Xiaoqiang Du

Water-soluble K7[CoIIICoII(H2O)W11O39] (1) was tested as the first cobalt-containing Keggin polyoxometalate catalyst for efficient O2 production via both visible-light driven and thermal water oxidation. Under the optimal photocatalytic conditions [photoirradiation at λ ≥ 420 nm, [Ru(bpy)3]Cl2 as the photosensor, Na2S2O8 as the oxidant in borate buffer (pH = 9.0)],the turnover number (TON) can reach as high as 360, the initial quantum yield and the initial turnover frequency (TOF) for the first 60 seconds was 27% and 0.5 s−1, respectively. Variables of the photocatalytic reaction, including catalyst concentrations, buffer types and concentrations, pHs, dye concentrations, oxidant concentrations, etc., were systemically studied. The oxygen atoms of the evolved oxygen came from water, as confirmed by isotope-labeled experiments. In the thermal water oxidation, the TON and oxygen yield were measured to be 15 and 60%, respectively. The stability of 1 was tested and confirmed with multiple experiments (laser flash photolysis, DLS, CV, FT-IR, EDX, and catalyst recycling) within the photocatalytic water oxidation duration, which ruled out the possibility that neither the free Co2+ ions were present in the reaction solution nor were cobalt oxide/hydroxide nanoparticles in situ formed from the assumed decomposition of 1. All the evidence stated here collectively supports that 1 is the true molecular catalyst. In addition, the recycled sample was reused for water oxidation catalysis and showed similar catalytic behaviors (kinetics and activity) to that of the freshly prepared catalyst. No insoluble forms where found when the borate buffer solution of 1 was aged for 2 months, whereas its analogue, K8[CoIICoII(H2O)W11O39] (4), is rapidly decomposed to a blue-purple cobalt oxide precipitate in borate buffer. The stability difference between 1 and 4 indicates that the +3 oxidation state of the central cobalt in 1 plays a vital role in maintaining the structural integrity. A series of other Keggin-type POMs, such as K6[CoIIW12O40] (2), K5[CoIIIW12O40] (3), K6[SiCoII(H2O)W11O39] (5), and K5[PCoII(H2O)W11O39] (6), were also evaluated for their catalytic activity by comparison with 1. Our study demonstrates that the unique structural features of the mixed-valent central and peripheral cobalt atoms are essential for 1 to maintain both catalytic stability and efficiency.

Using Ni(OH)₂/Ni₃S₂-12h as a bifunctional water splitting catalyst, with an overpotential of ∼340 mV, which is obtained at a very low cell voltage of 1.57 V with a current density of 10 mA cm⁻²in 1.0 M KOH.

Herein, transition metal chalcogenides of pristine cobalt sulfides are rationally designed to act as robust bifunctional photocatalysts for visible‐light‐driven water splitting for the first time. Through moderate solvothermal route, cobalt sulfides are synthesized in situ growth and observed by scanning electron microscope image analysis. Noteworthily, 3D hierarchical cobalt sulfides acting as bifunctional photocatalysts are implemented to catalyze the visible‐light‐driven oxygen evolution reaction and hydrogen evolution reaction. This efficient, earth‐abundant, and nonnoble water splitting catalyst for artificial photosynthesis is thoroughly analyzed by various spectroscopic techniques with the aim of investigating its photocatalytic mechanism under visible‐light illumination. The main catalyst of CoS‐2 exhibits considerable H 2 evolution rate of 1196 µmol h −1 g −1 and O 2 yield of 63.5%. The efficient activity is attributed to the effective electron transfer between the photosensitizer and catalyst, which is verified by transient absorption experiments. The effective electron transfer between the photosensitizer and catalyst during water oxidation is verified by the dramatic decline of [Ru(bpy) 3 ] 3+ concentration in the presence of the catalyst CoS‐2. At the same time, transient absorption experiments support a rapid electron transfers from 3 EY* (excited photosensitizer eosin‐Y) to the catalyst CoS‐2 for efficient hydrogen evolution.

One of the main goals of current renewable energy research is continuously developing low-cost, robust, and environmentally friendly electrocatalysts to replace scarce and unstable precious metal catalysts. Hence, a novel high-performance electrocatalytic water-splitting material based on a metal–organic framework (MOF)-derived Cu-doped Co9S8 nanorod array was successfully prepared. Using 1,3,5-benzenetricarboxylic acid (H3BTC) as a ligand, a CuCo MOF (CuCo-MOF) nanobelt array precursor was prepared and then transformed into a Cu-Co9S8 nanorod by a simple sulfurization process using thioacetamide as a sulfur source. It is worth noting that the intermediate CuCo-MOF nanobelts play a key role in the generation of nanostructures of the Cu-doped Co9S8 nanorod array. As one of the most promising bifunctional electrocatalysts reported, the Cu-doped Co9S8-6h only needs 260 mV to drive 50 mA cm–2 for oxygen evolution reaction and 62 mV to drive 10 mA cm–2 for hydrogen evolution reaction. When the Cu-doped Co9S8-6h was used as an electrode in a self-made two-electrode system, it requires only 1.49 V of cell voltage to drive 10 mA cm–2, which is one of the smallest open-circuit voltages reported. Density functional theory results show that the superior performance of the Cu-doped Co9S8-6h is attributed to increased conductivity and increased water adsorption energy because of Cu doping. By comparing the water adsorption energy of Co2+, Cu2+, and Co3+, it is proved that the active Cu2+ replaces the inert Co2+, and less low-valence Co2+ sites make the Cu-doped Co9S8-6h show superior catalytic activity. These results demonstrate that the Cu-doped Co9S8-6h nanorod array can be used as an excellent bifunctional electrocatalyst for overall water splitting, and this work will provide an excellent synergistic strategy to energy storage and conversion.

/NF catalyst is ascribed to the exposure of more active centers and a faster electron transfer rate. This work develops a novel method for exploiting Earth-abundant, robust and environmentally friendly OER and UOR electrocatalysts.