Redox-Mediated Electrocatalytic Ammonia Synthesis On Metalloporphyrin Metal-Organic Frameworks

Ammonia is a critical commodity for fertilisers and chemical industry but is currently synthesised by the carbon-intensive Haber-Bosch process. Ammonia electrosynthesis in aqueous conditions could allow low-emission ammonia to be generated using abundant starting materials and renewable energy. As such, both the electrocatalytic nitrogen reduction reaction (eNRR) and electrocatalytic nitrate reduction reaction (eNO3RR) are active areas of research. Unfortunately, most reported electrocatalysts for these reactions fail to achieve industrially relevant current densities (≥ 300 mA/cm^2). Metalloporphyrin metal-organic frameworks (MOFs) are nanoporous, 3D arrays which host a high density of metalloporphyrin active sites. Such materials could yield high ammonia production current densities, making them ideal electrocatalyst candidates. However, such MOFs generally exhibit poor electrical conductivity, limiting their activity.

This thesis investigates the activity and viability of the metalloporphyrin MOF PCN-222(M) for eNRR and eNO3RR, as well as the use of a molecular redox mediator to bypass the insulating framework. By using a redox mediator, electrons from the cathode can be shuttled to the metalloporphyrin active sites, potentially boosting the ammonia current density. After successfully exploring post-synthetic metalation techniques to synthesise PCN-222(M), the MOFs were investigated for both eNRR and eNO3RR. The latter proved successful, with PCN-222(Cu)-coated electrodes achieving an ammonia yield rate of up to 350 ± 40 μg/h/cm^2 and faradaic efficiency of up to 18.5 ± 0.6% in a membrane-free cell. Lastly, the interaction between the redox mediator methyl viologen and an iron(III) porphyrin was investigated.