Jacob Jorne

Study of the Exchange Current Density for the Hydrogen Oxidation and Evolution Reactions

The exchange current density for the hydrogen oxidation/evolution reactions was determined in a proton exchange membrane fuel cell. Ultralow Pt-loaded electrodes (0.003 mg pt /cm 2 ) were used to obtain measurable kinetic overpotential signals (50 mV at 2 A/cm 2 ). Using a simple Butler-Volmer equation, the exchange current density and transfer coefficient were determined to lie within the range of 235-600 mA/cm 2 Pt and 0.5-1, respectively. Due to the fast kinetics, no measurable voltage losses are predicted for pure-H 2 /air proton exchange membrane fuel cell applications when lowering the anode Pt loadings from its current value of 0.4 mg pt /cm 2 to the automotive target of 0.05 mg pt /cm 2 .

Effect of Relative Humidity on Oxygen Reduction Kinetics in a PEMFC

The theoretical contributions to overall cell voltage in a proton exchange membrane fuel cell (PEMFC) were calculated, accounted for, and compared to measured IR free cell voltage as relative humidity (RH) was varied from30-110%. The intrinsic oxygen reduction reaction (ORR) kinetics in a PEMFC, with regards to water and proton activity, were shown to be independent of RH above 50-60%, but significant losses in ORR kinetics were observed at lower RH values. The losses amounted to ca. 20 mV at 30% RH, corresponding to an approximate two-fold decrease in the ORR exchange current density. The dependence of ORR kinetics on water and proton activity is consistent with reduced H + activity at low RH.

Proton Conduction and Oxygen Reduction Kinetics in PEM Fuel Cell Cathodes: Effects of Ionomer-to-Carbon Ratio and Relative Humidity

The electrode in a proton exchange membrane (PEM) fuel cell is composed of a carbon-supported Pt catalyst coated with a thin layer of ionomer. At the cathode, where the oxygen reduction reaction occurs, protons arrive at the catalyst sites through the thin ionomer layer. The resistance to this protonic conduction (R H+,each ) through the entire thickness of the electrode can cause significant voltage losses, especially under dry conditions. The R H +, eath in the cathode with various ionomer/carbon weight ratios (I/C ratios) was characterized in a H 2 /N 2 cell using ac impedance under various operating conditions. AC impedance data were analyzed by fitting R H+,eath , cathode capacitance (C eath ), and high frequency resistance to a simplified transmission-line model with the assumption that the proton resistance and the pseudocapacitance are distributed uniformly throughout the electrode. The proton conductivity in the given types of electrode starts to drop at I/C ratios of approximately <0.6/1 or an ionomer volume fraction of ~ 13% in the electrode. The comparison to H 2 /O 2 fuel cell performance shows that the ohmic loss in the electrode can be quantified by this technique. The cell voltage corrected for ohmic losses is independent of relative humidity (RH) and the electrodes I/C ratio, which indicates that electrode proton resistivity ρ H+cath (ratio of R H+,cath over cathode thickness) is indeed an intrinsic RH-dependent electrode property. The effect of RH on the ORR kinetics was further identified to be rather small for the range of RH studied (≥35% RH).

Cathode Catalyst Utilization for the ORR in a PEMFC Analytical Model and Experimental Validation

The following model/experiment comparisons aid in predicting the maximum performance of a H 2 /oxygen proton exchange membrane fuel cell (PEMFC) for a cathode catalyst with known oxygen reduction reaction (ORR) kinetics in the absence of gas transport resistances. Specific focus was on modeling the voltage loss within the cathode catalyst layer, which results from a balance between slow ORR kinetics and resistance to proton transport. A unique plot of proton resistance correction vs the ratio of ohmic (expressed by the proton conduction sheet resistance, R sheet ) to charge transfer resistance (expressed by the ratio of the Tafel slope of the ORR over the current density, b/i) in a PEMFC cathode (iR sheet /b) was developed, based on an analytical solution to effective proton resistance in the porous cathode electrode of a proton exchange membrane fuel cell. Additionally, a plot of catalyst utilization (u) vs iR sheet /b was developed to serve as a guideline of experimental design parameters such that catalyst utilization is kept above 90%, which is a prerequisite for measuring the ORR kinetics. The model is applicable as long as the ORR follow simple Tafel kinetics.

Investigation of Low-Temperature Proton Transport in Nafion Using Direct Current Conductivity and Differential Scanning Calorimetry

The proton conductance of Nafion 117 was measured as a function of water content and temperature and compared to changes in the phase state of water. Conductance was measured using a direct current four-point probe technique, while the water phase was determined from differential scanning calorimetry of the melting transitions. Arrhenius plots of conductance show a crossover in the activation energy for proton transport for temperatures coinciding with the melting and freezing of water. This crossover temperature depends on the membranes water content per acid group, λ, and displays hysteresis between heating and cooling. Using calorimetry to estimate the fraction of the frozen water phase, both the crossover temperature and the hysteresis are found to correlate with the phase state of the water. For membranes starting with water contents above λ ∼ 8, the calorimetry and conductivity curves merge at low temperature, suggesting the formation of a common acid hydrate with similar network connectivity; for lower starting water contents, the low-temperature conductivity drops rapidly with λ. Based on Poisson-Boltzmann models, differences between the conductivity and calorimetry are attributed to gradients in the proton concentration that result in a proton-depleted core in the hydrated pores, which freezes first and contributes minimally to conductivity.

PEM Fuel Cell Operation at -20°C. I. Electrode and Membrane Water (Charge) Storage

An experimental procedure using isothermal galvanostatic operation was developed to quantify the charge (water) accumulation in proton exchange membrane (PEM) fuel cells at subfreezing conditions prior to voltage failure (i.e., zero cell voltage). The charge passed until voltage failure was compared to charge (water) storage estimates in the membrane phase and the cathode electrode void volume. Cryo-scanning electron microscope images of electrodes following voltage failure were used to assess ice filling of the cathode electrode void volume. At very low current densities, the membrane absorbs a maximum of ≈ 14 to ≈15 water molecules/per sulfonate group (λ max ≈ 14-15) and cathode electrode voids are completely ice filled. It is shown that the maximum charge storage of a membrane electrode assembly increases with electrode void volume and the difference between λ max and λ initial . With increasing current densities, decreasing fractions of the maximum charge storage can be utilized, which is shown to be related both to water transport resistances in the membrane phase and to reduced ice filling of the electrode void volume. Experimental results show that the charge storage utilization is mainly controlled by the current density and is less dependant on initial water content or electrode thickness.

Morphological Stability Analysis of Porous Silicon Formation

Morphological stability analysis for the photoelectrochemical etching of n-type Si is performed, in which the stability of the back-illuminated Si-electrolyte interface is theoretically investigated. Both the transport of holes in the semiconductor and ions in the diffusion layer are considered, together with the electrochemical reaction and surface energy. The roles and the effects of the various parameters, such as applied potential, dopant concentration, current density, illumination intensity, surface ener, and electrolyte concentration, on the morphology of porous silicon are studied. The results show that porous silicon is formed when the dissolution process is controlled by the supply of holes in the semiconductor, and the density of porous silicon is a property which is both material- and process-dependent

Oxygen Reduction Reaction Kinetics in Subfreezing PEM Fuel Cells

An experimental procedure was developed to measure oxygen reduction reaction kinetics in subfreezing polymer electrolyte membrane (PEM) fuel cells. The procedure was also used to measure kinetics at temperatures above 0°C, and results compared to those collected with a traditional kinetic measurement technique. In general, because of brief time durations in which PEMFCs can be operated below freezing temperatures, short equilibration times were required and thus, enhanced catalyst activity was observed. At progressively lower subfreezing temperatures, suspected mass transport or uncompensated ohmic losses resulted in nonlinear Tafel plots, which at lower decades of current density become linear with a slope close to that predicted by Tafel kinetics, 2.303RT/α c F. Consistent with results of other researchers at nonfrozen conditions, low water (or ice) content in the fuel cell results in lower catalyst activity and performance at subfreezing temperatures. Cyclic voltammograms indicate that the rate of oxide formation decreases at subfreezing temperatures and low water contents, indicating proton activity as a likely reason for reduced catalytic activity. Arrhenius plots of current density at a constant overpotential are linear (constant activation energy) over the temperature range from 55 to - 40°C, indicating no fundamental change in reaction mechanism at subfreezing temperatures.

PEM Fuel Cell Operation at − 20 ° C . II. Ice Formation Dynamics, Current Distribution, and Voltage Losses within Electrodes

Initially measured cell voltages during isothermal, galvanostatic operation at -20°C were analyzed. Individual contributions from various known voltage loss sources, including electrode resistances, were either measured a priori or modeled in order to quantify. Similar to nonfrozen conditions, voltage losses due to the sluggish kinetics of oxygen reduction reaction persist at subfreezing temperatures, contributing a majority of the overall losses. Membrane and cathode catalyst layer resistances also contribute a significant portion, due to decreased proton conductivity at subfreezing temperature. Catalyst utilization modeling within the cathode electrode thickness indicates that oxygen reduction reaction favors the electrode/membrane interface at moderate current densities. Cryo-scanning electron microscope (SEM) images confirm this prediction, which indicates that filling the electrode with ice occurs from the membrane outwards toward the diffusion medium. At lower currents, where the model predicts more uniform catalyst utilization across the thickness, more even filling of the electrode appears from cryo-SEM images. Due to the sensitive time-dependency of anode and cathode catalyst layer resistances, as well as membrane resistance, direct In Situ measurement of these parameters to elucidate voltage losses during operation of polymer electrolyte membrane fuel cells at subfreezing temperatures is presently difficult, if not impossible.

Proton Conduction in PEM Fuel Cell Cathodes: Effects of Electrode Thickness and Ionomer Equivalent Weight

The dependence of electrode proton resistivity on electrode thickness, Pt loading, ionomer loading, and ionomer equivalent weight (EW) in proton exchange membrane (PEM) fuel cell cathodes was investigated using a Pt/Vulcan catalyst. For uniform electrodes, the electrode proton resistivity is independent of the electrode thickness and Pt loading but depends on the ionomer loading and ionomer EW. There is a strong dependence on the ionomer EW when the ionomer/carbon weight (I/C) ratio is lower than 0.8. The electrode proton resistivity strongly depends on relative humidity (RH) and the density of ―SO 3 H groups in the electrode. The electrode proton resistivity becomes nearly independent of ionomer EW in electrodes when high I/C ratios are used. At low I/C ratios and low RH levels, electrodes with 850 EW ionomer exhibit better performance than those with 1050 EW. On the contrary, 850 EW electrodes give lower performance under overhumidified conditions due to electrode flooding.