High-temperature proton exchange membrane fuel cells (HT-PEMFCs) utilize a phosphoric acid- (PA-) doped polybenzimidazole (PBI) membrane as a polymer electrolyte. The PA concentration in the membrane can affect fuel cell performance, as a significant amount of PA can leak from the membrane electrode assembly (MEA) by dissolution in discharged water, which is a byproduct of cell operation. Spectrophotometric analysis of PA leakage in PA-doped polybenzimidazole membrane fuel cells is described here. This spectrophotometric analysis is based on measurement of absorption of an ion pair formed by phosphomolybdic anions and the cationoid color reagent. Different color reagents were tested based on PA detection sensitivity, stability of the formed color, and accuracy with respect to the amount of PA measured. This method allows for nondestructive analysis and monitoring of PA leakage during HT-PEMFCs operation.
Proton exchange membrane fuel cells (PEMFCs) have been of great interest to research, as these energy conversion devices pose a promising environmentally friendly alternative to fossil fuel-based technologies [
In HT-PEMFCs the usual solvent, water, is replaced with the proton conductive phosphoric acid (PA) in order to operate at such high temperatures (~200°C) and provide improved chemical and physical characteristics [
The membrane degradation mechanism has been studied in depth in the last few decades [
In this study, a simple colorimetric analysis of PA leakage in the PA-PBI membrane fuel cell is reported for the first time. The applicability of the colorimetric analysis method on HT-PEMFCs for PA at constant current density operation is presented. The selected method is shown to be a reliable and sensitive method of detection of PA, with a linear dynamic range of up to 1 × 10−5 M and a detection limit of 1 × 10−8 M.
All chemicals used were analytical grade reagents, and deionized water was used to produce all solutions.
For the molybdenum blue reaction (Table
Color reagent, molybdenum blue.
Compound | Quantity |
---|---|
Ammonium molybdate tetrahydrate | 0.6 g |
Potassium antimony tartrate hydrate | 0.024 g |
Deionized water | 30 mL |
Sulfuric acid (7.4 M) | 10 mL |
Ammonium sulfamate (0.44 M) | 10 mL |
L-Ascorbic acid (0.4 M) | 10 mL |
For the crystal violet reaction (Table
For the rhodamine B reaction (Table
Color reagent, rhodamine B.
Compound | Volume ratio |
---|---|
Ammonium molybdate tetrahydrate (7.4 × 10−3 M) | 1 |
Rhodamine B (1 × 10−3 M) | 1 |
Sulphuric acid (0.6 N) | 1 |
NaOCl solution (0.05 M) | 1 |
Color reagent, crystal violet.
Compound | Volume ratio |
---|---|
Ammonium molybdate tetrahydrate (1.2 × 10−2 M) | 1 |
Sulphuric acid (6.5 N) | 1 |
Crystal violet solution (1 × 10−3 M) | 1 |
Commercially available MEA (20.25 cm2 active area, Dongjin Semichem), Teflon gaskets, and graphite bipolar plates were assembled and used for HT-PEMFC operation. The unit cell was operated on constant current density at 1.1 A cm−2 for 84 hours, the cell operation temperature was 150°C, and the stoichiometry of hydrogen and air was 1.2 and 2.0, respectively. During the cell operation, the exhaust gases from both anode and cathode were cooled and collected in liquid state in the vials which contained 10 mL of distillated water. Then, the reagents were added to the vent solutions from the unit cell. After the color reaction, the solutions with added reagent were analyzed by UV/Vis spectroscopy. The spectrophotometer used was a LAMBDA 25 UV/Vis spectrophotometer from PerkinElmer.
The quantitative analysis of PA leakage during the PA-doped PBI membrane HT-PEMFCs operation was performed by spectrophotometric analysis of the colorimetric reaction of the PA-molybdenum compound [
A schematic depiction of the approach of colorimetric detection of PA leakage in HT-PEMFCs used in the experiment.
One of the PA detection methods used in this study was based on the analysis of the absorption spectrum of heteropoly molybdenum blue, the product of PA ions reacting with molybdate, followed by reduction with stannous chloride. The chemical equation is shown below [
This compound showed poor detection limit (>10 ppb) and low sensitivity. As an improvement to the heteropoly molybdenum blue method, stannous chloride was replaced with ascorbic acid to provide a stable reduction reaction, as well as a clear spectrum of molybdenum blue [
The sensitivity of the color reagent is defined in terms of apparent molar absorptivity. Color reagents must have a high apparent molar absorptivity and be a cationoid dye, which allows the formation of an ion pair with molybdophosphoric acid. Table
Molar absorptivity of color reagents [
Color reagent | Molar absorptivity (M−1 cm−1) |
---|---|
Molybdenum blue | 2.0 × 104 |
Brilliant green | 0.88 × 105 |
Crystal violet | 1.28 × 105 |
Malachite green | 7.8 × 104 |
Rhodamine B | 1.19 × 105 |
Servon red L | 0.7 × 104 |
Three major selection criteria are applied to the choice of color reagent for PA analysis: (1) the detection accuracy; (2) the color formation time and color stability; and (3) the detection limit at low PA concentration. There are many well-studied color reagents that can be applied to detecting low PA concentrations; however, the three chosen for this experiment were molybdenum blue [
Each color reagent was tested with PA standard solution with a concentration range of 0 M to 1 × 10−3 M. Gradation of each color dye is shown in Figure
PA analysis from model PA compound solution. (a) Molybdenum blue method, (b) rhodamine B, and (c) crystal violet color reagent.
The reaction with rhodamine B resulted in a distinct, light pink color. The chemical equation of the rhodamine B reaction is as follows [
The PA concentration was determined by the absorption spectra in standard solution, molybdenum blue, crystal violet, and rhodamine B solution. Clear absorption peaks were observed in each color reagent as shown in Figure
Spectrometric measurement of standard solution reacted with (a) molybdenum blue, (b) rhodamine B, and (c) crystal violet color reagent.
Relationship between PA concentration and the absorbance of standard solutions reacted with (a) molybdenum blue, (b) rhodamine B, and (c) crystal violet color reagent.
The molybdenum blue method was applied for quantitative analysis of leaked PA concentration from HT-PEMFCs (Figure
Applicability of the colorimetric method to real device. (a) Photograph of operating real device. (b) Histogram of PA contents and discharged water as a function of operation time.
Recently, various investigation methods for PA concentration in the HT-PEMFCs have been studied and proposed; however, the spectrometric quantification with a color reagent solution is the most efficient method. Color reagents which have high apparent molecular absorptivity were selected and tested. The molybdenum blue and ascorbic acid method provided the most stable and reliable results, especially with respect to detecting speed and accuracy. The application of spectrometric quantification of PA leakage on the HT-PEMFCs is a competitive method.
The authors declare no competing financial interests.
This study was supported by the Korean Government through the New and Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by MOTIE (no. 2010T100200501). This work was also financially supported by KIST through institutional project (no. 2E25411).