Proton Conductance Pathway Research Articles

Transmission Electron Microscopic Observation of Both Ionomer and Pt Distribution and Their Effects on Cathode Performance for Polymer Electrolyte Fuel Cells

In the electrode of a polymer electrolyte fuel cell (PEFC), the distribution of perfluorosulfonic ionomer on the Pt catalyst supported on carbon black (CB) significantly affects the fuel cell performance. The observation of the distribution of the ionomer and Pt catalyst on the carbon support are important for the improvement of PEFC performance. Recently, the direct observation of the ionomer with transmission electron microscopy (TEM) without damage by the electron beam has become possible due to technological improvements.In this study, four types of cathode catalyst inks were prepared in which the added amount of the perfluorosulfonic ionomer, prepared by AGC, was varied with respect to the CB (I/C ratio; I, weight of fluoro-ionomer; C, weight of graphitized CB (GCB)). The membrane electrode assemblies (MEAs) were made from catalyst layers (CLs), which were decals printed with these inks by the hot-press method. The catalyst powders collected from the cathodes were placed on a TEM grid without pretreatment. The ionomer distribution on GCB-supported Pt catalysts was observed with a high resolution TEM (Hitachi HT7700) with a low acceleration voltage (80 kV).1 The pore distribution of each sample was measured by use of N2 absorption analysis. The current-voltage (I-V) performances of the MEAs were also evaluated, and the effects of the ionomer distribution on the cell performance were investigated. A TEM image of the catalyst collected from the cathode (I/C = 0.8) is shown in Fig. 1a. The amorphous structure of the ionomer is able to be clearly observed on the surfaces of both the GCB and the Pt nanoparticles with the low acceleration voltage, high resolution TEM. A TEM image of the electrode catalyst of I/C = 1.6 sample is shown in Fig. 1b. The amorphous areas increased with increasing ionomer content and were observed more frequently. Agglomerations of Pt particles were also observed in some areas. In a previous study, Pt agglomeration and re-deposition due to sulfonate groups included in the high IEC ionomer were reported.2 These results suggested that the increased acid concentration of the ionomer around the Pt nanoparticles can cause severe Pt agglomeration. The mobilities of both ionomer and Pt particles might be influenced by the surface of the CB, and those on GCB would be expected to be larger than those on CB due to the smooth surface of graphite. The I-V performance of the MEAs is shown in Fig. 2. The performance increased with increasing I/C from 0.4 to 0.8 and 1.2 and then decreased for 1.6. Improvements in current density were considered to be due to increases in the number of proton-conductive pathways due to the ionomer in the CLs. The pore distribution of various the I/C samples were measured by use of N2 absorption analysis. The pore volumes of both nanopores of 2-5 nm and primary pores of 5-10 nm for the I/C = 1.6 cathode were decreased. The performance decrease for I/C = 1.6 is considered to be due to a decrease in the number of oxygen transport paths due to occupation of the pores of the carbon support with the ionomer. References (1) Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources, 315, 179 (2016). (2) R. Shimizu, Y.-C. Park, K. Kakinuma, A. Iiyama, M. Uchida, J. Electrochem. Soc., 165 (6) F3063-F3071 (2018) Figure 1

Enhancement in Proton Conductivity and Thermal Stability in Nafion Membranes Induced by Incorporation of Sulfonated Carbon Nanotubes.

Proton exchange membrane fuel cell (PEMFC) is one of the most promising green power sources, in which perfluorinated sulfonic acid ionomer-based membranes (e.g., Nafion) are widely used. However, the widespread application of PEMFCs is greatly limited by the sharp degradation in electrochemical properties of the proton exchange membranes under high temperature and low humidity conditions. In this work, the high-performance sulfonated carbon nanotubes/Nafion composite membranes (Su-CNTs/Nafion) for the PEMFCs were prepared and the mechanism of the microstructures on the macroscopic properties of membranes was intensively studied. Microstructure evolution in Nafion membranes during water uptake was investigated by positron annihilation lifetime spectroscopy, and results strongly showed that the Su-CNTs or CNTs in Nafion composite membranes significantly reinforced Nafion matrices, which influenced the development of ionic-water clusters in them. Proton conductivities in Su-CNTs/Nafion composite membranes were remarkably enhanced due to the mass formation of proton-conducting pathways (water channels) along the Su-CNTs. In particular, these pathways along Su-CNTs in Su-CNTs/Nafion membranes interconnected the isolated ionic-water clusters at low humidity and resulted in less tortuosity of the water channel network for proton transportation at high humidity. At a high temperature of 135 °C, Su-CNTs/Nafion membranes maintained high proton conductivity because the reinforcement of Su-CNTs on Nafion matrices reduced the evaporation of water molecules from membranes as well as the hydrophilic Su-CNTs were helpful for binding water molecules.

Proton Stabilization and Conduction Pathway in the Matrix Protein M2

The transmembrane channel M2 of Influenza A virus plays a critical role in its lifecycle, and serves primarily to acidify the viral interior for the release of ribonucleoproteins prior to viral-endosomal membrane fusion. Protons enter the channel via a narrow opening from the viral exterior and diffuse to the His37 tetrad, where multiple protonation events ultimately lead to conformational changes in the protein and dissociation of proton to the viral interior. The slow rate of proton transport in M2 suggests that conformational change of the protein is the rate-limiting step of the conduction process. However, it remains unclear whether these conformational changes in the protein are intimately correlated for the conduction of protons. We address this question by correlating conformational changes as measured by PET FCS with proton transport rates of full-length M2 measured using a well-established vesicle flux assay. Furthermore, recent computational models and high-resolution X-ray structures have shown that protons diffuse along Grotthuss water wires that line the inside of the pore leading to the His37 tetrad. While these studies offer insight into the proton conduction pathway through the channel, little is understood about the distribution and dynamics of the water within the pore. Using two-dimensional infrared (2D IR) spectroscopy, we aim to probe the dynamics of water and pore-lining carbonyls in the M2 channel. In applying 2D IR spectroscopy on microcrystals of isotopically labeled M2, we can probe the pH-dependent changes in the distribution and dynamics of the water independent of changes to the population of conformational states of the protein. This methodology will allow us to dissect the effects of pH and the protonation state of the His37 tetrad on the hydrogen-bonding network inside the channel.

Properties of the Voltage-Gated Proton Channel Gating Currents

The voltage-gated proton channel (Hv1) is a dimer of subunits containing four transmembrane domains (S1-S4) which play the double role of harboring the voltage-sensing charges and forming the proton conduction pathway. Some biophysical properties of Hv1, as the coupling between voltage-sensing and permeation pathway opening, the pH dependence of gating, the cooperativity between each subunit in the dimer, and the steps of charge movement during activation and deactivation can be better understood by measuring and characterizing the Hv1 gating currents. However, to measure gating currents, ionic currents needs to be eliminated or greatly reduced, a feat that is particularly challenging for Hv1 for which there is no effective blocker and proton concentration cannot be reduced more than 10 fold. Here, we expressed the low-conducting N264R Hv1 mutant from Ciona intestinalis in Xenopus laevis oocytes and succeeded in recording gating currents membrane patches with both, the monomeric and dimeric channels. In agreement with previous fluorescence measurements (Qiu et al., Neuron 77:288, 2013), the analysis of the gating currents kinetics and charge movement revealed that the ON component has a double exponential decay. Additionally, changes in pH modulate Hv1 gating currents kinetics and Q-V curves. This suggests that the pH sensors in Hv1 are located in the voltage-sensing domain. Finally, in our experimental conditions we observed charge immobilization after activation, which could be an intrinsic characteristic of Hv1 voltage-sensing domain. Supported by CONICYT-PFCHA/Doctorado Nacional/2017-21170395 to E.C., Fondecyt grants ACT 1104, Fondecyt 1160261 to C.G. and 1150273 to R.L.

Design of 3D continuous proton conduction pathway by controlling co-organization behavior of gemini amphiphilic zwitterions and acids

Gemini-type amphiphiles with benzylimidazolium-based zwitterionic head-groups have been newly designed and synthesized. They co-organize into liquid-crystalline ordered states with various acids owing to the nature of the zwitterionic parts to cause ion exchanges with these acids. Their co-organization behaviors can be tuned by various factors including the linker selection between the head-groups, the length of the long alkyl chains, the acid species, and the molar ratio between the zwitterions and acids. By tuning these factors, we have succeeded in preparing amphiphilic zwitterion/acid mixtures that form bicontinuous cubic liquid-crystalline assemblies with a 3D continuous hydrophilic surface. These cubic liquid crystals function as ion transporting matrices whose ionic conductivities are enhanced owing to the presence of a 3D continuous proton conduction pathway along the hydrophilic surface. The present material design would possess a great potential to bring an innovation to the design of proton conductive polymer electrolytes.

A flexible metal–organic framework with a high density of sulfonic acid sites for proton conduction

The design of stable electrolyte materials with high proton conductivity for use in proton exchange membrane fuel cells remains a challenge. Most of the materials explored have good conductivity at high relative humidity (RH), but significantly decreased conductivity at reduced RH. Here we report a chemically stable and structurally flexible metal–organic framework (MOF), BUT-8(Cr)A, possessing a three-dimensional framework structure with one-dimensional channels, in which high-density sulfonic acid (–SO3H) sites arrange on channel surfaces for proton conduction. We propose that its flexible nature, together with its –SO3H sites, could allow BUT-8(Cr)A to self-adapt its framework under different humid environments to ensure smooth proton conduction pathways mediated by water molecules. Relative to other MOFs, BUT-8(Cr)A not only has a high proton conductivity of 1.27 × 10−1 S cm−1 at 100% RH and 80 °C but also maintains moderately high proton conductivity at a wide range of RH and temperature. Proton-conducting metal-organic frameworks (MOFs) could be used as the electrolytes in proton exchange membrane fuel cells but chemically stable materials that perform well at low humidity are still sought. Here the authors prepare a stable, structurally flexible MOF that maintains high proton conductivity under a wide range of humidity.

Solution blown biofunctionalized poly(vinylidene fluoride) nanofibers for application in proton exchange membrane fuel cells

In this study, we prepared a novel type of biofunctionalized poly(vinylidene fluoride) (PVDF) nanofibers. Cysteine, a natural amino acid, was grafted onto the surfaces of PVDF nanofibers, The −SH groups of cysteine were then oxidized to −SO3H. The formation of proton-conducting pathways was induced by the −SO3H and COOH groups of the oxidized cysteine chains on the surfaces of the biofunctionalized PVDF nanofibers. Composite membranes were fabricated by impregnating the biofunctionalized PVDF nanofibers with Nafion. Then, the effects of incorporating nanofibers grafted with different amounts of oxidized cysteine on the thermal stability, water uptake dimensional stability, proton conductivity, and single-cell performance of Nafion composite membranes were investigated. The properties of the composite membranes were superior to those of the Nafion membrane. Furthermore, Nafion/PVDF–Cys-30 exhibited the highest proton conductivity of 0.22Scm−1 (80°C), and the maximum power density of 108.42mWcm−2 which was twice than the values of Nafion 117 membrane (51.2mWcm−2) at 60°C under 100% RH. The introduction of biofunctionalized nanofibers significantly improved cell performance, proton conductivity, dimensional stability, and methanol permeability of the membrane.

Hybrid composite membranes of chitosan/sulfonated polyaniline/silica as polymer electrolyte membrane for fuel cells

A series of novel ionic cross-linked chitosan (CS) based hybrid nanocomposites were prepared by using polyaniline/nano silica (PAni/SiO2) as inorganic filler and sulfuric acid as an ionic cross-linking agent. The CS-PAni/SiO2 nanocomposites show enhanced mechanical properties and improved oxidative stabilities. These nanocomposites can be effectively used as environmental friendly proton exchange membranes. Incorporation of PAni/SiO2 into CS matrix enhances water uptake and facilitates the phase separation which enables the formation of hydrophilic domains and improves the proton transport. Moreover, the doped polyaniline also provides some additional pathways for proton conduction. The membrane containing 3wt% loading of PAni/SiO2 in chitosan (CS-PAni/SiO2-3) exhibits high proton conductivity at 80°C (8.39×10−3Scm−1) in fully hydrated state due to its excellent water retention properties. Moreover, methanol permeability of the ionic cross-linked CS-PAni/SiO2 nanocomposite membranes significantly reduces with the addition of PAni/SiO2 nano particles. The CS-PAni/SiO2-3 composite membrane displays the best overall performance as a polymer electrolyte membrane.

Facile synthesis of metal oxide nanofibers and construction of continuous proton-conducting pathways in SPEEK composite membranes

In this study, a facile approach was used to prepare hydrophilic TiO2 nanofibers (TiNFs) by calcining electrospun polyacrylonitrile nanofibers embedded with titanium precursors. A high-performance proton exchange membrane (PEM) was achieved by incorporating the TiNFs into a sulfonated poly(ether ether ketone) (SPEEK) matrix. The high aspect ratios of the inorganic nanofibers permitted interconnecting of ion-conducting channels, thereby significantly improving proton conductivity in the membrane. In addition, the TiNFs provided tortuous pathways for methanol transport, suppressing the methanol permeability coefficient. The maximum power density of the SPEEK/TiNFs-1.0 composite membrane was 1.4 times higher than that of the pristine SPEEK membrane at 100% relative humidity and 60 °C. Furthermore, the stable fiber skeleton improved the mechanical stability of the composite membrane. The SPEEK/TiNFs-1.0 membrane exhibited superior mechanical properties with the tensile strength of 40.4 MPa and maximum elongation at break of 76%.

Improving proton conduction pathways in di- and triblock copolymer membranes: Branched versus linear side chains.

Phase separation within a series of polymer membranes in the presence of water is studied by dissipative particle dynamics. Each polymer contains hydrophobic A beads and hydrophilic C beads. Three parent architectures are constructed from a backbone composed of connected hydrophobic A beads to which short ([C]), long ([A3C]), or symmetrically branched A5[AC][AC] side chains spring off. Three di-block copolymer derivatives are constructed by covalently bonding an A30 block to each parent architecture. Also three tri-blocks with A15 blocks attached to both ends of each parent architecture are modeled. Monte Carlo tracer diffusion calculations through the water containing pores for 1226 morphologies reveal that water diffusion for parent architectures is slowest and diffusion through the di-blocks is fastest. Furthermore, diffusion increases with side chain length and is highest for branched side chains. This is explained by the increase of water pore size with 〈Nbond〉, which is the average number of bonds that A beads are separated from a nearest C bead. Optimization of 〈Nbond〉 within the amphiphilic parent architecture is expected to be essential in improving proton conduction in polymer electrolyte membranes.

Effect of Imidazole Arrangements on Proton-Conductivity in Metal-Organic Frameworks.

Imidazole molecules were frequently incorporated into porous materials to improve their proton conductivity. To investigate how different arrangements of imidazoles in metal-organic frameworks (MOFs) affect the overall proton conduction, we designed and prepared a MOF-based model system. It includes an Fe-MOF as the blank, an imidazole@Fe-MOF (Im@Fe-MOF) with physically adsorbed imidazole, and an imidazole-Fe-MOF (Im-Fe-MOF), which contains chemically coordinated imidazole molecules. The parent Fe-MOF, synthesized from the exchange of carboxylates in the preformed [Fe3(μ3-O)](carboxylate)6 clusters and multitopic carboxylate ligands, serves as a control. The Im@Fe-MOF was prepared by encapsulating free imidazole molecules into the pores of the Fe-MOF, whereas the Im-Fe-MOF was obtained in situ, in which imidazole ligands coordinate to the metal nodes of the framework. Proton-conductivity analyses revealed that the proton conductivity of Im-Fe-MOF was approximately two orders of magnitude greater than those of Fe-MOF and Im@Fe-MOF at room temperature. The high proton conductivity of 1.21 × 10-2 S cm-1 at 60 °C for Im-Fe-MOF ranks among the highest performing MOFs ever reported. The results of the density functional theory calculations suggest that coordinated imidazole molecules in Im-Fe-MOF provide a greater concentration of protons for proton transportation than do coordinated water molecules in Fe-MOF alone. Besides, Im-Fe-MOF exhibits steadier performance than Im@Fe-MOF does after being washed with water. Our investigation using the above ideal crystalline model system demonstrates that compared to disorderly arranged imidazole molecules in pores, the immobilized imidazole molecules by coordination bonds in the framework are more prone to form proton-conduction pathways and thus perform better and steadier in water-mediated proton conduction.

Controllable proton-conducting pathways via situating polyoxometalates in targeting pores of a metal–organic framework

Herein, two types of proton-conducting pathways with zigzag and linear profiles in MIL-101 were formed by loading Keggin-type polyoxometalate H3PW12O40 into larger and smaller cages, respectively.

Modification of Nafion membrane with biofunctional SiO2 nanofiber for proton exchange membrane fuel cells

Abstract Proton currents are an integral part of the most important energy-converting structures in biology. We prepared a new type of bioinspired Nafion (Bio-Nafion) membrane composited of biofunctional SiO2 (Bio-SiO2) nanofiber and Nafion matrix. SiO2 nanofibers were prepared by electrospinning silica sol prepared from tetraethyl orthosilicate. Meanwhile, Bio-SiO2 nanofibers were synthesized by immobilizing amino acids (cysteine, serine, lysine, and glycine) on SiO2 nanofibers, which acted as efficient proton-conducting pathways that involved numerous H+ transport sites. In our study, the SiO2 nanofibers biofunctionalized with cysteine were further oxidized, and the composite membranes were designated as Nafion-Cys, Nafion-Lys, Nafion-Ser, and Nafion-Gly, respectively. We then investigated the different polar groups ( SO3H, OH, and NH2) of the amino acids that contributed to membrane properties of thermal stability, water uptake (WU), dimensional stability, proton conductivity, and methanol permeability. Nafion-Cys exhibited the highest proton conductivity of 0.2424 S/cm (80 °C). Nafion-Gly showed the lowest proton conductivity and WU because glycine contains the least number of hydrophilic groups among the amino acids. Overall, the introduction of Bio-SiO2 nanofiber to composite membranes significantly improved proton conductivity, dimensional stability, and methanol permeability.

The Mg2+-containing Water Cluster of Mammalian Cytochrome c Oxidase Collects Four Pumping Proton Equivalents in Each Catalytic Cycle

Bovine heart cytochrome c oxidase (CcO) pumps four proton equivalents per catalytic cycle through the H-pathway, a proton-conducting pathway, which includes a hydrogen bond network and a water channel operating in tandem. Protons are transferred by H3O+ through the water channel from the N-side into the hydrogen bond network, where they are pumped to the P-side by electrostatic repulsion between protons and net positive charges created at heme a as a result of electron donation to O2 bound to heme a3 To block backward proton movement, the water channel remains closed after O2 binding until the sequential four-proton pumping process is complete. Thus, the hydrogen bond network must collect four proton equivalents before O2 binding. However, a region with the capacity to accept four proton equivalents was not discernable in the x-ray structures of the hydrogen bond network. The present x-ray structures of oxidized/reduced bovine CcO are improved from 1.8/1.9 to 1.5/1.6 Å resolution, increasing the structural information by 1.7/1.6 times and revealing that a large water cluster, which includes a Mg2+ ion, is linked to the H-pathway. The cluster contains enough proton acceptor groups to retain four proton equivalents. The redox-coupled x-ray structural changes in Glu198, which bridges the Mg2+ and CuA (the initial electron acceptor from cytochrome c) sites, suggest that the CuA-Glu198-Mg2+ system drives redox-coupled transfer of protons pooled in the water cluster to the H-pathway. Thus, these x-ray structures indicate that the Mg2+-containing water cluster is the crucial structural element providing the effective proton pumping in bovine CcO.

ChemInform Abstract: Hydrated Proton‐Conductive Metal—Organic Frameworks

AbstractReview: [37 refs.

1000-fold enhancement in proton conductivity of a MOF using post-synthetically anchored proton transporters.

Pyridinol, a coordinating zwitter-ionic species serves as stoichiometrically loadable and non-leachable proton carrier. The partial replacement of the pyridinol by stronger hydrogen bonding, coordinating guest, ethylene glycol (EG), offers 1000-fold enhancement in conductivity (10−6 to 10−3 Scm−1) with record low activation energy (0.11 eV). Atomic modeling coupled with 13C-SSNMR provides insights into the potential proton conduction pathway functionalized with post-synthetically anchored dynamic proton transporting EG moieties.

Hydrated Proton-Conductive Metal-Organic Frameworks.

Recent investigations into proton conduction in metal-organic frameworks (MOFs) indicate that MOFs are promising materials as a new class of proton conductors. Hydrated proton-conductive MOFs show not only high proton conductivity of approximately 10-2 S cm-1 , which is comparable to that of a practical organic polymer, but also structural visibility of proton-conducting pathways inside the materials owing to their high crystallinity. Herein, studies on the design, synthesis, and proton-conductive properties of MOFs with hydrated proton-conductive systems are introduced.

Preparations of an inorganic-framework proton exchange nanochannel membrane

In this work, a proton exchange membrane composed of straight and aligned proton conducting nanochannels is developed. Preparation of the membrane involves the surface sol-gel method assisted with a through-hole anodic aluminum oxide (AAO) template to form the framework of the PEM nanochannels. A monomolecular layer (SO3H(CH2)3Si(OCH3)3) is subsequently added onto the inner surfaces of the nanochannels to shape a proton-conducting pathway. Straight nanochannels exhibit long range order morphology, contributing to a substantial improvement in the proton mobility and subsequently proton conductivity. In addition, the nanochannel size can be altered by changing the surface sol-gel condition, allowing control of the active species/charge carrier selectivity via pore size exclusion. The proton conductivity of the nanochannel membrane is reported as high as 11.3 mS cm−1 at 70 °C with a low activation energy of 0.21 eV (20.4 kJ mol−1). First-principle calculations reveal that the activation energy for proton transfer is impressively low (0.06 eV and 0.07 eV) with the assistance of water molecules.

A new strategy for designing high-performance sulfonated poly(ether ether ketone) polymer electrolyte membranes using inorganic proton conductor-functionalized carbon nanotubes

Abstract Remarkable progress has been made on the use of polymer electrolyte membranes (PEMs) for renewable-energy-related research. In particular, carbon nanotubes (CNTs) have emerged as versatile nanomaterials to modify PEMs. However, the inert ionic conduction ability and possible short-circuiting risk are the two major obstacles to their further development. In this work, CNTs are firstly functionalized with an inorganic proton conductor, boron phosphate (BPO 4 ), using a facile polydopamine-assisted sol-gel method to yield BPO 4 @CNTs. This new additive is then used to modify sulfonated poly(ether ether ketone) (SPEEK). Polydopamine coating layer can act as an extraordinary glue to homogeneously adhere BPO 4 nanoparticles on CNTs, thereby not only reducing the risk of short-circuiting, but also fabricating new proton-conducting pathways in the composite membranes. A comprehensive characterization reveals that the thermal stability, tensile properties, and dimensional stability of PEMs are significantly improved. Compared with pure SPEEK, the proton conductivity of SPEEK/BPO 4 @CNTs-2 is improved by 45% and 150% at 20 °C and at 80 °C, respectively. Furthermore, the H 2 /O 2 cell performance of SPEEK/BPO 4 @CNTs-2 membrane exhibits a peak power density of 340.7 mW cm −2 at 70 °C, which is significantly better than that of pure SPEEK (254.2 mW cm −2 ), demonstrating the great potential of proton conductors-functionalized CNTs in PEMs.

Water Diffusion Dependence on Amphiphilic Block Design in (Amphiphilic-Hydrophobic) Diblock Copolymer Membranes.

Polyelectrolyte membranes (PEMs) are applied in polyelectrolyte fuel cells (PEFC). The proton conductive pathways within PEMs are provided by nanometer-sized water containing pores. Large-scale application of PEFC requires the production of low-cost membranes with high proton conductivity and therefore good connected pore networks. Pore network formation within four alternative model diblock (hydrophobic_amphiphilic) copolymers in the presence of water is studied by dissipative particle dynamics. Each hydrophobic block contains 50 consecutively connected hydrophobic (A) fragments, and amphiphilic blocks contain 40 hydrophobic A beads and 10 hydrophilic C beads. For one amphiphilic block the C beads are distributed uniformly along the backbone. For the other architectures C beads are located at the end of the side chains attached at regular intervals along the backbone. Water diffusion through the pores is modeled by Monte Carlo tracer diffusion through mapped morphologies. Diffusion is highest for the grafted architectures and increases with increase of length of the side chains. A consistent picture emerges in which diffusion strongly increases with the value of ⟨Nbond⟩ within the amphiphilic block, where ⟨Nbond⟩ is the average number of bonds between hydrophobic A beads and the nearest C bead.