Catalyst Layer Performance Research Articles

Guidance for targeted degradation analysis of OER kinetics of low-loading iridium-based catalysts in PEM water electrolysis cells

This paper critically evaluates three methods for determining metrics of the oxygen evolution reaction (OER) describing the performance and stability of low-loading iridium-based anode catalyst layers (CLs) in proton exchange membrane water electrolysis (PEMWE) cells. The methods applied are OER mass activity, voltage breakdown analysis (VBA), and constant Tafel slope VBA (CT-VBA). They are used to assess the OER metrics as a function of anode CL configurations, potential cycling, and level of degradation. Therefore, various accelerated stress tests (ASTs) are carried out for a targeted degradation of different anode CLs based on Ir black or Ir oxide. It turns out that the OER mass activity method is robust, straightforward and an ideal method for e.g., in-house screening tasks. On the other hand, the VBA method is suitable for comparative analysis across laboratories by distinguishing between the three main overpotentials. The CT-VBA method, however, offers improved accuracy, as it accounts for mass transport overpotential at low current density, and is particularly suitable for determining apparent exchange current density values further used in modelling approaches. This benefit comes with a drawback, as commonly accepted reference Tafel slopes for respective catalyst type and PTL configurations would be required within the PEMWE community. This guidance therefore helps to choose the right method for determining OER metrics depending on the research question.

Understanding the Effects of Anode Catalyst Conductivity and Loading on Catalyst Layer Utilization and Performance for Anion Exchange Membrane Water Electrolysis.

Anion exchange membrane water electrolysis (AEMWE) is a promising technology to produce hydrogen from low-cost, renewable power sources. Recently, the efficiency and durability of AEMWE have improved significantly due to advances in the anion exchange polymers and catalysts. To achieve performances and lifetimes competitive with proton exchange membrane or liquid alkaline electrolyzers, however, improvements in the integration of materials into the membrane electrode assembly (MEA) are needed. In particular, the integration of the oxygen evolution reaction (OER) catalyst, ionomer, and transport layer in the anode catalyst layer has significant impacts on catalyst utilization and voltage losses due to the transport of gases, hydroxide ions, and electrons within the anode. This study investigates the effects of the properties of the OER catalyst and the catalyst layer morphology on performance. Using cross-sectional electron microscopy and in-plane conductivity measurements for four PGM-free catalysts, we determine the catalyst layer thickness, uniformity, and electronic conductivity and further use a transmission line model to relate these properties to the catalyst layer resistance and utilization. We find that increased loading is beneficial for catalysts with high electronic conductivity and uniform catalyst layers, resulting in up to 55% increase in current density at 2 V due to decreased kinetic and catalyst layer resistance losses, while for catalysts with lower conductivity and/or less uniform catalyst layers, there is minimal impact. This work provides important insights into the role of catalyst layer properties beyond intrinsic catalyst activity in AEMWE performance.

Evolution of the network structure and voltage loss of anode electrode with the polymeric dispersion in PEM water electrolyzer

Exploring the intrinsic relationship between the network structure and the performance of catalyst layer (CL) by rational design its structure is of paramount importance for proton exchange membrane (PEM) electrolyzers. This study reveals the relative effect of polymeric dispersion evolution on oxygen evolution reaction (OER) performance and cell voltage loss and linked to CL network structure. The results show that although the dispersed particle size of the ionomer and ink increases with increasing the solubility parameter (δ) difference between the mixed solvent and the ionomer, MeOH-cat (ink from MeOH aqueous solution) has the largest ionomer and ink particle size resulting in the poorest stability, but has comparable OER overpotential to that of IPA-cat (249 mV@10 mA cm−2), which has the smallest dispersed size. While at 100 mA cm−2, the overpotential of the ink rises as the particle size increases, suggesting that the electrode structure becomes more influential as the current density increases. Quantitatively analyzed the electrolyzers’ voltage losses and determined that the CL from MeOH-cat has the lowest kinetic overpotential. However, its performance is the worst because of the insufficient network structure of CL, resulting in an output of 1.96 V at 1.5 A cm−2. Comparatively, the CL from IPA-cat has the highest kinetic overpotential yet can achieve the greatest performance of 1.76 V at 2 A cm−2 due to its homogeneous network structure and optimal mass transport. Furthermore, the performance variation becomes more pronounced as current density rises. Hence, this study highlights the significant impact of CL structure on electrolyzer’s performance. To improve performance in PEM water electrolysis technology, especially for large work current density, it is crucial to enhance the CL’s network structure.

Colloidal Stability of PFSA-Ionomer Dispersions Part II: Determination of Suspension pH Using Single-Ion Potential Energies.

Perfluorosulfonic acid (PFSA) ionomers serve a vital role in the performance and stability of fuel-cell catalyst layers. These properties, in turn, depend on the colloidal processing of precursor inks. To understand the colloidal structure of fuel-cell catalyst layers, we explore the aggregation of PFSA ionomers dissolved in water/alcohol solutions and relate the predicted aggregation to experimental measurements of solution pH. Not all side chains contribute to measured pH because of burying inside particle aggregates. To account for the measured degree of dissociation, a new description is developed for how PFSA aggregates interact with each other. The developed single-counterion electrostatic repulsive pair potential from Part I is incorporated into the Smoluchowski collision-based kinetics of interacting aggregates with buried side chains. We demonstrate that the surrounding solvent mixture affects the degree of aggregation as well as the pH of the system primarily through the solution dielectric permittivity, which drives the strength of the interparticle repulsive energies. Successful pH prediction of Nafion ionomer dispersions in water/n-propanol solutions validates the numerical calculations. Nafion-dispersion pH measurements serve as a surrogate for Nafion particle-size distributions. The model and framework can be leveraged to explore different ink formulations.

Effect of catalyst ink particle size on the structure of the catalyst layer and electrical performance in the process of ultrasonic spray manufacturing PEMFCs

Proton exchange membrane fuel cells (PEMFCs) have the advantage of high energy conversion efficiency. This study introduces a novel strategy aimed at enhancing the performance of fuel cells through the optimization of process parameters pertaining to the ultrasonic spraying of catalyst layers. This study found that with an increase in flow rate or a decrease in pressure, the atomization particle size of the catalyst ink enlarges, broadening the size distribution simultaneously. Larger ink droplets falling on the Nafion membrane result in a greater concentration of particles building up within the coffee ring, leading to deformities like orange peel texture, protrusions, and grooves within the catalyst layer. Consequently, the polarization curve deteriorates, and Pmax (peak power density) diminishes. Research has shown that optimizing the process parameters of ultrasonic spraying can optimize the catalyst layer structure, improve the mass transfer performance of membrane electrode assemblies (MEAs), and is an effective new solution for improving MEAs performance.

Impact of Low Catalyst Loading and Incorporation of Additives on Structure and Performance of PEM Electrolyzer Catalyst Layers

Polymer electrolyte membrane water electrolyzers (PEMWEs) are an important technology that can produce renewable hydrogen on demand, although with challenges posed by the slow kinetics of the anodic oxygen evolution reaction (OER) and degradation issues. The degradation of membrane electrode assemblies (MEAs) prepared with Ir-based catalysts is not well understood, especially with lower-loading catalyst layers and intermittent load inputs. This work investigates low-loading IrO2 catalyst layers made with different carbon-based additives, including high-surface-area carbon (HSC), graphitized HSC (gHSC), Vulcan, graphitic carbon nanofibers (gCNFs), graphitic nanoplatelets (gNPs), nitrogen-doped carbon spheres (NCs), Ti carbide (TiC) and Ti carbonitride (TiCN). While carbon-based supports have issues with corrosion at high oxidizing potentials, these materials provide a range of variations in morphology, shape, size, surface area, graphitization, and composition, allowing exploration of the impact of these parameters on catalyst layer structure to identify promising routes to improve catalyst layer structures. Catalyst layers were characterized with electron microscopy and x-ray spectroscopy to assess the distribution of catalyst, carbon additive, and ionomer. This information was correlated with electrochemical performance measurements to elucidate the impact on kinetic site access and mass transport. Selected samples were also subjected to more extended operation to investigate how the initial structures and their performance evolved.

X-Ray Photoelectron Spectroscopy Investigation of Catalyst-Ionomer Interactions for PEMFC Electrodes

Proton exchange membrane fuel cells (PEMFCs) are an increasingly attractive clean energy technology to meet stationary and transportation energy demands without contributing to greenhouse gases. Further developments to PEMFCs are primarily focused on improving the cathode catalyst layer which uses expensive platinum catalysts to compensate for the sluggish oxygen reduction reaction and a thin film of proton conductive ionomer to aid in mass transport. It is understood that the Pt-based carbon supported catalyst may experience heterogeneity in ionomer coverage motivating further studies of catalyst layers with focus on catalyst-ionomer interface. Specifically, it is important to improve understanding of how support morphology, Pt catalyst loading, amount of ionomer, and method of catalyst layer coating affect the catalyst-ionomer interface to identify potential pathways to further to improve catalyst utilization as the technology increases in scalability.This study focuses on investigating the catalyst-ionomer interface using x-ray photoelectron spectroscopy (XPS) as it is highly surface sensitive (top ~3-10 nm) and can capture subtle differences in the chemistry of the catalyst, ionomer, and support at the catalyst-ionomer interface. Due to the ionomer’s inherent susceptibility to x-ray induced damage, XPS analysis was conducted with a recently developed protocol to mitigate instrumental artefacts. In this work, elemental ratio quantification metrics derived from XPS are used to track surface ionomer content relative to amount of Pt nanoparticles and the amount of carbon support producing two parameters. These parameters are used to show how variations in carbon support graphitization, Pt loading, ionomer chemistry, and ionomer loading affect the catalyst-ionomer interface in unique ways. Additionally, changes to the catalyst-ionomer interface after degradation were also assessed, comparing beginning of life and end of life samples. Further, this presentation will also discuss the evolution of the catalyst-ionomer interface under humidified conditions which was assessed with in situ XPS. This talk highlights the potential of XPS to improve our understanding of relationships between surface properties, processing, performance, and durability and motivates future work in understanding catalyst layer fabrication, performance, and degradation.

Effects of agglomerate structure and operating humidity on the catalyst layer performance of PEM fuel cells

Understanding the transport mechanism in catalyst layer (CL) agglomerates is crucial to enhance Pt utilization and overall fuel cell performance. In this study, high-resolution porous agglomerates are stochastically reconstructed, and capillary condensation of water in agglomerates is directly simulated using a lattice Boltzmann pseudopotential model. On this basis, a pore-scale electrochemical model is developed to investigate the effects of agglomerate structure and operating humidity on local transport resistance and agglomerate effectiveness at different Pt loadings. The results reveal that liquid water formed above 70%RH improves the electrochemical surface area (ECSA) of agglomerates with low ionomer content. An optimum I/C of 0.4 in agglomerates is determined by balancing the activated ECSA and oxygen transport resistance. Through the comparison of local transport resistance with limiting current data, an additional oxygen dissolution resistance of 60–350 s m−1 at the pore/ionomer or ionomer/Pt interface is quantified. The lowest local transport resistance of the agglomerate is achieved at about 70%RH, while condensed water at higher humidities hampers oxygen diffusion within the agglomerate, leading to reduced Pt utilization. Agglomerate size significantly affects the local transport resistance only when excessive ionomer or liquid water severely blocks primary pores. Finally, a novel multiscale coupling strategy integrating the porous agglomerate sub-model with a continuous-scale CL model is proposed, offering innovative insights into comprehending the relationship between CL structure and the performance of proton exchange membrane fuel cells.

Effect of Dispersing Solvents for an Ionomer on the Performance of Copper Catalyst Layers for CO2 Electrolysis to Multicarbon Products

Effect of Dispersing Solvents for an Ionomer on the Performance of Copper Catalyst Layers for CO₂ Electrolysis to Multicarbon Products

Effect of Blended Perfluorinated Sulfonic Acid Ionomer Binder on the Performance of Catalyst Layers in Polymer Electrolyte Membrane Fuel Cells.

In this study, blended perfluorinated sulfonic acid (PFSA) ionomers with equivalent weights (EWs, g/mol) of ~1000, 980, and 830 are prepared. Catalyst layers (CLs), using blended PFSA ionomers, with different side chain lengths and EWs are investigated and compared to CLs using single ionomers. The ion exchange capacity results confirm that blended ionomers have the target EWs. As a result, blended ionomers exhibit higher ion conductivity than single ionomers at all temperatures due to the higher water uptake of the blended ionomers. This implies that blended ionomers have a bulk structure to form a competent free volume compared to single ionomers. Blended ionomers with short side chains and low EWs can help reduce the activation energy in proton conduction due to enhanced hydrophobic and hydrophilic segregation. In addition, when using the blended ionomer, the CLs form a more porous microstructure to help reduce the resistance of oxygen transport and contributes to lower mass transfer loss. This effect is proven in fuel cell operations at not a lower temperature (70 °C) and full humidification (100%) but at an elevated temperature (80 °C) and lower relative humidity (50 and 75%). Blended ionomer-based CLs with a higher water uptake and porous CL structure result in improved fuel cell performance with better mass transport than single ionomer-based CLs.

Effects of carbon aggregates and ionomer distribution on the performance of PEM fuel cell catalyst layer: A pore-scale study

An in-depth understanding of the structure-performance relationship of the catalyst layer is crucial for its optimal design. In this work, the effects of carbon aggregation and ionomer distribution morphologies on the performance of the catalyst layer are investigated by pore-scale simulation. First, an optimized stochastic algorithm that can control the carbon aggregation degree and ionomer morphology is proposed to reconstruct realistic catalyst layer structures. The structural characteristics of the catalyst layer, such as pore size distribution, agglomerate size, and ionomer connectivity, are analyzed. A pore-scale model based on the lattice Boltzmann method is developed to simulate the reactive transport processes in the catalyst layer. The results indicate that the reasonable aggregation degree of carbon supports can balance the pore and cross-ionomer transport resistances of the oxygen to achieve optimal catalyst layer performance. The uniform ionomer coating in the catalyst layer can further improve the performance, and the optimal ionomer content is determined by balancing the electrochemical active surface area and the oxygen transport resistance. Based on the simulation results, a key principle for the optimal design of the catalyst layer is also proposed.

Macroporous Carbon Films Support Vs. Conventional Carbon Particles with FeN4 Sites for Oxygen Reduction in Gas-Diffusion-Electrodes

Novel synthetic approaches have been developed to maximize the active site density of atomically dispersed Fe sites coordinated to nitrogen in carbon matrix. (Fe-N-C) (1,2) Nevertheless, the realistic maximum for single iron sites in these catalysts is deduced to be limited to 1021 sites per gram. (3) Therefore, for the application these Fe-N-C catalysts at the cathode for the oxygen reduction reaction in fuel cells, the catalyst material loading has to be increased to be able compete with the performance of Pt/C catalysts. Due to the resulting increased thickness of Fe-N-C catalyst layers, the oxygen available for the ORR at high current densities is dependent on the insufficient transport of oxygen across the catalyst layer. Freestanding catalyst nanostructured films, whose porosity is controlled across the entire thickness of the catalyst layer are expected to improve the mass transport at high current densities. Carbon nanostructures could provide the support substrate for such freestanding catalyst films. For instance, mesophase pitch-based films have been templated by silica (SiO2) nanoparticles have favourable morphologies for supporting Pt. (4) However, harsh conditions and long reaction times are required to etch away the SiO2. In our work, SiO2 nanoparticles are substituted by polystyrene nanoparticles, which decompose and tracelessly evaporate during the carbonization process. We have fabricated carbonized films of multiple thicknesses. We found 100 µm to be optimal, substantially thicker than Pt based electrodes to accommodate the lower density of sites and cheaper catalyst material. which is a usual thickness for Fe-N-C-based cathodes. Iron phthalocyanine (FePc) as a model active site is adsorbed on the freestanding carbon film. Gas-Diffusion-Electrodes (GDE) enable us to investigate the performance of such freestanding thin film catalysts at high current densities relevant for fuel cells. (5) A comparison of the ORR performance of catalyst layers, based on carbon films support (A) and conventional carbon particle support (B) are presented. We discuss the effect of adjusting the thickness of the two types of catalyst layers on the performance.1 Barrio J, Pedersen A, Feng J, Sarma S. C., Wang M, Li A. Y. , Luo, H. Ryan M. P., Titirici M.-M., Stephens I. E. L. J. Mater. Chem. A, 2022;10: 6023-302 Mehmood A, Pampel J, Ali G, Ha H Y, Ruiz-Zepeda F, Fellinger T P Adv. Energy Mater., 2018; 8: 11649-553 Mehmood A, Gong M, Jaouen F, Roy A, Zitolo A, Khan A, Sougrati M.-T., Primbs M., Martinez Bonastre A, Fongalland D, Drazic G, Strasser P, Kucernak A Nat. Cat., 2022;5, 311-234 Atwa M, Li X, Wang Z, Dull S, Xu S, Tong X, Tang R, Nishihara H, Prinz F, Birss V 2021;8: 2451-625 Inaba M, Jesen A W, Sievers G W, Escudero-Escribano M, Zana A, Arenz M Energy Environ. Sci,2018;11: 988-94

Catalyst Layers with Blended Ionomer Using Short- and Long-Side Chain Dispersions

Hydrogen-based energy conversion technology is based on the membrane electrode assembly (MEA) comprising of an electrolyte and a pair of catalyst layers. It should have high efficiency and allow dynamic operation at low temperature. Electrochemical reactions for hydrogen oxidation reaction and oxygen reduction reaction occur in the catalyst layers in the MEA. Catalyst layers for MEA is prepared by catalyst inks comprising of electrocatalyst, ionomer dispersion and additional solvents. Ionomers plays an important roles for the formation of porous structure of catalyst layer for transportation of gas and reactant and for the ionic conduction. The ionomers are divided that long side chain (LSC) and short side chain (SSC) depending on the length of side chain. Long side chain (LSC) ionomers shows high performance at low temperatures and high humidity, while short side chain (SSC) ionomers shows high performance at high temperatures and low humidity. It has been reported that the properties of ionomer dispersions substantially influence the performance of catalyst layers due to the different interaction between electrocatalysts and ionomer. In this study, to investigate the effect of the properties of ionomer, various contents of blended ionomer dispersions were prepared. Properties of single and blended ionomers were characterized using catalysts layers in terms of I-V polarization, cyclic voltammetry, electrochemical impedance spectroscopy and microscopic evaluation such as porosimetry. Acknowledgment This research was supported in part by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning(NRF-2019M3E6A1063677) and by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040520).

Textural effect of Pt catalyst layers with different carbon supports on internal oxygen diffusion during oxygen reduction reaction.

One way to address the cost issue of polymer electrolyte membrane fuel cells (PEMFCs) is to reduce the amount of platinum used in the cathode catalyst layers (CLs). The oxygen mass transfer resistance of the cathode CLs is the main bottleneck limiting the polarization performance of low Pt-loading membrane electrodes at high current densities. Pt nanoparticles, ionomers, carbon supports, and water in cathode CLs can all affect their oxygen mass transfer resistance. From the perspective of carbon supports, this paper changed the texture of CLs by adding carbon nanotubes (CNTs) or graphene oxide (GO) into carbon black (XC72) and studied its impact on the oxygen mass transfer resistance. A mathematical model was adopted to correlate total mass transfer resistance and internal diffusion efficiency factor with CL structure parameters in order to determine the dominant textural effect of a CL. The results show that adding 30%CNT or 20GO to carbon black of XC72 improved the electrocatalytic performance and mass transfer capability of the composite carbon-supported Pt catalyst layers during oxygen reduction reaction. The study further reveals that the smaller particle-sized carbon material with tiny Pt nanoparticles deposition can minimize the internal oxygen diffusion resistance. A less dense CL structure can reduce the oxygen transfer resistance through the secondary pores. The conclusion obtained can provide guidance for the rational design of optimal cathode CLs of PEMFCs.

Design of NiNC single atom catalyst layers and AEM electrolyzers for stable and efficient CO2-to-CO electrolysis: Correlating ionomer and cell performance

Deploying single-site NiNC catalysts in cathode catalyst layers of bipolar electrolyzer cells enables catalytic CO2 valorization to e-CO at industrially relevant yields and efficiencies. The performance of the cathode layers is controlled by the turnover frequency of the active sites as well as mass transfer to and from the active sites. While the atomic scale structure-reactivity relations of single-site NiNC catalysts have been extensively studied, the mass transfer characteristics of single atom catalyst layers were poorly discussed. In this work, we design, build, and test NiNC catalyst layers using a novel set of distinct ion exchange ionomer materials and correlate the performance of cathode catalyst layers with their reactivity and stability in full single MEA electrolyzer cells. The Sustainion anion exchange ionomer delivered optimal performance, yielding about 90% CO faradaic efficiency up to 300 mA cm−2 and 15 h stable performance at 200 mA cm−2. Our analysis attributes its favorable electrolyzer performance to its balanced conductivity and hydrophobicity, which mitigates electrode flooding while ensuring excellent ion and CO2 transfer rates even at high current densities.

Comparison of methods to determine electrocatalysts’ surface area in gas diffusion electrode setups: a case study on Pt/C and PtRu/C

In recent years, gas diffusion electrode (GDE) half-cell setups have attracted increasing attention, bridging the gap between fundamental and applied fuel cell research. They allow quick and reliable evaluation of fuel cell catalyst layers and provide a unique possibility to screen different electrocatalysts at close to real experimental conditions. However, benchmarking electrocatalysts’ intrinsic activity and stability is impossible without knowing their electrochemical active surface area (ECSA). In this work, we compare and contrast three methods for the determination of the ECSA: (a) underpotential deposition of hydrogen (Hupd); (b) CO-stripping; and (c) underpotential deposition of copper (Cuupd) in acidic and alkaline electrolytes, using representative electrocatalysts for fuel cell applications (Pt and PtRu-alloys supported on carbon). We demonstrate that, while all methods can be used in GDE setups, CO-stripping is the most convenient and reliable. Additionally, the application of Cuupd offers the possibility to derive the atomic surface ratio in PtRu-alloy catalysts. By discussing the advantages of each method, we hope to guide future research in accurately determining surface area and, hence, the intrinsic performance of realistic catalyst layers.

Scattering investigations into the structures of polymer-electrolyte-fuel-cell catalyst layers exhibiting robust performance against varying water fractions of catalyst ink solvents

Catalyst layers in polymer electrolyte fuel cells have a performance-influencing hierarchical porous structure comprising Pt-loaded carbon (Pt/C) particles and ionomer molecules. Ionomer thin films are formed on Pt/C particle aggregates by wet-coating solvent-based dispersions of Pt/C and an ionomer. Although the solvent composition critically influences the coatability of the dispersion, its effects on the catalyst layer performance remain unknown owing to hurdles in the evaluation of the ionomer distribution on the particles. We investigate the catalyst layers formed using dispersions of a water–ethanol solvent with varying water fractions. Electrochemical analyses indicate that the different ionomer structures in the dispersion minimally affect the catalyst layer performance after solvent drying. Small-angle scattering and neutron reflectivity measurements reveal the origin of the robust performance of the catalyst layers. The solvent-related difference in the catalyst inks marginally changes the hierarchical structure of the ionomer, resulting in a robust performance with tunable dispersion structures.

Investigation of Liquid Water Behavior and Performance of PEFC Catalyst Layers Using a Microdevice and in-Operando Infrared Microscopy

Studies on water behavior in the vicinity of a catalyst layer (CL) and its effect on the cell performance are important challenges because water management is a key factor to achieve higher performance of polymer electrolyte fuel cells (PEFCs). In this study, a microdevice that contains a catalyst coated membrane (CCM) was developed (1) and in-operando infrared (IR) microscopy was conducted to visualize water behavior on the CL. Two types of CLs contain either carbon (2) or tin oxide (3) as supporting material to investigate the effects of materials on the water behavior and cell performance.The microdevice, which was made of a silicon wafer, contains two microchannels (100 μm-width, 200 μm-pitch) and a current-collecting layer (30 μm-width) on a rib of the cathode plate. The CCM was sandwiched by two plates. The anode channel faced the rib of the cathode. The surface of a cathode catalyst layer was coated with thin Au by sputtering to enhance IR reflectance, which did not change the porous structure of the CL significantly. The surface of the cathode catalyst layer was observed by IR microscopy through the cathode plate.Figure 1 shows liquid water generated during the constant current operation of the device. The catalyst layer contains carbon as the supporting material in this case. The effects of the materials and flow conditions on the water behavior and cell performance were investigated.AcknowledgmentsThis work was supported by NEDO ECCEED’30.References(1) T. Suzuki et al., J. Therm. Sci. Technol., 11 (2016), 16-00370.(2) T. Suzuki et al., Int. J. Hydrogen Energy, 41 (2016), 20326.(3) Y. Chino et al., J. Electrochem. Soc., 162 (2015), F736. Figure 1

Role of Ionomer Dispersion in the Design of Microstructure of Catalyst Layers: Oxygen and Hydrogen Evolution Reactions

Green hydrogen production is importance in the upcoming hydrogen economy era. Proton exchange membrane (PEM) water electrolysis is one of the most important technology to produce the green hydrogen that requires only water and extra electricity supplied from renewable energy. Main component, i.e., membrane electrode assembly (MEA), is a key part consisting of polymer electrolyte membrane and two electrodes which are oxygen evolution reaction (OER) electrode and hydrogen evolution (HER) electrode. Both electrode should be coated on the surface of membranes for better performance and mass production. Catalyst layers for OER and HER electrodes are composed of an electrocatalyst (Pt/C) and proton conducting ionomer. In the previous literature, proton conducting ionomer directly affects their performance and durability. Thus, the optimized design of the microstructure of the catalyst layers is essential. In this study, for the reduction of hydrogen production cost, highly dispersed ionomers were introduced to develop the optimized catalyst layers for OER and HER. The effect of dispersing solvents for ionomers on the performance and durability of catalyst layers was mainly investigated. Developed ionomer dispersions showed higher performance and durability in PEM water electrolysis, which result was made by the electrochemical characterization such as I-V polarization, voltage increasing rate during durability test, and so on as well as the microscopic characterization such as SEM and TEM were carried out to evaluate the effect of ionomer dispersions on the performance and durability of HER electrode in PEM water electrolysis. Acknowledgments This research was supported in part by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT ＆ Future Planning (NRF-2019M3E6A1063677) and by 2022 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.

Effect of ionomer dispersions on the performance of catalyst layers in proton exchange membrane fuel cells

In this study, various ionomer dispersions having different equivalent weights, length of side chains, and dispersing solvents have been extensively analyzed to investigate the effect of the microstructure of catalyst layers on the performance of membrane-electrode assembly in proton exchange membrane fuel cells. Five different commercially available perfluorinated sulfonic acid ionomer dispersions have been used to prepare laboratory-made propylene glycol-based ionomer dispersions. The thickness of self-assembled ultra-thin films on silicon wafer for all types of ionomer dispersions has been measured and correlated mainly with the type of dispersing solvents (i.e., water, 2-propanol, and propylene glycol), the length of ionomer side chain, and the wettability. As a result, the short side chain and the propylene glycol-based ionomer dispersions form thicker films due to the smaller average size of their ionomer aggregates. The thick-ionomer wrapped catalyst layers especially prepared by the propylene glycol-based ionomer dispersions result in no influence on activation loss but approximately 42% less Ohmic loss and 43% higher limiting current density in proton exchange membrane fuel cells. In addition, it is confirmed that the slightly higher performance is exhibited as the thickness of ultra-thin ionomer film increases due to the effect of the length of side chains. The catalyst layers with the ionomer film over 100 nm show the higher performance than those below 100 nm. The MEA using Nafion D2020 having hydrophobic surface shows the lower increasing ratio (2.26%) in HFR than the one using Aquivion D98-25BS having hydrophilic surface due to the difference of the water-retaining ability.