Modulators are widely used in the synthesis of metal-organic frameworks (MOFs) for improving the porosity and morphology. For aluminum MOFs modulation has been seldom reported and as is shown here for the example of the aluminum furandicarboxylate MOF MIL-160 the positive effects of modulators are small and disadvantageous effects will be more likely. Formic acid as modulator can slightly increase the BET surface area and pore volume of MIL-160 up to a modulator:linker ratio of 1.25 : 1. Acetic acid only shows some increase in both surface area and pore volume at the smallest tested ratio of 0.125 : 1. The stronger acids oxalic acid and hydrochloric acid with the also more aluminum-coordinating anions have no positive porosity effect and decrease surface area and pore volume already at small amounts. At a 1 : 1 modulator:linker ratio for oxalic acid and at 0.75 : 1 for hydrochloric acid no porous MOF is formed anymore from the analysis by powder X-ray diffraction and nitrogen sorption. Further, thermogravimetric analysis and scanning electron microscopy suggests that none of the tested modulators has any noticeable positive effect on the introduction of linker defects or the improvement of crystallinity or crystal size.
Iron-containing nickel sulfides, selenides, and sulfoselenides were synthesized via a simple two-step hydrothermal reaction (temperature 160 °C) for their application as electrocatalysts in the oxygen evolution reaction (OER) in an alkaline solution (1 mol L–1 KOH). The study demonstrated that iron-containing nickel cobalt sulfides and selenides exhibit superior OER performance with lower overpotentials compared to iron-free nickel cobalt sulfide and selenide, which highlights the significant role of iron in enhancing OER nickel cobalt electrocatalysts: Fe0.1Ni1.4Co2.9(S0.87O0.13)4, η50 = 318 mV; Fe0.2Ni1.5Co2.8(S0.9O0.1)4, η50 = 310 mV; Fe0.3Ni1.2Co2.5(S0.9O0.1)4, η50 = 294 mV; Fe0.6Ni1.2Co2.5(S0.83O0.17)4, η50 = 294 mV, Fe0.4Ni0.7Co1.6(Se0.81O0.19)4, η50 = 306 mV compared to Ni1.0Co2.1(S0.9O0.1)4, η50 = 346 mV and Ni0.7Co1.4(Se0.85O0.15)4, η50 = 355 mV (all values at current densities η50 of 50 mA cm–2). Furthermore, the iron-containing nickel cobalt sulfoselenide Fe0.5Ni1.0Co2.0(S0.57Se0.25O0.18)4 displayed exceptional OER performance with η50 = 277 mV, surpassing the benchmark RuO2 electrode with η50 = 299 mV. The superior performance of the sulfoselenide was attributed to its low charge transfer resistance (Rct) of 0.8 Ω at 1.5 V vs. the reversible hydrogen electrode (RHE). Moreover, the sulfoselenide demonstrated remarkable stability, with only a minimal increase in overpotential (η50) from 277 mV to 279 mV after a 20 h chronopotentiometry test. These findings suggest that trimetallic iron, nickel and cobalt sulfide, selenide and especially sulfoselenide materials hold promise as high-performance, cost-effective, and durable electrocatalysts for sustainable OER reactions. This study provides a valuable approach for the development of efficient electrocatalytic materials, contributing to the advancement of renewable energy technologies.
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