KOH Medium Research Articles

Atomic Sulfur Filling Oxygen Vacancies Optimizes H Absorption and Boosts the Hydrogen Evolution Reaction in Alkaline Media

AbstractThe hydrogen evolution reaction (HER) usually has sluggish kinetics in alkaline solution due to the difficulty in forming binding protons. Herein we report an electrocatalyst in which sulfur atoms are doping in the oxygen vacancies (VO) of inverse spinel NiFe2O4 (S‐NiFe2O4) to create active sites with enhanced electron transfer capability. This electrocatalyst has an ultralow overpotential of 61 mV at the current density of 10 mA cm−2 and long‐term stability of 60 h at 1.0 Acm−2 in 1.0 M KOH media. In situ Raman spectroscopy revealed that S sites adsorb hydrogen adatom (H*) and in situ form S‐H*, which favor the production of hydrogen and boosts HER in alkaline solution. DFT calculations further verified that S introduction lowered the energy barrier of H2O dissociation. Both experimental and theoretical investigations confirmed S atoms are active sites of the S‐NiFe2O4.

Atomic Sulfur Filling Oxygen Vacancies Optimizes H Absorption and Boosts the Hydrogen Evolution Reaction in Alkaline Media.

The hydrogen evolution reaction (HER) usually has sluggish kinetics in alkaline solution due to the difficulty in forming binding protons. Herein we report an electrocatalyst in which sulfur atoms are doping in the oxygen vacancies (VO ) of inverse spinel NiFe2 O4 (S-NiFe2 O4 ) to create active sites with enhanced electron transfer capability. This electrocatalyst has an ultralow overpotential of 61 mV at the current density of 10 mA cm-2 and long-term stability of 60 h at 1.0 Acm-2 in 1.0 M KOH media. In situ Raman spectroscopy revealed that S sites adsorb hydrogen adatom (H*) and in situ form S-H*, which favor the production of hydrogen and boosts HER in alkaline solution. DFT calculations further verified that S introduction lowered the energy barrier of H2 O dissociation. Both experimental and theoretical investigations confirmed S atoms are active sites of the S-NiFe2 O4 .

Copper-Incorporated heterostructures of amorphous NiSex/Crystalline NiSe2 as an efficient electrocatalyst for overall water splitting

• Novel Cu-implanted heterostructure of amorphous NiSe x /crystalline NiSe 2 is prepared. • Catalyst needs overpotential of 156.9 mV for HER and 339 mV for OER at 10 mA cm −2. • The catalyst-based electrolyzer requires a low cell voltage of 1.62 V at 10 mA cm −2. • The catalyst-based electrolyzer shows a long-term stability of 21.5 h in 1.0 M KOH. In this research, we designed a novel heterostructure of porous amorphous-crystalline nickel selenide incorporated with copper (Cu-(a-NiSe x /c-NiSe 2 )) and shelled over one-dimensional TiO 2 nanorods (NRs) to simultaneously accelerate both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics in alkaline environment. The Cu-(a-NiSe x /c-NiSe 2 )/TiO 2 NRs supported by carbon cloth displayed as an effective bifunctional catalyst, which required low overpotentials of 156.9 mV for HER and 339 mV for OER to achieve a current response of 10 mA cm −2 in 1.0 M KOH medium. An electrolyzer derived from the Cu-(a-NiSe x /c-NiSe 2 )/TiO 2 NRs material allowed an operation voltage of 1.62 V at 10 mA cm −2 along with good long-term stability after 21.5 h operation towards water splitting in alkaline medium. This achievement was resulted from the fine-tuned 3D porous architecture of the amorphous NiSe x -crystalline NiSe 2 heterostructures doped by copper, which led to significant modulation of electronic properties as well as large surface of exposed electroactive site/types, thereby effectively promoting the catalytic performance. This study suggested a rational approach of structure and shape engineering to design a potential catalyst for producing green hydrogen via water spitting.

Comparative study on methylene blue adsorption behavior of coffee husk-derived activated carbon materials prepared using hydrothermal and soaking methods

Coffee husk, a carbonaceous precursor and common agricultural waste in Vietnam, was utilized to produce activated carbon by hydrothermal carbonization (HTC) with high adsorption capacity for pollutants. The characteristics and adsorption capacity of the hydrochar produced at the first HTC step (180 °C, 6 h) were subjected to further hydrothermal activation (130 °C for 2 h) with a low concentration of potassium hydroxide (ACHC-KOH 1 M), and directly soaked in KOH medium (ACHC-KOH 1:1). The surface area of the activated hydrochar obtained was significantly increased from 33.7 m2/g (raw hydrochar) to 703.9 m2/g for ACHC-KOH 1 M and 743.8 m2/g for ACHC- KOH 1:1, respectively. The maximum methylene blue (MB) adsorption achieved by the activated carbon materials were 357.38, 314.05, and 103.62 mg/g by ACHC-KOH 1 M, ACHC-KOH 1:1, and raw hydrochar, respectively. Adsorption kinetics and isotherms were employed to explain the MB adsorption process. Adsorption data of MB on ACHC-KOH 1 M and ACHC-KOH 1:1 fitted well with Langmuir adsorption isotherm and pseudo-second-order kinetic model. The results show that coffee husk-based activated carbon is a promising sorbent for the removal of MB from aqueous solutions. Compared to other carbonization methods, the HTC method has advantages of using a low temperature and concentration of KOH.

Tailoring the density of nanoflakes to enhance the hybrid battery performance of the NiS sheet array electrode

To produce three-dimensional (3D) low-inner-resistance electrode, nickel sulfide sheet-like nanoarray (NiS) with secondary nanoflakes vertically grown on primary nanosheets is fabricated via sulfurization of hierarchical structured Ni(OH)2 supported on carbon cloth substrate. Due to profuse exposed-edge planes, the obtained electrode exhibits remarkable hybrid properties with a highly specific capacity of 361 μA h cm−2 at 2 mA cm−2 in 3 M KOH medium, and excellent cycling stability (85 % capacitance retention after 5000 cycles). Further, the fabricated flexible and lightweight electrode is used to evaluate its performance in a solid-state flexible hybrid battery. The assembled flexible hybrid battery exhibits a high energy density of 10.97 Wh kg−1 with a power density of 54.86 W kg−1 at an operating voltage of 1.6 V. Thus, the designed NiS nanosheet arrays are considered as remarkable electrode material for energy storage applications.

Acid-induced topological morphology modulation of graphitic carbon nitride homojunctions as advanced metal-free catalysts for OER and pollutant degradation

Topological morphology that dominates the surface electronic properties of nanostructures plays a key role in producing desired materials for versatile functions and applications in many fields, but its modulation for specific functions remains a big challenge. Herein, we report an acid-induced method to prepare S-doped graphitic carbon nitride/graphitic carbon nitride (S-CN/CN) homojunction by simply pyrolyzing a supramolecular precursor synthesized from melamine and H2SO4. The topological morphology and electronic structure of CN homojunction can be easily adjusted only by changing the ratio of raw materials. Moreover, the topological morphology of S-CN/CN homojunction can be further adjusted from hollow cocoon to 2D nanosheets by changing the annealing conditions. The optimized S-CN/CN homojunction shows highly efficient in charge transfer and separation and exhibits superior OER activity and high ability to degrade organic pollutants. Impressively, S-CN/CN nanosheets only demand low overpotential of 301 mV to drive a current density of 10 mAcm−2 in 1 M KOH media, and the corresponding Tafel slope is only 57.71 mV/dec, which is superior to the most advanced precious metal IrO2 catalyst. Moreover, under visible light irradiation, its photodegradation kinetic rate of RhB is 2.38, which is 47.6 times higher than that of bulk CN. This work provides useful guidance for designing and developing efficient multifunctional metal-free catalysts.

Metal doped layered MgB2 nanoparticles as novel electrocatalysts for water splitting

Growing environmental problems along with the galloping rate of population growth have raised an unprecedented challenge to look for an ever-lasting alternative source of energy for fossil fuels. The eternal quest for sustainable energy production strategies has culminated in the electrocatalytic water splitting process integrated with renewable energy resources. The successful accomplishment of this process is thoroughly subject to competent, earth-abundant, and low-cost electrocatalysts to drive the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), preferably, in the same electrolyte. The present contribution has been dedicated to studying the synthesis, characterization, and electrochemical properties of newfangled electrocatalysts with the formal composition of Mg1−xTMxB2 (x = 0.025, 0.05, and 0.1; TM (transition metal) = Fe and Co) primarily in HER as well as OER under 1 M KOH medium. The electrochemical tests revealed that among all the metal-doped MgB2 catalysts, Mg0.95Co0.05B2 has the best HER performance showing an overpotential of 470 mV at − 10 mA cm−2 and a Tafel slope of 80 mV dec−1 on account of its high purity and fast electron transport. Further investigation shed some light on the fact that Fe concentration and overpotential for HER have adverse relation meaning that the highest amount of Fe doping (x = 0.1) displayed the lowest overpotential. This contribution introduces not only highly competent electrocatalysts composed of low-cost precursors for the water-splitting process but also a facile scalable method for the assembly of highly porous electrodes paving the way for further stunning developments in the field.

Aggregates of Ni/Ni(OH)2/NiOOH Nanoworms on Carbon Cloth for Electrocatalytic Hydrogen Evolution.

The development of an efficient electrocatalyst for hydrogen evolution reaction (HER) is essential to facilitate the practical application of water splitting. Here, we aim to develop an electrocatalyst, Ni/Ni(OH)2/NiOOH, via electrodeposition technique on carbon cloth, which shows efficient activity and durability for HER in an alkaline medium. Phase purity and morphology of the electrodeposited catalyst are determined using powder X-ray diffraction and electron microscopic techniques. The compositional and thermal stability of the catalyst is checked using X-ray photoelectron spectroscopy and thermogravimetry analysis. Electrodeposited Ni/Ni(OH)2/NiOOH material is an efficient, stable, and low-cost electrocatalyst for hydrogen evolution reaction in a 1.0 M KOH medium. The catalyst exhibits remarkable performance, achieving a current density of 10 mA/cm2 at a potential of -0.045 V vs reversible hydrogen electrode (RHE), and the Tafel slope value is 99.6 mV/dec. The overall electrocatalytic water splitting mechanism using Ni/Ni(OH)2/NiOOH catalyst is well explained, where formation and desorption of OH- ion on the catalyst surface are significant at alkaline pH. The developed electrocatalyst shows significant durability up to 200 h in a negative potential window in a highly corrosive alkaline environment along with efficient activity. The electrocatalyst can generate 165.6 μmol of H2 in ∼145 min of reaction time with 81.5% faradic efficiency.

Synthesis and characteristics of MMT reinforced chitosan nanocomposite

A chitosan (CN) biopolymer reinforced with montmorillonite (MMT) clay was prepared by solution mixing. The polymer solution was cast into films, dried at room temperature and characterized. The chemical structure and morphology were analyzed using FT-IR and XRD. The MMT was well dispersed and exfoliated in composites, though slight agglomeration was observed for composites with higher percentage of MMT. The mechanical properties of the composite were analyzed and the composite showed 150% increase in tensile strength over the virgin chitosan film. The thermal properties and degradation behavior were studied using DSC and TGA respectively. MMT reinforced chitosan composites had higher thermal degradation temperature than unfilled chitosan. The polymeric beads of CN-MMT were formed by suspension polymerization in KOH medium and sorption kinetic studies where performed on the same.

Heterostructural Ni3S2–Fe5Ni4S8 hybrids for efficient electrocatalytic oxygen evolution

Developing efficient and durable oxygen evolution reaction (OER) electrocatalysts with non-noble elements is particularly urgent for electrochemical water splitting. Herein, we report heterostructural Ni3S2–Fe5Ni4S8 hybrids directly grown on redox-NiFe foams (denoted as NFS/rNF) as excellent pre-catalysts for OER in 1 M KOH media. The electrochemical performance can be tuned by amount of sulfur source, and the optimized electrode only needs low overpotentials of 199 mV and 240 mV to attain 10 mA cm−2 and 100 mA cm−2, respectively. Moreover, the overpotential is further reduced to 231 mV to deliver 100 mA cm−2 after multi-cyclic voltammogram scans, due to the exposure of abundant in situ generated Ni(Fe)OOH species. Excellent stability with continuous 40 h operation is also demonstrated. Our study provides a new strategy for direct synthesis of self-standing integrated hybrid electrodes for efficient and durable water oxidation.

Efficient hydrogen evolution reaction in alkaline via novel hybrid of Pt deposited zinc phosphide nanosheets

In this study, the formation of a novel electrocatalyst originated from small Pt nanoparticles attached on surface of zinc phosphide nanosheets was successfully prepared by facile method without the binder usage for hydrogen evolution reaction (HER). The as-prepared catalyst was a highly porous structure with large active surface area, thereby effectively promoting the catalytic activities towards HER. The catalyst exhibited a small overpotential of 74 mV at a current density of 10 mA cm−2 in 1.0 M KOH medium, lower than other synthesized materials. The catalyst also showed a small Tafel slope of 55 mV dec-1 and long-term HER strength of 15 h at 20 mA cm−2. The excellent electrochemical behavior was primarily derived from the enhanced charge transfer and intrinsic activities of phases after Pt being inserted into structure of zinc phosphides. This study could offer an efficient approach for the synthesis of HER electrocatalysts in water splitting application.

High current density electrodeposition of NiFe/Nickel Foam as a bifunctional electrocatalyst for overall water splitting in alkaline electrolyte

Developing highly efficient and inexpensive bifunctional electrocatalysts is a grand challenge in water splitting process. In this article, NiFe electrocatalysts on nickel foam are prepared by electrodeposition method using high current density. The optimized NiFe electrocatalyst presented excellent electrocatalytic activities in 1.0 M KOH medium with low overpotentials of 100 mV for hydrogen evolution reaction and 230 mV for oxygen evolution reaction to achieve 10 mA·cm−2, respectively. Moreover, the optimized NiFe electrocatalyst as bifunctional electrocatalyst toward overall water splitting only needs a small cell voltage of 1.57 V to obtain 10 mA·cm−2. The extremely electrocatalytic performance of NiFe electrocatalyst is attributed to the fast charge transfer kinetics, large electrochemical surface area, and the conductive nickel foam.

Facile one-step in-situ encapsulation of non-noble metal Co2P nanoparticles embedded into B, N, P tri-doped carbon nanotubes for efficient hydrogen evolution reaction

Developing high performance, good stability and noble-metal-free electrocatalysts for renewable hydrogen evolution reaction (HER) remain a substantial challenge. Herein, we introduce a novel facile one-step in-situ strategy through pyrolysis for the synthesis of Co2P nanoparticles encapsulated Boron, Nitrogen, and Phosphorous tri-doped carbon nanotubes (Co2P/BNP-CNTs). The synergetic effect between Co2P nanoparticles and heteroatom doped CNTs contributes to the remarkable HER performance. The Co2P/BNP-CNT-900 electrocatalyst shows a low overpotential of 133 mV at a current density of 10 mA cm−2 and a small Tafel slope of 90 mV dec−1 in 0.1 M KOH media. More importantly, the Co2P/BNP-CNT-900 electrocatalyst exhibits superior long-term stability in alkaline solution at −0.25 V versus Reversible Hydrogen Electrode (RHE) for 15 h and up to 1000 cycles with negligible performance loss. Overall, our works suggest a one-pot facile synthesis strategy for rational designing high-performance electrocatalysts with enhanced HER performance.

Molybdenum and Phosphorous Dual Doping in Cobalt Monolayer Interfacial Assembled Cobalt Nanowires for Efficient Overall Water Splitting

AbstractThe necessity for better water splitting requires speedy development of efficient catalysts with high activity, long‐term stability, and cost effectiveness. In this work, a bifunctional catalyst originating from the interfacial assembly of a thin Mo,P‐codoped Co layer (≈50 nm) shelled Co nanowire (Co‐Mo‐P/CoNWs) network is fabricated via a facile approach. The catalyst exhibits low overpotentials of 0.08 and 0.27 V to reach a current response of 20 mA cm−2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively, together with long‐term stability in 1.0 m KOH medium. The outstanding performance is further demonstrated by a Co‐Mo‐P/CoNWs‐based electrolyzer, which enables a cell voltage of only 1.495 V to reach 10 mA cm−2, superior to one derived from commercial (Pt/C + RuO2/C) as well as to various reports recently published elsewhere. It is recognized that the formation of multiactive centers together with the increased active site number caused by Mo and P dual doping synergistically promote both hydrogen and oxygen evolution performance. Such a hybrid material opens a new approach for developing efficient and cost‐effective catalysts for water splitting application.

Chemical synthesis of NiO nanostructure by surfactant-assisted sol–gel methodology for urea electrocatalytic oxidation

NiO-nanoparticles were chemically designed via surfactant-assisted sol–gel methodology and investigated in terms of FESEM, BET, TEM, and XRD techniques which indicate the successful preparation of NiO-NPs. The electrocatalytic oxidation of urea was studied in the three-electrode cell to evaluate the efficiency of NiO-NPs material for urea electro-oxidation in the KOH medium. The as-prepared NPs demonstrated improved electrocatalytic oxidation of urea at various urea doses from 0.01 to 2.0 mol/l. The excellent electrocatalytic activity could be related to the successful synthesis of NiO-NPs which can easily form NiOOH and generate electrons from urea-bonds during the electrochemical process. This work will provide novel anode catalysts for urea fuel-cells.

An efficient and non-precious anode electrocatalyst of NiO-modified carbon nanofibers towards electrochemical urea oxidation in alkaline media

Non-precious NiO nanoparticles were combined with carbon fibers (CFs) to design the novel electrode material; NiO-CFs. The as-synthesized NiO-CFs material was investigated in terms of Field emission scanning electron microscope (FE-SEM), Fourier-transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) surface area, Energy-dispersive X-ray spectroscopy (EDX), UV–vis spectra, Transmission electron microscope (TEM), selected area diffraction (SAED) pattern and X-ray diffraction (XRD) techniques. These analyses indicate the successful synthesis of a nanocomposite of NiO-CFs. Cyclic voltammetry (CV) in addition to chronoamperometric (CA) and electrochemical impedance spectroscopy (EIS) methods were studied in the 3-electrodes system to examine the electrochemical performance of NiO-CFs material for urea oxidation in KOH medium. The synthesized nanocomposite showed improved electrochemical oxidation of urea at various urea concentrations up to 1.5 M. The decreasing of both charge transfer impedance and series resistance indicates the enhanced transfer of electrons in the occurrence of urea which could be related to the high electrochemical performance of NiO-CFs material as an electrocatalyst. The superior electrochemical activity can be due to the assembly of C-structure with NiO nanoparticles during the synthesis steps which enhance electrocatalysis, charge transfer, and structural defects.

Green sol-gel auto-combustion synthesis, characterization and investigation of the electrochemical hydrogen storage properties of barium cobalt oxide nanocomposites with maltose

Barium cobalt oxide nanocomposites (Ba2Co9O14/Co3O4 NCs) as potential hydrogen storage material fabricated by sol-gel auto-combustion method using maltose as reductant, for the first time. Three different ratios of Ba:maltose were applied, including 1:5.5, 1:11 and 1:22 for morphological engineering. X-Ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), along with Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission electron microscopy (TEM) images was applied for study the composition and structure of as-prepared samples. Also, the magnetic, optical and electrochemical properties of optimum sample were inquired using VSM, DRS and CV techniques. The porosity and surface properties of NCs were checked by Brunauer-Emmett-Teller (BET) measurements. FE-SEM micrographs of all maltose assisted-synthesis products showed formation of hexagonal nanoparticles on the surfaces of the microplates. According to FE-SEM, HRTEM and XRD results, 1:22 ratio of Ba:maltose and calcination process of 900 °C for 4 h, was selected as optimum condition. The electrochemical hydrogen sorption capability of obtained Ba2Co9O14/Co3O4 NCs was studied according to chronopotentiometry charge-discharge procedures in KOH medium and performed 1100 mAh/g discharge capacity. Based on the obtained results, Ba2Co9O14/Co3O4 NCs can be promising compounds to improve the electrochemical performance of hydrogen storage.

Iron sulfide nanosheets supported 3D foam: A binder-free electrocatalyst for sensitive and selective electrochemical H2O2 detection

Electrochemical nonenzymatic sensor has achieved numerous attentions from analytical chemistry communities. A facile and direct synthesis route for developing high-performance electrocatalyst is essential to improve efficiency of sensor in terms of cost and catalytic activities for reliable analysis applications. In this research, we introduced an easy synthesis approach for the in-situ fabrication of iron sulfide nanosheets (Fe–S NSs) vertically grown on a nickel foam (NF) via an electrodeposition step. Such a self-supported Fe–S NSs/NF demonstrated sensitive and selective determination towards hydrogen peroxide (H2O2) with a wide sensing region ranging from 10 μM to 3.3 mM in 0.1 M KOH medium. Besides, it’s excellent sensitivity of 4.48 μA μM−1 cm−2, low detection limit of 7.9 μM, good reproducibility, and stability have been successfully demonstrated as well. The facile fabrication process of Fe–S NSs/NF opened a novel and effective avenue for the development of non-precious metal-based electrocatalysts in analytical chemistry.

Facile Preparation of a Surface-Enriched Pt Layer Over Pd/C as an Efficient Oxygen Reduction Catalyst With Enhanced Activity and Stability

Abstract Pt-enriched surface layer formation on Vulcan carbon-supported Pd (Pt@Pd/C) was successfully prepared through a simple and one-pot formic acid reduction approach without any stabilizing agent. The electrocatalytic performance of Pt@Pd/C catalyst toward an oxygen reduction reaction (ORR) in alkaline medium was studied and also compared with standard carbon-supported Pt (Pt/C) and Pd (Pd/C) catalysts. The Pt@Pd/C exhibits higher electrochemical active surface area (74.7 m2/g) and mass activity (1.38 mA/µg) than Pt/C, Pd/C, and contending with standard reported catalysts. In durability tests, Pt@Pd/C showed negligible loss of intrinsic activity (∼10%) after 10,000 cycles which confirmed improved stability than Pt-based catalysts for ORR in KOH medium. This improved electrocatalytic performance could be attributed to their structural characteristics of the Pt-enriched surface layer on Pd/C-core and the compressive lattice strain on Pt. The present investigation demonstrates the simple preparation procedure for surface-enriched Pt on Pd/C and its improved performance for ORR, suggesting that it is a promising contender to benchmark ORR catalysts for alkaline fuel cells.

Hierarchical star-like molybdenum phosphides nanostructures decorated graphene on 3D foam as self-supported electrocatalyst for hydrogen evolution reaction

Fabrication of efficient electrocatalysts for hydrogen evolution reaction (HER) is important to produce hydrogen fuel on a large scale via electrocatalytic water splitting. In this research, a novel electrocatalyst based on star-like molybdenum phosphide structure vertically grown on highly conductive graphene supported by a three-dimensional nickel foam (MoP/GR/NF) was synthesized via a facile hydrothermal method followed by a phosphidization strategy at 300 °C. The exclusive hierarchical nanostructure with abundant porosity was beneficial for enhancing active site numbers and electrical conductivity. In particular, the formation of Mo–P impressively changed electronic structure of the resulting material, thereby accelerating electrocatalytic activity for HER in 1.0 M KOH medium. The MoP/GR/NF required small overpotentials of 95 and 189 mV to deliver current densities of 10 and 50 mA cm−2, respectively. In addition, it also demonstrated a Tafel slope of 54 mV dec−1, which was significantly smaller than that of the GR/NF, Mo–O/GR/NF, and Mo–P/NF. Furthermore, there was negligible decrease of current response and overpotential for a continuous HER during long-term operation, implying its excellent stability. The prospective catalytic activity and good stability of the MoP/GR/NF material could prospectively open a new aspect for developing effective electrocatalyst for hydrogen production in water splitting application.