Two-phase Mass Transport Research Articles

Engineering the PEM Water Electrolysis Anode for Better Interfacial Electrical Conductivity

In pursuing cost-effective green hydrogen production via proton exchange membrane water electrolysis (PEMWE), the high expense of the iridium (Ir)-based oxygen evolution reaction (OER) catalysts poses a significant challenge. To overcome this issue, researchers have focused on developing active OER catalysts and catalyst layers (CLs) with minimum uses of Ir. However, they frequently encountered performance problems at the single-cell level that originate not only from the kinetic polarization but also from the ohmic polarization. In this aspect, we have focused on identifying the reasons for the poor performance of low IrOx-loaded anodes, with loadings as low as 0.10 mg cm-2. We pinpoint the cause of the poor performance of the low loaded anodes to an electron transport problem within the native oxide on the Ti porous transport layer (PTL). Our analysis, grounded in the metal-insulator band model, reveals that the primary factors contributing to electron conductivity loss in the Ti oxide are the upward band bending induced by the Schottky contact with high work function IrOx and by the pinch-off effect from the extensive ionomer contact. Based on the above understanding, we propose an anode CL composed of a one-dimensional iridium catalyst, fabricated by electrospinning and calcination processes. Our nanofiber-based CL effectively mitigates the electron transport problem of the native Ti oxide. This improvement stems from the low work function of the Ir nanofiber catalyst and the highly porous structure that reduces the ionomer contact with the Ti oxide. Furthermore, the highly porous nature of the nanofiber-based CL enhances two-phase mass transport, facilitating the high current operation. These findings underscore the critical role of the CL/PTL interface of the PEMWE anode and present the necessity of a well-structured CL to achieve efficient and cost-effective water electrolysis.

Computational Model of Two-Phase Mass Transport Dynamics for pH-Buffered Hydrogen Evolution Reactions in Porous Electrodes

Computational Model of Two-Phase Mass Transport Dynamics for pH-Buffered Hydrogen Evolution Reactions in Porous Electrodes

Cross-dimensional multi-physics model and two-phase mass-transport enhancement for the hydrogen peroxide-based fuel cell

The direct liquid fuel cell utilizing hydrogen peroxide as an oxidant is considered to be the promising energy conversion technology, owing to its superior energy density and theoretical voltage. In the hydrogen peroxide-based fuel cells, the ever-changing two-phase mass transport and multi-component reaction processes resulting from the hydrogen peroxide self-decomposition exerts a pivotal influence on their performance. In this work, taken hydrogen peroxide self-decomposition into account, a cross-dimensional multi-physics model for the hydrogen peroxide-based fuel cell was proposed and developed to clarify the complex mechanisms of transports and reactions. In this ingeniously developed model, the multi-component reaction in the porous electrode is elucidated by a two-dimensional (2D) sub-model, the two-phase mass transport in the flow field is captured by a three-dimensional (3D) sub-model, and the one-dimensional (1D) correlation equation is served as a bridge between them. The uneven distribution of gas–liquid volume, velocity and current density and their relationships in fuel cell with serpentine flow field and dot matrix flow field were thus achieved. Furthermore, a novel gradient dot matrix flow field was developed to match the two-phase mass transport and multi-component reaction processes of hydrogen peroxide-based fuel cell. It is found that the continuously increasing pressure and velocity along flow path is balanced by the gradient-downsized ribs and the formation of large-volume oxygen bubbles is prevented by the gradient-densified ribs, achieving obviously uniformized current density distribution. Moreover, the gradient dot matrix flow field enables 54 % higher average hydrogen peroxide coverage contents (49 %) and 86 % lower average pressure drop (172 Pa) compared to the serpentine flow field, yielding 6.3 % higher peak power density (210 mW cm−2) and ultra-low extra pump power in fuel cell. This work will make contribution towards gaining profound insights into the intricate mechanisms underlying mass and charge transport, and offering invaluable guidance for the flow field design for the hydrogen peroxide-based fuel cells.

Enhancement of two-phase flow and mass transport by a two-dimensional flow channel with variable cross-sections in proton exchange membrane fuel cells

A novel two-dimensional flow channel (TDFCH) with variable cross-sections geometrically controlled by sinusoidal function is proposed for proton exchange membrane fuel cells (PEMFCs). Two-phase flow simulations and visualisation experiments are combined to reveal the mechanisms for improved water management and enhanced reactant transport. Results show that the centrifugal force generated by the vortex pushes the liquid water away from the central region of the flow channel, forming droplet and film flows to avoid blockage of the flow channel. The reactant gases accumulate on the windward side of the variable-diameter structures, leading to an increase of the local pressure and facilitating fuel diffusion into the porous electrode. The fluid inertia enhances mass transfer of reactants and removal of liquid water inside the porous electrode. The amplitude A of the flow channel based on sinusoidal curve plays a dominant role in improving the performance of the TDFCH. Under the same testing conditions, when amplitude A = 0.2 and period T = 2, the peak power density of the TDFCH-based cell reaches 517.9 mW cm−2, which is 26 % higher than the conventional straight flow channel. The newly designed TDFCH provides an efficient solution to realize performance improvement and long-term stable operation of PEMFCs.

New Analytical Expressions of Concentrations in Packed Bed Immobilized-Cell Electrochemical Photobioreactor

An electrochemical photobioreactor with a packed bed containing transparent gel granules and immobilized photosynthetic bacterial cells is shown with a one-dimensional two-phase flow and transport model. We consider the biological/chemical events in the electrochemical photobioreactor, the intrinsically connected two-phase flow and mass transport, and other factors. This model is based on a system of nonlinear equations. This paper applies Akbari-Ganji’s and Taylor series methods to find analytical solutions to nonlinear differential equations that arise in an immobilized-cell electrochemical photobioreactor. Approximate analytical expressions of the concentration of glucose and hydrogen are obtained in liquid and gas phases for different parameter values. Numerical simulations are presented to validate the theoretical investigations.

An improved strategy of passive micro direct methanol fuel cell: Mass transport mechanism optimization dominated by a single hydrophilic layer

Mass transport behaviors in passive micro direct methanol fuel cells (μDMFCs), including methanol mass transport at the anode and gas-liquid two-phase mass transport at the cathode, are essential during the actual working process. However, these mass transport behaviors also face serious problems, such as methanol crossover mass transport and gas-liquid two-phase mass transport obstruction, which significantly reduces the performance of passive μDMFCs, restricting their application greatly. Herein, we propose an improved multi-substance mass transport mechanism dominated by water mass transport. The proposed mechanism is established by constructing a hydrophilic mass transport layer (MTL). Theoretical analysis shows that mass transfer behaviors under the proposed mechanism are better than those under the conventional one, thus improving cell performance. Based on this, the proposed mechanism is applied to passive μDMFCs in real scenes by preparing hybrid nanomaterials to construct the MTL. As a result, the performance of novel μDMFCs with the proposed mechanism is greatly improved. The power density of the novel μDMFCs reaches nearly two times that of conventional ones, and the energy density reaches about 6.2 times under high-concentration methanol fuel. The comprehensive study on the mass transport mechanism of passive μDMFCs aims to provide theoretical reference and practical experience for promoting their wide application.

Correlating nanostructure features to transport properties of polymer electrolyte membrane electrolyzer anode catalyst layers

In this work, polymer electrolyte membrane water electrolyzer catalyst layers are stochastically generated with α pore regions (defined as regions with pores <250 nm in size), β pore regions (defined as regions with pores >250 nm in size) and an ionomer layer of uniform thickness. Pore network modelling simulations are performed to correlate structural characteristics (porosity, mean pore diameter, and β pore volume fraction) to single and two-phase mass transport properties. Porosity and two-phase mass permeability are well correlated, but order of magnitude variations are observed due to inhomogeneous pore size distributions at the same porosity. The effect of varying ionomer and iridium oxide material mass fractions on ion and electron transport properties is also explored. Electrical conductivity is well correlated with catalyst volume fraction, while ionic conductivity is less correlated, likely due to changes in connectivity resulting from the inhomogeneous structure of the porous phase. Due to the non-homogeneity of the pore sizes in the catalyst layer, existing correlations for porous media are insufficient predictors. As such, we recommend that both porosity and mean pore diameter are considered when estimating transport properties. It is additionally recommended that both α pore and β pore regions must be incorporated into catalyst layer designs to allow both enhanced connectivity and mass transport characteristics.

Two-phase mass transport model for microfluidic fuel cell with narrow electrolyte flow channel

Microfluidic fuel cells employ liquid electrolyte stream instead of conventional polymer electrolyte membrane to compartmentalize the anode and cathode, leading to the improvement of flexibility in practical applications. To boost the electrochemical performance and simplify operating conditions, a two-dimensional two-phase model is developed for a microfluidic fuel cell with a single narrow electrolyte channel, where a transport barrier layer is set close to the cathode to inhibit fuel crossover and remove the blank electrolyte stream. The shortened width of electrolyte channel decreases the distance between the anode and cathode, enhancing the fuel and proton transport and elevating the power density effectively. The generated gaseous CO2 via electrochemical reaction increases mass transfer resistance of liquid fuel through the anode catalyst layer, resulting in dramatic decrease in the fuel concentration and effective active area in the anode catalyst layer. Consequently, the transport of proton and fuel in the anode limits the output power density at high current densities. The numerical results provide a deep understanding of two-phase mass-transport in microfluidic fuel cell with a single narrow electrolyte channel and help better design and operation of this type microfluidic fuel cell.

Numerical Modelling of Mixing in a Microfluidic Droplet Using a Two-Phase Moving Frame of Reference Approach.

Droplets generated in microfluidic channels are effective self-contained micromixers and micro-reactors for use in a multiplicity of chemical synthesis and bioanalytical applications. Droplet microfluidic systems have the ability to generate multitudes of droplets with well-defined reagent volumes and narrow size distributions, providing a means for the replication of mixing within each droplet and thus the scaling of processes. Numerical modelling using computational fluid dynamics (CFD) is a useful technique for analysing and understanding the internal mixing in microfluidic droplets. We present and demonstrate a CFD method for modelling and simulating mixing between two species within a droplet travelling in straight microchannel, using a two-phase moving frame of reference approach. Finite element and level set methods were utilised to solve the equations governing the coupled physics between two-phase flow and mass transport of the chemical species. This approach had not been previously demonstrated for the problem of mixing in droplet microfluidics and requires less computational resources compared to the conventional fixed frame of reference approach. The key conclusions of this work are: (1) a limitation of this method exists for flow conditions where the droplet mobility approaches unity, due to the moving wall boundary condition, which results in an untenable solution under those conditions; (2) the efficiency of the mixing declines as the length of the droplet or plug increases; (3) the initial orientation of the droplet influences the mixing and the transverse orientation provides better mixing performance than the axial orientation and; (4) the recirculation inside the droplet depends on the superficial velocity and the viscosity ratio.

Modeling the Local Environment within Porous Electrode during Electrochemical Reduction of Bicarbonate

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO2 supply has been motivated as a means of integrating conversion with upstream CO2 capture. The way that CO2 is formed and transported during CO2-mediated bicarbonate reduction in flow cells is profoundly different from conventional CO2 saturated and gas-fed systems and a thorough understanding of the process would allow further advancements. Here, we report a comprehensive two-phase mass transport model to estimate the local concentration of species in the porous electrode resultant from homogeneous and electrochemical reactions of (bi)carbonate and CO2. The model indicates that significant CO2 is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO2. However, the in situ formation of CO2 and subsequent reduction to CO exhibits a plateau at high potentials due to neutralization of the protons by the alkaline reaction products, acting as the limiting step toward higher CO current densities. Nevertheless, the pH in the catalyst layer exhibits a relatively smaller rise, compared to conventional electrochemical CO2 reduction cells, because of the reaction between protons and CO32– and OH– that is confined to a relatively small volume. A large fraction of the CL exhibits a mildly alkaline environment at high current densities, while an appreciable amount of carbonic acid (0.1–1 mM) and a lower pH exist adjacent to the membrane, which locally favor hydrogen evolution, especially at low electrolyte concentrations. The results presented here provide insights into local cathodic conditions for both bicarbonate cells and direct-CO2 reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode.

Liquid transport in gas diffusion layer of proton exchange membrane fuel cells: Effects of micro-porous layer cracks

Micro porous layer (MPL) is a carbon layer (∼15 μm) that coated on the gas diffusion layer (GDL) to enhance the electrical conduction and membrane hydration of proton exchange membrane fuel cell (PEMFC). However, the liquid transport behavior from MPL to GDL and its impact on water management remain unclear. Thus, a three-dimensional volume of fluid (VOF) model is developed to investigate the effects of MPL crack properties on liquid water saturation, liquid pathway formation, and the two-phase mass transport mechanism in GDL. Firstly, a stochastic orientation method is used to reconstruct the fibrous structure of the GDL. After that, the liquid water saturation calculated from the numerical results agrees well with the experimental data. With considering the full morphology of the overlap between MPL and GDL, it's found that this overlap determines the preferred liquid emerging port of both MPL and GDL. Three crack design shapes in MPL are proposed on the base of the similarity crack formation processes of soil mud. In addition, the effects of crack shape, distance between cracks, and crack number on liquid water transport from MPL to GDL are investigated. It is found that the liquid water saturation of GDL increases with crack number and the distance between cracks, while presents little correlation to the crack shape. Hopefully, these results can help the development of PEMFC models without reconstructing full MPL morphology.

Numerically investigating two-phase reactive transport in multiple gas channels of proton exchange membrane fuel cells

Understanding the coupled mechanisms between two-phase flow and reactive transport in proton exchange membrane fuel cells is important for improving cell performance. In this study, a one-dimensional + three-dimensional model is developed to simulate the air–water two-phase flow, mass transport and electrochemical reaction in the cathode side of proton exchange membrane fuel cells. Dynamic water behaviors affected by several forces in three channels are well captured by the volume of fluid method. Effects of liquid water behaviors on pressure drop, mass flow rate, oxygen concentration distribution, current density and especially the distribution characteristics among different channels are discussed. It is found that the current density increases under certain flow patterns such as droplet detachment and water film on the top wall. Compared with single-phase flow, the two-phase flow magnifies the flow maldistribution in different channels, and such maldistribution in turn leads to different water detachment sequence in the three channels. The change of the mass flow rate in the second channel is more sensitive to that in the third channel. Effects of the gas channel wall wettability and channel structures are also investigated. The results show that changing the gas channel as hydrophobic will lead to higher pressure drop, lower current density and poorer flow distribution among different channels, which thus is not desirable in application. Modified structures of rectangle channel and tapered channel are finally explored. Compared with the base case, the tapered channel results in increased flow uniformity and averaged current density by 65.9% and 1.5%, respectively.

Gas distribution and droplet removal of metal foam flow field for proton exchange membrane fuel cells

Recently, porous metal foam has gained much attention as an alternative gas distributor of proton exchange membrane fuel cells. However, the gas distribution in the intricate porous flow field is different from the conventional flow channels and the liquid droplet behavior remains unclear. Thus, this study numerically investigated the two-phase mass transport capacities of the metal foam flow field. The metal foam morphology is reconstructed based on X-ray computational tomography technique and the two-phase interface is capture by volume of fluid method. A divergent gas transport mode is observed, which promotes the uniformity and convection of gas reactant flow with a much lower permeability than conventional flow channels. The heterogeneity of metal foam pore distribution should be minimized to reduce the pore-scale weak flow area. In addition, the air drag force on liquid droplet grows with droplet diameter in a similar way to that of a flow channel, but the resistance for liquid removal is no longer the shear force by wall surface but the adhesion by ligament surface. The hydrophobicity of ligaments is found necessary to reduce liquid retention phenomenon. In addition, the variation of gas velocity exhibits a stronger influence than droplet diameter on liquid removal, indicating that the metal foam flow field is applicable to high-current-density operation conditions for proton exchange membrane fuel cells.

Two-dimensional multi-physics modeling of porous transport layer in polymer electrolyte membrane electrolyzer for water splitting

Polymer electrolyte membrane (PEM) electrolyzers have received increasing attention for renewable hydrogen production through water splitting. In present work, a two-dimensional (2-D) multi-physics model is established for PEM electrolyzer to describe the two-phase flow, electron/proton transfer, mass transport, and water electrolysis kinetics with focus on the porous transport layer (PTL) and the channel-land structure. After comparing four sets of experimental data, the model is employed to investigate PTL thickness impact on liquid water saturation and local current density. It is found that the PTL under the land may have much lower liquid saturation than that under the channel due to land blockage. The PTL thickness may significantly impact liquid water access to the catalyst layer (CL) under the land. Specifically, the 100 μm thick PTL shows less than 1% liquid saturation at the CL-PTL interface under 4–5 A/cm2, leading to water starvation and electrolyzer voltage increase. As the operating current density decreases under 2–3.5 A/cm2, the liquid saturation recovers and increases to about 10–20%. In thicker PTLs, the liquid saturation is higher under the land reaching 30–40% at the CL-PTL interface under 5 A/cm2 for 200 and 500 μm thick PTLs. For the 100 μm thick PTL, the local current density drops to below 0.5 A/cm2 under the land with 5 A/cm2 average current density. For the 200 and 500 μm thick PTLs, the local current is almost uniform in the in-plane direction. The numerical model is extremely valuable to investigate PTL properties and dimensions to optimize channel-land design and configuration for high performing electrolyzers.

Controlled 3D nanoparticle deposition by drying of colloidal suspension in designed thin micro-porous architectures

ABSTRACT Nanoparticle deposition by drying of colloidal suspension in thin micro-porous architectures has attracted a lot of attention in scientific research as well as industrial applications. However, the underlying mechanisms of such three-dimensional (3D) deposition are not yet fully revealed due to the complexity of the co-occurring processes of two-phase fluid flows, phase change and mass transport. Consequently, the control of 3D nanoparticle deposition remains a challenge. We use a combined experimental and numerical approach to achieve controlled 3D nanoparticle deposition by drying of colloidal suspension in two pillar-based thin micro-porous architectures. By the design of pillar layout, rectangular-spiral and circular-spiral deposition configurations are obtained globally. By varying the surface wettability, vertically symmetric and sloped nanoparticle depositions can be achieved locally. While the numerical modeling reveals the mechanisms of liquid internal flow, as well as the impact of local drying rate on nanoparticle transport, accumulation and final deposition, the experimental results of deposition configurations validate the controlling strategies. This combined experimental and numerical work provides a framework to achieve desired 3D nanoparticle deposition in thin micro-porous architectures, with a thorough understanding of the underlying mechanisms of two-phase fluid flows, phase change and mass transport.

Two-phase computational modelling of a membraneless microfluidic fuel cell with a flow-through porous anode

Membraneless microfluidic fuel cells are miniaturized and integratable power sources as compared with conventional fuel cells based on membrane electrode assembly. To elucidate the interaction of two-phase flow, mass transport and electrochemical reactions, a two-dimensional two-phase model is developed for the microfluidic fuel cell with a flow-through porous anode. The two-phase flow in the anode and the microchannel is formulated by the two-fluid model and mixture multiphase flow theory, respectively. The modelling results suggest that the retention of gas phase in the anode catalyst layer and microchannel can reduce the effective active area and impede the proton conduction to limit the cell performance. However, the commonly used single-phase assumption fails to capture these effects, resulting in overestimated performance. The fuel crossover shows an opposite trend as compared with that predicted by the previous single-phase model, and is found to correlate with the two-phase flow in the microchannel. The flow rates of fuel and electrolyte on the fuel transport and gas-phase removal are also discussed. The present work highlights the significance of two-phase effects in the modelling of microfluidic fuel cells, and provides insights into the two-phase flow and mass transfer for future development and operation.

Modeling Nickel Electrowinning with Electrode Diaphragms Based on Nernst-Plank Equation and a Volume Force Form of Darcy's Law

The Ni electrowinning process with electrode diaphragms was simulated with an Electrochemical-Computational Fluid Dynamic model by considering a two-phase flow, mass transport, and electrochemical reactions. The effects of electrode diaphragms on the fluid flow, concentration, voltage drop, and current distribution were analyzed based on the simulation results. Results show that the electrode diaphragms have significant effects on the fluid flow and voltage drop in the electrolyte. The electrode diaphragms greatly reduced the strong convective flow caused by rising bubbles in the cell, and thus reduce the transport of H+ (generated at the anode) to the cathode. Based on this model, the effects of different operating parameters on the electrolyte potential and cathode current distribution have been analyzed. The results show that the cell temperature of 60°C, Ni concentration of 80 g/L, and a relatively low current density are the most favorable compared with other settings in the simulation for decreasing the electrolyte voltage drop and reducing the roughness of the deposition.

Two-phase hydrodynamic modelling and experimental characterization in an activated sludge electrooxidation flow reactor

Over the last decade, electrochemical processes involving solid–liquid hydrodynamic systems have been proposed for their tentative implementation in applications like water activated sludge electrooxidation as a pre-treatment for anaerobic digestion to produce biogas, mostly methane. However, electrooxidation needs to be optimized in terms of operating conditions, including improving operational flow. This work therefore, focuses on two-phase fluid dynamics and mass transport modeling of a Diaclean® cell, to establish a technical understanding of the electrochemical reactor behavior under two-phase hydrodynamics and improve operational conditions when solid–liquid systems are involved. Modelling was performed for two scenarios: (a) a single-phase flow to evaluate a solution without any solid particles using Navier–Stokes equations, and (b) a two-phase flow to analyze sludge using a Eulerian mixture model approach. These analyses of the proposed scenarios were validated with a simultaneous experimental study consisting of hydrodynamic studies (using mean residence time distributions tests) and mass transport characterization (by limiting electrochemical current techniques). The theoretical analysis presented here fits the experimental data obtained from residence time distribution and mass transport analysis at low Reynolds numbers quite well, but presents slight discrepancies at intermediate Re, in one and two-phase systems. This suggests refinement of the analyses is needed to consider some complex effects.

A Redox Fuel Cell Capable of Converting Ethanol to Electricity at a High Power Output

The use of hydrogen peroxide in fuel cells has recently received increasing attention, primarily due to its several unique characteristics when compared with the use of gaseous oxygen [1]. However, there are three issues associated with the use of hydrogen peroxide in fuel cells [2]. Firstly, the actual cathode potential is lower than the theoretical one, which is mainly attributed to the mixed potential resulting from the simultaneous hydrogen peroxide oxidation reaction on the cathode. Secondly, the hydrogen peroxide oxidation reaction releases gaseous oxygen, leading to a two-phase mass transport. Thirdly, the reduction of hydrogen peroxide in fuel cells has to use metal catalysts, such as platinum, palladium and gold [3]. In this work, we propose to create the cathode potential by introducing a redox couple to the cathode while to use hydrogen peroxide to chemically charge to redox ions. The redox cathode not only completely eliminates the mixed-potential problem associated with the direct reduction of hydrogen peroxide, but also enables a faster cathodic electrochemical kinetics even without noble metal catalysts [4]. It has been demonstrated that the direct ethanol fuel cell with a redox couple of V(IV)/V(V) yields a peak power density of 450 mW cm-2 at 60oC, which is 87.5% higher than that of the conventional cell with direct reduction of hydrogen peroxide [5]. References [2] L. An, T.S. Zhao, X.H. Yan, X.L. Zhou, P. Tan, Science Bulletin 60 (2015) 55-64. [3] L. An, T.S. Zhao, Z.H. Chai, L. Zeng, P. Tan. Int. J. Hydrogen Energy 39 (2014) 7407-7416. [4] L. An, T.S. Zhao, X.L. Zhou, L. Wei, X.H. Yan, RSC Adv. 4 (2014) 65031-65034. [5] L. An, T.S. Zhao, X.L. Zhou, X.H. Yan, C.Y. Jung, J. Power Sources 275 (2015) 831-834.

Numerical simulation of two-phase mass transport in three-dimensional naturally fractured reservoirs using discrete streamlines

ABSTRACTSimulation of multiphase mass transport in large field-scale heterogeneous, naturally fractured reservoirs with common grid-based methods is very time consuming and costly. An in-house streamline-based simulator is extended to simulate two-phase flow of oil and water in 3D heterogeneous, naturally fractured reservoirs. Different waterflooding problems have been simulated in 2D and 3D fractured reservoirs, and their results are compared with those of the grid-based commercial software. It has been shown that the streamline-based simulator can predict the results of the grid-based software with an acceptable accuracy, while its computational time and required memory is exceptionally less than the grid-based method.