Increase In Mass Transport Resistance Research Articles

Hydrothermal synthesis temperature induces sponge-like loose silica structure: A potential support for Fe2O3-based adsorbent in treating As(V)-contaminated water

Low cost Fe2O3-based sorbents with an exceptional selectivity toward the targeted As(V) pollutant have gained extensive attention in water treatment. However, their structural features often influence removal performance. In this respect, we present herein a rational design of silica-supported Fe2O3 sorbents with an enhanced morphological structure based on a simple temperature-induced process. Low-hydrothermal temperature synthesis (60 and 100 °C) provided a large silica-cluster size with a close packed structure (S-60 and S-100), contributing to an increase in mass transport resistance. Fe2O3/S-60 with 6.2-nm pore width silica achieved a maximum As(V) uptake capacity (qm) of only 3.5 mg g−1. Supporting Fe2O3 on S-100 with an approximately two-fold increase in the pore size (13 nm) did not lead to any evident enhancement in qe (3.7 mg g−1). However, expanding the pore window up to 22.6 nm (S-140) and 39.5 nm (S-180), along with changing from close-packed to sponge-like loose structures induced by high-temperature synthesis (140 °C and 180 °C), resulted in substantial increases in qm. Fe2O3/S-140 had 1.7 and 1.6 times higher qm (5.9 mg g−1) than Fe2O3/S-100 and Fe2O3/S-60, respectively. The highest qm (7.4 mg g−1) was achieved for Fe2O3/S-180, which was attributed to its relatively small-sized silica cluster and the largest cavities that facilitated easier access by As(V) to adsorbing sites.

Analysis of surface and interior degradation of gas diffusion layer with accelerated stress tests for polymer electrolyte membrane fuel cell

The effects of surface and interior degradation of the gas diffusion layer (GDL) on the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs) have been investigated using three freeze-thaw accelerated stress tests (ASTs). Three ASTs (ex-situ, in-situ, and new methods) are designed from freezing −30 °C to thawing 80 °C by immersing, supplying, and bubbling, respectively. The ex-situ method is designed for surface degradation of the GDL. Change of surface morphology from hydrophobic to hydrophilic by surface degradation of GDL causes low capillary pressure which decreased PEMFC performance. The in-situ method is designed for the interior degradation of the GDL. A decrease in the ratio of the porosity to tortuosity by interior degradation of the GDL deteriorates PEMFC performance. Moreover, the new method showed combined effects for both surface and interior degradation of the GDL. It was identified that the main factor that deteriorated the fuel cell performance was the increase in mass transport resistance by interior degradation of GDL. In conclusion, this study aims to investigate the causes of degraded GDL on the PEMFC performance into the surface and interior degradation and provide the design guideline of high-durability GDL for the PEMFC.

Identification of performance degradations in catalyst layer and gas diffusion layer in proton exchange membrane fuel cells

Degradations occur in both the catalyst layer (CL) and gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFC) due to different mechanisms. Identification of the locations and mechanisms of degradations is critical in mitigating them. In this study, performance degradation of a PEMFC is examined using an accelerated stress test (AST) protocol. Using a novel methodology of combining fresh and aged MEAs with either fresh or aged GDLs, the origins of losses in either CL or GDL are identified. Increase in mass transport resistance caused by increases in water saturation and in oxygen transport resistance are further isolated. The results show although kinetic degradation accounts for the primary performance decay, mass transport losses increase significantly due to the deterioration in water transport and increase in oxygen transport resistances. The increase in mass transport resistance in CL is greater than in GDL. The aged CL tends to retain more liquid water, causing significant decrease in actual porosity. Water transport in aged GDL also deteriorates and leads to increase in mass transport loss, especially under high relative humidity (RH). Oxygen transport resistance increases significantly in aged CL. However, there is no apparent increase in oxygen transport resistance in aged GDLs.

Understanding the interplay between cathode catalyst layer porosity and thickness on transport limitations en route to high-performance PEMFCs

We examine the interplay between cathode catalyst layer (CL) porosity/thickness on mass transport limitations in single cell fuel cells comprised of Pt/C-based CLs fabricated via ultrasonic spray deposition onto polymer electrolyte membranes. We determine that the pore size distribution remains unchanged as CL thickness increases from 3.8 to 11.8 μm, but porosity decreases with increasing CL thickness. The decrease in porosity results in an increase in mass transport resistance for thicker CLs, but does not result in an increase in charge transfer resistance for the oxygen reduction reaction. We found that a fuel cell comprising a 7.5 μm-thick cathode CL delivers the highest performance (1 A cm−2 at 0.60 V at 80 °C in H2|Air at a relative humidity of 50% under ambient pressure). We attribute this high performance to the CL striking an optimal balance between solid and void networks, with the solid networks facilitating transport of H+/e− to the Pt electrocatalyst, and the void network ensuring adequate transport of O2 to, and H2O away from, the Pt electrocatalyst.

Mass transport in anode gas diffusion layer of direct methanol fuel cell derived from compression effect

The anode gas diffusion layer plays an important role on mass transport. By adjusting the assembly pressure, the structure of gas diffusion layer can be controlled to investigate the two-phase behavior. The electrochemical test indicates that increasing the assembly pressure causes increase in mass transport resistance. Inhomogeneous compression of gas diffusion layer is observed by three-dimensional imaging technology. Based on the experimental data, a two-phase model is established. The simulated results show that the increment of surface roughness and the reduction of porosity, both derived from compression of gas diffusion layer, exacerbate CO2 blockage of the cell. Lowering polytetrafluoroethylene content of anode gas diffusion layer alleviates CO2 blockage by reducing the difference of surface tension between liquid-solid and gas-solid interface as well as the negative capillary pressure of gas diffusion layer. Thus, the peak power density climbs from 60.61 to 49.94 mW cm−2 to 74.79 and 70.47 mW cm−2 at the assembly pressure of 1.00 and 2.00 MPa, respectively. The optimal assembly pressure locates at where good contact between membrane electrode assembly and bipolar plate is achieved, with the least gas diffusion layer deformation possible.

Polytetrafluoroethylene content in standalone microporous layers: Tradeoff between membrane hydration and mass transport losses in polymer electrolyte membrane fuel cells

Fuel cell performance testing and in operando synchrotron radiography were used to investigate the effect of polytetrafluoroethylene (PTFE) in standalone microporous layers (MPLs) on mass transport and membrane hydration. Two standalone MPLs with 20 wt% and 30 wt% PTFE were fabricated and tested with inlet gas relative humidity (RH) between 50 and 100%. This study demonstrates that the performance of a fuel cell using a standalone MPL with 30 wt% PTFE decreases when the RH of the inlet gases is increased from 50% to 100%, whereas the performance of a fuel cell using a standalone MPL with 20 wt% PTFE remains stable over the same relative humidity range. Furthermore, this study demonstrates that a tradeoff between membrane hydration and mass transport losses must be considered when increasing the PTFE content within the MPL. Higher PTFE content led to greater liquid water accumulation adjacent to the catalyst layer. The greater liquid water accumulation adjacent to the catalyst layer improved membrane hydration and proton conductivity but also led to an increase in mass transport resistance. Standalone MPLs with higher PTFE content did not support high current density operation because the mass transport limitations outweighed the benefits of improved membrane hydration.

Polarization loss correction derived from hydrogen local-resistance measurement in low Pt-loaded polymer-electrolyte fuel cells

The reduction of platinum-loading on the cathode side of polymer-electrolyte fuel cells leads to a poorly understood increase in mass-transport resistance (MTR) at high current densities. This local resistance was measured using a facile hydrogen-pump technique with dilute active gases for membrane-electrode assemblies with catalyst layers of varying platinum-loading (0.03–0.40mgPt/cm2). Furthermore, polarization curves in H2/air were measured and corrected for the overpotential caused by the increased MTR for low loadings on the air side due to the reduced concentration of reactant gas at the catalyst surface. The difference in performance after correction for all resistances including the MTR is minor, suggesting its origin to be diffusive in nature, and proving the meaningfulness of the facile hydrogen-pump technique for the characterization of the cathode catalyst layer under defined operation conditions.

Evaluating in Situ Platinum Deposition As a Preparative Step for Fibsem Tomography of PEMFC Cathodes

In polymer electrolyte fuel cells (PEFC), cathode catalyst layers may experience high overpotentials during cell start up/shut down and even during normal operation. High overpotentials undesirably facilitate corrosion kinetics of the carbon catalyst support which results in a severe increase in mass transport resistance in the cathode. Focused ion beam and scanning electron microscope tomography (FIBSEM) is an emerging technique for nanometer-resolution direct observation of three-dimensional microstructure and shows promise for improving understanding of microstructural changes to PEFC cathodes in response to corrosion. However, the high porosity of PEFC cathodes leads to challenges in downstream image processing. Filling the pore space prior to tomography may allow circumvention of image processing challenges but has not yet been definitively shown to provide good results. The present paper assesses the viability of an organoplatinum electron beam induced deposition (PtEBID) step in order to fill the pore space prior to FIBSEM. PtEBID methods were investigated and validated with complementary AFM and STEM to quantify precursor diffusion into porous cathodes, kinetics of adsorption and cracking of organoplatinum on the carbon support surface, and damage induced by the focused ion beam used during FIBSEM.

Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers: Performance Degradation and Steady State Liquid Water Distributions with in Operando Synchrotron X-ray Radiography

As-received pristine gas diffusion layers (GDLs) were degraded through an accelerated artificial aging process by immersion into a 35% solution of H2O2 at 90°C for 12 hours. Polarization curves were obtained while synchrotron X-ray radiography was performed to investigate the effect of ageing on liquid water transport behavior. Peak output power density of the fuel cell composed of the aged GDL reached only 76% of that of the fuel cell composed of the pristine GDL. This performance degradation was attributed to an increase in mass transport resistance associated with liquid water accumulation at the aged GDL and flow field channel interface. The aged GDLs showed more liquid water accumulation at the microporous layer (MPL)/carbon substrate interface and carbon substrate regions than pristine GDLs at low current density operation. A fully developed water profile was established at lower current density for the aged GDLs compared to pristine GDLs.

Effect of cationic contaminants on polymer electrolyte fuel cell performance

The effect of cationic contaminants on polymer electrolyte fuel cell (PEFC) performance is investigated via in-situ injection of dilute cationic salt solutions. Four foreign cations (K+, Ba2+, Ca2+, Al3+) are chosen as contaminants in this study due to their prevalence and chemical structure (e.g. valence), however contaminants that have already received extensive coverage in the literature like sodium and iron are excluded. It is found that the cells with Ba(ClO4)2 and Ca(ClO4)2 injection exhibit little cell performance change during the current hold test, and the cells with Al(ClO4)3 and KClO4 injection show larger cell performance changes, i.e. decreasing cell voltage and increasing cell resistance. These cells with in-situ contaminant injection have a tendency to recover a portion of the lost performance after the recovery test when switched back to supersaturated air. The degradation in cell performance with the presence of cationic contaminants is mainly due, in addition to the membrane resistance increase associated with replacing protons on the sulfonate groups, to the increase in mass transport resistance and decrease in electrochemical surface area.

Effects of heat treatment time on electrochemical properties and electrode structure of polytetrafluoroethylene-bonded membrane electrode assemblies for polybenzimidazole-based high-temperature proton exchange membrane fuel cells

To improve cell performance, the effects of heat treatment time on the electrochemical properties and electrode structure of PTFE-bonded membrane electrode assemblies for PBI-based high-temperature proton exchange membrane fuel cells are investigated. The cell performance is observed to decrease in the high-current-density region rather than in the low-current-density region with increasing heat treatment time at 350 °C from 1 to 30 min. Microscopic studies reveal remarkable differences in the electrode structure by the agglomeration of dispersed PTFE and adjacent catalyst particles, depending on the heat treatment time. As the heat treatment time increases, only the large pore (secondary pore) volume in the electrode decreases, resulting in increase in mass transport resistance and concentration overpotential in the high-current-density region. Cell performance is not measured without heat treatment because the electrodes are not formed. When the electrodes are heat treated for 1 min at 350 °C, the best cell performance is obtained, 0.67 V at 200 mA cm−2.

Influence of the microporous layer on carbon corrosion in the catalyst layer of a polymer electrolyte membrane fuel cell

Corrosion of the catalyst support reduces PEM fuel cell performance via catalyst layer (CL) degradation (loss of porosity, catalyst connectivity, and active catalyst surface area). Carbon corrosion was investigated in a segmented cell for cathode gas diffusion layers (GDLs) with and without a microporous layer (MPL) to investigate the spatial aspects of GDL effect on corrosion. The cells were aged in situ using an accelerated stress test (AST) for carbon-support corrosion consisting of consecutive holds at 1.3 V. Carbon corrosion was quantified by measuring CO2 evolution during the AST.Performance degradation was substantial both with and without cathode MPL, but the degradation of the CL after prolonged corrosion was lower in the presence of an MPL. This was corroborated by better cell performance, higher remaining Pt active area, lower kinetic losses and smaller Pt particle size. The cell with an MPL showed increasingly nonuniform current distribution with corrosion time, which is correlated to the distribution of the Pt particle growth across the active area. This cell also showed an increase in mass-transport resistance due to MPL degradation. Without an MPL, GDL carbon fibers caused localized thinning in the cathode CL, originating from the combined effects of compression and corrosion.

Performance of PEM Fuel Cells at Sub-Freezing Temperatures

The performance of single-PEM fuel cells at sub-freezing temperatures has been characterized. Fully humidified fuel cells that were subjected to multiple freeze/thaw cycles down to -40 oC showed degradation in performance associated with mass transport problems at the gas diffusion layer. The performance loss was negligible for cloth GDLs and slow cooling rates while fast cooling rates and paper GDLs showed significant degradation. When a single-fuel cell was operated at -10 oC at a constant voltage, the current first increased to a maximum value, then started decaying due to ice formation. This ice formation was monitored using neutron radiography and was found to be concentrated near the inlets, outlets and cell edges resulting in a significant increase in mass transport resistance as revealed by AC impedance spectroscopy. Cyclic voltammograms at -10 oC show no change in catalyst electrochemical surface area associated with this decreased performance. Finally the cell did not exhibit any degradation in performance at 80 oC even after 5 separate operations at -10 oC, where the cell was allowed to completely clog with ice each time until no further current was produced.

Effect of open circuit voltage on performance and degradation of high temperature PBI–H 3PO 4 fuel cells

The impact of open circuit voltage (OCV) on the performance and degradation of polybenzimidazole–phosphoric acid (PBI–H 3PO 4) fuel cells operated at 180 °C was investigated. The OCV showed an initial quick increase in the first few minutes, followed by a much slower increase, and peaked after about 35 min. It then started an exponential and monotonous decline. Along with the decline of OCV, the performance of the fuel cell also declined. Operating the fuel cell with a load of 0.2 A cm −2 could temporarily boost the OCV and the fuel cell performance, but it could not recover the lost performance permanently. Electrochemical impedance spectroscopy (EIS) indicated significant loss of catalyst activity and increase in mass transport resistance due to the relatively high potential at OCV. X-ray diffraction (XRD) measurement showed that the cathode Pt crystallite size increased by as much as 430% after a total of 244.5 h of exposure to OCV.