Design equations for optimized PEM fuel cell electrodes

This paper presents simple mathematical expressions that can be used for optimizing fuel cell electrode structures, specifically polymer electrolyte membrane fuel cells (PEMFCs). Based on the effectiveness factor, equations relate current density to catalyst utilization and a mass transfer coefficient. These can be used to screen new materials or identify which specific processes need to be improved in an existing electrode design. The optimum thickness, or catalyst loading, and maximum current that can be achieved with a given set of materials can be calculated from a simple set of equations based on the mass transfer characteristics of the electrode materials. These methods can save considerable experimental time and cost during electrode development.     

燃料电池空气电极的孔道结构调控

燃料电池是将化学能转化成电能的装置,其空气电极催化层的设计,既要包含丰富的、易于接近的反应活性位,也要具备高度连通的电子、质子以及反应物、产物传质通道,因此电极必须具有特定三维几何结构形貌和有序分布的各功能化孔道,确保催化活性位得以充分利用以及反应可以连续进行。针对催化剂孔道的几何结构调控,本文调研了最近报道的一系列研究工作,从模板法、高温相变法、模板/相变复合方法以及基于金属有机框架材料进行孔道设计等四种主要方法出发,综述了该领域的最新研究进展。     

Electrochemical methods for the in situ regeneration of active surface area of aged fuel cell type electrodes

One of the main reasons for the decay in performance of phosphoric acid fuel cells (150° C) is the decrease in surface area of the platinum catalyst with time. It was found that fast potentiostatic cycling (50 mVs^–1) could regenerate the surface area of the Pt and recover the performance. The mechanism of regeneration is likely to be charge injection, resulting in electrostatic repulsion of the platinum particles in the loosely held agglomerates.This work was performed under the auspices of the US Department of Energy.Visiting scientist at the Brookhaven National Laboratory in the summers of 1976 and 1977.     

Three-dimensional random resistor-network model for solid oxide fuel cell composite electrodes

A three-dimensional reconstruction of solid oxide fuel cell (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of SOFC electrodes. Porosity of the electrode is controlled by adding pore former particles (spheres) to the electrode and ignoring them in analysis step. To enhance connectivity between particles and increase the length of triple-phase boundary (TPB), sintering process is mimicked by enlarging particles to certain degree after settling them inside the packing. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not uniform. Effects of electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.     

Modelling the 3D microstructure and performance of solid oxide fuel cell electrodes: Computational parameters

In this paper, the computational parameters for a 3D model for solid oxide fuel cell (SOFC) electrodes developed to link the microstructure of the electrode to its performance are investigated. The 3D microstructure model, which is based on Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials. Once formed, the synthetic electrodes are discretized into voxels (small cubes) of equal sizes from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three-phase boundary (TPB) length. Transport phenomena and electrochemical reactions taking place within the electrode are modelled so that the performance of the synthetic electrode can be predicted. The degree of microstructure discretization required to obtain reliable microstructural analysis is found to be related to the particle sizes used for generating the structure; the particle diameter should be at least 20–40 times greater than the edge length of a voxel. The structure should also contain at least 25³ discrete volumes which are called volume-of-fluid (VOF) units for the purpose of transport and electrochemical modelling. To adequately represent the electrode microstructure, the characterized volume of the electrode should be equivalent to a cube having a minimum length of 7.5 times the particle diameter. Using the modelling approach, the impacts of microstructural parameters on the electrochemical performance of the electrodes are illustrated on synthetic electrodes.     

Exceptionally stable nanostructured air electrodes for reversible solid oxide fuel cells via crystallization-assisted infiltration

Nanostructures present favorable prospects of manufacturing high-performing air electrodes for reversible solid oxide fuel cells (RSOFCs) with the potential to reduce their operating temperature. Here, we present trichloroacetic acid as an original infiltration agent for the facile nanoengineering of RSOFC electrodes. The new process relies on the thermal decomposition of trichloroacetic acid in water at temperatures above 70 °C, which causes intense CO₂ effervescence and crystallizes out the metal ions in the solution as slightly soluble carbonates. Essentially, this allows for the subsequent infiltration step to be performed immediately after drying, as opposed to conventional infiltration, which requires high-temperature calcination after each infiltration step. The anode-supported RSOFC consisting of a nanostructured LaCoO₃ air electrode permitted smooth switching between fuel cell and electrolysis cell modes with no evidence of degradation. In addition, the RSOFC presented exceedingly durable performance during accelerated stability tests.     

A method for rough estimation of the catalyst surface area in a fuel cell

A method for a rough estimation of the catalyst surface area in a fuel cell is developed. It is based on the deconvolution of experimental CO oxidation data by use of a mathematical model. The kinetic parameters of the model are determined by fitting the experimental curves. The experimental data are collected at different sweep rates (2–100 mV s⁻¹) and at different temperatures (room −60.0 °C). The model can predict the sweep rate dependence of the CO oxidation onset potential, the peak current, the peak potential and the peak broadness. The model is further used for the prediction of the baseline in the presence of CO and for calculation of the CO charge consumed up to half peak potential. It is obtained that the latter value is constant at different sweep rates and that the baseline deviates from linearity already at low sweep rates (2 mV s⁻¹), but not very significantly (2.0% in comparison to 8.8% at 100 mV s⁻¹, based on calculated CO charge). It is suggested that lower sweep rates should be used for experimental surface area determination.     

Comparative studies of configurations and preparation methods for direct methanol fuel cell electrodes

Catalyst-coated membrane (CCM) and catalyzed diffusion medium (CDM) prepared either by brush painting method or by spraying method were compared for direct methanol fuel cell (DMFC) anode and cathode. The pore structure and the morphology of the electrodes were characterized by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Internal resistance corrected polarization curves were employed to separate the contribution of each compartment of the membrane electrode assembly (MEA) to the overall polarization. It was shown that the increased mass transport resistance in the anode diffusion layer made the anode in CDM form act as the methanol barrier. The CCM configuration and the increased pores in micron scales in the catalyst layer were in favor of improving the performance of both anode and cathode. Accounting for the effect of methanol permeation, the combination of the anode in CDM form prepared by brush painting method and the cathode in CCM form prepared by spraying method was finally selected as the optimized configuration for MEA, which had the highest DMFC performance under near-ambient conditions.     

Effect of cerium oxide nanoparticles coating on the electrodes of benthic microbial fuel cell

Benthic microbial fuel cell is a power source for low-power devices. For enhancement of power, Cerium oxide (CeO₂) nanoparticles (NPs) were coated on anode and cathode electrodes and compared separately. CeO₂ NPs were synthesized by hydrothermal method and characterized by Dynamic light scattering. Polyvinylidene fluoride and graphite powder were used as a conductive matrix for binding CeO₂ NPs. Coated electrodes were characterized by physical and electrochemical analysis. Maximum power densities generated by CeO₂ coated cathode and anode were 60 and 43 mW/m³ respectively; whereas conductive matrix only produced 14 mW/m³. Results demonstrated that CeO₂ at cathode performed better than at anode. NPs show their effectiveness as an oxygen reduction reaction catalyst in the sea water.     

Symmetrical solid oxide fuel cells based on titanate nanocomposite electrodes

Nanocomposite electrodes of (Sr_0.7Pr_0.3)_0.95TiO_3±δ?Ce_0.9Gd_0.1O_1.95 are directly prepared by spray-pyrolysis deposition on Zr_0.82Y_0.16O_1.92 electrolytes and their properties are compared with those obtained by the traditional screen-printing powder method. The structural, microstructural and electrical characteristics are investigated for their potential use as both cathode and anode in Solid Oxide Fuel Cells. The nanocomposite electrodes with reduced particle size ～30 nm achieved a polarization resistance at 700 ºC of 0.50 and 0.46 Ω cm² in air and pure H₂, respectively, outperforming those obtained for the analogous screen-printed electrodes with particle size of 450 nm, i.e. 4.8 and 3.9 Ω cm², respectively. An electrolyte-supported cell with symmetrical electrodes reached a maximum and stable power density of 354 mW cm^-2 at 800 ºC. These results demonstrate that the performance of electrode materials with modest electrochemical properties but high phase stability, such as doped-SrTiO₃, can be highly improved by preparing nanocomposite electrodes directly on the electrolyte surface.     

电极改性强化微生物燃料电池产电同步降解有机污染物研究进展

微生物燃料电池(MFC)是一种利用微生物作为催化剂就能实现同步产电及降解有机污染物的绿色能源装置.电极作为MFC的重要组成部分,在提高污染物降解及产电能力方面发挥着至关重要的作用.介绍了MFC电极,主要包括碳基/合成材料修饰电极、导电聚合物/复合物修饰电极、金属/金属氧化物修饰电极及其他材料修饰电极及其最新研究进展,对...     

Using electrochemical impedance spectroscopy to compensate for errors when measuring polarisation curves during three-electrode measurements of solid oxide fuel cell electrodes

The use of three-electrode techniques involving an independent reference electrode is invaluable in determining the overpotential losses at solid oxide fuel cell (SOFC) electrodes. However, there are numerous barriers to achieve the accurate measurement of such overpotentials in an SOFC. Furthermore electrochemical impedance spectroscopy (EIS) is commonly used to analyse the processes occurring on SOFC electrodes, and there has been considerable work in establishing viable three-electrode techniques for EIS experiments under open circuit conditions. However, the three-electrode techniques currently developed for EIS experiments are not well suited for conditions of high load, or changing gas compositions; either intentionally or under diffusion limiting conditions. This paper reports a solution for commonly used pellet cells, which mitigates these problems. The paper presents a method using EIS to correct for errors when measuring the working electrode overpotential during polarisation arising from a shift in the electrolyte current distribution from the primary to the secondary current distribution under load. This technique enables meaningful overpotentials to be calculated using experimentally simple cell geometries under conditions where they cannot normally be accurately measured.     

Electrohydrodynamic dryer: Effect of emitters’ density and gap between discharge and collecting electrodes

Abstract

This paper presents the results of experimental study and mathematical simulation of multi-pin electrohydrodynamic dryer as a function of emitters’ density and a gap between discharge and collecting electrodes. Experimental data for multi-pin electrodes with different emitters’ spacing 1?×?1, 2?×?2, 3?×?3, 4?×?4, 6?×?6?cm and different gaps between electrodes at 2.0, 2.5, 3.0, 3.5, and 4.0?cm have been approximated with Warburg and Stuetzer models. Stuetzer model appears to be more accurate fit to experimental data for all electrode geometries. It also allowed decomposition of partial effects of emitters’ density and the gap between electrodes. Effect of emitters’ density was accounted through dimensionless geometry factor g_o. Using modified Stuetzer model, current density of the multi-pin electrode could be predicted as a function of voltage, gap and emitters’ density, which allows scaling up the multi-pin dryers for industrial applications.     

Improving electromechanical output of IPMC by high surface area Pd-Pt electrodes and tailored ionomer membrane thickness

In this study, we attempt to improve the electromechanical performance of ionic polymer–metal composites (IPMCs) by developing high surface area Pd-Pt electrodes and tailoring the ionomer membrane thickness. With proper electroless plating techniques, a high dispersion of palladium particles is achieved deep in the ionomer membrane, thereby increasing notably the interfacial surface area of electrodes. The membrane thickness is increased using 0.5 and 1 mm thick ionomer films. For comparison, IPMCs with the same ionomer membranes, but conventional Pt electrodes, are also prepared and studied. The electromechanical, mechanoelectrical, electrochemical and mechanical properties of different IPMCs are characterized and discussed. Scanning electron microscopy-energy dispersive X-ray (SEM-EDS) is used to investigate the distribution of deposited electrode metals in the cross section of Pd-Pt IPMCs. Our experiments demonstrate that IPMCs assembled with millimeter thick ionomer membranes and newly developed Pd-Pt electrodes are superior in mechanoelectrical transduction, and show significantly higher blocking force compared to conventional type of IPMCs. The blocking forces of more than 0.3 N were measured at 4V DC input, exceeding the force output of typical Nafion® 117-based Pt IPMCs more than two orders of magnitude. The newly designed Pd-Pt IPMCs can be useful in more demanding applications, e.g., in biomimetic underwater robotics, where high stress and drag forces are encountered.     

Anomalous transport phenomena in semihydrophobic fuel cell electrodes

The so-called bubbling and weeping effects observed with porous semihydrophobic fuel cell electrodes are discussed in terms of gas transport in porous media. It is shown that the bubbling of gas on the electrolyte side can be accounted for by transport equations valid in the intermediate region between ordinary and Knudsen diffusion. The anomalous transport of liquid toward the gas side is attributed to isothermal distillation proceeding in the electrode pores. A gradient of the partial pressure of water vapour in the pores is the principal cause for both effects.     

Systematic analysis of the direct methanol fuel cell

The dynamic operating behaviour of the direct methanol fuel cell (DMFC) is governed by several physico-chemical phenomena which occur simultaneously: double layer charging, electrode kinetics, mass transport inside the porous structures, reactant distributions in the anode and cathode flowbeds etc. Therefore it is essential to analyse the interactions of these phenomena in order to fully understand the DMFC. These phenomena were initially analysed independently by systematic experiments and model formulations. Electrode kinetics were determined by fitting models of varying complexity to electrochemical impedance spectroscopy (EIS) measurements. Reaction intermediates adsorbed on the catalyst seem to play a key role here. To describe mass transport across the DMFC a one-dimensional model was formulated applying the generalised Maxwell–Stefan equations for multi-component mass transport and a Flory–Huggins model for the activities of mobile species inside the membrane (PEM). Also swelling of the PEM as well as heat production and transport were considered. Finally, the anode flowbed was analysed by observing flow patterns in different flowbed designs and measuring residence time distributions (RTDs). Detailed CFD models as well as simpler CSTR network representations were used to compare to the experimental results. Even the simpler models showed good agreement with the experiments. After these investigations the results were combined: the electrode kinetics model was implemented in the mass transport model as well as in the CSTR network flowbed model. In both cases, good agreement, even to dynamic experiments, was obtained.     

Direct methanol fuel cell with non-equipotential electrodes

Our model of a direct methanol fuel cell with straight channels is extended to take into account finite conductivity of current collector plates and local contact resistances. Co-flow conditions are assumed and the two cases are considered: (i) when load resistor is connected at the inlet of the feed channels and (ii) when it is connected at the outlet. In both cases voltage loss in the plates induces growth of current production close to the point of load connection. The increase in local current j is accompanied by the growth of local polarization voltages η^a and η^c of the anode and the cathode sides, respectively. Moving the point of load connection one can change the distribution of j,η^a and η^c along the channel. Local “spot” of contact resistance decreases local current and polarization voltages; in the rest part of the cell these values increase.     

质子交换膜燃料电池双层扩散层特性三维分析

针对直流道质子交换膜燃料电池提出一种混合的两相三维非等温数学模型,考虑了液态水在多孔介质内的毛细流动和分布,分析了双层扩散层结构及碳纤维特性对电池性能的影响。结果表明,扩散层第一层（靠近催化层）厚度对质子膜电导率和气体传递特性有着相互制约的影响,需进行优化;在一定范围内,扩散层第一层碳纤维直径的减小可提高质子膜电导率,有利于电池性能的改善;在保持其他参数不变的前提下,应尽可能提高多孔介质的憎水性和孔隙率以提高电池输出性能。     

Mathematical model for a direct propane phosphoric acid fuel cell

A mathematical model was developed and used to predict the performance of direct propane phosphoric acid (PPAFC) fuel cells, utilizing both Pt/C state-of the-art electrodes and older Pt black electrodes. It was found that the overpotential caused by surface processes on the platinum catalyst in the anode is much greater than the potential losses caused by either ohmic resistance or propane diffusion in gas-filled and liquid-filled pores. In one comparison, the anode overpotential (0.5 V) was larger than the cathode overpotential (0.3 V) at a current density of 0.4 A cm⁻² for Pt loadings 4 mg Pt cm⁻². The need for sufficient water concentration at the anode, where water is a reactant, was indicated by the large effect of H₃PO₄ concentration. In another comparison, the model predicted that at 0.2 A cm⁻², modern carbon supported Pt catalysts would produce 0.35 V compared to 0.1 V for unsupported Pt black catalysts that were used several decades ago, when the majority of the research on direct hydrocarbon fuel cells was performed. The propane anode and oxygen cathode catalyst layers were modeled as agglomerates of spherical catalyst particles having their interior spaces filled with liquid electrolyte and being surrounded by gas-filled pores. The Tafel equation was used to describe the electrochemical reactions. The model incorporated gas and liquid-phase diffusion equations for the reactants in the anode and cathode and ionic transport in the electrolyte. Experimental data were used for propane and oxygen diffusivities, and for their solubilities in the electrolyte. The accuracy of the predicted electrical potentials and polarization curves were normally within ±0.02 V of values reported in experimental investigations of temperature and electrolyte concentration. Polarization curves were predicted as a function of temperature, pressure, electrolyte concentration, and Pt loading. A performance of 0.45 V at 0.5 A cm⁻² was predicted at some conditions.