Typical catalyst inks in proton exchange membrane fuel cells (PEMFCs) are composed of a catalyst, its support, an ionomer and a solvent and are used with solution processing approaches to manufacture conventional catalyst layers (CLs). Because of this, catalyst ink formulation and deposition processes are closely related to CL structure and performance. However, catalyst inks with ideal rheology and optimized electrochemical performances remain lacking in the large-scale application of PEMFCs. To address this, this review will summarize current progress in the formulation, characterization, modeling and deposition of catalyst inks. In addition, this review will highlight recent advancements in catalyst ink materials and discuss corresponding complex interactions. This review will also present various catalyst ink dispersion methods with insights into their stability and introduce the application of small-angle scattering and cryogenic transmission electron microscopy (cryo-TEM) technologies in the characterization of catalyst ink microstructures. Finally, recent studies in the kinetic modeling and deposition of catalyst inks will be analyzed. The formulation and the deposition process of catalyst inks determine the formation of catalyst. The interaction between the components of the catalyst ink governs the microstructure and processability of the ink, thereby affecting the microstructure and performance of the CL.

The Controllable Design of Catalyst Inks to Enhance PEMFC Performance: A Review

This study investigated the effects of the dielectric constant (e) 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 e 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 e of 40 tended to be more Newtonian fluid, with low yield stress (σy). The ionomer content altered the rheology of the ink by changing the internal interaction of inks. Solvents with e 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.

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

We investigate the effect of the poly(acrylic acid) (PAA) carrier polymer concentration on the microstructure and rheological properties of catalyst inks for electrospun polymer–electrolyte membran...

Toward Optimizing Electrospun Nanofiber Fuel Cell Catalyst Layers: Polymer–Particle Interactions and Spinnability

The catalyst layers of proton exchange membrane water electrolyzer are generally made by coating ink mixtures of catalyst and ionomer to the membrane. The particle size and stability of such inks are crucial to the formation of the catalyst layer's microstructure and overall cost. In this paper, the characteristics and stability of iridium oxide inks are investigated. The effects of inks dispersed by sonication and ball-milling on the particle size distribution, zeta potential, and viscosity of the catalyst inks are investigated. For both dispersion methods, it is found that increasing the dispersion time and strength effectively reduces the average particle size as expected. The inks prepared by ball-milling tend to have narrower and smaller size distribution than those by sonication. The rheological behavior of these inks is found to be slightly non-Newtonian. Ball-milling appears to increase the ink viscosity. A dual-dispersion technique that combines both dispersion steps is developed in the present study. The inks made with this procedure yield smaller size distributions than those with single dispersion, thus a higher specific catalyst area that can reduce catalyst loading and cost. These inks are also found to have a long shelf life remaining homogeneous. 

Fabrication of catalyst layer for proton exchange membrane water electrolyzer: I. Effects of dispersion on particle size distribution and rheological behavior


 MEAs with nanofiber mat electrodes containing Pt/C catalyst and Nafion binder were fabricated and evaluated. The electrodes were prepared by electrospinning a solution of catalyst powder, salt-form Nafion (with Na+, Li+, or Cs+ as the sulfonic acid counterion), and a carrier polymer of either polyethylene oxide or poly(acrylic acid). The carrier polymer was extracted prior to MEA testing by a hot water soaking step. The resulting fibers were 15-17% porous, with a core-shell-like morphology (a coating of primarily Nafion on the fiber surface). MEAs with anode/cathode catalyst loadings of 0.1 mgPt/cm2 each and a Nafion 211 membrane produced high power at both high and low relative humidity (RH) conditions in H2/air fuel cell tests, e.g., a maximum power density of 919 mW/cm2 at 100% RH and 832 mW/cm2 at 40% RH for a test at 80 °C and 200 kPaabs. The presence of nm-size pores within the fibers trapped water via capillary condensation during low RH feed gas testing, thus maintaining a high proton conductivity of the Nafion binder in the anode and cathode while minimizing/eliminating ionic isolation of catalyst particles in low water content, poorly conductive binder.

/pdf/electrospun-nanofiber-electrodes-for-high-and-low-humidity-2n6eqc85.pdf

Electrospun Nanofiber Electrodes for High and Low Humidity PEMFC Operation

Polymer electrolyte membrane fuel cells (PEMFCs) provide a renewable source of energy through the redox reaction of hydrogen and oxygen gas; however, operation relies on a costly platinum catalyst layer. This study investigates how electrospun catalyst layers may be employed to increase the surface area:volume ratio for catalysis to optimize PEMFC performance. When preparing electrospinning solutions, several base polymers were evaluated in varying concentrations to optimize fiber formation, with poly(acrylic acid) found to be preferable at a 12 wt% concentration. Ultimately, PEMFCs with electrospun catalyst layers achieved a 108% increase in power output compared to those air-sprayed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/8A100AF1F67DD7C55FCA8B19B8140E04/S2159685919001447a.pdf/div-class-title-electrospinning-deposition-of-poly-acrylic-acid-platinum-carbon-catalyst-ink-to-enhance-polymer-electrolyte-membrane-fuel-cell-performance-div.pdf

Electrospinning deposition of poly(acrylic acid): platinum/carbon catalyst ink to enhance polymer electrolyte membrane fuel cell performance

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Electrospinning deposition of poly(acrylic acid): platinum/carbon catalyst ink to enhance polymer electrolyte membrane fuel cell performance