Analysis of Ionic Domains on a Proton Exchange Membrane Using a Numerical Approximation Model Based on Electrostatic Force Microscopy

Abstract

Understanding the ionic channel network of proton exchange membranes that dictate fuel cell performance is crucial when developing proton exchange membrane fuel cells. However, it is difficult to characterize this network because of the complicated nanostructure and structure changes that depend on water uptake. Electrostatic force microscopy (EFM) can map surface charge distribution with nano-spatial resolution by measuring the electrostatic force between a vibrating conductive tip and a charged surface under an applied voltage. Herein, the ionic channel network of a proton exchange membrane is analyzed using EFM. A mathematical approximation model of the ionic channel network is derived from the principle of EFM. This model focusses on free charge movement on the membrane based on the force gradient variation between the tip and the membrane surface. To verify the numerical approximation model, the phase lag of dry and wet Nafion is measured with stepwise changes to the bias voltage. Based on the model, the variations in the ionic channel network of Nafion with different amounts of water uptake are analyzed numerically. The mean surface charge density of both membranes, which is related to the ionic channel network, is calculated using the model. The difference between the mean surface charge of the dry and wet membranes is consistent with the variation in their proton conductivity.

Quantitative Study of Charge Distribution Variations on Silica–Nafion Composite Membranes under Hydration Using an Approximation Model

“…Additionally, this result shows similarities with the proton conductivity under different hydration conditions. The proton conductivity under the fully hydrated condition was 60–70 times larger than that under room humidity in many studies [ 36 ]. These results agree well with the result from the surface charge calculation based on the model.…”

“…In this study, recast Nafion, which is exposed under ambient conditions, was prepared for measuring and analyzing surface charge distributions using the mathematical approximation model. In a previous study, the recast Nafion was quantitatively analyzed under dried and fully hydrated conditions [36]. The purpose of using recast Nafion in this study was to verify the reliability of the numerical In this study, recast Nafion, which is exposed under ambient conditions, was prepared for measuring and analyzing surface charge distributions using the mathematical approximation model.…”

Quantitative Understanding of Ionic Channel Network Variation in Nafion with Hydration Using Current Sensing Atomic Force Microscopy

“…Third, it works as a catalysis layer for oxidizing hydrogen. Son et al report current flow measurement by swiping the bias voltage from −1.5 V to 1.5 V, as shown in Figure 4 [ 27 ]. This result shows that the current increases considerably near to −1.5 V, and hydrolysis is increased and protons are generated when a Pt-coated tip is used.…”

Recent Developments on Bioinspired Cellulose Containing Polymer Nanocomposite Cation and Anion Exchange Membranes for Fuel Cells (PEMFC and AFC)

“…In both cases, the membrane is the centerpiece of the unit cells and plays the major role in completing the unit cell operation [ 46 , 47 ]. In PEMFC, the cation exchange membranes (CEM) play a significant role in transporting the proton (H + ) from the anode side to the cathode side ( Figure 1 c) [ 48 , 49 , 50 ]. As shown in Figure 1 c, the CEM transports the proton in two possible ways, the Grotthuss mechanism and the vehicle mechanism.…”

“…In the vehicle mechanism, the protons are diffused through a hydrated membrane using water molecules, where solvated protons form H 3 O + for diffusion. In the Grotthuss mechanism, proton transportation occurs via proton hopping by creating hydrogen bonds in the hydrolyzed ionic site of the membrane [ 48 , 49 , 50 ]. In AFC, the anion exchange membranes (AEM) play a major role in transporting the anion (OH − ) from the cathode side to the anode side ( Figure 1 d) [ 44 , 51 ].…”