NiFe-mixed metal porphyrin aerogels as oxygen evolution reaction catalysts in alkaline electrolysers.

Aerogels are a very interesting group of materials owing to their unique physical and chemical properties. In the context of electrocatalysis, the focus has been on their physical properties, and they have been used primarily as catalyst supports so far. In this work, we synthesized porphyrin aerogels containing Ni and NiFe mixed metal materials and studied them as catalysts for the oxygen evolution reaction (OER). Different Ni : Fe ratios were synthesized and studied in electrochemical cells, and DFT calculations were conducted in order to gain insight into their behavior. The activity trends were dependent on the metal ratios and differ from known NiFeOOH materials due to the change in the oxidation states of the metals to higher numbers. Herein, we show that Ni and Fe have a synergistic effect on the OER, despite being structurally separated. They are connected electronically, though, through a large organic aromatic system that facilitates electron sharing between them. Among the mixed metal porphyrin aerogels, the best ratio was found to be Ni : Fe = 35 : 65, in contrast to oxide/oxyhydroxide materials in which a ratio of 80 : 20 was found to be ideal.