Polymer and Composite Membranes for Proton-Conducting, High-Temperature Fuel Cells: A Critical Review

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.

Synthesis of self-assembled nucleobases and their anhydrous proton conductivity

We synthesized self-assembled nucleobases (SANs), such as 1-dodecylthymine (DOT) or 9-dodecyladenine (DOA), in which the nucleobase is immobilized on a long alkyl chain. The thermal stability of the SAN was increased by mixing with the acidic surfactant mono-dodecyl phosphate (MDP). Additionally, the SAN-MDP composite material showed proton conductivity of 4.62 × 10-4 S cm-1 at 160 °C under anhydrous conditions. Additionally, the activation energy of the proton conduction was approximately 0.2 eV and this value was one order of magnitude higher than that of a typical humidified perfluorinated membrane, in which the proton can be moved by vehicle molecules, such as water molecules. In contrast, when the nucleobase without the immobilization of a long alkyl chain was mixed with MDP, the proton conductivity of these composite materials was two orders of magnitude less than that of the SAN-MDP composite. Therefore, we measured the XRD spectra of the SAN-MDP composite material. As a result, the SAN-MDP composite material showed a self-assembled structure with a two-dimensional proton conducting pathway, such as a lamellar structure, and that the anhydrous proton conduction was related to the interaction between the nucleobase of the SAN and the phosphate group of MDP. Consequently, the self-assembled nucleobase derivatives have the potential for use as novel anhydrous proton conductors with a two-dimensional proton conducting pathway.

Preparation of Nanoporous PdIrZn Alloy Catalyst by Dissolving Excess ZnO for Cathode of High- Temperature Polymer Electrolyte Membrane Fuel Cells

Carbon-supported nanoporous palladium-iridium–zinc (NP-PdIrZn) electrocatalyst was prepared through the modification of the alcohol-reduction process following the selective dissolution of excess ZnO nanoparticles using NaOH solution. The electrocatalyst was applied successfully to the cathode for a high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC). X-ray diffraction (XRD) patterns of the NP-PdIrZn nanoparticles suggests formation of the ternary alloy and complete removal of ZnO without the formation of individual Pd, Ir, or Zn nanoparticles. Moreover, transmission electron microscopy (TEM) images displayed porous nanoparticles with an irregular shape, which was generated by removing the ZnO from the PdIrZn–ZnO nanocomposites, and was prepared by using the excessive Zn precursor. The electrochemical surface area (ECSA) of the NP-PdIrZn catalysts was estimated by cyclic voltammetry using a rotating disk electrode method , and the oxygen reduction reaction (ORR) activity was evaluated by a linear sweep method. The NP-PdIrZn catalysts showed larger ECSA and higher ORR activity than those of the PdIr and PdIrZn catalysts, which may be attributed to the increased exposed surface area by selective etching of the ZnO in the composites. Furthermore, the NP-PdIrZn catalyst exhibited excellent performance (0.66 V) in a single cell under the HT-PEMFC condition than those of the PdIr (0.58 V) and PdIrZn (0.62 V) catalysts, indicating that geometric and electronic control of Pd-based alloy can improve the single-cell performance for the HT-PEMFC.

α-ZrP Nanoreinforcement Overcomes the Trade-Off between Phosphoric Acid Dopability and Thermomechanical Properties: Nanocomposite HTPEM with Stable Fuel Cell Performance

In recent times, high-temperature polymer electrolyte membranes (HTPEMs) have emerged as viable alternatives to the Nafion-based low-temperature-operated polymer electrolyte membrane fuel cells. This is owing to their higher tolerance to fuel impurities, efficient water management, and higher cathode kinetics. However, the most efficacious HTPEMs such as poly(benzimidazole) (PBI) or 2,5-poly(benzimidazole) (ABPBI), which rely on the extent of phosphoric acid (PA) doping level for fuel cell performance, suffer from poor mechanical properties at higher acid doping levels and dopant leaching during continuous operation. To overcome these issues, we report the synthesis of ABPBI membranes and fabrication of ABPBI-zirconium pyrophosphate (α-ZrP)-based nanocomposite membranes by an ex situ methodology using methane sulfonic acid as the solvent. The incorporation of hydrophilic α-ZrP into the membrane resulted in higher dopability of PA (6.5 mol) and proton conductivity (46 mS/cm) of the membranes (10 wt % of α-ZrP) as against the corresponding values of 3.6 mol and 27 mS/cm, respectively, for the pristine membrane. More remarkably, these property improvements could be achieved while simultaneously augmenting the thermomechanical properties and oxidative stability of the membranes. The unit-cell tests showed a marked improvement in the maximum power density for the nanocomposite membrane (335 mW/cm² at 10 wt % α-ZrP content) over the pristine ABPBI membrane (200 mW/cm²). We also report for the first time the feasibility of a 100 W HTPEM fuel cell (HTPEMFC) stack operated with the nanocomposite membrane with an active area of 39 cm². The HTPEMFC stack delivered a stable voltage and power output, with a voltage drop rate of 0.84 μV/h over a run time of 730 h.

Preparation and thermal treatment influence on Pt-decorated electrospun carbon nanofiber electrocatalysts

Crystalline platinum nanoparticles supported on carbon nanofibers were synthesized for use as an electrocatalyst for polymer electrolyte membrane fuel cells. The nanofibers were prepared by a method of electrospinning from polymer solution with subsequent pyrolysis. Pt nanoneedles supported on polyacrylonitrile pyrolyzed electrospun nanofibers were synthesized by chemical reduction of H₂[PtCl₆] in aqueous solution. The synthesized electrocatalysts were investigated using scanning, high resolution transmission and scanning transmission electron microscopies, EDX analysis and electron diffraction. The shape and the size of the electrocatalyst crystal Pt nanoparticles were controled and found to depend on the method of H₂[PtCl₆] reduction type and on conditions of subsequent thermal treatment. Soft Pt reduction by formic acid followed by 100 °C thermal treatment produced needle-shape Pt nanoparticles with a needle length up to 25 nm and diameter up to 5 nm. Thermal treatment of these nanoparticles at 500 °C resulted in partial sintering of the Pt needles. When formic acid was added after 24 h from the beginning of platinization, Pt reduction resulted in small-size spherical Pt nanoparticle of less than 10 nm in diameter. Reduction of H₂[PtCl₆], preadsorbed on electrospun nanofibers in formic acid with further treatment in H₂ flow at 500 °C, resulted in intensive sintering of platinum particles, with formation of conglomerates of 50 nm in size, however, individual particles still retain a size of less than 10 nm. Electrochemically active surface area (ECSA) of Pt/C catalyst was measured by electrochemical hydrogen adsorption/desorption measurements in 0.5 M H₂SO₄. ECSA of needle-shape Pt nanoparticles was 25 m² g⁻¹. It increased up to 31 m² g⁻¹ after thermal treatment at 500 °C, likely, due to amorphous structures removal from carbon nanofibers and retaining of Pt nanoneedle morphology. ECSA of small-size spherical Pt nanoparticles was 26 m² g⁻¹. Further thermal treatment at 500 °C in vacuum decreased ECSA down to 20 m² g⁻¹ due to Pt sintering and Pt active sites deactivation. The thermal treatment of small-size spherical Pt nanoparticles in H₂ flow at 500 °C produced agglomerates of Pt nanoparticles with ECSA of 14 m² g⁻¹.

Platinum nanoparticle electrocatalysts on carbon nanofibers are engineered and optimized by varying reduction procedure and thermal treatment temperature and medium.

Proton Conductivity through Polybenzimidazole Composite Membranes Containing Silica Nanofiber Mats

The quest for sustainable and more efficient energy-converting devices has been the focus of researchers' efforts in the past decades. In this study, SiO2 nanofiber mats were fabricated through an electrospinning process and later functionalized using silane chemistry to introduce different polar groups -OH (neutral), -SO3H (acidic) and -NH2 (basic). The modified nanofiber mats were embedded in PBI to fabricate mixed matrix membranes. The incorporation of these nanofiber mats in the PBI matrix showed an improvement in the chemical and thermal stability of the composite membranes. Proton conduction measurements show that PBI composite membranes containing nanofiber mats with basic groups showed higher proton conductivities, reaching values as high as 4 mS·cm-1 at 200 °C.

Enhanced Conductivity of Composite Membranes Based on Sulfonated Poly(Ether Ether Ketone) (SPEEK) with Zeolitic Imidazolate Frameworks (ZIFs)

The zeolitic imidazolate frameworks (ZIFs) ZIF-8, ZIF-67, and a Zn/Co bimetallic mixture (ZMix) were synthesized and used as fillers in the preparation of composite sulfonated poly(ether ether ketone) (SPEEK) membranes. The presence of the ZIFs in the polymeric matrix enhanced proton transport relative to that observed for SPEEK or ZIFs alone. The real and imaginary parts of the complex conductivity were obtained by electrochemical impedance spectroscopy (EIS), and the temperature and frequency dependence of the real part of the conductivity were analyzed. The results at different temperatures show that the direct current (dc) conductivity was three orders of magnitude higher for composite membranes than for SPEEK, and that of the SPEEK/ZMix membrane was higher than those for SPEEK/Z8 and SPEEK/Z67, respectively. This behavior turns out to be more evident as the temperature increases: the conductivity of the SPEEK/ZMix was 8.5 × 10-3 S·cm-1, while for the SPEEK/Z8 and SPEEK/Z67 membranes, the values were 2.5 × 10-3 S·cm-1 and 1.6 × 10-3 S·cm-1, respectively, at 120 °C. Similarly, the real and imaginary parts of the complex dielectric constant were obtained, and an analysis of tan δ was carried out for all of the membranes under study. Using this value, the diffusion coefficient and the charge carrier density were obtained using the analysis of electrode polarization (EP).

Unique Proton Transportation Pathway in a Robust Inorganic Coordination Polymer Leading to Intrinsically High and Sustainable Anhydrous Proton Conductivity

Although comprehensive progress has been made in the area of coordination polymer (CP)/metal-organic framework (MOF)-based proton-conducting materials over the past decade, searching for a CP/MOF with stable, intrinsic, high anhydrous proton conductivity that can be directly used as a practical electrolyte in an intermediate-temperature proton-exchange membrane fuel cell assembly for durable power generation remains a substantial challenge. Here, we introduce a new proton-conducting CP, (NH₄)₃[Zr(H_2/3PO₄)₃] (ZrP), which consists of one-dimensional zirconium phosphate anionic chains and fully ordered charge-balancing NH₄⁺ cations. X-ray crystallography, neutron powder diffraction, and variable-temperature solid-state NMR spectroscopy suggest that protons are disordered within an inherent hydrogen-bonded infinite chain of acid-base pairs (N-H···O-P), leading to a stable anhydrous proton conductivity of 1.45 × 10^-3 S·cm^-1 at 180 °C, one of the highest values among reported intermediate-temperature proton-conducting materials. First-principles and quantum molecular dynamics simulations were used to directly visualize the unique proton transport pathway involving very efficient proton exchange between NH₄⁺ and phosphate pairs, which is distinct from the common guest encapsulation/dehydration/superprotonic transition mechanisms. ZrP as the electrolyte was further assembled into a H₂/O₂ fuel cell, which showed a record-high electrical power density of 12 mW·cm^-2 at 180 °C among reported cells assembled from crystalline solid electrolytes, as well as a direct methanol fuel cell for the first time to demonstrate real applications. These cells were tested for over 15 h without notable power loss.

On the nanosecond proton dynamics in phosphoric acid–benzimidazole and phosphoric acid–water mixtures

Combining ¹H-NMR, ¹⁷O-NMR, and high-resolution backscattering QENS hydrodynamic and structural proton transport in phosphoric acid is separated. The rate limiting steps for structural proton diffusion in mixtures of acid with Brønsted bases are found to occur below the nanosecond timescale.