Copper Cobalt Sulfide Nanosheets Realizing a Promising Electrocatalytic Oxygen Evolution Reaction

Nanostructured CuCo₂S₄, a mixed metal thiospinel, is found to be a benchmark electrocatalyst for oxygen evolution reaction (OER) in this study with a low overpotential, a low Tafel slope, a high durability, and a high turnover frequency (TOF) at lower mass loadings. Nanosheets of CuCo₂S₄ are realized from a hydrothermal synthesis method in which the average thickness of the sheets is found to be in the range of 8–15 nm. Aggregated nanosheets form a highly open hierarchical structure. When used as an electrocatalyst, CuCo₂S₄ nanosheets offer an overpotential value of 310 mV at a 10 mA cm^–2 current density, which remains consistent for 10000 measured cycles in a 1 M KOH electrolyte. A chronoamperometric study reveals constant oxygen evolution for 12 h at a 10 mV s^–1 scan rate without any degradation of the activity. Furthermore, the calculated mass activity of the CuCo₂S₄ electrocatalyst is found to be 14.29 A/g and to afford a TOF value of 0.1431 s^–1 at 310 mV at a mass loading of 0.7 mg cm^–2. For comparison, nanostructures of Co₃S₄ and Cu_0.5Co_2.5S₄ have been synthesized using a similar method followed for CuCo₂S₄. When compared to the OER activities among these three thiospinels and standard IrO₂, CuCo₂S₄ nanosheets offered the highest OER activities at the same mass loading (0.7 mg/cm²). Extensive X-ray photoelectron spectroscopy and electron paramagnetic resonance analyses for a mechanistic study reveal that introduction of Cu into the Co₃S₄ lattice enhances the oxygen evolution and kinetics by offering Cu²⁺ sites for utilitarian adsorption of OH, O, and OOH reactive species and also by offering a highly active high-spin state of octahedral Co³⁺ for OER catalysis

Efficient Organic Photovoltaics with Improved Charge Extraction and High Short-Circuit Current

Exciton generation, dissociation, free carrier transport, and charge extraction play an important role in the short-circuit current (<i>J</i>_sc) and power conversion efficiency of an organic bulk heterojunction (BHJ) solar cell (SC). Here we study the impact of band offset at the interfacial layer and the morphology of active layer on the extraction of free carriers. The effects are evaluated on an inverted BHJ SC using zinc oxide (ZnO) as a buffer layer, prepared via two different methods: ZnO nanoparticle dispersed in mixed solvents (ZnO A) and sol–gel method (ZnO B). The device with ZnO A buffer layer improves the charge extraction and <i>J</i>_sc. The improvement is due to the better band offset and morphology of the blend near the ZnO A/active layer interface. Further, the numerical analysis of current–voltage characteristics illustrates that the morphology at the ZnO A/active layer interface has a more dominant role in improving the performance of the organic photovoltaic than the band offset