Enhancing Energy Storage Capacity of 3D Carbon Electrodes Using Soft Landing of Molecular Redox Mediators.

The incorporation of redox-active species into the electric double layer is a powerful strategy for enhancing the energy density of supercapacitors. Polyoxometalates (POM) are a class of stable, redox-active species with multielectron activity, which is often used to tailor the properties of electrochemical interfaces. Traditional synthetic methods often result in interfaces containing a mixture of POM anions, unreactive counter ions, and neutral species. This leads to degradation in electrochemical performance due to aggregation and increased interfacial resistance. Another significant challenge is achieving the uniform and stable anchoring of POM anions on substrates to ensure the long-term stability of the electrochemical interface. These challenges are addressed by developing a mass spectrometry-based subambient deposition strategy for the selective deposition of POM anions onto engineered 3D porous carbon electrodes. Furthermore, positively charged functional groups are introduced on the electrode surface for efficient trapping of POM anions. This approach enables the deposition of purified POM anions uniformly through the pores of the 3D porous carbon electrode, resulting in unprecedented increase in the energy storage capacity of the electrodes. The study highlights the critical role of well-defined electrochemical interfaces in energy storage applications and offers a powerful method to achieve this through selective ion deposition.

Mitigating Interfacial Capacity Fading in Vanadium Pentoxide by Sacrificial Vanadium Sulfide Encapsulation for Rechargeable Mg-Ion Batteries.

Rechargeable Mg-ion Batteries (RMB) containing a Mg metal anode offer the promise of higher specific volumetric capacity, energy density, safety, and economic viability than lithium-ion battery technology, but their realization is challenging. The limited availability of suitable inorganic cathodes compatible with electrolytes relevant to Mg metal anode restricts the development of RMBs. Despite the promising capability of some oxides to reversibly intercalate Mg+2 ions at high potential, its lack of stability in chloride-containing ethereal electrolytes, relevant to Mg metal anode hinders the realization of a full practical RMB. Here the successful in situ encapsulation of monodispersed spherical V2O5 (≈200 nm) is demonstrated by a thin layer of VS2 (≈12 nm) through a facile surface reduction route. The VS2 layer protects the surface of V2O5 particles in RMB electrolyte solution (MgCl2 + MgTFSI in DME). Both V2O5 and V2O5@VS2 particles demonstrate high initial discharge capacity. However, only the V2O5@VS2 material demonstrates superior rate performance, Coulombic efficiency (100%), and stability (138 mA h g-1 discharge capacity after 100 cycles), signifying the ability of the thin VS2 layer to protect the V2O5 cathode and facilitate the Mg+2 ion intercalation/deintercalation into V2O5.

The inchoate horizon of electrolyzer designs, membranes and catalysts towards highly efficient electrochemical reduction of CO2 to formic acid.

The economic viability of CO2 reactors is contingent on the selectivity of the CO2 reduction reaction and the rate of product formation. For this, the rational design of electrolyzers also has a substantial impact on the figures of merit (current density, faradaic efficiency, cell durability). Thus, herein we portray a short review on the shortcomings, challenges and the recent developments on different reactor configurations, components and membrane structures for the efficient electrochemical CO2 reduction (CO2R) into HCOO-/HCOOH. Despite their low CO2 solubility and poor mass transport, H-type electrolyzers are commercialized due to their screening of a vast number of catalysts. In contrast, membrane-based gas and liquid phase flow reactors break the barriers faced by H-types through the incorporation of gas diffusion electrodes (GDEs) and the membrane electrode assembly (MEA). As the GDE forms the gas-liquid-solid interface, it allows the electrolyzers to generate current densities at the industrial level (200 mA cm-2). Intriguingly, a continuous liquid fed intermittent flow electrolyzer can control the electrolyte flow at a desired frequency and allow sufficient time for CO2 gas molecules to effectively reduce into HCOOH. Therefore, a high and stable faradaic efficiency (95%) is achieved in 4 h for HCOOH (576.98 mg) using the boron-doped diamond catalyst. Very recently, a novel strategy to enhance the CO2R to HCOO-/HCOOH has been adopted via the recirculation of by-products to the liquid phase MEA flow reactors, which substantially improves HCOO- selectivity, lowers material costs, and promotes CO2 mass transfer. In the end, the zero-gap electrolyzer has newly emerged and affords reduced ohmic losses, leading to a straight-forward implementation of industrial systems for CO2R to value-added products in the future. Besides, the efficiency of HCOO-/HCOOH production is also explored against proton exchange, anion exchange and bipolar membranes, and the pH of the electrolyte plays a dominant role in deciding the stability and characteristics of the membranes. It is also depicted that the product selectivity depends on different electrolyzer configurations. Recently, bimetallic alloys (Bi-Sn, Bi-In) and 2D layered composites (SnO2/rGO/CNT) have proven to be potential electrocatalysts (faradaic efficiency > 95%, highly selective and durable) assigned to the abundant active sites for CO2R. Based on the recent findings and future research directions, we draw reader's attention to construct economic, scalable and energy-efficient CO2R electrolyzers to realize the techno-economic predictions.

Calcination Strategy for Scalable Synthesis of Pithecellobium-Type Hierarchical Dual-Phase Nanostructured Cu x O to Columnar Self-Assembled CuO and Its Electrochemical Performances.

The search for low-cost environmentally benign promising electrode materials for high-performance electrochemical application is an urgent need for an applaudable solution for the energy crisis. For this, the present attempt has been made to develop a scalable synthetic strategy for the preparation of pure and dual-phase copper oxide self-hybrid/self-assembled materials from a copper oxalate precursor using the calcination route. The obtained samples were characterized by means of various physicochemical analytical techniques. Notably, we found that the BET surface area and pore volume of copper oxides measured by N2 adsorption-desorption decrease with the elevation of calcination temperature. From the XRD analysis, we observed the formation of a Cu2O cubic phase at low temperatures and a CuO monoclinic phase at high temperatures (i.e., 450 and 550 °C). FTIR and RAMAN spectroscopy were employed for bonding and vibrational structure analysis. The self-assembled dual-phase copper oxide particle as a pithecellobium-type hierarchical structure was observed through SEM of the sample prepared at 350 °C. The surface morphological structure for the samples obtained at 450 and 550 °C was a bundle-like structure developed though columnar self-assembling of the particles. All the above techniques confirmed the successful formation of Cu2O/CuO nanoparticles. Afterward, the electrochemical properties of the as-synthesized copper oxides reinforced by introducing carbon black (10% wt) were explored via cyclic voltammetry, electrochemical impedance spectroscopy, and galvanometric charge-discharge analysis. The Cu2O system exhibits the maximum specific capacitance performance value of 1355 F/g, whereas in the CuO system (at 450 and 550 °C), it possesses values of 903 and 724 F/g at a scan rate of 2 mV/s. This study reveals that the electrochemical properties of Cu2O are better than those of the CuO nanoparticles, which could be ascribed to the high surface area and morphology. The present assessment of the electrochemical properties of the developed material could pave the way to a low-cost electrode material for developing other high-performance hybrid electrodes for supercapacitor or battery applications.

Bimetallic nanoparticles for arsenic detection.

Effective and sensitive monitoring of heavy metal ions, particularly arsenic, in drinking water is very important to risk management of public health. Arsenic is one of the most serious natural pollutants in soil and water in more than 70 countries in the world. The need for very sensitive sensors to detect ultralow amounts of arsenic has attracted great research interest. Here, bimetallic FePt, FeAu, FePd, and AuPt nanoparticles (NPs) are electrochemically deposited on the Si(100) substrate, and their electrochemical properties are studied for As(III) detection. We show that trace amounts of As(III) in neutral pH could be determined by using anodic stripping voltammetry. The synergistic effect of alloying with Fe leads to better performance for Fe-noble metal NPs (Au, Pt, and Pd) than pristine noble metal NPs (without Fe alloying). Limit of detection and linear range are obtained for FePt, FeAu, and FePd NPs. The best performance is found for FePt NPs with a limit of detection of 0.8 ppb and a sensitivity of 0.42 µA ppb(-1). The selectivity of the sensor has also been tested in the presence of a large amount of Cu(II), as the most detrimental interferer ion for As detection. The bimetallic NPs therefore promise to be an effective, high-performance electrochemical sensor for the detection of ultratrace quantities of arsenic.

Size-selected TiO2 nanocluster catalysts for efficient photoelectrochemical water splitting.

Nanoclusters (NCs) are of great interest because they provide the link between the distinct behavior of atoms and nanoparticles and that of bulk materials. Here, we report precisely controlled deposition of size-selected TiO2 NCs produced by gas-phase aggregation in a special magnetron sputtering system. Carefully optimized aggregation length and Ar gas flow are used to control the size distribution, while a quadrupole mass filter provides precise in situ size selection (from 2 to 15 nm). Transmission electron microscopy studies reveal that NCs larger than a critical size (∼8 nm) have a crystalline core with an amorphous shell, while those smaller than the critical size are all amorphous. The TiO2 NCs so produced exhibit remarkable photoelectrochemical water splitting performance in spite of a small amount of material loading. NCs of three different sizes (4, 6, and 8 nm) deposited on H-terminated Si(100) substrates are tested for the photoelectrochemical catalytic performance, and significant enhancement in photocurrent density (0.8 mA/cm(2)) with decreasing NC size is observed with a low saturation voltage of -0.22 V vs Ag/AgCl (0.78 V vs RHE). The enhanced photoconductivity could be attributed to the increase in the specific surface area and increase in the number of active (defect) sites in the amorphous NCs. The unique advantages of the present technique will be further exploited to develop applications based on tunable, size-selected NCs.

In situ hybridization of superparamagnetic iron-biomolecule nanoparticles.

The increase in interest in the integration of organic-inorganic nanostructures in recent years has promoted the use of hybrid nanoparticles (HNPs) in medicine, energy conversion, and other applications. Conventional hybridization methods are, however, often long, complicated, and multistepped, and they involve biomolecules and discrete nanostructures as separate entities, all of which hinder the practical use of the resulting HNPs. Here, we present a novel, in situ approach to synthesizing size-specific HNPs using Fe-biomolecule complexes as the building blocks. We choose an anticancer peptide (p53p, MW 1.8 kDa) and an enzyme (GOx, MW 160 kDa) as model molecules to demonstrate the versatility of the method toward different types of molecules over a large size range. We show that electrostatic interaction for complex formation of metal hydroxide ion with the partially charged side of biomolecule in the solution is the key to hybridization of metal-biomolecule materials. Electrochemical deposition is then used to produce hybrid NPs from these complexes. These HNPs with controllable sizes ranging from 30 nm to 3.5 µm are found to exhibit superparamagnetic behavior, which is a big challenge for particles in this size regime. As an example of greatly improved properties and functionality of the new hybrid material, in vitro toxicity assessment of Fe-GOx HNPs shows no adverse effect, and the Fe-p53p HNPs are found to selectively bind to cancer cells. The superparamagnetic nature of these HNPs (superparamagnetic even above the size regime of 15-20 nm!), their biocompatibility, and the direct integration approach are fundamentally important to biomineralization and general synthesis strategy for bioinspired functional materials.

Bimetallic FeNi concave nanocubes and nanocages.

Concave nanostructures are rare because of their thermodynamically unfavorable shapes. We prepared bimetallic FeNi concave nanocubes with high Miller index planes through controlled triggering of the different growth kinetics of Fe and Ni. Taking advantage of the higher activity of the high-index planes, we then fabricated monodispersed concave nanocages via a material-independent electroleaching process. With the high-index facets exposed, these concave nanocubes and nanocages are 10- and 100-fold more active, respectively, toward electrodetection of 4-aminophenol than cuboctahedrons, providing a label-free sensing approach for monitoring toxins in water and pharmaceutical wastes.

Iron and aluminium based adsorption strategies for removing arsenic from water.

Arsenic is a commonly occurring toxic metal in natural systems and is the root cause of many diseases and disorders. Occurrence of arsenic contaminated water is reported from several countries all over the world. A great deal of research over recent decades has been motivated by the requirement to lower the concentration of arsenic in drinking water and the need to develop low cost techniques which can be widely applied for arsenic removal from contaminated water. This review briefly presents iron and aluminium based adsorbents for arsenic removal. Studies carried out on oxidation of arsenic(III) to arsenic(V) employing various oxidising agents to facilitate arsenic removal are briefly mentioned. Effects of competing ions, As:Fe ratios, arsenic(V) vs. arsenic(III) removal using ferrihydrite as the adsorbent have been discussed. Recent efforts made for investigating arsenic adsorption on iron hydroxides/oxyhydroxides/oxides such as granular ferric hydroxide, goethite, akaganeite, magnetite and haematite have been reviewed. The adsorption behaviours of activated alumina, gibbsite, bauxite, activated bauxite, layered double hydroxides are discussed. Point-of-use adsorptive remediation methods indicate that Sono Arsenic filter and Kanchan™ Arsenic filter are in operation at various locations of Bangladesh and Nepal. The relative merits and demerits of such filters have been discussed. Evaluation of kits used for at-site arsenic estimation by various researchers also forms a part of this review.

Synthesis of Mg(II) doped goethite and its cation sorption behaviour.

Modified goethite samples were prepared with Mg(II) content varying in the range of 0-1.36%. A typical TEM of Mg(II) doped sample showed needle shaped goethite particles having 10-30 nm width and 100-400 nm length. Sorption studies of cations namely Pb(II), Cu(II), Cd(II), Zn(II) and Fe(III) were conducted onto the Mg(II) doped modified goethite surface. Goethite doping with 0.18 M Mg (GMg(2)) showed better sorption capacity for Pb(II), Cu(II) and Cd(II). Therefore, the effect of contact time, solution pH, sorbate and sorbent concentrations on the sorption of various metal ions was studied on this sample in batch experiments. The time data fitted to pseudo-second-order kinetics for all the metal ions. Sorption on GMg(2) sample for the metal ions increased with the increase in pH from 2 to 4. The isothermic data showed good fit to both Langmuir and Freundlich isotherms except for Cd(II) which followed only the later model. The sorption capacities with respect to Pb(II), Cd(II), Zn(II), Cu(II) and Fe(III) were found to be 87.7, 153.25, 86.25, 33.4 and 72.5mg/g respectively. The results of sorption studies on GMg(2) from binary and ternary solutions have also been presented. The XRD patterns of metal ion loaded GMg(2) samples confirmed that metal ion adsorption resulted in shifting of d-values/altering of relative intensity (RI) of major planes of goethite.