High-temperature Polymer Electrolyte Fuel Cells Research Articles

Performance Analysis of Non-Humidified High-Temperature PEFC by Fuel Cell System Simulator

Polymer electrolyte fuel cells (PEFCs) are widely used for automobiles and other applications due to their excellent start-ability and load followability. For further popularization in the future, it is expected to be applied to commercial vehicles such as large trucks and buses. However, insufficient cooling of the PEFC becomes a major issue, for example when heavy-duty vehicle needs to operate on uphill roads at high-loads and high- temperatures. This is because conventional electrolyte membranes use water as a proton-conducting carrier, so PEFC’s operation temperature must be controlled below 100°C.In this study, we focus on high-temperature PEFC (HT-PEFC), which is expected to improve cooling performance by high-temperature operation (over 100°C). HT-PEFC can operate under non-humidified high-temperature environment by doping phosphoric acid as a proton-conducting carrier in the electrolyte membrane. Then, we evaluate the usefulness of the HT-PEFC by performance analysis using a FC system simulator.We use FC-DynaMo(1),(2), which is researched and developed by NEDO's industry-academia-government collaboration project, as the FC system simulator. FC-DynaMo is based on the FC stack system of Toyota motor corporation's 2nd generation MIRAI. As shown in Figure 1, in addition to the FC stack, hydrogen system, air system, and cooling system are also modeled, making it possible to simulate unsteady operation.The target vehicle for the simulation is a heavy-duty truck (Class8), and the results of a survey conducted by Zhang et al.(3) on the various uses of in the United States were used as a reference. From the results including output power, vehicle speed, and road gradient, we extracted and used the driving data of long-haul with continuous uphill and downhill roads, which is considered to be the high-load and high-temperature operation.For the membrane electrode assembly (MEA) of HT-PEFC, we used APM STD (PBI-MEA) manufactured by Advent technologies, which uses a polybenzimidazole membrane. We give the characteristic that the exchange current density increases exponentially as the temperature rises, referring to the study of Korsgaard et al.(4). In addition, proton conductivity is given as 10S/m published by Advent technologies. Figure 2(a) shows the calculated IV characteristic of MIRAI-MEA. And Figure 2(b) shows the that of PBI-MEA. As the temperature rises, the effect of reducing the activation overpotential by increasing the exchange current density can be confirmed. These results are in good agreement with the study which investigated the operation characteristics of PBI-MEA by Chen et al.(5) and Waller et al.(6). However, compared to the IV characteristic of MIRAI-MEA shown in Figure 2(a), the overpotential is too large, the maximum output is less than half, and the heat generation is nearly double. This is because it is nearly twice as large as the MIRAI-MEA’s activation overpotential, which causes a decrease in output power and an increase in heat generation. With this current PBI-MEA performance, it is difficult to evaluate the system compared with MIRAI-MEA. Therefore, in this study, we hypothetically set up a high-temperature MEA assuming future development (Virtual HT-MEA). Virtual HT-MEA is given MIRAI-MEA’s low activation overpotential and the characteristics of PBI-MEA that don’t dry-out at high-temperature. Figure 2(c) shows the Virtual HT-MEA’s IV characteristics. Here, at high current density, the performance at 120°C and 180°C is lower than that at 70°C because the concentration overpotential increases due to the decrease in the oxygen molar concentration at high-temperature.Figure 3 shows the results of Class8 driving simulation with each MEA adapted to the FC stack. In the MIRAI-MEA shown in Figure 3(a), a combination of 4-FC stack systems and 5-cooling systems is optimal. With this combination, the FC cell temperature can be controlled below 95°C. Figure 3(b) shows the analysis results of the FC stack using PBI-MEA. As shown in Figure 2(b), the PBI-MEA has a large overpotential, so a combination of 10-FC stack systems and 5-cooling systems is required to meet the request power. The current PBI-MEA’s performance cannot show the merits of non-humidified high-temperature operation. Figure 3(c) shows the analysis results of the FC stack using the Virtual HT-MEA. Stable operation is possible with 4-FC stack systems and 2-cooling systems. This means that 3-cooling systems can be reduced compared to the MIRAI-MEA’s system. This is because the cooling performance is improved due to the larger temperature difference between the outside air and the FC stacks.Reference(1)S.Hasegawa, et al., ECS Transactions,104, (2021)(2)S.Hasegawa, et al., ECS Transactions,109, (2022)(3)C.Zhang, et al., Transportation Research,Part D95, (2021)(4)A.R.Korsgaad, et al., Journal of Power Sources,162, (2006)(5)C.-Y.Chen, et al., Journal of Power Sources,195, (2010)(6)M.G.Waller, et al., International Journal of Hydrogen Energy,41, (2016) Figure 1

Simulation to analyze operation strategy of combined cooling and power system using a high-temperature polymer electrolyte fuel cell for data centers

With the development of the information and communication technology (ICT) industry, the energy consumption of data centers is continuously increasing. High-temperature polymer electrolyte fuel cells (HT-PEFC) have a high operating temperature of 120 °C or higher; thus, the heat generated from fuel cells stack is effectively used as a heat source for a absorption refrigerator (AR) to generate cooling. The combined cooling and power (CCP) system is proposed to satisfy the energy demand of data centers that require both power and cooling. The CCP system comprises an HT-PEFC and a double-effect AR that recovers the wasted heat of the fuel cell stack. As a result of analyzing the electrical and cooling efficiency of the CCP system according to the fuel cell operating conditions, the overall efficiency increased to 95%, which was significantly higher than the existing system. Based on the simulation of the developed model, coefficient of performance (COP) and cooling capacity depending on the temperature changes of chilled water and cooling water were calculated within the stack temperature range of 150–180 °C, and the COP changed from 1.1 to 1.58 depending on the conditions. The developed CCP system model can be used to plan strategies for flexible fuel cell operation according to the power and cooling demands required in the data center.

Study of Pt Alloy Catalysts for Oxygen Reduction Reaction on Rde and in MEA for High-Temperature Pemfcs

High-temperature polymer electrolyte fuel cells (HT-PEMFCs) have advantages in enhanced fuel impurities tolerance, easier heat rejection, and simpler water management[1]. Low-temperature PEMFCs with perfluorosulfonic acid electrolytes (e.g., Nafion) could not provide sufficient proton conductivity in the absence of water when T > 120 °C, limiting the application when a higher temperature is desired. High-temperature (HT-) PEMFCs have been developed by using phosphoric acid (H3PO4)-doped polymer electrolyte (e.g., polybenzimidazole, quaternary ammonium biphosphate ion pair coordinated polyphenylene, etc.) [2-4], where H3PO4 can conduct proton at high temperature up to 200 °C. Such change of the reaction environment, however, also affects oxygen reduction reaction (ORR) kinetics. Despite of a facilitated reaction kinetics under high temperature, a high Pt loading is still demanded in state-of-the-art HT-PEMFCs. To enhance ORR activity and increase Pt utilization, here we employ a strategy to alloy Pt with another transition metal (M), which has been demonstrated to be effective in LT-PEMFCs. In this study, Pt and Pt-M catalysts were used to investigate the alloy effect on the adsorption strength of H3PO4 and ORR in both half-cell rotating disk electrode (RDE) and membrane electrode assembly (MEA). In RDE study, the competition adsorption of phosphoric acid has led to a suppression on ORR activity on all catalysts, with less impact on ORR at 160 °C using pure H3PO4 than RT using 1 M H3PO4. Since alloying not just affects the O binding energy, but also H3PO4 adsorption, a different trend of ORR on Pt-M alloy catalysts was resulted in the presence of H3PO4. The alloy effect was further studied in MEA at 160 °C, which also indicates the effectiveness of using HT-RDE setup for HT-ORR catalyst screening. Reference: [1] Gittleman, C. S.; Jia, H.; De Castro, E. S.; Chisholm, C. R. I.; Kim, Y. S. Proton Conductors for Heavy-Duty Vehicle Fuel Cells. Joule 2021, 5 (7), 1660–1677.[2] Pingitore, A. T.; Huang, F.; Qian, G.; Benicewicz, B. C. Durable High Polymer Content m/ p-Polybenzimidazole Membranes for Extended Lifetime Electrochemical Devices. ACS Appl. Energy Mater. 2019, 2 (3), 1720–1726.[3] Lee, K. S.; Spendelow, J. S.; Choe, Y. K.; Fujimoto, C.; Kim, Y. S. An Operationally Flexible Fuel Cell Based on Quaternary Ammonium-Biphosphate Ion Pairs. Nat. Energy 2016, 1 (9).[4] 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. Nat. Energy 2022, 7 (3), 248–259.

Single-Layer-Graphene and Electrochemical Exfoliated Graphene Oxide for HT-PEMFC

High-temperature proton exchange membrane fuel cell (HT-PEMFC) is promising with its low demand for hydrogen purity, simple water management, and high waste heat utilization [1][2]. Although polybenzimidazole (PBI), as a typical high-temperature fuel cell membrane, has been developed for a long period, it still has defects such as phosphoric acid leaching and insufficient mechanical properties under high acid doping level [3][4]. With its unique 2D hexagonal grid structure and its electron cloud arrangement, single-layer graphene (SLG) can block the passage of any atoms [5]. However, hydrogen protons can easily penetrate SLG through a unique intermediate state mechanism to make it have excellent proton conductivity [6]. SLG's high proton conductivity combined with its excellent mechanical properties and thermal conductivity make it an ideal material for blocking phosphoric acid leaching. The SLG prepared by the chemical vapor deposition (CVD) method was transferred to the surface of catalyst layer of electrodes by the wet chemical transfer method. Due to the rupture and incompleteness of the SLG during the transfer process, the coverage rate of the SLG on the electrode surface is about 55%. This incomplete coverage facilitates small quantities of phosphoric acid leaching from PBI membrane to electrodes, which increases the three-phase boundary area and improves the performance of the HT-PEMFC. After 70 hours of accelerated stress testing (AST), the peak power density of membrane-electrode-assembly (MEA) with SLG on both sides of the cathode and anode can reach 480 mW cm-2, while the peak power density of MEA based on pure PBI membrane is only 249 mW cm-2. The quantification of phosphoric acid in the electrode by Lab based X-ray micro-computed tomography and the Raman spectroscopic mapping of the MEA cross-section shows that SLG can mitigate the leaching of phosphoric acid in the PBI film and improve the durability of HT-PEMFC. Graphene oxide (GO) is a low-cost application of graphene. Its doping in the PBI membrane can also reduce the leaching of phosphoric acid through enhancing the interaction between phosphoric acid and the PBI membrane [7]. A reactor based on 3D printing was designed so that natural graphite flakes can be used for electrochemical exfoliation to achieve rapid, safe, environmentally friendly, and mass production of GO. The electrochemically exfoliated (E)GO is doped in the PBI membrane. The 0.5%, 1% and 2% EGO loadings in the PBI membrane increased the peak power density by 13.8%, 24.4% and 29.2%, respectively.[1] Y.-L. Ma, J.S. Wainright, M.H. Litt, R.F. Savinell, Conductivity of PBI Membranes for High-Temperature Polymer Electrolyte Fuel Cells, Journal of The Electrochemical Society. 151 (2004) A8. https://doi.org/10.1149/1.1630037/XML.[2] H. Su, S. Pasupathi, B. Bladergroen, V. Linkov, B.G. Pollet, Optimization of gas diffusion electrode for polybenzimidazole-based high temperature proton exchange membrane fuel cell: Evaluation of polymer binders in catalyst layer, International Journal of Hydrogen Energy. 38 (2013) 11370–11378. https://doi.org/10.1016/J.IJHYDENE.2013.06.107.[3] S. Galbiati, A. Baricci, A. Casalegno, R. Marchesi, Degradation in phosphoric acid doped polymer fuel cells : A 6000 h parametric investigation, International Journal of Hydrogen Energy. 38 (2013) 6469–6480. https://doi.org/10.1016/j.ijhydene.2013.03.012.[4] S.H. Eberhardt, F. Marone, M. Stampanoni, F.N. Büchi, T.J. Schmidt, Quantifying phosphoric acid in high-temperature polymer electrolyte fuel cell components by X-ray tomographic microscopy, Journal of Synchrotron Radiation. 21 (2014) 1319–1326. https://doi.org/10.1107/S1600577514016348.[5] S. Hu, M. Lozada-Hidalgo, F.C. Wang, A. Mishchenko, F. Schedin, R.R. Nair, E.W. Hill, D.W. Boukhvalov, M.I. Katsnelson, R.A.W. Dryfe, I. V. Grigorieva, H.A. Wu, A.K. Geim, Proton transport through one-atom-thick crystals, Nature. 516 (2014) 227–230. https://doi.org/10.1038/nature14015.[6] S.M. Holmes, P. Balakrishnan, V.S. Kalangi, X. Zhang, M. Lozada-Hidalgo, P.M. Ajayan, R.R. Nair, 2D Crystals Significantly Enhance the Performance of a Working Fuel Cell, Advanced Energy Materials. 7 (2017) 1–7. https://doi.org/10.1002/aenm.201601216.[7] J. Li, X. Zeng, T. Ren, E. van der Heide, The Preparation of Graphene Oxide and Its Derivatives and Their Application in Bio-Tribological Systems, Lubricants 2014, Vol. 2, Pages 137-161. 2 (2014) 137–161. https://doi.org/10.3390/LUBRICANTS2030137.

Mechanism of action of polytetrafluoroethylene binder on the performance and durability of high-temperature polymer electrolyte fuel cells

In this work, new insights into impacts of the polytetrafluoroethylene (PTFE) binder on high temperature polymer electrolyte fuel cells (HT-PEFCs) are provided by means of various characterizations and accelerated stress tests. Cathodes with PTFE contents from 0 wt% to 60 wt% were fabricated and compared using electrochemical measurements. The results indicate that the cell with 10 wt% PTFE in the cathode catalyst layer (CCL) shows the best performance due to having the lowest mass transport resistance and cathode protonic resistance. Moreover, cyclic voltammograms show that Pt (100) edge and corner sites are significantly covered by PTFE and phosphate anions when the PTFE content is higher than 25 wt%. Open-circuit and low load-cycling conditions are applied to accelerate degradation processes of the HT-PEFCs. The PTFE binder shows a network structure in the pores of the catalyst layer, which reduces phosphoric acid leaching during the aging tests. In addition, the high binder HT-PEFCs more easily suffer from a mass transport problem, leading to more severe performance degradation.

Polybenzimidazole and ionic liquid composite membranes for high temperature polymer electrolyte fuel cells

Elevating the operating temperature of polymer electrolyte fuel cells (PEFCs) can facilitate the reaction kinetics and improve the CO tolerance of catalysts, and simplify the complex water-heat management system of fuel cells. In this study, we fabricated the ionic liquid-based polymer electrolyte that can operate up to 250 °C without external humidification. The membrane utilized polybenzimidazole (PBI) as the polymer matrix, and ionic liquids as the proton conductive fillers. Among all the investigated ionic liquids, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) shows favorable performance and stability. The PBI-[dema][TfO] composite membrane exhibits high proton conductivity of 108.9 mS cm−1 at 250 °C. The single cell with PBI-[dema][TfO] composite membrane and Pt/C as the electrode shows output performance of 144 and 62 mW/cm2 at 125 °C and 250 °C, respectively. This work demonstrates that it is very promising to fabricate high temperature PEFCs using the ionic liquid-based polymer electrolyte.

CrN/Cr-Coated Steel Plates for High-Temperature Polymer Electrolyte Fuel Cells: Performance and Durability

This paper investigates the general performance and durability performance of a high-temperature polymer electrolyte fuel cell (HT-PEFC) with CrN/Cr-coated SS316L as bipolar plates and compares this with the bare SS316L and graphite bipolar plates. Polarization curves are conducted every 200 h during the 1000 h durability tests for evaluating the overall performances of the HT-PEFCs with different types of bipolar plates. An electrochemical model based on polarization curves, combined with electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM), is used for understanding the degradation mechanisms at play. The CrN/Cr-coated SS316L bipolar plates show excellent corrosion resistance and performance in a real HT-PEFC. Thereby, the degradation rate decrease from ca. 16% of the highest output power with the bare metallic bipolar plates and graphite bipolar plates to almost zero with the CrN/Cr-coated steel bipolar plate.

Electrochemical Interface Optimization toward Low Oxygen Transport Resistance in High‐Temperature Polymer Electrolyte Fuel Cells

Due to the enhanced tolerance of CO‐containing species under elevated operation temperature around 150–220 °C, high‐temperature polymer electrolyte fuel cells (HT‐PEFCs) have drawn extensive interest in the recent decades. One of the major challenges remaining for HT‐PEFCs is the inferior cell performance ascribed to poor catalytic activity and mass transport, which are mainly determined by the electrochemical interface structure involving phosphoric acid electrolyte. Herein, the electrochemical interfaces are investigated and optimized by adjusting the hydrophilic phases within the catalyst layer (CL) to modulate phosphoric acid redistribution in the cathode. The electrochemical surface area and interfacial oxygen transport resistance in the CL are optimized by balancing the coverage of polytetrafluoroethylene (PTFE) and phosphoric acid on the catalyst surface, thus achieving an enhanced cell performance of the HT‐PEFC of up to 328.12 mW cm−2. This balancing strategy for optimizing the electrochemical interfacial structure could shed light on the development of electrode architecture design for energy conversion and storage devices.

The Influence of Acidity, Water Content and Temperature of Ionic Liquids on the Double Layer Properties of the Pt/Pil Interface

The use of green energy for transport applications is becoming ever more important, with fuel cells showing the potential to be a highly competitive candidate in the automobile industry. Traditional low temperature polymer electrolyte fuel cells (LT-PEFCs) can only be operated at temperatures below 80 °C. High temperature polymer electrolyte fuel cells (HT-PEFCs), based on phosphoric acid (H3PO4) doped polybenzimidazole membranes (PBI), can be operated in a temperature range of 160-180 °C. Because of the higher operating temperature, HT-PEFCs have many advantages, such as a much easier water and heat management, a more compact cooling system and a higher CO tolerance in the feed gas.[1] However, the major drawback of H3PO4 as an electrolyte is the poisoning effect on the redox catalyst and low O2 solubility. This leads to a significant decrease in the cathodic oxygen reduction reaction (ORR) kinetics.An approach to overcome these disadvantages is to substitute the electrolyte H3PO4 with a proton conducting ionic liquids (PILs). PILs exhibit a high ionic conductivity, a wide electrochemical window and a high chemical and thermal stability.[2] Therefore, they are promising candidates as alternative non-aqueous electrolytes in intermediate temperature fuel cells for operation temperatures at 120 °C.In this contribution, a study on the double layer properties of the interface between platinum and PILs, investigated by means of electrochemical impedance spectroscopy (EIS), is presented. Three PILs – [2-SEMA][TfO], [1-EIm][TfO] and [DEMA][TfO] – with different cation acidities are compared. Water is produced unavoidably during fuel cell operation at the anode side. Even at temperatures above 100 °C residual water will be present in the electrolyte. Thus, for all three ionic liquids, the layer structures at the interface of the electrodes are studied as a function of the water concentration and temperature by means of atomic force microscopy (AFM).Plots of the potential-dependent data from EIS measurements in the complex capacitance plane (CCP) show, that at least two differential double layer capacitances are obtained, depending on the potential (U = 0 – 1.5 V vs RHE), water concentration (c H 2 O = 0.6 – 7.2 wt%) and temperature (T = 30 – 90 °C). The differential capacitance of the fast process can mainly be attributed to charge redistributions at the interface and the ion transport in the double layer. The slow capacitive process is probably caused by charge redistributions in the innermost ion layer, which requires a higher activation energy for the reorientation of the ions, as the ions in this layer are strongly bound to the electrode.In addition, the PILs are studied as a function of c H 2 O and T by means of AFM to obtain a more direct image of the structure at the electrode interface. It is known that ionic liquids usually form a structure of alternating cation and anion layers, significantly different compared to (diluted) aqueous electrolytes. In the case of [DEMA][TfO] up to four distance dependent force maxima can be recognized, which indicate the ordered interfacial nanostructure. The structure at the interface is affected by the molecular structure and size of the cations (and anions) of the PIL, as well as by the water, absorbed to the surface and incorporated in the cation and anion layer, which results in a perturbance of the structure.In conclusion, the combined electrochemical double layer measurements and microscopic studies provide a deeper insight into the double layer structure at the interface between the Pt electrode and ionic liquids depending on the PIL acidity, water concentration and temperature. Figure 1

Fuel Cell Electrode Characterization Using Neutron Scattering.

Electrochemical energy conversion and storage is key for the use of regenerative energies at large scale. A thorough understanding of the individual components, such as the ion conducting membrane and the electrode layers, can be obtained with scattering techniques on atomic to molecular length scales. The largely heterogeneous electrode layers of High-Temperature Polymer Electrolyte Fuel Cells are studied in this work with small- and wide-angle neutron scattering at the same time with the iMATERIA diffractometer at the spallation neutron source at J-PARC, opening a view on structural properties on atomic to mesoscopic length scales. Recent results on the proton mobility from the same samples measured with backscattering spectroscopy are put into relation with the structural findings.

A Review of The Methanol Economy: The Fuel Cell Route

This review presents methanol as a potential renewable alternative to fossil fuels in the fight against climate change. It explores the renewable ways of obtaining methanol and its use in efficient energy systems for a net zero-emission carbon cycle, with a special focus on fuel cells. It investigates the different parts of the carbon cycle from a methanol and fuel cell perspective. In recent years, the potential for a methanol economy has been shown and there has been significant technological advancement of its renewable production and utilization. Even though its full adoption will require further development, it can be produced from renewable electricity and biomass or CO2 capture and can be used in several industrial sectors, which make it an excellent liquid electrofuel for the transition to a sustainable economy. By converting CO2 into liquid fuels, the harmful effects of CO2 emissions from existing industries that still rely on fossil fuels are reduced. The methanol can then be used both in the energy sector and the chemical industry, and become an all-around substitute for petroleum. The scope of this review is to put together the different aspects of methanol as an energy carrier of the future, with particular focus on its renewable production and its use in high-temperature polymer electrolyte fuel cells (HT-PEMFCs) via methanol steam reforming.

Tailoring Different Molecular Weight Phenylene-Polybenzimidazole Membranes with Remarkable Oxidative Stability and Conductive Properties for High-Temperature Polymer Electrolyte Fuel Cells.

Polybenzimidazole (ph-PBI) polymer was synthesized with different molecular weights (MWs) and casted into conductive films for use in high-temperature fuel cells (FCs). A comprehensive study on the influence of polymer MW on membrane cast efficiency, chemical stability, thermal behavior, tensile strength, conductivity, FC performance, and durability was reported. The synthesized materials were characterized by different techniques, including, nuclear magnetic resonance spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, impedance microscopy, and scanning electron microscopy. The results showed the importance of manufacturing ph-PBI membranes with controlled properties to achieve high efficiency FCs. High MW ph-PBI membrane (119 kDa) showed a slower rate of chemical degradation, remarkable mechanical properties, and an improved FC performance compared to low MW ph-PBI membrane (39 kDa), thanks to the architecture of high MW ph-PBI. A gain of 91% in proton conductivity with a 47% in FC power density was obtained for the ph-PBI membrane with MW 119 kDa.

Influence of operating conditions on the degradation mechanism in high-temperature polymer electrolyte fuel cells

The purpose of this study is to compare the degradation phenomena in high-temperature polymer electrolyte fuel cells (HT-PEFCs) under various operating conditions and investigate the degradation mechanism by using accelerated stress tests. Five stressors (i.e., high temperature, thermal cycling, open circuit voltage, high load cycling, and low load cycling) are applied to identical HT-PEFCs and compared to the standard operating condition. Extensive characterizations of the aged HT-PEFCs are then performed using techniques that include focused ion beam/scanning electron microscopy, inductively coupled plasma-mass spectrometry, energy dispersive X-ray mapping/line-scan analysis, polarization curves, electrochemical impedance spectroscopy and cyclic voltammetry. The results present a negligible amount of phosphoric acid leaching and notable degradation of the Pt/C catalyst. On the anode side, slight Pt corrosion is only visualized under open circuit conditions, while on the cathode side, evident Pt agglomeration in the catalyst layer and different Pt precipitation behaviors in the membrane are observed under all the conditions. The highest amount of Pt precipitation occurs in the cells held at open circuit voltage and cycled at low loads, which also exhibit the most pronounced performance degradation. In addition, the PA redistribution between the membrane and electrodes contributes to the cells’ resistance and performance differences.

The Influence of Acidity, Water Content and Temperature of Ionic Liquids on the Double Layer Properties of Pt/Pil Interface

The use of green energy for transport applications is becoming ever more important, with fuel cells showing the potential to be a highly competitive candidate in the automobile industry. Traditional low temperature polymer electrolyte fuel cells (LT-PEFCs) can only be operated at temperatures below 80 °C. High temperature polymer electrolyte fuel cells (HT-PEFCs), based on phosphoric acid (H3PO4) doped polybenzimidazole membranes (PBI), can be operated in a temperature range of 160-180 °C. Because of the higher operating temperature, HT-PEFCs have many advantages, such as a much easier water and heat management, a more compact cooling system and a higher CO tolerance in the feed gas.[1] However, the major drawback of H3PO4 as an electrolyte is the poisoning effect on the redox catalyst and low O2 solubility. This leads to a significant decrease in the cathodic oxygen reduction reaction (ORR) kinetics. An approach to overcome these disadvantages is to substitute the electrolyte H3PO4 with a proton conducting ionic liquids (PILs). PILs exhibit a high ionic conductivity, a wide electrochemical window and a high chemical and thermal stability.[2] Therefore, they are promising candidates as alternative non-aqueous electrolytes in intermediate temperature fuel cells for operation temperatures at 120 °C. In this contribution, a study on the double layer properties of the interface between platinum and PILs, investigated by means of electrochemical impedance spectroscopy (EIS), is presented. Three PILs – [2-SEMA][TfO], [1-EIm][TfO] and [DEMA][TfO] – with different cation acidities are compared. Water is produced unavoidably during fuel cell operation at the anode side. Even at temperatures above 100 °C residual water will be present in the electrolyte. Thus, for all three ionic liquids, the layer structures at the interface of the electrodes are studied as a function of the water concentration and temperature by means of atomic force microscopy (AFM). Plots of the potential-dependent data from EIS measurements in the complex capacitance plane (CCP) show, that at least two differential double layer capacitances are obtained, depending on the potential (U = 0 – 1.5 V vs RHE), water concentration (c H 2 O = 0.6 – 7.2 wt%) and temperature (T = 30 – 90 °C). The differential capacitance of the fast process can mainly be attributed to charge redistributions at the interface and the ion transport in the double layer. The slow capacitive process is probably caused by charge redistributions in the innermost ion layer, which requires a higher activation energy for the reorientation of the ions, as the ions in this layer are strongly bound to the electrode. In addition, the PILs are studied as a function of c H 2 O and T by means of AFM to obtain a more direct image of the structure at the electrode interface. It is known that ionic liquids usually form a structure of alternating cation and anion layers, significantly different compared to (diluted) aqueous electrolytes. In the case of [DEMA][TfO] up to four distance dependent force maxima can be recognized, which indicate the ordered interfacial nanostructure. The structure at the interface is affected by the molecular structure and size of the cations (and anions) of the PIL, as well as by the water, absorbed to the surface and incorporated in the cation and anion layer, which results in a perturbance of the structure. In conclusion, the combined electrochemical double layer measurements and microscopic studies provide a deeper insight into the double layer structure at the interface between the Pt electrode and ionic liquids depending on the PIL acidity, water concentration and temperature. [1] K. Wippermann, J. Wackerl, W. Lehnert, B. Huber, C. Korte, J. Electrochem., 2016, 162, F25-F37. [2] K. Wippermann, J. Giffin, S.Kuhri, W. Lehnert, C. Korte, Phys. Chem. Chem. Phys., 2017, 19, 24706-24723.

An online spatiotemporal temperature model for high temperature polymer electrolyte fuel cells

This paper focuses on the thermal behavior estimation for high temperature polymer electrolyte fuel cells (HT-PEFCs), which can be used for predicting stack temperature and developing a reliable thermal controller. To overcome the complex nonlinear, time-space-distributed properties in fuel cell stacks, an online spatiotemporal temperature model is developed here. This model is based on the least squares support vector machine (LS-SVM), which has the ability to approximate any nonlinear system by a simple model structure. In the proposed method, the spatial correlations across different locations are fully represented by the kernel function, and then the temporal features are further modeled as a function of the inlet flow rate, partial pressure on each side and current density. After that, integrating the spatial kernel function and temporal dynamics function, the spatiotemporal temperature model is then constructed. The proposed method is tested by simulation in Matlab, with a comparison of the experimental data collected from an HT-PEFC test rig. The modeling result demonstrates that the developed model can effectively and precisely predict HT-PEFC thermal behavior. The present study is of key importance and may help as a black box for future development of new optimization and control strategies.

Phosphoric Acid Loss Mechanism of High Temperature Proton Exchange Membranes

High temperature-polymer electrolyte fuel cells (HT-PEMFCs) traditionally utilize phosphoric acid-doped polybenzimidazole as proton exchange membrane.1 However, the retention of phosphoric acid within polymer matrix has been pointed out as one of the most critical factors that affect HT-PEMFC durability. Previously, we reported a new HT-PEMFC system in which a quaternary ammonium-functionalized polymer imparted a stronger ionic interaction which we deemed “ion-pair coordination” with phosphoric acid compared with that of polybenzimidazole,2 giving rise to improved water tolerance. In this study, we use a combination of 31P NMR and DFT computations to compare the ionic interactions between representative small molecule organic bases, benzimidazole or tetramethylammonium hydroxide, with phosphoric acid and water. Based of these results, we propose a new phosphoric acid loss mechanism in which the strongly interacting ion pairs can increase the phosphoric acid partition composition towards accommodating more phosphoric acid for better phosphoric acid retention. References Li, J.O. Jensen, R.F. Savinell, N.J. Bjerrum, Prog. Polym. Sci., 2009, 34, 449-477.-S. Lee, J.S. Spendelow, Y.K. Choe, C. Fujimoto, Y. S. Kim, Nature Energy, 2016, 1, 16120-16124.

Effect of Gas Humidification on Performance of High-Temperature Polymer Electrolyte Fuel Cells

Effect of Gas Humidification on Performance of High-Temperature Polymer Electrolyte Fuel Cells

Proton diffusion in the catalytic layer for high temperature polymer electrolyte fuel cells.

The present study focuses on quasielastic neutron scattering (QENS) of the proton dynamics in phosphoric acid (PA) inside the catalytic layer of high-temperature polymer electrolyte fuel cells (HT-PEFCs). The nanosecond proton dynamics is investigated on the local length scale around operating temperatures (300 K–430 K) using neutron backscattering spectroscopy. We have investigated the catalyst doped with different amounts of PA in order to understand the distribution of PA inside the layer. Three approaches are considered for the description of proton dynamics: the random jump diffusion model, distribution of diffusion constants and, finally, the trap model. Due to adsorption of the PA on the Pt particles the diffusion of protons in the catalytic layer is different in comparison to the bulk acid. The proton dynamics in the catalytic layer can be described by the random jump diffusion with traps. This diffusion is significantly slower than the diffusion of free PA; this also results in a lower conductivity, which is estimated from the obtained diffusion constant.

Stochastic Analysis of the Gas Flow at the Gas Diffusion Layer/Channel Interface of a High-Temperature Polymer Electrolyte Fuel Cell

Gas diffusion layers (GDLs) play a significant role in the efficient operation of high-temperature polymer electrolyte fuel cells. They connect the electrodes to the gas channels of the bipolar plate by porous material with a meso-scale geometric structure. The electrodes must be sufficiently supplied by gases from the channels to operate fuel cells efficiently. Furthermore, reaction products must be transported in the other direction. The gas transport is simulated in the through-plane direction of the GDL, and its microstructure created by a stochastic model is equivalent to the structure of real GDL material. Continuum approaches in cell-scale simulations have model parameters for porous regions that can be taken from effective properties calculated from the meso-scale simulation results, as one feature of multi-scale simulations. Another significant issue in multi-scale simulations is the interface between two regions. The focus is on the gas flow at the interface between GDL and the gas channel, which is analyzed using statistical methods. Quantitative relationships between functionality and microstructure can be detected. With this approach, virtual GDL materials can possibly be designed with improved transport properties. The evaluation of the surface flow with stochastic methods offers substantiated benefits that are suitable for connecting the meso-scale to larger spatial scales.

Fabrication of Stable and Well‐connected Proton Path in Catalyst Layer for High Temperature Polymer Electrolyte Fuel Cells

AbstractIt is of importance to establish stable and well‐connected proton path in the catalyst layer to promote the fuel cell performance. Here, we describe a novel method to fabricate stable and efficient proton path for high temperature polymer electrolyte fuel cells (HT‐PEFCs), in which the ionic liquid is doped into the platinum electrocatalyst. The electrochemical results depict that ionic liquid doped electrocatalyst exhibits comparable electrochemical surface area (ESA) and enhanced durability indicating that ionic liquid negligibly affects the hydrogen adsorption/desorption process and protects the electrocatalyst from carbon corrosion. Interestingly, the catalyzing activity toward oxygen reduction reaction (ORR) of Pt electrocatalyst is boosted after doping with ionic liquid mainly due to the modified electronic structures of Pt atoms induced by nitrogen atoms from ionic liquid resulting in weak interaction between Pt atoms and intermediates. The fuel cell performance of ionic liquid doped electrocatalyst is much improved ascribed to the homogeneously dispersed ionic liquid on the surface of Pt electrocatalyst facilitating the fabrication of triple phase boundaries (TPBs) as a result of efficient proton conduction in the catalyst layer. The fuel cell performance only decreases 10 % after 100,000 potential cycles from 1.0 to 1.5 V versus RHE suggesting that ionic liquid forms a stable proton path in the catalyst layer. Thus, the ionic liquid doped Pt electrocatalyst is applicable for the real HT‐PEFC operation.