Carina Lagergren’s research while affiliated with KTH Royal Institute of Technology and other places

The kinetics of oxygen reduction reaction (ORR) on Ag and Pt thin-layer electrodes was studied in an anion exchange membrane fuel cell (AEMFC). The two-dimensional nature of these layers minimizes the effects of current distribution and mass transport. The ORR activities were evaluated at 80 °C and 100% RH via polarization curves. Compared to Pt, Ag displays a lower open circuit potential and a lower performance at high voltages. For Ag an anodic peak at 0.82 V was obtained by cyclic voltammetry. This peak is related to the formation of Ag-oxides which were also observed in scanning electron microscopy images. At 100% O2, the Tafel slope for Ag was 160 mV dec⁻¹. For Pt above 0.8 V the slope was 75 mV dec⁻¹. By decoupling the first proton- and electron-transfer step of an associative ORR mechanism, a theoretical model captures the Tafel-slope response of Pt when the first proton transfer is the rate-determining step (rds). If the electron transfer is the rds, the theoretical slope fits well with the Tafel behavior of Ag. In fuel cell conditions, Ag performs better than Pt below 0.5 V, but the stability of Ag is compromised above 0.8 V.

Water is a key factor in anion-exchange membrane fuel cells, since it is both a product and a reactant, and humidifies the membrane and the ionomer phase. To optimize the operation conditions preventing cathode drying and anode flooding, better knowledge on the water transport is needed. In this work, the water transport across an AemionTM membrane is quantified for different applied water partial pressure differences and current densities. Two membrane thicknesses, 25 and 50 μm, are studied, as well as two gas diffusion layers (GDLs) of different hydrophobicity: the hydrophobic Sigracet 25BC treated with polytetrafluoroethylene (PTFE), and Freudenberg H23C2 being hydrophilic as it is not treated with PTFE. The measurements show that having a hydrophilic GDL on both electrodes results in poor electrochemical performance, and restricted water transport. Although the highest water molar flux was observed for hydrophilic GDL on cathode and hydrophobic GDL on anode, the best electrochemical performance was observed for the opposite combination. A water transport model considering absorption/desorption resistance, electroosmotic drag and diffusion was deployed. The best fit of the model to the experimental data was obtained with a water drag coefficient of 2, and almost about 30% difference in absorption/desorption coefficient due to different GDLs.

Low-temperature proton exchange membrane fuel cell (PEMFC) systems operating at <80 °C show limitations such as requirements for large cooling systems, employment of expensive catalysts and challenging water management. Operating the PEMFC at intermediate temperatures (80 - 120 °C) would reduce these limitations. As water is a product in the oxygen reduction reaction on the cathode it is important to understand its management at elevated temperatures to avoid flooding or drying out, especially due to exceeding water boiling temperature. In this work the water transport in PEMFC at 80 – 120 °C is examined by measuring the water content at anode and cathode during operation with sensors at inlets and outlets. This is examined under ambient and pressurized conditions and at relative humidities (RHs) of 40 % and 70 %. The results show that water content at the cathode always increases with rising current due to water production (Figure 1 a). However, the water content at the anode is dependent on the operating condition and is always lower compared to the cathode. Due to water diffusion from the cathode water is also slightly increased on anode at 80 °C, but at 100 and 120 °C negative values at the anode are noted. This means that the water dragged from the anode to the cathode by the migration of protons is larger than the water diffused from cathode to the anode at higher temperatures. This was further evaluated by performing hydrogen pump measurements and elucidated through a physics-based model using COMSOL Multiphysics software. The divergence due to temperature is smaller when the cell is pressurized (Figure 1 b), and negative values at the anode are observed only at 120 °C. Even though the water content is decreasing, the decrease of RH or drying out is never observed at either ambient or pressurized conditions. Water distribution was shown to be unaffected by changing RH of inlet gases, however, the divergence of RH between cathode and anode was not observed at 70 % RH as it is observed at 40 % RH. Overall, it is possible to run the cell at elevated temperatures without danger of drying out at tested pressures and RHs. Figure 1

The ongoing debate around the sustainability of per- and polyfluoroalkyl substances (PFASs) might restrict their use and the production in many sectors [1]. Particularly, in the field of proton exchange membrane fuel cells (PEMFCs), perfluorosulfonic acid (PFSA) ionomers, such as Nafion, represent the current state-of-the-art for membranes and ionomer in the catalyst layers (CLs). In fact, they provide an overall unmatched combination of properties such as ionic conductivity and stability [2,3]. In contrast, hydrocarbon (HC)-based polymers are an attractive alternative to PFSA-based materials, as they have been proven to be valid choices from a performance-standpoint and could potentially overcome the thermal stability limitations of PFSAs [4,5]. However, it has been reported that the conductivity of HC-based polymers is not surpassing that of PFSA-based polymers, especially in the CL, even upon optimization of the formulation [4,6]. Additionally, only a few studies report characterization of HC-MEAs above 80 °C, which is the targeted temperature range for some fuel cell applications [4,7]. In this work we used perfluorosulfonic acid (PFSA)- and sulfonated polyphenylene-based (HC)-polymers as ionomer in the CL and membrane in a fuel cell run above 80 °C. By spray-coating HC-based CLs onto either PFSA- or HC-membranes, we separated the contribution of the membrane from those of the electrodes and we analyzed the behaviour of each component individually. The electrochemical tests were performed between 80 and 120 °C, in a wide range of humidity and pressure levels. They included polarization curves and electrochemical impedance spectroscopy (EIS) in H 2 /air, similarly to our previous publications [8]. Additionally, we performed cyclic voltammetry and EIS to characterize the activity and conductivity of the cathode CL. Finally, by using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), we analyzed cross-sections of sprayed samples before and after the test, to investigate possible changes in the structure. A SEM/EDX cross-section of a sample is shown in Figure 1. The MEA consisted of a PFSA-based membrane (with a mechanical reinforcement) and HC-based CLs. Results suggested that the membrane thickness is unchanged after spraying and that the catalyst layers had a quite compact structure and a thickness of about 9 and 6 μm, for respectively the CL on the left and the one on the right. The EDX elemental analysis confirmed that the fluoride signal was solely present in the membrane, where a PFSA-based polymer was employed, while no appreciable signal was detected in the electrodes given the fluoride-free polymer chosen as ionomer. By methodically analyzing the performance losses of different polymers in the CL and in the membrane, we contribute to directing the research where the greatest limitations are still present, above 80 °C. Figure 1 SEM/EDX cross-section of an in-house sprayed sample (left) and a zoom on the line scan with the corresponding EDX spectra for Pt, F and C (right). While the membrane is PFSA-based, the CL is HC-based. [1] European Chemical Agency, Per- and polyfluoroalkyl substances (PFAS), Regist. Restrict. Intentions until Outcome. (2023). https://echa.europa.eu/registry-of-restriction-intentions/-/dislist/details/0b0236e18663449b (accessed April 10, 2024). [2] Y. Prykhodko, K. Fatyeyeva, L. Hespel, S. Marais, Progress in hybrid composite Nafion®-based membranes for proton exchange fuel cell application, Chem. Eng. J. 409 (2021) 127329. https://doi.org/10.1016/j.cej.2020.127329. [3] D.A. Cullen, K.C. Neyerlin, R.K. Ahluwalia, R. Mukundan, K.L. More, R.L. Borup, A.Z. Weber, D.J. Myers, A. Kusoglu, New roads and challenges for fuel cells in heavy-duty transportation, Nat. Energy. 6 (2021) 462–474. https://doi.org/10.1038/s41560-021-00775-z. [4] H. Nguyen, F. Lombeck, C. Schwarz, P.A. Heizmann, M. Adamski, H.F. Lee, B. Britton, S. Holdcroft, S. Vierrath, M. Breitwieser, Hydrocarbon-based Pemion TM proton exchange membrane fuel cells with state-of-the-art performance, Sustain. Energy Fuels. 5 (2021) 3687–3699. https://doi.org/10.1039/d1se00556a. [5] I. Innovations, Ionomr Innovations’ Pemion® hydrocarbon-based proton exchange membrane and polymer exceed industry durability targets, (n.d.). https://ionomr.com/wp-content/uploads/2023/01/Pemion-Durability-Data_News-Release-and-Technical-Backgrounder_For-Release-Jan19.pdf (accessed April 8, 2024). [6] A. Strong, B. Britton, D. Edwards, T.J. Peckham, H.-F. Lee, W.Y. Huang, S. Holdcroft, Alcohol-Soluble, Sulfonated Poly(arylene ether)s: Investigation of Hydrocarbon Ionomers for Proton Exchange Membrane Fuel Cell Catalyst Layers, J. Electrochem. Soc. 162 (2015) F513–F518. https://doi.org/10.1149/2.0251506jes. [7] Hydrogen and Fuel Cell Technologies Office Energy.gov, Fuel Cell 2016 Multi-Year Research, Development and Demonstration Plan, 2016. https://www.energy.gov/sites/default/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf. [8] M. Butori, B. Eriksson, N. Nikolić, C. Lagergren, G. Lindbergh, R. Wreland Lindström, R.W. Lindström, The Effect of Oxygen Partial Pressure and Humidification in Proton Exchange Membrane Fuel Cells at Intermediate Temperature (80 - 120 ◦C), J. Power Sources. 563 (2023). https://doi.org/10.1016/j.jpowsour.2023.232803. Figure 1

Ionomers based on poly(arylene piperidinium)s with varying ion exchange capacities are evaluated in different combinations of anode and cathode electrodes in anion exchange membrane fuel cells. The operational conditions are chosen with an asymmetrical regime including a dry anode with 50% relative humidity at the inlet, and full humidification at the cathode. Polarization and impedance measurements are carried out in potentiostatic steps within 0.3–0.9 V and distribution of relaxation times analysis is utilized to deconvolute resistance contributions. The results show that the best cell performance is achieved with both electrodes utilizing an ionomer with the highest ion exchange capacity (IEC) of 2.79 meq g⁻¹. Cells built exclusively from this ionomer achieved a peak power density of 1.01 W cm⁻². Deconvolution of the resistance contributions revealed the impact of water content on the effective charge transfer resistance in both electrodes and a diffusion resistance associated with the movement of water from anode to cathode side. The higher conductivity and water uptake of the high IEC ionomer resulted in a reduction of both resistance contributions, leading to the highest performance under the conditions evaluated. These findings provide important insights into how to tailor the electrode layers for optimum fuel cell output.

In the field of low temperature polymer electrolyte fuel cells, anion exchange membrane fuel cells (AEMFCs) have been considered a promising alternative by circumventing the obstacles to commercialization faced by the established PEMFC technology. The application of platinum group metal-free catalysts for the oxygen reduction reaction, the potential to utilize cheaper and more sustainable polymers than the industry benchmark Nafion®, and the generally less corrosive alkaline operation conditions, have all been identified as inherent advantages of the AEMFC technology, potentially enabling more cost-effective and environmental-friendly mass production of AEMFC system. The first major challenge in developing AEMFC systems was the lack of stable and conductive anion exchange polymers to be used as membrane and ionomer materials. This challenge was at least partly overcome by the successful development of a variety of next-generation polymers. Among those, poly(arylene piperidinium) show great potential through the combination an ether-free aromatic backbone for increased mechanical stability with a piperidinium-based cationic groups displaying excellent resistance to hydroxide attack in highly alkaline environment [1]. Furthermore, the crucial parameters ion exchange capacity (IEC), water uptake and dimensional swelling can be finely controlled via copolymerization and partial substitution of the cationic functional group in the repeating unit [2]. In a previous study using a series of poly(terphenyl piperidinium) membranes with different IECs, we established the importance of these three parameters, in particular for the ionomer used in the catalyst layer of the electrode [3]. Furthermore, it was established that water balance of AEMFC systems proved to be one of the greatest challenges in reaching and maintaining high cell performance. The asymmetrical nature of the simultaneous electrochemical production and consumption of water at the anode and cathode, respectively, necessitates fine-tuning the ionomer properties, as well as selecting operational parameters for each electrode individually in order to strike an optimal water balance [4]. In this study, we expand on our previous work by analyzing a matrix of electrode combination based on three poly(terphenyl piperidinium)ionomers with different IEC values. Furthermore, we complemented our analysis by evaluating the effects of substituting a fraction of regular dissolvable ionomer with insoluble non-conformal particles of cross-linked poly(terphenyl piperidinium). The beneficial effects of insoluble non-conformal ionomer particles in the catalyst layer has been previously shown by Varcoe and coworkers utilizing radion-grafted anion exchange membranes [5], and by Holdcroft and coworkers utilizing phenylated poly(phenylene) ionomers for PEMFCs [6]. Different electrode combinations were evaluated while varying operational parameters such as gas flow rates, gas relative humidities and back pressure. The recording of observed cell performances were complemented by in-situ analysis of in-let/out-let gas relative humidities, H 2 /O 2 electrochemical impedance spectroscopy for kinetic analysis and information on ohmic losses, H 2 /Ar electrochemical impedance spectroscopy to measure ionic conductivity, cyclic voltammetry to measure electrochemically active surface area as well as ex-situ characterization of the electrodes before/after the test run. The study was carried out with the aim to establish a correlation between the properties of individually tuned ionomers and operational parameters, and to investigate reasons for performance losses due to mass transport limitations, drop in ionomer conductivity, loss of active area, and decreased kinetics caused by phenyl adsorption. Finally, we demonstrate how the application of individually optimized electrodes using fine-tuned poly(terphenyl piperidinium) based ionomers in conjunction with suitable operating conditions will lead to a drastic increase in fuel cell performance compared to symmetrical non-optimized electrodes. A selection of experimental data is shown in Figure 1. [1] J. S. Olsson, T. H. Pham, P. Jannasch, Adv. Funct. Mater. 2017, 28, 1702758. [2] J. S. Olsson, T. H. Pham, P. Jannasch, Tuning poly(arylene piperidinium) anion-exchange membranes by copolymerization, partial quaternization and crosslinking, J. Membrane Sci., 2019, 578, 183-195. [3] T. Novalin, D. Pan, G. Lindbergh, C. Lagergren, P. Jannasch, R.W. Lindström, Electrochemical performance of poly(arylene piperidinium) membranes and ionomers in anion exchange membrane fuel cells, J. Power Sources, 2021, 507, 230287. [4] D. P. Leonard, S. Maurya , E. J. Park , S. Noh , C. Bae , E. D. Baca , C. Fujimoto and Y. S. Kim , J. Mater. Chem. A , 2020 , 8 , 14135-14144. [5] L. Q. Wang , J. J. Brink , Y. Liu , A. M. Herring , J. Ponce-Gonzalez , D. K. Whelligan and J. R. Varcoe, Energy Environ. Sci., 2017, 10 , 2154-2167. [6] E. Balogun, S. Cassegrain, P. Mardle, M. Adamski, T. Saatkamp, and S. Holdcroft, ACS Energy Lett. 2022, 7, 2070-2078. Figure 1