Coordination polymer-based supercapacitors with matched energy levels: enhanced capacity under visible light illumination in the presence of methanol.

Using 4-[5-(1H-pyrazol-4-yl)-4H-[1,2,4]triazol-3-yl]-pyridine (L1) and 2-(3-carboxypyridin-2-yl) nicotinic acid (H2L2), two coordination polymers (CPs) formulated as [Cu(HL1)2(Mo8O26)]·5H2O (1) and [{Cu4(L2)4(H2O)6}(H3PMo12O40)]·7H2O (2) were hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction. The two CPs exhibit a 2D architecture, but strong interlayer π-π stacking interactions are only observed in the structure of CP 1. Diffuse reflectance spectroscopy (DRS) and DFT calculations reveal that CP 1 is a semiconductor with a narrow band gap of 1.22 eV, and it showed a photocurrent response under visible light illumination (λ > 420 nm, 100 mW cm-2). Based on CP 1, a device FTO/CP 1/RuO2 was constructed, which showed a much enhanced photoresponse with a much larger specific capacity (120.7 C g-1) than the individual CP 1 (<1 C g-1) and RuO2 (10.5 C g-1) when irradiated by visible light in the presence of methanol. It was due to the matched energy levels between the components in the device and the oxidation potential of methanol in the electrolyte solution, leading to efficient separation of the photoinduced electrons and holes to give an improved photovoltaic effect. As for the FTO/CP 1/RuO2 device, its specific capacity (120.7 C g-1) obtained by the visible light illumination in the presence of methanol was much larger than that (11.5 C g-1) obtained in the dark without methanol, further suggesting the transformation among solar energy, chemical energy and electric energy.