Liquid Water Accumulation Research Articles

Neutron imaging of operando proton exchange membrane fuel cell with novel membrane

The performance of an alternative membrane based on aromatic multiblock copolymer was compared to that of the conventional perfluorosulfonic acid (PFSA) polymer membrane during fuel cell operation. The fuel cell with the alternative membrane exhibited more severe voltage loss than the conventional at current densities above 0.6 A/cm2 with fully saturated reactants. To rationalize this difference, operando high-resolution neutron imaging was used to investigate water distribution in the fuel cells, in parallel with impedance measurements. The type of membrane strongly affected water distribution, not only from anode to cathode, but also in-plane between ribs and channels. Specifically, in the alternative membrane, ionomer in cathode catalyst layer was less hydrated and dried as current density increased. The voltage loss with the alternative membrane was ascribed to ionomer drying rather than to the accumulation of liquid water, in accordance with the evolution of ohmic resistance. Furthermore, the alternative membrane experienced swell/shrinkage resulting in material shifts during the operation. These results pinpoint that developing new ionomers for fuel cells require not only to quantify the bulk transport properties of alternative membranes compared to PFSA, but also to evaluate their interfacial properties and the impact of ionomer on water management.

Numerical study on the impact of interface contact resistance on the performance of a PEMFC with serpentine flow field

The performance of a proton exchange membrane fuel cell (PEMFC) strongly relies on the flow-field design. In this study, a three-dimensional, multiphase, and non-isothermal PEMFC model with single serpentine flow field has been developed to investigate the effect of interface contact resistance (ICR) between the bipolar plate (BP) and gas diffusion layer (GDL) on the flow field dimension. In the cases of 25 different combinations of channel and rib width, polarization curves and power density curves of the PEMFC without ICR and with ICR of 7.418 mΩ.cm2 as well as 21 mΩ.2 are investigated. The results show that ICR has little effect on channel pressure drop but on liquid water accumulation. The value of ICR affects both the ohmic loss stage and concentration loss stage of the polarization curves. The power density of the PEMFC without ICR at the operating voltage 0.5V decreases as the rib width increases, while that with ICR increases as the rib width increases. Furthermore, the influence of invalid active area in BPs caused by flow field configuration on the cell performance should not be neglected when the ICR value is lower than 10 mΩ.2.

Effect of the porosity distribution on the liquid water transport in the gas diffusion layer of PEMFC

Water management significantly influences the life and power output of the proton exchange membrane fuel cell (PEMFC). The gas diffusion layer (GDL) provides key transport paths of reactant gas and liquid water in the PEMFC. This study uses the volume of fluid (VOF) method to numerically investigate the liquid water transport in the GDL with different porosity distributions. The GDL geometry is reconstructed based on the stochastic parameter method. Five GDL structures with different porosity distributions are investigated on the liquid water transport. And different contact angles are defined on the carbon fiber surface and their effect is discussed. The results show that the porosity distribution and contact angle have dramatic influence on the water distribution and saturation in the GDL. The liquid water accumulation tends to occur in the high porosity GDL region. The water saturation is higher and flow rate is lower in the GDL with inverted “V” shaped porosity distribution than “V” shaped porosity distribution. The water saturation and flow rate is smaller when the porosity gradient is greater for both the “V” and inverted “V” shaped porosity, due to fewer water penetration paths. The water saturation in the hydrophilic GDL is higher than in the hydrophobic GDL for all the five porosity distributions.

Investigation of Liquid Water Distribution under Temperature Gradient in PEFC Using in-situ X-ray Visualization

Polymer electrolyte fuel cells (PEFCs) are expected to be applied in wider range of applications including automobiles, and the operation at higher current density has been required. However, in the operation at high current densities, the performance of PEFCs is degraded due to liquid water accumulation in the gas diffusion layers (GDLs) which prevents the supply of oxygen to the catalyst. Furthermore, heat is generated in catalyst layer due to electrochemical reaction, leading temperature rise inside PEFCs. Since liquid water accumulation is closely related to temperature distribution, it is expected that temperature distribution alters the spots of liquid water accumulation. Therefore, it is necessary to understand the relationships between the liquid water distribution and the temperature distribution in the GDLs to improve the performance at high current density. This study intended to investigate the effects of temperature distribution on liquid water accumulation by in-situ X-ray visualization.Liquid water distribution of 60, 70, and 80 ℃ cell temperature cases were measured by X-ray radiography, and current density was kept 1.5 A/cm2 in all cases. We used a serpentine type cell with ribs and flow channels of 300 µm width with 1.5 x 3.0 mm2 active area. As for GDLs, SIGRACET 29BC was used. The humidity of hydrogen in the anode was fixed at 100%. While, the humidity of air in the cathode was adjusted so that the gap between water vapor pressure and saturation pressure becomes 4.0 kPa.Fig. 1 (a), (b) and (c) shows the liquid water distribution of the 60 ℃, 70 ℃, 80 ℃ cases. It is shown that the liquid water distribution in a region near the MPL/substrate boundary is clearly altered by the cell temperature. This effect is also observed in the averaged profiles of liquid water volume under rib areas (Fig. 1(d)). In Fig. 1(d), liquid water volume in the region near the MPL/substrate boundary (the region between 50 µm to 80 µm) of the 70 and 80 ℃ cases are clearly reduced compared to the 60 ℃ case. Since the overvoltage of each case is almost the same, the resultant temperature gradient inside GDLs due to the heat production is considered to be the same. Meanwhile, the saturated vapor pressure increases more abruptly at higher temperature, since the saturated vapor pressure is more affected by the initial cell temperature. The saturated vapor pressure of the 80 ℃ case is much larger than the other cases, therefore it is suggested that the lower amount of liquid water accumulation of the 80 ℃ case is caused by the larger saturated vapor pressure. Figure 1

Imaging and Quantification of Water inside Membrane-Electrode Assembly of Polymer Electrolyte Fuel Cell during Power Generation By Use of Neutron Beam

The polymer electrolyte fuel cells (PEFCs) have attracted attentions as energy-conversion devices of high power density with no emission of carbon dioxide. For the efficient and stable power generation of a PEFC, the water management inside the membrane-electrode assembly (MEA) is very important. A proper water management can adequately moisten the electrolyte membrane and improve the performance of a PEFC, while an excessive water can block the flow channels and the pores in gas diffusion layers (GDLs), as well as the catalyst layers; the mass transports are inhibited from the flow channels to the Pt catalysts resulting in a poor power generation. In case of large liquid water accumulations, the performance of the power generation is temporary decreased.1 Neutron imaging techniques have been used for understanding the distribution of liquid water inside PEFCs, and water droplets within the pores of the GDL and/or on the flow-channel walls have been mainly reported.2–4 However, the distribution of smaller amounts of water contained in catalyst layers or an electrolyte membrane has not been reported, which is very important because of the large effect on the mass transport inside the MEA.The visualization of water in an MEA was carried out using a pulse neutron beam at BL22 (RADEN) of MLF, J-PARC, Japan 5. The neutrons with the wavelength of 5-13.5 Å was separated and used to increase the attenuation of the neutron beam by water. A PEFC with 10 straight channels, 30 mm in length and 1 mm in width, was used. The endplates and current collectors were made of aluminum alloy (A7075) for the neutron penetration. The transmitted neutrons were converted to photons by a scintillator (ZnS(Li)) for imaging with a CCD camera. The neutron beam size was 102.4 mm ×102.4 mm (2048×2048 pixels), while a plate containing boron carbide (B4C) powders was used for a window of 50 mm × 50 mm to reduce the unpreferable radio activation of the cell. The cell temperature was kept at 80 oC. To obtain the background image, a dry N2 gas was supplied both to the anode and the cathode. Figures 1(a) and 1(b) show the background image under the dry condition and the image of water in the MEA at 100% RH (obtained by the division by the background image using an “ImageJ” software), respectively. The values at pixels aligned in the perpendicular direction in the area surrounded by yellow solid lines in Fig. 1(b) were integrated to be shown in Fig. 1(c). The neutron transmittance at the MEA shown by a red rectangle in Fig. 1(b) was lower than that in the other area. In this way, water in the MEA was detected by neutron imaging for the first time. The imaging of water during the power generation is now in progress, along with the qualification of water.Reference Hirakata, S. et al. Investigation of the effect of pore diameter of gas diffusion layers on cold start behavior and cell performance of polymer electrolyte membrane fuel cells. Electrochim. Acta 108, 304–312 (2013).Kramer, D. et al. In situ diagnostic of two-phase flow phenomena in polymer electrolyte fuel cells by neutron imaging: Part A. Experimental, data treatment, and quantification. Electrochim. Acta 50, 2603–2614 (2005).Turhan, A. et al. Quantification of liquid water accumulation and distribution in a polymer electrolyte fuel cell using neutron imaging. J. Power Sources 160, 1195–1203 (2006).Park, J. et al. Neutron imaging investigation of liquid water distribution in and the performance of a PEM fuel cell. Int. J. Hydrogen Energy 33, 3373–3384 (2008).Shinohara, T. et al. The energy-resolved neutron imaging system , RADEN. Rev. Sci. Instrum. 91, 043302 (2020). AcknowledgmentThis study was supported by Japan Science and Technology Agency (JST) and “SPer-FC Project” of New Energy and Industrial Technology Development Organization (NEDO). The neutron imaging was performed at BL22 (RADEN) of J-PARC (Proposal No. 2019A0217). Figure 1

Water Distribution in Fuel Cell Gas Channels Using a Mechanistic Discrete Particle Model

Water removal and accumulation in fuel cell gas channels is a complex phenomenon which is a consequence of the strength of the capillary forces and inertial forces (1 < Re < 1000). The presence of water in the channel restricts the access of oxygen to diffuse through the gas diffusion layers to the catalyst layer. The air supplied exchanges momentum to the droplets and water clusters, causing detachment when the adhesion force is overcome.Water emerging from discrete locations as droplets interact with both the channel walls and each discrete water cluster, depending on the configuration and operating conditions. Complexity increases as multiple detachment and coalescence events occur over large channel lengths and time scales. Modelling these systems with traditional method computational fluid dynamic (CFD) methods is computationally expensive so we have developed a new mechanistic discrete particle model to handle the liquid water two-phase flow.Taking inspiration from interface resolving volume of fluid (VoF) simulations of water flow in gas channels, we use analytically equations determined by the equilibrium contact angle in different configurations on the channel wall to predict detachment times and coalescence events. We compare the prediction of water area coverage ratio and channel saturation to the VoF simulations and can also estimate dynamic pressure drop.Using this model, we are able to investigate the dynamic effects of liquid water accumulation and distribution in the channel, with often periodic detachment events arising from the formation of liquid plugs. Consequently, the model is able to investigate the effect of the channel wall wettability, from hydrophilic to hydrophobic and the impact that has on the distribution and accumulation of water in the system.We compare the speed of this model against the VoF simulations to show that this type of modelling of two-phase flow in the channel can enable higher resolution accuracy when coupled with continuum scale approaches. Furthermore, with this approach it is possible to investigate the effect of: channel dimensions, liquid water flow rate and spatial location of liquid water emergence. This should allow for designed fuel cell structures to be evaluated. Figure 1

Investigation of Liquid Water Distribution under Temperature Gradient in PEFC Using in-situ X-ray Visualization

Polymer electrolyte fuel cells (PEFCs) are one of promising electric generating devices. It is necessary to understand the relationships between the water distribution and the temperature distribution in gas diffusion layers (GDLs) to improve the performance at high current density. To reveal the effects of cell temperature on liquid water accumulation inside PEFCs, in-situ X-ray measurement and numerical simulation were performed at 40, 60, 70, and 80°C. The measurement was carried out using a fuel cell having a serpentine-type flow field, ribs and flow channels of 300 μm width, and 1.5×3.0mm2 active area. Current density was kept 1.5 A/cm2 for 300 secs in all cases. Both the experimental results and the numerical results showed that the liquid water distribution in GDLs strongly depends on cell temperature. In the case of lower temperature, liquid water accumulated in a region near CL and MPL. While in the case of higher temperature, liquid water accumulated in a region near rib. In addition, the experimental results indicated that the vapor crossover depended on existence of liquid water in MPL, and it was shown that the amount of vapor crossover increased at lower temperature condition.

In Situ Measurement of Ionomer Water Content and Liquid Water Saturation in Fuel Cell Catalyst Layers by High-Resolution Small-Angle Neutron Scattering

In fuel cell catalyst layers (CLs), the ionomer plays roles as a binder and proton conductor. The ionomer requires hydration for enabling proton transport; however, the accumulation of excess liquid water in CL pores hinders oxygen diffusion. Therefore, a key to the improvement of CLs is understanding the distribution and the quantity of water in an operando fuel cell. However, there is a lack of information on the water content in conventional Pt-based CLs during operation, especially within the ionomer phase. Here, we present the application of high-resolution operando small-angle neutron scattering (SANS) on custom fuel cells to probe the CL water content. Using this technique, we quantified liquid water saturation and ionomer nanostructure (swelling) in in situ CLs, unique information not accessible via conventional diagnostic tools such as imaging. Even at a low current density (150 mA/cm2), water was observed in the cathode CL as liquid and adsorbed in the ionomer. Improving the understanding of water management in the CL is of high interest in fuel cell community, and our results confirm that operando SANS will be indispensable for the development of catalysts and ionomer-based CLs.

Using operando techniques to understand and design high performance and stable alkaline membrane fuel cells

There is a need to understand the water dynamics of alkaline membrane fuel cells under various operating conditions to create electrodes that enable high performance and stable, long-term operation. Here we show, via operando neutron imaging and operando micro X-ray computed tomography, visualizations of the spatial and temporal distribution of liquid water in operating cells. We provide direct evidence for liquid water accumulation at the anode, which causes severe ionomer swelling and performance loss, as well as cell dryout from undesirably low water content in the cathode. We observe that the operating conditions leading to the highest power density during polarization are not generally the conditions that allow for long-term stable operation. This observation leads to new catalyst layer designs and gas diffusion layers. This study reports alkaline membrane fuel cells that can be operated continuously for over 1000 h at 600 mA cm−2 with voltage decay rate of only 32-μV h−1 – the best-reported durability to date.

Impact of nonuniform reactant flow rate on the performance of proton exchange membrane fuel cell stacks

ABSTRACT In this study, a proton exchange membrane (PEM) fuel cell stack composed of five cells in series is numerically investigated to study the impact of the nonuniform reactant flow rate on the performance of the stack. A comparison of the water concentration, temperature, reaction heat source, and current density of change rule of two groups of fuel cell stacks with uniform and nonuniform reactant flow rate reveals the performance degradation mechanism caused by nonuniform reactant flow. The results indicate that while operating under low-voltage conditions, the nonuniform reactant flow rate will cause the accumulation of excess liquid water near the PEM that is near the cathode exhaust outlet, and the local area reacts strongly on the catalyst, whereas the local area reacts slowly. When the average voltage of the stack is 0.55 V, the current density under the nonuniform reactant flow rate condition is 12.9% lower than that of the uniform reactant flow rate condition. In the case of uniform and nonuniform reactant flow rate at low current densities, the performance difference is not evident, but it is expected to be pronounced with the increase in current density. The simulation results are compared with the experimental data reported in the literature through a polarization curve, and they turn out to be well correlated with the experimental results.

Experimental study on improving the dynamic characteristics of open-cathode PEMFC stack with dead-end anode by condensation and circulation of hydrogen

Water management is one of the most important issues for proton exchange membrane fuel cell stack. Liquid water accumulates in the stack may impede the transport of the reaction gas, resulting in unstable output performance and poor durability. In this study, Condensation mode and Condensation Circulation mode are proposed to reduce the accumulation of liquid water in the anode compartment, thus reducing the risk of flooding. Comparative research among the traditional dead-end anode (DEA) mode and presented modes are carried out on a ten-cell open-cathode PEMFC stack. The comparisons show that the proposed strategies can effectively alleviate the voltage decay caused by flooding and improve output stability. And the Condensation Circulation mode is more effective than the Condensation mode.

Understanding Fuel-Cell Impedance Spectra Via Numerical Modeling: A Single-Phase Water-Management Study

Cost of hydrogen-fueled proton-exchange-membrane fuel cells (PEMFCs) remains a challenge for the global commercialization of the technology and needs to be reduced to meet the U.S. Department of Energy goals [1]. Cost-reduction can be achieved by incorporating efficient water management, which helps keep the electrolyte hydrated while avoiding cell flooding by product water.Dynamic liquid-water accumulation and electrolyte hydration can be studied via transient polarization [2-6] and ohmic-resistance [2,4-7] curves, in which they induce hysteresis. Analyzing the polarization and resistance data may, however, be challenging due to the overlapping transients of various physical processes occurring in the cell during the voltage and current sweeps. Electrochemical impedance spectroscopy (EIS) allows the study of the dynamic processes of different time scales separately in the frequency domain and has been widely used for fuel-cell characterization [8-11].Fuel-cell impedance spectra often exhibit an inductive behavior at low frequencies, which has been shown to be affected by electrolyte hydration [10,12-14]. A recent experimental study [9] showed that the strength of this inductive behavior depends on the anode and cathode RH. Understanding the low-frequency inductance may help improve the water-management strategies for fuel cells. However, this inductive phenomenon has not been investigated in detail with a comprehensive numerical model.In this work, a transient 2D PEMFC model is developed in the open-source fuel-cell modeling software OpenFCST [17] with a thorough validation through dynamic polarization, resistance, and impedance-spectroscopy data at various operating conditions. Unlike other physical EIS models [13-16], the presented model does not rely on parameter fitting to reproduce the experimental impedance spectra. The model also takes the finite-rate water absorption/desorption kinetics of the electrolyte into account, which allows to investigate the effect of the interfacial transport of water on fuel-cell impedance and performance. With the presented model, water-management signatures are identified in fuel-cell impedance spectra under single-phase conditions that help elucidate electrolyte-hydration dynamics. The trends in the simulated effect of anode and cathode RH on the fuel-cell inductance are in agreement with experiments in [9], which verifies the model’s ability to correctly predict the inductive behavior. The low-frequency inductive behavior of fuel cells is shown to be related to the interfacial transport of water in the electrolyte and linked to the electrolyte-hydration and proton-conductivity dynamics in the membrane and in the catalyst layers. Additionally, an ohmic-resistance breakdown is performed with the model using the ohmic-heating-based approach proposed by Secanell et al. [18]. The high-frequency resistance obtained through EIS is shown to be comprised of the membrane resistance and the electronic resistance of other MEA components, but not the protonic resistance of the catalyst layers.Figure: Single-phase water-transport signatures in a fuel-cell-impedance spectrum.

Visualization study on enhancing water transport of proton exchange membrane fuel cells with a dead-ended anode by generating fluctuating flow at anode compartment

Anode water management is critical to the operating performance of proton exchange membrane fuel cells with dead-ended anodes. In this work, a new fuel supply mode, called two-way hydrogen supply mode, is proposed to improve the water management of an open cathode proton exchange membrane fuel cell stack with a dead-ended anode. In this mode, hydrogen can be supplied not only from the anode inlet to the anode side but also from the downstream to the anode side. The visualization technique is applied to investigate liquid water distribution and fluid dynamics in anode flow channels under two different operating modes. The experimental results show that the proposed mode can improve water distribution in the anode flow channels and reduce liquid water accumulation, resulting in more stable and higher output performance. The microscopic pictures further reveal the differences in water dynamics under two operating modes and show that the proposed mode can enhance the transport of droplets and suppress water vapor condensation, which significantly reduces the possibility of channel blockage downstream. Finally, the results also indicate that this mode is effective for the transport of water columns.

Ex-situ visualization of the wet domain in the microporous layer in a polymer electrolyte fuel cell by X-ray computed tomography under water vapor supply

The wet domain in a microporous layer (MPL) in a polymer electrolyte fuel cell (PEFC) was visualized in order to clarify water accumulation in the MPL. The performance of PEFC is affected by accumulation of liquid water in the cathode gas diffusion layer (GDL). The liquid water hinders oxygen transport to the catalyst layer, resulting in increased mass-transport loss. Although MPLs suppress water accumulation in the GDL, this mechanism has not yet been fully clarified. The present study used synchrotron X-ray micro computed tomography (CT) to visualize water accumulation in the MPL. The experiment was conducted while supplying water vapor to the MPL side of the GDL. The MPL was visualized to be wetted heterogeneously, and liquid water movement from the MPL to the GDL substrate was visualized. These observations were supported by plotting the volume fraction of either the wet domain of the MPL or the water cluster as a function of through-plane distance.

A two-dimensional numerical study of liquid water breakthrough in gas diffusion layer based on phase field method

Liquid water transport in gas diffusion layer is essential to water management in proton exchange membrane fuel cells. Based on the Phase-field method, the Cahn-Hilliard equation and Navier-Stokes equation are used to capture two-phase interfaces in the cathode of a fuel cell. Two kinds of porous media structures, i.e. regularized porous media and randomly generated porous media based on the quartet structure generation set, are constructed to make the representative two-dimensional geometry of a gas diffusion layer. The effects of contact angle, porosity and temperature on the liquid water breakthrough process are studied, whilst the shape and breakthrough path of the liquid water are successfully captured in the simulation. By regulating the distribution of contact angles, the accumulation of liquid water in a porous medium can be alleviated and the liquid water can be quickly broken out from the gas diffusion layer. The work is helpful for material design and water management of fuel cells.

A Discussion About Two-Phase Flow Pressure Drop in Proton Exchange Membrane Fuel Cells

A proton exchange membrane (PEM) fuel cell produces water during its operation which results in a liquid-gas two-phase flow within its flow channels. Because the length scales associated with flow channels are small, the two-phase flow in PEM fuel cell is mainly dominated by capillary forces. These capillary forces tend to hold droplets within the channels which eventually increase the pressure drop along the flow channels. The two-phase flow pressure drop along the flow channel can reveal information about the amount of liquid water accumulation. Therefore, a precise two-phase flow pressure drop model that can accurately predict the pressure drop in PEM fuel cell flow channels can be beneficial in estimating the amount of water content in flow channels. In the current study, liquid-gas two-phase flow pressure drops were measured in an ex-situ test section with liquid water and air flowing within the range of PEM fuel cell flow conditions. The measured pressure drops were then compared with nine existing pressure drop models developed for minichannels. Qualitative and quantitative comparisons are provided to compare the prediction capability of the models. Also, a discussion about capillary-scale two-phase flow systems and suggestions to improve prospective two-phase flow pressure drop models is provided.

Effect of PEFC Rib/Channel Width on Liquid Water Accumulation and Discharge by In-situ X-Ray Imaging

Water management within a polymer electrolyte fuel cell (PEFC) is one of the most important factor for improving the performance. Experimental studies have shown that liquid water tend to accumulate within catalyst layer (CL), gas diffusion layer (GDL) and on channel wall [1][2]. In this study, effects of PEFC rib/channel width on liquid water distribution and performance has been investigated by using in-situ X-ray imaging technique. For analyzing effects of rib/channel width, we used three flow channels with different rib/channel width (rib/channel: 0.2/0.4, 0.3/0/3, 0.4/0.2 mm), and in-situ X-ray visualization of liquid water was carried out. In addition, oxygen transport resistance was calculated by using a limiting current measurement, established by Baker et al [3]. In this study, we used the visualization cell with 4.5 mm2 active area and 1-path serpentine flow field. The cell was operated at cell temperature of 60 degC, relative humidity of 80 %, and various oxygen concentration (Oxygen concentration:2, 4, 6, 10, 15, 20 %, diluted by Nitrogen). As a result of performance test, the cell with wider channel width showed higher performance. Because liquid water was mainly accumulated in GDL under rib area, it suggested that the CL under rib area is not effectively used. To understand the effects of rib/channel with on liquid water distribution, oxygen transport resistance measurement was carried out. At low oxygen concentration condition (2, 4 and 6%), no liquid water was observed within PEFC, and oxygen diffusion resistance was approximately constant. On the other hand, liquid water accumulation within GDL under rib area, on channel wall and interface between micro porous layer (MPL) and substrate layer was observed at high oxygen concentration condition (10, 15 and 20%). As current increased, more liquid water accumulated, and it incur increase of oxygen diffusion resistance. In addition, fluctuation of cell voltage during liquid water accumulation and discharge behavior was observed at high current density. Figure 1 shows the result of liquid water visualization at cell voltage of 0.25 V. Liquid water is discharged from the GDL under rib area to the channel as a droplet (Figure 1 (b)). When growing liquid water droplet reached the bottom of channel, current density decreased immediately. At this moment, X-ray image showed that the rapidly growing water on another channel wall (Figure 1 (c)). Therefore, it can be concluded that water droplet continues to block gas flow until pressure difference before and after water drop grows up. So there is a possibility that flooding at one part induces another flooding at upper stream of the channel. These results show the importance to discharge a lot of liquid water from GDL to channel without flooding the channel. For improving PEFC performance while satisfying these conflicting factors, an appropriate combination of rib/channel width and channel wall wettability is essential.

Analysis of PEFC Gas Transport by Combination of X-Ray Radiography and Numerical Simulation

Polymer Electrolyte fuel cell ( PEFC ) is one of well established power-generating equipment and has become a focus of attention. The power generation process of PEFC includes a chemical reaction producing water. In a usual operating condition, water condenses inside MEA, and this liquid water accumulation is considered to affect the degradation of performance by preventing oxygen transport to the catalyst layer. In this study, to understand this phenomena, we have conducted a visualization of liquid water using X-ray microscope, and a two-dimensional numerical simulation of gas transport inside MEA. First, we have obtained liquid water distribution inside MEA by the X-ray visualization ( fig.1 ), and then incorporate the liquid water distribution into the numerical simulation as a reduced mean porosity ( i.e. reduced effective diffusion coefficient) as shown in fig.2. The resultant oxygen distribution with a condition of 3A/cm2current density, 40℃cell temperature, 80% relative humidity and 1atm gas pressure, shows lower oxygen concentration above land ( fig.3 ). In the presentation we will focus on effects of liquid water accumulation in detail by comparing with a simulation neglecting liquid water accumulation. Figure 1

Comprehensive Characterization of PGM-Free PEM Fuel Cells Using AC and DC Methods

In the past decade, a lot of efforts have been done to develop a platinum group metal-free (PGM-free), cathodic materials for proton exchange membrane fuel cells (PEMFCs) as alternative to classic Pt/C cathodes [1, 2]. At present, Fe-N-C systems are considered as one of the most promising catalysts for oxygen reduction reaction (ORR) with peak power density of about 0.5 W cm-2 [3]. The key problems in Fe-N-C electrodes relate to ORR mechanism which can proceeds through a combination of 2- and 4-electron pathways depending on operating current density, and to mass transport processes occurring in relatively thick electrodes structure. Herein, we report comprehensive study of Fe-N-C electrocatalyst integrated into cathodic catalyst layers of PEMFC with various composition configurations using AC and DC methods. We fit the physics-based impedance model, which takes into account oxygen transport in the cathode catalyst layer (CCL), gas diffusion layer (GDL) and flow-field channels to the experimental impedance spectra, and obtain main kinetic (Tafel slope) and transport parameters (proton conductivity and oxygen mass transport coefficients) of PGM-free PEMFCs in application-relevant conditions. Electrochemical evaluation has been performed using a segmented cell system and a test station developed at Hawaii Natural Energy Institute. Membrane electrode assemblies (MEAs) consisted of Fe-N-C (with varied catalyst and ionomer loadings) cathode gas diffusion electrodes (GDE), Pt anode GDE (0.1 mgPt cm-2, Alfa Aesar) and Nafion XL. The MEA was operated under galvanostatic control of the whole cell current, 80°C, 304 kPa back pressure, 100% relative humidity for the both electrodes and varied cathode stoichiometry. Details of the impedance model used in the present work have been reported in [4]. Fig. 1 a) shows a representative SEM image of the Fe-N-C GDE cross-section. The cathode thickness was found to be 120-140 μm for a sample with 3.0 mg cm- 2 catalyst loading (Fig. 1 a), while a catalyst content to 1.5 mg cm-2 caused appropriate decrease in the thickness to 70 μm. The independence of electrode thickness from ionomer content (35 vs 45 wt. %) may indicate the preferential distribution of Nafion® inside of large pores of Fe-N-C catalyst synthesized by sacrificial support method. The results obtained from AC-impedance and IV data clearly showed that the largest problem of all the electrodes is high absolute value of the kinetic Tafel slope, which varies from 100 to 250 mV/dec for current densities in the range of 0.025 to 0.4 A cm- 2. This parameter is responsible for faster decay of the cell polarization curve as compared to Pt/C-based PEMFCs. The mean proton conductivity σ p and CCL oxygen diffusivity Dox increase with the cell current density by a factor of ten (Fig. 1 b, c). Both effects take place also in Pt/C systems [5]; however, the absolute value of Dox in the Fe-N-C electrodes is an order of magnitude higher, than in the Pt/C cathodes (0.5-2.5×10- 4 cm2 s-1) [6]. The growth of σ p with current partly is due to accumulation of liquid water in the CCL; however, it may also be due to ionization of acidic molecules on carbon surface at lower cell potentials. A comparison of different CCL configurations shows that each catalyst loading value requires its own optimum ionomer content in the catalyst layer. Detailed analyses of the PGM-free PEMFC performance and its correlation with AC-impedance results will be presented and discussed. ACKNOWLEDGEMENTS We gratefully acknowledge ONR (N00014-18-1-2127), DOE EERE (DE-EE0008419) and Hawaiian Electric Company. The authors are thankful to Rohan Gokhale for preparing some GDE samples, Tina Carvalho for assisting with SEM, Günter Randolf and Jack Huizingh for valuable help in the system operation. REFERENCES S.T. Thompson, A.R. Wilson, P. Zelenay, D.J. Myers, K.L. More, K.C. Neyerlin, D. Papageorgopoulos, Solid State Ionics 319, 68-69 (2018).U. Martinez, S.K. Babu, E.F. Holby, P. Zelenay, Current Opinion Electrochem, 9, 224-232 (2018).A. Serov, K. Artyushkova, E. Niangar, C. Wang, N. Dale, F. Jaouen, M.-T. Sougrati, Q. Jia, S. Mukerjee, P. Atanassov, Nano Energy, 16, 293-300 (2015).A. Kulikovsky, J. Electroanal. Chem., 669, 28-34 (2012).T. Reshetenko, A. Kulikovsky. J. Electrochem. Soc., 163, F1100-F1106 (2016).T. Reshetenko, A. Kulikovsky, J. Electrochem. Soc., 164, F1633-F1640 (2017). Figure 1

Quantifying Water in PEM Fuel Cell Catalyst Layer with Small Angle Neutron Scattering

A polymer electrolyte membrane (PEM) fuel cell requires effective water management to achieve high performance. Balancing the water content and distribution is particularly important in a catalyst layer (CL), consisted of catalyst on carbon (typically Pt/C) covered by ionomer films. The accumulation of product water in the pores impedes reactant transport; however, simultaneously, the lack of water results in poor proton conductance in the ionomer. A diagnostic tool for measuring water content within the CL of an operating fuel cell is necessary for the development of the CL, especially for low Pt loading and non-precious metal group catalyst [1]. The common method for measuring water content in operando fuel cells is imaging based on synchrotron X-ray and neutron sources [2]. By using imaging method, water in the membrane electrode assembly (MEA) can be visualized on the micron-scale pixel resolution. However, this is not an ideal technique for probing water content in the CL for two main reasons: (1) the water in the ionomer cannot be distinguished from that in the pores, and (2) the spatial resolution is often not sufficient (as it is several times of the pixel resolution). At the French Alternative Energies and Atomic Energy Commission (CEA), we have developed in situ small angle scattering (SAS) techniques for fuel cell application based on neutron and synchrotron X-ray sources (Institut Laue-Langevin and European Synchrotron Radiation Facility). These techniques utilize focused neutron or X-ray beam to scan the areas of the MEA, each scanned point producing a scattering pattern. By radial averaging, the scattering pattern is converted into Q-range. The scattering due to the perfluorosulfonic acid (PFSA) ionomer produce a peak in the range of 0.1-0.25 Å-1, which arises from the separation between hydrophobic and hydrophilic domain. The water uptake in ionomer can be deduced from the shift of the peak position resulting from the swelling upon hydration. On the other hands, the overall water content can be obtained by either direct transmission or analyzing the high Q-range (0.4-0.45 Å-1). We previously measured liquid water accumulation (in the channels and the GDL) and water uptake in the membrane by through-plane SAS (i.e. the fuel cell is aligned perpendicular to the beam) [3]. Recently, we have further refined the custom fuel cell and SAS technique to probe water content only in the CLs. This was accomplished by developing a cell with an active area of 0.1 cm2 (0.1 mm wide in the path of the beam) and narrowing the beam size to 3 and 15 µm (for X-ray and neutron, respectively). This work will provide an overview of in situ SAS technique for fuel cells and focus on the comparison of two types of SAS investigations (neutron- vs. synchrotron X-ray-based).