The concept of using electrochemical reactions with polymer-based proton exchange membranes to separate, purify, and compress hydrogen was an unobvious outcome of the effort to develop power generation solutions for satellites and spacecraft in the United States in the 1950s. With the development of membrane-based fuel cell technology by General Electric in the 1960s and 1970s [1, 2], applications in hydrogen and related alkali ion-intensive processes [3] were a natural fallout of their programs. Included in the list of such applications besides fuel cells were the chlor-alkali membrane process, water electrolysis, oxygen concentrators, and electrochemical hydrogen pumping/concentrators (EHP). Furthermore, the development of Nafion (registered trademark of DuPont) ion exchange membranes was also an outcome of this period and remains the primary membrane separator for all of the above applications to date though this is being challenged by a new class of materials capitalizing on high-temperature membranes, the topic of this chapter. Electrochemical hydrogen pumping was first reported by Maget [4] while developing the SPE (Solid Polymer Electrolyte, the original description of the membrane and electrode technology prior to “PEM” terminology) family of electrochemical processes. Specifically it was found that a proton generated during the oxidation of hydrogen at the anode would recombine into “new” molecular hydrogen at the cathode after the proton was driven through the membrane by an applied potential (Fig. 24.1). For this process to work, other reaction chemistries at the cathode had to be eliminated, e.g., the oxygen or halide reduction reactions. Though this was a unique application of hydrogen oxidation-reduction chemistry, the hydrogen pump process never materialized as a useful concept and remained an intellectual and academic curiosity until recently. Applications and concepts which have emerged include hydrogen pumping, hydrogen separations, as well as adiabatic hydrogen compression. It has only been in recent years that practical applications have been, or are now under development for industrial and alternative energy applications. Furthermore, this technique has been found to be a useful electrochemical analytical tool for studying electrode performance in such devices as fuel cells and water electrolyzers [5]. K. Fishel • G. Qian • B.C. Benicewicz (*) Department of Chemistry and Biochemistry, University of South Carolina, 541 Main Street, Horizon 1 Room 232, Columbia, SC 29208, USA e-mail: benice@sc.edu

Electrochemical hydrogen pumping using a high temperature (>100°C) PBI membrane was demonstrated under non-humidified and humidified conditions at ambient pressures. Relatively low voltages were required to operate the pump over a wide range of hydrogen flow rates. The advantages of the high temperature capability were shown by operating the pump on reformate feed gas mixtures containing various amounts of CO and CO{sub 2}. Gas purity measurements on the cathode gas product were conducted and significant reductions in gas impurities were detected. The applicability of the PBI membrane for electrochemical hydrogen pumping and its durability under typical operating conditions was established with tests that lasted for nearly 4000 hours.

This work involves the study of the operational performance of phosphoric acid based electrochemical hydrogen pumps with a polybenzimidazole (PBI) electrolytic membrane. During characterization of the device, the power consumption was found to be highly sensitive to the water vapor pressure in the supply gas stream which affects the phosphoric acid concentration. The power requirement was 30 times higher when the supply gas stream was not humidified than when the supply gas stream was humidified to a dew point of 60°C. The increase in power consumption was caused by a decrease in platinum catalyst activity due to the adsorption of a species on the platinum catalyst when the phosphoric acid concentration exceeded 72.4 wt% P2O5 (100 wt% H3PO4). The increase in power consumption can be prevented by humidifying the supply gas stream.

Platinum-coated chromium nitride electrodes are deposited onto gas diffusion layers by normal and glancing angle deposition and are tested as cathodes for proton exchange membrane (PEM) fuel cells. X-ray diffraction and scanning electron microscopy show that the CrN forms 111-oriented nanoparticles with {100} facets that are covered by 3.4 x 10(11) Pt mounds/cm(2), independent of Pt loading 0.05 to 0.25 mg/cm(2). Polarization curves exhibit dE/d(log i) slopes b approximate to -100 and -150 mV/dec for high (E > 0.75 V) and low (E < 0.5 V) potentials, respectively, but show an anomalous drop with b approximate to -420 mV/dec in the intermediate voltage range. This is attributed to poor proton conduction associated with a reversible dewetting of electrode pores during low current operation. Quantitative analyses of rate-dependent polarization curves and electrochemical impedance spectra show that the time scale for pore filling by process water is 10(3) s, and that the ionic resistance R-C within the electrode increases by a factor of 4, from R-C approximate to 0.2 to 0.8 Omega cm(2), as E increases from 0.5 to 0.8 V. The increasing electrode resistance is attributed to a low water production rate at low current, which allows the relatively hydrophobic CrN to expel water from the electrode pores, resulting in a higher resistance for ionic transport. These results show that even ultrathin sputtered catalyst layers can exhibit incomplete flooding. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3247351] All rights reserved.

Mesophase pitch was investigated as a melt processable precursor to a compression or injection moldable all carbon bipolar plate. After shaping, carbonization to 1000°C or greater is required to achieve the desired electrical and mechanical properties, but gases evolved during this step lead to swelling. Carbon nanofiber was added to suppress swelling during carbonization and bypass the typical oxidation steps used when processing mesophase pitch. The addition of carbon nanofiber decreased swelling by increasing the viscosity of the melt. Carbonized materials with carbon nanofibers can show strengths (30–50MPa) and conductivities (20–80Scm−1) consistent with composite bipolar plate materials. The materials show conductivities below Department of Energy target values at the current carbonization temperatures, which were limited to 1000°C. The use of glass fibers as a secondary filler led to reduced gas permeability in porous samples.

Layers of 150 nm wide and 0.5-1.5 mu m long carbon nanorods were grown by glancing angle deposition on Si substrates, sputter-coated with 0.10 mg/cm(2) Pt, and transferred to polymer electrolyte membranes for testing as cathode electrodes in fuel cells. The rods were etched within fully assembled cells by applying a potential above the reversible H-2/O-2 voltage, which leads to polarization curves that show a 4-7 times higher current at 0.40 V. The current increase is attributed to the opening of pores within the electrode, which facilitates easy oxygen transport and leads to a reduction in mass transport resistance by a factor of 360, as determined by electrochemical impedance spectroscopy. Etching sequences with increasing voltage V-E indicate that V-E <= 1.6 V yields water electrolysis and Pt oxidation that facilitates Pt agglomeration and migration of Pt ions into the electrolyte, while V-E = 1.7 V results in removal of C and the formation of pores within rods that facilitate oxygen transport to reaction sites, yielding a 400-700% increase in fuel cell output current at low potential. These results suggest that the controlled etching of temporary scaffolds to create pores in an operating fuel cell may be an effective approach to reduce mass transport limitations. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3244589] All rights reserved.

Arrays of 1 mu m long C nanorods were grown by glancing angle deposition on flat and patterned Si wafers, coated with 0.1 mg/cm(2) Pt catalyst by magnetron sputtering, removed from the substrates, and tested as cathode electrodes in proton exchange membrane (PEM) fuel cells. Deposition on flat substrates yields a nearly fully dense nucleation layer with < 5 nm wide pores, followed by the formation of separated rods with an average width that strongly increases with rod height, from < 30 to 190 nm. In contrast, deposition on a patterned surface results in regularly spaced 50 nm wide pores and a rod width that only moderately increases with height, from 95 to 155 nm. Polarization curves on pure H-2 and O-2 for the two sample types are identical at high potential E>0.55 V. However, the cathodes deposited on the patterned substrates yield considerably higher currents at low potential, with a 2 times higher limiting current density i(L)=0.73 A/cm(2) than those grown on flat substrates. The higher current in the mass-transport-limited regime is attributed to the 10 times wider engineered pores that facilitate O-2 transport to the active catalyst sites, resulting in a 5 times lower mass transport resistance R-MT=1.5 cm(2) at E=0.50 V, as quantified by electrochemical impedance spectroscopy.

Platinum catalyst layers with Pt loadings w = 0.05-0.40 mg/cm(2) were deposited by magnetron sputtering from a variable deposition angle alpha onto gas diffusion layer (GDL) substrates and tested as cathode electrodes in proton exchange membrane (PEM) fuel cells using Nafion 1135 membranes and Teflon-bonded Pt-black electrode (TBPBE) anodes. Layers deposited at normal incidence (alpha = 0 degrees) are continuous and approximately replicate the rough surface morphology of the underlying GDL. In contrast, glancing angle deposition (GLAD) with alpha = 87 degrees and continuous substrate rotation yields highly porous layers consisting of vertically oriented Pt particles, 100-500 nm high and 100-300 nm wide, that are separated by 20-100 nm. The particle electrodes exhibit a higher (lower) mass-specific performance than the continuous-layer electrodes for a high (low) current density i. This is attributed to a higher porosity but lower overall electrochemically active surface area for the particles compared to the continuous layer. Increasing w in particle cells from 0.05 to 0.10 to 0.18 mg/cm(2) yields increasing potentials, but w = 0.40 mg/cm(2) causes a voltage drop at i > 0.4 A/cm(2), associated with the reduced pore density at large w. Comparison cells with a TBPBE cathode exhibit comparatively low Tafel slopes but a lower Pt mass specific performance than the sputtered catalysts. Quantitative analyses of kinetic and mass-transport losses in the polarization curves suggest a competing microstructural effect, favoring mass-transport performance and an efficient oxygen reduction reaction for particle and continuous layer electrodes, respectively. The overall results suggest that in addition to the well-known promise of sputter-deposited Pt catalysts as an approach to increase Pt utilization at low loading, GLAD provides the unique ability to control Pt porosity and to achieve efficient reactant flow for high-current-density operation. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3097188] All rights reserved.

Stress relaxation in the membrane electrode assemblies (MEA) in PEM fuel cells subjected to compressive loads is analyzed. This behavior is important because nonzero contact stress is required to maintain low electric resistivity in the fuel cell stack. Experimental results are used to guide the choice of the viscoelastic properties of the constituents of the MEA, the membrane and the gas diffusion layer (GDL), needed for the model. These properties are incorporated into the model that treats the membrane as a porous-viscoelastic solid, and the gas diffusion layer as a nonlinear elastic solid. Using numerical simulations (finite element method), the stress relaxation curves for the MEA are obtained for different fluid flow boundary conditions, variations in the material properties of the membrane and the GDL. The results are compared to experimental stress relaxation curves. Most of the experimental data were obtained at a temperature of 180°C, corresponding to operating conditions, so in the model the temperature was considered fixed and equal to this value.

This study focuses on the fabrication and performance testing of unsupported platinum black electrodes for proton exchange membrane fuel cells. Experiments with platinum black coated diffusion media of varying anode and cathode catalyst loadings with H(2)/air demonstrate successful performance and stability characteristics for anode catalyst loadings down to 0.25 mg/cm(2) while operating on pure H(2) and 0.62 mg/cm(2) cathode catalyst loadings, without significant voltage losses. The voltage losses as a result of reducing the platinum black cathode catalyst loadings from 2.6 to 0.62 mg/cm(2) are consistent with kinetic losses associated with the oxygen reduction reaction and lower electrocatalyst utilization. The study also highlights the durability and stability characteristics of these unsupported electrodes under extreme operating conditions. Optimization of the three-phase interface, namely electrode, electrolyte, and reactant gas, is shown to be dependent on the efficacy of the membrane-catalyst layer interface. (C) 2008 The Electrochemical Society.

Electrochemical hydrogen pumping using a high-temperature (>100°C) polybenzimidazole (PBI) membrane was demonstrated under non-humidified and humidified conditions at ambient pressures. Relatively low voltages were required to operate the pump over a wide range of hydrogen flow rates. The advantages of the high-temperature capability were shown by operating the pump on reformate feed gas mixtures containing various amounts of CO and CO2. Gas purity measurements on the cathode gas product were conducted and significant reductions in gas impurities were detected. The applicability of the PBI membrane for electrochemical hydrogen pumping and its durability under typical operating conditions were established with tests that lasted for nearly 4000h.