Comparing electrocatalytic hydrogen and oxygen evolution activities of first-row transition metal complexes with similar coordination environments

Developing cheap and efficient electrocatalysts for water splitting is required for energy conversion techniques. Many first-row transition metal complexes have been shown to be active for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Metal ions play crucial roles in these catalytic processes, but the activity dependence on the nature of metal ions has been rarely studied due to the difficulty to compare metal complexes with different coordination environments. We herein reported the synthesis of a series of metal complexes of azido-substituted porphyrin (1), in which metal ions have very similar coordination environments. By grafting 1-M (M = Mn, Fe, Co, Ni, and Cu) onto alkyne-functionalized carbon nanotubes (CNTs) through the same covalent connection, the resulted hybrids 1-M@CNT were all active and robust for both electrocatalytic HER and OER in alkaline aqueous solutions. Among these hybrids, 1-Fe@CNT displayed the highest electrocatalytic activity for HER, while 1-Co@CNT was the most active one for OER. Moreover, a two-electrode water electrolysis cell assembled with 1-Fe@CNT as the cathode and 1-Co@CNT as the anode required smaller applied bias potential by 210 mV to get 10 mA/cm2 current density as compared to that assembled with Pt/C and Ir/C with the same amount of metal loading. This work is significant to correlate HER and OER activity with the nature of first-row transition metal ions and to highlight promising potential applications of molecular electrocatalysis in water splitting.