Water removal characteristics of proton exchange membrane fuel cells using a dry gas purging method

Water removal from proton exchange membrane fuel cells (PEMFC) is of great importance to improve start-up ability and mitigate cell degradation when the fuel cell operates at subfreezing temperatures. In this study, we report water removal characteristics under various shut down conditions including a dry gas-purging step. In order to estimate the dehydration level of the electrolyte membrane, the high frequency resistance of the fuel cell stack was observed. Also, a novel method for measuring the amount of residual water in the fuel cell was developed to determine the amount of water removal. The method used the phase change of liquid water and was successfully applied to examine the water removal characteristics. Based on these works, the effects of several parameters such as purging time, flow rate of purging gas, operation current, and stack temperature on the amount of residual water were investigated.     

Effects of MEA fabrication method on durability of polymer electrolyte membrane fuel cells

To study the effects of fabrication methods on the durability of polymer electrolyte membrane fuel cells (PEMFCs), membrane-electrode assemblies (MEAs) were fabricated using a conventional method, a catalyst-coated membrane (CCM) method, and a CCM-hot pressed method. Single cells assembled with the prepared MEAs were operated galvanostatically at 600 mA cm⁻² for 1000 h for the conventional MEA and the CCM MEA and for 500 h for the CCM-hot pressed MEA. During operation, i-V curves, impedance spectra, and cyclic voltammograms were measured roughly every 100 h. Before and after long-term operation, the physical and chemical characteristics of the MEAs were analyzed using mercury porosimetry, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and Fourier transformation infrared spectroscopy (FTIR). Under the operating conditions, the CCM MEA exhibited the lowest degradation rate as well as the highest initial performance.     

Study of PEMFC ionomers through model molecules mimicking the ionomer repeat units

The thermal and electrochemical behaviours of PEMFC ionomers based on an aromatic backbone i.e., sulfonated polysulfones, sulfonated polyetheretherketones and sulfonated polybenzimidazoles, were investigated through their model molecules. Thermal degradation was characterized by weight losses and allowed inferring ionomer thermal degradations by chain breakings that cannot be detected by thermogravimetric analysis. In addition, cyclic voltammetry was used to characterize the electroactivity of some ionomers, related to their backbone and their ionic moieties. The use of model molecules is thus a useful tool for assessing both the thermal and electrochemical stability of ionomers. Lastly, due to the high concentrations used in liquid electrolytes as compared with an electrode/membrane interface it may be considered as an electrochemical aging test.     

Estimation of indirect CO2 emissions of a hydrogen‐powered medium size vehicle for the NEDC cycle

Reduction in greenhouse effect gases emission is a major source of concern nowadays. Internal combustion engines, as the most widely used power generation mean for transportation, represent a large share of such gases, which motivates active research efforts for alternative solutions. In this regard, PEM fuel cells represent a promising prospect and are thoroughly investigated, whether experimentally or through numerical simulation. The present work presents a simulation of the power potential of a PEM fuel cell, which is integrated to the full power electric traction chain of a medium size car. The cell potential is modelled by taking into account the different types of polarization. The driving performances of the vehicle and its hydrogen consumption are evaluated through a simple mathematical model and an application is performed for the New European Driving Cycle (NEDC) standard driving cycle. A preliminary sizing of the proton exchange membrane fuel cell (PEMFC) membrane area for the chosen vehicle is presented, along with that of a hydrogen storage tank for a typical autonomy. The main goal of the simulation is to estimate CO₂ indirect emissions due to the production of the needed hydrogen for the cycle via an electrolyser, compared with the case of a gasoline fueled vehicle. This is performed solely on a ‘fuel tank to wheel’ basis in order to have comparable figures. The results indicate that the environmental advantage of hydrogen cars is quite questionable if hydrogen is produced using carbon‐based energy sources. Copyright © 2009 John Wiley & Sons, Ltd.     

Reliability Evaluation of an Open‐Cathode PEMFC at Operating State and Longtime Vibration by Mechanical Loads

Reliability and durability of proton exchange membrane fuel cells (PEMFCs) are one of the most complex issues in real applications' environment. Especially, when it subjects to the various mechanical loads and vibrations. This topic requires more attention for research and experimental works. In this study, the effect of mechanical loads was investigated on an open‐cathode PEMFC in operating state. Several long time vibration tests have been applied on non‐operating PEMFC, and the performance of the fuel cell was evaluated during the test. Hydrogen leakage as a key parameter was investigated in performance monitoring instead of measuring torque on compression bolts. The vibration tests were consisted of sine, shock and random for X, Y and Z axes in operating state and random vibration in non‐operating state of PEMFC. The experimental results in operating state were indicated that the fuel cell performance has not been affected by the proposed vibrations. Furthermore, the test results of non‐operating state have been shown that the performance of PEMFC reduces about 0.6% in each four‐hour step of the vibration test. In addition, the experiments reveal that if the mechanical loads and vibrations cause physical damage on the fuel cell components, they can change the performance and reliability of the fuel cell.     

Pt-MoOx-carbon nanotube redox couple based electrocatalyst as a potential partner with polybenzimidazole membrane for high temperature Polymer Electrolyte Membrane Fuel Cell applications

A redox couple based electrocatalyst comprising of Pt-Multi Wall Carbon NanoTube (Pt-MWCNT) promoted with molybdenum oxide (MoO_x, 2 < x < 3) nanoparticles was prepared. The objective was to effectively organize the Pt-MoO_x interface on the smooth MWCNT surface to overcome the practical difficulties associated with establishing such interface with Pt dispersed on carbon morphologies possessing surface irregularities. The present study revealed the importance of stringent controlling of the additive level for maintaining a balanced bifunctional behavior of the catalyst combination through the synergistic effects by the components and the need of a proton conducting membrane operable at high temperature to get better output from the Polymer Electrolyte Membrane Fuel Cell (PEMFC) systems. An indigenously developed polybenzimidazole (PBI) membrane was used to fabricate a membrane electrode assembly (MEA) as it can be operated at higher temperatures compared to that of Nafion membranes. MoO_x additive level was carefully controlled by monitoring the active Pt area by cyclic voltammetry. All prepared electrocatalysts were characterized by using HRTEM, XRD and XPS to get information on dispersion and morphology, crystalinity and oxidation state of different elements, respectively. The system prepared with 5% MoO_x addition with respect to Pt (hereafter Pt-MoO_x(5%)-MWCNT) displayed balanced active Pt area and excellent oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities. Rotating Disk Electrode (RDE) system was extensively utilized to understand the ORR kinetics and the favorable role of MoO_x as the promoter in the reaction. The kinetic current (j_k) measured at 0.02 V vs. Hg/Hg₂SO₄ electrode from the Koutecky-Levich plots was 9 times higher and the apparent activation energy during single cell evaluation was 27 kJ/mol lower for the MoO_x promoted system, compared to the system without the additive. A higher operating temperature significantly favored the cell performance by a combined effect of enhancement in proton conductivity of the PBI membrane and possible kinetic benefit by the well postulated oxygen spill over effect by the MoO_x type systems in some combinations involving such systems.     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

CO oxidation and CO2 reduction on carbon supported PtWO3 catalyst

The activity of a carbon supported PtWO₃ (PtWO₃/C) catalyst in the CO oxidation and CO₂ reduction reactions was evaluated in sulfuric acid solution at room temperature.Cyclic voltammetry combined with on-line mass spectrometry shows that the oxidation of both saturated CO adlayer and dissolved CO on PtWO₃/C material commences at rather low potentials, ca. 0.18 and 0.12 V vs. RHE, respectively. However, the low-potential process seems to involve only a minor fraction of the CO adlayer, the major part of the adsorbed CO layer being oxidised at the potentials as high as those for pure Pt catalysts—ca. 0.7 V vs. RHE. PtWO₃/C material was found to reversibly de-activate upon a prolonged exposure to the CO-saturated solution due to the inhibition of the hydrogen tungsten bronze formation.The reduction of CO₂ on PtWO₃/C leads to the formation of an adsorbate - presumably CO - on the Pt sites of the catalyst. Although the rate of the adsorbate build-up on PtWO₃/C at 0.1 V is lower than that on pure Pt/C, our results indicate that upon a prolonged exposure of the PtWO₃/C electrode to a CO₂-saturated solution a complete poisoning of the Pt sites with the adsorbate is likely to occur at room temperature.     

质子膜燃料电池(PEMFC)多孔电极极化的两尺度分析

针对PEMFC(Proton Exchange Membrane Fuel Cell)多孔电极微结构的特征,提出一个分析该多孔电极极化的两尺度新方法,建立了能够同时在团簇微观和电极宏观两尺度上描述多孔电极内反应和传质、传荷耦联过程的理论模型。该两尺度模型方程为一非线性二阶微分方程组,给出了求解该数模的ADM(Adomian Decomposition Method)与有限差分(Newman′s BAND(J))相结合的便捷方法。还给出了氢PEMFC多孔阴极极化的计算实例,与传统的电极单尺度模型相比,包含微团尺度传质影响的两尺度模型预测值与极化实验数据能更好相符。提出的两尺度新方法也可被相似的多相催化反应系统理论分析所借鉴。     

用于PEMFC的介孔碳担载铂氧化钨电催化剂的制备与表征

利用介孔碳作为载体,制备介孔碳担载Pt-WO3复合催化剂应用于质子交换膜燃料电池（PEMFC）电极.以苯为碳源,采用气相沉积法复制介孔SiO2Al-SBA-15模板结构合成石墨化介孔碳Cg,采用浸渍法制备无定形介孔碳CMK-3.通过分步沉积,将Pt和WO3担载到介孔碳载体上,采用比表面分析（BET）、X线衍射（XRD）、透射电子显微镜（TEM）、循环伏安法以及单电池极化性能测试对介孔碳担载的复合催化剂进行表征.结果表明：介孔碳作为催化剂载体,其孔道结构有助于催化剂的均匀分散,从而提高催化剂的电催化剂活性.由于石墨化介孔碳的导电性能高于无定形介孔碳,因此Pt-WO3/Cg比Pt-WO3/CMK-3具有更好的电极催化活性.