Dynamic Metal-Coordinated Adhesive and Self-Healable Antifreezing Hydrogels for Strain Sensing, Flexible Supercapacitors, and EMI Shielding Applications.

Dynamic metal-coordinated adhesive and self-healable hydrogel materials have garnered significant attention in recent years due to their potential applications in various fields. These hydrogels can form reversible metal-ligand bonds, resulting in a network structure that can be easily broken and reformed, leading to self-healing capabilities. In addition, these hydrogels possess excellent mechanical strength and flexibility, making them suitable for strain-sensing applications. In this work, we have developed a mechanically robust, highly stretchable, self-healing, and adhesive hydrogel by incorporating Ca2+-dicarboxylate dynamic metal-ligand cross-links in combination with low density chemical cross-links into a poly(acrylamide-co-maleic acid) copolymer structure. Utilizing the reversible nature of the Ca2+-dicarboxylate bond, the hydrogel exhibited a tensile strength of up to ∼250 kPa and was able to stretch to 15-16 times its original length. The hydrogel exhibited a high fracture energy of ∼1500 J m-2, similar to that of cartilage. Furthermore, the hydrogel showed good recovery, fatigue resistance, and fast self-healing properties due to the reversible Ca2+-dicarboxylate cross-links. The presence of Ca2+ resulted in a highly conductive hydrogel, which was utilized to design a flexible resistive strain sensor. This hydrogel can strongly adhere to different substrates, making it advantageous for applications in flexible electronic devices. When adhered to human body parts, the hydrogel can efficiently detect limb movements. The hydrogel also exhibited excellent performance as a solid electrolyte for flexible supercapacitors, with a capacitance of ∼260 F/g at 0.5 A/g current density. Due to its antifreezing and antidehydration properties, this hydrogel retains its flexibility at subzero temperatures for an extended period. Additionally, the porous network and high water content of the hydrogel impart remarkable electromagnetic attenuation properties, with a value of ∼38 dB in the 14.5-20.5 GHz frequency range, which is higher than any other hydrogel without conducting fillers. Overall, the hydrogel reported in this study exhibits diverse applications as a strain sensor, solid electrolyte for flexible supercapacitors, and efficient material for electromagnetic attenuation. Its multifunctional properties make it a promising candidate for use in various fields as a state-of-the-art material.

Bimetallic Organic Frameworks via In Situ Solvothermal Sol-Gel-Crystal and Sol-Crystal Transformation as Durable Electrocatalysts for Oxygen Reduction Reaction.

The in situ solvothermal conversion of metal-organic gels (MOGs) to crystalline metal-organic frameworks (MOFs) represents a versatile and ingenious strategy that has been employed for the synthesis of MOF materials with specific morphologies, high yield, and improved functional properties. Herein, we have adopted an in situ solvothermal conversion of bimetallic MOGs to crystalline bimetallic MOFs with the aim of introducing a redox-active metal heterogeneity into the monometallic counterpart. The formation of bimetallic NiZn-MOF and CoZn-MOF via in situ solvothermal sol-gel-crystal and sol-crystal transformation is found to depend on the solvent systems used. The sol-to-gel-to-crystal transformation of NiZn-MOF via the formation of NiZn-MOG is found to occur through the gradual disruption of gel fibers leading to subsequent formation of microcrystals and single crystals of NiZn-MOF. These bimetallic MOFs and MOGs serve as promising electrocatalysts for oxygen reduction reaction (ORR) with an excellent methanol tolerance property, which can be attributed to the enhanced mass and charge transfer, higher oxygen vacancies, and bimetallic synergistic interactions among the heterometals. This work demonstrates a convenient strategy for producing bimetallic MOGs to MOFs through the introduction of a redox-active metal heterogeneity in the inorganic hybrid functional materials for fundamental and applied research. Our results connect MOGs and MOFs which have been regarded as having opposite physical states, that is, soft vs hard, and provide promising structural correlation between MOGs and MOFs at the molecular level.

Surface Tailoring-Modulated Bifunctional Oxygen Electrocatalysis with CoP for Rechargeable Zn-Air Battery and Water Splitting.

The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.

Dimension-Controlled Synthesis of Hybrid-Mixed Halide Perovskites for Solar Cell Application.

Despite the rapid improvement of photovoltaic (PV) efficiency in hybrid organic-inorganic metal halide perovskites (HOIPs), the fabrication procedure of a compact thin film in a large-area application is still a tedious work. Apart from the quality of the thin film, the stability of the perovskite materials and the expensive organic hole transport layer (HTL) within the HOIP-based PV device are the major issues that need to be addressed prior to their commercialization. Herein, a unique glass rod-based facile fabrication technique for producing a compact and stable thin film utilizing a mixed-halide-based perovskite precursor solution is demonstrated. The fabricated devices deliver high photoconversion efficiency (PCE) without the use of any HTL and show an excellent stability under ambient conditions. By varying the organic CH3NH3I (MAI) and inorganic PbBr2 content, perovskite materials with different dimensions, i.e., 3D, 2D, and 1D, are synthesized to produce an active layer for PV devices. Although a 2D single-halide perovskite is reported earlier, herein two different mixed-halide 2D perovskites, i.e., MA2PbI2Br2 and MAPb2IBr4, are synthesized successfully, and their performance is compared in detail along with that of 1D and 3D mixed-halide perovskites. The facile synthesized mixed-halide 2D-based MA2PbI2Br2 perovskite shows a PCE of 10.14% with a high stability of 92% after 100 days without encapsulation, which is much superior as compared to that of the mixed-halide 3D MAPbIBr2. The semiconducting behavior as well as the nature of the bandgap of the synthesized compounds is examined by pursuing density functional theory calculations. Specifically, the role of iodine doping to modify the electronic band structure is investigated, and introduction of iodine is found to reduce the effective masses of both electrons and holes in the perovskite material.

A core-shell magnet Mn70Bi30grown at seeds in magnetic fields and its impacts on its spin-dynamics, Curie point and other tailored properties.

The MnBi alloys is a model series of rare-Earth free magnets for surge of technologies of small parts of automobiles, power generators, medical tools, memory systems, and many others. The magnetics stem primarily at unpaired Mn-3d5spins (a 4.23µBmoment) align parallel via an orbital moment 0.27µBof Bi-5d106s2p3in a crystal lattice. Thus, using a surplus Mn (over Bi) in a Mn70Bi30type alloy designs a spin-rich system of duly tailored properties useful for magnetics and other devices. In this view, we report here a strategy of a refined alloy powder Mn70Bi30can grow into small crystals of hexagonal (h) plates at seeds as annealed in magnetic fields (in H2gas). So, small h-plates (30 to 50 nm widths) are grown up at (002) facets, wherein the edges are turned down in a spiral (≤2.1 nm thicknesses) in a core-shell structure. The results are described with x-ray diffraction, lattice images and magnetic properties of a powder Mn70Bi30(milled in glycine) is annealed at 573 K for different time periods, so to the Mn/Bi order at the permeable facets (seeds). Duly annealed samples exhibit an enhanced magnetization,Ms→ 70.8 emu g-1, with duly promoted coercivityHc→ 10.810 kOe (15.910 kOe at 350 K), energy-product 14.8 MGOe, and the crystal-field-anisotropy,K1→ 7.6 × 107erg cm-3, reported at room temperature. Otherwise,Msshould decline at any surplus 3d5-Mn spins order antiparallel at the antisites. Enhanced Curie point 658.1 K (628 K at Mn50Bi50alloy) anticipates that a surplus Mn does favor the Mn-Bi exchange interactions. Proposed spin models well describe the spin-dynamics and lattice relaxations (on anneals) over the lattice volume (with twins) and spin clusters.

Laterally Grown Strain-Engineered Semitransparent Perovskite Solar Cells with 16.01% Efficiency.

Hybrid organometallic halide perovskite-based semitransparent solar cell research has garnered significant attention recently due to their promising applications for smart windows, tandem devices, wearable electronics, displays, and sustainable internet-of-things. Though considerable progress has been made, stability, controlling the crystalline qualities, and growth orientation in perovskite thin films play crucial roles in improving the photovoltaic (PV) performance. Recently, strain modulation within the perovskite gathers an immense interest that is achieved by the ex situ process. However, little work is reported on in situ strain modulation, which is presented here. Apart from the challenges in the fabrication of high-efficiency perovskite solar cell (PSC) devices under ambient conditions, the stability of organic hole-transporting materials needs urgent attention. Herein, a single-step deposition of formamidiniumchloride (FACl)-mediated CH3NH3PbI3 (MAPbI3) thin films without an inert atmosphere and CuI as the inorganic hole-transporting material is demonstrated for their potential application toward semitransparent PSCs. The FACl amount in MAPbI3 (mg/mL) plays a critical role in controlling the crystallinity, growth orientations, and in situ strains, which modulate the charge carrier transport dynamics, thereby improving the efficiency of the PSC device. A photoconversion efficiency of 16.01% has been achieved from MAPbI3 with 20 mg/mL of FACl additive incorporation. The modification of the structural, electronic, and optical properties and the origin of strain in the as-synthesized MAPbI3 domains due to the addition of FACl are further validated with experimental findings in detail using density functional theory simulations.

Effect of Cooperative Redox Property and Oxygen Vacancies on Bifunctional OER and HER Activities of Solvothermally Synthesized CeO2/CuO Composites.

Herein, we report the synthesis of the CeO2/CuO composite as a bifunctional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) electrocatalyst in a basic medium. The electrocatalyst with an optimum 1:1 CeO2/CuO shows low OER and HER overpotentials of 410 and 245 mV, respectively. The Tafel slopes of 60.2 and 108.4 mV/dec are measured for OER and HER, respectively. More importantly, the 1:1 CeO2/CuO composite electrocatalyst requires only a 1.61 V cell voltage to split water to achieve 10 mA/cm2 in a two-electrode cell. The role of oxygen vacancies and the cooperative redox activity at the interface of the CeO2 and CuO phases is explained in the light of Raman and XPS studies, which play the determining factor for the enhanced bifunctional activity of the 1:1 CeO2/CuO composite. This work provides guidance for the optimization and design of a low-cost alternative electrocatalyst to replace the expensive noble-metal-based electrocatalyst for overall water splitting.

Recent advances in non-noble metal-based oxide materials as heterogeneous catalysts for C-H activation.

As the world moves towards a more sustainable future, it is desirable to replace the homogeneous catalytic processes and conventional industrial processes with energy-efficient, cost-effective, and greener heterogeneous processes. In heterogeneous catalysis, non-noble metal-based oxide catalysts have been gaining traction recently, as they are affordable, highly stable, and thus appealing for different industrial uses. C-H activation is crucial to numerous organic reactions such as hydrogenation, oxidation, reduction, oxidative coupling, substitution, and C-C coupling. However, it is a challenging task due to its high bond energy and high kinetic barrier of C-H bond cleavage, making it inert in most conventional processes, thereby requiring efficient catalysts. In recent years, non-noble metal-based oxides have made a huge difference in the reactivity of the C-H bond through their tailored surface area, surface charge, redox properties, and oxygen vacancies. In this perspective article, we discuss the application of nanostructured non-noble metal oxides with tailored surface properties through nano-structuring, doping, vacancy creation, and heterostructure formation in reactions involving C-H activation as the key step. Specifically, we focus on the essential surface properties of catalysts that play a major role in the C-H activation reaction for sustainable industrial applications.

Structural ordering at magnetic seeds with twins at boundaries of a core-shell alloy Mn60Bi40and tailored magnetic properties.

A spin Mn3d5-rich Mn60Bi40alloy reveals a model system in order to tailor profound magnetic properties at unpaired 3d5spins in such alloys of a core-shell structure. As annealed (at a critical temperature 573 K in H2gas), a refined powder (in glycine) grows onα-MnBi seeds (crystallites) present in it at Mn/Bi atoms order over topological layers, preferentially along (110) planes, at a self-confined structure at seeds of an anisotropic shape of hexagonal (h) plates (25-85 nm widths). In terms of the HRTEM images, the atoms turn down at edges (at the plates grow up) in a spiral layer, ≤ 2.1 nm thickness, of small core-shells. A spin model is proposed to delineate a way at the spins can pin down at the edges, form single magnetic domains, and raise coercivity (Hc), with no much loss of net magnetic moment. The X-ray diffraction and HRTEM images corroborate the results of topological pacing of atoms at the h-plates at anneals. A novelty is that a core-shell leads to tailor a superbHc, as much as 11.110 kOe (16.370 kOe at 350 K), with a fairly large magnetization, 76.5 emu g-1, at near 300 K. An enhanced Curie point 650.1 K (628 K at Mn50Bi50alloy) confers a surplus 3d5-Mn spin sensitively tunesα-MnBi stoichiometry and so its final magnetic structure. A refined alloy powder so made is useful to make powerful magnets and devices in the forms of films and bonded magnets in different shapes for uses as small tools, tweezers, and other devices.

In Situ Grown Mn(II) MOF upon Nickel Foam Acts as a Robust Self-Supporting Bifunctional Electrode for Overall Water Splitting: A Bimetallic Synergistic Collaboration Strategy.

The design of highly efficient, cost-effective non-noble metal-based electrocatalysts with superior stability for overall water splitting (OWS) reactions is of great importance as well as of immense challenge for the upcoming sustainable and green energy conversion technologies. Herein, a convenient and simple in situ solvothermal method has been adopted to fabricate a self-supported, binder-free 3D electrode (Mn-MOF/NF) by the direct growth of a newly synthesized carboxylate-based pristine Mn(II)-metal-organic framework (Mn-MOF) upon the conducting substrate nickel foam (NF). The binder-free Mn-MOF/NF electrode exhibits excellent performances toward OWS with ultralow overpotentials of 280 mV@20 mA cm-2 for the oxygen evolution reaction (OER) and 125 mV@10 mA cm-2 for the hydrogen evolution reaction (HER) with remarkable durability. Mn-MOF/NF can also attain a current density of 10 mA cm-2 with a low cell voltage of 1.68 V in a 0.1 M KOH solution in a two-electrode system for OWS. The direct growth of nonconducting electroactive Mn-MOF materials upon conducting substrate NF provides an excellent mass transport of the electrolyte with a relatively low contact resistance due to the strong catalyst-substrate contact and enhances the efficient electron transport for OWS. The redox chemical etching of the self-sacrificial substrate NF during solvothermal synthesis introduces redox-active Ni2+ in Mn-MOF/NF. Thus, the excellent OWS electrocatalytic activity can mainly be attributed to the bimetallic synergistic collaboration of the two redox active metal centers (Mn2+ and Ni2+) along with the excellent support surface of NF, which provides a high specific surface area and maximum utilization of the electroactive metal ion sites by preventing the self-aggregation of the active sites. The Mn-MOF/NF electrode also exhibits superb stability and durability for a prolonged time throughout the multiple cycles of full water splitting reactions. Therefore, this work elucidates a convenient and smart approach for constructing MOF-based bifunctional electrocatalysts for OWS.

Boosting the Photocatalytic H2 Evolution and Benzylamine Oxidation using 2D/1D g-C3N4/TiO2 Nanoheterojunction.

The present research aims at the elevation of solar-to-chemical energy conversion with extortionate performance and sustainability. The nanostructured materials are revolutionizing the water splitting technology into decoupled hydrogen with simultaneous value-added organic chemical production. Yet, the bottleneck in semiconductor photocatalysis is rapid charge recombination and sluggish reaction kinetics. Herein, we demonstrate an efficient and non-noble metal-based catalyst for successful redox reaction with a theoretical modeling through density functional theory (DFT) study. Implementing this robust approach on 2D/1D ultrathin g-C3N4 nanosheets and TiO2 nanowires heterojunction, we achieved H2 production of 5.1 mmol g-1 h-1 with apparent quantum efficiency of 7.8% under visible light illumination and 93% of benzylamine conversion to N-benzylidene benzylamine in situ. The interface of 2D g-C3N4 nanosheets and 1D nanowires provide ample active sites and extends the visible light absorption with requisite band edge position for the separation of photoinduced charge carriers with superior stability. The electronic properties, band structure, and stability of the heterojunction are further investigated via DFT calculations which corroborate the experimental results and in good agreement for the enhanced activity of the heterojunction.

Overcoming challenges due to enhanced biomedical waste generation during COVID-19 pandemic.

Biomedical wastes (BMWs) are potentially infectious to the environment and health. They are co-dependent and accumulative during the ongoing coronavirus disease-2019(COVID-19) pandemic. In India the standard treatment processes of BMWs are incineration, autoclaving, shredding, and deep burial; however, incineration and autoclaving are the leading techniques applied by many treatment providers. These conventional treatment methods have several drawbacks in terms of energy, cost, and emission. But the actual problem for the treatment providers is the huge and non-uniform flow of the BMWs during the pandemic. The existing treatment methods are lacking flexibility for the non-uniform flow. The Government of India has provisionally approved some new techniques like plasma pyrolysis, sharp/needle blaster, and PIWS-3000 technologies on a trial basis. But they are all found to be inadequate in the pandemic. Therefore, there is an absolute requirement to micromanage the BMWs based on certain parameters for the possible COVID-19 like pandemic in the future. Segregation is a major step of the BMW management. Its guideline may be shuffled as segregation at the entry points followed by collection instead of the existing system of the collection followed by segregation. Other steps like transportation, location of treatment facilities, upgradation of the existing treatment facilities, and new technologies can solve the challenges up to a certain extent. Technologies like microwave treatment, alkaline hydrolysis, steam sterilization, biological treatment, catalytic solar disinfection, and nanotechnology have a lot of scopes for the treatment of BMWs. Hi-tech approaches in handling and transportation are found to be fruitful in the initial steps of BMW management. End products of the treated BMWs can be potentially fabricated for the application in the built environment. Some policies need to be re-evaluated by the health care facilities or government administrations for efficient BMW management.

Structurally modified V2O5based extrinsic pseudocapacitor.

Rapidly changing demand on energy storage systems makes it essential to redesign the device architecture and materials required to fabricate the devices. It is crucial to introduce capacitive behaviour in a conventional energy storage device (batteries) to improve the lifetime and power efficiency of the hole energy storage system. The charge storing nature of electrode material primarily depends on particle size, grain size, the electrode's chemical structure, and effective diffusion lengths for electrolytes within the electrode. Here V2O5based Li-ion battery electrode is transformed into a Li-ion pseudocapacitive electrode by structural modifications. The modified structures are achieved by optimizing reaction pressure to obtain larger, medium and smaller V2O5particles (namely, V2O5-L, V2O5-M and V2O5-S). As a result, the plateau regions in galvanostatic charge-discharge plots and highly intense redox peaks in the CV plots of V2O5-L get flattened for V2O5-S. Also, the lucrative improvement in rate capabilities and stability for V2O5-S indicates induced pseudocapacitance in V2O5. Some devices are fabricated with the extrinsic pseudocapacitive material (V2O5-S), providing 4.36 mWh cm-3volumetric energy density with 125 mW cm-3volumetric power density. The device retains around 95% of its initial capacitance after 10k cycles and holds up to 63% after 25k stability cycles.

Room Temperature Acid-Free Greener Synthesis of Imine Using Cobalt-Doped Manganese Tungstate.

Facile synthesis of an imine compound through a greener route is still a challenging task. Industrial processes rely on the age-old Schiff reaction for the synthesis of imine, which are reversible and nongreen from an environmental viewpoint. Herein, cobalt-doped manganese tungstate with two different morphologies is synthesized and demonstrated as a recyclable catalyst for imine synthesis from the condensation of an aldehyde and an amine with 73% yield of an imine in a nonaqueous and nonacidic environment at room temperature. The high catalytic activity is attributed to cobalt doping, high surface area, strong acidic site, and the polar nature of the catalyst. The stability and recyclability test shows that the catalytic activity remains the same after several cycles, which is crucial from the industrial point of view. The formation of imine is found to follow an alternative mechanism in an irreversible manner with a polar four-membered intermediate unlike the conventional method. The demonstrated process has several advantages including irreversibility, "greener", environmental friendly, and energy-efficient.

Synthesis of V-shaped MnO2 nanostructure and its composites with reduced graphene oxide for supercapacitor application.

A unique V-shaped MnO2 nanostructure is synthesized with a weak acid (acetic acid) using the microwave-assisted hydrothermal technique. To improve the performance of MnO2 in supercapacitor applications, its composite was prepared with reduced graphene oxide (rGO), i.e., MnO2/rGO, with different weight ratios of MnO2 and rGO. The specific capacitance values of the as-synthesized V-shaped MnO2 nanostructure and MnO2/rGO nanocomposite were calculated to be 64.75 and 88.95 F g-1 at a current density of 0.5 A g-1, respectively, in 1 M Na2SO4 electrolyte. Furthermore, a two-electrode asymmetric supercapacitor device was fabricated using the MnO2/rGO nanocomposite as a positive electrode and activated carbon as a negative electrode. The device has shown energy densities of 25.14 and 17.95 W h kg-1 at 0.25 and 1 kW kg-1 power densities, respectively. These values suggest that the MnO2/rGO nanocomposite is a promising material for supercapacitor devices.

Small core-shell Mn0.5Bi0.5-Bi (⩽3 at%) magnets, the anisotropic growth of crystallite nanoplates, interface-bridging, and tailored magnetic properties.

The binary alloy Mn0.5+xBi0.5-x, x ⩽ 0.05, is a promising rare-earth-free magnetic material, with high-energy-density (a critical characteristic for electric motors and power electronics), low cost, and significant magnetic properties for multiple uses at room temperature. In this article, we report how a free Bi, when precipitated over Mn0.5+xBi0.5-x (x ⩽ 0.05) of small crystallites, diffuses back into a stable Mn0.5+xBi0.5-x, x → 0, via a peritectic reaction, which facilitates preferential growth of small core-shell crystallites with multiple facets, having the potential for tailored magnetic properties. This growth travels slowly in the anisotropic channels of vacancies on annealing the reactive nanopowder at a critical 573 K temperature in Ar gas. Thus, an initial crystallite size of D ∼ 27 nm grows to only 38 nm in a reaction extended over a period of 96 h. A transient phase, x > 0, which has Bi vacancies, primarily grows in the (101) and (110) facets, filling the vacancies over a 6.41% larger crystal density. If any excess Mn is present, it segregates over a saturated phase, combines with free Bi, and ultimately forms a stable alloy phase. The small crystallites contain an inbuilt surface Bi-layer (shell), with a 1-2 nm thickness, in a core-shell of nanoplates (20-60 nm width), as shown in the high resolution transmission electron microscope images. In the proposed microscopic model, with hybridized Mn-d5 and Bi-p3 electrons (also spins), the magnetic properties are readily controlled. Thus, at 300 K, a maximum coercivity Hc = 9.850 kOe (14.435 kOe at 350 K) develops (Hc = 5.010 kOe in the initial) in critical single domains (D ∼ 33 nm). A net 72.5 emu g-1 magnetization occurs, with an enhanced TC = 641.5 K (600.5 K at x ∼ 0.05) on an order of enhanced anisotropy constant K1, demonstrating the significant effects of this core-shell structure of small crystallites.

Tuning the morphology of CeO2 nanostructures using a template-free solvothermal process and their oxygen reduction reaction activity.

In fuel cells, the oxygen reduction reaction (ORR) at the cathode plays a crucial role in their performance. High cost, low abundance, catalyst poisoning, and poor durability of the pioneering ORR catalyst Pt make it less desirable for commercial fuel cells. Herein, we demonstrate a greener process to synthesize CeO2 nanostructures by varying reaction parameters in a single-step solvothermal route and provide a detailed mechanism for the formation of CeO2 nanostructures with different shapes. The shape and size of the CeO2 nanostructures such as hollow/solid spheres, triangular flakes, nanotubes, and flower-like structures result in a strong effect on their ORR activity. A normalized effect of effective surface area and oxygen vacancies in CeO2 nanostructures is found to govern the ORR activity order. Among the CeO2 nanostructures, hollow spheres exhibit the best ORR activity with a four-electron reduction pathway. Moreover, they show comparable ORR activity and outstanding stability as well as methanol fuel tolerance and are a substitute for Pt/C.

Illustrating the Role of Quaternary-N of BINOL Covalent Triazine-Based Frameworks in Oxygen Reduction and Hydrogen Evolution Reactions.

Defective nitrogen-doped carbon materials have shown a promising application as metal-free electrocatalysts in the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). However, there are still some challenges in the tuning of metal-free electrocatalysts and in understanding the roles of various nitrogen species in their electrocatalytic performance. Herein, we design a covalent triazine framework (CTF)-based material as an effective metal-free bifunctional electrocatalyst. We chose BINOL-CN (2,2'-dihydroxy-[1,1'-binaphthalene]-6,6'-dicarbonitrile) as both a carbon and a nitrogen source for the fabrication of N-containing CTF-based materials. Four BINOL-CTFs with varying N-functionalities (pyridinic-N/triazine-N, pyrrolic-N, quaternary-N, and pyridine-N-oxide) were successfully obtained. These materials were evaluated in the ORR and the HER in basic and acidic conditions, respectively. The best material has an onset potential of 0.793 V and a half-wave potential of 0.737 V, and it follows first-order kinetics in a 4e- pathway in the ORR reaction. The same material shows an impressive HER activity with an overpotential of 0.31 V to achieve 10 mA/cm2 and a small Tafel slope of 41 mV/dec, which is comparable to 31 mV/dec for Pt/C, making it a potential bifunctional electrocatalyst. We showed that the ORR and HER reactivity of CTF-based materials depends exclusively on the amount of quaternary-N species and on the available surface area and pore volume. This work highlights the engineering of CTF materials with varying amounts of N species as high-performance bifunctional electrocatalysts.

2D MOFs with Ni(II), Cu(II), and Co(II) as Efficient Oxygen Evolution Electrocatalysts: Rationalization of Catalytic Performance vs Structure of the MOFs and Potential of the Redox Couples.

Earth-abundant transition-metal-based metal-organic frameworks (MOFs) are of immense interest for the development of efficient and durable heterogeneous water splitting electrocatalysts. This repot explores the design of two-dimensional (2D) MOFs with redox-active metal centers (Ni(II), Co(II), and Cu(II)) containing two types of electron-rich linkers such as bis(5-azabenzimidazole), linear L1 and angular L2, and aromatic dicarboxylates. The electron-rich linkers are considered to stabilize the higher oxidation state of the redox-active metal centers in the course of the electrocatalytic oxygen evolution reaction (OER) process. The 2D MOFs of L1 and L2 with Co(II) (1 and 3) and Ni(II) (2 and 4) have been produced via the conventional hydrothermal synthesis, while the MOFs of Cu(II) (Cu@1 and Cu@3) are obtained by the postsynthetic transmetallation reaction of MOFs 1 and 3. The electrocatalytic OER activities of the six MOFs have been studied to explore the influence of the redox potential of the transition-metal quasi-reversible couples and the coordination environment around the redox-active metal centers in the electrocatalytic activity. The lowest overpotential of 370 mV exhibited by MOF 2 with the highest current density and TOF value indicates the importance of the presence of coordinated water molecules and the lowest redox potential value of the most favorable quasi-reversible couple Ni+2/Ni+3. These catalysts exhibit a remarkable stability up to 1000 OER cycles. These studies pave the way for the design of MOF materials toward the development of a promising heterogeneous OER electrocatalyst.

Engineering the Morphology and Crystal Phase of 3 D Hierarchical TiO2 with Excellent Photochemical and Photoelectrochemical Solar Water Splitting.

Owing to their unique characteristics, hierarchical TiO2 nanostructures have several advantages in solar-fuel production. In this work, a single-step approach has been developed to control both the crystal phase and morphology of TiO2 with 3 D urchin-like structure via a surfactant-free solvothermal route. The growth of 3 D hierarchical structure with phase-engineered band alignment, the role of the H2 O/HCl ratio, and fine-tuning of the reaction parameters are investigated systematically. An optimum ratio of anatase (41 %) to rutile (59 %) in the mixed-phase TiO2 (AR-2) results in excellent photocatalytic H2 generation activity of 5753 µmol g-1 after 5 h of irradiation with apparent quantum yields of 20.9 % at 366 nm and 4.5 % at 420 nm. The superior performance of AR-2, attributed to efficient separation of charge carriers through the phase junction, is apparent from the transient photocurrent response and photoluminescence studies. The 3 D urchin-like pure rutile TiO2 (R-1) composed of nanorods shows enhanced photocatalytic activity compared with pure anatase and pure rutile TiO2 nanoparticles, and this demonstrates the role of morphology. The best-performing mixed-phase 3 D TiO2 shows excellent durability up to 25 h and is shown to produce 3522 µmol g-1 of H2 under natural sunlight, which highlights its potential for long-term application.