Direct three-dimensional reconstruction of a nanoporous catalyst layer for a polymer electrolyte fuel cell

The direct three-dimensional reconstruction of a polymer electrolyte fuel cell cathode catalyst layer from focused ion beam/scanning electron microscope (FIB/SEM) images is presented. The carbon and pore distribution is shown and quantitatively analysed. A new catalyst layer sample (Fumapem-F950/HiSpec13100) is sliced with FIB and a series of SEM images is taken. The images are registered, segmented and a three-dimensional stack is reconstructed. The three-dimensional carbon and pore distribution is shown. Based on the reconstruction the pore size distribution is evaluated. The total porosity and the unconnected pores space is analysed. The fully segmented 2D images are provided as supplemental material to this paper for future analysis and modeling work.     

Numerical investigation of the effect of cathode catalyst layer structure and composition on polymer electrolyte membrane fuel cell performance

The effect of the cathode catalyst layer's structure and composition on the overall performance of a polymer electrolyte membrane fuel cell (PEMFC) is investigated numerically. The starting point of the sub-grid scale catalyst layer model is the well-known flooded agglomerate concept. The proposed model addresses the effects of ionomer (Nafion) loading, catalyst (platinum) loading, platinum/carbon ratio, agglomerate size and cathode layer thickness. The sub-grid scale model is first validated against experimental data and previously published results, and then embedded within a two-dimensional validated computational fluid dynamics code that can predict the overall performance of the fuel cell. The integrated model is then used to explore a wide range of the compositional and structural parameter space, mentioned earlier. In each case, the model is able to correctly predict the trends observed by past experimental studies. It is found that the performance trends are often different at intermediate versus high current densities—the former being governed by agglomerate-scale (or local) losses, while the latter is governed by catalyst layer thickness-scale (or global) losses. The presence of an optimal performance with varying Nafion content in the cathode is more due to the local agglomerate-scale mass transport and conductivity losses in the polymer coating around the agglomerates than due to the amount of Nafion within the agglomerate. It is also found that platinum mass loading needs to be at a moderate level in order to optimize fuel cell performance, even if cost is to be disregarded.     

The effects of non-uniform distribution of catalyst loading on polymer electrolyte membrane fuel cell performance

The catalyst layers are the most important part of the polymer electrolyte membrane (PEM) fuel cells, and the cell performance is highly related to its structure. The gas diffusion layers (GDLs) are also the essential components of the PEM fuel cell since the reactants should pass through these layers. Model prediction shows that electrical current in catalyst layer is non-uniform, influenced by the channel-land geometry. In addition, the compression effect of GDLs and water generation due to the electrochemical reaction may cause non-uniformity in porosity and, therefore, increases the non-uniformity in reactant concentration in GDL/catalyst layer interface. Simulation results suggest that non-uniform catalyst loading distribution in the catalyst layer will improve the performance of the whole catalyst layer by diminishing the variation in current density.     

Effects of gas-diffusion layer properties on the performance of the cathode for high-temperature polymer electrolyte membrane fuel cell

The role of the gas-diffusion layer (GDL) in high-temperature polymer electrolyte fuel cell (HT-PEMFC) differs from that in low-temperature PEMFC GDL due to operating conditions and environment. Determining the GDL's structural parameters that affect its transport properties, and how these properties impact HT-PEMFC performance was urgently required. Four commercial GDLs were employed in HT-PEMFC cathode's GDE and was examined using X-μCT, mercury intrusion porosimetry, and an optical microscope to analyze structural parameters and characteristics. Fractal theory was applied to comprehend the gas transmission property of GDL, and the validity of the theory was confirmed through ex-situ through-plane gas permeability measurement. The analysis indicated that the porosity of GDL influenced by the crack region of the MPL has more impact on the GDL's gas transmission than its thickness. After that, we established a correlation between HT-PEMFC cathode performance and GDL porosity and theoretical gas transmission properties using R² coefficient of determination.     

Improving open-cathode polymer electrolyte membrane fuel cell performance using multi-hole separators

We developed a new separator with a multi-hole structure (MHS) in the rib region for open-cathode polymer electrolyte membrane fuel cell (OC-PEMFC) stack to improve performance. The electrochemical current–voltage performance results clearly demonstrate that the performance of the OC-PEMFC stack using the MHS design was higher than that using the conventional parallel design at high current regions (i.e., over 7 A). The current increased by 11.24% at 12 V (i.e., 0.6 V/cell). The effects of supplying additional oxygen and removing generated water were identified as factors improving the performance. The individual cell voltages demonstrate that the initial value of standard deviation for the OC-PEMFC stack using MHS was somewhat high, but it exhibited better uniformity at higher current regions.     

Enhanced membrane electrode assembly performance by adding PTFE/Carbon black for high temperature polymer electrolyte membrane fuel cell

Phosphoric acid used as a proton-conductive medium in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) poisons the Pt surface and prevents oxygen transport in the cathode catalyst layer. The hydrophobic binders in the catalyst not only maintain the catalyst layer structure but also control the phosphoric acid distribution. In this study, polytetrafluoroethylene (PTFE)/carbon black (Vulcan XC-72R) added to the catalyst layer generates an oxygen transport channel. The catalyst layers coated on the gas diffusion layer by the bar-coating method serve as the cathode. High PTFE content causes hydrophobicity in the catalyst layer. The membrane electrode assembly (MEA) with 6 wt% PTFE/Vulcan results in the highest peak power density (0.347 W cm⁻²) and voltage (0.653 V) at 0.2 A cm⁻². A critical reason for its high performance is having the lowest R_ct + R_mt values measured at 0.6 V and 0.4 V. These results could contribute to improving the MEA performance for HT-PEMFCs.     

Direct preparation of core-shell platinum cathode in membrane electrode assembly catalyst layer for polymer electrolyte fuel cell

Core–shell catalyst has been attracting attention as a low-Pt catalyst for PEFC cathode. However, its mass production method has not yet been established. In this paper, a novel method suitable for continuous production of low-platinum catalyst layer for PEFC is proposed. Catalyst layer of carbon-supported Pd@Pt core–shell catalyst (Pd@Pt/C) is fabricated by using Cu underpotential deposition (UPD) followed by surface-limited redox replacement (SLRR) directly to the porous catalyst layer made of Pd core (Pd/C). The distribution of Pt corresponded well with that of Pd throughout the catalyst layer, indicating that the core–shell reaction occurs in the entire catalyst layer. Pd@Pt/C shows 1.8 times higher mass activity than Pt/C, which is comparable to Pd@Pt/C prepared by conventional microgram-scale method.     

Optimal catalyst layer structure of polymer electrolyte membrane fuel cell

In a membrane electrode assembly (MEA) of polymer electrolyte membrane fuel cells, the structure and morphology of catalyst layers are important to reduce electrochemical resistance and thus obtain high single cell performance. In this study, the catalyst layers fabricated by two catalyst coating methods, spraying method and screen printing method, were characterized by the microscopic images of catalyst layer surface, pore distributions, and electrochemical performances to study the effective MEA fabrication process. For this purpose, a micro-porous layer (MPL) was applied to two different coating methods intending to increase single cell performances by enhancing mass transport. Here, the morphology and structure of catalyst layers were controlled by different catalyst coating methods without varying the ionomer ratio. In particular, MEA fabricated by a screen printing method in a catalyst coated substrate showed uniformly dispersed pores for maximum mass transport. This catalyst layer on micro porous layer resulted in lower ohmic resistance of 0.087 Ω cm² and low mass transport resistance because of enhanced adhesion between catalyst layers and a membrane and improved mass transport of fuel and vapors. Consequently, higher electrochemical performance of current density of 1000 mA cm^-2 at 0.6 V and 1600 mAcm⁻² under 0.5 V came from these low electrochemical resistances comparing the catalyst layer fabricated by a spraying method on membranes because adhesion between catalyst layers and a membrane was much enhanced by screen printing method.     

Effects of operating conditions on durability of polymer electrolyte membrane fuel cell Pt cathode catalyst layer

In this study, we investigated the effects of humidity and oxygen reduction on the degradation of the catalyst of a polymer electrolyte membrane fuel cell (PEMFC) in a voltage cycling test. To elucidate the effect of humidity on the voltage cycling corrosion of a carbon-supported Pt catalyst with 3 nm Pt particles, voltage cycling tests based on 10,000 cycles were conducted using 100% relative humidity (RH) hydrogen as anode gas and nitrogen of varying humidities as cathode gas. The degradation rate of an electrochemical surface area (ECSA) was almost 50% under 189% RH nitrogen atmosphere and the Pt average particle diameter after 10,000 cycles under these conditions was about 2.3 times that of a particle of fresh catalyst because of the agglomeration of Pt particles.The oxygen reduction reaction (ORR) that facilitated Pt catalyst agglomeration when oxygen was employed as the cathode gas also demonstrated that Pt agglomeration was prominent in higher concentrations of oxygen. The ECSA degradation figure in 100% RH oxygen was similar to that in 189% RH nitrogen. It was concluded that liquid water, which was dropped under a supersaturated condition or generated by ORR, accelerated Pt agglomeration. In this paper, we suggest that the Pt agglomeration degradation occurs in a flooding area in a cell plane.     

Dependence of the performance of a high-temperature polymer electrolyte fuel cell on phosphoric acid-doped polybenzimidazole ionomer content in cathode catalyst layer

Phosphoric acid-doped polybenzimidazole is used as a fuel cell membrane and an ionomer in the catalyst layer of a high-temperature polymer electrolyte fuel cell. Single-cell tests are performed to find the optimum ionomer content in the cathode catalyst layer. To determine the effects of the ionomer in the catalyst layer, the potential loss in the cell is separated into activation, ohmic and concentration losses. Each of these losses is examined by means of impedance and morphological analyses. With the weight ratio of ionomer to Pt/C of 1:4 (20 wt.% ionomer in catalyst layer), the fuel cell shows the lowest ohmic resistance. The activation loss in the fuel cell is lowest when the ratio is 1:9 (10 wt.% ionomer in the catalyst layer). The cell performance is dependent on this ratio, and the best cell performance is obtained with a ratio of 1:4.     

Tunable aggregation of short-side-chain perfluorinated sulfonic acid ionomers for the catalyst layer in polymer electrolyte membrane fuel cells

We control the aggregation of short-side-chain (SSC) perfluorinated sulfonic acid (PFSA) ionomers for catalyst-layer (CL) inks by using a dispersion solvent of dipropylene glycol (DPG) and water. By increasing the fraction of PFSA backbone preferable DPG content in a dispersion solvent, the size of SSC-PFSA aggregates decreases exponentially from microscale to nanoscale, affecting the catalyst-ionomer agglomerates’ size and distribution in the CL inks. The surface morphology and porosity properties of the resulting CL are investigated, and the fuel cell performances are studied at two different humidity conditions (50 and 100% RH). Compared to the previous study with long-side-chain (LSC) PFSA ionomers, the SSC-PFSA ionomers show the optimized performance at higher DPG content, where the solvating power is intermediate for SSC-PFSA ionomers having shorter hydrophilic side chain than LSC-PFSA ionomers.     

Analysis of microporous layer characteristics of the anode for high-temperature polymer electrolyte membrane fuel cell

Four commercial gas diffusion layers (GDLs) are applied to high-temperature polymer electrolyte fuel cell anodes to investigate the relationship between microporous layer (MPL) properties and the performance of membrane electrode assembly. Subsequently, the physical properties such as the crack area and porosity of the MPL are analyzed via various methods. It is found that the capillary pressure of phosphoric acid (PA) from several theories is inversely related to the mass transfer capability (MTC) of the MPL. Finally, a new transport property named PA MTC of the MPL is proposed using the PA capillary pressure and MTC in MPL. The results indicate that the voltage at both low and high current densities increases separately as the PA MTC in the MPL adjusts to 0.5 MPa mm^-1. Subsequently, the voltage beyond 0.5 MPa mm^-1 increases in low current density contrary to the voltage at a high current density.     

Improvement of fuel cell performances through the enhanced dispersion of the PTFE binder in electrodes for use in high temperature polymer electrolyte membrane fuel cells

In high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), it is important that the structure of the electrode catalyst layer is formed uniformly. To achieve this, the binder must be well dispersed; however, polytetrafluoroethylene (PTFE), which is commonly employed in the preparation of HT-PEMFCs, is difficult to disperse during electrode manufacture due to its high hydrophobicity. In this study, we fabricate electrodes containing a surfactant to improve the dispersion of the PTFE binder and to enhance reproducibility during electrode manufacture. The electrodes are commonly prepared via a bar coating method, which is known to exhibit poor dispersion due to the small amounts of solvent employed compared to the spraying method. We then compare the properties of the obtained electrodes prepared in the presence and absence of the surfactant through physical and electrochemical characterization. It is found that the electrode containing the surfactant is structurally superior, and its single cell performance is significantly higher (i.e., 0.65 V at 0.2 Am cm⁻²). The single cells are suitable for operation at 150 °C using H₂/air at atmospheric pressure and a total platinum loading of 2.0 mg cm⁻².     

Improvement of cell performance in catalyst layers with silica-coated Pt/carbon catalysts for polymer electrolyte fuel cells

Carbon-supported Pt catalysts (Pt/Cs) for use of cathode catalyst layers (CLs) for PEFCs were covered with silica layers in order to improve performance. CLs with low ratio of ionomer to carbon (I/C) for Pt/C and silica-coated Pt/C were fabricated using an inkjet printing (denoted as Pt/C(IJ) and SiO₂-Pt/C(IJ)) to reduce oxygen diffusion resistance. Compared to Pt/C(IJ), SiO₂-Pt/C(IJ) ink maintained good dispersion and high stability under the lower I/C. The performance of SiO₂-Pt/C(IJ) was significantly higher than Pt/C(IJ) at 0.6 V under all humidity conditions. In particular, the performance of SiO₂-Pt/C(IJ) under low humidity conditions showed noticeable improvement regardless of current density area. From FIB-SEM, it was confirmed that the morphologies and porosities of both catalysts were the same. Thus, these results indicate that oxygen diffusion resistance, related to structure of CLs, hardly affects the performance, whereas improved performance is attributed to increased proton conductivity by silica layers containing hydrophilic groups.     

Electrochemical analysis of polymer electrolyte membrane fuel cell operated with dry-air feed

Electrochemical analysis of a commercial polymer electrolyte membrane fuel cell (PEMFC), operated at varying cathode relative humidity (RH) and current density, has been conducted to understand the factors that affect power performance when the PEMFC is operated with a dry-air feed. With a change in the cathode RH from 80 to 4%, the electrochemical area and double-layer capacitance of the cathode are reduced by 9 and 8%, respectively. This indicates that exclusion of the catalyst layer (CL) of the cathode from proton access occurs to some extent at low RH. It does not, however, explain the observed increase in activation loss. For the dry-air feed, the ionic resistances of the membrane and cathode CL are comparable in magnitude. Impedance analyses show that drying of the cathode at low RH and low current density leads not only to an increase in the ionic resistance of the CL, but also to increases in both charge-transfer and mass-transfer resistances. The simultaneous decrease in all the resistance components with decrease in the air permeability of the cathode diffusion layer highlights the importance of cathode design for operation with dry-air feed.     

Effect of platinum amount in carbon supported platinum catalyst on performance of polymer electrolyte membrane fuel cell

The performance of polymer electrolyte membrane fuel cells fabricated with different catalyst loadings (20, 40 and 60 wt.% on a carbon support) was examined. The membrane electrode assembly (MEA) of the catalyst coated membrane (CCM) type was fabricated without a hot-pressing process using a spray coating method with a Pt loading of 0.2 mg cm⁻². The surface was examined using scanning electron microscopy. The catalysts with different loadings were characterized by X-ray diffraction and cyclic voltammetry. The single cell performance with the fabricated MEAs was evaluated and electrochemical impedance spectroscopy was used to characterize the fuel cell. The best performance of 742 mA cm⁻² at a cell voltage of 0.6 V was obtained using 40 wt.% Pt/C in both the anode and cathode.     

Effects of cathode catalyst layer design parameters on cold start behavior of polymer electrolyte fuel cells (PEFCs)

We present a theoretical study on the effects of key catalyst layer (CL) design parameters on the cold start behavior of a polymer electrolyte fuel cell (PEFC) using a three-dimensional transient cold start model developed in a previous study 1 and 2. Among several CL design parameters, we adopt the ionomer fraction (?_I) and weight ratio of Pt to carbon support (wt%_Pt–C) in the cathode CL as CL design variables for this study. Therefore, other design parameters such as CL thickness and the oxygen reduction reaction (ORR) kinetic parameter are accordingly adjusted due to changes in ?_I and wt%_Pt–C for cold start simulations. The calculated results confirm that these two design parameters provide control of the ice storage capacity and water absorption potential of the cathode CL, and consequently have a substantial influence on the cold start behavior of a PEFC. We provide a guideline to design and optimize a cathode CL and membrane electrode assembly (MEA) for improved PEFC cold start capability.     

Nano mineral fiber enhanced catalyst coated membranes for improving polymer electrolyte membrane fuel cell durability

In order to protect the perfluorosulfonic acid (PFSA) ionomer from an attack of contaminant metal ions as well as to enhance the mechanical stability of catalyst layers, palygorskite (PGS) is introduced into the catalyst layer of polymer electrolyte membrane fuel cells. PGS is a widely used natural nano-sized silicate mineral fiber with unique nano-sized channel structure, has a strong absorption capacity for heavy metal ions. We identify a negative influence of Fe²⁺ on PFSA membranes to make a comparative study. Subsequently catalyst coated membranes (CCMs) prepared with a PGS-Pt/C composite catalyst show a great effect in reducing Fe²⁺ ion crossover. Results display that PGS absorbs Fe²⁺ in nano-structure channels, and effectively protect PFSA ionomer in both the catalyst layer and membrane from hydroxyl radicals (OH) attack. Thus, the chemical stability of PFSA ionomer in both the catalyst layer and membrane is greatly improved. Furthermore, the enhancement of the mechanical performance of catalyst layers is discussed.     

Optimization of graded catalyst layer to enhance uniformity of current density and performance of high temperature-polymer electrolyte membrane fuel cell

The optimal use of catalyst materials is essential to improve the performance, durability and reduce the overall cost of the fuel cell. The present study is related to spatial distributions of current and overpotential for various graded catalyst structures in a high temperature-polymer electrolyte membrane fuel cell (HT-PEMFC). The effect of catalyst gradient across the catalytic layer (CL) thickness and along the channel and their combination on cell performance and catalyst utilization is investigated. The graded catalytic structure comprises two, three, or multiple layers of catalyst distribution. For a total cathode catalyst loading of 0.35 mg/cm², higher loading near the membrane presents improved cell performance and catalyst utilization due to reduced limitations caused by oxygen and ion diffusions. However, non-uniformity in the current distribution is significantly increased. The increase in the catalyst loading along the reactant flow provides a substantially uniform current density but lower cell performance. The synergy of varying catalytic profiles across the CL thickness and along the cathode flow direction is investigated. The results emphasize the importance of a rational design of cathode structure and mathematical functions as a strategic tool for functional grading of a CL towards improved uniform current distribution and catalyst utilization.