Nanometer-scale mapping of irreversible electrochemical nucleation processes on solid Li-ion electrolytes.

Electrochemical processes associated with changes in structure, connectivity or composition typically proceed via new phase nucleation with subsequent growth of nuclei. Understanding and controlling reactions requires the elucidation and control of nucleation mechanisms. However, factors controlling nucleation kinetics, including the interplay between local mechanical conditions, microstructure and local ionic profile remain inaccessible.

Investigation of water and methanol sorption in monovalent- and multivalent-ion-exchanged nafion membranes using electron spin resonance.

Electron spin resonance (ESR) spectroscopy was used to monitor the local environment of 2,2,6,6-tetramethyl-4-piperidone N-oxide (TEMPONE) spin probe in Li(+), Ca(2+), and Al(3+) ion-exchanged Nafion 117 membranes swollen with mixed methanol/water solvent at varying compositions. The (14)N hyperfine splitting, a(N), which reflects the local polarity of the nitroxide probe, remains nearly steady at higher solvent contents but increases substantially at lower solvent contents, reflecting close contact with the ions. The rotational rate (R) of the probe increased with solvent content, depending strongly on the amount of solvent at low contents but increasing more gradually at higher solvent contents, similar to the behavior of previously measured solvent translation diffusion coefficients.

Electron spin resonance investigation of microscopic viscosity, ordering, and polarity in Nafion membranes containing methanol-water mixtures.

Electron spin resonance (ESR) was used to monitor the local environment of 2,2,6,6-tetramethyl-4-piperidone N-oxide (Tempone) spin probe in water and methanol mixtures in solution and in Li(+) ion exchanged Nafion 117 membranes. Solution spectra were analyzed using the standard fast-motion line width parameters, while membrane spectra were fitted using the microscopic order macroscopic disorder (MOMD) slow-motional line shape program of Freed and co-workers. The (14)N hyperfine splitting, aN, which reflects the local polarity of the nitroxide probe, decreases with increasing methanol concentration, consistent with the decrease in solvent polarity.

Effect of sorbed methanol, current, and temperature on multicomponent transport in nafion-based direct methanol fuel cells.

The CO2 in the cathode exhaust of a liquid feed direct methanol fuel cell (DMFC) has two sources: methanol diffuses through the membrane electrode assembly (MEA) to the cathode where it is catalytically oxidized to CO2; additionally, a portion of the CO2 produced at the anode diffuses through the MEA to the cathode. The potential-dependent CO2 exhaust from the cathode was monitored by online electrochemical mass spectrometry (ECMS) with air and with H2 at the cathode. The precise determination of the crossover rates of methanol and CO2, enabled by the subtractive normalization of the methanol/air to the methanol/H2 ECMS data, shows that methanol decreases the membrane viscosity and thus increases the diffusion coefficients of sorbed membrane components.