Electrochemical Reduction Of Water Research Articles

Titanium carbide sheet: metallic 2D structure with intrinsic and high catalytic activity for the hydrogen evolution reaction

Searching for a metallic material that possesses intrinsic and high catalytic activity is interesting for the hydrogen evolution reaction (HER) in the electrochemical reduction of water. In this work, via density functional theory, we identified a promising two-dimensional (2D) catalyst material, namely TiC (1 0 0) sheet, which is composed of two atomic layers. Phonon spectrum calculation shows that all vibrational modes of the TiC (1 0 0) sheet are real. Even at 1200 K, the geometrical structure of TiC (1 0 0) sheet is still well kept. Interestingly, TiC (1 0 0) sheet is metallic, which means that TiC (1 0 0) sheet can possess high conductivity. Moreover, the Gibbs free energy () of hydrogen on TiC (1 0 0) sheet at a low coverage is about 0.13 eV, which can be further optimized to an ideal value of 0 eV by introducing a strain. Besides, the effect of thickness on its catalytic activity is also assessed. We observed that TiC (1 0 0) sheet with a large thickness is also metallic. Calculated values of for four-layered and six-layered TiC (1 0 0) sheets at a moderate coverage can be about −0.06 and −0.04 eV, respectively. Our results highlight a new promising 2D catalyst material for the HER in the electrochemical reduction of water.

Reduced Faradaic Contributions and Fast Charging of Nanoporous Carbon Electrodes in a Concentrated Sodium Nitrate Aqueous Electrolyte for Supercapacitors.

The Faradaic processes related to electrochemical water reduction at the nanoporous carbon electrode under negative polarization are reduced when the concentration of aqueous sodium nitrate (NaNO3) is increased or the temperature is decreased. This effect enhances the relative contribution of ion electrosorption to the total charge storage process. Hydrogen chemisorption is reduced in aqueous 8.0 m NaNO3 due to the low degree of hydration of the Na+ cation; consequently, less free water is available for redox contributions, driving the system to exhibit electrical double‐layer capacitive characteristics. Hydrogen adsorption/desorption is facilitated in 1.0 m NaNO3 due to the high molar ratio. The excess of water shifts the local pH in carbon nanopores to neutral values, giving rise to a high overpotential for dihydrogen evolution in the latter. The dilution effect on local pH shift in 1.0 m NaNO3 can be reduced by decreasing the temperature. A symmetric activated carbon cell assembled with 8.0 m NaNO3 exhibits a high capacitance and coulombic efficiency, a larger contribution of ion electrosorption to the overall charge storage process, and a stable capacitance performance at 1.6 V.

Fabrication of bilayer Pd-Pt nanocages with sub-nanometer thin shells for enhanced hydrogen evolution reaction

The hydrogen evolution reaction (HER), which generates molecular hydrogen through the electrochemical reduction of water, is an important clean-energy technology. Platinum (Pt) is an ideal material for HER electrocatalysts in terms of low overpotential and fast kinetics. An effective method to improve the atom utilization efficiency of Pt is to fabricate Pt-based core-shell or nanocage structures with ultra-thin walls. This paper describes the construction of bilayer palladium (Pd)-Pt alloy nanocages catalyst with enhanced HER catalytic activity. The nanocages were fabricated by etching away the Pd templates of multishelled nanocubes composed of alternate shells of Pd and Pt with well-defined (100) facets. The bilayer Pd-Pt nanocages with sub-nanometer shells have a high dispersion of the active atoms on the outside and inside surfaces of outer layer and inner layer, respectively. Moreover, the Pd-Pt alloy lowers the overpotential for HER and speeds up the reaction rate of HER due to the synergies between Pd and Pt. The rational design of bilayer nanocages provided a novel route for boosting the atom utilization efficiency of Pt catalysts.

Design of Basal Plane Edges in Metal-Doped Nanostripes-Structured MoSe2 Atomic Layers To Enhance Hydrogen Evolution Reaction Activity

Hydrogen (H2) is a clean and renewable energy source with a vital role to reduce global dependence on fossil fuels. H2 evolution through electrochemical reduction of water is an essential eco-friendly strategy, and cost-effective electrocatalysts are critical for large scale manufacturing. This paper proposes metal (Cu, Fe, and Mn)-doped nanostripes-structured molybdenum selenide (MoSe2) electrocatalysts for the H2 evolution reaction (HER). For the first time, a solution deposition technique was successfully employed to architect the basal plane edges in nanostripes-structured MoSe2 through metal doping for enhanced HER. Cu-doped MoSe2 exhibited ∼86 mV overpotential with a small Tafel slope (∼44 mV/dec) which is greater among the available MoSe2-based catalysts. An outstanding robustness was observed in an acidic medium for constant HER over 20 h. First-principles density function theory (DFT) approximations showed that charge transfer between the doped metal and MoSe2 increased the density of states near...

Decorating cobalt phosphide and rhodium on reduced graphene oxide for high-efficiency hydrogen evolution reaction

Abstract Electrochemical reduction of water to hydrogen holds great promise for clean energy, while its widespread application relies on the development of efficient catalysts with large surface area, abundant exposed active sites and superior electron conductivity. Herein, we report a facile strategy to configure an electrocatalyst composed of cobalt phosphide and rhodium uniformly anchored on reduced graphene oxide for hydrogen generation. The hybrids effectively integrate the exposed active sites, electron conductivity and synergistic effect of the catalyst. Electrochemical tests exhibit that the catalyst shows superior hydrogen evolution reaction catalytic activity and stability, with a small Tafel slope of 43 mV dec−1. Over-potentials as low as 29 and 72 mV are required to achieve current densities of 2 and 10 mA cm−2 in 0.5M H2SO4, respectively. The hybrid constitution with highly active sites on conductive substrate is a new strategy to synthesize extremely efficient electrocatalysts. Especially, the efficient synergistic effect among cobalt phosphide, rhodium and reduced graphene oxide provides a novel approach for configuring electrocatalysts with high electron efficiency.

Catalytic H2 Evolution with CoO, Co(OH)2 and CoO(OH) Nanoparticles Generated from a Molecular Polynuclear Co Complex

Electrochemical water reduction by employing first‐row transition‐metal nanoparticles (NPs) constitutes a sustainable way for the generation of H2. We have synthesized Co‐based NPs from a molecular CoII/CoIII precursor after its reductive decomposition at –1.86 V versus NHE in different organic solvents. These NPs are able to electrochemically reduce water at pH 14. SEM, EDX and XPS analyses have allowed the determination of the chemical nature of the as‐deposited NPs: CoO when using MeCN as the solvent and CoO(OH) when employing either dichloromethane (DCM) or MeOH. After 2 h of constant polarization at 10 mA cm–2, the electrocatalytic activity of the NPs obtained in MeCN and DCM decreases, whereas it increases for those obtained in MeOH. In this solvent, the overpotential is reduced by 215–220 mV and the specific current density is tripled. Interestingly, during this activation process in MeOH, the precursor CoO(OH) NPs are converted into Co(OH)2. The implications of these results in the context of the current research in the field are also discussed.

Pyrolytic carbon supported alloying metal dichalcogenides as free-standing electrodes for efficient hydrogen evolution

Electrochemical reduction of water is a renewable way to produce clean hydrogen energy. In order to overcome the high-cost and shortage of noble metals, transition metal compounds involving earth-abundant elements, such as MoS2 and WS2, have been proposed as novel catalysts for hydrogen evolution reaction (HER). Efforts have been made to increase the intrinsic catalytic activity of transition metal dichalcogenides (TMDCs), while alloying same- group elements has not been vastly investigated. Moreover, besides the catalytic activity, the design of free-standing catalytic electrodes is also critical for HER. In this work, we synthesize pyrolytic carbon film with MoxW1-xS2 nanoflakes embedded as a free-standing flexible electrodes for HER catalysis. The pyrolytic carbon acts as conductive and flexible matrix for TMDC alloys with controlled composition, resulting in remarkably enhanced HER activity. The highest HER activity was observed for Mo0.37W0.63S2/C samples with an overpotential of 0.137 V at geometric current densities of jgeo = 10 mA cm−2 and a Tafel slope of 53 mV dec−1. These results now provide low-cost viable alternatives for the design and construction of catalysts based on alloyed TMDCs.

Ultrasound-assisted electrocatalytic hydrogenation in water

The optimum conditions for the electrocatalytic hydrogenation of (R)-(+)-pulegone in water, using ultrasound (continuous or pulsed mode) and a nickel sacrificial anode have been determined. The continuous ultrasound (14 W), without temperature control, showed to be a most effective technique if compared with pulses of 0.2 s applied after each second. The sonication of the reaction solution presented a slight stereochemical effect on the product distribution, and its magnitude depends on both the ultrasound power and the mode. Simultaneous ultrasound irradiation during electrochemical process showed a synergetic effect, able to generate H radical from water HO bond rupture, in addition to the H radicals formed by the electrochemical reduction of water on the electrode surface, increasing both current efficiency and conversion yields. Chitosan microspheres were use in the removal of nickel ions of the aqueous medium showing an adsorption efficiency of 64%.

Peculiar behavior of TiO2 tubular structure for water reduction in dark and under natural light exposure

TiO2 tubes were formed by electrochemical anodization process. The electrochemical reduction of water on TiO2 tubular structure (TiO2–TS) at various pH in dark and natural light was investigated by potentiodynamic cathodic polarization, cyclic voltammetry (CV) and impedance spectroscopy analyses. The negative shift in open circuit potential, relatively low Tafel slope and higher exchange current density presented by TiO2–TS under natural light depicted increased electrochemical activity for water reduction. Under light exposure, CV scan showed high cathodic current density with lower overpotential (0.3V) when compared to dark (>0.8V). Furthermore, under natural light, the decrease in charge transfer resistance and oxide film resistance also validated the increase in the electrochemical activity of TiO2–TS towards water dissociation.

Al-Doped CoP nanoarray: a durable water-splitting electrocatalyst with superhigh activity.

The scalable production of hydrogen fuel through electrochemical water reduction needs efficient Earth-abundant electrocatalysts to make the whole water-splitting process more energy efficient. In this Article, we report that an Al-doped CoP nanoarray on carbon cloth (Al-CoP/CC) behaves as a durable hydrogen evolution electrocatalyst with superhigh activity in 0.5 M H2SO4. It demands a pretty low overpotential of 23 mV to drive a geometrical catalytic current density of 10 mA cm-2, outperforming all reported non-precious metal catalysts. Density functional theory calculations reveal that Al-CoP has a more thermo-neutral hydrogen adsorption free energy than CoP. Notably, this Al-CoP/CC is also superior in activity and durability as a bifunctional catalyst for alkaline water electrolysis, and its two-electrode water electrolyser delivers 10 mA cm-2 water-splitting current at a cell voltage of 1.56 V in 1.0 M KOH. This work offers us an attractive cost-effective catalyst electrode in water-splitting devices for large-scale production of hydrogen fuels.

Novel layer-by-layer assembly of rGO-hybridised ZnO sandwich thin films for the improvement of photo-catalysed hydrogen production

Metal oxide semiconductor materials such as ZnO have tremendous potential as light absorbers for photo-catalysed electrodes in the electrochemical reduction of water. Promoters such as rGO have been added to reduce the recombination losses of charge carriers and improve its photoelectrochemical activity. In this study, the effect of layer ordering on the charge transfer properties of rGO-hybridised ZnO sandwich thin films for the photo-catalysed electrochemical reduction of water was investigated. rGO-hybridised ZnO sandwich thin films were prepared via a facile electrode position technique using a layer-by-layer approach. The thin films were analysed using FESEM, XRD, Raman, PL, UV–vis, EIS and CV techniques to investigate its morphological, optical and electrochemical properties. The FESEM images show the formation of distinct layers of rGO and ZnO nanorods/flakes via the layer-by-layer method. XRD confirmed the wurtzite structure of ZnO. PL spectroscopy revealed a reduction of photoemission intensity in the visible region (580nm) when rGO was incorporated into the ZnO thin film. Among the six thin films prepared, ZnO/rGO showed superior performance compared to the other thin films (0.964mA/cm) due to the presence of graphene edges which participate as heterogenous electrocatalysts in the photocatalysed electrolysis of water. rGO also acts as electron acceptor, forming an n-p heterojunction which improves the activity of ZnO to oxidise water molecules to O2. EIS revealed that the application of rGO as back contact (rGO/ZnO, rGO/ZnO/rGO) reduces the charge transfer resistance of a semiconductor thin film. Alternatively, rGO as front contact (ZnO/rGO, rGO/ZnO/rGO) improves the photo-catalysed electrolysis of water through the participation of the rGO edges in the chemical activation of water. The findings from this study indicate that the layer ordering significantly affects the thin film's electrostatic properties and this understanding can be further advantageous for tunable applications.

Ultrafine Metal Phosphide Nanocrystals in Situ Decorated on Highly Porous Heteroatom-Doped Carbons for Active Electrocatalytic Hydrogen Evolution.

In spite of being technologically feasible, electrochemical water reduction to facilitate hydrogen production is confronted with issues mainly due to the lack of affordable and efficient catalysts for the water reduction half reaction. Reported herein is the fabrication of metal phosphides nanocrystals uniformly loaded on highly porous heteroatom-modified carbons through one-step carbonization-phosphization methodology. Remarkably, the well-structured porosity and the increased electrochemically accessible active sites ensure the high catalytic efficiency for electrochemical hydrogen evolution in acidic medium in terms of small onset potentials (33 mV) and large cathodic current density (0.481 mA cm(-2)), even comparable to the state-of-the-art Pt/C benchmark. The easily prepared composite catalysts of structural and textural peculiarities may serve as promising non-noble metal catalysts for realistic hydrogen evolution.

3D architecture constructed via the confined growth of MoS2 nanosheets in nanoporous carbon derived from metal-organic frameworks for efficient hydrogen production.

The design and synthesis of robust, high-performance and low-cost three-dimensional (3D) hierarchical structured materials for the electrochemical reduction of water to generate hydrogen is of great significance for practical water splitting applications. In this study, we develop an in situ space-confined method to synthesize an MoS2-based 3D hierarchical structure, in which the MoS2 nanosheets grow in the confined nanopores of metal-organic frameworks (MOFs)-derived 3D carbons as electrocatalysts for efficient hydrogen production. Benefiting from its unique structure, which has more exposed active sites and enhanced conductivity, the as-prepared MoS2/3D nanoporous carbon (3D-NPC) composite exhibits remarkable electrocatalytic activity for the hydrogen evolution reaction (HER) with a small onset overpotential of ∼0.16 V, large cathodic currents, small Tafel slope of 51 mV per decade and good durability. We anticipate that this in situ confined growth provides new insights into the construction of high performance catalysts for energy storage and conversion.

A molecular copper catalyst for electrochemical water reduction with a large hydrogen-generation rate constant in aqueous solution.

The copper complex [(bztpen)Cu](BF4)2 (bztpen=N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen-generation rate constant (k(obs)) of over 10000 s(-1). A turnover frequency (TOF) of 7000 h(-1) cm(-2) and a Faradaic efficiency of 96% were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu](2+) in pH 2.5 buffer solution at -0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton-coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu](2+).

A Molecular Copper Catalyst for Electrochemical Water Reduction with a Large Hydrogen‐Generation Rate Constant in Aqueous Solution

AbstractThe copper complex [(bztpen)Cu](BF4)2 (bztpen=N‐benzyl‐N,N′,N′‐tris(pyridin‐2‐ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen‐generation rate constant (kobs) of over 10000 s−1. A turnover frequency (TOF) of 7000 h−1 cm−2 and a Faradaic efficiency of 96 % were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu]2+ in pH 2.5 buffer solution at −0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton‐coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu]2+.

Electrocatalytic and photocatalytic hydrogen production from acidic and neutral-pH aqueous solutions using iron phosphide nanoparticles.

Nanostructured transition-metal phosphides have recently emerged as Earth-abundant alternatives to platinum for catalyzing the hydrogen-evolution reaction (HER), which is central to several clean energy technologies because it produces molecular hydrogen through the electrochemical reduction of water. Iron-based catalysts are very attractive targets because iron is the most abundant and least expensive transition metal. We report herein that iron phosphide (FeP), synthesized as nanoparticles having a uniform, hollow morphology, exhibits among the highest HER activities reported to date in both acidic and neutral-pH aqueous solutions. As an electrocatalyst operating at a current density of -10 mA cm(-2), FeP nanoparticles deposited at a mass loading of ∼1 mg cm(-2) on Ti substrates exhibited overpotentials of -50 mV in 0.50 M H2SO4 and -102 mV in 1.0 M phosphate buffered saline. The FeP nanoparticles supported sustained hydrogen production with essentially quantitative faradaic yields for extended time periods under galvanostatic control. Under UV illumination in both acidic and neutral-pH solutions, FeP nanoparticles deposited on TiO2 produced H2 at rates and amounts that begin to approach those of Pt/TiO2. FeP therefore is a highly Earth-abundant material for efficiently facilitating the HER both electrocatalytically and photocatalytically.

Petaled Molybdenum Disulfide Surfaces: Facile Synthesis of a Superior Cathode for QDSSCs

Quantum dot sensitized solar cells (QDSSCs) are in need of a highly active, stable, and inexpensive cathode material for practical devices. Here, a new, facile, hydrothermal preparation of nanostructured MoS2 is shown. Grown directly from a planar Mo metal foil, the MoS2 films have a petaled morphology that exposes a large number of catalytically active Mo edge sites, and are highly active for the electrochemical reduction of water and aqueous polysulfide. Preliminary results of its performance in solar devices are further presented, demonstrating superior QDSSC efficiency compared to the use of Pt cathodes.

Electrochemical study of TiO2 modified with silver nanoparticles upon CO2 reduction

A systematic cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) study on titanium dioxide (TiO2) and silver–TiO2 surfaces was performed in order to decouple electrochemical reduction processes of carbon dioxide in aqueous solutions. CV studies indicate cathodic current increase on Ag–TiO2 compared to bare TiO2 surfaces. An equivalent circuit based on transmission line model was applied in order to adjust EIS data, and a modification of this model was made to account for Ag particle interaction with the electrolyte solution. Electrochemical processes were then decoupled upon applied potential where the role of TiO2 surface states was identified and separated from (a) silver reduction, (b) electronic transport on TiO2, and (c) charge transfer on TiO2 and Ag surfaces. The Ag–electrolyte interface impedance has considerably lower values than the TiO2–electrolyte interface, suggesting that the silver particles may be considered as favorable reaction sites for the electrochemical reduction of water and carbon dioxide.

Nano-cubic structured titanium nitride particle films as cathodes for the effective electrocatalytic debromination of BDE-47

An energetic TiN cathode was fabricated for effective electrocatalytic debromination of 2,2′,4,4′-tetrabromodiphenyl ethers (BDE-47); this was achieved by placing Ti foils in an aqueous suspension of TiN nanoparticles, then drying the system at 50°C for 12h. TEM and SEM characterization showed that the TiN nanoparticles—whose average size was approximately 50nm—were ideal nano-cubic structures and distributed uniformly on the Ti substrate. When applied as a cathode in cyclic voltammetry measurements, the TiN electrode exhibited stable electrochemical performance over 20 cycles, in the ∼1–10 pH range. The overpotential of the TiN cathode for electrochemical reduction of water (the main side reaction during the electrocatalytic reduction of pollutants in aqueous solution) was determined as 0.54V, which was much higher than the values for either a Pt wafer (0.12V), or a Pt film (0.07V). The TiN electrodes displayed superior electrocatalytic activity for the electrocatalytic debromination of BDE-47. The kinetic constant of BDE-47 degradation on TiN cathode is 0.65h−1, which was 2, 4 and 17 times as much as those on Pt film, Pt wafer and graphite cathodes, respectively. A pathway was proposed for the degradation of BDE-47, based on measurements of the intermediate products resulting from the removal of BDE-47 by GC–MS.

Rapid release of plasmid DNA from surfaces coated with polyelectrolyte multilayers promoted by the application of electrochemical potentials.

We report an approach to the rapid release of DNA based on the application of electrochemical potentials to surfaces coated with polyelectrolyte-based thin films. We fabricated multilayered polyelectrolyte films (or "polyelectrolyte multilayers", PEMs) using plasmid DNA and a model hydrolytically degradable cationic poly(β-amino ester) (polymer 1) on stainless steel substrates using a layer-by-layer approach. The application of continuous reduction potentials in the range of -1.1 to -0.7 V (vs a Ag/AgCl electrode) to film-coated electrodes in PBS at 37 °C resulted in the complete release of DNA over a period of 1-2 min. Film-coated electrodes incubated under identical conditions in the absence of applied potentials required 1-2 days for complete release. Control over the magnitude of the applied potential provided control over the rate at which DNA was released. The results of these and additional physical characterization experiments are consistent with a mechanism of film disruption that is promoted by local increases in pH at the film/electrode interface (resulting from electrochemical reduction of water or dissolved oxygen) that disrupt ionic interactions in these materials. The results of cell-based experiments demonstrated that DNA was released in a form that remains intact and able to promote transgene expression in mammalian cells. Finally, we demonstrate that short-term (i.e., non-continuous) electrochemical treatments can also be used to promote faster film erosion (e.g., over 1-2 h) once the potential is removed. Past studies demonstrate that PEMs fabricated using polymer 1 can promote surface-mediated transfection of cells and tissues in vitro and in vivo. With further development, the electrochemical approaches reported here could thus provide new methods for the rapid, triggered, or spatially patterned transfer of DNA (or other agents) from surfaces of interest in a variety of fundamental and applied contexts.