Microstructure and Transport Analyses in Polymer Electrolyte Membrane Fuel Cells: Comparing Modeling Approaches

Challenges and opportunities in modelling of proton exchange membrane fuel cells (PEMFC)

Imaging PEMFC performance heterogeneity by sensing external magnetic field

Recently, we have proposed a robust and computationally efficient numerical algorithm for the optimization of 2-D and 3 -D catalysts distributions in Proton Exchange Membrane Fuel Cells (PEMFCs) using a gradient-based technique [1-3]. The algorithm is based on the computation of the catalyst sensitivity functions of the cell current and cell voltage by using the adjoint method. This method was initially developed by the applied mathematics community to solve optimization problems in fluid dynamics, climate, and heat transfer problems and, later, by other research groups to solve optimization problems in magnetics, model parameter determination, and semiconductor devices [4-6]. It was only recently that COMSOL, a popular finite element solver, has introduced the adjoint method, which has started to be used by the electrochemical community for calculation of sensitivities of platinum distribution in PEMFCs [7,8]. In this presentation we analyze the cell current, cell voltage, and spectral electrochemical impendence properties of PEMFCs by using the catalyst sensitivity functions of cell voltage, γPt(r). These functions show how much the cell voltage is increasing by adding 1 unit amount of platinum at a given location inside the catalyst layer (and assuming the cell is operating at the same current density). For instance, the figure below shows the 2-D catalysis sensitivity functions of the cell voltage for two cells with membrane thicknesses of 60 mm and 25 mm, in which the catalyst is distributed uniformly inside the cell. It is apparent from these figures that in order to increase the cell voltage and consequently the power of the cell, one needs to increase the concentration of Pt particles in the regions under the oxygen openings. As it will be shown at the conference the optimum catalyst distribution also depends on the operating temperature, catalyst and gas diffusion layers porosities, Nafion content, and cell current. In addition to the above analysis, we will also introduce a technique for the analysis of variability of polarization curves induced by the randomness of pore sizes and pore shapes in the catalyst and gas diffusion layers in PEMFCs. The technique is based on the evaluation of sensitivity functions of the cell voltage and can also be extended to the analysis of variability induced by the random platinum distribution in the catalyst layers (which is assumed to have a uniform random distribution). More details about the method for the analysis of variability effects, as well as about the numerical implementation of the adjoint method in the case of finite element simulations and its computational efficiency will be presented at the conference. [1] J. Lamb, G. Mixon, and P. Andrei, “Optimization of polymer electrolyte membrane fuel cells with a large number of degrees of freedom”, ECS 231Meeting, New Orleans, LA, 2017. [2] J. Lamb, G. Mixon, and P. Andrei, “Mathematical optimization of the spatial distribution of platinum particles in the catalyst layer of PEMFCs”, PRIME 2016 Meeting, Honolulu, HI, 2016. [3] P. Andrei, G. Mixon, M. Mehta, and V. Bevara, “Design of the catalyst layers in PEMFCs using an adjoint sensitivity analysis approach”, ECS Transactions 66(8), 91-128, 2015. [4] D. Cacuci, M. Ionescu-Bujor and M. N. Ionel, Sensitivity and Uncertainty Analysis, Volume 2: Applications to Large-Scale Systems, Chapman & Hall/CRC (2005) [5] W. K. Anderson and V. Venkatakrishnan, “Aerodynamic design optimization on unstructured grids with a continuous adjoint formulation”, Computers & Fluids, 28, 443 (1999). [6] P. Andrei, I. Mayergoyz, “Quantum mechanical effects on random oxide thickness and random doping induced fluctuations in ultrasmall semiconductor devices”, Journal of Applied Physics, 94, 7163-7172 (2003). [7] COMSOL Multiphysics Software, https://www.comsol.com. [8] F. C. Cetinbas, S. G. Advani and A. K. Prasad, “Three dimensional proton exchange membrane fuel cell cathode model using a modified agglomerate approach based on discrete catalyst particles”, Electrochimica Acta, 174, 787 (2015). Figure 1

Due to its chemical stability in acidic environments and high ionic conductivity, Nafion has proven to be an essential material in proton exchange membrane fuel cell (PEMFC) designs. Nafion functions as both a membrane – separating the two electrodes – and as a conductive binder in each catalyst layer (CL). Nano-thin films of Nafion ionomer allow protons to reach carbon-supported Pt catalysts. In addition to protons, oxygen and water also move through the nano-thin Nafion ionomer to and from reactive Pt sites. These physical transport processes directly impact PEMFC performance. Therefore, measuring and determining ways to improve upon each of these processes is critical in the development of high performance PEMFCs.In regards to thick Nafion membranes, conductivity and diffusion measurements can be readily performed in most laboratories. Nano-thin Nafion films on the other hand prove more challenging to study due to experimental limitations. Nevertheless, neutron reflectometry (NR) has been reported in literature as a capable technique to provide simultaneous quantitative structural and composition profiles for thin-film Nafion [1-5]. Results for Nafion at native silicon oxide interfaces show complex multi-layered structures forming distinct water-rich and water-poor regions [1-3]. Despite these advancements in our understanding of thin-film Nafion, it is still unclear how these observed structures may impact PEMFC performance. This is because silicon does not interface with Nafion in PEMFC CLs. Instead, carbon and Pt are the primary and secondary materials that interface with the thin-film ionomer. In the latter case, thin-film Nafion at Pt interfaces has been briefly studied in literature and shows significantly less structure than Nafion at silicon oxide interfaces [4,5]. Beyond the limited number of studies for Pt-Nafion interfaces, carbon-Nafion interface studies are even more scarce in literature [5]. This is likely because carbon black (i.e. the electron conductor in PEMFC CLs) is too rough to be used with NR. Therefore, to confidently assume the structure of Nafion at carbon black interfaces, trends from NR work on other carbon-based samples need to be compiled and compared, while considering their bonding structures and surface chemistries.In this work, we extend the understanding of thin-film Nafion at carbon interfaces by testing four different carbon-based substrates. Using thin-film Nafion (< 100 nm) deposited on each carbon sample, NR was performed in both dry and humidified environments. Fitting the data to these experiments, results suggest a variety of polymer structures at the carbon-Nafion interface. These include complex multi-layered structures, similar to those at silicon oxide interfaces, as well as simple homogenous films with little-to-no interfacial structure. During this presentation, the implications of these results on species transport in PEMFC CLs will be discussed. Additionally, PEMFC performance predictions that incorporate learnings from this study will be made using an extended model from our earlier work [6].[1] S.C. DeCaluwe, A.M. Baker, P. Bhargava, J.E. Fischer, and J.A. Dura, “Structure-property Relationships at Nafion Thin-film Interfaces: Thickness Effects on Hydration and Anisotropic Ion Transport,” Nano Energy, vol. 46, pp. 91–100, 2018.[2] J.A. Dura, V.S. Murthi, M. Hartman, S.K. Satija, and C.F. Majkrzak, “Multilamellar Interface Structures in Nafion,” Macromolecules, vol. 42, no. 13, pp. 4769–4774, 2009.[3] U.N. Shrivastava, H. Fritzsche, and K. Karan, “Interfacial and Bulk Water in Ultrathin Films of Nafion, 3M PFSA, and 3M PFIA Ionomers on a Polycrystalline Platinum Surface,” Macromolecules, vol. 51, no. 23, pp. 9839–9849, 2018.[4] V.S. Murthi, J. Dura, S. Satija, and C. Majkrzak, “Water Uptake and Interfacial Structural Changes of Thin Film Nafion Membranes Measured by Neutron Reflectometry for PEM Fuel Cells,” ECS Transactions, vol. 16, no. 2, 2019.[5] D.L. Wood, J. Chlistunoff, J. Majewski, and R.L. Borup, “Nafion Structural Phenomena at Platinum and Carbon Interfaces,” Journal of the American Chemical Society, vol. 131, no. 50, pp. 18096–18104, 2009.[6] C.R. Randall and S.C. DeCaluwe, “Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure–Property Relationship Impacts,” Journal of Electrochemical Energy Conversion and Storage, vol. 17, no. 4, Jan. 2020.

As a critical component, the bipolar plate appreciably affects the performance of proton exchange membrane fuel cell (PEMFC). The flow field structure on the bipolar plate is significant in transporting the internal reaction gas and excess water. In the nature, the leaf veins provide efficient branch networks to the plant, contributing to distribute the nutrients in the whole system. By bionic means, a similar branching structure can be used to design flow channels on the bipolar plates for PEMFC, which helps distribute the reactant gas evenly and manage the water better. In this paper, a three-dimensional isothermal single phase model of the bio-inspired flow channel design based on the leaf vein pattern was presented. The effect of the angle between the main channel and sub-channels on the performance of PEMFC was studied at various cases. A PEMFC of 10.24cm2 activation area was assembled and examined experimentally. The results showed that increasing the angle appropriately was beneficial to improving the uniform oxygen distribution and excess water removal. After comprehensively analyzing the electrochemical performance, species distribution, and power loss of PEMFC, the optimal angle was presented.

Investigating and quantifying the effects of catalyst layer gradients, operating conditions, and their interactions on PEMFC performance through global sensitivity analysis

Vehicular application of Proton Exchange Membrane Fuel Cells (PEMFC) poses durability and stability issues toward the multiple start-stops operation. Start-stop cycle leads to performance decay for PEMFC over time. Considering its reliability, PEMFC's performance recovery becomes a major concern. In electrical vehicle which does not used hydrogen tank due to safety, PEMFC require fuel processor to produce hydrogen as fuel source from ethanol. Output temperature of hydrogen from fuel processor is about 80 °C, which is quite high for PEMFC. Temperature is alleged having an impact on the performance of PEMFC. This research is focused on studying the effect of start-stop cycle and hydrogen temperature on the performance of PEMFC under constant loads using electrochemical characterisation. The number of start-stop cycle was varied by 10, 20 and 30 cycles under constant loads of 1 and 0.5 ampere. The temperature that used for hydrogen was 40° C. The result shows that heating hydrogen to 40 °C does not have significant effect to PEMFC's performance. However, enhancing start-stop cycle number has increased degradation of PEMFC stack.

Gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs) are responsible for diffusion of reactant gases into the catalyst layers, current collection and the removal of produced water. An accumulation of generated liquid water within the GDL, known as flooding, impedes the supply of reactant gas and results in the increase of concentration overpotential. Therefore, understanding of oxygen transport and produced water removal characteristics is required to enhance cell performance. The objective of this study is to investigate the effect of GDL structure and operation conditions on PEMFC performance and liquid water removal, with a particular focus on comparing a novel GDL to a conventional GDL. The results of a simultaneous evaluation of cell performance and liquid water behavior in the GDLs by means of X-ray imaging under operational conditions are presented.

Proton Exchange Membrane Fuel Cells (PEMFCs) are widely regarded as promising clean energy technologies due to their high energy conversion efficiency, low environmental impact, and versatile application potential in transportation, stationary power, and portable devices. Central to the operation and performance of PEMFCs are advancements in materials and manufacturing processes that directly influence their efficiency, durability, and scalability. This review provides a comprehensive overview of recent progress in these areas, emphasizing the critical role of membrane electrode assembly (MEA) technology and its constituent components, including catalyst layers, membranes, and gas diffusion layers (GDLs). The MEA, as the heart of PEMFCs, has seen significant innovations in its structure and manufacturing methodologies to ensure optimal performance and durability. At the material level, catalyst layer advancements, including the development of platinum-group metal catalysts and cost-effective non-precious alternatives, have focused on improving catalytic activity, durability, and mass transport. Similarly, the evolution of membranes, particularly advancements in perfluorosulfonic acid membranes and alternative hydrocarbon-based or composite materials, has addressed challenges related to proton conductivity, mechanical stability, and operation under harsh conditions such as low humidity or high temperature. Additionally, innovations in gas diffusion layers have optimized their porosity, hydrophobicity, and structural properties, ensuring efficient reactant and product transport within the cell. By examining these interrelated aspects of PEMFC development, this review aims to provide a holistic understanding of the state of the art in PEMFC materials and manufacturing technologies, offering insights for future research and the practical implementation of high-performance fuel cells.

Minimizing mass-transport loss in proton exchange membrane fuel cell by freeze-drying of cathode catalyst layers

Non-uniform design of proton exchange membrane fuel cell and its matching with operations

Optimizing proton exchange membrane fuel cells (PEMFC) performance is crucial in order to attain sustainable commercialization. A major part of the issue involves understanding the role of Nafion in the catalyst layer. Nafion’s behavior on bulk mode has been extensively reported in the literature, though on ultra-thin layer mode (<20nm) its structural arrangement which dictates the transport properties highly depends on the type of substrate interfacing [1]. In PEMFC catalyst layers, Nafion covers carbon-supported platinum aggregates on ultra-thin layer mode. In these composite layers, Nafion works as a binder for the aggregates, at the same time as it is meant to ensure pathways for protons and let arrive to the catalyst surface.At high current densities, a steep performance drop is usually observed in PEMFC operation. A part of this drop is reported to be due to activation losses as a doubling of the Tafel slope is observed [2]. The origin of this Tafel slope doubling has been subject of multiple studies. Understanding the causes requires understanding the phenomena occurring at the electrode/electrolyte interface. A technique which is commonly employed is cyclic voltammetry (CV) as it can provide insights regarding these interactions.Comparative CV studies on monocrystalline platinum Pt (111) in a PFSI solution and solution exhibit distinct features, where oxide formation appears to be inhibited at the beginning of this range for PFSI [3]. These CV studies coupled with electrochemical quartz crystal microbalance (EQCM) measurements showed that when extending the potential range to 1.4V and holding the potential at 1.1V (place-exchanged oxide formation range) a large mass gain at 0.5V emerges. The origin of these features is found to be due to strongly adsorbed sulfonate groups on the platinum surface. Other works validate this behavior [4], where sulfonate groups are found to be adsorbed in both the double layer region (0.4-0.5V) and in the hydroxyl adsorption region (0.6-0.85V).In this work, the continuum model proposed by Huang et al. [5] is coupled with a reaction framework comprising multistep mechanism [6]. The adsorption of SO3- is expected to follow a Langmuirian behavior [7], indicating that the oxygen reduction reaction (ORR) is hindered through site blocking. For the adsorption of on Pt(111) a single electron transfer mechanism is assumed [8]. Kinetic parameters are obtained by fitting the model’s response to experimental data as in [6].The simulations allow quantifying the adsorption of sulfonate groups on the platinum surface and estimating its impact on the ORR.

To efficiently and extensively utilize hydrogen for transportation and stationary power generation, the development of low-cost and efficient proton exchange membrane fuel cells (PEMFC) is essential.1 Platinum (Pt) is used as catalyst in PEMFC due to its high performance, but its high cost has been one of the major barriers to the extensive use of PEMFC systems for transportation. In particular, high loadings of Pt are required at the PEMFC cathode due to the sluggish kinetics of the oxygen reduction reaction (ORR).2 One of the strategies adopted to overcome this barrier, is the development of low-cost platinum group metal (PGM)-free catalysts.3 Both the ORR activity and durability of PGM-free catalysts has improved considerably in recent years,4 but the mass activity of these materials remains much lower compared to Pt-based catalysts, requiring the use of higher catalyst loadings on the electrode. As a direct consequence, typical PGM-free catalyst layers (CL) are about 1 order of magnitude thicker than Pt-based ones (~100 µm vs. ~10 µm), creating more challenging conditions for transport of O2 and H+ to the active sites and removal of liquid water within the CL.5 A series of in-situ electrochemical diagnostics methods to measure the mass transport resistance in PGM-fee CLs based on H2 and O2 limiting currents have been developed within the DOE-sponsored ElectroCat consortium.4,6 We will show the application of these methods, in conjunction to other well-established in-situ and ex-situ characterizations (cyclic voltammetry, impedance spectroscopy, SEM, X-ray tomography), to explain the performance trend observed in different PGM-free CLs. We examined the impact of different CL fabrication variables like the ionomer-to-catalyst (I/C) ratio, the ink solvent composition, and the ionomer equivalent weight (EW), evidencing the ones providing harsher conditions for mass transport. The results show the importance achieving optimal transport conditions by selecting a proper combination of these fabrication parameters.7,8 With the aim of improving mass transport and ionic conductivity and expanding the CL operational robustness over a broader range of operating conditions, we developed an innovative electrode architecture having differentiated and ordered domains.9 In particular, we designed a CL divided into alternated catalyst and void domains (grooves). We investigated the fabrication of the groovy CL using different methods and tested the performance under different relative humidity conditions. The results show how the groovy CL structure provides performance enhancements compared to a traditional planar CL in conditions more challenging for mass transport, e.g., at high relative humidity and for electrodes prepared with high I/C and low EW ionomer. In addition, we demonstrated that filling the grooves with a material more hydrophobic than the main catalyst domain (e.g., catalyst mixed with ionomer with high EW and low I/C, or carbon mixed with PTFE) we can largely expand the operational robustness in oversaturated conditions. References D. A. Cullen et al., Nat. Energy (2021).D. Banham et al., Sci. Adv., 4, 1–7 (2018).L. Osmieri et al., Curr. Opin. Electrochem., 25, 100627 (2020).P. Zelenay and D. J. Myers, DOE Annual Merit Review - ElectroCat 2.0 (Electrocatalysis Consortium) (2021).L. Osmieri and Q. Meyer, Curr. Opin. Electrochem., 31, 100847 (2021).A. G. Star, G. Wang, S. Medina, S. Pylypenko, and K. C. Neyerlin, J. Power Sources, 450, 227655 (2020).L. Osmieri et al., Nano Energy, 75, 104943 (2020).G. Wang, L. Osmieri, A. G. Star, J. Pfeilsticker, and K. C. Neyerlin, J. Electrochem. Soc., 167, 044519 (2020).J. S. Spendelow, DOE Annual Merit Review - Accessible PGM-free Catalysts and Electrodes (2021).

In the recent years, by increasing the consumption of fossil fuels, the carbon dioxide level increases, which affects human health and climate change. One of the important technologies to solve these problems is fuel cell vehicles (FCVs), and, due to low operating temperature and high efficiency, the proton exchange membrane fuel cells (PEMFCs) have been more considered among various types of FCVs. Computational fluid dynamics (CFD) and numerical modeling have a great potential to study the detailed physical phenomena in the fuel cells such as mass, heat and energy transport, electrode kinetics, and potential fields and can reveal which factors in a real system are most important from a sensitivity point of view We reviewed several existing works relevant to CFD simulation and mathematical modeling of PEMFCs, which were performed by different software and codes like ANSYS Fluent, COMSOL, lattice Boltzmann, MATLAB codes, and OpenFOAM. The key factors that affect performance and failure modes of PEMFCs were discussed based on these simulations. One of these factors is water management that can be improved by using appropriate flow field configuration. The various configurations were reviewed. In addition, the effect of humidification of inlet reactants on the performance of PEMFCs was discussed.

Lattice Boltzmann Pore-Scale Investigation of Coupled Physical-electrochemical Processes in C/Pt and Non-Precious Metal Cathode Catalyst Layers in Proton Exchange Membrane Fuel Cells