Magnetic Resonance Imaging of Water in Operating Polymer Electrolyte Membrane Fuel Cells

Water management in polymer electrolyte membrane fuel cells (PEMFCs) is extremely important for the high performance and durable operation of fuel cells. Therefore, fundamental understanding of water transport involved in operating PEMFCs is necessary. This article presents a review of in situ magnetic resonance imaging (MRI) visualisation of water in operating PEMFCs, which is recognised as a powerful diagnostic tool for probing water behaviours, both in flow fields and in the membrane electrode assembly (MEA). The basic principles and hardware related to MRI visualisation are described with emphasis on the design, construction and material selection of a PEMFC for MRI experiments. The MRI results reported by several groups are outlined to illustrate the versatility and potential usefulness of in situ visualisation of water in operating PEMFCs using MRI.     

Current Advances in Polymer Electrolyte Fuel Cells Based on the Promotional Role of Under‐rib Convection

Literature data on the promotional role of under‐rib convection for polymer electrolyte fuel cells (PEFCs) fueled by hydrogen and methanol are structured and analyzed, thus providing a guide to improving fuel cell performance through the optimization of flow field interaction. Data are presented for both physical and electrochemical performance showing reactant mass transport, electrochemical reaction, water behavior, and power density enhanced by under‐rib convection. Performance improvement studies ranging from single cell to stack are presented for measuring the performance of real operating conditions and large‐scale setups. The flow field optimization techniques by under‐rib convection are derived from the collected data over a wide range of experiments and modeling studies with a variety of components including both single cell and stack arrangements. Numerical models for PEFCs are presented with an emphasis on mass transfer and electrochemical reaction inside the fuel cell. The models are primarily used here as a tool in the parametric analysis of significant design features and to permit the design of the experiment. Enhanced flow field design that utilizes the promotional role of under‐rib convection can contribute to commercializing PEFCs.     

Morphological Analyses of Polymer Electrolyte Fuel Cell Electrodes with Nano‐Scale Computed Tomography Imaging

We report a three‐dimensional (3D), pore‐scale analysis of morphological and transport properties for a polymer electrolyte fuel cell (PEFC) catalyst layer. The 3D structure of the platinum/carbon/Nafion electrode was obtained using nano‐scale resolution X‐ray computed tomography (nano‐CT). The 3D nano‐CT data was analyzed according to several morphological characteristics, with particular focus on various effective pore diameters used in modeling gas diffusion in the Knudsen transition regime, which is prevalent in PEFC catalyst layers. The pore diameter metrics include those based on chord length distributions, inscribed spheres, and surface area. Those pore diameter statistics are evaluated against computational pore‐scale diffusion simulations with local gas diffusion coefficients determined from the local pore size according to the Bosanquet formulation. According to our comparison, simulations that use local pore diameters defined by inscribed spheres provide effective diffusion coefficients that are consistent with chord‐length based estimations for an effective Knudsen length scale. By evaluating transport rates in regions of varying porosity within the nano‐CT data, we identified a Bruggeman correction scaling factor for the effective diffusivity.     

Multidimensional Modelling of Polymer Electrolyte Fuel Cells under a Current Density Boundary Condition

A three‐dimensional numerical model of the polymer electrolyte fuel cell (PEFC) is applied to study current distribution and cell performance under a current density boundary condition. Since the electronic resistance in the along‐channel direction in the current collector plate is much larger than in the other two directions, i.e., 50 mΩ cm² vs. 0.5 mΩ cm², it significantly affects current flow, and current and cell voltage distributions in a PEFC. An identical polarization curve results with two different boundary conditions, constant cell voltage and constant current density, however, the current density profiles in the along‐channel direction differ significantly; it is much flatter for the constant current boundary condition. Increasing the electronic conductivity of the bipolar plate diminishes the difference in the current density distribution under the two boundary conditions. The results also point out that an experimental validation of a PEFC model based on the polarization curve alone is insufficient, and that detailed current density distribution data in the along‐channel direction is essential.     

Novel Graphitised Carbonaceous Materials for Use as a Highly Corrosion‐Tolerant Catalyst Support in Polymer Electrolyte Fuel Cells

Cathode catalysts for polymer electrolyte fuel cells (PEFCs) are prepared by depositing Pt nanoparticles on carbon nanospheres (CNSs) and graphitised carbon nanospheres (GCNSs), and their corrosion‐tolerance and electrocatalytic activities for the oxygen reduction reaction are evaluated. Transmission electron micrographs show that the deposited Pt nanoparticles are well dispersed on CNSs. In Pt/GCNS, Pt nanoparticles accumulate selectively along the edges of GCNSs' polygonal surfaces. Electrochemical measurements with a rotating‐ring disk electrode in an O₂‐saturated H₂SO₄ solution show that Pt/GCNS and Pt/CNS produce less H₂O₂ during oxygen reduction, compared to that obtained with a Pt catalyst on carbon black (CB). Thermogravimetric analysis reveals that GCNSs show greater combustion‐tolerance than CNSs and CB. Furthermore, GCNSs show excellent electrochemical corrosion‐tolerance in a H₂SO₄ solution. These results indicate that GCNSs are superior for use as carbon supports, and can serve as cathode catalysts in PEFCs even under oxidative conditions.     

Impact of Compression on Effective Thermal Conductivity and Diffusion Coefficient of Woven Gas Diffusion Layers in Polymer Electrolyte Fuel Cells

The contrasting effect of compression on the ability of gas diffusion layer (GDL) in polymer electrolyte membrane fuel cell to conduct fluid, heat and electron implies that there is an optimal clamping force for cell performance. For a given GDL, understanding its associated optimal compression needs to know how its conductive ability changes with compressive pressure. In this paper we investigated the impact of compression on the effective diffusion coefficient and thermal conductivity of a carbon‐cloth GDL. The interior microstructures of the GDL under different compressions were acquired using X‐ray tomography; microscopic models were then developed to simulate gas diffusion and heat transfer in the microstructures in both in‐plane and through‐plane directions. The effective diffusion coefficient and thermal conductivity were calculated by volumetrically averaging the simulated gas diffusive and thermal flux rates at micron scale. The results show that both effective diffusion coefficient and thermal conductivity were anisotropic and their values in the in‐plane direction were higher than in the through‐plane direction. With porosity decreasing under the compression, the effective diffusion coefficient decreased faster in the through‐plane direction than in the in‐plane direction; the formula derived by Nam and Kaviany was capable of describing the change of the effective diffusion coefficient with porosity in the in‐plane direction but not in the through‐plane direction. For heat transfer, as the porosity decreased, the thermal conductivity increased faster in the through‐plane direction than in the in‐plane direction, and the increase in both directions could fit to the formula of Das et al.     

Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore‐Network Models

Water management remains a critical issue for polymer electrolyte fuel cell performance and durability, especially at lower temperatures and with ultrathin electrodes. To understand and explain experimental observations better, water transport in gas diffusion layers (GDLs) with macroscopically heterogeneous morphologies was simulated using a novel coupling of continuum and pore‐network models. X‐ray computed tomography was used to extract GDL material parameters for use in the pore‐network model. The simulations were conducted to explain experimental observations associated with stacking of anode GDLs, where stacking of the anode GDLs increased the limiting current density. Through imaging, it is shown that the stacked anode GDL exhibited an interfacial region of high porosity. The coupled model shows that this morphology allowed more efficient water movement through the anode and higher temperatures at the cathode compared to the single GDL case. As a result, the cathode exhibited less flooding and hence better low temperature performance with the stacked anode GDL.     

Performance Comparison of Pt1–xPdx/Carbon Nanotubes Catalysts in Both Electrodes of Polymer Electrolyte Membrane Fuel Cells

The catalytic activity of Pt_1–xPd_x nanoparticles supported on carbon nanotubes (CNTs) was evaluated for both the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs). Using a colloidal method, Pt_1–xPd_x/CNTs catalysts (x = 0, 0.46, 0.76, and 0.9) were prepared, and their physical and electrochemical characteristics were analyzed using a variety of characterization techniques, including XRD, TEM, energy dispersive spectrometer, cyclic voltammetry, and electrochemical impedance spectroscopy. Both Pt and Pd atoms formed a continuous solid solution and thus were randomly mixed in Pt_1–xPd_x nanoparticles. Due to the high hydrogen absorption of Pd, the use of Pd in the catalyst provided an advantage for HOR but a disadvantage for ORR. The Pt_0.53Pd_0.47/CNTs catalyst in the anode and cathode showed the best cell performance of PEMFCs.     

Non–Pt Catalyst Layer Operation in a PEM Fuel Cell: A Variable‐thickness Regime

A recently pubilshed experimental polarization curve of a PEM fuel cell with the non–Pt cathode catalyst layer (CCL) exhibits unusual feature: in the region of small current densities, the curve is close to linear. We report a model for the CCL performance which explains this effect. The model includes finite rate of the oxygen adsorption on the catalyst surface. Qualitatively, due to a very high exchange current density of the non–Pt catalyst, the ORR rate close to the membrane is determined by the potential–independent oxygen adsorption rate. This leads to a specific regime of the CCL operation, when only part of the CCL thickness contributes to current production, while the rest part is completely inactive. With the growth of the cell current, the active part increases in width, while the inactive part shrinks. The resulting polarization curve appears to be close to linear.     

PBI‐Based Polymer Membranes for High Temperature Fuel Cells – Preparation,Characterization and Fuel Cell Demonstration

Proton exchange membrane fuel cell (PEMFC) technology based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation above 100 °C is summarized and discussed. As one of the successful approaches to high operational temperatures, the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochemical characterization and fuel cell testing. A high temperature PEMFC system, operational at up to 200 °C based on phosphoric acid‐doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, has been demonstrated. A lifetime of continuous operation, for over 5000 h at 150 °C, and shutdown‐restart thermal cycle testing for 47 cycles has been achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, associated problems and further development are discussed.     

Thermo‐Mechanical Response of Fuel Cell Electrodes: Constitutive Model and Application in Studying the Structural Response of Polymer Electrolyte Fuel Cell

The characterization of the mechanical properties of fuel cell electrodes through the experimental techniques is a complex task due to the low thickness, constituents' heterogeneous composition, and fragile nature of the film. We present a preliminary investigation on the thermomechanical response of fuel cell catalyst layer (CL) obtained through the numerical experiment. Since the Nafion ionomer is one of the constituents' of the CL, a modified micromechanically motivated viscoplastic model is adopted to characterize the Nafion ionomer in terms of reduced density factor to account for the void content. The catalyst agglomerates are taken as inclusions in the ionomer matrix to form a composite unit which is used to plot the true stress–true strain response. Practicality of this work is tested by implementing the electrode layer as a separate component in the single fuel cell unit cell model. A remarkable difference in the magnitude of stress levels in the membrane is observed under thermal and hydrated conditions with the presence and absence of electrode layer in the simulation domain. The present work will assist in improved understanding of the localized stress distribution in the membrane, which is essential to understand its mechanical endurance.     

Doped Germanate‐Based Apatites as Electrolyte for Use in Solid Oxide Fuel Cells

Apatite ceramics, known for their good electrical conductivities, have garnered substantial attention as an alternative electrolyte for solid oxide fuel cells (SOFCs). However, studies focusing on the electrochemical performances of SOFCs with apatities as electrolytes remain rare, partly due to their high sintering temperature. In this study, the effects of Mg²⁺, Al³⁺, Ga³⁺, and Sn⁴⁺ dopants on the characteristics of La_9.5Ge₆O_{26 ± δ} are examined and their potential for use as SOFC electrolytes evaluated. The results indicate that La_9.5Ge_5.5Al_0.5O₂₆ is stabilized into a hexagonal structure, while the La_9.5Ge_5.5Sn_0.5O_26.25, La_9.5Ge_5.5Ga_0.5O₂₆, and La_9.5Ge_5.5Mg_0.5O_25.75 ceramics reveal triclinic cells accompanied with the second phase La₂Sn₂O₇ or La₂GeO₅. The study further demonstrates that a high sintering temperature is needed for both the La_9.5Ge_5.5Mg_0.5O_25.75 and the La_9.5Ge_5.5Sn_0.5O_26.25 ceramics, and the worst electrical conductivity among the examined systems appears in the La_9.5Ge_5.5Ga_0.5O₂₆ ceramic. The La_9.5Ge_5.5Al_0.5O₂₆ ceramic is accordingly selected for cell evaluation due to its ability to reach densification at 1,350 °C, its good electrical conductivity of 0.026 S cm^–1 at 800 °C, and its acceptable thermal expansion coefficient of 10.1 × 10^–6 K^–1. The maximum power densities of the NiO‐SDC/La_9.5Ge_5.5Al_0.5O₂₆/LSCF‐SDC single cell are found to be respectively 0.22, 0.16, 0.11, and 0.07 W cm^–2 at 950, 900, 850, and 800 °C.     

Dynamic Response of a Subsaturated Polymer Electrolyte Fuel Cell in Counterflow Mode

This work demonstrates that the operation of a subsaturated polymer electrolyte fuel cell in counterflow mode results in a significantly elongated relaxation time after a load change, if compared to coflow mode. This effect is investigated here by using combined dynamic locally resolved measurements of the current density, the high frequency resistance, and the relative humidity. It is shown that the elongated relaxation time is a consequence of slow membrane hydration in the region of the cell, downstream the anode flow field, where the diffusive flux of water across the membrane occurs from the anode to the cathode. Here, the anode gas stream, which is humidified upstream the anode flow field via back diffusion of water from the cathode to the anode, is the only source of water for both membrane hydration and the internal humidification of the cathode gas stream, which passes the cell in opposite direction.     

Effects of Microstructural Functional Layers on Flat‐Tubular Solid Oxide Fuel Cells

The functional layer of a flat‐tubular solid oxide fuel cell (SOFC) is examined using a three‐dimensional microscale electrode model. SOFC electrodes essentially include two types of layers: a structural layer and a functional layer. The structural layers, which are the anode support layer and the cathode current collector layer, are composed of large particles with a high porosity that facilitates gas diffusion. The functional layers consist of small particles with a low porosity that increases the triple phase boundary (TPB) reaction area and reduces the activation overpotential. In the model, the particle diameter and functional layer thickness are adjusted and analyzed. The effects of the two parameters on the performance of the functional layer are monitored in the contexts of several multilateral approaches. Most reactions occurred near the electrode–electrolyte interface; however, an electrode design that included additional TPB areas improved the electrode performance. The role of the functional layer in a flat‐tubular SOFC is examined as a function of the functional layer particle size and thickness. The performance of a cell could be enhanced by preparing a functional layer using particles of optimal size and thickness, and by operating the device under conditions optimized for these parameters.     

CO Tolerance of Commercial Pt and PtRu Gas Diffusion Electrodes in Polymer Electrolyte Fuel Cells

The CO tolerance of commercial Pt and PtRu anode electrodes from different suppliers (E‐Tek and Tanaka) has been examined in polymer electrolyte fuel cells (PEFC) using AC‐impedance spectroscopy along steady‐state current‐voltage curves. A simple mathematical model has been derived in order to extract important kinetic parameters for CO poisoning on different anode electrodes. The Tanaka PtRu (40:60) electrode demonstrated the best CO tolerance under the selected operating conditions. Inductive behavior in the low frequency region of the impedance spectra for the E‐Tek Pt and PtRu electrode proved to be characteristic for CO poisoning. However, the impedance spectra of the Tanaka PtRu electrode did not show any inductive behavior and its CO surface coverage, extracted by fitting the experimental data to the model, was lower than the surface CO coverage of the E‐Tek electrodes.     

In situ Synchrotron X‐ray Radiography Investigations of Water Transport in PEM Fuel Cells

Water transport in an operating PEM fuel cell was investigated with synchrotron X‐ray radiography with a spatial resolution of 3 μm and a temporal resolution of 5 s. This method allows for the detection of water accumulations with less than 10 μm diameter. We demonstrate that synchrotron X‐ray imaging can dramatically expand the possibilities of imaging with high spatial and time resolution, especially as a complement to neutron radiography. Water transport processes from the first appearance of small water accumulations in the gas diffusion layer to their transport into the channel system were analysed in situ. Correlations between local effects such as water formation and operating conditions of the whole system, e.g. power variations, were found. A recently described eruptive water transport mechanism is analysed in detail.     

Molecular‐Level Investigation of Critical Gap Size between Catalyst Particles and Electrolyte in Hydrogen Proton Exchange Membrane Fuel Cells

Molecular dynamics simulations have been performed to study the structure and transport at the electrode/electrolyte interface in hydrogen‐based proton exchange membrane fuel cells. We examine the wetting of catalyst surfaces that are not immediately adjacent to a Nafion membrane, but rather are separated from the membrane by a hydrophobic gap of carbon support surface (graphite). A mixture of Nafion, water and hydronium ions is able to wet small gaps (7.4 Å) of graphite and reach the catalyst surface, providing a path for proton transport from the catalyst to the membrane. However, for gaps of 14.8 Å, we observe no wetting of the graphite or the catalyst surface. Using a coarse‐grained model, we found that the presence of a graphite gap of 7.4 Å width slowed down the transport of water by at least an order of magnitude relative to a system with no gap. The implication is that catalyst particles that are not within nominally 1 nm of either the proton exchange membrane or recast ionomer in the electrode leading to the membrane do not possess a path for efficient proton transport to the membrane and consequently do not contribute significantly to power production in the fuel cell.     

Electrochemical Evaluation of Porous Non‐Platinum Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells

A porous non‐platinum electrocatalyst for the oxygen reduction reaction (ORR), obtained by pyrolysing a cobalt porphyrin precursor, was evaluated by electrochemical means. The reactivity of the non‐platinum ORR catalyst was investigated with a rotating disc electrode (RDE) experimental set up. RDE data were collected in an acidic electrolyte containing N₂, O₂, CO and under mixed reactant O₂/methanol conditions. The electrochemical performance of such‐obtained non‐platinum catalyst is discussed and compared to platinum‐based ORR catalysts. Based on the results collected here, we are able to propose and test possible proton exchange fuel cell (PEFC) operating conditions where non‐platinum ORR catalysts can be utilised. Direct methanol fuel cell (DMFC) data demonstrating a superior performance of the non‐platinum catalyst relative to platinum black, often perceived as the state‐of‐the‐art oxygen–reduction catalyst for the DMFC cathode is presented.     

Numerical and Experimental Verification of the Polymer Electrolyte Fuel Cell Performances Enhanced by Under‐Rib Convection

Understanding the current density distributions in polymer electrolyte fuel cells (PEFCs) is crucial for designing cell components, such as the flow field of bipolar plates. A new serpentine flow field equipped with sub‐channels and by‐passes (SFFSB) was numerically and experimentally confirmed to enhance the reactant transport rates and liquid removal efficiency compared with a conventional advanced serpentine flow field (CASFF). Consequently, the maximum current and the power densities of the SFFSB were increased due to the promotion of under‐rib convection. In this study, current density distributions are measured under transient conditions to verify the PEFC performances enhanced by under‐rib convection. The current density distributions of the SFFSB are compared with those of the CASFF. The results show that the SFFSB has a higher local current density and a more uniform distribution than the CASFF, therefore, the PEFC performances with the new flow field of SFFSB is enhanced by the better current density distributions.     

Water Free Operated Phosphoric Acid Doped Radiation‐Grafted Proton Conducting Membranes for High Temperature Polymer Electrolyte Membrane Fuel Cells

The present study uses the radiation‐induced grafting method and applies it onto poly(ethylene‐alt‐tetrafluoroethylene) (ETFE) for the synthesis of proton‐exchange membranes by using monomers 4‐vinyl pyridine (4VP), 2‐vinyl pyridine (2VP), N‐vinyl‐2‐pyrrolidone (NVP) followed by phosphoric acid doping. Phosphoric acid that provides Grotthuss mechanism in proton mobilization is used to transform the graft copolymers to a high temperature membrane state. Resultant proton‐exchange membranes are verified with their proton conductivity, water uptake, mechanical and thermal properties, and phosphorous distribution as ex situ characterization. Our most important finding as a novelty in literature is that ETFE‐g‐P4VP phosphoric acid doped proton‐exchange membranes exhibit proton conductivities as 66 mS cm^–1 at 130 °C, 53 mS cm^–1 at 120 °C, 45 mS cm^–1 at 80 °C at RH 100% and 55 mS cm^–1 at 130 °C, 40 mS cm^–1 at 120 °C, 35 mS cm^–1 at 80 °C at dry conditions. Moreover, ETFE‐g‐P4VP membranes still conserves the mechanical properties, i.e., tensile strength up to 48 MPa. ETFE‐g‐P4VP membranes were tested in PEMFC at 80, 100, and 120 °C and RH <2% and exhibit promising performance as an alternative to commercial Nafion® membranes. The single cell testing performance of ETFE‐g‐P4VP membranes is presented for the first time in literature in our study.