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This work describes the effects of catalyst layers (CLs) consisting of hydrophobic PTFE on the performance and water management of PEM fuel cells. Catalyst inks with various PTFE contents were coated on Nafion membranes and characterized using contact angle measurements, SEX-EDX, and mercury porosimetry. Fuel cell tests and electrochemical impedance spectroscopy (EIS) were conducted under varying operating conditions for the prepared materials. At dry conditions, CLs with 5 wt.% PTFE were advantageous for cell performance due to improved membrane hydration, whereas under humid conditions and high air flow rates CLs with 10 wt.% PTFE improved the performance in high current density region. Higher PTFE contents (≥20 wt.%) increased the mass transport resistance due to reduced porosity of the CLs structure. Operando neutron radiography was utilized to study the effects of hydrophobicity gradients within CLs and cathode microporous layer (MPLC) on liquid water distribution. More hydrophobic CLs increased the water content in adjacent layers and improved performance, especially at dry conditions. MPLC with higher PTFE contents increased the overall liquid water within the CLs and GDLs and escalated the water transfer to the anode side. Furthermore, the role of back-diffusion transport mechanism on water distribution was identified for the investigated cells. [ABSTRACT FROM AUTHOR]

The influence of hydrophobicity and porosity of the catalyst layer (CL) and cathode microporous layer (MPLC) on water distribution and performance of polymer electrolyte membrane fuel cell (PEMFC) is investigated. Hydrophobicity of the layers is altered with the addition of PTFE (polytetrafluoroethylene) and mono‐dispersed polymer particles are utilized to introduce the macro‐pores with a diameter of 0.5 µm and 30 µm within the CL and MPLC, respectively. The treated materials are implemented in a specially designed fuel cell with an active area of 8 cm2 to perform operando high‐resolution neutron tomography measurements. At high current density and humid operating conditions, MPLs with higher PTFE content increase the overall water content of the cell. The more hydrophobic MPL (40 wt.% PTFE) performs below the corresponding reference MPL (20 wt.% PTFE), whereas the performance result of double layer MPLC gives hint for further potential improvements of such design. The local water saturation beneath the land regions with the presence of perforated CL and MPLC is increased which is explained by lower capillary pressure barriers of bigger pores. Despite a higher water content, the perforated layers enhance the performance of the cell at both dry (RH 70%) and humid conditions (RH 120%), indicating that the parallel two‐phase flow is facilitated where the oxygen is transported through small pores and the water is preferentially transported through the bigger pores. [ABSTRACT FROM AUTHOR]

Liquid water saturation of pores in the gas diffusion layer (GDL) and the catalyst layer (CL) of polymer electrolyte membrane fuel cells (PEMFC) hinders the transport of the reactant gases, leading to inhomogeneous current density distribution, reduced overall cell performance, and accelerated material degradation. This effect restricts the PEMFC operation, specifically at high current densities. In this study, the simulation results illustrate how the interplay of wettability and pore sizes influences the water distribution within the CL, microporous layer (MPL) of the GDL, and specifically their interface. The liquid water distribution within the porous material is studied employing a voxel-based Monte Carlo (MC) model reflecting the effect of local thermodynamic boundary conditions and inner surface characteristics. Real material structures obtained with a focused ion beam - scanning electron microscope (FIB-SEM) are employed. Local temperature and relative humidity values required as inputs are obtained from sophisticated computational fluid dynamics (CFD) simulations comprising all relevant effects, including the electrochemical reactions. The results show that avoiding drastic changes in wettability at the CL-MPL interface can help to mitigate the possible detrimental water accumulations. Further developing and exploiting this study will contribute to facilitate the systematic material development for better PEMFC performance and durability. [ABSTRACT FROM AUTHOR]

Abstract: Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models. [Copyright &y& Elsevier]

Abstract: Handling of water accumulation is still a key issue in fuel cell research. The presented study evaluates the condensate removal capability of three different flow field designs. The designs are compared regarding cell voltage at different current densities using the same operating conditions. The investigated type of meander-shaped channels with a high degree of parallelization shows the best performance with stationary water thickness inside channels throughout the analyzed current densities. To develop and evaluate a condensate removal criterion for fuel cell construction, the pressure drop of each flow field is correlated to the water appearance visualized with neutron radiography. For a further insight, computational fluid dynamics simulation is used to calculate pressure drops present inside the characteristic regions of each flow field. Thus, a characteristic design limit of 20 mbar m−1 for meander-shaped channels is proven to ensure the absence of channel blockage. The meander-shaped channels show specific pressure drops around this limit depending on water production and gas supply. The two other analyzed flow fields suffer from higher channel filling rates: the investigated straight channels with less parallelization fill up with time, while the pattern-structured flow field demonstrates gravity as an additional influence on condensate removal. [Copyright &y& Elsevier]

Abstract: In this study, the influence of modifications of the porous transport layer (PTL) on the water management in polymer electrolyte membrane fuel cells is investigated. Laser perforation and milling are used to locally remove either the whole PTL or only the micro porous layer in the PTL. The changed liquid water distribution is visualized using synchrotron radiography and ESEM liquid water imaging. The observed effects are correlated with in-situ performance characteristics, pre-assembly and post-mortem analysis to examine on the beneficial and obstructive effects of the modifications. The analysis reveals that laser-perforation results in PTFE loss and hydrophilic regions which results in a good performance at dry conditions but serious flooding at high humidification conditions. Machined perforations show beneficial effects in the in-situ performance characteristics at high humidification conditions. A draining of the vicinity of the perforations is observed which results in an improved oxygen diffusivity. Water balancing and synchrotron visualization indicate that under the analyzed operating conditions the MPL increases the humidification of the ionomer on the one hand but creates an additional diffusion barrier by liquid water in the interface between the cathode catalyst layer and the MPL on the other hand. [Copyright &y& Elsevier]

Metal foam flow-fields have shown great potential in improving the uniformity of reactant distribution in polymer electrolyte fuel cells (PEFCs) by eliminating the 'land/channel' geometry of conventional designs. However, a detailed understanding of the water management in operational metal foam flow-field based PEFCs is limited. This study aims to provide the first clear evidence of how and where water is generated, accumulated and removed in the metal foam flow-field based PEFCs using in-operando neutron radiography, and correlate the water 'maps' with electrochemical performance and durability. Results show that the metal foam flow-field based PEFC has greater tolerance to dehydration at 1000 mA cm−2, exhibiting a ~50% increase in voltage, ~127% increase in total water mass and ~38% decrease in high frequency resistance (HFR) than serpentine flow-field design. Additionally, the metal foam flow-field promotes more uniform water distribution where the standard deviation of the liquid water thickness distribution across the entire cell active area is almost half that of the serpentine. These superior characteristics of metal foam flow-field result in greater than twice the maximum power density over serpentine flow-field. Results suggest that optimizing fuel cell operating condition and foam microstructure would partly mitigate flooding in the metal foam flow-field based PEFC. Image 1 • Neutron imaging applied to water management in metal foam flow-field based PEFC. • The metal foam flow-field enhances the uniformity of water distribution. • The metal foam flow-field improves the tolerance to dehydration. • The metal foam flow-field is susceptible to flooding at low current density. [ABSTRACT FROM AUTHOR]

Abstract In-depth understanding of water management is essential for the optimization of the performance and durability of polymer electrolyte fuel cells (PEFCs). Neutron imaging of liquid water has proven to be a powerful diagnostic technique, but it cannot distinguish between 'legacy' water that has accumulated in the system over time and 'nascent' water recently generated by reaction. Here, a novel technique is introduced to investigate the spatially resolved water exchange characteristics inside PEFCs. Hydro-electrochemical impedance imaging (HECII) involves making a small AC-sinusoidal perturbation to a cell and measuring the consequential water generated, using neutron radiographs, associated with the stimulus frequency. Subsequently, a least-squares estimation (LSE) analysis is applied to derive the spatial amplitude ratio and phase shift. This technique provides a complementary view to conventional neutron imaging and provides information on the source and 'history' of water in the system. By selecting a suitable perturbation frequency, HECII can be used to achieve an alternative image 'contrast' and identify different features involved in the water dynamics of operational fuel cells. Highlights • Hydro-electrochemical impedance imaging applied to water management of PEFC. • HECII distinguish between 'legacy' and 'nascent' water in the PEFC. • The perturbation frequency of HECII affects water dynamics features. [ABSTRACT FROM AUTHOR]

Abstract In-depth understanding of the effect of compression on the water management in polymer electrolyte fuel cells (PEFCs) is indispensable for optimisation of performance and durability. Here, in-operando neutron radiography is utilised to evaluate the liquid water distribution and transport within a PEFC under different levels of compression. A quantitative analysis is presented with the influence of compression on the water droplet number and median droplet surface area across the entire electrode area. Water management and performance of PEFCs is strongly affected by the compression: the cell compressed at 1.0 MPa demonstrates ∼3.2% and ∼7.8% increase in the maximum power density over 1.8 MPa and 2.3 MPa, respectively. Correlation of performance to neutron radiography reveals that the performance deviation in the mass transport region is likely due to flooding issues. This could be ascribed to the loss of the porosity and increased tortuosity factor of the gas diffusion layer under the land at higher compression pressure. The size and number of droplets formed as a function of cell compression was examined: with higher compression pressure, water droplet number and median droplet surface area rapidly increase, showing the ineffective water removal, which leads to fuel starvation and the consequent performance decay. Highlights • Mechanical compression affects water management in PEFCs. • Minimise compression for improved water management. • Increased compression leads to larger water droplet build-up. [ABSTRACT FROM AUTHOR]

Abstract: We present two different methods to increase the size of available neutron beams in order to allow for the investigation of large objects. Application of these methods is demonstrated for radiographic imaging of fuel cells. The first approach is a scanning procedure based on the coordinated translation of detector and sample through the beam. Further advancement was achieved by installing a focusing neutron guide, which offers an expanded neutron beam size after diverging from a focused point source. [Copyright &y& Elsevier]