Stack Performance of Proton Exchange Membrane Based Unitized Reversible Fuel Cells

Experimental study on laboratory scale Totalized Hydrogen Energy Utilization System using wind power data

If the necessary infrastructure can be provided, a practical future electric vehicle will be an efficient hydrogen-powered hybrid operating on a polymer fuel cell. Light weight, lowest cost and high efficiency imply an atmospheric pressure, rather than a pressurized, system. Polymer fuel cells require humidification for ionic conductivity, which means that product water is normally rejected as liquid. Atmospheric pressure systems of this type are difficult to operate. A self-humidified atmospheric pressure system operating at higher temperature (about 75-80 °C) on oxygen-enriched air rejecting product water in the vapour phase is described. It combines the advantages of unpressurized operation with the higher power density of a pressurized system, but with much lower parasitic power requirements.

Fuel cells – Proton-exchange membrane fuel cell | Cells

Recycling lithium-ion battery materials is a crucial step to achieving a circular economy. A hydrometallurgical recycling process of spent lithium-ion battery materials involves leaching by acids and precipitation by bases. A large amount of Li2SO4 leachate solution is generated at the end of the recycling process. Such Li2SO4 solution can be separated by electrodialysis (ED) to form H2SO4 and LiOH solutions, which are reused for leaching and precipitation, respectively, making the recycling a closed-loop process. In this study, an ‘electrodialysis stack constructed with bipolar membranes’ (EDBM) is built with repetitive unit cells consisting of a bipolar membrane, anion exchange membrane (AEM), and cation exchange membrane (CEM), see Fig. 1(a). The effective area for each membrane is 121.8 cm2 (70mm×174mm). The gap between these membranes is 0.8mm, with a mesh inserted to allow flow mixing. All experiments were operated at constant current mode. The feed, acid, and base solutions were recirculated through their tanks at initial concentrations of 1.1, 0.1, and 0.1 mol/L. The ion concentrations of the solutions were measured with Inductively coupled plasma mass spectrometry periodically. The weight and the pH value of the solution tanks were also recorded.The trend of concentration variations of the present stack with up to 6 cell pairs is found to be consistent with that observed with a three-compartment configuration in our previous study [1]. The concentration of H2SO4 increases with time linearly, whereas that of LiOH levels off due to a significant electro-osmosis that drags water through the CEM into the concentrate channel. The present setup differs from the three-compartment design in the bipolar membrane, which splits water to produce H+ and OH- at the internal interface, eliminating the formation of bubbly flows in the unit cell. It is found that the voltage loss across the bipolar membrane ca. 1V accounts for a major part of the unit cell of about 1.27V.The effects of current density, number of cell pairs, and flow rates on the overall stack performance are investigated. Increasing the stack current density accelerates the separation process at the cost of higher voltage losses. It is found that increasing the number of cell pairs can improve the overall production efficiency and reduce energy consumption per cell pair. For instance, at 800 A/m2, the specific energy consumption for LiOH production is reduced to 3.40 kWh/kg with five cell pairs versus 5.83 kWh/kg with two cell pairs. It is also found that increasing the solution flow rate can result into the increase of LiOH solution concentration, ion recovery rate, and current efficiency (Fig. 1b), although the impact of flow rate is low.A small EDBM stack is constructed and tested for Li2SO4 separation in the present study. This work demonstrates that EDBM can be a simple and energy-saving alternative to the current chemical precipitation method to produce LiOH from Li2SO4. The present study provides new insights into optimal operation and design for Li2SO4 EDBM. The findings are helpful in determining how the stack can be scaled up for practical application. Furthermore, a 2D multiphysics model is currently under development to elucidate the coupled transport processes within the unit cell. The model will be validated with experimental data of the present setup. Figure 1

A multifunctional electrocatalytic composite was synthesized for use as a generic catalytic material in the polymer electrolyte membrane (PEM) type reactors, working in either fuel cell, direct methanol fuel cell, or electrolyzer mode. It is constructed from graphene sheets evenly covered with TiONx on which PtCu, Ir, and Ru nanoparticles are immobilized. It enables high performances for oxygen reduction, hydrogen and methanol oxidation, and also hydrogen and oxygen evolution. Its activity is either higher or comparable to the monofunctional benchmark catalysts, namely, Pt, PtRu, and Ir nanoparticles. An important part of this study is the advanced electrochemical degradation investigation. With the use of an electrochemical flow cell coupled to ICP-MS and combining these results with identical location electron microscopy, we reveal not only potential shortcomings but also opportunities for our material. As a result, we also put forward general guidelines for the appropriate use of multifunctional electrocatalysts.

Reversible (unitised) PEM fuel cell devices

Thanks to recent considerable progress in proton exchange membrane (PEM) technology, fuel cells and electrolyzers are on the verge of widespread commercialization. When a fuel cell and an electrolyzer are combined, a regenerative fuel cell (RFC) system is formed. By using an auxiliary power supply, such as solar power, for recharging, an RFC provides a complete power system for niche markets such as low-earth-orbit (LEO) satellites. The thermodynamics of RFC systems are presented, and design tradeoffs are investigated: a unitized system, where the fuel cell and the electrolyzer are combined into a single electrochemical device, is compared to a discrete system, where the fuel cell and the electrolyzer are discrete components. The analyses show that the RFC is well suited for LEO space applications, due to an appropriate charge/discharge cycle, and represents a high-energy-density, high-efficiency power solution.

Unitized Regenerative fuel Cell (URFC) is an attractive and efficient method for producing hydrogen and clean energy. Nevertheless, to combine a polymer electrolyte water electrolyzer (PEMWE) and a polymer electrolyte fuel cell (PEMFC) is still a big challenge. Here, it is necessary to overcome several practical and structural features. For instance, the oxygen reduction (ORR) and the water oxidation (OER) are the limiting reaction steps at the oxygen electrode for PEMFC or PEMWE, respectively. Therefore, its high-efficiency depends on the type of electrocatalysts and the capability of the oxygen electrode to operate under the PEMFC or PEMWE conditions. As a consequence, a broad research is focused on developing a new design for the oxygen electrode in URFCs. In this work, several bifunctional electrodes for OER and ORR were designed by mixing electrocatalysts of Pt and IrO2 or Pt and RuO2 supported on Ebonex®. Elec- trochemical characterization by CV, LV and EIS in aqueous 0.5 M H2SO4 reveals that IrO2-Pt and RuO2-Pt supported on Ebonex®, exhibit high electrocatalytic properties for ORR and OER showing up a possible use in URFCs. In addition, IrO2 based electrodes display a higher stability than those based on RuO2.

Effect of titanium powder loading in gas diffusion layer of a polymer electrolyte unitized reversible fuel cell

The lifetime of proton exchange membrane (PEM) fuel cells depends largely on the durability of the PEM and is a critical issue in the commercialization of PEM fuel cell technology (1, 2). Perfluorosulfonated acid (PFSA) membranes, such as Nafion®, are the most widely used electrolyte material for PEM fuel cells.Many factors can cause PEM failure, including manufacturing and design issues, material properties, and fuel cell operation conditions. Each factor results in failure through a particular degradation mode. These modes can be classified as thermal, mechanical, and chemical modes (3-7). Because fuel cells typically operate at temperatures below that at which Nafion thermally degrades, chemical and mechanical degradation are generally considered the most important modes of membrane degradation.Although there has been a lot of progress in the area, the exact chemical membrane degradation mechanisms, the most susceptible points, and the impact of test conditions on degradation are not completely understood. The formation of radicals and their reactivity during the fuel cell operation have been confirmed as the main cause of PEM chemical degradation (8-11). These radicals are formed as a result of the reaction of crossover H2 and O2 on the surface of Pt or other metal ion contaminants (4). In addition to the electrodes, Pt particles have been observed to migrate into the membrane, producing another site for radical formation. Radical attack leads to loss of fluorine, sulfur, and organofluorine compounds from the PEM, and PEM thinning, all resulting in loss of mechanical integrity. The radical formation mechanisms and the impact of test conditions on radical formation are also not completely understood.It has been suggested that earlier versions of Nafion membranes chemically degrade through radical attack on carboxylic acid groups end groups (12-14) or at the C−S or O−C bonds in the side chain and main chain (12, 14-18), as shown in Figure 1. Chemically stabilized Nafion, e.g., Nafion 211, has been developed where the concentration of terminal carboxylic acid groups was decreased to negligible levels and, as a result, degradation decreased substantially (13). The use of radical scavengers such as ceria have increased the durability of PFSA membranes by as much as 1000-fold compared to unmitigated membranes (19). Despite the recent durability improvements, there is evidence that PFSA membranes are still susceptible to chemical damage.Typical indicators for chemical degradation of Nafion membranes include increased H2crossover, decreased ion exchange capacity, decreased proton conductivity, and decreased PEM thickness. Localized crossover tests have shown that membrane damage occurs preferably at the perimeter of the active area. The quantitative method of choice used for detecting chemical degradation is typically detection of fluoride in fuel cell outlets, reported as a fluoride emission rate (FER).Figure 1. Proposed locations of radical attack in NafionTo improve PEM chemical durability, a true understanding of the decay mechanisms is required. Models of some of the decay modes have been developed and they show a consistent but incomplete understanding of membrane decay. Through a combination of specific modeling and directed testing, an overall decay model can be developed.

Modeling is of key importance in fuel cell development beyond the current-state-of-the-art because it is beneficial to understand the mechanisms of various interacting phenomena and effects on the cell performance. It is also hard to measure the local parameters inside fuel cells, particularly inside the small scale functional materials. Modeling and simulation of charge transfer and electrochemical performance are critical to enable optimization of the geometry and performance. For macroscale modeling, the micro-and nano-structure related properties defining a porous media, e.g., the porosity and tortuosity and the specific area available for surface reactions are required. Thus the coupling of models valid at various scales is important for continued progress in the development of fuel cells. Within this paper both macroscale and micro-/nanoscale modeling approaches are presented for the fuel cell types SOFC (solid oxide fuel cell) and PEMFC (polymer electrolyte membrane fuel cell). In particular the electrodes, electrolyte and unit cells are considered. Some relevant results are provided. (Less)

Intermediate-temperature fuel cells have potential to be the cleanest and most efficient option for direct conversion to electricity of a wide variety of fuels, from hydrogen to hydrocarbons, coal gas, and bio-derived fuels. When operated in the reverse mode, on the other hand, they are very efficient for low-cost production of hydrogen from splitting water. Thus, a reversible fuel cell is ideally suited for large-scale energy storage and conversion, which is vital to the deployment of renewable energies. However, the commercialization of these systems hinges on rational design of novel materials of exceptional functionalities at lower temperatures to dramatically reduce the cost while enhancing performance and durability. A key challenge is the development of bifunctional catalysts, especially those for the oxygen electrode, for efficient dual-mode operation. Further, the reversible fuel cells must have high durability when cycled between the fuel cell and the electrolysis modes while maintaining high Faraday efficiency and roundtrip energy efficiency. The electrolyte materials must have high ionic transference number and excellent stability under the operating conditions. To accomplish these goals, it is imperative to gain a fundamental understanding of the mechanisms of charge and mass transport along surfaces, across interfaces, and through porous electrodes. Further, new protocols must be developed to control materials structure, composition, and morphology over multiple length scales. This presentation will highlight the critical scientific challenges facing the development of a new generation of reversible fuel cells based on oxygen ion- or proton- conducting electrolyte, the latest development of bifunctional electrodes and electrolyte materials, the strategies for enhancing electrode activity and durability of hydrogen evolution/oxidation, and oxygen reduction/evolution reactions in reversible fuel cells, the latest developments in modeling, simulation, and in situ characterization techniques for unraveling charge and mass transport mechanisms, and the outlook for future-generation energy storage systems that exploit nano-scale materials of significantly improved performance.

In recent years, the importance of sustainable energy sources in space exploration has been increasing, with regenerative fuel cells (RFCs) gaining attention. This study examines the design and performance evaluation of an RFC system with a focus on its application to lunar rovers. RFCs store energy in the form of hydrogen and oxygen and can efficiently generate electrical energy and water from the stored hydrogen and oxygen, making them particularly useful for long-duration missions.The Artemis program plans to explore the lunar south pole, with the possibility of future missions targeting the equatorial region. The lunar surface has a long day-night cycle, necessitating the storage of electricity generated by solar cells during the day for use during the long night. In this regard, RFCs, when combined with solar power generation, become a promising candidate for nighttime energy supply.Although RFCs are less efficient compared to lithium-ion batteries, they offer significant advantages in terms of energy storage density and mass per unit of stored energy. Additionally, the fact that the oxidizer and fuel can be stored independently from the cell enhances safety during large energy storage.At Toyota Motor Corporation, we have expertise and experience gained from our commercially available fuel cell electric vehicle (FCEV), the MIRAI, and we have developed water electrolysis technology in collaboration with Mitsubishi Heavy Industries (MHI) and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Based on these technologies, we have constructed an RFC system and conducted demonstration experiments.We will present the performance test results of the laboratory prototype and use these results to predict the performance of the RFC system for lunar rovers. Additionally, we will introduce several safety evaluation results, demonstrating that RFCs hold potential as a solution that meets the functional and safety requirements for lunar rovers.This research provides a new approach to energy storage and regenerative energy cycling for space vehicles and lays the foundation for future lunar exploration, long-duration missions, and the operation of lunar rovers.

To scale-up microbial fuel cells (MFCs), installing multiple unit cells in a common reactor has been proposed; however, there has been a serious potential drop when connecting unit cells in series. To determine the source of the loss, a basic stack-MFC (BS-MFC) has been devised, and the results show that the phenomenon is due to ions on the anode electrode traveling through the electrolyte to be reduced at the cathode connected in series. As calculated by means of the percentage potential drop, the degree of potential drop decreased with an increase in the unit-cell distance. When the distance was increased from 1 to 8 cm, the percentage potential drop in BS-MFC1 decreased from 46.76 ± 0.90 to 45.08 ± 0.70 % and in BS-MFC2 from 46.41 ± 0.95 to 43.82 ± 2.23 %. As the p-value of the t-test was lower than 0.05, the difference was considered significant; however, if the unit cells are installed far enough from each other to avoid the potential drop phenomenon, the system will be less dense, consequently reducing the ratio of electrode area per volume of anode compartment and decreasing the power density of the system. Finally, this study suggests design criteria for scaling-up MFC systems: Multiple-electrode-installed MFCs are modularized, and the unit cells are connected in series across the module (connecting each unit cell does not share the anolyte).

Introduction A pre-irradiation grafting method is a fascinating technique for direct introduction of a new functional polymer phase as a grafting chain into polymer films serving as a substrate which allows those films to retain their characteristics such as thermal stability, mechanical strength, and electronic properties. The radiation technique has been widely applied to the preparation of high performance fuel cell polymer electrolyte membranes (PEMs) for vehicles and domestic co-generation systems [1]. The PEMs of aromatic hydrocarbon polymers, so-called “super engineering plastics” including poly (ether ether ketone) (PEEK), have useful characteristics such as high mechanical strength, gas barrier property, and radiation resistance. However, PEEK films also have high chemical resistance; thus, it is difficult to introduce grafting monomers into the films. Recently, we succeeded in radiation-induced grafting of Ethyl 4-styrenesulfonate (E4S) into a PEEK film with grafting degrees around 100% [2]. We report herein the synthesis, characterization, and electrolyte properties of graft-type PEEK PEM with thin PEEK films less than 50 mm thickness. Fuel cell performance of graft-type PEEK PEM was estimated at 80 °C under 30 and 100% RH conditions. Experiments Graft-type PEEK PEM was synthesized by pre-irradiation grafting of E4S into PEEK films (Scheme 1) [3]. The PEEK films were immersed in a 10 wt% DVB solution of 1,4-dioxane at 50°C for 24h. The PEEK-g-DVB films were pre-irradiated with a 60Co γ-ray source (JAEA Takasaki, Gunma, Japan) at room temperature in argon atmosphere to a absorbed dose of 160 kGy. Then, the samples were immersed in 15 ml of the E4S solution of 1,4-dioxane (1/1 v/v) at 80 °C. The obtained PEEK-g-DVB-g-E4S film was hydrolyzed in 95 °C hot water for 24h (PEEK-g-DVB-g-PSSA PEM). The ion exchange capacity (IEC (mmol/g)), proton conductivity (S/cm) in-plane direction at room temperature, and water uptake (WU) was measured according to the previous report [2]. Proton conductivity was estimated at 80 °C in 30%RH; mechanical strength was measured at 80 °C in 100% RH, which is severe conditions for the fuel cell operation, was measured as previously reported [4]. The membrane/electrodes assembly (MEA) (Pt loading 0.5 mg/cm2) prepared by hot-press set into a 5 cm2. The fuel cell test was carried out at 80°C in a RH range of 30-100%. Results and Discussion According to the previously reported methods, we could prepare the PEEK-based PEM with grafting degrees of 88-107% and ion exchange capacity (IEC) of 2.3 – 2.5 mmol/g using the original PEEK substrates with film thickness of 6, 12, 16, 25, and 50 mm (Table 1). The variation of film thickness does not affect the conductivity and water uptake of the PEM. This is one of the thinnest PEM possessing sufficient conductivity as well as high mechanical strength (60 MPa of tensile strength at break). Then, we compare the fuel cell performance of the single fuel cell devices consisting of the PEEK-based PEM with various thicknesses in the operation condition at 80 ºC and 100%RH. As expected from the similar ion conductivity around 0.045 S/cm, the thinner PEM showed higher cell voltages at the current density in the range from 0.2 to 1.0 A/cm2. Especially, compared with a Nafion212 film (thickness: 50 μm), higher cell voltages (namely, power densities (W/cm2)) can be achieved in the PEM with film thickness less than 12 mm (Figure 1). The above results imply the advantages of mechanically tough PEEK as an original polymer substrate to be able to decrease the film thickness, resulting in higher power density owing to the less cell resistance. 19 μm PEEK PEM has good proton conductivity (0.0068 S/cm) at 80°C and 30%RH. The elastic modulus of the grafted film showed twice higher than that of Nafion 212 film at 80 degree and 100%RH. Acknowledgments For support of this work, the authors acknowledge the financial support from a Grant-in-Aid for Scientific Research (KAKENHI, No. 24550263) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.