Probing Ionomer Interactions with Electrocatalyst Particles in SolutionProbing Ionomer Interactions with Electrocatalyst Particles in Solution
The interaction between ionomer (ion-conducting polymer) and catalyst particles in porous electrodes of electrochemical-energy-conversion devices is a critical yet poorly understood phenomenon that determines device performance: electrode morphology is controlled by ionomer/particle interactions in precursor inks during electrode formation. In this Letter, we probe the origin of this interaction in inks to unravel the complexities of ionomer/particle adsorption interactions. Quartz-crystal microbalance studies detail ionomer adsorption (with a range of charge densities) to model surfaces under a variety of solvent environments, and isothermal-titration-calorimetry experiments extract thermodynamic binding information to platinum- and carbon-black nanoparticles. Results reveal that under the conditions tested, ionomer binding to platinum is similar to carbon, suggesting that adsorption to platinum-on-carbon catalyst particles in inks is likely dictated mostly by entropic interactions with the carbon surface. Furthermore, water-rich solvents (relative to mixed water/propanol) promote ionomer adsorption. Finally, ionomer dispersions change with time, yielding dynamic binding interactions.