Analysis of degradation in PEMFC caused by cell reversal during air starvation

The damage caused by cell reversal during proton exchange membrane fuel cells (PEMFCs) operation with air starvation was investigated by a single-cell experiment. Samples from degraded membrane–electrode assemblies (MEAs) were characterized. The loss of electrochemical surface area of the cathode platinum was detected by in situ cyclic voltammetry, and platinum sintering was detected by transmission electron microscopy (TEM) analysis. Degradation at the anode was not detected in the chemical analysis of the anode catalyst layer of MEA samples by energy dispersive X-ray analysis (EDX) and TEM. An obvious decrease in the performance of PEMFC was observed in a sample degraded by cell reversal for 120 min.     

Investigations of a platinum-ruthenium/carbon nanotube catalyst formed by a two-step spontaneous deposition method

Platinum (Pt) is a popular catalyst for hydrogen oxidation on the anode side of solid polymer fuel cells (SPFC). It increases the electrode activity, which catalyzes the reaction of the fuel cell. There are two methods commonly used to produce hydrogen for SPFC: fuel reforming and methanol decomposition. Both of these methods produce carbon monoxide, which is considered to be a poison for SPFC because it deactivates Pt easily. Adding ruthenium (Ru) to a Pt catalyst is an efficient way to improve the inhibition of carbon monoxide (CO) formation and reduce the Pt loading requirement.This study introduces a method to synthesize a bimetal catalyst that is suitable for SPFC. To improve the electrocatalyst activity, a new process with two spontaneous deposition steps is adopted. In the first step, Ru is deposited on the wall of carbon nanotubes (CNTs) to obtain Ru/CNTs. Pt is then added in the second deposition step to form Pt-Ru/CNTs. The morphology and microstructure of catalysts are characterized with microscopes, and the performance of membrane electrode assembly is evaluated by cyclic voltammetry method. Experimental results have proved that even with a lower Pt loading, this home-brewed bimetal catalyst performs a compatible electrocatalytic activity, and is capable of resisting attack from CO when a syngas (H₂ + 20 ppm CO) is provided.     

Experimental evaluation of CO poisoning on the performance of a high temperature proton exchange membrane fuel cell

We experimentally studied a high temperature proton exchange membrane (PEM) fuel cell to investigate the effects of CO poisoning at different temperatures. The effects of temperature, for various percentages of CO mixed with anode hydrogen stream, on the current-voltage characteristics of the fuel cell are investigated. The results show that at low temperature, the fuel cell performance degraded significantly with higher CO percentage (i.e., 5% CO) in the anode hydrogen stream compared to the high temperature. A detailed electrochemical analysis regarding CO coverage on electrode surface is presented which indicates that electrochemical oxidation is favorable at high temperature. A cell diagnostic test shows that both 2% CO and 5% CO can be tolerated equally at low current density (<0.3 A cm⁻²) with high cell voltage (>0.5 V) at 180 °C without any cell performance loss. At high temperature, both 2% CO and 5% CO can be tolerated at higher current density (>0.5 A cm⁻²) with moderate cell voltage (0.2-0.5 V) when the cell voltage loss within 0.03-0.05 V would be acceptable. The surface coverage of platinum catalyst by CO at low temperature is very high compared to high temperature. Results suggest that the PEM fuel cell operating at 180 °C or above, the reformate gas with higher CO percentage (i.e., 2-5%) can be fed to the cell directly from the fuel processor.     

Pt and PtRu nanoparticles deposited on single-wall carbon nanotubes for methanol electro-oxidation

Platinum (Pt) and platinum–ruthenium (PtRu) nanoparticles supported on Vulcan XC-72 carbon and single-wall carbon nanotubes (SWCNT) are prepared by a microwave-assisted polyol process. The catalysts are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The PtRu nanoparticles, which are uniformly dispersed on carbon, have diameters of 2–6 nm. All the PtRu/C catalysts display the characteristic diffraction peaks of a face centred cubic Pt structure, excepting that the 2θ values are shifted to slightly higher values. The results from XPS analysis reveal that the catalysts contain mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV) and Ru(IV). The electrooxidation of methanol is studied by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. Both PtRu/C catalysts have high and more durable electrocatalytic activities for methanol oxidation than a comparative Pt/C catalyst. Preliminary data from a single direct methanol fuel cell using the SWCNT supported PtRu alloy as the anode catalyst delivers high power density.     

Ruthenium-free,carbon-supported cobalt and tungsten containing binary & ternary Pt catalysts for the anodes of direct methanol fuel cells

Carbon supported Pt, PtCo, PtW and PtCoW catalysts were prepared by wet impregnation and their catalytic activities for electrooxidation of methanol in the acidic environment of direct methanol fuel cells (DMFCs) were determined at room temperature. The catalysts were characterized by X-ray powder diffraction and X-ray photoelectron spectroscopy and their electrochemical properties were measured by cyclic voltammetry and CO anodic stripping voltammetry. The ternary PtCoW/C catalyst was found to be the best in terms of activity and CO tolerance although the binary PtCo/C and PtW/C catalysts also outperformed Pt/C. The performance of PtCoW/C could be rationalized in terms of synergistic interactions where Co promoted the initiation of methanol dehydrogenation, W contributed to CO removal through the generation of oxygenated species and expedient removal of protons. The inclusion of base metals (Co and W) other than ruthenium provides additional options in the design of catalysts for DMFCs.     

Experimental factors that influence carbon monoxide tolerance of high-temperature proton-exchange membrane fuel cells

The poisoning effect of carbon monoxide (CO) on high-temperature proton-exchange membrane fuel cells (PEMFCs) is investigated with respect to CO concentration, operating temperature, fuel feed mode, and anode Pt loading. The loss in cell voltage when CO is added to pure hydrogen anode gas is a function of fuel utilization and anode Pt loading as well as obvious factors such as CO concentration, temperature and current density. The tolerance to CO can be varied significantly using a different experimental design of fuel utilization and anode Pt loading. A difference in cell performance with CO-containing hydrogen is observed when two cells with different flow channel geometries are used, although the two cells show similar cell performance with pure hydrogen. A different combination of fuel utilization, anode Pt loading and flow channel design can cause an order of magnitude difference in CO tolerance under identical experimental conditions of temperature and current density.     

Optimal topology and distribution of catalyst in PEMFC

The equations that govern the various transport phenomena occurring in a polymer electrolyte membrane fuel cell (PEMFC) were formulated and implemented in a commercial finite element software, in order to predict the fuel cell current density with respect to the operating conditions. The numerical model showed polarization curves in accordance with literature. The catalyst utilization was then improved by optimizing the platinum distribution (design variable) in the fuel cell, so as to maximize current density (objective function) for a fixed total amount of platinum (constraint). The first analysis showed that, for equal anode and cathode catalyst layer thicknesses, maximal current density was achieved by placing more catalyst in the cathode than in the anode. The second analysis showed that, for equal anode and cathode catalyst layer density, maximal current density was achieved by using a catalyst layer that is thicker on the cathode side than that on the anode side. Finally, a topological optimization of the platinum density within the cathode catalyst layer was performed with a gradient based algorithm, and the results showed that at a high stoichiometric ratio, the best design has most of its platinum placed where the reaction rate is the highest, i.e., close to the membrane layer.     

Effects of Nafion loading in anode catalyst inks on the miniature direct formic acid fuel cell

Nafion, within the anode and cathode catalyst layers, plays a large role in the performance of fuel cells, especially during the operation of the direct formic acid fuel cell (DFAFC). Nafion affects the proton transfer in the catalyst layers of the fuel cell, and studies presented here show the effects of three different Nafion loadings, 10 wt.%, 30 wt.% and 50 wt.%. Short term voltage-current measurements using the three different loadings show that 30 wt.% Nafion loading in the anode shows the best performance in the miniature, passive DFAFC. Nafion also serves as a binder to help hold the catalyst nanoparticles onto the proton exchange membrane (PEM). The DFAFC anode temporarily needs to be regenerated by raising the anode potential to around 0.8 V vs. RHE to oxidize CO bound to the surface, but the Pourbaix diagram predicts that Pd will corrode at these potentials. We found that an anode loading of 30 wt.% Nafion showed the best stability, of the three Nafion loadings chosen, for reducing the amount of loss of electrochemically active area due to high regeneration potentials. Only 58% of the area was lost after 600 potential cycles in formic acid compared to 96 and 99% for 10 wt.% and 50 wt.% loadings, respectively. Lastly we present cyclic voltammetry data that suggest that the Nafion adds to the production of CO during oxidation of formic acid for 12 h at 0.3 V vs. RHE. The resulting data showed that an increase in CO coverage was observed with increasing Nafion content in the anode catalyst layer.     

Accelerated test analysis of reversal potential caused by fuel starvation during PEMFCs operation

The performance of polymer electrolyte membrane fuel cells (PEMFCs) was reduced due to the degradation of the catalyst layer when reverse potential was generated by fuel starvation in PEMFCs. Detailed analysis was performed through accelerated reversal potential tests. The electrochemical impedance spectroscopy (EIS) measurement results showed that the charge transfer resistance increased and the electrochemical active surface area (ECSA) reduction was confirmed through the cyclic voltammetry (CV) measurements. Corrosion of the carbon used as the catalyst support was detected by confirming the CO peak in the 1st cyclic voltammogram in the CV measurements. Growth of the Pt catalyst due to the agglomeration and sintering of Pt was confirmed during increasing cycles of the accelerated reversal potential through X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses.     

Non-stoichiometric tungsten-carbide-oxide-supported Pt–Ru anode catalysts for PEM fuel cells – From basic electrochemistry to fuel cell performance

Durability and cost of Proton Exchange Membrane fuel cells (PEMFCs) are two major factors delaying their commercialization. Cost is associated with the price of the catalysts, while durability is associated with degradation and poisoning of the catalysts, primarily by CO. This motivated us to develop tungsten-carbide-oxide (W_xC_yO_z) as a new non-carbon based catalyst support for Pt–Ru–based anode PEMFC catalyst. The aim was to improve performance and obtain higher CO tolerance compared to commercial catalysts. The performance of obtained PtRu/W_xC_yO_z catalysts was investigated using cyclic voltammetry, linear scan voltammetry and rotating disk electrode voltammetry. Particular attention was given to the analysis of CO poisoning, to better understand how W_xC_yO_z species can contribute to the CO tolerance of PtRu/W_xC_yO_z. Improved oxidation of CO_ads at low potentials (E < 0.5 V vs. RHE) was ascribed to OH provided by the oxide phase at the interfacial region between the support and the PtRu particles. On the other hand, at high potentials (E > 0.5 V vs. RHE) CO removal proceeds dominantly via OH provided from the oxidized metal sites. The obtained catalyst with the best performance (30% PtRu/W_xC_yO_z) was tested as an anode catalyst in PEM fuel cell. When using synthetic reformate as a fuel in PEMFC, there is a significant power drop of 35.3 % for the commercial 30% PtRu/C catalyst, while for the PtRu/W_xC_yO_z anode catalyst this drop is around 16 %.     

Effect of CO in the anode fuel on the performance of PEM fuel cell cathode

Even trace amounts of CO in the fuel for a proton-exchange membrane fuel cell (PEMFC) could poison not only the anode, which is directly exposed to the fuel, but also the cathode, which is separated from the fuel by a proton-exchange membrane; and the performance decline of the cathode is sometimes more than that of the anode. Adsorption of CO on the cathode catalyst has been detected electrochemically, and this indicates that CO can pass through the membrane to reach the cathode. To reduce such a poisoning effect, fuel cell operation conditions (e.g. level of membrane humidification, gas pressure difference between cathode and anode), membrane and catalyst layer structures, and CO-tolerant cathode catalysts should be further explored.     

Degradation phenomena in PEM fuel cell with dead-ended anode

To improve the performance and durability of a dead-ended anode (DEA) fuel cell, it is important to understand and characterize the degradation associated with the DEA operation. To this end, the multiple degradation phenomena in DEA operation were investigated via systematic experiments. Three lifetime degradation tests were conducted with different cell temperatures and cathode relative humidities, during which the temporal evolutions of cell voltage and high frequency resistance (HFR) were recorded. When the cathode supply was fully humidified and the cell temperature was mild, the cathode carbon corrosion was the predominant degradation observed from scanning electronic microscopy (SEM) of postmortem samples. The catalyst layer and membrane thickness were measured at multiple locations across the cell active area in order to map the degradation patterns. These observations confirm a strong correlation between the cathode carbon corrosion and the anode fuel starvation occurring near the cell outlet. When the cathode supply RH reduced to 50%, membrane pin-hole failures terminated the degradation test. Postmortem analysis showed membrane cracks and delamination in the inlet region where membrane water content was the lowest.     

Anode bleeding experiments to improve the performance and durability of proton exchange membrane fuel cells

Cost, durability, efficiency and fuel utilization are important issues that remain to be resolved for commercialization of proton exchange membrane fuel cells (PEMFC). Anode flow mode, which includes recirculation, dead-ended and exit bleeding operation, plays an important role in fuel utilization, durability, performance and the overall cost of the fuel cell system. Depending on the flow mode, water and nitrogen accumulation in the anode leads to voltage transients and local fuel starvation, which causes cell potential reversal and carbon corrosion in the cathode catalyst layers. Controlled anode exit bleeding can avoid the accumulation of nitrogen and water and improve fuel utilization. In this study, we present a method to control the bleed rate with high precision in experiments and demonstrate that hydrogen utilization as high as 0.9988 for a 25 cm² single cell and 0.9974 for an 8.17 cm² single cell can be achieved without significant performance loss. In the experiments, anode pressure is kept at 1 bar higher than the cathode pressure to decrease nitrogen crossover from the cathode, decreasing the crossover from the cathode. Moreover, four load cycle profiles are applied to observe the cumulative loss in the electrochemical surface area (ECSA), which are acquired from cyclic voltammetry (CV) analysis. Experiments confirm that the ECSA loss and severe voltage transients are indicative of fuel starvation induced by prolonged dead-ended or low exit-bleed operation modes whereas bleed rates that are larger than the predicted crossover rate are sufficient to operate the fuel cell without voltage transients and detrimental ECSA loss.     

Composite anode for CO tolerance proton exchange membrane fuel cells

Fuel of proton exchange membrane fuel cells (PEMFC) mostly comes from reformate containing CO, which will poison the fuel cell electrocatalyst. The effect of CO on the performance of PEMFC is studied in this paper. Several electrode structures are investigated for CO containing fuel. The experimental results show that thin-film catalyst electrode has higher specific catalyst activity and traditional electrode structure can stand for CO poisoning to some extent. A composite electrode structure is proposed for improving CO tolerance of PEMFCs. With the same catalyst loading, the new composite electrode has improved cell performance than traditional electrode with PtRu/C electrocatalyst for both pure hydrogen and CO/H₂. The EDX test of composite anode is also performed in this paper, the effective catalyst distribution is found in the composite anode.     

Effect of CO and CO2 impurities on performance of direct hydrogen polymer-electrolyte fuel cells

Mechanisms by which trace amounts of CO and CO₂ impurities in fuel may affect the performance of direct hydrogen polymer-electrolyte fuel cell stacks have been investigated. It is found that the available data on CO-related polarization losses for Pt electrodes could be explained on the basis of CO adsorption on bridge sites, if the CO concentration is less than about 100 ppm, together with electrochemical oxidation of adsorbed CO at high overpotentials. The literature data on voltage degradation due to CO₂ is consistent with CO production by the reverse water–gas shift reaction between the gas phase CO₂ and the H₂ adsorbed on active Pt sites. The effect of oxygen crossover and air bleed in “cleaning” of poisoned sites could be modeled by considering competitive oxidation of adsorbed CO and H by gas phase O₂. A model has been developed to determine the buildup of CO and CO₂ impurities due to anode gas recycle. It indicates that depending on H₂ utilization, oxygen crossover and current density, anode gas recycle can enrich the recirculating gas with CO impurity but recycle always leads to buildup of CO₂ in the anode channels. The buildup of CO and CO₂ impurities can be controlled by purging a fraction of the spent anode gas. There is an optimum purge fraction at which the degradation in the stack efficiency is the smallest. At a purge rate higher than the optimum, the stack efficiency is reduced due to excessive loss of H₂ in purge gas. At a purge rate lower than the optimum, the stack efficiency is reduced due to the decrease in cell voltage caused by the excessive buildup of CO and CO₂. It is shown that the poisoning model can be used to determine the limits of CO and CO₂ impurities in fuel H₂ for a specified maximum acceptable degradation in cell voltage and stack efficiency. The impurity limits are functions of operating conditions, such as pressure and temperature, and stack design parameters, such as catalyst loading and membrane thickness.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Effect of polyoxometalate amount deposited on Pt/C electrocatalysts for CO tolerant electrooxidation of H2 in polymer electrolyte fuel cells

Polyoxometalate-deposited Pt/C electrocatalysts are prepared by impregnation with various amounts of polyoxometalate (POM) anions (from 2 to 16.7 wt.% PMo₁₂O₄₀^3–) on the Pt/C catalyst. The prepared electrocatalysts show a high CO electrooxidation performance over a half-cell system for CO stripping voltammetry, and CO tolerant electrooxidation of H₂ is further demonstrated over a proton exchange membrane fuel cell by using CO-containing H₂ gas feeds (0, 10, 50, and 100 ppm CO in H₂). In the CO stripping voltammograms, the onset and peak potentials for the CO oxidation appear to decrease as the POM deposition is increased, indicating that the electrooxidation of CO undergoes more efficiently on the catalyst surface with the deposited POMs on the Pt/C catalysts. In the single fuel cell tests with the CO-containing H₂ gases, the higher current density is also generated with the larger amounts of deposited POMs on the Pt/C catalysts. Importantly, the charge transfer resistance R_p appears to decrease monotonically with the POM amounts, which was measured by electrochemical impedance spectroscopy. Physico-chemical characterizations with electrocatalytic analyses show that the deposited POMs hardly affect the active phase of Pt catalyst itself but can help the electrooxidation of H₂ by efficiently oxidizing CO to prevent the Pt catalyst from poisoning. Consequently, this POM-deposited Pt/C catalyst can serve as a promising CO tolerant anode catalyst for the polymer electrolyte fuel cells that are operated with hydrocarbons-reformed H₂ fuel gases.     

The influence of carbon dioxide on PEM fuel cell anodes

The influence of CO₂ on the performance of PEM fuel cells was investigated by means of fuel cell experiments and cyclic voltammetry. Depending on the composition and microstructure of the fuel cell anode, the effect varies from small to significant. Adsorbed hydrogen plays a dominant role in the formation of CO-like species via the reverse water–gas shift reaction. Platinum sites which are not utilized in the electrochemical oxidation of hydrogen are thought to catalyze this reverse-shift reaction. Alloying with ruthenium suppresses the reverse-shift reaction.     

The effect of internal air bleed on CO poisoning in a proton exchange membrane fuel cell

It is found that carbon monoxide (CO) poisoning could be mitigated by increasing only cathode backpressure for a proton exchange membrane fuel cell (PEMFC) with ultra-thin membranes (≤25 μm). This mitigation can be explained by a heterogeneous oxidation of CO on a Pt-Ru/C anode by the permeated O₂ which is known as “internal air bleed” in his paper. A steady-state model which accounts for this internal air bleed has been developed to model the Pt-Ru/C anode polarization data when 50 ppm CO in H₂ is used as anode feed gas. The modeling results show that the mitigation of CO poisoning by the internal air bleed even exists at ambient conditions for a PEMFC with an ultra-thin membrane. Therefore, the effect of internal air bleed must be considered for modeling fuel cell performance or anode polarization data if an ultra-thin membrane and a low level of CO concentration are used for a Pt-Ru/C anode. An empirical relationship between the amount of internal air bleed used for the mitigation of CO poisoning and the fraction of free Pt sites is provided to facilitate the inclusion of an internal air bleed term in the modeling of anode polarization and the fuel cell performance.     

Iron phthalocyanine as a cathode catalyst for a direct borohydride fuel cell

A direct borohydride fuel cell (DBFC) is constructed using a cathode based on iron phthalocyanine (FePc) catalyst supported on active carbon (AC), and a AB₅-type hydrogen storage alloy (MmNi_3.55Co_0.75Mn_0.4Al_0.3) was used as the anode catalyst. The electrochemical properties are investigated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), etc. methods. The electrochemical experiments show that FePc-catalyzed cathode not only exhibits considerable electrocatalytic activity for oxygen reduction in the BH₄⁻ solutions, but also the existence of BH₄⁻ ions has almost no negative influences on the discharge performances of the air-breathing cathode. At the optimum conditions of 6 M KOH + 0.8 M KBH₄ and room temperature, the maximal power density of 92 mW cm⁻² is obtained for this cell with a discharge current density of 175 mA cm⁻² at a cell voltage of 0.53 V. The new type alkaline fuel cell overcomes the problem of the conventional fuel cell in which both noble metal catalysts and expensive ion exchange membrane were used.