Interplay between structure and properties in acid-base blend PBI-based membranes for HT-PEM fuel cells

Composite Proton-Conducting Membrane with Enhanced Phosphoric Acid Doping of Basic Films Radiochemically Grafted with Binary Vinyl Heterocyclic Monomer Mixtures

A composite proton conducting membrane (PCM) was prepared by radiation-induced grafting (RIG) of binary mixtures of 4-vinyl pyridine (4-VP) and 1-vinylimidazole (1-VIm) onto poly(ethylene-co-tetrafluoroethylene) (ETFE) film followed by phosphoric acid (PA) doping. The grafting parameters such as absorbed dose, temperature, monomer concentration, time, and monomer ratio were varied to control the degree of grafting (DG%). The effect of the reactivity ratio of 4-VP and 1-VIm on the composition and degree of monomer unit alternation in the formed graft copolymer was investigated. The changes in the chemical and physical properties endowed by grafting and subsequent PA acid doping were monitored using analytical instruments. The mechanical properties and proton conductivity of the obtained membrane were evaluated and its performance was tested in H₂/O₂ fuel cell at 120 °C under anhydrous and partially wet conditions. The acid doping level was affected by the treatment parameters and enhanced by increasing DG. The proton conductivity was boosted by incorporating the combination of pyridine and imidazole rings originating from the formed basic graft copolymer of 4-VP/1-VIm dominated by 4-VP units in the structure. The proton conductivity showed a strong dependence on the temperature. The membrane demonstrated superior properties compared to its counterpart obtained by grafting 4-VP alone. The membrane also showed a strong potential for application in proton exchange membrane fuel cells (PEMFC) operating at 120 °C.

Using Metal-Organic Framework HKUST-1 for the Preparation of High-Conductive Hybrid Membranes Based on Multiblock Copolymers for Fuel Cells

Novel proton-conducting hybrid membranes consisting of sulfonated multiblock copolymer of polysulfone and polyphenylsulfone (SPES) reinforced with a HKUST-1 metal-organic framework (MOF) (5, 10, and 20 wt. %) were prepared and characterized for fuel cell applications. The presence of the MOF in the copolymer was confirmed by means of FE-SEM and EDS. The hybrid membranes show a lower contact angle value than the pure SPES, in agreement with the water uptake (WU%), i.e., by adding 5 wt. % of the MOF, this parameter increases by 20% and 40% at 30 °C and 60 °C, respectively. Additionally, the presence of the MOF increases the ion exchange capacity (IEC) from 1.62 to 1.93 mequivH+ g−1. Thermogravimetric analysis reveals that the hybrid membranes demonstrate high thermal stability in the fuel cell operation temperature range (<100 °C). The addition of the MOF maintains the mechanical stability of the membranes (TS > 85 MPa in the Na+ form). Proton conductivity was analyzed using EIS, achieving the highest value with a 5 wt. % load of the HKUST-1. This value is lower than that observed for the HKUST-1/Nafion system. However, polarization and power density curves show a remarkably better performance of the hybrid membranes in comparison to both the pure SPES and the pure Nafion membranes.

A Novel High Temperature Fuel Cell Proton Exchange Membrane with Nanoscale Phase Separation Structure Based on Crosslinked Polybenzimidazole with Poly(vinylbenzyl chloride)

A semi-aromatic polybenzimidazole (DPBI) is synthesized via polycondensation of decanedioic acid (DCDA) and 3,3-diaminobenzidine (DAB) in a mixed phosphorus pentoxide/methanesulfonic acid (PPMA) solvent. Ascribing to in-situ macromolecular crosslinker of ploly((vinylbenzyl chloride) (PVBC), a robust crosslinked DPBI membrane (DPBI-xPVBC, x refers to the weight percentage of PVBC in the membrane) can be obtained. Comprehensive properties of the DPBI and DPBI-xPVBC membranes are investigated, including chemical structure, antioxidant stability, mechanical strength, PA uptake and electrochemical performances. Compared with pristine DPBI membrane, the PA doped DPBI-xPVBC membranes exhibit excellent antioxidative stability, high proton conductivity and enhanced mechanical strength. The PA doped DPBI-10PVBC membrane shows a proton conductivity of 49 mS cm^-1 at 160 °C without humidification. Particularly, it reveals an enhanced H₂/O₂ single cell performance with the maximum peak power density of 405 mW cm^-2, which is 29% higher than that of pristine DPBI membrane (314 mW cm^-2). In addition, the cell is very stable in 50 h, indicating the in-situ crosslinked DPBI with a macromolecular crosslinker of PVBC is an efficient way to improve the overall performance of HT-PEMs for high performance HT-PEMFCs.

Mesoscale Morphologies of Nafion-Based Blend Membranes by Dissipative Particle Dynamics

Polymer electrolyte membrane (PEM) composed of polymer or polymer blend is a vital element in PEM fuel cell that allows proton transport and serves as a barrier between fuel and oxygen. Understanding the microscopic phase behavior in polymer blends is very crucial to design alternative cost-effective proton-conducting materials. In this study, the mesoscale morphologies of Nafion/poly(1-vinyl-1,2,4-triazole) (Nafion-PVTri) and Nafion/poly(vinyl phosphonic acid) (Nafion-PVPA) blend membranes were studied by dissipative particle dynamics (DPD) simulation technique. Simulation results indicate that both blend membranes can form a phase-separated microstructure due to the different hydrophobic and hydrophilic character of different polymer chains and different segments in the same polymer chain. There is a strong, attractive interaction between the phosphonic acid and sulfonic acid groups and a very strong repulsive interaction between the fluorinated and phosphonic acid groups in the Nafion-PVPA blend membrane. By increasing the PVPA content in the blend membrane, the PVPA clusters’ size gradually increases and forms a continuous phase. On the other hand, repulsive interaction between fluorinated and triazole units in the Nafion-PVTri blend is not very strong compared to the Nafion-PVPA blend, which results in different phase behavior in Nafion-PVTri blend membrane. This relatively lower repulsive interaction causes Nafion-PVTri blend membrane to have non-continuous phases regardless of the composition.

NMR analysis of phosphoric acid distribution in porous fuel cell catalysts

Understanding the interaction of phosphoric acid and porous electrocatalysts is of vital importance for a rational design of high performance phosphoric acid based fuel cells on a molecular level. We demonstrate for the first time that NMR spectroscopy can be used for elucidating the preferred distribution sites of phosphoric acid in various nanoporous carbons. A pore size dependent negative chemical shift of 31P is observed, serving as a distinct signature of pore occupation, and applied for analysing a commercial catalyst (Pt@Vulcan) as a practical application.

Highly Conductive Polybenzimidazole Membranes at Low Phosphoric Acid Uptake with Excellent Fuel Cell Performances by Constructing Long-Range Continuous Proton Transport Channels Using a Metal-Organic Framework (UIO-66)

Phosphoric acid (PA)-doped polybenzimidazoles generally require high PA doping levels to achieve high conductivity as high-temperature proton exchange membranes. However, high PA doping levels result in a significant decrease in the mechanical properties of and PA leaching from the membranes. Herein, a Zr-based metal-organic framework material (UIO-66) was introduced into poly[2,2'-(p-oxydiphenylene)-5,5'-benzimidazole] (OPBI) membranes. The composite membranes exhibited long-range continuous proton transport channels when the mass ratio of UIO-66 to OPBI was ≥30 wt %. The long-range continuous proton transport channels endowed the composite membranes with high proton conductivity at low PA doping levels. When the doping of UIO-66 in the OPBI membrane reached 40 wt %, the membrane exhibited the highest proton conductivity (0.092 S cm^-1, at 160 °C) at a low PA uptake (73.25%), while the conductivity of the pristine OPBI membrane was 0.050 S cm^-1 with a high PA uptake (217.43%). Additionally, in the oxyhydrogen fuel cell test, 40%UIO-66@OPBI membranes exhibited excellent fuel cell performance with a peak power density of 583 mW cm^-2 at 160 °C, which is 50% higher than that of the pristine OPBI membrane (374 mW cm^-2). A single cell based on 40%UIO-66@OPBI also demonstrated good durability and could remain at about 600 mV after 500 h of operation under a constant load of 200 mA cm^-2.

Advancement toward Polymer Electrolyte Membrane Fuel Cells at Elevated Temperatures

Elevation of operational temperatures of polymer electrolyte membrane fuel cells (PEMFCs) has been demonstrated with phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. The technical perspective of the technology is simplified construction and operation with possible integration with, e.g., methanol reformers. Toward this target, significant efforts have been made to develop acid-base polymer membranes, inorganic proton conductors, and organic-inorganic composite materials. This report is devoted to updating the recent progress of the development particularly of acid-doped PBI, phosphate-based solid inorganic proton conductors, and their composite electrolytes. Long-term stability of PBI membranes has been well documented, however, at typical temperatures of 160°C. Inorganic proton-conducting materials, e.g., alkali metal dihydrogen phosphates, heteropolyacids, tetravalent metal pyrophosphates, and phosphosilicates, exhibit significant proton conductivity at temperatures of up to 300°C but have so far found limited applications in the form of thin films. Composite membranes of PBI and phosphates, particularly in situ formed phosphosilicates in the polymer matrix, showed exceptionally stable conductivity at temperatures well above 200°C. Fuel cell tests at up to 260°C are reported operational with good tolerance of up to 16% CO in hydrogen, fast kinetics for direct methanol oxidation, and feasibility of nonprecious metal catalysts. The prospect and future exploration of new proton conductors based on phosphate immobilization and fuel cell technologies at temperatures above 200°C are discussed.

Constructing anhydrous proton exchange membranes based on cadmium telluride nanocrystal-doped sulfonated poly(ether ether ketone)/polyurethane composites

Cadmium telluride (CdTe) nanocrystals with thiol stabilizers have been applied widely in the fields of energy storage and transformation. The aim of this work is to develop anhydrous proton exchange membranes (PEMs) by introducing CdTe nanocrystals bearing thioglycolic acid (tga) or mercaptopropionic acid (mpa) stabilizers into sulfonated poly(ether ether ketone) (SPEEK) and polyurethane (PU) systems. In the prepared SPEEK/PU/CdTe membranes, CdTe nanocrystals could provide desirable properties such as improving mechanical strength and enhancing proton conductivity by combining with phosphoric acid (PA) molecules. Successful preparation of SPEEK/PU/CdTe/PA membranes was demonstrated by the identification of high and stable proton conductivity and satisfactory thermal/chemical stability and mechanical properties. The fine appearance of membranes revealed uniform dispersion of components. Measurements of properties showed that the SPEEK(74%)/PU/CdTe-mpa(20/60/20)/100%PA membrane as a candidate anhydrous PEM is promising for use in high-temperature proton exchange membrane fuel cells. Specifically, the recommended membrane showed a proton conductivity of 1.18 × 10^-1 S cm^-1 at 160 °C and 3.96 × 10^-2 S cm^-1 at 100 °C, lasting for 600 h, and a tensile stress of 14.6 MPa at room temperature. Mixing inorganic CdTe nanocrystals with polymers to form inorganic/organic composite membranes is effective for producing anhydrous PEMs with cheaper polymers without functional groups to conduct protons.

High temperature composite membranes based on polybenzimidazole and dendrimer amine functionalized SBA-15 mesoporous silica for fuel cells

In this work SBA-15, a melamine-based functionalized SBA-15 mesoporous silica with a dicationic ionic liquid was used in order to improve the physicochemical properties of phosphoric acid doped polybenzimidazole membranes for application in HT-PEMFCs.

Fabrication of layered membrane electrolytes with spin coating technique as anhydrous proton exchange membranes

Spin coating technique is a simple and effective method to fabricate layered membranes and it has been widely used in the field of energy storage and transformation, biomaterials and electronics. The aim of this work is to develop anhydrous proton exchange membranes (PEMs) based on cheap polymers bearing the simple structure with spin coating technique. Successful fabrication of anhydrous PEMs based on polyvinylidene fluoride (PVDF) polymer, cadmium telluride (CdTe) nanocrystals and phosphoric acid (PA) molecules has been demonstrated by identification of high and stable proton conductivity. Specifically, (PVDF-CdTe-PA)₅/85%PA membranes present the maximum proton conductivity of 7.70 × 10^-2 S/cm at 160 °C and 1.42 × 10^-2 S/cm at 140 °C lasting 620 h. The decreased proton conduction resistance is revealed from the drastic reduction of activation energy (Ea) owing to the layered structure and the adsorption of PA molecules. The introduction of CdTe nanocrystals to form the organic/inorganic composite membranes that is substantially more effective at improving proton conductivity and stiffness, showing great promise in solving the dilemma of proton conductivity and mechanical property. This study provides the support to exploit anhydrous PEMs with more cheap polymers using spin coating technique.

Formation and investigation of dual cross-linked high temperature proton exchange membranes based on vinylimidazolium-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene

Dual-crosslinking provides a new strategy to enhance the dimensional and mechanical stabilities of membranes with high acid doping content and conductivity.