Atsushi Ohma

Membrane and Catalyst Performance Targets for Automotive Fuel Cells by FCCJ Membrane, Catalyst, MEA WG

The Fuel Cell Commercialization Conference of Japan (FCCJ) revised in 2011 a part of the target performance, durability, and cost of fuel cells for transportation application based on the latest technical data and knowledge obtained from the vehicle tests in the public thoroughfare and the simple systems to be expected in the commercialization stage. FCCJ also updated a methodology for testing membrane-electrode assemblies (MEAs) and its materials: electrolyte membranes and electrocatalysts. This revision is intended to assist the development of materials for industry, university, and research institutes from practicality, simplicity, and convenience points of view. The purpose of this paper is to describe the revised targets and the updated evaluation methods with the background technical information related to fuel cell vehicles (FCVs).

The Analysis of Performance Loss with Low Platinum Loaded Cathode Catalyst Layers

An analysis of the performance loss with low platinum loaded cathode catalyst layers (CCLs) was conducted. A modified 1-D calculation model in the CCL was developed in this study, where newly developed oxygen transport resistance in the direction from pores in the CCL to platinum surface (RO2,CCL-micro) and the effect of platinum oxide coverage on RO2,CCL-micro were taken into account. As a result, the increased voltage loss with low platinum loaded CCLs was demonstrated by the modified 1-D calculation model. The analysis results indicated that the oxygen transport loss caused by RO2,CCL-micro became a dominant factor of the performance loss with the low platinum loaded CCLs.

Analysis of Reactant Gas Transport in Catalyst Layers; Effect of Pt-loadings

An analytical in-situ technique was utilized to evaluate reactant gas transport resistance (Rother) in catalyst layers (CLs). It was found that Rother was increased with the decrease of Pt-loadings of the CLs although their thickness was decreased. Effective Knudsen diffusion coefficient was calculated from the pore size distribution and the porosity. It was estimated almost steady regardless of the Pt-loadings. To understand the increase of Rother, a simple transport model was established including Knudsen diffusion resistance in secondary pores and the local transport resistance around the Pt surface. Knudsen diffusion resistance should be decreased in lower Pt-loadings because of the shorter transport length. On the other hand, it was considered that the local transport resistance was increased with decreasing of the effective Pt surface area. Hence, that the effective Pt surface area might be one of the important factors for Rother in the CLs, especially in the lower Pt-loadings.

Fuel cell cathode catalyst layers from “green” catalyst inks

Fuel cell cathode catalyst layers deposited from a water-based catalyst ink formulation, using high water content and minimum volatile organic compounds, are investigated. Cathodes fabricated from a dispersion medium containing 96 wt% water are compared with cathodes fabricated from conventional alcohol-based inks containing 1-propanol–water 3 : 1 (w/w). The morphology of the two catalyst layers are similar, as are electrochemically-active surface areas at relative humidities of 100, 70 and 30% RH. Oxygen reduction kinetics obtained under fully humidified H2/O2 conditions exhibit similar Tafel slopes, 67 ± 3 mV per dec. However, cathodes prepared from water-based inks exhibit a lower H2/air fuel cell performance under 100, 70 and 30% RH while its porosity, obtained using mercury porosimetry, is slightly higher. EIS measurements obtained under high current density indicate that the mass transport resistance of the water-based catalyst layer is lower, which is consistent with porosimetric data, and suggests that factors other than mass transport limit the performance of the water-based cathode. The protonic resistance of the catalyst layers was found to be 105 and 145 mΩ cm2 for the propanol- and water-based catalyst layers, respectively. The differences are more pronounced when RH is decreased from 100 to 30%. This trend is consistent with the observed decrease in fuel cell performance under conditions of lower RH, and indicates that the higher proton resistance of the water-based catalyst layer is the cause of its lower fuel cell performance.

Analysis of Membrane Degradation Behavior During OCV Hold Test

Introduction Polymer electrolyte fuel cell (PEFC) is a very promising power source for automotive use. Durability of a membrane electrode assembly (MEA) is one of the vital issues to commercialize fuel cell vehicles (FCVs). In general, durability improvement of start-stop operation, load cycling, and high potential hold such as idling which is simulated in OCV hold, has been focused recently. In OCV hold, one of the main causes of the membrane degradation is believed due to deposited Pt (Pt band) in the membrane. In this study we investigated the membrane degradation behavior during OCV hold test by micro-Raman spectroscopy, which is a powerful tool to analyze morecular-structure change of the membrane, to validate the above hypothesis.

Surface oxide growth on platinum electrode in aqueous trifluoromethanesulfonic acid

Platinum in the form of nanoparticles is the key and most expensive component of polymer electrolyte membrane fuel cells, while trifluoromethanesulfonic acid (CF3SO3H) is the smallest fluorinated sulfonic acid. Nafion, which acts as both electrolyte and separator in fuel cells, contains -CF2SO3H groups. Consequently, research on the electrochemical behaviour of Pt in aqueous CF3SO3H solutions creates important background knowledge that can benefit fuel cell development. In this contribution, Pt electro-oxidation is studied in 0.1 M aqueous CF3SO3H as a function of the polarization potential (E(p), 1.10 ≤ E(p) ≤ 1.50 V), polarization time (t(p), 10(0) ≤ t(p) ≤ 10(4) s), and temperature (T, 278 ≤ T ≤ 333 K). The critical thicknesses (X1), which determines the applicability of oxide growth theories, is determined and related to the oxide thickness (d(ox)). Because X1 > d(ox) for the entire range of E(p), t(p), and T values, the formation of Pt surface oxide follows the interfacial place-exchange or the metal cation escape mechanism. The mechanism of Pt electro-oxidation is revised and expanded by taking into account possible interactions of cations, anions, and water molecules with Pt. A modified kinetic equation for the interfacial place exchange is proposed. The application of the interfacial place-exchange and metal cation escape mechanisms leads to an estimation of the Pt(δ+)-O(δ-) surface dipole (μ(PtO)), and the potential drop (V(ox)) and electric field (E(ox)) within the oxide. The Pt-anion interactions affect the oxidation kinetics by indirectly influencing the electric field within the double layer and the surface oxide.

Surface Electronic/Atomic Structure and Activation Energy on Pt(111), Pt3Cu(111), and PtCu(111) for PEFC Cathode

Theoretical analysis of oxygen reduction reaction (ORR) on cathode Pt(111), Pt3Cu(111), and PtCu(111) surfaces in a polymer electrolyte fuel cell (PEFC) is performed to investigate the ORR mechanisms on Pt(111) and the effect of alloying. From density functional theory (DFT) calculations, we found that the difference in adsorption energy of O2 molecules on the surface has a strong relation with the number of d-electrons and the position of the d-band center of the surface Pt atoms. From the results of the activation energy calculations using the unity bond index–quadratic exponential potential (UBI-QEP) method, we show that the oxygen atom coverage on Pt(111) surface has a strong influence on the ORR activity. In addition, it was found that Pt3Cu(111) and PtCu(111) surfaces have lower coverage compared with that of Pt(111) surface, which results in the enhancement of ORR activity.

Analysis of Proton Transport in Pseudo Catalyst Layers: Influence of Ionomer Content

Introduction A polymer electrolyte fuel cell (PEFC) is a promising primary power source and expected for automotive use. However, there are challenges still remaining for commercialization of fuel cell vehicles (FCVs) such as efficiency, durability, sub-zero startup, and cost reduction, etc. The greatest challenge of them is the cost reduction from the industrial point of view. Based on the cost reduction, an operation in low humidity and high current density is desirable. However, increase of IR loss is concerned in such condition. Especially, the decrease of proton conductivity in the catalyst layer could cause the decrease of the catalyst utilization. Therefore, it is important to understand proton transport phenomena in the catalyst layers in detail. In this study, we focused on proton the transport phenomena in the catalyst layers. We established an analysis technique using a pseudo catalyst layer (PCL) to understand proton transport phenomena in the layer. In this report, the influence of ionomer content on proton transport phenomena was examined with the analysis technique.

Microstructural observation of fuel cell catalyst inks by Cryo-SEM and Cryo-TEM

In order to improve the electricity generation performance of fuel cell electric vehicles, it is necessary to optimize the microstructure of the catalyst layer of a polymer electrolyte fuel cell. The catalyst layer is formed by a wet coating process using catalyst inks. Therefore, it is very important to observe the microstructure of the catalyst ink. In this study, the morphology of carbon-supported platinum (Pt/C) particles in catalyst inks with a different solvent composition was investigated by cryogenic scanning electron microscopy (cryo-SEM). In addition, the morphology of the ionomer, which presumably influences the formation of agglomerated Pt/C particles in a catalyst ink, was investigated by cryogenic transmission electron microscopy (cryo-TEM). The results of a cryo-SEM observation revealed that the agglomerated Pt/C particles tended to become coarser with a higher 1-propanol (NPA) weight fraction. The results of a cryo-TEM observation indicated that the actual ionomer dispersion in a catalyst ink formed a network structure different from that of the ionomer in the solvent.

Structural Effects of Cathode Catalyst Layer on the Performance of PEFC

The structure of cathode catalyst layer (CCL) has strong relationship with the performance of polymer electrolyte fuel cells (PEFCs). We investigated the relationship between the catalyst layer structure and the cell performance experimentally. Multi-layered CCL is used to investigate the effect of the layer design on the cell performance. Membrane side of CCL works, as a reaction area, more actively than the gas diffusion layer (GDL) side at low relative humidity (RH) due to the lower proton conductivity. On the other hand, when the cathode gas has less oxygen partial pressure at high RH, GDL side is more active than membrane side owing to low diffusivity of oxygen. We suggest that the volumetric catalyst concentration of the CCL membrane side should be higher at low RH, however at high RH with lower oxygen partial pressure in cathode gas, the GDL side should have higher concentration. Simple theoretical model is employed to see the behavior of the reaction distribution in the catalyst layer.Copyright