We present a rheological investigation of fuel cell catalyst inks. The effects of ink parameters, which include carbon black-support structure, Pt presence on carbon support (Pt−carbon), and ionomer (Nafion) concentration, on the ink microstructure of catalyst inks were studied using rheometry in combination with ultrasmall-angle X-ray scattering (USAXS) and dynamic light scattering (DLS). Dispersions of a high-surface-area carbon (HSC), or Ketjen black type, demonstrated a higher viscosity than Vulcan XC-72 carbon due to both a higher internal porosity and a more agglomerated structure that increased the effective particle volume fraction of the inks. The presence of Pt catalyst on both the carbon supports reduced the viscosity through electrostatic stabilization. For carbon-only dispersions (without Pt), the addition of ionomer up to a critical concentration decreased the viscosity due to electrosteric stabilization of carbon agglomerates. However, with Pt−carbon dispersions, the addition of ionomer showed contrasting behavior between Vulcan and HSC supports. In the Pt− Vulcan dispersions, the effect of ionomer addition on the rheology was qualitatively similar to Vulcan dispersions without Pt. The Pt−HSC dispersions showed an increased viscosity with ionomer addition and a strong shear-thinning nature, indicating that Nafion likely flocculated the Pt−HSC aggregates. These results were verified using DLS and USAXS. Further, the observations of the effect of ionomer:carbon ratio and a comparison between carbons of different surface areas provided insights on the microstructure of the catalyst ink corresponding to the optimized I/C ratio for fuel cell performance reported in the literature.

In situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with O 2 transport resistance for low-loaded (0.05 mg Pt cm −2 ) platinum-supported Vulcan carbon (Pt/Vu)-based electrodes in a polymer electrolyte fuel cell. Coupling the in situ diagnostic data with ex situ characterization of catalyst inks and electrode structures, the effect of ink composition is explained by both ink-level interactions that dictate the electrode microstructure during fabrication and the resulting local ionomer distribution near catalyst sites. Electrochemical techniques (CO displacement and ac impedance) show that catalyst inks with higher water content increase ionomer (sulfonate) interactions with Pt sites without significantly affecting ionomer coverage on the carbon support. Surprisingly, the higher anion adsorption is shown to have a minor impact on specific activity, while exhibiting a complex relationship with oxygen transport. Ex situ characterization of ionomer suspensions and catalyst/ionomer inks indicates that the lower ionomer coverage can be correlated with the formation of large ionomer aggregates and weaker ionomer/catalyst interactions in low-water content inks. These larger ionomer aggregates resulted in increased local oxygen transport resistance, namely, through the ionomer film, and reduced performance at high current density. In the water-rich inks, the ionomer aggregate size decreases, while stronger ionomer/Pt interactions are observed. The reduced ionomer aggregation improves transport resistance through the ionomer film, while the increased adsorption leads to the emergence of resistance at the ionomer/Pt interface. Overall, the high current density performance is shown to be a nonmonotonic function of ink water content, scaling with the local gas (H 2 , O 2 ) transport resistance resulting from pore, thin film, and interfacial phenomena.

Organic bulk‐heterojunction solar cells are being developed as a low‐cost alternative to inorganic photovoltaics. A key step to producing high‐efficiency bulk‐heterojunction devices is film curing using either heat or a solvent atmosphere. All of the literature examining the curing process have assumed that improvement of the bulk‐heterojunction morphology is the reason for the increased filling factor, short‐circuit current density, and efficiency following heat or solvent treatment. We show in this article that heat treatment causes the donor polymer (P3HT) and polymer electrode (PEDOT:PSS) to mix physically to form an interface layer. This interface layer is composed of a mixture of P3HT and PSS in which the P3HT is oxidized to P3HT+. This mixed layer affects the open‐circuit voltage and compensation voltage by limiting the dark current. This result implies that a simplistic description of the P3HT/PEDOT:PSS contact as a sharp interface between bulk P3HT and bulk PEDOT:PSS cannot adequately capture its electrical characteristics.

The composition of polymer-fullerene blends is a critical parameter for achieving high effi ciencies in bulk-heterojunction (BHJ) organic photovoltaics. Achieving the "right" materials distribution is crucial for device optimization as it greatly infl uences charge-carrier mobility. The effect of the vertical concentration profi le of materials in spin-coated BHJs on device properties has stirred particularly vigorous debate. Despite available literature on this subject, the results are often contradictory and inconsistent, likely due to differences in sample preparation and experimental considerations. To reconcile published results, the infl uence of heating, surface energy, and solvent additives on vertical segregation and doping in polymer-fullerene BHJ organic photovoltaics are studied using neutron refl ectometry and near edge X-ray absorption fi ne structure spectroscopy. It is shown that surface energies and solvent additives greatly impact heat-induced vertical segregation. Interface charging due to Fermi level mismatch increases (6,6)-phenyl-C 61 -butyric acid methyl ester (PCBM)-enrichment at the BHJ/cathode interface. Currentvoltage measurements show that self-assembly of interfaces affects the opencircuit voltage, resulting in clear changes to the power conversion effi ciency.

This work presents a study of the effects of ultrasonic dispersing methodology and time on catalyst agglomerate size in polymer electrolyte membrane fuel cell (PEMFC) catalyst ink dispersions. Cathode catalyst inks were prepared and characterized to elucidate the influences of ultrasonic dispersing method and time on catalyst ink particle size and CCL electrochemical properties. In-situ ultra-small-, small-, and wide-angle X-ray scattering (USAXS–SAXS–WAXS) analyses were used to study the impact of ultrasonication time and methodology on changes in the agglomerate, aggregate, and particle size and distribution during the dispersing process. Ex-situ transmission electron microscopy was also used to investigate the particle size of these inks. Fuel cell membrane electrode assemblies (MEAs) were prepared and tested to determine the influence of ink properties on CCL electrochemical properties, including the electrochemical active surface area (ECA), mass activity (MA), H2/air polarization curves, and oxygen mass-transport resistances. It was found that a combination of brief tip sonication followed by bath sonication was most effective at breaking up agglomerates, leading to maximum catalyst activity and MEA performance. Extended tip sonication was found to be too aggressive and resulted in detachment of the platinum nanoparticles from the carbon black support, which decreased electrochemical surface area and MEA performance. Quantification of oxygen mass transport resistance showed that electrodes with large catalyst agglomerates due to insufficient sonication had a higher non-Fickian (pressure independent) than properly dispersed catalyst. Through correlation of the performance with catalyst particle size, the desired CCL structure was proposed, which will provide insight into dispersion strategies for lab-scale spray coating and other processing techniques as well as for large-scale manufacturing.