In Situ Micro-Raman on the Membrane in a Working PEM Cell

Abstract

An electrochemical cell has been designed for in situ micro-Raman measurements on the polymer membrane in an operating polymer electrolyte cell ͑PEM͒. The method is applicable to studies of both the distribution of water and membrane structure in the working cell environment. An initial study of the water distribution across a Nafion 117 membrane in a cell working as a H 2 /H 2 pump cell at hydrogen flow and currents from 0 to 300 mA/cm 2 is presented. The results show that a hydration profile with a lower water content at the anode forms as current is applied to the cell.A critical part of a polymer electrolyte membrane fuel cell ͑PEMFC͒ and a direct methanol fuel cell ͑DMFC͒ is the polymer membrane electrolyte, which conducts protons from anode to cathode whilst blocking the transfer of electrons and reactants. The membrane material must combine high proton conductivity, low gas permeability, long term stability in the fuel cell environment, and of course low cost. 1 This paper presents a novel method to directly study the working polymer membrane in an operating electrochemical cell using confocal micro-Raman spectroscopy. In the field of proton conducting polymer electrolytes Raman spectroscopy can be used to study issues such as, membrane composition, molecular coordination, and polymer conformation. An added strength is the spatial resolution obtained with confocal micro-Raman spectroscopy, i.e., Raman microscopy, which under optimized conditions is on the level of micrometers both in horizontal and depth directions. The spatial resolution has, for instance, been exploited to map the distribution of functional molecular groups in a membrane 2 or to follow the extent of degradation of fuel cell tested membranes ex situ. [3][4][5] In the fuel cell application, high membrane proton conductivity is one of the key parameters for a good fuel cell performance. Conventional membranes, typically Nafion ͑DuPont͒ and other sulfonated polymers, combine hydrophobic and hydrophilic polymer parts that form a structure, phase separated on a nanometer scale. 6 The hydrophobic phase provides mechanical stability. The hydrophilic phase containing sulfonic acid groups will be proton conducting when sufficiently hydrated. The water content, commonly expressed as ϭ the number of water molecules per sulfonic acid group (H 2 O/SO 3 ), is therefore of great importance. In the range of hydration found in the fuel cell ϭ 2-14, the specific conductivity of Nafion 117 increase nearly an order of magnitude as the water content increases. 7 Proton transport through the membrane leads to a flow of water electro-osmotic drag 8 which is balanced by a concentration gradient induced back flow of water. A number of models describing the water distribution in fuel cell membranes have been put forward. 9,10 A common result of the different models is a depletion of water at the anode with increasing current, however the shape of the modelled concentration gradient varies. During the measurements presented here the cell is working as a hydrogen pu...

Operando Raman Micro-Spectroscopy of Polymer Electrolyte Fuel Cells

“…[28][29][30] Although there are reports of operando PEM fuel cell water management/transport studies, [31][32][33][34][35][36] no potential-dependent data was provided. This H 2 /air fuel cell spectroscopy uses density functional theory (DFT) calculated normal mode analysis of the Nafion side chain and perfluorinated small molecules to correlate variation of MEA Preparation.-Nafion 117 (Wilmington, DE) was pretreated by immersing sheets in boiling 8 M nitric acid followed by two hours in boiling Nanopure TM water.…”

Time/Space‐Resolved Studies of the Nafion Membrane Hydration Profile in a Running Fuel Cell

“…X-ray techniques, [18,19] small-angle neutron scattering (SANS), [20] magnetic-resonance imaging [21][22][23] and micro-Raman, [24] neutron radiography, [25] electrical-resistance measurements, [26][27][28][29] infrared absorption [30] and fluorescence spectroscopy [31] have been used for this purpose. Unfortunately, if one excludes micro-Raman measurements, [24] all of these techniques exhibit rather low spatial resolutions (if any). Moreover, they have other severe limitations, such as slow response to the hydration-degree variations, weak signals (resulting in poor accuracy), and only indirect dependence on the quantity of interest.…”

Raman Microspectroscopy as a Useful Tool for In Situ and Operando Studies of Water Transport in Perfluorosulfonic Membranes for PEMFCs

“…As far as we know, the first in situ Raman measurement of water concentration profiles crossing the membrane thickness under dynamic transport condition was reported by Matic et al [82] in 2005. A Nafion 117 membrane was placed in a H 2 /H 2 pump electrochemical cell and spectra were collected on the edge, in the cross-section mode (see Figure 2).…”