Porphyrin-based polyimide 2D porous organic polymers: band engineering for bifunctional electrocatalytic OER and HER.

The development of bifunctional catalysts for overall water splitting is a rapidly advancing area of research. In this study, we report the synthesis of novel porphyrin-based polyimide polymers for electrocatalytic oxygen (OER) and hydrogen evolution reactions (HER). By assembling aminophenyl porphyrins with naphthalenetetracarboxylic dianhydride (NTCDA) linkers, donor-acceptor (D-A) architectures that enable extended π-electron delocalization and tailored frontier molecular orbitals conducive to water splitting were formed. FTIR, XPS and XRD analyses confirmed successful imidization between aminophenyl porphyrins (TAPP, NiTAPP, and CuTAPP) and NTCDA, resulting in π-π stacked two-dimensional polymer networks POP-1, POP-2, and POP-3, respectively. Band structure studies via XPS revealed that both imidization and metal incorporation significantly influence the electronic properties and enable the fine-tuning of the HOMO-LUMO levels for enhanced electrocatalytic activity. Electrochemical evaluations demonstrated the bifunctional nature of the polymers, with the Ni(ii)-based polymer, i.e.POP-2, showing the best performance. POP-2 exhibited a lower OER onset potential (1.45 V vs. RHE) and smaller Tafel slope (167 mV dec^-1) compared to its metal-free (POP-1) and Cu(ii)-based (POP-3) analogues. Similarly, in the HER, POP-2 displayed a reduced onset potential (ca. 159 mV) and a lower Tafel slope (ca. 82 mV dec^-1), attributed to the favorable redox behavior of Ni, optimal hydrogen binding energy, and enhanced charge delocalization. Electrochemical impedance spectroscopy confirmed its superior conductivity, while Mott-Schottky and XPS analyses revealed beneficial band alignment and increased charge carrier density. Long-term stability tests further validated the durability of POP-2. This work highlights the potential of metal-coordinated conjugated polymers as efficient and robust heterogeneous bifunctional electrocatalysts for overall water splitting.