Electrochemical Energy Generation Research Articles

Graphene nanoplatelets with selectively functionalized edges as electrode material for electrochemical energy storage.

In recent years, graphene-based materials have been in the forefront as electrode material for electrochemical energy generation and storage. Despite this prevalent interest, synthesis procedures have not attained three important efficiency requirements, that is, cost, energy, and eco-friendliness. In this regard, in the present work, graphene nanoplatelets with selectively functionalized edges (XGnPs) are prepared through a simple, eco-friendly and efficient method, which involves ball milling of graphite in the presence of hydrogen (H2), bromine (Br2), and iodine (I2). The resultant HGnP, BrGnP, and IGnP reveal significant exfoliation of graphite layers, as evidenced by high BET surface area of 414, 595, and 772 m(2) g(-1), respectively, in addition to incorporation of H, Br, and I along with other oxygen-containing functional groups at the graphitic edges. The BrGnP and IGnP are also found to contain 4.12 and 2.20 at % of Br and I, respectively in the graphene framework. When tested as supercapacitor electrode, all XGnPs show excellent electrochemical performance in terms of specific capacitance and durability at high current density and long-term operation. Among XGnPs, IGnP delivers superior performance of 172 F g(-1) at 1 A g(-1) compared with 150 F g(-1) for BrGnP and 75 F g(-1) for HGnP because the large surface area and high surface functionality in the IGnP give rise to the outstanding capacitive performance. Moreover, all XGnPs show excellent retention of capacitance at high current density of 10 A g(-1) and for long-term operation up to 1000 charge-discharge cycles.

In-Operando X-ray Tomography Studies of Dendrite Formation in Zinc-Air Batteries

Rechargeable aqueous zinc-air systems are remarkably attractive power sources because of high specific energy, safety, environmental compatibility and low cost. However these systems are still under development and suffer from sufficiently short cycle life that prohibits their use for large electrical storage applications requiring several thousand cycles. One of the key factors responsible is dendritic growth when the system is recharged. Dendrites can grow sufficiently wide to penetrate the separator between the anode and the cathode and cause an internal short-circuit. Despite the substantial work accomplished in studying of zinc dendrite formation there are only few reports available on real time imaging of dendrite growth.Recent developments in X-ray micro/nano tomography have demonstrated the great advantage of the technique for non-destructive ex- and in-situ characterisation of materials and devices for electrochemical energy storage and generation. We used synchrotron x-ray tomography to investigate the mechanism and kinetics of dendrite formation during zinc-air system operation. The dendrite growth was analysed in-operando in radiography mode while three-dimensional image of dendrites covering the electrode surface was reconstructed from multiple tomographic projections ( Figure 1).

Ionic Liquid Electrolytes for Alkali Metal Electrochemical Devices: Low Melting Chloroaluminate Binary Solutions and the Alkali Trapping Problem

With an interest in developing low temperature versions of electrochemical energy storage and generation systems, like the ZEBRA (chloroaluminate electrolyte) cell, we have repeated and extended earlier studies of sodium chloroaluminate electrolytes whose liquidus temperatures have been lowered by mixing with chloroaluminates of weak-‐field, low-‐melting, chloroaluminate ionic liquids. We consider both organic and inorganic cations of low charge intensity, and here report on the case of dialkylimidazolium cations. The non-‐additive effects seen in conductivity, due to asymmetric cation-‐anion interactions and alkali cation trapping, are quantified and discussed in terms of cation competition models. Finally, the importance of these effects in controlling the kinetics of cell performance (to be reported separately) is assessed.

Atomic layer deposition for electrochemical energy generation and storage systems

Clean renewable energy sources (e.g., solar, wind, and hydro) offers the most promising solution to energy and environmental sustainability. On the other hand, owing to the spatial and temporal variations of renewable energy sources, and transportation and mobility needs, high density energy storage and efficient energy distribution to points of use is also critical. Moreover, it is challenging to scale up those processes in a cost-effective way. Electrochemical processes, including photoelectrochemical devices, batteries, fuel cells, super capacitors, and others, have shown promise for addressing many of the abovementioned challenges. Materials with designer properties, especially the interfacial properties, play critical role for the performance of those devices. Atomic layer deposition is capable of precise engineering material properties on atomic scale. In this review, we focus on the current state of knowledge of the applications, perspective and challenges of atomic layer deposition process on the electrochemical energy generation and storage devices and processes.

Electrochemical Properties of Carbon Nanoparticles Entrapped in a Silica Matrix

Carbon-based electrode materials have been widely used for many years for electrochemical charge storage, energy generation, and catalysis. We have developed an electrode material with high specific capacitance by entrapping graphite nanoparticles in a sol-gel network. Films from the resulting colloidal suspensions were porous due to the removal of the entrapped organic solvents from the sol-gel matrix giving rise to high Brunauer–Emmett–Teller specific surface areas and a high capacitance density . An exponential decrease of capacitance was observed with increasing scan rates in cyclic voltammetry studies on these films, suggesting the presence of pores ranging from micro- to mesopores. Nitrogen adsorption–desorption analysis and scanning electron microscope images of these films also confirmed the presence of the micropores as well as mesopores. A steep drop in the double-layer capacitance with polar electrolytes was observed when the films were rendered hydrophilic upon exposure to mild oxygen plasma. We propose a model whereby the microporous hydrophobic sol-gel matrix perturbs the hydration of ions which moves ions closer to the graphite nanoparticles, consequently increasing the capacitance of the film.

Electrochemical Energy Generation and Storage. Fuel Cells and Lithium-Ion Batteries

Abstract We illustrate our current work for electrochemical energy generation and storage (i.e., fuel cells and lithium-ion batteries, respectively). In fuel cell research, we have been developing Pt-based ordered intermetallic compounds as electrocatalysts towards anodic reactions of liquid fuels such as methanol, ethanol, and formic acid for proton-exchange membrane fuel cells (PEMFCs). Development of the intermetallic compounds ranges from bulk materials to nanoparticles. In our work, we have employed a combinatorial method to achieve rapid screening of a large number of potential electrocatalysts. Multi-element sputtering was employed to generate films with different compositions including most of a phase diagram and the resulting libraries were screened using a fluorescence assay. For portable applications, we have also developed a planar microfluidic membraneless fuel cell (PMMFC) device, which eliminates the need for a polyelectrolyte membrane (PEM) and takes advantage of the laminar flow of fuel and oxidant streams. Particularly, in order to increase power density (cell voltages) of a PMMFC device, we have focused on the development of a dual electrolyte PMMFC in which alkaline and acid electrolyte solutions are employed for fuel and oxidant streams, respectively. In lithium-ion battery research, we have been designing new cost-effective organic materials with high energy densities as cathode electroactive materials, targeting large applications such as electrically powered automotives. In particular, we have focused on organosulfur-based polymers, involving the redox chemistry of thiolates (RS−), capable of higher energy density than conventional lithium metal oxides such as LiCoO2. By combining electrochemical techniques with computational methods and organic synthesis, we have tried to establish an efficient procedure to develop novel organic-based electroactive materials suitable for the demands of energy storage materials for rechargeable lithium-ion batteries.

First-Principles Prediction of Equilibrium Potentials for Water Activation by a Series of Metals

In this paper, we describe and apply an ab initio quantum-mechanical model to examine the electrochemical dissociation of water over a number of close-packed and near-close-packed metal surfaces. We have examined a series of metals important for electrochemical energy generation and corrosion-resistant alloy development including Ni, Cu, Ru, Au, Mo, Pd, and Pt. Advances in electrochemical theory depend on a detailed understanding of the interplay between surface chemistry and electrochemical phenomena. Herein, we have used ab initio quantum-mechanical methods to examine the double-layer regions for over these metals and compare the equilibrium potentials for the initial steps of water reduction and oxidation at the surface with known experimental quantities. We find that in its current form, the model developed herein is semiquantitative: it allows for the correct prediction of trends and the size of the double-layer regions, but in some cases it results in significant deviation with known absolute equilibrium potentials. Additional discussion is provided that outlines steps that may be taken to improve the quantitative accuracy of this model.

50 Multiple steady states in electrochemical reactors

The uniqueness of the isothermal steady state operation of electrochemical reactors is examined for electrocatalytic reactions with simultaneous mass transport limitations. Analysis includes planar as well as porous, flow-by and packed-bed electrodes, used for electrochemical processing and energy generation. More than one steady state can exist with complex reaction kinetics in certain potential regimes, dependent also on the convective or diffusive transport conditions. From these states, at least one is unstable and it can result in potential or current (rate) oscillations, as indicated by reported experimental observations. In channel-flow electrochemical reactors instabilities can develop at some point within the reactor due to concentration and possibly potential changes. In order to determine conditions and regions of non-uniqueness, the non-linear kinetic and transport equations are solved by polynomial expansion and orthogonal collocation in the interior of the porous electrodes. While a first-order collocation is sufficient here to demonstrate multiplicity, higher order polynomial approximations are necessary to determine the exact potential and concentration regimes for the existence of multiple steady states.

Criteria for selective path promotion in electrochemical reaction sequences

AbstractAn analysis of consecutive electrochemical reactions is presented with some emphasis on organic electrocatalytic sequences. The effect of potential and electrode kinetic parameters on reaction selectivity and on current and rate distribution is examined in two model electrochemical reactors with channel flow or with complete mixing. Results and some examples demonstrate that the electrolyte potential is the most significant parameter for selectivity control of complex series reactions in electrochemical processing or energy generation. Criteria developed for selectivity and current or rate variation provide the basis for the design of electrochemical reactors and for optimal operation considerations with multiple electrode reactions. Such criteria and analyses apply to a number of working electrode configurations operating with ionic as well as molecular reactants.