PEFC Catalyst Layer Research Articles

Relative-Humidity Dependence of Electrochemically Active Surface Area in Porous Carbon Catalyst Layers

Polymer-electrolyte fuel cells (PEFCs) utilize porous catalyst layers (CLs) formed of carbon supports on which Pt particles are deposited and ionomer films are distributed. Carbon supports themselves have varying degrees of porosity, where high-surface-area carbon (HSC) supports possess nanometer-sized interior pores that are suitable for Pt nanoparticle deposition but prevent deleterious ionomer penetration. However, this requires protons to transport through water pathways inside the pores. To understand the generation of such pathways, we examine the various mechanisms of water uptake by PEFC CLs, and the subsequent impact of water uptake on Pt utilization through developing a multiphysics model of the water wetting phenomena as a function of relative humidity. The model details water uptake via ionomer absorption, capillary condensation in the hydrophilic pores, and surface adsorption using molecular potential that account for various water and surface dipole interactions. The results quantify how mesoporous carbons with highly hydrophilic pores increase Pt utilization through the development of wetted layers, which at the same time enable optimized gas-transport pathways. It also demonstrates the impact of pore-size distribution (PSD) and physical and chemical parameters on the water uptake phenomena, allowing for future CL particle and structure optimization.

Investigation of a Method of Evaluating Proton Transport Resistance in PEFC Catalyst Layers

Since there is no method to directly measure proton conductivity in the catalyst layer in PEFC even though proton conduction in the catalyst layer is one of the critical factors for high performance and durability. A method to directly measure proton transport resistance in the catalyst layers was investigated by sandwiching a sample layer by two Nafion® membranes to block electron transfer, and two gas diffusion electrodes to reduce gas diffusion resistance, in this study. Direct current was applied to the system in the hydrogen atmosphere, and measurements for proton transport resistance were performed in various sample layers. As a result, proton transport resistance in sample layers was successfully evaluated, and such measurements lead to improvement of current-voltage performance of PEFC.

Effect of Reaction Products on the PEFC Catalyst Ink Property and Catalyst Layer Quality

The catalyst layer (CL) is a critical component that determines the performance of proton exchange membrane fuel cells, and the production process of CL is important issue. A CL is fabricated from a catalyst ink mixed with platinum-supported carbon (Pt/C), ionomer solution, and a solvent. When the catalyst ink is coated on the sheet and dried, the CL is formed. By transferring the CL to the membrane, catalyst coated membrane can be made [1]. The process from the initial characteristics of the catalyst ink to the drying process determines the structure of CL and dominates the characteristics of the fuel cell. Thus, quality control of the catalyst ink is important to form a high performance and uniform CL [2].We have reported that alcohol oxidation reaction and condensation reaction, as catalyzed by platinum, occurred in the catalyst ink fabrication process [3]. Several reaction products, aldehydes, acids, and esters, were detected by gas chromatography mass spectrometry (GC/MS) [4]. In this study, concentration of those reaction products were measured quantitatively, and the effect of those reaction products on the catalyst ink property and CL quality was investigated.Catalyst ink was prepared by using the mixture of water and ethanol as a dispersant. We used platinum catalyst supported on carbon black and Nafion dispersion. The solvent was composed of ethanol and water. The weight ratio between ethanol and water was set to 46:54 or 60:40. Nonvolatile content was 10 wt%. Weight ratio between Nafion and the carbon support was 0.75. As a mixing process, a high-speed rotary type mixer was used, and mixing time was set to 10 min. The quantitative concentration of reaction products in each ink were analyzed by GC/MS. After concentration measurement, CL was fabricated using each ink. The crack formation was observed, and the correlation with the initial composition of the ink and the concentration of the reaction product was investigated.As a result of GC/MS, several reaction products (acetaldehyde, acetic acid, and ethyl acetate) were detected in the catalyst ink. Table 1 shows quantitative concentration of reaction products. Comparing the both catalyst inks, the concentration of acetaldehyde increased remarkably when the ethanol concentration increased. CLs were fabricated with using those catalyst inks and observed by microscope (Figure 1). There was no crack in the low ethanol concentration ink, and cracks occurred in the high ethanol concentration ink. Those results suggest that acetaldehyde may be a factor in causing cracks in the CL.To clarify the effect of acetaldehyde, acetic acid, and ethyl acetate on the crack formation in the CL, each substance was artificially added to the low ethanol concentration catalyst ink. In actual catalyst ink, it is considered that ethanol reacts to produce a reaction product. Therefore, in the modified catalyst ink, the amount of ethanol was reduced according to the amount of added substance. The CL was fabricated by using those catalyst ink and the observation results were shown in Figure 2. When the same amount of acetaldehyde contained in the high ethanol concentration catalyst ink was added, many cracks were generated in the CL. Several cracks occurred when ethyl acetate was added, but the required amount was significantly higher than the ethyl acetate concentration detected in the actual catalyst ink. No cracks were generated even if a large amount of acetic acid was added to the catalyst ink. These results indicate that the acetaldehyde is a factor of crack in the CL. It is considered that when the generated acetaldehyde exceeds a certain concentration in the catalyst ink, crack is caused in the CL. Acknowledgment: This study is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

The Development of Acquiring The Data with Micro Sensor in Drying Process of The PEFC Catalyst Layer

The Development of Acquiring The Data with Micro Sensor in Drying Process of The PEFC Catalyst Layer

Fabrication of Ionomer-free PEFC Catalyst Layer by Applying Graphene and Evaluation of the Oxygen Transport Resistance

Fabrication of Ionomer-free PEFC Catalyst Layer by Applying Graphene and Evaluation of the Oxygen Transport Resistance

Effect of Reaction Products on the PEFC Catalyst Ink Property and Catalyst Layer Quality

The catalyst layer is a critical component of proton exchange membrane fuel cells. Previous studies suggest that several reaction products were generated in the catalyst ink mixing process and it can effect on the structure of the catalyst layer. In this study, by identifying the reaction product contained in the catalyst ink and measuring those quantitative concentration, the effects on the catalyst ink and the catalyst layer were investigated. As a result, it was shown that acetaldehyde can affect the catalyst ink properties and the catalyst layer quality (micro-scale crack formation).

Effects of Solvent Composition on Viscosity and Dispersion Structure of PEFC Catalyst Ink

Structure and performance of PEFC catalyst layer (CL) is greatly influenced by dispersion structure of catalyst ink. Though the solvent composition of the catalyst ink affects the CL performance, correlation between solvent composition, CL structure, and performance has not been fully understood. The purpose of this study was to understand the effects of solvent compositions of the catalyst ink on the dispersion structure, the viscosity characteristics, and the CL structure. Four catalyst inks with different water/ethanol ratio were prepared, and the viscosity characteristics under different shear rate and the particle size distributions were measured without dilution. It was observed that the catalyst ink with higher ethanol ratio had higher viscosity and larger particle size, and it was suggested that decomposition product of ethanol by platinum catalyst affect the viscosity and the dispersion structure of the catalyst ink. Because the aggregates with in the catalyst ink strongly affect the CL structure, it is important to choose the solvent composition.

Fabrication of Pt-graded PEFC catalyst layers by intermittent double nozzle spray

Fabrication of Pt-graded PEFC catalyst layers by intermittent double nozzle spray

Investigation on structure and performance of PEFC catalyst layer prepared by inkjet printing and doctor blade method

Investigation on structure and performance of PEFC catalyst layer prepared by inkjet printing and doctor blade method

Nanoscale transport characteristics and catalyst utilization of vertically aligned carbon nanotube catalyst layers for fuel cell applications: Comprehensive stochastic modeling of composite morphological structures

Nanoscale transport characteristics and catalyst utilization of vertically aligned carbon nanotube (VACNT) catalyst layers (CLs) are evaluated using a fully statistical modeling approach based on the inherent random nature of the catalyst layer structures for fuel cell applications. Composite morphological structures of the catalyst layers are stochastically modeled with a 95% confidence level, and transport phenomena inside the catalyst layers are simulated using the D3Q19 lattice Boltzmann method (LBM). The effective diffusion coefficient of VACNT catalyst layers is predicted to be higher than that of the conventional catalyst layer, despite a relatively small pore diameter and a low-Knudsen diffusion coefficient. Consequently, the VACNT catalyst layers exhibit improved catalyst utilization compared to the conventional catalyst layers. These statistical results obtained from a series of numerical experiments confirm that the PEFC catalyst layers containing the VACNT catalyst supports can provide more efficient reactant transport, resulting in enhanced catalyst utilization for electrochemical reactions.

The Effect of Cracks on the In‐plane Electrical Conductivity of PEFC Catalyst Layers

AbstractFuel cell catalyst layers are a mixture of carbon, ionomer and platinum that are usually applied to the membrane during electrode assembly and can vary in structure for a number of reasons including the method of application, thickness of the layer, milling and heat treatments. Under some conditions significant cracking can occur which may or may not be beneficial to overall fuel cell performance but certainly has an effect on the catalyst's effective transport properties. The influence of cracking on the in‐plane electrical conductivity is studied experimentally and numerically through an image‐based approach. The conductivity is found to depend strongly on the crack structure which can be predicted with image based modelling. This fact is useful for determining the bulk conductivity from cracked samples for diagnostic purposes.

Ionomer Distribution Formation in PEFC Catalyst Layer by Pulse Injection and Dry Atmosphere Control

Ionomer Distribution Formation in PEFC Catalyst Layer by Pulse Injection and Dry Atmosphere Control

Analysis of Ionomer Distribution in PEFC Catalyst Layer with Different Ionomer-Carbon Ratio by Two-Stage Ion-Beam Processing

Analysis of Ionomer Distribution in PEFC Catalyst Layer with Different Ionomer-Carbon Ratio by Two-Stage Ion-Beam Processing

Measurement of Gas Transport Properties in PEFC Catalyst Layers Consisted of Different Carbon Materials by Using a Microfluidic Device

Gas transport properties in polymer electrolyte fuel cell catalyst layers consisted of different carbon materials were evaluated by using a microfluidic device. The microfluidic device consists of catalyst coated membrane and Si chip having parallel microchannels. Air and nitrogen were supplied to the two parallel channels in the device, respectively. Gas transport properties were evaluated by measuring outlet partial pressure of oxygen which was diffused through the catalyst layer. Adding multi-walled carbon nanotube in the catalyst layer instead of carbon black resulted in an increase of effective gas diffusivity, while geometrical porosity was decreased.

Non-Destructive Optical Measurement of Oxygen Concentration on PEFC Catalyst Layer

Tiny optical fiber sensor was applied to realize non-destructive measurement of oxygen concentration on PEFC catalyst layer.Insufficient supply of oxygen to the cathode catalyst layer caused by transport resistance is critical issue. Fundamental study in order to achieve efficient oxygen transport is required, however; non-destructive and in-situ measurement of oxygen concentration on the catalyst layer was not achieved. In this study, oxygen concentration on the catalyst layer under operating condition was measured by using a tiny optical fiber and oxygen indicator.Figure 1 shows experimental setup. Platinum tetrakis pentrafluoropheny porphine (PtTFPP) was employed as oxygen indicator[1]. When the PtTFPP is exposed to excitation light (l = 405 nm), phosphorescence emission (l = 650 nm) is produced. Phosphorescence intensity is determined by oxygen partial pressure. Thus, quantitative oxygen concentration can be obtained by measuring phosphorescence intensity and using calibration data. In this study, PtTFPP was painted at the edge of fused silica optical fiber of which diameter was 110 mm. In order to measure the phosphorescence intensity, bifurcated optical fibers were connected to excitation light source and spectrometer, respectively.The cell has an active area of 4 cm2 (2 × 2 cm) with straight channels. The channel width and depth were 1.0 and 1.0 mm, respectively, and the rib-to-channel ratio was 1. The carbon paper GDL without MPL (SIGRACET®24BA, SGL Group, USA) was used for both the anode and the cathode side. In order to measure the oxygen concentration on the catalyst layer with reducing insertion effect, optical fiber was inserted from the intrinsic large pore of GDL without boring a hole, and the edge of the fiber was touched on the catalyst layer surface. Flow rates of hydrogen and air were 50NmL/min and 125 NmL/min, respectively. Cell was operated under 33 °C because phosphorescence intensity becomes low under high temperature. Bubbler temperature was 22 °C (relative humidity was 50%)In the experiment, cell voltage was varied from 0.9 V to 0.2 V, and change of current density and oxygen concentration were measured. Figure 2 shows current-voltage (IV) characteristics. In the higher current density condition, variation of plots suggests emergence of flooding in the cathode. Since the flooding decreased gas diffusivity in the cathode catalyst layer, IV characteristics fluctuate at high current density and low cell voltage condition. As shown in Figure 3, change of oxygen concentration on the catalyst layer was measured successfully. Oxygen concentration decreases monotonically with decreasing cell voltage. Minimum value of the concentration was below 2 % at 0.2 V cell voltage. Figure 2 indicated that total cell performance was declined by flooding. On the other hand, local oxygen concentration of the catalyst layer was able to be measured by using a tiny optical fiber. Figure 3 shows large amount of oxygen was still consumed under high current density and it means that power generation concentrated on local surface of the catalyst layer where surface not covered with the liquid water.As discussed above, oxygen concentration on cathode catalyst layer was measured non-destructively. This technique holds further potential to measure oxygen transport phenomena on the catalyst layer under 60-80 °C cell operation by improving optical oxygen sensor sensitivity. AcknowledgementThis study is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Experimental setup in present study was assisted by Yoshihiko Aoki (Tokyo Institute of Technology). Reference[1] CS Chu., CA Lin, Sensors and Actuators B: Chemica, 2014, 195, 259-265. Figure 1

Non-Destructive Optical Measurement of Oxygen Concentration on PEFC Catalyst Layer

Insufficient supply of oxygen (O2) to the cathode Pt catalyst layer can be caused by transport resistance, and is a major problem in proton exchange membrane fuel cells. To clarify O2 transport phenomena, a non-destructive technique for measuring the O2 concentration is required. In this study, the O2 concentration at the catalyst layer was measured under operating conditions, using a tiny optical fiber and oxygen indicator. The O2 concentration decreased continuously with decreasing cell voltage. It was considered that a decrease in size of the reaction area was caused by water accumulation under the rib. Local O2 concentration measurements suggested that power generation was concentrated at the channel region.

Model and Experimental Analysis of Transport Resistances of Oxygen in Various Structures of PEFC Catalyst Layer

For efficient use of Pt catalyst in polymer electrolyte membrane fuel cells, the effects of cathode catalyst layer (CL) structure on the cell performance were evaluated by experiments and model analysis for various I/C ratios. To distinguish oxygen transport resistance in the CL from other resistances, IV characteristics were measured with different total pressures, and the limiting current density method was applied. Pore size distributions of the CLs were measured by nitrogen physisorption method, and the CL structural parameters were estimated using the estimation model developed by the authors. Using the model analysis of oxygen transport resistances in CL with the estimated parameters, the contributions of various oxygen transport resistances in CL were evaluated in further detail, and we discuss the relationship between the oxygen transport resistances and the cathode CL structure.

イオンビーム二段階加工による PEFC 触媒層内アイオノマー分布解析

イオンビーム二段階加工による PEFC 触媒層内アイオノマー分布解析

Tomographic Analysis of Polymer Electrolyte Fuel Cell Catalyst Layers: Methods, Validity and Challenges

Tomographic analysis of catalyst layers is one of the key methods to evaluate morphological properties in polymer electrolyte fuel cell PEFC catalyst layers. The most important applications for tomographic reconstruction are process control during manufacturing and investigations of ageing phenomena. We discuss all methods for tomographic imaging that are applicable for PEFC catalyst layers with an emphasis on focused ion beam / scanning electron microscopy tomography (FIB-SEMt). For now, no single tomographic method suffices to image all morphological properties of interest. Multi-tomographic approaches are therefore necessary. Phase fractions and phase-distributions are the most important properties to calculate within a tomographic reconstruction. We review further parameters that may be extracted from tomographic reconstructions and discuss open issues in first modeling approaches. Finally we discuss the validity of analyses within tomographic representations.

Stochastic Reconstruction and Transport Simulation of PEFC Catalyst Layers

A stochastic method for analysis and reconstruction of PEFC porous media is presented. The method uses statistical correlation function to analyze and parametrize the porous media structure. The improved reconstruction method uses a multigrid hierarchical simulated annealing technique to significantly reduce the reconstruction time and ascertains a long range connectivity. Analysis of the catalyst layers show it to be a homogeneous porous media with strong anisotropy in in-plane and through-plane directions. The stochastic reconstruction procedure is able to reconstruct 3D structures of the catalyst layer with similar statistical correlation functions as of the reference image. Molecular and Knudsen diffusion are simulated on the reconstructions using the open source package OpenFCST. The effective transport properties of the reconstructions is similar to that of the reference structure; however, the accuracy of the reconstructions has been limited by the imaging methods.