Robert F. Savinell

Acid doped polybenzimidazoles, a new polymer electrolyte

Polybenzimidazole films doped with phosphoric acid are being investigated as potential polymer electrolytes for use in hydrogen/air and direct methanol fuel cells. In this paper, we present experimental findings on the proton conductivity, water content, and methanol vapor permeability of this material, as well as preliminary fuel cell results. The low methanol vapor permeability of these electrolytes significantly reduces the adverse effects of methanol crossover typically observed in direct methanol polymer electrolyte membrane fuel cells.

Conductivity of PBI Membranes for High-Temperature Polymer Electrolyte Fuel Cells

Polybenzimidazole (PB1) film, a candidate polymer electrolyte membrane (PEM) for high-temperature (120-200°C) fuel cells, was cast from PBI/trifluoacetyl/H 3 PO 4 solution with constant molecular weight PBI powder and various acid doping levels. Conductivity measurements on these membranes were performed using an ac method under controlled temperature and relative humidity (RH). A complete set of conductivity data for H 3 PO 4 acid-doped PBI is presented as a function of temperature (60-200°C), RH (5-30%), and acid doping level (300-600 mol %). A mechanism of conductivity is proposed for the proton migration in this PBI/acid system based on this and previous work. Proton transfer in this system appears to occur along different paths for different doping levels, RHs, and temperatures. Hydrogen bonds immobilize the anions and form a network for proton transfer by a Grotthuss mechanism. The rate of proton transfer involving H 2 O is faster, leading to higher conductivity at higher RH. The order of the rate of proton transfer between various species is H 3 PO 4 (H 2 PO 4 -)...H-O-H> H 3 PO 4 ...H 2 PO - 4 > N-H + ...H 2 PO 4 - + N-H + ...H-O-H > N-H + ...N-H. The upper limit of proton conductivity is given by the conductivity of the liquid state H 3 PO 4 .

A H2O2 fuel cell using acid doped polybenzimidazole as polymer electrolyte

Phosphoric acid doped polybenzimidazole (PBI-poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole]) has been investigated for use in a H2O2 fuel cell. The prototype fuel cell test results show that the PBI fuel cell worked quite well at 150 °C with atmospheric pressure hydrogen and oxygen which were humidified at room temperature. No membrane dehydration was observed over 200 h operating. The maximum power density of this prototype fuel cell was 0.25 W cm−2 at current density of 700 mA cm2. Further improvement of the cell performance is to be anticipated by properly impregnating the electrode structure with the polymer electrolyte. The advantage of the H2O2 fuel cell using PBI as polymer electrolyte is that the cell design and the routine maintenance can be significantly simplified because of the low electro-osmotic drag number and good proton conductivity of the PBI membrane at elevated temperature.

Thermal stability of proton conducting acid doped polybenzimidazole in simulated fuel cell environments

Recently, polybenzimidazole membrane doped with phosphoric acid (PBI) was found to have promising properties for use as a polymer electrolyte in a high temperature (ca. 150 to 200 C) proton exchange membrane direct methanol fuel cell. However, operation at 200 C in strongly reducing and oxidizing environments introduces concerns of the thermal stability of the polymer electrolyte. To simulate the conditions in a high temperature fuel cell, PBI samples were loaded with fuel cell grade platinum black, doped with ca. 480 mole percent phosphoric acid (i.e., 4.8 H{sub 3}PO{sub 4} molecules per PBI repeat unit) and heated under atmospheres of either nitrogen, 5% hydrogen, or air in a thermal gravimetric analyzer. The products of decomposition were taken directly into a mass spectrometer for identification. In all cases weight loss below 400 C was found to be due to loss of water. Judging from the results of these tests, the thermal stability of PBI is more than adequate for use as a polymer electrolyte in a high temperature fuel cell.

Heat-treated iron(III) tetramethoxyphenyl porphyrin chloride supported on high-area carbon as an electrocatalyst for oxygen reduction: Part II. Kinetics of oxygen reduction

Oxygen reduction was investigated at FeTMPP–Cl adsorbed on the Black Pearls carbon and heat-treated at temperatures from 200 to 1000°C. Kinetic parameters of the reaction and the dependence of the reaction rate on the heat-treatment temperature were established in acid and alkaline media. It was found that the O2 reduction rate increases with increase in the heat-treatment temperature of the electrocatalyst, reaching a plateau at 700≤T≤1000°C. The reaction rate was higher in alkaline solution on all electrocatalysts examined, but its dependence on the heat-treatment temperature was the same as in acid media. Like similar other macrocycle catalysts, FeTMPP–Cl/BP was found to be methanol-tolerant. The effect of sulfate, perchlorate, and phosphate anions was also investigated and no influence on the reaction rate was found. Activities of FeTMPP–Cl/BP and Pt/BP electrocatalysts were compared. FeTMPP–Cl/BP heat-treated at 700≤T≤1000°C showed similar activity to Pt/BP if they are compared in alkaline solution, but in acid solution, the reaction rate on the Pt electrocatalyst was higher. The controversy in the literature on the role of metallic Fe particles in O2 reduction catalysis is discussed. Comparison of present results with previous cyclic voltammetry and TEM investigations of the same electrocatalyst gave further evidence that some other products of the heat-treatment are responsible for the high activity of the catalyst.

Evaluation of Ethanol, 1‐Propanol, and 2‐Propanol in a Direct Oxidation Polymer‐Electrolyte Fuel Cell A Real‐Time Mass Spectrometry Study

Ethanol, 1-propanol, and 2-propanol have been evaluated as alternative fuels for direct methanol/oxygen fuel cells. The relative product distributions for the electro-oxidation of these alcohols under fuel-cell conditions were determined using on-line mass spectrometry. For water/ethanol mole ratios between 5 and 2, ethanol is the main product, while CO{sub 2} is a minor product. However, an increase of the water/ethanol mole ratio increased the relative product distribution of CO{sub 2} slightly. Propanol was the main product of the electro-oxidation of 1-propanol with a similar percentage of CO{sub 2} being formed as for ethanol. In contrast, the electro-oxidation of 2-propanol yielded practically only acetone. Between 150 and 190 C, the product distributions for the electro-oxidation of ethanol, 1-propanol, and 2-propanol do not depend significantly on the temperature. No differences in the product selectivities of Pt-Ru and Pt-black were found. Ethanol is a promising alternative fuel for direct methanol fuel cells (DMFCs) with an electrochemical activity comparable to that of methanol. Conversely, 1-propanol and 2-propanol are not suitable as fuels for DMFCs due to their low electrochemical activity.

Thermal Stability of Nafion® in Simulated Fuel Cell Environments

Nafion{reg_sign} is an important polymer electrolyte for polymer electrolyte fuel cell applications due to its inertness and high proton conductivity. Operation of these fuel cells for extended periods of time at temperatures approaching 100 C introduces concerns of the thermal stability of the Nafion electrolyte. To simulate the conditions in a fuel cell, Nafion samples were loaded with fuel-cell grade platinum black and heated under atmospheres of nitrogen, 5% hydrogen, or air in a thermal gravimetric analyzer. The products of decomposition were taken directly into a mass spectrometer for identification. In all cases, Nafion was found to be thermally stable up to 280 C, at which temperature the sulfonic acid groups began to decompose. A mechanism for the decomposition is proposed which explains many of the evolved compounds observed during heating.

A Polymer Electrolyte for Operation at Temperatures up to 200°C

In developing advanced fuel cells and other electrochemical reactors, it is desirable to combine the advantages of solid polymer electrolytes with the enhanced catalytic activity associated with temperatures above 100 C. This will require polymer electrolytes which retain high ionic conductivity at temperatures above the boiling point of water. One possibility is to equilibrate standard perfluorosulfonic acid polymer electrolytes such as Nafion, with a high boiling point Bronsted base such as phosphoric acid. The Nafion/H[sub 3]PO[sub 4] electrolyte has been evaluated with respect to water content, ionic conductivity and transport of oxygen, and methanol vapor. The results show that at elevated temperatures reasonably high conductivity (>0.05 [Omega][sup [minus]1] cm[sup [minus]1]) can be obtained. Methanol permeability is shown to be proportional to the methanol vapor activity and thus decreases with increasing temperature for a given partial pressure. Comparisons and distinctions between this electrolyte and pure phosphoric acid are also considered.

Evaluation of a Sol-Gel Derived Nafion/Silica Hybrid Membrane for Polymer Electrolyte Membrane Fuel Cell Applications: II. Methanol Uptake and Methanol Permeability

Sol-gel derived Nafion/silica hybrid membranes were investigated as a potential polymer electrolyte for direct methanol fuel cell applications. Methanol uptake and methanol permeability were measured in liquid and vapor phase as a function of temperature, methanol vapor activity, and silica content. Decreased methanol uptake from liquid methanol was observed in the hybrid membranes with silica contents of 10 and 21 wt %. The hybrid membrane with silica content of ≈20 wt % showed a significant lower methanol permeation rate when immersed in a liquid methanol-water mixture at 25 and 80°C. Methanol uptake from the vapor phase by the hybrid membranes appears similar to that of unmodified Nafion. Methanol diffusion coefficients, as determined from sorption experiments, were slightly lower in the hybrid membranes than in unmodified Nafion. However, in direct permeation experiments, significantly lower methanol vapor permeability was seen only in the hybrid membrane with silica content of ≈20 wt %. Based on these results, Nafion/silica hybrid membranes with high silica content have potential as electrolytes for direct methanol fuel cells operating either on liquid or vapor-feed fuels.

Real‐Time Mass Spectrometric Study of the Methanol Crossover in a Direct Methanol Fuel Cell

The products of methanol crossover through the acid-doped polybenzimidazole polymer electrolyte membrane (PBI PEM) to the cathode of a prototype direct methanol fuel cell (DMFC) were analyzed using multipurpose electrochemical mass spectrometry (MPEMS) coupled to the cathode exhaust gas outlet. It was found that the methanol crossing over reacts almost quantitatively to CO 2 at the cathode with the platinum of the cathode acting as a heterogeneous catalyst. The cathode open-circuit potential is inversely proportional to the amount of CO 2 formed. A poisoning effect on the oxygen reduction also was found. Methods for the estimation of the methanol crossover rate at operating fuel cells are suggested.