Microscopy Supported Multi-Scale Modeling of PEM Fuel Cells

Hydrogen today plays a key role in everyday life. Nearly all of the hydrogen consumed in the United State alone are used for the treatment of metals, processing food, petroleum refining, and fuel for aircraft and vehicles [1]. However, the methods of generating this resource revolves around methane pyrolysis, partial oxidation, coal gasification, and electrolysis [2]. The green option of all these methods is electrolysis, but this process accounts for only 4% of the total hydrogen generated [3]. Within this study, Polymer Electrolyte Membrane Water Electrolyzers (PEMWEs) have been studied as alternative green energy technology for Hydrogen production.PEMWE technology is known for using a cell with solid polymer electrolytes to conduct protons and separating liquid water into product gases of hydrogen and oxygen [4]. A key component within a PEMWE is the Membrane Electrode Assembly (MEA). Within this MEA, there consists of carbon cloth, catalyst coated membrane (CCM), and a porous transport layer (PTL) [5, 6]. For the purposes of this research, we will focus on the PTL as it plays important roles in the cell since it contributes to charge transfer resistance, mass transport, and interfacial contact resistance. However, the PTL is subjected to the highly oxidative environment and overpotentials from the anode side of the electrolyzer which leads to passivation of surface layers and lowering the life expectancy of the PTL as well as cell performance [6]. To combat this, protective layers are applied to the surface of the PTL to mitigate the formation of titanium oxide layers (TiOx). Most coatings applied are platinum, iridium, or gold through magnetron sputter coating. The limitations of most commercial coatings lie in that only surfaces with a direct line of sight to the source material are coated, leaving much of the PTL exposed to oxidation. To combat this, we will use a technique called Reactive Spray Deposition Technology (RSDT). RSDT is a unique and novel open-to-air vapor-phase deposition method developed at UConn’s Center for Clean Energy Engineering [7]. By controlling parameters within the deposition such as quench flow rate, distance of flame, and propane concentrations; we can control particle size, distribution, and coagulation to the surface of the substrate with our precursor layers [8]. This technique allows for a deposition that penetrates deep into the volume of the PTL and provides layer coatings that cover all surfaces with a layer that is uniform, dense and continuous. These layers are further studied and optimized using Focused Ion Beam, Scanning Electron Microscopy, Inductively Coupled Plasma, and Interfacial Contact Resistance.

A parametric study of the effect of wetting and in proton exchange membrane (PFM) fuel cells morphological properties of the cathode porous transport layer (PTL), also known as the gas diffusion layer (GDL), on water transport in proton exchange membrane (PEM) fuel cells has been conducted using a two-dimensional network model that captures the two-phase capillary flow behavior. The effect of PTL wetting properties is explored by considering four contact angle values. The effect of the morphology is analyzed using six pore size distributions. These six pore size distributions are generated by varying the scale and shape parameters in Weibull probability distribution functions. Also, the effect of a microporous layer (MPL) on water transport in the PTL is considered. Inclusion of the MPL resulted in a significant increase in the percolation pressure and a reduction in the PTL water content due to the formation of a few localized fingers which serve as water conduits. When defects in the MPL were considered, the PTL water content remained low with percolation pressures reduced to values similar to a PTL without an MPL.

Thermal conductivity and temperature profiles of the micro porous layers used for the polymer electrolyte membrane fuel cell

In proton exchange membrane fuel cells (PEMFCs), appropriate water management is critical to achieve high power density operation with increased robustness. Proton exchange membranes (PEMs) require sufficient hydration to fulfill its function as proton conductor, while flooding at the cathode side can hamper the transport of reactant, resulting in deterioration of cell operation. Liquid water transport and accumulation should be affected by materials impregnated in membrane electrode assemblies (MEAs) and this causes variation of cell performance. This motivated us to investigate effects of impregnated materials in MEAs on liquid water accumulation behaviors and resultant cell performance. For this purpose, soft X-ray radiography has been proof it is a powerful incitement for observing liquid water in PEMFCs because of the high spatial and temporal resolution. [1-5]In this study, we focused our attention on micro porous layers (MPLs) with vapor growth carbon fiber (VGCF, Unitetek international co. LTD, Taiwan) impregnated in MEA, because VGCF-modified MPLs have been shown for better cell performance especially in high current density operation, suggesting an improvement on liquid water transport [6]. We first examined polarization curves of two types of MEAs with or without VGCF in MPL. In Fig.1, we observed better cell performance with a MEA with VGCF-modified MPL than those with a conventional MEA without VGCF. In the experiment, we fabricated VGCF MPL coated on gas diffusion substrate (SIGRACET® 24BA). We also used commercial gas diffusion layer (SIGRACET® 24BC) for comparison. In both MEAs, we fabricated catalyst coated membranes (CCMs) by decal transfer method. Nafion® 212 had been selected as the membrane and the same batch of catalyst layer with 0.27mgPt cm-2 loading had been used. The hydrogen and air were fed at 200mL min-1 and 500mL min-1 for a 5cm-2 (23 mm x 23 mm) active area normal cell, respectively. The channel width and depth were 1.0 and 1.0 mm. The rib-to-channel ratio was 1 for normal cell. For a X-ray cell (0.03cm-2, 1 mm x 3 mm), hydrogen and air were both fed at 15mL min-1. The channel width and depth were 0.5 and 0.5 mm. The rib-to-channel ratio was 1 for X-ray cell. Cell operating temperature was set at 80°C and relative humidity of the reactants was kept at 92%. In situ visualization of liquid water distribution in both MEAs by soft X-ray radiography clearly shows their difference as shown in Fig.2. Form the original image Fig.2 (a) and (b), we can found VGCF sample (Fig.2(b)) was difficult to recognize the MPL. The one of the reason is the MPL infiltrate into gas diffusion substrate and another reason is the VGCF MPL maybe has high porosity thus it didn’t absorb much soft X-ray cause to similar intensity of GDL and VGCF-MPL. In both cases (Fig.2(c) and (d)), liquid water was observed under the rib, however, for VGCF-modified MPL at 0.7A cm-2 (Fig.2(d)), less spatially-dispersed liquid water was observed under the rib, while liquid water closed to the catalyst layer was identified in the conventional MEA (Fig.2(c)). At high current density operation (1.2A cm-2), liquid water in the MEA with VGCF was still accumulated closed to the rib. These observations by soft X-ray radiography indicate that VGCF-MPL plays a role to guide liquid water removal in MEAs, which resulted in better cell performance, although physicochemical properties of VGCF-MPL should be unveiled in the future.This work has been supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

Imaging PEMFC performance heterogeneity by sensing external magnetic field

The poor interfacial contact between the low-iridium-loaded catalyst layer and the porous transport layer (PTL) remains a major bottleneck for limiting the performance of proton exchange membrane water electrolysis (PEMWE). In this work, a microporous layer (MPL) composed of CeO2 nanoparticles and microcrystalline cellulose (MCC) was developed to address this challenge. By rationally tuning the particle sizes of CeO2 and MCC, the resulting infill structure exhibited significantly improved interfacial compatibility and a tailored pore size distribution. Consequently, the ordered membrane electrode assembly (MEA) incorporating this optimized MPL demonstrated an electrochemically active surface area (ECSA) 2.8 times higher than that of a conventional MEA without an MPL filler. Furthermore, the fabricated ordered MEA achieved a high current density of 4.58 A cm-2 at 2.0 V with an ultralow Ir catalyst loading of 48.4 μg cm-2 while giving excellent activity retention for over 200 h at 0.5 A cm-2 current density. This study offers a promising strategy for designing high-performance MEA with ultralow precious metal loading.

Efficient water management is crucial for the good performances of proton-exchange membrane fuel cells (PEMFCs). The geometric and physical characteristics of the components of a PEMFC as well as operating conditions have an impact on the transport of water through the porous transport layer (PTL) and the two-phase flow regimes in the microchannels. One parameter of importance is the local temperature, which affects properties such as surface tension and is coupled with phase change. Indeed, a temperature difference of about 5K is expected across the PTL, with spatial variations due to the geometry of the flow field plate. We present preliminary results obtained with a first experimental setup for the ex-situ characterization of two-phase flow regimes in the flow channels. Water is pushed through the PTL, which is sandwiched between a porous metal foam and the flow field plate. The air flow rate, temperature and humidity can be controlled. The cell can be heated up by applying an electrical current through the metal foam. A transparent window is located on top of the flow channel. The two-phase flow within the micro-channels is visualized using a high-speed camera and laser-induced fluorescence. Preliminary results obtained under isothermal conditions at room temperature show that different two-phase flow regimes occur in the channels depending on the operating conditions, in good qualitative agreement with data from the literature. Eventually, a new visualization cell is presented that is expected to correct the flaws of the previous design and will allow a better thermal control. It will be possible to adjust the temperature gradient and the mean temperature in order to observe their impact on two-phase flow regimes for different types of PTL and flow rates. The results will provide a better understanding of water transport in PEMFC and benchmark data for the validation of numerical models.

Proton exchange membrane fuel cells (PEMFCs) can intensely lessen the emissions from the energy sector through environmentally friendly attributes. This evaluation paper summarizes the key additives of the PEMFC which are associated with water formation and delivery inside the cell. The gas diffusion layer (GDL) plays a key role in reactant gas diffusion and water control in proton exchange membrane (PEM) fuel cells. This paper also reviews updated literature regarding various hydrophobic and hydrophilic materials used for the improvement of the GDL and overall PEMFC performance. A style of carbon and steel-based microporous substrates (MPS) and microporous layer (MPL) and their impact in GDL overall performance are provided. Materials' properties that affect the performance of the MPL consisting of pore sizes, porosity, and permeability are additionally reviewed. Visualization of water in the flow channel and techniques to understand the mechanism of flow is reported. The failure modes related to the membrane electrode assembly (MEA) and typical PEMFC degradation are discussed. Prospects for development and addressing the water management and degradation of PEMFC through the exploration of further experimental and numerical studies are presented. Highlights Proton exchange membrane fuel cells (PEMFCs) can be a good candidate for several applications The material property of the GDL influences the overall fuel performance Water transport if not properly managed can cause poor performance and failure of the fuel cell There is need to understand how the fuel cell failure is related to the PEMFC component materials

Water transport in the Porous Transport Layer (PTL) plays an important role in the efficient operation of polymer electrolyte membrane fuel cells (PEMFC). Excessive water content as well as dry operating conditions are unfavorable for efficient and reliable operation of the fuel cell. The effect of thermal conductivity and porosity on water management are investigated by simulating two-phase ow in the PTL of the fuel cell using a network model. In the model, the PTL consists of a pore-phase and a solid-phase. Different models of the PTLs are generated using independent Weibull distributions for the pore-phase and the solid-phase. The specific arrangement of the pores and solid elements is varied to obtain different PTL realizations for the same Weibull parameters. The properties of PTL are varied by changing the porosity and thermal conductivity. The parameters affecting operating conditions include the temperature, relative humidity in the ow channel and voltage and current density. In addition, a novel high-speed capable Surface Plasmon Resonance (SPR) microscope was built based on Kretschmann's configuration utilizing a collimated Kohler illumination. The SPR allows thin film characterization in a thickness of approximately 0-200nm by measuring the changes in the refractive index. Various independent experiments were run to measure film thickness during droplet coalescence during condensation.

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) represent a promising technology for clean drivetrain solutions, in particular for heavy-duty applications. However, lifetime requirements demand high durability of each cell component.In this work, transport of liquid water through pristine and degraded gas diffusion layers (GDL) was simulated with a 3D Color-Gradient Lattice Boltzmann model. The GDL microstructure was reconstructed from high-resolution X-ray micro-computed tomography (μ-CT) of an impregnated Freudenberg H14. The effect of a microporous layer (MPL) was considered by reconstruction of an impregnated and MPL-coated H14. Aged microstructures were generated artificially, assuming loss of polytetrafluoroethylene (PTFE) within the GDL and increase of MPL macroporosity as main degradation mechanisms. Liquid water transport within aged microstructures was simulated by imposing a liquid phase flow rate until breakthrough was reached. Subsequently, the GDL microstructures were analyzed for their breakthrough characteristics by means of saturation and effective gas transport properties. When the MPL was pristine, no distinct GDL degradation effect was observable, this was attributed to the MPL dominating capillary transport. MPL aging, however, led to increased saturations and thus to a deterioration of the effective gas transport. With a partially degraded MPL, aging of the GDL then appeared to affect the breakthrough characteristics.

Proton Exchange Membrane Water Electrolysers (PEMWEs) is among the most promising approaches for green hydrogen production due to its rapid response times and high-power density operation, which make this electrolysis technology well-suited to harvest the intermittent power from wind and solar plants. Nevertheless, the commercial breakthrough of PEMWE technology continues to face barriers due to its costs [1]. The porous transport layer (PTL) is one of the PEMWE components that facilitates the transport of incoming water to the catalyst layer and the removal of the product gases, hence it is very important for good electrolyser performance. The state-of-the-art PTLs are manufactured by high temperature sintering of titanium fibers or powders, which can generate oxidized surfaces that will reduce PEMWE performance. Furthermore, titanium is listed by the European Union as a critical raw material since 2020 [2], so it is advisable to reduce the amounts of its use to produce PTLs. Additionally, the (micro)structure of state-of-the-art PTLs is not optimized for efficient liquid and gas transport in electrolysis. The manufacturing process plays a key role in achieving PTLs with optimized liquid and gas transport as well as reducing the usage of titanium.In this work, we used lithography to manufacture PTLs in which the (micro-)structure can be optimized and the usage of titanium metal can be significantly reduced. The lithography manufactured PTLs were used as substrates to produce catalyst-coated electrodes (CCE) with (ultra-)low loadings of iridium and the PEMWE performance was evaluated, respect to CCEs manufactured on commercial PTLs. We demonstrate similar performance of the CCEs manufactured with the engineered PTL, with the advantage of using less titanium and being able to tune the (micro-)structure of the component.

Introduction Recently, proton exchange membrane water electrolysis (PEMWE) has been expected as the hydrogen production technologies to convert the renewable energy with regional and temporal unevenness into hydrogen, which can be stored and transported. PEMWE has a high energy conversion efficiency at the high current density, however it is required to reduce the high material cost due to the utilization of noble metals. Currently, the development of new components and the reduction the amount of noble metals have been conducted. The porous transport layer (PTL) is one of the most important elements to improve the efficiency of PEMWE. PTLs have the roles such as the transporting electrons between the catalyst layer (CL) and the end plate, the discharging the generated gas, and supplying water to CL. Therefore, the accurate separation and determination of the polarization in the electrolyzer are essential technologies. In that case, the potential measurement using reference electrodes can be useful. In this study, we measured the anode and cathode polarizations by the elimination of membrane resistance, and evaluated the polarization of anode PTLs with a small size electrolyzer using double references. Experimental Figure 1 shows the schematic drawing of 1 cm2 electrolyzer with double references. Hydrogen storage Pd reference (PdH) were placed on the membrane by the contact with 0.5 M H2SO4. Anode and cathode electrocatalysts were IrOx (TKK) and Pt/C (TKK), respectively. The 1.0 mgIr/cm2 of anode catalyst was decaled on PEM (Nafion®115, DuPont) with Nafion ionomer, and the 1.0 mgPt/cm2 of cathode catalyst was assembled on the membrane as a catalyst coated electrode supported a carbon paper (39BC, SIGRACET®). Pt plated Ti (MMC) and the carbon paper were used as the anode and cathode PTL, respectively. These electrodes were fixed straight (x = 0 mm) or shift (x = 0.3, 0.5, 0.7, 1.0 mm) stacking as shown in Fig. 1. Operation temperature of the electrolyzer conducted at 80℃. DI water was pumped and induced at flow rates of 10 ml/min and 2 ml/min to the anode and cathode, respectively. The performance was evaluated by the chronoamperometry (CA) and the electrochemical impedance spectroscopy (EIS). Results and discussion Figure 2 shows the relationship between the inner resistance R ac obtained from high frequency resistance of EIS and the direct current resistance R dc between references. The slopes of line, which passed through the origin, became larger as the increase of shift amount. As the slope represented θ, they can be written by equation (1). Next, using the α, the relationship between R dc and R membrane is defined by equation (2). Here, assuming that the contact resistance between PTL and CL is sufficiently small, R ac is equal to membrane resistance R membrane. Hence, R membrane can be calculated as α is equal to θ.Figure 3 shows the Tafel plots of anode and cathode polarization curves using reference electrodes of anode or cathode sides, and the membrane iR removed polarization curves E corrected , assuming the reference potential was equivalent to 8 % (= (100-84)/2) from the electrode/electrolyte interface (x = 1.0 mm). E corrected on cathode side was approximately identical to the extrapolated Tafel slope, and there was no mass transfer effect, which is the difference between the two curves. The reason is that the cathode side was only hydrogen generation by supplying electrons. On the other hand, E corrected on anode side showed the overpotential for the extrapolation of Tafel slope above anode potential of 1.4 V vs. PdH, and especially in the high current density area, it was affected by larger mass transfer resistance than cathode side. This overpotential was considered the water/oxygen transfer resistance or/and the transfer resistance related to the electrons in CL around PTL.Figure 4 shows the comparison of overpotential for anode PTLs. In this current density range, these plots showed the linear relationship between the overpotential and the current density. To compare the anode PTLs, PTL A indicated smaller overpotential than PTL B with smaller pore size and bigger porosity.In conclusion, the double references potential measurement with shift arrangement of electrodes enabled to separate the polarization into anode and cathode, and remove the membrane iR. Based on this method, this measurement method using the developed electrolyzer indicated to be able to evaluate the difference of PTL overpotential for the several type PTLs. Acknowledgment This study was supported by the development of fundamental technology for advancement of water electrolysis hydrogen production in advancement of hydrogen technologies and utilization project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) in Japan. Figure 1

The use of expensive PGM catalysts for the OER and HER leads to high production costs of Polymer electrolyte water electrolysis (PEWE). Moving towards lower iridium oxide (IrOx) catalyst loadings for the anode side can severely affect performance and stability [1]. Recent studies revealed that the structure of the interface between the anodic porous transport layer (PTL) and the catalyst layer (CL) is a crucial factor limiting cell efficiency and contributes significantly to the performance and use of the CL. The coarse structure PTL can develop inactive zones of the catalyst layer, which happens when the catalyst layer is not “close” to the connecting titanium structure [2]. The situation becomes worse when the IrO2 catalyst loading is reduced, and the catalyst layers become thinner. Recent evidence has shown that having additional Microporous layers (MPL) can overcome some of these issues by maximizing catalyst utilization and increasing cell efficiency. However, at low Ir loadings, a thin protective Pt coating on MPL is needed to avoid formation of TiOx passivation layer on the MPL surface and increase surface conductivity [3].In this work, we attempt to replace the thin protective Pt coating by adding an additional layer of Ti or TiN on the existing MPL aiming when using low loadings of IrO2 (0.5 – 0.1 mgIr cm-2). Vacuum plasma spraying (VPS) technology was used to apply TiN on the surface of MPL, while Ti was spray-coated, and vacuum sintered to create a hierarchically structured 3-layer PTL. Electrochemical performances of both Ti and TiN coated MPLs were evaluated by polarization curves and Electrochemical Impedance Spectroscopy measurements. The results are compared with a Pt coated MPL reference.

Performance analysis of PEM fuel cell in mobile application under real traffic and environmental conditions

The most promising alternative to Platinum Group Metals (PGMs) Oxygen Reduction Reaction (ORR) catalysts for proton exchange membrane fuel cell (PEMFC) applications is a class of electrocatalysts consisting of atomically dispersed transition metals (Fe and Mn) in a matrix of carbon and nitrogen, so called M-N-C materials [1]. Currently, the emerging novel M-N-C materials demonstrated 30% of PGM catalyst activity [1]. Performance of PGM-free electrocatalysts can be improved by increasing density of M-Nx sites or optimizing their accessibility through proper integration into the triple phase catalyst layer in a membrane electrode assembly (MEA). Since overall MEA’s performance depends on properties of all its components: membrane, electrodes and gas diffusion layers (GDL), it is important to understand impact of these materials on PGM-free fuel cell operation. In this work we report effects of membrane thickness on performance of PGM-free PEMFCs using various electrochemical methods and modelling.The commercial, PGM-free electrocatalyst manufactured by Pajarito Powder and marketed as PMF-011904 was used in the present study. Catalyst coated membranes with active area of 23 cm2 were manufactured by IRD Fuel Cell by their proprietary digital printing method. For this work we used membranes with thickness of 15 (LYT0008), 25 (N211), 125 (N115) and 175 (N117) μm. The cathode PGM-free catalyst loading was 6.0 mg cm-2, while anode employed Pt/C catalyst with loading of 0.2 mgPt cm-2. Freudenberg H23C8 GDLs were applied for both electrodes. Electrochemical evaluation of PGM–free MEAs was performed using a custom test station, designed at the HNEI and characterized by dynamic response time <0.1 s. The anode and cathode were fed with H2 and O2 respectively, at constant flow rates of 0.5 slpm at 100% relative humidity and 150 kPa absolute backpressure for both electrodes, and the cell temperature was 80°C.Fig. 1 a) shows a representative SEM image of the MEA cross-section. The cathode thickness was found to be 140-160 μm for all samples. The IV data clearly showed that an increase in the membrane thickness led to an increase of HFR and caused poor performance (Fig. 1 b). The highest performance was achieved for MEAs with thin membranes 15 and 25 μm. At the same time application of thick membrane allowed us to reach the highest ocv values 0.979 V (175 μm) vs 0.929 V (15 μm). The impedance data were analyzed using physics-based model which includes proton and oxygen transport in the cathode layer [2]. The results demonstrated a rapid decay of proton conductivity from 15-18 to 5-10 mS cm-1 at current densities higher than 100 mA cm-2. Possible mechanism of the observed decay can be attributed to dissolution of Fe-N-C centers and formation of Fe3+ cations which reduce ionomer conductivity by screening SO3- groups [3-4].Due to relatively low proton conductivity and large electrode thickness, the ORR rate is strongly non–uniform through the cathode depth. Our estimates showed that this non-uniformity leads to 2 K overheat of the catalyst layer at the membrane surface. The overheat could induce pressure gradient, pushing liquid water out of the electrode and clearing void pores for gaseous transport of oxygen. This mechanism might explain explosive growth of the cathode oxygen diffusivity (5-7⋅10-3 cm2 s-1). Detailed analyses of the PGM-free PEMFC performance and its correlation with AC-impedance results will be presented and discussed.AcknowledgmentsWe gratefully acknowledge funding from ONR (N00014-19-1-2159) and DOE EERE (DE-EE0008419). The authors are thankful to T. Carvalho for assisting with SEM.Reference1. 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).2. A. Kulikovsky, J. Electroanal. Chem. 669, 28-34 (2012).3. T. Reshetenko, G. Randolf, M. Odgaard, B. Zulevi, A. Serov, A. Kulikovsky, J. Electrochem. Soc. 167, 084501 (2020).4. T. Reshetenko, A. Serov, A. Kulikovsky, P. Atanassov, J. Electrochem. Soc. 166, F653-F660 (2019). Figure 1