Oxygen Mass Transport Resistance Research Articles

Measuring the through-plane and in-plane oxygen apparent diffusion coefficients in the gas diffusion layer

Accurately predicting oxygen mass transport resistance and current distribution in fuel cells requires significant knowledge of oxygen diffusion coefficients in both in-plane and through-plane directions. However, there are few methods for measuring in-plane oxygen diffusion coefficients, which are important parameters for simulating oxygen flux distribution in gas diffusion layers (GDLs). In this study, we establish measurement methods and calculations for both in-plane and through-plane oxygen diffusion coefficients. Using in-house designed cells, we measure the in-plane and through-plane oxygen apparent diffusion coefficients of a commercial carbon paper (AvCarb EP40) at various torques and gas flow rates. We also simulate oxygen flux distributions in the GDL under each torque and gas flow rate. Our results show that the channel part is the major contributor to total oxygen flux at high torque conditions and that an increase in torque leads to a decrease in the contribution from the land part. Simulation results also suggest that a higher gas flow rate and lower torque contribute to a more uniform distribution of oxygen flux in the GDL.

In-situ Electrochemical Diagnostics for Morphological Study of CO2 Reduction Electrodes

Electrochemical CO2 reduction (CO2R) devices have immense potential to capture the effluent CO2 in hard-to-decarbonize sectors and convert them to valuable chemicals like CO, C2H4, CH4, alcohols etc. Inside these devices, CO2R occurs inside gas diffusion electrodes (GDE). The reaction rate strongly depends on the local environment at the catalyst-electrolyte interface like ionomer binders, pH, electrolyte cations etc[1]. The transport of reactants and products to and from the catalyst-electrolyte interface depends on the pore-space structure of the GDE, which comprises mostly of micro and smaller mesopores. Ionomer binders used in these GDEs also affect the Faradaic efficiencies (FE) of the CO2R products. Moreover, experimental results indicate that the KOH fed as the anolyte in alkaline CO2 electrolyzers impact the performance of CO2R at the cathode [2]. At present, there is a lack of simple experimental methods in the literature to systematically study the local electrochemical environments present in real decices and identify critical limiting phenomena.We have developed a novel setup to study the morphological aspects of CO2 electrodes at the electrode level without the complexities due to water splitting at the anode. A stable reference electrode, an ion-exchange membrane, and the CO2R cathode are integrated together to form a membrane electrode assembly (MEA) for the purpose of precise and reproducible electrochemical experimentation. AC impedance spectroscopy (EIS) is used to measure capacitance and ion-transport resistance of the CO2R electrodes. A novel MEA setup, previously developed in this group to study oxygen mass transport resistance (MTR) in non-Platinum group metal fuel cells electrodes, is used to measure MTR of CO2 in the CO2R electrodes [3]. We vary the experimental conditions used in the CO2R experiments, like feed gas RH, KOH flow rate etc., and measure the above-mentioned properties. In addition, we also used these diagnostic methods to study the role of ionomer, specifically the ionomer-catalyst interaction in these electrodes. Experiments done by our group members indicate that a suitable electrode ink recipe (catalyst and ionomer loading, organic solvent etc.) is required to optimize the performance of CO2R inside these electrodes. Overall, these techniques can be used to understand the CO2R trends of various electrodes and to identify key design parameters for more efficient CO2 reduction electrodes.Important references: Bui et al., Engineering Catalyst–Electrolyte Microenvironments to Optimize the Activity and Selectivity for the Electrochemical Reduction of CO2 on Cu and Ag; Acc. Chem. Res. 2022, 55, 484−494.Dinh et al., CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface; Science, (2018), 783-787, 360(6390).Star et a., Mass transport characterization of platinum group metal-free polymer electrolyte fuel cell electrodes using a differential cell with an integrated electrochemical sensor; Journal of Power Sources, (2020), 227655, 450.

Analysis of MEA’s durability under Accelerated Stress Tests that mimic realistic automotive operations

This work aims at studying MEA’s ageing under single operational “mode” accelerated stress tests (AST), that were specifically designed to replicate under hydrogen/air feeding the main stressors of realistic operations in the automotive sector. A methodology for developing AST is here presented and preliminary results about the activity included. In particular, low power and high power functioning have been mimicked in a Zero-Gradient hardware, which allows a reliable materials comparison. Quantities, measurable in-situ and operando, have been tracked during ageing, like cell power, polarization curves, Pt active area, oxygen mass transport resistance, Electrochemical Impedance Spectra. The final objective is to clarify the underlying ageing mechanisms and assess the contribution of each specific operation to the MEA lifetime, focusing in particular on the cathode catalyst layer durability. Moreover, the rate of voltage loss for the new ASTs has been successfully correlated to the degradation observed under a complete driving cycle protocol.

Exploration of operating conditions on oxygen mass transport resistance and performance of PEM fuel cells: Effects of inlet gas humidification

AbstractThis work presents comprehensive studies of inlet gas humidification on O2 mass transport and proton exchange membrane fuel cell performance to obtain a better understanding of transport processes at a cathode gas diffusion electrode. The O2 mass transport resistance was determined employing our segmented cell system and a novel method based on the distribution of the spatial limiting currents using 5% vol. O2 mixtures with various balance gases from He to C3H8. Variation of the molecular weight of the balance gas resulted in the determination of O2 mass transport resistance due to Knudsen diffusion as well as diffusion and dissolution through liquid and ionomer films in a cathode catalyst layer (RK+film) and gas phase (Rm, N2). The application of a gas diffusion layer with and without a microporous layer (MPL) allowed us to separate contributions from the cathode catalyst layer (RK+film, CCL) and MPL (RMPL). The data demonstrated that RK+film, CCL significantly decreased from 92.36 to 52.83 s/m at 32 and 100% relative humidity, respectively, due to the effect of humidification on gas permeability through the ionomer. At the same time, Rm, N2 decreased slightly with gas humidification, while RMPL increased slightly. The results showed that fuel cell performance improved with a decrease in permeability mass transport overpotentials and RK+film, CCL. An analysis of the data demonstrated correlations between operating conditions, the mass transport parameters of the cathode gas diffusion electrodes, fuel cell performance, and voltage losses.

Comprehensive Evaluation of Mass Transport Resistance in Pemfcs with Open Flow Field and Land/Channel Architectures

The deployment of fuel cell-powered vehicles to the consumer market clearly showed that continued performance improvement and cost reduction are further required. Proton exchange membrane fuel cell (PEMFC) performance can be improved not only by variation of a catalyst or electrode structure but also by modification of the flow field architecture. An open flow field design from Nuvera Fuel Cells enables power density higher than 1 W cm-2 and ensures excellent mass transfer and water management properties [1, 2]. Quantifying mass transport in the cell with open flow field architecture relative to the conventional land/channel benchmark presents high value to developers of fuel cell stacks for mobile applications, considering limited data available in the literature.Previously we developed a method for determining the oxygen mass transfer coefficient which is based on measurements of limiting current distributions using O2 mixtures with different diluents (from He to C3H8) [3]. The method can be modified and be applicable for a single cell and average limiting current density (iave ) can be described by Eq. 1. iave=ie (1-exp(-nFpr/RTiefRMT)) (1)where n is the number of electrons, F is the Faraday constant (96,485 C mol-1), pr is the dry inlet reactant pressure (Pa), R is the ideal gas constant (8.31 J mol-1 K-1), T is the temperature (K), f is the diluent-to-oxygen fraction in the dry inlet stream, and ie is the current density equivalent to the dry inlet oxygen concentration (A m-2), RMT is oxygen mass transport resistance (s m-1).In this work, a serpentine land/channel and single cell open flow field (SCOF) hardware from Nuvera Fuel Cells were employed to evaluate the feasibility of the methodology to determine the oxygen mass transport resistance, separate its components and compare two flow field designs.In this work, we evaluated 50 cm2 SCOF and 100 cm2 cell with a 10-channel serpentine configuration described in [4]. We employed commercially available MEAs from Gore with Pt loadings of 0.1 and 0.4 mgPt cm-2 for the anode and cathode, respectively. A GDL from Freudenberg with a thickness of 170 mm was used for the SCOF, while 25BC was applied for the serpentine design. The cells performance was studied at subsaturated conditions with 50% RH of feed gases and cell temperature of 80°C and at oversaturated conditions with 100% RH at cell temperature of 60°C.The serpentine cell showed better polarization performance at low current density (< 0.5 A cm-2), while at high current conditions, the SCOF demonstrated superior properties (Fig. 1 a). The oversaturated conditions caused a drastic drop in the serpentine cell potential with increasing current density, while a low humidity and higher cell temperature improved the cell performance. Polarization performance of the SCOF was similar under oversaturated and subsaturated conditions, and the cell reached 2.5 A cm-2 at a cell voltage of 0.58-0.6 V and generated a power density of 1.5 W cm-2.Using our method (Eq. 1 and Ref. 3), we determined the O2 mass transport resistances for SCOF and serpentine flow fields and deconvoluted them to contribution of a gas phase (Rm, N2 ) and a combination of Knudsen diffusion and transport through ionomer/water films in cathode electrode (RK+film ) (Fig. 1 b). Analysis of the the SCOF under subsaturated conditions showed that RK+film and Rm, N2 were 74.85 and 35.65 s m-1, respectively. These values were found to be lower than those for the serpentine cell and explained the lowest voltage losses and superior performance of the SCOF. The open field architecture also had better performance under oversaturated conditions, although it was characterized by the highest RK+film value of 144.09 s m-1 vs. 75.76 s m-1 for the serpentine cell. The SCOF had a lower Rm, N2 than the serpentine architecture, and the superior mass transport of the SCOF in the gas phase likely compensated for the hindered Knudsen diffusion and dissolution though films in the electrode. Detailed analysis of the obtained results will be presented and discussed.ACKNOWLEDGEMENTSWe gratefully acknowledge funding from ONR (N00014-19-1-2159) and ARO (W911NF-15-1-0188). The authors thank K. Bethune and J. Huizingh for valuable help and support in system operation.References K. Srouji, L.J. Zheng, R. Dross, A. Turhan, M.M. Mench, J. Power Sources, 218, 341-347 (2012).K. Srouji, L.J. Zheng, R. Dross, D. Aaron, M.M. Mench, J. Power Sources, 364, 92-100 (2017).V. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161, F1089-F1100 (2014).V. Reshetenko, G. Bender, K. Bethune, R. Rocheleau, Electrochim. Acta 88, 571-579 (2013). Figure 1

Carbon Corrosion in Polymer Electrolyte Fuel Cells: A Complex Interplay between Morphological Changes and Electrochemical Performance

Due to the high gravimetric energy density of hydrogen, the focus of implementation of polymer electrolyte fuel cells (PEFCs) has shifted from light duty passenger vehicles to heavy duty vehicles such as buses, trucks, locomotives and marine vessels1. A mechanistic understanding of degradation is therefore necessary to improve durability and efficiency. During start-up and shut-down (SUSD) of PEFC systems, the catalyst (Pt nanoparticles embedded on carbon support) undergoes local potentials ∼ 1 - 1.5 V caused by a combination of fuel (H2) starvation, mixed fuel region and cell reversal2. This leads to a series of degradation phenomenon including reduction in cathode catalyst layer (cCL) thickness and porosity, loss in electrochemical surface area (ECSA), ionomer degradation and loss in electrical contact, therefore resulting in severe performance loss2. The convoluted relationship between these individual degradation mechanisms, their chronology and their effects on electrochemical performance are yet unresolved.In this study, the complex interplay between morphological changes due to carbon corrosion and its effects on the electrochemical performance were analyzed using a combination of detailed electrochemical characterization, spectroscopy, and electron microscopy techniques. Commercially available catalyst coated membranes were subjected to the Department of Energy carbon corrosion accelerated stress test (AST) protocol. It was found that carbon corrosion in PEFCs can be divided into two phases. In the first phase (∼500 cycles), disordered domains of the carbon containing Pt nanoparticles oxidized faster with a severe loss of cCL thickness. Pt nanoparticles detached from the support and became electrically isolated and hence electrochemically inactive. Electrochemical surface area decreased rapidly in this phase with little change in the cCL pore structure. Both kinetic and mass transport overpotentials (due to decrease in roughness factor) increased. In the second phase (remaining 1500 cycles), a severe pore structure collapse was observed (see Figure 1). Porosity, mean pore size, and the pore connectivity decreased considerably. Pressure independent oxygen mass transport resistance showed anomalous increase. The oxygen-reduction reaction kinetics were impeded, reflected by an increase in the Tafel slope. This was caused due to severe mass transport limitation, even at low current densities of 0.1 A.cm-2 in oxygen environment owing to pore structure collapse. In addition, the AST did not change the concentration of oxides on carbon, however, the overall water management in the cCL deteriorated as the pores lost percolation. Ionomer degradation was observed in the second phase. Sheet capacitance and sheet resistance also decreased with carbon corrosion. Figure 1. a) 3D reconstructions of the cCL using FIB-SEM micrographs, b) porosity as a function of thickness. Scale bar is 1 µm.

Selective Oxidation of PEMFC Catalyst Supports: Overcoming the Trade-Off between Kinetics and Mass Transport Limitations

Widespread commercialization of PEM fuel cells and their cost parity with incumbent propulsion technologies depend in no small part on further reductions in platinum requirements (<0.125 gPt kW-1 rated). This even increases in importance as manufacturers move towards high volume production, because raw materials (such as Pt) do not benefit from economies of scale.[1] Lower cathode loadings (<0.1 mgPt cm-2), however, disparately impact high current density performance due to local mass transport limitations of oxygen and protons. Recent research efforts have demonstrated the key role of catalyst support structure and Pt nanoparticle location to mitigate these transport-related losses.[2-3] Typical high surface area carbons (HSCs) such as Ketjenblack host Pt nanoparticles in internal pores, resulting in obstructed gas diffusion towards the catalytically active sites. In contrast, Pt deposition onto carbons without appreciable mesoporosity (e.g. Vulcan) yields nanoparticles on the external surface of the primary carbon particle, where intimate contact with the ionomers’ sulfonate endgroups depresses the oxygen reduction reaction (ORR) kinetics due to ionomer poisoning. To overcome this trade-off, Yarlagadda et al. proposed an intermediate carbon morphology with accessible pores hosting Pt nanoparticles, shielding them from direct ionomer contact to avoid ionomer poisoning while impeding oxygen transport to the Pt surface as little as possible.[4] In this contribution, we systematically investigate an oxidative heat treatment suited to obtain such accessible carbon supports. We demonstrate that this methodology can selectively remove up to 14 wt% of carbon and analyze the concomitant changes to pore morphology using N2 physisorption. Catalysts with pristine and modified supports are fabricated into low-loaded membrane electrode assemblies (MEAs) and tested in 5 cm2 single cell PEM fuel cells. While our oxidative treatment leads to slightly reduced ORR mass activities, high current density performance in H2/air improves substantially compared to the pristine support (up to 90 mV at 2 A cm-2, Figure 1). We ascribe this voltage gain to significantly reduced oxygen mass transport resistance, in accordance with better pore accessibility.Lastly, we evaluate catalyst susceptibility to start-up/shut-down (SUSD) degradation. In SUSD events, H2/air fronts passing through the anode compartment induce oxidation currents in the cathode layer, leading to carbon corrosion and, ultimately, to a collapse of the electrode structure. It is thus conceivable that their selective pre-oxidation might make carbon supports more prone to this failure mode. Our experiments suggest that while higher degrees of oxidation modestly accelerate SUSD degradation, appreciable differences are only observed after both pristine and modified catalysts have already suffered SUSD-induced voltage losses that constitute end of life (50-100 mV loss versus initial performance). The performance gain conferred by the oxidative treatment is therefore not outweighed by lower resilience towards SUSD. ACKNOWLEDGMENTS We gratefully acknowledge funding from the Swiss National Foundation under the funding scheme Sinergia (project grant number 180335). We also express our gratitude to Ana Marija Damjanović for her help with the Start-Up/Shut-Down protocol and Mohammad Fathi Tovini for valuable consultations regarding thermogravimetric analysis. REFERENCES [1] C. S. Gittleman, A. Kongkanand, D. Masten, W. Gu, Current Opinion in Electrochemistry 2019, 18, 81-89.[2] G. S. Harzer, A. Orfanidi, H. El-Sayed, P. Madkikar, H. A. Gasteiger, Journal of The Electrochemical Society 2018, 165, F770-F779.[3] Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, Journal of Power Sources 2016, 315, 179-191.[4] V. Yarlagadda, M. K. Carpenter, T. E. Moylan, R. S. Kukreja, R. Koestner, W. Gu, L. Thompson, A. Kongkanand, ACS Energy Letters 2018, 3, 618-621. Figure 1

Determination of oxygen mass transport resistance in proton exchange membrane fuel cells with an open flow field architecture

In this work, we demonstrate the applicability and validity of our novel method in determining the oxygen mass transport resistance of proton exchange membrane fuel cells (PEMFCs) using a commercially relevant open flow field design. The method is based on the measurements of the limiting current when a cathode is fed by highly diluted O2 mixtures. Mass transport resistance originating from Knudsen diffusion and dissolution through liquid and ionomer films in a cathode catalyst layer (RK+film) can be separated from molecular diffusion in the gas phase (Rm, N2) by varying the diluent molecular weight. The performance and mass transport properties of the single cell open field (SCOF) design were evaluated and compared with a serpentine land/channel architecture under subsaturated and oversaturated conditions. Analysis of the SCOF under subsaturated conditions showed that RK+film and Rm, N2 were 74.85 and 35.65 s m−1, respectively. These values were found to be lower than those for the serpentine cell and explained the superior performance of the SCOF. The SCOF demonstrated better performance under oversaturated conditions as well due to the low Rm, N2, which compensated for the high RK+film. The results indicated that the open flow field architecture ensured good transport in the gas phase, uniform humidification of the catalyst layer and efficient oxygen diffusion through the ionomer.

Impact of a gas diffusion layer's structural and textural properties on oxygen mass transport resistance in the cathode and performance of proton exchange membrane fuel cells

In this work, we used our previously developed method for determination of oxygen mass transport coefficients (k) to evaluate the effects of the gas diffusion layer (GDL) structure and texture on oxygen mass transfer in a cathode electrode and on proton exchange membrane fuel cell (PEMFC) performance. The method is based on measurements of limiting current distributions using oxygen mixtures with different diluents, which allows us to separate a contribution from gas phase molecular diffusion (km) and a combination of Knudsen diffusion and transport through ionomer/water films (kK+film). GDLs with varying microporous layer (MPL) loadings from 50 to 150% were used for the cathode electrode. The lack of an MPL in the GDL resulted in the highest values of km and kK+film but led to poor performance since the MPL plays a critical role in water management. Moreover, the application of a GDL without an MPL allowed the oxygen mass transport coefficient in the catalyst layer (kK+film, CL) to be directly determined. The contribution from the MPL to oxygen transport (kK, MPL) was separated by comparing the results for a cell with a GDL with and without an MPL. An increase in MPL loading caused a gradual decrease in O2 mass transport coefficients and improvement in PEMFC performance in the high power regime. The obtained structure-to-property correlations showed a trade-off in MPL content among mass transport properties, texture and high performance.

Recent Advances in Electrode Design Based on One-Dimensional Nanostructure Arrays for Proton Exchange Membrane Fuel Cell Applications

Abstract One-dimensional (1D) Pt-based electrocatalysts demonstrate outstanding catalytic activities and stability toward the oxygen reduction reaction (ORR). Advances in three-dimensional (3D) ordered electrodes based on 1D Pt-based nanostructure arrays have revealed great potential for developing high-performance proton exchange membrane fuel cells (PEMFCs), in particular for addressing the mass transfer and durability challenges of Pt/C nanoparticle electrodes. This paper reviews recent progress in the field, with a focus on the 3D ordered electrodes based on self-standing Pt nanowire arrays. Nanostructured thin-film (NSTF) catalysts are discussed along with electrodes made from Pt-based nanoparticles deposited on arrays of polymer nanowires, and carbon and TiO2 nanotubes. Achievements on electrodes from Pt-based nanotube arrays are also reviewed. The importance of size, surface properties, and the distribution control of 1D catalyst nanostructures is indicated. Finally, challenges and future development opportunities are addressed regarding increasing electrochemical surface area (ECSA) and quantifying oxygen mass transport resistance for 1D nanostructure array electrodes.

Graded Microporous Layers for Enhanced Capillary‐Driven Liquid Water Removal in Polymer Electrolyte Membrane Fuel Cells

AbstractA novel microporous layer (MPL) is designed and fabricated with spatially graded poly(tetrafluoroethylene) (PTFE) to alleviate liquid water flooding in the cathode gas diffusion layer (GDL) of the polymer electrolyte membrane (PEM) fuel cell. In operando GDL liquid water distributions are examined using synchrotron X‐ray radiography and oxygen mass transport resistance via electrochemical characterizations, and it is found that the graded PTFE content in the MPL results in enhanced PEM fuel cell performance at high current densities (≥ 1.0 A cm−2). Specifically, less liquid water accumulates within the cathode GDL substrate, and the oxygen mass transport resistance is substantially lowered. This lower substrate water content is attributed to enhanced capillary‐driven removal of liquid water through the use of the graded MPL. This study demonstrates how strongly the spatial distributions of wettability and pore size of the MPL influence the performance of the PEM fuel cell.

Liquid water saturation and oxygen transport resistance in polymer electrolyte membrane fuel cell gas diffusion layers

Liquid water accumulation in the gas diffusion layers (GDLs) of polymer electrolyte membrane (PEM) fuel cells is governed by a complex interplay of factors, the full scope of which is not yet fully established in literature. This study presents the combined effects of relative humidity (RH) and current density on liquid water accumulation and oxygen mass transport resistance in the cathode GDLs of a PEM fuel cell. Through-plane liquid water saturation distributions were measured in situ using synchrotron X-ray radiography while simultaneously performing limiting current-based characterizations of oxygen transport resistance. At low current densities (< 0.5 A·cm−2), lower cathode RH levels resulted in the largest oxygen transport resistances due to reductions in ionomer hydration in the catalyst layer. In the intermediate current density range (1.5–2.1 A·cm−2), high RH levels resulted in the largest oxygen transport resistances due to the observed significant sensitivity of liquid water accumulation to cathode RH. At high current densities (>3.0 A·cm−2), cathode GDL liquid saturation levels were high regardless of cathode inlet RH, and the oxygen transport resistance was therefore less sensitive to RH. Furthermore, it was established that liquid water tends to preferentially accumulate in regions of higher local porosity within the GDL, identified by combining measured liquid water saturations with micro-computed tomography (μCT) characterizations of the through-plane porosity profile. Finally, the strong relationship between GDL oxygen transport resistance and liquid water-free (effective) pore space of the GDL was examined in order to consider the feasibility of predicting oxygen transport resistance based on overall liquid saturation.

Tailoring Catalyst Morphology towards High Performance for Low Pt Loaded PEMFC Cathodes

The effect of the catalyst synthesis method on the location of platinum nanoparticles on a high surface area Ketjenblack is investigated with respect to the high current density performance in low loaded proton exchange membrane fuel cells (PEMFC). Catalysts were prepared using various synthetic methods to deposit platinum nanoparticles at different locations on the carbon surface, e.g. inside or outside the pores of the primary particle. Transmission electron microscopy (TEM) suggested, that the Pt-particle deposition can be controlled to be preferentially on the outer carbon surface or within the pores. Electrochemical characterization was performed in thin-film rotating disk electrode (RDE) setup as well as in 5 cm2 single cell MEA tests. Although the carbon support was identical for all catalysts, the one with more Pt particles deposited on the outer carbon surface performed superior at high current which was attributed to a lower oxygen mass transport resistance. From the presented data, it can be concluded that not only the type or the surface area of the carbon black support affects the fuel cell performance, but that the synthesis approach is an additional parameter to tune the fuel cell performance at high current density.

Method of Measuring the Diffusion Resistance of the Structures of Porous Electrode Materials Based on a Proton-Conducting Ionomer and Carbon Nanomaterials

A method has been developed for measuring the oxygen mass-transport resistance of porous electrodes based on micro- and nanostructures of a Nafion proton-conducting ionomer, Pt/C, and carbon nanowires with the ultrahigh porosity. The method suggests measuring the limit oxygen electroreduction current density at the controlled oxygen flow through an electrochemical system and calculating the oxygen mass transport resistance using the ratio based on the Fick’s law. The method has been used to study the mass-transport loss of a membrane-electrode assembly electrode and electrode material based on a rotating disk electrode and can be applied in the development of new electrodes.

The Key to High Performance Low Pt Loaded Electrodes

The effect of ionomer distribution on the oxygen mass transport resistance, the proton resistivity of the cathode catalyst layer, and the H2/air fuel cell performance was investigated for catalysts with surface modified carbon supports. By introducing nitrogen containing surface groups, it was shown that the ionomer distribution in the cathodic electrode can be optimized to decrease mass transport related voltage losses at high current density. The in house prepared catalysts were fully characterized by TEM, TGA, elemental analysis, and XPS. Thin-film rotating disk electrode measurements showed that the carbon support modification did not affect the oxygen reduction activity of the catalysts, but exclusively affects the ionomer distribution in the electrode during electrode preparation. Limiting current measurements were used to determine the pressure independent oxygen transport resistance – primarily attributed to oxygen transport in the ionomer film – which decreases for catalysts with surface modified carbon support. Systematically lowering the ionomer to carbon ratio (I/C) from 0.65 to 0.25 revealed a maximum performance at I/C = 0.4, where an optimum between ionomer thickness and proton conductivity within the catalyst layer is obtained. From this work, it can be concluded that not only ionomer film thickness, but more importantly ionomer distribution is the key to high performance low Pt loaded electrodes.

Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers

A gas diffusion layer (GDL) was subjected to accelerated degradation (ageing) via complete submersion in hydrogen peroxide. The impacts of ageing were evaluated through deviations in limiting current densities, oxygen mass transport resistances, and liquid water saturation distributions in the aged GDL compared to pristine materials. For dry (volumetric oxygen concentration of 1%) and partially saturated cathode conditions (volumetric oxygen concentrations of 5%, 10%, and 21%), the fuel cell composed of aged GDLs reached limiting current densities up to 10.3% lower and experienced increases in overall oxygen mass transport resistance of up to 11.5% compared to the pristine GDLs. This performance reduction was attributed to a higher liquid water content in the aged GDL under the flow field channel compared to the pristine GDL. The results suggest that the higher saturation in the channel regions of the aged GDL led to increasingly restrictive gas transport due to the higher average water content and reduced pathways for reactant gas transport, therefore contributing to the reductions in the limiting current. In fuel cell builds with aged and pristine GDLs, up to 13.6 times more liquid water was present in the GDL under the rib compared to that under the channel at limiting current operation.

Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers: Mass Transport Resistance and Liquid Water Accumulation at Limiting Current Density with in operando Synchrotron X-ray Radiography

An SGL 29BC gas diffusion layer (GDL) was degraded (aged) in hydrogen peroxide. The impact of ageing was measured through deviations from pristine GDLs in terms of limiting current densities, oxygen mass transport resistances, and liquid water spatial distributions in the GDL. For dry and saturated cathode conditions (oxygen concentrations of 1% and 21%, respectively) the fuel cell containing the aged GDL reached limiting current densities that were up to 10.3% lower and experienced increases in oxygen mass transport resistance of up to 11.7% versus the pristine GDL. This performance reduction was attributed to a higher liquid water content (13% more at an oxygen concentration of 21%) in the aged GDL under the channel compared to the pristine GDL. In fuel cells built with aged and pristine GDLs, up to 200% more water was present in the GDL under the land compared to under the channel at limiting current operation.

Nanostructural Control and Performance Analysis of Carbon-Free Catalyst Layers Using Nanoparticle-Connected Hollow Capsules for PEFCs

In this study, the nanostructures of the carbon-free cathode catalyst layers using connected platinum–iron nanoparticle catalysts with porous hollow capsule structures are controlled by the ionomer morphologies (aggregated or not) formed by different treatments (ultrasonication or autoclaving) to the catalyst inks, and the relationship between the catalyst-layer nanostructures and the fuel-cell performances is investigated. Structural and electrochemical analyses of the capsule catalyst layers reveal that the nanosized ionomer morphologies on the capsules strongly influence the oxygen mass-transport resistance; the fully-swollen and thick ionomer layer would cause blocking of the interspaces between the capsules and slow oxygen-diffusion to the capsule surfaces. Thus, the uniform and thin ionomer coating formed by autoclaving produces a higher cell performance due to lower oxygen mass-transport resistance, compared with the nonuniform and partially thick coating formed by ultrasonication. Moreover, the thinner ionomer layer with a very small ionomer/capsule ratio reduces cell performance at high current densities when the humidity at the capsule catalyst layer increases, probably due to the increased oxygen mass-transport resistance through liquid water inside the catalyst layer. Therefore, in addition to the ionomer morphologies, water management in the capsule catalyst layers is important for further improving cell performance.

Determination of O₂ Mass Transport at the Pt | PFSA Ionomer Interface under Reduced Relative Humidity.

Oxygen mass transport resistance through the ionomer component in the cathode catalyst layer is considered to contribute overpotential losses in polymer electrolyte membrane fuel cells. Whereas it is known that water uptake, water transport, and proton conductivity are reduced upon reducing relative humidity, the effect on oxygen mass transport remains unknown. We report a two-electrode approach to determine mass transport coefficients for the oxygen reduction reaction in air at the Pt/perfluorosulfonic acid ionomer membrane interface between 90 and 30% RH at 70 °C using a Pt microdisk in a solid state electrochemical cell. Potential-step chronoamperometry was performed at specific mass-transport limiting potentials to allow for the elucidation of the oxygen diffusion coefficient (D(bO2)) and oxygen concentration (c(bO2)). In our efforts, novel approaches in data acquisition, as well as analysis, were examined because of the dynamic nature of the membrane under lowered hydration conditions. Linear regression analysis reveals a decrease in oxygen permeability (D(bO2c(bO2)) by a factor of 1.7 and 3.4 from 90 to 30% RH for Nafion 211 membrane and membranes cast from Nafion DE2020 ionomer solutions, respectively. Additionally, nonlinear curve fitting by way of the Shoup-Szabo equation is employed to analyze the entire current transient during potential step controlled ORR. We also report on the presence of an RH dependence of our previously reported time-dependency measurements for O2 mass transport coefficients.

Balancing mass transport resistance and membrane resistance when tailoring microporous layer thickness for polymer electrolyte membrane fuel cells operating at high current densities

Synchrotron X-ray radiography and electrochemical impedance spectroscopy (EIS) were concurrently used to quantify the effects of varying MPL thickness on PEMFC performance. Increasing the MPL thickness from 30μm to 50μm was found to lower the liquid water volume by 8% in the cathode substrate, which resulted in a 50% decrease in the oxygen mass transport resistance at a current density of 2.0A/cm2. Increasing the MPL thickness beyond 50μm at the same current density did not lead to any further substantial reductions of the liquid water volume in the cathode substrate (2%); however, the ohmic resistance increases significantly (16%). Among the four MPL thicknesses studied, the 50μm-thick MPL provides an optimized trade-off between the hydration of the membrane and the liquid water content in the fibrous substrate. A key observation of this work is the significant influence that high current density operation has on the heat and water management of polymer electrolyte membrane fuel cells.