Visible light driven water splitting through an innovative Cu-treated-δ-MnO2 nanostructure: probing enhanced activity and mechanistic insights

Ceria-doped SnO2 nanocubes for solar light-driven photocatalytic hydrogen production

The photocatalytic generation of hydrogen via solar energy using metal oxide semiconductor catalysts is a clean and renewable process which has the potential of solving the current energy nexus. SnO₂ is one such well-studied and established photocatalyst currently in practice but is only ultraviolet-light active which accounts for only 4% of the total incoming solar energy. The current study focuses on bringing this SnO₂ into the visible range using ceria as a dopant. Sol-gel and combustion methods were employed for synthesis and the as-synthesized catalysts were characterized using XRD, BET, UV diffuse reflectance spectra, PL spectra, and SEM micrographs. A unique cuboid type morphology was observed in 6% ceria-doped SnO₂ which provided more active sites for light absorption and thus reported a remarkable hydrogen production rate of 1.978 mmol/h under sunlight which was almost 346 times that of pure SnO₂ (5.71 µmol/h). Photoluminescence spectra of ceria-doped SnO₂ showed lower peak positions as compared to the pure SnO₂ indicating a reduction in charge recombination and an increase in the life time of the active species which explains the enhanced hydrogen production rates. The recyclability study of the catalysts showed that the hydrogen amount produced in the fifth recycle was nearly 80% as the first cycle showing that the catalyst can be used very effectively for more than five cycles without compromising on the yield.

Peroxymonosulfate activation using a composite of copper and nickel oxide coated on SBA-15 for the removal of sulfonamide antibiotics

The sluggish Ni(II)/Ni(III) redox cycle does not benefit perxymonosulfate (PMS) activation for recalcitrant pollutant degradation. To solve this problem, a heterogeneous catalyst, Cu_0.2Ni_0.8O/SBA-15 (CNS), was constructed to activate PMS for decomposing two sulfonamide antibiotics, sulfachlorpyridazine (SACP) and sulfapyridine (SAP). SACP and SAP were completely degraded over Cu_0.2Ni_0.8O/SBA-15/PMS (CNSP) after 90 min. O₂^.- was the dominant active species involved in the degradation of SACP and SAP. Structural analysis and elemental valence state observations indicated that Cu(Ⅰ) provided electrons through Cu-O-Ni bonds to realize the charge compensation for Ni(Ⅲ) in the CNSP system. Thus, the in situ Cu(I)/Cu(II) promoting the Ni(II)/Ni(III) cycle could accelerate the PMS activation. This work provides new insights into the electron transfer between transition metals and the charge compensation mechanism for PMS activation. The degradation mechanism was proposed based on the XPS results before and after the reaction, a radical quenching test, and an EPR test. Combined with the SACP and SAP degradation intermediates identified by LC-MS, we suggest that the choice of treatment process depends on the occurrence of a steric hindrance effect between the molecular structure of the degradation target and free radicals.

In Situ Grown CoMn2 O4 3D-Tetragons on Carbon Cloth: Flexible Electrodes for Efficient Rechargeable Zinc-Air Battery Powered Water Splitting Systems

The integration of energy conversion and storage systems such as electrochemical water splitting (EWS) and rechargeable zinc-air battery (ZAB) is on the vision to provide a sustainable future with green energy resources. Herein, a unique strategy for decorating 3D tetragonal CoMn₂ O₄ on carbon cloth (CMO-U@CC) via a facile one-pot in situ hydrothermal process, is reported. The highly exposed morphology of 3D tetragons enhances the electrocatalytic activity of CMO-U@CC. This is the first demonstration of such a bifunctional activity of CMO-U@CC in an EWS system; it achieves a nominal cell voltage of 1.610 V @ 10 mA cm^-2 . Similarly, the fabricated rechargeable ZAB delivers a specific capacity of 641.6 mAh g_zn ^-1 , a power density of 135 mW cm^-2 , and excellent cyclic stability (50 h @ 10 mA cm^-2 ). Additionally, a series of flexible solid-state ZABs are fabricated and employed to power the assembled CMO-U@CC-based water electrolyzer. To the best of the authors' knowledge, this is the first demonstration of an in situ-grown binder-free CMO-U@CC as a flexible multifunctional electrocatalyst for a built-in integrated rechargeable ZAB-powered EWS system.

Zn2+ Pre-Intercalation Stabilizes the Tunnel Structure of MnO2 Nanowires and Enables Zinc-Ion Hybrid Supercapacitor of Battery-Level Energy Density

Although there has been tremendous progress in exploring new configurations of zinc-ion hybrid supercapacitors (Zn-HSCs) recently, the much lower energy density, especially the much lower areal energy density compared with that of the rechargeable battery, is still the bottleneck, which is impeding their wide applications in wearable devices. Herein, the pre-intercalation of Zn²⁺ which gives rise to a highly stable tunnel structure of Zn_x MnO₂ in nanowire form that are grown on flexible carbon cloth with a disruptively large mass loading of 12 mg cm^-2 is reported. More interestingly, the Zn_x MnO₂ nanowires of tunnel structure enable an ultrahigh areal energy density and power density, when they are employed as the cathode in Zn-HSCs. The achieved areal capacitance of up to 1745.8 mF cm^-2 at 2 mA cm^-2 , and the remarkable areal energy density of 969.9 µWh cm^-2 are comparable favorably with those of Zn-ion batteries. When integrated into a quasi-solid-state device, they also endow outstanding mechanical flexibility. The truly battery-level Zn-HSCs are timely in filling up of the battery-supercapacitor gap, and promise applications in the new generation flexible and wearable devices.

Enhanced electrochemical oxygen and hydrogen evolution reactions using an NU-1000@NiMn-LDHS composite electrode in alkaline electrolyte

A well-designed NU-1000@NiMn-LDHS (NU@LDHS) composite can offer efficient electrocatalytic performance with ultralow HER and OER overpotentials of 93 and 129 mV, respectively, at a current density of 10 mA cm⁻² in 2 M KOH.

Mixed-Ligand-Architected 2D Co(II)-MOF Expressing a Novel Topology for an Efficient Photoanode for Water Oxidation Using Visible Light

The structural diversity of Co(II) metal centers is known to influence their physicochemical properties. A novel two-dimensional (2D) Co(II)-MOF {[Co₅(HL)₄(dpp)₂(H₂O)₂(μ-OH)₂]·21H₂O} n has been designed and synthesized by adopting a mixed-ligand strategy, using 1,3-di(4-pyridyl)propane (dpp) colinker with a flexible spacer H₃L (H₃L: 5-(2 carboxybenzyloxy)isophthalic acid). Co(II)-MOF features a 2D network, which is further interpenetrated among the equivalent sets and therefore results in a 3D supramolecular network. Topologically, the entire network can be viewed as a (3,4,8)-connected three-nodal net with the extended point symbol of {4.5.7}4{4¹².5².7¹⁰.9⁴}{5².8.9².10}2, duly assigned to the novel topological type smm2. The functional utility of Co(II)-MOF is demonstrated by employing it toward oxygen evolution reaction (OER) in a photoelectrochemical cell, exhibiting appreciable photocurrents of up to 5.89 mA/cm² when used as an anode in a photoelectrochemical cell, while also displaying encouraging electrocatalytic currents of 9.32 mA/cm² (at 2.01 V vs RHE) for the OER. Moreover, detailed electrochemical impedance spectroscopy studies confirm enhanced charge-transfer kinetics and improved conductivity under illumination with minimal effect of interfacial phenomena. This work provides a reference for the expanding field of research into applications of MOF materials and establishes MOF materials as favorable candidates for sustainable and efficient design of electrodes for water splitting.