electrocatalyst for electrochemical synthesis of ammonia. Ammonia formation rates of 9.19 × 10−7 mol s−1 m−2 and 1.53 × 10−6 mol s−1 m−2 were obtained at 400 °C when wet air and wet N2 were introduced into a simple single chamber reactor, respectively. The perovskite catalyst is low cost compared to the previously reported Ru/MgO and Pt/C catalysts. This experiment indicates that ammonia can be directly synthesised from wet air, a very promising simple technology for sustainable synthesis of ammonia in the future.

Electrochemical synthesis of ammonia from water vapour and nitrogen was investigated using an electrolytic cell based on CoFe2O 4-Ce0.8Gd0.18Ca0.02O 2-δ (CFO-CGDC), CGDC-ternary carbonate composite and Sm 0.5Sr0.5CoO3-δ-Ce0.8Gd 0.18Ca0.02O2-δ (SSCo-CGDC) as cathode, electrolyte and anode respectively. CoFe2O4, CGDC and SCCo were prepared via a combined EDTA-citrate complexing sol-gel and characterised by X-ray diffraction (XRD). The AC ionic conductivities of the CGDC-carbonate composite were investigated under three different atmospheres (air, dry O 2 and wet 5% H2-Ar). A tri-layer electrolytic cell was fabricated by a cost-effective one-step dry-pressing and co-firing process. Ammonia was successfully synthesised from water vapour and nitrogen under atmospheric pressure and the maximum rate of ammonia production was found to be 6.5 × 10-11 mol s-1 cm-2 at 400 C and 1.6 V which is two orders of magnitude higher than that of previous report when ammonia was synthesised from N2 and H2O at 650 C.

Single phase perovskite oxide La0.8Cs0.2Fe 0.8Ni0.2O3-δ was synthesised to be used as electrocatalyst for electrochemical synthesis of ammonia directly from wet air. It exhibits an orthorhombic structure with space group Pnma(62); a = 5.5493(5) A, b = 7.8352(10) A, c = 5. 5295(5) A, V = 240.42(4) A3. Composite made of Ce0.8Gd0.2O 2-δ (CGO) and (Li,Na,K)2CO3 was used as electrolyte. An ammonia formation rate of 9.21 × 10-7 mol s-1 m-2 was obtained at 400 C when applied a voltage of 1.4 V, while wet air (3 mol% H2O) was introduced to the single chamber reactor. This is just slightly lower than the value of 1.23 × 10-6 mol s-1 m-2 when wet N2 (3 mol% H2O) was fed under the same experimental conditions. These values are more than two orders of magnitude higher than the reported ammonia formation rates when synthesised from N2 and H2O at ∼ 600°C. The perovskite catalyst is also low cost compared to the Ru/MgO and Pt/C catalysts in previous reports. This experiment indicates that ammonia can be directly synthesised from wet air using low-cost catalysts. This is a very promising simple technology for sustainable synthesis of ammonia in the future.

Electrochemical synthesis of ammonia from wet nitrogen in an electrolytic cell using a La0.6Sr0.4FeO3−δ–Ce0.8Gd0.18Ca0.02O2−δ composite cathode and an oxide–carbonate composite electrolyte has been investigated. La0.6Sr0.4FeO3−δ was prepared via a combined EDTA–citrate complexing sol–gel process, characterised by X-ray diffraction and SEM. A tri-layer electrolytic cell was fabricated by a one-step dry-pressing and co-firing process. Ammonia was successfully synthesised from wet nitrogen under atmospheric pressure. Ammonia formation was observed at 375, 400, 425 and 450 °C and the maximum ammonia formation rate of 7 × 10−11 mol s−1 cm−2 was observed at 400 °C when a voltage of 1.4 V was applied. This ammonia formation rate corresponds to a Faradaic efficiency of ∼0.14% at a current density of 14.25 mA cm−2.

Electrochemical synthesis of ammonia was investigated using a cobalt-free La0.6Sr0.4Fe0.8Cu0.2O3-δ-Ce0.8Sm0.2O2-δ (LSFCu-SDC) composite cathode and SDC-ternary carbonate composite electrolyte. La0.6Sr0.4Fe0.8Cu0.2O3-δ and Ce0.8Sm0.2O2-δ were prepared via combined EDTA-citrate complexing sol–gel and glycine nitrate processes, respectively, and characterised by X-ray diffraction (XRD). Ammonia was successfully synthesised from wet hydrogen and dry nitrogen under atmospheric pressure using Ni-SDC, SDC-carbonate and LSFCu-SDC composites as anode, electrolyte and cathode respectively. Ammonia formation was observed at 400, 425, 450 and 475 °C and the maximum rate of ammonia production was found to be 5.39 × 10−9 mol s−1 cm−2 at 450 °C and 0.8 V. The AC impedance measurements were recorded before and after the ammonia synthesis in the range of temperature 400–475 °C. The formation of ammonia at the N2 side together with stable current at 450 °C under constant voltage demonstrates that SDC-(Li/Na/K)2CO3 composite electrolyte exhibits significant proton conduction at a temperature around 450 °C.

Ammonia is one of the most produced chemicals worldwide, and it is not only a major end product but also an important energy storage intermediate. The solid-state electrochemical synthesis of ammonia has the promise to overcome the limitations of the conventional catalytic reactors such as the limited conversion, severe environmental pollution and high energy consumption. Solid-state electrolytes either protonic or oxide ion conductors have been reviewed and particular emphasis is placed on their application to synthesise ammonia. The highest rate of ammonia formation according to the type of electrolyte utilised were in the following order; solid polymers > Ce0.8Gd0.2O2−δ-(Ca3(PO4)2-K3PO4) composites > fluorites > perovskites > pyrochlores although the catalysts in electrodes also play an important role. The highest rate reported so far is found to be 1.13 × 10−8 mol s−1 cm−2 at 80 °C with a potential of 2 V using Nafion membrane, SmFe0.7Cu0.1Ni0.2O3 (SFCN), and Ni-Ce0.8Sm0.2O2−δ as solid electrolyte, cathode and anode, respectively. Synthesising ammonia from steam and N2, by-passing H2 stage offers many advantages such as reduction of device numbers and then the overall costs. The factors affecting the rate of ammonia formation have been discussed as well.

Electrochemical synthesis of ammonia based on carbonate electrolyte is presented in this paper. Spinel oxide CoFe 2 O 4 was synthesised via a co-precipitation method and characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ammonia was successfully synthesised from hydrogen and nitrogen under atmospheric pressure using CoFe 2 O 4 as catalyst (primary particle size 10–20 nm) together with silver at the cathode, Ag–Pd at the anode and carbonate-LiAlO 2 composite as electrolyte respectively. Ammonia formation was observed at 400, 425 and 450 °C and the maximum rate of ammonia production was found to be 2.32 × 10 −10 mol s −1 cm −2 at 400 °C and 0.8 V. The AC impedance measurements were recorded before and after the ammonia synthesis at 400, 425 and 450 °C. The electrolytic cell during the ammonia synthesis process was more stable at 450 °C and 0.8 V over a period of 2 h. This experiment also indicates that the carbonate-LiAlO 2 composite electrolyte exhibits proton conduction.

Ammonia was directly synthesised from wet nitrogen at an intermediate temperature (375–425 °C) based on the oxygen-ion conduction of the Ce0.8Gd0.18Ca0.02O2−δ–((Li/Na/K)2CO3) composite electrolyte. A redox stable perovskite-based catalyst, La0.75Sr0.25Cr0.5Fe0.5O3−δ (LSCrF), was synthesised via a combined EDTA–citrate complexing sol–gel process to be used as a component of the La0.75Sr0.25Cr0.5Fe0.5O3−δ–Ce0.8Gd0.18Ca0.02O2−δ composite cathode for ammonia synthesis. Ammonia formation was studied at 375, 400 and 425 °C and a 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 °C when the applied voltage was 1.4 V. This is much higher than 7.0 × 10−11 mol s−1 cm−2 at 1.4 V and 400 °C when Cr-free Sr-doped LaFeO3−δ, La0.6Sr0.4FeO3−δ was used as the catalyst for the 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 Cr3+ ions at the B-site of doped LaFeO3 improves both the chemical stability and catalytic activity for ammonia synthesis.

Cobalt molybdenum nitride (Co3Mo3N) catalyst was synthesised through ammonolysis of the corresponding precursors by flowing pure ammonia gas. The catalyst was characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ammonia was successfully synthesised from wet hydrogen and dry nitrogen at atmospheric pressure using Co3Mo3N–Ag composite as cathode, Ag–Pd alloy as anode and LiAlO2–(Li/Na/K)2CO3 composite as an electrolyte. Ammonia formation was investigated at 400, 425 and 450 °C, and the maximum observed ammonia formation rate was 3.27×10−10 mol s−1 cm−2 at 450 °C when applied at 0.8 V. The catalytic activity of Co3Mo3N for electrochemical ammonia synthesis is lower than that of Pd. The successful synthesis of ammonia demonstrates that LiAlO2–(Li/Na/K)2CO3 composite exhibits protonic or/and oxygen ion conduction.

For the first time, ammonia has been directly synthesised from wet air at intermediate temperature. Ce0.8Gd0.2O2−δ (CGO)–(Li,Na,K)2CO3 electrolyte together with a new perovskite oxide Pr0.6Ba0.4Fe0.8Cu0.2O3−δ were used for electrochemical synthesis of ammonia. An ammonia formation rate of 1.07 × 10−6 mol s−1 m−2 was obtained at 400 °C when applied a voltage of 1.4 V, while wet air was introduced to the single chamber reactor. This is just slightly lower than the value of 1.83 × 10−6 mol s−1 m−2 when wet N2 was fed under the same experimental conditions. These values are two to three orders of magnitude higher than the reported ammonia formation rates when synthesised from N2 and H2O at ∼600 °C. The perovskite catalysts are also low cost compared to the Ru/MgO and Pt/C catalysts in previous reports.