The document discusses ion exchange membranes and their ionic resistance and permselectivity. It examines how varying properties of the membranes like water uptake, ion content, and surface structure impact resistance and permselectivity. The goal is to design membranes that can be used effectively in reverse electrodialysis to harvest energy from salinity gradients by having low resistance and high permselectivity. Testing of commercial and experimental membranes showed that as water uptake increased, resistance also increased, and resistance generally decreased with higher salt concentrations. Anion exchange membranes tended to have higher resistance than cation exchange membranes.

1. The PPG Undergraduate Research Fellowship Program is supported by The PPG Research Foundation.
2014 PPG Undergraduate Research
Fellowship Program
Ionic Resistance and Permselectivity of Ion Exchange Membranes
William R. Salem, Harrison J. Cassady, and Dr. Michael A. Hickner
Department of Materials Science and Engineering, The Pennsylvania State University
I. Introduction III. Materials and Methods IV. Results
Water and Our World
•Over one-third of the world lives in water-stressed regions.1
•By 2030, almost half of the world’s population will be living in water-stressed regions
making this a worldwide issue that needs to be addressed.1
•With only 2.5% of the world’s water supply being fresh water and only 1% of that fresh
water being accessible, there is a growing demand for the capability of purifying
contaminated water.2
Water Purification Techniques
Thermal: Multiple effect distillation, multiple stage flash
Membrane: Reverse osmosis (RO), electrodialysis (ED)
ED uses electric potential to desalinate water via polymeric ion exchange
membranes. Reverse electrodialysis (RED) can be used to harvest energy from salinity
gradients when salt water and fresh water are mixed. Knowing how ion exchange
membranes react in ionic resistance and permselectivity measurements can determine
which membranes will perform the best in the presence of a concentration gradient. ED
and RED, therefore, can be used to address two pressing and interrelated world-wide
shortages: water and energy.
II. Objective
The goal of this research is to determine the relationship between internal
and surface membrane properties of ion exchange membranes through membrane
selectivity (permselectivity) and resistance (inverse conductivity). The membrane
properties that were varied include ion exchange content, water uptake, and
surface structure.
Materials:
•Selemion™ (AGC Chemicals):
• AMV & CMV
•PCCell PC-SK
• AEM & CEM
Static Resistance:
An I-V sweep was used to measure the membrane potential
and current across a uniform concentration using Ag/AgCl
electrodes. The slope of the membrane potential versus current
was the membrane resistance.
Permselectivity:
Membrane potential was measured across a 0.5-0.1 M
concentration difference using Ag/AgCl double junction
electrodes.
V. Conclusions
• As water uptake increases area resistance increases because
ion mobility is unfavored in membranes with high water
uptake
• Area resistance decreases with increasing salt
concentration because ion mobility increases in higher
concentrated solutions
• Permselectivity increases based on the material properties
of the membrane
• Generally, AEMs had a higher resistance than CEMs
• Membranes with a lower ratio of ionic substituents had a
higher resistance because of poor ion mobility
As water uptake
increases, area resistance
increases. The correlation
suggests that as water
uptake values increase, ion
mobility is unfavored
through the membrane.
References
Note: C represents CEM and A represents AEM
RED Stack Image: Geise, Geoffrey M., Cassady, Harrison J. Ionic
resistance permselectivity of ion exchange membranes
Membranes
•Commercial membranes are too resistive and do not exclude ions effectively
•We have designed new anion exchange membranes (AEM) with fixed positive charges
(as shown above). Under an applied electric potential, AEMs permit anions to transport
through the membrane toward the anode
•The water uptake, ion content, and surface structure can be varied in the polymer
synthesis procedure
Operation of RED and Permselectivity
•The efficiency of power generation for RED and permselectivity is related to the
resistance of the membranes. A favorable RED stack contains membranes with low
resistance and high permselectivity. Presently, those membranes don’t exist.
•Power production also relies on the characteristics of the system, e.g., the number of
membranes in the stack, the electrical resistance of the membranes, and the resistance of
the membranes, and the resistances of other components of the system
•Kuraray Membranes
• AEMs & CEMs
•QA-PPO Membranes
• X20Y40 & X47Y13
Area resistance tends to
decrease with increasing salt
concentration for the
membranes, which may be due
to an increase in the
concentration of charge
carrying ions in the polymers
as salt concentration increases.
X47Y13 has higher
permselectivity and lower
intrinsic resistance than
X20Y40 due to its material
properties. The variation in
values with different salts is
because of the Ag/AgCl
electrode used.
Generally, AEMs
had a higher resistance
than CEMs. However,
not all CEMs were tested
so others with higher
resistances may be
present. A lower ratio of
ionic substituents (10,
12) led to higher
resistances because of
poor ion mobility.
1. "Scarcity, Decade, Water for Life, 2015, UN-Water, United Nations, MDG, Water, Sanitation, Financing, Gender,
IWRM, Human Right, Transboundary, Cities, Quality, Food Security." UN News Center. UN, Web. 16 July 2014.
2. "Clean Water Crisis, Water Crisis Facts, Water Crisis Resources - National Geographic." National Geographic.
National Geographic, Web. 17 July 2014.