EunAe Cho

Effects of Water Removal on the Performance Degradation of PEMFCs Repetitively Brought to < 0 ° C

For the mobile application, performance of polymer electrolyte membrane fuel cells (PEMFCs) should be maintained with being exposed to subzero temperatures in the winter time. To simulate the situation, a PEMFC was operated at 80°C, stopped, cooled to and kept at -10°C for I h, and heated to 80°C for the next operation. With the thermal cycle, cell performance was measured and found to degrade at a degradation rate of 2.3% based on current density at a cell voltage of 0.6 V. The degradation was attributed to freezing of water that was produced during operation and remained in the PEMFC after the operation. To prevent the performance degradation, water was removed from the PEMFC by supplying dry gases or an antifreeze solution to the PEMFC before the cell temperature fell to below 0°C. By using the gas-purging and the solution-purging method designed in this work, the performance degradation rate was successfully reduced to 0.06 and -0.47%, respectively.

Photoelectrochemical analysis on the passive film formed on Fe–20Cr in pH 8.5 buffer solution

Abstract The structure and composition of a passive film formed on Cr in pH 8.5 buffer solution were explored through the photo-electrochemical and impedance analysis of the film. The passive film on Cr was confirmed to be a single layer or duplex layers depending on the film formation potential. At low film formation potentials such as −300 mV versus saturated calomel electrode (SCE), a single Cr(OH) 3 layer was formed on Cr. In contrast, the passive film formed on Cr at potentials noble to 0 V SCE was composed of duplex layers; inner CrOOH and outer Cr(OH) 3 layers. Specifically, photocurrent spectra for the passive film could be resolved into two spectral components responsible, respectively, for the inner and the outer layers. The outer Cr(OH) 3 layer, exhibiting an n-type semiconductor with a band gap energy ( E g ) of 2.75 eV at 300 mV SCE , changed to a p-type semiconductor by decreasing the applied potential to 0 V SCE or lower. In contrast, the inner CrOOH layer revealed an n-type semiconductor with E g of 3.0 eV, irrespective of the applied potential. However, the Mott–Schottky behavior for the passive film on Cr revealed a p-type semiconductor above −100 mV SCE . An electronic band structure model for the passive film on Cr was proposed to explain the photocurrent spectral responses and the Mott–Schottky behavior of the film.

Effect of morphology of electrodeposited Ni catalysts on the behavior of bubbles generated during the oxygen evolution reaction in alkaline water electrolysis

We have investigated the release of active sites blocked by bubbles attached on the surface of catalysts during the oxygen evolution reaction (OER) in alkaline water electrolysis, via the modulation of the wetting properties of the four different morphologies of a nickel catalyst.

Prediction of Stress Corrosion Cracking Susceptibility of Stainless Steels Based on Repassivation Kinetics

Abstract Repassivation kinetics of rapidly scratched scars on the surface of type 304 (UNS S30400) stainless steel (SS) in a chloride solution was examined using an ampero-chronometric method. Its ...

Photoelectrochemical study on the passive film on Fe

Abstract Electronic properties of the passive film formed on Fe in pH 8.5 buffer solution were examined by the photoelectrochemical spectroscopy. Photocurrent responses for the passive film on Fe with film formation potential are explained by the modified band structure model developed on the basis of that for crystalline spinel (γ-Fe2O3) involving two types of electronic excitation processes. i.e., the p–d and the d–d transition. The photocurrent spectra for the passive film on Fe measured in a wide range of photon energy could be resolved into two components of photocurrent spectrum reflecting effects of each electronic transition on the photocurrent spectra. The band gap energy of each electronic transition process was determined from the resolved photocurrent spectrum; 2.87 eV for the d–d transition, and 3.67 eV for the p–d transition. A small photocurrent peak at 2.7 eV appeared at film formation potentials higher than 400 mV was associated with the formation of γ-FeOOH layer on γ-Fe2O3 film.

Supported core@shell electrocatalysts for fuel cells: close encounter with reality.

Core@shell electrocatalysts for fuel cells have the advantages of a high utilization of Pt and the modification of its electronic structures toward enhancement of the activities. In this study, we suggest both a theoretical background for the design of highly active and stable core@shell/C and a novel facile synthetic strategy for their preparation. Using density functional theory calculations guided by the oxygen adsorption energy and vacancy formation energy, Pd3Cu1@Pt/C was selected as the most suitable candidate for the oxygen reduction reaction in terms of its activity and stability. These predictions were experimentally verified by the surfactant-free synthesis of Pd3Cu1/C cores and the selective Pt shell formation using a Hantzsch ester as a reducing agent. In a similar fashion, Pd@Pd4Ir6/C catalyst was also designed and synthesized for the hydrogen oxidation reaction. The developed catalysts exhibited high activity, high selectivity, and 4,000 h of long-term durability at the single-cell level.

Effects of Cathode Inlet Relative Humidity on PEMFC Durability during Startup–Shutdown Cycling I. Electrochemical Study

This work investigated the effect of cathode inlet relative humidity (RH) on the durability of proton exchange membrane fuel cells (PEMFCs) during startup-shutdown cycling via single-cell experiments. Electrochemical techniques, including measurements of polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and linear sweep voltammetry, were performed to examine the effect of cathode inlet RH on the degradation of PEMFCs. The performance was better for PEMFCs cycled at a lower cathode inlet RH than for those cycled at a higher cathode inlet RH on the order of 0 > 50 > 100%. The CV and EIS results showed that as the cathode inlet RH increased, the loss of electrochemically active surface area and the increase in the charge-transfer resistance (R ct ) were faster during the startup-shutdown cycling. However, changes in ohmic resistance (R ohm ) and hydrogen crossover current density were not detectable, revealing that severe membrane degradation did not occur regardless of the cathode inlet RH during startup-shutdown cycles.

Development of a Durable PEMFC Startup Process by Applying a Dummy Load I. Electrochemical Study

Various polymer electrolyte membrane fuel cell (PEMFC) startup procedures were tested to explore possible techniques for reducing performance decay and improving durability during repeated startup-shutdown cycles. The effects of applying a dummy load, which prevents cell reversal by consuming the air at the cathode, on the degradation of a membrane electrode assembly were investigated via single-cell experiments. Electrochemical techniques, including measurement of polarization curves, electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry, were employed to investigate the degradation of the PEMFCs. The results showed that application of a dummy load during the startup procedure significantly reduced the performance decay, the decrease in the electrochemically active surface area, and the increase in charge-transfer resistance (R et ), which resulted in a dramatic improvement in durability. Our results suggest that starting up PEMFCs while applying a dummy load is an effective method for mitigating performance degradation caused by cell reversal under a repetition of unprotected startup cycles.

Effects of Purging on the Degradation of PEMFCs Operating with Repetitive On/Off Cycles

Degradation of polymer electrolyte membrane fuel cells (PEMFCs) that is facilitated by on/off cycles is one of the most important issues for commercialization of fuel cell vehicles. When a PEMFC stack is shut down, residual hydrogen could induce high voltage equivalent to open-circuit voltage to the cathode side that might cause sintering of the Pt catalyst and facilitate the formation of hydrogen peroxide radicals at the anode side that might decompose the Nafion membrane. In this study, the degradation of PEMFCs exposed to repetitive on/off cycles was investigated by measuring current density-voltage characteristics, ac impedance, cyclic voltammograms, gas leak, cross-sectional scanning electron microscopy images, and transmission electron microscopy images. To prevent the degradation of PEMFCs caused by the residual gases, hydrogen was removed from the anode gas channel by air-purging, which was found to be a very effective method.

Characteristics of membrane humidifiers for polymer electrolyte membrane fuel cells

Water management plays an important role in obtaining high performance from a polymer electrolyte membrane fuel cell (PEMFC). To reduce the volume and energy consumption of widely-used bubble humidifiers, membrane humidifiers were fabricated by using an ultrafiltration (UF) membrane and Nafion membranes. The performance of the membrane humidifiers was examined as a function of gas flow rate and operating temperature. A single cell was operated using the UF membrane humidifiers exhibiting almost the same performance with that employing bubble humidifiers.