(Plenary) Doing More with Less: Challenges for Catalyst Layer Design

Contamination of polymer electrolyte membrane (PEM) water electrolysis (WE) systems is one of the critical technical hurdles hindering mass adoption of PEM technology for green hydrogen production and has led to stringent water quality requirements and expensive water purification equipment to enable high efficiency operation and reach durability targets. The sources of contamination to the water supply to the electrolysis stack are numerous, and in the scope of this work, include leached metallic ions from balance-of-plant and stack components that are in direct contact with the de-ionized feedwater to the membrane electrode assembly (MEA) but downstream of the filtration system. These leachates are of critical interest, due to their extremely low concentration and detectability limitations, as well as their ability to accumulate over time to achieve significant uptakes in the MEA and cause significant performance loss and contribute to membrane aging/thinning. However, few published studies have investigated the effects of leached metal ions in PEM WE.In this investigation, we sought to characterize and understand the behavior of metallic contaminants in the MEA system, once absorbed, by adding the cations directly to the membrane prior to the deposition of the catalyst layers. This approach yields good control of the contamination level and the ability to study performance effects at low occupation fractions without requiring significant operational periods with which to accumulate the ions. We selected iron (Fe), aluminum (Al), and copper (Cu) as the metal ions of interest, given their prevalence in water systems and their unique expected behaviors in the acidic media of a PEM WE MEA. The results show clearly that the Fe and Al ions behave solely as mobile contaminants that accumulate in/near the cathode, while Cu shows the additional complexity of plating behavior in the cathode catalyst layer. Further, our results indicate the sensitivity of the performance and impedance signals to low-level contamination, indicating the possibility of early detection and mitigation prior to cell failure.

Proton exchange membrane fuel cells (PEMFCs) are being developed as alternative energy sources for both residential and automotive applications. For this technology to reach its full commercial potential, however, a significant reduction in cost is still required while the performance and durability of PEMFCs being maintained or improved. The key in this process is continuous improvement of membrane electrode assembly (MEA) through both the development of new materials and the optimization of the 3D structural arrangement of individual MEA components [1,2] . Structural optimization of MEA remains to be challenging due to multiple materials interacting across 6 orders of magnitude in their characteristic length scales, from several nanometers for the catalyst particle size to hundreds of micrometers for carbon fibers in porous transport layer. In recent years, several microscopy techniques have become widely available to characterize the three-dimensional (3D) structure of PEMFC catalyst layers. Due to the heterogeneous nature of the PEMFCs sample, a correlative, multi-scale imaging protocol is developed in this work. The following technology is used where their corresponding resolution limit is pushed. Transmission Electron Microscope tomography, TEMt [3] (0.60nm X 0.60nm X 0.60nm Voxel Size) Focussed Ion Beam - Scanning Electron Microscope tomography, FIB-SEM [4] (2.5nm X 2.5nm X20 nm Voxel Size) Micro and Nano X-ray Computed Tomography, NanoCT [5] (367nm X 367nm X 367nm Voxel Size) Each technique by itself supports valuable insight to be obtained on the investigated structure at the corresponding length scale. By combining 3D imaging data at multiple scales, an unified structural characterization workflow is developed. Furthermore, an up scaling approach is developed based on TEMt, SEM and NanoCT reconstruction of catalyst layer, micro-porous layer and porous gas transport layer of a PMEFC cathode. Structural properties and effective transport properties are computed at each scale via image-based numerical simulation. Effective transport properties are up-scaled. MEA performances are predicted based on the obtained effective quantities at micro-scale.

A parametric study of cathode catalyst layer structural parameters on the performance of a PEM fuel cell

The performance of PEMFCs has been linked to the porosity of the catalyst/carbon/ionomer composite in the cathode catalyst layer (CL), and this can be controlled to some degree by the equivalent weight (EW) of the ionomer.(1) Here, we study impact on PEMFC performance when the cathode CL contains a low equivalent weight (LEW) ionomer having short-side-chains (SSCs). Inks for the CLs are prepared using Pt/C electrocatalysts and either 27 wt.% Aquivion® SSC perfluorosulfonic acid ionomers (i.e. Aquivion® ionomer 980 EW, 830 EW and 700 EW) or a standard 33 wt.% long-side chain (LSC) Nafion® 1100 EW ionomer. The Pt/C/ionomer ink is directly deposited onto the Nafion proton exchange membrane by direct ultrasonic spray deposition to form catalyst-coated membranes (CCMs). The effects of ionomer EW on the surface area, pore structure and morphology of the prepared cathode CLs are surveyed by nitrogen adsorption and scanning electron microscopy (SEM). The CCMs are assembled into membrane electrode assemblies (MEAs), and the performance of the resulting MEAs are analyzed by I-V polarizations at different cell operating temperatures (i.e. 95, 80, 70 and 40°C) and relative humidity (i.e. 100, 70, 50 and 25% RH) in both air and oxygen environments. Cyclic voltammetry is used to determine the cathode CLs Pt ECSA and Pt utilization. In situ electrochemical impedance spectroscopy is used to understand the polarization losses in the cells, particularly in the mass transport region where electrode porosity has a large impact on flooding. Figure 1 compares the average polarization curves measured for 5 MEAs employing the SSC ionomer in their cathode CLs (i.e. Aquivion® 830 EW) to 5 MEAs employing the LSC ionomer in their cathode CLs (i.e. Nafion® 1100EW) at 80°C fed with ambient pressure air humidified at 100 % and 50 % RH. In the kinetic region (i.e. E ≥ 0.80 V) of the polarization curves, the cell performance of both set of MEAs are identical at both 100 and 50% RH. In the mass transport region at high current densities, the MEAs containing the SSC ionomer in their cathode CLs perform better than those containing the LSC ionomer in their cathode CLs under both moderate (50%) and high (100%) relative humidity. Our results appear to be unique because others have only seen improvement in PEMFCs with SSC LEW ionomers under low RH conditions, while we see performance improvements from the LEW SSC ionomers across the full range of RH. (1, 2) This presentation will discuss how the SSC ionomer affects the physical attributes of the MEA (porosity, impedance), resulting in higher performance at high current densities independent of the RH. Figure 1. Comparison of the average polarization curves measured for 5 Pt/VC MEAs employing the SSC ionomer (Aquivion® 830EW) in their cathode CLs to 5 Pt/VC MEAs employing the LSC ionomer (Nafion® 1100EW) in their cathode CLs at 80°C fed with ambient pressure air humidified at 50% and 100% RH. 1. C. Lei, D. Bessarabov, S. Y. Ye, Z. Xie, S. Holdcroft and T. Navessin, J Power Sources, 196, 6168 (2011). 2. Y. C. Park, K. Kakinuma, H. Uchida, M. Watanabe and M. Uchida, J Power Sources, 275, 384 (2015). Figure 1

Overview Polymer Electrolyte Fuel Cell (PEFC) is expected as a new power source in automobiles with high power density and low emission. Although PEFC has many advantages compared with internal combustion engines, the cost reduction and the improvement of performance and durability are needed for its practical application. For improvement of PEFC, it is important to understand the mechanism inside it under various operating and structural conditions and to feedback into the design of membrane electrode assembly (MEA) and bipolar plate constituting fuel cell stack.In order to visualize inside PEFC, we have been developed a numerical simulation tool calculating the cell and stack performance and physical distributions and a nondestructive, real-time/space visualization technique for oxygen partial pressure in the cathode flow channel and the GDL using oxygen-sensitive luminescent dye film [1].In this study, as for the first step of our numerical modeling development, the effects of operating condition, for example, cathode flow rate on the test cell as well as commercial-sized cell and stack were investigated. Numerical Modeling Inside the fuel cell, multi-physical and multi-scale of phenomena such as multi-component gas, liquid water and heat transport inside the flow channel and the GDL, electrochemical reactions and proton/water transport in the MEA are nonlinearly coupled [2]. These phenomena require too much calculation cost to be solved in high resolution by fully 3-dimensional equations based on computed fluid dynamics (CFD), so it is necessary to apply simplified models. In our modeling, macroscopic models are applied for microscopic phenomena in the catalyst layer and electrolyte such as electrochemical reactions (HOR/ORR) and proton/water transport. And these models are coupled with multi-dimensional heat, gas and liquid water transport equations in which effective parameters such as gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow field of bipolar plate and the manifold of stack are included [3]. Results First of all, the performance of 25cm2test cell with single serpentine flow channel was carried out. Hydrogen and air flow rate is 300ml/min, relative humidity is 60%RH, cell temperature is 353.15K and back pressure is 1atm, respectively.The distribution of oxygen partial pressure in cathode flow channel and on the GDL surface at cathode utilization 80% (0.58 A/cm2) are shown in Figure 1 compared to the measured data by a nondestructive visualization system. The results are good agreement with the measured data and indicate that the numerical prediction of current density distribution is valid in our model.Applying the validated model, the effect of cathode flow rate on the performance of short stack with 20 cells (max. 5kW) was carried out. As for the other operating conditions, relative humidity is 50%RH, cell temperature is 343.15K and back pressure is 1.5atm, respectively. Figure 2 shows the effect of cathode utilization on the distribution of normalized in-flow rate and ORR gain for each cell at 0.5A/cm2. As cathode utilization increases, in-flow rate decreases and ORR gain increases remarkably near the dead end of stack. This is because liquid water accumulates in the flow channel and the GDL.In addition, the performance of full-scale PEFC stack for automobiles under various operating conditions will also be discussed in this meeting.

Performance of an ultra-low platinum loading membrane electrode assembly prepared by a novel catalyst-sprayed membrane technique

Revolutionizing high-temperature polymer electrolyte membrane fuel cells: Unleashing superior performance with vertically aligned TiO2 nanorods supporting ordered catalyst layer featuring Pt nanowires

Balancing proton and mass transfers in cathode catalyst layer of high-temperature proton exchange membrane fuel cell via gradient porous structure design

Performance evaluation of platinum-molybdenum carbide nanocatalysts with ultralow platinum loading on anode and cathode catalyst layers of proton exchange membrane fuel cells

Design and optimization of gradient wettability pore structure of adaptive PEM fuel cell cathode catalyst layer

In polymer electrolyte fuel cells (PEFCs), the cathode catalyst layer (CL) needs a large amount of Pt because of the slow oxygen reduction reaction. Since electron, proton, and oxygen are necessary for the cathode reaction in PEFCs, achieving the optimum structure of cathode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. Some researches reported that when Aquivion is used as the ionomer of the CL, the output voltage increases as compared with the CL using Nafion in the intermediate current density region (1). However, dominant factors improving the cell performance have not been elucidated quantitatively. In this study, the oxygen transport resistance at the Pt surface, which is thought to have a large contribution to the oxygen transport resistance inside the CL (2), was investigated using CLs fabricated with different ionomers. In this study, catalyst coated membranes was fabricated with various cathode structure: 6 types of cathode side CLs using three kinds of Pt-supported carbons (TEC10E30E, TEC10E50E, TEC10E70TPM) having different Pt carrying densities and two types of ionomer, Nafion and Aquivion. All of these CLs were fabricated with I/C = 0.8. First, to distinguish the oxygen transport resistance in the CL from the other resistance such channels, I-V measurement experiments were conducted with different total pressures (0.10MPa, 0.15MPa, 0.20MPa), and the limiting current density method (2) was applied. In the I-V measurement, the flow rate of cathode gas (oxygen and nitrogen mixture) was 2000 sccm with different oxygen concentration (1%, 2%, 3%) and the flow rate of anode hydrogen was 100 sccm with RH 80% (80°C). Next, the oxygen transport resistance at the Pt surface was evaluated. (The details will be described later). In this evaluation, the electrochemical surface area (ECSA [m2/gPt]) in the CL is needed. The ECSA was estimated by CV measurement. In the CV measurement, cathode was filled with N2 gas with RH 100% (80°C). The flow rate of anode hydrogen was 100 sccm with RH100% (80°C). Figure 1 shows I-V characteristics of the CL with Pt loadings of approximately 0.20 mg/cm2. The output current density for the Aquivion based CL is higher than that of the Nafion based CL at a voltages around 0.5 V, whereas the Nafion based CL has superior performance in the operation at 0.1 V where oxygen supply is limited by various transport resistances. In this paper, we focus on the oxygen transport resistance at low voltage, in particular the resistance at the Pt surface, because the Pt surface resistance is considered to be dominant part of oxygen transport resistance under the low Pt loading CL. Figure 2 shows the ECSA estimated from the CV measurement. The values of ECSA are similar with Nafion and Aquivion based CLs. As the Pt carrying density increases, the ECSA decreases. This is because the Pt particles aggregate on the carbon and the surface area becomes smaller. Using the simplified evaluation formula of oxygen transport resistance (2), the total oxygen transport resistance in the CL can be expressed as RCL = RPt + Rdiss + Rpore . The RPt means the resistance at the Pt surface, Rdiss means the resistance of oxygen dissolves into ionomer, and Rpore means the oxygen diffusion resistance in the CL pores. The RPt is formulated as RPt = 1 / (APt × γ × kPt ) (2). Here, APt is the reactive surface area of catalyst Pt per unit area of the CL, γ is ratio of oxygen concentration in the ionomer and CL pore, and kPt [m/s] is the oxygen transfer rate constant at the Pt surface. Figure 3 shows the result evaluating RPt with both ionomer based CLs. The ordinate represents RCL and the abscissa represents 1/APt . Using this result, kPt can be calculated as kPt = 1.5×10 -2 s/m with the Nafion based CL and kPt = 9.1×10 -3 s/m with the Aquivion based CL. As the oxygen transport resistance at the Pt surface is inversely proportional to the kPt , the resistance of Aquivion based CL is estimated to be larger. This indicates a possibility that Aquivion based CL is inferior due to its larger Pt surface resistance. The larger Pt surface resistance may be due to the characteristics of Aquivion: the shorter side chain and the hydrophilic property induce strongly condensed structure of Aquivion at the surface of Pt, and that results in inhibiting the transport of oxygen. Reference (1)C. Park, et al., J. Power Sources, 315,179 (2016). (2)Tabe, et al., ECS Trans., 80(8), 205 (2017). Figure 1

Transport phenomena in proton exchange membrane fuel cells and over-potential distribution of membrane electrode assembly

Perfluorosulfonic acid (PFSA) polymer electrolytes are widely used in proton exchange membrane (PEM) fuel cell fabrication for the membrane separator and the electrode binder. The PEM serves to electrically insulate the cathode catalyst layer (CCL) from anode catalyst layer (ACL), provide proton conduction between both CCL and ACL, as well as prevent gas crossover. PFSA ionomers are also used in the CCL and ACL to mechanically bind the catalyst particles into thin-films, provide proton conduction between the PEM and catalyst surface, and produce a high proton reactant concentration at the oxygen reduction reaction catalyst. Despite their versatility to be used as a membrane and catalyst binder, the typical PFSA ionomers pose challenges due to the high oxygen transport resistance that is introduced by the thin ionomer films over the catalyst. In recent years, novel high oxygen permeability ionomers (HOPIs) have been developed as the successor to the traditional Nafion™ PFSA ionomers. Despite their remarkable results as a catalyst binder due to their increased oxygen transport rates, less is known regarding their degradation and functional lifetime. Although accelerated stress tests (ASTs) for platinum (Pt)/Pt-alloy catalysts, carbon supports, and membranes are well-established and commonly applied, there is no standard and easily implemented AST for electrode ionomers. One challenge is that membrane ASTs usually quantify the fluoride emission rate (FER) to assess chemical degradation, which is difficult to apply for electrode PFSA ionomer when also using a PFSA membrane because the FER is overwhelmed by membrane signal. In addition, changes in OCV and H2 crossover are not directly indicative of electrode ionomer degradation. Furthermore, concomitant degradation of the catalyst and support can interfere with interpreting electrode ionomer degradation. Here we present an effective and easily applied electrode ionomer chemical AST protocol that can be used to evaluate degradation in the functionality of the electrode ionomer. The approach borrows the chemical AST conditions of the chemical membrane AST’s open circuit voltage (OCV) hold and avoids excessive potential cycling to minimize catalyst and support degradation that will elicit changes in electrode properties separate from ionomer degradation. During the AST, we measure the cathode’s proton conduction resistance, oxygen permeability, and electrochemically surface area (ECSA) to evaluate the degradation of the ionomer’s functionality. Thus, this protocol can be used to assess if more complex and time consuming analyses of ionomer durability are needed as new chemistries emerge. Here we present results of this AST to both conventional PFSA ionomers and HOPIs.

Full-scale three-dimensional simulation of air cooling metal bipolar plate proton exchange membrane fuel cell stack considering a non-isothermal multiphase model

Proper water management in cathode porous electrode of polymer electrolyte fuel cells (PEFCs) is essential for achieving better performance. Especially at high current densities, excessive water produced by oxygen reduction reaction (ORR) on cathode side is rapidly condensed in catalyst layer (CL) and gas diffusion layer (GDL). When the open pores in cathode CL and the interface between CL and GDL are filled with liquid water, oxygen cannot be supplied to the reaction sites. This phenomenon, called “water flooding”, is a critical barrier for high efficiency and high power density operation. To alleviate this issue, it is necessary to understand and control liquid water transport inside cathode porous electrode. It is well known that microporous layer (MPL) plays a critical role in suppressing liquid water flooding in PEFCs [1,2]. MPL is a much thinner and finer porous layer, which is typically composed of carbon black (CB) particles and PTFE, and inserted between cathode CL and GDL. Nam et al. [2] experimentally and numerically investigated the vapor condensation and liquid water breakthrough in porous layers, and showed that inserting MPL between CL and GDL reduces the size and saturation level of interfacial water droplets on CL surface. However, there is a risk that MPL addition on cathode side increases oxygen transport resistance through porous layers. In general, MPL is formed by coating the slurry of CB powder and PTFE onto GDL substrate and then heat-treated. Its pore structure is complicatedly affected by various fabrication parameters such as CB loading, PTFE/CB ratio, coating and dry processes. To control interfacial water transport between cathode CL and GDL and minimize water flooding, it is essential to optimize the fabrication parameters of MPL determining its microstructure. In this study, we focused on CB loading as one of the major fabrication parameters of MPL, and analyzed the pore structure of MPLs with different CB loadings using SEM-EDX and mercury porosimetry. Furthermore, the operation test and the optical visualization of liquid droplet behavior in the cathode electrode were conducted using a transparent cell, and the influence of CB loading of MPL on its microstructure, the interfacial water transport and the performance characteristics of PEFC was discussed. In the experiment, the MPLs were coated on the hydrophobized carbon paper (Toray, TGP-H-060) by using the doctor-blade technique. The SEM-EDX observation revealed that the average thickness of two MPLs with 1.0 and 4.0 mg/cm2 CB are 33.7 and 123.6 um, respectively. When the CB loading is increased from 1.0 to 4.0 mg/cm2, the thickness of MPL is enlarged approximately 3.7 times. Subsequently, the operation test of a PEFC was carried out at 70 oC under high-humidity conditions, and the effect of CB loading of MPL on the I-V characteristics was investigated. The MPL addition on the cathode side drastically improves the limiting current density, because it suppresses the formation of liquid water film at the CL/GDL interface and reduces the concentration overpotential. In addition, the cell voltage in the high current density range increases with an increase in CB loading of MPL. Since the thick MPL raises the temperature near the cathode reaction sites, the rate of condensation of product water at the CL decreases in the case of 4.0 mg/cm2CB. Therefore, the thick MPL with high CB loading beneficially reduces water flooding at the cathode CL/MPL interface and improves the cell performance.