Proton exchange membrane water electrolysis at high current densities: Response time and gas-water distribution

Understanding the distribution of oxygen in proton exchange membrane water electrolysis (PEMWE) is crucial for improving electrolysis efficiency and gas removal. In this study, we developed a two-dimensional (2D) transient model that couples the Euler–Euler multiphase model with electric potential equations to investigate two-phase flow in PEMWE. Our simulation reveals that the system's response time initially decreases and then increases with current density, indicating longer response times at high current densities. Modifying the wetting properties of the porous transport layer (PTL) affects gas removal at low gas holdup, resulting in a maximum 15% decrease in gas holdup. However, at high gas holdup, the flow field in the channel predominantly governs bubble removal, making changes in PTL wetting properties less influential. With increasing gas production rate, an inverse gradient distribution of gas saturation appears, leading to uneven gas saturation and hindering efficient oxygen removal. This non-uniform gas saturation adversely affects electrolysis performance.     

质子交换膜水电解制氢膜电极研究进展

质子交换膜水电解（PEMWE）制氢具有可适用于风能太阳能等可再生能源的间歇性和波动性、能量转换效率高、启动快速、占地小等优点,成为目前绿氢制取重点关注的技术。膜电极作为水电解制氢关键核心部件,对于水电解制氢的性能、效率和寿命至关重要,并随着量产规模的扩大在系统成本中的占比越来越高。发展高性能、低成本和高耐久性的膜电极对于绿氢的低成本大规模制取具有重要意义。本文综述了近年来质子交换膜电解水制氢膜电极中质子交换膜、催化层、多孔传输层等关键材料部件以及膜电极制备技术的研究进展和成果,并进行了简要评述。从膜电极设计和开发的角度系统地梳理了如何提高电解制氢性能、降低水电解制氢膜电极成本等方面的进展。最后,就未来膜电极研发的方向提出了建议。     

Process based reconstruction and simulation of a three-dimensional fuel cell catalyst layer

Fundamental understanding of catalyst layer nanostructure of hydrogen polymer electrolyte membrane (PEM) fuel cells is critical for improvement in performance and durability. A process based 3D mathematical model has been developed to elucidate the effect of electrode composition, porosity and ionomer weight fraction in catalyst layers on electrochemical and nano-scale transport phenomena. Numerical reconstruction of catalyst layer random structure has been performed through a controlled random algorithm, mimicking the experimental fabrication process. Nano-scale species transport properties, e.g., Knudsen diffusion of oxygen in nano-pores and proton transport in thin-film electrolyte, have been included in the model, allowing for more rigorous study of the catalyst layer. It was found that there is a threshold in both porosity and ionomer weight fractions, below which species percolation through the random structure becomes difficult due to reduced connectivity and increased isolation. The degree of mixing or size of agglomerates has been studied and it was discovered that increasing or decreasing the agglomerate number from the optimum value reduces the electrochemically active area (ECA) and deteriorates species transport, suggesting an optimum level of stirring of the catalyst ink during catalyst layer preparation is critical.     

In situ scanning probe spectroscopy at nanoscale solid/liquid interfaces

Electrochemistry provides unique features for the preparation of low-dimensional structures, but in situ spectroscopy with atomic/molecular resolution at such structures is at present not well established yet. This paper shows that in situ scanning probe spectroscopy at solid/liquid interfaces can be utilized to study electronic properties at nanoscale, if appropriate conditions are applied. Tunneling spectroscopy provides information about tunneling barrier heights and electronic states in the tunneling gap, as shown on Au(1 1 1) substrates, contact spectroscopy allows for transport measurements at single nanostructures, as shown at Au/n-Si(1 1 1) nanodiodes. The influence of the electrolytic environment on spectroscopic investigations is not a principal limitation, but offers additional degrees of freedom, which allow, for example, spectroscopic studies of potential dependent surface phenomena at solid/liquid interfaces.     

Simulation of a high temperature electrolyzer

Based on Solid Oxide Fuel Cell (SOFC) technology, Solid Oxide Electrolysis Cell (SOEC) offers an interesting solution for mass hydrogen production. This study proposes a multiphysics model to predict the SOEC behavior, based on similar charge, mass, and heat transport phenomena as for SOFC. However, the mechanism of water steam reduction on Nickel/Yttria-Stabilized Zirconia (Ni/YSZ) cermet is not yet clearly identified. Therefore, a global approach is used for modeling. The simulated results demonstrated that a Butler–Volmer’s equation including concentration overpotential provides an acceptable estimation of the experimental electric performance under some operating conditions. These simulations highlighted three thermal operating modes of SOEC and showed that temperature distribution depends on gas feeding configurations.     

基于电荷模型的荷电膜传递现象的研究进展

根据电荷模型（空间电荷和固定电荷模型）,将荷电膜传递现象分为膜的分离性能、电性质及传递参数3类.其中膜的分离性能用截留率和通量来表征;膜的电性质包括膜的电化学性质和膜的动电性质,膜的电化学性质用膜电位来表征,膜的动电性质用流动电位、Zeta电位和电滞效应等参数来表征;膜的传递参数则用反射系数、溶质透过系数、纯水透过系数、电导率、离子的迁移数及电渗系数6个系数来表征.概述了空间电荷和固定电荷模型描述电解质溶液在荷电膜中的传递现象的研究进展,介绍了两种模型在荷电膜中应用时的各自优势,展望了电荷模型在荷电膜体系中应用的发展方向.     

Performance of stabilized zirconia nanofilters under dynamic conditions

The nanofiltration performance of stabilized zirconia nanofilters was tested under dynamic conditions in an electrolytic solution. Interaction between nanostructured materials and solutes may modify the rejection properties of such membranes in the dynamic range of the experiments. Optimal filtration performance can only be realized by a proper understanding of these interactions. At high pressures, a decrease in electrolyte rejection is observed and the membranes show a reduction in hydraulic permeability. Two approaches (one thermodynamic and the other electrokinetic) are considered to explain these phenomena, i.e. the concentration polarization model and the electroviscosity model. A combination of these two approaches is successfully developed to explain the observed nanofilter behaviour at high pressures in the case of electrolytes. Further investigations were performed using an uncharged solute (vitamin B₁₂) in an electrolyte buffer. Although a reduction in permeability was observed at high pressures, there was no important decrease in vitamin B₁₂ rejection because the optimal transmembrane pressure (where the rejection reaches a maximum) was above the pressure range studied experimentally or did not allow the rejection reducing effects of the concentration polarization layer to be observed more clearly.     

On the implementation of membrane models in computational fluid dynamics calculations of polymer electrolyte membrane fuel cells

The implementation of phenomenological membrane models within computational fluid dynamics (CFD) codes requires coupling of the conservation equation for water content within the membrane to the conservation equations for species mass outside the membrane. It is common practice to treat water and current transport within the membrane as one-dimensional (1D), i.e., normal to the membrane surface only. The purpose of this study is to investigate the accuracy and efficiency of various strategies of implementing a phenomenological membrane model within the framework of a two-dimensional (2D) CFD code. Springer's membrane model was compared against two other models available from the literature, and the accuracy of each model was assessed by comparing predicted results against experimental data. Results appear to indicate that the Springer model and the Nguyen and White model over-predict the drying of the membrane, while the Fuller and Newman model provides the best match with experimental data. Following these studies, three strategies for implementation of the membrane model were investigated: (1) 2D transport in membrane, (2) 1D transport in membrane and (3) 1D transport with approximate transport properties. Fuller and Newman's membrane model was used for these studies. The results obtained using the three approaches were found to be within 4% of each other, while there was no significant difference in the computational time required by the three models, indicating that an analytical 1D transport model for the membrane that uses approximate properties is adequate for describing transport through it.     

Analysis of proton exchange membrane fuel cell catalyst layers for reduction of platinum loading at Nissan

The biggest issue that must be addressed in promoting widespread use of fuel cell vehicles (FCVs) is to reduce the cost of the fuel cell system. Especially, it is of vital importance to reduce platinum (Pt) loading of catalyst layers (CLs) in the membrane electrode assembly (MEA) of a proton exchange membrane fuel cell (PEMFC). In order to lower the Pt loading of the MEA, mass transport of reactants related to the performance in high current density should be enhanced significantly as well as kinetics of the catalyst, which can result in the better Pt utilization and effectiveness. In this study, we summarized our analytical approach and methods for reduction of Pt loading in CLs. Microstructure, mass transport properties of the reactants, and their relation in CLs were elucidated by applying experimental analyses and computational methods. A simple CL model for I–V performance prediction was then established, where experimentally elucidated parameters of the microstructure and the properties in CLs were taken into account. Finally, we revealed the impact of lowering the Pt loading on the transport properties, polarization, and the I–V performance.     

Modelling mass transport in solid oxide fuel cell anodes: a case for a multidimensional dusty gas-based model

In this work the mass transport phenomena taking place in the fuel channel and the porous electrode of the anode of planar solid oxide fuel cells (SOFCs) are discussed. A comprehensive review of SOFC mass transport models in the literature is given and a new multidimensional, multicomponent, isothermal, dynamic model of the mass transport phenomena taking place in the fuel channel and the porous electrode of the anode of planar SOFCs is presented. The model can be used to predict species composition profiles and is based on the dusty-gas model (DGM) [Mason, E.A., Malinauskas, A.P., 1983. Gas Transport in Porous Media: The Dusty-Gas Model: Elsevier; Jackson, R., 1977. Transport in Porous Catalysts: Elsevier], which is considered to be the most accurate of the existing mass transfer models in porous media [Suwanwarangkul, R., Croiset, E., Fowler, M.W., Douglas, P.L., Entchev, E., Douglas, M.A., 2003. Performance comparison of Fick's, dusty-gas and Stefan-Maxwell models to predict the concentration overpotential of a SOFC anode. Journal of Power Sources 122, 9-18]. Our two-dimensional DGM is validated using experimental data [Yakabe, H., Hishinuma, M., Uratani, M., Matsuzaki, Y., Yasuda, I., 2000. Evaluation and modeling of performance of anode-supported solid oxide fuel cell. Journal of Power Sources 86, 423-431] and it is tested against a two-dimensional Stefan-Maxwell model (SMM) and against one-dimensional models (Fick's model, SMM and DGM) reported in the literature. It is shown that a detailed model is essential for the accurate prediction of concentration overpotentials especially at high fuel utilisation conditions, which are typical operating conditions for fuel cells [Hernández-Pacheco, E., Singh, D., Hutton, P.N., Patel, N., Mann, M.D., 2004. A macro-level model for determining the performance characteristics of solid oxide fuel cells. Journal of Power Sources 138, 174-186].     

Investigation of liquid water in gas diffusion layers of polymer electrolyte fuel cells using X-ray tomographic microscopy

In polymer electrolyte fuel cells (PEFCs), condensation of water within the pore network of the gas diffusion layers (GDL) can influence the gas transport properties and thus reduce the electrochemical conversion rates. The use of X-ray tomographic microscopy (XTM), which allows for a resolution in the order of one micrometer is investigated for studying ex situ the local saturation in GDL's. The strength of XTM is the high spatial resolution with simultaneous contrast for water and carbon, allowing for non-destructive 3D-imaging of the solid and the contained water. The application of this method for imaging the ex situ water intrusion into the porous network of GDLs is explored using absorption and phase contrast methods. It is shown that the inhomogeneous filling behavior of GDL materials can indeed be visualized with sufficient resolution. For Toray paper TGP-H-060 the local saturation was measured as function of the water pressure. The results, evaluated in 1D, 2D and 3D show a liquid water retention effect at the denser layers near the surface. A comparison with established capillary pressure functions is presented. Altogether, the results show the potential of the XTM-method as a tool for studying the liquid water behavior in PEFC on a microscopic scale.     

Comparison among structured and packed-bed reactors for the catalytic partial oxidation of CH4 at short contact times

Three types of catalyst support (foams, honeycomb monoliths with square channels and spheres with approximately equal values of specific geometric surface a_v) were examined and compared by simulation with a 1D, dynamic heterogeneous mathematical model for application to the autothermal partial oxidation of methane. Both cold start-up and steady-state behaviours were investigated.

It was found that mass and, particularly, heat transfer properties markedly affect the reactor behaviour, both at start up and at steady state. Thus, the choice of the catalyst support can lead to greatly different reactor performances. Concerning the reactor start-up, simulations revealed that better interphase heat transport properties and lower bed heat capacity are useful to minimize the total start-up time; on the other hand, more favourable transport properties reduce the maximum flow rate which allows to achieve and maintain an ignited steady state. At steady state, oxygen conversion is strictly governed by interphase mass transfer, while methane conversion depends on a more complex, mixed chemical-diffusional regime.     

A generalized reduced fluid dynamic model for flow fields and electrodes in redox flow batteries

High-dimensional models typically require a large computational overhead for multiphysics applications, which hamper their use for broad-sweeping domain interrogation. Herein, we develop a modeling framework to capture the through-plane fluid dynamic response of electrodes and flow fields in a redox flow cell, generating a computationally inexpensive two-dimensional (2D) model. We leverage a depth-averaging approach that also accounts for variations in out-of-plane fluid motion and departures from Darcy's law that arise from averaging across three-dimensions (3D). Our resulting depth-averaged 2D model successfully predicts the fluid dynamic response of arbitrary in-plane flow field geometries, with discrepancies of <5% for both maximum velocity and pressure drop. This corresponds to reduced computational expense, as compared to 3D representations (<1% of duration and 10% of RAM usage), providing a platform to screen and optimize a diverse set of cell geometries.     

The NETmix reactor: Pressure drop measurements and 3D CFD modeling

NETmix is a new static mixing technology based on a network of mixing chambers interconnected by channels. Three NETmix reactors with different geometries were used to obtain experimental data for pressure drop and a generalized model for pressure drop in NETmix reactors has been developed. This model features a single adjustable parameter and it is only dependent on the geometric configuration of the NETmix design. The Z factor and the power number were also determined to compare the performance of different NETmix configurations with other existing mixers. The dynamic measurement of pressure drop was used to evaluate the mixing dynamics in the NETmix chambers and, above the critical Reynolds number, the natural oscillation frequency was quantified. Furthermore, a three-dimensional computational fluid dynamic transport model was also developed and validated. The energy performance of the three NETmix prototypes was quantified and shown to be very competitive with the compared existing static mixers. The developed 3D CFD transport model, validated by the reported experimental data, enables the computation of transport properties for any geometrical design and fluid properties, and avoids the need for experimental data each time a new NETmix configuration is designed.     

A Non‐isothermal Model of a Laboratory Intermediate Temperature Fuel Cell Using PBI Doped Phosphoric Acid Membranes

A two‐dimensional non‐isothermal model developed for a single intermediate temperature fuel cell with a phosphoric acid (PA) doped PBI membrane is developed. The model of the experimental cell incorporates the external heaters, and the body of the fuel cell. The catalyst layers were treated as spherical catalyst particles agglomerates with porous inter‐agglomerate space. The inter‐agglomerate space is filled with a mixture of electrolyte (hot PA) and PTFE. All the major transport phenomena are taken into account except the crossover of species through the membrane. This model was used to study the influence of two different geometries (along the channel direction and cross the channel direction) on performance. It became clear, through the performance analyses, that the predictions obtained by along the channel geometry did not represent the general performance trend, and therefore this geometry is not appropriate for fuel cell simulations. Results also indicate that the catalyst layer was not efficiently used, which leads to large temperature differences through the MEA.     

An isothermal model of a laboratory intermediate temperature fuel cell using PBI doped phosphoric acid membranes

A two-dimensional isothermal model is described for an intermediate temperature fuel cell using a phosphoric acid doped polybenzimidazole (PBI) membrane. The model considered the membrane-electrode-assembly and gas flow channels. All the major transport phenomena were taken into account except the cross-over of species through the membrane. The catalyst layers were treated as spherical catalyst agglomerates with porous inter-agglomerate spaces. The inter-agglomerate spaces are filled with a mixture of electrolyte (hot phosphoric acid) and polytetrafluoroethylene (PTFE). The model was validated against experimental data and used to study the influence of the catalyst layer properties on performance. Through the analyses of the effectiveness factor the model showed that utilisation of catalyst particles was very low at high current densities. At these conditions, the reaction occurs mainly on the surface of the agglomerate. An optimum phosphoric acid loading was found from the model simulations. The model was also used to demonstrate the resistance of the intermediate temperature fuel cell to anode poisoning by CO.     

Modification of the carbon fiber surface with a copper coating for composite materials with epoxy

Surface treatment of carbon fibers is essential to provide adequate interfacial interaction, and strength in carbon fiber/epoxy composites. The electrodeposition of a metallic copper coating on the carbon fiber surface has been examined as an alternative method to improve carbon fiber-epoxy interfacial properties. The wettability of the carbon fiber by the epoxy resin was improved as a result of copper electrodeposition. As a consequence, the adhesion between the carbon fiber and epoxy was also greatly improved by the surface electrolytic treatment used. The electrodeposition conditions affected significantly both wettability and adhesion phenomena. The electrolytic current had a strong effect on the interface performance. It was found that there was an intermediate electrolytic current, within the range used, which promoted better wetting and composite strength, compared with conventionally surface-oxidized carbon fibers.     

A comprehensive CFD model of anode-supported solid oxide fuel cells

The two-dimensional comprehensive CFD model of anode-supported SOFCs operating at intermediate temperature has been presented. This model provides transport phenomena of gas species with electrochemical characteristics and micro-structural properties, and predicts SOFC performance. The mathematical model solves conservation of electrons and ions, continuity equation, conservation of momentum, conservation of mass, and conservation of energy. A continuum micro-scale model based on statistical properties together with a mole-based conservation model was employed. CFD technique was used to solve the set of governing equations. The cell performance was decomposed with contributions of each overpotential and was presented at several operating temperatures with analysis of effective diffusivity. It was found that the contribution of potential gain due to temperature rising was considerably high. However it became non-significant at high operating temperature due to decreasing of effective diffusivity in AFL. These results showed that the performance and the distributions of current density, overpotentials, and mole fractions of gas species have a strong dependence upon temperature. From these results, it was concluded that the conservation of energy should be accommodated in comprehensive SOFC model. Also the useful information for the effect of parameters on cell performance and transport phenomena was provided.     

A model of devolatilization behavior in lignite pyrolysis with solid heat carriers

To elucidate the influence of the process conditions on pyrolytic products, the interactions between transport phenomena and pyrolysis kinetics are quantitatively analyzed at the level of a single coal particle. A comprehensive mathematical model is formulated to predict intraparticle multiphysics and devolatilization behaviors; the model contains two correlative one-dimensional unsteady heat conservation equations and ternary mass conservation equations in conjunction with the simplified dusty-gas model. Moreover, a multistep kinetic model of coal devolatilization is adopted to predict the generating rates of the lumped pyrolytic products. Validation of the model against experimental and literature data showed that can predict the transient temperature profiles of a coal particle and the yields of volatiles. Finally, the effects of the main process conditions on the intra- and extra-temperature history of lignite during pyrolysis with solid heat carriers are analyzed. The interactions between physical transport and the pyrolysis reaction are also examined.     

Peculiar electric properties of polarized layer in alkaline silicate glasses

Broadband dielectric spectroscopy (BDS) was applied to study polarization phenomena in alkaline silicate glasses, in particular, properties and structure of subsurface (anodic) polarized layers forming in poling with deposited film electrodes of different structures. A model of poled glasses which does not contradict experimental data is proposed. In accordance with the model, a poled glass is presented as two resistor-capacitor circuits in a series connection, one of which is the polarized layer and another is the rest of the sample. It is found that the electric properties of the layers essentially depend on the structure of the anodic electrode used in glass poling. It is also shown that the dielectric response of poled glass samples is mainly determined by the electric properties of the submicron polarized layers and this gives an opportunity to reveal specific properties of the layers rather than ones of the glass sample bulk. Revealed temperature dependence of DC conductivity of the polarized layers obeys Arrhenius's law, and determining activation energy does not depend on the electrode. Finally, it is noted that today above-mentioned information about polarized layers can be obtained only by BDS.