Geometrical Current Density Research Articles

Low overpotential and high current CO2 reduction with surface reconstructed Cu foam electrodes

While recent reports have demonstrated that oxide-derived Cu-based electrodes exhibit high selectivity for CO2 reduction at low overpotential, the low catalytic current density (<2mA/cm2 at −0.45V vs. RHE) still largely limits its applications for large-scale fuel synthesis. Here we report an extremely high current density for CO2 reduction at low overpotential using a Cu foam electrode prepared by air-oxidation and subsequent electroreduction. Apart from possessing three-dimensional (3D) open frameworks, the resulting Cu foam electrodes prepared at higher temperatures exhibit enhanced electrochemically active surface area and distinct surface structures. In particular, the Cu foam electrode prepared at 500°C exhibits an extremely high geometric current density of ~9.4mA/cm2 in CO2-saturated 0.1M KHCO3 aqueous solution and achieving ~39% CO and ~23% HCOOH Faradaic efficiencies at −0.45V vs. RHE. The high activity and significant selectivity enhancement are attributable to the formation of abundant grain-boundary supported active sites and preferable (100) and (111) facets as a result of reconstruction of Cu surface facets. This work demonstrates that the structural integration of Cu foam with open 3D frameworks and the favorable surface structures is a promising strategy to develop an advanced Cu electrocatalyst that can operate at high current density and low overpotential for CO2 reduction.

Distribution of geometrical current density inside a flow-by porous electrode: Effect of electrode parameters and electrochemical reactions

The effect of the difference between the equilibrium potentials of the target and side reactions, the solution flow rate, and specific surface area of porous electrode (PE) on the distribution of the geometrical current density along the solution flow ig(y) at various average current densities is studied by the method of mathematical modeling. It is found that the largest range of the variation of ig(y) (the exponential decrease along the solution flow) is typical for the conditions that provide the limiting current mode of the target reaction on the entire PE surface in the absence of simultaneous hydrogen evolution. All changes in the operation conditions, which reduce the uniformity of the distribution of the current inside PE along the X axis and hamper reaching the limiting current mode (for example, a decrease in the equilibrium potential difference, an increase in the flow rate or specific surface area of PE) lead to more uniform distribution of ig(y).

Synergistic WO3·2H2O Nanoplates/WS2 Hybrid Catalysts for High-Efficiency Hydrogen Evolution.

Tungsten trioxide dihydrate (WO3·2H2O) nanoplates are prepared by in situ anodic oxidation of tungsten disulfide (WS2) film on carbon fiber paper (CFP). The WO3·2H2O/WS2 hybrid catalyst exhibits excellent synergistic effects which facilitate the kinetics of the hydrogen evolution reaction (HER). The electrochromatic effect takes place via hydrogen intercalation into WO3·2H2O. This process is accelerated by the desirable proton diffusion coefficient in the layered WO3·2H2O. Hydrogen spillover from WO3·2H2O to WS2 occurs via atomic polarization caused by the electric field of the charges on the planar defect or edge active sites of WS2. The optimized hybrid catalyst presents a geometrical current density of 100 mA cm(-2) at 152 mV overpotential with a Tafel slope of ∼54 mV per decade, making the materials one of the most active nonprecious metal HER catalysts.

Voltage Cycling Induced Losses in Electrochemically Active Surface Area and in H2/Air-Performance of PEM Fuel Cells

The influence of voltage cycling profiles on the degradation of MEAs (membrane electrode assemblies) for PEM (proton exchange membrane) fuel cells was investigated using MEAs with carbon supported catalysts. The loss of electrochemically active surface area (ECSA) of the cathode electrodes significantly increased with the upper potential limit. The ECSA loss was correlated with the performance decrease for hydrogen/air polarization curves, revealing purely kinetic losses at low and additional mass-transport related voltage losses at high platinum surface area normalized current densities (≡specific current densities). The absolute voltage losses only depended on the specific current density, independent of the extent of ECSA loss and the geometric current density.

Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution

Fe cations dramatically enhance oxygen evolution reaction (OER) activity when incorporated substitutionally into Ni or Co (oxy)hydroxides, serving as possible OER active sites. Pure Fe (oxy)hydroxides, however, are typically thought to be poor OER catalysts and are not well-understood. Here, we report a systematic investigation of Fe (oxy)hydroxide OER catalysis in alkaline media. At low overpotentials of ∼350 mV, the catalyst dissolution rate is low, the activity is dramatically enhanced by an AuOx/Au substrate, and the geometric OER current density is largely independent of mass loading. At higher overpotentials of ∼450 mV, the dissolution rate is high, the activity is largely independent of substrate choice, and the geometric current density depends linearly on loading. These observations, along with previously reported in situ conductivity measurements, suggest a new model for OER catalysis on Fe (oxy)hydroxide. At low overpotentials, only the first monolayer of the electrolyte-permeable Fe (oxy)hydro...

Analysis of the distribution of geometrical current density along the direction of solution flow inside flow-by porous electrodes

Abstract Calculations of electric field inside a flow-by porous electrode usually involve the assumption that the distribution of geometrical current density along the direction of solution flow inside the electrode is uniform. In the present work we propose the version of its successive refinement without using a priori assumptions. It is demonstrated that the assumption of the uniform current distribution along solution flow is quite acceptable under the conditions of the small changes of polarization resistance at the boundary between the porous electrode and the solution (small current density, high flow rate or prevalence of the side reaction of hydrogen evolution). Quite contrary, in the case of a sharp change of polarization resistance along solution flow this assumption is not fulfilled and causes quantitative distinctions of the spatial distribution of potential, current and concentration inside this electrode. An increase in non-uniformity of overall current distribution with an increase in solution depletion degree inside the porous electrode was confirmed experimentally for copper electrodeposition as an example.

Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide.

The scalable and sustainable production of hydrogen fuel through water splitting demands efficient and robust Earth-abundant catalysts for the hydrogen evolution reaction (HER). Building on promising metal compounds with high HER catalytic activity, such as pyrite structure cobalt disulphide (CoS2), and substituting non-metal elements to tune the hydrogen adsorption free energy could lead to further improvements in catalytic activity. Here we present a combined theoretical and experimental study to establish ternary pyrite-type cobalt phosphosulphide (CoPS) as a high-performance Earth-abundant catalyst for electrochemical and photoelectrochemical hydrogen production. Nanostructured CoPS electrodes achieved a geometrical catalytic current density of 10 mA cm(-2) at overpotentials as low as 48 mV, with outstanding long-term operational stability. Integrated photocathodes of CoPS on n(+)-p-p(+) silicon micropyramids achieved photocurrents up to 35 mA cm(-2) at 0 V versus the reversible hydrogen electrode (RHE), onset photovoltages as high as 450 mV versus RHE, and the most efficient solar-driven hydrogen generation from Earth-abundant systems.

Oxygen Evolution Reaction on Zirconium Oxide Film in Alkaline Medium

Introduction Alkaline water electrolysis (AWE) is one of the hydrogen production methods without any carbon dioxide emission with renewable electric power supply. The AWE has several advantages compared with polymer electrolyte water electrolysis such as utilize of inexpensive materials [1]. However, in the case of connection of the AWE to electric fluctuant power from renewable energy, the degradation of the electrodes is accelerated [2]. For the practical use for AWE with renewable energies, a durable alternative material should be required. We focused on zirconium compounds as the alternative material because of their high activity for oxygen reduction reaction and high chemical stability [3]. In this study, the relationship between the catalytic activity for oxygen evolution reaction (OER) and preparing method of the zirconia film has been investigated as a fundamental study of the OER activity in alkaline solution. Experimental A zirconium plate (Zr purity: 99.2%) was used as a starting material, and oxidized for 10 minutes in the range of temperature from 400 to 700oC in air to prepare ZrO­2 / Zr electrode (ZrO2 / Zr_Temp. of oxidation). To prepare ZrO2 / Ti electrode, a Zr target was used in deposition method of an arc plasma gun (APD). ZrO2 was deposited on Ti rods for 6000 shots applied 100 V with a 360 μF capacitor at room temperature (ZrO2/ Ti_APD). After evacuation of the chamber, oxygen was introduced with 1 mL / min during deposition. Oxygen partial pressure in the chamber was 0.083 Pa. The frequency of the pulse was 3 Hz. The ZrO2 electrode, a reversible hydrogen electrode (RHE), and a glassy carbon plate were used as a working, reference, and counter electrodes. All measurements were performed under N2 atmosphere in 0.1 M KOH at 30oC. Cyclic voltammetry (CV) was conducted for 100 cycles between 0 and 1.0 V vs. RHE with the scan rate of 100 mV s-1 as a pretreatment. The OER activity of the ZrO2 electrodes was evaluated by slow scan voltammetry (SSV) between 1.0 and 2.0 V vs. RHE with 5 mV s-1 of the scan rate. The onset potential (E onset) defined by the potential at (geometric current density) in Tafel region was used as the index for OER activity. Results and discussion Figure 1 shows the polarization curves of the ZrO2 / Zr and ZrO2 / Ti electrodes. The ZrO2 / Zr_550oC had the highest geometric current density (i­ geo) in this study. The ZrO2 / Ti_APD had smaller current density than the ZrO2 / Zr_550 and 600oC. Figure 2 shows the Tafel plot of the ZrO2 / Zr_550oC and the ZrO2 / Ti_APD. The Tafel slope of OER on the ZrO2 / Zr_550oC was 70 mV dec-1, and it was smaller than that of the ZrO2 / Ti_APD of 108 mV dec-1. However, Tafel region of the ZrO2 / Ti_APD was at almost same potential as that of the ZrO2 / Zr_550oC. The i geo of ZrO2 / Ti_APD was crossed that of the ZrO2 / Zr_550oC at about 1.7 V. This might be affected by contact resistance between ZrO2films and Ti substrates or oxide film resistance. Figure 3 shows the E onset of OER on the ZrO2 electrodes as a function of the oxidation temperature. The E onset decreased with the increase of oxidation temperature in the range from 400oC to 550oC. Above 550oC, the E onset was almost constant. Thus, the active site of OER on the ZrO2 / Zr would be formed above 550oC. The E onset of OER on the ZrO2 / Ti_APD was the smallest potential of 1.59 V vs. RHE in the ZrO2electrodes in this study. Though the ZrO2 / Ti had the lower i geo than the ZrO2 / Zr electrode in higher i geo region, the E onset of OER on the ZrO2 / Ti_APD was the lowest in this study. Therefore, the ZrO2/ Ti_APD has potential for the high OER catalytic activity in alkaline solution and would have higher current density by improvement conductivity. Reference [1]. S. Mitsushima and K. Matsuzawa, J. Hydr . E ner . Sys . Soc. Jpn., 36, 11 (2011) (in Japanese). [2]. H. Ichikawa, K. Matsuzawa, Y. Kohno, I. Nagashima, Y. Sunada, Y. Nishiki, A. Manabe, and S. Mitsushima, ECS Trans., 58(33), 9 (2014). [3]. A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim . Acta, 55, 8005 (2010). Figure 1

Photoelectrochemical Solar Energy Conversion and Electrocatalysis Using Earth-Abundant Nanomaterials

The scale of renewable energy challenge not only calls for highly efficient technologies but also abundant, inexpensive, and robust materials. Nanomaterials can help to mitigate the poor properties of earth-abundant semiconductors and catalysts to enhance solar energy conversion. We have controlled the nanostructures and polymorphs of layered MS2 (M = Mo, W) materials and other earth-abundant metal chalcogenides, such as metallic cobalt pyrite (CoS2), to significantly enhance their catalytic activity in hydrogen evolution reaction (HER). We further establish ternary pyrite-type cobalt phosphosulfide (CoPS) as a high-performance earth-abundant HER catalyst that achieves geometrical catalytic current density of 10 mA/cm2 at overpotentials as low as 48 mV with outstanding long-term operation stability. These earth-abundant catalysts and semiconductors have been integrated to enable efficient solar energy conversion. Integrated photocathodes of the novel earth-abundant catalysts on n +-p-p + silicon micropyramids achieved high photocurrents up to 35 mA/cm2 and onset photovoltages as high as 450 mV vs RHE, and 5-6% solar-to-hydrogen conversion efficiency.

High-performance electrocatalysis using metallic cobalt pyrite (CoS₂) micro- and nanostructures.

The development of efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is an ongoing challenge. We report metallic cobalt pyrite (cobalt disulfide, CoS2) as one such high-activity candidate material and demonstrate that its specific morphology--film, microwire, or nanowire, made available through controlled synthesis--plays a crucial role in determining its overall catalytic efficacy. The increase in effective electrode surface area that accompanies CoS2 micro- and nanostructuring substantially boosts its HER catalytic performance, with CoS2 nanowire electrodes achieving geometric current densities of -10 mA cm(-2) at overpotentials as low as -145 mV vs the reversible hydrogen electrode. Moreover, micro- and nanostructuring of the CoS2 material has the synergistic effect of increasing its operational stability, cyclability, and maximum achievable rate of hydrogen generation by promoting the release of evolved gas bubbles from the electrode surface. The benefits of catalyst micro- and nanostructuring are further demonstrated by the increased electrocatalytic activity of CoS2 nanowire electrodes over planar film electrodes toward polysulfide and triiodide reduction, which suggests a straightforward way to improve the performance of quantum dot- and dye-sensitized solar cells, respectively. Extension of this micro- and nanostructuring strategy to other earth-abundant materials could similarly enable inexpensive electrocatalysts that lack the high intrinsic activity of the noble metals.

Resolving Losses at the Negative Electrode in All-Vanadium Redox Flow Batteries Using Electrochemical Impedance Spectroscopy

We present an in situ electrochemical technique for the quantitative measurement and resolution of the ohmic, charge transfer and diffusion overvoltages at the negative electrode of an all-vanadium redox flow battery (VRFB) using electrochemical impedance spectroscopy (EIS). The mathematics describing the complex impedance of the V+2/V+3 redox reaction is derived and matches the experimental data. The voltage losses contributed by each process have been resolved and quantified at various flow rates and electrode thicknesses as a function of current density during anodic and cathodic polarization. The diffusion overvoltage was affected strongly by flow rate while the charge transfer and ohmic losses were invariant. On the other hand, adopting a thicker electrode significantly changed both the charge transfer and diffusion losses due to increased surface area. Furthermore, the Tafel plot obtained from the impedance resolved charge transfer overvoltage yielded the geometric exchange current density, anodic and cathodic Tafel slopes (135 ± 5 and 121 ± 5 mV/decade respectively) and corresponding transfer coefficients α = 0.45 ± 0.02 and β = 0.50 ± 0.02 in an operating cell.

Monometallic Cobalt–Trisglyoximato Complexes as Precatalysts for Catalytic H2 Evolution in Water

Electrochemical investigation on differently substituted boron-caped and glyoximato-strapped ligands of cobalt clathrochelate complexes in the presence of acid revealed the formation of cobalt-based nanoparticles at the surface of the electrode. The potential for the electrochemical deposition is a function of the chemical nature of the substituents. The overpotential for the catalytic proton reduction of the resulting cobalt particles at pH 7 was found to be ∼700 mV less negative than on the bare carbon electrode for a geometric current density of 5 mA/cm2.

Electrocatalytic performance of fuel cell reactions at low catalyst loading and high mass transport

An alternative approach to the rotating disk electrode (RDE) for characterising fuel cell electrocatalysts is presented. The approach combines high mass transport with a flat, uniform, and homogeneous catalyst deposition process, well suited for studying intrinsic catalyst properties at realistic operating conditions of a polymer electrolyte fuel cell (PEFC). Uniform catalyst layers were produced with loadings as low as 0.16 μgPt cm(-2) and thicknesses as low as 200 nm. Such ultra thin catalyst layers are considered advantageous to minimize internal resistances and mass transport limitations. Geometric current densities as high as 5.7 A cm(-2)Geo were experimentally achieved at a loading of 10.15 μgPt cm(-2) for the hydrogen oxidation reaction (HOR) at room temperature, which is three orders of magnitude higher than current densities achievable with the RDE. Modelling of the associated diffusion field suggests that such high performance is enabled by fast lateral diffusion within the electrode. The electrodes operate over a wide potential range with insignificant mass transport losses, allowing the study of the ORR at high overpotentials. Electrodes produced a specific current density of 31 ± 9 mA cm(-2)Spec at a potential of 0.65 V vs. RHE for the oxygen reduction reaction (ORR) and 600 ± 60 mA cm(-2)Spec for the peak potential of the HOR. The mass activity of a commercial 60 wt% Pt/C catalyst towards the ORR was found to exceed a range of literature PEFC mass activities across the entire potential range. The HOR also revealed fine structure in the limiting current range and an asymptotic current decay for potentials above 0.36 V. These characteristics are not visible with techniques limited by mass transport in aqueous media such as the RDE.

Dynamics of copper electrodeposition onto fibrous carbon electrodes with electrical conductivity varying across their thickness

Distribution of copper in its electrodeposition from a sulfuric acid solution onto fibrous carbon electrodes with electrical conductivity varying across their thickness, the deposition rate of copper, and the current efficiency by copper were studied in relation to the electrolysis duration, electrical conductivity of the electrode, geometric current density, and solution flow rate. The main factors governing the distribution of copper across the electrode thickness and the electrolysis parameters beginning from the process onset till the “clogging” of a part of the electrode by the metal were determined.

Dynamics of copper electrodeposition onto fibrous carbon electrodes

Distribution of copper electrodeposited from a sulfuric acid solution onto fibrous carbon electrodes, copper deposition rate, and current efficiency by the metal were studied in relation to the electrolysis duration, electrical conductivity of the electrode, geometric current density, and solution flow rate. The variation of the electrode thickness on which copper ions discharge at the limiting diffusion current at various solution flow rates and the electrode thickness on which the whole amount of oxygen dissolved in the electrolyte is reduced were calculated in relation to the solution flow rate and geometric current density. The main factors governing the distribution of copper across the electrode thickness and the electrolysis parameters from the beginning of the process till “clogging” of a part of the electrode by the metal were determined.

CO2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu2O Films

Modified Cu electrodes were prepared by annealing Cu foil in air and electrochemically reducing the resulting Cu(2)O layers. The CO(2) reduction activities of these electrodes exhibited a strong dependence on the initial thickness of the Cu(2)O layer. Thin Cu(2)O layers formed by annealing at 130 °C resulted in electrodes whose activities were indistinguishable from those of polycrystalline Cu. In contrast, Cu(2)O layers formed at 500 °C that were ≥~3 μm thick resulted in electrodes that exhibited large roughness factors and required 0.5 V less overpotential than polycrystalline Cu to reduce CO(2) at a higher rate than H(2)O. The combination of these features resulted in CO(2) reduction geometric current densities >1 mA/cm(2) at overpotentials <0.4 V, a higher level of activity than all previously reported metal electrodes evaluated under comparable conditions. Moreover, the activity of the modified electrodes was stable over the course of several hours, whereas a polycrystalline Cu electrode exhibited deactivation within 1 h under identical conditions. The electrodes described here may be particularly useful for elucidating the structural properties of Cu that determine the distribution between CO(2) and H(2)O reduction and provide a promising lead for the development of practical catalysts for electrolytic fuel synthesis.

Amorphous molybdenum sulfide films as catalysts for electrochemical hydrogen production in water

Amorphous molybdenum sulfide films are efficient hydrogen evolution catalysts in water. The films are prepared via simple electro-polymerization procedures and are characterized by XPS, electron microscopy and electronic absorption spectroscopy. Whereas the precatalysts could be MoS3 or MoS2, the active form of the catalysts is identified as amorphous MoS2. Significant geometric current densities are achieved at low overpotentials (e.g., 15 mA cm−2 at η = 200 mV) using these catalysts. The catalysis is compatible with a wide range of pHs (e.g., 0 to 13). The current efficiency for hydrogen production is quantitative. A 40 mV Tafel slope is observed, suggesting a rate-determining ion+atom step. The turnover frequency per active site is calculated. The amorphous molybdenum sulfide films are among the most active non-precious hydrogen evolution catalysts.

Starting regimes of plasma excitation in conducting aqueous solutions

The conditions for the formation of a plasma discharge in an oxygen-containing ion solution are analyzed. Starting regimes of plasma excitation, which depend on physicochemical properties of the electrolyte, electrode, released gas, and current density, are substantiated theoretically. The description of the process is based on electrode screening by gas bubbles. The critical values of the geometrical and true current density (9.7 and 143.5 × 104 A/m2), the extent of screening of the electrode by hydrogen bubbles (93.2%), the rate of emergence of bubbles (1.6 × 10-6 s-1), and other parameters preceding the evolution of the plasma are determined for the 1% aqueous solution of sulfuric acid. The results of calculations are confirmed in experiments. A method is proposed for determining the limiting wetting angle Θ of the electrode surface from the kinetics of emergence of bubbles. For the electrolyte and copper electrode used in experiments (T = 343 K, j = 4.18 A/m2), this angle was found to be 5.0° ± 0.1° and 175.0° ± 0.1° for hydrogen bubbles.

Biophysikalische Grundlagen der Anwendung elektromagnetischer Felder zur Beeinflussung der Osteogenese

The interactions between bone-forming cells and the extracellular processes determining the mineralisation of the osteoid with electric and/or magnetic fields is the elementary prerequisite. The magnetic or electric field or the combination of both, as well as their time related intensity are discussed as the stimulating factors. When using electric current there is a physical process to be considered. This process consists of changes of metabolites caused by the cathodic electrode reaction which may gain influence on the metabolism of bone-forming cells and the mineralisation. According to the results of this investigation the bone-producing effect of the cathodic electrode reaction saturates+ at a geometric current density of about 0.4 microA/mm2. Apart from the changes of metabolites produced by the electric current there are electric polarizations in front of interfaces and cell membranes due to local ion concentrations caused by even very weak electric fields.

Performance/design formulation for a solid polymer based acid electrolyte hydrogen/air fuel cell

Mathematical development of preliminary performance/design equations for a hydrogen/air, solid polymer acid electrolyte based fuel cell is presented. The development is based on the principles of transport phenomena, intrinsic electrochemical kinetics, and classical thermodynamics. The developed formulation is intended to quantitatively describe the mass fraction profiles of the chemical species, hydrogen and oxygen, in the cell anode and cathode diffusion and electrocatalytic reaction layers as a function of the distance in the proton transport direction at an axial distance parallel to the cell anode or cathode channel flow. Given the cell geometry, chemical species and charge transport, and intrinsic electrochemical kinetic parameters, the developed formulation can be employed to compute the species local mass fluxes and predict the cell anode and cathode cell overvoltages for a desired geometric current density. The presented single cell performance predictive formulation has also been linked to the formulation needed to predict the performance of a stack of a number of identical PEMFCs connected in series.