Metal-free quinoxaline-based donor–acceptor small molecule for efficient water splitting across universal pH conditions

Abstract

Designing efficient, stable, and environmentally friendly metal-free electrocatalysts is a significant challenge in the development of next-generation clean energy systems. In this study, a novel D–A type organic small molecule, 2,3,6,7-tetra(thiophen-2-yl)quinoxaline (TTQx), was designed and synthesized to act as a metal-free bifunctional electrocatalyst, which was effectively active in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This study experimentally investigated the thermal, optical, and electrochemical properties of the TTQx molecule, which were complemented by DFT-derived HOMO and LUMO energy levels. Electrostatic surface potential (ESP) mapping and structural analysis of a high-quality TTQx single crystal were performed to interpret the charge-density distributions and reveal the atomic-level characteristics governing the active sites for the HER and OER. TTQx demonstrated outstanding catalytic performances in various electrolytes, namely, 0.5 M H₂SO₄, 1.0 M KOH, and 1.0 M PBS, with low HER overpotentials of −108, −152, and −163 mV and OER overpotentials of 233, 251, and 271 mV at 10 mA cm⁻², respectively. Remarkably, TTQx maintained a high catalytic activity for over 24 hours of continuous operation in all electrolytes, with minimal degradation. Furthermore, TTQx-based devices delivered a current density of 10 mA cm⁻² at low cell voltages of 1.66 V in H₂SO₄, 1.76 V in KOH, and 1.79 V in PBS while maintaining a stable overall water splitting performance for over 70 hours. Notably, the superior performance of TTQx under acidic conditions highlighted the durability of this organic small molecular electrocatalyst. This work establishes an entirely metal-free electrocatalyst based on a π-extended organic small molecule that efficiently drives both the HER and OER, highlighting the potential of π-conjugated heterocycles for sustainable green hydrogen production via water splitting process.