Surface Of Porous Electrode Research Articles

NiO hybrid nanoarchitecture-based pseudocapacitor in organic electrolyte with high rate capability and cycle life

A 3D hierarchical NiO nanostructures with combined microstructure of nanoflakes and nanoflowers have been fabricated on carbon fibre cloth (CFC). Unique nano-micro structural features of NiO/CFC electrode showed an enhanced electrochemical activity in organic electrolyte (1 M tetraethylammonium tetrafluorborate (TEABF4) in propylene carbonate) in terms of rate capability, specific energy and power performance as well as potential limit. The electrode showed a specific capacitance of 170 Fg−1 for a current density of 5 Ag−1. Configured as a two-electrode symmetric supercapacitor, the device showed a specific capacitance of 34.9 Fg−1 at 1 Ag−1 current density. It delivered a maximum specific energy density of 19.4 Wh kg−1 at a high power density of 1002.8 W kg−1 at a constant current density of 1 Ag−1. The cell is also capable of long-term cycling stability with an efficiency of 58 % after 25,000 cycles with a potential window of 0 to ±2 V. This superior electrochemical activity of the NiO electrode is due to their structural benefits of well-connected hybrid nano/mesoporous structure and rapid ion intercalation within the porous electrode surface.

Quantifying the apparent ‘Catalytic’ effect of porous electrode surfaces

The voltammetry of porous electroactive surfaces is investigated by simulation. The surface is modelled as a solid block penetrated by a regularly distributed array of cylindrical pores. The effect of the voltage scan rate and the geometry (in terms of pore depth, radius, and inter-pore separation) of the porous surface on the voltammetry is examined for both fully reversible and fully irreversible electrode kinetics. In the latter case, it is demonstrated that when the pores are relatively shallow, a predictable shift in peak potential is observed corresponding to an apparent catalytic effect. For deeper pores, voltammetry corresponding to thin-layer behaviour is observed. The limits under which these behaviours operate are elucidated. This work builds on a previous investigation into the apparent catalytic effect exhibited by nanoparticle modified electrodes Ward et al. (2013) [11].

Analysis of Impedance Spectrum By Electrochemical Approach

Electrochemical impedance spectroscopy (EIS) and electrochemical approach are two major tools in analyzing the performance of a battery, where the former one is commonly used to measure frequency response of a battery. However, the impedance spectrum does not clearly reveal the information about the change in material parameters of electrochemical models like electrolyte conductivity, solid-phase diffusivity, and double-layer capacity. To gain more information about impedance spectrum, the link between the two methods should be established.By constructing an isothermal one-dimensional electrochemical model of lithium iron phosphate (LFP) cell, we have simulated the discharge curve of a single 26650 LFP cell, which agrees well with the experimental data [1], as shown in the inset of Fig. 1. Figure 1 also shows that the simulation physical model can be further used to calculate the impedance spectrum. Although there exists slight difference between the experiment and the simulation, which could be caused by the use of distinct electrode materials, using the physical model to explore battery impedance still provides a new way to have a deeper understanding of discharge behavior.It is known that double layer on the surface of solid electrodes significantly contributes to impedance spectrum, whereas it exerts little influence on the discharge curve whether it exists or not. Base on the electrochemical model, we find in Fig. 2 that there are few variations between the case which has double layer in cathode only and the other case which has double layer in both electrodes. This result shows an agreement with an experiment, which indicates that cathode contributes to impedance spectrum magnitude [2].In spectrum analysis the irreversible resistance, coming from electrolyte conductivity and resistance between the porous electrode surface and the current collector [3], is frequently treated as a constant value. Figure 3 shows that decreasing electrolyte conductivity in the electrochemical model drives the impedance curve to the right. This proves that the difference in electrolyte conductivity will lead to a change in the real-part resistance. More detailed discussion on the electrochemical parameters in the impedance spectrum will be presented in this work. REFERENCES [1] M. Safari and C. Delacourt, J. Electrochem. Soc.,158, A562 (2011).[2] G. Nagasubramanian, J. Power Sources, 87, 226 (2000)[3] D. D. Macdonald, Electrochemica Acta, 51, 1376, (2006)

A review of molecular modelling of electric double layer capacitors

Electric double-layer capacitors are a family of electrochemical energy storage devices that offer a number of advantages, such as high power density and long cyclability. In recent years, research and development of electric double-layer capacitor technology has been growing rapidly, in response to the increasing demand for energy storage devices from emerging industries, such as hybrid and electric vehicles, renewable energy, and smart grid management. The past few years have witnessed a number of significant research breakthroughs in terms of novel electrodes, new electrolytes, and fabrication of devices, thanks to the discovery of innovative materials (e.g. graphene, carbide-derived carbon, and templated carbon) and the availability of advanced experimental and computational tools. However, some experimental observations could not be clearly understood and interpreted due to limitations of traditional theories, some of which were developed more than one hundred years ago. This has led to significant research efforts in computational simulation and modelling, aimed at developing new theories, or improving the existing ones to help interpret experimental results. This review article provides a summary of research progress in molecular modelling of the physical phenomena taking place in electric double-layer capacitors. An introduction to electric double-layer capacitors and their applications, alongside a brief description of electric double layer theories, is presented first. Second, molecular modelling of ion behaviours of various electrolytes interacting with electrodes under different conditions is reviewed. Finally, key conclusions and outlooks are given. Simulations on comparing electric double-layer structure at planar and porous electrode surfaces under equilibrium conditions have revealed significant structural differences between the two electrode types, and porous electrodes have been shown to store charge more efficiently. Accurate electrolyte and electrode models which account for polarisation effects are critical for future simulations which will consider more complex electrode geometries, particularly for the study of dynamics of electrolyte transport, where the exclusion of electrode polarisation leads to significant artefacts.

Microscopic Insights into the Electrochemical Behavior of Nonaqueous Electrolytes in Electric Double-Layer Capacitors.

Electric double-layer capacitors (EDLCs) are electrical devices that store energy by adsorption of ionic species at the inner surface of porous electrodes. Compared with aqueous electrolytes, ionic liquid and organic electrolytes have the advantage of larger potential windows, making them attractive for the next generation of EDLCs with superior energy and power densities. The performance of both ionic liquid and organic electrolyte EDLCs hinges on the judicious selection of the electrode pore size and the electrolyte composition, which requires a comprehension of the charging behavior from a microscopic view. In this Perspective, we discuss predictions from the classical density functional theory (CDFT) on the dependence of the capacitance on the pore size for ionic liquid and organic electrolyte EDLCs. CDFT is applicable to electrodes with the pore size ranging from that below the ionic dimensionality to mesoscopic scales, thus unique for investigating the electrochemical behavior of the confined electrolytes for EDLC applications.

Electrochemistry at synchrotrons

Synchrotron techniques (X-ray and far infrared) have been used in electrochemical research for about the last 25 years. They are powerful new tools to investigate in situ the structure of the electrochemical interphase extending a few thousand angstrom in both phases around the solid/liquid interface. A brief description of the experimental techniques is given, with numerous examples of their applications to investigate a wide variety of phenomena, including measurements of the roughness of electrode surfaces such as relaxation/reconstruction of metal surfaces, the structure of the electrochemical double layer, adsorption/absorption at electrode surfaces, oxide film formation on metals and passivation, underpotential deposition of metals, electrode structures/compositions in batteries and fuel cells, and porous electrode surfaces such as anodic formation of porous silicon. Some historical notes and personal reminiscences are also included.

Nanostructured electrodes for biocompatible CMOS integrated circuits

This paper reports on the adaptation of standard complementary metal oxide semiconductor (CMOS) integrated circuit (IC) technology for biocompatibility, enabling a low-cost solution for drug discovery pharmacology, neural interface systems, cell-based biosensors and electrophysiology. The basis for the process is the anodisation of IC aluminium electrodes to form nanoporous alumina. The porous alumina was electrochemically thinned to reduce the alumina electrode impedance. For applications where a porous electrode surface is either preferred or acceptable, we demonstrated that porosity can be manipulated at room temperature by modifying the anodising electrolyte to include up to 40% polyethylene glycol and reducing the phosphoric acid concentration from 4% (w/v) to 1%. For applications requiring a planar microelectrode surface, a noble metal was electrodeposited into the pores of the alumina film. Limited success was achieved with a pH 7 platinum and pH 5 gold cyanide bath but good results were demonstrated with a pH 0.5 gold chloride bath which produced planar biocompatible electrodes. A further reduction in impedance was produced by deposition of platinum-black, which may be a necessary additional step for demanding applications such as neuronal recording. During this work a capability for real-time electrochemical impedance spectroscopy (EIS) was developed to study anodisation, barrier oxide thinning, oxide breakdown and electrodeposition processes. To study the pore morphology, focused ion beam (FIB) was employed to produce cross-sectional cuts of the IC features which were inspected by SEM with an ‘In-lens’ detector. The anodisation process and the optional electrodeposition steps require only simple bench equipment operated at room temperature and is therefore a viable route for manufacturing low-cost biocompatible electrodes from standard CMOS ICs.

Numerical Simulation of Reactive Transport Phenomena in the Hexis SOFC-System

An advanced computational reactive flow code has been implemented into Mathematica® to analyze (electro-) chemical conversions and various transport phenomena in the Hexis® SOFC-stack. The finite element approximation is applied for the space discretization of the continuity equations. The convection dominant cases are stabilized by the SUPG (Streamline Upwind Petrov Galerkin) method. The fluid flow in the interconnect channels is derived analytically with accurate representation of the slip velocity on the porous electrode surfaces. The mathematical system is linearized by means of a pseudo-time step discretization via a semi-implicit scheme. This stable iterative procedure is applied to an evolutionary problem and it converges unconditionally to the steady solution. Due to the low aspect ratio of the thickness to the cross-section of the cell, a one-dimensional model of the cell is coupled with the flow inside the gas channels. This approach reduces the computing time by retaining the main geometric details.

Formation of mediated biocatalytic cathodes by electrodeposition of a redox polymer and laccase

Mediated biocatalytic cathodes were fabricated via electrodeposition of an osmium-based redox polymer onto planar and porous electrode surfaces, incorporating the oxygen-reducing enzyme laccase. After one hour of electrodeposition time, bio-cathodes on glassy carbon substrate exhibited redox site loading of 6 nmol cm −2 and oxygen reduction current density of 1.4 mA cm −2. Those on carbon-fiber paper exhibited redox site loading of 46 nmol cm −2 and oxygen reduction current density of 15.7 mA cm −2. The current density and polarization behavior were comparable to those of bio-cathodes made by conventional chemical crosslinking. The specific activity in terms of current per mole of redox polymer or enzyme was significantly higher for electrodeposited films.

Electrochemical Study of High Electrochemical Double Layer Capacitance of Ordered Porous Carbons with Both Meso/Macropores and Micropores

Porous carbons with large meso/macropore surface areas were prepared by the colloidal-crystal-templating technique. The porous carbons exibited extremely high specific electrochemical double layer (EDL) capacitance of 200−350 F g-1 in an aqueous electrolyte (1 M H2SO4). The pore structure dependence of the capacitance was studied mainly by means of cyclic voltammetry and is discussed in detail. From the sweep rate dependence of the series resistance and capacitance, it was found that the ion-penetration depth at the porous electrode surface was finite and decreased with an increasing sweep rate. Peaks around the point of zero charge, which were observed in addition to typical rectangular voltammograms, were explained well by the potential drop in pores. The surface area dependence of the capacitance revealed that the contribution of the meso/macropore surface is as great as that of the plane electrodes and that only the part of the micropore surface adjacent to the opening mouths is effective.

Computer Simulation of Porous Electrodes with Immobilized Enzymes: The Percolation Properties of Multicomponent Structures

The percolation characteristics of porous electrodes with immobilized enzymes are calculated. It is presumed that active centers of the enzymes undergo direct oxidation or reduction on the electrode. Two versions of a three-dimensional electrode structure consisting of particles of identical size are studied. In one, the frame of the porous electrode (PE) comprises only those parts of the support that conduct electrons. In the other, the electrode is a two-component structure capable of ensuring the supply of two participants of the electrochemical process to the enzymes. Calculated are: the fraction of the support parts that conduct electrons (taken as a whole, they form an “electron cluster”); the electron cluster's transparency; and the number of exits the electron cluster has onto the rear surface of PE. The character of distribution of enzymes that are in contact with one or two macroscopic clusters, which comprise “conductive” particles, over the PE thickness is established. In doing so, it is assumed that the two clusters can be supplied either from one side (“parallel” clusters) or from two opposite sides (“collision” clusters) of PE. The electron cluster surface accessible to contact with enzymes and the number of enzymes in contact with such an electron cluster are determined. Two possibilities of the PE functioning are examined. In one, the electrochemical process proceeds at any contact of the enzyme with the support particles. In the other, a certain type of enzyme immobilization on the support is required. The region of optimum concentrations of components that make PE is established. Within this region the electrochemical activity may reach a maximum.

Ideal Conductivity Profile of a Porous Matrix for a Flow-through Porous Electrode with a High Conversion Degree of the Electroactive Component

The possibility of designing an equally accessible, in electric respect (η ≈ const), porous electrode (PE) is considered. The design involves creating a special conductivity profile of the solid phase, κs(x), in conditions of considerable variations of current density inside the electrode. A generalized expression for an ideal profile κs(x) at any current distribution is derived, and some concrete solutions for different PE operation modes at the limiting current of the target reaction are obtained. The profiles are exponential over a major fraction of PE and change to hyperbolic at a low conversion degree of the electroactive component. Similarities and differences of the profiles are analyzed. At a high conversion degree, only the exponential profile ensures effective operation of the entire surface of PE and selectivity of the target reaction.

Hydrogen evolution reaction on Ni-Al electrodes

The hydrogen evolution reaction (h.e.r) was studied in alkaline solutions on two types of electrodes: (1) obtained by alloying Raney nickel without or with nickel and (ii) by pressing Raney nickel and nickel powders at room temperature. The obtained electrodes are usually very active for the h.e.r. The most active electrode was obtained by pressing Raney nickel with nickel powder (50 wt %). It was characterized by a large roughness factor, R ∼ 10 000 and a very low overpotential at the current density of 250 mA cm−2, η250 = 56 mV. The mechanism of the h.e.r. was studied using a.c. impedance measurements. The high electrode activity is connected with the increase in the intrinsic activity of the porous electrode surface.