Low band gap π-conjugated porous polymers as bifunctional photoelectrocatalysts for overall water splitting

Conjugated microporous polymers (CMPs) are gaining attention for their unique structural properties and impressive charge transfer in electrochemical reactions. They are efficient bifunctional photoelectrocatalysts for crucial processes like the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting reaction (OWSR). The challenge in designing CMPs is achieving precise control of p-type and n-type behaviours within the polymer chain for specific photoelectrocatalytic reactions. We synthesized CMPs via Schiff-base condensation, tailoring their semiconducting properties and achieving band gaps under 1.6 eV by adjusting the acceptor to donor (A/D) ratio during synthesis. The CMP (designated as PDM1:2) with high donor content showed excellent properties for photoelectrocatalysis. It featured a semicrystalline morphology, resulting in impressive Tafel slopes (33 mV dec⁻¹ for the OER, 122 mV for the HER) and overpotentials (η₁₀) (230 mV for the OER, 229 mV for the HER, and 1.69 V for the OWSR). The CMPs showed better catalytic performance under light and consistently performed over 500 cycles with excellent stability for 24 hours. These findings emphasize the importance of optimizing the A/D molar ratio in the polymer framework to improve the separation and migration of photogenerated charge carriers. This optimization led to improved photoelectrocatalytic performance for prolonged OWSRs. We developed a prototype exhibiting the bulk photoelectrocatalytic behaviour of our CMPs in facilitating the OWSR.