Electrode Chemical Reaction Research Articles

Effects of Methane Pre-Reforming Percentage and Flow Arrangement on the Distribution of Temperature and Thermal Stress in Solid Oxide Fuel Cell

To obtain detailed information on the temperature field and thermal stress field inside the solid oxide fuel cell (SOFC) fueled with partially pre-reformed methane. A three-dimensional geometric and mathematical model of the SOFC is implemented by using the finite element method in the commercial software COMSOL Multiphysics®. The coupling characteristics were analyzed for electrode chemical reaction, multi-component mass transfer, and heat transfer process under typical operating conditions, which was further applied for predicting and analyzing the thermal stress distribution. After model validation, parametric simulations are conducted to investigate how the methane pre-reforming percentage and flow arrangement affect the temperature and the thermal stress of SOFC. The simulated results show that reducing the methane pre-reforming percentages can decrease the temperature maximum and the variation range of the first principal stress, but will increase the possibility of carbon deposition. The maximum temperature of the counter-flow is about 20 K lower than that of the co-flow, and the first principal stress variation range of the counter-flow is 8.6 Mpa lower than that of the co-flow. The methane pre-reforming percentages have a significant effect on the heat transfer and the thermal stress, and the counter-flow can improve the temperature uniformity and reduce the thermal stress variation range.

Investigation on the Pressure Rise and Energy Balance Due to Fault Arcs in a Closed Container Filled With Different Insulating Gases

Changes in the heat transfer of fault arcs in an electrical installation are closely connected with the energy balance and pressure rise. Their detailed analysis has not been carried out so far for various insulating gases used in installations. In this article, the thermal transfer coefficient kp, which acts as an important parameter assessing the pressure behavior, is investigated based on the analysis of energy balance of fault arcs in a closed container filled with air, SF6, CO2, and N2. A hybrid approach with a combination of a thermopile and a photomultiplier is proposed to measure the transient radiation energy from arcs. The energies related to electrode erosion, including electrode melting and vaporization as well as electrode chemical reaction, are all measured quantitatively. In particular, Joule heating and radiation as two key energy processes are compared and analyzed under different influencing factors. With various energy transfer processes in energy balance visualized, the influence of the arc current, electrode material, filling pressure, and gap distance between electrodes on kp is analyzed carefully for different insulating gases.

Effects of electrode chemical reactions on SF6 discharge characteristics in extremely inhomogeneous electric fields

SF6 is widely used in the gas-insulated metal-enclosed switchgear or the corona-stabilized gas spark switch applications as a gas dielectric. It is generally believed that the discharge characteristics are only related to the electric field distribution and gas molecular density; however, the electrode chemical reactions can indeed markedly affect the SF6 discharge characteristics in the extremely inhomogeneous electric fields under the steady-state voltage. In this study, we used a needle-plane electrode system to build an extremely inhomogeneous electric field and examined the discharge characteristics within it including U-p characteristics, corona appearance, and corona current. We also analyzed the micro-region characteristics of the electrode surface, including the surface morphology, elemental composition, and chemical state to fully qualitatively determine the role of the electrode chemical reactions in discharge behavior. We found that the N-shaped U-p curve widens, the filamentous leader channels disappear, and the corona current drops suddenly as the duration of the electrode chemical reactions increases. Varying the surface morphology, elemental composition, and chemical state was observed on the electrodes of different polarities through micro-region analysis. The metal fluoride or metal sulfide film on the electrode surface may serve as a resistive coating due to its low electrical conductivity, which obstructs the leader discharge while enhancing the streamer discharge, suppressing the transition from the streamer to leader discharge and altogether significantly altering the discharge characteristics. The resistive coating produced is the primary cause of the electrode chemical reaction effects on the discharge characteristics, primarily as it depresses the transition from the streamer to leader discharge. The results presented here may provide useful guidelines for further research on SF6 discharge under inhomogeneous electric fields.

Sulfur Pollutants Treatment Using Microbial Fuel Cells from Perspectives of Electrochemistry and Microbiology

Sulfur pollutants have negative impact on natural ecosystems and biological health in dynamic processes of migration and transformation on Earth. Microbial fuel cells (MFCs) have been shown to be a promising technique for the removal of sulfur pollutants in wastewater, and research efforts have been made for the development of this area. To date, some controversies still exist in the mechanism of sulfur pollutants transformation involved in electrochemical reactions and microbial effects, limiting the practical application. This review respectively discussed the roles of electrode chemical reaction and microorganisms in sulfur pollutants treatment using MFCs, demonstrated the treatment mechanism and effecting factors, and summarized the configurations, separator types, electrode materials and catalysts, as well as sulfur recovery and electrode regeneration. Furthermore, the feasibility of sulfur pollutants removal in MFCs was assessed by capital cost between MFCs and typical anaerobic biological treatment.

Application of Confocal 3D Micro-XRF for Solid/Liquid Interface Analysis

Solid/liquid interfaces are important locations for various chemical reactions, such as electrode chemical reactions and metal corrosions. Conventional surface analytical methods, such as XPS and SEM-EDS, have been applied to solid materials after being removed from the liquid phase. These methods do not involve direct observation, although useful information is available. It is important to directly observe surface reactions on solid materials in the liquid phase in order to understand the details of these reactions. One feasible method of doing this is 3D micro-XRF analysis. The confocal 3D micro XRF method enables nondestructive x-ray elemental analysis of localized microspace. We have applied a confocal 3D micro-XRF instrument for solid/liquid interface analysis. This technique was applied for direct observation of the chemical deposition of Cu on an Fe plate and the dissolution of Fe in a CuSO4 solution.

Prediction of submerged arc weld‐metal composition from flux ingredients with the help of statistical design of mixture experiment

A prediction model has been developed for submerged arc weld‐metal chemical composition in terms of flux ingredients with the help of statistical experiments for mixture (extreme vertices design). Bead‐on‐plate weld deposits as per statistical mixture design experiments were performed at the following welding parameters: current (400 A), voltage (26 V), speed (4.65 mm/s) and electrode extension (30 mm) using CaO–MgO–CaF2–Al2O3 flux system. The results show that some of the individual flux ingredients and their binary mixtures have a predominant effect on weld‐metal oxygen, manganese, silicon, sulphur, nickel and carbon content. The predicted results show a reasonably good agreement with the experimental results, which were obtained by performing the actual experiments based on a randomly designed flux. Analysis of the experimental data indicate that several mechanisms such as basicity index, oxygen potential, oxide stability, viscosity, electrode chemical reaction, kinetics of slag–metal reaction, etc. are operating simultaneously to yield the final weld‐metal composition.

Dissipations and polarizations in irreversible electrode processes: A unite formalism of stochastic thermodynamics of both concentration polarization and activation polarization

A generalized stochastic model of the irreversible electrode processes is proposed. By means of it the dissipation and the fluctuation around nonequilibrium steady states of the electrode chemical reaction systems is analyzed. It is shown that the polarizations, in spite of the concentration polarization or the electrochemical polarization, originate from the nontrivial dissipations together with the non-Poissonian fluctuations in irreversible electrode processes. Furthermore a dissipation-fluctuation formula of electrode polarization is deduced explicitly. This formula is a dominant nonequilibrium thermodynamic equality, useful for the theoretic analysis of the polarization in irreversible electrode processes when the validity of the classical equilibrium thermodynamics breaks down. It turns out that the well-known Nernst formula followed by concentration polarization and the famous Tafel equation valid for electrochemical polarization are only the macroscopic approximations of two special limit cases in the formalism presented in this paper. If the fluctuation departs from the Gaussian a revision will be inevitable.