PEM water electrolysis: Innovative approaches towards catalyst separation,recovery and recycling

We report the development of a facile recycling process for catalyst coated membranes (CCMs) used in polymer electrolyte membrane (PEM) water electrolyzers. After performance evaluation in an assembled electrolysis cell, ultrasonication is used to provide high-yield recovery of not only the noble-metal catalyst materials, but also of the fluoropolymer membrane itself, without the release of hazardous gases. Transmission electron microscopy (TEM) and electrochemical characterization are used to confirm the retention of catalyst particle size, and of the performance of the recycled CCMs. Furthermore, our projections indicate that, if this approach is widely employed, existing resources of noble metals will prove sufficient for the gigawatt-scale implementation of PEM water electrolyzers. This has profound implications for the achievement of current targets for reducing the consumption of precious metals for applications in electrolyzers, fuel cells and other energy storage devices.     

The effect of fuel cell operational conditions on the water content distribution in the polymer electrolyte membrane

Models play an important role in fuel cell design and development. One of the critical problems to overcome in the proton exchange membrane (PEM) fuel cells is the water management. In this work a steady state, two-dimensional, isothermal model in a single PEM fuel cell using individual computational fluid dynamics code was presented. Special attention was devoted to the water transport through the membrane which is assumed to be combined effect of diffusion, electro-osmotic drag and convection. The effect of current density variation distribution on the water content (λ) in membrane/electrode assembly (MEA) was determined. In this work the membrane heat conductivity is considered as a function of water content and the effect of temperature distribution in membrane is also analyzed. After that detail distributions of oxygen concentration, water content in membrane, net water flux and different overpotentials were calculated. Our simulation results show the reduction of reactant concentration in flow channels has a significant effect on electrochemical reaction in the gas diffusion and catalyst layer. Different fluxes are compared to investigate the effect of operating condition on the water fluxes in membrane. The amounts of different fluxes are strong function of current density, which is related to external load. The model also can use for simulating different kind of membranes. The model prediction of water content curves are compared with one-dimensional model predictions data reported in the validated open literature and a good compatibility were observed.     

The effect of operating parameters on water transport in PEM fuel cells

Water content in the membrane and the presence of liquid water in the catalyst layers (CL) and the gas diffusion layers (GDL) play a very important role in the performance of a PEM fuel cell. To study water transport in a PEM fuel cell, a two‐phase flow mathematical model is developed. This model couples the continuity equation, momentum conservative equation, species conservative equation, and water transport equation in the membrane. The modeling results of fuel cell performances agree well with measured experimental results. Then this model is used to simulate water transport and current density distribution in the cathode of a PEM fuel cell. The effects of operating pressure, cell temperature, and humidification temperatures on the net water transfer through the membrane, liquid water saturation, and current density distribution are studied. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(2): 89–100, 2006; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20107     

On water transport in polymer electrolyte membranes during the passage of current

This article discusses an approach to model the water transport in the membranes of PEM fuel cells during operation. Starting from a frequently utilized equation the various transport mechanisms are analyzed in detail. It is shown that the commonly used approach to simply balance the electro-osmotic drag (EOD) with counter diffusion and/or hydraulic permeation is flawed, and that any net transport of water through the membrane is caused by diffusion. Depending on the effective drag the cathode side of the membrane may experience a lower hydration than the anode side. The effect of a water-uptake layer on the net water transport will also be pictured. Finally, the effect of EOD is visualized using “Newton’s cradle”.     

Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode

Liquid water transport is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air–water flow patterns inside fuel cell gas flow channels with gas diffusion layer (GDL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents numerical investigations of air–water flow across an innovative GDL with catalyst layer and serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different static contact angles (hydrophilic or hydrophobic) were applied to the electrode (GDL and catalyst layer). The results showed that different wettabilities of cathode electrode could affect liquid water flow patterns significantly, thus influencing on the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown, several gas flow problems were observed, and some useful suggestions were given through investigating the flow patterns.     

Analysis of in situ water transport in Nafion by confocal micro-Raman spectroscopy

A proton exchange membrane fuel cell (PEMFC) is a promising alternative source of clean power for automotive applications, but its acceptance in such applications depends on reducing its costs and increasing its power density to achieve greater compactness. To meet these requirements, further improvements in cell performance are required. In particular, when the fuel cell is operating at high current density, the transport of water through the membrane has considerable impacts on the performance because of the large concentration gradient of water between the cathode and anode. Through-plane water permeation across the membrane is therefore a fundamental process in operational PEMFCs. Recently, resistance to water transport at the membrane-gas interface has been reported, and this is affected by the temperature and relative humidity. We investigated the distribution of water inside a proton exchange membrane during a water permeation test by using confocal micro-Raman spectroscopy with a fine spatial resolution (2-3 μm). In the presence of a water flux, the local water content is not necessarily in equilibrium with the water activity in the gas phase. Interfacial water-transport resistance due to the presence of a non-equilibrium membrane structure at the interface cannot be neglected.     

Contamination and moisture absorption effects on the mechanical properties of catalyst coated membranes in PEM fuel cells

Mechanical degradation of the catalyst coated membrane (CCM), which contains a perfluorosulfonic acid (PFSA) proton transport layer, can significantly deteriorate the performance of proton exchange membrane (PEM) fuel cells. We initially report on the adhesive and cohesive fracture properties of CCMs and show that failure occurs cohesively in the catalyst layer (CL). We then investigate the effects of foreign cations and chloride contamination and moisture absorption on the mechanical properties of CCMs. The fracture resistance of contaminated CCMs is significantly reduced and the time dependent growth of cracks in the CLs in moist air environments occurs at lower crack driving force thresholds. The deterioration in fracture resistance of the CCMs after foreign cation contamination is related to cation interaction with the molecular structure of PFSA polymer. The harmful effect of chloride contamination is attributed to chloride blocking on the surface of catalyst Pt particles, which tends to weaken the catalyst-polymer interface and induces crack initiation and subsequent propagation with lower energy. The accelerated time dependent crack growth at higher humidity is explained by the role of water molecules on weakening ionic interactions and the intermolecular strength of the PFSA polymer.     

Analysis of liquid water transport in cathode catalyst layer of PEM fuel cells

The performance of a polymer electrolyte membrane (PEM) fuel cell is significantly affected by liquid water generated at the cathode catalyst layer (CCL) potentially causing water flooding of cathode; while the ionic conductivity of PEM is directly proportional to its water content. Therefore, it is essential to maintain a delicate water balance, which requires a good understanding of the liquid water transport in the PEM fuel cells. In this study, a one-dimensional analytical solution of liquid water transport across the CCL is derived from the fundamental transport equations to investigate the water transport in the CCL of a PEM fuel cell. The effect of CCL wettability on liquid water transport and the effect of excessive liquid water, which is also known as “flooding”, on reactant transport and cell performance have also been investigated. It has been observed that the wetting characteristic of a CCL plays significant role on the liquid water transport and cell performance. Further, the liquid water saturation in a hydrophilic CCL can be significantly reduced by increasing the surface wettability or lowering the contact angle. Based on a dimensionless time constant analysis, it has been shown that the liquid water production from the phase change process is negligible compared to the production from the electrochemical process.     

Measurement of water transport rates across the gas diffusion layer in a proton exchange membrane fuel cell,and the influence of polytetrafluoroethylene content and micro-porous layer

Water management in a proton exchange membrane (PEM) fuel cell is one of the critical issues for improving fuel cell performance and durability, and water transport across the gas diffusion layer plays a key role in PEM fuel cell water management. In this work, we investigated the effects of polytetrafluoroethylene (PTFE) content and the application of a micro-porous layer (MPL) in the gas diffusion layer (GDL) on the water transport rate across the GDL. The results show that both PTFE and the MPL play a similar role of restraining water transport. The effects of different carbon loadings in the MPL on water transport were also investigated. The results demonstrate that the higher the carbon loading in the MPL, the more it reduces the water transport rate. Using the given cell hardware and components, the optimized operation conditions can be obtained based on a water balance analysis.     

Effective schemes to control the dynamic behavior of the water transport in the membrane of PEM fuel cell

We have investigated the transient behavior of the water transport across the membrane of the PEM fuel cell to seek for effective control schemes so that the best dynamic performance of the fuel cell can be obtained. It is found that both a larger starting operational current density i₀ and a smaller operational current density i can lead to a smaller dynamic response time t_ss, the time for the water distribution across the membrane to reach the steady state. Present results nevertheless point out that the most powerful as well as the most feasible control scheme is to control the humidification parameter k, i.e. to adjust the water content of the feeding fuel, so that the t_ss would remain steadily in a reasonably low value in a wide range of water flux fraction β, another control parameter of the membrane. The present conclusion can be useful for the design of the PEM fuel cell when its application on the dynamic mobile system is concerned.     

Experimental study on water transport in membrane humidifiers for polymer electrolyte membrane fuel cells

This paper presents an experimental setup for the measurement of water transfer in membrane humidifiers for automotive polymer electrolyte membrane (PEM) fuel cells at different process conditions. This setup was used to determine steady-state water permeation through perfluorinated sulfonic acid (PFSA)-based polymer membranes. The process conditions were varied within a relative humidity in the feed stream of RH = 30–90 %, absolute pressures of p = 1.25–2.5 bar, and temperatures of T = 320–360 K. The examined membranes are Nafion® membranes of different thicknesses (Nafion® 211, 212 and 115) and an experimental composite membrane manufactured by W. L. Gore & Associates. It was found that the overall water permeance is affected by both the mass transfer resistance of the membrane and the resistances in the boundary layers of the adjacent gas streams. The overall permeance is a strong function of water activity, with high levels of relative humidity showing the highest overall permeance. The absolute pressure only affects the overall permeance by affecting the diffusion in the boundary layers. Lower pressures are preferable for high overall water permeances. Increasing temperatures favor diffusion in the membrane and the boundary layers but lead to lower sorption into the membrane. The thicker Nafion® membranes show lower overall permeance at higher temperatures, while the overall permeance of the composite membrane shows no dependency on the temperature. Investigation of membrane humidifiers in counter-, co-, and cross-flow shows that the flow configuration in our setup has very little impact on the water flux in the humidifier.     

Numerical investigation of the optimal Nafion ionomer content in cathode catalyst layer: An agglomerate two-phase flow modelling

A two dimensional, across the channel, isothermal, two-phase flow model for a proton exchange membrane fuel cell is presented. Reactant transport in porous media, water phase transfer and water transport through the membrane are included. The catalyst layer is modelled as a spherical agglomerate structure. Liquid water occupies the secondary pores of the cathode catalyst layer to form a liquid water coating surrounding the agglomerate. The thickness is calculated by coupling the two-phase flow model with the agglomerate model. Ionomer swelling is associated with the non-uniform distribution of water in the ionomer determined from several processes occurring simultaneously, namely (1) water phase transfer between the vapour, dissolved and liquid water; (2) membrane/ionomer water content depending on the water vapour pressure; (3) a water film covering the catalyst agglomerate; (4) water transport through the membrane via electro-osmotic drag, back diffusion and hydraulic permeation. The model optimises the initial dry ionomer content in the cathode catalyst layer. The simulation results indicate that, to achieve the best fuel cell performance, the initial dry ionomer volume fraction should be controlled around 10%, corresponding to 0.3 mg cm⁻². By considering the effect of ionomer swelling on the reduction in CCL porosity and the increase in oxygen mass transport resistance, the accuracy of the model prediction is improved, especially at higher current densities.     

Bi-modal water transport behavior across a simple Nafion membrane

The development of predictive mathematical models for water management in polymer electrolyte membrane fuel cells requires detailed understanding of water distribution and water transport across the Nafion layer. The anisotropic microstructure of Nafion suggests the measurement of water content and mass transport should be along the fuel cell functional direction, i.e. across the membrane. Non-invasive, high resolution, microscopy measurements of this type are very challenging.We report here the calibration of a minimal mathematical model for diffusive water transport in Nafion against data from high-resolution water content maps determined with a new magnetic resonance imaging methodology developed for this purpose. A mock fuel cell was designed to permit well-controlled wetting and drying boundary conditions. With no chemical potential driving force involved, we assume the water transport behavior will be dominated by diffusion. Moreover we show that, in this context, our model is mathematically equivalent to the traditional permeation models based upon saturation dependent pressure gradients via a capillary pressure ansatz.The non-linear equilibrium water distribution across the Nafion membrane measured in this work suggests a bi-modal diffusivity. The model constructed associates distinct transport behaviors to water contents above and below a critical threshold, consistent with a rearrangement of a micro-structural pore network. The experimental observation and the model prediction agree with the primary features of Weber's model of Nafion, which predicts distinct modes of transport for hydration fronts traversing the through-plane direction of the membrane.     

Introducing a novel technique for measuring hydrogen crossover in membrane-based electrochemical cells

Hydrogen crossover that is the unwanted hydrogen permeation across the membrane driven by the difference of gas concentrations causes a critical concern of safety and efficiency for electrochemical cells, such as fuel cells and electrolyzer cells. Although the hydrogen crossover measurement in fuel cells that employ platinum based catalysts is simple and widely used in laboratory settings, it is questionable to apply existing limiting current method to water electrolyzer cells and alkaline exchange membrane (AEM) systems, which is due to the typical catalyst materials used and membrane properties, respectively. In this work, we demonstrate the operation of a compact and low-cost method of measuring hydrogen crossover that works for both AEM and proton exchange membrane (PEM) systems. The method entails a tandem configuration that utilizes an upstream crossover cell with a downstream cell in hydrogen pump configuration to measure the crossover in the cell of interest. We have successfully measured the hydrogen crossover with different membranes at various differential pressures. The developed method can be applied to catalyst-free membranes (both PEM and AEM) as well as PGM free catalyst containing cells. It will be a promising technique for measuring hydrogen crossover in-situ for a real operating membraned-based electrochemical cell or stack.     

The impact of thermal conductivity and diffusion rates on water vapor transport through gas diffusion layers

Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively.     

Multi-dimensional liquid water transport in the cathode of a PEM fuel cell with consideration of the micro-porous layer (MPL)

A multi-dimensional two-phase PEM fuel cell model, which is capable of handling the liquid water transport across different porous materials, including the catalyst layer (CL), the micro-porous layer (MPL), and the macro-porous gas diffusion medium (GDM), has been developed and applied in this paper for studying the liquid water transport phenomena with consideration of the MPL. Numerical simulations show that the liquid water saturation would maintain the highest value inside the catalyst layer while it possesses the lowest value inside the MPL, a trend consistent qualitatively with the high-resolution neutron imaging data. The present multi-dimensional model can clearly distinguish the different effects of the current-collecting land and the gas channel on the liquid water transport and distribution inside a PEM fuel cell, a feature lacking in the existing one-dimensional models. Numerical results indicate that the MPL would serve as a barrier for the liquid water transport on the cathode side of a PEM fuel cell.     

Innovative gas diffusion layers and their water removal characteristics in PEM fuel cell cathode

Liquid water transport is one of the key challenges regarding the water management in a proton exchange membrane (PEM) fuel cell. Conventional gas diffusion layers (GDLs) do not allow a well-organized liquid water flow from catalyst layer to gas flow channels. In this paper, three innovative GDLs with different micro-flow channels were proposed to solve liquid water flooding problems that conventional GDLs have. This paper also presents numerical investigations of air–water flow across the proposed innovative GDLs together with a serpentine gas flow channel on PEM fuel cell cathode by use of a commercial computational fluid dynamics (CFD) software package FLUENT. The results showed that different designs of GDLs will affect the liquid water flow patterns significantly, thus influencing the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown. Several gas flow problems for the proposed different kinds of innovative GDLs were observed, and some useful suggestions were given through investigating the flow patterns inside the proposed GDLs.     

Numerical investigations of effect of membrane electrode assembly structure on water crossover in a liquid-feed direct methanol fuel cell

A two-phase mass-transport model is employed to investigate the water transport behaviour through the membrane electrode assembly (MEA) of a liquid-feed direct methanol fuel cell (DMFC). Emphasis is placed on examining the effects of each constituent component design of the MEA, including catalyst layers, microporous layers and membranes, on each of the three water crossover mechanisms: electro-osmotic drag, diffusion, and convection. The results show that lowering the diffusion flux of water or enhancing the convection flux of water (termed as the back-flow flux) through the membrane are both feasible to suppress water crossover in DMFCs. It is found that the reduction in the diffusion flux of water can be mainly achieved through optimum design of the anode porous layers, as the effect of the cathode porous region on water crossover by diffusion is relatively smaller. On the other hand, the design of the cathode porous layers plays a more important role in increasing the back-flow flux of water from the cathode to anode.     

Ex situ characterization and modelling of fatigue crack propagation in catalyst coated membrane composites for fuel cell applications

Interactions between catalyst layers and membrane are known to influence the mechanical properties of catalyst coated membrane (CCM) composites used in fuel cells, and can further affect their fatigue-driven mechanical fracture — an important lifetime-limiting failure mode in automotive applications. Here, the fracture propagation phenomenon in CCMs is characterized through a series of ex situ experiments and microstructural investigations conducted across a range of stress, temperature (23-70 °C), and relative humidity (50–90%) conditions relevant to low-temperature polymer electrolyte fuel cells. In comparison to pure membranes, the crack propagation rates are slightly arrested in CCMs through mechanical reinforcement offered by the catalyst layers; however, the membrane layer still controls the overall crack growth trends through its temperature and humidity dependent ductile fracture characterized by confined yielding around the fracture surface. Local interfacial delamination and severe electrode cracking are found to accompany the CCM crack propagation, which aids membrane fracture by loss of local reinforcement. A Paris law based fracture modelling framework, incorporating the elastic-viscoplastic mechanical response of CCMs, is developed to semi-analytically evaluate one-dimensional crack growth rate during cyclic loading, and provides reasonably accurate predictions for the present ex situ problem.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.