Bryan S. Pivovar

Pervaporation membranes in direct methanol fuel cells

Abstract The membranes in direct methanol fuel cells must both conduct protons and serve as a barrier for methanol. Nafion, the most common fuel cell membrane, is an excellent conductor but a poor barrier. Polyvinyl alcohol pervaporation membranes are good methanol barriers but poor conductors. These and most other pervaporation membranes offer no significant advantages over Nafion in methanol fuel cell applications. However, polybenzimidazole membranes have demonstrated characteristics that suggest up to a 15-fold improvement in direct methanol fuel cells. This improvement may be due to an alternate form of proton conduction in which protons travel via a Grotthus or “hopping” mechanism.

The Membrane–Electrode Interface in PEFCs I. A Method for Quantifying Membrane–Electrode Interfacial Resistance

The interface between a membrane and electrode is a critical performance component for polymer electrolyte fuel cells (PEFCs) that to this point has gone largely unreported. This paper reports a method for quantifying interfacial resistance between a membrane and electrodes in polymer electrolyte membrane fuel cells. The interfacial resistances presented were obtained by subtracting electronic contributions from nonmembrane fuel cell resistance. Nonmembrane resistances have been determined from in situ fuel cell measurements of high-frequency resistance as a function of membrane thickness in direct methanol fuel cells. From these measurements, interfacial resistances were found to increase from a low of 8 mΩ cm 2 for commercial Nafion up to 57 mΩ cm 2 for recast Nafion membranes, both 1100 equivalent weight. Two aromatic hydrocarbon membranes were also tested and found to have interfacial resistances in between those of commercial and recast Nafion. The method by which electronic cell resistance has been estimated is presented. Membrane conductivity calculated from the study of interfacial resistance is presented as well, comparing fuel cell membrane conductivities to vapor and liquid equilibrated freestanding membranes. Finally, the impact of interfacial resistance on fuel cell performance is presented.

Platinum-Coated Copper Nanowires with High Activity for Hydrogen Oxidation Reaction in Base

Platinum (Pt)-coated copper (Cu) nanowires (Pt/CuNWs) are synthesized by the partial galvanic displacement of CuNWs and have a 100 nm diameter and are 25-40 μm length. Pt/CuNWs are studied as a hydrogen oxidation reaction (HOR) catalyst in base along with Cu templated Pt nanotubes (PtNT (Cu)), a 5% Cu monolayer on a bulk polycrystalline Pt electrode (5% ML Cu/BPPt), BPPt, and carbon supported Pt (Pt/C). Comparison of these catalysts demonstrates that the inclusion of Cu benefited the HOR activity of Pt/CuNWs likely by providing compressive strain on Pt; surface Cu further aids in hydroxyl adsorption, thereby improving the HOR activity of Pt/CuNWs. Pt/CuNWs exceed the area and mass exchange current densities of carbon supported Pt by 3.5 times and 1.9 times.

Moving Beyond Mass-Based Parameters for Conductivity Analysis of Sulfonated Polymers

The proton conductivity of polymer electrolytes is critical for fuel cells and has therefore been studied in significant detail. The conductivity of sulfonated polymers has been linked to material characteristics to elucidate trends. Mass-based measurements based on water uptake and ion exchange capacity are two of the most common material characteristics used to make comparisons between polymer electrolytes, but they have significant limitations when correlated to proton conductivity. These limitations arise in part because different polymers can have significantly different densities and because conduction occurs over length scales more appropriately represented by volume measurements rather than mass. Herein we establish and review volume-related parameters that can be used to compare the proton conductivity of different polymer electrolytes. Morphological effects on proton conductivity are also considered. Finally, the impact of these phenomena on designing next-generation sulfonated polymers for polymer electrolyte membrane fuel cells is discussed.

Electro-osmosis in Nafion 117, Polystyrene Sulfonic Acid, and Polybenzimidazole

Electro-osmotic drag coefficients for Nafion 117, polystyrene sulfonic acid (CR61 CMP), and polybenzimidazole were investigated in the presence of phosphoric, sulfuric, and hydrochloric acid. The coefficients vary with acid concentration and type with polymer structure and with water content, as illustrated by conductivity, transference number, and partition coefficient measurements. The results have implications for fuel cell performance and in the development of new polymer electrolyte membranes.

Stability of Cations for Anion Exchange Membrane Fuel Cells

The hydrothermal stability of quaternary ammonium hydroxides was evaluated to better understand the degradation of anion exchange membranes used in alkaline fuel cells. Benzyltrimethylammonium hydroxide and phenyltrimethylammonium hydroxide were examined as representative cations for membrane materials. The benzyltrimethylammonium hydroxides displayed much better stability than the phenyltrimethylammonium hydroxides under similar conditions. Additionally, as the concentration of the ammonium hydroxides in water increased, the stability of the cation decreased.

The Membrane–Electrode Interface in PEFCs IV. The origin and implications of interfacial resistance

Membrane-electrode interfacial resistance and deterioration over time was investigated using a series of sulfonated poly(arylene ether) membranes and Nafion. Dimensional mismatch due to swelling/deswelling, wetting/adhesion, and water transport mismatch between the electrodes and the polymer electrolyte membrane were all investigated as potential root causes of membrane-electrode interfacial resistance and durability. The data presented from a large number of diverse polymers strongly support dimensional mismatch as the primary cause of interfacial failure. Extended direct methanol fuel cell life tests, up to 3000 h, showed high performance and good durability for membranes with low water uptake. Polymer electrolyte membranes with ~35 vol % water uptake were the best in this study. However, a low water uptake coating layer greatly improved the performance and durability of a higher water uptake polymer electrolyte membrane. This study demonstrates the potential importance of the membrane-electrode interface on fuel cell performance and durability and provides a basis for implementing polymer electrolyte membranes effectively in high performance polymer electrolyte fuel cells (PEFCs).

Copoly(arylene ether nitrile)s—High-Performance Polymer Electrolytes for Direct Methanol Fuel Cells

Direct methanol fuel cell (DMFC) performance of sulfonated (arylene ether ether nitrile) (m-SPAEEN) copolymers is reported. Low water absorption of m-SPAEEN copolymers enabled increased proton-exchange concentrations in the hydrated polymer matrix, resulting in more desirable membrane properties for DMFC applications. The membrane electrode assemblies (MEAs) using m-SPAEENs showed improved cell properties which could not be obtained by the MEAs using sulfonated polysulfone or Nafion. The DMFC performance using an optimized m-SPAEEN membrane exceeded those of the other membrane systems. For example, 265 mA/cm 2 was obtained for an MEA using m-SPAEEN, compared to 230 and 195 mA/cm 2 for MEAs using sulfonated polysulfone and Nafion membranes, respectively, at 0.5 V, measured under identical conditions. In the comparative evaluations, membrane thickness was selected to give methanol crossover limiting currents that were similar for each of the polymer electrolyte types. Stable cell performance during extended operation (>100 h) suggested that interfacial compatibility between m-SPAEEN and Nafion-bonded electrodes was good.

Galvanic displacement as a route to highly active and durable extended surface electrocatalysts

Spontaneous galvanic displacement has been utilized in the development of novel electrocatalysts. The process occurs when a less noble metal template contacts a more noble metal cation and combines aspects of corrosion and electrodeposition. The cost of platinum (Pt) limits the commercial deployment of proton exchange membrane fuel cells. Although carbon-supported Pt typically has a moderate mass activity for oxygen reduction, it is limited by a relatively modest specific activity (activity per unit surface area). Conversely, extended Pt surfaces typically have high specific activity for oxygen reduction but commonly have low surface areas. Catalysts formed by spontaneous galvanic displacement are ideally situated, being able to take advantage of the specific activities generally associated with the catalyst type while significantly improving upon the surface area. In addition to acidic oxygen reduction, spontaneous galvanic displacement has been used in the development of catalysts for a variety of electrochemical reactions: hydrogen oxidation, alcohol oxidation, and basic oxygen reduction. Materials for these reactions have been incorporated into this perspective. Spontaneous galvanic displacement is a promising route in catalyst synthesis and cases exist where these electrocatalysts have demonstrated state-of-the-art performance.

Imaging of Water Profiles in PEM Fuel Cells Using Neutron Radiography: Effect of Operating Conditions and GDL Composition

The performance of polymer electrolyte membrane (PEM) fuel cells as a function of cathode inlet relative humidity (RH) and gas diffusion layer (GDL) properties has been characterized. The performance of 50 cm2 fuel cells at high current densities was a strong function of the polytetrafluoroethylene (PTFE) content in the cathode GDL microporous layer (MPL). The voltage at a current density of 1.4 A cm-2 decreased at all inlet RHs as the PTFE content in the cathode MPL increased from 5 % by weight to 23 % by weight. This was associated with a corresponding increase in the mass transport resistance as measured by AC impedance. The low frequency resistance also increased with increasing cathode inlet RH. These results were validated by high-resolution neutron radiography on specially designed 2.25 cm2 cells that showed increased water content in the GDLs at high inlet RHs and high microporous layer PTFE content. High-resolution neutron imaging also revealed higher water concentrations at the outlets, cathode GDL, anode flow channel, and MEA/GDL above the land when compared to the inlets, anode GDL, cathode flow channel, and MEA/GDL above the channel respectively.