Results 1 - 10 of 1930
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[en] Core@shell nanocrystals (NCs) have been widely explored for oxygen reduction reaction (ORR). In this work, monodisperse PdCu@PtCu NCs with various shell thicknesses and compositions have been synthesized through a two-step protocol. As-synthesized PdCu@PtCu core@shell catalysts show enhanced specific activities (SAs) and mass activities (MAs) towards ORR. When PdCu/PtxCuy (core/shell) atomic ratio and Pt/Cu (x/y) ratio both reach 1/1, the core@shell catalyst exhibits the highest SA and MA (normalized with total mass of Pt and Pd). It is also found that the core@shell catalysts show drastically enhanced stability compared with pure PtCu alloy catalyst. It is proposed that both the activity and stability enhancements can be ascribed to the electronic interaction or charge transfer between Pd atoms (in core) and the shell elements. This work demonstrates a new family of core@shell catalysts that can be potentially used as cathode electro-catalysts in fuel cells.
[en] Microbial electrode catalysis such as microbial fuel cells or electrosynthesis involves electron exchange with the electrodes located at the cell exterior; i.e., extracellular electron transport (EET). Despite the vast amount of research on the kinetics of EET to optimize the catalysis rate, the relevance of other factors, including upstream metabolic reactions, has scarcely been investigated. Herein, we report an in vivo electrochemical assay to confirm whether EET limits anodic current production (j) for the lactate oxidation of Shewanella oneidensis MR-1. Addition of riboflavin, which specifically enhances the EET rate, increased j only in the early phase before j saturation. In contrast, when we removed a trace metal ion necessary for upstream reactions from the electrolyte, a significant decrease in j and the lactate consumption rate was observed only after j saturation. These data suggest that the limiting factor for j shifted from EET to upstream reactions, highlighting the general importance of enhancing, for example, microbial metabolism, especially for long-standing practical applications. Our concept to specifically control the rate of EET could be applicable to other bioelectrode catalysis systems as a strategy to monitor their rate-limiting factors.
[en] Highlights: • Microbial anodes formed twice as fast at 40 °C as at 25 °C. • Bioanodes formed at 40 and 25 °C developed different electron transfer systems. • Bioanodes formed at 40 and 25 °C produced similar current densities when used at 25 °C. • Bioanodes formed at 40 and 25 °C showed similar redox systems when used at 25 °C. • Temperature impacted the biofilm structure. - Abstract: Reducing the time required for the formation of microbial anodes from environmental inocula is a great challenge. The possibility of reaching this objective by increasing the temperature during the bioanode preparation was investigated here. Microbial anodes were formed at 25 °C and 40 °C under controlled potential with successive acetate additions. At 25 °C, around 40 days were required to perform three acetate batches, which led to current density of 9.4 ± 2 A.m−2, while at 40 °C, 20 days were sufficient to complete three similar batches, leading to 22.9 ± 4.2 A.m−2. The bioanodes formed at 40 °C revealed three redox systems and those formed at 25 °C only one. The temperature also impacted the biofilm structure, which was less compact at 40 °C. When the bioanodes formed at 40 °C were switched to 25 °C, they produced current densities similar to those of bioanodes formed at 25 °C; they recovered the single redox system that was developed by the bioanodes formed at 25 °C and the difference in biofilm structures was mitigated. It is consequently fully appropriate to accelerate the formation of microbial anodes by increasing the temperatures to 40 °C even if they are finally intended to operate at room temperature.
[en] We comment on the value and nature of terms contributing to the Seebeck coefficient of a thermoelectric cell. Transported entropies of ions can be connected with thermodynamic entropies, but they cannot be compared directly to partial molar properties of ions, as they are transport properties, rather than equilibrium properties. Equilibrium thermodynamics is contained in the more general non-equilibrium thermodynamic theory as a special case, apart from that the two cannot be unified
[en] In spite of a high quantum efficiency in the bacterial photosynthetic reaction center (RC) the overall efficiency in a RC-based photovoltaic device is very poor partly because of an inefficient collection of charges by electrodes. To explain charge transport between the RC and an electrode a diffusion model is proposed. The numerical solution of the diffusion process describes the measured photocurrent well. An approximation of the initial condition is also made to obtain analytical expressions for the photocurrent. The model suggests that the slow transient response of the photocurrent is due to the diffusion in a biological photovoltaic device.
[en] Power level of a fuel cell depends on its operating condition, which is product of voltage and current-density the highest level of voltage is identified as reversible open circuit voltage (ROCV), which represents an ideal theoretical case [J. Larmin, A. Dicks, Fuel Cell System Explained, Willy, 2000 (ISBN)]. Compared to that is ideal operating voltage which is usually characterized as open circuit voltage (OCV) [J. Larmin, A. Dicks, Fuel Cell System Explained, Willy, 2000 (ISBN)]. An evaluation of deviation of operating voltage level from ideal operational case may provide information on the extent of improving efficiency and energy efficiency of a fuel cell. Therefore, quantification of operation deviation from OCV is the main point that is discussed in the present paper. The analysis procedure of voltage drop is based on step-by-step review of voltage drops over activation, internal currents (fuel-cross-over), Ohmic and mass-transport or concentration losses. Accumulated total voltage drops would be estimated as a sum of aforementioned losses. The accumulated voltage drops will then be reduced from OCV to obtain the operating voltage level. The above numerical analysis has been applied to identify the extents of voltage drop. The possible reducing variables in voltage drops reviewed and concluded that the activation loss has considerable impact on total voltage drops and it explains the most part of total losses. It is also found that the following correspondence parameters cause decrease in voltage drops: 1. Temperature increasing; 2. Pressure increasing; 3. Hydrogen or oxygen concentration increasing; 4. Electrode effective surface increasing; 5. Electrode and electrolyte, conductivity modification; 6. Electrolyte thickness reducing up to possible limitation; 7. Connection modification
[en] Vapor-feed microfluidic fuel cell (VF-MFC) has various advantages against the conventional liquid-feed microfluidic fuel cell, such as simpler fluidic management, higher fuel utilization, flow rate insensitiveness, and so on. To better understand the mechanisms behind its superiority and to further optimize its performance, a 3D isothermal numerical model has been developed in this work. The computational results agree very well with the previous and present experimental data, proving the validity of the current model for the VF-MFC simulation. Through this model, it is found that the dissolved fuel in the VF-MFC is well-controlled within a thin boundary layer nearby the anode catalyst surface, which can not only satisfy the demand of anode oxidation reaction but also greatly alleviate the wastage of fuel. In this manner, the VF-MFC can achieve satisfactory power output and high fuel utilization at the same time. In addition, the boundary layer effect on electrolyte flow rate can keep the fuel concentration in the thin layer relatively stable at different flow rates, which may be the reason behind the insensitiveness of VF-MFC performance to electrolyte flow rate. To further improve its power output and fuel efficiency, effects of the fuel evaporation area and the anode open ratio have also been thoroughly investigated with the present model. It is found that an evaporation-reaction area ratio of 11.1 is sufficient for the present VF-MFC, while a smaller fuel evaporation area can lead to improved fuel utilization at the expense of lower power output. To improve both the fuel utilization and power output, the electrode area towards the channel outlet is increased while keeping the vapor entrance area constant, i.e. the anode open ratio is reduced. By this strategy, the VF-MFC can achieve 48% higher power output and elevated fuel utilization from 27.5% to 41.8%, when an anode open ratio of 1:3 is adopted.
[en] Based on the unique catalytic properties of Pt by using its single layer on well-defined inexpensive nano-substrates one can maximize its activity at the oxygen fuel cell cathode. This illustrates an efficient way of using Pt while overcoming its limited supply. We present a highly active and stable ORR catalyst, consisting of PdNi core–shell nanoparticles, which was protected against decomposition in acid by Au atoms and activated for oxygen reduction with a Pt monolayer. The roles of each component in the catalyst is investigated and in the best case the catalyst showed a Pt group metal mass activity that was approximately 3 times higher than that of the commercial Pt/C electrocatalyst. The Au protected PdNi core–shell nanoparticles were found to be stable support for Pt under high oxidizing conditions
[en] The interest in solid state electrochemical devices including sensors, fuel cells, batteries, oxygen permeation membranes, etc. has grown rapidly in recent years. Many of the same figures of merit apply to these different applications, the key ones being ionic conduction in solid electrolytes, mixed ionic-electronic conduction (MIEC) in electrodes and permeation membranes, and gas-solid reaction kinetics in sensors and fuel cells. Optimization of device performance often relies on the careful understanding and control of both ionic and electronic defects in the materials that make up the key device components. To date, most materials in use have been discovered serendipitously. A key focus of this paper is on the tools available to scientists and engineers to practice 'defect engineering' for the purpose of optimizing the performance of such materials. Dopants, controlled structural disorder, and interfaces are examined in relation to increasing the conductivity of solid electrolytes. The creation of defect bands is demonstrated as a means for introducing high levels of electronic conductivity into a solid electrolyte for the purpose of creating a mixed conductor and thereby a monolithic fuel cell structure. Dopants are also examined as a means of reducing losses in a high temperature resonant sensor platform. The control of microstructure, down to the nano-scale, is shown capable of inverting the predominant ionic to an electronic charge carrier and thereby markedly modifying electrical properties. Electrochemical bias and light are also discussed in terms of creating defects locally thereby providing means for micromachining a broad range of materials with precise dimensional control, low residual stress and controlled etch rates
[en] Highlights: ► High magnetic strength reduces Rct and increases Cd in oxygen reduction reaction. ► Oxygen diffusion and transfer coefficient become large in high magnetic strength. ► The magnetic ZAFC discharge performance is better than the nonmagnetic ZAFC. ► Increased NdFeB/C load density improves the magnetic ZAFC discharge performance. ► Excess NdFeB/C load density decreases the magnetic ZAFC discharge performance. -- Abstract: This study investigates the effects of magnetic field on oxygen transfer and the correlations of electrochemical parameters in different magnetic strengths. The discharge performance of zinc–air fuel cell (ZAFC) was tested under magnetic and nonmagnetic conditions using neodymium–iron–boron/carbon (NdFeB/C) magnetic particles in ZAFC cathode. The results showed that the oxygen diffusion coefficient (DOi) and transfer coefficient (αi) increased by 102.14% and 52.38% when the magnetic strength increased from 0 mT to 5.0 mT, respectively. In addition, the electric double-layer capacitance (Cd) increased from 8.16 to 22.46 μF cm−2, the charge-transfer resistance (Rct) decreased from 9.43 Ω cm2 to 6.02 Ω cm2, and the oxygen reduction reaction (ORR) current was improved. With the NdFeB/C load density of 2.4 mg cm−2 in ZAFC cathode, the discharge current of magnetic ZAFC increased by 13.86% compared with the nonmagnetic ZAFC at the 0.80 V discharge voltage. These results indicate that magnetic strength has a positive correlation with DOi, αi, and the ORR current. Under magnetic attractions, the oxygen transfer process is easier at the Pt/C catalytic surface, and the discharge performance of magnetic ZAFC is superior to the nonmagnetic ZAFC. At lower NdFeB/C load density, increasing the NdFeB/C load density facilitates oxygen transfer and improves the discharge performance of ZAFC. However, the magnetic ZAFC discharge performance decreases at a higher NdFeB/C load density because of the blocked oxygen transfer channel and the magnetic field disorder caused by the magnetic interactions among different magnetic particles