Electrifying the synthesis of ammonia could help to decarbonize production of this important chemical and pave the way for its use as a low-carbon energy carrier. A promising approach in its early stages of development is the lithium-mediated method of N2 reduction. This process is thought to proceed by lithium metal reacting with N2 to form nitrides, which are then protonated to form ammonia and a lithium salt. The lithium salt is subsequently electrochemically reduced back to lithium metal. Despite the progress achieved in recent years, the method is still limited by poor gas transfer and a need for organic solvents, which are prone to degradation. Now, Shuiyun Shen, Junliang Zhang and colleagues at Shanghai Jiao Tong University seek to address these issues by demonstrating lithium-mediated ammonia synthesis in a membrane-electrode assembly (MEA) configuration.
The researchers build an MEA comprising gas diffusion electrodes with a lithium-decorated stainless-steel cloth at the cathode and a Pt/C catalyst at the anode. Rather than employing a commonly used liquid electrolyte (such as lithium salts dissolved in the solvent, tetrahydrofuran) the research team sandwich a lithium-doped polyethylene oxide electrolyte membrane between the electrodes; tetrahydrofuran is used only to wet the input N2 gases, lowering the amount needed. The use of the MEA, coupled with gas diffusion electrodes, enhances the mass transfer of N2 gas molecules to the reaction sites, improving the practicality of the lithium-mediated process. The team report a nitrogen fixation rate of 2.41 μmol h−1 cm−2 and a faradaic efficiency of 8.9% at a cell voltage of 3.6 V and suggest that higher efficiencies should be achievable in the future.
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