Porous zirconium oxide nanotube modified Nafion composite membrane for polymer electrolyte membrane fuel cells operated under dry conditions

Recent Advanced Synthesis Strategies for the Nanomaterial-Modified Proton Exchange Membrane in Fuel Cells

Hydrogen energy is converted to electricity through fuel cells, aided by nanostructured materials. Fuel cell technology is a promising method for utilizing energy sources, ensuring sustainability, and protecting the environment. However, it still faces drawbacks such as high cost, operability, and durability issues. Nanomaterials can address these drawbacks by enhancing catalysts, electrodes, and fuel cell membranes, which play a crucial role in separating hydrogen into protons and electrons. Proton exchange membrane fuel cells (PEMFCs) have gained significant attention in scientific research. The primary objectives are to reduce greenhouse gas emissions, particularly in the automotive industry, and develop cost-effective methods and materials to enhance PEMFC efficiency. We provide a typical yet inclusive review of various types of proton-conducting membranes. In this review article, special focus is given to the distinctive nature of nanomaterial-filled proton-conducting membranes and their essential characteristics, including their structural, dielectric, proton transport, and thermal properties. We provide an overview of the various reported nanomaterials, such as metal oxide, carbon, and polymeric nanomaterials. Additionally, the synthesis methods in situ polymerization, solution casting, electrospinning, and layer-by-layer assembly for proton-conducting membrane preparation were analyzed. In conclusion, the way to implement the desired energy conversion application, such as a fuel cell, using a nanostructured proton-conducting membrane has been demonstrated.

Sulfonated NbS2-based proton-exchange membranes for vanadium redox flow batteries

In this work, novel proton-exchange membranes (PEMs) based on sulfonated poly(ether ether ketone) (SPEEK) and two-dimensional (2D) sulfonated niobium disulphide (S-NbS₂) nanoflakes are synthesized by a solution-casting method and used in vanadium redox flow batteries (VRFBs). The NbS₂ nanoflakes are produced by liquid-phase exfoliation of their bulk counterpart and chemically functionalized with terminal sulfonate groups to improve dimensional and chemical stabilities, proton conductivity (σ) and fuel barrier properties of the as-produced membranes. The addition of S-NbS₂ nanoflakes to SPEEK decreases the vanadium ion permeability from 5.42 × 10^-7 to 2.34 × 10^-7 cm² min^-1. Meanwhile, it increases the membrane σ and selectivity up to 94.35 mS cm^-2 and 40.32 × 10⁴ S min cm^-3, respectively. The cell assembled with the optimized membrane incorporating 2.5 wt% of S-NbS₂ nanoflakes (SPEEK:2.5% S-NbS₂) exhibits high efficiency metrics, i.e., coulombic efficiency between 98.7 and 99.0%, voltage efficiency between 90.2 and 73.2% and energy efficiency between 89.3 and 72.8% within the current density range of 100-300 mA cm^-2, delivering a maximum power density of 0.83 W cm^-2 at a current density of 870 mA cm^-2. The SPEEK:2.5% S-NbS₂ membrane-based VRFBs show a stable behavior over 200 cycles at 200 mA cm^-2. This study opens up an effective avenue for the production of advanced SPEEK-based membranes for VRFBs.

Composite Polymers Development and Application for Polymer Electrolyte Membrane Technologies-A Review

Nafion membranes are still the dominating material used in the polymer electrolyte membrane (PEM) technologies. They are widely used in several applications thanks to their excellent properties: high proton conductivity and high chemical stability in both oxidation and reduction environment. However, they have several technical challenges: reactants permeability, which results in reduced performance, dependence on water content to perform preventing the operation at higher temperatures or low humidity levels, and chemical degradation. This paper reviews novel composite membranes that have been developed for PEM applications, including direct methanol fuel cells (DMFCs), hydrogen PEM fuel cells (PEMFCs), and water electrolysers (PEMWEs), aiming at overcoming the drawbacks of the commercial Nafion membranes. It provides a broad overview of the Nafion-based membranes, with organic and inorganic fillers, and non-fluorinated membranes available in the literature for which various main properties (proton conductivity, crossover, maximum power density, and thermal stability) are reported. The studies on composite membranes demonstrate that they are suitable for PEM applications and can potentially compete with Nafion membranes in terms of performance and lifetime.

Enhancing the mechanical properties of zirconia/Nafion® nanocomposite membrane through carbon nanotubes for fuel cell application

Membranes are widely used daily, such as for filtration in reverse osmosis, or in the form of electrolyte membrane fuel cells. Modified Nafion^® membranes were synthesised by impregnation and their mechanical properties were observed. The effect of the incorporation of a ZrO₂-CNT nano-filler within Nafion^® membrane on the thermal stability and crystallinity was investigated by TGA and XRD. Tensile test results show the increases in the mechanical properties of Nafion^® 117 membranes impregnated with ZrO₂-CNT when compared with that of commercial Nafion^® 117 membranes. The results also show that adding ZrO₂-CNT in Nafion^® 117 membranes improves the water contact angle and water uptake, as it enhances water retention within the membrane. The SEM results indicated that ZrO₂-CNT was well distributed in the Nafion^® 117 membrane pores through the impregnation method.

Synthesis and evaluation of HfO2 as a prospective filler in inorganic-organic hybrid membranes based on Nafion for PEM fuel cells

Hybrid inorganic-organic Nafion membranes modified with metal oxides (typically TiO₂, ZrO₂, WO₃) are a good alternative for fuel cell applications. However, one of their main limitations is associated with their relative low proton conductivity at temperatures above 80 °C. In this work, we overcome this issue using HfO₂ as a filler. HfO₂ was prepared by a sol-gel method, and it was compared with a recast Nafion membrane (named as recast). Deconvolved XPS spectra confirmed the presence of hafnia, while EDS analysis was used to determine its weight content resulting in a 1.88 wt%. FT-IR ATR experiments indicated that the HfO₂ hybrid membrane possess a higher capability to retain water than the recast. Thus, the water uptake, swelling degree, conductivity tests and fuel cell evaluations were performed. The water uptake analysis revealed that the hybrid membrane presented a higher retention percentage at 100 °C (61%) than recast (29%). This improvement enabled a higher ionic conductivity at 80 °C and 100 °C. The hybrid membrane displayed a higher conductivity at 100 °C than the recast membrane (112 versus 82 mS cm^-1), increasing the cell performance to 0.36 W cm^-2; being this performance almost two-fold higher to that obtained for the recast membrane. In summary, herein we demonstrated that HfO₂ can be considered as an excellent substitute to conventional fillers.

Composite Proton-Exchange Membrane with Highly Improved Proton Conductivity Prepared by in Situ Crystallization of Porous Organic Cage

Porous organic cage, a kind of newly emerging soluble crystalline porous material, is introduced to proton-exchange membrane by in situ crystallization. The crystallized Cage 3 with intrinsic water-meditated three-dimensional interconnected proton pathways working together with Nafion matrix generates a composite membrane with highly improved proton conductivity. Different from inorganic crystalline porous materials, like metal-organic frameworks, the organic porous material shows better compatibility with Nafion matrix due to the absence of inorganic elements. In addition, Cage 3 can absorb water up to 20.1 wt %, which effectively facilitates proton conduction under both high- and low-humidity conditions. Meanwhile, the selectivity of Nafion-Cage 3 composite membrane is also elevated upon the loading of Cage 3. The proton conductivity is evidently enhanced without obvious increased methanol permeability. At 90 °C and 95% RH, the proton conductivity of NC3-5 reaches 0.27 S·cm^-1, highly improved compared to 0.08 S·cm^-1 of recast Nafion under the same condition. This study offers a new strategy for modifying proton-exchange membrane with crystalline porous materials.

Sulfonated graphene oxide/Nafion composite membranes for high temperature and low humidity proton exchange membrane fuel cells

Preparation process of Nafion/Fe₃O₄–SGO composite membranes.

Poly(2,5-benzimidazole)-Grafted Graphene Oxide as an Effective Proton Conductor for Construction of Nanocomposite Proton Exchange Membrane

To improve proton conduction properties of conventional sulfonated poly(ether ether ketone) (SPEEK), poly(2,5-benzimidazole)-grafted graphene oxide (ABPBI-GO) was prepared to fabricate nanocomposite membranes, which then were further doped with phosphoric acid (PA). The ABPBI-GO was synthesized through the reaction of 3,4-diaminobenzoic acid with the carboxyl acid groups present on the GO surface. The simultaneous incorporation of ABPBI-GO and PA into SPEEK did not only improve the physicochemical performance of the membranes in terms of thermal stability, water uptake, dimensional stability, proton conductivity, and methanol permeation resistance but also relieve PA leaching from the membranes though acid-base interactions. The resulting composite membranes exhibited enhanced proton conductivities in extended humidity ranges thanks to the hygroscopic character of PA and the increased water uptake. Moreover, the unique self-ionization, self-dehydration, and nonvolatile properties of PA improved the high-temperature proton conductivities (σ) of PA-doped membranes. The PA-doped SPEEK/ABPBI-GO-3.0 delivered a σ of 7.5 mS cm^-1 at 140 °C/0% RH. This value was fourfold higher than that of pristine SPEEK membranes. The PA-doped SPEEK/ABPBI-GO-3.0 based fuel cell membranes delivered power densities of 831.06 and 72.25 mW cm^-2 at 80 °C/95% RH and 120 °C/0% RH, respectively. By contrast, the PA-doped SPEEK membrane generated only 655.63 and 44.58 mW cm^-2 under the same testing conditions.

Enhanced proton conductivity of proton exchange membranes by incorporating phosphorylated hollow titania spheres

The synergistic enhancement in proton conductivity of hybrid membranes by improving the water retention capacity and introducing additional proton transfer sites is reported.