Determining Extent of 

Ion-Exchange in Various 

Counterion Nafion Membranes 

Using Prompt Gamma Neutron 

Activation Analysis (PGAA) 

by Sandra K. Young, Samuel F. Trevino, 

Nora C. Beck Tan, and Rick L. Paul 

ARL-TR-2679 March 2002 

Approved for public release; distribution is unlimited. 

m203 14 043 
The findings in this report are not to be construed as an 

official Department of the Army position unless so 

designated by other authorized documents. 

Citation of manufacturer’s or trade names does not 

constitute an official endorsement or approval of the use 

thereof. 

Destroy this report when it is no longer needed. Do not 

return it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2679 March 2002 

. 

Determining Extent of 

Ion-Exchange in Various 

Counterion Nafion Membranes 

Using Prompt Gamma Neutron 

Activation Analysis (PGAA) 

Sandra K. Young, Samuel F. Trevino, 

and Nora C. Beck Tan 

Weapons and Materials Research Directorate, ARL 

Rick L. Paul 

National Institute of Standards and Technology 

Auvroved for uublic release; distribution is unlimited. 
Abstract 

Researchers have performed structural evaluations on Nafion membranes for 

many years. Various research groups have published a large volume of data; 

however, much of the work done by these groups is contradictory. One 

important aspect of the membranes is the chemical uniformity and quantity of 

water within the membrane under investigation. A new technique that has 

shown its usefulness for providing accurate and fast quantitative measurements 

of chemical composition quickly is prompt gamma neutron activation analysis 

(PGAA). In this work, a sulfonyl fluoride-form membrane, an as-received 

membrane from the manufacturer, and several differently pretreated H+-form 

Nafion membranes were examined using PGAA. The evaluation showed the 

necessity for pretreatment of membranes to eliminate contamination. A series of 

counterion-exchanged membranes were also examined to determine maximum 

conversion achieved and to identify possible limitations to complete conversion. 

The results show that it is possible to obtain nearly complete or complete 

conversion to the counterion from the H+-form Nafion samples. Water content 

can be limited within the membrane but not eliminated. In samples that were 

not pretreated, complete ion exchange was limited by the contamination. 

. 

ii 
Acknowledgments 

The authors would like to acknowledge the use of the facilities at the National 

Institute of Standards and Technology (NIST) Center for Neutron Research in 

performing all of the prompt gamma neutron activation analyses. The postdoctoral program, under the auspices of the American Association for 

Engineering Education, is acknowledged for financial support of Sandra K. 

Young. Dr. Donald Rivin of the U.S. Army Natick Soldier Center is also 

acknowledged for his helpful technical input. 

. . . 

111 
INTENTIONALLYLEFTBLANK. 

iv 
. 

. Acknowledgments 

Contents 

. . . 

111 

List of Figures vii 

List of Tables ix 

1. Introduction 1 

2. Experimental 4 

2.1 Materials .......................................................................................................... 4 

2.2 PFSI Membrane Pretreatments ..................................................................... 4 

2.3 Formulation of Ion-Exchanged Nafion Membranes ................................. 5 

2.4 Atmospheric Water Diffusion Into Nafion Analysis ................................. 5 

2.5 PGAA ............................................................................................................... 5. 

3. Results and Discussion 9 

3.1 Atmospheric Water Diffusion ...................................................................... 9 

3.2 Nafion Pretreatments ..................................................................................... 9 

3.3 Complete Counterion Exchanged Nafion ................................................. 10 

3.4 Limited Counterion Exchanged Nafion .................................................... 13 

3.5 As-Received Counterion Exchanged Nafion ............................................ 15 

4. Conclusions 16 

5. References 17 

Distribution List 21 

Report Documentation Page 23 

I . 

V 
. 

INTENTIONALLYLEFTBLANK. 

vi 
List of Figures 

. 

Figure 1. Nafion PFSI ...................................................................................................... 1 

Figure 2. Sample undergoing prompt gamma analysis.. .............. . ........................... .2 

Figure 3. Atmospheric water uptake into dry, initialized H+-form Nafion. .......... .6 

Figure 4. Prompt gamma spectrum for N117- H+ form. .......................................... 6 

vii 
. . . 

Vlll 
List of Tables 

. 

Table 1. Elements and detection limits for PGAA ...................................................... 3 

Table 2. Raw data from prompt gamma analysis ....................................................... 7 

Table 3. Prompt gamma comparison of H+-form Nafion membranes. ................ 

.10 

Table 4. Prompt gamma results from the counterion exchange 

procedures. ............................................................................................................... 11 

Table 5. Prompt gamma results for limited molar concentration 

solutions .................................................................................................................... 14 

Table 6. Prompt ganuna results for as-received H+-form Nafion ........................... 15 

ix 
X 
1. Introduction 

. 

. 

Nafion is a perfluorosulfonate ionomer (PFSI), consisting of a tetrafluoroethylene 

main chain and a perfluoroether sulfuric acid side chain [l] (Figure 1). 

Coulombic interactions between the ions in Nafion result in aggregation of the 

sulfonate groups into tightly packed ionic regions referred to as “clusters.” Ionic 

clustering in Nafion improves the material’s mechanical, thermal, and transport 

properties. The relationship between these properties and the membrane’s 

structure have made Nafion the focus of many structural studies [l-8]. 

-+&j--&-CF2Ity 

? 

CF2 

F-C- OCF2CF2- S&-H+ 

AF3 

Figure 1. Nafion PFSI. 

Critical to any property analysis of Nafion membranes is chemical knowledge of 

the counterion associated with the perfluorosulfonic acid group and the quantity 

of water within the membrane. The sulfonic group (-SOx-) is the sole site for 

variation in the membrane through simple chemical manipulations. The 

counterion, whether a proton, alkalai metal, or other metal cation, has an effect 

on the interactions of the sulfonic acid or sulfonic acid salt groups, which, in 

turn, affects the strength of the ionic aggregates. The presence of small amounts 

of ion exchange, while altering the chemical structure, can also change the 

interaction of the aggregates. Therefore, a method for identifying the ion(s) at 

the exchange site is needed to fully characterize the membranes. 

To date, changes regarding the sulfonic acid site, the resulting molecular 

structure, and molecular motions in addition to the presence and effect of water 

(and other solvents) regarding Nafion have been characterized through many 

means of chemical analysis. Research groups [9, 101 have utilized Fourier 

transform infrared (FTIR) spectroscopy for identification of the sulfonic acid side 

chains, the inter-associations with water, and the ii-&a-associations between 

waters within the Nafion membrane. Proton and deuterium nuclear magnetic 

resonance (NMR) have been used by many researchers [ll-151 for quantitative 

measurements of water and investigating the molecular motions within Nafion 

by monitoring relaxation times of the molecules. Dynamic mechanical analysis 

1 
(DMA) has long been used to follow the change in molecular motions within 

Nafion as a result of changes of the atom [16] associated with the sulfonic acid, 

mobility [17] on the sulfonic site, and solvent effects [18, 191 on the membrane. 

Wide-angle x-ray diffraction (XRD) has been used to determine the crystallinity 

of Nafion membranes [20], which affects mobility within the polymer chains. 

Following the significant theories reported in the field of ionomers [21,22], many 

researchers performing small-angle scattering (SAS) experiments continue 

discussions about the morphology of Nafion, counterion effects, and solvent 

effects on the morphology [23-271. 

Cold-neutron-capture prompt gamma neutron activation analysis (PGAA) is a 

new, ideal method for the evaluation of composition on Nafion membranes due 

to its nondestructive nature and sensitivity to trace ions, especially hydrogen. 

The technique for PGAA is described in detail elsewhere [28-291; the basics are 

as follows (see Figure 2): 

l When a sample is placed in a neutron beam, nuclei of many elements in the 

sample absorb neutrons and are transformed to an isotope of higher mass 

number in an excited state. 

l Prompt gamma rays, emitted by de-excitation of the compound nuclei, are 

then measured using a high-resolution gamma-ray detector. 

l Qualitative analysis is accomplished by identification of the gamma-ray 

energies, while comparison of gamma-ray intensities with those emitted by 

a standard yields quantitative analysis. The use of “cold” neutrons 

enhances the sensitivity. 

Detector 

Figure 2. Sample undergoing prompt gamma analysis. 

The limit of detection or the smallest possible detectable amount for prompt 

gamma analysis varies from element to element [28,29]. Limits of detection for 

many elements measured are summarized in Table 1. The table gives the range 

in which the detection limit for each element lies. These ranges do not represent 

the range of detectable quantities for these elements, as, for example, quantities 

2 
Table 1. Elements and detection limits for PGAA. 

Range 

(PR) 

0.01-0.1 

0.1-l 

l-10 

10-100 

Elements 

B, Cd, Sm, Gd 

Eu, Hg 

H, Cl, In, Nd 

Na, S, K, SC, Ti, V, Cr, Mn, Co, Ni, Cu, Ge, As, Se, 

Br, MO, Ag, Te, I, Au 

100-1000 

lOOc--10,000 

Mg, Al, Si, I’, Ca, Fe, Zn, Ga, Rb, Sr, Y, Zr, Nb, Sb, 

Ba, La 

C, N, F, Sn, Pb 

of hydrogen at concentrations much greater than 10 l&g can be detected. The 

limit of detection for any given element is not an absolute, but depends on the 

matrix of the sample (i.e., the detection limit for hydrogen measured in silicon is 

different from the detection limit for hydrogen measured in titanium). 

PGAA has many advantages to other experimental techniques. The real 

usefulness for prompt gamma is its ability to detect the light elements (such as 

hydrogen and carbon), which are not readily detectable by other methods. Other 

advantages of PGAA over other experimental techniques are: (1) the analysis is 

performed in situ (the element not extracted from matrix, i.e., nondestructive); (2) 

the signal arises from a nuclear rather than chemical process, hence results are 

not dependent on the chemical form of the element present; and (3) neutrons and 

gamma rays penetrate the whole sample, hence the bulk of the sample is 

analyzed and not just the surface, which is especially important in thicker films. 

When starting work on the Nafion program, several samples consisting of acidform, potassium-form, and calcium-form Nafion were analyzed using small 

angle neutron scattering (SANS). These samples were both samples made at the 

U.S. Army Research Laboratory CARL) and samples received from an external 

collaborator. When the SANS data was reduced, very different SANS patterns 

were observed for same counterion samples made at ARL when compared to the 

externally received samples. This was of some concern since it was not obvious 

why this would occur, whether it was due to contamination or other possible 

heat treatments incurred during transport of the samples. During deliberations 

of this problem with several National Institute of Standards and Technology 

(NIST) colleagues, the facile and practical use of PGAA in analysis of materials 

was discussed. While the SANS data will be discussed in a separate report, the 

PGAA analysis was used to elucidate these problems and work on other 

counterion issues involving Nafion. 

In this investigation, we apply PGAA to the characterization of ion and water 

content in Nafion membranes that have been subjected to a variety of drying, 
pretreatment, and counterion exchange conditions. These measurements are 

used to evaluate the effectiveness of various treatments for removing water and 

contaminants in the membranes and confirm the efficiency of ion-exchange 

reactions. 

2. Experimental 

2.1 Materials” 

PFSI membranes (1100 equivalent weight, 7 mil thick) in the SOaH-form (Nafion 

117) were obtained from C. G. Processing, Inc. Concentrated hydrochloric acid 

(HCI), concentrated nitric acid (HN03), potassium chloride (KCl), cesium 

chloride (CsCl), calcium chloride (CaClz), zinc chloride (ZnClz), magnesium 

chloride hexahydrate (MgClz - 6H20), cobalt chloride hexahydrate 

(CoCl2 . 6H20), copper chloride dihydrate (CuCl2 . 2H20), aluminum chloride 

hexahydrate (AlCl3 . 6H2O), and ferrous chloride (FeCla) were all obtained from 

VWR Scientific and used as received. All water utilized was distilled and 

deionized (DI/DS). 

2.2 PFSI Membrane Pretreatments 

Pretreatment 1 [30]: The membranes were cleaned in boiling concentrated nitric 

acid for 1 hr, rinsed several times with pure water, and dried overnight at 110 “C. 

Pretreatment 2 1311: The membranes were refluxed in 2 normal (N) HCl for 3 hr 

at 80 OC and then rinsed with Hz0 for 3 hr. 

Pretreatment 3 [32]: A common membrane pretreatment where the membranes 

were refluxed in 3% Hz02 for 1 hr, refluxed in DI Hz0 for 1 hr, refluxed in 

0.5 molar (M) H2S04 for 1 hr, and refluxed in DI Hz0 for 1 hr. 

Pretreatment 4 [33]: As-received membranes were exchanged to the acid form by 

refluxing in 50% HC1:50% HN03 solution (v:v), leaching out excess acid in 

deionized water reflux (3x), and finally vacuum drying at 125 “C. All 

membranes were exchanged to this standard initialized state prior to counterion 

exchange and prompt gamma analysis in order to assure maximum sample 

reproducibility. 

*Certain commercial products are identified in this report in order to specify the experimental 

procedures in adequate detail. This identification does not imply recommendation or endorsement 

by the authors or by NIST, nor does it imply that the products identified are necessarily the best 

available for the purpose. 

4 
2.3 Formulation of Ion-Exchanged Nafion Membranes 

2M and O.OlM solutions of each salt were made in l-liter volumetric flasks. 

Samples of initialized, H+-form Nafion were cut to approximately 3-cm x 3-cm 

squares. 

2M Salt Sohtions. Membranes were soaked in an excess of the 2M salt solutions 

for 24 hr (step 1). The solution was then brought to reflux and held at reflux 

temperature for 4 hr; afterwards, the membranes in the salt solution were 

allowed to cool overnight (step 2). The ion-exchanged samples were then 

refluxed twice in DI/DS H20 to flush the membrane of excess salts (step 3). 

O.OlM Salt Sdutions. Three separate ion exchanges were performed using the 

O.OlM solutions. The ion exchanges used three different mole ratios of salt to 

acid group in Nafion, 1:2,1:1, and 2:l. The membranes were preweighed, dried 

in a vacuum oven for 24 hr at 120 “C and 4 kPa to remove atmospheric water, 

cooled in a dessicator, and reweighed. This procedure allowed for the most 

accurate determination of the moles of sulfonic acid groups and subsequent 

calculation of the appropriate moles of salt used to exchange the sample. Nafion 

samples were then soaked in the dilute salt solution at room temperature for 

24 hr, removed from the solution and patted dry, placed back in the vacuum 

oven for 24 hr at 120 “C and 4 kPa to remove atmospheric and excess water, 

cooled in a dessicator, and reweighed. In the limited mole-ratio exchanges, we 

did not reflux the membranes in the salt solutions. This will be explained in 

detail in section 3. 

2.4 Atmospheric Water Diffusion Into Nafion Analysis 

A piece of initialized H+-form Nafion was weighed and placed in an oven at 

125 “C and 4 kPa for 24 hr to dry. The membrane was then removed from the 

oven, allowed to cool in a dessicator, and placed on an OHAUS GA200 analytical 

balance in a room at 22 “C and 55% relative humidity. The diffusion of 

atmospheric water into the membrane was observed and recorded over time 

until the amount of atmospheric water absorbed reached a plateau, typically at 

6.5 weight-percent uptake. Figure 3 describes the time dependence of this 

process. 

2.5 PGAA 

Teflon bags were prepared by cutting a Teflon sheet and heat sealing the edges; 

the precut ion-exchanged Nafion samples were placed in the bags. The sample 

and bag were then weighed and placed in an oven at 125 “C and 4 kPa for 24 hr 

to dry. Upon removal from the oven, the sample and bag were placed in a 

dessicator to cool and then reweighed before sealing the bag to obtain a measure 

of the overall atmospheric water loss. The sealed bags were stored in a 

dessicator prior to analysis. Figure 4 shows a typical spectrum from a sample of 

5 
1s 

5 10 15 20 25 30 

Time (minutes) 

Figure 3. Atmospheric water uptake into dry, initialized H+-form Nafion. 

70000 

60000 

50000 

v] 40000 

z 

s 

" 30000 

20000 

10000 

0 

I- 

--l 

S atoms 

840 keV 

F atoms 

1633 keV H atoms 

2223 keV 

I ,. I I I I I 

0 500 1000 1500 2000 2500 3000 

Energy (keV) 

Figure 4. Prompt gamma spectrum for N117- H+ form. 

6 
I . 

Nafion 117 that was exchanged to the H+ form. Spectrum is the plotted number 

of counts vs. energy in kiloelectronvolts (keV). Samples were analyzed for all 

ions present (K+, Ca 2+, Cl-, etc.), including Cs (308 keV), Fe (352 and 7634 keV), K 

(770 keV), Cl (787 and 1165 keV), S (841 keV), Zn (1077 keV), Al (1778 keV), Ca 

(1942 keV), H (2223 keV), Co (230,277, and 556 keV), Cu (278 and 7614 keV), and 

Mg (585 keV). Each line in the spectrum is a signature for a specific element. In 

addition to the gamma-ray peaks due to neutron capture in the sample, there are 

peaks at low energies in the prompt gamma-ray spectra resulting from neutron 

capture by germanium (in the detector), aluminum (in the sample holder, 

shielding, and detector endcap), cadminum (shielding), copper (shielding), and 

fluorine (from the Teflon bag). The background for these experiments was 

measured with an empty Teflon bag. For simplicity, only the peaks of interest 

relating to the sample were labeled. 

To quantify the elemental composition of the samples, the peak intensity for each 

atom detected is normalized to the peak intensity of sulfur. For these samples, 

sulfur is the internal sample reference because it is assumed that one sulfur atom 

would be present for each monovalent counterion, two sulfur atoms would be 

present for each divalent counterion, and three sulfur atoms would be present 

for each trivalent counterion. The uncertainties (2 sigma or 20) on the molar 

ratios were calculated from the uncertainties in the counting statistics and are 

equal to the square root of the total counts minus the background counts for a 

given peak. To calculate the uncertainties on the element ratios, we then add the 

uncertainties of the two elements and then take the square root of the sum of the 

squares. 

The raw results (see Table 2 for raw data from as-received H+-form Nafion) 

yielded information on any contamination present in the samples and a measure 

of the extent of counterion exchange achieved in each of the procedures 

previously mentioned. 

Table 2. Raw data from prompt gamma analysis. 

Sample H/S H/S Error 

H+ As-Received 2.27 0.07 

H+ Pretreatment 2 [31] 2.29 0.03 

Note: 20 uncertainties were evaluated based on counting statistics. 

K/S K/S Error 

0.05 0.01 

0 0 

To separate hydrogen contributions due to water from those due to acid the 

following calculations were used. The acid hydrogens per sulfur (H+/S) ratios 

were calculated using equation (1) for samples in the acid form and equation (2) 

for samples in the ion-exchanged form: 

acidH’ 

s 

= 1 - [Total ions/S]; (1) 

7 
acidH + 

= 1 - [Total ions/S - I, 

S 

(2) 

where 

the total ions/S is the ratio of total moles of all other ions measured to moles of 

sulfur, 

Cl-/S is the chlorine-to-sulfur molar ratio, and 

YZ is the charge of the contaminating ion or ion used in the ion-exchange 

procedure. 

For example, if K+ is the contaminant ion, such as in the case of the as-received 

sample, the charge is 1. For samples in the ion-exchange form, incomplete 

exchange would result in the presence of acid hydrogens and residual 

unassociated salt would result in the presence of chlorine. This equation is 

straightforward unless there is more than one metal ion involved and they are of 

different charges. For samples containing hydrogen plus two counterions, a 

different calculation was required for the reported H+/S ratios. Equation (3) was 

used: 

acidH + 

= 1 - [Total ions/S - 

S 

Cl- Ii! s * 

nl 

Ml 

S 

Ml f M2 

--~, 

s s 

t 

cl22% 

n2 

442 

S 

A41 I M2 

s s 

where 

the total ions/S is the ratio of total moles of all ions except hydrogen measured 

to moles of sulfur, 

U/S is the chlorine-to-sulfur molar ratio, which must be taken as a percent of 

the ions with which it is associated, 

nl is the charge of the ion Ml, and 

n2 is the charge of the ion M2. 

Once the acid H+/S ratio has been calculated, it is assumed that the remaining 

hydrogen is due to water, and equation (4) is used to calculate the HzO/S ratio: 

. 

8 
I ’ 

where 

H/S is the molar ratio of total hydrogen to sulfur, and 

acidH+/S has been calculated using equations (l), (2), or (3). 

(4) 

3. Results and Discussion 

3.1 Atmospheric Water Diffusion 

Because the diffusion of atmospheric water into Nafion could influence the 

results of the prompt gamma analysis, an evaluation of the atmospheric water 

absorbed by the membrane was conducted. The plot in Figure 3 shows the 

fraction of atmospheric water uptake in Nafion over time until the uptake 

reached a plateau of 6.508% atmospheric water uptake at about 27 min. The 

diffusion coefficient can be estimated from this curve by determining the time to 

attain half-time sorption equilibrium [34,35]: 

D = 0.0492*12 

I (5) 

t112 

where 

the half-time sorption value is the time it takes for half equilibrium sorption 

weight to be achieved, 

t1/2 is the half-time in seconds, and 

1 is the thickness of the membrane in centimeters. From the original data, D 

can then be calculated to be 2.1486 E-7 m/s. This value is in agreement 

with the other researchers [36]. 

3.2 Nafion Pretreatments 

Prompt gamma data was collected for a series of H+-form Nafion membranes. 

Several H+-form membranes were prepared using each of the four pretreatment 

techniques for as-received membranes described in the previous section. In 

9 
addition, two other samples were analyzed for comparison: a S02F precursor 

and a S03H as-received. The results are shown in Table 3. The prompt gamma 

analysis yielded information on the composition of the membranes-the only 

elements detected within the membranes were H, K, and S. Oxygen cannot be 

detected using the technique, and carbon and fluorine cannot be quantified 

because the samples were analyzed in Teflon. 

Table 3. Prompt gamma comparison of H+-form Nafion membranes. 

Sample Acid H+/S % Neutralization H20/S 

(Other Counterions) 

Pretreatment 1 1301” 1 + 0.022 0 0.685 rt 0.014 

Pretreatment 2 [311b lkO.013 0 0.635 + 0.008 

Pretreatment 3 [32]c 1 Z!Z 0.034 0 0.645 Y!I 0.023 

Pretreatment 4 [33]d I 15 0.029 I 0 1 0.056f0.016 1 

I -SOzF Precursor I Ok0 I 0 I Ok0 I 

Nafion, As-Received 0.95zko.033 5 0.660 kO.019 

aBoil in concentrated nitric acid; rinse with pure water. 

bReflux in 2N HCI; rinse with H20. 

CReflux in 3% H202; reflux in DI H20; reflux in 0.5M H2S04; reflux in H20. 

dReflux in 50% HC1:50% HN03 solution (v:v); reflux in DI/DS Hz0 (3x). 

Note: 2a uncertainties were evaluated based on counting statistics. 

The as-received sample revealed some contamination due to K+ ions, possibly as 

a result of processing. The pretreated samples were all completely converted to 

the sulfonic acid form; the contamination was removed from the membrane in 

the pretreatment processes. In addition, the drying procedure appears to be 

effective in removing most of the water from the membrane. Chemically, all of 

the pretreated membranes are the same. 

3.3 Complete Counterion Exchanged Nafion 

In addition to the H+ form membranes, several other counterion forms were 

prepared, including K+, Cs+, Mg 2+, Ca2+, Co2+, Zn*+, Cu2+, AP+, and Fe3+. In the 

ion-exchange procedure, three different membrane samples were prepared for 

each counterion using the following procedure: in the first step, samples were 

soaked in an excess of the 2M salt solution for 24 hr, and one sample was taken 

after step 1 was completed; in the second step, the two remaining samples were 

refluxed in the salt solution for 4 hr and one sample was taken after steps 1 and 2 

were completed; in the third step, the last sample was refluxed two times in 

DI H20 and the sample was taken after steps 1, 2, and 3 were completed. All 

10 
. 

samples were dried before prompt gamma analysis according to the procedure 

in section 2. The different ion-exchange samples were taken to determine what 

would be the maximum percent exchange for the ion, whether different 

conditions produced different degrees of exchange, how rigorous the conditions 

for exchange would need to be for maximum exchange, and whether there was 

any indication of reverse exchange back to the proton form due to refluxing with 

DI water, perhaps simulating some application processes such as in fuel cells. 

The results from the counterion exchange procedure are shown in Table 4. The 

H+/S ratios were the same regardless of the sample preparation (i.e., step 1 only, 

steps 1 and 2, or steps l-3). Samples from step 1 and from steps 1 and 2 of the 

treatment processes showed small amounts of chlorine within the membrane, 

while samples from steps l-3 processes showed no residual chlorine. While this 

was taken into account for the calculation of the H+/S-the degree of 

neutralization and the HzO/S - the presence of chlorine shows that there are still 

excess salts within the membrane before the final rinsing process. The final step 

in the neutralization process is necessary to remove these excess ions. 

Table 4. Prompt gamma results from the counterion exchange procedures. 

Counterion Neutralization Amount 

Sample Radius Size [371 Acid H+/S (To Counterion) H2O/S 

(Pm) (%) 

K 227 0.059 * 0.004 94 0.060 f 0.012 

Cs+/Step 1 265 0.065 + 0.006 93.5 0.033 z!z 0.017 

Cs+/Steps 1 and 2 265 0.060 f 0.005 94 0.060 rt 0.022 

Cs+ /Steps l-3 265 0.060 zk 0.005 94 0.040 f 0.023 

Mg2’ 160 0 100 0.725 

Ca2+ 197 0.008 It 0.0002 98.6 0.626 rt 0.010 

co2+ 125 0.020 If: 0.001 98 0.700 * 0.015 

I cu2+ I 128 I 0.022* 0.002 I 97.9 I 0.709 f 0.030 I 

I Zn2+/Sten 1 I 134 I 0 I 100 1 0.565 310.024 1 

I Znz+/Steos 1 and 2 I 134 I 0 I 100 1 0.675 3~ 0.015 1 I I I 

Zn2+/Steps l-3 134 0 100 0.669 zk 0.029 

Fe3’ 126 0 100 0.599 f 0.031 

AP’ 143 0 100 1.605 f 0.036 

Notes: 20 uncertainties were evaluated based on counting statistics. 

pm = picometers. 

Taken together, the results show that excess salts are picked up by the membrane 

but can be flushed out of the membrane through refluxing with DI/DS water. A 

rigorous procedure for exchange is not necessary since allowing the 

membrane(s) to sit in the saturated solution at room temperature is sufficient to 

allow for exchange, and a long-term, vigorous reflux in DI water does not appear 

to influence a reverse exchange to the protonated sulfonic groups. 

11 
Furthermore, a shortened procedure might be used to achieve the same results: 

soaking the membranes for 24 hr in the saturated salt solution (step 1) and then 

refluxing the ion-exchanged membranes in DI/DS water to remove excess ions 

(step 3). 

Because the acid H+/S, percent neutralization, and H20/S data are similar for 

the three ion-exchange procedures, only one set of results for the membranes 

soaked in the salt solutions (steps l-3) are shown in Table 4 for all membranes 

other than cesium and zinc. The amount of HzO/S in the sample fluctuated 

slightly from sample to sample, but showed less than one water molecule per 

sulfonic acid group remaining for membranes exchanged with all ions except for 

the AP+. All membranes were free from contamination except those exchanged 

with calcium, which revealed some potassium contamination. Because the 

membranes used were all pretreated, it is presumed that the potassium 

contamination originated from the calcium salt, which has potassium listed as a 

minor component. The reported H+/S ratios did require a different calculation 

than the one previously discussed for the calcium samples since there were two 

metal ions oresent and the ions contained different charges. The following 

equation wai used: 

acidH + 

s 

= 1 - [Total ions/S - ir cl- El S * 

n=l I K 

s 

K+C” 

s s 

t 

Cls * 

n=2 

Ca 

S 

- 1, 

K+C” 

s s 

\ 

1, (6) 

/ 

where 

the total ions/S is the ratio of total moles of all other ions measured to moles of 

sulfur, 

CT/S is the chlorine-to-sulfur molar ratio, which must be taken as a percent of 

the ions with which it is associated, and 

n is the charge of the contaminating ion or ion used in the ion-exchange 

procedure. 

The experiments show that the exchange procedures, used successfully, result in 

replacement of 94100% of acid protons. Conversions are slightly lower for 

monovalent cations than for di- and trivalent cations. Monovalent K+ and Cs+ 

ions exhibited the most difficulty in achieving maximum conversion. These ions 

are the largest in size and it is a possibility that in the membrane aggregates there 

is a limit to the accessibility of the ion for exchange. The accessibility limitation 

in the aggregates seems to be supported by solubility and diffusion 

measurements. Monovalent cations reduce the solubility and diffusion 

coefficients for water and simple alcohols much more than di- and trivalent ions 

1371. 

12 
3.4 Limited Counterion Exchanged Nafion 

The effects of stoichiometry on the exchange procedure were evaluated with the 

O.OlM salt solutions. Three separate ion exchanges were performed using mole 

ratios of salt to reactive group in Nafion of 1:2,1:1, and 21. These experiments 

were used to determine (1) whether a dilute solution having half the salt cations 

needed could force a partial exchange and if some control over the extent of 

exchange could be achieved, (2) whether an equal mole ratio of ions to sulfonic 

groups would yield the same extent of conversion as that of a saturated solution, 

and (3) what the limitations on the concentration of the exchange solution might 

be to obtain complete exchange. Partial exchange might be useful for situations 

where preferential solvent sorption is needed but total exchange is too limiting 

on the conductivity. In addition, the 1:l mole ratio samples and the 2x excess 

number of cations (2:l) were used to determine how easily the ions access the 

membrane, and for economic considerations, since some of the salts used for the 

exchange procedure are expensive, how few cations per sulfonic acid site could 

be used for the exchange procedure. 

For these experiments, the moles of sulfonic acid groups in the membrane 

(assuming one per equivalent) are estimated simply from the equation: 

W moles of -SO3H groups = 2 

EW, ’ 

where 

V&,,, is the weight of the sample, and 

EVVN is the equivalent weight of the Nafion sample (which, in our case, is 1100). 

For this evaluation, because the previous experiments showed that no rigorous 

exchange procedure was required to obtain maximum sample exchange, all 

exchanges were performed at room temperature by soaking the membrane in the 

solution for 24 hr. Recall for these experiments the membrane was preweighed, 

the approximate moles of sulfonic acid groups calculated, and an equivalent 

number of moles of salt solution used to exchange the sample. These 

experiments were performed on a much narrower scale with only K+, Ca2+, Znz+, 

and AP+. The results are shown in Table 5. Focusing strictly on the data for the 

monovalent cation, K+, the exchange is 33% for the 1:2 sample, 58.7% for the 1:l 

sample, and 82.6% for the 2:l sample. All samples are below the maximum 

exchange that was, in the other study using 2M salt solutions (which used about 

a 100x molar excess of salt ions), found to be 92%. An excess of greater than two 

times appears to be necessary to force complete exchange to occur. 

13 
Table 5. Prompt gamma results for limited molar concentration solutions 

Sample 

(mol salt:mol-S03H) Acid H+/S 

Neutralization 

(To Counterion) Amount HzO/S 

(%o) 

1:2-K+ Sol’n 0.661 f 0.042 33.094 1.038 f 0.011 

I 1:1-K+ Sol’n 1 0.437kO.023 1 56.347 1 0.748kO.015 1 

2:1--K+ Sol’n 0.205 IL 0.004 79.516 0.544 f 0.008 

1:2-Ca2+ Sol’n 0.566 + 0.030 42.220 2.486 f 0.012 

1:1-Ca2+ Sol’n 0.129 f 0.005 87.142 1.303 + 0.017 

2:1-Ca2+ Sol’n 0.015 + 0.001 98.468 1.786 I?Z 0.042 

1:2-Zn*+ Sol’n 0.550 2~ 0.082 45.009 2.784 f 0.034 

l:l-Zn*+ Sol’n 0.169 f 0.008 83.103 2.219 IL 0.021 

2:1- Zn2+ Sol’n 0.084k 0.007 91.560 2.911zk 0.044 

1:2-AP+ Sol’n 0.407f 0.041 59.253 3.019 f 0.030 

l:l-Al3+ Sol’n 0.083 rf: 0.008 91.716 2.024+ 0.045 

2:1-A13+ Sol’n 0.010 If: 0.002 100 

Note: 2a uncertainties were evaluated based on counting statistics. 

3.694If: 0.080 

These limited molar concentration solutions were used in the exchange 

procedures to determine if partial exchange could be achieved and to gain an 

idea of what the limits of the solution concentration for complete exchange might 

be. The samples that were swollen in 1:2 (mol:mol) solutions, containing about 

one half the number of moles of salt needed for complete exchange, did show 

limited exchange of the sulfonic acid sites (see Table 5). The samples also 

showed repeatability in the results of the limited exchange. In addition, while 

the amount of exchange was nearly twofold over the 1:2 (mol:mol) solutions, the 

1:l (mol:mol) solutions do not allow all counterions to access the membrane 

areas needed for complete exchange at the sulfonic acid sites. The divalent and 

the trivalent have more sites readily exchanged than the monovalent counterion. 

The 2:l (mohmol) solution exchange for the divalent and trivalent counterions 

show high degrees or complete exchange and the neutralization values are 

nearly identical to the saturated solution samples. However, the monovalent 

counterion would require a more concentrated salt solution to approach the 

maximum amount of exchange in the sample. The amount of water in the 

membranes is larger than in the other samples analyzed. However, this is not 

14 
significant since the membranes were not rigorously dried before analysis for 

this portion of the study due to its focus on stoichiometr c aspectsi of the 

exchange procedure. 

3.5 As-Received Counterion Exchanged Nafion 

Finally, as-received membranes were analyzed, focusing on the K+ 

contamination within the as-received membrane and the possibility that it could 

be exchanged out of the membrane when converting to a new counterion. For 

this study, the Zn2+ and A13+ cations were used since both exhibited high degrees 

of conversion to the counterion. Results are shown in Table 6. For both 

counterions, the amount of K+ ion contamination does not change significantly 

from the as-received H+-form sample, 5%, for the 1:2 (mol:mol) solutions but 

decreases by a factor of 2 for the 1:l (mol:mol) solutions. These results indicate 

that it is possible to displace some K+ ions with Zn2+ or AP+. The displacement of 

K+ contaminants with other counterions was not complete at the intermediate 

molar excess levels studied. The total ion-exchange levels achieved in these asreceived membranes are very similar to the counterion conversion achieved for 

initialized samples under the same exchange conditions. 

Table 6. Prompt gamma results for as-received H+-form Nafion. 

Sample 

(mol salt:molS03I-U 

As-Received 

Acid H+/S 

0.95 rt 0.033 

K+ Ion Neutralization Amount 

Contamination (To Counterion) H20/S 

(o/o) 

0.05 + 0.006 5 0.660 k 0.019 

Initialized 1 f 0.029 0 0 0.056 + 0.016 

1:2-Zn2+ Sol’n 0.486 310.083 0.050 f 0.009 46.426 3.094 l!I 0.085 

l:l-Zn2+ Sol’n 0.169 IL 0.025 0.025 + 0.008 81.051 3.112 -t 0.073 

1:2-A13+ Sol’n 0.413 -t 0.042 0.039 k 0.006 54.692 2.926 f 0.051 

I , 

l:l-AP+ Sol’n 1 0.010 + 0.001 1 0.013 + 0.005 1 98.725 I 3.439 If: 0.053 

Note: 20 uncertainties were evaluated based on counting statistics. 

15 
4. Conclusions 

PGAA was applied to the study of chemical composition in Nafion PFSI 

membranes. This technique is unique in its ability to quantitatively identify trace 

ion content and water content (through detection of hydrogen) in the 

membranes. The study was focused on identifying contamination, effectiveness 

of treatments to remove contamination, and extent of ion-exchange in 

membranes prepared using different procedures. The results established that 

as-received membranes contained small amounts of residual potassium 

contamination, which was effectively removed by any of a variety of standard 

pretreatments. Maximum extent of ion-exchange was also determined for a 

variety of mono-, di-, and trivalent counterions. Exchange levels for membranes 

prepared in excess ion solutions varied from 94-lOO%, increasing with valency. 

Partial exchange levels were achieved using mono-, di-, and trivalant cations 

introduced in ion-starved solutions. Results also indicated that ion-exchange can 

be used to reduce residual potassium contamination in as-received membranes. 

16 
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, 

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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Determining Extent of Ion-Exchange in Various Counterion Nafion Membranes AH42 

Using Prompt Gamma Neutron Activation Analysis (PGAA) 

6. AUTHOR(S) 

Sandra K. Young, Samuel F. Trevino, Nora C. Beck Tan, and Rick L. Paul* 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS 

U.S. Army Research Laboratory 

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Approved for public release; distribution is unlimited. 

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13. ABSTRACT(Maximum 200 words) 

Researchers have performed structural evaluations on Nafion membranes for many years. Various research groups 

have published a large volume of data; however, much of the work done by these groups is contradictory. One 

important aspect of the membranes is the chemical uniformity and quantity of water within the membrane under 

investigation. A new technique that has shown its usefulness for providing accurate and fast quantitative 

measurements of chemical composition quickly is prompt gamma neutron activation analysis (PGAA). In this work, 

a sulfonyl fluoride-form membrane, an as-received membrane from the manufacturer, and several differently 

pretreated H’-form Nation membranes were examined using PGAA. The evaluation showed the necessity for 

pretreatment of membranes to eliminate contamination. A series of counterion-exchanged membranes were also 

examined to determine maximum conversion achieved and to identify possible limitations to complete conversion. 

The results show that it is possible to obtain nearly complete or complete conversion to the counterion from the H+- 

form Nation samples. Water content can be limited within the membrane but not eliminated. In samples that were 

not pretreated, complete ion exchange was limited by the contamination. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

Nafion, perfluorosulfonate isonomer membrane, ion-exchange, prompt gamma neutron 28 

activation analysis, acid form membranes, counterion-form membranes 16. PRICE CODE 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT 

OF REPORT OF THIS PAGE OF ABSTRACT 

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL 

,IChl 7KAn n, 9an~smln Standard Form 298 (Rev ma\ 

23 Prescribed by ANSI Std. >sgi6 298-102 
, 

24