High Temperature Polymer Electrolyte Membrane Research Articles

A pore scale model study of mass transfer and electrochemical reaction in cathode catalytic layer of high-temperature polymer electrolyte membrane fuel cells

The mass transfer and related electrochemical reactions in the catalytic layer (CL) are crucial for the performance of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), but there is a lack of understanding of the optimal phosphoric acid content and Pt/C catalyst loading in the CL. In this work, we investigate the influences of Pt/C catalyst numbers (NPt/C) and phosphoric acid film thickness (δPA) on mass transfer and electrochemical reactions of the cathode CL using a pore-scale model (300 × 300 × 300 nm3). The results reveal that larger NPt/C and δPA can impede oxygen diffusion, while lower NPt/C and δPA lead to fewer reaction active sites and lower conductivity, both of which result in lower current density. Therefore, the balance of three-phase transport is extremely important for CL. There is a 3D volcanic relationship between current density, Pt/C catalysts, and phosphoric acid film. At NPt/C = 200 (7407 per μm3) and δPA = 4 nm (the volume fractions of 37.22 % and 17.87 %), the highest current density can be achieved at a potential of 0.5 V (vs. RHE), which increased with the decrease of potential. This work is valuable for understanding the optimizing the microstructure and three-phase transport pathway of the CL to improve the performance of HT-PEMFCs.

The characteristics of high temperature polymer electrolyte membranes for fuel cell based on 2-pyridene based polybenzimidazole blended with poly(vinyl-phosphonic acid)

The present study has focused on exploring new 2-pyridine-bridge-based polybenzimidazole (2-Py-PBIs) based materials for various energy-related uses in proton exchange membrane fuel cells (PEMFC). An electrochemical device, which transforms chemical energy into electrical energy, is known as fuel cell. Using solution polymerization with polyphosphoric acid as a solvent, a series of 2-Py-PBIs were synthesised from the 4,4'-([2,4′-bipyridine]-2′,6′-diyl)bis (benzene-1,2-diamine). 2-Pyridine bridge Polybenzimidazoles are cross-linked with poly (vinylphosphonic acid), which helps us to improve membrane properties like mechanical properties and proton conductivities. FT-IR was used to characterize chemical structure, Ubbelohde viscometer was employed to determine the inherent viscosity. Additionally investigated were the oxidative stability, swelling ratio, ion exchange capability, and water uptake for 2-Py-PBIs. Thermogravimetric analysis is used to evaluate thermal stability. The obtained 2-Py-PBIs membranes were thermally stable and mechanically strong when compared with conventional polybenzimidazole-based membranes. The 2-Py-PBIs:PVPA membranes showed proton conductivity between 0.10 µS/m to 4.65 µS/m.

Ultra‐low‐Potential Methanol Oxidation on Single‐Ir‐Atom Catalyst

Methanol oxidation plays a central role to implement sustainable energy economy, which is restricted by the sluggish reaction kinetics due to the multi‐electron transfer process accompanied by numerous sequential intermediate. In this study, an efficient cascade methanol oxidation reaction is catalyzed by single‐Ir‐atom catalyst at ultra‐low potential (< 0.1 V) with the promotion of the thermal and electrochemical integration in a high temperature polymer electrolyte membrane electrolyzer. At the elevated temperature, the electron deficient Ir site with higher methanol affinity could spontaneous catalyze the CH3OH dehydrogenation to CO under the voltage, then the generated CO and H2 was electrochemically oxidized to CO2 and proton. However, the methanol cannot thermally decompose with the voltage absence, which confirm the indispensable of the coupling of thermal and electrochemical integration for the methanol oxidation. By assembling the methanol oxidation reaction with hydrogen evolution reaction in the HT‐PEME with single‐Ir‐atom catalysts in the anode chamber, a max hydrogen production rate reaches 18 mol gIr‐1 h‐1, which is much greater than that of Ir nanoparticles and commercial Pt/C catalyst. This study also demonstrated the electrochemical MOR activity of the single atom catalysts, which broadens the renewable energy devices and the catalyst design by an integration concept.

Conquering Heavy-Loading Decay of High-Temperature Polymer Electrolyte Membrane Fuel Cells with a Conductive Polymer-Based Phosphoric Acid Reservoir

Conquering Heavy-Loading Decay of High-Temperature Polymer Electrolyte Membrane Fuel Cells with a Conductive Polymer-Based Phosphoric Acid Reservoir

Sulfonated poly(ether ether ketone)-Protic ionic Liquid-Based anhydrous Proton-Conducting composite polymer electrolyte membranes for High-Temperature fuel cell applications

A high-temperature polymer electrolyte membrane fuel cell (PEMFC) has enormous potential to produce clean energy with minimal environmental pollution along with high energy density and conversion efficiency. In the present work, anhydrous proton-conducting polymer electrolyte membranes for prospective high-temperature fuel cell application were successfully prepared using different protic ionic liquids (PILs) and sulfonated poly(ether ether ketone) (SPEEK). The protic ionic liquids (PILs), 2-hydroxyethylammonium formate, diethylmethyl ammonium triflate, 1-ethylimidazolium bis(trifluoromethylsulfonyl)imide, 1,2-dimethyl imidazolium bis(trifluoro methylsulfonyl)imide, and diethylmethylammonium methanesulfonate were selected based on their favorable physicochemical properties. The effect of the incorporation of the PIL on the structural, thermal, and electrical transport properties of the composite SPEEK polymer electrolyte membranes was studied. The prepared polymer electrolyte membranes (PEMs) were characterized using Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffractometry (XRD), and electrochemical impedance spectroscopy (EIS) techniques. FTIR results indicated an interaction between the PIL and the SPEEK, which resulted in homogeneous and flexible membranes. The DSC results confirmed good miscibility and elucidated plasticizing effect of the incorporated PIL on the SPEEK polymer matrix. All the PEMs showed good thermal stability and high-temperature anhydrous proton conductivity, achieving best proton conductivity, ≈8 × 10–3 S cm−1 at 120 °C and low activation energy ≈0.26 eV, making it suitable for prospective high-temperature PEMFC applications.

TiO2 nanolayer coated carbon support for highly durable high-temperature polymer electrolyte membrane fuel cell cathode

Carbon corrosion of Pt/C electrocatalyst in the cathode is a critical issue for the practical application of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Herein, we introduce a nanolayer composed of TiO2 nanoparticles between Pt and carbon support, which not only protects the carbon from corrosion but also forms a strong metal-support interaction between Pt and TiO2. The Pt nanoparticles are uniformly dispersed on the prepared C@TiO2 support (mean size: 2.27 nm). After potential sweeping for 5000 cycles in the acid solution, the electrochemical surface area loss of the homemade Pt/C@TiO2 is 10.9%, which is approximately half of that of commercial Pt/C (19.1%). In the practical HT-PEMFC single cell, Pt/C@TiO2 exhibits superior durability than Pt/C as well. After the accelerated durability test, the maximum power density of Pt/C@TiO2-MEA decreases by 8.6%, which is significantly lower than that of Pt/C-MEA (23.9%), suggesting a great potential application of Pt/C@TiO2 in HT-PEMFCs.

Comparative analysis of degradation mechanisms in HT-PEM fuel cells under start–stop and load cycling with hydrogen and nitrogen-diluted feeds

The integration of reformers into high-temperature polymer electrolyte membrane (HT-PEM) fuel cells presents a significant opportunity to improve fuel flexibility and cost-effectiveness. To understand their degradation, we applied two start–stop cycling procedures: one with pure H2 and another with an 80 vol % H2 and 20 vol % N2 mix to simulate reformate gas. We also performed a load cycling test with H2 as anode gas. The results indicated that degradation rate of pure H2 start–stop cycling showed 306 μV h−1, while load cycling demonstrated a degradation rate of 19.7 μV h−1 for the current density of 0.2 A cm−2, and 88.2 μV h−1 for the current density of 0.4 A cm−2. Start–stop cycling with diluted gas resulted in a constant voltage drop with a degradation rate of 3.129 mV h−1. Electrochemical impedance spectroscopy (EIS) indicated an increased ohmic resistance during start–stop cycling and higher charge transfer (high-frequency) and mass transport (low-frequency) resistance with nitrogen introduction. Scanning electron microscope results confirmed MEA degradation. In conclusion, start–stop cycling and anode gas dilution are stressors that accelerate HT-PEM fuel cell degradation, reducing its lifetime.

Using distribution of relaxation times to separate the impedances in the membrane electrode assembly for high-temperature polymer electrolyte membrane fuel cells

In the membrane electrode assembly (MEA) of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), many types of resistance existed, but they have not been distinctly characterized in previous research. In this study, the distribution of relaxation times (DRT) methodology is employed, in conjunction with a home-built electrochemical electrode analysis system (EEAS), to figure out the specific resistance components in the cathode gas diffusion electrode (GDE). The impedance analysis by the DRT method was conducted using the electrochemical impedance spectrum obtained at different temperatures and current densities of the cathode only, isolated from the MEA environment. The four peaks (designated as Peak 0 to 3) were depicted at DRT plots of cathode GDE below 103 Hz. By comparing the DRT plots for different conditions and considering the EEAS structure, the four peaks were explained to be caused by distinct resistances occurring at the electrode and interface. The regions of each peak were distinguished according to their position changes with temperature and current density. Furthermore, by contrasting the DRT plots of the cathode GDE with those of MEA containing the same cathode, an additional peak (referred to as Peak 4) is identified between 103 Hz and 104 Hz, which was attributed to resistance originating from the anode. This study could provide a new approach for future research of new catalysts and electrodes utilizing the resistance analysis of MEA for HT-PEMFC.

Elucidating the Complex Oxidation Behavior of Aqueous H3PO3 on Pt Electrodes via In Situ Tender X-ray Absorption Near-Edge Structure Spectroscopy at the P K-Edge.

In situ tender X-ray absorption near-edge structure (XANES) spectroscopy at the P K-edge was utilized to investigate the oxidation mechanism of aqueous H3PO3 on Pt electrodes under various conditions relevant to high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) applications. XANES and electrochemical analysis were conducted under different tender X-ray irradiation doses, revealing that intense radiation induces the oxidation of aqueous H3PO3 via H2O yielding H3PO4 and H2. A broadly applicable experimental procedure was successfully developed to suppress these undesirable radiation-induced effects, enabling a more accurate determination of the aqueous H3PO3 oxidation mechanism. In situ XANES studies of aqueous 5 mol dm-3 H3PO3 on electrodes with varying Pt availability and surface roughness reveal that Pt catalyzes the oxidation of aqueous H3PO3 to H3PO4. This oxidation is enhanced upon applying a positive potential to the Pt electrode or raising the electrolyte temperature, the latter being corroborated by complementary ion-exchange chromatography measurements. Notably, all of these oxidation processes involve reactions with H2O, as further supported by XANES measurements of aqueous H3PO3 of different concentrations, showing a more pronounced oxidation in electrolytes with a higher H2O content. The significant role of water in the oxidation of H3PO3 to H3PO4 supports the reaction mechanisms proposed for various chemical processes observed in this work and provides valuable insights into potential strategies to mitigate Pt catalyst poisoning by H3PO3 during HT-PEMFC operation.

Investigating the Impact of Catalyst Penetration into Gas Diffusion Layer on the Performance of High-Temperature Polymer Electrolyte Membrane Fuel Cells

The catalyst fabrication method, cell assembly, and operating conditions in polymer electrolyte membrane fuel cells (PEMFC) impact the catalyst penetration into the gas diffusion layer (GDL), alter its porous structure, and, consequently, the overall cell performance. This study investigates the effect of the catalyst layer (CL) penetration thickness, catalyst loading amount, and cell compression during assembly on species and current distributions, and overall cell performance. GDLs with large penetration thickness show a substantial resistance to reactant and proton transport, particularly at high current densities resulting in a drop in the cell performance. For zero, 50%, and 100% penetrations, the average current densities at an operating voltage of 0.4 V are 0.8329, 0.7920, and 0.71112 A cm−2, respectively. This indicates a performance loss of 5% and 15% for 50% and 100% penetrations in comparison to zero penetration. Higher catalyst loading results in greater penetration, negating the benefit of enhanced kinetics. Performance typically decreases by 3%–5% for 50% penetration and 12%–15% for 100% penetration when penetration levels increase for a certain Pt loading. An attempt is made to investigate the interplay between the effect of reactant and proton transport limitations on their distributions and cell performance. The combined effect of catalyst penetration and cell compression during the assembly has a crucial impact on cell performance with the starvation of reactants at high-density regions. The study highlights the necessity of optimizing the penetration thickness, catalyst loading, and cell assembly to achieve maximum cell performance.

Assessment of polybenzimidazole/MOF composite membranes for the improvement of high-temperature PEM fuel cell performance

This study aims to determine the most effective utilization of ZIF-8 type metal-organic framework (MOF) doped polybenzimidazole (PBI) composite membrane in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC) and investigate how ZIF-8 filler affects performance. ZIF-8 particles were prepared by solvothermal method and added to the PBI polymer using a weight percentage varying from 1 to 5 %. XRD, BET, and TEM examined the prepared ZIF-8. Composite membrane properties were investigated by XRD, SEM analysis, proton conductivity measurements, acid doping, and acid stripping tests. The HT-PEMFC performances of the membranes were carried out using Hydrogen and dry air at 150–180. The highest performance was acquired with the composite ZIF8/PBI-2 membrane as 0.432 W/cm2 at 170 °C. The obtained result is explained by easier proton transfer over ZIF-8's enlarged tunnel network. This study proposes a promising strategy to use ZIF-8 to prepare a PBI composite membrane with excellent proton conductivity, acid doping, and low acid leaching for HT-PEMFC application. The current study's findings can support future research on PBI/MOF-based composite membranes for HT-PEMFC applications.

High-performance and robust high-temperature polymer electrolyte membranes with moderate microphase separation by implementation of terphenyl-based polymers

Acid loss and plasticization of phosphoric acid (PA)-doped high-temperature polymer electrolyte membranes (HT-PEMs) are critical limitations to their practical application in fuel cells. To overcome these barriers, poly(terphenyl piperidinium)s constructed from the m- and p-isomers of terphenyl were synthesized to regulate the microstructure of the membrane. Highly rigid p-terphenyl units prompt the formation of moderate PA aggregates, where the ion-pair interaction between piperidinium and biphosphate is reinforced, leading to a reduction in the plasticizing effect. As a result, there are trade-offs between the proton conductivity, mechanical strength, and PA retention of the membranes with varied m/p-isomer ratios. The designed PA-doped PTP-20m membrane exhibits superior ionic conductivity, good mechanical strength, and excellent PA retention over a wide range of temperature (80–160 °C) as well as satisfactory resistance to harsh accelerated aging tests. As a result, the membrane presents a desirable combination of performance (1.462 W cm−2 under the H2/O2 condition, which is 1.5 times higher than that of PBI-based membrane) and durability (300 h at 160 °C and 0.2 A cm−2) in the fuel cell. The results of this study provide new insights that will guide molecular design from the perspective of microstructure to improve the performance and robustness of HT-PEMs.

Stability and performance of in-situ formed phosphosilicate nanoparticles in phosphoric acid-doped polybenzimidazole composite membrane fuel cells at elevated temperatures

One of the effective strategies to pursue the highly durable high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) is to introduce inorganic fillers to the phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. Among the inorganic fillers, phosphates such as phosphosilicate are effective in mitigating acid loss at elevated temperatures (200–300 °C). In this paper, the effect of in situ formed phosphosilicate on the performance and stability of SiO2/PA/PBI composite membranes is studied in detail. The mechanical properties and electrochemical performances of the in situ formed SiO2/PA/PBI membranes depend strongly on the content of in situ formed Si5P6O25 fillers and its distribution and microstructure in the membrane. Such in situ formed SiO2/PA/PBI composite membranes show a high conductivity of 53.5 mS cm−1 at 220 °C. The assembled single cell shows a maximum peak power density (PPD) of 530.6 mW cm−2 and excellent stability at elevated temperature of 220 °C for over 130 h. The exceptional stability at 220 °C is most likely due to the existence of predominant amorphous phosphosilicate phases in the in situ formed SiO2/PA/PBI composite membranes, which inhibits the evaporation and leaching of PA at elevated temperatures. The results indicate the practical application of in situ formed SiO2/PA/PBI composite membranes for HT-PEMFCs.

Effect of Polytetrafluorethylene Content in Fe‐N‐C‐Based Catalyst Layers of Gas Diffusion Electrodes for HT‐PEM Fuel Cell Applications

AbstractFe‐N‐C catalysts are a promising alternative to replace cost‐intensive Pt‐based catalysts in high temperature polymer electrolyte membrane fuel cell (HT‐PEMFC) electrodes. However, the electrode fabrication needs to be adapted for this new class of catalysts. In this study, gas diffusion electrodes (GDEs) are fabricated using a commercial Fe‐N‐C catalyst and different polytetrafluorethylene (PTFE) binder ratios, varying from 10 to 50 wt % in the catalyst layer (CL). The oxygen reduction reaction performance is investigated under HT‐PEMFC conditions (160 °C, conc. H3PO4 electrolyte) in a half‐cell setup. The acidophilic character of the Fe‐N‐C catalyst leads to intrusion of phosphoric acid electrolyte into the CL. The strength of the acid penetration depends on the PTFE content, which is visible via the contact angles. The 10 wt % PTFE GDE is less capable to withdraw product water and electrolyte and results into the lowest half‐cell performance. Higher PTFE contents counterbalance the acid drag into the CL and impede flooding. The power density at around 130 mA mgCatalyst−2 increases by 34 % from 10 to 50 wt % PTFE.

Illuminating Oxygen Reduction Reaction Kinetics in High Temperature Polymer Electrolyte Membrane Fuel Cells Using EIS

Owing to their notable power efficiency and zero greenhouse emissions, High Temperature Polymer Electrolyte Membrane Fuel Cells (HTPEMFCs) hold immense promise as a prospective candidate for generating clean energy. However, their commercialization is still impeded by the cathodic Oxygen Reduction Reaction (ORR) kinetics, which after almost half a century not only remain elusive, but constitute the most energy-intensive process, requiring large amounts of the expensive Pt catalyst to mitigate.In this work we provide solid experimental evidence, that the simple three elementary step Dissociative Adsorption Pathway (DAP) with two intermediate species (Oad and OHad), can accurately capture both the steady state (IV) and the Electrochemical Impedance Spectra (EIS) as well.Detailed EIS and IV experimental data were recorded at 180 °C under differential conditions, using a single serpentine HTPEMFC with a 4 cm2 active area. The fuel cell operated at the activation regime, where ORR potential losses dominate but mass transport limitations are negligible. The experimental data were successfully fitted with the derived analytic IV and EIS microkinetic model.The use of the DAP in accordance with the Transitions State Theory (TST), allowed the extraction and analysis of ORR kinetics and energetics. The deconvoluted EIS outlined the important role of the intrinsic kinetic inertia , i.e. the outcome of the competitive nature of the elementary reaction steps on the adsorbed intermediate species. Kinetic inertia not only dominated EIS and the polarization resistance, but demonstrated that the commonly used Butler Volmer and apparent Tafel slope kinetic approach is incapable of describing the multistep ORR kinetics. With the help of the Degree of Rate Control (DRC) analysis, we successfully identified the O2(g) dissociative adsorption as the rate limiting step.Finally, from an energetics perspective, by employing the TST kinetics formulation, we were able to calculate the free Gibbs’ transition states activation (kinetics) and reaction steps’ energies (thermodynamics). These results clarified that, the high ORR overpotential losses stem from the combined strength of both kinetically and thermodynamically imposed barriers, which originate from the high bonding strength of Oad on the Pt surface and the rather high activation energy of the O2(g) adsorption step.

Deconvolution of Charge-Transfer, Mass Transfer, and Ohmic Resistances of Phosphonic Acid-Sulfonic Acid Ionomer Binders in Electrochemical Hydrogen Pumps

Ion-pair high-temperature polymer electrolyte membranes (HT-PEMs) paired with phosphonic acid ionomer electrode binders have substantially improved the performance of HT-PEM electrochemical hydrogen pumps (EHPs)1, 2 and fuel cells3. Recently, blending poly(pentafluorstyrene-co-tetrafluorostyrene phosphonic acid) (PTFSPA) with NafionTM improved ionomer conductivity under anhydrous conditions in the temperature range of 100 °C to 250 °C. Using the said polymer blend as an electrode binder resulted in a 2 W.cm-2 peak power density of fuel cells4 at 240 °C (a HT-PEM fuel cell record). However, much is still unknown about how phosphonic acid ionomers blended with perfluorosulfonic acid (PFSA) materials affect electrode kinetics and gas transport in porous electrodes. In this work, we studied the ionic conductivity, electrode kinetics, and gas transport resistance of 3 types of phosphonic acid ionomers, poly(vinyl phosphonic acid), poly(vinyl benzyl phosphonic acid) by themselves and when blended with Aquivion®. These studies were performed in the context of an EHP platform – both membrane electrode assemblies (MEAs) and interdigitated electrode arrays (IDAs) decorated with nanoscale platinum electrocatalysts with thin film ionomer electrolytes. For all phosphoric acid ionomer types, the addition of Aquivion® promoted ionic conductivity, hydrogen oxidation/evolution reaction kinetics (HOR/HER), and hydrogen gas permeability. Solid-state 31P NMR revealed that the addition of Aquivion® significantly reduced phosphate ester formation in phosphoric acid ionomers and this played a vital role in enhancing ionomer blend conductivity. Using the best blend variant, PTFSPA-Aquivion®, a remarkable EHP performance of 5.1 A.cm-2 at 0.4 V at T = 200 °C was attained. Density functional theory (DFT) simulations identified that phosphonic acids with electron-withdrawing moieties reduced the propensity of the phosphonic acid to adsorb to platinum electrocatalyst surfaces. All phosphonic acid ionomers are anticipated to have a greater affinity for adsorption on platinum when compared to sulphonic acid ionomers (PFSAs). The relative adsorption affiliation of the various phosphoric acid ionomers from DFT is consistent with an explanation that stronger anion-specific adsorption has a detrimental impact and is commensurate with experimentally obtained MEA charge-transfer kinetics. A machine learning-aided compositional model5 revealed that the addition of Aquivion® reduced activation and concentration overpotentials in EHP MEAs and improved exchange current density and diffusivity in EHP IDAs. Future work involves employing the EHPs in a real-world application for separating and compressing hydrogen from the natural gas mixture.References Venugopalan, G.; Bhattacharya, D.; Andrews, E.; Briceno-Mena, L.; Romagnoli, J.; Flake, J.; Arges, C. G., Electrochemical Pumping for Challenging Hydrogen Separations. ACS Energy Letters 2022, 7 (4), 1322-1329.Venugopalan, G.; Bhattacharya, D.; Kole, S.; Ysidron, C.; Angelopoulou, P. P.; Sakellariou, G.; Arges, C. G., Correlating high temperature thin film ionomer electrode binder properties to hydrogen pump polarization. Materials Advances 2021, 2 (13), 4228-4234.Atanasov, V.; Lee, A. S.; Park, E. J.; Maurya, S.; Baca, E. D.; Fujimoto, C.; Hibbs, M.; Matanovic, I.; Kerres, J.; Kim, Y. S., Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells. Nat Mater 2021, 20 (3), 370-377.Lim, K. H.; Lee, A. S.; Atanasov, V.; Kerres, J.; Park, E. J.; Adhikari, S.; Maurya, S.; Manriquez, L. D.; Jung, J.; Fujimoto, C.; Matanovic, I.; Jankovic, J.; Hu, Z.; Jia, H.; Kim, Y. S., Protonated phosphonic acid electrodes for high power heavy-duty vehicle fuel cells. Nature Energy 2022, 7 (3), 248-259.Briceno-Mena, L. A.; Venugopalan, G.; Romagnoli, J. A.; Arges, C. G., Machine learning for guiding high-temperature PEM fuel cells with greater power density. Patterns 2021, 2 (2), 100187. Figure 1

Efficient High Temperature PEMFC Metallic Stack

High temperature (HT) polymer electrolyte membrane fuel cells (PEMFCs) are highly beneficial for high power applications [1-3]. They can be used both in stationary, as well as portable power devices [1,4,5] combining the advantages of the PEM type cells (flexibility in dynamic operation, compactness) and high temperature operation (elimination of fuel humidification and carbon monoxide clean up).This work focuses on the development, construction and testing of an efficient all-metallic HT-PEMFC stack with a power output of 1 kWe [6]. The selection of the proper polymer electrolytes allowed the operation of the stack within the range of 160-190oC. The Membrane-Electrode-Assemblies (MEA) and all components of the stack were designed and prepared in-house. The fuel cell stack characteristics and performance are affected by many parameters such as the materials, components’ design and proper assembly of the stack, as well as the operating conditions.The stack accomplished 50% electrical efficiency with power output of 1070 W and a power density of about 0.18 W/cm2 at 180°C under pure H2 feed. The same performance was reached with reformate hydrogen feeds. The 1 kW electrical and 1 kW thermal power HT-PEMFC stack presented herein consists of thirty MEAs, metallic bipolar plates and an innovative cost-efficient internal pumpless two-phase equilibrium water-steam cooling. The effective design and construction of the latter secures temperature gradientless and isothermal operation of the stack and the production of high temperature superheated steam, which is an effective carrier of high-quality thermal energy with several commercial and industrial applications. Acknowledgments Funding from Region of Western Greece and European Regional Development Fund (RIS3) through DemoCHP DЕР7-0019758 and the Operational Program “Competitiveness, Entrepreneurship & Innovation” (EPAnEK), NSRF 2014-2020 through Solar2HyP Τ2ΕΔΚ-01877.

Nanoscale Investigations of the Electric Double Layer in Protic Ionic Liquids

A hydrogen-based energy storage system will be the backbone of a future energy grid using renewable energies. Polymer electrolyte membrane fuel cells (PEMFCs) are a key element in this energy system as they convert chemical energy stored as hydrogen into electrical energy on demand. PEMFC systems, especially for automotive application, could be significantly improved by increasing the operation temperature above 100 °C. Protic ionic liquids are promising candidates as non-aqueous protic electrolytes for next-generation high-temperature polymer electrolyte membrane fuel cells. These fuel cells have a target operation temperature of 160 °C and allowing for a more efficient water and heat management compared to conventional Nafion®-based PEMFCs, which operate at temperatures below 80 °C [1].In order to ensure a reliable and efficient operation an electrolyte with a high electrochemical performance and stability has to be selected. For this purpose, protic ionic liquids have been proposed and first fuel cell tests have shown promising results [2]. Hence, we aim on understanding the properties of this class of novel electrolytes on an atomistic level, which would allow designing suitable material combinations and predicting their properties for an efficient fuel cell operation. As ionic liquids are molten salts, which are liquid below 100 °C, their electrochemical properties differ significantly from those of aqueous solutions. Instead of a classical electric double layer, which can be described by the models provided by Helmholtz, Gouy-Chapman and Stern, the interface structure formed between the electrolyte and a charged electrode is governed by the interplay between coulomb interaction and steric effects between the (large) molecular ions [3]. In order to understand the formation of this double layer on a metallic electrode, we employ atomic force microscopy and infrared spectroscopy in combination with molecular dynamics simulations. Our results show that in the interface region between the prototype protic ionic liquid diethylmethylammonium triflate ([Dema][TfO]) and a Pt electrode, a dense layered structure consisting of alternating anion and cation layers is present, that extends several nanometres into the bulk of the electrolyte [4]. The composition and structure changes with applied potential due to a preferential attraction of anions or cations depending on the electrode charge. When water is added to the ionic liquid, the layered structure becomes distorted and water molecules appear near the electrode. Since the presence of water will also influence the relevant electrochemical processes such as the oxygen reduction reaction (ORR), the analysis of the double layer structure on an atomistic scale is necessary in order to understand the subtle interactions between the molecules in the electrolyte and to propose design routes for novel more efficient ionic liquid-based electrolytes. Wippermann, K.; Suo, Y.; Korte, C. Oxygen Reduction Reaction Kinetics on Pt in Mixtures of Proton-Conducting Ionic Liquids and Water: The Influence of Cation Acidity. J. Phys. Chem. C 2021, 125, 4423–4435, doi:10.1021/acs.jpcc.0c11374.Skorikova, G.; Rauber, D.; Aili, D.; Martin, S.; Li, Q.; Henkensmeier, D.; Hempelmann, R. Protic Ionic Liquids Immobilized in Phosphoric Acid-Doped Polybenzimidazole Matrix Enable Polymer Electrolyte Fuel Cell Operation at 200 °C. Journal of Membrane Science 2020, 608, 118188, doi:10.1016/j.memsci.2020.118188.Rodenbücher, C.; Wippermann, K.; Korte, C. Atomic Force Spectroscopy on Ionic Liquids. Applied Sciences 2019, 9, 2207, doi:10.3390/app9112207.Rodenbücher, C.; Chen, Y.; Wippermann, K.; Kowalski, P.M.; Giesen, M.; Mayer, D.; Hausen, F.; Korte, C. The Structure of the Electric Double Layer of the Protic Ionic Liquid [Dema][TfO] Analyzed by Atomic Force Spectroscopy. International Journal of Molecular Sciences 2021, 22, 12653, doi:10.3390/ijms222312653. Figure 1

Characterization of Catalyst Layer Using Gas Diffusion Electrode Half-Cell System for High Temperature Polymer Electrolyte Membrane Fuel Cell

High-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is being developed for the heavy-duty vehicle which requires high power density and operation under extreme conditions. Conventional HT-PEMFC using phosphoric acid (PA) immersed PBI membranes and polytetrafluoroethylene (PTFE) binder faced issues such as unstable durability and low performance of membrane electrode assembly (MEA) due to PA leakage and uneven distribution within the catalyst layer.In recent years, ion-pair membranes and ionomers for high temperatures have been developed, which can reduce PA leakage in MEA and improve ionic conduction within the catalyst layer. Incorporation of a new material into the MEA requires further research to enhance electrode design, catalyst utilization, and performance for the appropriate ionomer, as this material exhibits different characteristics from the conventional PTFE binder.In this study, various gas diffusion electrode (GDE) was fabricated with the new ionomer, and GDE half-cell setups were introduced for rapid and efficient characterization of performance in the oxygen reduction reaction. This system serves as an intermediate step between rotating disk electrodes and single cells, providing a powerful tool for quickly screening and evaluating GDEs using small amounts of catalyst. Recent publications from Wilkinson [1] and Gasteiger [2] groups reviewed this new system, proposed new measurement protocols, and suggested improvements. Accordingly, GDE half-cell setups are designed to simulate HT-PEMFC operating conditions at 150 ℃ temperature with 85 wt.% PA electrolyte.Electrochemical characteristics were evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) following the protocols in previous papers with GDE half-cell. Resistance values obtained through EIS were further differentiated by peaks and frequency-based responses through the distribution of relaxation times transformation. Performance comparison between GDEs was carried out by measuring the voltage at 0.2 A/cm² and peak power density at 150 ℃, simulating a single cell-like environment. GDEs were fabricated with variables such as Pt loading, thickness, and porosity. And their physical properties were confirmed through mercury intrusion porosimetry and scanning electron microscopy. Ultimately, the study elucidated the correlation between the electrochemical data and physical properties.

Performance and durability of high temperature proton exchange membrane fuel cells with silicon carbide filled polybenzimidazole composite membranes

High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are used for stationary to mobile applications and show increased tolerances to fuel impurities like CO compared to their low temperature counterpart. However, conventional HT-PEMFC membranes, based on polybenzimidazole (PBI), suffer from creeping during long-term operation and thus a significant increase in surface resistance. In this study, inorganic fillers in terms of silicon carbide (SiC) are incorporated into the standard PBI-based Celtec®-P membrane by a well-established large-scale polyphosphoric acid process. Operations over 1000 h of load cycling between 0.6 and 1.0 A cm−2 of SiC and for conventional HT-PEMFCs reveal differences in long-term durability. The ohmic resistance of the SiC-based single cell is 1/3 lower, while the membrane has a higher thickness retention. Cell performances are improved with lower degradation rates (<65 μV h−1 for both SiC and >100 μV h−1 for both conventional HT-PEMFCs) after 1000 h. However, SiC particle mobility and increased hydrogen permeability are observed after testing.