Effects of fluid slippage on pressure-driven electrokinetic energy conversion in conical nanochannels

The effects of fluid slippage on the pressure-driven electrokinetic energy conversion in conical nanochannels are systematically investigated in this paper. We present a multiphysical model that couples the Planck–Nernst–Poisson equations and the Navier–Stokes equation with a Navier slip condition to fulfill this purpose. We systematically look into the variation of various performance indicators of electrokinetic energy conversion, for example, streaming current, streaming potential, generation power, energy conversion efficiency, regulation parameter, and enchantment ratio, with the conicity of nanochannels and the slip length for two pressure differences of the same magnitude but opposite directions. Particularly, enhancement ratios related to streaming current, streaming potential, generation power, and energy conversion efficiency are defined to comprehensively measure the enhancement of the performance of electrokinetic energy conversion due to the slip length. The results demonstrate that a combination of large slip length and small conicity enhances the electrokinetic energy conversion performance significantly. Furthermore, the fluid slippage-induced enhancement of the electrokinetic energy conversion in the backward pressure difference mode is stronger than that in the forward pressure difference mode. Our results provide design and operation guidelines for pressure-driven electrokinetic energy conversion devices.     

Combinatorial approach toward high-throughput analysis of direct methanol fuel cells

A 40-member array of direct methanol fuel cells (with stationary fuel and convective air supplies) was generated by electrically connecting the fuel cells in series. High-throughput analysis of these fuel cells was realized by fast screening of voltages between the two terminals of a fuel cell at constant current discharge. A large number of voltage-current curves (200) were obtained by screening the voltages through multiple small-current steps. Gaussian distribution was used to statistically analyze the large number of experimental data. The standard deviation (sigma) of voltages of these fuel cells increased linearly with discharge current. The voltage-current curves at various fuel concentrations were simulated with an empirical equation of voltage versus current and a linear equation of sigma versus current. The simulated voltage-current curves fitted the experimental data well. With increasing methanol concentration from 0.5 to 4.0 M, the Tafel slope of the voltage-current curves (at sigma=0.0), changed from 28 to 91 mV.dec-1, the cell resistance from 2.91 to 0.18 Omega, and the power output from 3 to 18 mW.cm-2.     

Hydrogen fuel electrode based on bioelectrocatalysis by the enzyme hydrogenase

Our aim is to show, that the enzymes as electrocatalysts are able to improve the performance characteristics of the fuel cells. The hydrogen fuel electrode based on hydrogenase from Thiocapsa roseopersicina immobilized directly on carbon filament material has been made. The enzyme electrode has operated according to electron tunneling between the enzyme active site and the electrode support; this mechanism is called direct bioelectrocatalysis. Under pure hydrogen the efficiencies in energy conversion of the enzyme electrode and the noble metal based commercial fuel electrode are similar concerning both the hydrogen equilibrium potential achieved and the current densities in H₂ oxidation. However, the use of the enzyme electrodes completely avoids the problem of poisoning the anode by the impurities of carbon monoxide present in reforming gas, which limits the use of cheap hydrogen containing fuel. The stability of the biological catalysts can be drastically improved by their immobilization on electrode supports, which provide the development of commercially competitive biofuel cells.     

采用甲烷燃料的管状固体氧化物燃料电池浸渍阳极电化学性能

以NiO和8％（摩尔分数）氧化钇稳定的氧化锆为原料,采用注凝成型工艺制备了管状同体氧化物燃料电池阳极支撑体．用离子浸渍法对阳极支撑体进行表面修饰．用电化学工作站测单电池交流阻抗和输出性能并且用化学气相色谱仪对电池尾气进行分析．测试结果表明修饰后的阳极在通甲烷的情况下出现了一定程度的积炭,但是积炭现象在一定的测试时间内达到平衡,没有对电池造成破坏,并且显著地提高了电池阳极的电化学性能．单电池存通入氯气和甲烷的情况下最大输出功率密度分别达到了225和400mW／cm^2．     

The voltage–current curve of a polymer electrolyte fuel cell: “exact” and fitting equations

An analytical solution to the problem of the performance of the cathode electrocatalyst layer in the polymer electrolyte fuel cell is obtained. This solution describes the effect of limiting current density due to imperfect oxygen transport in backing layer. Simple formulas for electrocatalyst layer polarization voltage are derived in the limiting cases of small and large currents. Based on exact formulas, the fitting equation for the polarization voltage is obtained. All fitting parameters are expressed in terms of transport and kinetic parameters. Comparison of theoretical and experimental voltage–current curves of a fuel cell shows reasonable agreement. The fitting equation furnishes an opportunity to characterize fuel cells using experimental voltage–current curves.     

A comprehensive three-dimensional modeling of an internal reforming planar solid oxide fuel cell with different interconnect designs

Development of low-emission or zero-emission power generation systems is one of the most important subjects humanity is dealing with. Among different under development technologies and energy systems, a solid oxide fuel cell (SOFC) is an efficient and low-emission energy conversion device that is passing its research and development career. The current study aims to investigate a hydrocarbon fueled anode-supported planar-type SOFC due to simpler geometry, higher power density, and low overpotentials. In this study, electric performance of a SOFC with different interconnect designs under different operating conditions, such as operating voltage, channel inlet temperature, pre-reforming rate of methane, and inlet fuel and air velocity, has been investigated by use of a three-dimensional model considering complicated systems of equations: species mass conservation, first law of thermodynamics, conservation of momentum, and non-linear electrochemical models including multi-specious diffusion. It has been concluded that at a given voltage, inlet temperature, inlet air and fuel velocity, and pre-reforming rate, wider gas channels help to more uniform distribution and better penetration of reactant gases. Therefore, considering low-concentration polarization as an object, narrow ribs are preferred over wide ribs. By increasing the rate of the electrochemical reaction, the current and power density and subsequently the temperature difference increase but the fuel consumption in all cases has almost a decreasing trend. Also, it has been found that increasing inlet air velocity has little effect on current and power density but because of more cooling effect, it reduces the temperature difference and fuel consumption coefficient. On the other hand, increasing the inlet temperature has no meaningful effect on the temperature difference along the channels.

     

Tests and optimization of low–power power plant based on solid–oxide fuel cells

The paper presents the results of experimental development of the fuel processor of natural gas steam–air conversion and power plants with different design layouts based on solid–polymer and solid–oxide fuel cells. The preferability of using solid–oxide fuel cells in stationary power plants with natural gas as fuel is confirmed. The test results confirm the working efficiency and safety of the chosen solutions. Directions for the future activity in the field of design and development of low–power power plants based on solid–oxide fuel cells are formulated.     

直接甲醇燃料电池的甲醇浓度控制方法

直接甲醇燃料电池(DMFC)直接以甲醇为阳极燃料,具有系统结构简单、体积能量密度高、燃料补充方便等特点,非常适合用于小型移动电源。甲醇浓度对DMFC性能和燃料利用效率的影响非常大,甲醇浓度高低直接决定DMFC输出性能的好坏,控制好DMFC中的甲醇浓度,对其寿命长短起着至关重要的作用。本文将目前已有的甲醇浓度控制方法分为有甲醇浓度传感器和无甲醇浓度传感器两大类,评述了这些浓度控制方法的研究现状和优缺点,并展望了甲醇浓度控制方法的趋势。     

用于质子交换膜燃料电池催化剂结构研究的原位XAFS实验方法

依托上海光源的X射线吸收精细结构(XAFS)谱学线站(BL14W1),建立并发展了用于氢-氧质子交换膜燃料电池(PEMFC)原位XAFS实验的测试装置,以Pt/C纳米催化剂作为PEMFC的阴极催化剂, Pd/C作为燃料电池的阳极催化剂,采集在工作状态下的阴极催化剂的XAFS数据,同步监测燃料电池的电流-电压(J-V)曲线和功率密度曲线,观察到Pt/C催化剂在反应过程中不同电位下氧化态的变化,在高电位下Pt/C催化剂的表面存在较强的Pt-O键,降低了Pt/C催化剂的性能.本文同时也验证了我们所建立的实验装置和研究方法的可行性和可靠性.     

旋转磁场对质子交换膜燃料电池工作性能的影响

外加磁场可以提高PEM（Proton Exchange Membrane,质子交换膜）燃料电池的工作性能,本文通过在PEM燃料电池表面分别附加正方形梯度磁场、同极性组合圆柱形磁场以及异极性组合圆柱形磁场,分析旋转磁场、静态磁场以及未加磁场3种磁场环境下燃料电池的输出功率密度变化. 研究发现,外部磁场的分布规律不同,磁场的变动对燃料电池的影响也不相同,尤其是异极性组合磁场,旋转的磁场使燃料电池的最大功率密度提高了21.27%,明显高于加载静态磁场时提高的11.70%. 旋转磁场产生的效果与磁场旋转速度有关,提高转速有利于增强磁场对燃料电池工作性能的影响,当转速为30 r·min^-1时影响最大,随着转速进一步提高,影响效果逐渐变差。     

Performance improvement of air-breathing proton exchange membrane fuel cell (PEMFC) with a condensing-tower-like curved flow field

Air-breathing proton exchange membrane fuel cells (PEMFCs) are very promising portable energy with many advantages. However, its power density is low and many additional supporting parts affect its specific power. In this paper, we aim to improve the air diffusion and fuel cell performance by employing a novel condensing-tower-like curved flow field rather than an additional fan, making the fuel cell more compact and has less internal power consumption. Polarization curve test and galvanostatic discharge test are carried out and proved that curved flow field can strengthen the air diffusion into the PEMFC and improve its performance. With appropriate curved flow field, the fuel cell peak power can be 55.2% higher than that of planar flow field in our study. A four-layer stack with curved cathode flow field is fabricated and has a peak power of 2.35 W (120 W/kg).     

固体氧化物燃料电池直接以焦炭为燃料的电性能

直接碳固体氧化物燃料电池（DC-SOFC）是一种潜在的固体碳燃料高效率、低污染发电技术。本研究报道了将工业焦炭直接用作管式DC-SOFC燃料的研究。制备了电极材料为Ag-GDC （钆掺杂氧化铈）的YSZ （钇稳定化氧化锆）电解质支撑型管式固体氧化物燃料电池（SOFC）。采用拉曼光谱、扫描电镜和X射线能谱仪对焦炭燃料进行了性质表征。结果表明,焦炭燃料呈微米级的颗粒状,并含有大量对Boudouard反应有利的缺陷结构。电池以纯焦炭为燃料在850℃取得的最大功率密度为149mW/cm²,在碳燃料表面负载能提高Boudouard反应速率的Fe催化剂后,最大功率密度提高至217mW/cm²。通过电化学测试和尾气表征,分析了恒电流放电过程中电池的性能衰减机制。测试结果证明了将焦炭直接用作全固态DC-SOFC的燃料产生电能的可行性。     

Comparison of two types of vertically aligned ZnO NRs for highly efficient polymer solar cells

Vertically aligned ZnO nanorods (NR) are prepared by two different syntheses methods and applied on polymer solar cells (PSCs). The ZnO electrodes work as the electron transport layer with the P3HT:PCBM blend acting as the active material. Several organic blend solution conditions are optimized: concentration, solvent, and deposition speed. The effect of different NR electrode morphologies is analyzed on the solar cell performance and characterized by current–voltage curves and IPCE analyses. The photovoltaic performance of the solar cells was observed to be influenced by many factors, among them infiltration of the organic P3HT:PCBM blend within the ZnO NR layer. The infiltration of the active layer was monitored by cross section SEM and energy dispersive X-ray spectroscopy analyses. Our results show that higher power conversion efficiencies are achieved when shorter NRs lengths are applied. The best power conversion efficiency obtained was 2.0% for a 400 nm ZnO NR electrode. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

A microfluidic fuel cell with flow-through porous electrodes

A microfluidic fuel cell architecture incorporating flow-through porous electrodes is demonstrated. The design is based on cross-flow of aqueous vanadium redox species through the electrodes into an orthogonally arranged co-laminar exit channel, where the waste solutions provide ionic charge transfer in a membraneless configuration. This flow-through architecture enables improved utilization of the three-dimensional active area inside the porous electrodes and provides enhanced rates of convective/diffusive transport without increasing the parasitic loss required to drive the flow. Prototype fuel cells are fabricated by rapid prototyping with total material cost estimated at 2 USD/unit. Improved performance as compared to previous microfluidic fuel cells is demonstrated, including power densities at room temperature up to 131 mW cm-2. In addition, high overall energy conversion efficiency is obtained through a combination of relatively high levels of fuel utilization and cell voltage. When operated at 1 microL min-1 flow rate, the fuel cell produced 20 mW cm-2 at 0.8 V combined with an active fuel utilization of 94%. Finally, we demonstrate in situ fuel and oxidant regeneration by running the flow-through architecture fuel cell in reverse.     

Tafel slopes from first principles

Tafel slopes for multistep electrochemical reactions are derived from first principles. The derivation takes place in two stages. First, Dirac’s perturbation theory is used to solve the Schrödinger equation. Second, current–voltage curves are obtained by integrating the single-state results over the full density of states in electrolyte solutions. Thermal equilibrium is assumed throughout. Somewhat surprisingly, it is found that the symmetry factor that appears in the Butler–Volmer equation is different from the symmetry factor that appears in electron transfer theory, and a conversion formula is given. Finally, the Tafel slopes are compiled in a convenient look-up table.     

Enhance cathodic capacitance to eliminate power overshoot in microbial fuel cells

Journal of Solid State Electrochemistry - Power overshoot that presents in the power curves of microbial fuel cells (MFCs) is one of the main reasons for the deterioration of MFC performance over...     

Thermal Dissociation in Dynamic Conditions by Modeling Thermogravimetric Curves Using The Logarithm of Conversion Degree

The dependence of conversion degree estimated from the (TG) curve of the mass loss on heating of temperature has been analyzed. It has been shown that dynamic TG curve can be modeled by an equation relating to the logarithm of conversion degree as a function of temperature. A coefficient in the equation developed provides information on the distance from the equilibrium, therefore, the coefficient a ₂=0–50 implies equilibrium, while a ₂>50 informs about some distance from the equilibrium. Further possibilities for the use of the models of lnα vs. 1/T in the analyses of thermodynamics and kinetics of thermal dissociation of solids has been shown. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Sodium-Ion-Conducting Direct Formate Fuel Cell: Generating Electricity and Producing Base

A barrier that limits the development of the conventional cation-exchange membrane direct liquid fuel cells (CEM-DLFCs) is that the CEM-DLFCs need additional base to offer both alkaline environment and charge carriers. Herein, we propose a Na⁺-conducting direct formate fuel cell (Na-DFFC) that is operated in the absence of added base. A proof-of-concept Na-DFFC yields a peak power density of 33 mW cm⁻² at 60 °C, mainly because the hydrolysis of sodium formate provides enough OH⁻ and Na⁺ ions, proving the conceptual feasibility. Moreover, contrary to the conventional chlor-alkali process, this Na-DFFC enables to generate electricity and produce NaOH simultaneously without polluting the environment. The Na-DFFC runs stably during 13 hours of continuous operation at a constant current of 10 mA, along with a theoretical production of 195 mg NaOH. This work presents a new type of electrochemical conversion device that possesses a wide range of potential applications.     

The Open Circuit Voltage in Biofuel Cells: Nernstian Shift in Pseudocapacitive Electrodes

In the development of biofuel cells great effort is dedicated to achieving outstanding figures of merit, such as high stability, maximum power output, and a large open circuit voltage. Biofuel cells with immobilized redox mediators, such as redox polymers with integrated enzymes, show experimentally a substantially higher open circuit voltage than the thermodynamically expected value. Although this phenomenon is widely reported in the literature, there is no comprehensive understanding of the potential shift, the high open circuit voltages have not been discussed in detail, and hence they are only accepted as an inherent property of the investigated systems. We demonstrate that this effect is the result of a Nernstian shift of the electrode potential when catalytic conversion takes place in the absence or at very low current flow. Experimental evidence confirms that the immobilization of redox centers on the electrode surface results in the assembled biofuel cell delivering a higher power output because of charge storage upon catalytic conversion. Our findings have direct implications for the design and evaluation of (bio)fuel cells with pseudocapacitive elements.