Polymer Electrolyte Fuel Cell Electrodes Research Articles

High-Resolution Mapping of the PFSA Polymer Distribution in PEFC Electrode Layers

Perfluorosulfonic acid (PFSA) ionomer is a key component within electrode layers for the polymer electrolyte fuel cell (PEFC) since the solid electrolyte enables proton transport to the active catalyst site. Both the through-layer loading on a 100-nm length scale as well as the local film thickness distribution on a 1-nm scale of the ionomer in the porous layer structure are important to electrode performance so quantitative measurements of both properties are desired. In this paper, we present ionomer mapping measurements at both length scales using two common PEFC electrode constructions.

Modeling Hierarchical Non-Precious Metal Catalyst Cathodes for PEFCs Using Multi-Scale X-ray CT Imaging

This paper reports the development of a model for simulating polymer electrolyte fuel cells (PEFCs) with non-precious metal catalyst (NPMC) cathodes. NPMCs present an opportunity to dramatically reduce the cost of PEFC electrodes by removing the costly Pt catalyst. To address the significant transport losses in thick NPMC cathodes (ca. >60 µm), we developed a hierarchical electrode model that resolves the unique structure of the NPMCs we studied. A unique feature of the approach is the integration of the model with morphology data extracted from nano-scale resolution X-ray computed tomography (nano-CT) imaging of the electrodes. A notable finding is the impact of the liquid water accumulation in the electrode and the significant performance improvement possible if electrode flooding is mitigated.

Morphological Analysis of Polymer Electrolyte Fuel Cell Electrode Using High Resolution X-Ray Computed Tomography and Subsequent Performance Analysis

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Morphological Analysis of Polymer Electrolyte Fuel Cell Electrode Using High Resolution X-ray Computed Tomography and Subsequent Performance Analysis

Here we present a study on the morphology of the porous polymer electrolyte fuel cell (PEFC) electrodes as well as the individual catalyst agglomerates that assemble into the electrode structure, and how those characteristics influence fuel cell performance. The agglomerates and the porous electrode structure were three-dimensionally imaged using non-invasive and non-destructive nano-scale resolution X-ray computed tomography (nano-CT). The GDE fabricated from the same catalyst layer slurry (ink) as the imaged samples were electrochemically evaluated in a PEFC. By using the same ink batch in the two imaging steps, and in the fuel cell testing and then systematically varying ink processing, we have delineated the complex relationships between catalyst ink, electrode structure, and PEFC performance. At the higher current densities, we found that the electrode comprising smaller agglomerates performed better than the one comprising relatively larger agglomerates.

Morphological Analyses of Polymer Electrolyte Fuel Cell Electrodes with Nano‐Scale Computed Tomography Imaging

AbstractWe report a three‐dimensional (3D), pore‐scale analysis of morphological and transport properties for a polymer electrolyte fuel cell (PEFC) catalyst layer. The 3D structure of the platinum/carbon/Nafion electrode was obtained using nano‐scale resolution X‐ray computed tomography (nano‐CT). The 3D nano‐CT data was analyzed according to several morphological characteristics, with particular focus on various effective pore diameters used in modeling gas diffusion in the Knudsen transition regime, which is prevalent in PEFC catalyst layers. The pore diameter metrics include those based on chord length distributions, inscribed spheres, and surface area. Those pore diameter statistics are evaluated against computational pore‐scale diffusion simulations with local gas diffusion coefficients determined from the local pore size according to the Bosanquet formulation. According to our comparison, simulations that use local pore diameters defined by inscribed spheres provide effective diffusion coefficients that are consistent with chord‐length based estimations for an effective Knudsen length scale. By evaluating transport rates in regions of varying porosity within the nano‐CT data, we identified a Bruggeman correction scaling factor for the effective diffusivity.

Evaluation of the Pt/C Catalyst for Fuel Cells Prepared by a NanoparticleFormation Pulsed Arc Plasma Source

We have proposed a new platinum catalyst supporting method using a nano particles formation pulse arc plasma source.This supporting method has the following features.1) Distribution of nano particle diameter is small. 2) Nano particle diameter can be easily controlled by changing discharge parameter. 3) Nano particle diameter can be kept constant while its deposition rate is changed.4) Since nano particles are adhered to surface of the supporting material strongly, aggregation of the catalysts can be suppressed even at high temperature..5) Platinum catalyst on the carbon powder for polymer electrolyte fuel cell electrodes was prepared using this method, and current / voltage characteristic of its MEA was evaluated. Although its performance is a little inferior compared with the conventional catalyst market prepared by a wet process , it may be concluded that a good characteristic is acquired taking account of platinum quantity of 1/10 of the conventional one.

Systematic Evaluation of Polymer Electrolyte Fuel Cell Electrodes with Hydrocarbon Polyelectrolytes by Considering the Polymer Properties

When using a hydrocarbon polyelectrolyte, such as sulfonated poly(arylene ether sulfone) (SPES), in a catalyst layer of a polymer electrolyte fuel cell, significantly decreased performance was observed. We clarified the main reason for this decrease from a systematic investigation into the humidity dependence of the polymer properties, such as the swelling ratio and oxygen permeability, and by comparison with the performance of a membrane electrode assembly (MEA) with SPES electrodes. The swelling ratio of SPES increased markedly at >90% relative humidity (RH), which led to blocking of the catalyst layer pores. This tendency semiquantitatively corresponds to the decreased performance of MEA at >90% RH, considering the water generated by the reaction. Therefore, the main cause of the decrease is the blocking of oxygen diffusion in the pores from the swelling of SPES in the catalyst layer rather than oxygen permeation through the SPES layer, as previously suggested.

Resolving the Three‐Dimensional Microstructure of Polymer Electrolyte Fuel Cell Electrodes using Nanometer‐Scale X‐ray Computed Tomography

AbstractThe electrodes of a polymer electrolyte fuel cell (PEFC) are composite porous layers consisting of carbon and platinum nanoparticles and a polymer electrolyte binder. The proper composition and arrangement of these materials for fast reactant transport and high electrochemical activity is crucial to achieving high performance, long lifetimes, and low costs. Here, the microstructure of a PEFC electrode using nanometer‐scale X‐ray computed tomography (nano‐CT) with a resolution of 50 nm is investigated. The nano‐CT instrument obtains this resolution for the low‐atomic‐number catalyst support and binder using a combination of a Fresnel zone plate objective and Zernike phase contrast imaging. High‐resolution, non‐destructive imaging of the three‐dimensional (3D) microstructures provides important new information on the size and form of the catalyst particle agglomerates and pore spaces. Transmission electron microscopy (TEM) and mercury intrusion porosimetry (MIP) is applied to evaluate the limits of the resolution and to verify the 3D reconstructions. The computational reconstructions and size distributions obtained with nano‐CT can be used for evaluating electrode preparation, performing pore‐scale simulations, and extracting effective morphological parameters for large‐scale computational models.

Spatially Resolved, In Situ Potential Measurements through Porous Electrodes As Applied to Fuel Cells

We report the development and use of a microstructured electrode scaffold (MES) to make spatially resolved, in situ, electrolyte potential measurements through the thickness of a polymer electrolyte fuel cell (PEFC) electrode. This new approach uses a microfabricated apparatus to analyze the coupled transport and electrochemical phenomena in porous electrodes at the microscale. In this study, the MES allows the fuel cell to run under near-standard operating conditions, while providing electrolyte potential measurements at discrete distances through the electrode's thickness. Here we use spatial distributions of electrolyte potential to evaluate the effects of Ohmic and mass transport resistances on the through-plane reaction distribution for various operating conditions. Additionally, we use the potential distributions to estimate the ionic conductivity of the electrode. Our results indicate the in situ conductivity is higher than typically estimated for PEFC electrodes based on bulk polymer electrolyte membrane (PEM) conductivity.

Tungsten oxide in polymer electrolyte fuel cell electrodes—A thin-film model electrode study

Thin films of WO x and Pt on WO x were evaporated onto the microporous layer of a gas diffusion layer (GDL) and served as model electrodes in the polymer electrolyte fuel cell (PEFC) as well as in liquid electrolyte measurements. In order to study the effects of introducing WO x in PEFC electrodes, precise amounts of WO x (films ranging from 0 to 40 nm) with or without a top layer of Pt (3 nm) were prepared. The structure of the thin-film model electrodes was characterized by scanning electron microscopy and X-ray photoelectron spectroscopy prior to the electrochemical investigations. The electrodes were analyzed by cyclic voltammetry and the electrocatalytic activity for hydrogen oxidation reaction (HOR) and CO oxidation was examined. The impact of Nafion in the electrode structure was examined by comparing samples with and without Nafion solution sprayed onto the electrode. Fuel cell measurements showed an increased amount of hydrogen tungsten bronzes formed for increasing WO x thicknesses and that Pt affected the intercalation/deintercalation process, but not the total amount of bronzes. The oxidation of pre-adsorbed CO was shifted to lower potentials for WO x containing electrodes, suggesting that Pt-WO x is a more CO-tolerant catalyst than Pt. For the HOR, Pt on thicker films of WO x showed an increased limiting current, most likely originating from the increased electrochemically active surface area due to proton conductivity and hydrogen permeability in the WO x film. From measurements in liquid electrolyte it was seen that the system behaved very differently compared to the fuel cell measurements. This exemplifies the large differences between the liquid electrolyte and fuel cell systems. The thin-film model electrodes are shown to be a very useful tool to study the effects of introducing new materials in the PEFC catalysts. The fact that a variety of different measurements can be performed with the same electrode structure is a particular strength.

Relative Humidity Dependence of Pt Utilization in Polymer Electrolyte Fuel Cell Electrodes: Effects of Electrode Thickness, Ionomer-to-Carbon Ratio, Ionomer Equivalent Weight, and Carbon Support

Pt utilization, i.e., the ratio of the electrochemically active Pt surface area of an MEA to the total Pt surface area of the catalyst, was analyzed at 20–100% RH, and 40–80°C by a newly developed technique using CO as a probe. The Pt utilization significantly decreased with the decreasing relative humidity (RH) for a Pt/C catalyst in which the Pt particles are supported not only on the outer surface, but also within the pores of the carbon particles, while it remained high even at a low RH for the samples with Pt particles supported only on the outer surface of the carbon particles. We presume that this is because the Pt particles within the pores are not in contact with the ionomer therefore, the protons cannot access them at a low RH.

Influence of arc duration time on the synthesis of carbon nanohorns by a gas-injected arc-in-water system: application to polymer electrolyte fuel cell electrodes

Single-walled carbon nanohorns (SWCNH) can be synthesized by a gas-injected arc-in-water (GI-AIW) method at low cost. This study revealed the influence of arc discharge duration time on the yield of SWCNH in the synthesis using this method. As a result, the yield of SWCNH increased with the arc discharge duration time, while an increase in arc plasma temperature was observed. This result can be explained by considering that the increased temperature of the reaction zone can lead to an increase in carbon radical density, which can positively affect the production yield. Here, two arc discharge modes, continuous and discrete, were generated so that modes having different temperatures with the same discharge current could be compared. From this investigation, it was suggested that an excessive ion density may cause a decrease in SWCNH production. The as-grown SWCNH possessed a high specific surface area when the arc discharge current was optimized with continuous arc mode using the GI-AIW method. Such a product can support Pt catalyst with high dispersivity, leading to high performance in polymer electrolyte fuel cells as catalyst layers.

Simulated start–stop as a rapid aging tool for polymer electrolyte fuel cell electrodes

The corrosion stability of supported catalysts as employed in state of the art intermediate temperature polymer electrolyte fuel cells has been studied by means of simulated start–stop cycling (150 cycles). The carbon dioxide formation from the air electrode has been monitored during repeated cycling runs and the loss of catalyst support has been correlated with performance drops. Degradation effects have been studied at different current densities in order to differentiate between kinetic and mass transport effects. Finally, correlations of this accelerated aging tool with a more realistic durability test over 4000 h and 157 start–stop cycles have been made and the good agreement between simulated and realistic approaches has been confirmed, demonstrating the high value of the experimental approach and analysis.

3D Imaging of Polymer Electrolyte Fuel Cell Electrodes

The degradation of Polymer Electrolyte Fuel Cell (PEFC) electrodes was examined by electrochemical and 3D imaging methods. A potential cycling test was conducted with a commercial catalyst coated membrane (CCM) where the cathode side of the cell was subjected to a potential square wave of 0.6V and open circuit voltage. After 24000 cycles, the PEFC performance remains nearly unchanged except for minor current losses in the low current regime. According to hydrogen under potential desorption measurements the electrochemically active Pt area decreases to 25% of the initial value. Synchrotron-based X-Ray tomography of the CCMs yields thickness distributions of catalyst layers and the membrane. A bubble like structure of the cathode layer is observed. Focused Ion Beam/Scanning Electron Microscopy-serial sectioning was employed to determine the pore structure of the cathode. The porosity before and after degradation was almost unaltered by the cycling test. TEM lamellas of the pristine and degraded cathode catalyst layers show an increase of the Pt particle size from 2.2nm to 4.6nm. Under the given conditions, an increase of hydrogen permeability across the membrane and a loss of Pt surface area are the predominant degradation mechanisms.

Stability Enhancement of Pd Catalysts by Compositing with Polypyrrole Layer for Polymer Electrolyte Fuel Cell Electrodes

Pd catalyst composited with a conducting polymer of polypyrrole is prepared and investigated as a stable H2 electrooxidation catalyst in an acidic condition. A significant stability enhancement of Pd by compositing electropolymerized polypyrrole can be observed and its mechanistic scheme for the stability is discussed through several electrochemical and physicochemical characterizations on the polypyrrole-composited Pd catalysts.

PEDOT-PSSA as an alternative support for Pt electrodes in PEFCs

Poly (3,4-ethylenedioxythiophene) (PEDOT) and poly (styrene sulphonic acid) (PSSA) supported platinum (Pt) electrodes for application in polymer electrolyte fuel cells (PEFCs) are reported. PEDOT-PSSA support helps Pt particles to be uniformly distributed on to the electrodes, and facilitates mixed electronic and ionic (H+-ion) conduction within the catalyst, ameliorating Pt utilization. The inherent proton conductivity of PEDOT-PSSA composite also helps reducing Nafion content in PEFC electrodes. During prolonged operation of PEFCs, Pt electrodes supported onto PEDOT-PSSA composite exhibit lower corrosion in relation to Pt electrodes supported onto commercially available Vulcan XC-72R carbon. Physical properties of PEDOT- PSSA composite have been characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. PEFCs with PEDOT-PSSA-supported Pt catalyst electrodes offer a peak power-density of 810 mW cm−2 at a load current-density of 1800 mA cm−2 with Nafion content as low as 5 wt.% in the catalyst layer. Accordingly, the present study provides a novel alternative support for platinized PEFC electrodes.

PEFC electrodes based on carbon black supporting electrocrystallised nanostructured Pt particles

This work is focused on the study of electro-crystallisation of Pt nanostructured particles used as catalyst on carbon black support for polymer electrolyte fuel cells electrodes. Electrochemical single and multiple pulse galvanostatic depositions have been applied defining the best operational parameters leading to a highly nanostructured electro-catalysts morphology. Electrochemical measurements such as cyclic voltammetries have been carried out in order to determine the electrochemical active surface together with morphological analysis by means of scanning electron microscopy. The influence of electro-deposition parameters on the Pt loading have been studied and optimised. The investigated materials have been then characterised by means of polarisation curves. The electrodeposited catalyst cathodes present performances and maximum specific power density values similar or better than those of standard E-TEK electrode, prepared by conventional powder-type technique. These results strongly encourage the implementation of cathodes obtained by electro-deposition in a real-world device.

Influence of binder properties on kinetic and transport processes in polymer electrolyte fuel cell electrodes

The objectives of this study are to estimate the contributions of kinetic, ohmic and mass transport overpotentials to the overall voltage loss in polymer electrolyte membrane fuel cell (PEMFC) electrodes and to relate these overpotentials to electrode binder properties such as ionic conductivity, ion exchange capacity (IEC) and O(2) permeability. The model electrode binders studied were perfluorosulfonic acid ionomers (PFSA; of IECs 1.35 meq g(-1) and 0.95 meq g(-1)), sulfonated poly ether ether ketone (SPEEK; of IECs 1.35, 1.75 and 2.1 meq g(-1)) and sulfonated poly sulfone (SPSU; of IEC 1.5 meq g(-1)). The O(2) permeability of these binders varied from 0.15 x 10(-12) mol cm(-1) s(-1) for SPSU to 6 x 10(-12) mol cm(-1) s(-1) for PFSA IEC 0.95 meq g(-1) at 80 degrees C and 75%RH. The electrodes prepared were characterized by cyclic voltammetry to estimate electrochemically active surface area (ECA) of platinum. Steady state polarization (V-I) experiments were performed with hydrogen as fuel and oxidants including O(2), 21% O(2)/N(2) (air), 21% O(2)/He (Helox) and 4% O(2)/N(2). The V-I data obtained was analyzed to determine the relative contributions of the different sources of polarization in the electrode. Electrodes prepared with PFSA binders had similar ECAs of 28 m(2) g(-1)-Pt, while those prepared using hydrocarbon binders had an ECA of 10 to 14 m(2) g(-1)-Pt at 80 degrees C and 75%RH. The same trend was seen in mass activity. At optimized binder loadings, a semi-quantitative relationship was obtained relating binder O(2) permeability to the mass transport losses within the electrode. Furthermore, a novel semi-quantitative method of plotting helox-air voltage gain against O(2)-air gain was employed to probe the O(2) transport limitations in the electrodes. Based on this analysis, it is suggested that the SPEEK and SPSU bound electrodes suffered from binder phase diffusion limitations in addition to gas phase diffusion limitation, while the PFSA bound electrodes predominantly exhibited gas-phase diffusion limitations.

Nafion Structural Phenomena at Platinum and Carbon Interfaces

Neutron reflectometry was used to examine the interactions of polymer electrolyte fuel cell (PEFC) materials that comprise the triple-phase interface. Smooth, idealized layers of Nafion on glassy carbon (GC) and Pt surfaces were used to experimentally model the PEFC electrode interfaces. Different multilayer structures of Nafion were found in contact with the Pt or GC surfaces. These structures showed separate hydrophobic and hydrophilic domains formed within the Nafion layer when equilibrated with saturated D(2)O vapor. A hydrophobic Nafion region was formed adjacent to a Pt film. However, when Nafion was in contact with a PtO surface, the Nafion at the Pt interface became hydrophilic. The adsorbed oxide layer caused a long-range restructuring of the perfluorosulfonic acid polymer chains that comprise Nafion. The thicknesses of the hydrophobic and hydrophilic domains changed to the same magnitude when the oxide layer was present compared to a thin hydrophobic domain in contact with Pt. A three-layer Nafion structure was formed when Nafion was in direct contact with GC. The findings in this research are direct experimental evidence that both the interfacial and long-range structural properties of Nafion are affected by the material with which it is in contact. Evidence of physical changes of aged Nafion films was obtained, and the results showed a permanent increase in the thickness of the Nafion film and a decrease in the scattering length density (SLD), which are attributed to irreversible swelling of the Nafion film. The aging also resulted in a decrease in the SLD of the GC substrate, which is likely due to either an increase in surface oxidation of the carbon or loss of carbon mass at the GC surface.

NMR Study on PEFC Electrodes

Water transport within a polymer electrolyte fuel cell (PEFC) electrode has been probed. A method to reveal tortuosity within the PEFC electrode is demonstrated. NMR relaxation and diffusion measurements are applied to yield information about molecular local interaction and long range diffusion, respectively, within the electrode sample. The diffusion coefficient (D) and relaxation time (T1) are measured from electrode samples which have been prepared using a complete decal process followed by equilibration under various RH conditions. Tortuosity within the electrode can be quantified by comparing the diffusion coefficient and relaxation result.