Polymer Electrolyte Fuel Cell Electrodes Research Articles

Three-dimensional nanostructure analysis of non-stained Nafion in fuel-cell electrode by combined ADF-STEM tomography.

The polymer electrolyte fuel cell (PEFC) is one of the strongest candidates for a next-generation power source for vehicles which do not emit CO2 gas as exhaust gas. The key factor in PEFCs is the nano-scaled electrochemical reactions that take place on the catalyst material and an ionomer supported by a carbon support. However, because the nano-scaled morphological features of the key materials in the catalyst compound cannot be observed clearly by transmission electron microscopy, improvement of PEFC performance had been approached by an imaginal schematic diagram based on an electrochemical analysis. In this study, we revealed the nano-scaled morphological features of the PEFC electrode in three dimensions and performed a quantitative analysis of the nanostructure by the newly developed 'Combined ADF-STEM tomography technique'. This method combines information from plural annular darkfield detectors with different electron collection angles and can emphasize the difference of the electron scattering intensity between the ionomer and carbon in the cross-sectional image of the reconstructed three-dimensional (3D) data. Therefore, this segmentation method utilizing image contrast does not require a high electron beam current like that used in energy dispersive X-ray analysis, and thus is suitable for electron beam damage-sensitive materials. By eliminating the process of manually determining the thresholds for obtaining classified component data from grayscale data, the obtained 3D structures have sufficient accuracy to allow quantitative analysis and specify the nano-scaled structural parameters directly related to power generation characteristics.

Investigating Nano-Sized Dispersity and High Crystallinity of Perfluoro-Sulfonic Acidionomer in Polymer Electrolyte Fuel Cell Electrodes Via Electrospray Deposition

Polymer electrolyte fuel cells (PEFCs) hold great promise for mitigating the worldwide climate crisis by tapping into the potential of producing clean energy from renewable sources. However, widespread commercial adoption of PEFCs requires addressing the challenges associated with PEFC performance and durability, particularly concerning electrode materials and design. The PEFC electrode is a crucial component of PEFC performance, with the ionomer playing a vital role as both an electrode binder and proton conductor. Nano-sized dispersity and high crystallinity of the ionomer are vital properties that are crucial for achieving enhanced performance and durability. However, existing methods for fabricating PEFC electrodes have been unable to attain these desirable ionomer characteristics. In this work, we propose a novel approach to fabricating PEFC electrode architectures with ionomers possessing nano-sized dispersity and high crystallinity using the electrospray deposition (ESD) technique. In the ESD process, a precursor solution is atomized into charged droplets, while evaporating the solvent, and results in the formation of advanced electrode with vertical architecture that exhibits high crystallinity of ionomer, in contrast to much lower crystallinity of those fabricated by conventional decal methods. The ionomer characteristics significantly impact the overall fuel cell performance and durability. Therefore, controlling the residual solvent content during the ESD process is a crucial aspect that is vital for achieving the desired ionomer characteristics. The residual solvent content is tuned by investigating the initial mixed solvent ratio, substrate temperature, and distance between the nozzle to substrate. By fine-tuning the residual solvent content and optimizing the deposition parameters, the ESD technique can overcome the limitations of traditional fabrication methods, resulting in PEFC electrodes that offer enhanced performance, increased durability, and improved energy efficiency. The electrospray deposition (ESD) technique provides precise control over the amount of residual solvent required for deposition, making it a scalable technique. This work shows that the residual solvent content of PEFC electrodes significantly impacts the ionomer crystallinity and the electrode architecture using a well-controlled and optimized electrospray technique, without the use of any additives or special ionomer. In summary, this work demonstrates a novel approach to fabricating PEFC electrode architectures with ionomers possessing nano-sized dispersity and high crystallinity using the electrospray deposition technique. This approach provides precise control over the residual solvent content, allowing for enhanced performance and increased durability of PEFC electrodes due to the high-crystallinity of the ionomer. These findings have important implications for the development of sustainable energy technologies and contribute to the ongoing efforts towards mitigating the worldwide climate crisis.

Performance and Durability of Carbon-Free Ionomer-Free PEFC Cathode Using Pt Nanosheet Catalyst

Polymer electrolyte fuel cells (PEFCs) have been attracting attention as highly efficient power sources and have been commercialized for use in residential power plants and automobiles. However, further widespread adoption requires the development of low-platinum catalysts. Additionally, for installation in heavy duty vehicles such as large trucks, high durability at high temperatures is necessary. Pt nanosheets (Pt(ns)), consisting of several atomic layers of Pt with a flat structure, possess high mass activity and durability due to the high specific surface area and low surface energy [1]. Conventional PEFC electrodes consisting of carbon-supported platinum (Pt/C) catalyst and ionomer suffer from degradation due to dissolution/redeposition, Ostwald ripening, and agglomeration of Pt, corrosion of the carbon support, and Pt poisoning by ionomer. Therefore, using unsupported Pt(ns) for an electrode that consists of neither carbon nor ionomer is expected to eliminate the degradation factors, resulting in a highly active and durable electrode. The purpose of this study is to develop highly active and durable electrodes by using Pt(ns) and eliminating degradation factors by making them carbon-free and ionomer-free.Polycrystalline Pt(ns) was synthesized by interlayer oxidation of graphene, as described elsewhere [2]. Catalyst ink was prepared, and cathode catalyst layers were created by dropping or spraying it onto carbon paper. A membrane-electrode assembly (MEA) was made by hot-pressing the cathode catalyst layer together with an anode made of Pt/C onto an electrolyte membrane. Power generation was conducted at 80˚C by supplying H2 to the anode and O2 or air to the cathode, each humidified under ambient pressure. Durability was evaluated by repeatedly applying cyclic potentials between 1.0 V and 1.5 V, supplying N2 to the anode and cathode, and examining changes in the cyclic voltammogram.The I-V curves of MEAs with ionomer-free carbon-free Pt(ns) cathodes fabricated using drop-coating and spray-coating methods and measured using air and oxygen as the cathode supply gases are shown in Fig. 1. As demonstrated in the figure, the activation overpotential at low current density was high, suggesting a low Pt utilization. The shape of the I-V curve of MEA using the spray method at high current density indicated that the concentration overpotential was suppressed under oxygen conditions but was high under air conditions. On the other hand, the concentration overpotential of the MEA using the drop method was high even under oxygen conditions. This may be attributed to the small spaces between the restacked nanosheets, which inhibited diffusion.A start-stop simulated potential cycle test was conducted for the MEA using ionomer-free carbon-free Pt(ns) cathode. The CV curves measured after a certain number of cycles are shown in Fig. 2. As shown in the figure, the change in the shape of the CV was small, and the ECSA remaining rate of the electrode was above 80% even after 8000 cycles, whereas that of the Pt/C cathode was less than 50% after 4000 cycles. This suggests that carbon-free ionomer-free Pt(ns) electrode has the potential as highly durable low Pt electrodes.AcknowledgementPart of this paper is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

Polymer Electrolyte Fuel Cell Electrodes with Improved Durability

Proton exchange membrane fuel cells (PEMFC) are a promising clean energy conversion device for both transportation and stationary power applications. Current PEMFCs contain high loadings of expensive and rare platinum (Pt) based catalysts, which presents an obstacle to widespread commercialization in clean energy applications. While extensive efforts have been devoted to increase the catalytic activity of Pt while reducing the loading and to stabilize Pt particles1, state of the art Pt catalysts typically consist of 2-5 nm Pt nanoparticles supported on carbon (Pt/C). Small Pt particles are vulnerable to degradation due to Ostwald ripening, coalescence, and support corrosion during dynamic operation of the fuel cell. Therefore, before the broader commercialization of PEMFCs can be achieved, the durability and robustness of fuel cell electrodes need to be improved. In this work, we present an alternative novel strategy for improving fuel cell performance durability with a new electrode architecture containing carbon-free catalyst. Acknowledgement: We gratefully acknowledge support from the Laboratory Directed Research & Development Program at Los Alamos National Laboratory.

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.

Optimized Decal Transfer Method for the Mitigation of Incidental Particle Deposition at the Interface of Proton Exchange Membranes and Catalyst Layers

The membrane electrolyte assembly (MEA) is the key component of a proton exchange membrane fuel cell (PEMFC). The MEA usually consists of gas diffusion layers as outer layers to the inner catalyst coated membrane (CCM). There are various methods to prepare CCMs and decal transfer is currently a common method which was first introduced by Wilson and Gottesfeld [1] and further developed by other researchers [2]–[4]. The initial step of this method is that the catalyst ink is coated onto an intermediate substrate material creating a catalyst coated film (CCF). This step is followed by a transfer of the active layer onto the membrane by hot pressing the membrane and CCF between heated press platens maintained at a specific high temperature for an optimized time and pressure to ensure bonding.Extensive studies on the procedure of decal transfer, substrate material and its preparation, catalyst solvents, etc. have increased the transfer yield of decal transfer to more than 95% [5]. Additionally, in the last decade, industrial development has reduced its cost; however, the total cost for fuel cell manufacturing is still relatively high. One reason for this high production cost is the contamination sensitivity of the CCMs by external particles that can potentially be detrimental to fuel cell operation [6]. This requirement necessitates the use of cleanrooms and quality control equipment to prevent contaminants entering the manufactured components, thus increasing the overall costs. The purpose of the present research is to improve the understanding of the interactions between foreign contaminants and the manufacturing process in the context of MEA production quality. The specific objectives are to i) understand the impact of external particles on the catalyst layer decal transfer process and ii) improve the robustness of the decal transfer method in the presence of external particles.This research investigates the impact of incidental external particles that may be found on the membrane surface or the CCF prior to hot pressing. 60µm Silica microspheres (Si-M) were selected as a representative of solid particles and CCMs with purposely introduced Si-Ms were fabricated and several decal transfer methods and support materials were tested and imaged using the X-ray computed tomography (XCT) technique to analyze the impact on the fuel cell integrity and operation at the presence of the Si-M. While regular decal transfer protocols result in excessive membrane thinning in the presence of these particles, it was observed that by changing the rate of applied pressure and using alternative support materials when transferring the cathode catalyst onto a half-coated membrane, it was possible to reduce membrane thinning under the particles by more than 20% while maintaining transfer quality and cell performance (Fig. 1). In addition, it was observed that although anisotropic mechanical properties of the membrane can adversely affect the CCM topography after the decal transfer, a tuned protocol is able to prevent unwanted deformations and potential stress concentrations. Overall, it is envisioned that the outcomes of this work may enable relaxed quality control measures and manufacturing site cleanroom standards by reducing the potential effects of external particles on MEA production quality. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, Ballard Power Systems, and W.L. Gore & Associates. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. Keywords: fuel cell; membrane durability; X-ray computed tomography; decal transfer; manufacturing References Wilson, M. S. & Gottesfeld, S. Thin-film catalyst layers for polymer electrolyte fuel cell electrodes. J. Appl. Electrochem. 22, 1–7 (1992).Shahgaldi, S., Alaefour, I. & Li, X. Impact of manufacturing processes on proton exchange membrane fuel cell performance. Appl. Energy 225, 1022–1032 (2018).Cho, H. J. et al. Development of a novel decal transfer process for fabrication of high-performance and reliable membrane electrode assemblies for PEMFCs. Int. J. Hydrogen Energy 36, 12465–12473 (2011).Thanasilp, S. & Hunsom, M. Effect of MEA fabrication techniques on the cell performance of Pt-Pd/C electrocatalyst for oxygen reduction in PEM fuel cell. Fuel 89, 3847–3852 (2010).Liang, X., Pan, G., Xu, L. & Wang, J. A modified decal method for preparing the membrane electrode assembly of proton exchange membrane fuel cells. Fuel 139, 393–400 (2015).James, B. D., Moton, J. M. & Colella, W. G. Mass Production Cost Estimation of Direct H2 PEM Fuel Cell Systems for Transportation Applications: 2018 Update. ASME 2014 12th Int. Conf. Fuel Cell Sci. Eng. Technol. collocated with ASME 2014 8th Int. Conf. Energy Sustain. V001T07A002–V001T07A002 (2018). Figure 1

The Ionomer Molecular Structure Effect in the PEFC Ink & Applications

The interaction between the perfluorosulfonic acid (PFSA) ionomer and platinum group metal (PGM) catalyst in polymer electrolyte fuel cell (PEFC) inks is a critical issue for designing high-performance PEFC electrodes. During the ink fabrication, the complex interparticle interaction of the ink components determines the agglomerate morphology and size distribution. Among the ink components, the ionomer, which has amphiphilic in nature due to its hydrophobic backbone and the hydrophilic ionic group, mostly effective parameter for interparticle interaction. Therefore, understanding the interaction and molecular structure of ionomer is required for a high-performance catalyst layer. The backbone length of the ionomer which controls the spacing between ionic pendant groups is the main parameter of the ionomer character. In this study, the three different ionic spacing ionomers, made in the Solvay industry: Aquivion D72-25BS, Aquivion D83-24BS, and D98-25BS, were used for investigating the adsorption behavior of the catalyst ink and PEFC catalyst layer morphology. During the fabrication of PEFC ink and catalyst layer, all samples have same sulfonic acid molality to avoid the ionic clustering effect in the side chain. Ex-situ measurements were carried out to quantify the interparticle interaction and agglomerate size distribution in the PEFC ink. The dynamic lights scattering (DLS) carry out to determine the aggregation size distribution. The rheological behavior was conducted to understand the interparticle interactions and the agglomeration behavior of the inks by steady-shear and dynamic-oscillatory-shear measurements. It is noted that increasing the ionomer backbone length, which favors interaction via hydrophobic interaction on a carbon support, suppresses the agglomeration of ink components and decreases ink viscosity. The in situ electrochemical tests were carried out to figure out the relationship between ink formulation and catalyst layer morphology. It was found that the long ionomer backbone length helps to suppress the O2 transport resistance and ORR kinetic loss by uniform distribution from ionomer backbone-carbon supports interaction.

Influence of Carbon Support Type, Presence of Platinum and Ionomer Content on Agglomeration in Catalyst Layers

Polymer electrolyte fuel cell (PEFC) energy conversion systems have found application in transportation applications due to their high power density, low operating temperature, high energy conversion efficiency, and speed of refueling.1 One of the key components of a PEFC is the membrane electrode assembly (MEA), which consists of a proton exchange membrane (PEM), such as a perfluorosulfonic acid (PFSA), sandwiched between two catalyst layers (CLs) serving as anode and cathode. Several studies of CL microstructure−performance relationships suggest that the electrode microstructure characteristics, such as the extent of the catalyst−ionomer interaction, continuity of the ionomer phase, and pore size distribution are critical for performance.2-4 However, successful implementation of novel materials for PEFC electrodes is hindered by the lack of a fundamental understanding of the complex, multicomponent nature of the CL, the interplay of its components, and their independent and cooperative effects on the effective properties and performance.In this work, ultra-small angle X-ray scattering (USAXS) was employed to investigate the effects of carbon support type, presence of platinum, and ionomer loading on catalyst agglomeration in CLs. Particle size distributions (PSDs), obtained from fitting the scattering data using the maximum entropy (MaxEnt) method, were used to determine the size of carbon aggregates and agglomerates from carbon-ionomer and platinum-carbon-ionomer CLs. Three types of carbon supports were chosen for the investigation: high surface area carbon (HSC), Vulcan XC-72, and graphitized Vulcan XC-72. CLs with a range of perfluorosulfonic acid (PFSA) ionomer loadings (0.2-1.0 ionomer to carbon ratio, I/C) were studied in order to evaluate the effect of ionomer on the CL agglomerate structure. The CL agglomerate structure will be correlated with electrochemical performance to reveal the interplay of the components, and their independent and cooperative effects on performance. References B.D. McNicol, D. A. J. Rand, K. R. Williams, J. Power Sources 100 (2001) 47–59.F. C. Cetinbas, X. Wang, R. K. Ahluwalia, N. N. Kariuki, R. P. Winarski, Z. Yang, J. Sharman, and D. J. Myers, J. Electrochem. Soc., 164, (2017) F1596–F1607.Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources 315 (2016) 179-191.S. Holdcroft, Chem. Mater., 26 (2014) 381−393. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the Fuel Cell Performance and Durability Consortium (FC-PAD). The Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science User Facility. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.

Ionomer immobilized onto nitrogen-doped carbon black as efficient and durable electrode binder and electrolyte for polymer electrolyte fuel cells

An implementation of the polymer electrolyte fuel cell (PEFC) electrode, composed of the perfluorinated sulfonic acid (PFSA) ionomer with unprecedented uniform distribution, has recently gained a great attention due to its positive impact on the cell performance as well as durability. However, an issue with the conventional electrodes is a stronger adsorption of sulfonate terminal groups specifically onto Pt catalyst than carbon black (CB) support, that accelerates an inhomogeneous distribution of the PFSA ionomer. In this work, we report a novel PEFC electrode by introducing the ionomer immobilized onto nitrogen-doped carbon black (I/NCB) as electrode binder as well as electrolyte. The carbon atoms adjacent to nitrogen allow a considerable attraction with sulfonated groups over the NCB surfaces, which creates more homogeneous distribution of ionomer when compared with Pt/CB in the conventional electrode. As compared with the conventional electrode, the electrode employing I/NCB presents a superior activity towards oxygen reduction reaction (Pt mass activity increased from 91.3 to 139.6 A g−1) and mass-transport characteristics (mass-transport resistance decreased from 65.9 to 45.5 mΩ cm2), with the cell current density enhanced by 23% at 0.6 V. The enhanced cell durability is confirmed by less degradation in the electrochemically available surface area after the accelerated stress test of carbon support, mainly attributed to the promoted stability of NCB over CB. Given the advances in both performance and durability, the proposed electrode architecture based on I/NCB may provide an alternative pathway in obtaining better electrode kinetics and O2 transport, along with an exceptional resistivity towards the carbon support degradation.

Parameter Optimization in the Drying Process of Catalyst Ink for PEFC Electrode Films with Few Cracks

The fabrication of polymer electrolyte fuel cell (PEFC) electrodes is complicated, and consists of four major steps: dispersion, mixing, coating, and drying. Each step is realized by a combination of various phenomena; hence, it is highly challenging to investigate the entire process in detail. The phenomena occurring during drying are particularly complex, and are primarily responsible for the difficulty with control of the PEFC electrode fabrication process [1]. Such complex factors affect the component distribution and structure of the electrode, and consequently, the performance of the fabricated PEFCs. Optimal drying conditions need to be determined for the stable fabrication of high-quality electrodes; however, it is not practical to simulate all complicated phenomena during the process. This leads to reliance on empirical rules put forth by researchers and engineers.Similar problems have long been encountered in the field of materials. In recent years, the application of machine learning, particularly active learning methods such as Bayesian optimization, has achieved some success [2]. In this study, Bayesian optimization was applied to the drying process of PEFC catalyst ink to determine optimum drying conditions in a few trials. A schematic of the workflow for parameter optimization using the Bayesian method is shown in Fig. 1.The ink was fabricated by dispersing carbon particles (Vulcan XC-72R) and Nafion in water and 2-propanol (IPA). Subsequently, 30 μL of the ink was dropped onto a glass slide placed on a rubber heater. The temperature history to be set for the heater was the parameter to be optimized, and drying was performed with the temperature history indicated by the machine learning system. The heater temperature was controlled for 10 min after the drop, and the temperature was specified in eight steps ranging from 30 °C to 100 °C every 2 min. Thus, there were a total of 85 = 32,768 parameter sets to be explored. The captured images of the dried droplets were binarized to detect cracked areas, and the ratio of the cracked areas to the droplet area was defined as the “crack ratio” and used for evaluation.The typical temperature histories and appearances of the samples with high and low crack ratios are shown in Fig. 2. The temperature history that resulted in a low crack ratio was confirmed by active search to be a globally optimal parameter set. Fig. 2 shows that the samples with high crack ratios exhibit distinctive radial cracks and low-thickness-film regions that spread to the periphery, resulting in a high crack ratio. In the samples with a lower crack ratio, there was no region with a low-thickness film. Examining the temperature history revealed that the crack ratio tended to be lower when the temperature was kept low for the first 2 min, then increased, and subsequently decreased again after another 2 min. We proposed the mechanism shown in Fig. 3 to explain why this temperature history resulted in a low crack ratio. Rapid heating immediately after the drop caused non-uniformity in the film thickness because of the flow of solute particles, but refraining from heating during this period may have allowed the concentration to increase gradually and suppress the free flow of particles. If the concentration was sufficiently high, acceleration of heating had little effect on the film thickness, but if heating continued even at the end of drying, the position of the particles may have been rapidly fixed, and cracks caused by stress in the film may have occurred.Thus, using Bayesian optimization, a highly efficient search was achieved for the drying process parameters of the PEFC catalyst ink, which were previously difficult to adjust. Moreover, the process window discovered by the search was limited, and could not have been found by repeating conventional hypothesis-driven experiments. Using the Bayesian method, we obtained the optimal parameters for a certain condition and examined the mechanism of drying using the information obtained in the process window. There can be further investigations of the drying process to acquire more knowledge about the drying phenomenon; the scope of future studies can also be expanded to include other processes in PEFC electrode film formation or a wider range of parameters.This work was supported by JST-Mirai Program Grant Number JP19216242, Japan. References T. Kusano et al. Structural evolution of a catalyst in for fuel cells during the drying process investigated by CV-SANS. Polym. J. 47, 546-555 (2015).A. G. Kusne et al. On-the-fly closed-loop materials discovery via Bayesian active learning. Nat. Commun. 11, 5966 (2020). Figure 1

Scalable Sacrificial Templating to Increase Porosity and Platinum Utilisation in Graphene-Based Polymer Electrolyte Fuel Cell Electrodes.

Polymer electrolyte fuel cells hold great promise for a range of applications but require advances in durability for widespread commercial uptake. Corrosion of the carbon support is one of the main degradation pathways; hence, corrosion-resilient graphene has been widely suggested as an alternative to traditional carbon black. However, the performance of bulk graphene-based electrodes is typically lower than that of commercial carbon black due to their stacking effects. This article reports a simple, scalable and non-destructive method through which the pore structure and platinum utilisation of graphene-based membrane electrode assemblies can be significantly improved. Urea is incorporated into the catalyst ink before deposition, and is then simply removed from the catalyst layer after spraying by submerging the electrode in water. This additive hinders graphene restacking and increases porosity, resulting in a significant increase in Pt utilisation and current density. This technique does not require harsh template etching and it represents a pathway to significantly improve graphene-based electrodes by introducing hierarchical porosity using scalable liquid processes.

Engineering ionomer homogeneously distributed onto the fuel cell electrode with superbly retrieved activity towards oxygen reduction reaction

Polymer electrolyte fuel cell electrode, especially at lower Pt loadings, suffer from an inevitable performance reduction, primarily as a result of reduced catalytic activity and enlarged mass-transport resistance triggered by ionomer layer enveloping Pt. In course of the conventional slurry method, severe agglomeration leads to formation of larger ionomer particles attributed to electrostatic attraction between sulfonic acid groups and Pt, effectuating catalytic poisoning and creating substantially tortuous pathways for O2. Here, we propose an electrode architecture with infinitesimally thin ionomer deposited onto Pt using electrostatic spray deposition under hot and humid environments. The resultant electrode possesses a hierarchical structure with distinctively homogeneous distribution of ionomer, enabling tripled mass activity and doubled cell performance when compared to the state-of-the-art decal-transferred electrode, and concurrently, improved stability towards carbon degradation. Given the cost-effectiveness and excellent scalability, this work leads the way towards a sustainable trajectory in the long-term targets on global hydrogen mobility.

Inexpensive gram scale synthesis of porous Ti4O7 for high performance polymer electrolyte fuel cell electrodes.

As a fuel cell catalyst support, more than 2 g of Magnéli phase Ti4O7 fine-particles were synthesized in a single reaction via an inexpensive route. The single-cell performance reached that of commercial carbon-supported platinum, with an excellent load cycle durability, one of the highest ever reported for oxide-supported platinum catalysts.

Simultaneous in situ measurements and numerical analysis of mass transfer in polymer electrolyte fuel cell electrode slurries during drying

Simultaneous in situ measurements and numerical analysis of mass transfer in polymer electrolyte fuel cell electrode slurries during drying

Synthesis of Metal and Metal Oxide Nanoparticles Encapsulated in Electron-Conductive Hollow Three-Dimensional Inorganic Structures

Nanoporous materials have attracted substantial interest due to the wide range of applications of these materials in sensing, catalysis, electrocatalysis, water purification, separation, and energy conversion and storage.1-3 There is a persistent need for the development of new heterogenous catalysts, fuel cell cathode materials and materials for Li-ion battery electrodes to address some of the problems encountered with state-of-the-art materials, such as the ionomer poisoning of the polymer electrolyte membrane fuel cell (PEMFC) cathode catalysts and sintering and aggregation of heterogeneous catalysts.4,5 This talk introduces a new class of sintering and aggregation-resistant materials termed “metal and metal oxide nanoparticles encapsulated in electron-conductive hollow 3D inorganic structure” for electrocatalysis and energy storage applications. The developed synthetic strategy is generic, reliable, and scalable. It is a combination of seed-mediated growth, sol-gel chemistry, and chemical vapor deposition (CVD). The prepared nanomaterials are electron conductive and have yolk-like structure. They serve as nanoreactors and have been proven to be accessible to reactant molecules in gaseous and liquid phases and sinter resistant. The electronic structure of the conductive layer has been identified by means of electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), and thermal gravimetric analysis (TGA) to be graphene-like structure with sp2 hybridized bonds.References Freeman, Christopher J., Ahmed A. Farghaly, Hajira Choudhary, Amy E. Chavis, Kyle T. Brady, Joseph E. Reiner, and Maryanne M. Collinson. "Microdroplet-Based Potentiometric Redox Measurements on Gold Nanoporous Electrodes." Analytical Chemistry 88, no. 7 (2016): 3768-3774.Farghaly, Ahmed A., Rezaul K. Khan, and Maryanne M. Collinson. "Biofouling-resistant platinum bimetallic alloys." ACS Applied Materials & Interfaces 10, no. 25 (2018): 21103-21112.Khan, Rezaul K., Ahmed A. Farghaly, Tiago A. Silva, Dexian Ye, and Maryanne M. Collinson. "Gold-Nanoparticle-Decorated Titanium Nitride Electrodes Prepared by Glancing-Angle Deposition for Sensing Applications." ACS Applied Nano Materials 2, no. 3 (2019): 1562-1569.Cetinbas, Firat C., Rajesh K. Ahluwalia, Nancy N. Kariuki, and Deborah J. Myers. "Agglomerates in Polymer Electrolyte Fuel Cell Electrodes: Part I. Structural Characterization." Journal of The Electrochemical Society 165, no. 13 (2018): F1051-F1058.Kongkanand, Anusorn, and Mark F. Mathias. "The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells." The Journal of Physical Chemistry Letters 7, no. 7 (2016): 1127-1137. AcknowledgementsThis work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.

Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells

One of the main challenges of mass deployment of fuel cell electrical cars is the limited power density of the polymer electrolyte fuel cell (PEFC), requiring high loadings of precious Pt metal to drive the oxygen reduction reaction (ORR). Within the catalyst layer of PEFC electrodes, Pt nanoparticles are typically dispersed on porous Ketjenblack carbon due to its high surface area, and the ability of Pt to be deposited into the smaller mesopores of the carbon support; so that they can be protected from ionomer poisoning. Up to 62% of Pt nanoparticles can be buried in the interior of the mesopores1. As the cathode ORR takes place at the triple-phase boundary of the liquid electrolyte, oxygen gas, and solid Pt deposited on the carbon support, a sufficient diffusion of oxygen and protons in the liquid media is vital for the ORR reaction. However, Nafion as the proton conductor cannot penetrate into the micro and mesopores of the support due to the size exclusions; therefore, Pt nanoparticles buried within those pores will not be accessed by the reactants at dry conditions. Studies have shown that electric double layers formed at Pt-water interface can help mediate proton transport increasing proton conductivity by three orders of magnitude. Although, this conductivity is still about two orders of magnitude lower than that of Nafion2 , 3.The principle of electrocatalyst interface design in this work is to impregnate the high surface area (HSA) Ketjenblack carbon pores with a secondary phase, imidazolium-derived ionic liquids (ILs), that can fill the mesopores of the carbon support and provide satisfactory oxygen solubility and proton transfer due to high ΔpKa value and hydrogen-bonded network formation4 , 5. Pt/C-IL catalyst systems are prepared with 1-alkyl-3-metylimidazolium bis(trifluoromethylsulfonyl)imides with short cationic alkyl chain lengths including ([C2mim][NTf2]) and ([C4mim][NTf2]), along with 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim][NTf2]) in order to study the effect of methyl group attachment to the carbon in the 2-position.The effect of the IL molecular structure on the electrochemical behavior of Pt/C catalyst is investigated in half-cell electrochemical set-up, rotating disk electrode (RDE). Figure 1a contains the cyclic voltammogram of pristine and IL-modified Pt/C samples. Pt/C catalyst exhibits electrochemical surface area (ECSA) loss after the IL modification. IL-modified Pt/C samples including Pt/C-[C2mim][NTf2], [C4mim][NTf2], and [C4dmim][NTf2] show ECSA values of 81.29, 78.1 and 78.9 m2 g-1 Pt’ respectively, which are lower than that of the pristine Pt/C. Moreover, IL molecules create a hydrophobic microenvironment that can lead to the repelling of the water molecules from Pt active sites protecting them from being oxidized; therefore, the onset potential of Pt-oxide formation is shifted to higher potentials and the coverage of the non-reactive oxygenated species is decreased, improving the specific activity of the electrocatalysts.Electrochemical impedance spectroscopy (EIS) is also implemented to measure overall effective proton diffusion resistance within the catalyst layers of Pt/C-IL-Nafion and Pt/C-Nafion. The linear portion of the Nyquist plot data at high frequencies shows a noticeably enhanced proton accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion (Figure 1b). References Huang, J., Li, Z. & Zhang, J. Review of characterization and modeling of polymer electrolyte fuel cell catalyst layer: The blessing and curse of ionomer. Front. Energy 11, 334–364 (2017). Zenyuk, I. V & Litster, S. Modeling ion conduction and electrochemical reactions in water films on thin-film metal electrodes with application to low temperature fuel cells. Electrochim. Acta 146, 194–206 (2014). Thompson, E. L. & Baker, D. Proton Conduction on Ionomer-Free Pt Surfaces. (2011). doi:10.1149/1.3635605 Miran, M. S. et al. Key factor governing the physicochemical properties and extent of proton transfer in protic ionic liquids: ΔpKa or chemical structure? Phys. Chem. Chem. Phys. 21, 418—426 (2018). Moschovi, A. M., Dracopoulos, V. & Nikolakis, V. Inter- and Intramolecular Interactions in Imidazolium Protic Ionic Liquids. J. Phys. Chem. B 118, 8673–8683 (2014). Figure 1

(Invited) Towards Addressing Fundamental Manufacturing Questions for Polymer Electrolyte Fuel Cell Electrodes

As a result of the expansion of material handling, backup power, and now, automotive markets for low-temperature fuel cells, the need to understand and overcome challenges to high-volume electrode production has never been greater. OEMs and system integrators globally are addressing these challenges specific to their systems and materials. However, fundamental questions remain to be answered, to the benefit of the entire industry. The interactions between the supported catalyst, ionomer, and dispersing media in the electrode ink, the method of dispersion of the solid constituents of the ink, and the interaction of the ink with the substrate, strongly impact the morphology and ultimately performance of the electrode. The ink formulation must also be appropriate for the coating and drying processes, the physics and parameters of which also impact electrode morphology and performance. Research efforts to address these questions are complicated by multiple pathways to construct the membrane electrode assembly (MEA) – including decal-transfer, catalyst-coated membrane, and gas-diffusion electrode structures – as well as a great variety of coating processes that could be used. These different structures not only potentially affect the performance of the MEA – and differently under different operating conditions – but they affect cost. In this talk, manufacturing pathways and challenges for electrodes will be discussed, and an overview of activities at the National Renewable Energy Laboratory to address these fundamental questions will be given.

Mass transport characterization of platinum group metal-free polymer electrolyte fuel cell electrodes using a differential cell with an integrated electrochemical sensor

A method to enable in place measurements of the mass transport resistance for platinum group metal (PGM)-free polymer electrolyte fuel cell electrodes is presented. Thin platinum black layers deposited at the membrane interface served as electrochemical sensors, performing hydrogen oxidation, for hydrogen probe gas molecules while PGM-free catalyst materials, being electro-inactive to hydrogen oxidation, were probed for mass transport resistance through the full layer. Theoretical considerations, assumptions, and future applications of the methodology are discussed. The method is demonstrated on catalyst layers fabricated from a commercially available PGM-free oxygen reduction catalyst. Effective diffusivity measurements using hydrogen were made which were then used to estimate the effective diffusivity of air of the full layer without the confounding effect of electrode flooding. This method complements alternative techniques such as mercury porosimetry or x-ray computed tomography and can be easily adopted by research groups everywhere to study MEA-level properties of PGM-free catalyst layers and accelerate the development and deployment of PGM-free PEFCs.

(Invited) Structural Characterization of Polymer Electrolyte Fuel Cell Electrodes Using Nano-Scale Synchrotron X-Ray Computed Tomography

With improvements in the intrinsic oxygen reduction reaction activity of both Pt-based and platinum group metal-free (PGM-free) catalysts for polymer electrolyte fuel cells, in recent years more attention is being focused on utilization of the active sites in these catalysts under conditions relevant to the automotive application. Taking full advantage of the performance offered by these high activity catalysts requires comprehensive knowledge of the electrode microstructure and the spatial distribution of the catalyst, support, ionomer, and pores. Nano-scale resolution X-ray computed tomography (nano-CT) has been used as a powerful tool for non-destructive quantification of three-dimensional pore morphology and estimation of ionomer distribution in both Pt-based and PGM-free cathode catalyst layers (CCL) [1, 2]. On the other hand, due to insufficient resolution (20 nm voxel size) this technique cannot resolve the detailed structures of support, ionomer, and catalyst. We developed the “hybrid computational method” [2] combining statistical information from transmission electron microscopy (TEM), ultra-small angle X-ray scattering (USAXS), and BET with nano-CT to reconstruct the features below the nano-CT resolution. In this study, an example will be given of application of this method to a Pt alloy-based electrode. Various shape, size and composition agglomerates are reconstructed to 1 nm voxel resolution using the hybrid methodology. It is shown that ionomer film thickness is spatially non-uniform inside the agglomerates and strongly correlates with ionomer to carbon weight ratio (I/C) while it is independent of the agglomerate size. Compared to experimental observations, the application of capillary condensation theory to the reconstructed agglomerate structure is shown to accurately represent the relative humidity (RH) dependence of the electrochemically-active surface area (ECA). Furthermore, direct numerical simulations are performed to quantify localized transport losses. Effectiveness factor correlations are derived for use in macroscale models. The effects of RH, agglomerate structural properties, and pores in the carbon support on the local oxygen transport resistance are examined. It is demonstrated that low catalyst utilization is responsible for the experimentally-observed high local transport resistance under dry conditions and that agglomerate shape and size affect the local oxygen transport resistance only if the primary pores are poorly accessible.

In-Situ Electrochemical Techniques to Determine Ionomer Coverage in PEFC Electrodes

Many electrochemical devices such as low temperature fuels utilize ionomers to improve efficiency and overall performance. In polymer electrolyte fuel cells (PEFCs), perflurosulfonic acid ionomers improve ionic (protonic) conductivity and Pt utilization within the catalyst layer. However, the strong interaction between sulfonate (-SO3 -) functional groups and Pt nanoparticles have been linked lower catalytic rates and higher O2 transport resistances.1,2 There have been numerous strategies implemented to control the local distribution of ionomer and Pt/C catalyst particles, but limitations in the resolution of state-of-the-art characterization techniques make it difficult to assess how ink formulations and/or electrode fabrication techniques control ionomer-catalyst interactions and its impact device performance.3–5 Herein, the development and utilization of in situ electrochemical techniques designed to probe the local ionomer interactions will be utilized in conjunction with established testing protocols to relate ionomer coverage (on Pt and/or C support) with kinetic performance and O2 transport resistance of fully-conditioned PEFC electrodes. Specifically, CO displacement chronoamperometry and electrochemical impedance spectroscopy techniques will be described to determine local ionomer coverages across a series of Pt/C MEAs. These results will promote a deeper understanding of ionomer-catalyst interactions which in turn will inform future materials development and integration necessary to achieve high performance PGM (e.g. Pt/C and Pt alloy/C) electrocatalyst-based electrodes. Schuler, T. et al. Fuel-Cell Catalyst-Layer Resistance via Hydrogen Limiting-Current Measurements. J. Electrochem. Soc. 166, F3020–F3031 (2019).Subbaraman, R., Strmcnik, D., Paulikas, A. P., Stamenkovic, V. R. & Markovic, N. M. Oxygen Reduction Reaction at Three-Phase Interfaces. ChemPhysChem 11, 2825–2833 (2010).Berlinger, S. A., McCloskey, B. D. & Weber, A. Z. Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation. J. Phys. Chem. B 122, 7790–7796 (2018).Orfanidi, A., Rheinländer, P. J., Schulte, N. & Gasteiger, H. A. Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC. J. Electrochem. Soc. (2018). doi:10.1149/2.1251814jesKhandavalli, S. et al. Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks. ACS Appl. Mater. Interfaces (2018). doi:10.1021/acsami.8b15039