Nafion Loading Research Articles

Effect of pH on the Selectivity of γ-MnO2 Electrocatalysts Towards Oxygen Evolution Reaction in the Presence of Chloride Ions in Alkaline Environment

This paper reports on progress in development of electrocatalysts intended for use in seawater, which optimize the balance of competing reactions between chlorine evolution and oxygen evolution. The oxygen evolution reaction (OER) from H2O is thermodynamically more favorable due to a lower equilibrium potential [1]. However, chlorine evolution successfully competes with the OER due to faster kinetics. One of a few catalysts that demonstrates high selectivity towards the OER in the presence of chloride ions in the alkaline environment (i.e. pH 12) is γ-MnO2 [2].In this presentation, we (1) compare activities and selectivities towards the OER of electrodeposited and chemically synthesized γ-MnO2 and (2) determine how pH affects selectivity in the pH range from 11 to 13.g-MnO2 powder was synthesized using hydrothermal method. The synthesis resulted in fabrication of material with a flower-like morphology, characterized by a crystallite size of 5-15 nm and BET surface area of 75 m2/g. In parallel, MnO2 films were prepared by electrodeposition on a Pt RDE substrate, following the procedure outlined by Tsagareli et al. [3]. The electrocatalytic activity of both γ-MnO2 powder and electrodeposited MnO2 was investigated by the thin-film RDE method. The thin films of γ-MnO2 powder were obtained by drop-casting MnO2/VC ink [1] on glassy carbon substrates.We found that the shape of polarization curves on MnO2/VC films is strongly dependent on the Nafionä loading. At low Nafion loadings, we observe diffusion-limited currents, which we attribute to OH- diffusion through the Nafion. At higher Nafion loadings, neither diffusion-limited currents, nor the currents’ dependence on the rotation rate are observed. Our data indicate that at low Nafion loadings, MnO2/VC films selectively produce O2 in 0.5M NaCl at pH 12 and 13 and current densities up to 15 mA/cm2. Because of the potential effect of Nafion on the permeability of Cl- ions to the catalyst’s active sites, a set of parallel experiments is being carried out on the electrolytically deposited MnO2 films. The results of the latter experiments will be compared to those obtained on MnO2/VC films in the presentation.

Influence of hybrid filler on charge conduction and storage performance of polyvinyl chloride/nitrocellulose blend for hybrid electrolyte application

AbstractPolymer blends of polyvinyl chloride/nitrocellulose/MoS2‐Nafion (PVC/NC/MoS2‐Nafion) were prepared using a casting technique. FTIR analysis revealed chemical interactions between the polymer matrix and filler. XRD analysis was employed to investigate changes in structural phases, while SEM illustrated the agglomerated morphology of the modified blends. The critical role of ionic interaction between Nafion and polymer system in enhancing electrical properties was confirmed through impedance analysis. The introduction of oxygenated functionalities improved surface topography and roughness, as observed by AFM. Electrical conductivity increased from 10−6 S/cm to 10−4 S/cm, accompanied by a decrease in activation energy (Ea). The addition of Nafion decreased bulk resistance, indicating enhanced ionic mobility, as supported by impedance analysis. TG‐DSC analysis demonstrated that MoS2 and Nafion improved thermal properties of polymer blends. The glass transition temperature (Tg) increased by approximately 13 °C and 21 °C with MoS2 and Nafion, respectively. Universal testing machine (UTM) results indicated moderate tensile strength and Young's modulus achieved by MoS2 and Nafion. With increasing Nafion loading, elongation at break rose to around 71 %, showcasing excellent interfacial interaction in polymer‐filler system. The resulting polymer blends, modified by Nafion, exhibited high ionic/electrical conductivity and show promise for hybrid polymer electrolyte applications.

Turning copper into gold — Tuning a gas diffusion electrode catalyst for the efficient conversion of CO[formula omitted] to C[formula omitted]H[formula omitted] or CO

Copper is regarded as a unique electrocatalyst for the CO2 reduction reaction as it is able to produce products that require more than two electrons, such as C2H4 and C2H5OH. When combined with the high activity of a gas diffusion electrode, copper becomes a great choice of catalyst for the CO2 reduction reaction, however, it has a history of poor selectivity for a given product. In this work, we have synthesised a CuO electrocatalyst that has typical copper-like selectivity initially, however, as we tune the Nafion loading, we are able to shift the selectivity towards that of a gold-based catalyst. We propose that this is due to the Nafion providing an in-penetrable barrier to the electrolyte that prevents the catalyst from being flooded, which is supported by electrochemical, XAS, and TEM analysis.

Design of multi-layered gradient catalysts for efficient proton exchange membrane fuel cells

Efficient catalyst layers (CLs) in proton exchange membrane fuel cells (PEMFCs) should possess a highly active surface for electrochemical reaction and an ideal platform for simultaneous mass transfer. Efforts have been devoted to lower the widely used Pt and Nafion loading due to their high cost. However, the increase in gas transport resistance and decrease in transfer ability of protons and electrons result in a degraded fuel cell performance. Therefore, CL design with optimal efficiency is important for high performance and stability of PEMFCs. In this study, we design gradient multi-layered CLs in which the loadings of Pt–C catalyst and Nafion ionomer are set differently for each layer to increase efficiency. For ideal cell performance, the ionomer loading should be higher near the membrane layer and Pt–C loading should be higher near the gas diffusion layer. In addition, we confirm that if the loading difference between layers in the gradient CL becomes excessively large, performance can degrade due to the non-uniform Pt active surface. Thus, we compared 1-layer, 3-layer and 5-layer CLs, and designing more layers to reduce the difference in loading between layers results in higher electrochemical surface area (ECSA) and improved performance.

Electrochemical Characterisation of the Oxygen Reduction Reaction in Realistic Catalyst Layers with a Gas Diffusion Electrode (GDE)

The extremely promising activities of advanced fuel cell catalyst materials achieved in an ideal rotating disk electrode (RDE) environment can frequently not be transferred to technologically relevant membrane electrode assemblies (MEAs). This can mainly be ascribed to a non-optimal catalyst layer composition, which significantly affects the management of educts and products on the catalyst layer surface for the oxygen reduction reaction (ORR). Therefore, the composition of the catalyst layer has enormous potential to significantly improve the performance of these systems in MEA. However, it has to be optimized for each individual catalyst system. Currently, MEA experiments have to be employed for catalyst layer optimization.[1] However, such investigations are time consuming, expensive (large quantities of catalyst and extended test equipment needed) and do not allow independent investigation of either cathode or anode catalyst layer. In order to accelerate catalyst layer optimization screening, the gas diffusion electrode (GDE) half-cell setup for investigating realistic catalyst layers was recently proposed.[2] In a Pt loading study, it was previously shown that similar performance trends compared to MEA experiments can be achieved.[2] In the present work, we introduce advanced electrochemical characterization methods, such as Oxygen Transport Resistance [3, 4] and CO-Displacement [5] , which have been developed for MEA technique, to the GDE method. By using a commercial Pt on Vulcan catalyst system, we investigate the impact of Nafion loading on the electrochemical performance. The results show, that high ionomer loadings lead to severe O2 mass transport limitations, whereas for small loadings, lower ionomer coverage are measured. Both result in a significant performance loss at high current densities. Therefore, an intermediate ionomer loading which forms a thin layer of ionomer leads to an optimal performance for the Vulcan carbon support.This work demonstrates that advanced electrochemical methods can also be applied to GDE setups to shed light on the optimal composition of the triple phase interface of catalyst layers. This is an innovative step for the future to efficiently optimise catalyst systems and to gain fundamental insight into the understanding of catalyst layers.Literature[1] K. Ehelebe, D. Escalera-López, S. Cherevko, Current Opinion in Electrochemistry 2021, 29, 100832.[2] K. Ehelebe, D. Seeberger, M. T. Y. Paul, S. Thiele, K. J. J. Mayrhofer, S. Cherevko, Journal of The Electrochemical Society 2019, 166, F1259-F1268.[3] D. R. Baker, C. Wieser, K. C. Neyerlin, M. W. Murphy, ECS Transactions 2006, 3, 989-999.[4] D. R. Baker, D. A. Caulk, K. C. Neyerlin, M. W. Murphy, Journal of The Electrochemical Society 2009, 156, B991.[5] T. R. Garrick, T. E. Moylan, V. Yarlagadda, A. Kongkanand, Journal of The Electrochemical Society 2016, 164, F60-F64.

Part II: NiMoO4 Nanostructures Synthesized by the Solution Combustion Method: A Parametric Study on the Influence of Material Synthesis and Electrode-Fabrication Parameters on the Electrocatalytic Activity in the Hydrogen Evolution Reaction.

Earth-abundant NiMo-oxide nanostructures were investigated as efficient electrocatalytic materials for the hydrogen evolution reaction (HER) in acidic media. Synthesis and non-synthesis parameters were thoroughly studied. For the non-synthesis parameters, the variation in Nafion loading resulted in a volcano-like trend, while the change in the electrocatalyst loading showed that the marginal benefit of high loadings attenuates due to mass-transfer limitations. The addition of carbon black to the electrocatalyst layer improved the HER performance at low loadings. Different carbon black grades showed a varying influence on the HER performance. Regarding the synthesis parameters, a calcination temperature of 500 °C, a calcination time between 20 and 720 min, a stoichiometric composition (Ni/Mo = 1), an acidic precursor solution, and a fuel-lean system were conditions that yielded the highest HER activity. The in-house NiMoO4/CB/Nafion electrocatalyst layer was found to offer a better long-term performance than the commercial Pt/C.

Influence of Nafion loading in the anode catalyst layer on the performance of anion‐exchange membrane direct formate fuel cells

Anion-exchange membrane direct formate fuel cells (AEM DFFCs) were fabricated and tested at various fuel and electrolyte concentrations ranging from 1 to 5 M, flow rates ranging from 1 to 5 mL/min, and cell temperatures ranging from room temperature to 60°C with varying Nafion ionomer contents in the anode. A mixture of potassium formate (HCOOK) and potassium hydroxide (KOH) was fed into the serpentine anolyte flow field, while the air was forced into the serpentine air flow field at 100 sccm. The results revealed that feeding DFFCs with an anolyte solution containing 3-M HCOOK in 3-M KOH yielded the best cell output, compared to using an anolyte solution containing higher fuel and electrolyte concentrations. According to an empirical model investigation of the AEM DFFCs performance, reducing the Nafion ionomer content induced a rise in the exchange current density and the fuel crossover equivalent current density but resulted in a drop in the DFFCs' area-specific resistance. In this investigation, the optimum Nafion ionomer content in the anode was 1.87 mg/cm2. The maximum power density of the DFFCs tested at ambient temperature and 60°C were 94.78 mW/cm2 and 115.3 mW/cm2, respectively, as the AEM DFFCs were fed with anolyte containing 3-M HCOOK in 3-M KOH at 5 mL/min with anode Nafion ionomer content of 1.87 mg/cm2.

Measurement of the Protonic and Electronic Conductivities of PEM Water Electrolyzer Electrodes.

Reducing anode catalyst layer proton- and electron-transport resistances in polymer electrolyte membrane water electrolyzers is critical to improving its performance and maximizing catalyst utilization at high current density. A hydrogen pump technique is adapted to measure the protonic conductivity of IrOx-based catalyst layers. The protonic resistance of the catalyst layer is obtained by subtracting the protonic resistance of an assembly of two NRE211 membranes hot-pressed together from an assembly of two NRE211 membranes with an IrOx intermediate layer. The through-plane and in-plane electronic conductivities were also measured using two- and four-probe methods, respectively. Using these techniques, the protonic and electronic conductivities of the IrOx catalyst layers with varying Nafion loading were measured. The results show that the limiting charge-transport phenomena in the IrOx catalyst layer can be either proton or electron transport, depending on the ionomer loading in the catalyst layer. These results are validated by numerical simulation, as well as by comparison to the high-frequency resistance of an electrolyzer with the same layer.

Effect of Nafion loading and the novel flow field designs on innovative anode electrocatalyst for improved Direct Methanol Fuel cells performance

Efficiency of a Direct methanol fuel cells (DMFCs) not just merely depends on the catalyst, but also on the flow channels, physical/electrochemical kinetics, Membrane Electrode Assembly (MEA) preparation and other operating parameters. This article is focused on the influence of novel flow field design and the effect of Nafion loading during MEA preparation on the performance of DMFC. A novel Pt/NiTiO3 was used as the anode electrocatalyst in the DMFC. The results show a 27.8% & 5.1% increment in cell performance due to changes in Nafion loading and flow field designs, respectively. A Nafion loading of 0.5 mg/cm2 and the sinuous flow field gave the highest performance. Thus, Pt-NiTiO3/C performed to be an encouraging anode catalyst with the new sinuous flow field.

Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation.

Perfluorosulfonic acid (PFSA) dispersions are used as components in a variety of electrochemical technologies, particularly in fuel-cell catalyst-layer inks. In this study, we characterize dispersions of a common PFSA, Nafion, as well as inks of Nafion and carbon. It is shown that solvent choice affects a dispersion's measured pH, which is found to scale linearly with Nafion loading. Dispersions in water-rich solvents are more acidic than those in propanol-rich solvents: a 90% water versus 30% water dispersion can have up to a 55% measured proton deviation. Furthermore, because electrostatic interactions are a function of pH, these differences affect how particles aggregate in solution. Despite having different water contents, all inks studied demonstrate the same particle size and surface charge trends as a function of pH, thus providing insights into the relative influence of solvent and pH effects on these properties.

Electrochemical study of temperature and Nafion effects on interface property for oxygen reduction reaction

With the growth of new energy economy, proton exchange membrane fuel cell (PEMFC) has great potential to be success. However, the lack of high-performance catalyst layer (CL) especially at cathode limits its applications. It is becoming increasingly important to understand interface property during electrocatalytic oxygen reduction reaction (ORR). Here, the rotating disk electrode (RDE) method is developed to study the temperature and Nafion ionomer content effects on interface formed between Nafion and Pt/C. The results show that the temperature has the significant influence on electrochemical active sites, charging double capacitance, and reaction polarization resistance at low Nafion content region. Excess Nafion loaded in CLs will turn to self-reunion and increase the exposed active sites. We find that the optimum Nafion loading is in the range of 30 to 40 wt.%. The highest specific activity we achieve is 107.8 μA/cm2.Pt at 60 °C with 0.4 of ionomer/catalyst weight ratio, corresponding to the kinetic current 283.5 μA at 0.9 V. This finding provides new insights into enhancing the Pt utilization and designing high-efficiency catalysts for ORR.Graphical abstractThe schematic diagram of the predicated interface and structure models of different Nafion content CLs under various temperatures.

Improved PEM fuel cell performance with hydrophobic catalyst layers

Flooding of catalyst layers is one of the major issues, which effects performance of low temperature proton exchange membrane fuel cells (PEMFC). Rendering catalyst layers hydrophobic one may improve the performance of PEMFC depending on Pt percentage in the catalyst and Polytetrafluoroethylene (PTFE) loading on the electrode. In this study, effect of hydrophobicity in catalyst layers on performance has been investigated by comparing performances of membrane electrode assemblies prepared with 48% Pt/C. Ultrasonic coating technique was used to manufacture highly efficient electrodes. Power density at 0.45 V increased by the addition of PTFE, from 0.95 to 1.01 W/cm2 with H2/O2 feed; while it slightly increased from 0.52 W/cm2 to 0.53 W/cm2 with H2/Air feed. Addition of PTFE to catalyst layers while keeping Pt loading constant, enhanced performance providing improved water management. Kinetic activity increased by decreasing Nafion loading from 0.37 mg/cm2 to 0.25 mg/cm2 while introducing PTFE (0.12 mg/cm2) to the electrode. Electrochemical impedance spectroscopy (EIS) results proved that charge transfer resistance decreased with hydrophobic catalyst layers for H2/O2 feed. This is attributed to enhanced water management due to PTFE presence.

Impedance of a PEM fuel cell cathode with nonuniform ionomer loading: Analytical and numerical study

We report a physics-based model for impedance of the cathode catalyst layer (CCL) with nonuniform Nafion loading in a PEM fuel cell. The general system of equations for the perturbation amplitudes is derived and solved analytically for the cases of either fast oxygen transport, or fast proton transport in the CCL. The expressions for the respective CCL impedances are obtained. In the case of large cell current, we analyse the numerical solutions for the CCL impedance.

Rapid Analysis of Platinum and Nafion Loadings Using Laser Induced Breakdown Spectroscopy

Pt and Nafion are materials widely used in electrochemical devices including PEM fuel cells, flow batteries, electrolyzers, and electrochemical sensors. In PEM fuel cells, the catalyst layer consists of multiple materials including catalyst Platinum (Pt) or Pt-based alloys, carbon, and ionomer (usually Nafion). Laser induced breakdown spectroscopy (LIBS) is a promising analytical tool for probing the elemental composition in a sample, offering advantages of fast, in-situ, and online analysis, minimal sample preparation requirement, and the capability of spontaneous multi-element analysis. In this paper, we present a study of probing the Pt and Nafion loadings in their mixtures with graphite carbon, respectively, using LIBS technology. Pt-carbon and Nafion-carbon samples of various material loading are prepared for the LIBS testing. Plasma plume and shadowgraphic images are presented to show plasma and associated shockwave evolution. It takes a few milliseconds for spectrum collection per laser shot. Each sample analysis contains 49 shots in the present study and takes less than 1 minute. The results show LIBS is capable of probing the Pt and Nafion loadings in the mixtures. The method is useful in rapid analysis of catalyst layer composition and examination of material loss due to degradation.

Modeling Electrochemical Performance of the Hierarchical Morphology of Precious Group Metal-Free Cathode for Polymer Electrolyte Fuel Cell

This paper presents a two-dimensional (2D) computational model of a polymer electrolyte fuel cell (PEFC) with a platinum group metal-free (PGM-free) catalyst cathode that can significantly reduce PEFC costs by eliminating the need for expensive platinum catalysts. Due to their comparatively low volumetric activity, PGM-free cathodes are an order of magnitude thicker than their Pt-based counterpart. The resulting need for greater electrode thickness to achieve sufficient power density requires careful attention to the transport losses across the thicker cathodes. The presented model is used to correlate the composition and morphology of the cathode to PEFC performance. The model is a complete cell, continuum model that includes an advanced agglomerate model for a microstructurally consistent representation of the cathode. A unique feature of the approach is the integration of morphology and transport parameter statistics extracted from nano-scale resolution X-ray computed tomography (nano-CT) imaging of PGM-free cathodes. The model was validated with experimental results of PGM-free cathodes with varying Nafion loading. Our key findings are a need for increased cathode hydrophobicity and increased ionomer conductivity through either reduced tortuosity or increased bulk conductivity. We further use the model to evaluate targets for the volumetric activity and active site density for future catalysts.

Impedance Spectroscopy Study of the PEM Fuel Cell Cathode with Nonuniform Nafion Loading

We report modeling and experimental study of impedance of the PEM fuel cell cathode with nonuniform ionomer loading. A physics–based model for the high–frequency impedance is developed and analytical solution for impedance is derived. Assuming that the CCL proton conductivity σp exponentially decays from the membrane surface, we fit the model to experimental spectra of the cell measured at the open circuit conditions. Fitting gives the characteristic scale of the σp decay, the average CCL proton conductivity and the double layer capacitance.

Resolving Electrode Morphology's Impact on Platinum Group Metal-Free Cathode Performance Using Nano-CT of 3D Hierarchical Pore and Ionomer Distribution.

This article reports on the characterization of polymer electrolyte fuel cell (PEFC) cathodes featuring a platinum group metal-free (PGM-free) catalyst using nanoscale resolution X-ray computed tomography (nano-CT) and morphological analysis. PGM-free PEFC cathodes have gained significant interest in the past decade since they have the potential to dramatically reduce PEFC costs by eliminating the large platinum (Pt) raw material cost. However, several challenges remain before they are commercially viable. Since these catalysts have lower volumetric activity, the PGM-free cathodes are thicker and subject to increased gas and proton transport resistances that reduce the performance. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding the correlation between electrode fabrication, morphology, and performance is crucial. In this work, the pore/solid structure and the ionomer distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The associated transport properties were evaluated from pore/particle-scale simulations within the nano-CT-imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion ionomer loading. We show that this is primarily a result of distinct changes in ionomer's spatial distribution. The significant impact of electrode morphology on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability.

Stable Li–Organic Batteries with Nafion‐Based Sandwich‐Type Separators

Similar to Li–S batteries, Li–organic batteries have also been plagued by the dissolution of active materials and the resulting shuttle effect for many years. An effective strategy to eliminate the shuttle effect is adopting solid electrolytes or Li–ion permselective separators to prohibit the dissolved electroactive species from migrating to the Li anode. A polypropylene/Nafion/polypropylene (PNP) sandwich‐type separator is reported with many advantages in comparison with previously reported LISICON, polymer electrolyte, and other Nafion utilization forms. The physical and chemical properties of PNP separators are studied in detail by cross‐section scanning electron microscopy (SEM), infrared spectroscopy (IR), and electrochemical impedance spectroscopy. 1,1′‐Iminodianthraquinone (IDAQ), a novel organic cathode, is taken as an example to quantitatively investigate the function of PNP separators. In the presence of PNP5 with the most appropriate Nafion loading of 0.5 mg cm–2, IDAQ is able to achieve dramatically improved cycling stability with capacity retention of 76% after 400 cycles and Coulombic efficiency above 99.6%, which reaches the highest level for reported soluble organic electrode materials. Besides Li–organic batteries, such kind of Nafion‐based sandwich‐type separators are also promising for Li–S batteries and other new battery designs involving dissolved electroactive species.

Less-Nafion electrodes based on PSSA-Pt/Vulcan catalyst for PEM fuel cells

Functionalization on the carbon material of PEM fuel cells providing them an ionic conductivity, has been thought as a promising way to improve the catalyst activity. Pt/Vulcan catalysts grafted with polystyrene sulfonate (PSSA) groups from 5 to 20wt.% have been prepared. Owing to the ion-exchange properties of the PSSA grafted on carbon support, we developed less-Nafion electrodes, with only 0.25 or 0.50mgcm−2 Nafion loading. The effect of PSSA graft ratio and the Nafion amount was studied in a single cell at 30°C by polarization curves and impedance measurements. It was found that 5 and 10wt.% PSSA with 0.50mgcm−2 Nafion allow high efficiency, however lower than 10wt.% grafted Pt/C with 0.25mgcm−2 Nafion only. The performance of the various electrodes could be correlated to the overall amount of sulfonate introduced: highest yields were obtained with an overall sulfonate concentration near 0.7–0.8μmolcm−2.

Probing rough composite surfaces with atomic force microscopy: Nafion ionomer in fuel cell electrodes

Optimizing Nafion loading and surface distribution of Nafion in the fuel cell electrode is critical for the fuel cell performance for minimizing ohmic and mass transport overpotentials. An atomic force microscopy method is used here for a qualitative and a quantitative discrimination between the ionomer and Pt in the fuel cell electrode. This work describes a methodology for the analysis of complex composite surface of fuel cell electrodes and discrimination of different materials on the electrode surface. The reported methodology could be extended for imaging composite rough surfaces when contrast is based on mechanical properties, adhesion and electrical conductivity.