NMR and Electrochemical Investigation of the Transport Properties of Methanol and Water in Nafion and Clay-Nanocomposites Membranes for DMFCs

Elucidating the Water and Methanol Dynamics in Sulfonated Polyether Ether Ketone Nanocomposite Membranes Bearing Layered Double Hydroxides

Conventional Nafion membranes demonstrate a strong affinity for methanol, resulting in a high fuel crossover, poor mechanical stability, and thus poor performance in direct methanol fuel cells (DMFCs). This study involves the synthesis and physiochemical characterization of an alternative polymer electrolyte membrane for DMFCs based on sulfonated poly(ether ether ketone) and a layered double hydroxide (LDH) material. Nanocomposite membranes (sPL), with filler loading ranging between 1 wt% and 5 wt%, were prepared by simple solution intercalation and characterized by XRD, DMA, swelling tests, and EIS. For the first time, water and methanol mobility inside the hydrophilic channels of sPEEK-LDH membranes were characterized by NMR techniques. The introduction of LDH nanoplatelets improved the dimensional stability while having a detrimental effect on methanol mobility, with its self-diffusion coefficient almost two orders of magnitude lower than that of water. It is worth noting that anionic lamellae are directly involved in the proton transport mechanism, thus enabling the formation of highly interconnected paths for proton conduction. In this regard, sPL₃ yielded a proton conductivity of 110 mS cm⁻¹ at 120 °C and 90% RH, almost attaining the performance of the Nafion benchmark. The nanocomposite membrane also showed an excellent oxidative stability (over more than 24 h) during Fenton’s test at 80 °C. These preliminary results demonstrate that an sPL₃ nanocomposite can be potentially and successfully applied in DMFCs.

Mapping the Design of Electrolyte Materials for Electrically Rechargeable Zinc-Air Batteries

Electrically rechargeable zinc-air batteries (ERZABs) have attracted substantial research interest as one of the best candidate power sources for electric vehicles, grid-scale energy storage, and portable electronics owing to their high theoretical capacity, low cost, and environmental benignity. However, the realization of ERZABs with long cycle life and high energy and power densities is still a considerable challenge. The electrolyte, which serves as the ionic conductor, is one of the core components of ERZABs, as it plays a significant role during the discharge-charge process and greatly influences the rechargeability, operating voltage, lifespan, power density, and safety of ERZABs. Herein, the fundamental electrochemistry of electrolyte materials for ERZABs and the associated challenges are presented. Furthermore, recent advances in electrolyte materials for ERZABs, including alkaline aqueous electrolytes, nonalkaline electrolytes, ionic liquids, and semisolid-state electrolytes are discussed. This work aims to provide insights into the future exploration of high-performance electrolytes and thus promote the development of ERZABs.

Swelling layered minerals applications: A solid state NMR overview

Swelling layered clay minerals form an important sub-group of the phyllosilicate family. They are characterized by their ability to expand or contract in the presence or absence of water. This property makes them useful for a variety of applications, ranging from environmental technologies to heterogeneous catalysis, and including pharmaceutical and industrial applications. Solid State Nuclear Magnetic Resonance (SS-NMR) has been extensively applied in the characterization of these materials, providing useful information on their dynamics and structure that is inaccessible using other characterization methods such as X-ray diffraction. In this review, we present the key contributions of SS-NMR to the understanding of the mechanisms that govern some of the main applications associated to swelling clay minerals. The article is divided in two parts. The first part presents SS-NMR conventional applications to layered clay minerals, while the second part comprises an in-depth review of the information that SS-NMR can provide about the different properties of swelling layered clay minerals.

New Insights into Properties of Methanol Transport in Sulfonated Polysulfone Composite Membranes for Direct Methanol Fuel Cells

Methanol crossover through a polymer electrolyte membrane has numerous negative effects on direct methanol fuel cells (DMFCs) because it decreases the cell voltage due to a mixed potential (occurrence of both oxygen reduction and methanol oxidation reactions) at the cathode, lowers the overall fuel utilization and contributes to long-term membrane degradation. In this work, an investigation of methanol transport properties of composite membranes based on sulfonated polysulfone (sPSf) and modified silica filler is carried out using the PFG-NMR technique, mainly focusing on high methanol concentration (i.e., 5 M). The influence of methanol crossover on the performance of DMFCs equipped with low-cost sPSf-based membranes operating with 5 M methanol solution at the anode is studied, with particular emphasis on the composite membrane approach. Using a surface-modified-silica filler into composite membranes based on sPSf allows reducing methanol cross-over of 50% compared with the pristine membrane, making it a good candidate to be used as polymer electrolyte for high energy DMFCs.

Fluid Behavior in Nanoporous Silica

We investigate dynamics of water (H2O) and methanol (CH3OH and CH3OD) inside mesoporous silica materials with pore diameters of 4.0, 2.5, and 1.5 nm using low-field (LF) nuclear magnetic resonance (NMR) relaxometry. Experiments were conducted to test the effects of pore size, pore volume, type of fluid, fluid/solid ratio, and temperature on fluid dynamics. Longitudinal relaxation times (T1) and transverse relaxation times (T2) were obtained for the above systems. We observe an increasing deviation in confined fluid behavior compared to that of bulk fluid with decreasing fluid-to-solid ratio. Our results show that the surface area-to-volume ratio is a critical parameter compared to pore diameter in the relaxation dynamics of confined water. An increase in temperature for the range between 25 and 50°C studied did not influence T2 times of confined water significantly. However, when the temperature was increased, T1 times of water confined in both silica-2.5 nm and silica-1.5 nm increased, while those of water in silica-4.0 nm did not change. Reductions in both T1 and T2 values as a function of fluid-to-solid ratio were independent of confined fluid species studied here. The parameter T1/T2 indicates that H2O interacts more strongly with the pore walls of silica-4.0 nm than CH3OH and CH3OD.

Transport Properties and Mechanical Features of Sulfonated Polyether Ether Ketone/Organosilica Layered Materials Nanocomposite Membranes for Fuel Cell Applications

In this work, we study the preparation of new sulfonated polyether ether ketone (sPEEK) nanocomposite membranes, containing highly ionic silica layered nanoadditives, as a low cost and efficient proton exchange membranes for fuel cell applications. To achieve the best compromise among mechanical strength, dimensional stability and proton conductivity, sPEEK polymers with different sulfonation degree (DS) were examined. Silica nanoplatelets, decorated with a plethora of sulfonic acid groups, were synthesized through the one-step process, and composite membranes at 1, 3 and 5 wt% of filler loadings were prepared by a simple casting procedure. The presence of ionic layered additives improves the mechanical strength, the water retention capacity and the transport properties remarkably. The nanocomposite membrane with 5% wt of nanoadditive exhibited an improvement of tensile strength almost 160% (68.32 MPa,) with respect to pristine sPEEK and a ten-times higher rate of proton conductivity (12.8 mS cm⁻¹) under very harsh operative conditions (i.e., 90 °C and 30% RH), compared to a filler-free membrane. These findings represent a significant advance as a polymer electrolyte or a fuel cell application.

UV-crosslinked poly(arylene ether sulfone) – LAPONITE® nanocomposites for proton exchange membranes

UV-crosslinked sulfonated poly(arylene sulfone)/clay nanocomposites are fabricated by incorporating UV-crosslinkable monomers, bridge molecules, and clay nanofillers for high performance proton exchange membrane fuel cells.

Selectivity of Direct Methanol Fuel Cell Membranes

Sulfonic acid-functionalized polymer electrolyte membranes alternative to Nafion^® were developed. These were hydrocarbon systems, such as blend sulfonated polyetheretherketone (s-PEEK), new generation perfluorosulfonic acid (PFSA) systems, and composite zirconium phosphate–PFSA polymers. The membranes varied in terms of composition, equivalent weight, thickness, and filler and were investigated with regard to their methanol permeation characteristics and proton conductivity for application in direct methanol fuel cells. The behavior of the membrane electrode assemblies (MEA) was investigated in fuel cell with the aim to individuate a correlation between membrane characteristics and their performance in a direct methanol fuel cell (DMFC). The power density of the DMFC at 60 °C increased according to a square root-like function of the membrane selectivity. This was defined as the reciprocal of the product between area specific resistance and crossover. The power density achieved at 60 °C for the most promising s-PEEK-based membrane-electrode assembly (MEA) was higher than the benchmark Nafion^® 115-based MEA (77 mW·cm⁻²vs. 64 mW·cm⁻²). This result was due to a lower methanol crossover (47 mA·cm⁻² equivalent current density for s-PEEK vs. 120 mA·cm⁻² for Nafion^® 115 at 60 °C as recorded at OCV with 2 M methanol) and a suitable area specific resistance (0.15 Ohm cm² for s-PEEK vs. 0.22 Ohm cm² for Nafion^® 115).