Impact of Solvent on Ionomer Structure and Fuel Cell Durability

Abstract

Electrode structure within PEFCs, including the Pt-ionomer interface, is created while making electrodes from catalyst inks based on ionomer dispersed in solvent. The relationship between final electrode structure and processing conditions is poorly understood. We have varied the solvent used in cathode catalyst inks, and then subjected the resulting MEAs to hydrogen-air performance and durability testing. Specifically, cathodes cast from inks based on inonomer dispersions in water-propanol-isopropanol (W/P cathode) initially perform better than cathodes cast from glycerol-based dispersions (Gly cathode), but are far less durable. After 10,000 potential cycles from 0.60 V to 1.0 V in N2, the performance on air of the W/P cathode falls significantly below that of the Gly cathode. NMR and neutron scattering measurements of ionomer dispersions, as well as AFM and TEM data from cast ionomer films, offer insight into how the effect of solvent choice on the ionomer structure may impact durability.

Effect of Dispersion Solvents in Catalyst Inks on the Performance and Durability of Catalyst Layers in Proton Exchange Membrane Fuel Cells

“…Recent studies, reporting that the solvent type significantly affected the shape of the ionomer dispersed in the solvents, pointed out that polar protic solvents, such as water, n-propanol, isopropanol, ethanol, or methanol, result in highly-solvated large particles, while polar aprotic solvents, such as NMP, dimethylformamide, or dimethyl sulfoxide, result in a random-coil conformation (true solution behavior) [21][22][23][24]. Table 1 summarizes the size distributions of the five different ionomer dispersions measured by DLS.…”

“…The various shapes of Nafion dispersed in these three types of solvent could influence the performance and durability of the PEMFCs. Johnston and co-workers reported that cathodes cast from inks based on ionomer dispersions in water-propanol-isopropanol initially performed better than those cast from glycerol-based dispersions, but were far less durable because of a higher degree of phase separation, which resulted in faster Pt particle growth [22]. The types of solvents used affected the surface morphology of the CLs.…”

Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation

“…5 The water-alcohol-PFSA mixture is particularly relevant because it is the typical solvent of commercial dispersions and CL inks. It has been demonstrated that these different solution-phase morphologies directly impact cast film properties; [6][7][8][9][10][11][12][13][14] ionomers maintain a memory of these structures. 15 These solvent effects must also change the interactions driving ionomer-particle aggregation/stability in an ink, because different water:alcohol ratio inks each exhibit different aggregate sizes, 7,[16][17][18][19][20][21][22] and once dried, different CL morphology, water uptake, conductivity, etc.…”

Probing Ionomer Interactions with Electrocatalyst Particles in Solution

“…17 Additionally, there seems to be a hydrophobic interaction between the ionomer backbone and the carbon surface. The impact of solvent 13,[18][19][20][21] and particle type [22][23] on ink aggregation is attributed to differences in ionomer/particle interactions, which propagate to impact device performance 20,[24][25][26][27][28][29][30][31][32] : ionomer coverage and electrochemical performance of the CL are dependent on the ink water:propanol ratio 32 . Clearly, ionomer/particle agglomerates are impacted by the interactions between these components in solution.…”

“…Additionally, there seems to be a hydrophobic interaction between the ionomer backbone and the carbon surface. The impact of solvent ,,− and particle type , on ink aggregation is attributed to differences in ionomer/particle interactions, which propagate to impact the overall current–voltage behavior of the device. ,,,− Metrics like ink zeta potential have been shown to correlate well with mass activity, non-Fickian resistance, and limiting current density and to be dependent on ink water/propanol ratios: the maximum performance of each of these parameters is observed at intermediate water concentrations . This can be explained by competing microstructural changes: increasing the ionomer coverage of the agglomerates and increasing the agglomerate size as ink water content increases .…”