Proton Conductance Pathway Research Articles

Super Proton Conductive Nafion/Short Fiber/Nanosilica/Deep Eutectic Solvent (DES) Composite Membrane for Application in Anhydrous Fuel Cells

AbstractThe aim of present study is to reduce the tortuosity of proton conductive pathways within the polymer electrolyte membranes (PEMs) for application in anhydrous PEM fuel cells. For this purpose, for the first time, microsize chopped fibers (modified glass wool fibers), modified nanosilica, and a deep eutectic solvent (DES) are incorporated into the Nafion matrix to extend proton conductive pathways. The long length of the fibers as new and cheap additives creates a continuous interface within the polymer matrix which significantly improves its proton conductivity, likely by reducing the tortuosity of proton conductive pathways. While nanosilica promotes the connectivity of the short fibers, EG/NaBr‐based DES is also used as a nonaqueous electrolyte to maintain proton conductivity at anhydrous conditions. The measurements show super proton conductivity beyond 1400 mS cm−1 at 100 °C and anhydrous conditions for the Nafion/DES composite membrane containing 5 wt% short fibers and 2 wt% nanosilica. Furthermore, fuel cell performance of the prepared membranes shows a promising result by providing a maximum power density of 715 mW cm−2 at 95 °C. This study clearly shows the important role of ionic paths within the PEMs to achieve desirable properties in the real condition of a fuel cell system.

Anhydrous Proton Conductivity in HAp-Collagen Composite

It is well known that a proton conductor is needed as an electrolyte of hydrogen fuel cells, which are attracting attention as an environmentally friendly next-generation device. In particular, anhydrous proton-conducting electrolytes are highly desired because of their advantages, such as high catalytic efficiency and the ability to operate at high temperatures, which will lead to the further development of fuel cells. In this study, we have investigated the proton-conducting properties of the hydroxyapatite (HAp)-collagen composite without external humidification conditions. It was found that, by injecting HAp into collagen, the electrical conductivity becomes higher than that of the HAp or the collagen. Moreover, the motional narrowing of the proton NMR line is observed above 130 °C. These results indicate that the electrical conductivity observed in the HAp-collagen composite is caused by mobile protons. Furthermore, we measured the proton conduction of HAp-collagen composite films with different HAp contents and investigated the necessity of the appearance of proton conductivity in HAp-collagen composites. HAp content (n = 0–0.38) is the number of HAp per collagen peptide representing Gly-Pro-Hyp. These results indicate that injection of HAp into collagen decreases the activation energy of proton conduction which becomes almost constant above a HAp content n of 0.3. It is deduced that the proton-conduction pathway in the HAp-collagen composite is fully formed above n = 0.3. Furthermore, these results indicate that the value of the activation energy of proton conductivity was lowered, accompanied by the formation of the HAp-collagen composite, and saturated at n > 0.3. From these results, the HAp-collagen composite forms the proton-conduction pathway n > 0.3 and becomes the proton conductor with no external humidification in the condition of n > 0.3 above 130 °C.

Insight into the High Proton Conductivity of One-/Two-Dimensional Cadmium Phosphites.

The study describes the synthesis and structural attributes of two new cadmium phosphites, [Cd{OP(O)(OH)H}2(4,4'-bipy)] (1) and [H2pip][Cd(HPO3)2(H2O)]·H2O (2). The structure of 1 adopts a two-dimensional motif featuring alternate [Cd-μ2-O]2 and [Cd-O-P-O]2-cyclic rings, while the inorganic chains are held together by 4,4'-bipyridine. The presence of strong hydrogen bonding interactions between the appended H2PO3 groups (O---O = 2.55 Å) provides a facile proton conduction pathway and results in a proton conductivity of 3.2 × 10-3 S cm-1 at 75 °C under 77% relative humidity (RH). Compound 2 comprises an anionic framework formed by vertex-shared [Cd-O-P-O]2-cyclic rings, while the [H2pip] cations between the adjacent chains assist a well-directed O-H---O hydrogen-bonded network between coordinated water, lattice water, and phospite groups. The bulk proton conductivity value under conditions as in 1 reaches 4.3 × 10-1 S cm-1. For both 1 and 2, the proton conductivity remains practically unchanged under ambient temperatures (25-35 °C), suggesting their potential in low-temperature fuel cells.

Synergistically promoted proton conduction of proton exchange membrane by phosphoric acid functionalized carbon nanotubes and graphene oxide

One dimensional (1D) and two dimensional (2D) nanomaterials with suitable functional groups possess great potential as proton conduction accelerators due to their long-range proton conductive feature. Herein, two outstanding proton conduction accelerators, phosphoric acid functionalized 1D carbon nanotubes and 2D graphene oxide (PCNT and PGO) were prepared, and then incorporated into Nafion matrix to obtain the codoped proton exchange membrane (PEM). Compared with common sulfonation, phosphorylation exhibits the prospect of more efficient proton conduction due to the amphoteric characteristic, large hydration energy and polarizability of phosphoric acid group (–PO3H2). Introduction of PCNT and PGO provided extra excellent proton conductive sites and improved water retention capacity. More importantly, codoping of PCNT and PGO made their 1D and 2D proton conduction pathways combine as hierarchical proton conduction pathways for better interconnecting ionic clusters, which further generated synergistic effect to form more consecutive proton transfer channels in the codoped PEM, whose proton conductivity climbed to 0.282 S/cm under 90 °C, 95% RH, distinctly greater than that of the recast Nafion (0.130 S/cm). Meanwhile, the codoped PEM attained a peak power density of 493.8 mW/cm2, which was approximately 72% larger than that of the recast Nafion (286.8 mW/cm2).

Novel bifunctional fillers (ATP/P–CNOs) for sulfonated poly(aryl ether sulfone) matrix for improved power output and durability of H2/O2 fuel cell at low humidity

The preparation of novel composite proton exchange membranes (PEMs) with homogeneous structures and efficient proton-conducting sites is of great importance for the development of PEM fuel cells. In this work, a double-filler of adenosine triphosphate (ATP) and phosphorylated carbon nano-onions (P–CNOs) is incorporated into sulfonated poly(aryl ether sulfone) (SPAES) by simple solution mixing to design a bionic proton-conduction pathways for high-performance PEMs. A series of uniformly distributed, dense and flexible SPAES/ATP/P–CNOs membranes are prepared and characterized for their membrane morphology, water uptake, dimensional stability, electrochemical properties and durability. Both the ATP and P–CNOs fillers can provide abundant proton conducting sites through hydrogen-bonding networks and acid-base pairs, thus improving the membrane stability and proton conductivity. Thanks to the formation of ionic interaction between ATP/P–CNOs and their interaction with SPAES substrates, the composite membranes demonstrate higher ATP retention, better dimensional-mechanical-chemical stability, higher proton conductivity and power output than the pristine and single-filler SPAES membranes. The SPAES/ATP/P–CNOs-2 membrane displays high proton conductivity (196 mS/cm at 90 °C in water and 33 mS/cm at 80 °C/51% relative humidity, RH) and power output (752 mW/cm2 at 80 °C/100% RH and 302 mW/cm2 at 80 °C/50% RH) in a H2/O2 fuel cell. It also experiences the minimal cell performance loss and lowest hydrogen crossover during 6 days of operation at 60% RH. The proposed ATP/P–CNOs channel approach can provide a reference for the development of multi-component composite high-efficiency PEMs for fuel cells.

Water cluster encapsulated polyoxometalate-based hydrogen-bonded supramolecular frameworks (PHSFs) as a new family of high-capacity electrode materials

Water cluster encapsulated polyoxometalate (POM)-based hydrogen-bonded supramolecular frameworks (PHSFs) are novel advanced high-capacity electrode materials for supercapacitors thanks to synergistic effects from internal components as well as the existence of numerous electronic transports and proton conductive pathways formed by hydrogen bond interactions. In this work, three new water cluster encapsulated PHSFs: [Cu2(H2O)4H2(imbta)4](PMo12O40)2·6H2O (1), [Cu (H2O)2H4(pybta)4](PMo12O40)2·2H2O (2) and [Ag2H7(pybta)6(PMo12O40)3]·12H2O (3) as well as one new PHSF: [AgH2(imbta)2](PMo12O40) (4) were hydrothermally synthesized, where the imbta and pybta are 1-imidazol-1-ylmethyl-1H-benzotriazole and 1-pyridin-3-ylmethyl-1H-benzotriazole, respectively. The electrochemical properties of the four compound-based electrodes were estimated by cyclic voltammetry, galvanostatic charge-discharge, electrochemically active surface area, and electrochemical impedance spectroscopy. In particular, due to the unique water-cluster-encapsulated PHSF structure with a large number of O–H⋯O interactions, the 1-based electrode has the highest specific capacitance of 710 F·g−1 at 1 A·g−1 among all the four electrodes, which is superior to that of most POM-based electrodes to date. Furthermore, the 1-based electrode exhibits an excellent capacitance retention about 91.2 % after 1000 cycles at a high charge/discharge current density of 10 A·g−1. The results of this study provide inspiration and guidance for the construction of PHSF-based electrode materials, which have broad application potential in a new-generation of high-performance energy storage devices.

Molecular dynamics simulation analysis of sulfonated polybenzimidazole/[formula omitted]: A potential polymer electrolyte membrane for high-temperature fuel cells

In this research work, the ionic conductivity performance of a prospective candidate polymer electrolyte membrane, sulfonated polybenzimidazole/[DEMA+][NTf2-], was investigated using the molecular dynamics simulation technique. At first, a unit cell consisting of three chains (each one consisting 30 monomers) of polybenzimidazole polymer was determined as the optimum amorphous unit cell by comparing the simulated solubility parameter and glass transition temperature with available experimental data. Furthermore, the behavior of the ionic self-diffusion coefficients was investigated using the mean square displacement curves. It was found that the diffusion of [DEMA+] and [NTf2-] depends on the degrees of sulfonation (DS) values: with increasing DS, the ionic self-diffusion coefficients enhance, which could be attributed to a reduction in [DEMA+][NTf2-]- polymer chains interactions. The results revealed that the simulated ionic conductivities of this ionic liquid-based membrane is comparable with other electrolyte membranes and would be suitable for high-temperature applications. In addition, the investigation was complemented with analysis of plausible proton conduction pathways by evaluating the radial/angular distribution function and cationic/anionic clusters. The results suggest that proton hopping through very short ionic bridges is possible but could not be dominated. This work would pave the way for further research into the new protic ionic liquid -doped sulfonated polybenzimidazole membranes for proton exchange membrane fuel cells.

Nonconformal Particles of Hyperbranched Sulfonated Phenylated Poly(phenylene) Ionomers as Proton-Conducting Pathways in Proton Exchange Membrane Fuel Cell Catalyst Layers

Characteristic poor electrochemical kinetics, high ionic resistance, and high mass transport resistance within the catalyst layer (CL) are chief among parameters that cause poor performance of proton exchange membrane fuel cells (PEMFCs) utilizing hydrocarbon-based proton-conducting ionomers. Herein, the design and addition of nondimensionally swellable, nonconformal, hyperbranched sulfo-phenylated poly(phenylene) ionomer particles (HB-sPPT-H+) are reported to introduce a direct pathway for proton conduction in hydrocarbon ionomer-based CLs, resulting in an eight times reduction in ionic resistance of the CL, a 71% increase in catalyst mass activity, and a >90% increase in power at 0.6 V (H2/air) compared to state-of-the-art hydrocarbon ionomer-based CLs. The benefits of incorporating HB-sPPT-H+ ionomer particles are also shown when employed in perfluorosulfonic acid (PFSA) ionomer-based PEMFCs. These results dispel a commonly held conception that hydrocarbon ionomers possess limitations of gas permeability and electrochemical activity and open up previously unexplored avenues of ionomer development for nonfluorous, wholly hydrocarbon PEMFCs.

Molecular-Level Control over Ionic Conduction and Ionic Current Direction by Designing Macrocycle-Based Ionomers.

Poor ionic conductivity of the catalyst-binding, sub-micrometer-thick ionomer layers in energy conversion and storage devices is a huge challenge. However, ionomers are rarely designed keeping in mind the specific issues associated with nanoconfinement. Here, we designed nature-inspired ionomers (calix-2) having hollow, macrocyclic, calix[4]arene-based repeat units with precise, sub-nanometer diameter. In ≤100 nm-thick films, the in-plane proton conductivity of calix-2 was up to 8 times higher than the current benchmark ionomer Nafion at 85% relative humidity (RH), while it was 1–2 orders of magnitude higher than Nafion at 20–25% RH. Confocal laser scanning microscopy and other synthetic techniques allowed us to demonstrate the role of macrocyclic cavities in boosting the proton conductivity. The systematic self-assembly of calix-2 chains into ellipsoids in thin films was evidenced from atomic force microscopy and grazing incidence small-angle X-ray scattering measurements. Moreover, the likelihood of alignment and stacking of macrocyclic units, the presence of one-dimensional water wires across this macrocycle stacks, and thus the formation of long-range proton conduction pathways were suggested by atomistic simulations. We not only did see an unprecedented improvement in thin-film proton conductivity but also saw an improvement in proton conductivity of bulk membranes when calix-2 was added to the Nafion matrices. Nafion–calix-2 composite membranes also took advantage of the asymmetric charge distribution across calix[4]arene repeat units collectively and exhibited voltage-gating behavior. The inclusion of molecular macrocyclic cavities into the ionomer chemical structure can thus emerge as a promising design concept for highly efficient ion-conducting and ion-permselective materials for sustainable energy applications.

A Preinstalled Protic Cation as a Switch for Superprotonic Conduction in a Metal-Organic Framework.

Metal–organic frameworks (MOFs), made from various metal nodes and organic linkers, provide diverse research platforms for proton conduction. Here, we report on the superprotonic conduction of a Pt dimer based MOF, [Pt2(MPC)4Cl2Co(DMA)(HDMA)·guest] (H2MPC, 6-mercaptopyridine-3-carboxylic acid; DMA, dimethylamine). In this framework, a protic dimethylammonium cation (HDMA+) is trapped inside a pore through hydrogen bonding with an MPC ligand. Proton conductivity and X-ray measurements revealed that trapped HDMA+ works as a preinstalled switch, where HDMA+ changes its relative position and forms an effective proton-conducting pathway upon hydration, resulting in more than 105 times higher proton conductivity in comparison to that of the dehydrated form. Moreover, the anisotropy of single-crystal proton conductivity reveals the proton-conducting direction within the crystal. The present results offer insights into functional materials having a strong coupling of molecular dynamic motion and transport properties.

Brown Adipose Tissue: A Short Historical Perspective.

Brown adipose tissue (BAT) was first identified by Conrad Gessner in 1551, but it was only in 1961 that it was firmly identified as a thermogenic organ. Key developments in the subsequent two decades demonstrated that: (1) BAT is quantitatively important to non-shivering thermogenesis in rodents, (2) uncoupling of oxidative phosphorylation through a mitochondrial proton conductance pathway is the central mechanism by which heat is generated, (3) uncoupling protein-1 is the critical factor regulating proton leakage in BAT mitochondria. Following pivotal studies on cafeteria-fed rats and obese ob/ob mice, BAT was then shown to have a central role in the regulation of energy balance and the etiology of obesity. The application of fluorodeoxyglucose positron emission tomography in the late 2000s confirmed that BAT is present and active in adults, resulting in renewed interest in the tissue in human energetics and obesity. Subsequent studies have demonstrated a broad metabolic role for BAT, the tissue being an important site of glucose disposal and triglyceride clearance, as well as of insulin action. BAT continues to be a potential target for the treatment of obesity and related metabolic disorders.

Post-synthesis functionalization of ZIF-90 with sulfonate groups for high proton conduction.

Introducing sulfonic acid groups into MOF materials is one of the effective approaches to enhance proton conduction. Here, we attempted to prepare a new post-modified ZIF-90-based material by addition reaction of the aldehyde group with bisulfite to obtain partially functionalized ZIF-90-SO3Na(2.3). ZIF-90-SO3Na(2.3) exhibits a high proton conductivity of 2.26 × 10-2 S cm-1 at 98% RH and 100 °C.

PANa/Covalent organic framework composites with improved water uptake and proton conductivity.

It is a challenge to effectively load proton carriers in COFs to improve their proton conductivity. Herein, we report a series of COF-based composites, PANa@UCOF-x (PANa: sodium polyacrylate, x: weight percentage of PANa), which were prepared by coating different proportions of superabsorbent PANa on the COF surface based on an in situ reaction strategy. Since PANa can greatly enhance the enrichment of water molecules in one-dimensional (1D) channels of COFs, these COF-based composites exhibit superprotonic conduction. At 80 °C and 95% relative humidity (RH), the proton conductivity of PANa@UCOF-10, PANa@UCOF-28 and PANa@UCOF-40 reaches 1.6 × 10-2, 5.1 × 10-2, and 1.1 × 10-1 S cm-1, respectively, which is 4-5 orders of magnitude higher than 7.4 × 10-7 S cm-1 of the original UCOF. This work not only develops a new method to improve the water content of the COF channels, but also proves the important role of ordered channels in constructing effective proton conduction pathways.

Molecular design of a series of gemini-type zwitterionic amphiphiles with various linker lengths: control of their self-organisation for developing gyroid nanostructured proton conductive membranes

We have developed our strategy to create gyroid nanostructured polymer membranes having a 3D continuous proton conduction pathway.

A Nafion/polybenzimidazole composite membrane with consecutive proton-conducting pathways for aqueous redox flow batteries

A composite membrane with consecutive proton-conducting pathways is designed for aqueous redox flow batteries. The high proton conductivity and ion selectivity are endowed respectively by the interconnected Nafion nanofibers and PBI matrix.

Special Section on Mass and Charge Transport in Fuel Cells and Metal-Ion Batteries

Abstract Given the increasing energy demand and carbon dioxide emission, countries all over the world are vigorously developing sustainable and clean energy. Fuel cells and metal-ion batteries that directly convert chemical energy into electric energy have been receiving ever-increasing attention for energy conversion and storage in several applications such as portable, mobile, and stationary applications. Nowadays, not understanding mass and charge transport in fuel cells and metal-ion batteries, which results in low performance and durability, are still challenges for their large-scale commercialization. For example, the insufficient interaction of catalyst/ionomer/reactant as a result of fuel cells lacking the ion-conducting, reactant-delivering, or proton-conducting pathways leads to the deactivated triple-phase boundary. Meanwhile, the metal-ions transport in the interface of solid active materials and electrolyte, and the charge transport including ions transport in the electrolyte, and electron transport in the solid phase, are not well known in advanced metal-ion batteries. An ideal electrode architecture that boosts the performance and durability of cells and batteries needs the electrode design to meet all the requirements of electrochemical kinetics and mass and charge transport characteristics.

Development of Polymer Electrolyte Fuel Cell (PEFC) Cathodes with High Oxygen Permeability Ionomer (HOPI) for High Performance and Durability

The electrolyte-catalyst interface in the cathodes of polymer electrolyte fuel cells (PEFCs) has a significant impact on the utilization, performance, and durability of fuel cell systems. The ionomer plays a critical role in providing a facile proton conduction pathway from the membrane to the Pt-based catalyst and creating an acidic local environment for the oxygen reduction reaction with high specific activity. However, the commonly used perfluorosulfonic acid (PFSAs) ionomers also negatively impact performance by hindering oxygen transport and partial poisoning of the catalyst by the anionic acid groups. There is a growing consensus that the densification of the PFSA ionomers at the Pt-alloy catalyst greatly increases the local oxygen transport resistance relative to permeation through bulk PFSA films. To address this issue, new high oxygen permeability ionomers (HOPIs) that resist the densification are being developed. This presentation will present our development of membrane electrode assemblies (MEAs) with HOPI in the cathode. The results include a model-based analysis, ink and coating optimization, and experimental fuel cell and transport characterization. The results highlight the significant increase in voltage efficiency and maximum power density when fabricating cathodes with HOPI as well as the significant positive impact on performance durability.This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office, Award Number DE-EE0008822.

Multiple Strategies to Fabricate a Highly Stable 2D CuIICuI–Organic Framework with High Proton Conductivity

Using multifunctional organic ligands with multiple acidic groups (carboxylate and sulfonate groups) to synthesize metal-organic frameworks (MOFs) bearing effective H-bond networks is a promising strategy to obtain highly proton conductive materials. In this work, a highly stable two-dimensional MOF, [CuII5CuI2(μ3-OH)4(H2O)6(L)2(H2L)2]·3H2O (denoted as YCu161; H3L = 6-sulfonaphthalene-1,4-dicarboxylic acid) containing mixed-valence [CuII5CuI2(μ3-OH)4]8+ subunits, was successfully prepared. It exhibited excellent stability and temperature- and humidity-dependent proton conduction properties. Its optimal proton conductivity reached 1.84 × 10-3 S cm-1 at 90 °C and 98% relative humidity. On the basis of a crystal structure analysis, water vapor adsorption test results, and activation energy calculations, we deduced the proton conduction pathway and mechanism. Apparently, uncoordinated sulfonic and carboxyl groups and a network of abundant H-bonds inside the framework were responsible for the efficient proton transfer. Therefore, the strategy of selecting suitable bifunctional ligands to construct two-dimensional Cu-cluster-based MOFs with excellent proton conductivity is feasible.

Surface architecture and proton conduction in SPVDF-co-HFP based nanocomposite membrane for fuel cell applications: Influence of aprotic solvent mixture

In the present study, the prime focus is to study the effect of a new solvent mixture on the formation of PVDF-co-HFP based nanocomposite membrane by solution casting method and their applicability to perform as a proton conducting membrane for fuel cell applications. A definite proportion two dipolar aprotic solvent mixture N-Methyl-2-pyrrolidone (NMP) and Acetonitrile (ACN) was used as the solvent mixture and it has been observed that the use of these solvents results in a porous structure with smaller micro domains within it, as evidenced from the FESEM micrographs. The mechanical resistance of the membranes also significantly improved with the change of solvent resulting in the formation of a ductile membrane. Incorporation of in-situ synthesized silanized silica nanoparticles into the polymer backbone following the sulfonation of the membranes using chlorosulfonic acid, also contributed in the improved physicochemical and the electrochemical properties with respect to higher water retention, better ion exchange capability (0.98 meq. g−1) and proton conductivities. Additionally, the silanized silica nanoparticles showed improved dispersibility within the polymer membrane thereby resulting in absorption of more sulfonic acid groups and shortened interchain separation distance which has been confirmed from XRD analysis. Thus, an interconnected proton conduction pathway which was formed, enhanced the proton conductivity to 0.113 S cm−1. Further, the barrier properties of the silica nanoparticles additionally suppressed the methanol crossover as compared with Nafion, while improving the membrane selectivity (3.65 × 104 S.scm−3).

Exploring H-bonding interaction to enhance proton permeability of an acid-selective membrane

Acid recovery from industrial aqueous streams is an essential process, which commonly requires a cost-effective, energy-efficient, and straightforward approach, such as membrane technology. However, the limited acid/salt separation performance of the membrane materials makes the use of membrane technology very challenging. In this study, we fabricated acid-selective membranes by grafting urea groups and quaternary ammoniums (QA) containing side chains on the hydrophobic poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbones. As confirmed by Nano-scale microscopy and Small-angle X-ray scattering techniques, the intramolecular H-bonding interactions between urea units facilitate self-assembly of the side chains and the formation of continuous acid transport channels. The produced QA clusters also enabled fast anion transportation but reject cations transport, while the H-bonding networks provide a proton conduction pathway to endow high acid/salt selectivity. The resultant acid selective membranes showed a higher H+ dialysis coefficient (0.081 m h−1) and better selectivities for the separation of HCl from FeCl2 aqueous solution (31.1) than the recently reported commercial membranes. Moreover, ten consecutive cycles of acid recovery tests demonstrated excellent operational stability of the representative membrane.