Anodic Electrochemical Reactions Research Articles

Fuels for direct carbon fuel cells: Present status and development prospects

The direct carbon fuel cell (DCFC) has been considered an efficient way to mitigate the energy crisis and environmental pollution issues, owing to its high energy conversion efficiency, low pollution, and abundant and readily available resources, but its performance is closely related to the fuel used. We give a brief introduction to the development of DCFCs and a review of the current status of DCFC fuels, with an emphasis on the impact of different carbonaceous materials, such as graphite, coal, activated carbon, solid waste, and coke on DCFC performance. We also systematically analyze the relation between the electrochemical reaction activity of the fuels and their characteristics. For carbonaceous fuels, defects in the crystal structure and surface oxygen functional groups can promote the anodic electrochemical reaction, and their wettability by the electrolyte. Their pore structure, conductivity and particle size can affect mass transfer and charge transfer in the anodic electrochemical reaction. We discuss the function of anodic catalysts in the DCFC, which accelerate the conversion of carbon from solid to gas and improve the anodic electrochemical reaction rate. Based on current research on DCFC fuels, we forecast development trends and key issues relating to DCFC fuels. [New Carbon Materials 2015, 30(1): 12–18]

Self-Driven Bioelectrochemical Mineralization of Azobenzene by Coupling Cathodic Reduction with Anodic Intermediate Oxidation

Bioelectrochemical systems have been intensively studied as a promising technology for wastewater treatment and environment remediation. Coupling of the anodic and cathodic electrochemical reactions allows an enhanced degradation of recalcitrant organics, but external power supply is usually needed to overcome the thermodynamic barrier. In this work, we report a self-driven degradation of azobenzene in a microbial fuel cell (MFC), where the cathodic reduction of azobenzene was effectively coupled with the anodic oxidation of its reduction degradation intermediate (i.e., aniline). The anodic degradation rate of aniline, as the sole carbon source, was significantly higher than that under open-circuit conditions, suggesting a considerable bioelectrochemical oxidation of aniline. Output voltages up to 8mV were obtained in the MFC. However, a shift of cathodic electron acceptor from oxygen to azobenzene resulted in a decreased aniline degradation rate and output voltage. The present work may provide valuable implications for development of sustainable bioelectrochemical technologies for environmental remediation.

Effect of thermal decomposition products of coal on anodic reactions in direct carbon fuel cells

The effect of inherent volatile matter in fuels on anodic electrochemical reactions was investigated in a direct carbon fuel cell (DCFC). Through thermal decomposition, raw coals, which are used as power source in the DCFC system, release gases under an operating temperature of 700°C. These exhaust gases change the gas composition around the anode and affect the system’s electrochemical reaction. To investigate the effect of produced gases, a comparative study was conducted between Indonesian sub-bituminous coal and char obtained through thermal treatment. The open circuit voltage of raw coal increased to 0.23 V compared with that of char. The maximum power density of raw coal (52 mW/cm2) was also higher than that of char (37 mW/cm2) because of the enhanced electron transport caused by the additional reaction of hydrogen, methane, and carbon monoxide. The influence of volatile matter on the anodic electrolyte-electrode interface was analyzed with the use of the equivalent circuit induced from fitting the impedance data.

Tailoring nanostructured catalysts for electrochemical energy conversion systems

AbstractThis review covers topics related to the synthesis of nanoparticles, the anodic and cathodic electrochemical reactions and low temperature electrochemical energy devices. The thermodynamic aspects of nucleation and growth of nanoparticles are discussed. Different methods of chemical synthesis such as w/o microemulsion, Bönnemann, polyol and carbonyl are presented. How the electrochemical reactions take place on the surface of the catalytic nanoparticles and the importance of the substrate is put in evidence. The use of nanomaterials in low temperature energy devices such as H2/O2 polymer electrolyte or proton exchange membrane fuel cell (PEMFC) and micro-direct methanol fuel cell (μDMFC), as well as recent progress and durability, is discussed. Special attention is given to the novel laminar flow fuel cell (LFFC). This review starts with the genesis of catalytic nanoparticles, continues with the surface electrochemical reactions that occur on them, and finally it discusses their application in electrochemical energy devices such as low temperature fuel cells or Li-air batteries.

Multilayered SOFC Anode Structure with Electroless Ni-YSZ for Enhancement of Cell Performance

High performance SOFC is fabricated using multilayer anode support with graded porosity. The graded anode fulfills the requirement of proper fuel oxidation at anode, effective electrochemical reaction at anode/electrolyte interface, thermal compatibility among cell components and long term stability. In such multilayer anode, conventional cermet (40 vol % Ni) helps in proper fuel diffusion and catalytic oxidation. Novel electroless anode (28-32 vol % Ni) is employed adjacent to YSZ electrolyte for effective electrochemical activity and thermal compatibility. Because of the presence of unique core (YSZ)-shell (Ni) microstructure, electroless anode is found to extend the anodic electrochemical reaction from electrolyte/anode interface to the bulk. Significant performance enhancement with current density of ~3.3 Acm-2 is observed for single cells fabricated with multilayer anode at 800degC, 0.7V. The multilayer anode is found to reduce the cell degradation from ~11 % (conventional anode) to ~1.6 % /1000 h during long term performance (~2000 h).

Experimental characterization and mechanistic modeling of carbon monoxide fueled solid oxide fuel cell

The paper presents an elementary reaction based solid oxide fuel cell (SOFC) model coupled with anodic elementary heterogeneous reactions and electrochemical charge transfer reactions for CO/CO 2 fuel based on an anode supported button cell. The model is calibrated and validated using experimental data obtained for various CO/CO 2 fuel compositions at 750, 800 and 850 °C. The comparison shows that the modeling results agree well with the experimental data. The effects of operating conditions on the cell performance and the detailed species concentration distribution are predicted. Then, the carbon deposition on the SOFC anode with CO/CO 2 fuel is experimentally measured and simulated using the elementary reaction model. The results indicate that lower temperature and lower operation voltage are helpful to reduce the possibilities of carbon deposition on Ni particle surfaces.

Correlation of interfacial electrode potential and corrosion resistance of plasma polymer coated galvanized steel. Part 1: Ultra-thin plasma polymer films of varying thickness

Ultra-thin SiO 2-like plasma polymer films were deposited on zinc coated steel. Such films led to a strong inhibition of cathodic and anodic electrochemical reactions and a negative shift in the electrode potential. When the SiO 2-like films are additionally coated with a few micron thick organic film, the resulting interface electrode potential is further shifted cathodically down to −0.8 V SHE as measured by means of a scanning Kelvin probe. This interface potential is about the same as the free corrosion potential of zinc in a chloride containing electrolyte. Accordingly, the interface proved to be extremely resistant to cathodic de-adhesion processes.

Surface micromachining via solid electrochemical reaction on oxide ion conductors

We propose a simple and novel route for the surface microstructuring of tough materials (glassy carbon, SiC, W and Mo) through a solid electrochemical reaction at a microcontact between the oxide ion conductor and target substrate. More specifically, the surface of materials was locally oxidized by an anodic electrochemical reaction with the oxide ion conductor via the microcontact under a dc bias. Since the oxidation products of the targets were gaseous or sublimable, a fine-patterned surface was obtained as a result of the continuous consumption of the oxidation products formed at the microcontact. The success of the proposed method may stimulate such conventional applications of oxide ion conductors.

Passive direct methanol fuel cells fed with methanol vapor

A vapor fed passive direct methanol fuel cell (DMFC) is proposed to achieve a high energy density by using pure methanol for mobile applications. Vapor is provided from a methanol reservoir to the membrane electrode assembly (MEA) through a vaporizer, barrier and buffer layer. With a composite membrane of lower methanol cross-over and diffusion layers of hydrophilic nanomaterials, the humidity of the MEA was enhanced by water back diffusion from the cathode to the anode through the membrane in these passive DMFCs. The humidity in the MEA due to water back diffusion results in the supply of water for an anodic electrochemical reaction with a low membrane resistance. The vapor fed passive DMFC with humidified MEA maintained 20–25 mW cm −2 power density for 360 h and performed with a 70% higher fuel efficiency and 1.5 times higher energy density when compared with a liquid fed passive DMFC.

Solid Electrolyte Membrane Reactors: Status and Trends

AbstractFor Abstract see ChemInform Abstract in Full Text.

Solid electrolyte membrane reactors: Status and trends

A survey of recently published research work on solid electrolyte (SE) membrane reactors is given, with focus on high-temperature oxygen ion conductors, high-temperature proton conductors and low-temperature proton conductors. In these three material classes, the current status and the future trends of membrane reactor development are briefly elucidated. SE membrane reactor principles are realized in gas sensors, fuel cells, electrolyzers and reactors for partial oxidation. In all these fields SE membranes are in contact with porous electrolyte layers at which anodic or cathodic electrochemical reactions take place. In the area of membrane reactors using high-temperature oxygen ion conductors, there is a trend towards lower operating temperatures on order to ensure stable long-term operation of the membrane materials, and to match the optimal temperature window of the applied catalysts. As a younger generation of ion conducting ceramics, high-temperature proton conductors offer new possibilities for the implementation of electrochemical membrane reactors. Finally, current trends in the application of low-temperature proton conductors being based on polymeric materials are discussed. These materials can not only be used for fuel cells but also as membranes in hydrogenation or oxidation reactors.

Electrochemical oxidation of a carbon black loaded polymer electrode in aqueous electrolytes

The suitability of a polymeric composite material for use as part of an anode structure in a cathodic protection system has been examined. The composite material was a conductive blend (volume resistivity typically 1.5 Ω cm) of carbon black in a polyethylene binder. A long operational lifetime for the material demands that the rate of carbon loss must be low. In the work reported here, electrochemical and in situ analytical techniques were employed to characterise the performance of the material over a wide range of anodic current densities in a variety of aqueous electrolytes. The predominant anodic electrochemical reaction on the polymeric material is CO2 formation in acid and neutral solutions, which causes loss of carbon from the surface and the development of a non-conducting layer of polyethylene. The characteristics of the reaction suggest that it occurs via the discharge of H2O. In alkaline pH, however, the anodic reactions are more complex. A high OH− concentration (pH 12 or higher) favours the formation of oxygen rather than CO2, particularly at low anodic potentials. The presence of CO3 2− in the electrolyte catalyses the evolution of oxygen at pH values as low as 9. The electrochemical formation of oxygen always occurs in parallel with the generation of some humic acid in the solution.

Reaction mechanism for the ferric chloride leaching of sphalerite

Reaction mechanisms for the ferric chloride leaching of sphalerite are proposed based on data obtained in leaching and dual cell experiments presented in this work and in a previous study. The results from the leaching experiments show that at low concentrations the rate is proportional to [Fe3+]T 0.5 and [Cl-]T 0.43 but at higher concentrations the reaction order with respect to both [Fe3+]T and [Cl-]T decreases. Using dual cell experiments which allow the half cell reactions to be separated, increased rates are observed when NaCl is added to the anolyte and to the catholyte. The increase in rate is attributed to a direct, anodic electrochemical reaction of Cl- with the mineral. When NaCl is added only to the catholyte, a decrease in the rate is observed due to a decrease in theE 0 of the cathode which is attributed to the formation of ferric-chloro complexes. Several possible electrochemical mechanisms and mathematical models based on the Butler-Volmer relation are delineated, and of these, one model is selected which accounts for the experimentally observed changes in reaction order for both Fe3+ and Cl-. This analysis incorporates a charge transfer process for each ion and an adsorption step for ferric and chloride ions. The inhibiting effect of Fe2+ noted by previous investigators is also accounted for through a similar model which includes back reaction kinetics for Fe2+. The proposed models successfully provide a theoretical basis for describing the role of Cl-, Fe3+, and Fe2+ as well as their interrelationship in zinc sulfide leaching reactions. Possible applications of these results to chloride leaching systems involving other sulfides or complex sulfides are considered.

The effects of water and chloride ions on the electrochemical behaviour of iron and 304l stainless steel in alcohols

The electrochemical behaviour of pure iron and 304L stainless steel in protic organic media has been characterized by determination of current-potential potentiodynamic curves. The media studied was methyl, ethyl, isopropyl, n-butyl and 2-chloroethyl alcohols. The influence of water and chloride ion concentration on the cathodic and anodic electrochemical reactions has been investigated. The presence of water, even at a very low concentration, strongly influences the passivation of iron in protic alcoholic solutions. The kinetics of the oxidation reactions, when water and/or chloride ions were present, are a function of the specific alcohol. This has been interpreted in terms of the different protic and stereo chemistry properties of the alcohols considered.

Anodic Oxide Films and Electrochemical Reactions on Cd0.2Hg0.8Te

The formation and reduction of anodic oxide on (CMT) are studied by electrochemical methods and x‐ray photoelectron spectroscopy (XPS). The anodic oxide is formed at for pH 4.9 and for pH 13 (SHE). The and contents relative to increase within the oxide with the pH value. In a small potential range, , pH 4.9, and , pH 13, is oxidized separately. This behavior leads to a oxide surface layer in alkaline solutions whereas in acidic solutions the oxidation products are dissolved completely and the oxide is stabilized only by an content for . In alkaline solutions, CMT shows a small Cd deficiency at the semiconductor‐oxide interface according to an increased Cd content for the oxide. Ethylene glycol is a necessary additive to to form a well‐passivating film presumably by the reduction of the solubility of the oxide. The presence of Cd and Hg in the bulk and and within the oxide is a necessary condition from the side of the solid phase. The formation of tellurites and ditellurites seems reasonable to explain the stability of the passivating oxide layer.

Anodic Oxide Films and Electrochemical Reactions on HgTe

Anodic oxidation processes on surfaces in acetate buffer pH 4.9, borate buffer pH 8.4, and pH 13.0, with and without ethylene glycol, have been studied. Pt‐split electrode experiments permit the simultaneous examination of formation and reduction of oxides and of soluble reaction products. The composition and thickness of the anodic oxide layers are evaluated coulometrically. XPS measurements yield the chemical composition of the surface after the different electrochemical treatments. They essentially confirm the electrochemical analysis. Optically homogeneous oxide layers of up to ∼1000Å thickness are grown in acetate buffer, borate buffer, and ethylene glycol. The layers are composed of mixed oxides, , with . The molar ratio levels off to a value of for high oxidation potentials in acetate buffer and reaches almost for the glycol mixture. The missing Hg is dissolved as during anodic oxidation with only a small amount of Te compounds. The first cathodic reduction step of the oxides in the same electrolytes reduces to metallic Hg. As a consequence is dissolved, demonstrating the importance of the for the chemical stability of the mixed oxide layer. Special cathodic reduction conditions (−0.6 VH in acetate buffer) produce a stoichiometrically clean surface, removing any oxide or elemental Te at the surface due to previous chemical treatments. The electrochemical reactions are similar for all three electrolytes mentioned above. However, in pure the disk and ring currents are more than one order of magnitude larger (>1 mA cm−2) than those in acetate and boric borate buffer (∼0.1 mA cm−2), indicating a higher dissolution rate of these oxide layers. The presence of 90% ethylene glycol in reduces the current density again to the low levels (<0.1 mA cm−2).

Dynamic scattering life in the nematic compound p-methoxybenzylidene-p′-amino phenyl acetate as influenced by current density

The time to failure of a compound exhibiting dynamic scattering, operated under dc excitation, is shown to be inversely proportional to current density. Two failure modes have been observed, both related to an anodic electrochemical reaction. Failure occurs in the compound p-methoxybenzylidene-p′-amino phenyl acetate (APAPA) after the passage of 1.2 × 106 C/mole through the operating cell. The quantity of charge passed through, in C/mole of cell material, is recommended as a figure of merit for materials which fail due to electrochemical processes.

Kinetics of Iron Corrosion in Concentrated Acidic Chloride Solutions

The kinetics of active iron dissolution at 25°C has been studied in deaerated 1N and 6N chloride solutions for hydrogen ion concentrations of 0.1–6.0N. For 1N chloride solutions with , the dissolution reaction is described by anodic Tafel slope and electrochemical reaction order with respect to hydrogen ion activity . These parameters are characteristic of a dissolution mechanism in which chemisorbed halide ions interact with adsorbed hydroxyl ions in a two‐electron rate‐determining step. For 6N chloride solutions, this mechanism holds only over a narrower (H+) region up to 0.24N, in which . With increased ; and for , indicating that the hydrogen ion (and no longer the hydroxyl ion) catalyzed the dissolution reaction. A positive value of ensues for synergistic adsorption of hydrogen ions on the halide ion covered surface. A formal detailed dissolution mechanism is proposed, giving and , in agreement with experiment. Alternate dissolution mechanisms including one‐electron transfer schemes are discussed.