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Flywheel energy storage systems (FESS) are devices that are used in short duration grid-scale energy storage applications such as frequency regulation and fault protection. The energy storage component of the FESS is a flywheel rotor, which can store mechanical energy as the inertia of a rotating disk. This article explores the interdependence of key rotor design parameters, i.e., shape, operating speed, rotor radius, standby losses, and choice of material, and their influence on the energy storage characteristics of the FESS. Two commercially manufactured metal flywheels with distinct energy storage characteristics are used as case studies to examine the potential benefit of using shape optimization in combination with operating speed, size, and material selection for rotor design. A sequential hybrid optimization strategy that combines a global genetic algorithm with a gradient-based local method is used to solve the rotor shape optimization problem. The choice of an optimal combination of operating speed and rotor radius, together with shape optimization, is demonstrated to provide 21–46% improvements in the energy capacity of two existing commercial FESS designs. Results show that self discharge losses in the rotor can be reduced by designing optimally shaped rotors with large radii operating at low speeds. It is advantageous, on an “energy-per-cost of material” basis, to use steel as the rotor material for optimally shaped flywheels with large radii operated at low speeds. Conversely, aluminium is a better choice of material for flywheels with smaller radii operated at high speeds.

With the increasing demand for clean hydrogen production, both as a fuel and an indispensable reagent for chemical industries, acidic water electrolysis has attracted considerable attention in academic and industrial research. Iridium is a well-accepted active and corrosion-resistant component of catalysts for oxygen evolution reaction (OER). However, its scarcity demands breakthroughs in catalyst preparation technologies to ensure its most efficient utilisation. This minireview focusses on the wet-chemistry synthetic methods of the most active and (potentially) durable iridium catalysts for acidic OER, selected from the recent publications in the open literature. The catalysts are classified by their synthesis methods, with authors’ opinion on their practicality. The review may also guide the selection of the state-of-the-art iridium catalysts for benchmarking purposes.

A multi-dimensional, compressible fluid flow solver, valid in both channel and porous media, is derived by volume-averaging the Navier–Stokes equations. By selecting an appropriate average density/velocity pair, a continuous, stable solution is obtained for both pressure and velocity. The proposed model is validated by studying the pressure drop of two commonly used experimental setups to measure in-plane and through-plane permeability of fuel cell porous media. Numerical results show that the developed model is able to reproduce the experimentally measured pressure drop at varying flow rates. Further, it highlights that previously used methods of extracting permeability, which rely on the use of simplified one-dimensional models, are not appropriate when high flow rates are used to study the porous media. At high flow rates, channel–porous media interactions cannot be neglected and can result in incorrect permeability estimations. For example, at flow rates of 1 SLPM a discrepancy of 12% in pressure drop was observed when using previous permeability values instead of the values obtained in the article using the proposed 3D model. Given that at high flow rate one-dimensional models might not be appropriate, previous estimations of Forchheimer permeability might not be accurate. To illustrate the suitability of the numerical model to fuel cell applications, fluid flow bypass in serpentine and interdigitated fuel cell flow channels is also investigated.

As the proton exchange membrane fuel cell (PEMFC) has improved its performance and power density, the efficiency has remained unchanged. With around half the reaction enthalpy released as heat, thermal gradients grow. To improve the understanding of such gradients, PEMFC component thermal conductivity has been increasingly investigated over the last ten years, and the catalyst layer (CL) is one of the components where thermal conductivity values are still scarce. CLs in PEMFC are where the electrochemical reactions occur and most of the heat is released. The thermal conductivity in this region affects the heat distribution significantly within a PEMFC. Thermal conductivities for a graphitized and a non-graphitized CL were measured for compaction pressures in the range of 3 and 23 bar. The graphitized CL has a thermal conductivity of 0.12 ± 0.05 WK–1m–1, whilst the non-graphitized CL conductivity is 0.061 ± 0.006 WK–1m–1, both at 10 bar compaction pressure. These results suggest that the graphitization of the catalyst material causes a doubling of the thermal conductivity of the CL. This important finding bridges the very few existing studies. Additionally, a 2D thermal model was constructed to represent the impact of the results on the temperature distribution inside a fuel cell.

Geometric tolerance characteristics of metal additive manufactured (AM) parts play a significant role in ensuring the part functionality. In such cases, prior estimation of geometric tolerances, i.e. geometric dimensioning and tolerancing (GD&T) characteristics, can prove vital to reduce part rejection and to minimize material wastage and cost. This article presents a framework to estimate geometric tolerances in laser powder bed fusion (LPBF) processes. For a given geometry, skin model shapes are generated based on material shrinkage and thermo-mechanical simulation. Samples from skin model shapes are utilized for geometric tolerance estimation. A case study is presented to validate the developed framework and demonstrate its applicability in metal AM.

The prediction of pressure and flow distributions inside porous membranes is important if the geometry deviates from single-bore tubular geometries. This task remains challenging, especially when considering local porosity variations caused by lumen- and shell-side membrane skins and macro- and micro-void structures, all of them present in multibore membranes. This study analyzes pure water forward and reverse permeation and backwashing phenomena for a polymeric multibore membrane with spatially-varying porosity and permeability properties using computational fluid dynamics simulations. The heterogeneity of porosity distribution is experimentally characterized by scanning electron microscopy scans and reconstructed cuboids of X-ray micro-computed tomography scans. The reconstructed cuboids are used to determine porosity, pore size distribution, and intrinsic permeability in the membrane’s porous structure in all spatial directions. These position-dependent properties are then applied to porous media flow simulations of the whole membrane domain with different properties for separation layer, support structure, and outside skin layer. Various cases mimicking the pure water permeation, fouling, and backwashing behavior of the membrane are simulated and compared to previously obtained MRI measurements. This work reveals (a) anisotropic permeability values and isoporosity in all directions and (b) differing contributions of each lumen channel to the total membrane performance, depending on the membrane-skin’s properties. This study encourages to pertain the quest of understanding the interaction of spatially distributed membrane properties and the overall membrane module performance of multibore membranes.

Development of an active and durable catalyst for oxygen evolution reaction (OER) is a pivotal part of designing an efficient PEM water electrolyser. Iridium is one of the best available catalysts because of its corrosion resistance and activity [1]. Use of Ir is however, limited by its scarcity, limited durability and high cost. Its deposition on a support is a popular way to improve the available surface area, activity and stability [2]. For applications in acidic water electrolysis, supports with the highest chemical resistance and conductivity are preferred. While different methods have been studied to impregnate a support with catalyst nanoparticles, such as Adams’s fusion method and polyol method, their scalability remains questionable due to intensive resource requirements. In this work, Ir oxide based supported catalysts have been synthesized using the incipient wetness method (IWM), one of the most commonly used method of catalyst synthesis in the industry due to its ease of scalability, limited resource requirement and the opportunity to explore the effect of strong metal support interactions. This method however has not been discussed in literature. In this study, Ir oxide (IrOx) (Ir loading on support = 20 wt. percent) was dispersed on commercial supports (ZrO2, Nb2O5, Ta2O5, ATO) using H2IrCl6.xH2O precursor (99.9% trace metal basis, Sigma Aldrich). The catalysts were calcined at 400oC for 2 h in a muffle furnace to produce Ir oxide based catalysts. Electrochemical measurements were carried out on a standard rotating-disk electrode (RDE) system (PINE Research MSR Rotator), and a three-electrode electrochemical cell in 1.0 M sulfuric acid electrolyte (H2SO4 optima grade, Fisher Scientific). The performance of the aforementioned supported Ir oxide catalyst was compared among themselves, and to benchmark commercial catalysts (Umicore Ir Black and IrOx TKK) using metrics such as mass normalized activity (A/g-1 Ir), ECSA normalized activity (mA/cm2 Ir ECSA), Tafel slope (mV/dec) and charge transfer resistance (Rct) measured during the electrochemical test. It was observed that the activity of IrOx/ZrO2 (415 A/gIr, 5.2 mA/cm2 Ir ECSA) was the best among all the 4 impregnated catalysts, and was in fact, more than an order of magnitude greater than that of IrOx/ATO (30 A/gIr, 0.3 mA/cm2 Ir ECSA) at a potential of 1.53 VRHE. A significant drop in the Tafel slope measured in the potential range of 1.45-1.55 VRHE was observed upon changing the support from ATO (80 mV/dec) to ZrO2 (60 mV/dec) hinting towards a change in the reaction mechanism. IrOx/ATO was used as a baseline due to prevalent recognition of ATO as an excellent support for OER catalysts in the literature. Upon comparison with commercial benchmark catalysts it was observed that the Ir ECSA normalized activity of IrOx/Nb2O5, IrOx/Ta2O5, IrOx/ZrO2 surpasses both Umicore Ir black ( 2.23 mA/cm2 Ir ECSA) and IrOx TKK (1.23 mA/cm2 Ir ECSA) with IrOx/ZrO2 providing the highest activity. While Yttria-stabilized zirconia (YSZ) has been a popular choice as an electrolyte and anode for high temperature SOFC due to its non-reducing nature, high thermal stability and mechanical strength, and acceptable oxygen ion conductivity [3], its application in PEM water electrolysis as catalyst support has not been discussed. In this work, we focus on finding the causes for superior performance of ZrO2 as a support for Ir oxide based OER reaction in acidic conditions through the lens of electrocatalysis. References: [1] X. Li, X. Hao, A. Abudula, and G. Guan, “Nanostructured catalysts for electrochemical water splitting: Current state and prospects,” Journal of Materials Chemistry A, vol. 4, no. 31, pp. 11973–12000, 2016, doi: 10.1039/c6ta02334g. [2] H. Dhawan, M. Secanell, and N. Semagina, “State-of-the-art iridium-based catalysts for acidic water electrolysis: a minireview of wet-chemistry synthesis methods,” Jan. 01, 2021. https://www.ingentaconnect.com/content/matthey/jmtr/pre-prints/content-jm_jmtr_semagapr21 (accessed Mar. 26, 2021). [3] T. K. Maiti et al., “Zirconia- and ceria-based electrolytes for fuel cell applications: critical advancements toward sustainable and clean energy production,” Environ Sci Pollut Res, vol. 29, no. 43, pp. 64489–64512, Sep. 2022, doi: 10.1007/s11356-022-22087-9. Figure 1

Proton exchange membrane (PEM) and anion exchange membrane (AEM) fuel cells (FCs) are the two types of fuel cell devices that electrochemically convert the chemical energy of hydrogen into electricity and heat with water as the only by-product. Due to no requirement of precious and non-renewable platinum as the catalyst material, AEMFCs have attracted great attention in recent years [1,2]. However, water balance between anode and cathode in AEMFCs is more crucial than in PEMFCs, as water not only is produced in the anode, hindering hydrogen transport to the anode catalyst layer, but also functions as reactant in the cathode. Water transport properties of AEMs is one of the key factors affecting water balance between anode and cathode [1]. Accurate measurement of AEM water transport properties is paramount for AEM design and manufacturing to improve AEMFC water management and, in turn, performance and durability. AEMFCs with recently developed PiperION AEMs have been shown to achieve good AEMFC performance [3,4]; however, there is no available study in the literature measuring its water transport properties. To the best of the authors' knowledge, there are only a few studies reporting the measurement of AEMs water diffusivity, such as Fumapem FAA-3 [5,6], Aemion [5], Tokuyama A201 [7,8] and SnowPure Excellion I-200 [9]. Even in those limited studies, interfacial transport rates were either not considered in the data analysis [6,8,9] or not given as a function of water activity [5,7,8]. In this work, the interfacial desorption rate of AEMs is determined from a liquid-vapor permeation setup by measuring the water flux through the membrane at different relative humidity (RH). To quantify the interfacial exchange rate and determine which mode of transport is dominant (bulk or interfacial), a novel approach involving three different mathematical models was used: a diffusion-dominant model, a desorption-dominant model, and a combined diffusion-desorption model. By analyzing the sensitivity of the modeling results to the individual transport process, the dominant mode was identified. The model correctly identified the limiting transport mode in Nafion membranes, and suggested that interfacial transport was also limiting in AEMs of Aemion AH1-HNN8-50-X, Fumapem FAA-3-30/50 and PiperION-A40. With the developed model, semi-empirical relationships for the water desorption rate from AEMs and Nafion membranes as functions of the water content and temperature were obtained. These relationships can be readily used in AEMFCs and PEMFCs models. References [1] K. Yassin, et al., Quantifying the critical effect of water diffusivity in anion exchange membranes for fuel cell applications, Journal of Membrane Science 608 (2020) 118206. [2] X. Luo, et al., Structure-transport relationships of poly (aryl piperidinium) anion-exchange membranes: Eeffect of anions and hydration, Journal of Membrane Science 598 (2020) 117680. [3] J. Wang, et al., Poly (aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells, Nature Energy 4(5) (2019) 392-398. [4] T. Wang, et al., High-performance hydroxide exchange membrane fuel cells through optimization of relative humidity, backpressure and catalyst selection, Journal of The Electrochemical Society 166(7) (2019) F3305. [5] X. Luo, et al., Water permeation through anion exchange membranes, Journal of Power Sources 375 (2018) 442-451. [6] M. Marino, et al., Hydroxide, halide and water transport in a model anion exchange membrane, Journal of Membrane Science 464 (2014) 61-71. [7] Y. Li, et al., Measurements of water uptake and transport properties in anion-exchange membranes, International Journal of Hydrogen Energy 35 (11) (2010) 5656-5665. [8] B. Eriksson, et al., Quantifying water transport in anion exchange membrane fuel cells, International Journal of Hydrogen Energy 44 (10) (2019) 4930–4939. [9] T.D. Myles, et al., Calculation of water diffusion coefficients in an anion exchange membrane using a water permeation technique, Journal of the Electrochemical Society 158(7) (2011) B790.

Lithium-ion battery (LIB) cathodes are porous electrodes made of active material (AM) that stores lithium, a composite of carbon additives and polymeric binder (CBD) that facilitates electron transport and ensures the mechanical integrity of the electrode, and electrolyte-filled pore space that facilitates lithium-ion transport [1]. The volume fraction and morphology of the different constituents at the microscale necessarily determine transport properties and influence the measurable performance [2]. In this work, the impact of NMC electrode microstructure on the effective transport properties is studied using FIB-SEM-based three-dimensional (3D) particle-resolved microscale simulations. The effect of AM and CBD bulk electronic conductivity on the effective electronic conductivity is first studied and used to highlight that the impact of the AM bulk conductivity is negligible compared to that of the CBD. Next, the impact of CBD volume fraction, in isolation from its morphology, is studied using morphological operations by eroding and dilating the CBD phase in the FIB-SEM images and analyzing its impact on the effective conductivity using multiple 3D reconstructions. Increasing the CBD volume fraction results in a nonlinear increase in the effective electronic conductivity. To study the effect of CBD morphology, two stochastic CBD reconstruction techniques are proposed. The first method places new CBD voxels preferentially next to existing CBD voxels, and the second method deposits the CBD randomly in the pore space. The effective electronic conductivity for microstructures containing stochastic CBD morphologies is calculated and compared to that evaluated for microstructures with eroded and dilated CBD. The CBD generated stochastically results in a predicted higher effective electronic conductivity primarily due to a lower CBD tortuosity when compared to the CBD generated with morphological operations. Finally, the impact of CBD porosity on electrode tortuosity is studied by estimating the pore-phase tortuosity considering a solid and a porous CBD. The diffusivity of the porous CBD is estimated using multi-resolution FIB-SEM images. Results show that not accounting for the CBD porosity increases the electrode tortuosity by a factor of up to three at low electrode porosities. References [1] B. L. Trembacki, A. N. Mistry, D. R. Noble, M. E. Ferraro, P. P. Mukherjee, S. A. Roberts, Mesoscale analysis of conductive binder domain morphology in lithium-ion battery electrodes, Journal of The Electrochemical Society 165 (13) (2018) E725–E736 [2] Xu, Hongyi, et al. ‘Guiding the Design of Heterogeneous Electrode Microstructures for Li‐Ion Batteries: Microscopic Imaging, Predictive Modeling, and Machine Learning’. Advanced Energy Materials, vol. 11, no. 19, May 2021, p. 2003908. DOI.org (Crossref), https://doi.org/10.1002/aenm.202003908. Figure 1

In this paper, the hydrogen pump technique is used to study the proton-transport resistance of polymer electrolyte membrane water electrolyzer electrodes. Three catalyst coated membranes made by sandwiching two membranes together were prepared, with two of them including an intermediate pseudo catalyst layer (PCL) with 35 and 55 %wt. ionomer loadings. The proton-transport resistance was calculated by subtracting the overall resistance of the cell without a PCL from that with a PCL. The effect of the ionomer loading on the PCL proton conductivity was studied. As expected, the proton conductivity increased with increasing ionomer loading. The results are in line with the expectation based on the literature data and show that the hydrogen pump technique can be used to obtain the proton-transport resistance of the electrodes.

To help design future amperometric NOx sensors, a physics-based sensor model that includes diffusion and electrochemical submodels is developed. It is shown that NO is partly reduced in the O2 sensing chamber which affects NO concentration in the O2 sensing and in the NOx sensing chamber. Therefore, the electrochemical model is developed to simulate partial reduction of NOx on the O2 sensing electrode and reduction of NOx on the NOx sensing electrode. A transport model that simulates diffusion of the gas species through the sensor diffusion barriers and sensor chambers is coupled to the electrochemical submodels. A fully controlled sensor test-rig that provides controlled gas mixtures is employed to carry out experiments to estimate model parameters. Then, the sensor is installed on the exhaust system of a medium duty Diesel engine and then on a port injection spark ignition engine. Experiments at different engine operating conditions with different NOx concentrations from 0 to 2820 ppm have been performed to validate the model accuracy at different operating conditions. Through the validation process, the NOx sensing cell voltage is changed experimentally at different NOx concentrations to evaluate the model accuracy at different cell voltages. The model results closely match the experiments with the maximum 12% error for the NOx sensing pumping current.

Deterioration of intrinsic activity of Ir-based electrocatalysts during the oxygen evolution reaction (OER) has received much less attention compared to the metal dissolution. Combining chronoamperometry with operando electrochemical impedance spectroscopy and cyclic voltammetry, we show that the deactivation via active site phase transformation from hydrous Ir oxide/hydroxide into anhydrous Ir oxide, is concomitant with the dissolution-induced loss of electrochemical surface area. The relative contributions from these deactivation paths were found to be structure sensitive. Systematic evaluation of different Ir-based catalysts at identical electro-oxidative conditions showed that hydrous IrOx with structural short-range order exhibited an initial minor degradation of intrinsic activity but the most significant dissolution after the extended stability test. In contrast, newly-reported Ir superstructures with higher crystallinity and larger proportion of low-index crystal terminations exhibited enhanced resistance to dissolution but a major degradation of intrinsic activity, as the performance-relevant hydrous oxide/hydroxide species developed only on the surface of metallic Ir. The Ir/IrOx catalyst regeneration was demonstrated.

The precision of LPBF manufactured parts is quantified by characterizing the geometric tolerances based on the ISO 1101 standard. However, there are research gaps in the characterization of geometric tolerance of LPBF parts. A literature survey reveals three significant research gaps: (1) systematic design of benchmarks for geometric tolerance characterization with minimum experimentation, (2) holistic geometric tolerance characterization in different orientations and with varying feature sizes, and (3) a comparison of results, with and without the base plate. This research article focuses on addressing these issues by systematically designing a benchmark that can characterize geometric tolerances in three principal planar directions. The designed benchmark was simulated using the finite element method, manufactured using a commercial LPBF process using stainless steel (SS 316L) powder, and the geometric tolerances were characterized. The effect of base plate removal on the geometric tolerances was quantified. Simulation and experimental results were compared to understand tolerance variations using process variations such as base plate removal, orientation, and size. The tolerance zone variations not only validate the need for systematically designed benchmarks, but also for tri-planar characterization. Simulation and experimental result comparisons provide quantitative information about the applicability of numerical simulation for geometric tolerance prediction for the LPBF process.

Analysis of drop spreading and sliding on solid substrates is critical for many industrial applications, such as microfluidic devices, cooling towers, and fuel cells. A new three-dimensional model is proposed for droplet dynamics. Its numerical solution is obtained by the particle finite element method, based on an updated Lagrangian framework to accurately track the deformation of the droplet. The model hinges on boundary conditions at the solid–liquid interface to account for viscous dissipation and retention forces. These conditions are essential to obtain mesh-independent solutions and a realistic spatiotemporal evolution of the droplet deformation. Several numerical simulations are performed to assess the performance of the model for spreading and sliding drops, and results are compared to experimental data found in the literature. Good agreement is obtained with the available data. Simulations performed in two dimensions show striking discrepancies with the experimental data, thus demonstrating the need for three-dimensional simulations.

Flywheel energy storage systems (FESS) are an excellent short duration grid energy storage solution; however, their cost and energy storage capacity are typical barriers to their widespread commercialization. FESS can be designed by optimizing the shape of the flywheel rotor, choice of rotor material, operating speed and rotor radius. This study optimizes the flywheel rotor shape at various operating speeds and outer radii. It is found that the energy capacity of the rotor can be improved by choosing an ideal combination of operating speed and rotor radius. Our earlier work showed that including the cost of the FESS as an optimization objective could significantly alter the FESS design [1]. Therefore, the cost effectiveness of the FESS is also studied by comparing rotors made from different materials on an energy-per-cost basis, while the cost ratio of the materials is varied.

Metal additive manufacturing (AM) has become a predominant process for manufacturing complex metal parts. However, research on controlling the geometric tolerances of the metal AM printed parts and assemblies is scarce. This paper presents a methodology to conduct a geometric tolerance and manufacturing assemblability study of the parts manufactured by metal AM. An assembly benchmark test artifact (ABTA) is designed to include mating features with given assembly conditions based on geometric tolerancing quantifiers. For virtual analysis, prediction phase ABTA samples are generated by using systematic and random field theory deviations. The prediction phase deviations are then calibrated using deviations from a numerical simulation based on thermo-mechanical finite element model of the part. These samples or ‘skin model shapes’ are subjected to geometric tolerance and assemblability study. For experimental validation of the method, geometric tolerance quantification and actual assembly was conducted on laser powder bed fusion (LPBF) fabricated parts. The comparative analysis of the experimental and virtual results validates the new methodology and its ability to provide reliable information regarding assemblability, size dimensions and geometric tolerances. The method can be extended to any AM process for performing a virtual tolerance and manufacturing assemblability study.

The Particle Finite Element Method (PFEM) is used to develop a model to study two-phase flow in fuel cell gas channels. First, the PFEM is used to develop the model of free and sessile droplets. The droplet model is then coupled to an Eulerian, fixed-grid, model for the airflow. The resulting coupled PFEM-Eulerian algorithm is used to study droplet oscillations in an air flowand droplet growth in a lowtemperature fuel cell gas channel. Numerical results show good agreement with predicted frequencies of oscillation, contact angle, and deformation of injected droplets in gas channels. The PFEM-based approach provides a novel strategy to study droplet dynamics in fuel cells.

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Mahrous, Elaf, et al. “A Particle Finite Element-Based Model for Droplet Spreading Analysis.” Physics of Fluids, vol. 32, no. 4, American Institute of Physics, Apr. 2020, p. 42106, doi:10.1063/5.0006033. and may be found at https://aip.scitation.org/doi/abs/10.1063/5.0006033 A particle finite element method-based model is proposed to analyze droplet dynamics problems, particularly droplet spreading on solid substrates (wetting). The model uses an updated Lagrangian framework to formulate the governing equations of the liquid. The curvature of the liquid surface is tracked accurately using a deforming boundary mesh. In order to predict the spreading rate of the droplet on the solid substrate and track the corresponding contact angle evolution, dissipative forces at the contact line are included in the formulation in addition to the Navier-slip boundary conditions at the solid–liquid interface. The inclusion of these boundary conditions makes it possible to account for the induced Young’s stress at the contact line and for the viscous dissipation along the solid–liquid interfacial region. These are found to be essential to obtain a mesh-independent physical solution. The temporal evolution of the contact angle and the contact line velocity of the proposed model are compared with spreading droplets and micro-sessile droplet injection experiments and are shown to be in good agreement. We are grateful to Dr. Howard Stone, Dr. James Bird, and Dr. Shreyas Mandre for their permission to use Fig. 11(a) published in Ref. 53. We thank the reviewers for their feedback and constructive comments. E.M. is thankful to Dr. Ajay Prasad for the fruitful discussion about the effect of shear stresses on droplet spreading phenomena. E.M. acknowledges the financial support by Jubail University College and the Royal Commission for Jubail and Yanbu of Saudi Arabia. M.S. and A.J. acknowledge financial support from the Natural Science and Engineering Research Council of Canada (NSERC) Collaborative Research and Development, Grant No. NSERC CRDPJ 445887-12, and the NSERC Discovery grant. P.R. was supported by the AMADEUS project (Grant No. PGC2018- 101655-B-I00) funded by the Spanish Ministry of Science, Innovation and Universities. T.C. and A.Z.W. acknowledge financial support by the Fuel Cell Performance and Durability Consortium (FC-PAD) and by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy, under Contract No. DE-AC02-05CH11231.

Most recent research on flywheel rotors has focused on high-speed composite rotors as the storage element of the flywheel energy storage system (FESS). Literature research indicates that this is primarily due to the high specific energy of composites compared to metals. However, a quantitative comparison of the performance of flywheels made from these materials has not been conducted. This paper aims to answer the question - ‘Are composite flywheels better suited for energy storage than metal flywheels?’. This study uses three different performance indices: kinetic energy; specific energy; and, energy per cost, to compare the corresponding rotor designs. A plain-stress, linear elastic mathematical model of the flywheel rotor described by Krack et al. (2010) is used for analysis. Different optimization formulations corresponding to performance indices chosen based on the FESS application are then solved to study optimal FESS designs. The study indicates that for applications where the energy-per-cost is to be maximized, metals are superior to composite rotor materials. On a total energy basis, metals and composites are on par with each other. Composite rotors are however, superior for applications requiring high specific energy. A hybrid rotor, with a metallic energy storage element and a thin composite burst-rim, is also optimally designed and found to be a viable solution, because it offers the cost benefit of metal rotors, as well as the burst-safety provided by composites. Keywords: Flywheel energy storage, Optimization, Rotor materials, Kinetic energy, Specific energy, Energy per cost

Mechanical bearings in a flywheel energy storage system (FESS) may experience unique wear patterns due to the vacuum condition that such systems operate under. The FESS discussed herein uses an aluminum flywheel rotor hub with an integrated shaft and full silicon nitride ceramic bearings. The bearings experienced fretting wear, as is common to many bearing–shaft systems, which eroded the naturally forming oxide layer on the surface of the shaft which was not replaced due to the lack of oxygen. This exposed the soft aluminum surface below creating the opportunity for material transfer between the surfaces and cold welding between components to occur. The existence of fretting and material transfer is demonstrated, and the opportunity for cold welding between components is discussed. The effects of these processes on system components are described. Recommendations to avoid or mitigate fretting and adhesion damage to the system are made for the studied FESS in particular, and, more generally, similar systems operating in vacuum conditions.

Improvements in imaging techniques have enabled the reconstruction of complex porous media which can be analyzed by computer simulations. The two most popular methods for numerical analysis of transport in porous media are direct numerical simulation (DNS) and pore network modeling (PNM). This work aims at assessing the suitability of these techniques to study dry and wet transport properties of porous transport layers for fuel cells and electrolyzers by comparing numerical predictions to experimental data for mercury intrusion, and transport properties. The microstructures of different materials are obtained using micro X-ray computed tomography and characterized by measuring mercury intrusion porosimetry (MIP) curves, dry permeability and diffusivity. Their results are compared to numerically predicted MIP, and dry and wet permeability and diffusivity. Results show that DNS is capable of accurately predicting intrusion, and transport properties without using any fitting parameters. Accurate predictions could be achieved with a PNM when the inscribed diameter method was used for pore size distribution, and the equivalent diameter was used to estimate pore transport properties. While DNS provides more accurate results without necessitating any calibration, a properly constructed PNM is shown to provide relatively good estimations of transport properties at a reduced computational expense.

The performance of platinum group metal-free (PGM-free) catalyst layers suffers from mass transport limitations owing to the thickness required to achieve sufficiently high loading to match the performance of the Pt-based electrodes. A more detailed understanding of the PGM-free electrode structure is of a great importance to further improve their performance, but the nanoscale structure presents a challenge. In the present study, a non-PGM catalyst was synthesized by the sacrificial support method, and the electrospraying technique was used to fabricate catalyst layer electrodes. Electrodes with substantially different structural properties were obtained by varying the electrospraying parameters such as ink flow rate and the distance between the needle and the substrate. A wide range of structural properties of these non-PGM electrodes were experimentally measured, including thickness, porosity, pore size distribution, specific surface area, and the mass transport characteristics in the form of tortuosity. In general, the non-PGM catalyst layers fabricated by the electrospraying technique had much lower tortuosity than conventional catalyst layers due to a combination of highly porous structure and larger interagglomerate pores reducing the impact of the Knudsen effect. Geometric tortuosity was also obtained by adjusting the measured effective diffusivity values to remove the Knudsen effect, and it was found that electrosprayed and conventional layers follow a similar trend with porosity.

An indirectly heated tubular reactor is fabricated and used to study methane thermal decomposition conversion and determine kinetic parameters. A combined perfectly mixed reactor with bypass (CPMR) is proposed as an alternative to the traditional perfectly mixed and plug flow reactors. The CPMR model is used in order to account for buoyancy flow in the reactor. Results comparing the numerical predictions from all three models to experimental data show that buoyancy effects are significant in the reactor under study and also in most reactors in the literature. Including this effect might significantly improve the accuracy of the model predictions. The CPMR reactor model with a reaction rate constant of 5.43 × 1015 1/s and an activation energy of 420.7 kJ/mol is capable of reproducing the obtained experimental data in this study and in the literature.

Since a considerable amount of polymer electrolyte fuel cell (PEFC) stack cost is associated with platinum (Pt) that serves as a catalyst, reduction of Pt loading in the catalyst layers (CLs) without affecting the performance is a major research goal of the fuel cell industry. One of the essential factors affecting the Pt loading in the CLs is the fabrication technique used. Inkjet printing (IJP) is one such technique that can produce CLs with low Pt loading and a high Pt utilization [1-3]. However, a drawback for the IJP CLs, was found to be a reduction in porosity at higher Pt loadings, which corresponds to the increased number of deposited layers [2]. This was a major limiting factor affecting the performance of IJP electrodes, when comparing to conventionally fabricated electrodes [2]. In this work, a modified catalyst ink formulation is used, resulting in CLs that have a higher porosity compared to previous layers. Further, a methodology to determine the CL porosity, based on Archimedes law, is reported. The catalyst ink consisted of 50 wt. % Pt/C catalyst dispersed in a mixture of Isopropanol, 5 wt. % ionomer solution and a diol solvent to increase the ink viscosity. The improvement in porosity was achieved using a higher volatility solvent in the ink. A Fujifilm Dimatix inkjet printer was used for the CL fabrication process. Details regarding the fabrication process can be found in our earlier work [2]. The Pt loading in the CLs was determined by X-ray fluorescence analyzer (Themo Scientific Niton XL3t). The target Pt loading was 0.15 mg/cm2, corresponding to approximately 16 printed layers. For comparison, electrodes using the previous ink recipe were also fabricated and analyzed [2]. The CLs were imaged using a laser scanning microscope to estimate the crack density and a scanning electron microscope (SEM) to determine the CL thickness. In-situ characterization of the electrodes was done in a 40 cm2 cell to compare the performance at different conditions, electrochemical active area and oxygen transport resistance using AFCC testing protocols. For determination of CL porosity, a commercially available density determination kit (Sartorius mechatronics) was modified in-house and used in tandem with a Python based GUI to monitor the sample weight. The setup resembled the one reported by Rashapov et al. [4]. A PTFE substrate was used for the CLs when measuring porosity. Weight of the sample (CL + substrate) was taken in air, n-octane and deionized water. The total pore volume is obtained with the hypothesis that octane intrudes all the pores in the sample, thus reflecting its solid volume fraction, whereas water does not intrude any pores owing to its high surface tension, reflecting the CL bulk volume. To reduce the measurement error, an average of at least three weight readings was taken in each medium. The overall porosity of the IJP CL using the modified recipe was found to be 69.5±2.3 % compared to 51.3±2.2 % using the previous ink recipe, indicating a substantial improvement in the porosity [2]. This is reflected in the preliminary in-situ testing where a gain between 20 – 50 mV is achieved at different operating conditions compared to previous IJP electrodes. CLs using the modified ink recipe showed a power density of 1 W/cm2 at 0.6 V at normal operating conditions (680C, 70% RH), indicating that inkjet printing appears to be a feasible technique for electrode fabrication. Further testing of the CLs is underway. References: 1. Shukla, S., Domican, K., Karan, K., Bhattacharjee, S., & Secanell, M. Electrochimica Acta, 156, 289-300 (2015) 2. Shukla, S., Stanier, D., Saha, M. S., Stumper, J., & Secanell, M. Journal of The Electrochemical Society, 163(7), F677-F687 (2016) 3. Saha, M. S., Malevich, D., Halliop, E., Pharoah, J. G., Peppley, B. A., & Karan, K. Journal of The Electrochemical Society, 158(5), B562-B567 (2011) 4. Rashapov, R. R., Unno, J., & Gostick, J. T. Journal of The Electrochemical Society, 162(6), F603-F612 (2015) Figure 1

Improvements in imaging techniques, such as micro- and nano-computer tomography (CT) and focus ion beam scanning electron microscopy (FIB-SEM), have enabled the reconstruction of complex porous media that can then be analyzed by computer simulation to estimate effective transport properties, e.g., ref. [1, 2]. Experimental setups have also been developed to measure dry and partially-saturated gas diffusivity and permeability of gas diffusion layers (GDLs) and catalyst layer (CLs), e.g., ref. [3, 4]. Based on the available literature, a comparison between micro-scale simulation results and experimental data must be performed to assess the accuracy of direct numerical simulation CT image analysis for dry and partially-saturated transport property estimation. In the later case, the validity of computational methods to predict water intrusion, such as full morphology/morphological image opening [1] and watershed segmentation [5], should also be assessed by comparing mercury intrusion experiments to numerical predictions, and by direct comparison of numerical results to CT images. Finally, these transport properties should be integrated into complete fuel cell models where volume-averaging techniques are used to properly estimate channel-porous media interactions. Such interactions however are seldom studied in literature. The proposed contribution aims at first studying the validity of two popular methods for numerical estimation of effective transport properties from three-dimensional reconstructed porous media, i.e., direct numerical simulation (DNS) and pore network modeling (PNM). Two fuel cell gas diffusion media and an electrolyzer porous transport layer are analyzed by CT and characterized by measuring mercury intrusion porosimetry (MIP), and dry permeability and diffusivity. A comparison of numerical and experimental results shows that DNS tools in OpenFCST [6] are capable of accurately predicting intrusion, and transport properties without using any fitting parameters. Accurate predictions are also achieved with the PNM implementation in OpenPNM [7] when the inscribed diameter method is used to estimate the pore size distribution, and the equivalent diameter is used to estimate pore transport properties. These tools open an avenue for computational design of porous materials. Effective transport properties must be integrated into volume-averaged fuel cell models where porous media-channel interactions are of paramount importance. These interactions however, as well as the method used for volume averaging fuel cell equations, are seldom studied in detail. Therefore, the second contribution of the talk aims at analyzing channel-porous media interactions by developing a compressible volume-averaged channel-porous media model in OpenFCST, and validating it with respect to permeability and diffusion bridge experiments in the literature. Comparison of numerical and experimental results show that, depending on the volume-averaging methodology used, experimentally obtained transport properties do not correspond to the input parameters needed in volume-averaged models. Finally, the numerical model is used to estimate flow by-pass in serpentine and inter-digitated channels, as well as compressibility effects [8]. References: [1] M Sabharwal, JT Gostick, M Secanell Virtual liquid water intrusion in fuel cell gas diffusion media, Journal of the Electrochemical Society 165 (7) (2018) F553. [2] I. V. Zenyuk, D. Y. Parkinson, L. G. Connolly, A. Z. Weber, Gas-diffusion-layer structural properties under compression via X-ray tomography, Journal of Power Sources 328 (2016) 364–376. [3] P. Mangal, L. M. Pant, N. Carrigy, M. Dumontier, V. Zingan, S. Mitra, M. Secanell, Experimental study of mass transport in PEMFCs: Through plane permeability and molecular diffusivity in GDLs, Electrochimica Acta 167 (2015) 160–171. [4] T. G. Tranter, P. Stogornyuk, J. T. Gostick, A. D. Burns, W. F. Gale, A method for measuring relative in-plane diffusivity of thin and partially saturated porous media: An application to fuel cell gas diffusion layers, International Journal of Heat and Mass Transfer 110 (2017) 132–141. [5] J. T. Gostick, Versatile and efficient pore network extraction method using marker-based watershed segmentation, Physical Review E 96 (2) (2017) 1–15 [6] M Secanell, A Putz, P Wardlaw, V Zingan, M Bhaiya, M Moore, J Zhou, Chad Balen, Kailyn Domican, Openfcst: An open-source mathematical modelling software for polymer electrolyte fuel cells, ECS Transactions 64 (3), 655, 2014 and www.openfcst.org [7] Gostick et al. OpenPNM: A pore network modeling package. Computing in Science & Engineering. 18(4), p60-74 (2016) and http://openpnm.org/. [8] A Jarauta, V Zingan, P Minev, M Secanell A Compressible Fluid Flow Model Coupling Channel and Porous Media Flows and Its Application to Fuel Cell Materials, Transport in Porous Media 134 (2) (2020) 351-386.

Due to the high scarcity and cost of the catalysts used in proton exchange membrane water electrolysis (PEMWE), i.e. platinum and iridium, it is of paramount importance to maximise their utilisation and lifespan, particularly for the anode catalyst layer (ACL) where iridium is commonly used to catalyse the oxygen evolution reaction (OER). Maximising utilisation requires understanding how the reaction is distributed within the catalyst layer (CL), which is affected by the layer electronic and protonic conductivity, in addition to the activity of the catalyst [1]. Recently, it has been shown that a CL composed of a commonly used IrOx catalyst from Tanaka Kikinzoku Kogyo (TKK) has an electronic conductivity that is three orders of magnitude lower than the protonic [2]. Such a low conductivity may result in the reaction being extremely concentrated in the ACL and therefore allow for a reduction of the catalyst loading. Such a reduction has been demonstrated in the literature [3], where ACLs with loadings of the order of 0.1 mg/cm2 still provide excellent performance when compared to ACLs with the more commonly used loadings of 1-5 mg/cm2 [4]. The improved performance was attributed to the improved distribution of the catalyst due to the use of an optimised deposition method. The impact of the low electronic conductivity was not studied, as the measured ohmic resistance was dominated by the NRE-117 membrane, and the through plane reaction distribution cannot be determined experimentally. As such, this work uses numerical modelling to investigate the impact of the low electronic conductivity on the ohmic resistance of the cell and on the reaction distribution in the ACL. A two-dimensional, macro-homogeneous PEMWE model is implemented in OpenFCST [5]. Charge transport is accounted for using Ohm’s Law, and multi-step reaction kinetic models are used for the hydrogen evolution reaction [6] and the OER [7]. The conductivities of the protonic and electronic phases are taken from recently published ex-situ measurements [2]. The ohmic heating method [8] is used to compute the voltage losses incurred from charge transport. The numerical model is compared to in-house experimentally obtained polarisation curves using a 5 cm2 cell, using a TKK IrOx catalyser in the ACL and an NRE 211 membrane. The results show a close agreement between the experimentally and numerically obtained polarisation curves, with the electronic transport in the ACL incurring the highest voltage loss in the cell. The reaction distribution shows that it is strongly concentrated at the ACL/porous transport layer interface, due to the low electronic conductivity of the IrOx. The model shows that the catalyst loading of the layer to be reduced from 1 mg/cm2 to 0.025 mg/cm2, without significantly reducing the kinetic performance. The overall resistance of the layer was reduced, though further reductions in loading causes kinetic losses to dominate. These trends are in agreement with the data shown by Taie et al. [3]. However, the concentrated reaction distribution causes large gradients in electronic potential within the ACL. As such, part of the CL experiences potential differences between the phases as large as 1.6 V at 1.8 A/cm2, creating a strongly oxidising environment for the catalyst. Tan et al. [9] showed the TKK catalyst degrades significantly faster at 1.6 V compared to 1.53 V, so high current density operation with this catalyst may cause shorter lifespans. The maximum potential difference experienced by the ACL can be reduced if the conductivities of the phases are of a similar order of magnitude. For example, if the ACL has an electronic conductivity ten times smaller than the protonic, instead of one thousand times [2], but still has the same performance at 1.8 A/cm2, the maximum potential difference is reduced to 1.51 V, which could result in a significantly lower degradation rate [9]. This suggests that the conductivity of the catalyst may be crucial to achieving lower degradation rates. References: [1] K. Neyerlin et al., J. Electrochem. Soc., 2007, 154 B631. [2] M. Mandal et al., ACS Appl. Mater. Interfaces, 2020, 12, 44, 49549–49562 [3] Z. Taie et al. ACS Appl. Mater. Interfaces, 2020, 12, 47, 52701–52712 [4] M. Carmo et al., Int. J. Hy. Ener., 2013, 38(12), 4901–4934 [5] M. Secanell, et al., ECS Trans, 2014, 64 (3) , 655 [6] K. Elbert et al., ACS Catalysis, 2015, 5(11), 6764–6772 [7] Z. Ma et al., J. of Electroanalytical Chem., 2018819, 296–305 [8] A. Kosakian et al., Electro. Acta, 2020, 350, 136204 [9] X. Tan et al., Journal of Catalysis, 2019 371, 57–70 Figure 1

The open circuit voltage (OCV) in a proton exchange membrane fuel cell (PEMFC) is typically recorded as being approximately 300 mV lower than the equilibrium voltage computed by the Nernst equation. While a number of causes have been proposed, the voltage drop is generally attributed to the oxidation of crossover hydrogen in the cathode. A single phase, through-the-channel model is presented that includes hydrogen transport across the membrane, an empirical model for the hydrogen oxidation reaction (HOR) fit to experimental data obtained at high potentials and a multi-step kinetic model to describe the oxygen reduction reaction (ORR). Model predictions were compared to experimentally obtained OCVs and the results show that the model is capable of capturing the experimentally observed changes in OCV with platinum loading, as well as fuel cell performance; and that, at low Pt loadings, small quantities of unreacted hydrogen leave the cathode because the HOR is kinetically limited by oxide blocking and anion adsorption. A parametric study is used to show that a minimum OCV is achieved at ultra-low loadings. Results also show that only a multi-step ORR model can simultaneously capture polarization data and the OCV.

Liquid drops sliding on surfaces are ubiquitous both in the natural and industrial world. The prediction of such drop motions has far-reaching implications in many fields of application, including microfluidics, phase change heat transfer, or coating technology. We present a numerical model based on the particle finite element method for the prediction of the sliding motion of liquid drops. The model includes the effect of a retention force which acts in the vicinity of the drop's contact line. This effect is found to be essential to obtain realistic spatiotemporal evolution of the drop. Thus far limited to two-dimensional simulations, the proposed model is validated by using experimental data found in the published literature, covering a wide range of drop size and physical properties. The numerical results are found to be mesh-independent and in good agreement with the experiments.

Mass transport of oxygen through an ionomer contained within the cathode catalyst layer in an anion exchange membrane fuel cell is critical for a functioning fuel cell, yet is relatively unexplored. Moreover, because water is a reactant in the oxygen reduction reaction (ORR) in alkaline media, an adequate supply of water is required. In this work, ORR mass transport behavior is reported for methylated hexamethyl-p-terphenyl polymethylbenzimidazoles (HMT-PMBI), charge balanced by hydroxide ions (IEC from 2.1 to 2.5 mequiv/g), and commercial Fumatec FAA-3 membranes. Electrochemical mass transport parameters are determined by potential step chronoamperometry using a Pt microdisk solid-state electrochemical cell, in air at 60 °C, with relative humidity controlled between 70% and 98%. The oxygen diffusion coefficient (DbO2), oxygen concentration (cbO2), and oxygen permeability (DbO2·cbO2) were obtained by nonlinear curve fitting of the current transients using the Shoup–Szabo equation. Mass transport parameter...

Image analysis and numerical simulation algorithms are introduced to analyze the micro-structure, transport, and electrochemical performance of thin, low platinum loading inkjet printed electrodes. A local thresholding algorithm is used to extract the catalyst layer pore morphology from focused ion beam scanning electron microscopy (FIB-SEM) images. n-point correlation functions, such as auto-correlation, chord length, and pore-size distribution are computed to interpret the micro-structure variations between different images of the same catalyst layer. Pore size distributions are in agreement with experimental results. The catalyst layer exhibits anisotropy in the through-plane direction, and artificial anisotropy in the FIB direction due to low slicing resolution. Microscale numerical mass transport simulations show that transport predictions are affected by image resolution and that a minimum domain size of 200 nm is needed to estimate transport properties. A micro-scale electrochemical model that includes a description of the ionomer film resistance and a multi-step electrochemical reaction model for the oxygen reduction reaction is also presented. Results show that the interfacial mass transport resistance in the ionomer film has the largest effect on the electrochemical performance.

The introduction of flywheel energy storage systems in a light rail transit train is analyzed. Mathematical models of the train, driving cycle and flywheel energy storage system are developed. These models are used to study the energy consumption and the operating cost of a light rail transit train with and without flywheel energy storage. Results suggest that maximum energy savings of 31% can be achieved using a flywheel energy storage systems with an energy and power capacity of 2.9 kWh and 725 kW respectively. Cost savings of 11% can be obtained by utilizing different flywheel energy storage systems with 1.2 kWh and 360 kW. The introduction of flywheel energy storage systems in a light rail transit train can therefore result in substantial energy and cost savings.