Increasing PEMFC Performance Using Nafion-Hybrid and Alternative Membrane Materials

Executive Summary

Proton Exchange Membrane fuel cells (PEMFCs) most typically use a material called Nafion for their electrolyte membranes to transfer protons from the anode to the cathode to produce a current flow. Nafion has been used for a few decades quite successfully however it suffers from a few major problems. Proper hydration of Nafion is vital to the operation of the cell; water molecules help transport the hydrogen ions through the membrane. This means the gasses entering the cell must be hydrated which adds to the complexity and expense of the system. Operating temperatures must be kept below 125°C to maintain proper hydration and structural properties of the membrane, and to improve operating life. By introducing alternatives to Nafion membranes the hydration, temperature, and lifetime issues can be reduced while allowing for a higher proton exchange rate which leads to higher power densities and ultimately better and longer cell performance.

Alternative membranes can be made by adding small amounts of platinum which helps to produce water and keep it hydrated, or by altering a Nafion membrane’s chemical makeup, or by replacing the Nafion all together. All three of these processes are considered in this paper as well as their added benefits. The additional cost compared to a Nafion membrane is quite low; many of the added materials are low in cost and require little additional processing, however the hydration and temperature improvements can allow for smaller systems which cost less to produce.

A comparative experiment between Nafion and these three leading alternatives is proposed to determine the benefits and possible draw backs of each of alternatives discussed. The experiment examines important characteristics desirable to PEMFCs electrolytes such as electrical resistance, chemical and temperature related resilience, structural strength, hydrogen permeability, dimensional changes, water absorption, and proton exchange. An operational test of one cell is also conducted to show overall performance of each membrane in both hydrated and dry conditions.

Introduction

Perfluorosulfonic acid membranes such as Nafion must be well hydrated and kept at low temperatures to maintain high ionic conductivity. Several alternatives to Nafion membranes have been produced to lessen these issues; this paper will concentrate three types: a PTFE/Nafion Pt-PDDA membrane, a polysiloxane Nafion hybrid membrane, and polyimide-silica membranes produced by Sandia National Laboratories.

In the past self-humidifying membranes have been constructed by introducing nanometer sized Pt particulates into the Nafion resin to catalyze the water production reaction. This however created electron conduction paths and short circuited the cell. Layering this Nafion-Pt mix with layers of pure Nafion reduced this problem though it involves a complicated and thus expensive casting process. A thin cast Nafion electrolyte was also conceived to enable self hydration through water production at the cathode yet could only be used at near ambient temperatures and suffered from reactant gas crossover and structural failure.

Materials

Y. Liu et al. have proposed a new self-hydrating Nafion/PTFE membrane which utilizes a bonded solution of Pt dispersed in poly-diallyldimethylammonium chloride. The Pt-PDDA solution electrostatically attaches to the pores of the PTFE film before the Nafion is introduced. This creates a strong membrane while eliminating the electron conduction issue and is simpler to produce than the multilayered membrane.

Organic-inorganic hybrid materials are becoming more common in many industries. They are produced using solvent gel methods relying on hydrolysis and condensation reactions. The inclusion of a polysiloxane precursor (about 15% by weight) in Nafion membranes demonstrated by Lavorgna et al. allows for higher water condensation and absorption rates in the electrolyte as well as a higher thermal stability.

Sandia National Laboratories has been working towards a Nafion alternative that is similar to the organic-inorganic hybrids. Polyimide-silica membranes can be formed with the sol-gel method using alkoxide precursors and network polymers or oligomers. By careful selection of the polyimide and alkoxide a membrane can be produced with high structural and thermal properties and low gas permeabilities while maintaining higher water absorption rates.

Experiment

These three proposed membrane alternatives show great potential for the improvement of a PEMFC. However their operational parameters need to be categorized and tested against Nafion to show how much benefit a replacement can provide and to determine its weak points. To accomplish this each membrane needs to be tested in several categories: electrical insulation, chemical stability, thermal stability, mechanical strength, hydrogen permeability, dimensional stability, water content, proton conductivity, and operating performance within a cell under humidified conditions and with dry gas.

Two samples of each membrane for each test will be film cast into 10cm x 10cm specimens, unless otherwise specified by the test. One sample should have a thickness of no more than 50µm and the other should be cast at a thickness of 25µm. Each sample will be prepared to its full operating condition including activation and oxidation. For proper formation of each of the three alternative membranes see Y. Liu, M. Lavorgna, and C. J. Cornelius.

Electrical Insulation

Resistance measurements will be calculated using the fast current pulse Automated Membrane Resistance method as described by F.N. Buchi et al.

Chemical Stability

Samples will be weighed and then soaked in a 3% aqueous solution of Fenton’s reagent containing 2ppm FeSO4 at 80°C for 1 hour. After measuring the final weight the percent weight reduction can be calculated by:

Thermal Stability

The thermal stability of each sample will be calculated by using a TA Instruments 2950 thermogravimetric analyzer or equivalent at a scan rate of 20°C/min in both argon and oxygen atmospheres with a platinum sample pan. The samples will be run from room temperature to 600°C. The weight reduction equation above can be reused.

Mechanical Strength

Samples will be dehydrated at negative pressure at 80°C for 10 hours and then tested at room temperature using a tensile tester according to the Standard Test Method for Tensile Properties of Thin Plastic Sheeting ASTM D882-02.

Hydrogen Permeability

Hydrogen permeability will be conducted using the gas chromatograph method described by K. Broka et al. Both the 50µm and 25µm samples should be tested.

Dimensional Stability

Membrane samples will be kept at negative pressure at 80°C for 24 hours to allow for complete dehydration. Samples will then be measured to determine an initial width L1. Each sample will then be immersed in deionized water at 80°C for 24 hours, after which the final length L2 should be measured. The percent of dimensional change can then be calculated by:

Water Content

Samples will soak in water at 80°C for 24 hours then will be given an initial weight W1. After being completely dehydrated at negative pressure at 80°C for 24 hours a second weight W2 will be taken. The percent of water content can then be calculated by:

Proton Conductivity

Proton conductivity will be measured for each sample using the ac impedance four electrode method described by Y. Sone et al. Samples should be 3cm x 1cm, affixed in a cell with two platinum foil electrodes spaced 3cm apart. Two platinum needles attached 1cm apart in the center of the cell will measure voltage drop at temperatures ranging from 35°C to 80°C. Temperature and relative humidity should be regulated per the test run by Y. Sone et al. Proton conductivity at varying temperatures should resemble the plot given by the equation:

Where \(A\) is the frequency factor, \(E_a\) is the activation energy, \(R\) is the universal gas constant, and \(T\) is temperature.

Operating Performance

A 40% weight Pt/C catalyst will be loaded to an anode and cathode of 5cm2 at 0.4mg Pt/cm2 and should be hot pressed to each sample at 140°C and 2 MPa for 1 minute. A carbon paper diffusion layer wil be used in the MEA. A single cell will be tested at 65°C at 0.1 bar hydrogen and 1 bar air in a capable apparatus such as the FuelCon Evaluator C50-LT or equivalent. Current and voltage measurements should be collected up to 1000mA/cm2. The cell will then be dried with nitrogen for 12 hours and the current-voltage measurements will be collected again using dry gasses at 1.2 times stoichiometric values for hydrogen and 2 times that for air.

Citations

[1] Lavorgna, Marino, Gilbert, Marianne, Mascia, Leno, Mensitieri, Giuseppe, Scherillo, Giuseppe, & Ercolano, Giorgio. Hybridization of Nafion membranes with an acid functionalised polysiloxane: Effect of morphology on water sorption and proton conductivity. J Membrane Sci 2009; 330: 214-226.

[2] Cornelius, Chris J., Marand, Eva. Hybrid inorganic-organic materials based on a 6FDA-6FpDA-DABA polimide and silica: physical characterization studies. Polymer 2002; 43: 2385-2400.

[3] Liu, Yonghau, Nguyen, Tienhoa, Kristian, Noel, Yu, Yaolun, & Wang, Xin. Reinforced and self-humidifying composite membrane for fuel cell applications. J Membrane Sci 2009; 330: 357-362.

[4] Sone, Yoshitsugu, Ekdunge, Per, Simonsson, Daniel. Proton Conductivity of Nafion 117 as Measured by a Four-Electrode AC Impedance Method. J Electrochem Soc 1996; 143: 1254-1259.

[5] Buchi, Felix N., Scherer, Gunther G. In-situ resistance measurements of Nafion 117 membranes in polymer electrolyte fuel cells. J Electroanalytical Chem 1994; 404: 37-43.

[6] Broka, K, Ekdunge, P,. Oxygen and hydrogen permeation properties and water uptake of Nafion 117 membrane and recast film for PEM fuel cell. J Applied Electrochemistry 1997; 27: 117-123.