CHEMICAL SCIENCE

Synthesis of ammonia by high-efficiency electrochemical membrane reactor at atmospheric pressure and medium temperature


On June 5, 2023, Associate Professor Xue Jian of South China University of Technology and Professor Wang Haihui of Tsinghua University published a research result entitled “A high-efficiency electrochemical protonconducting membrane reactor for ammonia production at intermediate temperatures” in the journal Joule.

This achievement reports a membrane reactor that can achieve efficient electrochemical ammonia synthesis under atmospheric pressure and medium temperature conditions, which can couple the hydrogen purification process with the ammonia synthesis reaction process in situ, provide highly active protons for nitrogen reduction reaction, obtain excellent electrochemical ammonia synthesis performance at medium temperature 350 °C, the highest Faraday efficiency can reach 43.8%, and the corresponding ammonia production rate is 231.1 μg h-1 cm-2. The atmospheric medium temperature electrochemical membrane reactor provides a new research idea for the development of ammonia synthesis.

The corresponding author of the paper is Associate Professor Xue Jian and Professor Wang Haihui, and the first author is Weng Guowei.

Ammonia is one of the most basic raw materials in industrial and agricultural production, and it is also an ideal hydrogen carrier. In the past hundred years, the production of ammonia has mainly relied on the energy-intensive Haber-Bosch process, due to the operation of high temperature and high pressure reaction conditions, the energy consumption of the process accounts for 1~2% of the total global energy consumption, and the CO2 emissions account for 1.5% of the total global emissions, so it is urgent to develop a more energy-saving and environmentally friendly ammonia synthesis technology. At present, researchers have developed a technology for nitrogen fixation through electrochemical reactions using water as a proton source at room temperature, but the electrochemical nitrogen fixation technology still faces great challenges: 1) low nitrogen concentration, solubility in water is only 6.8×10-4 mol L-1; 2) The kinetics of nitrogen reduction reaction at room temperature are extremely slow, resulting in slow ammonia production rate; 3) The nitrogen reduction reaction (NRR) and hydrogen evolution reaction (HER) potential are close, so it is easy to occur serious hydrogen evolution competition reaction, resulting in low current Faraday efficiency. In contrast, the electrochemical ammonia synthesis technology at high temperature (>500 °C) is beneficial to the reaction kinetics of the nitrogen fixation process, but because the ammonia synthesis is an exothermic reaction, the ammonia generated is easily decomposed by thermodynamic equilibrium at high temperature, resulting in a decrease in ammonia production rate. Therefore, in order to take into account the kinetics and thermodynamics of ammonia synthesis, it is urgent to develop an electrochemical ammonia synthesis technology that can operate at atmospheric medium temperature.

In this work, Associate Professor Xue Jian of South China University of Technology and Professor Wang Haihui of Tsinghua University collaborated to construct a membrane reactor that can achieve efficient electrochemical ammonia synthesis under atmospheric pressure and medium temperature (300 °C~400 °C) for the first time, the electrochemical membrane reactor can couple the hydrogen purification process and the ammonia synthesis reaction process in situ, realize the coupling strengthening of the reaction process, use the anode catalyst and proton conductor membrane to activate hydrogen molecules into protons in situ, and under the drive of electric field, protons can be transferred to the nitrogen side of the cathode through the membrane controllably The active proton directly “attacks” the nitrogen molecules adsorbed on the surface of the cathode catalyst to generate ammonia, thereby achieving efficient electrochemical synthesis of ammonia.

Figure 1: Physical characteristics of LWO membrane reactor: (A) schematic diagram of LWO proton conduction membrane reactor for electrocatalytic N2 hydrogenation to NH3; (B) SEM cross-sectional view of LWO membrane reactor; (C) HRTEM diagram of Ru@LWO; Corresponding inverse FFT mode diagram of the yellow area in (D)(C); Intensity distribution plot of lattice spacing in (E)(D); (F) HRTEM diagram of Pt@LWO; Corresponding inverse FFT mode plot of the yellow region in (G)(F); Intensity distribution plot of lattice spacing in (H)(G).

Figure 2: CO2 resistance stability and hydrogen separation performance of LWO membranes: (A) in situ XRD pattern of LWO samples from 30 °C to 800 °C in a pure CO2 atmosphere; (B) Raman spectra of LWO, BCZYYb7111 and BCY10 samples treated in pure CO2 at 500 °C for 100 hours; (C) The amount of hydrogen permeability when different volumes of CO2/H2 mixture were added to the anode side of the LWO membrane at 350 °C, and different steam reforming (SR) reactions were simulated: (methane SR: CO2/H2=1/4; methanol SR: CO2/H2=1/3; coal SR: CO2/H2=1/2); (D) The corresponding current Faraday efficiency when different volumes of CO2/H2 mixture (methane SR: CO2/H2=1/4; methanol SR: CO2/H2=1/3; coal SR: CO2/H2=1/2) is added to the anode side of the LWO membrane at 350 °C; (E) At a temperature of 350 °C and a current density of 2500 mA cm-2, a mixture with a volume composition of CO2/H2=1/4 was added to the anode side of the LWO film for long-term stability test of H2 production.

Figure 3: Electrochemical ammonia synthesis performance of LWO membrane reactor: (A) at 300 °CAmmonia production rate at different current densities at 400 °C; (B) Current Faraday efficiency at different current densities at 300 °C~400 °C; (C) Comparison of LWO electrochemical membrane reactor and “fixed bed” reactor modes in terms of ammonia production rate; (D) Compared with the ammonia production rate and current Faraday efficiency of LWO electrochemical membrane reactor at medium temperature 350 °C with the performance of electrochemical ammonia synthesis at other atmospheric pressures, the blue sphere label corresponds to room temperature (Ru-based catalyst) and the yellow sphere marker corresponds to high temperature (>500 °C); (E) Long-term stability test of ammonia synthesized by LWO electrochemical membrane reactor at medium temperature 350 °C and 2500 mA cm-2.

Fig. 4: DFT calculation results for electrocatalytic nitrogen reduction reaction at 350 °C: (A) symmetric nitrogen hydrogenation pathway in an electrocatalyzed membrane reactor at 350 °C and 1.2 V; (B) asymmetric nitrogen hydrogenation pathway in an electrocatalytic membrane reactor at 350 °C and 1.2 V; (C) Ru (101) indicating the free energy evolution of three possible reaction pathways on N2 reduction; and (D) comparison of free energy of the fast-controlled steps of the three possible reaction pathways at 350 °C.

The high-efficiency electrochemical membrane reactor successfully realized the in-situ coupling of hydrogen purification process and ammonia synthesis reaction process at atmospheric pressure and medium temperature, and obtained excellent hydrogen separation efficiency and ammonia synthesis performance, which provided a new research direction for the development of electrochemical ammonia synthesis technology. This work was supported by the National Key Research and Development Program of China (2022YFB4002602), the National Natural Science Foundation of China (22278150, 22075086, 22138005, 22141001), the Guangdong Basic and Applied Basic Research Fund (2022A1515010980) and the Science Exploration Award. (Source: Science Network)

Related paper information:https://doi.org/10.1016/j.joule.2023.05.013



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