An Overview of Metal-Organic Framework Based Electrocatalysts: Design and Synthesis for Electrochemical Hydrogen Evolution, Oxygen Evolution, and Carbon Dioxide Reduction Reactions.

Due to the increasing global energy demands, scarce fossil fuel supplies, and environmental issues, the pursued goals of energy technologies are being sustainable, more efficient, accessible, and produce near zero greenhouse gas emissions. Electrochemical water splitting is considered as a highly viable and eco-friendly energy technology. Further, electrochemical carbon dioxide (CO2 ) reduction reaction (CO2 RR) is a cleaner strategy for CO2 utilization and conversion to stable energy (fuels). One of the critical issues in these cleaner technologies is the development of efficient and economical electrocatalyst. Among various materials, metal-organic frameworks (MOFs) are becoming increasingly popular because of their structural tunability, such as pre- and post- synthetic modifications, flexibility in ligand design and its functional groups, and incorporation of different metal nodes, that allows for the design of suitable MOFs with desired quality required for each process. In this review, the design of MOF was discussed for specific process together with different synthetic methods and their effects on the MOF properties. The MOFs as electrocatalysts were highlighted with their performances from the aspects of hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and electrochemical CO2 RR. Finally, the challenges and opportunities in this field are discussed.

Lanthanum Thin Film on Boron-Doped Diamond, Glassy Carbon and Platinum Electrodes-An Electrochemical Approach.

Electrodeposition of lanthanum (La) thin film had been successfully carried out in the triethylmethy-lammonium methyl sulfate (TEMAMS) ionic liquid on boron-doped diamond (BDD), glassy carbon (GC) and platinum (Pt) electrodes. The electrolyte shows highest electrochemical stability window of 4.88 V on the BDD, followed by the GC (4.5 V) and Pt (4.1 V). The electrochemical reduction of La(III) ions takes place significantly at a less positive potential on the BDD than the GC, while on the Pt electrode, predominant decomposition current associated with electrochemical reduction/oxidation of TEMAMS dominates the overall process. Surface morphologic characteristics of La thin film deposits obtained on the three electrodes were characterized by scanning electron microscope (SEM) and deposited lanthanum thin film was confirmed by energy-dispersive X-ray spectroscopy (EDS) analysis.

Surfactant Tween 20 Controlled Perovskite Film Fabricated by Thermal Blade Coating for Efficient Perovskite Solar Cells.

In recent years, additive engineering has received considerable attention for the fabrication of high-performance perovskite solar cells (PSCs). In this study, a non-ionic surfactant, polyoxyethylene (20) sorbitan monolaurate (Tween 20), was added as an additive into the MAPbI3 perovskite layer, and the thermal-assisted blade-coating method was used to fabricate a high-quality perovskite film. The Tween 20 effectively passivated defects and traps in the MAPbI3 perovskite films. Such a film fabricated with an appropriate amount of Tween 20 on the substrate showed a higher photoluminescence (PL) intensity and longer carrier lifetime. At the optimal concentration of 1.0 mM Tween 20, the performance of the PSC was apparently enhanced, and the champion PSC demonstrated a PCE of 18.80%. Finally, this study further explored and compared the effect on the device performance and ambient stability of the MAPbI3 perovskite film prepared by the spin-coating method and the thermal-assisted blade coating.

Controlled Photoanode Properties for Large-Area Efficient and Stable Dye-Sensitized Photovoltaic Modules.

Nowadays, a dye-sensitized solar cell (DSSC) attracts attention to its development widely due to its several advantages, such as simple processes, low costs, and flexibility. In this work, we demonstrate the difference in device structures between small size and large size cells (5 cm × 5 cm, 10 cm × 10 cm and 10 cm × 15 cm). The design of the photoanode and dye-sensitized process plays important roles in affecting the cell efficiency and stability. The effects of the TiO2 electrode, using TiCl4(aq) pretreatment and post-treatment processes, are also discussed, whereas, the open-circuit voltage (Voc), short-circuit current density (Jsc), and module efficiency are successfully improved. Furthermore, the effects on module performances by some factors, such as dye solution concentration, dye soaking temperature, and electrolyte injection method are also investigated. We have demonstrated that the output power of a 5 cm × 5 cm DSSC module increases from 86.2 mW to 93.7 mW, and the module efficiency achieves an outstanding performance of 9.79%. Furthermore, enlarging the DSSC modules to two sizes (10 cm × 10 cm and 10 cm × 15 cm) and comparing the performance with different module designs (C-DSSC and S-DSSC) also provides the specific application of polymer sealing and preparing high-efficiency large-area DSSC modules.

Electrochemical mineralization of iron-tannate stain on HAp and bovine enamel-A non-peroxide approach.

Prolonged treatments for the destaining of teeth using high concentrations of hydrogen peroxide may cause secondary unwanted effects such as tooth hypersensitivity and gingival irritation. Hence, it is aimed to develop a non-peroxide-based method to oxidize iron-tannate (Fe-TA) stained hydroxyapatite (HAp) and bovine enamel (BE) samples. Constant current electrolysis (CCE) experiments were carried out on Pt working electrode in aqueous NaCl, KCl and KI solutions at discrete concentrations under continuous experiment and a non-continuous experiment. CCE shows that in the presence of iron tannate (Fe-TA) stained HAP, approximately 30 ppm of iodine was generated using 0.1M KI and nearly 40 ppm was produced with 0.2 M KI. By using a non-continuous CCE process, the lowest amount of chlorine was generated from NaCl solution, which was well within the safety limits for oral applications. Depending on the experimental conditions used, between 13 ppm and 124 ppm of chlorine was generated. CCE of Fe-TA stained on HAp using KCl reveals that at the lowest current density of 10 mA/cm2, the amount of hypochlorite generated was 20 ppm on Pt electrode having a surface area of 6 cm2. Ion chromatographic (IC) analysis revealed that non-continuous CCE of Fe-TA-BE in NaCl generated a low concentration of sodium perchlorate (0.8 ppm), whereas the continuous process generated no perchlorate, but a considerable higher quantity of chlorate for Fe-TA-BE (37 ppm) and Fe-TA-HAp (140 ppm) samples.

Reducing Defects in Organic-Lead Halide Perovskite Film by Delayed Thermal Annealing Combined with KI/I2 for Efficient Perovskite Solar Cells.

This study improved quality of CH3NH3PbI3 (MAPbI3) perovskite films by delaying thermal annealing in the spin coating process and introducing KI and I2 to prepare MAPbI3 films that were low in defects for high-efficiency perovskite solar cells. The influences of delayed thermal annealing time after coating the MAPbI3 perovskite layer on the crystallized perovskite, the morphology control of MAPbI3 films, and the photoelectric conversion efficiency of solar cells were investigated. The optimal delayed thermal annealing time was found to be 60 min at room temperature. The effect of KI/I2 additives on the growth of MAPbI3 films and the corresponding optimal delayed thermal annealing time were further investigated. The addition of KI/I2 can improve perovskite crystallinity, and the conductivity and carrier mobility of MAPbI3 films. Under optimized conditions, the photoelectric conversion efficiency of MAPbI3 perovskite solar cells can reach 19.36% under standard AM1.5G solar illumination of 100 mW/cm2.

Improved electrocatalytic activity of Au@Fe3O4 magnetic nanoparticles for sensitive dopamine detection.

Substantially, noble metals are important for the development of low-cost, sensitive, selective, superior performance, and portable electrochemical sensors. Herein, we describe gold (Au) nanoparticles (NPs) systematically decorated with magnetic Fe3O4 nanocomposites on the fabrication of sensitive dopamine sensor is described. Magnetic Au@Fe3O4 nanocomposites were prepared by reducing HAuCl4 on the surfaces of Fe3O4 nanoparticles. The surface morphology of Au@Fe3O4 nanocomposites was characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical behaviour of the modified electrode was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and amperometric techniques, in which it was shown to be highly sensitive and selective towards DA. The amperometric detection of dopamine sensor, using this sensing element, exhibits a wide linear response of 0-0.8 µM with a low detection limit of 2.7 nM. In addition, the fabricated electrode showed an excellent stability and good reproducibility. The proposed analytical method was successfully applied to determine the concentration of dopamine in human urine samples and the unknown concentration of DA in human urine samples No. 1, 2 and 3 were determined as 0.056 ± 0.82 × 10-3, 0.037 ± 0.87 × 10-3 and 0.020 ± 0.94 × 10-3 µM, respectively, with recoveries ranging from 97.2% to 103.4%, suggesting that the fabricated electrode can effectively detect DA in human urine samples.

Promising post-consumer PET-derived activated carbon electrode material for non-enzymatic electrochemical determination of carbofuran hydrolysate.

In this work, activated carbon (AC) materials, prepared from polyethylene terephthalate (PET) waste bottles were used as the sensing platform for the indirect detection of carbofuran. The morphology and surface properties of the PET-derived AC (PET-AC) were characterized by N2 adsorption/desorption isotherm, X-ray diffraction (XRD), field-emission scanning/transmission electron microscopy (FE-SEM/TEM) and Raman spectroscopy. The electrochemical activity of the PET-AC modified glassy carbon electrode (GCE) (PET-AC/GCE) was measured by cyclic voltammetry and amperometry. The enhanced surface area and desirable porosities of PET-AC are attributed for the superior electrocatalytic activity on the detection of carbofuran phenol, where, the proposed sensor shows low detection limit (0.03 µM) and remarkable sensitivity (0.11 µA µM-1 cm-2). The PET-AC/GCE holds high selectivity towards potentially interfering species. It also provides desirable stability, repeatability and reproducibility on detection of carbofuran phenol. Furthermore, the proposed sensor is utilized for the detection of carbofuran phenol in real sample applications. The above mentioned unique properties and desirable electrochemical performances suggest that the PET-derived AC is the most suitable carbonaceous materials for cost-effective and non-enzymatic electrochemical sensor.

Ag x Cu y Ni z Trimetallic Alloy Catalysts for the Electrocatalytic Reduction of Benzyl Bromide in the Presence of Carbon Dioxide.

Different compositions of trimetallic alloy containing silver, copper, and nickel (Ag x Cu y Ni z ) were electrodecorated in a protic ionic liquid medium on glassy carbon electrodes in order to investigate the suitability of the materials as catalysts for electrochemical reduction of carbon dioxide (CO2). Surface characteristic morphology obtained by scanning electron microscopy shows cauliflower crystallites for the deposit of Ag, whereas materials of Cu and Ni exhibit cubic grains and fine particles, respectively. Deposits of trimetallic alloy containing Ag, Cu, and Ni exhibit the mixture of the three characteristic features. Further, trimetallic alloy containing a large amount of Ag provides high crystallinity, whereas predominance of Cu as well as Ni results in porous structures, as revealed by X-ray diffraction analysis. Atomic absorption spectroscopy () was used to determine the compositions of different alloy materials. The suitability of nanomaterials as cathodes for electroreduction of benzyl bromide in CO2 containing 0.1 M tetra-n-butylammonium tetrafluoroborate (DMF/TBABF4)/N,N-dimethylformamide medium was explored. The linear sweep voltammogram reveals that Ag46Cu40Ni14 shows higher cathodic peak current and lower cathodic peak potential than those of other deposited nanomaterials as well as alloys, indicating its higher catalytic activity for such an electroreduction process, whereas potentiostatic electrolysis confirms the abovementioned results.

Sequential Preparation of Dual-Layer Fluorine-Doped Tin Oxide Films for Highly Efficient Perovskite Solar Cells.

A dual-layer fluorine-doped tin oxide (FTO) film has been fabricated by means of sequential spray pyrolysis for high-efficiency perovskite solar cells (PSCs). The H-FTO/L-FTO dual layer film consists of a H-FTO layer prepared at high deposition temperature (≈450 °C) and a L-FTO layer, fabricated at low deposition temperature (≈150 °C), which is used to replace the traditional compact TiO2 /FTO layer. The effects of F/Sn molar ratio, precursor solution concentration, and deposition temperature on the electrical, optical, surface morphological, and grain structural characteristics of H-FTO layers have been studied systematically. With an increase in precursor solution concentration, the mobility and carrier concentration of H-FTO increases; however, the grain size and sheet resistance decreases as the precursor solution concentration increases. A high deposition temperature results in a large grain size and enhanced haze value. The L-FTO layer expresses compact layer growth, inconsistent with the H-FTO surface structure, and possesses excellent electron collection and transport efficiency. The effect of the hole-blocking characteristics of L-FTO on the PSC performance is studied. This study provides a novel dual-layer FTO film to replace the traditional compact TiO2 /FTO layer, which is usually prepared by coating TiO2 precursor on FTO following calcination at 450 °C. The H-FTO/L-FTO dual-layer film can simplify the fabrication process and maintain a high power conversion efficiency (PCE); this results in more efficient electron transportation and blocking of holes. The champion device of PSCs with H-FTO/L-FTO shows the highest PCE of 17.37 % under the illumination of 100 mW cm-2 (AM1.5G).

Detection of nicotine based on molecularly imprinted TiO2-modified electrodes.

Amperometric detection of nicotine (NIC) was carried out on a titanium dioxide (TiO(2))/poly(3,4-ethylenedioxythiophene) (PEDOT)-modified electrode by a molecular imprinting technique. In order to improve the conductivity of the substrate, PEDOT was coated onto the sintered electrode by in situ electrochemical polymerization of the monomer. The sensing potential of the NIC-imprinted TiO(2) electrode (ITO/TiO(2)[NIC]/PEDOT) in a phosphate-buffered saline (PBS) solution (pH 7.4) containing 0.1M KCl was determined to be 0.88 V (vs. Ag/AgCl/saturated KCl). The linear detection range for NIC oxidation on the so-called ITO/TiO(2)[NIC]/PEDOT electrode was 0-5mM, with a sensitivity and limit of detection of 31.35 microA mM(-1)cm(-2) and 4.9 microM, respectively. When comparing with the performance of the non-imprinted one, the sensitivity ratio was about 1.24. The sensitivity enhancement was attributed to the increase in the electroactive area of the imprinted electrode. The at-rest stability of the ITO/TiO(2)[NIC]/PEDOT electrode was tested over a period of 3 days. The current response remained about 85% of its initial value at the end of 2 days. The ITO/TiO(2)[NIC]/PEDOT electrode showed reasonably good selectivity in distinguishing NIC from its major interferent, (-)-cotinine (COT). Moreover, scanning electrochemical microscopy (SECM) was employed to elucidate the surface morphology of the imprinted and non-imprinted electrodes using Fe(CN)(6)(3-)/Fe(CN)(6)(4-) as a redox probe on a platinum tip. The imprinted electrode was further characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR).