Equation Elucidating the Catalyst-Layer Proton Conductivity in a Polymer Electrolyte Fuel Cell Based on the Ionomer Distribution Determined Using Small-Angle Neutron Scattering

The performance of a polymer electrolyte fuel cell can be enhanced by improving the proton conductivity of the catalyst layer, where the oxygen reduction reaction generates electrochemical power. Protons are conducted through the ionomer coatings on catalyst-supporting carbon particles, which form porous structures that facilitate oxygen diffusion during the reaction within the catalyst layer. Therefore, while a higher ionomer content in the catalyst layer is favorable, the proton conductivity is additionally governed by the type of carbon support. As the influence of the ionomer distribution is not fully understood, we introduce a novel proton conductivity model for use in simulating catalyst layers with various amounts of ionomers and different carbon types. This proton conductivity model considers that several ionomers occur as thin films with drastically suppressed proton conductivities. Although evaluating the thin-film ionomer fraction is challenging, proton-conducting ion clusters in thick-film ionomers have been detected by characterizing the catalyst layers via small-angle neutron scattering. Our model reveals that reducing the fraction of the thin-film ionomer or avoiding factors that suppress its proton conduction improves the performance of the catalyst layer.

Effects of Electrostatic Force and Network Structure on the Stability of Proton-Exchange Membrane Fuel Cell Catalyst Ink

The stability of the catalyst slurry of a proton-exchange membrane fuel cell (PEMFC) is of great significance to its large-scale production and commercialization. In this study, three kinds of slurries with different stabilities were prepared using different probe ultrasonic powers. The influence of electrostatic force and network structure on slurry stability was also studied. In addition, the catalyst layer (CL) and membrane electrode assembly (MEA) were further tested to determine the relationship between slurry stability, CL, and MEA performance. The results showed that the slurry prepared with 600 W dispersion power had the least agglomeration on day 12, which is due to the clusters in the slurry having the smallest average particle size and the largest surface area, thereby allowing them to absorb the most Nafion and have the largest electrostatic force to inhibit agglomeration. However, the slurry with 1200 W dispersion power had the least sedimentation after 9.4 days because the strength of the network structure in the slurry strengthened the most, resulting in a significant increase in viscosity and inhibition of sedimentation. Electrochemical tests showed that the MEA gradually exhibited worse electrical performance and higher impedance due to the agglomeration of catalyst particles caused by the standing process. Altogether, this study provides insights to better understand and regulate the stability of catalyst slurries.

Interfacial Distribution of Nafion Ionomer Thin Films on Nitrogen-Modified Carbon Surfaces

Chemically modified carbon supports for the cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) show considerable promise for boosting the oxygen reduction reaction. This study evaluated the ionomer distribution of Nafion ionomer thin films on nitrogen (N)-modified carbon surfaces along their depth direction. Neutron reflectivity (NR) measurements performed using the double-contrast technique with H2O and D2O revealed that the introduction of N functional groups to carbon thin films promoted ionomer adsorption onto the surface under wet conditions (22 °C, 85% relative humidity). Molecular dynamics (MD) simulations conducted to verify the origin of the robust contact between the ionomer and N-modified carbon surface revealed an ionomer adsorption mechanism on the N-modified carbon surfaces, which involved Coulomb interactions between the positively charged carbon surface and the ionomer side chains with negatively charged sulfonic acid groups. The positive surface charge, which was determined using the contents of the N functional groups estimated by X-ray photoelectron spectroscopy, was found to be sufficient as an impetus for ionomer adsorption. This strategy involving NR measurements and MD simulations can provide insights into the solid-ionomer interfacial structures in a cathode catalyst layer and can therefore be extensively employed in studies on PEFCs.

Effect of Catalyst Ink and Formation Process on the Multiscale Structure of Catalyst Layers in PEM Fuel Cells

The structure of a catalyst layer (CL) significantly impacts the performance, durability, and cost of proton exchange membrane (PEM) fuel cells and is influenced by the catalyst ink and the CL formation process. However, the relationship between the composition, formulation, and preparation of catalyst ink and the CL formation process and the CL structure is still not completely understood. This review, therefore, focuses on the effect of the composition, formulation, and preparation of catalyst ink and the CL formation process on the CL structure. The CL structure depends on the microstructure and macroscopic properties of catalyst ink, which are decided by catalyst, ionomer, or solvent(s) and their ratios, addition order, and dispersion. To form a well-defined CL, the catalyst ink, substrate, coating process, and drying process need to be well understood and optimized and match each other. To understand this relationship, promote the continuous and scalable production of membrane electrode assemblies, and guarantee the consistency of the CLs produced, further efforts need to be devoted to investigating the microstructure of catalyst ink (especially the catalyst ink with high solid content), the reversibility of the aged ink, and the drying process. Furthermore, except for the certain variables studied, the other manufacturing processes and conditions also require attention to avoid inconsistent conclusions.

Bio-interactive nanoarchitectonics with two-dimensional materials and environments

Like the proposal of nanotechnology by Richard Feynman, the nanoarchitectonics concept was initially proposed by Masakazu Aono. The nanoarchitectonics strategy conceptually fuses nanotechnology with other research fields including organic chemistry, supramolecular chemistry, micro/nanofabrication, materials science, and bio-related sciences, and aims to produce functional materials from nanoscale components. In this review article, bio-interactive nanoarchitectonics and two-dimensional materials and environments are discussed as a selected topic. The account gives general examples of nanoarchitectonics of two-dimensional materials for energy storage, catalysis, and biomedical applications, followed by explanations of bio-related applications with two-dimensional materials such as two-dimensional biomimetic nanosheets, fullerene nanosheets, and two-dimensional assemblies of one-dimensional fullerene nanowhiskers (FNWs). The discussion on bio-interactive nanoarchitectonics in two-dimensional environments further extends to liquid-liquid interfaces such as fluorocarbon-medium interfaces and viscous liquid interfaces as new frontiers of two-dimensional environments for bio-related applications. Controlling differentiation of stem cells at fluidic liquid interfaces is also discussed. Finally, a conclusive section briefly summarizes features of bio-interactive nanoarchitectonics with two-dimensional materials and environments and discusses possible future perspectives.

Control of Cluster Structures in Catalyst Inks by a Dispersion Medium

The cluster structure in the catalyst ink of a proton exchange membrane fuel cell determines its performance. The interaction among solvent, ionomer, and catalyst in ink determines the cluster structure and affects the microstructure and surface morphology of the catalyst layer, which is of great significance to improve the conductivity of the catalyst layer to protons, electrons, and water. First, the dissolved state of the main chain and the side chain of the ionomer in solvent was characterized. The results of relative viscosity, ζ-potential, effective proton fraction, and nuclear magnetic resonance (NMR) showed that the alcohol aqueous solution promoted the stretching electrolysis of the main chain and the side chain of the ionomer more than the pure aqueous solvent, making the ionomer clusters smaller. The rheological test of the ink shows that the pure water solvent ink has the largest cluster and the strongest network structure. Under the test conditions, the clusters in the ink can be reconstructed quickly after breakage through viscous shearing. The addition of alcohols will make the clusters in the ink smaller and the network structure brittle. After the clusters and the network structure are damaged, they will slowly recombine and the viscosity in the ink will gradually recover. Ethanol will minimize the clusters in the ink, and the network structure in the ink is the weakest. The effect of the network strength on the cluster structure is weakened by reducing the solid content in the ink. The amplitude scanning test shows that the network structure in the slurry is almost eliminated after reducing the solid content, the storage modulus of ink with water, 50 wt % isopropyl alcohol (IPA), 50 wt % n-propanol (NPA), and 50 wt % ethanol (ET) decreases in turn, as well as the liquid viscosity behavior increases and the cluster particle size in the ink decreases. In conclusion, more dispersed ionomers and alcohol molecules with smaller molecular structures are more conducive to the dispersion of clusters in the ink.

Interplay between Interparticle Potential and Adsorption Structure in Nanoparticle Dispersions with Polymer Addition as Displayed by Small-Angle Scattering

The effect of polymer adsorption on the dispersion stability of particles is an important subject applicable to various manufacturing processes. In this study, small-angle scattering was used to examine the relationship between interparticle potential and polymer adsorption in dispersions of nanoparticles with an 81 Å radius containing two types of polymers. Small-angle X-ray scattering (SAXS) measurements in a silica/polyacrylamide (PAAm) system showed an increase in interparticle attractive interactions as PAAm concentration was increased. In a silica/poly(ethylene oxide) (PEO) system, the correlation between PEO concentration and interparticle potential strength became negligible at higher concentrations. Hence, the contrast variation small-angle neutron scattering (CV-SANS) method was employed to evaluate the interparticle potential and polymer adsorption simultaneously. CV-SANS revealed that PAAm was adsorbed to silica particles with a polymer shell layer thickness of 186 Å. The attractive potential observed in the absorbed layer region can be attributed to bridging PAAm molecules between the silica particles. By contrast, CV-SANS of the silica/PEO system indicated a low-polymer-concentration layer with a thickness of 34 Å around silica particles, indicating weak adsorption of PEO molecules. Negligible interaction between PEO and silica particles was assumed to be the origin of the depletion stabilization from excess polymer addition. Thus, quantitative analyses conducted using SAXS and CV-SANS measurements for the first time clearly demonstrated a difference in the adsorption structure of the polymer, which induces changes in the interaction potential between nanoparticles.

Effect of Dispersion Solvents and Ionomers on the Rheology of Catalyst Inks and Catalyst Layer Structure for Proton Exchange Membrane Fuel Cells

This study investigated the effects of the dielectric constant (ε) of a dispersion solvent and ionomer content on the rheology of graphitized carbon (GC)-supported Pt catalyst ink and the structure of catalyst layers (CLs). The ionomer dispersions and catalyst inks were tested using rheological techniques, zeta (ξ) potential, and dynamic light scattering measurements. Results showed that increases in the solvent ε or ionomer content increased the ξ-potential of catalyst particles in the ink, which reduced the catalyst agglomerate size. Steady-state and oscillation scans showed that the Pt/GC catalyst ink had shear-thinning properties and gel-like behavior. The ink with a solvent ε of 40 tended to be more Newtonian fluid, with low yield stress (σ_y). The ionomer content altered the rheology of the ink by changing the internal interaction of inks. Solvents with ε of 70 and 55 enhanced the adsorption of ionomers onto catalysts, thereby increasing the adhesion between ink particles and reducing the risk of CL cracking. As the ionomer content increased, the catalyst absorbed more ionomers in inks, increasing the fracture toughness of CLs, which reduced the crack width.

Distinguishing Adsorbed and Deposited Ionomers in the Catalyst Layer of Polymer Electrolyte Fuel Cells Using Contrast-Variation Small-Angle Neutron Scattering

The ionomers distributed on carbon particles in the catalyst layer of polymer electrolyte fuel cells (PEFCs) govern electrical power via proton transport and oxygen permeation to active platinum. Thus, ionomer distribution is a key to PEFC performance. This distribution is characterized by ionomer adsorption and deposition onto carbon during the catalyst-ink coating process; however, the adsorbed and deposited ionomers cannot easily be distinguished in the catalyst layer. Therefore, we identified these two types of ionomers based on the positional correlation between the ionomer and carbon particles. The cross-correlation function for the catalyst layer was obtained by small-angle neutron scattering measurements with varying contrast. From fitting with a model for a fractal aggregate of polydisperse core-shell spheres, we determined the adsorbed-ionomer thickness on the carbon particle to be 51 Å and the deposited-ionomer amount for the total ionomer to be 50%. Our technique for ionomer differentiation can be used to optimally design PEFC catalyst layers.

In Situ Small-Angle X-ray Scattering Studies on the Growth Mechanism of Anisotropic Platinum Nanoparticles

Shape-controlled platinum nanoparticles exhibit extremely high oxygen reduction activity. Platinum nanoparticles were synthesized by the reduction of a platinum complex in the presence of a soft template formed by organic surfactants in oleylamine. The formation of platinum nanoparticles was investigated using in situ small-angle X-ray scattering experiments. Time-resolved measurements revealed that different particle shapes appeared during the reaction. After the nuclei were generated, they grew into anisotropic rod-shaped nanoparticles. The shape, size, number density, reaction yield, and specific surface area of the nanoparticles were successfully determined using small-angle X-ray scattering profiles. Anisotropic platinum nanoparticles appeared at a low reaction temperature (∼100 °C) after a short reaction time (∼30 min). The aspect ratio of these platinum nanoparticles was correlated with the local packing motifs of the surfactant molecules and their stability. Our findings suggest that the interfacial structure between the surfactant and platinum nuclei can be important as a controlling factor for tailoring the aspect ratio of platinum nanoparticles and further optimizing the fuel cell performance.

Effect of Dispersion Medium Composition and Ionomer Concentration on the Microstructure and Rheology of Fe-N-C Platinum Group Metal-free Catalyst Inks for Polymer Electrolyte Membrane Fuel Cells

We present an investigation of the microstructure and rheological behavior of catalyst inks consisting of Fe-N-C platinum group metal-free catalysts and a perfluorosulfonic acid ionomer in a dispersion medium (DM) of water and 1-propanol (nPA). The effects of the ionomer-to-catalyst (I/C) ratio and weight percentage of water (H₂O %) in the DM on the ink microstructure were studied. Steady-shear and dynamic-oscillatory-shear rheology, in combination with synchrotron X-ray scattering, was utilized to understand interparticle interactions and the level of agglomeration of the inks. In the absence of the ionomer, the inks were significantly agglomerated, approaching a gel-like microstructure for catalyst concentrations as low as 2 wt %. The effect of H₂O % in the DM on particle agglomeration was found to vary with particle concentration. In concentrated inks (≥2 wt % catalyst), increasing H₂O % was found to increase agglomeration because of the hydrophobic nature of the catalysts. In dilute inks (<1 wt % catalyst), the trend was reversed with increasing H₂O %, suggesting that electrostatic interactions are dominating the behavior. In inks with 5 wt % catalyst, the addition of an ionomer was found to significantly stabilize the catalyst against agglomeration. Maximum stability was observed at 0.35 I/C for all DM H₂O % studied. At high ionomer concentrations (I/C > 0.35), interesting differences were observed between nPA-rich inks (H₂O % ≤ 50%) and H₂O-rich (82% H₂O) inks. The nPA-rich inks remained predominantly stable-ink viscosity only weakly increased with I/C and the Newtonian behavior was maintained for I/C up to 0.9. In contrast, the H₂O-rich inks exhibited a significant increase in viscoelasticity with increasing I/C, suggesting flocculation of the catalyst by the ionomer. These differences suggest that the nature of the interactions between the ionomer and catalyst is highly dependent on the H₂O % in the DM.