Durability study of HTPEMFC through current distribution measurements and the application of a model

In this work, a life study of a single 50 cm² high temperature proton exchange membrane fuel cell (HTPEMFC) has been carried out and experimental results are used to assess the causes of performance degradation in PBI HTPEMFC. Current distribution measurements, electrochemical characterization and physicochemical post-mortem analyses are combined with a CFD model that uses local parameters to determine that coalescence of catalyst particles (catalyst agglomeration) can be considered one of the main reasons responsible for the PBI fuel cell short life, according to electrochemically active area (ESA) and charge transfer resistance measurements. According to the current density distribution information, this agglomeration apparently occurs over the whole electrode surface. Increase of membrane resistance also contributes to the loss of fuel cell performance, but the significance is not as large as the effect of the catalyst. The model has been demonstrated to be a suitable diagnostic tool to identify degradation causes.     

Recent advances in polybenzimidazole/phosphoric acid membranes for high‐temperature fuel cells

Proton exchange membrane fuel cells are one of the most promising technologies for sustainable power generation in the future. In particular, high‐temperature proton exchange membrane fuel cells (HT‐PEMFCs) offer several advantages such as increased kinetics, reduced catalyst poisoning and better heat management. One of the essential components of a HT‐PEMFC is the proton exchange membrane, which has to possess good proton conductivity as well as stability and durability at the required operating temperatures. Amongst the various membrane candidates, phosphoric acid‐impregnated polybenzimidazole‐type polymer membranes (PBI/PA) are considered the most mature and some of the most promising, providing the necessary characteristics for good performance in HT‐PEMFCs. This review aims to examine the recent advances made in the understanding and fabrication of PBI/PA membranes, and offers a perspective on the future and prospects of deployment of this technology in the fuel cell market. © 2014 Society of Chemical Industry     

Improving of Micro Porous Layer based on Advanced Carbon Materials for High Temperature Proton Exchange Membrane Fuel Cell Electrodes

This work reports the study of four different carbon materials for their application as carbon material in microporous layers for high temperature proton exchange membrane fuel cells electrodes. The microporous layers were prepared with carbon black (a commercial one, Vulcan XC72), two different carbon nanofibers, CNF, (Ribbon and Platelet structure) and carbon nanospheres, all of them prepared in our lab. The microporous layers were characterized by XRD. The hydrophobicity, electrical conductivity, and permeability to different gases were also evaluated. The stability is an important issue to be overcome in the field of proton exchange membrane fuel cells. Thus, accelerated thermal and electrochemical degradation tests in phosphoric acid media were carried out to evaluate the stability of the different advanced materials tested under the same conditions. From all the performed essays, the carbon nanospheres were the best nano‐carbon materials because of the lower degradation degree shown by the microporous layer prepared with them and the good conductivity and permeability achieved, whereas CNF with a Platelet structure showed a low electrochemical stability due to their greater edge plane exposure which favors their corrosion.     

高温PEMFC的性能衰减研究与一维数值模拟

以PBI/H3PO4体系高温质子交换膜燃料电池为研究对象,进行了500h恒负载条件下的连续寿命实验,运用电化学电位扫描技术和交流阻抗技术测试了电池在连续运行过程中的电化学活性面积(AEC)和电池内阻的变化,实验结果表明在500h的连续寿命实验中,电池性能衰减的主要原因是高温操作条件下由于催化剂的烧结造成的电化学活性面积的损失。同时,建立了基于高温体系的一维数学模型,利用电化学方法测试得到的两个重要参数——体交换电流密度和电池内阻作为模型输入参数,计算了电极扩散层的反应气体浓度分布,运用模型模拟了电池在连续寿命实验中不同时间的稳态极化曲线,模拟结果和实验结果基本一致。     

Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane

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 4000 h.     

Polymer electrolyte membrane fuel cells based on Nafion and acid-doped PBI: State-of-the-art and recent progress

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Thermal aging of a blend of high‐performance fibers

The focus of this work is the study of the thermal aging of high‐performance fibers used in the making of fire protective garments. Accelerated thermal aging tests were carried out on fabric samples made up of a blend of Kevlar® (poly p‐phenylene terephthalamide) and PBI (poly benzimidazole) staple fibers, as well as on yarns pulled from this fabric, by means of exposure to elevated temperatures, comprised between 190°C and 320°C. All samples underwent loss of breaking force retention. The material thermal life, defined as the time required for the fibers to attain a 50% reduction of the original breaking force, ranged between a dozen of days at the lowest exposure temperature, to less than an hour at the highest. Breaking force data were fitted using the Arrhenius model following two different approaches, namely the extrapolated thermal life value and the shift factors yielded by the time‐temperature superposition (TTS). The Arrhenius model seemed to describe appropriately the overall aging process, as inferred from the excellent fit obtained when using both approaches, although activation energies provided from both approaches are different. To follow the chemical evolution of the material with thermal aging, Fourier‐transform infrared (FTIR) analyses were conducted. The qualitative analysis of the FTIR spectra showed little evidence of chemical changes between the aged and the nonaged samples, indicating either that the aging process carries on without significant modification of the chemical structure of the fibers, or that FTIR is not an appropriate method to spot such a modification. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

Coupled mechanical stress and multi-dimensional CFD analysis for high temperature proton exchange membrane fuel cells (HT-PEMFCs)

We use a combined finite element method (FEM)/computational fluid dynamics (CFD) methodology to numerically investigate the effects of gas diffusion layer (GDL) compression/intrusion on the performance of a phosphoric acid-doped polybenzimidazole (PBI) membrane-based high temperature proton exchange membrane fuel cell (HT-PEMFC). Three-dimensional (3-D) FEM simulations are conducted under various displacement clamping conditions to analyze cell deformation characteristics. Then, a multi-dimensional HT-PEMFC CFD model is applied to the deformed cell geometries to study transport and electrochemical processes during HT-PEMFC operations. Our numerical simulation results reveal that the maximum stresses in the deformed GDLs always occur near the edge of the ribs. The combined effects of GDL compression/intrusion considerably increase spatial non-uniformity in the species and current density distributions, and reduce cell performance.