Covalent Metalloporphyrin Polymer Coated on Carbon Nanotubes as Bifunctional Electrocatalysts for Water Splitting

Metalloporphyrins have exhibited excellent electrocatalytic activities for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In order to improve the efficiency and conductivity, these molecular catalysts need to be immobilized on conductive electrode materials. Herein, a facile "one-pot" strategy was developed to coat a covalent metalloporphyrin polymer on a carbon nanotube (CNT) as bifunctional catalysts [denoted as MTIPP@CNTs, H₂TIPP = 5,10,15,20-tetra(4-(imidazole-1-yl)phenyl)porphyrin)] for water splitting in alkaline solution. MTIPP@CNTs have shown excellent electrocatalytic activities for both the HER and OER when metalloporphyrin's central metal is optimized as well as the amount of catalysts that is loaded on the CNT. The overpotential (η₁₀) of NiTIPP@CNT-2 for the OER is only 320 mV at a current density of 10 mA cm^-2 in 1.0 M KOH, and CoTIPP@CNT-1 exhibited an excellent electrocatalytic activity for the HER (η₁₀ = 450 mV for 10 mA cm^-2). Furthermore, the remarkable bifunctional electrocatalytic performance (a cell voltage of 2.04 V with a current density of 10 mA cm^-2) was also explored in the overall water splitting test.